http://arxiv.org/abs/2304.09219

A blue depression is found in the spectra of M dwarfs from 4000 to 4500A. This depression shows an increase toward lower temperatures though is particularly sensitive to gravity and metallicity. It is the single most sensitive feature in the optical spectra of M dwarfs. The depression appears as centered on the neutral calcium resonance line at 4227A and leads to nearby features being weaker by about two orders of magnitude than predicted. We consider a variety of possible causes for the depression including temperature, gravity, metallicity, dust, damping constants, and atmospheric stratification. We also consider relevant molecular opacities which might be the cause identifying AlH, SiH, and NaH in the spectral region. However, none of these solutions are satisfactory. In the absence of a more accurate determination of the broadening of the calcium line perturbed by molecular hydrogen, we find a promising empirical fit using a modified Lorentzian line profile for the calcium resonance line. Such fits provide a simplistic line-broadening description for this calcium resonance line and potentially other un-modelled resonance lines in cool high-pressure atmospheres. Thus we claim the most plausible cause of the blue depression in the optical spectra of M dwarfs is a lack of appropriate treatment of line broadening for atomic calcium. The broad wings of the calcium resonance line develop at temperatures below about 4000K and are analogous to the neutral sodium and potassium features which dominate the red optical spectra of L dwarfs.

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H. Jones, Y. Pavlenko, Y. Lyubchik, et. al.
Thu, 20 Apr 23
49/57

Comments: Accepted in MNRAS

http://arxiv.org/abs/2304.08578

We report results of an observing campaign to study the dwarf M flare star EV Lacertae. Between October 2021 and January 2022 we obtained concurrent B band photometry and low resolution spectroscopy of EV Lac on 39 occasions during 10 of which we observed flares with amplitude greater than 0.1 magnitude. Spectra were calibrated in absolute flux using concurrent photometry and flare-only spectra obtained by subtracting mean quiescent spectra. We measured B band flare energies between Log E = 30.8 and 32.6 erg. In the brightest flares we measured temporal development of flare flux in H I and He I emission lines and in the adjacent continuum and found that flux in the continuum subsided more rapidly than in the emission lines. Although our time resolution was limited, in our brightest flare we saw flux in the continuum clearly peaking before flux in the emission lines. We observed a progressive decrease in flare energy from H\b{eta} to H{\delta}. On average we found 37% of B band flare energy appeared in the H\b{eta} to H{\epsilon} emission lines with the remainder contributing to a rise in continuum flux. We measured black-body temperatures for the brightest flares between 10,500 +- 700 K and 19,500 +- 500 K and found a linear relationship between flare temperature and continuum flux at 4170 {\AA}. Balmer lines in flare-only spectra were well fitted by Gaussian profiles with some evidence of additional short-lived blue-shifted emission at the flare peak.

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D. Boyd, R. Buchheim, S. Curry, et. al.
Wed, 19 Apr 23
43/58

Comments: 12 pages, 13 figures, accepted for publication in the Journal of the AAVSO

http://arxiv.org/abs/2304.07397

We present measurements of the interferometrically-resolved binary star system 12 Com and the single giant star 31 Com in the cluster Coma Berenices. 12 Com is a double-lined spectroscopic binary system consisting of a G7 giant and an A3 dwarf at the cluster turnoff. Using an extensive radial velocity dataset and interferometric measurements from PTI and the CHARA array, we measured masses $M_1 =2.64 \pm 0.07 M_\odot$ and $M_2 =2.10 \pm 0.03 M_\odot$. Interferometry also allows us to resolve the giant, and measure its size as $R_1 = 9.12 \pm 0.12 \pm 0.01 R_\odot$. With the measured masses and radii, we find an age of $533 \pm 41 \pm 42$ Myr. For comparison, we measure the radius of 31 Com to be $8.36 \pm 0.15 R_\odot$. Based on the photometry and radius measurements, 12 Com A is likely the most evolved bright star in the cluster, large enough to be in the red giant phase, but too small to have core helium burning. Simultaneous knowledge of 12 Com A’s mass and photometry puts strong constraints on convective core overshooting during the main sequence phase, which in turn reduces systematic uncertainties in the age. Increased precision in measuring this system also improves our knowledge of the progenitor of the cluster white dwarf WD1216+260.

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R. Lam, E. Sandquist, G. Schaefer, et. al.
Tue, 18 Apr 23
17/80

Comments: N/A

http://arxiv.org/abs/2304.07970

Swirls are ubiquitous in the solar atmosphere. They are believed to be related to the excitation of different modes of magnetohydrodynamic waves and pulses, as well as spicules. However, statistical studies of their collective behaviour are rare. In this paper, we aim to study the collective, as well as the behaviour of individual photospheric and chromospheric swirls detected by the automated swirl detection algorithm (ASDA) from observations obtained by the Swedish 1-m Solar Telescope and the Hinode satellite. Detailed analysis of six different parameters of photospheric and chromospheric swirls is performed employing the wavelet analysis. Two clusters of periods with significant wavelet power, one from $3-8$ minutes and the other from $10-14$ minutes, have been found. The former coincides with the dominant period of the global $p$-mode spectrum. Wavelet and Fast Fourier Transform (FFT) analysis of example swirls also reveals similar periods. These results suggest that global $p$-modes might be important for triggering photospheric and thus chromospheric swirls. A novel scenario of global $p$-modes providing energy and mass fluxes to the upper solar atmosphere via generating swirls, Alfv\’en pulses and spicules is then proposed.

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J. Liu, D. Jess, R. Erdélyi, et. al.
Tue, 18 Apr 23
26/80

Comments: 8 figures and 3 tables, to be published in A&A

http://arxiv.org/abs/2304.07649

The solar magnetic fields emerging from the photosphere into the chromosphere and corona are comprised of a combination of “closed” and “open” fields. The closed magnetic field lines are defined as those having both ends rooted in the solar surface, while the open field lines are those having one end extending out into interplanetary space and the other rooted at the Sun’s surface. Since the early 2000’s, the amount of total unsigned open magnetic flux estimated by coronal models have been in significant disagreement with in situ spacecraft observations, especially during solar maximum. Estimates of total open unsigned magnetic flux using coronal hole observations (e.g., using extreme ultraviolet (EUV) or Helium (He) I) are in general agreement with the coronal model results and thus show similar disagreements with in situ observations. While several possible sources producing these discrepancies have been postulated over the years, there is still no clear resolution to the problem. This paper provides a brief overview of the problem and summarizes some proposed explanations for the discrepancies. In addition, two different ways of estimating the total unsigned open magnetic flux are presented, utilizing the Wang-Sheeley-Arge (WSA) model, and one of the methods produce surprisingly good agreement with in situ observations. The findings presented here suggest that active regions residing near the boundaries of mid-latitude coronal holes are the probable source of the missing open flux. This explanation also brings in line many of the seemly contradictory facts that have made resolving this problem so difficult.

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C. Arge, A. Leisner, S. Wallace, et. al.
Tue, 18 Apr 23
41/80

Comments: 14 pages, 5 figures

http://arxiv.org/abs/2304.08089

In this work, we present Stellar Spectra Factory (SSF), a tool to generate empirical-based stellar spectra from arbitrary stellar atmospheric parameters. The relative flux-calibrated empirical spectra can be predicted by SSF given arbitrary effective temperature, surface gravity, and metallicity. SSF constructs the interpolation approach based on the SLAM, using ATLAS-A library as the training dataset. SSF is composed of 4 data-driven sub-models to predict empirical stellar spectra. SSF-N can generate spectra from A to K type and some M giant stars, covering 3700 < Teff < 8700 K, 0 < logg < 6 dex, and -1.5 < [M/H] < 0.5 dex. SSF-gM is mainly used to predict M giant spectra with 3520 < Teff < 4000K and -1.5 < [M/H] < 0.4 dex. SSF-dM is for generating M dwarf spectra with 3295 < Teff < 4040K, -1.0 < [M/H] < 0.1 dex. And SSF-B can predict B-type spectra with 9000 < Teff < 24000K and -5.2< MG < 1.5 mag. The accuracy of the predicted spectra is validated by comparing the flux of predicted spectra to those with same stellar parameters selected from the known spectral libraries, MILES and MaStar. The averaged difference of flux over optical wavelength between the predicted spectra and the corresponding ones in MILES and MaStar is less than 5%. More verification is conducted between the magnitudes calculated from the integration of the predicted spectra and the observations in PS1 and APASS bands with the same stellar parameters. No significant systematic difference is found between the predicted spectra and the photomatric observations. The uncertainty is 0.08mag in r band for SSF-gM when comparing with the stars with the same stellar parameters selected from PS1. And the uncertainty becomes 0.31mag in i band for SSF-dM when comparing with the stars with the same stellar parameters selected from APASS.

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W. Ji, C. Liu and B. Zhang
Tue, 18 Apr 23
46/80

Comments: 16 pages, 13 figures

http://arxiv.org/abs/2304.07695

We analyzed Asteroid Terrestrial-impact Last Alert System (ATLAS), Zwicky Transient Facility (ZTF) and All-Sky Automated Survey for Supernovae (ASAS-SN) data of MASTER OT J055845.55+391533.4 and found that this object repeats superoutburst with a dip in the middle of the outburst followed by long and sometimes oscillating rebrightening, just like a WZ Sge-type dwarf nova or an AM CVn-type object. The mean supercycle was 298(8) d, too short for a WZ Sge star, but with only a few normal outbursts. We also observed the 2023 February-March superoutburst and established the superhump period of 0.05509(2) d. This period appears to exclude the possibility of an AM CVn star. Although the 2023 observations could not detect superhumps after the dip, the 2014, 2016 and 2021 data seem to suggest that low-amplitude superhumps were present during the rebrightening phase. We note that a dip during a superoutburst is a feature common to the unusual SU UMa-type dwarf nova MASTER OT J172758.09+380021.5 during some of its superoutbursts. These objects may comprise a new class of rebrightening phenomenon in SU UMa-type dwarf novae.

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T. Kato, H. Itoh, T. Vanmunster, et. al.
Tue, 18 Apr 23
64/80

Comments: 15 pages, 8 figures, VSOLJ Variable Star Bulletin No. 113

http://arxiv.org/abs/2304.07158

Gaia EDR3 has provided unprecedented data that generate a lot of interest in the astrophysical community, despite the fact that systematics affect the reported parallaxes at the level of ~ 10 muas. Independent distance measurements are available from asteroseismology of red-giant stars with measurable parallaxes, whose magnitude and colour ranges more closely reflect those of other stars of interest. In this paper, we determine distances to nearly 12,500 red-giant branch and red clump stars observed by Kepler, K2, and TESS. This is done via a grid-based modelling method, where global asteroseismic observables, constraints on the photospheric chemical composition, and on the unreddened photometry are used as observational inputs. This large catalogue of asteroseismic distances allows us to provide a first comparison with Gaia EDR3 parallaxes. Offset values estimated with asteroseismology show no clear trend with ecliptic latitude or magnitude, and the trend whereby they increase (in absolute terms) as we move towards redder colours is dominated by the brightest stars. The correction model proposed by Lindegren et al. (2021) is not suitable for all the fields considered in this study. We find a good agreement between asteroseismic results and model predictions of the red clump magnitude. We discuss possible trends with the Gaia scan law statistics, and show that two magnitude regimes exist where either asteroseismology or Gaia provides the best precision in parallax.

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S. Khan, A. Miglio, E. Willett, et. al.
Mon, 17 Apr 23
11/51

Comments: 11 pages, 8 figures, Accepted for publication in A&A

http://arxiv.org/abs/2304.06770

Asteroseismology has been used extensively in recent years to study the interior structure and physical processes of main sequence stars. We consider prospects for using pressure modes (p-modes) near the frequency of maximum oscillation power to probe the structure of the near-core layers of main sequence stars with convective cores by constructing stellar model tracks. Within our mass range of interest, the inner turning point of p modes as determined by the JWKB approximation evolves in two distinct phases during the main sequence, implying a sudden loss of near-core sensitivity during the discontinuous transition between the two phases. However, we also employ non-JWKB asymptotic analysis to derive a contrasting set of expressions for the effects that these structural properties will have on the mode frequencies, which do not encode any such transition. We show analytically that a sufficiently near-core perturbation to the stellar structure results in non-oscillatory, degree-dependent perturbations to the star’s oscillation mode frequencies, contrasting with the case of an outer glitch. We also demonstrate numerically that these near-core acoustic glitches exhibit strong angular degree dependence, even at low degree, agreeing with the non-JWKB analysis, rather than the degree-independent oscillations which emerge from JWKB analyses. These properties have important implications for using p-modes to study near-core mixing processes for intermediate-mass stars on the main sequence, as well as for the interpretation of near-center acoustic glitches in other astrophysical configurations, such as red giants.

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C. Lindsay, J. Ong and S. Basu
Mon, 17 Apr 23
18/51

Comments: 14 pages, 5 figures, Accepted for publication in ApJ: April 12, 2023

http://arxiv.org/abs/2304.07192

In helioseismic studies, an observational parameter of primary concern is the P-angle, the angle along which lies the solar axis of rotation for a given image. For the six observing sites employed by The Global Oscillation Network Group (GONG), this angle acts additionally as a marker of relative image orientation, allowing concurrent images to be precisely aligned and merged to provide the highest possible quality data. In this report, we present and investigate two methods of determining the P-angle via the rotational signature embedded in solar Dopplergram images by examining the large-scale structure of the observed velocity field. As with other studies, we find that the Dopplergram produces a time-varying ‘P-angle’ signature according to the presentation of various physical phenomena across the solar surface, but with the potential for sub-degree identification of the axis of rotation. However, close agreement between separate P-angle-finding techniques also reveals current limitations to P-angle determination that are imposed by the calibration state of the GONG-site Dopplergrams, leaving these P-angle-finding methods for GONG with errors on the scale of less than a degree between two site.

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A. Hughes, I. Gonzalez-Hernandez, S. McManus, et. al.
Mon, 17 Apr 23
22/51

Comments: 36 pages, 32 figures

http://arxiv.org/abs/2304.07038

The use of the coronal approximation to model line emission from the solar transition region has led to discrepancies with observations over many years, particularly for Li- and Na-like ions. Studies have shown that a number of atomic processes are required to improve the modelling for this region, including the effects of high densities, solar radiation and charge transfer on ion formation. Other non-equilibrium processes, such as time dependent ionisation and radiative transfer, are also expected to play a role. A set of models which include the three relevant atomic processes listed above in ionisation equilibrium has recently been built. These new results cover the main elements observed in the transition region. To assess the effectiveness of the results, the present work predicts spectral line intensities using differential emission measure modelling. Although limited in some respects, this differential emission measure modelling does give a good indication of the impact of the new atomic calculations. The results are compared to predictions of the coronal approximation and to observations of the average, quiet Sun from published literature. Significant improvements are seen for the line emission from Li- and Na-like ions, inter-combination lines and many other lines. From this study, an assessment is made of how far down into the solar atmosphere the coronal approximation can be applied, and the range over which the new atomic models are valid.

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R. Dufresne, G. Zanna and H. Mason
Mon, 17 Apr 23
23/51

Comments: Accepted by MNRAS, 16 pages, 3 figures

http://arxiv.org/abs/2304.07115

The star CQ Tau belongs to the family of UX Ori type stars. It has very complex photometric behavior and complex structure of the circumstellar environment. In our paper we constructed the historical 125 years light curve of this star basing on the published photometric observations. It follows that besides a random component characteristic of UX Ori type stars, the large amplitude periodic component with the 10 year period is also present. Its existence was suspected earlier in [11]. New observations confirm its reality. It points to an existence of the second component close to the star. The density waves and matter flows caused by the companion motion lead to periodic changes in the circumstellar extinction and brightness of the star. This result is discussed in context of the recent observations of CQ Tau with high angular resolution.

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V. Grinin, L. Tambovtseva, O. Barsunova, et. al.
Mon, 17 Apr 23
45/51

Comments: 7 pages, 4 figures, accepted by Astrophysics

http://arxiv.org/abs/2304.06355

Common envelope events have been associated with the formation of a planetary nebulae since its proposition more than forty five years ago. However, until recently there have been doubts as to whether a common envelope while the donor is ascending the red giant branch, rather than the subsequent asymptotic red giant branch, would result in a planetary nebula. There is now strong theoretical and observational evidence to suggest that some planetary nebulae are, indeed, the products of common envelope phases which occurred while the nebular progenitor was on the red giant branch. The characterisation of these systems is challenging but has the potential to reveal much about the common envelope — a critical evolutionary phase in the formation of a plethora of interesting astrophysical phenomena.

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D. Jones, T. Hillwig and N. Reindl
Fri, 14 Apr 23
6/64

Comments: 6 pages, to appear in Highlights of Spanish Astrophysics XI, Proceedings of the XV Scientific Meeting of the Spanish Astronomical Society held on September 4 0 9, 2022, in La Laguna, Spain. M. Manteiga, L. Bellot, P. Benavidez, A. de Lorenzo-Caceres, M. A. Fuente, M. J. Martinez, M. Vazquez- Acosta, C. Dafonte (eds.), 2023

http://arxiv.org/abs/2304.06301

Relying on precise observations for the CPD-54 810 binary system, we investigate the robustness of the estimated age and convective core overshooting for a system with both stars on the main sequence (MS). […] We adopt the SCEPtER pipeline, based on grids of stellar models computed for a different initial chemical composition and convective core overshooting efficiency. The base fit suggests a common age of $3.02 \pm 0.15$ Gyr, in agreement with recent literature. This estimated convective core overshooting parameter is $\beta = 0.09 \pm 0.01$, with a corresponding convective core mass $M_c = 0.059^{+0.017}{-0.021}$ $M{\odot}$. The robustness of these estimates were tested assuming a narrow constraint on the helium-to-metal enrichment ratio. The chemical solution of the system changes, but the age and the overshooting parameter are almost unchanged ($3.08^{+0.17}_{-0.14}$ Gyr and $0.09 \pm 0.01$). In a further test, we halved the uncertainty as to the effective temperature of both stars and again the estimated parameter shows only small variations ($3.02 \pm 0.12$ Gyr and $0.09 \pm 0.01$). This low variability suggests that the age of the system with both stars in the MS can be reliably estimated at a 5\% level, but it also indicates that the power of the investigation is probably low. […] Despite the great increase in the observational constraints’ precision, the results support the conclusions of previous theoretical works on the stellar parameter calibration with double MS star binary systems.

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G. Valle, M. Dell’Omodarme, P. Moroni, et. al.
Fri, 14 Apr 23
51/64

Comments: Accepted for publication in A&A. Abstract abridged

http://arxiv.org/abs/2304.05926

Inertial modes have been observed on the Sun at low longitudinal wavenumbers. These modes probe the dynamics and structure of the solar convection zone down to the tachocline. While linear analysis allows the complex eigenfrequencies and eigenfunctions of these modes to be computed, it gives no information about their excitation nor about their amplitudes.
We tested the hypothesis that solar inertial modes are stochastically excited by the turbulent motions entailed by convection. We have developed a theoretical formalism where the turbulent velocity fluctuations provide the mechanical work necessary to excite the modes. The modes are described by means of a 2D linear wave equation, relevant for the quasi-toroidal modes observed on the Sun, with a source term, under the beta plane approximation. Latitudinal differential rotation is included in the form of a parabolic profile that approximates the solar differential rotation at low and mid latitudes.
We obtain synthetic power spectra for the wave’s latitudinal velocity, longitudinal velocity, and radial vorticity, with azimuthal orders between 1 and 20. The synthetic power spectra contain the classical equatorial Rossby modes, as well as a rich spectrum of additional modes. The mode amplitudes are found to be of the same order of magnitude as observed on the Sun (~ 1 m/s). There is a qualitative transition between low and high azimuthal orders: the power spectra for m < 5 show modes that are clearly resolved in frequency space, while the power spectra for m > 5 display regions of excess power that consist of many overlapping modes.
The general agreement between the predicted and observed inertial mode amplitudes supports the assumption of stochastic excitation by turbulent convection. Our work shows that the power spectra are not easily separable into individual modes, thus complicated the interpretation of the observations.

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J. Philidet and L. Gizon
Thu, 13 Apr 23
5/59

Comments: 19 pages, accepted for publication in Astronomy & Astrophysics

http://arxiv.org/abs/2304.05630

Using a sample of 361 nearby brown dwarfs, we have searched for 4.6$\mu$m variability indicative of large-scale rotational modulations or large-scale long-term changes on timescales of over 10 years. Our findings show no statistically significant variability in \textit{Spitzer} ch2 or \textit{WISE} W2 photometry. For \textit{Spitzer} the ch2 1$\sigma$ limits are $\sim$8 mmag for objects at 11.5 mag and $\sim$22 mmag for objects at 16 mag. This corresponds to no variability above 4.5$\%$ at 11.5 mag and 12.5$\%$ at 16 mag. We conclude that highly variable brown dwarfs, at least two previously published examples of which have been shown to have 4.6$\mu$m variability above 80 mmag, are very rare. While analyzing the data, we also developed a new technique for identifying brown dwarfs binary candidates in \textit{Spitzer} data. We find that known binaries have IRAC ch2 PRF (point response function) flux measurements that are consistently dimmer than aperture flux measurements. We have identified 59 objects that exhibit such PRF versus apertures flux differences and are thus excellent binary brown dwarf candidates.

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H. Brooks, J. Kirkpatrick, A. Meisner, et. al.
Thu, 13 Apr 23
9/59

Comments: 18 pages, 11 figures, accepted for publication in AJ, and 1 figure set of 361 figures with the example being figure 8

http://arxiv.org/abs/2304.05692

We present light curves and flares from a seven day, multi-wavelength observational campaign of AU Mic, a young and active dM1e star with exoplanets and a debris disk. We report on 73 unique flares between the X-ray to optical data. We use high-time resolution NUV photometry and soft X-ray (SXR) data from XMM-Newton to study the empirical Neupert effect, which correlates the gradual and impulsive phase flaring emissions. We find that 65% (30 of 46) flares do not follow the Neupert effect, which is three times more excursions than seen in solar flares, and propose a four part Neupert effect classification (Neupert, Quasi-Neupert, Non-Neupert I & II) to explain the multi-wavelength responses. While the SXR emission generally lags behind the NUV as expected from the chromospheric evaporation flare models, the Neupert effect is more prevalent in larger, more impulsive flares. Preliminary flaring rate analysis with X-ray and U-band data suggests that previously estimated energy ratios hold for a collection of flares observed over the same time period, but not necessarily for an individual, multi-wavelength flare. These results imply that one model cannot explain all stellar flares and care should be taken when extrapolating between wavelength regimes. Future work will expand wavelength coverage using radio data to constrain the nonthermal empirical and theoretical Neupert effects to better refine models and bridge the gap between stellar and solar flare physics.

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I. Tristan, Y. Notsu, A. Kowalski, et. al.
Thu, 13 Apr 23
12/59

Comments: 46 pages, 18 figures, 15 tables. Accepted for publication in The Astrophysical Journal

http://arxiv.org/abs/2304.05499

A Ribbon of enhanced energetic neutral atom (ENA) emissions was discovered by the Interstellar Boundary Explorer (IBEX) in 2009, redefining our understanding of the heliosphere boundaries and the physical processes occurring at the interstellar interface. The Ribbon signal is intertwined with that of a globally distributed flux (GDF) that spans the entire sky. To a certain extent, Ribbon separation methods enabled examining its evolution independent of the underlying GDF. Observations over a full solar cycle revealed the Ribbon’s evolving nature, with intensity variations closely tracking those of the solar wind (SW) structure after a few years delay accounting for the SW-ENA recycling process. In this work, we examine the Ribbon structure, namely, its ENA fluxes, angular extent, width, and circularity properties for two years, 2009 and 2019, representative of the declining phases of two adjacent solar cycles. We find that, (i) the Ribbon ENA fluxes have recovered in the nose direction and south of it down to ~ 25{\deg} (for energies below 1.7 keV) and not at mid and high ecliptic latitudes; (ii) The Ribbon width exhibits significant variability as a function of azimuthal angle; (iii) Circularity analysis suggests that the 2019 Ribbon exhibits a statistically consistent radius with that in 2009. The Ribbon’s partial recovery is aligned with the consensus of a heliosphere with its closest point being southward of the nose region. The large variability of the Ribbon width as a function of Azimuth in 2019 compared to 2009 is likely indicative of small-scale processes within the Ribbon.

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M. Dayeh, E. Zirnstein, P. Swaczyna, et. al.
Thu, 13 Apr 23
25/59

Comments: 5 figures

http://arxiv.org/abs/2304.04973

We analyzed All-Sky Automated Survey for Supernovae (ASAS-SN), Asteroid Terrestrial-impact Last Alert System (ATLAS) and Transiting Exoplanet Survey Satellite (TESS) observations of CM Mic and found that this object belongs to a small group of ER UMa stars showing standstills. In addition to typical ER UMa-type cycles, the object showed standstills between 2017 and 2019 July, and in 2022. The supercycles varied between 49 and 83 d. In 2015, the object showed outbursts with a cycle length of ~35 d. An analysis of TESS observations during the 2020 July outburst detected superhumps with a mean period of 0.080251(6) d (value after the full development of superhumps). We also studied other ER UMa stars showing standstills mainly using Zwicky Transient Facility (ZTF) data. DDE 48, MGAB-V728 and ZTF18abmpkbj mostly showed ER UMa-type supercycles but showed one or two standstills. MGAB-V3488 was mostly in ER UMa states with short (~25 d) supercycles in 2020-2022 similar to RZ LMi. This object also showed long standstills. PS1-3PI J181732.65+101954.6 showed ER UMa-type supercycles up to 2020 May and entered a long standstill. ZTF18abncpgs showed standstills most of the time, but also showed ER UMa-type supercycles occasionally between standstills. ZTF19aarsljl is a likely member of this group. MGAB-V284 showed a pattern similar to ER UMa stars showing standstills but with a longer time-scale of normal outbursts. This object seems to be an ER UMa star with standstills above the period gap. None of the objects we studied showed a superoutburst arising from a long standstill, as recorded in NY Ser in 2018, although the 2019 June-July superoutburst of PS1-3PI J181732.65+101954.6 might have been an exception.

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T. Kato and N. Kojiguchi
Wed, 12 Apr 23
40/45

Comments: 33 pages, 20 figures, VSOLJ Variable Star Bulletin No. 112

http://arxiv.org/abs/2304.03909

To address the problem of the low accuracy of transverse velocity field measurements for small targets in high-resolution solar images, we proposed a novel velocity field measurement method for high-resolution solar images based on PWCNet. This method transforms the transverse velocity field measurements into an optical flow field prediction problem. We evaluated the performance of the proposed method using the Ha and TiO datasets obtained from New Vacuum Solar Telescope (NVST) observations. The experimental results show that our method effectively predicts the optical flow of small targets in images compared with several typical machine- and deep-learning methods. On the Ha dataset, the proposed method improves the image structure similarity from 0.9182 to 0.9587 and reduces the mean of residuals from 24.9931 to 15.2818; on the TiO dataset, the proposed method improves the image structure similarity from 0.9289 to 0.9628 and reduces the mean of residuals from 25.9908 to 17.0194. The optical flow predicted using the proposed method can provide accurate data for the atmospheric motion information of solar images. The code implementing the proposed method is available on https://github.com/lygmsy123/transverse-velocity-field-measurement.

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Z. Shang, S. Mu, K. Ji, et. al.
Tue, 11 Apr 23
1/63

Comments: 14 pages, 10 figures, 4 tables. Accepted for publication in Research in Astronomy and Astrophysics

http://arxiv.org/abs/2304.03466

Power-law distributions have been studied as a significant characteristic of non-linear dissipative systems. Since discovering the power-law distribution of solar flares that was later extended to nano-flares and stellar flares, it has been widely accepted that different scales of flares share the same physical process. Here, we present the newly developed Semi-Automated Jet Identification Algorithm (SAJIA) and its application for detecting more than 1200 off-limb solar jets during Solar Cycle 24. Power-law distributions have been revealed between the intensity/energy and frequency of these events, with indices found to be analogous to those for flares and coronal mass ejections (CMEs). These jets are also found to be spatially and temporally modulated by the solar cycle forming a butterfly diagram in their latitudinal-temporal evolution, experiencing quasi-annual oscillations in their analysed properties, and very likely gathering in certain active longitudinal belts. Our results show that coronal jets display the same nonlinear behaviour as that observed in flares and CMEs, in solar and stellar atmospheres, strongly suggesting that they result from the same nonlinear statistics of scale-free processes as their counterparts in different scales of eruptive events. Although these jets, like flares and other large-scale dynamic phenomena, are found to be significantly modulated by the solar cycle, their corresponding power-law indices still remain similar.

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J. Liu, A. Song, D. Jess, et. al.
Mon, 10 Apr 23
22/36

Comments: 10 figures, 1 table, to be published in ApJ Supplement

http://arxiv.org/abs/2304.03604

We present a complete analysis of the individual components of the ABC visual triple system HIP 32475. AB pair was discovered during the Hipparcos mission, with a separation of 412 mas. Later, in 2015, a third component was added to the system by discovering it at a small angular distance from B. In our analysis, we follow Al-Wardat’s method for analyzing binary and multiple stellar systems, which is a computational spectrophotometric method. Using estimated parameters, the components’ positions on the H-R diagram, evolutionary tracks, and isochrones are defined. Depending on the analysis, we estimate the age of the system as 1.259 Gyr with a metallicity of $Z=0.019$. The results show that component A started to evolve from the main sequence to the sub-giants stage, while components B and C are still in the main sequence stage.

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A. Hussein, E. Abu-Alrob, F. Alkhateri, et. al.
Mon, 10 Apr 23
28/36

Comments: N/A

http://arxiv.org/abs/2304.02837

The commonly accepted mechanism governing the formation of the nascent wind in oxygen-rich AGB stars combines an initial boost above the photosphere, given by shock waves resulting from stellar pulsations and convective cell granulation, with a subsequent acceleration fuelled by the radiation pressure of the star on dust grains. We use six nearby stars, for which detailed studies of visible and infrared observations at the VLT and millimetre observations at ALMA are available, to assess the extent to which the validity of this picture is currently corroborated. We show that while providing a very useful guide to current research and having received general support and suffered no contradiction, it still requires many additional observations to be reliably validated. In particular, observations of the highest possible angular resolution at both millimetre and visible/infrared wavelengths, performed in conjunction with measurements of the light curve, are necessary to tell apart the respective roles played by convection and stellar pulsations. A major unanswered question is the lack of understanding of the apparent contradiction between the observed high variability near the photosphere and the persistence over decades, or even centuries, of the global anisotropy displayed by the CSE. New observations of the close neighbourhood of the star are required to elucidate the mechanism that governs rotation, in particular in the cases of R Dor, L2 Pup and EP Aqr. We argue that the presence of stellar or planetary companions does not seriously impact the formation of the nascent wind and only modifies its subsequent evolution.

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P. Darriulat, D. Hoai, P. Nhung, et. al.
Fri, 7 Apr 23
37/50

Comments: 22 pages,16 figures

http://arxiv.org/abs/2304.02667

The magnetic processes associated with the non-thermal broadening of optically thin emission lines appear to carry enough energy to heat the corona and accelerate the solar wind. We investigate whether non-thermal motions in cool stars exhibit the same behaviour as on the Sun by analysing archival stellar spectra taken by the Hubble Space Telescope, and full-disc Solar spectra taken by the Interface Region Imaging Spectrograph. We determined the non-thermal velocities by measuring the excess broadening in optically thin emission lines formed in the stellar atmosphere; the chromosphere, the transition region and the corona. Assuming the non-thermal broadening is caused by the presence of Alfv\’en waves, we also determined the associated wave energy densities. Our results show that, with a non-thermal velocity of $\sim$23 kms$^{-1}$ the Sun-as-a-star results are in very good agreement with values obtained from spatially-resolved solar observations. The non-thermal broadening in our sample show correlation to stellar rotation, with the strength of the non-thermal velocity decreasing with decreasing rotation rate. Finally, the non-thermal velocity in cool Sun-like stars varies with atmospheric height or temperature of the emission lines, and peaks at transition region temperatures. This points towards a solar-like Alfv\’en wave driven heating in stellar atmospheres. However, the peak is at a lower temperature in some cool stars suggesting that, other magnetic process such as flaring events could also dominate.

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S. Saikia, T. Lueftinger, V. Airapetian, et. al.
Fri, 7 Apr 23
39/50

Comments: 13 pages, 8 Figures, accepted for publication in The Astrophysical Journal

http://arxiv.org/abs/2304.02706

Results of a large program of spectroscopic monitoring of nearby solar-type stellar hierarchical systems using the CHIRON echelle spectrograph at the 1.5 m telescope are summarized. Ten papers of this series contain 102 spectroscopic orbits and substantially contribute to the knowledge of periods and eccentricties, providing input for the study of their formation and early evolution. Radial velocities of additional 91 targets without CHIRON orbits (members of wide physical pairs) are published here. Our results are compared to the recent Gaia Non-Single Star (NSS) catalog, revealing its strengths and weaknesses. The NSS provides orbital periods for 31 objects of the CHIRON sample (about one third). Of the 22 spectroscopic NSS orbits in common, 14 are in good agreement with CHIRON, the rest have reduced velocity amplitudes or other problems. Hence ground-based monitoring gives, so far, a more accurate and complete picture of nearby hierarchies than Gaia. The distribution of inner periods in hierarchical systems is non-monotonic, showing a shallow minimum in the 30-100 days bin and a strong excess at shorter periods, compared to the smooth distribution of simple binaries in the field. The period-eccentricity diagram of inner subsystems updated by this survey, recent literature, and Gaia, displays an interesting structure.

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A. Tokovinin
Fri, 7 Apr 23
42/50

Comments: 11 pages, 3 figures, 4 tables. Accepted by The Astronomical Journal

http://arxiv.org/abs/2304.03165

The photometric time series of solar-like stars can exhibit rotational modulation due to active regions co-rotating with the stellar surface, allowing us to constrain stellar rotation and magnetic activity. In this work we investigate the behavior, particularly the variability, of the photometric magnetic activity of Kepler solar-like stars and compare it with that of the Sun. We adopted the photometric magnetic activity proxy Sph, which was computed with a cadence of 5 x the rotation period, Prot. The average Sph was taken as the mean activity level, and the standard deviation was taken as a measure of the temporal variation of the magnetic activity over the observations. We also analyzed Sun-as-a-star photometric data from VIRGO. Sun-like stars were selected from a very narrow parameter space around the solar properties. We also looked into KIC 8006161 (HD 173701), an active metal-rich G dwarf, and we compared its magnetic activity to that of stars with similar stellar parameters. We find that the amplitude of Sph variability is strongly correlated with its mean value, independent of spectral type. An equivalent relationship has been found for ground-based observations of chromospheric activity emission and magnetic field strength, but in this work we show that photometric Kepler data also present the same behavior. While, depending on the cycle phase, the Sun is among the less active stars, we find that the solar Sph properties are consistent with those observed in Kepler Sun-like stars. KIC 8006161 is, however, among the most active of its peers, which tend to be metal-rich. This results from an underlying relationship between Prot and metallicity and supports the following interpretation of the magnetic activity of KIC 8006161: its strong activity is a consequence of its high metallicity, which affects the depth of the convection zone and, consequently, the efficiency of the dynamo.

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A. Santos, S. Mathur, R. García, et. al.
Fri, 7 Apr 23
45/50

Comments: Published in A&A; 12 pages including 11 figures and 3 tables (main text); 10 additional pages including 17 figures and 5 tables (appendix)

http://arxiv.org/abs/2304.02552

Sun-as-a-star coronal plasma composition, derived from full-Sun spectra, and the F10.7 radio flux (2.8 GHz) have been shown to be highly correlated (r = 0.88) during solar cycle 24. However, this correlation becomes nonlinear during increased solar magnetic activity. Here, we use co-temporal, high spatial resolution, multi-wavelength images of the Sun to investigate the underlying causes of the non-linearity between coronal composition (FIP bias) and F10.7 solar index correlation. Using the Karl G. Jansky Very Large Array (JVLA), Hinode/EIS (EUV Imaging Spectrometer), and the Solar Dynamic Observatory (SDO), we observed a small active region, AR 12759, throughout the solar atmosphere from the photosphere to the corona. Results of this study show that the magnetic field strength (flux density) in active regions plays an important role in the variability of coronal abundances, and it is likely the main contributing factor to this non-linearity during increased solar activity. Coronal abundances above cool sunspots are lower than in dispersed magnetic plage regions. Strong magnetic concentrations are associated with stronger F10.7 cm gyroresonance emission. Considering that as the solar cycle moves from minimum to maximum, the size of sunspots and their field strength increase with gyroresonance component, the distinctly different tendencies of radio emission and coronal abundances in the vicinity of sunspots is the likely cause of saturation of Sun-as-a-star coronal abundances during solar maximum, while the F10.7 index remains well correlated with the sunspot number and other magnetic field proxies.

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A. To, A. James, T. Bastian, et. al.
Thu, 6 Apr 23
1/76

Comments: 15 pages, 5 figures, 2 tables, accepted for publication in The Astrophysical Journal

http://arxiv.org/abs/2304.02615

The binary systems consisting of a Be star and a white dwarf (BeWDs) are very interesting.They can originate from the binaries composed of a Be star and a subdwarf O or B star (BesdOBs), and they can merge into red giants via luminous red nova or can evolve into double WD potentially detected by $LISA$ mission. Using the method of population synthesis, we investigate the formation and the destiny of BeWDs,and discuss the effects of the metallicity ($Z$) and the common envelope evolution parameters. We find that BesdOBs are significant progenitors of BeWDs. About 30\% ($Z=0.0001$)-50\% ($Z=0.02$) of BeWDs come from BesdOBs. About 60\% ($Z=0.0001$) -70\% ($Z=0.02$) of BeWDs turn into red giants via a merger between a WD and a non-degenerated star. About 30\% ($Z=0.0001$) -40\% ($Z=0.02$) of BeWDs evolve into double WDs which are potential gravitational waves of $LISA$ mission at a frequency band between about $3\times10^{-3}$ and $3\times10^{-2}$ Hz. The common envelope evolution parameter introduces an uncertainty with a factor of about 1.3 on BeWD populations in our simulations.

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C. Zhu, G. Lü, X. Lu, et. al.
Thu, 6 Apr 23
17/76

Comments: 17 pages, 12 figures, 2 table, accepted for publication in RAA

http://arxiv.org/abs/2304.02543

While many M dwarfs are known to have strong magnetic fields and high levels of magnetic activity, we are still unsure about the properties of their starspots and the origin of their magnetic dynamos. Both starspots and chromospheric heating are generated by the surface magnetic field; they produce photometric variability and Halpha emission, respectively. Connecting brightness variations to magnetic activity therefore provides a means to examine M dwarf magnetism. We survey 30 M dwarfs previously identified as fast rotating stars (Prot < 10 days). We present time-series optical photometry from the Transiting Exoplanet Survey Satellite (TESS) and contemporaneous optical spectra obtained using the Ohio State Multi-Object Spectrograph (OSMOS) on the 2.4m Hiltner telescope at MDM Observatory in Arizona. We measure rotation periods and photometric amplitudes from TESS light curves using Gaussian Processes. From the OSMOS spectra, we calculate the equivalent width of Halpha, and LHalpha/Lbol. We find a weak positive correlation between Halpha luminosity and the semi-amplitude, Rvar p=0.005_{-0.005}^{+0.075}. We also observe short-term variability (between 20-45 minutes) in Halpha equivalent widths and possible enhancement from flares consistent to recent literature.

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A. Soto, E. Newton, S. Douglas, et. al.
Thu, 6 Apr 23
20/76

Comments: 19 pages, 9 Figures, 2 Tables, Poster Presented at Cool Stars 21, Publication post-copy editing

http://arxiv.org/abs/2304.02618

Context: Solar flares are the result of the sudden release of magnetic energy in the corona. Much of this energy goes into accelerating charged particles to high velocity. These particles travel along the magnetic field and the energy is dissipated when the density gets high enough, primarily in the solar chromosphere. Modelling this region is difficult because the radiation energy balance is dominated by strong, optically thick spectral lines.
Aims: Our aim is to provide the community with realistic simulations of a flaring loop with an emphasis on the detailed treatment of the chromospheric energy balance. This will enable a detailed comparison of existing and upcoming observations with synthetic observables from the simulations, thereby elucidating the complex interactions in a flaring chromosphere.
Methods: We used the 1D radiation hydrodynamics code RADYN to perform simulations of the effect of a beam of electrons injected at the apex of a solar coronal loop. A grid of models was produced, varying the total energy input, the steepness, and low-energy cutoff of the beam energy spectrum.
Results: The full simulation results for a grid of models are made available online. Some general properties of the simulations are discussed.

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M. Carlsson, L. Fletcher, J. Allred, et. al.
Thu, 6 Apr 23
59/76

Comments: 11 pages, 9 figures. Accepted for publication in Astronomy & Astrophysics

http://arxiv.org/abs/2304.02158

Using time-distance local helioseismology flow maps within 1 Mm of the solar photosphere, we detect inflows toward activity belts that contribute to solar cycle scale variations in near-surface meridional flow. These inflows stretch out as far as 30 degrees away from active region centroids. If active region neighborhoods are excluded, the solar cycle scale variation in background meridional flow diminishes to below 2~m~s$^{-1}$, but still shows systematic variations in the absence of active regions between Sunspot Cycles 24 and 25. We, therefore, propose that the near-surface meridional flow is a three component flow made up of: a constant baseline flow profile that can be derived from quiet Sun regions, variations due to inflows around active regions, and solar cycle scale variation of the order of 2~m~s$^{-1}$. Torsional oscillation, on the other hand, is found to be a global phenomenon i.e. exclusion of active region neighborhoods does not affect its magnitude or phase significantly. This non-variation of torsional oscillation with distance away from active regions and the three-component breakdown of the near-surface meridional flow serve as vital constraints for solar dynamo models and surface flux transport simulations.

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S. Mahajan, X. Sun and J. Zhao
Thu, 6 Apr 23
60/76

Comments: 14 pages, 9 figures, accepted for publication in the Astrophysical Journal

http://arxiv.org/abs/2304.02057

Previous studies analyzing the evanescent nature of acoustic waves in the lower solar atmosphere, up to 300\,km above the photosphere, have shown an unexpected phase shift of an order of 1\,s between different heights. Those studies investigated the spectral line \ion{Fe}{1} 6173.3\,\AA, commonly used for helioseismic measurements. Such phase-shifts can contribute to a misinterpretation of the measured travel times in local helioseismology, complicating inferences of, e.g., the deep meridional flow. In this study, we carry out phase-shift computations using a simulated, fully radiative, and convective atmosphere from which the \ion{Fe}{1} 6173.3\,\AA\ line is synthesized. The resulting phase-shifts as functions of frequency across multiple heights show non-zero values in evanescent waves, similar to what was found in observational data. Comparing the Doppler-velocities estimated from the synthesized absorption line with the true velocities directly obtained from the simulated plasma motions, we find substantial differences in phase-shifts between the two. This leads us to hypothesize that the non-adiabaticity of the solar atmosphere yields extra phase-shift contributions to Doppler velocities. Finally, computing phase-differences for different viewing angles reveals a systematic center-to-limb variation, similar to what is present in observations. Overall, this study helps to improve our understanding of the physical cause of the helioseismic center-to-limb effect.

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M. Waidele, J. Zhao and I. Kitiashvili
Thu, 6 Apr 23
67/76

Comments: N/A

http://arxiv.org/abs/2304.01520

New analyses of earlier ALMA observations of oxygen-rich AGB star EP Aquarii are presented, which contribute major progress to our understanding of the morpho-kinematics of the circumstellar envelope (CSE). The birth of the equatorial density enhancement (EDE) is shown to occur very close to the star where evidence for rotation has been obtained. High Doppler velocity wings are seen to consist of two components, the front end of the global wind, reaching above $\pm$12 \kms, and an effective line broadening, confined within 200 mas from the centre of the star, reaching above $\pm$20 \kms\ and interpreted as caused by the pattern of shock waves resulting from the interaction between stellar pulsation and convective cell granulation. Close to the star, episodic and lumpy mass ejections are observed, and their interaction with the gas of the nascent EDE, first rotating and later slowly expanding, is seen to play an important role in the development of the wind and the evolution of its radial velocity from 8-10 \kms\ on the polar symmetry axis to $\sim$2 \kms\ at the equator. It implies a very complex morpho-kinematics, which prevents making reliable interpretations with reasonable confidence. In particular, it sheds serious doubts on an earlier interpretation implying the presence of a white dwarf companion orbiting the star at an angular distance of $\sim$0.4 arcsec from its centre and currently west of it.

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P. Nhung, D. Hoai, P. Darriulat, et. al.
Wed, 5 Apr 23
8/62

Comments: 17 pages, 22 figures

http://arxiv.org/abs/2304.01863

Orbital motions in four hierarchical stellar systems discovered by speckle interferometry are studied. Their inner orbits are relatively well constrained, while the long outer orbits are less certain. The eccentric and misaligned inner orbits in the early-type hierarchies Epsilon Cha (B9V, central star of the 5 Myr old association, P=6.4 yr, e=0.73), and I~385 (A0V, P~300 yr, e~0.8) suggest past dynamical interactions. Their nearly equal masses could be explained by a dynamical decay of a 2+2 quadruple progenitor consisting of four similar stars. However, there is no evidence of the associated recoil, so similar masses could be just a consequence of accretion from the same core. The other two hierarchies, HIP 32475 (F0IV, inner period 12.2 yr) and HIP 42910 (K7V, inner period 6.8 yr), have smaller masses and are double twins where both inner and outer mass ratios are close to one. A double twin could either result from a merger of one inner pair in a 2+2 quadruple or can be formed by a successive fragmentation followed by accretion.

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A. Tokovinin
Wed, 5 Apr 23
18/62

Comments: 11 pages, 8 figures, 5 tables

http://arxiv.org/abs/2304.01835

We assemble the most complete and clean red supergiant (RSG) sample (2,121 targets) so far in the Small Magellanic Cloud (SMC) with 53 different bands of data to study the MLR of RSGs. In order to match the observed spectral energy distributions (SEDs), a theoretical grid of 17,820 Oxygen-rich models (normal'' anddusty” grids are half-and-half) is created by the radiatively-driven wind model of the DUSTY code, covering a wide range of dust parameters. We select the best model for each target by calculating the minimal modified chi-square and visual inspection. The resulting MLRs from DUSTY are converted to real MLRs based on the scaling relation, for which a total MLR of $6.16\times10^{-3}$ $M_\odot$ yr$^{-1}$ is measured (corresponding to a dust-production rate of $\sim6\times10^{-6}$ $M_\odot$ yr$^{-1}$), with a typical MLR of $\sim10^{-6}$ $M_\odot$ yr$^{-1}$ for the general population of the RSGs. The complexity of mass-loss estimation based on the SED is fully discussed for the first time, indicating large uncertainties based on the photometric data (potentially up to one order of magnitude or more). The Hertzsprung-Russell and luminosity versus median absolute deviation diagrams of the sample indicate the positive relation between luminosity and MLR. Meanwhile, the luminosity versus MLR diagrams show a “knee-like” shape with enhanced mass-loss occurring above $\log_{10}(L/L_\odot)\approx4.6$, which may be due to the degeneracy of luminosity, pulsation, low surface gravity, convection, and other factors. We derive our MLR relation by using a third-order polynomial to fit the sample and compare our result with previous empirical MLR prescriptions. Given that our MLR prescription is based on a much larger sample than previous determinations, it provides a more accurate relation at the cool and luminous region of the H-R diagram at low-metallicity compared to previous studies.

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M. Yang, A. Bonanos, B. Jiang, et. al.
Wed, 5 Apr 23
41/62

Comments: 16 pages, 19 figures, accepted by A&A

http://arxiv.org/abs/2304.01606

Alfv\’enic motions are ubiquitous in the solar atmosphere and their observed properties are closely linked to those of photospheric p-modes. However, it is still unclear how a predominantly acoustic wave driver can produce these transverse oscillations in the magnetically dominated solar corona. In this study we conduct a 3D ideal MHD numerical simulation to model a straight, expanding coronal loop in a gravitationally stratified solar atmosphere which includes a transition region and chromosphere. We implement a driver locally at one foot-point corresponding to an acoustic-gravity wave which is inclined by $\theta = 15^{\circ}$ with respect to the vertical axis of the magnetic structure and is similar to a vertical driver incident on an inclined loop. We show that transverse motions are produced in the magnetic loop, which displace the axis of the waveguide due to the breaking of azimuthal symmetry, and study the resulting modes in the theoretical framework of a magnetic cylinder model. By conducting an azimuthal Fourier analysis of the perturbed velocity signals, the contribution from different cylindrical modes is obtained. Furthermore, the perturbed vorticity is computed to demonstrate how the transverse motions manifest themselves throughout the whole non-uniform space. Finally we present some physical properties of the Alfv\’enic perturbations and present transverse motions with velocity amplitudes in the range of $0.2-0.75$ km s$^{-1}$ which exhibit two distinct oscillation regimes corresponding to $42$ s and $364$ s, where the latter value is close to the period of the p-mode driver in the simulation.

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S. Skirvin, Y. Gao and T. Doorsselaere
Wed, 5 Apr 23
45/62

Comments: 15 pages, 8 figures, accepted in ApJ

http://arxiv.org/abs/2304.00087

The NASA K2 mission that succeeded the nominal Kepler mission observed several hundreds of thousands of stars during its operations. While most of the stars were observed in single campaigns of 80 days, some of them were targeted for more than one campaign. We perform an asteroseismic study of a sample of eight solar-like stars observed during K2 Campaigns 6 and 17. We first extract the light curves for the two campaigns using two different pipelines, EVEREST and Lightkurve. The seismic analysis is done on the combined light curve of C6 and C17 where the gap between them was removed and the two campaigns were stitched together. We determine the global seismic parameters of the solar-like oscillations using two different methods (A2Z pipeline and the apollinaire code). We perform the peak-bagging of the modes to characterize their individual frequencies. By combining the frequencies with the Gaia DR2 effective temperature and luminosity, and metallicity for five of the targets, we determine the fundamental parameters of the targets using the IACgrids based on the MESA code. While the masses and radii of our targets probe a similar parameter space compared to the Kepler solar-like stars with detailed modeling, we find that for a given mass our more evolved stars seem to be older compared to previous seismic stellar ensembles. We calculate the stellar parameters using two different grids of models, incorporating and excluding the treatment of diffusion, and find that the results agree generally within the uncertainties, except for the ages. The seismic radii and the Gaia DR2 radii present an average difference of 4% with a dispersion of 5%. Although the agreement is quite good, the seismic radii are slightly underestimated compared to Gaia DR2 for our stars, the disagreement being greater for the more evolved ones.

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L. González-Cuesta, S. Mathur, R. García, et. al.
Tue, 4 Apr 23
10/111

Comments: 10 pages (without annexes), 13 figures. Accepted for publication in A&A

http://arxiv.org/abs/2304.00422

Solar filaments are dense and cool plasma clouds in the solar corona. They are supposed to be supported in a dip of coronal magnetic field. However, the models are still under argument between two types of the field configuration; one is the normal polarity model proposed by Kippenhahn & Schlueter (1957), and the other is the reverse polarity model proposed by Kuperus & Raadu (1974). To understand the mechanism that the filaments become unstable before the eruption, it is critical to know the magnetic structure of solar filaments. In this study, we performed the spectro-polarimetric observation in the He I (10830 angstrom) line to investigate the magnetic field configuration of dark filaments. The observation was carried out with the Domeless Solar Telescope at Hida Observatory with a polarization sensitivity of 3.0×10^-4. We obtained 8 samples of filaments in quiet region. As a result of the analysis of full Stokes profiles of filaments, we found that the field strengths were estimated as 8 – 35 Gauss. By comparing the direction of the magnetic field in filaments and the global distribution of the photospheric magnetic field, we determined the magnetic field configuration of the filaments, and we concluded that 1 out of 8 samples have normal polarity configuration, and 7 out of 8 have reverse polarity configuration.

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D. Yamasaki, Y. Huang, Y. Hashimoto, et. al.
Tue, 4 Apr 23
42/111

Comments: 27 pages, 17 figures. Accepted for the Publications of the Astronomical Society of Japan

http://arxiv.org/abs/2304.00674

Context. Asteroseismic studies showed that cores of post main-sequence stars rotate slower than theoretically predicted by stellar models with purely hydrodynamical transport processes. Recent studies on main sequence stars, particularly Gamma Doradus ($\gamma$ Dor) stars, revealed their internal rotation rate for hundreds of stars, offering a counterpart on the main sequence for studies of angular momentum transport. Aims. We investigate whether such a disagreement between observed and predicted internal rotation rates is present in main sequence stars by studying angular momentum transport in $\gamma$ Dor stars. Furthermore, we test whether models of rotating stars with internal magnetic fields can reproduce their rotational properties. Methods. We compute rotating models with the Geneva stellar evolution code taking into account meridional circulation and the shear instability. We also compute models with internal magnetic fields using a general formalism for transport by the Tayler-Spruit dynamo. We then compare these models to observational constraints for $\gamma$ Dor stars that we compiled from the literature, combining so the core rotation rates, projected rotational velocities from spectroscopy, and constraints on their fundamental parameters. Results. We show that combining the different observational constraints available for $\gamma$ Dor stars enable to clearly distinguish the different scenarios for internal angular momentum transport. Stellar models with purely hydrodynamical processes are in disagreement with the data whereas models with internal magnetic fields can reproduce both core and surface constraints simultaneously. Conclusions. Similarly to results obtained for subgiant and red giant stars, angular momentum transport in radiative regions of $\gamma$ Dor stars is highly efficient, in good agreement with predictions of models with internal magnetic fields.

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F. Moyano, P. Eggenberger, S. Salmon, et. al.
Tue, 4 Apr 23
71/111

Comments: Submitted to Astronomy & Astrophysics. 15 pages, 16 figures

http://arxiv.org/abs/2304.00297

V606 Centauri (V606 Cen) is an early B-type close binary with an orbital period of 1.4950935\,d and the complete light curves are very difficult to be observed on the ground. By analyzing the unbroken and continuous light curve obtained by TESS, we found that it is a marginal contact binary with a very low fill-out factor of about 2\%. The O-C diagram of V606 Cen is constructed for the first time based on the 118.8-years eclipse times. It is found that the O-C diagram shows a downward parabolic change together with a cyclic oscillation with an amplitude of 0.0544\, d and a period of 88.8\, yr. The downward parabolic variation reveals a linear period decrease at a rate of $dP/dt = -2.06 \times{10^{-7}} d \cdot yr^{-1}$ that can be explained by the mass transfer from the more massive component to the less massive one. Both the marginal contact configuration and the continuous period decrease suggest that V606 Cen is a newly formed contact binary via Case A mass transfer. Meanwhile, the cyclic change in the O-C diagram can be explained by the Light-Travel Time Effect via the presence of a third body. The lowest mass of the tertiary companion is determined as M${3}$ = 4.51($\pm0.38$)M${\odot}$ that is orbiting around the central eclipsing binary in a nearly circular orbit (e=0.32). All the results indicate that V606 Cen is a newly formed massive contact binary and just reaches the contact configuration during the mass transfer in a hierarchical triple system.

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F. Li, W. Liao, S. Qian, et. al.
Tue, 4 Apr 23
72/111

Comments: N/A

http://arxiv.org/abs/2304.01048

Solar oxygen abundance measurements based on the O I near-infrared triplet have been a much-debated subject for several decades since non-local thermodynamic equilibrium (NLTE) calculations with 3D radiation-hydrodynamics model atmospheres introduced a large change to the 1D LTE modelling. In this work, we aim to test solar line formation across the solar disk using new observations obtained with the SST/CRISP instrument. The observed dataset is based on a spectroscopic mosaic stretching from disk center to the solar limb. By comparing the state-of-the-art 3D NLTE models with the data, we find that the 3D NLTE models provide an excellent description of line formation across the disk. We obtain an abundance value of $A(\mathrm{O}) = (8.73 \pm 0.03)$ dex, with a very small angular dispersion across the disk. We conclude that spectroscopic mosaics are excellent probes for geometric and physical properties of hydrodynamics models and non-LTE line formation.

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A. Pietrow, R. Hoppe, M. Bergemann, et. al.
Tue, 4 Apr 23
95/111

Comments: Accepted in A&A

http://arxiv.org/abs/2303.18098

Coronal jets are eruptions identified by a collimated, sometimes twisted spire. They are small-scale energetic events compared with flares. Using multi-wavelength observations from the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) and a magnetogram from Hinode/Spectro-Polarimeter (Hinode/SP), we study the formation and evolution of a jet occurring on 2019 March 22 in the active region NOAA 12736. A zero-$\beta$ magnetohydrodynamic (MHD) simulation is conducted to probe the initiation mechanisms and appearance of helical motion during this jet event. As the simulation reveals, there are two pairs of field lines at the jet base, indicating two distinct magnetic structures. One structure outlines a flux rope lying low above the photosphere in the north of a bald patch region and the other structure shows a null point high in the corona in the south. The untwisting motions of the observed flux rope was recovered by adding an anomalous (artificial) resistivity in the simulation. A reconnection occurs at the bald patch in the flux rope structure, which is moving upwards and simultaneously encounters the field lines of the null point structure. The interaction of the two structures results in the jet while the twist of the flux rope is transferred to the jet by the reconnected field lines. The rotational motion of the flux rope is proposed to be an underlying trigger of this process and responsible for helical motions in the jet spire.

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J. Zhu, Y. Guo, M. Ding, et. al.
Mon, 3 Apr 23
2/53

Comments: 17pages, 9 figures. Accepted for publication in The Astrophysical Journal