I remember hearing about Herbig-Haro objects from my early years at grad school, working around many astronomers interested in star formation and stars in the early stages of their lives. Herbig-Haro objects are impressive jets emanating from stars in formation. The material in these jets propagates in opposite directions and along a straight line from the central star.
Citation: ESA/Webb, NASA, CSA, Tom Ray (Dublin)
The name Herbig-Haro, or HH objects, is given after the two astronomers George Herbig and Guillermo Haro, who first observed them in the Orion Nebula during the 1940s. Both astronomers were not interested in these objects but rather in the central stars from which these originate. These stars are called T-Tauri stars[1] -- actually, these stars are in the late stages of formation that would eventually evolve to form a star like our sun. In that stage, their light is not produced by the fusion of hydrogen in the core but rather by a magnetic flux that channels ionized gas (hot gas where atoms have lost electrons) from the disk onto the star’s surface. The complex interaction between the magnetic field, the corona, and the disk in the T-Tauri stars is likely responsible for the observed variability in these objects, for the production of the jets, and ultimately for the final mass of the resulting star. This complex interaction continues even after hydrogen is ignited in the center. At this point, it becomes a Class 0 star but is still considered a protostar because it is still gaining mass.
Caption: Steve Bowers
The study of the HH objects concentrates on explaining how these are formed, why these are so aligned, and why the material seems to be ejected in periodic bursts. All studies agree that the presence of the magnetic fields makes these jets to be aligned or collimated in a straight line. The explanation for the periodicity of the burst events varies, but all include magnetic fields. One of these studies attributes the periodicity to the pinching of the magnetic field lines and to the differential rotation between the star and the disk, which results in the growth of toroidal field pressure[2][3]. This process leads to the inflation of the field lines attached close to the internal edge of the disk, which will eventually reconnect and produce plasmoids that propagate ballistically outward[4].-- which facilitates the formation of the collimated flows. In all cases, the accelerated particles from the stellar wind and magnetized jets heat the molecular and atomic material, making them observable in the visible, infrared, and radio frequencies. Each type of light provides different information about these objects and sheds some light on how stars form.
Credito: Entstehung von Herbig-Haro-Objekten
Here, we look at two HH objects observed by the James Webb Space Telescope (JWST). Probably the most spectacular of this pair is the image of Herbig-Haro 211, which shows amazing details of the jets emanating from the central star[5]. These are not a single stream of gas but periodic bursts that astronomers try to explain with magnetic fields. The star giving rise to this object is known as a Class 0 protostar, about 3.5 times the sun’s luminosity but very cold, just 33 K, compared to the 5500 K of the Sun, and surrounded by ∼0.8 solar masses in dust.[6]
We also need telescopes like James Webb to peer inside the jets in HH 211 and see their structure. This is because the light of this jet is produced by hydrogen, carbon monoxide, and silicon monoxide molecules, which are heated by the ejected material colliding with the ambient gas or slower-moving gas. Because the velocities of the gas in the jet are not energetic enough, these molecules cannot be broken into simpler atoms and ions but heated enough to emit the light we can observe with Webb.
Citation: ESA/Webb, NASA, CSA, Tom Ray (Dublin)
The detailed image taken by Web also reveals a series of bow shocks, moving away from the central star and produced by the periodic ejection events. The separation of the bright and dark regions in these jets provides information to confirm the proposed physical mechanisms that give rise to intermittent bursts of material from the central protostar. From this image, astronomers concluded that the velocities of the innermost outflow structures are roughly 48-60 miles per second (80 to 100 kilometers per second), where the bow shock structures are produced by the slower material these jets encounter along their path. The three bow shocks observed might have a dynamical age of about ∼1000 years, with outflow events producing the observed shocks at 425 and 290 years[6]. In this image of Webb, we can also see thin silicon monoxide bipolar jets with a knotted structure inside a cavity of hydrogen and CO, which is close to the star and appears as a small blue-hue nebulosity. With these observations, astronomers also found that the outflow is relatively slow compared to more evolved protostars with similar jets.
Herbig-Haro 46/47, at 1,470 light-years from us in the Vela Constellation, is another of the objects observed by JWST[7]. As its name indicates, the jets are powered by two protostars. In this image, the protostars shine bright in the center, but part of their light is buried deep in the disk of gas and dust, which provides more material and helps them to gain more mass. The disk is not visible, but its shadow can be seen in the two dark conical regions surrounding the central stars.
Credits: NASA, ESA, CSA
In this case, the Herbig-Haro objects are produced by two T-Tauri stars, making the pattern of the jets more complex. The first difference we notice when compared with other single-star HH objects is that the jet bifurcates in two strands [8], which can be clearly seen in the jet with direction to the lower left of the image. This is not what we observe in the right-side jet. It shows two bow shocks, one with a lot of material glowing behind the crest, while the other is just a faint red nebulosity in the image. The jet at the left-hand side does not show clear bow shocks, and several possible explanations have been proposed. For example, a substantial bend of one of the jets exciting shockwaves within and in the surrounding medium, or the combination of a shock wave produced by the material ejected and another by the pressure difference in the surrounding gas [8]. One contributing factor to the difference between the jet to the right and the jet to the left is that the jet to the left is within what we call the Bok globule, which in the image appears as a very dim cloud-like elliptical structure, offset by 30 degrees from the line of the jet. This globule is a cloud of dense dust and gas that is just a dark region when viewed in visible light [9]. Because the left-side jets are within this globule, we need telescopes like James Webb to peer inside and see the structure of the jet with great detail. With Webb, we also see the stars and galaxies that lie well beyond it. [7]. In this image, the more recent ejection appears in the right-hand side jet as a faint blue line parallel to the diffraction spike at 2 o’clock. One of the oldest ejections is barely seen as a red bow shock at the top right corner of the image.
Credit:ESO/Bo Reipurth
As we can see, both of these objects are spectacularly full of details that help to shed light on when and how the ejected material is produced and how these relate to star formation.
References:
[1] https://www.orionsarm.com/eg-article/47a1353481fb2
[2] C. Fendt 2009 ApJ 692 346
[3] AP. Goodson & R.M. Winglee 1999, The Astrophysical Journal, 524,159
[4] G. Pantolmos, C. Zanni and J. Bouvier, A&A 643, A129 (2020)
[5] https://webbtelescope.org/contents/media/images/2023/141/01H9NWH9JEBFPKVD3M1RRTGGQJ
[6] https://www.aanda.org/articles/aa/pdf/2005/07/aa1821.pdf
[7] https://www.nasa.gov/universe/webb-snaps-highly-detailed-infrared-image-of-actively-forming-stars/
[8] https://articles.adsabs.harvard.edu/pdf/1991A%26A...246..511R
[9] GroundBased ops https://www.eso.org/public/images/eso1336c/