J. G. Robertson

Starlight demonstration of the Dragonfly instrument: an integrated photonic pupil-remapping interferometer for high-contrast imaging

In the two decades since the first extra-solar planet was discovered, the detection and characterization of extra-solar planets has become one of the key endeavours in all of modern science. Recently, direct detection techniques such as interferometry or coronagraphy have received growing attention because they reveal the population of exoplanets inaccessible to Doppler or transit techniques, and moreover they allow the faint signal from the planet itself to be investigated. Next-generation stellar interferometers are increasingly incorporating photonic technologies due to the increase in fidelity of the data generated. Here, we report the design, construction and commissioning of a new high-contrast imager, the integrated pupil-remapping interferometer, an instrument we expect will find application in the detection of young faint companions in the nearest star-forming regions. The laboratory characterization of the instrument demonstrated high-visibility fringes on all interferometer baselines in addition to stable closure phase signals. We also report the first successful on-sky experiments with the prototype instrument at the 3.9-m Anglo-Australian Telescope. Performance metrics recovered were consistent with ideal device behaviour after accounting for expected levels of decoherence and signal loss from the uncompensated seeing. The prospect of complete Fourier coverage coupled with the current performance metrics means that this photonically enhanced instrument is well positioned to contribute to the science of high-contrast companions.

Multiwavelength diameters of nearby Miras and semiregular variables

ABSTRACT We have used optical interferometry to obtain multi-wavelength visibility curves for eightred giants over the wavelength range 650–1000 nm. The observations consist of wavelength-dispersed fringes recorded with MAPPIT (Masked APerture-Plane Interference Telescope) atthe 3.9-mAnglo-AustralianTelescope.We present results for fourMiras (R Car, oCet, R Hya,R Leo) and four semi-regularvariables (R Dor, W Hya, L 2 Pup, γ Cru). All stars except γ Crushow strong variations of angular size with wavelength. A uniform-disk model was found tobe a poor fit in most cases, with Gaussian (or other more tapere d profiles) preferred. This,together with the fact that most stars showed a systematic increase in apparent size towardthe blue and a larger-than-expected linear size, even in thered, all point toward significantscattering by dust in the inner circumstellar environment. Some stars showed evidence forasymmetric brightness profiles, while L 2 Pup required a two-component model, indicating anasymmetrical circumstellar dust shell.Key words: techniques: interferometry – stars: AGB and post-AGB

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.

The angular diameter of R Doradus: a nearby Mira-like star

We find the angular diameter of R Doradus to be 57 ± 5 mas, exceeding that of Betelgeuse and implying that R Dor is larger in apparent size than every star except the Sun. R Dor is shown to be closely related to the Mira variables. We estimate an effective temperature of 2740 ± 190 K, a distance of 61 ± 7 pc, a luminosity of 6500 ± 1400 L0 and a radius of 370 ± 50 R 0 . The characteristics of R Dar are consistent with it being near the edge of a Mira instability strip. We detect non-zero closure phases from R Dor, indicating an asymmetric brightness distribution. We also observed W Hydrae, a small-amplitude Mira, for which we find an angular diameter of 44 ± 4 mas.

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.

Wavelength dependence of angular diameters of M giants: an observational perspective

ABSTRACTWe discuss the wavelength dependence of angular diameters of M giants from an observationalperspective. Observers cannot directly measure an optical-depth radius for a star, despite thisbeing a common theoretical definition. Instead, they can use an interferometer to measure thesquare of the fringe visibility. We present new plots of the wavelength-dependent centre-to-limb variation (CLV) of intensity of the stellar disk as well as visibility for Mira and non-MiraM giant models. We use the terms “CLV spectra” and “visibilit y spectra” for these plots. Wediscuss a model-predicted extreme limb-darkening effect (also called the narrow-bright-coreeffect) in very strong TiO bands which can lead to a misinterpretation of the size of a star inthese bands. We find no evidence as yet that this effect occurs in real stars. Our CLV spectracan explain the similarity in visibilities of R Dor (M8IIIe) that have been observed recentlydespite the use of two different passbands. We compare several observations with models andfind the models generally under-estimate the observed variation in visibility with wavelength.We present CLV and visibility spectra for a model that is applicable to the M supergiant α Ori.Key words: techniques:interferometric,stars: fundamentalparameters, stars: variables: Miras,stars: individual: R Dor, α Ori1 INTRODUCTIONThe radius of a star is considered to be one of its fundamen-tal properties. Stellar radii may be determined from interfer-ometric observations of stellar angular diameters, but starsdo not necessarily appear to us as sharp-edged disks of uni-formbrightness and size. Rather, two effects may be present:limb-darkeningwhich tapers the surface brightness down to-wards the star’s edge and line absorption which changes thestar’s apparent size. Both effects depend on the wavelengthof the observation and are particularly significant in M gi-ants.To determine the radii of M giants from interferomet-ric data, observers have relied upon comparisons with mod-els of the centre-to-limb variation (CLV) of the intensity ofthe stellar disk. The simplest CLV, a uniform disk, is oftenused for this. Alternatively we can use CLVs derived fromappropriate model photospheres of late M giants, since thisprocedure can lead to improvements in these models and agreater understandingof these stars. The model-photosphereCLVs are defined for a particular filter or spectral passband

Optical interferometry of early-type stars with pavo@chara - i.fundamental stellar properties

We present interferometric observations of 7 main-sequence and 3 giant stars with spectral types from B2 to F6 using the PAVO beam combiner at the CHARA array. We have directly determined the angular diameters for these objects with an average precision of 2.3%. We have also computed bolometric fluxes using available photometry in the visible and infrared wavelengths, as well as space-based ultraviolet spectroscopy. Combined with precise \textit{Hipparcos} parallaxes, we have derived a set of fundamental stellar properties including linear radius, luminosity and effective temperature. Fitting the latter to computed isochrone models, we have inferred masses and ages of the stars. The effective temperatures obtained are in good agreement (at a 3% level) with nearly-independent temperature estimations from spectroscopy. They validate recent sixth-order polynomial (B-V)-

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

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