Lithium abundance and 6Li/7Li ratio in the active giant HD123351 I. A comparative analysis of 3D and 1D NLTE line-profile fits [SSA]

http://arxiv.org/abs/1704.06460


Current three-dimensional (3D) hydrodynamical model atmospheres together with NLTE spectrum synthesis, permit to derive reliable atomic and isotopic chemical abundances from high-resolution stellar spectra. Not much is known about the presence of the fragile 6Li isotope in evolved solar-metallicity RGB stars, not to mention its production in magnetically active targets like HD123351. From fits of the observed CFHT spectrum with synthetic line profiles based on 1D and 3D model atmospheres, we seek to estimate the abundance of the 6Li isotope and to place constraints on its origin. We derive A(Li) and the 6Li/7Li isotopic ratio by fitting different synthetic spectra to the Li-line region of a high-resolution CFHT spectrum (R=120 000, S/R=400). The synthetic spectra are computed with four different line lists, using in parallel 3D hydrodynamical CO5BOLD and 1D LHD model atmospheres and treating the line formation of the lithium components in non-LTE (NLTE). We find A(Li)=1.69+/-0.11 dex and 6Li/7Li=8.0+/-4.4 % in 3D-NLTE, using the line list of Mel\’endez et al. (2012), updated with new atomic data for V I, which results in the best fit of the lithium line profile of HD123351. Two other line lists lead to similar results but with inferior fit qualities. Our 2-sigma detection of the 6Li isotope is the result of a careful statistical analysis and the visual inspection of each achieved fit. Since the presence of a significant amount of 6Li in the atmosphere of a cool evolved star is not expected in the framework of standard stellar evolution theory, non-standard, external lithium production mechanisms, possibly related to stellar activity or a recent accretion of rocky material, need to be invoked to explain the detection of 6Li in HD123351.

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A. Mott, M. Steffen, E. Caffau, et. al.
Mon, 24 Apr 17
1/54

Comments: 16 pages, 11 figures. Accepted for publication in A&A

Testing a one-dimensional prescription of dynamical shear mixing with a two-dimensional hydrodynamic simulation [SSA]

http://arxiv.org/abs/1704.06261


The treatment of mixing processes is still one of the major uncertainties in 1D stellar evolution models. This is mostly due to the need to parametrize and approximate aspects of hydrodynamics in hydrostatic codes. In particular, the effect of hydrodynamic instabilities in rotating stars, for example, dynamical shear instability, evades consistent description. We intend to study the accuracy of the diffusion approximation to dynamical shear in hydrostatic stellar evolution models by comparing 1D models to a first-principle hydrodynamics simulation starting from the same initial conditions. We chose an initial model calculated with the stellar evolution code GENEC that is just at the onset of a dynamical shear instability but does not show any other instabilities (e.g., convection). This was mapped to the hydrodynamics code SLH to perform a 2D simulation in the equatorial plane. We compare the resulting profiles in the two codes and compute an effective diffusion coefficient for the hydro simulation. Shear instabilities develop in the 2D simulation in the regions predicted by linear theory to become unstable in the 1D model. Angular velocity and chemical composition is redistributed in the unstable region, thereby creating new unstable regions. Eventually the 2D simulation settles in a symmetric, steady state, which is Richardson stable everywhere, whereas the instability remains for longer in the 1D model due to current limitations in the 1D code. A spatially resolved diffusion coefficient is extracted by comparing the initial and final profiles of mean atomic mass. The presented simulation gives a first insight on hydrodynamics of shear instabilities in a real stellar environment and even allows us to directly extract an effective diffusion coefficient. We see evidence for a critical Richardson number of 0.25 as regions above this value remain stable for the course of the simulation.

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P. Edelmann, F. Roepke, R. Hirschi, et. al.
Mon, 24 Apr 17
9/54

Comments: 15 pages, 14 figures, accepted for publication by A&A, movie available at this https URL

Hydrodynamic ablation of protoplanetary disks via supernovae [SSA]

http://arxiv.org/abs/1704.06308


We present three-dimensional simulations of a protoplanetary disk subject to the effect of a nearby (0.3pc distant) supernova, using a time-dependent flow from a one dimensional numerical model of the supernova remnant (SNR), in addition to constant peak ram pressure simulations. Simulations are performed for a variety of disk masses and inclination angles. We find disk mass-loss rates that are typically 1e-7 to 1e-6 Msol/yr (but peak near 1e-5 Msol/yr during the “instantaneous” stripping phase) and are sustained for around 200 yr. Inclination angle has little effect on the mass loss unless the disk is close to edge-on. Inclined disks also strip asymmetrically with the trailing edge ablating more easily. Since the interaction lasts less than one outer rotation period, there is not enough time for the disk to restore its symmetry, leaving the disk asymmetrical after the flow has passed. Of the low-mass disks considered, only the edge-on disk is able to survive interaction with the SNR (with 50% of its initial mass remaining). At the end of the simulations, disks that survive contain fractional masses of SN material up to 5e-6. This is too low to explain the abundance of short-lived radionuclides in the early solar system, but a larger disk and the inclusion of radiative cooling might allow the disk to capture a higher fraction of SN material.

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J. Close and J. Pittard
Mon, 24 Apr 17
16/54

Comments: 16 pages, 16 figures, 2 tables, accepted by MNRAS

Spectral Evidence for an Inner Carbon-Rich Circumstellar Dust Belt in the Young HD36546 A-Star System [SSA]

http://arxiv.org/abs/1704.06348


Using the NASA/IRTF SpeX & BASS spectrometers we have obtained novel 0.7 – 13 um observations of the newly imaged HD36546 debris disk system. The SpeX spectrum is most consistent with the photospheric emission expected from an Lstar ~ 20 Lsun, solar abundance A1.5V star with little/no extinction and excess emission from circumstellar dust detectable beyond 4.5 um. Non-detections of CO emission lines and accretion signatures point to the gas poor circumstellar environment of a very old transition disk. Combining the SpeX and BASS spectra with archival WISE/AKARI/IRAS/Herschel photometery, we find an outer cold dust belt at ~135K and 20 – 40 AU from the primary, likely coincident with the disk imaged by Subaru (Currie et al. 2017), and a new second inner belt with temperature ~570K and an unusual, broad SED maximum in the 6 – 9 um region, tracing dust at 1.1 – 2.2 AU. An SED maximum at 6 – 9 um has been reported in just two other A-star systems, HD131488 and HD121191, both of ~10 Myr age (Melis et al. 2013). From Spitzer, we have also identified the ~12 Myr old A7V HD148567 system as having similar 5 – 35 um excess spectral features (Mittal et al. 2015). The Spitzer data allows us to rule out water emission and rule in carbonaceous materials – organics, carbonates, SiC – as the source of the 6 – 9 um excess. Assuming a common origin for the 4 young Astar systems’ disks, we suggest they are experiencing an early era of carbon-rich planetesimal processing.

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C. Lisse, M. Sitko, R. Russell, et. al.
Mon, 24 Apr 17
42/54

Comments: 14 Pages, 2 Figures

High frequency waves in the corona due to null points [SSA]

http://arxiv.org/abs/1704.06551


This work aims to understand the behavior of non-linear waves in the vicinity of a coronal null point. In previous works we have showed that high frequency waves are generated in such magnetic configuration. This paper studies those waves in detail in order to provide a plausible explanation of their generation. We demonstrate that slow magneto-acoustic shock waves generated in the chromosphere propagate through the null point and produce a train of secondary shocks that escape along the field lines. A particular combination of the shock wave speeds generates waves at a frequency of 80 mHz. We speculate that this frequency may be sensitive to the atmospheric parameters in the corona and therefore can be used to probe the structure of this solar layer.

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I. Santamaria, E. Khomenko, M. Collados, et. al.
Mon, 24 Apr 17
53/54

Comments: N/A

Emergence of magnetic flux generated in a solar convective dynamo. I: Formation of Sunspots and Active regions, and Origin of Their Asymmetries [SSA]

http://arxiv.org/abs/1704.05999


We present a realistic numerical model of sunspot and active region formation through the emergence of flux bundles generated in a solar convective dynamo. The magnetic and velocity fields in a horizontal layer near the top boundary of the solar convective dynamo simulation are used as a time-dependent bottom boundary to drive realistic radiative-magnetohydrodynamic simulations of the upper most layer of the convection zone. The main results are: (1) The emerging flux bundles rise with the mean speed of convective upflows, and fragment into small-scale magnetic elements that further rise to the photosphere, where bipolar sunspot pairs are formed through the coalescence of the small-scale magnetic elements. (2) Filamentary penumbral structures form when the sunspot is still growing through ongoing flux emergence. In contrast to the classical Evershed effect, the inflow seems to prevail over the outflow in a large part of the penumbra. (3) A well formed sunspot is a mostly monolithic magnetic structures that is anchored in a persistent deep-seated downdraft lane. The flow field outside the spot shows a giant vortex ring that comprises of an inflow below 15 Mm depth and an outflow above 15 Mm depth. (4) The sunspots successfully reproduce the fundamental properties of the observed solar active regions, including the more coherent leading spots with a stronger field strength, and the correct tilts of bipolar sunspot pairs. These asymmetries can be linked to the intrinsic asymmetries in the emerging magnetic and flow fields adapted from the convective dynamo simulation.

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F. Chen, M. Rempel and Y. Fan
Fri, 21 Apr 17
1/73

Comments: 27 pages, 13 figures, submitted to ApJ

Cosmic Rays near Proxima Centauri b [SSA]

http://arxiv.org/abs/1704.06168


Cosmic rays are an important factor of space weather determining radiation conditions near the Earth and it seems to be essential to clarify radiation conditions near extrasolar planets too. Last year a terrestrial planet candidate was discovered in an orbit around Proxima Centauri. Here we present our estimates on parameters of stellar wind of the Parker model, possible fluxes and fluencies of galactic and stellar cosmic rays based on the available data of the Proxima Centauri activity and its magnetic field. We found that galactic cosmic rays will be practically absent near Proxima~b up to energies of 1~TeV due to the modulation by the stellar wind. Stellar cosmic rays may be accelerated in Proxima Centauri events, which are able to permanently maintain density of stellar cosmic rays in the astrosphere comparable to low energy cosmic ray density in the heliosphere. Maximal proton intensities in extreme Proxima events should be by 3–4 orders more than in solar events.

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A. Struminsky, A. Sadovski and A. Belov
Fri, 21 Apr 17
10/73

Comments: N/A