1,160 research outputs found
SDSS J080531.84+481233.0: An Unresolved L Dwarf/T Dwarf Binary
SDSS J080531.84+481233.0 is a peculiar L-type dwarf that exhibits unusually
blue near-infrared and mid-infrared colors and divergent optical (L4) and
near-infrared (L9.5) spectral classifications. These peculiar spectral traits
have been variously attributed to condensate cloud effects or subsolar
metallicity. Here I present an improved near-infrared spectrum of this source
which further demonstrates the presence of weak CH4 absorption at 1.6 micron
but no corresponding band at 2.2 micron. It is shown that these features can be
collectively reproduced by the combined light spectrum of a binary with L4.5
and T5 components, as deduced by spectral template matching. Thus, SDSS
J080531.84+481233.0 appears to be a new low-mass binary straddling the L
dwarf/T dwarf transition, an evolutionary phase for brown dwarfs that remains
poorly understood by current theoretical models. The case of SDSS
J080531.84+481233.0 further illustrates how a select range of L dwarf/T dwarf
binaries could be identified and characterized without the need for high
angular resolution imaging or radial velocity monitoring, potentially
alleviating some of the detection biases and limitations inherent to such
techniques.Comment: 11 pages, 4 figures, accepted by A
The Evolution of L and T Dwarfs in Color-Magnitude Diagrams
We present new evolution sequences for very low mass stars, brown dwarfs and
giant planets and use them to explore a variety of influences on the evolution
of these objects. We compare our results with previous work and discuss the
causes of the differences and argue for the importance of the surface boundary
condition provided by atmosphere models including clouds.
The L- to T-type ultracool dwarf transition can be accommodated within the
Ackerman & Marley (2001) cloud model by varying the cloud sedimentation
parameter. We develop a simple model for the evolution across the L/T
transition. By combining the evolution calculation and our atmosphere models,
we generate colors and magnitudes of synthetic populations of ultracool dwarfs
in the field and in galactic clusters. We focus on near infrared color-
magnitude diagrams (CMDs) and on the nature of the ``second parameter'' that is
responsible for the scatter of colors along the Teff sequence. Variations in
metallicity and cloud parameters, unresolved binaries and possibly a relatively
young population all play a role in defining the spread of brown dwarfs along
the cooling sequence. We find that the transition from cloudy L dwarfs to
cloudless T dwarfs slows down the evolution and causes a pile up of substellar
objects in the transition region, in contradiction with previous studies. We
apply the same model to the Pleiades brown dwarf sequence. Taken at face value,
the Pleiades data suggest that the L/T transition occurs at lower Teff for
lower gravity objects. The simulated populations of brown dwarfs also reveal
that the phase of deuterium burning produces a distinctive feature in CMDs that
should be detectable in ~50-100 Myr old clusters.Comment: Accepted for publication in the ApJ. 52 pages including 20 figure
Influence of interstellar and atmospheric extinction on light curves of eclipsing binaries
Interstellar and atmospheric extinctions redden the observational photometric
data and they should be handled rigorously. This paper simulates the effect of
reddening for the modest case of two main sequence T1 = 6500K and T2 = 5500K
components of a detached eclipsing binary system. It is shown that simply
subtracting a constant from measured magnitudes (the approach often used in the
field of eclipsing binaries) to account for reddening should be avoided.
Simplified treatment of the reddening introduces systematics that reaches
\~0.01mag for the simulated case, but can be as high as ~0.2mag for e.g.
B8V--K4III systems. With rigorous treatment, it is possible to uniquely
determine the color excess value E(B-V) from multi-color photometric light
curves of eclipsing binaries.Comment: 6 pages, 9 figures, 1 table, Kopal's Binary Star Legacy conference
contribution (Litomysl 2004), to be published by Kluwer A&S
Spectral Classification and Effective Temperatures of L and T Dwarfs Based of Near-Infrared Spectra
We have obtained near-infrared spectra of L dwarfs, L/T transition objects
and T dwarfs using Subaru. Resulting spectra are examined in detail to see
their dependence on the spectral types. We have obtained bolometric
luminosities of the objects with known parallaxes in our sample, first by
integrating the spectra and second by K band bolometric correction. We derive
the relation between effective temperature and spectral type.Comment: To appear in May 20, 2004 issue of ApJ There is a companion paper by
Tsuji, Nakajima and Yanagisaw
Cool Customers in the Stellar Graveyard IV: Spitzer Search for Mid-IR excesses Around Five DAs
Hydrogen atmosphere white dwarfs with metal lines, so-called DAZs, require
external accretion of material to explain the presence of weak metal line
absorption in their photospheres. The source of this material is currently
unknown, but could come from the interstellar medium, unseen companions, or
relic planetesimals from asteroid belt or Kuiper belt analogues. Accurate
mid-infrared photometry of these white dwarfs provide additional information to
solve the mystery of this accretion and to look for evidence of planetary
systems that have survived post main sequence evolution. We present {\em
Spitzer} IRAC photometry accurate to 3% for four DAZs and one DA with
circumstellar absorption lines in the UV. We search for excesses due to unseen
companions or circumstellar dust disks. We use {\em Hubble Space Telescope}
NICMOS imaging of these white dwarfs to gauge the level of background
contamination to our targets as well as rule out common proper motion
companions to WD 1620-391. All of our targets show no excesses due to
companions 20 M, ruling out all but very low mass companions to these
white dwarfs at all separations. No excesses due to circumstellar disks are
observed, and we place limits on what types of disks may still be present.Comment: 18 pages, 8 figures, Accepted to A
The Interstellar N/O Abundance Ratio: Evidence for Local Infall?
Sensitive measurements of the interstellar gas-phase oxygen abundance have
revealed a slight oxygen deficiency ( 15%) toward stars within 500 pc of
the Sun as compared to more distant sightlines. Recent observations of
the interstellar gas-phase nitrogen abundance indicate larger variations, but
no trends with distance were reported due to the significant measurement
uncertainties for many sightlines. By considering only the highest quality
( 5 ) N/O abundance measurements, we find an intriguing trend in
the interstellar N/O ratio with distance. Toward the seven stars within
500 pc of the Sun, the weighted mean N/O ratio is 0.217 0.011, while for
the six stars further away the weighted mean value (N/O = 0.142 0.008) is
curiously consistent with the current Solar value (N/O =
0.138). It is difficult to imagine a scenario invoking
environmental (e.g., dust depletion, ionization, etc.) variations alone that
explains this abundance anomaly. Is the enhanced nitrogen abundance localized
to the Solar neighborhood or evidence of a more widespread phenomenon? If it is
localized, then recent infall of low metallicity gas in the Solar neighborhood
may be the best explanation. Otherwise, the N/O variations may be best
explained by large-scale differences in the interstellar mixing processes for
AGB stars and Type II supernovae.Comment: accepted for publication in the Astrophysical Journal Letter
GAIA accuracy on radial velocities assessed from a synthetic spectra database
Spectrograph aboard the GAIA satellite operates in the near-IR, in the 8490--
8740 \AA window accessible also from the ground. The most important parameter
yet to be determined is the spectral resolution. Realistic estimates of the
zodiacal light background are obtained and a total of
correlation runs are used to study the accuracy of radial velocity measured by
the spectrograph as a function of resolution, magnitude of the target, its
spectral type and luminosity class. Accuracy better than 2 km/s is achievable
for bright stars if a high enough dispersion is chosen. Radial velocity error
of 5 km/s is at for Cepheids and at 17.7 for horizontal branch stars.
Even for very faint objects, with spectra dominated by background and readout
noise, the optimal dispersion is still in the 0.25 / 0.75 \AA/pix range. This
is also true for complicated cases such as spectroscopic binaries or if
information other than radial velocity, i.e. abundances of individual elements
or stellar rotation velocity, is sought after. The results can be scaled to
assess performance of future ground based instruments.Comment: accepted for publication in Astronomy and Astrophysics, 8 pages, 4
figure
A Cold Nearby Cloud Inside the Local Bubble
The high-latitude Galactic H I cloud toward the extragalactic radio source 3C
225 is characterized by very narrow 21 cm emission and absorption indicative of
a very low H I spin temperature of about 20 K. Through high-resolution optical
spectroscopy, we report the detection of strong, very narrow Na I absorption
corresponding to this cloud toward a number of nearby stars. Assuming that the
turbulent H I and Na I motions are similar, we derive a cloud temperature of 20
(+6, -8) K (in complete agreement with the 21 cm results) and a line-of-sight
turbulent velocity of 0.37+/-0.08 km/s from a comparison of the H I and Na I
absorption linewidths. We also place a firm upper limit of 45 pc on the
distance of the cloud, which situates it well inside the Local Bubble in this
direction and makes it the nearest-known cold diffuse cloud discovered to date.Comment: 11 pages, 3 figures, accepted for publication in ApJ Letter
Ammonia as a tracer of chemical equilibrium in the T7.5 dwarf Gliese 570D
We present the first analysis of an optical to mid-infrared spectrum of the
T7.5 dwarf Gliese 570D with model atmospheres, synthetic spectra, and brown
dwarf evolution sequences. We obtain precise values for the basic parameters of
Gl 570D: Teff=800 - 820K, log g (cm/s^2)=5.09 - 5.23, and log L/Lsun= -5.525 to
-5.551. The Spitzer IRS spectrum shows prominent features of ammonia (NH3) that
can only be fitted by reducing the abundance of NH3 by about one order of
magnitude from the value obtained with chemical equilibrium models. We model
departures from chemical equilibrium in the atmosphere of Gl 570D by
considering the kinetics of nitrogen and carbon chemistry in the presence of
vertical mixing. The resulting model spectrum reproduces the data very well.Comment: Accepted for publication in the ApJ. 10 pages, including 3 figure
Bayesian analysis of exoplanet and binary orbits
We introduce BASE (Bayesian astrometric and spectroscopic exoplanet detection
and characterisation tool), a novel program for the combined or separate
Bayesian analysis of astrometric and radial-velocity measurements of potential
exoplanet hosts and binary stars. The capabilities of BASE are demonstrated
using all publicly available data of the binary Mizar A.Comment: Accepted for publication in Astronomy & Astrophysic
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