Scientific Results in Press

Spectroscopic observations of comet C/1999 H1 (Lee) with the SEST, JCMT, CSO, IRAM and Nançayradio telescopes

N. Biver(¹), D. Bockelée-Morvan(²), J. Crovisier(²), F. Henry(²), J.K. Davies(³), H.E. Matthews(³), P. Colom(²), E. Gérard(²), D.C. Lis(⁴), T.G. Phillips(⁴), F. Rantakyrö(⁵), L. Haikala(⁵), and H.A. Weaver(⁶)
(¹)Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA, (²)Observatoire de Paris-Meudon, 5, place J. Janssen, F-92195 Meudon, France, (³)Joint Astronomy Centre, 660 N. A'ohoku Place, Hilo, HI 96720, USA, (⁴)California Institute of Technology, MS 320-47, Pasadena, CA 91125, USA, (⁵)Swedish-ESO Submillimetre Telescope, ESO, Alonso de Cordova 3107, Vitacura, Santiago, Chile, (⁶)Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218-2686, USA

Abstract:
Coordinated spectroscopic radio observations of comet C/1999 H1 (Lee) were undertaken between May 4 and October 26, 1999, 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 Millimétrique, and the Nançayradio telescope.

We report on observations of OH, HCN, CH₃OH, H₂CO, 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 $1.6\times10^{29}$ 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 comets Hyakutake and Hale-Bopp. An upper limit of D/H $< 300\times 10^{-5}$ 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 to previous comets.

Astron. J., in press

CO band emission from MWC349 : I. First overtone bands from a disk or from a wind?

M. Kraus (¹), E. Krügel(¹), C. Thum(²), and T. Geballe(³)
(¹)Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany, (²)Institut de Radio Astronomie Millimetrique, F-38406 Saint Martin d'Hères, France, (³)Gemini Observatory, 670 North A'ohoku Place, University Park, Hilo, Hawaii 96720, USA

Abstract:
We observed the near infrared emission in the wavelength range 2.28-2.5$\mu$m from the peculiar B[e]-star MWC349. The spectra contain besides the strong IR continuum the first overtone CO bands and most of the hydrogen recombination lines of the Pfund series, both in emission. We also modeled the spectra. The Pfund lines have a gaussian profile with a FWHM of $\sim 100$km/s, and it turned out that their emission is in LTE and optically thin. To explain the CO bands, several scenarios were investigated. We found that the CO band heads are formed under LTE and that the gas must have a temperature of 3500 to 4000K. The width of the $2\rightarrow 0$ band head indicates kinematical broadening of 50 to 60km/s. We can obtain fits to the measured spectra assuming that the CO gas has a column density of $5\cdot 10^{20}$cm^-2 and is located either at the inner edge of the rotating circumstellar disk. In this case, the disk must have a bulge which partly blocks the radiation so that the observer sees only a sector on the far side where the radial velocities are small. Or the CO emission originates in a wind with gaussian line profiles. Both fits are of equal quality and satisfactory. In a third alternative where the fit is less convincing, the CO emission is optically thin and comes from an extended Keplerian disk.

A&A, in press

The Bright Gamma-Ray Burst 991208 - Tight Constraints on Afterglow Models from Observations of the Early-Time Radio Evolution

T. J. Galama(¹), M. Bremer(²), F. Bertoldi(³), K.M. Menten(³), U. Lisenfeld(⁴), D. S. Shepherd(⁵), B. Mason(⁶), F. Walter(⁶), G. G. Pooley(⁷), D. A. Frail(⁵), R. Sari(⁸), S. R. Kulkarni(¹), E. Berger(¹), J.S. Bloom(¹), A. J. Castro-Tirado(⁹), J. Granot(¹⁰),
(¹)Division of Physics, Mathematics and Astronomy, California Institute of Technology, MS 105-24, Pasadena, CA 91125, (²)Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, F-38406 Saint-Martin d'Hères, France, (³)Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany, (⁴)Instituto de Radioastronomia Milimetrica, Avenida Pastora 7, Nucleo Central, E-18012 Granada, Spain, (⁵)National Radio Astronomy Observatory, P.O. Box 0, Socorro, NM 87801, (⁶)California Institute of Technology, Owens Valley Radio Observatory 105-24, Pasadena, CA 91125, (⁷)Mullard Radio Astronomy Observatory, Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0HE, UK, (⁸)California Institute of Technology, Theoretical Astrophysics 103-33, Pasadena, CA 91125, (⁹)LAEFF-INTA, Villafranca del Castillo, PO Box 50.727, E-28080 Madrid, Spain , (¹⁰)AC Racah Institute, Hebrew University, Jerusalem 91904, Israel

Abstract:
The millimeter wavelength emission from GRB991208 is the second brightest ever detected, yielding a unique data set. We present here well-sampled spectra and light curves over more than two decades in frequency for a two-week period. This data set has allowed us for the first time to trace the evolution of the characteristic synchrotron self-absorption frequency $\nu_{\rm a}$ and peak frequency $\nu_{\rm m}$, and the peak flux density F$_{\rm m}$: we obtain $\nu_{\rm a} \propto t^{-0.15 \pm 0.12}$, $\nu_{\rm m} \propto t^{-1.7 \pm 0.4}$, and F $_{\rm m} \propto t^{-0.47 \pm 0.11}$. From the radio data we find that models of homogeneous or wind-generated ambient media with a spherically symmetric outflow can be ruled out. A model in which the relativistic outflow is collimated (a jet) can account for the observed evolution of the synchrotron parameters, the rapid decay at optical wavelengths, and the observed radio to optical spectral flux distributions that we present here, provided that the jet transition has not been fully completed in the first two weeks after the event. These observations provide additional evidence that rapidly decaying optical/X-ray afterglows are due to jets and that such transitions either develop very slowly or perhaps never reach the predicted asymptotic decay F $(t) \propto t^{-p}$.

Accepted for publication in ApJ Letters

High-resolution observations at $\lambda = 3$ mm of the OH 231.8+4.2 molecular outflow

C. Sanchez-Contreras(¹),(²), V. Bujarrabal(¹), R. Neri(³), J. Alcolea(¹)
(¹)Observatorio Astronomico Nacional (IGN), Ap. 1143, 28800 Alcala de Henares, Spain, (²)Departamento de Astrofisica, Facultad CC. Fisicas, Universidad Complutense, 28040 Madrid, Spain, (³)IRAM, 300 rue de la Piscine, 38406 St Martin d'Hères, France

Abstract:
We present high spatial resolution observations of HCO⁺(J=1 $\rightarrow$0), SO (J=22 $\rightarrow$11), H¹³CN (J=1 $\rightarrow$0), SiO (v=1, J=2 $\rightarrow$1), and the continuum at 3mm from OH231.8+4.2, taken with the IRAM interferometer at Plateau de Bure. We also report the first detection of NS in circumstellar envelopes. The overall distribution of the emission of all molecules (except for HCO⁺ and the SiO maser) is similar to that of CO. The most intense emission arises from a compact, slowly-expanding component around the central star. The rest of the emission comes from gas distributed in a narrow region along the symmetry axis, that flows outwards following a velocity gradient also similar to that found in CO. Our observations show with high accuracy the distribution of the HCO⁺ intensity, that is found to be very clumpy and strongly enhanced in the shock-accelerated lobes. We argue that such a distribution is due to the efficient formation of this molecule by shock-induced reactions. An expanding disk or ring around the central star is detected from the SO emission. The characteristic radius and expansion velocity of this structure are 210¹⁶cm and 6-7kms^-1, respectively. The SiO maser emission could arise from the innermost parts of such a disk. The 3mm continuum emission seems to be due to cold dust ($\sim$20K) distributed in the lobes of object OH 231.8+4.2 as well as from warmer ($\sim$55K) dust located in a compact region surrounding the central star.

Appeared in A&A, 357, 651, 2000