Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1977
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1977apj...215l..83f&link_type=abstract
Astrophysical Journal, Part 2 - Letters to the Editor, vol. 215, July 15, 1977, p. L83-L86.
Astronomy and Astrophysics
Astrophysics
11
Abundance, Gas Density, Helium, Interstellar Gas, Ultraviolet Radiation, Apollo Soyuz Test Project, Hot Stars, Spaceborne Telescopes
Scientific paper
During the Apollo-Soyuz Test Project, an extreme-ultraviolet telescope observed the region of the sky from which the interstellar medium approaches the sun. The instrument had a tin filter whose bandpass included the 584-A line of neutral helium. The observations set an upper bound for the number density of neutral helium in the local interstellar medium of 0.004 + or - 0.0022 per cu cm, which is significantly lower than previously reported. The stated error is dominated by the present uncertainty in the solar 584-A flux, which is taken to be approximately 200 million photons/sq cm per sec at earth's orbit. The result is not consistent with previous resonant-scattering Ly-alpha observations of interstellar hydrogen passing through the solar system, which generally yield a hydrogen number density of about 0.1 per cu cm, unless interstellar helium is locally depleted below its cosmic abundance. The result is consistent with cosmic abundances if the much lower average hydrogen densities inferred from recent column-density measurements to nearby hot stars are in fact representative of the immediate solar neighborhood.
Bowyer Stuart
Freeman John J.
Lampton Michael
Margon Bruce
Paresce Francesco
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