One of the more complex areas of study in stellar evolution focuses on the young stars that form within the various regions of our universe. These newborn stars are created out of the gas and dust of giant molecular clouds in the interstellar medium, formed in a process known as contraction, like those within the star-forming region known as R Corona Australis, seen at left. This protostellar phase is an early stage in the process of star formation and for a solar-mass star, it lasts about 100,000 years. It is within this time frame and beyond that the short-lived Herbig-Haro objects are created, forming from individual stars that begin to develop within the stellar gas and dust clouds, sometimes ejecting matter in the form of jets or protoplanetary disks. At right is one such object in the constellation of Taurus. "This view of a protostellar object called HH-30 reveals an edge-on disk of dust encircling a newly forming star. Light from the forming star illuminates the top and bottom surfaces of the disk, making them visible, while the star itself is hidden behind the densest parts of the disk.The reddish jet emanates from the inner region of the disk, and possibly directly from the star itself. Hubble's detailed view shows, for the first time, that the jet expands for several billion miles from the star, but then stays confined to a narrow beam. The protostar is 450 light-years away in the constellation Taurus." Image of R Corona Australis: European Space Agency's XMM-Newton X-ray observatory. Image of HH-30: Hubblesite C. Burrows (STScI & ESA), the WFPC 2 Investigation Definition Team, and NASA

Herbig-Haro 5B & 6 discovered by Herbig 1974 ( see J. Bally, D. Devine & B. Reipurth 1996 )

Image (R-band) of Haro 6-5B. This image is one of several obtained by the Wide Field and Planetary Camera 2 Investigation Definition Team's research around the star and nebulosity of FS Tauri A, conducted using the "Hubble Space Telescope (HST) to image circumstellar material around young stellar objects"   [9]

The Hubble Space Telescope, as well as others, have greatly increased our understanding of a wide range of stellar objects and those of the Herbig-Haro class are no exception. Yet, our knowledge of the origins and composition of Herbig-Haro jets goes back further, a direct results of numerous studies started in the late 1970's, e.g. — Schwartz, 1975; Raymond, 1979; Herbig and Jones 1981 & Herbig and Jones 1983; Finkenzeller, U and Mundt, 1984; Storm et al., 1986. These surveys have directly contributed to the understanding that Hebig-Haro objects are shock-waves, occurring under a wide range of enviromental conditions, including their occurrence in HII regions like the Orion Nebula ( Guido Münch, 1977; Axon and Taylor, 1984 ) and which eventually lead to the acceptance that they develope from the bipolar outflows of young stars and protostars ( see Reipurth, 2000 ).   [10]

Herbig-Haro 111 Shock Fronts, originally found by Bo Reipurth with the Very Large Array (VLA) ( see A. C. Raga et al. 2002 )

Image far Left: In their paper Kinematics of the HH 111 jet from STIS spectra ( Reipurth & Raga, 2002 ) the research team conducted a long-slit spectrum of the Herbig-Haro object HH 111, obtained with STIS on the Hubble Space Telescope — the area of the spectrograph's data collecting slit is marked in gray lines running top to bottom. They found that knots along the jet ( H, F in image ) exhibit a sudden drop in their radial velocity (modulus) thus confirming that the emission from the knots is formed in shocks. Though noting the need for further study of their conclusions, the research team found "that at the positions of knots F and H, sharp drops in the radial velocity are present. These sharp drops in velocity at the positions of the knots are in qualitative agreement with what one would expect from an interpretation of the knots as "internal working surfaces"* resulting from an ejection velocity time-variability (see, e.g., Raga et al. 1990; de Gouveia dal Pino & Benz 1994).   *See section on Herbig-Haro 34 for an explaination of "working surface"

We find that the regions in between the knots have somewhat more confusing kinematic characteristics. Part of the inter-knot region kinematics can indeed be interpreted in terms of a ballistic jet from a variable source, but other parts clearly do not correspond to material following a ballistic motion."

Image at left, HH111 from APOD. "The complex interactions of three stars in the Orion B molecular cloud complex have resulted in the ejection of particles along a 12 light-year long jet. One of the stars in the HH111 system has apparently also been ejected leaving two stars tightly bound in a binary orbit. The spectacular jet emanates from one of these two stars. The complete system is quite complex and involves another less-luminous jet and disks of rapidly rotation gas. In the top part of this Hubble Space Telescope image taken in visible light, successive bubbles are seen to move along the jet. At the bottom in infrared light, a torus of dark dust and gas from which the stars formed millions of years ago can be seen. Protons and electrons zip along near 500 kilometers per second in the HH111 jet, which is about 1500 light-years away." Credit: Bo Reipurth (CASA/U. Colorado) et al., HST, NASA.


As noted by Bo Reipurth, John Bally and Jon Morse, "The ultimate goal of research on jets is to probe the nature of physical processes that operate within several AU of a young star, to learn about the potential existence of physical conditions that might lead to the formation of planets, and to understand how stars and planetary systems form." [12]   This goal was the result of ongoing surveys of T Tauri stars, began in earnest shortly after George Herbig met Guillermo Haro at an astronomy conference in Tucson, Arizona in which the objects were discussed.

Images Aladin Previewer: (l) HH111 ( RGB, SERC & POSSII ). (r) Also located in this region is HH110j just northwest of HH111 ( north is up in the images ).

But as we shall see, Herbig-Haro objects have recently thrown science a few curves and surprises, leading to the conclusion that these objects have a mind of their own which seems to defy current thinking as to how they should behave.

Herbig-Haro 49 & 50, first noted by Schwartz ( see W. J. Zealey et al. 1998 )

One of the more recent and dramatic images of HH objects appeared on the APOD website on August 11, 2007 and it gives an insight into the turbulent forces within these objects. "Light-years in length, this cosmic tornado is actually a powerful jet cataloged as HH (Herbig-Haro) 49/50 blasting down from the top of a Spitzer Space Telescope view. Though such energetic outflows are well known to be associated with the formation of young stars, the exact cause of the spiraling structures apparent in this case is still mysterious. The embryonic star responsible for the 100-kilometer per second jet is located just off the top of the picture, while the bright star seen near the tip of the jet may just by chance lie along the line of sight. In the false-color infrared image, the tornado glows with infrared light generated as the outflow heats surrounding dust clouds. The color coding shows a trend from red to blue hues at the tornado's tip indicating a systematic increase in emission at shorter wavelengths. The trend is thought to indicate an increase in molecular excitation closer to where the head of the jet is impacting interstellar gas. HH49/50 is about 450 light-years distant, located in the Chamaeleon I molecular cloud." Credit: J. Bally (Univ. of Colorado) et al., JPL-Caltech, NASA, image from APOD

But it seems that the more we discover about these objects, the less we seem to know. In commenting on HH49/50's shape and apparent composition Giovanni Fazio, a physicist at the Harvard-Smithsonian Center for Astrophysics said, "I've never seen anything quite like this one...We were really quite stunned by it," and in a news article about HH49/50 msnbc.com's Technology & Space website noted that:

"Scientists aren't sure what is behind HH 49/50's unique spiral shape. One hypothesis is that magnetic fields in the region are somehow twisting the particle jets; another idea is that the shock waves are creating eddies in the dust clouds which then glow and become visible."

In their 2006 article A "Tornado" in Space the thunderbolts.info website also noted this, adding "In fact, astronomers express great astonishment at such formations. Gravitational models featured in twentieth century astronomy never envisioned narrow jets of anything streaming away from stellar bodies. Neither gravity nor standard gas laws would allow it." These are not the only sources pointing to some very big unknowns in regard to Herbig-Haro objects:

• Böhm, Karl-Heinz The Observational Study of Herbig-Haro Shock Waves from ADS, 1995
• Goodman, Alyssa A.; Arce, Héctor G. PV Cephei: Young Star Caught Speeding? from ADS, 2004
• Bacciotti et al., Indications for Rotation in DG Tau Jet The Astrophysical Journal, 576:222-231, 2002 September 1
• René Liseau et el. Physics of Outflows: the Binary Protostar L 1551 IRS 5 and its Jets arXiv:astro-ph/0410405v1 18 Oct 2004

On the topic of jets and outflows, the last of these papers ( René Liseau et el. ) noted that "[s]ince their discovery nearly three decades ago, the unexpected phenomenon of outflows in star forming regions has remained essentially unexplained. In particular, the processes responsible for the acceleration and collimation of the flows present one of the major unsolved problems of modern astrophysics". However, the researchers were able to state that with recent observational results now in hand, detailed comparison with theoretical models could be had. While HH 49, 50 and 111 have all seen intense study into the struture of their respective jets extending behind the shock front ( where, in the cooling gas, electrons and ions recombine to give an emission line spectrum that is characteristic of Herbig-Haro objects ), so too has HH34 and it's bipolar outflowing jets.

Herbig-Haro 34, first noted by Haro ( see Reipurth et el. 2001 )

One of the interesting characteristics of HH34 ( and of many others for that matter ) is in the deceleration of the shocks within the bipolar jet's outflow. In Dal Pino, 2000 it was noted,

"The deceleration occurs as momentum is transferred by gas expelled sideways from the traveling pulses. The simulation of a pressure-confined, steady state jet with similar initial conditions to those of the pulsed jet shows that the flow in this case experiences acceleration. This result is thus an additional indication that the primary source of deceleration in the giant flows cannot be attributed to braking of the jet head against the external medium." In other words, the observed slowing within the jet is a mechanism within the jet itself rather than an external slowing due to collisions with the surrounding interstellar medium: "the symmetric pattern of deceleration in both lobes of the jet indicates that the process is rather internal to the flow. Our three-dimensional simulations show, in fact, that deceleration can be naturally produced in jets formed by long-period, large-amplitude velocity variations, where momentum is progressively lost from the central axis and transferred sideways by the traveling pulses." This was one of the jet's unknowns — that radial velocity models did not account for certain observed deceleration within the jet itself. [13]   Also see: A. Riera et al. The high-velocity outflow in the proto-planetary nebula Hen 3-1475; R. López et al. 3-D kinematics of the HH 110 jet

Two opposite jets ram into the surrounding interstellar matter forming HH34, another object with some very distinct characteristics and unknowns. "Some features of HH-34 are understood -- some are not. At the core of Herbig-Haro 34 lies a seemingly typical young star. This star, though, somehow ejects energetic "bullets" of high-energy particles, appearing as red streaks toward the lower right of the this image. Astronomers speculate that a burst of these particles might rebound when gas from a disk surrounding the star momentarily collapses onto the star. Visible near the end of each light-year long jet is a glowing cap. HH-34 lies about 1500 light-years away in the Orion Nebula star-forming region. The cause of the large arc of gas on the upper left known as the waterfall remains unexplained." Image from APOD, credit: FORS Team, 8.2-meter VLT, ESO.

At right is a drawing created by Gregory Maxwell, illustrating the basic mechanics of HH objects. The images shows material being ejected ( the jet ) by a nearby young star ( protostar ) and colliding at supersonic speeds with the interstellar medium ( ISM ). The resulting shock waves of fast moving and heated gases responsible for generating the visible light we observed as the HH object.

On the physics of Herbig Haro Shocks and "working surface", the Jetset website explains,

"As the collimated protostellar jet interacts with the undisturbed ambient medium, it generates what is known as a "working surface" that the observer then sees as an HH object. The most violent interaction occurs at the initial point of contact between the jet and the ambient medium resulting in a terminal working surface, which is often bow-shaped (for example HH 34S shown [left]). Internal working surfaces trace milder internal collisions within the outflow itself. Each HH object or knot represents a major event of mass ejection and it is therefore likely that these internal shocks occur when fast moving gas from a fresh ejection episode catches up with slower moving material ejected earlier. Hence the archetypal HH jet mentioned above is traced by a series of knots. This is the exact morphology of the HH 34 jet as imaged in [SII] [not shown]. Indeed several of the knots in this flow appear to be bow-shaped, consistent with the idea that they are internal working surfaces." The image at left is a close-up from the larger image, Protostar HH-34 in Orion from the ESO website showing HH34S. The image on right is from a publication ( A. C. Raga et al., 2002 ) discussing the different approaches that have been used for modeling the emission of Herbig-Haro objects, in this case, Herbig-Haro34. The "[s]chematic diagram showing the filamentary structure observed in H HST images of HH 34S and the proper motions derived by Reipurth et al. (2002). The x-axis approximately coincides with the outflow axis. We have chosen 5 of the cross-correlation boxes of Reipurth et al. (2002), which lie along the edge of the bowshock and which include filamentary structures with well defined orientations (the boxes are numbered 6, 13, 18, 19, and 21).   [14]

Herbig-Haro 30 & XZ Tauri