Posted: July 25, 2008
For the first time, astronomers have witnessed the expulsion of a shell of dusty gas around a freshly erupted nova and tracked its evolution for over 200 days, providing a new and invaluable way of estimating the distances to nearby novae.
Nova Scorpii 2007a, which was discovered on 4 February 2007 in the constellation Scorpius, provided the focus for the Very Large Telescope Interferometer (VLTI) study. Novae are understood to occur in a double star system once a dense white dwarf star has over-fed on material from a companion red dwarf star, causing the white dwarf to explode in a bright display of fireworks often visible to the naked eye.
Observations were carried out at the Very Large Array on the Paranal Mountain in Chile. Image: ESO.
A high resolution survey was carried out by astronomers using the VLTI over five months following the explosion, who witnessed and tracked the formation and evolution of dust around the object, the first time such measurements have been made. The first observations were secured 23 days after the nova’s discovery and showed that the stellar source was extremely compact, less than 1 milli-arcsecond in diameter, comparable to viewing one grain of sand from about 100 kilometres away. A few days later the source measured 13 milli-arcseconds.
"It is most likely that the latter size corresponds to the diameter of the dust shell in expansion, while the size previously measured was an upper limit of the erupting source," explains lead author Olivier Chesneau.
The ring of dusty debris was present for more than 200 days, during which it expanded at a rate close to 2 million kilometres per hour. The erupting source was screened from view, becoming more than 10,000 times dimmer at visual wavelengths over those 200 days.
"This is the first time that the dust shell of a nova is spatially
The astronomers speculate that the mass of dust ejected from the star could be equivalent to over 30 times the mass of Earth, an impressive feat when considering that the star itself was less than the radius of the Earth, with the dust component comprising less than one percent the mass of the star. However, factors such as the stability of the dust, and the duration of dust formation will effect this value, and “the error bar is large” says Chesneau, “because it is difficult to know exactly when the dust production stops”.
The measurement of the angular expansion rate away from the source enabled the astronomers to derive an accurate distance to the object, in this case about 5500 light-years, providing a new and promising technique for obtaining distances of nearby novae. The observations also provided new details on the feeding mechanisms of these stellar vampires.
Chesneau reveals that the next step will be to study the emission lines of the erupting star, and to look for other novae events to learn more about their first moments. “Each line form carries a lot of information about the local density and temperature,” he says. “As interferometrists, our speciality is to spatially resolve the first moments; the goal is to see the line forming regions as early as possible.”
For Nova Scorpii 2007a, further analysis can be performed with other telescopes - such as Hubble - since the ejecta can now be more easily spatially resolved.
An animation of the evolution of the dust shell can be found on the ESO website.