60 research outputs found
Turbulent Velocity Structure in Molecular Clouds
We compare velocity structure in the Polaris Flare molecular cloud at scales
ranging from 0.015 pc to 20 pc to simulations of supersonic hydrodynamic and
MHD turbulence computed with the ZEUS MHD code. We use several different
statistical methods to compare models and observations. The Delta-variance
wavelet transform is most sensitive to characteristic scales and scaling laws,
but is limited by a lack of intensity weighting. The scanning-beam
size-linewidth relation is more robust with respect to noisy data. Obtaining
the global velocity scaling behaviour requires that large-scale trends in the
maps not be removed but treated as part of the turbulent cascade. We compare
the true velocity PDF in our models to velocity centroids and average line
profiles in optically thin lines, and find that the line profiles reflect the
true PDF better unless the map size is comparable to the total line-of-sight
thickness of the cloud. Comparison of line profiles to velocity centroid PDFs
can thus be used to measure the line-of-sight depth of a cloud. The observed
density and velocity structure is consistent with supersonic turbulence with a
driving scale at or above the size of the molecular cloud and dissipative
processes below 0.05 pc. Ambipolar diffusion could explain the dissipation. The
velocity PDFs exclude small-scale driving such as that from stellar outflows as
a dominant process in the observed region. In the models, large-scale driving
is the only process that produces deviations from a Gaussian PDF shape
consistent with observations. Strong magnetic fields impose a clear anisotropy
on the velocity field, reducing the velocity variance in directions
perpendicular to the field. (abridged)Comment: 21 pages, 24 figures, accepted by A&A, with some modifications,
including change of claimed direct detection of dissipation scale to an upper
limi
Spatially extended OH+ emission from the Orion Bar and Ridge
We report the first detection of a Galactic source of OH+ line emission: the
Orion Bar, a bright nearby photon-dominated region. Line emission is detected
over ~1' (0.12 pc), tracing the Bar itself as well as the Southern tip of the
Orion Ridge. The line width of ~4 km/s suggests an origin of the OH+ emission
close to the PDR surface, at a depth of A_V ~0.3-0.5 into the cloud where most
hydrogen is in atomic form. Steady-state collisional and radiative excitation
models require unrealistically high OH+ column densities to match the observed
line intensity, indicating that the formation of OH+ in the Bar is rapid enough
to influence its excitation. Our best-fit OH+ column density of ~1x10^14 cm^-2
is similar to that in previous absorption line studies, while our limits on the
ratios of OH+/H2O+ (>~40) and OH+/H3O+ (>~15) are higher than seen before.
The column density of OH+ is consistent with estimates from a thermo-chemical
model for parameters applicable to the Orion Bar, given the current
uncertainties in the local gas pressure and the spectral shape of the ionizing
radiation field. The unusually high OH+/H2O+ and OH+/H3O+ ratios are probably
due to the high UV radiation field and electron density in this object. In the
Bar, photodissociation and electron recombination are more effective destroyers
of OH+ than the reaction with H2, which limits the production of H2O+. The
appearance of the OH+ lines in emission is the result of the high density of
electrons and H atoms in the Orion Bar, since for these species, inelastic
collisions with OH+ are faster than reactive ones. In addition, chemical
pumping, far-infrared pumping by local dust, and near-UV pumping by Trapezium
starlight contribute to the OH+ excitation. Similar conditions may apply to
extragalactic nuclei where OH+ lines are seen in emission.Comment: Accepted by A&A; 10 pages, 5 figure
The fine structure line deficit in S 140
We try to understand the gas heating and cooling in the S 140 star forming
region by spatially and spectrally resolving the distribution of the main
cooling lines with GREAT/SOFIA. We mapped the fine structure lines of [OI] (63
{\mu}m) and [CII] (158 {\mu}m) and the rotational transitions of CO 13-12 and
16-15 with GREAT/SOFIA and analyzed the spatial and velocity structure to
assign the emission to individual heating sources. We measure the optical depth
of the [CII] line and perform radiative transfer computations for all observed
transitions. By comparing the line intensities with the far-infrared continuum
we can assess the total cooling budget and measure the gas heating efficiency.
The main emission of fine structure lines in S 140 stems from a 8.3'' region
close to the infrared source IRS 2 that is not prominent at any other
wavelength. It can be explained by a photon-dominated region (PDR) structure
around the embedded cluster if we assume that the [OI] line intensity is
reduced by a factor seven due to self-absorption. The external cloud interface
forms a second PDR at an inclination of 80-85 degrees illuminated by an UV
field of 60 times the standard interstellar radiation field. The main radiation
source in the cloud, IRS 1, is not prominent at all in the fine structure
lines. We measure line-to-continuum cooling ratios below 10^(-4), i.e. values
lower than in any other Galactic source, rather matching the far-IR line
deficit seen in ULIRGs. In particular the low intensity of the [CII] line can
only be modeled by an extreme excitation gradient in the gas around IRS 1. We
found no explanation why IRS 1 shows no associated fine-structure line peak,
while IRS 2 does. The inner part of S 140 mimics the far-IR line deficit in
ULIRGs thereby providing a template that may lead to a future model.Comment: Accepted for publication in Astronomy & Astrophysic
An overview of the planned CCAT software system
CCAT will be a 25m diameter sub-millimeter telescope capable of operating in
the 0.2 to 2.1mm wavelength range. It will be located at an altitude of 5600m
on Cerro Chajnantor in northern Chile near the ALMA site. The anticipated first
generation instruments include large format (60,000 pixel) kinetic inductance
detector (KID) cameras, a large format heterodyne array and a direct detection
multi-object spectrometer. The paper describes the architecture of the CCAT
software and the development strategy.Comment: 17 pages, 6 figures, to appear in Software and Cyberinfrastructure
for Astronomy III, Chiozzi & Radziwill (eds), Proc. SPIE 9152, paper ID
9152-10
CMR exploration II -- filament identification with machine learning
We adopt magnetohydrodynamics (MHD) simulations that model the formation of
filamentary molecular clouds via the collision-induced magnetic reconnection
(CMR) mechanism under varying physical conditions. We conduct radiative
transfer using RADMC-3D to generate synthetic dust emission of CMR filaments.
We use the previously developed machine learning technique CASI-2D along with
the diffusion model to identify the location of CMR filaments in dust emission.
Both models showed a high level of accuracy in identifying CMR filaments in the
test dataset, with detection rates of over 80% and 70%, respectively, at a
false detection rate of 5%. We then apply the models to real Herschel dust
observations of different molecular clouds, successfully identifying several
high-confidence CMR filament candidates. Notably, the models are able to detect
high-confidence CMR filament candidates in Orion A from dust emission, which
have previously been identified using molecular line emission.Comment: ApJ accepte
CMR exploration I -- filament structure with synthetic observations
In this paper, we carry out a pilot parameter exploration for the
collision-induced magnetic reconnection (CMR) mechanism that forms filamentary
molecular clouds. Following Kong et al. (2021), we utilize Athena++ to model
CMR in the context of resistive magnetohydrodynamics (MHD), considering the
effect from seven physical conditions, including the Ohmic resistivity
(), the magnetic field (), the cloud density (), the cloud
radius , the isothermal temperature , the collision velocity , and
the shear velocity . Compared to their fiducial model, we consider a
higher and a lower value for each one of the seven parameters. We quantify the
exploration results with five metrics, including the density probability
distribution function (-PDF), the filament morphology (250 m dust
emission), the - relation, the dominant fiber width, and the ringiness
that describes the significance of the ring-like sub-structures. The
exploration forms straight and curved CMR-filaments with rich sub-structures
that are highly variable in space and time. The variation translates to
fluctuation in all the five metrics, reflecting the chaotic nature of magnetic
reconnection in CMR. A temporary relation is noticeable during
the first 0.6 Myr. Overall, the exploration provides useful initial insights to
the CMR mechanism.Comment: 31 pages, 20 figures, 1 tabl
The water abundance behind interstellar shocks: results from /PACS and /IRS observations of HO, CO, and H
We have investigated the water abundance in shock-heated molecular gas,
making use of measurements of far-infrared CO and HO line
emissions in combination with measurements of mid-IR H rotational
emissions. We present far-infrared line spectra obtained with 's PACS
instrument in range spectroscopy mode towards two positions in the protostellar
outflow NGC 2071 and one position each in the supernova remnants W28 and 3C391.
These spectra provide unequivocal detections, at one or more positions, of 12
rotational lines of water, 14 rotational lines of CO, 8 rotational lines of OH
(4 lambda doublets), and 7 fine-structure transitions of atoms or atomic ions.
We first used a simultaneous fit to the CO line fluxes, along with H
rotational line fluxes measured previously by , to constrain the
temperature and density distribution within the emitting gas; and we then
investigated the water abundances implied by the observed HO line fluxes.
The water line fluxes are in acceptable agreement with standard theoretical
models for nondissociative shocks that predict the complete vaporization of
grain mantles in shocks of velocity km/s, behind which the
characteristic gas temperature is K and the HO/CO ratio is 1.2Comment: 42 pages, 15 figures, accepted for publication in the Astrophysical
Journa
Herschel/HIFI Spectral Mapping of C, CH, and CH in Orion BN/KL: The Prevailing Role of Ultraviolet Irradiation in CH Formation
The CH ion is a key species in the initial steps of interstellar carbon
chemistry. Its formation in diverse environments where it is observed is not
well understood, however, because the main production pathway is so endothermic
(4280 K) that it is unlikely to proceed at the typical temperatures of
molecular clouds. We investigation CH formation with the first
velocity-resolved spectral mapping of the CH rotational
transitions, three sets of CH -doubled triplet lines, C and
C, and CHOH 835~GHz E-symmetry Q branch transitions, obtained
with Herschel/HIFI over 12 arcmin centered on the Orion BN/KL
source. We present the spatial morphologies and kinematics, cloud boundary
conditions, excitation temperatures, column densities, and C optical
depths. Emission from C, CH, and CH is indicated to arise in the
diluted gas, outside of the explosive, dense BN/KL outflow. Our models show
that UV-irradiation provides favorable conditions for steady-state production
of CH in this environment. Surprisingly, no spatial or kinematic
correspondences of these species are found with H S(1) emission tracing
shocked gas in the outflow. We propose that C is being consumed by rapid
production of CO to explain the lack of C and CH in the outflow, and
that fluorescence provides the reservoir of H excited to higher
ro-vibrational and rotational levels. Hence, in star-forming environments
containing sources of shocks and strong UV radiation, a description of CH
formation and excitation conditions is incomplete without including the
important --- possibly dominant --- role of UV irradiation.Comment: Accepted for publication in The Astrophysical Journa
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