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
giampapa@noao.edu
Abstract
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.
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