Alpha Centauri: Dust and Its Significance

by Paul Gilster on February 5, 2014

When I was growing up, Alpha Centauri was utterly dismissed as a possible location for planets. A binary system couldn’t possibly produce them, I read, and it was assumed that planets could only be found around single stars like our own Sun. How times have changed. Now we know of plenty of multiple star systems with planets — the number is over ten percent of all known exoplanets — and while many of these are widely spaced, we’ve nonetheless found a few in tight circumstances indeed. HD196885 (Gamma Cephei) is an example, where the separation between the two stars is on the order of 20 AU, much like Centauri A and B.

How planets form around such stars is an interesting issue because, as a new paper considering dust in the Alpha Centauri system explains, the standard core-accretion model runs into problems with environments as perturbed as these. We can see the results in existing observations: Radial velocity methods detect no planets more massive than 2.5 Jupiter masses inside 4 AU of Centauri A or B, and large regions seem, from theoretical models, to be a real challenge to planet formation, with an outer limit for the process perhaps in the 2 AU region.

Centauri_Orbits

Image: Trajectory of Alpha Centauri B relative to A (fixed to the coordinate origin) as seen from the Earth (inclined ellipse) and face-on (horizontal ellipse). The orbit parameters are taken from Pourbaix et al. (2002). Credit: SiriusB/Wikimedia Commons.

What’s interesting about a new paper called “How dusty is α Centauri?” is that it gets into issues raised by the large number of planet-bearing stars known to show far-infrared emission from cool circumstellar dust. The authors, led by Joachim Wiegert (Onsala Space Observatory, Sweden), observed the Centauri stars to look for dust emissions there, presumably the results of collisions between objects up to planetesimal size. The team sought to understand the properties of the dust and to gauge the minimum temperature (Tmin) of the stellar chromospheres, a layer in the stars’ atmospheres that plays a role in how we determine the emission from cool dust in such systems.

The close-in Centauri stars, then, are templates that may help us find undetected dust disks around other stars. The team notes that it is::

…primarily concerned with the possible effects Tmin might have on the estimation of very low emission levels from exo-Edgeworth-Kuiper belt dust. The intensity of the stellar model photosphere beyond 20 to 40 µm is commonly estimated from the extrapolation of the spectral energy distribution (SED) into the Rayleigh-Jeans (RJ) regime at the effective temperature Teff. There is a potential risk that this procedure will overestimate actual local stellar emissions, which may be suppressed at the lower radiation temperatures. In those cases, where the SEDs are seemingly well fit by the RJ-extrapolations, the differences may in fact be due to emission from cold circumstellar dust (exo-Edgeworth-Kuiper belts) and, here, we wish to quantify the magnitude of such an effect.

As to Alpha Centauri itself, a slight infrared excess could be interpreted as dust emissions around both Centauri A and B. Disk modeling then produces the possibility of two circumstellar disks, the one around Centauri A being no larger than 2.78 ± 1.48 AU, while around Centauri B the limits are 2.52 ± 1.60 AU (the planet tentatively identified as Centauri Bb in 2012 is small enough and close enough to the star that it is inside the range of the authors’ disk models, and is thus ignored in their modelling). And while a circumbinary dust disk is possible at distances greater than 75 or so AU from the barycenter of the stars, no such disk was detected by these observations, and the authors note that other studies have made this scenario unlikely.

And this is interesting: “These size limits are reminiscent of the inner solar system, i.e., this opens the possibility for an asteroid belt-analogue for each star which forms dust discs through the grinding of asteroids and comets.” The results, displaying the possible disks and their snow lines, are shown in the figure below:

Centauri_dust_2

Image: Face-on circumstellar test-particle discs after ~ 103 periods shown with the stars close to periapsis. α Cen A is colour-coded blue (the left star and its orbit) and α Cen B is colour-coded red (the right star and its orbit). The green circles represent acrit around the stars and the magenta circles show estimates of their respective snow lines. Credit: Wiegert et al., taken from the paper.

We don’t yet know whether the excess at 24 µm the researchers found is a detection of warm dust or not — the paper describes the data as “marginal excesses” for both Centauri A and B, adding that “If due to circumstellar emission from dust discs, fractional luminosity and dust mass levels would be some 10 to 100 times those of the solar zodiacal cloud.” As we push deeper into the study of the nearest stars, we’ll use what we learn about dust emissions and their relation to stellar temperatures to study cool circumstellar disk possibilities around more distant targets.

The paper is Wiegert et al., “How dusty is α Centauri?” accepted at Astronomy & Astrophysics and available as a preprint. Thanks to Andy Tribick for the pointer to this one.

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ljk February 5, 2014 at 12:33

High Potential for Life Circling Alpha Centauri B, our Nearest Neighbor

by SHANNON HALL on FEBRUARY 5, 2014

While exoplanets make the news on an almost daily basis, one of the biggest announcements occurred in 2012 when astronomers claimed the discovery of an Earth-like planet circling our nearest neighbor, Alpha Centauri B, a mere 4.3 light-years away. That’s almost close enough to touch.

Of course such a discovery has led to a heated debate over the last three years. While most astronomers remain skeptical of this planet’s presence and astronomers continue to study this system, computer simulations from 2008 actually showed the possibility of 11 Earth-like planets in the habitable zone of Alpha Centauri B.

Now, recent research suggests that five of these computer-simulated planets have a high potential for photosynthetic life.

Full article here:

http://www.universetoday.com/108865/high-potential-for-life-circling-alpha-centauri-b-our-nearest-neighbor/

RobFlores February 5, 2014 at 13:24

Simple Question: What makes astronomers think A & B centauri Formed
together. Are the odds for Stars existing billions of years in time passing
so close so as to capture each other that low. I agree their Metalicity is close to each other. If that is the main evidence for their forming together, then isn’t possible that they may have been born separately in the same stellar nursery and later encounter brought them close enough for capture.
This would give a limited time to create planets separately, w/o having
to account for the turbulence in the accretion disk of a dual star formation.

Tarmen February 5, 2014 at 19:22

It is good that there is at least some raw material at these two stars. Even an asteroid belt would be 100 percent better than nothing. Hop scotcher colonists through the Oort Cloud(s) may prefer asteroids and planetoids over planets anyway. And colonies at these two stars could be mutually supportive of each other. ABC is still a long way from here though. We have to walk before we can run.

Astronist February 5, 2014 at 20:14

RobFlores: the odds for capture are extremely low. Remember firstly that encounters as close as hundreds or tens of AU will be extremely rare, because of the vastness of space. Secondly, that if two stars are passing each other, because of the conservation of momentum they cannot capture into an orbit around each other unless one of them has a companion which is ejected carrying off the excess angular momentum. So in order to create a binary by capture, you need one of the stars to be a binary in the first place.

Stephen A.

kzb February 6, 2014 at 8:41

First let me say I hope this is not the case, but I need to ask the question: does the existence of this dust where it is PRECLUDE the presence of planets in these zones?

Or is the resolution too poor to show gaps in the disks where planets are? It sounds like it is, so I am hopeful this is the case.

(And, going forward, being able to detect gaps in the dust would give us a good way of detecting planets.)

Ronald February 6, 2014 at 12:02

kzb: very good question. I would think (think, not jnow for sure) that the presence of such a dust and debris disk precludes at least the presence of a giant planet *within that region*.
At the same time it could be a good telltale sign for (small, terrestrial) planets in such a system, maybe not in the dusty region itself, but in the system.
Anyone else more clarity on this?

James M. Essig February 6, 2014 at 13:25

Got to go to the Centauri System. I want especially to visit Pandora.

Oddly enough, if a planet or moon in the Centauri System is found to support life, the planet will likely be named Pandora.

kzb February 7, 2014 at 8:37

Hi Ronald, part of the definition of a planet is that it clears its orbit. But this dust detection is apparently a marginal detection, i.e it is very difficult to observe. In fact, since Herschel is now over with, perhaps there is no chance of these observations being refined in the near future.

But I still think this could be a way forward with finding planets in this system. Someone just has to find a way of observing and measuring the gaps in the dust disk. From that we’d have the orbits and mass estimates of any planets.

Ronald February 7, 2014 at 10:55

kzb: again good point, measuring planetary sizes indirectly by measuring the gaps in the dust disk.
However, inward migration, praticularly of giant planets, which would scoop up dust like a snowball, might spoil this measurement. Or not?

kzb February 7, 2014 at 12:49

Another question about the AC system. I realise that it is not certain that Proxima is coeval with A and B. However, if it is, and it has been thrown into such a wide orbit, is it not reasonable to think that there could be planets in similar far-flung orbits?

andy February 7, 2014 at 18:38

Well the inner planets in our solar system are embedded within the zodiacal dust cloud, pedantic semantics of the IAU definition of “planet” notwithstanding. So the presence of dust around the Alpha Centauri stars (if confirmed) doesn’t preclude a planetary system also being there.

So to turn kzb’s question around: what kind of structures are present in the zodiacal dust cloud in our solar system, and if we were at Alpha Centauri would they be observable?

kzb February 10, 2014 at 9:04

The paper says the AC dust is an order of mag denser that the solar system zodiacal dust. More importantly, it implies the source of this dust is asteroids colliding with each other.

Dust particles will not stay put, I expect any signal in the distribution will get erased pretty quickly. It takes the continued presence of larger bodies to maintain any structure. Large gas giants in this system (excluding my second question) I believe are already excluded by observations. So even if there were any migrating gas giants originally, they are long gone, and any signal in the dust distribution will have long gone too.

That is what I think for what it is worth, getting on to Andy’s last point, I really do not know how feasible this idea is. I do suspect it is not easy, because the dust in question was a marginal (and uncertain) detection in the first place. So trying to detect any patterns in it is a tough ask.

Ronald February 11, 2014 at 8:55

kzb: “AC dust is an order of mag denser that the solar system zodiacal dust”.

I hope I am wrong here, but this may not bode well for planets there. I think I understood (or not?) that a dense dust cloud and/or asteroid belt precludes the presence of planets *in that particular region*.

On the other hand, Tau Ceti also has a very massive debris disc and yet it also has planets: http://en.wikipedia.org/wiki/Tau_Ceti#Debris_disk
However, the debris disk extends approx. from 10 – 55 AU, with the bulk of it from 35 – 50 AU, rather analogous to our Kuiper Belt.

61 Virginis also has a rather massive debris disk between 30 – 100 AU and a planetary system: http://en.wikipedia.org/wiki/61_Virginis#Planetary_system
So does 82 Eridani.

My impression now is that an outer dust/debris disk can be an indication for the presence or at least the possible formation of planets, but such a disk in the inner region would rather preclude the presence of planets in that region.

Anyone with more knowledge on this topic?

kzb February 12, 2014 at 9:01

Ronald, your first para is one of the things I’ve been trying to say. It’s not clear to me if this debris disk starts outside the HZs or not. If it extends close to the stars, then the only hope for any planets in the HZs is if there are gaps in the disks, analogous to the structure in Saturn’s rings.

If these gaps do not exist, that surely means there are no planets in the debris disk regions. These AC debris disks are definitely more compact than either of the other two systems you mention. In fact they are constrained to be less than 4AU outer radius, according to the paper.

Looking again at their Figure 9, maybe it can be discerned that the inner boundary for both disks is around 1AU (also, maybe we are over-interpreting the data). If this is the case, then there is still the possibilty of planets within this radius, and in the case of AC-B, that includes the HZ.

Ronald February 12, 2014 at 12:51

kzb: very clear and entirely agreed. With regard to AC, it looks like our hope is now mainly on B. And without wanting to be a heretic on this wonderful website, even for B the chances seem rather slim. Personally, my hope is rather on single solar type stars (like the 3 I mentioned above). But time will tell. Hopefully soon.

andy February 12, 2014 at 19:06

I still don’t see that the planet necessarily has to clear a gap in the disc (if one exists). It depends on the rate of dust production and how efficiently dust grains migrate through the system (the dust does not have to remain in the region in which it is produced). One might expect that an asteroid belt that is being perturbed by an eccentric binary star system might have a rather different collision rate to one being perturbed by a giant planet on a circular orbit. Then there’s the relative timescale for a planet to clear a dust grain from its vicinity versus the timescale for which a dust grain would move through that region, e.g. due to Poynting-Robertson drag.

So while I would anticipate that a planet would induce structures in the dust disc, the assertion that it must be in a cleared-out gap does not seem to me to be necessarily correct.

kzb February 13, 2014 at 9:03

Andy, you are of course absolutely correct. Any gap will be the result of a balancing act between the rate of dust dispersion into the orbital area being considered and the rate of clearing by a large body. The situation would have to be modelled to assess the feasibility.

I imagine there are no absolute gaps. The gaps are simply more rarified than on either side, and the boundaries will be diffuse, not sharp. But the gaps should be larger and easier to detect as the planetary mass increases. There will undoubtedly be a detection limit for the planetary mass. As to what that might be, and whether it is in a useful range, I really have little handle on.

As things stand, I am sure this detection limit is very large, because the dust is only a marginal detection. I just wonder if it could be improved on in the foreseeable future.

kzb February 13, 2014 at 14:31

Ronald, also we need to beware of reading too much into this paper. Given the detection problems, how well are the boundary radia known? Their Figure 9 might give a false impression.

Also, Andy’s point that dust might be dispersed into the planetary orbits on a rapid timescale (faster than any planet would clear it). That is, there are no asteroids in the planetary orbits (well, usually), but there IS dust. This seems a real possibilty; maybe we should not conclude anything either way until someone can model what is happening.

ljk August 14, 2014 at 15:19

NASA Probe May Have Caught Dust from Interstellar Space, a First

By Charles Q. Choi, Space.com Contributor | August 14, 2014 02:01 pm ET

Seven tiny grains of rock captured by NASA’s comet-chasing Stardust probe in 2004 may be visitors from the vast reaches of interstellar space, researchers say.

These interstellar dust motes from Stardust are fluffier and more diverse than expected, findings that could one day shed light on the origins of the solar system, scientists added.

Interstellar dust motes are bits of rock that permeate the enormous spaces between the stars. Supernovas and ancient stars produce interstellar dust, which contains elements such as carbon, nitrogen and oxygen that are necessary for life.

Full article here:

http://www.space.com/26833-nasa-stardust-spacecraft-captured-interstellar-dust.html

To quote:

The scientists enlisted the aid of volunteers around the world in the Stardust@home project. These citizen scientists, who called themselves “Dusters,” helped study more than a million digital images of the microscopic impacts that particles made on the aerogel and on pieces of aluminum foil on Stardust located between the aerogel tiles on the collector tray.

“The Dusters as a community are really good at finding tracks, much better than we are,” Westphal said.

The researchers and citizen scientists analyzed 71 tracks that particles made as they crashed into the aerogel tiles. The analysis was unable to identify two of the tracks, but revealed that 66 were caused by spacecraft debris, leaving three potential grains of interstellar dust. Their discoverers named these particles Orion, Hylabrook and Sorok.

Bruce Hudson, a retired carpenter in Ontario, Canada, chose the name Orion due to his affinity with space; Naomi Wordsworth, in Buckinghamshire in England, took Hylabrook from a poem by Robert Frost; and Westphal and his colleagues named Sorok.

“Sorok was track 40, and ‘sorok’ means 40 in Russian,” Westphal said.

ljk August 18, 2014 at 11:06

Dust in the (Interstellar) Wind: Seven Particles of Possible Interstellar Origin Identified in Samples Returned by NASA’s Stardust Spacecraft

By Leonidas Papadopoulos

Following a preliminary analysis of the dust grains that had been returned to Earth by NASA’s Stardust spacecraft in 2006, a research team identified seven particles that might have originated from the interstellar medium that permeates the entire Milky Way galaxy. If confirmed, this discovery will represent the first time ever that humanity has sampled the stardust itself—the stuff the Solar System and all life on Earth are made of.

Even though we may be tempted to think of the interstellar medium as being a vast expanse of empty space, in reality it is far from a perfect vacuum. Despite having an average density which is lower than that of any artificial vacuum created on Earth, the interstellar medium is consisted for the most part of a rarefied gas that permeates the entire galaxy and is made up of atomic and molecular hydrogen and helium as well as ionized particles.

Besides these huge amounts of gas, the interstellar medium also contains trace amounts of solid microscopic particles which are believed to be a few molecules to a few tenths of a micrometer (millionth of a metre) in size and are composed of heavier elements like silicates, carbon, oxygen, silicon, nickel, and iron.

These heavier elements are the products of stellar alchemy—the ashes left behind by the fusion of lighter elements like hydrogen and helium inside the cores of every star throughout the galaxy. When stars reach the end of their lives, they eject most of their mass into space, the more massive ones in violent supernova explosions and the lighter ones in a slow blowing off of their outer layers, enriching the interstellar medium in the process with all the heavier elements they had cooked inside their stellar furnaces.

The death throes of these older stars also form the catalyst with which newer generations of stars and planetary systems are formed, now enriched in these heavy elements from their stellar progenitors. Our own Solar System also came into existence through this endless cosmic cycle of creation and destruction approximately 4.5 billion years ago, when a giant molecular gas cloud gravitationally collapsed, possibly from the explosion of a neighboring supernova.

Full article here:

http://www.americaspace.com/?p=65936

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