Star Formation in the VELA Molecular Clouds - Part Two - the Luminosity Function of the Class-I Sources

Details Star Formation in the VELA Molecular Clouds - Part Two - the Luminosity Function of the Class-I Sources Star Formation in the VELA Molecular Clouds - Part Two - the Luminosity Function of the Class-I Sources

Aug 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...275..489l&link_type=abstract

Astronomy and Astrophysics, Vol.275, NO.2/AUG(II), P. 489, 1993

Astronomy and Astrophysics

Astrophysics

26

Stars: Formation, Stars: Pre-Main-Sequence, Stars: Luminosity Function, Mass Function, Ism: Individual Objects: Vela Clouds

Scientific paper

We present the results of an unbiased survey of a flux limited sample of red IRAS point sources [Fν(12 μm) ≥ 1.0 Jy] in a region of the southern sky being situated close to the plane of the Galaxy and containing the Giant Molecular Clouds (GMCs) of the Vela Molecular Ridge (VMR). From multifrequency observations we identify the Lada & Wilking (LW) Class I sources of low to intermediate luminosity in the actively star forming VMR. These clouds have probably been forming stars for at least some 106 years and the overall efficiency is estimated to lie in the range 5 - 15%, i.e. being comparable to the average value found for the solar neighbourhood. The shape of the bolometric luminosity function of the Class I sources of the VMR is similar to that observed in other star forming regions of the solar neighbourhood. These LFs are commonly very fiat and can be fit by a mean power law of index 0.4±0.2 over five orders of magnitude in luminosity, i.e. the form of the Class I LFs appears invariant under luminosity translation. Such similarity suggests that the shape of the Class I LFs is not strongly dependent on the different cloud properties, which can vary appreciably from cloud to cloud. For the VMR, we have also obtained a statistically more reliable Class I LF, containing more than 102 sources and which is the result of subtracting the source content of an adjacent reference field being void of molecular material at the LSR-velocities of the VMR. Besides of also being fiat, this LF does furthermore reveal that a single power law representation seems inadequate over the observed range in luminosity. We identify the indicated power law break tentatively as the result of a turnover of the Class I LF at lower luminosities. Such turnover has been predicted by some recent theories of protostellar evolution and is introduced by the development of a characteristic mass scale during the mass accretion time of the star forming cloud. Our observations would thus lend support to the evolutionary model according to which the LW-Class I sources, as a class, are protostars acereting matter at (normally) relatively high rates. It would further be implied that the prevailing accretion rates are not predominantly determined by the mass of the acereting protostar, but largely by the effective isothermal sound speeds of the cloud. For a given star forming cloud, this would determine the exact location of the peak of the LF, thus introducing a dependence on global properties of the cloud. We argue that in typical GMCs like the VMR a Salpeter-like IMF is determined already by the time of appearance of the Class I sources, whose mass spectrum is that observed for molecular cloud clumps. Any initial differences in local IMFs produced by, e.g., less massive molecular clouds would become `ironed out' with time, as the GMCs develop stellar mass functions similar to the field star IMF at comparable efficiency. We present, finally, some heuristic arguments which indicate that the average mass column through a molecular cloud provides an observable measure of the efficiency at which the cloud is producing stars.

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