A relationship between iron spread and variability in giant stars of globular clusters = Una relación entre dispersión de hierro y variabilidad en estrellas gigantes de cúmulos globulares.

Abstract

The work presented here is the results from an analysis of 34 light curves, 17 in the V filter and 17 in the I filter of 17 giant stars of NGC 3201, spanning 4 nights of observation carried out on March 20-23, 2013. The objective of this work was to check and prove if there is a relationship between the variability of a giant star and its iron abundance. To analyze the light curves the Generalized Lomb-Scargle periodogram and the Phase Dispersion Minimization method were computed, and used in order to check if each giant was a variable star. Then a revision of the [Fe/H] values of each variable star from the literature was carried out. From the period analysis made in the sample of 17 stars, 8 were non-variable, 3 were considered as possible variables and 6 stars were variable. From the sample of the discovered variable stars, they displayed different periods ranging from 0.0881±0.0001 to 0.5418±0.0027 days. These 6 variable stars showed distinct values of [Fe I/H], the 3 most metal-rich stars being RGB, one star with [Fe I/H] = −1.37 and two with [Fe I/H] = −1.31 dex. The two most metal-poor variables have [Fe I/H] = −1.61 and [Fe I/H] = −1.62 dex and are AGB stars and one variable showed [Fe I/H] = −1.50 dex also being an AGB star. Meanwhile stars that were non-variables showed −1.56 ≤ [Fe I/H] ≤ −1.40. The conclusion brought from this work is that there is a relationship between the iron abundance and variability of giant stars, where stars that are variable present extreme values [Fe/H] compared with the stars that are non-variable and that variability affects somehow the spectroscopic determination of the iron content of giant stars in globular clusters.