Hot bubbles of planetary nebulae with hydrogen-deficient winds

Abstract

Context. The first high-resolution X-ray spectroscopy of a planetary nebula, BD +30 • 3639, opened the possibility to study plasma conditions and chemical compositions of X-ray emitting "hot" bubbles of planetary nebulae in much greater detail than before. Aims. We investigate (i) how diagnostic line ratios are influenced by the bubble's thermal structure and chemical profile, (ii) whether the chemical composition inside the bubble of BD +30 • 3639 is consistent with the hydrogen-poor composition of the stellar photosphere and wind, and (iii) whether hydrogen-rich nebular matter has already been added to the bubble of BD +30 • 3639 by evaporation. Methods. We applied an analytical, one-dimensional (1D) model for wind-blown bubbles with temperature and density profiles based on self-similar solutions including thermal conduction. We also constructed heat-conduction bubbles with a chemical stratification. The X-ray emission was computed using the well-documented CHIANTI code. These bubble models are used to re-analyse the high-resolution X-ray spectrum from the hot bubble of BD +30 • 3639. Results. We found that our 1D heat-conducting bubble models reproduce the observed line ratios much better than plasmas with single electron temperatures. In particular, all the temperature-and abundance-sensitive line ratios are consistent with BD +30 • 3639 X-ray observations for (i) an intervening column density of neutral hydrogen, N H = 0.20 +0.05 −0.10 ×10 22 cm −2 , (ii) a characteristic bubble X-ray temperature of T X = 1.8 ± 0.1 MK together with (iii) a very high neon mass fraction of about 0.05, virtually as high as that of oxygen. For lower values of N H , we cannot exclude the possibility that the hot bubble of BD +30 • 3639 contains a small amount of "evaporated" (or mixed) hydrogen-rich nebular matter. Given the possible range of N H , the fraction of evaporated hydrogen-rich matter cannot exceed 3% of the bubble mass. Conclusions. The diffuse X-ray emission from BD +30 • 3639 can be well explained by models of wind-blown bubbles with thermal conduction and a chemical composition equal to that of the hydrogen-poor and carbon-, oxygen-, and neon-rich stellar surface.Article published by EDP Sciences A98, page 1 of 23 A&A 620, A98 (2018)

FAST Search for Circumstellar Atomic Hydrogen. II. Is BD+30°3639 an Interacting Planetary Nebula?

“…An optical spectroscopic study has shown that the N + and O ++ ionized regions have an expansion velocity of 28 km s −1 and 35.5 km s −1 , respectively (Bryce & Mellema 1999). It is well known that BD+30 contains a central hot bubble (e.g., Heller et al 2018), which is also revealed as an extended X-ray emission (Leahy et al 2000). The ionization and shock heating may result in a radial pressure gradient, causing an acceleration of the ionized gas.…”

Discovery of Radial Spectral Hardening in the Hot Bubble of Planetary Nebula BD+30°3639 with Median Energy Imaging

“…However, when there is sufficient signal to constrain the hot bubble abundances, the chemical composition is often found to be discrepant with both the central star and nebular abundances (e.g., Toalá et al 2019). Mixing between the central star and nebular abundances is included in theoretical efforts to understand hot bubble X-ray emission (Steffen et al 2008;Toalá & Arthur 2016;Heller et al 2018;Toalá & Arthur 2018). In the X-ray-emitting temperature ranges of hot bubbles, the abundances of carbon, nitrogen, oxygen, neon, and iron have the strongest influence on the spectral shape of the X-ray emission.…”

“…Hot bubbles are expected to scale with the square of the fast wind velocity; however, Chandra and XMM-Newton observations indicate that plasma temperatures are much cooler than expected (e.g., Kastner et al 2008;Ruiz et al 2013). Understanding the origin of these cooler plasma temperatures is an unresolved area of research being explored with a variety of approaches (Soker & Kastner 2003;Akashi et al 2007;Steffen et al 2008;Toalá & Arthur 2016;Heller et al 2018;Toalá & Arthur 2018).…”

Stellar wind models of central stars of planetary nebulae

“…Some planetary nebulae are sources of X-ray emission, which may have a diffuse component coming from the nebula (e.g., Chu et al 2001, Heller et al 2018) and a point-source component associated with the central star (Freeman et al 2014, Montez et al 2015. X-ray emission of O stars, which is supposed to originate in their winds (Feldmeier et al 1997), shows a strong correlation between the X-ray luminosity and stellar luminosity roughly as L X ≈ 10 −7 L (Nazé 2009).…”