Is Proxima Centauri a Bound Star?

by Paul Gilster on January 19, 2016

About 1.4 million years from now, the K-class star Gliese 710, now 64 light years distant in the constellation Serpens, will brush past our Solar System. Moving to within 50,000 AU, the star could be expected to have an unsettling effect on cometary orbits in the Oort Cloud, perhaps dislodging some of these comets to cause them to move into our inner planetary system. An interesting scenario, particularly remembering speculation that comets were a source of water for the early Earth, and may perform a similar function in other young systems.

So just how common are such celestial encounters? We may have one at our very doorstep in the form of Proxima Centauri. The Pale Red Dot campaign that began yesterday is focusing on a red dwarf that is roughly 15,000 AU from the close binary stars Centauri A and B. If you think about what our system would look like with a red dwarf at the inner edge of the Oort Cloud, you can see that Proxima may play a large role in the evolution of the triple-star system.

proxima_image

Image: Proxima Centauri, indicated by the arrow, in an image from the first night’s run at the Pale Red Dot observing campaign at La Silla. Credit: ESO/Guillem Anglada-Escudé.

Assuming, that is, that this is a bound system. It was back in 1993 that Robert Matthews and Gerard Gilmore (Cambridge University) studied kinematic data on Proxima and concluded that the star was not necessarily gravitationally bound. In fact, it was on the borderline, and could well be a star that was simply moving past Centauri A and B, having been formed elsewhere. The researchers cited the need for further kinematic data to resolve the matter.

The following year, J. Anosova (Physical Research Laboratory, Ahmedabad, India) proposed that the three Centauri stars were part of a stellar moving group, with Proxima drifting close but not necessarily bound. But the problem would not go away. Proxima’s small relative velocity in relation to Centauri A and B (0.53 ± 0.14 km s-1) makes the likelihood that it is unbound quite small, which is why the assumption of a bound triple system has generally been the rule since the star’s discovery in 1915.

Jeremy Wertheimer and Gregory Laughlin (UC-Santa Cruz) have had the most recent word, finding in 2006 that Proxima is indeed bound to Centauri A and B:

The availability of Hipparcos data has provided us with the ability to implement a significant improvement over previous studies of the α Cen system. Our results indicate that it is quite likely that Proxima Cen is gravitationally bound to the α Cen A-B pair, thus suggesting that they formed together within the same birth aggregate and that the three stars have the same ages and metallicities. As future observations bring increased accuracy to the kinematic measurements, it will likely become more obvious that Proxima Cen is bound to the α Cen A-B binary and that Proxima Cen is currently near the apastron of an eccentric orbit…

The researchers are also able to make a prediction: When we can get a more refined look at Proxima Centauri’s absolute radial velocity, we should see a value of -22.3 km s-1 < vr < -22.0 km s-1. Hence the importance of our continuing investigation of this small star. As we proceed with projects like Pale Red Dot and beyond, we should be able to eventually declare the question of Proxima’s bound status closed.

Ramifications of a Bound Proxima

This is of more than passing interest, because Proxima Centauri could well be influencing the planetary systems (if any) of the two primary Centauri stars. As I mentioned above, a star like Gl 710 passing by our system could cause disruptions in the Oort Cloud, and a passing Proxima Centauri could certainly serve the same function in that system. But a bound Proxima — one that is, as Wertheimer and Laughlin believe, currently located near the apastron position of its orbit, could have useful effects on an infant system. As the paper notes:

If Proxima were bound to the system during its formation stages, then it may have gravitationally stirred the circumbinary planetesimal disk of the α Cen system, thereby increasing the delivery of volatile-rich material to the dry inner regions.

A significant factor indeed as we ponder the question of astrobiology around these stars. Laughlin has written elsewhere about the fact that Alpha Centauri planets will likely be dry because of the close orbit of Centauri A and B — the period of the AB binary pair is 79 years, and the stars make a close approach of 11.2 AU. Silicates and metals condense out of the protoplanetary disks around Centauri A and B, but to get water, we need to go further out, into the circumbinary disk surrounding both stars. A bound Proxima Centauri, then, becomes the mechanism for driving water-laden materials inward to any dry, terrestrial-class planets.

And there is another factor that makes the question of Proxima’s relation to Centauri A and B significant. If Proxima is bound, the implication is that all three stars were formed out of the same molecular cloud, and that would imply not just the same age but the same metallicity (metals being elements higher than hydrogen and helium). We know that both Centauri A and B are richer in metals than our own Sun, and if Proxima is likewise metal-rich, it may have an elevated chance of having planets. The link between gas-giant formation and metals has been well explored, though such a link for rocky worlds is not established.

What all this comes down to is that a gravitationally bound Proxima Centauri would exist as the third member of a system that should be rich in the materials needed to form terrestrial planets. The problem now is to find them using searches like the ongoing effort at La Silla, where the Pale Red Dot campaign will be in progress until April. As mentioned yesterday, you can follow the effort on Twitter: @Pale_red_dot, and use the hashtag #PaleRedDot. Conditions were problematic last night at the observatory but it looks like the first observations were made.

The paper by Matthews and Gilmore is “Is Proxima really in orbit about Alpha CEN A/B?,” Monthly Notices of the Royal Astronomical Society Vol. 261, No. 2 (1993), p. L5-L7 (abstract). The Anosova paper is “Dynamics of nearby multiple stars. The α Centauri system,” Astronomy & Astrophysics 292 (1994), 115-118 (full text). The Wertheimer and Laughlin paper is “Are Proxima and Alpha Centauri Gravitationally Bound?” The Astronomical Journal 132:1995-1997 (2006), available online.

tzf_img_post

{ 19 comments… read them below or add one }

Michael January 19, 2016 at 12:58

Although it is indicated that Proxima is bound it may have been more tightly bound or even could have been captured by the Centauri main stars in the past, it is that close to the ‘bound’ mark.

Reply

Joy January 19, 2016 at 14:37

Another plug here for Cixin Liu, “The Three Body Problem”, and the two sequels. AFAIK Liu is the first writer to think about the implications of the instability of the Centauri system. An enjoyable and provocative read.

Reply

DJ Kaplan January 19, 2016 at 15:52

It’s possible that Proxima attracted all the comets into its orbit (or devoured them), keeping water from becoming available for AC’s exoplanets.

Reply

P January 19, 2016 at 18:34

I’m amazed the metallicity of Prox hasn’t been determined/estimated! Surely this isn’t true? If not why not?

P

Reply

Paul Gilster January 19, 2016 at 21:08

P writes:

I’m amazed the metallicity of Prox hasn’t been determined/estimated! Surely this isn’t true? If not why not?

This may help, from a previous post here:

If we thought Proxima were not bound by Centauri A and B, we’d have a problem, because as Laughlin noted on systemic, it’s tricky to figure out the metallicity of a solitary red dwarf:

“Metallicities for red dwarf stars are notoriously difficult to determine. Low-mass red dwarfs are cool enough so that molecules such as titanium oxide, water, and carbon monoxide are able to form in the stellar atmospheres. The presence of molecules leads to a huge number of lines in the spectra, which destroys the ability to fix a continuum level, and makes abundance determinations very difficult.”

But if you have a red dwarf within a multiple star system, you can use the metallicity of the more massive primary star(s) to infer the metallicity of the red dwarf, a method that has produced a metallicity calibration for red dwarfs that thus far has proven useful. All of this means that if Proxima Centauri is indeed bound to Centauri A and B, then its metallicity is on the same order as theirs. When Xavier Bonfils (Observatoire de Grenoble) and colleagues went to work on red dwarf metallicity in 2008, they examined 20 red dwarfs whose metallicity could be estimated in this way. Of those 20 stars, reports Laughlin, only five were higher than the Sun in metallicity, and only one star, GL 324, proved to be as rich in metals as Proxima Centauri.

Reply

Kristophr January 19, 2016 at 23:32

” … Moving to within 50,000 AU, the star could be expected to have an unsettling effect on cometary orbits in the Oort Cloud, …”

Ermmm, that will mean that our Sun will be moving directly through Gliese 710’s Oort cloud.

This could get exciting, for some very scary levels of “exciting”.

Reply

Eniac January 22, 2016 at 10:59

I think the Oort cloud is a little less exciting than you think. Objects are few and far between. We would probably not notice anything other than a greater incidence of comets. And a very bright star, of course.

Reply

Janw Pool January 20, 2016 at 6:25

@DJ Kaplan: I don’t see how Proxima could only attract them, instead of changing their orbits in different directions due to the close encounters.
That said, if the encounters were at a rather slow velocity, attraction could be an important issue nevertheless.

But what about Proxima’s flares? Does this mean that the star should be relatively young? The age of AC is approx. 5b and that is quite old. In that case, Proxima could not have been created at the same time as AC.
I think this is a simple way of reasoning and astronomers certainly would have come to such a conclusion a long time ago.

Reply

Bynaus January 20, 2016 at 6:33

To some of the posters here: Note that “capture” of an object by another (e.g., a star by a star, a comet by a star, but also a moon by a planet, etc.) is by no means a simple feat. In a simple two-body system, it is impossible, because the momentum a gravitating object gathers when approaching another will carry it away again. Capture is only possible if, e.g., there is a third body onto which some of the momentum can be imparted (e.g., a binary companion to the body to be catched, which is then ejected – in fact, this is the preferred mode of capture for all the irregular moons of the gas giants), or if the gravity field surrounding the gravitating system changes (e.g., in a dispersing young star cluster).

It is interesting to note that the gravitational acceleration experienced by Proxima towards the Alpha Centauri A&B stars is on the same order of magnitude as the anomalous gravitational acceleration of stars experienced in the outer part of the galaxy (usually ascribed to dark matter) and also the Pioneer anomaly. As such, the question of Proxima’s boundness might also serve as a test of new theories of gravity.

Reply

Michael Spencer January 20, 2016 at 8:05

That PaleRedDot image reminds me of a similar image- does anybody remember a program initiates by the editors of APOD? A program that placed fish-eye sky observers around the planet? This could have been twenty years ago. I wonder what happened?

Reply

tom January 20, 2016 at 10:29

Considering all the ambitious space telescopes and ground based networks, data collection shouldn’t be that difficult?
But it never is that easy.
A couple of years back I thought some science writer should’ve done a popular book on the history of our closets stellar neighbors?
The catalogue for Alpha Centauri didn’t occur ’til until astronomic records we’re being kept for Australia?

Reply

Michael January 20, 2016 at 10:52

@Bynaus January 20, 2016 at 6:33

‘To some of the posters here: Note that “capture” of an object by another (e.g., a star by a star, a comet by a star, but also a moon by a planet, etc.) is by no means a simple feat. In a simple two-body system, it is impossible, because the momentum a gravitating object gathers when approaching another will carry it away again. Capture is only possible if, e.g., there is a third body onto which some of the momentum can be imparted (e.g., a binary companion to the body to be catched, which is then ejected’

During Proximas formation the stars would have been much closer together and therefore much more likely to have interacted and been disrupted, the chances Proxima could have been captured or pulled away by A.B Centauri is much higher. In a star forming region there could be hundreds of stars jostling about in close proximity and about as far away from a two bodied analogy as you can get. A two body problem could also change depending on the masses involved as gravity waves can remove energy from the system allowing capture but it would have to be on a knife edge as the gravity force is very small.

Reply

Ronald January 20, 2016 at 13:15

The same issue/question is valid for other very wide binaries, such as notably Zeta 1 and 2 Reticuli, which are separated by at least 3,750 AU.

Reply

FrankH January 20, 2016 at 16:47

@Janw Pool
Red dwarf flare activity tapers off as they age, but that doesn’t mean it disappears completely – Barnard’s Star (8-10Gyr) had a huge flare in 1998.

Reply

andy January 20, 2016 at 18:49

@Janw Pool:
The lower-mass M-dwarfs remain active for much longer times: there is a break around spectral type M4, see this paper. From the abstract:

Over all M spectral types that we probe, we find that the presence of magnetic activity is tied to rotation, including for late-type, fully convective M dwarfs. We also find evidence that the fraction of late-type M dwarfs that are active may be higher at longer rotation periods compared to their early-type counterparts, with several active, late-type, slowly rotating stars present in our sample. Additionally, we find that all M dwarfs with rotation periods shorter than 26 days (early-type; M1-M4) and 86 days (late-type; M5-M8) are magnetically active.

Stars below the M4 break, including Proxima Centauri (M5.5Ve) need to spin down to much longer rotation periods before the activity switches off, so even old stars can be sufficiently active. There’s also evidence that the rate of spindown for such stars is slower than for more massive M dwarfs above the break.

Reply

Michael January 21, 2016 at 1:42

The likelihood of planets around Proxima is higher due to a low rotation rate inferring more momentum could be in planets in orbit and the higher metallicity inferring more dense material been available to form planetary cores quicker. My hopes for planets around Proxima have never been higher.

Reply

andy January 21, 2016 at 13:05

Even pinning down the properties of a well-studied binary like Alpha Centauri AB is quite difficult! There’s a recent orbit determination which finally seems to bring the visual and radial velocity-derived orbit in line with the asteroseismometry-derived properties of the system. The resultant parallax is more in line with the first revision of the Hipparcos catalogue than the later one.

Pourbaix & Boffin (2016) “Parallax and masses of ɑ Centauri revisited

Reply

Navneeth January 23, 2016 at 8:21

But if you have a red dwarf within a multiple star system, you can use the metallicity of the more massive primary star(s) to infer the metallicity of the red dwarf, a method that has produced a metallicity calibration for red dwarfs that thus far has proven useful.

But isn’t that sort of assuming the result beforehand? Or am I misunderstanding the procedure? What if the RD is a captured star and happens to have a similar metalliticity?

Reply

Michael January 23, 2016 at 12:56

You have a valid point, one is assuming the other first. Gaia may give us a more precise orbital position and velocity to answer the question once and for all. But it may have been captured as well as the orbital velocity is cutting it kind of close! I wonder if this lensing event in Feb could be used to narrow down the mass, position and velocity better, after all the rate of change of the deflection should give it away.

Reply

Leave a Comment

{ 2 trackbacks }