190 research outputs found
Detection of H2 pure rotational line emission from the GG~Tau binary system
We present the first detection of the low-lying pure rotational emission
lines of H2 from circumstellar disks around T~Tauri stars, using the Short
Wavelength Spectrometer on the Infrared Space Observatory. These lines provide
a direct measure of the total amount of warm molecular gas in disks. The J=2->0
S(0) line at 28.218 mum and the J=3->1 S(1) line at 17.035 mum have been
observed toward the double binary system GG Tau. Together with limits on the
J=5->3 S(3) and J=7->5 S(5) lines, the data suggest the presence of gas at
T_kin=110+-10 K with a mass of (3.6+-2.0)x10^-3 M_sol (3sigma). This amounts to
~3% of the total gas + dust mass of the circumbinary disk as imaged by
millimeter interferometry, but is larger than the estimated mass of the
circumstellar disk(s). Possible origins for the warm gas seen in H2 are
discussed in terms of photon and wind-shock heating mechanisms of the
circumbinary material, and comparisons with model calculations are made.Comment: 14 pages including 1 figure. To appear in Astrophysical Journal
Letter
Shapes of Molecular Cloud Cores and the Filamentary Mode of Star Formation
Using recent dust continuum data, we generate the intrinsic ellipticity
distribution of dense, starless molecular cloud cores. Under the hypothesis
that the cores are all either oblate or prolate randomly-oriented spheroids, we
show that a satisfactory fit to observations can be obtained with a gaussian
prolate distribution having a mean intrinsic axis ratio of 0.54. Further, we
show that correlations exist between the apparent axis ratio and both the peak
intensity and total flux density of emission from the cores, the sign of which
again favours the prolate hypothesis. The latter result shows that the mass of
a given core depends on its intrinsic ellipticity. Monte Carlo simulations are
performed to find the best-fit power law of this dependence. Finally, we show
how these results are consistent with an evolutionary scenario leading from
filamentary parent clouds to increasingly massive, condensed, and roughly
spherical embedded cores.Comment: 16 pages, incl. 11 Postscript figures. Accepted by Ap
Gas Dynamics in the Luminous Merger NGC 6240
We report 0.5"x0.9" resolution, interferometric observations of the 1.3 mm CO
J=2-1 line in the infrared luminous galactic merger NGC 6240. About half of the
CO flux is concentrated in a rotating but highly turbulent, thick disk
structure centered between the two radio and near-infrared nuclei. A number of
gas features connect this ~500 pc diameter central disk to larger scales.
Throughout this region the molecular gas has local velocity widths which exceed
300 km/s FWHM and even reach FWZP line widths of 1000 km/s in a number of
directions. The mass of the central gas concentration constitutes a significant
fraction of the dynamical mass, M_gas(R<470 pc) ~ 2-4x10^9 M_o ~ 0.3-0.7 M_dyn.
We conclude that NGC 6240 is in an earlier merging stage than the prototypical
ultraluminous galaxy, Arp 220. The interstellar gas in NGC 6240 is in the
process of settling between the two progenitor stellar nuclei, is dissipating
rapidly and will likely form a central thin disk. In the next merger stage, NGC
6240 may well experience a major starburst like that observed in Arp 220.Comment: To be published in Ap.J.; 7 figure
Massive Protoplanetary Disks in the Trapezium Region
(abridged) We determine the disk mass distribution around 336 stars in the
young Orion Nebula cluster by imaging a 2.5' x 2.5' region in 3 mm continuum
emission with the Owens Valley Millimeter Array. For this sample of 336 stars,
we observe 3 mm emission above the 3-sigma noise level toward ten sources, six
of which have also been detected optically in silhouette against the bright
nebular background. In addition, we detect 20 objects that do not correspond to
known near-IR cluster members. Comparisons of our measured fluxes with longer
wavelength observations enable rough separation of dust emission from thermal
free-free emission, and we find substantial dust emission toward most objects.
For the ten objects detected at both 3 mm and near-IR wavelengths, eight
exhibit substantial dust emission. Excluding the high-mass stars and assuming a
gas-to-dust ratio of 100, we estimate circumstellar masses ranging from 0.13 to
0.39 Msun. For the cluster members not detected at 3 mm, images of individual
objects are stacked to constrain the mean 3 mm flux of the ensemble. The
average flux is detected at the 3-sigma confidence level, and implies an
average disk mass of 0.005 Msun, comparable to the minimum mass solar nebula.
The percentage of stars in Orion surrounded by disks more massive than ~0.1
Msun is consistent with the disk mass distribution in Taurus, and we argue that
massive disks in Orion do not appear to be truncated through close encounters
with high-mass stars. Comparison of the average disk mass and number of massive
dusty structures in Orion with similar surveys of the NGC 2024 and IC 348
clusters constrains the evolutionary timescales of massive circumstellar disks
in clustered environments.Comment: 27 pages, including 7 figures. Accepted by Ap
Resolving Molecular Line Emission from Protoplanetary Disks: Observational Prospects for Disks Irradiated by Infalling Envelopes
Molecular line observations that could resolve protoplanetary disks of ~100
AU both spatially and kinematically would be a useful tool to unambiguously
identify these disks and to determine their kinematical and physical
characteristics. In this work we model the expected line emission from a
protoplanetary disk irradiated by an infalling envelope, addressing the
question of its detectability with subarcsecond resolution. We adopt a
previously determined disk model structure that gives a continuum spectral
energy distribution and a mm intensity spatial distribution that are consistent
with observational constraints of HL Tau. An analysis of the capability of
presently working and projected interferometers at mm and submm wavelengths
shows that molecular transitions of moderate opacity at these wavelengths
(e.g., C17O lines) are good candidates for detecting disk lines at subarcsecond
resolution in the near future. We suggest that, in general, disks of typical
Class I sources will be detectable.Comment: 41 pages, 16 figures. To be published in The Astrophysical Journa
Tracing the envelopes around embedded low-mass young stellar objects with HCO+ and millimeter-continuum observations
Interferometer observations of millimeter-continuum (OVRO) and single-dish
observations of HCO+ and H13CO+ J=1-0, 3-2, and 4-3 (JCMT, IRAM 30m) are
presented of nine embedded low-mass young stellar objects (YSOs) in Taurus. All
nine objects are detected at 3.4 and 2.7 mm, with fluxes of 4-200 mJy, and
consist of unresolved (<3 arcsec) point sources, plus, toward about half of the
objects, an extended envelope. The point sources likely are circumstellar
disks, showing that these are established early in the embedded phase.
Literature values of 1.1 mm continuum emission are used to trace the envelopes,
carrying 0.001-0.26 M(sol). In HCO+, the 1-0 lines trace the surrounding
clouds, while the 3-2 and 4-3 are concentrated toward the sources with
intensities well correlated with the envelope flux. An HCO+/H2 abundance of
1.2e-8 is derived. The HCO+ line strengths and envelope fluxes can be fit
simultaneously with the simple collapse model of Shu (1977), and related
density power laws with slopes p=1-3. As an indicator of the relative
evolutionary phase of a YSO, the ratio of HCO+ 3-2 line intensity over
bolometric luminosity is proposed, which is roughly proportional to the current
ratio of envelope over stellar mass. It is concluded that HCO+ 3-2 and 4-3 are
excellent tracers of the early embedded phase of star formation.Comment: 45 pages, 10 figures, ApJ/AASLaTeX. To be published in The
Astrophysical Journa
Warm Molecular Layers in Protoplanetary Disks
We have investigated molecular distributions in protoplanetary disks,
adopting a disk model with a temperature gradient in the vertical direction.
The model produces sufficiently high abundances of gaseous CO and HCO+ to
account for line observations of T Tauri stars using a sticking probability of
unity and without assuming any non-thermal desorption. In regions of radius R >
10 AU, with which we are concerned, the temperature increases with increasing
height from the midplane. In a warm intermediate layer, there are significant
amounts of gaseous molecules owing to thermal desorption and efficient
shielding of ultraviolet radiation by the flared disk. The column densities of
HCN, CN, CS, H2CO, HNC and HCO+ obtained from our model are in good agreement
with the observations of DM Tau, but are smaller than those of LkCa15.
Molecular line profiles from our disk models are calculated using a
2-dimensional non-local-thermal-equilibrium (NLTE) molecular-line radiative
transfer code for a direct comparison with observations. Deuterated species are
included in our chemical model. The molecular D/H ratios in the model are in
reasonable agreement with those observed in protoplanetary disks.Comment: 11 pages, Latex (aa.cls), to be published in Astronomy and
Astrophysic
Time aspects of the European Complement to GPS: Continental and transatlantic experimental phases
The CNES project of a European Complement to GPS (CE-GPS) is conceived to fulfill the needs of Civil Aviation for a non-precise approach phase with GPS as sole navigation means. This generates two missions: a monitoring mission - alarm of failure - ,and a navigation mission - generating a GPS-like signal on board the geostationary satellites. The host satellites will be the Inmarsat constellation. The CE-GPS missions lead to some time requirements, mainly the accuracy of GPS time restitution and of monitoring clock synchronization. To demonstrate that the requirements of the CE-GPS could be achieved, including the time aspects, an experiment has been scheduled over the Last two years, using a part of the Inmarsat II F-2 payload and specially designed ground stations based on 10 channels GPS receivers. This paper presents a review of the results obtained during the continental phase of the CE-GPS experiment with two stations in France, along with some experimental results obtained during the transatlantic phase (three stations in France, French Guyana, and South Africa). It describes the synchronization of the monitoring clocks using the GPS Common-view or the C- to L-Band transponder of the Inmarsat satellite, with an estimated accuracy better than 10 ns (1 sigma)
SiO Emission in the Multi-Lobe Outflow associated with IRAS 16293-2422
We have mapped the thermal emission line of SiO (v = 0; J = 2-1) associated
with the quadrupolar molecular outflow driven by the very cold far-infrared
source IRAS 16293-2422. The SiO emission is significantly enhanced in the
northeastern red lobe and at the position ~50" east of the IRAS source. Strong
SiO emission observed at ~50" east of the IRAS source presents evidence for a
dynamical interaction between a part of the eastern blue lobe and the dense
ambient gas condensation, however, such an interaction is unlikely to be
responsible for producing the quadrupolar morphology. The SiO emission in the
northeastern red lobe shows the spatial and velocity structure similar to those
of the CO outflow, suggesting that the SiO emission comes from the molecular
outflow in the northeastern red lobe itself. The observed velocity structure is
reproduced by a simple spatio-kinematic model of bow shock with a shock
velocity of 19-24 km/s inclined by 30-45 deg from the plane of the sky. This
implies that the northeastern red lobe is independent of the eastern blue lobe
and that the quadrupolar structure is due to two separate bipolar outflows.
The SiO emission observed in the western red lobe has a broad pedestal shape
with low intensity. Unlike the SiO emission in the northeastern red lobe, the
spatial extent of the SiO emission in the western red lobe is restricted to its
central region. The spatial and velocity structures and the line profiles
suggest that three different types of the SiO emission are observed in this
outflow; the SiO emission arises from the interface between the outflowing gas
and the dense ambient gas clump, the SiO emission coming from the outflow lobe
itself, and the broad SiO emission with low intensity observed at the central
region of the outflow lobe.Comment: 14 pages, 6 figures (figures 1 and 4 are color), gzipped tar file, To
appear in the Ap
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
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