Spectroscopic distances to late-type stars

AbstractA common approach to determining distances to stars without astrometric information is to compare stellar evolution models with parameters obtained from spectroscopic techniques. This method is routinely applied in the context of large-scale stellar surveys out to distances of several kpc. However, systematic errors may arise because of inaccurate spectroscopic parameters. We explore the effects of non-local thermodynamic equilibrium (NLTE) on the determination of surface gravities and metallicities for a large sample of metal-poor stars within approximately 10 kpc of the Sun. Using the improved Teff scale, we then show that stellar parameters estimated based on the widely used method of 1D LTE excitation-ionization balance of Fe results in distances which are systematically in error. For metal-poor giants, [Fe/H] ~ −2 dex, the distances can be overestimated by up to 70%. We compare the results with those from the Radial Velocity Experiment Survey catalogue (rave) for the stars in common, and find similar offsets.

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Inclusion of a time-dependent, non-local convection theory in a stellar evolution code

Proceedings of the International Astronomical Union ◽

10.1017/s1743921307000646 ◽

2006 ◽

Vol 2 (S239) ◽

pp. 314-316 ◽

Cited By ~ 1

Author(s):

Achim Weiss ◽

Martin Flaskamp

Keyword(s):

Velocity Field ◽

Local Time ◽

Stellar Evolution ◽

Time Dependent ◽

Test Cases ◽

The Sun ◽

Specific Test ◽

Non Local ◽

Convective Boundaries

AbstractThe non-local, time-dependent convection theory of Kuhfuß (1986) in both its one- and three-equation form has been implemented in the Garching stellar evolution code. We present details of the implementation and the difficulties encountered. Specific test cases have been calculated, among them a 5 M⊙ star and the Sun. These cases point out deficits of the theory. In particular, the assumption of an isotropic velocity field leads to too extensive overshooting and has to be modified at convective boundaries. Some encouraging aspects are indicated as well.

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Determination of corona, LTE, and NLTE regimes of optically thin carbon plasmas

Laser and Particle Beams ◽

10.1017/s0263034608000013 ◽

2008 ◽

Vol 26 (1) ◽

pp. 21-32 ◽

Cited By ~ 14

Author(s):

J.M. Gil ◽

R. RodrÍguez ◽

R. Florido ◽

J.G. Rubiano ◽

P. Martel ◽

...

Keyword(s):

Steady State ◽

Plasma Density ◽

Thermodynamic Equilibrium ◽

Local Thermodynamic Equilibrium ◽

Radiative Properties ◽

Thin Carbon ◽

Non Local

AbstractIn this work is accomplished the determination of the corona, local and non-local thermodynamic equilibrium regimes for optically thin carbon plasmas in steady state, in terms of the plasma density and temperature using the ABAKO code. The determination is made through the analysis of the plasma average ionization and ion and level populations. The results are compared whit those obtained applying Griem's criterion. Finally, it is made a brief analysis of the effects of the calculation of level populations assuming different plasma regimes in radiative properties, such as emissivities and opacities.

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H and He in stripped-envelope SNe – how much can be hidden?

Proceedings of the International Astronomical Union ◽

10.1017/s1743921312012793 ◽

2011 ◽

Vol 7 (S279) ◽

pp. 122-125

Author(s):

S. Hachinger ◽

P. A. Mazzali ◽

S. Taubenberger ◽

W. Hillebrandt ◽

K. Nomoto ◽

...

Keyword(s):

Radiative Transfer ◽

Thermodynamic Equilibrium ◽

Stellar Evolution ◽

Local Thermodynamic Equilibrium ◽

Explosion Energy ◽

High Mass ◽

Synthetic Spectra ◽

Non Local ◽

Moderate Mass

AbstractH and He features in photospheric spectra have rarely been used to constrain the structure of Type IIb/Ib/Ic supernovae (SNe IIb/Ib/Ic). The lines have to be modelled with a detailed non-local-thermodynamic-equilibrium (NLTE) treatment, including effects uncommon in stars. Once this is done, however, one obtains valuable hints on the characteristics of progenitors and explosions (composition, explosion energy, . . .). We have extended a radiative transfer code to compute synthetic spectra of SNe IIb, Ib and Ic. Here, we discuss our first larger set of models, focusing on the question: How much H/He can be hidden (i.e. remain undetected in photospheric spectra) in SNe Ib/Ic? For the SNe studied (relatively low Mej = 1. . .3 M⊙), we find a limit of MHe ≲ 0.1 M⊙ in SNe Ic (no unambiguous He lines). Stellar evolution models for single stars normally always yield higher masses. We suggest that low- or moderate-mass SNe Ic result from efficient envelope stripping in binaries. We propose similar studies on H/He in high-mass and extremely aspherical SNe, and observations covering the region of He I λ 20581.

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Ionization balance of Ti in the photospheres of the Sun and four late-type stars

Monthly Notices of the Royal Astronomical Society ◽

10.1111/j.1365-2966.2011.18295.x ◽

2011 ◽

Vol 413 (3) ◽

pp. 2184-2198 ◽

Cited By ~ 68

Author(s):

Maria Bergemann

Keyword(s):

The Sun ◽

Late Type ◽

Ionization Balance

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Fe I/Fe II ionization equilibrium in cool stars: NLTE versus LTE

Proceedings of the International Astronomical Union ◽

10.1017/s1743921310000530 ◽

2009 ◽

Vol 5 (S265) ◽

pp. 197-200 ◽

Cited By ~ 8

Author(s):

Lyudmila Mashonkina ◽

Thomas Gehren ◽

Jianrong Shi ◽

Andreas Korn ◽

Frank Grupp

Keyword(s):

Local Thermodynamic Equilibrium ◽

The Sun ◽

Line Formation ◽

Formation Model ◽

Hydrogen Atoms ◽

Cool Stars ◽

Solar Metallicity ◽

Non Local ◽

The Difference ◽

Model Atom

AbstractNon-local thermodynamic equilibrium (NLTE) line formation for neutral and singly-ionized iron is considered through a range of stellar parameters characteristic of cool stars. A comprehensive model atom for Fe I and Fe II is presented. Our NLTE calculations support the earlier conclusions that the statistical equilibrium (SE) of Fe I shows an underpopulation of Fe I terms. However, the inclusion of the predicted high-excitation levels of Fe I in our model atom leads to a substantial decrease in the departures from LTE. As a test and first application of the Fe I/II model atom, iron abundances are determined for the Sun and four selected stars with well determined stellar parameters and high-quality observed spectra. Within the error bars, lines of Fe I and Fe II give consistent abundances for the Sun and two metal-poor stars when inelastic collisions with hydrogen atoms are taken into account in the SE calculations. For the close-to-solar metallicity stars Procyon and β Vir, the difference (Fe II - Fe I) is about 0.1 dex independent of the line formation model, either NLTE or LTE. We evaluate the influence of departures from LTE on Fe abundance and surface gravity determination for cool stars.

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Deuterium Observations in our Galaxy - View A

Symposium - International Astronomical Union ◽

10.1017/s0074180900166525 ◽

2000 ◽

Vol 198 ◽

pp. 141-150 ◽

Cited By ~ 2

Author(s):

Jeffrey L. Linsky ◽

Brian E. Wood

Keyword(s):

Chemical Evolution ◽

Systematic Errors ◽

Interstellar Cloud ◽

The Sun ◽

Galactic Chemical Evolution ◽

Late Type ◽

High Quality ◽

Ultraviolet Spectra ◽

Local Interstellar Cloud ◽

Very High

Accurate measurements of the D/H ratio in our Galaxy provide critical tests of Galactic chemical evolution and constrain the primordial value of D/H. Very high quality ultraviolet spectra from the GHRS and STIS instruments on HST have been analyzed for lines of sight toward both early and late-type stars and hot white dwarfs. We will summarize the results that are being obtained for D/H along sightlines through the Local Interstellar Cloud (LIC) and other nearby warm clouds. All sightlines through the LIC are consistent with D/H = (1.53 ± 0.18) × 10−5. Whether or not significantly different values of D/H are present in other clouds within 100 pc of the Sun is not yet settled, but there is evidence that D/H is significantly lower in Orion (500 pc). We will describe the likely sources of systematic errors in determining D/H that must be understood and quantified when analyzing such ultraviolet spectra.

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Radio Emission from the Sun and Stars: New Insights into Energetic Phenomena

Symposium - International Astronomical Union ◽

10.1017/s0074180900182063 ◽

2004 ◽

Vol 219 ◽

pp. 145-158

Author(s):

T. S. Bastian

Keyword(s):

Radio Emission ◽

Energy Release ◽

Coronal Mass Ejections ◽

Magnetic Energy ◽

Spectroscopic Techniques ◽

The Sun ◽

Late Type ◽

Radio Observations ◽

Dwarf Stars ◽

Magnetic Energy Release

Energetic phenomena on the Sun and late-type stars pose a number of fascinating puzzles. These include coronal heating, flares, and coronal mass ejections, all believed to be manifestations of magnetic energy release. Radio radiation is a sensitive tracer of energetic phenomena on both the Sun and stars. Radio observations of the Sun over the past decade have produced new insights into the physics of magnetic energy release in flares and coronal mass ejections. Radio observations of late-type stars have exploited sensitive imaging and spectroscopic techniques to further constrain the nature of the relevant emission mechanisms. A surprise has been the recent discovery of radio emission from brown dwarf stars, implying the existence of substantial magnetic fields and a means of dissipating magnetic energy, neither of which are understood.

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The Opacity Project and the Iron Project

International Astronomical Union Colloquium ◽

10.1017/s0252921100036551 ◽

1996 ◽

Vol 152 ◽

pp. 569-576

Author(s):

Anil K. Pradhan

Keyword(s):

Recent Work ◽

Thermodynamic Equilibrium ◽

Large Scale ◽

Local Thermodynamic Equilibrium ◽

Atomic Data ◽

Data Sets ◽

Iron Ions ◽

Theoretical Methods ◽

Work Related ◽

Ionization Balance

Systematic and large-scale calculation of accurate plasma opacities and atomic data by the Opacity Project and the Iron Project has applications in many areas of astrophysics. Analysis of EUVE observations using monochromatic opacities of elements calculated by the Opacity Project is described. Theoretical methods and atomic calculations are discussed briefly. Recent work related to ionization balance, photoionization and recombination, and the modeling of plasmas in local thermodynamic equilibrium (LTE) and non-LTE are discussed. New calculations for the important iron ions, under the Iron Project, are also described and their relevance to the more extensive non-LTE calculations is pointed out. The comprehensive radiative and collisional data sets from the Opacity and the Iron Projects should be applicable to a large number of sources in the EUV and other wavelength regions.

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The influence of NLTE effects in Fe I lines on an inverted atmosphere

Astronomy and Astrophysics ◽

10.1051/0004-6361/201937041 ◽

2020 ◽

Vol 633 ◽

pp. A157 ◽

Cited By ~ 2

Author(s):

H. N. Smitha ◽

R. Holzreuter ◽

M. van Noort ◽

S. K. Solanki

Keyword(s):

Local Thermodynamic Equilibrium ◽

Solar Physics ◽

Accurate Determination ◽

Line Of Sight ◽

Line Pair ◽

Line Profiles ◽

Magnetic Elements ◽

Non Local ◽

Magnetohydrodynamic Simulation

Context. Ultraviolet overionisation of iron atoms in the solar atmosphere leads to deviations in their level populations based on Saha-Boltzmann statistics. This causes their line profiles to form in non-local thermodynamic equilibrium (NLTE) conditions. When inverting such profiles to determine atmospheric parameters, the NLTE effects are often neglected and other quantities are tweaked to compensate for deviations from the LTE. Aims. We investigate how the routinely employed LTE inversion of iron lines formed in NLTE underestimates or overestimates atmospheric quantities, such as temperature (T), line-of-sight velocity (vLOS), magnetic field strength (B), and inclination (γ) while the earlier papers have focused mainly on T. Our findings has wide-ranging consequences since many results derived in solar physics are based on inversions of Fe I lines carried out in LTE. Methods. We synthesized the Stokes profiles of Fe I 6301.5 Å and 6302.5 Å lines in both LTE and NLTE using a snapshot of a 3D magnetohydrodynamic simulation. The profiles were then inverted in LTE. We considered the atmosphere inferred from the inversion of LTE profiles as the fiducial model and compared it to the atmosphere resulting from the inversion of NLTE profiles. The observed differences have been attributed to NLTE effects. Results. Neglecting the NLTE effects introduces errors in the inverted atmosphere. While the errors in T can go up to 13%, in vLOS and B, the errors can go as high as 50% or above. We find these errors to be present at all three inversion nodes. Importantly, they survive degradation from the spatial averaging of the profiles. Conclusions. We provide an overview of how neglecting NLTE effects influences the values of T, vLOS, B, and γ that are determined by inverting the Fe I 6300 Å line pair, as observed, for example, by Hinode/SOT/SP. Errors are found at the sites of granules, intergranular lanes, magnetic elements, and basically in every region susceptible to NLTE effects. For an accurate determination of the atmospheric quantities and their stratification, it is, therefore, important to take the NLTE effects into account.

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