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) 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.

14 pages, 5 figures, accepted to ApJ. Corrected typos, minor modifications to eq. 5 and corresponding text

References in corpus (12)

• A Compact X-ray Source and Possible X-ray Jets within the Planetary Nebula Menzel 3
• Wind clumping and the wind-wind collision zone in the Wolf-Rayet binary gamma Velorum
• Detailed Far-UV to Optical Analysis of Four [WR] Stars
• The X-Ray Spectrum of a Planetary Nebula at High Resolution: Chandra Gratings Spectroscopy of BD+30 3639
• X-Ray Emission from the Pre-Planetary Nebula Henize 3-1475
• FUSE Observations of Nebular O VI Emission from NGC 6543
• Structure and clumping in the fast wind of NGC6543
• Suzaku Reveals Helium-burning Products in the X-ray Emitting Planetary Nebula BD+303639
• X-Ray Emission from Planetary Nebulae Calculated by 1D Spherical Numerical Simulations
• Redshifted emission lines and radiative recombination continuum from the Wolf-Rayet binary theta Muscae: evidence for a triplet system?
• X-Ray Emission by A Shocked Fast Wind from the Central Stars of Planetary Nebulae
• Morphology of synchrotron emission in young supernova remnants

Cited by in corpus (8)

• Highlights and Discoveries from the Chandra X-ray Observatory
• Photoionization Emission Models for the Cyg X-3 X-ray Spectrum
• Suzaku Observations of the Prototype Wind-Blown Bubble NGC 6888
• New Insights into SNR Evolution Revealed by the Discovery of Recombining Plasmas
• Comparing shocks in planetary nebulae with the solar wind termination shock
• Hot bubbles of planetary nebulae with hydrogen-deficient winds - II. Analytical approximations with application to BD+30${}^{\circ }$3639
• Mixing of hot shocked plasma with cold gas in Nova YZ Ret 2020
• Analysis of the X-ray spectrum of the hot bubble of BD+30°3639