A Ca ii H&K survey of γ Doradus candidates

In an investigation of the starspot hypothesis as it applies to the 'slowly variable' F-type dwarfs, we spectroscopically observed eight promising γ Doradus candidates to search for Ca  ii H&K emission. We found that there are no significant emission reversals in the cores of these resonance lines. Based on the ceiling flux calculations of the Ca  ii K line and on calculations of the Rossby number, we conclude that there is no support for the presence of strong magnetic activity and the starspot hypothesis in these objects.     

Chromospheric Ca II H and K emission among subdwarfs

Echelle spectra have been obtained of the Ca  II H and K lines for a sample of metal-poor subdwarf stars as well as for a number of nearby Population I dwarfs selected from among those included in the Mount Wilson HK survey. The main conclusion of this paper is that Ca  II H- and K-line emission does occur among subdwarfs. It is particularly notable among those subdwarfs with colours of B − V ≥0.75; all such stars observed exhibit chromospheric emission, although emission is observed among some subdwarfs bluer than this colour. The Ca  II K emission profile in most subdwarfs exhibits an asymmetry of V / R >1, similar to that seen in the integrated light of the solar disc. Two quantitative indicators of the contrast between the peaks in the emission profile and the neighbouring photospheric line profile are introduced. Measurements of these indicators show that the level of Ca  II emission among the subdwarfs is similar to that among low-activity Population I dwarfs.     

O i line emission in cool stars: calculations using partial redistribution

Radiative transfer calculations have been performed for five cool stars: α Tau, β Gem, Procyon, ε Eri and the Sun, for the purpose of investigating the behaviour of the O  i emission over a wide range of stellar types, and its dependence on coherent photon scattering. These stars span a range of spectral types from F5 iv – v to K5 iii and surface gravities  1.25 < log  g * < 4.75  . Particular attention has been paid to the calculation of the flux in the resonance triplet around 1305 Å which is pumped by H Lyβ, including the effects of partial redistribution (PRD) and cross-redistribution of photons. These are the first calculations for the resonance triplet in giant stars using a full PRD treatment. Calculations of the predominantly collisionally excited intersystem doublet at 1355, 1358 Å are included, and it is found that the ratio of these fluxes shows the effects of opacity. The flux in the forbidden line at 1641 Å is calculated for the giant stars and the effects of coherent scattering on this line are investigated. The discrepancy between the calculated and observed fluxes in the O  i lines is used to infer the inadequacy of single-component chromospheric models.     

HD 9770, a southern active-chromosphere system

The visual triple system HD 9770 (BB Scl) has been the subject of a four-year programme of UVB ( RI )_C photometry and H α échelle spectroscopy. Analysis of the data obtained over that period shows that star B, and probably also star A, of HD 9770 is a binary. The A system comprises a K1V star, which may be in a binary system with another K dwarf. The B system is an eclipsing binary of the BY Dra type in which both stars are chromospherically active. An orbital period of 0.476 525±0.000 013 d has been derived from the light curve in V . Physical parameters derived from analysis of the light curves in UBV ( RI )_C are presented.     

Modelling the chromosphere and transition region of ε Eri (K2 V)

Measurements of ultraviolet line fluxes from Space Telescope Imaging Spectrograph and Far-Ultraviolet Spectroscopic Explorer spectra of the K2-dwarf ε Eri are reported. These are used to develop new emission measure distributions and semi-empirical atmospheric models for the chromosphere and lower transition region of the star. These models are the most detailed constructed to date for a main-sequence star other than the Sun. New ionization balance calculations, which account for the effect of finite density on dielectronic recombination rates, are presented for carbon, nitrogen, oxygen and silicon. The results of these calculations are significantly different from the standard Arnaud & Rothenflug ion balance, particularly for alkali-like ions. The new atmospheric models are used to place constraints on possible first ionization potential (FIP)-related abundance variations in the lower atmosphere and to discuss limitations of single-component models for the interpretation of certain optically thick line fluxes.     

3‐D hydrodynamic simulations of the solar chromosphere

We present first results of three‐dimensional numerical simulations of the non‐magnetic solar chromosphere, computed with the radiation hydrodynamics code CO⁵BOLD. Acoustic waves which are excited at the top of the convection zone propagate upwards into the chromosphere where the waves steepen into shocks. The interaction of the waves leads to the formation of complex structures which evolve on short time scales. Consequently, the model chromosphere is highly dynamical, inhomogeneous, and thermally bifurcated.     

The southern active-chromosphere star HD 6628

The star HD 6628, heretofore classified as a G5 subgiant, is shown to be a chromospherically active single-lined spectroscopic binary with a period of 27.332±0.008 d. From high-resolution spectra, the system is found to consist of a late F-type dwarf and an active G8–K1 bright subgiant, the latter having a rotation period of not more than 14.8±3.8 d derived from the width of metal lines. Further stellar and orbital parameters are derived and presented.     

The Na i D resonance lines in main-sequence late-type stars

We study the sodium D lines (D1: 5895.92 Å; D2: 5889.95 Å) in late-type dwarf stars. The stars have spectral types between F6 and M5.5 ( B − V between 0.457 and 1.807) and metallicity between  [Fe/H]=−0.82  and 0.6. We obtained medium-resolution echelle spectra using the 2.15-m telescope at the Argentinian observatory Complejo Astronómico El Leoncito (CASLEO). The observations have been performed periodically since 1999. The spectra were calibrated in wavelength and in flux. A definition of the pseudo-continuum level is found for all our observations. We also define a continuum level for calibration purposes. The equivalent width of the D lines is computed in detail for all our spectra and related to the colour index ( B − V ) of the stars. When possible, we perform a careful comparison with previous studies. Finally, we construct a spectral index  ( R '_D)  as the ratio between the flux in the D lines and the bolometric flux. We find that, once corrected for the photospheric contribution, this index can be used as a chromospheric activity indicator in stars with a high level of activity. Additionally, we find that combining some of our results, we obtain a method to calibrate in flux stars of unknown colour.