Subluminous Stars in the HR Diagram

1978 ◽  
Vol 80 ◽  
pp. 129-132
Author(s):  
C. C. Dahn ◽  
R. S. Harrington
Keyword(s):  
Main Sequence ◽  
Main Sequence Stars ◽  
Naval Observatory ◽  
Nuclear Burning ◽  
Pass Through ◽  
Hr Diagram

Eggen (1969) has defined a subluminous star “… as one that is fainter by at least a factor of 10 in visual luminosity than the main sequence stars of the same(U-V).” Implicit is the suggestion that single stars might exist in the region of the HR diagram between the subdwarfs and the normal degenerate sequence. While the evolutionary calculations of Chin and Stothers (1971) indicate that stars with masses <0.2 M⊙will pass through this region following the completion of nuclear burning, these models have not yet been confirmed. We summarize Naval Observatory astrometry and photometry as it pertains to the existence of such stars.

scholarly journals The Impact of Starspots on Mass and Age Estimates for Pre-main Sequence Stars

2015 ◽  
Vol 10 (S314) ◽  
pp. 91-94
Author(s):  
Garrett Somers ◽  
Marc H. Pinsonneault
Keyword(s):  
Binary Systems ◽  
Main Sequence ◽  
Late Type ◽  
Main Sequence Stars ◽  
Eclipsing Binary ◽  
Lithium Depletion ◽  
The Impact ◽  
Age Estimates ◽  
Hr Diagram

AbstractWe investigate the impact of starspots on the evolution of late-type stars during the pre-main sequence (pre-MS). We find that heavy spot coverage increases the radii of stars by 4–10%, consistent with inflation factors in eclipsing binary systems, and suppresses the rate of pre-MS lithium depletion, leading to a dispersion in zero-age MS Li abundance (comparable to observed spreads) if a range of spot properties exist within clusters from 3-10 Myr. This concordance with data implies that spots induce a range of radii at fixed mass during the pre-MS. These spots decrease the luminosity and Teff of stars, leading to a displacement on the HR diagram. This displacement causes isochrone derived masses and ages to be systematically under-estimated, and can lead to the spurious appearance of an age spread in a co-eval population.

scholarly journals Open issues in probing interiors of solar-like oscillating main sequence stars: 2. Diversity in the HR diagram

2011 ◽  
Vol 271 ◽  
pp. 012032 ◽  
Author(s):  
M J Goupil ◽  
Y Lebreton ◽  
J P Marques ◽  
S Deheuvels ◽  
O Benomar ◽  
...  
Keyword(s):  
Main Sequence ◽  
Main Sequence Stars ◽  
Open Issues ◽  
Hr Diagram

scholarly journals Mathematical Assessment of Physical and Chemical Processes from the Middle B to the Early F Type Main Sequence Stars

2017 ◽  
Vol 12 (S330) ◽  
pp. 362-363
Author(s):  
Kutluay Yüce ◽  
Saul J. Adelman
Keyword(s):  
Main Sequence ◽  
Stellar Atmospheres ◽  
Stellar Winds ◽  
Main Sequence Stars ◽  
Energy Distributions ◽  
Elemental Abundances ◽  
Effective Temperatures ◽  
Evolutionary Paths ◽  
Physical And Chemical ◽  
Hr Diagram

AbstractThe middle B to the early F Main Sequence stars have some of the most quiet stellar atmospheres. In this part of the HR diagram we find stars with atmospheres in radiative equilibrium. They lack the convective circulations of the middle F and cooler stars and the massive stellar winds of hotter stars. When stars of different mass evolve off the Main Sequence in this part of the HR Diagram their evolutionary paths do not cross initially. Thus stars with the same effective temperature and surface gravity have the same luminosity and mass. By comparing their elemental abundances, we might be able to identify physical processes which cause any differences in their abundances. Here we begin with stars whose effective temperatures and surface gravities are similar, and which have been analyzed by us using spectra obtained from the Dominion Astrophysical Observatory (DAO). Improvements in our knowledge of the energy distributions of stars (for example via GAIA measurements) should lead to improved estimates of stellar effective temperatures and surface gravities.

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 Variation in convective properties across the HR diagram

2010 ◽  
Vol 6 (S271) ◽  
pp. 401-402
Author(s):  
Joel Tanner ◽  
Sarbani Basu ◽  
Pierre Demarque ◽  
Frank Robinson
Keyword(s):  
Main Sequence ◽  
Mixing Length ◽  
Main Sequence Stars ◽  
Hydrodynamic Simulations ◽  
Structural Differences ◽  
Low Mass ◽  
Stellar Models ◽  
Mixing Length Theory ◽  
T Tau ◽  
Hr Diagram

AbstractWe perform 3D radiative hydrodynamic simulations to study convection in low-mass main-sequence stars with the aim of improving stellar models. Comparing models from a 0.90 M⊙ evolutionary track with 3D simulations reveals distinct differences between simulations and mixing length theory. The simulations show obvious structural differences throughout the superadiabatic layer where convection is inefficient at transporting energy. The discrepancy between MLT and simulation changes as the star evolves and the dynamical effects of turbulence increase. Further, the simulations reveal a T-tau relation that is sensitive to the strength of the turbulence, which is in contrast to 1D stellar models that use the same T-tau relation across the HR diagram.

scholarly journals Colours and Teff of Main Sequence Stars Covering the Galactic Metallicity Range

1995 ◽  
Vol 164 ◽  
pp. 368-368
Author(s):  
A. Alonso ◽  
S. Arribas ◽  
C. Martínez-Roger
Keyword(s):  
Temperature Scale ◽  
Main Sequence ◽  
Main Sequence Stars ◽  
Flux Method ◽  
Absolute Flux ◽  
The Galaxy ◽  
Effective Temperatures ◽  
Bolometric Correction ◽  
Flux Calibration ◽  
Hr Diagram

We present the results of a general programme aimed to study the effects of metallicity on the temperature scale of late main sequence stars (F0-K5). A sample of approximately 400 stars with published UBV(RI) and ubvy – β photometry has been collected from the literature. A three years campaign of observations (Alonso et al. 1994b) was carried out to obtain JHK photometry in order to apply the InfraRed Flux Method (IRFM) to derive effective temperatures. The effect of metallicity on Colour-Colour IR diagrams is discussed. The absolute flux calibration in the IR was revised in Alonso et al. (1994a). The effect of metallicity on the bolometric correction has been studied in Alonso et al. (1995) in order to derive bolometric fluxes. The temperatures have been derived by applying the IRFM using new Kurucz models. Teff = f(Colours, [Fe/H], log(g)) relations are obtained for dwarfs covering the ranges 4000K ≤ Teff ≤ 8000K, 3.5 ≤ logg ≤ 5.00, +0.5 ≤ [Fe/H] ≤ −3.00 which expands considerably the database of previous works. These relations are used to check atmosphere models through the analysis of UBV RIJH Kubvy – β synthetic photometry in combination with the IRFM. The transformation from the theoretical HR diagram into an observational one is analyzed with the new relations. The influence of these points on the study of the evolution of the Galaxy is briefly discussed.

scholarly journals Spectral classification and HR diagram of pre-main sequence stars in NGC 6530

2012 ◽  
Vol 546 ◽  
pp. A9 ◽  
Author(s):  
L. Prisinzano ◽  
G. Micela ◽  
S. Sciortino ◽  
L. Affer ◽  
F. Damiani
Keyword(s):  
Main Sequence ◽  
Spectral Classification ◽  
Main Sequence Stars ◽  
Hr Diagram

Chromospheric activity as a function of age in main-sequence stars

1966 ◽  
Vol 24 ◽  
pp. 40-43
Author(s):  
O. C. Wilson ◽  
A. Skumanich
Keyword(s):  
Main Sequence ◽  
The Sun ◽  
Main Sequence Stars ◽  
Tentative Conclusion ◽  
Chromospheric Activity ◽  
General Field ◽  
Large Clusters

Evidence previously presented by one of the authors (1) suggests strongly that chromospheric activity decreases with age in main sequence stars. This tentative conclusion rests principally upon a comparison of the members of large clusters (Hyades, Praesepe, Pleiades) with non-cluster objects in the general field, including the Sun. It is at least conceivable, however, that cluster and non-cluster stars might differ in some fundamental fashion which could influence the degree of chromospheric activity, and that the observed differences in chromospheric activity would then be attributable to the circumstances of stellar origin rather than to age.

On The Radii of Ap Stars

1976 ◽  
Vol 32 ◽  
pp. 49-55 ◽  
Author(s):  
F.A. Catalano ◽  
G. Strazzulla
Keyword(s):  
Observational Data ◽  
Line Width ◽  
Main Sequence ◽  
Main Sequence Stars ◽  
Ap Stars

SummaryFrom the analysis of the observational data of about 100 Ap stars, the radii have been computed under the assumption that Ap are main sequence stars. Radii range from 1.4 to 4.9 solar units. These values are all compatible with the Deutsch's period versus line-width relation.

Pre–Main-Sequence Stars in the Young Galactic Cluster IC 4996: A CCD Photometric Study

1998 ◽  
Vol 116 (4) ◽  
pp. 1801-1809 ◽  
Author(s):  
Antonio J. Delgado ◽  
Emilio J. Alfaro ◽  
André Moitinho ◽  
José Franco
Keyword(s):  
Main Sequence ◽  
Photometric Study ◽  
Main Sequence Stars ◽  
Galactic Cluster
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