S. Leurini

Spectral imaging of the Central Molecular Zone in multiple 3-mm molecular lines

We have mapped 20 spectral lines in the Central Molecular Zone (CMZ) around the Galactic Centre, emitting from 85.3 to 93.3 GHz. This work used the 22-m Mopra radio telescope in Australia, equipped with the 8-GHz bandwidth University of New South Wales-Mopra Spectrometer (UNSW-MOPS) digital filter bank, obtaining ∼2 km s−1 spectral and ∼40 arcsec spatial resolution. The lines measured include emission from the c-C3H2, CH3CCH, HOCO+, SO, H13CN, H13CO+, SO, H13NC, C2H, HNCO, HCN, HCO+, HNC, HC3N, 13CS and N2H+ molecules. The area covered is Galactic longitude −0bsl000647 to 1bsl000648 and latitude −0bsl000643 to 0bsl000642, including the bright dust cores around Sgr A, Sgr B2, Sgr C and G1.6−0.025. We present images from this study and conduct a principal component analysis on the integrated emission from the brightest eight lines. This is dominated by the first component, showing that the large-scale distribution of all molecules is very similar. We examine the line ratios and optical depths in selected apertures around the bright dust cores, as well as for the complete mapped region of the CMZ. We highlight the behaviour of the bright HCN, HNC and HCO+ line emission, together with that from the 13C isotopologues of these species, and compare the behaviour with that found in extragalactic sources where the emission is unresolved spatially. We also find that the isotopologue line ratios (e.g. HCO+/H13CO+) rise significantly with increasing redshifted velocity in some locations. Line luminosities are also calculated and compared to that of CO, as well as to line luminosities determined for external galaxies.

On the metallicity gradient of the Galactic disk

Aims. The iron abundance gradient in the Galactic stellar disk provides fundamental constraints on the chemical evolution of this important Galaxy component, however the spread around the mean slope is, at fixed Galactocentric distance, more than the estimated uncertainties. Methods. To provide quantitative constraints on these trends, we adopted iron abundances for 265 classical Cepheids (more than 50% of the currently known sample) based either on high-resolution spectra or on photometric metallicity indices. Homogeneous distances were estimated using near-infrared period-luminosity relations. The sample covers the four disk quadrants, and their Galactocentric distances range from similar to 5 to similar to 17 kpc. We provided a new theoretical calibration of the metallicity-index-color (MIC) relation based on Walraven and NIR photometric passbands. Results. We estimated the photometric metallicity of 124 Cepheids. Among them 66 Cepheids also have spectroscopic iron abundances and we found that the mean difference is -0.03 +/- 0.15 dex. We also provide new iron abundances, based on high-resolution spectra, for four metal-rich Cepheids located in the inner disk. The remaining iron abundances are based on high-resolution spectra collected by our group (73) or available in the literature (130). A linear regression over the entire sample provides an iron gradient of -0.051 +/- 0.004 dex kpc(-1). The above slope agrees quite well, within the errors, with previous estimates based either on Cepheids or on open clusters covering similar Galactocentric distances. However, Cepheids located in the inner disk systematically appear more metal-rich than the mean metallicity gradient. Once we split the sample into inner (R(G) <8 kpc) and outer disk Cepheids, the slope (-0.130 +/- 0.015 dex kpc(-1)) in the former region is approximate to 3 times steeper than the slope in the latter one (-0.042 +/- 0.004 dex kpc(-1)). In the outer disk the radial distribution of metal-poor (MP, [Fe/H] <-0.02 dex) and metal-rich (MR) Cepheids across the four disk quadrants does not show a clear trend when moving from the innermost to the external disk regions. The relative fractions of MP and MR Cepheids in the 1st and in the 3rd quadrants differ at the 8 sigma (MP) and 15 sigma (MR) levels. Finally, we found that iron abundances in two local overdensities of the 2nd and of the 4th quadrant cover individually a range in iron abundance of approximate to 0.5 dex. Conclusions. Current findings indicate that the recent chemical enrichment across the Galactic disk shows a clumpy distribution.

Methanol as a diagnostic tool of interstellar clouds - I. Model calculations and application to molecular clouds

We present a detailed analysis of the diagnostic properties of methanol, (CH3OH), in dense molecular clouds, made possible by the availability of new (CH3OH-He) collisional rate coefficients. Using a spherical Large Velocity Gradient (LVG) model, the dependence on kinetic temperature and spatial density of various millimeter and submillimeter line bands is inves- tigated over a range of physical parameters typical of high- and low-mass star-forming regions. We find CH3OH to be a good tracer of high-density environments and sensitive to the kinetic temperature. Using our LVG model, we have also developed an innovative technique to handle the problem of deriving physical parameters from observed multi-line spectra of a molecule, based on the simultaneous fit of all the lines with a synthetic spectrum, finding the best physical parameters using numerical methods.

Deuteration as an evolutionary tracer in massive-star formation

Context. Theory predicts, and observations confirm, that the column density ratio of a molecule containing D to its counterpart containing H can be used as an evolutionary tracer in the low-mass star formation process. Aims. Since it remains unclear if the high-mass star formation process is a scaled-up version of the low-mass one, we investigated whether the relation between deuteration and evolution can be applied to the high-mass regime. Methods. With the IRAM-30 m telescope, we observed rotational transitions of N 2 D + and N 2 H + and derived the deuterated fraction in 27 cores within massive star-forming regions understood to represent different evolutionary stages of the massive-star formation process. Results. The abundance of N 2 D + is higher at the pre-stellar/cluster stage, then drops during the formation of the protostellar object(s) as in the low-mass regime, remaining relatively constant during the ultra-compact HII region phase. The objects with the highest fractional abundance of N 2 D + are starless cores with properties very similar to typical pre-stellar cores of lower mass. The abundance of N 2 D + is lower in objects with higher gas temperatures as in the low-mass case but does not seem to depend on gas turbulence. Conclusions. Our results indicate that the N 2 D + -to-N 2 H + column density ratio can be used as an evolutionary indicator in both low-and high-mass star formation, and that the physical conditions influencing the abundance of deuterated species likely evolve similarly during the processes that lead to the formation of both low- and high-mass stars.

Deuterium chemistry in the Orion Bar PDR. "Warm" chemistry starring CH2D+

Context. High levels of deuterium fractionation in gas-phase molecules are usually associated with cold regions, such as prestellar cores. Significant fractionation ratios are also observed in hot environments such as hot cores or hot corinos, where they are believed to be produced by the evaporation of the icy mantles surrounding dust grains, and are thus remnants of a previous cold (either gasphase or grain surface) chemistry. The recent detection of DCN towards the Orion Bar, in a clump at a characteristic temperature of 70 K, has shown that high deuterium fractionation can also be detected in PDRs. The Orion Bar clumps thus appear to be a good environment for the observational study of deuterium fractionation in luke warm gas, allowing us to validate chemistry models for a different temperature range, where dominating fractionation processes are predicted to differ from those in cold gas (< 20K). Aims. We aimed to study observationally in detail the chemistry at work in the Orion Bar PDR, to understand whether DCN is either produced by ice mantle evaporation or is the result of warm gas-phase chemistry, involving the CH_2D^+ precursor ion (which survives higher temperatures than the usual H_2D^+ precursor). Methods. Using the APEX and the IRAM 30m telescopes, we targeted selected deuterated species towards two clumps in the Orion Bar. Results. We confirmed the detection of DCN and detected two new deuterated molecules (DCO^+ and HDCO) towards one clump in the Orion Bar PDR. Significant deuterium fractionations are found for HCN and H_2CO, but we measured a low fractionation in HCO^+. We also provide upper limits to other molecules relevant to deuterium chemistry. Conclusions. We argue that grain evaporation in the clumps is unlikely to be a dominant process, and we find that the observed deuterium fractionation ratios are consistent with predictions of pure gas-phase chemistry models at warm temperatures (T ~ 50K). We show evidence that warm deuterium chemistry driven by CH_2D^+ is at work in the clumps.

Methanol as a diagnostic tool of interstellar clouds - II. Modelling high-mass protostellar objects

Context. Fundamental properties of interstellar clouds must be known to investigate the initial conditions of star formation within them and the interaction of newly formed stars with their environment. Methanol has proven to be useful to probe densities and temperatures of various environments within interstellar clouds. Aims. We aim to explore the potential of methanol as a tracer molecule for regions in which high-mass stars are forming or have recently formed, in particular so-called high-mass protostellar objects (HMPOs) and infrared dark clouds (IRDCs). Methods. We present and analyse multi-frequency centimetre and (sub)millimetre single-dish observations of methanol toward a sample of 13 sources that are in the poorly understood earliest phases of evolution of high-mass stars (HMPOs and IRDCs). For each source in our sample, we derive physical parameters such as the kinetic temperature, the spatial density. and the methanol column density. We apply our large velocity gradient modelling and fitting technique that involves fitting a synthetic spectrum to all the measured lines simultaneously. Results. In several sources, we find that more than one physical component is necessary to fit the spectra; moreover, broad non-Gaussian linewidths suggest outflows in many sources from both the IRDC and the HMPO subsamples. Kinetic temperatures are found to be between 10 and 60 K and spatial densities in the range 10 5 -10 6 cm -3 . Hotter, denser cores are found in a few HMPOs, indicating that these sources already harbour hot cores heated by protostars.

Dust and gas emission in the prototypical hot core G29.96–0.02 at sub-arcsecond resolution

Context. Hot molecular cores are an early manifestation of massive star formation where the molecular gas is heated to temperatures >100 K undergoing a complex chemistry. Aims. One wants to better understand the physical and chemical processes in this early evolutionary stage. Methods. We selected the prototypical hot molecular core G29.96−0.02 being located at the head of the associated ultracompact Hii region. The 862 µm submm continuum and spectral line data were obtained with the Submillimeter Array (SMA) at sub-arcsecond spatial resolution. Results. The SMA resolved the hot molecular core into six submm continuum sources with the finest spatial resolution of 0.36 �� ×0.25 �� (∼1800 AU) achieved so far. Four of them located within 7800 (AU) 2 comprise a proto-Trapezium system with estimated protostellar densities of 1.4×10 5 protostars/pc 3 . The plethora of ∼80 spectral lines allows us to study the molecular outflow(s), the core kinematics, the temperature structure of the region as well as chemical effects. The derived hot core temperatures are of the order 300 K. We find interesting chemical spatial differentiations, e.g., C 34 S is deficient toward the hot core and is enhanced at the UCHii/ hot core interface, which may be explained by temperature sensitive desorption from grains and following gas phase chemistry. The SiO(8−7) emission outlines likely two molecular outflows emanating from this hot core region. Emission from most other molecules peaks centrally on the hot core and is not dominated by any individual submm peak. Potential reasons for that are discussed. A few spectral lines that are associated with the main submm continuum source, show a velocity gradient perpendicular to the large-scale outflow. Since this velocity structure comprises three of the central protostellar sources, this is not a Keplerian disk. While the data are consistent with a gas core that may rotate and/or collapse, we cannot exclude the outflow(s) and/or nearby expanding UCHii region as possible alternative causes of this velocity pattern.

APEX 1 mm line survey of the Orion Bar

Context. Unbiased molecular line surveys are a powerful tool for analyzing the physical and chemical parameters of astronomical objects and are the only means for obtaining a complete view of the molecular inventory for a given source. The present work stands for the first such investigation of a photon-dominated region. Aims. The first results of an ongoing millimeter-wave survey obtained towards the Orion Bar are reported. Methods. The APEX telescope in combination with the APEX-2A facility receiver was employed in this investigation. Results. We derived the physical parameters of the gas through LVG analyses of the methanol and formaldehyde data. Information on the sulfur and deuterium chemistry of photon-dominated regions is obtained from detections of several sulfur-bearing molecules and DCN.

Observations and modelling of CO and [C i] in protoplanetary disks - First detections of [C i] and constraints on the carbon abundance

Context. The gas-solid budget of carbon in protoplanetary disks is related to the composition of the cores and atmospheres of the planets forming in them. The principal gas-phase carbon carriers CO, C0, and C+ can now be observed regularly in disks. Aims: The gas-phase carbon abundance in disks has thus far not been well characterized observationally. We obtain new constraints on the [C]/[H] ratio in a large sample of disks, and compile an overview of the strength of [C i] and warm CO emission. Methods: We carried out a survey of the CO 6-5 line and the [C i] 1-0 and 2-1 lines towards 37 disks with the APEX telescope, and supplemented it with [C ii] data from the literature. The data are interpreted using a grid of models produced with the DALI disk code. We also investigate how well the gas-phase carbon abundance can be determined in light of parameter uncertainties. Results: The CO 6-5 line is detected in 13 out of 33 sources, [C i] 1-0 in 6 out of 12, and [C i] 2-1 in 1 out of 33. With separate deep integrations, the first unambiguous detections of the [C i] 1-0 line in disks are obtained, in TW Hya and HD 100546. Conclusions: Gas-phase carbon abundance reductions of a factor of 5-10 or more can be identified robustly based on CO and [C i] detections, assuming reasonable constraints on other parameters. The atomic carbon detection towards TW Hya confirms a factor of 100 reduction of [C]/[H]gas in that disk, while the data are consistent with an ISM-like carbon abundance for HD 100546. In addition, BP Tau, T Cha,

The jet and the disk of the HH 212 low-mass protostar imaged by ALMA: SO and SO2 emission

Context. The investigation of the disk formation and jet launching mechanism in protostars is crucial to understanding the earliest stages of star and planet formation. Aims. We aim to constrain the physical and dynamical properties of the molecular jet and disk of the HH 212 protostellar system at unprecedented angular scales, exploiting the capabilities of the Atacama Large Millimeter Array (ALMA). Methods. The ALMA observations of HH 212 in emission lines from sulfur-bearing molecules, SO 98−87 ,S O 10 11−1010, SO2 82,6−71,7, are compared with simultaneous CO 3−2, SiO 8−7 data. The molecules column density and abundance are estimated using simple radiative transfer models. Results. SO 98−87 and SO2 82,6−71,7 show broad velocity profiles. At systemic velocity, they probe the circumstellar gas and the cavity walls. Going from low to high blue- and red-shifted velocities the emission traces the wide-angle outflow and the fast (∼100−200 km s −1 ), collimated (∼90 AU) molecular jet revealing the inner knots with timescales ≤50 yr. The jet transports a mass− −−