Gravitational fragmentation caught in the act: the filamentary Musca molecular cloud

Abstract

Context. Filamentary structures are common in molecular clouds. Explaining how they fragment to dense cores is a missing step in understanding their role in star formation. Aims. We perform a case study of whether low-mass filaments are close to hydrostatic prior to their fragmentation, and whether their fragmentation agrees with gravitational fragmentation models. To accomplish this, we study the ∼6.5 pc long Musca molecular cloud, which is an ideal candidate for a filament at an early stage of fragmentation. Methods. We employ dust extinction mapping, in conjunction with near-infrared JHK S -band data from the CTIO/NEWFIRM instrument, and 870 μm dust continuum emission data from the APEX/LABOCA instrument to estimate column densities in Musca. We use the data to identify fragments from the cloud and to determine the radial density distribution of its filamentary part. We compare the cloud's morphology with 13 CO and C 18 O line emission observed with the APEX/SHeFI instrument. Results. The Musca cloud is pronouncedly fragmented at its ends, but harbors a remarkably well-defined, ∼1.6 pc long filament in its center region. The line mass of the filament is 21-31 M pc −1 and the full width at half maximum (FWHM) 0.07 pc. The radial profile of the filament can be fitted with a Plummer profile, which has the power-index of 2.6 ± 11% and is flatter than that of an infinite hydrostatic filament. The profile can also be fitted with a hydrostatic cylinder truncated by external pressure. These models imply a central density of ∼5-10 × 10 4 cm −3 . The fragments in the cloud have a mean separation of ∼0.4 pc, in agreement with gravitational fragmentation. These properties, together with the subsonic and velocity-coherent nature of the cloud, suggest a scenario in which an initially hydrostatic cloud is currently gravitationally fragmenting. The fragmentation started a few tenths of a Myr ago from the ends of the cloud, leaving its center still relatively nonfragmented, possibly because of gravitational focusing in a finite geometry.

The Link Between Turbulence, Magnetic Fields, Filaments, and Star Formation in the Central Molecular Zone Cloud G0.253+0.016

“…Here we find that G0.253 +0.016 in the CMZ has similar filament properties as seen in spiral-arm clouds, e.g., that over-dense regions are located along filaments (see Figure 1). It is remarkable that the filament width of 0.05 0.15 pc -found in observations of nearby spiralarms clouds in the Milky Way is close to universal (Arzoumanian et al 2011;Benedettini et al 2015;Federrath 2016;Kainulainen et al 2016). It is even more remarkable that we find here a filament width of =  W 0.17 0.08 pc fil for the CMZ cloud G0.253+0.016, consistent with W fil in the solar neighborhood.…”

Slingshot mechanism in Orion: Kinematic evidence for ejection of protostars by filaments

“…For Musca, we derive λ(0.1 pc) = 25 M /pc from Fig. 5 of Kainulainen et al (2016). That is, the ISF has a factor 3−6 higher line density that these nearby filaments.…”

Resolving the fragmentation of high line-mass filaments with ALMA: the integral shaped filament in Orion A

“…Another possibility could be large-enough, pre-existing density fluctuations that can grow and collapse faster than the global, longitudinal collapse ensues (Larson 1985;Pon et al 2011). Supporting this possibility, filaments with low star formation activity have been observed to contain significant density fluctuations (e.g., Roy et al 2015;Kainulainen et al 2016). Developing this cartoon framework into a coherent theory is beyond this paper, but it would be a crucial topic for future works that aim at understand the fragmentation of highly super-critical filaments.…”

“…Filamentary structures are fundamental building blocks of the molecular clouds of the interstellar medium (ISM), manifesting themselves over wide ranges of sizes (∼0.1-100 pc), masses (∼1-10 5 M ⊙ ), and line-masses ( 1 000 M ⊙ pc −1 ) (e.g., Bally et al 1987;Hacar et al 2013;Alves de Oliveira et al 2014;Kainulainen et al 2013Kainulainen et al , 2016Abreu-Vicente et al 2016). Specifically, filaments that have line-masses greatly in excess to the critical value of the self-gravitating, thermally supported, non-magnetised, infinitely long equilibrium model, i.e., ≫16 M ⊙ pc −1 (Ostriker 1964), contain large enough mass reservoirs to give birth to high-mass stars and star clusters (e.g., Pillai et al 2006;Beuther et al 2010Beuther et al , 2015aHenning et al 2010;Schneider et al 2012;Kainulainen et al 2013;Stutz & Gould 2016;Contreras et al 2016).…”