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Register a new userMulti-Wavelength Observations of Comet C/2011 L4 (Pan-Starrs)
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Dynamically new comet C/2011 L4 (PanSTARRS) is one of the brightest comets since the great comet C/1995 O1 (Hale-Bopp). Here, we present our multi-wavelength observations of C/2011 L4 during its in-bound passage to the inner Solar system. A strong absorption band of water ice at 2.0 $mu$m was detected in the near infrared spectra, taken with the 8-m Gemini-North and 3-m IRTF telescopes. The companion 1.5 $mu$m band of water ice, however, was not observed. Spectral modeling show that the absence of the 1.5 $mu$m feature can be explained by the presence of sub-micron-sized fine ice grains. No gas lines (i.e. CN, HCN or CO) were observed pre-perihelion either in the optical or in the sub-millimeter. 3-$sigma$ upper limits to the CN and CO production rates were derived. The comet exhibited a very strong continuum in the optical and its slope seemed to become redder as the comet approached the Sun. Our observations suggest that C/2011 L4 is an unusually dust-rich comet with a dust-to-gas mass ratio $>$ 4.
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We present pre-perihelion infrared 8 to 31 micron spectrophotometric and imaging observations of comet C/2012 K1 (Pan-STARRS), a dynamically new Oort Cloud comet, conducted with NASAs Stratospheric Observatory for Infrared Astronomy (SOFIA) facility (+FORCAST) in 2014 June. As a new comet (first inner solar system passage), the coma grain population may be extremely pristine, unencumbered by a rime and insufficiently irradiated by the Sun to carbonize its surface organics. The comet exhibited a weak 10 micron silicate feature ~1.18 +/- 0.03 above the underlying best-fit 215.32 +/- 0.95 K continuum blackbody. Thermal modeling of the observed spectral energy distribution indicates that the coma grains are fractally solid with a porosity factor D = 3 and the peak in the grain size distribution, a_peak = 0.6 micron, large. The sub-micron coma grains are dominated by amorphous carbon, with a silicate-to-carbon ratio of 0.80 (+0.25) (- 0.20). The silicate crystalline mass fraction is 0.20 (+0.30) (-0.10), similar to with other dynamically new comets exhibiting weak 10 micron silicate features. The bolometric dust albedo of the coma dust is 0.14 +/- 0.01 at a phase angle of 34.76 degrees, and the average dust production rate, corrected to zero phase, at the epoch of our observations was Afrho ~ 5340~cm.
A sequence of events, dominated by two outbursts and ending with the preperihelion disintegration of comet C/2017 S3, is examined. The onset times of the outbursts are determined with high accuracy from the light curve of the nuclear condensation before it disappeared following the second outburst. While the brightness of the condensation was declining precipitously, the total brightness continued to grow in the STEREO-As HI1 images until two days before perihelion. The red magnitudes measured in these images refer to a uniform cloud of nuclear fragments, 2200 km^2 in projected area, that began to expand at a rate of 76 m s^(-1) at the time of the second outburst. A tail extension, detected in some STEREO-A images, consisted of dust released far from the Sun. Orbital analysis of the ground-based observations shows that the comet had arrived from the Oort Cloud in a gravitational orbit. Treating positional residuals as offsets of a companion of a split comet, we confirm the existence of the cloud of radiation-pressure driven millimeter-sized dust grains emanating from the nucleus during the second outburst. We detect a similar, but compact and much fainter cloud (or a sizable fluffy dust aggregate fragment) released at the time of the first outburst. --- The debris would make a sphere of 140 m across and its kinetic energy is equivalent to the heat of crystallization liberated by 100 tons of amorphous water ice. Ramifications for short-lived companions of the split comets and for 1I `Oumuamua are discussed.
P/2011 S1 (Gibbs) is an outer solar system comet or active Centaur with a similar orbit to that of the famous 29P/Schwassmann-Wachmann 1. P/2011 S1 (Gibbs) has been observed by the Pan-STARRS 1 (PS1) sky survey from 2010 to 2012. The resulting data allow us to perform multi-color studies of the nucleus and coma of the comet. Analysis of PS1 images reveals that P/2011 S1 (Gibbs) has a small nucleus $< 4$ km radius, with colors $g_{P1}-r_{P1} = 0.5 pm 0.02$, $r_{P1}-i_{P1} = 0.12 pm 0.02$ and $i_{P1}-z_{P1} = 0.46 pm 0.03$. The comet remained active from 2010 to 2012, with a model-dependent mass-loss rate of $sim100$ kg s$^{-1}$. The mass-loss rate per unit surface area of P/2011 S1 (Gibbs) is as high as that of 29P/Schwassmann-Wachmann 1, making it one of the most active Centaurs. The mass-loss rate also varies with time from $sim 40$ kg s$^{-1}$ to 150 kg s$^{-1}$. Due to its rather circular orbit, we propose that P/2011 S1 (Gibbs) has 29P/Schwassmann-Wachmann 1-like outbursts that control the outgassing rate. The results indicate that it may have a similar surface composition to that of 29P/Schwassmann-Wachmann 1. Our numerical simulations show that the future orbital evolution of P/2011 S1 (Gibbs) is more similar to that of the main population of Centaurs than to that of 29P/Schwassmann-Wachmann 1. The results also demonstrate that P/2011 S1 (Gibbs) is dynamically unstable and can only remain near its current orbit for roughly a thousand years.
High-quality white-light images from the SECCHI/HI-1 telescope onboard STEREO-B reveal high-velocity evanescent clumps [HVECs] expelled from the coma of the C/2011 L4 [Pan-STARRS] comet. Animated images provide evidence of highly dynamic ejecta moving near-radially in the anti-sunward direction. The bulk speed of the clumps at their initial detection in the HI1-B images range from $200-400$ km s$^{-1}$ followed by an appreciable acceleration up to speeds of $450-600$ km s$^{-1}$, which are typical of slow to intermediate solar wind speeds. The clump velocities do not exceed these limiting values and seem to reach a plateau. The images also show that the clumps do not expand as they propagate. Order of magnitude calculations show that ionized single atoms or molecules accelerate too quickly compared to observations, while dust grains micron sized or larger accelerate too slowly. We find that neutral Na, Li, K, or Ca atoms with $beta>50$ could possibly fit the observations. Just as likely, we find that an interaction with the solar wind and the heliospheric magnetic field (HMF) can cause the observed clump dynamical evolution, accelerating them quickly up to solar wind velocities. We thus speculate that the HVECs are composed of charged particles (dust particles) or neutral atoms accelerated by radiation pressure at $beta>50$ values. In addition, the data suggest that clump ejecta initially move along near-radial, bright structures, which then separate into HVECs and larger dust grains that steadily bend backwards relative to the comets orbital motion due to the effects of solar radiation and gravity. These structures gradually form new striae in the dust tail. The near-periodic spacing of the striae may be indicative of outgassing activity modulation due to the comet nucleus rotation. It is, however, unclear whether all striae are formed as a result of this process.
We present our results on the Chandra X-ray Observatory Advanced CCD Imaging Spectrometer (ACIS) observations of the bright Oort Cloud comets C/2012 S1 (ISON) and C/2011 L4 (PanSTARRS). ISON was observed between 2013 October 31-November 06 during variable speed solar wind (SW), and PanSTARRS was observed between 2013 April 17-23 during fast SW. ISON produced an extended parabolic X-ray morphology consistent with a collisionally thick coma, while PanSTARRS demonstrated only a diffuse X-ray-emitting region. We consider these emissions to be from charge exchange (CX) and model each comets emission spectrum from first principles accordingly. Our model agrees with the observational spectra and also generates composition ratios for heavy, highly charged SW ions interacting with the cometary atmosphere. We compare our derived SW ion compositions to observational data and find a strong agreement between them. These results further demonstrate the utility of CX emissions as a remote diagnostics tool of both astrophysical plasma interaction and SW composition. In addition, we observe potential soft X-ray emissions via ACIS around 0.2 keV from both comets that are correlated in intensity to the hard X-ray emissions between 0.4-1.0 keV. We fit our CX model to these emissions, but our lack of a unique solution at low energies makes it impossible to conclude if they are cometary CX in origin. We lastly discuss probable emission mechanism sources for the soft X-rays and explore new opportunities these findings present in understanding cometary emission processes via Chandra.
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