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

Context. We present the first high spatial resolution monitoring of th e dust forming nova V1280 Sco performed with the Very Large Telescope Interferometer (VLTI). Aims. These observations aim at improving the distance determination of such events and constraining the mechanisms leading to very effi cient dust formation under the harsh physical conditions encountered in novae ejecta. Methods. Spectra and visibilities were regularly obtained from the onset of the dust formation 23 days after discovery (or 11 days after maximum) till day 145, using the beam-combiner instruments AMBER (near-IR) and MIDI (mid-IR). These interferometric observations are complemented by near-infrared data from the 1.2m Mt. Abu Infrared Observatory, India. The observations are first interpreted with simple uniform and Gaussian models but more complex models, involving a second shell, are necessary to explain the data obtained from t=110d after outburst. This behavior is in accordance with the light curve of V1280 Sco which exhibits a secondary peak around t=106d, followed by a new steep decline, suggesting a new dust forming event. Spherical dust shell models generated with the DUSTY code are also used to investigate the parameters of the main dust shell. Results. Using uniform disk and Gaussian models, these observations allow us to determine an apparent linear expansion rate for the dust shell of 0.35± 0.03 mas day −1 and the approximate time of ejection of the matter in which dust formed as te jec = 10.5± 7d, i.e. close to the maximum brightness. This information, combined with the expansion velocity of 500±100km.s −1 , implies a distance estimate of 1.6±0.4kpc. The sparse uv coverage does not allow to get clear indications of deviation from spherical symmetry. The dust envelope parameters were determined. The dust mass generated was typically 2-8 10 −9 M⊙ day −1 , with a probable peak in production at about 20 days after the detection of dust and another peak shortly after t=110d, when the amount of dust in the shell was estimated as 2.2 10 −7 M⊙. Considering that the dust forming event lasted at least 200-250d, the mass of the ejected material is likely to have exceeded 10 −4 M⊙. The conditions for the formation of multiple shells of dust are also discussed.