Imaging the Surface of Altair Monnier, John D; Zhao, M; Pedretti, E ...
Science,
07/2007, Letnik:
317, Številka:
5836
Journal Article
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Spatially resolving the surfaces of nearby stars promises to advance our knowledge of stellar physics. Using optical long-baseline interferometry, we constructed a near-infrared image of the rapidly ...rotating hot star Altair with a resolution of <1 milliarcsecond. The image clearly reveals the strong effect of gravity darkening on the highly distorted stellar photosphere. Standard models for a uniformly rotating star cannot explain our findings, which appear to result from differential rotation, alternative gravity-darkening laws, or both.
We present the results of long-baseline optical interferometry observations using the Precision Astronomical Visual Observations (PAVO) beam combiner at the Center for High Angular Resolution ...Astronomy (CHARA) Array to measure the angular sizes of three bright Kepler stars: θ Cygni, and both components of the binary system 16 Cygni. Supporting infrared observations were made with the Michigan Infrared Combiner (MIRC) and Classic beam combiner, also at the CHARA Array. We find limb-darkened angular diameters of 0.753 ± 0.009 mas for θ Cyg, 0.539 ± 0.007 mas for 16 Cyg A and 0.490 ± 0.006 mas for 16 Cyg B. The Kepler Mission has observed these stars with outstanding photometric precision, revealing the presence of solar-like oscillations. Due to the brightness of these stars the oscillations have exceptional signal-to-noise, allowing for detailed study through asteroseismology, and are well constrained by other observations. We have combined our interferometric diameters with Hipparcos parallaxes, spectrophotometric bolometric fluxes and the asteroseismic large frequency separation to measure linear radii (θ Cyg: 1.48 ± 0.02 R, 16 Cyg A: 1.22 ± 0.02 R, 16 Cyg B: 1.12 ± 0.02 R), effective temperatures (θ Cyg: 6749 ± 44 K, 16 Cyg A: 5839 ± 42 K, 16 Cyg B: 5809 ± 39 K) and masses (θ Cyg: 1.37 ± 0.04 M, 16 Cyg A: 1.07 ± 0.05 M, 16 Cyg B: 1.05 ± 0.04 M) for each star with very little model dependence. The measurements presented here will provide strong constraints for future stellar modelling efforts.
Sunspots are cool areas caused by strong surface magnetic fields that inhibit convection. Moreover, strong magnetic fields can alter the average atmospheric structure, degrading our ability to ...measure stellar masses and ages. Stars that are more active than the Sun have more and stronger dark spots than does the Sun, including on the rotational pole. Doppler imaging, which has so far produced the most detailed images of surface structures on other stars, cannot always distinguish the hemisphere in which the starspots are located, especially in the equatorial region and if the data quality is not optimal. This leads to problems in investigating the north-south distribution of starspot active latitudes (those latitudes with more starspot activity); this distribution is a crucial constraint of dynamo theory. Polar spots, whose existence is inferred from Doppler tomography, could plausibly be observational artefacts. Here we report imaging of the old, magnetically active star ζ Andromedae using long-baseline infrared interferometry. In our data, a dark polar spot is seen in each of two observation epochs, whereas lower-latitude spot structures in both hemispheres do not persist between observations, revealing global starspot asymmetries. The north-south symmetry of active latitudes observed on the Sun is absent on ζ And, which hosts global spot patterns that cannot be produced by solar-type dynamos.
ABSTRACT We have observed and spatially resolved a set of seven A-type stars in the nearby Ursa Major moving group with the Classic, CLIMB, and PAVO beam combiners on the Center for High Angular ...Resolution Astronomy Array. At least four of these stars have large rotational velocities ( 170 ) and are expected to be oblate. These interferometric measurements, the stars' observed photometric energy distributions, and values are used to computationally construct model oblate stars from which stellar properties (inclination, rotational velocity, and the radius and effective temperature as a function of latitude, etc.) are determined. The results are compared with MESA stellar evolution models to determine masses and ages. The value of this new technique is that it enables the estimation of the fundamental properties of rapidly rotating stars without the need to fully image the star. It can thus be applied to stars with sizes comparable to the interferometric resolution limit as opposed to those that are several times larger than the limit. Under the assumption of coevality, the spread in ages can be used as a test of both the prescription presented here and the MESA evolutionary code for rapidly rotating stars. With our validated technique, we combine these age estimates and determine the age of the moving group to be 414 23 Myr, which is consistent with, but much more precise than previous estimates.
The CHARA Array is a six 1 m telescope optical/IR interferometric array located on Mount Wilson, California, designed and built by the Center for High Angular Resolution Astronomy of Georgia State ...University. In this paper we describe the main elements of the Array hardware and software control systems, as well as the data reduction methods currently being used. Our plans for upgrades in the near future are also described.
A classical nova occurs when material accreting onto the surface of a white dwarf in a close binary system ignites in a thermonuclear runaway. Complex structures observed in the ejecta at late stages ...could result from interactions with the companion during the common-envelope phase. Alternatively, the explosion could be intrinsically bipolar, resulting from a localized ignition on the surface of the white dwarf or as a consequence of rotational distortion. Studying the structure of novae during the earliest phases is challenging because of the high spatial resolution needed to measure their small sizes. Here we report near-infrared interferometric measurements of the angular size of Nova Delphini 2013, starting one day after the explosion and continuing with extensive time coverage during the first 43 days. Changes in the apparent expansion rate can be explained by an explosion model consisting of an optically thick core surrounded by a diffuse envelope. The optical depth of the ejected material changes as it expands. We detect an ellipticity in the light distribution, suggesting a prolate or bipolar structure that develops as early as the second day. Combining the angular expansion rate with radial velocity measurements, we derive a geometric distance to the nova of 4.54 ± 0.59 kiloparsecs from the Sun.
Abstract
Presented are the first interferometric images of cool starspots on the chromospherically active giant
λ
Andromedae. Using the Michigan Infra-Red Combiner coupled to the Center for High ...Angular Resolution Astronomy Array, 26 interferometric observations were made between 2008 August 17 and 2011 September 24. The photometric time series acquired at Fairborn Observatory spanning 2008 September 20 to 2011 January 20 is also presented. The angular diameter and power-law limb-darkening coefficient of this star are 2.759 ± 0.050 mas and 0.229 ± 0.111, respectively. Starspot properties are obtained from both modeled and SQUEEZE reconstructed images. The images from 2010 through 2011 show anywhere from one to four starspots. The cadence in the data for the 2010 and 2011 data sets is sufficient to measure a stellar rotation period based on apparent starspot motion. This leads to estimates of the rotation period (
P
2010
= 61 ± 4.0 days,
P
2011
= 54.0 ± 2.4 days) that are consistent with the photometrically determined period of 54.8 days. In addition, the inclination and position angle of the rotation axis are computed for both the 2010 and 2011 data sets; values (
Ψ
¯
= 21.°5,
i
¯
= 78.°0) for each are nearly identical between the two years.
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 interferomettic 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 (v sub(max)) and the large frequency separation ( Delta v). We find excellent agreement within the observational uncertainties, and empirically show that simple estimates of asteroseismic radii for main-sequence stars are accurate to lap4%. We furthermore find good agreement of our measured effective temperatures with spectroscopic and photometric estimates with mean deviations for stars between T sub(eff) = 4600-6200 K of -22 + or - 32 K (with a scatter of 97 K) and -58 + or - 31 K (with a scatter of 93 K), 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 HD 173701. 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.
We have measured the angular diameters of six M dwarfs with the CHARA Array, a long-baseline optical interferometer located at Mount Wilson Observatory. Spectral types range from M1.0 V to M3.0 V and ...linear radii from 0.38 to 0.69 R sub( ). These results are consistent with the seven other M dwarf radii measurements from optical interferometry and with those for 14 stars in eclipsing binary systems. We compare all directly measured M dwarf radii to model predictions and find that current models underestimate the true stellar radii by up to 15%-20%. The differences are small among the metal-poor stars but become significantly larger with increasing metallicity. This suggests that theoretical models for low-mass stars may be missing some opacity source that alters the computed stellar radii.
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