338 research outputs found
The long-term evolution of photoevaporating transition discs with giant planets
Photo-evaporation and planet formation have both been proposed as mechanisms
responsible for the creation of a transition disc. We have studied their
combined effect through a suite of 2d simulations of protoplanetary discs
undergoing X-ray photoevaporation with an embedded giant planet. In a previous
work we explored how the formation of a giant planet triggers the dispersal of
the inner disc by photo-evaporation at earlier times than what would have
happened otherwise. This is particularly relevant for the observed transition
discs with large holes and high mass accretion rates that cannot be explained
by photo-evaporation alone. In this work we significantly expand the parameter
space investigated by previous simulations. In addition, the updated model
includes thermal sweeping, needed for studying the complete dispersal of the
disc. After the removal of the inner disc the disc is a non accreting
transition disc, an object that is rarely seen in observations. We assess the
relative length of this phase, to understand if it is long lived enough to be
found observationally. Depending on the parameters, especially on the X-ray
luminosity of the star, we find that the fraction of time spent as a
non-accretor greatly varies. We build a population synthesis model to compare
with observations and find that in general thermal sweeping is not effective
enough to destroy the outer disc, leaving many transition discs in a relatively
long lived phase with a gas free hole, at odds with observations. We discuss
the implications for transition disc evolution. In particular, we highlight the
current lack of explanation for the missing non-accreting transition discs with
large holes, which is a serious issue in the planet hypothesis.Comment: 11 pages, 5 figures; accepted by MNRA
Empirical constraints on turbulence in proto-planetary discs
Proto-planetary discs, the birth environment of planets, are an example of a
structure commonly found in astrophysics, accretion discs. Identifying the
mechanism responsible for accretion is a long-standing problem, dating back
several decades. The common picture is that accretion is a consequence of
turbulence, with several instabilities proposed for its origin. While
traditionally this field used to be a purely theoretical endeavour, the
landscape is now changing thanks mainly to new observational facilities such as
the ALMA radio interferometer. Thanks to large improvements in spatial and
spectral resolution and sensitivity (which have enabled the study of disc
substructure, kinematics and surveys of large disc populations), multiple
techniques have been devised to observationally measure the amount of
turbulence in discs. This review summarises these techniques, ranging from
attempts at direct detection of turbulence from line broadening, to more
indirect approaches that rely on properties of the dust or consider the
evolution of global disc properties (such as masses, radii and accretion rates)
for large samples, and what their findings are. Multiple lines of evidence
suggest that discs are in fact not as turbulent as thought one decade ago. On
the other hand, direct detection of turbulence in some discs and the finite
radial extent of dust substructures and in some cases the finite vertical
extent strongly indicate that turbulence must be present at some level in
proto-planetary discs. It is still an open question whether this amount of
turbulence is enough to power accretion or if this is instead driven by other
mechanisms, such as MHD winds.Comment: 24 pages, 7 figures. Accepted for publication on New Astronomy
Review
The interplay between X-ray photoevaporation and planet formation
We assess the potential of planet formation instigating the early formation
of a photoevaporation driven gap, up to radii larger than typical for
photoevaporation alone. For our investigation we make use of hydrodynamics
models of photoevaporating discs with a giant planet embedded. We find that, by
reducing the mass accretion flow onto the star, discs that form giant planets
will be dispersed at earlier times than discs without planets by X-ray
photoevaporation. By clearing the portion of the disc inner of the planet
orbital radius, planet formation induced photoevaporation (PIPE) is able to
produce transition disc that for a given mass accretion rate have larger holes
when compared to standard X-ray photoevaporation. This constitutes a possible
route for the formation of the observed class of accreting transition discs
with large holes, which are otherwise difficult to explain by planet formation
or photoevaporation alone. Moreover, assuming that a planet is able to filter
dust completely, PIPE produces a transition disc with a large hole and may
provide a mechanism to quickly shut down accretion. This process appears to be
too slow however to explain the observed desert in the population of transition
disc with large holes and low mass accretion rates.Comment: 11 pages, 10 figures, accepted by MNRAS on 31/12/201
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The evolution of photoevaporating viscous discs in binaries
A large fraction of stars are in binary systems, yet the evolution of proto-planetary discs in binaries has been little explored from the theoretical side. In this paper we investigate the evolution of the discs surrounding the primary and secondary components of binary systems on the assumption that this is driven by photoevaporation induced by X-rays from the respective star. We show how for close enough separations (20-30 AU for average X-ray luminosities) the tidal torque of the companion changes the qualitative behaviour of disc dispersal from inside out to outside in. Fewer transition discs created by photoevaporation are thus expected in binaries. We also demonstrate that in close binaries the reduction in viscous time leads to accelerated disc clearing around both components, consistent with unresolved observations. When looking at the differential disc evolution around the two components, in close binaries discs around the secondary clear first due to the shorter viscous timescale associated with the smaller outer radius. In wide binaries instead the difference in photo-evaporation rate makes the secondaries longer lived, though this is somewhat dependent on the assumed scaling of viscosity with stellar mass. We find that our models are broadly compatible with the growing sample of resolved observations of discs in binaries. We also predict that binaries have higher accretion rates than single stars for the same disc mass. Thus binaries probably contribute to the observed scatter in the relationship between disc mass and accretion rate in young stars.This work has been supported by the DISCSIM project, grant agreement 341137 funded by the European Research Council under ERC-2013-ADG, and by the Munich Institute for Astro- and Particle Physics (MIAPP) of the DFG cluster of excellence ‘Origin and Structure of the Universe’
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