The ALMA Lupus protoplanetary disk survey: evidence for compact gas disks and molecular rings from CN
Leiden Observatory, Leiden University,
PO Box 9513, 2300 RA Leiden, The Netherlands
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
Accepted: 6 November 2018
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 Rc ≤ 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.
Key words: astrochemistry / protoplanetary disks / stars: pre-main sequence / stars: individual: Sz 98 / stars: individual: Sz 71 / techniques: interferometric
© ESO 2019