Other
Scientific paper
Jul 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996hst..prop.6514d&link_type=abstract
HST Proposal ID #6514
Other
Hst Proposal Id #6514 Stellar Populations
Scientific paper
The dwarf spheroidal {dSph} companions to the Galaxy show a large diversity in their horizontal branch {HB} morphologies, with many possessing redder HBs than would be expected from their metal abundances. This variety of HB types, which is also seen in the outer halo globular clusters, is often taken as indicating a large age range in the outer galactic halo. Our Cycle 4 results for And I, a dSph in the outer halo of M31, reveal that this dSph, like many of its galactic counterparts, also has a redder HB than expected for its abundance. This new result naturally leads to the speculation that the outer halo of M31 may also contain a significant range of ages, in a similar fashion to our Galaxy. However, determination of the HB morphologies of the two other dSphs in the outer halo of M31, And II and And III, is an essential requirement before this speculation can be pursued. Indeed with results only for And I, we cannot know whether the M31 outer halo contains the same diversity of HB types as the outer galactic halo, or is instead more homogeneous. We therefore sought HST time with WFPC2 primarily in order to determine the HB morphologies of both And II and And III. The TAC, however, granted time for observations of only one system which we have chosen to be And II. Additional results from this project will include comparison of HB and giant-branch- tip distances, improved estimates of abundance dispersions, estimates of the RR Lyrae star content, and limits on the fraction of younger stars since the observations will reveal any main sequence stars younger than 2-3 Gyr.
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