Young Stellar Objects

Young stellar objects (YSOs) are stars in the earliest stages of development. There are two principal kinds of YSOs: protostars and pre-main sequence (PMS) stars. YSOs are almost always found within or near interstellar gas and dust, most often embedded or partially embedded in molecular clouds. They are also intimately associated with the other manifestations of the star formation process such as bipolar outflows, Herbig-Haro objects, jets, water masers and circumstellar disks. Protostars are the most mysterious and exotic YSOs being so heavily embedded in surrounding gas and dust that they are invisible at the traditional optical wavelengths and can only be studied in the infrared, millimeter or sub-millimeter wavelength bands. It is during the protostellar stage of stellar evolution that a star acquires the bulk of its mass via the infall and accretion of surrounding gas and dust. Most of the material that goes into forming a star is accreted through a circumstellar disk and in this process the protostellar system drives an energetic bipolar jet and outflow into its surroundings. PMS stars are low mass stars in the post-protostellar stage of evolution. They have not evolved to the point where nuclear burning can begin in their cores. The least evolved PMS stars are surrounded by remnant accretion disks. These disks are also known as protoplanetary disks since it is believed that planetary systems are formed within them during this phase of evolution. Understanding the nature of YSOs is an essential step toward deciphering the origins of stars and planets. Members of the RG Division have made major contributions to and continue to shape the development of YSO studies.