# A likely flyby of binary protostar Z CMa caught in action

Ruobing Dong, Hauyu Baobab Liu, Nicolás Cuello, Christophe Pinte, Péter Ábrahám, Eduard Vorobyov, Jun Hashimoto, Ágnes Kóspál, Eugene Chiang, Michihiro Takami, et al.

► To cite this version:

Ruobing Dong, Hauyu Baobab Liu, Nicolás Cuello, Christophe Pinte, Péter Ábrahám, et al. A likely flyby of binary protostar Z CMa caught in action. Nature Astronomy, 2022, 6, pp.331-338. 10.1038/s41550-021-01558-y. insu-03705388

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# A likely flyby of binary protostar Z CMa caught in action

Ruobing Dong ID<fnref n="1" />✉, Hauyu Baobab Liu ID²✉, Nicolás Cuello ID³✉, Christophe Pinte ID⁴✉,
Péter Ábrahám ID⁵,⁶, Eduard Vorobyov⁷,⁸, Jun Hashimoto ID⁹, Ágnes Kóspál ID⁵,⁶,¹⁰, Eugene Chiang¹¹,
Michihiro Takami ID², Lei Chen ID⁵, Michael Dunham¹², Misato Fukagawa ID¹³, Joel Green¹⁴,
Yasuhiro Hasegawa ID¹⁵, Thomas Henning ID¹⁰, Yaroslav Pavlyuchenkov ID¹⁶, Tae-Soo Pyo ID¹⁷ and
Motohide Tamura ID⁹,¹³,¹⁸

Close encounters between young stellar objects in star-forming clusters are expected to markedly perturb circumstellar disks. Such events are witnessed in numerical simulations of star formation<fnref n="1" />⁻³, but few direct observations of ongoing encounters have been made. Here we report sub-0.1"-resolution Atacama Large Millimeter/Submillimeter Array and Jansky Very Large Array observations towards the million-year-old binary protostar Z Canis Majoris in dust continuum and molecular line emission. A point source ~4,700 au from the binary has been discovered at both millimetre and centimetre wavelengths. It is located along the extension of a ~2,000 au streamer structure previously found in scattered light imaging, whose counterpart in dust and gas emission is also newly identified. Comparison with simulations shows signposts of a rare flyby event in action. Z CMa is a 'double burster', as both binary components undergo accretion outbursts⁴, which may be facilitated by perturbations to the host disk by flybys⁵⁻⁸.

Z Canis Majoris is a pre-main sequence binary system located at ~1,125 pc in the Canis Major R1 association (Methods). The south-east component belongs to a class of outbursting protostars called FUors, inferred on the basis of the characteristic broad doubled optical absorption lines⁹. The northwest component 0.1" apart has displayed repeated outbursts in the past few decades, resembling the EXor-type outbursting protostars¹⁰. The pair has been spatially resolved in adaptive optics observations at multiple epochs¹⁰,¹¹, and their motion is consistent with a face-on circular orbit composed of a 5 $M_\odot$ star (EXor component) and a 1 $M_\odot$ star (FUor component)¹²,¹³.

Previous spatially unresolved millimetre observations reported a total flux density of 26 ± 1 mJy at 1.4 mm and 2.1 ± 0.1 mJy at 7 mm (ref. ¹⁴). We observed the Z CMa system in continuum and molecular line emission at around 224 GHz (1.3 mm) using the Atacama Large Millimeter/Submillimeter Array (ALMA; Methods), and in continuum emission at 33 GHz (9 mm) using the Jansky Very Large Array (JVLA; Methods). Figure 1 shows the synthesized 1.3 and

9 mm continuum emission images. In total we detect four point-like sources (A–D) at 1.3 mm. Two of them, A and B, are the central binary. Gaussian fitting of the flux indicates that both are surrounded by disks ~20 au in size, potentially containing millimetre-sized dust particles. This is in line with millimetre observations of outbursting protostars in general¹⁵,¹⁶, which often show compact dust emission. At 9 mm, both sources are detected too, as well as a jet-like structure 0.1" in length probably originating from B. Gas observations show prominent outflow and cavity structures on large scales (Extended Data Figs. 1 and 2).

The other two 1.3 mm sources, C and D, were previously unknown. In the continuum map imaged with robust=0, C is detected with a flux density of 700 ± 40 mJy and a signal-to-noise ratio of 27 at 4.2" (4,725 au), while D is detected with a flux density of 550 ± 24 mJy and a signal-to-noise ratio of 21 at 0.8" (900 au; Methods and Extended Data Fig. 3). They have spectral line counterparts, as shown in Fig. 2 (most noticeable in C¹⁸O and SO emission; Methods), that appear to connect to the circumbinary environment of Z CMa. We conclude that both are probably physically associated with the system rather than being foreground or background sources. In addition, component C is detected at 9 mm by JVLA with signal-to-noise ratio=5 (Methods). Gaussian fitting of its flux indicates that C is marginally resolved at both 1.3 and 9 mm with a deconvolved size of 40–50 au. Its spectral index $\alpha$ between the two wavelengths is 1.6 ± 0.1. This low index may be explained by non-thermal contributions at 9 mm, a hot central source embedded in an optically thick medium¹⁷ or strongly scattering dust at millimetre wavelengths¹⁸. Component D cannot be recovered in the JVLA observations. It may be a fragment that has recently formed from turbulent fragmentation in the disk or surrounding cloud¹⁹,²⁰, in which scenario it is only visible at millimetre and not centimetre wavelength due to its youth and associated insufficient dust growth. Further characterizations of these sources are presented in Methods and Supplementary Information, including the intensity-weighted

<footnote n="1">¹Department of Physics & Astronomy, University of Victoria, Victoria, British Columbia, Canada. ²Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, Republic of China. ³Université Grenoble Alpes, CNRS, IPAG, Grenoble, France. ⁴Monash Centre for Astrophysics (MoCA) and School of Physics and Astronomy, Monash University, Clayton, Victoria, Australia. ⁵Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Eötvös Loránd Research Network (ELKH), Budapest, Hungary. ⁶Institute of Physics, ELTE Eötvös Loránd University, Budapest, Hungary. ⁷Department of Astrophysics, University of Vienna, Vienna, Austria. ⁸Research Institute of Physics, Southern Federal University, Rostov-on-Don, Russia. ⁹Astrobiology Center, National Institutes of Natural Sciences, Mitaka, Tokyo, Japan. ¹⁰Max-Planck-Institut für Astronomie, Heidelberg, Germany. ¹¹Department of Astronomy, University of California at Berkeley, Berkeley, CA, USA. ¹²Department of Physics, State University of New York at Fredonia, Fredonia, NY, USA. ¹³National Astronomical Observatory of Japan, Mitaka, Tokyo, Japan. ¹⁴Space Telescope Science Institute (STScI), Baltimore, MD, USA. ¹⁵Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA. ¹⁶Institute of Astronomy, Russian Academy of Sciences, Moscow, Russia. ¹⁷Subaru Telescope, National Astronomical Observatory of Japan, National Institutes of Natural Sciences, Hilo, HI, USA. ¹⁸Department of Astronomy, The University of Tokyo, Tokyo, Japan. ¹⁹e-mail: rbdong@uvic.ca; hyliu@asiaa.sinica.edu.tw; nicolas.cuello@univ-grenoble-alpes.fr; christophe.pinte@monash.edu</footnote>
<figure><img src="image_3.png" /><figcaption>**Fig. 1 | Observations of the Z CMa system.** The centre is at component A (ALMA coordinates RA, dec. = 07 h 03 min 43.154 s, −11° 33′ 06.13″). Left: Subaru H-band scattered-light image with an angular resolution of 0.07″ (ref. ²¹). The polarized intensity (PI) is normalized to the stellar intensity flux (I*). Middle: ALMA 1.3 mm dust continuum emission with Briggs robust = 2 (beam size 115 mas × 95 mas, position angle = P.A. = 74°). The inset is a zoom-in of the inner 0.6″ × 0.6″ region (dotted box) with Briggs robust = −2 (beam size 71 mas × 42 mas, P.A. = 68°). Right: JVLA 9 mm dust continuum emission with Briggs robust = 2 (beam size 100 mas × 66 mas, P.A. = 4°). The inset is a zoom-in of the inner 0.6″ × 0.6″ region (dotted box) with Briggs robust = −2 (beam size 62 mas × 45 mas, P.A. = 1°). In all panels, the angular resolution of the beam is marked in the lower left corner. The four point sources detected in the ALMA observations, and the counterparts of A, B, and C and a jet detected in the JVLA observations, are labelled.</figcaption></figure>

velocity (moment 1) maps. No other point sources have been detected in 1.3 mm continuum emission imaged with robust=0 with a 3σ detection limit of 78 µJy, equivalent to a dust mass of 3 $M_\odot$ (Methods).

Near-infrared imaging observations have revealed a large disk in scattered light extending to ~1,500 au (ref. ²¹). Remarkably, the disk has a ~2"-long filamentary structure in the southwest, discovered in Keck adaptive optics observations¹² and confirmed by VLT and Subaru in polarized light imaging¹³,²¹ (Fig. 1). In our observations the inner ~1" of this streamer is detected in the 1.3 mm continuum, with a total flux density of 3 ± 0.6 mJy and an estimated solid mass of 119 ± 24 $M_\odot$ (Methods). It is detected in ¹³CO and C¹⁸O emission as well (Fig. 3). The detection in dust and gas emission suggests that the streamer is a dust surface-density enhancement.

The origin of the streamer has been a long-standing puzzle. It has been variously hypothesized to be the wall of a cavity created by a molecular outflow¹³, a spiral arm produced by gravitational instability²¹,²² or the relic of a recently captured low-mass (~0.01 $M_\odot$) cloudlet⁷. The newly discovered source C at the end of the streamer points to another possible scenario, which we consider the most likely: C may be a young stellar object (YSO) that has recently performed a flyby around Z CMa, and the streamer is a structure produced by this tidal interaction event.

Tidal interactions between a protoplanetary disk and a flyby intruder are known to strongly perturb the disk²³. Depending on the angle $\beta$ between the angular momentum vector of the disk and that of the intruder (with respect to the host), flyby events can be categorized as prograde ($\beta$<90°) or retrograde (90°< $\beta$≤180°). Prograde flybys typically induce a pair of spiral arms in the disk on a global scale, with the primary arm pointing towards the intruder and the secondary arm roughly 180° away in the azimuth. These arms are due to the combined effect of the intruder's gravitational perturbation and the displacement of the centre of mass of the system²³. They are most prominent and symmetric when $\beta$=0° (copla-

nar and prograde flyby), and their strength and symmetry decrease as $\beta$ increases. Retrograde flybys have much weaker effects on the disk, unless it is deeply penetrating⁶. In non-coplanar flybys, the primary arm can be lifted above the disk plane to keep pointing to the intruder, while the secondary arm is embedded in the disk⁶.

Figure 4 shows a simulated flyby event (Methods and Extended Data Fig. 4), in which an intruder flies by a single disk-hosting primary with an intruder/primary mass ratio $q$=0.3. We do not expect the binary nature of Z CMa to substantially affect the outcome of the flyby at distances more than ten times the binary separation. The intruder approaches the host on a prograde parabolic orbit inclined with respect to the host's disk by $\beta$=45°. The pericentre is at $r_{peri}$=3,000 au and in the disk plane. The size of the initial disk surrounding the host is $R_{out}$=840 au, thus the flyby is non-penetrating. A pair of spiral arms is produced in the density space with comparable strengths. The two arms, however, have different illuminations, because they are in different planes. The primary arm pointing to the flyby is in the orbital plane of the flyby, and the light path from the host star is unobscured (the host star is assumed to be much more luminous than the intruder at near-infrared wavelengths). The arm is expected to be present in dust and gas emission due to its enhanced density²⁴. In contrast, the secondary arm is in the plane of the circumprimary disk and is much less well illuminated by the host star²⁴. The overall morphology of the model, with one prominent arm extending beyond the main disk and pointing towards the intruder, resembles the observations of the Z CMa system (Fig. 4). We note that the model does not represent a unique solution; instead, it is one representative example that produces an overall match to the observations (Methods).

Component C has not been identified at optical and infrared wavelengths in the SDSS, Gaia, 2MASS or WISE databases. Analysis of archival Keck adaptive optics imaging observations of Z CMa yields an upper limit of 21.9 mag for C in H-band apparent magnitude (Methods and Extended Data Fig. 5), corresponding
<figure><img src="image_4.png" /><figcaption>**Fig. 2** | Integrated intensity (moment 0) maps of the <sup>12</sup>CO (2-1), <sup>13</sup>CO (2-1), C<sup>18</sup>O (2-1) and SO <sup>3Σ</sup> 6(5)-5(4) transitions. The beam size (0.19" × 0.18"; P.A. = 88°) is marked in the lower left corner. The robust = 0 weighted 224 GHz continuum image (beam size 0.075" × 0.047"; P.A. = 65°) is shown in green contours at 0.13 and 1.3 mJy per beam levels (5 and 50× the root mean square noises) to highlight the four compact continuum sources. ALMA/JVLA source C is labelled.</figcaption></figure>

to the photosphere of a million-year-old planetary-mass object at 1,125 pc, or a solar-mass YSO experiencing ~10 mag of extinction²⁵. Past simulations and our tests indicate that an intruder with $q \lesssim 0.1$ has difficulties in exciting the observed streamer, putting a lower limit of ~0.6 $M_\odot$ in mass for C (Methods). Therefore, C is probably heavily extincted. A flyby intruder may capture material from the host disk to form an envelope or a disk, depending on the angle and the level of penetration of the flyby⁶,²⁴,²⁶. The intruder may also possess a natal disk before the flyby. The detection of C in both dust and gas emission indicates its possession of circumstellar material, which may originate from either scenario (Methods). A heavy extinction may be caused by, for example, an edge-on circumstellar

disk. Adopting conventional assumptions, its 1.3 mm flux density translates to ~28 $M_\odot$ of solids in the circumstellar environment. Future observations at mid- to far-infrared wavelengths are needed to probe the spectral energy distribution of C to further constrain its properties.

Simulations have shown that a prograde flyby may exert gravitational torques and lead to shocks and angular-momentum redistribution in the disk, and drive material to smaller radii around the binary⁵,²³,²⁷⁻²⁹. This may trigger accretion outbursts with the accretion rate jumping up by one order of magnitude shortly after pericentre passage⁶. This appears consistent with the double-burster nature of Z CMa. Isolated protostars with masses from 0.1 to 40 $M_\odot$ spend
<figure><img src="image_5.png" /><figcaption>**Fig. 3** | Subaru H-band image and ¹³CO moment 0 map overlaid with the 5σ contours in ALMA 1.3 mm continuum emission and in JVLA 9 mm continuum emission. Left: Subaru H-band image. Right: ¹³CO moment 0 map. Both the ALMA and JVLA maps are imaged with robust = 2. ALMA/JVLA source C is labelled.</figcaption></figure>

on average 1–10% of the time in self-induced outbursts³⁰,³¹. Thus, the chance of witnessing a binary in this mass range undergoing accretion outbursts simultaneously is expected to be ≤1% if the two outbursts are independent of each other. Given the small number of protostar binaries that have been monitored with sufficient depth and temporal coverage to enable detections of outbursts, the double-burster nature of Z CMa implies that whatever has triggered the outbursts in one may have triggered those in the other. This is plausible if both are facilitated by the flyby.

While our multiwavelength observations of the Z CMa system lend support for a possible recent flyby event, alternative scenarios accounting for the observations exist. For example, it is possible that C is a self-gravitating clump previously formed in the dense environment at smaller radii that has subsequently been ejected due to dynamical interactions with the stars and/or other clumps³². Such clumps may form by fragmentation of gravitationally unstable disks³³,³⁴. In such an event, the ejecta drive a bow shock, which may appear to be filamentary streamers in observations²⁶. The binary nature of Z CMa, which promotes dynamical scattering, encourages this scenario. However, the bow shock behind the ejecta typically has two edges with roughly equal strengths (and both of them are expected to be within the disk plane). In addition, the resulting streamers are more prominent closer to the ejecta²⁶, instead of the other way around as observed. We thus conclude that the ejecta scenario is less likely. Recent observations have shown multiple filamentary structures as infall and accretion streamers connecting a cloud to the disk in a few systems³⁵⁻³⁹. However, unlike those cases, Z CMa's streamer is not known to be connected to a large-scale cloud, and no other tails similar in strength and size have been found. Three-dimensional gravomagnetohydrodynamic simulations have also shown that streamers and clumps can form in turbulent fragmentation in filamentary giant molecular clouds¹⁹,²⁰. While it is possible that the observed streamer and source C form in this manner, the fact that the streamer points toward the source C makes the flyby scenario compelling.

Our discovery of C at a special location with respect to the streamer, the unique morphology of the disk in multiwavelength imaging observations and the double-burster nature of Z CMa show signposts of a flyby in action around a binary YSO. These set the Z

CMa system apart from a handful of other flyby candidate systems (Methods). Flybys during star formation are expected to markedly affect the disk and the planet formation processes within it. For example, the extremely long-period object Sedna⁴⁰ has prompted the hypothesis that the solar system may have experienced close encounters with intruders in its infancy⁴¹. Future high-spatial-resolution observations at both millimetre and near-infrared wavelengths may find other systems similar to Z CMa by identifying an intruder and its dynamically induced structures, thus establishing an empirical estimate of the frequency of flybys and better assessing their role in the evolution of protoplanetary disks. In particular, it would be exciting to search for flyby events in outbursting protostar systems, to test the hypothesis that flybys may trigger accretion outbursts. Such studies will also shed light on the formation and evolution of binary and multiple systems⁴².

## Methods

**Distance to Z CMa.** The distance to Z CMa is controversial. The Gaia Early Data Release 3 (EDR3) measured a parallax of 2.035 ± 0.632 mas (ref. ⁴³), corresponding to a distance of 491 ± 221 pc. However, the renormalised unit weight error (ruwe) parameter is 36.798, a value far exceeding the generally accepted upper limit of 1.4. This is a strong implication for an unreliable parallax solution⁴³, possibly caused by the photometric variabilities of this system, or by the imperfect modelling of a disk-bearing binary system as a single star.

To clarify the distance to Z CMa, we performed a Gaia EDR3-based analysis of Canis Major R1, the cluster of which Z CMa is considered to be a member⁴⁴⁻⁴⁶. We compiled a list of 62 young stars in CMa R1, all with ruwe ≤ 1.4, from the literature⁴⁷⁻⁴⁹. The distribution of their distances exhibits a strong single peak at 1,125 ± 30 pc, with only one star located closer than 650 pc. The area occupied by CMa R1 members on the sky coincides well with the position of Z CMa. The selected cluster members have a tight distribution in proper motion: pm<sub>RA</sub> = -4.49 ± 0.2 mas yr<sup>-1</sup>, pm<sub>DE</sub> = +1.59 ± 0.19 mas yr<sup>-1</sup>. The proper motion of Z CMa, taken either from Gaia EDR3 or from ref. ⁵⁰, matches that of CMa R1 within 3σ in right ascension, and is higher by not more than 2.25 mas yr<sup>-1</sup> in declination. We conclude that Z CMa fits in the CMa R1 cluster in both coordinates and proper motion, and that there is no hint of additional star formation activity in the foreground of the cluster. Thus, we assume that Z CMa is a member of Canis Major R1, and adopt a distance of 1,125 pc.

**ALMA observations.** We performed ALMA observations on 2016 October 21 in the C40-6 array configuration, and on 2017 September 07 in the C40-8 configuration (project 2016.1.00110.S). We performed the Atacama Compact Array observations on 2017 July 09 (project 2016.2.00168.S). We manually
<figure><img src="image_6.png" /><figcaption>**Fig. 4** | Comparing simulations with observations. Top row: two views of the gas column density in the hydrodynamical simulation of a stellar flyby. The system is shown 14 kyr after the perturber has gone through the pericentre. Bottom row: the resulting H-band synthetic image in polarized light obtained through radiative transfer simulation (left) is to be compared with the Subaru H-band polarized light observation (right). In the synthetic image the line of sight is in the z direction.</figcaption></figure>

calibrated the data using the CASA v5.1.1 software package⁵¹. Spectral line imaging was performed using the Miriad software package⁵². In continuum emission, we detect four compact sources. Two of them (A and B) compose the binary. The other two, C and D, are to the southwest and northeast of the binary, respectively. We performed two-dimensional Gaussian fitting using the task imfit in the image domain to measure their properties, and list them in Extended Data Fig. 3. Gas observations show prominent outflow and cavity structures on large scales, which dominate the signals in both the moment 0 (Extended Data Fig. 1) and moment 1 maps (Extended Data Fig. 2). ¹³CO and C¹⁸O do not trace the central binary very well, while the SO line does. All of these continuum sources have spectral line counterparts, so they are probably parts of the same system. The previously detected streamer in scattered light in the south of the binary has counterparts in both continuum and line emission. Further description of the observations and data processing can be found in Supplementary Information.

**JVLA observations.** We performed JVLA Ka-band (29–37 GHz) standard continuum-mode observations toward Z CMa in the A array configurations on 2016 October 07 and 10 (project 16B-080). We manually followed the standard data calibration strategy using the CASA package, release 5.3.0. The ALMA

components A, B and C are detected at 9 mm. We performed two-dimensional Gaussian fitting in the image domain using the CASA task imfit to measure their properties, and list them in Extended Data Fig. 3. ALMA source D was not detected. Further description of the observations and data processing can be found in Supplementary Information.

**Dust mass estimates.** We adopt fluxes in continuum maps imaged with robust=0 for dust mass estimates. Assuming that the circumstellar material is optically thin at 1.3 mm, we estimate its dust mass on the basis of the integrated flux density as

$$M_{\text{dust}} = \frac{f_{\nu} d^2}{B_{\nu} (T_{\text{dust}}) \kappa_{\nu}}, \quad (1)$$

where d is the distance to the source, $f_ν$ is the measured flux density, $B_ν(T_{\text{dust}})$ is the Planck function at the dust temperature $T_{\text{dust}}$, assumed to be 20 K, and $κ_ν$ is the dust opacity, assumed to be 2.2 cm² g⁻¹ at 224 GHz (ref. ⁵³). The dust masses around C and D are estimated to be 28 ± 1 M☉ and 22 ± 1 M☉, respectively (the uncertainty only reflects the uncertainty in the flux measurement). For the streamer, given its measured total flux density in 1.3 mm continuum emission of 3 ± 0.6 mJy, we
estimate $M_{\text{dust}} = 119 \pm 24 M_{\odot}$. As a reference, the $3\sigma$ detection limit in our 1.3 mm continuum emission is $78 \mu Jy$, corresponding to a dust mass of $3 M_{\odot}$. We note that the derived dust mass has large uncertainties due to the uncertainties in the dust temperature and opacity and whether the emission is optically thin or thick.

**Hydrodynamical and radiative transfer simulations.** We model the hydrodynamical evolution of the disk around Z CMa using the smoothed particle hydrodynamics (SPH) PHANTOM code$^{54}$. The disk is modelled using $10^6$ gas SPH particles and assuming a total disk mass of $0.3 M_{\odot}$ (thus a disk-to-star mass ratio of 5%). The initial disk surface density follows a power-law profile $\Sigma \propto R^{-1}$ and we assume that the disk is vertically isothermal. We adopt a mean Shakura–Sunyaev disk viscosity $\alpha_{SS} \approx 0.005$ by setting a fixed artificial viscosity parameter $\alpha_{AV} = 0.2$ and using the 'disk viscosity' flag of PHANTOM$^{55}$. The initial disk inner and outer radii are $R_{\text{in}} = 120$ au and $R_{\text{out}} = 840$ au. $R_{\text{in}}$ is not critical in this study since the main focus is on the arm morphology at large distances. The measured accretion rate onto the primary strongly depends on the choice of $R_{\text{in}}$ and it is expected to increase during the encounter, especially for prograde inclined flybys$^{56}$. During the tidal encounter, material in the disk is spread to ~2,500 au.

We model component C as the intruder. We consider D as a low-mass clump that does not have an important impact on the disk dynamics, and thus is ignored in the simulations. As mentioned in the main text, D may be a fragment that has recently formed from gravitoturbulent fragmentation in the disk or the surrounding cloud, in which scenario it is only visible at millimetre and not centimetre wavelength due to its youth and associated insufficient dust growth. For prograde flybys and companion–disk interaction in general$^{56}$, more prominent spiral arms form interior to the orbit of the perturber. The fact that D appears to be closer to the primaries than the streamer suggests that it is unlikely to be the driver of the streamer. Moreover, if D were a massive perturber deeply penetrating the disk, it would be expected to leave strong signatures in the disk, which is not observed in our multiwavelength observations. In contrast, C is a much more robust candidate, since it leaves the expected dynamical signatures in the disk.

We place the perturber on a parabolic prograde orbit at an angle of $\beta = 45^\circ$ with respect to the disk plane as shown in the sketch in Extended Data Fig. 4. The pericentre of the orbit is fixed along the line of nodes and at a distance of $r_{\text{peri}} = 3,000$ au from the disk-hosting star. The perturber is initially placed at ten times $r_{\text{peri}}$. The masses of the host star and the perturber are set to $6 M_{\odot}$ and $1.8 M_{\odot}$, which give $q = 0.3$. Previous simulations$^{5,23}$ have shown that as the mass ratio increases the flyby event produces more prominent streamers, while the intruder rips off a larger fraction of the circumprimary disk, leaving a more compact disk afterwards. The exact radial extent and morphology of the streamer depend on $q$, $r_{\text{peri}}$, initial disk size and $\beta$. The mass of C is poorly constrained from the observations. On the basis of a limited number of tests we find that a minimum mass ratio of ~0.1 is needed to roughly reproduce the streamer with a companion at the current distance. We model both the star and the perturber as sink particles that interact with gas particles via gravity and accretion$^{57}$. Given the large spatial scales considered and the computational costs, we set the sink particle accretion radius to 10 au on both stars.

The flyby setup is publicly available in the PHANTOM repository. Here we consider a non-penetrating prograde flyby to trigger prominent spirals without capturing material around the perturber. The circumprimary disk is initially in the x–y plane. To simultaneously match the appearance of the disk in scattered light and the orientation of C, we rotate the system by $105^\circ$, $30^\circ$ and $180^\circ$ around the $z$, $y$ and $x$ axes (in clockwise sense and in that order). The line of sight is in the z direction (axes are shown in red in Fig. 4 and Extended Data Fig. 4).

We use the radiative transfer code MCFOST$^{58,59}$ to generate synthetic polarized scattered-light images. We build a Voronoi tessellation where each cell is associated with one SPH particle. The disk is passively heated by the two stellar sources (the host and the intruder); however, the flyby star is turned off in simulating the scattered-light image to mimic extinction by its circumstellar material (see below). We use isochrones$^{60,61}$ at 3 Myr for the two sink particles of 6 and $1.8 M_{\odot}$, resulting in effective temperatures of 18,400 and 4,800 K and radii of 3.00 and $2.36 M_{\odot}$, respectively. We used $10^8$ packets to compute the dust temperature structure and sample the specific intensity before generating the polarized scattered-light images at 1.6 µm via a ray-tracing algorithm. The dust grain population is assumed to follow a power law $dn(a) \propto a^{-3.5} da$ between 0.03 µm and 1 µm, and the total dust mass is set to 0.01 times the gas mass from the SPH simulation. While grains are known to be able to grow to larger sizes in early-stage disks$^{62}$, sub-micrometre-sized grains dominate the scattering of starlight. The presence of larger grains, not included in our simulations, is not expected to affect scattered-light observations. The dust properties are calculated using the Mie theory assuming astrosilicate composition$^{63}$. The resulting synthetic observation in the H band is shown in Fig. 4.

A prograde perturber may cause a strong gravitational pull able to excite the eccentricity of the material in the disk. This translates into enhanced stellar accretion and outbursts. The time delay between the pericentre passage and the peak of the accretion outbursts, as well as their duration, depend on a number of parameters including the intruder–flyby mass ratio, the pericentre distance, the angle of the pericentre and the inclination of the intruder’s orbit relative to the circumprimary disk plane, and the size of the circumprimary disk. Cuello et al.$^6$

carried out a sample of tests and found that for prograde flyby events the delay between the pericentre passage and the peak of the outbursts is of the order of 10% of the Keplerian orbital period at the pericentre, and the outbursts temporally coincide with prominent spiral arms in the disk (they both fade away with time). Meanwhile, the outbursts can last for thousands of years or more. This is broadly consistent with our model, in which the intruder passed the pericentre of the order of 10% of the Keplerian orbital period ago, or a few thousand years. We caution that in current simulations$^6$ the stellar accretion is modelled as material crossing the inner boundary of the disk at 10 au. Future investigations are needed to model the disk dynamics in the inner disk to better quantify the accretion outburst timescales.

Our model demonstrates that a flyby event can produce one prominent arm extending beyond the outer edge of the circumprimary disk and pointing to the intruder. We note that the model is not designed to perfectly reproduce the observations, and no extensive parameter space survey has been carried out to find a best-fit model or to constrain the parameters in the flyby. Despite a match in the overall shape, there are a number of discrepancies between the model and the observations. For example, the streamer extends to a larger distance and continuously fades outward in the synthetic near-infrared image, while the observed streamer bends at its end and ends more sharply. Moreover, the synthetic image shows another spiral arm in the north side of the disk interior to $1''$, which is not seen in observed scattered light (although there are hints of a secondary arm at this distance in the ALMA dust and gas emission maps, Figs. 1 and 2). The observed streamer is also more than one order of magnitude brighter than that in the model. These discrepancies are probably the result of the model having a different orbit of the intruder, a different initial disk size, a different viewing orientation and/or different scattering properties of the dust from the Z CMa system. In addition, the signal-to-noise ratio interior to $1''$ in the Subaru observation may not be high enough to reveal structures.

**Keck/NIRC2 archive data.** ALMA source C is located outside the field of view in Subaru/HiCIAO observations (Fig. 1a). In Extended Data Fig. 4 we present H-band Keck/NIRC2 observations taken on 2005 October 21 (Keck programme U14N2), which have a field of view large enough to cover C ($10^6 \times 10^7$). The Keck data were taken with a coronagraph. The total exposure time is 10 s. We conducted the standard calibration procedure: dark subtraction, flat fielding and hot/bad-pixel correction. No point sources are detected at the position of source C. According to the NIRC2 observer’s manual, the zero-point magnitude at the H band is 25.4 mag. The background noise including photon noise of the central star and the sky noise at the position of the point source C is 23.7 mag, and thus the $5\sigma$ detection limit of the point source C at the H band is 21.9 mag in apparent magnitude.

**Circumstellar extinction of component C.** Our experiments show that a flyby object with $q \gtrsim 0.1$ is needed to induce the observed streamer, corresponding to $M_{\odot} \gtrsim 0.6 M_{\odot}$. A 2 Myr, 0.6 $M_{\odot}$ YSO has a photospheric luminosity of 13.3 mag at 1,125 pc in apparent magnitude at the H band$^{25}$. Thus, an extinction of $A_{H} > 8$ mag is needed to render component C undetected in the Keck observations. A flyby object on a grazing or penetrating orbit may capture material from the primary disk$^{24,26,64}$. It may also possess circumstellar material before the flyby. The estimated dust mass in the circumstellar material around C is $28 \pm 1 M_{\odot}$ on the basis of its 1.3 mm flux, which may provide sufficiently large extinction. For example, if one lunar mass of interstellar-medium dust with $\kappa = 5,300 \text{ cm}^{-2} \text{ g}^{-1}$ at the H band$^{65}$ is distributed spherically symmetrically with a radial density profile $\rho \propto r^{-1.5}$ between 1 and 10 au, the resulting extinction at $A_{H}$ reaches 10 mag. Alternatively, if C acquires an edge-on disk after the flyby, the stellar component can be completely obscured$^{66}$ too. Modelling the specific circumstellar environment around C after the flyby is difficult, due to our ignorance of its state before the flyby.

We note that class 0 YSOs, protostars embedded in a substantial natal envelope, have spectral energy distributions peaking around or longward of 100 µm (ref. $^{67}$). Such objects are often detectable only at far-infrared and millimetre wavelengths$^{68}$. However, source C is unlikely to belong to this class. Its gas emission is confined to a region 200 au in size (Fig. 2), inconsistent with the thousands of astronomical units size of the envelope of class 0 objects$^{69}$. In addition, the envelope would probably be distorted and stretched due to the gravitational interaction with the primary, which is not seen. Class 0 objects usually have $M_{\odot} < M_{\text{envelope}}$ (ref. $^{70}$). The observed 1.3 mm flux from C corresponds to a total gas + dust mass of 0.01 $M_{\odot}$, assuming a gas-to-dust mass ratio of 100, which is two orders of magnitude smaller than the estimated mass of component C.

**Comparisons of the Z CMa system with other flyby candidate systems.** A handful of systems with disks undergoing possible stellar flybys have been proposed before, such as RW Aurigae$^{71,72}$, AS 205$^{73}$ and UX Tauri$^{74,75}$. However, the unbound nature of the perturber in them does not have strong observational support and remains to be confirmed. For example, latest observations suggest that the two components in RW Aur form a bound binary, as the disk morphology is inconsistent with a one-off encounter$^{76}$. In the bound companion case the circumprimary disk is expected to be severely truncated and the spirals tightly wound and symmetric$^{77,78}$. The radial extent and the shape of the spiral and the disk in Z CMa strongly favour an unbound perturber. In addition, multiple
encounters with a bound companion drive a pair of symmetric arms even if the orbit of the companion is misaligned from the circumprimary disk<sup>29,30</sup>. This is not seen in Z CMa but is seen in other proposed flyby systems. The streamer in Z CMa is convincingly detected in all three tracers (scattered light, and gas and dust emission), which is not the case in other systems. Moreover, the more compact streamer in the millimetre dust continuum than in scattered light, as well as the burster nature of the hosts in Z CMa, support a recent flyby as shown by simulations<sup>6,24</sup>.

In Supplementary Information we provide an estimate of the occurrence rate of Z CMa-like flyby events.

**Data availability**

Raw ALMA data are publicly available via the ALMA archive https://almas science. eso.org/aq/ under project IDs 2016.1.00110.S and 2016.2.00168.S. Raw JVLA data are publicly available via the JVLA archive https://archive.nrao.edu/archive/ advquery.jsp under project code 16B-080. Raw Keck data are publicly available via the KOA Data Access Service http://koa.ipac.caltech.edu/ under the programme ID U14N2. Final reduced and calibrated image files are available at https://doi. org/10.6084/m9.fishshare.16915327.

**Code availability**

The PHANTOM code is made available at https://github.com/danieljprice/phantom by D. Price. The MCFOST code is made available at https://github.com/cpinte/ mcfost by C.P.

Received: 15 April 2021; Accepted: 9 November 2021;
Published online: 13 January 2022

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## Acknowledgements

We thank C.-H. Kim, J. Lee and H. Deng for helpful discussions. R.D. would like to thank L. Xu for support and encouragement in the period of this work.

R.D. acknowledges financial support provided by the Natural Sciences and Engineering Research Council of Canada through a Discovery Grant, as well as the Alfred P. Sloan Foundation through a Sloan Research Fellowship. N.C. acknowledges support from the European Union's Horizon 2020 research and innovation programme under Marie Skłodowska-Curie grant agreement 210021. H.B.L. acknowledges support from the Ministry of Science and Technology (MoST) of Taiwan (grant 108-2112-M-001-002-MY3). E.V. acknowledges support from the Russian Fund for Fundamental Research, Russian–Taiwanese project 19-52-52011. Y.H. is supported by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. A.K. and L.C. acknowledge support from the European Research Council under the European Union's Horizon 2020 research and innovation programme, grant agreement 716155 (SACCRED). L.C. is supported by the Hungarian OTKA grant K132406. M. Takami is supported by the Ministry of Science and Technology (MoST) of Taiwan (grants 106-2119-M-001-026-MY3 and 109-2112-M-001-019). H.B.L. and M.T. are supported by MoST of Taiwan 108-2923-M-001-006-MY3 for the Taiwanese–Russian collaboration project. The Geryon cluster at the Centro de Astro-Ingenieria UC was extensively used for the calculations performed in this paper. BASAL CATA PFB-06, the Anillo ACT-86, FONDEQUIP AIC-57 and QUIMAL 130008 provided funding for several improvements to the Geryon cluster. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2016.1.00110.S and #2016.2.00168.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan) and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, auI/NRAO and NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

## Author contributions

R.D. led the ALMA proposals. H.B.L. led the JVLA proposal and processed the ALMA and JVLA data. N.C. and C.P. performed the hydrodynamics and radiative transfer simulations. R.D., H.B.L., N.C. and C.P. wrote the manuscript. All coauthors provided input to the observational proposals and/or the manuscript.

## Competing interests

The authors declare that they have no competing interests.

## Additional information

Extended data is available for this paper at https://doi.org/10.1038/s41550-021-01558-y.

Supplementary information The online version contains supplementary material available at https://doi.org/10.1038/s41550-021-01558-y.

Correspondence and requests for materials should be addressed to Ruobing Dong, Hauyu Baobab Liu, Nicolás Cuello or Christophe Pinte.

Peer review information Nature Astronomy thanks Thomas Haworth, Michael Kuffmeier and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Reprints and permissions information is available at www.nature.com/reprints.

Publisher's note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

© The Author(s), under exclusive licence to Springer Nature Limited 2022
<figure><img src="image_7.png" /></figure>

**Extended Data Fig. 1** | Integrated intensity maps of the <sup>12</sup>CO (2-1), <sup>13</sup>CO (2-1), C<sup>18</sup>O (2-1), and SO <sup>3Σ</sup> 6(5)-5(4) transitions. The beam size (0.19" × 0.18"; P.A.=88') is marked at the lower left corner. The Robust=0 weighted 224 GHz continuum image (beam size 0" × 0".047; P.A.=65') is shown in green contours at 0.13 and 1.3 mJy beam<sup>-1</sup> levels (5 and 50 × the root mean square noises) to highlight the four compact continuum sources. The ALMA/VLA source C is labeled.
<figure><img src="image_8.png" /></figure>

**Extended Data Fig. 2** | Moment-1 maps (intensity-weighted mean velocity) of the gas observations. The Robust=0 weighted 224 GHz continuum image (beam size 0.075 × 0.047; P.A.=65°) is shown in white contours at 0.13 and 1.3 mJy beam⁻¹ levels (5 and 50 × the root mean square noises). The systematic velocity is ~14 km/s (ref. ⁸¹). The ALMA/VLA source C is labeled.
<table><thead><tr><th>#</th><th>R.A.<br>(J2000)</th><th>Decl.<br>(J2000)</th><th>Peak intensity<br>(µJy beam<sup>-1</sup>)</th><th>Gaussian fitted flux<br>(µJy)</th><th>a<sup>a</sup><br>(mas)</th><th>b<sup>b</sup><br>(mas)</th><th>P.A.<sup>c</sup><br>(degree)</th></tr></thead><tbody><tr><td colspan="8">ALMA 224.247 GHz, Briggs Robust = 0 weighted (θ<sub>maj</sub> × θ<sub>min</sub>=0".078×0".046; P.A.=66°)</td></tr><tr><td>A</td><td>07<sup>h</sup>03<sup>m</sup>43<sup>s</sup>.154</td><td>-11°33'06".13</td><td>21000±410</td><td>24000±810</td><td>23±6.8</td><td>22±9.1</td><td>82±86</td></tr><tr><td>B</td><td>07<sup>h</sup>03<sup>m</sup>43<sup>s</sup>.159</td><td>-11°33'06".22</td><td>4100±62</td><td>4700±120</td><td>26±5.8</td><td>20±5.9</td><td>71±61</td></tr><tr><td>C</td><td>07<sup>h</sup>03<sup>m</sup>42<sup>s</sup>.976</td><td>-11°33'09".43</td><td>480±26</td><td>700±40</td><td>43±8.5</td><td>35±7.6</td><td>51±84</td></tr><tr><td>D</td><td>07<sup>h</sup>03<sup>m</sup>43<sup>s</sup>.191</td><td>-11°33'05".56</td><td>560±26</td><td>550±24</td><td>...</td><td>...</td><td>...</td></tr><tr><td colspan="8">JVLA 32.998 GHz, Briggs Robust = 0 weighted (θ<sub>maj</sub> × θ<sub>min</sub>=0".070×0".049; P.A.=2.5°)</td></tr><tr><td>A</td><td>07<sup>h</sup>03<sup>m</sup>43<sup>s</sup>.153</td><td>-11°33'06".14</td><td>630±28</td><td>780±57</td><td>37±7.7</td><td>18±13</td><td>80±49</td></tr><tr><td>B</td><td>07<sup>h</sup>03<sup>m</sup>43<sup>s</sup>.158</td><td>-11°33'06".22</td><td>260±11</td><td>390±29</td><td>59±9.8</td><td>33±12</td><td>38±23</td></tr><tr><td>C</td><td>07<sup>h</sup>03<sup>m</sup>42<sup>s</sup>.975</td><td>-11°33'09".46</td><td>24±5.2<sup>d</sup></td><td>27±5.2</td><td>...</td><td>...</td><td>...</td></tr></tbody></table>

**Extended Data Fig. 3** | Gaussian fitting of point-like components detected around Z CMa. Gaussian fittings may be confused by extended dust emission. While we treat the Gaussian fitted flux as the total flux and use it in dust mass estimates, the true total flux from each source may be in between the peak intensity and the Gaussian fitted flux. (a) Gaussian deconvolved major axis FWHM. (b) Gaussian deconvolved minor axis FWHM. (c) Gaussian deconvolved position angle. (d) Only detected from the Robust = 2 weighted image, with beam=100×66 mas (P.A.=19°). (e) Gaussian fitting results indicate that the source is consistent with a δ function with non-detectable size.
<figure><img src="image_9.png" /></figure>

**Extended Data Fig. 4 | Sketch of the flyby simulation geometry.** The perturber's orbital plane is in orange. The plane of the initial circumprimary disk is in blue. The primary spiral pointing towards the perturber is in solid red and is in the plane of the perturber. The secondary spiral is in dashed red and in the plane of the circumprimary disk. The line of sight is in the z direction.
<figure><img src="image_10.png" /></figure>

**Extended Data Fig. 5** | Keck/NIRC2 H-band archive data of Z CMa. The data were taken in 2005 Oct 21. White circle indicates the position of the point source C.