T. Kasuga

Observations of 1999 YC and the Breakup of the Geminid Stream Parent

The Apollo asteroid 1999 YC may share a dynamical association with the Phaethon-Geminid Stream Complex (PGC) (Ohtsuka et al. 2008). Here, we present photometric observations taken to determine the physical properties of 1999 YC. The object shows a nearly neutral reflection spectrum, similar to but slightly redder than the related objects 3200 Phaethon and 2005 UD. Assuming an albedo equal to that of 3200 Phaethon, we find that the diameter is 1.4 ± 0.1 km. Time-resolved broadband photometry yields a double-peaked rotational period of 4.4950 ± 0.0010 h, while the range of the light curve indicates an elongated shape having a projected axis ratio of ~1.9. Surface brightness models provide no evidence of lasting mass loss of the kind seen in active short-period cometary nuclei. An upper limit to the mass loss is set at ~10–3 kg s–1, corresponding to an upper limit on the fraction of the surface that could be sublimating water ice of <10–3. If sustained over the 1000 yr age of the Geminid stream, the total mass loss from 1999 YC (3 × 107 kg) would be small compared to the reported stream mass (~1012-1013 kg), suggesting that the stream is the product of catastrophic, rather than steady-state, breakup of the parent object.

Thermal desorption of Na in meteoroids: Dependence on perihelion distance of meteor showers

Context. The observed sodium abundance of meteoroids in meteor showers might differ from the original abundance because of processing in interplanetary space after ejections from their parent bodies. Among various processes, thermal alteration of alkali silicate is most likely the major process of Na depletion. Aims. We clarify at which perihelion distances the thermal desorption alters the Na content of meteoroids that are observed as meteor showers. Methods. We compile Na abundances of meteoroids in meteor showers at each perihelion distance and compare them to the sublimation temperatures of alkali silicates. Results. Na abundances of meteoroids do not depend on their perihelion distances at 0.14 ≤ q ≤ 0.99 AU. No Na depletion in these distances constrains the temperature of meteoroids at q = 0.14AU to be lower than the sublimation temperature of alkali silicates ∼900 K. Conclusions. Meteoroid particles are characterized as large, compact, blackbody- like particles. On orbit with perihelion distances q < 0.1 AU, meteoroids would show evidence of thermal desorption of metals, in particular, Na.

Metallic abundances of the 2002 Leonid meteor deduced from high-definition TV spectra

High-definition TV spectra in the ultraviolet-visible region were obtained during the 2002 Leonid aircraft campaign. We analyze the spectra of the brightest fireball that appeared at 03 h 47 m 54 s UT on Nov. 19, 2002 and identify the neutral atoms, mainly MgI, FeI, CaI, and NaI in the observed wavelengths between 300-650 nm. The singly ionized atomic emissions, CaII and MgII lines, also appeared in the spectrum in several epochs during the series of video frames. From analysis of the spectra, time variation in the abundances of metallic atoms, along with their electronic excitation and blackbody temperatures, were obtained assuming the Local Thermal Equilibrium (LTE) condition. Both Fe and Ca abundances relative to Mg are lower than the solar abundance, while Na is slightly higher. We found correlation between the excitation temperature and the abundance of Ca, which suggests incomplete evaporation of the Ca due to intrinsic refractoriness. A search for bands of CHON-related molecules, such as OH and CN, is not successful in the brightest fireball in this study.

First result of June Boötid meteor spectrum

This paper shows the first observational result of a spectrum of a June Bootid meteor obtained by the High-Definition TV observational system. The very faint spectrum appeared at 15 h 59 m 29.693 s on 2004 June 23 UT, which was about 5th mag- nitude. Wavelengths between 360−620 nm were observed, and the strong emissions of neutral atoms as MgI, FeI and NaI were identified. Emissions of single ionized atoms were not observed. The abundances of metallic atoms, their excitation tempera- ture were obtained under the Local Thermal Equilibrium (LTE) conditions. The results, Fe/Mg = 0.15 and Na/Mg = 0.00027 suggest the possibility that the abundances of June Bootid meteor are extremely different from the solar system abundances. The excitation temperature value, 3867 ± 13 K is low in agreement with their slow moving velocity.

Metallic abundances of 2002 leonid meteoroids in two dust trails formed in different epochs : No evidence of solar heating effect

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.

Is a 2004 Leonid meteor spectrum captured in a 182cm telescope

Context. It has been thought that fast-moving meteor spectra consist of only two excitation temperature regimes: the 5000 K main component and another hot component at 10 000 K. This belief does not always satisfy observed spectra due to the lack of sufficient physical correlation among derived excitation temperatures, observed fluxes, upper energy levels (E u ), and Einstein A coefficients (A ul ) of each spectral emission line. Aims. This work tries to correlate them and discovery new excitation temperature regimes in meteor spectra in the visual to near IR wavelength region. Methods. We focus on the upper energy levels and Einstein A coefficients of observed spectral emission lines. A model fitting the first positive band of nitrogen (N 2 ) and total number of Si II under quasi-neutral conditions proved the key to identifying of new components. Results. We have identified two new excitation temperature regions in meteor spectra. One is a Mid component at 8000 K for N 2 and another a Jet component above 10 000 K for Si II. This breakthrough has allowed us to reproduce the meteor spectrum. Conclusions. The spectra of high-speed meteors may consist of more than two excitation temperature regions including the main, Mid, hot, and Jet components.