The physical nature of HH objects, the scientific significance of HH objects

A typical and complete HH object is a jet structure composed of many knots arranged in dense clusters or arranged in a straight line, and this jet structure actually reflects the essence of the HH object: it is a kind of ejecta from a young star, to be precise, a gas mass that is excited when the high-velocity gas ejected by the young star rushes into the peristellar gas. When the high-speed star wind blows into the peristellar gas from the poles of the young star, it will form a shock wave, and in the area that gradually cools down after the shock wave, under the conditions of suitable temperature (TE 7000K) and density (Ne n 103-N 104cm-3), some gas clumps will excite some special spectral lines, and the optical bands are mainly some collision excitation lines, thus forming an optically visible HH object with specific excitation lines.

Currently, the widely accepted definition of an HH object is: a small-scale shock wave excitation region closely associated with the star-forming region, which usually has some specific spectrum to distinguish it from the photoionization region (Reipurth, 1999).

HH objects are currently thought to be stars in the process of forming, or protostellar blanks.

The HH object (Herbig-Haro object) is a semi-star-semi-cloud-like luminous visible object that appears in the star-forming region, and it is a nebula-like object formed by newborn stars in the universe. The newly born star is constantly spewing gas at a speed of nearly hundreds of kilometers per second, and these gases collide violently with the gas and dust clouds around the star, producing light. HH objects are ubiquitous in the star-forming region, and multiple HH objects are often seen in a row near the polar axis of a single nascent star.

During the first few hundred thousand years of the birth of a new star, it was usually surrounded by an accretion disk formed by a mass of gaseous matter; The substance on the inner side of the accretion disk is ionized due to the energy of high-speed rotation, and the resulting plasma is ejected on the vertical plane of the accretion disk, which is called pole jetting; When these ionized materials collide with gases in interstellar space at high speeds, generating shock waves and bright radiation, they become the Herbig-Harrow objects we observe. Protostars exist inside it.

HH objects are fairly short-lived astronomical phenomena that do not last more than thousands of years. As the gas continues to diverge into the interstellar matter, the HH object becomes obscure.

HH objects are produced in the early stages of star formation, i.e., the bipolar jet phase of young stars, and most HH objects appear with very short time scales, so HH objects become a direct and accurate tracer of the star-forming activity being experienced. It is of great significance to study the morphology, structure, spectral lines, kinematics and large-scale distribution of HH objects to reveal the formation of single stars, the mechanism of jets, the characteristics of star formation in small regions, and even the law of star formation at large scales.

HH is a kind of half-star, half-cloud-like, optically visible object that occurs in the star-forming region. "Half-star, half-cloud" is due to the fact that many HH objects appear to look like stars on the negatives, but their half-width is at least twice as large as that of a real star, and they are often surrounded by cloud-like structures that make them appear somewhat looming. HH objects exhibit a variety of morphological characteristics:

Some are like the knots of a tree; Some are bow-shaped; Some are in the shape of short rods; Some have bright heads and diffuse tails like comets; Others are like a small scattered nebula. Although the scale of the HH objects themselves varies, the scale of the jets of the HH objects composed of them can reach a few parsecs (PC) (Bally & Devine 1997).

Astronomical spectroscopy observations estimate that the HH object is moving away from the jet parent star at a high speed of 100 to 1,000 kilometers per second. In recent years, continuous observations by the Harper Space Telescope have clearly captured high-resolution images of the self-motion of HH objects. By analyzing these images using the parallax method, we can determine the distance between these buried HH objects and the Earth.

Liquid Bu. As matter moves away from the jet source, HH objects that enter interstellar matter slowly change in appearance and morphology over the course of several years; Some clumps in the jet stream may increase or decrease in brightness, or dissipate completely; There may also be new clumps. The difference in velocity of jet matter may also cause changes in the appearance of HH objects.

Instead of ejecting matter continuously and steadily, the jet parent star releases gas and dust into the universe in the same direction in a pulse. The velocity of each jet pulse may vary and cause the jet material to collide with each other, creating a shock wave on the surface of the clump.

HH celestial bodies show a variety of morphological characteristics: some resemble the knots of trees; Some are bow-shaped; Some are in the shape of short rods; Some have bright heads, annihilation parts, and diffuse tails, like comets; Others are like a small cluster of diffuse nebulae. Although the scale of the HH objects themselves varies, the scale of the jets of the HH objects composed of them can reach a few parsecs (pc) (Bally&Devine 1997).