Comet C/2011 W3 (Lovejoy): Orbit Determination, Outbursts, Disintegration of Nucleus, Dust-Tail Morphology, and Relationship to New Cluster of Bright Sungrazers

Abstract

We describe the physical and orbital properties of C/2011 W3. After surviving the perihelion passage, the comet was observed to undergo major physical changes. The permanent loss of the nuclear condensation and the formation of a narrow spine tail was observed first at Malargue, Argentina, on December 20 and then systematically at Siding Spring, Australia. The process of disintegration culminated with an outburst (terminal fragmentation event) on December 17.6 UT. The postperihelion tail, observed for ∼3 months, was the product of activity over <2 days. Because of the delayed response to the hostile environment in the immediate proximity of the Sun, the nucleus' breakup and crumbling was probably caused by thermal stress due to the penetration of intense heat pulse deep into the nucleus' interior after perihelion. The same mechanism may be responsible for cascading fragmentation of sungrazers at large heliocentric distances. The observed behavior is at odds with the rubble-pile model, since the residual mass of the nucleus after perihelion, estimated at ∼10 12 g (a sphere ∼150-200 m across), still possessed significant cohesive strength. The spine tail -the product of the terminal outburst -was a synchronic feature, whose brightest part contained submillimeter-sized dust particles, released at velocities not exceeding 30 m s −1 . The loss of the nuclear condensation prevented an accurate orbital-period determination by traditional techniques. Since the missing nucleus must have been located on the synchrone, whose orientation and sunward tip have been measured, we compute the astrometric positions of this missing nucleus as the coordinates of the points of intersection of the spine tail's axis with the lines of forced orbital-period variation, derived from the orbital solutions based on high-quality preperihelion astrometry from the ground. The resulting orbit gives 698 ± 2 years for the osculating orbital period, which proves that C/2011 W3 is the first major member of the expected new, 21st-century cluster of bright Kreutz-system sungrazers, whose existence was predicted by these authors in 2007. From the spine tail's evolution, we determine that its measured tip, populated by dust particles 1-2 mm in diameter, receded antisunward from the computed position of the missing nucleus. The bizarre appearance of the comet's dust tail in images taken only hours after perihelion with the coronagraphs on board the SOHO and STEREO spacecraft is readily understood. The disconnection of the comet's head from the tail released before perihelion and an apparent activity attenuation near perihelion have a common cause -sublimation of all dust at heliocentric distances smaller than about 1.8 solar radii. The tail's brightness is strongly affected by forward scattering of sunlight by dust. From an initially broad range of particle sizes, the grains that were imaged the longest had a radiation-pressure parameter β ≃ 0.6, diagnostic of submicron-sized silicate grains and consistent with the existence of the dust-free zone around ...

Extreme-Ultraviolet and X-Ray Observations of Comet Lovejoy (C/2011 W3) in the Lower Corona

“…We find the AIA/171 brightness to be characterized by three distinct peaks and leave their interpretation to the proceeding section, where we discuss the profiles shown in Figure 7 in the context of the outburst model proposed by Sekanina & Chodas (2012). The total integrated emission under the flux versus time plot in Figure 7 is log(I) = 7.609 DN, which implies to a total outgassing of 10 35.53 oxygen atoms based on the 171 Å production rates listed in Table 2.…”

“…We discuss our work in the context of others in Section 4. This includes commentary on the Sekanina & Chodas (2012) outburst model, the source of neutral oxygen (water ice versus dust grains), how our results compare with those of Schrijver et al (2012) for comet C/2011 N3, and what our results imply about the size of the nucleus. Finally, Section 5 summarizes our findings.…”

“…In this scenario, the comet is destroyed by successive, large fragmentation events rather than steady outgassing. Sekanina & Chodas (2012) include a discussion of preliminary results from the work presented here, which they explain in the context of their fragmentation model. We will revisit this in Section 4.1.…”

“…Note that in applying the Table 2 rates to the integrated flux, we are in effect assuming that the corona is of uniform temperature and density throughout the AIA FOV. If entirely from water, our total outgassing result translates to a mass loss of ∼1.0 × 10 13 g. For comparison, Sekanina & Chodas (2012) used dust observations to estimate a residual mass of the nucleus of ∼1 × 10 12 g after its destruction 1.6 days post-perihelion at 31 R . We will discuss the implications of our mass loss estimate on the expected size of the nucleus in Section 4.4.…”

Comets as solar probes

“…Even Lovejoy lasted only 2-3 days after passing through the Sun's corona. 13 But before Lovejoy's 4.5-billion-year history ended, both the descent and ascent phases of its path were visible to spacecraft looking from three very different perspectives. 2 The idea of using comets to learn about the Sun and its surroundings is not new.…”