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Balloon-borne Large-Aperture Submillimeter Telescope

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Star Formation in Cold Cores

Introduction

Much of the interstellar medium (ISM) is filled with a mix of optically-opaque dust and molecular gas. The diffuse dust attenuates optical and ultraviolet (UV) light from background stars. At certain locations instabilities in the ISM allow it to gravitationally collapse — increasing the density drastically, in some cases enough for a star (or group of stars) to form in the middle. Once a star has formed the increase in power output can raise the temperature of its immediate surroundings. Young stars can produce outflows, and push the parent cloud material outward. Very massive, short-lived O and B stars produce enough UV radiation to photodissociate molecular gas around them. Over time these young stellar objects either destroy the clouds from which they were born, or simply drift away from them, eventually becoming typical “naked” main-sequence stars. All of these processes make it difficult to learn about the “pre-conditions” required for star-formation to occur.

Axhead Molecular Cloud
Revealing hidden dust: In the left picture, taken in visible light as part of the Palomar all sky survey, a subtle shadow from an invisible object blocks starlight from stars behind it. In the right picture, taken with BLAST in the submillimeter at 0.35mm wavelength, we clearly see the source of the shadow - a giant molecular cloud has formed. This future birthplace of stars has cooled to about 10 K, and is filled with hundreds of dense sources at the earliest stages of star formtion.

At optical wavelengths the very first stages of this process are completely invisible. At the densities of star-forming cores all of the optical-UV light is completely absorbed by the dust, heating it to ~10–30 K. At these temperatures the the dust radiates most of its power as a near-blackbody at long wavelengths (> 100 μm), where the dust is completely transparent. Stars only become optically visible once the material in their vicinity reaches sufficiently low densities (and have therefore already evolved for some time). Radio observations can detect free-free emission from slightly younger (and massive) objects whose first UV light ionizes their immediate environment. This light is also emitted at long wavelengths where the dust is optically thin, but of course requires the presence of a young massive star capable of producing ionizing radiation. The only way to see the very earliest phase of star-formation that precedes this is to observe the cold-dust re-processed light directly, which is emitted at wavelengths ~1000–100 μm, the submillimeter band.

Over the last 15 years submillimeter imaging arrays operating from the ground, such as SCUBA on the JCMT and MAMBO on the IRAM 30-m, have enabled the first detailed studies of these pre-stellar collapsed (or collapsing) cores. Surveys now covering several tens of square degrees across a number of known star-forming regions in the Galactic plane have uncovered large numbers of compact peaks in the submm emission, many of which are new objects, and which may form (or are forming) new stars. However, this first generation of surveys is limited by the inability to measure total luminosities and temperatures (and hence masses) of cores accurately, since they typically only sample a single point on their SEDs. While plausible temperatures can be assumed in order to extrapolate from these single-wavelength measurements to produce order-of-magnitude results, multi-band imaging near the peak of the thermal SED (~100–500 μm) is required to accurately constrain their SEDs. This is where BLAST comes in.

BLAST surveyed ~20 deg² of the Galactic plane visible from the northern hemisphere during its 2005 flight, and over 200 deg² of the southern plane during its 2006 flight (including a particularly deep map covering 50 deg² in the Vela molecular ridge). First results from the both flights have now been published, and may be accessed from the results page.

BLAST map of the VMR
A new view of star formation: a 50 square degree submillimeter map of a portion of the Galactic Plane made with BLAST. It is the result of 21 hours of observing spaced throughout the 2006 flight of BLAST. This field is dominated by relatively local emission, with the Perseus arm approximatly 10 kpc behind it. 

The image contains objects at all stages of evolution, including: a large cool cloud with little evidence for recent star formation over much of it (region C); cool dust arranged in linear structures (e.g., region Y); clouds which have been substantially heated by star formation (e.g., region X); and the clumpy interstellar medium warmed up in a roughly circular structure by massive young stars (e.g., region Z). Also very striking in the maps are the large numbers of compact sources throughout the image --- particularly in cooler regions. Given the large size of the map, the wide range of physical conditions, and the unbiased nature of the field selection, it is reasonable to take the molecular gas normalized core counts and mass function for this field as representative of the Galaxy as a whole. For details and results see the Vela paper.

Results

Our unbiased, 50 deg2 submillimeter Galactic survey at 250, 350, and 500 µm from the 2006 flight of the BLAST has resolution ranging from 36 to 60 arceconds in the three submillimeter bands spanning the thermal emission peak of cold pre-stellar cores. From it, we have determined the temperature, luminosity, and mass of more than a thousand compact sources in a range of evolutionary stages and an unbiased statistical characterization of the population. From comparison with C18O data, we find the dust opacity per gas mass, κ/R = 0.16 cm2g-1 at 250 µm, for cold clumps. We find that 2% of the mass of the molecular gas over this diverse region is in cores colder than 14 K, and that the mass function for these cold cores is consistent with a power law with index α= −3.22+/−0.14 over the mass range 14MSun < M < 80 MSun, steeper than the Salpeter α= −2.35 for the initial mass function of stars. Additionally, we infer a mass dependent cold core lifetime of τ(M)= 4 ×106(M/20MSun)−0.9 years – longer than what has been found in previous surveys of either low or high mass cores, and significantly longer than free fall or turbulent decay time scales. This implies some form of non-thermal support for cold cores during this early stage of star formation.

 

Send questions or comments to Mark Devlin