Astronomy and Astrophysics – Astrophysics
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995a%26a...293..107d&link_type=abstract
Astronomy and Astrophysics 293, 107-126 (1995)
Astronomy and Astrophysics
Astrophysics
37
Stars: Individual: {Lambda} And, Arcturus, Stars: Activity Of, Stars: Fundamental Parameters, Stars: Abundances, Stars: Close Binaries
Scientific paper
With very high resolution and high S/N spectra as well as multicolour photometric light curves, we have studied the activity, rotation and evolution of the RS CVn system λ And (HD 222107). Maximum entropy modelling of Johnson BV light curves tells us that two spots are present on the star around epoch 1991.5. One spot is large and rather stable, located at high latitude and covering about 8% of the total stellar surface, while the other one, sitting on the equator about 140deg away in longitude, is roughly twice smaller and varies significantly on timescales of a few months. From the B-V colour index, we infer a spot temperature of T_s_=4000+/-300K. A side result is that the most probable value for the angle between the line of sight and the rotation axis of λ And is 60deg^+30^_-15_. Using a two-component atmospheric structure, we computed a model spectrum for λ And between 6218 A and 6270 A which fits the observations at an accuracy better than half a percent. We find that the quiet atmospheric structure has an effective temperature of T_p_=4750+/-30K, a gravity of logg=2.5+/-0.2 and a microturbulence of ξ=1.6+/-0.3km/s. The chemical abundances of λ And are found to agree with those of Arcturus within 0.1dex. In particular, oxygen and light metals are similarly enhanced. From a Fourier analysis of the spectrum broadening, we derive a projected rotational velocity of vsini=6.5+/-0.3km/s and radial-tangential macroturbulence velocities of ζ_p_=5.5+/-0.6km/s for the photosphere and ζ_s_=10.0+/-2.0km/s for the active regions. We find this picture to be compatible with the rotational modulation of line depths, line widths and line bisectors observed in the spectra. This enhanced macroturbulence in spots still calls for a detailed physical interpretation. These spectroscopic constraints imply that λ And has a mass of 0.65^+0.6^_-0.3_Msun_ and a radius of 7.5+/-1.0Rsun_, and is thus very similar to Arcturus though somewhat less evolved. We demonstrate that the unseen secondary component of the system is a low mass main sequence star or a massive brown dwarf of mass 0.08+/-0.02Msun_, rather than a cool white dwarf. Evolutionary models for iron-deficient and oxygen enhanced stars indicate that the primary star of λ And should have completed the CN dredge up about 100Myr ago, and that its convective envelope has already started to recede. Arcturus lies further away on the same track. Comparing rotation periods of both stars tells that Arcturus has twice more angular momentum in its convective envelope that λ And. Since this difference is most likely the result of evolution, it suggests that strong radial differential rotation exists in λ And, whose radiative core should then store at least 50% of the total angular momentum. It also indicates that large amounts of angular momentum are transported to the envelope after dredge up completion, resulting probably from meridional circulation or/and shear instabilities in the radiative core. We then demonstrate that tidal forces in λ And are likely responsible for circularising the orbit, but should have also synchronised rotation with revolution. We show that the strong global fields recently detected in similarly active RS CVn system can cause a strong enough magnetic braking to explain the observed non synchronisation. Finally, given the low angular momentum content of the system at present time, we predict that λ And should start collapsing within the next 70Myr, and in particular before mass is exchanged between system components.
Donati Jean-Francois
Hall David S.
Henry Gregory W.
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