‘Oumuamua’s Star Trek: Potential Origin in a Giant Molecular Cloud?

Title: Evidence Suggesting that ‘Oumuamua is the ~ 30 Myr-old product of a Molecular Cloud

Authors: Cheng-Han Hsieh, Gregory Laughlin, Hector G. Arce

First Author’s Institution: Department of Astronomy, Yale University, New Haven, CT 06511, USA

Status: available as an arXiv preprint; submitted to The Astrophysical Journal [open access]

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Ever since it was first observed in October 2017, the interstellar interloper 1I/‘Oumuamua has sparked numerous debates in both the astronomical community and in the eyes of the public: on its nature and composition, on where it came from, and possibly most importantly, on whether it had a natural origin. Although a majority of researchers agree that ‘Oumuamua is most likely not an alien artifact, the question of its origin remains one of the biggest mysteries and hotly discussed topics in astrophysics today. The issue was made even more compelling by the discovery in August 2019 of a second interstellar visitor, comet 2I/Borisov, and astronomers have been grappling with a number of questions since then:

  • How many freely orbiting interstellar objects are there?
  • Where did ‘Oumuamua and Borisov originate?
  • Could we predict the arrival of future interstellar interlopers?

The authors of today’s paper focus on ‘Oumuamua, and investigate the hypothesis that it might have originated in a nearby Giant Molecular Cloud (GMC). GMCs are the sites of star formation in galaxies as the birthplace of star clusters or stellar associations. To test the GMC-origin theory for ‘Oumuamua, the authors compare its orbital dynamics to that of known stellar associations.

Dialing Back the (Orbital) Clock

To study ‘Oumuamua’s orbital dynamics relative to the centre of our Galaxy, the authors relied on orbit modelling codes. Orbit modelling is one of the most powerful tools in the theoretical astrophysicist’s arsenal, provided one knows the gravitational potential of the object governing the orbits (eg. the Sun or the Milky Way) – because we know exactly the equations of gravity on these scales and can solve them numerically! 
First, the authors used the code REBOUND (which is better suited for star-planet systems) to integrate ‘Oumuamua’s orbit back from present day to when it first entered the Sun’s gravitational potential almost 100 years ago. This allowed them to compute its position and velocity back then relative to the Milky Way, which could then be used as initial conditions for orbit modelling even further back in time. To extend the modelling even further into the past, when the predominant gravitational influence on ‘Oumuamua would have been from the Milky Way as a whole instead of just the Sun, the authors used the code Gala (better suited for galactic systems) to integrate ‘Oumuamua’s orbit 500 million years (Myr) into the past.

Two-panel plot showing orbits of 'Oumuamua, Borisov, and the Sun. Left hand panel shows orbits in cartesian coordinates in the plane of the Milky Way, while the right hand panel shows the same for cylindrical coordinates.
Figure 1: Galactic orbits for ‘Oumuamua, Borisov, and the Sun shown up to 500 Myr back in time from present day. Left: in Cartesian coordinates. Right: in cylindrical coordinates. The red point shows where ‘Oumuamua and Borisov entered our solar system – roughly the current position of the Sun. (Source: figure 1 in today’s paper.)

Figure 1 highlights the results from their orbit modelling for ‘Oumuamua, with orbits for the Sun and Borisov shown for comparison. A striking feature is that ‘Oumuamua seems to have entered the Solar System at almost exactly the same moment as it reached the maximum radial and vertical extents in its orbit. Even more importantly, the vertical extent of its orbit never goes beyond 0.05 kiloparsecs above or below the Galactic midplane. Both of these features together suggest that ‘Oumuamua is a relatively young object and probably originated in a short-lived population! 
In order to obtain an estimate on ‘Oumuamua’s age, the authors compared its orbit to those of ~800,000 stars in the Solar neighbourhood  observed by the Gaia survey. In general, the vertical orbital extent of these stars increased with age due to the accumulated effect of gravitational interactions, and through this comparison the authors estimated ‘Oumuamua’s age to be ~ 35 Myr – next to nothing on Galactic timescales!

Ghosts of GMC’s Past?

Ultimately, the authors wish to test the theory that ‘Oumuamua formed in a nearby GMC. Since stellar associations, like all star clusters, usually represent the latter stage of GMC evolution, and tend to be bright and easily identified in surveys, they decided to try and match ‘Oumuamua to a nearby association to see if they could have originated in the same GMC. The authors took a sample of 27 stellar associations within 150 kiloparsecs of the Sun (from the Gaia dataset) and integrated their orbits back 500 Myr into the past using the same procedure as before. Of these 27 tested, the Carina (CAR) and Columba (COL) associations showed the best kinematic agreement with the orbit of ‘Oumuamua.

Figure 2: Galactic orbit of ‘Oumuamua (grey curves in each panel) compared to those of three different young associations, shown in cylindrical coordinates. The red point shows the Sun’s current position in each panel while the black point shows the current position of the stellar association. Each panel shows the orbits integrated back 500 Myr into the past from present day. The ‘CAR’ and ‘COL’ associations show closest agreement with ‘Oumuamua, with CAR in particular having really strong agreement, raising the possibility of them having the same origin. Conversely, 118Tau is shown as an example of disagreement between ‘Oumuamua’s orbit and the association’s orbit. (Source: figure 3 in today’s paper.)

As shown in Figure 2, the orbits of CAR and COL agree quite well with that of ‘Oumuamua, both in terms of their radial/vertical extent and in the timing of their nearest passage to our Solar System. In particular, the maximum likelihood of CAR’s orbit intersecting with ‘Oumuamua is at 34 Myr ago from present day, while the same for COL’s orbit is at 42 Myr ago, albeit with a lower likelihood of intersection. Given their independent estimate of 35 Myr for ‘Oumuamua’s age (almost exactly the same as its intersection time with CAR – 34 Myr ago), the authors claim that the Carina stellar association, and its parent GMC, were likely the place of origin for our favourite interstellar interloper! However, they clarify that the evidence is not strong enough to completely rule out COL, and their main conclusion is that CAR and COL are both likely candidates for ‘Oumuamua’s origin, with CAR being slightly more preferred by orbit modelling.

It has now been almost four years since we first discovered ‘Oumuamua, and although we have learned quite a bit about this interstellar visitor since then, there are still numerous mysteries yet to be solved. The authors of today’s paper showed strong evidence of its origin and co-evolution with either the CAR or COL stellar associations. However, a detailed understanding of its precise formation mechanism within a GMC remains elusive. As ‘Oumuamua and Borisov continue on their trek through the stars, astronomers and planetary scientists await with bated breath the arrival of additional interlopers, seeking to understand where they came from and how they happened upon our neck of the woods.

Astrobite edited by Gloria Fonseca Alvarez

Featured image credit: European Southern Observatory / M. Kornmesser

About Pratik Gandhi

I'm a 3rd year astrophysics PhD student at UC Davis, originally from Mumbai, India. I study galaxy formation and evolution, and am really excited about the use of both simulations and observations in the study of galaxies. I am interested in science communication, teaching, and social issues in academia. Also a huge fan of Star Trek, with Deep Space Nine and The Next Generation being my favourites!

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