Absorption effects in the spectra of radio stars produced by interstellar hydrogen.

Abstract

view Abstract Citations (3) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Absorption effects in the spectra of radio stars produced by interstellar hydrogen. Lilley, A. E. Abstract Observations show that 21 cm hydrogen line profiles for directions which contain radio stars are very unusual in shape and amplitude. When one obtains a 21 cm profile for the exact direction which contains a radio star, the profile is considerably distorted compared to the profiles obtained for directions immediately adjacent to the radio star. The effect is due to the absorption of continuum radiation from the radio star by interstellar hydrogen which lies between the observer and the radio source. If the radio star lies in the center of the antenna reception beam, the absorption is effective only over the small cone subtended by the radio star, whereas the emission contribution originates over the entire cone subtended by the antenna reception beam. With the 50 foot NRL antenna and radio stars such as Cassiopeia A and Taurus A, the cone subtended by the radio star is quite small compared to the reception cone of the antenna. This provides a unique increase in the effective angular resolution in the absorption observations. By obtaining hydrogen line profiles for selected positions very close to the radio star, one can readily determine the "expected" 21 cm profile for the direction of the radio star. The expected profile is the profile that would have been obtained in the absence of the radio star, and is taken as the mean of the profiles obtained from the adjacent comparison positions. The comparison profiles show only minor differences and their mean is an acceptable measure of the "expected" profile. If we call this expected profile AT(v), then it can be shown that the profile obtained when the radio star is in the antenna beam is given by: AT'(v) = AT(v) - TA(i - e-rl(P)) - ETk(i - e-r2(c))e-rl(c)(i - e7c)J~ where TA is the apparent antenna temperature of the radio star, r,(v) and r,(v) are Hi optical depths within the small cone subtended by the radio star, T,(v) the optical depth to the radio star and r2 (P) the optical depth from the radio star to the most distant limit of Hi. Tk is the temperature which defines the population distribution over the hydrogen hyperfine levels, rs is the continuum optical depth of the radio star, and ~s/~B is the ratio of the solid angles subtended by the radio star and the antenna beam. For an antenna only 50 feet in diameter, ~s/~B is of the order of 10-2 or less and the last term in the equation above may be safely neglected. This enables one to determine the approximate Hi optical depth from the observer to the radio star along the small cone subtended by the radio star: r,(v) -ln (i AT(v) fA AT'(v) The observations obtained for Cassiopeia A (Hagen, Lilley and McClain, 1954) show three sharp maxima in the Hi optical depth suggesting Hi cloud structure. One peak is near the local standard of rest in radial velocity, and two peaks occupy the radial velocity range of the second spiral arm near galactic longitude 800. Van de Hulst, Muller and Oort (1954) place this second arm at a distance of approximately 3 kiloparsecs, which becomes the minimum distance to the Cassiopeia radio star. If a different model of galactic rotation were assumed, the computed distance to the second arm would be adjusted, but the minimum distance of the radio source remains that of the second arm, whatever its true distance. It is interesting that for radio stars and interstellar hydrogen clouds, the effect is very similar to optical interstellar absorption lines in stellar spectra. Hagen, Lilley and McClain I954, Naval Research Laboratory Report No. 4448, Washington, D. C. Van de Hulst, Muller and Oort I954, B. A. N. 12, "7. Naval Research Laboratory, Washington, D. C. Publication: The Astronomical Journal Pub Date: 1955 DOI: 10.1086/107154 Bibcode: 1955AJ.....60R.167L full text sources ADS |