New kinds of interactions in the interstellar medium

Throughout our Galaxy, the interstellar medium (ISM) is distributed quite inhomogeneously - but strongly concentrated to the Galactic plane - and appears in a rich variety of forms and conditi-ons. The morphological and physical structure of the ISM (being gas and dust) is commonly belie-ved to be largely determined by the energy feedback from massive stars, including intense UV radiation, fast stellar winds and supernova shock waves. - Dust coexists with gas except in the hot-test regions, and is frequently a tracer of the ISM easier observable than gas, due to its emission in the infrared. Since the mission of the InfraRed Astronomical Satellite (IRAS), it is known that dust is present in bewildering morphological complexity also away from the Galactic plane, ap-pearing there as "cirrus", heated by the interstellar radiation field. Nonetheless, dust at higher alti-tudes seems to be more uniformly distributed with Galactic latitude; in general, most ISM appears to be in the form of discrete clouds with an average number density of 0.1-0.5 atoms per cm3. One might think that both the objects responsible for the morphological complexity and the physical states of the ISM as well as the structural composition of the latter are well known. We believe, however, to be able to demonstrate that a certain fraction of e.g. the numerous cavities in the ISM are created by a hitherto neglected mechanism, namely the interactions of material blown off by the predecessors of planetary nebulae (PNe) and/or the winds of the central stars of PNe with the ISM. Possibly, this new phenomenon - detected by us when examining NASA SkyView maps based on IRAS data - is a significant contributor in shaping interstellar matter. - Another kind of interaction with the ISM - theoretically predicted many years ago and sought after since that time - could be small ISM clouds of low mass hit by supernova remnants. Such clouds could exist in interstellar space in vast numbers, but would remain undetectable unless they are heated up by shock fronts. Each of such cases - only recently there was no convincing one known - would represent an interesting astrophysical laboratory. We believe that we have found such a case. For both detections - i.e. the holes in the ISM (and other large dust emission structures) and the case of what we think is the hitherto best example for a small isolated ISM cloud hit by a super-nova blast wave - detailed studies are planned on the basis of this application. The studies com-prise above all extended observations (spectra, images), mainly in the optical wavelength range, but also theoretical investigations, both aiming at understanding the detected phenomena in general (the holes) and in great detail (the cloud), and to work out their significance in the context of an improved insight into the interstellar matter.

The interstellar medium - gas and dust particles between the stars, frequently accumulating in clouds and nebulae of various morphologies and densities - is the basis for processes of large astrophysical significance: It provides, for example, the birth material of stars and their planetary system, and is the site of bewildering physical processes which largely cannot be imitated in terrestrial laboratories. Hence, any discovery of new kinds of processes or objects in the interstellar medium act as enrichments in the understanding of the (astro)physical world. In our project, we were able to detect a couple of different, hitherto unknown, manifestations of interstellar matter. The most spectacular of our discoveries - mostly found during our careful screening of infrared images of the sky obtained in 1983 with a satellite ("IRAS") - is represented by two adjacent filaments of bipolar shape and truly huge projected dimensions (9 degrees each, that is about 18 times the apparent diameter of the moon). They shine at 60 and 100 micrometer, that is at far infrared wavelenghts. Objects of this shape and apparent size have no been detected before. We found hints of their nature: We think that both objects have the same origin, stemming from an old, decayed system of at least three stars: One of these stars, when dying, has blown out stellar gases partly captured by the two other stars; the latter then were ejected from the system and on their part started to blow away this material in two bipolar jet streams, respectively. Since this event may have started more that 100.000 years ago, in the meanwhile these two jets grew tremendously large and cooled. This possible pair of very old dust-rich (?) jets could not be detected by us in any other wavelengths - neither in the optical, nor in the radio regime etc. Hence, a definite proof for the nature of this pair of objects is still lacking. An other object, detected by us about 1 decade ago and only provisionally studied by us then, appears to belong to a class of objects with few examples known - a former small interstellar cloud interacting with a shock front, which first squeezes the cloud and then destroys it via the formation of strong vorticity. The object - which displays a striking morphology and was named "Criss-Cross Nebula" by us - is outstanding due to its closeness and due to the presence of its recently detected umbrella-like faint nebula. The overall morpology and the information we got from a recently obtained optical spectrum leads us to suggest that this nebula will turn out to be a "model example" for the above mentioned interactions, since it looks very similar to numerical 3-D simulations done by other authors. Last not least we discovered and studied (by optical imaging and spectroscopy) an optical filament at the borders of a gigantic, nearby supernova remnant, which to date was known to glow in X-rays only. Finally, we found a number of regions poor in interstellar matter around planetary nebulae. Unfortunately we hitherto cannot prove whether these regions are real - probably generated by winds of the central stars of the planetary nebulae - or are only chance projections of in fact unrelated phenomena.