Gone in a Blaze of Glory: The Demise of Comet C/2015 D1 (SOHO)

We present studies of C/2015 D1 (SOHO), the first sunskirting comet ever seen from ground stations over the past half century. The Solar and Heliospheric Observatory (SOHO) witnessed its peculiar light curve with a huge dip followed by a flare-up around perihelion: the dip was likely caused by sublimation of olivines, directly evidenced by a coincident temporary disappearance of the tail. The flare-up likely reflects a disintegration event, which we suggest was triggered by intense thermal stress established within the nucleus interior. Photometric data reveal an increasingly dusty coma, indicative of volatile depletion. A catastrophic mass-loss rate of ∼10⁵ kg s^-1 around perihelion was seen. Ground-based Xingming Observatory spotted the post-perihelion debris cloud. Our morphological simulations of post-perihelion images find newly released dust grains of size a ≳ 10 μm in radius however, a temporal increase in a_min was also witnessed, possibly owing to swift dispersions of smaller grains swept away by radiation forces without replenishment. Together with the fading profile of the light curve, a power-law dust size distribution with index γ = 3.2 ± 0.1 is derived. We detected no active remaining cometary nuclei over ∼0.1 km in radius in post-perihelion images acquired at Lowell Observatory. Applying a radial nongravitational parameter, {{A}}₁=≤ft(1.209+/- 0.118\right)× {10}^-6 AU day^-2, from an isothermal water-ice sublimation model to the SOHO astrometry significantly reduces residuals and sinusoidal trends in the orbit determination. The nucleus mass ∼10⁸-10⁹ kg and the radius ∼50-150 m (bulk density ρ_d = 0.4 g cm^-3 assumed) before the disintegration are deduced from the photometric data; consistent results were determined from the nongravitational effects.