External photoevaporation of protoplanetary discs in sparse stellar groups: the impact of dust growth

Abstract

We estimate the mass loss rates of photoevaporative winds launched from the outer edge of protoplanetary discs impinged by an ambient radiation field. We focus on mild/moderate environments (the number of stars in the group/cluster is N 50), and explore disc sizes ranging between 20 and 250 AU. We evaluate the steady-state structures of the photoevaporative winds by coupling temperature estimates obtained with a PDR code with 1D radial hydrodynamical equations. We also consider the impact of dust dragging and grain growth on the final mass loss rates. We find that these winds are much more significant than have been appreciated hitherto when grain growth is included in the modelling: in particular, mass loss rates 10 −8 M /yr are predicted even for modest background field strengths ( 30 G 0 ) in the case of discs that extend to R > 150 AU. Grain growth significantly affects the final mass loss rates by reducing the average cross section at FUV wavelengths, and thus allowing a much more vigorous flow. The radial profiles of observable quantities (in particular surface density, temperature and velocity patterns) indicate that these winds have characteristic features that are now potentially observable with ALMA. In particular, such discs should have extended gaseous emission that is dust depleted in the outer regions, characterised by a non-Keplerian rotation curve, and with a radially increasing temperature gradient.

First evidence of external disc photoevaporation in a low mass star forming region: the case of IM Lup

“…They suggested that this halo might be a remnant structure rather than being material driven out of the disc by photoevaporation. The photoevaporation interpretation was disfavoured based on the inferred low UV field and outer disk temperatures, which were well below those which had been previously considered by external photoevaporation models (Adams et al 2004;Facchini et al 2016). However, since this regime is previously unexplored it is difficult to conclude this with any certainty.…”

“…For some time there has also been the theoretical expectation that protoplanetary discs might be significantly affected by more canonical radiation field strengths found in a cluster environment (e.g. Scally & Clarke 2001;Adams et al 2004;Holden et al 2011;Facchini et al 2016;Haworth et al 2016). This is now being directly supported by recent observations, such E-mail: t.haworth@imperial.ac.uk † Hubble fellow as those by Kim et al (2016) who identify proplyds irradiatied by a 3000 G 0 1 radiation field -approximately 100 times weaker than the field strengths irradiating classical proplyds (Störzer & Hollenbach 1999).…”

Where can a Trappist-1 planetary system be produced?

“…The dominant heating contribution in a PDR can readily be photoelectric heating from atomic layers of polycyclic aromatic hydrocarbons (PAHs, see e.g. Figure 2 of Facchini et al 2016), the abundance of which is highly uncertain, particularly towards the outer regions of discs (e.g. Geers et al 2006;Oliveira et al 2010;Perez-Becker & Chiang 2011).…”

“…the O stars). Furthermore there is the theoretical expectation of effective photoevaporation in lower radiation regimes (Adams et al 2004;Holden et al 2011;Facchini et al 2016). In particular, where the gravitational potential from the star is shallow, even very low UV fields can drive significant mass loss, as is expected for the large disc in IM Lup which is being irradiated by a field of only ∼ 4 G 0 (Cleeves et al 2016;Haworth et al 2017).…”

A Candidate Planetary-Mass Object With a Photoevaporating Disk in Orion

“…The main physical mechanism driving disk dispersal in these harsh environments is thought to be photoevaporation driven by UV photons from the massive stars in the clusters (Johnstone et al 1998;Störzer & Hollenbach 1999;Richling & Yorke 2000;Clarke 2007;Anderson et al 2013;Facchini et al 2016). UV photons from massive stars ionize and heat the gas in the disk surface and induce a gas flow away from the disk when the sound speed of the gas exceeds the escape velocity (Johnstone et al 1998;Hollenbach et al 2000, p. 401).…”