大尺度赫比格-哈罗天体的光学研究进展

近期大视场CCD成像观测表明，由年轻星激发的部分赫比格－哈罗（HH）天体的尺度范围超过1pc，比过去认识的HH天体的尺度大了一个量级。目前的观测已经发现pc尺度的HH天体有数十个，表明HH天体达到pc尺度是很普遍的，这将深刻地改变人们对年轻星与其周围环境作用的认识。综述评述了pc尺度HH天体的观测性质，讨论了巨HH天体对恒星形成的重大意义及其对周围星际介质的物理和化学影响。     

Large－Scale Distribution of Herbig－Haro Objects in Taurus

We report our new results on Herbig-Haro (HH) objects in the star forming region of Taurus from a wide-field survey with the 60/90 cm Schmidt telescope of the Beijing Astronomical Observatory. This survey using CCD imaging with a narrow band [SII] filter and an intermediate band [BATC10] filter covered approximately 30 square degrees in Taurus. Besides confirming the known HH Objects in the region, we discovered seven new HH candidates, and groups. Six of these are HH 701A-B, HH 702A-D, HH 703, HH 704A-D, HH 705, HH 706 and the seventh is a group, a new component of HH 319, labeled HH 319B-D. Based on the large-scale distribution of pre-main-sequence (PMS) stars in Taurus, we analyze statistically the most probable distance from the HH objects to each PMS star, from which we estimate the typical timescale of these HH objects to be between (1.3 - 2.0) ×104 yrs; and we also obtain the birth rates of HH objects: 0.447±0.198 for Class Ⅰ PMS stars, 0.360±0.222 for Class Ⅱ PMS stars, and -0.148±0.234 fo     

Spectroscopic constraints on outflows from BN-type objects

New high-quality high spectral resolution observations of the HI line emission from massive young stellar objects are described and discussed. It is proposed that two distinct physical components contribute to the observed emission. One of these is an optically-thick high-velocity stellar wind, the other a more slowly moving optically-thin volume of gas that may, in the case of S106IR at least, be caused by mass loading of the stellar wind. This decomposition is shown to resolve a long-standing problem regarding the relative widths of high and low opacity lines.     

Herbig-Haro objects—tracers of the formation of low-mass stars and sub-stellar objects

Herbig-Haro objects (HHOs) are caused by outflows from young objects. Since the outflow relies on mass accretion from a circumstellar disk, it indicates ongoing growth. Recent results of infrared observations yielded evidence for disks around brown dwarfs. This suggests that at least a certain fraction of brown dwarfs forms like stars. Thus, young sub-stellar objects might cause HHOs as well. We present selected results of a general survey for HHOs based on DSS-II plates and CCD images taken with the Tautenburg Schmidt telescope. Numerous young objects could be identified due to their association with newly detected HHOs. In some cases the luminosity is consistent with very low-mass stars or close to sub-stellar values. This holds for L1415-IRS and a few infrared sources embedded in other dark clouds (e.g., GF9, BHR111). The question on the minimum mass for outflow activity is addressed.     

Influence of molecular outflows from young stellar objects on molecular clouds

In this paper it is investigated whether molecular outflows can support the turbulence in molecular clouds against decay. In the momentum-driven regime of molecular flows the time scale of the replenishment of the fluctuations is longer than the dissipation time scale of pure hydrodynamical supersonic turbulence.     

Numerical Simulations and Astrophysical Applications of Laboratory Jets at Omega

We have conducted experiments on the Omega laser at the University of Rochester that have produced jets of supersonic Ti impacting and being deflected by a ball of high density plastic. These mm-sized jets of dense plasma are highly complex, have large Reynolds numbers, and, given sufficient time and shear, should produce a fully turbulent flow. The experiments are diagnosed with a point-projection backlighter, resulting in a single image per shot. Simulations of the 3D hydrodynamics capture the large-scale features of the experimental data fairly well while missing some of the smaller scale turbulent-like phenomena. This is to be expected given the limited characterization of the targets as well as the finite resolution of the 3D simulations. If Euler scaling holds, these experiments should model larger astrophysical jets in objects such as HH 110 where an outflow can be seen colliding with a molecular cloud. However, Euler scaling demands that not only the isothermal internal Mach numbers of the two systems be similar but also that any dissipative mechanisms, such as radiative cooling or viscous dissipation, be of equal importance relative to each other. Similar equations of state are required as well. We discuss such issues in the context of these experiments and simulations.     

Interactions between molecular outflows and optical jets

Young stars produce both molecular outflows and, at a later evolutionary stage, well-collimated optical jets. The simplest explanation is that the molecular outflows are driven byobscured optical jets, rather than directly, by a disk wind for example, but the optical jets appear to have too small a momentum flux. Recent statistical studies however show that the molecular flows must be quasi-stationary, which means that the dynamical lifetime is a gross underestimate of the true age. As a consequence much less thrust is required. We present recent observations of RNO 43, which has well-defined optical and molecular outflows lying close to the plane of the sky. Excellent agreement with the observations is obtained with a simple kinematic model for the molecular material, which supposes that it lies in a parabolic shell around the optical jet with the highest velocities at the working surface. Together with our modelling of the NGC2024 outflow, this is very strong evidence that molecular outflows are produced by prompt entrainment of molecular material in a neutral or weakly-ionized jet.     

Near-infrared imaging of H2 in molecular outflows from young stars

In an attempt to identify the molecular shocks associated with the entrainment of ambient gas by collimated stellar winds from young stars, we have imaged a number of known molecular outflows in H₂ v=1-0 S(1) and wide-band K. In each flow, the observed H₂ features are closely associated with peaks in the CO outflow maps. We therefore suggest that the H₂ results from shocks associated with the acceleration or entrainment of ambient, molecular gas. This molecular material may be accelerated either in a bow shock at the head of the flow, or along the length of the flow through a turbulent mixing layer.     

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.     

Search for HH objects and emission stars in star formation regions. IV. New Herbig-Haro flows and objects associated with cometary nebulae

As part of a project to search for new Herbig-Haro (HH) objects in star formation regions, observations are reported on the neighborhoods of five cometary nebulae: MacC H12, MacC sH15, GM 1-14, RNO 33, and Pars 17. We have been able to identify 9 previously unknown HH objects in those regions. Almost all of these objects belong to directed flows whose sources are, with high probability, the central stars of these nebulae. In the cases of MacC H12 and GM 1-14, the outflows have a distinct bipolar structure. The sources of the outflows are located on a J-H/H-K diagram in order to classify them. __________ Translated from Astrofizika, Vol. 51, No. 1, pp. 15–27 (February 2008).     

Virtual MHD Jets on Grids

As network performance has outpaced computational power and storage capacity, a new paradigm has evolved to enable the sharing of geographically distributed resources. This paradigm is known as Grid computing and aims to offer access to distributed resource irrespective of their physical location. Many national, European and international projects have been launched during the last years trying to explore the Grid and to change the way we are doing our everyday work. In Ireland, we have started the CosmoGrid project that is a collaborative project aimed to provide high performance super-computing environments. This will help to address complex problems such as magnetohydrodynamic outflows and jets in order to model and numerically simulate them. Indeed, the numerical modeling of plasma jets requires massive computations, due to the wide range of spatial-temporal scales involved. We present here the first jet simulations and their corresponding models that could help to understand results from laboratory experiments.     

Near-infrared imaging in H2 of molecular (CO) outflows from young stars

We have imaged several known molecular (CO) outflows in H₂ v=1-0 S(1) and wide-band K in order to identify the molecular shocks associated with the acceleration of ambient gas by outflows from young stars. We detected H₂ line emission in all the flows we observed: L 1157, VLA 1623, NGC 6334I, NGC 2264G, L 1641N and Haro 4-255. A comparison of the H₂ data with CO outflow maps strongly suggests that prompt entrainment near the head of a collimated jet probably is the dominant mechanism for producing the CO outflows in these sources.     

Gas-Dynamical Models of Star-forming Regions

This article describes some of the techniques used to include various micro-processes in numerical gas-dynamic simulations. These techniques are then used in simulations of photo-evaporating clumps, H₂ level populations and time dependent shocks/jets. This revised version was published online in July 2006 with corrections to the Cover Date.     

The physical conditions in the BHR71 outflows

Highly-collimated outflows are believed to be the earliest stage in outflow evolution, so their study is essential for understanding the processes driving outflows. The BHR71 Bok globule is known to harbour such a highly-collimated outflow, which is powered by a protostar belonging to a protobinary system. Using the APEX telescope on Chajnantor, we mapped the BHR71 highly-collimated outflow in CO(3-2), and observed several bright points of the outflow in the molecular transitions CO(4-3), CO(7-6), ¹³CO(3-2), C¹⁸O(3-2), CH₃OH(7-6) and H₂CO(4-3). We use an LVG code to characterise the temperature enhancements in these regions. These observations are particularly interesting for investigating the interaction of collimated outflows with the ambient molecular cloud. In our CO(3-2) map, the second outflow driven by IRS2, which is the second source of the binary system, is completely revealed and shown to be bipolar. We also measure temperature enhancements in the lobes. The CO and methanol LVG modelling points to temperatures between 30 and 50 K in the two lobes. The methanol emission in the southern lobe bright knot is barely resolved with the APEX single-dish. ALMA will thus be a central tool to study the shock chemistry in these regions.     

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.     

Formation of Working Surfaces in Radiatively Cooled Laboratory Jets

Whilst observations provide many examples of collimated outflows or jets from astrophysical bodies, there remain unresolved questions relating to their formation, propagation and stability. The ability to form scaled jets in the laboratory has provided many useful insights. Experiments (Lebedev et al.: 2002, ApJ 564, 113) using conical arrays of fine metallic wires on the MAGPIE generator (1MA in 240 ns) have produced radiatively cooled collimated jets in vacuum using the redirection of convergent flows by a conical shock. Here we present results of a jet produced by this method propagating through a photo-ionized, quasi-stationary gas cloud. A working surface is observed at the head of the jet. The velocity of this working surface is lower than the velocity of a jet tip in vacuum.     

X-ray Flashes from Off-axis Nonuniform Jets

It has been widely believed that the outflows in gamma-ray bursts are jetted and some jets may have structures like ε(θ) ∝ θ^-k. We check the possibility that X-ray flashes come from such jets. Both qualitative and quantitative analyses have shown that this model can reproduce most of the observational features of both X-ray flashes and gamma-ray bursts. Using the usual parameters of gamma-ray bursts, we have carried out numerical calculations for both uniform and nonuniform jets, of their fluxes, spectra and peak energies. It seems that nonuniform jets are more appropriate to these observational properties than uniform jets.We have also shown that in our model the observational ratio of gamma-ray bursts to X-ray flashes is about a few units.