Long-term outburst activity of comet 17P/Holmes and constraints on ejecta size distributions

Abstract

A quantitative understanding of cometary outbursts requires robust constraints on the size distribution of ejected particles, which governs outburst dynamics and underpins estimates of released gas and dust. In the absence of direct measurements of particle sizes, assumptions about the size distribution play a central role in modelling dust-trail formation, their dynamical evolution and observability, and the potential production of meteor showers following encounters with Earth. We analyse brightness amplitude variations associated with outbursts of comet 17P/Holmes from 1892 to 2021, with particular emphasis on the exceptional 2007 mega-outburst. During this event the comet underwent a rapid and substantial brightening; at its peak, the expanding coma reached a diameter larger than that of the Sun and briefly became the largest object in the Solar System visible to the naked eye. We constrain the size distribution and total mass of porous agglomerates composed of ice, organics, and dust ejected during the outburst. The inferred particle size distribution is consistent with a power law of index q, yielding effective particle sizes between 1.15 x 10^-6 m for q = 4 and 5 x 10^-3 m for q = 2. Accounting for effective particle size, sublimation flux, and bulk density, we find that the total number of ejected particles increases with both q and sublimation flux. These results place quantitative constraints on the physical properties of outburst ejecta and provide physically motivated initial conditions for long-term dust-trail evolution modelling, relevant to the origin of meteoroid streams and the interplanetary dust population.

Figures (13)

• Figure 1: Image acquired on 2011 October 30 with the 0.8-m Jacobus Kapteyn Telescope (IAC80). (a) Overall view of the coma, showing pronounced asymmetries and an apparent diameter of approximately 7 arcmin ($\sim$ 505,000 km). (b) Larson–Sekanina filtered image at a position angle of 9$^{\circ}$, revealing multiple expanding jets emerging from the false nucleus, as well as a system of concentric rings. These structures are likely the result of multiple dust-producing episodes. The prominent jet and the condensation visible below it support a scenario involving the massive release of large fragments, which subsequently fragmented within the coma, giving rise to the observed layered morphology.
• Figure 2: Comet 17P/Holmes 2007-10-28 22:49 UT. Image obtained using 0.07-m f/2.8 Camera lens + CCD Starlight HX916 at K02 Eastwood Observatory, Leigh on sea, Essex, UK. 24 $\times$ 10-second exposures through UV/IR cut filter. Orientation = N up and E to the left. Field of view = 165' $\times$ 130'. Image scale 7.6" per pixel.
• Figure 3: Comet 17P/Holmes 2007-11-15 20:09UT. Image obtained using 0.07-m f/2.8 Camera lens + CCD Starlight HX916 at K02 Eastwood Observatory, Leigh on sea, Essex UK. 10 $\times$ 60-second exposures through UV/IR cut filter. Orientation = N up and E to the left. Field of view = 165' $\times$ 130'. Image scale 7.6" per pixel.
• Figure 4: Comet 17P/Holmes 2021-08-10 03:18 UT. Image obtained remotely using 0.32-m f/8 Corrected Dall-Kirkham astrograph + CMOS QHY600M at Z10 PGC Fregenal de la Sierra observatory, Extremadura, Spain. 20 $\times$ 60-second exposures through UV/IR cut filter. m$_{1}$=13.9 against Gaia DR2 G in 45.1" rad aperture m$_{2}$=16.4 against Gaia DR2 G in 5.1" rad aperture. Orientation = N up and E to the left. Field of view = 30.6' $\times$ 20.4' Image scale 1.22" per pixel.
• Figure 5: Comet 17P/Holmes 2021-08-17 02:59 UT. Image obtained remotely using 0.32-m f/8 Corrected Dall-Kirkham astrograph + CMOS QHY600M at Z10 PGC Fregenal de la Sierra observatory, Extremadura, Spain. 19 $\times$ 60 second exposures through UV/IR cut filter. m$_{2}$=17.2 against Gaia DR2 G in 5.1" rad aperture. Orientation = N up and E to the left. Field of view = 30.6' $\times$ 20.4'. Image scale 1.22" per pixel.