We describe the physical and orbital properties of C/2011 W3. After surviving 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 were observed first at Malargue, Argentina, on December 20 and then systematically at Siding Spring, Australia. The process of disintegration culminated with a terminal fragmentation event on December 17.6 UT. The postperihelion dust tail, observed for ~3 months, was the product of activity over <2 days. The nucleus' breakup and crumbling were probably caused by thermal stress due to the penetration of the 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 delayed response to the hostile environment in the solar corona is at odds with the rubble-pile model, since the residual mass of the nucleus, estimated at ~1012 g (equivalent to a sphere 150–200 m across) just before the terminal event, still possessed nontrivial cohesive strength.

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 sime 0.6, diagnostic of submicron-sized silicate grains and consistent with the existence of the dust-free zone around the Sun. The role and place of C/2011 W3 in the hierarchy of the Kreutz system and its genealogy via a 14th-century parent suggest that it is indirectly related to the celebrated sungrazer X/1106 C1, which, just as the first-generation parent of C/2011 W3, split from a common predecessor during the previous return to perihelion.

When discovered on November 27, C/2011 W3 had only ~18 days to reach perihelion (which occurred on 2011 December 16.0 UT), and there was little hope for an accurate determination of the orbital period from preperihelion data, regardless of their quality. Observing circumstances were unfavorable for ground-based imaging, since the comet's elongation from the Sun was rapidly decreasing, from 50° at discovery to merely 17fdg6 on December 10, when its position was measured from the ground for the last time before perihelion. Still, more than 100 astrometric positions were obtained during this two-week period (Spahr et al. 2011, 2012), the great majority of which were sufficiently accurate and mutually consistent to be used for deriving a high-quality set of elements, except for the orbital period. For example, two sets of elliptical elements computed by Williams (2011a, 2011b), from 91 observations between November 27 and December 8 and from 94 observations between November 27 and December 10, gave osculating orbital periods of, respectively, 376 ± 51yr (leaving a mean residual of ±0farcs7) and 680 ± 64yr (leaving ±0farcs8).

Figure 1. Some of the earliest postperihelion images of Comet C/2011 W3 taken—at the request of J. Černý—by J. Ebr, M. Prouza, P. Kubánek, and M. Jelínek with the FRAM 30 cm f/10 Schmidt–Cassegrain reflector, a robotic, remotely controlled telescope at the Pierre Auger Observatory, Malargue, Argentina. Each frame is approximately 11' on a side, corresponding to some 430,000 km in the first image and 375,000 km in the last image. North is up and east is to the left. The comet was 5fdg8 from the Sun during the first exposure, 8fdg5 during the second, 11fdg0 during the third, and 13fdg3 during the last one. These images provided the first evidence of the major physical changes in the comet's morphological appearance, which culminated with the sudden, complete loss of the nuclear condensation on December 20.

Preliminary light curve of comet C/2011 W3 based on visual and CCD total brightness estimates made from the ground. The magnitudes plotted have been normalized to 1 AU from Earth with the inverse-square power law and to a zero phase angle with the "compound" Henyey–Greenstein law, as modified by Marcus. Personal and instrumental effects have been corrected to the limited degree possible. The comet is shown to have been much brighter after perihelion than before it.

Comet C/2011 W3 with its dust tail shortly before reaching perihelion, as imaged by the C3 coronagraph on board the SOHO spacecraft. The upper image was taken on December 15.504 UT, or 0.508 days before perihelion, the lower image on December 15.796 UT, or 0.216 days before perihelion. In the panels next to the images, the observations are compared with two syndynames; the one for β = 2.5 is slightly to the left of the other, which refers to β = 0.6. The syndynames are calibrated by the times of dust release, reckoned in days from the time of the comet's perihelion passage (the negative numbers indicate days before the comet's perihelion). The panels also provide the scale and orientation of the images. We note that in both images the tail displays a slightly lesser curvature than the syndynames but lies generally between them, which implies that the comet released submicron-sized dust of both dielectric and strongly absorbing nature.

Appearance of comet C/2011 W3 and its dust tail in an image taken with the C2 coronagraph on board the SOHO spacecraft on December 16.117 UT, or 0.105 days after perihelion. The tail, to the south-southeast of the Sun, is seen to be completely disconnected from the comet's head, to the west of the Sun. The head's image is saturated (with some blooming being apparent) but with no tail extension. The tail lies entirely between the syndynames β = 0.6 and 0.8, as shown in the panel to the right. The first of the two syndynames is plotted thicker and is calibrated by the times of dust release, reckoned in days from the time of the comet's perihelion passage, pinpointing the locations of particles released at perihelion and 0.1, 1, and 2 days before perihelion. The dot on the segment of the syndyname β = 0.8 identifies the location of dust released 1 day before perihelion. We note that the entire northwestern branch of the tail is missing. The tail's brightness is largely determined by forward scattering of sunlight by dust, as the phase angle increases toward the bottom of the image. The scale and orientation of the image are the same as those of the panel. (Image credit: ESA/NASA/LASCO consortium.)

Peculiar appearance of comet C/2011 W3 and its dust tail in an image taken with the COR2 coronagraph on board the STEREO-A spacecraft on December 16.246 UT, or 0.234 days after perihelion. The tail has two branches located, respectively, to the southeast and northwest of the Sun, while the comet head's site is nearly to the east of the Sun. The tail's southeastern branch is faint because of backscatter of sunlight, while the blob-shaped northwestern branch appears to be at least in part contaminated by a relatively weak but broad coronal mass ejection. The panel to the right identifies the locations on the syndyname β = 0.6, which are populated by dust released from the nucleus at six different times between 8 days before perihelion and 0.03 days after perihelion. Three additional release times—0.1, 0.3, and 0.7 days before perihelion—are identified by the synchrones drawn by the broken curves. The panel also shows the area of the tail's blob to the northwest of the Sun and short segments of the syndynames for β equaling 0.5 and 0.8. No tail is detected for release times between 0.1 days before perihelion and about 0.1 days after perihelion, but the head's image is elongated in the direction away from the Sun, suggesting that a postperihelion tail already began to develop during a couple of hours before the frame was taken. (Image credit: NASA/SECCHI consortium.)

Bizarre appearance of comet C/2011 W3 and its dust tail in an image taken with the COR2 coronagraph on board the STEREO-B spacecraft on December 16.517 UT, or 0.505 days after perihelion. The tail has two branches located, respectively, to the southwest and northeast of the Sun, while the comet's head is to the west-southwest of the Sun and north of the brightest part of the tail's southwestern branch. Careful inspection shows that the head's image is elongated along the line with the Sun, evidence that a new, postperihelion tail is being developed. The entire tail is fitted with a syndyname of β = 0.6, as shown in the panel that also provides the scale and orientation of the image. Only the heavily drawn segments of the syndyname are seen as the tail in the image. The syndyname is calibrated by the times of dust release, reckoned in days from the time of the comet's perihelion passage (a negative number means before perihelion, and vice versa). Brightness-wise, the southwestern branch dominates because of forward scattering of sunlight. A broad secondary maximum is noted on the northeastern branch at a location corresponding to a release time of about 0.3 days before perihelion (December 15.7 UT). The tail then rapidly fades and disappears for a release time near 0.1 days before perihelion, suggesting an attenuation of activity hours before perihelion. Short segments of the syndynames β = 0.4, 0.5, and 0.8 are also plotted, marked as A, B, and C, respectively, to illustrate a general deficit of dust with accelerations different from 0.6. The dots on these syndyname segments show the locations of particles released 0.1 days before perihelion. The northeastern tail's boundary is sharper to the south than to the north, indicating that dust with β Gt 0.6 is more scarce than that with β < 0.6. No tail is apparent along the wide swath of space referring to release times during the first hours after perihelion. (Image credit: NASA/SECCHI consortium.)

Relationship between the parameter Λμ1/2ρ−1—a function of the sublimation-pressure constant, molar weight, and bulk density—and the latent heat of sublimation L. The uninterrupted thick curve is the set of solutions that refer to the onset of profuse sublimation of dust in the tail of comet C/2011 W3 at 1.73$R_{\mbox{\scriptsize \boldmath $\odot $}}$ from the Sun. The dashed thick curve is the set of solutions that refer to the onset of profuse sublimation at 1.88$R_{\mbox{\scriptsize \boldmath $\odot $}}$. The two squares are the locations in the plot of the two end members of the olivine solid solution system, based on the sublimation-pressure constants by Nagahara et al. (1994): forsterite (Fo; Mg-endmember) and fayalite (Fa; Fe-endmember). The dots on the Fo–Fa connecting line refer, from left to right, to the members of the olivine system, whose formula is, respectively, Mg1.6Fe0.4SiO4, Mg1.2Fe0.8SiO4, Mg0.8Fe1.2SiO4, and Mg0.4Fe1.6SiO4. For reference and orientation, we also plot the data points for "silicates," as used by Kimura et al. (1997, KIM), and for quartz, SiO2 (Haynes 2011), and an extrapolated relationship between Λ and L, as used by Sekanina (2003) for some metallic elements in his study of the light curves of SOHO sungrazers, with an average value of 4 for the ratio $\sqrt{\mu }/\rho$. The constants for olivine used by Kimura et al. (2002) are not shown, as they nearly coincide with those for forsterite.

Temperature variations with depth beneath the surface derived for the nucleus of comet C/2011 W3 with a maximum diameter of 600 m. The one-dimensional heat-transfer equation has been solved, averaging the incident solar energy over the surface, assuming that the energy is spent only on the conduction into the nucleus and thermal reradiation (bare surface, devoid of ices or other substances that would sublimate), and adopting a Bond albedo of 0.03, an emissivity of unity, a coefficient of effective thermal conductivity of 2 × 104 erg cm−1 s−1 K−1, a specific heat capacity of 8 × 106 erg g−1 K−1, and a bulk density of 0.4 g cm−3. The individual curves are identified by the time from perihelion: 0.6 days before perihelion, at perihelion, and 0.1 and 1.6 days after perihelion, when comet C/2011 W3 was at heliocentric distances of, respectively, 0.0731, 0.00555, 0.0196, and 0.144 AU. Also marked are the crystallization temperature of amorphous water ice and the melting points of forsterite and fayalite.

Relative thermal stress in the interior of comet C/2011 W3 as a function of depth beneath the surface. Derived from the temperature variations 0.6 days before perihelion for the nucleus 400 m in diameter (solid), at perihelion for 280 m in diameter (broken), and 1.6 days after perihelion for 150 m in diameter (solid), the stress curves are normalized to thermal stress at the nucleus' center at perihelion. The segments of the curves are marked to distinguish between the radial and tangential stress components and whether the stress was a compression or tension.

Computed thermal stress near the nucleus' center of C/2011 W3. The solid curves show the stress growth through perihelion for the two adopted values of Young's modulus; the horizontal dashed lines mark the estimated limits on the cohesive strength of the nucleus (from Lasue et al. 2009, 2011). The stress curves represent a conservative estimate; because the thermal expansion coefficient increases with the temperature, the actual postperihelion growth of thermal stress in the comet's interior is even steeper. The time of the terminal outburst of C/2011 W3, 1.6 ± 0.2 days after perihelion, is shown by the solid vertical line, its uncertainty by the parallel broken lines.

Temperature variations with heliocentric distance at the surface and at 10 depths beneath the surface derived for the nucleus of comet C/2011 W3. The computations are based on the model and assumptions described in the caption to Figure 12. The surface-temperature variations are shown for preperihelion (broken) and postperihelion branches of the orbit; all other curves are postperihelion. The broken line parallel to the axis of ordinates shows the heliocentric distance 0.144 AU, at which the cataclysmic fragmentation event occurred on December 17.6 UT.