Alpha Centauri

Abstract: We have made velocity observations of the star alpha Cen B from two sites, allowing us to identify 37 oscillation modes with l=0-3. Fitting to these modes gives the large and small frequency separations as a function of frequency. The mode lifetime, as measured from the scatter of the oscillation frequencies about a smooth trend, is similar to that in the Sun. Limited observations of the star delta Pav show oscillations centred at 2.3 mHz with peak amplitudes close to solar. We introduce a new method of measuring oscillation amplitudes from heavily-smoothed power density spectra, from which we estimated amplitudes for alpha Cen A and B, beta Hyi, delta Pav and the Sun. We point out that the oscillation amplitudes may depend on which spectral lines are used for the velocity measurements.

Abstract: A map of the surface of a brown dwarf reveals features that suggest patchy clouds, providing the mechanism for the dispersal of atmospheric dust as brown dwarfs cool with age. The recently discovered system known as Luhman 16AB is a binary consisting of two brown dwarfs — objects much bigger than planets but not big enough to become stars — and is a mere six light years from us. Only Alpha Centauri and Barnard's star are closer. Ian Crossfield et al. have now mapped the surface of brown dwarf Luhman 16B in the infrared and find large-scale surface patterns indicative of patchy clouds. Monitoring suggests that the characteristic timescale for the evolution of global weather patterns is about one day. Further observations of the evolution of weather patterns on brown dwarfs could provide a new benchmark for understanding how global circulation conditions affect dusty atmospheres on brown dwarfs and giant extrasolar planets. Brown dwarfs—substellar bodies more massive than planets but not massive enough to initiate the sustained hydrogen fusion that powers self-luminous stars1,2—are born hot and slowly cool as they age. As they cool below about 2,300 kelvin, liquid or crystalline particles composed of calcium aluminates, silicates and iron condense into atmospheric ‘dust’3,4, which disappears at still cooler temperatures (around 1,300 kelvin)5,6. Models to explain this dust dispersal include both an abrupt sinking of the entire cloud deck into the deep, unobservable atmosphere5,7 and breakup of the cloud into scattered patches6,8 (as seen on Jupiter and Saturn9). However, hitherto observations of brown dwarfs have been limited to globally integrated measurements10, which can reveal surface inhomogeneities but cannot unambiguously resolve surface features11. Here we report a two-dimensional map of a brown dwarf’s surface that allows identification of large-scale bright and dark features, indicative of patchy clouds. Monitoring suggests that the characteristic timescale for the evolution of global weather patterns is approximately one day.

Abstract: We compare the first direct angular diameter measurements obtained on our closest stellar neighbour, Centauri, to recent model diameters constrained by asteroseismic observations. Using the VINCI instrument installed at ESO's VLT Interferometer (VLTI), the angular diameters of the two main components of the system, Cen A and B, were measured with a relative precision of 0.2% and 0.6% respectively. Particular care has been taken in the calibration of these measurements, considering that VINCI is estimating the fringe visibility using a broadband K filter. We obtain uniform disk angular diameters for Cen A and B ofUD(A) = 8:314 0:016 mas andUD(B) = 5:856 0:027 mas, and limb darkened angular diameters ofLD(A)= 8:511 0:020 mas andLD(B)= 6:001 0:034 mas. Combining these values with the parallax from Soderhjelm (1999), we derive linear diameters of D(A)= 1:224 0:003 D and D(B)= 0:863 0:005 D. These values are compatible with the masses published by Thevenin et al. (2002) for both stars.

Abstract: New radial velocities of Cen A & B obtained in the framework the Anglo-Australian Planet Search programme as well as in the CORALIE programme are added to those by Endl et al. (2001) to improve the pre- cision of the orbital parameters. The resulting masses are 1:105 0:0070 M and 0:934 0:0061 M for A and B respectively. The factors limiting how accurately these masses can be derived from a combined visual-spectroscopic solution are investigated. The total eect of the convective blueshift and the gravitational redshift is also inves- tigated and estimated to dier by 215 8m s 1 between the components. This suggests that the dierence in convective blueshift between the components is much smaller than predicted from current hydrodynamical model atmosphere calculations.