LETTERS https://doi.org/10.1038/s41550-018-0667-x Corrected: Publisher Correction A circumbinary protoplanetary disk in a polar configuration Grant M. Kennedy 1,2*, Luca Matrà3, Stefano Facchini 4,5, Julien Milli6, Olja Panić7, Daniel Price 8,9, David J. Wilner 3, Mark C. Wyatt10 and Ben M. Yelverton10 Nearly all young stars are initially surrounded by ‘protoplan- circumbinary debris disk 99 Herculis is thought to have a polar con- etary’ disks of gas and dust, and in the case of single stars at figuration13, this disk is four times larger than Neptune’s orbit and, least 30% of these disks go on to form planets1. The process with an age similar to the Solar System, does not provide evidence of protoplanetary disk formation can result in initial misalign- that young gas-rich polar circumbinary disks exist. ments, where the disk orbital plane is different from the stel- Here, we report that the circumbinary disk in the young HD lar equator in single-star systems, or different from the binary 98800 system is strongly misaligned with the binary orbital plane. orbital plane in systems with two stars2. A quirk of the dynam- We infer that the disk is in the polar configuration by simulating the ics means that initially misaligned ‘circumbinary’ disks— disk dynamics. We further show that despite this misalignment, the those that surround two stars—are predicted to evolve to disk shows physical properties similar to disks seen around young one of two possible stable configurations: one where the disk single stars. and binary orbital planes are coplanar and one where they are The HD 98800 system is a well-known hierarchical quadruple perpendicular (a ‘polar’ configuration)3–5. Previous work has star system 44.9 parsecs from Earth14, and a member of the ~10-Myr- found coplanar circumbinary disks6, but no polar examples old TW Hydrae association15,16. It consists of two pairs of binaries were known until now. Here, we report the first discovery of (called ‘A’ and ‘B’, or equally ‘AaAb’ and ‘BaBb’) with semi-major a protoplanetary circumbinary disk in the polar configura- axes of about 1 astronomical unit (au), which themselves orbit tion, supporting the predictions that such disks should exist. each other with a semi-major axis of 54 au. The binary BaBb is well The disk shows some characteristics that are similar to disks characterized17, with an eccentricity of 0.785 ± 0.005, an ascending around single stars, and that are attributed to dust growth. node of 337.6 ± 2.4° anti-clockwise from north and an inclination of Thus, the first stages of planet formation appear able to pro- 67 ± 3°. Using the new data presented here, we derive a new orbit for ceed in polar circumbinary disks. AB (see Supplementary Information), which has an eccentricity of The process of star and disk formation is not guaranteed to yield 0.52 ± 0.01 and a period of 246 ± 5 years, with an ascending node of a simple coplanar circumstellar disk, as might be surmised from the 4.7 ± 0.2° and an inclination of 88.4 ± 2°. These orbits, as projected coplanar nature of the Solar System. Simulations suggest that once a on the sky plane, are shown in Fig. 1. The AaAb orbit is less certain, protoplanetary disk has initially formed around a star or stars, fur- but the details do not affect our conclusions. ther accretion of material with different angular momenta can result The northern pair known as HD 98800BaBb has been known to in disks with multiple orbital planes, or disks whose orbital planes host a bright circumbinary disk since its discovery in the 1980s18. are different from the binary stars they orbit2. IRS 43 appears to be a The disk is thought to be influenced by the stellar system19, with very young system with a misaligned circumbinary disk7—evidence the inner edge of the disk truncated by the inner binary BaBb, and that the initial conditions suggested by simulations do indeed occur. the outer edge externally truncated by A20,21. The orientation of the Theory suggests that circumbinary disks should then evolve to disk was initially thought to be coplanar with the inner binary20, one of two possible configurations—a quirk dictated by the orbital but higher-resolution observations suggest a different orientation21 dynamics around two stars. For small initial misalignments, the (see Methods for a comparison with our results). Whether the disk angular momentum vector of circumbinary orbits Lc precesses harbours a significant mass of gas has been unclear, meaning that it 21,22 about the binary angular momentum vector Lb (a ‘coplanar’ family has been interpreted as both a gas-rich ‘protoplanetary’ disk and 23 of orbits), but for larger misalignments Lc precesses about a vector in a gas-poor ‘debris’ disk . The detection of oxygen towards the sys- 8 24 25 the binary’s pericentre direction ϖb (a ‘polar’ family) . Because the tem , and molecular hydrogen emission towards B , suggests that disks that orbit young stars are gas rich and hence dissipative, an ini- this pair is accreting from a gas-rich disk, favouring the gas-rich tially misaligned circumbinary disk will generally evolve to an end ‘protoplanetary’ disk interpretation. state that belongs to one of these two families3–5,9–11. The expected To ascertain the disk orientation, size, structure and evolution- timescale for this reorientation is generally shorter than the typical ary status, we observed the HD 98800 system with the Atacama ~3 Myr disk lifetime12, meaning that young gas-rich circumbinary Large Millimeter/submillimeter Array (ALMA; see Methods). Data disks are most likely to be observed to be in one of the two possible were taken at 230 GHz (1.3 mm), to image dust continuum emis- orientations, rather than at an intermediate orientation. While the sion and the carbon monoxide J = 2–1 rotational transition, and 1Department of Physics, University of Warwick, Coventry, UK. 2Centre for Exoplanets and Habitability, University of Warwick, Coventry, UK. 3Harvard– Smithsonian Center for Astrophysics, Cambridge, MA, USA. 4Max-Planck-Institut für Extraterrestrische Physik, Garching, Germany. 5European Southern Observatory, Garching, Germany. 6European Southern Observatory, Santiago, Chile. 7School of Physics and Astronomy, University of Leeds, Leeds, UK. 8Monash Centre for Astrophysics, Monash University, Melbourne, Victoria, Australia. 9School of Physics and Astronomy, Monash University, Melbourne, Victoria, Australia. 10Institute of Astronomy, University of Cambridge, Cambridge, UK. *e-mail: [email protected] 230 NATURE ASTRonomy | VOL 3 | MARCH 2019 | 230–235 | www.nature.com/natureastronomy NATURE ASTRONOMY LETTERS 2.0 5 AU 1 AU –24° 46′ 38.5″ 1.5 –24° 46′ 39.4″ –24° 46′ 39.0″ Flux density (mJy beam 1.0 Ba –24° 46′ 39.5″ –24° 46′ 39.5″ Bb Declination (J2000) 0.5 –1 ) –24° 46′ 40.0″ AaAb –24° 46′ 39.6″ 0 –24° 46′ 40.5″ ′′ ′′ ′′ 170° 31′ 17.8″ 170° 31′ 17.7″ 170° 31′ 17.6″ 170° 31′ 17.5″ 170° 31′ 17.4″ 17.8 17.6 17.4 ′ ′ ′ Right ascension (J2000) 170° 31 170° 31 170° 31 Right ascension (J2000) Fig. 1 | ALMA 1.3 mm continuum image of the HD 98800 dust disk, showing a narrow dust ring 3.5 AU in radius that is 2 AU wide. White semi-transparent lines show the orbits of the inner binary (BaBb) and the path of the outer binary (AaAb) with respect to BaBb, with dots at the star locations at the time of the ALMA observation. The resolution of these ‘uniformly weighted’ images (32 × 25 milliarcsec2, or 1.4 × 1.1 AU2) is given by the ellipse in the lower left corner. Left, entire system. Right, magnification of BaBb. 14 dust and carbon monoxide are at 2.5 ± 0.02 and 1.6 ± 0.3 au, respec- 1 AU tively, while the outer edges are at 4.6 ± 0.01 and 6.4 ± 0.5 au (see 12 Methods). These models show that the disk is largely axisymmetri- cal, although a small asymmetry in the dust distribution at the inner 10 edge may be a sign of interaction with the binary. The dust and carbon monoxide components are consistent with having the same 8 Velocity (km s orientation; the disk has a position angle of 16 ± 1° (measured anti- clockwise from north) and is inclined by either 26 or 154° (±1°) 6 from the sky plane. While the Doppler shifts seen in carbon mon- oxide show that the north side of the disk is rotating towards us 4 –1 (Fig. 2), thus constraining the ascending node to be north of the ) Declination (J2000) star, the inclination remains ambiguous because the disk could be 2 rotating either clockwise or anti-clockwise as projected on the sky. That is, these observations do not distinguish whether the east or 0 west side of the disk is closer to Earth. Of the two possible disk orientations, the 154° case is only 4° away –2 from the polar configuration (that is, it is perpendicular to both the BaBb orbital plane and the BaBb pericentre direction), while the 26° case is inclined 48° from the BaBb binary plane (which we refer to as Right ascension (J2000) the ‘moderately’ misaligned case). The uncertainty on these relative angles is about 4°, so the polar orientation is consistent with being Fig. 2 | Carbon monoxide gas velocity map. Colours show the intensity- perfectly polar. Given the small chance that a randomly chosen ori- weighted carbon monoxide velocity, and contours the dust. The gas velocity entation should appear to be in the polar configuration, which is structure is consistent with that expected if the carbon monoxide disk has the expected based on the dynamics and models discussed above, this same orientation as the dust.