Deep direct 1 μm images and polarization maps have been taken for several objects of bipolar molecular outflow, using a CCD camera at the prime-focus of the 3.5 m telescope on Calar Alto/Spain. Compact reflection nebulae are found near GL2591, GL2884 = S140/IRS1, GL2789 = V645 Cyg, L1551/IRS5 and within the red reflection nebula in Serp. The position angles of polarization vectors coincide in all cases with that of the 2.2 μm polarization of the central source itself, and it is perpendicular to the direction of mass outflow (if it is well defined). The visual extinction in front of the central source, is at least 20 mag higher than that which is affecting the scattered light. This is consistent with a model of bipolar outflow, where the active source is surrounded by a dust torus or ring.

Deep CCD imaging of the Serpens bipolar nebula shows it to be surrounded by molecular cloud material having spiral density structure. Polarization mapping indicates that the magnetic field in this material also exhibits spiral structure and we interpret this as the remains of the magnetically-braked collapse of a protostellar cloud. A binary star system has formed in the cloud core.

We report the detection of circular polarization in three T Tauri stars with known intrinsic linear polarization. A circumstellar origin is required.

During the course of a monitoring programme of the linear polarization of various T Tauri stars, UY Aur was observed to undergo a strong polarization burst between 1984 October and 1985 January. The linear polarization rised from about 1.5% early in 1984 to a maximum of 7.6% in 1984 October and declined back to the earlier value after 3 to 4 months. It is the first time that such a large increase in linear polarization is reported in a T Tauri star. However the interpretation is complicated by the fact that UY Aur is a visual binary with a separation of 0.8 and both components were included in the measurements.

T Tauri stars show linear polarization typically between 1–3%. A two band linear polarization survey of some T Tauri star was reported earlier by Bastien (1982). Most of the stars show pronounced time variability in polarization and position angle (Bastien, 1980; 1982). Wavelength dependence of polarization is important in determining the specific mechanism(s) producing polarization. For a systematic study of polarization in T Tauri stars, we have taken up an observing programme to measure linear polarization of some stars in Taurus-Auriga region. Polarization measurements of 9 T Tauri stars are reported here. Observations were made on January 8–11, 1984 with MINIPOL (Frecker and Serkowski, 1976) on 61“ telescope of University of Arizona.

Polarization measurement of the background stars in the region of dark globules is important to study the magnetic field geometry and grants characteristics in the globule. These parameters are important for the formation and evolution of dark globules. We made Polarimetric observations of stars in three nearby dark clouds - B5, L134 and Heiles Cloud 2. Polarization measurements of stars in the region of B5 were made with ‘MINIPOL’ (Frecker and Serkowski, 1976) on 61″ telescope of University of Arizona. Observations for the stars in the region of L134 and Heiles Cloud 2 were made using PRL Polarimeter (Deshpande et al. 1985) on 1 meter telescope of Indian Institute of Astrophysics, Bangalore. Results are presented and discussed here. Figure 1 shows the polarization vectors projected on the sky plane for the above globules.

Most sources of B2O maser radio emission at 1.35 cm, associated with star formation regions, show strong variability with, sometimes, rapid bursts of emission (see, e.g., Liljeström 1984, Rowland and Cohen 1986, and references therein). A preliminary conclusion on the possible cyclicity of H2O maser variability can be drawn (Lekht et al. 1982, 1983), with a quasiperiod of several years. The “quiet” state of a maser source, with moderate, slowly varying values of the line flux density, turns to the “active” phase with H2O line bursts (Lekht et al. 1983). The H2O maser generation region is probably located in a rotating gas-and-dust disc (torus) around a protostar (or young star). This is pointed to by VLBI observations showing in some sources maser features arranged in an ellipsoidal structure around a common centre (presumably, the protostellar object - see Downes et al. 1979), as well as by symmetrical character of E2O line profiles of many masers (Lekht et al. 1982). As an excitation mechanism for H2O, collisional pumping in two-temperature medium behind a shock front (with hot heavy particles and cold free electrons or vice versa) is widely accepted (Bolgova et al. 1982, Kylafis and Norman 1986).

The role of the magnetic field and the possibility of its leakage during the contraction of an interstellar cloud need more studies both quantitatively and qualitatively.

Recent observations of Herbig-Haro objects and optical jets are reviewed, including observations of the stellar objects responsible for these and related outflow phenomena. The review discusses observations obtained in the following wavelength bands: radio, far-infrared, infrared, optical, and ultraviolet.

A brief overview of the observational characteristics of HH objects is given. Current models for their production by the interaction of stellar winds and jets with interstellar gas are critically discussed. Models for two specific systems of HH objects, namely, the Orion HH objects and the HH46-47 system are described with reference to the general production mechanisms.

A bipolar Herbig-Haro jet system associated with the emission-line object Th 28 has been discovered from direct CCD-imaging, long-slit spectroscopy, and broadband infrared observations. The observations were carried out using the facilities of the European Southern Observatory, La Silla, Chile. A direct CCD-image taken through a narrow band Hα interference filter is shown in Figure 1.

Deep CCD imaging of HH 34 in H∝ shows that the HH-object has a bow shock-like structure, of which the wings can be traced over about 1 arcmin ( pc). A knotty jet is pointing towards the apex of the bow shock structure. Long-slit spectroscopy reveals that 1) the jet has approximately a constant radial velocity and electron density. 2). The spectrum of the jet is of a much lower excitation than that of HH 34. 3) HH 34 has a complex velocity and line excitation structure. The extended bow shock is interpreted by a jet of which the working surface is propagating with high velocity (≈ 200 km/s) through a partially ionized medium.

Optical jets, and collimated outflows, are now recognized as a common phenomena associated with young stars (Mundt 1985, Strom et al. 1986). Presented here are the results of new CCD imaging and spatially resolved spectroscopy for ten such objects. Using these and previously published data on twenty known jets, we compiled a set of observational criteria describing the phenomena. From this compilation we addressed several physical questions pertaining to the nature of collimated outflows associated with young stars.

We have made observations of the (1,1) and (2,2) inversion lines of ammonia (NH3) towards the dark cloud region containing the Herbig-Haro (HH) objects 24–27. These transitions are only excited at H2 densities >104 cm−3, and thus probe high density gas. From the observed hyperfine splitting one can calculate optical depths. The optical depth ratio can be used to determine the rotational temperature T21 which is equal to the kinetic temperature under dark cloud conditions.

CCD images show that the reflection nebula associated with PV Cep is bipolar. From spectroscopy of this object as well as of the bipolar Boomerang Nebula we find low excited Herbig-Haro emission and indications for collimated high velocity flows along the polar axes of both nebulae. The central star of the Boomerang Nebula is probably double.

Near infrared and Ha CCD images of the newly found cometary reflection nebula GN 20.18.3. led to the discovery of its illuminating star and of a bright Ha knot located on its axis of symmetry. The knot has a Herbig-Haro spectrum indicating the presence of a collimated outflow from the central star. Also in the Bok globule L 810 a red CCD image reveals a cometary reflection nebula surrounding the central star, from which a shock excited filament is emanating. These observations emphasize the common origin of cometary nebulae and collimated high velocity flows.

It has recently been found that models of a radiating bow shock can explain qualitatively the strange emission line profiles observed in some Herbig-Haro (H-H) objects. It is also possible to compare directly the emission line intensity maps predicted from these models with CCD images of H-H objects. Such a comparison between our models and observations of HH 46/47 is presented, showing that the condensation HH 47A may tentatively be identified with a bow shock formed at the “head” of a jet.

We discuss four different tests of the bow-shock theory of Herbig-Haro objects, emphasizing especially tests based on position-velocity diagrams and on the appearance of “double layer” structures in the spatial maxima of the high- and low-velocity components of the emission lines. Though this latter effect is surprising, it is a fundamental consequence of the bow shock theory.

It is currently believed that Herbig-Haro (HH) objects are a consequence of a high-velocity (up to at least 200 km s−1) outflow of material from a young embedded star. These flows can often be detected by deep observations of optical emission lines using CCD cameras.

High-resolution long-slit spectroscopy of HH24 has revealed the presence of collimated bipolar mass flow originating from the “central” infrared source SSV63. The bipolar system consists of the well known jet-like HH24C (mean VLSR −190 km s−1 and a newly detected weak component, HH24E (~ + 170 km s−1).