Imaging the Surface of Altair Monnier, John D; Zhao, M; Pedretti, E ...
Science (American Association for the Advancement of Science),
07/2007, Volume:
317, Issue:
5836
Journal Article
Peer reviewed
Open access
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 comprehensive models for the Herbig Ae stars MWC 275 and AB Aur that aim to explain their spectral energy distribution (from UV to millimeter) and long-baseline interferometry (from ...near-infrared to millimeter) simultaneously. Data from the literature, combined with new mid- infrared (MIR) interferometry from the Keck Segment Tilting Experiment, are modeled using an axisymmetric Monte Carlo radiative transfer code. Models in which most of the near-infrared (NIR) emission arises from a dust rim fail to fit the NIR spectral energy distribution (SED) and sub-milliarcsecond NIR CHARA interferometry. Following recent work, we include an additional gas emission component with similar size scale to the dust rim, inside the sublimation radius, to fit the NIR SED and long-baseline NIR interferometry on MWC 275 and AB Aur. In the absence of shielding of starlight by gas, we show that the gas-dust transition region in these YSOs will have to contain highly refractory dust, sublimating at similar to 1850 K. Despite having nearly identical structure in the thermal NIR, the outer disks of MWC 275 and AB Aur differ substantially. In contrast to the AB Aur disk, MWC 275 lacks small grains in the disk atmosphere capable of producing significant 10-20 mum emission beyond similar to 7 AU, forcing the outer regions into the "shadow" of the inner disk.
Context. Accurate stellar parameters are needed in numerous domains of astrophysics. The position of stars on the Hertzsprung-Russell diagram is an important indication of their structure and ...evolution, and it helps improve stellar models. Furthermore, the age and mass of stars hosting planets are required elements for studying exoplanetary systems. Aims. We aim at determining accurate parameters of a set of 18 bright exoplanet host and potential host stars from interferometric measurements, photometry, and stellar models. Methods. Using the VEGA/CHARA interferometer operating in the visible domain, we measured the angular diameters of 18 stars, ten of which host exoplanets. We combined them with their distances to estimate their radii. We used photometry to derive their bolometric flux and, then, their effective temperature and luminosity to place them on the H-R diagram. We then used the PARSEC models to derive their best fit ages and masses, with error bars derived from Monte Carlo calculations. Results. Our interferometric measurements lead to an average of 1.9% uncertainty on angular diameters and 3% on stellar radii. There is good agreement between measured and indirect estimations of angular diameters (either from SED fitting or from surface brightness relations) for main sequence (MS) stars, but not as good for more evolved stars. For each star, we provide a likelihood map in the mass-age plane; typically, two distinct sets of solutions appear (an old and a young age). The errors on the ages and masses that we provide account for the metallicity uncertainties, which are often neglected by other works. From measurements of its radius and density, we also provide the mass of 55 Cnc independently of models. From the stellar masses, we provide new estimates of semi-major axes and minimum masses of exoplanets with reliable uncertainties. We also derive the radius, density, and mass of 55 Cnc e, a super-Earth that transits its stellar host. Our exoplanetary parameters reflect the known population of exoplanets. Conclusions. This work illustrates how precise interferometric measurements of angular diameters and detailled modeling allow fundamental parameters of exoplanet host stars to be constrained at a level permiting analysis of the planet’s parameters.
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.
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.
We present the first 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 B-type stars and have detected 24 companions at separations ranging from 7 to 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
γ with γ = −0.46, agreeing with previous Sco-Cen high-mass work and differing significantly 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 per cent 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.
ABSTRACT
We present interferometric observations of the
σ
Orionis triple system using the CHARA Array, NPOI, and VLTI. Using these measurements, we spatially resolve the orbit of the close ...spectroscopic binary (Aa,Ab) for the first time and present a revised orbit for the wide pair (A,B). Combining the visual orbits with previously published radial velocity measurements and new radial velocities measured at CTIO, we derive dynamical masses for the three massive stars in the system of
= 16.99 ± 0.20
,
= 12.81 ± 0.18
, and
= 11.5 ± 1.2
. The inner and outer orbits in the triple are not coplanar, with a relative inclination of
–127
. The orbital parallax provides a precise distance of 387.5 ± 1.3 pc to the system. This is a significant improvement over previous estimates of the distance to the young
σ
Orionis cluster.
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.
We present interferometric measurements obtained with the CHARA Array of 13 adolescent-age stars in nearby moving groups. The motivation was to spatially resolve the largest stars and to search for ...binary companions. Nine stars have diameters smaller than the resolution limit and no evidence for companions within 0.5-50 mas and ΔH < 2.0 mag. The diameters of three stars were spatially resolved: GJ 159 (0.582 0.016 mas) and GJ 393 (0.564 0.021 mas) in the AB Dor moving group, and former member HD 89744 (0.556 0.032 mas). Combining the angular diameters with their distances and bolometric fluxes, we measured radii and effective temperatures. The temperatures of GJ 159 (6286 123 K) and GJ 393 (3515 68 K) are consistent with spectroscopic measurements. Comparisons with evolutionary models show that HD 89744 has evolved off the main sequence. GJ 159 and GJ 393 lie within 1.5 of the zero-age main sequence, complicating their age estimates because it is unclear whether the stars are contracting or expanding. GJ 159 has a mass of 1.2 0.1 with an age spanning 0.021-3.0 Gyr. Its debris disk and lithium abundance favor a young age. GJ 393 has a mass of 0.42 0.03 and a lower limit on its age 0.06 Gyr. This overlaps with the age of the moving group; however, an older age would be more consistent with its slow rotation, low activity, and luminosity, suggesting that GJ 393 is a kinematic interloper.
Stellar activity observed as large surface spots, radio flares, or emission lines is often found in binary systems. UX Arietis exhibits these signs of activity, originating on the K0 subgiant primary ...component. Our aim is to resolve the binary, measure the orbital motion, and provide accurate stellar parameters such as masses and luminosities to aid in the interpretation of the observed phenomena. Using the CHARA six-telescope optical long-baseline array on Mount Wilson, California, we obtained amplitudes and phases of the interferometric visibility on baselines up to 330 m in length, resolving the two components of the binary. We reanalyzed archival Center for Astrophysics spectra to disentangle the binary component spectra and the spectrum of the third component, which was resolved by speckle interferometry. We also obtained new spectra with the Nordic Optical Telescope, and we present new photometric data that we use to model stellar surface spot locations. Both interferometric visibilities and spectroscopic radial velocities are modeled with a spotted primary stellar surface using the Wilson-Devinney code. We fit the orbital elements to the apparent orbit and radial velocity data to derive the distance (52.1 0.8 pc) and stellar masses ( , ). The radius of the primary can be determined to be and that of the secondary to be . The equivalent spot coverage of the primary component was found to be 62% with an effective temperature 20% below that of the unspotted surface.
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