E. Gerard

Observations at Nançay of the OH 18-cm lines in comets. The data base. Observations made from 1982 to 1999

Since the apparition of comet Kohoutek 1973 XII, the 18-cm lines of the OH radical have been systematically observed in a number of comets with the Nancay radio telescope. Between 1973 and 1999, 52 comets have been successfully detected. This allowed an evaluation of the cometary water production rates and their evolution with time, as well as a study of several physical processes such as the excitation mechanisms of the OH radio lines, the expansion of cometary atmospheres, their anisotropy in relation to non-gravitational forces, and the Zeeman effect in relation to the cometary magnetic field. Part of these observations and their analysis have already been published. The bulk of the results are now organized in a data base. The present paper is a general presentation of the Nancay cometary data base and a more specific description of the observations of 53 cometary apparitions between 1982 and 1999. Comets observed before 1982 are only partly incorporated in the data base. Observations of comets since 2000 have benefited from a major upgrade of the telescope; they will be presented in forthcoming publications.

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.

Observations of the OH Radical in Comet C/1995 O1 (Hale-Bopp) with the Nançay Radio Telescope

We present OH 18-cm observations of comet Hale-Bopp (C/1995 O1) at the Nançay radio telescope. On nucleus and offset position observations allowed us to obtain both OH production rates and quenching radii. The maximum OH production rate was reached around perihelion, at about1031 s-1.

Polarimetric observations of OH masers in proto-planetary nebulae

The 1612 and 1667 MHz OH maser lines have been measured in all four Stokes parameters in 47 proto-planetary nebula (PPN) candidates. Out of 42 objects detected, 40 and 34 are 1612 and 1667 MHz emitters, respectively. The spectral extent of the 1667 MHz line overshoots that of the 1612 MHz line in about 80% of the targets. 52% and 26% of the 1612 and 1667 MHz sources, respectively, show linear polarization in at least some features. Circular polarization is more frequent, occurring in 78% and 32% of sources of the respective OH lines. The percentage polarization is usually small (<15%) reaching up to 50-80% in a few sources. Features of linearly polarized emission are usually weak (0.5-4 Jy) and narrow (0.3-0.5 km s -1 ). The strength of the magnetic field inferred from likely Zeeman pairs in two sources of a few mG is consistent with values reported elsewhere for those classes of objects. An upper limit of the electron density in the envelope of OH17.7-2.0 derived from the difference in the position angle of polarization vectors for the two OH lines is about 1 cm -3 . Distinct profiles of polarization position angle at 1612 and 1667 MHz are seen in about one third of the sources and strongly suggest that the envelopes are permeated by structured magnetic fields. The geometry of the magnetic field is implicated as an important cause of the depolarization found in some PPN candidates. For the subset of targets which show axisymmetric shells in the optical or radio images we found a dominance of magnetic field components which are orthogonal to the long axis of the nebulae. This finding supports the hypothesis that such bipolar lobes are shaped by the magnetic field.

Observations of hydroxyl ground state transitions in a complete sample of methanol sources

High sensitivity observations of all four transitions of the ground state 2 Π3/2, J = 3/ 2o f OH in both senses of circular polarization have been carried out with the Nancay radio telescope. The sample was a set of 100 star forming regions detected in a recent unbiased survey of 6668 MHz CH3OH masers. OH maser emission was found in 55 objects of which 31 were not previously catalogued. The 1665 MHz line was seen in almost all OH maser sources and was accompanied by the 1667 MHz line in about 75% of cases. Respectively 7% and 11% of OH 1665 MHz masers were accompanied by maser lines at 1612 and 1720 MHz. These two satellite line masers never occurred simultaneously in the same source nor at the same radial velocity, suggesting mutually exclusive physical conditions as predicted by models. OH maser emission usually shared the same velocity range as the 6668 MHz CH3OH maser. The intensity ratio of the 6668 MHz and 1665 MHz lines clearly divides the sample into methanol- and hydroxyl-rich sources and could be controlled by the abundance of maser molecules and the kinetic temperature. The OH maser emission was substantially polarized with a mean fractional circular polarization of 0.30 and Zeeman pair candidates were found in 15 targets. 36 sources were found in OH absorption at the main lines and 24 of them were also accompanied by OH maser emission. OH absorption features were blueshifted with regard to the related OH masers, indicating that they were formed in front of the central continuum sources. Absorption at 1720 MHz was always accompanied by emission at 1612 MHz and vice versa. The behaviour of stimulated emission and absorption in both satellite lines was thus conjugated and the 1720 MHz emission features seem to be signatures of regions of low hydrogen density and OH column density. The correlation of OH and CH3OH flux densities with the IRAS flux densities found for our sample appears to support pumping schemes of both molecules by infrared photons. Statistics of masers in the sample appear to be consistent with the scenario that the CH3OH masers appeared earlier than the OH masers.

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 Millimétrique (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.

Discovery of a detached H i gas shell surrounding α Orionis

We report the detection of the HI line at 21 cm in the direction of alpha Ori with the Nancay Radiotelescope and with the Very Large Array. The observations confirm the previous detection of HI emission centered on alpha Ori, but additionally reveal for the first time a quasi-stationary detached shell of neutral atomic hydrogen ~4 arcmin. in diameter (0.24 pc at a distance of 200 pc). The detached shell appears elongated in a direction opposite to the stars space motion. A simple model shows that this detached atomic gas shell can result from the collision of the stellar wind from alpha Ori with the local interstellar medium (ISM). It implies that alpha Ori has been losing matter at a rate of ~ 1.2x10^-6 solar masses per year for the past 8x10^4 years. In addition, we report the detection of atomic hydrogen associated with the far-infrared arc located 6 arcmin. north-east of alpha Ori, that has been suggested to trace the bow shock resulting from the motion of the star through the ISM. We report also the detection by the Galaxy Evolution Explorer (GALEX) of a far-UV counterpart to this arc.

Discovery of a detached HI gas shell surrounding alpha Orionis

We report the detection of the HI line at 21 cm in the direction of alpha Ori with the Nancay Radiotelescope and with the Very Large Array. The observations confirm the previous detection of HI emission centered on alpha Ori, but additionally reveal for the first time a quasi-stationary detached shell of neutral atomic hydrogen ~4 arcmin. in diameter (0.24 pc at a distance of 200 pc). The detached shell appears elongated in a direction opposite to the stars space motion. A simple model shows that this detached atomic gas shell can result from the collision of the stellar wind from alpha Ori with the local interstellar medium (ISM). It implies that alpha Ori has been losing matter at a rate of ~ 1.2x10^-6 solar masses per year for the past 8x10^4 years. In addition, we report the detection of atomic hydrogen associated with the far-infrared arc located 6 arcmin. north-east of alpha Ori, that has been suggested to trace the bow shock resulting from the motion of the star through the ISM. We report also the detection by the Galaxy Evolution Explorer (GALEX) of a far-UV counterpart to this arc.

Observations of the OH radical in comet C/1996 B2 (Hyakutake) with the Nançay radio telescope

Abstract The 18-cm lines of the OH radical were monitored in C/1996 B2 (Hyakutake) with the Nancay radio telescope between 1 March and 18 May 1996. Besides providing regular measurements of the water production rate, the C/1996 B2 campaign was useful to test and improve existing models of the OH density, excitation and tranfer because the comet passed successively close to the Earth and close to the Sun. At the time of closest approach to Earth, a rough mapping was made by observing at offset positions several beamwidths (3.5′) from the nucleus together with the centre position. The OH line intensity at the centre was comparble to those at offset positions, providing direct evidence for collisional quenching of the OH maser in the inner coma. After removing the Greenstein effect, the line profiles suggest a slight excess outgassing towards the Sun. We are able to detect the OH satellite lines at 1612 and 1721 MHz. A weak Zeeman splitting of the 1667 and 1665 MHz transitions was observed corresponding to an average line-of-sight magnetic field in the range +5 to +10 nT.

The formation of a detached shell around the carbon star Y CVn

Y CVn is a carbon star surrounded by a detached dust shell that has been imaged by the Infrared Space Observatory at 90 μm. With the Nancay Radio Telescope, we have studied the gaseous counterpart in the 21-cm H I emission line. New data have been acquired and allow to improve the signal-to-noise ratio on this line. The high spectral resolution line profiles obtained at the position of the star and at several offset positions set strong constraints on the gas temperature and kinematics within the detached shell; the bulk of the material should be at ∼100-200 K and in expansion at 1∼2 km s _1 . In addition, the line profile at the central position shows a quasi-rectangular pedestal that traces an 8 km s _1 outflow of ∼ 1.0 x 10 _7 M ⊙ yr _1 , stable for about 2 x 10 4 yr, which corresponds to the central outflow already studied with CO rotational lines. We present a model in which the detached shell results from the slowing down of the stellar wind by surrounding matter. The inner radius corresponds to the location where the stellar outflow is abruptly slowed down from ∼8 to 2 km s _1 (termination shock). The outer radius corresponds to the location where external matter is compressed by the expanding shell (bow shock). In this model, the mass-loss rate of YCVn has been set constant, at the same level of 1.0 x 10- 7 M ⊙ yr _1 , for ∼4.5 x 10 5 yr. The gas temperature varies from ∼1800 K at the inner limit to 165 K at the interface between circumstellar matter and external matter. Our modelling shows that the presence of a detached shell around an asymptotic giant branch star may not mean that a drastic reduction of the mass-loss rate has occurred in the past. The inner radius of such a shell might only be the effect of a termination shock rather than of an interruption of the mass-loss process.