447 research outputs found
An L-type substellar object in Orion: reaching the mass boundary between brown dwarfs and giant planets
We present J-band photometry and low-resolution optical spectroscopy
(600-1000 nm) for one of the faintest substellar member candidates in the young
sigma Ori cluster, SOri 47 (I=20.53, Bejar et al. 1999). Its very red
(I-J)=3.3+/-0.1 color and its optical spectrum allow us to classify SOri 47 as
an L1.5-type object which fits the low-luminosity end of the cluster
photometric and spectroscopic sequences. It also displays atmospheric features
indicative of low gravity such as weak alkaline lines and hydride and oxide
bands, consistent with the expectation for a very young object still undergoing
gravitational collapse. Our data lead us to conclude that SOri 47 is a true
substellar member of the sigma Ori cluster. Additionally, we present the
detection of LiI in its atmosphere which provides an independent confirmation
of youth and substellarity. Using current theoretical evolutionary tracks and
adopting an age interval of 1-5 Myr for the sigma Ori cluster, we estimate the
mass of SOri 47 at 0.015+/-0.005 Msun, i.e. at the minimum mass for deuterium
burning, which has been proposed as a definition for the boundary between brown
dwarfs and giant planets. SOri 47 could well be the result of a natural
extension of the process of cloud fragmentation down to the deuterium burning
mass limit; a less likely alternative is that it has originated from a
protoplanetary disc around a more massive cluster member and later ejected from
its orbit due to interacting effects within this rather sparse (~12
objects/pc^3) young cluster.Comment: 9 pages, 3 figures, accepted for publication in ApJ Letter
Low-Mass Star Formation and the Initial Mass Function in the Rho Ophiuchi Cloud Core
We have obtained moderate-resolution (R=800-1200) K-band spectra for ~100
stars within and surrounding the cloud core of rho Oph. We have measured
spectral types and continuum veilings and have combined this information with
results from new deep imaging. The IMF peaks at about 0.4 M_sun and slowly
declines to the hydrogen burning limit with a slope of ~-0.5 in logarithmic
units (Salpeter is +1.35). Our lower limits on the numbers of substellar
objects demonstrate that the IMF probably does not fall more steeply below the
hydrogen burning limit, at least down to ~0.02 M_sun. We then make the first
comparison of mass functions of stars and pre-stellar clumps (Motte, Andre, &
Neri) measured in the same region. The similar behavior of the two mass
functions in rho Oph supports the suggestion of Motte et al. and Testi &
Sargent that the stellar mass function in young clusters is a direct product of
the process of cloud fragmentation. After considering the effect of extinction
on the SED classifications of the sample, we find that ~17% of the rho Oph
stars are Class I, implying ~0.1 Myr for the lifetime of this stage. In spectra
separated by two years, we observe simultaneous variability in the Br gamma
emission and K-band continuum veiling for two stars, where the hydrogen
emission is brighter in the more heavily veiled data. This behavior indicates
that the disk may contribute significantly to continuous K-band emission, in
contrast to the proposal that the infalling envelope always dominates. Our
detection of strong 2 micron veiling (r_K=1-4) in several Class II and III
stars, which should have disks but little envelope material, further supports
this proposition.Comment: 35 pages, 14 figures, accepted to Ap
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
Search for young stars among ROSAT All-Sky Survey X-ray sources in and around the R CrA dark cloud
We present the ROSAT All-Sky Survey data in a 126 deg^2 area in and around
the CrA star forming region. With low-resolution spectroscopy of unidentified
ROSAT sources we could find 19 new pre-main sequence stars, two of which are
classical T Tauri stars, the others being weak-lined. The spectral types of
these new T Tauri stars range from F7 to M6. The two new classical T Tauri
stars are located towards two small cloud-lets outside of the main CrA cloud.
They appear to be ~10 Myrs old, by comparing their location in the H-R diagram
with isochrones for an assumed distance of 130 pc, the distance of the main CrA
dark cloud. The new off-cloud weak-line T Tauri stars may have formed in
similar cloud-lets, which have dispersed recently. High-resolution spectra of
our new T Tauri stars show that they have significantly more lithium absorption
than zero-age main-sequence stars of the same spectral type, so that they are
indeed young. From those spectra we also obtained rotational and radial
velocities. For some stars we found the proper motion in published catalogs.
The direction and velocity of the 3D space motion - south relative to the
galatic plane - of the CrA T Tauri stars is consistent with the dark cloud
being formed originally by a high-velocity cloud impact onto the galactic
plane, which triggered the star formation in CrA. We also present VRIJHK
photometry for most of the new T Tauri stars to derive their luminosities,
ages, and masses.Comment: A&A Suppl. in pres
A Young Very Low-Mass Object surrounded by warm dust
We present a complete low-resolution (R~100) near-infrared spectrum of the
substellar object GY11, member of the rho-Ophiuchi young association. The
object is remarkable because of its low estimated mass and age and because it
is associated with a mid-infrared source, an indication of a surrounding dusty
disk. Based on the comparison of our spectrum with similar spectra of field
M-dwarfs and atmospheric models, we obtain revised estimates of the spectral
type, effective temperature and luminosity of the central object. These
parameters are used to place the object on a Hertzprung-Russell diagram and to
compare with the prediction of pre-main sequence evolutionary models. Our
analysis suggests that the central object has a very low mass, probably below
the deuterium burning limit and in the range 8-12MJupiter, and a young age,
less than 1Myr. The infrared excess is shown to be consistent with the emission
of a flared, irradiated disk similar to those found in more massive brown dwarf
and TTauri systems. This result suggests that substellar objects, even the
so-called isolated planetary mass objects, found in young stellar associations
are produced in a similar fashion as stars, by core contraction and
gravitational collapse.Comment: Accepted by ApJ Letters, 13 pages 4 figure
First Detection of Millimeter Dust Emission from Brown Dwarf Disks
We report results from the first deep millimeter continuum survey targeting
Brown Dwarfs (BDs). The survey led to the first detection of cold dust in the
disks around two young BDs (CFHT-BD-Tau 4 and IC348 613), with deep JCMT and
IRAM observations reaching flux levels of a few mJy. The dust masses are
estimated to be a few Earth masses assuming the same dust opacities as usually
applied to TTauri stars.Comment: 5 pages, accepted for ApJ
The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. XI. Lupus Observed With IRAC and MIPS
We present c2d Spitzer/IRAC observations of the Lupus I, III and IV dark
clouds and discuss them in combination with optical and near-infrared and c2d
MIPS data. With the Spitzer data, the new sample contains 159 stars, 4 times
larger than the previous one. It is dominated by low- and very-low mass stars
and it is complete down to M 0.1M. We find 30-40 % binaries
with separations between 100 to 2000 AU with no apparent effect in the disk
properties of the members. A large majority of the objects are Class II or
Class III objects, with only 20 (12%) of Class I or Flat spectrum sources. The
disk sample is complete down to ``debris''-like systems in stars as small as M
0.2 M and includes sub-stellar objects with larger IR
excesses. The disk fraction in Lupus is 70 -- 80%, consistent with an age of 1
-- 2 Myr. However, the young population contains 20% optically thick accretion
disks and 40% relatively less flared disks. A growing variety of inner disk
structures is found for larger inner disk clearings for equal disk masses.
Lupus III is the most centrally populated and rich, followed by Lupus I with a
filamentary structure and by Lupus IV, where a very high density core with
little star-formation activity has been found. We estimate star formation rates
in Lupus of 2 -- 10 M Myr and star formation efficiencies of a
few percent, apparently correlated with the associated cloud masses.Comment: Accepted for publication in the ApJS. Contains 101 pages, 23 figures,
and 13 tables. A version with full resolution figures can be found at
http://peggysue.as.utexas.edu/SIRTF/PAPERS/pap102.pub.pd
Mass Flows in Cometary UCHII Regions
High spectral and spatial resolution, mid-infrared fine structure line
observations toward two ultracompact HII (UCHII) regions (G29.96 -0.02 and Mon
R2) allow us to study the structure and kinematics of cometary UCHII regions.
In our earlier study of Mon R2, we showed that highly organized mass motions
accounted for most of the velocity structure in that UCHII region. In this
work, we show that the kinematics in both Mon R2 and G29.96 are consistent with
motion along an approximately paraboloidal shell. We model the velocity
structure seen in our mapping data and test the stellar wind bow shock model
for such paraboloidal like flows. The observations and the simulation indicate
that the ram pressures of the stellar wind and ambient interstellar medium
cause the accumulated mass in the bow shock to flow along the surface of the
shock. A relaxation code reproduces the mass flow's velocity structure as
derived by the analytical solution. It further predicts that the pressure
gradient along the flow can accelerate ionized gas to a speed higher than that
of the moving star. In the original bow shock model, the star speed relative to
the ambient medium was considered to be the exit speed of ionized gas in the
shell.Comment: 34 pages, including 14 figures and 1 table, to be published in ApJ,
September 200
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