Blowing a Standard Candle: The Disappearing Mass of Delta Cephei
|Date/Time:||Monday, 27 Feb 2012 - Wednesday, 29 Feb 2012|
|Location:||Physics Hall Room 5|
Cepheid stars hold the keys of the cosmological distance scale. Thanks to a well known relation between their luminosity and period (Levitt law), they are the primary "standard candles" used to measure the size and age of the universe. They are also the benchmark for stellar evolution models of intermediate mass stars. Despite their importance, there are still outstanding puzzles in the theoretical understanding of Cepheids. Namely, the mass predicted by evolutionary models is significantly larger than the mass estimated by pulsation theory, or directly measured in binary systems. A possible solution is that these stars lose mass while they are in the Cepheid phase.
I will present the first direct observation that one Cepheid star, the class namesake delta Cephei, is currently losing mass. These observations are based on data obtained with the Spitzer Space Telescope and with the Very Large Array radio-telescope. We found that delta Cephei is associated to a vast circumstellar structure, reminiscent of a bow shock. This structure is created as a strong wind from the star pushes against the local interstellar medium. We measure a velocity of the wind of ~ 36 km/s, and a mass loss rate in the range 1E-7 - 1E-6 Mo/yr. I will discuss the importance of this discovery in the context of the Cepheid mass discrepancy, and the cosmological distance scale.