- Current Research
- Older Stuff
Stellar Waves & Oscillations
As fluids, stars support a variety of propagating disturbances that take the form of traveling waves when localized, or standing waves (also known as oscillations or pulsations) when global. At the surface of a star, these disturbances cause fluctuations in the star's brightness and color that can be measured via time-series photometry and spectroscopy. Analyzing these measurements can in turn shed light on the internal structure of the star — the discipline of asteroseismology.
Starting with my PhD thesis, I've been studying waves and oscillations in stars for almost three decades, primarily from a theoretical and computational perspective. My highest-profile work has been the development of an open-source stellar oscillation code, GYRE, that predicts the oscillation-mode frequencies and wavefunctions of an input stellar model (see Townsend & Teitler 2013). GYRE has been used in hundreds of papers since its release, and continues to be actively developed by my Mad Star group. Recent enhancements include
- the ability to model non-adiabatic oscillations (see Townsend et al. 2018)
- integration into MESA (see below)
- a new contour-mapping approach for finding modes (see Goldstein & Townsend 2020)
- solution of forced oscillation problems (see below)
Stellar Structure & Evolution
Developing a foundational understanding of the structure and evolution of stars is arguably the crown jewel of 20th-century Astrophysics. However, there are still many areas where the details — both coarse and fine — need to be figured out.
Over the past decade, much progress has been driven by MESA (Modules for Experiments in Stellar Astrophysics), an open-source software instrument for solving the coupled system of equations governing quasi-1D stellar structure and evolution (see Paxton et al. 2011). Since its first public release, MESA has been adopted by over a thousand researchers worldwide to explore ideas at and beyond the cutting edge of stellar astrophysics.
I've been involved in the MESA project since soon after its inception; my contributions include
- developing and maintaining the MESA software development kit (SDK) (introduced in Appendix C of Paxton et al. 2013)
- integrating the GYRE oscillation code (above) into MESA (described in Section 3 of Paxton et al. 2015)
- with Anne Thoul, developing new 'predictive mixing' and 'convective premixing' schemes for handling convective boundaries in MESA (described in Section 2 of Paxton et al. 2018 and Section 5 of Paxton et al. 2019, respectively)
- rebuilding MESA's atmosphere (atm) module to improve accuracy and flexibility (to be described in an upcoming paper)
As a spin-off of my work on stellar oscillations, I've recently become interested in stellar tides — the periodic distortions caused by the gravitational field of a binary companion. Tides can be viewed as an instance of forced oscillations, and much of the formalism for tides carries over from the study of stellar oscillations.
With this in mind, I've worked recently with my Mad Star group to extend GYRE so that it can model tides. We are currently writing up our work as a series of papers; however, here is a movie visualization produced from a GYRE simulation of a heartbeat system (an eccentric binary whose light curve resembles the human heartbeat).
In addition to the focused research described dbove, I'm often involved in offshoot projects arising from my 40-year interest in scientific computation. Highlights include
- implementing the Lomb-Scargle periodogram on graphics processing units (see Townsend 2009)
- developing a new exact algorithm for radiative coooling in hydro codes (see Townsend 2010)
- creating the new Multidimensional Spectral Grids (MSG) project, a software and data infrastructure for interpolating stellar spectra and photometric colors.
Here, I mention some projects I've been involved in over the years. These are no longer my main research focus, but I still dabble in them from time to time.
Unexpectedly (from a dynamo standpoint), strong (~kG) fields are observed in a number of massive stars, including the helium-strong chemically peculiar stars (e.g., σ Ori E), the helium-weak stars (e.g., 36 Lyn) and a few more-massive O-type stars (e.g., θ1 Ori C). My past research has focused on creating models for the magnetospheres of these stars — circumstellar environments where the dynamics of wind outflows are significantly affected by the presence of the magnetic field. These include
- the Rigidly Rotating Magnetosphere (RRM) model (see Townsend & Owocki 2004)
- the Rigid-Field Hydrodynamics (RFHD) model (see Townsend, Owocki & ud-Doula 2007)
- the Analytical Dynamical Magnetosphere (ADM) model (see Owocki et al. 2016)
I've also contributed toward our observational exploration of these systems, in particular leading the team that discovered the spin-down of σ Ori E — arguably the first direct measurement of magnetic braking in a main-sequence star (see Townsend et al. 2011).