The low-level radial velocity variability in Barnard's star (= GJ 699) ? Secular acceleration, indications for convective redshift, and planet mass limits

We report results from 2 1=2 yr of high precision radial velocity (RV) monitoring of Barnard's star. The high RV measurement precision of the VLT-UT2+UVES of 2:65 m s 1 made the following findings possible. (1) The first detection of the change in the RV of a star caused by its space motion (RV secular acceleration). (2) An anti-correlation of the measured RV with the strength of the filling-in of the H line by emission. (3) Very stringent mass upper limits to planetary companions. Using only data from the first 2 years, we obtain a best-fit value for the RV secular acceleration of 5:15 0:89 m s 1 yr 1 .T his agrees within 0:95 with the predicted value of 4:50 m s 1 yr 1 based on the Hipparcos proper motion and parallax combined with the known absolute radial velocity of the star. When the RV data of the last half-year are added the best-fit slope is strongly reduced to 2:97 0:51 m s 1 yr 1 (3:0 away from the predicted value), clearly suggesting the presence of additional RV variability in the star. Part of it can be attributed to stellar activity as we demonstrate by correlating the residual RVs with an index that describes the filling-in of the H line by emission. A correlation coecient of 0:50 indicates that the appearance of active regions causes a blueshift of photospheric absorption lines. Assuming that active regions basically inhibit convection we discuss the possibility that the fundamental (inactive) convection pattern in this M4V star produces a convective redshift which would indicate that the majority of the absorption lines relevant for our RV measurements is formed in a region of convective overshoot. This interpretation could possibly extend a trend indicated in the behaviour of earlier spectral types that exhibit convective blueshift, but with decreasing line asymmetries and blueshifts as one goes from G to K dwarfs. Based on this assumption, we estimate that the variation of the visible plage coverage is about 20%. We also determine upper limits to the projected mass m sin i and to the true mass m of hypothetical planetary companions in circular orbits. For the separation range 0:017 0:98 AU we exclude any planet with m sin i> 0:12 MJupiter and m> 0:86 MJupiter. Throughout the habitable zone around Barnard's star, i.e. 0:034 0:082 AU, we exclude planets with m sin i> 7:5 MEarth and m> 3:1 MNeptune.

[1]  F. Allard,et al.  Numerical simulations of surface convection in a late M-dwarf , 2002, astro-ph/0208584.

[2]  A. P. Hatzes,et al.  Starspots and exoplanets , 2002 .

[3]  W. Cochran,et al.  The planet search program at the ESO Coudé Echelle spectrometer. III. The complete Long Camera survey results , 2002, astro-ph/0207512.

[4]  R. P. Butler,et al.  On the Double-Planet System around HD 83443 , 2002, astro-ph/0206178.

[5]  S. Saar,et al.  Searching for Planets in the Hyades. II. Some Implications of Stellar Magnetic Activity , 2002, astro-ph/0204216.

[6]  R. P. Butler,et al.  Radial Velocities for 889 Late-Type Stars , 2001, astro-ph/0112477.

[7]  Dayton L. Jones,et al.  Stellar encounters with the solar system , 2001 .

[8]  R. Paul Butler,et al.  A Pair of Resonant Planets Orbiting GJ 876 , 2001 .

[9]  Fischer,et al.  Correcting Radial Velocities for Long-Term Magnetic Activity Variations , 2000, The Astrophysical journal.

[10]  R. P. Butler,et al.  The Lick Planet Search: Detectability and Mass Thresholds , 1999, astro-ph/9906466.

[11]  Otto G. Franz,et al.  Interferometric Astrometry of Proxima Centauri and Barnard's Star Using HUBBLE SPACE TELESCOPE Fine Guidance Sensor 3: Detection Limits for Substellar Companions , 1999, astro-ph/9905318.

[12]  Wolfgang Voges,et al.  THE ROSAT ALL-SKY SURVEY CATALOGUE OF THE NEARBY STARS , 1999 .

[13]  Julián Chela Flores,et al.  Exobiology: Matter, Energy, and Information in the Origin and Evolution of Life in the Universe , 2012, Springer Netherlands.

[14]  W. Jefferys,et al.  Photometry of Proxima Centauri and Barnard's Star Using Hubble Space Telescope Fine Guidance Sensor 3: A Search for Periodic Variations , 1998, astro-ph/9806276.

[15]  Robert A. Donahue,et al.  Activity-Related Radial Velocity Variation in Cool Stars , 1997 .

[16]  J. Gizis.  M-subdwarfs: spectroscopic classification and the metallicity scale , 1996, astro-ph/9611222.

[17]  M. A. C. Perryman,et al.  The Hipparcos and Tycho catalogues : astrometric and photometric star catalogues derived from the ESA Hipparcos Space Astrometry Mission , 1997 .

[18]  R. P. Butler,et al.  A Planetary Companion to 70 Virginis , 1996 .

[19]  M. Mayor,et al.  A Jupiter-mass companion to a solar-type star , 1995, Nature.

[20]  George Gatewood,et al.  A study of the astrometric motion of Barnard's star , 1995 .

[21]  S. Majewski.  Galactic Structure Surveys and the Evolution of the Milky Way , 1993 .

[22]  J. Kasting,et al.  Habitable zones around main sequence stars. , 1993, Icarus.

[23]  J. Scargle.  Studies in astronomical time series analysis. II - Statistical aspects of spectral analysis of unevenly spaced data , 1982 .

[24]  D. F. Gray.  Observations of spectral line asymmetries and convective velocities in F, G and K stars. , 1982 .

[25]  P. V. D. Kamp.  The planetary system of Barnard's star , 1982 .

[26]  P. V. D. Kamp,et al.  Perspective secular changes in stellar proper motion, radial velocity and parallax , 1977 .

[27]  N. Lomb.  Least-squares frequency analysis of unequally spaced data , 1976 .

[28]  P. V. D. Kamp.  Astrometric study of Barnard's star from plates taken with the 24-inch Sproul refractor. , 1963 .

[29]  E. Barnard.  Observations of the small star with large proper-motion , 1917 .

[30]  E. Barnard.  A small star with large proper-motion , 1916 .