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Stellar Observations with UVCS

 

Any star within of the ecliptic passes within 10 of the sun for about 5 days once per year. There are 10-20 UV bright stars which pass close enough to the sun to be observed and which have been observed with Voyager and IUE. A star will move across the slit or a resolution element in the spatial direction in about 5 minutes. If the pointing can be accurately enough predicted, we can let a star move along the length of the slit to build up counts and evaluate the detector uniformity.

It may be possible to measure the O VI absorption profile in the solar wind. The O VI absorption of the solar wind will be weak, but if measurable it provides a way to see O VI after Doppler dimming has made the scattered intensity too small to detect. An estimate for a streamer is (with very large uncertainty) at 1.5 , so high signal-to-noise will be needed.

An example of a possible target is Sco, which passes 1.7 from sun center on Nov. 27. Its brightness is about 36 at 1032 , or about 36 counts per pixel during the 300 seconds the star takes to cross a resolution element (for the occulter position at 1.7 ). By allowing the star to slide along the slit, one acquires about 4000 counts per wavelength pixel, which makes implies a detection limit of about . Unfortunately, a star moves along the length of the slit only in passing more or less over the solar pole, where the O VI optical depth will be smaller. This observation will also challenge the flat field accuracy. Observations farther from sun center will acquire more counts, but the predicted optical depth drops faster than 1/r. The numbers in the tables pertain to Sco.

It will also be possible to observe a few stars, such as V471 Tau, for their own sake. This is probably only worthwhile for stars detectable in the O VI channel, as IUE or HST have larger collecting areas than UVCS at 1200 . One might observe the O VI emission profile of a solar-type star, but a rough search in the Bright Star Catalogue did not turn up any very bright candidates. Most of the hundreds of stars in the Bright Star Catalogue within of the ecliptic are A stars (including chemically peculiar and magnetic stars) or red giants. The main usefulness of UVCS stellar observations may be interstellar absorption line studies on stars such as Sco, because dozens of strong lines of ions such as O VI and molecules such as and CO lie below the wavelength range accessible to HST.

Stellar Observations with UVCS

** For this example, the slit is placed at 5 at a roll angle of from ecliptic north, and the star drifts across the middle of the slit in about 6 minutes. The start time for this exposure must be specified carefully.



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Next: UV Observations of Up: Examples of UVCS Previous: Coordinated Observations with



Peter Smith
Fri Jan 17 12:11:15 EST 1997