45 research outputs found
A Constraint on the Amount of Hydrogen from the CO Chemistry in Debris Disks
The faint CO gases in debris disks are easily dissolved into C by UV
irradiation, while CO can be reformed via reactions with hydrogen. The
abundance ratio of C/CO could thus be a probe of the amount of hydrogen in the
debris disks. We conduct radiative transfer calculations with chemical
reactions for debris disks. For a typical dust-to-gas mass ratio of debris
disks, CO formation proceeds without the involvement of H because a small
amount of dust grains makes H formation inefficient. We find that the CO to
C number density ratio depends on a combination of
, where is the hydrogen nucleus
number density, is the metallicity, and is the FUV flux normalized
by the Habing flux. Using an analytic formula for the CO number density, we
give constraints on the amount of hydrogen and metallicity for debris disks. CO
formation is accelerated by excited H either when the dust-to-gas mass
ratio is increased or the energy barrier of chemisorption of hydrogen on the
dust surface is decreased. This acceleration of CO formation occurs only when
the shielding effects of CO are insignificant. In shielded regions, the CO
fractions are almost independent of the parameters of dust grains.Comment: 29pages, 13figures, accepted for Ap
Physical conditions of gas components in debris disks of 49 Ceti and HD 21997
Characterization of gas component in debris disks is of fundamental
importance for understanding its origin. Toward this goal, we have conducted
non-LTE (local thermodynamic equilibrium) analyses of the rotational spectral
lines of CO including those of rare isotopologues (CO and CO)
observed toward the gaseous debris disks of 49 Ceti and HD 21997 with the
Atacama Large Millimeter/submillimeter Array (ALMA) and Atacama Compact Array
(ACA). The analyses have been carried out for a wide range of the H
density, and the observed line intensities are found to be reproduced, as far
as the H density is higher than 10 cm. The CO column density
and the gas temperature are evaluated to be (1.8-5.9)10
cm and 8-11 K for 49 Ceti and (2.6-15)10 cm and
8-12 K for HD 21997, respectively, where the H collision is assumed for
the rotational excitation of CO. The results do not change significantly even
if electron collision is considered. Thus, CO molecules can be excited under
environments of no H or a small number of H molecules, even where
the collision with CO, C, O, and C would make an important contribution
for the CO excitation in addition to H. Meanwhile, our result does not
rule out the case of abundant H molecules. The low gas temperature
observed in the debris disks is discussed in terms of inefficient heating by
interstellar and stellar UV radiation.Comment: 25 pages, 5 figures, 9 tables, accepted for publication in Ap
Rotation in the NGC 1333 IRAS 4C Outflow
We report molecular line observations of the NGC 1333 IRAS 4C outflow in the
Perseus Molecular Cloud with the Atacama Large Millimeter/Submillimeter Array.
The CCH and CS emission reveal an outflow cavity structure with clear
signatures of rotation with respect to the outflow axis. The rotation is
detected from about 120 au up to about 1400 au above the envelope/disk
mid-plane. As the distance to the central source increases, the rotation
velocity of the outflow decreases while the outflow radius increases, which
gives a flat specific angular momentum distribution along the outflow. The mean
specific angular momentum of the outflow is about 100 au km/s. Based on
reasonable assumptions on the outward velocity of the outflow and the protostar
mass, we estimate the range of outflow launching radii to be 5-15 au. Such a
launching radius rules out that this outflow is launched as an X-wind, but
rather, it is more consistent to be a slow disk wind launched from relatively
large radii on the disk. The radius of the centrifugal barrier is roughly
estimated, and the role of the centrifugal barrier in the outflow launching is
discussed.Comment: Accepted to ApJ. 29 pages, 8 figure
Physical Conditions of Gas Components in Debris Disks of 49 Ceti and HD 21997
Characterization of the gas component in debris disks is of fundamental importance for understanding their origin. To address this goal, we have conducted non-LTE (local thermodynamic equilibrium) analyses of the rotational spectral lines of CO including those of rare isotopologues (13CO and C18O) that have been observed toward the gaseous debris disks of 49 Ceti and HD 21997 with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Atacama Compact Array (ACA). The analyses have been carried out for a wide range of H2 density, and the observed line intensities are found to be reproduced as long as the H2 density is higher than 103 cm-3. The CO column density and the gas temperature are evaluated to be (1.8-5.9) × 1017 cm-2 and 8-11 K for 49 Ceti and (2.6-15) × 1017 cm-2 and 8-12 K for HD 21997, respectively, where the H2 collision is assumed for the rotational excitation of CO. The results do not change significantly even if electron collision is considered. Thus, CO molecules can be excited under environments containing no H2 or a small number of H2 molecules, even where collisions with CO, C, O, and C+ would make an important contribution to the CO excitation in addition to H2. Meanwhile, our result does not rule out the case of abundant H2 molecules. The low gas temperature observed in the debris disks is discussed in terms of inefficient heating by interstellar and stellar UV radiation