212 research outputs found
Detection of abundant solid methanol toward young low mass stars
We present detections of the absorption band at 3.53 micron due to solid
methanol toward three low-mass young stellar objects located in the Serpens and
Chameleon molecular cloud complexes. The sources were observed as part of a
large spectroscopic survey of ~40 protostars. This is the first detection of
solid methanol in the vicinity of low mass (M <1 Msol) young stars and shows
that the formation of methanol does not depend on the proximity of massive
young stars. The abundances of solid methanol compared to water ice for the
three sources are in the range 15-25% which is comparable to those for the most
methanol-rich massive sources known. The presence of abundant methanol in the
circumstellar environment of some low mass young stars has important
consequences for the formation scenarios of methanol and more complex organic
species near young solar-type stars.Comment: Accepted for publication in A&A letter
The Disk Population of the Chamaeleon I Star-Forming Region
We present a census of circumstellar disks in the Chamaeleon I star-forming
region. Using the Infrared Array Camera and the Multiband Imaging Photometer
onboard the Spitzer Space Telescope, we have obtained images of Chamaeleon I at
3.6, 4.5, 5.8, 8.0, and 24 um. To search for new disk-bearing members of the
cluster, we have performed spectroscopy on objects that have red colors in
these data. Through this work, we have discovered four new members of
Chamaeleon I with spectral types of M4, M6, M7.5, and L0. The first three
objects are highly embedded (A_J~5) and reside near known protostars,
indicating that they may be among the youngest low-mass sources in the cluster
(<1 Myr). The L0 source is the coolest known member of Chamaeleon I. Its
luminosity implies a mass of 0.004-0.01 M_sun, making it the least massive
brown dwarf for which a circumstellar disk has been reliably detected. To
characterize the disk population in Chamaeleon I, we have classified the
infrared spectral energy distributions of the 203 known members that are
encompassed by the Spitzer images. Through these classifications, we find that
the disk fraction in Chamaeleon I is roughly constant at ~50% from 0.01 to 0.3
M_sun. These data are similar to the disk fraction of IC 348, which is a denser
cluster at the same age as Chamaeleon I. However, the disk fraction at M>1
M_sun is significantly higher in Chamaeleon I than in IC 348 (65% vs. 20%),
indicating longer disk lifetimes in Chamaeleon I for this mass range. Thus,
low-density star-forming regions like Chamaeleon I may offer more time for
planet formation around solar-type stars than denser clusters
ISOCAM observations of the rho Ophiuchi cloud: Luminosity and mass functions of the pre-main sequence embedded cluster
We present the results of the first extensive mid-infrared (IR) imaging
survey of the rho Ophiuchi embedded cluster, performed with the ISOCAM camera
on board the ISO satellite. The main molecular cloud L1688, as well as L1689N
and L1689S, have been completely surveyed for point sources at 6.7 and 14.3
micron. A total of 425 sources are detected including 16 Class I, 123 Class II,
and 77 Class III young stellar objects (YSOs). Essentially all of the mid-IR
sources coincide with near-IR sources, but a large proportion of them are
recognized for the first time as YSOs. Our dual-wavelength survey allows us to
identify essentially all the YSOs with IR excess in the embedded cluster down
to Fnu ~ 10 - 15 mJy. It more than doubles the known population of Class II
YSOs and represents the most complete census to date of newly formed stars in
the rho Ophiuchi central region. The stellar luminosity function of the
complete sample of Class II YSOs is derived with a good accuracy down to L=
0.03 Lsun. A modeling of this lumino- sity function, using available pre-main
sequence tracks and plausible star for- mation histories, allows us to derive
the mass distribution of the Class II YSOs which arguably reflects the IMF of
the embedded cluster. We estimate that the IMF in rho Ophiuchi is well
described by a two-component power law with a low- mass index of -0.35+/-0.25,
a high-mass index of -1.7 (to be compared with the Salpeter value of -1.35),
and a break occurring at M = 0.55+/-0.25 Msun. This IMF is flat with no
evidence for a low-mass cutoff down to at least 0.06 Msun.Comment: A&A Document Class -- version 5.01, 27 pages, 10 figures v2: typos
added including few changes in source numberin
Photoevaporation of protoplanetary discs I: hydrodynamic models
In this paper we consider the effect of the direct ionizing stellar radiation
field on the evolution of protoplanetary discs subject to photoevaporative
winds. We suggest that models which combine viscous evolution with
photoevaporation of the disc (e.g. Clarke, Gendrin & Sotomayor 2001)
incorrectly neglect the direct field after the inner disc has drained, at late
times in the evolution. We construct models of the photoevaporative wind
produced by the direct field, first using simple analytic arguments and later
using detailed numerical hydrodynamics. We find that the wind produced by the
direct field at late times is much larger than has previously been assumed, and
we show that the mass-loss rate scales as (where is the
radius of the instantaneous inner disc edge). We suggest that this result has
important consequences for theories of disc evolution, and go on to consider
the effects of this result on disc evolution in detail in a companion paper
(Alexander, Clarke & Pringle 2006b).Comment: 13 pages, 9 figures. Accepted for publication in MNRA
A multifrequency study of the active star forming complex NGC6357. I. Interstellar structures linked to the open cluster Pis24
We investigate the distribution of the gas (ionized, neutral atomic and
molecular), and interstellar dust in the complex star forming region NGC6357
with the goal of studying the interplay between the massive stars in the open
cluster Pis24 and the surrounding interstellar matter. Our study of the
distribution of the ionized gas is based on narrow-band Hhalfa, [SII], and
[OIII] images obtained with the Curtis-Schmidt Camera at CTIO, Chile, and on
radio continuum observations at 1465 MHz taken with the VLA with a synthesized
beam of 40 arcsec. The distribution of the molecular gas is analyzed using
12CO(1-0) data obtained with the Nanten radiotelescope, Chile (angular
resolution = 2.7 arcmin). The interstellar dust distribution was studied using
mid-infrared data from the GLIMPSE survey and far-infrared observations from
IRAS. NGC6357 consists of a large ionized shell and a number of smaller optical
nebulosities. The optical, radio continuum, and near- and mid-IR images
delineate the distributions of the ionized gas and interstellar dust in the HII
regions and in previously unknown wind blown bubbles linked to the massive
stars in Pis24 revealing surrounding photodissociation regions. The CO line
observations allowed us to identify the molecular counterparts of the ionized
structures in the complex and to confirm the presence of photodissociation
regions. The action of the WR star HD157504 on the surrounding gas was also
investigated. The molecular mass in the complex is estimated to be (4+/-2)X10^5
Mo. Mean electron densities derived from the radio data suggest electron
densities > 200 cm^-3, indicating that NGC6357 is a complex formed in a region
of high ambient density. The known massive stars in Pis24 and a number of newly
inferred massive stars are mainly responsible for the excitation and
photodissociation of the parental molecular cloud.Comment: 16 pages, 9 figures. Accepted for publication in MNRA
The Stellar Content of Obscured Galactic Giant H II Regions IV.: NGC3576
We present deep, high angular resolution near-infrared images of the obscured
Galactic Giant H II region NGC3576. Our images reach objects to ~3M_sun. We
collected high signal-to-noise K-band spectra of eight of the brightest
objects, some of which are affected by excess emission and some which follow a
normal interstellar reddening law. None of them displayed photospheric features
typical of massive OB type stars. This indicates that they are still enshrouded
in their natal cocoons. The K-band brightest source (NGC3576 #48) shows CO 2.3
micron bandhead emission, and three others have the same CO feature in
absorption. Three sources display spatially unresolved H_2 emission, suggesting
dense shocked regions close to the stars. We conclude that the remarkable
object NGC3576 #48 is an early-B/late-O star surrounded by a thick
circumstellar disk. A number of other relatively bright cluster members also
display excess emission in the K-band, indicative of reprocessing disks around
massive stars (YSOs). Such emission appears common in other Galactic Giant H II
regions we have surveyed. The IMF slope of the cluster, Gamma = -1.51, is
consistent with Salpeter's distribution and similar to what has been observed
in the Magellanic Cloud clusters and in the periphery of our Galaxy.Comment: 14 pages, 11 figures, accepted for publication in A
ISOCAM observations of the L1551 star formation region
The results of a deep mid-IR ISOCAM survey of the L1551 dark molecular cloud
are presented. The aim of this survey is a search for new YSO (Young Stellar
Object) candidates, using two broad-band filters centred at 6.7 and 14.3
micron. Although two regions close to the centre of L1551 had to be avoided due
to saturation problems, 96 sources were detected in total (76 sources at 6.7
micron and 44 sources at 14.3 micron). Using the 24 sources detected in both
filters, 14 were found to have intrinsic mid-IR excess at 14.3 micron and were
therefore classified as YSO candidates. Using additional observations in B, V,
I, J, H and K obtained from the ground, most candidates detected at these
wavelengths were confirmed to have mid-IR excess at 6.7 micron as well, and
three additional YSO candidates were found. Prior to this survey only three
YSOs were known in the observed region (avoiding L1551 IRS5/NE and HL/XZ Tau).
This survey reveals 15 new YSO candidates, although several of these are
uncertain due to their extended nature either in the mid-IR or in the
optical/near-IR observations. Two of the sources with mid-IR excess are
previously known YSOs, one is a brown dwarf MHO 5 and the other is the well
known T Tauri star HH30, consisting of an outflow and an optically thick disk
seen edge on.Comment: 14 Pages, 8 Figure
The IC2118 association: new T Tauri stars in high-latitude molecular clouds
We identified new pre-main sequence stars in the region of high-latitude
molecular clouds associated with the reflection nebula IC2118, around l = 208
degr and b = -27 degr. The stars were selected as T Tauri candidates in
objective prism plates obtained with the Schmidt telescope of Konkoly
Observatory. Results of spectroscopic follow-up observations, carried out with
the FLAIR spectrograph installed on the UK Schmidt and with ALFOSC on Nordic
Optical Telescope, are presented in this paper. Based on spectral types,
presence of emission lines and lithium absorption line, we identified five
classical T Tauri stars and a candidate weak-line T Tauri star projected on the
molecular clouds, as well as two candidate pre-main sequence stars outside the
nebulous region. Using the near infrared magnitudes obtained from the 2MASS All
Sky Catalog. we determined the masses and ages of these stars. We found that
the five classical T Tauri stars projected on the clouds are physically related
to them, whereas the other stars are probably background objects. Adopting a
distance of 210 pc for IC2118 (Kun et al. 2001) and using Palla & Stahler's
(1999) evolutionary tracks we derived an average age of 2.5 million yrs and a
mass interval of 0.4--1.0 M_sun for the members of the IC2118 association.Comment: 11 pages, 6 figures, accepted for publication by Astronomy and
Astrophysic
Observations of Massive Star Forming Regions with Water Masers: Mid-Infrared Imaging
We present here a mid-infrared imaging survey of 26 sites of water maser
emission. Observations were obtained at the InfraRed Telescope Facility 3-m
telescope with the University of Florida mid-infrared imager/spectrometer
OSCIR, and the JPL mid-infrared camera MIRLIN. The main purpose of the survey
was to explore the relationship between water masers and the massive star
formation process. It is generally believed that water masers predominantly
trace outflows and embedded massive stellar objects, but may also exist in
circumstellar disks around young stars. We investigate each of these
possibilities in light of our mid-infrared imaging. We find that mid-infrared
emission seems to be more closely associated with water and OH maser emission
than cm radio continuum emission from UC HII regions. We also find from the
sample of sources in our survey that, like groups of methanol masers, both
water and OH masers have a proclivity for grouping into linear or elongated
distributions. We conclude that the vast majority of linearly distributed
masers are not tracing circumstellar disks, but outflows and shocks instead.Comment: 49 pages; 23 figures; To appear in February 2005 ApJS; To download a
version with better quality figures, go to
http://www.ctio.noao.edu/~debuizer
Spectroscopic Confirmation of the Least Massive Known Brown Dwarf in Chamaeleon
We present spectroscopy of two candidate substellar members of the Chamaeleon
I star-forming region. The candidates, which were identified photometrically by
Oasa, Tamura, & Sugitani, have been observed at 1-2.5 micron during
commissioning of the Gemini Near-Infrared Spectrograph. The late-type nature of
one of the candidates, OTS 44, is confirmed through the detection of strong
steam absorption bands. The other object, OTS 7, exhibits no late-type features
and is likely a background star or galaxy. The gravity-sensitive shape of the
H- and K-band continua demonstrate that OTS 44 is a young, pre-main-sequence
object rather than a field dwarf. We measure a spectral type of M9.5 for OTS 44
based on a comparison of its spectrum to data for optically-classified young
late-type objects. Because OTS 44 is the coolest and faintest object with
confirmed membership in Chamaeleon I, it is very likely the least massive known
member of the cluster. By comparing the position of OTS 44 on the H-R diagram
to the evolutionary models of Chabrier & Baraffe, we infer a mass of ~0.015
M_sun. Although this estimate is uncertain by at least a factor of two, OTS 44
is nevertheless one of the least massive free-floating brown dwarfs confirmed
spectroscopically to date.Comment: 10 pages, The Astrophysical Journal, 2004, v617 (December 10
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