CDROM/AJ/V109/P1797 Cool Giants (Eaton 1995) ================================================================================ H-Alpha Measurements for Cool Giants Joel A. Eaton <1995, AJ, 109, 1797> =1995AJ....109.1797E ================================================================================ 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. File Summary: -------------------------------------------------------------------------------- File Name Lrecl Records Explanations -------------------------------------------------------------------------------- table1.dat 90 254 Measurements of H-alpha in Red Giants table1.tex 101 395 TeX version of table1.tex -------------------------------------------------------------------------------- Byte-by-byte Description of file: table1.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 8 A8 --- Star Harvard-Revised (HR) star number 9 1X --- --- Blank 10-21 A12 --- Name Other name 22 1X --- --- Blank 23 A1 --- Note *[ab] Table note letter 24 1X --- --- Blank 25-36 A12 --- Sp Spectral type 37-42 F6.2 mag V []? V magnitude 43-48 F6.2 mag B-V []? B-V magnitude 49-55 F7.3 --- Rc []? Residual intensity of H-alpha line 56 1X --- --- Blank 57 A1 --- l_FWHD [<] FWHD limiting character 58-61 F4.2 --- FWHD []? Full-width of H-alpha at half-depth 62 1X --- --- Blank 63 A1 --- l_EW [<] EW limiting character 64-69 F6.3 --- EW []? Equivalent width of H-alpha 70-75 F6.1 --- dRV []? Diff. in radial velocity between H-alpha and strong metallic lines 76-82 F7.3 --- EW_FeI []? Equivalent width of Fe I lambda 6546 83 A1 --- n_EW_FeI *[T] Possible telluric line contamination 84-89 F6.3 --- EW_CaI []? Equivalent width of Ca I lambda 6572 90 A1 --- n_EW_CaI *[T] Possible telluric line contamination -------------------------------------------------------------------------------- Notes for file: table1.dat -------------------------------------------------------------------------------- Note: a. Observed with Kitt Peak coude feed telescope only b. from Kurucz et al. 1984, Solar Flux Atlas from 296 to 1300 nm n_EW_FeI, n_EW_CaI: A "T" indicates that the equivalent widths possibly are contaminated by telluric absorption lines. See the printed paper, section 2 for a discussion of the analysis for marking the equivalent widths as possibly contaminated. -------------------------------------------------------------------------------- ================================================================================ (End) Lee Brotzman [ADS] 04-May-1995