1,165 research outputs found
Low-Mass Star Formation and the Initial Mass Function in the Rho Ophiuchi Cloud Core
We have obtained moderate-resolution (R=800-1200) K-band spectra for ~100
stars within and surrounding the cloud core of rho Oph. We have measured
spectral types and continuum veilings and have combined this information with
results from new deep imaging. The IMF peaks at about 0.4 M_sun and slowly
declines to the hydrogen burning limit with a slope of ~-0.5 in logarithmic
units (Salpeter is +1.35). Our lower limits on the numbers of substellar
objects demonstrate that the IMF probably does not fall more steeply below the
hydrogen burning limit, at least down to ~0.02 M_sun. We then make the first
comparison of mass functions of stars and pre-stellar clumps (Motte, Andre, &
Neri) measured in the same region. The similar behavior of the two mass
functions in rho Oph supports the suggestion of Motte et al. and Testi &
Sargent that the stellar mass function in young clusters is a direct product of
the process of cloud fragmentation. After considering the effect of extinction
on the SED classifications of the sample, we find that ~17% of the rho Oph
stars are Class I, implying ~0.1 Myr for the lifetime of this stage. In spectra
separated by two years, we observe simultaneous variability in the Br gamma
emission and K-band continuum veiling for two stars, where the hydrogen
emission is brighter in the more heavily veiled data. This behavior indicates
that the disk may contribute significantly to continuous K-band emission, in
contrast to the proposal that the infalling envelope always dominates. Our
detection of strong 2 micron veiling (r_K=1-4) in several Class II and III
stars, which should have disks but little envelope material, further supports
this proposition.Comment: 35 pages, 14 figures, accepted to Ap
VLA 3.5 cm continuum sources in the Serpens cloud core
We present VLA 3.5 cm continuum observations of the Serpens cloud core. 22
radio continuum sources are detected. 16 out of the 22 cm sources are suggested
to be associated with young stellar objects (Class 0, Class I, flat-spectrum,
and Class II) of the young Serpens cluster. The rest of the VLA sources
plausibly are background objects. Most of the Serpens cm sources likely
represent thermal radio jets; on the other hand, the radio continuum emission
of some sources could be due to a gyrosynchroton mechanism arising from
coronally active young stars. The Serpens VLA sources are spatially distributed
into two groups; one of them located towards the NW clump of the Serpens core,
where only Class 0 and Class I protostars are found to present cm emission, and
a second group located towards the SE clump, where radio continuum sources are
associated with objects in evolutionary classes from Class 0 to Class II. This
subgrouping is similar to that found in the near IR, mid-IR and mm wavelength
regimes.Comment: 2 figures, accepted by Astronomical journa
Colors and K-Band Counts of Extremely Faint Field Galaxies
We combine deep K-band (Keck) with V- and I-band (NTT) observations of two
high-Galactic latitude fields, surveying a total of ~2 sq. arcmin. The K-band
galaxy counts continue to rise above K=22, reaching surface densities of few x
10^5 per sq. degree. The slope for the counts is (d log(N) per mag per sq.
degree) = 0.23 +/- 0.02 between K=18-23, consistent with other deep K surveys.
The numbers of galaxies in each mag bin is about two times greater than the
galaxy counts of Djorgovski et al. (1995).
The optical and near infrared magnitudes of all objects detected in the V+I+K
image are discussed in the context of grids of isochrone synthesis galaxy
evolutionary models (Bruzual & Charlot 1993, 1995). The colors of most of the
observed galaxies are consistent with a population drawn from a broad redshift
distribution. A few galaxies at K=19-20 are red in both colors (V-I>3; I-K>2),
consistent with being early-type galaxies having undergone a burst of star
formation at z>5 and viewed at z~1. At K>20, we find ~8 ``red outlier''
galaxies with I-K>4 and V-I<2.5, whose colors are difficult to mimic by a
single evolving or non-evolving stellar population at any redshift. They are
likely either low-metallicity, dusty dwarf galaxies, or old galaxies at high
redshift (z>1.2). Their surface density is several per square arcminute, which
is so high that they are probably common objects of low luminosity .Comment: 28 pages, 9 Figures (2 Plates), uses aaspp4.sty. The complete paper
(including large figures and the plates) are available via anonymous ftp at
ftp://coma.berkeley.edu/pub/lmoustakas/ . To appear in the Astrophysical
Journal, Feb 1 1997, vol. 47
JHK Standard Stars for Large Telescopes: the UKIRT Fundamental and Extended Lists
We present high-precision JHK photometry with the 3.8m UK Infrared Telescope
(UKIRT) of 83 standard stars, 28 from the widely used preliminary list known as
the "UKIRT Faint Standards" (Casali & Hawarden, 1992), referred to here as the
Fundamental List, and 55 additional stars referred to as the Extended List. The
stars have 9.4<K<15.0 and most should be readily observable with imaging array
detectors in normal operating modes on telescopes of up to 10m aperture. Many
are accessible from the southern hemisphere. Arcsec-accuracy positions (J2000,
Epoch ~1998) are given, together with optical photometry and spectral types
from the literature, where available, or inferred from the J-K colour. Finding
charts are provided for stars with proper motions exceeding 0.3"/yr. On 30
nights between late 1994 and early 1998 the stars from the Fundamental List,
which were used as standards for the whole programme, were observed on an
average of 10 nights each, and those from the Extended List 6 nights. The
average internal standard error of the mean results for K is 0.005 mag; for J-H
it is 0.003 mag for the Fundamental List stars and 0.006 mag for the Extended
List; for H-K the average is 0.004 mag. The results are on the natural system
of the IRCAM3 imager, which used a 256x256 InSb detector array with "standard"
JHK filters, behind gold-coated fore-optics and a gold- or silver-dielectric
coated dichroic. We give colour transformations onto the CIT, Arcetri and
LCO/Palomar NICMOS systems, and preliminary transformations onto the system
defined by the new Mauna Kea Observatory filter set.Comment: 13 pages includes one figure, accepted by MNRAS Feb 2001 Revised
Version with a transformation typo correcte
QSO Absorbing Galaxies at z<~1: Deep Imaging and Spectroscopy in the Field of 3C 336
We present very deep WFPC2 images and FOS spectroscopy from the Hubble Space
Telescope (HST) together with numerous supporting ground-based observations of
the field of the quasar 3C 336 (). The observations are designed
to investigate the nature of galaxies producing metal line absorption systems
in the spectrum of the QSO. Along a single line of sight, we find at least 6
metal line absorption systems (of which 3 are newly discovered) ranging in
redshift from 0.317 to 0.892. Through an extensive program of optical and IR
imaging, QSO spectroscopy, and faint galaxy spectroscopy, we have identified 5
of the 6 metal line absorption systems with luminous (L_K > 0.1 L*_K) galaxies.
These have morphologies ranging from very late-type spiral to S0, and exhibit a
wide range of inclination and position angles with respect to the QSO
sightline. The only unidentified absorber, despite our intensive search, is a
damped Lyman system at . Analysis of the absorption
spectrum suggests that the metal abundances ([Fe/H]) in this system are
similar to those in damped systems at , and to the two other damped
systems for which abundances have been determined at . We have found no
examples of intrinsically faint galaxies () at small impact
parameters that might have been missed as absorber candidates in our previous
ground-based imaging and spectroscopic programs on MgII absorbing galaxies.
There are no bright galaxies (L > 0.1 L_K) within 50h^{-1} kpc which do not
produce detectable metal lines (of Mg II 2796, 2803 and/or C IV 1548, 1550) in
the QSO spectrum. All of these results generally support the inferences which
we have previously reached from a larger survey for absorption-selected
galaxies at z\simlt 1.Comment: 32 pages latex (AAS v4.0 style). 8 Postscript figures (including HST
plate) available at ftp://astro.caltech.edu/users/ccs/3c336_figs.ps.gz .
Submitted to Ap
Does Infall End Before the Class I Stage?
We have observed HCO+ J=3-2 toward 16 Class I sources and 18 Class 0 sources,
many of which were selected from Mardones et al. (1997). Eight sources have
profiles significantly skewed to the blue relative to optically thin lines. We
suggest six sources as new infall candidates. We find an equal "blue excess"
among Class 0 and Class I sources after combining this sample with that of
Gregersen et al. (1997). We used a Monte Carlo code to simulate the temporal
evolution of line profiles of optically thick lines of HCO+, CS and H2CO in a
collapsing cloud and found that HCO+ had the strongest asymmetry at late times.
If a blue-peaked line profile implies infall, then the dividing line between
the two classes does not trace the end of the infall stage.Comment: 21 pages, 8 figures, accepted by ApJ for April 20, 2000, added
acknowledgmen
Variability of the extreme z=4.72 blazar, GB 1428+4217
We report X-ray and radio variability of GB 1428+4217 which confirm its
blazar nature. IR observations reveal a powerful optical-UV component, not
obscured by dust, which is suggestive of the presence of a billion solar mass
black hole, already formed by z ~ 5. A detailed comparison of the broad band
spectral properties of GB 1428+4217 with those of nearby blazars shows it to be
extreme, but nevertheless consistent with the trend found for nearby sources.Comment: MNRAS, in press - 5 pages, 5 figure
Detection of a Lyman-alpha Emission-Line Companion to the z=4.69 QSO BR1202-0725
We report the detection of a nearby emission-line companion to the z=4.695
quasar BR1202-0725. Deep narrow-band exposures on this field from the UH 2.2 m
show a Ly alpha flux of 1.5\times\ten{-16} ergs cm^{-2} s^{-1}. High-resolution
HST WFC2 imaging in the F814W filter band shows continuum structure near the
emission position, at 2.6" NW of the quasar, corresponding to a projected
separation of ~7.5 h^{-1} kpc for \qnought=0.5, where h=\hnought/100\kms
Mpc^{-1}. We discuss possible explanations for the combined line and color
properties. The ionization is most likely produced by the quasar, but if due to
underlying star formation would require a star formation rate of ~7 h^2 solar
masses per yr.Comment: 12 pages LaTeX (aastex4.0 macros: aasms4.sty, flushrt.sty) plus 3
figures. To appear in the Astrophysical Journal Letters. Typo in original
submission ("2" should have been "7" in luminosity: 7e42 h^{-2} ergs and in
the star formation rate: 7 h^{-2} \msun per yr). Resubmitted with figure
Infrared Colors at the Stellar/Substellar Boundary
We present new infrared photometry for 61 halo and disk stars around the
stellar/substellar boundary. These data are combined with available optical
photometry and astrometric data to produce color--color and absolute
magnitude--color diagrams. The disk and halo sequences are compared to the
predictions of the latest model atmospheres and structural models.
We find good agreement between observation and theory except for known
problems in the V and H passbands probably due to incomplete molecular data for
TiO, metal hydrides and HO. The metal--poor M subdwarfs are well matched by
the models as oxide opacity sources are less important in this case. The known
extreme M subdwarfs have metallicities about one--hundredth solar, and the
coolest subdwarfs have T K with masses 0.09M/M. The grainless models are not able to reproduce the flux
distributions of disk objects with T 2500 K, however a preliminary
version of the NextGen--Dusty models which includes homogeneous formation and
extinction by dust grains {\it is} able to match the colors of these very cool
objects. The least luminous objects in this sample are GD165B, three DENIS
objects --- DBD0205, DBD1058 and DBD1228 --- and Kelu-1. These have
T 2000 K and are at or below the stellar limit with masses
0.075M/M. Photometry alone cannot constrain these parameters
further as the age is unknown, but published lithium detections for two of
these objects (Kelu-1 and DBD1228) imply that they are young (aged about 1 Gyr)
and substellar (mass 0.06M/M).Comment: ApJ, in press. 18 pages. Also available at
ftp://ftp.jach.hawaii.edu/pub/ukirt/skl/dM_preprint
Evolution of dust and ice features around FU Orionis objects
(abridged) We present spectroscopy data for a sample of 14 FUors and 2 TTauri
stars observed with the Spitzer Space Telescope or with the Infrared Space
Observatory (ISO). Based on the appearance of the 10 micron silicate feature we
define 2 categories of FUors. Objects showing the silicate feature in
absorption (Category 1) are still embedded in a dusty and icy envelope. The
shape of the 10 micron silicate absorption bands is compared to typical dust
compositions of the interstellar medium and found to be in general agreement.
Only one object (RNO 1B) appears to be too rich in amorphous pyroxene dust, but
a superposed emission feature can explain the observed shape. We derive optical
depths and extinction values from the silicate band and additional ice bands at
6.0, 6.8 and 15.2 micron. In particular the analysis of the CO_2 ice band at
15.2 micron allows us to search for evidence for ice processing and constrains
whether the absorbing material is physically linked to the central object or in
the foreground. For objects showing the silicate feature in emission (Category
2), we argue that the emission comes from the surface layer of accretion disks.
Analyzing the dust composition reveals that significant grain growth has
already taken place within the accretion disks, but no clear indications for
crystallization are present. We discuss how these observational results can be
explained in the picture of a young, and highly active accretion disk. Finally,
a framework is proposed as to how the two categories of FUors can be understood
in a general paradigm of the evolution of young, low-mass stars. Only one
object (Parsamian 21) shows PAH emission features. Their shapes, however, are
often seen toward evolved stars and we question the object's status as a FUor
and discuss other possible classifications.Comment: accepted for publication in ApJ; 63 pages preprint style including 8
tables and 24 figure
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