323 research outputs found
Effects of X-ray irradiation and disk flaring on the [NeII] 12.8 micron emission from young stellar objects
The [Ne II] fine-structure emission line at 12.8 micron has been detected in
several young stellar objects (YSO) spectra. This line is thought to be
produced by X-ray irradiation of the warm protoplanetary disk atmospheres,
however the observational correlation between [Ne II] luminosities and measured
X-ray luminosities shows a large scatter. Such spread limits the utility of
this line as a probe of the gaseous phase of disks, as several authors have
suggested pollution by outflows as a probable cause of the observed scatter. In
this work we explore the possibility that the large variations in the observed
[Ne II] luminosity may be caused instead by different star-disk parameters. In
particular we study the effects that the hardness of the irradiating source and
the structure (flaring) of the disk have on the luminosity and spectral profile
of the [Ne II] 12.8 micron line. We find that varying these parameter can
indeed cause up to an order of magnitude variation in the emission luminosities
which may explain the scatter observed, although our models predict somewhat
smaller luminosities than those recently reported by other authors who observed
the line with the Spitzer Space Telescope. Our models also show that the
hardness of the spectrum has only a limited (undetectable) effect on the line
profiles, while changes in the flaring power of the disk significantly affect
the size of the [Ne II] emission region and, as a consequence, its line
profile. In particular we suggest that broad line profiles centred on the
stellar radial velocity may be indicative of flat disks seen at large
inclination angles.Comment: 9 pages, 8 figures. accepted for publication in MNRA
The imprint of photoevaporation on edge-on discs
We have performed hydrodynamic and radiative transfer calculations of a
photoevaporating disc around a Herbig Ae/Be star to determine the evolution and
observational impact of dust entrained in the wind. We find that the wind
selectively entrains grains of different sizes at different radii resulting in
a dust population that varies spatially and increases with height above the
disc at radii > 10 AU. This variable grain population results in a 'wingnut'
morphology to the dust density distribution. We calculate images of this dust
distribution at NIR wavelengths that also show a wingnut morphology at all
wavelengths considered. We have also considered the contribution that
crystalline dust grains will have in the wind and show that a photoevaporative
wind can result in a significant crystallinity fraction at all radii, when the
disc is edge-on. However, when the disc's photosphere is unobscured, a
photoevaporative wind makes no contribution to the observable crystallinity
fraction in the disc. Finally, we conclude that the analysis of extended
emission around edge-on discs could provide a new and independent method of
testing photoevaporation models.Comment: 8 pages, 6 figures, accepted for publication in MNRA
Ionization--induced star formation V: Triggering in partially unbound clusters
We present the fourth in a series of papers detailing our SPH study of the
effects of ionizing feedback from O--type stars on turbulent star forming
clouds. Here, we study the effects of photoionization on a series of initially
partially unbound clouds with masses ranging from --M
and initial sizes from 2.5-45pc. We find that ionizing feedback profoundly
affects the structure of the gas in most of our model clouds, creating large
and often well-cleared bubble structures and pillars. However, changes in the
structures of the embedded clusters produced are much weaker and not well
correlated to the evolution of the gas. We find that in all cases, star
formation efficiencies and rates are reduced by feedback and numbers of objects
increased, relative to control simulations. We find that local triggered star
formation does occur and that there is a good correlation between triggered
objects and pillars or bubble walls, but that triggered objects are often
spatially-mixed with those formed spontaneously. Some triggered objects acquire
large enough masses to become ionizing sources themselves, lending support to
the concept of propagating star formation. We find scant evidence for spatial
age gradients in most simulations, and where we do see them, they are not a
good indicator of triggering, as they apply equally to spontaneously-formed
objects as triggered ones. Overall, we conclude that inferring the global or
local effects of feedback on stellar populations from observing a system at a
single epoch is very problematic.Comment: 17 pages, 11 figures (mostly degraded to get under the submission
size limit), accepted by MNRA
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
Ionizing feedback from massive stars in massive clusters III: Disruption of partially unbound clouds
We extend our previous SPH parameter study of the effects of photoionization
from O-stars on star-forming clouds to include initially unbound clouds. We
generate a set of model clouds in the mass range M
with initial virial ratios =2.3, allow them to form
stars, and study the impact of the photoionizing radiation produced by the
massive stars. We find that, on the 3Myr timescale before supernovae are
expected to begin detonating, the fractions of mass expelled by ionizing
feedback is a very strong function of the cloud escape velocities. High-mass
clouds are largely unaffected dynamically, while lower-mass clouds have large
fractions of their gas reserves expelled on this timescale. However, the
fractions of stellar mass unbound are modest and significant portions of the
unbound stars are so only because the clouds themselves are initially partially
unbound. We find that ionization is much more able to create well-cleared
bubbles in the unbound clouds, owing to their intrinsic expansion, but that the
presence of such bubbles does not necessarily indicate that a given cloud has
been strongly influenced by feedback. We also find, in common with the bound
clouds from our earlier work, that many of the systems simulated here are
highly porous to photons and supernova ejecta, and that most of them will
likely survive their first supernova explosions.Comment: 14 pages, 13 figures (some degraded and greyscaled), accepted by
MNRA
Testing protoplanetary disc dispersal with radio emission
We consider continuum free-free radio emission from the upper atmosphere of
protoplanetary discs as a probe of the ionized luminosity impinging upon the
disc. Making use of previously computed hydrodynamic models of disc
photoevaporation within the framework of EUV and X-ray irradiation, we use
radiative transfer post-processing techniques to predict the expected free-free
emission from protoplanetary discs. In general, the free-free luminosity scales
roughly linearly with ionizing luminosity in both EUV and X-ray driven
scenarios, where the emission dominates over the dust tail of the disc and is
partial optically thin at cm wavelengths. We perform a test observation of GM
Aur at 14-18 Ghz and detect an excess of radio emission above the dust tail to
a very high level of confidence. The observed flux density and spectral index
are consistent with free-free emission from the ionized disc in either the EUV
or X-ray driven scenario. Finally, we suggest a possible route to testing the
EUV and X-ray driven dispersal model of protoplanetary discs, by combining
observed free-free flux densities with measurements of mass-accretion rates. On
the point of disc dispersal one would expect to find a M_dot^2 scaling with
free-free flux in the case of EUV driven disc dispersal or a M_dot scaling in
the case of X-ray driven disc dispersal.Comment: Accepted MNRAS, 12 pages, 11 figures, (pdf generation fixed
Protoplanetary disc evolution and dispersal: the implications of X-ray photoevaportion
(Abridged) We explore the role of X-ray photoevaporation in the evolution and
dispersal of viscously evolving T-Tauri discs. We show that the X-ray
photoevaporation wind rates scale linearly with X-ray luminosity, such that the
observed range of X-ray luminosities for solar-type T-Tauri stars (10e28-10e31
erg\s) gives rise to vigorous disc winds with rates of order 10e-10-10e-7
M_sun/yr. We use the wind solutions from radiation-hydrodynamic models, coupled
to a viscous evolution model to construct a population synthesis model so that
we may study the physical properties of evolving discs and so-called
`transition discs'. Current observations of disc lifetimes and accretion rates
can be matched by our model assuming a viscosity parameter alpha = 2.5e-3. Our
models confirm that X-rays play a dominant role in the evolution and dispersal
of protoplanetary discs giving rise to the observed diverse population of inner
hole `transition' sources which include those with massive outer discs, those
with gas in their inner holes and those with detectable accretion signatures.
To help understand the nature of observed transition discs we present a
diagnostic diagram based on accretion rates versus inner hole sizes that
demonstrate that, contrary to recent claims, many of the observed accreting and
non accreting transition discs can easily be explained by X-ray
photoevaporation. Finally, we confirm the conjecture of Drake et al. (2009),
that accretion is suppressed by the X-rays through `photoevaporation starved
accretion' and predict this effect can give rise to a negative correlation
between X-ray luminosity and accretion rate, as reported in the Orion data.Comment: Figure 12 and 13 have been updated. In the original version the
results from an unused model run were plotted by mistak
Theoretical spectra of photoevaporating protoplanetary discs: An atlas of atomic and low-ionisation emission lines
We present a calculation of the atomic and low-ionisation emission line
spectra of photoevaporating protoplanetary discs. Line luminosities and
profiles are obtained from detailed photoionisation calculations of the disc
and wind structures surrounding young active solar-type stars. The disc and
wind density and velocity fields were obtained from the recently developed
radiation-hydrodynamic models of Owen et al., that include stellar X-ray and
EUV irradiation of protoplanetary discs at various stages of clearing, from
primordial sources to inner hole sources of various hole sizes.
Our models compare favourably with currently available observations, lending
support to an X-ray driven photoevaporation model for disc dispersal. In
particular, we find that X-rays drive a warm, predominantly neutral flow where
the OI 6300A line can be produced by neutral hydrogen collisional excitation.
Our models can, for the first time, provide a very good match to both
luminosities and profiles of the low-velocity component of the OI 6300A line
and other forbidden lines observed by Hartigan et al., which covered a large
sample of T-Tauri stars.
We find that the OI 6300A and the NeII 12.8um lines are predominantly
produced in the X-ray-driven wind and thus appear blue-shifted by a few km/s
for some of the systems when observed at non-edge-on inclinations. We note
however that blue-shifts are only produced under certain conditions: X-ray
luminosity, spectral shape and inner hole size all affect the location of the
emitting region and the physical conditions in the wind. We caution therefore
that while a blueshifted line is a tell-tale sign of an outflow, the lack of a
blueshift should not be necessarily interpreted as a lack of outflow.Comment: 18 pages, 7 figures, accepted to be published in MNRAS - changes in
the revised version: reference list update
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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