Jul 1984
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984iue..prop.1752a&link_type=abstract
IUE Proposal ID #MLGTA
Physics
Scientific paper
One of the surprising discoveries by IUE is that of redshifted emission lines of Si IV and C IV in the farultraviolet spectra of several late-type giants and supergiants. In the solar atmosphere, redshifts of high excitation emissions frequently are observed over magnetic active regions, and are thought to be associated with downflows of 10^5 K plasma in magnetic filaments. Nevertheless, the giants and supergiants that exhibit redshifted Si IV and C IV emissions differ substantially from dwarf stars like the Sun in terms of their fundamental stellar properties. It is possible, therefore, that the redshifts of the giants and supergiants have an entirely different origin than suggested by the solar analogy. For example, the appearance of a redshifted emission profile could result from an expanding, optically thick wind owing to the "P-Cygni" mechanism. We propose to continue, in the 7th year of IUE, the exploration of redshifts of high excitation emissions by addressing the question of whether the phenomenon in the supergiants is produced by an outflow of optically thick plasma (a "wind") or by a downflow of material (an "antiwind"). The test requires observations, using a precise radial velocity technique, of the intersystem lines of Si III (1892) and C III (1909) in the spectrum of the most luminous star, beta Draconis (G2 Ib-II), for which redshifts have been detected previously. In particular, the intersystem lines are optically thin, and therefore cannot exhibit a P-Cygni effect, whereas the Si IV and C IV doublets might be optically thick and therefore susceptible to the P-Cygni phenomenon. Furthermore, owing to different sensitivities to density, the C III] emission will be more heavily weighted towards a low-density, extended wind, while the Si III] emission will tend to be formed in the higher density structures from which the permitted lines of Si IV and C IV arise. Accordingly, the absolute velocity of Si III] 1892, and the difference in the velocities of 1892 and C III] 1909, can be used to test the wind and antiwind hypotheses. The highest precision in the measurement of the velocity differences can be achieved by taking a minimum of four separate spectra of the 1900A region of Beta Dra. The optimum exposures of the 1900 A region of Beta Ori are 7 hours, but the precise measurement of radial velocities requires that the satellite be thermally stable prior to the observations. The necessary stability can be attained, and maintained, only by coordinating US2, Vilspa, and US1 shifts on consecutive days. An important practical byproduct of the requirement for thermal stability is the opportunity to take Intensity Transfer Function calibrations in the LWR or LWP cameras during the long SWP exposures.
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