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Publications (21)46.49 Total impact

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    ABSTRACT: Bright-rimmed clouds (BRCs) are clouds that have been compressed by an external ionization shock front. We present the first high-resolution VLA observations of 20 of these BRCs in the northern hemisphere. We detected water maser emission from three objects: IRAS 21346+5714 (BRC 36), IRAS 21388+5622 (BRC 37), and IRAS 21445+5712 (BRC 39). The low detection rate supports the evidence that BRCs produce mostly low-luminosity objects, for which maser emission is weak and episodic, and suggests that the embedded sources are in a more advanced evolutionary phase than Class 0 objects.
    The Astrophysical Journal 12/2008; 675(2):1352. · 6.73 Impact Factor
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    ABSTRACT: 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 in the on-line appendix. A discussion of the main properties of the H2O variability in our sample will be presented in a forthcoming paper. Comment: 11 pages, 9 figures, to be published in Astronomy and Astrophysics; all plots in appendix (not included) can be downloaded from http://www.arcetri.astro.it/~starform/water_maser_v2.html or http://www.ira.inaf.it/papers/masers/water_maser_v2.html
    Astronomy and Astrophysics 10/2007; · 5.08 Impact Factor
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    ABSTRACT: The Arcetri/Bologna H2O maser group has been monitoring the 1.3-cm water maser emission from a sample of 43 star-forming regions (SFRs) and 22 late-type stars for about 20 years at a sampling rate of 4-5 observations each year, using the 32-m Medicina Radio Telescope (HPBW 1.′9 at 22 GHz). For the late-type stars we observe representative samples of OH/IR-stars, Mira's, semi-regular variables, and supergiants. The SFR-sample spans a large interval in FIR luminosity of the associated Young Stellar Object (YSO), from 20 L to 1.5 × 106 L, and offers a unique data base for the study of the long-term (years) variability of the maser emission in regions of star formation.This presentation concerns only the masers in SFRs. The information obtained from single-dish monitoring is complementary to what is extracted from higher-resolution (VLA and VLBI) observations, and can better explore the velocity domain and the long-term variability therein.We characterize the variability of the sources in various ways and we study how it depends on the luminosity and other properties of the associated YSO and its environment.
    Proceedings of the International Astronomical Union 02/2007; 3:223 - 227.
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    ABSTRACT: Context: Water maser emission is a powerful tracer of the presence of embedded sources in dense clouds since it requires elevated temperatures (>100 K) and densities (>107 cm-3) that can be found in circumstellar disks and/or jets/outflows associated with Young Stellar Objects. Bright rimmed clouds compressed by ionization fronts from nearby massive stars are considered good examples of externally triggered star formation, possibly resulting in the formation of massive stars. Aims: We aim to determine the water maser emission frequency and characteristics of 45 bright rimmed clouds in the southern hemisphere identified by Sugitani & Ogura (1994, ApJS, 92, 163). Methods: We have used the Tidbinbilla 70-m radiotelescope to perform a high sensitivity survey at 22.2 GHz of the maser emission from the 616-523 rotational transition of H2O molecules. Results: We found 7 water maser sources out of 44 (16% detection rate), 5 being new detections. With the exception of the maser associated with BRC 68, all the other maser are characterized by low integrated fluxes and luminosities. Conclusions: Most maser sources fall below the correlation between the H2O and far-infrared luminosity found in other studies towards a variety of star forming regions. These results are similar to those found in the companion survey of BRCs in the northern hemisphere by Valdettaro et al. (2005, A&A, 443, 535). The low detection frequency and the properties of water maser emission from BRCs indicate that low-mass star formation is the most natural outcome of the external compression induced by the ionization front from nearby massive stars. Based on observations obtained with the 70-m Tidbinbilla radiotelescope.
    Astronomy & Astrophysics - ASTRON ASTROPHYS. 01/2007; 466(1):247-253.
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    ABSTRACT: It is well known that water vapor maser emission at 22.2 GHz is associated with the earliest stages of both low- and high-mass star formation and it can be considered a reliable diagnostic of their evolutionary state. Bright Rimmed Clouds (BRCs) are clouds that have been compressed by an external ionization-shock front which focuses the neutral gas into compact globules. The boundary layer between the neutral gas and the gas ionized by the incident photons is often called ``bright rim'' but the clumps are sometimes classified also as speck globules or cometary globules depending on their appearance. Small globules with bright rims have been considered to be potential sites of star formation and have been studied in several individual regions. We present results from high resolution VLA observations searching for new candidates of recent star formation in bright-rimmed clouds/globules associated with IRAS point sources.
    Journal of the Korean Astronomical Society 01/2007; 40:127. · 0.91 Impact Factor
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    ABSTRACT: Bright Rimmed Clouds (BRCs) are clouds that have been compressed by an external ionization-shock front which focuses the neutral gas into compact globules. The boundary layer between the neutral gas and the gas ionized by the incident photons is often called "bright rim" but the clumps are sometimes classified also as speck globules or cometary globules depending on their appearance. Small globules with bright rims have been considered to be potential sites of star formation and have been studied in several individual regions as in Orion and M17. Some systematic surveys of bright-rimmed clouds/globules associated with IRAS point sources have been made searching for new candidates of recent star formation. For some of the 44 objects in the northern sky, evidence has been found for the presence of small clusters of embedded sources of intermediate and high-far-infrared luminosity (L_{FIR} > 10^2 Lsun). We present the first high resolution VLA observations for the three detected sources, from the 20 BRCs observed during the months of February and March 2005: IRAS 21346+5714 (BRC36), IRAS 21388+5622 (BRC37) and IRAS 21445+5712 (BRC39). The low detection rate seems to support the idea that BRCs produce mostly low-luminosity objects, for which maser emission is weak and episodic, and that the embedded sources are in a more advanced evolutionary phase than class 0 objects.
    Revista mexicana de astronomía y astrofísica 01/2006; 26q:161. · 1.20 Impact Factor
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    ABSTRACT: Bright Rim Clouds (BRCs) are clouds that have been compressed by an external ionization-shock front which focuses the neutral gas into compact globules. The boundary layer between the neutral gas and the gas ionized by the incident photons is often called ``bright rim" but the clumps are sometimes classified also as speck globules or cometary globules depending on their appearance. Small globules with bright rims have been considered to be potential sites of low mass star formation and have been studied in several regions, as in Orion and M17. Some systematic surveys of bright-rimmed clouds associated with IRAS point sources have been made searching for new candidates of recent star formation. For some objects in the northern sky, evidence has been found for the presence of small clusters of embedded sources of intermediate and high-far-infrared luminosity (LFIR>102 L&sun;). We present the first VLA observations which have detected H2O maser emission towards 3 sources. The low detection rate seems to support the idea that BRCs produce mostly low-luminosity objects, for which maser emission is weak and episodic, and that the embedded sources are in a more advanced evolutionary phase.
    12/2005;
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    R. Valdettaro, F Palla, J. Brand, R. Cesaroni
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    ABSTRACT: We report the results of a multi-epoch survey of water maser observations at 22.2 GHz with the Medicina radiotelescope from 44 bright rimmed clouds (BRCs) of the northern hemisphere identified by Sugitani et al. (1989) as potential sites of star formation. The data span 16 years of observations and allow to draw conclusions about the maser detection rate in this class of objects. In spite of the relatively high far-infrared luminosities of the embedded sources ($L_{\rm FIR}\ga 10^2$ L$_\odot$), H$_2$O maser emission was detected towards three globules only. Since the occurrence of water masers is higher towards bright IRAS sources, the lack of frequent H$_2$O maser emission is somewhat surprising if the suggestion of induced intermediate- and high-mass star formation within these globules is correct. The maser properties of two BRCs are characteristic of exciting sources of low-mass, while the last one (BRC~38) is consistent with an intermediate-mass object. We argue that most BRCs host young stellar objects of low-luminosity, likely in an evolutionary phase later than the protostellar Class 0 sources, and that a significant contribution to the observd IRAS luminosity comes from warm dust heated by the radiation from the bright rim. Comment: Astronomy and Astrophysics, in press
    Astronomy and Astrophysics 08/2005; · 5.08 Impact Factor
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    ABSTRACT: An overview is given of the analysis of more than a decade of H2O maser data from our monitoring program. We find the maser emission to generally depend on the luminosity of the YSO as well as on the geometry of the SFR. There appears to be a threshold luminosity of a few times 10**4 Lsol above and below which we find different maser characteristics. Comment: 9 pages, 1 table, 11 figures. Accepted for publ. in Ap&SpSci. Refereed contribution to "Dense Molecular Gas around Protostars and in Galactic Nuclei", European Workshop 2004 on Astronomical Molecules. Eds. Y. Hagiwara, W.A. Baan, H.J. van Langevelde
    Astrophysics and Space Science 07/2004; · 2.06 Impact Factor
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    ABSTRACT: (Abridged) H2O masers in 14 SFRs have been monitored once every 2-3 months for up to 13 years. We investigate the dependence of the overall spectral morphology of the maser emission and its variability on the luminosity of the YSO, and look for bursts and gradients in individual components. We find that higher-luminosity sources tend to be associated with stronger and more stable masers.Higher-luminosity YSOs can excite more emission components over a larger range in velocity, yet the emission that dominates the spectra is at a velocity very near that of the molecular cloud in which the objects are embedded. For Lfir > 3E+04 Lo the maser emission becomes increasingly structured and more extended in velocity with increasing Lfir. Below this limit the maser emission shows the same variety of morphologies, but without a clear dependence on Lfir and with a smaller velocity extent. Also, for sources with Lfir above this limit, the water maser is always present above the 5sigma-level; below it, the typical 5sigma detection rate is 75-80%. We conclude that there must be a lower limit to Lfir, below which the associated maser is below the detection level most of the time. These results can be understood in terms of scaled versions of similar SFRs with different YSO luminosities, each with many potential sites of maser amplification, which can be excited provided there is sufficient energy to pump them, i.e. the basic pumping process is identical regardless of the YSO luminosity. In SFRs with lower input energies, the conditions of maser amplification are much closer to the threshold conditions, and consequently more unstable. We find indications that the properties of the maser emission may be determined also by the geometry of the SFR, specifically by the beaming and collimation properties of the outflow driven by the YSO. Comment: 15 pages, with 15 figs, 2 tables. Accepted for publ. in Astron. & Astroph. A version of this paper with higher-resolution versions of Figs. 10-15 can be found on http://www.arcetri.astro.it/~starform/publ2003.htm
    Astronomy and Astrophysics 06/2003; · 5.08 Impact Factor
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    Proceedings of the International Astronomical Union 01/2002; 206:51.
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    ABSTRACT: 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.
    Astronomy and Astrophysics 01/2002; 383(1):244-266. · 5.08 Impact Factor
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    Proceedings of the International Astronomical Union 01/2002; 206:72.
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    Proceedings of the International Astronomical Union 01/2002; 206.
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    ABSTRACT: Table 2 reports the completely updated and revised version of the Arcetri H2O maser Catalogue. It contains all the 1013 sources observed at Medicina (423 detected) and lists one entry per source, usually the one with the highest flux density observed at Medicina. (2 data files).
    VizieR Online Data Catalog. 03/2001;
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    ABSTRACT: We present a second update of the Arcetri Catalog of water masers (Comoretto et al. \cite{Comoretto90}; Brand et al. \cite{Brand94}). 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 his 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{Comoretto90}), Brand et al. (\cite{Brand94}) 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. Based on observations collected with the Medicina 32-m radiotelescope. The Medicina telescope is operated by the Istituto di Radioastronomia, C.N.R., Bologna. Table 2 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/368/845
    Astronomy and Astrophysics 01/2001; 368(3):845-865. · 5.08 Impact Factor
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    F. Palla, C. Codella, R. Valdettaro, C. Baffa
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    ABSTRACT: A high-sensitivity search for water maser emission at 22.2 GHz has been performed on a sample of 91 IRAS point sources at high-latitudes. The aim of the survey is to verify if these clouds are capable of star formation as indicated by the presence of water masers. The sample is based on the recent work of Magnani et al. (1995ApJS...96..159M) who have identified 192 infrared objects from the IRAS Faint Source Survey possibly associated with translucent clouds at galactic latitudes |b|>=30°. These IRAS sources have far-infrared colours typical of young stellar objects and pre-main-sequence stars and thus provide a starting list for further studies about their actual nature. H_2_O maser emission is a good diagnostic of the presence of dense gas and of recent star formation. We did not find water maser emission at a level of 0.2-0.5Jy (3σ) in any of the 91 objects. The negative result indicates that these high-latitude sources do not represent potential sites of star formation, consistent with the fact that most high-latitude molecular clouds do not appear gravitationally bound.
    Astronomy and Astrophysics 10/1997; 327:755-757. · 5.08 Impact Factor
  • F. Palla, C. Codella, R. Valdettaro, C. Baffa
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    ABSTRACT: A high-sensitivity search for water maser emission at 22.2 GHz has been performed on a sample of 91 IRAS point sources at high-latitudes. The aim of the survey is to verify if these clouds are capable of star formation as indicated by the presence of water masers. The sample is based on the recent work of Magnani et al. (1995ApJS...96..159M) who have identified 192 infrared objects from the IRAS Faint Source Survey possibly associated with translucent clouds at galactic latitudes |b|>=30°. These IRAS sources have far-infrared colours typical of young stellar objects and pre-main-sequence stars and thus provide a starting list for further studies about their actual nature. H2O maser emission is a good diagnostic of the presence of dense gas and of recent star formation. We did not find water maser emission at a level of 0.2-0.5Jy (3sigma) in any of the 91 objects. The negative result indicates that these high-latitude sources do not represent potential sites of star formation, consistent with the fact that most high-latitude molecular clouds do not appear gravitationally bound. (2 data files).
    VizieR Online Data Catalog. 07/1997;
  • M. Felli, L. Testi, R. Valdettaro, J.-J. Wang
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    ABSTRACT: We present near infrared broad band (J,H and K) and narrow band (H_2_ S(1)1->0 and Brγ) images, and high resolution molecular observations (C^34^S(2-1), (3-2), (5-4) and ^13^CO(2-1)) around the highly variable H_2_O maser located between the S 235 A and B optical nebulosities. These observations are part of an on-going search for the sources of excitation of H_2_O masers in regions of star formation or, alternatively, for the earliest evolutionary phases of massive stars. We confirm the presence of a highly obscured stellar cluster between S 235 A and B and, from the colour-colour analysis, we show that the cluster contains many sources with infrared excess, which are believed to be Young Stellar Objects (YSOs) in an early evolutionary stage. Diffuse Brγ emission is found mainly in the vicinity of S 235 A, and unresolved Brγ emission is found coincident with S 235 B. Hot molecular hydrogen emission is distributed around the S 235 A nebula, especially in a belt-like region to the south of S 235 A, at the edge of the HII region. The driving source of the H_2_O maser does not appear to be either the YSO inside S 235 A or S 235 B, but is identified with a faint near infrared member of the cluster, with a large (H-K) colour excess, located near the position of the maser. A hot dust envelope around an early type star may be the source of the near IR emission. This identification is supported by the coincidence of the maser and the near IR source with the center of a high density and compact molecular core observed in C^34^S and ^13^CO. The lack of radio continuum emission from the area around the maser suggests that the star powering the maser and responsible for the near IR emission must be in a very early evolutionary stage, highly obscured even at K band and surrounded by an envelope with such a high density that any radio continuum emission is strongly self-absorbed. In any case the evolutionary status of such a star is much earlier than those of the exciting stars of S 235 A and S 235 B. Strong variability of the maser emission and large velocity differences of the maser features with respect to the molecular cloud velocity imply the presence of highly collimated, energetic and short duration jet activity in this YSO. The more evolved members of the cluster S 235 A and S 235 B lie on the sides of the molecular core, suggesting that star formation in the cluster is not coeval but proceeds from the outside towards the core of the molecular cloud.
    Astronomy and Astrophysics 03/1997; 320:594-604. · 5.08 Impact Factor
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    ABSTRACT: 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.
    http://dx.doi.org/10.1051/0004-6361:20011631.