305 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
On the N=2 Supersymmetric Camassa-Holm and Hunter-Saxton Equations
We consider N=2 supersymmetric extensions of the Camassa-Holm and
Hunter-Saxton equations. We show that they admit geometric interpretations as
Euler equations on the superconformal algebra of contact vector fields on the
1|2-dimensional supercircle. We use the bi-Hamiltonian formulation to derive
Lax pairs. Moreover, we present some simple examples of explicit solutions. As
a by-product of our analysis we obtain a description of the bounded
traveling-wave solutions for the two-component Hunter-Saxton equation.Comment: 1+19 pages, 3 figures; v2: reference added; v3: more references
added, published in LM
On the circum(sub)stellar environment of brown dwarfs in Taurus
Aims : We want to investigate whether brown dwarfs (BDs) form like stars or
are ejected embryos. We study the presence of disks around BDs in the Taurus
cloud, and discuss implications for substellar formation models. Methods : We
use photometric measurements from the visible to the far infrared to determine
the spectral energy distributions (SEDs) of Taurus BDs. Results: We use Spitzer
color indices, Halpha as an accretion indicator, and models fit to the SEDs in
order to estimate physical parameters of the disks around these BDs. We study
the spatial distribution of BDs with and without disks across the Taurus
aggregates, and we find that BDs with and without disks are not distributed
regularly across the Taurus cloud. Conclusions: We find that 48%+/- 14% of
Taurus BDs have a circumstellar disk signature, a ratio similar to recent
results from previous authors in other regions. We fit the SEDs and find that
none of the disks around BDs in Taurus can be fitted convincingly with a
flaring index beta = 0, indicating that heating by the central object is
efficient and that the disks we observe retain a significant amount of gas. We
find that BDs with disks are proportionally more numerous in the northern
Taurus filament, possibly the youngest filament. We do not find such a clear
segregation for classical T Tauri stars (CTTS) and weak-lined T Tauri stars
(WTTS), suggesting that, in addition to the effects of evolution, any
segregation effects could be related to the mass of the object. A by-product of
our study is to propose a recalibration of the Barrado y Navascues & Martin
(2003) accretion limit in the substellar domain. The global shape of the limit
fits our data points if it is raised by a factor 1.25-1.30.Comment: 11 pages, 5 figures, A&A accepte
New Young Star Candidates in CG4 and Sa101
The CG4 and Sa101 regions together cover a region of ~0.5 square degree in
the vicinity of a "cometary globule" that is part of the Gum Nebula. There are
seven previously identified young stars in this region; we have searched for
new young stars using mid- and far-infrared data (3.6 to 70 microns) from the
Spitzer Space Telescope, combined with ground-based optical data and
near-infrared data from the Two-Micron All-Sky Survey (2MASS). We find infrared
excesses in all 6 of the previously identified young stars in our maps, and we
identify 16 more candidate young stars based on apparent infrared excesses.
Most (73%) of the new young stars are Class II objects. There is a tighter
grouping of young stars and young star candidates in the Sa101 region, in
contrast to the CG4 region, where there are fewer young stars and young star
candidates, and they are more dispersed. Few likely young objects are found in
the "fingers" of the dust being disturbed by the ionization front from the
heart of the Gum Nebula.Comment: Accepted for publication in A
Direct evidence of dust growth in L183 from mid-infrared light scattering
Context. Theoretical arguments suggest that dust grains should grow in the dense cold parts of molecular clouds. Evidence of larger grains has so far been gathered in near/mid infrared extinction and millimeter observations. Interpreting the data is, however, aggravated by the complex interplay of density and dust properties (as well as temperature for thermal emission).
Aims. Direct evidence of larger particles can be derived from scattered mid-infrared (MIR) radiation from a molecular cloud observed in a spectral range where little or no emission from polycyclic aromatic hydrocarbons (PAHs) is expected.
Methods. We present new Spitzer data of L183 in bands that are sensitive and insensitive to PAHs. The visual extinction AV map derived in a former paper was fitted by a series of 3D Gaussian distributions. For different dust models, we calculate the scattered MIR radiation images of structures that agree with the AV map and compare them to the Spitzer data.
Results. The Spitzer data of L183 show emission in the 3.6 and 4.5 μm bands, while the 5.8 μm band shows slight absorption. The emission layer of stochastically heated particles should coincide with the layer of strongest scattering of optical interstellar radiation, which is seen as an outer surface on I band images different from the emission region seen in the Spitzer images. Moreover, PAH emission is expected to strongly increase from 4.5 to 5.8 μm, which is not seen. Hence, we interpret this emission to be MIR scattered light from grains located further inside the core, and call it ”coreshine”. Scattered light modeling when assuming interstellar medium dust grains without growth does not reproduce flux measurable by Spitzer. In contrast, models with grains growing with density yield images with a flux and pattern comparable to the Spitzer images in the bands 3.6, 4.5, and 8.0 μm.
Conclusions. There is direct evidence of dust grain growth in the inner part of L183 from the scattered light MIR images seen by Spitzer
Spectroscopic identification of DENIS-selected brown dwarf candidates in the Upper Scorpius OB association
We present low-resolution (R=900) optical (576.1--1,051.1 nm) spectroscopic
observations of 40 candidate very low-mass members in the Upper Scorpius OB
association. These objects were selected using the , and photometry
available in the DENIS database. We have derived spectral types and we have
measured H and NaI doublet (at 818.3 and 819.5 nm) equivalent widths.
We assess the youth of the objects by comparing them to their older
counterparts of similar spectral type in the Pleiades cluster and the field.
Our analysis indicates that 28 of our targets are young very low-mass objects,
and thus they are strong candidate members of the OB association. The other 12
DENIS sources are foreground M dwarfs or background red giants. Our sample of
spectroscopic candidate members includes 18 objects with spectral types in the
range M6.5 and M9, which are likely young brown dwarfs. We classify these
candidates as accreting/non accreting using the scheme proposed by Barrado y
Navascu\'es & Mart\'\i n (2003). We find 5 substellar-mass candidate cluster
members that are still undergoing mass accretion, indicating that the timescale
for accretion onto brown dwarfs can be as long as 5 Myr in some cases.Comment: Accepted for publication in The Astronomical Journal, January 200
Masses of the components of SB2 binaries observed with Gaia. II. Masses derived from PIONIER interferometric observations for Gaia validation
In anticipation of the Gaia astrometric mission, a sample of spectroscopic
binaries is being observed since 2010 with the Sophie spectrograph at the
Haute--Provence Observatory. Our aim is to derive the orbital elements of
double-lined spectroscopic binaries (SB2s) with an accuracy sufficient to
finally obtain the masses of the components with relative errors as small as 1
% when combined with Gaia astrometric measurements. In order to validate the
masses derived from Gaia, interferometric observations are obtained for three
SB2s in our sample with F-K components: HIP 14157, HIP 20601 and HIP 117186.
The masses of the six stellar components are derived. Due to its edge-on
orientation, HIP 14157 is probably an eclipsing binary. We note that almost all
the derived masses are a few percent larger than the expectations from the
standard spectral-type-mass calibration and mass-luminosity relation. Our
calculation also leads to accurate parallaxes for the three binaries, and the
Hipparcos parallaxes are confirmed.Comment: 10 pages, 3 figures, accepted by MNRA
Spitzer Observations of IC 2118
IC 2118, also known as the Witch Head Nebula, is a wispy, roughly cometary,
~5 degree long reflection nebula, and is thought to be a site of triggered star
formation. In order to search for new young stellar objects (YSOs), we have
observed this region in 7 mid- and far-infrared bands using the Spitzer Space
Telescope and in 4 bands in the optical using the U. S. Naval Observatory
40-inch telescope. We find infrared excesses in 4 of the 6 previously-known T
Tauri stars in our combined infrared maps, and we find 6 entirely new candidate
YSOs, one of which may be an edge-on disk. Most of the YSOs seen in the
infrared are Class II objects, and they are all in the "head" of the nebula,
within the most massive molecular cloud of the region.Comment: Accepted to Ap
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