S. Štefl

PIONIER: a 4-telescope visitor instrument at VLTI

PIONIER stands for Precision Integrated-Optics Near-infrared Imaging ExpeRiment. It combines four 1.8m Auxilliary Telescopes or four 8m Unit Telescopes of the Very Large Telescope Interferometer (ESO, Chile) using an integrated optics combiner. The instrument has been integrated at IPAG starting in December 2009 and commissioned at the Paranal Observatory in October 2010. It provides scientific observations since November 2010. In this paper, we detail the instrumental concept, we describe the standard operational modes and the data reduction strategy. We present the typical performance and discuss how to improve them. This paper is based on laboratory data obtained during the integrations at IPAG, as well as on-sky data gathered during the commissioning at VLTI. We illustrate the imaging capability of PIONIER on the binaries deltaSco and HIP11231. PIONIER provides 6 visibilities and 3 independent closure phases in the H band, either in a broadband mode or with a low spectral dispersion (R=40), using natural light (i.e. unpolarized). The limiting magnitude is Hmag=7 in dispersed mode under median atmospheric conditions (seeing 3ms) with the 1.8m Auxiliary Telescopes. We demonstrate a precision of 0.5deg on the closure phases. The precision on the calibrated visibilities ranges from 3 to 15% depending on the atmospheric conditions. PIONIER has been installed and successfully tested as a visitor instrument for the VLTI. It permits high angular resolution imaging studies at an unprecedented level of sensitivity. The successful combination of the four 8m Unit Telescopes in March 2011 demonstrates that VLTI is ready for 4-telescope operation.

Cyclic variability of the circumstellar disk of the Be star zeta Tauri II. Testing the 2D global disk oscillation model

Context. About 2/3 of the Be stars present the so-called V/R variations, a phenomenon characterized by the quasi-cyclic variation in the ratio between the violet and red emission peaks of the H i emission lines. These variations are generally explained by global oscillations in the circumstellar disk forming a one-armed spiral density pattern that precesses around the star with a period of a few years. Aims. This paper presents self-consistent models of polarimetric, photometric, spectrophotometric, and interferometric observations of the classical Be star ζ Tauri. The primary goal is to conduct a critical quantitative test of the global oscillation scenario. Methods. Detailed three-dimensional, NLTE radiative transfer calculations were carried out using the radiative transfer code HDUST. The most up-to-date research on Be stars was used as input for the code in order to include a physically realistic description for the central star and the circumstellar disk. The model adopts a rotationally deformed, gravity darkened central star, surrounded by a disk whose unperturbed state is given by a steady-state viscous decretion disk model. It is further assumed that this disk is in vertical hydrostatic equilibrium. Results. By adopting a viscous decretion disk model for ζ Tauri and a rigorous solution of the radiative transfer, a very good fit of the time-average properties of the disk was obtained. This provides strong theoretical evidence that the viscous decretion disk model is the mechanism responsible for disk formation. The global oscillation model successfully fitted spatially resolved VLTI/AMBER observations and the temporal V/R variations in the Hα and Brγ lines. This result convincingly demonstrates that the oscillation pattern in the disk is a one-armed spiral. Possible model shortcomings, as well as suggestions for future improvements, are also discussed.

Non-radially pulsating Be stars

Based on more than 3000 high-resolution echelle spectra of 27 early-type Be stars, taken over six years, it is shown that the short-term periodic line profile variability of these objects is due to non-radial pulsation. The appearance of the line profile variability depends mostly on the projected rotational velocity v sini and thus, since all Be stars rotate rapidly, on the inclination i. The observed variability of the investigated stars is described, and for some of them line profile variability periods are given for the first time. For two of the investigated stars the line profile variability was successfully modeled as non-radial pulsation with � = m =+ 2 already in previous works. Since Be stars with similarly low v sini share the same variability properties, these are in general explainable under the same model assumptions. The line profile variability of stars with higher v sini is different from the one observed in low v sini stars, but can be reproduced by the same model, if only the model inclination is modified to more equatorial values. Only for a few stars with periodic line profile variability the � = m = 2 non-radial pulsation mode is not able to provide a satisfying explanation. These objects might pulsate in different modes (e.g. tesseral ones, � |m|). Almost all stars in the sample show traces of outburst-like variability, pointing to an ephemeral nature of the mass-loss phenomenon responsible for the formation of the circumstellar disk of early-type Be stars, rather than a steady star-to-disk mass transfer. In addition to the variability due to non-radial pulsation present in most stars, several objects were found to show other periods residing in the immediate circumstellar environment. The presence of these secondary periods is enhanced in the outburst phases. Short-lived aperiodic phenomena were clearly seen in two stars. But, given the unfavourable sampling of our database to follow rapid variability of transient nature, they might be more common. Only in two out of 27 stars short-term spectroscopic variability was not detected at all.

Optical atmospheric extinction over Cerro Paranal

Aims. The present study was conducted to determine the optical extinction curve for Cerro Paranal under typical clear-sky observing conditions, with the purpose of providing the community with a function to be used to correct the observed spectra, with an accuracy of 0.01 mag airmass −1 . Additionally, this work was meant to analyze the variability of the various components, to derive the main atmospheric parameters, and to set a term of reference for future studies, especially in view of the construction of the Extremely Large Telescope on the nearby Cerro Armazones. Methods. The extinction curve of Paranal was obtained through low-resolution spectroscopy of 8 spectrophotometric standard stars observed with FORS1 mounted at the 8.2 m Very Large Telescope, covering a spectral range 3300–8000 A. A total of 600 spectra were collected on more than 40 nights distributed over six months, from October 2008 to March 2009. The average extinction curve was derived using a global fit algorithm, which allowed us to simultaneously combine all the available data. The main atmospheric parameters were retrieved using the LBLRTM radiative transfer code, which was also utilised to study the impact of variability of the main molecular bands of O2 ,O 3 ,a nd H 2O, and to estimate their column densities. Results. In general, the extinction curve of Paranal appears to conform to those derived for other astronomical sites in the Atacama desert, like La Silla and Cerro Tololo. However, a systematic deficit with respect to the extinction curve derived for Cerro Tololo before the El Chichon eruption is detected below 4000 A. We attribute this downturn to a non standard aerosol composition, probably revealing the presence of volcanic pollutants above the Atacama desert. An analysis of all spectroscopic extinction curves obtained since 1974 shows that the aerosol composition has been evolving during the last 35 years. The persistence of traces of non meteorologic haze suggests the effect of volcanic eruptions, like those of El Chichon and Pinatubo, lasts several decades. The usage of the standard CTIO and La Silla extinction curves implemented in IRAF and MIDAS produce systematic over/under-estimates of the absolute flux.

Cyclic variability of the circumstellar disk of the Be star

Context. Emission lines formed in decretion disks of Be stars often undergo long-term cyclic variations, especially in the violet-to-red (V/R) ratio of their primary components. The underlying structural and dynamical variations of the disks are only partly understood. From observations of the bright Be-shell star ζ Tau, the possibly broadest and longest data set illustrating the prototype of this behaviour was compiled from our own and archival observations. It comprises optical and infrared spectra, broad-band polarimetry, and interferometric observations. Aims. The dense, long-time monitoring permits a better separation of repetitive and ephemeral variations. The broad wavelength coverage includes lines formed under different physical conditions, i.e. different locations in the disk, so that the dynamics can be probed throughout much of the disk. Polarimetry and interferometry constrain the spatial structure. All together, the objective is a better understand the dynamics and life cycle of decretion disks. Methods. Standard methods of data acquisition, reduction, and analysis were applied. Results. From 3 V/R cycles between 1997 and 2008, a mean cycle length in Hα of 1400–1430 days was derived. After each minimum in V/R, the shell absorption weakens and splits into two components, leading to 3 emission peaks. This phase may make the strongest contribution to the variability in cycle length. There is no obvious connection between the V/R cycle and the 133-day orbital period of the not otherwise detected companion. V/R curves of different lines are shifted in phase. Lines formed on average closer to the central star are ahead of the others. The shell absorption lines fall into 2 categories differing in line width, ionization/excitation potential, and variability of the equivalent width. They seem to form in separate regions of the disk, probably crossing the line of sight at different times. The interferometry has resolved the continuum and the line emission in Brγ and HeI 2.06. The phasing of the Brγ emission shows that the photocenter of the line-emitting region lies within the plane of the disk but is offset from the continuum source. The plane of the disk is constant throughout the observed V/R cycles. The observations lay the foundation for the fully self-consistent, one-armed, disk-oscillation model developed in Paper II.

\zeta

We present a radial velocity study of the rapidly rotating B star Regulus that indicates the star is a single-lined spectroscopic binary. The orbital period (40.11 days) and probable semimajor axis (0.35 AU) are large enough that the system is not interacting at present. However, the mass function suggests that the secondary has a low mass (M2 > 0.30 M☉), and we argue that the companion may be a white dwarf. Such a star would be the remnant of a former mass donor that was the source of the large spin angular momentum of Regulus itself.

Tauri - I. Long-term monitoring observations

The B2Vn star HR 7355 is found to be a He-rich magnetic star. Spectropolarimetric data were obtained with FORS1 at UT2 on Paranal Observatory to measure the disc-averaged longitudinal magnetic field at various phases of the presumed 0.52 d cycle. A variable magnetic field with strengths betweenB z �=− 2200 and +3200 G was found, with confidence limits of 100 to 130 G. The field topology is that of an oblique dipole, while the star itself is seen about equator-on. In the intensity spectra, the He I lines show the typical equivalent width (EW) variability of He-strong stars, usually attributed to surface abundance spots. The amplitudes of the EW variability of the He I lines are extraordinarily strong compared to other cases. These results not only put HR 7355 unambiguously among the early-type magnetic stars, but confirm its outstanding nature: with v sin i = 320 km s −1 , the parameter space in which He-strong stars are known to exist has doubled in terms of rotational velocity.

A Spectroscopic Orbit for Regulus

Echelle spectroscopy and mostly unaided-eye photometry of the southern Be star ω CMa were obtained in the period 1996-2003. The monitoring is bracketed by two brightenings by 0. m 4-0. m 5. The results of a literature search suggest that such phases occur about once a decade and have various commonalities. Along with these photometric events goes enhanced line emission. This is due to an increased total mass of the disk as well as to a change in its density profile. The models by Poeckert & Marlborough (1978, 1979) imply that the enhanced continuum flux originates from the inner disk. Higher-order Balmer line emission is correlated with brightness. The increase in Hα is retarded by some months, possibly indicating a time delay in filling up and ionizing the outer disk. In the (U − B) vs. (B −V) colour diagram and the D54 vs. D34 Balmer decrement diagram the path from the ground to the bright state is distinct from the return path. This could result from the bulk of the disk matter being in the outer (inner) disk during the photometric ground (high) state, while the two transitions between the two states are both due to changes progressing radially outward. Some µ Cen-like outbursts (Rivinius et al. 1998c) seem to occur in all phases. It is conceivable that the build-up of the inner disk is caused by more frequent or more effective outbursts. During the photometric bright state various other phenomena gain in prominence and suggest this to be a phase of increased activity. Of particular interest, but possibly only apparently related to this phase, are absorption components at redshifts well beyond the range covered by the combination of rotation and nonradial pulsation.

Magnetic field detection in the B2 Vn star HR 7355

The rapid line-profile variability of the early-type and pole-on Be star ω CMa between 1996 and 2002 is characterized across the complete optical spectrum, for quiescent phases as well as for outbursts. Owing to differentand changing line-profile variability patterns, amplitudes and γ-velocities are different from line to line and are variable on a time scale of months. A comprehensive time series analysis was performed on the modes of a set of selected lines (after individual seasonal normalization to avoid biases). At a high level of confidence, only the well-known 1.37-d period could be found in photospheric lines not contaminated by the disk. Outside major outbursts, when the star is at its photometric ground state, the phase coherence of the variability is very robust. During strong outbursts, when the star is visually bright, the period may either be very slightly different or phase jumps may occur. The present observations do not have the sampling necessary to distinguish between these possibilities. Harmanecs (1998) report of continuous, cyclic period variations cannot be confirmed. Arguments are presented that temporary period changes may be related to interactions between the photospheric non-radial pulsation and the disk when (during outbursts) these two domains are in contact with one another. This result does not seem to be an artifact of the also previously reported transient periodicities near 1.49-d, which are prominent during outbursts and seem to be anchored in the exo-photosphere. However, if not properly taken into account, they may easily lead to false conclusions about multiple or variable periods. In the Be star μ Cen, which has a similar spectral type, outbursts are triggered by the beating of two or more non-radial pulsation modes (Rivinius et al. 1998b). Since ω CMa, too, undergoes outbursts although its photospheric variability is single-periodic, the case of p Cen cannot be generalized to the activity of all early-type Be stars or to the Be phenomenon at large.

Stellar and circumstellar activity of the Be star ω CMa I. Line and continuum emission in 1996-2002

Aims. We searched for massive stars with Balmer emission consistent with magnetically confined circumstellar material. Methods. Archival spectroscopic and photometric data were investigated. Results. HR 7355 is a formerly unknown He-strong star showing Balmer emission. At