Interstellar Interloper 1I/2017 U1: Observations from the NOT and WIYN Telescopes

Abstract: 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 paper reveals that 1I/2017 U1 exhibits a highly elongated shape and reddish surface, challenging traditional notions of interstellar object composition.
• It uses precise photometric data from the NOT and WIYN telescopes to estimate a 55-meter effective radius with significant lightcurve variations.
• The study implies that many similar interstellar objects may exist, urging a re-evaluation of comet-asteroid classification frameworks.

Interstellar Interloper 1I/2017 U1: Observational Analysis and Implications

The paper under discussion is an observational study on the first interstellar object detected in the solar system, 1I/2017 U1 ('Oumuamua), utilizing data from both the Nordic Optical Telescope (NOT) and the WIYN telescope. The significance of this work lies in the opportunity to analyze an object not only from outside the solar system but also potentially from a different planetary system, thereby enabling astrochemical and dynamical studies that can broaden our understanding of such objects in the galactic context.

Observational Results

The study focuses on data gathered during 'Oumuamua's flyby of Earth in October 2017. Detailed photometric analysis reveals some intriguing characteristics:

• Optical Colors: The colors B-V = 0.70±0.06 and V-R = 0.45±0.05 are noted to overlap those of the D-type asteroids, particularly Jovian Trojans, suggesting a slightly reddish surface. However, these values are inconsistent with the ultrared objects found in the Kuiper Belt, indicating that 'Oumuamua's surface composition might be different or modified during its interstellar journey.
• Physical Characteristics: The absolute magnitude $H_V = 22.95$ allows a size estimation—assuming a typical geometric albedo ($p_V = 0.1$)—resulting in an effective radius of about 55 meters. Notably, the lightcurve variation is substantial, suggesting an elongated shape with a semi-axis ratio estimated at 6:1, which is considerable compared to typical small solar system bodies.
• Lack of Coma: Despite expectations of sublimation, especially considering its close perihelion, no cometary activity (coma) was observed. This suggests a sturdy non-volatile surface layer of around 0.5 m might be present, possibly formed by cosmic ray exposure during its interstellar travel.

Theoretical and Practical Implications

The absence of a visible coma yet highly elongated shape indicates 'Oumuamua may not exhibit traditional cometary activity possibly necessitating a re-evaluation of current comet-asteroid classification schemes, particularly for interstellar objects. The upper limit on dust production rate further strengthens the argument against any significant recent outgassing.

In terms of dynamics, 1I/2017 U1's high eccentricity ($e = 1.197$) and perihelion distance ($q = 0.254$ AU) illustrate its interstellar nature conclusively, differentiating it from even the most eccentric Oort cloud comets. The study estimates roughly 10,000 similar interstellar objects could exist within Neptune's orbit, suggesting a potential transient population traversing the solar system at any given time.

This research also raises questions on the origin and evolutionary history of 1I/2017 U1. An estimate of the number density suggests such bodies, though rare in number density per cubic parsec, may be ubiquitous enough that solar systems will interact with a number over cosmic timescales. The inclined orbit ($i = 122.6^\circ$) reinforces possibilities of a tumultuous ejection history from its original system, potentially offering insights into the dynamics of planetary system formation and evolution.

Concluding Remarks

Overall, the detection and subsequent study of 'Oumuamua represents a crucial step for astrophysical research into the properties and behaviors of interstellar solids. It marks the beginning of a new phase of observational astronomy, where systematic searches might yield recurring data on similar interstellar objects, thereby opening new frontiers in the understanding of the galaxy's small body population. Continued observation and research are essential to improve the characterization of such enigmatic bodies, which may, in turn, demand revisions of existing paradigms in planetary science and cosmogony.