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Register a new userUnmixing the Galactic Halo with RR Lyrae tagging
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We show that tagging RR Lyrae stars according to their location in the period-amplitude diagram can be used to shed light on the genesis of the Galactic stellar halo. The mixture of RR Lyrae of ab type, separated into classes along the lines suggested by Oosterhoff, displays a strong and coherent evolution with Galactocentric radius. The change in the RR Lyrae composition appears to coincide with the break in the halos radial density profile at ~25 kpc. Using simple models of the stellar halo, we establish that at least three different types of accretion events are necessary to explain the observed RRab behavior. Given that there exists a correlation between the RRab class fraction and the total stellar content of a dwarf satellite, we hypothesize that the field halo RRab composition is controlled by the mass of the progenitor contributing the bulk of the stellar debris at the given radius. This idea is tested against a suite of cosmological zoom-in simulations of Milky Way-like stellar halo formation. Finally, we study some of the most prominent stellar streams in the Milky Way halo and demonstrate that their RRab class fractions follow the trends established previously.
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We present the analysis of 12227 type-ab RR Lyrae found among the 200 million public lightcurves in the Catalina Surveys Data Release 1 (CSDR1). These stars span the largest volume of the Milky Way ever surveyed with RR Lyrae, covering ~20,000 square degrees of the sky (0 < RA < 360, -22 < Dec < 65 deg) to heliocentric distances of up to 60kpc. Each of the RR Lyrae are observed between 60 and 419 times over a six-year period. Using period finding and Fourier fitting techniques we determine periods and apparent magnitudes for each source. We find that the periods at generally accurate to sigma = 0.002% by comparison with 2842 previously known RR Lyrae and 100 RR Lyrae observed in overlapping survey fields. We photometrically calibrate the light curves using 445 Landolt standard stars and show that the resulting magnitudes are accurate to ~0.05 mags using SDSS data for ~1000 blue horizontal branch stars and 7788 of the RR Lyrae. By combining Catalina photometry with SDSS spectroscopy, we analyze the radial velocity and metallicity distributions for > 1500 of the RR Lyrae. Using the accurate distances derived for the RR Lyrae, we show the paths of the Sagittarius tidal streams crossing the sky at heliocentric distances from 20 to 60 kpc. By selecting samples of Galactic halo RR Lyrae, we compare their velocity, metallicity, and distance with predictions from a recent detailed N-body model of the Sagittarius system. We find that there are some significant differences between the distances and structures predicted and our observations.
For the first time accurate pulsation properties of the ancient variable stars of the Fornax dwarf spheroidal galaxy (dSph) are discussed in the broad context of galaxy formation and evolution. Homogeneous multi-band $BVI$ optical photometry of spanning {it twenty} years has allowed us to identify and characterize more than 1400 RR Lyrae stars (RRLs) in this galaxy. Roughly 70% are new discoveries. We investigate the period-amplitude distribution and find that Fornax shows a lack of High Amplitude (A$_Vgsim$0.75 mag) Short Period fundamental-mode RRLs (P$lsim$0.48 d, HASPs). These objects occur in stellar populations more metal-rich than [Fe/H]$sim$-1.5 and they are common in the Galactic halo (Halo) and in globulars. This evidence suggests that old (age older than 10 Gyr) Fornax stars are relatively metal-poor. A detailed statistical analysis of the role of the present-day Fornax dSph in reproducing the Halo period distribution shows that it can account for only a few to 20% of the Halo when combined with RRLs in massive dwarf galaxies (Sagittarius dSph, Large Magellanic Cloud). This finding indicates that Fornax-like systems played a minor role in building up the Halo when compared with massive dwarfs. We also discuss the occurrence of HASPs in connection with the luminosity and the early chemical composition of nearby dwarf galaxies. We find that, independently of their individual star formation histories, bright (M$_Vlsim$-13.5 mag) galaxies have HASPs, whereas faint ones (M$_Vgsim$-11 mag) do not. Interestingly enough, Fornax belongs to a luminosity range (--11$<$M$_V<$--13.5 mag) in which the occurrence of HASPs appears to be correlated with the early star formation and chemical enrichment of the host galaxy.
We present a catalog of 5,290 RR Lyrae stars (RRLs) with metallicities estimated from spectra of the LAMOST Experiment for Galactic Understanding and Exploration (LEGUE) and the Sloan Extension for Galactic Understanding and Exploration (SEGUE) surveys. Nearly 70 per cent of them (3,642 objects) also have systemic radial velocities measured. Given the pulsating nature of RRLs, metallicity estimates are based on spectra of individual exposures, by matching them with the synthetic templates. The systemic radial velocities are measured by fitting the observed velocity as a function of phase assuming an empirical pulsating velocity template curve. Various tests show that our analyses yield metallicities with a typical precision of 0.20,dex and systemic radial velocities with uncertainties ranging from 5 to 21,km,s$^{-1}$ (depending on the number of radial velocity measurements available for a given star). Based on the well calibrated near-infrared $PM_{W1}Z$ or $PM_{K_{rm s}}Z$, and $M_{V}$-[Fe/H] relations, precise distances are derived for these RRLs. Finally, we include Gaia DR2 proper motions in our catalog. The catalog should be very useful for various Galactic studies, especially of the Galactic halo.
We present a catalog of RR Lyrae stars (RRLs) observed by the Xuyi Schmidt Telescope Photometric Survey (XDSS). The area we consider is located in the North Galactic Cap, covering 376.75 sq deg at RA $approx$ 150 deg and Dec $approx$ 27 deg down to a magnitude limit of i $approx$ 19. Using the variability information afforded by the multi-epoch nature of our XDSS data, combined with colors from the Sloan Digital Sky Survey, we are able to identify candidate RRLs. We find 318 candidates, derive distances to them and estimate the detection efficiency. The majority of our candidates have more than 12 observations and for these we are able to calculate periods. These also allows us to estimate our contamination level, which we predict is between 30% to 40%. Finally we use the sample to probe the halo density profile in the 9-49 kpc range and find that it can be well fitted by a double power law. We find good agreement between this model and the models derived for the South Galactic Cap using the Watkins et al. (2009) and Sesar et al. (2010) RRL data-sets, after accounting for possible contamination in our data-set from Sagittarius stream members. We consider non-spherical double power law models of the halo density profile and again find agreement with literature data-sets, although we have limited power to constrain the flattening due to our small survey area. Much tighter constraints will be placed by current and future wide-area surveys, most notably ESAs astrometric Gaia mission. Our analysis demonstrates that surveys with a limited number of epochs can effectively be mined for RRLs. Our complete sample is provided as accompanying online material.
We characterize the spatial density of the Pan-STARRS1 (PS1) sample of RR Lyrae stars, to study the properties of the old Galactic stellar halo as traced by RRab stars. This sample of 44,403 sources spans Galactocentric radii of $0.55 ; mathrm{kpc} leq R_{mathrm{gc}} leq 141 ; mathrm{kpc}$ with a distance precision of 3% and thus is able to trace the halo out to larger distances than most previous studies. After excising stars that are attributed to dense regions such as stellar streams, the Galactic disc and bulge as well as halo globular clusters, the sample contains ${sim}11,000$ sources within $20 ; mathrm{kpc} leq R_{mathrm{gc}} leq 131 ; mathrm{kpc}$. We then apply forward modeling using ellipsoidal stellar density models $rho(l,b,R_{mathrm{gc}})$ both with a constant and a radius-dependent halo flattening $q(R_{mathrm{gc}})$. Assuming constant flattening $q$, the distribution of the sources is reasonably well fit from $20 ; mathrm{kpc}$ to $131 ; mathrm{kpc}$ by a single power law with $n=4.40^{+0.05}_{-0.04}$ and $q=0.918^{+0.016}_{-0.014}$. The distance distribution is fit comparably well by an Einasto profile with $n=9.53^{+0.27}_{-0.28}$, an effective radius $r_{mathrm{eff}}=1.07 pm 0.10 ; mathrm{kpc}$ and a halo flattening of $q=0.923 pm 0.007$. If we allow for a radius-dependent flattening $q(R_{mathrm{gc}})$, we find evidence for a distinct flattening of $q{sim}0.8$ of the inner halo at ${sim} 25 ; mathrm{kpc}$. Additionally, we find that the south Galactic hemisphere is more flattened than the north Galactic hemisphere. The results of our work are largely consistent with many earlier results, e.g. cite{Watkins2009}, cite{Iorio2017}. We find that the stellar halo, as traced in RR Lyrae stars, exhibits a substantial number of further significant over- and underdensities, even after all known overdensities have been masked.
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