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On the photodissociation of interstellar H2 and CH molecules
Physica 41 (1969) 27-28
ON THE
o North-Holland Publishing Co., Amsterdam
PHOTODISSOCIATION AND
OF INTERSTELLAR
Hs
CH MOLECULES K. DRESSLER
Princeton University Observatory, Princeton, New Jersey, U.S.A. * Received 5 November
1968
Synopsis The photodissociation of molecular hydrogen in H I regions by the mechanism discussed by Stecher and Williams has been examined quantitatively in more detail by Liszt. Numerical results are presented and discussed in connection with the observations of CH and other interstellar molecules.
The mechanism for photodissociation of Hz discussed by Stecher and Williams 1) has been examined quantitatively in more detail by Liszt 2) who confirms the resulting high dissociation rate of Hz in HI regions. The process involves absorption in discrete lines of the molecular Lyman transition, followed by emission into the dissociation continuum of the electronic ground state. On the basis of reliable absorption f-values, emission lifetimes and vibrational branching ratios of Hz, and of realistic stellar fluxes in the 9 12-1100 A range, including blanketing by stellar lines, Liszt has calculated an Hs dissociation rate of 2 x IO-7 parsec-s s-1 per molecule near a star of type BOV. A cloud of 10 parsec diameter would have to contain 10-s Hs molecule per ems to produce the 1049 A line of Hs with an equivalent width of 50 mA, which is marginally detectable with instruments on sounding rockets, and such an equilibrium density would require an Hz association rate of 2 X 1O-l4 cm-ss-1, or approximately 1 molecule per cm3 in 106 year, in a cloud at 100 parsec from the nearest B star. Theoretical discussions of molecule formation processes generally result in much lower estimated association rates. On the other hand, the optical absorption spectra of the interstellar molecules CH, CH+ and CN are observable although these molecules must also be affected by high photodissociation rates due to observed and predicted strong, diffuse absorption lines in the far ultraviolet. Quantitative measurements of the probability * Present address: Physical Chemistry Laboratory, 27
E.T.H.,
Zurich, Switzerland.
28
PHOTODISSOCIATION
OF INTERSTELLAR
Hz AND
CH MOLECULES
for predissociation in the upper state of the interstellar CH line at 3143 A have been made by Hesser and Lutz 3). The results show that essentially each photon absorbed in that line leads to the dissociation of the absorbing CH molecule. These data indicate that either the molecular formation rates in the interstellar medium are higher than generally estimated, or that substantial regions of space are effectively shielded from the dissociating radiation.
REFERENCES 1) Stecher, T. P. and Williams,
D. A., Astrophys.
J. 149 (1967) L 29.
2)
Liszt, H. S., (1968) unpublished.
3)
Hesser, J. E. and Lutz, B. L., Phys. Rev. Letters 20 (1968) 363.
ON THE
o North-Holland Publishing Co., Amsterdam
PHOTODISSOCIATION AND
OF INTERSTELLAR
Hs
CH MOLECULES K. DRESSLER
Princeton University Observatory, Princeton, New Jersey, U.S.A. * Received 5 November
1968
Synopsis The photodissociation of molecular hydrogen in H I regions by the mechanism discussed by Stecher and Williams has been examined quantitatively in more detail by Liszt. Numerical results are presented and discussed in connection with the observations of CH and other interstellar molecules.
The mechanism for photodissociation of Hz discussed by Stecher and Williams 1) has been examined quantitatively in more detail by Liszt 2) who confirms the resulting high dissociation rate of Hz in HI regions. The process involves absorption in discrete lines of the molecular Lyman transition, followed by emission into the dissociation continuum of the electronic ground state. On the basis of reliable absorption f-values, emission lifetimes and vibrational branching ratios of Hz, and of realistic stellar fluxes in the 9 12-1100 A range, including blanketing by stellar lines, Liszt has calculated an Hs dissociation rate of 2 x IO-7 parsec-s s-1 per molecule near a star of type BOV. A cloud of 10 parsec diameter would have to contain 10-s Hs molecule per ems to produce the 1049 A line of Hs with an equivalent width of 50 mA, which is marginally detectable with instruments on sounding rockets, and such an equilibrium density would require an Hz association rate of 2 X 1O-l4 cm-ss-1, or approximately 1 molecule per cm3 in 106 year, in a cloud at 100 parsec from the nearest B star. Theoretical discussions of molecule formation processes generally result in much lower estimated association rates. On the other hand, the optical absorption spectra of the interstellar molecules CH, CH+ and CN are observable although these molecules must also be affected by high photodissociation rates due to observed and predicted strong, diffuse absorption lines in the far ultraviolet. Quantitative measurements of the probability * Present address: Physical Chemistry Laboratory, 27
E.T.H.,
Zurich, Switzerland.
28
PHOTODISSOCIATION
OF INTERSTELLAR
Hz AND
CH MOLECULES
for predissociation in the upper state of the interstellar CH line at 3143 A have been made by Hesser and Lutz 3). The results show that essentially each photon absorbed in that line leads to the dissociation of the absorbing CH molecule. These data indicate that either the molecular formation rates in the interstellar medium are higher than generally estimated, or that substantial regions of space are effectively shielded from the dissociating radiation.
REFERENCES 1) Stecher, T. P. and Williams,
D. A., Astrophys.
J. 149 (1967) L 29.
2)
Liszt, H. S., (1968) unpublished.
3)
Hesser, J. E. and Lutz, B. L., Phys. Rev. Letters 20 (1968) 363.