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First Estimate of Water Production Rates of Comet C/2019 Y4 (ATLAS) from SWIFT/UVOT observations

ATel #13634; Kumar Venkataramani (Auburn University), Zexi Xing (University of Hong Kong, Auburn University), Emanuele Bonamente (Auburn University), Dennis Bodewits (Auburn University)
on 14 Apr 2020; 18:01 UT
Credential Certification: Kumar Venkataramani (kumar@auburn.edu)

Subjects: Optical, Ultra-Violet, Comet, Solar System Object

We report on observations by the Neil Gehrels-Swift Observatory of comet C/2019 Y4 (ATLAS) obtained on UT 2020 April 12.84 when the comet was at a heliocentric distance of 1.26 AU and 1.01 AU from Earth. The comet was imaged using the UVW1 and V filters on Swift’s Ultraviolet-Optical Telescope, which are centered at 2600 A (FWHM = 693 A) and 5468 A (FWHM = 769 A), respectively. The UVW1 image shows diffuse coma extending to more than 100,000 km. The V filter image showed an elongated coma with the V-band magnitude of 9.4 +/- 0.01 measured within the 100,000 km circular aperture centered on the photo-center. The median combined images in the V filter along with the continuum subtracted OH image are available at https://auburn.box.com/s/olfbe8wfunfuz3oipfnneclmgm59zqjm . The fragmentation and elongated nucleus of this comet was reported earlier by Ye & Qichen, Atel #13620, followed by Steele et al., ATel #13622 and Zhong-Yi Lin et al., ATel #13629. Emission from OH (A-X) transition is a major contributor to the flux in the UVW1 images (D. Bodewits et al 2014 ApJ 786 48). The V filter images were used to remove the continuum from the UVW1 images in order to isolate the OH flux. The contamination factor was calculated by measuring the ratio of the solar flux in both the filters, assuming a reddened solar spectrum of 15% per 100 nm between the centers of the UVW1 ( 2600 A) and the V band filter (5468 A). The measured surface brightness was then converted into the column density of OH molecules using fluorescence efficiencies of OH (Schleicher & A’Hearn, 1988, ApJ, 331, 1058) at the comet’s heliocentric velocity of -33 km/s. Comparing this distribution with the vectorial model and assuming that all of the OH is derived from H_2O, we obtain a water production rate of 1.25E28 +/- 5E25 mol/sec within an aperture of 100,000 km. This 1-sigma error only includes the photon statistics; the absolute error of the water production rate is likely ~25%, caused by modeling assumptions and data processing effects (cf. Xing et al. 2020).

Comet C/2019 Y4 (Atlas) : OH (Continuum Subtracted from UVW1 filter) and V filter images from SWIFT/UVOT