CHEMICAL IMAGING OF THE CO SNOW LINE IN THE HD 163296 DISK

Chunhua Qi,Michiel Hogherheijde,Edwin A Bergin,Karin I Öberg,David J Wilner,A Meredith Hughes,Paola D’Alessio,Sean M Andrews

doi:10.1088/0004-637x/813/2/128

Abstract

The condensation fronts (snow lines) of H2O, CO and other abundant volatiles in the midplane of a protoplanetary disk affect several aspects of planet formation. Locating the CO snow line, where the CO gas column density is expected to drop substantially, based solely on CO emission profiles is challenging. This has prompted an exploration of chemical signatures of CO freeze-out. We present ALMA Cycle 1 observations of the N2H+ J=3-2 and DCO+ J=4-3 emission lines toward the disk around the Herbig Ae star HD~163296 at ~0.5" (60 AU) resolution, and evaluate their utility as tracers of the CO snow line location. The N2H+ emission is distributed in a ring with an inner radius at 90 AU, corresponding to a midplane temperature of 25 K. This result is consistent with a new analysis of optically thin C18O data, which implies a sharp drop in CO abundance at 90 AU. Thus N2H+ appears to be a robust tracer of the midplane CO snow line. The DCO+ emission also has a ring morphology, but neither the inner nor the outer radius coincides with the CO snow line location of 90 AU, indicative of a complex relationship between DCO+ emission and CO freeze-out in the disk midplane. Compared to TW Hya, CO freezes out at a higher temperature in the disk around HD 163296 (25 vs. 17 K in the TW Hya disk), perhaps due to different ice compositions. This highlights the importance of actually measuring the CO snow line location, rather than assuming a constant CO freeze-out temperature for all disks.

Highlights

A condensation front is the two-dimensional surface in a protoplanetary disk where abundant volatiles freeze out of the gas phase onto solid particles: the “snow line” marks this condensation front at the disk midplane
Neither the inner nor the outer DCO+ emission edges coincide with the CO snow line location, suggesting a more complicated relationship between CO freeze-out and DCO+ emission than was proposed by Mathews et al (2013), perhaps relating to the “higher temperature” chemical channel discussed by Favre et al (2015)
We have presented and analyzed new Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 1 observations of N2H+ and DCO+ emission, along with archival CO isotopologue data toward the disk around HD 163296 with the aim to constrain the CO snow line location

Summary

Introduction

A condensation front is the two-dimensional surface in a protoplanetary disk where abundant volatiles freeze out of the gas phase onto solid particles: the “snow line” marks this condensation front at the disk midplane. Based on observations of protostars and comets, as well as theoretical models of disk chemistry, the most important snow lines are due to H2O, CO2, CO and N2 freezeout (Oberg et al 2011a; Mumma & Charnley 2011) At each of these snow lines there will be a substantial increase in particle size and solid surface density, augmented by cold finger and pressure trap effects, which may speed up planetesimal formation (Ciesla & Cuzzi 2006; Johansen et al 2007; Chiang & Youdin 2010; Gundlach et al 2011; Ros & Johansen 2013). That is, where a planet accretes its core and envelope with respect to different snow lines will affect its solid and atmospheric C/O ratio, and the planetary chemistry (Oberg et al 2011b)

Results

Discussion

Conclusion