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Register a new user1I/`Oumuamua As Debris of Dwarf Interstellar Comet That Disintegrated Before Perihelion
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Zdenek Sekanina
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Intrinsically faint comets in nearly-parabolic orbits with perihelion distances much smaller than 1 AU exhibit strong propensity for suddenly disintegrating at a time not long before perihelion, as shown by Bortle (1991). Evidence from available observations of such comets suggests that the disintegration event usually begins with an outburst and that the debris is typically a massive cloud of dust grains that survives over a limited period of time. Recent CCD observations revealed, however, that also surviving could be a sizable fragment, resembling a devolatilized aggregate of loosely-bound dust grains that may have exotic shape, peculiar rotational properties, and extremely high porosity, all acquired in the course of the disintegration event. Given that the brightness of 1I/`Oumuamuas parent could not possibly equal or exceed the Bortle survival limit, there are reasons to believe that it suffered the same fate as do the frail comets. The post-perihelion observations then do not refer to the object that was entering the inner Solar System in early 2017, as is tacitly assumed, but to its debris. Comparison with C/2017 S3 and C/2010 X1 suggests that, as a monstrous fluffy dust aggregate released in the recent explosive event, `Oumuamua should be of strongly irregular shape, tumbling, not outgassing, and subjected to effects of solar radiation pressure, consistent with observation. The unknown timing of the disintegration event may compromise studies of the parents home stellar system. Limited search for possible images of the object to constrain the time of the (probably minor) outburst is recommended.
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1I/`Oumuamua is the first confirmed interstellar body in our Solar System. Here we report on observations of `Oumuamua made with the Spitzer Space Telescope on 2017 November 21--22 (UT). We integrated for 30.2~hours at 4.5 micron (IRAC channel 2). We did not detect the object and place an upper limit on the flux of 0.3 uJy (3sigma). This implies an effective spherical diameter less than [98, 140, 440] meters and albedo greater than [0.2, 0.1, 0.01] under the assumption of low, middle, or high thermal beaming parameter eta, respectively. With an aspect ratio for `Oumuamua of 6:1, these results correspond to dimensions of [240:40, 341:57, 1080:180] meters, respectively. We place upper limits on the amount of dust, CO, and CO2 coming from this object that are lower than previous results; we are unable to constrain the production of other gas species. Both our size and outgassing limits are important because `Oumuamuas trajectory shows non-gravitational accelerations that are sensitive to size and mass and presumably caused by gas emission. We suggest that `Oumuamua may have experienced low-level post-perihelion volatile emission that produced a fresh, bright, icy mantle. This model is consistent with the expected eta value and implied high albedo value for this solution, but, given our strict limits on CO and CO2, requires another gas species --- probably H2O --- to explain the observed non-gravitational acceleration. Our results extend the mystery of `Oumuamuas origin and evolution.
1I/Oumuamua is the first interstellar object observed passing through the Solar System. Understanding the nature of these objects will provide crucial information about the formation and evolution of planetary systems, and the chemodynamical evolution of the Galaxy as a whole. We obtained the galactic orbital parameters of this object, considering 8 different models for the Galaxy, and compared it to those of stars of different ages from the Geneva-Copenhagen Survey (GCS). Assuming that the galactic orbital evolution of this object is similar to that of stars, we applied a Bayesian analyses and used the distribution of stellar velocities, as a function of age, to obtain a probability density function for the age of Oumuamua. We considered two models for the age-velocity dispersion relation (AVR): the traditional power law, fitted using data from the GCS; and a model that implements a second power law for younger ages, which we fitted using a sample of 153 Open Clusters (OCs). We find that the slope of the AVR is smaller for OCs than it is for field stars. Using these AVRs, we constrained an age range of 0.01-1.87 Gyr for Oumuamua and characterized a most likely age ranging between 0.20-0.45 Gyr, depending on the model used for the AVR. We also estimated the intrinsic uncertainties of the method due to not knowing the exact value of the Solar motion and the particularities of 1I/Oumuamuas ejection.
The initial Galactic velocity vector for the recently discovered hyperbolic asteroid 1I/Oumuamua (A/2017 U1) is calculated for before its encounter with our solar system. The latest orbit (JPL-13) shows that Oumuamua has eccentricity > 1 at 944sigma, significance (1.19936 +- 0.00021), i.e. clearly unbound. Assuming no non-gravitational forces, the objects inbound Galactic velocity was U, V, W = -11.457, -22.395, -7.746 (+-0.009, +-0.009, +-0.011) km/s (U towards Galactic center), with total heliocentric speed 26.32 +- 0.01 km/s. When the velocity is compared to the local stars, Oumuamua can be ruled out as co-moving with any of the dozen nearest systems, i.e. it does not appear to be associated with any local exo-Oort clouds (most notably that of the Alpha Centauri triple system). Oumuamuas velocity is within 5 km/s of the median Galactic velocity of the stars in the solar neighborhood (<25 pc), and within 2 km/s of the mean velocity of the local M dwarfs. Its velocity appears to be statistically too typical for a body whose velocity was drawn from the Galactic velocity distribution of the local stars (i.e. less than 1 in 500 field stars in the solar neighborhood would have a velocity so close to the median UVW velocity). In the Local Standard of Rest frame (circular Galactic motion), Oumuamua is remarkable for showing both negligible radial (U) and vertical (W) motion, while having a slightly sub-Keplerian circular velocity (V; by ~11 km/s). These calculations strengthen the interpretation that A/2017 U1 has a distant extrasolar origin, but not among the very nearest stars. Any formation mechanism for this interstellar asteroid should account for the coincidence of Oumuamuas velocity being so close to the LSR.
We study the origin of the interstellar object 1I/2017 U1 Oumuamua by juxtaposing estimates based on the observations with simulations. We speculate that objects like Oumuamua are formed in the debris disc as left over from the star and planet formation process, and subsequently liberated. The liberation process is mediated either by interaction with other stars in the parental star-cluster, by resonant interactions within the planetesimal disc or by the relatively sudden mass loss when the host star becomes a compact object. Integrating backward in time in the Galactic potential together with stars from the Gaia-TGAS catalogue we find that about 1.3Myr ago Oumuamua passed the nearby star HIP 17288 within a mean distance of $1.3$pc. By comparing nearby observed L-dwarfs with simulations of the Galaxy we conclude that the kinematics of Oumuamua is consistent with relatively young objects of $1.1$--$1.7$Gyr. We just met Oumuamua by chance, and with a derived mean Galactic density of $sim 3times 10^{5}$ similarly sized objects within 100,au from the Sun or $sim 10^{14}$ per cubic parsec we expect about 2 to 12 such visitors per year within 1au from the Sun.
The results of the photometric observations of comet C/2009 P1 (Garradd) performed at the 60-cm Zeiss-600 telescope of the Terskol observatory have been analyzed. During the observations, the comet was at the heliocentric and geocentric distances of 1.7 and 2.0 AU, respectively. The CCD images of the comet were obtained in the standard narrowband interference filters suggested by the International research program for comet Hale-Bopp and correspondingly designated the Hale-Bopp (HB) set. These filters were designed to isolate the BC ($lambda$4450/67 {AA}), GC ($lambda$5260/56 {AA}) and RC ($lambda$7128/58 {AA}) continua and the emission bands of C2 ($lambda$5141/118 {AA}), CN ($lambda$3870/62 {AA}), and C3 ($lambda$4062/62 {AA}). From the photometric data, the dust production rate of the comet and its color index and color excess were determined. The concentration of C2, CN, and C3 molecules and their production rates along the line of sight were estimated. The obtained results show that the physical parameters of the comet are close to the mean characteristics typicalof the dynamically new comets.
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