D. Buhl and L. E. Snyder
detected an
Unidentified Interstellar Microwave Line
Nature 228, 267–269 (1970)
strongly in emission at 89190 ± 2 MHz toward the high-mass star-forming regions W3(OH), Orion, Sgr A NH₃A) and W51 and toward the dark cloud L134 using the 11 m NRAO telescope.

W. Klemperer
concomitantly suggested the
Carrier of the Interstellar 89.190 GHz Line
Nature 227, 1230 (1970)
to be the ground state transition of HCO⁺ based on structure considerations.

R. C. Woods, T. A. Dixon, R. J. Saykally, and P. G. Szanto
proved this suggestion by measuring the
Laboratory Microwave Spectrum of HCO⁺
Phys. Rev. Lett. 35, 1269–1272 (1975)
at the frequency derived from radio astronomical observations.

L. E. Snyder, J. M. Hollis, F. J. Lovas, and B. L. Ulich
reported slightly later on the
Detection, Identification, and Observations of Interstellar H¹³CO⁺
Astrophys. J. 209, 67–74 (1976)
at 86754 MHz, close to the predicted frequencies. The NRAO 11 m telecope was used, and the line was detected in several sources: DR21, DR21(OH), NGC 2264, NGC 7538, Rrion A, Sgr B2(OH), W3(OH), W49, W51, and W75N.

The Radio Detection of Interstellar DCO⁺
Astrophys. J. 209, L83–L85 (1976)
with the NRAO 11 m telecope toward DR21(OH), L134, and NGC 2264 was reported by
J. M. Hollis, L. E. Snyder, F. J. Lovas, and D. Buhl.

M. Guélin and P. Thaddeus
reported on the
Detection of HC¹⁸O⁺ in Sagittarius B2
Astrophys. J. 209, L139–L141 (1979)
again using the NRAO 11 m telecope.

M. Guélin, W. D. Langer, and R. W. Wilson
studied the distribution of 5 isotoplogs of HCO⁺ in several sources:
The State of Ionization in Dense Molecular Clouds
Astron. Astrophys. 107, 107–127 (1982).
The observations were performed with the Bell Lab. 7 m telescope. In the course of the investigations, D¹³CO⁺ was detected in TMC-1 and in L183.

M. Guélin, J. Cernicharo, and R. A. Linke
described the
Detection of HC¹⁷O⁺ in Sagittarius&nsp;B2
Astrophys. J. 263, L89–L93 (1982)
using the Bell Lab. 7 m telescope.

HCO⁺ was also detected in the diffuse medium toward the extragalactic contiunuum sources QSO B0212+735, $C 50.11 (NRAO150), and BL Lac by
R. Lucas and H. Liszt,
Plateau de Bure Observations of mm-wave Molecular Absorption from ¹³CO, HCO⁺, and HCN
Astron. Astrophys. 282, L5–L8 (1994).

L. M. Ziurys, E. D. Tenenbaum, R. L. Pulliam, N. J. Woolf, and S. N. Milam,
Carbon Chemistry in the Envelope of VY Canis Majoris: Implications for Oxygen-Rich Evolved Stars
Astrophys. J. 695, 1604–1613 (2009);
detected HCO⁺ in the circumstellar envelope of this star using the ARO 12 m dish as well as the ARO SMT.

R. L. Pulliam, J. L. Edwards, and L. M. Ziurys,
studied the
Circumstellar Ion-Molecule Chemistry: Observations of HCO⁺ in the Envelopes of O-rich Stars and IRC+10216
Astrophys. J. 743, Art. No. 36 (2011).
CW Leo, also known as IRC+10216, is a carbon-rich AGB star. Again, the ARO 12 m dish as well as the ARO SMT were employed.

J. R. Najita, J. S. Carr, S. E. Strom, D. M. Watson, I. Pascucci, D. Hollenbach, U. Gorti, and L. Keller
reported on
//Spitzer// Spectroscopy of the Transition Object TW Hya
Astrophys. J. 712, 274–286 (2010).
The observation of overlapping Q-branch transitions of the ν₂ band of HCO⁺ near 829.2 cm^–1 appears to be not only the first observation of this band in a young stellar object, but probably the first infrared observation of HCO⁺ in space.

N. S. Hakobian and R. M. Crutcher
detected the
First Interstellar HCO⁺ Maser
Astrophys. J. 712, 274–286 (2010)
through CARMA observations of its 1 – 0 transition in direction of the DR21(OH)-1 methanol maser region.

Contributor(s): H. S. P. Müller; 11, 2012