AMBER-NACO aperture-synthesis imaging of the half-obscured central star and the edge-on disk of the red giant L₂Puppis

Abstract

Aims. The red giant L 2 Pup started a dimming event in 1994, which is considered to be caused by the ejection of dust clouds. We present near-IR aperture-synthesis imaging of L 2 Pup achieved by combining data from VLT/NACO and the AMBER instrument of the Very Large Telescope Interferometer (VLTI). Our aim is to spatially resolve the innermost region of the circumstellar environment. Methods. We carried out speckle interferometric observations at 2.27 µm with VLT/NACO and long-baseline interferometric observations with VLTI/AMBER at 2.2-2.35 µm with baselines of 15-81 m. We also extracted an 8.7 µm image from the mid-IR VLTI instrument MIDI.Results. The diffraction-limited image obtained by bispectrum speckle interferometry with NACO with a spatial resolution of 57 mas shows an elongated component. The aperture-synthesis imaging combining the NACO speckle data and AMBER data with a spatial resolution of 5.6 × 7.3 mas further resolves not only this elongated component, but also the central star. The reconstructed image reveals that the elongated component is a nearly edge-on disk with a size of ∼180 × 50 mas lying in the E-W direction, and furthermore, that the southern hemisphere of the central star is severely obscured by the equatorial dust lane of the disk. The angular size of the disk is consistent with the distance that the dust clouds that were ejected at the onset of the dimming event should have traveled by the time of our observations, if we assume that the dust clouds moved radially. This implies that the formation of the disk may be responsible for the dimming event. The 8.7 µm image with a spatial resolution of 220 mas extracted from the MIDI data taken in 2004 (seven years before the AMBER and NACO observations) shows an approximately spherical envelope without a signature of the disk. This suggests that the mass loss before the dimming event may have been spherical.

ALMA observations of the nearby AGB star L2 Puppis

“…and the dust disk at a resolution corresponding to 0.9 AU at L 2 Pup. The east-west elongation of the diffuse emission is consistent with the major axis of the dust disk observed by Kervella et al (2014Kervella et al ( , 2015, Lykou et al (2015a,b), and Ohnaka et al (2015) at infrared and visible wavelengths. The visible image of L 2 Pup from Kervella et al (2015) is shown in Fig.…”

“…Ohnaka et al (2015) also concluded a high inclination from the partial obscuration of the stellar disk by the edge of the dust disk. We therefore adopt a value of i = 82 ± 5…”

“…From NACO aperture masking and long-baseline interferometry, Lykou et al (2015a,b) and Ohnaka et al (2015) confirmed the overall geometry and extension of the disk. Kervella et al (2015) identified the polarimetric signature of the circumstellar disk and detected bipolar "hourglass" cones in L 2 Pup's envelope using the VLT/SPHERE AO imaging polarimeter.…”

“…The angular diameter of the photosphere of L 2 Pup A has been measured by Kervella et al (2014) using near-infrared interferometry at θ LD = 17.9 ± 1.6 mas (see also Ohnaka et al 2015). The major axis of the central source in the ALMA continuum map is 20.8 mas at a position angle of 51…”

Mass loss of stars on the asymptotic giant branch

“…Indeed, the presence of a disk has been inferred in a number of post-AGB objects (Bujarrabal et al 2013), while disks around AGB stars have remained elusive. However, clear evidence of a dust disk with an inner and outer edge lying at ≈ 6 and ≈ 13 au, respectively, has recently been found towards the M-star L 2 Pup Lykou et al 2015;Ohnaka et al 2015). This could be the first evidence of disk formation around a star at the end of its evolution on the AGB.…”