Spectroscopy of blue horizontal branch stars in NGC 6656 (M 22) ^{⋆,⋆⋆,⋆⋆⋆}

¹ Departamento de Astronomía, Universidad de Concepción, Casilla 160-C, Concepción, Chile
e-mail: carosalgado@udec.cl
² Instituto de Astronomía, Universidad Católica del Norte, Av. Angamos 0610, Antofagasta, Chile
³ The Milky Way Millennium Nucleus, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
⁴ Departamento de Astronomía y Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile

Received: 14 March 2013
Accepted: 5 July 2013

Abstract

Context. Recent investigations revealed very peculiar properties of blue horizontal branch (HB) stars in ω Centauri, which show anomalously low surface gravity and mass compared to other clusters and to theoretical models. ω Centauri, however, is a very unusual object, hosting a complex mix of multiple stellar populations with different metallicity and chemical abundances.

Aims. We measured the fundamental parameters (temperature, gravity, and surface helium abundance) of a sample of 71 blue HB stars in M 22, with the aim of clarifying if the peculiar results found in ω Cen are unique to this cluster. M 22 also hosts multiple sub-populations of stars with a spread in metallicity, analogous to ω Cen.

Methods. The stellar parameters were measured on low-resolution spectra fitting the Balmer and helium lines with a grid of synthetic spectra. From these parameters, the mass and reddening were estimated.

Results. Our results on the gravities and masses agree well with theoretical expectations, matching the previous measurements in three “normal” clusters. The anomalies found in ω Cen are not observed among our stars. A mild mass underestimate is found for stars hotter than 14 000 K, but an exact analogy with ω Cen cannot be drawn. We measured the reddening in the direction of M 22 with two independent methods, finding E(B − V) = 0.35 ± 0.02 mag, with semi-amplitude of the maximum variation Δ(E(B − V)) = 0.06 mag, and an rms intrinsic dispersion of σ(E(B − V)) = 0.03 mag.