554 research outputs found
CORMASS: A Compact and Efficient NIR Spectrograph for Studying Low-Mass Objects
CorMASS (Cornell Massachusetts Slit Spectrograph) is a compact,
low-resolution (R=300), double-pass prism cross-dispersed near-infrared (NIR)
spectrograph in operation on the Palomar Observatory 60-inch telescope. Its
2-dimensional spectral format provides simultaneous coverage from lambda ~ 0.75
microns to lambda ~ 2.5 microns (z'JHK bands). A remotely operated cold flip
mirror permits its NICMOS3 detector to function as a K_s slit viewer to assist
object placement into the 2 arcsec x 15 arcsec slit. CorMASS was primarily
designed for the rapid spectral classification of low-mass stellar and
sub-stellar objects identified by the Two-Micron All Sky Survey (2MASS).
CorMASS' efficiency and resolution also make it a versatile instrument for the
spectral observation and classification of many other types of bright objects
(K<14) including quasars, novae, and emission line objects.Comment: To be published in Feb 2001 PASP, 19 pages, 12 Figures, High
Resolution file can be retrieved from
ftp://iras2.tn.cornell.edu/pub/wilson/papers/cormass.ps.g
On high proper motion white dwarfs from photographic surveys
The interpretation of high proper motion white dwarfs detected by Oppenheimer
et al (2001) was the start of a lively controversy. While the discoverers
identify a large fraction of their findings as dark halo members, others
interpret the same sample as essentially made of disc and/or thick disc stars.
We use the comprehensive description of Galactic stellar populations provided
by the "Besancon" model to produce a realistic simulation of Oppenheimer et al.
data, including all observational selections and calibration biases. The
conclusion is unambiguous: Thick disc white dwarfs resulting from ordinary
hypotheses on the local density and kinematics are sufficient to explain the
observed objects, there is no need for halo white dwarfs. This conclusion is
robust to reasonable changes in model ingredients. The main cause of the
misinterpretation seems to be that the velocity distribution of a proper motion
selected star sample is severely biased in favour of high velocities. This has
been neglected in previous analyses. Obviously this does not prove that no such
objects like halo white dwarfs can exist, but Oppenheimer et al. observations
drive their possible contribution in the dark matter halo down to an extremely
low fraction.Comment: 4 pages, 1 figure, A&A Letters, accepte
Transit Target Selection Using Reduced Proper Motions
In searches for planetary transits in the field, well over half of the survey
stars are typically giants or other stars that are too large to permit
straightforward detection of planets. For all-sky searches of bright V<~11
stars, the fraction is ~90%. We show that the great majority of these
contaminants can be removed from the sample by analyzing their reduced proper
motions (RPMs): giants have much lower RPMs than dwarfs of the same color. We
use Hipparcos data to design a RPM selection function that eliminates most
evolved stars, while rejecting only 9% of viable transit targets. Our method
can be applied using existing or soon-to-be-released all-sky data to stars
V<12.5 in the northern hemisphere and V<12 in the south. The method degrades at
fainter magnitudes, but does so gracefully. For example, at V=14 it can still
be used to eliminate giants redward of V-I~0.95, that is, the blue edge of the
red giant clump.Comment: Submitted to ApJ, 17 pages including 6 figure
Chandra Observations of Galaxy Cluster Abell 2218
We present results from two observations (combined exposure of ~17 ks) of
galaxy cluster A2218 using the Advanced CCD Imaging Spectrometer on board the
Chandra X-ray Observatory that were taken on October 19, 1999. Using a
Raymond-Smith single temperature plasma model corrected for galactic absorption
we find a mean cluster temperature of kT = 6.9+/-0.5 keV, metallicity of
0.20+/-0.13 (errors are 90 % CL) and rest-frame luminosity in the 2-10 keV
energy band of 6.2x10^{44} erg/s in a LambdaCDM cosmology with H_0=65 km/s/Mpc.
The brightness distribution within 4'.2 of the cluster center is well fit by a
simple spherical beta model with core radius 66".4 and beta = 0.705 . High
resolution Chandra data of the inner 2' of the cluster show the x-ray
brightness centroid displaced ~22" from the dominant cD galaxy and the presence
of azimuthally asymmetric temperature variations along the direction of the
cluster mass elongation. X-ray and weak lensing mass estimates are in good
agreement for the outer parts (r > 200h^{-1}) of the cluster; however, in the
core the observed temperature distribution cannot reconcile the x-ray and
strong lensing mass estimates in any model in which the intracluster gas is in
thermal hydrostatic equilibrium. Our x-ray data are consistent with a scenario
in which recent merger activity in A2218 has produced both significant
non-thermal pressure in the core and substructure along the line of sight; each
of these phenomena probably contributes to the difference between lensing and
x-ray core mass estimates.Comment: 33 pages, 6 figures, uses AASTeX 5.02, ApJ submitte
The X-ray luminosity function of AGN at z~3
We combine Lyman-break colour selection with ultradeep (> 200 ks) Chandra
X-ray imaging over a survey area of ~0.35 deg^2 to select high redshift AGN.
Applying careful corrections for both the optical and X-ray selection
functions, the data allow us to make the most accurate determination to date of
the faint end of the X-ray luminosity function (XLF) at z~3. Our methodology
recovers a number density of X-ray sources at this redshift which is at least
as high as previous surveys, demonstrating that it is an effective way of
selecting high z AGN. Comparing to results at z=1, we find no evidence that the
faint slope of the XLF flattens at high z, but we do find significant (factor
~3.6) negative evolution of the space density of low luminosity AGN. Combining
with bright end data from very wide surveys we also see marginal evidence for
continued positive evolution of the characteristic break luminosity L*. Our
data therefore support models of luminosity-dependent density evolution between
z=1 and z=3. A sharp upturn in the the XLF is seen at the very lowest
luminosities (Lx < 10^42.5 erg s^-1), most likely due to the contribution of
pure X-ray starburst galaxies at very faint fluxes.Comment: 16 pages, 9 figures, accepted for publication in MNRA
Multi-wavelength properties of the high-energy bright Seyfert 1 galaxy IGR J18027-1455
A new sample of hard X-ray sources in the Galactic Plane is being revealed by
the regular observations performed by the INTEGRAL satellite. The full
characterization of these sources is mandatory to understand the hard X-ray
sky. Here we report new multifrequency radio, infrared and optical observations
of the source IGR J18027-1455, as well as a multi-wavelength study from radio
to hard X-rays. The radio counterpart of IGR J18027-1455 is not resolved at any
observing frequency. The radio flux density is well fitted by a simple power
law with a spectral index alpha=-0.75+/-0.02. This value is typical of
optically thin non-thermal synchrotron emission originated in a jet. The NIR
and optical spectra show redshifted emission lines with z=0.034, and a broad
Halpha line profile with FWHM ~3400 km/s. This suggests an Active Galactic
Nucleus (AGN) of type 1 as the optical counterpart of IGR J18027-1455. We
confirm the Seyfert 1 nature of the source, which is intrinsically bright at
high energies both in absolute terms and when scaled to a normalized 6 cm
luminosity. Finally, comparing its X-ray luminosity with isotropic indicators,
we find that the source is Compton thin and AGN dominated. This indicates that
INTEGRAL might have just seen the tip of the iceberg, and several tens of such
sources should be unveiled during the course of its lifetime.Comment: 6 pages, 6 figures. Accepted for publication in A&A. Minor changes
according to referee repor
Discovery of a Brown Dwarf Companion to Gliese 570ABC: A 2MASS T Dwarf Significantly Cooler than Gliese 229B
We report the discovery of a widely separated (258\farcs3\pm0\farcs4) T
dwarf companion to the Gl 570ABC system. This new component, Gl 570D, was
initially identified from the Two Micron All Sky Survey (2MASS). Its
near-infrared spectrum shows the 1.6 and 2.2 \micron CH absorption bands
characteristic of T dwarfs, while its common proper motion with the Gl 570ABC
system confirms companionship. Gl 570D (M = 16.470.07) is nearly a
full magnitude dimmer than the only other known T dwarf companion, Gl 229B, and
estimates of L = (2.80.3)x10 L_{\sun} and T = 75050
K make it significantly cooler and less luminous than any other known brown
dwarf companion. Using evolutionary models by Burrows et al. and an adopted age
of 2-10 Gyr, we derive a mass estimate of 5020 M for this object.Comment: 13 pages, 2 figures, 2 tables, accepted by ApJ
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