14 research outputs found
H3+ in Diffuse Interstellar Clouds: a Tracer for the Cosmic-Ray Ionization Rate
Using high resolution infrared spectroscopy we have surveyed twenty
sightlines for H3+ absorption. H3+ is detected in eight diffuse cloud
sightlines with column densities varying from 0.6x10^14 cm^-2 to 3.9x10^14
cm^-2. This brings to fourteen the total number of diffuse cloud sightlines
where H3+ has been detected. These detections are mostly along sightlines
concentrated in the Galactic plane, but well dispersed in Galactic longitude.
The results imply that abundant H3+ is common in the diffuse interstellar
medium. Because of the simple chemistry associated with H3+ production and
destruction, these column density measurements can be used in concert with
various other data to infer the primary cosmic-ray ionization rate, zeta_p.
Values range from 0.5x10^-16 s^-1 to 3x10^-16 s^-1 with an average of 2x10^-16
s^-1. Where H3+ is not detected the upper limits on the ionization rate are
consistent with this range. The average value of zeta_p is about an order of
magnitude larger than both the canonical rate and rates previously reported by
other groups using measurements of OH and HD. The discrepancy is most likely
due to inaccurate measurements of rate constants and the omission of effects
which were unknown when those studies were performed. We believe that the
observed column density of H3+ is the most direct tracer for the cosmic-ray
ionization rate due to its simple chemistry. Recent models of diffuse cloud
chemistry require cosmic-ray ionization rates on the order of 10^-16 s^-1 to
reproduce observed abundances of various atomic and molecular species, in rough
accord with our observational findings.Comment: Accepted to ApJ, 35 pages, 5 figures, 5 table
Chemical telemetry of OH observed to measure interstellar magnetic fields
We present models for the chemistry in gas moving towards the ionization
front of an HII region. When it is far from the ionization front, the gas is
highly depleted of elements more massive than helium. However, as it approaches
the ionization front, ices are destroyed and species formed on the grain
surfaces are injected into the gas phase. Photodissociation removes gas phase
molecular species as the gas flows towards the ionization front. We identify
models for which the OH column densities are comparable to those measured in
observations undertaken to study the magnetic fields in star forming regions
and give results for the column densities of other species that should be
abundant if the observed OH arises through a combination of the liberation of
H2O from surfaces and photodissociation. They include CH3OH, H2CO, and H2S.
Observations of these other species may help establish the nature of the OH
spatial distribution in the clouds, which is important for the interpretation
of the magnetic field results.Comment: 11 pages, 2 figures, accepted by Astrophysics and Space Scienc
Cold gas in the Intra Cluster Medium: implications for flow dynamics and powering optical nebulae
We show that the mechanical energy injection rate generated as the
intra-cluster medium (ICM) flows around cold clouds may be sufficient to power
the optical and near infra-red emission of nebulae observed in the central
regions of a sample of seven galaxy clusters. The energy injection rate is
extremely sensitive to the velocity difference between the ICM and cold clouds,
which may help to explain why optical and infra-red luminosity is often larger
than expected in systems containing AGNs. We also find that mass recycling is
likely to be important for the dynamics of the ICM. This effect will be
strongest in the central regions of clusters where there is more than enough
cold gas for its evaporation to contribute significantly to the density of the
hot phase.Comment: 8 pages, 2 figures, accepted for publication in MNRA
Hot and Diffuse Clouds near the Galactic Center Probed by Metastable H3+
Using an absorption line from the metastable (J, K) = (3, 3) level of H3+
together with other lines of H3+ and CO observed along several sightlines, we
have discovered a vast amount of high temperature (T ~ 250 K) and low density
(n ~ 100 cm-3) gas with a large velocity dispersion in the Central Molecular
Zone (CMZ) of the Galaxy, i.e., within 200 pc of the center. Approximately
three fourths of the H3+ along the line of sight to the brightest source we
observed, the Quintuplet object GCS 3-2, is inferred to be in the CMZ, with the
remaining H3+ located in intervening spiral arms. About half of H3+ in the CMZ
has velocities near ~ - 100 km s-1 indicating that it is associated with the
180 pc radius Expanding Molecular Ring which approximately forms outer boundary
of the CMZ. The other half, with velocities of ~ - 50 km s-1 and ~ 0 km s-1, is
probably closer to the center. CO is not very abundant in those clouds. Hot and
diffuse gas in which the (3, 3) level is populated was not detected toward
several dense clouds and diffuse clouds in the Galactic disk where large column
densities of colder H3+ have been reported previously. Thus the newly
discovered environment appears to be unique to the CMZ. The large observed H3+
column densities in the CMZ suggests an ionization rate much higher than in the
diffuse interstellar medium in the Galactic disk. Our finding that the H3+ in
the CMZ is almost entirely in diffuse clouds indicates that the reported volume
filling factor (f ≥ 0.1) for n ≥ 104 cm-3 clouds in the CMZ is an
overestimate by at least an order of magnitude.Comment: 33 pages, 5 figures, 3 table
The Internal Energy for Molecular Hydrogen in Gravitationally Unstable Protoplanetary Disks
The gas equation of state may be one of the critical factors for the disk
instability theory of gas giant planet formation. This letter addresses the
treatment of H in hydrodynamical simulations of gravitationally unstable
disks. In our discussion, we point out possible consequences of erroneous
specific internal energy relations, approximate specific internal energy
relations with discontinuities, and assumptions of constant . In
addition, we consider whether the ortho/para ratio for H in protoplanetary
disks should be treated dynamically as if the species are in equilibrium.
Preliminary simulations indicate that the correct treatment is particularly
critical for the study of gravitational instability when -50 K.Comment: 13 pages, 3 figures. To appear in ApJ
The Formation of Fragments at Corotation in Isothermal Protoplanetary Disks
Numerical hydrodynamics simulations have established that disks which are
evolved under the condition of local isothermality will fragment into small
dense clumps due to gravitational instabilities when the Toomre stability
parameter is sufficiently low. Because fragmentation through disk
instability has been suggested as a gas giant planet formation mechanism, it is
important to understand the physics underlying this process as thoroughly as
possible. In this paper, we offer analytic arguments for why, at low ,
fragments are most likely to form first at the corotation radii of growing
spiral modes, and we support these arguments with results from 3D hydrodynamics
simulations.Comment: 21 pages, 1 figur
Absorption Line Survey of H3+ toward the Galactic Center Sources II. Eight Infrared Sources within 30 pc of the Galactic Center
Infrared absorption lines of H3+, including the metastable R(3,3)l line, have
been observed toward eight bright infrared sources associated with hot and
massive stars located in and between the Galactic Center Cluster and the
Quintuplet Cluster 30 pc to the east. The absorption lines with high velocity
dispersion arise in the Galaxy's Central Molecular Zone (CMZ) as well as in
foreground spiral arms. The temperature and density of the gas in the CMZ, as
determined from the relative strengths of the H3+ lines, are T=200-300K and
n=50-200cm^-3. The detection of high column densities of H3+ toward all eight
stars implies that this warm and diffuse gaseous environment is widespread in
the CMZ. The products of the ionization rate and path length for these sight
lines are 1000 and 10 times higher than in dense and diffuse clouds in the
Galactic disk, respectively, indicating that the ionization rate, zeta, is not
less than 10^-15 s^-1 and that L is at least on the order of 50 pc. The warm
and diffuse gas is an important component of the CMZ, in addition to the three
previously known gaseous environments: (1) cold molecular clouds observed by
radio emission of CO and other molecules, (2) hot (T=10^4-10^6K) and highly
ionized diffuse gas (n_e=10-100cm^-3) seen in radio recombination lines, far
infrared atomic lines, and radio-wave scattering, and (3) ultra-hot
(T=10^7-10^8K) X-ray emitting plasma. Its prevalence significantly changes the
understanding of the environment of the CMZ. The sight line toward GC IRS 3 is
unique in showing an additional H3+ absorption component, which is interpreted
as due to either a cloud associated with circumnuclear disk or the "50 km s^-1
cloud" known from radio observations. An infrared pumping scheme is examined as
a mechanism to populate the (3,3) metastable level in this cloud.Comment: 15 pages, 5 figures, 4 tables Accepted for publication in the
Astrophysical Journa