arXiv subjects

S. Bagnulo

Publications and source records attributed to S. Bagnulo.

At least 127 records · Page 7Linked to original sources

arXiv2007

Dust observations of Comet 9P/Tempel 1 at the time of the Deep Impact

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 km/s. The material ejected by the impact expanded into the normal coma, produced by ordinary cometary activity. The characteristics of the non-impact coma and cloud produced by the impact were studied by observations in the visible wavelengths and in the near-IR. The scattering characteristics of the "normal" coma of solid particles were studied by comparing images in various spectral regions, from the UV to the near-IR. For the non-impact coma, a proxy of the dust production has been measured in various spectral regions. The presence of sublimating grains has been detected. Their lifetime was found to be about 11 hours. Regarding the cloud produced by the impact, the total geometric cross section multiplied by the albedo was measured as a function of the color and time. The projected velocity appeared to obey a Gaussian distribution with the average velocity of the order of 115 m/s. By comparing the observations taken about 3 hours after the impact, we have found a strong decrease in the cross section in J filter, while that in Ks remained almost constant. This is interpreted as the result of sublimation of grains dominated by particles of sizes of the order of some microns.

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arXiv2007

Late stages of the evolution of A-type stars on the main sequence: comparison between observed chemical abundances and diffusion models for 8 Am stars of the Praesepe cluster

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. Methods. We have obtained high resolution, high signal-to-noise ratio spectra of eight previously-classified Am stars, two normal A-type stars and one Blue Straggler, considered to be members of the Praesepe cluster. For all of these stars we have determined fundamental parameters and photospheric abundances for a large number of chemical elements, with a higher precision than was ever obtained before for this cluster. For seven of these stars we also obtained spectra in circular polarization and applied the LSD technique to constrain the longitudinal magnetic field. Results. No magnetic field was detected in any of the analysed stars. HD 73666, a Blue Straggler previously considered as an Ap (Si) star, turns out to have the abundances of a normal A-type star. Am classification is not confirmed for HD 72942. For HD 73709 we have also calculated synthetic Delta-a photometry that is in good agreement with the observations. There is a generally good agreement between abundance predictions of diffusion models and values that we have obtained for the remaining Am stars. However, the observed Na and S abundances deviate from the predictions by 0.6 dex and >0.25 dex respectively. Li appears to be overabundant in three stars of our sample.

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arXiv2007

Searching for links between magnetic fields and stellar evolution. II. The evolution of magnetic fields as revealed by observations of Ap stars in open clusters and associations

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. We are carrying out a large survey of magnetic fields in cluster Ap stars with the goal of obtaining a sample of these stars with well-determined ages. In this paper we analyse the information available from the survey as it currently stands. We select from the available observational sample the stars that are probably (1) cluster or association members and (2) magnetic Ap stars. For the stars in this subsample we determine the fundamental parameters T_eff, log(L/L_o), and M/M_o. With these data and the cluster ages we assign both absolute age and fractional age (the fraction of the main sequence lifetime completed). For this purpose we have derived new bolometric corrections for Ap stars. Magnetic fields are present at the surfaces of Ap stars from the ZAMS to the TAMS. Statistically for the stars with M > 3 M_o the fields decline with advancing age approximately as expected from flux conservation together with increased stellar radius, or perhaps even faster than this rate, on a time scale of about 3 10^7 yr. In contrast, lower mass stars show no compelling evidence for field decrease even on a timescale of several times 10^8 yr. Study of magnetic cluster stars is now a powerful tool for obtaining constraints on evolution of Ap stars through the main sequence. Enlarging the sample of known cluster magnetic stars, and obtaining more precise RMS fields, will help to clarify the results obtained so far. Further field observations are in progress.

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arXiv2007

VIMOS total transmission profiles for broad-band filters

VIMOS is a wide-field imager and spectrograph mounted on UT3 at the VLT, whose FOV consists of four 7'x8' quadrants. Here we present the measurements of total transmission profiles -- i.e. the throughput of telescope + instrument -- for the broad band filters U, B, V, R, I, and z for each of its four quadrants. Those measurements can also be downloaded from the public VIMOS web-page. The transmission profiles are compared with previous estimates from the VIMOS consortium.

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arXiv2007

Ca isotopic anomaly in the atmospheres of Ap stars

We present results of the Ca stratification analysis in the atmospheres of 21 magnetic chemically peculiar (Ap) stars. This analysis was based on the spectral observations carried out with the UVES spectrograph attached to the 8-m VLT telescope. Ca was found to be strongly stratified in all stars with different effective temperatures and magnetic field strengths. This element is overabundant by 1-1.5 dex below logtau_5000~-1 and strongly depleted above logtau_5000=-1.5. Based on the overall Ca abundance distributions, we modelled a profile of the IR-triplet Ca II 8498 line. It shows a significant contribution of the heavy isotopes 46Ca and 48Ca, which represent less than 1% of the solar Ca isotopic mixture. In Ap stars with the relatively small surface magnetic fields (<= 4-5 kG) the light 40Ca isotope is concentrated close to the photosphere, while the heavy isotopes are pushed towards the outer layers. Isotopic separation disappears in the atmospheres of stars with magnetic fields above 6-7 kG. The observed overall Ca stratification and isotopic anomalies may be explained by a combined action of the radiatively-driven diffusion and the light-induced drift.

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arXiv2007

The Optical Polarisation of the Vela Pulsar revisited

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). Our measurement has been obtained through an accurate reanalysis of archival polarisation observations obtained with the FORS instrument at the VLT. We have measured a phase-averaged linear polarisation degree P=9.4% +/- 4% and a position angle 146 +/- 11 deg, very close to the ones of the axis of symmetry of the X-ray arcs and jets detected by Chandra and of the pulsar proper motion.We have compared the measured phase-averaged optical polarisation with the expectations of different pulsars' magnetosphere models. We have found that all models consistently predict too large values of the phase-averaged linear polarization with respect to the observed one. This is probably a consequence of present models' limitations which neglect the contributions of various depolarisation effects. Interestingly, for the outer gap model we have found that, assuming synchrotron radiation for the optical emission, the observed polarisation position angle also implies an alignment between the pulsar rotational axis and the axis of symmetry of the X-ray arcs and jets.

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arXiv2007

A search for strong, ordered magnetic fields in Herbig Ae/Be stars

The origin of magnetic fields in intermediate-mass and high-mass stars is fundamentally a mystery. Clues toward solving this basic astrophysical problem can likely be found at the pre-main sequence (PMS) evolutionary stage. With this work, we perform the largest and most sensitive search for magnetic fields in pre-main sequence Herbig Ae/Be (HAeBe) stars. Sixty-eight observations of 50 HAeBe stars have been obtained in circularly polarised light using the FORS1 spectropolarimeter at the ESO VLT. An analysis of both Balmer and metallic lines reveals the possible presence of weak longitudinal magnetic fields in photospheric lines of two HAeBe stars, HD 101412 and BF Ori. The intensity of the longitudinal fields detected in HD 101412 and BF Ori suggest that they correspond to globally-ordered magnetic fields with surface intensities of order 1 kG. Monte Carlo simulations of the longitudinal field measurements of the undetected stars allow us to place an upper limits of about 300 G on the general presence of aligned magnetic dipole magnetic fields, and of about 500 G on perpendicular dipole fields. We find that the observed bulk incidence of magnetic HAeBe stars in our sample is 8-12%, in good agreement with that of magnetic main sequence stars of similar masses. We also find that the rms longitudinal field intensity of magnetically-detected HAeBe stars is similar to that of Ap stars and consistent with magnetic flux conservation during stellar evolution. These results are all in agreement with the hypothesis that the magnetic fields of main sequence Ap/Bp stars are fossils, which already exist within the stars at the pre-main sequence stage. Finally, we explore the ability of our new magnetic data to constrain magnetospheric accretion in Herbig Ae/Be stars.

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arXiv2007

Magnetic fields in Herbig Ae/Be stars

Studies of stellar magnetism at the pre-main sequence phase can provide important new insights into the detailed physics of the late stages of star formation, and into the observed properties of main sequence stars. This is especially true at intermediate stellar masses, where magnetic fields are strong and globally organised, and therefore most amenable to direct study. This paper reviews recent high-precision spectropolarimetric observations of pre-main sequence Herbig Ae/Be stars, which are yielding qualitatively new information about intermediate-mass stars: the origin and evolution of their magnetic fields; the role of magnetic fields in generating their spectroscopic activity and in mediating accretion in their late formative stages; the factors influencing their rotational angular momentum; and the development and evolution of chemical peculiarity in their photospheres.

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arXiv2006

Rotation and chemical abundances of Ap/Bp stars in the open cluster NGC 6475

The connection between age, rotation and chemical abundance of magnetic Ap stars is poorly understood. Using open clusters, we are able to study samples of stars that are both co-eval and co-environmental. By determining rotation and chemical abundance for Ap star members of clusters with various ages, the variations of these properties as a function of age and environment can be derived. All four probable Ap star members of the open cluster NGC 6475, as well as one normal late B star, were studied using detailed spectrum synthesis of high resolution UVES-POP spectra. Probable cluster membership was confirmed for all five stars, however chemical abundance anomalies only appear to be present in spectra of three. Projected rotational velocity and chemical abundances for 21 elements ranging from C to Eu are presented for the 5 stars. In the three peculiar stars we find overabundances of Si, Cr, Mn, Fe and rare earths such as Nd, characteristic of Ap stars. The set of chemically peculiar stars show fairly homogeneous abundance tables, however notable differences exist for a few elements. There also exist appreciable differences in the v sin i and main sequence evolutionary stage of the chemically peculiar stars. This may hint at the underlying processes giving rise to the observed abundance anomalies. With this first detailed study of chemical abundances of a complete sample of magnetic Ap/Bp stars in an open cluster, we have initiated an exploration of the environmental and evolutionary influence on chemical peculiarity.

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arXiv2006

Investigating the pre-main sequence magnetic chemically peculiar system HD 72106

The origin of the strong magnetic fields observed in chemically peculiar Ap and Bp stars stars has long been debated. The recent discovery of magnetic fields in the intermediate mass pre-main sequence Herbig Ae and Be stars links them to Ap and Bp stars, providing vital clues about Ap and Bp stars and the origin and evolution of magnetic fields in intermediate and high mass stars. A detailed study of one young magnetic B star, HD 72106A, is presented. This star appears to be in a binary system with an apparently normal Herbig Ae star. A maximum longitudinal magnetic field strength of +391 +/- 65 G is found in HD 72106A, as are strong chemical peculiarities, with photospheric abundances of some elements ranging up to 100x above solar.

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arXiv2006

Characterisation of the magnetic fields of the Herbig Be stars HD 200775 and V380 0ri

The origin of the magnetic fields of the chemically peculiar main sequence Ap/Bp stars is still matter of intense debate. The recent discoveries of magnetic fields in Herbig Ae/Be stars using high resolution data obtained with the spectropolarimeter ESPaDOnS at CFHT provide a strong argument in favour of the fossil field hypothesis. Using a simple oblique rotator model of a centered dipole, we fit the Stokes V LSD profiles of two of these magnetic HAeBe stars, HD 200775 and V380 Ori, as well as their variations on timescales from days to months. We find that in both cases the dipole hypothesis is acceptable and we determine the rotation period, the angle between rotation and magnetic axes and the intensity of the magnetic field at pole.

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arXiv2006

Pulsation in the atmosphere of the roAp star HD 24712. I. Spectroscopic observations and radial velocity measurements

We have investigated the structure of the pulsating atmosphere of one of the best studied rapidly oscillating Ap stars, HD 24712. For this purpose we analyzed spectra collected during 2001-2004. An extensive data set was obtained in 2004 simultaneously with the photometry of the Canadian MOST mini-satellite. This allows us to connect directly atmospheric dynamics observed as radial velocity variations with light variations seen in photometry. We directly derived for the first time and for different chemical elements, respectively ions, phase shifts between photometric and radial velocity pulsation maxima indicating, as we suggest, different line formation depths in the atmosphere. This allowed us to estimate for the first time the propagation velocity of a pulsation wave in the outer stellar atmosphere of a roAp star to be slightly lower than the sound speed. We confirm large pulsation amplitudes (150-400 m/s) for REE lines and the Halpha core, while spectral lines of the other elements (Mg, Si, Ca, and Fe-peak elements) have nearly constant velocities. We did not find different pulsation amplitudes and phases for the lines of rare-earth elements before and after the Balmer jump, which supports the hypothesis of REE concentration in the upper atmosphere above the hydrogen line-forming layers. We also discuss radial velocity amplitudes and phases measured for individual spectral lines as tools for a 3D tomography of the atmosphere of HD 24712.

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arXiv2006

Standard stars for linear polarization

We present the analysis of the observations of standard stars for linear polarization obtained from 1999 to 2005 within the context of the calibration plan of the FORS1 instrument of the ESO VLT. We have considered observations carried out both in imaging polarimetric and in spectropolarimetric mode. Broadband polarization was obtained in the Bessel BVRI filters; spectropolarimetry was obtained with various grisms covering different optical ranges and with a typical resolution of a few hundreds. Spectropolarimetric data have been convolved with the transmission functions of the Bessel filters, which enabled us to calculate polarization values equivalent to broadband polarization measurements in imaging mode. Finally, for each star, instrument mode, and for each Bessel filter band, we have calculated an average polarization value obtained after filtering all available data with a k-sigma clipping algorithm.

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arXiv2006

The magnetic field of the pre-main sequence Herbig Ae star HD 190073

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. The particular aim of this paper is to announce the detection of a structured magnetic field in the Herbig Ae star HD 190073, and to discuss various scenarii for the geometry of the star, its environment and its magnetic field. We have used the ESPaDOnS spectropolarimeter at CFHT in 2005 and 2006 to obtain high-resolution and signal-to-noise circular polarization spectra which demonstrate unambiguously the presence of a magnetic field in the photosphere of this star. Nine circular polarization spectra were obtained, each one showing a clear Zeeman signature. This signature is suggestive of a magnetic field structured on large scales. The signature, which corresponds to a longitudinal magnetic field of 74+- 10 G, does not vary detectably on a one-year timeframe, indicating either an azimuthally symmetric field, a zero inclination angle between the rotation axis and the line of sight, or a very long rotation period. The optical spectrum of HD 190073 exhibits a large number of emission lines. We discuss the formation of these emission lines in the framework of a model involving a turbulent heated region at the base of the stellar wind, possibly powered by magnetic accretion. This magnetic detection brings an important element for our understanding of stellar magnetism at intermediate masses.

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arXiv2006

Polarimetric Standardization

The use of polarimetric techniques is nowadays widespread among solar and stellar astronomers. However, notwithstanding the recommandations that have often been made about the publication of polarimetric results in the astronomical literature, we are still far from having a standard protocol on which to conform. In this paper we review the basic definitions and the physical significance of the Stokes parameters, and we propose a standardization of the measurement of polarized radiation.

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arXiv2006

Chemical stratification in the atmosphere of Ap star HD 133792. Regularized solution of the vertical inversion problem

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 produced by the selective radiative levitation and gravitational settling of chemical species. Here we present a new approach to mapping the vertical chemical structures in stellar atmospheres. We have developed a regularized chemical inversion procedure that uses all information available in high-resolution stellar spectra. The new technique for the first time allowed us to recover chemical profiles without making a priori assumptions about the shape of chemical distributions. We have derived average abundances and applied the vertical inversion procedure to the high-resolution VLT UVES spectra of the weakly magnetic, cool Ap star HD 133792. Our analysis yielded improved estimates of the atmospheric parameters of HD 133792. We show that this star has negligible vsini and the mean magnetic field modulus =1.1+/-0.1 kG. We have derived average abundances for 43 ions and obtained vertical distributions of Ca, Si, Mg, Fe, Cr, and Sr. All these elements except Mg show high overabundance in the deep layers and solar or sub-solar composition in the upper atmosphere of HD 133792. In contrast, the Mg abundance increases with height. We find that transition from the metal-enhanced to metal-depleted zones typically occurs in a rather narrow range of depths in the atmosphere of HD 133792. Based on the derived photospheric abundances, we conclude that HD 133792 belongs to the rare group of evolved cool Ap stars, which possesses very large Fe-peak enhancement, but lacks a prominent overabundance of the rare-earth elements.

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arXiv2006

Studies of Neutron Stars at Optical/IR Wavelengths

In the last years, optical studies of Isolated Neutron Stars (INSs) have expanded from the more classical rotation-powered ones to other categories, like the Anomalous X-ray Pulsars (AXPs) and the Soft Gamma-ray Repeaters (SGRs), which make up the class of the magnetars, the radio-quiet INSs with X-ray thermal emission and, more recently, the enigmatic Compact Central Objects (CCOs) in supernova remnants. Apart from 10 rotation-powered pulsars, so far optical/IR counterparts have been found for 5 magnetars and for 4 INSs. In this work we present some of the latest observational results obtained from optical/IR observations of different types of INSs.

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arXiv2006

A search for kilogauss magnetic fields in white dwarfs and hot subdwarf stars

We present new results of a survey for weak magnetic fields among DA white dwarfs with inclusion of some brighter hot subdwarf stars. We have detected variable circular polarization in the Halpha line of the hot subdwarf star Feige 34 (SP: sdO). From these data, we estimate that the longitudinal magnetic field of this star varies from -1.1 +/- 3.2 kG to +9.6 +/- 2.6 kG, with a mean of about +5 kG and a period longer than 2 h. In this study, we also confirm the magnetic nature of white dwarf WD1105-048 and present upper limits of kilogauss longitudinal magnetic fields of 5 brightest DA white dwarfs. Our data support recent finding that 25% of white dwarfs have kilogauss magnetic fields. This frequency also confirms results of early estimates obtained using the magnetic field function of white dwarfs.

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