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Register a new userThe Orbit and Size-Frequency Distribution of Long Period Comets Observed by Pan-STARRS1
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We introduce a new technique to estimate the comet nuclear size frequency distribution (SFD) that combines a cometary activity model with a survey simulation and apply it to 150 long period comets (LPC) detected by the Pan-STARRS1 near-Earth object survey. The debiased LPC size-frequency distribution is in agreement with previous estimates for large comets with nuclear diameter $>sim 1$~km but we measure a significant drop in the SFD slope for small objects with diameters $<1$~km and approaching only $100$~m diameter. Large objects have a slope $alpha_{big} = 0.72 pm 0.09 (stat.) pm 0.15 (sys.)$ while small objects behave as $alpha_{small} = 0.07 pm 0.03 (stat.) pm 0.09 (sys.)$ where the SFD is $propto 10^{alpha H_N}$ and $H_N$ represents the cometary nuclear absolute magnitude. The total number of LPCs that are $>1$~km diameter and have perihelia $q<10$~au is $0.46 pm 0.15 times 10^9$ while there are only $2.4 pm 0.5 (stat.) pm 2 (sys.) times 10^9$ objects with diameters $>100$~m due to the shallow slope of the SFD for diameters $<1$~m. We estimate that the total number of `potentially active objects with diameters $ge 1$~km in the Oort cloud, objects that would be defined as LPCs if their perihelia evolved to $<10$~au, is $(1.5pm1)times10^{12}$ with a combined mass of $1.3pm0.9 , M_{Earth}$. The debiased LPC orbit distribution is broadly in agreement with expectations from contemporary dynamical models but there are discrepancies that could point towards a future ability to disentangle the relative importance of stellar perturbations and galactic tides in producing the LPC population.
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We determine the absolute magnitude (H) distribution (or size-frequency distribution, SFD; $N(H) propto 10^{alpha H}$ where $alpha$ is the slope of the distribution) for near-Earth objects (NEO) with $13<H<30$ and Asteroid Retrieval Mission (ARM) targets with $27<H<31$ that were detected by the 1st telescope of the Panoramic Survey Telescope and Rapid Response System - Pan-STARRS1 (e.g. Kaiser et al. 2002, Kaiser 2004, Hodapp et al. 2004). The NEO and ARM target detection efficiencies were calculated using the Greenstreet et al. (2012) NEO orbit distribution. The debiased Pan-STARRS1 NEO absolute magnitude distribution is more complex than a single slope power law - it shows two transitions - at H$sim$16 from steep to shallow slope, and in the $21<H<23$ interval from a shallow to steep slope, which is consistent with other recent works (e.g. Mainzer et al. 2011c, Brown et al. 2013, Harris and D`Abramo 2015). We fit $alpha = 0.48pm0.02$ for NEOs with $13<H<16$, $alpha = 0.33pm0.01$ for NEOs with $16<H<22$, and $alpha = 0.62pm0.03$ for the smaller objects with $H>22$. There is also another change in slope from steep to shallow around H=27. The three ARM target candidates detected by Pan-STARRS1 in one year of surveying have a corrected SFD with slope $alpha = 0.40^{+0.33}_{-0.45}$. We also show that the window for follow up observations of small (H$gtrsim$22) NEOs with the NASA IRTF telescope and Arecibo and Goldstone radars are extremely short - on order of days, and procedures for fast response must be implemented in order to measure physical characteristics of small Earth-approaching objects. CFHTs MegaCam and Pan-STARRS1 have longer observing windows and are capable of following-up more NEOs due to their deeper limiting magnitudes and wider fields of view.
The seven known main belt comets (MBCs) have orbital characteristics of main belt asteroids yet exhibit dust ejection like comets. In order to constrain their physical and orbital properties we searched the Thousand Asteroid Light Curve Survey (TALCS; Masiero et al. 2009) for additional candidates using two diagnostics: tail and coma detection. This was the most sensitive MBC survey effort to date, extending the search from MBCs with H~18 (D~1 km) to H~21 (D~150 m). We fit each of the 924 TALCS objects to a PSF model incorporating both a coma and nuclear component to measure the fractional contribution of the coma to the total surface brightness. We determined the significance of the coma detection using the same algorithm on a sample of comparable null detections. We did not identify any MBC candidates with this technique to a sensitivity limit on the order of cometary mass loss rate of about 0.1 kg/s. Our tail detection algorithm identified statistically significant flux in a segmented annulus around the candidate object. We show that the technique can detect tail activity throughout the asteroid belt to the level of the currently known MBCs. Although we did not identify any MBC candidates with this technique, we find a statistically significant detection of faint activity in the entire ensemble of TALCS asteroids. This suggests that many main belt asteroids are active at very low levels. We set 90% upper confidence limits on the number distribution of MBCs as a function of absolute magnitude, semimajor axis, eccentricity, and inclination. There are <~ 400000 MBCs in the main belt brighter than H_V=21 (~150 m) and the MBC:MBA ratio is <~ 1:400. We further comment on the ability of observations to meaningfully constrain the snow lines location. Under some reasonable and simple assumptions we claim 85% confidence that the contemporary snow line lies beyond 2.5 AU.
The solar systems Oort cloud can be perturbed by the Galactic tide and by individual passing stars. These perturbations can inject Oort cloud objects into the inner parts of the solar system, where they may be observed as the long-period comets (periods longer than 200 years). Using dynamical simulations of the Oort cloud under the perturbing effects of the tide and 61 known stellar encounters, we investigate the link between long-period comets and encounters. We find that past encounters were responsible for injecting at least 5% of the currently known long-period comets. This is a lower limit due to the incompleteness of known encounters. Although the Galactic tide seems to play the dominant role in producing the observed long-period comets, the non-uniform longitude distribution of the cometary perihelia suggests the existence of strong -- but as yet unidentified -- stellar encounters or other impulses. The strongest individual future and past encounters are probably HIP 89825 (Gliese 710) and HIP 14473, which contribute at most 8% and 6% to the total flux of long-period comets, respectively. Our results show that the strength of an encounter can be approximated well by a simple proxy, which will be convenient for quickly identifying significant encounters in large data sets. Our analysis also indicates a smaller population of the Oort cloud than is usually assumed, which would bring the mass of the solar nebula into line with planet formation theories.
Remote investigations of the ancient solar system matter has been traditionally carried out through the observations of long-period (LP) comets that are less affected by solar irradiation than the short-period counterparts orbiting much closer to the Sun. Here we summarize the results of our decade-long survey of the distant activity of LP comets. We found that the most important separation in the dataset is based on the dynamical nature of the objects. Dynamically new comets are characterized by a higher level of activity on average: the most active new comets in our sample can be characterized by afrho values >3--4 higher than that of our most active returning comets. New comets develop more symmetric comae, suggesting a generally isotropic outflow. Contrary to this, the coma of recurrent comets can be less symmetrical, ocassionally exhibiting negative slope parameters, suggesting sudden variations in matter production. The morphological appearance of the observed comets is rather diverse. A surprisingly large fraction of the comets have long, teniouos tails, but the presence of impressive tails does not show a clear correlation with the brightness of the comets.
We simulate the passage through the Sun-Jupiter system of interstellar objects (ISOs) similar to 1I/`Oumuamua or 2I/Borisov. Capture of such objects is rare and overwhelmingly from low incoming speeds onto orbits akin to those of known long-period comets. This suggests that some of these comets could be of extra-solar origin, in particular inactive ones. Assuming ISOs follow the local stellar velocity distribution, we infer a volume capture rate of $0.051,mathrm{au}^3 mathrm{yr}^{-1}$. Current estimates for orbital lifetimes and space densities then imply steady-state captured populations of $sim10^2$ comets and $sim10^5$ `Oumuamua-like rocks, of which 0.033% are within 6au at any time.
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