Cannot find file /Users/jewitt/Dropbox/HST_Ciffreo/Ciffreo.pdf0Assigned ID: 2654
----- Attempting Submission 1 (Fri Jan 07 18:55:11 GMT 2022) -----This is the initial Phase2 submission. Moving target orbital elements are from JPL Horizons, and the "no gyro bias" button has been selected. Exposure times of 360s could all be reduced as needed for scheduling, down to the minimum 348s for hiding readouts. Timing constraints are tight to both sample the ongoing comet disintigration while avoiding the brightest field stars, and is also negotiable as needed. -- Max Mutchler-1647244258----- Attempting Submission 1 (Tue Jan 18 20:26:41 GMT 2022) -----Disintegrating Comet 108P/CiffreoAs was first noticed on 2022 January 2, the short-period (Kuiper belt) comet 108P/Ciffreo is undergoing a fragmentation event. In low-resolution ground-based data, the ejected fragments appear only as a diffuse, low surface brightness blob separated from the main nucleus. Given such data, neither photometric nor dynamical studies are possible. We propose early-time measurements at the high angular resolution offered by HST to characterize these embedded fragments in terms of their brightness and velocity distributions, with the aim being to determine the process behind the fragmentation. Three orbits are requested to determine the sky-plane motion of the fragments so that their velocities and ejection dates can be determined. Cometary fragmentation events are both unpredictable and short-lived and, as a result, few have been studied in detail. DDT observations are needed to obtain data while the fragment swarm is fresh and before it dissipates and fades.We request three orbits with WFC3 spread over about a week in order to achieve our two main science objectives: fragment size distribution and dynamics. This timing will permit the measurement of fragment velocities and their extrapolation to determine the release dates, assuming nominal ejection speeds of order 1 m/s. The ejected fragments may themselves be sublimating. Recoil forces from asymmetric sublimation drive a non-gravitational (rocket) acceleration of magnitude inversely proportional to fragment size. If the rocket force from sublimation in the subsolar region dominates, the fragments will remain in the orbital plane, because like radiation pressure this force is directed away from the Sun and acts only to reduce the effective solar gravity. If outgassing is negligible and instead the fragment motion is governed by the initial velocity, the fragments will generally occupy a more three- dimensional distribution, as in active asteroid P/2010 A2. Analysis of the dynamical evolution of the fragments will therefore provide a separate indication of whether sublimation played a major role in the break-up event, or if other (e.g., tidal or centrifugal) forces were prominently involved. Our basic observing strategy is to take multiple long exposures (348-400s) using WFC3 and the wide bandpass filter (F350LP) for maximum sensitivity. We use F350LP because it provides a count rate 1.6 higher for a target with a solar-type spectrum than other commonly used broadband filters (e.g., F606W). We will also dither the exposures to mitigate the effects from bad pixels, cosmic rays, and the inter-chip gap. The WFC3 Exposure Time Calculator indicates that, for a point source, solar-type spectrum, a signal-to-noise ratio of 6 is reached in 348 s at V = 26.5 in the F350LP filter (ETC request ID WFC3UVIS.im.765972; optimal extraction yields SNR=13). We gain an additional 0.9 magnitudes by combining five images from each orbit, giving V = 27.4. For example, on UT 2021 March 1, when heliocentric distance rH = 2.31 AU, geocentric distance 1.68 AU and phase angle 22, magnitude V = 27.4 corresponds to an object radius 65 m (geometric albedo 0.04 and C-type asteroid phase function assumed). These are upper limits to the point source thresholds because at least some of the fragments are embedded in their own dust comae. However, it is clear that HST offers unprecedented sensitivity to faint fragments. Gas emission from 108P/Ciffreo is likely to be negligible, as it is in all low activity comets at 2 AU. Even if gas drives the expulsion of matter from the nucleus, its rapid expansion velocity and intrinsically small resonance fluorescence scattering cross- section (compared to continuum scattering by dust) renders gas optically unimportant in observations of comets much beyond Earths orbit. The apparent rates of motion relative to sidereal are easily within Hubbles tracking capabilities. This rate of motion is also slow enough to keep a single pair of guide stars within the FGS pickles for an entire visibility window. The ephemeris uncertainties (from JPL Horizons) on 108P/Ciffreo are sub-arcsecond and therefore of no consequence for these observations. We understand that we will have essentially no control over the spacecraft roll angle. Observations are requested as early as possible in order to capture the early-time development of the ejected material, before it has had a change to fade and disperse. However, we do not request disruptive scheduling of these observations. In the February - March period, 108P/Ciffreo will be about 2.2 AU from the Sun 1.5 AU from the Earth and at a phase angle near 20 degrees. Observations from about -8 below the plane of the orbit of 108P will provide a suitable perspective view of the distribution of bodies and dust in the orbit plane, which we will interpret using an existing Monte-Carlo dust dynamics model.D. Jewitt has a long record of solar system observations with ground-based telescopes and HST. J. Li is experienced with data reduction and analysis pertaining to space-based observations of comets. M. Mutchler is an expert on the optimized use of HST for solar system observations. J. Agarwal and Y. Kim are experts in the dynamical modeling of dust under the action of solar gravity and radiation pressure.Solar System Astronomy0falseSUPPORTEDSOLAR SYSTEMYESCOMETDisintegrating cometTYPE=COMET,Q=1.676339083838281,E=0.549930518846976,I=13.31246416501656
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