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HR 6902 was the first target of a systematic study by Griffin (1986, JApA, 7, 195) of binaries showing composite spectra. It is also a well-studied member of the ζ Aur class. ζ Aur systems are long-period eclipsing binaries that are comprised of an evolved giant primary and a hot dwarf companion. Although those component stars have very different effective temperatures they have similar luminosities in the blue and near-UV regions, and hence display a composite spectrum at those wavelengths. In principle the ζ Aur systems are excellent tests of evolutionary and structural stellar models. In recent years the somewhat fragmentary eclipse photometry of HR 6902 has been out-classed by the high-precision continuous monitoring by the space mission CoRoT. HR 6902 was selected as a primary target of its seismology field, because the possible detection of solar-like pulsations in a giant component of a double-lined eclipsing binary could help to calibrate the scaling relation of giant pulsators. Our poster reported the results of a new analysis based on the CoRoT observations and follow-up spectroscopy with HARPS at the ESO 3.6-m telescope at La Silla.
The unprecedented accuracy of the CoRoT photometry enabled us to:
improve drastically the accuracy of the binary orbit and stellar parameters (by a factor ~10 for the radii)
extend the test of validity/calibration of the scaling relations to high stellar mass and radius, and put constraints on the evolutionary state (particularly since this binary is certainly free from tidal effects).
Λ cold-dark-matter hierarchical models of galaxy formation suggest that the halo of the Milky Way (MW) has been assembled, at least in part, through accretion of protogalactic fragments partially resembling the present-day dwarf spheroidal (dSph) satellites of the MW. Investigation of the stellar populations of the MW's globular clusters (GCs) and dSph companions can thus provide excellent tests to infer the dominant Galaxy-formation scenario, whether merger/accretion or cloud collapse. Pulsating variable stars offer a very powerful tool in this context, since variables of different types allow tracing the different stellar generations in a galaxy and to reconstruct the galaxy's star-formation history and assembly back to the first epochs of galaxy formation. In particular, the RR Lyrae stars, belonging to the old population (t > 10 Gyr), witnessed the epoch of halo formation, and thus hold a crucial role to identify the MW satellites that may have contributed to build up the Galactic halo. In the MW, most GCs with an RR Lyrae population sharply divide into two distinct groups (Oosterhoff types I and II) based on the mean periods and relative proportion of fundamental-mode (RRab) and first-overtone (RRc) RR Lyrae stars. On the other hand, the Galactic-halo field RR Lyrae stars show a dominance of Oosterhoff I properties. Here, we investigate the Oosterhoff properties of a number of different stellar systems, starting from relatively undisturbed dwarf galaxies (the Fornax dSph and its globular clusters), through distorted and tidally disrupting ones (the Bootes and Canes Venatici II dSphs), to possible final relics of the disruption process (the Galactic globular cluster NGC 2419). We are addressing the crucial question of whether the RR Lyrae pulsation properties in these systems conform to the Oosterhoff dichotomy characterizing the MW variables. If they do not, the Galaxy's halo cannot have been assembled by dSph-like protogalactic fragments resembling the present-day dSph companions of the MW. We have reduced and combined long time series from different telescopes, both ground- and space-based. Variable stars have been detected with image-subtraction techniques using the package isis2.1. Periods, amplitudes and Oosterhoff type for all variable stars, as well as color–magnitude diagrams of the stellar populations are discussed for each stellar cluster analyzed.
We have applied the Image Subtraction to B,V time-series photometry of four globular clusters of the Fornax dwarf spheoridal galaxy, which were observed with the 2.2m ESO-MPI, the 6.5m Magellan/Clay, the 4m CTIO/Blanco telescopes and the WFPC2 on board of HST. We have identified RR Lyrae stars and obtained well sampled light curves for the clusters' variables. From the periods and pulsation properties of their RR Lyrae stars Fornax globular clusters seem to be of Oosterhoff intermediate type.
The CNES/European space mission COROT will monitor asteroseismic targets located in selected fields to probe stellar interiors. Therefore, suitable candidate targets have to be searched for in order to optimize the scientific return of the mission. However, to be able to use the asteroseismic tools on the stars, their physical parameters must be known in advance. In this work, we detail the process of building a photometric database of all the stars brighter than V = 8.0 in the field of view of COROT and the process of selecting suitable δ Sct and γ Dor-type stars for the mission.
For an optimal selection of the seismology targets (for both COROT programs devoted to asteroseismology, i.e., the core and exploratory ones), it is essential to gather a priori as much information as possible on all potential candidates. With this aim, Strömgren-Crawford uvby-Hβ and Ca II H&K photometry were obtained for all of them. These data have been used to derive estimates of their effective temperatures, surface gravities and metallicities. These observations, together with high resolution echelle spectroscopy and high angular resolution imaging observations, are components of an ambitious ground-based program.
We present an overview of our study of the short period variable stars in the Large Magellanic Cloud, and in the dwarf galaxies Fornax, Leo I, and NGC 6822. Light curves are presented for RR Lyrae stars, Anomalous Cepheids and, for the first time, for Dwarf Cepheids in the field and in the globular cluster #3 of the Fornax galaxy.
We discuss the results of a systematic search for multi-periodic pulsations in Galactic Bulge δ Scuti and RR Lyrae stars. Six “normal” double-mode variables pulsating in two radial modes have been identified (5 δ Scuti-type and 1 RR Lyrae-type). In 37 RR Lyrae stars secondary periodicities very close to the primary pulsation frequency have been detected. These periodicities correspond to nonradial modes of oscillation. They are found in ∼ 23% of RRab and in ∼ 3% of RRc variables of our sample.
Soon after launch in March 1999, the primary science instrument onboard the Wide-Field Infrared Explorer (WIRE) satellite failed due to loss of coolant. However, it proved possible to begin an asteroseismology program using the 52-mm aperture star camera. A few bright stars were monitored with the 512x512 SITe CCD in a bandpass approximately equivalent to V + R; further details about the orbit, the detector and the raw data reduction can be found in Buzasi et al. (2000) and Buzasi (2000). We included the binary star θ2 Tauri among the targets. It is composed of an A7IV primary and an A5V secondary (P = 140.728 d). The primary is a a δ Scuti star which has been observed several times in the last twenty years. Five terms have been determined in its light curve (Breger, 1989 and references therein); Li et al. (1997) demonstrated the amplitude variability of some of these terms by comparing different campaigns.
δ Sct stars are among the most promising targets to perform ground-based asteroseismology. High resolution spectroscopy offers us a powerful technique to identify radial and nonradial pulsation modes, since we can easily detect oscillations and travelling features in the line profiles.
HR 2740 was a target for a photometric campaign carried out at La Silla (ESO) and Sutherland (SAAO) from 1997 January 14 to 1997 February 11 (Poretti et al., 1997). The campaign revealed that HR 2740 is one of the brightest γ Dor stars, a class of variable stars located near the cool border of the instability strip, and in which gravity pulsation modes are excited. Four frequencies were identified (f1=1.0434, f2=0.9951, f3=1.1088, f4=0.9019 c d−1), which together yield a satisfactory solution to the observed light curve. The frequency analysis was not simple, but thanks to the large coverage in longitude we could separate the effect of aliasing on the two terms f3=1.1088 c d−1 and f4=0.9019 c d−1, linked by the relationship f4 = 2 - f3. Moreover, only the long time baseline allowed us to resolve the two close terms f1=1.0434 c d−1 and f2=0.9951 c d−1. See Poretti et al. (1997) for a detailed discussion.
The absolute magnitudes of δ Scuti stars derived from parallaxes measured by the Hipparcos satellite were compared with the previous estimates based on photometric uvby² indices, and significant differences were found which are related to photometric effects of metallicity and rotational velocity. A reliable calibration of Mv in terms of the photometric indices shall include an estimate of these effects. It is important also to take into account the possible presence of unresolved close companions in order to fully exploit the accuracy of Mv of nearby stars derived from the trigonometric parallaxes.
The Mv of few bright SX Phe stars support the period-luminosity relation obtained with ground based observations of globular clusters, while it does not seem to confirm the empirical dependence of this relation on the metallicity.
Some high amplitude δ Scuti stars with intermediate or normal metallicity and small and uncertain parallax have apparently a very low luminosity. Simulations of Mv determinations from observed parallaxes based on the discussion of observational errors by Lutz and Kelker (1973) have shown that the low luminosity could be an effect related to these errors.
The Joint European X-ray Telescope (JET-X) is one of the core scientific instruments of the SPECTRUM RONTGEN-γ astrophysics mission. The project is a collaboration of British, Italian and Russian consortia, with the participation of the Max Planck Institut (Germany). JET-X was designed to study the emission from X-ray sources in the band of 0.3-10 keV. Citterio et al. (1996 and references therein) describe its structure, composed by two identical and coaligned Wolter I telescopes. Focal plane imaging is provided by cooled X-ray sensitive CCD detectors which combine high spatial resolution with good spectral resolution, including coverage of the iron line complex around 7 keV at a resolution of ΔE/E ~ 2%.
In two recent papers (Pardo & Poretti 1997; Poretti & Pardo 1997) we analyzed all the available photometry of galactic double-mode Cepheids (DMCs) with the aim of detecting in each case the importance of the harmonics and of the cross coupling terms. We found that no a priori fit can be reliably applied to the measurements of a DMC, but a careful frequency analysis must be done to evaluate the importance of each term. As a further application of this technique, we obtained very precise indications about the properties of the Fourier parameters. When discussing the generalized phase differences Gi,j we demonstrated that plotting them as a function of the order |i|+|j|, there are well-defined regions where they are confined: the second order terms have π < Gi,j < 3π/2; the third order terms have π/2 < Gi,j < π; the fourth order terms cluster around 2π.
The stars HD 224638 and HD 224945 belong to the new class of F0 V spectral type stars which show an unusual kind of variability for that region of the HR diagram (Mantegazza et al. 1993).
The variability of these two stars has been discovered and studied by Mantegazza, Poretti & Zerbi (1994). They show small amplitude light variations with characteristic time scales of the order of one day. The light curves are not periodic and it is difficult to satisfactorily fit them even with several periodic terms.
In order to explain our 1991 B colour observations we tentatively suggested that in both stars two close periodic terms were present with a characteristic frequency of about 0.8 c/d, and that these terms had a double wave shape for HD 224638 and a triple wave one for HD 224945.
Extensive photometric monitoring of bright δ Scuti stars, made in the last years, allowed us to get reliable frequency determinations also in very complicated cases. Table 1 gives a full description of our photometric targets. Recently, we realized that combining photometry with simultaneous high resolution spectroscopy, in order to study line profile variations, offers the possibility to perform a much more reliable mode identification. Hence, the photometric runs of FG Vir, X Cae, HD 2724 were paired with spectroscopic ones obtained at the Coudé Auxiliary Telescope (ESO, La Silla, Chile). The stellar spectra are centred at 4508 Å and they cover a range of 37.6 Å; the resolving power is about 50000-60000 and the linear dispersion is 2.4 Å/mm.
In the past years, in a series of papers (Antonello & Poretti 1986, Antonello et al. 1990, Mantegazza & Poretti 1992, Poretti 1994) the Fourier decomposition was successfully applied to the light curves of galactic Cepheids with P < 8 d. The separation of these Cepheids into two groups was evident. The first, more numerous (195 stars after the latest inclusion) group is constituted by the classical Cepheids, i.e., the Cepheids which follow the Hertzsprung progression: they occupy a narrow region in the ϕ21 – P plane and are also characterised by a R21 ratio larger than 0.20. The second group, containing 30 objects, is constituted by Cepheids which deviate from the Hertzsprung progression, describing a ‘Z’ crossing the classical sequence in the ϕ21 – P plane; their R21 values are smaller than 0.20. The relation between stars of the upper and lower sequence of the ‘Z’ is confirmed by the ϕ31 – P plane, where these stars describe an unique sequence. As first suggested by Antonello & Poretti (1986), the splitting of Cepheids into two groups can be explained by a different pulsation mode: while the classical Cepheids are pulsating in the fundamental radial mode, the other Cepheids are pulsating in the first overtone radial mode.
IUE observations of δ Scuti variables were planned to study the correlations between chromospheric activity and dynamics of pulsations, convection, rotation and to search for evidence of mass loss. So far we observed the following stars: ρ Pup, β Cas, o1 Eri, K2 Boo, τ Peg, 69 Tau, 71 Tau and τ Cyg. Results and discussions on our survey may be found in Pasinetti Fracassini et al. (1990) and Fracassini et al. (1991).
Ultraviolet spectroscopic data (6 LWP and 3 SWP spectra) of 71 Tau were obtained with IUE in the year 1990, spanning an interval of 5h35rn and covering about 1.5 cycles of the pulsation period. The period, derived from new photometric observations, is 4h32m with an cimplitude of 0m.028. This variable is the most intense X-ray source in the Hyades cluster according to the results of Einstein Observatory.
In the last decade stars showing variability of unclear origin have been detected among early F-type stars mainly as secondary results in the context of observational programmes devoted to the study of δ-Scuti and CP stars.
For this reason only a small number of these objects has been thoroughly studied: their positions in HR diagram is shown in fig.l.
Figure 1: HR diagram of the observed stars.
The stars in NGC 2516 have been singled out in a survey work by Antonello and Mantegazza (1986), HD 164615 has been studied by Abt et al.(1983), the variability in 9 Aur has been discovered by Krischinas et al.(1990,1991) and HD 23375 in Pleiades has been pointed out by Breger (1972). The solid line in Fig. 1 is the ZAMS and the dashed lines are the borders of the i-Scuti stars instability strip.
At Merate Observatory the study of δ Sct stars began in the sixties and spectroscopie and photometric campaigns were continuously undertaken in order to clarify the controversial points. As an obvious extension of the research, the observation of δ Sct stars was proposed for telescope-time allocation at European Southern Observatory, in order to take advantage of the ESO facilities and of the considerably better sky of La Silla. We put in our observing programme δ Sct stars showing cycle-to-cycle variations and, possibly, an amplitude larger than 0.05 mag in order to have a better signal-to-noise ratio; they are listed in the table. It is important to notice that we always tried to collect a number of measurements as large as possible (from 1988 we collected more than 1000 measurements per star), because it is a well-established fact that periodicities with
an amplitude of a few thousandths of magnitude can be present in δ Sct stars. Moreover, when several modes are excited, an adequate frequency resolution becomes necessary and, therefore, a sufficient time baseline is requested (Poretti and Mantegazza 1992). A poor data sampling also gives an apparent good fitting for several solutions, all leaving a small residual rms, and there is no possibility to choose among them: different authors can pick up different solutions, generating the conflicting interpretations often found in δ Sct star literature.
We report some of the recent results of our studies on δ Scuti star pulsation which are based on observations made at Merate and La Silla Observatories, sometimes in two site campaigns. Our recent experiences on X Caeli and 44 Tauri have shown that quite complicate light curves can be resolved even with observations obtained from one site only, if compact and accurate datasets are collected on sufficiently long time baselines (Mantegazza and Poretti, 1992; Poretti et al., 1992). Here we report the preliminary results of the light curve analysis of three more stars surveyed for more than 120 hours each: BI CMi, HD 18878 and HD 224639.
BI CMi has been observed in a two-site campaign in January-February 1991, the observations being collected during 17 nights. The frequency analysis of the light curve allowed us to identify unambiguously 4 pulsation modes at 8.247, 8.863, 8.514 and 7.424 c/d with semi-amplitudes of 22, 19, 5 and 5 mmag respectively.
The Fourier decomposition has been successfully applied to several classes of pulsating variables. Antonello and Poretti (1986) and Antonello et al. (1990a) applied it to the Cepheids with P < 8 d. The latter authors redefined the s – Cepheids a Population I Cepheids that do not follow the Hertzsprung progression, but have progression of their own. The same authors proposed a new denomination (Antonell et al., 1990b): C-a stars to indicate the Classical Cepheids and C-b stars to indicate the redefined s–Cepheids.
The new photometric data obtained at La Silla and Merate Observatories (Mantegazza and Poretti, 1992) increase the evidence of a separation of Cepheids into two well defined subclasses on the basis of the Fourier parameters of their light curves.
In the ϕ21 – P plane, the s– and Classical Cepheids are characterized by two sequences well separated for P<5.5 d.