A List of Emission Lines in Capella

Detected with FUSE

B. E. Wood and T. B. Ake

We present here a list of strong, relatively unblended emission lines in Capella (α Aurigae; G8 III + G1 III) as discussed by Young et al. (2001) in the paper "FUSE Observations of Capella". Although dominated by the resonance lines of C III and O VI, the spectrum exhibits a wealth of weaker lines from the chomospheric, transition, and coronal regions of the stellar components. Identifications are based on the solar spectrum (Feldman & Doscheck 1991; Curdt et al. 1997) and predictions using CHIANTI (Dere et al. 1997) using the emission measure distribution and densities from UV and EUV observations of Capella (Brickhouse et al. 2000). Our line list should be useful for those working with FUSE spectra of cool stars.

One complication with the Capella spectrum is that the high density and intrinsic width of the lines leads to substantial blending. In many cases it is not easy to determine which lines are dominating the emission features and which are either minor contributors to a blend or not present at all. In addition, because of the low satellite orbit, FUSE spectra are contaminated with airglow or geocoronal emission lines (Feldman et al. 2001 ), particularly in the LWRS. We use two methods to identify where airglow contamination is significant in the data. One method is to separate the time-tagged FUSE spectra into ''day'' and ''night'' components, as Ake et al. (2000) did in their analysis of the AB Dor spectrum. Geocoronal emission is not as strong when FUSE is in the nighttime part of its orbit, so the ''night'' spectrum will show reduced fluxes where airglow is present. Secondly, we have obtained an observation taken through the MDRS aperture where, thanks to its smaller width, there is less airglow emission than in the LWRS spectrum. By comparing the combined day and night LWRS data with both the nighttime-only spectrum and the MDRS spectrum, we can identify regions of the Capella spectrum that are contaminated with airglow emission.

The table below lists the strong, relatively unblended lines that we believe can be identified unambiguously with known atomic transitions. Blends of lines from the same ion are generally included in this category only if the number of lines is three or less, with the strong C III λ1176 multiplet being an exception. Therefore the S I blend near 1162 Å is included, but the highly blended C I features between 1155 Å and 1160 Å are not included, for example. In the interest of brevity, we list only the emitting atom or ion and the rest wavelength of each line. We refer the reader to the Feldman & Doschek (1991) and Curdt et al. (1997) papers for information on the atomic transitions responsible for these lines. In some cases the rest wavelengths were changed to match wavelengths listed in Morton (1991). Lines contaminated by airglow are noted. Many of the O I lines are partially or entirely airglow emission, as are the N I lines near 1135 Å and the second order He I λ584 emission at 1169 Å. Airglow also severely contaminates the H I Ly β line at 1025 Å. Note too that solar scattered light, particularly in C III λ977 and O VI λ1032, has been known to appear in the SiC channels. Users are warned to exercise caution when analyzing these features.

Ake, T. B., et al. 2000, ApJ, 538, L87
Brickhouse, N. S., et al. 2000, ApJ, 530, 387
Curdt, W., et al.  1997, A&AS, 126, 281
Dere, K. P., et al., 1997, A&AS, 125, 149
Feldman, P. D., et al. 2001, J. Geophys. Res., in press
Feldman, U., & Doschek, G. A. 1991, ApJS, 75, 925
Morton, D. C. 1991, ApJS, 77, 119
Young, P. R., et al. 2001, ApJL, submitted

A Capella Line List: Relatively Unblended, Positively Detected Lines

Ion  λrest  (Å)  Ion  λrest  (Å)  Ion  λrest  (Å)  Ion  λrest  (Å)  Ion  λrest  (Å) 

N IV 924.283  H I 972.537  Si III   997.389  O I 1041.688a N I 1134.165a
H I 926.226  O I 973.885a C II 1009.859  S IV 1062.662  N I 1134.415a
O I 929.517a Fe XVIII 974.86  C II 1010.083  S IV 1072.974  N I 1134.980a
H I 930.748  C III 977.020  C II 1010.372  S IV 1073.516  Ne V 1136.51 
S VI 933.378  O I 978.617  S III 1012.502  S III 1077.13  Fe III 1142.956 
O I 936.630a N III 979.842  S III 1015.574  N II 1083.990  Ne V 1145.61 
H I 937.804  N III 979.919  S III 1015.784  N II 1084.562  O I 1152.151 
He II 942.490  Fe III 981.373  Fe III 1017.254  N II 1084.580  S I 1161.35 
He II 942.538  Fe III 983.860  Fe III 1017.746  He II 1084.914  S I 1161.57 
S VI 944.523  Fe III 985.824  Fe III 1018.286  N II 1085.529  S I 1161.72 
C II 945.977  O I 988.578a S III 1021.115  N II 1085.546  He I 1168.66/2a,b
C II 946.199  O I 988.655a S III 1021.328  N II 1085.701  C III 1174.934 
O I 948.686a O I 988.773a H I 1025.722a Si III 1108.368  C III 1175.263 
H I 949.743  N III 989.799  O I 1027.431a Si III 1109.966  C III 1175.591 
O I 950.733a O I 990.127a O I 1028.157a Si III 1113.228  C III 1175.711 
O I 950.885a O I 990.204a O VI 1031.925  Fe III 1122.526  C III 1175.987 
He II 958.675 O I 990.801a C II 1036.337  Fe III 1124.881  C III 1176.371 
He II 958.725  N III 991.511  C II 1037.018  Fe III 1126.729     
Fe III 967.197  N III 991.577  O VI 1037.614  Fe III 1128.050     
Fe III 968.955  He II 992.338  O I 1039.230a Fe III 1128.724     
O I 971.738a S II 996.00  O I 1040.943a Fe III 1129.191     

aPartially or entirely airglow emission.
bSecond order line.

The figure at left is the LWRS FUSE spectrum of Capella derived from observation P1041301 as found in Young et al. (2001). A full size version can be obtained here either as a PostScript or PDF file. The quiet solar spectrum shown below in each panel was obtained by the SUMER instrument on SOHO (Curdt et al. 1997; W. Curdt, private communication). The shaded regions above each spectrum denote areas contaminated by airglow lines. The Capella C III λλ, and O VI λλ lines, reduced by a factor 18, are plotted as shaded profiles.