A Subarcsecond ALMA Molecular Line Imaging Survey of the Circumbinary, Protoplanetary Disk Orbiting V4046 Sgr

Joel H Kastner,P Hily-Blant,T Forveille,D Wilner,K Öberg,D A Dickson-Vandervelde,U Gorti,C Qi,S Andrews

doi:10.3847/1538-4357/aacff7

Abstract

Abstract We present a suite of Atacama Large Millimeter Array (ALMA) interferometric molecular line and continuum images that elucidate, on linear size scales of ∼30–40 au, the chemical structure of the nearby, evolved, protoplanetary disk orbiting the close binary system V4046 Sgr. The observations were undertaken in the 1.1–1.4 mm wavelength range (ALMA Bands 6 and 7) with antenna configurations involving maximum baselines of several hundred meters, yielding subarcsecond-resolution images in more than a dozen molecular species and isotopologues. Isotopologues of CO and HCN display centrally peaked morphologies of integrated emission-line intensity, whereas the line emission from complex nitrile group molecules (HC3N, CH3CN), deuterated molecules (DCN, DCO+), hydrocarbons (as traced by C2H), and potential CO ice line tracers (N2H+, and H2CO) appears as a sequence of sharp and diffuse rings. The dimensions and morphologies of HC3N and CH3CN emission are suggestive of photodesorption of organic ices from the surfaces of dust grains, while the sequence of increasing radius of peak intensity represented by DCN (smallest), DCO+, N2H+, and H2CO (largest) is qualitatively consistent with the expected decline of midplane gas temperature with increasing disk radius. Empirical modeling indicates that the sharp-edged C2H emission ring lies at relatively deep disk layers, leaving open the question of the origin of C2H abundance enhancements in evolved disks. This study of the “molecular anatomy” of V4046 Sgr should serve as motivation for additional subarcsecond ALMA molecular line imaging surveys of nearby, evolved protoplanetary disks aimed at addressing major uncertainties in protoplanetary disk physical and chemical structure and molecular production pathways.

Highlights

Contemporary models describing viscously heated, irradiated protoplanetary disks orbiting solar-mass pre-main sequence (T Tauri) stars typically invoke a combination of gas-phase, gas-grain, and grain surface processes, some of which are driven by the intense dissociating and ionizing radiation from the central stars (e.g., Cleeves et al 2013; Walsh et al 2015)
We recently carried out an Atacama Large Millimeter Array (ALMA) line imaging study of V4046 Sgr that was aimed at investigating the origin of the large CN and C2H abundances that are characteristic of evolved molecular disks. All of these ALMA line surveys of V4046 Sgr have been undertaken in the 1.1–1.4 mm wavelength range (ALMA Bands 6 and 7) with antenna configurations involving maximum baselines of several hundred meters, yielding subarcsecond-resolution images in more than a dozen molecular species and isotopologues
The resulting ALMA 235 GHz and 264 GHz continuum images and moment 0 line images for the molecular species and transitions listed in Table 1, as well as a moment 1 12CO(2–1) image, are presented in Figure 1 in 12′′ × 12′′ fields of view and in Figures 2 and 3 in 6′′ × 6′′ fields of view

Summary

Introduction

Contemporary models describing viscously heated, irradiated protoplanetary disks orbiting solar-mass pre-main sequence (T Tauri) stars typically invoke a combination of gas-phase, gas-grain, and grain surface processes, some of which are driven by the intense dissociating and ionizing radiation from the central stars (e.g., Cleeves et al 2013; Walsh et al 2015). The handful of examples of nearby (D 100 pc), evolved (age ∼5–20 Myr) pre-main-sequence stars of roughly solar mass that are orbited by and actively accreting from primordial circumstellar disks offer unparalleled opportunities to investigate these and other late-stage planet formation processes (Kastner 2016). Two of these star–disk systems, TW Hya and V4046 Sgr—which lie at D = 60.09 ± 0.14 pc and 72.47 ± 0.34 pc, respectively (Gaia Collaboration et al 2018) —have been among the most popular subjects for early millimeter-wave interferometric imaging studies of disks carried out with the Atacama Large Millimeter Array (ALMA)

Results

Discussion

Conclusion