Time Variability of the Ca II K Emission Line of Giants

Abstract

ABSTRACTThis paper reports the results of a program to search for evidence of time variability in the profile of the central emission component of the Ca ii K line of a sample of Population I field giant stars. High resolution spectra of this line have been obtained using echelle and coudé spectrographs on the 82-inch (2.1-m) and 107-inch (2.7-m) telescopes of McDonald Observatory. Data are drawn from previous papers of Smith & Shetrone and combined with new observations obtained during the period 2004–2007 to give multiple observations for a sample of 31 evolved giant stars extending over the period 1998–2007. The asymmetry of the Ca ii K2 emission profile has been quantified by the parameter V/R, which is defined to be the ratio of the intensity in the violet emission peak to the intensity in the red emission peak. Among the giants in our sample with 0.85 < (B - V) < 1.25 the individual V/R values fall mainly in the range 0.97 < (V/R) < 1.27, while for those giants with (B - V) > 1.3 this range broadens to 0.7 < (V/R) < 1.27. A number of the giants in our sample are found to have temporal variations in V/R. The range observed in this parameter for any given star is Δ(V/R) < 0.26, with the largest variations being around the 3.5 σ level in our data. Evidence for variability in V/R is found among both giants with (B - V) < 1.2 and (B - V) > 1.3. By contrast, some of the giants in our sample showed no evidence of variability in their emission asymmetry. Most of the giants in our sample that show V/R < 1.0 on at least one occasion have colors of (B - V) > 1.3. Nonetheless, some of the stars of such color do show blue-dominant asymmetry. Furthermore, some giants of this color are clearly variable in their emission asymmetry and are seen to change from blue-dominant emission profiles (V/R > 1.0) to red-dominant (V/R < 1.0) or vice versa. Thus, in terms of the behavior of V/R as a function of position in the color-magnitude diagram, there is a transition in a statistical sense from mainly blue-dominant asymmetry at (B - V) < 1.3 to red-dominant at around (B - V) ∼ 1.4. However, within the color range 1.3 < (B - V) < 1.4, a wide range of asymmetries occurs, as found originally by Stencel, and time variability appears to be one explanation for both blue and red-dominant asymmetries occurring in this domain. Upon dividing our sample according to metallicity, those giants with [Fe/H] < -0.2 evince a transition to red-dominant asymmetry (i.e., V/R < 1.0) at slightly bluer B - V color than giants with [Fe/H] > -0.1. Among giants with 0.8 < (B - V) < 1.2 the mean values of V/R found in our sample range from 1.05 to 1.20, whereas the individual V/R values at any given time shower a broader scatter from ∼0.97 to 1.27. Among such stars there is also evidence of time variability, although in most cases it does not include a transition to a red-dominant V/R value of <1.0.