http://www.researchonline.mq.edu.au/vital/access/services/Feed ${session.getAttribute("locale")} 5 http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:23311 We present results of a long-baseline interferometry campaign using the PAVO beam combiner at the CHARA Array to measure the angular sizes of five main-sequence stars, one subgiant and four red giant stars for which solar-like oscillations have been detected by either Kepler or CoRoT. By combining interferometric angular diameters, Hipparcos parallaxes, asteroseismic densities, bolometric fluxes, and high-resolution spectroscopy, we derive a full set of near-model-independent fundamental properties for the sample. We first use these properties to test asteroseismic scaling relations for the frequency of maximum power (ν max) and the large frequency separation (Δν). We find excellent agreement within the observational uncertainties, and empirically show that simple estimates of asteroseismic radii for main-sequence stars are accurate to ≲ 4%. We furthermore find good agreement of our measured effective temperatures with spectroscopic and photometric estimates with mean deviations for stars between T eff = 4600-6200 K of -22 ± 32 K (with a scatter of 97K) and -58 ± 31 K (with a scatter of 93K), respectively. Finally, we present a first comparison with evolutionary models, and find differences between observed and theoretical properties for the metal-rich main-sequence star HD173701. We conclude that the constraints presented in this study will have strong potential for testing stellar model physics, in particular when combined with detailed modeling of individual oscillation frequencies. 2012-12-10T01:50:28.605Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:22783 We present an analysis of the WISE photometric data for 829 stars in the Sco-Cen OB2 association, using the latest high-mass membership probabilities. We detect infrared excesses associated with 135 BAF-type stars, 99 of which are secure Sco-Cen members. There is a clear increase in excess fraction with membership probability, which can be fitted linearly. We infer that 41 +/- 5 per cent of Sco-Cen OB2 BAF stars have excesses, while the field star excess fraction is consistent with zero. This is the first time that the probability of non-membership has been used in the calculation of excess fractions for young stars. We do not observe any significant change in excess fraction between the three subgroups. Within our sample, we have observed that B-type association members have a significantly smaller excess fraction than A-and F-type association members. 2012-11-11T22:21:02.382Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:19547 21 page(s) 2012-06-04T20:43:52.456Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:17036 We present four model series of the CODEX dynamical opacity-sampling models of Mira variables with solar abundances, designed to have parameters similar to o Cet, R Leo and R Cas. We demonstrate that the CODEX models provide a clear physical basis for the molecular shell scenario used to explain interferometric observations of Mira variables. We show that these models generally provide a good match to photometry and interferometry at wavelengths between the near-infrared and the radio, and make the model outputs publicly available. These models also demonstrate that, in order to match visible and infrared observations, the Fe-poor silicate grains that form within 3 continuum radii must have small grain radii and therefore cannot drive the winds from O-rich Mira variables. 2012-01-20T14:33:07.879Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:15838 We present a new high-mass membership of the nearby Sco OB2 association based on Hipparcos positions, proper motions and parallaxes, and radial velocities taken from the 2nd Catalogue of Radial Velocities with Astrometric Data (CRVAD-2). The Bayesian membership selection method developed makes no distinction between subgroups of Sco OB2 and utilizes linear models in calculation of membership probabilities. We select 436 members, 88 of which are new members not included in previous membership selections. We include the classical non-members α-Cru and β-Cru as new members as well as the pre-main-sequence stars HIP 79080 and 79081. We also show that the association is well mixed over distances of 8° on the sky, and hence no determination can be made as to the formation process of the entire association. 2011-11-09T03:16:51.588Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:15594 In 2005 we obtained very precise interferometric measurements of the pole-on rapid rotator Vega (A0 V) with the longest baselines of the Center for High Angular Angular Resolution (CHARA) Array and the Fiber Linked Unit for Optical Recombination (FLUOR). For the analysis of these data, we developed a code for mapping sophisticated PHOENIX model atmospheres on to the surface of rotationally distorted stars described by a Roche-von Zeipel formalism. Given a set of input parameters for a star or binary pair, this code predicts the interferometric visibility, spectral energy distribution and high-resolution line spectrum expected for the system. For the gravity-darkened Vega, our model provides a very good match to the K-band interferometric data, a good match to the spectral energy distribution except below 160 nm and a rather poor match to weak lines in the high dispersion spectrum where the model appears overly gravity darkened. In 2006, we used the CHARA Array and FLUOR to obtain high precision measurements of the massive, non-eclipsing, double-line spectroscopic binary Spica, a 4-day period system where both components are gravity darkened rapid rotators. These data supplement recent data obtained with the Sydney University Stellar Interferometer (SUSI). Our study follows the classic 1971 study by Herbison-Evans et al. who resolved Spica as a binary with the Narrabri Stellar Intensity Interferometer (NSII). We will report on our progress modeling the new interferometric and archival spectroscopic data, with the goal towards better constraining the apsidal constant. 2011-10-22T20:11:15.105Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:15312 The growing interest in solar twins is motivated by the possibility of comparing them directly to the Sun. To carry on this kind of analysis, we need to know their physical characteristics with precision. Our first objective is to use asteroseismology and interferometry on the brightest of them: 18 Sco. We observed the star during 12 nights with HARPS for seismology and used the PAVO beam-combiner at CHARA for interferometry. An average large frequency separation 134.4 ± 0.3 μHz and angular and linear radiuses of 0.6759 ± 0.0062 mas and 1.010 ± 0.009 R⊙ were estimated. We used these values to derive the mass of the star, 1.02 ± 0.03 M⊙. 2011-10-08T08:50:10.811Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:15101 We present preliminary asteroseismic results from Kepler on three G-type stars. The observations, made at one-minute cadence during the first 33.5days of science operations, reveal high signal-to-noise solar-like oscillation spectra in all three stars: about 20 modes of oscillation may be clearly distinguished in each star. We discuss the appearance of the oscillation spectra, use the frequencies and frequency separations to provide first results on the radii, masses, and ages of the stars, and comment in the light of these results on prospects for inference on other solar-type stars that Kepler will observe. 2011-09-29T06:52:07.263Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:15057 The mid-infrared properties of pre-planetary disks are sensitive to the temperature and flaring profiles of disks for the regions where planet formation is expected to occur. In order to constrain theories of planet formation, we have carried out a mid-infrared (lambda = 10.7 mu m) size survey of young stellar objects using the segmented Keck telescope in a novel configuration. We introduced a customized pattern of tilts to individual mirror segments to allow efficient sparse-aperture interferometry, allowing full aperture synthesis imaging with higher calibration precision than traditional imaging. In contrast to previous surveys on smaller telescopes and with poorer calibration precision, we find that most objects in our sample are partially resolved. Here, we present the main observational results of our survey of five embedded massive protostars, 25 Herbig Ae/Be stars, 3 T Tauri stars, 1 FU Ori system, and five emission-line objects of uncertain classification. The observed mid-infrared sizes do not obey the size-luminosity relation found at near-infrared wavelengths and a companion paper will provide further modeling analysis of this sample. In addition, we report imaging results for a few of the most resolved objects, including complex emission around embedded massive protostars, the photoevaporating circumbinary disk around MWC 361A, and the subarcsecond binaries T Tau, FU Ori, and MWC 1080. 2011-09-27T06:11:05.822Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:15059 We present results from a spectro-interferometric study of the Miras o Cet, R Leo, and W Hya obtained with the Keck Aperture Masking Experiment from 1998 September to 2002 July. The spectrally dispersed visibility data permit fitting with circularly symmetric brightness profiles such as a simple uniform disk (UD). The stellar angular diameter obtained over up to ∼ 450 spectral channels spanning the region 1.1–3.8 μm is presented. Use of a simple UD brightness model facilitates comparison between epochs and with existing data and theoretical models. Strong size variations with wavelength were recorded for all stars, probing zones of H2O, CO, OH, and dust formation. Comparison with contemporaneous spectra extracted from our data shows a strong anticorrelation between the observed angular diameter and flux. These variations consolidate the notion of a complex stellar atmosphere consisting of molecular shells with time-dependent densities and temperatures. Our findings are compared with existing data and pulsation models. The models were found to reproduce the functional form of the wavelength versus angular diameter curve well, although some departures are noted in the 2.8–3.5 μm range. 2011-09-27T06:11:01.769Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:15077 We have measured solar-like oscillations in red giants using time-series photometry from the first 34 days of science operations of the Kepler Mission. The light curves, obtained with 30 minute sampling, reveal clear oscillations in a large sample of G and K giants, extending in luminosity from the red clump down to the bottom of the giant branch. We confirm a strong correlation between the large separation of the oscillations (Δν) and the frequency of maximum power (νmax). We focus on a sample of 50 low-luminosity stars (νmax > 100 μHz, L ≲ 30L) having high signal-to-noise ratios and showing the unambiguous signature of solar-like oscillations. These are H-shell-burning stars, whose oscillations should be valuable for testing models of stellar evolution and for constraining the star formation rate in the local disk. We use a new technique to compare stars on a single échelle diagram by scaling their frequencies and find well-defined ridges corresponding to radial and non-radial oscillations, including clear evidence for modes with angular degree l = 3. Measuring the small separation between l = 0 and l = 2 allows us to plot the so-called C-D diagram of δν02 versus Δν. The small separation δν01 of l = 1 from the midpoint of adjacent l = 0 modes is negative, contrary to the Sun and solar-type stars. The ridge for l = 1 is notably broadened, which we attribute to mixed modes, confirming theoretical predictions for low-luminosity giants. Overall, the results demonstrate the tremendous potential of Kepler data for asteroseismology of red giants. 2011-09-27T06:10:25.951Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:15041 A new beam-combination and detection system has been installed in the Sydney University Stellar Interferometer working at the red end of the visual spectrum (λλ500-950 nm) to complement the existing blue-sensitive system (λλ430-520 nm) and to provide an increase in sensitivity. Dichroic beam-splitters have been introduced to allow simultaneous observations with both spectral systems, albeit with some restriction on the spectral range of the longer wavelength system (λλ550-760 nm). The blue system has been upgraded to allow remote selection of wavelength and spectral bandpass, and to enable simultaneous operation with the red system with the latter providing fringe-envelope tracking. The new system and upgrades are described and examples of commissioning tests presented. As an illustration of the improvement in performance the measurement of the angular diameter of the southern F supergiant δ CMa is described and compared with previous determinations. 2011-09-23T14:20:29.360Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:15047 Observations of the southern Cepheid ℓ 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. 2011-09-23T14:20:10.905Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:15048 We have used the Sydney University Stellar Interferometer (SUSI) to measure the angular diameter of the F9 V star β Virginis (β Vir). After correcting for limb darkening and combining with the revised Hipparcos parallax, we der ive a radius of 1.703 ± 0.022 R⊙ (1.3 per cent). We have also calculated the bolometric flux from published measurements 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. and 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 the theoretical models of this star and its oscillations. 2011-09-23T14:20:10.685Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:15008 New angular diameter determinations for the bright southern F8 supergiant ? CMa enable the bolometric emergent flux and effective temperature of the star to be determined with improved accuracy. The spectral flux distribution and bolometric flux have been determined from published photometry and spectrophotometry and combined with the angular diameter to derive the bolometric emergent flux Ϝ =(6.50 ± 0.24) × 10⁷ Wm ⁻² and the effective temperature Teff =5818 ± 53 K. The new value for the effective temperature is compared with previous interferometric and infrared flux method determinations. The accuracy of the effective temperature is now limited by the uncertainty in the bolometric flux rather than by the uncertainty in the angular diameter. 2011-09-21T23:10:10.474Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:15010 Observations of the southern Cepheids ℓ Car and β Dor to yield the mean angular diameters and angular pulsation amplitudes have been made with the Sydney University Stellar Interferometer (SUSI)at a wavelength of 700 nm. The results of a preliminary analysis are compared with those obtained with the VLTI at 2.2 μm and excellent agreement between the results from the two instruments is found for ℓ Car but there are significant differences for β Dor. 2011-09-21T23:10:07.515Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:14995 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. 2011-09-21T05:20:25.609Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:14975 CoKu Tau/4 has been labeled as one of the very few known transition disk objects-disks around young stars that have their inner disks cleared of dust, arguably as a result of planetary formation. We report aperture-masking inter-ferometry and adaptive optics imaging observations showing that CoKu Tau/4 is in fact a near-equal binary star of projected separation ∼53 mas (∼8 AU). The spectral energy distribution of the disk is then naturally explained by the inner truncation of the disk through gravitational interactions with the binary star system. We discuss the possibility that such "unseen" binary companions could cause other circumbinary disks to be labeled as transitional. 2011-09-19T10:00:12.547Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:14977 We describe the Cool Opacity-sampling Dynamic EXtended (codex) atmosphere models of Mira variable stars, and examine in detail the physical and numerical approximations that go in-to the model creation. The codex atmospheric models are obtained by computing the temperature and the chemical and radiative states of the atmospheric layers, assuming gas pressure and velocity profiles from Mira pulsation models, which extend from near the H-burning shell to the outer layers of the atmosphere. Although the code uses the approximation of Local Thermodynamic Equilibrium (LTE) and a grey approximation in the dynamical atmosphere code, many key observable quantities, such as infrared diameters and low-resolution spectra, are predicted robustly in spite of these approximations. We show that in visible light, radiation from Mira variables is dominated by fluorescence scattering processes, and that the LTE approximation likely underpredicts visible-band fluxes by a factor of 2. 2011-09-19T10:00:00.710Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:14935 The formation of dust with temperature-dependent non-grey opacity is considered in a series of self-consistent model atmospheres at different phases of an O-rich Mira variable of mass 1.2M0. Photometric and interferometric properties of these models are predicted under different physical assumptions regarding the dust formation. The iron content of the initial silicate that forms and the availability of grain nuclei are found to be critical parameters that affect the observable properties. For certain plausible parameter values, dust would form at 2-3 times the average continuum photospheric radius. This work provides a consistent physical explanation for the larger apparent size of Mira variables at wavelengths shorter than 1 μm than that predicted by dust-free fundamental-mode pulsation models. 2011-09-16T07:31:10.822Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:14936 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 al so 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 A. 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: MB/MA = 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 ai 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: MA = 10.4 ±1.3 and MB = 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. 2011-09-16T07:31:07.308Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:14937 We present optical interferometric polarimetry measurements of the Mira-like variables R Car and RR Sco, using the Sydney University Stellar Interferometer. By making visibility measurements in two perpendicular polarizations, the relatively low-surface brightness light scattered by atmospheric dust could be spatially separated from the bright Mira photospheric flux. This is the first reported successful use of long-baseline optical interferometric polarimetry. Observations were able to place constraints on the distribution of circumstellar material in R Car and RR Sco. The inner radius of dust formation for both stars was found to be less than 3 stellar radii: much closer than the expected innermost stable location for commonly assumed astrophysical 'dirty silicate' dust in these systems (silicate dust with a significant iron content). A model with the dust distributed over a shell which is geometrically thin compared to the stellar radius was preferred over an outflow. We propose dust components whose chemistry and opacity properties enable survival at these extreme inner radii. 2011-09-16T07:31:05.859Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:14950 The Mira AB system is a nearby (∼ 107 pc) example of a wind accreting binary star system. In this class of system, the wind from a mass-losing red giant star (Mira A) is accreted onto a companion (Mira B), as indicated by an accretion shock signature in spectra at ultraviolet and X-ray wavelengths. Using novel imaging techniques, we report the detection of emission at mid-infrared wavelengths between 9.7 and 18.3 μm from the vicinity of Mira B but with a peak at a radial position about 10 AU closer to the primary Mira A. We interpret the mid-infrared emission as the edge of an optically-thick accretion disk heated by Mira A. The discovery of this new class of accretion disk fed by M-giant mass loss implies a potential population of young planetary systems in white dwarf binaries, which has been little explored despite being relatively common in the solar neighborhood. 2011-09-16T07:30:40.995Z ]]>