Les observations des nuages moléculaires diffus en absorption millimétrique

For sensitivity reasons, absorption spectroscopy was not possible at millimeter wavelengths until recently. While emission measurements need collisional excitation of the molecular levels above the cosmic background temperature, absorption measurements are able to detect molecules in low-density regions. However, large collecting area instruments are required. In diffuse clouds, the HCO⁺ ion is frequently seen, more easily than the CO molecule. At the photodissociation transition of CO, around N_H2 ~ 10²¹ cm⁻², the CO column density varies at a much higher rate than the HCO⁺ column density, for values of N_HCO⁺ ~ 10¹² cm⁻². The HCO⁺ to OH abundance ratio is remarkably constant (0.03 to 0.05). Other species such as H₂CO, HCN, HNC, CN, C₂H, C₂H₂, CS, SO, H₂S, can now also be studied in diffuse clouds.