Gabriel Paubert

Long-term Evolution of the Outgassing of Comet Hale-Bopp From Radio Observations

C/1995 O1 (Hale-Bopp) has been observed on a regular basis since August 1995 at millimetre and submillimetre wavelengths using IRAM, JCMT, CSO and SEST radio telescopes. The production rates of eight molecular species (CO, HCN, CH3OH, H2CO,H2S, CS, CH3CN,HNC) have been monitored as a function of heliocentric distance(rh from 7 AU pre-perihelion to 4 AU post-perihelion. As comet Hale-Bopp approached and receded from the Sun, these species displayed different behaviours. Far from the Sun, the most volatile species were found in general relatively more abundant in the coma. In comparison to other species, HNC, H2CO and CS showed a much steeper increase of the production rate with decreasing rh. Less than 1.5 AU from the Sun, the relative abundances were fairly stable and approached those found in other comets near 1 AU.The kinetic temperature of the coma, estimated from the relative intensities of the CH3OH and CO lines, increased with decreasing rh, from about10 K at 7 AU to 110 K around perihelion. The expansion velocity of the gaseous species, derived from the line shapes, also increased with a law close torh3.

CN Zeeman measurements in star formation regions

Aims. Magnetic fields play a primordial role in the star formation process. The Zeeman effect on the CN radical lines is one of the few methods of measuring magnetic fields in the dense gas of star formation regions. Methods. We report new observations of the Zeeman effect on seven hyperfine CN N = 1−0 lines in the direction of 14 regions of star formation. Results. We have improved the sensitivity of previous detections, and obtained five new detections. Good upper limits are also achieved. The probability distribution of the line-of-sight field intensity, including non-detections, provides a median value of the total field Btot = 0.56 mG while the average density of the medium sampled is n(H2) = 4.5 × 10 5 cm −3 . We show that the CN line probably samples regions similar to those traced by CS and that the magnetic field observed mostly pervades the dense cores. The dense cores are found to be critical to slightly supercritical with a mean mass-to-flux ratio M/Φ ∼ 1 to 4 with respect to critical. Their turbulent and magnetic energies are in approximate equipartition.

The structure, stability, and global distribution of Io's atmosphere

Millimeter-wave observations of SO2 have allowed the first groundbased direct detection of Ios neutral atmosphere. From observations of two SO2 rotational lines, at 221.965 and 143.057 GHz, tentative detection of a third SO2 line, at 346.652 GHz, and upper limits on two other lines, basic properties of Ios atmosphere are inferred. The SO2 atmosphere appears to have global temporal stability and can be represented by a collisionally thick 1011−1012 cm−3 atmosphere (p = 3−40 nbar) covering a limited fraction (5–20%) of Ios surface, with possibly larger pressures on the trailing side than on the leading. The horizontal distribution of gaseous SO2 is best described as the result of discrete distribution of (equilibrium or volcanic) sources rather than by vapor pressure equilibrium over a smooth distribution of surface frosts. The lower atmosphere seems surprisingly hot, about 500–600 K at 40 km. A reanalysis of the IRIS/Voyager observation of the ν3 SO2 band at 7 μm over Loki, using a NLTE transfer model, suggests temperature/pressure conditions at Loki consistent with those derived for the global atmosphere from the millimeter-wave data. High temperatures in the lower scale height, however, are not accommodated by simple thermal models. Our results suggest that Ios atmosphere may be best described by a “volcanic source” atmospheric model, although some aspects of the “equilibrium” models, notably the temporal stability, are also present. While the primary problem remains the need to unambigously determine and explain the vertical thermal structure, it must be noted that if the atmosphere is hot, the concept of an atmosphere in dynamical equilibrium with one or more volcanic sources may provide a reasonable explanation to the Pioneer 10 occultation. Finally, new upper limits on atmospheric H2S, SO, and CO were obtained.

Detection of the CN Zeeman Effect in Molecular Clouds

Observations of the Zeeman effect in the 3 mm lines of CN have been carried out with the 30 m IRAM telescope toward seven dense molecular clouds. Detections were achieved toward the Orion Molecular Cloud 1 (OMC1), toward two cores in the DR21OH molecular cloud, and probably toward the M17SW molecular cloud. The line-of-sight magnetic field strengths inferred are Blos(OMC1)=-0.36±0.08, Blos(DR21OH1)=-0.36±0.10, Blos(DR21OH2)=-0.71±0.12, and Blos(M17SW)=-0.33±0.14 mG. The theoretical implications of these results are discussed.

Chemical Composition Diversity Among 24 Comets Observed at Radio Wavelengths

We present a comparative study on molecular abundances in comets based on millimetre/submillimetre observations made with the IRAM 30-m, JCMT, CSO and SEST telescopes. This study concerns a sample of 24 comets (6 Jupiter-family, 3 Halley-family, 15 long-period) observed from 1986 to 2001 and 8 molecular species (HCN, HNC, CH3CN, CH3OH, H2CO, CO, CS, H2S). HCN was detected in all comets, while at least 2 molecules were detected in 19 comets.

Volcanically emitted sodium chloride as a source for Io's neutral clouds and plasma torus

The atmosphere of Jupiters satellite Io is extremely tenuous, time variable and spatially heterogeneous. Only a few molecules—SO2, SO and S2—have previously been identified as constituents of this atmosphere, and possible sources include frost sublimation, surface sputtering and active volcanism. Io has been known for almost 30 years to be surrounded by a cloud of Na, which requires an as yet unidentified atmospheric source of sodium. Sodium chloride has been recently proposed as an important atmospheric constituent, based on the detection of chlorine in Ios plasma torus and models of Ios volcanic gases . Here we report the detection of NaCl in Ios atmosphere; it constitutes only ∼0.3% when averaged over the entire disk, but is probably restricted to smaller regions than SO2 because of its rapid photolysis and surface condensation. Although the inferred abundance of NaCl in volcanic gases is lower than predicted, those volcanic emissions provide an important source of Na and Cl in Ios neutral clouds and plasma torus.

Detection of Sulfur Monoxide in Io's Atmosphere

Millimeter-wave observations of Io with the IRAM 30 m telescope have allowed the detection of two rotational lines of SO at 219.949 and 138.178 GHz in Ios atmosphere. The observations can be fitted by assuming that gaseous SO is collocated with SO2 on a restricted fraction of Ios surface. In this case, the SO/SO2 mixing ratio is 3%-10%, in agreement with predictions from one-dimensional photochemical models and suggesting a vertical eddy diffusion coefficient in the range 3 × 107 to 3 × 108 cm2 s-1. Alternatively, SO could constitute a tenuous, global atmosphere with column density in the range (2-6) × 1014 cm-2. Photochemistry in SO2 hydrostatic and/or volcanic plume atmospheres and horizontal transport can conceivably produce such an extended SO atmosphere.

Stratospheric profile of HCN on Titan from millimeter observations

Abstract Measurements of the (1−0) line of hydrogen cyanide at 88.6 GHz in the Titan atmosphere are reported. Synthetic spectra were fitted to the observations to derive the vertical distribution of HCN in the stratosphere. The observed line is significantly narrower than that computed for constant stratospheric mixing ratios, implying an increase in the HCN concentration with altitude. From a least-squares analysis taking into account measurement noise and calibration uncertainties, a mean mixing ratio scale height of 47 −11 +36 km is derived for the 100- to 300-km region. The HCN abundance is found to be best constrained around the 170-km level where the inferred mixing ratio is 3.3 −0.8 +0.9 × 10 −7 . The results are consistent with recent analyses of Voyager infrared measurements. The inferred vertical concentration gradient is much steeper and the abundance in the lower stratosphere smaller than predicted by current photochemical models. Theoretical HCN profiles may, however, be brought into agreement with the present results by reducing the magnitude of the vertical eddy mixing assumed in the stratosphere.

Millimeter-wave observations of Saturn, Uranus, and Neptune - CO and HCN on Neptune

Saturn, Uranus, and Neptune were observed at millimeter wavelengths with the IRAM 30 m telescope. The major result is the detection of CO and HCN in Neptunes stratosphere, with respective mixing ratios of (6.5 +/- 3.5) x 10 exp -7 and (3 +/- 1.5) x 10 exp -10. CO seems to be present in Neptunes troposphere as well and to slowly decrease with altitude (scale height about 200 km). HCN is probably formed from reactions between CH3 and N, which can be supplied in sufficient amounts by escape from Tritons atmosphere. The origin of CO, however, is more problematic, because: (1) thermochemical models fail to reproduce the observed abundance by a factor of about 1000; and (2) an external source would require a very large flux of oxygen. CO appears to be at least 15 times less abundant on Uranus than on Neptune. Finally, an upper limit of 10 exp -7 for CO in Saturns stratosphere suggests an internal origin for Saturnian CO.

The 1995-2002 Long-Term Monitoring of Comet C/1995 O1 (HALE-BOPP) at Radio Wavelength

The bright comet Hale-Bopp provided the first opportunity to follow the outgassing rates of a number of molecular species over a large range of heliocentric distances. We present the results of our observing campaign at radio wavelengths which began in August 1995 and ended in January 2002. The observations were carried out with the telescopes of Nancay, IRAM, JCMT, CSO and, since September 1997, SEST. The lines of nine molecules (OH, CO, HCN, CH3OH, H2CO, H2S, CS, CH3CN and HNC) were monitored. CS, H2S, H2CO, CH3CN were detected up to r h = 3–4 AU from the Sun, while HCN and CH3OH were detected up to 6 AU. CO, which is the main driver of cometary activity at heliocentric distances larger than 3–4 AU, was last detected in August 2001, at r h = 14 AU.