Spectroscopy of blue horizontal branch stars in NGC 6656 (M22)

TL;DR

This study investigates blue horizontal branch stars in M22 to compare their properties with those in en, aiming to understand if peculiar features are unique or common among clusters with multiple stellar populations.

Contribution

The paper provides the first detailed spectroscopic analysis of blue HB stars in M22, showing their properties align with theoretical models and differ from peculiar stars in en.

Findings

M22 blue HB stars have normal gravities and masses.

No anomalies similar to en are observed in M22.

Reddening in M22 is measured as E(B-V)=0.35 1.02 mag.

Abstract

Recent investigations revealed very peculiar properties of blue horizontal branch (HB) stars in \omega Centauri, which show anomalously low surface gravity and mass compared to other clusters and to theoretical models. \omega Centauri, however, is a very unusual object, hosting a complex mix of multiple stellar populations with different metallicity and chemical abundances. We measured the fundamental parameters (temperature, gravity, and surface helium abundance) of a sample of 71 blue HB stars in M22, with the aim of clarifying if the peculiar results found in \omega Cen are unique to this cluster. M22 also hosts multiple sub-populations of stars with a spread in metallicity, analogous to \omega Cen. 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. Our results on the gravities and masses agree well with theoretical expectations, matching the previous measurements in three "normal" clusters. The anomalies found in \omega 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 \omega Cen cannot be drawn. We measured the reddening in the direction of M22 with two independent methods, finding E(B-V)=0.35 \pm 0.02 mag, with semi-amplitude of the maximum variation \Delta(E(B-V))=0.06 mag, and an rms intrinsic dispersion of \sigma(E(B-V))=0.03 mag.