221 research outputs found
A Disk Census for Young Brown Dwarfs
Recent surveys have identified sub-stellar objects down to planetary masses
in nearby star-forming regions. Reliable determination of the disk frequency in
young brown dwarfs is of paramount importance to understanding their origin.
Here we report the results of a systematic study of infrared L'-band
(3.8-micron) disk excess in ~50 spectroscopically confirmed objects near and
below the sub-stellar boundary in several young clusters. Our observations,
using the ESO Very Large Telescope, Keck I and the NASA Infrared Telescope
Facility, reveal that a significant fraction of brown dwarfs harbor disks at a
very young age. Their inner disk lifetimes do not appear to be vastly different
from those of disks around T Tauri stars. Our findings are consistent with the
hypothesis that sub-stellar objects form via a mechanism similar to solar-mass
stars.Comment: accepted for publication in The Astronomical Journa
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
Four Brown Dwarfs in the Taurus Star-Forming Region
We have identified four brown dwarfs in the Taurus star-forming region. They
were first selected from and CCD photometry of 2.29 square degrees
obtained at the Canada-France-Hawaii Telescope. Subsequently, they were
recovered in the 2MASS second incremental data release point source catalog.
Low-resolution optical spectra obtained at the William Herschel telescope allow
us to derive spectral types in the range M7--M9. One of the brown dwarfs has
very strong H emission (EW=-340 \AA). It also displays Br
emission in an infrared spectrum obtained with IRCS on the Subaru telescope,
suggesting that it is accreting matter from a disk. The \ion{K}{1} resonance
doublet and the \ion{Na}{1} subordinate doublet at 818.3 and 819.5 nm in these
Taurus objects are weaker than in field dwarfs of similar spectral type,
consistent with low surface gravities as expected for young brown dwarfs. Two
of the objects are cooler and fainter than GG Tau Bb, the lowest mass known
member of the Taurus association. We estimate masses of only 0.03 M for
them. The spatial distribution of brown dwarfs in Taurus hints to a possible
anticorrelation between the density of stars and the density of brown dwarfs.Comment: ApJ Letters (in press
Supplementation of Dairy Cows under Alfalfa Grazing Conditions with Ground Corn
Two trials were carried out during the Autumn of 1991 and 1992 to investigate the effects of corn supplementation on lactational performance of dairy cows under alfalfa grazing. Forty two multiparous Holstein cows with 30-60 days in milk were used in a randomized continuous design with covariance analysis. The treatments were 0.0, 3.5 and 7.0 kg.day-1 of corn grain supplementation (T1, T2 and T3 respectively) in 1991 and 0.0, 3.0, 6.0 and 9.0 kg.day-1 (T1, T2, T3 and T4 respectively) in 1992. Six cows per treatment were used divided in 3 cows per grazing paddock. Dry matter (DM) intake (DMI) was estimated weekly for each group of cows. The pasture allowance was between 22-26 kg DM.cow-1.day-1. Pasture and grain DMI were 16.6 and 0.0, 13.5 and 3.2, 13.3 and 6.1 kg.cow-1.day-1 (T1, T2 and T3 respectively) in 1991; 16.8 and 0.0, 15.7 and 2.1, 14.0 and 4.1, 12.2 and 6.3 kg.cow-1.day-1 (T1, T2, T3 and T4 respectively) in 1992. The substitution rate was 0.66 kgDM pasture per kgDM corn. There was a lineal effect of supplementation on milk production the responses were 0.936 (1991) and 1.173 (1992) kg milk kgDM-1 corn. However, non significant effects (P \u3e0.05) on fat content (32.8 and 31.9 g. kg-1 milk) and protein contents (30.7 and 30.7 g. kg-1 milk) in milk during both years (1991 and 1992 respectively) were observed
Young Low-Mass Stars and Brown Dwarfs in IC 348
I present new results from a continuing program to identify and characterize
the low-mass stellar and substellar populations in the young cluster IC 348
(1-10~Myr). Optical spectroscopy has revealed young objects with spectral types
as late as M8.25. The intrinsic J-H and H-K colors of these sources are
dwarf-like, whereas the R-I and I-J colors appear intermediate between the
colors of dwarfs and giants. Furthermore, the spectra from 6500 to 9500 A are
reproduced well with averages of standard dwarf and giant spectra, suggesting
that such averages should be used in the classification of young late-type
sources. An H-R diagram is constructed for the low-mass population in IC 348
(K6-M8). The presumably coeval components of the young quadruple system GG~Tau
(White et al.) and the locus of stars in IC 348 are used as empirical
isochrones to test the theoretical evolutionary models. For the models of
Baraffe et al., an adjustment of the temperature scale to progressively warmer
temperatures at later M types, intermediate between dwarfs and giants, brings
all components of GG~Tau onto the same model isochrone and gives the population
of IC 348 a constant age and age spread as a function of mass. When other
observational constraints are considered, such as the dynamical masses of
GM~Aur, DM~Tau, and GG~Tau~A, the models of Baraffe et al. are the most
consistent with observations of young systems. With compatible temperature
scales, the models of both D'Antona & Mazzitelli and Baraffe et al. suggest
that the hydrogen burning mass limit occurs near M6 at ages of <10 Myr. Thus,
several likely brown dwarfs are discovered in this study of IC 348, with masses
down to ~20-30 M_J.Comment: 23 pages, 9 figures, accepted to Ap
The Galactic 26Al Problem and The Close Binary SNIb/c Solution?
The origin of the long-lived radioactive 26Al, which has been observed in the
Galactic interstellar medium from its 1.809 MeV decay gamma-ray line emission,
has been a persistent problem for over twenty years. Wolf-Rayet (WR) winds were
thought to be the most promising source, but their calculated 26Al yields are
not consistent with recent analyses of the 1.809 MeV emission from the nearest
WR star and nearby OB associations. The expected 26Al yield from the WR star
exceeds by as much as a factor of 3, that set by the 2-sigma upper limit on the
1.809 MeV emission, while the WR yields in the OB associations are only about
1/3 of that required by the 1.809 MeV emission. We suggest that a solution to
these problems may lie in 26Al from a previously ignored source: explosive
nucleosynthesis in the core collapse SNIb/c supernovae of WR stars that have
lost most of their mass to close binary companions. Recent nucleosynthetic
calculations of SNIb/c suggest that their 26Al yields depend very strongly on
the final, pre-supernova mass of the WR star, and that those with final masses
around 6 to 8 solar masses are expected to produce as much as 0.01 solar masses
of 26Al per supernova. Such binary SNIb/c make up only a small fraction of the
current SNIb/c and only about 1% of all Galactic core collapse supernovae. They
appear to be such prolific sources that the bulk of the present 26Al in the
Galaxy may come from just a few hundred close binary SNIb/c and the intense
1.809 MeV emission from nearby OB associations may come from just one or two
such supernova.Comment: Accepted for publication in Astrophysical Journal Letters, 611,10
August 200
Exploring Brown Dwarf Disks in rho Oph
This paper discusses evidence for and properties of disks associated to brown
dwarfs in the star-forming region rho-Oph. We selected nine objects fr om the
ISOCAM survey of Bontemps et al. (2001) that have detections in the two
mid-infrared bands (6.7 and 14.3 um), relatively low extinction and low
luminosity. We present low-resolution near-infrared spectra in the J, H and K
bands, and determine for each source spectral type, extinction, effective
temperature a nd luminosity by comparing the spectra to those of field dwarfs
and to the most recent model stellar atmospheres. The results indicate that
eight objects have spectral types M6-M7.5, effective temperature of 2600-2700
K, one has a later spectral type (M8.5) and lower temperature (about 2400 K).
The derived extinctions range between Av~2 and 8 mag. The location of the
objects on the HR diagram, in spite of the uncertainties of the evolutionary
tracks for young objects of substellar mass, indicates that all the objects are
very young and have masses below about 0.08 Msun. The coolest object in our
sample has mass in the range 8-12 Mjup (0.008-0.012 Msun). In all cases, the
mid-infrared excess is consistent with the predictions of mode ls of disks
irradiated by the central object, showing that circumstellar disks are commonly
associated to young brown dwarfs and plan etary-mass objects. Finally, we
discuss possible variations of the disk geometry among different objects, as
well as the possibility of using these data to discriminate between various f
ormation scenarios.Comment: 15 pages, A&A in press, full resolution color figures for appendix A
and B are available at
http://www.arcetri.astro.it/~lt/preprints/preprints.htm
The Stellar Population of the Chamaeleon I Star-Forming Region
I present a new census of the stellar population in the Chamaeleon I
star-forming region. Using optical and near-IR photometry and followup
spectroscopy, I have discovered 50 new members of Chamaeleon I, expanding the
census of known members to 226 objects. Fourteen of these new members have
spectral types later than M6, which doubles the number of known members that
are likely to be substellar. I have estimated extinctions, luminosities, and
effective temperatures for the known members, used these data to construct an
H-R diagram for the cluster, and inferred individual masses and ages with the
theoretical evolutionary models of Baraffe and Chabrier. The distribution of
isochronal ages indicates that star formation began 3-4 and 5-6 Myr ago in the
southern and northern subclusters, respectively, and has continued to the
present time at a declining rate. The IMF in Chamaeleon I reaches a maximum at
a mass of 0.1-0.15 M_sun, and thus closely resembles the IMFs in IC 348 and the
Orion Nebula Cluster. In logarithmic units where the Salpeter slope is 1.35,
the IMF is roughly flat in the substellar regime and shows no indication of
reaching a minimum down to a completeness limit of 0.01 M_sun. The low-mass
stars are more widely distributed than members at other masses in the northern
subcluster, but this is not the case in the southern subcluster. Meanwhile, the
brown dwarfs have the same spatial distribution as the stars out to a radius of
3 deg (8.5 pc) from the center of Chamaeleon I
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