Ambruster and collaborators A. Brown (U. CO) and F. Fekel (COE, TN
State U.) have been granted time with the GHRS on Hubble Space Telescope
for observations of all 6 solar-neighborhood, zero-age-main-sequence
(ZAMS) K dwarfs which they have identified over the last 7 years. These
stars all have spectral types between K0-K2 V, near-primordial lithium
abundances, and all are single. The only variable is rotation rate,
which varies between 8 hr and 6.9 d. The importance of the stars is that
the nearest cluster ZAMS stars, the early K dwarfs in the
Pleiades cluster, are too distant for spectroscopic observations of
higher temperature spectral lines with IUE, EUVE, or even HST. As a
result, the initial upper atmospheric temperature structure of a cool
star just beginning core hydrogen burning is not known. However,
chromospheric, transition region, and coronal temperature (and sometimes
density) diagnostics can be obtained for the 6 local stars (
d
pc) using IUE, EUVE, and now HST data, and will provide,
through Volume Emission Measure modelling, the upper atmospheric
structure as a function of rotation for ZAMS K dwarfs.
As of Sep 1995, IUE observations have been made of all 6 stars, and
EUVE observations of 4 (one star, HD 197890 ('Speedy Mic') is too faint
for EUVE and is very marginal with IUE). Simultaneous IUE, EUVE, and
HST observations of HD 1405 (P
d) were made in late
September 1995. HST observations of the remaining stars will take place
in Cycle 5.