Chromospherically active stars. XX. The giant single-lined binary HD 161570

Spectroscopy of HD 161570 shows it to be a single-lined spectroscopic binary with a period of 45.623 days and a nearly circular orbit. The primary star has spectral type G7 III, and the secondary is most likely a G or K dwarf. A high-resolution spectrum of the 3950 Angstrom region confirms that the primary has AZ Ca II H and K emission lines. The logarithm of the giant's lithium abundance is less than 0.9, indicating that it is not lithium-rich. From photometric observations covering six seasons, we detected periodic short-term light variability with an amplitude ranging from 0.02 to 0.04 mag. We interpret these shortterm variations as due to stellar rotation and find that the rotation period varied somewhat from season to season, with a representative value of 64.1 days. Thus, the very different orbital and rotation periods make HD 161570 an asynchronous rotator. Low-amplitude, long-term photometric variability was also seen. The giant's weak Ca II H and K emission, low-amplitude photometric variations, and relatively low v sin i of 7.0 km s(-1) indicate that this star is only modestly chromospherically active. This result, plus the spectral type and asynchronous rotation of the primary, suggest that it is just beginning to ascend the red giant branch.