Spitzer Observations of Interstellar Object 1I/‘Oumuamua

Abstract

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 µm (IRAC channel 2). We did not detect the object and place an upper limit on the flux of 0.3 µJy (3σ). 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 η, 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 CO 2 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 'Oumuamua's 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 η value and implied high albedo value for this solution, but, given our strict limits on CO and CO 2 , requires another gas species -probably H 2 O -to explain the observed non-gravitational acceleration. Our results extend the mystery of 'Oumuamua's origin and evolution.

Modelling the light curve of ‘Oumuamua: evidence for torque and disc-like shape

“…Most of observations of 'Oumuamua were done during the 5-day interval between October 25 and October 30 (Fraser et al 2018;Drahus et al 2018), with a few more observations over the next two months until its final sighting on January 2, 2018 (Micheli et al 2018). The observations were primarily done in visible light, though two very interesting non-detections in other wavelengths were also reported: ⋆ E-mail: syam@physics.mcmaster.ca in infrared by Spitzer (Trilling et al 2018) and in radio by SETI (Harp et al 2019).…”

Capture of interstellar objects: a source of long-period comets

“…The discovery of our first known interstellar visitor 1I/'Oumuamua (Meech et al 2017;'Oumuamua ISSI Team et al 2019) raised the intriguing possibility that we might one day be able to understand the planet formation environment around alien stars by studying asteroids or comets ejected from them. The nature of 'Oumuamua was immediately questioned with some studies suggesting that the object displayed minor levels of cometary activity (Micheli et al 2018), some suggesting it did not (Rafikov 2018) and some setting relatively low upper limits on any potential activity (Trilling et al 2018). Unfortunately, the transient nature of 'Oumuamua means that it is impossible to gather further data, and curious parties were forced to await the chance detection of another Interstellar Object (ISO).…”

Detection of CN Gas in Interstellar Object 2I/Borisov

“…Assuming it was dark, 1I was very red and small with an average diameter of 200m (Meech et al 2017). However, it could possibly be as small as 100 m across if it has a higher albedo (Trilling et al 2018). 'Oumuamua's rotational light curve was extraordinary, with a brightness range of over 2.5 magnitudes, implying that it had a very elongated nucleus with an axis ratio >5:1, perhaps as large as 10:1.…”