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Wheatley J.,University of California at Berkeley | Welsh B.Y.,University of California at Berkeley | Browne S.E.,Eureka Scientific
Publications of the Astronomical Society of the Pacific | Year: 2012

We present well-sampled ultraviolet (UV) light curves of seven RR Lyrae stars (SW Aqr, HL Her, SDSS J100133:3+014328:1, SDSS J104844:1+581539:4, SDSS J105513:7+564747:0, SDSS J105622:2+570520:6, and USNO-B1.0 1024-0010686) serendipitously obtained with the NASA GALEX satellite that show changes of up to AB = 6:5 mag in the far-ultraviolet (FUV; 1350-1800 Å) wavelength band. For the test case SW Aqr, we combine the GALEX FUV and NUV (1800-2800 Å) observations with visible U, B, and V photometry to determine the variation of physical parameters such as temperature, surface gravity, and radius during the RR Lyrae pulsation cycle of 0.459299 days. Assuming that SWAqr changes temperature from 6100 K to 8400 K over this cycle, we find that the FUVand NUV light curves can be well matched using the 2004 stellar atmosphere models of Castelli and Kurucz (CK) and a best-fit metallicity of [Fe=H] = -1:5. Pulsation periods are also found for the remaining six stars from their UV light curves, and we have shown that the UV data are consistent with the predictions of CK models using an assumed temperature variation of 6100 K to 8000 K and a log g variation of 2.15 to 3.75 over the pulsation cycle for assumed metallicity values of -2:0 < [Fe=H] < 0:0. Our main finding from the present analysis is that CK models can be successfully used to predict the light-curve behavior of RR Lyrae stars at UV wavelengths if the minimum and maximum temperatures are known from (preferably) visible light-curve observations, such that the stellar metallicity values can be derived. Finally, we note that the model fits to the GALEX data are quite sensitive to the light curves around minimum light, such that the best-fit metallicity is well constrained over this region of the pulsation cycle. © 2012. The Astronomical Society of the Pacific. All rights reserved. Source


Blackburne J.A.,Ohio State University | Blackburne J.A.,Massachusetts Institute of Technology | Pooley D.,Eureka Scientific | Rappaport S.,Massachusetts Institute of Technology | Schechter P.L.,Massachusetts Institute of Technology
Astrophysical Journal | Year: 2011

Microlensing perturbations to the flux ratios of gravitationally lensed quasar images can vary with wavelength because of the chromatic dependence of the accretion disk's apparent size. Multiwavelength observations of microlensed quasars can thus constrain the temperature profiles of their accretion disks, a fundamental test of an important astrophysical process which is not currently possible using any other method. We present single-epoch broadband flux ratios for 12 quadruply lensed quasars in 8 bands ranging from 0.36 to 2.2 μm, as well as Chandra 0.5-8keV flux ratios for five of them. We combine the optical/IR and X-ray ratios, together with X-ray ratios from the literature, using a Bayesian approach to constrain the half-light radii of the quasars in each filter. Comparing the overall disk sizes and wavelength slopes to those predicted by the standard thin accretion disk model, we find that on average the disks are larger than predicted by nearly an order of magnitude, with sizes that grow with wavelength with an average slope of ∼0.2 rather than the slope of 4/3 predicted by the standard thin disk theory. Though the error bars on the slope are large for individual quasars, the large sample size lends weight to the overall result. Our results present severe difficulties for a standard thin accretion disk as the main source of UV/optical radiation from quasars. © 2011. The American Astronomical Society. All rights reserved. Source


Stark C.C.,NASA | Schneider G.,University of Arizona | Weinberger A.J.,Carnegie Institution of Washington | Debes J.H.,US Space Telescope Science Institute | And 3 more authors.
Astrophysical Journal | Year: 2014

New multi-roll coronagraphic images of the HD 181327 debris disk obtained using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope reveal the debris ring in its entirety at high signal-to-noise ratio and unprecedented spatial resolution. We present and apply a new multi-roll image processing routine to identify and further remove quasi-static point-spread function-subtraction residuals and quantify systematic uncertainties. We also use a new iterative image deprojection technique to constrain the true disk geometry and aggressively remove any surface brightness asymmetries that can be explained without invoking dust density enhancements/deficits. The measured empirical scattering phase function for the disk is more forward scattering than previously thought and is not well-fit by a Henyey-Greenstein function. The empirical scattering phase function varies with stellocentric distance, consistent with the expected radiation pressured-induced size segregation exterior to the belt. Within the belt, the empirical scattering phase function contradicts unperturbed debris ring models, suggesting the presence of an unseen planet. The radial profile of the flux density is degenerate with a radially varying scattering phase function; therefore estimates of the ring's true width and edge slope may be highly uncertain. We detect large scale asymmetries in the disk, consistent with either the recent catastrophic disruption of a body with mass >1% the mass of Pluto, or disk warping due to strong interactions with the interstellar medium. © 2014. The American Astronomical Society. All rights reserved. Source


Miles B.E.,University of California at Los Angeles | Roberge A.,NASA | Welsh B.,Eureka Scientific
Astrophysical Journal | Year: 2016

We present the analysis of time-variable Doppler-shifted absorption features in far-UV spectra of the unusual 49 Ceti debris disk. This nearly edge-on disk is one of the brightest known and is one of the very few containing detectable amounts of circumstellar (CS) gas as well as dust. In our two visits of Hubble Space Telescope STIS spectra, variable absorption features are seen on the wings of lines arising from Cii and Civ but not for any of the other CS absorption lines. Similar variable features have long been seen in spectra of the well-studied Pictoris debris disk and attributed to the transits of star-grazing comets. We calculated the velocity ranges and apparent column densities of the 49 Cet variable gas, which appears to have been moving at velocities of tens to hundreds of km s-1 relative to the central star. The velocities in the redshifted variable event seen in the second visit show that the maximum distances of the infalling gas at the time of transit were about 0.05-0.2 au from the central star. A preliminary attempt at a composition analysis of the redshifted event suggests that the C/O ratio in the infalling gas is super-solar, as it is in the bulk of the stable disk gas. © 2016. The American Astronomical Society. All rights reserved. Source


Rappaport S.,Kavli Institute for Astrophysics and Space Research | Levine A.,Kavli Institute for Astrophysics and Space Research | Pooley D.,Eureka Scientific | Steinhorn B.,Kavli Institute for Astrophysics and Space Research
Astrophysical Journal | Year: 2010

The Chandra X-Ray Observatory was used to image the collisional ring galaxy Arp 147 for 42 ks. We detect nine X-ray sources with luminosities in the range of (1.4-7) × 1039 erg s-1 (assuming that the sources emit isotropically) in or near the blue knots of star formation associated with the ring. A source with an X-ray luminosity of 1.4 × 1040 erg s-1 is detected in the nuclear region of the intruder galaxy. X-ray sources associated with a foreground star and a background quasar are used to improve the registration of the X-ray image with respect to Hubble Space Telescope (HST) high-resolution optical images. The intruder galaxy, which apparently contained little gas before the collision, shows no X-ray sources other than the one in the nuclear bulge which may be a poorly fed supermassive black hole. These observations confirm the conventional wisdom that collisions of gas-rich galaxies trigger large rates of star formation which, in turn, generate substantial numbers of X-ray sources, some of which have luminosities above the Eddington limit for accreting stellar-mass black holes (i.e., ultraluminous X-ray sources, "ULXs").We also utilize archival Spitzer and Galex data to help constrain the current star formation rate in Arp 147 to ∼7 M⊙ yr-1. All of these results, coupled with binary evolution models for ULXs, allow us to tentatively conclude that the most intense star formation may have ended some 15 Myr in the past. © 2010. The American Astronomical Society. All rights reserved. Source

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