G. Davignon

Long term photometric monitoring with the Mercator telescope - Frequencies and mode identification of variable O-B stars

Aims. We selected a large sample of O-B stars that were considered as (candidate) slowly pulsating B, beta Cep, and Maia stars after the analysis of their hipparcos data. We analysed our new seven passband geneva data collected for these stars during the first three years of scientific operations of the mercator telescope. We performed a frequency analysis for 28 targets with more than 50 high-quality measurements to improve their variability classification. For the pulsating stars, we tried both to identify the modes and to search for rotationally split modes. Methods: We searched for frequencies in all the geneva passbands and colours by using two independent frequency analysis methods and we applied a 3.6 S/N-level criterion to locate the significant peaks in the periodograms. The modes were identified by applying the method of photometric amplitudes for which we calculated a large, homogeneous grid of equilibrium models to perform a pulsational stability analysis. When both the radius and the projected rotational velocity of an object are known, we determined a lower limit for the rotation frequency to estimate the expected frequency spacings in rotationally split pulsation modes. Results: We detected 61 frequencies, among which 33 are new. We classified 21 objects as pulsating variables (7 new confirmed pulsating stars, including 2 hybrid beta Cep/SPB stars), 6 as non-pulsating variables (binaries or spotted stars), and 1 as photometrically constant. All the Maia candidates were reclassified into other variability classes. We performed mode identification for the pulsating variables for the first time. The most probable l value is 0, 1, 2, and 4 for 1, 31, 9, and 5 modes, respectively, including only 4 unambiguous identifications. For 7 stars we cannot rule out that some of the observed frequencies belong to the same rotationally split mode. For 4 targets we may begin to resolve close frequency multiplets. Based on observations collected with the p7 photometer attached to the Flemish 1.2-m mercator telescope situated at the Roque de los Muchachos observatory on La Palma (Spain). Section [see full text], including Figs. is only available in electronic form at http://www.aanda.org, and Tables 2 and 3 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/243

An isothermal section at 550 C in the Al-rich corner of the Al-Fe-Mn-Si system

Phase equilibria in the Al corner of the Al-Fe-Mn-Si system at 550 °C have been explored. Twenty-six quaternary alloys were prepared and analyzed by microprobe analysis. Elemental powders were mixed under air and molten at 1000 °C under argon flux. These specimens received a heat treatment at 550 °C during 4 to 12 weeks and finally were water quenched. A four-phase region, where Al, Si,α-Al(Fe,Mn)Si, andβ-Al(Fe,Mn)Si are in equilibrium with each other, was found. The presence of a single-phaseα-Al(Fe,Mn)Si region, starting in the Al-Mn-Si subsystem and extending toward the Al-Fe-Si subsystem, could be confirmed. Some problems concerning the fit of the quaternary system with its ternary Al-Fe-Mn subsystem remain to be solved.

Photometric multi-site campaign on the open cluster NGC 884 - I. Detection of the variable stars

Context. Recent progress in the seismic interpretation of field β Cep stars has resulted in improvements of the physics in the stellar structure and evolution models of massive stars. Further asteroseismic constraints can be obtained from studying ensembles of stars in a young open cluster, which all have similar age, distance and chemical composition. Aims. To improve our comprehension of the β Cep stars, we studied the young open cluster NGC 884 to discover new B-type pulsators, besides the two known β Cep stars, and other variable stars. Methods. An extensive multi-site campaign was set up to gather accurate CCD photometry time series in four filters (U, B, V, I )o f a field of NGC 884. Fifteen different instruments collected almost 77 500 CCD images in 1286 h. The images were calibrated and reduced to transform the CCD frames into interpretable differential light curves. Various variability indicators and frequency analyses were applied to detect variable stars in the field. Absolute photometry was taken to deduce some general cluster and stellar properties. Results. We achieved an accuracy for the brightest stars of 5.7 mmag in V, 6.9 mmag in B, 5.0 mmag in I and 5.3 mmag in U .T he noise level in the amplitude spectra is 50 μmag in the V band. Our campaign confirms the previously known pulsators, and we report more than one hundred new multi- and mono-periodic B-, A- and F-type stars. Their interpretation in terms of classical instability domains is not straightforward, pointing to imperfections in theoretical instability computations. In addition, we have discovered six new eclipsing binaries and four candidates as well as other irregular variable stars in the observed field.

Mercator and the P7-2000 photometer

We present the Mercator Telescope together with the P7-2000 photometer as its first-light instrument. Mercator is a 1.2-meter Ritchey-Chretien telescope, installed at the Roque De Los Muchachos Observatory on La Palma and fully operational since Spring 2001. The Geneva Observatory developed this telescope and a twin, known as the Euler Telescope, was already inaugurated at the La Silla Observatory in 1998. Mercator is an alt-azimuthal telescope designed for semi-automatic operation and high operational robustness. P7 is a high-precision 2-channel differential photometer, built by the Geneva Observatory and in permanent use for over 25 years on various telescopes. It allows quasi-simultaneous observations in the 7 filters of the Geneva photometric system with a variable sampling rate up to 100 samples per second. This vintage instrument was completely refurbished in 2000 to function in an automatic mode on the Mercator Telescope. Electronics were completely renewed and are now based on a digital signal processor (DSP), which controls the instrument and performs basic data reduction. The optical system was left unmodified, apart from the addition of a field camera that is also used for auto-guiding. We also added instrument temperature control and a mechanical derotator. Since the 7 filters are acquired simultaneously and the absolute calibration of the colors is strictly homogeneous, the Mercator-P7 combination is a unique tool to study stellar variability on many different timescales. The current scientific program focuses on multi-periodic phenomena in early-type stars with the goal to identify the frequency spectrum and to constrain stellar models by asteroseismology studies. More than 43000 observations have been performed since 2001 and a precision of few milli-magnitudes is routinely achieved. Our photometric measurements result in the continuous calculation of the atmospheric extinction coefficients and these data are available online for other observers as well. In this paper, we describe the telescope, the photometer and their software, followed by the presentation of some first results. Finally, we discuss an upcoming upgrade and the complete instrumentation plan for Mercator.

Multiperiodicity in the large-amplitude rapidly-rotating beta Cephei star HD 203664

Aims. We made a seismic study of the young massive β Cephei star HD 203664 with the goal of constraining its interior structure. Methods. Our study is based on a time series of 328 new Geneva 7-colour photometric data of the star spread over 496.8 days. Results. The data confirm the frequency of the dominant mode of the star, which we refined to f 1 = 6.02885 c d -1 . The mode has a large amplitude of 37 mmag in V and is unambiguously identified as a dipole mode (l = 2) from its amplitude ratios and non-adiabatic computations. Besides f 1 , we discovered two additional new frequencies in the star with amplitudes above 4σ: f 2 = 6.82902 c d -1 and f 3 = 4.81543 c d -1 , or one of their daily aliases. The amplitudes of these two modes are only between 3 and 4 mmag, which explains why they were not detected before. Their amplitude ratios are too uncertain for mode identification. Conclusions. We show that the observed oscillation spectrum of HD 203664 is compatible with standard stellar models but that we have insufficient information for asteroseismic inferences. Among the large-amplitude β Cephei stars, HD 203664 stands out as the only one rotating at a significant fraction of its critical rotation velocity (∼40%).

CCD camera and automatic data reduction pipeline for the Mercator telescope on La Palma

In this paper we present the development of a CCD imager for the modern 1.2m MERCATOR telescope dedicated to long term monitoring of variable astrophysical phenomena. This instrument is a result of the collaboration of the Observatory of Geneva with the Institute of Astronomy in Leuven. After a technical description of the main components of the CCD camera system, the text will focus on the automatization of the observations and subsequent data reduction. The telescope itself is an altazimuth mounted 1.2 m Ritchey-Chretien telescope and is operated in a semi-automatic mode. The system executes a predefined sequence of observations, that only need occasional checking of data quality by the astronomer. The observation software is written in a FORTRAN based interpreter language (INTER) running on a UNIX system that communicates with the astronomer via GUIs implemented in Perl/Tk. The data reduction is integrated into one package and includes pre-reduction, photometric and astrometric calibration, extraction, catalogue preparation and archiving. This allows to have a GUI driven reduction that is both flexible and robust. The preliminary reduced data give the astronomers an indication of the quality of their observations, so that they can adjust their program or camera settings during the same night.

HERMES: a high-resolution spectrograph for the Mercator Telescope

Hermes, a high-resolution fiber-fed echelle spectrograph is currently under development for the 1.2-meter Mercator Telescope at the Roque De Los Muchachos Observatory on La Palma. The optical design is based on a large R2.6 echelle grating, operating in quasi-Littrow and white-pupil configuration, and a double-prism cross-disperser. This instrument records the complete optical spectrum from 380 to 875 nm in one single exposure on a 2k x 4k CCD. Order separation is sufficiently large to record two interleaved spectra simultaneously through two 60-mm core fibers: one spectrum of the object and one of the nearby sky or, alternatively, of a wavelength calibration lamp for high-precision radial velocity measurements. This type of observations also benefits from the high instrumental stability, owing to bench-mounting the spectrograph with a fixed configuration in a precisely temperature and humidity controlled chamber. Modest telescope size calls for high detection efficiency and great efforts in the design of both the fiber link and the spectrograph itself go in that direction. We aim at a peak-efficiency larger than 25% for the complete system. With a large fiber aperture of 2.3 arcsec, the resolving powerλ/ΔλΑ is 50000, a value that can be increased to 90000 with an adjustable slit at the fiber exit. This instrument mounted on a flexible telescope has a wide astronomical scope, going from asteroseismology to binary star research and chemical studies of stars and circumstellar material. We present the spectrograph design and we report on the project status.

Towards ensemble asteroseismology of the young open clusters χ Persei and NGC 6910

As a result of the variability survey in χ Persei and NGC 6910, the number of β Cep stars that are members of these two open clusters is increased to twenty stars, nine in NGC 6910 and eleven in χ Persei. We compare pulsational properties, in particular the frequency spectra, of β Cep stars in both clusters and explain the differences in terms of the global parameters of the clusters. We also indicate that the more complicated pattern of the variability among B-type stars in χ Persei is very likely caused by higher rotational velocities of stars in this cluster. We conclude that the sample of pulsating stars in the two open clusters constitutes a very good starting point for the ensemble asteroseismology of β Cep-type stars and maybe also for other B-type pulsators (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Fully automated high-resolution spectroscopy at Swiss 1.2 m La Silla telescope

A Ritchey-Chretien 1.2 m telescope (EULER) and the High- Resolution echelle Spectrograph (CORALIE), a new Swiss observing facility at ESO La Silla Observatory, are operational and since Summer 1998. The Observatory operation is fully automated and supports the unattended, attended and interactive mode of operation under local or remote control. The control hardware is based on Local Control Units (LCU) built from PC/RedHat Linux computers and a Unix Computing Server. The Operational Software is built around INTER, a command language interpreter featuring communication control, data access, image processing functions and easy access to external resources. The general SW architecture is a non-hierarchical tree of pairs made of hardware- independent interpreters running on the Observing Server and hardware dependent servers running on the LCUs. The Operational Software includes the full access (Creation/Modification/Retrieval) to the input/output databases, telescope, instrument and auxiliary set-up and control files as well as a full data-reduction pipeline. We describe briefly the system architecture, summarize the performances and the experience gained over 18 months of operation and we discuss some critical issues: use of standard components, parallel operation, real-time requirements, system upgrade and maintenance.

Photometric multi-site campaign on massive B stars in the open cluster χ Persei (NGC 884)

In 2005 a photometric observation campaign started on the open cluster χ Persei, involving 13 telescopes spread over the whole northern hemisphere. After two years we gathered almost 1200 hours of data. We present here preliminary results on the variability search, especially from the 60-cm telescope in Bialkow (Poland), which show seven confirmed β Cephei stars, four candidate B-type pulsators and other interesting variable stars.