Letter to the Editor

A Herschel study of YSO evolutionary stages and formation timelines in two fields of the Hi-GAL survey^*

¹ Istituto di Fisica dello Spazio Interplanetario – INAF, via Fosso del Cavaliere 100, 00133 Rome, Italy 
e-mail: davide.elia@ifsi-roma.inaf.it
² Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal

³ Dipartimento di Scienze Fisiche, Università di Napoli “Federico II”, Naples, Italy

⁴ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

⁵ Department of Physics, University of Arizona, 1118 E. 4th Street, Tucson, AZ 85721, USA 

⁶ Center for Astrophysics and Space Astronomy, University of Colorado, Boulder, CO 80309-0389, USA

⁷ NASA Herschel Science Center, IPAC, Caltech, Pasadena, CA 91125, USA

⁸ ASI Science Data Center, 00044 Frascati, Rome, Italy 

⁹ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China 

¹⁰ Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8, Canada

¹¹ Department of Astronomy & Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada

¹² Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD, UK 

¹³ Laboratoire AIM, CEA/IRFU CNRS Université Paris Diderot, Service d'Astrophysique, 91191 Gif-sur-Yvette, France

¹⁴ School of Physics, University of Exeter, Stocker Road, Exeter, EX4 4QL, UK

¹⁵ Dipartimento di Fisica, Università di Roma 2 “Tor Vergata”, Rome, Italy

¹⁶ University of Puerto Rico, Rio Piedras Campus, Physics Dept., Box 23343, UPR station, San Juan, Puerto Rico, USA

¹⁷ INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125, Firenze, Italy

¹⁸ Spitzer Science Center, IPAC, MS 220-6, California Institute of Technology, Pasadena, CA 91125, USA

¹⁹ Dipartimento di Fisica, Università di Roma 1 “La Sapienza”, Rome, Italy 

²⁰ Centre for Astrophysics and Planetary Science, University of Kent, Canterbury CT2 7NH, UK

²¹ Dipartimento di Fisica, Università del Salento, CP 193, 73100 Lecce, Italy

²² ESO, Karl Schwarzschild-Strasse 2, 85748 Garching bei München, Germany

²³ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK


Received: 31 March 2010
Accepted: 16 April 2010

Abstract

We present a first study of the star-forming compact dust condensations revealed by Herschel in the two 2° × 2° Galactic Plane fields centered at [, b] = [30°, 0°] and [, b] =[59°, 0°] , respectively, and observed during the science demonstration phase for the Herschel Infrared GALactic plane survey (Hi-GAL) key-project. Compact source catalogs extracted for the two fields in the five Hi-GAL bands (70, 160, 250, 350 and 500 μm) were merged based on simple criteria of positional association and spectral energy distribution (SED) consistency into a final catalog which contains only coherent SEDs with counterparts in at least three adjacent Herschel bands. These final source lists contain 528 entries for the = 30° field, and 444 entries for the = 59° field. The SED coverage has been augmented with ancillary data at 24 μm and 1.1 mm. SED modeling for the subset of 318 and 101 sources (in the two fields, respectively) for which the distance is known was carried out using both a structured star/disk/envelope radiative transfer model and a simple isothermal grey-body. Global parameters like mass, luminosity, temperature and dust properties have been estimated. The L_bol/M_env ratio spans four orders of magnitudes from values compatible with the pre-protostellar phase to embedded massive zero-age main sequence stars. Sources in the = 59° field have on average lower L/M, possibly outlining an overall earlier evolutionary stage with respect to the sources in the = 30° field. Many of these cores are actively forming high-mass stars, although the estimated core surface densities appear to be an order of magnitude below the 1 g cm^-2 critical threshold for high-mass star formation.