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Register a new userImaging the spotty surface of Betelgeuse in the H band
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This paper reports on H-band interferometric observations of Betelgeuse made at the three-telescope interferometer IOTA. We image Betelgeuse and its asymmetries to understand the spatial variation of the photosphere, including its diameter, limb darkening, effective temperature, surrounding brightness, and bright (or dark) star spots. We used different theoretical simulations of the photosphere and dusty environment to model the visibility data. We made images with parametric modeling and two image reconstruction algorithms: MIRA and WISARD. We measure an average limb-darkened diameter of 44.28 +/- 0.15 mas with linear and quadratic models and a Rosseland diameter of 45.03 +/- 0.12 mas with a MARCS model. These measurements lead us to derive an updated effective temperature of 3600 +/- 66 K. We detect a fully-resolved environment to which the silicate dust shell is likely to contribute. By using two imaging reconstruction algorithms, we unveiled two bright spots on the surface of Betelgeuse. One spot has a diameter of about 11 mas and accounts for about 8.5% of the total flux. The second one is unresolved (diameter < 9 mas) with 4.5% of the total flux. Resolved images of Betelgeuse in the H band are asymmetric at the level of a few percent. The MOLsphere is not detected in this wavelength range. The amount of measured limb-darkening is in good agreement with model predictions. The two spots imaged at the surface of the star are potential signatures of convective cells.
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Algol (Beta Per) is an extensively studied hierarchical triple system whose inner pair is a prototype semi-detached binary with mass transfer occurring from the sub-giant secondary to the main-sequence primary. We present here the results of our Algol observations made between 2006 and 2010 at the CHARA interferometer with the Michigan Infrared Combiner in the H band. The use of four telescopes with long baselines allows us to achieve better than 0.5 mas resolution and to unambiguously resolve the three stars. The inner and outer orbital elements, as well as the angular sizes and mass ratios for the three components are determined independently from previous studies. We report a significantly improved orbit for the inner stellar pair with the consequence of a 15% change in the primary mass compared to previous studies. We also determine the mutual inclination of the orbits to be much closer to perpendicularity than previously established. State-of-the-art image reconstruction algorithms are used to image the full triple system. In particular an image sequence of 55 distinct phases of the inner pair orbit is reconstructed, clearly showing the Roche-lobe-filling secondary revolving around the primary, with several epochs corresponding to the primary and secondary eclipses.
The dynamics of the surface and inner atmosphere of the red supergiant star Betelgeuse are the subject of numerous high angular resolution and spectroscopic studies. Here, we present three-telescope interferometric data obtained at 11.15 microns wavelength with the Berkeley Infrared Spatial Interferometer (ISI), that probe the stellar surface continuum. We find striking variability in the size, effective temperature, and degree of asymmetry of the star over the years 2006-2009. These results may indicate an evolving shell of optically thick material close to the stellar photosphere.
Context: Betelgeuse is one the largest stars in the sky in terms of angular diameter. Structures on the stellar photosphere have been detected in the visible and near-infrared as well as a compact molecular environment called the MOLsphere. Mid-infrared observations have revealed the nature of some of the molecules in the MOLsphere, some being the precursor of dust. Aims: Betelgeuse is an excellent candidate to understand the process of mass loss in red supergiants. Using diffraction-limited adaptive optics (AO) in the near-infrared, we probe the photosphere and close environment of Betelgeuse to study the wavelength dependence of its extension, and to search for asymmetries. Methods: We obtained AO images with the VLT/NACO instrument, taking advantage of the cube mode of the CONICA camera to record separately a large number of short-exposure frames. This allowed us to adopt a lucky imaging approach for the data reduction, and obtain diffraction-limited images over the spectral range 1.04-2.17 $mu$m in 10 narrow-band filters. Results: In all filters, the photosphere of Betelgeuse appears partly resolved. We identify an asymmetric envelope around the star, with in particular a relatively bright plume extending in the southwestern quadrant up to a radius of approximately six times the photosphere. The CN molecule provides an excellent match to the 1.09 mic bandhead in absorption in front of the stellar photosphere, but the emission spectrum of the plume is more difficult to interpret. Conclusions: Our AO images show that the envelope surrounding Betelgeuse has a complex and irregular structure. We propose that the southwestern plume is linked either to the presence of a convective hot spot on the photosphere, or to the rotation of the star.
We accurately determine the fundamental system parameters of the neutron-star X-ray transient Cen X-4 solely using phase-resolved high-resolution UVES spectroscopy. We first determine the radial-velocity curve of the secondary star and then model the shape of the phase-resolved absorption line profiles using an X-ray binary model. The model computes the exact rotationally broadened phase-resolved spectrum and does not depend on assumptions about the rotation profile, limb-darkening coefficients and the effects of contamination from an accretion disk. We determine the secondary star-to-neutron star binary mass ratio to be 0.1755+/-0.0025, which is an order of magnitude more accurate than previous estimates. We also constrain the inclination angle to be 32 (+8; -2) degrees, Combining these values with the results of the radial velocity study gives a neutron star mass of 1.94 (+0.37; -0.85) Msun consistent with previous estimates. Finally, we perform the first Roche tomography reconstruction of the secondary star in an X-ray binary. The tomogram reveals surface inhomogeneities that are due to the presence of cool starspots. A large cool polar spot, similar to that seen in Doppler images of rapidly-rotating isolated stars is present on the Northern hemisphere of the K7 secondary star and we estimate that about 4 per cent of the total surface area of the donor star is covered with spots. This evidence for starspots supports the idea that magnetic braking plays an important role in the evolution of low-mass X-ray binaries.
Betelgeuse is one of the most magnificent stars in the sky, and one of the nearest red supergiants. Astronomers gathered in Paris in the Autumn of 2012 to decide what we know about its structure, behaviour, and past and future evolution, and how to place this in the general context of the class of red supergiants. Here I reflect on the discussions and propose a synthesis of the presented evidence. I believe that, in those four days, we have achieved to solve a few riddles.
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