665 research outputs found
Mid-infrared observations of the SGR 1900+14 error box
We report on mid-infrared observations of the compact stellar cluster located
in the proximity of SGR 1900+14, and the radio/X-ray position of this
soft-gamma repeater. Observations were performed in May and June of 2001 when
the bursting source was in an active state. At the known radio and X-ray
position of the SGR we did not detect transient mid-IR activity, although the
observations were performed only hours before and after an outburst in the
high-energy band.Comment: 4 pages, 3 figures, to appear in "Gamma-Ray Burst and Afterglow
Astronomy 2001", Woods Hole; 5-9 Nov, 200
The triple-mode pulsating variable V823 Cas
Based on extended multicolour CCD photometry of the triple-mode radial
pulsator V823 Cas we studied the properties of the coupling frequencies invoked
by nonlinear processes. Our results support that a resonance connection as
suggested by Antonello & Aikawa (1998) affects the mode coupling behaviour. The
P1/P0 period ratio of V823 Cas has an "out of range" value if compared with the
period ratios of the known double mode pulsators, while the P2/P1 period ratio
is normal. The periods and period ratios cannot be consistently interpret
without conflict with pulsation and/or evolution models. We attempt to
interpret this failure by the suggestion that at present, the periods of V823
Cas are in a transient, resonance affected state, thus do not reflect the true
parameters of the object. The anomalous period change behaviour of the
fundamental and second overtone modes supports this idea. We have also raised
the possibility that a f0 + f2 = 2f1 resonance may act in triple mode
pulsators.Comment: 10 pages, 7 figures, 5 tables. Accepted for publication in Astronomy
and Astrophysic
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
Extraction of level density and gamma strength function from primary gamma spectra
We present a new iterative procedure to extract the level density and the
gamma strength function from primary gamma spectra for energies close up to the
neutron binding energy. The procedure is tested on simulated spectra and on
data from the Yb-173(He-3,alpha)Yb-172 reaction.Comment: 23 pages including 1 table and 7 figure
A 10-micron Search for Inner-Truncated Disks Among Pre-Main-Sequence Stars With Photometric Rotation Periods
We use mid-IR (primarily 10 m) photometry as a diagnostic for the
presence of disks with inner cavities among 32 pre-main sequence stars in Orion
and Taurus-Auriga for which rotation periods are known and which do not show
evidence for inner disks at near-IR wavelengths. Disks with inner cavities are
predicted by magnetic disk-locking models that seek to explain the regulation
of angular momentum in T Tauri stars. Only three stars in our sample show
evidence for excess mid-IR emission. While these three stars may possess
truncated disks consistent with magnetic disk-locking models, the remaining 29
stars in our sample do not. Apparently, stars lacking near-IR excesses in
general do not possess truncated disks to which they are magnetically coupled.
We discuss the implications of this result for the hypothesis of
disk-regulated angular momentum. Evidently, young stars can exist as slow
rotators without the aid of present disk-locking, and there exist very young
stars already rotating near breakup velocity whose subsequent angular momentum
evolution will not be regulated by disks. Moreover, we question whether disks,
when present, truncate in the manner required by disk-locking scenarios.
Finally, we discuss the need for rotational evolution models to take full
account of the large dispersion of rotation rates present at 1 Myr, which may
allow the models to explain the rotational evolution of low-mass pre-main
sequence stars in a way that does not depend upon braking by disks.Comment: 20 pages, 4 figure
SDSS J210014.12+004446.0: A New Dwarf Nova with Quiescent Superhumps?
We report follow-up observations of the Sloan Digital Sky Survey Cataclysmic
Variable SDSS J210014.12+004446.0 (hereafter SDSS J2100). We obtained
photometry and spectroscopy in both outburst and quiescent states, providing
the first quiescent spectrum of this source. In both states, non-sinusoidal
photometric modulations are apparent, suggestive of superhumps, placing SDSS
J2100 in the SU UMa subclass of dwarf novae. However, the periods during
outburst and quiescence differ significantly, being 2.099 plus or minus 0.002
hr and 1.96 plus or minus 0.02 hr respectively. Our phase-resolved spectroscopy
during outburst yielded an estimate of about 2 hr for the orbital period,
consistent with the photometry. The presence of the shorter period modulation
at quiescence is unusual, but not unique. Another atypical feature is the
relative weakness of the Balmer emission lines in quiescence. Overall, we find
a close similarity between SDSS J2100 and the well-studied superhump
cataclysmic Variable V503 Cygni. By analogy, we suggest that the quiescent
modulation is due to a tilted accretion disk -- producing negative superhumps
-- and the modulation in outburst is due to positive superhumps from the
precession of an elliptical disk.Comment: 6 pages, 5 eps figures, accepted by PASP Dec. 16th, 200
- …