338 research outputs found
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
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
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 Survey for Circumstellar Disks Around Young Substellar Objects
(Abridged) We have completed the first systematic survey for disks around
spectroscopically identified young brown dwarfs and very low mass stars. We
have obtained L'-band (3.8 um) imaging for 38 very cool objects in IC 348 and
Taurus. Our targets span spectral types from M6 to M9.5 (~100 to ~15 Mjup).
Using the objects' measured spectral types and extinctions, we find that most
of our sample (77%+/-15%) possess intrinsic IR excesses, indicative of disks.
Because the excesses are modest, conventional analyses using only IR colors
would have missed most of the sources with excesses. The observed IR excesses
are correlated with Halpha emission, consistent with a common accretion disk
origin. The excesses can be explained by disk reprocessing of starlight alone;
the implied accretion rates are at least an order of magnitude below typical
values for classical T Tauri stars. The observed distribution of IR excesses
suggests the presence of inner disk holes. The disk frequency appears to be
independent of the mass and age. In the same star-forming regions, disks around
brown dwarfs are at least as long-lived (~3 Myr) as disks around the T Tauri
stars. Altogether, the frequency and properties of young circumstellar disks
appear to be similar from the stellar regime down to the substellar and
planetary-mass regime. This provides prima facie evidence of a common origin
for most stars and brown dwarfs.Comment: ApJ, in press, 28 pages. Minor change to the online, abridged version
of the abstract. No change to the actual pape
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
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 Abundance of Interstellar Nitrogen
Using the HST Goddard High Resolution Spectrograph (GHRS), we have obtained
high S/N echelle observations of the weak interstellar N I 1160, 1161 A
absorption doublet toward the stars Gamma Cas, Lambda Ori, Iota Ori, Kappa Ori,
Delta Sco, and Kappa Sco. In combination with a previous GHRS measurement of N
I toward Zeta Oph, these new observations yield a mean interstellar gas phase
nitrogen abundance (per 10 H atoms) of 10 N/H = 75 +/- 4. There are no
statistically significant variations in the measured N abundances from
sightline to sightline and no evidence of density-dependent depletion from the
gas-phase. Since N is not expected to be depleted much into dust grains in
these diffuse sightlines, its gas-phase abundance should reflect the total
interstellar abundance. Consequently, the GHRS observations imply that the
abundance of interstellar nitrogen (gas plus grains) in the local Milky Way is
about 80% of the solar system value of 10 N/H = 93 +/- 16. Although this
interstellar abundance deficit is somewhat less than that recently found for
oxygen and krypton with GHRS, the solar N abundance and the N I oscillator
strengths are too uncertain to definitively rule out either a solar ISM N
abundance or a 2/3 solar ISM N abundance similar to that of O and Kr.Comment: 14 pages, LaTeX, 2 Postscript figures; ApJ Letters, in pres
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
An Interaction of a Magellanic Leading Arm High Velocity Cloud with the Milky Way Disk
The Leading Arm of the Magellanic System is a tidally formed HI feature
extending \sim 60\arcdeg from the Magellanic Clouds ahead of their direction
of motion. Using atomic hydrogen (HI) data from the Galactic All Sky-Survey
(GASS), supplemented with data from the Australia Telescope Compact Array, we
have found evidence for an interaction between a cloud in the Leading Arm and
the Galactic disk where the Leading Arm crosses the Galactic plane. The
interaction occurs at velocities permitted by Galactic rotation, which allows
us to derive a kinematic distance to the cloud of 21 kpc, suggesting that the
Leading Arm crosses the Galactic Plane at a Galactic radius of
kpc.Comment: 14 pages, 5 figures, accepted to Astrophysical Journal Letters. Full
resolution version available at
ftp://ftp.atnf.csiro.au/pub/people/nmcclure/papers/LeadingArm_apjl.pd
- âŠ