167 research outputs found
Sensitive survey for 13CO, CN, H2CO, and SO in the disks of T Tauri and Herbig Ae stars II: Stars in Oph and upper Scorpius
We attempt to determine the molecular composition of disks around young
low-mass stars in the Oph region and to compare our results with a
similar study performed in the Taurus-Auriga region. We used the IRAM 30 m
telescope to perform a sensitive search for CN N=2-1 in 29 T Tauri stars
located in the Oph and upper Scorpius regions. CO J=2-1 is
observed simultaneously to provide an indication of the level of confusion with
the surrounding molecular cloud. The bandpass also contains two transitions of
ortho-HCO, one of SO, and the CO J=2-1 line, which provides
complementary information on the nature of the emission. Contamination by
molecular cloud in CO and even CO is ubiquitous. The CN detection
rate appears to be lower than for the Taurus region, with only four sources
being detected (three are attributable to disks). HCO emission is found
more frequently, but appears in general to be due to the surrounding cloud. The
weaker emission than in Taurus may suggest that the average disk size in the
Oph region is smaller than in the Taurus cloud. Chemical modeling shows
that the somewhat higher expected disk temperatures in Oph play a direct
role in decreasing the CN abundance. Warmer dust temperatures contribute to
convert CN into less volatile forms. In such a young region, CN is no longer a
simple, sensitive tracer of disks, and observations with other tracers and at
high enough resolution with ALMA are required to probe the gas disk population.Comment: 18 pages, 5 figures, accepted for publication in A&
A sensitive survey for 13CO, CN, H2CO and SO in the disks of T Tauri and Herbig Ae stars
We use the IRAM 30-m telescope to perform a sensitive search for CN N=2-1 in
42 T Tauri or Herbig Ae systems located mostly in the Taurus-Auriga region.
CO J=2-1 is observed simultaneously to indicate the level of confusion
with the surrounding molecular cloud. The bandpass also contains two
transitions of ortho-HCO, one of SO and the CO J=2-1 line which
provide complementary information on the nature of the emission.
While CO is in general dominated by residual emission from the cloud,
CN exhibits a high disk detection rate % in our sample. We even report CN
detection in stars for which interferometric searches failed to detect
CO, presumably because of obscuration by a foreground, optically thick,
cloud. Comparison between CN and o-HCO or SO line profiles and intensities
divide the sample in two main categories. Sources with SO emission are bright
and have strong HCO emission, leading in general to [HCO/CN].
Furthermore, their line profiles, combined with a priori information on the
objects, suggest that the emission is coming from outflows or envelopes rather
than from a circumstellar disk. On the other hand, most sources have
[HCO/CN], no SO emission, and some of them exhibit clear
double-peaked profiles characteristics of rotating disks. In this second
category, CN is likely tracing the proto-planetary disks. From the line flux
and opacity derived from the hyperfine ratios, we constrain the outer radii of
the disks, which range from 300 to 600 AU. The overall gas disk detection rate
(including all molecular tracers) is , and decreases for fainter
continuum sources.
This study shows that gas disks, like dust disks, are ubiquitous around young
PMS stars in regions of isolated star formation, and that a large fraction of
them have AU.Comment: 31 pages (including 59 figures
Searching for sub-stellar companion into the LkCa15 proto-planetary disk
Recent sub-millimetric observations at the Plateau de Bure interferometer
evidenced a cavity at ~ 46 AU in radius into the proto-planetary disk around
the T Tauri star LkCa15 (V1079 Tau), located in the Taurus molecular cloud.
Additional Spitzer observations have corroborated this result possibly
explained by the presence of a massive (>= 5 MJup) planetary mass, a brown
dwarf or a low mass star companion at about 30 AU from the star. We used the
most recent developments of high angular resolution and high contrast imaging
to search directly for the existence of this putative companion, and to bring
new constraints on its physical and orbital properties. The NACO adaptive
optics instrument at VLT was used to observe LkCa15 using a four quadrant phase
mask coronagraph to access small angular separations at relatively high
contrast. A reference star at the same parallactic angle was carefully observed
to optimize the quasi-static speckles subtraction (limiting our sensitivity at
less than 1.0). Although we do not report any positive detection of a faint
companion that would be responsible for the observed gap in LkCa15's disk
(25-30 AU), our detection limits start constraining its probable mass,
semi-major axis and eccentricity. Using evolutionary model predictions, Monte
Carlo simulations exclude the presence of low eccentric companions with masses
M >= 6 M Jup and orbiting at a >= 100 AU with significant level of confidence.
For closer orbits, brown dwarf companions can be rejected with a detection
probability of 90% down to 80 AU (at 80% down to 60 AU). Our detection limits
do not access the star environment close enough to fully exclude the presence
of a brown dwarf or a massive planet within the disk inner activity (i.e at
less than 30 AU). Only, further and higher contrast observations should unveil
the existence of this putative companion inside the LkCa15 disk.Comment: 6 pages, 4 figures, accepted for publication in A&
Sub-arcsec imaging of the AB Aur molecular disk and envelope at millimeter wavelengths: a non Keplerian disk
We present sub-arcsecond images of AB Auriga obtained with the IRAM Plateau
de Bure interferometer in the isotopologues of CO, and in continuum at 3 and
1.3 mm. Instead of being centrally peaked, the continuum emission is dominated
by a bright, asymmetric (spiral-like) feature at about 140 AU from the central
star. The large scale molecular structure suggests the AB Aur disk is inclined
between 23 and 43 degrees, but the strong asymmetry of the continuum and
molecular emission prevents an accurate determination of the inclination of the
inner parts. We find significant non-Keplerian motion, with a best fit exponent
for the rotation velocity law of 0.41 +/- 0.01, but no evidence for radial
motions. The disk has an inner hole about 70 AU in radius. The disk is warm and
shows no evidence of depletion of CO. The dust properties suggest the dust is
less evolved than in typical T Tauri disks. Both the spiral-like feature and
the departure from purely Keplerian motions indicates the AB Aur disk is not in
quasi-equilibrium. Disk self-gravity is insufficient to create the
perturbation. This behavior may be related either to an early phase of star
formation in which the Keplerian regime is not yet fully established and/or to
a disturbance of yet unknown origin. An alternate, but unproven, possibility is
that of a low mass companion located about 40 AU from AB Aur.Comment: 10 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
Millimetre continuum observations of comet C/2009 P1 (Garradd)
Little is known about the physical properties of the nuclei of Oort cloud
comets. Measuring the thermal emission of a nucleus is one of the few means for
deriving its size and constraining some of its thermal properties. We attempted
to measure the nucleus size of the Oort cloud comet C/2009 P1 (Garradd). We
used the Plateau de Bure Interferometer to measure the millimetric thermal
emission of this comet at 157 GHz (1.9 mm) and 266 GHz (1.1 mm). Whereas the
observations at 266 GHz were not usable due to bad atmospheric conditions, we
derived a 3-sigma upper limit on the comet continuum emission of 0.41 mJy at
157 GHz. Using a thermal model for a spherical nucleus with standard thermal
parameters, we found an upper limit of 5.6 km for the radius. The dust
contribution to our signal is estimated to be negligible. Given the water
production rates measured for this comet and our upper limit, we estimated that
Garradd was very active, with an active fraction of its nucleus larger than
50%.Comment: Accepted for publication in Astronomy & Astrophysics. 5 pages, 2
figure
High resolution imaging of the GG Tau system at 267 GHz
Studying circumbinary disks is critical to understanding the formation
mechanisms of binary stars. While optical or mid-infrared images reveal the
scattered mission, millimeter observations provide direct measurements of the
dust thermal emission. We study the properties of the circumbinary disk around
the well-known, multiple young stellar object GG Tau with the highest possible
sensitivity and spatial resolution. We mapped the continuum emission of GG Tau
at 267 GHz using the IRAM Plateau de Bure interferometer equipped with upgraded
receivers and LO systems. An angular resolution of 0.45"x0.25" was achieved,
corresponding to a linear resolution of 65x35 AU. The GG Tau A circumbinary
disk is observed as an extremely clearly defined narrow ring. The width of the
ring is not resolved. Emission from the central binary is detected and clearly
separated from the ring: it coincides with the GG Tau Aa position and may
therefore trace a circumstellar disk around this star. The mass ratio of the
circumbinary to circumprimary material is ~80.Comment: 5 pages, 2 figures, accepted for publication in A&
Probing the structure of protoplanetary disks: a comparative study of DM Tau, LkCa 15 and MWC 480
We report sub-arcsec CO observations of the disks around MWC 480, LkCa 15 and
DM Tau, and simultaneous measurements of HCO+ J=1-0. We derive the disk
properties by fitting a standard disk model, with all parameters power laws of
the distance to the star. Biases are explained and discussed. We find evidence
for vertical temperature gradient in the disks of MWC 480 and DM Tau, as in AB
Aur, but not in LkCa 15. The disks temperature increase with stellar effective
temperature. Most of the CO gas is at temperatures smaller than 17 K, the
condensation temperature on grains. The scale height of the CO distribution
appears larger (by 50%) than the hydrostatic scale height. The more UV luminous
stars have more CO, but there is no simple dependency of CO abundance and
isotopologue ratio with stellar type. The 13CO/HCO+ ratio is around 600. The
temperature behaviour is consistent with expectations, but published chemical
models have difficulty reproducing the observed CO quantities. Vertical mixing
and photo-dissociation at the disk outer edge seem important chemical agents.
The CO data suggest a more complex surface density distribution than assumed in
models.Comment: Astronomy and Astrophysics (2007) accepted in 200
Dynamical Masses of Low Mass Stars in the Taurus and Ophiuchus Star Forming Regions
We report new dynamical masses for 5 pre-main sequence (PMS) stars in the
L1495 region of the Taurus star-forming region (SFR) and 6 in the L1688 region
of the Ophiuchus SFR. Since these regions have VLBA parallaxes these are
absolute measurements of the stars' masses and are independent of their
effective temperatures and luminosities. Seven of the stars have masses
solar masses, thus providing data in a mass range with little data, and of
these, 6 are measured to precision . We find 8 stars with masses in the
range 0.09 to 1.1 solar mass that agree well with the current generation of PMS
evolutionary models. The ages of the stars we measured in the Taurus SFR are in
the range 1-3 MY, and MY for those in L1688. We also measured the
dynamical masses of 14 stars in the ALMA archival data for Akeson~\&~Jensen's
Cycle 0 project on binaries in the Taurus SFR. We find that the masses of 7 of
the targets are so large that they cannot be reconciled with reported values of
their luminosity and effective temperature. We suggest that these targets are
themselves binaries or triples.Comment: 20 page
Chemistry in Disks. II. -- Poor molecular content of the AB Aur disk
We study the molecular content and chemistry of a circumstellar disk
surrounding the Herbig Ae star AB Aur at (sub-)millimeter wavelengths. Our aim
is to reconstruct the chemical history and composition of the AB Aur disk and
to compare it with disks around low-mass, cooler T Tauri stars. We observe the
AB Aur disk with the IRAM Plateau de Bure Interferometer in the C- and D-
configurations in rotational lines of CS, HCN, C2H, CH3OH, HCO+, and CO
isotopes. Using an iterative minimization technique, observed columns densities
and abundances are derived. These values are further compared with results of
an advanced chemical model that is based on a steady-state flared disk
structure with a vertical temperature gradient, and gas-grain chemical network
with surface reactions. We firmly detect HCO+ in the 1--0 transition,
tentatively detect HCN, and do not detect CS, C2H, and CH3OH. The observed HCO+
and 13CO column densities as well as the upper limits to the column densities
of HCN, CS, C2H, and CH3OH are in good agreement with modeling results and
those from previous studies. The AB Aur disk possesses more CO, but is less
abundant in other molecular species compared to the DM Tau disk. This is
primarily caused by intense UV irradiation from the central Herbig A0 star,
which results in a hotter disk where CO freeze out does not occur and thus
surface formation of complex CO-bearing molecules might be inhibited.Comment: Accepted by A&
Disks around CQ Tau and MWC 758: dense PDR or gas dispersal?
The overall properties of disks surrounding intermediate PMS stars (HAe) are
not yet well constrained by current observations. The disk inclination, which
significantly affect SED modeling, is often unknown. We attempted to resolve
the disks around CQ Tau and MWC 758, to provide accurate constraints on the
disk parameters, in particular the temperature and surface density
distribution. We report arcsecond resolution observations of dust and CO line
emissions with the IRAM array. The disk properties are derived using a standard
disk model. We use the Meudon PDR code to study the chemistry. The two disks
share some common properties. The mean CO abundance is low despite disk
temperatures above the CO condensation temperature. Furthermore, the CO surface
density and dust opacity have different radial dependence. The CQ Tau disk
appears warmer, and perhaps less dense than that of MWC 758. Modeling the
chemistry, we find that photodissociation of CO is a viable mechanism to
explain the low abundance. The photospheric flux is not sufficient for this: a
strong UV excess is required. In CQ Tau, the high temperature is consistent
with expectation for a PDR. The PDR model has difficulty explaining the mild
temperatures obtained in MWC 758, for which a low gas-to-dust ratio is
preferred. A yet unexplored alternative could be that, despite currently high
gas temperatures, CO remains trapped in grains, as the models suggest that
large grains can be cold enough to prevent thermal desorption of CO. The low
inclination of the CQ Tau disk, ~30^\circ, challenges previous interpretations
given for the UX Ori - like luminosity variations of this star. We conclude
that CO cannot be used as a simple tracer of gas-to-dust ratio, the CO
abundance being affected by photodissociation, and grain growth.Comment: Accepted for publication in Astronomy & Astrophysic
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