Fragmentation of star-forming filaments in the X-shape Nebula of the California molecular cloud

Abstract

Aims. We aim to further investigate the fragmentation of star-forming filaments in the X-shape Nebula of the California molecular cloud, so as to better understand the exact role of filaments in the early stages of star formation. Methods. We applied getsources and getfilaments extraction methods to the multi-wavelength images observed with Herschel in order to produce background-subtracted images of candidate cores and filaments, respectively. The properties of the filaments and cores were determined from such an analysis. A map of 13 CO(2 − 1) emission from the SMT 10m telescope was also used to constrain the dynamical state of the filaments. Results. We obtained a complete sample of filamentary structures and dense cores from the Herschel high-resolution (18.2 ) H 2 column density map of the region. We selected 10 filaments with elongation factors E > 4 and column density contrasts C > 0.5 from the filamentary network identified with getfilaments. All 10 filaments have roughly the same deconvolved FWHM width, with a median value 0.12 ± 0.03 pc, independently of their column densities ranging from < 10 21 cm −2 to > 10 22 cm −2 . We also identified 2 protostellar cores, 20 robust prestellar cores, 11 candidate prestellar cores, and 27 unbound starless cores. Two star-forming filaments stand out based on the Herschel data: Filaments 8 and 10 harbor quasi-periodic chains of dense cores with a typical projected core spacing of ∼ 0.15 pc. These two filamentary structures have supercritical line masses and are not static. Filament 8 exhibits a prominent transverse velocity gradient, suggesting that it is accreting gas from the parent cloud gas reservoir. The estimated mass accretion rate is ∼ 40 ± 10 M Myr −1 pc −1 . Filament 10 includes two embedded protostars with outflows and is likely at a later evolutionary stage than filament 8. Our findings support the notion that dense molecular filaments play a crucial role in the star formation process. We suggest that accretion onto the two star-forming filaments, as well as geometrical bending, explains why the observed core spacing along them is significantly shorter than the canonical separation of ∼ 4 times the filament width predicted by classical cylinder fragmentation theory.

The Gravitational Instability of Adiabatic Filaments

“…In particular, if the filament has a diameter D = 2H, then Figure 4 (see also Table 2) predicts that star-forming cores are preferentially separated by a length 2πH/k max ≃ πD/2 ≃ 1.5D. Interestingly, this prediction is consistent with the findings of Zhang et al (2020), who investigated the properties of a set of star-forming filaments and found that the average spacing between cloud cores was ∼ 0.15 pc, while the full-width half-max width of the filament was measured to be 0.1 pc.…”

Ubiquitous velocity fluctuations throughout the molecular interstellar medium

“…6,108 This periodicity has been noted both in direct tracers of star formation, 6,14,[109][110][111] as well as in molecular gas. 112 Analogously, on small scales, both dust continuum 26,[113][114][115][116] and molecular line observations 19,117,118 of molecular clouds in the Milky Way have revealed quasi-periodically spaced chains of dense clumps and pre-stellar cores along the crests of dense filaments.…”

The hierarchical fragmentation of filaments and the role of sub-filaments

“…This result may appear in contradiction to recent observational work (Jackson et al 2010;Busquet et al 2013;Lu et al 2014;Beuther et al 2015;Henshaw et al 2016;Teixeira et al 2016;Kainulainen et al 2017;Ladjelate et al 2020;Zhang et al 2020). These works used a variety of methods to investigate the presence of a characteristic fragmentation length-scale; some used null hypothesis tests and others did not, some relied on clustering in the core separations and others used more advanced techniques such as the two-point correlation function and the N th nearest neighbour method.…”