344 research outputs found
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
Inner rings in disc galaxies: dead or alive
In this Letter, I distinguish "passive" inner rings as those with no current star formation as distinct from "active" inner rings that have undergone recent star formation. I built a sample of nearby galaxies with inner rings observed in the near-and mid-infrared from the NIRS0S and the S(4)G surveys. I used archival far-ultraviolet (FUV) and H alpha imaging of 319 galaxies to diagnose whether their inner rings are passive or active. I found that passive rings are found only in early-type disc galaxies (-3 <= T <= 2). In this range of stages, 21 +/- 3% and 28 +/- 5% of the rings are passive according to the FUV and H alpha indicators, respectively. A ring that is passive according to the FUV is always passive according to H alpha, but the reverse is not always true. Ring-lenses form 30-40% of passive rings, which is four times more than the fraction of ring-lenses found in active rings in the stage range -3 <= T <= 2. This is consistent with both a resonance and a manifold origin for the rings because both models predict purely stellar rings to be wider than their star-forming counterparts. In the case of resonance rings, the widening may be at least partly due to the dissolution of rings. If most inner rings have a resonance origin, I estimate 200 Myr to be a lower bound for their dissolution time-scale. This time-scale is of the order of one orbital period at the radius of inner rings
Search for the companions of Galactic SNe Ia
The central regions of the remnants of Galactic SNe Ia have been examined for
the presence of companion stars of the exploded supernovae. We present the
results of this survey for the historical SN 1572 and SN 1006. The spectra of
the stars are modeled to obtain Teff, log g and the metallicity. Radial
velocities are obtained with an accuracy of 5--10 km s. Implications for
the nature of the companion star in SNeIa follow.Comment: 8 pages, 2 Postscript figures. Appeared in "From Twilight to
Highlight: the Physics of Supernovae", ed. W. Hillebrandt & B. Leibundgut
(Springer), pp. 140-14
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
On the origin of the neutral hydrogen supershells: the ionized progenitors and the limitations of the multiple supernovae hypothesis
Here we address the question whether the ionized shells associated with giant
HII regions can be progenitors of the larger HI shell-like objects found in the
Milky Way and other spiral and dwarf irregular galaxies. We use for our
analysis a sample of 12 HII shells presented recently by Rela\~no et al. (2005,
2007). We calculate the evolutionary tracks that these shells would have if
their expansion is driven by multiple supernovae explosions from the parental
stellar clusters. We find, contrary to Rela\~no et al. (2007), that the
evolutionary tracks of their sample HII shells are inconsistent with the
observed parameters of the largest and most massive neutral hydrogen
supershells. We conclude that HII shells found inside giant HII regions may
represent the progenitors of small or intermediate HI shells, however they
cannot evolve into the largest HI objects unless, aside from the multiple
supernovae explosions, an additional energy source contributes to their
expansion.Comment: Accepted for publication in ApJ, tentatively scheduled for the ApJ
July 1, 2008, v681n1 issue. 19 pages, 4 figure
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
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
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
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
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