The age and structure of the Galactic bulge from Mira variables

Robin M. Catchpole , Patricia A. Whitelock , Michael W. Feast , Shaun M. G. Hughes , Mike Irwin , Christophe Alard (1)

1 IAP - Institut d'Astrophysique de Paris

We report periods and JHKL observations for 643 oxygen-rich Mira variables found in two outer bulge fields at b = -7° and l = ±8° and combine these with data on 8057 inner bulge Miras from the Optical Gravitational Lensing Experiment, MACHO and Two Micron All Sky Survey surveys, which are concentrated closer to the Galactic Centre. Distance moduli are estimated for all these stars. Evidence is given showing that the bulge structure is a function of age. The longer period Miras (log P > 2.6, age ∼5 Gyr and younger) show clear evidence of a bar structure inclined to the line of sight in both the inner and outer regions. The distribution of the shorter period (metal-rich globular cluster age) Miras appears spheroidal in the outer bulge. In the inner region these old stars are also distributed differently from the younger ones and possibly suggest a more complex structure. These data suggest a distance to the Galactic Centre, R₀, of 8.9 kpc with an estimated uncertainty of ∼0.4 kpc. The possible effect of helium enrichment on our conclusions is discussed.

Mots clés: stars: AGB and post-AGB; stars: variables: general; Galaxy: bulge; Galaxy: structure; infrared: stars; Astrophysics - Solar and Stellar Astrophysics; Astrophysics - Astrophysics of Galaxies

Domaines: Planète et Univers [physics]; Astrophysique [astro-ph]

Robin M. Catchpole, Patricia A. Whitelock, Michael W. Feast, Shaun M. G. Hughes, Mike Irwin, et al.. The age and structure of the Galactic bulge from Mira variables. Monthly Notices of the Royal Astronomical Society, 2016, 455, pp.2216-2227. ⟨10.1093/mnras/stv2372⟩. ⟨insu-03747405⟩