227 research outputs found
The Galactic disk mass-budget : II. Brown dwarf mass-function and density
In this paper, we extend the calculations conducted previously in the stellar
regime to determine the brown dwarf IMF in the Galactic disk. We perform Monte
Carlo calculations taking into account the brown dwarf formation rate, spatial
distribution and binary fraction. Comparison with existing surveys seems to
exclude a power-law MF as steep as the one determined in the stellar regime
below 1 \msol and tends to favor a more flatish behaviour. Comparison with
methane-dwarf detections tends to favor an eventually decreasing form like the
lognormal or the more general exponential distributions determined in the
previous paper. We calculate predicting brown dwarf counts in near-infrared
color diagrams and brown dwarf discovery functions. These calculations yield
the presently most accurate determination of the brown dwarf census in the
Galactic disk. The brown dwarf number density is comparable to the stellar one,
pc. The corresponding brown dwarf mass
density, however, represents only about 10% of the stellar contribution, i.e.
\rho_{BD}\simle 5.0\times 10^{-3} \mvol. Adding up the local stellar density
determined previously yields the density of star-like objects, stars and brown
dwarfs, in the solar neighborhood \rho_\odot \approx 5.0\times 10^{-2} \mvol.Comment: 39 pages, Latex file, uses aasms4.sty, to be published in ApJ,
corrected version with correct figure
IR Colors and Sizes of Faint Galaxies
We present J and Ks band galaxy counts down to J=24 and Ks=22.5 obtained with
the new infrared imager/spectrometer, SOFI, at the ESO New Technology
Telescope. The co-addition of short, dithered, images led to a total exposure
time of 256 and 624 minutes respectively, over an area of arcmin
centered on the NTT Deep Field. The total number of sources with S/N is
1569 in the J sample and 1025 in the Ks-selected sample. These are the largest
samples currently available at these depths. A d/d relation with slope
of in J and in Ks is found with no evident sign of a
decline at the magnitude limit. The observed surface density of ``small''
sources is much lower than ``large'' ones at bright magnitudes and rises more
steeply than the large sources to fainter magnitudes. Fainter than
and Ks, small sources dominate the number counts. Galaxies get redder
in J-K down to
J and Ks. At fainter magnitudes, the median color becomes
bluer with an accompanying increase in the compactness of the galaxies. We show
that the blue, small sources which dominate the faint IR counts are not
compatible with a high redshift () population. On the contrary, the
observed color and compactness trends, together with the absence of a turnover
at faint magnitudes and the dominance of small sources, can be naturally
explained by an increasing contribution of sub- galaxies when going to
fainter apparent magnitudes. Such evidence strongly supports the existence of a
steeply rising () faint end of the local infrared luminosity
function of galaxies - at least for luminosities .Comment: Accepted for publication on A&A; 15 pages, 13 figure
An Improved Red Spectrum of the Methane or T-dwarf SDSS 1624+0029: Role of the Alkali Metals
A Keck~II low resolution spectrum shortward of ome-micron is presented for
SDSS 1624+0029, the first field methane or T dwarf discovered in the Sloan
Digital Sky Survey. Significant flux is detected down to the spectrum's short
wavelength limit of 6200\AA. The spectrum exhibits a broad absorption feature
centered at 7700\AA, which we interpret as the K~I 7665/7699 resonance doublet.
The observed flux declines shortward of 7000\AA, due most likely to the red
wing of the Na~I doublet. Both Cs~I doublet lines are detected more strongly
than in an earlier red spectrum. Neither Li~I absorption nor H emission
are detected. An exploratory model fit to the spectrum suggests that the shape
of the red spectrum can be primarily accounted for by the broad wings of the
K~I and Na~I doublets. This behavior is consistent with the argument proffered
by Burrows, Marley and Sharp that strong alkali absorption is principally
responsible for depressing T dwarf spectra shortward of 1m. In particular,
there seems no compelling reason at this time to introduce dust or an
additional opacity source in the atmosphere of the SDSS object. The width of
the K~I and strengths of the Cs~I lines also indicate that the Sloan object is
warmer than Gl~229B.Comment: accepted March 3, 2000 for Ap.J. Letters, LaTeX, 2 figure
The circumstellar environment of the YSO TMR-1 and a revisit to the candidate very low-mass object TMR-1C
TMR-1 (IRAS~04361+2547) is a class~I proto-stellar source located in the
nearby Taurus star-forming region. Its circumstellar environment is
characterized by extended dust emission with complex structures and conspicuous
filaments. A faint companion, called TMR-1C, located near the proto-star had
been detected in previous studies, but its nature as a very young substellar
object remained inconclusive. To improve the constraints on the nature of
TMR-1C, and to investigate the process of very low-mass star formation in the
TMR-1 system we use very sensitive infrared imaging observations as well as NIR
spectroscopy. We construct the SED of TMR-1C over a much larger wavelength
range as had been possible in previous work and compare it with models of
extincted background stars, young sub-stellar objects, and very low-mass stars
with circumstellar disk and envelope emission. We also search for additional
low-luminosity objects in the immediate environment of the TMR-1, study the
surrounding NIR dust morphology, and analyse the emission line spectrum of a
filamentary structure in the physical context of a bow-shock model. We find
that the observed SED of TMR-1C is inconsistent with an extincted background
star, nor can be fitted with available models for a young extremely low-mass
(<12M_Jup) object. Our near-IR spectrum indicates an effective temperature of
at least ~3000K. Based on a good match of TMR-1C's SED with radiation transfer
models of young stellar objects with circumstellar disks, we propose that
TMR-1C is most likely a very low-mass star with M~0.1-0.2M_sun surrounded by a
circumstellar disk with high inclination, i>80deg. Moreover, we detect an
additional very faint source, which we call TMR-1D, and that shows a quite
striking symmetry in position with TMR-1C. TMR-1C and TMR-1D may have been
formed from a common triggered star-formation event, caused by... (abstract
abridged)Comment: 15 pages, 11 figures, accepted for publication in A&
A narrow-band search for Ly alpha emitting galaxies at z = 8.8
Aims: The first star forming galaxies in the early universe should be copious
Ly alpha emitters, and may play a significant role in ionizing the
intergalactic medium (IGM). It has been proposed that the luminosity function
of Lya emitting galaxies beyond z~6 may be used to constrain the neutral
fraction of the IGM during this epoch. In this work we report on a search for
Ly alpha emitters at redshift 8.8.
Methods: We performed a narrow band imaging programme using ISAAC at the ESO
VLT. Seven fields, covering a total area of 31sq. arcmin and for which optical
and broad band infra-red images have been obtained in the GOODS survey, were
imaged to a limiting flux (respectively luminosity) of ~ 1.3 x 10^{-17}
ergs.s^{-1}.cm^{-2} (respectively ~ 1.3 x 10^{43} ergs.s^{-1} in a narrow band
filter centered in a region of low OH sky emission at 1.19 micron. Candidate
Lyman alpha emitters are objects that are detected in the ISAAC NB images and
undetected in the visible broad band images.
Results: No z=8.8 Ly alpha emitting galaxies were detected to a limit
approaching recent estimates of the luminosity function at z ~ 6. Our results
do suggest, however, that detections or substantial constraints could be
achieved by this method in the near future with larger field instruments
planned for various telescopes.Comment: 7 pages, accepted for publication in A&
Limits on the luminosity function of Ly-alpha emitters at z = 7.7
The Ly-alpha luminosity function (LF) of high-redshift Ly-alpha emitters
(LAEs) is one of the few observables of the re-ionization epoch accessible to
date with 8-10 m class telescopes. The evolution with redshift allows one to
constrain the evolution of LAEs and their role in re-ionizing the Universe at
the end of the Dark Ages.
We have performed a narrow-band imaging program at 1.06 microns at the CFHT,
targeting Ly-alpha emitters at redshift z ~ 7.7 in the CFHT-LS D1 field. From
these observations we have derived a photometric sample of 7 LAE candidates at
z ~ 7.7.
We derive luminosity functions for the full sample of seven objects and for
sub-samples of four objects. If the brightest objects in our sample are real,
we infer a luminosity function which would be difficult to reconcile with
previous work at lower redshift. More definitive conclusions will require
spectroscopic confirmation.Comment: 12 pages, accepted to Astronomy and Astrophysic
Evolutionary models for very-low-mass stars and brown dwarfs with dusty atmospheres
We present evolutionary calculations for very-low-mass stars and brown dwarfs
based on synthetic spectra and non-grey atmosphere models which include dust
formation and opacity, i.e. objects with \te\simle 2800 K. The interior of
the most massive brown dwarfs is shown to develop a conductive core after Gyr which slows down their cooling. Comparison is made in optical and
infrared color-magnitude diagrams with recent late-M and L-dwarf observations.
The saturation in optical colors and the very red near-infrared colors of these
objects are well explained by the onset of dust formation in the atmosphere.
Comparison of the faintest presently observed L-dwarfs with these dusty
evolutionary models suggests that dynamical processes such as turbulent
diffusion and gravitational settling are taking place near the photosphere. As
the effective temperature decreases below \te\approx 1300-1400 K, the colors
of these objects move to very blue near-infrared colors, a consequence of the
ongoing methane absorption in the infrared. We suggest the possibility ofa
brown dwarf dearth in color-magnitude diagrams around this temperature.Comment: 38 pages, Latex file, uses aasms4.sty, accepted for publication in
Ap
A Methane Isolated Planetary Mass Object in Orion
We report on the discovery of a free-floating methane dwarf toward the
direction of the young star cluster sigma Orionis. Based on the object's
far-red optical and near-infrared photometry and spectroscopy, we conclude that
it is a possible member of this association. We have named it as S Ori
J053810.1-023626 (S Ori 70 is the abridged name). If it is a true member of
sigma Orionis, the comparison of the photometric and spectroscopic properties
of S Ori 70 with state-of-the-art evolutionary models yields a mass of 3
(+5/-1) Jupiter mass for ages between 1 Myr and 8 Myr. The presence of such a
low-mass object in our small search area (55.4 sq. arcmin) would indicate a
rising substellar initial mass function in the sigma Orionis cluster even for
planetary masses.Comment: Accepted for publication in the ApJ. Twelve pages, figures and tables
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