Precise radial velocity measurements of G and K giants^*

Multiple systems and variability trend along the Red Giant Branch

¹ Max-Planck-Institut für Astronomie, Koenigstuhl 17, 69117 Heidelberg, Germany
² Kiepenheuer-Institut für Sonnenphysik, Schoeneckstr. 6, 79104 Freiburg (Brsg), Germany
³ European Southern Observatory, Karl-Schwarzschild-Str. 3, 85748 Garching bei München, Germany
⁴ Observatório Nacional, R. Gal. José Cristino 77, 20921-400 Sao Cristavao, Rio de Janeiro, Brazil
⁵ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁶ Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy
⁷ Departamento de Física, Universidade Federal do Rio Grande do Norte, 59072-970 Natal, Brazil

Corresponding author: J. Setiawan, setiawan@mpia-hd.mpg.de

Received: 15 January 2004
Accepted: 14 March 2004

Abstract

We present the results of our radial velocity (RV) measurements of G and K giants, concentrating on the presence of multiple systems in our sample. Eighty-three giants have been observed for 2.5 years with the fiber-fed echelle spectrograph FEROS at the 1.52 m ESO telescope in La Silla, Chile. Seventy-seven stars (93%) of the targets have been analyzed for RV variability using simultaneous Th–Ar calibration and a cross-correlation technique. We estimate the long-term precision of our measurement as better than 25 . Projected rotational velocities have been measured for most stars of the sample. Within our time-base only 21 stars (or 27%) show variability below 2σ, while the others show RV variability with amplitudes up to several . The large amplitude (several ) and shape (high eccentricity) of the RV variations for 11 of the program stars are consistent with stellar companions, and possibly brown dwarf companions for two of the program stars. In those systems for which a full orbit could be derived, the companions have minimum masses from ~0.6  down to 0.1 . To these multiple systems we add the two candidates of giant planets already discovered in the sample. This analysis shows that multiple systems contribute substantially to the long-term RV variability of giant stars, with about 20% of the sample being composed of multiple systems despite screening our sample for known binary stars. After removing binaries, the range of RV variability in the whole sample clearly decreases, but the remaining stars retain a statistical trend of RV variability with luminosity: luminous cool giants with show RV variations with  , while giants with including those in the clump region exhibit less variability or they are constant within our accuracy. The same trend is observed with respect to absolute visual magnitudes: brighter stars show a larger degree of variability and, when plotted in the RV variability vs. magnitude diagram a trend of increasing RV scatter with luminosity is seen. The amplitude of RV variability does not increase dramatically, as predicted, for instance, by simple scaling laws. At least two luminous and cooler stars of the sample show a correlation between RV and chromospheric activity and bisector asymmetry, indicating that in these two objects RV variability is likely induced by the presence of (chromospheric) surface structures.