3,550 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
Organ building in New England in the eighteenth and nineteenth centuries
Thesis (M.M.)--Boston University
Bibliography: p. 199-207
Far-infrared signatures and inner hole sizes of protoplanetary discs undergoing inside-out dust dispersal
By means of radiative transfer simulation we study the evolution of the
far-infrared colours of protoplanetary discs undergoing inside-out dispersal,
often referred to as transition discs. We show that a brightening of the mid
and far-infrared emission from these objects is a natural consequence of the
removal of the inner disc. Our results can fully explain recent observations of
transition discs in the Chamaleon and Lupus star forming regions from the
Herschel Gould Belt Survey, which show a higher median for the 70?um (Herschel
PACS 1) band of known transition objects compared with primordial discs. Our
theoretical results hence support the suggestion that the 70?um band may be a
powerful diagnostic for the identification of transition discs from photometry
data, provided that the inner hole is larger than tens of AU, depending on
spectral type. Furthermore we show that a comparison of photometry in the K ,
12?um and 7u0?m bands to model tracks can provide a rough, but quick estimate
of the inner hole size of these objects, provided their inclination is below
?85 degrees and the inner hole size is again larger than tens of AU.Comment: 8 pages, figure 4, accepted for 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
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
Activism's Impact on Diversified Investors and the Market
We model activism as it affects the future distribution of prices in a
portfolio context with risk-averse expected utility of end-of-period
wealth maximizing investors. We characterize activism as affecting the
mean, the variance, and/or the covariance of the target firm’s
price with the prices of the other firms. This characterization allows
us to investigate conditions under which the activist would choose to
become an activist and, subsequently, to derive the sequence of
equilibria that begins with the surreptitious acquisition of shares by
the activist and ends at the moment of the activist’s divestiture
of these shares. We investigate the impact of activism not only on the
target firm’s price over time and the activist’s profit, but
also on the redistribution of portfolio holdings of all market
participants that this activism induces. We propose a method to evaluate
activism and show that, while activism may augment the share price of
the target firm, there exist conditions under which activism would not
necessarily increase the value of the market. Furthermore, we show that
the pro.t of the activist is at the expense of the group of other
investors. We compare our results to recent empirical findings.Statistics Working Papers Serie
Exploring Tax Evasion in the Context of Political Uncertainty
We present a model of agents facing the uncertainty of two future forms
of government who are able to insure against this uncertainty by hiding
funds from taxation. In order to choose whether or not to hide funds
from taxation, agents need to know policy choices that each government
would make should it come to power. But each government, before it could
make its decision, would need to know the choices of the agents who
would, for example, produce tax revenues. This informational tension is
resolved endogenously. We derive the resulting level of tax evasion in
society and the optimal choices made by the potential governments. We
examine how changes in governmental structure would affect the level of
tax evasion, and how that, in turn, would affect a particular form of
capital flight
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