The Dwarf Spheroidal Companions to M31: Variable Stars in Andromeda VI

Abstract

We have surveyed Andromeda VI, a dwarf spheroidal galaxy companion to M31, for variable stars using F450W and F555W observations obtained with the Hubble Space Telescope. A total of 118 variables were found, with 111 being RR Lyrae, 6 anomalous Cepheids, and 1 variable we were unable to classify. We find that the Andromeda VI anomalous Cepheids have properties consistent with those of anomalous Cepheids in other dwarf spheroidal galaxies. We revise the existing period-luminosity relations for these variables. Further, using these and other available data, we show that there is no clear difference between fundamental and first-overtone anomalous Cepheids in a period-amplitude diagram at shorter periods, unlike the RR Lyrae. For the Andromeda VI RR Lyrae, we find that they lie close to the Oosterhoff type I Galactic globular clusters in the period-amplitude diagram, although the mean period of the RRab stars, <P_ab> = 0.588 d, is slightly longer than the typical Oosterhoff type I cluster. The mean V magnitude of the RR Lyrae in Andromeda VI is 25.29+/-0.03, resulting in a distance 815+/-25 kpc on the Lee, Demarque, & Zinn distance scale. This is consistent with the distance derived from the I magnitude of the tip of the red giant branch. Similarly, the properties of the RR Lyrae indicate a mean abundance for Andromeda VI which is consistent with that derived from the mean red giant branch color.Comment: 23 pages, including 13 figures and 6 tables, emulateapj5/apjfonts style. Accepted by the Astronomical Journal. We recommend the interested reader to download the preprint with full-resolution figures, which can be found at http://www.noao.edu/noao/staff/pritzl/M31dwarfs.htm

The Carina Project. I. Bright Variable Stars

“…According to this circumstantial evidence, the variables brighter than hV i ¼ 20 mag might be tentatively classified as anomalous Cepheids. Data plotted in the top panel of Figure 5 indicate that the anomalous Cepheids can be divided into two period-luminosity relations, perhaps signifying different pulsation modes as suggested by empirical (Nemec et al 1994;Pritzl et al 2002) and theoretical (Bono et al 1997b) evidence. Note that according to this selection criterion the two variables V158 and V182 appear to be evolved RR Lyrae stars, since they present (BÀV ) colors quite similar to V149, the faintest AC, but their periods are significantly longer (see below).…”

The Variable Stars of the Draco Dwarf Spheroidal Galaxy: Revisited

“…We have calculated absolute magnitudes for the Draco AC stars assuming a distance modulus of (m − M) 0 = 19.49 and an E(B − V ) = 0.03 (Pritzl et al 2002a) in order to incorporate our results with their empirical P-L relations. Figure 11 shows the location of the Draco AC stars with respect to the AC stars found in other Local Group dwarf galaxies.…”

Dwarf Spheroidal Satellites of M31. I. Variable Stars and Stellar Populations in Andromeda Xix

“…The left panel of Figure 3 shows the comparison of And XIX variables with the period-amplitude diagram of the RR Lyrae stars detected in other four M31 dwarf satellites (namely, And I, And II, And III and And VI). Data for the RR Lyrae stars in And I and And III are from Pritzl et al (2005), for And II from Pritzl et al (2004), and for And VI from Pritzl et al (2002). The variables appear to concentrate mainly towards the Oo-I locus or in the region between Oo I and Oo-Int lines, thus suggesting that there is paucity of Oo-II satellites around M31.…”

“…However, the CMDs currently available for the M31 dSphs generally sample only the brightest stars (e.g. Zucker et al 2004;McConnachie et al 2008;Richardson et al 2011;Bell et al 2011), and even when Hubble Space Telescope (HST) data are available (Pritzl et al 2002(Pritzl et al , 2004(Pritzl et al , 2005Mancone & Sarajedini 2008;Yang & Sarajedini 2012) The pulsating variable stars are a powerful alternative tool to investigate the different stellar generations occurred in the M31 dSphs, as Classical Cepheids can be used to trace the young stars (typical ages ranging from a few to a few hundred Myr), and the RR Lyrae stars, which are comparably old but about 3 magnitude brighter than the MSTO, can allow to unravel the oldest stars born more than 10 Gyr ago. Using characteristics like the mean period of the fundamental-mode (RRab) and first-overtone (RRc) pulsators and the ratio of number of RRc (N c ) to total number of RR Lyrae stars (N ab+c ) (f c = N c /N ab+c ) the MW field and cluster RR Lyrae are divided into two different groups (Oosterhoff 1939): Oosterhoff type I (Oo I) clusters have P ab ≃ 0.55 d, P c ≃ 0.32 d, and f c ∼ 0.17; and Oosterhoff type II (Oo II) clusters have P ab ≃ 0.65 d, P c ≃ 0.37 d, and f c ∼ 0.44.…”

“…Of the M31 globular clusters only two had the RR Lyrae stars fully characterized, B514 was found to have Oo-Int properties (Clementini et al 2009) and G11 to be an Oo II GC (Contreras Ramos et al 2013). Six of the M31 dSphs have been analysed so far for variability and RR Lyrae stars have been identified in all of them (Pritzl et al 2002(Pritzl et al , 2004(Pritzl et al , 2005Mancone & Sarajedini 2008;Yang & Sarajedini 2012). According to these studies all three Oosterhoff types (Oo I, Oo II, and Oo-Int) seem to be present among the M31 satellites.…”