Speaker:	Massimo Marengo (Iowa State University)
Title:	Blowing a Standard Candle: the Disappearing Mass of delta Cephei
Date (JST):	Mon, May 28, 2012, 14:00 - 15:30
Place:	Seminar Room A
Abstract:	Cepheid stars hold the keys of the cosmological distance scale. Thanks to their period-luminosity relation (Leavitt law), they are the first rung in the ladder we use to measure the size and age of the universe. They are also the benchmark for intermediate-mass stellar evolution models. 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 prototype delta Cephei, is currently losing mass. These observations are based on data obtained with the Spitzer Space Telescope in the infrared, and with the Extended Very Large Array in the radio. We found that delta Cephei is associated to a vast circumstellar structure, reminiscent of a termination shock. This structure is created as the wind from the star interacts with 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.

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