{'fraction_non_alphanumeric': 0.04431866723622384, 'fraction_numerical': 0.03139683895771038, 'mean_word_length': 4.277261200338123, 'pattern_counts': {'":': 0, '<': 3, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 1, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
GG Tau: the fifth element

We aim at unveiling the observational imprint of physical mechanisms that govern planetary formation in young, multiple systems. In particular, we investigate the impact of tidal truncation on the inner circumstellar disks. We observed the emblematic system GG Tau at high-angular resolution: a hierarchical quadruple system composed of low-mass T Tauri binary stars surrounded by a well-studied, massive circumbinary disk in Keplerian rotation. We used the near-IR 4-telescope combiner PIONIER on the VLTI and sparse-aperture-masking techniques on VLT/NaCo to probe this proto-planetary system at sub-AU scales. We report the discovery of a significant closure-phase signal in H and Ks bands that can be reproduced with an additional low-mass companion orbiting GG Tau Ab, at a (projected) separation rho = 31.7 +/- 0.2mas (4.4 au) and PA = 219.6 +/- 0.3deg. This finding offers a simple explanation for several key questions in this system, including the missing-stellar-mass problem and the asymmetry of continuum emission from the inner dust disks observed at millimeter wavelengths. Composed of now five co-eval stars with 0.02<= Mstar<= 0.7 Msun, the quintuple system GG Tau has become an ideal test case to constrain stellar evolution models at young ages (few 10^6yr).


Introduction
Planet formation is a common process that can occur in different environments. While the first decade of planet searches has preferentially focused on single, solar-like host stars, it has been more recently shown that a large proportion of extrasolar giant planets are born in binary systems (Udry & Santos 2007). The recent diskoveries of transiting circumbinary planets in close binary systems (Kepler 16, 34,35, Doyle et al. 2011;Welsh et al. 2012), as well as the planet candidate directly imaged around the young low-mass binary 2MASS J0103 (Delorme et al. 2013) have proven that planets can also appear in a circumbinary disk, despite the strong dynamical mechanisms that shape the disk and can rapidly clear out its inner region. Stars in young binary systems are expected to be surrounded by two inner disks, located inside the Roche lobes and an outer circumbinary ring or disk outside the outer Lindblad resonances (e.g., Artymowicz & Lubow 1994). Persistent signs of accretion in binary systems, as well as direct imaging of residual gas in the inner region, demonstrate that gas and dust can flow from the outer reservoir through this gravitationally unstable zone to nurture inner circumstellar disks (where planet formation may also occur), which otherwise would not survive. Understanding how the inner disks are re-Based on ESO programs 090.C-0339 (A,F) plenished is also important in the general context of planetary system formation, since binary stars provide a scaled-up version of a proto-planet environment in a circumstellar (CS) disk. Finally, mutiple systems can provide essential clues in testing stellar evolution models, as they provide a set of co-eval stars with different masses at a common distance (e.g, White et al. 1999) In the past two decades, the young hierarchical quadruple system GG Tau, composed of two low-mass binary systems, has been subject of many detailed studies. With its relatively massive (0.15 M ) and bright outer ring, GG Tau A is one of the best known nearby (140 pc) T Tauri binaries, with a 0.26 separation (36 au on the sky plane). The circumbinary disk (R in = 180 au) has been observed in thermal dust emission (Dutrey et al. 1994;Guilloteau et al. 1999;Piétu et al. 2011) and in scattered light (Roddier et al. 1996;Silber et al. 2000), and is in Keplerian rotation (Dutrey et al. 1994). The scattered-light images proved that the gravitationally unstable zone is not empty of dust. Indirect evidence for gas flow from the ring towards the inner system(s) has been found from 12 CO J=2-1 gas image (Guilloteau & Dutrey 2001) and from near-IR H 2 transitions (Beck et al. 2012). The warm H 2 gas may be heated by shocks, as material from the circumbinary ring is accreted onto material close to the stars. The existence of inner CS disks is independently attested by mm excess emission on GG Tau Aa (Piétu et al. 2011  H α accretion signature separately detected around Aa and Ab, [OI] line detection around Ab (White et al. 1999;Hartigan & Kenyon 2003), and 10µm silicate feature from hot grains in both Aa and Ab environments (Skemer et al. 2011). We have recently undertaken a very high-spatial resolution observing program of GG Tau A, from UV to mm wavelengths. In this letter, we report near-IR VLT interferometric observations of the inner region of the GG Tau A system, where we detect a new component and direct evidence for resolved circumstellar dust emission.

Observations and data analysis
We observed the GG Tau  We also observed GG Tau A on 2012 December 6 and 7 with the diffraction-limited imager VLT/NAOS-CONICA (Lenzen et al. 2003). We performed sparse-aperture-masking (SAM) observations on NaCo with a 7-hole mask (Tuthill et al. 2000). The mask at the pupil-plane blocks most of the light from the centered target and resamples the primary mirror into a set of smaller subapertures that form a sparse interferometric array with 21 baselines. The 0.26 close binary Aa-Ab is resolved in the NaCo field of view, but not by the individual subpupils. Measurements of closure-phases (CP) allow one to detect highcontrast companions and proved to be more efficient than classical AO-imaging within ∼ 20 − 300 mas at H and K s -bands (Lacour et al. 2011). We used a 27 mas/pixel plate scale and highcadence frame acquisition mode, the science target itself being used for the IR wavefront sensing. On Dec. 6, data were recorded with the H and K s bands (2.0 − 2.1 µm) filters, while H and L filters were used on Dec. 7. Atmospheric conditions were stable (θ 0 ∼ 0.5 − 0.8 , τ 0 ∼ 3 − 6 ms). The same two calibrator sources as for PIONIER were selected, with a fast-switch pointing sequence. The observations were reduced using the Paris SAMP pipeline as described in Lacour et al. (2011).

Results
The PIONIER measurement for GG Tau Aa is consistent with a marginally resolved, symmetric emission, as attested by the fringe visibility (V 2 ) and CP functions displayed in Fig.2. Around the dimmer GG Tau Ab, we report the detection of a CP signal as large as 30 deg, which reveals an asymmetric brightness distribution. We attribute this feature to a third companion (Ab2) in the main binary system GG Tau A. Square visibilities also show oscillation-like structures, but we chose to rely on the CP values to derive the binary characteristics, because they are less affected by atmospheric phase fluctuations and known telescope vibrations. We used the LITPRO software 1 (Tallon-Bosc et al. 2008) to extract the Ab1-Ab2 binary flux ratio and astrometric position by independently fitting the PIONIER and NaCo-SAM data (Table 1). A simple model with two unresolved stars yields a good fit to the CP values (χ 2 red =1.5 vs 7.1 for a single star), with a raw flux ratio of 8.1 ± 2.1 (H-band). We derive a separation of 31.7 ± 0.2 mas and a position angle PA= 219.6 ± 0.3 deg (Fig.1). 2 Because the IR photo-center of Ab1-Ab2 is closer to Ab1 and their separation represents about 30 % of the instrumental FOV (i.e., an 8 m telescope Airy disk), we applied a correction for the FOV attenuation. The correction factor (based on a Gaussian-profile attenuation of FWHM = 41 mas, centered on the K s -band centroid) amounts to f corr = 0.53. (The uncertainty on f corr depends on the AO and tip-tilt stabilizer performance; we evaluate δ f corr ∼ 0.2 for a residual uncertainty 4 mas rms on the position of the Ab1-Ab2 centroid). The attenuationcorrected PIONIER flux ratio for a two-point source-model be- The two components of GG TAu A were simultaneously observed with NaCo. The CP values are dominated by the Aa-Ab binary, and we first fit its position: the 256 mas separation is consistent with long-term astrometric studies (Köhler 2011), and the flux ratios agree with literature data. We then added a third component Ab2 by fitting its location relative to Aa and its flux ratio. The Ab1-Ab2 close binary is well constrained by the 1 www.jmmc/litpro 2 Because of the limited (u, v) coverage with VLTI, a second position for Ab2 is possible (ρ = 32.5±0.2 mas, PA = 230.0±0.4 deg), although less likely (χ 2 r = 2.0). NaCo data, and the three-component model always yields a better fit. The best-fit positions are consistent with each other at H and K s bands, as well as with the VLTI estimate (although with larger uncertainties, see Table 1). We adopted an averaged separation of 31.6 ± 3.3 mas and PA= 222.6 ± 6.0 deg with NaCo.
In the less angularly resolved L band, we fixed the binary separation to its mean value and only fit the flux ratio (2.6 ± 0.4). The mask subaperture sizes correspond to an equivalent pupil diameter of 1.2 m, hence a 350 mas point spread function (PSF) in H band. To properly account for the resulting flux attenuation, we oversampled the Fourier plane by a factor of 3. The derived NaCo-SAM flux ratios are then 1.6±0.4 and 2.4±0.3 at H and K s bands, respectively. These values were used to estimate the spectral type of Ab2. Surprisingly, the inferred Ab1/Ab2 H-band flux ratio appears to be lower than our PIONIER estimate (4.3 ± 2.3). This discrepancy is discussed in Appendix A.

Nature of the detected component
A non zero closure-phase indicates a non axisymmetric brightness distribution, which may either be interpreted as the presence of a stellar companion, or as a brightness asymmetry in the disk emission. Similar detections with SAM techniques have recently been reported for a few other young sources with known protoplanetary disks (e.g., T Cha, Huélamo et al. 2011). For these transitional disks, it has been demonstrated that starlight scattered off the inner edge of the outer disk (typically at R ∼ 10 au) can equally well reproduce the detected closure-phase as a (sub)stellar companion (Olofsson et al. 2013;Cieza et al. 2013). However, the CP amplitudes in these systems are much smaller than for GG Tau Ab ( 1 deg, implying much higher IR contrasts ∆mag ∼ 5). It also requires the disk to have a high inclination (at least 60 deg) for forward-scattering by µm-sized grains to be efficient at the gap outer edge. We judge a disk-origin to be highly unlikely, since we detect a CP as large as 30 deg. We calculated the probability P for the CP to be caused by contamination from a background source. P is evaluated from the surface density ρ(m) of sources brighter than m = H + ∆H = 9.1 + 2.5 = 11.6 (worst case) around GG Tau A: P(Θ, m) = 1 − exp −πρ(m)Θ 2 (Brandner et al. 2000). With 591 sources within a Θ 1 deg radius circle around GG Tau A (after 2MASS, Cutri et al. (2003)) , P is only 10 −7 at a maximum separation of 0.032 .
Finally, re-examining archival NaCo data from December 2003 brings another argument in favor of a companion. We employed a procedure similar to that described in Köhler et al. (2000): we extracted subframes around GG Tau Aa and Ab and applied speckle-interferometric techniques to compute the modulus of the visibility of Ab, using Aa as a PSF reference. The visibility shows clear signs of a close companion. A binary model was fitted, resulting in an Ab1-Ab2 separation of 28 ± 3 mas, PA = 65 ± 5 deg [mod 180 deg], and a flux ratio of 4.6 ± 1.0 (K s -band). This a posteriori detection further supports the bound-companion hypothesis. Assuming a circular orbit in the sky plane, we derive a crude estimate of the orbital period P = 16 ± 1 yr for clockwise (CW) rotation from the 9 yr separated positions. Interestingly, this result agrees well with the orbital period derived according to the spectral type and stellar mass estimates (see Sect.4.2 below), and CW rotation was also independently derived for the outer disk (from the CO velocity field, Guilloteau et al. 1999) and for the Aa-Ab orbit.

Characteristics of the new triple stellar system
If we assume that the close binary orbit is coplanar with the outer disk, the de-projected separation of Ab1 and Ab2 was 5.1 au in Dec. 2012. Following the spectroscopic analysis of Hartigan & Kenyon (2003), the bright system GG Tau A is composed of Aa, an M0 star (∼ 0.6 M ), and of Ab, which displays a M2V spectral type (∼ 0.38 M for a single star). Hartigan & Kenyon (2003) noted that the inferred spectral type for Ab leads to an insufficient stellar mass in the system in comparison with the dynamical mass estimate of 1.28 ± 0.07 M (for a distance of 140 pc) derived from the CO gas kinematics in the ring by Guilloteau et al. (1999). We assume here that this spectral type M2 can be attributed to the brighter component Ab1.
If we assume that the relative flux excesses around Ab1 and Ab2 are roughly similar, the reported contrast approximates the stars contrast. Combining all flux ratio constraints in H, K s and L bands (see Table 1), and using the spectral type-color relations (Bessell et al. 1991;Leggett 1992), we propose that Ab2 is an M3±0.5 dwarf star, with a mass 0.25 − 0.35 M (Hartigan & Kenyon 2003) leading to a binary mass ratio q Ab1−Ab2 = 0.8±0.2. If the orbit of Ab1-Ab2 is circular and coplanar with the outer ring, with a total stellar (dynamical) mass of 0.58 − 0.75 M and a semi-major axis of 5.1±0.4 au, its orbital period is estimated to P = 14 +3 −2 yr from Kepler laws. If the relative IR flux excess were much higher around Ab1 than Ab2, it would bias the spectral type estimate for Ab2 towards earlier types (i.e., q ∼ 1).
We note that a slightly better solution to the PIONIER CP fit might be found with a third component in the GG Tau Ab system, that is, if Ab2 were itself a close binary. Such models are degenerated: several solutions exist with Ab2a-Ab2b separations of 10-15 mas (1.5-2.3 au, i.e., about a third of the distance between Ab1 and the Ab2 barycenter), yielding χ 2 red ∼ 1 instead of 1.5 (do f = 27), with a flux ratio F Ab2a /F Ab2b ∼ 1.5 − 3. A sig-  Table 1. Relative astrometric position and flux contrast for the main binary Aa-Ab (photo-center of Ab1-Ab2), and for the close binary Ab1-Ab2, as derived from our best-fit of the closure phases measured by VLT/NaCo-SAM and VLTI/PIONIER. The formal errors on NaCo-SAM PA do not include uncertainties due the detector orientation. ( * ): after correction for FOV attenuation (see text), the related uncertainty dominates nificantly improved (u, v) coverage would be necessary to decide for one of the two scenarios. Finally, the analysis of the PIONIER visibilities, in combination with an independent estimate of the excess flux around Aa and Ab from literature photometry, allows one to partly constrain the presence and characteristics of circumstellar material in the two systems. The related discussion can be found in Appendix A, where we show that Aa may be surrounded by a CS disk revealed by its marginally resolved bright inner rim, while the dust distribution around Ab1 and Ab2 might be more complicated.

Consequences for the dynamics of the system
In addition to solving the missing-stellar-mass problem, the discovery of a fifth component in this system provides a logical explanation for the lack of submm/mm continuum emission around Ab attested by PdBI (Piétu et al. 2011) and ALMA observations (Dutrey et al. 2014, submitted). The Ab1 and Ab2 components are indeed surrounded by Roche lobes of radius ∼ 2 au. Tidal truncation effects naturally prevent the existence of stable disk(s) larger than this limit, and thus not massive enough to be strong mm emitters, in contrast to Aa. The 10µm silicate emission probably arises from warm grains located within 2 au (i.e., well inside the Roche lobes). The resulting gravitational disturbance may also affect the supply of material from the outer regions through dust and gas streamers, as it breaks the symmetry of the Aa-Ab system. The close binary GG Tau Ab also strengthens the impact of tidal truncation onto the circumbinary ring. However, the location of its inner rim (180 au), which is much more distant than the apparent separation of the Aa and Ab(1+2) components, remains a puzzling characteristics of the system.

Conclusion
The emblematic, young binary system GG Tau A (0.26 separation) has been successfully observed with interferometric and AO / sparse-aperture-masking techniques (SAM) at the Very Large Telescope. We found that: 1. The secondary GG Tau Ab is itself a close binary, with a projected separation of 0.032 (or 4.5 au) and PA = 220 • (end 2012). It is consistent with a M3V (Ab2) and M2V (Ab1) low-mass binary. This finding solves the discrepancy between the dynamical stellar mass derived from CO gas kinematics (Guilloteau et al. 1999) and the most recent spectraltype estimate of Aa and Ab (Hartigan & Kenyon 2003). Based on a tentative a posteriori identification in archival (2003) VLT/NACO images, its orbital period is estimated to P Ab1−Ab2 ∼ 16 yr, a value consistent with the period derived from the binary mass and separation. 2. All stars in this triple system present significant IR excesses, confirming the presence of circumstellar material. Around GG Tau Aa, the NIR emission is partly resolved at 1.65 µm and the derived geometrical ring radius is typical of protoplanetary disks around low-luminosity stars. For GG Tau Ab, due to tidal truncation, a (deprojected) separation of ∼ 5.1 au sets a strong constraint on the maximum radial extent of any circumstellar disk surrounding Ab1 and/or Ab2 (R out < ∼ 2 au). The binary nature of this system also provides a simple explanation to the intriguing non detection of mm continuum emission at the location of Ab. 3. With five coeval low-mass stars, this young multiple system becomes an ideal test case to constrain evolutionary models, provided that future astrometric studies will refine the stars physical parameters of the Ab system.  Table 2. Photometry of the triple system: observed apparent magnitudes from Hartigan & Kenyon (2003) for Aa and (Ab1+Ab2), extinctioncorrected magnitudes, expected magnitudes from color-indices and the spectral-type estimate, expected flux ratios between stellar components for the closest spectral types, and estimated flux excess from circumstellar material for the adopted M2/M3V spectral types.
GG Tau Table 3. Summary of geometrical model fits to the VLTI-PIONIER data on 2012-10-30 for GG Tau Aa and Ab(1+2) respectively. ( a ): a circular ring and an elongated ring (with an aspect ratio, an inclination and orientation forced to the known circumbinary ring values) yield similar results. ( b ): fit to closure-phase values alone, a single-star model would yield χ 2 red = 7.