Clustering Properties of Far-Infrared Sources in Hi-GAL Science Demonstration Phase Fields

The Astrophysical Journal (Impact Factor: 6.73). 04/2011; 735. DOI: 10.1088/0004-637X/735/1/28
Source: arXiv

ABSTRACT We use a Minimum Spanning Tree algorithm to characterize the spatial
distribution of Galactic Far-IR sources and derive their clustering properties.
We aim to reveal the spatial imprint of different types of star forming
processes, e.g. isolated spontaneous fragmentation of dense molecular clouds,
or events of triggered star formation around HII regions, and highlight global
properties of star formation in the Galaxy. We plan to exploit the entire
Hi-GAL survey of the inner Galactic plane to gather significant statistics on
the clustering properties of star forming regions, and to look for possible
correlations with source properties such as mass, temperature or evolutionary
stage. In this paper we present a pilot study based on the two 2x2 square
degree fields centered at longitudes l=30 and l=59 obtained during the Science
Demonstration Phase (SDP) of the Herschel mission. We find that over half of
the clustered sources are associated with HII regions and infrared dark clouds.
Our analysis also reveals a smooth chromatic evolution of the spatial
distribution where sources detected at short-wavelengths, likely proto-stars
surrounded by warm circumstellar material emitting in the far-infrared, tend to
be clustered in dense and compact groups around HII regions while sources
detected at long-wavelengths, presumably cold and dusty density enhancements of
the ISM emitting in the sub-millimeter, are distributed in larger and looser

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    Monthly Notices of the Royal Astronomical Society 09/2011; · 5.52 Impact Factor
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    ABSTRACT: Context. Stars are born deeply embedded in molecular clouds. In the earliest embedded phases, protostars emit the bulk of their radiation in the far-infrared wavelength range, where Herschel is perfectly suited to probe at high angular resolution and dynamic range. In the high-mass regime, the birthplaces of protostars are thought to be in the high-density structures known as infrared-dark clouds (IRDCs). While massive IRDCs are believed to have the right conditions to give rise to massive stars and clusters, the evolutionary sequence of this process is not well-characterized. Aims: As part of the Earliest Phases of Star formation (EPoS) Herschel guaranteed time key program, we isolate the embedded structures within IRDCs and other cold, massive molecular clouds. We present the full sample of 45 high-mass regions which were mapped at PACS 70, 100, and 160 μm and SPIRE 250, 350, and 500 μm. In the present paper, we characterize a population of cores which appear in the PACS bands and place them into context with their host molecular cloud and investigate their evolutionary stage. Methods: We construct spectral energy distributions (SEDs) of 496 cores which appear in all PACS bands, 34% of which lack counterparts at 24 μm. From single-temperature modified blackbody fits of the SEDs, we derive the temperature, luminosity, and mass of each core. These properties predominantly reflect the conditions in the cold, outer regions. Taking into account optical depth effects and performing simple radiative transfer models, we explore the origin of emission at PACS wavelengths. Results: The core population has a median temperature of 20 K and has masses and luminosities that span four to five orders of magnitude. Cores with a counterpart at 24 μm are warmer and bluer on average than cores without a 24 μm counterpart. We conclude that cores bright at 24 μm are on average more advanced in their evolution, where a central protostar(s) have heated the outer bulk of the core, than 24 μm-dark cores. The 24 μm emission itself can arise in instances where our line of sight aligns with an exposed part of the warm inner core. About 10% of the total cloud mass is found in a given cloud's core population. We uncover over 300 further candidate cores which are dark until 100 μm. These are possibly starless objects, and further observations will help us determine the nature of these very cold cores.
    Astronomy and Astrophysics 11/2012; 547:A49. · 5.08 Impact Factor

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