A. P. Jacob

Multisite campaign on the open cluster M67 – II. Evidence for solar-like oscillations in red giant stars

Measuring solar-like oscillations in an ensemble of stars i n a cluster, holds promise for testing stellar structure and evolution more stringently than just fitting parameters to single field stars. The most ambitious attempt to pursue these prospects was by Gilliland et al. (1993) who targeted 11 turn-off stars in the open cluster M67 (NGC 2682), but the oscillation amplitudes were too small (< 20µmag) to obtain unambiguous detections. Like Gilliland et al. (1993) we also aim at detecting solar-like oscillations in M67, but we target red giant stars with expected amplitudes in the range 50‐500µmag and periods of 1 to 8 hours. We analyse our recently published photometry measurements, obtained during a six-week multisite campaign using nine telescopes around the world. The observations are compared with simulations and with estimated properties of the stellar oscillations. Noise le vels in the Fourier spectra as low as 27µmag are obtained for single sites, while the combined data reach 19µmag, making this the best photometric time series of an ensemble of red giant stars. These data enable us to make the first test of the scaling relations (used to estimate frequency and amplitude) with an homogeneous ensemble of stars. The detected excess power is consistent with the expected signal from stellar oscillations, both in terms of its frequ ency range and amplitude. However, our results are limited by apparent high levels of non-white noise, which cannot be clearly separated from the stellar signal.

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.

Asteroseismological Studies of Long-Period Variable Subdwarf B Stars. I. A Multisite Campaign on PG 1627+017

We present the results of an extensive multisite campaign on the long-period variable subdwarf B star PG 1627+017. We gathered 300 hr of useful R-band and ~50 hr of simultaneous U/R differential photometry. From the R-band data we were able to extract 23 periodicities in the 4500-9000 s range with amplitudes between 0.05% and 0.5% of the stars mean brightness. The oscillations with the highest amplitudes cluster between 6300 and 7050 s and are thought to exhibit frequency splitting due to binary-synchronous stellar rotation. Interestingly, we find the observed period distribution to be extremely nonuniform, with dense frequency multiplets occurring in several narrow band passes. In order to compare the observed period spectrum to theoretical predictions, we constructed a set of newly updated and improved subdwarf B star models. We find that by invoking degree indices of l = 2, 3, and 4, nonadiabatic calculations can qualitatively reproduce the range of periodicities measured for PG 1627+017 if its atmospheric parameters are pushed to the lower end of their spectroscopic temperature uncertainties. However, the exploitation of rotational splitting and the U/R photometry, as well as the mean spacing between periodicities, indicate that at least the four highest amplitude peaks probably correspond to modes with l = 1. While this points to deficiencies in our models at the nonadiabatic level, the resulting constraints on mode identification are invaluable to first attempts at asteroseismology. Indeed, we identify only a few families of models that can closely reproduce the main periodicities observed in terms of dipole modes. This leaves us hopeful that, given a larger number of partially identified observed frequencies, asteroseismology may be achieved for long-period variable subdwarf B stars.

Period and chemical evolution of SC stars

The SC and CS stars are thermal-pulsing asymptotic giant branch stars with a C/O ratio close to unity. Within this small group, the Mira variable BH Cru recently evolved from spectral type SC (showing ZrO bands) to CS (showing weak C2). Wavelet analysis shows that the spectral evolution was accompanied by a dramatic period increase, from 420 to 540 d, indicating an expanding radius. The pulsation amplitude also increased. Old photographic plates are used to establish that the period before 1940 was around 490 d. Chemical models indicate that the spectral changes were caused by a decrease in stellar temperature, related to the increasing radius. There is no evidence for a change in C/O ratio. The evolution in BH Cru is unlikely to be related to an ongoing thermal pulse. Periods of the other SC and CS stars, including nine new periods, are determined. A second SC star, LX Cyg, also shows evidence for a large increase in period, and one further star shows a period inconsistent with a previous determination. Mira periods may be intrinsically unstable for C/O ≈ 1; possibly because of a feedback between the molecular opacities, pulsation amplitude, and period. LRS spectra of 6 SC stars suggest a feature at λ> 15 µm, which resembles one recently attributed to the iron-sulphide troilite. Chemical models predict a large abundance of FeS in SC stars, in agreement with the proposed

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

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.

Long-baseline interferometric multiplicity survey of the Sco-Cen OB association

We present the rst multiplicity-dedicated long baseline optical interferometric survey of the Scorpius-Centaurus-Lupus-Crux association. We used the Sydney University Stellar Interferometer to undertake a survey for new companions to 58 Sco-Cen Btype stars and have detected 24 companions at separations ranging from 7-130 mas, 14 of which are new detections. Furthermore, we use a Bayesian analysis and all available information in the literature to determine the multiplicity distribution of the 58 stars in our sample, showing that the companion frequency is F = 1:35 0:25 and the mass ratio distribution is best described by q with = 0:46, agreeing with previous Sco-Cen high mass work and diering signicantly from lower-mass stars in Tau-Aur. Based on our analysis, we estimate that among young B-type stars in moving groups, up to 23% are apparently single stars. This has strong implications for the understanding of high-mass star formation, which requires angular momentum dispersal through some mechanism such as formation of multiple systems.