THE ASTRONOMICAL JOURNAL    VOLUME 109, NUMBER 4, PAGE 1797          APRIL 1995

                     H-ALPHA MEASUREMENTS FOR COOL GIANTS

                                 JOEL A. EATON

   Center of Excellence in Information Systems, Tennessee State University,
      Suite 265G, 330 Tenth Avenue North, Nashville, Tennessee 37203-3401
                    Electronic mail: eaton@coe.tnstate.edu


                                   ABSTRACT

    The H-alpha line in a cool star is usually an indication of the
    conditions in its chromosphere. I have collected H-alpha spectra of
    many northern G-M stars, which show how the strength and shape of the
    H-alpha line change with spectral type.  These observations detect
    surprisingly little variation in absorption-line depth (Rc ~ 0.23 +/-
    0.08), linewidth (FWHD ~ 1.44 +/- 0.22 A), or equivalent width (EW ~
    1.12 +/- 0.17 A) among G5-M5 III giants.  Lines in the more luminous
    stars tend to be broader and stronger by 30%-40% than in the Class III
    giants, while the H-alpha absorption tends to weaken among the cooler M
    giants.  Velocities of H-alpha and nearby photospheric lines are the
    same to within 1.4 +/- 4.4 km/s for the whole group.  To interpret
    these observations, I have calculated H-alpha profiles, Ly-alpha
    strengths and C II] strengths for a series of model chromospheres
    representing a cool giant star like Alpha Tau.  Results are sensitive
    to the mass of the chromosphere, to chromospheric temperature, to
    clumping of the gas, and to the assumed physics of line formation.  The
    ubiquitous nature of H-alpha in cool giants and the great depth of
    observed lines argue that chromospheres of giants cover their stellar
    disks uniformly and are homogeneous on a large scale.  This is quite
    different from conditions on a small scale:  To obtain a high enough
    electron density with the theoretical models, both to explain the
    excitation of hydrogen and possibly also to give the observed C II]
    multiplet ratios, the gas is probably clumped.  The 6540-6580 A spectra
    of 240 stars are plotted in an Appendix, which identifies the date of
    observation and marks positions of strong telluric lines on each
    spectrum.  Section 2 assesses the effects of telluric lines and
    estimates that the strength of scattered light is ~5% of the continuum
    in these spectra.  Table 1 gives the measurements of H-alpha as well as
    equivalent widths of two prominent photospheric lines, Fe I lambda 6546
    and Ca I lambda 6572, which strengthen with advancing spectral type.