Letter | Published:

Charged dust grains and excitation of rotational levels of interstellar molecular hydrogen

Naturevolume 254pages203205 (1975) | Download Citation



THE strengths of ultraviolet absorption lines of interstellar hydrogen have been measured in the spectra of several reddened early-type stars1–3. Interstellar H2 molecules are observed mainly in the two lowest rotational levels J = 0 (para-H2) and J = 1 (ortho-H2) with a ratio of populations corresponding to an average temperature 80 K. This temperature is consistent with the mean HI kinetic temperature determined in 21-cm absorption studies4,5, whereas higher rotational levels up to J = 5 or 6 are populated corresponding to significantly higher excitation temperatures3,6 in the range 150–200K. These observations indicate that a process other than thermal gas collisions is responsible for populating the higher rotational levels. The precise nature of the excitation mechanism is as yet unknown. A process which could contribute to non-thermal excitation of rotational levels involves encounters of charged dust grains with interstellar hydrogen. We have examined this mechanism and find that it could be of comparable, if not greater, importance in relation to other processes which have been proposed.

Access optionsAccess options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.


  1. 1

    Carruthers, G., Astrophys. J. Lett., 161, L81 (1970).

  2. 2

    Smith, A. M., Astrophys. J. Lett., 179, L11 (1973).

  3. 3

    Spitzer, L., et al., Astrophys. J. Lett., 181, L116 (1973).

  4. 4

    Hughes, M. P., Thompson, A. R., and Colvin, R. S., Astrophys. J. Suppl., 23, 323 (1971).

  5. 5

    Radhakrishnan, V., Murray, J. D., Lockhart, P., and Whittle, R. P. S., Astrophys. J. Suppl., 24, 15 (1972).

  6. 6

    Spitzer, L., and Chochran, H. D., Astrophys. J. Lett., 186, L123 (1973).

  7. 7

    Wickramasinghe, N. C., Mon. Not. R. astr. Soc., 159, 269 (1972).

  8. 8

    Chiao, R. Y., and Wickramasinghe, N. C., Mon. Not. R. astr. Soc., 159, 361 (1972).

  9. 9

    Salpeter, E. E., and Wickramasinghe, N. C., Nature, 222, 442 (1972).

  10. 10

    Hayakawa, S., Proc. Symp. Solid State Astrophys. (Cardiff, 1974).

  11. 11

    Watson, W. D., Astrophys. J., 176, 103 (1972).

  12. 12

    Ferguson, E., Atomic and Nuclear Data Tables, 12, 159 (1973).

  13. 13

    Black, J. H., and Dalgarno, A., Astrophys. J. Lett., 184, L101 (1973).

  14. 14

    Dalgarno, A., Black, J. H., and Weisheit, J. C., Astrophys. J. Lett., 14, 77 (1973).

  15. 15

    Busby, M. R., and Trilling, L., Adv. appl. Mech., Rarified Gas Dynamics Suppl., 5, 1135 (1969).

  16. 16

    Dalgarno, A., and Wright, E. L., Astrophys. J. Lett., 174, L49 (1972).

Download references

Author information


  1. Department of Applied Mathematics and Astronomy, University College, PO Box 78, Cardiff, CF1 1XL, UK

    • S. P. TARAFDAR


  1. Search for S. P. TARAFDAR in:

  2. Search for N. C. WICKRAMASINGHE in:

About this article

Publication history



Issue Date




By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.