Lithium, exoplanets and 16 Cygni

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Lithium, exoplanets and 16 Cygni

Post by Lazarus on 18th November 2009, 6:19 pm

Don't think this one's been mentioned here, but it seems exoplanet host stars are lithium-poor. I've linked to systemic instead of the paper because the paper's behind the Nature paywall so I haven't read it. :-)

So an obvious test would be a multiple star system where one of the stars is an exoplanet host. You'd assume that the stars should have similar compositions because they probably formed from the same nebula at roughly the same time.

So let's take the 16 Cygni system which contains three stars, of which two (A,B) are well-studied solar analogues. The third star (C) is a red dwarf about which not much is known, so I'll ignore it. 16 Cygni B is an exoplanet host. 16 Cygni A is radial velocity stable.

While the abundances for most of the elements in 16 Cyg A and B agree quite well, 16 Cygni B is depleted in lithium by about a factor of five relative to 16 Cygni A.

Hmmmm...


Last edited by Lazarus on 18th November 2009, 6:46 pm; edited 1 time in total
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Re: Lithium, exoplanets and 16 Cygni

Post by exoplanet on 18th November 2009, 6:41 pm

Hi,

this is indeed a very interesting and intriguing observation. However I felt that the way it was announced was a bit misleading. The sample of stars that "do not have planets", that was compared to the sample "with planets", is, I gather, a sample with radial velocity stable stars. This does not mean that these stars do not have planets, since they may be below the discovery threshold of the radial velocity technique.

So "exoplanet host stars are lithium-poor" should perhaps be re-phrased as "stars with giant exoplanets are lithium poor". Or am I missing something ?

If so, lithium depletion is perhaps telling us something about the *kind* of planetary system a star has, more than whether it has one (or not).

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Re: Lithium, exoplanets and 16 Cygni

Post by Lazarus on 18th November 2009, 6:46 pm

Biases and observational limits are definitely things to consider here. To take a different example, the well-known planet-metallicity correlation seems to be a lot stronger for hot Jupiters than long-period gas giants. But the hot Jupiters are easiest to detect.

This is why it is frustrating that the paper is behind a Nature paywall... difficult to find out what the detection limits were for the planetless stars. Given the nature of Nature papers (ha ha), it is unlikely there is too much detail either, thanks to the very short article length limits.
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Re: Lithium, exoplanets and 16 Cygni

Post by Sirius_Alpha on 18th November 2009, 6:47 pm

I was about to post on it, but decided to do some reading on it first. Seems that planet formation helps to alter the convective histories of their host stars, allowing for lithium to be destroyed.

That's really interesting about that striking difference between the lithium abundances in the two main 16 Cygni stars. How well does this hold up for other multi-stellar planet host systems?

exoplanet wrote:This does not mean that these stars do not have planets, since they may be below the discovery threshold of the radial velocity technique.
Oh definitely. If they are outside the detection threshold, then they're probably sub-Jovian.

Exoplanet wrote:If so, lithium depletion is perhaps telling us something about the *kind* of planetary system a star has, more than whether it has one (or not).
You may well be right. It's definitely telling us that the planet populations around low-lithium stars are different than those of high-lithium stars. What this relation is, as you point out, is that giant, easily detectable planets don't seem as common. It's too early to tell if lithium-rich stars lack the large population of low-mass terrestrial planets that has been proposed to exist around sun-like stars.

Edit:
Lazarus wrote:Given the nature of Nature papers (ha ha)

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Re: Lithium, exoplanets and 16 Cygni

Post by Lazarus on 18th November 2009, 7:12 pm

Sirius_Alpha wrote:How well does this hold up for other multi-stellar planet host systems?
Good question - the problem is a lot of the companions are red dwarfs, and that makes metallicity determinations difficult: the star is dim, and M dwarfs have extremely complex spectra in the first place because their atmospheres are cool enough that molecules can form. I think 16 Cygni is the best studied example, it received attention even before the discovery of the planet because the stars are solar analogues. Haven't done a decent search though... it's difficult to keep track of the multi-star exoplanet-host systems.

Hopefully the announcement of this anticorrelation between lithium and (detectable?) planetary systems will encourage some more investigation of planet-hosting binaries.
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Re: Lithium, exoplanets and 16 Cygni

Post by exoplanet on 18th November 2009, 8:34 pm

Another famous pair of sun-like stars, one of which host a planet, is HD80606/7. This paper:

http://www.aanda.org/articles/aa/pdf/2001/32/aade293.pdf

indicates (last paragraph of section 2) that both are lithium depleted.

Another article on a multiple system showing the "16Cyg effect" is :

http://www.iop.org/EJ/article/0004-637X/568/1/363/54915.text.html

on HD178911A,B (part of Struve 2474 in Lyra).

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Re: Lithium, exoplanets and 16 Cygni

Post by Lazarus on 19th November 2009, 6:07 pm

Thanks for the info!

Haven't managed to find anything aside from upper limits for the lithium abundances of HD 80606/7. Not sure what kind of RV data is available for HD 80607... could be an interesting target if its lithium is indeed comparable to HD 80606.

Definitely there are a few binaries with similar stars in the exoplanets list: XO-2 is another example, which hasn't received much attention beyond the presence of the planet around XO-2N.
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Re: Lithium, exoplanets and 16 Cygni

Post by exoplanet on 20th November 2009, 5:55 pm

This paper contains very relevant information for this thread:

http://arxiv.org/abs/astro-ph/0206473

Cheers,

Luis

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Re: Lithium, exoplanets and 16 Cygni

Post by Lazarus on 20th November 2009, 6:23 pm

Nice find there Luis... these binary systems with stars with very similar properties but differing lithium abundances should provide an important test for the lithium/planets anticorrelation.

Also interesting about the open cluster lithium patterns that are mentioned: the Hyades has a close relationship between temperature of the star and amount of lithium present, whereas the solar age open cluster M67 has a larger spread.
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Re: Lithium, exoplanets and 16 Cygni

Post by Sirius_Alpha on 21st November 2009, 1:30 am

Regarding 16 Cygni, from what I've read (I can't recall the source off the top of my head but if you want, I'll go dig it up), the red dwarf companion in 16 Cygni is a 0.17 solar-mass star in a fairly close (a few tens of AU) orbit around 16 Cyg A.

If the lithium depletion observed in stars with planets is a result of increased mixing of the star due to interaction with planets and the disk, then how would the interaction between the red dwarf and 16 Cyg A fit in to all this?

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Re: Lithium, exoplanets and 16 Cygni

Post by Lazarus on 21st November 2009, 5:42 am

According to this source the projected separation of 16 Cygni C from 16 Cygni A is 73 AU.

This contrasts quite strongly with the HD 178911A/C system which is a system with orbital period 3.55 years, eccentricity 0.589, semimajor axis 2.9 AU (from Kepler's third law and stellar masses of 1.07 and 0.84 times solar)

It would seem therefore that the binary formation process, even at separations on the scale of planetary systems, doesn't deplete the lithium in the star. That's going to make things difficult...
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Re: Lithium, exoplanets and 16 Cygni

Post by Sirius_Alpha on 21st November 2009, 10:09 am

If HD 178911 C formed from gravitational collapse as I think is expected for stars, would this result in a different effect on the convective history of the star?

We seem to agree that planet-formation affects lithium abundance, but would it be more accurate to say core-accretion specifically? I'm unsure how it would make a significant difference though.

If no one objects, I think I may move this to Extrasolar Mechanics, as the topic of the lithium/planets anti-correlation would perhaps better fit there.

Edit: Hmm, 73 AU is what I had as well but I didn't believe it due to system stability. What's the periastron of 16 Cyg A and B?

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Re: Lithium, exoplanets and 16 Cygni

Post by exoplanet on 21st November 2009, 3:43 pm

With the ongoing efforts to detect planets around Alpha Centauri B, it is interesting to look at the lithium abundances for both stars. According to this (somewhat dated) article, Alpha Centauri B is lithium depleted whereas A has twice the Sun's abundance.

http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1984A%26A...140..427S&db_key=AST&page_ind=0&data_type=GIF&type=SCREEN_VIEW&classic=YES

So far, radial velocity studies have basically ruled out the existence of giant planets in orbit...

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Re: Lithium, exoplanets and 16 Cygni

Post by Lazarus on 21st November 2009, 5:39 pm

exoplanet wrote:According to this (somewhat dated) article, Alpha Centauri B is lithium depleted whereas A has twice the Sun's abundance.
Nice that it is the B component that is depleted: that's the one that's being most heavily targeted because of the K-dwarf RV "sweet spot".
Sirius_Alpha wrote:What's the periastron of 16 Cyg A and B?
Not particularly well determined. There are limits on the outer orbit but nothing particularly solid: it is a very long-period system!
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Re: Lithium, exoplanets and 16 Cygni

Post by ciceron on 21st November 2009, 6:31 pm

Now , this is very interesting , didn't know that fact about the Centauri system.

Anyone can point me to the current programs searching our neigbords for planets? I have just this feeling we are close , very close to find some earth-sized body just there... i'll go as far as to hope that one of then is in retrograde orbit Wink

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Re: Lithium, exoplanets and 16 Cygni

Post by Sirius_Alpha on 21st November 2009, 6:57 pm

HARPS is scanning a lot of nearby systems with the high-precision GTO Programme. Alf Cen is always being monitored now days, I heard the usual bright near stars (Tau Cet et al) are being monitored.

Thread moved to Mechanics.

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Re: Lithium, exoplanets and 16 Cygni

Post by Sirius_Alpha on 22nd November 2009, 10:47 pm

Oklo.org has quite an interesting blog entry on the issue.

http://oklo.org/2009/11/22/lithium-induced-speculations/

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Re: Lithium, exoplanets and 16 Cygni

Post by ciceron on 23rd November 2009, 3:57 am

Very interesting article. I wonder if relative abundance of other metals are being considered separately , instead of the general "metalicy" or its just Lithium the only element that stands up in correlations.

So far , it seems that high metalicy and low Lithium are two markers for selecting stars. Low Lithium is spected on slow rotating stars so it really only tells us that something dampened the angular moment of the system, be it a long lived protoplanetary dust disk in the youth of the star or another kind of transfer (like a close encounter with another star). High metallicy on the other side gives us some insigth in what the composition of the primordial cloud was at the birth of the star , so as far as the metallicy of the star goes up , heavy rocky planetesimals in the protodisk form in greater numbers.

That's what the process looks like in my mind. So far , then , Lithium depletion in solar-like stars seems to me like a easier way to determine the stelar rotation , not really an indicative of planet bearing status.

EDIT : seems systemic beat me to the question of other elements Smile

A less-well-known prediction is that one also expects that stars with
high oxygen (and by proxy, silicon) abundances relative to iron will
also show increased planet fractions at given metallicity. Sarah
Dodson-Robinson showed this was true as part of her Ph.D. Thesis.
Here’s the the key diagram from her paper on the topic:
http://oklo.org/2009/11/18/lithium/


Last edited by ciceron on 23rd November 2009, 4:36 am; edited 1 time in total (Reason for editing : Read the answer to my question.)

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Re: Lithium, exoplanets and 16 Cygni

Post by Sirius_Alpha on 24th November 2009, 10:28 am

Enhanced lithium depletion in Sun-like stars with orbiting planets
http://arxiv.org/abs/0911.4198

Abstract wrote:The surface abundance of lithium on the Sun is 140 times less than protosolar, yet the temperature at the base of the surface convective zone is not hot enough to burn Li. A large range of Li abundances in solar type stars of the same age, mass and metallicity is observed, but theoretically difficult to understand. An earlier suggestion that Li is more depleted in stars with planets was weakened by the lack of a proper comparison sample of stars without detected planets. Here we report Li abundances for an unbiased sample of solar-analogue stars with and without detected planets. We find that the planet-bearing stars have less than 1 per cent of the primordial Li abundance, while about 50 per cent of the solar analogues without detected planets have on average 10 times more Li. The presence of planets may increase the amount of mixing and deepen the convective zone to such an extent that the Li can be burned.

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Re: Lithium, exoplanets and 16 Cygni

Post by Lazarus on 24th November 2009, 3:34 pm

Nice to be able to get the paper. So this effect is restricted to only a narrow effective temperature range 5600-5900K, probably thanks to the different structure of the outer layers of stars outside this range. Places rather more stringent requirements on the kinds of binary star systems which would be useful to test such effects.

As Greg Laughlin says, the high lithium stars may potentially have very different planetary system architectures... already this seems to be the case with the [Fe/H] correlation (decreasing metallicity gets you fewer inner-system Jupiters), definitely worth trying to figure out what high-lithium planetary systems (if they exist) are doing.

Also relates to the question of what proportion of stars don't form planets at all...
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Re: Lithium, exoplanets and 16 Cygni

Post by Sirius_Alpha on 10th December 2009, 4:00 pm

Parent Stars of Extrasolar Planets. X. Lithium Abundances and vsini Revisited
http://fr.arxiv.org/abs/0912.1621

Abstract wrote:We determine Li abundances and vsini values from new spectra of 53 stars with Doppler-detected planets not included in our previous papers in this series. We also examine two sets of stars without detected planets, which together serve as our comparison sample. Using the method of comparison of Li abundances and vsini values between two sets of stars we introduced in Gonzalez (2008), we confirm that these two quantities are smaller among stars with planets compared to stars without detected planets near the solar temperature. The transition from low to high Li abundance among SWPs occurs near 5850 K, a revision of about 50 K from our previous determination. The transition from low to high vsini occurs near 6000 K, but this temperature is not as well constrained.

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Re: Lithium, exoplanets and 16 Cygni

Post by Sirius_Alpha on 2nd July 2010, 4:58 am

From the EPE's bibliography:

Lithium depletion in solar-like stars: no planet connection
Astron. & Astrophys. , - , -
accepted
preprint

It's behind a paywall. Unfortunate, as it looks like it'll be a really interesting paper.

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Re: Lithium, exoplanets and 16 Cygni

Post by Sirius_Alpha on 3rd August 2010, 8:08 pm

It's on ArXiv now.
http://arxiv.org/abs/1008.0575

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Re: Lithium, exoplanets and 16 Cygni

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