Context. Red giant branch (RGB) stars are very bright objects in galaxies and are often used as standard candles. Interferometry is the ideal tool to characterize the dynamics and morphology of their ...atmospheres. Aims. We aim at precisely characterising the surface dynamics of a sample of RGB stars. Methods. We obtained interferometric observations for three RGB stars with the MIRC instrument mounted at the CHARA interferometer. We looked for asymmetries on the stellar surfaces using limb-darkening models. Results. We measured the apparent diameters of HD 197989 (ϵ Cyg) = 4.61 ± 0.02 mas, HD 189276 (HR 7633) = 2.95 ± 0.01 mas, and HD 161096 (β Oph) = 4.43 ± 0.01 mas. We detected departures from the centrosymmetric case for all three stars with the tendency of a greater effect for lower logg of the sample. We explored the causes of this signal and conclude that a possible explanation to the interferometric signal is the convection-related and/or the magnetic-related surface activity. However, it is necessary to monitor these stars with new observations, possibly coupled with spectroscopy, in order to firmly establish the cause.
Context. There is still a debate about the nature of the mechanism that causes the pulsation excitation of the rapidly oscillating Ap stars that oscillate above the highest theoretically acoustic ...frequency. HD 24712 is a good test case for such a study because it is bright, its parallax accurately determined, and its frequency spectrum is well known. Aims. Visible long-baseline interferometry is a unique technique for measuring accurate angular diameters of targets as small as the brightest roAp stars, and thus estimating accurate radii by a method as independent as possible of atmosphere models. Methods. We used the visible spectrograph VEGA at the CHARA long-baseline optical array to observe HD 24712, and we derived its limb-darkened diameter. We also estimated its bolometric flux from spectroscopic data in the literature and determined its radius, luminosity, and effective temperature. Results. We determined a limb-darkened angular diameter of 0.335 ± 0.009 mas for HD 24712 and derived a radius of R = 1.772 ± 0.057 R⊙, a luminosity of L = 7.2 ± 1.8 L⊙, and an effective temperature of Teff = 7235 ± 280 K, which is in very close agreement with the values provided by the self-consistent stratified model developed for this star. We used these fundamental parameters to set HD 24712 in the Hertzsprung-Russell diagram. Its position is marginally consistent with the region where high radial order modes are predicted to be excited by the κ-mechanism. Conclusions. We conclude that oscillations in this star are most likely not driven by the κ-mechanism.
We report on a high angular resolution survey of circumstellar disks around 24 northern sky Be stars. The K-band continuum survey was made using the CHARA Array long baseline interferometer ...(baselines of 30-331 m). The interferometric visibilities were corrected for the flux contribution of stellar companions in those cases where the Be star is a member of a known binary or multiple system. For those targets with good (u, upsilon) coverage, we used a four-parameter Gaussian elliptical disk model to fit the visibilities and to determine the axial ratio, position angle, K-band photospheric flux contribution, and angular diameter of the disk's major axis. For the other targets with relatively limited (u, upsilon) coverage, we constrained the axial ratio, inclination angle, and/or disk position angle where necessary in order to resolve the degeneracy between possible model solutions. We also made fits of the ultraviolet and infrared spectral energy distributions (SEDs) to estimate the stellar angular diameter and infrared flux excess of each target. The mean ratio of the disk diameter (measured in K-band emission) to stellar diameter (from SED modeling) is 4.4 among the 14 cases where we reliably resolved the disk emission, a value which is generally lower than the disk size ratio measured in the higher opacity H alpha emission line. We estimated the equatorial rotational velocity from the projected rotational velocity and disk inclination for 12 stars, and most of these stars rotate close to or at the critical rotational velocity.
Metal-poor halo stars are important astrophysical laboratories that allow us to unravel details about many aspects of astrophysics, including the chemical conditions at the formation of our Galaxy, ...understanding the processes of diffusion in stellar interiors, and determining precise effective temperatures and calibration of colour-effective temperature relations. To address any of these issues the fundamental properties of the stars must first be determined. HD 140283 is the closest and brightest metal-poor Population II halo star (distance = 58 pc and V = 7.21), an ideal target that allows us to approach these questions, and one of a list of 34 benchmark stars defined for Gaia astrophysical parameter calibration. In the framework of characterizing these benchmark stars, we determined the fundamental properties of HD 140283 (radius, mass, age, and effective temperature) by obtaining new interferometric and spectroscopic measurements and combining them with photometry from the literature. The interferometric measurements were obtained using the visible interferometer VEGA on the CHARA array and we determined a 1D limb-darkened angular diameter of θ1D = 0.353 ± 0.013 milliarcsec. Using photometry from the literature we derived the bolometric flux in two ways: a zero reddening solution (AV = 0.0 mag) of Fbol of 3.890 ± 0.066 × 10-8 erg s-1 cm-2, and a maximum of AV = 0.1 mag solution of 4.220 ± 0.067 × 10-8 erg s-1 cm-2. The interferometric Teff is thus between 5534 ± 103 K and 5647 ± 105 K and its radius is R = 2.21 ± 0.08R⊙. Spectroscopic measurements of HD 140283 were obtained using HARPS, NARVAL, and UVES and a 1D LTE analysis of Hα line wings yielded Teffspec = 5626 ± 75 K. Using fine-tuned stellar models including diffusion of elements we then determined the mass M and age t of HD 140283. Once the metallicity has been fixed, the age of the star depends on M, initial helium abundance Yi, andmixing-length parameter α, only two of which are independent. We derive simple equations to estimate one from the other two. We need to adjust α to much lower values than the solar one (~2) in order to fit the observations, and if AV = 0.0 mag then 0.5 ≤ α ≤ 1. We give an equation to estimate t from M, Yi (α), and AV. Establishing a reference α = 1.00 and adopting Yi = 0.245 we derive a mass and age of HD 140283: M = 0.780 ± 0.010M⊙ and t = 13.7 ± 0.7 Gyr (AV = 0.0 mag), or M = 0.805 ± 0.010M⊙ and t = 12.2 ± 0.6 Gyr (AV = 0.1 mag). Our stellar models yield an initial (interior) metal-hydrogen mass fraction of Z/X i = −1.70 and log g = 3.65 ± 0.03. Theoretical advances allowing us to impose the mixing-length parameter would greatly improve the redundancy between M,Yi, and age, while from an observational point of view, accurate determinations of extinction along with asteroseismic observations would provide critical information allowing us to overcome the current limitations in our results.
The Best Interest of the Child − Self-report questionnaire (BIC-S) is designed as a tool to ensure young people in secure residential care have a voice about their living environment. The primary aim ...of this study was to determine the psychometric properties of the BIC-S. Second, we wanted to map the experiences of young people with their current residential living environment. Therefore, both quantitative and qualitative methods were used to assess the experiences of young people with the BIC-S instrument in this cross-sectional study. Based on a sample of 74 young people staying in secure residential care in the Netherlands (2015), we explored the construct validity and reliability of the BIC-S through a Mokken Scale Analysis. The sample consisted of 38 boys and 36 girls (mean age 15.5). After evaluating the psychometric properties of the instrument, we focused on the experiences of young people with their current residential living environment. The results show that eleven out of the fourteen BIC-S conditions form a moderate scale to measure how young people perceive the quality of their living environment (H = .40; Rho = .86). With regard to the residential environment, the participants often raise issues which relate to personal safety, to activities within the institute, or to the relationship they have with care professionals. The results indicate that the BIC-S has the potential to serve as an instrument for young people to voice opinions on their living environment while in residential care.
Aims. We aim at directly detecting the presence of optically thin circumstellar dust emission within the terrestrial planetary zone around main sequence stars known to harbour cold debris discs. The ...present study focuses on a sample of six bright A- and early F-type stars. Methods. High-precision interferometric observations have been obtained in the near-infrared K band with the FLUOR instrument installed on the CHARA Array. The measured squared visibilities are compared to the expected visibility of the stellar photospheres based on theoretical photospheric models taking into account rotational distortion. We search for potential visibility reduction at short baselines, a direct piece of evidence for resolved circumstellar emission. Results. Our observations bring to light the presence of resolved circumstellar emission around one of the six target stars (\zeta Aql) at the 5\sigma level. The morphology of the emission source cannot be directly constrained because of the sparse spatial frequency sampling of our interferometric data. Using complementary adaptive optics observations and radial velocity measurements, we find that the presence of a low-mass companion is a likely origin for the excess emission. The potential companion is characterised by a K-band contrast of four magnitudes. It has a most probable mass of about 0.6 similar to M_{\odot} and is expected to orbit between about 5.5 AU and 8 AU from its host star assuming a purely circular orbit. Nevertheless, by adjusting a physical debris disc model to the observed Spectral Energy Distribution of the zeta Aql system, we also show that the presence of hot dust within 10 AU from zeta Aql, producing a total thermal emission equal to 1.69 pm 0.31% of the photospheric flux in the K band, is another viable explanation for the observed near-infrared excess. Our re-interpretation of archival near- to far-infrared photometric measurements shows however that cold dust is not present around zeta Aql at the sensitivity limit of the IRS and MIPS instruments onboard Spitzer, and urges us to remove zeta Aql from the category of bona fide debris disc stars. Conclusions. The hot debris disc around Vega (Absil et al. 2006) currently remains our only secure resolved detection within the context of this survey, with six genuine early-type debris disc stars observed so far. Further observations will be needed to assess whether zeta Aql also belongs to this hot debris disc category.
Context. The radius of a star is a very important constraint to evolutionary models, particularly when combined with asteroseismology. Diameters can now be measured interferometrically with great ...precision (better than 1%), but the center-to-limb darkening (CLD) remains a potential source of bias. Measuring this bias is possible by completely resolving the star using long-baseline interferometry, and has only been achieved for a handful of stars. Aims. The red giant η Ser (K0III-IV) is a particularly interesting target, as asteroseismic oscillations have recently been detected in this star by spectroscopy. We aim to measure its radius with high accuracy, debiased from limb darkening, in order to bring new constraints to its models. Methods. We obtained interferometric observations of η Ser in the near-infrared using the CHARA/FLUOR instrument, in particular in the so-called second lobe of visibility in order to constrain the CLD and debias our diameter estimation. Results. The limb darkened angular diameter of η Ser is 2.944 ± 0.010 mas (using spherical photosphere models PHOENIX and MARCS for the limb darkening), that converts into a radius of 5.897 ± 0.028 $R_\odot$ with the Hipparcos parallax. Thanks to a precise visibility measurement in the second lobe of the visibility function of η Ser and a one-parameter limb-darkened visibility profile, we were able to show that the photosphere models have the best agreement possible. Conclusions. Our limb darkening measurement of η Ser is in agreement with existing atmosphere models of this star, with a slightly better agreement for models using spherical geometry. This is a strong indication that interferometric angular diameter measurements for red giants, corrected for the CLD using models, are unbiased at a very small level (a fraction of 1%). In particular, this strengthens our confidence in the existing catalogues of calibrator stars for interferometry that are based on giant stars similar to η Ser. The high accuracy of our measurement brings a new and strong constraint for the asteroseismic modeling of this star.
We have measured the angular diameter of the transiting extrasolar planet host star HD 189733 using the CHARA optical/IR interferometric array. Combining our new angular diameter of 0.377 plus or ...minus 0.024 mas with the Hipparcos parallax leads to a linear radius for the host star of 0.779 plus or minus 0.052 R sub( )and a radius for the planet of 1.19 plus or minus 0.08 R sub(Jup). Adopting the mass of the planet as derived by its discoverers, we derive a mean density of the planet of 0.91 plus or minus 0.18 g cm super(-3). This is the first determination of the diameter of an extrasolar planet through purely direct means.
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⊙.
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