分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We present detailed C, O, Na, Mg, Si, Ca, Ti, V, Fe, Zr, Ba, and Eu abundance measurements for 20 red giant branch (RGB) stars in the LMC star cluster NGC 1846 ([Fe/H] = -0.59). This cluster is 1.95 Gyr old and lies just below the supposed lower age limit (2 Gyr) for the presence of multiple populations in massive star clusters. Our measurements are based on high and low-resolution VLT/FLAMES spectra combined with photometric data from HST. Corrections for non-local thermodynamic equilibrium effects are also included for O, Na, Mg, Si, Ca, Fe and Ba. Our results show that there is no evidence for multiple populations in this cluster based on the lack of any intrinsic star-to-star spread in the abundances of Na and O: we place 95 \% confidence limits on the intrinsic dispersion for these elements of $\leq 0.07$ and $\leq 0.09$ dex, respectively. However, we do detect a significant spread in the carbon abundances, indicating varying evolutionary mixing occurring on the RGB that increases with luminosity. Overall, the general abundance patterns for NGC 1846 are similar to those seen in previous studies of intermediate-age LMC star clusters and field stars.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Piyush Sharda
Anish M. Amarsi
Kathryn Grasha
Mark R. Krumholz
David Yong
Gen Chiaki
Arpita Roy
Thomas Nordlander
摘要： Star formation models predict that the metal-poor initial mass function (IMF) can be substantially different from that observed in the metal-rich Milky Way. This changeover occurs because metal-poor gas clouds cool inefficiently due to their lower abundance of metals and dust. However, predictions for the metal-poor IMF to date rely on assuming Solar-scaled abundances, that is, [X/O] = 0 at all [O/H]. There is now growing evidence that elements such as C and O that dominate metal line cooling in the ISM do not follow Solar scaling at low metallicities. In this work, we extend models that predict the variation in the characteristic (or, the peak) IMF mass as a function of metallicity using [C/O] ratios derived from observations of metal-poor Galactic stars and of H II regions in dwarf galaxies. These data show [C/O] < 0 at sub-Solar [O/H], which leads to a substantially different metal-poor IMF in the metallicity range where C I and C II cooling dominate ISM thermodynamics, resulting in an increase in the characteristic mass by a factor as large as 7. An important consequence of this difference is a shift in the location of the transition from a top- to a bottom-heavy IMF upwards by 0.5 $-$ 1 dex in metallicity. Our findings indicate that the IMF is very sensitive to the assumptions around Solar-scaled ISM compositions in metal-poor systems (e.g., dwarf galaxies, the Galactic halo and metal-poor stars) that are a key focus of JWST.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
G. S. Da Costa
M. S. Bessell
Thomas Nordlander
Arvind C. N. Hughes
Sven Buder
A. D. Mackey
Lee R. Spitler
D. B. Zucker
摘要： The advent of large-scale stellar spectroscopic surveys naturally leads to the implementation of machine learning techniques to isolate, for example, small sub-samples of potentially interesting stars from the full data set. A recent example is the application of the t-SNE statistical method to $\sim$600,000 stellar spectra from the GALAH survey in order to identify a sample of candidate extremely metal-poor (EMP, [Fe/H] $\leq$ -3) stars. We report the outcome of low-resolution spectroscopic follow-up of 83 GALAH EMP candidates that lack any previous metallicity estimates. Overall, the statistical selection is found to be efficient ($\sim$one-third of the candidates have [Fe/H] $\leq$ -2.75) with low contamination ($$ -2), and with a metallicity distribution function that is consistent with previous work. Five stars are found to have [Fe/H] $\leq$ -3.0, one of which is a main sequence turnoff star. Two other stars are revealed as likely carbon-enhanced metal-poor (CEMP) stars of type CEMP-$s$, and a known carbon star is re-identified. The results indicate that the statistical selection approach employed was successful, and therefore it can be applied to forthcoming even larger stellar spectroscopic surveys with the expectation of similar positive outcomes.
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