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Title: Long-lived, long-period radial velocity variations in Aldebaran: A planetary companion and stellar activity
Author: A. P. Hatzes, W. D. Cochran, M. Endl, E. W. Guenther, P. MacQueen, M. Hartmann, M. Zechmeister, I. Han, B.-C. Lee, G.A.H. Walker, S. Yang, A.M. Larson, K.-M. Kim, D. E. Mkrtichian, M. Doellinger, A.E. Simon, L. Girardi

We investigate the nature of the long-period radial velocity variations in Alpha Tau first reported over 20 years ago. We analysed precise stellar radial velocity measurements for Alpha Tau spanning over 30 years. An examination of the Halpha and Ca II 8662 spectral lines, and Hipparcos photometry was also done to help discern the nature of the long-period radial velocity variations. Our radial velocity data show that the long-period, low amplitude radial velocity variations are long-lived and coherent. Furthermore, Halpha equivalent width measurements and Hipparcos photometry show no significant variations with this period. Another investigation of this star established that there was no variability in the spectral line shapes with the radial velocity period. An orbital solution results in a period of P = 628.96 0.90 d, eccentricity, e = 0.10 0.05, and a radial velocity amplitude, K = 142.1 7.2 m/s. Evolutionary tracks yield a stellar mass of 1.13 0.11 M_sun, which corresponds to a minimum companion mass of 6.47 0.53 M_Jup with an orbital semi-major axis of a = 1.46 0.27 AU. After removing the orbital motion of the companion, an additional period of ~ 520 d is found in the radial velocity data, but only in some time spans. A similar period is found in the variations in the equivalent width of Halpha and Ca II. Variations at one-third of this period are also found in the spectral line bisector measurements. The 520 d period is interpreted as the rotation modulation by stellar surface structure. Its presence, however, may not be long-lived, and it only appears in epochs of the radial velocity data separated by ~ 10 years. This might be due to an activity cycle. The data presented here provide further evidence of a planetary companion to Alpha Tau, as well as activity-related radial velocity variations.

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Aldebaran in the constellation of Taurus.

Picture 003


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Title: Spatially resolved, high-spectral resolution observation of the K giant Aldebaran in the CO first overtone lines with VLTI/AMBER
Authors: Keiichi Ohnaka

Aim: We present a high-spatial and high-spectral resolution observation of the well-studied K giant Aldebaran with AMBER at the Very Large Telescope Interferometer (VLTI). Our aim is to spatially resolve the outer atmosphere (so-called MOLsphere) in individual CO first overtone lines and derive its physical properties, which are important for understanding the mass-loss mechanism in normal (i.e., non-Mira) K--M giants.
Methods: Aldebaran was observed between 2.28 and 2.31 micron with a projected baseline length of 10.4m and a spectral resolution of 12000.
Results: The uniform-disk diameter observed in the CO first overtone lines is 20--35% larger than is measured in the continuum. We have also detected a signature of inhomogeneities in the CO-line-forming region on a spatial scale of ~45 mas, which is more than twice as large as the angular diameter of the star itself. While the MARCS photospheric model reproduces the observed spectrum well, the angular size in the CO lines predicted by the MARCS model is significantly smaller than observed. This is because the MARCS model with the parameters of Aldebaran has a geometrical extension of only ~2% (with respect to the stellar radius). The observed spectrum and interferometric data in the CO lines can be simultaneously reproduced by placing an additional CO layer above the MARCS photosphere. This CO layer is extended to 2.5 0.3 stellar radii with CO column densities of 5x10^{19}--2x10^{20} cm^-2 and a temperature of 1500 200 K.
Conclusions: The high spectral resolution of AMBER has enabled us to spatially resolve the inhomogeneous, extended outer atmosphere (MOLsphere) in the individual CO lines for the first time in a K giant. Our modelling of the MOLsphere of Aldebaran suggests a rather small gradient in the temperature distribution above the photosphere up to 2--3 stellar radii.

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P1010013b.jpg
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Aldebaran

Date: 2:30 GMT, 26 August, 2011
Exposure: 10 seconds



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P8090006
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Aldebaran

Date: 2:46 GMT, 07 September, 2007
Exposure: 10 seconds

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