Formation and X-ray emission from hot bubbles in planetary nebulae – II. Hot bubble X-ray emission

Abstract

We present a study of the X-ray emission from numerical simulations of hot bubbles in planetary nebulae (PNe). High-resolution, two-dimensional, radiation-hydrodynamical simulations of the formation and evolution of hot bubbles in PNe, with and without thermal conduction, are used to calculate the X-ray emission and study its timedependence and relationship to the changing stellar parameters. Instabilities in the wind-wind interaction zone produce clumps and filaments in the swept-up shell of nebular material. Turbulent mixing and thermal conduction at the corrugated interface can produce quantities of intermediate temperature and density gas between the hot, shocked wind bubble and the swept-up photoionized nebular material, which can emit in soft, diffuse X-rays. We use the chianti software to compute synthetic spectra for the models and calculate their luminosities. We find that models both with conduction and those without can produce the X-ray temperatures and luminosities that are in the ranges reported in observations, although the models including thermal conduction are an order of magnitude more luminous than those without. Our results show that at early times the diffuse X-ray emission should be dominated by the contribution from the hot, shocked stellar wind, whereas at later times the nebular gas will dominate the spectrum. We analyse the effect of sampling on the resultant spectra and conclude that a minimum of 200 counts is required to reliably reproduce the spectral shape. Likewise, heavily smoothed surface-brightness profiles obtained from low-count detections of PNe do not provide a reliable description of the spatial distribution of the X-ray emitting gas.

On the X-ray temperature of hot gas in diffuse nebulae

“…The hot bubble loses heat to the surrounding shell, and the inner surface of the dense shell evaporates into the bub-ble, raising the density in the interface region. This scenario has been explored analytically and numerically in the context of main-sequence stellar wind bubbles (Weaver et al 1977;Reyes-Iturbide et al 2009), PNe (Soker 1994;Steffen et al 2008;Toalá & Arthur 2016) and WR nebulae (Toalá & Arthur 2011). Although these models with thermal conduction can produce gas at a few million degrees, the Xray luminosity for long-lived objects such as main-sequence stellar wind bubbles is orders of magnitude higher than observations suggest.…”

X-ray Shaping of Planetary Nebulae

“…Previous models [23,37] have been superseded by the recent 2-D hydro-dynamical simulations presented by Toalá & Arthur [38]. These new models show the relevance of turbulent mixing and heat conduction on the evolution of the hot gas in PNe [39]. Rayleigh-Taylor instabilities at the interface are shown to be responsible for shadowing instabilities, which have notable effects in the mid-IR (dust) and near-IR H 2 (molecular) morphologies [6].…”

“…The current Chandra and XMM-Newton CCD spectroscopic observations of PNe have limited spectral resolution, typically ≈70 eV, which is degraded in cases of low count rate [39]. High-dispersion grating spectroscopic observations of the two X-ray brightest PNe, BD+30 • 3639 and NGC 6543, have provided interesting insights on the physical processes associated with the production of hot gas in PNe.…”

“…We have seen above that magnetic fields may change this situation, but certainly the interaction of the hot bubble with the AGN envelope is complex and can produce short-lived non-isobaric regions. The 2-D radiative hydro-dynamical simulations of Toalá & Arthur [39] predict the presence of pockets of plasma with varying physical conditions at the interface between the hot bubble and the nebular rim.…”

X-ray Observations of Planetary Nebulae since WORKPLANS I and Beyond

“…On the other hand, the combined effects of heat conduction and turbulent mixing have been tested using high-resolution, two-dimensional, 1 The sensitivity of Chandra HRC has remained basically unchanged in this time period, but the spectral capability of this instrument, which is otherwise very well suited for high-resolution imaging studies, is very limited. radiation-hydrodynamical simulations [20,21]. In these detailed models, instabilities in the wind-wind interaction zone produce clumps and filaments in the swept-up shell of nebular material with notable effects in the time-dependent X-ray emission as the stellar parameters change.…”