V605 Aquilae

Lore & Background

V605 Aquilae was discovered by Max Wolf in 1920 on photographic plates taken July 4, 1919. Initially thought to be a nova, it was later recognized as a very unusual variable. At its peak it reached magnitude 10.4; prior to 1918 it was magnitude 15 or fainter, then brightened to 12th magnitude in 1918 and stayed at 11th or brighter for over a year before fading. It brightened again to 12th magnitude in late 1921 and 1923, then disappeared. Its spectral type during outbursts was R0, a cool hydrogen-deficient carbon star similar to some R Coronae Borealis stars.

Subsequent detections at magnitudes 18–20 likely captured only a small knot of nebulosity around the star. Hubble images showed the star itself fainter than magnitude 23, while the nebulosity was a bright irregular infrared object 2.5" across. Scattered light revealed a [WC4] spectral type, quite different from the outburst spectrum. In 2013, the central star was detected at magnitude 20.2 with an estimated four magnitudes of extinction, and its spectral type is now [WC4], a hydrogen-deficient, helium and carbon-rich object with strong emission lines.

In 1921, the surface was estimated to consist of 98% helium and 1% carbon, typical of an RCB star. By 2006, abundances were 55% helium, 45% carbon, and 5% oxygen, typical of a WC star. Starting around 1970, the temperature began to increase and is now over 90,000 K. The star is widely believed to be a born-again star, a post-asymptotic-giant-branch star that experienced a very late thermal pulse. An alternative explanation is a nova from an oxygen-neon white dwarf, and a merger between a white dwarf and a normal companion has also been proposed.

Reader's Guide

V605 Aquilae is significant as a rare example of a born-again star, a post-asymptotic-giant-branch object that underwent a very late thermal pulse and resumed fusion. Its dramatic change in spectral type from cool R0 to hot [WC4] over decades, along with its shift in surface composition from nearly pure helium to carbon-rich, provides direct evidence of nuclear processing and mixing in evolved stars. The star's outbursts, initially mistaken for a nova, and its subsequent fading behind dense nebulosity illustrate the complex interplay between stellar eruptions and surrounding material. The detection of a small non-spherical nebula from the 1919 outburst, distinct from the older spherical planetary nebula, offers clues to the geometry of mass loss. The distance of 4,600 parsecs, derived from angular size and radial velocity comparisons, anchors its physical parameters. V605 Aquilae challenges simple stellar evolution models and is a key object for understanding late thermal pulses, white dwarf mergers, and the origins of hydrogen-deficient carbon stars.

Did You Know?

• V605 Aquilae was discovered by Max Wolf on photographic plates taken July 4, 1919.
• Its surface composition changed from 98% helium and 1% carbon in 1921 to 55% helium, 45% carbon, and 5% oxygen by 2006.
• The star's temperature began increasing around 1970 and is now over 90,000 K.
• The visible planetary nebula is spherical and far older than the 1919 outburst, while the outburst produced a smaller non-spherical nebula.