Chemistry in disks

S Guilloteau,D Semenov,E Chapillon,V Piétu,Th Henning,A Dutrey,V Wakelam,L Reboussin,E Di Folco

doi:10.1051/0004-6361/201527088

Abstract

(abridged) We used the IRAM 30-m to perform a sensitive wideband survey of 30 protoplanetary disks in the Taurus Auriga region. We simultaneously observed HCO$^+$(3-2), HCN(3-2), C$_2$H(3-2), CS(5-4), and two transitions of SO. We combine the results with a previous survey which observed $^{13}$CO (2-1), CN(2-1), two o-H$_2$CO lines and one of SO. We use available interferometric data to derive excitation temperatures of CN and C$_2$H in several sources. We determine characteristic sizes of the gas disks and column densities of all molecules using a parametric power-law disk model. Our study is mostly sensitive to molecules at 200-400 au from the stars. We compare the derived column densities to the predictions of an extensive gas-grain chemical disk model, under conditions representative of T Tauri disks. This survey provides 20 new detections of HCO$^+$ in disks, 18 in HCN, 11 in C$_2$H, 8 in CS and 4 in SO. HCO$^+$ is detected in almost all sources, and its J=3-2 line is essentially optically thick, providing good estimates of the disk radii. The other transitions are (at least partially) optically thin. Variations of the column density ratios do not correlate with any specific property of the star or disk. Disks around Herbig Ae stars appear less rich in molecules than those around T Tauri stars, although the sample remains small. SO is only found in the (presumably younger) embedded objects, perhaps reflecting an evolution of the S chemistry due to increasing depletion with time. Overall, the molecular column densities, and in particular the CN/HCN and CN/C$_2$H ratios, are well reproduced by gas-grain chemistry in cold disks. This study provides a census of simple molecules in disks of radii $> 200-300$ au. Extending that to smaller disks, or searching for less abundant or more complex molecules requires a much more sensitive facility, i.e. NOEMA and ALMA.

Highlights

One of the fundamental problems of modern astrophysics is to comprehend the formation and evolution of planets and to discern their physical and chemical structures (e.g., Burke et al 2014; Marcy et al 2014; Raymond et al 2014) The initial conditions in protoplanetary disks play a crucial role in determining the properties of the emerging planetary systems (Öberg et al 2011; Moriarty et al 2014; Mordasini et al 2015; Thiabaud et al 2015)
We used the IRAM 30 m radio telescope to perform a sensitive wideband survey of 30 stars in the Taurus Auriga region known to be surrounded by gaseous circumstellar disks
Combined with the previous study by Guilloteau et al (2013), our survey provides an important quantitative step in the number of molecules detected in disks with 20 new detections of HCO+, 18 of HCN, 19 of CN, 11 of C2H, 10 of H2CO, 8 of CS, 4 of SO, and 4 of C17O in a sample of 30 disks, of which only 5 had already been studied extensively

Summary

Introduction

One of the fundamental problems of modern astrophysics is to comprehend the formation and evolution of planets and to discern their physical and chemical structures (e.g., Burke et al 2014; Marcy et al 2014; Raymond et al 2014) The initial conditions in protoplanetary disks play a crucial role in determining the properties of the emerging planetary systems (Öberg et al 2011; Moriarty et al 2014; Mordasini et al 2015; Thiabaud et al 2015). Since the initial discovery of ∼10 molecules in protoplanetary disks almost 20 yr ago (Dutrey et al 1997; Kastner et al 1997), the chemical composition of disks still remains largely a mystery

Results

Discussion

Conclusion