Carbon enrichment of the evolved stars in the Sagittarius dwarf spheroidal

TL;DR

This study analyzes the spectra of stars in the Sagittarius dwarf galaxy to identify carbon and S stars, examine their properties, and understand their role in stellar evolution and chemical enrichment.

Contribution

It provides new spectroscopic data on carbon and S stars in the Sagittarius dwarf galaxy and investigates their lifetimes and contribution to interstellar medium enrichment.

Findings

Identification of 23 carbon stars, mostly new discoveries.

Average lifetimes of carbon and S stars estimated at 130-500 kyr.

The bulk of the population can produce observed planetary nebulae.

Abstract

We present spectra of 1142 colour-selected stars in the direction of the Sagittarius Dwarf Spheroidal (Sgr dSph) galaxy, of which 1058 were taken with VLT/FLAMES multi-object spectrograph and 84 were taken with the SAAO Radcliffe 1.9-m telescope grating spectrograph. Spectroscopic membership is confirmed (at >99% confidence) for 592 stars on the basis of their radial velocity, and spectral types are given. Very slow rotation is marginally detected around the galaxy's major axis. We identify five S stars and 23 carbon stars, of which all but four carbon stars are newly-determined and all but one (PQ Sgr) are likely Sgr dSph members. We examine the onset of carbon-richness in this metal-poor galaxy in the context of stellar models. We compare the stellar death rate (one star per 1000-1700 years) to known planetary nebula dynamical ages and find that the bulk population produce the observed (carbon-rich) planetary nebulae. We compute average lifetimes of S and carbon stars as 60-250 and 130-500 kyr, compared to a total thermal-pulsing asymptotic giant branch lifetime of 530-1330 kyr. We conclude by discussing the return of carbon-rich material to the ISM.