1,341 research outputs found
Sensitive Limits on the Water Abundance in Cold Low Mass Molecular Cores
We present SWAS observations of water vapor in two cold star-less clouds, B68
and Core D in rho Ophiuchus. Sensitive non-detections of the 1(10)-1(01)
transition of o-H2O are reported for each source. Both molecular cores have
been previously examined by detailed observations that have characterized the
physical structure. Using these rather well defined physical properties and a
Monte-Carlo radiation transfer model we have removed one of the largest
uncertainties from the abundance calculation and set the lowest water abundance
limit to date in cold low-mass molecular cores. These limits are < 3 x 10^{-8}
(relative to H2) and < 8 x 10^{-9} in B68 and rho Oph D, respectively. Such low
abundances confirm the general lack of ortho-water vapor in cold (T < 20 K)
cores. Provided that the ortho/para ratio of water is not near zero, these
limits are well below theoretical predictions and appear to support the
suggestion that most of the water in dense low-mass cores is frozen onto the
surfaces of cold dust grains.Comment: 12 pages, 3 figures, accepted by Astrophysical Journal Letter
Current Star Formation in the Ophiuchus and Perseus Molecular Clouds: Constraints and Comparisons from Unbiased Submillimeter and Mid-Infrared Surveys. II
We present a census of the population of deeply embedded young stellar
objects (YSOs) in the Ophiuchus molecular cloud complex based on a combination
of Spitzer Space Telescope mid-infrared data from the "Cores to Disks" (c2d)
legacy team and JCMT/SCUBA submillimeter maps from the COMPLETE team. We have
applied a method developed for identifying embedded protostars in Perseus to
these datasets and in this way construct a relatively unbiased sample of 27
candidate embedded protostars with envelopes more massive than our sensitivity
limit (about 0.1 M_sun). Embedded YSOs are found in 35% of the SCUBA cores -
less than in Perseus (58%). On the other hand the mid-infrared sources in
Ophiuchus have less red mid-infrared colors, possibly indicating that they are
less embedded. We apply a nearest neighbor surface density algorithm to define
the substructure in each of the clouds and calculate characteristic numbers for
each subregion - including masses, star formation efficiencies, fraction of
embedded sources etc. Generally the main clusters in Ophiuchus and Perseus
(L1688, NGC1333 and IC348) are found to have higher star formation efficiencies
than small groups such as B1, L1455 and L1448, which on the other hand are
completely dominated by deeply embedded protostars. We discuss possible
explanations for the differences between the regions in Perseus and Ophiuchus,
such as different evolutionary timescales for the YSOs or differences, e.g., in
the accretion in the two clouds.Comment: Accepted for publication in ApJ (56 pages, 13 figures; abstract
abridged). Version with full-resolution figures available at
http://www.astro.uni-bonn.de/~jes/paper120.pd
Hot Organic Molecules Toward a Young Low-Mass Star: A Look at Inner Disk Chemistry
Spitzer Space Telescope spectra of the low mass young stellar object (YSO)
IRS 46 (L_bol ~ 0.6 L_sun) in Ophiuchus reveal strong vibration-rotation
absorption bands of gaseous C2H2, HCN, and CO2. This is the only source out of
a sample of ~100 YSO's that shows these features and the first time they are
seen in the spectrum of a solar-mass YSO. Analysis of the Spitzer data combined
with Keck L- and M-band spectra gives excitation temperatures of > 350 K and
abundances of 10(-6)-10(-5) with respect to H2, orders of magnitude higher than
those found in cold clouds. In spite of this high abundance, the HCN J=4-3 line
is barely detected with the James Clerk Maxwell Telescope, indicating a source
diameter less than 13 AU. The (sub)millimeter continuum emission and the
absence of scattered light in near-infrared images limits the mass and
temperature of any remnant collapse envelope to less than 0.01 M_sun and 100 K,
respectively. This excludes a hot-core type region as found in high-mass YSO's.
The most plausible origin of this hot gas rich in organic molecules is in the
inner (<6 AU radius) region of the disk around IRS 46, either the disk itself
or a disk wind. A nearly edge-on 2-D disk model fits the spectral energy
distribution (SED) and gives a column of dense warm gas along the line of sight
that is consistent with the absorption data. These data illustrate the unique
potential of high-resolution infrared spectroscopy to probe organic chemistry,
gas temperatures and kinematics in the planet-forming zones close to a young
star.Comment: 4 pages, 4 figures; To appear in Astrophysical Journal Letter
Quiescent Dense Gas in Protostellar Clusters: the Ophiuchus A Core
We present combined BIMA interferometer and IRAM 30 m Telescope data of N2H+
1-0 line emission across the nearby dense, star forming core Ophiuchus A (Oph
A) at high linear resolution (e.g., ~1000 AU). Six maxima of integrated line
intensity are detected which we designate Oph A-N1 through N6. The N4 and N5
maxima are coincident with the starless continuum objects SM1 and SM2
respectively but the other maxima are not coincident with previously-identified
objects. In contrast, relatively little N2H+ 1-0 emission is coincident with
the starless object SM2 and the Class 0 protostar VLA 1623. The FWHM of the
N2H+ 1-0 line, Delta V, varies by a factor of ~5 across Oph A. Values of Delta
V < 0.3 km/s are found in 14 locations in Oph A, but only that associated with
N6 is both well-defined spatially and larger than the beam size. Centroid
velocities of the line, V_LSR, vary relatively little, having an rms of only
\~0.17 km/s. Small-scale V_LSR gradients of <0.5 km/s over ~0.01 pc are found
near SM1, SM1N, and SM2, but not N6. The low N2H+ abundances of SM2 or VLA 1623
relative to SM1, SM1N, or N6 may reflect relatively greater amounts of N2
adsorption onto dust grains in their colder and probably denser interiors. The
low Delta V of N6, i.e., 0.193 km/s FWHM, is only marginally larger than the
FWHM expected from thermal motions alone, suggesting turbulent motions in the
Oph A core have been reduced dramatically at this location. The non-detection
of N6 in previous thermal continuum maps suggests that interesting sites
possibly related to star formation may be overlooked in such data.Comment: LaTex with 7 figures, produces 36 pages. Accepted for publication in
ApJ. Typo related to Equation 3 fixed, caused derived values of N(N2H+) and
X(N2H+) to be low by factors of ~40%. Conclusions of paper are unchange
Triggered Star Formation by Massive Stars
We present our diagnosis of the role that massive stars play in the formation
of low- and intermediate-mass stars in OB associations (the Lambda Ori region,
Ori OB1, and Lac OB1 associations). We find that the classical T Tauri stars
and Herbig Ae/Be stars tend to line up between luminous O stars and
bright-rimmed or comet-shaped clouds; the closer to a cloud the progressively
younger they are. Our positional and chronological study lends support to the
validity of the radiation-driven implosion mechanism, where the Lyman continuum
photons from a luminous O star create expanding ionization fronts to evaporate
and compress nearby clouds into bright-rimmed or comet-shaped clouds. Implosive
pressure then causes dense clumps to collapse, prompting the formation of
low-mass stars on the cloud surface (i.e., the bright rim) and
intermediate-mass stars somewhat deeper in the cloud. These stars are a
signpost of current star formation; no young stars are seen leading the
ionization fronts further into the cloud. Young stars in bright-rimmed or
comet-shaped clouds are likely to have been formed by triggering, which would
result in an age spread of several megayears between the member stars or star
groups formed in the sequence.Comment: 2007, ApJ, 657, 88
The CO Molecular Outflows of IRAS 16293-2422 Probed by the Submillimeter Array
We have mapped the proto-binary source IRAS 16293-2422 in CO 2-1, 13CO 2-1,
and CO 3-2 with the Submillimeter Array (SMA). The maps with resolution of
1".5-5" reveal a single small scale (~3000 AU) bipolar molecular outflow along
the east-west direction. We found that the blueshifted emission of this small
scale outflow mainly extends to the east and the redshifted emission to the
west from the position of IRAS 16293A. A comparison with the morphology of the
large scale outflows previously observed by single-dish telescopes at
millimeter wavelengths suggests that the small scale outflow may be the inner
part of the large scale (~15000 AU) E-W outflow. On the other hand, there is no
clear counterpart of the large scale NE-SW outflow in our SMA maps. Comparing
analytical models to the data suggests that the morphology and kinematics of
the small scale outflow can be explained by a wide-angle wind with an
inclination angle of ~30-40 degrees with respect to the plane of the sky. The
high resolution CO maps show that there are two compact, bright spots in the
blueshifted velocity range. An LVG analysis shows that the one located 1" to
the east of source A is extremely dense, n(H_2)~10^7 cm^-3, and warm, T_kin >55
K. The other one located 1" southeast of source B has a higher temperature of
T_kin >65 K but slightly lower density of n(H_2)~10^6 cm^-3. It is likely that
these bright spots are associated with the hot core-like emission observed
toward IRAS 16293. Since both two bright spots are blueshifted from the
systemic velocity and are offset from the protostellar positions, they are
likely formed by shocks.Comment: 27 pages, 8 figures, accepted for publication in ApJ, minor typos
correcte
Cloud Structure and Physical Conditions in Star-Forming Regions from Optical Observations. II. Analysis
To complement the optical absorption-line survey of diffuse molecular gas in
Paper I, we obtained and analyzed far ultraviolet H and CO data on lines of
sight toward stars in Cep OB2 and Cep OB3. Possible correlations between column
densities of different species for individual velocity components, not total
columns along a line of sight as in the past, were examined and were
interpreted in terms of cloud structure. The analysis reveals that there are
two kinds of CH in diffuse molecular gas: CN-like CH and CH-like CH.
Evidence is provided that CO is also associated with CN in diffuse molecular
clouds. Different species are distributed according to gas density in the
diffuse molecular gas. Both calcium and potassium may be depleted onto grains
in high density gas, but with different dependences on local gas density. Gas
densities for components where CN was detected were inferred from a chemical
model. Analysis of cloud structure indicates that our data are generally
consistent with the large-scale structure suggested by maps of CO
millimeter-wave emission. On small scales, the gas density is seen to vary by
factors greater than 5.0 over scales of 10,000 AU. The relationships
between column densities of CO and CH with that of H along a line of sight
show similar slopes for the gas toward Cep OB2 and OB3, but the CO/H and
CH/H ratios tend to differ which we ascribe to variation in average density
along the line of sight.Comment: 49 pages, 9 figures, accepted by Ap
Photometric Accretion Signatures Near the Substellar Boundary
Multi-epoch imaging of the Orion equatorial region by the Sloan Digital Sky
Survey has revealed that significant variability in the blue continuum persists
into the late-M spectral types, indicating that magnetospheric accretion
processes occur below the substellar boundary in the Orion OB1 association. We
investigate the strength of the accretion-related continuum veiling by
comparing the reddening-invariant colors of the most highly variable stars
against those of main sequence M dwarfs and evolutionary models. A gradual
decrease in the g band veiling is seen for the cooler and less massive members,
as expected for a declining accretion rate with decreasing mass. We also see
evidence that the temperature of the accretion shock decreases in the very low
mass regime, reflecting a reduction in the energy flux carried by the accretion
columns. We find that the near-IR excess attributed to circumstellar disk
thermal emission drops rapidly for spectral types later than M4. This is likely
due to the decrease in color contrast between the disk and the cooler stellar
photosphere. Since accretion, which requires a substantial stellar magnetic
field and the presence of a circumstellar disk, is inferred for masses down to
0.05 Msol we surmise that brown dwarfs and low mass stars share a common mode
of formation.Comment: 37 pages, 14 figures, accepted by A
Large Silicon Abundance in Photodissociation Regions
We have made one-dimensional raster-scan observations of the rho Oph and
sigma Sco star-forming regions with two spectrometers (SWS and LWS) on board
the ISO. In the rho Oph region, [SiII] 35um, [OI] 63um, 146um, [CII] 158um, and
the H2 pure rotational transition lines S(0) to S(3) are detected, and the PDR
properties are derived as the radiation field scaled by the solar neighborhood
value G_0~30-500, the gas density n~250--2500 /cc, and the surface temperature
T~100-400 K. The ratio of [SiII] 35um to [OI] 146um indicates that silicon of
10--20% of the solar abundance must be in the gaseous form in the
photodissociation region (PDR), suggesting that efficient dust destruction is
undergoing even in the PDR and that part of silicon atoms may be contained in
volatile forms in dust grains. The [OI] 63um and [CII] 158um emissions are too
weak relative to [OI] 146um to be accounted for by standard PDR models. We
propose a simple model, in which overlapping PDR clouds along the line of sight
absorb the [OI] 63um and [CII] 158um emissions, and show that the proposed
model reproduces the observed line intensities fairly well. In the sigma Sco
region, we have detected 3 fine-structure lines, [OI] 63um, [NII] 122um, and
[CII] 158um, and derived that 30-80% of the [CII] emission comes from the
ionized gas. The upper limit of the [SiII] 35um is compatible with the solar
abundance relative to nitrogen and no useful constraint on the gaseous Si is
obtained for the sigma Sco region.Comment: 25 pages with 7 figures, accepted in Astrophysical Journa
Tracing the Mass during Low-Mass Star Formation. II. Modelling the Submillimeter Emission from Pre-Protostellar Cores
We have modeled the emission from dust in pre-protostellar cores, including a
self-consistent calculation of the temperature distribution for each input
density distribution. Model density distributions include Bonnor-Ebert spheres
and power laws. The Bonnor-Ebert spheres fit the data well for all three cores
we have modeled. The dust temperatures decline to very low values (\Td \sim 7
K) in the centers of these cores, strongly affecting the dust emission.
Compared to earlier models that assume constant dust temperatures, our models
indicate higher central densities and smaller regions of relatively constant
density. Indeed, for L1544, a power-law density distribution, similar to that
of a singular, isothermal sphere, cannot be ruled out. For the three sources
modeled herein, there seems to be a sequence of increasing central
condensation, from L1512 to L1689B to L1544. The two denser cores, L1689B and
L1544, have spectroscopic evidence for contraction, suggesting an evolutionary
sequence for pre-protostellar cores.Comment: 22 pages, 9 figures, Ap. J. accepted, uses emulateapj5.st
- …