Theoretical stellar evolution models are constructed and tailored to the best known, observationally derived characteristics of metal-poor (Fe/H ∼ −2.3) stars representing a range of evolutionary ...phases: subgiant HD 140283, globular cluster M92, and four single, main sequence stars with well-determined parallaxes: HIP 46120, HIP 54639, HIP 106924, and WOLF 1137. It is found that the use of a solar-calibrated value of the mixing length parameter MLT in models of these objects is ineffective at reproducing their observed properties. Empirically calibrated values of MLT are presented for each object, accounting for uncertainties in the input physics employed in the models. It is advocated that the implementation of an adaptive mixing length is necessary in order for stellar evolution models to maintain fidelity in the era of high-precision observations.
Abstract
The bright, nearby binary
α
Centauri provides an excellent laboratory for testing stellar evolution models, because it is one of the few stellar systems for which we have high-precision ...classical (mass, radius, luminosity) and asteroseismic (
p
-mode) observations. Stellar models are created and fit to the classical and seismic observations of both stars by allowing for the free variation of the convective mixing length parameter
α
MLT
. This system is modeled using five different sets of assumptions about the physics governing the stellar models. There are 31 pairs of tracks (out of ∼150,000 generated) that fit the classical, binary, and seismic observational constraints of the system within 3
σ
. Models with each tested choice of input physics are found to be viable, but the optimal mixing lengths for
α
Cen A and
α
Cen B remain the same regardless of the physical prescription. The optimal mixing lengths are
α
MLT,A
/
α
⊙
= 0.932 and
α
MLT,B
/
α
⊙
= 1.095. That
α
Cen A and
α
Cen B require subsolar and supersolar mixing lengths, respectively, to fit the observations is a trend consistent with recent findings, such as those of Kervella et al., Joyce & Chaboyer, and Viani et al. The optimal models find an age for
α
Centauri of 5.3 ± 0.3 Gyr.
ABSTRACT Synthetic Red Giant Branch Bump (RGBB) magnitudes are generated with the most recent theoretical stellar evolution models computed with the Dartmouth Stellar Evolution Program (DSEP) code. ...They are compared to the observational work of Nataf et al., who present RGBB magnitudes for 72 globular clusters. A DSEP model using a chemical composition with enhanced capture /Fe = +0.4 and an age of 13 Gyr shows agreement with observations over metallicities ranging from Fe/H = 0 to Fe/H −1.5, with discrepancy emerging at lower metallicities.
Context. The fraction of binary stars is an important ingredient to interpret globular cluster dynamical evolution and their stellar population. Aims. We investigate the properties of main-sequence ...binaries measured in a uniform photometric sample of 59 Galactic globular clusters that were observed by HST WFC/ACS as a part of the Globular Cluster Treasury project. Methods. We measured the fraction of binaries and the distribution of mass-ratio as a function of radial location within the cluster, from the central core to beyond the half-mass radius. We studied the radial distribution of binary stars, and the distribution of stellar mass ratios. We investigated monovariate relations between the fraction of binaries and the main parameters of their host clusters. Results. We found that in nearly all the clusters, the total fraction of binaries is significantly smaller than the fraction of binaries in the field, with a few exceptions only. Binary stars are significantly more centrally concentrated than single MS stars in most of the clusters studied in this paper. The distribution of the mass ratio is generally flat (for mass-ratio parameter q > 0.5). We found a significant anti-correlation between the binary fraction in a cluster and its absolute luminosity (mass). Some, less significant correlation with the collisional parameter, the central stellar density, and the central velocity dispersion are present. There is no statistically significant relation between the binary fraction and other cluster parameters. We confirm the correlation between the binary fraction and the fraction of blue stragglers in the cluster.
The open cluster M67 has solar metallicity and an age of about 4 Gyr. The turnoff (TO) mass is close to the minimum mass for which solar metallicity stars develop a convective core during main ...sequence evolution as a result of the development of hydrogen burning through the CNO cycle. The morphology of the color-magnitude diagram (CMD) of M67 around the TO shows a clear hook-like feature, a direct sign that stars close to the TO have convective cores. VandenBerg et al. investigated the possibility of using the morphology of the M67 TO to put constraints on the solar metallicity, particularly CNO elements, for which solar abundances have been revised downward by more than 30% over the last few years. Here, we extend their work, filling the gaps in their analysis. To this aim, we compute isochrones appropriate for M67 using new (low metallicity) and old (high metallicity) solar abundances and study whether the characteristic TO in the CMD of M67 can be reproduced or not. We also study the importance of other constitutive physics on determining the presence of such a hook, particularly element diffusion, overshooting and nuclear reaction rates. We find that using the new solar abundance determinations, with low CNO abundances, makes it more difficult to reproduce the characteristic CMD of M67. This result is in agreement with results by VandenBerg et al. However, changes in the constitutive physics of the models, particularly overshooting, can influence and alter this result to the extent that isochrones constructed with models using low CNO solar abundances can also reproduce the TO morphology in M67. We conclude that only if all factors affecting the TO morphology are completely under control (and this is not the case), M67 could be used to put constraints on solar abundances.
Abstract
We collected the largest spectroscopic catalog of RR Lyrae (RRLs) including ≈20,000 high-, medium-, and low-resolution spectra for ≈10,000 RRLs. We provide the analytical forms of radial ...velocity curve (RVC) templates. These were built using 36 RRLs (31 fundamental—split into three period bins—and five first-overtone pulsators) with well-sampled RVCs based on three groups of metallic lines (Fe, Mg, Na) and four Balmer lines (H
α
, H
β
, H
γ
, H
δ
). We tackled the long-standing problem of the reference epoch to anchor light-curve and RVC templates. For the
V
-band, we found that the residuals of the templates anchored to the phase of the mean magnitude along the rising branch are ∼35% to ∼45% smaller than those anchored to the phase of maximum light. For the RVC, we used two independent reference epochs for metallic and Balmer lines and we verified that the residuals of the RVC templates anchored to the phase of mean RV are from 30% (metallic lines) up to 45% (Balmer lines) smaller than those anchored to the phase of minimum RV. We validated our RVC templates by using both the single-point and the three phase point approaches. We found that barycentric velocities based on our RVC templates are two to three times more accurate than those available in the literature. We applied the current RVC templates to Balmer lines RVs of RRLs in the globular NGC 3201 collected with MUSE at VLT. We found the cluster barycentric RV of
V
γ
= 496.89 ± 8.37(error) ± 3.43 (standard deviation) km s
−1
, which agrees well with literature estimates.
Photometry with the Hubble Space Telescope Advanced Camera for Surveys (HST ACS) reveals that the subgiant branch (SGB) of the globular cluster NGC 1851 splits into two well-defined branches. If the ...split is due only to an age effect, the two SGBs would imply two star formation episodes separated by similar to 1 Gyr. We discuss other anomalies in NGC 1851 that could be interpreted in terms of a double stellar population. Finally, we compare the case of NGC 1851 with the other two globulars known to host multiple stellar populations, and show that all three clusters differ in several important respects.
Abstract
We provide the largest and most homogeneous sample of
α
-element (Mg, Ca, Ti) and iron abundances for field RR Lyrae (RRLs; 162 variables) by using high-resolution spectra. The current ...measurements were complemented with similar abundances available in the literature for 46 field RRLs brought to our metallicity scale. We ended up with a sample of old (
t
≥ 10 Gyr), low-mass stellar tracers (208 RRLs: 169 fundamental, 38 first overtone, and 1 mixed mode) covering 3 dex in iron abundance (−3.00 ≤ Fe/H ≤ 0.24). We found that field RRLs are ∼0.3 dex more
α
poor than typical halo tracers in the metal-rich regime (Fe/H ≥ −1.2), while in the metal-poor regime (Fe/H ≤ −2.2) they seem to be on average ∼0.1 dex more
α
enhanced. This is the first time that the depletion in
α
elements for solar iron abundances is detected on the basis of a large, homogeneous, and coeval sample of old stellar tracers. Interestingly, we also detected a close similarity in the
α
/Fe trend between
α
-poor, metal-rich RRLs and red giants (RGs) in the Sagittarius dwarf galaxy as well as between
α
-enhanced, metal-poor RRLs and RGs in ultrafaint dwarf galaxies. These results are supported by similar elemental abundances for 46 field horizontal branch stars. These stars share with RRLs the same evolutionary phase and the same progenitors. This evidence further supports the key role that old stellar tracers play in constraining the early chemical enrichment of the halo and, in particular, in investigating the impact that dwarf galaxies have had in the mass assembly of the Galaxy.
Abstract
We developed a new approach to provide accurate estimates of the metal content, reddening, and true distance modulus of RR Lyrae stars (RRLs). The method is based on homogeneous optical (
...BVI
) and near-infrared (
JHK
) mean magnitudes and on predicted period–luminosity–metallicity relations (
IJHK
) and absolute mean magnitude–metallicity relations (
BV
). We obtained solutions for three different RRL samples in
ω
Cen: first overtone (RRc, 90), fundamental (RRab, 80), and global (RRc+RRab) in which the period of first overtones were fundamentalized. The metallicity distribution shows a well defined peak at Fe/H∼−1.98 and a standard deviation of
σ
= 0.54 dex. The spread is, as expected, metal-poor (Fe/H ≤ −2.3) objects. The current metallicity distribution is ∼0.3 dex more metal-poor than similar estimates for RRLs available in the literature. The difference vanishes if the true distance modulus we estimated is offset by −0.06/−0.07 mag in true distance modulus. We also found a cluster true distance modulus of
μ
= 13.720 ± 0.002 ± 0.030 mag, where the former error is the error on the mean and the latter is the standard deviation. Moreover, we found a cluster reddening of
E
(
B
−
V
) = 0.132 ± 0.002 ± 0.028 mag and spatial variations of the order of a few arcmin across the body of the cluster. Both the true distance modulus and the reddening are slightly larger than similar estimates available in the literature, but the difference is within 1
σ
. The metallicity dependence of distance diagnostics agrees with theory and observations, but firm constraints require accurate and homogeneous spectroscopic measurements.
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