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The Astrophysical Journal, 638:1176–1179, 2006 February 20 # 2006. The American Astronomical Society. All rights reserved. Printed in U.S.A.

METALLIC ABUNDANCES OF 2002 LEONID METEOROIDS IN TWO DUST TRAILS FORMED IN DIFFERENT EPOCHS: NO EVIDENCE OF SOLAR HEATING EFFECT T. Kasuga,1, 2 J. Watanabe,2 T. Yamamoto,3 N. Ebizuka,4 and H. Kawakita5 Received 2005 June 25; accepted 2005 October 18

ABSTRACT High-definition TV spectra in the ultraviolet to visible region were obtained during the 2002 Leonid aircraft campaign. We analyzed 20 meteor spectra obtained from the 1767 (seven revolution) and 1866 (four revolution) trails on 2002 November 19 and identified neutral atoms, mainly Mg i, Fe i, Ca i, and Na i, in the observed wavelengths between 300 and 650 nm. The singly ionized atomic emissions, Ca ii and Mg ii lines, also appeared in the spectrum. The abundances of the metallic atoms, the electronic excitation temperature, and the electron density are obtained for each spectrum, assuming the Boltzmann distribution for the number at each energy level. The metallic abundances of Fe, Ca, and Na relative to Mg are slightly lower than solar abundances on average. We could not find any evidence of the solar heating effect on Leonid meteoroids between the 1767 and 1866 trails on orbit with their perihelion (q
1 AU). We can support the idea that silicate and carbon-mixed silicate are preserved in interplanetary space for at least several hundred years. Bands of CHON-related molecules, such as OH and CN, are not detected in this study. Subject headinggs: meteors, meteoroids — Sun: abundances

1. INTRODUCTION

heating effect, we need samples of the same shower with short or long perihelion but originating from clearly known epochs of several dust trails. Thus, it is still an open question whether the solar heating effect on meteoroids causes the change of their metallic abundances during orbital motion in interplanetary space. The recent activities of the Leonid meteor showers, which are the fastest (72 km s1) of all meteor showers, are caused by the Earth intersecting dense trails of dust ejected from the parent comet 55P/Tempel-Tuttle (q
1 AU ). After the return of the parent comet in 1998, several theoretical calculations predicted that peak activities could reach storm levels in the next 5 yr (McNaught & Asher 1999, 2001). The Leonid Multi-Instrument Aircraft (Leonid MAC) campaign, which started in 1998, has brought the greatest advances in meteor astronomy (Jenniskens & Butow 1999; Jenniskens et al. 2000a; Jenniskens 2002a). This mission’s goal was to bring together scientists in different disciplines and from all over the world to cooperate on observations of the Leonid meteors using wide-ranging techniques in 1998– 2002. The 2002 Leonid storms were considered to be the first instance of such storms during which we would be ‘‘lucky’’ enough to gain an understanding of the solar heating effect on meteoroids because the two different epochs of ejection of the meteoroids and the duration of their exposure to space had already been predicted. Two storm peaks were expected for the Leonid meteor shower on 2002 November 19 (Lyytinen & Van Flandern 2000; McNaught & Asher 2002; Jenniskens 2002b; Vaubaillon 2002). The first, the 1767 (seven revolution) dust trail, was predicted to cause a strong display over Europe at 4:06 UT, and the second, the 1866 (four revolution) dust trail, was predicted to appear over the United States at 10:47 UT with zenithal hourly rate (ZHR) in the 2000– 4000 range (Gural et al. 2004). This was a rare chance to focus on any difference between two dust trails, which are evaluated in this paper by the same instruments under similar observing conditions. During the 2002 Leonid MAC mission, we succeeded in obtaining 20 spectra of the Leonid meteors using the airborne highdefinition television (HDTV) spectroscopic system focused on the near-ultraviolet wavelength range on 2002 November 19 (Kasuga et al. 2005b). The ultraviolet-visible region is confirmed to contain many lines of metallic atoms (e.g., Borovicˇka 1993), and bands of

Generally, meteor showers originate in comet dust. Comets are exposed to solar radiation and heated every return, and their dust particles are emitted into interplanetary space. They, as well as their dust particles, experience weathering due to heating and cooling cycles, solar wind affection, and dust-dust collisions during their orbital revolution (Mann & Czechowski 2005). Thus, the solar heating effect and space weathering on meteoroids may facilitate fragmentation of the grains, make the remaining units more (or less) cohesive, and change the volatile elements’ contribution (Na, sodium-containing minerals, and, if possible, organics) as suggested by Jenniskens (2002c). The solar heating effect on meteoroids may strongly depend on their perihelion distance. For example, a 2004 Geminid meteor with a small perihelion (q
0:14 AU) shows extreme Na depletion (Kasuga et al. 2005a). Borovicˇka et al. (2005) suggested that the wide variation in Na content of Geminid meteors, from Na-poor to solar abundances, is correlated with the time the meteoroid is orbiting the Sun as a separate body. Thus, short-perihelion meteoroids are believed to be susceptible to the solar heating effect. However, Evidence of the solar heating effect from the Geminid meteor is inconclusive. Its parent is thought to be the asteroid 3200 Phaethon (e.g., Williams & Wu 1993). The dust trails are not detected even now (e.g., Hsieh & Jewitt 2005). Furthermore, it is not clear from their formation epochs whether the dust trails existed because the period is very short (
1.5 yr). It is also unknown whether some asteroids have unusual Na content. In order to investigate the solar 1 Department of Astronomical Science, School of Physical Science, Graduate University for Advanced Studies, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan; [email protected]. 2 National Astronomical Observatory of Japan ( NAOJ), National Institute of Natural Science, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan; jun.watanabe@ nao.ac.jp. 3 Institute of Low Temperature Science, Department of Earth and Planetary Sciences, Hokkaido University, Kita-19, Nishi-8, Kita-ku, Sapporo, Hokkaido 060-0819, Japan; [email protected]. 4 Institute of Physical and Chemical Research ( RIKEN ), Wako, Saitama 3510198, Japan; [email protected]. 5 Department of Physics, Kyoto Sangyo University, Motoyama, Kamigamo, Kita-ku, Kyoto 603-8555, Japan; [email protected].

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METALLIC ABUNDANCES OF 2002 LEONID METEOROIDS CHON-related molecules such as OH and CN have also been anticipated by previous research (Jenniskens et al. 2002). The number of spectra originating in the 1767 trail was seven, and 13 spectra were deduced from the 1866 trail. In this paper, we first describe the metallic abundances deduced from two dust trails formed in different epochs and then discuss the solar heating effects on the cometary meteoroids of dust trails with a perihelion distance of q
1 AU. 2. OBSERVATIONS The HDTV near-ultraviolet spectroscopic observation was performed on board a NASA DC-8 airplane during the 2002 Leonid MAC mission. At the same time, a TV observation was also performed on board the Flying Infrared Signature Technology Aircraft ( FISTA). The final mission of the NASA- and USAFsponsored 2002 Leonid MAC project of two instrumented aircraft flew from Madrid, Spain, to Omaha, Nebraska, putting 38 participating researchers from seven nations in a position to study Leonid meteor storms under excellent observing conditions on 2002 November 19 (Jenniskens 2003). The flight schedule was set in order to ensure detection of both predicted Leonid meteor storm peaks deduced from the dust trails formed in 1767 and 1866 (Gural et al. 2004). The HDTV spectroscopic efficiency of the airborne observation using the 600 groove mm1 reflective grating and detailed calibration method was described in Kasuga et al. (2005b). The diagonal coverage of the field of view (FOV ) was 30 , and the observable bands were in the 300–650 nm range. Unfortunately, the efficiency for the wavelength range below 350 nm has large error bars because the flux of the calibrated source reflection emission was weak. A maximum spectral resolution of
1.0 nm (k/
k
300) was achieved for the reflective grating mentioned above. 3. ANALYSIS 3.1. Line Identification During the 2002 Leonid MAC mission, the Japanese team obtained 20 spectra of Leonid meteors. In this paper, we focus on all spectrum data obtained from both the first and the second peak activities of the 2002 Leonids (Table 1). At wavelengths longer than 600 nm, most of the emission features originated in the Earth’s atmosphere, whereas most of the features below 600 nm originated in the meteor. Many lines of metallic elements appear that can be identified by the line catalog as shown in Kasuga et al. (2005b). Metallic atom emission lines at 518 nm ( Mg i triplet) and 589 nm ( Na i doublet) are typical for the Leonid meteors. Unfortunately, Na i lines (589 nm) are obtained only from four meteors during our observation because the coverage of the wavelength is limited by the FOV (Table 1). The lines at 358 nm (Fe i), 374 nm (Fe i), 383 nm (Mg i, Fe i), 404 nm (Fe i), 423 nm (Ca i), and 438 nm (Fe i) were also identified. The lines at around 393– 396 and 448 nm are recognized. These lines can be interpreted as the ionized emissions of Ca ii (393, 396 nm) and Mg ii. Ionized emissions are important for deriving the electron density, as described in x 3.2. In the near-ultraviolet region around 300–400 nm, bands of CHON-related molecules, such as OH and CN, are expected, as well as many lines of metallic atoms. In the lines near 309 nm, bands of interesting molecules, such as OH A2+ –X 2 (0–0), were possibly detected in another Leonid meteor in a different year (Jenniskens et al. 2002). However, because of poor efficiency in the wavelength region below 350 nm, we could not detect any

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TABLE 1 Analyzed Meteors of the 2002 Leonids

UT

Te (K)

ne (m3)

Ca ii (393, 396 nm) Component Type

Na i (589 nm) within FOV?

First Peak (1767 Trail ) 3:47:54a ....... 3:49:57 ........ 4:05:24 ........ 4:21:36 ........ 4:54:38 ........ 5:15:30 ........ 5:16:42 ........

5784 5353 4929 5618 5392 5287 5240

3.6 1.6 6.1 3.2 1.5 7.0 1.3

; ; ; ; ; ; ;

1021 1021 1019 1021 1021 1020 1020

Main Main Hot Main Main Hot Hot

Yes No No No Yes No No

Second Peak (1866 Trail ) 10:25:53 ...... 10:27:39 ...... 10:34:04 ...... 10:35:26 ...... 10:35:44 ...... 10:41:07 ...... 10:41:25 ...... 10:42:05 ...... 10:50:08 ...... 10:52:50 ...... 10:54:30 ...... 11:14:52 ...... 11:15:19 ......

5860 5124 5120 5208 5336 5146 5497 5118 5149 5305 5135 5929 5280

5.5 1.3 3.2 4.4 4.7 5.5 4.4 6.0 4.1 6.0 9.2 6.4 1.3

; ; ; ; ; ; ; ; ; ; ; ; ;

1021 1021 1020 1020 1020 1021 1021 1020 1020 1020 1020 1021 1021

Main Main Main Main Main Main Main Main Main Hot Main Main Main

No No No Yes No No No No No No No No Yes

Note.—Derived values of excitation temperature Te , electron density ne , and component type of Ca ii (393, 396 nm) and Na i (589 nm) in the FOV. a Fireball in Kasuga et al. (2005b).

emissions related to OH or CN in any observed spectra listed in Table 1. In this study, we chose the highest quality spectrum frame for each meteor spectrum under the condition of being nonsaturated and in which the widest wavelength range is taken in Table 1, although we already know that the metallic abundances may vary slightly along the meteor trajectory; for example, the relative variations of Fe/Mg and Na/Mg reach
20%, and that of Ca/Mg is
50% along the fireball trajectory (Kasuga et al. 2005b). 3.2. Abundances of Metallic Elements In this study, we assume a Boltzmann distribution for the number at each energy level and that the excitation temperature, Te , is the same for all neutral metallic species and blackbody. The analysis methods for obtaining the metallic abundances are described in Kasuga et al. (2004, 2005b). The procedure yielded the excitation temperature Te listed as in Table 1 and the total number ratio relative to Mg i, namely, Fe i/Mg i, Ca i/Mg i, and Na i/Mg i, for four neutral atoms. Here we have to consider the degree of ionization of the atoms in order to obtain the elemental abundances derived by the Saha equation (Allen 1999). For the total number of atomic species X, NXtotal is expressed as NXtotal ¼ NX i þ NX ii ;

ð1Þ

where NX i is the number of neutral atoms and NX ii is the number of singly ionized atoms. The number of more highly ionized atoms is thought to be negligible (e.g., Borovicˇka 1993). When first considering ionized atoms, we applied ionized emissions (Mg ii and Ca ii) for the hot-component theory to obtain Ca ii/Mg ii as described in Kasuga et al. (2005b). However,

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KASUGA ET AL.

Fig. 1.—Derived metallic abundances of Fe (crosses), Ca ( plus signs), and Na (asterisks) relative to Mg for 20 Leonid meteors. The first peak contains seven meteor data points belonging to the 1767 trail, while the second peak contains 13 data points belonging to the 1866 trail. The horizontal lines are the solar abundances (Anders & Grevesse 1989).

the application of this analysis process failed for some obtained spectra. Nevertheless, the temperature of the hot component could be fitted to 10,000 K. According to this theory, two types of electron densities are obtained as solutions using the Saha function and the relation between the pressure of the radiant gas on the main component and the hot component. However, both of the derived values of the electron density were negative, which is an unrealistic situation for observed spectra in most cases. In those cases, we cannot avoid the conclusion that they are not in a hot-component condition. Here we assume that the Ca ii (393, 396 nm) was under the main-component condition instead of the hot-component condition for some spectra in order to derive the electron density (Kasuga et al. 2005b, 2006). We derived the total metallic abundances using this electron density. The component type of Ca ii (393, 396 nm) and the electron density for each spectrum are listed in Table 1. The mean excitation temperature in Table 1, 5340  270 K, roughly agrees with Te of the other cases of Leonid meteors estimated by Trigo-Rodrı´guez et al. (2003). The Leonids show the highest excitation temperature because of the highest kinetic energy caused by the retrograde trajectory ( Trigo-Rodrı´guez et al. 2003). The excitation temperature value,
3900 K, of the June Boo¨tid meteor is low, which agrees with their slow-moving velocity (18 km s1; Kasuga et al. 2004). 4. RESULTS AND DISCUSSION The results are plotted in Figure 1, showing the metallic abundances of Fe, Ca, and Na relative to Mg, together with their solar abundances, shown as horizontal lines (Anders & Grevesse 1989). The mean values for each trail are listed in Table 2. 4.1. Metallic Abundances One of the remarkable features in Figure 1 is that the abundance ratio of Fe/Mg is almost lower than the solar abundance in both trails. This trend in the 2002 Leonids has already been suggested by Kasuga et al. (2005b), which indicates the presence of Mg-rich silicate meteoroids, as observed in comets. The mean value of the 1767 trail is (Fe/ Mg)1st /(Fe/ Mg) ¼ 0:55, and that of the 1866 trail is (Fe/ Mg)2nd /(Fe/ Mg) ¼ 0:60, as shown in Table 2. In our data, Ca/Mg is always lower than the solar abundance ratio in both trails: (Ca /Mg)1st /(Ca /Mg) ¼ 0:25, and

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(Ca /Mg)2nd /(Ca / Mg) ¼ 0:19. Ca can be easily affected by incomplete evaporation among Mg, Fe, and Na, as described in Trigo-Rodrı´guez et al. (2003) and Borovicˇka et al. (1999). The Ca trend is also confirmed in the evaporation process along the fireball trajectory, which strongly suggests incomplete evaporation of Ca due to refractoriness ( Kasuga et al. 2005b). Na/Mg has a value slightly lower than that of the solar abundance in our data. The mean is ( Na /Mg)1st /( Na / Mg) ¼ 0:72 in 1767 and (Na/ Mg)2nd /( Na/Mg) ¼ 0:57 in 1866. Recent work by Trigo-Rodrı´guez et al. (2003, 2004) shows that the mean metallic abundances of Leonid meteors almost agree with the solar abundances except for Ca and Na; the Ca abundance is lower than the solar abundance, while Na is more abundant than the solar value. Kasuga et al. (2005b) also show that the Na abundance is slightly higher than the solar abundances. A noteworthy fact is that Kasuga et al. (2005b) focused on just one of the brightest fireballs in the 2002 Leonids along its trajectory. Generally, faint meteors tend to be poor in Na, which is true for Leonid meteors (Borovicˇka et al. 1999). Errors of the fitted metallic abundances and excitation temperatures are not included in Figure 1 because they are unrealistically small, while errors of the metallic abundances correspond to several kelvins for excitation temperatures (Kasuga et al. 2005b; Borovicˇka 1993). On the other hand, population standard deviations for each abundance are added in Tables 1 and 2 for statistical evaluation. 4.2. Solar Heating Effect on Leonid Meteoroids Here we concentrate on the solar heating effect on meteoroids. The metallic abundances of meteoroids may vary with their perihelion distance because of the solar heating effect. However, this effect is not clearly understood yet. The comets’ perihelion passages are believed to be special places where meteoroids are strongly heated by the Sun. Following in situ research, Fomenkova et al. (1992) concluded that dust in comets, for example, P/Halley, consists of (1) CHON, (2) carbonmixed silicate, and (3) silicate particles. If meteoroids are affected by solar heating, then variation of the metallic abundance between the 1767 (seven revolution) and 1866 (four revolution) trails is expected. For example, we might easily expect that Na in the 1866 trail has evaporated less, so the Na abundance in the 1866 trail could be larger than that in the 1767 trail. On the other hand, the Ca abundance would be large in the 1767 trail because of its refractoriness relative to Mg (Field 1974, Table 3). However, the metallic abundances in both trails, listed in Table 2, do not show any clear difference. Each trail shows almost the same abundances. We can conclude that the solar heating effect on the metallic composition is not confirmed in Leonid meteoroids after at least 100 yr; i.e., silicate, even after ejection from a comet, either is not affected by solar heating or is protected by something from solar exposure TABLE 2 Comparison of Metallic Abundances Meteoric Value Abundance Ratio

First Peak

Second Peak

Solar Value

Fe/Mg......................... Ca/Mg........................ Na /Mg .......................

0.46  0.23 0.014  0.007 0.039  0.012

0.50  0.45 0.011  0.009 0.031  0.005

0.84 0.057 0.054

Note.—Comparison of the metallic abundances originating in 1767 (first peak) with those of the 1866 trail (second peak) and the solar abundances (taken from Anders & Grevesse 1989).

No. 2, 2006

METALLIC ABUNDANCES OF 2002 LEONID METEOROIDS

in interplanetary space. This result concentrating on silicate particles deduced from two Leonid trails may encourage confirmation of previous research efforts on carbon-mixed silicate in cometary dust. Mann & Czechowski (2005) indicate that cometary dust contains carbon in the form of organic refractory material that can survive high temperatures in the inner solar system, and Carbary et al. (2003) have already detected carbon in a Leonid meteor. If carbon is mixed in silicate, it also can be preserved in comet dust with a metallic composition. Our study strongly supports previous research carried out by Mann & Czechowski (2005) and Carbary et al. (2003). We concluded that carbon-mixed silicate and silicate are not affected by the solar heating effect for at least 100 yr on orbits with a perihelion distance of
1 AU. If a preservation model for CHON in Leonid meteors is developed and more observational results, as well as possible detection of OH (Jenniskens et al. 2002), are reported, then meteors will be a very important key to astrobiology (Jenniskens et al. 2000b). However, the discussion of a CHON-preservation model in meteoroids is beyond the scope of our paper. Metallic (or carbon-mixed) compositions of Leonid meteoroids are found to be preserved in interplanetary space at a perihelion distance
1 AU. They are not affected by solar heating. From another view, silicate cores might be protected by organic refractory mantles from solar heating in interplanetary space (Greenberg 2000). If the perihelion distances of meteoroids were smaller, this concept might be reconsidered. The Sun-approaching meteor showers, originating from comets, are suitable for this investigation in future work. 5. CONCLUSIONS We carried out HDTV spectroscopic observations of the 2002 Leonid meteor shower on the 2002 Leonid MAC mission and analyzed all meteor spectra obtained from the 1767 (seven revolution) and 1866 (four revolution) trails on 2002 November 19. From analysis of these spectra, we obtained the abundances of metallic atoms, the electronic excitation temperature, and the

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electron density. It was found that the metallic abundances in this Leonid meteor shower, under the assumption of the Boltzmann distribution for the number at each energy level, differed slightly from the solar abundances and that their abundances were almost the same in each trail. These results indicate that all 2002 Leonid meteor spectra data obtained with the HDTV system helped us to clarify the fact that solar heating does not affect Leonid meteoroids in interplanetary space at a perihelion distance of
1 AU and that metals in silicate dust are preserved at least for 100 yr. The Sun-approaching meteor showers with small perihelion distances may show different results. This matter will be addressed in future work. We have reached the following conclusions about these meteoroids: 1. The solar heating effect on Leonid meteoroids is not confirmed. 2. Silicate composition and carbon-mixed silicate are preserved in Leonid meteoroids for 100 yr. 3. Bands of interesting molecules, such as OH and CN, were not confirmed in this study.

The authors are grateful to Peter Jenniskens for organizing the 2002 Leonid MAC mission. The mission was successful with the support of NASA Ames Research Center (USA), Edwards Air Force Base ( USA), Offutt Air Force Base (USA), Torrejon Air Force Base (Spain), and the Center for Astrobiology (Spain). The Leonid project in Japan was supported by grants from the National Astronomical Observatory of Japan ( NAOJ) and the Japan Space Forum (JSF). T. K. thanks Y. Hirahara (Nagoya University), H. Yano (ISAS/JAXA), and S. Abe (Ondrejov, Czech Republic) for their kind support. T. Y. acknowledges support by a grant from the Institute of Low Temperature Science, Hokkaido University, and by Grants-in-Aid from the Japan Society for the Promotion of Science.

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