Disc clearing of young stellar objects: evidence for fast inside-out dispersal

Abstract

The time-scale over which and the modality by which young stellar objects (YSOs) disperse their circumstellar discs dramatically influences the eventual formation and evolution of planetary systems. By means of extensive radiative transfer (RT) modelling, we have developed a new set of diagnostic diagrams in the infrared colour-colour plane (K -[24] vs. K -[8]), to aid with the classification of the evolutionary stage of YSOs from photometric observations. Our diagrams allow the differentiation of sources with unevolved (primordial) discs from those evolving according to different clearing scenarios (e.g. homologous depletion vs. inside-out dispersal), as well as from sources that have already lost their disc. Classification of over 1500 sources in 15 nearby starforming regions reveals that approximately 39 % of the sources lie in the primordial disc region, whereas between 31 % and 32 % disperse from the inside-out and up to 22 % of the sources have already lost their disc. Less than 2 % of the objects in our sample lie in the homogeneous draining regime. Time-scales for the transition phase are estimated to be typically a few 10 5 years independent of stellar mass. Therefore, regardless of spectral type, we conclude that currently available infrared photometric surveys point to fast (of order 10 % of the global disc lifetime) inside-out clearing as the preferred mode of disc dispersal. Muzerolle et al. (2010) by using IRAC data and the 24 µmfluxes obtained with MIPS (Rieke et al. 2004). Furlan et al.investigated the disc evolution in the three prominent star-forming regions Taurus, Ophiuchus, and Cha I and found similar transitional disc fractions in all three regions. This study is based on spectroscopic data taken with the 'Spitzer Infrared Spectrograph ' (Houck et al. 2004). A spectroscopic study of the young stellar objects in Taurus has been published by Furlan et al. (2011). The mid-infrared variability of (pre-)transitional disks was investigated with Spitzer (e.g. Espaillat et al. (2011)).The interpretation of infrared colours in relation to the evolutionary state of a disc is, however, far from being trivial. This is particularly true with regards to the classification of transition discs, the latter being intended as objects caught in the act of dispersal, i. e. going from discbearing to disc-less stars. The modality and time-scales of disc dispersal are especially relevant to the planet formation process. In particular, inside-out dispersal from internal = [3.14, 5.75, 6.8, 8.9] EN V K − [8] I = [0.73, 0.91, 1.28, 3.] EN V K − [24] II = [3.22, 5.3, 6.9, 9.] EN V K − [8] II = [0.82, 0.85, 1.38, 3.] EN V K − [24] III = [3.73, 4.75, 6.7, 8.9] EN V K − [8] III = [0.94, 1.0, 1.65, 3.] EN V K − [24] IV = [3.95, 5.2, 7., 8.5] EN V K − [8] IV = [1.23, 1.27, 2.05, 3.]

The long-term evolution of photoevaporating transition discs with giant planets

“…Clarke et al (2001) showed that when the mass accretion rate becomes comparable with the mass loss by photoevaporation, a gap opens at around a few AUs, rapidly shutting down accretion (the so-called UV switch) and producing a transition disc. The outer disc left is quickly eroded from the inside out by photoevaporation, which agrees with observational findings Koepferl et al 2013). While this explains some transition discs, it does not explain all of them.…”

“…This is further reinforced by the fact that the viscous time-scale in the outer regions of protoplan-etary discs ( 100 AU) is Myr, which implies that some other process must drive disc dispersal at these radii. Observations currently show that approximately 10 percent of all observed discs (Kenyon & Hartmann 1995;Koepferl et al 2013) are "transition" discs (TD). If all discs go through the transition disc phase, this implies that this phase must be short-lived (of order of 10 5 years), implying that disc evolution observes a "two-time scale behaviour".…”

Accretion of Jupiter-Mass Planets in the Limit of Vanishing Viscosity

“…If one requires that Jupiter takes a million years to accrete its envelope, then its runaway growth needs to be limited by a stellar accretion rate of 10 −9 M yr −1 . However, at this very low rate, the disk photoevaporates rapidly (i.e., a few 10 5 yr, see, e.g., Koepferl et al 2013;Gorti et al 2009;Szulágyi et al 2012). Thus a Jupiter mass of gas is unlikely to be accreted by the planet.…”

A photoevaporative gap in the closest planet-forming disc

“…Luhman et al 2010;Koepferl et al 2013), and it can reproduce the intensities and profiles of emission lines produced in the wind . However, the direct observation of a disc undergoing gap formation via photoevaporation is still lacking.…”