The UK Infrared Telescope M33 monitoring project – IV. Variable red giant stars across the galactic disc

Abstract

We have conducted a near-infrared monitoring campaign at the UK InfraRed Telescope (UKIRT), of the Local Group spiral galaxy M 33 (Triangulum). The main aim was to identify stars in the very final stage of their evolution, and for which the luminosity is more directly related to the birth mass than the more numerous less-evolved giant stars that continue to increase in luminosity. In this fourth paper of the series, we present a search for variable red giant stars in an almost square degree region comprising most of the galaxy's disc, carried out with the WFCAM instrument in the K band. These data, taken during the period 2005-2007, were complemented by J-and H-band images. Photometry was obtained for 403 734 stars in this region; of these, 4643 stars were found to be variable, most of which are Asymptotic Giant Branch (AGB) stars. The variable stars are concentrated towards the centre of M 33, more so than low-mass, less-evolved red giants. Our data were matched to optical catalogues of variable stars and carbon stars and to mid-infrared photometry from the Spitzer Space Telescope. Most dusty AGB stars had not been previously identified in optical variability surveys, and our survey is also more complete for these types of stars than the Spitzer survey. The photometric catalogue is made publicly available at the Centre de Données astronomiques de Strasbourg.

THE IDENTIFICATION OF EXTREME ASYMPTOTIC GIANT BRANCH STARS AND RED SUPERGIANTS IN M33 WITH 24 μm VARIABILITY

“…Javadi et al (2014) find a non-variable star with moderate red colors, which they believe to be the NIR counterpart to HBS 220970. The i′ light curve from the Hartman et al (2006) survey is presented in the left side of Figure 6 and a folded light curve, assuming the E. Polomski et al (2014, in preparation) classification, on a period of 142.857 ± 0.7568 days on the right.…”

“…These also have J-K s < 2.4 mag. The remaining VCs, excluding 1, 5, 7, and 14, which are discussed in their respective sections, are not found to be variable by Javadi et al (2014). This implies that these VCs are possibly not evolved stars (see discussion in Section 6).…”

“…Further, Khan et al (2010) as well as our own color-based XAGB classification demonstrates the problem in assuming that AGB stars with very red colors are most likely C-rich (see Section 6). This object was found by Javadi et al (2014) to not be variable in their NIR monitoring of M33.…”

“…Our VCs were also cross matched to the catalog of a NIR monitoring survey for variable stars across the entire disk of M33 by Javadi et al (2014). VCs 6,8,9,13,16,21,and 23 were matched to strongly confident variable sources with J-K s < 2.4 mag.…”

Pulsating stars and the distance scale

“…We applied the technique to I-band observations of M33 ( [37,38]) and discovered over 1800 Mira candidates in this galaxy ( [35]). We matched our optical catalog with NIR and MIR observations from [39,40] and obtained PLRs that compare favorably to their LMC counterparts (see Fig. 10).…”