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 stellarwind (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 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 ∆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.

X-ray Shaping of Planetary Nebulae

“…These can cross the contact discontinuity into the cold nebula to produce the continuum emission excess attributed to recombination lines of C VI in BD+30 • 3639 [29].…”

Highlights and discoveries from theChandraX-ray Observatory

“…The Chandra X-ray Observatory, the third of NASA's four Great Observatories and its flagship mission for X-ray astronomy, was launched by NASA's Space Shuttle Columbia, Eileen Collins commanding. The launch took place on July 23,1999 and Chandra was boosted to a high earth elliptical orbit by a separable Inertial Upper Stage followed by several burns of engines integral to the spacecraft. The other Great Observatories are the Hubble Space Telescope (HST), the Compton Gamma-Ray Observatory (no longer operating but succeeded in 2008 by the Fermi Gamma-ray Telescope) and the Spitzer Infrared Space Telescope.…”

“…Higher spectral resolution LETG data further reveal that the low-energy end of the hot bubble spectrum is dominated by emission lines of C and O, and constrain its temperature to lie in the range 1.7-2.9 MK [22]. The forest of lines near 0.5 keV is likely due to C6+ ions from the hot bubble penetrating exterior (cooler) nebular gas before recombining [23]. Credit: Adapted from [21] and [23].…”

Thermal instability in X-ray photoionized media in active galactic nuclei

“…The issue was discussed in other astrophysical contexts like wind collisions, but the theory is rather complex and uncertain. Recent Chandra observations of a planetary nebula PN BD+30 • 3639 by Nordon et al (2009) suggest complex structure of such a zone, with ions penetrating the contact discontinuity boundary. The count rate in AGN observation is much lower, so at present the search for such traces in AGN warm absorber is likely to be premature.…”