A Survey of Chromospheric Activity
in the Solar-Type Stars in M67

Mark S. Giampapa

National Optical Astronomy Observatories, National Solar Observatory
950 N. Cherry Ave, POB 26732, Tucson, AZ 85726-6732
The NOAO is operated for the National Science Foundation by the Association of Universities for Research in Astronomy

We discuss our program to measure the level of chromospheric activity in the solar-type stars in the galactic cluster M67. We describe the observational approach and the data reduction methods adopted, and the selection criteria for the stellar sample.

1. Introduction

We have initiated a survey to measure the level of chromospheric Ca II H&K emission in ~100 solar analogs in the galactic cluster M67. The primary objective of this program is to begin a long-term investigation of the nature of solar variability. M67 is an especially appropriate target of observation since it is approximately the same age (about 5 Gyr +/- 1 Gyr; Demarque et al. 1992) and of the same metallicity as the Sun (Barry & Cromwell 1974). Our working hypothesis is that a single `snap-shot' of a large sample of solar analogs will reveal the potential range of solar chromospheric activity. In this way, we immediately obtain information on the potential long-term variability of the Sun that would not otherwise be possible (or practically feasible) with the modern solar Ca II data-base of only two decades (e.g., White & Livingston 1978; Livingston 1994). This is especially important given that the amplitude of long-term, solar and stellar variations in brightness are correlated with cycle variations in chromospheric emission (Hudson 1988; Radick 1991). In view of the fact that the Sun is the engine that drives climate on the Earth, any variation in the solar `constant' must be taken into account in the investigation of the long-term behavior of the global climate.

The final results of this survey are expected to reveal the range of Ca II H+K emission observed in the sun-like stars in M67. We will interpret this range as indicative of the potential cycle-related variability that can occur in solar-type stars and the Sun. For example, is the range in chromospheric activity among the `Suns' in M67 similar to the 11% excursion in Ca II emission levels seen thus far between the minimum and the maximum in the solar cycle? Those stars that exhibit Ca II emission levels that are less than that seen at solar minimum will confirm, as suggested by Baliunas & Jastrow (1990), the occurrence of exceptionally quiescent phases of magnetic activity analogous to the so-called `Maunder-minimum' episode of the Sun during A. D. 1645 - 1715 when visible manifestations of solar activity vanished. This period corresponds to a time of reduced average global temperatures on the earth known as the "Little Ice Age" (Foukal & Lean 1990).

An additional objective of the survey is to determine if the solar-age stars in M67 conform to the proposed empirical relationships between age and activity (that have been inferred mainly from young clusters of known ages), and the extent to which activity cycle-related variability among stars of the same age contribute to the scatter in age-activity calibrations.