Creating White Dwarf Photospheres in the Laboratory
Sandia National Laboratories
White dwarf (WD) atmospheric parameters (i.e., effective temperature, T_eff, and surface gravity, log(g) ) are a foundation for many research threads in astrophysics, including: constraining the age of our Galaxy through cosmochronology, probing stellar interiors with asteroseismology, and determining intergalactic distances using type 1a supernovae. Astronomers infer WD atmospheric parameters using atmosphere models, of which theoretical line profiles are critical ingredients. The standard in the WD-astronomy community for theoretical hydrogen line profiles came from the University of Montreal in the work by Tremblay & Bergeron (2009), whose improved profiles resulted in a systematic increase in the inferred T_eff and log(g) (and hence, mass) of observed WDs. Falcon et al. (2010) used an observational mass-determination method mostly independent from line profiles that incorporates gravitational redshifts to also infer an increased WD mean mass. The significance of these works motivates our laboratory experiments to create plasmas at white dwarf photospheric conditions (electron temperature, T_e ~ 1 eV or 12,000 K; electron density, n_e ~ 10^17 cm^-3) to test the theoretical profiles used in WD atmosphere models. We use x-ray radiation produced at the Sandia National Laboratories Z Pulsed Power Facility to initiate plasma formation in a large (120x120x10-mm) gas cell and spectroscopically measure multiple hydrogen Balmer lines. I summarize the plasma formation, present measured profiles of H Balmer lines, and discuss plasma-condition diagnosis. I point out intriguing results suggesting that WD atmospheric parameters inferred from H-beta may be more accurate than those inferred from higher-order lines. The consequence of this follows that of Tremblay & Bergeron by trending spectroscopically inferred surface gravities (and masses) to higher values.
Date: Thursday, 23 April 2015 Time: 11:30 Where: Université de Montréal Pavillon Roger-Gaudry, local D-460 Contact: Patrick Dufour