An SMA Survey of Chemistry in Disks Around Herbig AeBe Stars

Abstract

Protoplanetary disks around Herbig AeBe stars are exciting targets for studying the chemical environments where giant planets form. Save for a few disks, however, much of Herbig AeBe disk chemistry is an open frontier. We present a Submillimeter Array ∼213–268 GHz pilot survey of millimeter continuum CO isotopologs and other small molecules in disks around five Herbig AeBe stars (HD 34282, HD 36112, HD 38120, HD 142666, and HD 144432). We detect or tentatively detect 12CO 2–1 and 13CO 2–1 from four disks, C18O 2–1 and HCO+ 3–2 from three disks, HCN 3–2, CS 5–4, and DCO+ 3–2 from two disks, and C2H 3–2 and DCN 3–2 from one disk each. H2CO 3–2 is undetected at the sensitivity of our observations. The millimeter continuum images of HD 34282 suggest a faint, unresolved source ∼5.″0 away, which could arise from a distant orbital companion or an extended spiral arm. We fold our sample into a compilation of T Tauri and Herbig AeBe/F disks from the literature. Altogether, most line fluxes generally increase with millimeter continuum flux. Line flux ratios between CO 2–1 isotopologs are nearest to unity for the Herbig AeBe/F disks. This may indicate emitting layers with relatively similar, warmer temperatures and more abundant CO relative to the disk dust mass. Lower HCO+ 3–2 flux ratios may reflect lower ionization in Herbig AeBe/F disks. Lower detection rates and flux ratios for DCO+ 3–2, DCN 3–2, and H2CO 3–2 suggest smaller regimes of cold chemistry around the luminous Herbig AeBe/F stars.