843 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
17 new very low-mass members in Taurus. The brown dwarf deficit revisited
Recent studies of the substellar population in the Taurus cloud have revealed
a deficit of brown dwarfs (BD) compared to the Trapezium cluster population
(Briceno et al 1998; Luhman 2000; Luhman et al 2003a; Luhman 2004). However,
these works have concentrated on the highest stellar density regions of the
Taurus cloud. We have performed a large scale optical survey of this region,
covering a total area of 30 deg^2, and encompassing the densest part of the
cloud as well as their surroundings, down to a mass detection limits of 15
Jupiter Masses (MJ). In this paper, we present the optical spectroscopic
follow-up observations of 97 photometrically selected potential new low-mass
Taurus members, of which 27 are strong late-M (SpT < M4V) candidates. These
observations reveal 5 new very low mass (VLM) Taurus members and 12 new BDs.
Combining our observations with previously published results, we derive an
updated substellar to stellar ratio in Taurus of Rss =0.23 +/- 0.05. This ratio
now appears consistent with the value previously derived in the Trapezium
cluster under similar assumptions of 0.26 +/- 0.04. We find strong indication
that the relative numbers of BDs with respect to stars is decreased by a factor
2 in the central regions of the aggregates with respect to the more distributed
population. Our findings are best explained in the context of the
embryo-ejection model where brown dwarfs originate from dynamical interactions
in small N unstable multiple systems.Comment: 20 pages, 15 figure
Improved radial velocity orbit of the young binary brown dwarf candidate ChaHa8
The very young brown dwarf candidate ChaHa8 was recently discovered to have a
close (~1AU) companion by means of radial velocity monitoring. We present
additional radial velocity data obtained with UVES/VLT between 2007 and 2010,
which significantly improve the orbit of the system. The combined data set
spans ten years of radial velocity monitoring for ChaHa8. A Kepler fit to the
data yields an orbital period of 5.2 yrs, an eccentricity of 0.59, and a radial
velocity semi-amplitude of 2.4 km/s. A companion mass M2sini (which is a lower
limit due to the unknown orbital inclination) of 25 Mjup and of 31 Mjup is
derived when using model-dependent mass estimates for the primary of 0.07 and
0.10 Msun, resp. Assuming random orientation of orbits in space, we find a very
high probability that the companion of Chaha8 is of substellar nature: With a
greater than 87% probability, the companion mass is between 30 and 69 Mjup and
the mass ratio < 0.7. The absence of any evidence of the companion in the
cross-correlation function together with the size of the radial velocity
amplitude also indicate a mass ratio of at most 0.7, and likely smaller.
Furthermore, the new data exclude the possibility that the companion has a mass
in the planetary regime (<13 Mjup). We show that the companion contributes
significantly to the total luminosity of the system: model-dependent estimates
provide a minimum luminosity ratio L2/L1 of 0.2. ChaHa8 is the 4th known
spectroscopic brown dwarf or very low-mass stellar binary with determined
orbital parameters, and the 2nd known very young one. With an age of only ~3
Myr it is of particular interest to very low-mass formation and evolution
theories. In contrast to most other spectroscopic binaries, it has a relatively
long period and it might be possible to determine the astrometric orbit of the
primary and, thus, the orbital inclination.Comment: Accepted for publication in A&A main Journal, minor changes (language
editing
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
New Brown Dwarfs and an Updated Initial Mass Function in Taurus
I have performed a search for young low-mass stars and brown dwarfs (BDs) in
2 regions encompassing a total area of 4 deg^2 in the Taurus star-forming
region, discovering 15 new members of Taurus. In addition, I present 7 new
members outside of these areas from the initial stage of a survey of all of
Taurus. These 22 objects exhibit spectral types of M4.5-M9.25 and masses of
0.3-0.015 M_sun according to the theoretical evolutionary models of Baraffe and
Chabrier, 7 of which are likely to be BDs. Emission in H(alpha), He I, Ca II,
[O I], and [S II] and excess emission in optical and near-IR bands among some
of these objects suggest the presence of accretion, outflows, and circumstellar
disks. The results from the 4 deg^2 survey have been combined with previous
studies of Taurus to arrive at an IMF for a total area of 12.4 deg^2. As in the
previous IMFs for Taurus, the updated IMF peaks at a higher mass (0.8 M_sun)
than the mass functions in IC 348 and Orion (0.1-0.2 M_sun). Meanwhile, the
deficit of BDs in Taurus appears to be less significant (x1.4-1.8) than found
in earlier studies (x2) because of a slightly higher BD fraction in the new IMF
for Taurus and a lower BD fraction in the new spectroscopic IMF for the
Trapezium from Slesnick and coworkers. The spatial distribution of the low-mass
stars and BDs discovered in the two new survey areas closely matches that of
the more massive members. Thus, on the degree size scales (~3 pc) probed to
date, there is no indication that BDs form through ejection.Comment: 35 pages, The Astrophysical Journal, 2004, v617 (December 20
Young Brown Dwarfs in the Core of the W3 Main Star-Forming Region
We present the results of deep and high-resolution (FWHM ~ 0".35) JHK NIR
observations with the Subaru telescope, to search for very low mass young
stellar objects (YSOs) in the W3 Main star-forming region. The NIR survey
covers an area of ~ 2.6 arcmin^2 with 10-sigma limiting magnitude exceeding 20
mag in the JHK bands. The survey is sensitive enough to provide unprecedented
details in W3 IRS 5 region and reveals a census of the stellar population down
to objects below the hydrogen-burning limit. We construct JHK color-color (CC)
and J-H/J and H-K/K color-magnitude (CM) diagrams to identify very low
luminosity YSOs and to estimate their masses. Based on these CC and CM
diagrams, we identified a rich population of embedded YSO candidates with
infrared excesses (Class I and Class II), associated with the W3 Main region. A
large number of red sources (H-K > 2) have also been detected around W3 Main.
We argue that these red stars are most probably pre-main-sequence (PMS) stars
with intrinsic color excesses. Based on the comparison between theoretical
evolutionary models of very low-mass PMS objects with the observed CM diagram,
we find there exists a substantial substellar population in the observed
region. The mass function (MF) does not show the presence of cutoff and sharp
turnover around the substellar limit, at least at the hydrogen-burning limit.
Furthermore, the MF slope indicates that the number ratio of young brown dwarfs
and hydrogen-burning stars in the W3 Main is probably higher than those in
Trapezium and IC 348. The presence of mass segregation, in the sense that
relatively massive YSOs lie near the cluster center, is seen. The estimated
dynamical evolution time indicates that the observed mass segregation in the W3
Main may be the imprint of the star formation process.Comment: 39 pages, 15 figures. Accepted for publication in the Astrophysical
Journa
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
Chemical composition of the Taurus-Auriga association
The Taurus-Auriga association is perhaps the most famous prototype of a
low-mass star forming region, surveyed at almost all wavelengths.
Unfortunately, like several other young clusters/associations, this T
association lacks an extensive abundance analysis determination. We present a
high-resolution spectroscopic study of seven low-mass members of Taurus-Auriga,
including both weak-lined and classical T Tauri stars designed to help robustly
determine their metallicity. After correcting for spectral veiling, we
performed equivalent width and spectral synthesis analyses using the GAIA set
of model atmospheres and the 2002 version of the code MOOG. We find a solar
metallicity, obtaining a mean value of [Fe/H]=0.05. The
-element Si and the Fe-peak one Ni confirm a solar composition. Our
work shows that the dispersion among members is well within the observational
errors at variance with previous claims. As in other star forming regions, no
metal-rich members are found, reinforcing the idea that old planet-host stars
form in the inner part of the Galactic disc and subsequently migrate.Comment: In press on A\&
A Near-Infrared Spectroscopic Survey of Class I Protostars
We present the results of a near-IR spectroscopic survey of 110 Class I
protostars observed from 0.80 microns to 2.43 microns at a spectroscopic
resolution of R=1200. We find that Class I objects exhibit a wide range of
lines and the continuum spectroscopic features. 85% of Class I protostars
exhibit features indicative of mass accretion, and we found that the veiling
excess, CO emission, and Br Gamma emission are closely related. We modeled the
spectra to estimate the veiling excess (r_k) and extinction to each target. We
also used near-IR colors and emission line ratios, when available, to also
estimate extinction. In the course of this survey, we observed the spectra of
10 FU Orionis-like objects, including 2 new ones, as well as 3 Herbig Ae type
stars among our Class I YSOs. We used photospheric absorption lines, when
available, to estimate the spectral type of each target. Although most targets
are late type stars, there are several A and F-type stars in our sample.
Notably, we found no A or F class stars in the Taurus-Auriga or Perseus star
forming regions. There are several cases where the observed CO and/or water
absorption bands are deeper than expected from the photospheric spectral type.
We find a correlation between the appearance of the reflection nebula, which
traces the distribution of material on very large scales, and the near-IR
spectrum, which probes smaller scales. The spectra of the components of
spatially resolved protostellar binaries tend to be very similar. In particular
both components tend to have similar veiling and H_2 emission, inconsistent
with random selection from the sample as a whole. There is a strong correlation
between [Fe II] and H_2 emission, supporting previous results showing that H_2
emission in the spectra of young stars is usually shock excited by stellar
winds.Comment: 89 pages, 13 figures, 7 Table
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