64 research outputs found
Variables in Globular Cluster NGC 5024
We present the results of a commissioning campaign to observe Galactic
globular clusters for the search of microlensing events. The central 10' X 10'
region of the globular cluster NGC 5024 was monitored using the 2-m Himalayan
Chandra Telescope in R-band for a period of about 8 hours on 24 March 2010.
Light curves were obtained for nearly 10,000 stars, using a modified Difference
Image Analysis (DIA) technique. We identified all known variables within our
field of view and revised periods and status of some previously reported
short-period variables. We report about eighty new variable sources and present
their equatorial coordinates, periods, light curves and possible types. Out of
these, 16 are SX Phe stars, 10 are W UMa-type stars, 14 are probable RR Lyrae
stars and 2 are detached eclipsing binaries. Nine of the newly discovered SX
Phe stars and two eclipsing binaries belong to the Blue Straggler Star (BSS)
population.Comment: 29 pages, 22 figures, replaced with rewritten data reduction par
KIC 8410637: a 408-day period eclipsing binary containing a pulsating red giant
Detached eclipsing binaries (dEBs) are ideal targets for accurate measurement
of masses and radii of ther component stars. If at least one of the stars has
evolved off the main sequence (MS), the masses and radii give a strict
constraint on the age of the stars. Several dEBs containing a bright K giant
and a fainter MS star have been discovered by the Kepler satellite. The mass
and radius of a red giant (RG) star can also be derived from its asteroseismic
signal. The parameters determined in this way depend on stellar models and may
contain systematic errors. It is important to validate the asteroseismically
determined mass and radius with independent methods. This can be done when
stars are members of stellar clusters or members of dEBs. KIC 8410637 consists
of an RG and an MS star. The aim is to derive accurate masses and radii for
both components and provide the foundation for a strong test of the
asteroseismic method and the accuracy of the deduced mass, radius and age. We
analyse high-resolution spectra from three different spectrographs. We also
calculate a fit to the Kepler light curve and use ground-based photometry to
determine the flux ratios between the component stars in the BVRI passbands. We
measured the masses and radii of the stars in the dEB, and the classical
parameters Teff, log g and [Fe/H] from the spectra and ground-based photometry.
The RG component of KIC 8410637 is most likely in the core helium-burning red
clump phase of evolution and has an age and composition very similar to the
stars in the open cluster NGC 6819. The mass of the RG in KIC 8410637 should
therefore be similar to the mass of RGs in NGC 6819, thus lending support to
the most up-to-date version of the asteroseismic scaling relations. This is the
first direct measurement of both mass and radius for an RG to be compared with
values for RGs from asteroseismic scaling relations.Comment: Accepted 20.6.2013 for publication in Astronomy and Astrophysic
Discovery of a Variable Star Population in NGC 2808
We have applied the image subtraction method to images of the peculiar,
bimodal-horizontal branch globular cluster NGC 2808, taken over a total of six
nights over a range of five months. As a result, we have found, for the first
time, a sizeable population of variable stars in the crowded inner regions of
the cluster, thus raising the known RR Lyrae population in the cluster to a
total of 18 stars. In addition, an eclipsing binary and two other variables
with periods longer than 1 day were also found. Periods, positions and
(differential) light curves are provided for all the detected variables. The
Oosterhoff classification of NGC 2808, which has recently been associated with
a previously unknown dwarf galaxy in Canis Major, is briefly discussed.Comment: 8 pages, 4 figures. A&A, in pres
The Carina Project: I. Bright Variable Stars
We present new BV time series data of the Carina dwarf spheroidal galaxy.
Current data cover an area of about 0.3 square degrees around the center of the
galaxy and allowed us to identify 92 variables. Among them 75 are RR Lyrae
stars, 15 are "bona fide" Anomalous Cepheids, one might be a Galactic field RR
Lyrae, and one is located along the Carina Red Giant Branch. Expanding upon the
seminal photographic investigation by Saha, Monet & Seitzer (1986) we supply
for the first time accurate estimates of their pulsation parameters (periods,
amplitudes, mean magnitude and colors) on the basis of CCD photometry.
Approximately 50% of both RR Lyrae and Anomalous Cepheids are new
identifications. Among the RR Lyrae sample 6 objects are new candidate
double-mode variables. On the basis of their pulsation properties we estimated
that two variables (V152, V182) are about 50% more massive than typical RR
Lyrae stars, while the bulk of the Anomalous Cepheids are roughly a factor of
two more massive than fundamental mode RR Lyrae stars. This finding supports
the evidence that these objects are intermediate-mass stars during central He
burning phases. The data for Carina, together with data available in the
literature, strongly support the conclusion that dSph galaxies can barely be
classified into the classical Oosterhoff dichotomy. The mean period of
fundamental mode RR Lyrae in Carina resembles that found for Oosterhoff type II
clusters, whereas the ratio between first overtones and total number of RR
Lyrae is quite similar to that found in Oosterhoff type I clusters.Comment: 54 pages, 12 figures, accepted for publications on The Astronomical
Journal. Note that some figures are not included: a complete version with all
the 20 figures is available at this website:
http://snoopy.mporzio.astro.it/carinaproject
- …