Issue |
A&A
Volume 508, Number 2, December III 2009
|
|
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Page(s) | 1067 - 1072 | |
Section | Atomic, molecular, and nuclear data | |
DOI | https://doi.org/10.1051/0004-6361/200913274 | |
Published online | 21 October 2009 |
A&A 508, 1067-1072 (2009)
Rotational spectroscopy of AlO
Low-N transitions of astronomical interest in the
state
O. Launila1 - D. P. K. Banerjee2
1 - KTH AlbaNova University Center,
Department of Applied Physics,
106 91 Stockholm, Sweden
2 - Astronomy and Astrophysics Division,
Physical Research Laboratory,
Ahmedabad 380009, Gujarat, India
Received 9 September 2009 / Accepted 9 October 2009
Abstract
Aims. The detection of rotational transitions of the AlO
radical at millimeter wavelengths from an astronomical source has
recently been reported. In view of this, rotational transitions in the
ground
state of AlO have been reinvestigated.
Methods. Comparisons between Fourier transform and microwave data indicate a discrepancy regarding the derived value of
in the v
= 0 level of the ground state. This discrepancy is discussed in the
light of comparisons between experimental data and synthesized
rotational spectra in the v = 0, 1 and 2 levels of
.
Results. A list of calculated rotational lines in v = 0, 1 and 2 of the ground state up to N'
= 11 is presented which should aid astronomers in analysis and
interpretation of observed AlO data and also facilitate future searches
for this radical.
Key words: molecular data - molecular processes - ISM: molecules - radio lines: stars
1 Introduction
The ground
state of the AlO radical has been studied with microwave spectroscopy in the v = 0, 1 and 2 levels. Törring et al. (1989) recorded the
transition near 76 GHz. In their Table 1, frequencies for several
and
transitions are shown. Yamada et al. (1990) and Goto et al. (1994) recorded several
rotational transitions, resulting in a set of accurate molecular constants, including
and
.
Launila et al. (1994) have performed a Fourier transform study of the
transition of AlO in the 2
m region. In that work, some discrepancies were pointed out regarding their derived
values, as compared to those found in the microwave work. While Yamada et al. (1990) had given a positive value for
for v = 0, the sign was in fact found to be negative in the light of high-N data of Launila et al. (1994). One of the aims of the present paper is to reinvestigate this discrepancy more closely. The work by Goto et al. (1994), dealing with the v = 1, 2 vibrational levels of the ground state of AlO, does not show the same discrepancy, however.
In a theoretical work, Ito et al. (1994)
have discussed and explained the observed vibrational anomalies in the
spin-rotation constants of the ground state in the light of spin-orbit
interaction with the
and
states.
Recently, Tenenbaum & Ziurys (2009) reported three rotational transitions of AlO from the supergiant star VY Canis Majoris. In order to facilitate future millimeter-wave search for rotational transitions of AlO, tables containing expected frequencies in the v = 0, 1 and 2 levels of the ground state are useful. In the present work, such tables are presented.
2 Analysis and results
The
ground state of the AlO radical represents a good example of a Hund's case (
)
coupling. Here, the nuclear and molecular spins
and
couple to form an intermediate vector
,
which subsequently couples with
to form
.
Although the energy levels can be calculated with a standard hyperfine Hamiltonian based on
formalism, the J quantum number is to be considered as a bookkeeping device only. This is the case in Table I of Yamada et al. (1990), where both J' and J'' have been included. Goto et al. (1994) only specify the ``good'' quantum numbers N and F in their Tables 1 and 2.
In the work of Launila et al. (1994) it was found that the positive sign of
for v = 0 was in conflict with the high-N
behavior of the spin doubling of the ground state, as shown in
Figs. 3 and 4 of their study. We have plotted in the present
work the observed
and R2 branches in the (2, 0) band of the
transition from N = 3 to N = 104 (Fig. 1). Here, we are using standard spectroscopic notations, where superscripts in branch designations refer to N-numbered branch types, while subscripts refer to the spin components. For instance,
denotes a Q-branch of R-type, going from the upper state spin component 2 to the lower state component 1. Vibrational bands are denoted as (
,
). From the plot in Fig. 1 we can see that the spin doubling of the ground state v = 0 level increases with N, up to about 0.1 cm-1 for N = 50, after which it starts to decrease until it finally reaches the value of about 0.07 cm-1 for N = 104. According to the constants of Yamada et al. (1990), the spin doubling would have to increase monotonically to about 0.3 cm-1, which is in contrast with the observations.
![]() |
Figure 1:
The
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Open with DEXTER |
We have also recalculated the rotational transition
shown in Fig. 2 of Yamada et al. (1990), using the constants given by them (
MHz,
MHz), and also using the same constants, but with a reversed sign of
.
The Hamiltonian used was the same as in Launila et al. (1994). The results are presented in Fig. 2.
![]() |
Figure 2:
The
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Open with DEXTER |
The uppermost trace of Fig. 2
shows the experimental data, while the middle one shows the
recalculated rotational spectrum using the constants of Yamada
et al. (1990). The lower trace shows the same spectrum calculated with reversed sign of
.
The calculated spectra have been convoluted with a Gaussian profile of 0.75 MHz FWHM. It is clear from this comparison that the reversed sign of
results in an almost perfect agreement with the experimental data, while the unaltered constants by Yamada et al. (1990) give rise to deviations of up to
1 MHz. Our conclusion is that the data fit in the work of Yamada et al. (1990) is essentially correct, although the sign of
has to be changed. This sign change alone results in overall deviations of less than
0.1 MHz. However, no new least-squares fit of the data in Yamada et al. (1990)
has been performed in the present work. The intensities in the
calculated spectra were derived using an intensity formula (B1) from
Bacis et al. (1973). Although this formula had actually been written for a Hund's case
transition, the agreement with experimental data is surprisingly good.
The calculated frequencies and relative intensities (at 230 K) for all
rotational transitions in v = 0, 1 and 2 up to N' = 11 are given in Tables 1-3.
Table 1:
Calculated frequencies and intensities of
rotational transitions in the
(v=0) ground state of AlO. The constants from Yamada et al. (1990) have been used, with the exception that the sign of
has been reversed. The intensities are calculated according to a Hund's case
formula (B1) from Bacis et al. (1973), at a temperature of 230 K.
Table 2:
Calculated frequencies and intensities of
rotational transitions in the
(v=1) ground state of AlO. The constants from Goto et al. (1994) have been used. The intensities are calculated according to a Hund's case
formula (B1) from Bacis et al. (1973), at a temperature of 230 K.
Table 3:
Calculated frequencies and intensities of
rotational transitions in the
(v=2) ground state of AlO. The constants from Goto et al. (1994) have been used. The intensities are calculated according to a Hund's case
formula (B1) from Bacis et al. (1973), at a temperature of 230 K.
3 Discussion
The determination of the correct constants for a molecule/radical has
its own intrinsic value. Additionally, the present study is also
relevant in an astronomical context. Tenenbaum & Ziurys (2009) have recently made the first radio/mm detection of the AlO (
)
radical toward the envelope of the oxygen rich supergiant star VY Canis Majoris (VY CMa). They observed the
and
rotational transitions of AlO at 268 and 230 GHz and the
line at 153 GHz. While their search for the
hyperfine transitions was based on direct laboratory measured frequencies by Yamada et al. (1990), the search for the
and
transitions was based on frequencies calculated from spectroscopic
constants of those authors. As has been pointed out, an error is
present in one of these constants (
in the v
= 0 level of the ground state), which leads to deviations between the
calculated and true frequencies. While these deviations are small and
may not affect the final result of a line search too seriously, it is
still desirable to have accurate frequencies to facilitate future
millimeter-wave searches for rotational transitions of AlO. It is
planned for several such searches to be taken place in the near future
as a consequence of the recent detection in VY CMa. AlO is slowly
emerging as a molecule which could attract a fair deal of interest
among astronomers. In the VY CMa detection for example it is shown how
its study could lead to a better understanding of the gas-phase
refractory chemistry in oxygen-rich envelopes. AlO has also been
proposed to be a potential molecule in the formation of alumina, which
is one of the earliest and most vital dust condensates in oxygen rich
circumstellar environments (Banerjee et al. 2007).
The mineralogical dust condensation sequence and processes involved are
issues of considerable interest to astronomers. The AlO radical also
received considerable attention after the strong detection of several
bands in the near-infrared (1-2.5 microns) in the eruptive variables V4332 Sgr and V838 Mon (Banerjee et al. 2003; Evans et al. 2003). Other IR detections of AlO include IRAS 08182-6000 and IRAS 18530+0817 (Walker et al. 1997). In the optical, the
bands have been prominently detected in U Equulei (Barnbaum et al. 1996) and in several cool stars and Mira variables including Mira itself (Keenan et al. 1969; Garrison 1997).
The line lists presented here should also aid in analysis of mm line data for aspects related to kinematics. Since the rotational levels of AlO species are split by both fine and hyperfine interactions, each rotational transition consists of several closely spaced hyperfine components. Figure 2 exemplifies this. If line broadening is small and the spectral resolution of the observations is adequate to resolve these components, then different components will yield different Doppler velocities if the corresponding reference or rest frequencies are in error. This could lead to ambiguity in interpreting the data. Even if the hyperfine components are not distinctly resolved but rather blended to give a composite line profile (as in the observed profiles in VY CMa), modelling of such composite profiles using wrong rest frequencies could lead to errors in estimating the composite line centre, half width of the composite profile and half-widths of the individual hyperfine components. Proper estimates of such kinematic parameters are important as they help determine the size and site of origin of a molecular species. The detailed discussion by Tenenbaum & Ziurys (2009) in the case of VY CMa, which has three distinctly different kinematic flows in the system, illustrates this.
4 Summary
Rotational transitions of AlO have been reinvestigated. The discrepancy regarding the derived value of
of the
ground state v = 0 level has been shown to be due to a sign error in the microwave work by Yamada et al. (1990). A list of calculated rotational lines in v = 0, 1 and 2 of the ground state up to N' = 11 has been presented.
References
- Bacis, R., Collomb, R., & Bessis, N. 1973, Phys. Rev. A, 8(5), 2255
- Banerjee, D. P. K., Misselt, K. A., Su, K. Y. L., Ashok, N. M., & Smith, P. S. 2007, ApJ, 666, L25 [NASA ADS] [CrossRef]
- Banerjee, D. P. K., Varricatt, W. P., Ashok, N. M., & Launila, O. 2003, ApJ, 598, L31 [NASA ADS] [CrossRef]
- Barnbaum, C., Omont, A., & Morris, M. 1996, A&A, 310, 259 [NASA ADS]
- Evans, A., Geballe, T. R., Rushton, M. T., et al. 2003, MNRAS, 343, 1054 [NASA ADS] [CrossRef]
- Garrison, R. F. 1997, JAAVSO, 25, 70 [NASA ADS]
- Goto, M., Takano, S., Yamamoto, S., Ito, H., & Saito, S. 1994, Chem. Phys. Lett., 227, 287 [NASA ADS] [CrossRef]
- Ito, H., & Goto, M. 1994, Chem. Phys. Lett., 227, 293 [NASA ADS] [CrossRef]
- Keenan, P. C., Deutsch, A. J., & Garrison, R. F. 1969, ApJ, 158, 261 [NASA ADS] [CrossRef]
- Launila, O., & Jonsson, J. 1994, J. Mol. Spectrosc., 168, 1 [NASA ADS] [CrossRef]
- Tenenbaum, E. D., & Ziurys, L. M. 2009, ApJ, 694, L59 [NASA ADS] [CrossRef]
- Törring, T., & Herrmann, R. 1989, Mol. Phys., 68(6), 1379
- Walker, H. J., Tsikoudi, V., Clayton, C. A., et al. 1997, A&A, 323, 442 [NASA ADS]
- Yamada, C., Cohen, E. A., Fujitake, M., & Hirota, E. 1990, J. Chem. Phys., 92(4), 2146
All Tables
Table 1:
Calculated frequencies and intensities of
rotational transitions in the
(v=0) ground state of AlO. The constants from Yamada et al. (1990) have been used, with the exception that the sign of
has been reversed. The intensities are calculated according to a Hund's case
formula (B1) from Bacis et al. (1973), at a temperature of 230 K.
Table 2:
Calculated frequencies and intensities of
rotational transitions in the
(v=1) ground state of AlO. The constants from Goto et al. (1994) have been used. The intensities are calculated according to a Hund's case
formula (B1) from Bacis et al. (1973), at a temperature of 230 K.
Table 3:
Calculated frequencies and intensities of
rotational transitions in the
(v=2) ground state of AlO. The constants from Goto et al. (1994) have been used. The intensities are calculated according to a Hund's case
formula (B1) from Bacis et al. (1973), at a temperature of 230 K.
All Figures
![]() |
Figure 1:
The
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Open with DEXTER | |
In the text |
![]() |
Figure 2:
The
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Open with DEXTER | |
In the text |
Copyright ESO 2009
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