Activity evolution, outbursts, and splitting events of comet 73P/Schwassmann-Wachmann 3

Context. The split Jupiter family comet 73P/Schwassmann-Wachmann 3 was monitored between January 21 and May 25 2006, for 24 nights. Aims. The goals of the campaign were to characterize the two principal comet components (C and B) and the smaller component G during their approach to perihelion, and study differences and commonalities in their evolution to obtain insight into the nature of the nuclei (gas and dust). We aimed to assess the chemical homogeneity/heterogeneity of the different components, the presence of jets and other coma structures, the rotation axis, the long-term activity evolution, and the detection of new fragmentation events. Methods. Long-slit spectra and optical broadband images were acquired using the CAFOS instrument at the 2.2-m telescope at Calar Alto Observatory (CSIC-MPG). Data obtained in service mode consisted of spectra and Johnson R filter images. By observing for four nights close to perigee, the comet could be imaged in the Johnson UBVRI filters. When possible, we analyzed the radial profile of the dust brightness and we derived the dust and gas production rates, the dust reddening, and the N-S profiles of the CN, C2, and C3 column densities. The analysis of the morphological evolution of coma structures and the determination of the rotation axis is performed in a separate paper. Results. We found that components C and B behave differently during most of our observations. While component C did not show any sudden increase in dust productivity, as measured by Afp, component B was characterized by a higher activity variation, exhibiting two outburst peaks and fragmentation events. Excluding outburst dates, component B always had lower dust productivity than component C. We also found differences in the behavior of the dust brightness radial profiles and in the dust colors. Differences in the dust colors are found also with respect to component G. In the spectral analysis, we found that both C and B components seem to be carbon-chain depleted, their compositions being almost the same. This indicates that the pre-split original, intact nucleus probably had a homogenous composition. A two-dimensional color map of component B on May 13 shows relative color variations in the inner coma that can be interpreted qualitatively in terms of Mie theory as fragmentation of silicate dust particles emanating from the nucleus. © ESO 2009

Coma structures in comet 73P/schwassmann-wachmann 3, components B and C, between january and may 2006

The Jupiter family comet 73P/Schwassmann-Wachmann 3 has been widely observed since 1995 after a nucleus break-up event produced at least five components labeled 73P-A to E. During the 2006 appearance, two of them (B and C) showed very strong coma activity. Our R-filter imaging of 73P-B & C from 21 January to 25 May 2006 revealed the presence of fan-like structures in the comae of both components and evidence for further fragmentation events in component B. As of early April 2006, component C showed two jets emanating from the nucleus, with one continuously visible. Through a simulation of the orbital geometry we infer that the rotation axis of 73P-C has an inclination of 20° to the orbital plane and a longitude of 45° at perihelion. The coma activity of component B was highly variable, displaying signatures of at least 3 fragmentation events. The coma was characterized by the continuous presence of a jet roughly in sunward direction, starting from the beginning of May. The first fragmentation event of component B may have happened between April 16 and April 26, leading to the presence of at least 6 fragments detected in images of May 2. The second one happened on or shortly before May 8, the third one between May 18 and 24. For the rotation axis of 73P-B we infer an inclination of 5°-15° to the orbital plane and a longitude of 20°-30° at perihelion. © 2009 Springer Science+Business Media B.V

Photometric observations of comet 81P/Wild 2 during the 2010 perihelion passage

Context. The Jupiter-family comet 81P/Wild 2, target of the NASA Stardust mission, is very important in the context of the studies of pristine objects in the solar system. First, it was only recently deflected into the present orbit, having spent at least 300 yr at higher heliocentric distance prior to the orbital change in 1974. It is therefore likely that the comet experienced a recent activation with consequent low alteration of its original material. Second, it is the only comet whose coma material was brought back to Earth for laboratory analysis. We observed the object between 2010 February 9 and September 9 for a total of 11 nights during the 2010 perihelion passage. Aims. The goals of the campaign were the characterization of the comet's dust activity and the comparison with previous apparitions to derive hints on the secular behavior of the object. Methods. Broadband R-and I-images were acquired using three instruments: ALFOSC, CAMELOT, and TCP. The first one is mounted at the Nordic Optical Telescope on La Palma, while the second and the third are mounted at the Instituto de Astrofisica de Canarias 0.82-m telescope on Tenerife. We analyzed the presence and variability of dust structures in the coma with image-enhancing techniques, the radial profile of the dust brightness, and we measured the dust production rate and the dust reddening. Results. We found evidence of a long-lasting sunward fan and anti-solar tail activity throughout all our observations up to a heliocentric distance of 2.42 AU. Afρ measurements suggest a pre-perihelion peak of the activity, caused by a seasonal effect, plus two post-perihelion outbursts. Both spatial and Afρ radial profiles indicate a steady-state coma at nucleocentric distances greater than ~1000-2000 km. The color analysis reveals a moderately reddened dust with a 6-9%/1000 reddening, consistent with the current picture of cometary dust. The second outburst emitted dust with lower reddening. Conclusions. The comparison with previous perihelion passages points toward a recurrent main activity always driven by the same areas on the nucleus, producing dust with similar characteristics and in similar coma structures in different years. Our Afρ measurement at the longest heliocentric distance suggests the comet was less dust-productive in 2010, pointing toward a possible secular aging of the object and its activity. The change of dust colors during the unusual second outburst suggests that an internal part of the nucleus has different physical properties compared with those that produce the recurrent main activity, pointing toward a heterogeneous comet. © ESO, 2012

The Distribution, Excitation and Formation of Cometary Molecules: Methanol, Methyl Cyanide and Ethylene Glycol

We present an interferometric and single dish study of small organic species toward Comets C/1995 O1 (Hale-Bopp) and C/2002 T7 (LINEAR) using the BIMA interferometer at 3 mm and the ARO 12m telescope at 2 mm. For Comet Hale-Bopp, both the single-dish and interferometer observations of CH3OH indicate an excitation temperature of 105+/-5 K and an average production rate ratio Q(CH3OH)/Q(H2O)~1.3% at ~1 AU. Additionally, the aperture synthesis observations of CH3OH suggest a distribution well described by a spherical outflow and no evidence of significant extended emission. Single-dish observations of CH3CN in Comet Hale-Bopp indicate an excitation temperature of 200+/-10 K and a production rate ratio of Q(CH3CN)/Q(H2O)~0.017% at ~1 AU. The non-detection of a previously claimed transition of cometary (CH2OH)2 toward Comet Hale-Bopp with the 12m telescope indicates a compact distribution of emission, D<9'' (<8500 km). For the single-dish observations of Comet T7 LINEAR, we find an excitation temperature of CH3OH of 35+/-5 K and a CH3OH production rate ratio of Q(CH3OH)/Q(H2O)~1.5% at ~0.3 AU. Our data support current chemical models that CH3OH, CH3CN and (CH2OH)2 are parent nuclear species distributed into the coma via direct sublimation off cometary ices from the nucleus with no evidence of significant production in the outer coma.Comment: accepted for publication in Ap

Fermi surfaces of single layer dielectrics on transition metals

Single sheets of hexagonal boron nitride on transition metals provide a model system for single layer dielectrics. The progress in the understanding of h-BN layers on transition metals of the last 10 years are shortly reviewed. Particular emphasis lies on the boron nitride nanomesh on Rh(111), which is a corrugated single sheet of h-BN, where the corrugation imposes strong lateral electric fields. Fermi surface maps of h-BN/Rh(111) and Rh(111) are compared. A h-BN layer on Rh(111) introduces no new bands at the Fermi energy, which is expected for an insulator. The lateral electric fields of h-BN nanomesh violate the conservation law for parallel momentum in photoemission and smear out the momentum distribution curves on the Fermi surface.Comment: 14 pages, 6 figures, 1 table, 1 equation, Accepted for publication in the Special Surface Science issue in honor of Gerhard Ertl's Nobel Priz

Spectrophotometric investigation of Phobos with the Rosetta OSIRIS-NAC camera and implications for its collisional capture

TheMartian satellite Phobos has been observed on 2007 February 24 and 25, during the pre- and post-Mars closest approach (CA) of the ESA Rosetta spacecraftMars swing-by. The goal of the observations was the determination of the surface composition of different areas of Phobos, in order to obtain new clues regarding its nature and origin. Near-ultraviolet, visible and near-infrared (263.5-992.0 nm) images of Phobos's surface were acquired using the Narrow Angle Camera of the OSIRIS instrument onboard Rosetta. The six multi-wavelength sets of observations allowed a spectrophotometric characterization of different areas of the satellite, belonging respectively to the leading and trailing hemisphere of the anti-Mars hemisphere, and also of a section of its sub-Mars hemisphere. The pre-CA spectrophotometric data obtained with a phase angle of 19° have a spectral trend consistent within the error bars with those of unresolved/disc-integrated measurements present in the literature. In addition, we detect an absorption band centred at 950 nm, which is consistent with the presence of pyroxene. The post-CA observations cover from NUV to NIR a portion of the surface (0° to 43°E of longitude) never studied before. The reflectance measured on our data does not fit with the previous spectrophotometry above 650 nm. This difference can be due to two reasons. First, the OSIRIS observed area in this observation phase is completely different with respect to the other local specific spectra and hence the spectrum may be different. Secondly, due to the totally different observation geometry (the phase angle ranges from 137° to 140°), the differences of spectral slope can be due to phase reddening. The comparison of our reflectance spectra, both pre-and post-CA, with those of D-type asteroids shows that the spectra of Phobos are all redder than the mean D-type spectrum, but within the spectral dispersion of other D-types. To complement this result, we performed an investigation of the conditions needed to collisionally capture Phobos in a way similar to that proposed for the irregular satellites of the giant planets. Once put in the context of the current understanding of the evolution of the early Solar system, the coupled observational and dynamical results we obtained strongly argue for an early capture of Phobos, likely immediately after the formation of Mars. © 2012 The Authors

Pre-hibernation performances of the OSIRIS cameras onboard the Rosetta spacecraft

Context. The ESA cometary mission Rosetta was launched in 2004. In the past years and until the spacecraft hibernation in June 2011, the two cameras of the OSIRIS imaging system (Narrow Angle and Wide Angle Camera, NAC and WAC) observed many different sources. On 20 January 2014 the spacecraft successfully exited hibernation to start observing the primary scientific target of the mission, comet 67P/Churyumov-Gerasimenko. Aims. A study of the past performances of the cameras is now mandatory to be able to determine whether the system has been stable through the time and to derive, if necessary, additional analysis methods for the future precise calibration of the cometary data. Methods. The instrumental responses and filter passbands were used to estimate the efficiency of the system. A comparison with acquired images of specific calibration stars was made, and a refined photometric calibration was computed, both for the absolute flux and for the reflectivity of small bodies of the solar system. Results. We found a stability of the instrumental performances within ±1.5% from 2007 to 2010, with no evidence of an aging effect on the optics or detectors. The efficiency of the instrumentation is found to be as expected in the visible range, but lower than expected in the UV and IR range. A photometric calibration implementation was discussed for the two cameras. Conclusions. The calibration derived from pre-hibernation phases of the mission will be checked as soon as possible after the awakening of OSIRIS and will be continuously monitored until the end of the mission in December 2015. A list of additional calibration sources has been determined that are to be observed during the forthcoming phases of the mission to ensure a better coverage across the wavelength range of the cameras and to study the possible dust contamination of the optics

Optimality of mutation and selection in germinal centers

The population dynamics theory of B cells in a typical germinal center could play an important role in revealing how affinity maturation is achieved. However, the existing models encountered some conflicts with experiments. To resolve these conflicts, we present a coarse-grained model to calculate the B cell population development in affinity maturation, which allows a comprehensive analysis of its parameter space to look for optimal values of mutation rate, selection strength, and initial antibody-antigen binding level that maximize the affinity improvement. With these optimized parameters, the model is compatible with the experimental observations such as the ~100-fold affinity improvements, the number of mutations, the hypermutation rate, and the "all or none" phenomenon. Moreover, we study the reasons behind the optimal parameters. The optimal mutation rate, in agreement with the hypermutation rate in vivo, results from a tradeoff between accumulating enough beneficial mutations and avoiding too many deleterious or lethal mutations. The optimal selection strength evolves as a balance between the need for affinity improvement and the requirement to pass the population bottleneck. These findings point to the conclusion that germinal centers have been optimized by evolution to generate strong affinity antibodies effectively and rapidly. In addition, we study the enhancement of affinity improvement due to B cell migration between germinal centers. These results could enhance our understandings to the functions of germinal centers.Comment: 5 figures in main text, and 4 figures in Supplementary Informatio

European consensus statement on essential colposcopy

Peer reviewedPostprin