X-ray emission from planetary nebulae calculated by 1D spherical numerical simulations

Abstract

We calculate the X‐ray emission from both constant and time‐evolving shocked fast winds blown by the central stars of planetary nebulae (PNe) and compare our calculations with observations. Using spherically symmetric numerical simulations with radiative cooling, we calculate the flow structure and the X‐ray temperature and luminosity of the hot bubble formed by the shocked fast wind. We find that a constant fast wind gives results that are very close to those obtained from the self‐similar solution. We show that in order for a fast shocked wind to explain the observed X‐ray properties of PNe, rapid evolution of the wind is essential. More specifically, the mass‐loss rate of the fast wind should be high early on when the speed is ∼300–700 km s−1, and then it needs to drop drastically by the time the PN age reaches ∼1000 yr. This implies that the central star has a very short pre‐PN (post‐asymptotic giant branch) phase.

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

“…This is consistent with the previous theoretical work of Akashi et al (2007), who showed that diffuse X-ray emission can come from a shocked stellar wind whose velocity is rapidly increasing in the first 1000 yrs of post-AGB evolution.…”

“…In the post-AGB stage the stellar parameters are determined by the mass of the central hot object that remains after the hydrogen envelope has been expelled. Previous 1D numerical work by Akashi et al (2007) and Steffen et al (2008) has shown that it is important to take into account the time history of the stellar wind as it increases in velocity and decreases in mass-loss rate over the few hundred years after the AGB envelope is ejected. In order to explain the low X-ray temperatures, either the X-rays come from wind shocked while the velocity was still low (vw < 500 km s −1 ), or some physical process operates to produce large quantities of gas at about a million degrees even when the stellar wind velocity is higher than 1000 km s −1 .…”

“…Mellema & Frank 1995;Stute & Sahai 2006;Akashi et al 2007;Lou & Zhai 2010), even for the case of non-isotropic wind ejections (see Akashi et al 2008). Given that diffuse X-ray emission is a relatively common phenomenon in planetary nebulae, any theoretical model should be able to explain all the features of the X-ray emission for a wide range of model parameters.…”

Supernova remnants, planetary nebulae and superbubbles: Prospects for new XMM‐Newton observations

“…Other possibilities include a rapid weakening of the fast wind in terms of mass loss, a slower moderate velocity wind of ≃ 500 km/s (post-AGB wind), or two opposite jets (collimated fast wind) as discussed in Akashi et al (2007).…”

“…However, the understanding of their origin, structures and evolution is just beginning with a better coverage of the entire electromagnetic spectrum (Kwok 2005). In particular, the nature and origin of their extended, spatially resolved X-ray emission is under close investigation since the launch of Chandra and XMM-Newton X-ray observatories (Akashi et al 2007 and references therein). PNe are formed by the interaction of a tenuous fast wind with a copious slow Asymptotic Giant Branch (AGB) wind, that produces either elliptical or bipolar PNe (Stute & Sahai ⋆ Corresponding author: e-mail: adecourchelle@cea.fr Frank & Mellema 1994).…”