A Remarkable Oxygen-rich Asymptotic Giant Branch Variable in the Sagittarius Dwarf Irregular Galaxy

TL;DR

This study identifies and characterizes three large amplitude AGB variables in the metal-deficient Sagittarius Dwarf Irregular Galaxy, revealing a rare oxygen-rich star with unusual properties and comparing stellar populations across Local Group galaxies.

Contribution

First detailed infrared study of AGB variables in Sgr dIG, highlighting a rare oxygen-rich star and providing insights into stellar evolution in low-metallicity environments.

Findings

Discovered an oxygen-rich AGB variable with a 950-day period in Sgr dIG.

Identified two carbon-rich AGB variables with periods of 670 and 503 days.

Compared AGB populations in Sgr dIG, NGC6822, and IC1613, linking differences to star formation and metallicity.

Abstract

We report and discuss JHKs photometry for Sgr dIG, a very metal-deficient galaxy in the Local Group, obtained over 3.5 years with the Infrared Survey Facility in South Africa. Three large amplitude asymptotic giant branch variables are identified. One is an oxygen-rich star that has a pulsation period of 950 days, that was until recently undergoing hot bottom burning, with Mbol~-6.7. It is surprising to find a variable of this sort in Sgr dIG, given their rarity in other dwarf irregulars. Despite its long period the star is relatively blue and is fainter, at all wavelengths shorter than 4.5microns, than anticipated from period-luminosity relations that describe hot bottom burning stars. A comparison with models suggests it had a main sequence mass Mi~5 times solar and that it is now near the end of its AGB evolution. The other two periodic variables are carbon stars with periods of 670 and 503 days (Mbol~-5.7 and -5.3). They are very similar to other such stars found on the AGB of metal deficient Local Group Galaxies and a comparison with models suggests Mi~3 times solar. We compare the number of AGB variables in Sgr dIG to those in NGC6822 and IC1613, and suggest that the differences may be due to the high specific star formation rate and low metallicity of Sgr dIG.