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Contributed by Dr. Burkhard Wolff, University of Kiel (Germany) Based on paper: Wolff, B., D. Koester, and R. Lallement, "Evidence for an Ionization Gradient in the Local Interstellar Medium: EUVE Observations of White Dwarfs", A&A, 346, 969, 1999.
The Local Interstellar Medium (LISM) is composed of two components: the Local Bubble - a 50-200 pc wide cavity that is filled with hot ionized gas (T ~ 106 K) - and the Local Cloud/Local Fluff, which extends to 10-20 pc from the sun and is composed of warm (T ~ 5000-10000 K), partially neutral, gas. A major question is what provides the ionization in the Local Fluff?
EUVE observations of hot DA white dwarfs (WDs) - i.e., only Balmer lines in the optical spectrum - provides an excellent means to answer this question. Hot DA WDs (Teff > 25000 K) emit strongly in the EUV; if not too hot (Teff < 50000 K) these sources exhibit a featureless EUV spectrum that is not contaminated by trace metals in its atmosphere. These conditions make hot DA WDs ideal background sources for detecting the LISM ionization edges in the EUV: HI (912 Å), HeI (504 Å), HeII (228 Å), and the HeI autoionization feature (206 Å).
Column density measurements (of HI, HeI, and HeII) for 29 DA WDs along with assumed cosmic H/He abundances were used to determination the ionization degrees for H and He. The results from the comparison of WD distance with extension of the Local Bubble (as determined from soft X-ray background by Snowden et al., 1998) shows NHI < 5 x 1018/cm2 for stars within the Bubble.
The ionization degree for H is between 0 and 70%, with an even higher ionization towards Canis Major, but a best fit for the projection in the direction of (l,b)=(228,-22). The assumption that the Local Fluff is ionized by epsilon CMa - which is located at (l,b)=(240,-11) and determines the EUV radiation field at lambda > 540 Å (Vallerga, 1998) - is consistent with observed gradient in H ionization.
The ionization degree for He is ~40% (mean) and shows no correlation with distance or position of the WD. The stellar EUV radiation is too weak to cause such ionization, and that radiation from the interface between the Local Fluff and Hot Bubble may be important (Slavin & Frisch, 1998).
Slavin, J.D., & P.C. Frisch, in D. Breidtschwerdt, M.J. Freyberg, & J. Trumper (eds.), "The Local Bubble and Beyond," Springer, Berlin, p. 305, 1998.
Snowden, S.L. et al., ApJ 493, 715, 1998.
Vallerga, J.V., ApJ 497, 921, 1998.