Herbig–Haro object

Herbig–Haro object. These are small, bright patches of nebula associated with newly born stars, formed when fast-moving collimated jets of ionized gas ejected from protostars collide with surrounding clouds of gas and dust at speeds of several hundred kilometres per second. The resulting shocks heat the material, causing it to glow brightly across the electromagnetic spectrum, from radio waves to X-rays. They are transient phenomena, lasting only a few tens of thousands of years, and serve as key signposts and probes of the energetic processes inherent in star formation.

Discovery and history

The first such objects were observed in the late 19th century by Sherburne Wesley Burnham, who noted a small patch of emission near the star T Tauri. However, their nature remained mysterious for decades. In the 1940s and 1950s, astronomers George Herbig and Guillermo Haro independently conducted systematic studies of these nebulous knots while working at the Lick Observatory and the Tonantzintla Observatory, respectively. They recognized them as a distinct class of objects not associated with reflection nebulae or planetary nebulae. The seminal work of Alfvén and von Weizsäcker on magnetohydrodynamics later provided a theoretical framework, but it was the Very Large Array and subsequent space telescope observations that confirmed their origin in protostellar jets.

Formation and characteristics

Formation begins within the dense, rotating accretion disk surrounding a young stellar object. Intense magnetic fields, coupled with the disk's rotation, channel material into powerful, opposing jets along the system's polar axes. These jets, composed primarily of molecular hydrogen and ions like S II and O III, travel at supersonic velocities. When they slam into slower-moving ambient material, such as the remnants of the progenitor molecular cloud, they create bow shocks and Mach disks where kinetic energy is converted into radiation. Characteristic emission lines, particularly from hydrogen-alpha and forbidden transitions of iron and nitrogen, dominate their spectra, distinguishing them from other nebular types.

Observational methods and studies

Early studies relied on photographic plates and spectroscopy with instruments at places like the Mount Wilson Observatory. The advent of charge-coupled devices and adaptive optics on ground-based telescopes such as the Keck Observatory and the Very Large Telescope allowed for detailed imaging of their intricate structures. Observations in the infrared with the Spitzer Space Telescope and the James Webb Space Telescope peer through obscuring dust to study their driving sources. Crucially, the Hubble Space Telescope has provided high-resolution images revealing knots, filaments, and time-variable motions, while XMM-Newton and the Chandra X-ray Observatory have detected high-energy emission from the hottest shock regions.

Notable examples

Some of the most extensively studied objects include HH 1/2 in the Orion Nebula, a pair of bright knots driven by the same invisible protostar. HH 34 in Orion exhibits a spectacular symmetric jet and bow shock structure. HH 46/47, located in the Gum Nebula, showcases a complex jet interacting with a cavity wall. HH 211, imaged by the Atacama Large Millimeter Array, is a prime example of a very young, molecular jet. The HH 212 system, also in Orion, reveals a remarkably symmetric series of knots along a jet, providing a clear laboratory for studying jet pulsation and accretion bursts.

Role in star formation

These objects are critical to understanding the dynamics of star birth. Their jets carry away excess angular momentum, allowing material from the accretion disk to spiral onto the growing protostar, a process essential for the star to gain mass. By studying their proper motion and kinematics, astronomers can trace the history of mass ejection events and accretion variability. Furthermore, the jets inject energy and momentum into the surrounding cloud, potentially triggering or quenching further star formation and influencing the overall efficiency of the star formation process within regions like the Taurus molecular cloud or the Carina Nebula.

Category:Astronomical objects Category:Star formation Category:Nebulae