COlijnen

COlijnen are spectral lines produced by rotational transitions in carbon monoxide (CO) molecules. They appear primarily in the millimeter and submillimeter part of the spectrum, with the most prominent transition being the J=1→0 line at 115.271 GHz. In astronomy, CO is the most common tracer of molecular gas because molecular hydrogen (H2), the most abundant molecule, has no permanent dipole moment and is difficult to detect directly under cold interstellar conditions.

CO emission originates in molecular clouds, protoplanetary disks, and circumstellar envelopes, and is routinely observed with radio and millimeter telescopes such as ALMA, NOEMA, and the JCMT. Observations commonly target different isotopologues, notably 12CO, 13CO, and C18O. The 12CO line is usually optically thick and traces the overall distribution and kinematics of molecular gas, while 13CO and C18O, being less abundant, trace denser or deeper layers and help estimate column densities.

The strength of a CO line depends on gas temperature and density, through excitation conditions and radiative transfer. The J=1→0 line has a relatively low critical density (a few hundred cm⁻³) but higher-J transitions (2→1, 3→2) require progressively denser or warmer gas. Line profiles yield velocity information via Doppler shifts, enabling maps of galactic rotation, turbulence, and inflows or outflows.

Applications include estimating molecular gas masses using the CO-to-H2 conversion factor X_CO, studying star formation, and mapping large-scale gas distributions in galaxies. Limitations include variations of X_CO with metallicity and environment, optical depth effects, and potential CO-dark H2 gas in some regions.