A clash of disks: the traumatic life of the HV-DO Tau system

This guest post was written by Arianna Musso Barcucci. Arianna is a 3rd year PhD student in the Planet and Star Formation department at Max Planck Institute for Astronomy in Heidelberg, Germany. She works on direct imaging of extrasolar planets and their environments, with a particular interest in the early stages of planetary formation and evolution. When she is not hunting for planets, she enjoys reading, hiking and painting.

Title: Evidence of a past disc-disc encounter: HV and DO Tau
Authors: Andrew J. Winter, Richard A. Booth, Cathie J. Clarke
First Author’s Institution: Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB30HA, UK
Status: open access on arXiv

An extended family of stars

Astronomy 101 tells us that stars are born in molecular clouds. While this statement is certainly true, many details of how exactly this happens are still nebulous. For example, hierarchical formation theory suggests that stars prefer to be born in big families, comprising of several stars. These young multiple systems are often unstable, and over time they tend to gravitationally interact and disperse, losing all evidence of their common past.

However, if some of the family members were surrounded by protoplanetary disks at the time of this dynamical restructuring, it is possible that some evidence of their past got imprinted in the disks. Finding disks with such ’fingerprints’ could help us cast light on the initial phases of stellar formation, and could support the hierarchical formation theory.

The peculiar configuration of the HV-DO Tau system

The authors of today’s paper believe to have found a system that shows evidence of a past as part of a higher multiplicity system!

As shown in Figure 1, on the left we have HV Tau, a triple system composed of the very close binary AB (10 au of separation), and the additional third component C at a distance of ~550 au. On the other side of Figure 1, more than 10,000 au away, is the solitary star DO Tau. The authors used Herschel observations of this system at three different wavelengths (70, 100 and 160 microns) to study its very peculiar disks.

Figure 1. Schematic configuration of the HV-DO Tau system as shown in Figure 2 in the paper. The tight AB binary is shown (and treated) as a single component.

Not only do both HV Tau-C and DO Tau possess a protoplanetary disk (of 50 au and 75 au in radius, respectively), but the two disks also show a very elongated structure, arching towards each other, creating a bridge spanning more than 12,000 au!

Moreover, the disk size does not depend on the wavelength of the observation, suggesting a traumatic event in its past that somehow truncated the disks (in an unperturbed case, particles of different size would be blown away at different distances, and the disk size would depend on the wavelength).

Such a remarkable system immediately raises some questions: were these four stars once part of a single stellar family? Is the peculiar disk structure the last surviving evidence of this long-forgotten union? And if that is the case, what exactly happened in the past, and how much did the single components change in the process?

Simulate it until you make it!

The only way to answer these questions is to have a “sneak peak” into the past of these stars, and the only way to achieve that (except for a time machine), is a detailed simulation of the system.

To reduce the computational time, the authors limited the range of initial parameters by placing constraints based on the observations. Even though this is a reasonable approach, it prevents them from exploring all possible solutions, and they caution that the “best-fitting model” should be treated simply as one possible scenario.

In the simulations, they considered the HV Tau-AB binary as a single object, and assigned the same mass of 0.7M to each component of the triple system. They required DO and HV to be initially bound, with a closest possible interaction of 50 au (since any closer encounter would have disrupted the disks almost completely, and nothing would be visible today). In addition, after the encounter, the DO component must be unbound or have an extremely wide orbit in order to match the present day configuration. Similarly, every final configuration must see the C and AB components bound to each other (as they are today).

The hydrodynamical simulation of the disk-disk interaction is described by three main parameters: the temperature of the disk (which is a function of the distance to the nearest star), the mass content of the disk as a function of distance from its primary star (treated as a power law), and the relative mass between the two disks (which was allowed to vary during the simulations). For every simulation they took a snapshot after 40,000 years (this being a limit of the simulation itself, rather than physically justified).

The simulations that best resembles the actual observations is shown in Figure 2, where the authors made a one-to-one comparison between the observations (on the left) and the best model (on the right) for two different wavelengths, showing how well the simulation manages to reproduce the actual data. As explained above, the parameter space explored during these simulations was limited to reduce the total computational time. This means that the best-fitting model is the ’best one’ only relatively to this restrained parameter space. Nevertheless, its initial parameters can help us unveil the history of this peculiar system. As it turned out, HV-DO Tau hides a traumatic event in its past.

Figure 2. Left. Herschel observations in the far-infrared at 100μm (upper panel) and 160μm (lower panel). Right. Best simulation shown for the same wavelengths. The overall configuration of the stellar components is clearly reproduced, as well as the disks structure and orientation. Figure 7 in the paper.

The life story of HV-DO Tau

The authors concluded that the most likely scenario is the following: sometime in the past, the four stars were indeed part of a single quadruple system, with DO Tau being on a highly eccentric orbit. Around 100,000 years ago, DO tau came too close to the other three stars, and in particular to HV Tau-C. The encounter was so close that the two stars likely penetrated each others disks! Given what we know of the disk inclinations, it is reasonable to assume that they were perpendicular to each other, leading to what must have been a spectacular clash!

The consequences were quite dramatic: due to the encounter, the initial quadrupole system lost much of its angular momentum (the ’glue’ that was keeping it together), resulting in DO Tau being ejected (or at least, sent far far away) to its present distance of more than 10,000 au. During this separation, both DO Tau and HV Tau C tried to hold on to their disks, which were being dragged away by the opposite component, resulting in the quite spectacular disk bridge observed today.

The disks’ collision and subsequent restructuring of the entire system happened roughly 100,000 years ago and is still visible today! This is thanks to the low density of stars in the Taurus association, which means that no further encounters happened to erase this evidence. The authors suggest that similar encounters are not uncommon in the past of many stars, but in most of the cases all the evidence is lost.

In this sense, the HV-DO Tau system is a lucky find. Not only does it have a spectacular history, but it also shows that the theory of hierarchical star formation is a possible and plausible explanation!

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