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Two components in meteor spectra
Planet. Space Sci., Vol. 42, No. 2, pp. 145-150,
Pergamon Printed
1994 (0 1994 Elsevier Science Ltd in GreatBritain. All rights reserved 0032-0633/94 $7.00+ 0.00
0032-0633(93)E0023-6
Two components in meteor spectra Jiii Borovirka Astronomical
Institute,
25 1 65 Ondfejov
Observatory,
Received 8 July 1993 ; revised and accepted
Czech Republic
10 November
1993
Abstract. Through an analysis of fireball spectra it was found that meteor heads consist of two parts with quite different temperatures. The spectra of both parts can be fitted with a simple thermal equilibrium model. The temperature of the main spectrum is about 4000 K, and that of the second spectrum is about 10,000 K. There is little evidence for a dependence of temperatures on the meteor velocity. However, the mass and luminosity of the second part relative to the main part grows rapidly with meteor velocity. The high temperature part forms only 0.02% of the meteor vapor envelope in slow meteors but accounts for more than 5% in fast meteors. In fast meteors most of the light is produced in this part, mainly by the CaII lines. This fact influences the meteor luminous efficiency and color index. The traditional classification of meteor spectra reflects the variable intensity of the second spectrum. In the high temperature region the density is also enhanced. This region is very probably related to the meteor shock wave.
Introduction events consist of three parts : the head, the wake and the train. All these parts can be resolved spatially (by visual, televisual or photographic observations) and exhibit different spectra (e.g. Halliday, 1968). The brightest part is the meteor head. Recently, a simple physical model, assuming thermal equilibrium, was applied to the spectrum of the head of the EN 151058 fireball, aiming to fit the spectrum and to determine the temperature and chemical composition of the radiating gas (Borovicka, 1993). This attempt was successful, nearly 300 lines of the spectrum were explained within this model and a temperature of 4400 K was determined at the meteor’s brightest point. Nevertheless, several lines remained unexplained. It was found that these Meteor
Correspondence
to: J. Borovitka.
lines form another spectral component of the head spectrum, with a temperature of about 10,000 K. This high temperature component will be called “the second spectrum”. An important conclusion was also made, that the gases forming the main and the second spectrum have the same chemical composition, the only difference being the temperature (and perhaps the density, which cannot be determined directly from the spectrum). Indirect evidence allowed us to estimate that air represents about 95% of the gases. The EN 151068 fireball was relatively slow, the velocity at the brightest point being 17 km/s. The main purpose of this paper is to study the relation of the main and the second spectrum for fireballs of various velocities. For this purpose, three other fireballs-with velocities of 32-67 km/s-were studied.
Formation of the second spectrum In this section we describe both spectral components and the physical conditions under which they arise. The theoretical background can be found in BoroviEka (1993). Thermal equilibrium (i.e. the Boltzmann and Saha equations for excitation and ionization) is assumed and, for simplicity, the same relative elemental abundances as in carbonaceous chondrites are assumed. The exceptions are nitrogen and oxygen, which come mainly from the atmosphere and are the most abundant elements in the radiating gas. We assumed the atomic ratios N/Fe = 120 and O/Fe = 35, as resulted from the pressure balance for the EN 151068 fireball (BoroviEka, 1993). The main parameters of a radiating volume are temperature, density and size. Here, we summarize how these parameters can be derived from the spectrum. Temperature can be determined unambiguously, provided that there is a sufficient number of suitable lines of one atom. This is usually true for the main spectrum (the lines of FeI), but not for the second spectrum. Density cannot be derived directly. If the radiation is optically thick in some lines, the column density, N (the density
146
J. Boroviekn
integrated along the line of sight, cm-“), can be separated from the visible surface area of the radiating region, P (cm”). Assuming that the geometrical depth is comparable to the transverse dimensions, the size and the density can then be estimated. The product NP is the total number of atoms in the radiating region. If the radiation is optically thin, only this product can be determined. Usually the radiation is optically thick for the main spectrum and optically thin for the second spectrum. Note also that the total number of atoms can only be determined from a spectrum calibrated on an absolute scale, while for the temperature the relative line intensities are sufficient. An important Factor for the appearance of the spectrum is the ionization of atoms. The degree of ionization depends on temperature and density. Although density has no direct influence on the spectrum, it does in fact affect line intensity ratios by changing the abundances of neutral atoms and ions. The situation for the main spectrum is usually clear. There are enough lines in the spectra of bright fireballs to determine all the main parameters of the radiating region. The degree of ionization is computed for the resulting temperature and density. The chemical composition need not be assumed since it can be computed from the abundances of neutral atoms. Typical lines of the main spectrum are given in Table 1. They are comprised mainly of Table 1. Typical lines of meteor main spectrum 2, (A)
I
Atom-m.
3608.9 3647.8 3479.9 3705.6 3719.9 3722.6 3733.3 3734.9 3137.1 3745.6 3748.3 3749.5 3758.2 3763.8 3799.5 3815.8 3820.4 3824.4 3825.9 3829.4 3832.3 3834.2 3838.3 3856.4 3859.9 3878.6 3886.3 3899.7 3905.5
5 5 5 5 7 5 5 7 7 12 6 7 6 6 7 6 8 6 7 6 8 6 10 6 8 6 8 6 3
Fe 1-23 Fe 1-23 Fe 1-5 Fe I-5 Fe 1-5 Fe l-5 Fe I-5 Fe 1-2 I Fe I-5 Fe I-5 Fe I-5 Fe I-2 1 Fe I-21 Fe I-21 Fe I-2 1 Fe I-45 Fe I-20 Fe 1-4 Fe I-20 Mg I-3 Mg I-3 Fe I-20 Mg I-3 Fe I-4 Fe 1-4 Fe I-4 Fe I-4 Fe I-4 Si I-3
in meteor spectra
lines of neutral Na, Mg, Fe, Ca, Cr and ionized Ca. The typical temperature is deduced to be 4000-4500 K (see below). The atoms of Na, Al and Ca are already highly ionized for the conditions of the main spectrum. The derived chemical composition for EN 15 1068 was consistent with the composition of chondrites. except for the depletion of refractory elements caused by an incomplete evaporation (Borovicka. 1993). For the second spectrum the situation is more complicated. The chemical composition cannot be derived exactly but, nevertheless, the chondritic values are consistent with the observations of the present fireballs. The typical lines for the second spectrum are given in Table 2. The high excitation lines of Mg II, Si II, N I and 0 I belong purely to the second spectrum. The lines of CaII are bright in both spectra. In fact. Table 2 contains almost all iines observed in the second spectrum. They are insufficient for a direct determination of temperature. Also, the atomic density is uncertain. As a startingvalue for our consideration we can choose nF, = 3 x 10’ ’ iron atoms per cm3. This corresponds to the density of the main spectrum region determined for the EN 151068 fireball. The key for the estimation of the temperature is the ratio of intensity of the Fe II lines to the Mg II and Si II (multiplet 2) lines, which have almost the same excitation potential. The Fe II lines have lower excitation potential
in A/i 3600-9000 8, with computed
intensities
E,
ah
1
Atom-m.
E,
1.01
3922.9 3927.9 3930.3 3933.7 3961.5 3968.5 4030.8 4033.1 4045.8 4063.6 4071.7 4132.1 4143.9 4202.0 4226.7 4254.4 4271.X 4274.8 4289.7 4307.9 4325.8 4375.9 4383.6 4404.x 4415.1 4891.5 4920.5 4957.6 5167.3
6 6 6 10 2 10 5 4 8 7 6 2 4 3 IO 7 7 6 5 8 7 2 11 8 4 1 1.5 2 8
Fe I-4 Fe 1-4 Fe I-4 Ca II-I Al l-l Ca II-1 Mn I-2 Mn I-2 Fe I-43 Fe 1-43 Fe I-43 Fe 1-43 Fe I-43 Fe 1-42 Ca I-2 Cri-1 Fe I-42 CrI-1 Crl-I Fe 1-42 Fe I-42 Fe I-2 Fe l-41 Fe I-41 Fe I-41 FeI-318 Fel-318 FeI-318 Mg I-2
0.05 0.11 0.09 0.00 0.01 0.00 0.00 0.00 1.49 1.56 1.61
0.91 0.00 0.05 0.00 0.09 0.11 0.86 0.05 0.09 a 0.11 0.91 0.96 0.99 0.96 a 1.49 0.86 0.00 0.91 2.71 2.71 0.96 2.71 0.05 0.00 0.09 0.05 0.09 1.90
: Two components
a.4
5167.5 5172.7 5183.6 5206.0 5208.4 5227.2 5269.5 5328.0 5371.5 5397.1 5405.8 5424.7 1.61 5434.5 1.56 5446.‘) 1.49 5455.6 0.00 5506.8 0.00 5528.4 I .49 0.00 5615.7 0.00 5889.9 5895.9 1.56 6162.2 1.61 O.OOW 6495.0 8183.3 I .49 S194.8 1.56 8387.8 1.61 2.85 a 8542.1 2.83 a 8662.1 2.81 a 86X8.6 2.71 8806.8
I ._
Atomm
4 17 22 3 4 3 10 8 5 3 3 3 2 2 2 0.5 0.5 0.5 40 34 0.5 0.5 2 3
Fe 1-37 Mg 1-2 Mg I-2 Cr 1-7 Cr I-7 Fe l-37 FeI-15 FeI-I5 FeI-15 Fe I-15 Fel-15 Fef-15 Fcl-I5 Fe I-15 Fcl-tS Fei-15 Mg I-Y Fe I-686 Na I-l Na I-1 Ca 1-3 FeI-168 Na I-4 Na 1-4 Fe I-60 Ca IL-2
I 4 3 1.5 2
Ca II-2 Fe I-60 Mg l-7
I:>I
1.49 w 2.71 3.12 0.94 0.94 1.56~ 0.86 w 0.91 0.96 0.91 0.99 0.96 1.OI 0.99 1.01 0.99 4.34 3.33 0.00 0.00 1.YO 2.39 2.lOp 2.lOp 2.18p 1.70 1.69 2.lXp 4.35 p
Z, The computed relative intensity. In the main spectrum, I of the red and infrared lines depends strongly on column density. These lines can be brighter than given here for bright fireballs and fainter for faint meteors; m, multiplet number: E,. lower excitation potential (in eV). In crowded regions, only the brightest lines are given. Remarks: a, the intensity includes nearby iinc(s) of the same multiple1 : p, predicted line. not yet reported in meteors; I-.not observable. masked by the lines of the main spectrum; w, may be stronger, if there is a substantial contribution of wake radiation.
J. Borovitka : Two components
2. Typical lines of meteor second spectrum
i(A)
I
Atom-m.
3706.0 3736.9 3759.3 3761.3 3856.1 3862.5 3933.7 3968.5 4131.0 4233.2 4481.2 4583.8 4923.9 5018.4 5169.0
2 4 3 3 2 1 400 250 I 1 20 1 3 5 3
Ca II- 3 Ca II-3 Ti II-13 Ti II-13 Si 11-l Si II-1 Ca II-I Ca II-1 Si II-3 Fe II-27 Mg II-4 Fe (I-38 Fe II 42 Fe II-42 Fe II-42
147
in meteor spectra
Ei 3.12r 3.15r 0.61 r 0.57 r 6.83 6.83 0.00 0.00 9.80 2.58 8.86 2.81 2.89 2.89 2.89r
in ii 3600&9000 A with computed
i(A) 53 16.6 5616.5 6158.2 6347.1 6371.4 6482.7 6562.8 7423.6 7442.3 7468.3 7711.9 1714.2 7775.4 7896.4 8184.8
I
1.5
2 4 6 13 9 6 2
intensities
Atom-m.
E,
J.(A)
Fe II-49
3.15 11.76 10.74a 8.09 8.09 11.76a 10.15 10.33 10.33 10.33 9.14 9.14 9.14
8188.0 8216.3 8223.1 8242.5 8446.4 8446.8 8498.0 8542.1 8662.1 8680.2 8683.4 8686.1 8703.2 8711.6 8718.8
N I-24 01-10 Si 11-2 Si KI-2 N I-21 HI-l N I-3 N I-3 N 1-3 0 1-l 01-l 01-l Mg II-8 N 1-2
10.00p 10.33
I
2 14 8 15 8 3 4
Atom-m.
E,
N I-2 N I-2 N I-2 N I-2 01-4 0 1-4 Ca II-2 Ca II-2 Ca II-2 N I-l NI-1 NI-1 NI-1 NI-1 NI-1
10.33 10.33 10.33 IO.33 9.52 9.52 I .69 1.70 1.69 10.34 10.33 10.33 10.33 10.33 10.34
See footnote to the Table I for explanation of symbols. and their intensity relative to the Mg II and Si II lines decreases with increasing temperature. The observed ratio corresponds nearly to 10,000 K. The ionization potentials of Mg, Si and Fe are similar, so that the abundance ratio Mg IIjSi II/Fe II is close to the ratio Mg/Si/Fe as a whole and the correction for ionization is not needed for this consideration. In fact, most Mg, Si and Fe atoms are expected to be just singly ionized for the conditions in question. Only for higher temperature (-_ 15,000 K) or lower density would most of the Mg and Fe atoms become doubly ionized, and the ratios Si II/Fe II and Si II/Mg II would increase correspondingly. Nitrogen and oxygen are both partly ionized. N I and 0 I have bright lines in the infrared. They have actually been observed in meteor spectra (Millman and Halliday, 1961; Nagasawa, 197 1; Halliday, 1987 ; Murayama. 1990) but our spectra do not cover the infrared region. The N I and 0 I lines in the visual part of the spectrum are fainter, but are observable weakly in some of our spectra. They confirm the temperature of about 10.000 K. because they would be stronger for higher temperatures (unless the atomic density was very low and N and 0 were highly ionized). The lines of N II and 0 11 have such high excitation potentials that they cannot be visible at temperatures below 20,000 K. They are actually not visible in our spectra (except for EN 210463, where their presence is uncertain). Their possible occurrence in Perseid spectra (Halliday, I96 1) may be probably attributed to some nonequilibriutn effects. The intensity of hydrogen lines depends on the actual abundance of this element, which probably varies in meteors. H lines are visible in only one of our fireballs (EN 210463). As for atomic density, values lower than nPe = 10’.‘cm-’ are improbable, because the abundance of CaII would become very small due to the transition to Ca II1 and the very bright H and K lines of Ca II could not be produced. For iron densities higher than lOI cmv3, elements like Na, Mg, Ca, Cr, Mn and Fe will not becompletely ionized, a non-negIi~ible alnoLlnt of neutral atoms will remain. and the same lines as in the main spectrum will be produced, of course with different intensity ratios. In that case it would be difficult to distinguish which part of a given line
comes from the main spectrum and which part originates from the second spectrum. Nevertheless, it seems that the computed main spectrum fits the observed spectrum quite well and that a substantial part of the lines of the above mentioned neutral atoms originate in the main spectrum. The iron density is therefore not higher than IO’” cm-‘. Only the lines of the high multiplets of Fe I (multiplets I 146. 1163, 1165) can also be formed in the second spectrum. These iron lines are relatively faint. The Mg I lines, which also have a relatively high excitation potential, may also play a role in the production of the second spectrum. We can therefore conclude that a typical second spectrum is formed in the region with temperature of 10,000 i 1000 K and atomic density of iron atoms of lo”10’” cm- ‘. The mass of this region in comparison with the main spectrum region will be studied in the next section.
Dependence on velocity Even from a spectrum calibrated on a relative scale, the ratio of numbers of atoms (NP) in both radiating regions can be determined. This represents the mass ratio of the region forming the main spectrum to that forming the second spectrum, ~~~.~~. Four fireballs of different velocities were studied in this way : EN 15 IO68 [ 17 km/s, data taken from Borovirka (1993)], EN 210463 [32 km/s, line intensities taken from Ceplecha (1971)], EN 300768 (45 km/s) and EN 23 1068 (67 km/s, Orionid). All spectra were obtained with grating cameras at the OndPejov Observatory. More details about the fireballs can be found in Ceplecha (1977). The results are given in Table 3. The following quantities are given in Table 3 : The veiocity 11, height h and absolute magnitude, mag, at the fireball’s brightest point (where the spectrum was analysed), the temperatures of the main and the second spectrum, T, and r,, the mass ratio M,/AJ, and the ratios of luminosities in the blue and the visual spectral region (B and V bands), ZfB’and Zfv). The temperature of the main spectrum is, within the limits of the errors, the same for all fireballs. For EN
J. BorovZka : Two components in meteor spectra
14X Table 3. The relation of the main and the second spectrum in four fireballs 1’ (km/s)
(k$)
mag
(2)
EN 151068 EN 2 IO463
17 32
45 77
-9 -12
4400 4300
( 10.000) (10,000)
3000 600
EN 30076X EN 231068
45 67
76 85
- 10 -7
(4100)$ 4100
( I0,000) ( 10.500)
250 15
Fireball
*Parentheses
indicate
that the temperature
-1
60
20 %xi~;
70
[k$S]
Fig. 1. The share of the second spectrum as a function of meteor velocity
I,‘B’!‘I,‘fi’ 20 6 3 0.3
I1\), [ (V) I ,‘2 500 I 00 80 4
values are not certain.
300768 the temperature is somewhat uncertain due to wake radiation, which is hard to separate from that of the meteor head. Note also that the temperature in EN I5 1068 in fact varied from 3500 to 4700 K along the fireball path (Borovieka, 1993). The temperature of 10,000 + 1000 K, as discussed in the previous section, is applicable to the second spectra of all fireballs studied. The temperature for the high velocity Orionid is only slightly enhanced. There are some individual differences; for example, in the Orionid the Si II lines are brighter relative to Mg II and Fe I I in comparison with the other three fireballs, but these differences cannot be unambiguously explained as differences in temperature and/or density (a discrepancy in the Ca II and N I lines would be the result for the Orionid). Rather, the differences are more likely to have been caused by differences in chemical composition or deviations from thermal equilibrium. For all velocities the gas forming the main spectrum dominates (M, > Mi). However, the mass ratio M,/M? decreases strongly with velocity, meaning that the share of the high temperature gas grows rapidly with velocity (the velocity is, of course, also closely related to the height-fast meteors have greater height). The dependence is displayed in Fig. I. It can be fitted with the exponential function
10
M,I’M?
M2 = 5 x IO-"M,e".'"',
where 1’ is in km/s. This empirical relation is, of course, preliminary. We cannot be sure that the ratio A4,iMz depends only on velocity. In principal, it may also depend on meteoroid mass or height or chemical composition. However, the dependence on velocity is certainly dominant. The changing ratio M,/M: is the main factor which influences the appearance of spectra of meteors of difl‘erent velocities. While the shape of the second spectrum probably remains nearly the same in all meteors and the main spectrum changes only gradually with increasing meteoroid mass as more lines become optically thick (major differences in chemical composition are not considered here), the ratio of the brightness of both spectra is a steep function of velocity. The Si II lines are dominant in the red part of the spectrum of fast meteors but are very inconspicuous in slow meteors. For fast meteors. the H and K lines of Ca II are by far the most intense lines. while for slow meteors this is not the case. In Fig. 2 the main spectrum and the second spectrum of the Orionid EN 231068 are compared. The spectra were computed for the parameters (temperature, abundances) derived from the observed spectrum. The role of the second spectrum should also be taken into account in association with its contribution to meteor luminous efficiency. In the last two columns of Table 3. the ratios of luminosities of the two spectra are given for two spectral bands. They were obtained by integrating the synthetic spectra (computed for the parameters derived from the observed spectra) multiplied by the transmissivity functions of the B and V bands of the UBV international photometric system. The luminosities do not include molecular emissions (almost negligible in the present spectra) and wake radiation. The second spectrum is always much brighter in the B band (due to strong Ca II lines) than in the V band, which is most effective between 5000 and 6000 A. where no strong emissions are present. The main spectrum is nearly color-neutral with B-V 2 0. The intensities of bright lines (including violet Call) are reduced by self-absorption effects to nearly the same level. For the Orionid case, more than three quarters of the light in the B band comes from the second spectrum. For velocities of about 30 km/s, the contribution of the second spectrum is nearly 10% in the B band. In V. the second spectrum is important only for high velocities (20% for the Orionid). The second spectrum is again dominant in fast meteors in the infrared due to the strong lines of 0 I and N I.
J. BoroviEka : Two components
0 . 35i-i
:uI~
I I II /lh
4000
4500
4.000
4500
149
in meteor spectra
I , i I I I ‘1 I .I I ‘t ‘14
5000
5500
6000
6500
6000
6600
6000
6500
0 3500
wavelength
Fig. 2. The main spectrum and the second spectrum of the Orionid EN 231068 on the same scale. The H and K lines in the second spectrum have been truncated; their intensities are II and 6, respectively
Conclusions observational facts given in this paper can be summarized as follows. Meteor heads consist of two diverse parts which were not resolved spatially but exhibit different spectra. For all velocities, most of the radiating gas around the meteoroid has a temperature of about 4000 K. A part of the gas has a temperature of about 10,000 K, again almost independently of the velocity. Differences in chemical composition between the two parts were not distinguished. These parts are not related to the two chemical components with different boiling points assumed in their dustball model for faint meteors by Hawkes and Jones (1975). The share of the second, high temperature, part grows rapidly, nearly exponentially, with meteor velocity. This part forms only * 0.02% of the meteor gas in slow meteors (2’ < 15 km/s), but contributes more than 5% of the gas in fast meteors (2: > 65 km/s}. in fast meteors most of the light is produced in the high temperature region. Besides the velocity, the share of the second spectral part may also depend on meteoroid mass, i.e. on meteor brightness. The various ratios of the infrared CaII lines (produced in both parts) and 0 I lines (produced in the second part) observed in Perseids of different brightness by Millman and HaiIiday (196 1) may be evidence for this effect. An alternative explanation is that the portion of air in the meteor gas changed among these meteors. Note that the 01 lines were bright even in the TV spectra of
The
very faint meteors (+4 mag) obtained by Mukhamednazarov and Mal’tseva (1989). The ratio of intensity of the second spectrum and the main spectrum is the key factor which differentiates the spectra of meteors of different velocities and similar chemical composition. Both spectra themselves do not differ so much from case to case. Millman’s standard classification of meteor spectra (e.g. Millman and McKinley. 1963) expresses mainty just the different intensity of the second spectrum. Type Y, where the H and K lines are the strongest lines. represents meteors with a strong second spectrum, whereas type X, where the strongest lines are of Na I or Mg 1, has a weak second spectrum. The rare type Z (strongest Fe I or Cr I) probably represents meteors with different chemical composition. The interpretation of the observational facts seems to be straightforward. Since not only temperature, but also density is very probably higher in the second part, the most likely situation is that this part is related to the meteor shock wave or to a relaxation region just behind the shock wave. The turbulent gas forming the main spectrum is not directly ill~Ltenced by the on-coming flow of air and its temperature does not depend on velocity. A somewhat surprising fact is that the temperature of the shock wave region is only very weakly influenced by velocity in our current observations. The cause of the steep decrease of the MI/Ml ratio is not so obvious. Preliminary considerations suggest that the thickness of the shock wave region does not change dramatically, with the geometrical thickness being of the order of 1 mm (0.553 mm) in the present four fireballs, assuming the density nFe = IO” cm-‘. (This iron density corresponds to the total atomic density n = 1.6 x 10’” cin3. including the air. and the free electron density n, = 3 x lOI cme3.) The size and density of the main spectrum region exhibits larger changes. There is no evidence for an intermediate region with a temperature between 5000 and 10,000 K. More meteor spectra should be studied to examine the relationship between the second spectrum and the main spectrum in order to improve our knowledge of its dependency on velocity. A dependency on brightness, height or other quantities should be examined as well. At the same time the observational constraints given here should be used in theoretical considerations, including calculations of shock wave formation, energy transport and of the luminous efficiency. A~,knolz,k~~lyerfl~nr.I am indebted to Dr D. 0. RcVelle for the improvement of the language style of this paper and for his valuable comments.
References Borovitka, J., A fireball spectrum
analysis. Asrro~. Astrophys. 279,627~645, 1993. Ceplecha, Z., Spectral data on terminal flare and wake of double station meteor no. 38421. Bull. ustron. fm-t. Czech. 22, 219304,1971. Ceplecha, Z., Fireballs photographed in central Europe. Bull. astron. Inst. Cxch. 28, 328 340, 1977. Halliday, I., A study of spectral line identifications in Perseid
150
meteor spectra. Puhl. Dominion Ohs. Ottuwu 25, 3-16, 1961. Halliday, I., The influence of exposure duration and trail orientation on photographic meteor spectra. in Phy.sirs and D~wamics of’Metcor,c (edited by L. Kresak and P. M. Millman), IAU Symp. no. 33, pp. 91~104. Reidel, Dordrecht, 1968. Halliday, I., The spectra of meteors from Halley’s comet. Astroll. AstrophJx. 187, 921-924. 1987. Hawkes, R. L. and Jones, J., A quantitative model for the ablation of dustball meteors. Mon. Not. R. astron. SW. 175, 339-356, 1975.
J. Borovicka
: Two components
in meteor spectra
Millman, P. M. and Halliday, I., The near infra-red spectrum of meteors. Plmwt. Spucc Sci. 5, 137-140, 1961. Millman, P. M. and McKinley, D. W. R., Meteors, in The Moon. Meteorites. und L’onx~t.s. The S&r Systen~, Vol. IV (edited by Middlehurst and Kuiper). pp. 674-773. The University of Chicago Press. 1963. Mukhamednazarov, S. and Mal’tseva, N. V., Study of the TV spectrograms of meteors. Astron. Ve.stnik 23, 297-303, 1989. Murayama, H., Near-infrared meteor spectra obtained in 1985. Tok~*o Mrtror Nrtwwrk Report 9, 53 51, 1990. Nagasawa, K., A meteor spectrum in the infrared region. Tok~v ustron. Bull.. 2nd Ser. 213, 2505-2513. 1971.
Pergamon Printed
1994 (0 1994 Elsevier Science Ltd in GreatBritain. All rights reserved 0032-0633/94 $7.00+ 0.00
0032-0633(93)E0023-6
Two components in meteor spectra Jiii Borovirka Astronomical
Institute,
25 1 65 Ondfejov
Observatory,
Received 8 July 1993 ; revised and accepted
Czech Republic
10 November
1993
Abstract. Through an analysis of fireball spectra it was found that meteor heads consist of two parts with quite different temperatures. The spectra of both parts can be fitted with a simple thermal equilibrium model. The temperature of the main spectrum is about 4000 K, and that of the second spectrum is about 10,000 K. There is little evidence for a dependence of temperatures on the meteor velocity. However, the mass and luminosity of the second part relative to the main part grows rapidly with meteor velocity. The high temperature part forms only 0.02% of the meteor vapor envelope in slow meteors but accounts for more than 5% in fast meteors. In fast meteors most of the light is produced in this part, mainly by the CaII lines. This fact influences the meteor luminous efficiency and color index. The traditional classification of meteor spectra reflects the variable intensity of the second spectrum. In the high temperature region the density is also enhanced. This region is very probably related to the meteor shock wave.
Introduction events consist of three parts : the head, the wake and the train. All these parts can be resolved spatially (by visual, televisual or photographic observations) and exhibit different spectra (e.g. Halliday, 1968). The brightest part is the meteor head. Recently, a simple physical model, assuming thermal equilibrium, was applied to the spectrum of the head of the EN 151058 fireball, aiming to fit the spectrum and to determine the temperature and chemical composition of the radiating gas (Borovicka, 1993). This attempt was successful, nearly 300 lines of the spectrum were explained within this model and a temperature of 4400 K was determined at the meteor’s brightest point. Nevertheless, several lines remained unexplained. It was found that these Meteor
Correspondence
to: J. Borovitka.
lines form another spectral component of the head spectrum, with a temperature of about 10,000 K. This high temperature component will be called “the second spectrum”. An important conclusion was also made, that the gases forming the main and the second spectrum have the same chemical composition, the only difference being the temperature (and perhaps the density, which cannot be determined directly from the spectrum). Indirect evidence allowed us to estimate that air represents about 95% of the gases. The EN 151068 fireball was relatively slow, the velocity at the brightest point being 17 km/s. The main purpose of this paper is to study the relation of the main and the second spectrum for fireballs of various velocities. For this purpose, three other fireballs-with velocities of 32-67 km/s-were studied.
Formation of the second spectrum In this section we describe both spectral components and the physical conditions under which they arise. The theoretical background can be found in BoroviEka (1993). Thermal equilibrium (i.e. the Boltzmann and Saha equations for excitation and ionization) is assumed and, for simplicity, the same relative elemental abundances as in carbonaceous chondrites are assumed. The exceptions are nitrogen and oxygen, which come mainly from the atmosphere and are the most abundant elements in the radiating gas. We assumed the atomic ratios N/Fe = 120 and O/Fe = 35, as resulted from the pressure balance for the EN 151068 fireball (BoroviEka, 1993). The main parameters of a radiating volume are temperature, density and size. Here, we summarize how these parameters can be derived from the spectrum. Temperature can be determined unambiguously, provided that there is a sufficient number of suitable lines of one atom. This is usually true for the main spectrum (the lines of FeI), but not for the second spectrum. Density cannot be derived directly. If the radiation is optically thick in some lines, the column density, N (the density
146
J. Boroviekn
integrated along the line of sight, cm-“), can be separated from the visible surface area of the radiating region, P (cm”). Assuming that the geometrical depth is comparable to the transverse dimensions, the size and the density can then be estimated. The product NP is the total number of atoms in the radiating region. If the radiation is optically thin, only this product can be determined. Usually the radiation is optically thick for the main spectrum and optically thin for the second spectrum. Note also that the total number of atoms can only be determined from a spectrum calibrated on an absolute scale, while for the temperature the relative line intensities are sufficient. An important Factor for the appearance of the spectrum is the ionization of atoms. The degree of ionization depends on temperature and density. Although density has no direct influence on the spectrum, it does in fact affect line intensity ratios by changing the abundances of neutral atoms and ions. The situation for the main spectrum is usually clear. There are enough lines in the spectra of bright fireballs to determine all the main parameters of the radiating region. The degree of ionization is computed for the resulting temperature and density. The chemical composition need not be assumed since it can be computed from the abundances of neutral atoms. Typical lines of the main spectrum are given in Table 1. They are comprised mainly of Table 1. Typical lines of meteor main spectrum 2, (A)
I
Atom-m.
3608.9 3647.8 3479.9 3705.6 3719.9 3722.6 3733.3 3734.9 3137.1 3745.6 3748.3 3749.5 3758.2 3763.8 3799.5 3815.8 3820.4 3824.4 3825.9 3829.4 3832.3 3834.2 3838.3 3856.4 3859.9 3878.6 3886.3 3899.7 3905.5
5 5 5 5 7 5 5 7 7 12 6 7 6 6 7 6 8 6 7 6 8 6 10 6 8 6 8 6 3
Fe 1-23 Fe 1-23 Fe 1-5 Fe I-5 Fe 1-5 Fe l-5 Fe I-5 Fe 1-2 I Fe I-5 Fe I-5 Fe I-5 Fe I-2 1 Fe I-21 Fe I-21 Fe I-2 1 Fe I-45 Fe I-20 Fe 1-4 Fe I-20 Mg I-3 Mg I-3 Fe I-20 Mg I-3 Fe I-4 Fe 1-4 Fe I-4 Fe I-4 Fe I-4 Si I-3
in meteor spectra
lines of neutral Na, Mg, Fe, Ca, Cr and ionized Ca. The typical temperature is deduced to be 4000-4500 K (see below). The atoms of Na, Al and Ca are already highly ionized for the conditions of the main spectrum. The derived chemical composition for EN 15 1068 was consistent with the composition of chondrites. except for the depletion of refractory elements caused by an incomplete evaporation (Borovicka. 1993). For the second spectrum the situation is more complicated. The chemical composition cannot be derived exactly but, nevertheless, the chondritic values are consistent with the observations of the present fireballs. The typical lines for the second spectrum are given in Table 2. The high excitation lines of Mg II, Si II, N I and 0 I belong purely to the second spectrum. The lines of CaII are bright in both spectra. In fact. Table 2 contains almost all iines observed in the second spectrum. They are insufficient for a direct determination of temperature. Also, the atomic density is uncertain. As a startingvalue for our consideration we can choose nF, = 3 x 10’ ’ iron atoms per cm3. This corresponds to the density of the main spectrum region determined for the EN 151068 fireball. The key for the estimation of the temperature is the ratio of intensity of the Fe II lines to the Mg II and Si II (multiplet 2) lines, which have almost the same excitation potential. The Fe II lines have lower excitation potential
in A/i 3600-9000 8, with computed
intensities
E,
ah
1
Atom-m.
E,
1.01
3922.9 3927.9 3930.3 3933.7 3961.5 3968.5 4030.8 4033.1 4045.8 4063.6 4071.7 4132.1 4143.9 4202.0 4226.7 4254.4 4271.X 4274.8 4289.7 4307.9 4325.8 4375.9 4383.6 4404.x 4415.1 4891.5 4920.5 4957.6 5167.3
6 6 6 10 2 10 5 4 8 7 6 2 4 3 IO 7 7 6 5 8 7 2 11 8 4 1 1.5 2 8
Fe I-4 Fe 1-4 Fe I-4 Ca II-I Al l-l Ca II-1 Mn I-2 Mn I-2 Fe I-43 Fe 1-43 Fe I-43 Fe 1-43 Fe I-43 Fe 1-42 Ca I-2 Cri-1 Fe I-42 CrI-1 Crl-I Fe 1-42 Fe I-42 Fe I-2 Fe l-41 Fe I-41 Fe I-41 FeI-318 Fel-318 FeI-318 Mg I-2
0.05 0.11 0.09 0.00 0.01 0.00 0.00 0.00 1.49 1.56 1.61
0.91 0.00 0.05 0.00 0.09 0.11 0.86 0.05 0.09 a 0.11 0.91 0.96 0.99 0.96 a 1.49 0.86 0.00 0.91 2.71 2.71 0.96 2.71 0.05 0.00 0.09 0.05 0.09 1.90
: Two components
a.4
5167.5 5172.7 5183.6 5206.0 5208.4 5227.2 5269.5 5328.0 5371.5 5397.1 5405.8 5424.7 1.61 5434.5 1.56 5446.‘) 1.49 5455.6 0.00 5506.8 0.00 5528.4 I .49 0.00 5615.7 0.00 5889.9 5895.9 1.56 6162.2 1.61 O.OOW 6495.0 8183.3 I .49 S194.8 1.56 8387.8 1.61 2.85 a 8542.1 2.83 a 8662.1 2.81 a 86X8.6 2.71 8806.8
I ._
Atomm
4 17 22 3 4 3 10 8 5 3 3 3 2 2 2 0.5 0.5 0.5 40 34 0.5 0.5 2 3
Fe 1-37 Mg 1-2 Mg I-2 Cr 1-7 Cr I-7 Fe l-37 FeI-15 FeI-I5 FeI-15 Fe I-15 Fel-15 Fef-15 Fcl-I5 Fe I-15 Fcl-tS Fei-15 Mg I-Y Fe I-686 Na I-l Na I-1 Ca 1-3 FeI-168 Na I-4 Na 1-4 Fe I-60 Ca IL-2
I 4 3 1.5 2
Ca II-2 Fe I-60 Mg l-7
I:>I
1.49 w 2.71 3.12 0.94 0.94 1.56~ 0.86 w 0.91 0.96 0.91 0.99 0.96 1.OI 0.99 1.01 0.99 4.34 3.33 0.00 0.00 1.YO 2.39 2.lOp 2.lOp 2.18p 1.70 1.69 2.lXp 4.35 p
Z, The computed relative intensity. In the main spectrum, I of the red and infrared lines depends strongly on column density. These lines can be brighter than given here for bright fireballs and fainter for faint meteors; m, multiplet number: E,. lower excitation potential (in eV). In crowded regions, only the brightest lines are given. Remarks: a, the intensity includes nearby iinc(s) of the same multiple1 : p, predicted line. not yet reported in meteors; I-.not observable. masked by the lines of the main spectrum; w, may be stronger, if there is a substantial contribution of wake radiation.
J. Borovitka : Two components
2. Typical lines of meteor second spectrum
i(A)
I
Atom-m.
3706.0 3736.9 3759.3 3761.3 3856.1 3862.5 3933.7 3968.5 4131.0 4233.2 4481.2 4583.8 4923.9 5018.4 5169.0
2 4 3 3 2 1 400 250 I 1 20 1 3 5 3
Ca II- 3 Ca II-3 Ti II-13 Ti II-13 Si 11-l Si II-1 Ca II-I Ca II-1 Si II-3 Fe II-27 Mg II-4 Fe (I-38 Fe II 42 Fe II-42 Fe II-42
147
in meteor spectra
Ei 3.12r 3.15r 0.61 r 0.57 r 6.83 6.83 0.00 0.00 9.80 2.58 8.86 2.81 2.89 2.89 2.89r
in ii 3600&9000 A with computed
i(A) 53 16.6 5616.5 6158.2 6347.1 6371.4 6482.7 6562.8 7423.6 7442.3 7468.3 7711.9 1714.2 7775.4 7896.4 8184.8
I
1.5
2 4 6 13 9 6 2
intensities
Atom-m.
E,
J.(A)
Fe II-49
3.15 11.76 10.74a 8.09 8.09 11.76a 10.15 10.33 10.33 10.33 9.14 9.14 9.14
8188.0 8216.3 8223.1 8242.5 8446.4 8446.8 8498.0 8542.1 8662.1 8680.2 8683.4 8686.1 8703.2 8711.6 8718.8
N I-24 01-10 Si 11-2 Si KI-2 N I-21 HI-l N I-3 N I-3 N 1-3 0 1-l 01-l 01-l Mg II-8 N 1-2
10.00p 10.33
I
2 14 8 15 8 3 4
Atom-m.
E,
N I-2 N I-2 N I-2 N I-2 01-4 0 1-4 Ca II-2 Ca II-2 Ca II-2 N I-l NI-1 NI-1 NI-1 NI-1 NI-1
10.33 10.33 10.33 IO.33 9.52 9.52 I .69 1.70 1.69 10.34 10.33 10.33 10.33 10.33 10.34
See footnote to the Table I for explanation of symbols. and their intensity relative to the Mg II and Si II lines decreases with increasing temperature. The observed ratio corresponds nearly to 10,000 K. The ionization potentials of Mg, Si and Fe are similar, so that the abundance ratio Mg IIjSi II/Fe II is close to the ratio Mg/Si/Fe as a whole and the correction for ionization is not needed for this consideration. In fact, most Mg, Si and Fe atoms are expected to be just singly ionized for the conditions in question. Only for higher temperature (-_ 15,000 K) or lower density would most of the Mg and Fe atoms become doubly ionized, and the ratios Si II/Fe II and Si II/Mg II would increase correspondingly. Nitrogen and oxygen are both partly ionized. N I and 0 I have bright lines in the infrared. They have actually been observed in meteor spectra (Millman and Halliday, 1961; Nagasawa, 197 1; Halliday, 1987 ; Murayama. 1990) but our spectra do not cover the infrared region. The N I and 0 I lines in the visual part of the spectrum are fainter, but are observable weakly in some of our spectra. They confirm the temperature of about 10.000 K. because they would be stronger for higher temperatures (unless the atomic density was very low and N and 0 were highly ionized). The lines of N II and 0 11 have such high excitation potentials that they cannot be visible at temperatures below 20,000 K. They are actually not visible in our spectra (except for EN 210463, where their presence is uncertain). Their possible occurrence in Perseid spectra (Halliday, I96 1) may be probably attributed to some nonequilibriutn effects. The intensity of hydrogen lines depends on the actual abundance of this element, which probably varies in meteors. H lines are visible in only one of our fireballs (EN 210463). As for atomic density, values lower than nPe = 10’.‘cm-’ are improbable, because the abundance of CaII would become very small due to the transition to Ca II1 and the very bright H and K lines of Ca II could not be produced. For iron densities higher than lOI cmv3, elements like Na, Mg, Ca, Cr, Mn and Fe will not becompletely ionized, a non-negIi~ible alnoLlnt of neutral atoms will remain. and the same lines as in the main spectrum will be produced, of course with different intensity ratios. In that case it would be difficult to distinguish which part of a given line
comes from the main spectrum and which part originates from the second spectrum. Nevertheless, it seems that the computed main spectrum fits the observed spectrum quite well and that a substantial part of the lines of the above mentioned neutral atoms originate in the main spectrum. The iron density is therefore not higher than IO’” cm-‘. Only the lines of the high multiplets of Fe I (multiplets I 146. 1163, 1165) can also be formed in the second spectrum. These iron lines are relatively faint. The Mg I lines, which also have a relatively high excitation potential, may also play a role in the production of the second spectrum. We can therefore conclude that a typical second spectrum is formed in the region with temperature of 10,000 i 1000 K and atomic density of iron atoms of lo”10’” cm- ‘. The mass of this region in comparison with the main spectrum region will be studied in the next section.
Dependence on velocity Even from a spectrum calibrated on a relative scale, the ratio of numbers of atoms (NP) in both radiating regions can be determined. This represents the mass ratio of the region forming the main spectrum to that forming the second spectrum, ~~~.~~. Four fireballs of different velocities were studied in this way : EN 15 IO68 [ 17 km/s, data taken from Borovirka (1993)], EN 210463 [32 km/s, line intensities taken from Ceplecha (1971)], EN 300768 (45 km/s) and EN 23 1068 (67 km/s, Orionid). All spectra were obtained with grating cameras at the OndPejov Observatory. More details about the fireballs can be found in Ceplecha (1977). The results are given in Table 3. The following quantities are given in Table 3 : The veiocity 11, height h and absolute magnitude, mag, at the fireball’s brightest point (where the spectrum was analysed), the temperatures of the main and the second spectrum, T, and r,, the mass ratio M,/AJ, and the ratios of luminosities in the blue and the visual spectral region (B and V bands), ZfB’and Zfv). The temperature of the main spectrum is, within the limits of the errors, the same for all fireballs. For EN
J. BorovZka : Two components in meteor spectra
14X Table 3. The relation of the main and the second spectrum in four fireballs 1’ (km/s)
(k$)
mag
(2)
EN 151068 EN 2 IO463
17 32
45 77
-9 -12
4400 4300
( 10.000) (10,000)
3000 600
EN 30076X EN 231068
45 67
76 85
- 10 -7
(4100)$ 4100
( I0,000) ( 10.500)
250 15
Fireball
*Parentheses
indicate
that the temperature
-1
60
20 %xi~;
70
[k$S]
Fig. 1. The share of the second spectrum as a function of meteor velocity
I,‘B’!‘I,‘fi’ 20 6 3 0.3
I1\), [ (V) I ,‘2 500 I 00 80 4
values are not certain.
300768 the temperature is somewhat uncertain due to wake radiation, which is hard to separate from that of the meteor head. Note also that the temperature in EN I5 1068 in fact varied from 3500 to 4700 K along the fireball path (Borovieka, 1993). The temperature of 10,000 + 1000 K, as discussed in the previous section, is applicable to the second spectra of all fireballs studied. The temperature for the high velocity Orionid is only slightly enhanced. There are some individual differences; for example, in the Orionid the Si II lines are brighter relative to Mg II and Fe I I in comparison with the other three fireballs, but these differences cannot be unambiguously explained as differences in temperature and/or density (a discrepancy in the Ca II and N I lines would be the result for the Orionid). Rather, the differences are more likely to have been caused by differences in chemical composition or deviations from thermal equilibrium. For all velocities the gas forming the main spectrum dominates (M, > Mi). However, the mass ratio M,/M? decreases strongly with velocity, meaning that the share of the high temperature gas grows rapidly with velocity (the velocity is, of course, also closely related to the height-fast meteors have greater height). The dependence is displayed in Fig. I. It can be fitted with the exponential function
10
M,I’M?
M2 = 5 x IO-"M,e".'"',
where 1’ is in km/s. This empirical relation is, of course, preliminary. We cannot be sure that the ratio A4,iMz depends only on velocity. In principal, it may also depend on meteoroid mass or height or chemical composition. However, the dependence on velocity is certainly dominant. The changing ratio M,/M: is the main factor which influences the appearance of spectra of meteors of difl‘erent velocities. While the shape of the second spectrum probably remains nearly the same in all meteors and the main spectrum changes only gradually with increasing meteoroid mass as more lines become optically thick (major differences in chemical composition are not considered here), the ratio of the brightness of both spectra is a steep function of velocity. The Si II lines are dominant in the red part of the spectrum of fast meteors but are very inconspicuous in slow meteors. For fast meteors. the H and K lines of Ca II are by far the most intense lines. while for slow meteors this is not the case. In Fig. 2 the main spectrum and the second spectrum of the Orionid EN 231068 are compared. The spectra were computed for the parameters (temperature, abundances) derived from the observed spectrum. The role of the second spectrum should also be taken into account in association with its contribution to meteor luminous efficiency. In the last two columns of Table 3. the ratios of luminosities of the two spectra are given for two spectral bands. They were obtained by integrating the synthetic spectra (computed for the parameters derived from the observed spectra) multiplied by the transmissivity functions of the B and V bands of the UBV international photometric system. The luminosities do not include molecular emissions (almost negligible in the present spectra) and wake radiation. The second spectrum is always much brighter in the B band (due to strong Ca II lines) than in the V band, which is most effective between 5000 and 6000 A. where no strong emissions are present. The main spectrum is nearly color-neutral with B-V 2 0. The intensities of bright lines (including violet Call) are reduced by self-absorption effects to nearly the same level. For the Orionid case, more than three quarters of the light in the B band comes from the second spectrum. For velocities of about 30 km/s, the contribution of the second spectrum is nearly 10% in the B band. In V. the second spectrum is important only for high velocities (20% for the Orionid). The second spectrum is again dominant in fast meteors in the infrared due to the strong lines of 0 I and N I.
J. BoroviEka : Two components
0 . 35i-i
:uI~
I I II /lh
4000
4500
4.000
4500
149
in meteor spectra
I , i I I I ‘1 I .I I ‘t ‘14
5000
5500
6000
6500
6000
6600
6000
6500
0 3500
wavelength
Fig. 2. The main spectrum and the second spectrum of the Orionid EN 231068 on the same scale. The H and K lines in the second spectrum have been truncated; their intensities are II and 6, respectively
Conclusions observational facts given in this paper can be summarized as follows. Meteor heads consist of two diverse parts which were not resolved spatially but exhibit different spectra. For all velocities, most of the radiating gas around the meteoroid has a temperature of about 4000 K. A part of the gas has a temperature of about 10,000 K, again almost independently of the velocity. Differences in chemical composition between the two parts were not distinguished. These parts are not related to the two chemical components with different boiling points assumed in their dustball model for faint meteors by Hawkes and Jones (1975). The share of the second, high temperature, part grows rapidly, nearly exponentially, with meteor velocity. This part forms only * 0.02% of the meteor gas in slow meteors (2’ < 15 km/s), but contributes more than 5% of the gas in fast meteors (2: > 65 km/s}. in fast meteors most of the light is produced in the high temperature region. Besides the velocity, the share of the second spectral part may also depend on meteoroid mass, i.e. on meteor brightness. The various ratios of the infrared CaII lines (produced in both parts) and 0 I lines (produced in the second part) observed in Perseids of different brightness by Millman and HaiIiday (196 1) may be evidence for this effect. An alternative explanation is that the portion of air in the meteor gas changed among these meteors. Note that the 01 lines were bright even in the TV spectra of
The
very faint meteors (+4 mag) obtained by Mukhamednazarov and Mal’tseva (1989). The ratio of intensity of the second spectrum and the main spectrum is the key factor which differentiates the spectra of meteors of different velocities and similar chemical composition. Both spectra themselves do not differ so much from case to case. Millman’s standard classification of meteor spectra (e.g. Millman and McKinley. 1963) expresses mainty just the different intensity of the second spectrum. Type Y, where the H and K lines are the strongest lines. represents meteors with a strong second spectrum, whereas type X, where the strongest lines are of Na I or Mg 1, has a weak second spectrum. The rare type Z (strongest Fe I or Cr I) probably represents meteors with different chemical composition. The interpretation of the observational facts seems to be straightforward. Since not only temperature, but also density is very probably higher in the second part, the most likely situation is that this part is related to the meteor shock wave or to a relaxation region just behind the shock wave. The turbulent gas forming the main spectrum is not directly ill~Ltenced by the on-coming flow of air and its temperature does not depend on velocity. A somewhat surprising fact is that the temperature of the shock wave region is only very weakly influenced by velocity in our current observations. The cause of the steep decrease of the MI/Ml ratio is not so obvious. Preliminary considerations suggest that the thickness of the shock wave region does not change dramatically, with the geometrical thickness being of the order of 1 mm (0.553 mm) in the present four fireballs, assuming the density nFe = IO” cm-‘. (This iron density corresponds to the total atomic density n = 1.6 x 10’” cin3. including the air. and the free electron density n, = 3 x lOI cme3.) The size and density of the main spectrum region exhibits larger changes. There is no evidence for an intermediate region with a temperature between 5000 and 10,000 K. More meteor spectra should be studied to examine the relationship between the second spectrum and the main spectrum in order to improve our knowledge of its dependency on velocity. A dependency on brightness, height or other quantities should be examined as well. At the same time the observational constraints given here should be used in theoretical considerations, including calculations of shock wave formation, energy transport and of the luminous efficiency. A~,knolz,k~~lyerfl~nr.I am indebted to Dr D. 0. RcVelle for the improvement of the language style of this paper and for his valuable comments.
References Borovitka, J., A fireball spectrum
analysis. Asrro~. Astrophys. 279,627~645, 1993. Ceplecha, Z., Spectral data on terminal flare and wake of double station meteor no. 38421. Bull. ustron. fm-t. Czech. 22, 219304,1971. Ceplecha, Z., Fireballs photographed in central Europe. Bull. astron. Inst. Cxch. 28, 328 340, 1977. Halliday, I., A study of spectral line identifications in Perseid
150
meteor spectra. Puhl. Dominion Ohs. Ottuwu 25, 3-16, 1961. Halliday, I., The influence of exposure duration and trail orientation on photographic meteor spectra. in Phy.sirs and D~wamics of’Metcor,c (edited by L. Kresak and P. M. Millman), IAU Symp. no. 33, pp. 91~104. Reidel, Dordrecht, 1968. Halliday, I., The spectra of meteors from Halley’s comet. Astroll. AstrophJx. 187, 921-924. 1987. Hawkes, R. L. and Jones, J., A quantitative model for the ablation of dustball meteors. Mon. Not. R. astron. SW. 175, 339-356, 1975.
J. Borovicka
: Two components
in meteor spectra
Millman, P. M. and Halliday, I., The near infra-red spectrum of meteors. Plmwt. Spucc Sci. 5, 137-140, 1961. Millman, P. M. and McKinley, D. W. R., Meteors, in The Moon. Meteorites. und L’onx~t.s. The S&r Systen~, Vol. IV (edited by Middlehurst and Kuiper). pp. 674-773. The University of Chicago Press. 1963. Mukhamednazarov, S. and Mal’tseva, N. V., Study of the TV spectrograms of meteors. Astron. Ve.stnik 23, 297-303, 1989. Murayama, H., Near-infrared meteor spectra obtained in 1985. Tok~*o Mrtror Nrtwwrk Report 9, 53 51, 1990. Nagasawa, K., A meteor spectrum in the infrared region. Tok~v ustron. Bull.. 2nd Ser. 213, 2505-2513. 1971.