Scientific Results in Press

New molecules found in comet C/1995 O1 (Hale-Bopp) - Investigating the link between cometary and interstellar material

D. Bockelée-Morvan (¹), D.C. Lis(²), J.E. Wink(³), D. Despois(⁴), J. Crovisier(¹), R. Bachiller(⁵), D.J. Benford(²), N. Biver(⁶), P. Colom(¹), J.K. Davies(⁷), E. Gérard(¹), B. Germain(¹), M. Houde(⁸), D. Mehringer(⁹), R. Moreno(³), G. Paubert(¹⁰), T.G. Phillips(²) and H. Rauer(¹¹)
(¹)Observatoire de Paris, F-92195 Meudon, France (²)California Institute of Technology, MS 320-47, Pasadena, CA 91125, USA, (³)Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, F-38406 Grenoble, France, (⁴)Observatoire de Bordeaux, B.P. 89, F-33270 Floirac, France (⁵)Instituto Geográfico Nacional, Observatorio Astronómico Nacional, Apartado 1143, E-28800 Alcalá de Henares, Spain, (⁶)Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA, (⁷)Joint Astronomy Centre, 680 North A'Ohoku Place, University Park, Hilo, HI 96720-6030, USA, (⁸)Caltech Submillimeter Observatory, 111 Novelo Street, Hilo, HI 96720, USA, (⁹)University of Illinois, Department of Astronomy, 1002 W. Green St. Urbana, IL 61801, USA, (¹⁰)Instituto de Radioastronomía Millimétrica, Avenida Divina Pastora 7, Núcleo Central, E-18012 Granada, Spain, (¹¹)DLR, Institute of Space Sensor Technology and Planetary Exploration, Rutherfordstraße, D-12484 Berlin, Germany

Abstract:
We present millimetre and submillimetre observations of comet C/1995 O1 (Hale-Bopp) undertaken near perihelion with the Caltech Submillimeter Observatory and the 30-m telescope and Plateau de Bure interferometer of the Institut de Radioastronomie Millimétrique. From a spectral molecular survey, six new cometary molecular species have been identified for the first time in a comet: SO, SO₂, HC₃N, NH₂CHO, HCOOH, and HCOOCH₃. Relative abundances with respect to water are 0.3% (SO), 0.2% (SO₂), 0.02% (HC₃N), 0.01-0.02% (NH₂CHO), 0.09% (HCOOH), and 0.08% (HCOOCH₃). Several rotational transitions of OCS and HNCO, whose first identifications were made previously in comet C/1996 B2 (Hyakutake), have also been detected, confirming that these molecular species are ubiquitous compounds of cometary atmospheres. Inferred abundances of OCS and HNCO relative to water in comet Hale-Bopp are 0.4% and 0.1%, respectively. During this observational campaign, we also observed rotational lines of HCN, HNC, CH₃CN, CO, CH₃OH, H₂CO, H₂S, and CS. In combination with results of other observations, a comprehensive view of the volatile composition of the coma of comet Hale-Bopp is obtained. A quantitative comparison shows that chemical abundances in comet Hale-Bopp parallel those inferred in interstellar ices, hot molecular cores and bipolar flows around protostars. This suggests that the processes at work in the interstellar medium, in particular grain surface chemistry, have played a major role in the formation of cometary ices. It supports models in which cometary volatiles formed in the interstellar medium and suffered little processing in the Solar Nebula.

Published in A&A 2000, 353, 1101

Dense gas in nearby galaxies.
XIII. CO submillimeter line emission from the starburst galaxy M82

R.Q. Mao (^1,2,3), C. Henkel(¹), A. Schulz(^4,5), M. Zielinsky(⁶), R. Mauersberger (^7,8,9), H. Störzer(⁶), T.L. Wilson(^1,7), - P. Gensheimer(⁷)

(¹) Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany, (²) Purple Mountain Observatory, Chinese Academy of Sciences, 210008 Nanjing, PR China, (³) National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100018, PR China, (⁴) Institut für Physik und Didaktik, Universität zu Köln, Gronewaldstr. 22, D-50931 Köln, Germany, (⁵) Institut für Astrophysik und Extraterrestrische Forschung der Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany, (⁶) I. Physikalisches Institut der Universität zu Köln, Zülpicher Straße 77, D-50937 Köln, Germany, (⁷) Submillimeter Telescope Observatory, The University of Arizona, Tucson AZ 85721, U.S.A., (⁸) Steward Observatory, The University of Arizona, Tucson AZ 85721, U.S.A. and (⁹) Instituto de Radioastronomia Milimétrica, Avda. Divina Pastora, 7NC, E-18012 Granada, Spain

Abstract:
¹²CO J = 1-0, 2-1, 4-3, 7-6, and ¹³CO 1-0, 2-1, and 3-2 line emission was mapped with angular resolutions of 13'' - 22'' toward the nuclear region of the archetypical starburst galaxy M82. There are two hotspots on either side of the dynamical center, with the south-western lobe being slightly more prominent. Lobe spacings are not identical for all transitions: For the submillimeter CO lines, the spacing is $\sim$15''; for the millimeter lines (CO J = 2-1 and 1-0) the spacing is $\sim$26'', indicating the presence of a `low' and a `high' CO excitation component. A Large Velocity Gradient (LVG) excitation analysis of the submillimeter lines leads to inconsistencies, since area and volume filling factors are almost the same, resulting in cloud sizes along the lines-of-sight that match the entire size of the M82 starburst region. Nevertheless, LVG column densities agree with estimates derived from the dust emission in the far infrared and at submillimeter wavelengths. 22'' beam averaged total column densities are N(CO) $\sim$ 510¹⁸ and N(H₂) $\sim$ 10²³ $\hbox{{\rm cm}}^{-2}$; the total molecular mass is a few 10⁸M$_\odot$. Accounting for high UV fluxes and variations in kinetic temperature and assuming that the observed emission arises from photon dominated regions (PDRs) resolves the problems related to an LVG treatment of the radiative transfer. Spatial densities are as in the LVG case ( $n({\rm H}_2$) $\sim$ 10^3.7 $\hbox{{\rm cm}}^{-3}$ and $\sim$10³ $\hbox{{\rm cm}}^{-3}$ for the high and low excitation component, respectively), but ¹²CO/¹³CO intensity ratios $\ga$10 indicate that the bulk of the CO emission arises in UV-illuminated diffuse cloud fragments of small column density (N(H₂) $\sim$ 510²⁰ $\hbox{{\rm cm}}^{-2}$/ ${\rm km\,s}^{-1}$) and sub-parsec cloud sizes with area filling factors $\gg$1. Thus CO arises from quite a different gas component than the classical high density tracers (e.g. CS, HCN) that trace star formation rates more accurately. The dominance of such a diffuse molecular interclump medium also explains observed high [C I]/CO line intensity ratios. PDR models do not allow a determination of the relative abundances of ¹²CO to ¹³CO. Ignoring magnetic fields, the CO emitting gas appears to be close to the density limit for tidal disruption. Neither changes in the ¹²C/¹³C abundance ratio nor variations of the incident far-UV flux provide good fits to the data for simulations of larger clouds. A warm diffuse ISM not only dominates the CO emission in the starburst region of M82 but is also ubiquitous in the central region of our Galaxy, where tidal stress, cloud-cloud collisions, shocks, high gas pressure, and high stellar densities may all contribute to the formation of a highly fragmented molecular debris. ¹²CO, ¹²CO/¹³CO, and [C I]/CO line intensity ratios in NGC253 (and NGC4945) suggest that the CO emission from the centers of these galaxies arises in a physical environment that is similar to that in M82. Starburst galaxies at large distances (z$\sim$ 2.2-4.7) show ¹²CO line intensity ratios that are consistent with those observed in M82. PDR models should be applicable to all these sources. ¹²CO/¹³CO line intensity ratios $\gg$10, sometimes observed in nearby ultraluminous mergers, require the presence of a particularly diffuse, extended molecular medium. Here [C I]/CO abundance ratios should be as large or even larger than in M82 and NGC253.
Acknowledgements: Based on observations with the Heinrich-Hertz-Telescope (HHT) and the IRAM 30-m telescope. The HHT is operated by the Submillimeter Telescope Observatory on behalf of Steward Observatory and the Max-Planck-Institut für Radioastronomie.

Accepted for publication in A&A. Send offprint requests to C. Henkel; p220hen@mpifr-bonn.mpg.de

The structure of the collapsing envelope
around the low-mass protostar IRAS 16293-2422

C. Ceccarelli(¹), A. Castets(²), E. Caux(³), D. Hollenbach(⁴), L. Loinard(⁵), S. Molinari(⁶) and A.G.G.M. Tielens(⁷)
(¹) Laboratoire d'Astrophysique, Observatoire de Grenoble, BP 53, F-38041 Grenoble Cédex 09, France, (²) Observatoire de Bordeaux, BP 89, 33270 Floirac, France, (³) CESR CNRS-UPS, BP 4346, 31028 - Toulouse Cédex 04, France, (⁴) NASA Ames Research Center, MS 245-3, Moffett Field, CA 94035-1000, USA, (⁵) Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, F-38406 St. Martin d'Hères, France, (⁶) IPAC, California Institute of Technology, MS 100-22, Pasadena CA 91125, USA, (⁷) SRON/Kaypten Institute, P.O. Box 800, 9700 AV, Groningen, The Netherlands

Abstract:
Using H₂O, OI and SiO data, we derive the structure of the collapsing envelope around the low-mass protostar IRAS 16293-2422 down to $r \sim 30$ AU. With an accurate model which computes self-consistently the chemical composition, thermal balance and line emission from a collapsing envelope (Ceccarelli, Hollenbach & Tielens 1996), we find that IRAS 16293-2422 is a 0.8 M$_\odot$ protostar accreting from the surrounding envelope at a rate of $3.5 \times 10^{-5}$ M$_\odot$ yr^-1, in good agreement with previous studies. The model predicts that the water abundance in the outer (r $\geq$ 150 AU) part of the envelope is $5 \times 10^{-7}$ with respect to H nuclei, while it is a few times larger at smaller radii (r $\leq$ 150 AU). This enhancement results from the evaporation of icy grain mantles when the temperature exceeds $\sim$ 100 K. The same model can reproduce the observations of the SiO J=2-3 to J=8-7 lines provided the abundance of SiO is $1.5 \times 10^{-8}$ in the inner region, while it is only $4 \times 10^{-12}$ in the rest of the envelope. The SiO abundance enhancement in the inner regions is likely due to the evaporation of the grain mantles also responsible for the abundance enhancement of H₂O. The cooling and heating mechanisms of the gas throughout the envelope as derived from the model are discussed, and used to derive the gas temperature profile.
Acknowledgements: Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA.

Accepted for publication in A&A

The hot core of the solar-type protostar IRAS 16293-2422: H₂CO emission

C. Ceccarelli(¹), L. Loinard(²), A. Castets(³), A.G.G.M. Tielens(⁴), and E. Caux(⁵)
(¹) Laboratoire d'Astrophysique, Observatoire de Grenoble - BP 53, F-38041 Grenoble Cédex 09, France, (²) Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, F-38406 St. Martin d'Hères, France, (³) Observatoire de Bordeaux, BP 89, 33270 Floirac, France, (⁴) SRON, P.O. Box 800, NL-9700 AV Groningen , The Netherlands and (⁵) CESR CNRS-UPS, BP 4346, 31028 Toulouse Cédex 04, France

Abstract:
We model the H₂CO and H₂¹³CO line emission observed towards the solar-type protostar IRAS 16293-2422. Based upon previous analysis of the physical structure of the envelope surrounding IRAS 16293-2422, we develop a model in which the H₂CO lines are emitted by two components: a cold H₂CO-poor outer envelope and a warm H₂CO-rich core. We find that the model reproduces successfully all the available H₂CO and H₂¹³CO data for a H₂CO abundance equal to $(1.1 \pm 0.3) \times 10^{-9}$ in the outer and $(1.1 \pm 0.4) \times 10^{-7}$ in the inner regions of the envelope respectively. We interpret this increase of the H₂CO abundance as due to the evaporation of the grain mantles when the dust temperature exceeds 100 K at about 150 AU from the center, forming a hot core like region. Assuming that all mantle constituents evaporate and are detected in the gas phase, we derive that the H₂CO-ice abundance is about 3% of the H₂O-ice abundance. This is the first measurement of the H₂CO abundance in grain mantles around a low-mass protostar.

Accepted for publication in A&A Letters. Send offprint requests to: Ceccarelli.Cecilia@obs.ujf-grenoble.fr

  

Figure 2: Lindqvist et al.: CO(1-0) and CO(2-1) maps of the shell around U Cam

Molecular Line Observations of Proto-planetary Nebulae

J. Alcolea(¹), V. Bujarrabal(¹), A. Castro-Carrizo(¹), C. Sanchez-Contreras(¹), R. Neri(²), J. Zweigle(²)
(¹)OAN (IGN), Apartado 1143, E-28800 Alcalá de Henares, Madrid, Spain, (²)IRAM, 300 rue de la Piscine, F-38406 St. Martin d'Hères, France

Abstract:
We present our recent results on mm-wave CO observations of proto-planetary nebulae. These include high-resolution interferometric maps of various CO lines in three well known bipolar PPNe: M1-92, M2-56 and OH231.8+4.2. The global properties of the high velocity molecular emission in post-AGB sources have been also studied, by means of high-sensitivity single dish observations of the J= 1-0 and J=2-1 lines of ¹²CO and ¹³CO. We discuss the implications of these results to constrain the origin of the post-AGB molecular high-velocity winds and the shaping of bipolar PPNe and PNe. In addition, we also present the results of an interferometric map of the molecular envelope around the luminous high-latitude star 89Her, a low mass post-AGB source which is also a close binary system.

Appeared in Asymmetrical Planetary Nebulae II: ASP Conference Series, Vol. 199

Molecular Emission from the Shocked Bipolar Outflow in OH 231.8+4.2

C. Sanchez-Contreras(¹), V. Bujarrabal(¹), R. Neri(²), J. Alcolea(¹)
(¹)OAN (IGN), Apartado 1143, E-28800 Alcalá de Henares, Madrid, Spain,(²)IRAM, 300 rue de la Piscine, F-38406 St. Martin d'Hères, France

Abstract:
We present high-resolution observations of several molecular lines in OH 231.8+4.2 taken with the IRAM interferometer. All molecules are distributed in a narrow region along the symmetry axis, and flow outwards following a velocity gradient similar to that found in CO. The HCO+ emission is found to be very clumpy and strongly enhanced in the shock-accelerated lobes, indicating that the formation of this molecule is probably dominated by shock induced reactions. SO is present in the axial outflow as well as in an expanding equatorial disk. The SiO maser emission seems to arise from the innermost parts of such a disk. We also report the first detection of NS in circumstellar envelopes.

Appeared in Asymmetrical Planetary Nebulae II:ASP Conference Series, Vol. 199

The young detached CO shell around U Camelopardalis

M. Lindqvist(¹), H. Olofsson(²), R. Lucas(³), F.L. Schøier(²), R. Neri(³), V. Bujarrabal(⁴) and C. Kahane(⁵)
(¹)Onsala Space Observatory, 43992 Onsala, Sweden, (²)Stockholm Observatory, 13336, Saltsjøbaden, Sweden, (³)IRAM, 300 rue de la Piscine, 38406 St Martin d'Hères Cédex, France, (⁴)OAN, Apartado 1143, 28800 Alcalá de Henares, Spain, (⁵)Observatoire de Grenoble, B.P. 53, 38041 Grenoble Cédex 9, France

Abstract:
We report IRAM Plateau de Bure interferometer observations of the carbon star U Cam in the CO(1-0) and CO(2-1) lines. The remarkable images (Fig. 2) show that U Cam is surrounded by a geometrically thin, $\sim 10^{16}$cm, shell of gas at a distance of $\sim 6\times 10^{16}$cm from the star, that expands with a velocity of $\sim 23$kms^-1. The estimated mass of the shell is low, $\sim 10^{-3}$M$_\odot$. In addition, we detect emission that peaks at the stellar position. From this we estimate a present mass loss rate and gas expansion velocity of $\sim 2.5\times 10^{-7}$M$_\odot$/yr and $12\,$kms^-1, respectively. One possible explanation to the structure of the circumstellar medium is that the shell was produced during a very short period, $\sim 150\,$yr, of high mass loss rate, $\sim 10^{-5}$M$_\odot$/yr, about 800yr ago. U Cam may fit into the scenario where a helium-shell flash modulates the mass loss rate on short times scales.

Appeared in A&A, 351, L1, 1999

The enormous abundance of D₂CO in IRAS16293-2422

L. Loinard(¹), A. Castets(²), C. Ceccarelli(³), A.G.G.M. Tielens(⁴), A. Faure(³), E. Caux(⁵) and G. Duvert(³)
(¹)Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, F-38406 St. Martin d'Hères, France, (²)Observatoire de l'Université de Bordeaux I, B.P. 89, F-33270 Floirac, France, (³)Laboratoire d'Astrophysique, Observatoire de Grenoble - BP 53, F-38041 Grenoble Cédex 09, France, (⁴)SRON, P.O. Box 800, NL-9700 AV Groningen , The Netherlands, (⁵)CESR, CNRS-UPS, BP 4346, 31028 - Toulouse Cédex 04, France

Abstract:
Ceccarelli et al. (1998) recently reported the detection of D₂CO in the low-luminosity protostar IRAS 16293-2422. Using the data available at the time, they found that the abundance of D₂CO might be as high as 1/10th that of its hydrogenated counterpart H₂CO. Here we describe and analyse new multi-transition observations of D₂CO, HDCO, H₂CO, and H₂¹³CO towards IRAS 16293-2422. Correcting for the opacity of the H₂CO lines, we find that the abundance of D₂CO is $\sim$ 5$\%$ that of H₂CO. In addition, we find a component in absorption - also associated to IRAS 16293-2422, but at larger radius - where the abundance of D₂CO compared to H₂CO could be even higher. Though slightly lower than initially claimed, the abundance of D₂CO in IRAS 16293-2422 is extremely high, more than one order of magnitude higher than in Orion KL, the only other source where D₂CO has ever been detected. Because the gas temperature (T = 20-50 K) is too high, deuteration in the gas-phase is very insufficient to explain such high abundances. We conclude that D₂CO is most likely not currently formed in the gas phase, but is evaporated from the dust grains, where it has been accumulating during the cold, dense pre-collapse period.

Accepted for publication in A&A

Starburst in the Ultraluminous Galaxy Arp 220 - Constraints from Observations of Radio Recombination Lines and Continuum

K.R. Anantharamaiah(^1,2), F. Viallefond(³), Niruj R, Mohan (^1,2,4), W.M. Goss(¹) & J.H. Zhao(⁵)
(¹)National Radio Astronomy Observatory, Socorro, NM 87801, USA, (²)Raman Research Institute, Bangalore 560 080, India, (³)DEMIRM, Observatoire de Paris-Meudon, Paris, France, (⁴)Joint Astronomy Program, Indian Institute of Science, Bangalore 560 012, (⁵)Center for Astrophysics, Cambridge, MA, USA

Abstract:
We present observations of recombination lines (RRL) from Arp 220 near 8.1 GHz (H92$\alpha$) and 1.4 GHz (H167$\alpha$ and H165$\alpha$) made using the Very Large Array and near 84 GHz (H42$\alpha$), 96 GHz (H40$\alpha$) and 207 GHz (H31$\alpha$) made using the IRAM 30 m telescope (Fig. 3). RRLs were detected at all the frequencies except at 1.4 GHz where a sensitive upper limit was obtained. We also present continuum flux measurements at these frequencies as well as at 327 MHz made with the VLA. The continuum spectrum which has a spectral index $\alpha \sim -0.6$ ( $S_\nu \propto \nu^{-\alpha}$) between 5 and 10 GHz, shows a break near 1.5 GHz, a complete turnover below 500 MHz and a higher spectral index above 50 GHz.

  

Figure: Recombination lines from Arp 220 in the 3mm (H40$\alpha$ and H42$\alpha$) and the 1.2 mm (H31$\alpha$) bands observed using the IRAM-30m. Solid lines are the observed line profiles corrected for a linear baseline. The dashed lines represent statistical rms noise level in each channel including the uncertainty in the baseline level. The dotted line in the top frame is the integrated H92$\alpha$ line profile from the VLA observations

We show that a model with three components of ionized gas with different densities and area covering factors can consistently explain both RRL and continuum data. The total mass of the ionized gas in the three components is $3.2\times 10^7$ $M_{\odot}$ requiring $4.2\times 10^5$ O5 stars with a total Lyman continuum (Lyc) production rate N $_{\sc Lyc}$  $\sim 1.3\times 10^{55}$ photons s^-1. The ratio of the expected Br$\alpha$ and Br$\gamma$ fluxes implies a dust extinction corresponding to $A_v \sim 45$ magnitudes. The derived Lyc photon production rate implies a continuous star formation rate (SFR) averaged over the life time of OB stars of $\sim$ 240 $M_{\odot}$ yr^-1. The Lyc photon production rate of $\sim 3\%$ associated with the high density HII regions implies similar SFR at recent epochs (t < 10⁵yrs). An alternative model of high density gas, which cannot be excluded on the basis of the available data, predicts ten times higher SFR at recent epochs. If confirmed, this result implies that star formation in Arp 220 consists of multiple starbursts of very high SFR (few $\times 10^3$ $M_{\odot}$ yr^-1) and short durations ($\sim 10^5$ yrs). The similarity of IR-excess, $L_{IR}/L_{L_\alpha} \sim 24$, in Arp 220 to the values observed in starburst galaxies shows that most of the high luminosity of Arp 220 is due to the on-going starburst, rather than due to any hidden AGN. A comparison of the IR-excesses in Arp 220, the Galaxy and M33 indicates that the starburst in Arp 220 has an IMF which is similar to that in normal galaxies and has a duration longer than 10⁷ yrs. If there was no infall of gas during this period, then the star formation efficiency (SFE) in Arp 220 is $\sim 50\%$. The high SFR and SFE in Arp 220 is consistent with their known dependences on mass and density of gas in star forming regions of normal galaxies.

ApJ, in press