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

TL;DR

This paper provides spectroscopic evidence of ions crossing the contact discontinuity in astrophysical shocks, revealing a significant temperature jump and offering insights into magnetic field effects and heat conduction in planetary nebulae.

Contribution

It presents the first direct spectroscopic evidence of ions crossing the contact discontinuity in a planetary nebula, constraining the temperature jump and magnetic field influence.

Findings

Detection of narrow radiative recombination continua indicating ions crossing the contact discontinuity.

Quantitative constraint on the temperature jump across the contact discontinuity (>80 eV).

Implication that ion crossing and temperature disparity are common in stellar wind 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) 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.