347 research outputs found
Models of turbulent dissipation regions in the diffuse interstellar medium
Supersonic turbulence is a large reservoir of suprathermal energy in the
interstellar medium. Its dissipation, because it is intermittent in space and
time, can deeply modify the chemistry of the gas. We further explore a hybrid
method to compute the chemical and thermal evolution of a magnetized
dissipative structure, under the energetic constraints provided by the observed
properties of turbulence in the cold neutral medium. For the first time, we
model a random line of sight by taking into account the relative duration of
the bursts with respect to the thermal and chemical relaxation timescales of
the gas. The key parameter is the turbulent rate of strain "a" due to the
ambient turbulence. With the gas density, it controls the size of the
dissipative structures, therefore the strength of the burst. For a large range
of rates of strain and densities, the models of turbulent dissipation regions
(TDR) reproduce the CH+ column densities observed in the diffuse medium and
their correlation with highly excited H2. They do so without producing an
excess of CH. As a natural consequence, they reproduce the abundance ratios of
HCO+/OH and HCO+/H2O, and their dynamic range of about one order of magnitude
observed in diffuse gas. Large C2H and CO abundances, also related to those of
HCO+, are another outcome of the TDR models that compare well with observed
values. The abundances and column densities computed for CN, HCN and HNC are
one order of magnitude above PDR model predictions, although still
significantly smaller than observed values
Measurement of two-halo neutron transfer reaction p(Li,Li)t at 3 MeV
The p(\nuc{11}{Li},\nuc{9}{Li})t reaction has been studied for the first time
at an incident energy of 3 MeV delivered by the new ISAC-2 facility at
TRIUMF. An active target detector MAYA, build at GANIL, was used for the
measurement. The differential cross sectionshave been determined for
transitions to the \nuc{9}{Li} ground andthe first excited states in a wide
range of scattering angles. Multistep transfer calculations using different
\nuc{11}{Li} model wave functions, shows that wave functions with strong
correlations between the halo neutrons are the most successful in reproducing
the observation.Comment: 6 pages, 3 figures, submitted to Physical Review Letter
CO/H2 Abundance Ratio ~ 10^{-4} in a Protoplanetary Disk
The relative abundances of atomic and molecular species in planet-forming
disks around young stars provide important constraints on photochemical disk
models and provide a baseline for calculating disk masses from measurements of
trace species. A knowledge of absolute abundances, those relative to molecular
hydrogen (H2), are challenging because of the weak rovibrational transition
ladder of H and the inability to spatially resolve different emission
components within the circumstellar environment. To address both of these
issues, we present new contemporaneous measurements of CO and H2 absorption
through the "warm molecular layer" of the protoplanetary disk around the
Classical T Tauri Star RW Aurigae A. We use a newly commissioned observing mode
of the Hubble Space Telescope-Cosmic Origins Spectrograph to detect warm H2
absorption in this region for the first time. An analysis of the emission and
absorption spectrum of RW Aur shows components from the accretion region near
the stellar photosphere, the molecular disk, and several outflow components.
The warm H2 and CO absorption lines are consistent with a disk origin. We model
the 1092-1117A spectrum of RW Aur to derive log10
N(H2)~=~19.90 at T(H2) ~=~440~+/-~39 K. The CO
~--~ bands observed from 1410-1520A are best fit by log10
N(CO)~=~16.1~ at T(CO) ~=~200 K.
Combining direct measurements of the HI, H2, and CO column densities, we find a
molecular fraction in the warm disk surface of ~>=~0.47 and derive a
molecular abundance ratio of CO/H2~=~1.6~x~10, both
consistent with canonical interstellar dense cloud values.Comment: ApJ - accepted. 13 pages, 8 figure
The Dark Molecular Gas
The mass of molecular gas in an interstellar cloud is often measured using
line emission from low rotational levels of CO, which are sensitive to the CO
mass, and then scaling to the assumed molecular hydrogen H_2 mass. However, a
significant H_2 mass may lie outside the CO region, in the outer regions of the
molecular cloud where the gas phase carbon resides in C or C+. Here, H_2
self-shields or is shielded by dust from UV photodissociation, where as CO is
photodissociated. This H_2 gas is "dark" in molecular transitions because of
the absence of CO and other trace molecules, and because H_2 emits so weakly at
temperatures 10 K < T < 100 K typical of this molecular component. This
component has been indirectly observed through other tracers of mass such as
gamma rays produced in cosmic ray collisions with the gas and
far-infrared/submillimeter wavelength dust continuum radiation. In this paper
we theoretically model this dark mass and find that the fraction of the
molecular mass in this dark component is remarkably constant (~ 0.3 for average
visual extinction through the cloud with mean A_V ~ 8) and insensitive to the
incident ultraviolet radiation field strength, the internal density
distribution, and the mass of the molecular cloud as long as mean A_V, or
equivalently, the product of the average hydrogen nucleus column and the
metallicity through the cloud, is constant. We also find that the dark mass
fraction increases with decreasing mean A_V, since relatively more molecular
H_2 material lies outside the CO region in this case.Comment: 38 page, 11 figures, Accepted for Publication in ApJ, corrected
citation and typo in Appendix
CO-dark gas and molecular filaments in Milky Way type galaxies
We use the moving mesh code AREPO coupled to a time-dependent chemical
network to investigate the formation and destruction of molecular gas in
simulated spiral galaxies. This allows us to determine the characteristics of
the gas that is not traced by CO emission. Our extremely high resolution AREPO
simulations allow us to capture the chemical evolution of the disc, without
recourse to a parameterised `clumping factor'. We calculate H2 and CO column
densities through our simulated disc galaxies, and estimate the CO emission and
CO-H2 conversion factor. We find that in conditions akin to those in the local
interstellar medium, around 42% of the total molecular mass should be in
CO-dark regions, in reasonable agreement with observational estimates. This
fraction is almost insensitive to the CO integrated intensity threshold used to
discriminate between CO-bright and CO-dark gas, as long as this threshold is
less than 10 K km/s. The CO-dark molecular gas primarily resides in extremely
long (>100 pc) filaments that are stretched between spiral arms by galactic
shear. Only the centres of these filaments are bright in CO, suggesting that
filamentary molecular clouds observed in the Milky Way may only be small parts
of much larger structures. The CO-dark molecular gas mainly exists in a
partially molecular phase which accounts for a significant fraction of the
total disc mass budget. The dark gas fraction is higher in simulations with
higher ambient UV fields or lower surface densities, implying that external
galaxies with these conditions might have a greater proportion of dark gas.Comment: Accepted by MNRA
Critical angular momentum distributions in collapsars: quiescent periods from accretion state transitions in long gamma-ray bursts
The rotation rate in pre-supernova cores is an important ingredient which can
profoundly affect the post-collapse evolution and associated energy release in
supernovae and long gamma ray bursts (LGRBs). Previous work has focused on
whether the specific angular momentum is above or below the critical value
required for the creation of a centrifugally supported disk around a black
hole. Here, we explore the effect of the distribution of angular momentum with
radius in the star, and show that qualitative transitions between high and low
angular momentum flow, corresponding to high and low luminosity accretion
states, can effectively be reflected in the energy output, leading to
variability and the possibility of quiescent times in LGRBs.Comment: 22 pages, 6 figures, 2 Tables, accepted for publication in Ap
Loopy Cuts: Surface-Field Aware Block Decomposition for Hex-Meshing.
We present a new fully automatic block-decomposition hexahedral meshing
algorithm capable of producing high quality meshes that strictly preserve
feature curve networks on the input surface and align with an input surface
cross-field. We produce all-hex meshes on the vast majority of inputs, and
introduce localized non-hex elements only when the surface feature network
necessitates those. The input to our framework is a closed surface with a
collection of geometric or user-demarcated feature curves and a feature-aligned
surface cross-field. Its output is a compact set of blocks whose edges
interpolate these features and are loosely aligned with this cross-field. We
obtain this block decomposition by cutting the input model using a collection
of simple cutting surfaces bounded by closed surface loops. The set of cutting
loops spans the input feature curves, ensuring feature preservation, and is
obtained using a field-space sampling process. The computed loops are uniformly
distributed across the surface, cross orthogonally, and are loosely aligned
with the cross-field directions, inducing the desired block decomposition. We
validate our method by applying it to a large range of complex inputs and
comparing our results to those produced by state-of-the-art alternatives.
Contrary to prior approaches, our framework consistently produces high-quality
field aligned meshes while strictly preserving geometric or user-specified
surface features
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