855 research outputs found
The Structure of the DoAr 25 Circumstellar Disk
We present high spatial resolution (< 0.3" = 40\Sigma
\propto r^{-p}$ with p = 0.34, significantly less steep than a steady-state
accretion disk (p = 1) or the often adopted minimum mass solar nebula (p =
1.5). Even though the total mass of material is large (M_d = 0.10 M_sun), the
densities inferred in the inner disk for such a model may be too low to
facilitate any mode of planet formation. However, alternative models with
steeper density gradients (p = 1) can explain the observations equally well if
substantial grain growth in the planet formation region (r < 40 AU) has
occurred. We discuss these data in the context of such models with dust
properties that vary with radius and highlight their implications for
understanding disk evolution and the early stages of planet formation.Comment: ApJL in pres
Increased HCO production in the outer disk around HD 163296
Three formaldehyde lines were observed (HCO 3--2, HCO
3--2, and HCO 3--2) in the protoplanetary disk
around the Herbig Ae star HD 163296 with ALMA at 0.5 arcsecond (60 AU) spatial
resolution. HCO 3--2 was readily detected via imaging, while
the weaker HCO 3--2 and HCO 3--2 lines
required matched filter analysis to detect. HCO is present throughout most
of the gaseous disk, extending out to 550 AU. An apparent 50 AU inner radius of
the HCO emission is likely caused by an optically thick dust continuum. The
HCO radial intensity profile shows a peak at 100 AU and a secondary bump at
around 300 AU, suggesting increased production in the outer disk. Different
parameterizations of the HCO abundance were compared to the observed
visibilities with minimization, using either a characteristic
temperature, a characteristic radius or a radial power law index to describe
the HCO chemistry. Similar models were applied to ALMA Science Verification
data of CO. In all modeling scenarios, fits to the HCO data show an
increased abundance in the outer disk. The overall best-fit HCO model shows
a factor of two enhancement beyond a radius of 27020 AU, with an inner
abundance of . The HCO emitting region has a lower
limit on the kinetic temperature of K. The CO modeling suggests
an order of magnitude depletion in the outer disk and an abundance of in the inner disk. The increase in HCO outer disk emission
could be a result of hydrogenation of CO ices on dust grains that are then
sublimated via thermal desorption or UV photodesorption, or more efficient
gas-phase production beyond about 300 AU if CO is photodisocciated in this
region
Exploring DCO as a tracer of thermal inversion in the disk around the Herbig Ae star HD163296
We aim to reproduce the DCO emission in the disk around HD163296 using a
simple 2D chemical model for the formation of DCO through the cold
deuteration channel and a parametric treatment of the warm deuteration channel.
We use data from ALMA in band 6 to obtain a resolved spectral imaging data cube
of the DCO =3--2 line in HD163296 with a synthesized beam of
0."53 0."42. We adopt a physical structure of the disk from the
literature that reproduces the spectral energy distribution. We then apply a
simplified chemical network for the formation of DCO that uses the physical
structure of the disk as parameters along with a CO abundance profile, a
constant HD abundance and a constant ionization rate. Finally, from the
resulting DCO abundances, we calculate the non-LTE emission using the 3D
radiative transfer code LIME. The observed DCO emission is reproduced by a
model with cold deuteration producing abundances up to .
Warm deuteration, at a constant abundance of , becomes
fully effective below 32 K and tapers off at higher temperatures, reproducing
the lack of DCO inside 90 AU. Throughout the DCO emitting zone a CO
abundance of is found, with 99\% of it frozen out below
19 K. At radii where both cold and warm deuteration are active, warm
deuteration contributes up to 20\% of DCO, consistent with detailed
chemical models. The decrease of DCO at large radii is attributed to a
temperature inversion at 250 AU, which raises temperatures above values where
cold deuteration operates. Increased photodesorption may also limit the radial
extent of DCO. The corresponding return of the DCO layer to the
midplane, together with a radially increasing ionization fraction, reproduces
the local DCO emission maximum at 260 AU.Comment: 9 pages, 5 figures, accepted 7th July 201
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