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Herbig-Haro 110 in Context: A Conversation with Astronomer
Bo Reipurth from the University of Hawaii
by Joshua Sokol (STScI)


The Danish astronomer Bo Reipurth has been investigating how stars form for over three decades. In addition to authoring numerous scientific papers, he has facilitated communication between like-minded researchers by founding The Star Formation Newsletter and the Center for Star and Planet Formation.  Dr. Reipurth was gracious enough to answer my questions, enriching my appreciation of Herbig-Haro (HH) 110. Top Image: The view from the European Southern Observatory's (ESO) La Silla site in Chile on a typical, unclouded night. (ESO/Y. Beletsky)

JS: You discovered HH 110, right? How did you find it?

BR: During the late 80s I worked at ESO's observatory La Silla in Chile for several years. Although it is among the best places in the world to observe the sky, even there the weather can turn bad. So during cloudy nights when we were waiting for the weather to improve, I spent much time in the library at a light table examining the deep wide-field photographic atlases that at that time had recently been completed of the sky. While examining a little-known region in northern Orion, I came across some faint smudges that looked tantalizingly like a Herbig-Haro object. As soon as the sky cleared I took images of the object, and it turned out to be the beautiful flow we now know as HH 110.

A wider view of HH 110, also showing its progenitor jet, HH 270 . (Subaru/Bo Reipurth)


JS: What makes HH 110 special, from a scientific standpoint?

BR: HH 110 is unlike any other HH object known. For years I believed the driving source had to be hidden somewhere along the well-defined flow axis. But after failing to find the source at optical and infrared wavelengths, I realized that we needed to think “outside the box” to understand this peculiar object. And with the discovery of the nearby HH 270 that points to HH 110, and the embedded source associated with HH 270, it became clear that we are dealing with the very unusual situation of an HH flow that collides with and is deflected by a cloud.


JS: How has the landscape of HH study shifted since they were first discovered? How has knowledge about them grown — what do we know now that was once mysterious — and how has the methodology developed to enable these insights?

BR: Broadly speaking, subfields in astronomy typically go through three phases: an early phase when basic data are accumulated and various ideas are tried out, then a main phase when numerous and exhaustive investigations provide a detailed and widely accepted framework for understanding, and finally an extended period when finer details are worked out (occasionally turning the accepted wisdom on its head).

The large-scale structure of a bipolar HH jet observed in the Carina Nebula.
(NASA, ESA, and M. Livio and the Hubble 20th Anniversary Team)

Herbig-Haro objects were discovered by George Herbig and Guillermo Haro more than sixty years ago, and for thirty years a number of fundamental aspects, such as their fast motions and their shocked nature, were gradually unraveled. In the eighties, with the advent of routine infrared and millimeter observations, Herbig-Haro objects found their place in the larger picture of star formation, and most of what we know today was learned.

We are now in a phase of consolidation, where the question of the nature of Herbig-Haro objects has been replaced by intense efforts to understand the engine near the driving source that launches the jet. The angular size scale of this formation region is so small at the typical distances of many hundred parsecs for star forming regions that we cannot resolve the jet engine even with our most powerful observing techniques. So we rely on studies of Herbig-Haro objects in the vicinity of their driving sources to indirectly infer something about the jet engine through physical and mathematical models. This is difficult, but a necessity if we want to claim we understand how stars form.

JS: Are there other open problems? What else are you excited to learn about HH objects in the future?

BR: In addition to the abovementioned problem of the jet engine, perhaps the most exciting — and also most difficult — problem at the moment is whether jets rotate. Because stars rotate, and the surrounding disks (out of which planets form) rotate, we would expect that jets streaming from such star-disk systems should also rotate. But we cannot resolve the regions near the star where this effect would be strongest, and in the regions that we can resolve the expected rotational velocities are barely measurable. So this is an issue that is likely to confound us for a while. But we can already celebrate that Herbig-Haro objects have provided unique and fundamental insights into the way that stars form.