Viscous Evolution and Photoevaporation of Circumstellar Disks Due to External Far Ultraviolet Radiation Fields

Abstract

This paper explores the effects of FUV radiation fields from external stars on circumstellar disk evolution. Disks residing in young clusters can be exposed to extreme levels of FUV flux from nearby OB stars, and observations show that disks in such environments are being actively photoevaporated. Typical FUV flux levels can be factors of ∼ 10 2 − 10 4 higher than the interstellar value. These fields are effective in driving mass loss from circumstellar disks because they act at large radial distance from the host star, i.e., where most of the disk mass is located, and where the gravitational potential well is shallow. We combine viscous evolution (an α-disk model) with an existing FUV photoevaporation model to derive constraints on disk lifetimes, and to determine disk properties as functions of time, including mass loss rates, disk masses, and radii. We also consider the effects of X-ray photoevaporation from the host star using an existing model, and show that for disks around solar-mass stars, externally-generated FUV fields are often the dominant mechanism in depleting disk material. For sufficiently large viscosities, FUV fields can efficiently photoevaporate disks over the entire range of parameter space. Disks with viscosity parameter α = 10 −3 are effectively dispersed within 1−3 Myr; for higher viscosities (α = 10 −2 ) disks are dispersed within ∼ 0.25−0.5 Myr. Furthermore, disk radii are truncated to less than ∼ 100 AU, which can possibly affect the formation of planets. Our model predictions are consistent with the range of observed masses and radii of proplyds in the Orion Nebula Cluster.

Circumstellar discs in Galactic centre clusters: Disc-bearing B-type stars in the Quintuplet and Arches clusters

“…Previous observational studies of Herbig Ae/Be stars suggested that photoevaporation destroys primordial discs around high-mass stars within less than 1 Myr (Hillenbrand et al 1998;Hernández et al 2005;Alonso-Albi et al 2009;Gorti et al 2009). A short survival timescale of less than 1-3 Myr in UV-rich clusters is confirmed by numerical simulations of disc survival (Anderson et al 2013, and references therein). When FUV, EUV, and X-ray luminosities are taken into account, stars with M < 3 M have simulated disc lifetimes of several 10 6 years.…”

“…For Herbig Ae/Be stars in nearby OB associations, Hernández et al find intermediate-mass disc fractions of only 2-5% at ages 3-10 Myr. In particular in the dense environment of starburst clusters such as the Arches and Quintuplet clusters near the Galactic centre, star-disc interactions (Olczak et al 2012) and the external UV radiation field (Anderson et al 2013;Fatuzzo & Adams 2008;Adams et al 2004;Scally & Clarke 2001;Richling & Yorke 2000;Johnstone et al 1998) might contribute to an even more rapid depletion of circumstellar material.…”

Disk Dispersal: Theoretical Understanding and Observational Constraints

“…In this case, the viscous timescale decreases with time and hence the clearing is accelerated. This effect results in the disc clearing an order of magnitude faster than t w (Clarke 2007, Anderson et al 2013 and it is obvious that in this case most of the disc mass is cleared by accretion rather than photoevaporation. Nevertheless photoevaporation is playing a decisive role in preventing disc spreading and thus keeping the viscous timescale short.…”

Magnetic and Gravitational Disk-Star Interactions: An Interdependence of PMS Stellar Rotation Rates and Spin-Orbit Misalignments

“…In this work, we adopt a out = 50 AU as a characteristic size of a typical protoplanetary disk (e.g., Anderson et al 2013). With these specifications, the Hamiltonian takes on a simple form:…”