Mg II h + k emission lines as stellar activity indicators of main sequence F-K stars

Context. The largest dataset of stellar activity measurements available at present is the one obtained at the Mount Wilson Observatory, where high-precision Ca II H+K uxes have been measured from 1966 for about 2200 stars. Since the Mg II h and k lines at 2800 A are formed in a similar way to the Ca II H+K emission lines, they are also good indicators of chromospheric structure. The International Ultraviolet Explorer (IUE) provides a large database of UV spectra in the band 1150-3350 A, from 1978 to 1995, which can also be used to study stellar activity. Aims. The main purpose of this study is to use the IUE spectra in the analysis of magnetic activity of main sequence F-K stars. Combining IUE observations of Mg II and optical spectroscopy of Ca II, the registry of activity of stars can be extended in time. Methods. We retrieved all the high-resolution spectra of F, G, and K main sequence stars observed by IUE (i.e. 1623 spectra of 259 F to K dwarf stars). We obtained the continuum surface ux near the Mg II h+k lines near 2800 A and the Mg II line-core surface ux from the IUE spectra. Results. We obtained a relation between the mean continuum ux near the Mg II lines with the colour B V of the star. For a set of 117 nearly simultaneous observations of Mg II and Ca II uxes of 21 F5 to K3 main sequence stars, we obtained a colour dependent relation between the Mount Wilson Ca II S-index and the Mg II emission line-core ux. As an application of this calibration, we computed the Mount Wilson index for all the dF to dK stars which have high resolution IUE spectra. For some of the most frequently observed main sequence stars, we analysed the Mount Wilson index S from the IUE spectra, together with the ones derived from visible spectra. We conrm the cyclic chromospheric activity of Eri (HD 22049) and Hydri (HD 2151), and we nd a magnetic cycle in Cen B (HD 128621)

[1]  R. Díaz,et al.  Hα and the Ca II H and K lines as activity proxies for late-type stars , 2007 .

[2]  Jeffrey C. Hall,et al.  The Activity and Variability of the Sun and Sun-like Stars. I. Synoptic Ca II H and K Observations , 2007 .

[3]  C. Saffe,et al.  On the ages of exoplanet host stars , 2005, astro-ph/0510092.

[4]  Noordwijk,et al.  X-rays from α Centauri – The darkening of the solar twin , 2005, astro-ph/0508260.

[5]  S. Saar,et al.  A Comparison of the Outer Atmosphere of the “Flat Activity” Star τ Ceti (G8 V) with the Sun (G2 V) and α Centauri A (G2 V) , 2004 .

[6]  E. H. Olsen,et al.  The Geneva-Copenhagen survey of the Solar neighbourhood - Ages, metallicities, and kinematic properties of ~14 000 F and G dwarfs , 2004, astro-ph/0405198.

[7]  J. Schmitt,et al.  NEXXUS: A comprehensive ROSAT survey of coronal X-ray emission among nearby solar-like stars , 2003, astro-ph/0308510.

[8]  E. Guinan,et al.  Magnetic activity of six young solar analogues I. Starspot cycles from long-term photometry , 2002 .

[9]  P. Demarque,et al.  α Centauri AB , 2000 .

[10]  W. Wamsteker,et al.  II. Ripple correction and absolute calibration for IUE high resolution spectra , 1999, astro-ph/9910520.

[11]  A. Dupree,et al.  Extreme Ultraviolet Explorer Observations of the W Ursae Majoris Contact Binary 44i Bootis: Coronal Structure and Variability , 1998 .

[12]  Manuel Güdel,et al.  The X-Ray Sun in Time: A Study of the Long-Term Evolution of Coronae of Solar-Type Stars , 1997 .

[13]  S. Saar,et al.  Rotation, turbulence and evidence for magnetic fields in southern dwarfs , 1997 .

[14]  K. Levay,et al.  International Ultraviolet Explorer New Spectral Image Processing System Information Manual: Version 2.0 , 1997 .

[15]  P. Flower,et al.  Transformations from Theoretical Hertzsprung-Russell Diagrams to Color-Magnitude Diagrams: Effective Temperatures, B-V Colors, and Bolometric Corrections , 1996 .

[16]  S. Baliunas,et al.  A Relationship between Mean Rotation Period in Lower Main-Sequence Stars and Its Observed Range , 1996 .

[17]  Sallie L. Baliunas,et al.  A Survey of CA II H and K Chromospheric Emission in Southern Solar-Type Stars , 1996 .

[18]  D. F. Gray,et al.  Magnetic Activity Variations of epsilon Eridani , 1995 .

[19]  D. F. Gray,et al.  THE ACTIVITY CYCLE OF TAU CETI , 1994 .

[20]  William H. Press,et al.  Numerical recipes in FORTRAN (2nd ed.): the art of scientific computing , 1992 .

[21]  A. Endal,et al.  Rotational modulation in IUE spectra of cool dwarf chromospheres. I, Data analysis and period search techniques , 1991 .

[22]  D. Dravins,et al.  The Distant Future of Solar Activity: A Case Study of beta Hydri. II. Chromospheric Activity and Variability , 1990 .

[23]  C. Schrijver,et al.  The magnetic, basal, and radiative-equilibrium components in Mount Wilson CA II H + K fluxes , 1989 .

[24]  William H. Press,et al.  Numerical Recipes in FORTRAN - The Art of Scientific Computing, 2nd Edition , 1987 .

[25]  Donald F. Heath,et al.  The Mg 280‐nm doublet as a monitor of changes in solar ultraviolet irradiance , 1986 .

[26]  S. Baliunas,et al.  A Prescription for period analysis of unevenly sampled time series , 1986 .

[27]  William H. Press,et al.  Numerical recipes in C. The art of scientific computing , 1987 .

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

[29]  E. Bohm-Vitense.  The Effective Temperature Scale , 1981 .

[30]  A. Wesselink,et al.  Alpha and Proxima Centauri. , 1978 .

[31]  O. C. Wilson,et al.  Chromospheric variations in main-sequence stars , 1978 .

[32]  Walter L. Smith.  Probability and Statistics , 1959, Nature.