My long-term research program is focused on pursuing new frontiers in stellar astrophysics by approaching massive stars as fundamental cosmological tools. By combining multi-wavelength observations and theoretical modeling of massive stars, star-forming galaxies, and transient phenomena such as SNe and GRBs, we can make new strides towards understanding these critical objects and effectively utilizing them to take full advantage of the frontiers that will be opened up by future facilities such as the ELTs, JWST, LSST, and WFIRST.
Previous group members:
*Graduate students: Erika Zetterlund (2014-2015; currently at CU Boulder)
*Undergraduate: Jeffrey Jennings (2015-2016; currently a graduate student at LMU Munich); Nathen Nguyen (2016; currently a graduate student at Universidad de Chile); Michelle Villenueve (2014-2015; currently a graduate student at CU Boulder)
Massive Stars as Engines: Stellar Evolution, Population Models, and the UV Regime
The massive star population is a crucial ingredient in the study and modeling star-forming galaxies. In recent years my collaborators and I have used the Starburst99 stellar population synthesis and Mappings III photoionization codes to generate a new grid of model spectra for star-forming galaxies (available for download here) and are currently updating these models using the new Geneva evolutionary tracks examining the effects of rotation on the ionizing capabilities of a stellar population.
My latest work is focused on the UV spectra of star-forming galaxies, as this is the key regime dominated by the galaxies’ youngest population of stars. HST SNAP program 13481 was a pilot program for acquiring COS spectra of star-forming galaxies in advance of a large-scale survey. With a large library of UV spectra for star-forming galaxies we can test galaxy models and calibrate ISM diagnostics across multiple wavelengths, unifying observations of star-forming in our universe across an unprecedented wavelength range.
- Zetterlund, E., Levesque, E. M., Leitherer, C., & Danforth, C. W. “Ultraviolet ISM Diagnostics for Star-Forming Galaxies I. Tracers of Metallicity and Extinction” 2015, ApJ, 805, 151
- Levesque, E. M. & Richardson, M. L. A. “[NeIII]/[OII] as a Useful Diagnostic of Ionization Parameter in Star-Forming Galaxies“ 2014, ApJ, 780, 100
- Levesque, E. M. & Leitherer, C. 2013, “Modeling Tracers of Young Stellar Population Age in Star-Forming Galaxies“ 2013, ApJ, 779, 170
- Levesque, E. M., Leitherer, C., Ekstrom, S., Meynet, G., & Schaerer, D. “The Effects of Stellar Rotation. I. Impact on the Ionizing Spectra and Integrated Properties of Stellar Populations” 2012, ApJ, 751, 67
Massive Stars as Lighthouses: Progenitors of the Transient Universe
Time-domain studies of massive stars have highlighted their value as beacons thanks to their spectacular core-collapse deaths and other luminous transient phenomena. Long-duration gamma-ray bursts (LGRBs) are observable across the visible universe and can sample faint far-away star-forming galaxy populations. Closer to home, core-collapse SNe and transients alert us to the death throes of massive stars, helping us to better understand these stars’ final evolutionary stages and explosion mechanisms. Nearby events can be studied through pre-explosion imaging or remnant observations, while at greater distances we depend on studies of the progenitors’ host environments.
My current research is simultaneously focused on both approaches: as a continuation of research begun during my PhD thesis I am continuing to expand the host galaxy sample for LGRBs and core-collapse SNe using the Magellan telescopes at Las Campanas and the Keck telescopes at Mauna Kea. At the same time, several recent compelling events, such as the 2012 eruptions of SN 2009ip, are highlighting the importance of refining spectroscopic follow-up techniques for the wealth of nearby transients being discovered by current and future survey programs.
- Levesque, E. M. “The Host Galaxies of Long-Duration Gamma-Ray Bursts”, PASP, 2014, 126, 1 (invited review)
- Levesque, E. M., Stringfellow, G. S., Ginsburg, A. G., Bally, J., & Keeney, B. A. “The Peculiar Balmer Decrement of SN 2009ip: Constraints on Circumstellar Geometry” 2014, AJ, 147, 23
- Levesque, E. M., Chornock, R., Soderberg, A. M., Berger, E., & Lunnan, R., “Host Galaxy Properties of the Subluminous GRB 120422A/SN 2012bz” 2012, ApJ, 758, 92
Massive Stars as Laboratories: Stellar Astrophysics in (and beyond) the Local Group
All of the work described above depends on an accurate picture of massive stars’ physical properties and evolution. Fortunately, massive star populations in nearby galaxies are uniquely available as local laboratories, allowing us to observe their physical properties and evolutionary behavior in exceptional detail. Given the current impossibility of directly observing individual stars at high redshifts, understanding the massive stars of the early universe is particularly dependent on the study of their analogs in neighboring low-metallicity galaxies. My previous research has determined the physical properties of red supergiants – a critical mass-losing phase in the evolution of moderately-massive stars – in the Milky Way, Magellanic Clouds, and low-metallicity dwarfs NGC 6822 and WLM. By further expanding this work to greater distances and to other phases of massive star evolution, we can further diversity our sample of well-defined stellar populations that can then be applied to research at greater distances.
Finally, the ability to study extragalactic massive stars in such detail has opened the door to some remarkable new discoveries. My colleagues and I recently detected the first robust Thorne-Zytkow object (TZO) candidate in the SMC (paper, press). The confirmed existence of TZOs would have profound implications for the field of stellar astronomy, and we are continuing this work with new observations of our candidate and cutting-edge models of these stars’ interiors.
- Levesque, E. M., Massey, P., Zytkow, A., & Morrell, N. “Discovery of a Thorne-Żytkow Object Candidate in the Small Magellanic Cloud“, 2014, MNRAS Letters, 443, L94
- Levesque, E. M. & Massey, P. “Spectral Types of Red Supergiants in NGC 6822 and the Wolf-Lundmark-Melotte Galaxy” 2012, AJ, 144, 2
- Levesque, E. M. “The Physical Properties of Red Supergiants” 2010, New Astronomy Reviews, 54, 1 (invited review)