The Dwarf Spheroidal Companions to M31: WFPC2 Observations of Andromeda I

Images have been obtained with the Hubble Space Telescope WFPC2 camera of Andromeda I, a dwarf spheroidal (dSph) galaxy that lies in the outer halo of M31. The resulting color-magnitude diagrams reveal for the first time the morphology of the horizontal branch in this system. We find that, in a similar fashion to many of the galactic dSph companions, the horizontal branch (HB) of And~I is predominantly red. Combined with the metal abundance of this dSph, this red HB morphology indicates that And I can be classified as a ``second parameter'' system in the outer halo of M31. This result then supports the hypothesis that the outer halo of M31 formed in the same extended chaotic manner as is postulated for the outer halo of the Galaxy.Comment: 26 pages using aas2pp4.sty, including 2 tables and 7 figures, to be published in AJ. Figure 1 is in gif form. To include in main ps file, use xv to create a ps file called Da_Costa.fig1.ps and uncomment appropriate lines in .tex fil

The Dwarf Spheroidal Companions to M31: WFPC2 Observations of Andromeda III

The Hubble Space Telescope WFPC2 camera has been used to image Andromeda III, a dwarf spheroidal companion (dSph) to M31. The resulting color-magnitude (c-m) diagrams reveal the morphology of the horizontal branch (HB) in this dwarf galaxy. We find that like Andromeda I and Andromeda II, and like most of the Galactic dSph companions, the HB morphology of And III is predominantly red, redder than that of both And I and And II despite And III having a lower mean metallicity. We interpret this red HB morphology as indicating that the bulk of the And III population is ~3 Gyr younger than the age of the majority of Galactic globular clusters. Nevertheless, the And III c-m diagram does reveal a few blue HB stars, and a number of RR Lyrae variables are also evident in the data. This indicates that And III does contain an `old' population of age comparable to that of the Galactic globular clusters. There is no evidence, however, for any young stars in And III despite a claimed association between this dSph and an HI cloud. As was the case for And II, but not And I, no radial gradient was detected in the And III HB morphology. And III is ~75 kpc from the center of M31, comparable to the Galactocentric distances of Sculptor and Draco. Comparison with standard globular cluster red giant branches indicates = -1.88 +/-0.11, consistent with the absolute-magnitude - mean abundance relation followed by dSph galaxies. The same comparison yields an intrinsic abundance dispersion of sigma([Fe/H]) = 0.12, a low value compared to the Galactic dSphs of comparable luminosity to And III. The list of candidate variables reveals one definite and one probable Anomalous Cepheid variables.Comment: 30 pages including 1 table, 10 figures, Fig 1 as jpeg to save space. Accepted for publication in the Astronomical Journal, July 2002 issu

Star Formation in the Gulf of Mexico

We present an optical/infrared study of the dense molecular cloud, L935, dubbed "The Gulf of Mexico", which separates the North America and the Pelican nebulae, and we demonstrate that this area is a very active star forming region. A wide-field imaging study with interference filters has revealed 35 new Herbig-Haro objects in the Gulf of Mexico. A grism survey has identified 41 Halpha emission-line stars, 30 of them new. A small cluster of partly embedded pre-main sequence stars is located around the known LkHalpha 185-189 group of stars, which includes the recently erupting FUor HBC 722.Comment: Submitted to A&A, 14 pages, 18 figure