scholarly journals Stellar Abundance Analyses in the Light of 3D Hydrodynamical Model Atmospheres

2003 ◽  
Vol 210 ◽  
pp. 273-286 ◽  
Author(s):  
Martin Asplund
Keyword(s):  
Recent Progress ◽  
Hydrodynamical Model ◽  
Observational Constraints ◽  
The Sun ◽  
Mixing Length ◽  
Line Formation ◽  
Late Type ◽  
Line Shapes ◽  
Free Parameters

I describe recent progress in terms of 3D hydrodynamical model atmospheres and 3D line formation and their applications to stellar abundance analyses of late-type stars. Such 3D studies remove the free parameters inherent in classical 1D investigations (mixing length parameters, macro- and microturbulence) yet are highly successful in reproducing a large arsenal of observational constraints such as detailed line shapes and asymmetries. Their potential for abundance analyses is illustrated by discussing the derived oxygen abundances in the Sun and in metal-poor stars, where they seem to resolve long-standing problems as well as significantly alter the inferred conclusions.

scholarly journals The light elements in the light of 3D and non-LTE effects

2009 ◽  
Vol 5 (S268) ◽  
pp. 191-200 ◽  
Author(s):  
Martin Asplund ◽  
Karin Lind
Keyword(s):  
Systematic Errors ◽  
Hydrodynamical Model ◽  
Line Formation ◽  
Light Elements ◽  
Late Type ◽  
Halo Stars ◽  
Isotopic Abundances ◽  
Hydrogen Lines ◽  
The Impact

AbstractIn this review we discuss possible systematic errors inherent in classical 1D LTE abundance analyses of late-type stars for the light elements (here: H, He, Li, Be and B). The advent of realistic 3D hydrodynamical model atmospheres and the availability of non-LTE line formation codes place the stellar analyses on a much firmer footing and indeed drastically modify the astrophysical interpretations in many cases, especially at low metallicities. For the Teff-sensitive hydrogen lines both stellar granulation and non-LTE are likely important but the combination of the two has not yet been fully explored. A fortuitous near-cancellation of significant but opposite 3D and non-LTE effects leaves the derived 7Li abundances largely unaffected but new atomic collisional data should be taken into account. We also discuss the impact on 3D non-LTE line formation on the estimated lithium isotopic abundances in halo stars in light of recent claims that convective line asymmetries can mimic the presence of 6Li. While Be only have relatively minor non-LTE abundance corrections, B is sensitive even if the latest calculations imply smaller non-LTE effects than previously thought.

A Search for Dyson Spheres around Late-Type Stars in the Solar Neighborhood II

1997 ◽  
Vol 161 ◽  
pp. 707-709 ◽  
Author(s):  
Jun Jugaku ◽  
Shiro Nishimura
Keyword(s):  
Solar Neighborhood ◽  
The Sun ◽  
Late Type ◽  
Solar Type

AbstractWe continued our search for partial (incomplete) Dyson spheres associated with 50 solar-type stars (spectral classes F, G, and K) within 25 pc of the Sun. No candidate objects were found.

scholarly journals Are “realistic” model atmospheres realistic enough?

2009 ◽  
Vol 5 (S265) ◽  
pp. 187-196 ◽  
Author(s):  
Bengt Gustafsson
Keyword(s):  
Realistic Model ◽  
Considerable Progress ◽  
Observational Methods ◽  
Construction Technology ◽  
Mixing Length ◽  
Late Type ◽  
Chemical Abundances ◽  
Stellar Spectra ◽  
Analysis Methods ◽  
Major Break

AbstractDuring the latest decades the number of papers on stellar chemical abundances has increased dramatically. This is basically reflecting the very great achievements in telescope- and spectrometer-construction technology. The analysis of the resulting stellar spectra, however, is still not up to the standard that is offered by the observational methods. Recent significant advances in the analysis methods (i.e., in constructing model atmospheres and model spectra to compare with the observed ones) is reviewed with the emphasis on the application to abundance analysis of late-type stars. It is found that the very considerable progress that have been made beyond mixing-length convection and LTE is a major break-through for physically consistent modeling. Still, however, further steps must be taken, in particular for the cooler stars, before the situation is fully satisifactory.

scholarly journals Modeling the center-to-limb variation of the Ca i 4227 Å line using FCHHT models

2014 ◽  
Vol 10 (S305) ◽  
pp. 381-386
Author(s):  
H. D. Supriya ◽  
H. N. Smitha ◽  
K. N. Nagendra ◽  
J. O. Stenflo ◽  
M. Bianda ◽  
...  
Keyword(s):  
Solar Spectrum ◽  
Visible Spectrum ◽  
Observational Constraints ◽  
Solar Model ◽  
The Sun ◽  
Dimensional Modeling ◽  
Height Dependence ◽  
Computationally Expensive ◽  
Chromospheric Line ◽  
The Magnetic Field

AbstractThe Ca i 4227 Å is a chromospheric line exhibiting the largest degree of linear polarization near the limb, in the visible spectrum of the Sun. Modeling the observations of the center-to-limb variations (CLV) of different lines in the Second Solar Spectrum helps to sample the height dependence of the magnetic field, as the observations made at different lines of sight sample different heights in the solar atmosphere. Supriya et al. (2014) attempted to simultaneously model the CLV of the (I, Q/I) spectra of the Ca i 4227 Å line using the standard 1-D FAL model atmospheres. They found that the standard FAL model atmospheres and also any appropriate combination of them, fail to simultaneously fit the observed Stokes (I, Q/I) profiles at all the limb distances (μ) satisfying at the same time all the observational constraints. This failure of 1-D modeling approach can probably be overcome by using multi-dimensional modeling which is computationally expensive. To eliminate an even wider choice of 1-D models, we attempt here to simultaneously model the CLV of the (I, Q/I) spectra using the FCHHT solar model atmospheres which are updated and recent versions of the FAL models. The details of our modeling efforts and the results are presented.

scholarly journals The confrontation of the shock-powered synchrotron maser model with the Galactic FRB 200428

2020 ◽  
Vol 500 (2) ◽  
pp. 2704-2710 ◽  
Author(s):  
Yun-Wei Yu ◽  
Yuan-Chuan Zou ◽  
Zi-Gao Dai ◽  
Wen-Fei Yu
Keyword(s):  
Line Of Sight ◽  
Model Parameters ◽  
Observational Constraints ◽  
Synchrotron Emission ◽  
Physical Processes ◽  
X Ray ◽  
Emission Efficiency ◽  
Free Parameters ◽  
Fast Radio Burst ◽  
Parameter Constraints

ABSTRACT The association of FRB 200428 with an X-ray burst (XRB) from the Galactic magnetar SGR 1935+2154 offers important implications for the physical processes responsible for the fast radio burst (FRB) phenomena. By assuming that the XRB emission is produced in the magnetosphere, we investigate the possibility that the FRB emission is produced by shock-powered synchrotron maser (SM), which is phenomenologically described with a number of free parameters. The observational constraints on the model parameters indicate that the model can in principle be consistent with the FRB 200428 observations, if the ejecta lunched by magnetar activities can have appropriate ingredients and structures and the shock processes occur on the line of sight. To be specific, a complete burst ejecta should consist of an ultra-relativistic and extremely highly collimated e± component and a sub-relativistic and wide-spreading baryonic component. The internal shocks producing the FRB emission arise from a collision between the e± ejecta and the remnant of a previous baryonic ejecta at the same direction. The parameter constraints depend on the uncertain spectrum and efficiency of the SM emission. While the spectrum is tentatively described by a spectral index of −2, we estimate the emission efficiency to be around 10−4 by requiring that the synchrotron emission of the shocked material cannot be much brighter than the magnetosphere XRB emission.

Galactic Infrared Astronomy

1971 ◽  
Vol 2 (1) ◽  
pp. 14-20 ◽  
Author(s):  
A. R. Hyland
Keyword(s):  
Interstellar Extinction ◽  
Infrared Astronomy ◽  
The Sun ◽  
Late Type ◽  
Infrared Observations ◽  
Effective Temperatures

Until the last six years few infrared observations of astronomical objects (other than the Sun and planets) had been attempted. Those few were primarily concerned with obtaining bolometric corrections and effective temperatures, the variation of bolometric magnitudes of late-type variables, and the derivation of the laws of interstellar extinction.

scholarly journals Sun-like Stars: magnetic fields, cycles and exoplanets

2015 ◽  
Vol 11 (A29A) ◽  
pp. 360-364
Author(s):  
Rim Fares
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Observational Constraints ◽  
The Sun ◽  
Field Generation ◽  
Large Scale Magnetic Field ◽  
Planetary Orbits ◽  
Activity Cycles ◽  
Magnetic Cycles

AbstractIn Sun-like stars, magnetic fields are generated in the outer convective layers. They shape the stellar environment, from the photosphere to planetary orbits. Studying the large-scale magnetic field of those stars enlightens our understanding of the field properties and gives us observational constraints for field generation dynamo models. It also sheds light on how “normal” the Sun is among Sun-like stars. In this contribution, I will review the field properties of Sun-like stars, focusing on solar twins and planet hosting stars. I will discuss the observed large-scale magnetic cycles, compare them to stellar activity cycles, and link that to what we know about the Sun. I will also discuss the effect of large-scale stellar fields on exoplanets, exoplanetary emissions (e.g. radio), and habitability.

scholarly journals IUE Observations of the Chromospheric Activity-Age Relation in Young Solar-Type Stars

1983 ◽  
Vol 102 ◽  
pp. 161-164
Author(s):  
Theodore Simon ◽  
Ann Merchant Boesgaard
Keyword(s):  
Magnetic Fields ◽  
Flare Activity ◽  
The Sun ◽  
Late Type ◽  
Cool Star ◽  
Opportune Time ◽  
Solar Type ◽  
Age Relation ◽  
Relationship Of ◽  
The Relationship

The difficulties of measuring magnetic fields in late-type stars other than the sun are well known, as one is reminded by other contributions to these Proceedings. This Symposium nevertheless comes at a very opportune time, as we are now at the point where we can begin to explore the relationship of stellar magnetism to flare activity and quiescent cool star chromospheres, transition regions (TRs), and coronae.

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