BRITE-Constellation photometry of π5 Orionis, an ellipsoidal SPB variable

ABSTRACT Results of an analysis of the BRITE-Constellation photometry of the SB1 system and ellipsoidal variable π5 Ori (B2 III) are presented. In addition to the orbital light-variation, which can be represented as a five-term Fourier cosine series with the frequencies forb, 2forb, 3forb, 4forb, and 6forb, where forb is the system’s orbital frequency, the star shows five low-amplitude but highly significant sinusoidal variations with frequencies fi (i = 2, .., 5, 7) in the range from 0.16 to 0.92 d−1. With an accuracy better than 1σ, the latter frequencies obey the following relations: f2 − f4 = 2forb, f7 − f3 = 2forb, f5 = f3 − f4 = f7 − f2. We interpret the first two relations as evidence that two high-order ℓ = 1, m = 0 gravity modes are self-excited in the system’s tidally distorted primary component. The star is thus an ellipsoidal SPB variable. The last relations arise from the existence of the first-order differential combination term between the two modes. Fundamental parameters, derived from photometric data in the literature and the Hipparcos parallax, indicate that the primary component is close to the terminal stages of its main-sequence (MS) evolution. Extensive Wilson–Devinney modelling leads to the conclusion that best fits of the theoretical to observed light curves are obtained for the effective temperature and mass consistent with the primary’s position in the HR diagram and suggests that the secondary is in an early MS evolutionary stage.

A METHOD FOR ESTIMATING PARALLAXES OF VCBS: MODIFICATION TO HIPPARCOS PARALLAX MEASUREMENTS

We presented a method useful for estimating parallaxes of visually close binary stars (VCBS). The method depends on atmospheric modeling of the components of the VCBS. We construct model atmospheres by using a grid of Kurucz solar metalicity blanketed models1, which can be applied to calculate the synthetic spectral energy distribution for each component of the binary separately. In addition to study the entire system. However, the entire observational spectral energy distribution of the system was used as a reference for the comparison with the synthetic ones. We choose Hip4809 system to investigate the method, and we also study its physical and geometrical parameters. The new parallax of the system was estimated as π = 40.32 ± 5.00 mas, which disagrees with the Hipparcos parallax measurement (13.94 ± 0.90 mas). Furthermore the model atmosphere parameters of the components of the system were derived as: [Formula: see text], [Formula: see text], log ga = 4.50 ± 0.13, log gb = 4.50 ± 0.13, Ra = 0.94 ± 0.12R⊙, Rb = 0.93 ± 0.12R⊙, Depending on the derived parameters, the masses of the system's components were estimated as 1.03 ± 0.02M⊙ and 1.01 ± 0.02M⊙ for the primary and secondary components respectively, and their spectral types were concluded as G6V for both of them.

The Angular Diameter and Fundamental Parameters of Sirius A

The Sydney University Stellar Interferometer (SUSI) has been used to make a new determination of the angular diameter of Sirius A. The observations were made at an effective wavelength of 694.1 nm and the new value for the limb-darkened angular diameter is 6.048 ± 0.040 mas (± 0.66%). This new result is compared with previous measurements and is found to be in excellent agreement with a conventionally calibrated measurement made with the European Southern Observatory's Very Large Telescope Interferometer (VLTI) at 2.176 μm (but not with a second globally calibrated VLTI measurement). A weighted mean of the SUSI and first VLTI results gives the limb-darkened angular diameter of Sirius A as 6.041 ± 0.017 mas (± 0.28%). Combination with the Hipparcos parallax gives the radius equal to 1.713 ± 0.009 R⊙. The bolometric flux has been determined from published photometry and spectrophotometry and, combined with the angular diameter, yields the emergent flux at the stellar surface equal to (5.32 ± 0.14) × 108 W m−2 and the effective temperature equal to 9845 ± 64 K. The luminosity is 24.7 ± 0.7 L⊙.

Systematic biases and uncertainties of Hipparcos parallax

Ground-based optical/IR interferometers have provided strong support to the space-based astrometric mission Hipparcos ever since the Hipparcos instrument was in operation in 1989. Interferometric observations also produced critical corrections of orbital motion to many targets, including radio stars, which link the Hipparcos system to the International Celestial Reference Frame (ICRF). In particular, orbital parallax from interferometers confirmed the 10% bias of the Pleiades distance from Hipparcos, and thus avoids revision of classical astronomy. Significant offsets and errors of Hipparcos parallax introduced by binary jitters are demonstrated in this work. By comparing the Hipparcos results with long baseline interferometry and other techniques including spectroscopy, multi-color photometry, Main-Sequence fitting, light curve measurements, Lunar occultation, Fine Guidance Sensor, etc., systematic biases and uncertainties of Hipparcos parallaxes are investigated and analyzed. We have established good models for major error sources of Hipparcos parallax, such as zonal bias, binary jitters, and luminosity-dependent errors. The lessons learned from the systematic biases of Hipparcos parallax are valuable to future space missions like SIM and Gaia.

Hipparcos Positions of B/Be Stars on the HR Diagram

Absolute magnitudes of Be and B stars are computed for each spectral type and luminosity class V and IV, using the Hipparcos parallax measurements. Some simulations have been carried out in order to estimate the effects which could bias the mean absolute magnitude calculations. As a result, only stars with σπ/π < 15% have been used. A first result is that B stars are fainter than previous estimations by about 0.5 magnitude on average. We then observe that on average Be stars are brighter than B stars of the same spectral type and this over-luminosity increases with the spectral type. A possible interpretation is proposed based on the fact that the rotational velocity of the late Be stars is near the critical rotational velocity.

Two Aspects of Using Hipparcos Data for Studying Multiperiodic Stellar Pulsations

Two aspects of using Hipparcos data for studying multiperiodic stellar pulsations involve (1) deriving frequencies of multiperiodic pulsators from Hipparcos Epoch Photometry, and (2) using Hipparcos parallaxes together with other data, such as the frequencies, frequency ratios and multicolor photometry, to identify pulsation modes. Details are examined by looking at the δ Scuti star DK Virginis. From Hipparcos Epoch Photometry, two frequencies are derived. These frequencies are then verified by showing that they also fit all available ground-based observations of the star. In addition, Hipparcos parallax and a photometric effective temperature are used to compare the position of DK Vir in the HR diagram with evolutionary tracks. The star turns out to be at the end of core-hydrogen-burning stage of its evolution or slightly beyond, with a mass of 2.2 M⊙, or somewhat smaller. The observed frequencies indicate that the two detected modes are p3 and p2, while the relatively large photometric amplitudes imply ℓ ≲ 2.

Radial Velocities Without Spectroscopy

We discuss non-spectroscopic (astrometric) ways to determine radial velocities and their potentials in future astrometric missions. Radial-velocity accuracies are presented, based on Hipparcos parallax and proper motion data for several open clusters.

Globular Cluster Ages from Hipparcos Subdwarfs

The new Hipparcos parallax data for local subdwarfs allow a much more reliable determination of the distance to globular clusters (by direct sequence fitting) than was previously possible. Earlier studies (Reid 1997, Gratton et al. 1997) have reported larger distances than expected, especially for the most metal-poor clusters, implying much younger ages. In our study of Hipparcos subdwarfs applied to M92 — representative of the oldest, most metal-poor clusters — we find however a distance only slightly in excess of previous expectations. We show, using Monte Carlo simulations, that most of the difference may be explained by our more detailed treatment of the Lutz-Kelker and selection biases. With up-to-date stellar evolution models, we derive a minimum age of 13 Gyr for the Universe. This value, although lower than previous estimates, still imposes a rather strict upper limit to Ho in the context of standard cosmological models.

Suggestion to Revise the Hipparcos-Parallax of Cepheids Belonging to Binary Systems

It has been suspected (Szabados 1997) that the apparent orbit of previously unresolved binaries involving a Cepheid component falsifies the Hipparcos-parallaxes deduced for these Cepheids. Since the incidence of binaries among Cepheids exceeds 50 per cent, this adverse effect on the parallax determination is a major error source. An obvious remedy is to repeat the determination of the five astrometric parameters for the Cepheids, members in binary systems, taking into accountthe orbital elements known from spectroscopic studies. This will result in more precise parallaxes and cosmic distance scale. Moreover, these Cepheidsare ideal targets for the future microarcsec astrometric missions. This study was partly supported by OTKA-grants T 014852 and T 022946.