Two-colour photometry of the binary planetary nebula nuclei UU Sagitte and V477 Lyrae: oversized secondaries in post-common-envelope binaries

Abstract

We present new V‐ and R‐passband CCD photometry of UU Sge and V477 Lyr, the eclipsing binary nuclei of the planetary nebulae Abell 63 and Abell 46, respectively. We have performed a simultaneous analysis of VR light curves and estimated the effective temperatures for the primary and secondary stars to be 78 000 ± 3000 and 6136 ± 240 K for UU Sge, 49 500 ± 4500 and 3874 ± 350 K for V477 Lyr. We have also reanalysed the previously measured radial velocities and combined the results with those obtained from the analysis of the light curves to derive absolute parameters of the components. The secondary stars have larger radii than expected from their main‐sequence counterparts at the same masses. We have determined the post‐common‐envelope ages and the thermal time‐scales of the systems and examined the possible reasons of expanded radius of the secondary components, together with some selected post‐common envelope binaries. We conclude that the secondary components of the nuclei of the planetary nebulae are still out of thermal equilibrium along with two post‐common envelope systems: HS 1136+6646 and RE 1016−053. For other systems, magnetic activity has been suggested as the more plausible reason for their expanded radii. We have also estimated the common‐envelope efficiency parameters of UU Sge and V477 Lyr.

Structure and spectra of irradiated secondaries in close binaries

“…Increased heating results in more N iii and C iii, but the higher continuum changes the shape and hence does not reduce this problem. Combining these two considerations results in a T eff, p value of 80 000−85 000 K, comparable to the light curve analysis of Pustynski & Pustylnik (2005); Afşar & Ibanoǧlu (2008) and the revised value of BPH94. Table 3 lists the model prediction for T eq, s on the day-side of the secondary depending on T eff, p for a separation of 2.46 R and T eff, s = 3400 K. Results vary on the order of 50−100 K for 1 dex abundance changes.…”

“…7), even though this requires an enrichment of approximately +1.5 dex in N on the primary surface. This result resembles the light-curve solution by Pustynski & Pustylnik (2005) and the new values by Afşar & Ibanoǧlu (2008).…”

“…The thermal relaxation time-scale of the disturbed outer layers of the secondary, once the common envelope is ejected, is ∼10 4 years, comparable to the estimated age of the PN. This could explain why the secondary is oversized for its mass (PB93; Afşar & Ibanoǧlu 2008) and could mean that the internal structure of the secondary is inhomogeneous, i.e., the accreted material has not mixed in (Prialnik & Livio 1985;Sarna & Ziolkowski 1988). Probably the heating on the day-side of the secondary also contributes to the inflation of the star.…”

“…There is an ongoing effort to constrain system parameters further (e.g. more recent work by Pustynski & Pustylnik 2005;Afşar & Ibanoǧlu 2008) since the uncertainties still do not allow the evolutionary status to be pinpointed. Table 1 shows a compilation of previously derived parameters of UU Sge from Bond et al (1978), de Kool & Ritter (1993, Walton et al (1993), PB93/BPH94, Pustynski & Pustylnik (2005), Afşar & Ibanoǧlu (2008), and our results for comparison.…”

The post-common envelope central stars of the planetary nebulae Henize 2-155 and Henize 2-161

“…Afşar & Ibanoǧlu (2008) considered that the inflated secondaries in Abell 46 and Abell 63 may originate from magnetic activity, ruling this possibility out due to the also increased temperatures (magnetic activity has been shown to result in increased radii and reduced temperatures). De Marco et al (2008) state that the extreme heating of the secondaries by the much hotter primaries could lead to them "puffing up" as the star is irradiated by leakage of radiation from the "day side" to the "night side", the level of this effect is, however, unclear and may not contribute sufficiently to increase the radii by the levels seen.…”