Physical properties and radiant distribution of the Orionids as observed by the Canadian Automated Meteor Observatory’s mirror tracking system

AbstractFourteen Orionids were observed by the Canadian Automated Meteor Observatory’s (CAMO) mirror tracking system. Their radiants were measured with an average precision of 3' and a possible radiant structure was revealed. Ablation modelling shows that light curves, decelerations, and wakes of the observed Orionids can be well modelled using a similar bulk density to the in-situ measurements of dust ejected by the comet 1P/Halley.IntroductionThe Orionids are an annual meteor shower whose parent body is the comet 1P/Halley. The shower mostly has mm-sized particles, but cm-sized Orionid fireball outbursts have been observed in the past (Spurný & Shrbený, 2007). Many previous studies attempted to characterize the radiant dispersion of fainter Orionids, but in most cases the precision of their observations was likely on the same order as the measured dispersion (Kresák & Porubčan, 1970; Hajduk, 1970). Kresák & Porubčan (1970) measured the dispersion of 0.84° (median offset from the mean radiant), while Spurný & Shrbený (2007) measured a dispersion of the resonant Orionid branch to be only 0.12°. Cm-sized meteoroids that are not very affected by non-gravitational forces and are locked in a resonance are naturally expected to have smaller dispersions, but it is not clear whether the dispersion of smaller non-resonant meteoroids was really resolved by Kresák & Porubčan (1970).Dynamical models are usually utilized to predict and understand the activity and evolution of meteoroid streams, including the Orionids (e.g. Sato & Watanabe, 2007; McIntosh & Jones, 1988). The accuracy of such models is dependent on knowing the physical properties of the parent body and the dust it produces, especially the bulk density of the ejected dust. The in-situ investigation of physical properties of dust ejected from 1P/Halley was done by the Vega-2 spacecraft – during its 1986 flyby it measured the dust bulk density of 300 kg/m3 (Krasnopolsky et al., 1988).In this work we use high-precision measurements of the 2019 Orionids and fit a meteoroid ablation model to them. We successfully fit the light curve and deceleration, and for the first time the wake of the observed meteoroids.Methods14 Orionids were observed by high-resolution narrow field CAMO cameras (6 arcseconds per pixel, 3 m/px at 100 km precision). The data was manually calibrated and reduced, and the trajectories were computed using the Monte Carlo meteor trajectory estimation method by Vida et al. (2020).The observed light curve, high-resolution meteoroid deceleration, and wake were fit using the Borovička et al. (2007) meteoroid ablation model which models meteoroid fragmentation as a continuous release of μm-sized grains.ResultsRadiant structureThe measured CAMO radiant dispersion was compared to radiant measurements by the Cameras for All-sky Meteor Surveillance (CAMS; Jenniskens et al., 2011), and the Global Meteor Network[1] (GMN) cameras with 16mm lenses. The comparison is shown in Fig 1. The CAMS and GMN data were filtered by excluding all trajectories with the convergence angles smaller than 15° and a velocity error higher than 15%. Furthermore, all radiants with radiant errors higher than 30 arc minutes for CAMS, and 5 arc minutes for GMN were excluded from the analysis. The radiant error cutoff reflects the stated errors in the datasets themselves and is chosen so that the 25% most precise radiants are used.Figure 1: Comparison of CAMO, CAMS, and GMN Orionid radiants. Error bars are shown for the GMN and CAMO data, while for CAMS are on the order of 0.5 deg.Fig 1. shows that both the CAMO and GMN data sets are small in number, which raises concerns about small number statistics. Nevertheless, they are consistent among themselves and the observed radiant dispersion is an order of magnitude higher than the stated radiant measurement precision. Interestingly, the radiants appear to the organized into two possible distinct groups and have a very low variation of the ecliptic latitude of only ~0.1°. In Fig 2., we show how we attempted to separate the radiants into two groups: one cut by ecliptic latitude at β = -7.5°, and the other cut by Sun-centered ecliptic longitude at λg – λs = 246°. After the radiant drift correction, the latitude cut does not seem to drastically reduce the radiant dispersion of individual groups below the overall dispersion of ~0.4°. Nevertheless, the cut by the Sun-centered longitude reduced the drift-corrected dispersion of the branch with λg – λs < 246° to only 0.1°. Although further measurements and dynamical modelling are needed to confirm the existence of the two separate groups, we find strong evidence that we have resolved the radiant structure of the Orionids.Figure 2: CAMO Orionid radiants color coded by the solar longitude. Figure 3: Dispersion analysis of the two groups split by the Sun-centred ecliptic longitude. [1] Global Meteor Network data: https://globalmeteornetwork.org/data/ Physical properties of the Orionids Although our modelling efforts are still in the initial stage, we were able to fit the ablation model to all observations quite well with very similar physical properties. Figures 4 and 5 show an example of the fit to the light curve, dynamics, and the wake to an Orionid observed on 2019/10/23 09:13:10 UTC. For this particular event, we used the initial velocity of 67.6 km/s at the beginning of the simulation at 180 km, a bulk density of 300 kg/m3, a grain density of 3000 kg/m3, initial mass of 3.1x10-6 kg, intrinsic ablation coefficient of 0.025 s2/km2, initial height of erosion of 114 km, and erosion coefficient of 0.45 s2/km2, a grain mass index of 2.15, an grain sizes between 19 – 317 μm. A detailed analysis will be done in a future paper. Figure 4: Light curve, velocity, and the wake fit for the example CAMO Orionid. Figure 5: Lag (“the distance that the meteoroid falls behind an object with a constant velocity that is equal to the initial meteoroid velocity”; Subasinghe et al, 2017) fit for the example CAMO Orionid.

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