[Paper Review] An edge-on translucent dust disk around the nearest AGB star L2 Puppis - VLT/NACO spectro-imaging from 1.04 to 4.05 microns and VLTI interferometry

P. Kervella, M. Montargès+7 more|arXiv (Cornell University)|Apr 11, 2014

Stellar, planetary, and galactic studies20 citations

TL;DR

This study presents the first direct imaging of an edge-on, translucent dust disk around L2 Puppis, the nearest AGB star, using VLT/NACO spectro-imaging (1.04–4.05 μm) and VLTI interferometry. The disk's high opacity in the J band and increasing transparency toward longer wavelengths—combined with thermal emission in the L band—were reproduced via Monte Carlo radiative transfer modeling (RADMC-3D), revealing a geometrically thick, edge-on disk that explains the star's long-term variability and challenges the binary companion hypothesis, suggesting instead variable lighting effects on asymmetric circumstellar material.

ABSTRACT

As the nearest known AGB star (d=64pc) and one of the brightest (mK-2), L2 Pup is a particularly interesting benchmark object to monitor the final stages of stellar evolution. We report new lucky imaging observations of this star with the VLT/NACO adaptive optics system in twelve narrow band filters covering the 1.0-4.0 microns wavelength range. These diffraction limited images reveal an extended circumstellar dust lane in front of the star, that exhibits a high opacity in the J band and becomes translucent in the H and K bands. In the L band, extended thermal emission from the dust is detected. We reproduce these observations using Monte-Carlo radiative transfer modeling of a dust disk with the RADMC-3D code. We also present new interferometric observations with the VLTI/VINCI and MIDI instruments. We measure in the K band an upper limit to the limb-darkened angular diameter of theta_LD = 17.9 +/- 1.6 mas, converting to a maximum linear radius of R = 123 +/- 14 Rsun. Considering the geometry of the extended K band emission in the NACO images, this upper limit is probably close to the actual angular diameter of the star. The position of L2 Pup in the Herzsprung-Russell diagram indicates that this star has a mass around 2 Msun and is probably experiencing an early stage of the asymptotic giant branch. We do not detect any stellar companion of L2 Pup in our adaptive optics and interferometric observations, and we attribute its apparent astrometric wobble in the Hipparcos data to variable lighting effects on its circumstellar material. We however do not exclude the presence of a binary companion, as the large loop structure extending to more than 10 AU to the North-East of the disk in our L band images may be the result of interaction between the stellar wind of L2 Pup and a hidden secondary object. The geometric configuration that we propose, with a large dust disk seen almost edge-on, appears particularly favorable to test and develop our understanding of the formation of bipolar nebulae.

Motivation & Objective

• To resolve the circumstellar environment of L2 Puppis, the nearest AGB star, to understand its mass-loss processes and variability.
• To determine whether the observed astrometric wobble in Hipparcos data is due to a binary companion or circumstellar effects.
• To investigate the geometry and physical properties of the extended dust emission seen in infrared imaging.
• To test the hypothesis that the long-term brightness variability of L2 Puppis results from variable dust obscuration in a disk.

Proposed method

• Acquired diffraction-limited, narrow-band imaging across 1.04–4.05 μm using the VLT/NACO adaptive optics system.
• Conducted interferometric observations with VLTI/VINCI and MIDI to measure the limb-darkened angular diameter of the star.
• Applied Monte Carlo radiative transfer modeling using the RADMC-3D code to simulate dust scattering, absorption, and thermal emission in a geometrically thick, edge-on disk.
• Fitted the model to the observed spectral energy distribution and spatially resolved images to constrain dust density, temperature, and disk geometry.
• Used the measured angular diameter (17.9 ± 1.6 mas) to derive a linear radius of 123 ± 14 R☉, consistent with the star's evolutionary status.
• Evaluated the possibility of a hidden binary companion by analyzing asymmetries in the L-band image, particularly a 10 AU loop structure.

Experimental results

Research questions

• RQ1 What is the true nature of the circumstellar material responsible for the long-term brightness variability of L2 Puppis?
• RQ2 Is the 141-day astrometric wobble in Hipparcos data caused by a stellar companion or by variable illumination of asymmetric circumstellar dust?
• RQ3 What is the geometric and physical structure of the extended infrared emission around L2 Puppis, particularly in the L band?
• RQ4 Can the observed wavelength-dependent opacity and transparency of the disk be explained by a single, edge-on dust disk with varying optical depth?
• RQ5 What role might a hidden companion play in shaping the observed disk morphology and driving mass-loss asymmetries?

Key findings

• The VLT/NACO images reveal a high-opacity dust lane in the J band, becoming translucent in H and K bands, with detectable thermal emission in the L band.
• The limb-darkened angular diameter of L2 Puppis is constrained to 17.9 ± 1.6 mas, corresponding to a linear radius of 123 ± 14 R☉.
• Monte Carlo radiative transfer modeling with RADMC-3D successfully reproduces the wavelength-dependent morphology and spectral energy distribution, confirming an edge-on, geometrically thick dust disk.
• The long-term brightness minimum since ~1995 is best explained by the transit of a dense, clumpy region of the disk across the line of sight, consistent with variable dust obscuration.
• No stellar companion was detected in NACO or VINCI observations, and the Hipparcos astrometric wobble is attributed to variable lighting effects on asymmetric circumstellar material rather than orbital motion.
• The 10 AU loop structure in the L-band image suggests possible interaction with a hidden companion, supporting the hypothesis that binarity may be responsible for disk formation.