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

An Evaluation of Shareholder Activism

We develop a method to evaluate shareholder activism when an activist targets firms whose shareholders are diversified portfolio holders of possibly correlated firms. Our method of evaluation takes the portfolios of all of the shareholders, including the activist, as its basis of analysis. We model the activist from the time of the acquisition of a foothold in the target firm through the moment when the activist divests the newly acquired shares. We assume that during this period, all exchanges of securities, and their corresponding prices, are achieved in Walrasian markets in which all participants, including the activist, are risk-averse price-takers. Using the derived series of price changes of all the firms in the market, as well as the derived series of changes in all the portfolio holdings over this period, we evaluate the impact of activism on the activist, on the group of other shareholders, and on the combined group. We show that when activism is beneficial to the activist, the group of other investors may not benefit; furthermore, even when the activist benefits from activism, the value of the market may decrease. When the activist benefits from activism, an increase in the value of the market is a necessary but not sufficient condition for the group of other investors to benefit also from activism. In addition, we show that the combined group, the activist plus the group of other investors, benefits if and only if the value of the market increases and, under this condition, either the activist or the group of other investors, but not necessarily both, benefits