Narrow Radiative Recombination Continua: A
Signature of Ions Crossing the Contact Discontinuity of
Astrophysical Shocks
Raanan Nordon, Ehud
Behar, Noam
Soker, Joel
H. Kastner and Young
Sam Yu
Abstract. X-rays
from planetary nebulae (PNs) are believed to originate from a
shock driven into the fast stellar wind (v ~ 1000 km/s) as it
collides with an earlier circumstellar slow wind (v ~ 10 km/s).
In theory, the shocked fast wind (hot bubble) and the ambient
cold nebula can remain separated by magnetic fields along a
surface referred to as the contact discontinuity (CD) that
inhibits diffusion and heat conduction. The CD region is
extremely difficult to probe directly owing to its small size
and faint emission. This has largely left the study of CDs,
stellar-shocks, and the associated micro-physics in the realm of
theory. This paper presents spectroscopic evidence for ions from
the hot bubble (kT ~ 100 eV) crossing the CD and penetrating the
cold nebular gas (kT ~ 1 eV). Specifically, a narrow radiative
recombination continuum (RRC) emission feature is identified in
the high resolution X-ray spectrum of the PN BD+30 3639
indicating bare C VII ions are recombining with cool electrons
at kT_e=1.7+-1.3 eV. An upper limit to the flux of the narrow
RRC of H-like C VI is obtained as well. The RRCs are interpreted
as due to C ions from the hot bubble of BD+30 3639 crossing the
CD into the cold nebula, where they ultimately recombine with
its cool electrons. The RRC flux ratio of C VII to C VI
constrains the temperature jump across the CD to Delta kT > 80
eV, providing for the first time direct evidence for the stark
temperature disparity between the two sides of an astrophysical
CD, and constraining the role of magnetic fields and heat
conduction accordingly. Two colliding-wind binaries are noted to
have similar RRCs suggesting a temperature jump and CD crossing
by ions may be a common feature of stellar wind shocks.
Categories. astro-ph
Comment. 14
pages, 5 figures, accepted to ApJ