1,331 research outputs found
Modeling molecular hyperfine line emission
In this paper we discuss two approximate methods previously suggested for
modeling hyperfine spectral line emission for molecules whose collisional
transitions rates between hyperfine levels are unknown. Hyperfine structure is
seen in the rotational spectra of many commonly observed molecules such as HCN,
HNC, NH3, N2H+, and C17O. The intensities of these spectral lines can be
modeled by numerical techniques such as Lambda-iteration that alternately solve
the equations of statistical equilibrium and the equation of radiative
transfer. However, these calculations require knowledge of both the radiative
and collisional rates for all transitions. For most commonly observed radio
frequency spectral lines, only the net collisional rates between rotational
levels are known. For such cases, two approximate methods have been suggested.
The first method, hyperfine statistical equilibrium (HSE), distributes the
hyperfine level populations according to their statistical weight, but allows
the population of the rotational states to depart from local thermodynamic
equilibrium (LTE). The second method, the proportional method approximates the
collision rates between the hyperfine levels as fractions of the net rotational
rate apportioned according to the statistical degeneracy of the final hyperfine
levels. The second method is able to model non-LTE hyperfine emission. We
compare simulations of N2H+ hyperfine lines made with approximate and more
exact rates and find that satisfactory results are obtained.Comment: 34 pages. Pages 22-34 are data tables. For ApJ
The standard model of star formation applied to massive stars: accretion disks and envelopes in molecular lines
We address the question of whether the formation of high-mass stars is
similar to or differs from that of solar-mass stars through new molecular line
observations and modeling of the accretion flow around the massive protostar
IRAS20126+4104. We combine new observations of NH3(1,1) and (2,2) made at the
Very Large Array, new observations of CHCN(13-12) made at the Submillimeter
Array, previous VLA observations of NH(3,3), NH(4,4), and previous Plateau de
Bure observations of C34S(2-1), C34S(5-4), and CHCN(12-11) to obtain a data set
of molecular lines covering 15 to 419 K in excitation energy. We compare these
observations against simulated molecular line spectra predicted from a model
for high-mass star formation based on a scaled-up version of the standard
disk-envelope paradigm developed for accretion flows around low-mass stars. We
find that in accord with the standard paradigm, the observations require both a
warm, dense, rapidly-rotating disk and a cold, diffuse infalling envelope. This
study suggests that accretion processes around 10 M stars are similar to those
of solar mass stars.Comment: Accepted MNRA
High Resolution CO Observations of Massive Star Forming Regions
Context. To further understand the processes involved in the formation of
massive stars, we have undertaken a study of the gas dynamics surrounding three
massive star forming regions. By observing the large scale structures at high
resolution, we are able to determine properties such as driving source, and
spatially resolve the bulk dynamical properties of the gas such as infall and
outflow. Aims. With high resolution observations, we are able to determine
which of the cores in a cluster forming massive stars is responsible for the
large scale structures. Methods. We present CO observations of three massive
star forming regions with known HII regions and show how the CO traces both
infall and outflow. By combining data taken in two SMA configurations with JCMT
observations, we are able to see large scale structures at high resolution.
Results. We find large (0.26-0.40 pc), massive (2-3 M_sun) and energetic (13-17
\times 10^44 erg) outflows emanating from the edges of two HII regions
suggesting they are being powered by the protostar(s) within. We find infall
signatures in two of our sources with mass infall rates of order 10-4 M_sun/yr.
Conclusions. We suggest that star formation is ongoing in these sources despite
the presence of HII regions. We further conclude that the source(s) within a
single HII region are responsible for the observed large scale structures; that
these large structures are not the net effect of multiple outflows from
multiple HII regions and hot cores.Comment: 8 pages,2 figures, accepted for publication in A&
Interferometric array design: optimizing the locations of the antenna pads
The design of an interferometric array should allow optimal instrumental
response regarding all possible source positions, times of integration and
scientific goals. It should also take into account constraints such as
forbidden regions on the ground due to impracticable topography. The complexity
of the problem requires one to proceed by steps. A possible approach is to
first consider a single observation and a single scientific purpose. A new
algorithm is introduced to solve efficiently this particular problem called the
configuration problem. It is based on the computation of pressure forces
related to the discrepancies between the model (as determined by the scientific
purpose) and the actual distribution of Fourier samples. The flexibility and
rapidity of the method are well adapted to the full array design. A software
named APO that can be used for the design of new generation interferometers
such as ALMA and ATA has been developed.Comment: 9 pages, 7 figure
The Evolution of Cloud Cores and the Formation of Stars
For a number of starless cores, self-absorbed molecular line and column
density observations have implied the presence of large-amplitude oscillations.
We examine the consequences of these oscillations on the evolution of the cores
and the interpretation of their observations. We find that the pulsation energy
helps support the cores and that the dissipation of this energy can lead toward
instability and star formation. In this picture, the core lifetimes are limited
by the pulsation decay timescales, dominated by non-linear mode-mode coupling,
and on the order of ~few x 10^5--10^6 yr. Notably, this is similar to what is
required to explain the relatively low rate of conversion of cores into stars.
For cores with large-amplitude oscillations, dust continuum observations may
appear asymmetric or irregular. As a consequence, some of the cores that would
be classified as supercritical may be dynamically stable when oscillations are
taken into account. Thus, our investigation motivates a simple hydrodynamic
picture, capable of reproducing many of the features of the progenitors of
stars without the inclusion of additional physical processes, such as
large-scale magnetic fields.Comment: 12 pages, 7 figures, submitted to Ap
Dynamic analysis of space structures including elastic, multibody, and control behavior
The problem is to develop analysis methods, modeling stategies, and simulation tools to predict with assurance the on-orbit performance and integrity of large complex space structures that cannot be verified on the ground. The problem must incorporate large reliable structural models, multi-body flexible dynamics, multi-tier controller interaction, environmental models including 1g and atmosphere, various on-board disturbances, and linkage to mission-level performance codes. All areas are in serious need of work, but the weakest link is multi-body flexible dynamics
Chemistry and Radiative Transfer of Water in Cold, Dense Clouds
The Herschel Space Observatory's recent detections of water vapor in the
cold, dense cloud L1544 allow a direct comparison between observations and
chemical models for oxygen species in conditions just before star formation. We
explain a chemical model for gas phase water, simplified for the limited number
of reactions or processes that are active in extreme cold ( 15 K). In this
model, water is removed from the gas phase by freezing onto grains and by
photodissociation. Water is formed as ice on the surface of dust grains from O
and OH and released into the gas phase by photodesorption. The reactions are
fast enough with respect to the slow dynamical evolution of L1544 that the gas
phase water is in equilibrium for the local conditions thoughout the cloud. We
explain the paradoxical radiative transfer of the HO ()
line. Despite discouragingly high optical depth caused by the large Einstein A
coefficient, the subcritical excitation in the cold, rarefied H causes the
line brightness to scale linearly with column density. Thus the water line can
provide information on the chemical and dynamical processes in the darkest
region in the center of a cold, dense cloud. The inverse P-Cygni profile of the
observed water line generally indicates a contracting cloud. This profile is
reproduced with a dynamical model of slow contraction from unstable
quasi-static hydrodynamic equilibrium (an unstable Bonnor-Ebert sphere).Comment: submitted to MNRA
Is protostellar heating sufficient to halt fragmentation? A case study of the massive protocluster G8.68-0.37
If star formation proceeds by thermal fragmentation and the subsequent
gravitational collapse of the individual fragments, how is it possible to form
fragments massive enough for O and B stars in a typical star-forming molecular
cloud where the Jeans mass is about 1Msun at the typical densities (10^4 cm^-3)
and temperatures (10K)? We test the hypothesis that a first generation of
low-mass stars may heat the gas enough that subsequent thermal fragmentation
results in fragments >=10Msun, sufficient to form B stars. We combine ATCA and
SMA observations of the massive star-forming region G8.68-0.37 with radiative
transfer modeling to derive the present-day conditions in the region and use
this to infer the conditions in the past, at the time of core formation.
Assuming the current mass/separation of the observed cores equals the
fragmentation Jeans mass/length and the region's average density has not
changed, requires the gas temperature to have been 100K at the time of
fragmentation. The postulated first-generation of low-mass stars would still be
around today, but the number required to heat the cloud exceeds the limits
imposed by the observations. Several lines of evidence suggest the observed
cores in the region should eventually form O stars yet none have sufficient raw
material. Even if feedback may have suppressed fragmentation, it was not
sufficient to halt it to this extent. To develop into O stars, the cores must
obtain additional mass from outside their observationally defined boundaries.
The observations suggest they are currently fed via infall from the very
massive reservoir (~1500Msun) of gas in the larger pc scale cloud around the
star-forming cores. This suggests that massive stars do not form in the
collapse of individual massive fragments, but rather in smaller fragments that
themselves continue to gain mass by accretion from larger scales.Comment: 23 pages, 14 figures. Accepted for publication in Ap
Understanding hydrogen recombination line observations with ALMA and EVLA
Hydrogen recombination lines are one of the major diagnostics of H II region
physical properties and kinematics. In the near future, the Expanded Very Large
Array (EVLA) and the Atacama Large Millimeter Array (ALMA) will allow observers
to study recombination lines in the radio and sub-mm regime in unprecedented
detail. In this paper, we study the properties of recombination lines, in
particular at ALMA wavelengths. We find that such lines will lie in almost
every wideband ALMA setup and that the line emission will be equally detectable
in all bands. Furthermore, we present our implementation of hydrogen
recombination lines in the adaptive-mesh radiative transfer code RADMC-3D. We
particularly emphasize the importance of non-LTE (local thermodynamical
equilibrium) modeling since non-LTE effects can drastically affect the line
shapes and produce asymmetric line profiles from radially symmetric H II
regions. We demonstrate how these non-LTE effects can be used as a probe of
systematic motions (infall & outflow) in the gas. We use RADMC-3D to produce
synthetic observations of model H II regions and study the necessary conditions
for observing such asymmetric line profiles with ALMA and EVLA.Comment: MNRAS in pres
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