J. R. North

γ2 Velorum: orbital solution and fundamental parameter determination with SUSI

The first complete orbital solution for the double-lined spectroscopic binary system γ 2 Velorum, obtained from measurements with the Sydney University Stellar Interferometer (SUSI), is presented. This system contains the closest example of a Wolf-Rayet star and the promise of full characterization of the basic properties of this exotic high-mass system has subjected it to intense study as an archetype for its class. In combination with the latest radial-velocity results, our orbital solution produces a distance of 336 +8 -7 pc, significantly more distant than the Hipparcos estimation. The ability to fully specify the orbital parameters has enabled us to significantly reduce uncertainties and our result is consistent with the Very Large Telescope Interferometer (VLTI) observational point, but not with their derived distance. Our new distance, which is an order of magnitude more precise than prior work, demands critical reassessment of all distance-dependent fundamental parameters of this important system. In particular, membership of the Vela OB2 association has been re-established, and the age and distance are also in good accord with the population of young stars reported by Pozzo et al. We determine the 0-star primary component parameters to be M v (O) = -5.63 ± 0.10mag, R(O) = 17 ± 2 R ⊙ and M(0) = 28.5 ± 1.1 M ⊙ . These values are consistent with calibrations found in the literature if a luminosity class of II-III is adopted. The parameters of the Wolf-Rayet (WR) component are M v (WR) = -4.33 ±0.17 mag and M(WR) = 9.0 ±0.6M ⊙ .

The fundamental parameters of the roAp star α Circini

We have used the Sydney University Stellar Interferometer to measure the angular diameter of α Cir. This is the first detailed interferometric study of a rapidly oscillating A (roAp) star, α Cir being the brightest member of its class. We used the new and more accurate Hipparcos parallax to determine the radius to be 1.967 ± 0.066 R⊙ . We have constrained the bolometric flux from calibrated spectra to determine an effective temperature of 7420 ± 170 K . This is the first direct determination of the temperature of an roAp star. Our temperature is at the low end of previous estimates, which span over 1000 K and were based on either photometric indices or spectroscopic methods. In addition, we have analysed two high-quality spectra of α Cir, obtained at different rotational phases and we find evidence for the presence of spots. In both spectra we find nearly solar abundances of C, O, Si, Ca and Fe, high abundance of Cr and Mn, while Co, Y, Nd and Eu are overabundant by about 1 dex. The results reported here provide important observational constraints for future studies of the atmospheric structure and pulsation of α Cir.

Orbital solution and fundamental parameters of σ Scorpii

The first orbital solution for the spectroscopic pair in the multiple star system sigma Scorpii, determined from measurements with the Sydney University Stellar Interferometer (SUSI), is presented. The primary component is of beta Cephei variable type and has been one of the most intensively studied examples of its class. The orbital solution, when combined with radial velocity results found in the literature, yields a distance of 174(+23,-18) pc, which is consistent with, but more accurate than the Hipparcos value. For the primary component we determine 18.4+/-5.4 M_sun, -4.12+/-0.34 mag and 12.7+/-1.8 R_sun for the mass, absolute visual magnitude and radius respectively. A B1 dwarf spectral type and luminosity class for the secondary is proposed from the mass determination of 11.9+/-3.1 M_sun and the estimated system age of 10 Myr.

Orbital Solution & Fundamental Parameters of sigma Scorpii

The first orbital solution for the spectroscopic pair in the multiple star system sigma Scorpii, determined from measurements with the Sydney University Stellar Interferometer (SUSI), is presented. The primary component is of beta Cephei variable type and has been one of the most intensively studied examples of its class. The orbital solution, when combined with radial velocity results found in the literature, yields a distance of 174(+23,-18) pc, which is consistent with, but more accurate than the Hipparcos value. For the primary component we determine 18.4+/-5.4 M_sun, -4.12+/-0.34 mag and 12.7+/-1.8 R_sun for the mass, absolute visual magnitude and radius respectively. A B1 dwarf spectral type and luminosity class for the secondary is proposed from the mass determination of 11.9+/-3.1 M_sun and the estimated system age of 10 Myr.

The radius and mass of the subgiant star β Hyi from interferometry and asteroseismology

We have used the Sydney University Stellar Interferometer to measure the angular diameter of β Hydri. This star is a nearby G2 subgiant the mean density of which was recently measured with high precision using asteroseismology. We determine the radius and effective temperature of the star to be 1.814 ± 0.017 R⊙ (0.9 per cent) and 5872 ± 44 K (0.7 per cent) respectively. By combining the radius with the mean density, as estimated from asteroseismology, we make a direct estimate of the stellar mass. We find a value of 1.07 ± 0.03 M⊙ (2.8 per cent), which agrees with published estimates based on fitting in the Hertzsprung–Russell diagram, but has much higher precision. These results place valuable constraints on theoretical models of β Hyi and its oscillation frequencies.

Observations of the pulsation of the Cepheid l Car with the Sydney University Stellar Interferometer

Observations of the southern Cepheid t Car to yield the mean angular diameter and angular pulsation amplitude have been made with the Sydney University Stellar Interferometer at a wavelength of 696 nm. The resulting mean limb-darkened angular diameter is 2.990 ± 0.017 mas (i.e. ± 0.6 per cent) with a maximum-to-minimum amplitude of 0.560 ± 0.018 mas corresponding to 18.7 ± 0.6 per cent in the mean stellar diameter. Careful attention has been paid to uncertainties, including those in measurements, in the adopted calibrator angular diameters, in the projected values of visibility squared at zero baseline, and to systematic effects. No evidence was found for a circumstellar envelope at 696 nm. The interferometric results have been combined with radial displacements of the stellar atmosphere derived from selected radial velocity data taken from the literature to determine the distance and mean diameter of l Car. The distance is determined to be 525 ± 26 pc and the mean radius 169 ± 8 R ⊙ . Comparison with published values for the distance and mean radius shows excellent agreement, particularly when a common scaling factor from observed radial velocity to pulsation velocity of the stellar atmosphere (the p-factor) is used.

A new determination of the orbit and masses of the Be binary system δ Scorpii

The binary starδ Sco (HD143275) underwent remarkable brightening in the visible in 2000, and continues to be irregularly variable. The system was observed with the Sydney University Stellar Interferometer (SUSI) in 1999, 2000, 2001, 2006 and 2007. The 1999 observations were consistent with predictions based on the previously published orbital elements. The subsequent observations can only be explained by assuming that an optically bright emission region with an angular size of 2 ± 1 mas formed around the primary in 2000. By 2006/2007 the size of this region grew to an estimated 4m as. We have determined a consistent set of orbital elements by simultaneously fitting all the published interferometric and spectroscopic data as well as the SUSI data reported here. The resulting elements and the brightness ratio for the system measured prior to the outburst in 2000 have been used to estimate the masses of the components. We find MA = 15 ± 7M � and MB = 8.0 ± 3.6 M� . The dynamical parallax is estimated to be 7.03 ± 0.15 mas, which is in good agreement with the revised Hipparcos parallax.

The radius and other fundamental parameters of the F9 V star β Virginis

We have used the Sydney University Stellar Interferometer (SUSI) to measure the angular diameter of the F9 V star β Virginis. After correcting for limb darkening and combining with the revised Hipparcos parallax, we derive a radius of 1.703 ± 0.022 R ⊙ (1.3 per cent). We have also calculated the bolometric flux from published meas urements which, combined with the angular diameter, implies an effective temperature of 6059 ± 49 K (0.8 per cent). We also derived the luminosity of β Vir to be L = 3.51 ± 0.08 L ⊙ (2.1 per cent). Solar-like oscillations were measured in this star by Carrier et al. (2005) an d using their value for the large frequency separation yields the mean stellar density with an uncertainty of about 2 per cent. Our constraints on the fundamental parameters of β Vir will be important to test theoretical models of this star and its oscillations.

Orbital elements, masses and distance of λ Scorpii A and B determined with the Sydney University Stellar Interferometer and high-resolution spectroscopy

The triple system HD 158926 (λ Sco) has been observed interferometrically with the Sydney University Stellar Interferometer, and the elements of the wide orbit have been determined. These are significantly more accurate than the previous elements found spectroscopically. The inclination of the wide orbit is consistent with the inclination previously found for the orbit of the close companion. The wide orbit also has low eccentricity, suggesting that the three stars were formed at the same time. The brightness ratio between the two B stars was also measured at λ= 442 and 700 nm. The brightness ratio and colour index are consistent with the previous classification of λ Sco A as B1.5 and λ Sco B as B2. Evolutionary models show that the two stars lie on the main sequence. Since they have the same age and luminosity class (IV), the mass–luminosity relation can be used to determine the mass ratio of the two stars: M_B/M_A= 0.76 ± 0.04. The spectroscopic data have been reanalyzed using the interferometric values for P, T, e and ω, leading to revised values for a_1 sin i and the mass function. The individual masses can be found from the mass ratio, the mass function, spectrum synthesis and the requirement that the age of both components must be the same: M_A= 10.4 ± 1.3 and M_B= 8.1 ± 1.0 M_⊙. The masses, angular semimajor axis and the period of the system can be used to determine the dynamical parallax. We find the distance to λ Sco to be 112 ± 5 pc, which is approximately a factor of 2 closer than the Hipparcos value of 216 ± 42 pc.

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⊙.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 2.176μm (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±0.009R . 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)×10 Wm−2 and the effective temperature equal to 9845±64 K. The luminosity is 24.7±0.7L .