K. Birkle

Visible and near-IR observations of transneptunian objects - Results from ESO and Calar Alto Telescopes

We present visible (BVRI) and near-IR (JHKs) broadband photometry and visible low-dispersion spec- troscopy of Transneptunian Objects (TNOs) and Centaurs. In total, 16 TNOs and 1 Centaur were observed over the past two years at ESO telescopes in La Silla and Paranal in Chile as well as at the Calar Alto Observatory in Spain. The sample consists of objects measured for the rst time and those for which comparison data is available from literature. The targets were: 1992QB1, 1993RO, 1994EV3, 1995HM5, 1995SM55, 1996RQ20, 1996TL66, 1996TO66, 1996TP66, 1997CQ29, 1997CS29, 1998HK151, 1998TF35, 1998VG44, 1998WH24, 1998XY95, 1999TC36. The spectra of 5 TNOs (1995SM55, 1996TO66, 1997CQ29, 1997CS29, 1998HK151) show almost constant gradients over the visible wavelength range with only marginal indication for a flatter slope beyond 750{800 nm. The photometric colour gradients obtained quasi-simultaneously are in good agreement with the spectral data. This suggests that in general photometric colour gradients are a valuable diagnostic tool for spec- tral type classication of TNOs. The photometric study revealed a number of new objects with neutral and red colours. For re-measured objects the published broadband colours were { in general { conrmed, although a few remarkable exceptions exist. Two TNOs appear to be outlyers according to the available broadband colours: 1993EV3 and 1995HM5. 1995SM55 is the bluest TNO measured so far. No clear global correlation between V I colour and absolute R lter brightness of our TNO targets is found. However, the data for the 5 brightest TNOs (brighter than 5 mag absolute magnitude) could also be interpreted with a linear increase of V I colour by about 0.75 mag per brightness magnitude. The colour-colour diagrams show continuous reddening of the TNOs in V R vs. B V , R I vs. B V and R I vs. V R. The bimodality suggested from earlier measurements of Tegler & Romanishin (1998) is not conrmed. According to our colour gradient statistics (number of objects per gradient interval) most of the TNOs have surface reddening between 0 and 40%/100 nm. For the Cubewanos the major population falls between 20{40%/100 nm. The Plutinos and Centaurs show a bifold grouping, i.e. a neutral/slightly reddish group (reddening <20%/100 nm) and a red group (reddening 30{40%/100 nm). The statistical signicance of the various populations found is suering { for the Centaurs and scattered disk objects very severely { from the small number of objects measured. However, the diversity of the reddening distribution of Centaurs/Plutinos and Cubewanos, if conrmed by new observations, may indicate a dierent balancing of resurfacing processes for these object types: for instance, for Centaurs a possibility is that re-condensed frost from coma activity may be dominant over impact re-surfacing and high energy surface processing.

ESO large program on physical studies of Transneptunian Objects and Centaurs: Visible photometry – First results

We present the first results of BVRI photometry of Transneptunian Objects (TNOs) and Centaurs obtained through the ESO Large Program on physical studies of these icy bodies in the outer solar system. In total 28 objects were observed of which 18 are new measurements. Combining our new BVRI photometry with the data summary published by Hainaut & Delsanti (2002) results in a database of 94 objects: 45 Cubewanos, 22 Plutinos, 13 scattered disk objects, 14 Centaurs. The reddening range seems to be similar among the four dynamical classes (− 5t o 55%/l00 nm) and only one outlier (1994 ES2) exists. The spectral gradient distribution of the Cubewanos peaks between 25 to 35%/l00 nm, while for the three other types the maximum seems to fall below 20%/l00 nm. A clustering of red Cubewanos with perihelia beyond ∼41 AU in low eccentricity and low inclination orbit suggests that these objects are less affected by the physical processes that potentially produce neutral colors, i.e. resurfacing by collision and by intrinsic activity. For Cubewanos and scattered disk objects, the range of reddening increases with decreasing perihelion distance and with increasing orbital excitation. A correlation of the spectral slope with inclination is present for Cubewanos and scattered disk objects, and is non-existent for the other dynamical types. It is unclear whether these trends (or their absence) are discriminative for the correctness of the resurfacing scenarios. If intrinsic activity is responsible for resurfacing, the start of the effect inside ∼41 AU from the Sun may be indicative for the driving agent, while in the collision scenario the survival of the red Cubewano cluster in the central region of the Kuiper-Belt argues for the existence of a population of bodies the surface of which is heavily radiation processed without impact resurfacing.

Visible and near-infrared spectroscopy of the Centaur 32532 (2001 PT13). ESO Large Programm on TNOs and Centaurs: First spectroscopy results

We present photometric and spectroscopic data obtained through visible and near-infrared observations of the Centaur 32532 (2001 PT13) performed with FORS1 and ISAAC at the Very Large Telescope (VLT-ESO) within the framework of an ESO large program on the Trans-Neptunian objects (TNOs) and Centaurs. The results show evidence for a difference in the near-infrared spectral behaviour obtained during two observations carried out one month apart. In one spectrum there is the possible presence of signatures of water ice in small amounts. Two models have been proposed to interpret the surface composition of this Centaur.

A long-term photometric study of the FU Orionis star V 733 Cephei

Context. The FU Orionis candidate V733 Cep was discovered by Roger Persson in 2004. The star is located in the dark cloud L1216 close to the Cepheus OB3 association. Because only a small number of FU Orionis stars have been detected to date, photometric and spectral studies of V733 Cep are of great interest. Aims. The studies of the photometrical variability of PMS stars are very important to the understanding of stellar evolution. The main purpose of our study is to construct a long-time light curve of V733 Cep. On the basis of BVRI monitoring we also study the photometric behavior of the star. Methods. We gather data from CCD photometry and archival photographic plates. The photometric BVRI data (Johnson-Cousins system) that we present were collected from June 2008 to October 2009. To facilitate transformation from instrumental measurements to the standard system, fifteen comparison stars in the field of V733 Cep were calibrated in BVRI bands. To construct a historical light curve of V733 Cep, a search for archival photographic observations in the Wide-Field Plate Database was performed. As a result, 192 photographic plates containing the field of V733 Cep were found. Some plates were analyzed at our request to estimate the magnitude of V733 Cep. Results. Our photometric study confirms the affiliation of V733 Cep to the group of FU Orionis objects. An outburst in the optical and a slow rise in brightness during the period 1971‐1993 are well documented. During the period 1993‐2004, V733 Cep exhibited its maximum brightness and the amplitude of the observed outburst exceeded 4. m 5( R). TheBVRI photometric data imply that from February 2007 to October 2009, a slow decrease in brightness of V733 Cep was observed. The observed color evolution of V − I index also suggest that V733 Cep is currently fading. The long-time light curve of V733 Cep is similar to the light curves of other FU Orionis objects.

A long-term photometric study of V 1184 Tauri

Aims. The main purpose of our investigation is to construct the long-time light curve of the PMS star V 1184 Tau. We consider the study of the photometrical variability of PMS stars as very important for understanding stellar evolution. The unusual photometric variability of V 1184 Tau was reported in previous studies as well, but the nature of the observed deep minima is still under discussion. Methods. We present recent data from CCD photometry and from archival photographic plates. The photometric BVRI data presented in this paper were collected from November 2005 to November 2007. To construct the historical light curve of V 1184 Tau, a search for archived photographic observations in the Wide-Field Plate Database (WFPDB) was made. As a result, 412 photographic plates were found containing the field of V 1184 Tau. A part of the plates were scanned at our request and a magnitude estimation was made of V 1184 Tau. Results. Our recent photometric data suggest that the star brightness is still near the minimum. Thus the period of strong light variations that started in 2003 continues up to the present, hence more than 4 years. Our data from the archival photographic plates suggest that an unknown minimum of brightness exists in the approximate period 1980-1985. Taking all available photometric and spectroscopic data into account we must reject the hypothesis that V 1184 Tau is an FU Orionis type object. V 1184 Tau is a G type low-mass star whose spectrum is similar to WTT stars, but its photometric behavior is typical of the UX Orionis variable stars. Assuming the obscuration from orbiting dust clouds as a reason for a deep minimum, we estimate the approximate period of obscurations and the interval between two deep minima. Our calculations give a 25-28 year period between the two minima and approximately 8-10 year duration of the minima.

An Early Investigation Of The Striated Tail Of Comet Hale-Bopp (C/1995 O1)

The Sekanina-Farrell particle fragmentation model for the striated tails of dust comets is successfully applied to two images of comet Hale-Bopp to study the motions of 12 striae in a time span of March 12–15, 1997. There is evidence for recurring outbursts with a periodicity of 11h21m, consistent with results based on analysis of dust jets. The ejecta in all the striae appear to have been released from one source on the nucleus between the end of January and the second half of February 1997, some 60 to 40 days before perihelion. The parent particles were subjected to a radiation pressure acceleration of βp ≃ 0.55 and their fragmentation lifetimes in 11 of the 12 striae were practically constant and equal to 13–15 days, when normalized to 1 AU from the Sun. Brief analysis of Watanabe et al.s measurements of striae on their images from March 5–9, 1997 shows even shorter fragmentation lifetimes for the parent particles, mostly about 7–11 days at1 AU.

Nucleus and tail studies of comet P/Swift-tuttle

CCD images of comet P/Swift-Tuttle, obtained in April 1994 with the 2.2m telescope at ESO La Silla/Chile, showed a comaless stellar nucleus. From absolute photometry we estimated the equivalent radius of the cometary nucleus to be about 11 km (assuming an albedo of 0.04 as for P/Halley) for two rotation phase angles which differ by about 75 deg. From that we conclude that the nucleus is either of rather spherical shape or that the viewing geometry was almost pole-on during our observations.An analysis of the plasma tail and inner coma of the comet by means of photographic plates and CCD images through IHW and BVR filters, obtained with the 80cm Schmidt camera and the 1.2m telescope at Calar Alto/Spain in November 1992, revealed several tail rays, head streamers and substructures in brightness excess areas in the coma. While some of the tail rays extended to several million km nuclear distance, most of them can be traced to starting points which lie in a region just 20000–35000 km projected distance tailward from the nucleus.

Dust Morphology Of Comet Hale-Bopp (C/1995 O1): I. Pre-Perihelion Coma Structures In

In 1996 comet Hale-Bopp exhibited a porcupine-like coma with straight jets of dust emission from several active regions on the nucleus. The multi-jet coma geometry developed during the first half of 1996. While the jet orientation remained almost constant over months, the relative intensity of the jets changed with time. By using the embedded fan model of Sekanina and Boehnhardt (1997a) the jet pattern of comet Hale-Bopp in 1996 can be interpreted as boundaries of dust emission cones (fans) from four — possibly five — active regions on the nucleus (for a numerical modelling see part II of the paper by Sekanina and Boehnhardt, 1997b).

Observations of the GIOTTO target comet P/Grigg-Skjellerup at the Calar Alto Observatory

CCD images of comet P/Grigg-Skjellerup, obtained for astrometric purposes with the 3.5 m telescope at the Calar Alto Observatory/Spain, were used for an analysis of the activity status of the nucleus and for a search of faint coma structures. The nucleus was found essentially inactive beyond 2.7 AU solar distance both inbound and outbound (observations on 12–13 August, 1986, 21–23 October, 1986, 22 August, 1988, 18 October, 1988, 9 and 12 September, 1991 and 3 December, 1991). The coma of the comet was well developed in May and July 1987 with a diameter of at least 190 000 km on 24 May, 1987 and of at least 80 000 km on 24 July 1987. The coma showed a cone of diffuse brightness enhancement in the sunward hemisphere. The orientation of the cone axis changed from the Sun direction in May 1987 towards about North in July 1987, i.e., it was almost perpendicular to the projected Sun-nucleus line on the sky. The cone opening angle became smaller from about 100‡ in May to about 50 ‡ in July 1987. A weak and narrow plasma tail was found in the images of May 1987.

Physical studies of KuiperBelt objects: an ESO VLT large program

Since April 2001 a Large Program for the study of physical properties of Trans-Neptunian Objects (TNO) and Centaurs is underway at the Paranal (Very Large Telescope VLT) and La Silla (New Technology Telescope NTT) observatories of the European Southern Observatory (ESO) in Chile. Combining service (SM) and visitor mode (VM) observations multi-wavelength imaging (BVRIJHK filters) and low-dispersion spectroscopy is performed in the visible and near-IR on a sample of objects that should allow a better and more consistent taxonomic characterization and classification of these pristine bodies in our solar system. Starting with a summary of the current knowledge on the Kuiper-Belt and the populations of objects, the paper presents the goals of this project and its scientific and organizational implementation. It illustrates the progress and the scientific achievements by a hynoptic view of results from photometry and spectroscopy of these Solar System objects.