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This paper reports the first OH 18-cm line observation of the first detected interstellar object 1I/2017 U1 (`Oumuamua) using the Green Bank Telescope. We have observed the OH lines at 1665.402 MHz, 1667.359, and 1720.53 MHz frequencies with a spectral resolution of 357 Hz (approximately 0.06 km-s^{-1}). At the time of the observation, `Oumuamua was at topocentric distance and velocity of 1.07 au and 63.4 km-s^{-1}, respectively, or at heliocentric distance and velocity of 1.8 au and 39 km-s^{-1}, respectively. Based on a detailed data reduction and an analogy-based inversion, our final results confirm the asteroidal origin of `Oumuamua (as discussed in Meech et al., 2017) with an upper bound of OH production of Q[OH] < 0.17 x 10^{28} s^{-1}.
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The recently discovered minor body 1I/2017 U1 (`Oumuamua) is the first known object in our Solar System that is not bound by the Suns gravity. Its hyperbolic orbit (eccentricity greater than unity) strongly suggests that it originated outside our Solar System; its red color is consistent with substantial space weathering experienced over a long interstellar journey. We carry out an simple calculation of the probability of detecting such an object. We find that the observed detection rate of 1I-like objects can be satisfied if the average mass of ejected material from nearby stars during the process of planetary formation is ~20 Earth masses, similar to the expected value for our Solar System. The current detection rate of such interstellar interlopers is estimated to be 0.2/year, and the expected number of detections over the past few years is almost exactly one. When the Large Synoptic Survey Telescope begins its wide, fast, deep all-sky survey the detection rate will increase to 1/year. Those expected detections will provide further constraints on nearby planetary system formation through a better estimate of the number and properties of interstellar objects.
We present observations of the interstellar interloper 1I/2017 U1 (Oumuamua) taken during its 2017 October flyby of Earth. The optical colors B-V = 0.70$pm$0.06, V-R = 0.45$pm$0.05, overlap those of the D-type Jovian Trojan asteroids and are incompatible with the ultrared objects which are abundant in the Kuiper belt. With a mean absolute magnitude $H_V$ = 22.95 and assuming a geometric albedo $p_V$ = 0.1, we find an average radius of 55 m. No coma is apparent; we deduce a limit to the dust mass production rate of only $sim$ 2$times$10$^{-4}$ kg s$^{-1}$, ruling out the existence of exposed ice covering more than a few m$^2$ of the surface. Volatiles in this body, if they exist, must lie beneath an involatile surface mantle $gtrsim$0.5 m thick, perhaps a product of prolonged cosmic ray processing in the interstellar medium. The lightcurve range is unusually large at $sim$2.0$pm$0.2 magnitudes. Interpreted as a rotational lightcurve the body has semi-axes $sim$230 m $times$ 35 m. A $sim$6:1 axis ratio is extreme relative to most small solar system asteroids and suggests that albedo variations may additionally contribute to the variability. The lightcurve is consistent with a two-peaked period $sim$8.26 hr but the period is non-unique as a result of aliasing in the data. Except for its unusually elongated shape, 1I/2017 U1 is a physically unremarkable, sub-kilometer, slightly red, rotating object from another planetary system. The steady-state population of similar, $sim$100 m scale interstellar objects inside the orbit of Neptune is $sim$10$^4$, each with a residence time $sim$10 yr.
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.
We examine data from the Murchison Widefield Array (MWA) in the frequency range 72 -- 102 MHz for a field-of-view that serendipitously contained the interstellar object Oumuamua on 2017 November 28. Observations took place with time resolution of 0.5 s and frequency resolution of 10 kHz. %This observation was undertaken for another purpose but due to the MWAs extremely large field-of-view, Oumuamua was serendipitously observed simultaneously. Based on the interesting but highly unlikely suggestion that Oumuamua is an interstellar spacecraft, due to some unusual orbital and morphological characteristics, we examine our data for signals that might indicate the presence of intelligent life associated with Oumuamua. We searched our radio data for: 1) impulsive narrow-band signals; 2) persistent narrow-band signals; and 3) impulsive broadband signals. We found no such signals with non-terrestrial origins and make estimates of the upper limits on Equivalent Isotropic Radiated Power (EIRP) for these three cases of approximately 7 kW, 840 W, and 100 kW, respectively. These transmitter powers are well within the capabilities of human technologies, and are therefore plausible for alien civilizations. While the chances of positive detection in any given Search for Extraterrestrial Intelligence (SETI) experiment are vanishingly small, the characteristics of new generation telescopes such as the MWA (and in the future, the Square Kilometre Array) make certain classes of SETI experiment easy, or even a trivial by-product of astrophysical observations. This means that the future costs of SETI experiments are very low, allowing large target lists to partially balance the low probability of a positive detection.
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