HNCO, was among the very early molecules to be detected in space. The 4_0,4 – 3_0,3 transition at 3.4 mm was the first one to be detected in the Galactic center source Sgr B2(OH) by
L. E. Snyder and D. Buhl,
Interstellar Isocyanic Acid
Astrophys. J. 177, (1972), 619.
The 1_0,1 – 0_0,0 transition was detected subsequently by
D. Buhl, L. E. Snyder, and J. Edrich,
An Interstellar Emission Line from Isocyanic Acid at 1.4 Centimeters
Astrophys. J. 177, (1972), 625.

The molecule is also abundant in dark clouds, such as TMC-1, where the molecule was detected by
R. L. Brown,
Isocyanic Acid in the Taurus Molecular Cloud 1
Astrophys. J. 248, (1981), L119.

It was also found in three translucent clouds (CB 17 CB 24, and CB 228):
B. E. Turner, R. Terzieva, and E. Herbst,
The Physics and Chemistry of Small Translucent Molecular Clouds. XII. More Complex Species Explainable by Gas-Phase Processes
Astrophys. J. 518, 699–732 (1999).

F. Wyrowski, P. Schilke, and C. M. Walmsley
not only reported on
Vibrationally Excited HC₃N toward Hot Cores
Astron. Astrophys. 341, 882–895 (1999);
but also on the detection of vibrationally excited HNCO with the IRAM 30 m telescope in Gal 10.47+0.03, which is also known as W31C (or part of the same cloud complex). Several J = 5 – 4 and 7 – 6 transitions pertaining to the ground state and the three lowest excited vibrational states were detected near 110 and 154 GHz.

L. Velilla Prieto, C. Sánchez Contreras, J. Cernicharo, M. Agúndez, G. Quintana-Lacaci, J. Alcolea, V. Bujarrabal, F. Herpin, K. M. Menten, and F. Wyrowski
report on
New N-bearing species towards OH 231.8+4.2 — HNCO, HNCS, HC3N, and NO
Astron. Astrophys. 575, (2015), Art. No. A84.
The molecules were identified in the course of a line survey of the circumstellar envelope of this O-rich AGB star, aka QX Pup, with the IRAM 30 m dish at 3 to 1 mm.

J. L. Neill, E. A. Bergin, D. C. Lis, P. Schilke, N. R. Crockett, C. Favre, M. Emprechtinger, C. Comito, S.-L. Qin, D. E. Anderson, A. M. Burkhardt, J.-H. Chen, B. J. Harris, S. D. Lord, B. A. McGuire, T. D. McNeill, R. R. Monje, T. G. Phillips, A. L. Steber, T. Vasyunina, and S. Yu
reported on
Herschel Observations of Extraordinary Sources: Analysis of the Full Herschel/HIFI Molecular Line Survey of Sagittarius B2(N)
Astrophys. J. 789, Art. No. 8 (2014).
They observed b-type transitions of the ¹³C species with K_a = 1 − 0 between 899 and 902 GHz in absorption. HN¹³CO is difficult to detect because the C atom is very close to the center of mass, such that transitions of the ¹³C species are frequently overlapped by those of the ¹²C species.

A. Coutens, J. K. Jørgensen, M. H. D. van der Wiel, H. S. P. Müller, J. M. Lykke, P. Bjerkeli, T. L. Bourke, H. Calcutt, M. N. Drozdovskaya, C. Favre, E. C. Fayolle, R. T. Garrod, S. K. Jacobsen, N. F. W. Ligterink, K. I. Öberg, M. V. Persson, E. F. van Dishoeck, and S. F. Wampfler
reported on
The ALMA-PILS Survey: First Detections of Deuterated Formamide and Deuterated Isocyanic Acid in the Interstellar Medium
Astron. Astrophys. 590, Art. No. L6 (2016).
Carrying out a line survey toward IRAS 16293-2422 between 329 and 363 GHz, a deuteration degree of about one percent was derived for source B assuming a ¹²C/¹³C ratio of 68 because lines of the main isotopolog are frequently affected by optical depth effects.

J. Cernicharo, M. Agúndez, C. Cabezas, B. Tercero, R. Fuentetaja, N. Marcelino, and P. de Vicente
announced the
Discovery of thiofulminic acid with the QUIJOTE line survey: A study of the isomers of HNCS and HNCO in TMC-1
Astron. Astrophys. 682, (2024), Art. No. L4.
The authors detect H¹⁵NCO about a factor of 330 lower in abundance that the main isotopolog.

The detection of HCNO the second isomer, has been reported by
N. Marcelino, J. Cernicharo, B. Tercero, and E. Roueff,
Discovery of Fulminic Acid, HCNO, in Dark Clouds
Astrophys. J. 690, (2009), L27.
The IRAM 30 m telescope has been employed to detect the J = 4 – 3 and 5 – 4 transitions in B1 and toward the protostar position of L1527. The former transition was also detected toward the carbon chain peak of L1527, L1544, and L183. Interestingly, it was searched for, but not found toward the cyanopolyyne peak of the O-deficient dark cloud TMC-1. Moreover, it was not found in Orion KL. The HNCO isomer was found in all these sources.

Obviously, the detection of three transitions would have been more convincing. However, there are several reasons why the detection probably can be viewed as quite secure: the number of lines is quite low in dark clouds, in particular that of U-lines; the rotational temperatures in the two sources, in which both transitions have been detected, is identical within error bars with that of HNCO; the HNCO/HCNO ratio in the sources in which HCNO line(s) have been detected, fall in a fairly narrow range of about 20 – 60.

J. Cernicharo, M. Agúndez, C. Cabezas, B. Tercero, R. Fuentetaja, N. Marcelino, and P. de Vicente
announced the
Discovery of thiofulminic acid with the QUIJOTE line survey: A study of the isomers of HNCS and HNCO in TMC-1
Astron. Astrophys. 682, (2024), Art. No. L4.
The authors detect DCNO about a factor of 25 lower in abundance that the main isotopolog.

HOCN was identified tentatively in two previously published line surveys of Sgr B2(OH) and in one unpublished survey of the same source:
S. Brünken, C. A. Gottlieb, M. C. McCarthy, and P. Thaddeus,
Laboratory Detection of HOCN and Tentative Identification in Sgr B2
Astrophys. J. 697, (2009), 880.
The transitions detected were the K_a = 0, J = 4 – 3 and 5 – 4; the 5 – 4 and 6 – 5; and the 7 – 6. The first two pairs of transitions have fairly similar intensities, the last, singly observed transition had not been calibrated. Even though the detection was deemed tentative, it is probably fairly secure.

More recently, the Sgr B2 region was studied more thoroughly by:
S. Brünken, A. Belloche, S. Martín, L. Verheyen, and K. M. Menten,
Interstellar HOCN in the Galactic Center Region
Astron. Astrophys. 516, (2010), Art. No. A109.
Data from a line survey of Sgr B2(M) and (N) were supplemented by dedicated searches toward several other positions in the Sgr B2 region. Up to four K_a = 0 transitions were detected with the IRAM 30 m telescope. The transitions had J“ = 3, 4, 6, and 7 and extend from 83 to 168 GHz. Interestingly, they authours found a nearly contant HNCO/HOCN ratio of around 200. Moreover, the molecules were less abundant toward the hot and dense regions.

N. Marcelino, M. Agúndez, J. Cernicharo, E. Roueff, and M. Tafalla
not only reported on the
Discovery of the Elusive Radical NCO and Confirmation of H₂NCO⁺ in Space
Astron. Astrophys. 612, Art. No. L10 (2018).
They also observed HOCN along with HCNO and HNCO toward the dense core L483 in the course of a 3 mm line survey carried out with the IRAM 30 m telescope.

Contributor(s): H. S. P. Müller; 02, 2009; 05, 2009; 05, 2010; 03, 2012; 06, 2016; 05, 2018; 09, 2019; 02, 2024