Circumstellar matter exists in many forms, such as winds from early and from late type stars, in infra-red sources and OH and H2O masers, in X-ray sources and jets, and molecular flows and Herbig-Haro objects. There is considerable diversity in the nature of the starlike bodies that underlie these phenomena. Here an attempt is made to trace the connection between some of the observations related to pre-main sequence stars and their surroundings, with emphasis on jets and molecular flows.

At the end of this year, the Sonderforschungsbereich (Special Collaborative Programme) no. 132 on “Theoretical and Observational Stellar Astronomy” in Heidelberg will terminate after fifteen years. Although 15 years are only 2 1/2 per cent of the age of the University of Heidelberg, which is celebrating its 600 th anniversary this year, they are nevertheless a long and important time for astronomical research in Heidelberg. On the occasion of the termination of the Sonderforschungsbereich, we are now given the opportunity to present an essential part of its research, namely that on circumstellar matter at an international conference, and we are grateful to the International Astronomical Union that this could be realized, and that we can welcome here so many participants to this Symposium. As the Speaker of our Sonderforschungsbereich I would like to briefly introduce to you the general concept of the institution of an SFB and give an overview over its structure and research activities.

We propose that the formation of low mass stars in molecular clouds takes place in four stages. The first stage is the formation of slowly rotating cloud cores through the slow leakage of magnetic (and turbulent) support by ambipolar diffusion. These cores asymptotically approach quasistatic states resembling singular isothermal spheres, but such end states cannot actually be reached because they are unstable. The second phase begins when a condensing cloud core passes the brink of instability and collapses dynamically from “inside-out,” building up a central protostar and nebular disk. The emergent spectral energy distributions of theoretical models in the infall stage are in close agreement with those of recently found infrared sources with steep spectra. As the rotating protostar gains mass, deuterium will eventually ignite in the central regions and drive the star nearly completely convective if its mass is less than about 2 M ⊙. This initiates the next step of evolution - the bipolar outflow phase - in which a stellar wind pushes outward and breaks through the infalling envelope. The initial breakout is likely to occur along the rotational poles, leading to collimated jets and bipolar outflows. The intense stellar wind eventually widens to sweep out gas in nearly all 4π steradian, revealing the fourth stage - a T Tauri star with a surrounding remnant nebular disk. Radiation from a disk adds an infrared excess to the expected spectral energy distribution of the revealed source. The detailed shape of this infrared excess depends on whether the disk is largely passive and merely reprocesses stellar photons, or is relatively massive and actively accreting. Both extremes of spectral shapes are observed in T Tauri stars; the amount of circumstellar material in the form of disks around nearly formed stars may be related to the dual issues of the origins of binary-star and planetary systems.

After presenting NGC 7129 as a prototypical star-forming region, I discuss what can be learned from the radio spectra of embedded infrared sources. I then review available observational evidence for disks around young stellar objects, with emphasis on accretion disks around T Tauri stars. Finally, new results on the role of magnetic fields in the circumstellar activity of T Tauri stars are presented.

The fact that bipolar flows are widespread among stars of very different spectral types is emphasized. First, the stars associated with the phenomenon are divided into broad types: protostars and pre-main-sequence stars; red giants; symbiotic objects; protoplanetaries; planetaries; novae and cataclysmic variables; and peculiar hot stars. Second, the evidence for circumstellar “disks” or toroids is considered among these different categories of star. Finally, the possible role of binarity is discussed.

Current thinking on the origin, evolution and stability of stellar jets and bipolar flows is reviewed. Particular emphasis is given to the driving mechanisms of bipolar molecular and ionised gas outflows from young stellar objects. General constraints on both hydrodynamic and magnetohydrodynamic flows are presented. The interrelationship between the protostellar outflows and others such as those associated with powerful, highly collimated outflows in active galactic nuclei are discussed. Intimately connected phenomena such as maser sources and Herbig-Haro objects are briefly treated and finally, areas of potentially interesting future research are indicated.

Following an Einstein X-ray survey of the rho Oph molecular cloud, uncovering several dozen young stellar objects (YSO) (Montmerle et al. 1983), an extensive radio continuum VLA(C) survey was conducted in 1983 to look for counterparts to these sources at 1.4 and 5 GHz (André et al. 1986) To shed light on emission mechanisms, we performed follow-up VLA(A/B) observations of identified radio stars in March 1985 at 1.4, 5 and 15 GHz.

VLA radio continuum observations have been obtained for a number of pre-main sequence (PMS) stars in Corona Australis, Lupus, Scorpius and Taurus. A variety of PMS sources were detected and for other stars upper limits to the ionized mass loss rates were determined. A strong double source, showing two radio jets, was found associated with an embedded infra-red source in the R CrA molecular cloud. Some of the PMS stars show extended radio emission associated with ionized circumstellar envelopes, even though photoionization by EUV photons is not sufficient to produce the ionized regions.

Infrared speckle interferometric observations at 3.6 μm carried out at the ESO 3.6m telescope have indicated that EL29, a deeply embedded infrared source in the Ophiuchus dark cloud (Elias 1978), appears to have a core-halo structure (Zinnecker, Chelli and Perrier 1989). A fit to the data gave angular scales of order of 0.05 arcsec for the core and 3.5 arcsec for the halo (Gaussian FWHM) corresponding to linear diameters of 8 AU and 620 AU, respectively (for a distance to the cloud of 160 pc). New infrared speckle observations confirm that the core is resolved at the 2 sigma level, and carries about 90% of the total 3.6 μm flux (Fig. 1). Furthermore, there is evidence from a recent IRTF lunar occultation experiment in the K-band (2.2 μm) for an additional inner 6 milli-arcsec (~ 1 AU) core which carries about 75–85% of the total K-flux from this source (Simon et al 1986). Perhaps this inner core is the dust shell inside of which grains are destroyed by the radiation from a central object.

It is immediately apparent from IRAS and other far infrared data (figure 1) that all known FU Orionis stars, or fuors, are associated with substantial amounts of cool circumstellar dust.

The pre-outburst magnitudes for V1057 Cyg indicate that this star was immersed in circumstellar dust already before its flare-up in 1969–1970, and hence that this dust did not - as has been claimed - appear only after the outburst; regrettably there exist no far infrared data from that time. The colours also exclude the possibility that the star then was of the alleged spectral type KO, but point to it being of a rather early type and obscured by both interstellar, local, and circumstellar matter. For details, see Welin 1985.

The infrared bolometric luminosity of the extended infrared emission from the L1551 flow is estimated as 19 (−4 +10) L⊙. Ultraviolet radiation from the shock associated with the flow appears to heat the surrounding dust. The extended infrared emission raises the total energy requirement for the flow over a 10^4 year lifetime to 10^46–47 ergs. If gravitational in origin, this energy likely originates from a region <10^13 cm. Infrared radiation offers a new probe for interstellar shocks by sampling the ultraviolet halo surrounding the shock.

Recent submillimeter observations of infrared sources embedded in dark clouds have uncovered the existence of a few highly compact objects. Two examples of this phenomenon are the infrared sources observed towards B335 and L1551 (Keene et al. (1983), Davidson and Jaffe (1984)). One can estimate for these regions gas masses of approximately one solar mass, densities of at least 106 cm−3, and diameters less than 1017 cm. Both infrared sources are associated with bipolar outflows observed in CO with mass loss rates of order 10−6 solar masses per year. It is reasonable in both cases to suspect the existence of matter in some form of disc around the infrared source.

One might also expect to observe molecular counterparts to the submillimeter compact objects. In an effort to do this, we have mapped the C18O J = 1 → 0 transition in a small area surrounding the infrared sources in B335 and L1551 using the IRAM 30-m telescope. The beamwidth was 22 arc sec. which corresponds to 5 1016 cm at the distance of L1551 and 9 1016 cm at the distance of B335. In each case, we do find a compact structure which we believe to be the molecular counterparts of the compact submillimeter sources. We estimate the abundance ratio CO/H2 to be of order 10−4 in L1551 and at least 10−5 in B335. Further details of this work are contained in an article submitted to Astronomy and Astrophysics.

We argue that the narrow jets which are sometimes seen to escape from YSOs into an ambient molecular cloud consist of e±-plasma which can be created in stellar magnetospheric discharges and subsequently centrifugally post-accelerated. This high-pressure pair plasma is squeezed into two jets which ram cocoons into the molecular cloud, observed in the form of molecular lobes (outflows).

An analysis of published observations of the L1551 optical jet leads to the conclusion that the driving region is very small, possibly little larger than the star itself. Further the data suggest that the jet is precessing.

A search for the helical velocity field that had been predicted in a magnetodynamic theory of Uchida and Shibata was made in the bipolar flows L1551 by using 12CO 115 GHz line, and evidence was obtained for it in the low velocity maps as the skew inter-invasion of the root part of the blue- and red shifted lobes into different sides of the opposite lobes. Theoretical implications of this and other findings are discussed, and the advantage of models with magnetic field is stressed.