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.

Star Clusters in M31: I. A Catalog and a Study of the Young Clusters

Abstract: We present an updated catalog of 1300 objects in the field of M31, including 670 likely star clusters of various types, the rest being stars or background galaxies once thought to be clusters. The coordinates in the catalog are accurate to 02, and are based on images from the Local Group Survey (LGS) or from the Digitized Sky Survey (DSS). Archival Hubble Space Telescope (HST) images and the LGS were inspected to confirm cluster classifications where possible, but most of the classifications are based on spectra taken of ~1000 objects with the Hectospec fiber positioner and spectrograph on the 6.5 m MMT. The spectra and images of young clusters are analyzed in detail in this paper; analysis of older clusters will appear in a later paper. Ages and reddenings of 140 young clusters are derived by comparing the observed spectra with model spectra. Seven of these clusters also have ages derived from HST color-magnitude diagrams (two of which we present here); these agree well with the spectroscopically determined ages. Combining new V-band photometry with the M/L values that correspond to the derived cluster ages, we derive masses for the young clusters, finding them to have masses as great as 105 with a median of 104 M ☉, and a median age of 0.25 Gyr. In comparison, therefore, Milky Way (MW) open clusters have the lowest median mass, the MW and M31 globulars the highest, and the LMC young massive clusters and the M31 young clusters are in between. The young clusters in M31 show a range of structures. Most have the low concentration typical of MW open clusters, but there are a few which have high concentrations. We expect that most of these young clusters will be disrupted in the next Gyr or so; however, some of the more massive and concentrated of the young clusters will likely survive for longer. The spatial distribution of the young clusters is well correlated with the star-forming regions as mapped out by mid-IR emission. A kinematic analysis likewise confirms the spatial association of the young clusters with the star-forming young disk in M31.

The Elm Survey. II. Twelve Binary White Dwarf Merger Systems

Abstract: We describe new radial velocity and X-ray observations of extremely low-mass white dwarfs (ELM WDs, {approx}0.2 M{sub sun}) in the Sloan Digital Sky Survey Data Release 4 and the MMT Hypervelocity Star survey. We identify four new short period binaries, including two merger systems. These observations bring the total number of short period binary systems identified in our survey to 20. No main-sequence or neutron star companions are visible in the available optical photometry, radio, and X-ray data. Thus, the companions are most likely WDs. Twelve of these systems will merge within a Hubble time due to gravitational wave radiation. We have now tripled the number of known merging WD systems. We discuss the characteristics of this merger sample and potential links to underluminous supernovae, extreme helium stars, AM CVn systems, and other merger products. We provide new observational tests of the WD mass-period distribution and cooling models for ELM WDs. We also find evidence for a new formation channel for single low-mass WDs through binary mergers of two lower mass objects.

Prospects for detection of catastrophic collisions in debris disks

Abstract: We investigate the prospects for detecting dust from two-body collisions during the late stages of planet formation at 1–150 AU. We develop an analytic model to describe the formation of a dusty cloud of debris and use numerical coagulation and N-body calculations to predict observable signals from these events. In a minimum mass solar nebula, collisions of 100–1000 km objects at distances of 3–5 AU or less from the parent star are observable at mid-IR wavelengths as bright clumps or rings of dust. At 24 μm, the clumps are ~0.1–1 mag brighter than emission from dust in the background debris disk. In edge-on systems, dusty clumps produce eclipses with depths of 1.0 mag that last for ~100 orbital periods. Large-scale surveys for transits from exosolar planets, using satellites such as Kepler, can plausibly detect these eclipses and provide important constraints on the terrestrial environment for ages of 100–300 Myr.

Chandra X-Ray Observations of NGC 1316 (Fornax A)

Abstract: We report the results of the Chandra ACIS subarcsecond resolution X-ray observation of the archetypal merger radio galaxy NGC 1316 (Fornax A). We confirm the presence of fine substructures in the hot interstellar medium (ISM). Some of these are likely to result from interaction with the radio jets, while others may be related to a complex intermingling of different phases of the ISM. We detect a low-luminosity X-ray active galactic nucleus (AGN) with LX = 5 × 1039 ergs s-1 (in 0.3-8 keV) and a Γ = 1.7 power-law energy spectrum. We also detect 81 point sources within the 25th magnitude isophotal ellipse of NGC 1316 (LX in the range of 2 × 1037 to 2 × 1039 ergs s-1), with hard (kT ~ 5 keV) X-ray spectra, typical of X-ray binaries, and a spatial radial distribution consistent with that of the optical (i.e., stellar) surface brightness. We derive the X-ray luminosity function (XLF) of these sources, correcting for the incompleteness at the faint end caused by the presence of the diffuse emission from the hot ISM in the central regions of NGC 1316 and by the widening of the Chandra point-spread functions at increasing distance from the aim point. With these corrections, the XLF is well reproduced by a single—unbroken—power law with a slope of -1.3 down to our threshold luminosity of ~3 × 1037 ergs s-1. The hot ISM has temperatures in the 0.5-0.6 keV range, its surface brightness distribution is more centrally concentrated than that of the point sources, and its temperature appears to decrease at larger radii. These properties suggest that the ISM may be subject to partial winds. Taking into account the spectral complexity of the ISM, and the presence of unresolved low luminosity X-ray sources (which can be inferred from the spectra), we constrain the metal abundance of the hot ISM to be Z = 0.25-1.3 Z☉ (90% confidence).