Person:

Guillochon, James

Stream-stream collisions play an important role for the circularization of highly eccentric streams resulting from tidal disruption events (TDEs). We perform three dimensional radiation hydrodynamic simulations to show that stream collisions can contribute significant optical and ultraviolet light to the flares produced by TDEs, and can sometimes explain the majority of the observed emission. Our simulations focus on the region near the radiation pressure dominated shock produced by a collision and track how the kinetic energy of the stream is dissipated by the associated shock. When the mass flow rate of the stream M˙ is a significant fraction of the Eddington accretion rate, ≳2% of the initial kinetic energy is converted to radiation directly as a result of the collision. In this regime, the collision redistributes the specific kinetic energy into the downstream gas and more than 16% of the mass can become unbound. The fraction of unbound gas decreases rapidly as M˙ drops significantly below the Eddington limit, with no unbound gas being produced when M˙ drops to 1% of Eddington; we find however that the radiative efficiency increases slightly to ≲8% in these low M˙ cases. The effective radiation temperature and size of the photosphere is determined by the stream velocity and M˙, which we find to be a few times 104~K and 1014~cm in our calculations, comparable to the inferred values of some TDE candidates. The photosphere size is directly proportional to M˙, which can explain the rapidly changing photosphere sizes seen in TDE candidates such as PS1-10jh.

The primary challenge of rocket propulsion is the burden of needing to accelerate the spacecraft's own fuel, resulting in only a logarithmic gain in maximum speed as propellant is added to the spacecraft. Light sails offer an attractive alternative in which fuel is not carried by the spacecraft, with acceleration being provided by an external source of light. By artificially illuminating the spacecraft with beamed radiation, speeds are only limited by the area of the sail, heat resistance of its material, and power use of the accelerating apparatus. In this paper, we show that leakage from a light sail propulsion apparatus in operation around a solar system analogue would be detectable. To demonstrate this, we model the launch and arrival of a microwave beam-driven light sail constructed for transit between planets in orbit around a single star, and find an optimal beam frequency on the order of tens of GHz. Leakage from these beams yields transients with flux densities of Jy and durations of tens of seconds at 100 pc. Because most travel within a planetary system would be conducted between the habitable worlds within that system, multiply-transiting exoplanetary systems offer the greatest chance of detection, especially when the planets are in projected conjunction as viewed from Earth. If interplanetary travel via beam-driven light sails is commonly employed in our galaxy, this activity could be revealed by radio follow-up of nearby transiting exoplanetary systems. The expected signal properties define a new strategy in the search for extraterrestrial intelligence (SETI).