MIT Astrophysics Brown Bag Lunch Series: Spring 2018

Mondays at 12:05 PM in Marlar Lounge 37-252 (unless otherwise noted)
MIT Kavli Institute for Astrophysics and Space Research
70 Vassar Street, Cambridge, MA

The Brown Bag lunch is a forum for visiting astronomers/astrophysicists to speak about their research. Talks begin at 12:05 and speakers should plan 20 minutes of material, to leave room for questions during and after the presentation. If you are visiting MIT and would like to give a lunch talk, you may either contact the organizers directly or have your local colleagues arrange a time. The present organizers of the series are Rana Ezzeddine and Paul Torrey.

Monday January 22 -- Two Talks!


Supermassive Black Hole Binary Candidates from the Pan-STARRS1 Medium Deep Survey

Tingting Liu  
University of Maryland

We present a systematic search for periodically varying quasar and supermassive black hole binary (SMBHB) candidates in the Pan-STARRS1 Medium Deep Survey. From ~9,000 color-selected quasars in a ~50 deg^2 sky area, we identify 26 candidates with more than 1.5 cycles of variation. We extend the baseline of observations via our imaging campaign with the Discovery Channel Telescope and the Las Cumbres Observatory network and reevaluate the candidates using a more rigorous, maximum likelihood method; 3 candidates remained significant, if we assume the variability of all normal quasars are characterized by the Damped Random Walk process. The down-selected "gold" sample translates to an SMBHB rate of 0.3 per 10^3 quasars out to z~2, in tension with previous work by Graham et al. (2015) and Charisi et al. (2016). We also find that the binary residence times of the “gold" sample are best matched to the expected distribution if the mass ratio q = 0.3. Using our study with MDS as a benchmark, we estimate that ~10,000 periodic quasars with more than 3 cycles could be discovered by the Large Synoptic Survey Telescope, paving the way for the direct detection of individual gravitational wave sources by future pulsar timing array experiments.

Characterising exoplanet atmospheres as part of the LRG-BEASTS survey

James Kirk
University of Warwick

Studies of exoplanet atmospheres have revealed a startling diversity between systems, with many showing clouds and hazes which mask pressure-broadened absorption features. In the small sample of planets studied to date, no strong correlation has emerged between key planetary parameters and the presence, or absence, of clouds and hazes, although there is evidence that temperature might play a role. In order to characterise this diversity and unravel the underlying physical processes, it is essential that we expand the current sample of studied planets. This is the focus of the Low Resolution Ground-Based Exoplanet Atmosphere Survey using Transmission Spectroscopy (LRG-BEASTS, “large beasts”). I will present the latest results from LRG-BEASTS which is pioneering the use of 4-metre class telescopes for transmission spectroscopy.

Monday January 28


Knottiness of X-ray jets: testing the obstacle-in-jet model  

Sarka Wykes  

Recently, it has become evident that X-ray jets are ubiquitous in low-power radio galaxies. In addition to diffuse X-ray emission, many of the jets feature compact X-ray brightness enhancements ('knots'). Their radio morphology is often similar, but not identical, to the X-ray morphology, and in some cases optical counterparts are observed as well. The issue of the knot origin(s) is yet undecided, although models exist suggesting that the knots with non-zero proper motion represent moving plasma packets while the knots without detected proper motions would originate from obstacles in the jet (stars and/or clouds) in interaction with the jet plasma flow. I will show some results from our 2D hydrodynamical simulations and analytical calculations of Centaurus A's jet interacting with winds of its internal, evolved stars, with a view to investigating the properties of shocks generated in these interactions and their match to the observed X-ray knots. The fate of the material released by the embedded stars, which is carried away by the jet at a rate of ~0.002 solar masses per year, will be also briefly addressed.

Monday March 26 -- MIT Spring Vacation Week

Monday April 2

Suppressing cooling flows in massive galaxies with turbulent stirring

Kung-Yi Su

We investigate the cooling flow problem of massive galaxies with halo masses of 10^12 - 10^14 solar masses using high-resolution idealized, non-cosmological simulations with the FIRE-2 (Feedback In Realistic Environments) stellar feedback model. We find that various solutions proposed in the literature, including Type Ia supernovae, AGB winds, other stellar feedback, morphological quenching, magnetic fields, and cosmic ray feedback, do not suppress cooling flows. On the other hand, for the ~10^14 Msun halo, our toy model for AGN feedback, where we include turbulent stirring, does significantly reduce the cooling flow and quenches the galaxy with an energy input rate ∼ 10^43 erg s^−1 . The run with turbulent stirring has higher ratios of cooling time to dynamical time (tcool/tdynamical) and cooling time to turbulent mixing time (tcool/tturb), consistent with the weaker cooling flow. On the contrary, if the same amount of energy is injected as thermal heat, it has a much weaker effect on the cooling flow.

Monday April 23


Amplification of magnetic field at inward shocks of supernova remnant Cassiopeia A

Federico Fraschetti
University of Arizona

Multi-epoch X-ray observations of Supernova Remnants (SNRs) are crucial to probe the presence of accelerated electrons at shocks and the observed rapid variability has been long indicating a magnetic field far exceeding the theoretically predicted strength by the compression of the shock alone. Within the SNR Cassiopeia A, joint Chandra and NuSTAR observations recently allowed to single out six inward moving shocks arguably originated from the collision of the forward shock with a molecular cloud in the interstellar medium. Such shocks exhibit an unexpected variability (both increase and decrease) of the flux up to 50% on timescale of a few years. We show that such variability can be interpreted as the result of small-scale dynamo amplifying the magnetic field at astrophysical shocks, demonstrating for the first time that the unfolding of a dynamo process formerly theoretically identified can be observed in astrophysical systems.

Monday April 30


Stellar parameters determination using Slice Inverse Regression 

Marwan Gebran 
Columbia University & Notre Dame University, LB

Sky and On-ground spectroscopic surveys are challenging our capabilities of performing fast and efficient analysis techniques. Our understanding of stars and exoplanets hosts highly depend on the stellar properties. In that context, I will present a new spectroscopic automated procedure that simultaneously derives the effective temperature Teff , surface gravity log g, metallicity [Fe/H], and equatorial projected rotational velocity vsin i for stars. It is based on a combination of the principal component analysis (PCA) inversion and a Regularized Slice inverse regression method (RSIR). The efficiency and accuracy of this procedure have been proven for FGK, A , and late type M dwarf stars. The technique requires a learning database that is generated using observed and/or synthetic data.

Monday May 14

The Habitable Exoplanet Observatory (HabEx):
Science Goals and Projected Capabilities  

Scott Gaudi 
Ohio State University

The Habitable-Exoplanet Observatory (HabEx) is a candidate flagship mission being studied by NASA and the astrophysics community in preparation of the 2020 Decadal Survey. The HabEx mission concept is a large (~4m) diffraction-limited space telescope, providing unprecedented resolution and contrast in the optical, with likely extensions into the near UV and near infrared domains.

The primary goal of HabEx is to answer fundamental questions in exoplanet science, such as searching for and characterizing potentially habitable worlds, providing the first complete “family portraits” of planets around our nearest Sun-like neighbors, and placing the solar system in the context of a diverse set of exoplanets.  At the same time, HabEx will enable a broad range of Galactic, extragalactic, and solar system astrophysics, from resolved stellar population studies that inform the stellar formation history of nearby galaxies, to characterizing the life cycle of baryons as they flow in and out of galaxies, to detailed studies of bodies in our own solar system.

We report here on our team’s efforts in defining a scientifically compelling HabEx mission that is technologically executable, affordable within NASA’s expected budgetary envelope, and timely for the next decade. In particular, we motivate the architecture, quantify the technical requirements, and predict the science yield, for a small number of design reference missions, all with broad capabilities in exoplanet, cosmic origins, and planetary science.

Monday May 21

How Black Holes Shape Globular Clusters 

Kyle Kremer 
Northwestern University

Recent numerical simulations and observational indications have suggested that globular clusters (GCs) may contain large populations of stellar-mass black holes (BHs). This has important implications to gravitational wave astronomy, as GCs are understood to be efficient factories of merging BH-BH binaries. In this talk, I will examine ways BH populations influence the dynamical evolution and observable properties of GCs. Additionally, I will explore the dynamical formation of gravitational wave sources in GCs that may be detectable by LISA.