494 research outputs found
The large-scale disk fraction of brown dwarfs in the Taurus cloud as measured with Spitzer
Aims. The brown dwarf (BD) formation process has not yet been completely
understood. To shed more light on the differences and similarities between star
and BD formation processes, we study and compare the disk fraction among both
kinds of objects over a large angular region in the Taurus cloud. In addition,
we examine the spatial distribution of stars and BD relative to the underlying
molecular gas Methods. In this paper, we present new and updated photometry
data from the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope
on 43 BDs in the Taurus cloud, and recalculate of the BD disk fraction in this
region. We also useed recently available CO mm data to study the spatial
distribution of stars and BDs relative to the cloud's molecular gas. Results.
We find that the disk fraction among BDs in the Taurus cloud is 41 \pm 12%, a
value statistically consistent with the one among TTS (58 \pm 9%). We find that
BDs in transition from a state where they have a disk to a diskless state are
rare, and we study one isolated example of a transitional disk with an inner
radius of \approx 0.1 AU (CFHT BD Tau 12, found via its relatively small mid-IR
excess compared to most members of Taurus that have disks. We find that BDs are
statistically found in regions of similar molecular gas surface density to
those associated with stars. Furthermore, we find that the gas column density
distribution is almost identical for stellar and substellar objects with and
without disks.Comment: 8 page, 6 figures, Accepted in Astronomy & Astrophysics
X-ray view of IC348 in the light of an updated cluster census
We study the properties of the coronae of the low-mass stars in the young
(~2-3Myr), nearby (~310pc) open cluster IC348 combining X-ray and
optical/infrared data. The four existing Chandra observations of IC348 are
merged, thus providing a deeper and spatially more complete X-ray view than
previous X-ray studies of the cluster. We have compiled a comprehensive catalog
of IC348 members taking into account recent updates to the cluster census. Our
data collection comprises fundamental stellar parameters, infrared excess
indicating the presence of disks, Halpha emission as a tracer of chromospheric
emission or accretion and mass accretion rates. We have detected 290 X-ray
sources in four merged Chandra exposures, of which 187 are associated with
known cluster members. Only four of the X-ray sources are brown dwarfs
(spectral type M6 and later). The detection rate is highest for diskless Class
III stars and increases with stellar mass. This may be explained with higher
X-ray luminosities for higher mass and later evolutionary stage that is evident
in the X-ray luminosity functions. In particular, we find that for the lowest
examined masses (0.1-0.25 Msun) there is a difference between the X-ray
luminosity functions of accreting and non-accreting stars (classified on the
basis of their Halpha emission strength) as well as those of disk-bearing and
diskless stars (classified on the basis of the slope of the spectral energy
distribution). These differences disappear for higher masses. This is related
to our finding that the L_x/L_bol ratio is non-constant across the
mass/luminosity sequence of IC348 with a decrease towards lower luminosity
stars. Our analysis of an analogous stellar sample in the Orion Nebula Cluster
suggests that the decline of L_x/L_ bol for young stars at the low-mass end of
the stellar sequence is likely universal.Comment: Accepted for publication in Astronomy & Astrophysic
A Spitzer IRAC Imaging Survey for T Dwarf Companions Around M, L, and T Dwarfs: Observations, Results, and Monte Carlo Population Analyses
We report observational techniques, results, and Monte Carlo population
analyses from a Spitzer Infrared Array Camera imaging survey for substellar
companions to 117 nearby M, L, and T dwarf systems (median distance of 10 pc,
mass range of 0.6 to \sim0.05 M\odot). The two-epoch survey achieves typical
detection sensitivities to substellar companions of [4.5 {\mu}m] \leq 17.2 mag
for angular separations between about 7" and 165". Based on common proper
motion analysis, we find no evidence for new substellar companions. Using Monte
Carlo orbital simulations (assuming random inclination, random eccentricity,
and random longitude of pericenter), we conclude that the observational
sensitivities translate to an ability to detect 600-1100K brown dwarf
companions at semimajor axes greater than ~35 AU, and to detect 500-600K
companions at semimajor axes greater than ~60 AU. The simulations also estimate
a 600-1100K T dwarf companion fraction of < 3.4% for 35-1200 AU separations,
and < 12.4% for the 500-600K companions, for 60-1000 AU separations.Comment: 35 pages, 6 figure
Spitzer/IRAC Photometry of the Eta Chameleontis Association
We present IRAC 3.6, 4.5, 5.8 and 8 micron photometry for the 17 A, K and M
type members of the Eta Chameleontis association. These data show infrared
excesses toward six of the 15 K and M stars, indicating the presence of
circumstellar disks around 40% of the stars with masses of 0.1-1 solar mass.
The two A-stars show no infrared excesses. The excess emission around one of
the stars is comparable to the median excess for classical T Tauri stars in the
Taurus association; the remaining five show comparatively weak excess emission.
Taking into account published Halpha spectroscopy that shows that five of the
six stars are accreting, we argue that the disks with weak mid-infrared
excesses are disks in which the inner disks have been largely depleted of small
grains by grain growth, or, in one case, the small grains have settled to the
midplane. This suggests that Eta Cha has a much higher fraction of disks caught
in the act of transitioning into optically thin disks than that measured in
younger clusters and associations.Comment: Accepted to ApJ letter
Accretion-ejection connection in the young brown dwarf candidate ISO-Cha1 217
As the number of observed brown dwarf outflows is growing it is important to
investigate how these outflows compare to the well studied jets from young
stellar objects. A key point of comparison is the relationship between outflow
and accretion activity and in particular the ratio between the mass outflow and
accretion rates (/). The brown dwarf candidate
ISO-ChaI 217 was discovered by our group, as part of a spectro-astrometric
study of brown dwarfs, to be driving an asymmetric outflow with the
blue-shifted lobe having a position angle of 20. The aim here
is to further investigate the properties of ISO-ChaI 217, the morphology and
kinematics of its outflow, and to better constrain
(/). The outflow is spatially resolved in the
lines and is detected out to 1\farcs6
in the blue-shifted lobe and ~ 1" in the red-shifted lobe. The asymmetry
between the two lobes is confirmed although the velocity asymmetry is less
pronounced with respect to our previous study. Using thirteen different
accretion tracers we measure log() [M/yr]= -10.6
0.4. As it was not possible to measure the effect of extinction on the ISO-ChaI
217 outflow was derived for a range of values of A, up to
a value of A = 2.5 mag estimated for the source extinction. The logarithm
of the mass outflow () was estimated in the range -11.7 to -11.1
for both jets combined. Thus / [\Msun/yr] lies
below the maximum value predicted by magneto-centrifugal jet launching models.
Finally, both model fitting of the Balmer decrements and spectro-astrometric
analysis of the H line show that the bulk of the H I emission comes
from the accretion flow.Comment: accepted by Astronomy & Astrophysic
Discovery of a Planetary-Mass Brown Dwarf with a Circumstellar Disk
Using the Hubble Space Telescope, the 4 m Blanco telescope at the Cerro
Tololo Inter-American Observatory, and the Spitzer Space Telescope, we have
performed deep imaging from 0.8 to 8 um of the southern subcluster in the
Chamaeleon I star-forming region. In these data, we have discovered an object,
Cha 110913-773444, whose colors and magnitudes are indicative of a very
low-mass brown dwarf with a circumstellar disk. In a near-infrared spectrum of
this source obtained with the Gemini Near-Infrared Spectrograph, the presence
of strong steam absorption confirms its late-type nature (>=M9.5) while the
shapes of the H- and K-band continua and the strengths of the Na I and K I
lines demonstrate that it is a young, pre-main-sequence object rather than a
field dwarf. A comparison of the bolometric luminosity of Cha 110913-773444 to
the luminosities predicted by the evolutionary models of Chabrier and Baraffe
and Burrows and coworkers indicates a mass of 8+7/-3 M_Jup, placing it fully
within the mass range observed for extrasolar planetary companions (M<=15
M_Jup). The spectral energy distribution of this object exhibits mid-infrared
excess emission at >5 um, which we have successfully modeled in terms of an
irradiated viscous accretion disk with M'<=10e-12 M_sun/year. Cha 110913-773444
is now the least massive brown dwarf observed to have a circumstellar disk, and
indeed is one of the least massive free-floating objects found to date. These
results demonstrate that the raw materials for planet formation exist around
free-floating planetary-mass bodies.Comment: 5 pages, accepted to Astrophysical Journal Letter
New Low-Mass Stars and Brown Dwarfs with Disks in Lupus
Using the Infrared Array Camera and the Multiband Imaging Photometer aboard
the {\it Spitzer Space Telescope}, we have obtained images of the Lupus 3
star-forming cloud at 3.6, 4.5, 5.8, 8.0, and 24 \micron. We present photometry
in these bands for the 41 previously known members that are within our images.
In addition, we have identified 19 possible new members of the cloud based on
red 3.6-8.0 \micron colors that are indicative of circumstellar disks. We have
performed optical spectroscopy on 6 of these candidates, all of which are
confirmed as young low-mass members of Lupus 3. The spectral types of these new
members range from M4.75 to M8, corresponding to masses of 0.2-0.03
for ages of Myr according to theoretical evolutionary models. We also
present optical spectroscopy of a candidate disk-bearing object in the vicinity
of the Lupus 1 cloud, 2M 1541-3345, which Jayawardhana & Ivanov recently
classified as a young brown dwarf ( ) with a spectral type
of M8. In contrast to their results, we measure an earlier spectral type of
M5.750.25 for this object, indicating that it is probably a low-mass star
( ). In fact, according to its gravity-sensitive absorption
lines and its luminosity, 2M 1541-3345 is older than members of the Lupus
clouds ( Myr) and instead is probably a more evolved
pre-main-sequence star that is not directly related to the current generation
of star formation in Lupus.Comment: 18 pages, 3 tables, 6 figure
On the MBM12 Young Association
I present a comprehensive study of the MBM12 young association (MBM12A). By
combining infrared (IR) photometry from the Two-Micron All-Sky Survey (2MASS)
survey with new optical imaging and spectroscopy, I have performed a census of
the MBM12A membership that is complete to 0.03 Msun (H~15) for a 1.75deg X
1.4deg field encompassing the MBM12 cloud. I find five new members with masses
of 0.1-0.4 Msun and a few additional candidates that have not been observed
spectroscopically. From an analysis of optical and IR photometry for stars in
the direction of MBM12, I identify M dwarfs in the foreground and background of
the cloud. By comparing the magnitudes of these stars to those of local field
dwarfs, I arrive at a distance modulus 7.2+/-0.5 (275 pc) to the MBM12 cloud;
it is not the nearest molecular cloud and is not inside the local bubble of hot
ionized gas as had been implied by previous distance estimates of 50-100 pc. I
have also used Li strengths and H-R diagrams to constrain the absolute and
relative ages of MBM12A and other young populations; these data indicate ages
of 2 +3/-1 Myr for MBM12A and 10 Myr for the TW Hya and Eta Cha associations.
MBM12A may be a slightly evolved version of the aggregates of young stars
within the Taurus dark clouds (~1 Myr) near the age of the IC 348 cluster (~2
Myr).Comment: to be published in The Astrophysical Journal, 41 pages, 14 figures,
also found at http://cfa-www.harvard.edu/sfgroup/preprints.htm
IRAC Observations of Taurus Pre-Main Sequence Stars
We present infrared photometry obtained with the IRAC camera on the Spitzer
Space Telescope of a sample of 82 pre-main sequence stars and brown dwarfs in
the Taurus star-forming region. We find a clear separation in some IRAC
color-color diagrams between objects with and without disks. A few
``transition'' objects are noted, which correspond to systems in which the
inner disk has been evacuated of small dust. Separating pure disk systems from
objects with remnant protostellar envelopes is more difficult at IRAC
wavelengths, especially for objects with infall at low rates and large angular
momenta. Our results generally confirm the IRAC color classification scheme
used in previous papers by Allen et al. and Megeath et al. to distinguish
between protostars, T Tauri stars with disks, and young stars without (inner)
disks. The observed IRAC colors are in good agreement with recent improved disk
models, and in general accord with models for protostellar envelopes derived
from analyzing a larger wavelength region. We also comment on a few Taurus
objects of special interest. Our results should be useful for interpreting IRAC
results in other, less well-studied star-forming regions.Comment: 29 pages 10 figures, to appear in Ap
AzTEC 1.1 mm Observations of the MBM12 Molecular Cloud
We present 1.1 mm observations of the dust continuum emission from the MBM12
high-latitude molecular cloud observed with the Astronomical Thermal Emission
Camera (AzTEC) mounted on the James Clerk Maxwell Telescope on Mauna Kea,
Hawaii. We surveyed a 6.34 deg centered on MBM12, making this the largest
area that has ever been surveyed in this region with submillimeter and
millimeter telescopes. Eight secure individual sources were detected with a
signal-to-noise ratio of over 4.4. These eight AzTEC sources can be considered
to be real astronomical objects compared to the other candidates based on
calculations of the false detection rate. The distribution of the detected 1.1
mm sources or compact 1.1 mm peaks is spatially anti-correlated with that of
the 100 micronm emission and the CO emission. We detected the 1.1 mm
dust continuum emitting sources associated with two classical T Tauri stars,
LkHalpha262 and LkHalpha264. Observations of spectral energy distributions
(SEDs) indicate that LkHalpha262 is likely to be Class II (pre-main-sequence
star), but there are also indications that it could be a late Class I
(protostar). A flared disk and a bipolar cavity in the models of Class I
sources lead to more complicated SEDs. From the present AzTEC observations of
the MBM12 region, it appears that other sources detected with AzTEC are likely
to be extragalactic and located behind MBM12. Some of these have radio
counterparts and their star formation rates are derived from a fit of the SEDs
to the photometric evolution of galaxies in which the effects of a dusty
interstellar medium have been included.Comment: 8 pages, 6 figures, The Astrophysical Journal, in pres
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