Olivier Marco

On 4 July 2005 at 05:52 UT, the impactor of NASA's Deep Impact (DI) mission crashed into comet 9P/Tempel 1 with a velocity of about 10 . The material ejected by the impact expanded into the normal coma, produced by ordinary cometary activity. The La Silla and Paranal sites of the European Southern Observatory (ESO) in Chile participated in the world-wide campaign to observe this event.

Context.On 4 July 2005 at 05:52UT the impactor of NASA's DeepImpact (DI) mission exploded at comet 9P/Tempel 1. The ejecta material of the impact expanded into the coma that is produced by normal cometary activity. The La Silla and Paranal sites of the European Southern Observatory ESO in Chile participated in the world-wide campaign to observe this event.Aims.The gas and dust content of the cometary coma is observed around the time of the DI event to identify signatures of the impact, like changes in the gas production (CN, C3, C2, NH2 and Na) and in the dust properties. The study also describes the normal activity pattern in the coma and the expanding ejecta cloud.Methods.The gas production rates and the dust reddening slope are measured in images obtained through narrowband cometary filters. The ejecta cloud and the features of normal cometary activity, as imaged in narrow- and broadband filter images, are studied with respect to geometry, intensity, and persistence.Results.The production of CN, C3, and C2 gas by the nucleus is at similar level on 3 July and 9+10 July 2005. The mixing ratios of these gases are in the range of those for “typical” comets. NH2 and Na gas are not detected above the dust continuum flux in the coma. The reddening slope of the dust continuum changes from a value of 9%/100 nm, constant throughout 20 000 km distance around the nucleus, on 3 July to about 19–20%/100 nm on 9+10 July 2005. An increase of the slope with radial distance is found. Both changes might be due to the presence of dust from the ejecta cloud. The dust coma of the comet showed a porcupine pattern of 9 coma jets before impact which remained intact after impact. It can be interpreted as being due to embedded fan structures produced by at least 4 active regions on the rotating nucleus. The ejecta cloud contains dust grains of likely absorbing material and possibly dielectric admixtures with solar radiation pressure factors β of 0.2–1.9 and ejection velocities of 160–370 m s-1. The ejecta cloud is optically thin 21 h after impact and shows a higher surface brightness and redder color in the sunward sector compared to the tailward side.

Adaptive optics (AO) allows one to derive the point spread function (PSF) simultaneously to the science image, which is a major advantage in post-processing tasks such as astrometry/photometry or deconvolution. Based on the algorithm of Veran et al. (1997), PSF reconstruction has been developed for four different AO systems so far: PUEO, ALFA, Lick-AO and Altair. A similar effort is undertaken for NAOS/VLT in a collaboration between the group PHASE (Onera and Observatoire de Paris/LESIA) and ESO. In this paper, we first introduce two new algorithms that prevent the use of the so-called "U_ij functions" to: (1) avoid the storage of a large amount of data (for both new algorithms), (2) shorten the PSF reconstruction computation time (for one of the two) and (3) provide an estimation of the PSF variability (for the other one). We then identify and explain issues in the exploitation of real-time Shack-Hartmann (SH) data for PSF reconstruction, emphasising the large impact of thresholding in the accuracy of the phase residual estimation. Finally, we present the data provided by the NAOS real-time computer (RTC) to reconstruct PSF ((1) the data presently available, (2) two NAOS software modifications that would provide new data to increase the accuracy of the PSF reconstruction and (3) the tests of these modifications) and the PSF reconstruction algorithms we are developing for NAOS on that basis.

Stellar occultations, when a Solar System object passes between us and a star and blocks its light, are eagerly awaited by astronomers as they provide a chance to make measurements that are not normally possible. It had been 25 years since a solitary observation of a stellar occultation by Pluto's moon Charon. But on 11 July 2005 another occurred and this time observatories across South America were ideally placed to track it. The resulting haul of data has been used to obtain an accurate measure of Charon's radius, of close to 605 km, and to establish an upper limit (a rather low one) on the density of its atmosphere. Visit tinyurl.com/9c56s for a QuickTime movie of the event. Pluto and its satellite, Charon (discovered in 1978; ref. 1), appear to form a double planet, rather than a hierarchical planet/satellite couple. Charon is about half Pluto's size and about one-eighth its mass. The precise radii of Pluto and Charon have remained uncertain, leading to large uncertainties on their densities2. Although stellar occultations by Charon are in principle a powerful way of measuring its size, they are rare, as the satellite subtends less than 0.3 microradians (0.06 arcsec) on the sky. One occultation (in 1980) yielded a lower limit of 600 km for the satellite's radius3, which was later refined to 601.5 km (ref. 4). Here we report observations from a multi-station stellar occultation by Charon, which we use to derive a radius, RC = 603.6 ± 1.4 km (1σ), and a density of ρ = 1.71 ± 0.08 g cm-3. This occultation also provides upper limits of 110 and 15 (3σ) nanobar for an atmosphere around Charon, assuming respectively a pure nitrogen or pure methane atmosphere.

We present new images of the coronal-line region, as traced by [Si vii] 2.48 μm, in some of the nearest Seyfert 2 galaxies. In each of these galaxies, the coronal-line emission comprises a bright, compact central source and extended emission showing broad alignment along a particular direction, usually coinciding with that defined by the radio emission or the extended narrow-line region. The full extent of the coronal-line emission ranges from a few tens of parsecs to ∼150 pc radius from the nucleus and is a factor of ∼10 smaller than that seen in the extended, lower ionization gas. With a spatial resolution of 10 pc or better, the coronal region shows diffuse and filamentary structure in all cases, and it is difficult to see whether it breaks down into discrete blobs such as those seen in lower ionization lines or radio images of comparable resolution. The extent of the coronal-line emission is larger than would be predicted by photoionization models, which argues for additional in situ gas excitation, the most plausible energy source being shock excitation.

On 4 July 2005, many observatories around the world and in space observed the collision of Deep Impact with comet 9P/Tempel 1 or its aftermath. This was an unprecedented coordinated observational campaign. These data show that (i) there was new material after impact that was compositionally different from that seen before impact; (ii) the ratio of dust mass to gas mass in the ejecta was much larger than before impact; (iii) the new activity did not last more than a few days, and by 9 July the comet's behavior was indistinguishable from its pre-impact behavior; and (iv) there were interesting transient phenomena that may be correlated with cratering physics.

We have collected in 2004 adaptive optics corrected L′ images of the Galactic Center region with NAOS-CONICA at VLT. A strong variability was observed as well as a correlation between the photocenter positions and fluxes of the L′-band counterpart of Sgr A*. It is interpreted as the combined emission of a point-like flaring source at the position of Sgr A*/IR itself and an extended dust structure, 75 mas south west of Sgr A*/IR, which we name Sgr A*-f. We examine the different possible mechanisms to explain this dual Sgr A* L′ emission and conclude it is likely a flaring emission associated to energetic events in the close environment of the black hole plus a quiescent emission resulting from the collision of Sgr A*-f by a jet from Sgr A*.