Salt, Hot Water, and Silicon Compounds Tracing Massive Twin Disks

Figure 1 shows the 1.3 and 3 mm continuum maps. The dust emission dominates the 1.3 mm continuum, highlighting the circumbinary disk and outflow cavities, while the 3 mm continuum reveals the jet structures. The structures seen in the 1.3 mm continuum are very similar to those in 0.85 mm continuum (Appendix Figure B1), which was first reported by Zapata et al. (2019). Three protostars are prominent at all wavelengths, namely IRAS 16547-Ea and IRAS 16547-Eb (hereafter, sources A and B) forming the proto-binary with an apparent separation of $300\,\mathrm{au}$, and a much weaker third source IRAS 16547-W. Using the 0.85 mm fluxes, which are less affected by the free–free emission than the 1.3 and 3 mm fluxes, we evaluate circumstellar disk masses of $0.19\ {M}_{\odot }$ and $0.035\ {M}_{\odot }$ around sources A and B within a radius of $0\buildrel{\prime\prime}\over{.} 05$ ($150\,{\rm{au}}$) assuming a dust temperature of $350\,{\rm{K}}$ (Appendix B).

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The proto-binary is surrounded by a circumbinary disk of $2500\,\mathrm{au}$, outflow cavities are seen on the northern and southern sides of the circumbinary disk (see also Zapata et al. 2019).

The 3 mm continuum newly reveals jet knots from source A aligned in a northwest–southeast direction, which is consistent with the orientation of the central radio source detected by centimeter observations (${\rm{P}}.{\rm{A}}.\,=\,-16^\circ$; Rodríguez et al. 2005, 2008). The resolution of the ALMA observation is an order of magnitude higher than those in the previous radio observations, which allows us to spatially resolve this central radio source into sources A and B and several jet knots, and to determine that the jet originates from source A. The jet orientation is also close to the elongated distribution of water masers (Franco-Hernández et al. 2009). The prominence of the proto-binary and jet knots in 3 mm continuum suggests that they are dominated by free–free emissions, and the jet knots may also contain significant synchrotron contributions. We leave the detailed analysis of the multi-band continuum to a forthcoming paper.

Rich molecular lines are detected in IRAS 16547, especially in Bands 6 and 7. Figure 2 shows the integrated intensity maps of representative emission lines, which trace different components in the proto-binary system from the circumbinary disk to the individual circumstellar disks (see Appendix A for the summary of the lines presented in this work). Methyl cyanide CH₃CN, which is commonly used as a disk tracer toward massive protostars (e.g., Johnston et al. 2015, 2020; Beuther et al. 2017), associates with the circumbinary disk and the outflow cavity at the 1000 au scale (panels a and b). We detect the CH₃CN $({12}_{K}$–${11}_{K})$ K-ladder from K = 0 to K = 11 with excitation temperatures from ∼60 to $\sim 600\,{\rm{K}}$. Here as representatives, K = 4 and K = 8 lines are shown as they are less contaminated from neighboring lines. The emission of sulfur dioxide SO₂, another typical hot-core molecule, with ${E}_{u}/k=403\ {\rm{K}}$, also traces the circumbinary disk and the outflow cavity (panel c). However, peaks of these lower-energy transitions of CH₃CN and SO₂ with ${E}_{u}/k\lesssim 1000\ {\rm{K}}$ do not coincide with the positions of sources A and B, due to self-absorption and/or absorption against the compact continuum sources in slightly redshifted velocities, indicating that they trace the outer cooler infalling material. This wide distribution makes it difficult to study the innermost regions of a few hundred au by these lines.

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On the other hand, the vibrationally excited transitions of SO₂, CS, and H₂O with upper-state energies of ${E}_{u}/k\,\gtrsim 1000\ {\rm{K}}$ trace the innermost region of the circumbinary disk and the individual protostellar disks (panels d–f). In particular, the H₂O ${v}_{2}=1$ emission with ${E}_{u}/k=3464\ {\rm{K}}$ is concentrated at the positions of sources A and B (panel f; the extended emission comes from contamination of other lines; see Figure 3(d)). Such a high upper-state energy reflects the high temperature of protostellar disks in massive star formation at several hundred au. With lower E_u, the SO₂ ${v}_{2}=1$ and CS v = 1 lines also trace the rotation of the circumbinary disk on the 1000 au scale (see below). We note that Zapata et al. (2019) first reported the CS v = 1 emission tracing the rotating circumbinary disk, but its connection to the individual circumstellar disks were not known.

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Furthermore, we found that the emissions of NaCl, SiO, and SiS are also concentrated in the vicinity of the protostars (panels g–i; again the extended emissions come from contamination of other lines). These lines are not detected in the 1000 au scale, although they have the low upper-state energies of ${E}_{u}/k\lt 100\,{\rm{K}}$, and their critical densities of $\sim {10}^{6}$–${10}^{7}\,{\mathrm{cm}}^{-3}$ are not high. This fact indicates that these refractory molecules are enhanced only in the innermost regions of several hundred au. It is worth noting that this is the second reported detection of the alkali metal halide, NaCl, in protostellar systems after the Orion Source I disk (Ginsburg et al. 2019; Wright et al. 2020).

Appendix Table C1 summarizes the emission lines presented in this Letter. We note that some other transitions of NaCl, SiO, SiS, and vibrationally excited H₂O are also detected, which will be reported in a future paper.

Using these lines, we can illustrate the kinematics from the circumbinary disk to the individual circumstellar disks. Figure 3(a) presents the moment 1 map of the SO₂ ${v}_{2}=1$ line, showing the rotation of the circumbinary disk as reported by Zapata et al. (2015, 2019). The systemic velocity of IRAS 16547 is about $-31\,\mathrm{km}\ {{\rm{s}}}^{-1}$ (Garay et al. 2003). The rotation direction is consistent with the elongation of the circumbinary structure. Following Zapata et al. (2015, 2019), we plot the position–velocity (PV) diagrams along the major axis of the circumbinary disk (P.A. = $50^\circ$), passing between sources A and B (panel b). The PV diagram of the SO₂ ${v}_{2}=1$ line shows a rotational profile with velocity increasing toward the center. However, inside $0\buildrel{\prime\prime}\over{.} 1$, or $300\,\mathrm{au}$, the SO₂ ${v}_{2}=1$ emission does not show the high-velocity component, which is expected for the Keplerian disk. Instead, the SiO emission nicely traces the central high-velocity components up to ${\rm{\Delta }}{v}_{\mathrm{lsr}}\simeq \pm 30\ {{\rm{km}}{\rm{s}}}^{-1}$.

Figure 3(c) shows the PV diagrams of the SO₂ ${v}_{2}=1$ and SiO emissions along a slit passing through sources A and B (P.A. $=\,-65^\circ$). This PV diagram is clearly not a simple Keplerian profile inside $0\buildrel{\prime\prime}\over{.} 1$, suggesting that the two protostars dominate the dynamics at this scale. Not only the SiO line but also the SO₂ ${v}_{2}=1$ line shows the high-velocity components (especially in redshifted velocities) associated with sources A and B. The same is also seen in the PV diagrams of the H₂O ${v}_{2}=1$ emission with P.A. $=\,-65^\circ$ (panel d), where the hot-water emission prominently shows two circumstellar components. These indicate that the rotation around the binary system is smoothly connected to the rotation around the individual protostars. The two circumstellar components are not quite parallel in the PV diagrams, judging from the different direction of velocity gradient. This suggests misalignment of the rotation directions between the twin disks (see below). We note that the contaminations by other lines are seen, extending to the southeast direction at ${v}_{\mathrm{lsr}}\simeq -20$ and $-27\ {{\rm{km}}{\rm{s}}}^{-1}$. We have not been able to identify these contamination lines. This source is rich in complex organic molecules, and a simultaneous check through the whole spectrum ranges is required for accurate line identification, which is left for future work.

We note that the blueshifted emission of SiO in source A is missing, probably due to self-absorption (see Figure C1 in Appendix C), indicating that the SiO emission traces the outflowing material. However, as opposed to the commonly seen extended SiO emissions tracing shocked regions along the outflow, here the compact morphology of SiO and its close association with the two protostars suggest that it traces the material just launched from the disks or the surface layers of the disks, which can show both rotation and outflowing motions (e.g., Hirota et al. 2017; Maud et al. 2019; Zhang et al. 2019a).

Figure 4 shows the blueshifted and redshifted emissions of selected lines of water (panel a), silicon-compound (panels b–f), and sodium chloride (panels g–h). To better resolve the kinematics of the individual circumstellar disks, for the lines in Band 6, we further improve the resolution to $\sim 0\buildrel{\prime\prime}\over{.} 035$ ($\sim 100\,\mathrm{au}$), by emphasizing data with longer baselines using a robust parameter of −0.5. For source A, the H₂O and NaCl emissions show velocity gradients in a northeast–southwest direction, which is similar to the rotation direction of the circumbinary disk (P.A. $=\,50^\circ$). Therefore, we interpret this velocity gradient as the disk rotation of source A. This orientation is also consistent with the rotating structure traced by water maser (Franco-Hernández et al. 2009) around source A. We note that this disk feature is not affected by the extended contaminations seen at ${v}_{\mathrm{lsr}}\simeq -20$ and $-27\ {{\rm{km}}{\rm{s}}}^{-1}$ in Figure 3(d), because they are significantly weakened by emphasizing the longer baselines. The disk A rotation is more difficult to identify in silicon-compounds emissions because these emissions could be blended with the outflowing motion. The velocity gradients of the v = 0 emissions of SiO and ³⁰SiO are ambiguous due to the strong absorption in the blueshifted component. For the SiO emission, comparing the redshifted emission position and the continuum peak position gives a velocity gradient direction in ${\rm{P}}.{\rm{A}}.\,\sim \,10^\circ$, which could be the outflow direction (or the direction between the outflow and the disk rotation). If this is the case, resembling the typical star formation picture, the outflow direction would be nearly perpendicular to the disk rotation, and close to parallel with the jet-knot orientation. On the other hand, the velocity gradient of the vibrationally excited ²⁹SiO line is consistent with the disk A rotation, as this line is optically thinner than the SiO v = 0 line due to its rarity and high-excitation state. For SiS, redshifted components of v = 0 and 1 lines roughly follow the same velocity gradients seen by H₂O and NaCl, suggesting the existence of SiS in the disk. However, the blueshifted component is missing in the low excitation (v = 0) map, probably due to the similar reason for SiO. The NaCl lines trace the disk components even for the lower excitations, suggesting NaCl does not exist in the outflow unlike silicon compounds.

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In source B, the velocity gradients seen in the emissions of H₂O, ³⁰SiO, ²⁹SiO, and SiS v = 0 are close to parallel to the disk A rotation, but in the opposite direction, suggesting that the circumstellar disk of source B is rotating in the opposite direction to the disk A and the circumbinary disk. The high-velocity component of the SiO v = 0 emission again shows a gradient perpendicular to the disk rotation, which may also trace the outflowing motion, similar to source A.

Based on the new high-resolution ALMA observations, we identify two groups of molecular lines tracing the innermost 100 au scale of the massive binary system IRAS 16547. The first group is the vibrationally excited "hot" lines with ${E}_{u}/k\gt 1000\ {\rm{K}}$. Especially, the H₂O line with ${E}_{u}/k\gt 3000\ {\rm{K}}$ nicely traces the individual circumstellar disks. The second group is the refractory molecules, i.e., alkali halides (NaCl) and silicon compounds (SiO and SiS) in the case of IRAS 16547. The lines of refractory species do not necessarily have high excitation of $\gt 1000\ {\rm{K}}$, but they trace only the innermost regions around the circumstellar disks. This fact indicates they are released to the gas phase within the disks on the 100 au scale. The production pathway of these gas-phase refractory molecules is probably through the destruction of dust grains, e.g., sputtering under strong shocks or radiation, and thermal desorption from grain surfaces. SiO and SiS are likely released to the gas phase in the disk and then launched to the outflow, as they are associated with both the disks and the outflows. Further multi-line high-resolution observations and their excitation analysis will be crucial to investigate chemical origin of those species.

Some previous observations have reported similar molecular lines tracing the innermost regions of massive protostellar sources. Orion Source I has been intensively studied as it is the closest massive protostar candidate at a distance of $415\,\mathrm{pc}$. All the disk-tracing molecules reported here, i.e., H₂O, SiO, SiS, and NaCl, are detected in Orion Source I's disk and wind launched from the disk (Hirota et al. 2012, 2014, 2017; Ginsburg et al. 2019; Wright et al. 2020). Moreover, emission lines of aluminum monoxide (AlO), one of the most refractory materials, are also found to be associated with the central region of Orion Source I (Tachibana et al. 2019). However, Orion Source I might not be the prototypical massive protostar because it has some peculiar features, e.g., the lack of an envelope, and the association of extremely strong SiO masers. Some claimed that Orion Source I could be an evolved star rather than a protostar (see Báez-Rubio et al. 2018). In either case, the single example could not establish these molecules as common disk probes of massive star formation.

Recently, in the B-type protostar G339.88–1.26, Zhang et al. (2019a) found that the disk and envelope can be disentangled not only by kinematics but also by chemical signatures. In particular, they found that the Keplerian disk is traced by SiO emission. Maud et al. (2018, 2019) also presented the SiO and vibrationally excited H₂O emissions tracing a Keplerian disk around the O-type protostar G17.64 + 0.16 at $\lesssim 100\,\mathrm{au}$. Moreover, the lines of sodium chloride are detected in the disk of G17.64 + 0.16 (L. Maud 2020, private communication). In addition, in both G339.88–1.26 and G17.64 + 0.16, these authors noted that the complex organic molecules and typical hot-core lines (such as CH₃CN) trace the envelopes rather than the disks, similar to our case of IRAS 16547. Consistent with these previous findings, in the case of IRAS 16547, the individual circumstellar disks on $100\,\mathrm{au}$ scale are traced by the NaCl, SiS, and vibrationally excited H₂O lines. These studies suggest that hot water, silicon compounds, and alkali halides could be commonly present in dynamical and hot massive protostellar sources, and can be used to trace the inner disk and/or the material just launched from the disk. Further systematic observations are needed to confirm the common presence of those molecular lines in massive protostellar sources. Developing from the conventional hot-core chemistry, the "hot-disk" chemistry would be an essential avenue for future research of massive star formation. This work has demonstrated the usefulness of the hot-disk lines for understanding the dynamics down to $\sim 100\,\mathrm{au}$ from the massive protostars. We also note that the lower-energy transitions of refractory molecules are excellent targets for future radio observations by the Square Kilometre Array and the Next Generation Very Large Array, which will be able to resolve the sublimation fronts of solid materials at a 10 au scale.

An additional importance of the hot-disk chemistry around protostars is its unique link to meteoritics. The oldest materials contained in primitive meteorites, i.e., Ca-Al-rich inclusions (CAIs) and chondrules, have been sublimated or molten once in the proto-solar disk. This fact suggests that at least some materials in the pre-solar nebula must be heated to $\gtrsim 1500\ {\rm{K}}$, although protoplanetary disks are typically as cool as few hundred Kelvin in planet-forming regions of several au scale (e.g., Bell et al. 2000). Therefore, how and where CAIs and chondrules formed is still a matter of debate. Further observations of hot-disk chemistry could provide important constraints on the gas-phase conditions of refractory species, and might give unique insights into the formation of high-temperature meteoritic components.

Finally, we discuss the unveiled picture of the massive proto-binary IRAS 16547, and a possible scenario of its origin (see the schematics in Figure 5). The orbital dynamics could be constrained based on the systemic-velocity difference between two sources (Zhang et al. 2019b). We find the velocity difference is as small as ${\rm{\Delta }}{v}_{\mathrm{lsr}}\lesssim 2\,\mathrm{km}\ {{\rm{s}}}^{-1}$ based on the available inner-disk tracing lines (Figure C1 in Appendix C). If the two protostars are coplanar with the circumbinary disk, and orbiting the same circular path following the Keplerian profile of the circumbinary disk (the enclosed mass of $25\ {M}_{\odot };$ the inclination of $55^\circ$ by Zapata et al. 2019), the expected velocity difference is about $4\,\mathrm{km}\ {{\rm{s}}}^{-1}$. The fact that the observed ${\rm{\Delta }}{v}_{\mathrm{lsr}}$ is smaller than the simple Keplerian velocity suggests that the protostars are gravitationally bound.

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The ionized state of surrounding environments provide hints of the evolutionary stage of massive protostars (Tanaka et al. 2016; Rosero et al. 2019; Zhang et al. 2019c). Based on the free–free fluxes at 3 mm, we estimate ionizing photon rates of $9.6\times {10}^{45}\,{{\rm{s}}}^{-1}$ and $4.3\times {10}^{45}\,{{\rm{s}}}^{-1}$ for sources A and B, respectively (Appendix B). Note that those are upper limits as we ignore the contribution from dust emission. The estimated ionizing-photon rates are several orders of magnitude lower than that of zero-age main-sequence (ZAMS) stars with $\gt 2\times {10}^{4}\ {L}_{\odot }$ (Davies et al. 2011), suggesting that the binary stars are at protostar phase with large radii of $\sim 20\ {R}_{\odot }$. The evolutionary calculations of protostars proposed that such large radii of massive protostars are the consequence of high accretion rates of $\gtrsim {10}^{-4}{M}_{\odot }\,{\mathrm{yr}}^{-1}$ (e.g., Hosokawa & Omukai 2009; Haemmerlé et al. 2016).

Although the free–free emission has been detected in IRAS 16547 at radio wavelengths (Rodríguez et al. 2005, 2008), we do not detect hydrogen recombination lines such as H40α and H42α. The non-detection of recombination lines can be explained by the line broadening with a width of $\gtrsim 100\,\mathrm{km}\ {{\rm{s}}}^{-1}$ (Appendix B), which is consistent with the presence of jets seen in the 3 mm continuum. Additionally, the northern jet knots are located with approximately equal intervals of $0\buildrel{\prime\prime}\over{.} 4$, or $1000\,\mathrm{au}$, which may be evidence of periodic accretion induced by a hidden companion or some disk instability around source A. Assuming a total mass of $20\,{M}_{\odot }$ and a jet velocity of $100\,{{\rm{km}}{\rm{s}}}^{-1}$ on the sky plane, we estimate this hidden companion should have a period of $50\,\mathrm{yr}$ and a semimajor axis of $40\,{\rm{au}}$. The proper motion of the jet knots would be detectable by follow-up observations with similar resolutions, which will provide important clues for testing the companion's presence and for understanding jet launching and precession (Rodríguez et al. 2008).

The two protostars have similar continuum fluxes and line emissions, and look coplanar with the circumbinary disk. Those features superficially link to the disk fragmentation as the origin of the binary system (e.g., Krumholz et al. 2009). A puzzling finding, however, is the tentative detection of the counter-rotating disks (Figure 4), which are difficult to form by disk fragmentation. An alternative mechanism is turbulent fragmentation at the molecular cloud-core scale (e.g., Offner et al. 2010; Bate 2012; Kuffmeier et al. 2019). Although their birthplaces may be distant, some pairs of protostars can migrate to as close as $\lesssim 100\,\mathrm{au}$, forming binary systems. The presence of turbulence leads to the random rotation of protostellar disks, which remains even after the migration (Offner et al. 2016). The turbulent fragmentation scenario would go well with the small-cluster nature of IRAS 16547 on the scale of $\lesssim 0.1\,\mathrm{pc}$, seen in the misalignments of several outflows and jets (Higuchi et al. 2015). However, considering the actual origin of binary systems could be much more complicated, e.g., the combination of both fragmentation processes (Rosen & Krumholz 2020) and the dynamical interactions with highly eccentric orbits (Saiki & Machida 2020), it is difficult to conclude the formation process based on the currently available information. We want to emphasize that the detection of the counter-rotation is still tentative, and follow-up high-resolution observations are required to conclude the disk orientations of IRAS 16547.