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Scientific Results in Press

Interferometric imaging of carbon monoxide in comet C/1995 O1 (Hale-Bopp): evidence for a strong rotating jet

D. Bockelée-Morvan$(^{1})$, F. Henry$(^{1})$, N. Biver$(^{1})$, J. Boissier$(^{2})$, P. Colom$(^{1})$, J. Crovisier$(^{1})$, D. Despois$(^{3})$, R. Moreno$(^{1})$, and J. Wink$(^{2})$
$(^{1})$ Observatoire de Paris, F-92195 Meudon, France, $(^{2})$ IRAM, F-38406 Saint Martin d'Hères, France, $(^{3})$ Observatoire de Bordeaux, BP 89, F-33270 Floirac, France

Context. Observations of the CO $J(1-0)$ 115 GHz and $J(2-1)$ 230 GHz lines in comet C/1995 O1 (Hale-Bopp) were performed with the IRAM Plateau de Bure interferometer on 11 March, 1997. The observations were conducted in both single-dish (ON-OFF) and interferometric modes with 0.13 km s$^{-1}$ spectral resolution. Images of CO emission with 1.7 to $3\hbox{$^{\prime\prime}$}$ angular resolution were obtained.
Aims. The ON-OFF and interferometric spectra show a velocity shift with sinusoidal time variations related to the Hale-Bopp nucleus rotation of 11.35 h. The peak position of the CO images moves perpendicularly to the spin axis direction in the plane of the sky. This suggests the presence of a CO jet, which is active night and day at about the same extent, and is spiralling with nucleus rotation. The high quality of the data allows us to constrain the characteristics of this CO jet.
Methods. We have developed a 3-D model to interpret the temporal evolution of CO spectra and maps. The CO coma is represented as the combination of an isotropic distribution and a spiralling gas jet, both of nucleus origin.
Results. Spectra and visibilities (the direct output of interferometric data) analysis shows that the CO jet comprises $\sim
40$% the total CO production and is located at a latitude $\sim
20^\circ$ North on the nucleus surface. Our inability to reproduce all observational characteristics shows that the real structure of the CO coma is more complex than assumed, especially in the first thousand kilometres from the nucleus. The presence of another moving CO structure, faint but compact and possibly created by an outburst, is identified.

Accepted for publication in A&A

Millimeter HEMT amplifier measurements at cryogenic temperatures

P. Serres$(^{1})$, Y. Bortolotti$(^{1})$, G. Buttin$(^{1})$, B. Pissard$(^{1})$, G. Valente$(^{1})$, F. Mattiocco$(^{1})$, B. Lazareff$(^{1})$
$(^{1})$IRAM, 300 rue de la Piscine, 38406 St Martin d'Hères, France

IRAM is designing an 84-116 GHz dual polarisation HEMT receiver in order to get experience in using such receivers at the Pico Veleta Observatory, which can then be applied to a future project of building a 3 mm HEMT focal plane array. Preliminary measurements of HEMT MMIC amplifiers in terms of gain, noise temperature, saturation, and stability at 300 K and at 4 K will be described. Two solutions are presented for the down converting of the HEMT amplifier output: the first uses a very large (4-36 GHz) IF band and a fixed tuned LO at 80 GHz while the second is designed with a smaller IF band (4-12 GHz) and a 67-91 GHz tunable LO. In the 84-116 GHz range, receiver noise temperatures between 32 and 51 K were obtained (see Fig. 4). The measured stability ( $1s > \tau > 100s$) is below $4\times 10^{-4}$ at 101 GHz.

Presentation at the 1st Radionet Engineering Forum Workshop (Chalmers, Gothenburg, Sweden, 23-24 June 2009)

Figure 4: P. Serres et al.: Receiver noise temperature in the 84-116 GHz range, based on HEMT MMIC amplifiers.

Intermittency of interstellar turbulence: Parsec-scale coherent structure of intense, velocity shear

P. Hily-Blant$(^{1})$ and E. Falgarone$(^{2})$
$(^{1})$LAOG, CNRS & Université Joseph Fourier, UMR 5571, 414 Rue de la Piscine BP 53 F-38041 Grenoble Cedex 09 $(^{2})$LRA/LERMA, CNRS & École normale supérieure & Observatoire de Paris, UMR 8112, 24 rue Lhomond, 75231 Paris Cedex 05, France

Aims. Benefitting from the duality of turbulence (random versus coherent motions), we search for coherent structures in the turbulent velocity field of molecular clouds, anticipating their importance in cloud evolution.
Methods. We analyse a large map ( $40\hbox{$^{\prime}$}$ by $20\hbox{$^{\prime}$}$) obtained with the HERA multibeam receiver (IRAM-30m telescope) in a high latitude cloud of the Polaris Flare at unprecedented spatial (11) and spectral (0.05 km s$^{-1}$) resolution for the $^{12}$CO$(2-1)$ line.
Results. We find that two parsec-scale components of velocities differing by $\sim 2$ kms$^{-1}$, share a narrow interface ($<0.15$ pc) that appears to be an elongated structure of intense velocity-shear, $\sim 15$ to 30 kms$^{-1}$ pc$^{-1}$. The locus of the extrema of line-centroidvelocity increments (E-CVI) in that field follows this intense-shear structure as well as that of the $^{12}$CO$(2-1)$ high-velocity line wings. The tiny spatial overlap in projection of the two parsec-scale components implies that they are sheets of CO emission and that discontinuities in the gas properties (CO enrichment and/or increase in gas density) occur at the position of the intense velocity shear.
Conclusions. These results identify spatial and kinematic coherence on scales of between 0.03 pc and 1 pc. They confirm that the departure from Gaussianity of the probability density functions of E-CVIs is a powerful statistical tracer of the intermittency of turbulence. They provide support for a link between large-scale turbulence, its intermittent dissipation rate and low-mass dense core formation.

Appeared in: A&A 500, L29

The modulation of SiO maser polarization by Jovian planets

Wiesemeyer, H.$(^{1})$
$(^{1})$IRAM, 300 rue de la Piscine, Domaine Universitaire, 38406 Saint Martin d'Hères, France

Aims. Searching for planets in the atmosphere of AGB stars is difficult, due to confusion with the stellar wind and pulsations. The aim here is to provide a complementary strategy for planet searches in such a dense environment.
Methods. The polarization properties of SiO masers, especially their circular polarization, are, under certain conditions, good tracers of rapid magnetospheric events. A Jovian planet with a magnetosphere whose dipole axis is misaligned with its rotation axis naturally provides such conditions. Here I present several models showing that the polarization will be periodically modulated.
Results. The linear and circular polarization of an SiO maser in a planetary magnetosphere is modulated by the precessing dipole component of the latter. The effect is measurable in saturated masers, while unsaturated masers only exhibit weak changes, because of dilution effects, and because the circular polarization there stems from the Zeeman effect making it as weak as for thermal radiation. The situation would change if anisotropic pump- and loss-rates were included, which would increase the fractional linear and, via magnetorotation, the circular polarization of the modulation.
Conclusions. Single-dish monitoring with a dense enough time sampling and a carefully calibrated polarimeter, in combination with VLBI observations, are suited to detecting and locating a periodic modulation of the circular maser polarization due to a precessing Jovian magnetosphere. The phenomenon will be rare, because a favorable arrangement of maser and magnetosphere is needed. Otherwise the polarization may be below the detection threshold, especially if the maser is unsaturated. Though exhibiting a qualitatively similar modulation, linear polarization is likely to suffer more from confusion due to dilution of the magnetosphere within the maser cross section, even in VLBI observations.

Appeared in: A&A 501, 647

Precessing planetary magnetospheres in SiO stars? First detection of quasi-periodic polarization fluctuations in RLeonis and VCamelopardalis

H. Wiesemeyer$(^{1})$, C. Thum$(^{1})$, A. Baudry$(^{2,3})$, and F. Herpin$(^{2,3})$
$(^{1})$IRAM, 300 rue de la Piscine, 38406 Saint Martin d'Hères, France, $(^{2})$Université de Bordeaux, Laboratoire d'Astrophysique de Bordeaux, 33000 Bordeaux, France, $(^{3})$CNRS/INSU, UMR 5804, BP 89, 33270 Floirac, France

Context. The origin of magnetism around asymptotic giant branch (AGB) stars remains an uncertainty. These stars may drive an important dynamo, but if the magnetic energy dissipates entirely into X-rays, the observed X-ray luminosities are too low to maintain a strong, dynamically important global field. Other explanations of the circular polarization in SiO masers in AGB atmospheres may thus be required.
Aims. The interaction of the AGB wind with both previously ejected matter and planets is expected to produce complex magnetohydrodynamic phenomena on a short timescale, such that strong magnetic fields can be maintained locally. Here, we provide observational evidence of the corresponding magnetic fluctuations.
Methods. We use the circular polarization of the $v = 1$, $J = 2-1$ SiO masers as a tracer of magnetic activity. A correlation polarimeter allows us to record simultaneously all Stokes parameters. An SiO maser survey of 77 AGB stars was performed from which eight sources of the strongest circular polarization were selected for further monitoring. Results. In two AGB stars, V Cam and R Leo, we find evidence of pseudo-periodic fluctuations in the fractional circular polarization (Fig. 5) on a timescale of a few hours, from which we infer magnetic fluctuations of $\sim 1$ G. The phenomenon is rare and, if detected in an SiO star, restricted to a narrow range of velocities. It seems to be associated with planetary wake flows suggested by VLBI maps.
Conclusions. While scenarios involving magnetic activity in the extended stellar atmosphere have problems explaining all observed features, precessing Jovian magnetospheres predict all of them without difficulty. For the case of R Leo, we constrain the orbit of the planet (estimated period 5.2 years), derive a stellar mass estimate of $0.7~\mbox{M$_\odot$}$ from it, and discuss the impact of planetary magnetism on the survival of planets. Smooth velocity variations in the fluctuating circular polarization feature are predicted as the planet moves along its orbit.

Appeared in A&A 498, 801

Figure 5: Wiesemeyer et al.: Time series of polarization measurements. From top to bottom: Stokes $I$ in Jansky, the fractional linear and circular polarizations, and the polarization angle (i.e. the position angle of linear polarization, in degree E from N), for the SiO maser spots from V Cam at 7.5 km s$^{-1}$ (LSR velocity, left) and R Leo at 4.4 km s$^{-1}$ (LSR velocity, right). The continuous heavy lines in the plots for circular polarization show model results (Wiesemeyer 2009, this newsletter) for a saturated maser in the equatorial plane, at 3 Jupiter radii from the planet, with a magnetic dipole field of eight times Jupiter's and misaligned with the rotation axis (in the sky plane) by 10 degrees, and rotation periods of 10.8 hours (V Cam, left) and 6.3 hours (R Leo, right).

The Beam Pattern of Reflector Antennas with Buckled Panels

A. Greve$(^{1})$, D. Morris$(^{1})$, J. Peñalver$(^{2})$, C. Thum$(^{1})$, and M. Bremer$(^{1})$
$(^{1})$IRAM, 300 rue de la Piscine, 38406 St. Martin d'Hères, France, $(^{2})$IRAM, Av. Divina Pastora 7, Núcleo Central, 18012 Granada, Spain

On high precision reflector telescopes the transient thermal panel buckling canhave an effective rms-value comparable to the errors in the adjustment of the reflector panels. Under this condition, high signal-to-noise radio holography of high spatial resolution can reveal the characteristic signature of panel buckling in the beam pattern and can map the surface deformation of the buckling, while lower signal-to-noise Moon limb scans may see the buckling onlyunder favourable conditions. Detailed diffraction calculations, and some observations, indicate (1) that the panel buckling produces diffraction rings and/or diffraction spokes, (2) that panel buckling in azimuthal direction may have a smaller degrading effect than panel buckling in radial direction becausefor azimuthal buckling the energy is spread more uniformly over a large solid angle, and (3) that the coverage of the reflector aperture with buckled panels determines the multiplicity of the diffraction rings and/or diffraction spokes.

Accepted for publication in IEEE Trans. on Antennas and Propagation

CO line emission in the halo of a radio galaxy at $z = 2.6$

N.P.H. Nesvadba, R. Neri$(^{3})$, C. De Breuck$(^{4})$, M.D. Lehnert$(^{2})$, D. Downes$(^{3})$, F. Walter$(^{5})$, A. Omont$(^{6})$, F. Boulanger$(^{1})$ and N. Seymour$(^{7})$
$(^{1})$Institut d'Astrophysique Spatiale, Université Paris Sud 11, Orsay, France, $(^{2})$GEPI, Observatoire de Paris, CNRS, Université Denis Diderot, Meudon, France, $(^{3})$Institut de Radio Astronomie Millimétrique (IRAM), St. Martin d'Hères, France, $(^{4})$European Southern Observatory, Karl-Schwarzschild Straße, Garching bei München, Germany, $(^{5})$Max Planck Institut für Astronomie, Heidelberg, Germany, $(^{6})$Institut d'Astrophysique de Paris, CNRS, Université Pierre et Marie Curie, Paris, $(^{7})$Mullard Space Science Laboratory, UCL, Holmbury St. Mary, Dorking, Surrey RH5 6NT

We report the detection of luminous CO$(3-2)$ line emission in the halo of the $z = 2.6$ radio galaxy (HzRG) TXS0828+193, which has no detected counterpart at optical to mid-infrared wavelengths implying a stellar mass $\lower.5ex\hbox{$\; \buildrel < \over \sim \;$}$ few $\times10^9\mbox{M$_\odot$}$ and relatively low star formation rates. With the IRAM Plateau de Bure Interferometer (PdBI), we find two CO emission-line components at the same position at $\sim
80$ kpc distance from the HzRG along the axis of the radio jet, with different blueshifts of few 100 km s$^{-1}$ relative to the HzRG and a total luminosity of $\sim 2 \times 10^{10}$ K km s$^{-1}$ pc$^2$ detected at a total significance of $\sim 8\sigma$. HzRGs have significant galaxy overdensities and extended haloes of metal-enriched gas often with embedded clouds or filaments of denser material, and likely trace very massive dark matter haloes. The CO emission may be associated with a gas-rich, low-mass satellite galaxy with very little ongoing star formation, in contrast to all previous CO detections of galaxies at similar redshifts. Alternatively, the CO may be related to a gas cloud or filament and perhaps jet-induced gas cooling in the outer halo, somewhat in analogy with extended CO emission found in low-redshift galaxy clusters.

Appeared in: MNRAS 395, L16

Present and Future Cometary Science with the IRAM Plateau de Bure Interferometer

Boissier J.$(^{1})$, Bockelée-Morvan D.$(^{2})$, Biver N.$(^{2})$, Crovisier J.$(^{2})$, Moreno R.$(^{2})$, Lellouch E.$(^{2})$ and Neri R.$(^{1})$
$(^{1})$IRAM Grenoble, France, $(^{2})$LESIA Paris, France

Interferometric observations are essential to probe the molecular emission in the inner cometary atmospheres and study the outgassing from the nucleus. Mapping the continuum emission can provide information about the dust and/or nucleus properties. We present here a summary of the observations of the dust and gas coma of comet 17P/Holmes and nuclear observations of 8P/Tuttle, both carried out with the IRAM interferometer at Plateau de Bure (PdBI) in 2007 - 2008. The observations of these two comets demonstrate the ability of the PdBI in terms of cometary science. In the near future, several improvements will be made (new receivers at 0.8 mm, a new wide-band correlator) allowing more frequent and more detailed studies of comets. On the long term, NOEMA, an expansion project, may add up to six antennas to the Plateau de Bure Interferometer, and increase the baseline lengths. Such an instrument would offer a complement to ALMA to track comets of the northern hemisphere with about half the sensitivity of ALMA for continuum studies.

Appeared in: Earth, Moon, and Planets 105, 89

The Chemical Diversity of Comets: Synergies Between Space Exploration and Ground-based Radio Observations

Crovisier J.$(^{1})$, Biver N.$(^{1})$, Bockelée-Morvan D.$(^{1})$, Boissier J.$(^{2})$, Colom P.$(^{1})$ and Lis D.C.$(^{3})$
$(^{1})$LESIA, Observatoire de Paris, France, $(^{2})$IRAM Grenoble, France, $(^{3})$California Institute of Technology, USA

A fundamental question in cometary science is whether the different dynamical classes of comets have different chemical compositions, which would reflect different initial conditions. From the ground or Earth orbit, radio and infrared spectroscopic observations of a now significant sample of comets indeed reveal deep differences in the relative abundances of cometary ices. However, no obvious correlation with dynamical classes is found. Further results come, or are expected, from space exploration. Such investigations, by nature limited to a small number of objects, are unfortunately focussed on short-period comets (mainly Jupiter-family). But these in situ studies provide ``ground truth'' for remote sensing. We discuss the chemical differences in comets from our database of spectroscopic radio observations, which has been recently enriched by several Jupiter-family and Halley-type comets.

Appeared in: Earth, Moon, and Planets 105, 267

The physical conditions in Gomez's Hamburger (IRAS 18059-3211), a pre-MS rotating disk

V. Bujarrabal$(^{1})$, K. Young$(^{2})$, and A. Castro-Carrizo$(^{3})$
$(^{1})$Observatorio Astronómico Nacional (OAN-IGN), Apartado 112, 28803 Alcalá de Henares, Spain, $(^{2})$Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA, $(^{3})$IRAM, 300 rue de la Piscine, 38406 St. Martin d'Hères, France

Aims. We aim to study the structure, dynamics, and physical conditions of Gomez's Hamburger (IRAS 18059-3211; GoHam), and in particular confirm that GoHam mainly consists of a flaring disk in Keplerian rotation around a young, probably pre-MS star.
Methods. We present high resolution SMA maps of $^{12}$CO $J = 2-1$, $^{13}$CO $J = 2-1$, $^{12}$CO $J = 3-2$, and C$^{17}$O $J = 3-2$, as well as data on $^{12}$CO $J = 6-5$ and the continuum flux at these wavelengths. Spatial resolutions as high as 1 are attained. Except for the C$^{17}$O data, the dynamical ranges are larger than 10. The maps are compared with a numerical model, which simulates the emission of a rotating disk with the expected general properties of such objects, and a very satisfactory fitting of our maps is obtained. The meaning and reliability of our results are thoroughly discussed.
Results. Our observations allow measurement of the main properties of GoHam on scales of between $\sim 1\hbox{$^{\prime\prime}$}$ ( $\sim 5
\times 10^{15}$ cm, for the assumed distance, 300 pc) and the total extent of the nebula, $14\hbox{$^{\prime\prime}$}$. We are able to measure the global structure of the gas-rich disk, which is found to be flaring, and its dynamics, which is clearly dominated by Keplerian rotation, with a small degree of turbulence. The combination of different lines, in particular of different opacities, allows us to estimate reasonably the distributions of gas temperature and density. We clearly find a significant and sharp increase in temperature at large distances from the equator, accompanied by a decrease in density of the same order. Finally, we identify a condensation in the southern part of the disk that has no counterparts in the northern nebula. This condensation is quite extended (about $5 \times 10^{15}$ cm), contains a significant amount of mass (roughly, $\sim 6 \times 10^{-3} \mbox{M$_\odot$}$), and seems to be associated with a detectable distortion of the global rotation kinematics. We discuss several possible interpretations of that feature.

Appeared in: A&A 500, 1077

Rotating molecular outflows: the young T Tauri star in CB 26

Launhardt R.$(^{1})$, Pavlyuchenkov Ya., Gueth F.$(^{3})$, Chen X.$(^{1})$, Dutrey A.$(^{4,5})$, Guilloteau S.$(^{4,5})$, Henning Th.$(^{1})$, Piétu V.$(^{3})$, Schreyer K.$(^{6})$, Semenov D.$(^{1})$
$(^{1})$Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany, $(^{2})$Institute of Astronomy, Russian Academy of Sciences, Pyatnitskaya 48, Moscow 109117, Russia, $(^{3})$IRAM, 300 rue de la Piscine, 38406 Saint-Martin-d'Hères, France, $(^{4})$Université Bordeaux 1, Laboratoire d'Astrophysique de Bordeaux (LAB), France, $(^{5})$CNRS/INSU - UMR5804, BP 89, 33270 Floirac, France, $(^{6})$Astrophysikalisches Institut und Universitäts-Sternwarte, Schillergäßchen 2-3, 07745 Jena, Germany

Context: The disk-outflow connection is thought to play a key role in extracting excess angular momentum from a forming proto-star. Although jet rotation has been observed in a few objects, no rotation of molecular outflows has been unambiguously reported so far.
Aims: We report new millimeter-interferometric observations of the edge-on T Tauri star - disk system in the isolated Bok globule CB 26. The aim of these observations was to study the disk-outflow relation in this 1 Myr old low-mass young stellar object.
Methods: The IRAM PdBI array was used to observe $^{12}$CO$(2-1)$ at 1.3 mm in two configurations, resulting in spectral line maps with $1{\farcs}5$ resolution. We use an empirical parameterized steady-state outflow model combined with 2-D line radiative transfer calculations and $\chi^2$-minimization in parameter space to derive a best-fit model and constrain parameters of the outflow.
Results: The data reveal a previously undiscovered collimated bipolar molecular outflow of total length $\approx 2000$ AU, escaping perpendicular to the plane of the disk. We find peculiar kinematic signatures that suggest that the outflow is rotating with the same orientation as the disk. However, we could not ultimately exclude jet precession or two misaligned flows as possible origins of the observed peculiar velocity field. There is indirect indication that the embedded driving source is a binary system, which, together with the youth of the source, could provide a clue to the observed kinematic features of the outflow.
Conclusions: CB 26 is so far the most promising source in which to study the rotation of a molecular outflow. Assuming that the outflow is rotating, we compute and compare masses, mass flux, angular momenta, and angular momentum flux of the disk and outflow and derive disk dispersal timescales of $0.5 \ldots 1$ Myr, comparable to the age of the system.
Based on observations carried out with the IRAM Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). Also based on observations collected at the Centro Astronómico Hispano Alemán (CAHA) at Calar Alto, operated jointly by the Max-Planck Institut für Astronomie and the Instituto de Astrofísica de Andalucía (CSIC). A complete set of channel maps is available in electronic form at the CDS via anonymous ftp to ( or via

Appeared in: A&A 494, 147

A CO Emission Line from the Optical and Near-IR Undetected Submillimeter Galaxy GN10

E. Daddi$(^{1})$, H. Dannerbauer$(^{2})$, M. Krips$(^{3})$, F. Walter$(^{2})$, M. Dickinson$(^{4})$, D. Elbaz$(^{1})$, and G. E. Morrison$(^{5,6})$
$(^{1})$CEA, Laboratoire AIM, Irfu/SAp, F-91191 Gif-sur-Yvette, France, $(^{2})$MPIA, Königstuhl 17, D-69117 Heidelberg, Germany, $(^{3})$IRAM, St. Martin d'Hères, France, $(^{4})$NOAO, 950 N. Cherry Avenue, Tucson, AZ 85719, USA. $(^{5})$IfA, University of Hawaii, Honolulu, HI 96822, USA, $(^{6})$CFHT, Kamuela, HI 96743, USA

We report the detection of a CO emission line from the submillimeter galaxy (SMG) GN10 in the GOODS-N field. GN10 lacks any counterpart in extremely deep optical and near-IR imaging obtained with the Hubble Space Telescope and ground-based facilities. This is a prototypical case of a source that is extremely obscured by dust, for which it is practically impossible to derive a spectroscopic redshift in the optical/near-IR. Under the hypothesis that GN10 is part of a proto-cluster structure previously identified at $z \sim 4.05$ in the same field, we searched for CO$(4-3)$ at 91.4 GHz with the IRAM Plateau de Bure Interferometer, and successfully detected a line. We find that the most likely redshift identification is $z = 4.0424 \pm
0.0013$, based on: (1) the very low chance that the CO line is actually serendipitous from a different redshift; (2) a radio-IR photometric redshift analysis; (3) the identical radio-IR spectral energy distribution, within a scaling factor of 2 other SMGs at the same redshift. The faintness at optical/near-IR wavelengths requires an attenuation of $A_V \sim 5-7.5$ mag. This result supports the case that a substantial population of very high-$z$ SMGs exists that had been missed by previous spectroscopic surveys. This is the first time that a CO emission line has been detected for an SMG that is invisible in the optical and near-IR. Our work demonstrates the power of existing and planned facilities for completing the census of star formation and stellar mass in the distant universe by measuring redshifts of the most obscured galaxies through millimeter spectroscopy.

Appeared in: APJ Letters 695, L176

Hyperfine structure in the $J = 1- 0$ transitions of DCO$^+$, DNC, and HN$^{13}$C: astronomical observations and quantum-chemical calculations

Floris F. S. van der Tak$(^{1,3})$, Holger S. P. Müller$(^{2,3})$, Michael E. Harding$(^{4,5})$, and Jürgen Gauss$(^{4})$
$(^{1})$SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands, $(^{2})$I. Physikalisches Institut der Universität, Zülpicher Straße 77, 50937 Köln, Germany, $(^{3})$Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany, $(^{4})$Institut für Physikalische Chemie, Universität Mainz, 55099 Mainz, Germany, $(^{5})$Department of Chemistry and Biochemistry, University of Texas, Austin, TX 78712, U.S.A.

Context. Knowledge of the hyperfine structure of molecular lines is useful for estimating reliable column densities from observed emission, and essential for the derivation of kinematic information from line profiles.
Aims. Deuterium bearing molecules are especially useful in this regard, because they are good probes of the physical and chemical structure of molecular cloud cores on the verge of star formation. However, the necessary spectroscopic data are often missing, especially for molecules which are too unstable for laboratory study.
Methods. We have observed the ground-state $(J = 1-0)$ rotational transitions of DCO$^+$, HN$^{13}$C and DNC with the IRAM 30m telescope toward the dark cloud LDN 1512 which has exceptionally narrow lines permitting hyperfine splitting to be resolved in part. The measured splittings of $50-300$ kHz are used to derive nuclear quadrupole and spin-rotation parameters for these species. The measurements are supplemented by high-level quantum-chemical calculations using coupled-cluster techniques and large atomic-orbital basis sets.
Figure 6: Van der Tak et al.: Spectrum of the DNC $J = 1- 0$ line, observed toward the dark cloud LDN 1512 with the IRAM 30m telescope. The extremely small line width of this cloud allow the resolution of six hyperfine components and an accurate determination of the molecular hyperfine parameters.
Results. We find  kHz and $C_I = -1.12
(43)$ kHz for DCO$^+$, $eQq = 272.5 (51)$ kHz for HN$^{13}$C, and $eQq(D) =
265.9(83)$ kHz and $eQq(N) = 288.2 (71)$ kHz for DNC see Fig. 6. The numbers for DNC are consistent with previous laboratory data, while our constants for DCO$^+$ are somewhat smaller than previous results based on astronomical data. For both DCO$^+$ and DNC, our results are more accurate than previous determinations. Our results are in good agreement with the corresponding best theoretical estimates, which amount to $eQq = 156.0$ kHz and $C_I = -0.69$ kHz for DCO$^+$, $eQq = 279.5$ kHz for HN$^{13}$C, and $eQq(D) = 257.6$ kHz and $eQq(N) = 309.6$ kHz for DNC. We also derive updated rotational constants for HN$^{13}$C: $B=43545.6000(47)$ MHz and $D=93.7(20)$ kHz.
Conclusions. The hyperfine splittings of the DCO$^+$, DNC and HN $^{13}C J = 1-0$ lines range over $0.47-1.28$ km s$^{-1}$, which is comparable to typical line widths in pre-stellar cores and to systematic gas motions on $\sim 1000$ AU scales in protostellar cores. We present tabular information to allow inclusion of the hyperfine splitting in astronomical data interpretation. The large differences in the $^{14}$N quadrupole parameters of DNC and HN$^{13}$C have been traced to differences in the vibrational corrections caused by significant non-rigidity of these molecules, particularly along the bending coordinate.

Accepted for publication in A&A

New CO detections of lensed submillimetre galaxies in A2218: probing molecular gas in the LIRG regime at high redshift

K.K. Knudsen$(^{1})$, R. Neri$(^{2})$, J.-P. Kneib$(^{3})$, and P.P. van der Werf$(^{4})$
$(^{1})$Argelander-Institut für Astronomie, Auf dem Hügel 71, 53123 Bonn, Germany, $(^{2})$Institut de Radio Astronomie Millimétrique (IRAM), 300 rue de la Piscine, Domaine Universitaire de Grenoble, St. Martin d'Hères 38406, France $(^{3})$Laboratoire d'Astrophysique de Marseille, OAMP, Université Aix-Marseille & CNRS, 38 rue F. Joliot-Curie, 13388 Marseille Cedex 13, France $(^{4})$Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

Context. Submillimetre galaxies (SMGs) are distant, dusty galaxies undergoing star formation at prodigious rates. Recently there has been major progress in understanding the nature of the bright SMGs (i.e. $S_{850 \mu{\rm m}} > 5$ mJy). The samples for the fainter SMGs are small and are currently in a phase of being built up through identification studies.
Aims. We study the molecular gas content in the two SMGs, SMMJ163555 and SMMJ163541, at redshifts $z = 1.034$ and $z
= 3.187$ with unlensed submillimetre fluxes of 0.4 mJy and 6.0 mJy. Both SMGs are gravitationally lensed by the foreground cluster Abell 2218.
Methods. We used the IRAM Plateau de Bure Interferometer to obtain observations at 3 mm of the lines CO$(2-1)$ for SMMJ163555 and CO$(3-2)$ for SMMJ163541. Additionally, we obtained CO$(4-3)$ observations for the candidate $z = 4.048$ SMMJ163556 with an unlensed submillimetre flux of 2.7 mJy.
Results. The CO$(2-1)$ line was detected for SMMJ163555 at redshift 1.0313 with an integrated line intensity of $1.2 \pm 0.2$ Jy km s$^{-1}$ and a line width of $410 \pm 120$ km s$^{-1}$. From this a gas mass of $1.6 \times 10^9 \; \mbox{M$_\odot$}$ is derived and a star formation efficiency of $440 \; \mbox{L$_\odot$}/\mbox{M$_\odot$}$ estimated. The CO$(3-2)$ line was detected for SMMJ163541 at redshift 3.1824, possibly with a second component at redshift 3.1883, with an integrated line intensity of $1.0 \pm 0.1$ Jy km s$^{-1}$ and a line width of $280 \pm
50$ km s$^{-1}$ From this a gas mass of $2.2 \times 10^{10} \;
\mbox{M$_\odot$}$ is derived and a star formation efficiency of $1000 \; \mbox{L$_\odot$}/\mbox{M$_\odot$}$ is estimated. For SMMJ163556, the CO$(4-3)$ is undetected within the redshift range $4.035-4.082$ down to a sensitivity of 0.15 Jy km s$^{-1}$.
Conclusions. Our CO-line observations confirm the optical redshifts for SMMJ163555 and SMMJ163541. The CO-line luminosity $L'_{CO}$ for both galaxies is consistent with the $L_{FIR} - L'_{CO}$ relation. SMMJ163555 has the lowest far-infrared luminosity of all SMGs with a known redshift and is one of the few high-redshift LIRGs whose properties can be estimated prior to ALMA.

Appeared in: A&A 496, 45

Circumstellar disks around Herbig Be stars

T. Alonso-Albi$(^{1})$, A. Fuente$(^{1})$, R. Bachiller$(^{1})$, R. Neri$(^{2})$, P. Planesas$(^{1,3})$, L. Testi$(^{4,6})$, O. Berné$(^{5})$, and C. Joblin$(^{5})$
$(^{1})$Observatorio Astronómico Nacional, Apdo. 112, 28803 Alcalá de Henares (Madrid), Spain, $(^{2})$Institut de Radio Astronomie Milimétrique, 300 rue de la Piscine, Domaine Universitaire de Grenoble, 38406 St. Martin d'Hères, France, $(^{3})$Atacama Large Millimeter/Submillimeter Array, Joint ALMA Office, Santiago, Chile, $(^{4})$INAF - Osservatorio Astrofisico de Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy, $(^{5})$Centre d'Étude Spatiale des Rayonnements, CNRS et Université Paul Sabatier Toulouse 3, Observatoire Midi-Pyrénées, 9 Av. du Colonel Roche, 31028 Toulouse Cedex 04, France, $(^{6})$European Southern Observatory, Karl Schwarzschild Straße 2, 85748 Garching, Germany

Aims. Our goal is to investigate the properties of the circumstellar disks around intermediate mass stars to determine their occurrence, lifetime and evolution.
Methods. We completed a search for circumstellar disks around Herbig Be stars using the NRAO Very Large Array (VLA) and the IRAM Plateau de Bure (PdB) interferometers. Thus far, we have observed 6 objects with 4 successful detections. The results towards 3 of these stars (R Mon, MWC 1080, MWC 137) were presented elsewhere. We present our new VLA and PdBI data for the three objects MWC 297, Z CMa, and LKH$\alpha$ 215. We constructed the SED from near-IR to centimeter wavelengths by adding our millimeter and centimeter data to the available data at other wavelengths, mainly Spitzer images. The entire SED was fitted using a disk+envelope model. In addition, we compiled all the disk millimeter observations in the literature and completed a statistical analysis of all the data.
Results. We show that the disk mass is usually only a small percentage (less than 10%) of the mass of the entire envelope in HBe stars. For the disks, there are large source-to-source variations. Two disks in our sample, R Mon and Z CMa, have similar sizes and masses to those found in T Tauri and Herbig Ae stars. The disks around MWC 1080 and MWC 297 are, however, smaller ($r_{out} <
100$ AU). We did not detect the disks towards MWC 137 and LkH$\alpha$ 215 at millimeter wavelengths, which limits the mass and the size of the possible circumstellar disks.
Conclusions. A comparison between our data and previous results for T Tauri and Herbig Ae stars indicates that although massive disks ( $\sim 0.1 \; \mbox{M$_\odot$}$) are found in young objects ($\sim 10^4$ yr), the masses of the disks around Herbig Be stars are usually $5-10$ times lower than those around lower mass stars. We propose that disk photoevaporation is responsible for this behavior. In Herbig Be stars, the UV radiation disperses the gas in the outer disk on a timescale of a few $10^5$ yr. Once the outer part of the disk has vanished, the entire gaseous disk is photoevaporated on a very short timescale ($\sim 10^5$ yr) and only a small, dusty disk consisting of large grains remains.

Appeared in: A&A 497, 117

Molecular gas in NUclei of GAlaxies (NUGA)
XII. The head-on collision in NGC 1961

F. Combes$(^{1})$, A. J. Baker$(^{2})$, E. Schinnerer$(^{3})$, S. García-Burillo$(^{4})$, L. K. Hunt$(^{5})$, F. Boone$(^{1})$, A. Eckart$(^{6})$, R. Neri$(^{7})$, and L. J. Tacconi$(^{8})$
$(^{1})$Observatoire de Paris, LERMA, 61 Av. de l'Observatoire, 75014 Paris, France, $(^{2})$Rutgers, The State University of NJ, 136 Frelinghuysen Road, Piscataway, NJ 08854-8019, USA, $(^{3})$MPIA, Königstuhl 17, 69917 Heidelberg, Germany, $(^{4})$Observatorio Astronómico Nacional (OAN), Observatorio de Madrid, Alfonso XII, 3, 28014 Madrid, Spain, $(^{5})$INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy, $(^{6})$Universität zu Köln, I. Physikalisches Institut, Zülpicher Straße 77, 50937 Köln, Germany, $(^{7})$IRAM, 300 rue de la Piscine, 38406 St. Martin d'Hères, France, $(^{8})$Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany

We present high-resolution maps of the CO$(1-0)$ and CO$(2-1)$ emission from the LINER 2 galaxy NGC 1961. This galaxy is unusual among late-type (Sc) disk galaxies in having a very large radial extent and inferred dynamical mass. We propose a head-on collision scenario to explain the perturbed morphology of this galaxy - both the off-centered rings and the inflated radius. This scenario is supported by the detection of a steep velocity gradient in the CO$(1-0)$ map at the position of a southwest peak in radio continuum and near-infrared emission. This peak would represent the remnant of the disrupting companion. We use numerical models to demonstrate the plausibility of the scenario. While ram pressure stripping could in principle be important for shocking the atomic gas and produce the striking head-tail morphology, the non detection of this small galaxy group in X-ray emission suggests that any hot intragroup medium has too low a density. A prediction of the collision model is the propagation of ring waves from the center to the outer parts, superposed on a probable pre-existing $m = 2$ barred spiral feature, accounting for the observed complex structure of rings and spokes. This lopsided wave accounts for the sharp boundary observed in the atomic gas on the southern side. Through dynamical friction, the collision finishes quickly in a minor merger, the best fit being for a companion with a mass ratio 1:4. We argue that NGC1961 has a strongly warped disk, which gives the false impression of a nearly face-on system; the main disk is actually more edge-on, and this error in the true inclination has led to the surprisingly high dynamical mass for a morphologically late-type galaxy. In addition, the outwardly propagating ring artificially enlarges the disk. The collision de-stabilizes the inner disk and can provide gas inflow to the active nucleus.

Appeared in: A&A 503, 73

HCO mapping of the Horsehead: tracing the illuminated dense molecular cloud surfaces

M. Gerin$(^{1})$, J. R. Goicoechea$(^{1})$, J. Pety$(^{2,1})$, and P. Hily-Blant$(^{3})$
$(^{1})$LERMA-LRA, UMR 8112, CNRS, Observatoire de Paris and École Normale Supérieure, 24 Rue Lhomond, 75231 Paris, France, $(^{2})$Institut de Radio Astronomie Millimétrique (IRAM), 300 rue de la Piscine, 38406 Saint Martin d'Hères, France, $(^{3})$Laboratoire d'Astrophysique, Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 09, France

Context. Far-UV photons (FUV) strongly affect the physical and chemical state of molecular gas in the vicinity of young massive stars.
Aims. Finding molecular tracers of the presence of FUV radiation fields in the millimeter wavelength domain is desirable because IR diagnostics (for instance PAHs) are not easily accessible along high extinction line-of-sights. Furthermore, gas phase diagnostics provide information on the velocity fields.
Methods. We have obtained maps of the HCO and H$^{13}$CO$^+$ ground state lines towards the Horsehead edge at $5\hbox{$^{\prime\prime}$}$ angular resolution with a combination of Plateau de Bure Interferometer (PdBI) and the IRAM-30m telescope observations. These maps have been complemented with IRAM-30m observations of several excited transitions at two different positions.
Results. Bright formyl radical emission delineates the illuminated edge of the nebula, with a faint emission remaining towards the shielded molecular core. Viewed from the illuminated star, the HCO emission almost coincides with the PAH and CCH emission. HCO reaches a similar abundance to HCO$^+$ in the photon dissociation region (PDR), $1-2 \times 10^{-9}$ with respect to H$_2$. To our knowledge, this is the highest HCO abundance ever measured. Pure gas-phase chemistry models fail to reproduce the observed HCO abundance by $\sim 2$ orders of magnitude, except if reactions of atomic oxygen with carbon radicals abundant in the PDR (i.e., CH$_2$) play a significant role in the HCO formation. Alternatively, HCO could be produced in the PDR by non-thermal processes such as photo-processing of ice mantles and subsequent photo-desorption of either HCO or H$_2$CO, and further gas phase photodissociation.
Conclusions. The measured HCO/H$^{13}$CO$^+$ abundance ratio is large towards the PDR ($\simeq 50$), and much lower toward the gas shielded from FUV radiation ( $\lower.5ex\hbox{$\; \buildrel < \over \sim \;$}1$). We propose that high HCO abundances ( $\raisebox{-0.6ex}{$\: \stackrel{>}{\scriptstyle \sim} \:$}10^{10}$) together with large HCO/H$^{13}$CO$^+$ abundance ratios ( $\raisebox{-0.6ex}{$\: \stackrel{>}{\scriptstyle \sim} \:$}1$) are sensitive diagnostics of the presence of active photochemistry induced by FUV radiation.

Appeared in: A&A 494, 977

The ionization fraction gradient across the Horsehead edge: an archetype for molecular clouds

J.R. Goicoechea$(^{1})$, J. Pety$(^{2,3})$, M. Gerin$(^{3})$, P. Hily-Blant$(^{4})$, and J. Le Bourlot$(^{5})$
$(^{1})$Laboratorio de Astrofísica Molecular, Centro de Astrobiología. CSIC-INTA, Carretera de Ajalvir, Km 4. Torrejón de Ardoz, 28850 Madrid, Spain, $(^{2})$IRAM, 300 rue de la Piscine, 38406 Grenoble Cedex, France, $(^{3})$LERMA - LRA, UMR 8112, CNRS, Observatoire de Paris and École Normale Supérieure, 24 rue Lhomond, 75231 Paris, France, $(^{4})$ Laboratoire d'Astrophysique, Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 09, France, $(^{5})$LUTH, UMR 8102 CNRS, Université Paris 7 and Observatoire de Paris, Place J. Janssen, 92195 Meudon, France

Context. The ionization fraction (i.e., the electron abundance) plays a key role in the chemistry and dynamics of molecular clouds.
Aims. We study the H$^{13}$CO$^+$, DCO$^+$ and HOC$^+$ line emission towards the Horsehead, from the shielded core to the UV irradiated cloud edge, i.e., the photodissociation region (PDR), as a template to investigate the ionization fraction gradient in molecular clouds.
Methods. We analyze an IRAM Plateau de Bure Interferometer map of the H$^{13}$CO$^+$ $J = 1- 0$ line at a $6{\farcs}8 \times
4{\farcs}7$ resolution, complemented with IRAM-30m H$^{13}$CO$^+$ and DCO$^+$ higher$-J$ line maps and new HOC$^+$ and CO$^+$ observations. We compare self-consistently the observed spatial distribution and line intensities with detailed depth-dependent predictions of a PDR model coupled with a nonlocal radiative transfer calculation. The chemical network includes deuterated species, $^{13}$C fractionation reactions and HCO$^+$/HOC$^+$ isomerization reactions. The role of neutral and charged PAHs in the cloud chemistry and ionization balance is investigated.
Results. The detection of the HOC$^+$ reactive ion towards the Horsehead PDR proves the high ionization fraction of the outer UV irradiated regions, where we derive a low [HCO$^+$]/[HOC$^+$]$ \simeq
75-200$ abundance ratio. In the absence of PAHs, we reproduce the observations with gas-phase metal abundances, [Fe+Mg+...], lower than $4\times 10^9$ (with respect to H), and a cosmic-ray ionization rate of $\xi = (5\pm 3) \times 10^{-17} {\rm s}^{-1}$. The inclusion of PAHs modifies the ionization fraction gradient and increases the required metal abundance.
Conclusions. The ionization fraction in the Horsehead edge follows a steep gradient, with a scale length of $\sim 0.05$ pc (or $\sim 25\hbox{$^{\prime\prime}$}$), from [e$^-$] $\simeq 10^{-4}$ (or $n_e \sim 1-5$ cm$^{-3}$) in the PDR to a few times $\sim 10^{-9}$ in the core. PAH$^{-1}$ anions play a role in the charge balance of the cold and neutral gas if substantial amounts of free PAHs are present ([PAH]$ > 10^{-8}$).

Appeared in: A&A 498, 771

Imaging galactic diffuse clouds: CO emission, reddening and turbulent flow in the gas around $\zeta $ Ophiuchi

H.S. Liszt$(^{1})$, J. Pety$(^{2,3})$, and K. Tachihara$(^{4})$
$(^{1})$National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA, USA 22903-2475, USA $(^{2})$Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint-Martin d'Hères, France $(^{3})$Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France $(^{4})$National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, Tokyo 181-8588, Japan

Context. Most diffuse clouds are only known as kinematic features in absorption spectra, but those with appreciable H2 content may be visible in the emission of such small molecules as CH, OH, and CO.
Aims. We interpret in greater detail the extensive observations of $^{12}$CO emission from diffuse gas seen around the archetypical line of sight to $\zeta $ Oph.
Methods. The $^{12}$CO emission is imaged in position and position-velocity space, analyzed statistically, and then compared with maps of total reddening $E^\infty_{B-V}$ and with models of the C$^+ - $CO transition in H$_2$-bearing diffuse clouds.
Results. Around $\zeta $ Oph, $^{12}$CO emission appears in two distinct intervals of reddening centered near $E^\infty_{B-V} \approx
0.4$ and 0.65 mag, of which $\lower.5ex\hbox{$\; \buildrel < \over \sim \;$}0.2$ mag is background material.Within either interval, the integrated $^{12}$CO intensity varies up to $6-12$ K km s$^{-1}$, compared to 1.5 K km s$^{-1}$ toward $\zeta $ Oph. Nearly 80% of the individual profiles have velocity dispersions $\sigma_v < 0.6$ km s$^{-1}$, which are subsonic at the kinetic temperature derived from H$_2$ toward $\zeta $ Oph, 55 K. Partly as a result, $^{12}$CO emission exposes the internal, turbulent, supersonic ($1-3$ km s$^{-1}$) gas flows with especial clarity in the cores of strong lines. The flows are manifested as resolved velocity gradients in narrow, subsonically-broadened line cores.
Conclusions. The scatter between N(CO) and $E_{B-V}$ in global, CO absorption line surveys toward bright stars is present in the gas seen around $\zeta $ Oph, reflecting the extreme sensitivity of N($^{12}$CO) to ambient conditions. The two-component nature of the optical absorption toward $\zeta $ Oph is coincidental and the star is occulted by a single body of gas with a complex internal structure, not by two distinct clouds. The very bright $^{12}$CO lines in diffuse gas arise at $N(H_2) \approx 10^{21}$ cm$^{-2}$ in regions of modest density $n(H) \approx 200-500$ cm$^{-3}$ and somewhat more complete C$^+ - $CO conversion. Given the variety of structure in the foreground gas, it is apparent that only large surveys of absorption sightlines can hope to capture the intrinsic behavior of diffuse gas.

Appeared in: A&A 499, 503

CO emission and variable CH and CH+ absorption towards HD 34078: evidence for a nascent bow shock?

P. Boissé$(^{1})$, E. Rollinde$(^{1})$, P. Hily-Blant$(^{2})$, J. Pety$(^{3})$, S. R. Federman$(^{4})$, Y. Sheffer$(^{4})$, G. Pineau des Forêts$(^{5})$, E. Roueff$(^{6})$, B.-G. Andersson$(^{7})$, and G. Hébrard$(^{1})$
$(^{1})$Institut d'Astrophysique de Paris (IAP), UMR7095 CNRS, Université Pierre et Marie Curie-Paris 6, 98 bis boulevard Arago, 75014 Paris, France, $(^{3})$IRAM, 300 rue de la Piscine, 38406 Saint-Martin-d'Hères; Laboratoire d'Astrophysique, Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 9, France, $(^{4})$IRAM, 300 rue de la Piscine, 38406 Saint Martin d'Hères; Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France, $(^{5})$Department of Physics and Astronomy, University of Toledo, Toledo, OH 43606, USA, $(^{6})$IAS, Université d'Orsay, 91405 Orsay Cedex, France, $(^{7})$LUTH, Observatoire de Paris-Meudon, 92195 Meudon Cedex, France, $(^{8})$NASA Ames Research Center, Moffett Field, CA 94035, USA

Context. The runaway star HD 34078, initially selected to investigate small scale structure in a foreground diffuse cloud, has been shown to be surrounded by highly excited H$_2$, the origin of which is unclear.
Aims. We first search for an association between the foreground cloud and HD 34078. Second, we extend previous investigations of temporal absorption line variations (CH, CH$^+$, H$_2$) in order to better characterize them and understand their relation to small-scale structure in the molecular gas.
Methods. We have mapped the $^{12}$CO$(2-1)$ emission $at 12\hbox{$^{\prime\prime}$}$ resolution around HD 34078's position, using the 30 m IRAM antenna. The follow-up of CH and CH$^+$ absorption lines has been extended over 5 more years: 26 visible spectra have been acquired since 2003 at high or intermediate resolution. In parallel, CH absorption towards the reddened star $\zeta $ Per has been monitored to check the instrumental stability and homogeneity of our measurements. Three more FUSE spectra have been obtained to search for N(H$_2$) variations.
Results. CO observations show a pronounced maximum near HD 34078's position, clearly indicating that the star and diffuse cloud are associated. The optical spectra confirm the reality of strong, rapid and correlated CH and CH$^+$ fluctuations (up to 26% for N(CH$^+$) between 2007 and 2008). On the other hand, N(H$_2, J = 0$) has varied by less than 5% over 4 years, indicating the absence of marked density structure at scales below 100 AU. We also discard N(CH) variations towards $\zeta $ Per at scales less than 20 AU.
Conclusions. Observational constraints from this work and from $24 \mu$m dust emission appear to be consistent with H$_2$ excitation but inconsistent with steady-state bow shock models and rather suggest that the shell of compressed gas surrounding HD 34078 or lying at the boundary of a small foreground clump is seen at an early stage of the interaction. The CH and CH$^+$ time variations as well as their high abundances are likely due to chemical structure in the shocked gas layer located at the stellar wind/ambient cloud interface. Finally, the lack of variation in both N(H$_2, J = 0$) towards HD 34078 and N(CH) towards $\zeta $ Per suggests that quiescent molecular gas is not subject to pronounced small-scale structure.

Appeared in: A&A 501, 221

Why did Comet 17P/Holmes burst out? Nucleus splitting or delayed sublimation?

W.J. Altenhoff$(^{1})$, E. Kreysa$(^{1})$, K.M. Menten$(^{1})$, A. Sievers$(^{3})$, C. Thum$(^{2})$, and A. Weiss$(^{1})$
$(^{1})$Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany $(^{2})$IRAM, Domaine Universitaire, 38406 St. Martin d'Hères, France, $(^{3})$IRAM, Pico Veleta, Granada, Spain

Based on millimeter-wavelength continuum observations we suggest that the recent ``spectacle'' of comet 17P/Holmes can be explained by a thick, air-tight dust cover and the effects of H$_2$O sublimation, which started when the comet arrived at the heliocentric distance $\leq
2.5$ AU. The porous structure inside the nucleus provided enough surface for additional sublimation, which eventually led to the break up of the dust cover and to the observed outburst. The magnitude of the particle burst can be explained by the energy provided by insolation, stored in the dust cover and the nucleus within the months before the outburst: the subliming surface within the nucleus is more than one order of magnitude larger than the geometric surface of the nucleus - possibly an indication of the latter's porous structure. Another surprise is that the abundance ratios of several molecular species with respect to H$_2$O are variable. During this apparition, comet Holmes lost about 3% of its mass, corresponding to a ``dirty ice'' layer of 20 m.

Appeared in: A&A 495, 975

CO and HI observations of an enigmatic interstellar cloud

Y. Libert$(^{1})$, E. Gérard$(^{2})$, T. LeBertre$(^{1})$, L. Matthews$(^{3})$, C. Thum$(^{4})$, and J.M. Winters$(^{4})$
$(^{1})$LERMA, UMR 8112, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France, $(^{2})$GEPI, UMR 8111, Observatoire de Paris, 5 Place J. Janssen, 92195 Meudon Cedex, France, $(^{3})$MIT Haystack Observatory, Off Route 40, Westford, MA 01886, USA, $(^{4})$IRAM, 300 rue de la Piscine, 38406 St. Martin d'Hères, France

Context. An isolated HI cloud with peculiar properties has recently been discovered by Dedes et al. (2008, A&A, 491, L45) with the 300-m Arecibo telescope, and subsequently imaged with the VLA. It has an angular size of $\sim 6\hbox{$^{\prime}$}$, and the Hi emission has a narrow line profile of width $\sim 3$ km s$^{-1}$.
Aims. We explore the possibility that this cloud could be associated with a circumstellar envelope ejected by an evolved star.
Methods. Observations were made in the rotational lines of CO with the IRAM-30m telescope, on three positions in the cloud, and a total-power mapping in the HI line was obtained with the Nançay Radio Telescope.
Results. CO was not detected and seems too underabundant in this cloud to be a classical late-type star circumstellar envelope. On the other hand, the HI emission is compatible with the detached-shell model that we developed for representing the external environments of AGB stars.
Conclusions. We propose that this cloud could be a fossil circumstellar shell left over from a system that is now in a post-planetarynebula phase. Nevertheless, we cannot rule out that it is a Galactic cloud or a member of the Local Group, although the narrow line profile would be atypical in both cases.

Appeared in: A&A 500, 1133

Heracles: The HERA CO Line Extragalactic Survey

Adam K. Leroy$(^{1})$, Fabian Walter$(^{1})$, Frank Bigiel$(^{1})$, Antonio Usero$(^{2,3})$, Axel Weiss$(^{4})$, Elias Brinks$(^{2})$, W.J.G. de Blok$(^{5,6})$, Robert C. Kennicutt$(^{7})$, Karl-Friedrich Schuster$(^{8})$, Carsten Kramer$(^{9})$, H. W. Wiesemeyer$(^{8,9})$, and Hélène Roussel$(^{1,10})$
$(^{1})$Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117, Heidelberg, Germany, $(^{2})$Centre for Astrophysics Research, University of Hertfordshire, Hatfield AL10 9AB, UK, $(^{3})$Observatorio Astronómico Nacional, C/ Alfonso XII, 3, 28014, Madrid, Spain, $(^{4})$MPIfR, Auf dem Hügel 69, 53121, Bonn, Germany, $(^{5})$Research School of Astronomy & Astrophysics, Mount Stromlo Observatory, Cotter Road, Weston ACT 2611, Australia, $(^{6})$Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa, $(^{7})$University of Cambridge, Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK, $(^{8})$IRAM, 300 rue de la Piscine, 38406 St. Martin d'Hères, France, $(^{9})$IRAM, Avenida Divina Pastora 7, E-18012 Granada, Spain, $(^{10})$Institut d'Astrophysique de Paris, CNRS et Université P. & M. Curie, 98 bis Blvd Arago, 75014 Paris, France

We present the Heterodyne Receiver Array CO Line Extragalactic Survey, an atlas of CO emission from 18 nearby galaxies that are also part of The HI Nearby Galaxy Survey and the Spitzer Infrared Nearby Galaxies Survey. We used the HERA multipixel receiver on the IRAM 30-m telescope to map the CO $J = 2 \rightarrow 1$ line over the full optical disk (defined by the isophotal radius $r_{25}$) of each target, at $13\hbox{$^{\prime\prime}$}$ angular resolution and 2.6 km s$^{-1}$ velocity resolution. Here we describe the observations and reduction of the data and show channel maps, azimuthally averaged profiles, integrated intensity maps, and peak intensity maps. The implied H$_2$ masses range from $7 \times 10^6$ to $6 \times 10^9\mbox{M$_\odot$}$, with four low metallicity dwarf irregular galaxies yielding only upper limits. In the cases where CO is detected, the integrated H$_2$-to-HI ratios range from 0.02 to 1.13 and H$_2$-to-stellar mass ratios from 0.01 to 0.25. Exponential scale lengths of the CO emission for our targets are in the range $0.8-3.2$ kpc, or $0.2 \pm 0.05 \; r_{25}$. The intensity-weighted mean velocity of CO matches that of HI very well, with a $1\sigma$ scatter of only 6 km s$^{-1}$. The CO $J = 2 \rightarrow 1/J = 1 \rightarrow 0$ line ratio varies over a range similar to that found in the Milky Way and other nearby galaxies, $\sim 0.6 - 1.0$, with higher values found in the centers of galaxies. The typical line ratio, $\sim 0.8$, could be produced by optically thick gas with an excitation temperature of $\sim 10$ K.

Appeared in: AJ 137, 4670

Unveiling the main heating sources in the Cepheus A HW2 region

I. Jiménez-Serra$(^{1})$, J. Martín-Pintado$(^{2})$, P. Caselli$(^{1})$, S. Martín$(^{3})$, A. Rodríguez-Franco$(^{2,4})$, C. Chandler$(^{5})$ and J.M. Winters$(^{6})$
$(^{1})$School of Physics & Astronomy, E.C. Stoner Building, The University of Leeds, Leeds, LS2 9JT, UK, $(^{2})$Centro de Astrobiología (CSIC/INTA), Ctra. de Torrejón a Ajalvir km 4, E-28850 Torrejón de Ardoz, Madrid, Spain, $(^{3})$Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA, $(^{4})$Escuela Universitaria de Óptica, Departamento de Matemática Aplicada (Biomatemática), Universidad Complutense de Madrid, Avda. Arcos de Jalón s/n, E-28037 Madrid, Spain, $(^{5})$National Radio Astronomy Observatory, P.O. Box O Socorro NM 87801, USA, $(^{6})$IRAM, 300 Rue de la Piscine, F-38406 St. Martin d'Hères, France

We present high angular resolution PdBI images (beam of $\sim
0{\farcs}33$) of the $J=27\rightarrow 26$ line from several vibrational levels ($v_7=1$ and $v_6=1$) of HC$_3$N toward Cepheus A HW2. These images reveal the two main heating sources in the cluster: one centered in the disk collimating the HW2 radio jet (the HW2 disk), and the other associated with a hot core $0{\farcs}3$ northeast HW2 (the HC). This is the first time that vibrationally excited emission of HC$_3$N is spatially resolved in a disk. The kinematics of this emission shows that the HW2 disk rotates following a Keplerian law. We derive the temperature profiles in the two objects from the excitation of HC$_3$N along the HW2 disk and the HC. These profiles reveal that both objects are centrally heated and show temperature gradients. The inner and hotter regions have temperatures of $350\pm 30$ K and $270\pm 20$ K for the HW2 disk and the HC, respectively. In the cooler and outer regions, the temperature drops to $250\pm 30$ K in the HW2 disk, and to $220\pm 15$ K in the HC. The estimated luminosity of the heating source of the HW2 disk is $\sim 2.2 \times 10^4 \; \mbox{L$_\odot$}$, and $\sim 3000 \; \mbox{L$_\odot$}$ for the HC. The most massive protostar in the HW2 region is the powering source of the HW2 radio jet. We discuss the formation of multiple systems in this cluster. The proximity of the HC to HW2 suggest that these sources likely form a binary system of B stars, explaining the observed precession of the HW2 radio jet.

Accepted for publication in ApJL

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