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Scientific Results in Press

Dust formation and mass-loss in long-period variable stars

Winters, J.M. $(^{1})$
$(^{1})$Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, 38406 St Martin d'Hères, France

An approach to describe dust forming circumstellar shells around pulsating Asymptotic Giant Branch stars is presented with the aim of understanding the mass-loss mechanism of these objects. The resulting model structures are discussed and the model predictions are compared with suitable observations. Finally, some implications of the mass-loss process on the evolution of the mass-losing star are discussed.

Appeared in: EAS Publications Series, Vol. 19, 167

Adapting and Expanding Interferometric Arrays

Karastergiou A.$(^{1})$, Neri R.$(^{1})$, Gurwell M. A.$(^{2})$
$(^{1})$IRAM, 300 rue de la Piscine, Domaine Universitaire, Saint Martin d'Hères, France, $(^{2})$Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138

We outline here a simple yet efficient method for finding optimized configurations of the elements of radioastronomical interferometers with fixed pad locations. The method can be successfully applied, as we demonstrate, to define new configurations when changes to the array take place. This may include the addition of new pads or new antennas, or the loss of pads or antennas. Our method is based on identifying which placement of elements provides the most appropriate u-v plane sampling for astronomical imaging.

Appeared in: ApJS 164, 552

First Detection of HCO+ Emission at High Redshift

Riechers D. A.$(^{1})$, Walter F.$(^{1})$, Carilli C. L.$(^{2})$, Weiss A.$(^{3})$, Bertoldi F.$(^{4})$, Menten K. M.$(^{3})$, Knudsen K. K.$(^{1})$, Cox P.$(^{5})$
$(^{1})$Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany, $(^{2})$National Radio Astronomy Observatory, PO Box O, Socorro, NM 87801, $(^{3})$Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany, $(^{4})$Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany, $(^{5})$Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, Domaine Universitaire, F-38406 Saint Martin d'Hères, France

We report the detection of HCO $^+ (J=1\leftarrow 0)$ emission toward the Cloverleaf quasar ($z=2.56$) through observations with the Very Large Array. This is the first detection of ionized molecular gas emission at high redshift ($z>2$). HCO$^+$ emission is a star formation indicator similar to HCN, tracing dense molecular hydrogen gas [ $n(H2)\simeq 10^5$ cm$^{-3}$] within star-forming molecular clouds. We derive a lensing-corrected HCO$^+$ line luminosity of $L'_{HCO^+}=3.5\times10^9$ K km s$^{-1}$ pc$^2$. Combining our new results with CO and HCN measurements from the literature, we find an HCO$^+$/CO luminosity ratio of 0.08 and an HCO$^+$/HCN luminosity ratio of 0.8. These ratios fall within the scatter of the same relationships found for low-z star-forming galaxies. However, an HCO$^+$/HCN luminosity ratio close to unity would not be expected for the Cloverleaf if the recently suggested relation between this ratio and the far-infrared luminosity were to hold. We conclude that a ratio between HCO$^+$ and HCN luminosity close to 1 is likely due to the fact that the emission from both lines is optically thick and thermalized and emerges from dense regions of similar volume. The CO, HCN, and HCO$^+$ luminosities suggest that the Cloverleaf is a composite active galactic nucleus-starburst system, in agreement with the previous finding that about 20% of the total infrared luminosity in this system results from dust heated by star formation rather than heating by the active nucleus. We conclude that HCO$^+$ is potentially a good tracer for dense molecular gas at high redshift.

Appeared in: ApJ 645, L13

Molecular gas in the Andromeda galaxy

Nieten Ch.$(^{1})$; Neininger, N.$(^{1,2,3})$, Guélin M.$(^{3})$, Ungerechts H.$(^{4})$, Lucas R.$(^{3})$; Berkhuijsen E. M.$(^{1})$, Beck R.$(^{1})$, Wielebinski R.$(^{1})$
$(^{1})$Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany, $(^{2})$Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany $(^{3})$Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 St. Martin d' Hères, France, $(^{4})$Instituto de Radioastronomía Milimétrica, Avenida Divina Pastora 7, 18012 Granada, Spain


Aims. We study the distribution of the molecular gas in the Andromeda galaxy (M 31) and compare this with the distributions of the atomic gas and the emission from cold dust at $\lambda = 175 \mu$m.

Methods. We obtained a new $^{12}$CO($J = 1-0$)-line survey of the Andromeda galaxy with the highest resolution to date ( $23\hbox{$^{\prime\prime}$}$, or 85 pc along the major axis), observed On-the-Fly with the IRAM 30-m telescope. We fully sampled an area of $2^{\circ} \times 0{\fdg}5$ with a velocity resolution of 2.6 km s$^{-1}$. In several selected regions we also observed the $^{12}$CO(2-1)-line. Results.Emission from the $^{12}$CO(1-0) line was detected from galactocentric radius $R=3$ kpc to $R=16$ kpc with a maximum in intensity at $R\sim 10$ kpc. The molecular gas traced by the (velocity-integrated) (1-0)-line intensity is concentrated in narrow arm-like filaments, which often coincide with the dark dust lanes visible at optical wavelengths. Between R=4 kpc and R=12 kpc the brightest CO filaments define a two-armed spiral pattern that is described well by two logarithmic spirals with a pitch angle of $7\degr-8\degr$. The arm-interarm brightness ratio averaged over a length of 15 kpc along the western arms reaches about 20 compared to 4 for H I at an angular resolution of $45\hbox{$^{\prime\prime}$}$. For a constant conversion factor $X_{CO}$, the molecular fraction of the neutral gas is enhanced in the spiral arms and decreases radially from 0.6 on the inner arms to 0.3 on the arms at $R\simeq 10$ kpc. The apparent gas-to-dust ratios $N(H I)/I_{175}$ and $(N(H
I)+2N(H_2))/I_{175}$ increase by a factor of 20 between the centre and $R \simeq 14$ kpc, whereas the ratio $2N(H_2)/I_{175}$ only increases by a factor of 4.

Conclusions. Either the atomic and total gas-to-dust ratios increase by a factor of 20 or the dust becomes colder towards larger radii. A strong variation of $X_{CO}$ with radius seems unlikely. The observed gradients affect the cross-correlations between gas and dust. In the radial range $R=8-14$ kpc total gas and cold dust are well correlated; molecular gas correlates better with cold dust than atomic gas. The mass of the molecular gas in M 31 within a radius of 18 kpc is $M (H_2)
= 3.6\times 10^8 \mbox{M$_\odot$}$ at the adopted distance of 780 kpc. This is 7% of the total neutral gas mass in M 31.

Appeared in: A&A 453, 459

Dissipative structures of diffuse molecular gas. I. Broad HCO+ (J=1-0) emission

Falgarone E.$(^{1})$, Pineau Des Forêts G.$(^{2,3})$, Hily-Blant P.$(^{4})$, Schilke P.$(^{5})$
$(^{1})$LERMA/LRA, CNRS - UMR 8112, École Normale Supérieure, 24 rue Lhomond, 75005 Paris, France, $(^{2})$IAS, CNRS - UMR 8617, Université Paris-Sud, 91405 Orsay, France, $(^{3})$LUTH, CNRS - UMR 8102, Observatoire de Paris, 92195 Meudon, France, $(^{4})$IRAM, 300 rue de la Piscine, 38406 St. Martin d'Hères, France, $(^{5})$MPIfR, Auf den Hügel 69, 53121 Bonn, Germany

Aims: Specific chemical signatures of the intermittent dissipation of turbulence were sought in diffuse molecular clouds.

Methods: We observed HCO$^+$(1-0) lines and the two lowest rotational transitions of $^{12}$CO and $^{13}$CO with an exceptional signal-to-noise ratio in the translucent environment of low-mass dense cores, where turbulence dissipation is expected to take place. Some of the observed positions belong to a new kind of small-scale structure identified in CO(1-0) maps of these environments as the locus of non-Gaussian velocity shears in the statistics of their turbulent velocity field, i.e. singular regions generated by the intermittent dissipation of turbulence.

Results: We report the detection of broad HCO$^+$(1-0) lines (10 mK $< T_{A^*}
< 0.5$ K). We achieve the interpretation of ten HCO$^+$ velocity components by conducting it in conjunction with that of the associated optically thin $^{13}$CO emission. The derived HCO$^+$ column densities span a broad range, $10^{11}< $N(HCO $^+)/\Delta v <4 \times 10^{12}$ cm$^{-2}$/km s$^{-1}$, and the inferred HCO$^+$ abundances, $2 \times 10^{-10}< X ($HCO$^+) <
10^{-8}$, are more than one order of magnitude above those produced by steady-state chemistry in gas that is weakly shielded from UV photons, even at large densities. We compare our results with predictions of non-equilibrium chemistry, swiftly triggered in bursts of turbulence dissipation and followed by a slow thermal and chemical relaxation phase, assumed to be isobaric. The set of values derived from observations, i.e. large HCO$^+$ abundances, temperatures in the range of 100-200 K, and densities in the range $100-10^3$ cm$^{-3}$, unambiguously belongs to the relaxation phase. In contrast, the kinematic properties of the gas suggest that the observed HCO$^+$ line emission results from a space-time average in the beam of the whole cycle followed by the gas and that the chemical enrichment is made at the expense of the non-thermal energy. Last, we show that the "warm chemistry" signature (i.e. large abundances of HCO$^+$, CH$^+$, H$_2$O, and OH) acquired by the gas within a few hundred years, which is the duration of the impulsive chemical enrichment, is kept over more than a thousand years. During the relaxation phase, the H$_2$O/OH abundance ratio stays close to the value measured in diffuse gas by the SWAS satellite, while the OH/HCO$^+$ ratio increases by more than one order of magnitude.

Appeared in: A&A 452, 511

Probing isotopic ratios at z = 0.89: molecular line absorption in front of the quasar PKS 1830-211

Sebastien Muller$(^{1,2})$, Michel Guelin$(^{2})$, Michael Dumke$(^{2})$, Robert Lucas$(^{1})$, Françoise Combes$(^{3})$
$(^{1})$ASIAA, P.O. Box 23-141, Taipei, 106 Taiwan, $(^{2})$IRAM, 300 rue de la Piscine, 38406 St Martin d'Hères, France, $(^{3})$ESO, Alonso de Cordova 3107, Casilla 19001, Santiago 19, Chile, $(^{4})$LERMA, 61 av. de l'Observatoire, F-75014 Paris, France

With the Plateau de Bure interferometer, we have measured the C, N, O and S isotopic abundance ratios in the arm of a spiral galaxy with a redshift of 0.89. The galaxy is seen face-on according to HST images. Its bulge intercepts the line of sight to the radio-loud quasar PKS 1830-211, giving rise at mm wavelengths to two Einstein images located each behind a spiral arm. The arms appear in absorption in the lines of several molecules, giving the opportunity to study the chemical composition of a galaxy only a few Gyr old. The isotopic ratios in this spiral galaxy differ markedly from those observed in the Milky Way. The $^{17}$O/$^{18}$O and $^{14}$N/$^{15}$N ratios are low, as one would expect from an object too young to let low mass stars play a major role in the regeneration of the gas.

Accepted for publication in A&A

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Up: IRAM Newsletter 67 (August 2006) Previous: A new CLASS directed