Spectroscopy of horizontal branch stars in NGC6752 - Anomalous results on atmospheric parameters and masses

TL;DR

This study used ESO VLT-FORS2 spectra to analyze horizontal branch stars in NGC6752, revealing anomalies in atmospheric parameters and masses, especially for certain temperature groups, challenging existing models and assumptions.

Contribution

It provides new spectroscopic data on horizontal branch stars, identifies peculiar groups with anomalous properties, and questions the adequacy of standard atmospheric models for certain temperature ranges.

Findings

Systematic underestimation of color excess compared to literature

Identification of two peculiar star groups at 14000 K and 27000 K

Unusually high masses in some Extreme Horizontal Branch stars

Abstract

We used the ESO VLT-FORS2 facility to collect low-resolution spectra of 51 targets distributed along the Horizontal Branch. We determined atmospheric parameters by comparison with theoretical models through standard fitting routines, and masses by basic equations. Results are in general in good agreement with previous works, although not always with theoretical expectations for cooler stars (Teff<15000 K). The calculated color excess is systematically lower than literature values, pointing towards a possible underestimation of effective temperatures. Moreover, we find two groups of stars at Teff=14000 K and at Teff=27000$ K that present anomalies with respect to the general trend and expectations. We suppose that the three peculiar bright stars at Teff=14000 K are probably affected by an enhanced stellar wind. For the eight Extreme Horizontal Branch stars at Teff=27000 K which show unusually high masses we find no plausible explanation. While most of our results agree well with the predictions of standard horizontal branch evolution, we still have problems with the low masses we derive in certain temperature ranges. We believe that Kurucz ATLAS9 LTE model atmospheres with solar-scaled abundances are probably inadequate for these temperature ranges. Concerning the group of anomalous stars at Teff=27000 K, a Kolmogorov-Smirnov test indicates that there is only an 8.4% probability that these stars are randomly drawn from the general distribution in the color-magnitude diagram. This is not conclusive but points out that these stars could be both (and independently) spectroscopically and photometrically peculiar with respect to the general Extreme Horizontal Branch population.