Isotope Chemistry

Deuterated species, i.e., chemical species with deuterium nuclei substituted in place of hydrogen nuclei, become abundant at low temperature (T ~ 10 K) and high density (nH ~ 105 cm-3) where otherwise common molecules such as CO and N2 freeze onto the surfaces of interstellar dust grains. This opens up new possibilities to trace the gas in cold and dense environments, such as in the centers of star-forming cores. Because both the proton and the deuteron (deuterium nucleus) have non-zero spin, species with multiple protons and/or deuterons can exist in different spin states. These spin states can be considered as distinct species at low temperature because radiative decay from one spin state to the other is forbidden by selection rules. Deuterium chemistry is fundamentally connected with spin-state chemistry through the H3+ + H2 reacting system. Therefore, simultaneous studies of deuterium fractionation and spin-state chemistry yield important information on the properties of star-forming material, and shed light on the initial conditions of star formation. The CAS deuterium chemistry model is used by several members of the CAS group in observational and theoretical studies. A recent research highlight pertains to constraining the chemical age of the envelope surrounding the prostellar system IRAS 16293-2422 A/B to approximately one million years (Harju et al. 2017), made possible by the combination of chemical and radiative transfer models and the simultaneous observation of both the ortho and para spin states of D2H+ (Figure 1), and the coupling to previous observations of H2D+ toward the same target (Brünken et al. 2014). This also marks the first detection of para-D2H+ in space.

The modeling effort carried out to explain the abundance distributions of H2D+ and D2H+ toward IRAS 16293-2422 A/B envelope led to the suggestion that rotational excitation could influence the abundances of these species in a measurable way in physical conditions relevant to star-forming cores. This is made possible by the fact that the lowest rotational levels of H2D+ and D2H+ are so close to each other that a significant degree of rotational excitation in these species can occur even below 20 K, depending on the medium density. Making use of the state-to-state rate coefficients for the H3+ + H2 reacting system calculated by Hugo et al. (2009), we constructed a new chemical model where rotational excitation is taken into account when determining the rate coefficients associated with the H3+ + H2 reacting system (Sipilä et al. 2017). We found that both deuterium-to-hydrogen abundance ratios and spin-state abundance ratios are modified in protostellar disk conditions, implying that the rate coefficient modifications should be taken into account when modeling the chemistry of H2D+ and D2H+, and other chemical species linked to these molecules, in protostellar disks. Figure 2 illustrates some of our results.

The isotopes of carbon (13C) and nitrogen (15N) are subject to increasing interest in the star-formation community, because tracing the chemical evolution of isotopes through the star formation process allows us to understand the isotopic abundance ratios observed in the Solar System, for example. 13C has received little attention on the theoretical side, despite its relatively high abundance in the ISM. Only a handful of models investigating carbon isotopes exist in the literature. A new model for the carbon isotope chemistry is in development at CAS since 2018 in collaboration with experts in Paris and Florence. The first publication on this project is currently in the writing stage. The new model – which will be extended to 15N in the future – greatly expands the number of tracer molecules that we can study, providing unique insight into the chemistry in star-forming regions.

Figure 1: Observed (blue) and modeled (red) spectra for for ortho and para H 2D + and D 2H + as presented in Harju et al. (2017). Lines without the effect of dust continuum are shown in green. The match between the observations and the model is very good, and constrains the chemical age of the envelope surrounding IRAS 16293-2422 A/B to ~10 6 yr.

Figure 2:Time-evolution of the H/D ratios of H3+ and its deuterated isotopologs at constant density n(H2) = 107 cm-3 for three different temperatures. The dashed and solid lines correspond to the new model where rotational excitation is taken into account, while the dotted lines show results from the old model where only the rotational ground state of each species is populated.

References

  • Brünken, S. et al. 2014, Nature 516, 219
  • Harju, J. et al. 2017, ApJ 840, 63
  • Hugo, E. et al. 2009, J. Chem. Phys. 130, 164302
  • Sipilä, O. et al. 2017, A&A 607, A26
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