43 research outputs found
Discovery of > 200 RR Lyrae Variables in M62: An Oosterhoff I Globular Cluster with a Predominantly Blue HB
We report on the discovery of a large number of RR Lyrae variable stars in
the moderately metal-rich Galactic globular cluster M62 (NGC 6266), which
places it among the top three most RR Lyrae-rich globular clusters known.
Likely members of the cluster in our studied field, from our preliminary number
counts, include about 130 fundamental-mode (RRab) pulsators, with =
0.548 d, and about 75 first-overtone (RRc) pulsators, with = 0.300 d.
The average periods and the position of the RRab variables with well-defined
light curves in the Bailey diagram both suggest that the cluster is of
Oosterhoff type I. However, the morphology of the cluster's horizontal branch
(HB) is strikingly similar to that of the Oosterhoff type II globular cluster
M15 (NGC 7078), with a dominant blue HB component and a very extended blue
tail. Since M15 and M62 differ in metallicity by about one dex, we conclude
that metallicity, at a fixed HB type, is a key parameter determining the
Oosterhoff status of a globular cluster and the position of its variables in
the Bailey diagram.Comment: 5 pages, 4 figures. ApJ Letters, in pres
The Dwarf Spheroidal Companions to M31: Variable Stars in Andromeda VI
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, =
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
Variable Stars in the Unusual, Metal-Rich Globular Cluster NGC 6388
We have undertaken a search for variable stars in the metal-rich globular
cluster NGC 6388 using time-series BV photometry. Twenty-eight new variables
were found in this survey, increasing the total number of variables found near
NGC 6388 to ~57. A significant number of the variables are RR Lyrae (~14), most
of which are probable cluster members. The periods of the fundamental mode RR
Lyrae are shown to be unusually long compared to metal-rich field stars. The
existence of these long period RRab stars suggests that the horizontal branch
of NGC 6388 is unusually bright. This implies that the metallicity-luminosity
relationship for RR Lyrae stars is not universal if the RR Lyrae in NGC 6388
are indeed metal-rich. We consider the alternative possibility that the stars
in NGC 6388 may span a range in [Fe/H]. Four candidate Population II Cepheids
were also found. If they are members of the cluster, NGC 6388 would be the most
metal-rich globular cluster to contain Population II Cepheids. The mean V
magnitude of the RR Lyrae is found to be 16.85+/-0.05 resulting in a distance
of 9.0 to 10.3 kpc, for a range of assumed values of for RR Lyrae. We
determine the reddening of the cluster to be E(B-V)=0.40+/-0.03 mag, with
differential reddening across the face of the cluster. We discuss the
difficulty in determining the Oosterhoff classification of NGC 6388 and NGC
6441 due to the unusual nature of their RR Lyrae, and address evolutionary
constraints on a recent suggestion that they are of Oosterhoff type II.Comment: 35 pages, 16 figures, emulateapj5/apjfonts style. Astronomical
Journal, in press. We recommend the interested reader to download instead the
preprint with full-resolution figures, which can be found at
http://www.noao.edu/noao/staff/pritzl/clusters.htm