1,317 research outputs found
Parallax and Luminosity Measurements of an L Subdwarf
We present the first parallax and luminosity measurements for an L subdwarf,
the sdL7 2MASS J05325346+8246465. Observations conducted over three years by
the USNO infrared astrometry program yield an astrometric distance of
26.7+/-1.2 pc and a proper motion of 2.6241+/-0.0018"/yr. Combined with
broadband spectral and photometric measurements, we determine a luminosity of
log(Lbol/Lsun) = -4.24+/-0.06 and Teff = 1730+/-90 K (the latter assuming an
age of 5-10 Gyr), comparable to mid-type L field dwarfs. Comparison of the
luminosity of 2MASS J05325346+8246465 to theoretical evolutionary models
indicates that its mass is just below the sustained hydrogen burning limit, and
is therefore a brown dwarf. Its kinematics indicate a ~110 Myr, retrograde
Galactic orbit which is both eccentric (3 <~ R <~ 8.5 kpc) and extends well
away from the plane (Delta_Z = +/-2 kpc), consistent with membership in the
inner halo population. The relatively bright J-band magnitude of 2MASS
J05325346+8246465 implies significantly reduced opacity in the 1.2 micron
region, consistent with inhibited condensate formation as previously proposed.
Its as yet unknown subsolar metallicity remains the primary limitation in
constraining its mass; determination of both parameters would provide a
powerful test of interior and evolutionary models for low-mass stars and brown
dwarfs.Comment: Accepted to ApJ 10 September 2007; 13 pages, 5 figures, 3 tables,
formatted in emulateapj styl
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
High-Redshift Quasars Found in Sloan Digital Sky Survey Commissioning Data III: A Color Selected Sample at i^*<20 in the Fall Equatorial Stripe
This is the third paper in a series aimed at finding high-redshift quasars
from five-color (u'g'r'i'z') imaging data taken along the Celestial Equator by
the SDSS during its commissioning phase. In this paper, we first present the
observations of 14 bright high-redshift quasars (3.66<z<4.77, i^*<20)
discovered in the SDSS Fall Equatorial Stripe, and the SDSS photometry of two
previously known high-redshift quasars in the same region of the sky. Combined
with the quasars presented in previous papers, we define a color-selected
flux-limited sample of 39 quasars at 3.6 < z < 5.0 and i^*<20, covering a total
effective area of 182 deg^2. From this sample, we estimate the average spectral
power law slope in the rest-frame ultraviolet for quasars at z~4 to be -0.79
with a standard deviation of 0.34, and the average rest-frame equivalent width
of the Ly alpha+N V emission line to be 69 A with a standard deviation of 18 A.
The selection completeness of this multicolor sample is determined from the
model colors of high-redshift quasars, taking into account the distributions of
emission line strengths, intrinsic continuum slope, the line and continuum
absorption from intervening material, and the effects of photometric errors.
The average completeness of this sample is about 75%. The selection function
calculated in this paper will be used to correct the incompleteness of this
color-selected sample and to derive the high-redshift quasar luminosity
function in a subsequent paper. In the Appendix, we present the observations of
an additional 18 faint quasars (3.57<z<4.80, 20.1<i^*<20.8) discovered in the
region on the sky that has been imaged twice. Several quasars presented in this
paper exhibit interesting properties, including a radio-loud quasar at z=4.77,
and a narrow-line quasar (FWHM = 1500 km s^-1) at z=3.57.Comment: AJ accepted (Jan 2001), with minor changes; high-resolution finding
charts available at http://www.sns.ias.edu/~fan/papers/q3.p
What's in a name?: a discussion paper on the labels and location of self-organising community groups, with particular reference to mental health and Black groups
This article discusses the labelling and location of self-organising community groups â âself-helpâ, âpeer supportâ
and âservice userâ. It notes the increasingly close relationship between these groups and statutory authorities,
and how this relationship may put the benefits of the groups at risk. Historical, cultural and social factors are
discussed to help explain differences and separate developments within African, Caribbean and other Black
communities
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