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Register a new userPrecision distances to dwarf galaxies and globular clusters from Pan-STARRS1 3$pi$ RR Lyrae
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We present new spatial models and distance estimates for globular clusters (GC) and dwarf spheroidals (dSphs) orbiting our Galaxy based on RR Lyrae (RRab) stars in the Pan-STARRS1 (PS1) 3$pi$ survey. Using the PS1 sample of RRab stars from Sesar et al. (2017) in 16 globular clusters and 5 dwarf galaxies, we fit structural models in $(l,b,D)$ space; for 13 globular clusters and 6 dwarf galaxies, we give only their mean heliocentric distance $D$. We verify the accuracy of the period-luminosity (PL) relations used in Sesar et al. (2017) to constrain the distance to those stars, and compare them to period-luminosity-metallicity (PLZ) relations using metallicities from Carretta et al. (2009). We compare our Sesar et al. (2017) distances to the parallax-based textit{Gaia} DR2 distance estimates from Bailer-Jones et al. (2018), and find our distances to be consistent and considerably more precise.
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We present a comprehensive and precise description of the Sagittarius (Sgr) stellar streams 3D geometry as traced by its old stellar population. This analysis draws on the sample of ${sim}44,000$ RR Lyrae (RRab) stars from the Pan-STARRS1 (PS1) 3$pi$ survey (Hernitschek et al. 2016,Sesar et al. 2017b), which is ${sim}80%$ complete and ${sim}90%$ pure within 80~kpc, and extends to ${gtrsim} 120$~kpc with a distance precision of ${sim} 3%$. A projection of RR Lyrae stars within $|tilde{B}|_{odot}<9^circ$ of the Sgr streams orbital plane reveals the morphology of both the leading and the trailing arms at very high contrast, across much of the sky. In particular, the map traces the stream near-contiguously through the distant apocenters. We fit a simple model for the mean distance and line-of-sight depth of the Sgr stream as a function of the orbital plane angle $tilde{Lambda}_{odot}$, along with a power-law background-model for the field stars. This modeling results in estimates of the mean stream distance precise to ${sim}1%$ and it resolves the streams line-of-sight depth. These improved geometric constraints can serve as new constraints for dynamical stream models.
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.
The most common methods to derive the distance to globular clusters using RR Lyrae variables are reviewed, with a special attention to those that have experienced significant improvement in the past few years. From the weighted average of these most recent determinations the absolute magnitude of the RR Lyrae stars at [Fe/H]=-1.5 is Mv = 0.59 +/- 0.03 mag, corresponding to a distance modulus for the LMC (m-M)o = 18.48 +/- 0.05.
We present the discovery of a faint Milky Way satellite, Laevens 2/Triangulum II, found in the Panoramic Survey Telescope And Rapid Response System (Pan-STARRS 1) 3 pi imaging data and confirmed with follow-up wide-field photometry from the Large Binocular Cameras. The stellar system, with an absolute magnitude of M_V=-1.8 +/-0.5, a heliocentric distance of 30 +2/-2 kpc, and a half-mass radius of 34 +9/-8 pc, shows remarkable similarity to faint, nearby, small satellites such as Willman 1, Segue 1, Segue 2, and Bootes II. The discovery of Laevens 2/Triangulum II further populates the region of parameter space for which the boundary between dwarf galaxies and globular clusters becomes tenuous. Follow-up spectroscopy will ultimately determine the nature of this new satellite, whose spatial location hints at a possible connection with the complex Triangulum-Andromeda stellar structures.
We search for RR Lyrae stars in 27 nearby ($<100$ kpc) ultra-faint dwarf satellite galaxies using the Gaia DR2 catalog of RR Lyrae stars. Based on proper motions, magnitudes and location on the sky, we associate 47 Gaia RR Lyrae stars to 14 different satellites. Distances based on RR Lyrae stars are provided for those galaxies. We have identified RR Lyrae stars for the first time in the Tucana II dwarf galaxy, and find additional members in Ursa Major II, Coma Berenices, Hydrus I, Bootes I and Bootes III. In addition we have identified candidate extra-tidal RR Lyrae stars in six galaxies which suggest they may be undergoing tidal disruption. We found 10 galaxies have no RR Lyrae stars neither in Gaia nor in the literature. However, given the known completeness of Gaia DR2 we cannot conclude these galaxies indeed lack variable stars of this type.
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