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H2D+ observations give an age of at least one million years for a cloud core forming Sun-like stars

Abstract

The age of dense interstellar cloud cores, where stars and planets form, is a crucial parameter in star formation and difficult to measure. Some models predict rapid collapse1,2, whereas others predict timescales of more than one million years (ref. 3). One possible approach to determining the age is through chemical changes as cloud contraction occurs, in particular through indirect measurements of the ratio of the two spin isomers (ortho/para) of molecular hydrogen, H2, which decreases monotonically with age4,5,6. This has been done for the dense cloud core L183, for which the deuterium fractionation of diazenylium (N2H+) was used as a chemical clock to infer7 that the core has contracted rapidly (on a timescale of less than 700,000 years). Among astronomically observable molecules, the spin isomers of the deuterated trihydrogen cation, ortho-H2D+ and para-H2D+, have the most direct chemical connections to H2 (refs 8, 9, 10, 11, 12) and their abundance ratio provides a chemical clock that is sensitive to greater cloud core ages. So far this ratio has not been determined because para-H2D+ is very difficult to observe. The detection of its rotational ground-state line has only now become possible thanks to accurate measurements of its transition frequency in the laboratory13, and recent progress in instrumentation technology14,15. Here we report observations of ortho- and para-H2D+ emission and absorption, respectively, from the dense cloud core hosting IRAS 16293-2422 A/B, a group of nascent solar-type stars (with ages of less than 100,000 years). Using the ortho/para ratio in conjunction with chemical models, we find that the dense core has been chemically processed for at least one million years. The apparent discrepancy with the earlier N2H+ work7 arises because that chemical clock turns off sooner than the H2D+ clock, but both results imply that star-forming dense cores have ages of about one million years, rather than 100,000 years.

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Figure 1: Observed and modelled H2D+ spectra.
Figure 2: Modelled ortho/para H2D+ abundance ratio.

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Acknowledgements

GREAT is a development by the MPI für Radioastronomie and the KOSMA/Universität zu Köln, in cooperation with the MPI für Sonnensystemforschung and the DLR Institut für Planetenforschung. SOFIA is jointly operated by the Universities Space Research Association, Inc. (USRA), under NASA contract NAS2-97001, and the Deutsches SOFIA Institut (DSI) under DLR contract 50 OK 0901 to the University of Stuttgart. APEX, the Atacama Pathfinder Experiment, is a collaboration between the Max Planck Institut für Radioastronomie (MPIfR), the Onsala Space Observatory (OSO), and the European Southern Observatory (ESO). This work has been supported by the Collaborative Research Centre 956, funded by the Deutsche Forschungsgemeinschaft (DFG). O.S. and J.H. acknowledge support from the Academy of Finland grants 132291 and 250741. P.C. acknowledges the financial support of the European Research Council (ERC; project PALs 320620).

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Authors and Affiliations

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Contributions

S.S., S.B., O.A., P.C., J.H., O.S. and J.S. jointly designed the study and proposed the SOFIA observations. E.T.C. performed the calibration and the analysis of the SOFIA data. C.E.H. was instrumental in developing the GREAT receiver. T.K. and K.M.M. made the APEX observations and analysed these data. O.S. carried out the chemistry and radiative transfer modelling with help from J.H. The paper was jointly written by S.B., J.H., O.S., P.C. and S.S. All authors discussed the results and commented on the manuscript.

Corresponding authors

Correspondence to Sandra Brünken or Stephan Schlemmer.

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Extended data figures and tables

Extended Data Figure 1 Temperature and density distribution of the source model.

Physical model of IRAS 16293-2422 A/B, consisting of a widely used core model22 and a low-density ambient cloud. a, The number density n(H2) as a function of radius. b, The radial profile of the kinetic temperature, T. The ambient cloud is assumed to have n(H2) = 104 cm−3 and T = 10 K. The shaded interval, between a radius of 3,000 and 6,100 au, represents the outer envelope of the core, which dominates the observed para-H2D+ absorption and ortho-H2D+ emission.

Extended Data Figure 2 The relationship between ortho/para-H2D+ and ortho/para-H2.

The ortho/para-H2D+ ratio as a function of ortho/para-H2 resulting from chemistry simulations for different values of the kinetic temperature T, indicated with colours. The dashed curves represent the approximation given by the analytical formula from Hugo et al.10.

Extended Data Figure 3 N2D+/N2H+ and ortho/para-H2D+ as functions of ortho/para-H2, for different values of T and n(H2).

a, The N2D+/N2H+ abundance ratio versus the ortho/para H2 ratio for selected values of the kinetic temperature, T, and the H2 number density, n(H2). b: The ortho/para H2D+ ratio versus the ortho/para H2 ratio for different temperatures and densities. One can see that this relationship depends on T but not on n(H2).

Extended Data Figure 4 N2D+/N2H+ and ortho/para-H2D+ as functions of ortho/para-H2, for different values of T and ζ.

a, The N2D+/N2H+ abundance ratio versus the ortho/para H2 ratio for selected values of the kinetic temperature, T, and the cosmic ray ionization rate, ζ. b, The same for the ortho/para H2D+ ratio versus the ortho/para H2 ratio for different temperatures and densities n(H2). Hardly any dependence on ζ is seen except at the lowest temperatures.

Extended Data Figure 5 The H2 spin temperature.

Variation of the H2 spin temperature Tspin as a function of kinetic temperature and time in a dark cloud according to our gas-grain chemistry model. The corresponding ortho/para-H2 is indicated on the right. The gas density, n(H2) = 105 cm−3, and the visual extinction, AV = 10 mag, are kept constant. Ortho/para-H2 tends for long evolutionary times towards the thermal values (dashed line) above Tkin ≈ 12 K. The blue-hatched region indicates the T range applicable to the dense core surrounding IRAS 16293-2422 A/B (between a radius of 3,000 and 6,100 au).

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Brünken, S., Sipilä, O., Chambers, E. et al. H2D+ observations give an age of at least one million years for a cloud core forming Sun-like stars. Nature 516, 219–221 (2014). https://doi.org/10.1038/nature13924

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