Neutron-capture element deficiency of the Hercules dwarf spheroidal galaxy

TL;DR

This study finds that the Hercules dwarf spheroidal galaxy has extremely low barium abundances, indicating limited chemical enrichment by massive stars and low star formation efficiency, based on spectroscopic analysis of its red giant stars.

Contribution

First comprehensive measurement of very low barium abundance across multiple stars in the Hercules dSph galaxy, revealing unique chemical enrichment history.

Findings

Hercules dSph shows Ba abundance log epsilon < 0.7 dex.

Heavy element depletion suggests enrichment by massive stars.

Low star formation efficiency inferred from chemical signatures.

Abstract

We present an assessment of the barium abundance ratios for red giant member stars in the faint Hercules dwarf spheroidal (dSph) galaxy. Our results are drawn from intermediate-resolution FLAMES/GIRAFFE spectra around the Ba II 6141.71 AA absorption line at low signal-to-noise ratios. For three brighter stars we were able to gain estimates from direct equivalent-width measurements, while for the remaining eight stars only upper limits could be obtained. These results are investigated in a statistical manner and indicate very low Ba abundances of log epsilon (Ba) < 0.7 dex (3 sigma). We discuss various possible systematic biasses, first and foremost, a blend with the Fe I 6141.73 AA-line, but most of those would only lead to even lower abundances. A better match with metal-poor halo and dSph stars can only be reached by including a large uncertainty in the continuum placement. This contrasts with the high dispersions in iron and calcium (in excess of 1 dex) in this galaxy. While the latter spreads are typical of the very low luminosity, dark-matter dominated dSphs, a high level of depletion in heavy elements suggests that chemical enrichment in Hercules was governed by very massive stars, coupled with a very low star formation efficiency. While very low abundances of some heavy elements are also found in individual stars of other dwarf galaxies, this is the first time that a very low Ba abundance is found within an entire dSph over a broad metallicity range.