Max Planck Institute for Astronomy - Annual Report 2005

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22 II. Highlights AB Dor C AB Dor A 50 m� Fig. II.2.2: The orbits of AB Dor A and C, determined from astrometric data. The supposed binary therefore actually is a quadruple system composed of two close pairs orbiting one another at a distance of about 135 AE. The orbit of AB Dor C around A (Fig. II.2.2) allows the orbital parameters to be determined and thus, for the first time, enabling the mass of such a low-mass star to be calculated from astrometric observations. AB Dor C gravitationally acts on its central star while both stars are moving around a common center of gravity. Such observations of the so-called reflex motion had already been performed in previous years with VLBI, and hipparCoS and could now be used to reconstruct the orbit. As it turns out, AB Dor C is moving on a highly elliptical orbit (e � 0.59), taking 11.75 years to complete one period. The known mass of AB Dor A of 0.865 solar masses and the orbital parameters yielded a mass of 0.09 solar masses for AB Dor C (corresponding to 93 Jupiter masses). Thus the object is just above the upper limit of 0.07 solar masses for brown dwarfs. From the magnitudes measured at three near-infrared wavelengths, a spectral type of M 7 – M 9 could be derived for AB Dor C. In order to determine the type more precisely, the astronomers took a spectrum of the object using the naCo adaptive optics spectrograph. The spectrum best matched a spectral type of M 8 (Fig. II.2.3) which is assigned a temperature of 2600 K. The luminosity of AB Dor C was determined from the measured infrared fluxes to be 0.0018 solar luminosities. Normalized flux In this manner, the physical parameters of this lowmass star were determined and could be compared to the predictions made by theoretical models. Surprisingly, the companion was cooler by 400 degrees and 2.5 times fainter than expected for the given mass based on the latest theoretical models (Fig. II.2.4). In other words: current models yield masses that are only half as large for such young, low-mass objects. This significantly affects the interpretation of a number of observations. Thus many young, low-mass objects were found by infrared surveys in the area of the Orion star-forming region. With the help of theoretical evolutionary tracks, a majority of them had been classified as brown dwarfs. According to the new findings obtained for AB Dor C, many of them should now be considered to be low-mass stars. The existence of so-called free floating planetary objects should also now be considered with caution. These low-luminosity objects also located in the Orion Nebula had been classified as planets. If the masses were underestimated here, too, by a factor of two, these objects may be misidentified brown dwarfs. This work illustrates the danger of using evolutionary tracks for low-mass stars and brown dwarfs. In order to obtain a reliable calibration of the mass-luminosityrelation, more cases such as AB Dor C must be found. However, such objects are extremely rare. Only one percent of all stars have close low-mass companions, and only one percent of all stars in the solar neighborhood are young. Therefore, the fact that this measurement could be performed at all can be considered a real stroke of luck. A verification of these results with more objects would be desirable. Fig. II.2.3: Spectrum of AB Dor C in the near-infrared range. 1 0.8 0.6 0.4 2.0 Na I 2.1 2.2 Wavelength [�m] CO 2.3 2.4
M J [mag] T eff [K] 2800 2600 2400 9 10 11 0.01 M H [mag] M KS [mag] 8 9 10 11 8 9 10 11 0.1 1 10 0.01 0.1 1 10 Age [Gyr] Age [Gyr] Fig. II.2.4: Comparison of the determined physical quantities of AB Dor C (star symbol) with the results of theoretical models (lines). II.2 AB Doradus C: Young, Low-mass Star is Twice as Massive as Expected 23 (Rainer Lenzen, Wolfgang Brandner. Participating institutes: Steward Observatory, University of Arizona, Universitad de Valencia, Harvard-Smithsonian Center for Astrophysics, European Southern Observatory.)
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Page 5: Contents Preface ..................
Page 8 and 9: 6 I. General I.1 Scientific Goals U
Page 10 and 11: 8 I. General massive protostars and
Page 12 and 13: 10 I. General achieve higher angula
Page 14 and 15: 12 I. General The institute is co-l
Page 16 and 17: 14 I. General Point Source sensitiv
Page 18 and 19: 16 I. General Edinburgh Groningen D
Page 20 and 21: 18 II. Highlights II.1 Light Echoes
Page 22 and 23: 20 II. Highlights 10 arcsec Fig. II
Page 26 and 27: 24 II. Highlights II.3 The First He
Page 28 and 29: 26 II. Highlights II.4 Planetesimal
Page 30 and 31: 28 II. Highlights azimuthal directi
Page 32 and 33: 30 II. Highlights ticles fall faste
Page 34 and 35: 32 II. Highlights ANTU Seyfert 2 ga
Page 36 and 37: 34 II. Highlights cretion disk, one
Page 38 and 39: 36 II. Highlights 8.5 �m 2.3 ly 0
Page 40 and 41: 38 II. Highlights II.6 Massive Star
Page 42 and 43: 40 II. Highlights log M� /pc2 �
Page 44 and 45: 42 II. Highlights II.7 Observations
Page 46 and 47: 44 II. Highlights log IR luminosity
Page 48 and 49: 46 II.8 Dynamics, Dust, and Young S
Page 50 and 51: 48 II. Highlights and we see a temp
Page 52 and 53: 50 III. Selected Research Areas III
Page 54 and 55: 52 III. Scientific Work Fig. III.1.
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72 III.3 The Galaxy-Dark Matter Con
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74 III. Scientific Work redshift su
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76 III. Scientific Work Mocking the
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78 III. Scientific Work tion rate o
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82 III. Scientific Work DDO 53 HO I
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84 III. Scientific Work Fig. III.4.
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86 III. Scientific Work tribution o
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88 IV. Instrumental Development All
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102 IV. Instrumental Development IV
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106 IV. Instrumental Development IV
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108 IV. Instrumental Development jo
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110 V. People and Events 1� 10�
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112 V. People and Events V.2 Open H
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114 V. People and Events V.3 Furthe
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116 V. People and Events presented.
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118 V. People and Events The IMPRS
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124 V. People and Events These oppo
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138 V. People and Events Fig. V.13.
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140 V. People and Events V.14 Where
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142 V. People and Events against sp
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144 Staff Directors: Henning (Actin
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146 Working Groups Rainer Lenzen, H
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148 Cooperation with industrial Fir
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150 Conferences Conferences and Mee
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152 Conferences D. Lemke: JWST-miri
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154 Conferences/Service in Comitees
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156 Publications Szalay, I. Szapudi
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158 Publications Hamilton, C. M., W
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160 Publications Brinkmann, D. G. Y
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162 Publications Zheng, X. Z., F. H
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164 Publications of the wheel mecha
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166 Publications ASP Conf. Ser. 331
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The Max Planck Society The Max Plan
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Max Planck Institute for Astronomy - Annual Report 2005

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