My research
interests focus on the structure and evolution of stars by modelling them using evolution codes, pulsation codes, and stellar atmosphere codes. I find it best to discuss my current research under three separate themes: 1) Cepheids, 2) Stellar Atmospheres, 3) Other fun stuff.*** Links to come ****
Cepheids (What is a Classical Cepheid?)
Classical Cepheids are one of the most well-studied objects in the night sky because they are useful laboratories for understanding the structure and evolution of stars in general, but they are also one of the best standard candles. Stellar pulsation allows us to measure the properties of Cepheids, but the pulsation period is also related to a Cepheids absolute brightness or luminosity. Henrietta Leavitt [1], more than one hundred years ago, determined this relation between period and luminosity, now called the Leavitt Law.
As a probe of stellar structure and evolution, pulsation models by Robert Christy [2] and others allowed astronomers to predict how massive Cepheids are. However, these masses did not agree with masses from stellar evolution calculations [3], where evolution calculations predicted more massive Cepheids for the same luminosity and temperature. This mass difference became known as the Cepheid mass discrepancy [4]. A number of solutions proposed, including mass loss, extra mixing such as convective core overshooting, and even missing opacities [5]. Resolving this discrepancy and understanding what physics is missing from pulsation and evolution calculations and is still a significant challenge. Read more here.
Another part of my research related to Cepheids is understanding the structure of the Leavitt Law as a function of wavelength and metallicity. The Leavitt Law is an important tool for measuring distances to far off galaxies and for measuring the expansion of the universe, denoted by the Hubble Constant, H0 [6]. However, the ability of the Leavitt Law to measure the Hubble Constant with a precision to constrain cosmological parameters is limited by its unknown dependence on Cepheid composition, potential infrared excess, and other unknown physics. Therefore, it is important to better understand the underlying physics of Classical Cepheids to better constrain the Leavitt Law. Read more here.
- Leavitt, Henrietta S. 1908, AnHar, 60, 87
- Christy, Robert F. 1966, ApJ, 145, 340
- Stobie, R.S. 1969, MNRAS, 144, 511
- Cox, A.N. 1980, ARA&A, 18,15
- Bono, G; Caputo, F.; & Castellani, V. 2006, MmSAI, 77, 207
- Freedman, W.L.; Madore, B.F.; Gibson, B.K.; Ferrarese, L.; Kelson, D.D.; Sakai, S.; Mould, J.R.; Kennicutt, R.C.; Ford, H.C.; Graham, J.A.; Huchra, J.P.; Hughes, S.M.G.; Illingworth, G.D.; Macri, L.M.; & Stetson, P.B. 2001, ApJ, 553, 47
Stellar Atmospheres
The light we see from stars is all emitted from the outer layers near the surface of stars. We call these layers the stellar atmosphere, and the study of stellar atmospheres is also more than 100 years old. In fact, stellar atmospheres are so well modelled that they have been called on the great successes in computational astrophysics. However, there are still a number of problems that can be addressed.
One such issue is the study of stellar limb-darkening and how it contributes to observations of stars in binary systems, stars with transiting exoplanets, and measurements from optical interferometry. Stellar limb-darkening is the change of the amount light, as seen by an observer, as one looks from the center of the star towards the edge or limb. Traditionally, observations have not been precise enough to constrain stellar limb-darkening until now, with the precision of observations from the Kepler satellite, and from microlensing observations and interferometry. As such, it is now important to better model this limb-darkening, especially if we want to better understand the properties of exoplanets. Read more here.
Another problem is how stellar atmospheres vary as a function of stellar mass. Earlier stellar atmosphere models did not account for stellar mass as a way to speed up computing time, but now, we can model atmospheres as functions of mass and radius and luminosity. In this way, we can explore how light from these stars change for different masses. In particular, we can look at predicted stellar spectra from model atmospheres and see how particular absorption lines depend on stellar mass.
Other Fun Stuff
There are more projects that I am interested in such as:
- Binarity in Type II Cepheids
- The progenitors of long gamma-ray bursts
- Rotation, pulsation, and mass loss in Be and B[e] stars
- Starspots in mercury-manganese stars
- The Blazkho effect in pulsating variable stars
- Pulsation and rotation in variable white dwarf stars
D’OH! Still under construction, more to come, check back soon.