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.

Mutual phenomena of the Galilean satellites in 1973. III. Final results from 91 light curves.

Abstract: This paper presents an analysis of photoelectric light curves of 91 mutual eclipses and occultations of the Galilean satellites that occurred from June to December 1973. From the deepest curves, radii and standard errors are deduced for J2, J3, and J4. It is shown that the substantial differences between the depths and midtimes observed and those predicted by Sampson's theory can be used in a clear and definite way to revise the orbital constants of that theory. The present analysis, which reduces all the data provided by these mutual events in one body, diminishes the latitude and longitude residuals of the unrevised theory by as much as a factor of about 10. The resulting overall consistency and the demonstrated inadequacy of an explanation based solely on surface-brightness variations argue that these corrections do represent the dominant effects required to account for the light curves. In order that these mutual events can be used with other astrometric data, the separations between a satellite pair at midevent are given. A small systematic (with solar phase angle) and unexplained difference in the longitude corrections is found for Io and Europa as obtained from nearly simultaneous pairs of occultations and eclipses.

INTEGRAL and Swift observations of EXO 2030+375 during a giant outburst

Abstract: INAF IASF-Bo, via Gobetti 101, 40129 Bologna, ItalyABSTRACTAims.We investigate the X-ray spectral and timing properties of the high mass X-ray binary EXO 2030+375 observed during itsJune–September 2006 giant (type II) outburst.Methods.The data analyzed in this work are from partly simultaneous observations with INTEGRALand Swift. The pulse period Pand its temporal derivative P˙ are measured. X-ray pulse profiles in different energy ranges and time intervals are constructed. Pulseaveraged X-ray spectra for different time intervals are studied.Results.We report a strong spin-up of the source during the outburst,comparable to that observed in 1985 during the previous giantoutburst when the source was discovered. The value of P˙ is found to be linearly related to the X-ray luminosity of thesource duringthe outburst. For the first time the hard X-ray (> 25 keV) characteristics of the source during a type II outburst are studied. The X-raypulse profiles apparently change with luminosity. The X-ray spectral continuum in the 3–120 keV energy range is modeled with anabsorbed power law with an exponential cutoff around E ∼ 26 keV. An iron emission line at ∼6–7 keV is observed. The spectrumreveals some features between 10 and 20 keV which can be modeled either by a broad emission line at ∼13–15 keV (a “bump”) or bytwo absorption lines at ∼10 and ∼20 keV.Key words. X-ray binaries; neutron stars; accretion disks

Sdss j074511.56+194926.5: discovery of a metal-rich and tidally distorted extremely low mass white dwarf

Abstract: We present the discovery of an unusual, tidally distorted extremely low mass white dwarf (WD) with nearly solar metallicity. Radial velocity measurements confirm that this is a compact binary with an orbital period of 2.6975 hr and a velocity semi-amplitude of K = 108.7 km s–1. Analysis of the hydrogen Balmer lines yields an effective temperature of T eff = 8380 K and a surface gravity of log g = 6.21 that in turn indicate a mass of M = 0.16 M ☉ and a cooling age of 4.2 Gyr. In addition, a detailed analysis of the observed metal lines yields abundances of log (Mg/H) = –3.90, log (Ca/H) = –5.80, log (Ti/H) = –6.10, log (Cr/H) = –5.60, and log (Fe/H) = –4.50, similar to the sun. We see no evidence of a debris disk from which these metals would be accreted, though the possibility cannot entirely be ruled out. Other potential mechanisms to explain the presence of heavy elements are discussed. Finally, we expect this system to ultimately undergo unstable mass transfer and merge to form a ~0.3-0.6 M ☉ WD in a few Gyr.

The Physical Properties of the Red Supergiant WOH G64: The Largest Star Known?

Abstract: WOH G64 is an unusual red supergiant (RSG) in the Large Magellanic Cloud (LMC), with a number of properties that set it apart from the rest of the LMC RSG population, including a thick circumstellar dust torus, an unusually late spectral type, maser activity, and nebular emission lines. Its reported physical properties are also extreme, including the largest radius for any star known and an effective temperature that is much cooler than other RSGs in the LMC, both of which are at variance with stellar evolutionary theory. We fit moderate-resolution optical spectrophotometry of WOH G64 with the MARCS stellar atmosphere models, determining an effective temperature of 3400 ? 25?K. We obtain a similar result from the star's broadband V ? K colors. With this effective temperature, and taking into account the flux contribution from the asymmetric circumstellar dust envelope, we calculate log(L/L ?) = 5.45 ? 0.05 for WOH G64, quite similar to the luminosity reported by Ohnaka and collaborators based on their radiative transfer modeling of the star's dust torus. We determine a radius of R/R ? = 1540, bringing the size of WOH G64 and its position on the Hertzsprung-Russell diagram into agreement with the largest known Galactic RSGs, although it is still extreme for the LMC. In addition, we use the Ca?II triplet absorption feature to determine a radial velocity of 294 ? 2 km s?1 for the star; this is the same radial velocity as the rotating gas in the LMC's disk, which confirms its membership in the LMC and precludes it from being an unusual Galactic halo giant. Finally, we describe the star's unusual nebula emission spectrum; the gas is nitrogen-rich and shock-heated, and displays a radial velocity that is significantly more positive than the star itself by 50 km s?1.

A decline in the nonthermal x-ray emission from cassiopeia a

Abstract: We present new Chandra ACIS-S3 observations of Cassiopeia A which, when combined with earlier ACIS-S3 observations, show evidence for a steady ~1.5%-2% yr–1 decline in the 4.2-6.0 keV X-ray emission between the years 2000 and 2010. The computed flux from exposure corrected images over the entire remnant showed a 17% decline over the entire remnant and a slightly larger (21%) decline from regions along the remnant's western limb. Spectral fits of the 4.2-6.0 keV emission across the entire remnant, forward shock filaments, and interior filaments indicate that the remnant's nonthermal spectral power-law index has steepened by about 10%, with interior filaments having steeper power-law indices. Since TeV electrons, which give rise to the observed X-ray synchrotron emission, are associated with the exponential cutoff portion of the electron distribution function, we have related our results to a change in the cutoff energy and conclude that the observed decline and steepening of the nonthermal X-ray emission is consistent with a deceleration of the remnant's 5000 km s–1 forward shock of 30-70 km s–1 yr–1.