arXiv:2012.06578v1 [astro-ph.SR] 5 Oct 2020 Nxne l 02 03 aciie l 03 08.In fundamen- pre- simulations, the 2018). hydrodynamic calibrate 2013, in to involved al. and parameters tal theories et such Facchini test objects 2013, to disk central 2012, order the the al. from around et separate independently (Nixon to tear precess rings to could cause and and in torques apart proposed gravitational disk been resulting disk. the the has the of it that axis simulations 2012 rotation computer are the on vectors to Based momentum respect angular with orbital misaligned whose tiple example One disks. is these the from affect forming and populations structures that planet disk processes protoplanetary dynamical shape new sim- might unveil numerical Furthermore, to in- continue planet processes. de- ulations a other to by by or, opened of is disk, gap stead protoplanetary a a whether certainty in morphol- with observed the rive esti- gap from to a planet difficult of gap-opening still a ogy instance, of knowl- for mass limited is, the it our mate areas simulating of these result when in a edge included As be interactions. gas+dust to disk-body and need viscosity that about microphysics material knowledge de- disk with further be interactions can require gravity, sources simple the systems. point by planetary between understanding scribed interactions of to the and key While systems the stellar and of holds other architecture material each with disk interact with bodies how Understanding hydrodynamics disk for benchmark as multiples Pre-main-sequence iktearing disk apdds n onaatrn as ring torn-apart a and disk warped ecmr o ikhydrodynamics disk for benchmark r-ansqec rpewt a with triple pre-main-sequence A tfnKas nvriyo Exeter of University Kraus, Stefan eal [email protected]) (email: htmgtocri h ik rudmul- around disks the in occur might that WOrionis: GW 1 tde ar utpesses ti oae nthe in of located age is an It has and systems. a best- and multiple such λ brightest Tauri the as of T one serve studied is system to triple This PMS potential the Orionis. GW is the studies, hydrodynamic for has stone” “rosetta that the how system image One can perturbation. the and to to bodies able responds perturbing disk are dynamical the we and of systems masses these 3-dimensional For the measure 2014). directly and al. binaries 2013 et Kraus PMS Dˆuchene & Reipurth instance on for reviews see multiples general and a (for with laboratory us provide unique systems multiple (PMS) main-sequence ihobtlprosof periods orbital With -adflxrtoaddrvdmr qa asrto (e.g. ratios mass equal the more derived from and mass ratio the flux H-band estimated workers Other respectively. h eutn risaesoni iue1adcorrespond 2020a). and ( al. 1 near-circular Figure et a in (Kraus to shown tertiary are the orbits resulting and The binary astrometric inner the the the monitor of and to Interferometer used the VLT were array the by CHARA 2019, covered fits. and orbit arc the 2008 in Between orbital degeneracies in small resulted the IOTA although and orbits disk, hinted stellar and the the between tertiary misalignment the signficant for in- a period solutions at orbit orbit These 11.5 stars. a 3 dicated all derived for solutions velocities colleagues orbit and radial first Czekala these astrometry, IOTA Using for the velocities and secondary. radial and Oak provide primary and the that Observatory and Whipple over Observatory L. in obtained Fred Ridge were presented the that work with al. spectra the of 35 et set on Czekala impressive Building an coworkers, 2017 and 2011). Mathieu al. by third et a discovered (Berger and component binary inner the inter- resolved infrared IOTA ferometer with the associated with Prato Observations lines of secondary. 1991). detection the al. spectroscopic the et reported single-lined (2018) (Mathieu al. a period et day be 242 a to with known binary long is Ori GW system triple The the and disk the between interactions stars. the at strong expecting to time while range same characterisation, right orbit the full in a just are enable separations and periods bital i o h ne iayada4216 a and binary inner the for bit hr h uuliciainbtenteobtlpae is planes orbital the 13 between inclination mutual the where e a ogbe atro eae ahe tal. et Mathieu debate. M of 2.5 yield- tracks, matter ing evolutionary using a mass sys- the Ori been estimated GW (1995) the long in has components the tem of masses precise The etaywt infiatecnrct ( eccentricity significant with tertiary roi trfrigrgo 38p;Kukle l 2017) al. et Kounkel pc; (388 region star-forming Orionis . 9 ± 1 . 1 ⊙ ◦ . n . M 0.5 and e ∼ 0 = ilo er Cle ta.2004). al. et (Calvet years million 1 ⊙ . ∼ 069 o h rmr n secondary, and primary the for otsad1 er,teor- the years, 11 and months 9 ± 0 . 0) 241 009), . 8 ± e 4 . dyobtfrthe for orbit 6day 0 = . 62 . ± 379 0 . 5a or- 05day ± 0 . 003), 20 20 2010.0 C D 2009.9 2011.0 4 A B 2009.0 10 10 2005.9 2011.9 2003.9 2009.9

2019.7 [km/s] [km/s] 2011.9 0 0 A,A-B 2 A,AB-C RV 2010.0 C RV 2012.8 -10 -10 2019.7 A A+B 0 -20 -20 2015.0 B E F 2018.1 2015.0 30 30 dDEC[mas] 2003.9 20 20 2011.0 -2 10 10

2005.9 [km/s] [km/s] 0 0 2005.0 2018.1 B,A-B 2009.0 2012.8 2005.0 -10 -10 B,AB-C RV RV -20 -20 -4 GWOriC GWOriB GWOriB -30 -30 4 2 0 -2 -4 30 20 10 0 -10 -20 -30 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Phase A-B Phase AB-C dRA [mas] dRA [mas]

Figure 1: Top: GW Ori orbit fit to the astrometry of the inner binary (A-B; panel A) and the tertiary (AB-C; panel B), where the primary is in the origin of the coordinate sys- tem (Kraus et al. 2020a). Panels C-F show the fit to the data presented by Czekala et al. 2017. Left: Aperture-synthesis image of GW Ori, obtained with the MIRC-X instrument and CHARA on 2019 August 27. The white dot marks the location of the center-of-mass of the system, while the blue/brown/white curves give the best-fit orbits derived for the component A/B/C. From Kraus et al. (2020).

3.2 M⊙ and 2.7 M⊙; Prato et al. 2018). These discrepan- To fit the mid-infrared SED, Fang et al. (2014, 2017) de- cies can likely be explained by the non-negligle & time- rived a dust-depleted gap at ∼ 45 au. Further evidence variable contributions from circumbinary/circumtertiary for a truncated or gapped disk structure comes from the dust emission biasing the near-infrared flux ratio (Kraus line profile of the CO fundamental lines, where Najita et al. 2020a). The dynamical masses resulting from the et al. (2003) noted that the line profile exhibits a nar- orbit solution are MA = 2.47 ± 0.33, MB = 1.43 ± 0.18, row+broad emission component and that the line width and MC =1.36 ± 0.28 M⊙ (Kraus et al. 2020a). increases towards the more energetic transitions. The sys- tem also shows signposts of active accretion, in particular Brγ-emission (Folha & Emerson 2001) and strong UV veil- A highly dynamical environment in the in- −7 −1 ing (M˙ acc =3 × 10 M⊙ yr , Calvet et al. 2004). ner few au There has been a long debate on the viewing geometry The spectral energy distribution (SED) of GWOri fea- of the system: Based on measurements of the stellar rota- P . v i tures strong excess emission from mid-infrared to millime- tion period ( =3 25 days), the rotation velocity ( sin = . ± . −1 ter wavelengths, indicating the presence of circumstellar 43 0 2 5kms ) and estimates of the stellar radius, Bou- vier & Bertout 1989 estimated the inclination of the sys- dust. SED modeling suggested the presence of a circumbi- ∼ ◦ nary disk extending from around 1.2 au (Fang et al. 2017), tem to 15 , i.e. close to face-on. Using a similar method, but other observational data, Mathieu et al. 1995 obtained 2.1au (Artymowicz & Ludow 1994) or 3.3au (Mathieu et ◦ al. 1995) outwards. This circumbinary disk was also re- an inclination of 30 . However, photometric observations solved spatially, where the VLTI and CHARA visibilities also reported Algol-like eclipses (e.g. Shevchenko et al. associate 16 ± 2% of the H-band flux with circumbinary 1992, Lamzin et al. 1998, Czekala et al. 2017) that have +2.5 been interpreted as evidence for a nearly edge-on disk ori- material located 2−0.8 au from the inner binary (Kraus et al. 2020a). entation. Variability on longer time scales has been reported as well,

2 ABThermal!dust!emission!1.3mm Thermal!dust!emission!1.3mm

R3asym

DC

DAB

R3 R1 R2

N

100!au 20!au E

C 12CO! moment! 0! D 12CO! moment! 1! Bi! et! al.! 2020 +! continuum! (contours) +! continuum! (contours) 12CO! mom.! 1

C2

C1

Figure 2: ALMA observations of GW Ori at 0.12” (right, Bi et al. 2020a) and 0.02” resolution (left, panels A-D, Kraus et al. 2020a). The top row shows continuum images, while the bottom row shows 12CO moment maps.

including dramatic changes in the near-infrared SED on and seen under intermediate inclination 37 ± 1◦ (Fig. 2A), timescales of ∼ 20 yrs (Fang et al. 2014). This variabil- representing the angular moment vector of the cloud that ity might be linked with a 0.2 mag-amplitude sinusoidal feeds the disk. The Eastern side of the disk is tilted to- variation in the V-band light curve (Czekala et al. 2017) wards us, as indicated by the strong forward-scattered that is phased with the of the tertiary. The light from that side of the disk (Fig. 3). origin of the long-term variability has not been answered The inner submillimeter ring, R3, appears much more cir- conclusively yet, but might be due to changes in the view- cular in the images – which could be interpreted as a more ing geometry or accretion rate on the circumtertiary disk. face-on viewing geometry. However, from extrapolating The circumtertiary disk has been detected as submillime- the center of the outer rings (Bi et al. 2020)and from direct ter emission in the ALMA 0.02” images (Fig. 2B) and as imaging (Kraus et al. 2020a) it is evident that the inner near-infrared excess emission near the location of the ter- ring is not centered on the position of the stars (Fig. 2B). tiary in infrared interferometry data (Kraus et al. 2020a). This can be explained best if the ring is intrinsically ec- centric but seen under significant inclination and appears Misaligned rings near-circular in projection. The following additional in- formation can be used to derive the 3-dimensional shape The intermediate/outer dust disk has been probed with and orientation of the ring:

JCMT (Mathieu et al. 1995) and SMA millimeter interfer- 12 ometry, where Fang et al. 2017 highlighted the large spa- (a) Gas kinematics: CO moment 1 maps show that ∼ the rotation axis of the outer disk is oriented roughly tial extent ( 400au) and high mass (0.12M⊙) of the disk. ◦ Bi et al. 2020 and Kraus et al. 2020a acquired ALMA data in East-West direction (position angle 90 ; e.g. Fang with different baseline configurations and detected three et al. 2017, Czekala et al. 2017) with a ’twist’ in the dust rings. The two outer rings, R1 and R2, have radii velocity field in the inner 0.2” (Bi et al. 2020; Fig. 2, of about 350 and 180au and are oriented North-South right). The twist might follow a spiral-arm pattern, with the position angle of the rotation axis changing

3 Figure 3: Top-left: Overlay of the ALMA continuum image (blue) and the SPHERE scattered light image (red; credit: ESO/Exeter/Kraus et al.). Top-right: SPHERE H-band polarimetric image and model image. Bottom: Sketch of the 3-dimensional disk geometry of GW Ori. From Kraus et al. 2020a.

to 180◦ at 100au and ∼ 210◦ at ∼ 30au (Kraus et not follow a straight line but changes direction at al. 2020a, Fig. 2D). ∼ 100 au separation (Fig. 3, top-right), indicating that the shadow falls onto a curved surface in the (b) Warm gas at the inner surface of the ring: 12 inner 100 au. Simultaneous modeling of the shadow The CO moment 0 map shows that the CO sur- morphology and of the ALMA continuum geometry face brightness is low at the location of the ring R3, yields an eccentricity e =0.3 and a semi-major axis which can be explained with the high optical depth of 43 au radius for ring R3 and that the ring is seen and low gas temperature within the ring. However, under inclination of 155◦ (Fig. 3; Kraus et al. 2020a). there is strong CO emission near the inner edge of ring R3 on the Eastern side (labeled C1 in Fig. 2C), indicating that the Eastern side of the ring is farther Broken & warped disk geometry away from the observer and that we see warm gas at the illuminated inner surface of the ring (Fig. 3, Shadows have been observed in several protoplanetary disks, bottom-right; Kraus et al. 2020a). but GW Ori is (to my knowledge) the first case where the ring casting the shadow has been spatially resolved. This (c) Shadows cast by the ring: SPHERE scattered enables tight constraints on the shape and 3-dimensional light imagery (Figs. 3 and 4, top) exhibits several orientation of the misaligned ring as well as the curva- shadows, including narrow shadows in south-east and ture of the warped disk surface inside of the middle ring north-west direction (S1 and S2; Fig. 3, top) and R2 (r . 182au). The scattered-light morphology shows broader shadows extending in north and south-west a strong East-West asymmetry, where the bright Eastern direction (S3 and S4). Remarkably, shadow S1 does arc A3 and the fainter Western arc A4 form together an

4 Artist!impression (adopting!derived!3D!geometry) SPHERE!(H−band)

Figure 4: Artist impression of the 3-dimensional geometry of the GW Ori disk (left) and comparison with the SPHERE image (right). Credit: ESO/Cal¸cada, Exeter/Kraus et al.

apparent ellipse with semi-major axis of 90 au and high The ’sphNG’ simulation presented in Kraus et al. (2020a), eccentricity (e =0.65; Fig. 4). Kraus et al. (2020a) iden- on the other hand, shows the disk tearing effect, where the tifies this ellipse as the point where the disk breaks due gravitational torque of the three stars tears the disk apart to the gravitational torque from the central triple system. into distinct rings that precess independently around the The bright arc A3 constitutes the side of the warped disk central objects. After letting the dust distribution evolve surface that is facing away from Earth and that appears for a few thousand years, a ring breaks out of the disk bright in scattered light due to the direct illumination from plane, whose radius (∼ 40 au), eccentricity (e ∼ 0.2), and the stars. Arc A4 corresponds to the side facing towards extreme misalignment agree well with the observed prop- us, where we see only the self-shadowed outer side of the erties of ring R3. Also, the disk breaks and a warp forms warped surface (Fig. 3). The shadows from the misaligned just beyond the break radius, whose dimension, geometry, ring are cast onto this warped surface and appear as shad- and low column density agree reasonably well with the ows S1 and S2, while the broad shadows S3 and S4 cor- properties of the warp derived from the GWOri observa- respond to the regions where the break orbit crosses the tions (Fig. 5). plane of the outer disk, which coincides with the direction Both simulations adopt similar Shakura-Sunyaev viscosi- in the warp with the highest radial column density. ties (αSS =0.008 − 0.013 for Bi et al.; 0.01-0.02 for Kraus et al.). Therefore, it appears more likely that the different Origin of the disk misalignments outcomes might be related to the setup of the simulation. There are differences concerning the number of stars in- To determine the origin of the extreme disk misalignments cluded in the simulation (2 stars in Bi et al. simulation; observed in GW Ori, two teams recently presented smoothed 3 stars in Kraus et al. simulation) and the orbit solution particle hydrodynamic simulations. Bi et al. conducted that is adopted for the simulation (Czekala et al. 2017 SPH simulations using the ’phantom’ code and concluded solution and Kraus et al. 2020a solution, respectively). that the gravitational torque of the stars alone cannot ex- plain the observed large misalignment between the dust ring. Instead, they propose that an undiscovered compan- ion located between the inner and middle ring that might have broken the disk and induced the misalignments.

5 A B C

Warp

100!au 100!au 100!au Dec z z

RA RA RA

0 2 4 6 −1 0 1 2 −17 −15 −13 −11 column!density!/!g!cm−2 log!(column!density!/!g!cm−2 ) log!(density!/!g!cm−3 )

Figure 5: SPH model, computed with the sphNG code developed by Matthew Bate and collaborators. The simulation adopts the triple star orbits shown in Fig. 1 and an initial disk orientation that corresponds to the outer ALMA rings R1+R2. The snapshot shows the gas density after 9500 years. Panel (A) shows the column density along the line-of- sight seen from Earth; in (B) and (C) the z-axis indicates the direction towards the observer.

Outlook tearing and dust filtration processes near the disk warp re- gion. It is unclear whether the outer-most ring can also be Over the last few decades several exciting pre-main-sequence explained by the dynamical interplay between the central multiple systems have been found and extensively stud- triple system and the disk, or whether dust trapping near ied, including GG Tau, HD142527, HD98800, and T Tauri. a planet-induced density gap might be required to explain GW Ori stands out with respect to the tight constraints the high submillimeter surface brightness in this region. on the full 3-dimensional orbits for all components in the But what caused the misalignment between the disk and system, the dynamical masses, and our knowledge on the the orbits in the first place? Possibilities include turbulent 3-dimensional geometry of the strongly distorted disk (for disk fragmentation (Offner et al. 2010), perturbation by a visualization of the deduced disk geometry & orbits, other stars in a stellar cluster (Clarke & Pringle 1993), the see the interactive 3-dimensional model in Fig. 6). Due capture of disk material during a stellar flyby (Clarke & to this unique information, the system could serve as a Pringle 1991), or the infall of material with a different an- valuable benchmark for calibrating hydrodynamic mod- gular momentum vector from that of the gas that formed els and fundamental parameters under well-defined condi- the stars initially (Bate et al. 2010, Bate 2018). To answer tions. This could provide the validation & refinement that this question, statistical information will be of essence, is needed before applying the models to the much less-well- both on the disk-orbit misalignment in pre-main-sequence constrained planet formation case, where the masses and multiples and on the orbital architecture and spin-orbit orbits of the gap-opening bodies are not known in general. alignment in main-sequence systems. Obtaining such con- The disk-tearing effect that we might witness in GW Ori in straints on a large sample of stars is a key science objec- action, constitutes an important new mechanism for mov- tive for the proposed VLTI instrument BIFROST (Kraus ing disk material onto highly oblique or retrograde orbits, et al. 2019, 2020b) and could offer important new insights even at very wide separations from the star. At the same on both the star- and planet-formation processes. time, the observed torn ring seems to be sufficiently mas- Acknowledgement: sive and might be sufficiently stable for planet formation I would like to thank my collaborators on our recent paper on GWOri to occur, potentially giving rise to an yet-undiscovered and the members of the team around Jiaqing Bi for delightful dis- population of circum-multiple planets on highly oblique, cussions on this fascinating object. Also I acknowledge support from long-period orbits. an ERC Starting Grant (Grant Agreement No. 639889).

An important open question concerns the origin of the out- References: ermost ring in GW Ori. The high submillimeter brightness Artymowicz & Ludow 1994, ApJ 421, 651 of the inner and middle ring can likely be explained by disk Bate, M.R., et al. 2010, MNRAS 401, 1505

6 Figure 6: The SFN article contains here an interactive 3-dimensional model of the disk geometry in GWOri, as derived in Kraus et al. 2020a. Unfortunately, it was not possible to upload the graphics to arxiv – please retrieve the SFN article from http://www.ifa.hawaii.edu/users/reipurth/newsletter/newsletter333.pdf or the 3-dimensional graphics from https://www.eso.org/public/archives/releases/pdf/eso2014a.pdf .

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