56 research outputs found
Radiative transfer in protoplanetary disks
We present a new 3D continuum radiative transfer code, MCFOST, based on a
Monte-Carlo method. The reliability and efficiency of the code is tested by
comparison with five different radiative transfer codes previously tested by
Pascucci et al., 2004, using a 2D disk configuration. When tested against the
same disk configuration, no significant difference is found between the
temperature and SED calculated with MCFOST and with the other codes. The
computed values are well within the range of values computed by the other
codes. The code-to-code differences are small, they rarely exceed 10% and are
usually much smaller.Comment: 20 pages, 11 figures, GRETA conference : "Radiative transfer and
Applications to Very Large Telescopes
A Herschel PACS survey of the dust and gas in Upper Scorpius disks
We present results of far-infrared photometric observations with Herschel
PACS of a sample of Upper Scorpius stars, with a detection rate of previously
known disk-bearing K and M stars at 70, 100, and 160 micron of 71%, 56%, and
50%, respectively. We fit power-law disk models to the spectral energy
distributions of K & M stars with infrared excesses, and have found that while
many disks extend in to the sublimation radius, the dust has settled to lower
scale heights than in disks of the less evolved Taurus-Auriga population, and
have much reduced dust masses. We also conducted Herschel PACS observations for
far-infrared line emission and JCMT observations for millimeter CO lines. Among
B and A stars, 0 of 5 debris disk hosts exhibit gas line emission, and among K
and M stars, only 2 of 14 dusty disk hosts are detected. The OI 63 micron and
CII 157 micron lines are detected toward [PZ99] J160421.7-213028 and [PBB2002]
J161420.3-190648, which were found in millimeter photometry to host two of the
most massive dust disks remaining in the region. Comparison of the OI line
emission and 63 micron continuum to that of Taurus sources suggests the
emission in the former source is dominated by the disk, while in the other
there is a significant contribution from a jet. The low dust masses found by
disk modeling and low number of gas line detections suggest that few stars in
Upper Scorpius retain sufficient quantities of material for giant planet
formation. By the age of Upper Scorpius, giant planet formation is essentially
complete.Comment: 48 pages, 14 figures, accepted A&
The Nature of Class I Sources: Periodic Variables in Orion
We present a quantitative, empirically based argument that at least some
Class I sources are low-mass, pre-main-sequence stars surrounded by spatially
extended envelopes of dusty gas. The source luminosity arises principally from
stellar gravitational contraction, as in optically visible pre-main-sequence
stars that lack such envelopes. We base our argument on the fact that some
Class I sources in Orion and other star-forming regions have been observed by
Spitzer to be periodic variables in the mid-infrared, and with periods
consistent with T Tauri rotation rates. Using a radiative transfer code, we
construct a variety of dust envelopes surrounding rotating, spotted stars, to
see if an envelope that produces a Class I SED at least broadly matches the
observed modulations in luminosity. Acceptable envelopes can either be
spherical or flattened, and may or may not have polar cavities. The key
requirement is that they have a modest equatorial optical depth at the Spitzer
waveband of 3.6 m, typically 0.6. The total
envelope mass, based on this limited study, is at most about 0.1
, less than a typical stellar mass. Future studies should
focus on the dynamics of the envelope, to determine whether material is
actually falling onto the circumstellar disk.Comment: 17 pages, 16 figures. Additional light curve figures and associated
data table referred to in Appendix B available as online dat
The disk around the brown dwarf KPNO Tau 3
We present submillimeter observations of the young brown dwarfs KPNO Tau 1,
KPNO Tau 3, and KPNO Tau 6 at 450 micron and 850 micron taken with the
Submillimeter Common-User Bolometer Array on the James Clerke Maxwell
Telescope. KPNO Tau 3 and KPNO Tau 6 have been previously identified as Class
II objects hosting accretion disks, whereas KPNO Tau 1 has been identified as a
Class III object and shows no evidence of circumsubstellar material. Our 3
sigma detection of cold dust around KPNO Tau 3 implies a total disk mass of
(4.0 +/- 1.1) x 10^{-4} Msolar (assuming a gas to dust ratio of 100:1). We
place tight constraints on any disks around KPNO Tau 1 or KPNO Tau 6 of <2.1 x
10^{-4} Msolar and <2.7 x 10^{-4} Msolar, respectively. Modeling the spectral
energy distribution of KPNO Tau 3 and its disk suggests the disk properties
(geometry, dust mass, and grain size distribution) are consistent with
observations of other brown dwarf disks and low-mass T-Tauri stars. In
particular, the disk-to-host mass ratio for KPNO Tau 3 is congruent with the
scenario that at least some brown dwarfs form via the same mechanism as
low-mass stars.Comment: 18 pages (preprint format), 3 figures, published in Ap
Discovery of an extended debris disk around the F2V star HD 15745
Using the Advanced Camera for Surveys aboard the Hubble Space Telescope, we
have discovered dust-scattered light from the debris disk surrounding the F2V
star HD 15745. The circumstellar disk is detected between 2.0" and 7.5" radius,
corresponding to 128 - 480 AU radius. The circumstellar disk morphology is
asymmetric about the star, resembling a fan, and consistent with forward
scattering grains in an optically thin disk with an inclination of ~67 degrees
to our line of sight. The spectral energy distribution and scattered light
morphology can be approximated with a model disk composed of silicate grains
between 60 and 450 AU radius, with a total dust mass of 10E-7 M_sun (0.03
M_earth) representing a narrow grain size distribution (1 - 10 micron).
Galactic space motions are similar to the Castor Moving Group with an age of
~10E+8 yr, although future work is required to determine the age of HD 15745
using other indicators.Comment: 7 pages, 4 figures, ApJ Letters, in pres
A search for passive protoplanetary disks in the Taurus-Auriga star-forming region
We conducted a 12-month monitoring campaign of 33 T Tauri stars (TTS) in
Taurus. Our goal was to monitor objects that possess a disk but have a weak
Halpha line, a common accretion tracer for young stars, to determine whether
they host a passive circumstellar disk. We used medium-resolution optical
spectroscopy to assess the objects' accretion status and to measure the Halpha
line. We found no convincing example of passive disks; only transition disk and
debris disk systems in our sample are non-accreting. Among accretors, we find
no example of flickering accretion, leading to an upper limit of 2.2% on the
duty cycle of accretion gaps assuming that all accreting TTS experience such
events. Combining literature results with our observations, we find that the
reliability of traditional Halpha-based criteria to test for accretion is high
but imperfect, particularly for low-mass TTS. We find a significant correlation
between stellar mass and the full width at 10 per cent of the peak (W10%) of
the Halpha line that does not seem to be related to variations in free-fall
velocity. Finally, our data reveal a positive correlation between the Halpha
equivalent width and its W10%, indicative of a systematic modulation in the
line profile whereby the high-velocity wings of the line are proportionally
more enhanced than its core when the line luminosity increases. We argue that
this supports the hypothesis that the mass accretion rate on the central star
is correlated with the Halpha W10% through a common physical mechanism.Comment: accepted for publication in MNRAS; 26 pages, 9 figures, 3 table
Continuum and line modelling of discs around young stars. I. 300000 disc models for Herschel/GASPS
We have combined the thermo-chemical disc code ProDiMo with the Monte Carlo
radiative transfer code MCFOST to calculate a grid of ~300000 circumstellar
disc models, systematically varying 11 stellar, disc and dust parameters
including the total disc mass, several disc shape parameters and the
dust-to-gas ratio. For each model, dust continuum and line radiative transfer
calculations are carried out for 29 far IR, sub-mm and mm lines of [OI], [CII],
12CO and o/p-H2O under 5 inclinations. The grid allows to study the influence
of the input parameters on the observables, to make statistical predictions for
different types of circumstellar discs, and to find systematic trends and
correlations between the parameters, the continuum fluxes, and the line fluxes.
The model grid, comprising the calculated disc temperatures and chemical
structures, the computed SEDs, line fluxes and profiles, will be used in
particular for the data interpretation of the Herschel open time key programme
GASPS. The calculated line fluxes show a strong dependence on the assumed UV
excess of the central star, and on the disc flaring. The fraction of models
predicting [OI] and [CII] fine-structure lines fluxes above Herschel/PACS and
Spica/SAFARI detection limits are calculated as function of disc mass. The
possibility of deriving the disc gas mass from line observations is discussed.Comment: accepted by MNRAS. 5 pages, 4 figures, 3 table
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