Large Grains in our and External Galaxies

We review some of the evidences that dust properties are dominated by large grains in some astronomical objects. We emphasize the effects of large grains on the extinction and emissivity, but we also discuss some dynamical implications. We also propose a density sequence, where objects characterized by denser environments are also associated with average larger grains. This revised version was published online in September 2006 with corrections to the Cover Date.     

On the structure of the globular cluster M4

On the basis of the work by Greenstein on the globular cluster M 4, the spatial distribution for groups of cluster stars in different stages of evolution is investigated. The comparison between the various samples is made using the apparent distributions. The results are discussed.

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Riassunto Sulla base dello studio di Greenstein sull'ammasso globulare M4, è stata riesaminata la distribuzione spaziale di gruppi di stelle di questo ammasso evolutivamente significativi. Si puntualizza come sia possibile operare un confronto tra le distribuzioni dei vari gruppi sulla base delle distribuzioni apparenti anzichè su quelle reali senza alterare la sostanza dei risultati. Si discutono i risultati.

     

Star formation

Various topics on star formation, centered on the observed properties of young stars and their environment, are reviewed. (a) In our Galaxy, young stellar objects are generally associated with giant molecular clouds. (b) Giant molecular clouds cannot be in free-fall collapse. They are probably stabilized by magnetic fields, which are then likely to dominate the dynamical evolution of the clouds themselves. (c) Star formation occurs mostly in spiral arms. The role of spiral density waves is however not yet clearly understood. (d) The formation of massive stars can perturb the evolution of the progenitor cloud, and possibly trigger the sequential formation of OB subgroups. (e) There is a large number of clouds in the Galaxy associated only with low and intermediate mass young stars. These clouds are not perturbed by the presence of massive stars, and are probably the best source of information on the primary triggering mechanism, active on a galactic scale, and on the initial conditions for star formation.Paper presented at the European Workshop on Planetary Sciences, organised by the Laboratorio di Astrofisica Spaziale di Frascati, and held between April 23–27, 1979, at the Accademia Nazionale del Lincei in Rome, Italy.     

Statistics on the solar spectrum suitable for the study of the blanketing effect in stars of spectral types F,G and K

A complete set of statistics useful to the study of the blanketing effect in stars of spectral type F, G and K have been obtained on the basis of the solar spectrum. In the spectral range 2250 Å-6400 Å the following distributions have been derived: distribution of lines with chemical elements, percentage of lines originating from ionized atoms, distribution of lines with the excitation potential and with the oscillator strength. Also the distribution with the equivalent width has been obtained in the range 3060 Å-6400 Å. With these distributions, samples of lines have been derived for intervals of about 200 Å in the range 2250 Å-6400 Å. Evaluations for the spectral density of lines and the percentage of non-overlapping lines are also included.     

On the relations between theoretical and observed colours in theBVRI system

Transformation relations from theoretical apparent magnitude and (b-v), (v-r), (v-i) colurs to the observedm _v and (B-V), (V-R), (V-I) quantities are derived.Black body colours in theBVRI system are also computed.     

Search for Circumstellar Disks Around Herbig Be Stars

We have carried out a high-sensitivity search for circumstellar disks around Herbig Be stars in the continuum at 1.3 mm and 2.7 mm using the IRAM interferometer at the Plateau de Bure (PdBI). We report data on three Herbig Be stars MWC1080, MWC137 and R Mon. We have observed two of them, MWC137 and R Mon, in the continuum at 0.7 cm and 1.3 cm using the NRAO Very Large Array (VLA). This allows us to build the complete SED at mm and cm wavelengths and fit a simple disk model. We report the detection of circumstellar disks around MWC1080 with M _d = 0.003M _⊙ and R Mon with M _d = 0.01M _⊙. A significant upper limit to the mass of the circumstellar disk around MWC137 has been obtained. Our results show that the ratio M _d/M * is at least an order of magnitude lower in Herbig Be stars than in Herbig Ae and T Tauri stars.     

Current problems on horizontal branch stars

In the light of our present knowledge on stellar evolution, it is shown that red ZAHB will not develop for very low metal Globular Clusters ifY is appreciably greater than 0.1.Recent determinations of the mass-sizes of the He-cores at the time of the He-flash support independently conclusion that, if no increase in surface He-abundance occurs between the turn-off evolutionary phase and the H.B. stages,Y-values lower than 0.2 imply ages of the clusters larger than 20 b.y. By comparing evolutionary paths with the observed populations of the Horizontal Branches we find evidence for mass loss in blue H.B., and evidence for mass dispersion in red H.B. not depopulated in RR Lyrae. A few clusters (M3, M15) have their H.B. dominated by mass dispersion. Finally the expected correlations between mass loss and spatial distribution of H.B. stars are briefly discussed in order to point out some observational possibilities.     

The study of young substellar objects with ALMA

We discuss the potential of ALMA for studying the formation of substellar objects. We first review briefly the various formation mechanisms proposed so far and stress the unique capability of ALMA to detect and study pre-brown dwarf cores and to confirm the core-collapse scenario to the lowest possible masses. We then discuss the properties of disks around substellar objects. We show how it will be possible to detect with ALMA most disks around objects with mass as low as few Jupiter masses, and to resolve spatially their emission in the more favorable cases.     

An X‐shooter survey of star forming regions: Low‐mass stars and sub‐stellar objects

We present preliminary results of our X‐shooter survey in star forming regions. In this contribution we focus on subsamples of young stellar and sub‐stellar objects (YSOs) in the Lupus star forming region and in the TW Hya association. We show that the X‐shooter spectra are suitable for conducting several parallel studies such as YSO + disk fundamental parameters, accretion and outflow activity in the very low‐mass (VLM) and sub‐stellar regimes, as well as magnetic activity in young VLM YSOs, and Li abundance determinations. The capabilities of X‐shooter in terms of wide spectral coverage, resolution and limiting magnitudes, allow us to assess simultaneously the accretion/outflow, magnetic activity, and disk diagnostics, from the UV and optical to the near‐IR, avoiding ambiguities due to possible YSO variability (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Stellar parameters of young brown dwarfs

We present X‐shooter observations of two brown dwarf candidates. We focus on the determination of stellar parameters and their errors. The targets, an accreting class II and a non‐accreting class III objects, are members of the σ Orionis star‐forming region. We derive the spectroscopic spectral types from the VIS spectrum and the stellar parameters. We find that the uncertainties on the stellar parameters have a minor effect on the determination of the mass accretion rate for the accreting star, thus confirming that the discrepancies between the mass accretion rate estimates found with different (simultaneous) tracers are probably due to different physical conditions where the accretion/wind indicators are produced (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)