A false-color image map of the gas density in the Musca star-forming filament (the highest densities are shown in red). New theoretical work on the structure of these long filaments proposes several kinds of star-forming zones along the length and successfully reproduces many of the features seen in filaments like this one in Musca.
Credit: Kainulainen, 2016
Interstellar molecular clouds are often seen to be elongated and "filamentary" in shape, and come in a wide range of sizes. In molecular clouds, where stars form, the filamentary structure is thought to play an important role in star formation as the matter collapses to form protostars. Filamentary clouds are detected because the dust they contain obscures the optical light of background stars while emitting at infrared and submillimeter wavelengths. Observations of some filaments indicate that they are themselves composed of bundles of closely spaced fibers with distinct physical properties.
Computer simulations are able to
reproduce some of these filamentary structures, and astronomers
generally agree that turbulence in the gas combined with gravitational
collapse can lead to filaments and protostars within them, but the exact
ways in which filaments form, make stars, and finally dissipate are not
understood. The number of new stars that develop, for example, varies
widely between filaments for reasons that are not known.
The usual model for a star forming filament is a cylinder whose
density increases towards the axis according to a specific profile, but
which otherwise is uniform along its length. CfA astronomer Phil Myers
has developed a variant of this model in which the filament has a
star-forming zone along its length where the density and diameter are
higher, with three generic profiles to describe their shapes. Besides
being a more realistic description of a filament's structure, the
different density profiles develop different strength gravitational
"wells" naturally leading to different numbers of stars forming within
them.
Myers compares the star formation properties of these three kinds of
zones with the properties of observed star formation filaments, with
excellent results. The filament in the molecular cloud in Musca has
relatively little star formation, and can be reasonably well explained
with one of the three profiles indicative of an early stage of
evolution. A small cluster of young stars in the Corona Australis
constellation fits a second model that has evolved for longer, while
Ophiuchus hosts a filament that may be near the end of its star forming
lifetime and resembles the third type. The three profiles so far seem
able to account for the full range of conditions. The new results are an
important step in bringing more sophistication and realism to the
theory of star forming filaments. Future work will probe the specific
processes that fragment the various star-forming zones into their stars.
