Possible interstellar meteoroids detected by the Canadian Meteor Orbit Radar
Introduction
Interstellar meteoroids offer a source of direct sampling of material that originated from beyond our solar system. Presolar grains embedded within meteorites already provide samples of small solids formed in other star systems (Zinner, 2014), but the particular system where such grains are produced is unknown. Observations having trajectory information of large (>10 μm) interstellar meteoroids may retain information on their source region as well as providing clues as to the environment through which they traversed before being detected, as such large particles are not coupled to the local gas flow. Even larger interstellar particles (ISPs) on the order >100 μm in radius (Murray et al., 2004) may travel through the interstellar medium (ISM) for great distances with little perturbation by Lorentz forces. Integrating the motion of these meteoroids may allow for their original sources to be determined. Unique identification of the stellar system from which a particular interstellar meteoroid originates can help constrain the planet formation process, provide limits of the spatial density of larger grains in the interstellar medium, and probe debris disks.
Small interstellar particles have been directly detected in-situ in the solar system by various spacecraft impact ionization sensors, including the Ulysses (Grün et al., 1993), Galileo (Baguhl et al., 1995) and Cassini spacecraft (Altobelli, 2003), though the particle masses derived from impact sensors are very uncertain. The impact-detected particles were on the order of 1 μm in radius (10−14 kg) and smaller, assuming an average density of 3000 kg/m3. Plasma producing EM emission from impacts of micron-sized and smaller interstellar particles have been detected by antennas on the STEREO (Zaslavsky et al., 2012) and WIND (Wood et al., 2015) spacecraft. These measurements showed that sub-micron interstellar dust shows a strong flux variation with time and orbital location. Such small particles are coupled to the local gas flow in the solar neighborhood, which has an upstream flow direction in galactic coordinates of lgal = 3°, bgal = 16° (Frisch et al., 1999) with a speed of 25 km/s. A good review summarizing our understanding of the small interstellar dust detected in the solar system is given by Sterken et al. (2019).
The radar detection of interstellar meteoroids was reported by Baggaley (2000) using the Advanced Meteor Orbit Radar (AMOR) in New Zealand and by Meisel et al. (2002a) using the Arecibo radio telescope in Puerto Rico. Masses and sizes of these detections were 9 10−9 kg (89 μm) and 7 10−11 kg (18 μm), respectively, again assuming a meteoroid density of 3000 kg/m3. However, the veracity of these detections as real interstellar meteoroids has been questioned by other authors [eg. Hajduk, 2001; Murray et al., 2004; Musci et al., 2012].
Hajduková et al. (2013) examined a catalogue comprised of 64650 meteors observed by a multi-station video meteor network in Japan between 2007 and 2008 (SonotaCo, 2009). Of these detections, 7489 appeared to have hyperbolic orbits. After filtering for meteors with low error in measured velocity and rejecting meteors associated with showers, 238 retained hyperbolic orbits and showed no prior significant gravitational perturbations from close encounters with planets within our solar system. Their main conclusion, that most apparently hyperbolic optically measured meteoroid orbits are due to measurement error, is similar to the main conclusion from other similar studies [eg. Musci et al. (2012); Hajduková (2012)].
Murray et al. (2004) provided estimates for the flux of interstellar particles and examined potential ISP-producing sources. They suggested the most prolific source would be dust grains produced as condensates in the atmospheres of asymptotic giant branch stars, which are blown into the ISM by stellar winds. Other sources they considered included dust grains from young main-sequence stars ejected by dynamical interactions with planets, and dust ejected due to radiation pressure from a host star. Additionally, they identified dust grains emitted from high-speed narrow jets formed during the accretion phase of young stellar objects to be a possible source of interstellar meteoroids. Weryk and Brown (2004) analyzed data collected by the Canadian Meteor Orbit Radar (CMOR) between May 2002 and September 2004, where more than 1.5 million meteor orbits were computed. From this study, 40 meteoroids were found to have heliocentric speeds 2σ above the hyperbolic limit, and 12 of these meteoroids had heliocentric speeds 3σ above the hyperbolic limit.
The present study may be considered an extension to the work of Weryk and Brown (2004). Here we expand on that study, in particular, by examining CMOR echoes collected on six receiver stations (five remote, as compared to only two available remote stations for the previous study). These additional stations significantly increase the confidence in time of flight velocity solutions, compared to the minimum three required for a unique time of flight solution as was the case for the earlier Weryk and Brown (2004) study. We also develop an improved velocity correction for atmospheric deceleration for CMOR echoes based on examination of shower-associated meteors. This improves our confidence in derived out-of-atmosphere velocities and therefore calculated heliocentric orbits.
The discovery of 1I/‘Oumuamua, in 2017 (Meech et al., 2017) was the first definitive observation of a large interstellar object transiting through our solar system. A two day observation arc established that the orbit of ‘Oumuamua was hyperbolic. Over 200 subsequent observations over 34 days provided a more accurate assessment of its orbit and detailed lightcurve measurements provide information on its size, morphology (Trilling et al., 2018), and clues to its composition (Jewitt et al., 2017). The present study uses the orbital elements of ‘Oumuamua, as a test case to model whether CMOR could detect a hypothetical hyperbolic meteoroid stream of ‘Oumuamua-associated meteoroids.
Section snippets
Instrumentation and initial data processing
The Canadian Meteor Orbit Radar (CMOR) is a multi-frequency HF/VHF radar array located in Tavistock, Ontario, Canada (43.26°N, 80.77°W). It consists of a main site three-element Yagi-Uda vertically directed antenna as a transmitter, five co-located receiving antennas, and five additional remote station receivers. All receiver antennas are two-element vertically directed Yagi-Uda antennas. The five receiving antennas at the main site are arranged as an interferometer, allowing for positional
Filtering
CMOR multi-station echoes collected between January 2012 and June 2019 were examined to find evidence of interstellar meteoroids. This data set comprised over 11 million individual multi-station echoes for which time of flight velocities could be determined. We restricted our search to those echoes recorded on all six receiver stations - this yielded 395973 echoes. This was done as using all receiver stations gives the most accurate velocities.
Among this dataset, we further selected only events
Deceleration correction
A more accurate deceleration correction for meteor echoes detected by CMOR allows for improved pre-atmosphere velocity determination leading to more accurate orbit calculations. In particular, this correction provides our best statistical estimate to the true top of atmosphere velocity, recognizing that all our candidate events have measured in-atmosphere speeds which already produce hyperbolic orbits.
Shower meteors, being on common orbits (by definition) should have similar entry velocities.
CMOR detection sensitivity to hyperbolic meteors - case study of a hypothetical ‘Oumuamuids shower
The foregoing analysis suggests that some already detected CMOR meteors may be hyperbolic, but more generally can CMOR detect interstellar (hyperbolic) meteoroids and with what degree of confidence? Without a clear positive detection of many hyperbolic events, answering this question requires simulating CMOR’s response to meteoroids moving on hyperbolic orbits.
1I/‘Oumuamua was the first large interstellar objected detected transiting through our solar system. It was discovered by the Panoramic
Conclusion
We used the Canadian Meteor Orbit Radar to detect in excess of 160000 six station meteor echoes over a 7.5 year timespan in a search for potential interstellar meteoroids. Our integrated collecting area time product for the survey was 7106 km2 hours.
Among this sample, we found five candidate interstellar meteoroids with eccentricities 2σ above the hyperbolic limit based purely on their in-atmosphere speeds, by restricting our search to high-quality, 6-station events with low velocity
CRediT authorship contribution statement
Mark Froncisz: Formal analysis, Investigation, Data curation, Writing - original draft. Peter Brown: Funding acquisition, Methodology, Project administration, Writing - original draft, Writing - review & editing, Supervision. Robert J. Weryk: Software, Visualization, Writing - original draft, Writing - review & editing, Methodology.
Acknowledgements
This work was supported in part by the NASA Meteoroid Environment Office under cooperative agreement 80NSSC18M0046. PGB also acknowledges funding support from the Natural Sciences and Engineering Research council of Canada and the Canada Research Chairs program. We thank Denis Vida for providing model estimates for the deceleration of CMOR-sized meteoroids for comparison to our empirical values. An earlier version of this manuscript was improved considerably through comments of two anonymous
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