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X-rays from planetary nebulae (PNs) are believed to originate from a shockdriven into the fast stellar wind (v ~ 1000 km/s) as it collides with anearlier circumstellar slow wind (v ~ 10 km/s). In theory, the shocked fast wind(hot bubble) and the ambient cold nebula can remain separated by magneticfields along a surface referred to as the contact discontinuity (CD) thatinhibits diffusion and heat conduction. The CD region is extremely difficult toprobe directly owing to its small size and faint emission. This has largelyleft the study of CDs, stellar-shocks, and the associated micro-physics in therealm of theory. This paper presents spectroscopic evidence for ions from thehot 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 PNBD+30 3639 indicating bare C VII ions are recombining with cool electrons atkT_e=1.7+-1.3 eV. An upper limit to the flux of the narrow RRC of H-like C VIis obtained as well. The RRCs are interpreted as due to C ions from the hotbubble of BD+30 3639 crossing the CD into the cold nebula, where theyultimately recombine with its cool electrons. The RRC flux ratio of C VII to CVI constrains the temperature jump across the CD to Delta kT > 80 eV, providingfor the first time direct evidence for the stark temperature disparity betweenthe two sides of an astrophysical CD, and constraining the role of magneticfields and heat conduction accordingly. Two colliding-wind binaries are notedto have similar RRCs suggesting a temperature jump and CD crossing by ions maybe a common feature of stellar wind shocks.

Narrow Radiative Recombination Continua: A Signature of Ions Crossing the Contact Discontinuity of Astrophysical Shocks