scholarly journals Turbulence and the Lithium Abundance in Giants and Main-Sequence Disk and Halo Stars

1991 ◽  
Vol 145 ◽  
pp. 327-340
Author(s):  
S. Vauclair
Keyword(s):  
Main Sequence ◽  
Present Situation ◽  
Point Of View ◽  
Theoretical Point ◽  
Main Sequence Stars ◽  
Lithium Abundance ◽  
Giant Stars ◽  
Halo Stars

The present situation concerning the observations of lithium in subgiant and giant stars is reviewed. These observations, compared to the determinations of lithium in main-sequence stars, can be used to obtain more stringents constraints on the dynamics of the stellar gas in the main-sequence as well as in the subgiant and giant stages. The macroscopic motions expected in stars are discussed from a theoretical point of view and compared to the observations, with special focusing on the theories of rotation-induced turbulence.

scholarly journals The Frequency of Faint M Giant Stars at High Galactic Latitudes

1977 ◽  
Vol 4 (2) ◽  
pp. 35-36 ◽  
Author(s):  
N. Sanduleak
Keyword(s):  
Main Sequence ◽  
Galactic Plane ◽  
Absolute Magnitude ◽  
Main Sequence Stars ◽  
Giant Stars ◽  
The North ◽  
Objective Prism ◽  
M Stars ◽  
Low Dispersion ◽  
North Galactic

Based on the observations of M giant stars in the north galactic polar objective-prism survey of Upgren (1960) and the data summarized by Blanco (1965) the overall space density of all M-type giants as a function of distance from the galactic plane at the position of the sun can be approximated by,where z is in kpc and ρ(z) is the number of stars per 106 pc3. This relationship is derived from the observed fall-off in space densities up to a distance of about 2 kpc.The question arises as to the validity of extrapolation equation (1) to larger z distances so as to predict the number of faint M giants expected per unit area near the galactic poles. Adopting for the M giants a mean visual absolute magnitude of −1.0 (Blanco 1965), one finds that equation (1) predicts that less than one giant fainter than V~12 should be expected in a region of 200 square degrees. This expectation formed the hypothesis of a thesis study (Sanduleak 1965) in which it was assumed that the very faint M stars detected in a deep, infrared objective-prism survey at the NGP were main-sequence stars, since this could not be ascertained spectroscopically on the very low-dispersion plates used.

scholarly journals Discussion D: Observational problems with Li, Be and B

2009 ◽  
Vol 5 (S268) ◽  
pp. 493-497
Author(s):  
Poul Erik Nissen
Keyword(s):  
Globular Clusters ◽  
Heavy Element ◽  
Main Sequence ◽  
Downward Trend ◽  
Element Abundance ◽  
Theoretical Studies ◽  
Dwarf Stars ◽  
Giant Stars ◽  
Halo Stars ◽  
Solar Type

AbstractIn Discussion D the following problems were addressed: Has 6Li really been detected in the atmospheres of metal-poor halo stars? Is there a downward trend or increased scatter of Li abundances in stars on the ‘Li-plateau’ at metallicities [Fe/H] ≲ −2.5? Are there significant differences of Li abundances in main-sequence, turn-off, and sub-giant stars in globular clusters? Is the Li abundance in solar-type stars related to the presence of planets? How does the Be abundance in dwarf stars increase with the heavy-element abundance, and is there a cosmic scatter in Be at a given [Fe/H]? The discussion of these problems is summarized and some suggestions for future observational and theoretical studies are mentioned.

scholarly journals Main-sequence and sub-giant stars in the globular cluster NGC 6397: The complex evolution of the lithium abundance

2009 ◽  
Vol 5 (S268) ◽  
pp. 257-261
Author(s):  
J. I. González Hernández ◽  
P. Bonifacio ◽  
E. Caffau ◽  
M. Steffen ◽  
H.-G. Ludwig ◽  
...  
Keyword(s):  
Globular Cluster ◽  
Main Sequence ◽  
3D Models ◽  
Stellar Atmospheres ◽  
Evolutionary Status ◽  
Thermodynamical Equilibrium ◽  
Lithium Abundance ◽  
Giant Stars ◽  
Effective Temperatures ◽  
Hydrodynamical Simulations

AbstractThanks to the high multiplex and efficiency of Giraffe at the VLT we have been able for the first time to observe the Li I doublet in the Main Sequence stars of a globular cluster. At the same time we observed Li in a sample of Sub-Giant stars of the same B-V colour.Our final sample is composed of 84 SG stars and 79 MS stars. In spite of the fact that SG and MS span the same temperature range we find that the equivalent widths of the Li I doublet in SG stars are systematically larger than those in MS stars, suggesting a higher Li content among SG stars. This is confirmed by our quantitative analysis carried out making use of 1D hydrostatic plane-parallel models and 3D hydrodynamical simulations of the stellar atmospheres.We derived the effective temperatures of stars in our the sample from Hα fitting. Theoretical profiles were computed using 3D hydrodynamical simulations and 1D ATLAS models. Therefore, we are able to determined 1D and 3D-based effective temperatures. We then infer Li abundances taking into account non-local thermodynamical equilibrium effects when using both 1D and 3D models.We find that SG stars have a mean Li abundance higher by 0.1 dex than MS stars. This result is obtained using both 1D and 3D models. We also detect a positive slope of Li abundance with effective temperature, the higher the temperature the higher the Li abundance, both for SG and MS stars, although the slope is slightly steeper for MS stars. These results provide an unambiguous evidence that the Li abundance changes with evolutionary status.The physical mechanisms responsible for this behaviour are not yet clear, and none of the existing models seems to describe accurately these observations. Based on these conclusions, we believe that the cosmological lithium problem still remains an open question.

scholarly journals The Rotation of the Ap Stars From the Point of View of the Rigid Rotator Model

1970 ◽  
Vol 4 ◽  
pp. 254-263
Author(s):  
George W. Preston
Keyword(s):  
Distribution Function ◽  
Main Sequence ◽  
Point Of View ◽  
Main Sequence Stars ◽  
Rigid Rotator ◽  
Long Period ◽  
Rotator Model ◽  
Rotational Distribution ◽  
The Mean ◽  
Ap Stars

AbstractDeutsch’s period vs. line-width relation for the periodic Ap stars is re-examined from the point of view of the rigid rotator with the aid of recently determined values of v sin i for these objects. The agreement between computed rotational velocities and observed values of v sin i is satisfactory if the radii of the Ap stars are twice those of zero-age main sequence stars of the same color. The computed rotational velocities also agree with the mean rotational velocities of all Ap stars if suitable allowances are made for observational limitations. However, a difficulty arises in connection with the number of ‘long-period’ stars that have been discovered. Their small computed rotational velocities and their frequency are such that they cannot be regarded as part of any rotational distribution function that describes normal stars or even most Ap stars. It is concluded that if the periods of these objects are rotational periods, then a powerful rotational deceleration mechanism must be operative in at least some Ap stars, primarily those of the SrCrEu group.

scholarly journals Spectroscopic and Photometric Observations of the Southern, Intermediate-Age Clusters NGC 2477 and NGC 2660

1980 ◽  
Vol 85 ◽  
pp. 235-236
Author(s):  
James E. Hesser ◽  
Horace A. Smith
Keyword(s):  
Radial Velocity ◽  
Main Sequence ◽  
Meridional Circulation ◽  
Main Sequence Stars ◽  
Image Tube ◽  
Giant Stars ◽  
Photometric Observations ◽  
Blue Straggler ◽  
The Mean

47 A mm−1 image-tube spectra show that the turnoff region and blue straggler stars in NGC 2477 are not, in general, rapid rotators, as postulated by Hartwick and Hesser (1974) to explain, via mixing induced by meridional circulation, the widely differing photometric metallicity estimates based upon uvbyβ photometry of the main-sequence stars and DDO photometry of the giant stars. However, the discrepant metallicity estimates are largely reconciled by recent revisions in the reddening corrections for the DDO system (Boyle 1979, Janes 1979); and by small color corrections of the type described by Muzzio (1978) and Schmidt and Taylor (1979) applied to the β-photometry. The mean radial velocity is found to be 5.6 kms−1 while the mean Vsini is 105 km s−1. Star “m” is spectroscopically confirmed to be an Am star.

scholarly journals The Gaia-ESO Survey: Lithium enrichment histories of the Galactic thick and thin disc

2018 ◽  
Vol 610 ◽  
pp. A38 ◽  
Author(s):  
X. Fu ◽  
D. Romano ◽  
A. Bragaglia ◽  
A. Mucciarelli ◽  
K. Lind ◽  
...  
Keyword(s):  
Main Sequence ◽  
Big Bang ◽  
Galactic Cosmic Rays ◽  
Asymptotic Giant Branch ◽  
Red Giants ◽  
Main Sequence Stars ◽  
Lithium Abundance ◽  
Thin Disc ◽  
Relative Contribution ◽  
Solar Metallicity

Lithium abundance in most of the warm metal-poor main sequence stars shows a constarnt plateau (A(Li) ~ 2.2 dex) and then the upper envelope of the lithium vs. metallicity distribution increases as we approach solar metallicity. Meteorites, which carry information about the chemical composition of the interstellar medium (ISM) at the solar system formation time, show a lithium abundance A(Li) ~ 3.26 dex. This pattern reflects the Li enrichment history of the ISM during the Galaxy lifetime. After the initial Li production in big bang nucleosynthesis, the sources of the enrichment include asymptotic giant branch (AGB) stars, low-mass red giants, novae, type II supernovae, and Galactic cosmic rays. The total amount of enriched Li is sensitive to the relative contribution of these sources. Thus different Li enrichment histories are expected in the Galactic thick and thin disc. We investigate the main sequence stars observed with UVES in Gaia-ESO Survey iDR4 catalogue and find a Li- [α/Fe] anticorrelation independent of [Fe/H], Teff, and log (g). Since in stellar evolution different α enhancements at the same metallicity do not lead to a measurable Li abundance change, the anticorrelation indicates that more Li is produced during the Galactic thin disc phase than during the Galactic thick disc phase. We also find a correlation between the abundance of Li and s-process elements Ba and Y, and they both decrease above the solar metallicity, which can be explained in the framework of the adopted Galactic chemical evolution models.

scholarly journals Lithium Abundance, Diffusion and Turbulence

1995 ◽  
Vol 10 ◽  
pp. 459-460
Author(s):  
G. Michaud ◽  
G. Beaudet
Keyword(s):  
Main Sequence ◽  
Normal Values ◽  
Present Theory ◽  
Constant Mass ◽  
Physical Processes ◽  
Main Sequence Stars ◽  
Hydrogen Burning ◽  
Lithium Abundance ◽  
Calculated Curve ◽  
Fully Coupled

Richer & Michaud (1993) calculated a series of envelopes fully coupled to non-rotating, constant mass, stellar evolution models of hydrogen burning stars with masses in the range of 1.2 to 2.2 M⊙, typical of A and F main sequence stars. They included He settling. The location of the theoretically predicted gap of the Hyades agrees quite well with the observed one, a result obtained without the introduction of any free parameter. At temperatures above the gap, while the observed lithium abundances are within a factor of 2-3 of normal values, the theoretical calculated curve drops to very low values. Diffusion velocities being fairly small, any other physical process with larger or similar velocities can reduce the effect of diffusion and produce the observed results. Mass loss is one such process. Another difficulty with the present theory is the width of the gap. Observations show that the observed gap is wider than the calculated one in the Hyades. This also suggests that other physical processes play an important role.

scholarly journals Testing planet formation theories with giant stars

2007 ◽  
Vol 3 (S249) ◽  
pp. 209-222
Author(s):  
Luca Pasquini ◽  
M.P. Döllinger ◽  
A. Hatzes ◽  
J. Setiawan ◽  
L. Girardi ◽  
...  
Keyword(s):  
Main Sequence ◽  
Planet Formation ◽  
Mass Range ◽  
Stellar Mass ◽  
Giant Planets ◽  
Main Sequence Stars ◽  
Giant Stars ◽  
Solar Type ◽  
The Subject ◽  
Host Stars

AbstractPlanet searches around evolved giant stars are bringing new insights to planet formation theories by virtue of the broader stellar mass range of the host stars compared to the solar-type stars that have been the subject of most current planet searches programs. These searches among giant stars are producing extremely interesting results. Contrary to main sequence stars planet-hosting giants do not show a tendency of being more metal rich. Even if limited, the statistics also suggest a higher frequency of giant planets (at least 10%) that are more massive compared to solar-type main sequence stars.The interpretation of these results is not straightforward. We propose that the lack of a metallicity-planet connection among giant stars is due to pollution of the star while on the main sequence, followed by dillution during the giant phase. We also suggest that the higher mass and frequency of the planets are due to the higher stellar mass. Even if these results do not favor a specific formation scenario, they suggest that planetary formation might be more complex than what has been proposed so far, perhaps with two mechanisms at work and one or the other dominating according to the stellar mass. We finally stress as the detailed study of the host stars and of the parent sample is essential to derive firm conclusions.

The occurrence of lithium in F5-G8 main-sequence stars

1966 ◽  
Vol 24 ◽  
pp. 13-14
Author(s):  
G. H. Herbig
Keyword(s):  
Main Sequence ◽  
Physical Significance ◽  
Lithium Content ◽  
Main Sequence Stars ◽  
Lithium Abundance ◽  
Major Variation ◽  
Classification Parameter

The discovery that there is a major variation in the strength of the Li1λ6707 line among F and G dwarfs adds another classification parameter in this region of the H-R diagram. (We do not consider here the Li situation in stars of higher luminosity.) But the immediate question is: What is the physical significance of this dispersion in lithium content? The details are now in press (1), but here we review the evidence to the effect that the lithium abundance, at least in G stars, can be considered as an index of evolutionary age.

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