We quantify the variability of faint unresolved optical sources using a catalog based on multiple SDSS imaging observations. The catalog covers SDSS Stripe 82, and contains 58 million photometric ...observations in the SDSS ugriz system for 1.4 million unresolved sources. In each photometric bandpass we compute various low-order lightcurve statistics and use them to select and study variable sources. We find that 2% of unresolved optical sources brighter than g=20.5 appear variable at the 0.05 mag level (rms) simultaneously in the g and r bands. The majority (2/3) of these variable sources are low-redshift (<2) quasars, although they represent only 2% of all sources in the adopted flux-limited sample. We find that at least 90% of quasars are variable at the 0.03 mag level (rms) and confirm that variability is as good a method for finding low-redshift quasars as is the UV excess color selection (at high Galactic latitudes). We analyze the distribution of lightcurve skewness for quasars and find that is centered on zero. We find that about 1/4 of the variable stars are RR Lyrae stars, and that only 0.5% of stars from the main stellar locus are variable at the 0.05 mag level. The distribution of lightcurve skewness in the g-r vs. u-g color-color diagram on the main stellar locus is found to be bimodal (with one mode consistent with Algol-like behavior). Using over six hundred RR Lyrae stars, we demonstrate rich halo substructure out to distances of 100 kpc. We extrapolate these results to expected performance by the Large Synoptic Survey Telescope and estimate that it will obtain well-sampled 2% accurate, multi-color lightcurves for ~2 million low-redshift quasars, and will discover at least 50 million variable stars.
Using effective temperature and metallicity derived from SDSS spectra for ~60,000 F and G type main sequence stars (0.2<g-r<0.6), we develop polynomial models for estimating these parameters from the ...SDSS u-g and g-r colors. We apply this method to SDSS photometric data for about 2 million F/G stars and measure the unbiased metallicity distribution for a complete volume-limited sample of stars at distances between 500 pc and 8 kpc. The metallicity distribution can be exquisitely modeled using two components with a spatially varying number ratio, that correspond to disk and halo. The two components also possess the kinematics expected for disk and halo stars. The metallicity of the halo component is spatially invariant, while the median disk metallicity smoothly decreases with distance from the Galactic plane from -0.6 at 500 pc to -0.8 beyond several kpc. The absence of a correlation between metallicity and kinematics for disk stars is in a conflict with the traditional decomposition in terms of thin and thick disks. We detect coherent substructures in the kinematics--metallicity space, such as the Monoceros stream, which rotates faster than the LSR, and has a median metallicity of Fe/H=-0.96, with an rms scatter of only ~0.15 dex. We extrapolate our results to the performance expected from the Large Synoptic Survey Telescope (LSST) and estimate that the LSST will obtain metallicity measurements accurate to 0.2 dex or better, with proper motion measurements accurate to ~0.2 mas/yr, for about 200 million F/G dwarf stars within a distance limit of ~100 kpc (g<23.5). abridged
Astrophys.J. 588 (2003) 824; Erratum-ibid. 605 (2004) 575-577 We present evidence for a ring of stars in the plane of the Milky Way,
extending at least from l = 180 deg to l = 227 deg with turnoff ...magnitude $g
\sim 19.5$; the ring could encircle the Galaxy. We infer that the low Galactic
latitude structure is at a fairly constant distance of R = 18 +/- 2 kpc from
the Galactic Center above the Galactic plane, and has R = 20 +/- 2 kpc in the
region sampled below the Galactic plane. The evidence includes five hundred
SDSS spectroscopic radial velocities of stars within 30 degrees of the plane.
The velocity dispersion of the stars associated with this structure is found to
be 27 km/s at (l,b) = (198, -27), 22 km/s at (l,b) = (225, 28), 30 km/s at
(l,b) = (188, 24), and 30 km/s at (l,b) = (182, 27) degrees. The structure
rotates in the same prograde direction as the Galactic disk stars, but with a
circular velocity of 110+/-25 km/s. The narrow measured velocity dispersion is
inconsistent with power law spheroid or thick disk populations. We compare the
velocity dispersion in this structure with the velocity dispersion of stars in
the Sagittarius dwarf galaxy tidal stream, for which we measure a velocity
dispersion of 20 km/s at (l, b) = (165, -55) deg. We estimate a preliminary
metallicity from the Ca II (K) line and color of the turnoff stars of Fe/H =
-1.6 with a dispersion of 0.3 dex and note that the turnoff color is consistent
with that of the spheroid population. We interpret our measurements as evidence
for a tidally disrupted satellite of $2 \times 10^7$ to $5 \times 10^8 \rm
M_\odot$ which rings the Galaxy.
