489 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&
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
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
Planetary Nebulae as standard candles XI. Application to Spiral Galaxies
We report the results of an [O III] lambda 5007 survey for planetary nebulae
(PN) in three spiral galaxies: M101 (NGC 5457), M51 (NGC 5194/5195) and M96
(NGC 3368). By comparing on-band/off-band [O III] lambda 5007 images with
images taken in H-alpha and broadband R, we identify 65, 64 and 74 PN
candidates in each galaxy, respectively. From these data, an adopted M31
distance of 770 kpc, and the empirical planetary nebula luminosity function
(PNLF), we derive distances to M101, M51, and M96 of 7.7 +/- 0.5, 8.4 +/- 0.6,
and 9.6 +/- 0.6 Mpc. These observations demonstrate that the PNLF technique can
be successfully applied to late-type galaxies, and provide an important overlap
between the Population I and Population II distance scales. We also discuss
some special problems associated with using the PNLF in spiral galaxies,
including the effects of dust and the possible presence of [O III] bright
supernova remnants.Comment: 38 pages, TeX, with tables included but not figures. Uses epsf.tex
and kpnobasic.tex. To be published in the Astophysical Journal. Full paper is
available at http://www.astro.psu.edu/users/johnf/Text/research.htm
Clues to Nuclear Star Cluster Formation from Edge-on Spirals
We find 9 nuclear cluster candidates in a sample of 14 edge-on, late-type
galaxies observed with HST/ACS. These clusters have magnitudes (M_I ~ -11) and
sizes (r_eff ~ 3pc) similar to those found in previous studies of face-on,
late-type spirals and dE galaxies. However, three of the nuclear clusters are
significantly flattened and show evidence for multiple, coincident structural
components. The elongations of these three clusters are aligned to within 10
degrees of the galaxies' major axes. Structurally, the flattened clusters are
well fit by a combination of a spheroid and a disk or ring. The nuclear cluster
disks/rings have F606W-F814W (~V-I) colors 0.3-0.6 magnitudes bluer than the
spheroid components, suggesting that the stars in these components have ages <
1 Gyr. In NGC 4244, the nearest of the nuclear clusters, we further constrain
the stellar populations and provide a lower limit on the dynamical mass via
spectroscopy. We also present tentative evidence that another of the nuclear
clusters (in NGC 4206) may also host a supermassive black hole. Based on our
observational results we propose an in situ formation mechanism for nuclear
clusters in which stars form episodically in compact nuclear disks, and then
lose angular momentum or heat vertically to form an older spheroidal structure.
We estimate the period between star formation episodes to be 0.5 Gyr and
discuss possible mechanisms for tranforming the disk-like components into
spheroids. We also note the connection between our objects and massive globular
clusters (e.g. Cen), UCDs, and SMBHs. (Abridged)Comment: Accepted for publication in the A
Complex molecules toward low-mass protostars: the Serpens core
Gas-phase complex organic molecules are commonly detected toward high-mass
protostellar hot cores. Detections toward low-mass protostars and outflows are
comparatively rare, and a larger sample is key to investigate how the chemistry
responds to its environment. Guided by the prediction that complex organic
molecules form in CH3OH-rich ices and thermally or non-thermally evaporate with
CH3OH, we have identified three sight-lines in the Serpens core - SMM1, SMM4
and SMM4-W - which are likely to be rich in complex organics. Using the IRAM
30m telescope, narrow lines (FWHM of 1-2 km s-1) of CH3CHO and CH3OCH3 are
detected toward all sources, HCOOCH3 toward SMM1 and SMM4-W, and C2H5OH not at
all. Beam-averaged abundances of individual complex organics range between 0.6
and 10% with respect to CH3OH when the CH3OH rotational temperature is applied.
The summed complex organic abundances also vary by an order of magnitude, with
the richest chemistry toward the most luminous protostar SMM1. The range of
abundances compare well with other beam-averaged observations of low-mass
sources. Complex organic abundances are of the same order of magnitude toward
low-mass protostars and high-mass hot cores, but HCOOCH3 is relatively more
important toward low-mass protostars. This is consistent with a sequential ice
photochemistry, dominated by CHO-containing products at low temperatures and
early times.Comment: 20 pages, including 5 figures. Accepted for publication in Ap
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
Physical Sources of Scatter in the Tully-Fisher Relation
We analyze residuals from the Tully-Fisher relation for the emission-line
galaxies in the Nearby Field Galaxy Survey, a broadly representative survey
designed to fairly sample the variety of galaxy morphologies and environments
in the local universe. For spirals brighter than M_R^i=-18, we find strong
correlations between Tully-Fisher residuals and both B-R color and EW(Halpha).
The extremes of the correlations are populated by Sa galaxies, which show
consistently red colors, and spirals with morphological peculiarities, which
are often blue. If we apply an EW(Halpha)-dependent or B-R color-dependent
correction term to the Tully-Fisher relation, the scatter in the relation no
longer increases from R to B to U but instead drops to a nearly constant level
close to the scatter we expect from measurement errors. We argue that these
results probably reflect correlated offsets in luminosity and color as a
function of star formation history. Broadening the sample in morphology and
luminosity, we find that most non-spirals brighter than M_R^i=-18 follow the
same correlations as do spirals, albeit with greater scatter. However, the
color and EW(Halpha) correlations do not apply to galaxies fainter than
M_R^i=-18 or to emission-line S0 galaxies with anomalous gas kinematics. For
the dwarf galaxy population, the parameters controlling Tully-Fisher residuals
are instead related to the degree of recent disturbance: overluminous dwarfs
have higher rotation curve asymmetries, brighter U-band effective surface
brightnesses, and shorter gas consumption timescales than their underluminous
counterparts. As a result, sample selection strongly affects the measured
faint-end slope of the Tully-Fisher relation. Passively evolving, rotationally
supported galaxies display a break toward steeper slope at low luminosities.Comment: 58 pages including 21 figures, AJ, accepte
Distribution of dust clouds around the central engine of NGC 1068
We studied the distribution of dust clouds around the central engine of NGC
1068 based on shifted-and-added 8.8 - 12.3 micron (MIR) multi-filter images and
3.0 - 3.9 micron (L-band) spectra obtained with the Subaru Telescope. In a
region of 100 pc (1.4") around the central peak, we successfully constructed
maps of color temperatures and emissivities of the MIR and L-band continua as
well as the 9.7 micron and 3.4 micron dust features with spatial resolutions of
26 pc (0.37") in the MIR and 22 pc (0.3") in the L-band. Our main results are:
1) color temperature of the MIR continuum scatters around the thermal
equilibrium temperature with the central engine as the heat source while that
of the L-band continuum is higher and independent upon distance from the
central engine; 2) the peak of the 9.7 micron silicate absorption feature is
shifted to a longer wavelength at some locations; 3) the ratio of the optical
depths of the dust features is different from the Galactic values and show
complicated spatial distribution; and 4) there is a pie shaped warm dust cloud
as an enhancement in the emissivity of the MIR continuum extending about 50 pc
to the north from the central engine. We speculate that material falls into the
central engine through this cloud.Comment: 26 pages, 9 figures. Accepted for publication on Ap
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