In a word, my research provides new insights into the physics of classical Cepheid variable stars with the aim of improving the foundation of the cosmic distance scale.
The distance scale is a crucial element of astronomy and cosmology that defines our understanding of the Universe’s size and age. Specifically, a highly accurate distance scale is required to measure the present-day expansion rate of the universe (the Hubble constant) in order to learn more about Dark Energy, a kind of anti-gravity that accelerates this expansion.
The cosmic distance scale is often referred to as a distance “ladder”, since no single method of measuring astronomical distances applies to all astronomical scales. Hence, the distance ladder is composed of different “rungs” (or methods) that build on top of one another. The best available distance ladder to measure the Hubble constant consists of two rungs: classical Cepheid variable stars (closer) and type Ia supernovae (farther). Since Cepheids set the luminosity zero-point for the supernovae, it is crucial to achieve an accurate calibration of Cepheid distances to ensure that the Hubble constant is accurately measured. The state-of-the-art precision in terms of a Hubble constant measurement is nowadays a staggering 2.4% (Riess et al. 2016). Achieving even better accuracy as needed to investigate Dark Energy will require a careful analysis of intervening sources of error that have thus far been negligible or are not yet well-understood, such as binarity or effects related to chemical composition.
This is where my research comes in. As a Swiss National Science Foundation Postdoctoral Fellow, I investigate the binarity, variability, cluster membership, and evolution of Cepheid variable stars with the aim of identifying avenues for further improving the accuracy of the distance scale. As a recent example, we (Anderson et al., ApJS in press) have recently investigated the impact of binarity (orbital motion) on parallax measurements made with the Hubble Space Telescope. Even delta Cephei, prototype of the classical Cepheids and one of the most studied variable stars, possesses such a companion that must be taken into account by Gaia when measuring its parallax (Anderson et al. 2015).
Aiming to assist the calibration of the distance scale, my research has been successful in providing new insights into the physics of these important stars. Two avenues of research have been particularly exciting and fruitful:
- providing the first detailed analyses of the effects of rotation on the evolution of Cepheids together with the Geneva stellar evolution group (Anderson et al. 2014, 2016b) and
- discovering unexpected irregularity in the radial velocity variability of Cepheids (Anderson 2014, Anderson et al. 2016a).
In the near future, I am particularly excited about the data releases from the ESA space mission Gaia (first one this September), which will provide highly accurate astrometric measurements (positions, proper motions, parallaxes) of more than one billion stars in the Milky Way. Among these will be up to ten thousand Cepheids, for which these measurements will be invaluable for better understanding the physics of these stars (such as the effects of rotation and convection) and improving their ability to calibrate the distance scale. Moreover, Gaia will provide the same information for hundreds of thousdands of RR Lyrae stars, type-II Cepheids, and other pulsating variable stars that may be used as cosmic distance tracers. I am thus very excited to expand my horizon and apply my expertise in stellar physics to these other groups of stars, leveraging Gaia’s potential for serendipitous discovery as well as revolutionizing stellar physics.