Marcello Felli

Solar system-sized condensations in the Orion Nebula

New observations of 22 ultracompact radio sources toward the Trapezium cluster and toward the core of the Kleinmann-Low nebula are reported. The diameters of the sources range from less than 3.7 x 10 to the 14th cm to about 3.4 x 10 to the 15th cm. Their emission measures are typically greater than 10 to the 8th pc/cm exp 6, their electron densities are greater than or equal to 10 to the 6th/cu cm, and most are optically thin at 2 cm. Fifteen are associated with stars. Two interpretations for the optically visible nebular condensations are discussed. They may be dense, molecular globules embedded in the diffuse H II region with envelopes ionized by the Trapezium stars, or they may be low-mass, premain-sequence stars whose accretion disks are slowly being evaporated by the Trapezium stars. Properties and mass-loss rates are derived for the objects not associated with visible stars. 39 references.

The Arcetri Catalog of H2O maser sources: Update 2000

We present a second update of the Arcetri Catalog of water masers (Comoretto et al. [CITE]; Brand et al. [CITE]). The present study reports the results of the observations carried out with the Medicina 32-m radiotelescope from January 1993 to April 2000 on a sample of 300 sources. This compilation consists of newly discovered maser sources that did not appear in the previous Arcetri Catalogs and is made of: a) detections from the literature, and b) unpublished detections obtained with the Medicina antenna. Overall, 83 out of 300 sources were detected. The detection rate is low (28% ) and we attribute this result to the inclusion in our survey of a rather large number of spurious maser detections that have appeared in one particular paper. The observational parameters are reported in tabular form for all the 300 sources and the spectra of the detected masers are presented. We discuss the global properties of the complete Arcetri Catalog based on Comoretto et al. ([CITE]), Brand et al. ([CITE]) and the present observations, which now contains 1013 galactic water maser sources. Of these, 937 have an IRAS counterpart within 1 arcmin from the nominal position of the maser. We establish a classification scheme based on the IRAS flux densities which allows to distinguish between water masers associated with star forming regions and late-type stars. The Arcetri Catalog represents a useful data base for systematic studies of galactic water maser sources.

Mass-Losing Semiregular Variable Stars in Baade's Windows

By cross-correlating the results of two recent large-scale surveys, the general properties of a well-defined sample of semiregular variable stars have been determined. ISOGAL mid-infrared photometry (7 and 15 ?m) and MACHO V and R light curves are assembled for approximately 300 stars in the Baades windows of low extinction toward the Galactic bulge. These stars are mainly giants of late M spectral type, evolving along the asymptotic giant branch (AGB). They are found to possess a wide and continuous distribution of pulsation periods and to obey an approximate log P-Mbol relation or set of such relations. Approximate mass-loss rates in the range of ~1 ? 10-8 to 5 ? 10-7 M? yr-1 are derived from ISOGAL mid-infrared photometry and models of stellar spectra adjusted for the presence of optically thin circumstellar silicate dust. Mass-loss rates depend on luminosity and pulsation period. Some stars lose mass as rapidly as short-period Mira variables but do not show Mira-like amplitudes. A period of 70 days or longer is a necessary but not sufficient condition for mass loss to occur. For AGB stars in the mass-loss ranges that we observe, the functional dependence of mass-loss rate on temperature and luminosity can be expressed as T?L?, where ? = -8.80 and ? = +1.74, in agreement with recent theoretical predictions. If we include our mass-loss rates with a sample of extreme mass-losing AGB stars in the Large Magellanic Cloud and ignore T as a variable, we get the general result for AGB stars that L2.7, valid for AGB stars with 10-8 < < 10-4 M? yr-1.

Water maser variability over 20 years in a large sample of star-forming regions: the complete database

Context. Water vapor emission at 22 GHz from masers associated with star-forming regions is highly variable. Aims. We present a database of up to 20 years of monitoring of a sample of 43 masers within star-forming regions. The sample covers a large range of luminosities of the associated IRAS source and is representative of the entire population of H2O masers of this type. The database forms a good starting point for any further study of H2O maser variability. Methods. The observations were obtained with the Medicina 32-m radiotelescope, at a rate of 4−5 observations per year. Results. To provide a database that can be easily accessed through the web, we give for each source: plots of the calibrated spectra, the velocity-time-flux density plot, the light curve of the integrated flux, the lower and upper envelopes of the maser emission, the mean spectrum, and the rate of the maser occurrence as a function of velocity. Figures for just one source are given in the text for representative purposes. Figures for all the sources are given in electronic form the appendix. A discussion of the main properties of the H2O variability in our sample will be presented in a forthcoming paper.

Young massive stars in the ISOGAL survey ?;??;??? II. The catalogue of bright YSO candidates

The 7 and 15m observations of selected fields in the Galactic Plane obtained with ISOCAM during the ISOGAL program oer a unique possibility to search for previously unknown YSOs, undetected by IRAS because of lower sensitivity or confusion problems. In a previous paper (Felli et al. 2000) we established criteria of general validity to select YSOs from the much larger population of Post Main Sequence (Post-MS) stars present in the ISOGAL fields by comparing radio and IR observations of five fields located at l +45. The selection was based primarily on the position of the point sources in the (15)-(7)-(15) diagram, which involves only ISOGAL data and allows to find possible YSOs using the survey data alone. In the present work we revise the adopted criteria by comparing radio-identified UC HII regions and ISOGAL observations over a much larger region. The main indications of the previous analysis are confirmed, but the criteria for selecting YSO candidates had to be revised to select only bright objects, in order to limit the contamination of the sample by Post-MS stars. The revised criteria ((15) 4:5, (7) (15) 1:8) are then used to extract YSO candidates from the ISOGAL Point Source Catalogue in preparation. We select a total of 715 YSO candidates, corresponding to2% of the sources with good detections at 7 and 15m. The results are presented in a table form that provides an unique input list of small diameter,6 00 , Galactic YSO candidates. The global properties of the sample of YSO candidates are briefly discussed.

New signposts of massive star formation in the S235A-B region

Aims. We report on new aspects of the star-forming region S235AB revealed through high-resolution observations at radio and midinfrared wavelengths. Methods. Using the Very Large Array, we carried out sensitive observations of S235AB in the cm continuum (6, 3.6, 1.3, and 0.7) and in the 22 GHz water maser line. These were complemented with Spitzer Space Telescope Infrared Array Camera archive data to clarify the correspondence between radio and IR sources. We made also use of newly presented data from the Medicina water maser patrol, started in 1987, to study the variability of the water masers found in the region. Results. S235A is a classical HII region whose structure is now well resolved. To the south, no radio continuum emission is detected either from the compact molecular core or from the jet-like structure observed at 3.3 mm, suggesting emission from dust in both cases. We find two new compact radio continuum sources (VLA-1 and VLA-2) and three separate maser spots. VLA-1 coincides with one of the maser spots and with a previously identified IR source (M 1). VLA-2 lies towards S235B and represents the first radio detection from this peculiar nebula that may represent an ionized wind from a more evolved star. The two other maser spots coincide with an elongated structure previously observed within the molecular core in the C 34 S line. This structure is perpendicular to a bipolar molecular outflow observed in HCO + (1−0) and may trace the associated equatorial disk. The Spitzer images reveal a red object towards the molecular core. This is the most viable candidate for the embedded source originating the outflow and maser phenomenology. Conclusions. The picture emerging from these and previous data shows the extreme complexity of a small (≤0.5 pc) star-forming region where widely different stages of stellar evolution are present.

Long-term study of water maser emission associated with young stellar objects - I. The database

We present the results of more than 10 years of monitoring of the water vapor maser emission in 14 star forming regions obtained with the Medicina 32-m radiotelescope. The sample of objects covers a large range of luminosities of the associated FIR sources. In order to present in a compact and representative way the large amount of data available, we give for each source: a time-velocity-flux density plot, the time dependent integrated flux, the lower and upper envelopes of the maser emission, the frequency of the maser occurrence as a function of velocity, and the mean velocity averaged over the observing period. A brief morphological description of the environment of the maser source is also given. The present data form the basis for a discussion of the main properties of the water vapor maser emission to be presented in a forthcoming paper.

New light on the S235A-B star forming region

The S235A-B star forming region has been extensively observed in the past from the radio to the near IR, but what was happening in the immediate surroundings of the water maser, placed in between the two nebulosities, was still unclear because of insufficient resolution especially in the spectral range from the Far IR to the mm, even though there were sound indications that new young stellar objects (YSOs) are being formed there. We present here new high resolution maps at mm wavelengths in different molecules (HCO + ,C 34 S, H2CS, SO2 and CH3CN), as well as in the 1.2 and 3.3 mm continuum obtained with the Plateau de Bure interferometer, and JCMT observations at 450 µm and 850 µm that unambiguously reveal the presence of new YSOs placed in between the two H regions S235A and S235B and associated with the water maser. A molecular core and an unresolved source in the mm and in the sub-mm are centred on the maser, with indication of mass infall onto the core. Two molecular bipolar outflows and a jet originate from the same position. Weak evidence is found for a molecular rotating disk perpendicular to the direction of the main bipolar outflow. The derived parameters indicate that one of the YSOs is an intermediate luminosity object (L ∼ 10 3 L� ) in a very early evolutionary phase, embedded in a molecular core of ∼100 M� , with a temperature of 30 K. The main source of energy for the YSO could come from gravitational infall, thus making of this YSO a rare example of intermediate luminosity protostar representing a link between the earliest evolutionary phases of massive stars and low mass protostars of class 0-I.

The structure and intensity evolution of a solar burst at 2.8 cm and the relation with the soft X-ray emitting region

The decay phase of a microwave burst was observed with a one-dimensional angular resolution of 16″ at 2.8 cm. The structure was found to be composed of several bursting regions with different evolution. The event was also observed in soft X-rays by full-disk detectors. The joint analysis of these data suggests that the complete event has thermal origin and that the soft X-ray emission is associated with the rapidly varying component of the radio structure. From these results the average values of the electron temperatures and densities were computed for each component.

YOUNG MASSIVE STARS IN THE ISOGAL SURVEY. I. VLA OBSERVATIONS OF THE ISOGAL L = +45 FIELD

We present VLA radio continuum observations at 3.6 and 6 cm of a ~0.65 sq. deg. field in the galactic plane at l = +45°. These observations are meant to be used in a comparison with ISO observations at 7 and 15 μm of the same region. In this paper we compare the radio results with other radio surveys and with the IRAS-PSC. At 3.6 and/or 6 cm we detect a total of 34 discrete sources, 13 of which are found in five separate extended complexes. These are all multiple or single extended thermal ultra-compact HII (UCHII) regions. While for each of these complexes an IRAS counterpart could be reliably found, no IRAS counterpart could be reliably identified for any of the remaining 21 sources. Of these 21 compact sources, six are candidate UCHII regions, and the other 15 are most probably background extragalactic non-thermal sources. The five IRAS sources associated with the radio continuum complexes all satisfy the [Wood & Churchwell (1989a]WC89; WC89) color criteria for UCHII. None of the other 38 IRAS point sources present in our surveyed field show the same colors. This fraction of WC89 type to total IRAS sources is consistent with what is found over the entire galactic plane. The fact that, when observed with a compact VLA configuration, the IRAS sources with “UCHII colors” are found to be associated with arcminute-scale extended sources, rather than with compact or unresolved radio sources, may have important implications on the estimated lifetime of UCHII regions.