62nd Annual Meeting of the APS Division of Plasma Physics
Volume 65, Number 11
Monday–Friday, November 9–13, 2020;
Remote; Time Zone: Central Standard Time, USA
Session ZI01: Invited: ICF and HEDP Diagnostics
9:30 AM–12:30 PM,
Friday, November 13, 2020
Chair: Johan Frenje, MIT
Abstract: ZI01.00002 : Constraining Stellar Evolution through Helium Spectral Line Broadening Experiments at Sandia National Laboratories' Z-Machine*
10:00 AM–10:30 AM
Live
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Abstract
Author:
Marc-Andre Schaeuble
(Sandia National Laboratories)
White Dwarfs (WDs) are the final evolutionary state of nearly all stars in
the sky, including our Sun. Fundamental WD parameters such as surface
temperature (T$_{\mathrm{eff}})$ and gravity (log $g)$ can constrain many
aspects stellar evolution. The origin of helium atmosphere WDs (DBs) is
unknown and thus highlights deficiencies in current stellar evolution
models. Several DB evolutionary channels have been proposed, but a lack of
accurate DB log $g$ measurements makes discriminating between different models
difficult. DB log $g$ values are obtained by fitting model atmospheres to
observed spectra. The derived log $g$ strongly depends on our understanding of
line broadening in DB atmospheres. Results presented in Bergeron et al.
(2011) and Kepler et al. (2015) show an unexpected DB log $g$ increase at
T$_{\mathrm{eff\thinspace \thinspace }}$\textless 16,000 K, indicating that
the modeled line widths at those temperatures are severely underestimated.
An incomplete understanding of neutral broadening has been identified as the
leading hypothesis for this behavior. We investigated this phenomenon by
performing first-of-their-kind at-parameter neutral broadening experiments
at Sandia National Laboratories' Z-machine, the most energetic pulsed x-ray
source on earth. Our line width measurements of He I at 5875 \textunderscore
, the strongest optical transition in DB spectra, are at least a factor of
1.5 wider than that of any previous experiment or Stark broadening
prediction for this feature. The varying neutral helium fraction in these
experiments provides evidence that the extra broadening is most likely
caused by neutrals in the plasma. Derrider et al. (1975), the neutral
broadening theory used by the WD community, underpredicts the neutral
broadening contribution by at least an order of magnitude. This experimental
evidence suggests that the DB log $g$ upturn could result from an incomplete
neutral broadening theory and could thereby also constrain DB evolution.
*SNL is managed and operated by NTESS under DOE NNSA contract DE-NS0003525. MS acknowledges the support of WCAPP and the Z Fundamental Science Program.