Abell 35

Lore & Background

The nebula is characterised by its unusual appearance, which features a central bow shock surrounded by symmetric emission. In the centre lies a binary star composed of a G-type star and a white dwarf. The optical spectrum of the central star is dominated by a G8 III–IV star that travels through the interstellar medium with a transverse velocity of 150 km/s. The star has a radius of 1.5–3.5 R☉, indicating it is a subgiant, and a projected rotational velocity of 55 ± 10 km/s, showing it rotates fast. Its visual apparent magnitude is 9.6, and it exhibits short-term variability every 0.765 days, which is its rotational period; it has the variable star designation LW Hydrae. When observed in ultraviolet by the International Ultraviolet Explorer, a very hot companion was discovered, categorised as a DAO white dwarf based on strong absorption features of HI and He II. Its effective temperature is about 80,000 Kelvin and its mass is 0.48 M☉, too low to form a planetary nebula. Using the Hubble Space Telescope, the separation of the two stars was estimated to be between 0.08 and 0.14 arcseconds in 1998.

Reader's Guide

Abell 35 is significant as a prototype for a class of planetary nebulae with binary systems comprising a fast-rotating late-type subgiant or giant star and a very hot companion, referred to as Abell 35-type nuclei. Its disputed origin—whether it is a true planetary nebula or interstellar medium photoionised by the passing binary—highlights the complexity of classifying such objects. The bow shock and symmetric emission provide clues to the interaction between the binary system and the surrounding medium. The white dwarf's low mass (0.48 M☉) is noted as too low to form a planetary nebula, supporting the alternative formation hypothesis. The binary's separation measured by the Hubble Space Telescope and the fast rotation of the G-type star (rotational period 0.765 days) are key characteristics that define this system and its legacy in the study of evolved binary stars.

Did You Know?

• The nebula's central star has the variable star designation LW Hydrae.
• The white dwarf companion has an effective temperature of about 80,000 Kelvin.
• The binary system's separation was measured between 0.08 and 0.14 arcseconds using the Hubble Space Telescope.
• The G-type star has a projected rotational velocity of 55 ± 10 km/s.