69 research outputs found
Individual element sensitivity for stellar evolutionary isochrones
Stellar evolution calculations with variable abundance ratios were used to gauge the effects on temperatures, luminosities, and lifetimes in various phases. The individual elements C, N, O, Mg, Si, and Fe were included. Most of the effect relevant to integrated light models is contained in the temperature variable, as opposed to the time-scale or luminosity. We derive a recipe for including abundance-sensitive temperature effects that is applicable to existing isochrone grids. The resultant enhanced isochrones are incorporated into composite stellar population models and compared with galaxy data from the Sloan Digital Sky Survey. A severe oxygen–age degeneracy is apparent, 2–3 Gyr per 0.1 dex in [O/R], where R represents a heavy element such as Fe. Over the range of early-type galaxy velocity dispersion, the spans of all abundance ratios are reduced but the age range increases and becomes systematically older. Allowing Fe-peak elements the freedom to vary accentuates this increase of age span. Overall, these results sharpen the age–mass correlation known as downsizing but decrease the steepness of abundance ratio gradients. Both of these observations, in turn, imply a more robust contribution from gas-free mergers in the histories of typical elliptical galaxies
Near-infrared chemical abundances of stars in the Sculptor dwarf galaxy
Owing to the recent identification of major substructures in our Milky Way
(MW), the astronomical community has started to reevaluate the importance of
dissolved and existing dwarf galaxies. In this work, we investigate up to 13
elements in 43 giant stars of the Sculptor dwarf galaxy (Scl) using
high-signal-to-noise-ratio near-infrared (NIR) APOGEE spectra. Thanks to the
strong feature lines in the NIR, we were able to determine high-resolution O,
Si, and Al abundances for a large group of sample stars for the first time in
Scl. By comparing the [/Fe] (i.e., O, Mg, Si, Ca, and Ti) of the stars
in Scl, Sagittarius, and the MW, we confirm the general trend that less massive
galaxies tend to show lower [/Fe]. The low [Al/Fe] () in Scl
demonstrates the value of this ratio as a discriminator with which to identify
stars born in dwarf galaxies (from MW field stars). A chemical-evolution model
suggests that Scl has a top-light initial mass function (IMF), with a high-mass
IMF power index of , and a minimum Type Ia supernovae delay time of
Myr. Furthermore, a linear regression analysis indicates a negative
radial metallicity gradient and positive radial gradients for [Mg/Fe] and
[Ca/Fe], in qualitative agreement with the outside-in formation scenario.Comment: A&A accepte
Multiple stellar populations at less evolved stages: detection of chemical variations among main-sequence dwarfs in NGC 1978
Multiple stellar populations (MPs) with different chemical compositions are
not exclusive features of old GCs (older than 10 Gyr). Indeed, recent studies
reveal that younger clusters (2--6 Gyr-old) in the Magellanic Clouds also
exhibit star-to-star chemical variations among evolved stars. However, whether
MPs are present among less evolved dwarfs of these intermediate-age clusters is
still unclear. In this work, we search for chemical variations among GK-type
dwarfs in the 2 Gyr-old cluster NGC 1978, which is the youngest cluster
with MPs. We exploit deep ultraviolet and visual observations from the Hubble
Space Telescope to constrain the nitrogen (N) and oxygen (O) variations among
MS stars. To do this, we compare appropriate photometric diagrams that are
sensitive to N and O with synthetic diagrams of simple stellar populations and
MPs. We conclude that the G- and K-type MS stars in NGC\,1978 host MPs. Our
statistical analysis shows that the fraction of N-rich stars ranges from
40\% to 80\%, depending on the detailed distributions of nitrogen
and oxygen.Comment: 16 pages, 10 figures, ApJ accepte
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