The chemical composition of TS 01, the most oxygen-deficient planetary nebula

Abstract

The planetary nebula TS 01 (also called PN G 135.9+55.9 or SBS 1150+599A) with its record-holding low oxygen abundance and its double degenerate close binary core (period 3.9 h) is an exceptional object located in the Galactic halo. We have secured observational data in a complete wavelength range to pin down the abundances of half a dozen elements in the nebula. The abundances are obtained via detailed photoionization modelling which takes into account all the observational constraints (including geometry and aperture effects) using the pseudo-3D photoionization code Cloudy_3D. The spectral energy distribution of the ionizing radiation is taken from appropriate model atmospheres. Incidentally we find from the new observational constraints that both stellar components contribute to the ionization: the "cool" one provides the bulk of hydrogen ionization, while the "hot" one is responsible for the presence of the most highly charged ions, which explains why previous attempts to model the nebula experienced difficulties. The nebular abundances of C, N, O, and Ne are found to be 1/3.5, 1/4.2, 1/70, and 1/11 of the solar value respectively, with uncertainties of a factor 2. Thus the extreme O deficiency of this object is confirmed. The abundances of S and Ar are less than 1/30 of solar. The abundance of He relative to H is 0.089 ± 0.009. Standard models of stellar evolution and nucleosynthesis cannot explain the abundance pattern observed in the nebula. To obtain an extreme oxygen deficiency in a star whose progenitor has an initial mass of about 1 M requires an additional mixing process, which can be induced by stellar rotation and/or by the presence of the close companion. We have computed a stellar model with an initial mass of 1 M , appropriate metallicity, and initial rotation of 100 km s −1 , and find that rotation greatly improves the agreement between the predicted and observed abundances.

Heavy elements in planetary nebulae: A theorist's gold mine

“…However, at 1 M we find log (O) = 6.93, with log (C) = 8.16 and log (N) = 6.55. Our predicted O abundance is consistent with the O abundance of the halo PN TS 01, which has the very low O abundance of 1/70 th of the solar value (log (O) ≈ 6.9, Stasińska et al 2010). However, the C abundance is near the derived upper limit for TS 01 and our predicted N is too lower.…”

“…However, the C abundance is near the derived upper limit for TS 01 and our predicted N is too lower. Note that Stasińska et al (2010) find that rapid rotation greatly improves the agreement between the predicted and observed abundances, and in particular enhances the N abundance.…”

The evolution of planetary nebulae

“…We conclude from the discussion in this section that any method to estimate empirically the contribution of dynamics to the total cooling function from an otherwise static model, as done by Stasińska et al (2010) in their study of the metalpoor PN G135.9+55.9, will not provide a sound result. These authors used Eq.…”

“…The metal-poorer sequences are thought to demonstrate nonequilibrium effects more clearly. They are useful for interpreting objects with very low metallicity, like, for instance, the Galactic halo object PN G135.9+55.9 which has a mean metal content below 1/10 solar (Péquignot & Tsamis 2005;Stasińska et al 2010;Sandin et al 2010b). …”

Photoionization models of Planetary Nebulae

“…Another example is the detailed model of TS01 by Stasińska et al (2010). Without any observational constraint on the electron temperature (i.e.…”