The cooling of neutral gas of primordial composition, or with very low levels of metal enrichment, depends crucially on the formation of molecular coolants, such as H2 and HD within the gas. Although the chemical reactions involved in the formation and destruction of these molecules are well known, the same cannot be said for the rate coefficients of these reactions, some of which are uncertain by an order of magnitude. Here we discuss two reactions for which large uncertainties exist the formation of H2 by associative detachment of H- with H and the destruction of H- by mutual neutralization with protons. We show that these uncertainties can have a dramatic impact on the effectiveness of cooling during protogalactic collapse

Glover, S. C. O.

Jappsen, A.-K.

Savin, D. W.

NASA Technical Reports Server

NASA LAW, February 14-16, 2006, UNLV, Las Vegas
Cosmological Implications of the Uncertainty in Astrochemical
Rate Coefficients
S. C. O. Glover1,2, D. W. Savin3, and A.-K. Jappsen2
1American Museum of Natural History, New York, NY, USA
2Astrophysikalisches Institut Potsdam, Germany
3Columbia Astrophysics Laboratory, New York, NY, USA
ABSTRACT
The cooling of neutral gas of primordial composition, or with very low levels
of metal enrichment, depends crucially on the formation of molecular coolants,
such as H2 and HD within the gas. Although the chemical reactions involved
in the formation and destruction of these molecules are well known, the same
cannot be said for the rate coefficients of these reactions, some of which are
uncertain by an order of magnitude. Here we discuss two reactions for which
large uncertainties exist – the formation of H2 by associative detachment of H
−
with H and the destruction of H− by mutual neutralization with protons. We
show that these uncertainties can have a dramatic impact on the effectiveness of
cooling during protogalactic collapse.
1. Primordial H2 Chemistry
In the early universe, during the epoch of first star formation H2 forms primarily via
the radiative association process
H + e− → H− + hν (1)
followed by the associative detachment reaction
H− +H→ H2 + e
−. (2)
For gas with a high fractional ionization, H− is also destroyed rapidly by
H− +H+ → H+H (3)
However, the rate coefficients for reactions 2 and reaction 3 are both highly uncertain, as
shown respectively in figure 1 below. Here we report on some of the cosmological implications
of these uncertainties. A more detailed discussion is given in Glover et al. (2006).
https://ntrs.nasa.gov/search.jsp?R=20060052453 2019-08-29T18:46:43+00:00Z
– 249 –
Fig. 1.— A summary of the various values found in the literature for the associative detach-
ment rate coefficient for reaction 2 (left) and the mutual neutralization rate coefficient for
reaction 3 (right). See Glover et al. (2006) for a fuller discussion.
2. Simulations
To study the impact of these astrochemical rate coefficient uncertainties, we have sim-
ulated the chemistry, cooling, and collapse of initially ionized gas into small protogalactic
halos. Our simulations use a modified version of the GADGET smoothed particle hydrody-
namics (SPH) code (Springer et al. 2001), to which we have added a treatment of primordial
cooling and chemistry. For full details of the code see Glover et al. (2006). We simulate
collapse at z = 20 into 107 M⊙ dark matter halos, with various levels of background radia-
tion. For each set of parameters, we perform 9 runs, with different combinations of values
for the associative detachment and mutual neutralization rate coefficients. Between them,
these combinations span the full range of plausible values.
We initialized each of our simulations at a redshift z = 20 and allowed them to run for 220
Myr; given our adopted cosmological parameters, this interval corresponds to approximately
1.25 Hubble times, with the simulations terminating at a redshift z = 11.2. Protogalaxies
that fail to cool and collapse during this interval are unlikely to get the chance to do so
thereafter, as the typical interval between major mergers of dark matter halos is of the order
of a Hubble time (Lacey & Cole 1993).
3. Results
Simulations were carried out for a wide range of initial conditions. A full discussion and
presentation of our results is given in Glover et al. (2006). Representative results are shown
in figure 2 for the model predicted central H2 fractional abundance of the primordial gas
– 250 –
Fig. 2.— The evolution of the central H2 abundance (left) and density (right) of a primordial
gas cloud in a set of runs performed without an ultraviolet background. Mutual neutraliza-
tion rates for these runs were taken from: Dalgarno & Lepp (1987) - solid lines; Croft et al.
(1999) - dashed lines; and Peterson et al. (1971) - dotted lines. For each choice of mutual
neutralization rate, three different associative detachment rates were used – the measured
value of Schmeltekopf et al. (1967), which has a factor of at least two uncertainty, along with
values 3.85 times larger and 2 times smaller to represent the range of published values for
this reaction.
cloud and for the central density of the gas.
4. Summary
We have found that uncertainties in the associative detachment and mutual neutraliza-
tion rate coefficients lead to uncertainties in the H2 formation rate and the final H2 fraction.
These uncertainties have a measurable impact on the thermal and dynamical evolution of
the collapsing gas. Though not shown here, the effect is particularly large when a UV field
is present. In those cases, the final H2 abundance may be uncertain by as much as a factor
of 100. In summary, the predicted ability of the gas to cool in a given model protogalaxy
depends in part on the choice of chemical rate coefficients used.
SCOG was supported in partly by a NASA Education grant and an NSF AST grant.
DWS was supported in part by a NASA APRA grant. AKJ was supported in part by the
Emmy Noether Program of the Deutsche Forschungsgemeinschaft.
– 251 –
REFERENCES
Bates, D. R., & Lewis, J. T. 1955, Proc. Phys. Soc. A, 68, 173
Bieniek, R. J., & Dalgarno, A. 1979, ApJ, 228, 635
Browne, A., & Dalgarno, A. 1969, J. Phys. B, 2, 885
Cˇ´ızˇek, M., Hora´cˇek, J., & Domcke, W. 1998, J. Phys. B, 31, 2571
Croft, H., Dickinson, A. S., & Gadea, F. X. 1999, MNRAS, 304, 327
Dalgarno, A., & McCray, R. A. 1973, ApJ, 181, 95
Dalgarno, A., & Lepp, S. 1987, in Astrochemistry, ed. M. S. Vardya & S. P. Tarafdar,
Dordrecht: Reidel, 109
Glover, S. C. O., Savin, D. W., & Jappsen, A.-K. 2006, ApJ, 640, 553
Lacey, C., & Cole, S. 1993, MNRAS, 262, 627
Launay, J. M., Le Dourneuf, M., & Zeippen, C. J. 1991, A&A, 252, 842
Moseley, J., Aberth, W., & Peterson, J. R. 1970, Phys. Rev. Lett. 24, 435
Peterson, J. R., Aberth, W. H., Moseley, J. T., & Sheridan, J. R. 1971, Phys. Rev. A, 3,
1651
Sakimoto, K. 1989, Chem. Phys. Lett., 164, 294
Schmeltekopf, A. L., Fehsenfeld, F. C., & Ferguson, E. E. 1967, ApJ, 118, L155
Springel, V., Yoshida, N., & White, S. D. M. 2001, NewA, 6, 79


Cosmological Implications of the Uncertainty in Astrochemical Rate Coefficients

The cooling of neutral gas of primordial composition, or with very low levels of metal enrichment, depends crucially on the formation of molecular coolants, such as H_2 and HD within the gas. Although the chemical reactions involved in the formation and destruction of these molecules are well known, the same cannot be said for the rate coefficients of these reactions, some of which are uncertain by an order of magnitude. Here we discuss two reactions for which large uncertainties exist -- the formation of H_2 by associative detachment of H^- with H and the destruction of H^- by mutual neutralization with protons. We show that these uncertainties can have a dramatic impact on the effectiveness of cooling during protogalactic collapse

Savin, Daniel Wolf

Jappsen, A. K.

Columbia University Academic Commons

NASA LAW, February 14-16, 2006, UNLV, Las VegasCosmological Implications of the Uncertainty in AstrochemicalRate CoefficientsS. C. O. Glover1,2, D. W. Savin3, and A.-K. Jappsen21American Museum of Natural History, New York, NY, USA2Astrophysikalisches Institut Potsdam, Germany3Columbia Astrophysics Laboratory, New York, NY, USAABSTRACTThe cooling of neutral gas of primordial composition, or with very low levelsof metal enrichment, depends crucially on the formation of molecular coolants,such as H2 and HD within the gas. Although the chemical reactions involvedin the formation and destruction of these molecules are well known, the samecannot be said for the rate coefficients of these reactions, some of which areuncertain by an order of magnitude. Here we discuss two reactions for whichlarge uncertainties exist – the formation of H2 by associative detachment of H−with H and the destruction of H− by mutual neutralization with protons. Weshow that these uncertainties can have a dramatic impact on the effectiveness ofcooling during protogalactic collapse.1. Primordial H2 ChemistryIn the early universe, during the epoch of first star formation H2 forms primarily viathe radiative association processH + e− → H− + hν (1)followed by the associative detachment reactionH− +H→ H2 + e−. (2)For gas with a high fractional ionization, H− is also destroyed rapidly byH− +H+ → H+H (3)However, the rate coefficients for reactions 2 and reaction 3 are both highly uncertain, asshown respectively in figure 1 below. Here we report on some of the cosmological implicationsof these uncertainties. A more detailed discussion is given in Glover et al. (2006).– 249 –Fig. 1.— A summary of the various values found in the literature for the associative detach-ment rate coefficient for reaction 2 (left) and the mutual neutralization rate coefficient forreaction 3 (right). See Glover et al. (2006) for a fuller discussion.2. SimulationsTo study the impact of these astrochemical rate coefficient uncertainties, we have sim-ulated the chemistry, cooling, and collapse of initially ionized gas into small protogalactichalos. Our simulations use a modified version of the GADGET smoothed particle hydrody-namics (SPH) code (Springer et al. 2001), to which we have added a treatment of primordialcooling and chemistry. For full details of the code see Glover et al. (2006). We simulatecollapse at z = 20 into 107 M⊙ dark matter halos, with various levels of background radia-tion. For each set of parameters, we perform 9 runs, with different combinations of valuesfor the associative detachment and mutual neutralization rate coefficients. Between them,these combinations span the full range of plausible values.We initialized each of our simulations at a redshift z = 20 and allowed them to run for 220Myr; given our adopted cosmological parameters, this interval corresponds to approximately1.25 Hubble times, with the simulations terminating at a redshift z = 11.2. Protogalaxiesthat fail to cool and collapse during this interval are unlikely to get the chance to do sothereafter, as the typical interval between major mergers of dark matter halos is of the orderof a Hubble time (Lacey & Cole 1993).3. ResultsSimulations were carried out for a wide range of initial conditions. A full discussion andpresentation of our results is given in Glover et al. (2006). Representative results are shownin figure 2 for the model predicted central H2 fractional abundance of the primordial gas– 250 –Fig. 2.— The evolution of the central H2 abundance (left) and density (right) of a primordialgas cloud in a set of runs performed without an ultraviolet background. Mutual neutraliza-tion rates for these runs were taken from: Dalgarno & Lepp (1987) - solid lines; Croft et al.(1999) - dashed lines; and Peterson et al. (1971) - dotted lines. For each choice of mutualneutralization rate, three different associative detachment rates were used – the measuredvalue of Schmeltekopf et al. (1967), which has a factor of at least two uncertainty, along withvalues 3.85 times larger and 2 times smaller to represent the range of published values forthis reaction.cloud and for the central density of the gas.4. SummaryWe have found that uncertainties in the associative detachment and mutual neutraliza-tion rate coefficients lead to uncertainties in the H2 formation rate and the final H2 fraction.These uncertainties have a measurable impact on the thermal and dynamical evolution ofthe collapsing gas. Though not shown here, the effect is particularly large when a UV fieldis present. In those cases, the final H2 abundance may be uncertain by as much as a factorof 100. In summary, the predicted ability of the gas to cool in a given model protogalaxydepends in part on the choice of chemical rate coefficients used.SCOG was supported in partly by a NASA Education grant and an NSF AST grant.DWS was supported in part by a NASA APRA grant. AKJ was supported in part by theEmmy Noether Program of the Deutsche Forschungsgemeinschaft.– 251 –REFERENCESBates, D. R., & Lewis, J. T. 1955, Proc. Phys. Soc. A, 68, 173Bieniek, R. J., & Dalgarno, A. 1979, ApJ, 228, 635Browne, A., & Dalgarno, A. 1969, J. Phys. B, 2, 885Cˇ´ızˇek, M., Hora´cˇek, J., & Domcke, W. 1998, J. Phys. B, 31, 2571Croft, H., Dickinson, A. S., & Gadea, F. X. 1999, MNRAS, 304, 327Dalgarno, A., & McCray, R. A. 1973, ApJ, 181, 95Dalgarno, A., & Lepp, S. 1987, in Astrochemistry, ed. M. S. Vardya & S. P. Tarafdar,Dordrecht: Reidel, 109Glover, S. C. O., Savin, D. W., & Jappsen, A.-K. 2006, ApJ, 640, 553Lacey, C., & Cole, S. 1993, MNRAS, 262, 627Launay, J. M., Le Dourneuf, M., & Zeippen, C. J. 1991, A&A, 252, 842Moseley, J., Aberth, W., & Peterson, J. R. 1970, Phys. Rev. Lett. 24, 435Peterson, J. R., Aberth, W. H., Moseley, J. T., & Sheridan, J. R. 1971, Phys. Rev. A, 3,1651Sakimoto, K. 1989, Chem. Phys. Lett., 164, 294Schmeltekopf, A. L., Fehsenfeld, F. C., & Ferguson, E. E. 1967, ApJ, 118, L155Springel, V., Yoshida, N., & White, S. D. M. 2001, NewA, 6, 79

Cosmological Implications of the Uncertainty in Astrochemical Rate Coefficients

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server

Columbia University Academic Commons