375 research outputs found
Radiation hydrodynamics of triggered star formation: the effect of the diffuse radiation field
We investigate the effect of including diffuse field radiation when modelling
the radiatively driven implosion of a Bonnor-Ebert sphere (BES).
Radiation-hydrodynamical calculations are performed by using operator splitting
to combine Monte Carlo photoionization with grid-based Eulerian hydrodynamics
that includes self-gravity. It is found that the diffuse field has a
significant effect on the nature of radiatively driven collapse which is
strongly coupled to the strength of the driving shock that is established
before impacting the BES. This can result in either slower or more rapid star
formation than expected using the on-the-spot approximation depending on the
distance of the BES from the source object. As well as directly compressing the
BES, stronger shocks increase the thickness and density in the shell of
accumulated material, which leads to short, strong, photo-evaporative ejections
that reinforce the compression whenever it slows. This happens particularly
effectively when the diffuse field is included as rocket motion is induced over
a larger area of the shell surface. The formation and evolution of 'elephant
trunks' via instability is also found to vary significantly when the diffuse
field is included. Since the perturbations that seed instabilities are smeared
out elephant trunks form less readily and, once formed, are exposed to enhanced
thermal compression.Comment: Accepted for publication in MNRAS. 19 pages, 14 figures, 8 table
The first multidimensional view of mass loss from externally FUV irradiated protoplanetary discs
Computing the flow from externally FUV irradiated protoplanetary discs
requires solving complicated and expensive photodissociation physics
iteratively in conjunction with hydrodynamics. Previous studies have therefore
been limited to 1D models of this process. In this paper we compare
2D-axisymmetric models of externally photoevaporating discs with their 1D
analogues, finding that mass loss rates are consistent to within a factor four.
The mass loss rates in 2D are higher, in part because half of the mass loss
comes from the disc surface (which 1D models neglect). 1D mass loss rates used
as the basis for disc viscous evolutionary calculations are hence expected to
be conservative. We study the anatomy of externally driven winds including the
streamline morphology, kinematic, thermal and chemical structure. A key
difference between the 1D and 2D models is in the chemical abundances. For
instance in the 2D models CO can be dissociated at smaller radial distances
from the disc outer edge than in 1D calculations because gas is
photodissociated by radiation along trajectories that are assumed infinitely
optically thick in 1D models. Multidimensional models will hence be critical
for predicting observable signatures of environmentally photoevaporating
protoplanetary discs.Comment: 15 pages, 12 figures. Accepted for publication in the MNRAS main
journa
The theory of globulettes: candidate precursors of brown dwarfs and free-floating planets in H II regions
Large numbers of small opaque dust clouds - termed `globulettes' by Gahm et al - have been observed in the H II regions surrounding young stellar clusters. With masses typically in the planetary (or low mass brown dwarf) regime, these objects are so numerous in some regions (e.g. the Rosette) that, if only a small fraction of them could ultimately collapse, then they would be a very significant source of free-floating planets. Here we review the properties of globulettes and present a theoretical framework for their structure and evolution. We demonstrate that their interior structure is well described by a pressure-confined isothermal Bonnor-Ebert sphere and that the observed mass-radius relation (M α R^2:2) is a systematic consequence of a column density threshold below which components of the globulette are not identified. We also find that globulettes with this interior structure are very stable against collapse within H II regions. We follow Gahm et al in assuming that globulettes are detached from the tips of pillars protruding in from the swept-up shell that borders the expanding HII region and produce a model for their dynamics, finding that globulettes will eventually impact the shell. We derive an expression for the time it takes to do so and show that dissipation of energy via dust cooling allows all globulettes to survive this encounter and escape into the wider ISM. Once there the ambient pressure drops and they disperse on timescales around 30-300 kyr and should be observable using ALMA out to distances of order a parsec. Since we find that globulettes are stable, the only route via which they might still form brown dwarfs or planets is during their collision with the shell or some other violent perturbative event.This is the final published version of the article. This article has been published in MNRAS © 2014 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved
Dispersal of protoplanetary discs: How stellar properties and the local environment determine the pathway of evolution
We study the evolution and final dispersal of protoplanetary discs that
evolve under the action of internal and external photoevaporation, and
different degrees of viscous transport. We identify five distinct dispersal
pathways, which are i) very long lived discs (Myr), ii) inside-out
dispersal where internal photoevaporation dominates and opens inner holes, iii)
outside-in dispersal where external photoevaporation dominates through disc
truncation and two intermediate regimes characterised by lingering material in
the inner disc with the outer disc dispersed predominantly by either internal
or external photoevaporation. We determine how the lifetime, relative impact of
internal and external winds and clearing pathway varies over a wide, plausible,
parameter space of stellar/disc/radiation properties. There are a number of
implications, for example in high UV environments because the outer disc
lifetime is shorter than the time-scale for clearing the inner disc we do not
expect transition discs to be common, which appears to be reflected in the
location of transition disc populations towards the Orion Nebular Cluster.
Irrespective of environment, we find that ongoing star formation is required to
reproduce observed disc fractions as a function of stellar cluster age. This
work demonstrates the importance of including both internal and external winds
for understanding protoplanetary disc evolution.Comment: Submitted to MNRAS. 19 pages, 15 figure
Rapid radiative clearing of protoplanetary discs
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. We investigate the role of
photoevaporation in the final clearing of gas from low mass discs with inner
holes. In particular, we study the so-called "thermal sweeping" mechanism which
results in rapid clearing of the disc. Thermal sweeping was originally thought
to arise when the radial and vertical pressure scale lengths at the X-ray
heated inner edge of the disc match. We demonstrate that this criterion is not
fundamental. Rather, 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 (which depends on the local X-ray flux). We derive new
critical peak volume and surface density estimates for rapid radiative clearing
which, in general, result in rapid dispersal happening less readily than in
previous estimates. This less efficient clearing of discs by X-ray driven
thermal sweeping leaves open the issue of what mechanism can clear gas from the
outer disc sufficiently quickly to explain the non-detection of cold gas around
weak line T Tauri stars.Comment: 13 pages, Accepted for publication in MNRA
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Photochemical-dynamical models of externally FUV irradiated protoplanetary discs
There is growing theoretical and observational evidence that protoplanetary disc evolution
may be significantly affected by the canonical levels of far ultraviolet (FUV) radiation found
in a star forming environment, leading to substantial stripping of material from the disc outer
edge even in the absence of nearby massive stars. In this paper we perform the first full radiation
hydrodynamic simulations of the flow from the outer rim of protoplanetary discs externally
irradiated by such intermediate strength FUV fields, including direct modelling of the
photon dominated region (PDR) which is required to accurately compute the thermal properties.
We find excellent agreement between our models and the semi–analytic models of
Facchini et al. (2016) for the profile of the flow itself, as well as the mass loss rate and location
of their “critical radius”. This both validates their results (which differed significantly
from prior semi–analytic estimates) and our new numerical method, the latter of which can
now be applied to elements of the problem that the semi–analytic approaches are incapable of
modelling. We also obtain the composition of the flow, but given the simple geometry of our
models we can only hint at some diagnostics for future observations of externally irradiated
discs at this stage. We also discuss the potential for these models as benchmarks for future
photochemical–dynamical codes
The external photoevaporation of structured protoplanetary disks
The dust in planet-forming disks evolve rapidly through growth and radial
drift, and external photoevaporation also contributes to this evolution in
massive star-forming regions. We test whether the presence of substructures can
explain the survival of the dust component and observed millimeter continuum
emission in protoplanetary disks located within massive star-forming regions.
We also characterize the dust content removed by the photoevaporative winds.
For this, we performed hydrodynamical simulations of protoplanetary disks
subject to irradiation fields of , , and ,
with different dust trap locations. We used the FRIED grid to derive the mass
loss rate for each irradiation field and disk properties, and then measure the
evolution of the dust mass over time. For each simulation we estimate continuum
emission at along with the radii encompassing
of the continuum flux, and characterize the dust size distribution
entrained in the photoevaporative winds, along with the resulting
far-ultraviolet (FUV) cross section. Our simulations show that the presence of
dust traps can extend the lifetime of the dust component of the disk to a few
millionyears if the FUV irradiation is , but only if
the dust traps are located inside the photoevaporative truncation radius. The
dust component of a disk quickly disperse if the FUV irradiation is strong
() or if the substructures are located outside the photoevaporation
radius. We do find however, that the dust grains entrained with the
photoevaporative winds may result in an absorption FUV cross section of at early times of evolution (0.1 Myr),
which is enough to trigger a self-shielding effect that reduces the total mass
loss rate, and slow down the disk dispersal in a negative feedback loop
process.Comment: Accepted for publication in A&
Planet formation via pebble accretion in externally photoevaporating discs
We demonstrate that planet formation via pebble accretion is sensitive to
external photoevaporation of the outer disc. In pebble accretion, planets grow
by accreting from a flux of solids (pebbles) that radially drift inwards from
the pebble production front. If external photoevaporation truncates the outer
disc fast enough, it can shorten the time before the pebble production front
reaches the disc outer edge, cutting off the supply of pebble flux for
accretion, hence limiting the pebble mass reservoir for planet growth.
Conversely, cloud shielding can protect the disc from strong external
photoevaporation and preserve the pebble reservoir. Because grain growth and
drift can occur quickly, shielding even on a short time-scale (<1 Myr) can have
a non-linear impact on the properties of planets growing by pebble accretion.
For example a planetary seed at 25 au stays at 25 au with a
lunar mass if the disc is immediately irradiated by a G field, but
grows and migrates to be approximately Earth-like in both mass and orbital
radius if the disc is shielded for just 1 Myr. In NGC 2024, external
photoevaporation is thought to happen to discs that are <0.5 Myr old, which
coupled with the results here suggests that the exact planetary parameters can
be very sensitive to the star forming environment. Universal shielding for
time-scales of at least Myr would be required to completely nullify
the environmental impact on planetary architectures.Comment: Accepted for publication in mnras, 12 pages, 8 figure
Determining the mid-plane conditions of circumstellar discs using gas and dust modelling: a study of HD 163296
The mass of gas in protoplanetary discs is a quantity of great interest for
assessing their planet formation potential. Disc gas masses are, however,
traditionally inferred from measured dust masses by applying an assumed
standard gas-to-dust ratio of . Furthermore, measuring gas masses
based on CO observations has been hindered by the effects of CO freeze-out.
Here we present a novel approach to study the mid-plane gas by combining
CO line modelling, CO snowline observations and the spectral energy
distribution (SED) and selectively study the inner tens of au where freeze-out
is not relevant. We apply the modelling technique to the disc around the Herbig
Ae star HD 163296 with particular focus on the regions within the CO snowline
radius, measured to be at 90 au in this disc. Our models yield the mass of
CO in this inner disc region of
M. We
find that most of our models yield a notably low , especially in the
disc mid-plane (). Our only models with a more interstellar medium
(ISM)-like require CO to be underabundant with respect to the ISM
abundances and a significant depletion of sub-micron grains, which is not
supported by scattered light observations. Our technique can be applied to a
range of discs and opens up a possibility of measuring gas and dust masses in
discs within the CO snowline location without making assumptions about the
gas-to-dust ratio.This work has been supported by the DISCSIM project, grant agreement 341137 funded by the European Research Council under ERC-2013-ADG. DMB is funded by this ERC grant and an STFC studentship. OP is supported by the Royal Society Dorothy Hodgkin Fellowship. During a part of this project OP was supported by the European Union through ERC grant number 279973. TJH is funded by the STFC consolidated grant ST/K000985/1.This is the final version of the article. It first appeared from Oxford University Press via http://dx.doi.org/10.1093/mnras/stw132
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