Narrow radiative recombination continua: A signature of ions crossing the contact discontinuity of astrophysical shocks

Abstract

X-rays from planetary nebulae (PNs) are believed to originate from a shock driven into the fast stellar wind (v ∼ 1000 km s-1) as it collides with an earlier circumstellar slow wind (v ∼ 10 km s-1). 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 microphysics in the realm of theory. This paper presents spectroscopic evidence of 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 recombine with cool electrons at kTe = 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 ?kT > 80 eV, providing for the first time direct evidence of 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.