VLT/FLAMES high-resolution chemical abundances in Sculptor: a textbook dwarf spheroidal galaxy

Abstract

We present detailed chemical abundances for 99 red-giant branch stars in the centre of the Sculptor dwarf spheroidal galaxy, which have been obtained from high-resolution VLT/FLAMES spectroscopy. The abundances of Li, Na, α-elements (O, Mg, Si, Ca Ti), iron-peak elements (Sc, Cr, Fe, Co, Ni, Zn), and r-and s-process elements (Ba, La, Nd, Eu) were all derived using stellar atmosphere models and semi-automated analysis techniques. The iron abundances populate the whole metallicity distribution of the galaxy with the exception of the very low metallicity tail, −2.3 ≤ [Fe/H] ≤ −0.9. There is a marked decrease in [α/Fe] over our sample, from the Galactic halo plateau value at low [Fe/H] and then, after a 'knee', a decrease to sub-solar [α/Fe] at high [Fe/H]. This is consistent with products of core-collapse supernovae dominating at early times, followed by the onset of supernovae type Ia as early as ∼12 Gyr ago. The s-process products from low-mass AGB stars also participate in the chemical evolution of Sculptor on a timescale comparable to that of supernovae type Ia. However, the r-process is consistent with having no time delay relative to core-collapse supernovae, at least at the later stages of the chemical evolution in Sculptor. Using the simple and well-behaved chemical evolution of Sculptor, we further derive empirical constraints on the relative importance of massive stars and supernovae type Ia to the nucleosynthesis of individual iron-peak and α-elements. The most important contribution of supernovae type Ia is to the iron-peak elements: Fe, Cr, and Mn. There is, however, also a modest but non-negligible contribution to both the heavier α-elements: S, Ca and Ti, and some of the iron-peak elements: Sc and Co. We see only a very small or no contribution to O, Mg, Ni, and Zn from supernovae type Ia in Sculptor. The observed chemical abundances in Sculptor show no evidence of a significantly different initial mass function, compared to that of the Milky Way. With the exception of neutron-capture elements at low [Fe/H], the scatter around mean trends in Sculptor for [Fe/H] > −2.3 is extremely low, and compatible with observational errors. Combined with the small scatter in the age-elemental abundances relation, this calls for an efficient mixing of metals in the gas in the centre of Sculptor since ∼12 Gyr ago.

Carbon, oxygen, and iron abundances in disk and halo stars

“…The trends inferred here for oxygen in the low-α halo population are similar to those found in dwarf satellite galaxies via red giant stars (e.g. Hill et al 2019). The same is true for carbon A104, page 16 of 19 A. M. Amarsi et al: Carbon, oxygen, and iron abundances in disk and halo stars (e.g.…”

Spatial variations in the Milky Way disc metallicity–age relation

“…The high metallicity of the knee, relative to local dwarf galaxies, indicates that the star formation rate was high at that time (see Tolstoy et al 2009, and references therein). The steeper slope of the highα sequence compared to the low-α sequence is likely caused by a decline in star formation, which results in a dominant contribution of the delayed type Ia supernovae over type II (see Hill et al 2019). This is consistent with derivations of the local star formation history that find a burst of significant star formation at early time followed by a lull in star formation using thick disc stars (Snaith et al 2014;Haywood et al 2015) and a two-infall model (Spitoni et al 2019).…”

“…The steeper slope of the highα sequence compared to the low-α sequence is likely caused by a decline in star formation, which results in a dominant contribution of the delayed type Ia supernovae over type II (see Hill et al 2019). This is consistent with derivations of the local star formation history that find a burst of significant star formation at early time followed by a lull in star formation using thick disc stars (Snaith et al 2014;Haywood et al 2015) and a two-infall model (Spitoni et al 2019). Most GCE models find the Milky Way disc star formation history can be approximated by a peak in star formation at around 9-10 Gyr ago, followed by a exponential decline in star formation (e.g.…”

Neutron-capture elements in dwarf galaxies

“…1, with a total stellar mass ∼10 6 M (de Boer et al 2012a). High-resolution (HR) spectroscopic analysis of some heavy element abundances in Sculptor has been performed by different authors for ≤5 stars each (Shetrone et al 2003;Geisler et al 2005;Tafelmeyer et al 2010;Frebel et al 2010a;Kirby & Cohen 2012;Skúladóttir et al 2015a;Jablonka et al 2015;Simon et al 2015), and for 99 stars in Hill et al (2019). Recently, Ba in this galaxy has also been studied by Duggan et al (2018), and Starkenburg et al (2013) included Sr and Ba measurements and upper limits for seven stars at low metallicity, [Fe/H] < −3, using spectra of intermediate resolution.…”

“…Recently, Ba in this galaxy has also been studied by Duggan et al (2018), and Starkenburg et al (2013) included Sr and Ba measurements and upper limits for seven stars at low metallicity, [Fe/H] < −3, using spectra of intermediate resolution. For A171, page 2 of 13 a general and more complete overview of the previous studies of Sculptor, see Hill et al (2019) and references therein.…”