Searching for a predicted metallicity variation in the type II globular cluster NGC 362 = Buscando una variación de metalicidad predicha en el cúmulo globular de tipo II NGC 362.
Loading...
Date
2021
Journal Title
Journal ISSN
Volume Title
Publisher
Universidad de Concepción
Abstract
According to Milone et al. (2017), NGC 362 is a rare Type II Galactic globular cluster,
showing a complex pseudo two-color diagram or ’Chromosome Map’ based on their
HST UV Legacy photometry. The clear separation of its stellar populations in the
color-magnitude diagram and the distribution of the giant stars in the chromosome
map strongly suggests that NGC 362 could host stars with both cluster-nominal as
well as enhanced heavy-element abundances, in particular iron, leading to an intrinsic
metallicity spread, as shown by at least some other Type II globulars. However,
despite previous spectroscopic observations of NGC 362, no such metallicity variation
has been detected, but previous studies did not pre-select potential normal and
anomalous stars. Our main goal is to search for any internal variation of [Fe/H]
which would give us insight into the formation and evolution of this interesting
globular cluster and Type II clusters in general. In this work, we present the
abundance analysis for a sample of 11 red giant branch members based on high resolution and high S/N spectra obtained with the MIKE echelle spectrograph
mounted at the Magellan-Clay telescope. The stars were carefully selected from the
HST photometry to maximize the possibility of including a metallicity variation based
on the Milone et al. (2017) scenario. HST and GAIA photometry and astrometry has
been used to determine atmospheric parameters and membership. We obtained Teff,
log(g) and vt
for our target stars and measured the mean iron content of the sample
and its dispersion with three different methods, which lead to [Fe/H]1=-1.10±0.02,
[Fe/H]2=-1.09±0.01 and [Fe/H]3=-1.10±0.01, while the internal dispersion turned
out to be σ[Fe/H]1=0.06±0.01, σ[Fe/H]2=0.03±0.01 and σ[Fe/H]3=0.05±0.01 respectively.
The error analysis gives an internal dispersion due to observational error of 0.05
dex. Comparing the observed dispersion with the internal errors, we conclude that
NGC 362 does not show any trace of an internal iron spread, in particular between
the anomalous red stars with presumably higher metallicities and the bulk of the
stars that fall along the nominal first and second generation sequences, based on
the chromosome map. Thus, our findings do not support the metallicity spread
predicted by Milone et al. (2017). This makes NGC 362 the first Type II cluster
which definitely does not show such a spread, thus requiring a modification to their
prediction
Description
Tesis presentada para optar al grado de Magíster en Astronomía.
Keywords
Estrellas, Cúmulos Globulares, Estrellas Supergigantes