Brad Hansen


Office: 3-913, Physics and Astronomy
Phone: (310) 825-5924
Fax: (310) 206-2096


Physics and Astronomy, and
Institute of Geophysics and Planetary Physics ,

Positions held in the past:

Hubble Fellow, Princeton University
Research Associate, Canadian Institute for Theoretical Astrophysics


Ph.D., 1996, California Institute of Technology  
Undergraduate: University of Natal, Durban
High School: St Henry's College, Durban


Curriculum vitae (PDF)

Popular Accounts

Spitzer Science Center release on Extrasolar Planets

Hubble Space Telescope release on White Dwarfs in Star Clusters

New Scientist article on Black Holes in the Galactic Center


Research Interests: Theoretical Astrophysics

White Dwarfs[1] are the end product of stellar evolution for most stars (except for those massive enough to explode as supernovae). As such, the study of old white dwarfs can tell us a lot about the history of star formation in our Galaxy. By the time a star has reached the white dwarf stage, it has exhausted all the nuclear burning resources with which it was born. With no remaining reservoir of energy supply white dwarfs slowly cool and fade over time. I have worked a lot on theoretical models for the evolution and appearance of white dwarfs of different kinds. With my observational collaborators, I have used these models to estimate the ages of various stellar components of our Galaxy, including both the stars in the immediate vicinity of the sun and some of the nearest globular clusters. One of our recent results is a setting a lower limit on the age of our Galaxy of approximately 11 billion years.

Extrasolar Planets [2] are also a subject of some interest to me, both the theoretical and observational side. I am interested in both the dynamics and structure of extrasolar planets. Most of my recent work has been focussed on understanding various issues related to giant planets that orbit close to their parent stars. The structure of these planets is strongly affected by the irradiation they receive from the star and this leads to a variety of interesting questions. A recent highlight includes the first detection of a phase variable brightness from one of these systems, which suggests that the planet has different temperatures on the side facing the star and on the side facing away from the star. We are working to understand how this comes about, what it says about the nature of the atmosphere and how this might be reflected in other observations.
Another interest is how these two different subjects (planets and white dwarfs) can be linked. In particular, one of my long term goals is to understand how planetary systems are affected by stellar evolution. We have several observational indicates that planets can exist around evolved stars like pulsars and white dwarfs, but there are still many holes in the theory of how this comes about.

Neutron Stars and Black Holes[3] are the first objects I worked on, during my PHD. My most recent interest in this subject concerns the black hole at the center of our Galaxy and the immediate environment. With my students Steve Berukoff and Elliot Koch, I have been studying the dynamics of thousand-solar mass black holes orbiting the main black hole. We believe these `intermediate mass' black holes may have played an important role in the transport of young stars to the Galactic center and they will have an important influence on the resulting dynamics.

Selected Recent Publications:
Planet Atmospheres:

I. J. Crossfield, T. Barman, B. Hansen, T. Tanaka & T. Kodama,
``Re-evaluating WASP-12b: Strong Emission at 2.315 μm, Deeper Occultations, and an Isothermal Atmosphere''
Astrophysical Journal, Vol 760, Page 140 (2012)

I. J. Crossfield, T. Barman & B. Hansen
``High-resolution, Differential, Near-infrared Transmission Spectroscopy of GJ 1214b''
Astrophysical Journal, Vol 736, Page 132 (2011)

I. J. Crossfield, B. Hansen, J. Harrington, J. Cho , D. Deming, K. Menou, S. Seager
``A new 24 micron Phase Curve for upsilon Andromedae b''
Astrophysical Journal, Vol 723, Page 1436 (2010)

Planet Dynamics :

B. Hansen & N. Murray
``Testing In Situ Assembly with the Kepler Planet Candidate Sample''
Astrophysical Journal, Vol 775, Page 53 (2013)

B. Hansen & N. Murray
``Migration then Assembly: Formation of Neptune-mass Planets inside 1 AU''
Astrophysical Journal, Vol 751, Page 158 (2012)

B. Hansen
``Calibration of Equilibrium Tide Theory for Extrasolar Planet Systems''
Astrophysical Journal, Vol 723, Page 285 (2010)

White Dwarf Cooling:

B. Hansen et al.
``An Age Difference of 2 Gyr between a Metal-Rich and a Metal-Poor Globular Cluster''
Nature, Vol 500, Page 51 (2013)

E. Y. Chen & B. Hansen
``Cooling curves and chemical evolution curves of convective mixing white dwarf stars''
Monthly Notices of Royal Astronomical Society, Vol 413, Page 2828 (2011)

B. Hansen et al.
``The White Dwarf Cooling Sequence of NGC6397''
Astrophysical Journal, Vol 671, Page 380 (2007)