18606865
Galactic interstellar 18 O/ 17 O ratios - a radial gradient?
Context. The determination of interstellar abundances is essential for a better understanding of stellar nucleosynthesis and the “chemical” evolution of the Galaxy. Aims. The aim is to determine 18 O/ 17 O abundance ratios across the entire Galaxy. These provide a measure of the amount of enrichment by high-mass versus intermediate-mass stars. Methods. Such ratios, derived from the C 18 O and C 17 O $J = 1$–0 lines alone, may be affected by systematic errors. Therefore, the C 18 O and C 17 O (1–0), (2–1), and (3–2), as well as the 13 CO(1–0) and (2–1) lines, were observed towards 18 prominent galactic targets (a total of 25 positions). The combined dataset was analysed with a large velocity gradient model, accounting for optical depth effects. Results. The data cover galactocentric radii between 0.1 and 16.9 kpc (solar circle at 8.5 kpc). Near the centre of the Galaxy, 18 O/ 17 O = 2.88  ±  0.11. For the galactic disc out to a galactocentric distance of ~10 kpc, 18 O/ 17 O = 4.16  ±  0.09. At ~16.5 kpc from the galactic centre, 18 O/ 17 O = 5.03  ±  0.46. Assuming that 18 O is synthesised predominantly in high-mass stars ($M >8$ $M_{\odot}$), while C 17 O is mainly a product of lower mass stars, the ratio from the inner Galaxy indicates a dominance of CNO-hydrogen burning products that is also apparent in the carbon and nitrogen isotope ratios. The high 18 O/ 17 O value of the solar system (5.5) relative to that of the ambient interstellar medium suggests contamination by nearby high-mass stars during its formation. The outer Galaxy poses a fundamental problem. High values in the metal-poor environment of the outer Galaxy are not matched by the low values observed towards the even more metal-poor Large Magellanic Cloud. Apparently, the outer Galaxy cannot be considered as an intermediate environment between the solar neighbourhood and the interstellar medium of small metal-poor galaxies. The apparent 18 O/ 17 O gradient along the galactic disc and the discrepancy between outer disc and LMC isotope ratios may be explained by different ages of the respective stellar populations. More data from the central and far outer parts of the Galaxy are, however, needed to improve the statistical significance of our results.
115454830
Clouds, filaments, and protostars: TheHerschel Hi-GAL Milky Way
We present the first results from the science demonstration phase for the Hi-GAL survey, the Herschel key program that will map the inner Galactic plane of the Milky Way in 5 bands. We outline our data reduction strategy and present some science highlights on the two observed 2° × 2° tiles approximately centered at l = 30° and l = 59°. The two regions are extremely rich in intense and highly structured extended emission which shows a widespread organization in filaments. Source SEDs can be built for hundreds of objects in the two fields, and physical parameters can be extracted, for a good fraction of them where the distance could be estimated. The compact sources (which we will call cores' in the following) are found for the most part to be associated with the filaments, and the relationship to the local beam-averaged column density of the filament itself shows that a core seems to appear when a threshold around AV ~ 1 is exceeded for the regions in the l = 59° field; a AV value between 5 and 10 is found for the l = 30° field, likely due to the relatively higher distances of the sources. This outlines an exciting scenario where diffuse clouds first collapse into filaments, which later fragment to cores where the column density has reached a critical level. In spite of core L/M ratios being well in excess of a few for many sources, we find core surface densities between 0.03 and 0.5 g cm-2. Our results are in good agreement with recent MHD numerical simulations of filaments forming from large-scale converging flows.
118239150
The exciting future of (sub-)millimeter interferometry: ALMA
The Atacama Large Millimeter/submillimeter Array (ALMA), presently under construction, is a revolutionary astronomical interferometer, that will operate at (sub)millimeter wavelengths. With unprecedented sensitivity, resolution, and imaging capability, ALMA will explore the (sub-)mm Universe, one of astronomy's last frontiers. ALMA is expected to provide insight in star- and galaxy formation in the early Universe and to image local star- and planet formation in great detail. The ALMA Commissioning and Science Verification phase is currently in course, preparing the path for Early Science. The Call for ALMA Early Science proposals is expected to be released before the end of 2010. In this contribution we will describe the ALMA project, the array and its receivers, its science goals, and its scientific and technological potential. We will outline the organizational structure of the ALMA Regional Centres, that will play an important role in providing support to the users, with particular attention to the Italian ALMA Regional Centre in Bologna. Finally, we will illustrate what ALMA can contribute to the specific science case of AGN fueling.
14067221
Search for massive protostar candidates in the southern hemisphere - II. Dust continuum emission
In an ongoing effort to identify and study high-mass protostellar candidates we have observed in various tracers a sample of 235 sources selected from the IRAS Point Source Catalog, mostly with $\delta Low sources and 93 High , which are believed to be in different evolutionary stages. Both sub-samples have been studied in detail by comparing their physical properties and morphologies. Massive dust clumps have been detected in all but 8 regions, with usually more than one clump per region. The dust emission shows a variety of complex morphologies, sometimes with multiple clumps forming filaments or clusters. The mean clump has a linear size of ~$ 0.5$ pc, a mass of ~$ 320~M_\odot$ for a dust temperature $T_{\rm d} = 30$ K, an H 2 density of $9.5\times10^5$ cm -3 , and a surface density of 0.4 g cm -2 . The median values are 0.4 pc, $102~M_\odot$, $4\times10^4$ cm -3 , and 0.14 g cm -2 , respectively. The mean value of the luminosity-to-mass ratio, $L/M \simeq 99~L_\odot/M_\odot$, suggests that the sources are in a young, pre-ultracompact Hii  phase. We have compared the millimeter continuum maps with images of the mid-IR MSX emission, and have discovered 95 massive millimeter clumps non-MSX emitters, either diffuse or point-like, that are potential prestellar or precluster cores. The physical properties of these clumps are similar to those of the others, apart from the mass that is ~3 times lower than for clumps with MSX counterpart. Such a difference could be due to the potential prestellar clumps having a lower dust temperature. The mass spectrum of the clumps with masses above $M \sim 100~M_\odot$ is best fitted with a power-law d$N/{\rm d}M \propto M^{-\alpha}$ with $\alpha = 2.1$, consistent with the Salpeter (1955) stellar IMF, with $\alpha=2.35$. On the other hand, the mass function of clumps with masses $10~M_\odot\la M \la 120~M_\odot$ is better fitted with a power law of slope $\alpha = 1.5$, more consistent with the mass function of molecular clouds derived from gas observations.
16713830
Highly deuterated pre-stellar cores in a high-mass star formation region ⋆
Aims. We have observed the deuterated gas in the high-mass star formation region IRAS 05345+3157 at high-angular resolution, in order to determine the morphology and the nature of this gas. Methods. We have mapped the N2H + (1–0) line with the Plateau de Bure Interferometer, and the N2D + (3–2) and N2H + (3–2) lines with the Submillimeter Array.
59289400
Molecular gas and stars in the translucent cloud MBM 18 (LDN 1569)
Context. We investigate star formation in translucent, high-latitu de clouds. Aims. Our aim is to understand the star-formation history and rate in the solar neighbourhood. Methods. We used spectroscopic observations of newly found candidate Hα emission-line stars to establish their pre-main-sequence nature. The environment was studied through molecular line observations of the cloud (MBM 18/LDN 1569) in which the stars are presumably embedded. Results. Ten candidate Hα emission-line stars were found in an objective grism survey of a∼ 1 square degree region in MBM 18, of which seven have been observed spectroscopically in this study. Four of these have weak (| W(Hα)|< 5 A) Hα emission, and six out of seven have spectral types M1−M4 V. One star is of type F7-G1 V, and has Hα in absorption. The spectra of three of the M-stars may show an absorption line of LiI, although none of these is an unambiguous detection. The M-stars lie at distances between∼60 pc and 250 pc, while most distance determinations of MBM 18 found in the literature agree on 120− 150 pc. For the six M-stars a good fit is obtained with pre-main-sequence isochrones indi cating ages between 7.5 and 15 Myr. The mass of the molecular material, derived from the integrated 12 CO(1‐0) emission, is∼ 160 M⊙ (for a distance of 120 pc). This is much smaller than the viria l mass (∼ 10 3 M⊙), and the cloud is not gravitationally bound. Using a clump-finding routine, we identify 12 clumps from the CO-data, with masses between 2.2 and 22 M⊙. All clumps have a virial mass at least six times higher than t heir CO-mass, and thus none are in gravitational equilibrium. A similar situation is found fr om higher-resolution CO-observations of the northern part of the cloud. Conclusions. Considering the relative weakness or absence of the Hα emission, the absence of other emission lines, and the lack of clear LiI absorption, the targets are not T Tauri stars. Wi th ages between 7.5 and 15 Myr they are old enough to explain the lack of lithium in their spectra. Based on the derived distances, some of the stars may lie inside the molecular cloud. From the fact that the cloud as a whole, as well as the individual clumps, are not gravitationally bound, in combination with the ages of the stars we conclude that it is not likely that (these) stars were formed in MBM 18.
120833712
Subterranean ground motion studies for the Einstein Telescope
Seismic motion limits the low-frequency sensitivity of ground-based gravitational wave detectors. A conceptual design study into the feasibility of a future-generation gravitational wave observatory, coined the Einstein Telescope, has been completed. As part of this design phase, we performed a ground motion study to determine the seismic noise characteristics at various sites across the globe. This investigation focused on underground sites and encompassed a variety of geologies, including clay, salt, and hard rock, at 15 locations in nine European countries, the USA, and Japan. In addition, we analyzed data from the Virtual European Broadband Seismograph Network to characterize European seismic motion. We show that, in the region of interest for future-generation gravitational wave detectors (1–10 Hz), seismic motion is dominated by activity from anthropogenic sources. A number of sites were found that exhibited a reduction in seismic power of several orders of magnitude with respect to current detector sites, thus making it possible to set requirements for the Einstein Telescope seismic noise environment.
123641202
Seismic Attenuation System (SAS) in the Kamioka Mine