Smithsonian Astrophysical Observatory

About: Smithsonian Astrophysical Observatory is a facility organization based out in Cambridge, Massachusetts, United States. It is known for research contribution in the topics: Galaxy & Stars. The organization has 1665 authors who have published 3622 publications receiving 132183 citations. The organization is also known as: SAO.

SDSS J184037.78+642312.3: The First Pulsating Extremely Low Mass White Dwarf

Abstract: We report the discovery of the first pulsating extremely low mass (ELM) white dwarf (WD), SDSS J184037.78+642312.3 (hereafter J1840). This DA (hydrogen-atmosphere) WD is by far the coolest and the lowest-mass pulsating WD, with Teff = 9100 \pm 170 K and log g = 6.22 \pm 0.06, which corresponds to a mass ~ 0.17 Msun. This low-mass pulsating WD greatly extends the DAV (or ZZ Ceti) instability strip, effectively bridging the log g gap between WDs and main sequence stars. We detect high-amplitude variability in J1840 on timescales exceeding 4000 s, with a non-sinusoidal pulse shape. Our observations also suggest that the variability is multi-periodic. The star is in a 4.6 hr binary with another compact object, most likely another WD. Future, more extensive time-series photometry of this ELM WD offers the first opportunity to probe the interior of a low-mass, presumably He-core WD using the tools of asteroseismology.

Small-Scale X-Ray Variability in the Cassiopeia A Supernova Remnant

Abstract: A comparison of X-ray observations of the Cassiopeia A supernova remnant taken in 2000, 2002, and 2004 with the Chandra ACIS-S3 reveals the presence of several small-scale features (≤10'') that exhibit significant intensity changes over a 4 yr time frame. Here we report on the variability of six features, four of which show count rate increases from ~10% to over 90%, and two of which show decreases of ~30%-40%. While extracted 1-4.5 keV X-ray spectra do not reveal gross changes in emission-line strengths, spectral fits using nonequilibrium-ionization, metal-rich plasma models indicate increased or decreased electron temperatures for features showing increasing or decreasing count rates, respectively. Based on the observed count rate changes and the assumption that the freely expanding ejecta has a velocity of ~5000 km s-1 at the reverse shock front, we estimate the unshocked ejecta to have spatial scale variations of 0.02-0.03 pc, which is consistent with the X-ray-emitting ejecta belonging to a more diffuse component of the supernova ejecta than that seen in the optically emitting ejecta, which have spatial scales of ~10-3 pc.

Extended H I Disks in Dust Lane Elliptical Galaxies

Abstract: We present the results of H I observations of five dust lane elliptical galaxies with the Australia Telescope Compact Array. Two galaxies (NGC 3108 and NGC 1947) are detected, and sensitive upper limits are obtained for the other three. In the two detected galaxies, the H I is distributed in a regular, extended, and warped disklike structure of low surface brightness. Adding data from the literature, we find that several more dust lane elliptical galaxies have regular H I structures. This H I is likely to be a remnant of accretions and/or mergers that took place a considerable time ago and in which a significant fraction of the gas survived to form a disk. The presence of regular H I structures suggests that some mergers lead to galaxies with extended low surface brightness density gas disks. These gas disks will evolve very slowly, and these elliptical galaxies will remain gas-rich for a long period of time. One of the galaxies we observed (NGC 3108) has a very large amount of neutral hydrogen (MH I = 4.5 × 109 M⊙; MH I/LB ~ 0.09), which is very regularly distributed in an annulus extending to a radius of ~6 Reff. The kinematics of the H I distribution suggest that the rotation curve of NGC 3108 is flat out to at least the last observed point. We estimate a mass-to-light ratio of M/LB ~ 18 M⊙/LB,⊙ at a radius of ~6Reff from the center. Several of the galaxies we observed have an unusually low gas-to-dust ratio MH I/Mdust, suggesting that their cold interstellar medium, if present as expected from the presence of dust, may be mainly in molecular rather than atomic form.