Submitted Manuscript: Confidential

Direct Imaging Discovery of a Low- Between Three

Kevin Wagner1,7,8*, Dániel Apai1,2,8, Markus Kasper3, Kaitlin Kratter1, Melissa McClure3,

Massimo Robberto4, Jean-Luc Beuzit5,6

1Department of Astronomy and Steward Observatory, The University of Arizona, 933 N. Cherry

Avenue, Tucson, AZ 85721, USA.

2Lunar and Planetary Laboratory, The University of Arizona, 1640 E. University Boulevard,

Tucson, AZ 85718, USA.

3European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany.

4Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA.

5Université Grenoble Alpes, IPAG, F-38000 Grenoble, France.

6CNRS, IPAG, F-38000 Grenoble, France.

7National Science Foundation Graduate Research Fellow.

8Earths in Other Solar Systems Team, NASA Nexus for Exoplanet System Science.

*Correspondence to: [email protected]

Abstract: We present the discovery of a young giant in the HD131399 triple system through imaging and spectroscopy with VLT/SPHERE. HD131399Ab is one of the lowest mass and coldest directly imaged with a mass of 4±1 times that of and an of 850±50 . The planet is seen at 82 au projected separation from the primary , which is surprisingly comparable to the ~300–400 au semi-major axis of the hierarchical triple and dynamically unlike any other known exoplanet. Through N-body simulations we confirm that a range of stable exist for the planet, and that the gravitational influence of the binary excites an orbital evolution that is compatible with a wide range of plausible formation scenarios and evolutionary histories. The location of HD131399Ab on a wide in a triple system is unexpected and demonstrates that massive planets may form or migrate to long and possibly unstable orbits in multi-star systems.

One Sentence Summary: This article presents the discovery and characterization of a dynamically active young exoplanet within a triple star system.

Main Text:

Thousands of planets around other stars have recently been discovered (e.g. 1, 2), revealing a greater diversity than predicted by traditional planet formation models based on the solar system. Extreme examples of this are planets within binary and multiple star systems, which form and evolve in time-evolving radiation and gravitational fields. Direct imaging allows for the detection and characterization through spectroscopy of long-period giant planets – enabling constraints to be placed on planet formation models via predictions of planet population statistics and atmospheric properties. However, most direct imaging surveys have traditionally excluded visual binary or multiple systems whose separations are less than a few hundred astronomical units1 due to the assumption that such planetary systems would either be disrupted or never form. As a result of this observational bias, most directly imaged have been found around single stars.

A surprising result from the handful of confirmed directly imaged planets is that long- period giant planets appear to be more common around A-type stars (M~1.6–2.4 M⊙, where M⊙

1Where one , or au, stands for the mean Earth– distance. = 1 ) than around solar-type and lower mass stars (3–14). Since roughly half of A-stars belong to binary or multiple systems (15, 16), to build a complete census of long-period giant planets requires investigation of both single and multiple systems. In the latter, a variety of long- period orbits might arise as a result of planet-planet or planet-star scattering events (17) and even some with chaotic orbits that wander between the stellar components (18, 19) – all of which may be readily detected by direct imaging. In our ongoing adaptive optics imaging campaign using

VLT/SPHERE (20) we are sampling a coeval population of ~100 young single and multiple A- type stars in the Upper Scorpius--Lupus association to place constraints on the primordial occurrence rate of long-period giant planets.

HD131399ABC is a 16±1 Myr old triple system in Upper Centaurus Lupus (UCL; 21,

22) at a distance of 98±7 pc (23) whose basic properties are given in Table 1. Despite its young age, the system shows no evidence of infrared excess and thus its primordial disk has likely been depleted to beneath detectable levels (24). We observed HD131399 on June 12, 2015 with

SPHERE, obtaining a wide range of near-infrared spectral coverage from Y– to K-band (0.95–

2.25 µm) and diffraction-limited imaging with an 8.2-meter telescope aperture. Our observations, described in detail in the supplementary online materials (SOM), resulted in the discovery of

HD131399Ab, a point source with a contrast to HD131399A of ΔK1(2.1 µm)=12.5±0.1 magnitudes and projected separation of 0.84 arcsec, or 82±6 au (see Fig. 1 and Table 1). After the initial discovery, we obtained follow-up observations with SPHERE to verify common and to improve the quality of the near-infrared spectrum, enabling the characterization of the planet’s atmospheric properties.

HD131399A HD131399Ab HD131399B HD131399C

Spectral Type A1V T3±1 G3 K5

Mass 1.82 M⊙ 4±1 MJup 0.96 M⊙ 0.57 M⊙

Temperature 9300 K 850±50 K 5700 K 4400 K

(TEff)

Distance (pc) 98±7

Projected 0.839±0.004 2015 June 3.149±0.006 3.215±0.006 separation from 2016 March A (arcsec) 0.834±0.004 3.150±0.006 3.220±0.006

Projected 82±6 309±22 315±22 separation from A (au)

Position angle 194.2±0.3 2015 June 221.9±0.3 222.0±0.3 (Degrees E of N 2016 March from A) 194.1±0.3 221.5±0.3 221.9±0.3

J magnitude 6.772±0.018 20.0±0.2

H magnitude 6.708±0.034 19.7±0.2

K-band K=6.643±0.026 K1=19.1±0.1 K1=8.47±0.04 K1=10.49±0.06 magnitude

Table 1. Basic parameters of the stars and directly imaged planet in HD131399. The , effective temperatures, and spectral types were found through comparison of their K1 to evolutionary models (23). J, H, and K magnitudes for HD131399A were obtained from the

2MASS All Sky Catalog of point sources. A description of our astrometric calibrations and associated uncertainties can be found in the SOM.

Fig. 1. VLT/SPHERE images revealing HD131399Ab and the stellar components of the hierarchical triple system. The central regions that are affected by the coronagraph and residual scattered starlight are blocked by a mask, with the location of HD131399A indicated by the crosshairs. The right image shows a composite of the PSF subtracted aperture around the planet (dashed) superposed on the wide field image showing the stellar components of the system whose are suppressed to the level of the planet for clarity.

We detect HD131399Ab with a signal to noise ratio in Y– (1.04 µm), J– (1.25 µm), and

H-band (1.62 µm) of 8–10 and in K1 and K2 (2.11 µm and 2.25 µm) of 23.5 and 11.3, respectively. We extract the position of the planet via injection of a negative Gaussian PSF over a grid of 0.01-pixel spacing, from which we determine its position as the location with the minimum square of residual intensity in an aperture of one FWHM diameter around the planet.

This method is typically accurate to determine the center of a well-sampled PSF to within ~0.1 pixels (25), which enables precision for the planet’s confirmation and orbital analysis.

Following astrometric calibrations (see SOM for details), we measure a positional displacement to HD131399A of Δα = 2.7±8.7 mas and Δδ = 5.0±8.0 mas over the nine-month baseline, where the uncertainties are dominated by the calibration of the instrument orientation across the two epochs. This allows us to reject the hypothesis of a background object with 95% confidence, which would have moved relative to HD131399A by Δα = 22.3±0.6 mas and Δδ = 23.6±0.6 mas due to relatively high proper motion of the system (23). Assuming a Keplerian orbit for the planet with a semi-major axis of 82 au yields a period of roughly 550 , which for a face-on circular orbit over nine months is expected to produce ~7 mas of relative motion or less for eccentric or inclined orbits, which is consistent with the observations.

We convert the object’s absolute K1 magnitude to a mass estimate via comparison to widely used evolutionary tracks calculated for “hot-start” initial conditions (26), in which the planet retains its initial entropy of formation. Systematic interpolation between evolutionary tracks argues for a mass of M=4±1 MJup, which places HD131399Ab firmly in the planetary mass regime. Since at a given luminosity age and mass are degenerate, the well-constrained distance and age of the system (98±7 pc and 16±1 Myr, respectively) make HD131399Ab an important example as a low-mass directly imaged exoplanet. The closest low-mass analogue to

HD131399Ab is , which has an even lower possible minimum mass of M=2–12

MJup. Unlike other more luminous directly imaged planets, HD131399Ab and 51 Eridani b are also consistent with less-widely used cold-start core-accretion models (noted for 51 Eri b in 14), in which an arbitrary cooling parameter is introduced to allow the planet to lose some fraction of its initial entropy throughout its formation due to inefficient accretion (27). These models are less sensitive to mass since the initial energy loss is currently unconstrained, but within these systematic uncertainties HD131399Ab and 51 Eridani b could both be as massive as 12 MJup in the most extreme scenario.

Using the integral field spectrograph (IFS, 28) on SPHERE we also obtained a 0.95–1.65

µm spectrum. The quality of this spectrum allows the characterization of the and methane absorption bands within the spectral range of 1.4–1.6 µm. In K-band, where the contrast with the star is more favorable, the dual-band images also probe the 2.2 µm methane absorption. Like the exoplanet 51 Eridani b and other field (non-exoplanet) T-type brown dwarfs, the near-infrared spectrum of HD131399Ab displays prominent methane and water absorption bands (see Fig. 2).

The data are in good agreement with models of exoplanetary (29), allowing us to estimate the atmospheric properties of effective temperature and surface . Systematic

2 !!.! exploration of interpolated atmospheric models argue for TEff = 850±50 K and log(g) = !. !!!.!

(cm/s2), where the uncertainty in is mostly dominated by systematic uncertainties within the models and not by the model-data fit. These values are consistent with the mass estimates from the planet’s K1-band luminosity and are similar to the TEff = 600–750K and log(g)

= 3.0–5.5 (cm/s2) estimates for 51 Eridani b.

Fig. 2: Near-infrared spectrum of HD131399Ab. (a) Data (black) alongside the best-fit model

2 in red, with TEff = 850 K and log(g) = 3.8 cm/s , showing water and methane

2 All atmospheric models used in this study can be found on online at http://svo2.cab.inta- csic.es/theory/newov/. absorption in the atmosphere with the approximate absorption regions indicated by the dashed lines. The spectrum of 51 Eri b (14) is shown in blue, scaled by 50% to match the luminosity of

HD131399Ab, which is three times farther away though intrinsically more luminous. (b) Near- infrared spectrum of HD131399Ab and field brown dwarfs with spectrum normalized independently. Comparison to spectra of standard classifications of field brown dwarfs argues for a spectral type of T3±1.

The transition between L and T spectral types (TEff ~2100–1300 K and TEff ~1300–600 K, respectively) is marked by the appearance of methane absorption at H– and K-bands in the atmospheres of the cooler T dwarfs. In the J vs. J–H color-magnitude diagram this appears as bluer (more negative J–H color) compared to the hotter L dwarfs. At the threshold of the L/T transition the becomes brighter in the J-band, seen as the kink in the blue shaded region of the color magnitude diagram (Fig. 3), as the silicate clouds transition from above to below the photosphere. During this transition the objects become brighter and bluer. The fact that cloudy directly imaged exoplanets (such as HR8799bcde, β Pic b, or ) appear at the bottom of the L-dwarf sequence argues for cloud layers in these low-gravity objects that are thicker than in their higher-gravity counterparts. In contrast, HD131399Ab and the two other directly imaged T-type exoplanets follow the T-dwarf sequence, which we interpret as evidence for a similarity between the mostly or fully cloud-free atmospheres of long-period planets and cool field brown dwarfs. HD131399Ab is also the directly imaged exoplanet closest to the L/T transition, and thus an important object for further spectral characterization to elucidate the differences in the formation pathways and compositions of planets and field dwarfs.

Fig. 3. J–H color-magnitude diagram of brown dwarfs and directly imaged giant exoplanets. HD131399Ab falls among the methane dominated T dwarfs near the L/T transition.

The L and T dwarf data were obtained from (30), while the directly imaged exoplanet data are from (31–34). Parallax measurements were used to convert apparent to absolute magnitudes.

A particularly interesting and unique property of HD131399Ab is that it orbits within a triple system whose semi-major axis is comparable to that of the planet (see Fig. 4, 5). Because the presence of a second and third star can greatly limit the phase space where planetary orbits are stable, observing a system in this configuration is thought to be unlikely (e.g. 18, 35). In fact, most direct imaging exoplanet surveys exclude known binaries and triples. In our ongoing survey, we have imaged 18 single A-type stars and 15 binary or triple star systems with separations similar to the HD131399 system. Although the sample size is small, it is surprising that the first planet detected in our survey is in a close triple system. The discovery of

HD131399Ab suggests that direct imaging surveys may have been excluding viable targets around which long-period giant exoplanets might be more frequent than around single stars.

Fig. 4: The components of the HD131399 hierarchical triple star system and comparison to the Solar System. This composite image shows the ~4 MJup exoplanet, HD131399Ab, between the 1.82 M⊙ and 0.96 M⊙ stellar components A and B, as well as the 0.57 M⊙ star HD131399C in a tight orbit with B. The image is composed of J–, H–, and K-band colors for components A and Ab (colored as blue, green, and red, respectively) and the monochromatic K-band image of components B and C. For clarity, the luminosity of the planet is enhanced by a factor of 105, and since only K-band photometry exists for B and C their colors here are adjusted to be representative of typical G3 and K5 stars. (a) The dashed line shows our best-fit orbit of the BC pair and a preliminary orbit for the planet. The orbit shown for the planet has orbital elements that are consistent with the data, although the astrometric measurements permit a significant range of orbits, with the parameter ranges given in the SOM. (b) The image is reproduced with the orbits of the Solar System planets overlaid for perspective.

The orbit of the stellar components is constrained by observations dating back to 1897

(36). We fit these measurements to a grid of orbital models for a binary system using the center of mass of the BC system, which is motivated by the system’s hierarchical nature and the fact that most previous data could not resolve the B and C components. Our best-fit model (see SOM for details) argues for a semi-major axis of a★ = 349 ± 28 au, eccentricity of e★ = 0.13±0.05, and inclination of i★ = 45°– 65° with respect to the plane of the sky. Using only the newer, more reliable data permits a wider range of a★ = 270–390 au, e★ = 0.1–0.3, and i★ = 30°– 70°. The two epochs of observations of the planet are less constraining, though we perform a preliminary orbit-fit to obtain a plausible range of parameters. Using the same method described in the SOM,

!!" !!" our parameter retrievals suggest ap = 82!!" au, ep = 0.35±0.25, and ip = 40!!"°, with no single solution being strongly preferred.

Fig. 5: Ratio of semi-major axes of planets that orbit one star of a multiple system (satellite, or S-type planets) to the semi-major axes of their host systems. The line of one-third times the binary separation represents the approximate critical radius of tidal truncation and orbital stability (35) in the coplanar case. Although the critical radius varies somewhat for different parameters of ratio, eccentricity, and inclination, HD131399Ab is much closer to the critical radius than any other known exoplanet. Note that for the long-period directly imaged planets and long-period binaries the orbital semi-major axes bear significant uncertainties or lack estimates all together, and hence their present projected separation is plotted instead. This includes HD131399Ab, although from the results of the orbit fitting the semi-major axes of this system are indeed similar to the projected separations.

The constraints on the semi-major axis of the triple system results in a more dynamically extreme configuration than for any known exoplanet within a binary or multiple system, with the ratio of semi-major axes q = ap/a★ = 0.14–0.38. Note that q<0.23 requires higher eccentricities

(ep>0.3) to maintain the ≥82 au observational constraint on the planet’s apocenter. The most dynamically similar planets to HD131399Ab are the radial-velocity discovered γ Cep Ab (37),

HD41004Ab (38) and HD142Ac (39) in all of which q ~ 0.1. Perhaps the most similar well- studied example is the transiting system Kepler-444, which hosts five sub-Earth sized planets within 0.1 au from the primary Kepler-444A (40). This stellar system is likewise a hierarchical triple, with a tight M-dwarf binary at 66 au from the planet hosting primary star. This pair of stars is on a highly eccentric orbit which brings them within ~5 au of the planetary system where they have a significant dynamical influence on the planets. While similar, HD131399 stands out due to the smaller relative separations of the planet and the stars.

We confirm through N-body simulations (see SOM for details) that stable configurations do exist within the astrometric constraints, and that even for stable orbits the planet’s orbital parameters (a,e,i) undergo complex evolution due to the influence of the BC pair. Follow-up observations over the next decade will be able to constrain the orbital plane, eccentricity, and period of HD131399Ab, allowing a more thorough exploration of its origin and analysis of its orbital stability. At present, we cannot state with certainty that component Ab is on a presently stable orbit. Given the young age of the system, it could still be undergoing scattering and dynamical evolution. While possible, this is not the most likely scenario as unstable configurations typically lead to an ejection (or even a collision) on timescales of a few Myr (35).

Given its location in a triple system, a broad set of formation pathways and orbits are possible for HD131399Ab. We speculate that the planet may have arrived at its present orbit through one of three possible scenarios: Scenario A) the planet formed on a short orbit around component A, and underwent a planet-planet scattering event that ejected it to its current long period orbit. This scenario requires the presence of a massive planet on a shorter period orbit.

Such a planet could have evaded detection if it were beneath our sensitivity limits (see SOM for details). As a consequence we would also expect the Ab orbit to be rather eccentric, which is consistent with our stability constraints. Scenario B) HD131399Ab has formed in isolation on a long-period orbit around HD131399A –– perhaps as a result of the dynamical influence of the B and C components on the protoplanetary disk –– and is now on a stable orbit around

HD131399A, though several works have shown this to be unlikely (e.g. 41, 42). Scenario C) the planet formed early while the stellar orbits were still evolving, and through interactions with A-

BC arrived at its present long-period orbit. This scenario differs in predictions from Scenario A as it does not require the presence of a second close-in massive planet. In both scenarios A and

C, there exists the possibility that the planet is no longer tied to the star around which it formed, allowing it to have formed as a either circumprimary planet around star A or as a circumbinary planet around stars B and C, although the unstable nature of such a system makes this possibility less likely. Follow-up observations of the planetary and stellar orbits will be able to determine between scenarios A, B, and C, and whether HD131399Ab orbits one or three stars.

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Acknowledgments: This work is based on observations performed with VLT/SPHERE under program IDs 095.C-0389A (PI: D. Apai) and 296.C-5036A (PI: K. Wagner). KRW is supported by the National Science Foundation Graduate Research Fellowship Program under grant No.

2015209499. The results reported herein benefited from collaborations and/or information exchange within NASA’s Nexus for Exoplanet System Science (NExSS) research coordination network sponsored by NASA’s Science Mission Directorate. This research has benefitted from the SpeX Prism Spectral Libraries maintained by Adam Burgasser, and the Washington Double

Star Catalog maintained at the U.S. Naval Observatory.

Supplementary Materials:

Materials and Methods

Supplementary Text

Figures S1-S3

Tables S1-S2

References (43-54)