Spectroscopy of the Chromospheric Activity of Cool Stars in Young Clusters

    Main sequence stars with convective envelopes show evidence of activity  similar to that seen in the
Solar chromosphere. The phenomenon has been  well known since pioneering the work of Olin Wilson
(1978). It continues to be monitored in a few score stars by a specialized instrument  of the SAO on the
Mt. Wilson 60-inch telescope. The activity is monitored by measuring the flux in the emission core of
the H- and K-lines of singly ionized Calcium. As in the Sun, the activity manifests itself on a variety of
time scales by variations in this flux. Individual flare events cause substantial increases in the flux on
scales of minutes to hours.  The activity tends to be concentrated in active regions of limited longitudinal
extent and the flux is modulated by the stellar rotation on scales of days to weeks. The active regions
form, grow then decay on time scales of weeks to months. The overall level of activity is further
modulated by long term cycles like the Sun Spot cycle on scales of years to decades. An understanding
of the influence of such stellar parameters as mass, age, chemical composition and rotation rate will
greatly aid in understanding and testing models of the magnetic dynamo mechanism which is thought to
be the root cause.  Furthermore chromospheric activity in the pre-main sequence phase may well play an
important role in the establishment of the final main sequence rotation rate spectrum for stars which will
ultimately have radiative envelopes. But  dependence of  the effect on a variety of stellar  parameters is
mapped out very poorly and a large statistical base is not  available. 
   The result of the initial survey  is that the level of chromospheric activity in NGC2264 is much
lower than anticipated, more than half of the stars in the sample do NOT show the Ca II emission
features.  Linsky (private communication) has  remarked on how unusual this seems to be.  However,
in order to determine whether this is common among stars of this sort, it has been suggested that a
sampling of other young clusters be accomplished.  There have been surveys of chromospheric activity
in other young clusters.  Stauffer et al. (1997) and  Reid et al. (1995) studied the M-dwarfs in the
Hyades while Hodgkins et al. (1995) observed similar stars in the Pleiades.  These studies utilized Hà
equivalent width data in conjunction with ROSAT X-ray fluxes and focused on the later K and M type
stars which show X-ray emission and strong Hà emission. They do not thus provide the needed
comparison. Only Beasley and Cram (1993), who used the AAT MOS for a study of the CaII H & K
lines in two southern clusters, is somewhat analogous.  Their data is of much lower resolution and
signal-to-noise that we have achieved with the WIYN/Hydra/MOS and is thus of little use for
comparison with our NGC2264 data.  The G, K and early M stars in the  Pleiades are of visual
magnitudes similar to those of the NGC2264 sample and provide excellent examples of more nearly
Solar stars at a different, very much younger age than does the field. The Pleiades was observed in the
fall of 1997 and indeed most of the stars at B-V > 0.5 had moderate to strong H&K emission.  
   Another cluster with an age of about 10 times that of the Pleiades is Praesepe, M44, NGC 2632. 
It's extent and the brightness of its G, K and M stars are similar to the Pleiades.  The common wisdom
would be that its stars should be somewhat reduced in H&K emission strength relative to the Pleiades
and we propose to check this expectation.  For the 1998B semester we would propose to  observe of
NGC 2632 at (RA,Dec)=(8:37,+19:52) in the last third  of the nights.  If this goes as well as did the
Pleiades in 1997, we might also be able to revisit either NGC2264 or the Pleiades for a variability check. 
   The first two thirds of each night are being requested for our Solar System programs.   

References

Beasley, A.J.  and Cram, L.E.  1993, Ap.J.,418. 157.
Hodgkins , S.T., Jamesen, R.F.,& Steele, I.A. 1995, MNRAS, 274, 869.
Reid,N.,, Hawley, S.L. & Mateo, M. 1995, MNRAS 272, 828.
Stauffer, J.R., Balachandran, S.C., Krishnamurthi, A., Pisonneault, M., Terndrup, D.M. & Stern, R.A.
1997, Ap.J. 475, 604. 
Wilson O.C. 1978, ApJ, 226, 379