My principal area of research involves the development and application of numerical modeling to interpret the spectra of the most massive and luminous stars. Due to their high luminosities massive stars shed a significant fraction of their mass during their lifetime. As a consequence material that has undergone nuclear processing is exposed at the stellar surface. Through spectroscopic analyses we can learn about the composition of this material, and hence gain insights into nuclear processes, convection, and stellar evolution, and consequently into the evolution of our galaxy. The research utilizes many different scientific fields including numerical modeling, radiative transfer, radiation–hydrodynamics, polarization, and atomic physics.
To model the spectrum produced by a star we need to determine the density and temperature structure of the star's atmosphere. These depend on the opacity which is a measure of the resistance of the gas to the flow of radiation. In many stars the opacity can be simply determined from local quantities — the composition of the gas, its density, and its temperature. In such a case we can use local thermodynamic equilibrium (LTE) to fully specify the ionization state of the gas, and the populations in excited states of atoms and ions. However hot stars have an intense radiation field, and this radiation affects the gas. Because the radiation propagates through the atmosphere it couples together regions of different temperature. As a consequence the state of the gas is not in LTE (i.e., non–LTE), and we have to determine the ionization state of the gas, and the populations of excited levels, from first principles (i.e., we solve the equations that describe how individual atomic levels are populated and depopulated). This problem is nontrivial and requires iteration. The state of the gas is determined by the radiation field which itself is determined by the populations. Some elements, such as iron, have thousands of lines which impede the flow of radiation through the atmosphere. The is referred to as line blanketing
To perform my research I have developed a large radiative transfer code, CMFGEN, which is now used by many different investigators. CMFGEN allows non–LTE line blanketed model atmospheres to be constructed and used for detailed spectroscopic analyses. It has been used to study O stars and their descendents — Luminous Blue Variables and Wolf–Rayet Stars.
In collaboration with Luc Dessart (University of Arizona) CMFGEN is currently being used to model Type II SN. The debris of supernovae explosions can be utilized to study their nuclear evolution. We can also use supernovae studies to place constraints on hydrodynamical models — there is still considerable uncertainty in hydrodynamical models of supernovae explosions. Further, supernovae can be used to measure distances. Measuring distances in astrophysics is always difficult — supernovae have advantages since they are very bright and hence can be seen to large distances. We are exploring the accuracy of a distance measuring technique called the expanding photosphere method. This method does not make any assumption about supernovae being standard candles — rather it relies on the expansion of the supernovae to determine distances. In principle the technique is straightforward — in practice there are uncertainties because of radiation transfer effects which we are addressing using CMFGEN.
- "Radiative-transfer models for supernovae IIb/Ib/Ic from binary-star progenitors," Dessart, L, Hillier, D. J., Woosley, S., Livne, E., Waldman, R., Yoon, S.-C., Langer, N., 2015, MNRAS, 453, 2189
- "Critical ingredients of Type Ia supernova radiative-transfer modelling" Dessart, L., Hillier, D. J., Blondin, S., Khokhlov, A., 2014, MNRAS, 441, 3249.
- "Properties of Galactic early-type O-supergiants. A combined FUV-UV and optical analysis," Bouret, J.-C., Hillier, D. J., Lanz, T., Fullerton, A. W., 2012, A&A, 544, 67
- "Time-dependent radiative transfer calculations for supernovae," Hillier, D. J., Dessart, L., 2012, MNRAS, 424, 252.
- "A tale of two stars: The extreme O7 Iaf+ supergiant AV83 and the OC7.5 III((f)) star AV69," Hillier, D.J., Lanz, T., Heap, S., Hubeny, I., Smith, L.J., Evans, C.J., Lennon, D.J., Bouret, J.C., ApJ, 588, 1039, 2003.
A full list of my publications can be found here.