# Detection Method for Binary Star Planets

Astrometry, using the position and motion of celestial objects to further astronomical research, is ever more useful in the study of extrasolar planets. If you can measure how much a given star is displaced by the presence of a planet, you have a valuable adjunct to existing radial velocity and transit methods. Now a new study has examined astrometry’s uses with binary stars, using the Hale telescope on Mt. Palomar and the Keck II instruments on Mauna Kea. And with certain restrictions, adaptive optics may allow us to detect binary star planets.

The targets were seventeen binary or multiple star systems, most of them M-dwarf binaries closer than 20 parsecs from the Sun. Observations were conducted in the near infrared, with relative separations and position angles carefully measured. Study authors Krzysztof Helminiak and Maciej Konacki (Nicolaus Copernicus Astronomical Center, Poland) note the advantage of close systems:

The closure of companions allows one to observe visual binaries in a smaller ﬁeld, where atmospheric distortions are weakly affecting the measurements, [the] star’s image is better sampled, and the measurements errors in pixels transfer to smaller errors in arcseconds. But, instead of parallax and proper motion, one have to take into account the orbital motion. Note also that in case of a positive detection of a 3-rd body, relative astrometry does not give the information around which particular star of the system the body is orbiting.

The funding-challenged Space Interferometry Mission, unlikely to launch before 2016, will use astrometric techniques in the hunt for terrestrial worlds around nearby stars, a search that will demand an accuracy of one millionth of an arcsecond. A characteristic ‘wobble’ in relation to nearby reference stars may indicate that the target star has a companion; indeed, the mission has the potential of detecting planets smaller than Earth around the closest stars. While we’re waiting for missions like SIM, though, the current paper shows that careful CCD imaging with adaptive optics on Earth can be a successful strategy.

Image: Astrometry focuses on measuring how the position of a star in the sky varies as the star wobbles. If the extrasolar system is seen face-on, this variation will take the form of a circular motion. If the system is seen edge-on, the variation will appear as a back-and-forth motion on the sky. Credit: Rich Townsend/University College London.

Don’t expect the detection of terrestrial worlds from this method — the authors are talking about “…much better precision than 1 milliarcsecond,” or a thousandth of an arcsecond. SIM beats that by far, but what the authors outline may be sufficient for detecting larger worlds around small stars and, of course, it’s doable today. The paper is Helminiak and Konacki, “Precise Astrometry of Visual Binaries with Adaptive Optics. A Way for Finding Exoplanets?” slated to appear in the proceedings of the Les Houches Winter School Physics and Astrophysics of Planetary Systems, (EDP Sciences: EAS Publications Series), and available online.

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• MaDeR July 30, 2008, 2:40

You would want to look into ESA Gaia project. It will do high precision parallax measurements. Sufficiently high to discover many planets that “tug” their stars. Unfortunately it will be huge bath of hot jupiters (mainly). Well, better that than nothing, of course – there are many questions surrounding hot jupiters. For example, why some HJ are more puffed up than expected (in comparsion with theory)? Assessing population of hot jupiters in our neighborhood will allow constraint many models of planet formation. And more. :)

• Administrator July 30, 2008, 14:43

Yes, Gaia is coming up in the queue. For those who want to follow up on this right away, here’s a link to the project’s home page:

http://gaia.esa.int/science-e/www/area/index.cfm?fareaid=26

• nardo July 30, 2008, 14:46

ESA’s Gaia project will be an impressive tour de force for Milky Way structure & evolution. For planets, however, it has been obscenely oversold. Individual astrometric measures will be at the 200 microarcsecond level – no better than picking off Jupiters at 10 parsecs – and they cannot be scheduled. One is at the mercy of a preset scan pattern, which cannot be optimized for planets of various periods. Such a sampling function will lose many of the ‘cool’ jupiters in the data set, and the ‘hot’ jupiters will have astrometric signatures that are too small. Gaia: good for stars, bad for planets.

• Administrator July 30, 2008, 15:00

200 micro-arcseconds? I hadn’t heard that, and it certainly does impact planetary measurements. Interesting… Of course, Gaia aims at many other interesting projects, but we’ll talk about them in an upcoming post.

• daniel July 31, 2008, 10:46

nardo said: 200 micro-arcseconds!
where did get this information? i’m sure the you are absolutely wrong! http://gaia.esa.int/science-e/www/object/index.cfm?fobjectid=40577 the gaia go to be a great mission to detect extrasolar planet

• Administrator July 31, 2008, 14:11

Is the 200 micro-arcseconds figure incorrect? I can’t find the relevant numbers on the Gaia site — would appreciate any further reference. The site daniel lists references “…Gaia’s microarcsecond-precision…” Are they saying the exoplanet work will take place at the 1 microarcsecond level?

• daniel August 1, 2008, 13:07

this a interest abstract that i found about GAIA ” GAIA AND THE HUNT FOR EXTRA-SOLAR PLANETS” of 1997 http://arxiv.org/PS_cache/astro-ph/pdf/9707/9707019v1.pdf and this abstract of 2007 http://arxiv.org/PS_cache/arxiv/pdf/0711/0711.4903v1.pdf on the simulations the presion its between 5 to 100 μas GAIA will be a great planet hurt indeed

• Administrator August 1, 2008, 13:30

daniel, thanks very much. I’ll give both these files a look.

• ljk August 15, 2008, 7:55

Planetary systems around close binary stars: the case of the very dusty, Sun-like, spectroscopic binary BD+20 307

Authors: B. Zuckerman, Francis C. Fekel, Michael H. Williamson, Gregory W. Henry, M. P. Muno

(Submitted on 13 Aug 2008)

Abstract: Field star BD+20 307 is the dustiest known main sequence star, based on the fraction of its bolometric luminosity, 4%, that is emitted at infrared wavelengths. The particles that carry this large IR luminosity are unusually warm, comparable to the temperature of the zodiacal dust in the solar system, and their existence is likely to be a consequence of a fairly recent collision of large objects such as planets or planetary embryos. Thus, the age of BD+20 307 is potentially of interest in constraining the era of terrestrial planet formation.

The present project was initiated with an attempt to derive this age using the Chandra X-ray Observatory to measure the X-ray flux of BD+20 307 in conjunction with extensive photometric and spectroscopic monitoring observations from Fairborn Observatory. However, the recent realization that BD+20 307 is a short period, double-line, spectroscopic binary whose components have very different lithium abundances, vitiates standard methods of age determination.

We find the system to be metal-poor; this, combined with its measured lithium abundances, indicates that BD+20 307 may be several to many Gyr old. BD+20 307 affords astronomy a rare peek into a mature planetary system in orbit around a close binary star (because such systems are not amenable to study by the precision radial velocity technique).

Comments: accepted for ApJ, December 10, 2008

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0808.1765v1 [astro-ph]

Submission history

From: Benjamin Zuckerman [view email]

[v1] Wed, 13 Aug 2008 01:21:22 GMT (38kb)

http://arxiv.org/abs/0808.1765

• murison September 28, 2008, 22:25

Regarding the 200 microarcsec question: Perhaps the confusion is over single-observation accuracy versus mission accuracy. The two can be very different. For astrometric effects of planets orbiting stars, you may be largely stuck with something like the single-measurement accuracy, depending on planetary orbit period, observation cadence, and how difficult the orbit is to characterize (and hence model). nardo does specifically mention single-measurement accuracy in his post. I do not offhand know what is the single-measurement accuracy being claimed for GAIA. 200 uas does seem large, but perhaps that is for faint stars. For the brighter stars it would/should be an order of magnitude better than that.

• ljk October 6, 2008, 0:05

Stellar wobble caused by a binary system: Can it really be mistaken as an extra-solar planet?

Authors: Maria H. M. Morais, Alexandre C. M. Correia

(Submitted on 2 Oct 2008)

Abstract: The traditional method for detecting extra-solar planets relies on measuring a small stellar wobble which is assumed to be caused by a planet orbiting the star.

Recently, it was suggested that a similar stellar wobble could be caused by a close binary system (Schneider and Cabrera, 2006). Here we show that, although the effect of a close binary system can at first sight be mistaken as a planetary companion to the star, more careful analysis of the observational data should allow us to distinguish between the two effects.

Comments: 9 pages, 6 figures. Astronomy & Astrophysics, in press

Subjects: Astrophysics (astro-ph)

DOI: 10.1051/0004-6361:200810741

Cite as: arXiv:0810.0506v1 [astro-ph]

Submission history

From: Alexandre Correia [view email]

[v1] Thu, 2 Oct 2008 18:18:36 GMT (631kb)

http://arxiv.org/abs/0810.0506

• ljk November 25, 2008, 0:43

arXiv:0811.3807

Date: Mon, 24 Nov 2008 06:33:46 GMT (743kb)

Title: The sdB+M Eclipsing System HW Virginis and its Circumbinary Planets

Authors: Jae Woo Lee, Seung-Lee Kim, Chun-Hwey Kim, Robert H. Koch, Chung-UkLee, Ho-Il Kim, and Jang-Ho Park

Categories: astro-ph

Comments: 26 pages, including 5 figures and 8 tables, accepted for publicationin AJ

For the very short-period sdB eclipsing binary HW Vir, we present new CCD photometry made from 2000 through 2008. In order to obtain consistency of the binary parameters, our new light curves were analyzed simultaneously with previously published radial-velocity data. The secondary star parameters of $M_2$=0.14 M$_\odot$, $R_2$=0.18 R$_\odot$, and $T_2$=3,084 K are consistent with those of an M6-7 main sequence star.

More than 250 times of minimum light, including our 41 timings and spanning more than 24 yrs, were used for a period study. From a detailed analysis of the $O$–$C$ diagram, it emerged that the orbital period of HW Vir has varied as a combination of a downward-opening parabola and two sinusoidal variations, with cycle lengths of $P_3$=15.8 yr and$P_4$=9.1 yr and semi-amplitudes of $K_3$=77 s and $K_4$=23 s, respectively.

The continuous period decrease with a rate of $-8.28\times10^{-9}$ d yr$^{-1}$ may be produced by angular momentum loss due to magnetic stellar wind braking but not by gravitational radiation. Of the possible causes of the cyclical components of the period change, apsidal motion and magnetic period modulation can be ruled out.

The most reasonable explanation of both cyclical variationsis a pair of light-travel-time effects driven by the presence of two substellarcompanions with projected masses of $M_3 \sin i_3$=19.2 M$\rm_{Jup}$ and $M_4\sin i_4$=8.5 M$\rm_{Jup}$.

The two objects are the first circumbinary planets known to have been formed in a protoplanetary disk as well the first ones discovered by using the eclipse-timing method.

The detection implies that planets could be common around binary stars just as are planets around single stars and demonstrates that planetary systems formed in a circumbinary disk can survive over long time scales.