Spectroscopic Survey of Faint Planetary-Nebula Nuclei. V. The EGB 6-Type Central Star of Abell 57

TL;DR

This paper reports the discovery and analysis of an EGB 6-type planetary nebula nucleus in Abell 57, revealing a dense, compact emission knot and a dusty companion, with implications for nebula evolution and rare phenomena.

Contribution

It provides detailed spectroscopic and imaging analysis of Abell 57's nucleus, identifying its unique dense emission knot and proposing a new scenario for EGB 6-type nebulae formation.

Findings

The emission knot is extremely dense (~1.6x10^7 cm^-3) and compact (~4.5 AU).

The central star has a near-IR excess indicating a dusty companion (~1800 K).

EGB 6-type nuclei are rare, possibly due to a transient phenomenon.

Abstract

During our spectroscopic survey of central stars of faint planetary nebulae (PNe), we found that the nucleus of Abell 57 exhibits strong nebular emission lines. Using synthetic narrow-band images, we show that the emission arises from an unresolved compact emission knot (CEK) coinciding with the hot (90,000 K) central star. Thus Abell 57 belongs to the rare class of "EGB 6-type" PNe, characterized by dense emission cores. Photometric data show that the nucleus exhibits a near-IR excess, due to a dusty companion body with the luminosity of an M0 dwarf but a temperature of ~1800 K. Emission-line analysis reveals that the CEK is remarkably dense (electron density ~1.6x10**7 cm**-3), and has a radius of only ~4.5 AU. The CEK suffers considerably more reddening than the central star, which itself is more reddened than the surrounding PN. These puzzles may suggest an interaction between the knot and central star; however, Hubble Space Telescope imaging of EGB 6 itself shows that its CEK lies more than ~125 AU from the PN nucleus. We discuss a scenario in which a portion of the AGB wind that created the PN was captured into a dust cloud around a distant stellar companion; this cloud has survived to the present epoch, and has an atmosphere photoionized by radiation from the hot central star. However, in this picture EGB 6-type nuclei should be relatively common, yet they are actually extremely rare; thus they may arise from a different transitory phenomenon. We suggest future observations of Abell 57 that may help unravel its mysteries.