5,358 research outputs found
ISM properties in hydrodynamic galaxy simulations: Turbulence cascades, cloud formation, role of gravity and feedback
We study the properties of ISM substructure and turbulence in hydrodynamic
(AMR) galaxy simulations with resolutions up to 0.8 pc and 5x10^3 Msun. We
analyse the power spectrum of the density distribution, and various components
of the velocity field. We show that the disk thickness is about the average
Jeans scale length, and is mainly regulated by gravitational instabilities.
From this scale of energy injection, a turbulence cascade towards small-scale
is observed, with almost isotropic small-scale motions. On scales larger than
the disk thickness, density waves are observed, but there is also a full range
of substructures with chaotic and strongly non-isotropic gas velocity
dispersions. The power spectrum of vorticity in an LMC-sized model suggests
that an inverse cascade of turbulence might be present, although energy input
over a wide range of scales in the coupled gaseous+stellar fluid could also
explain this quasi-2D regime on scales larger than the disk scale height.
Similar regimes of gas turbulence are also found in massive high-redshift disks
with high gas fractions. Disk properties and ISM turbulence appear to be mainly
regulated by gravitational processes, both on large scales and inside dense
clouds. Star formation feedback is however essential to maintain the ISM in a
steady state by balancing a systematic gas dissipation into dense and small
clumps. Our galaxy simulations employ a thermal model based on a barotropic
Equation of State (EoS) aimed at modelling the equilibrium of gas between
various heating and cooling processes. Denser gas is typically colder in this
approach, which is shown to correctly reproduce the density structures of a
star-forming, turbulent, unstable and cloudy ISM down to scales of a few
parsecs.Comment: MNRAS in pres
Blind decomposition of Herschel-HIFI spectral maps of the NGC 7023 nebula
Large spatial-spectral surveys are more and more common in astronomy. This
calls for the need of new methods to analyze such mega- to giga-pixel
data-cubes. In this paper we present a method to decompose such observations
into a limited and comprehensive set of components. The original data can then
be interpreted in terms of linear combinations of these components. The method
uses non-negative matrix factorization (NMF) to extract latent spectral
end-members in the data. The number of needed end-members is estimated based on
the level of noise in the data. A Monte-Carlo scheme is adopted to estimate the
optimal end-members, and their standard deviations. Finally, the maps of linear
coefficients are reconstructed using non-negative least squares. We apply this
method to a set of hyperspectral data of the NGC 7023 nebula, obtained recently
with the HIFI instrument onboard the Herschel space observatory, and provide a
first interpretation of the results in terms of 3-dimensional dynamical
structure of the region.Comment: Proceedings of the 2012 meeting of the french astronomical society
(SF2A) in Nic
The Footprint Database and Web Services of the Herschel Space Observatory
Data from the Herschel Space Observatory is freely available to the public
but no uniformly processed catalogue of the observations has been published so
far. To date, the Herschel Science Archive does not contain the exact sky
coverage (footprint) of individual observations and supports search for
measurements based on bounding circles only. Drawing on previous experience in
implementing footprint databases, we built the Herschel Footprint Database and
Web Services for the Herschel Space Observatory to provide efficient search
capabilities for typical astronomical queries. The database was designed with
the following main goals in mind: (a) provide a unified data model for
meta-data of all instruments and observational modes, (b) quickly find
observations covering a selected object and its neighbourhood, (c) quickly find
every observation in a larger area of the sky, (d) allow for finding solar
system objects crossing observation fields. As a first step, we developed a
unified data model of observations of all three Herschel instruments for all
pointing and instrument modes. Then, using telescope pointing information and
observational meta-data, we compiled a database of footprints. As opposed to
methods using pixellation of the sphere, we represent sky coverage in an exact
geometric form allowing for precise area calculations. For easier handling of
Herschel observation footprints with rather complex shapes, two algorithms were
implemented to reduce the outline. Furthermore, a new visualisation tool to
plot footprints with various spherical projections was developed. Indexing of
the footprints using Hierarchical Triangular Mesh makes it possible to quickly
find observations based on sky coverage, time and meta-data. The database is
accessible via a web site (http://herschel.vo.elte.hu) and also as a set of
REST web service functions.Comment: Accepted for publication in Experimental Astronom
The Horsehead mane: Towards an observational benchmark for chemical models
After a discussion about the need for observational benchmark for chemical
models, we explain 1) why the Horsehead western edge is well suited to serve as
reference for models and 2) the steps we are taking toward this goal. We
summarize abundances obtained to date and we show recent results
Connection between PAHs and small hydrocarbons in the Horsehead Nebula Photo-Dissociation Region
We present recent observations of small hydrocarbons (C3H2, C2H, C4H) with
high abundances in the Photo-Dissociation Region of the Horsehead nebula. Our
results show for the first time observational indications that the small
hydrocarbon distribution follows the Aromatic Infrared Bands (AIBs) emission
traced by ISO-LW2 (5-8.5 microns), whereas it does not coincide with the CO and
isotopes large-scale distribution. The derived abundances are significantly
higher than in local clouds. This enhancement might be explained by an in situ
formation assisted by the release of carbonaceous molecules from UV-irradiated
aromatic particles.Comment: 3 pages, 2 figures. To appear in the proceedings of "Chemistry as a
Diagnostic of Star Formation", University of Waterloo, Canada, August 2002
(C.L. Curry and M. Fich eds
ISM properties in hydrodynamic galaxy simulations: turbulence cascades, cloud formation, role of gravity and feedback
We study the properties of interstellar medium (ISM) substructure and turbulence in hydrodynamic [adaptive mesh refinement (AMR)] galaxy simulations with resolutions up to 0.8 pc and 5 Ă 103 Mâ. We analyse the power spectrum of the density distribution, and various components of the velocity field. We show that the disc thickness is about the average Jeans scalelength, and is mainly regulated by gravitational instabilities. From this scale of energy injection, a turbulence cascade towards small scale is observed, with almost isotropic small-scale motions. On scales larger than the disc thickness, density waves are observed, but there is also a full range of substructures with chaotic and strongly non-isotropic gas velocity dispersions. The power spectrum of vorticity in a Large Magellanic Cloud sized model suggests that an inverse cascade of turbulence might be present, although energy input over a wide range of scales in the coupled gaseous+stellar fluid could also explain this quasi-two-dimensional regime on scales larger than the disc scaleheight. Similar regimes of gas turbulence are also found in massive high-redshift discs with high gas fractions. Disc properties and ISM turbulence appear to be mainly regulated by gravitational processes, both on large scales and inside dense clouds. Star formation feedback is however essential to maintain the ISM in a steady state by balancing a systematic gas dissipation into dense and small clumps. Our galaxy simulations employ a thermal model based on a barotropic equation of state aimed at modelling the equilibrium of gas between various heating and cooling processes. Denser gas is typically colder in this approach, which is shown to correctly reproduce the density structures of a star-forming, turbulent, unstable and cloudy ISM down to scales of a few parsec
Star Formation Near Photodissociation Regions: Detection of a Peculiar Protostar Near Ced 201
We present the detection and characterization of a peculiar low-mass
protostar (IRAS 22129+7000) located ~0.4 pc from Ced 201 Photodissociation
Region (PDR) and ~0.2 pc from the HH450 jet. The cold circumstellar envelope
surrounding the object has been mapped through its 1.2 mm dust continuum
emission with IRAM-30m/MAMBO. The deeply embedded protostar is clearly detected
with Spitzer/MIPS (70 um), IRS (20-35 um) and IRAC (4.5, 5.8, and 8 um) but
also in the K_s band (2.15 um). Given the large "near- and mid-IR excess" in
its spectral energy distribution, but large submillimeter-to-bolometric
luminosity ratio (~2%), IRAS 22129+7000 must be a transition Class 0/I source
and/or a multiple stellar system. Targeted observations of several molecular
lines from CO, 13CO, C18O, HCO+ and DCO+ have been obtained. The presence of a
collimated molecular outflow mapped with the CSO telescope in the CO J=3-2 line
suggests that the protostar/disk system is still accreting material from its
natal envelope. Indeed, optically thick line profiles from high density tracers
such as HCO+ J=1-0 show a red-shifted-absorption asymmetry reminiscent of
inward motions. We construct a preliminary physical model of the circumstellar
envelope (including radial density and temperature gradients, velocity field
and turbulence) that reproduces the observed line profiles and estimates the
ionization fraction. The presence of both mechanical and (non-ionizing)
FUV-radiative input makes the region an interesting case to study triggered
star formation
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