HST Observations of the Chromosphere of a Carbon Star

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Session 5 -- Chromospheres/Coronae/Active Binaries
Display presentation, Monday, 30, 1994, 9:20-6:30

[5.06] HST Observations of the Chromosphere of a Carbon Star

L. M. Ensman, H. R. Johnson (Indiana Univ.), K. G. Carpenter (LASP at NASA/GSFC), R. D. Robinson, D. Luttermoser (CSCN at NASA/GSFC)

Ultraviolet spectra from the first Hubble Space Telescope observations of a carbon star are presented, as well as line identifications and an initial analysis of the velocity structure of the emitting region.

Two spectra of UU Aur (HD 46687, type N3; C5,3) have been obtained. In the lower-resolution FOS spectrum (2310 -- 3275\AA), UV emission lines of Mg~I, Mg~II, and Fe~II, fluoresced lines of Fe~I and II, semi-forbidden emission from C~II, Si~II, and Al~II, absorption lines of Fe~I and Ti~II, and absorption by CH and CaCl have been identified. In the high resolution GHRS spectrum, which covers a 50\AA\ region around the Mg~II h and k emission lines, overlying Mg~II and Mn~I absorption and two fluoresced Fe~I lines have been resolved. The Mg~II, and perhaps the Ti~II, absorption appears to be interstellar.

UV emission lines of predominantly singly ionized metals indicate the existence of a chromosphere at $\sim 5000$ K. In UU Aur, the Mg II emission clearly arises in material which is flowing away from the star at 35--60 km/s. Shifts of the Fe~II emission lines support the conclusion that the chromosphere is expanding at highly supersonic velocities. However, overlying Mn I absorption must be due to stationary material further out. The forbidden and fluoresced lines also come from a region(s) with negligible velocity. These data imply that shocks may form in the atmospheres of carbon stars and thus be a mechanism for chromospheric heating and mass loss. In fact, previous semi-empirical chromospheric models of the carbon star TX Psc (Luttermoser, Johnson, Avrett, and Loeser 1989) indicate a similar shock-like velocity field. \vskip20pt Luttermoser, D., Johnson, H. R., Avrett, G., and Loeser, R. 1989, ApJ 345, 543.

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