Interferometry and asteroseismology: The radius of τ Cet
TL;DR: In this article, the radius of the nearby star τ Cet has been determined from interferometry using the VINCI instrument on VLTI using the siderostats.
Abstract: We have determined from interferometry the radius of the nearby star τ Cet, using recent observations with the VINCI instrument on VLTI using the siderostats. The limb-darkened disk diameter is determined, with an unprecedented internal precision of 0.5%, to be 1.971 ± 0.009(int.) ± 0.05(ext.) mas, corresponding to a physical radius of 0.773 ± 0.004(int.) ± 0.02(ext.) R� . With this determination τ Cet becomes a prime target for asteroseismic campaigns to determine its internal structure, and thereby test stellar evolution theory. We discuss implications for asteroseismology and present predictions for oscillation properties.
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TL;DR: In this paper, the authors search the Hubble Space Telescope (HST) archive for previously unanalyzed observations of stellar H I Lyα emission lines, their primary purpose being to look for new detections of Lyα absorption from the outer heliosphere and to also search for analogous absorption of the astrospheres surrounding the observed stars.
Abstract: We search the Hubble Space Telescope (HST) archive for previously unanalyzed observations of stellar H I Lyα emission lines, our primary purpose being to look for new detections of Lyα absorption from the outer heliosphere and to also search for analogous absorption from the astrospheres surrounding the observed stars. The astrospheric absorption is of particular interest because it can be used to study solar-like stellar winds that are otherwise undetectable. We find and analyze 33 HST Lyα spectra in the archive. All the spectra were taken with the E140M grating of the Space Telescope Imaging Spectrograph (STIS) instrument on board HST. The HST STIS spectra yield four new detections of heliospheric absorption (70 Oph, ξ Boo, 61 Vir, and HD 165185) and seven new detections of astrospheric absorption (EV Lac, 70 Oph, ξ Boo, 61 Vir, δ Eri, HD 128987, and DK UMa), doubling the previous number of heliospheric and astrospheric detections. When combined with previous results, 10 of 17 lines of sight within 10 pc yield detections of astrospheric absorption. This high detection fraction implies that most of the ISM within 10 pc must be at least partially neutral, since the presence of H I within the ISM surrounding the observed star is necessary for an astrospheric detection. In contrast, the detection percentage is only 9.7% (3 out of 31) for stars beyond 10 pc. Our Lyα analyses provide measurements of ISM H I and D I column densities for all 33 lines of sight, and we discuss some implications of these results. Finally, we measure chromospheric Lyα fluxes from the observed stars. We use these fluxes to determine how Lyα flux correlates with coronal X-ray and chromospheric Mg II emission, and we also study how Lyα emission depends on stellar rotation.
290 citations
TL;DR: In this article, the authors derived an empirical effective temperature and bolometric luminosity calibration for G and K dwarfs, by applying their own implementation of the Infrared Flux Method to multiband photometry.
Abstract: We derive an empirical effective temperature and bolometric luminosity calibration for G and K dwarfs, by applying our own implementation of the Infrared Flux Method to multiband photometry. Our study is based on 104 stars for which we have excellent BV(RI)CJHKS photometry, excellent parallaxes and good metallicities.
Colours computed from the most recent synthetic libraries (ATLAS9 and MARCS) are found to be in good agreement with the empirical colours in the optical bands, but some discrepancies still remain in the infrared. Synthetic and empirical bolometric corrections also show fair agreement.
A careful comparison to temperatures, luminosities and angular diameters obtained with other methods in the literature shows that systematic effects still exist in the calibrations at the level of a few per cent. Our Infrared Flux Method temperature scale is 100-K hotter than recent analogous determinations in the literature, but is in agreement with spectroscopically calibrated temperature scales and fits well the colours of the Sun. Our angular diameters are typically 3 per cent smaller when compared to other (indirect) determinations of angular diameter for such stars, but are consistent with the limb-darkening corrected predictions of the latest 3D model atmospheres and also with the results of asteroseismology.
Very tight empirical relations are derived for bolometric luminosity, effective temperature and angular diameter from photometric indices.
We find that much of the discrepancy with other temperature scales and the uncertainties in the infrared synthetic colours arise from the uncertainties in the use of Vega as the flux calibrator. Angular diameter measurements for a well-chosen set of G and K dwarfs would go a long way to addressing this problem.
204 citations
TL;DR: In this paper, the authors compare the differences in temperature between the standard spectroscopic technique based on iron lines and the infrared flux method (IRFM) and show that the differences for the cooler stars are significantly smaller and more homogeneously distributed than in previous studies.
Abstract: Context. Temperature, surface gravity, and metallicitity are basic stellar atmospheric parameters necessary to characterize a star. There are several methods to derive these parameters and a comparison of their results often shows considerable discrepancies, even in the restricted group of solar-type FGK dwarfs.Aims. We want to check the differences in temperature between the standard spectroscopic technique based on iron lines and the infrared flux method (IRFM). We aim to improve the description of the spectroscopic temperatures especially for the cooler stars where the differences between the two methods are higher, as presented in a previous work.Methods. Our spectroscopic analysis was based on the iron excitation and ionization balance, assuming Kurucz model atmospheres in LTE. The abundance analysis was determined using the code MOOG. We optimized the line list using a cool star (HD 21749) with high resolution and high signal-to-noise spectrum, as a reference in order to check for weak, isolated lines.Results. We test the quality of the new line list by re-deriving stellar parameters for 451 stars with high resolution and signal-to-noise HARPS spectra, that were analyzed in a previous work with a larger line list. The comparison in temperatures between this work and the latest IRFM for the stars in common shows that the differences for the cooler stars are significantly smaller and more homogeneously distributed than in previous studies for stars with temperatures below 5000 K. Moreover, a comparison is presented between interferometric temperatures with our results that shows good agreement, even though the sample is small and the errors of the mean differences are large. We use the new line list to re-derive parameters for some of the cooler stars that host planets. Finally, we present the impact of the new temperatures on the [Cr i/Cr ii] and [Ti i/Ti ii] abundance ratios that previously showed systematic trends with temperature. We show that the slopes of these trends for the cooler stars become drastically smaller.
170 citations
TL;DR: In this article, the angular diameter of five nearby Vega-like stars, α PsA, β Leo, β Pic, � Eri and τ Cet, was estimated with a 1 to 2% accuracy.
Abstract: We report in this paper the direct interferometric measurement of the angular diameter of five nearby Vega-like stars: α PsA, β Leo, β Pic, � Eri and τ Cet. The near-infrared (K and H bands) observations were conducted at the VLTI during the commissioning period with the VINCI instrument and three different baselines ranging from 66 m to 140 m. The five stellar photospheres are resolved and we derive their angular diameters with a 1 to 2% accuracy, except for β Pic (14%). We discuss the detectability and the influence of a possible small amount of warm circumstellar dust on our measurements. In addition, we have used the stellar evolution code CESAM (Morel 1997) to compare the computed fundamental parameters to the observed values (linear diameter, luminosity, temperature and chemical abundance). As a result of the simulation, the age of the stars is inferred and found to be in good agreement with previous estimates from various other methods.
167 citations
Cites background or methods from "Interferometry and asteroseismology..."
...The simulation converges with a luminosity log (L/L ) = −0.296 (to be compared with log (L/L ) = −0.284±0.0240.026, Pijpers et al. 2003), an effective temperature Teff = 5377 K and a linear radius of 0.821 R in agreement with the VLTI determination....
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...Pijpers et al. (2003) have chosen to adopt a larger helium content but with a much smaller mass (0.77 M ) to account for both spectroscopic observations and their interferometric measurement....
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...12 to 16. τCeti: The size of τCet has recently been estimated by Pijpers et al. (2003) from the same initial set of observations....
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...Its size has already been inferred from the VLTI data by Pijpers et al. (2003) and we will give in this section a revised value for its angular diameter, on the basis of a more extended calibration procedure and after the revision of its main calibrator’s size, θCet....
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...Indeed, we cannot confirm from our analysis the discrepancy observed by Pijpers et al. (2003) between the interferometric and spectroscopic diameters....
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TL;DR: In this paper, the semimajor axes of the associated planets can be estimated from the dust temperature of the disks and rings of a distant star using the 160 μm filter.
Abstract: Submillimeter and near-infrared images of cool dusty debris disks and rings suggest the existence of unseen planets. At dusty but nonimaged stars, semimajor axes of associated planets can be estimated from the dust temperature. For some young stars these semimajor axes are greater than 1'' as seen from Earth. Such stars are excellent targets for sensitive near-infrared imaging searches for warm planets. To probe the full extent of the dust and hence of potential planetary orbits, Spitzer observations should include measurements with the 160 μm filter.
150 citations
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TL;DR: The updated OPAL Rosseland mean opacities for Population I stars have been presented in this paper, where the main opacity changes are increases of as much as 20% for population I stars due to the explicit inclusion of 19 metals (compared to 12 metals in the earlier calculations).
Abstract: The reexamination of astrophysical opacities has eliminated gross discrepancies between a variety of observations and theoretical calculations; thus allowing for more detailed tests of stellar models. A number of such studies indicate that model results are sensitive to modest changes in the opacity. Consequently, it is desirable to update available opacity databases with recent improvements in physics, refinements of element abundance, and other such factors affecting the results. Updated OPAL Rosseland mean opacities are presented. The new results have incorporated improvements in the physics and numerical procedures as well as corrections. The main opacity changes are increases of as much as 20{percent} for Population I stars due to the explicit inclusion of 19 metals (compared to 12 metals in the earlier calculations) with the other modifications introducing opacity changes smaller than 10{percent}. In addition, the temperature and density range covered by the updated opacity tables has been extended. As before, the tables allow accurate interpolation in density and temperature as well as hydrogen, helium, carbon, oxygen, and metal mass fractions. Although a specific metal composition is emphasized, opacity tables for different metal distributions can be made readily available. The updated opacities are compared to other work. {copyright} {ital 1996 Themore » American Astronomical Society.}« less
2,869 citations
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...The evolution tracks were computed using the evolution code of Christensen-Dalsgaard (1982), with the EFF equation of state (Eggleton et al. 1973), OPAL opacities (Iglesias & Rogers 1996), Bahcall & Pinsonneault (1995) nuclear cross sections, and the mixing-length formalism (MLT) for convection....
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TL;DR: In this article, the effects on neutrino fluxes of each change in the input physics are evaluated separately by constructing a series of solar models with one additional improvement added at each stage.
Abstract: Helium and heavy-element diffusion are both included in precise calculations of solar models. In addition, improvements in the input data for solar interior models are described for nuclear reaction rates, the solar luminosity, the solar age, heavy-element abundances, radiative opacities, helium and metal diffusion rates, and neutrino interaction cross sections. The effects on the neutrino fluxes of each change in the input physics are evaluated separately by constructing a series of solar models with one additional improvement added at each stage. The effective 1 σ uncertainties in the individual input quantities are estimated and used to evaluate the uncertainties in the calculated neutrino fluxes and the calculated event rates for solar neutrino experiments. The calculated neutrino event rates, including all of the improvements, are 9.3-1.4+1.2 SNU for the 37Cl experiment and 137-7+8 SNU for the 71Ga experiments. The calculated flux of 7Be neutrinos is 5.1 (1.00-0.07+0.06)×10^9 cm^-2 s^-1 and the flux of 8B neutrinos is 6.6(1.00-0.17+0.14)×10^6 cm^-2 s^-1. The primordial helium abundance found for this model is Y=0.278. The present-day surface abundance of the model is Ys=0.247, in agreement with the helioseismological measurement of Ys=0.242±0.003 determined by Hernandez and Christensen-Dalsgaard (1994). The computed depth of the convective zone is R=0.712R⊙, in agreement with the observed value determined from p-mode oscillation data of R=0.713±0.003R⊙ found by Christensen-Dalsgaard et al. (1991). Although the present results increase the predicted event rate in the four operating solar neutrino experiments by almost 1 σ (theoretical uncertainty), they only slightly increase the difficulty of explaining the existing experiments with standard physics (i.e., by assuming that nothing happens to the neutrinos after they are created in the center of the sun). For an extreme model in which all diffusion (helium and heavy-element diffusion) is neglected, the event rates are 7.0-1.0+0.9 SNU for the 37Cl experiment and 126-6+6 SNU for the 71Ga experiments, while the 7Be and 8B neutrino fluxes are, respectively, 4.5(1.00-0.07+0.06)×10^9 cm^-2 s^-1 and 4.9(1.00-0.17+0.14)×10^6 cm^-2 s^-1. For the no-diffusion model, the computed value of the depth of the convective zone is R=0.726R⊙, which disagrees with the observed helioseismological value. The calculated surface abundance of helium, Ys=0.268, is also in disagreement with the p-mode measurement. The authors conclude that helioseismology provides strong evidence for element diffusion and therefore for the somewhat larger solar neutrino event rates calculated in this paper.
704 citations
TL;DR: In this article, the authors start from six absolutely calibrated continuous stellar spectra from 1.2 to 35?m for K0, K1.5, K3, K5, and M0 giants, and normalize each template by photometry for the individual stars using published and/or newly secured near and mid-infrared photometry obtained through fully characterized, absolutely calibrated, combinations of filter passband, detector radiance response, and mean terrestrial atmospheric transmission.
Abstract: We start from our six absolutely calibrated continuous stellar spectra from 1.2 to 35 ?m for K0, K1.5, K3, K5, and M0 giants. These were constructed as far as possible from actual observed spectral fragments taken from the ground, the Kuiper Airborne Observatory, and the IRAS Low Resolution Spectrometer, and all have a common calibration pedigree. From these we spawn 422 calibrated spectral templates for stars with spectral types in the ranges G9.5?K3.5 III and K4.5?M0.5 III. We normalize each template by photometry for the individual stars using published and/or newly secured near- and mid-infrared photometry obtained through fully characterized, absolutely calibrated, combinations of filter passband, detector radiance response, and mean terrestrial atmospheric transmission. These templates continue our ongoing effort to provide an all-sky network of absolutely calibrated, spectrally continuous, stellar standards for general infrared usage, all with a common, traceable calibration heritage. The wavelength coverage is ideal for calibration of many existing and proposed ground-based, airborne, and satellite sensors, particularly low- to moderate-resolution spectrometers. We analyze the statistics of probable uncertainties, in the normalization of these templates to actual photometry, that quantify the confidence with which we can assert that these templates truly represent the individual stars. Each calibrated template provides an angular diameter for that star. These radiometric angular diameters compare very favorably with those directly observed across the range from 1.6 to 21 mas.
585 citations
Additional excerpts
...For θ Cet the uniform disk diameter has been estimated to be 2.69± 0.03mas, using absolutely calibrated spectra over a wide spectral range (Cohen et al. 1999)....
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Book•
01 Dec 1991
TL;DR: In this article, observational data derived from the world's largest solar telescopes are correlated with theoretical discussions in nuclear and atomic physics by contributors representing a wide range of interests in solar research, focusing both on processes occurring at the interior of the Sun and on complicated features observed at its surface.
Abstract: In this volume, observational data derived from the world's largest solar telescopes are correlated with theoretical discussions in nuclear and atomic physics by contributors representing a wide range of interests in solar research. Focusing both on processes occurring at the interior of the Sun and on complicated features observed at its surface, these chapters offer both basic explanations of solar phenomena and an overview of present controversies. Three areas of research covered in the volume are of particular interest: the pulsating nature of the Sun and how these oscillations facilitate the measurement of temperature, density, and pressure of its interior, thus revealing the depth of the surface convection zone and the composition of central regions; confirmation of the predicted flux of neutrinos via solar oscillation observations, yielding new speculations that they are produced at the solar centre but are converted to unobservable forms in passing through the Sun as they escape; and the interpretation of magnetic surface structures, based on both groundbased and space observations, in terms of chromosphere heating.
459 citations