The Central Star of NGC 2346 as a Clue to Binary Evolution through the Common Envelope Phase

We present an analysis of the binary central star of the planetary nebula NGC 2346 based on archival data from the International Ultraviolet Explorer, and new low- and high-resolution optical spectra (3700–7300 Å). By including in the spectral analysis the contribution of both stellar and nebular continuum, we reconciled long-time discrepant UV and optical diagnostics and derive E(B–V) = 0.18 ± 0.01. We reclassified the companion star as A5IV by analyzing the wings of the Balmer absorption lines in the high-resolution (R = 67,000) optical spectra. Using the distance to the nebula of 1400 pc from Gaia DR2, we constructed a photoionization model based on abundances and line intensities derived from the low-resolution optical spectra, and obtained a temperature of Teff = 130,000 K and a luminosity of L = 170 L⊙ for the ionizing star, consistent with the UV continuum. This analysis allows us to better characterize the binary system’s evolution. We conclude that the progenitor star of NGC 2346 has experienced a common envelope phase, in which the companion star has accreted mass and evolved off the main-sequence.