Morphology and photometry of comet C/2019 Y4 (ATLAS) from STEREO-A HI-1
ATel #13813; Matthew Knight (United States Naval Academy/University of Maryland), Karl Battams (Naval Research Laboratory)
on 17 Jun 2020; 14:27 UT
Credential Certification: Matthew Knight (astromatthew@gmail.com)
We report on comet C/2019 Y4 (ATLAS) during it transit of NASA/STEREO-A's Heliospheric Imager-1 (HI-1) field of view from 2020 May 25.9 until 2020 June 6.8. These observations encompassed the comet's perihelion passage on 2020 May 31.0 and occurred at a different viewing geometry than from Earth since STEREO-A is in a trailing Earth-like orbit, with a ~70° STEREO-Sun-Earth separation angle at the time of observations. HI-1 has a square field of view that is 20° wide and centered 14° from the Sun along the Sun-Earth line, and records broadband white-light observations (630-730 nm) of the extended solar corona and solar outflow. Observations are typically 40-min stacked exposures having a resolution of 70 arcsec/pixel.
Throughout its passage, the comet maintained a consistent morphology as an extended, thin, and slightly diffuse trail that closely aligned with its orbital path. No nuclear condensation or discrete fragments (cf. CBET #4751, ATEL #13620, ATEL #13629, ATEL #13651) were evident, though the brightness density was generally greater near its leading edge. No gas or ion tail was observed, and there was no obvious interaction between the comet tail and the solar wind, as is commonly observed with active comets in the HI-1 field of view (e.g., Vourlidas et al. 2007, ApJ 668, L79-L82). The visible portion of the trail was approximately 1° in length throughout the passage, and 3-4 arcmin in width, though it appeared narrower and slightly more diffuse at the end of the sequence.
Photometry was measured in a circular aperture of radius 350 arcsec, with sky removed via subtraction of a median daily background, and calibration coefficients used in Knight & Battams (2014, ApJL 782, L37, 5pp). Images obviously contaminated by background objects were excluded. Owing to the lack of a prominent central condensation, the photometric aperture was centered by-eye at an approximately consistent location near the leading edge of the comet. The comet had an apparent V magnitude of ~9.0 when first visible in HI-1, brightened gradually to a peak at ~8.7 from 2020 May 27-30, and faded to ~9.9 when it left HI-1. The photometric measurements had an envelope of about +/-0.3 mag, generally larger than the formal uncertainties and likely attributable to frequent fainter background objects, transient solar activity, and inconsistencies in centering the aperture.