585 research outputs found
Modelling the 3D physical structure of astrophysical sources with GASS
The era of interferometric observations leads to the need of a more and more
precise description of physical structures and dynamics of star-forming
regions, from pre-stellar cores to protoplanetary discs. The molecular emission
can be traced in multiple physical components such as infalling envelopes,
outflows and protoplanetary discs. To compare with the observations, a precise
and complex radiative transfer modelling of these regions is needed. We present
GASS (Generator of Astrophysical Sources Structure), a code that allows us to
generate the three-dimensional (3D) physical structure model of astrophysical
sources. From the GASS graphical interface, the user easily creates different
components such as spherical envelopes, outflows and discs. The physical
properties of these components are modelled thanks to dedicated graphical
interfaces that display various figures in order to help the user and
facilitate the modelling task. For each component, the code randomly generates
points in a 3D grid with a sample probability weighted by the molecular
density. The created models can be used as the physical structure input for 3D
radiative transfer codes to predict the molecular line or continuum emission.
An analysis of the output hyper-spectral cube given by such radiative transfer
code can be made directly in GASS using the various post-treatment options
implemented, such as calculation of moments or convolution with a beam. This
makes GASS well suited to model and analyse both interferometric and
single-dish data. This paper is focused on the results given by the association
of GASS and LIME, a 3D radiative transfer code, and we show that the complex
geometry observed in star-forming regions can be adequately handled by
GASS+LIME
An interferometric study of the low-mass protostar IRAS 16293-2422: small scale organic chemistry
Aims: To investigate the chemical relations between complex organics based on
their spatial distributions and excitation conditions in the low-mass young
stellar objects IRAS 16293-2422 A and B. Methods: Interferometric observations
with the Submillimeter Array have been performed at 5''x3'' resolution
revealing emission lines of HNCO, CH3CN, CH2CO, CH3CHO and C2H5OH. Rotational
temperatures are determined from rotational diagrams when a sufficient number
of lines are detected. Results: Compact emission is detected for all species
studied here. For HNCO and CH3CN it mostly arises from source A, CH2CO and
C2H5OH have comparable strength for both sources and CH3CHO arises exclusively
from source B. HNCO, CH3CN and CH3CHO have rotational temperatures >200 K. The
(u,v)-visibility data reveal that HNCO also has extended cold emission.
Conclusions: The abundances of the molecules studied here are very similar
within factors of a few to those found in high-mass YSOs. Thus the chemistry
between high- and low-mass objects appears to be independent of luminosity and
cloud mass. Bigger abundance differences are seen between the A and B source.
The HNCO abundance relative to CH3OH is ~4 times higher toward A, which may be
due to a higher initial OCN- ice abundances in source A compared to B.
Furthermore, not all oxygen-bearing species are co-existent. The different
spatial behavior of CH2CO and C2H5OH compared with CH3CHO suggests that
hydrogenation reactions on grain-surfaces are not sufficient to explain the
observed gas phase abundances. Selective destruction of CH3CHO may result in
the anti-coincidence of these species in source A. These results illustrate the
power of interferometric compared with single dish data in terms of testing
chemical models.Comment: 11 pages, 15 figures, accepeted by A&
Continuous star cluster formation in the spiral NGC 45
We determined ages for 52 star clusters with masses < 10^6 solar masses in
the low surface brightness spiral galaxy NGC 45. Four of these candidates are
old globular clusters located in the bulge. The remaining ones span a large age
range. The cluster ages suggest a continuous star/cluster formation history
without evidence for bursts, consistent with the galaxy being located in a
relatively unperturbed environment in the outskirts of the Sculptor group.Comment: 4 pages, 3 figures. To appear in "Island Universes - Structure and
Evolution of Disk Galaxies", Terschelling (Netherlands), July 200
Cold gas as an ice diagnostic toward low mass protostars
Up to 90% of the chemical reactions during star formation occurs on ice
surfaces, probably including the formation of complex organics. Only the most
abundant ice species are however observed directly by infrared spectroscopy.
This study aims to develop an indirect observational method of ices based on
non-thermal ice desorption in the colder part of protostellar envelopes. For
that purpose the IRAM 30m telescope was employed to observe two molecules that
can be detected both in the gas and the ice, CH3 OH and HNCO, toward 4 low mass
embedded protostars. Their respective gas-phase column densities are determined
using rotational diagrams. The relationship between ice and gas phase
abundances is subsequently determined. The observed gas and ice abundances span
several orders of magnitude. Most of the CH3OH and HNCO gas along the lines of
sight is inferred to be quiescent from the measured line widths and the derived
excitation temperatures, and hence not affected by thermal desorption close to
the protostar or in outflow shocks. The measured gas to ice ratio of ~10-4
agrees well with model predictions for non-thermal desorption under cold
envelope conditions and there is a tentative correlation between ice and gas
phase abundances. This indicates that non-thermal desorption products can serve
as a signature of the ice composition. A larger sample is however necessary to
provide a conclusive proof of concept.Comment: accepted by A&A letters, 10 pages including 5 figure
A Broad Search for Counterrotating Gas and Stars: Evidence for Mergers and Accretion
We measure the frequency of bulk gas-stellar counterrotation in a sample of
67 galaxies drawn from the Nearby Field Galaxy Survey, a broadly representative
survey of the local galaxy population down to M_B-15. We detect 4
counterrotators among 17 E/S0's with extended gas emission (24% +8 -6). In
contrast, we find no clear examples of bulk counterrotation among 38 Sa-Sbc
spirals, although one Sa does show peculiar gas kinematics. This result implies
that, at 95% confidence, no more than 8% of Sa-Sbc spirals are bulk
counterrotators. Among types Sc and later, we identify only one possible
counterrotator, a Magellanic irregular. We use these results together with the
physical properties of the counterrotators to constrain possible origins for
this phenomenon.Comment: 19 pages, 4 figures, AJ, accepte
Imaging of star clusters in unperturbed spiral galaxies with the Advanced Camera for Surveys. I. The low luminosity galaxy NGC 45
We present results from ACS and WFPC observations in the low luminosity
galaxy NGC 45. We identified 28 young star cluster candidates. While the exact
values of age, mass, and extinction depend somewhat on the choice of SSP
models, we find no young clusters with masses higher than a few 1000 Msun for
any model choice. We derive the luminosity function of young star clusters and
find a slope of alpha=-1.94+-0.28. We also identified 19 old globular clusters
and we estimate a specific frequency of globular clusters of S_N=1.4-1.9 which
is significantly higher than observed for other late-type galaxies (e.g. SMC,
LMC, M33). Most of these globular clusters appear to belong to a metal-poor
population, although they coincide spatially with the location of the bulge of
NGC 45.Comment: 16 pages,18 figures, accepted for publication in A&
TIMASSS : The IRAS16293-2422 Millimeter And Submillimeter Spectral Survey: Tentative Detection of Deuterated Methyl Formate (DCOOCH3)
High deuterium fractionation is observed in various types of environment such
as prestellar cores, hot cores and hot corinos. It has proven to be an
efficient probe to study the physical and chemical conditions of these
environments. The study of the deuteration of different molecules helps us to
understand their formation. This is especially interesting for complex
molecules such as methanol and bigger molecules for which it may allow to
differentiate between gas-phase and solid-state formation pathways. Methanol
exhibits a high deuterium fractionation in hot corinos. Since CH3OH is thought
to be a precursor of methyl formate we expect that deuterated methyl formate is
produced in such environments. We have searched for the singly-deuterated
isotopologue of methyl formate, DCOOCH3, in IRAS 16293-2422, a hot corino
well-known for its high degree of methanol deuteration. We have used the
IRAM/JCMT unbiased spectral survey of IRAS 16293-2422 which allows us to search
for the DCOOCH3 rotational transitions within the survey spectral range (80-280
GHz, 328-366 GHz). The expected emission of deuterated methyl formate is
modelled at LTE and compared with the observations.} We have tentatively
detected DCOOCH3 in the protostar IRAS 16293-2422. We assign eight lines
detected in the IRAM survey to DCOOCH3. Three of these lines are affected by
blending problems and one line is affected by calibration uncertainties,
nevertheless the LTE emission model is compatible with the observations. A
simple LTE modelling of the two cores in IRAS 16293-2422, based on a previous
interferometric study of HCOOCH3, allows us to estimate the amount of DCOOCH3
in IRAS 16293-2422. Adopting an excitation temperature of 100 K and a source
size of 2\arcsec and 1\farcs5 for the A and B cores, respectively, we find that
N(A,DCOOCH3) = N(B,DCOOCH3) ~ 6.10^14 /cm2. The derived deuterium fractionation
is ~ 15%, consistent with values for other deuterated species in this source
and much greater than that expected from the deuterium cosmic abundance.
DCOOCH3, if its tentative detection is confirmed, should now be considered in
theoretical models that study complex molecule formation and their deuteration
mechanisms. Experimental work is also needed to investigate the different
chemical routes leading to the formation of deuterated methyl formate
Near-arcsecond resolution observations of the hot corino of the solar type protostar IRAS 16293-2422
Complex organic molecules have previously been discovered in solar type
protostars, raising the questions of where and how they form in the envelope.
Possible formation mechanisms include grain mantle evaporation, interaction of
the outflow with its surroundings or the impact of UV/X-rays inside the
cavities. In this Letter we present the first interferometric observations of
two complex molecules, CH3CN and HCOOCH3, towards the solar type protostar
IRAS16293-2422. The images show that the emission originates from two compact
regions centered on the two components of the binary system. We discuss how
these results favor the grain mantle evaporation scenario and we investigate
the implications of these observations for the chemical composition and
physical and dynamical state of the two components.Comment: 5 pages (apjemulate), 2 figures; accepted by ApJ
The Baryon Content of Extremely Low Mass Dwarf Galaxies
We investigate the gas content and baryonic Tully-Fisher relationship for
extremely low luminosity dwarf galaxies in the absolute magnitude range -13.5 >
Mr > -16. The sample is selected from the Sloan Digital Sky Survey and consists
of 101 galaxies for which we have obtained follow-up HI observations using the
Arecibo Observatory and Green Bank Telescope. This represents the largest
homogeneous sample of dwarfs at low luminosities with well-measured HI and
optical properties. The sample spans a range of environments, from dense groups
to truly isolated galaxies. The average neutral gas fraction is f_gas=0.6,
significantly exceeding that of typical gas-rich galaxies at higher
luminosities. Dwarf galaxies are therefore less efficient at turning gas into
stars over their lifetimes. The strong environmental dependence of the gas
fraction distribution demonstrates that while internal processes can reduce the
gas fractions to roughly f_gas=0.4, external processes are required to fully
remove gas from a dwarf galaxy. The average rotational velocity of our sample
is vrot=50 km/s. Including more massive galaxies from the literature, we fit a
baryonic Tully-Fisher slope of M_baryon \propto vrot^(3.70+/- 0.15). This slope
compares well with CDM models that assume an equal baryon to dark matter ratio
at all masses. While gas stripping or other processes may modify the baryon to
dark matter ratio for dwarfs in the densest environments, the majority of dwarf
galaxies in our sample have not preferentially lost significant baryonic mass
relative to more massive galaxies.Comment: 33 pages, 8 figures. Accepted to ApJ. Data available at
http://www.ociw.edu/~mgeha/researc
Broadening of Spectral Lines due to Dynamic Multiple Scattering and the Tully-Fisher Relation
The frequency shift of spectral lines is most often explained by the Doppler
Effect in terms of relative motion, whereas the Doppler broadening of a
particular line mainly depends on the absolute temperature. The Wolf effect on
the other hand deals with the correlation induced spectral change and explains
both the broadening and shift of the spectral lines. In this framework a
relation between the width of the spectral line is related to the redshift z
for the line and hence with the distance. For smaller values of z a relation
similar to the Tully-Fisher relation can be obtained and for larger values of z
a more general relation can be constructed. The derivation of this kind of
relation based on dynamic multiple scattering theory may play a significant
role in explaining the overall spectra of quasi stellar objects. We emphasize
that this mechanism is not applicable for nearby galaxies, .Comment: 18 pages, 5 figures, revised Version has been submitted to Physical
Review A. (2nd author's affiliation corrected
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