One of the key mission objectives of Herschel is to observe water in various environments, but in protoplanetary disks in particular. Water has an immediate r^ance to the formation of planets and to our own origin. We propose to obtain deep PACS/SPIRE line spectra of water, OH and CO in protoplanetary disks already known to have strong rotational water vapor emission at mid-infrared (10-36 micron) wavelengths, as detected by Spitzer (the newly discovered mid-infrared molecular forest). The mid-infrared lines trace warm to hot gas formed within a few AU from the central star. The proposed observations will relate the oxygen chemistry and transport of water in the inner few AU with that of the cooler outer disk - 1-100 AU - as traced by the PACS water lines. The line ratios of water in the Spitzer spectral range suggest that water vapor may be strongly depleted in the disk surface beyond 1 AU although gas and dust temperatures are high enough to maintain abundant water out to at least 10 AU. It was suggested by Meijerink et al. 2009 that this depletion of water could be due to a ..cold finger effect., in which the surface water vapor is carried downwards into colder regions of the disk where it freezes out and is bound in the disk mid-plane as part of the general dust growth and settling. If so, the surface abundance structure of water is a direct tracer of the mid-plane snow-line. Based on model fits to the mid-infrared lines, we predict very strong differences in the 50-200 micron water lines, depending on the distribution of water in the outer disk surface. Our program is unique in that 1) we specifically target disks known to have strong water emission from the inner disk and 2) we go significantly deeper than others ensuring that we can constrain the abundance of water vapor down to 10^-10 per hydrogen in the outer disk, thus putting very strong const...raints of the efficacy of the proposed cold finger effect.
Publication
The Herschel SPIRE Fourier Transform Spectrometer Spectral Feature Finder - II. Estimating radial velocity of SPIRE spectral observation sources . Scott Jeremy P. et al. . Monthly Notices of the Royal Astronomical Society . null . null . 2020MNRAS.496.4894S , Signatures of warm carbon monoxide in protoplanetary discs observed with Herschel SPIRE . van der Wiel M. H. D. et al. . Monthly Notices of the Royal Astronomical Society, Volume 444, Issue 4, p.3911-3925 . 444 . 10.1093\/mnras\/stu1462 . 2014MNRAS.444.3911V , The Herschel SPIRE Fourier Transform Spectrometer Spectral Feature Finder - III. Line identification and off-axis spectra . Benson Chris S. et al. . Monthly Notices of the Royal Astronomical Society . null . null . 2020MNRAS.496.4906B , The Herschel SPIRE Fourier Transform Spectrometer Spectral Feature Finder I. The Spectral Feature Finder and Catalogue . Hopwood R. et al. . Monthly Notices of the Royal Astronomical Society . null . null . 2020MNRAS.496.4874H , Measurements of Water Surface Snow Lines in Classical Protoplanetary Disks . Blevins Sandra M. et al. . The Astrophysical Journal, Volume 818, Issue 1, article id. 22, 22 pp. (2016). . 818 . 10.3847\/0004-637X\/818\/1\/22 . 2016ApJ...818...22B ,
Herschel was launched on 14 May 2009! It is the fourth 'cornerstone' mission in the ESA science programme. With a 3.5 m Cassegrain telescope it is the largest space telescope ever launched. It is performing photometry and spectroscopy in approximately the 55-671 µm range, bridging the gap between earlier infrared space missions and groundbased facilities.
European Space Agency, 2013, Cool Herschel/Hot Spitzer: The Distribution Of Water In Protoplanetary Disks, SPG v14.2.0, European Space Agency, https://doi.org/10.5270/esa-eg2mush
