Ion chemistry in the early universe:
Key Words.:molecular processes, atomic processes – cosmology: Early universe – ISM: molecules
Aims:The role of HeH has been newly assessed with the aid of newly calculated rates which use entirely ab initio methods, thereby allowing us to compute more accurately the relevant abundances within the global chemical network of the early universe. A comparison with the similar role of the ionic molecule LiH is also presented.
Methods:Quantum calculations have been carried out for the gas-phase reaction of HeH with H atoms with our new in-house code, based on the negative imaginary potential method. Integral cross sections and reactive rate coefficients obtained under the general conditions of early universe chemistry are presented and discussed.
Results:With the new reaction rate, the abundance of HeH in the early universe is more than one order of magnitude larger than in previous studies. Our more accurate findings further buttress the possibility to detect cosmological signatures of HeH.
The existence of HeH in astrophysical environments has been extensively discussed in several papers. Roberge & Dalgarno (1982) surmised that HeH is produced in planetary nebulae and in dense molecular clouds under the presence of X-Ray and extreme UV ionization sources. Liu et al. (1997) investigated the possible detection of HeH in NGC 7027, a very high electron density nebular object, via its = 1–0 strong rotational line at 149.14 m, concluding that its identification is complicated by the accidental near-coincidence of that transition frequency with the 149.09 and 149.39 m lines of CH.
In the last years the presence of HeH in low temperature and low density helium-rich white dwarfs has further been investigated: Harris et al. (2004) demonstrated that this molecule is the dominant positive ion in such objects and strongly affects their opacity. Engel et al. (2005), finally, showed that it could also be possible to detect HeH in cool helium stars as those studied by Saio et al. (2000).
Additionally, the discovery of the very metal-poor stars (Christlieb et al. 2002, Frebel et al. 2005) strengthened the quest for Population III stars. It then follows that to study the evolution of primordial molecules formed during post-recombination era is a fundamental step to gain a better understanding of the “how and when” such stars were formed.
To establish the role of the main molecular species (LiH, LiH, HeH, H, HD, HD) during the gravitational collapse which led to the first stars formation, it is however well-known that their opacity data are indeed needed. It is also just as important to know the elementary mechanisms which lead to the formation and destruction of such molecules and the corresponding rates that were present at the conditions of the astrophysical environment. In other words, we have to know as much as possible the chemical history of all the above species.
An extensive analysis of the lithium chemistry has been reported in a recent paper (Bovino et al. 2011) while an analogously accurate study of the reactions which involve helium-based molecular species, and in particular the HeH molecular cation in reaction with H, is still missing. The latter is indeed considered to be the first molecule to appear in the Universe (Lepp et al. 2002): in an environment dominated by a low-density regime where 3-body collisions are unlikely to occur, the radiative association is thus the main path to form that molecule. Hydrogen and helium, which are the most abundant species which would have existed at the early universe epoch, can therefore combine to form HeH by the familiar reaction:
for which the rate coefficient has been obtained from Zygelman et al. (1998) and fitted by Stancil et al. (1998). On the other hand, the main destruction path for the formed HeH molecules is still the reaction with H:
which however has never been studied with sufficient accuracy from a theoretical point of view. Crossed-beam experimental data for energies larger than 0.2 eV have been presented by Rutherford & Vroom (1973), fitted by Linder et al. (1995) and then included in the early universe model calculations of Schleicher et al. (2008), but no direct calculations have been made available thus far.
In this paper we therefore report an accurate analysis, from ab initio data, which deals with the above reaction. We present new quantum calculations for process (2) over a broad range of collision energies: the corresponding rate coefficient has been obtained from the calculated reactive cross sections and is presented in the following sections, where newly computed abundance of HeH is also reported. Finally, we additionally compare it with the known behavior of LiH cations under the same environmental conditions and can therefore draw some conclusions on their relative roles during the early universe chemical evolution.
2 The quantum reactive calculations
The elementary reactions indicated in either direction by process (2) have been studied both experimentally (Rutherford & Vroom 1973, Karpas et al. 1979, Tang et al. 2005) and theoretically (Aquilanti et al. 2000, Palmieri et al. 2000, Xu et al. 2008, Ramachandran et al. 2009), because of its peculiar resonant features detected experimentally. Several potential energy surfaces (PES’s) have been obtained and employed, using quasi-classical trajectories, close coupling (time-independent and time-dependent) methods to study the inverse of the reaction (2).
The polar cation of the present study, HeH, has a large dipole moment (1.66 Debye) and has about 11 vibrational levels (Zygelman et al. 1998). For this reason it is therefore considered of great importance in the early universe as possible coolant during the post-recombination era. As suggested by Dubrovich (1994), molecules with large dipole moments could interact with the cosmic microwave background radiation (CMB) inducing both spatial and spectral distorsions (Maoli et al. 1994). The interaction with the CMB is therefore crucial to determine the evolution and abundance of primordial molecules. The study of the HeH chemical evolution, therefore, strongly depends on the accuracy of the main reaction rate coefficients which lead to its destruction or formation. Formation mechanisms for HeH have already been extensively investigated (Zygelman et al. 1998) while we wish here to focus on its destruction mechanisms, and in particular on the direct exothermic (0.748 eV) reaction (2). In our calculations we employ one of the latest of the available, calculated PES (Ramachandran et al. 2009). Since it is also known that, under early universe conditions molecules are most likely to be in their roto-vibrational ground state ( = 0, = 0), we have further decided to study reaction (2) starting from this level of the target cationic molecule.
The exact quantum study of reactions involving an ionic system is usually rather difficult due to the presence of a long range ”tail” in the interaction which therefore requires the use of a large basis set to generate numerically converged cross sections and rates. The use of an accurate computational method which is less costly in terms of resources is therefore mandatory. In a recent paper (Tacconi et al. 2011) we have presented a novel computational implementation of the negative imaginary potential (NIP) scheme proposed earlier by Baer and coworkers (1990) and we shall use it for the present study. We start by solving the Schrödinger equation in a Body-Fixed (BF) frame of reference, in the presence of an additional NIP. Hence we write the Hamiltonian of the triatomic system in the BF frame, within the coupled states (CS) (McGuire 1976) approximation
where , , and are the Jacobi coordinates, and are the reduced masses of the triatomic complex and diatomic molecule respectively. and are the total angular momentum and its projection on the relative axis while is the diatomic rotational angular momentum. is the interaction potential and has the functional form given in Bovino et al. (2010b). The CS approximation provides the exact dynamics when the target rotational angular momentum is chosen to be equal to 0 (MgGuire 1976). Thus, it is here equivalent to the full coupled-channel (CC) formulation. Because of the flux-absorbing effect coming from the NIP, the resulting S-matrix is now non-unitary and its default to unitarity yields here the (state-to-all) reaction probability, as discussed in Tacconi et al. (2011). From the reaction probability one can in turn obtain the reactive cross section
while the rate coefficients are computed by averaging the appropriate reactive cross sections over a Boltzmann distribution of velocities for the incoming atom:
where is the gas temperature, is the Boltzmann constant and the reduced mass of the system. In the following section we shall present the effects from the accurate rates computed above on the final chemical evolution.
3 The chemical network and the evolutionary modelling
The evolution of the pregalactic gas is usually considered within the framework of a Friedmann cosmological model and the primordial abundances of the main gas components are taken from the standard big bang nucleosynthesis results (Smith et al. 1993). The numerical values of the cosmological parameters used in the calculation are obtained from WMAP5 data (Komatsu et al. 2009) and are listed in Table 1 (for details, see Coppola et al. 2011).
The abundance of HeH is obtained by solving a set of differential coupled rate equations of the form
where and are formation and destruction rates for the reactant species with number density , which in general depend on the number densities of the other species and on the radiation field. The rate coefficients involved in the helium chemistry network are the same as those reported in Galli and Palla (1998) (hereafter GP98), with the exception of the destruction reaction (2) which indeed now comes from the present quantum calculations. The additional equations which govern the temperature and redshift evolutions have been reported in several earlier papers (see e.g. GP98, Bovino et al. 2011) and therefore we shall not discuss them within the present context.
|km s Mpc|
4 Results and comparisons
As already outlined in the Introduction, our understanding of the early universe evolution strongly depends on our knowledge of the elementary processes involving atomic and molecular species taken to be present at that time. At the molecular level, therefore the observables that one needs to obtain are the various cross sections related to their mechanisms of occurrence. In the present work we have accurately calculated the cross sections for HeH () in reaction with H() using quantum dynamics and ab initio data which provide the interaction forces.
In the following we shall report and discuss our results and further compare the new HeH abundances produced by the new quantum reaction data with those obtained for another molecular ion present in the chemical network, LiH (Bovino et al. 2011).
4.1 The reaction cross sections
A set of three hundreds coupled equations which describe reaction (2) have been solved using our recent, in-house code (Tacconi et al. 2011). A direct product of a Colbert-Miller (Colbert & Miller 1992) discrete variable representation (DVR) of 100 points ranging from 0.5 to 12 for the reagent ion vibrational coordinate and a rotational basis of 40 spherical harmonics has been employed to describe the reactive complex. The solution matrix has been propagated up to 50 and the convergence was checked as a function of the propagator step-size, yielding a final convergence of cross section values within about 1%.
The calculations were carried out over a wide range of energies, from 10 to 1.0 eV, and for total angular momentum values () ranging from 10 to 90 for the highest energy. The NIP parameters were tested following the Baer criteria (Baer et al. 1990): the NIP stability has been reached for , and the NIP order (see Tacconi et al. 2011).
Our computed, -converged integral cross sections are presented in Figure 1. It is clear from the Figure the non-Langevin behavior of the cross sections for this ionic reaction since, instead of becoming independent of E at lower energies, they go through a marked maximum around 10 eV. In the same figure we report the experimental (thick line, from Rutherford et al. 1973) and the extrapolated (dotted line, from Linder et al. 1995) cross sections which reach down to collision energy values around 10 eV. The agreement between our theoretical results and the experimental findings, both measured and extrapolated, is reassuringly good and confirms the physical reliability of the present NIP method for handling ionic reactions. No measured data are available at the lower collision energies as crossed beam experiments with ionic partners become increasingly difficult in that range. It is, however, worth noting here that the results of Figure 1 are very different from the findings for LiH (Bovino et al. 2010a) in the same range of energies. Such differences are related to different microscopic mechanisms of the reaction pathways between the two systems, an aspect of the problem which we are planning to analyse more in detail in further work. Qualitatively, the two systems, although both undergoing no barrier, exothermic reactions, have very different exothermicity values that affect product separations at low energy.
4.2 The depletion rates
The new rate coefficients are reported in Table 2 and further fitted with the following formula:
valid for K. The experimental value obtained by Karpas et al. (1979) is also reported for comparison in the same Table. It is again very reassuring to see that the only existing experimental datum at 300 K is indeed very close to our computational value at the same temperature.
In order to more clearly evaluate the significance of the newly computed cross sections we need to view the process (2) within the broader context of the variety of elementary events involved with this molecular cation. To that end, Figure 2 shows the main processes leading to the formation and the destruction of HeH. As clearly shown by the figure, the radiative association process () dominates the formation of HeH over the whole range of redshifts, while the association involving the He ion () and the one involving the H molecule (), have negligible contributions at the lower redshift values, the most important range for the present study.
From the same figure one further realizes that the photodissociation mechanism () is the dominant destruction path of the HeH for while, on the other hand, the chemical path which involves the reaction (2), the one which destroys HeH () in collision with H, is seen to be the most important process for . A contribution also comes from the electron-assisted dissociation reaction () that neutralizes the charged partners and dominates over () for .
experimental (Karpas et al. 1979)
4.3 A comparison with LiH
We do know that during the Dark Ages of the universe, the existing molecules interacted with CMB photons and with the present atoms. What we do not know is if these species existed with large enough abundances to produce now detectable signals. Because of its large dipole moment (1.66 Debye), and the large abundances of its atomic components (H and He), HeH, is certainly one of the prime candidates amenable to observations (Persson et al. 2010). This is even more important as our recent ab-initio results (Bovino et al, 2009, 2010a, 2010b, 2011) showed that lithium-containing molecules such as LiH and LiH, in spite of their significant dipole moments that would favor transitions (5.9 and 0.6 Debye), have very low abundances over the whole range of redshifts studied.
In Figure 3 we report the calculated rate coefficients for the depletion reaction (2) over a wide range of temperatures (redshifts) together with the earlier calculations for LiH depletion (Bovino et al. 2010a). As it is clear from that Figure, the HeH + H reaction turns out to be very efficient at the redsfhifts above 100, with rate values comparable to those of the LiH + H destruction reaction given in the same figure (dot-dashed line). However, while the latter remains essentially constant over the whole range of examined redshifts, the efficiency of reaction (2) quickly decreases for , coming down by about two orders of magnitude by the time unitary redshift values are reached. This behavior therefore favors the HeH as a more likely survival candidate vis-á-vis the LiH case, thereby providing a good molecular coolant acting during the primordial star formation. In the same figure we also report the experimental rate at 300 K given by Karpas et al. (1979); the rates included in the GP98 model are taken from the experimental data of Rutherford & Vroom (1973). As seen earlier for the cross sections, the experimental rates are in good agreement with our quantum results (see also Table 2).
In Figure 4 we report the calculated abundances for HeH obtained using our new quantum rates discussed above: they are compared with the old ones obtained by GP98 and also with our newest results for LiH (Bovino et al., 2011). The scenario emerging from the above calculations indicates that for redshift around , the molecular HeH abundance has now increased by more than one order of magnitude with respect to the earlier calculations. It thus remains a more abundant ionic species LiH, hence becoming an interesting molecular candidate for actual detection. As shown by Maoli et al. (1994) (see also Schleicher et al. 2008), the relevant quantity for determining the imprint of molecules on the CMB is the optical depth due to line absorption. In the case of HeH, the optical depth has a broad peak around a frequency of GHz, due to redshifted rotational transitions with -6 of the ground vibrational level. With the new rate coefficient derived in this work, the maximum value of the optical depth, , is about one order of magnitude larger than the value computed earlier by Schleicher et al. (2008), an encouraging result for observational perspectives. The new reaction rate has also an effect on the abundance of H, that is reduced by a factor of in the range with respect to earlier results. However this reduction has no significant effect on the abundance of H because the H channel of production of H is no longer active at these redshifts.
We have carried out a new set of ab initio, quantum calculations for the rate coefficient of the reaction (2), which therefore permit us a revision of the abundance of HeH molecule within the expected conditions operating during the early universe. One of the main results from the present reactive calculations is that the chemical destruction rate of this polar cation exhibits a temperature dependence that turns out to be very different from previous data, both experimental and theoretical, especially for temperatures smaller than 100 K.
These new results obtained for the HeH abundance now indicate that this molecule should be much more likely to have survived at low redshift, since its fractional abundance goes up by more than one order of magnitude from previous estimates, becoming of the order of 10. This new findings should be considered within the broader context whereby the fractional abundance of LiH, recently calculated by Bovino et al. (2011), turns out to be significant mostly in the low-redshift region of . The comparison between the specific fractional abundances of the two species therefore suggests rather compellingly that the HeH ionic molecule, if more abundant than the LiH, would become more likely to be observed experimentally, even more so than what was suggested by previous works (GP98, Persson et al. 2010).
It is also worth to mention that, due to its larger dipole moment and to its greater number of roto-vibrational levels, when compared with LiH, the molecular cation HeH turns out to be the best candidate to date for experimental observations in different astrophysical environments, a result now supported by the present ensemble of accurate computational studies from quantum, ab initio methods.
Acknowledgements.We thank the CINECA and CASPUR consortia for providing us with the necessary computational facilities and the University of Roma “Sapienza” for partial financial support. We are grateful to Dr. D. De Fazio for providing us with the potential energy surface of Ramachandran et al. (2009). D.G. also thanks F. Palla for discussions on this subject.
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