scholarly journals The Lithium Isotope Ratio In Old Stars

1995 ◽  
Vol 10 ◽  
pp. 443-444
Author(s):  
P.E. Nissen
Keyword(s):  
Isotope Ratio ◽  
Main Sequence ◽  
Cosmic Ray ◽  
Big Bang ◽  
Stellar Atmosphere ◽  
Lithium Isotope ◽  
Stellar Rotation ◽  
Main Sequence Stars ◽  
Big Bang Nucleosynthesis ◽  
Upper Limits

The lithium isotope ratio in stars can be determined from high resolution observations of the profile of the Li I 6707 Å absorption line. Earlier studies of old F and G stars (Andersen et al. 1984, Maurice et al. 1984, Pilachowski et al. 1989) have led to upper limits of 6Li/7Li ranging from 0.05 to 0.10. Recently, Smith, Lambert & Nissen (1993) seem to have detected 6Li in HD 84937 - a metal-poor turnoff star with Teff ⋍ 6200 K and [Fe/H] ⋍ —2.4. An isotope ratio 6Li/7Li = 0.05 ± 0.02 was determined (see Fig. 1) The detection has been confirmed by Hobbs & Thorburn (1994), who derived 6Li/7Li = 0.07 ± 0.03. The main contribution to the quoted (1σ) errors comes from the noise in the spectrum (S/N = 400) and possible errors in the Doppler broadening of the Li line (Nissen 1994). This broadening is due to stellar rotation and macro-turbulent motions in the stellar atmosphere and can be determined from the profiles of unblended metallic absorption lines.As discussed in detail by Steigman et al. (1993) the presence of 6Li in the atmosphere of HD 84937 is consistent with the measured Be abundance (Boesgaard & King 1993) within the context of i) Standard Big Bang nucleosynthesis, ii) Pop. II cosmic ray nucleosynthesis and iii) standard (non-rotating) models for Li depletion. In particular, Steigman et al. derive D6 > 0.2, where D6 is the depletion factor for 6Li. As shown by Chaboyer (1994) standard stellar evolution models with new opacities predict D6≃ 0.4 for turnoff stars and subgiants with Teff > 5900 K. The same models predict D7 ≃ 1.0, i.e. no 7Li depletion for main sequence stars as well as subgiants with Teff ≥ 5800 K.

scholarly journals Lithium evolution from Pre-Main Sequence to the Spite plateau: an environmental solution to the cosmological lithium problem

2015 ◽  
Vol 11 (S317) ◽  
pp. 300-301
Author(s):  
Xiaoting Fu ◽  
Alessandro Bressan ◽  
Paolo Molaro ◽  
Paola Marigo
Keyword(s):  
Main Sequence ◽  
Big Bang ◽  
Stellar Atmosphere ◽  
Main Sequence Stars ◽  
Lower Mass ◽  
Big Bang Nucleosynthesis ◽  
Lithium Abundance ◽  
Wide Range ◽  
Nuclear Burning ◽  
And Diffusion

AbstractLithium abundance derived in metal-poor main sequence stars is about three times lower than the primordial value of the standard Big Bang nucleosynthesis prediction. This disagreement is referred to as the lithium problem. We reconsider the stellar Li evolution from the pre-main sequence to the end of main sequence phase by introducing the effects of overshooting and residual mass accretion. We show that 7Li could be significantly depleted by convective overshooting in the pre-main sequence phase and then partially restored in the stellar atmosphere by residual accretion which follows the Li depletion phase and could be regulated by EUV photo-evaporation. By considering the conventional nuclear burning and diffusion along the main sequence we can reproduce the Spite plateau for stars with initial mass m0=0.62–0.80 M⊙, and the Li declining branch for lower mass dwarfs, e.g, m0=0.57–0.60 M⊙, for a wide range of metallicities (Z=0.00001 to Z=0.0005), starting from an initial Li abundance A(Li) = 2.72.

scholarly journals Proton-Induced Reactions of Astrophysical Interest

2018 ◽  
Vol 184 ◽  
pp. 02001
Author(s):  
Thomas Chillery
Keyword(s):  
Cross Sections ◽  
Main Sequence ◽  
Nuclear Astrophysics ◽  
Big Bang ◽  
High Accuracy ◽  
Low Energy ◽  
Current Status ◽  
Main Sequence Stars ◽  
Big Bang Nucleosynthesis ◽  
Proton Induced Reactions

A discrepancy exists between the 6Li abundances predicted from big bang nucleosynthesis models and those measured in pre-main sequence stars. To further constrain the predicted abundances of 6Li in these stars, high accuracy measurements are required of reactions destroying 6Li. Namely 6Li(p,γ)7Be and 6Li(p,α) 3He. These have recently been studied at the Laboratory for Underground Nuclear Astrophysics (LUNA) to measure their low energy cross sections. I present both the campaign’s experimental setup and current status of the data analysis.

Stellar rotation in lower main-sequence stars measured from time variations in H and K emission-line fluxes. I - Initial results

1981 ◽  
Vol 250 ◽  
pp. 276 ◽  
Author(s):  
A. H. Vaughan ◽  
G. W. Preston ◽  
S. L. Baliunas ◽  
L. W. Hartmann ◽  
R. W. Noyes ◽  
...  
Keyword(s):  
Emission Line ◽  
Main Sequence ◽  
Stellar Rotation ◽  
Main Sequence Stars ◽  
Time Variations ◽  
Initial Results

scholarly journals Long rotation period main-sequence stars from Kepler SAP light curves

2019 ◽  
Vol 489 (4) ◽  
pp. 5513-5529 ◽  
Author(s):  
Kaiming Cui ◽  
Jifeng Liu ◽  
Shuhong Yang ◽  
Qing Gao ◽  
Huiqin Yang ◽  
...  
Keyword(s):  
Light Curve ◽  
Main Sequence ◽  
Rotation Period ◽  
Light Curves ◽  
Stellar Rotation ◽  
Main Sequence Stars ◽  
Found Objects ◽  
Period Detection ◽  
Search Data

ABSTRACT Stellar rotation plays a key role in stellar activity. The rotation period could be detected through light curve variations caused by star-spots. Kepler provides two types of light curves: one is the Pre-search Data Conditioning (PDC) light curves, and the other is the Simple Aperture Photometer (SAP) light curves. Compared with the PDC light curves, the SAP light curves keep the long-term trend, relatively suitable for searches of long-period signals. However, SAP data are inflicted by some artefacts such as quarterly rolls and instrumental errors, making it difficult to find the physical periods in the SAP light curves. We explore a systematic approach based on the light curve pre-processing, period detection, and candidate selection. We also develop a simulated light curve test to estimate our detection limits for the SAP-like LCs. After applying our method to the raw SAP light curves, we found more than 1000 main-sequence stars with periods longer than 30 d; 165 are newly discovered. Considering the potential flaw of the SAP, we also inspect the newly found objects with photometry methods, and most of our periodical signals are confirmed.

scholarly journals Photospheric and “Chromospheric” CIV Lines in B-Main-Sequence Stars

1986 ◽  
Vol 116 ◽  
pp. 113-116
Author(s):  
Fiorella Castelli ◽  
Carlo Morossi ◽  
Roberto Stalio
Keyword(s):  
High Temperature ◽  
Stellar Wind ◽  
High Velocity ◽  
Main Sequence ◽  
Stellar Atmosphere ◽  
Uv Spectra ◽  
Spectral Lines ◽  
Early Type ◽  
Main Sequence Stars ◽  
Early Type Stars

The presence in the far-UV spectra of early-type stars of spectral lines of superionized atoms is argument of controversial debate among astronomers. Presently there is agreement on the non-radiative origin of these ions but not on the proposed mechanisms for their production nor on the proposed locations in the stellar atmosphere where they are abundant. Cassinelli et al. (1978) suggest that the Auger mechanism is operative in a cool wind blowing above a narrow corona to produce these ions; Lucy and White (1980) introduce radiative instabilities growing into hot blobs distributed across the stellar wind; Doazan and Thomas (1982) make these ions to be formed in both pre- and post-coronal, high temperature regions at low and high velocity respectively.

scholarly journals The Galactic Evolution of Boron

2000 ◽  
Vol 198 ◽  
pp. 405-414 ◽  
Author(s):  
Francesca Primas
Keyword(s):  
Hubble Space Telescope ◽  
Cosmic Ray ◽  
Big Bang ◽  
Stellar Structure ◽  
Current Status ◽  
Big Bang Nucleosynthesis ◽  
Medium Resolution ◽  
The Galaxy ◽  
Stellar Interiors ◽  
Astrophysical Research

Boron, together with lithium and beryllium, belongs to the group of the so-called light elements, the importance of which ranges from providing important tests to Big Bang nucleosynthesis scenarios to being useful probes of stellar interiors and useful tools to further constrain the chemical evolution of the Galaxy.Since it became operative in the late eighties, the Hubble Space Telescope (HST) and its high- and medium-resolution spectrographs have played a key role in analyzing boron. Boron has now been observed in several stars and in the interstellar medium (ISM), providing important information in different fields of astrophysical research (nucleosynthesis, cosmic-ray spallation, stellar structure). In particular, determinations of boron in unevolved stars of different metallicity have allowed to study how boron evolves with iron.After a general review of the current status of boron observations and of the major uncertainties affecting the measurements of its abundance, I will mainly concentrate on unevolved stars and discuss the ‘evolutionary’ picture emerging from the most recent analyses and how its interpretation compares with theoretical expectations. A brief discussion on future prospects will conclude this contribution, showing how the field may evolve and improve.

scholarly journals Revisiting the connection between magnetic activity, rotation period, and convective turnover time for main-sequence stars

2018 ◽  
Vol 618 ◽  
pp. A48 ◽  
Author(s):  
M. Mittag ◽  
J. H. M. M. Schmitt ◽  
K.-P. Schröder
Keyword(s):  
Main Sequence ◽  
Rotation Period ◽  
Magnetic Activity ◽  
Turnover Time ◽  
Rossby Number ◽  
Stellar Rotation ◽  
Main Sequence Stars ◽  
Rotation Periods ◽  
Period Distribution ◽  
Cool Stars

The connection between stellar rotation, stellar activity, and convective turnover time is revisited with a focus on the sole contribution of magnetic activity to the Ca II H&K emission, the so-called excess flux, and its dimensionless indicator R+HK in relation to other stellar parameters and activity indicators. Our study is based on a sample of 169 main-sequence stars with directly measured Mount Wilson S-indices and rotation periods. The R+HK values are derived from the respective S-indices and related to the rotation periods in various B–V-colour intervals. First, we show that stars with vanishing magnetic activity, i.e. stars whose excess flux index R+HK approaches zero, have a well-defined, colour-dependent rotation period distribution; we also show that this rotation period distribution applies to large samples of cool stars for which rotation periods have recently become available. Second, we use empirical arguments to equate this rotation period distribution with the global convective turnover time, which is an approach that allows us to obtain clear relations between the magnetic activity related excess flux index R+HK, rotation periods, and Rossby numbers. Third, we show that the activity versus Rossby number relations are very similar in the different activity indicators. As a consequence of our study, we emphasize that our Rossby number based on the global convective turnover time approaches but does not exceed unity even for entirely inactive stars. Furthermore, the rotation-activity relations might be universal for different activity indicators once the proper scalings are used.

scholarly journals NLTE spectroscopic analysis of the 3He anomaly in subluminous B-type stars

2018 ◽  
Vol 618 ◽  
pp. A86 ◽  
Author(s):  
D. Schneider ◽  
A. Irrgang ◽  
U. Heber ◽  
M. F. Nieva ◽  
N. Przybilla
Keyword(s):  
Main Sequence ◽  
Spectroscopic Analysis ◽  
Stellar Atmosphere ◽  
Atomic Diffusion ◽  
Horizontal Branch ◽  
Line Profiles ◽  
Main Sequence Stars ◽  
Sdb Stars ◽  
Non Local ◽  
First Time

Several B-type main-sequence stars show chemical peculiarities. A particularly striking class are the 3He stars, which exhibit a remarkable enrichment of 3He with respect to 4He. This isotopic anomaly has also been found in blue horizontal branch (BHB) and subdwarf B (sdB) stars, which are helium-core burning stars of the extreme horizontal branch. Recent surveys uncovered 11 3He sdBs. The 3He anomaly is not due to thermonuclear processes, but caused by atomic diffusion in the stellar atmosphere. Using a hybrid local/non-local thermodynamic equilibrium (LTE/NLTE) approach for B-type stars, we analyzed high-quality spectra of two known 3He BHBs and nine known 3He sdBs to determine their isotopic helium abundances and 4He/3He abundance ratios. We redetermined their atmospheric parameters and analyzed selected He I lines, including λ4922 Å and λ6678 Å, which are very sensitive to 4He/3He. Most of the 3He sdBs cluster in a narrow temperature strip between 26000 K and 30000 K and are helium deficient in accordance with previous LTE analyses. BD+48° 2721 is reclassified as a BHB star because of its low temperature (Teff = 20700 K). Whereas 4He is almost absent (4He/3He < 0.25) in most of the known 3He stars, other sample stars show abundance ratios up to 4He/3He ∼2.51. A search for 3He stars among 26 candidate sdBs from the ESO SPY survey led to the discovery of two new 3He sdB stars (HE 0929–0424 and HE 1047–0436). The observed helium line profiles of all BHBs and of three sdBs are not matched by chemically homogeneous atmospheres, but hint at vertical helium stratification. This phenomenon has been seen in other peculiar B-type stars, but is found for the first time for sdBs. We estimate helium to increase from the outer to the inner atmosphere by factors ranging from 1.4 (SB 290) up to 8.0 (BD+48° 2721).

Temperatures and radii of O stars

1979 ◽  
Vol 83 ◽  
pp. 103-108
Author(s):  
A. B. Underhill ◽  
L. Divan ◽  
V. Doazan ◽  
M.L. Prévot-Burnichon
Keyword(s):  
Main Sequence ◽  
Near Infrared ◽  
Stellar Atmosphere ◽  
Class Iii ◽  
Main Sequence Stars ◽  
Class V ◽  
Intermediate Band ◽  
B Stars ◽  
Luminosity Class ◽  
Effective Temperatures

Angular diameters have been estimated for 18 O and 142 B stars using absolute intermediate-band photometry in the near infrared and they have been combined with integrated fluxes to yield effective temperatures. The effective temperatures of the O stars lie in the range 30000 K to about 47000 K. For a given subtype, the luminosity class I stars have lower effective temperatures than the main-sequence stars by about 1000 K. The absorption-line spectral types of the supergiants of types O and B reflect electron temperatures which are higher than can be maintained by the integrated flux which flows through the stellar atmosphere. Distances have been estimated for all the stars and linear diameters found. The average radius for an 08 to 09.5 supergiant is about 23.3 R⊙; the radii for luminosity class III and Class V O stars lie in the range 6.8 to 10.7⊙ R.

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