Tightening the Parameters for Centauri A and B

by Paul Gilster on February 10, 2017

When it comes to the nearest stars, our focus of late has been on Proxima Centauri and its intriguing planet. But of course the work on Centauri A and B continues at a good clip. The prospects in this system are enticing — a G-class star like our own, a K-class dwarf likewise capable of hosting planets, and the red dwarf Proxima a scant 15000 AU away. Project Blue examines how we might image planets here as our radial velocity studies proceed.

But we have much to learn, and not just about possible planets. A new paper by Pierre Kervella (Observatoire de Paris), working with Lionel Bigot and Fréderic Thévenin (both at the Observatoire de la Cote d’Azur), reminds us of the importance of firming up our stellar data.

We need to learn as much as possible about Centauri A and B not just because we’d like to find planets there but also because the work has implications for space missions, including the ESA’s Gaia, which will tighten our distance measurements to many stars. The Alpha Centauri stars are important benchmarks for Gaia, putting the emphasis on an accurate calibration of the basic stellar parameters in this system.

Alpha Cen sizes

Image: The Alpha Centauri stars in comparison to our Sun and other Solar System objects. Credit: Pierre Kervella.

Kervella and team have used new observations of Centauri A and B with the Very Large Telescope Interferometer equipped with the PIONIER (Precision Integrated Optics Near-infrared Imaging ExpeRiment) beam combiner to operate in the near infrared. Their paper reports on improved measurements of both stars’ angular diameters in relation to the phenomenon known as limb darkening. The latter results point to the need to improve our models as we study the photospheres of stars including our own Sun.

Limb darkening refers to the gradual decrease in brightness that we see as we look away from the center of a star toward its outer edge, or limb. Have a look at the image below to see the effect, which is easily visible in photographs of the Sun. When we look at the center of the Sun’s disk, we see the greatest light emission because we are viewing the deepest, and hottest layers, while at the limb, we are seeing only cooler layers that produce less light.

The phenomenon is important because we can use it to study how a star’s atmosphere is structured, but it turns up as a factor in everything from eclipsing binary stars to gravitational microlensing. Moreover, limb darkening will affect the shape of the transit curve produced by a planet moving in front of its star. The planet blocks a smaller part of the star’s light when it is near the limb, and a greater fraction as it moves toward the center of the star. The center of a transit, in other words, is always going to be deeper than the edges, something that would not happen if a star had a uniform brightness (there the transit ‘curve’ would appear flat).

800px-2012_Transit_of_Venus_from_SF

Image: A filtered image of the Sun in visible light, showing the limb-darkening effect as a dimmer luminosity towards the edge or limb of the solar disk. The image was taken during the 2012 transit of Venus (seen here as the dark spot at the upper right). Credit: Brocken Inaglory / Wikimedia Commons.

What the researchers find is that there is a discrepancy between the measured limb darkening parameters and the models previously discussed in the literature — these overestimate limb darkening for both Centauri A and B. While the difference is small and does not affect the team’s measurements of the stars’ angular diameter, the limb darkening models need adjustment, a problem that is not limited to Alpha Centauri. From the paper:

This implies that the underlying atmosphere models deviate from the real intensity profiles of α Cen, and we note that similar discrepancies are observed on the Sun. The observed discrepancies indicate that the predictive accuracy of the current generation of model atmospheres may be significantly lower than expected. This is likely to be more critically the case for stars with parameters that are very different from those of the Sun (e.g., cooler stars with molecular envelopes) and for wavelength regions more complex to model than the near-infrared (e.g., the ultraviolet and visible).

Our interest in cooler stars like Proxima Centauri, and the useful transit depths that could be observed with exoplanets around nearby red dwarfs mean we need as accurate a limb darkening model as possible in order to measure their transits precisely. These observations for Centauri A and B also offer us benchmarks we can use to firm up our atmospheric models for more distant stars.

Proxima-planets

Image: Proxima Centauri in relation to familiar objects in our own system. Credit: Pierre Kervella.

Meanwhile, the radii of Centauri A and B are shown to line up with earlier work. For Centauri A, we get a radius of 1.2234 ± 0.0013 ± 0.0051 R (where R is the radius of the Sun). Centauri B yields 0.8632 ± 0.0009 ± 0.0036R. As the paper notes, “Together with the parallax and masses recently reported by Kervella et al. (2016), as well as spectroscopic studies, the determined radii complete the calibration of the fundamental parameters of both components of α Centauri.”

The paper is Kervella, Bigot and Thévenin “The radii and limb darkenings of α Centauri A and B: Interferometric measurements with VLTI/PIONIER,” Astronomy & Astrophysics 597, A137 (2017). Abstract / preprint.

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{ 19 comments… read them below or add one }

Mark February 10, 2017 at 12:32

Alpha Centauri is way too bright to be observed with Gaia. Using a special technique for heavily saturated images they can measure stars as bright as magnitude +3, but that still makes the individual stars of the system a factor of 10 too bright to measure.

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Paul Gilster February 10, 2017 at 13:38

From Kervella et al.:

“In addition, the α Cen pair is one of the principal benchmark stars of the Gaia mission (Heiter et al. 2015; Jofré et al. 2015). An extremely accurate calibration of its fundamental parameters is essential for the validation of the data analysis methods that are currently applied to the fainter targets of the Gaia catalog (see e.g. Bailer-Jones et al. 2013).”

So Centauri A and B have their uses for Gaia.

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Jeff Wright February 10, 2017 at 16:56

Has Hubble ever imaged our closest neighbors?

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Paul Gilster February 10, 2017 at 21:26
Neil S February 10, 2017 at 13:37

Could someone explain the extra level of ± please. E.g. in 0.8632 ± 0.0009 ± 0.0036R☉ do those represent errors from different sources, and why not combine them into 0.8632 ± ( 0.0009 + 0.0036R☉). And why not put a value in for R☉, which must be known very well now.

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Paul Gilster February 10, 2017 at 13:46

I had this from an email this morning from Pierre Kervella:

“The precision on the radii of A and B are currently limited by the accuracy with which we know the wavelength of our PIONIER observations (0.4%). This sets the systematic error bars of 0.0051 and 0.0036 Rsun on their radii. We expect to improve this absolute accuracy with the GRAVITY instrument of the VLT Interferometer soon.”

And re the solar radius (this is from the paper):

“We adopt the IAU convention (Prša et al. 2016) for the nominal solar radius,” here listed as 695 700 km.

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Neil S February 10, 2017 at 14:14

Ah, so the second ± quantity is actually a variable quantity because R☉is variable. Thanks.

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George King February 10, 2017 at 14:25

So would the $2 million Centauri “stare” mission discussed in this prior piece still potentially add something meaningful to the mix?

http://www.centauri-dreams.org/?p=32845

Or have subsequently planned projects substantially reduced its net overall likely benefit, such as the project discussed in this piece?

http://www.centauri-dreams.org/?p=36925

I’m still looking for that spare $2 million behind the seat cushions, but the low cost stare project intrigued me at the time.

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Scott Guerin February 10, 2017 at 15:56

Non-scientific aside but I’m getting through the last of the Dark Forest Trilogy and for anyone who doesn’t know, it’s a subtle adventure through space-time centered on the relationship between Humans, the Trisolarians in the Centauri system, and a very populous and aggressive intelligent universe beyond.

I highly recommend it!

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Clark L. Zeuss February 11, 2017 at 2:15

An excellent series! And with as close as Cen A and B come to each other, it does make you think about the climate on a planet in between, as the first book deals with.

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jonW February 10, 2017 at 16:29

Is it just “luck” that the closest stellar system has such similar star(s) to our own sun, when as many as 90% of the galaxy’s stars are red dwarves, whose planets as we’ve seen here recently are likely to have significant problems of habitability? Or is there an explanation for why the nearest G-class star is also part of the nearest stellar system?

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David W Cummings February 11, 2017 at 16:07

jonW, why would it be luck, one way or another, what kind of stars are near to us (as long as they aren’t about to explode and kill us)?

We are lucky to live on a planet that has suffered all the right accidents to make life and evolution possible, and we are lucky to not live near a star that’s about to go super-nova tomorrow, and for that matter, we are lucky to live in a universe whose value of dark energy is 10^-120 and not 10^-116 or 10^-124… but I just don’t see how the lineup of stars 4.4 light years away from us (be they G, be they K) contributes to our luck one way or another.

I’m not trying to be argumentative here, really. I’m just trying to understand your question.

And BTW, I think we are very, very, very lucky to have a large moon. I think our moon, as much as it does get mentioned, is still greatly under-rated by a lot of people when considering the subject of all the lucky accidents our tree-of-life here on Earth owes its existence to.

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jonW February 13, 2017 at 20:44

Yes, maybe I wasn’t clear: I don’t mean that the presence of a nearby G- or K-type star contributed to our being here, the way the things you mentioned did (and I concur about our large moon btw). I mean: if we are interested in particular in exoplanets around a star similar to our own sun, above and beyond our interest in exoplanets in general, aren’t we lucky that the closest system to us features two relatively sun-like stars? You would expect, since over 90% of stars in our galaxy are red dwarves, that we would be closest to a red dwarf. Since red dwarves have problems of habitability then we are naturally primarily interested in habitable zones around sun-like stars and therefore we are lucky that we have such a system close at hand (well, relatively close). My question as to whether it is just pure luck was meant to suggest there could be another explanation, like shared origins of the sun and Alpha Centauri A and B, and we have stayed close together. If this were the case, then it is not just chance that they are like us, any more than it is not chance when two siblings look alike.

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Robert L February 12, 2017 at 12:44

A simplification of the nomenclature here would be really beneficial: we have α and β star systems, A B and C stars and a b planet (with no hint of what became of planet a), and I have to remind myself how it all works every time I revisit the subject. Calling C “Proxima” was a start, albeit short-sighted in that it is only true for another 25,00 years.

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Bruce Mayfield February 12, 2017 at 19:08

I think the convention is that the first exoplanet discovered around any star is always called lowercase “b”, then any additional planets found would be c, d and so on, with lower case a not used. Uppercase letters are used for stars. (What the heck about BDs? IAU ruling needed? Yuck. ;)

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LocalFluff February 12, 2017 at 17:27

270,000 AU to Proxima. If it had been only 15,000 AU away it would’ve been an easy target to reach ;-)

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Paul Gilster February 12, 2017 at 20:36

You mistake my meaning. Proxima is 15000 AU away from Centauri A and B. I was trying to say that getting to A and B also gives us an inviting target within reach of the binary stars. I was not saying that Proxima is 15000 AU from Earth. If only it were…

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Harry R Ray February 21, 2017 at 11:45

“NASA will hold a news conference at 1 PM EST Wedensday, Feb 22 to present news on planets that orbit stars other than our own sun, known as exoplanets. The event will air live on NASA television and the agency’s website. Details of these findings are embargoed by the journal Nature until 1 PM…”. This could be one of three things: ONE: Either ANOTHER megadump of new planets from the original mission or (a) new potentially habitable planet(s) from the K2 mission, TWO(and the reason I am posting this comment HERE)CONFIRMATION of Alpha Centauri Bc(which seems to be the CONSENCUS from the NASAwatch “rumor mill”. THREE(and what I BELIEVE to be the case)the long-awaited release of the Spitzer observations of the TRAPPIST-1 system! The reason I am going with “THREE” OVER “TWO” or “ONE” is that one of the breifing participants is Michael Gillon, the PRINCIPAL INVESTIGATER of the TRAPPIST-1 discovery paper. He was also marginally involved in the paper perporting a possible transit of an Earth sized planet of Alpha Centauri B. Demory was the principal investigator of THAT paper, and he is NOTICABLY ABSENT from this press conference. We’ll know soon enough which one it is.

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Harry R Ray February 21, 2017 at 12:54

Did some more checking. Here is what I have found. Jason Wright tweeted that he has no idea about what it is, but Steinn Sigurdsson told him it has something to do with the Spitzer Space Telescope. ALSO: According to Abel Mendez, REVISIONS will be made to the HEC Catalog TOMORROW! VERY EXCITING STUFF!!!!!!!!

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