[Baps] two talks this week: Wed 3/27 and Thu 3/28, both at 2pm

[Amanda S. Bosh]

We’ll have two special spring break talks this week, please join us!  Snacks provided (no lunch).

Hendrik Burghaus (Deutsches SOFIA Institut and NASA Ames Research Center)
Wednesday, 27 March 2019, 2pm in 54-419

DISTOPIA - A New Tool for Stellar Occultation Predictions

A well-established technique to study the size and other properties of bodies in our solar system is the observation of stellar occultations. To reliably predict these events, it is crucial to have stellar positions and apparent ephemerides of highest precision available, requiring demanding astrometric measurements. With the publication of the Gaia star catalog by the European Space Agency (ESA), a new era of positional astronomy has begun, offering new opportunities for occultation science particularly in the outer solar system. I describe the development of a novel occultation prediction tool in Python that uses Gaia data. After introducing the fundamental concepts, I will cover the program's functionalities and different modes of operation. The code has been validated with the help of a successful observation of an occultation by the TNO Varda back in September 2018. Aside of generating occultation predictions, I will also provide an example how the developed code can be used to support data analysis of a successfully observed event. Finally, I will provide a live demo to the interested audience.

Taylor Safrit (MIT EAPS)
Thursday, 28 March 2019, 2pm in 54-419

The Formation of Bilobate Comet Shapes through Sublimative Torques

Approximately 70 percent of observed cometary nuclei are bilobate (made of two primary masses connected by a narrow neck) (Hirabayashi et al. 2016). Subcatastrophic collisions between cometary bodies can result in these shapes, but require impact speeds an order of magnitude smaller than typical impact velocities in the scattered disk (Gomes et al. 2008), the reservoir from which most Jupiter-family comets (JFCs) originate. Additionally, only 10–20 percent of similarly-sized asteroids are bilobate (Benner et al. 2015), suggesting that the mechanism producing bilobate shapes must be unique to comets. We investigate a novel mechanism for bilobate comet nucleus formation in JFCs, in which sublimative torques acting on comet nuclei during their dynamical migration through the Centaur region spin them up to disruption. Simulations of rotationally disrupted, comet-like rubble-piles (with strengths of 1–10 Pa and internal friction angles of 35°) (Steckloff and Samarasinha 2018) find that rotationally disrupted nuclei reform as bilobate objects (Sánchez and Scheeres 2016; Sánchez and Scheeres 2018). Although centaurs are too distant for H2O ice to sublimate vigorously, they are near enough to the Sun for CO2 and CO ices to do so. We therefore focus on whether CO2- and CO-driven sublimative torques are sufficient to rotationally disrupt centaurs. We combine simulations of the dynamical evolution of centaurs with our SYORP sublimative torque model to compute the torques created by the sublimation of CO2 and CO ices. We find that JFCs smaller than 100 kilometers in radius typically experience sufficient sublimative torques during their migration through the Centaur region to be restructured into bilobate shapes. This suggests that the observed bilobate distribution of comet shapes is likely the result of cometary sublimative evolution, rather than a primordial property of objects in cometary reservoirs. Thus, we expect to observe more bilobate shapes as comets migrate inward. This population-scale shape evolution could be detected with occultation studies.
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