High-precision 2MASS JHKs light curves and other data for RR Lyrae star SDSS J015450+001501: strong constraints for non-linear pulsation models

We present and discuss an extensive data set for the non-Blazhko ab-type RR Lyrae star SDSSJ015450+001501, including optical SDSS ugriz light curves and spectroscopic data, LINEAR and CSS unfiltered optical light curves, and infrared 2MASS JHKs and WISE W1 and W2 light curves. Most notably, light curves obtained by 2MASS include close to 9000 photometric measures collected over 3.3 years and provide exceedingly precise view of near-IR variability. These data demonstrate that static atmosphere models are insufficient to explain multi-band photometric light curve behavior and present strong constraints for non-linear pulsation models for RR Lyrae stars. It is a challenge to modelers to produce theoretical light curves that can explain data presented here, which we make publicly available.

Modeling the Time Variability of SDSS Stripe 82 Quasars as a Damped Random Walk

We model the time variability of ~9,000 spectroscopically confirmed quasars in SDSS Stripe 82 as a damped random walk. Using 2.7 million photometric measurements collected over 10 years, we confirm the results of Kelly et al. (2009) and Koz{l}owski et al. (2010) that this model can explain quasar light curves at an impressive fidelity level (0.01-0.02 mag). The damped random walk model provides a simple, fast [O(N) for N data points], and powerful statistical description of quasar light curves by a characteristic time scale (tau) and an asymptotic rms variability on long time scales (SF_inf). We searched for correlations between these two variability parameters and physical parameters such as luminosity and black hole mass, and rest-frame wavelength. We find that tau increases with increasing wavelength with a power law index of 0.17, remains nearly constant with redshift and luminosity, and increases with increasing black hole mass with power law index of 0.21+/-0.07. The amplitude of variability is anti-correlated with the Eddington ratio, which suggests a scenario where optical fluctuations are tied to variations in the accretion rate. The radio-loudest quasars have systematically larger variability amplitudes by about 30%, when corrected for the other observed trends, while the distribution of their characteristic time scale is indistinguishable from that of the full sample. We do not detect any statistically robust differences in the characteristic time scale and variability amplitude between the full sample and the small subsample of quasars detected by ROSAT. Our results provide a simple quantitative framework for generating mock quasar light curves, such as currently used in LSST image simulations. (abridged)

SDSS Observations of the Milky Way vs. N-body Models: A Comparison of Stellar Distributions in the Position-Velocity-Metallicity Space

The data obtained by the recent modern sky surveys enable detailed studies of the stellar distribution in the multi-dimensional space spanned by spatial coordinates, velocity and metallicity, from the solar neighborhood all the way out to the outer Milky Way halo. While these results represent exciting observational breakthroughs, their interpretation is not simple. For example, traditional decomposition of the thin and thick disks predicts a strong correlation in metallicity and kinematics at $sim$1 kpc from the Galactic plane; however, recent SDSS--based work has demonstrated an absence of this correlation for disk stars. Instead, the variation of the metallicity and rotational velocity distributions can be modeled using non--Gaussian functions that retain their shapes and only shift as the distance from the mid--plane increases. To fully contextualize these recent observational results, a detailed comparison with sophisticated numerical models is necessary. Modern simulations have sufficient resolution and physical detail to study the formation of stellar disks and spheroids over a large baseline of masses and cosmic ages. We discuss preliminary comparisons of various observed maps and N--body model predictions and find them encouraging. In particular, the N--body disk models of Rov{s}kar et al. cite{Roskar 2008} reproduce a change of disk scale height reminiscent of thin/thick disk decomposition, as well as metallicity and rotational velocity gradients, while not inducing a correlation of the latter two quantities, in qualitative agreement with SDSS observations.

Time Variability of Quasars: the Structure Function Variance

Significant progress in the description of quasar variability has been recently made by employing SDSS and POSS data. Common to most studies is a fundamental assumption that photometric observations at two epochs for a large number of quasars will reveal the same statistical properties as well-sampled light curves for individual objects. We critically test this assumption using light curves for a sample of $sim$2,600 spectroscopically confirmed quasars observed about 50 times on average over 8 years by the SDSS stripe 82 survey. We find that the dependence of the mean structure function computed for individual quasars on luminosity, rest-frame wavelength and time is qualitatively and quantitatively similar to the behavior of the structure function derived from two-epoch observations of a much larger sample. We also reproduce the result that the variability properties of radio and X-ray selected subsamples are different. However, the scatter of the variability structure function for fixed values of luminosity, rest-frame wavelength and time is similar to the scatter induced by the variance of these quantities in the analyzed sample. Hence, our results suggest that, although the statistical properties of quasar variability inferred using two-epoch data capture some underlying physics, there is significant additional information that can be extracted from well-sampled light curves for individual objects.