Issue |
A&A
Volume 552, April 2013
|
|
---|---|---|
Article Number | A33 | |
Number of page(s) | 3 | |
Section | Astrophysical processes | |
DOI | https://doi.org/10.1051/0004-6361/201220699 | |
Published online | 20 March 2013 |
Research Note
Excitation and deexcitation processes in atom-Rydberg atom collisions in helium-rich white dwarf atmospheres
1
University of Belgrade, Institute of physics,
PO Box 57,
11001
Belgrade,
Serbia
e-mail:
vlada@ipb.ac.rs
2
Institute Isaac Newton of Chile, Yugoslavia Branch, 11060
Belgrade,
Serbia
3
Astronomical Observatory, Volgina 7, 11060
Belgrade 74, Serbia
4
Observatoire de Paris, 92195
Meudon Cedex,
France
Received:
6
November
2012
Accepted:
14
February
2013
We aim to show the importance of non-elastic excitation and deexcitation processes in He∗(n) + He(1s2) collisions with the principal quantum number n ≥ 3 for helium-rich white dwarf atmospheres. We compare the efficiencies of these processes with those of the known non-elastic electron-He∗(n) atom processes in the atmospheres of some DB white dwarfs. We show that in significant parts of the considered atmospheres, which contain weakly ionized layers (the ionization degree ≲10-3), the influence of the studied atom-Rydberg atom processes on excited helium atom populations is dominant or at least comparable to the influence of the concurrent electron-He∗(n)-atom processes.
Key words: atomic processes / astrochemistry / stars: atmospheres / white dwarfs
© ESO, 2013
1. Introduction
We here continue our previous investigations of inelastic A + A∗(n) atom-Rydberg atom collision processes (where the principal quantum number was n ≫ 1 and A was the ground-state atom), which influence the excited atom populations in stellar atmospheres. We consider the processes that can be interpreted as resulting from a resonant energy exchange between the subsystem A + A+, in which A+ is the core of the Rydberg atom A∗(n), and the outer electron of this atom. There are two groups of such processes. The first group involves the chemi-ionization processes (Janev & Mihajlov 1980; Mihajlov et al. 1996, 1997), which connect the block of atomic Rydberg states with the continuum. The second group comprises the so-called (n − n′)-mixing processes within the above mentioned block of atomic Rydberg states (Mihajlov 1982; Mihajlov et al. 2004, 2008), i.e. the excitation and deexcitation processes that imply transitions between the Rydberg states with the principal quantum numbers n and n′ ≠ n.
The chemi-ionization processes in A∗(n) + A collisions, together with their inverse processes, have been studied for both A = H and A = He cases, which are especially important for stellar atmospheres. Mihajlov et al. (2003, 2007) studied the influence of these processes on the excited hydrogen atom populations and the spectral line profiles in the atmosphere of an M red dwarf (Teff = 3800 K) using the PHOENIX code. Mihajlov et al. (1997, 2011, 2003) studied the potential influence of these processes on the excited hydrogen and helium atom populations in the atmospheres of Sun and of some DB white dwarfs.
However, the possible influence of the (n − n′)-mixing processes in A∗(n) + A collisions on the excited-atom populations in stellar atmospheres has only been studied for the A = H case in the context of the atmosphere of the Sun (Mihajlov et al. 2005). In the above mentioned studies concerning the solar atmosphere, the necessary calculations were performed within the corresponding non-local thermodynamic equilibrium (non-LTE) model proposed by Vernazza et al. (1981). The findings of these studies have shown that the efficiency of these processes in a major part of the solar photosphere is dominant with respect to the efficiency of the relevant concurrent excitation and deexcitation processes for n = 4,5 and 6, and is comparable with them for n = 7 and 8. The above mentioned studies concluded that the excitation and deexcitation processes in H∗(n) + H collisions should be included ab initio into future non-LTE models of the solar atmosphere. The importance and effects of the atom-Rydberg atom (n − n′)-mixing processes have also been analyzed in Barklem (2007) and Mashonkina (2009).
We found no non-LTE models similar to that of Vernazza et
al. (1981) for the Sun that would have provided all data necessary for calculations
as in Mihajlov et al. (2005), in relation to the
atmospheres of DB white dwarfs and other similar helium-rich stellar atmospheres. However,
since the influence of strong electromagnetic emission is the same in helium-rich
atmospheres as in the Sun, the distribution shape of the helium excited-atom population
probably deviates considerably from the Boltzmann distribution. Taking this into account and
because the data on the relative efficiencies of atom-Rydberg atom processes and the
relevant concurrent processes would be useful for future non-LTE stellar-atmosphere models,
the chemi-ionization processes in He∗(n) + He collisions were
studied in Mihajlov et al. (2003) in the context of
DB white dwarf atmospheres, within the models proposed by Koester (1980). In the current study, for the same reason and using the same
models, the efficiencies of the excitation processes (1)and
the inverse deexcitation processes
(2)(where
n + p and n − p are
the principal quantum numbers of the final Rydberg states) are compared for
3 ≤ n ≤ 8 with the efficiencies of the relevant concurrent processes.
Here, we consider the well-known electron-excited-atom collision excitation/deexcitation
processes
(3)where
and
denote a free electron
in the initial and final states, respectively. The necessary calculations were performed
using the models in Koester (1980) for DB white dwarf
atmospheres with log g = 8 and
Teff = 12 000 K and
Teff = 14 000 K.
2. Theoretical remarks
As several of the above mentioned studies have reported, the same dipole resonant mechanism
(DRM) causes the non-elastic (n − n′)-mixing
and chemi-ionization processes in
A∗(n) + A collisions. The
DRM was also discussed in detail in Mihajlov et al.
(2012). For the (n − n′)-mixing
processes (1) and (2), with A = He, the DRM assumes that they occur in the
region
Rn;n ± p ≪ rn,
where R is the internuclear distance and
rn ~ n2 is the
characteristic radius of atom He∗(n), in the vicinity of the
corresponding resonant points
Rn;n ± p.
These parameters are found to be the roots of the equation
U2(R) − U1(R) = εn ± p − εn,
where εn and
εn ± p denote the energies
of the Rydberg states | n > and
| n ± p > with the principal
quantum numbers n and n ± p; and
U1(R) and
U2(R) are the adiabatic energies of the
electronic ground state | 1;R > and the first
excited state | 2;R > of the molecular ion
. We took the
potential curves U1(R) and
U2(R) from Ignjatović et al. (2009) and following the foundational work of Janev & Mihajlov (1979), which focuses on the
(n − n′)-mixing processes, we assumed that
the processes (1) and (2) are caused exclusively by the transitions
| n > → |n + p >
and
| n > → |n − p >
of the outer electron, which occur simultaneously with the transitions
| 2;R > → |1;R >
and
| 1;R > → |2;R >,
respectively, in the subsystem He+ + He, close to the resonant points
Rn;n ± p.
This fact is responsible for the resonant character of the processes in (1) and (2).
For A = H only chemi-ionization processes with n ≥ 4 and (n − n′)-mixing processes with min(n,n′) ≥ 4 can be described on the basis of the DRM, which caused by the existence of the stable negative ion H−. In contrast, for A = He, the excited helium state with n = 3 can be taken as the lower boundary of the block of Rydberg states. This was also used in Mihajlov et al. (2003), who focused on the chemi-ionization processes in DB white dwarf atmospheres.
The processes (1) and (2) are characterized by the excitation and deexcitation rate coefficients Kn;n + p(T) and Kn;n − p(T), where T is the local temperature of the atomic particles in the considered stellar atmosphere. In accordance with the aim of this work, these rate coefficients are defined as the quantities that determine the mean rates of transition (per single excited atom) caused by the processes (1) and (2) between whole shells with a given n and n ± p. They are determined here using a method similar to that described in Mihajlov et al. (2008): the excitation rate coefficients Kn;n + p(T) are calculated directly and numerically in the semi-classical way, while the deexcitation rate coefficients Kn;n − p(T) are determined later according to the principle of thermodynamical balance. All necessary expressions are given in Mihajlov et al. (2008).
Excitation rate coefficients K3;3 + p(T) for 1 ≤ p ≤ 5 in [10-9 cm3 s-1].
For the DB white dwarf atmospheres considered here, the values of Kn;n + p(T) and Kn;n − p(T), which are used in Eqs. (1) and (2) for all necessary temperatures, can be calculated in principle by fitting the values of Kn;n + p(T) presented in Table 2 of Mihajlov et al. (2008), but only for the region 4 ≤ n ≤ 8. Bearing this in mind, Table 1 shows the values of the excitation rate coefficients K3;n + p(T) when 1 ≤ p ≤ 5. These values, together with Table 1 in Mihajlov et al. (2008), enable the calculation of Kn;n + p(T) and Kn;n − p(T) for the entire block, 3 ≤ n ≤ 8.
The concurrent electron-Rydberg atom processes (3) are characterized here by the rate coefficients αn;n ± p(Te = T), where Te denotes the electron temperature, which in the general case is not equal to the atomic temperature. In the current study, the values of αn;n ± p(T) are determined from the corresponding expressions given in Vriens & Smeets (1980) and Johnson (1972). We note that ion-atom non elastic collisional processes, although they are also characterized by long-range interaction, cannot be concurrent. The reason is the huge difference in masses of electron and ion, which causes the impact ion-atom velocity to be several orders of magnitudes lower than electron-atom impact velocity.
To estimate the relative efficiency of processes (1) and (2) and the concurrent
processes (3), it is sufficient to study the
behavior of quantities
and
in the considered DB white dwarf atmospheres, which are given by the expressions
(4)and
(5)where
N(n), N(1), and
Ne are the local densities of all Rydberg helium atoms in the
states with a given n ≥ 3, of a helium atom in the ground state, and of
free electrons, respectively; and the products
Kn,n ± p·N(n)·N(1)
and
αn,n ± p·N(n)·Ne
are the partial atom- and electron-Rydberg atom excitation/deexcitation fluxes. The factor
η is defined here by the relation
(6)and its behavior in
the atmospheres of the considered DB white dwarfs (which is very important for the studied
processes) is presented in Fig. 1.
![]() |
Fig. 1 Parameters (temperature T – full line and η = N(1)/Ne – dotted) for DB white dwarf atmosphere models with log g = 8 and Teff = 12 000 K and 14 000 K from Koester (1980) as functions of log τ, where τ is the Rosseland optical depth. |
![]() |
Fig. 2 Behavior of quantities |
3. Results and discussion
Figure 2 illustrates the behavior of the quantity
with 3 ≤ n ≤ 8 in the DB white dwarf atmospheres described by the models
presented in Koester (1980), for
log g = 8 and Teff = 12 000 K and 14 000
K. All results were obtained using the data from Koester
(1980), and these quantities are presented as functions of log τ,
where τ is the Rosseland optical depth. Figure 2 unambiguously shows that for DB white dwarf atmospheres with
Teff ≲ 14 000 K, the excitation processes (1) in the lower part of the Rydberg block
(3 ≤ n ≤ 8) must be taken into account as a new important factor that
influences the helium Rydberg-state populations. Figure 2 shows that in the parts of the atmosphere where the ionization degree is
≲10-3, the processes (1) with
n = 3,4, and 5 are dominant with
respect to the known concurrent processes (3); for n = 6 and 7 they are dominant in the parts where the
ionization degree is ≲10-4; and in the remaining parts their efficiency is
comparable with the efficiency of the processes (3). Even the efficiency of the processes (1) with n = 8 is close to, or at least comparable with, the
efficiency of the processes (3) in the parts
where the ionization degree is <10-4.
These facts are especially important for the considered DB white dwarf atmospheres, because for any temperature from the relevant temperature regions, the corresponding Boltzman distribution of the excited helium atom populations will have a distinct minimum (known as the “bottleneck”) only in the region 3 ≤ n < 8. Thus, it is correct to expect that even a considerable perturbation of the mentioned distribution would not result in a significant change in the position of this minimum. Hence, it follows that the excitation processes (1) should by all means be taken into account in any future non-LTE model of DB white dwarf atmospheres with similar parameters.
Examining the behavior of the quantities with
4 ≤ n ≤ 8 in the same DB white dwarf atmospheres would lead to the same
conclusions, including the necessity of introducing the deexcitation processes (2) into the mentioned models. For this reason,
the efficiencies of the considered
(n − n′)-mixing processes are only illustrated
in Fig. 2.
Finally, we emphasize that these results are relevant not only to the atmospheres of the DB white dwarfs mentioned here, but also to a considerably wider range of helium-rich stellar atmospheres. For instance, an entire array of white dwarfs of other types, as described in Wegner & Koester (1985), Dufour et al. (2006, 2007), have similar parameters. Consequently, processes (1) and (2) must be just as important as the concurrent processes (3). Overall, the findings suggest that the considered atom-Rydberg atom (n − n′)-mixing processes (1) and (2) should be included ab initio in the modeling of the helium-rich white dwarf atmospheres.
Acknowledgments
The authors are grateful to the Ministry of Education, Science and Technological Development of the Republic of Serbia for the support of this work within the projects 176002, III4402.
References
- Barklem, P. S. 2007, A&A, 466, 327 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Dufour, P., Bergeron, P., Schmidt, G. D., et al. 2006, ApJ, 651, 1112 [NASA ADS] [CrossRef] [Google Scholar]
- Dufour, P., Bergeron, P., Liebert, J., et al. 2007, ApJ, 663, 1291 [NASA ADS] [CrossRef] [Google Scholar]
- Ignjatović, L. M., Mihajlov, A. A., Sakan, N. M., Dimitrijević, M. S., & Metropoulos, A. 2009, MNRAS, 396, 2201 [NASA ADS] [CrossRef] [Google Scholar]
- Janev, R. K., & Mihajlov, A. A. 1979, Phys. Rev. A, 20, 1890 [NASA ADS] [CrossRef] [Google Scholar]
- Janev, R. K., & Mihajlov, A. A. 1980, Phys. Rev. A, 21, 819 [NASA ADS] [CrossRef] [Google Scholar]
- Johnson, L. C. 1972, ApJ, 174, 227 [NASA ADS] [CrossRef] [Google Scholar]
- Koester, D. 1980, A&AS, 39, 401 [NASA ADS] [Google Scholar]
- Mashonkina, L. 2009, Phys. Scr. T, 134, 014004 [NASA ADS] [CrossRef] [Google Scholar]
- Mihajlov, A. A. 1982, in Proceedings of international conference on plasma physics. G [Google Scholar]
- Mihajlov, A. A., Dimitrijević, M. S., & Djurić, Z. 1996, Phys. Scri., 53, 159 [Google Scholar]
- Mihajlov, A. A., Ignjatović, L. M., Vasilijević, M. M., & Dimitrijević, M. S. 1997, A&A, 324, 1206 [NASA ADS] [Google Scholar]
- Mihajlov, A. A., Jevremović, D., Hauschildt, P., et al. 2003, A&A, 403, 787 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Mihajlov, A. A., Ignjatović, L. M., Djurić, Z., & Ljepojević, N. N. 2004, J. Phys. B At. Mol. Phys., 37, 4493 [Google Scholar]
- Mihajlov, A. A., Ignjatović, L. M., & Dimitrijević, M. S. 2005, A&A, 437, 1023 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Mihajlov, A. A., Jevremović, D., Hauschildt, P., et al. 2007, A&A, 471, 671 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Mihajlov, A. A., Ignjatović, L. M., Srećković, V. A., & Djurić, Z. 2008, J. Quant. Spec. Radiat. Transf., 109, 853 [NASA ADS] [CrossRef] [Google Scholar]
- Mihajlov, A. A., Ignjatović, L. M., Srećković, V. A., & Dimitrijević, M. S. 2011, ApJS, 193, 2 [NASA ADS] [CrossRef] [Google Scholar]
- Mihajlov, A. A., Srećković, V. A., Ignjatović, L. M., & Klyucharev, A. N. 2012, J. Cluster Sci., 23, 47 [CrossRef] [Google Scholar]
- Vernazza, J. E., Avrett, E. H., & Loeser, R. 1981, ApJS, 45, 635 [NASA ADS] [CrossRef] [Google Scholar]
- Vriens, L., & Smeets, A. H. M. 1980, Phys. Rev. A, 22, 940 [NASA ADS] [CrossRef] [Google Scholar]
- Wegner, G., & Koester, D. 1985, ApJ, 288, 746 [NASA ADS] [CrossRef] [Google Scholar]
All Tables
All Figures
![]() |
Fig. 1 Parameters (temperature T – full line and η = N(1)/Ne – dotted) for DB white dwarf atmosphere models with log g = 8 and Teff = 12 000 K and 14 000 K from Koester (1980) as functions of log τ, where τ is the Rosseland optical depth. |
In the text |
![]() |
Fig. 2 Behavior of quantities |
In the text |
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