May 06, 2013

Briana Indahl, UW Madison

"Search for Carbon-Rich Asymptotic Giant Branch Stars in Milky Way Globular Clusters"

From our current understanding of stellar evolution, it would not be expected to find carbon rich
asymptotic giant branch (AGB) stars in Milky Way globular clusters. Due to the low metallicity
of the population II stars making up the globular clusters and the age of the clusters, stars large
enough to fuse carbon should have already evolved off of the asymptotic giant branch. Recently,
however, there have been serendipitous discoveries of these types of stars. Carbon rich AGB
stars could exist either as a member of a mass transfer binary system where the star has obtained
carbon from its carbon white dwarf companion, or as a product of a stellar merger. Matsunaga et
al. (2006) discovered a Mira variable in the globular cluster Lynga 7. It was later confirmed by
Feast et al. (2012) that the star is a member of the cluster and it must be a product of a steller
merger. In the same year, Sharina et al. (2012) made a serendipitous discovery of a carbon star in
the low metallicity globular cluster NGC6426 and reports it to be a CH star (a population II
carbon star that is a member of a mass transfer binary system). Five more of these types of stars
have been made as serendipitous discoveries and have been reported by Harding (1962), Dickens
(1972), Cote et al. (1997), and Van Loon (2007). The abundance of these types of carbon stars in
Milky Way globular clusters has been unknown because the discovery of these types of objects
has only ever been a product of a serendipitous discovery. These stars could have been easily
overlooked in the past as they are outside the typical parameter space of galactic globular
clusters. Also advances in near-infrared instruments and observing techniques have made it
possible to detect the fainter carbon stars in binary systems. Having an understanding of the
abundances of carbon stars in galactic globular clusters will aid in the modeling of globular
cluster and galaxy formation leading to a better understanding of these processes. To get an
understanding of the abundances of these stars we conducted the first comprehensive search for
AGB carbon stars into all Milky Way globular clusters listed in the Harris Catalog (expect for
Pyxis). I have found 128 carbon star candidates using methods of comparing the color
magnitude diagrams of the clusters with the carbon stars of the Large Magellenic Clouds and
picking out very red stars in the red giant branch range. Observations will need to be done of
these candidates to further confirm if they are carbon stars and are member of their respective
globular cluster

Event Details

May 06, 2013


Sterling 4421 (Colloquium Room)

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