Comparing Young Stellar Populations Across Active Regions in the M31 Disk

I present preliminary BV resolved stellar photometry of the M31 disk measured from the data set of the NOAO/CTIO Local Group Survey. I have performed detailed analyses of the star formation histories in and around three currently active regions in the M31 disk: OB 78, OB 102 and the northeast spiral arm. The results suggest that low H$\alpha$ emission from OB 78 compared to other active regions is directly related to the details of the recent star formation histories of the regions. In addition, while every active region I analyzed shows a recent increase in star formation activity, some active regions also contain overdensities of stars with ages <~100 Myr compared to adjacent regions. The adjacent regions show a relatively constant, low star formation rate over the past 100 Myr. There is no significant difference between the stellar populations on either side of the active regions. This symmetry provides no obvious signature of recent propagation of star formation near these currently active regions of the M31 disk.Comment: 32 pages, 11 figures, accepted for publication in MNRA

A Panoply of Cepheid Light Curve Templates

We have generated accurate V and I template light curves using a combination of Fourier decomposition and principal component analysis for a large sample of Cepheid light curves. Unlike previous studies, we include short period Cepheids and stars pulsating in the first overtone mode in our analysis. Extensive Monte Carlo simulations show that our templates can be used to precisely measure Cepheid magnitudes and periods, even in cases where there are few observational epochs. These templates are ideal for characterizing serendipitously discovered Cepheids and can be used in conjunction with surveys such as Pan-Starrs and LSST where the observational sampling may not be optimized for Cepheids.Comment: 12 pages, 14 figures. Accepted for publication in AJ fixed embarrassing typo

Numerical Simulation of the Korteweg–de Vries Equation with Machine Learning

A machine learning procedure is proposed to create numerical schemes for solutions of nonlinear wave equations on coarse grids. This method trains stencil weights of a discretization of the equation, with the truncation error of the scheme as the objective function for training. The method uses centered finite differences to initialize the optimization routine and a second-order implicit-explicit time solver as a framework. Symmetry conditions are enforced on the learned operator to ensure a stable method. The procedure is applied to the Korteweg–de Vries equation. It is observed to be more accurate than finite difference or spectral methods on coarse grids when the initial data is near enough to the training set

Mapping Low-Density Intergalactic Gas: a Third Helium Lyman-alpha Forest

We present a new HST/STIS spectrum of the z=3.18 quasar PKS 1935-692 and summarize the spectral features shortwards of 304A in the rest frame likely to be caused by foreground HeII Lyman-alpha absorption. In accord with previous results on two other quasars at similar redshifts, we demonstrate a correlation with the HI Lyman-alpha forest absorption, and show that much of the helium absorption is caused by a comparable quantity of more diffuse gas with Omega~0.01, that is not detected in HI. The helium ionization zone around the quasar is detected as well as a void seen in both HI and HeII. The properties of the absorption are in broad agreement with those of the other quasars and with models of the protogalactic gas distribution and ionization at this redshift.Comment: 17 pages including 5 figures. As accepted for publication in The Astronomical Journal (minor revisions