VLTI monitoring of the dust formation event of the Nova V1280 Scorpii^*

¹ UMR 6525 H. Fizeau, Univ. Nice Sophia Antipolis, CNRS, Observatoire de la Côte d'Azur, Av. Copernic, 06130 Grasse, France e-mail: Olivier.Chesneau@obs-azur.fr
² Physical Research Laboratory, Navrangpura, Ahmedabad, Gujarat, India
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁴ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁵ Jodrell Bank Center for Astrophysics, The University of Manchester, Oxford Street, Manchester M13 9PL, UK
⁶ European Southern Observatory, Casilla 19001, Santiago 19, Chile

Received: 30 January 2008
Accepted: 25 April 2008

Abstract

Context. We present the first high spatial-resolution monitoring of the dust-forming nova V1280 Sco, performed with the Very Large Telescope Interferometer (VLTI).

Aims. These observations promise to improve the distance determination of such events and constrain the mechanisms leading to very efficient dust formation under the harsh physical conditions encountered in novae ejecta.

Methods. Spectra and visibilities were regularly acquired between the onset of dust formation, 23 days after discovery (or 11 days after maximum), and day 145, using the beam-combiner instruments AMBER (near-IR) and MIDI (mid-IR). These interferometric observations were complemented by near-infrared data from the 1.2 m Mt. Abu Infrared Observatory, India. The observations are initially interpreted in terms of simple uniform models; however more complex models, probably involving a second shell, are required to explain data acquired following  d after outburst. This behavior is in accordance with the light curve of V1280 Sco, which exhibits a secondary peak at about  d, followed by a new, steep decline, suggesting a new dust-forming event. Spherical dust shell models generated with the DUSTY code are used to investigate the parameters of the main dust shell.

Results. Using uniform disk models, these observations allow us to determine an apparent linear expansion rate for the dust shell of  mas day^-1 and the approximate ejection time of the matter in which dust formed of t_ejec = 10.5 ± 7 d, i.e. close to the maximum brightness. This information, combined with the expansion velocity of 500 ± 100 km s^-1, implies a distance estimate of 1.6 ± 0.4 kpc. The sparse uv coverage does not enable deviations from spherical symmetry to be clearly discerned. The dust envelope parameters were determined. The dust mass generated was typically 2–8 10^-9  day^-1, with a probable peak in production at about 20 days after the detection of dust and another peak shortly after  d, when the amount of dust in the shell was estimated as 2.2 10^-7 . Considering that the dust-forming event lasted at least 200–250 d, the mass of the ejected material is likely to have exceeded 10^-4 . The conditions for the formation of multiple shells of dust are also discussed.