Deep optical imaging of asymptotic giant branch circumstellar envelopes

Abstract

We report results of a program to image the extended circumstellar envelopes of asymptotic giant branch (AGB) stars in dust-scattered Galactic light. The goal is to characterize the shapes of the envelopes to probe the mass-loss geometry and the presence of hidden binary companions. The observations consist of deep optical imaging of 22 AGB stars with high mass loss rates: 16 with the ESO 3.6 m NTT telescope, and the remainder with other telescopes. The circumstellar envelopes are detected in 15 objects, with mass loss rates > ∼ 2 × 10 −6 M yr −1 . The surface brightness of the envelopes shows a strong decrease with Galactic radius, which indicates a steep radial gradient in the interstellar radiation field. The envelopes range from circular to elliptical in shape, and we characterize them by the ellipticity (E = major/minor axis) of iso-intensity contours. We find that ∼50% of the envelopes are close to circular with E < ∼ 1.1, and others are more elliptical with ∼20% with E > ∼ 1.2. We interpret the shapes in terms of populations of single stars and binaries whose envelopes are flattened by a companion. The distribution of E is qualitatively consistent with expectations based on population synthesis models of binary AGB stars. We also find that ∼50% of the sample exhibit small-scale, elongated features in the central regions. We interpret these as the escape of light from the central star through polar holes, which are also likely produced by companions. Our observations of envelope flattening and polar holes point to a hidden population of binary companions within the circumstellar envelopes of AGB stars. These companions are expected to play an important role in the transition to post-AGB stars and the formation of planetary nebulae.

Extended Dust Emission from Nearby Evolved Stars★

“…1a shows extended emission up to a projected radius of ∼ 1 at the two SCUBA-2 wavelengths and ∼ 4.5 at the PACS wavelengths. The SCUBA-2 extent seen is comparable to the optical extent seen by Mauron et al (2013). In addition, more than half of the emission is from the extended component at all wavelengths.…”

Mass loss of stars on the asymptotic giant branch

“…This is determined by photodissociation and hence the strength of the interstellar UV field. Mauron et al (2013) have shown, using dust-scattered light, that there is a steep gradient in the strength of the interstellar radiation field with Galactic radius, and the distribution of OB associations causes substantial local variations. In lower-metallicity and/or extragalactic environments, the interstellar UV field may be very different (e.g., McDonald et al 2015).…”

“…Our knowledge of the morphology of the dust CSEs has improved significantly due to observations in dust-scattered light (e.g., Mauron et al 2013;Sahai and Mack-Crane 2014), and through observations with the Herschel Space Observatory of thermal dust emission (e.g., Cox et al 2012a). In particular, the latter shows that on the larger scales there are significant departures from sphericity, but these are not necessarily connected to a non-isotropic mass loss.…”

HST IMAGES REVEAL DRAMATIC CHANGES IN THE CORE OF IRC+10216

“…To date, binary companions have been detected for only a small number of AGB stars with low mass-loss rates through composite spectra or light curves (Jorissen et al 2003). There are indications that a significant fraction of AGB stars have binary companions, such as elliptical CSEs (Mauron et al 2013) as expected from binary AGB stars (Huggins et al 2009), and spiral-shell patterns in the CSEs (Mauron & Huggins 2006;Mayer et al 2011Mayer et al , 2014Maercker et al 2012;Kim et al 2013;Cernicharo et al 2015;Decin et al 2015) modeled by AGB winds under the gravitational influence of binary companions (Theuns & Jorissen 1993;Soker 1994;Mastrodemos & Morris 1999;Kim & Taam 2012a, 2012b, 2012cKim et al 2013).…”