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A Microlensing Opportunity for Centauri A

First light for the European Extremely Large Telescope (E-ELT) is scheduled for 2024, a useful fact given that a few years later, we may be able to use the instrument in a gravitational lensing opportunity involving Alpha Centauri. Specifically, Centauri A is expected to align with the star 2MASS 14392160-6049528, thought to be a red giant or supergiant and far more distant than Alpha Centauri. This will create an event that not just the E-ELT but other instruments, like the GRAVITY instrument on the Very Large Telescope Interferometer (VLTI), will be able to study — GRAVITY is capable of extremely high accuracy astrometry.

A team of French astronomers led by Pierre Kervella (CNRS/Universidad de Chile) is behind this new study, which involved fine-tuning our knowledge of the trajectories of Centauri A and B. Remember that we see gravitational lensing when a massive object like a star distorts the spacetime around it, so that light from the more distant object must follow a curved path to reach us. The amount of mass in the closer star affects the extent of this deflection, and when one or more planets orbit the star, they become theoretically detectable.


Image: The predicted trajectory of Alpha Centauri A and B. Credit: ESO.

The Centauri A event is to occur in 2028, by which time we may well have knowledge of other planets around one or more of the primary Alpha Centauri stars. But a particularly useful aspect of microlensing is that it is not reliant on proximity to the star. Unlike transit studies or radial velocity analysis, which can produce evidence for a close-in planet quickly but require lengthy study for planets further out in the system, microlensing allows us to spot planets in wider orbits, even if the observation in which we find them is transitory and does not repeat.

So this lensing event is another potential window into this intriguing system, around which at present we know only of the world orbiting Proxima Centauri. Recall that the latter has also been studied in two microlensing events, one in October of 2014, the second in February of 2016. We found Proxima b through radial velocity methods (although the star was also the subject of transit studies), but microlensing is useful even when it fails to turn up planets, since it gives us a way of refining our estimates of a given star’s mass.

Alpha Centauri gives us serious opportunities for microlensing because the star field behind it is densely populated, thanks to the system’s location near the plane of the galaxy as seen from Earth. The Kervella paper studies conjunctions with background stars that will occur in coming decades, based on observations of the field surrounding Alpha Centauri that Kervella and co-author Fréderic Thévenin (Observatoire de la Côte d’Azur) began over a decade ago.


Image: An enlargement of the conjunction that will occurs in 2028, with the Einstein ring of Alpha Cen A represented in cyan color. Credit: Pierre Kervella.

The conjunction of Centauri A with 2MASS 14392160-6049528 is the most favorable of the conjunctions involving Alpha Centauri over the next three decades. From the paper, which references the background star as S5; i.e., one of the background stars numbered in the survey:

During the approaches, astrometric measurements will reveal the relativistic deflection of the background star light with a high signal to noise ratio. For the conjunction with S5, we may be able to directly observe the gravitational splitting of the distant source image using the E-ELT, VLTI/GRAVITY or ALMA. The astrometric monitoring of the relative positions of α Cen and the S stars may reveal the presence of planets through secondary gravitational lensing. In addition, the light from the background star will possibly be subject to photometric variations induced by transits of low mass objects present in the α Cen system.

Potential results include, in other words, everything from high and low mass planets to asteroids and comets. The authors believe the conjunctions in coming decades will give us highly accurate information about the Alpha Centauri stars’ proper motion, orbital parameters and parallax values. And note this: “This accuracy will be valuable for high-precision modeling of the two stars of α Cen, and for the preparation of the recently announced Breakthrough Starshot initiative to send ultra-fast light-driven nanocrafts to α Centauri.”

The Kervella paper is “Close stellar conjunctions of α Centauri A and B until 2050,” Astronomy & Astrophysics 594 (2016), A107 (abstract). On microlensing and Proxima Centauri, see Sahu et al., “Microlensing Events by Proxima Centauri in 2014 and 2016: Opportunities for Mass Determination and Possible Planet Detection,” Astrophysical Journal Volume 782, Issue 2 (2014). Abstract available.


Comments on this entry are closed.

  • DJ Kaplan October 25, 2016, 12:11

    May I clarify that we are using the “microlensing” properties of Centauri A to look at “2MASS 14392160-6049528” and not the other way around?

    • Paul Gilster October 25, 2016, 15:44

      Centauri A’s mass curves spacetime near it and produces the lensing effect on the background star’s light. The nature of that distortion may tell us much about the foreground system, including whether it has planets.

      • DJ Kaplan October 25, 2016, 18:59

        Thanks, that would be two birds with one stone, so to speak!

  • Sean Robert Meaney October 25, 2016, 13:37

    Hope I live long enough to see it… or we develop gravity source detection technology to an awesome level where we can see individual planets rotating around stars.

  • Michael October 25, 2016, 14:07

    If we get starshot up and running we can send out probes that can be positioned to use other stars as microlenses greatly increasing our view of the universe.

    • Andrew Palfreyman October 25, 2016, 16:51

      Let us count the “if”s there, for there are many! Perhaps chief among them being the inability to stop.

      • Michael October 26, 2016, 1:02

        We can stop by using fission decay coatings to slow the probe down.

  • andy October 25, 2016, 18:28

    Sounds like some pretty precise astrometry should be possible, which could help narrow down the properties of the system.

    On a similar note, considering that Matvienko & Orlov (2014) noted that the key factor in determining whether or not Proxima is a bound member of Alpha Centauri is the radial velocity of Proxima, I would be interested to see an investigation of this question using the Pale Red Dot campaign’s data that was used to discover Proxima b.

  • Marshall Eubanks October 25, 2016, 18:37

    As the Gaia catalog (including good parallaxes and proper motions) starts to firm up in the next few years, you can expect to see a lot more of these predicted microlensing events – they happen, but you can’t predict them well without a good sky catalog.

  • David Cummings October 26, 2016, 9:25

    The total count of confirmed exoplanets is now up to 3397


    In the next ten years, with JWST, the developing microlensing technologies and other emerging and developing detection techniques, I wonder if the count will be in the multiple tens of thousands.

  • Harry R Ray October 26, 2016, 9:26

    Paul Gilster(or ANY reader): Has there been any UPDATE re the SECOND microlensing event at Proxima Centauri last February? Is a paper due out sometime in the near future?

    • Paul Gilster October 26, 2016, 9:35

      Harry, I wrote Kailash Sahu at the Space Telescope Science Institute about this yesterday while working on the microlensing post. Will report when I hear more.

  • ljk October 26, 2016, 9:52
  • Spaceman October 26, 2016, 10:12

    It would be interesting if microlensing, as described in this article, turns up evidence for a debris belt but no evidence for planets around Alpha Centauri A. Perhaps this might provide clues about the dynamical history of the system. In other words, the revelation of a debris belt could give hints as to what happened to circumstellar material that may have never successfully coalesced into planets due to the gravitational perturbations of a close binary system. Of course, I hope more planets are discovered in this trinary system in addition to Proxima centauri b: especially terrestrial sized orbs in the HZ of the A and/or B stars!

  • Harry R Ray November 28, 2016, 19:32

    Paul Gilster: You have probably heard by now that the IAU has NIXED the moniker “Alpha Centauri” and replaced it with the older version “Regil Kentauris”. YUCK!!!!! Thank our lucky stars they did NOT mess with Proxima Centauri for NOW(since it is now ALMOST CERTAIN that Proxima Centauri IS bound to(UGH!!!!)Regil Kentauris, who knows in the not too distant future what they’ll do with it, but for NOW, your website can still be called “Centauri Dreams” instead of(GAAAAH!!!!)Kentauris Dreams”.

    • Paul Gilster November 28, 2016, 21:39

      Well, I plan to keep the name in any case! Wow, what a change that would be…

  • Harry R Ray November 28, 2016, 19:38

    Sorry, I meant KentaurUs, not Kentauris.

  • Marshall Eubanks November 29, 2016, 1:45

    Rigil Kentaurus is supposed to be from the Arabic; I am not sure if Arabic astronomers came up with the name, or if it is some sort of back formation. The IAU WGSN recognizes the Bayer designations, and Alpha Centauri is the system’s Bayer designation, so I see no reason (even from the IAU perspective) that this name can’t be used. Note, BTW, that Proxima Centauri was explicitly approved.

    Here is the current full list.