Perturbation growth in accreting filaments

Abstract

We use smoothed particle hydrodynamic simulations to investigate the growth of perturbations in infinitely long filaments as they form and grow by accretion. The growth of these perturbations leads to filament fragmentation and the formation of cores. Most previous work on this subject has been confined to the growth and fragmentation of equilibrium filaments and has found that there exists a preferential fragmentation length scale which is roughly 4 times the filament's diameter. Our results show a more complicated dispersion relation with a series of peaks linking perturbation wavelength and growth rate. These are due to gravo-acoustic oscillations along the longitudinal axis during the sub-critical phase of growth. The positions of the peaks in growth rate have a strong dependence on both the mass accretion rate onto the filament and the temperature of the gas. When seeded with a multi-wavelength density power spectrum there exists a clear preferred core separation equal to the largest peak in the dispersion relation. Our results allow one to estimate a minimum age for a filament which is breaking up into regularly spaced fragments, as well as an average accretion rate. We apply the model to observations of filaments in Taurus by Tafalla & Hacar (2015) and find accretion rates consistent with those estimated by Palmeirim et al. (2013).

Gravity drives the evolution of infrared dark hubs: JVLA observations of SDC13

“…Based on the observed longitudinal velocity gradients and assuming that the gas is freefalling, Peretto et al (2014) estimated that the SDC13 filaments had been collapsing for ∼ 1 to 4 Myr. This is consistent with the estimate based on the Clarke et al (2016Clarke et al ( , 2017 model. However, in the calculation of τ crit we have not taken into account projection effects in the estimate of λ core .…”

“…We find that all filaments are supercritical, with an average transonic non-thermal velocity dispersion, and that core spacing is typically regular along them (∼ 0.37 ± 0.16 pc). Using semi-analytical models of nonequilibrium filaments (Clarke et al 2016(Clarke et al , 2017 we determine that the filaments are a few Myrs old, consistent with the SDC13 dynamical timescale derived by Peretto et al (2014). We find that the large radial velocity gradients across two of the four filament cannot be due to gravity nor rotation, but most likely due to the compression caused by a nearby Hii region.…”

“…However, in a turbulent medium, filaments in equilibrium are likely to be rare objects. Interestingly, Clarke et al (2016Clarke et al ( , 2017 studied the fragmentation of non-equilibrium, accreting filaments and showed that the fastest growing mode of density perturbations in such systems, λ core , is a function of the time it takes to build-up a critical filament through accretion, τ crit , and the effective sound speed, a e f f,0 :…”

Filamentary fragmentation in a turbulent medium

“…Filaments assembled from gas with subsonic turbulence, or where gravity dominates over turbulence, therefore possess two preferential fragmentation length scales: a large fragmentation length scale which is consistent with a modified version of the filamentary fragmentation model presented in Clarke, Whitworth & Hubber (2016); and a small fragmentation length scale which is consistent with the effective Jeans length in these filaments.…”

“…Instead, filament and perturbations will co-evolve during a non-equilibrium accretion stage until the filament becomes unstable and fragments. This scenario has been investigated in an earlier work, Clarke, Whitworth & Hubber (2016) (hereafter CWH16). Using numerical simulations to investigate the perturbation growth in an accreting filament, CWH16 find that the fastest growing mode is the result of a resonance between radial accretion and longitudinal oscillations.…”