RADIATIVE CLEARING OF PROTOPLANETARY DISCS

Image: protoplanetary disc surrounding the young star HL Tauri, a very young T Tauri star in the constellation Taurus, approximately 450 light-years (140 pc) from Earth in the Taurus Molecular Cloud. These new ALMA observations reveal substructures within the disc that have never been seen before and even show the possible positions of planets forming in the dark patches within the system. Credit: ALMA (ESO/NAOJ/NRAO)

T Tauri stars are pre-main-sequence stars in the process of contracting to the main sequence. Their central temperatures are too low for hydrogen fusion. Instead, they are powered by gravitational energy released as the stars contract, while moving towards the main sequence, which they reach after about 100 million years. Roughly half of T Tauri stars have circumstellar disks, which in this case are called protoplanetary discs because they are probably the progenitors of planetary systems like the Solar System.

This image compares the size of the Solar System with HL Tauri and its surrounding protoplanetary disc. Although the star is much smaller than the Sun, the disc around HL Tauri stretches out to almost three times as far from the star as Neptune is from the Sun.
Credit: ALMA (ESO/NAOJ/NRAO)

Understanding the mechanism by which protoplanetary discs are dispersed is important, in particular, because it constrains the timescale within which planets can form. It is now generally thought that disc dispersal happens from the inside out. Such discs with inner holes have thus been labelled 'transition discs'. Multiple explanations for the appearance of inner holes have been proposed; however the most promising are either clearing by a planet (or planets) or photoevaporation. Once photoevaporation halts the accretion on to the star, a few Jupiter masses of gas should be left at radii beyond 10 AU (astronomical unit - 1AU=150 million kilometers ~ the distance from Earth to the Sun) and that this should survive for of order half a Myr thereafter before ultimate photoevaporation. Once accretion ceases, the reservoir of gas at large radii must either be small or else then rapidly cleared by an unidentified mechanism.

This is a composite image of the young star HL Tauri and its surroundings using data from ALMA (enlarged in box at upper right) and the NASA/ESA Hubble Space Telescope (rest of the picture). This is the first ALMA image where the image sharpness exceeds that normally attained with Hubble. Credit: ALMA (ESO/NAOJ/NRAO)/NASA/ESA

The lack of observed transition discs with inner gas holes of radii greater than ~50AU implies that protoplanetary discs dispersed from the inside out must remove gas from the outer regions rapidly. In a recent paper (Haworth et al. 2016) published in MNRAS, the authors investigate the role of photoevaporation in the final clearing of gas from low mass discs with inner holes. In particular, they study the so-called 'thermal sweeping' mechanism which results in rapid clearing of the disc. Thermal sweeping occurs when the pressure maximum at the inner edge of the dust heated disc falls below the maximum possible pressure of X-ray heated gas.

Image: Artistic illustration of a protoplanetary disk.
Credit: ALMA (ESO/NAOJ/NRAO), Mark Garlick

They found that the radiative clearing happens less readily than previously believed and that the X–ray driven thermal sweeping does not appear to be the solution to the lack of non-accreting transition discs with large holes.
They conclude that it is possible that far-ultraviolet heating could play an important role in the final clearing of protoplanetary discs and can clear gas from the outer disc sufficiently quickly to explain the non-detection of cold gas around weak line T Tauri stars.


The paper (Haworth et al. 2016) is available online>>
http://mnras.oxfordjournals.org/content/457/2/1905.short
http://arxiv.org/pdf/1512.02234v2.pdf

More images and information on HL Tauri are available here>>
http://www.almaobservatory.org/en/press-room/press-releases/771-revolutionary-alma-image-reveals-planetary-genesis