Probing the final stages of protoplanetary disk evolution with ALMA

Abstract

Context. The evolution of a circumstellar disk from its gas-rich protoplanetary stage to its gas-poor debris stage is not understood well. It is apparent that disk clearing progresses from the inside-out on a short time scale and models of photoevaporation are frequently used to explain this. However, the photoevaporation rates predicted by recent models differ by up to two orders of magnitude, resulting in uncertain time scales for the final stages of disk clearing. Aims. Photoevaporation theories predict that the final stages of disk-clearing progress in objects that have ceased accretion but still posses considerable material at radii far from the star. Weak-line T Tauri stars (WTTS) with infrared emission in excess of what is expected from the stellar photosphere are likely in this configuration. We aim to provide observational constraints on theories of disk-clearing by measuring the dust masses and CO content of a sample of young (1.8−26.3 Myr) WTTS. Methods. We used ALMA Band 6 to obtain continuum and 12 CO(2−1) line fluxes for a sample of 24 WTTS stars with known infrared excess. For these WTTS, we inferred the dust mass from the continuum observations and derived disk luminosities and ages to allow comparison with previously detected WTTS. Results. We detect continuum emission in only four of 24 WTTS, and no 12 CO(2−1) emission in any of them. For those WTTS where no continuum was detected, their ages and derived upper limits suggest they are debris disks, which makes them some of the youngest debris disks known. Of those where continuum was detected, three are possible photoevaporating disks, although the lack of CO detection suggests a severely reduced gas-to-dust ratio. Conclusions. The low fraction of continuum detections implies that, once accretion onto the star stops, the clearing of the majority of dust progresses very rapidly. Most WTTS with infrared excess are likely not in transition but are instead young debris disks, whose dust is either primordial and has survived disk-clearing, or is of second-generation origin. In the latter case, the presence of giant planets within these WTTS might be the cause.

Disk Dispersal: Theoretical Understanding and Observational Constraints

“…In the inner disk (∼ 1AU), however, there appears to be clearer evidence of simultaneous dispersal. The fraction of accreting disks (withṀ acc > 10 −11 M ⊙ yr −1 ) in stellar groups declines on timescales similar to those of NIR excesses (Fedele et al 2010), with some non-accreting sources (withṀ acc < 10 −11 M ⊙ yr −1 ) still showing IR excesses (also Ingleby et al 2013, Hardy et al 2015. This may indicate that gas in the inner disk is removed first, consistent with dispersal scenarios (see Alexander et al 2014).…”

“…Sub-millimeter emission is rarely seen from disks without NIR excesses (Andrews & Williams 2005), indicating either that the entire disk is depleted simultaneously or that the larger grains are lost earlier due to some combination of drift, fragmentation and/or planetesimal formation. Dust at mid-infrared wavelengths appears to last slightly longer (e.g., Wahhaj et al 2010, Hardy et al 2015; see Figure 1), however, debris disks may contaminate emission statistics at these wavelengths. Nevertheless, the transition from optically thick to optically thin disksbelieved to be represented by a class of objects called transition disks-appears to proceed from inside-out, .i.e., the inner dust is depleted first.…”

Alma Observations of Circumstellar Disks in the Upper Scorpius Ob Association

“…Based on a lack of CI emission toward the disk around CQ Tau, Chapillon et al (2010) concluded that the weak CO emission previously observed for this disk is due to depletion of the gas as a whole, not just of CO. Focusing on disks later in their evolution, Hardy et al (2015) observe 24 sources with ALMA lacking signs of ongoing accretion, but still showing infrared excesses indicative of dust. While four of these sources are detected in the 1.3 mm continuum, none are detected in 12 CO J=2-1.…”

Fingerprints of giant planets in the photospheres of Herbig stars

“…Several sources are in the 10 Myr regime. At such ages, many systems already show low emission levels consistent with debris disks (e.g., Hardy et al 2015;Wyatt et al 2015). This is particularly surprising since A stars are thought to lose their disks faster (Ribas et al 2015) and suggests that the giant planet systems responsible for clearing large portions of the inner disk may contribute to disk longevity by trapping material in the outer disk.…”