The ALMA Lupus protoplanetary disk survey: evidence for compact gas disks and molecular rings from CN

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: terwisga@strw.leidenuniv.nl
² Max-Planck-Institut für Extraterrestrische Physik, Gießenbachstraße, 85741 Garching bei München, Germany
³ Institute for Astronomy, University of Hawai‘i at Mānoa, 2680 Woodlawn Dr., Honolulu, HI, USA
⁴ Herzberg Astronomy & Astrophysics Programs, NRC of Canada, 5017 West Saanich Road, Victoria, BC V9E 2E7, Canada
⁵ Department of Astronomy, University of California, Berkeley, CA 94720, USA
⁶ Institute of Astronomy, University of Cambridge, Madingley Road, CB3 0HA, Cambridge, UK
⁷ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
⁸ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
⁹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

Received: 17 September 2018
Accepted: 6 November 2018

Abstract

Context. The cyanide radical CN is abundant in protoplanetary disks, with line fluxes often comparable to those of ¹³CO. It is known to be sensitive to UV irradiation of the upper disk atmosphere, with models predicting ring-shaped emission.

Aims. We seek to characterize the CN emission from 94 Class-II disks in the Lupus star-forming region, compare it to observations in other regions, and interpret our observations with a grid of models. The CN emission morphology is discussed for two primordial disks, Sz 71 and Sz 98, and is modeled in more detail.

Methods. ALMA observed CN N = 3−2 in Lupus disks down to sensitivities better than previous surveys. Models constructed with the physico-chemical code DALI are used to study the integrated fluxes of the disks and resolved emission of CN in disks without (dust) substructures.

Results. CN N = 3−2 is bright, and detected in 38% of sources, but its disk-integrated flux is not strongly correlated to either ¹³CO or continuum flux. Compared to pre-ALMA single-dish surveys, no significant difference in the CN flux distributions in Lupus and Taurus–Auriga is found, although ρ Ophiuchus disks may be fainter on average. We find ring-shaped CN emission with peak radii of ~50 AU in two resolved disks.

Conclusions. A large fraction of sources are faint in CN; only exponential gas surface density cutoffs at R_c ≤ 15 AU can reconcile observations with models. This is the first observational evidence of such a compact gas disk population in Lupus. Absolute intensities and the emission morphology of CN are reproduced by DALI models without the need for any continuum substructure; they are unrelated to the CO snowline location. The observations presented here, together with the modeling of these rings, provide a new probe of the structure and conditions in disks, and particularly their incident UV radiation field, if disk size is determined from the data.