A Survey of C₂H, HCN, and C¹⁸O in Protoplanetary Disks

Abstract

Molecular lines observed towards protoplanetary disks carry information about physical and chemical processes associated with planet formation. We present ALMA Band 6 observations of C 2 H, HCN, and C 18 O in a sample of 14 disks spanning a range of ages, stellar luminosities, and stellar masses. Using C 2 H and HCN hyperfine structure fitting and HCN/H 13 CN isotopologue analysis, we extract optical depth, excitation temperature, and column density radial profiles for a subset of disks. C 2 H is marginally optically thick (τ ∼1-5) and HCN is quite optically thick (τ ∼ 5-10) in the inner 200 AU. The extracted temperatures of both molecules are low (10-30K), indicative of either sub-thermal emission from the warm disk atmosphere or substantial beam dilution due to chemical substructure. We explore the origins of C 2 H morphological diversity in our sample using a series of toy disk models, and find that disk-dependent overlap between regions with high UV fluxes and high atomic carbon abundances can explain a wide range of C 2 H emission features (e.g. compact vs. extended and ringed vs. ringless emission). We explore the chemical relationship between C 2 H, HCN, and C 18 O and find a positive correlation between C 2 H and HCN fluxes, but no relationship between C 2 H or HCN with C 18 O fluxes. We also see no evidence that C 2 H and HCN are enhanced with disk age. C 2 H and HCN seem to share a common driver, however more work remains to elucidate the chemical relationship between these molecules and the underlying evolution of C, N, and O chemistries in disks.

An Evolutionary Study of Volatile Chemistry in Protoplanetary Disks

“…C 18 O, C 2 H, H 13 CN, DCN, and HC 15 N observations towards the Class 0/I sources are presented in this work, along with HCN isotopologue observations for a subset of the Class II sources. C 2 H, HCN, and C 18 O observations towards the Class II disks were previously presented in Bergner et al (2019b). HC 15 N and DCN were also previously targeted towards six disks in our sample in Guzmán et al (2017) and Huang et al (2017).…”

“…We therefore focus on transitions of the CO isotopologues CO, 13 CO, and C 18 O; the HCN isotopologues HCN, H 13 CN, HC 15 N, and DCN; and C 2 H. Table 1 summarizes the previously unpublished molecular line observations within each ALMA program. We note that this data set is complementary to that presented in Bergner et al (2019b), which included observations of CO or C 18 O, HCN or H 13 CN, and C 2 H towards each Class II…”

“…The full Class II disk sample is described in Bergner et al (2019b), and source properties are shown in Table 2. The sample spans a range of physical properties including age (0.4-14 Myr), stellar mass (0.5-2 M ), and stellar luminosity (0.2-25 L ).…”

“…The HC 15 N 3-2 transition was observed towards CI Tau, DM Tau, DO Tau, LkCa 15, and MWC 480 as part of project 2016.1.00627.S. For sources with DCN or HC 15 N observations, we also re-analyze the HCN and H 13 CN 3-2 observations originally presented in Bergner et al (2019b) to facilitate a consistent analysis of isotopic ratios. Lastly, we include previously unpublished observations of 13 CO 2-1 towards J1609, J1612, and J1614, for which C 18 O emission was not detected.…”

Detection of Phosphorus-bearing Molecules toward a Solar-type Protostar

“…CO AND H 2 COLUMN DENSITIES CO column densities are estimated by fitting the C 17 O 1-0 hyperfine structure. The procedure used to model the hyperfine spectrum is described in detail in Bergner et al (2019). C 17 O spectral line parameters are taken from the CDMS catalog using measurements from Klapper et al (2003), and are listed in Table 2.…”