Institut De Radioastronomie Millimetrique Research Articles

Quiescent Giant Molecular Cloud Cores in the Galactic Center

We have used the Long Wavelength Spectrometer (LWS) aboard the Infrared Space Observatory (ISO) to map the far-infrared continuum emission (45-175 micrometer) toward several massive Giant Molecular Cloud (GMC) cores located near the Galactic center. The observed far-infrared and submillimeter spectral energy distributions imply low temperatures (approx. 15 - 22 K) for the bulk of the dust in all the sources, consistent with external heating by the diffuse ISRF and suggest that these GMCs do not harbor high- mass star-formation sites, in spite of their large molecular mass. Observations of FIR atomic fine structure lines of C(sub II) and O(sub I) indicate an ISRF enhancement of approx. 10(exp 3) in the region. Through continuum radiative transfer modeling we show that this radiation field strength is in agreement with the observed FIR and submillimeter spectral energy distributions, assuming primarily external heating of the dust with only limited internal luminosity (approx. 2 x 10(exp 5) solar luminosity). Spectroscopic observations of millimeter-wave transitions of H2CO, CS, and C-34S carried out with the Caltech Submillimeter Observatory (CSO) and the Institut de Radio Astronomie Millimetrique (IRAM) 30-meter telescope indicate a gas temperature of approx. 80 K, significantly higher than the dust temperatures, and density of approx. 1 x 10(exp 5)/cc in GCM0.25 + 0.01, the brightest submillimeter source in the region. We suggest that shocks caused by cloud collisions in the turbulent interstellar medium in the Galactic center region are responsible for heating the molecular gas. This conclusion is supported by the presence of wide-spread emission from molecules such as SiO, SO, and CH3OH, which are considered good shock tracers. We also suggest that the GMCs studied here are representative of the typical, pre-starforming cloud population in the Galactic center.

Spectroscopic Observations of Comet C/1999 H1 (Lee) with the SEST, JCMT, CSO, IRAM, and NanÇay Radio Telescopes

Coordinated spectroscopic radio observations of comet C/1999 H1 (Lee) were undertaken between 1999 May 4 and 1999 October 26, using the Swedish-ESO Submillimetre Telescope, the James Clerk Maxwell Telescope, the Caltech Submillimeter Observatory, the 30 m telescope of the Institut de Radio Astronomie Millimetrique, and the Nancay radio telescope. We report on observations of OH, HCN, CH3OH, H2CO, and CS and on the evolution of their production rates with heliocentric distance between 0.8 and 1.7 AU, where the total outgassing rate ranged between 0.2 and 1.6 × 1029 molecules s-1. HNC was detected unexpectedly in this medium-activity comet with a relatively large HNC/HCN mixing ratio of 12%, close to that measured in comet C/1995 O1 (Hale-Bopp), which cannot be explained by current chemical models of the coma. CO was tentatively detected with a low abundance around 4% relative to water and is clearly underabundant in comparison to that in comets Hyakutake and Hale-Bopp. An upper limit of D/H less than 300 × 10-5 in water was found from a brief search for HDO. Molecular abundances relative to water of the other species around 1 AU are similar to those observed in other comets, although CH3OH (4%) and H2CO (1%) exhibit some of the largest abundances compared with previous comets.

Spectroscopic Monitoring of Comet C/1996 B2 (Hyakutake) with the JCMT and IRAM Radio Telescopes

Molecular radio lines were monitored in comet C/1996 B2 (Hyakutake) from 1996 February 10 to June 23, using the James Clerk Maxwell Telescope and the 30 m telescope and the Plateau de Bure interferometer of the Institut de Radioastronomie Millimetrique. We report on observations of HCN, CH3OH, CO, H2CO, CS, and H2S and on the evolution of their production rates with heliocentric distance (rh), from 1.86 down to 0.24 AU at perihelion. Most production rates increased roughly as r down to 0.6 AU preperihelion. Closer to the Sun, they stalled before decreasing beyond 0.6 AU postperihelion, when observations resumed. The CS/HCN ratio varied as r from 1.2 to 0.24 AU. A rapid increase of the mean gas temperature in the coma is measured, and the gas expansion velocity increased from 0.55 to 1.6 km s-1, as the comet approached the Sun from 1.6 to 0.3 AU. Molecular abundances of the minor species around 1 AU are similar to those observed in other comets, while the CO abundance relative to water is high (≈22%). Coarse mapping was used to check the comet's position and to investigate the density distribution of the molecules within the coma. It provides constraints on the size of the extended source of formaldehyde, found to be between 1.2 and 2 times the scale length of H2CO itself. The density distribution of CS is compatible with its production from the photodissociation of a short-lived molecule such as CS2. The density distribution observed for CO can be mostly explained by a nuclear source.

The Structure and Dynamics of the Proto–Planetary Nebula M1‐92

We present high-resolution (10) maps of 13CO J = 2-1 in the proto-planetary nebula M1-92, Minkowski's Footprint, obtained with the Institut de Radioastronomie Millimetrique (IRAM) interferometer at Plateau de Bure. We confirm the main components found in our previous works: a central disklike condensation, a bipolar double-shell structure with axial outward velocity increasing with the distance to the star, and two opposed features at the tips of the nebula where the maximal deprojected velocity is attained, ~70 km s-1. The major quality of the present data allows us to estimate the very small width of the double-shell walls, ~06 (2 × 1016 cm), and the diameter of the central disk, 2''-3'' (1017 cm). The whole structure is probably the remnant of the previous asymptotic giant branch (AGB) shell after being shocked by the bipolar post-AGB jets. The mass of the molecular envelope is about 0.9 M☉, and its kinetic momentum and energy (released by the wind interaction) are ~3 × 1039 g cm s-1 and ~7 × 1045 ergs, respectively. Since the interaction time must be significantly smaller than the age of the nebula, 900 yr, these figures imply very energetic post-AGB jets that cannot be driven by radiation pressure. We also notice that the inner disklike structure is too large for collimating the very narrow post-AGB jets. We propose that reaccretion of material, ejected during the previous AGB phase, is the most likely mechanism to explain the strongly bipolar and very energetic post-AGB ejections.

The Simultaneous Spectrum of Sagittarius A* from 20 Centimeters to 1 Millimeter and the Nature of the Millimeter Excess

We report results of a multiwavelength campaign to measure the simultaneous spectrum of the supermassive black hole candidate Sgr A* in the Galactic center from centimeter to millimeter wavelengths using the Very Large Array, the Berkeley-Illinois-Maryland Array (BIMA), the Nobeyama 45 m, and the Institut de Radioastronomie Millimetrique (IRAM) 30 m telescopes. The observations confirm that the previously detected millimeter excess is an intrinsic feature of the spectrum of Sgr A*. The excess can be interpreted as and effect of the presence of an ultracompact component of relativistic plasma with a size of a few Schwarzschild radii near the black hole. If so, Sgr A* might offer a unique possibility to image the putative black hole against the background of this component with future millimeter VLBI experiments.

A multibeam SIS mixer module for a focal plane array receiver

The integration of many receiver units into a receiver array is a common method of improvement of imaging systems. This approach, well known in the mm band for Schottky mixer arrays, has not so far been developed for Superconductor - Insulator - Superconductor (SIS) junction mixers, which give the best sensitivity in the short mm wave range and in the submm range. We demonstrate for the first time a practical low noise multibeam receiver module using SIS mixer technology. The basis for the integration of several SIS mixers with a common local oscillator source is given by the saturation of the SIS receiver noise dependence upon local oscillator power. The module comprises three identical SIS mixers integrated with a common local oscillator, coupled through a three branch waveguide directional coupler. The multibeam module has been developed for a focal plane array receiver of the 30 meter radio telescope of the Institut de Radioastronomie Millimetrique (IRAM).

Evidences for extended sources and temporal modulations in molecular observations of C/1995 O1 (Hale-Bopp) at the IRAM interferometer

Comet C/1995 O1 (Hale-Bopp) has been observed on October 5 and 25, 1996 and from March 6 to March 22, 1997 with the Institut de Radioastronomie Millimetrique (IRAM) interferometer at Plateau de Bure (France). Millimetre lines of HCN,HNC, CO, H2CO, CH3OH, H2S, CS and SO were mapped with spatial resolutions of 1.5–3.5 arc sec. These observations allow us to investigate whether these species are released by the nucleus or produced in the coma by extended sources or photo-processes. The brightness distribution of the HCN J (1-0) line is consistent with release from the nucleus. The HNC J (1-0) distribution deviates from that of HCN in the innermost coma, and indicates production of HNC in the coma. This is in agreement with the heliocentric variation of the HNC/HCN ratio (Biver et al., 1997, Science 275, 1915; Irvine et al., 1998, this issue) and formation by chemical reactions (Rodgers and Charnley, 1998, Ap. J. 501, L227; Irvine et al., 1998, Nature 393, 547). There is clear evidence that SO is a photo dissociation product. The observations also confirm that H2CO is mainly produced by an extended source, as first evidenced in comet P/Halley. The contribution of the nucleus to the total H2CO production rate does not exceed 6%. The molecular lines have also been monitored hourly with the five antennas of the interferometer in single-dish mode. The line velocity shifts show aperiodic modulation linked to the nucleus rotation. The amplitude of the modulation differs from one species to another. The periodic modulation seen for the CO J (2-1) line on March 11 suggests that a significant fraction of CO is released continuously night and day by an active source situated at equatorial latitudes on the nucleus surface.

A molecular survey of comet C/1995 O1 (Hale-Bopp) at the IRAM telescopes

Spectroscopic observations of comet Hale-Boppwere undertaken near perihelion at millimetre wavelengths with the Institut de Radioastronomie Millimetrique (IRAM) telescopes at Plateau de Bure (France)and Pico Veleta (Spain). They resulted in the first detections of HCOOH,SO2, NH2CHO and HCOOCH3 in a comet. HDO was detected through its312–221 line at 225.897 GHz,complementing the observation of the 101–000 line at 464.925 GHz at the James Clerk Maxwell Telescope (Meier et al., 1998,Science 279, 842).Several unidentified lines are present in the spectra. Observations of HC3N, HNCO, OCS, SO, CN, CO+, HCO+, in addition to more ‘classical’species CO, HCN, HNC, CH3CN, CH3OH, H2CO, CS and H2S (Biveret al., this issue) permit us to make out an extensive inventory of the composition of the coma of comet Hale-Bopp at its perihelion. It presents strong analogies with gas-phase abundances measured in interstellar hot cores and bipolar flows, which are believed to reflect the composition of interstellar grains.

The IRAS 2 and IRAS 4 Outflows and Star Formation in NGC 1333

We report the first detection of the western bow shock component from IRAS 2 in NGC 1333 along with observations of previously detected shocks and outflow winds from this source and those from IRAS 4. We compare the shock and outflow distributions from these two young stellar objects (YSOs), and the locations of other YSOs, with the overall distribution of the dense molecular gas in the star-forming core using high spatial resolution observations of CS (J = 2 → 1, 3 → 2, and 5 → 4) emission made with the Institut de Radio Astronomie Millimetrique 30 m antenna. These comparisons provide a new picture of the morphology and dynamics of the star-forming core of NGC 1333. The CS maps show (1) a large cavity with many YSOs just at the inner edge of the cavity, (2) a dense, compressed shell at 8 km s-1; and (3) a gas layer at 7 km s-1 probably located inside the cavity. We find that the IRAS 2 and IRAS 4 outflows impact different gas layers as indicated by the spatial association of the red- and blueshifted lobes, and that IRAS 2 is located near the front edge of the CS shell. The burst of star formation that has shaped NGC 1333 is occurring in the compressed shell traced by CS and now appears to be in a late evolutionary stage.

Near–focus active optics: An inexpensive method to improve millimeter–wavelength radio telescopes

The application of active and adaptive optics allows the construction of large diameter lightweight optical telescopes for observations below the seeing limit of the atmosphere. Active wavefront correction in a Cassegrain/Gregorytype radio telescope can be made with a deformable main reflector or deformable subreflector. Here we suggest the possibility of correcting spatially large‐scale wavefront deformations with a small size corrector located near the focus of the telescope. Using representative examples of the Institut de Radioastronomie Millimetrique (IRAM, Spain) 30‐meter diameter, millimeter‐wavelength telescope, we calculate the improvement expected from the correction of (1) the systematic component in homology deformations, (2) the large‐scale residual errors of a reflector adjustment, and (3) the beam degradation experienced in observations with a wobbling subreflector. The improvement in surface/wavefront precision obtained from piston correction with a corrector of some 50 elements is of the order of 30 – 40%. We investigate in particular the systematic component of homology deformations, their representation by low‐order Zernike polynomials, and their elimination by near‐focus correction. We study in detail the homology deformations of the IRAM 30‐m reflector.

Pb/Bi/In and Nb/Al junctions for a 345 GHz open structure astronomical receiver

The authors report on the preparation and properties of Pb/Bi/In-oxide-Pb/Bi and Nb-Al oxide-Nb junctions prepared for a 345-GHz open-structure heterodyne receiver. The lowest receiver noise temperature with the Pb/Bi/In junctions is 215 K DSB. In this case, no matching network is integrated to tune out the junction capacitance. For the Nb/Al junctions, a microwave transmission line compensates the capacitance around 345 GHz. With the Nb-Al oxide-Nb two-junction array at 4.2 K, the lowest receiver noise temperature is 80 K at 330 GHz. The receiver is in use at the IRAM (Institut de Radio Astronomie Millimetrique) 30-m radio telescope in Spain.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Microwave detection of hydrogen sulphide and methanol in comet Austin (1989c1)

INFRARED and microwave spectroscopy can be used to identify and study the parent molecules directly sublimed from cometary nuclei. Here we report spectroscopic observations of comet Austin (1989c1) at millimetre wavelengths using the 30-m telescope of the Institut de Radio Astronomie Millimetrique (IRAM). We detected the rotational transitions of hydrogen cyanide and form-aldehyde, which were previously observed in comets Halley and Brorsen–Metcalf1. In addition, we identified hydrogen sulphide and methanol, neither of which has previously been detected in a comet. The presence of hydrogen sulphide provides severe constraints on the formation of cometary nuclei, whereas that of methanol supports the hypothesis that cometary nuclei have retained, at least in part, some primitive material originating from the solar nebula.