A magnetic reconnection event in the solar atmosphere driven by relaxation of a twisted arch filament system [SSA]


We present high-resolution observations of a magnetic reconnection event in the solar atmosphere taken with the New Vacuum Solar Telescope, AIA and HMI. The reconnection event occurred between the threads of a twisted arch filament system (AFS) and coronal loops. Our observations reveal that the relaxation of the twisted AFS drives some of its threads to encounter the coronal loops, providing inflows of the reconnection. The reconnection is evidenced by flared X-shape features in the AIA images, a current-sheet-like feature apparently connecting post-reconnection loops in the \halpha$+$1 \AA\ images, small-scale magnetic cancellation in the HMI magnetograms and flows with speeds of 40–80 km/s along the coronal loops. The post-reconnection coronal loops seen in AIA 94 \AA\ passband appear to remain bright for a relatively long time, suggesting that they have been heated and/or filled up by dense plasmas previously stored in the AFS threads. Our observations suggest that the twisted magnetic system could release its free magnetic energy into the upper solar atmosphere through reconnection processes. While the plasma pressure in the reconnecting flux tubes are significantly different, the reconfiguration of field lines could result in transferring of mass among them and induce heating therein.

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Z. Huang, C. Mou, H. Fu, et. al.
Fri, 19 Jan 18

Comments: 5 figs, accepted for publication in ApJL

Iron-peak elements Sc, V, Mn, Cu and Zn in Galactic bulge globular clusters [SSA]


Globular clusters are tracers of the history of star formation and chemical enrichment in the early Galaxy. Their abundance pattern can help understanding their chemical enrichment processes. In particular, the iron-peak elements have been relatively little studied so far in the Galactic bulge. The abundances of iron-peak elements are derived for the sample clusters, and compared with bulge field, and thick disk stars. We derive abundances of the iron-peak elements Sc, V, Mn, Cu, and Zn in individual stars of five bulge globular clusters (NGC 6528, NGC 6553, NGC 6522, NGC 6558, HP1), and of the reference thick disk/inner halo cluster 47 Tucanae (NGC 104). High resolution spectra were obtained with the UVES spectrograph at the Very Large Telescope over the years. The sample globular clusters studied span metallicities in the range -1.2<Fe/H]<0.0. V and Sc appear to vary in lockstep with Fe, indicating that they are produced in the same supernovae as Fe. We find that Mn is deficient in metal-poor stars, confirming that it is underproduced in massive stars; Mn-over-Fe steadily increases at the higher metallicities due to a metallicity-dependent enrichment by supernovae of type Ia. Cu behaves as a secondary element, indicating its production in a weak-s process in massive stars. Zn has an alpha-like behaviour at low metallicities, which can be explained in terms of nucleosynthesis in hypernovae. At the metal-rich end, Zn decreases with increasing metallicity, similarly to the alpha-elements.

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H. Ernandes, B. Barbuy, A. Alves-Brito, et. al.
Fri, 19 Jan 18

Comments: article resubmitted to Astronomy & Astrophysics, taking into account referee’s comments

An L+T Spectral Binary with Possible AB Doradus Kinematics [SSA]


We present the identification of WISE~J135501.90$-$825838.9 as a spectral binary system with a slight possibility of planetary-mass components in the $130-200$\,Myr AB~Doradus moving group. Peculiarities in the near-infrared spectrum of this source suggest it to be a blended-light binary with L6.0$\pm$1.0 and T3.0$\pm$1.8 or L7.0$\pm$0.6 and T7.5$\pm$0.4 components. Its proper motion and radial velocity as a combined-light source yield a high membership probability for AB~Doradus. While the young L6+T3 case is underluminous in a color-magnitude diagram at the AB~Doradus kinematic distance, the young L7+T7.5 case could be viable. Gravity-sensitive indicators are more consistent with a field-age binary. If confirmed as a young object, member of AB~Doradus, we estimate masses of 11$\pm$1\,\mjup\ and 9$\pm$1\,\mjup\ with both component masses below the Deuterium burning mass limit. Otherwise, we find masses of $72^{+4}{-5}$ and $61^{+6}{-8}$ for the field L6+T3 case and $70^{+2}{-4}$ and $42^{+5}{-6}$ for the field L7+T7.5 case. Our identification of WISE~J135501.90$-$825838.9 as a candidate young spectral binary introduces a new technique for detecting and characterizing planetary-mass companions to young brown dwarfs.

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D. Gagliuffi, J. Gagne, J. Faherty, et. al.
Fri, 19 Jan 18

Comments: 12 pages, 2 figures, 7 subfigures

The atmospheric parameters of FGK stars using wavelet analysis of CORALIE spectra [SSA]


Atmospheric properties of F-,G- and K-type stars can be measured by spectral model fitting. These methods require data with good signal-to-noise ratio and reliable continuum normalisation. This is particularly challenging for the spectra we have obtained with the CORALIE \'{e}chelle spectrograph for FGK stars with transiting M-dwarf companions. The spectra tend to have low signal-to-noise ratios, which makes it difficult to analyse them using existing methods. Our aim is to create a reliable automated spectral analysis routine to determine $T_{\rm eff}$, [Fe/H], $V \sin i$ from the CORALIE spectra of FGK stars. We use wavelet decomposition to distinguish between noise, continuum trends, and stellar spectral features in the CORALIE spectra. A subset of wavelet coefficients from the target spectrum are compared to those from a grid of models in a Bayesian framework to determine the posterior probability distributions of the atmospheric parameters. By testing our method using synthetic spectra we found that our method converges on the best fitting atmospheric parameters. We test the wavelet method on 20 FGK exoplanet host stars for which higher quality data have been independently analysed using equivalent width measurements. We find that we can determine $T_{\rm eff}$ to a precision of $85$K, [Fe/H] to a precision of 0.06 dex and $V \sin i$ to a precision of 1.35 kms$^{-1}$ for stars with $V \sin i$ $\geq$ 5 kms$^{-1}$. We find an offset in metallicity $\approx -$0.18 dex relative to the equivelent width fitting method. We can determine $\log g$ to a precision of $0.13$ dex but measurements are only reliable to confirm dwarf-like surface gravity. The wavelet method can be used to determine $T_{\rm eff}$, [Fe/H] and $V \sin i$ for FGK stars from \'{e}chelle spectra. We find that our method is self consistent, and robust for spectra with SNR$\gtrapprox 40$.

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S. Gill, P. Maxted and B. Smalley
Fri, 19 Jan 18

Comments: 14 pages, 13 figures, 6 tables. Accepted for publication in A&A

The "FIP Effect" and the Origins of Solar Energetic Particles and of the Solar Wind [SSA]


We find that the element abundances in solar energetic particles (SEPs) and in the slow solar wind (SSW), relative to those in the photosphere, show different patterns as a function of the first ionization potential (FIP) of the elements. Generally, the SEP and SSW abundances reflect abundance samples of the solar corona, where low-FIP elements, ionized in the photosphere, are more efficiently conveyed upward to the corona than high-FIP elements that are initially neutral atoms. Abundances of the elements, especially C, P, and S show a crossover from low to high FIP at ~10 eV in the SEPs but ~14 eV for the solar wind, suggesting that cooler photospheric plasma, perhaps from cool sunspots beneath active regions, supply the coronal source material that eventually will be shock-accelerated as SEPs, while the source of the SSW may lie at the base of diverging open-field lines surrounding but outside active regions. Meanwhile, energetic particles accelerated from the solar wind itself by shock waves at corotating interaction regions (CIRs), generally beyond 1 AU, confirm the FIP pattern of the solar wind.

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D. Reames
Fri, 19 Jan 18

Comments: 11 pages, 2 figures, submitted to Solar Physics

OGLE-LMC-ECL-09937: The Most Massive Algol-Type Binary System With A Mass Measurement Accurate to 2% [SSA]


This paper presents a detailed analysis of the light and radial velocity curves of the semi-detached eclipsing binary system OGLE-LMC-ECL-09937. The system is composed of a hot, massive and luminous primary star of a late-O spectral type, and a more evolved, but less massive and luminous secondary, implying an Algol-type system that underwent a mass transfer episode. We derive masses of 21.04 +/- 0.34 M_Sun and 7.61 +/- 0.09 M_Sun and radii of 9.93 +/- 0.06 R_Sun and 9.18 +/- 0.04 R_Sun, for the primary and the secondary component, respectively, which make it the most massive known Algol-type system with masses and radii of the components measured with <2% accuracy. Consequently, the parameters of OGLE-LMC-ECL-09937 provide an important contribution to the sparsely populated high-mass end of the stellar mass distribution, and an interesting object for stellar evolution studies, being a possible progenitor of a binary system composed of two neutron stars.

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D. Skowron, M. Kourniotis, J. Prieto, et. al.
Fri, 19 Jan 18

Comments: 16 pages, 7 figures, 4 Tables, published in Acta Astronomica

Rates and delay times of Type Ia supernovae in the helium-enriched main-sequence donor scenario [SSA]


The nature of the progenitors of Type Ia supernovae (SNe Ia) remains a mystery. Comparing theoretical rates and delay-time distributions of SNe Ia with those inferred observationally can constrain their progenitor models. In this work, taking thermohaline mixing into account in the helium-enriched main-sequence (HEMS) donor scenario, we address rates and delay times of SNe Ia in this channel by combining the results of self-consistent binary evolution calculations with population synthesis models. We find that the Galactic SN Ia rate from the HEMS donor scenario is around 0.6-1.2 per century, which is about 30% of the observed rate. Delay times of SNe Ia in this scenario cover a wide range of 0.1-1.0 Gyr. We also present the pre-explosion properties of companion stars in the HEMS donor scenario, which will be helpful for placing constraints on SN Ia progenitors through analyzing their pre-explosion images.

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Z. Liu and R. Stancliffe
Fri, 19 Jan 18

Comments: 11 pages, 8 figures, Accepted for publication in MNRAS