We analyzed the Subaru/Suprime-Cam images of 73P/Schwassmann-Wachmann 3B and detected no fewer than 154 mini-comets. We applied synchrone-syndyne analysis, modified for rocket effect analysis, to the mini-fragment spatial distribution. We found that most of these mini-comets were ejected from fragment B by an outburst occurring around 1 April 2006. The ratio of the rocket force to solar gravity was 7 to 23 times larger than that exerted on fragment B. No significant color variation was found. We examined the surface brightness profiles of all detected fragments and estimated the sizes of 154 fragments. We found that the radius of these mini-fragments was in the 5- to 108-m range (equivalent size of Tunguska impactor). The power-law index of the differential size distribution was q = -3.34 +/- 0.05. Based on this size distribution, we found that about 1-10% of the mass of fragment B was lost in the April 2006 outbursts. Modeling the cometary fragment dynamics revealed that it is likely that mini-fragments smaller than ~10-20 m could be depleted in water ice and become inactive, implying that decameter-sized comet fragments could survive against melting and remain as near-Earth objects. We attempted to detect the dust trail, which was clearly found in infrared wavelengths by Spitzer. No brightness enhancement brighter than 30.0 mag arcsec^-2 (3sigma) was detected in the orbit of fragment B.
Deep Dive into 2006 Fragmentation of Comet 73P/Schwassmann-Wachmann 3B Observed with Subaru/Suprime-Cam.
We analyzed the Subaru/Suprime-Cam images of 73P/Schwassmann-Wachmann 3B and detected no fewer than 154 mini-comets. We applied synchrone-syndyne analysis, modified for rocket effect analysis, to the mini-fragment spatial distribution. We found that most of these mini-comets were ejected from fragment B by an outburst occurring around 1 April 2006. The ratio of the rocket force to solar gravity was 7 to 23 times larger than that exerted on fragment B. No significant color variation was found. We examined the surface brightness profiles of all detected fragments and estimated the sizes of 154 fragments. We found that the radius of these mini-fragments was in the 5- to 108-m range (equivalent size of Tunguska impactor). The power-law index of the differential size distribution was q = -3.34 +/- 0.05. Based on this size distribution, we found that about 1-10% of the mass of fragment B was lost in the April 2006 outbursts. Modeling the cometary fragment dynamics revealed that it is likely
1. Introduction 73P/Schwassmann-Wachmann 3 (hereafter 73P/S-W3) is a member of the Jupiter-family comets (JFCs), orbiting the Sun with a 5.4-year period. During the apparition of 1995, 73P/S-W3 showed a huge outburst in activity. Afterward, four separate nuclei were confirmed and labeled A, B, C, and D. Of the four, fragment C was the largest and the presumed principal remnant of the original nucleus. The size of the nucleus was studied based on the standard assumption for a geometric albedo of 0.04 and a linear phase coefficient of 0.04 mag deg -1 ; the upper limit of the pre-breakup radius was 1.1 km (Boehnhardt et al. 1999), and the radius of fragment C was 0.68 ± 0.04 km. Although the radius of fragment B was estimated to be 0.68 km from Hubble Space Telescope (HST) observations (Toth et al. 2003), Boehnhardt et al. (2002) established an upper limit of 0.2-0.3 km. Due to poor observing conditions, fragments A and D were not found in the 2001 apparition.
We had a good opportunity to observe these broken comet fragments during its 2006 return. From near-infrared spectroscopy, no remarkable differences between fragment B and fragment C were found (Kobayashi et al. 2007;Villanueva et al. 2006). HST photographed two fragments, B and G, on 18-20 April 2006. These images revealed several dozen mini-fragments. The Spitzer Space Telescope showed not only many fragments distributed nearly on orbit but also the debris trail between them. The debris trail (dust trail) is composed of large dust particles ejected before the last perihelion passage (Vaubaillon and Reach 2006;Reach et al. 2007). Fuse et al. (2007) made optical observations of fragment B on 3 May 2006 using the wide-field optical camera attached to the Subaru 8.2-m telescope. R-band images confirmed 58 mini-comets in the vicinity of fragment B. No fragments were found along the orbit of fragment B in their Subaru images (Fuse et al. 2007).
This spectacular Subaru image presents several concerns. We noticed that most of these fragments were distributed between the anti-solar direction from fragment B and the negative orbit velocity vector. This positioning was quite interesting because these mini-comets behaved dynamically like dust particles pressed back by solar radiation pressure against the solar gravity. No obvious dust trail was found in the Subaru optical image, even though it was clear in the Spitzer infrared image. In this study, we re-analyzed the Subaru/Suprime-Cam images using the masking method developed for the detection of faint cometary dust clouds (Ishiguro et al. 2007;Sarugaku et al. 2007;Ishiguro 2008) and constructed a comet image without contaminants (e.g., stars and galaxies). This technique enabled us to detect mini-comets brighter that 26.5 mag and diffuse light sources associated with the comet brighter than 30.0 mag arcsec -2 .
Applying a unique method of examining fragment size and onset time (modified synchrones and syndynes), we examined the dynamical properties of the mini-fragments.
We also studied the brightness profile of these mini-comets and deduced their sizes.
Given the dynamical properties and sizes, we considered the activity of the mini-comets.
We re-analyzed the Subaru data provided by the SMOKA data server, which is operated by the Astronomy Data Center, National Astronomical Observatory of Japan (Baba et al. 2002). Observations of 73P/S-W3 were carried out by Fuse et al. (2007) using the Subaru 8.2-m telescope on Mauna Kea, Hawaii, on a single day, 3 May 2006, when fragment B was at a heliocentric distance r h = 1.070 AU, a geocentric distance Δ = 0.112 AU, and a solar phase angle α = 54°. Fuse et al. (2007) used an optical CCD camera, Suprime-Cam, attached to the prime focus of Subaru. This combination provided wide-field imaging capability, 34’ × 27’, with a pixel resolution of 0.20" pixel -1 . The seeing was about 0.7" (FWHM), which projects to 57 km at the position of the comet. The exposure time and filters are summarized in Table 1. All comet images were taken in comet-tracking mode. Although Fuse et al. (2007) did not use the short exposure-time R-band images (10-30 s) and V-band images, we found that these were essential to (i) determining the brightness of mini-comets near fragment B, (ii) improving the signal-to-noise ratio, and (iii) identifying detected sources as having a cometary origin. Fragment B was so bright that a large area of sky near B was saturated in 120-s exposures. The signal-to-noise ratio was improved by combining all images. We used the V and R composite image to confirm the mini-fragments because the color index V-R avoids false detections (see Section 2.2). Further explanations of the Suprime-Cam and 73P/S-W3B observations appear in Miyazaki et al. (2002) and Fuse et al. (2007), respectively.
[Table 1]
As a first step, the obtained data were reduced in the standard way with bias and flat-field corrections. These ancillary data were also provided through the SMOKA system. Be
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