A Comprehensive, Wide-Field Study of Pulsating Stars in the Carina Dwarf Spheroidal Galaxy

We report the detection of 388 pulsating variable stars (and some additional miscellaneous variables) in the Carina dSph galaxy over an area covering the full visible extent of the galaxy and extending a few times beyond its photometric (King) tidal radius along the direction of its major axis. Included in this total are 340 newly discovered dwarf Cepheids which are mostly located ~2.5 magnitudes below the horizontal branch and have very short periods (<0.1 days) typical of their class and consistent with their location on the upper part of the extended main sequence of the younger populations of the galaxy. Several extra-tidal dwarf cepheids were found in our survey up to a distance of ~1 degree from the center of Carina. Our sample also includes RR Lyrae stars and anomalous Cepheids some of which were found outside the galaxy's tidal radius as well. This supports past works that suggests Carina is undergoing tidal disruption. We use the period-luminosity relationship for dwarf Cepheids to estimate a distance modulus of 20.17 +/- 0.10 mags, in very good agreement with the estimate from RR Lyrae stars. We find some important differences in the properties of the dwarf Cepheids of Carina and those in Fornax and the LMC, the only extragalactic samples of dwarf Cepheids currently known. These differences may reflect a metallicity spread, depth along the line of sight and/or, different evolutionary paths of the dwarf Cepheid stars.Comment: Accepted for publication in A

A Hybrid Algorithm for Period Analysis from Multi-band Data with Sparse and Irregular Sampling for Arbitrary Light Curve Shapes

Ongoing and future surveys with repeat imaging in multiple bands are producing (or will produce) time-spaced measurements of brightness, resulting in the identification of large numbers of variable sources in the sky. A large fraction of these are periodic variables: compilations of these are of scientific interest for a variety of purposes. Unavoidably, the data-sets from many such surveys not only have sparse sampling, but also have embedded frequencies in the observing cadence that beat against the natural periodicities of any object under investigation. Such limitations can make period determination ambiguous and uncertain. For multi-band data sets with asynchronous measurements in multiple pass-bands, we want to maximally utilize the information on periodicity in a manner that is agnostic of differences in the light curve shapes across the different channels. Given large volumes of data, computational efficiency is also at a premium. This paper develops and presents a computationally economic method for determining periodicity which combines the results from two different classes of period determination algorithms. The underlying principles are illustrated through examples. The effectiveness of this approach for combining asynchronously sampled measurements in multiple observables that share an underlying fundamental frequency is also demonstrated.Comment: accepted for publication in the Astronomical Journa

Velocities of RR Lyrae Stars in the Sagittarius Tidal Stream

We have measured radial velocities and metallicities of 16 RR Lyrae stars, from the QUEST survey, in the Sagittarius tidal stream at 50 kpc from the galactic center. The distribution of velocities is quite narrow (std dev=25 km/s) indicating that the structure is coherent also in velocity space. The mean heliocentric velocity in this part of the stream is 32 km/s. The mean metallicity of the RR Lyrae stars is [Fe/H]=-1.7. Both results are consistent with previous studies of red giant stars in this part of the stream. The velocities also agree with a theoretical model of the disruption of the Sagittarius galaxy.Comment: 4 pages, 3 figures, to be published in the proceedings of the conference "Satellite and Tidal Streams", La Palma 26-30 May 2003, Spain, ASP Conference Serie

RR Lyrae Stars in the Halo: Tracers of Streams of Debris of Disrupted Galaxies

We discuss the first part of a survey for RR Lyrae variables in the galactic halo that is being made with the 1m Schmidt telescope at the Venezuelan National Observatory. So far the survey has discovered 497 variables in 380 sq. degrees, lying from 4 to 60 kpc from the Sun. It has detected three statistically significant clumps of variables, which shows that outer halo does not have smooth density contours. One clump is located at 50 kpc from the galactic center, and it is probably tidal debris from the Sagittarius dwarf spheroidal galaxy. A second structure, at 17 kpc from the galactic center, appears to be due to the tidal disruption of the globular cluster Pal 5. The third group, at 19 kpc, is not related with any known globular cluster or dwarf galaxy. The little that is known about its properties is consistent with it being debris from a disrupted galaxy.Comment: To appear in the proceedings of the workshop Variability with Wide Field Imagers (6 pages

The Galactic Thick Disc density profile traced with RR Lyrae stars

We used a combination of public RR Lyrae star catalogs and a Bayesian methodology to derive robust structural parameters of the inner Halo (<25 kpc) and Thick Disc of the Milky Way. RR Lyrae stars are an unequivocal tracer of old metal-poor populations, for which accurate distances and extinctions can be individually estimated and so, are a reliable independent means of tracing the population of the old high-[\alpha/Fe] disc usually associated to the Thick Disc. In particular, the chosen RR Lyrae sample spans regions at low galactic latitude toward the anti-center direction, allowing to probe the outermost parts of the disc. Our results favour a Thick Disc with short scale height and short scale length, $h_z=0.65_{-0.05}^{+0.09}$ kpc, $h_R=2.1_{-0.25}^{+0.82}$ kpc, for a model in which the inner Halo has a constant flattening of $q=0.90_{-0.03}^{+0.05}$ and a power law index of $n=-2.78_{-0.05}^{+0.05}$. Similar short scales for the Thick Disc are also found when considering an inner Halo with flattening dependent on radius. We also explored a model in which the Thick Disc has a flare and, although this is only mildly constrained by our data, a flare onset in the inner $\sim11$ kpc is highly disfavoured.Comment: 14 pages, 8 figures. Updated to match MNRAS accepted version. No results or conclusions change

Halo Substructure in the QUEST RR Lyrae Survey

A survey of 380 sq. deg. of the sky with the 1m Schmidt telescope at the Observatorio Nacional de Llano del Halo and the QUEST camera has found 498 RR Lyrae variables lying from 4 to 60 kpc from the Sun. We describe the halo substructure revealed by these data and the results of measuring some of the stars' radial velocities and metal abundances.Comment: 8 pages, 5 figures, proceedings of the conference "Satellites and Tidal Streams", La Palma 26-30 May 2003, Spain, ASP Conference Serie

The Selection of RR Lyrae Stars Using Single-epoch Data

We utilize a complete sample of RR Lyrae stars discovered by the QUEST survey using light curves to design selection criteria based on SDSS colors. Thanks to the sensitivity of the u-g color to surface gravity and of g-r color to effective temperature, and to the small photometric errors (~0.02 mag) delivered by SDSS, RR Lyrae stars can be efficiently and robustly recognized even with single-epoch data. In a 100% complete color-selected sample, the selection efficiency (the fraction of RR Lyrae stars in the candidate sample) is 6%, and, by adjusting color cuts, it can be increased to 10% with a completeness of 80%, and to 60% with 28% completeness. Such color selection produces samples that are sufficiently clean for statistical studies of the Milky Way's halo substructure, and we utilize it to select 3,643 candidate RR Lyrae stars from SDSS Data Release 1. We demonstrate that this sample recovers known clumps of RR Lyrae stars associated with the Sgr dwarf tidal tail, and Pal 5 globular cluster, and use it to constrain the halo substructure away from the Sgr dwarf tidal tail. These results suggest that it will be possible to study the halo substructure out to ~70 kpc from the Galactic Center in the entire area imaged by the SDSS, and not only in the multiply observed regions.Comment: submitted to AJ (18 pages, 7 figures

The CIDA-QUEST Large Scale Variability Survey in the Orion OB Association: initial results

Using the 8k x 8k CCD Mosaic Camera on the 1m Schmidt telescope in Venezuela, we are conducting a large-scale, deep optical, multiepoch, photometric (BVRIHa) survey over 120 sq.deg. in the Orion OB association, aimed at identifying the low mass stellar populations with ages less than about 10 Myr. We present initial results for a 34 sq.deg. area spanning Orion 1b, 1a and the B Cloud. Using variability as our main selection criterion we derive much cleaner samples than with the usual single-epoch photometric selection, allowing us to attain a much higher efficiency in follow up spectroscopy and resulting in an preliminary list of 74 new low-mass (~ 0.4 Msun) pre-main sequence stars. Though preliminary, this list of new T Tauri stars already suggests that the fraction of accreting young stars in 1a is much lower than in 1b, which would be expected if 1a is indeed older than 1b. We are analyzing in detail the light curves of these new stars, and spectroscopy of further candidates is under way.Comment: 6 pages, 4 postscript figures; contributed article, to appear in the Proceedings of the 11th Cambridge Cool Stars Workshop, Tenerife, Spain, Oct.4-8, 1999 (ASP Conference Series

VLT Spectroscopy of RR Lyrae Stars in the Sagittarius Tidal Stream

Sixteen RR Lyrae variables from the QUEST survey that lie in the leading arm of the tidal stream from the Sagittarius dSph galaxy have been observed spectroscopically to measure their radial velocities and metal abundances. The systemic velocities of 14 stars, which were determined by fitting a standard velocity curve to the individual measurements, have a sharply peaked distribution with a mean of 33 km/s and a standard deviation of only 25 km/s. The [Fe/H] distribution of these stars has a mean of -1.76 and a standard deviation of 0.22. These measurements are in good agreement with previous ones from smaller samples of stars. The mean metallicity is consistent with the age-metallicity relation that is observed in the main body of the Sgr dSph galaxy. The radial velocities and the distances from the Sun of these stars are compared with recent numerical simulations of the Sgr streams that assume different shapes for the dark matter halo. Models that assume a oblate halo do not fit the data as well as ones that assume a spherical or a prolate distribution. However, none of the fits are completely satisfactory. Every model fails to reproduce the long extent of the stream in right ascension (36 degr) that is seen in the region covered by the QUEST survey. Further modeling is required to see if this and the other mismatches between theory and observation can be removed by judicial choices for the model parameters or instead rule out a class of models.Comment: 35 pages. Accepted for publication in A

New variable stars in NGC 6652 and its background Sagittarius stream

We conducted a variable star search on the metal-rich Galactic globular cluster NGC 6652 using archival Gemini-S/GMOS data. We report the discovery of nine new variable stars in the NGC 6652 field, of which we classify six as eclipsing binaries and one as SX Phoenicis stars, leaving two variables without classification. Using proper motions from Gaia DR2 and HST, albeit with some uncertainties, we find that the cluster, the field, and the background Sagittarius stream, have 3 of these variables each. We also reassess the membership of known variables based on the Gaia proper motions, confirming the existence of one RR Lyrae star in the cluster.Comment: AJ accepte