45 research outputs found
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 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
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
Revealing signatures of planets migrating in protoplanetary discs with ALMA multi-wavelength observations
Recent observations show that rings and gaps are ubiquitous in protoplanetary
discs. These features are often interpreted as being due to the presence of
planets; however, the effect of planetary migration on the observed morphology
has not been investigated hitherto. In this work we investigate whether
multiwavelength mm/submm observations can detect signatures of planet
migration, using 2D dusty hydrodynamic simulations to model the structures
generated by migrating planets and synthesising ALMA continuum observations at
0.85 and 3 mm. We identify three possible morphologies for a migrating planet:
a slowly migrating planet is associated with a single ring outside the planet's
orbit, a rapidly migrating planet is associated with a single ring inside the
planet's orbit while a planet migrating at intermediate speed generates one
ring on each side of the planet's orbit. We argue that multiwavelength data can
distinguish multiple rings produced by a migrating planet from other scenarios
for creating multiple rings, such as multiple planets or discs with low
viscosity. The signature of migration is that the outer ring has a lower
spectral index, due to larger dust grains being trapped there. Of the recent
ALMA observations revealing protoplanetary discs with multiple rings and gaps,
we suggest that Elias 24 is the best candidate for a planet migrating in the
intermediate speed regime.Comment: Accepted for publication in MNRA
The minimum mass of detectable planets in protoplanetary discs and the derivation of planetary masses from high-resolution observations.
We investigate the minimum planet mass that produces observable signatures in infrared scattered light and submillimetre (submm) continuum images and demonstrate how these images can be used to measure planet masses to within a factor of about 2. To this end, we perform multi-fluid gas and dust simulations of discs containing low-mass planets, generating simulated observations at 1.65, 10 and 850 μm. We show that the minimum planet mass that produces a detectable signature is ∼15 M⊕: this value is strongly dependent on disc temperature and changes slightly with wavelength (favouring the submm). We also confirm previous results that there is a minimum planet mass of ∼20 M⊕ that produces a pressure maximum in the disc: only planets above this threshold mass generate a dust trap that can eventually create a hole in the submm dust. Below this mass, planets produce annular enhancements in dust outwards of the planet and a reduction in the vicinity of the planet. These features are in steady state and can be understood in terms of variations in the dust radial velocity, imposed by the perturbed gas pressure radial profile, analogous to a traffic jam. We also show how planet masses can be derived from structure in scattered light and submm images. We emphasize that simulations with dust need to be run over thousands of planetary orbits so as to allow the gas profile to achieve a steady state and caution against the estimation of planet masses using gas-only simulations.We thank an anonymous referee for a careful reading of our manuscript and many useful comments. We thank Leonardo Testi for a stimulating discussion that started this work, Sijme-Jan Paardekooper and Richard Alexander for their constructive criticism, Judith Ngoumou and the Munich Star Formation Coffee for a very lively discussion. This work has been supported by the DISCSIM project, grant agreement 341137 funded by the European Research Council under ERC-2013-ADG. This work used the DIRAC Shared Memory Processing system at the University of Cambridge, operated by the COSMOS Project at the Department of Applied Mathematics and Theoretical Physics on behalf of the STFC DiRAC HPC Facility (www.dirac.ac.uk). This equipment was funded by BIS National E-infrastructure capital grant ST/J005673/1, STFC capital grant ST/H008586/1, and STFC DiRAC Operations grant ST/K00333X/1. DiRAC is part of the National E-Infrastructure.This is the final version of the article. It first appeared from Oxford University Press via http://dx.doi.org/10.1093/mnras/stw69
The efficiency of dust trapping in ringed proto-planetary discs
When imaged at high-resolution, many proto-planetary discs show gaps and
rings in their dust sub-mm continuum emission profile. These structures are
widely considered to originate from local maxima in the gas pressure profile.
The properties of the underlying gas structures are however unknown. In this
paper we present a method to measure the dust-gas coupling and the
width of the gas pressure bumps affecting the dust distribution, applying
high-precision techniques to extract the gas rotation curve from emission lines
data-cubes. As a proof-of-concept, we then apply the method to two discs with
prominent sub-structure, HD163296 and AS 209. We find that in all cases the gas
structures are larger than in the dust, confirming that the rings are pressure
traps. Although the grains are sufficiently decoupled from the gas to be
radially concentrated, we find that the degree of coupling of the dust is
relatively good (). We can therefore reject scenarios in
which the disc turbulence is very low and the dust has grown significantly. If
we further assume that the dust grain sizes are set by turbulent fragmentation,
we find high values of the turbulent parameter (). Alternatively, solutions with smaller turbulence are still
compatible with our analysis if another process is limiting grain growth. For
HD163296, recent measurements of the disc mass suggest that this is the case if
the grain size is 1mm. Future constraints on the dust spectral indices will
help to discriminate between the two alternatives
Observing planetesimal formation under streaming instability in the rings of HD 163296
We introduce a new technique to determine the gas turbulence and surface
density in bright disc rings, under the assumption that dust growth is limited
by turbulent fragmentation at the ring centre. We benchmark this prescription
in HD 163296, showing that our measurements are consistent with available
turbulence upper limits and agree with independent estimates of the gas surface
density within a factor of two. We combine our results with literature
measurements of the dust surface density and grain size to determine the
dust-to-gas ratio and Stokes number in the 67 au and 100 au rings. Our
estimates suggest that particle clumping is taking place under the effect of
streaming instability (SI) in the 100 au ring. Even though in the presence of
external isotropic turbulence this process might be hindered, we provide
evidence that turbulence is non-isotropic in both rings and likely originating
from mechanisms (such as ambipolar diffusion) that could ease particle clumping
under SI. Finally, we determine the mass accretion rate under the assumption
that the disc is in steady state and turbulence regulates angular momentum
transport. Our results are in tension with spectroscopic measurements and
suggest that other mechanisms might be responsible for accretion, in
qualitative agreement with the detection of a magneto-centrifugal wind in this
system. Applying our method to larger samples can be used to statistically
assess if SI is a viable mechanism to form planetesimals in bright rings.Comment: 13 pages, 4 figures; accepted for publication on ApJ