Molecules with ALMA at Planet-forming Scales (MAPS) II: CLEAN Strategies\n for Synthesizing Images of Molecular Line Emission in Protoplanetary Disks

Abstract

The Molecules with ALMA at Planet-forming Scales large program (MAPS LP)\nsurveyed the chemical structures of five protoplanetary disks across more than\n40 different spectral lines at high angular resolution (0.15" and 0.30" beams\nfor Bands 6 and 3, respectively) and sensitivity (spanning 0.3 - 1.3 mJy/beam\nand 0.4 - 1.9 mJy/beam for Bands 6 and 3, respectively). In this article, we\ndescribe our multi-stage workflow -- built around the CASA tclean image\ndeconvolution procedure -- that we used to generate the core data product of\nthe MAPS LP: the position-position-velocity image cubes for each spectral line.\nOwing to the expansive nature of the survey, we encountered a range of imaging\nchallenges; some are familiar to the sub-mm protoplanetary disk community, like\nthe benefits of using an accurate CLEAN mask, and others less well-known, like\nthe incorrect default flux scaling of the CLEAN residual map first described in\nJorsater & van Moorsel 1995 (the "JvM effect"). We distill lessons learned into\nrecommended workflows for synthesizing image cubes of molecular emission. In\nparticular, we describe how to produce image cubes with accurate fluxes via the\n"JvM correction," a procedure that is generally applicable to any image\nsynthesized via CLEAN deconvolution but is especially critical for low S/N\nemission. We further explain how we used visibility tapering to promote a\ncommon, fiducial beam size and contextualize the interpretation of signal to\nnoise ratio when detecting molecular emission from protoplanetary disks. This\npaper is part of the MAPS special issue of the Astrophysical Journal\nSupplement.\n