39 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
Tortuosity of the brachiocephalic artery complicated with arterial injury after tracheotomy: a case report
Tracheotomy is an operation of the airway performed even on critical care patients. Surgical complications of tracheotomies are fatal. In this study, tortuosity of the brachiocephalic artery complicated with arterial injury was observed in a patient after tracheotomy. A 95-year-old woman in coma was admitted to our medical center. The patient needed airway management, and tracheal intubation was performed. The cause of the coma was extensive cerebral infarction of the right middle cerebral artery. It was expected that the coma would be prolonged, and a tracheotomy was performed after 7 days. Tortuosity of the brachiocephalic artery was confirmed with cervical computed tomography before surgery. The patient bled through the tracheostomy after 30 days. To arrest bleeding from the right common carotid artery, a vascular repair surgery was performed. There was no recurrent bleeding after surgery. After 37 days, the patient died of deteriorating primary disease. Although tracheotomy is a common operation, attention should be paid to abnormalities of blood vessels including tortuosity of the brachiocephalic artery
Primordial or Secondary? Testing models of debris disk gas with ALMA
The origin and evolution of gas in debris disks is still not well understood.
Secondary gas production from cometary material or a primordial origin have
been proposed. So far, observations have mostly concentrated on CO, with only
few C observations available. We create an overview of the C and CO content of
debris disk gas and use it test state-of-the-art models. We use new and
archival ALMA observations of CO and CI emission, complemented by CII data from
Herschel, for a sample of 14 debris disks. This expands the number of disks
with ALMA measurements of both CO and CI by ten disks. We present new
detections of CI emission towards three disks: HD 21997, HD 121191 and HD
121617. We use a simple disk model to derive gas masses and column densities.
We find that current state-of-the-art models of secondary gas production
overpredict the neutral carbon content of debris disk gas. This does not rule
out a secondary origin, but might indicate that the models require an
additional C removal process. Alternatively, the gas might be produced in
transient events rather than a steady-state collisional cascade. We also test a
primordial gas origin by comparing our results to a simplified thermo-chemical
model. This yields promising results, but more detailed work is required before
a conclusion can be reached. Our work demonstrates that the combination of C
and CO data is a powerful tool to advance our understanding of debris disk gas.Comment: 90 pages, 60 figures. Accepted for publication in ApJ. version 2:
additional acknowledgement. versions 3, 4: minor edit
Numerical Examination of the Stability of an Exact Two-dimensional Solution for Flux Pile-up Magnetic Reconnection
The Kelvin--Helmholtz (KH) and tearing instabilities are likely to be
important for the process of fast magnetic reconnection that is believed to
explain the observed explosive energy release in solar flares. Theoretical
studies of the instabilities, however, typically invoke simplified initial
magnetic and velocity fields that are not solutions of the governing
magnetohydrodynamic (MHD) equations. In the present study, the stability of a
reconnecting current sheet is examined using a class of exact global MHD
solutions for steady state incompressible magnetic reconnection, discovered by
Craig & Henton. Numerical simulation indicates that the outflow solutions where
the current sheet is formed by strong shearing flows are subject to the KH
instability. The inflow solutions where the current sheet is formed by a fast
and weakly sheared inflow are shown to be tearing unstable. Although the
observed instability of the solutions can be interpreted qualitatively by
applying standard linear results for the KH and tearing instabilities, the
magnetic field and plasma flow, specified by the Craig--Henton solution, lead
to the stabilization of the current sheet in some cases. The sensitivity of the
instability growth rate to the global geometry of magnetic reconnection may
help in solving the trigger problem in solar flare research.Comment: Accepted for publication in ApJ. Associated movie files and a PDF
with high-resolution figures are available at
http://www.pha.jhu.edu/~shirose/Craig