Properties of molecules in space

Caution!! Although I am happy to share my research notes below with all visitors to my webpages, these pages are mainly designed for my own use and subject to change without warning. I do not guarantee the correctness of all contents as well.

This page summarize the relation of different molecular species in space with various astrophysical environments. Their typical properties such as abundances, excitation temperatures, isotopic ratios and so on are collected from literature or other sources to help us understand which species are good tracers of which astronomical objects or phenomena.

Table of Contents:

• Carbon hydrides: C2H,C3H2, CH3C2H,
• Cyanides and Nitrogen Species: NH3, CN, HCN, HNC,HC3N, CH3CN, CH3CH2CN,
• Sulfur species: CS,
• Molecular ions: HCO+,DCO+, N2H+, N2D+,
• Oxides and organic species: SiO, H2O, HDO, HCOOH, c-C2H4O, CH3CHO
• Other species: HNCO,

• --------------------------------- Carbon hydrides ----------------------------------
• C2H: a tracer of young cold core but destroyed in older cores. (Vasyunina et al., 2010arXiv1012.0961V)
• C3H2: It is a early-time chemical tracer of star formation, a dense gas tracer. (Frau et al., 2010ApJ...723.1665F, Caselli & Myers 1995ApJ...446..665C)
• CH3C2H: a good tracer of dense gas in early stages of star formation and a good thermometer. (Vasyunina et al., 2010arXiv1012.0961V)
• --------------------------------- Cyanides and Nitrogen Species ----------------------------------
• NH3: dense cold gas tracer. (Caselli & Myers 1995ApJ...446..665C)
• CN: It is a early-time chemical tracer of star formation. (Frau et al., 2010ApJ...723.1665F)
• HCN
  • H¹²CN: a ubiquitous dense gas tracer, also trace infall motion in low-mass cores, but NOT in massive cores, because of high level of turbulence and the occurence of outflows. (Vasyunina et al., 2010arXiv1012.0961V) It is a early-time chemical tracer of star formation. (Frau et al., 2010ApJ...723.1665F)
  • H¹³CN: the same as HCN.
• HNC: a tracer of dense gas. The HCN/HNC ratio is a sensitive probe of cloud conditions (~1 in cold clouds, and varies from 5 to 80 in OMC from its edge to core) (Vasyunina et al., 2010arXiv1012.0961V)
• CH3CN: a tracer of warm and dense gas, its asymmetri top line series are a good thermometer. (partly from Vasyunina et al., 2010arXiv1012.0961V)
• HC3N: a valuable tracer of physical conditions of molecular clouds, can be found in clouds of all sizescales. It is usually optically thin and trace very dense gas. (Vasyunina et al., 2010arXiv1012.0961V)
• CH3CH2CN: a tracer of hot cores. (Mehringer et al., 2004ApJ...608..306M) Produced on icy grain surface and released by grain mantle evaporation.
• --------------------------------- Sulfur species ----------------------------------
• CS: a tracer of dense gas (due to its high dipole moment), it's depleted toward the core of cold dark clouds. (Vasyunina et al., 2010arXiv1012.0961V). It is a early-time chemical tracer of star formation. (Frau et al., 2010ApJ...723.1665F)
• --------------------------------- Molecular Ions ----------------------------------
• HCO+
  • H¹²CO+: a good tracer of dense gas. It is an abundant species. It abundance is enhanced in regions with higher fraction of ionization and shock regions of outflows. (Vasyunina et al., 2010arXiv1012.0961V). It is a late-time chemical tracer of star formation regions. (Frau et al., 2010ApJ...723.1665F)
  • H¹³CO+: the same as H¹²CO+, but is usually optically thin. Comparison of the line profile of the two isotopic species can reveal bulk motion in clouds.
• DCO+: It is a late-time chemical tracer of star formation regions. (Frau et al., 2010ApJ...723.1665F)
• N2H+: a selective tracer of quiescent gas (such as cold star forming cores), it is depleted later than most other species at low temperatures. Its hyperfine structure lines can be used to determine its optical depth. (Vasyunina et al., 2010arXiv1012.0961V) It is a late-time chemical tracer of star formation regions. (Frau et al., 2010ApJ...723.1665F) and a dense gas tracer (Caselli & Myers 1995ApJ...446..665C)
• N2D+: It is a late-time chemical tracer of star formation regions. (Frau et al., 2010ApJ...723.1665F)
• --------------------------------- Oxides ----------------------------------
• SiO: a tracer of active shocks. (Vasyunina et al., 2010arXiv1012.0961V)
• H2O
  • H₂¹⁸O: a tracer of hot core or outflows. (Marseille et al., 2010A&A...522A..40M)
• HDO: (Marseille et al., 2010A&A...522A..40M)
• HCOOH: formic acid, a cold gas tracer (Ikeda et al., 2001ApJ...560..792I)
• c-C2H4O: cyclic-ethylene oxide, a cold gas tracer (Ikeda et al., 2001ApJ...560..792I)
• CH3CHO: iso-acetaldehyde, a cold gas tracer (Ikeda et al., 2001ApJ...560..792I)
• --------------------------------- Other species ----------------------------------
• HNCO: could be a tracer of shock chemistry, because its strength correlates with that of SiO.