4,556 research outputs found
Serendipitous Discovery and Parallax of a Nearby L Dwarf
A field star serendipitously observed in a parallax program proved to have a
proper motion of 562 mas/yr and a parallax of 82 +- 2 mas. The star is
identified with 2MASS J07003664+3157266. A Keck LRIS spectrum shows its
spectral type to be L3.5, as expected from its infrared and optical colors and
absolute magnitude. This object had not been previously recognized as an L
dwarf, perhaps because of crowding at its relatively low Galactic latitude (b =
+15.8 degrees).Comment: PASP, in press. 8 pages incl.2 postscript figures, plus one jpeg
figur
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
Preliminary Parallaxes of 40 L and T Dwarfs from the U.S. Naval Observatory Infrared Astrometry Program
We present preliminary trigonometric parallaxes and proper motions for 22 L
dwarfs and 18 T dwarfs measured using the ASTROCAM infrared imager. Relative to
absolute parallax corrections are made by employing 2MASS and/or SDSS
photometry for reference frame stars. We combine USNO infrared and optical
parallaxes with the best available CIT system photometry to determine M_J, M_H,
and M_K values for 37 L dwarfs between spectral types L0 to L8 and 19 T dwarfs
between spectral types T0.5 and T8 and present selected absolute magnitude
versus spectral type and color diagrams, based on these results. Luminosities
and temperatures are estimated for these objects. Of special interest are the
distances of several objects which are at or near the L-T dwarf boundary so
that this important transition can be better understood. The previously
reported early-mid T dwarf luminosity excess is clearly confirmed and found to
be present at J, H, and K. The large number of objects that populate this
luminosity excess region indicates that it cannot be due entirely to selection
effects. The T dwarf sequence is extended to M_J~16.9 by 2MASS J041519-0935
which, at d = 5.74 pc, is found to be the least luminous [log(L/L_sun)=-5.58]
and coldest (T_eff~760 K) brown dwarf known. Combining results from this paper
with earlier USNO CCD results we find that, in contrast to the L dwarfs, there
are no examples of low velocity (V_tan < 20 km/s) T dwarfs. We briefly discuss
future directions for the USNO infrared astrometry program.Comment: 73 pages, 9 figures, 9 tables, accepted for publication in The
Astronomical Journa
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
Constraints on the disk geometry of the T Tauri star AA Tau from linear polarimetry
We have simultaneously monitored the photometric and polarimetric variations
of the Classical T Tauri star AA Tau during the fall of 2002. We combine these
data with previously published polarimetric data covering two earlier epochs.
The phase coverage is complete, although not contiguous. AA Tau clearly shows
cyclic variations coupled with the rotation of the system. The star-disk system
produces a repeatable polarisation curve where the polarisation increases with
decreasing brightness. The data fit well with the model put forward by Bouvier
et al. (1999) where AA Tau is viewed almost edge-on and its disk is actively
dumping material onto the central star via magnetospheric accretion. The inner
edge of the disk is deformed by its interaction with the tilted magnetosphere,
producing eclipses as it rotates and occults the photosphere periodically. From
the shape of the polarisation curve in the QU-Plane we confirm that the
accretion disk is seen at a large inclination, almost edge-on, and predict that
its position angle is PA~90 deg., i.e., that the disk's major axis is oriented
in the East-West direction.Comment: Astron. Astrophys., in pres
Periodic Bursts of Coherent Radio Emission from an Ultracool Dwarf
We report the detection of periodic (p = 1.96 hours) bursts of extremely
bright, 100% circularly polarized, coherent radio emission from the M9 dwarf
TVLM 513-46546. Simultaneous photometric monitoring observations have
established this periodicity to be the rotation period of the dwarf. These
bursts, which were not present in previous observations of this target, confirm
that ultracool dwarfs can generate persistent levels of broadband, coherent
radio emission, associated with the presence of kG magnetic fields in a
large-scale, stable configuration. Compact sources located at the magnetic
polar regions produce highly beamed emission generated by the electron
cyclotron maser instability, the same mechanism known to generate planetary
coherent radio emission in our solar system. The narrow beams of radiation pass
our line of sight as the dwarf rotates, producing the associated periodic
bursts. The resulting radio light curves are analogous to the periodic light
curves associated with pulsar radio emission highlighting TVLM 513-46546 as the
prototype of a new class of transient radio source.Comment: 12 pages, 3 figures, accepted for publication in ApJ Letter
- …