Degree Name

Master of Science (Hons.)


Department of Physics


An exciting area of research in star formation is tlie study of outflows driven by mass loss from Young Stellar Objects (YSOs). This mass loss can be as much as 10"^ MQ yr 1 with wind velocities of the order 300 km s'^. The nature of the wind that drives this Tnass loss is still uncertain. Evidence that a YSO is undergoing mass loss is provided through the effect the stellar wind has on the surrounding medium. Phenomena such as bipolar molecular outflows, masers and the Herbig-Haro (HH) objects are all indicators of a YSO undergoing mass loss. Visible observations have identified the following morphological features to be present in most outflows: • An embedded YSO which drives the outflow; • A collimated jet and counter jet; • A section where the jet disappears, but isolated emission knots remain; • A working surface (bow shock) with associated HH Object(s). Imaging at optical wavelengths has shown that the HH objects and HH jets are regions of shock-excited gas emitting in Ha and forbidden excitation lines such as [O I] (U6300,6363) and [S II] (U6716,6731). Therefore the different shock structures of the HH complex can be distinguished by comparing images at optical emission lines with differing excitation parameters ie: [S II] ("u ~ 100 km s"^). Due to heavy extinction from star forming clouds, many YSOs are obscured at optical wavelengths. Near-infrared images often provide the first identification of the driving source of the outflow as the optical extinction becomes less important at longer wavelengths. HH objects/jets are associated with both low-luminosity (Lbol -1-100 Lq) T Tauri stars and high-luminosity YSOs (Lbol ~ Lq) such as IRAS sources. The visibility of the source depends the amount of obscuration, the orientation of the flow and circumstellar disk. Digitised images from the ESO/SERC southern sky survey plates are presented, and when combined with [S II], Ha and H2 images from the literature, are useful for identifying features within the outflow complex, and identifying the driving YSO. This will be beneficial when looking for new HH objects from the ESO/SERC plates in areas of known star formation.