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Scientific Results in Press

Detection of the Linear Radical HC$_4$N in IRC+10216

Cernicharo J.$(^{1})$, Guélin M.$(^{2})$, Pardo J. R.$(^{1})$
$(^{1})$Department of Molecular and Infrared Astrophysics, Instituto de Estructura de la Materia, CSIC, C/Serrano 121, 28006 Madrid, Spain, $(^{2})$Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, F-38406 St. Martin d'Hères, France

We report the detection of the linear radical HC$_4$N in the C-rich envelope of IRC +10216. After HCCN, HC$_4$N is the second member of the allenic chain family HC$_{2n}$N observed in space. The column density of HC$_4$N is found to be $1.5\times10^{12}$ cm$^{-2}$. The abundance ratio HC$_2$N/HC$_4$N is 9, a factor of 2 larger than the decrement observed for the cyanopolyynes (HC$_{2n+1}$N/HC$_{2n+3}$N). Linear HC$_4$N has a $^3\Sigma$ electronic ground state and is one of the three low-energy isomeric forms of this molecule. We have searched for the bent and ringed HC4N isomers but could only derive an upper limit to their column densities: $\leq 3\times10^{12}$ cm$^{-2}$ (at $3 \sigma$).

Appeared in: ApJ 615, p. L145

AGB mass-loss and recycling

T. Le Bertre$(^{1})$, E. Gérard$(^{2})$ and J.M. Winters$(^{3})$
$(^{1})$LERMA, UMR 8112, Observatoire de Paris, 61 av. de l'Observatoire, F-75014 Paris, France $(^{2})$GEPI, UMR 8111, Observatoire de Paris, 5 place J. Janssen, F-92195 Meudon Cedex, France $(^{3})$IRAM, 300 rue de la Piscine, F-38406 Saint-Martin-d'Hères, France

Mass-loss from AGB stars is a highly variable process which renders the determination of its rate and of its balance as a function of the stars' evolutionary stage a difficult task. Several diagnostic tools can be considered. The most commonly used are the rotational lines of molecules, like CO or SiO, and the emission by dust at infrared wavelengths. However they are sensitive to only a part of the material, sometimes in limited regions of the circumstellar shells. Other tracers, which may be very useful like lines of HI and H$_2$, should also be considered in order to get a more representative picture of these shells which extend out to the Interstellar Medium.

High spatial and spectral resolution, large fields of view and high dynamical range are all important to reveal the 3-D structure of the outflows and to identify the relevant driving process(es) involved, and finally to determine the mass-loss history for individual sources and the contribution of the AGB star population to the recycling of matter.

Appeared in: Proceedings of the dusty and molecular universe: a prelude to Herschel and ALMA, 27-29 October 2004, Paris, France. Ed. by A. Wilson. ESA SP-577, Noordwijk, Netherlands: ESA Publications Division, ISBN 92-9092-855-7, 2005, p. 217 - 222

The millimeter and submillimeter spectrum of CRL618

Pardo J. R.$(^{1})$, Cernicharo J. R.$(^{1})$, Goicoechea J. R.$(^{1})$, Guélin M.$(^{2})$, Phillips, T. G$(^{3})$
$(^{1})$Department of Molecular and Infrared Astrophysics, Instituto de Estructura de la Materia, CSIC, C/Serrano 121, 28006 Madrid, Spain $(^{2})$Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, F-38406 St. Martin d'Hères, France $(^{3})$California Institute of Technology, Downs Laboratory of Physics 320-47, Pasadena, CA 91125

We present the millimeter and submillimeter line survey of the proto-planetary nebula CRL618. The survey has been carried out with the 30-m IRAM radiotelescope (80-115, 129-180, and 202-278 GHz) and the CSO telescope (280-350 GHz). The line survey shows the rotational lines of several molecular species that are produced in the proto-planetary nebula stage and that are absent in the AGB phase represented by IRC+10216. The wide frequency coverage allows a precise modelling of the molecular emission and permits to derive the physical parameters of the inner regions of CRL618 at angular scales below $5\hbox{$^{\prime\prime}$}$. The abundances of HC$_3$N and HC$_5$N, the dominant species in number of detected lines, are enhanced by a factor larger than 100 relative to those found in red giants and AGB stars.

Appeared in: Proceedings of the dusty and molecular universe: a prelude to Herschel and ALMA, 27-29 October 2004, Paris, France. Ed. by A. Wilson. ESA SP-577, Noordwijk, Netherlands: ESA Publications Division, ISBN 92-9092-855-7, 2005, p. 455 - 456

Q0957+561 revised: CO emission from a disk at $z=1.4$

Krips M.$(^{1,2})$, Neri R.$(^{2})$, Eckart A.$(^{1})$, Downes D.$(^{2})$, Martín-Pintado J.$(^{3})$ Planesas, P.$(^{4})$
$(^{1})$I. Physikalisches Institut, University of Cologne, Zülpicherstr. 77, 50937 Köln, $(^{2})$Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, 38406 Saint Martin d'Hères, France, $(^{3})$Instituto de Estructura de la Materia (CSIC), Serrano 121, 28006 Madrid, Spain, $(^{4})$Observatorio Astronómico Nacional (IGN), Apartado 112, 28800 Alcalá de Henares, Spain

Based on additional interferometric observations, we reanalysed the CO(2-1) and 3 mm continuum emission of Q0957+561, a lensed QSO at a redshift of z=1.4141. The emission in the CO(2-1) lines reveals a gas-rich host galaxy with a peculiar double-peaked profile at one of the two lensed images. Our new interferometric CO maps of the host galaxy agree well with HST images obtained by Keeton et al. (2000) and we thus argue that the two velocity components arise from molecular gas in the disk of the host galaxy. We also present new model calculations, all in excellent agreement with recent time delay measurements and simulations.

Based on observations carried out with the IRAM Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).

Appeared in: A&A 431, 879

Chemical differentiation along the CepA-East outflows

C. Codella$(^{1})$, R. Bachiller$(^{2})$, M. Benedettini$(^{3,4})$, P. Caselli$(^{5})$, S. Viti$(^{4})$ and V. Wakelam$(^{6,7})$
$(^{1})$Istituto di Radioastronomia, INAF, Sezione di Firenze, Largo E. Fermi 5, 50125 Firenze, Italy, $(^{2})$Observatorio Astronómico Nacional (IGN), Apartado 1143, E-28800, Alcalá de Henares (Madrid), Spain, $(^{3})$Istituto di Fisica dello Spazio Interplantario, INAF, Area di Ricerca Tor Vergata, Via Fosso del Cavaliere 100, 00133 Roma, Italy, $(^{4})$Department of Physics and Astronomy, University College London, Gower Street WC1E6 BT London, UK, $(^{5})$Osservatorio Astrofisico di Arcetri, INAF, Largo E. Fermi 5, 50125 Firenze, Italy, $(^{6})$Observatoire de Bordeaux, BP 89, 33270 Floirac, France, $(^{7})$The Ohio State University, Department of Physics, 174 W. 18th Ave., Columbus, OH 43210-1106 USA

We present the results of a multiline survey at mm-wavelengths of the Cepheus A star forming region. Four main flows have been identified: three pointing in the SW, NE, and SE directions and accelerating high density CS clumps. The fourth outflow, revealed by high-sensitivity HDO observations, is pointing towards South and is associated with conditions particularly favourable to a chemical enrichment. At the CepA-East position the emissions due to the ambient clump and to the outflows coexist and different molecules exhibit different spectral behaviours. Some species (C$^{13}$CH, C$_3$H$_2$, CH$_2$CO, CH$_3$C$_2$H, HC$^{18}$O$^+$) exhibit relatively narrow lines at ambient velocities (ambient peak). Other molecules (CO, CS, H$_2$S, SiO, SO, SO$_2$) show extended wings tracing the whole range of the outflow velocities. Finally, OCS, H$_2$CS, HDO, and CH$_3$OH are associated with wings and, in addition, show wings and in addition reveal a bright high velocity redshifted spectral peak (outflow peak) which can be used to investigate the southern outflows. At ambient velocities the gas is dense ($>$ 10$^5$ cm$^{-3}$) and different components at distinct temperatures coexist, ranging from the relatively low kinetic temperatures ($\le$ 50 K) measured with H$_2$S, CH$_3$OH, H$_2$CS, and CH$_3$C$_2$H, to definitely higher temperature conditions, $\sim$ 100-200 K, obtained from the SiO, SO, and SO$_2$ spectra. For the outflow peak we derive densities between $\sim$ 10$^4$ cm$^{-3}$ to $\sim$ 10$^7$ cm$^{-3}$ and high temperatures, $\simeq$ 100-200 K, indicating regions compressed and heated by shocks.

The analysis of the line profiles shows that the SiO molecule dominates at the highest velocities and at the highest excitation conditions, confirming its close association with shocks. H$_2$S, SO$_2$, and SO preferentially trace more quiescent regions than SiO, and in particular a lack of bright H$_2$S emission at the highest velocities is found. OCS and H$_2$CS emit at quite high velocities, where the abundances of three shock tracers like SiO, CH$_3$OH, and HDO are higher. These results may indicate that H$_2$S is not the only major sulphur carrier in the grain mantles, and that OCS and H$_2$CS may probably play an important role on the grains; or that alternatively they rapidly form once the mantle is evaporated after the passage of a shock. Finally, the outflow peak emission has been compared with recent time-dependent sulphur chemistry models: the results indicate that, if associated with accurate measurements of the physical conditions, the CH$_3$OH/H$_2$CS column density ratio can be used as an effective chemical clock to date the age of shocked gas.

MNRAS, in press

The orbiting gas disk in the Red Rectangle

V. Bujarrabal$(^{1})$, A. Castro-Carrizo$(^{2})$, J. Alcolea$(^{3})$, R. Neri$(^{2})$
$(^{1})$Observatorio Astronómico Nacional (IGN), Apdo. 112, 28803 Alcalá de Henares, Spain, $(^{2})$Institute de Radio Astronomie Millimétrique, 300 rue de la Piscine, 38406 St. Martin d'Hères, France, $(^{3})$Observatorio Astronómico Nacional (IGN), c/ Alfonso XII 3, 28014, Madrid, Spain

We present accurate maps of the CO $J$=2-1 and 1-0 lines made with the Plateau de Bure interferometer of the gas disk around the central star(s) of the Red Rectangle, a well known protoplanetary nebula. We confirm that the molecular gas in this source forms a disk perpendicular to the conspicuous axis of symmetry of the optical nebula and that this disk is in rotation. We present detailed modeling of the CO emission and extensive discussion on the accuracy of the values fitted for the different parameters. The outer radius of the disk is $\sim$ 2.7 10$^{16}$ ( $\frac{D({\rm pc})}{710}$) cm, as a function of the assumed distance $D$, which is thought to vary between 380 and 710 pc. The rotation is found to be keplerian, at least in the inner disk. From this velocity field, we derive a central mass between 0.9 M$_\odot$, for a distance of 380 pc, and 1.7 M$_\odot$, for 710 pc. Previous studies on the nature of the stellar component favor the highest value. In the outer disk, we deduce the presence of a slow expansion velocity ($\sim$ 0.8 kms$^{-1}$), superimposed to rotation. We find gas temperatures decreasing from $\sim$ 400 to 30 K across the disk and densities $\mathrel{\rlap{\lower4pt\hbox{\hskip1pt$\sim$}}
\raise1pt\hbox{$>$}}3 \cdot 10^4$.

A&A, in press

Warm gas in the cold diffuse interstellar medium: Spectral signatures in the H$_2$ pure rotational lines

Falgarone E.$(^{1})$, Verstraete L.$(^{2})$, Pineau Des Forêts G.$(^{2})$, Hily-Blant P.$(^{3})$
$(^{1})$Laboratoire de Radioastronomie, LERMA, École Normale Supérieure, 24 rue Lhomond, 75231 Paris Cedex 05, France, $(^{2})$Institut d'Astrophysique Spatiale, Bât. 121, Université de Paris XI, 91405 Orsay Cedex, France, $(^{3})$Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, 38406 Grenoble, France

We present ISO-SWS observations of five pure rotational lines of H$_2$ along a line of sight through the Galaxy which avoids regions of massive star formation. It samples 30 mag of gas, half of it (i.e. 15 mag) being diffuse gas running from the solar neighbourhood to the molecular ring, up to the far side of the Galaxy. The intensities of the S(1) and S(2) lines are too large relative to S(0) to be produced by UV excitation in the known radiation field of the Galaxy. The excitation of these transitions has to tap a more powerful source of energy. We investigate the possibility that it takes place in a large number of magneto-hydrodynamic (MHD) shocks or coherent small-scale vortices, two processes responsible for the intermittent dissipation of MHD turbulence. These dissipation bursts locally and temporarily heat the diffuse gas to temperatures ( $T_k \approx 10^3$ K) well above that of the ambient diffuse gas. We compute the spectroscopic signatures of these processes in the H$_2$ lines. Not only are the computed relative line intensities in good agreement with the observations, but the few percent of warm gas involved is consistent with other independent determinations. We find that the fraction of warm H$_2$ in the diffuse gas (i.e. H$_2$ molecules in $J_u \geq 3$ levels) on that line of sight, $N(H_2^*)/A_v \approx 4 \times 10^{17} cm^{-2} mag^{-1}$, is the same as that found from far UV spectroscopy in the direction of nearby stars. It is also the same as that estimated in the solar neighbourhood to reproduce the large observed abundances of molecules like CH$^+$. These results suggest that the existence, within the cold neutral medium (CNM), of a few percent of warm gas, for which UV photons cannot be the sole heating source, is ubiquitous and presumably traces the intermittent dissipation of MHD turbulence in the cold diffuse gas.

Appeared in: A&A 433, 997

A study of the Keplerian accretion disk and precessing outflow in the massive protostar IRAS 20126+4104

Cesaroni R.$(^{1})$, Neri R.$(^{2})$, Olmi L.$(^{3})$, Testi L.$(^{1})$, Walmsley C. M.$(^{1})$, Hofner P.$(^{4})$
$(^{1})$Osservatorio Astrofisico di Arcetri, INAF, Largo E. Fermi 5, 50125 Firenze, Italy, $(^{2})$IRAM, 300 rue de la Piscine, Domaine Universitaire, 38406 St. Martin d'Hères Cedex, France, $(^{3})$Istituto di Radioastronomia, CNR, Sezione di Firenze, Largo E. Fermi 5, 50125 Firenze, Italy, $(^{4})$National Radio Astronomy Observatory, PO Box O, Socorro, NM 87801, USA

We report on interferometric observations at 3.2 and 1.3 mm of the massive young stellar object IRAS 20126+4104 obtained in the C$^{34}$S and CH$_3$OH lines and in the continuum emission. The C$^{34}$S data confirm the existence of a Keplerian disk, as already suggested by various authors. However, the mass of the central object is $\approx 7
\mbox{M$_\odot$}$, significantly less than previous estimates. We believe that such a discrepancy is due to the fact that the rotation curve is affected not only by the star but also by the mass in the innermost regions of the disk itself: this leads to an overestimate of the stellar mass when low-density tracers are used to study the velocity field over regions larger than a few seconds of arc (i.e. a few 0.01 pc). On the basis of the line profiles we speculate that accretion onto the star might be still occurring through the disk. This seems consistent with current models of high-mass star formation which predict an accretion luminosity equal to that of IRAS 20126+4104 for a $7 \mbox{M$_\odot$}$ protostar. The CH$_3$OH lines trace both the disk and the bipolar outflow previously detected in other molecules such as HCO$^+$, SiO, and H$_2$. New H$_2$ images obtained at $2.2
\mu$m confirm that the outflow axis is undergoing precession. We elaborate a simple model that suitably fits the data thus allowing derivation of a few basic parameters of the precession.

Appeared in A&A 434, 1039

A detailed study of the rotating toroids in G31.41+0.31 and G24.78+0.08

Beltrán M. T.$(^{1})$, Cesaroni R.$(^{1})$, Neri R.$(^{2})$, Codella C.$(^{3})$, Furuya R. S.$(^{4})$, Testi, L.$(^{1})$ and Olmi L.$(^{3})$
$(^{1})$INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy,
$(^{2})$IRAM, 300 rue de la Piscine, 38406 Saint Martin d'Hères, France $(^{3})$Istituto di Radioastronomia, INAF, Sezione di Firenze, Largo E. Fermi 5, 50125 Firenze, Italy,
$(^{4})$Division of Physics, Mathematics, and Astronomy, California Institute of Technology, MS 105-24, Pasadena, CA 91125, USA

We present the results of high angular resolution millimeter observations of gas and dust toward G31.41+0.31 and G24.78+0.08, two high-mass star forming regions where four rotating massive toroids have been previously detected. The CH$_3$CN (12-11) emission of the toroids in G31.41+0.31 and core A1 in G24.78+0.08 has been modeled assuming that it arises from a disk-like structure seen edge-on, with a radial velocity field. For G31.41+0.31 the model properly fits the data for a velocity $v_{rot} \simeq 1.7$ km s$^{-1}$ at the outer radius $R_{out} \simeq 13400$ AU and an inner radius $R_{inn} \simeq
1340$ AU, while for core A1 in G24.78+0.08 the best fit is obtained for $v_{rot} \simeq 2.0$ km s$^{-1}$ at $R_{out} \simeq 7700$ AU and $R_{inn} \simeq 2300$ AU. Unlike the rotating disks detected around less luminous stars, these toroids are not undergoing Keplerian rotation. From the modeling itself, however, it is not possible to distinguish between constant rotation or constant angular velocity, since both velocity fields suitably fit the data. The best fit models have been computed adopting a temperature gradient of the type $T
\propto R^{-3/4}$, with a temperature at the outer radius $T_{out}
\simeq 100$ K for both cores. The $M_{dyn}$ needed for equilibrium derived from the models is much smaller than the mass of the cores, suggesting that such toroids are unstable and undergoing gravitational collapse. The collapse is also supported by the CH$_3^13$CN or CH$_3$CN line width measured in the cores, which increases toward the center of the toroids. The estimates of $v_{inf}$ and $\dot{M}_{acc}$ are 2 km s$^{-1}$ and $\propto 3\times 10^{-2} \mbox{M$_\odot$}$ yr$^{-1}$ for G31.41+0.31, and 1.2 km s$^{-1}$ and $\propto 9\times 10^{-3} \mbox{M$_\odot$}$ yr$^{-1}$ for G24.78+0.08 A1. Such large accretion rates could weaken the effect of stellar winds and radiation pressure and allow further accretion on the star. The values of $T_{rot}$ and N$_{CH_3CN}$, derived by means of the RD method, for both G31.41+0.31 and the sum of cores A1 and A2 (core A of Codella et al. 1997, A&A, 325, 282) in G24.78+0.08 are in the range 132-164 K and $2-8\times 10^{16}$ cm$^{-2}$. For G31.41+0.31, the most plausible explanation for the apparent toroidal morphology seen in the lower K transitions of CH$_3$CN (12-11) is self-absorption, which is caused by the high optical depth and temperature gradient in the core.

Appeared in: A&A, 435, 901

An interferometric CO survey of luminous submillimetre galaxies

Greve T. R.$(^{1,5})$, Bertoldi F.$(^{2})$, Smail Ian$(^{3})$, Neri R.$(^{4})$, Chapman S. C.$(^{5})$, Blain A. W.$(^{5})$, Ivison R. J.$(^{1,6})$, Genzel R.$(^{7,8})$, Omont A.$(^{9})$, Cox P.$(^{10})$, Tacconi L.$(^{7})$ and Kneib J.-P.$(^{5,11,12})$
$(^{1})$Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, $(^{2})$Max-Planck Institut für Radioastronomie(MPIfR), Bonn, Germany, $(^{3})$Institute for Computational Cosmology, University of Durham, South Road, Durham DH1 3LE, $(^{4})$Institut de Radio Astronomie Millimétrique (IRAM), St Martin d'Hères, France, $(^{5})$California Institute of Technology, Pasadena, CA 91125, USA, $(^{6})$Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, $(^{7})$Max-Planck Institut für extraterrestrische Physik (MPE), Garching, Germany $(^{8})$Department of Physics, University of California, Berkeley, USA, $(^{9})$Institut d'Astrophysique de Paris, CNRS, Université de Paris, Paris, France, $(^{10})$Institut d'Astrophysique Spatiale, Université de Paris Sud, Orsay, France, $(^{11})$Observatoire Midi-Pyrénées, UMR5572, 14 Avenue Edouard Belin, 31400 Toulouse, France $(^{12})$Laboratoire d'Astrophysique de Marseille, UMR 6110, CNRS-Université de Provence, Traverse du Siphon-Les trois Lucs, 13012 Marseille, France

In this paper, we present results from an Institut de Radio Astronomie Millimétrique (IRAM) Plateau de Bure millimetre-wave Interferometer (PdBI) survey for carbon monoxide (CO) emission towards radio-detected submillimetre galaxies (SMGs) with known optical and near-infrared spectroscopic redshifts. Five sources in the redshift range $z\approx
1-3.5$ were detected, nearly doubling the number of SMGs detected in CO. We summarize the properties of all 12 CO-detected SMGs, as well as six sources not detected in CO by our survey, and use this sample to explore the bulk physical properties of the submillimetre galaxy (SMG) population as a whole. The median CO line luminosity of the SMGs is $\langle L'_{CO}\rangle = (3.8 \pm 2.0)\times
10^{10}$Kkms$^{-1}$pc$^2$. Using a CO-to-H$_2$ conversion factor appropriate for starburst galaxies, this corresponds to a molecular gas mass $\langle M(H_2)\rangle = (3.0 \pm 1.6) \times 10^{10} \mbox{M$_\odot$}$ within an $\approx 2$kpc radius, approximately 4 times greater than the most luminous local ultraluminous infrared galaxies (ULIRGs) but comparable to that of the most extreme high-redshift radio galaxies (HzRGs) and quasi-sellar objects (QSOs). The median CO FWHM linewidth is broad, $\langle FWHM \rangle = 780 \pm 320$kms$^{-1}$, and the SMGs often have double-peaked line profiles, indicative of either a merger or a disc. From their median gas reservoirs ( $\approx 3\times
10^{10}\mbox{M$_\odot$}$) and star formation rates ( $\raisebox{-0.6ex}{$\: \stackrel{>}{\scriptstyle \sim} \:$}700\mbox{M$_\odot$}$ yr$^{-1}$), we estimate a lower limit on the typical gas-depletion time-scale of $\raisebox{-0.6ex}{$\: \stackrel{>}{\scriptstyle \sim} \:$}40$Myr in SMGs. This is marginally below the typical age expected for the starbursts in SMGs and suggests that negative feedback processes may play an important role in prolonging the gas consumption time-scale. We find a statistically significant correlation between the far-infrared and CO luminosities of the SMGs, which extends the observed correlation for local ULIRGs to higher luminosities and higher redshifts. The non-linear nature of the correlation implies that SMGs have higher far-infrared to CO luminosity ratios and possibly higher star formation efficiencies (SFEs), than local ULIRGs. Assuming a typical CO source diameter of $\Theta \approx 0.5$ arcsec ($D\approx 4$kpc), we estimate a median dynamical mass of $\langle M_{dyn} \rangle \approx (1.2 \pm 1.5) \times
10^{11}\mbox{M$_\odot$}$ for the SMG sample. Both the total gas and stellar masses imply that SMGs are very massive systems, dominated by baryons in their central regions. The baryonic and dynamical properties of these systems mirror those of local giant ellipticals and are consistent with numerical simulations of the formation of the most massive galaxies. We have been able to impose a lower limit of $\raisebox{-0.6ex}{$\: \stackrel{>}{\scriptstyle \sim} \:$}5 \times 10^{-6}Mpc^{-3}$ to the comoving number density of massive galaxies in the redshift range $z\approx 2-3.5$, which is in agreement with results from recent spectroscopic surveys and the most recent model predictions.

Appeared in: MNRAS 359, 1165

Molecular gas in a $z \approx 2.5$ triply-imaged, sub-mJy submillimetre galaxy typical of the cosmic far-infrared background

Kneib J.-P.$(^{1,2,3})$, Neri R.$(^{4})$, Smail I.$(^{5})$, Blain A.$(^{2})$, Sheth K.$(^{2})$, van der Werf P.$(^{6})$, Knudsen K. K.$(^{6})$
$(^{1})$Observatoire Midi-Pyrénées, CNRS-UMR5572, 14 avenue E. Belin, 31400 Toulouse, France $(^{2})$Caltech-Astronomy, MC105-24, Pasadena, CA 91125, USA, $(^{3})$OAMP, Laboratoire d'Astrophysique de Marseille, traverse du Siphon, 13012 Marseille, France, $(^{4})$IRAM, 300 rue de la Piscine, 38640 Saint Martin d'Hères, France, $(^{5})$Institute for Computational Cosmology, University of Durham, South Road, Durham DH1 3LE, UK, $(^{6})$Leiden Observatory, PO Box 9513, NL - 2300 RA Leiden, The Netherlands

We present the results of observations from the Plateau de Bure IRAM interferometric array of the submillimetre (submm) galaxy SMMJ16359+6612 lying at $z=2.516$ behind the core of the massive cluster A 2218. The foreground gravitational lens produces three images with a total magnification of 45 of this faint submm galaxy, which has an intrinsic submm flux of just $f_{850 \mu m}=0.8$ mJy - placing it below the confusion limit of blank-field surveys. The substantial magnification provides a rare opportunity to probe the nature of a distant sub-mJy submm-selected galaxy, part of the population which produces the bulk of the cosmic far-infrared background at submm wavelengths. Our observations detect the CO(3-2) line in all three images, as well as the CO(7-6) line and the dust continuum at 1.3 mm for the brightest image but only at a $3 \sigma$ level. The velocity profile of the CO(3-2) line displays a double-peak profile which is well fit by two Gaussians with FWHM of 220 km s$^{-1}$ and separated by 280 km s$^{-1}$. We estimate the dynamical mass of the system to be $\approx 1.5 \times 10^{10} \mbox{M$_\odot$}$ and an H$_2$ gas mass of $4.5
\times 10^9\mbox{M$_\odot$}$. We identify a spatial offset of $\approx 1$ $^{\prime\prime}$between the two CO(3-2) velocity components, again benefiting from the magnification due to the foreground lens, modeling of which indicates that the offset corresponds to just $\approx 3$ kpc in projection at $z=2.516$. The spatial and velocity properties of these two components are closely related to features detected in previously published H$\alpha$ spectroscopy. We propose that this source is a compact merger of two typical Lyman-break galaxies with a maximal separation between the two nuclei of about 3 kpc, although a dusty disk explanation is not excluded. This system is much less luminous and massive than other high-redshift submillimetre galaxies studied to date, but it bears a close similarity to similarly luminous, dusty starburst resulting from lower-mass mergers in the local Universe.

Appeared in:A&A, 434, 819

Velocity field and star formation in the Horsehead nebula

P. Hily-Blant$(^{1,2})$, D. Teyssier$(^{3,4})$, S. Philipp$(^{5})$ and R. Güsten$(^{5})$
$(^{1})$LRA-LERMA, École normale supérieure et Observatoire de Paris, 24 rue Lhomond, 75231 Paris cedex 05, France, $(^{2})$Institut de Radio Astronomie Millimétrique, 300 Rue de la Piscine, F-38406 Saint Martin d'Hères, France, $(^{3})$Space Research Organization Netherlands, P.O.Box 800, 9700 AV Groningen, The Netherlands, $(^{4})$Departamento de Astrofisica Molecular e Infrarroja, Instituto de Estructura de la Materia, CSIC, Serrano 121, 28006, Madrid, Spain $(^{5})$Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

Using large scale maps in ${\rm C^{18}O(2-1)}$ and in the continuum at 1.2mm obtained at the IRAM-30m antenna with the Heterodyne Receiver Array (HERA) and MAMBO2, we investigated the morphology and the velocity field probed in the inner layers of the Horsehead nebula. The data reveal a non-self-gravitating ( $m/m_{\rm vir}\approx 0.3$) filament of dust and gas (the ``neck'', $\emptyset = 0.15-0.30\,\rm pc$) connecting the Horsehead western ridge, a Photon-Dominated Region illuminated by $\sigma$Ori, to its parental cloud L1630. Several dense cores are embedded in the ridge and the neck. One of these cores appears particularly peaked in the 1.2mm continuum map and corresponds to a feature seen in absorption on ISO maps around 7$\mu$m. Its $\rm C^{18}O$ emission drops at the continuum peak, suggestive of molecular depletion onto cold grains. The channel maps of the Horsehead exhibit an overall north-east velocity gradient whose orientation swivels east-west, showing a somewhat more complex structure than was recently reported by Pound 2003 et al. using BIMA CO$(1-0)$ mapping. In both the neck and the western ridge, the material is rotating around an axis extending from the PDR to L1630 (angular velocity $=1.5-4.0\,\rm km\cdot s^{-1}$). Moreover, velocity gradients along the filament appear to change sign regularly ( $3\,\rm km\cdot s{-1} \cdot pc{-1}$, period=0.30pc) at the locations of embedded integrated intensity peaks. The nodes of this oscillation are at the same velocity. Similar transverse cuts across the filament show a sharp variation of the angular velocity in the area of the main dense core. The data also suggest that differential rotation is occurring in parts of the filament. We present a new scenario for the formation and evolution of the nebula and discuss dense core formation inside the filament.

Accepted by A&A

Photon dominated regions in the spiral arms of M83 and M51

C.Kramer$(^{1})$, B.Mookerjea$(^{1})$, E.Bayet$(^{2})$, S.Garcia-Burillo$(^{3})$, M.Gerin$(^{2})$, F.P. Israel$(^{4})$, J.Stutzki$(^{1})$, and J.G.A. Wouterloot$(^{5})$
$(^{1})$KOSMA, I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany, $(^{2})$Radioastronomie Millimetrique: UMR 8540 du CNRS, Laboratoire de Physique de l'ENS, 24 Rue Lhomond, 75231 Paris cedex 05, France, $(^{3})$Centro Astronomico de Yebes, IGN, E-19080 Guadalajara, Spain, $(^{4})$Sterrewacht Leiden, P.O. Box 9513, 2300 RA Leiden, The Netherlands, $(^{5})$Joint Astronomy Centre, 660 N. A'ohoku Place, Hilo, HI, USA

We present [CI] $^3$P$_1$-$^3$P$_0$ spectra at four spiral arm positions and the nuclei of the nearby galaxies M83 and M51 obtained at the JCMT. The spiral arm positions lie at galacto-centric distances of between 2kpc and 6kpc. This data is complemented with maps of CO 1-0, 2-1, and 3-2, and ISO/LWS far-infrared data of [CII] (158$\mu$m), [OI] (63$\mu$m), and [NII] (122$\mu$m) allowing for the investigation of a complete set of all major gas cooling lines. From the intensity of the [NII] line, we estimate that between 15% and $30$% of the observed [CII] emission originate from the dense ionized phase of the ISM. The analysis indicates that emission from the diffuse ionized medium is negligible. In combination with the FIR dust continuum, we find gas heating efficiencies below $\sim 0.21\%$ in the nuclei, and between 0.25 and 0.36% at the outer positions. Comparison with models of photon-dominated regions (PDRs) of Kaufman et al. (1999) with the standard ratios [OI](63)/[CII]$_{\rm PDR}$ and (OI(63)$+$[CII]$_{\rm PDR}$) vs. TIR, the total infrared intensity, yields two solutions. The physically most plausible solution exhibits slightly lower densities and higher FUV fields than found when using a full set of line ratios, [CII]$_{\rm PDR}$/[CI](1-0), [CI](1-0)/CO(3-2), CO(3-2)/CO(1-0), [CII]/CO(3- 2), and, [OI](63)/[CII]$_{\rm PDR}$. The best fits to the latter ratios yield densities of $10^4$cm$^{-3}$ and FUV fields of $\sim\,G_0=$20-30 times the average interstellar field without much variation. At the outer positions, the observed total infrared intensities are in perfect agreement with the derived best fitting FUV intensities. The ratio of the two intensities lies at 4-5 at the nuclei, indicating the presence of other mechanisms heating the dust. [CI] area filling factors lie below 2% at all positions, consistent with low volume filling factors of the emitting gas. The fit of the model to the line ratios improves significantly if we assume that [CI] stems from a larger region than CO 2-1. Improved modelling would need to address the filling factors of the various submm and FIR tracers, taking into consideration the presence of density gradients of the emitting gas by including cloud mass and size distributions within the beam.

Accepted by A&A

GRB 050509b: the elusive optical/nIR/mm afterglow of a short-duration GRB

A. J. Castro-Tirado$(^{1})$, A. de Ugarte Postigo$(^{1})$, J. Gorosabel$(^{1})$, T. Fathkullin$(^{2})$, V. Sokolov$(^{2})$, M. Bremer$(^{3})$, I. Márquez$(^{1})$, A. J. Marín$(^{1})$, S. Guziy$(^{1,4})$, M. Jelínek$(^{1})$, P. Kubánek$(^{5})$, R. Hudec$(^{5})$, S. Vitek$(^{6})$, T. J. Mateo Sanguino$(^{7})$, A. Eigenbrod$(^{8})$, M. D. Pérez-Ramírez$(^{9})$, A. Sota$(^{1})$, J. Masegosa$(^{1})$, F. Prada$(^{1})$, and M. Moles$(^{1})$
$(^{1})$Instituto de Astrofísica de Andalucía (IAA-CSIC), P.O. Box 3.004, E-18.080 Granada, Spain, $(^{2})$Special Astrophysical Observatory, Russian Academy of Sciences, Karanchai-Cherkessia, Nizniy-Arkhyz, 357147, Russia, $(^{3})$Institute de Radioastronomie Milimétrique (IRAM), 300 rue de la Piscine, 38406 Saint Martin d'Hères, France, $(^{4})$Nikolaev State University, Nikolskaya 24, 54.030 Nikolaev, Ukraine, $(^{5})$Astronomical Institute, Academy of Sciences of the Czech Republic, 25165 Ondrejov, Czech Republic, $(^{6})$Fakulta electrotechnická, Czech Technical University, 121 35 Praha, Czech Republic, $(^{7})$Dept. de Ing. Electrónica, Sistemas Informáticos y Automática, Univ. de Huelva, 21.819 Palos de la Frontera (Huelva), Spain, $(^{8})$Ecole Polytechnique Fédérale de Lausanne, Lab. d'Astrophysique, Observatoire, CH-1290 Chavannes-des-Bois, Switzerland, $(^{9})$Departamento de Física (EPS), Univ. de Jaén, Campus Las Lagunillas, E-23,071 Jaén, Spain

We present multiwavelength (optical/near infrared/millimetre) observations of a short duration gamma-ray burst detected by Swift (GRB 050509b) collected between 0 seconds and $\approx 18.8$ days after the event. No optical, near infrared or millimetre emission has been detected in spite of the well localised X-ray afterglow, confirming the elusiveness of the short duration events. We also discuss the possibility of the burst being located in a cluster of galaxies at z = 0.225 or beyond. In the former case, the spectral energy distribution of the neighbouring, potential host galaxy, favours a system harbouring an evolved dominant stellar population (age $\approx 360$ Myr), unlike most long duration GRB host galaxies observed so far, i.e. thus giving support to a compact binary merger origin. Any underlying supernova that could be associated with this particular event should have been at least 3 magnitudes fainter than the type Ib/c SN 1998bw and 2.3 magnitudes fainter than a typical type Ia SN.

Accepted for publication in A&A letters

A New Intermediate-Mass Protostar in the Cepheus A HW2 Region

Jesús Martín-Pintado$(^{1})$, Izaskun Jiménez-Serra$(^{1})$, Artur Rodríguez-Franco$(^{1})$, Sergio Martín$(^{2})$, Clemens Thum$(^{3})$
$(^{1})$Departamento de Astrofísica Molecular e Infrarroja, Instituto de Estructura de la Materia, CSIC, Calle Serrano 121, E-28006 Madrid, Spain, $(^{2})$Instituto de Radioastronomía Milimétrica, Local 20, Avenida Divina Pastora 7, E-18012 Granada, Spain, $(^{3})$Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, F-38406 Saint Martin d'Hères, France

We present the discovery of the first molecular hot core associated with an intermediate-mass protostar in the Cep A HW2 region. The hot condensation was detected from single-dish and interferometric observations of several high-excitation rotational lines (from 100 to 880 K above the ground state) of SO$_2$ in the ground vibrational state and of HC$_3$N in the vibrationally excited states $v_7=1$ and $v_7=2$. The kinetic temperature derived from both molecules is $\approx 160$ K. The high angular resolution observations ( $1.25\hbox{$^{\prime\prime}$}\times 0.99 \hbox{$^{\prime\prime}$}$) of the SO$_2$ $J=28_{7,21}\rightarrow 29_{6,24}$ line (488 K above the ground state) show that the hot gas is concentrated in a compact condensation with a size of $\approx 0.6\hbox{$^{\prime\prime}$}$ ($\approx 430$ AU), located $0.4\hbox{$^{\prime\prime}$}$ (300 AU) east from the radio jet HW2. The total SO$_2$ column density in the hot condensation is $\approx 10^{18}$ cm$^{-2}$, with an H$_2$ column density ranging from $\approx 10^{23}$ to $6\times 10^{24}$ cm$^{-2}$. The H$_2$ density and the SO$_2$ fractional abundance must be larger than $10^7$ cm$^{-3}$ and $2\times
10^{-7}$, respectively. The most likely alternatives for the nature of the hot and very dense condensation are discussed. From the large column densities of hot gas, the detection of the HC$_3$N vibrationally excited lines, and the large SO$_2$ abundance, we favor the interpretation of a hot core heated by an intermediate-mass protostar of $10^3$ L$_\odot$. This indicates that the Cep A HW2 region contains a cluster of very young stars.

Appeared in: ApJ 628, L61

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