A nova occurs following the build-up of a thin layer of hydrogen on the surface of a white dwarf -  a highly evolved star with the diameter of the Earth and the mass of the sun.  The hydrogen is provided by a close binary companion.  As the pressure in the layer of accreted hydrogen builds and the temperature reaches a critical level, it triggers a thermonuclear runaway.  The light from the explosion significantly exceeds the star's normal brightness and the outer layers are ejected away at high velocities. Over time, the star slowly fades as the fireball expands and cools.  The CHARA Array can image the expanding fireball at the earliest stages after a nova explosion.

 

Nova Delphinus 2013 (V339 Del)

On Aug. 14, 2013, the Japanese amateur astronomer Koichi Itagaki discovered a “new” star that was subsequently named Nova Delphinus 2013 (otherwise known as V339 Del). Within 15 hours of the discovery of Nova Del 2013 and within 24 hours of the actual explosion, astronomers pointed the telescopes of the CHARA Array toward the nova to image the fireball and measure its size and shape. The size of Nova Del 2013 was measured on 27 nights over the course of two months. The observations produced the first images of a nova during the early fireball stage and revealed how the structure of the ejected material evolves as the gas expands and cools. It appears the expansion is more complicated than simple models previously predicted.  The first measurement represents the earliest size yet obtained for a nova event.

Expansion curve of Nova Del 2013. The measured angular diameters in milli-arcseconds are plotted against the day of observation following the detonation of the nova (one milli-arcsecond is a thousandth of an arcsecond; 1 arcsecond is 1/60 of an arcminute; 1 arcminute is 1/60 of a degree). Changes in the expansion rate reveal how the structure of the ejected material evolves as the gas expands and cools. The three inserted panels show images of the fireball produced by observations with the CHARA Array on days 3, 5, and 7 after the outburst. The color indicates the relative brightness, not the actual color; the horizontal bar at the bottom right of each image corresponds to an angular size of 0.5 milli-arcseconds.  Image credit: Schaefer et al. 2014, Nature, 515, 243.


Measuring the expansion of the nova allowed the researchers to determine that Nova Del 2013 is at a distance of 14,800 light years from the sun. This means that, while the explosion was witnessed here on Earth last August, it actually took place nearly 15,000 years ago. During the first CHARA observation, the physical size of the fireball was roughly the size of the Earth’s orbit. When last measured 43 days after the detonation, it had expanded nearly 20-fold, at a velocity of more than 600 kilometers per second, to nearly the size of Neptune’s orbit, the outermost planet in our solar system.

Images of the fireball were created from the interferometric measurements using the University of Michigan Infrared Beam Combiner (MIRC), an instrument that combines all six telescopes of the CHARA Array simultaneously to create images. The observations reveal the explosion was not precisely spherical and the fireball had a slightly elliptical shape. This provides clues to understanding how material is ejected from the surface of the white dwarf during the explosion. The CHARA observations also showed the outer layers became more diffuse and transparent as the fireball expanded. After about 30 days the researchers saw evidence for a brightening in the cooler, outer layers, potentially caused by the formation of dust grains that emit light at infrared wavelengths. Studying how the structure of novae changes at the earliest stages brings new insights to theoretical models of novae eruptions.

References:

Schaefer et al. 2014, Nature, 515, 234