赫比格-哈罗天体高分辨观测研究进展

赫比格－哈罗天体（ＨＨ天体）包含了有关原恒星吸积和抛射过程的许多重要信息,ＨＨ天体高分辨观测研究取得了一系列新进展：分辨出激波峰面、马赫盘和辐射冷却区;分辨出喷流节点的结构,发现它们大多是内工作面,而不是由Ｋｅｌｖｉｎ－Ｈｅｌｍｈｏｌｔｚ不稳定性所产生的斜激波;发现喷流宽度随到激发源距离的减小仅缓慢减小,对喷流的准直和加速模型提供了限制条件;ＨＨ天体在小尺度上尚有复杂的激发结构。对这些进展进行了评     

Herbig-Haro jets from time-dependent sources

A considerable amount of effort has been made towards obtaining a theoretical understanding of the collimated, optically detected outflows (Herbig-Haro objects) ejected by young stars. The most clear results have been obtained for the case of the Herbig-Haro jets, a loosely defined category which groups the Herbig-Haro (HH) objects with jet-like structures of aligned knots. In particular, it has recently been shown that at least some of the characteristics of the HH jets can be straightforwardly explained in terms of models of jets from variable sources. This paper presents a review of the properties of models of jet flows from sources with a variability in the ejection velocity, in the ejection direction, and with a general velocity+direction variability. Also, a comparison between the observational characteristics of HH jets and the predictions from variable source jet models is carried out.     

The emergence of a neutral Herbig–Haro jet into a photoionized nebula

Recent observations show the existence of an increasing number of collimated outflows ejected by young, low-mass stars which are embedded in H  ii regions. At distances of a few tens of au from the star, at least one lobe of these outflows will be shielded from the ambient ionizing radiation by the compact, high-extinction circumstellar disc. Within these shielded regions, the jets are probably mostly neutral, similar to the jets in 'normal' Herbig–Haro (HH) objects. At larger distances, these jets emerge into the photoionized nebula, and start to be photoionized by the radiation from the ionizing photon source of the nebula.
In this paper, we model the photoionization of an initially neutral HH jet. This process begins as an ionization front at the side of the jet, which is directed towards the ionizing star of the nebula, and progresses into the beam of the jet. There are two possible solutions. In the first solution, the jet beam becomes fully ionized through the passage of an R-type ionization front. In the second solution, the ionization front slows down enough to become a D-type front (or is already a D-type front at the point in which the jet emerges into the photoionized nebula), forming a partially ionized jet beam, with an expanding photoionized region and a compressed neutral region.
We explore these two types of solutions both analytically and numerically, and discuss the observational effects introduced by this jet photoionization process, concentrating in a region of parameter space that straddles the parameters deduced for HH 444 (the jet from V 510 Orionis).     

The observational study of Herbig-Haro shock waves

We review the basic shock properties and the origin and the geometry of Herbig-Haro (H-H) shock waves. We first discuss different aspects of “normal” H-H objects which are connected with working surfaces (including internal working surfaces) of jets from young stellar objects. The emphasis is on unsolved problems of the H-H shock waves and not on the problems of the jet. We study the line flux ratios of high excitation H-H objects (high velocity shocks) and low excitation HH objects (low velocity shocks) and carry out a comparison with theoretical predictions in both cases. We emphasize an unexplained deficit of higher ions (especially OIII and SIII, but also various other ions) in high excitation objects. This lets the line flux ratios of HH objects appear as if their shock velocities are almost never above 100 km s^?1, while other shock diagnostics (position-velocity diagrams, integrated line profiles, distributions of fluxes along the axis of the bow shock, etc.) definitely indicate higher shock velocities. Some aspects of the spectrum interpretation of the very low velocity shocks (like HH7) are explained quite well by the theory. A basic unsolved problem is, however, the explanation of the CI lines whose flux is up to a factor 10 times stronger than predicted for any model. Obviously we are very far from correctly predicting the ionization of C in shock models. In the last chapter we discuss, as one example of a very unusual HH-object, HH255 (Burnham's nebula). Detailed line fluxes in the immediate environment of T Tauri (the source of HH255) have shown that HH255 has a shock wave spectrum and is definitely an HH object. In the very narrow region between 3″ and 4″ S of T Tauri we find a sharp peak of the velocity dispersion, the centroid velocity, and N_e. In the same region there is an almost discontinous increase in ionization. Between 4″ and 10″ S (corresponding to 600-1600 a.u.) of T Tauri (the source of HH255) the ionization remains high but the centroid velocity is zero (with respect to T Tauri) and the velocity dispersion is very small. This result is completely surprising for a shock wave which according to the flux ratios must have ～90 km s^?1-1 shock velocity. Why should a cooling region of a shock have a centroid velocity of ～0 km s^?1 over a large range of distance from the stellar source? At present the geometry of the HH255 is enigmatic.     

HH 111 STIS Observations and their Analysis using Analytical and Numerical Models

The internal structure of stellar jets arising from young stellar objects is characterized by a series of `knots' or condensations which have highly supersonic proper motions. These structures, we believe, are the result of a variable ejection from the source, which leads to the formation of internal working surfaces or small bowshock within the jet beam. In this paper we present a long-slit spectrum of Herbig-Haro object HH 111 obtained with STIS and an interpretation of this observation in terms of an ejection velocity variability model.     

Simulations of the supersonic radiative jet propagation in plasmas

Supersonic plasma jets are ubiquitous in astrophysics. Our study focus on the jets emanated from Herbig-Haro (HH) objects. They have velocities of a few hundred km/s and are extending over the distances more than a parsec. Interaction of the jets with surrounding matter produces two specific structures in the jet head: the bow shock and the Mach disk. The radiative cooling of these shocks affects strongly the jet dynamics. A tool to understand the physics of these jets is the laboratory experiment. A supersonic jet interaction with surrounding plasma was studied on the PALS laser facility. A collimated high-Z plasma jet with a velocity exceeding 400 km/s was generated and propagated over a few millimeters length. Here we report on study the effect of radiative cooling on the head jet structure with a 2D radiative hydrodynamic code. The simulation results demonstrated the scalability of the experimental observations to the HH jets.     

Chemistry ahead of Herbig-Haro objects

There is now compelling evidence that dark molecular clouds are clumpy. Much of the clumpiness is unresolved by single-dish telescopes but is apparent in the data from array telescopes. Molecular clumps may also be observed close to Herbig-Haro (HH) objects. These clumps are easily observable because they are `illuminated' due to the UV radiation from the shock front of the HH jet. A detailed observational and theoretical study of one HH clump has been performed and it indicates that this clump must be transient and has a similar density and temperature to those clumps detected in the cloud interior. Thus, HH clumps may be used as an independent method of determining physical parameters of the clumpiness of molecular clouds.     

Kinematics of the high-excitation HH 890 jet in the Rosette Nebula

Photoionized jets immersed in HII regions display special properties, which made them a distinctive category of Herbig-Haro (HH) flows. Detailed studies of such jet systems became one of the key issues in our understanding of jet production and evolution. HH 890, initially called the Rosette HH2 jet, is the second photoionized jet discovered in the spectacular HII region of the Rosette Nebula. Contrary to conventional impres-sions of a jet, its discrete components are found to be unexpectedly broad and spatially detached from the proposed energy source. The jet displays additional unusual features which point to the disputable nature of the system. Here, we investigate the kinematics of the jet through high-quality echelle spectrograms. It is distinctively resolved into a fast component with a mean approaching velocity of-39 km s-1 with respect to the systemic rest frame and a slow component with radial velocity centered at -9 km s-1. The slow component indicates an apparently larger dispersion in radial velocity in various emission lines and is likely dissolving at roughly the speed of sound, which favors a photoevap-orated origin. The [SII] doublet ratios indicate an electron density of～1.1×103 cm-3in the collimated jet and ～9×102cm-3 in the HII region. This, along with the diffuse appearance of the extensive part of the jet, leads to a dissipation of the jet in the fully ionized medium of Rosette. In addition, time series of photometric observations provide evidence for remarkable light variations of the energy source. Its amplitudes of variation amount to1 mag in both R and I, which is commensurate with the young evolutionary status of the source as indicated by a red, late type optical spectrum.     

Interpreting the Observations of Herbig-Haro Jets

This paper reviews the numerical simulations of radiative jets with concrete predictions of the emitted radiation, which can be compared directly with observations of individual HH objects. The only models that have been developed to this point are the “internal working surface model” (in which the structures along HH jets are interpreted as working surfaces resulting from a time-variability in the ejection) or the “Kelvin-Helmholtz instability model” (in which the HH knots are associated with shocks resulting from K-H instabilities in the jet beam/environment boundary). The predictions of intensity maps, line ratios, line profiles and proper motions are discussed.     

Star formation region in Vela

A star formation region connected with SNO 41 is investigated. The observations of this region were carried out in the ¹²CO (1-0) line and in the 1.2-mm (with SIMBA) with the 15-m SEST mm telescope (Cerro La Silla, Chile). A blue shifted outflow is revealed from the ¹²CO(1-0) observations, while a bipolar outflow is apparent from the 1.2-mm SIMBA image. In CO it seems that a very faint dust envelope around SNO 41 probably exists, which is expanding with a velocity of ∼10.5 km/s. The distance to SNO 41 is estimated as ∼1500 pc. There are outflows also present in 2MASS images. A spiral jet has a condensation (resembling a HH object) at the end. Another jet has a discontinuity and a bow-shock-like structure on it. In 2MASS images there are also spots resembling HH objects. In this region there is also a rather luminous point source (IRAS 08546-4254), which has IR colors typical for an YSO connected with a water maser. The detection of a strong CS (2-1) line emission toward IRAS 08546-4254, with the same velocity as the CO line, shows the existence of a high density core of molecular gas associated to this source. A methanol maser is also associated with that IRAS source. The existence of CS line emission and a methanol maser (at 6.669 Ghz) is an indication of the presence of a very young massive star. It is not excluded that this IRAS source is the center of outflows mentioned above, because this source coincides with the center of the 1.2-mm SIMBA image and also with the place of origin of the jet with bow-shock-like structure. Published in Astrofizika, Vol. 50, No. 1, pp. 5–15 (February 2007).     

Discovery of an Optical Jet in the Rosette Nebula： Rosette HH2

We report on the discovery of an optical jet-Rosette HH2-in the Rosette Nebula. The jet system bears unique features for residing at the center of a giant HII region, and its energy source is visible with apparently very low extinction along the line of sight. Unlike most other Herbig-Haro jets, this jet indicates a high-excitation origin, and its extended portion shows a seemingly intact structure, instead of normally a shocked working surface, which is attributed to photoablation.     

The physics of turbulent and dynamically unstable Herbig–Haro jets

The overall properties of the Herbig–Haro objects such as centerline velocity, transversal profile of velocity, flow of mass, and flow of energy are explained adopting two models for the turbulent jet. The complex shapes of the Herbig–Haro objects, such as the arc in HH34, can be explained by introducing the combination of different kinematic effects such as velocity behavior along the main direction of the jet and the velocity of the star in the interstellar medium. The behavior of the intensity or brightness of the line of emission is explored in three different cases: transversal one-dimensional (1D) cut, longitudinal 1D cut, and 2D map. An analytical explanation for the enhancement in intensity or brightness such as usually modeled by the bow shock is given by a careful analysis of the geometrical properties of the torus.     

Combined fitting of Ulysses/COMPTEL GRB spectra

This paper describes a comparison of observations of the HH 30 jet/counterjet system and theoretical models of jets propagating in a strongly stratified medium. We find that the observed westward bending of the HH 30 jet and counterjet can be explained as the result of a plane-parallel pressure stratification of the surrounding environment. This model predicts specific properties for the kinematics of the outflow, that could be straight-forwardly checked with future spectroscopic and proper motion studies of HH 30.     

The Dynamics of Stellar Jets

This paper discusses the formation of two-shock working surfaces in Herbig-Haro (HH) jets. These working surfaces can be formed either at the leading edge of the jet flow, or inside the body of the jet beam (as a result of variabilities in the jet flow velocity), and depending on the parameters of the flow can be either massless or mass conserving. It is shown that observations might indicate that these two regimes actually occur in some HH jets. This revised version was published online in July 2006 with corrections to the Cover Date.     

A Study of the Energy Sources of Herbig—Haro Objects

1 INTRODUCTIONHerbig-Haro objects are a kind of semi-sta]c, semi-nebuIa objects associated with star fOrm-ing regions. Although such objects were noticed by Burnham as early as l890s, they did notattract much attention until the late 1940s until their independent rediscovery by Herbig andHaro in NGC 1999. A few years later, these objects were named Herbig-Haro objects fOr thefirst time by Ambartsumian (Reipurth 1997). In the fOllowing half century, the observationand theoretical res…     

Shocked molecular hydrogen in the LkHα234 region

We present near-IR images of the shocked gas in in HH 167 (the LkH234 jet) and HH 103. The H₂ 1-0 S(1) (2.122µm) and the 1.644µm [FeII] lines were observed. The relative spatial distribution of these two lines provides some insight into the nature of the shocks producing the objects.     

The spatial structure of the HH 30 jet

New, deep, wide-field [SII] images of the HL Tauri region show the extended spatial structure of the HH 30 jet and counter-jet. At an angular distance of 300 arcsec toward the NE, the HH 30 jet ends in a group of scattered condensations. This previously undetected structure might correspond to a broken-up working surface. Our images also include HH 262, which is shown to have a previously undetected extended emission region.     

Spectroscopic Studies of the Herbig-Haro Objects Hh84 and Hh85

Spectroscopic studies have been made of the two Herbig-Haro objects HH84 and HH85. Isocontours of the [SII] emission lines, the radial velocities, and the relative line intensities and electron densities are given for the A, B, C, E clusters of HH84 and the A, B1, and B2 clusters of HH85. Comparisons with earlier results are made. Significant variations in the radial velocities and line widths are observed within the confines of HH84. It is concluded that HH84 may actually be a shock wave at the termination of a collimated outflow, but it is still not known whether this flow is coupled to HH83. HH85 is definitely a part of the HH34 giant outflow.     

Spectral study of the object HH12

Spectral studies of one of the brightest Herbig-Haro objects, HH12, using a multiaperture (multi-pupil) spectrograph are reported. We identify nine knots (densification nodes) in intensity diagrams. Hα emission mainly predominates in this object, except in two of the knots (C and M) which have a lower excitation level, given their high [SII] line intensity. The average electron temperature across the object is 6700 K. It is shown that the radial velocity of the object as a whole is low, i.e., its motion is mostly in the celestial plane. The excitation source for HH12 is also discussed. __________ Translated from Astrofizika, Vol. 49, No. 1, pp. 71–79 (February 2006).