139 research outputs found
Revisiting the theory of interferometric wide-field synthesis
After several generations of interferometers in radioastronomy, wide-field
imaging at high angular resolution is today a major goal for trying to match
optical wide-field performances. All the radio-interferometric, wide-field
imaging methods currently belong to the mosaicking family. Based on a 30 years
old, original idea from Ekers & Rots, we aim at proposing an alternate
formalism. Starting from their ideal case, we successively evaluate the impact
of the standard ingredients of interferometric imaging. A comparison with
standard nonlinear mosaicking shows that both processing schemes are not
mathematically equivalent, though they both recover the sky brightness. In
particular, the weighting scheme is very different in both methods. Moreover,
the proposed scheme naturally processes the short spacings from both
single-dish antennas and heterogeneous arrays. Finally, the sky gridding of the
measured visibilities, required by the proposed scheme, may potentially save
large amounts of hard-disk space and cpu processing power over mosaicking when
handling data sets acquired with the on-the-fly observing mode. We propose to
call this promising family of imaging methods wide-field synthesis because it
explicitly synthesizes visibilities at a much finer spatial frequency
resolution than the one set by the diameter of the interferometer antennas.Comment: 22 pages, 6 PostScript figures. Accepted for publication in Astronomy
& Astrophysics. Uses aa LaTeX macros
The Horsehead nebula, a template source for interstellar physics and chemistry
We present a summary of our previous investigations of the physical and
chemical structure of the Horsehead nebula, and discuss how these studies led
to advances on the understanding of the impact of FUV radiation on the
structure of dense interstellar clouds. Specific molecular tracers can be used
to isolate different environments, that are more sensitive to changes in the
FUV radiation or density than the classical tracers of molecular gas : the CO
isotopologues or the dust (sub)millimeter continuum emission. They include the
HCO or CCH radicals for the FUV illuminated interfaces, or the molecular ions
HCO, DCO and other deuterated species (DNC, DCN) for the cold
dense core. We discuss future prospects in the context of Herschel and ALMA
Plateau de Bure Interferometer Observations of the Disk and Outflow of HH30
HH30 is a well-known Pre-Main-Sequence star in Taurus. HST observations have
revealed a flared, edge-on disk driving a highly-collimated optical jet, making
this object a case study for the disk-jet-outflow paradigm. We obtained high
angular resolution (about 1") observations of the dust continuum at 2.7 and 1.3
mm, and of the 12CO(2-1), 13CO(2-1) & (1-0), C18O(1-0) emissions around HH30. A
standard disk model is used to fit the 13CO(2-1) uv-plane visibilities and
derive the disk properties, and the stellar mass. It results that HH30 is a low
mass TTauri of spectral type around M1 and age 1 to 4 Myrs, surrounded by a
medium size Keplerian disk, of mass around 4e-3 Msun and outer radius 420 AU.
The disk rotation vector points toward the North-Eastern jet. Using a distance
of 140 pc, we deduce a stellar mass of 0.45 Msun. A highly asymmetric outflow
originates from the inner parts of the disk. It presents to first order a
conical morphology with a 30 degree half opening angle and a constant (12 km/s)
radial velocity field. Outflow rotation was searched for but not found. These
observations do not enable to assign the origin of the molecular outflow to
entrainment by the optical jet or to a disk wind. In the latter case, the upper
limit of the outflow rotation velocity implies an origin in the inner 15 AU of
the disk.Comment: 20 pages, 15 PostScript figures. Accepted for publication in
Astronomy & Astrophysics. Uses aa LaTeX macro
Tracing Dense Gas in Six Resolved GMCs of the Andromeda Galaxy
We present dense-gas--tracing molecular observations of six resolved Giant
Molecular Clouds (GMCs) in the Andromeda Galaxy (M31). Using the NOEMA
interferometer, we observed the transitions of HCN(1-0), HCO(1-0), and
HNC(1-0), as well as CO(1-0) and 100 GHz continuum emission. This
complements our earlier work with the Submillimeter Array (SMA), including
resolved dust continuum detections of these clouds at 230 GHz. In this work, we
first compare different continuum measurements to conclude that the average
free-free contamination of the observed flux is 71% at 3 mm but only 13% at 1
mm, confirming that emission at 3 mm is less reliable than that at 1 mm for
calculating dust masses of star-forming clouds. While the CO emission is
more extended than both HCN and HCO emission, which in turn is more
extended than HNC emission, we find that both HCN and HCO are spatially
coincident with, and similarly extended as, the 230 GHz dust emission. This
suggests that both the 230 GHz dust continuum and most importantly the HCN
emission traces the dense gas component of these GMCs. From comparison of the
molecular emission with dust masses derived from the 230 GHz continuum
emission, we obtain the first direct measurements of the dust-mass-to-light
ratios ( and ) in GMCs of an
external galaxy. For HCN, the result is broadly similar to a measurement in the
local Perseus cloud suggesting that these are indeed dense gas conversion
factors. A larger cloud sample will be required to assess whether HCN is
tracing comparable cloud-scale density regimes across the environments of M31.Comment: MNRAS, in pres
Resolving the inner dust disks surrounding LkCa 15 and MWC 480 at mm wavelengths
International audienceWe performed sub-arcsecond high-sensitivity nterferometric observations of the thermal dust emission at 1.4 mm and 2.8 mm in the disks surrounding LkCa 15 and MWC 480, with the new 750 m baselines of the IRAM PdBI array. This provides a linear resolution of about 60 AU at the Taurus distance. We report the existence of a cavity of about 50 AU radius in the inner disk of LkCa 15. Whereas LkCa 15 emission is optically thin, the optically thick core of MWC 480 is resolved at 1.4 mm with a radius of about 35 AU, constraining the dust temperature. In MWC 480, the dust emission is coming from a colder layer than the CO emission, most likely the disk mid-plane. These observations provide direct evidence of an inner cavity around LkCa 15. Such a cavity most probably results from the tidal disturbance created by a low mass companion or large planet at about 30 AU from the star. These results suggest that planetary system formation is already at work in LkCa 15. They also indicate that the classical steady-state viscous disk model is a too simplistic description of the inner 50 AU of ''proto-planetary'' disks, and that the disk evolution is coupled to the planet formation process. The MWC 480 results indicate that a proper estimate of the dust temperature and size of the optically thick core are essential to determine the dust emissivity index
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
Chemical complexity in the Horsehead photodissociation region
The interstellar medium is known to be chemically complex. Organic molecules
with up to 11 atoms have been detected in the interstellar medium, and are
believed to be formed on the ices around dust grains. The ices can be released
into the gas-phase either through thermal desorption, when a newly formed star
heats the medium around it and completely evaporates the ices; or through
non-thermal desorption mechanisms, such as photodesorption, when a single
far-UV photon releases only a few molecules from the ices. The first one
dominates in hot cores, hot corinos and strongly UV-illuminated PDRs, while the
second one dominates in colder regions, such as low UV-field PDRs. This is the
case of the Horsehead were dust temperatures are ~20-30K, and therefore offers
a clean environment to investigate what is the role of photodesorption. We have
carried-out an unbiased spectral line survey at 3, 2 and 1mm with the IRAM-30m
telescope in the Horsehead nebula, with an unprecedented combination of
bandwidth high spectral resolution and sensitivity. Two positions were
observed: the warm PDR and a cold condensation shielded from the UV field
(dense core), located just behind the PDR edge. We summarize our recently
published results from this survey and present the first detection of the
complex organic molecules HCOOH, CH2CO, CH3CHO and CH3CCH in a PDR. These
species together with CH3CN present enhanced abundances in the PDR compared to
the dense core. This suggests that photodesorption is an efficient mechanism to
release complex molecules into the gas-phase in far-UV illuminated regions.Comment: 15 pages, 7 figures, 7 tables, Accepted in Faraday discussions 16
The IRAM-30m line survey of the Horsehead PDR: II. First detection of the l-C3H+ hydrocarbon cation
We present the first detection of the l-C3H+ hydrocarbon in the interstellar
medium. The Horsehead WHISPER project, a millimeter unbiased line survey at two
positions, namely the photo-dissociation region (PDR) and the nearby shielded
core, revealed a consistent set of eight unidentified lines toward the PDR
position. Six of them are detected with a signal-to-noise ratio from 6 to 19,
while the two last ones are tentatively detected. Mostly noise appears at the
same frequency toward the dense core, located less than 40" away. We
simultaneously fit 1) the rotational and centrifugal distortion constants of a
linear rotor, and 2) the Gaussian line shapes located at the eight predicted
frequencies. The observed lines can be accurately fitted with a linear rotor
model, implying a 1Sigma ground electronic state. The deduced rotational
constant value is Be= 11244.9512 +/- 0.0015 MHz, close to that of l-C3H. We
thus associate the lines to the l-C3H+ hydrocarbon cation, which enables us to
constrain the chemistry of small hydrocarbons. A rotational diagram is then
used to infer the excitation temperature and the column density. We finally
compare the abundance to the results of the Meudon PDR photochemical model.Comment: 9 pages, 7 PostScript figures. Accepted for publication in Astronomy
\& Astrophysics. Uses aa LaTeX macro
Benchmarking PDR models against the Horsehead edge
To prepare for the unprecedented spatial and spectral resolution provided by
ALMA and Herschel/HIFI, chemical models are being benchmarked against each
other. It is obvious that chemical models also need well-constrained
observations that can serve as references. Photo-dissociation regions (PDRs)
are particularly well suited to serve as references because they make the link
between diffuse and molecular clouds, thus enabling astronomers to probe a
large variety of physical and chemical processes. At a distance of 400 pc (1"
corresponding to 0.002 pc), the Horsehead PDR is very close to the prototypical
kind of source (i.e. 1D, edge-on) needed to serve as a reference to models.Comment: 4 pages, 2 PostScript figures. To appear in the proceedings of
"SF2A-2006: Semaine de l'Astrophysique Francaise", SF2A, 2006 (D. Barret, F.
Casoli, T. Contini, G. Lagache, A. Lecavelier, and L. Pagani eds). Uses
LaTeX2
A 850-GHz waveguide receiver employing a niobium SIS junction fabricated on a 1-μm Si_3N_4 membrane
We report on a 850-GHz superconducting-insulator-superconducting (SIS) heterodyne receiver employing an RF-tuned niobium tunnel junction with a current density of 14 kA/cm^2, fabricated on a 1-µm Si_3N_4 supporting membrane. Since the mixer is designed to be operated well above the superconducting gap frequency of niobium (2Δ/h ≈ 690 GHz), special care has been taken to minimize niobium transmission-line losses. Both Fourier transform spectrometer (FTS) measurements of the direct detection performance and calculations of the IF output noise with the mixer operating in heterodyne mode, indicate an absorption loss in the niobium film of about 6.8 dB at 822 GHz. These results are in reasonably good agreement with the loss predicted by the Mattis-Bardeen theory in the extreme anomalous limit. From 800 to 830 GHz, we report uncorrected receiver noise temperatures of 518 or 514 K when we use Callen and Welton's law to calculate the input load temperatures. Over the same frequency range, the mixer has a 4-dB conversion loss and 265 K ± 10 K noise temperature. At 890 GHz, the sensitivity of the receiver has degraded to 900 K, which is primarily the result of increased niobium film loss in the RF matching network. When the mixer was cooled from 4.2 to 1.9 K, the receiver noise temperature improved about 20% 409-K double sideband (DSB). Approximately half of the receiver noise temperature improvement can be attributed to a lower mixer conversion loss, while the remainder is due to a reduction in the niobium film absorption loss. At 982 GHz, we measured a receiver noise temperature of 1916 K
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