Analysis of far-UV data of central stars of planetary nebulae: Occurrence and variability of stellar winds

Abstract

The occurrence of stellar wind in the central star of a planetary nebula (CSPN) can be revealed by the presence of P Cygni profiles of high-excitation lines overimposed on its stellar continuum. We examined the entire Far-Ultraviolet Spectroscopic Explorer (FUSE) archive and merged all useful spectroscopic observations of CSPNe to produce the highest quality spectra that can be used to assess the frequency of stellar winds. Furthermore, the individual spectra of each CSPN were compared to search for variability in the P Cygni profile. P Cygni profiles of high-excitation lines have been found in 44 CSPNe, with a clear correlation between the ionization potential of the lines and the effective temperature of the star. We introduce a prescription to derive the terminal wind velocity (v ∞ ) from saturated and unsaturated P Cygni profiles and provide new values of v ∞ for these stars. Another 23 CSPNe do not show P Cygni profiles, or else their data in the FUSE archive are not conclusive enough to determine the occurrence of P Cygni profiles. Variability in the P Cygni profile of high-excitation, far-UV lines is found for the first time in six CSPNe, namely Hen 2-131, NGC 40, NGC 1535, NGC 2392, Sp 3, and SwSt 1. This increases up to 13 the number of CSPNe with variable P Cygni profiles in the UV, including those previously reported using IUE or FUSE observations. Variability is seen primarily in the unsaturated P v and Si iv lines, but also in saturated C iii and O vi lines. The CSPNe with variable P Cygni profiles have similar stellar properties (relatively low log (g) and T eff ) that suggest they are less evolved CSPNe. Some of the CSPNe with variable P Cygni profiles show O vi lines, while their effective temperature is insufficient to produce this ion. We suggest that this ion is produced by Auger ionization from X-rays associated to shocks in their stellar winds, as is the case in massive OB stars of high ionization potential ions that cannot be abundantly produced by photoionizations.

THE CHANDRA PLANETARY NEBULA SURVEY (ChanPlaNS). III. X-RAY EMISSION FROM THE CENTRAL STARS OF PLANETARY NEBULAE

“…This is further supported by the fact that during the high-luminosity phase of post-AGB evolution, where a majority of our detections are made, stellar winds are strongest and subsequently fade after the temperature of the central star has peaked (Cerruti-Sola & Perinotto 1985). Further evidence for self-shocking winds comes from UV spectroscopy where CSPNe with variable P Cygni profiles are observed with high ionization potential ions (e.g., O vi) despite low effective temperatures, which is interpreted as Auger ionization from X-ray emission due to shocks in their stellar winds (Guerrero & De Marco 2013). For X-ray generation via self-shocking winds, the variation in the wind speed is more important than the absolute value.…”

Absence of hot gas within the Wolf-Rayet bubble around WR 16

“…For both observations, the quality of the spectrum registered by SiC2B was limited and the resulting spectrum was not used. Details of the data processing and merging are similar as those described by Guerrero & De Marco (2013). Figure 3 shows the final spectrum obtained from the FUSE observations.…”

TheHerschelPlanetary Nebula Survey (HerPlaNS)

“…While PN investigations have been traditionally done through diagnostics of optical emission lines, PNe are bright sources at a wide range of wavelengths from the radio through the UV, and in some cases, even in the X-ray (e.g., Pottasch et al 1984;Zijlstra et al 1989;Siódmiak & Tylenda 2001;Corradi et al 2003;Schönberner et al 2005;Sandin et al 2008;Sahai et al 2011;Kastner et al 2012;Guerrero & De Marco 2013). Investigations using far-infrared (far-IR) radiation are especially critical to comprehend PNe as complex physical systems in their entirety, because a large fraction of the nebula mass may reside outside the central ionized region (e.g., Villaver et al 2002).…”