95 research outputs found
The Dwarf Nova PQ Andromedae
We report a photometric study of the WZ Sagittae-type dwarf nova PQ
Andromedae. The light curve shows strong (0.05 mag full amplitude) signals with
periods of 1263(1) and 634(1) s, and a likely double-humped signal with
P=80.6(2) min. We interpret the first two as nonradial pulsation periods of the
underlying white dwarf, and the last as the orbital period of the underlying
binary. We estimate a distance of 150(50) pc from proper motions and the two
standard candles available: the white dwarf and the dwarf-nova outburst. At
this distance, the K magnitude implies that the secondary is probably fainter
than any star on the main sequence -- indicating a mass below the Kumar limit
at 0.075 M_sol. PQ And may be another "period bouncer", where evolution now
drives the binary out to longer period.Comment: PDF, 13 pages, 2 figures; accepted, in press, to appear September
2005, PASP; more info at http://cba.phys.columbia.edu
The Discovery of an Embedded Cluster of High-Mass Stars Near SGR 1900+14
Deep I-band imaging to approximately I = 26.5 of the soft gamma-ray repeater
SGR 1900+14 region has revealed a compact cluster of massive stars located only
a few arcseconds from the fading radio source thought to be the location of the
SGR (Frail, Kulkarni, & Bloom 1999). This cluster was previously hidden in the
glare of the pair of M5 supergiant stars (whose light was removed by PSF
subtraction) proposed by Vrba et al. (1996) as likely associated with the SGR
1900+14. The cluster has at least 13 members within a cluster radius of
approximately 0.6 pc, based on an estimated distance of 12-15 kpc. It is
remarkably similar to a cluster found associated with SGR 1806-20 (Fuchs et al.
1999). That similar clusters have now been found at or near the positions of
the two best-studied SGRs suggests that young neutron stars, thought to be
responsible for the SGR phenomenon, have their origins in proximate compact
clusters of massive stars.Comment: 5 pages, 3 figures, accepted Astrophysical Journal Letter
V1647 Ori: The X-ray Evolution of a Pre-main Sequence Accretion Burst
We present Chandra X-ray Observatory monitoring observations of the recent
accretion outburst displayed by the pre-main sequence (pre-MS) star V1647 Ori.
The X-ray observations were obtained over a period beginning prior to outburst
onset in late 2003 and continuing through its apparent cessation in late 2005,
and demonstrate that the mean flux of the spatially coincident X-ray source
closely tracked the near-infrared luminosity of V1647 Ori throughout its
eruption. We find negligible likelihood that the correspondence between X-ray
and infrared light curves over this period was the result of multiple X-ray
flares unrelated to the accretion burst. The recent Chandra data confirm that
the X-ray spectrum of V1647 Ori hardened during outburst, relative both to its
pre-outburst state and to the X-ray spectra of nearby pre-MS stars in the L1630
cloud. We conclude that the observed changes in the X-ray emission from V1647
Ori over the course of its 2003-2005 eruption were generated by a sudden
increase and subsequent decline in its accretion rate. These results for V1647
Ori indicate that the flux of hard X-ray emission from erupting low-mass,
pre-MS stars, and the duration and intensity of such eruptions, reflect the
degree to which star-disk magnetic fields are reorganized before and during
major accretion events.Comment: 12 pages, 3 figs.; accepted by the Astrophysical Journal (Letters
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