Comparative statistics and origin of triple and quadruple stars

Aims. We report the discovery of very shallow (ΔF/F ≈ 3.4× 10 −4 ), periodic dips in the light curve of an active V = 11.7 G9V star observed by the CoRoT satellite, which we interpret as caused by a transiting companion. We describe the 3-colour CoRoT data and complementary ground-based observations that support the planetary nature of the companion. Methods. We used CoRoT colours information, good angular resolution ground-based photometric observations in-and out-of transit, adaptive optics imaging, near-infrared spectroscopy, and preliminary results from radial velocity measurements, to test the diluted eclipsing binary scenarios. The parameters of the host star were derived from optical spectra, which were then combined with the CoRoT light curve to derive parameters of the companion. Results. We examined all conceivable cases of false positives carefully, and all the tests support the planetary hypothesis. Blends with separation >0.40 or triple systems are almost excluded with a 8 × 10 −4 risk left. We conclude that, inasmuch we have been exhaustive, we have discovered a planetary companion, named CoRoT-7b, for which we derive a period of 0.853 59 ± 3 × 10 −5 day and a radius of R p = 1.68 ± 0.09 R Earth . Analysis of preliminary radial velocity data yields an upper limit of 21 M Earth for the companion mass, supporting the finding. Conclusions. CoRoT-7b is very likely the first Super-Earth with a measured radius. This object illustrates what will probably become a common situation with missions such as Kepler, namely the need to establish the planetary origin of transits in the absence of a firm radial velocity detection and mass measurement. The composition of CoRoT-7b remains loosely constrained without a precise mass. A very high surface temperature on its irradiated face, ≈1800-2600 K at the substellar point, and a very low one, ≈50 K, on its dark face assuming no atmosphere, have been derived.

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“…A detailed study of stars by Tokovinin (2008) shows that at least 8% of the solar-type stars have three or more components.…”

Section: Corot Coloursmentioning

confidence: 99%

Transiting exoplanets from the CoRoT space mission

Léger¹,

Rouan²,

Schneider³

et al. 2009

A&A

499

378

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“…If we consider the canonical 12th magnitude G-star, this means that for the given semi-major axis, the ratio of the ratio of semi-major axis to distance can produce a maximum separation as small as 0.01 arcsec. Using the results obtained by Tokovinin (2008), we find that the probability of a triple system containing two M-stars with an orbital period of one year is between 0.1 and 0.4%. The probability of a triple system containing two eclipsing M-stars with an orbital period of up to one year is ≥10 −4 .…”

Section: The Brightness Difference Between the Primarymentioning

confidence: 99%

“…Since the CoRoT and Kepler mission focus on main-sequence F, G, and K stars (M * = 0.5 to 1.7 M ), only these stars are discussed in the following. According to Tokovinin (2008) at least 8% of the solar-type stars are in systems containing three or more stars. Dynamically stable triple systems have to be hierarchical with P L /P S > 5 (Eggleton 2006).…”

Section: The Brightness Difference Between the Primarymentioning

confidence: 99%

High-resolution infrared spectroscopy as a tool to detect false positives of transit search programs

Guenther

Tal-Or

2010

A&A

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Context. Transit search programs such as CoRoT and Kepler now have the capability of detecting planets as small as the Earth. The detection of these planets however requires the removal of all false positives. Although many false positives can be identified by a detailed analysis of the light-curves (LCs), the detections of others require additional observations. An important source of false positives are faint eclipsing binaries within the point spread function (PSF) of the target star. For example, triple stars are an important source of false positives. Unfortunately, most of the methods previously applied have difficulties in detecting these objects. Aims. We develop a new method that allows us to detect faint eclipsing binaries with a separation smaller than one arcsec from target stars. We thereby focus on binaries that mimic the transits of terrestrial planets. These binaries can be either at the same distance as the target star (triple stars), or at either larger, or smaller distances. Methods. A close inspection of the problem indicates that these systems contain either late-type stars, or stars of high extinction. Thus, in both cases the binaries are brighter in the infrared than in the optical regime. We show how high resolution infrared (hereafter IR) spectroscopy can be used to remove these false positives. Results. For the triple star case, we find that the brightness difference between a primary and an eclipsing secondary is about 9−10 mag in the visual but only about 4.5−5.9 mag in the K-band. In the next step, we demonstrate how the triple star hypothesis can be excluded by taking a high-resolution IR spectrum. Simulations of these systems show that the companions can be detected with a false-alarm probability of ∼2%, if the spectrum has a signal-to-noise ratio (S /N) ≥ 100. We subsequently show that high-resolution IR spectra also allows to detect most of the false positives caused by foreground or background binaries. Conclusions. If high resolution IR spectroscopy is combined with photometric methods, virtually all false positives can be detected without radial velocity (RV) measurements. It is thus possible to confirm transiting terrestrial planets with a modest investment of observing time.

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“…We also tested e out = 0.5 but found that almost all systems were unstable in this configuration so we have omitted these results for simplicity. The requirement that e out < 0.5 for stability is restrictive because observations show this to be a typical outer eccentricity (Tokovinin 2008).…”

Section: A Primordial Inner Binarymentioning

confidence: 99%

“…The inner eccentricity was set to zero initially, although it rose dramatically during Kozai cycles. The tertiary star was given a mass M 3 = 0.6M , corresponding to the median observed outer mass ratio q out = 0.4, and placed at a typical separation of 100 AU (Tokovinin 2008). The mutual inclination ∆I in,out = 74 • was chosen such that the inner binary would ultimately shrink to a 5 d period, according to Eq.…”

Section: A Primordial Inner Binarymentioning

confidence: 99%

No circumbinary planets transiting the tightestKeplerbinaries – a possible fingerprint of a third star

Martin

Mazeh

Fabrycky

2015

Mon. Not. R. Astron. Soc.

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The Kepler mission has yielded the discovery of eight circumbinary systems, all found around eclipsing binaries with periods greater than 7 d. This is longer than the typical eclipsing binary period found by Kepler, and hence there is a dearth of planets around the closest binaries. In this paper we suggest how this dearth may be explained by the presence of a distant stellar tertiary companion, which shrunk the inner binary orbit by the process of Kozai cycles and tidal friction, a mechanism that has been implicated for producing most binaries with periods below 7 d. We show that the geometry and orbital dynamics of these evolving triple-star systems are highly restrictive for a circumbinary planet, which is subject itself to Kozai modulation, on one hand, and can shield the two inner stars from their Kozai cycle and subsequent shrinking, on the other hand. Only small planets on wide and inclined orbits may form, survive and allow for the inner binary shrinkage. Those are difficult to detect.

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Comparative statistics and origin of triple and quadruple stars

Cited by 199 publications

References 44 publications

Transiting exoplanets from the CoRoT space mission

Transiting exoplanets from the CoRoT space mission

High-resolution infrared spectroscopy as a tool to detect false positives of transit search programs

No circumbinary planets transiting the tightestKeplerbinaries – a possible fingerprint of a third star

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