Novae and Peculiar Eruptive Variables

A nova is a cataclysmic nuclear explosion in a white dwarf star (WD) in a close binary stellar system. It is caused by the accretion of matter from the companion star on to the WD. Once a certain fraction of matter is collected, the

Credit: NASA/CXC/M.Weiss Artist's conception of a white dwarf star accreting material from a larger companion star in a close binary system.

pressure and temperature is high enough to ignite a nuclear explosion. This nova event then is seen as a sudden brightening of a star. Such events should occur again and again. A few systems are known to have several such events in recent history (timescales of a few tens to a hundred years). This is then the called recurrent novae. But in fact all novae should be recurrent. It only takes often thousands of years between the events.

Novae – especially recurrent Novae with several outbursts known – represent an extreme astrophysical laboratory. They are very bright during the explosion phase. As the occurrence can’t be predicted and as they are often too bright for large telescopes, they are perfect targets for small facilities which are available nearly all the time for the research group (e.g. the UCN – Bochum observatory at Cerro Armazones). But many investigations on the details of the objects can be only obtained after the glow of the explosive cloud has dimmed. This then requires large telescopes and their spectrographs.
We carry out both – observations at large facilities like e.g. ESO and at our own small telescopes. We further do numerical calculations to determine the structure and the physics of these objects. Novae are also indicators of stellar populations in general. Due to the brightness during the outburst they can be observed far out into space. We intend to use them also to study the intergalactic stellar population.

Sometimes novae do not behave normal. For example Nova Cygni 2006 (V2362 Cyg) re-brightened due to the fact, that it ejected a second shell of material after a few hundred days. Thus regularly following such events

The spectra of V2362 Cyg obtained from October 2006 to January 2007 with a 60 cm telescope. The marked transitions of elements give direct evidence on density and temperature of the material (e.g. ionized nitrogen N II from allowed to a forbidden state - green; Neutral oxygen O I to forbidden emission of single ionized O II and further to double ionized O III - red). (Credit: Kimeswenger et al. doi 10.1051/0004-6361:20078821)

spectroscopically during the long decline is crucial to obtain information about unusual physical processes. E.g. the appearance of so called forbidden lines – these are transition in atoms which has extremely large lifetimes and thus only can occur in very low density regions – give us direct measurements of densities without assumptions about the environment.

Some eruptive Variables do not follow any known scheme. V838 Mon – originally classified as Nova – attracted the attention of the astrophysical community after its second outburst, which was discovered at a small observatory. The spectral evolution of V838 Mon was as peculiar as its light curve. In spite of large ejection velocities at the outburst onset (500 km/sec), the expanding material never reached optically thin conditions. It remained optically thick and got cooler and cooler with time, initially mimicking a K giant, then making a complete excursion along the whole sequence of M giant spectra down to M10, and finally entering the realm of L-type supergiants: A new spectral type never seen before anywhere, and characterized by temperatures so low that they were previously measured only in brown dwarf stars. Little consensus has been reached so far on the nature and causes of the outburst of V838 Mon. The interpretations published in the literature cover a wide range of possibilities such as the swallowing of giant planets, merging of the components of a binary star, surface helium flash in a highly evolved and very massive star and a highly degenerate hydrogen flash in a low mass, cool and very slowly accreting white dwarf. Further investigations of the slowly dimming star and it’s environment and his (possible) relatives in the Milky Way (Nova Sgr 1999, Nova Sgr 1943) are on-going.

Faculty member active in this area: Stefan Kimeswenger

The (unspectacular) discovery image of the second outburst of V838 Mon at a small telescope (left) and the widely known spectacular views by the Hubble Space Telescope afterwards (right). (Credit: left panel - Kimeswenger, right panel – NASA/HST)