95 research outputs found
Probing MHD Shocks with high-J CO observations: W28F
Context. Observing supernova remnants (SNRs) and modelling the shocks they
are associated with is the best way to quantify the energy SNRs re-distribute
back into the Interstellar Medium (ISM). Aims. We present comparisons of shock
models with CO observations in the F knot of the W28 supernova remnant. These
comparisons constitute a valuable tool to constrain both the shock
characteristics and pre-shock conditions. Methods. New CO observations from the
shocked regions with the APEX and SOFIA telescopes are presented and combined.
The integrated intensities are compared to the outputs of a grid of models,
which were combined from an MHD shock code that calculates the dynamical and
chemical structure of these regions, and a radiative transfer module based on
the 'large velocity gradient' (LVG) approximation. Results. We base our
modelling method on the higher J CO transitions, which unambiguously trace the
passage of a shock wave. We provide fits for the blue- and red-lobe components
of the observed shocks. We find that only stationary, C-type shock models can
reproduce the observed levels of CO emission. Our best models are found for a
pre-shock density of 104 cm-3, with the magnetic field strength varying between
45 and 100 {\mu}G, and a higher shock velocity for the so-called blue shock
(\sim25 km s-1) than for the red one (\sim20 km s-1). Our models also
satisfactorily account for the pure rotational H2 emission that is observed
with Spitzer.Comment: 8 pages, 6 figures, 1 table, accepted for A&A SOFIA/GREAT Special
Issu
High-resolution wide-band Fast Fourier Transform spectrometers
We describe the performance of our latest generations of sensitive wide-band
high-resolution digital Fast Fourier Transform Spectrometer (FFTS). Their
design, optimized for a wide range of radio astronomical applications, is
presented. Developed for operation with the GREAT far infrared heterodyne
spectrometer on-board SOFIA, the eXtended bandwidth FFTS (XFFTS) offers a high
instantaneous bandwidth of 2.5 GHz with 88.5 kHz spectral resolution and has
been in routine operation during SOFIA's Basic Science since July 2011. We
discuss the advanced field programmable gate array (FPGA) signal processing
pipeline, with an optimized multi-tap polyphase filter bank algorithm that
provides a nearly loss-less time-to-frequency data conversion with
significantly reduced frequency scallop and fast sidelobe fall-off. Our digital
spectrometers have been proven to be extremely reliable and robust, even under
the harsh environmental conditions of an airborne observatory, with
Allan-variance stability times of several 1000 seconds. An enhancement of the
present 2.5 GHz XFFTS will duplicate the number of spectral channels (64k),
offering spectroscopy with even better resolution during Cycle 1 observations.Comment: Accepted for publication in A&A (SOFIA/GREAT special issue
GREAT [CII] and CO observations of the BD+40{\deg}4124 region
The BD+40\degree4124 region was observed with high angular and spectral
resolution with the German heterodyne instrument GREAT in CO J = 13 \rightarrow
12 and [CII] on SOFIA. These observations show that the [CII] emission is very
strong in the reflection nebula surrounding the young Herbig Ae/Be star
BD+40\degree4124. A strip map over the nebula shows that the [CII] emission
approximately coincides with the optical nebulosity. The strongest [CII]
emission is centered on the B2 star and a deep spectrum shows that it has faint
wings, which suggests that the ionized gas is expanding. We also see faint CO J
= 13 \rightarrow 12 at the position of BD+40\degree4124, which suggests that
the star may still be surrounded by an accretion disk.We also detected [CII]
emission and strong CO J = 13 \rightarrow 12 toward V1318 Cyg. Here the [CII]
emission is fainter than in BD+40\degree4124 and appears to come from the
outflow, since it shows red and blue wings with very little emission at the
systemic velocity, where the CO emission is quite strong. It therefore appears
that in the broad ISO beam the [CII] emission was dominated by the reflection
nebula surrounding BD+40\degree4124, while the high J CO lines originated from
the adjacent younger and more deeply embedded binary system V1318 Cyg
Mid- and high-J CO observations towards UCHIIs
A study of 12 ultracompact HII regions was conducted to probe the physical
conditions and kinematics in the inner envelopes of the molecular clumps
harboring them. The APEX telescope was used to observe the sources in the CO
(4-3) and 13CO (8-7) lines. Line intensities were modeled with the RATRAN
radiative transfer code using power laws for the density and temperature to
describe the physical structure of the clumps. All sources were detected in
both lines. The optically thick CO (4-3) line shows predominantly blue skewed
profiles reminiscent of infall. Line intensities can be reproduced well using
the physical structure of the clumps taken from the literature. The optically
thick line profiles show that CO is a sensitive tracer of ongoing infall in the
outer envelopes of clumps harboring ultracompact HII regions and hot molecular
cores.Comment: APEX A&A special issue, accepte
GREAT: the SOFIA high-frequency heterodyne instrument
We describe the design and construction of GREAT, the German REceiver for
Astronomy at Terahertz frequencies operated on the Stratospheric Observatory
for Infrared Astronomy (SOFIA). GREAT is a modular dual-color heterodyne
instrument for highresolution far-infrared (FIR) spectroscopy. Selected for
SOFIA's Early Science demonstration, the instrument has successfully performed
three Short and more than a dozen Basic Science flights since first light was
recorded on its April 1, 2011 commissioning flight.
We report on the in-flight performance and operation of the receiver that -
in various flight configurations, with three different detector channels -
observed in several science-defined frequency windows between 1.25 and 2.5 THz.
The receiver optics was verified to be diffraction-limited as designed, with
nominal efficiencies; receiver sensitivities are state-of-the-art, with
excellent system stability. The modular design allows for the continuous
integration of latest technologies; we briefly discuss additional channels
under development and ongoing improvements for Cycle 1 observations.
GREAT is a principal investigator instrument, developed by a consortium of
four German research institutes, available to the SOFIA users on a
collaborative basis
High-resolution absorption spectroscopy of the OH 2Pi 3/2 ground state line
The chemical composition of the interstellar medium is determined by gas
phase chemistry, assisted by grain surface reactions, and by shock chemistry.
The aim of this study is to measure the abundance of the hydroxyl radical (OH)
in diffuse spiral arm clouds as a contribution to our understanding of the
underlying network of chemical reactions. Owing to their high critical density,
the ground states of light hydrides provide a tool to directly estimate column
densities by means of absorption spectroscopy against bright background
sources. We observed onboard the SOFIA observatory the 2Pi3/2, J = 5/2 3/2 2.5
THz line of ground-state OH in the diffuse clouds of the Carina-Sagittarius
spiral arm. OH column densities in the spiral arm clouds along the sightlines
to W49N, W51 and G34.26+0.15 were found to be of the order of 10^14 cm^-2,
which corresponds to a fractional abundance of 10^-7 to 10^-8, which is
comparable to that of H_2O. The absorption spectra of both species have similar
velocity components, and the ratio of the derived H_2O to OH column densities
ranges from 0.3 to 1.0. In W49N we also detected the corresponding line of
^18OH
Dimethyl ether in its ground state, v=0, and lowest two torsionally excited states, v11=1 and v15=1, in the high-mass star-forming region G327.3-0.6
The goal of this paper is to determine the respective importance of solid
state vs. gas phase reactions for the formation of dimethyl ether. This is done
by a detailed analysis of the excitation properties of the ground state and the
torsionally excited states, v11=1 and v15=1, toward the high-mass star-forming
region G327.3-0.6. With the Atacama Pathfinder EXperiment 12 m submillimeter
telescope, we performed a spectral line survey. The observed spectrum is
modeled assuming local thermal equilibrium. CH3OCH3 has been detected in the
ground state, and in the torsionally excited states v11=1 and v15=1, for which
lines have been detected here for the first time. The emission is modeled with
an isothermal source structure as well as with a non-uniform spherical
structure. For non-uniform source models one abundance jump for dimethyl ether
is sufficient to fit the emission, but two components are needed for the
isothermal models. This suggests that dimethyl ether is present in an extended
region of the envelope and traces a non-uniform density and temperature
structure. Both types of models furthermore suggest that most dimethyl ether is
present in gas that is warmer than 100 K, but a smaller fraction of 5%-28% is
present at temperatures between 70 and 100 K. The dimethyl ether present in
this cooler gas is likely formed in the solid state, while gas phase formation
probably is dominant above 100 K. Finally, the v11=1 and v15=1 torsionally
excited states are easily excited under the density and temperature conditions
in G327.3-0.6 and will thus very likely be detectable in other hot cores as
well.Comment: 12 pages (excluding appendices), 8 figures, A&A in pres
High-J CO emission in the Cepheus E protostellar outflow observed with SOFIA/GREAT
We present and analyze two spectrally resolved high-J CO lines towards the
molecular outflow Cep E, driven by an intermediate-mass class 0 protostar.
Using the GREAT receiver on board SOFIA, we observed the CO (12--11) and
(13--12) transitions (E_u ~ 430 and 500 K, respectively) towards one position
in the blue lobe of this outflow, that had been known to display high-velocity
molecular emission. We detect the outflow emission in both transitions, up to
extremely high velocities (~ 100 km/s with respect to the systemic velocity).
We divide the line profiles into three velocity ranges that each have
interesting spectral features: standard, intermediate, and extremely
high-velocity. One distinct bullet is detected in each of the last two. A large
velocity gradient analysis for these three velocity ranges provides constraints
on the kinetic temperature and volume density of the emitting gas, >~ 100 K and
> ~ 10^4 cm^-3, respectively. These results are in agreement with previous ISO
observations and are comparable with results obtained by Herschel for similar
objects. We conclude that high-J CO lines are a good tracer of molecular
bullets in protostellar outflows. Our analysis suggests that different physical
conditions are at work in the intermediate velocity range compared with the
standard and extremely high-velocity gas at the observed position.Comment: Accepted for publication in A&A (SOFIA/GREAT special issue
CI observations in the CQ Tau proto-planetary disk: evidence for a very low gas-to-dust ratio ?
Gas and dust dissipation processes of proto-planetary disks are hardly known.
Transition disks between Class II (proto-planetary disks) and Class III (debris
disks) remain difficult to detect. We investigate the carbon chemistry of the
peculiar CQ Tau gas disk. It is likely a transition disk because it exhibits
weak CO emission with a relatively strong millimeter continuum, indicating that
the disk might be currently dissipating its gas content. We used APEX to
observe the two CI lines at 492GHz and 809 GHz in the disk orbiting CQ Tau. We
compare the observations to several chemical model predictions. We focus our
study on the influence of the stellar UV radiation shape and gas-to-dust ratio.
We did not detect the CI lines. However, our upper limits are deep enough to
exclude high-CI models. The only available models compatible with our limits
imply very low gas-to-dust ratio, of the order of a few, only. These
observations strengthen the hypothesis that CQ Tau is likely a transition disk
and suggest that gas disappears before dust.Comment: 5 pages, 5 figures, accepted for publication in A&
[12CII] and [13CII] 158 mum emission from NGC 2024: Large column densities of ionized carbon
Context: We analyze the NGC 2024 HII region and molecular cloud interface
using [12CII] and [13CII] observations. Aims: We attempt to gain insight into
the physical structure of the interface layer between the molecular cloud and
the HII region. Methods. Observations of [12CII] and [13CII] emission at 158
{\mu}m with high spatial and spectral resolution allow us to study the detailed
structure of the ionization front and estimate the column densities and
temperatures of the ionized carbon layer in the PDR. Results: The [12CII]
emission closely follows the distribution of the 8 mum continuum. Across most
of the source, the spectral lines have two velocity peaks similar to lines of
rare CO isotopes. The [13CII] emission is detected near the edge-on ionization
front. It has only a single velocity component, which implies that the [12CII]
line shape is caused by self-absorption. An anomalous hyperfine line-intensity
ratio observed in [13CII] cannot yet be explained. Conclusions: Our analysis of
the two isotopes results in a total column density of N(H)~1.6\times10^23 cm^-2
in the gas emitting the [CII] line. A large fraction of this gas has to be at a
temperature of several hundred K. The self-absorption is caused by a cooler
(T<=100 K) foreground component containing a column density of N(H)~10^22
cm^-2
- …