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تسجيل مستخدم جديدSelected topics in the evolution of low-mass stars
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M. Catelan
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Low-mass stars play a key role in many different areas of astrophysics. In this article, I provide a brief overview of the evolution of low-mass stars, and discuss some of the uncertainties and problems currently affecting low-mass stellar models. Emphasis is placed on the following topics: the solar abundance problem, mass loss on the red giant branch, and the level of helium enrichment associated to the multiple populations that are present in globular clusters.
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Open clusters (OC) of 1-3 Gyr age contain intermediate-to-low-mass stars in evolutionary phases of multiple relevance to understanding Li evolution. Stars leaving the main sequence (MS) from the hot side of the Lithium dip (LD) at a fixed age can inc
lude a range of mass, varying degrees of core degeneracy, and helium ignition under quiescent or flash conditions. An ongoing survey of a significant sample of stars from the giant branch to below the LD in key open clusters has revealed patterns that supply critical clues to the underlying source of Li variation among stars of differing mass and age. While the LD is well established in OC of this age, stars on the hot side of the LD can exhibit Li ranging from the apparent primordial cluster value to upper limits similar to those found at the LD center, despite occupying the same region of the color-magnitude diagram (CMD). Stars on the first-ascent giant branch show a dramatic decline in measurable Li that correlates strongly with increasing age and reduced turnoff mass. We discuss how these trends can be explained in the context of the existence of the LD itself and the temporal evolution of individual stars.
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nt histories and the spectral energy distributions of their host galaxies. The ability to accurately model TP-AGB stars is critical to the interpretation of the integrated light of distant galaxies, especially in redder wavelengths. We continue previous efforts to constrain the evolution and lifetimes of TP-AGB stars by modeling their underlying stellar populations. Using Hubble Space Telescope (HST) optical and near-infrared photometry taken of 12 fields of 10 nearby galaxies imaged via the ACS Nearby Galaxy Survey Treasury and the near-infrared HST/SNAP follow-up campaign, we compare the model and observed TP-AGB luminosity functions as well as the number ratio of TP-AGB to red giant branch stars. We confirm the best-fitting mass-loss prescription, introduced by Rosenfield et al. 2014, in which two different wind regimes are active during the TP-AGB, significantly improves models of many galaxies that show evidence of recent star formation. This study extends previous efforts to constrain TP-AGB lifetimes to metallicities ranging -1.59 < [Fe/H] < -0.56 and initial TP-AGB masses up to ~ 4 Msun, which include TP-AGB stars that undergo hot-bottom burning.
We present an analysis of K2 light curves (LCs) from Campaigns 4 and 13 for members of the young ($sim$3 Myr) Taurus association, in addition to an older ($sim$30 Myr) population of stars that is largely in the foreground of the Taurus molecular clou
ds. Out of 156 of the highest-confidence Taurus members, we find that 81% are periodic. Our sample of young foreground stars is biased and incomplete, but nearly all (37/38) are periodic. The overall distribution of rotation rates as a function of color (a proxy for mass) is similar to that found in other clusters: the slowest rotators are among the early M spectral types, with faster rotation towards both earlier FGK and later M types. The relationship between period and color/mass exhibited by older clusters such as the Pleiades is already in place by Taurus age. The foreground population has very few stars, but is consistent with the USco and Pleiades period distributions. As found in other young clusters, stars with disks rotate on average slower, and few with disks are found rotating faster than $sim$2 d. The overall amplitude of the light curves decreases with age and higher mass stars have generally lower amplitudes than lower mass stars. Stars with disks have on average larger amplitudes than stars without disks, though the physical mechanisms driving the variability and the resulting light curve morphologies are also different between these two classes.
[Abridged] The stellar Initial Mass Function (IMF) suggests that sub-solar stars form in very large numbers. Most attractive places for catching low-mass star formation in the act are young stellar clusters and associations, still (half-)embedded in
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We have undertaken a systematic study of pre-main sequence (PMS) stars spanning a wide range of masses (0.5 - 4 Msolar), metallicities (0.1 - 1 Zsolar) and ages (0.5 - 30 Myr). We have used the Hubble Space Telescope (HST) to identify and characteris
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