107 research outputs found
Deuterated molecules in DM Tau: DCO+, but no HDO
We report the detection of the J=2-1 line of DCO+ in the proto-planetary disk
of DM Tau and re-analyze the spectrum covering the 465 GHz transition of HDO in
this source, recently published by Ceccarelli et al. (2005). A modelling of the
DCO+ line profile with the source parameters derived from high resolution HCO+
observations yields a DCO+/HCO+ abundance ratio of about 0.004, an order of
magnitude smaller than that derived in the low mass cores. The re-analysis of
the 465 GHz spectrum, using the proper continuum flux (0.5 Jy) and source
systemic velocity (6.05 km/s), makes it clear that the absorption features
attributed to HDO and C6H are almost certainly unrelated to these species. We
show that the line-to-continuum ratio of an absorption line in front of a
Keplerian disk can hardly exceed the ratio of the turbulent velocity to the
projected rotation velocity at the disk edge, unless the line is optically very
thick (tau > 10 000). This ratio is typically 0.1-0.3 in proto-planetary disks
and is about 0.15 in DM Tau, much smaller than that for the alleged absorption
features. We also show that the detection of H2D+ in DM Tau, previously
reported by these authors, is only a 2-sigma detection when the proper velocity
is adopted. So far, DCO+ is thus the only deuterated molecule clearly detected
in proto-planetary disks
Dust continuum and Polarization from Envelope to Cores in Star Formation: A Case Study in the W51 North region
We present the first high-angular resolution (up to 0.7", ~5000 AU)
polarization and thermal dust continuum images toward the massive star-forming
region W51 North. The observations were carried out with the Submillimeter
Array (SMA) in both the subcompact (SMA-SubC) and extended (SMA-Ext)
configurations at a wavelength of 870 micron. W51 North is resolved into four
cores (SMA1 to SMA4) in the 870 micron continuum image. The associated dust
polarization exhibits more complex structures than seen at lower angular
resolution. We analyze the inferred morphologies of the plane-of-sky magnetic
field (B_bot) in the SMA1 to SMA4 cores and in the envelope using the SMA-Ext
and SMA-SubC data. These results are compared with the B_bot archive images
obtained from the CSO and JCMT. A correlation between dust intensity gradient
position angles (phi_{nabla I}) and magnetic field position angles (phi_B) is
found in the CSO, JCMT and both SMA data sets. This correlation is further
analyzed quantitatively. A systematically tighter correlation between
phi_{nabla I} and phi_B is found in the cores, whereas the correlation
decreases in outside-core regions. Magnetic field-to-gravity force ratio
(Sigma_B) maps are derived using the newly developed polarization - intensity
gradient method by Koch, Tang & Ho 2012. We find that the force ratios tend to
be small (Sigma_B <= 0.5) in the cores in all 4 data sets. In regions outside
of the cores, the ratios increase or the field is even dominating gravity
(Sigma_B > 1). This possibly provides a physical explanation of the tightening
correlation between phi_{nabla I} and phi_B in the cores: the more the B field
lines are dragged and aligned by gravity, the tighter the correlation is.
Finally, we propose a schematic scenario for the magnetic field in W51 North to
interpret the four polarization observations at different physical scales.Comment: Accepted for publication in ApJ. 10 pages. 7 figure
The circumstellar disk of AB Aurigae: evidence for envelope accretion at late stages of star formation?
The circumstellar disk of AB Aurigae has garnered strong attention owing to
the apparent existence of spirals at a relatively young stage and also the
asymmetric disk traced in thermal dust emission. However, the physical
conditions of the spirals are still not well understood. The origin of the
asymmetric thermal emission is unclear.
We observed the disk at 230 GHz (1.3 mm) in both the continuum and the
spectral line ^12CO J=2-1 with IRAM 30-m, the Plateau de Bure interferometer,
and the Submillimeter Array to sample all spatial scales from 0.37" to about
50". To combine the data obtained from these telescopes, several methods and
calibration issues were checked and discussed.
The 1.3 mm continuum (dust) emission is resolved into inner disk and outer
ring. Molecular gas at high velocities traced by the CO line is detected next
to the stellar location. The inclination angle of the disk is found to decrease
toward the center. On a larger scale, based on the intensity weighted
dispersion and the integrated intensity map of ^12CO J=2-1, four spirals are
identified, where two of them are also detected in the near infrared. The total
gas mass of the 4 spirals (M_spiral) is 10^-7 < M_spiral < 10^-5 M_sun, which
is 3 orders of magnitude smaller than the mass of the gas ring. Surprisingly,
the CO gas inside the spiral is apparently counter-rotating with respect to the
CO disk, and it only exhibits small radial motion.
The wide gap, the warped disk, and the asymmetric dust ring suggest that
there is an undetected companion with a mass of 0.03 M_sun at a radius of 45
AU. Although an hypothetical fly-by cannot be ruled out, the most likely
explanation of the AB Aurigae system may be inhomogeneous accretion well above
or below the main disk plane from the remnant envelope, which can explain both
the rotation and large-scale motions detected with the 30-m image.Comment: 17 pages, 13 figures, accepted for publication in A&A journal. Typos
are correcte
Plateau de Bure Interferometer Observations of the Disk and Outflow of HH30
HH30 is a well-known Pre-Main-Sequence star in Taurus. HST observations have
revealed a flared, edge-on disk driving a highly-collimated optical jet, making
this object a case study for the disk-jet-outflow paradigm. We obtained high
angular resolution (about 1") observations of the dust continuum at 2.7 and 1.3
mm, and of the 12CO(2-1), 13CO(2-1) & (1-0), C18O(1-0) emissions around HH30. A
standard disk model is used to fit the 13CO(2-1) uv-plane visibilities and
derive the disk properties, and the stellar mass. It results that HH30 is a low
mass TTauri of spectral type around M1 and age 1 to 4 Myrs, surrounded by a
medium size Keplerian disk, of mass around 4e-3 Msun and outer radius 420 AU.
The disk rotation vector points toward the North-Eastern jet. Using a distance
of 140 pc, we deduce a stellar mass of 0.45 Msun. A highly asymmetric outflow
originates from the inner parts of the disk. It presents to first order a
conical morphology with a 30 degree half opening angle and a constant (12 km/s)
radial velocity field. Outflow rotation was searched for but not found. These
observations do not enable to assign the origin of the molecular outflow to
entrainment by the optical jet or to a disk wind. In the latter case, the upper
limit of the outflow rotation velocity implies an origin in the inner 15 AU of
the disk.Comment: 20 pages, 15 PostScript figures. Accepted for publication in
Astronomy & Astrophysics. Uses aa LaTeX macro
Chemistry in Disks. IX. Observations and modeling of HCO+ and DCO+ in DM Tau
We present resolved Plateau de Bure Array observations of DM Tau in lines of
HCO+ (3-2), (1-0) and DCO+ (3-2). A power-law fitting approach allowed a
derivation of column densities of these two molecules. A chemical inner hole of
~50 AU was found in both HCO+ and DCO+ with DCO+ emission extending to only 450
AU. An isotopic ratio of R_D = N(DCO+) / N(HCO+) was found to range from 0.1 at
50 AU and 0.2 at 450 AU. Chemical modeling allowed an exploration of the
sensitivity of these molecular abundances to physical parameters out with
temperature, finding that X-rays were the domination ionization source in the
HCO+ molecular region and that R_D also is sensitive to the CO depletion. The
ionization fraction, assuming a steady state system, was found to be x(e-) ~
10. Modeling suggests that HCO+ is the dominant charged molecule in the
disk but its contribution to ionization fraction is dwarfed by atmoic ions such
as C+, S+ and H+.Comment: 13 pages with 8 figures, to be published in A&A, accepted 29/12/1
Resolving the inner dust disks surrounding LkCa 15 and MWC 480 at mm wavelengths
International audienceWe performed sub-arcsecond high-sensitivity nterferometric observations of the thermal dust emission at 1.4 mm and 2.8 mm in the disks surrounding LkCa 15 and MWC 480, with the new 750 m baselines of the IRAM PdBI array. This provides a linear resolution of about 60 AU at the Taurus distance. We report the existence of a cavity of about 50 AU radius in the inner disk of LkCa 15. Whereas LkCa 15 emission is optically thin, the optically thick core of MWC 480 is resolved at 1.4 mm with a radius of about 35 AU, constraining the dust temperature. In MWC 480, the dust emission is coming from a colder layer than the CO emission, most likely the disk mid-plane. These observations provide direct evidence of an inner cavity around LkCa 15. Such a cavity most probably results from the tidal disturbance created by a low mass companion or large planet at about 30 AU from the star. These results suggest that planetary system formation is already at work in LkCa 15. They also indicate that the classical steady-state viscous disk model is a too simplistic description of the inner 50 AU of ''proto-planetary'' disks, and that the disk evolution is coupled to the planet formation process. The MWC 480 results indicate that a proper estimate of the dust temperature and size of the optically thick core are essential to determine the dust emissivity index
Circumbinary Ring, Circumstellar disks and accretion in the binary system UY Aurigae
Recent exo-planetary surveys reveal that planets can orbit and survive around
binary stars. This suggests that some fraction of young binary systems which
possess massive circumbinary disks (CB) may be in the midst of planet
formation. However, there are very few CB disks detected. We revisit one of the
known CB disks, the UY Aurigae system, and probe 13CO 2-1, C18O 2-1, SO
5(6)-4(5) and 12CO 3-2 line emission and the thermal dust continuum. Our new
results confirm the existence of the CB disk. In addition, the circumstellar
(CS) disks are clearly resolved in dust continuum at 1.4 mm. The spectral
indices between the wavelengths of 0.85 mm and 6 cm are found to be
surprisingly low, being 1.6 for both CS disks. The deprojected separation of
the binary is 1.26" based on our 1.4 mm continuum data. This is 0.07" (10 AU)
larger than in earlier studies. Combining the fact of the variation of UY Aur B
in band, we propose that the CS disk of an undetected companion UY Aur Bb
obscures UY Aur Ba. A very complex kinematical pattern inside the CB disk is
observed due to a mixing of Keplerian rotation of the CB disk, the infall and
outflow gas. The streaming gas accreting from the CB ring toward the CS disks
and possible outflows are also identified and resolved. The SO emission is
found to be at the bases of the streaming shocks. Our results suggest that the
UY Aur system is undergoing an active accretion phase from the CB disk to the
CS disks. The UY Aur B might also be a binary system, making the UY Aur a
triple system.Comment: 14 pages, 11 figures; accepted for publication in Ap
Mapping CO Gas in the GG Tauri A Triple System with 50 AU Spatial Resolution
We aim to unveil the observational imprint of physical mechanisms that govern
planetary formation in the young, multiple system GG Tau A. We present ALMA
observations of CO and CO 3-2 and 0.9 mm continuum emission with
0.35" resolution. The CO 3-2 emission, found within the cavity of the
circumternary dust ring (at radius AU) where no CO emission is
detected, confirms the presence of CO gas near the circumstellar disk of GG Tau
Aa. The outer disk and the recently detected hot spot lying at the outer edge
of the dust ring are mapped both in CO and CO. The gas emission
in the outer disk can be radially decomposed as a series of slightly
overlapping Gaussian rings, suggesting the presence of unresolved gaps or dips.
The dip closest to the disk center lies at a radius very close to the hot spot
location at ~AU. The CO excitation conditions indicate that the
outer disk remains in the shadow of the ring. The hot spot probably results
from local heating processes. The two latter points reinforce the hypothesis
that the hot spot is created by an embedded proto-planet shepherding the outer
disk.Comment: 8 pages, 4 figures. Accepted by Ap
New Millimeter CO Observations of the Gas-rich Debris Disks 49 Cet and HD 32297
Previous observations revealed the existence of CO gas at nearly protoplanetary level in several dust-rich debris disks around youn A-type stars. Here we used the Atacama Large Millimeter/submillimete Array (ALMA) 7 m Array to measure 13CO and C18 emission toward two debris disks, 49 Cet and HD 32297, and detecte similarly high CO content (>0.01 M ⊕). These high C masses imply a highly efficient shielding of CO molecules agains stellar and interstellar ultraviolet photons. Adapting a recen secondary gas disk model that considers both shielding by carbon atom and self-shielding of CO, we can explain the observed CO level in bot systems. Based on the derived gas densities we suggest that, in the H 32297 disk, dust and gas are coupled and the dynamics of small grains i affected by the gaseous component. For 49 Cet, the question of couplin remains undecided. We found that the main stellar and disk properties o 49 Cet and HD 32297 are very similar to those of previously identifie debris disks with high CO content. These objects constitute together th first known representatives of shielded debris disk
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