The Free-fall Time of Finite Sheets and Filaments

Details The Free-fall Time of Finite Sheets and Filaments The Free-fall Time of Finite Sheets and Filaments

Jan 2012

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012aas...21934118v&link_type=abstract

American Astronomical Society, AAS Meeting #219, #341.18

Astronomy and Astrophysics

Astronomy

Scientific paper

Molecular clouds often exhibit filamentary or sheet-like shapes. We compute the free-fall time (\tff) for finite, uniform, self-gravitating circular sheets and filamentary clouds of small but finite thickness, so that their volume density ρ can still be defined. We find that, for thin sheets, the free-fall time is larger than that of a uniform sphere with the same volume density by a factor proportional to \sqrt{A}, where the aspect ratio A is given by A=R/h, R being the sheet's radius and h is its thickness. For filamentary clouds, the aspect ratio is defined as A=L/R, where L is the filament's half length and R is its (small) radius, and the modification factor is a more complicated, although in the limit of large A it again reduces to nearly \sqrt{A}. We propose that our result for filamentary shapes naturally explains the ubiquitous configuration of clumps fed by filaments observed in the densest structures of molecular clouds. Also, the longer free-fall times for non-spherical geometries in general may contribute towards partially alleviating the ``star-formation conundrum'', namely, that the star formation rate in the Galaxy appears to be proceeding in a timescale much larger than the total molecular mass in the Galaxy divided by its typical free-fall time. If molecular clouds are in general formed by thin sheets and long filaments, then their relevant free-fall time may have been systematically underestimated, possibly by factors of up to one order of magnitude.

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