RR Lyrae Variables in M32 and the Disk of M31
Abstract
We observed two fields near M32 with the Advanced Camera for Surveys/High Resolution Channel (ACS/HRC) on board the Hubble Space Telescope. The main field, F1, is 1farcm8 from the center of M32; the second field, F2, constrains the M31 background, and is 5farcm4 distant. Each field was observed for 16 orbits in each of the F435W (narrow B) and F555W (narrow V) filters. The duration of the observations allowed RR Lyrae stars to be detected. A population of RR Lyrae stars determined to belong to M32 would prove the existence of an ancient population in that galaxy, a subject of some debate. We detected 17 RR Lyrae variables in F1 and 14 in F2. A 1σ upper limit of 6 RR Lyrae variables belonging to M32 is inferred from these two fields alone. Use of our two ACS/WFC parallel fields provides better constraints on the M31 background, however, and implies that 7+4 -3 (68% confidence interval) RR Lyrae variables in F1 belong to M32. We have therefore found evidence for an ancient population in M32. It seems to be nearly indistinguishable from the ancient population of M31. The RR Lyrae stars in the F1 and F2 fields have indistinguishable mean V-band magnitudes, mean periods, distributions in the Bailey diagram, and ratios of RRc to RRtotal types. However, the color distributions in the two fields are different, with a population of red RRab variables in F1 not seen in F2. We suggest that these might be identified with the detected M32 RR Lyrae population, but the small number of stars rules out a definitive claim.
Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with GO proposal 10572.- Publication:
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The Astrophysical Journal
- Pub Date:
- January 2010
- DOI:
- 10.1088/0004-637X/708/1/817
- arXiv:
- arXiv:0911.0355
- Bibcode:
- 2010ApJ...708..817F
- Keywords:
-
- galaxies: elliptical and lenticular;
- cD;
- galaxies: individual: M32 M31;
- Local Group;
- stars: Population II;
- stars: variables: other;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- 19 pages, 18 figures, accepted ApJ