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Register a new userConstraints on the distance moduli, helium and metal abundances, and ages of globular clusters from their RR Lyrae and non-variable horizontal-branch stars. II. Multiple stellar populations in 47Tuc, M3, and M13
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Pavel Denissenkov
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We present a new set of horizontal-branch (HB) models computed with the MESA stellar evolution code. The models adopt $alpha$-enhanced cite{ags09} metals mixtures and include the gravitational settling of He. They are used in our HB population synthesis tool to generate theoretical distributions of HB stars in order to describe the multiple stellar populations in the globular clusters 47Tuc, M3, and M13. The observed HB in 47Tuc is reproduced very well by our simulations for [Fe/H] $= -0.70$ and [$alpha$/Fe] $= +0.4$ if the initial helium mass fraction varies by $Delta Y_0 sim 0.03$ and approximately 21%, 37%, and 42% of the stars have $Y_0 = 0.257$, 0.270, and 0.287, respectively. These simulations yield $(m-M)_V = 13.27$, implying an age near 13.0 Gyr. In the case of M3 and M13, our synthetic HBs for [Fe/H] $= -1.55$ and [$alpha$/Fe] $= 0.4$ match the observed ones quite well if M3 has $Delta Y_0 sim 0.01$ and $(m-M)_V = 15.02$, resulting in an age of 12.6 Gyr, whereas M13 has $Delta Y_0 sim 0.08$ and $(m-M)_V = 14.42$, implying an age of 12.9 Gyr. Mass loss during giant-branch evolution and $Delta Y_0$ appear to be the primary second parameters for M3 and M13. New observations for 7 of the 9 known RR Lyrae in M13 are also reported. Surprisingly, periods predicted for the $c$-type variables tend to be too high (by up to $sim 0.1$~d).
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It has recently been suggested that the presence of multiple populations showing various amounts of helium enhancement is the rule, rather than the exception, among globular star clusters. An important prediction of this helium enhancement scenario is that the helium-enhanced blue horizontal branch (HB) stars should be brighter than the red HB stars which are not helium-enhanced. In this Letter, we test this prediction in the case of the Galactic globular cluster M3 (NGC 5272), for which the helium-enhancement scenario predicts helium enhancements of > 0.02 in virtually all blue HB stars. Using high-precision Stroemgren photometry and spectroscopic gravities for blue HB stars, we find that any helium enhancement among most of the clusters blue HB stars is very likely less than 0.01, thus ruling out the much higher helium enhancements that have been proposed in the literature.
We have derived relations between full-width-half-maxima and equivalent widths of metallic absorption lines in the spectra of RR~Lyrae stars to estimate new upper limits on the axial equatorial rotational velocities of RR~Lyrae and metal-poor red horizontal branch stars (RHB). We also have derived the variations of RR~Lyrae macroturbulent velocities during the pulsation cycles. In RRab cycles the line widths are dominated by phase-dependent convolutions of axial rotation and macroturbulence, which we designate as V_macrot. The behavior of V_macrot is remarkably uniform among the RRab stars, but the behavior of V_macrot among RRc stars varies strongly from star to star. The RRab stars exhibit an upper limit on V_macrot of 5 +/- 1 km/s with weak evidence of an anti-correlation with T_eff. The RRc minima range from 2 to 12 km/s. The abrupt decline in large rotations with decreasing T_eff at the blue boundary of the instability strip and the apparently smooth continuous variation among the RRab and RHB stars suggests that HB stars gain/lose surface angular momentum on time scales short compared to HB lifetimes. V_macrot values for our metal-poor RHB stars agree well with those derived by Fourier analysis of an independent but less metal-poor sample of Carney et al. (2008); they conform qualitatively to the expectations of Tanner et al. (2013). A general conclusion of our investigation is that surface angular momentum as measured by V_rot*sini is not a reliable indicator of total stellar angular momentum anywhere along the HB.
Multiple stellar populations (MPs) are a distinct characteristic of Globular Clusters (GCs). Their general properties have been widely studied among main sequence, red giant branch (RGB) and horizontal branch (HB) stars, but a common framework is still missing at later evolutionary stages. We studied the MP phenomenon along the AGB sequences in 58 GCs, observed with the Hubble Space Telescope in ultraviolet (UV) and optical filters. By using UV-optical color-magnitude diagrams, we selected the AGB members of each cluster and identified the AGB candidates of the metal-enhanced population in type II GCs. We studied the photometric properties of AGB stars and compared them to theoretical models derived from synthetic spectra analysis. We observe the following features: i) the spread of AGB stars in photometric indices sensitive to variations of light-elements and helium is typically larger than that expected from photometric errors; ii) the fraction of metal-enhanced stars in the AGB is lower than in the RGB in most of the type II GCs; iii) the fraction of 1G stars derived from the chromosome map of AGB stars in 15 GCs is larger than that of RGB stars; v) the AGB/HB frequency correlates with the average mass of the most helium-enriched population. These findings represent a clear evidence of the presence of MPs along the AGB of Galactic GCs and indicate that a significant fraction of helium-enriched stars, which have lower mass in the HB, does not evolve to the AGB phase, leaving the HB sequence towards higher effective temperatures, as predicted by the AGB-manque scenario.
We present a detailed abundance study of 11 RR Lyrae ab-type variables: AS Vir, BS Aps, CD Vel, DT Hya, RV Oct, TY Gru, UV Oct, V1645 Sgr, WY Ant, XZ Aps, and Z Mic.High resolution and high S/N echelle spectra of these variables were obtained with 2.5 m du Pont telescope at the Las Campanas Observatory. We obtained more than 2300 spectra, roughly 200 spectra per star, distributed more or less uniformly throughout the pulsational cycles. A new method has been developed to obtain initial effective temperature of our sample stars at a specific pulsational phase. We find that the abundance ratios are generally consistent with those of similar metallicity field stars in different evolutionary states and throughout the pulsational cycles for RR Lyrae stars. TY Gru remains the only n-capture enriched star among the RRab in our sample. A new relation is found between microturbulence and effective temperature among stars of the HB population. In addition, the variation of microturbulence as a function of phase is empirically shown to be similar to the theoretical variation. Finally, we conclude that the derived teffand log g values of our sample stars follow the general trend of a single mass evolutionary track.
The location of Galactic Globular Clusters (GC) stars on the horizontal branch (HB) should mainly depend on GC metallicity, the first parameter, but it is actually the result of complex interactions between the red giant branch (RGB) mass loss, the coexistence of multiple stellar populations with different helium content, and the presence of a second parameter which produces dramatic differences in HB morphology of GCs of similar metallicity and ages (like the pair M3--M13). In this work, we combine the entire dataset from the Hubble Space Telescope Treasury survey and stellar evolutionary models, to analyse the HBs of 46 GCs. For the first time in a large sample of GCs, we generate population synthesis models, where the helium abundances for the first and the extreme second generations are constrained using independent measurements based on RGB stars. The main results are: 1) the mass loss of first generation stars is tightly correlated to cluster metallicity. 2) the location of helium enriched stars on the HB is reproduced only by adopting a higher RGB mass loss than for the first generation. The difference in mass loss correlates with helium enhancement and cluster mass. 3) A model of pre-main sequence disc early loss, previously developed by the authors, explains such a mass loss increase and is consistent with the findings of multiple-population formation models predicting that populations more enhanced in helium tend to form with higher stellar densities and concentrations. 4) Helium-enhancement and mass-loss both contribute to the second parameter.
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