J. R. D. Lépine

The spiral structure of the Galaxy revealed by CS sources and evidence for the 4:1 resonance

We present a map of the spiral structure of the Galaxy, as traced by molecular carbon monosulphide (CS) emission associated with IRAS sources which are believed to be compact Hxa0ii regions. The CS line velocities are used to determine the kinematic distances of the sources in order to investigate their distribution in the galactic plane. This allows us to use 870 objects to trace the arms, a number larger than that of previous studies based on classical Hxa0ii regions. The distance ambiguity of the kinematic distances, when it exists, is solved by different procedures, including the latitude distribution and an analysis of the longitude–velocity diagram. The study of the spiral structure is complemented with other tracers: open clusters, Cepheids, methanol masers and Hxa0ii regions. The well-defined spiral arms are seen to be confined inside the corotation radius, as is often the case in spiral galaxies. We identify a square-shaped sub-structure in the CS map with that predicted by stellar orbits at the 4:1 resonance (four epicycle oscillations in one turn around the galactic centre). The sub-structure is found at the expected radius, based on the known pattern rotation speed and epicycle frequency curve. An inner arm presents an end with strong inwards curvature and intense star formation that we tentatively associate with the region where this arm surrounds the extremity of the bar, as seen in many barred galaxies. Finally, a new arm with concave curvature is found in the Sagitta to Cepheus region of the sky. The observed arms are interpreted in terms of perturbations similar to grooves in the gravitational potential of the disc, produced by crowding of stellar orbits.

Oxygen abundance distribution in the Galactic disc

Weperformedanon-localthermodynamicequilibrium(NLTE)analysisoftheinfraredoxygen tripletforalargenumberofCepheidspectraobtainedwiththeHobby–EberlyTelescope.These data were combined with our previous NLTE results for stars observed with the Max Planck Gesellschaft Telescope with the aim of investigating the oxygen abundance distribution in the Galactic thin disc. We found the slope of the radial (O/H) distribution to be equal −0.058 dexkpc −1 . However, we found some evidence that the distribution might become flatter in the outer parts of the disc. This is supported by the results of other authors who have studied open clusters, planetary nebulae and H II regions. Some mechanisms of flattening are discussed.

A mechanism for the formation of oxygen and iron bimodal radial distribution in the disc of our Galaxy

Recently, it has been proposed that there are two type Ia supernova progenitors: short-lived and long-lived. On the basis of this idea, we develop a theory of a unified mechanism for the formation of the bimodal radial distribution of iron and oxygen in the Galactic disc. The underlying cause for the formation of the fine structure of the radial abundance pattern is the influence of the spiral arms, specifically the combined effect of the corotation resonance and turbulent diffusion. From our modelling, we conclude that in order to explain the bimodal radial distributions simultaneously for oxygen and iron and to obtain approximately equal total iron output from different types of supernovae, the mean ejected iron mass per supernova event should be the same as quoted in the literature if the maximum mass of stars, which eject heavy elements, is 50 M� . For the upper mass limit of 70 M� , the production of iron by a type II supernova explosion should increase by about 1.5 times.

Phosphorus-bearing molecules in solar-type star forming regions: First PO detection

As part of the Large Program ASAI (Astrochemical Surveys At IRAM), we have used the IRAM 30m telescope to lead a systematic search for the emission of rotational transitions of P-bearing species between 80 and 350 GHz towards L1157-B1, a shock position in the solar-type star forming region L1157. We report the detection of several transitions of PN and, for the first time, of prebiotic molecule PO. None of these species are detected towards the driving protostar of the outflow L1157-mm. Analysis of the line profiles shows that PN arises from the outflow cavity, where SiO, a strong shock tracer, is produced. Radiative transfer analysis yields an abundance of 2.5 × 10 and 0.9 × 10 for PO and PN, respectively. These results imply a strong depletion (≈ 100) of Phosphorus in the quiescent cloud gas. Shock modelling shows that atomic N plays a major role in the chemistry of PO and PN. The relative abundance of PO and PN brings constraints both on the duration of the pre-shock phase, which has to be ∼ 10 yr, and on the shock parameters. The maximum temperature in the shock has to be larger than 4000K, which implies a shock velocity of 40 km s.

H i aperture synthesis and optical observations of the pair of galaxies NGC 6907 and 6908

NGCxa06908, an S0 galaxy situated in the direction of NGCxa06907, was only recently recognized as a distinct galaxy, instead of only a part of NGCxa06907. We present 21-cm radio synthesis observations obtained with the Giant Metrewave Radio Telescope (GMRT) and optical images and spectroscopy obtained with the Gemini-North telescope of this pair of interacting galaxies. From the radio observations, we obtained the velocity field and the Hxa0i column density map of the whole region containing the NGCxa06907/8 pair, and by means of the Gemini multi-object spectroscopy we obtained high-quality photometric images and 5 A resolution spectra sampling the two galaxies. By comparing the rotation curve of NGCxa06907 obtained from the two opposite sides around the main kinematic axis, we were able to distinguish the normal rotational velocity field from the velocity components produced by the interaction between the two galaxies. Taking into account the rotational velocity of NGCxa06907 and the velocity derived from the absorption lines for NGCxa06908, we verified that the relative velocity between these systems is lower than 60 kmxa0s−1. The emission lines observed in the direction of NGCxa06908, not typical of S0 galaxies, have the same velocity expected for the NGCxa06907 rotation curve. Some emission lines are superimposed on a broader absorption profile, which suggests that they were not formed in NGCxa06908. Finally, the Hxa0i profile exhibits details of the interaction, showing three components: one for NGCxa06908, another for the excited gas in the NGCxa06907 disc and a last one for the gas with higher relative velocities left behind NGCxa06908 by dynamical friction, used to estimate the time when the interaction started in (3.4 ± 0.6) × 107 yr ago.

Corotation resonance and the radial distribution of oxygen in a spiral galaxy

A theory is proposed for the formation of the bimodal radial distribution of oxygen in the Galaxy, which displays a steep abundance gradient in the inner region and becomes flat in the middle and outer regions. The formation of this structure is associated with the following effects. First, type II supernovae, which are sources of oxygen, are concentrated in the spiral arms. Second, the enrichment rate is proportional to the rate at which a chemical element enters the spiral arm, which has the form of a density wave. This implies that the rate of oxygen production is substantially suppressed in most of the corotation circle due to the low relative velocity of the Galactic material with respect to the spiral arms and the low rate at which the arms are crossed by the Galactic material. Third, the corotation radius is near the Sun. The net result of these effects combined with turbulent diffusion is the development of the structure discussed in the paper.

A search for cyanopolyynes in L1157-B1

We present here a systematic search for cyanopolyynes in the shock region L1157-B1 and its associated protostar L1157-mm in the framework of the Large Program Astrochemical Surveys At IRAM (ASAI), dedicated to chemical surveys of solar-type star forming regions with the IRAM 30m telescope. Observations of the millimeter windows between 72 and 272 GHz permitted the detection of HC

Statistics of initial velocities of open clusters

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The role of spiral arms in the chemical evolution of galactic disks

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Survey of Young Stellar Objects Associated with Molecular Clouds

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