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Title: Salt, Hot Water, and Silicon Compounds Tracing Massive Twin Disks
Authors: Kei E. I. Tanaka et al.
First Author’s Institution: National Astronomical Observatory of Japan
Status: Submitted to ApJL
How Do Massive Stars Form?

This Hubble image shows N159, a nursery for massive star formation within one of the Milky Way’s satellite galaxies, the Large Magellanic Cloud. [ESA/Hubble & NASA]
While recent theoretical work and simulations support this disk accretion model, detecting the presence of such disks is not free from observational difficulties. To do so, observers seek to identify the signatures of rotating gas within these disks by using molecular emission lines at millimeter wavelengths. But high spatial resolutions are required to correctly disentangle emission from molecules in the inner disk versus those associated with surrounding gas structures, such as protostellar envelopes and outflows. The advent of interferometers, such as the Atacama Large Millimeter/Submillimeter Array (ALMA), has provided the necessary angular resolutions and led to the detections of an increasing number of disk-like structures around massive protostars. But, despite this progress, there is no consensus as to which molecular lines uniquely trace these massive circumstellar disks. Moreover, few studies have been conducted at sufficiently small spatial scales to directly probe the structure of these disks.
In today’s astrobite, we take a look at new high spatial resolution observations of massive protostellar object IRAS 16547-4247, which reveal the presence of two rotating massive disks and identify a potentially universal “hot-disk” chemistry found in the innermost disks around massive protostars.
Massive Twin Disks in IRAS 16547-4247
Today’s authors used ALMA to observe the massive protostellar system IRAS 16547-4247, which is located over 9,000 light-years from Earth. Previous radio observations revealed the presence of jets and indicated that accretion is currently ongoing in the vicinity of the protostar. IRAS 16547 is also known to be a binary system, comprised of two compact dusty objects with a separation of 300 au, surrounded by a larger, rotating circumbinary disk. By observing IRAS 16547 at a resolution of only a few hundreds of au, today’s authors are able to investigate the gas dynamics of both massive protostellar disks in detail.
Figure 1 shows the continuum images of IRAS 16547 taken with ALMA. Emission from dust dominates the 1.3-mm continuum, highlighting the circumbinary disk and outflow cavities, while the 3-mm continuum reveals the jet structures. Three individual protostars are seen at both wavelengths: IRAS 16547-Ea (source A) and IRAS 16547-Eb (source B), which form the protobinary, and a weaker third source IRAS 16547-W. The protobinary comprised of sources A and B is surrounded by a circumbinary disk, while outflow cavities are located to the north and south.

Figure 1: An image of the 1.3-mm (color scale and grey contours) and 3-mm (cyan contours) continuum toward IRAS 16547. The cyan and grey circles in the lower left indicates the resolutions of the observations, while the scale bar shows a physical distance of 1,000 au, or about 20x the size of our solar system. [Adapted from Tanaka et al. 2020]

Figure 2: Map of total emission detected from various emission lines (color scale and black contours) overlaid with the 1.3-mm continuum emission (grey contours). Molecule names, transitions, and integrated velocity ranges are show in the upper left of each panel. Red crosses indicate the continuum peaks of sources A and B. The black circle in the lower left indicates the resolution of the radio observations. [Adapted from Tanaka et al. 2020]
Inner Disk Tracers: Hot Water and Salt
Figure 3 shows the velocity structure of selected lines that trace the rotation of the individual disks. In source B, the velocity gradients are close to parallel to disk A’s rotation, but lie in the opposite direction, suggesting that the circumstellar disk of source B is rotating in the opposite direction as disk A and the circumbinary disk.

Figure 3: Map of the velocity structure of selected molecular lines that trace the inner disks (blue and red contours) overlaid on the 1.3-mm continuum emission (grey scale and black contours). Molecule names, transitions, and integrated velocity ranges are shown in the upper left of each panel. Stars mark the continuum peaks of sources A and B. Cyan lines indicate the orientations of disk rotation (panels a, and c–e), and yellow lines show the outflows (panel b). The white circle in the lower left indicates the resolution of the radio observations. [Adapted from Tanaka et al. 2020]

Figure 4: Schematic view of the massive protobinary in IRAS 16547. The central twin disks are seen in high-energy H2O transitions (“hot H2O”), as well as NaCl and silicon-compound lines that are produced by the destruction of dust grains. The circumbinary disk, dusty outflow cavity, and jet knots are indicated. The blue and red colors show the rotation of the gas in the disks. [Adapted from Tanaka et al. 2020]
Implications of “Hot-disk” Chemistry
These results suggest that hot water, silicon compounds, and salts may be common in hot massive protostellar sources and serve as valuable tracers of inner disk material. The presence of this “hot-disk” chemistry provides a promising path for future studies of massive star formation.
In addition, hot-disk chemistry has an important link to meteoritics in our solar system. The oldest materials contained in primitive meteorites are those associated with Ca-Al-rich inclusions (CAIs), which were either sublimated or molten at some point in our protosolar disk. This means that the presolar nebula had to be heated to at least 1500 K, which is in apparent contrast with the low temperatures of a few hundred Kelvin typically associated with protoplanetary disks. Thus, it is still unclear how and where CAIs formed. Further observations of hot-disk chemistry may provide important constraints on gas-phase conditions of refractory molecules and provide insight into the formation of high-temperature meteoritic components.
About the author, Charles Law:
Hi! I’m a third-year graduate student at Harvard/CfA. I study chemical complexity in protoplanetary disks and star-forming regions using telescopes such as the SMA, VLA, and ALMA. In my free time, I enjoy hiking, bicycling, and traveling.