Tuesday, October 15, 2013

Ancient Nova Sell around Z Camelopardalis

Z Camelopardalis is a dwarf nova located at a distance of about 530 light years. It consists of a white dwarf accreting hydrogen-rich matter from a companion red dwarf star. The accreted matter forms an accretion disk around the white dwarf where instability in the accretion disk causes it to episodically dump much of itself onto the white dwarf. The hydrogen-rich matter gradually accumulates on the white dwarf’s surface. Each dump liberates a large amount of gravitational potential energy and causes Z Camelopardalis to brighten by up to a factor of 40. This process repeats itself every 20 days or so.

Figure 1: Artist’s concept of Z Camelopardalis, a stellar system featuring a white dwarf accreting hydrogen-rich matter from a companion star. Credit: NASA/JPL-Caltech.

It is predicted that white dwarfs like Z Camelopardalis will eventually accumulate sufficient hydrogen-rich matter to undergo classical nova eruptions. During a classical nova, the accumulated hydrogen-rich matter on the white dwarf ignites and fuses hydrogen into other heavier elements in a thermonuclear runaway process. The surface of the white dwarf explodes and creates clearly visible shells of ejected material in the aftermath. A classical nova is thousands of times more luminous than a dwarf nova.

In January 2004, an image of Z Camelopardalis and its surroundings was acquired by NASA’s Galaxy Evolution Explorer (GALEX) satellite, a space-based ultraviolet (UV) telescope. The image shows an arc of UV emitting material centred on, and located southwest (SW) of Z Camelopardalis. Linear nebulosities to the northeast (NE) and southeast (SE) are also visible. These features appear to extend out to a distance of up to ~2 light years around Z Camelopardalis and are believed to be part of a shell of ejected material that was formed when Z Camelopardalis underwent a classical nova eruption in the past.

Figure 2: Image of the dwarf nova Z Camelopardalis and its surroundings. Z Camelopardalis is circled. (M. Shara et al., 2012)

Observations of other classical novae show ejection velocities in the range 300 to 3000 km/s. Given the physical size of the classical nova ejecta around Z Camelopardalis, it allows the age range of the ejecta to be set at 2,400 to 240 years. A larger age is favoured due to the “snowplough effect” observed with other classical novae where the ejecta sweeps up the interstellar medium and decelerates. In fact, an age of more than 1,300 years is most likely for the ejecta around Z Camelopardalis. This means Z Camelopardalis underwent a classical nova eruption over 1,300 years ago. During such an event, Z Camelopardalis would have been one of the brightest stars in the sky for at least a few days. It is possible that historical observations of such an event might exist.

- M. Shara et al., “An ancient nova shell around the dwarf nova Z Camelopardalis”, Nature 446, 159-162 (8 March 2007)
- M. Shara et al., “The Inter-Eruption Timescale of Classical Novae from Expansion of the Z Camelopardalis Shell”, ApJ 756 (2012) 107 arXiv:1205.3531 [astro-ph.SR]