VLT/FLAMES spectroscopy of red giant branch stars in the Fornax dwarf spheroidal galaxy

Abstract

Context. Fornax is one of the most massive dwarf spheroidal galaxies in the Local Group. The Fornax field star population is dominated by intermediate age stars but star formation was going on over almost its entire history. It has been proposed that Fornax experienced a minor merger event. Aims. Despite recent progress, only the high metallicity end of Fornax field stars ([Fe/H] > -1.2 dex) has been sampled in larger number via high resolution spectroscopy. We want to better understand the full chemical evolution of this galaxy by better sampling the whole metallicity range, including more metal poor stars. Methods. We use the VLT-FLAMES multi-fibre spectrograph in high-resolution mode to determine the abundances of several α, iron-peak and neutron-capture elements in a sample of 47 individual red giant branch stars in the Fornax dwarf spheroidal galaxy. We combine these abundances with accurate age estimates derived from the age probability distribution from the colour-magnitude diagram of Fornax. Results. Similar to other dwarf spheroidal galaxies, the old, metal-poor stars of Fornax are typically α-rich while the young metal-rich stars are α-poor. In the classical scenario of the time delay between Type II (SNe II) and Type Ia Supernovae (SNe Ia), we confirm that SNe Ia started to contribute to the chemical enrichment at [Fe/H] [Eu/Mg] are compatible with an initial mass function that lacks the most massive stars and with star formation that kept going on throughout the whole history of Fornax. We find that massive stars kept enriching the interstellar medium in α-elements, although they were not the main contributor to the iron enrichment.

High-precision abundances of elements in solar twin stars

“…A Ni-Na relation has, however, been found for thick-disk and halo stars with −1.6 < [Fe/H] − 0.4 (Nissen & Schuster 1997, 2010, 2011 and for stars in dwarf spheroidal galaxies in the same metallicity interval (Venn et al 2004;Letarte 2010;Lemasle 2014). For these metal-poor stars, the amplitude of the variations is larger (∼0.…”

An r-process Enhanced Star in the Dwarf Galaxy Tucana III*

“…In summary, only one of the 21 stars in Sculptor with Eu measurements qualifies as an r-I star and none are r-II stars. Fornax: The dSph Fornax is well-studied, with neutroncapture element abundances published for 147 stars (Shetrone et al 2003;Letarte et al 2010;Tafelmeyer et al 2010;Lemasle et al 2014). The majority of these are enhanced not only in Eu but also in s-process elements such as Ba and La, such that only six qualify as r-I stars, three from the sample of Lemasle et al (2014) and three from the sample of Letarte et al (2010).…”

“…Several long-studied dwarfs such as Draco, Ursa Minor (UMi), Sculptor, Fornax, and Carina also contain rprocess enhanced stars (e.g., Shetrone et al 2003;Cohen & Huang 2010;Lemasle et al 2014;Tsujimoto et al 2015), although the implications of these stars have received relatively little attention in the literature. These dwarf galaxies, including both the ultra-faint dwarfs and the classical dwarf spheroidals, provide a unique environment for exploring nucleosynthesis events in the early universe.…”

Four and one more: The formation history and total mass of globular clusters in the Fornax dSph

“…Figure 4 shows the resulting F814W and F555W-F814W CMD for each cluster, overlaid with isochrones from the Dartmouth Stellar Evolution Database isochrone set (Dotter et al 2008) tracing metal-poor ([Fe/H] = −2.5 dex) populations at a fixed age of 14 Gyr. For each metallicity, we adopted a value of [α/Fe] consistent with the α-element distribution determined from high-resolution spectroscopic measurements (Letarte et al 2006;Lemasle et al 2014;Hendricks et al 2016). Confirmed RR Lyrae stars for each cluster are indicated with red circles, according to data collected from Mackey & Gilmore (2003a), Greco et al (2007).…”