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Context. A high percentage of the astrophysically important RR Lyrae stars show a periodic amplitude and/or phase modulation of their pulsation cycles. More than a century after its discovery, this “Blazhko effect” still lacks acceptable theoretical understanding. In one of the plausible models for explaining the phenomenon, the modulation is caused by the effects of a magnetic field. So far, the available observational data have not allowed us to either support nor rule out the presence of a magnetic field in RR Lyrae stars.

Aims. We study how chemical abundances of late B-, A-, and early F-type stars evolve with time, and we search for correlations between the abundance of chemical elements and other stellar parameters, such as effective temperature and ${\upsilon}\sin i$.

Context. Recent spectropolarimetric observations of Herbig Ae/Be stars have yielded new arguments in favour of a fossil origin for the magnetic fields of intermediate mass stars.

Aims.We have completed an accurate investigation of the Ca isotopic composition and stratification in the atmospheres of 23 magnetic chemically peculiar (Ap) stars of different temperature and magnetic field strength.

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 ${\rm km\,s}^{-1}$. 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.

Aims.We aim to provide observational constraints on diffusion models that predict peculiar chemical abundances in the atmospheres of Am stars. We also intend to check if chemical peculiarities and slow rotation can be explained by the presence of a weak magnetic field.

Aims.We investigated a sample of 28 well-known spectroscopically-identified magnetic Ap/Bp stars, with weak, poorly-determined or previously undetected magnetic fields. The aim of this study is to explore the weak part of the magnetic field distribution of Ap/Bp stars.

Context.The evolution of magnetic fields in Ap stars during the main sequence phase is presently mostly unconstrained by observation because of the difficulty of assigning accurate ages to known field Ap stars.

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.In this work we present a revised measurement of the phase-averaged optical polarisation of the Vela pulsar (PSR B0833-45), for which only one value has been published so far (Wagner & Seifert 2000, ASP Conf. Ser., 202, 315).

Aims.We have investigated the structure of the pulsating atmosphere of one of the best studied rapidly oscillating Ap stars, HD 24712.

Context.The general context of this paper is the study of magnetic fields in the pre-main sequence intermediate mass Herbig Ae/Be stars. Magnetic fields are likely to play an important role in pre-main sequence evolution at these masses, in particular in controlling the gains and losses of stellar angular momentum.

Context.High spectral resolution studies of cool Ap stars reveal conspicuous anomalies of the shape and strength of many absorption lines. This is a signature of large atmospheric chemical gradients (chemical stratification) produced by the selective radiative levitation and gravitational settling of chemical species.

Context.The chemically peculiar stars of the upper main sequence represent a natural laboratory for the study of rare-earth elements (REE).

Context.About 5% of upper main sequence stars are permeated by a strong magnetic field, the origin of which is still matter of debate.

Context.The photospheres of about 5–10% of the upper main sequence stars exhibit remarkable chemical anomalies. Many of these chemically peculiar (CP) stars have a global magnetic field, the origin of which is still a matter of debate.

Aims.It is unclear whether chemically peculiar stars of the upper main sequence represent a class completely distinct from normal A-type stars, or whether there exists a continuous transition from the normal to the most peculiar late F- to early B-type stars. A systematic abundance analysis of open cluster early-type stars would help to relate the observed differences of the chemical abundances of the photospheres to other stellar characteristics, without being concerned by possible different original chemical composition. Furthermore, if a continuous transition region from the very peculiar to the so called normal A–F stars exists, it should be possible to detect objects with mild peculiarities.

Context.The chemically peculiar HgMn stars are a class of Bp stars which have historically been found to be both non-magnetic and non-variable. Remarkably, it has recently been demonstrated that the bright, well-studied HgMn star αAnd exhibits clear Hg iiline profile variations indicative of a non-uniform surface distribution of this element.

We report a discovery of the Zeeman resolved spectral lines, corresponding to the extremely large magnetic field modulus $\langle B_{\rm s}\rangle$= 17.5 kG, in the cool Ap star HD 178892. The mean longitudinal field of this star reaches 7.5 kG, and its rotational modulation implies the strength of the dipolar magnetic component $B_{\rm p}\ge$23 kG. We have revised rotation period of the star using the All Sky Automated Survey photometry and determined P= 8.2478 d. Rotation phases of the magnetic and photometric maxima of the star coincide with each other. We obtained Geneva photometric observation of HD 178892 and estimated Teff= $7700\pm250$K using photometry and the hydrogen Balmer lines. Preliminary abundance analysis reveals abundance pattern typical of rapidly oscillating Ap stars.

We report the discovery, using FORS1 at the ESO-VLT and ESPaDOnS at the CFHT, of magnetic fields in the young A-type stars HD 101412, V380 Ori and HD 72106A. Two of these stars (HD 101412 and V380 Ori) are pre-main sequence Herbig Ae/Be (HAeBe) stars, while one (HD 72106A) is physically associated with a HAeBe star. Remarkably, evidence of surface abundance spots is detected in the spectra of HD 72106A. The magnetic fields of these objects display intensities of order 1 kG at the photospheric level, are ordered on global scales, and appear in approximately 10% of the stars studied. Based on these properties, the detected stars may well represent pre-main sequence progenitors of the magnetic Ap/Bp stars. The low masses inferred for these objects (2.6, 2.8 and 2.4 $M_\odot$) represent additional contradictions to the hypothesis of Hubrig et al. (2000, ApJ, 539, 352), who claim that magnetic fields appear in intermediate-mass stars only after 30% of their main sequence evolution is complete. Finally, we fail to confirm claims by Hubrig et al. (2004, A&A, 428, L1) of magnetic fields in the Herbig Ae star HD 139614.

During a survey of field strengths in upper main sequence stars in open clusters, we observed the star NGC 2244-334 in the Rosette Nebula cluster and discovered an extraordinarily large mean longitudinal field of about -9 kG, the second largest longitudinal field known in a non-degenerate star. This star appears to be a typical Ap He-wk (Si) star of about $4~M_\odot$. Spectrum synthesis using a line synthesis code incorporating the effects of the strong magnetic field indicates that He is underabundant by about 1.5 dex, and C, O and Mg by about 0.1–0.4 dex, while Si, Mn and Fe are overabundant by about 1 dex, and Cr and Ti are nearly 2 dex overabundant.
Cluster membership for this star is secure, so its age is about $2\times 10^6$yr, which is less than 3% of its main sequence lifetime. This star is one of the very youngest magnetic upper main sequence stars with a well-determined age, and confirms that both magnetic fields and strong chemical peculiarity can appear in stars which are both extremely young and very close to the ZAMS.

We present the first investigation of the structure of the stellar surface magnetic field using line profiles in all four Stokes parameters. We extract the information about the magnetic field geometry and abundance distributions of the chemically peculiar star 53 Cam by modelling time-series of high-resolution spectropolarimetric observations with the help of a new magnetic Doppler imaging code. This combination of the unique four Stokes parameter data and state-of-the-art magnetic imaging technique makes it possible to infer the stellar magnetic field topology directly from the rotational variability of the Stokes spectra. In the magnetic imaging of 53 Cam we discard the traditional multipolar assumptions about the structure of magnetic fields in Ap stars and explore the stellar magnetic topology without introducing any global a priori constraints on the field structure. The complex magnetic model of 53 Cam derived with our magnetic Doppler imaging method achieves a good fit to the observed intensity, circular and linear polarization profiles of strong magnetically sensitive $\ion{Fe}{ii}$spectral lines. Such an agreement between observations and model predictions was not possible with any earlier multipolar magnetic models, based on modelling Stokes Ispectra and fitting surface averaged magnetic observables (e.g., longitudinal field, magnetic field modulus, etc.). Furthermore, we demonstrate that even the direct inversion of the four Stokes parameters of 53 Cam assuming a low-order multipolar magnetic geometry is incapable of achieving an adequate fit to our spectropolarimetric observations. Thus, as a main result of our investigation, we discover that the magnetic field topology of 53 Cam is considerably more complex than any low-order multipolar expansion, raising a general question about the validity of the multipolar assumption in the studies of magnetic field structures of Ap stars. In addition to the analysis of the magnetic field of 53 Cam, we reconstruct surface abundance distributions of Si, Ca, Ti, Fe and Nd. These abundance maps confirm results of the previous studies of 53 Cam, in particular dramatic antiphase variation of Ca and Ti abundances.

The Plutino 28978 Ixion, one of the largest objects in the Kuiper Belt, is measured by Rfilter photometry and linear polarimetry over the phase angle range as well as by BVRIcolours and visible spectroscopy. Ixion is a medium red object with spectral slope of 17.7%/100 nm (400-900 nm). While its opposition phase curve surge (0.2 mag/deg) is within the typical range found for other solar system objects, it displays unusually high negative polarization (min. -1.3%). Comparison with model spectra and numerical modeling of the opposition brightening and polarization effects suggest an areal mixture of absorbing (dark) and icy (bright) compounds on its surface: the mixing ratio is about 6:1 for dark and bright material of 0.21 and 0.79 single-scattering albedo and of 250 and 33 dimensionless mean free path (length multiplied by wave number), respectively.

With FORS 1 at the VLT we have tried for the first time to measure the magnetic field variation over the pulsation cycle in six roAp stars to begin the study of how the magnetic field and pulsation interact. For the star HD 101065, which has one of the highest photometric pulsation amplitudes of any roAp star, we found a signal at the known photometric pulsation frequency at the 3σlevel in one data set; however this could not be confirmed by later observations. A preliminary simple calculation of the expected magnetic variations over the pulsation cycle suggests that they are of the same order as our current noise levels, leading us to expect that further observations with increased S/N have a good chance of achieving an unequivocal detection.

We have been searching for magnetic Ap stars in open clusters, in order to clarify the time evolution of magnetic fields in middle main sequence stars from the ZAMS to the TAMS. We have discovered that the star HD 66318 in the open cluster NGC 2516 has an extraordinarily large magnetic field: the measured mean longitudinal component $B_{\rm l} \approx 4.5$kG, and the mean field modulus $B_{\rm s} \approx 14.5$kG. This star thus has one of the largest fields so far discovered in a non-degenerate star, and the largest field known in a current Ap star cluster member.?We estimate that HD 66318 has completed about $16 \pm 5$% of its main sequence life. It thus appears to contradict the hypothesis of Hubrig et al. that magnetic fields are only found in stars that have completed at least 30% of their main sequence lifetimes.?There is no indication that the spectrum or brightness of the star is variable, and the spectral lines are very sharp. The star probably has a very long rotation period (years). ?We have modelled some parts of the observed spectrum, assuming that the chemical composition is uniform both horizontally and vertically, and using a simple multipolar expansion for the field structure; although our model does not reproduce exactly the observed spectrum, it is clear that the atmospheric chemical composition of the star is very peculiar, with Ti, Cr and Fe overabundant by between 1.5 and 2.5 dex. Both La iiand iiiare apparently about 4 dex overabundant. In contrast, evidence for the presence of other rare earths is difficult to find in the spectrum. It appears that Ce iii, Pr iii, Nd iiand iii, and Eu iiare detected with inferred overabundances ranging between 1.5 and 5 dex, but for most of these elements, the abundance of the non-detected ionization state is significantly lower than that inferred for the detected state. HD 66318 thus seems to exhibit strong discrepancies between abundances deduced for different ionization states of rare earths, a phenomenon so far found only in somewhat cooler stars. ?Even within one ionization state it has not been found possible to fit most observed lines with a single value of abundance. For example, when we fit medium strength lines of Fe ii, the calculated strong lines of this ion are deeper than observed, and the calculated weak lines are less deep than observed. This situation is probably due to strong vertical abundance stratification of most of the elements studied.?Finally, HD 66318 also shows a new form of core-wing anomaly in Hain which the observed line profile falls belowthe computed one in the core. These characteristics clearly suggest that the atmospheric structure of HD 66318 is not closely similar to that of a normal main sequence A star of similar parameters.?

We present the results of a statistical study of the magnetic structure of upper main sequence chemically peculiar stars. We have modelled a sample of 34 stars, assuming that the magnetic morphology is described by the superposition of a dipole and a quadrupole field, arbitrarily oriented. In order to interpret the modelling results, we have introduced a novel set of angles that provides one with a convenient way to represent the mutual orientation of the quadrupolar component, the dipolar component, and the rotation axis. Some of our results are similar to what has already been found in previous studies, e.g., that the inclination of the dipole axis to the rotation axis is usually large for short-period stars and small for long-period ones – see Landstreet & Mathys ([CITE]). We also found that for short-period stars (approximately $P<10\,\mathrm{days}$) the plane containing the two unit vectors that characterise the qua dru po le is almost coincident with the plane containing the stellar rotation axis and the dipole axis. Long-period stars seem to be preferentially characterised by a qua dru po le orientation such that the planes just mentioned are perpendicular. There is also some loose indication of a continuous transition between the two classes of stars with increasing rotational period.

We report new high resolution Stokes Vspectropolarimetry and ultra-high precision longitudinal magnetic field measurements of the pulsating classical Cepheid ηAql which fail to corroborate the report by Plachinda ([CITE]) of a ~100 G magnetic field in this star.

This paper describes an experiment aimed at evaluating the capability of the FORS1instrument at the VLTfor measuring weak ($\simeq $0.1%) Zeeman circular polarization signatures in stellar H Balmer lines. We have obtained low-resolution polarized spectra at 3500–5800 Å of two bright A-type stars, HD 94660, and HD 96441. The former is a well known magnetic star, the latter an apparently non-magnetic, unpolarized star that was observed for comparison purposes. In order to test the possibility of performing multi-object spectropolarimetric measurements (e.g., for cluster studies), observations were taken both on-axis (i.e., at the CCD field center) and off-axis (i.e., at the edges of the CCD). In HD 94660 (the magnetic star), we detected a clear signal of circular polarization in all observed hydrogen Balmer lines (i.e., from Hβto the Balmer limit), with a typical peak-to-peak amplitude of about 0.8%. From the analysis of Stokes Vunder the weak-field approximation, we have estimated a mean longitudinal field of $-2085 \pm 85$G, a value fully consistent with previous studies of this star. Notably, we found that at wavelengths shorter than about 4100 Å, the polarization signal detected off-axis is about 20–30% smaller than that detected on-axis. No polarization due to the Zeeman effect was detected in the comparison star HD 96441, and we estimate that the contribution due to instrumental circular polarization is limited to the order of $0.01\%$. This experiment demonstrates the effectiveness of FORS1at the VLTas a tool for high-sensitivity diagnosis of the magnetic field in upper main sequence stars, providing the potential for measuring fields in fainter and more rapidly rotating stars than has previously been possible.

We present observations in the optical, near-infrared and millimetre bands of the Ultraluminous Infrared galaxy IRAS 19254-7245, also known as “The Superantennae”. This galaxy is an interacting system with a double nucleus and long tails extending for about 350 kpc. We studied in detail the southern component of the system which is optically classified as a Sy2 galaxy. We have developed a method to determine the parameters of the emission lines in a spectrum in the case of multiple components and severe blending. Our data allow us to build a picture of the environment around the nucleus of the galaxy with unprecedented detail. The optical lines show a complex dynamical structure that is not observable in the near-infrared. In addition we find typical features of AGN such as the coronal lines of [FeVII]5721 and [SiVI]1.96. We also detect strong emission from [FeII]1.64 and H2.

With the aim of establishing a benchmark for the detailed calculation of the polarised line profiles of magnetic stars, we describe an intercomparison of LTE Stokes profiles calculated using three independent, state-of-the-art magnetic spectrum synthesis codes: Cossam, ı10 and Zeeman2. We find, upon establishing a homogeneous basis for the calculations (identical definitions of the Stokes parameters and the magnetic and stellar reference frames, identical input model stellar atmosphere, identical input atomic data, and identical chemical element abundances and magnetic field distributions), that local and disc-integrated Stokes IQUVprofiles of Fe ${\scriptstyle {\rm II}} \lambda 4923.9$calculated using the three codes agree very well. For the illustrative case of disc-integrated profiles calculated for abundance $\log {n_{\rm Fe}}/{n_{\rm tot}}=-4.60$, dipole magnetic field intensity $B_{\rm d}=5$kG, and projected rotational velocity $v_{\rm e}\sin i=20$km s-1, Stokes Iprofiles (depth ∼40% of the continuum flux Ic) agree to within about 0.05% rms of Ic, Stokes Vprofiles (full amplitude ∼10% ) to within about 0.02% rms of Ic, and Stokes Qand Uprofiles (full amplitudes ∼2% ) at the sub-0.01% rms level. These differences are sufficiently small so as to allow for congruent interpretation of the best spectropolarimetric data available, as well as for any data likely to become available during the near future. This indicates that uncertainties in modeling Stokes profiles result overwhelmingly from uncertainties in input atomic and physical data, especially the state and structure of model stellar atmospheres.

We present a comparison of observed and calculated Stokes IQUVspectra of two well-known magnetic chemically peculiar stars, βCoronae Borealis and 53 Camelopardalis. The observed Stokes spectra were recently described by Wade et al. ([CITE]), and have been complemented with additional circularly polarized spectra obtained at the Special Astrophysical Observatory. The calculated spectra represent the predictions of new and previously published magnetic field models derived from the analysis of some surface averaged field estimates (e.g., longitudinal field, magnetic field modulus, etc.). We find that these magnetic models are not sufficient to account fully for the observed Stokes profiles -particularly remarkable is the disagreement between the predicted and observed Stokes Qand Uprofiles of 53 Cam. We suggest that this should be interpreted in terms of magnetic morphologies which are significantly more complex than the second-order multipolar expansions assumed in the models. However, it is clear that some of our inability to reproduce the detailed shapes of the Stokes IQUVprofiles is unrelated to the magnetic models. For many metallic ions, for both stars, we found it impossible to account for the strengths and shapes of the observed spectral line profiles when we adopted a unique value for the individual ion abundance. We suggest that this results from strongly non-uniform distributions of these ions as a function of optical depth (i.e., chemical stratification), a hypothesis that is supported by comparison with simple chemically stratified models.

We present the discovery of pulsational variations in the cool magnetic Ap star HD?115226 ? the first high-amplitude rapidly oscillating Ap (roAp) star discovered with time-series spectroscopy. Using high-resolution spectra obtained with the HARPS instrument at the European Southern Observatory 3.6-m telescope, we detect radial velocity variations with a period of 10.86?min in Priii, Ndiii, Dyiii lines, and in the narrow cores of hydrogen lines. Pulsational amplitudes exceed 1?km?s-1?in individual lines of Ndiii. The presence of running waves in the stellar atmosphere is inferred from a phase shift between the radial velocity maxima of rare-earth and hydrogen lines. Our abundance analysis demonstrates that HD?115226 exhibits a typical roAp spectroscopic signature, notably ionization anomaly of Pr, Nd, and Dy. We discuss the discovery of pulsations in HD?115226 in the context of recent spectroscopic studies of roAp stars and point to the existence of a correlation between spectroscopic pulsational amplitude and the stellar rotation rate.

Nighttime polarimetric measurements are often obtained very close to the limits of the instrumental capabilities. It is important to be aware of the possible sources of spurious polarization, and to adopt data reduction techniques that best compensate for the instrumental effects intrinsic to the design of the most common polarimeters adopted for nighttime observations. We define a self-consistent framework starting from the basic definitions of the Stokes parameters, and we present an analytical description of the data reduction techniques commonly used with a polarimeter (consisting of a retarder wave plate and a Wollaston prism) to explore their advantages and limitations. We first consider an ideal polarimeter in which all optical components are perfectly defined by their nominal characteristics. We then introduce deviations from the nominal behavior of the polarimetric optics, and develop an analytical model to describe the polarization of the outgoing radiation. We study and compare the results of two different data reduction methods, one based on the differences of the signals, and one based on their ratios, to evaluate the residual amount of spurious polarization. We show that data reduction techniques may fully compensate for small deviations of the polarimetric optics from their nominal values, although some important (first-order) corrections have to be adopted for linear polarization data. We include a detailed discussion of quality checking by means of null parameters. We present an application to data obtained with the FORS1 instrument of the ESO VLT, in which we have detected a significant amount of cross talk between circular and linear polarization. We show that this cross-talk effect is not due to the polarimetric optics themselves, but is most likely caused by spurious birefringence due to the instrument's collimator lens.

Context. Modern spectroscopy of early-type stars often aims at studying complex physical phenomena such as stellar pulsation, the peculiarity of the composition of the photosphere, chemical stratification, the presence of a magnetic field, and its interplay with the stellar atmosphere and the circumstellar environment. Comparatively less attention is paid to identifying and studying the ?normal? A- and B-type stars and testing how the basic atomic parameters and standard spectral analysis allow one to fit the observations. By contrast, this kind of study is paramount for eventually allowing one to correctly quantify the impact of the various physical processes that occur inside the atmospheres of A- and B-type stars.Aims. We wish to establish whether the chemical composition of the solar photosphere can be regarded as a reference for early A- and late B-type stars.Methods. We have obtained optical high-resolution, high signal-to-noise ratio spectra of three slowly rotating early-type stars (HD?145788, 21?Peg and ??Cet) that show no obvious sign of chemical peculiarity, and performed a very accurate LTE abundance analysis of up to 38 ions of 26 elements (for 21?Peg), using a vast amount of spectral lines visible in the spectral region covered by our spectra.Results. We provide an exhaustive description of the abundance characteristics of the three analysed stars with a critical review of the line parameters used to derive the abundances. We compiled a table of atomic data for more than 1100 measured lines that may be used in the future as a reference. The abundances we obtained for He, C, Al, S, V, Cr, Mn, Fe, Ni, Sr, Y, and Zr are compatible with the solar ones derived with recent 3D radiative-hydrodynamical simulations of the solar photosphere. The abundances of the remaining studied elements show some degree of discrepancy compared to the solar photosphere. Those of N, Na, Mg, Si, Ca, Ti, and Nd may well be ascribed to non-LTE effects; for P, Cl, Sc and Co, non-LTE effects are totally unknown; O, Ne, Ar, and Ba show discrepancies that cannot be ascribed to non-LTE effects. The discrepancies obtained for O (in two stars) and Ne agree with very recent non-LTE?abundance analysis of early B-type stars in the solar neighbourhood.

We present our discovery of pulsational radial-velocity variations in the cool Ap?star HD?75445, an object spectroscopically similar to the bright, rapidly-oscillating Ap?(roAp) star???Equ. Based on high-resolution time-series spectroscopy obtained with the HARPS spectrometer at the European Southern Observatory 3.6-m telescope, we detected oscillations in Nd?II?and Nd?III?lines with a period close to 9?min and amplitudes of 20-30?m?s-1. Substantial variation in the pulsational amplitude during our 3.8?h observing run reveals the presence of at least three excited non-radial modes. The detection of extremely low amplitude pulsations in HD?75445?indicates that the roAp?excitation mechanism produces variability in the radial velocity amplitude of between a few tens m?s-1?and several km?s-1. This supports the idea that many, if not all, cool Ap?stars occupying the roAp?instability strip may harbour non-radial pulsations, which currently remain undetected due to their small photometric and radial-velocity amplitudes.

Context. Trans-Neptunian objects (TNOs) contain the most primitive and thermally least-processed materials from the early accretional phase of the solar system. They allow us to study interrelations between various classes of small bodies, their origin and evolution. Aims. We exploit the use of polarimetric techniques as a remote-sensing tool to characterize the surface of TNOs. Methods. Using FORS1 of the ESO VLT, we have obtained linear-polarization measurements in the Bessell R?filter for five TNOs at different values of their phase angle (i.e., the angle between the Sun, the object, and the Earth). Due to the large distance of the targets (?30?AU), the observed range of phase angles is limited to about 0??2?. Results. We have analyzed our new observations of five TNOs, and those of another four TNOs obtained in previous works, and discovered that there exist two classes of objects that exhibit different polarimetric behaviour. Objects with a diameter >1000?km, such as, e.g., Pluto and Eris, show a small polarization in the scattering plane (~0.5%) which slowly changes in the observed phase angle range. In smaller objects such as, e.g., Ixion and Varuna, linear polarization changes rapidly with the phase angle, and reaches ~1% (in the scattering plane) at phase angle 1?. The larger objects have a higher albedo than the smaller ones, and have the capability of retaining volatiles such as CO, N2and CH4. Both of these facts can be linked to their different polarimetric behaviour compared to smaller objects. Conclusions. In spite of the very limited range of observable phase angles, ground-based polarimetric observations are a powerful tool to identify different properties of the surfaces of TNOs. We suggest that a single polarimetric observation at phase angle ~1? allows one to determine whether the target albedo is low or high.

Context. Comet 2P/Encke, a short-period comet, is a favorable target for studies of light scattering by bare cometary nuclei. These studies enable assessment of the nucleus size, the material albedo, and the light-scattering properties of the cometary surface. Observations of 2P/Encke were completed between October to December?2006 when the comet approached the Sun between 2.7 and 2.1?AU.Aims. We characterize the activity of the cometary coma and the physical properties of the nucleus of 2P/Encke such as its size, albedo, colors, spectral slope, surface roughness, porosity, and single-particle properties. Methods. Broadband imaging photometry and broadband and narrowband linear polarimetry is measured for the nucleus of 2P/Encke over the phase-angle range 4-28?deg. Results. An analysis of the point spread function of the comet in polarimetric images reveal only weak coma activity in 2P/Encke, corresponding to dust production of the order of 0.05?kg/s. Over the measured phase-angle range the nucleus displays a color independent photometric phase function of almost linear slope (??= 0.050?? 0.004?mag/deg). The absolute R?filter magnitude at zero phase angle is 15.05?? 0.05?mag and corresponds to an equivalent radius for the nucleus of 2.43?? 0.06?km (adopted albedo of 0.047). The nucleus color?V-R was measured to be 0.47?? 0.07, suggesting a spectral slope?S' of 11?? 8%/100?nm. The phase function of linear polarimetry in the V and R?filter shows a widely color independent linear increase with phase angle (0.12?? 0.02%/deg). The test of the empirical albedo-polarization relationship for asteroids reveals unreasonably high albedo values for the nucleus. We find discrepancies in the photometric and polarimetric parameters between 2P/Encke and other minor bodies in the solar system, which may indicate significant differences in the surface material properties and light-scattering behavior of the bodies.Conclusions. The linear polarimetric phase function of 2P/Encke presented here is the first ever measured for a cometary nucleus. It provides encouragement to future studies of cometary nuclei in order to characterize the light-scattering behavior of comets on firm empirical grounds and provide suitable input to a comprehensive modeling of the light scattering by cometary surfaces.

Context. A small fraction of upper main sequence stars have strong, highly structured magnetic fields. The origin and evolution of these fields are not adequately understood.Aims. We are carrying out a survey of magnetic fields in Ap stars in open clusters in order to obtain the first sample of magnetic upper main sequence stars with precisely known ages. These data will constrain theories of field evolution in these stars. Methods. A survey of candidate open cluster magnetic Ap stars was carried out using the new ESPaDOnS spectropolarimeter at the CFHT. This instrument provides an alternative to the FORS1 spectropolarimeter used up to now for this survey.Results. We have obtained 44 measurements of the mean longitudinal fields ?Bz? of 23 B6?A2 stars that have been identified as possible Ap stars and that are possible members of open clusters, with a median uncertainty of about 45?G. Of these stars, 10 have definite field detections. Nine stars of our sample are found not to be magnetic Ap stars. These observations significantly increase the information available about low-mass stars near the TAMS compared to our previous sample. Conclusions. We find that ESPaDOnS?provides field measurements comparable to those that we have previously obtained with FORS1, and that these data also contain a large amount of useful information not readily obtained from lower resolution spectropolarimetry. With the new data we are able to expand the available data on low-mass, relatively evolved Ap stars, and identify more robustly which observed stars are actually magnetic Ap stars and cluster members. Re-analysis of the enlarged data set of cluster Ap stars indicates that such stars with masses in the range of 2?5?M?? show rms fields larger than about 1?kG only when they are near the ZAMS. The time scale on which these large fields disappear varies strongly with mass, ranging from about 250?Myr for stars of 2?3?M??to 15?Myr for stars of 4?5?M?. Our data are consistent either with emergent flux conservation for most (but not all) Ap stars, or with modest decline in flux with age.

Context.We study the surface properties of the transneptunian population of Solar-system bodies.Aims. We investigate the surface characteristics of the large dwarf planet (136199)?Eris. Methods. With the FORS1 instrument of the ESO Very Large Telescope, we have obtained Bessell broadband R linear polarimetry and broadband V and I photometry of Eris. We have modelled the observations in terms of the coherent-backscattering mechanism to constrain the surface properties of the object.Results. Polarimetric observations of Eris show a small negative linear polarization without opposition surge in the phase angle range of 0.15?-0.5?. The photometric data allow us to assume a brightness opposition peak at phase angles below 0.2?-0.3?. The data obtained suggest a possible similarity to the polarimetric and photometric phase curves of Pluto. The measured absolute magnitude and broadband colors of Eris are HV=-1.15?mag, V-R=0.41?mag, and V-I=0.75?mag.Conclusions.The observational data with theoretical modelling are in agreement with the surface of Eris being covered by large inhomogeneous particles.

Context.Polarization is a powerful remote-sensing method to investigate solar system bodies. It is an especially sensitive diagnostic tool to reveal physical properties of the bodies whose observational characteristics are governed by small scatterers (dust, regolith surfaces). For these objects, at small phase angles, a negative polarization is observed, i.e., the electric vector?E oscillates predominantly in the scattering plane, contrary to what is typical for rather smooth homogeneous surfaces. The behavior of negative polarization with phase angle depends on the size, composition and packing of the scatterers. These characteristics can be unveiled by modelling the light scattering by the dust or regolith in terms of the coherent backscattering mechanism. Aims.We investigate the surface properties of TNOs and Centaurs by means of polarimetric observations with FORS1 of the ESO VLT. Methods. We have obtained new broadband polarimetric measurements over a range of phase angles for a TNO, 50000?Quaoar (in the R Bessel filter), and a Centaur, 2060?Chiron (in the BVR Bessel filters). Simultaneously to the polarimetry, we have obtained R?broadband photometry for both objects. We have modelled these new observations of Quaoar and Chiron, and revised the modelling of previous observations of the TNO?28978?Ixion using an improved value of its geometric albedo. Results. TNOs Ixion and Quaoar, and Centaur Chiron show a negative polarization surge. The Centaur Chiron has the deepest polarization minimum (?1.5?1.4%). The two TNOs show differing polarization curves: for Ixion, the negative polarization increases rapidly with phase; for Quaoar, the polarization is relatively small (?-0.6%), and nearly constant at the observed phase angles. For all three objects, modelling results suggest that the surface contains an areal mixture of at least two components with different single-scatterer albedos and photon mean-free paths.

We report the discovery, using FORS1 at the ESO-VLT and ESPaDOnS at the Canada-France-Hawaii Telescope, of magnetic fields and chemical peculiarities in the Herbig Ae/Be stars HD 72106 and HD 101412. These stars may well represent pre-main sequence progenitors of the magnetic Ap/Bp stars. At the same time, we fail to confirm claims by Hubrig et al.(2004) of the presence of magnetic fields in the Herbig Ae star HD 139614.

Four probable Ap members of the open cluster NGC 6475, were investigated through detailed spectrum synthesis. Cluster membership was confirmed for all four stars, however chemical abundance analysis shows that only three are truly Ap. Derived $v \sin i$values and chemical abundances for 20 elements ranging form C to Eu are presented.

We explore the problems of determining the abundance of Si in a number of hot Ap stars.

Background/Purpose: The growing use of routine ultrasonography during pregnancy is leading to an increasing number of prenatally diagnosed neuroblastomas. Optimal strategy has not yet been defined for these patients, because knowledge on this particular neuroblastoma (NB) population is still limited. However, definite guidelines are needed to avoid inadequate treatment. The authors analyzed the cases of antenatally detected NB (ADNB) reported in the Italian Neuroblastoma Registry during the past 6 years to elucidate the features of this subset of NB. Methods: The Italian Neuroblastoma Registry was reviewed for the period January 1993 to December 1998 to collect clinical, radiographic, surgical, and histopathological data on ADNB cases. NB stage was evaluated according to INSS criteria. All patients had undergone imaging (computed tomography or magnetic resonance imaging) of the primary tumor and bone marrow biopsy before surgical resection. Results: Seventeen patients were identified. Primary tumour site was adrenal glands in 16 cases and retroperitoneal ganglia in 1. Stage distribution was stage 1, 13 cases; stage II-A, 1 case; stage II-B, 1 case; stage IV-S, 2 cases. All cases underwent primary tumour resection. Mean age at surgery was 4 weeks. Resection of primary tumor was radical in 16 cases, partial in 1. All tumors were characterised by favourable histology according to Shimada classification. N-myc gene amplification was studied in 14 patients. N-myc amplification was detected only in a newborn with stage II-A NB, who died of massive bleeding 2 days after tumor resection. DNA index and 1p deletion were studied in 11 and 8 patients, respectively. Both diploidy and deletion of 1p were observed in a newborn who subsequently died of disease progression despite surgery, chemotherapy, and radiation therapy. Fourteen of 17 patients currently are alive and free of disease, and one with IV-S NB and short follow-up is alive with disease. Conclusions: Our data give evidence that in most cases infants with ADNB represent a subset of patients with excellent outcome. Aggressive treatment may not always be necessary. Infants with ADNB with unfavorable features should undergo early surgical excision, whereas patients with favourable features could be observed awaiting spontaneous regression of the mass, reserving delayed surgery for tumors that increase in size or do not regress.

Without Abstract:

Between October, 1977 and December, 1984, 61 children aged 5 months to 18 years (median 6.5) with solid tumours have been treated with high dose melphalan (HDM) 100-240 mg/m2 with autologous non-cryo-preserved bone marrow "rescue" to increase the rate of haematological recovery. Of these 61, 60 were evaluable on December 31st 1984; one patient was lost to follow up. Thirty-two patients received HDM and marrow autograft after conventional chemotherapy with or without radiotherapy (16 neuroblastoma, 6 rhabdomyosarcoma, 9 Ewings, 1 osteosarcoma) while 28 patients received this treatment after relapse following conventional chemotherapy (7 neuroblastoma, 9 rhabdomyosarcoma, 6 Wilms, 2 Ewings, 1 histiocytosis X, 1 carcinoma of the ovary, 1 yolk sac tumour, and 1 nasopharyngeal carcinoma). The median duration of leucopenia was 11 days (range 5-36 days). The other immediate toxicities due to melphalan were severe vomiting and nausea, mucositis in most patients and convulsions in 6. Of the 32 patients treated in first remission, 9 are now disease-free at 1 to 82 months. Of the 28 patients treated after relapse, 3 are alive without disease at 6-86 months. HDM produces responses in patients with disease resistant to other agents, particularly with Wilms' tumours and rhabdomyosarcoma, and occasionally these responses are durable. It seems to be less effective for children with Stage IV neuroblastoma.

We present broad‐band photometry in the optical, near‐infrared and submillimetre, and mid‐infrared spectrophotometry of a selection of carbon stars with optically thin envelopes. Most of the observations were carried out simultaneously.  Beside the emission feature at 11.3 μ m due to silicon carbide grains in the circumstellar environment, many of our mid‐infrared spectra show an emission feature at 8.6 μ m. All the observed spectral energy distributions exhibit a very large far‐infrared flux excess. Both these features are indeed common to many carbon stars surrounded by optically thin envelopes.  We have modelled the observed spectral energy distributions by means of a full radiative transfer treatment, paying particular attention to the features quoted above. The peak at 8.6 μ m is usually ascribed to the presence of hydrogenated amorphous carbon grains. We find also that the feature at 8.6 μ m might be reproduced by assuming that the stars have a circumstellar environment formed of both carbon‐ and oxygen‐rich dust grains, although this is in contrast with what one should expect in a carbon‐rich environment. The far‐infrared flux excess is usually explained by the presence of a cool detached dust shell. Following this hypothesis, our models suggest a time‐scale for the modulation of the mass‐loss rate of the order of some 103yr.

A generalized two-streamed Eddington aproximation is applied to the problem of radiative transfer in a spherically symmetric dust shell. We investigate whether our technique allows us to draw conclusions about the size distribution of the grains.