Young Stars

Stars form out of large collections of gas and dust known as molecular clouds when the densest parts of these clouds collapse gravitationally. One consequence of this collapse is that young stars (T Tauri stars) are usually surrounded by massive, opaque, circumstellar disks. These disks gradually accrete onto the stellar surface, and thereby radiate energy both from the disk (infrared wavelengths), and from the position where material falls onto the star at (optical and ultraviolet wavelengths). Somehow a fraction of the material accreted onto the star is ejected perpendicular to the disk plane in a highly collimated stellar jet. The circumstellar disk eventually dissipates, probably when planets begin to form. Young stars also have dark spots on their surfaces which are analogous to sunspots but cover a much larger fraction of the surface area of the star. The figure above illustrates the current conceptual picture of a young star.

The T Tauri phase of stellar evolution lasts 1 - 10 million years. By studying these objects we gain some insight into what conditions must have been like in our solar system soon after the formation of our Sun, but before the formation of the Earth. My research in this area has focussed on several aspects of T Tauri star research. I have been particularly interested in learning how to use the optical and ultraviolet excess emission (called `veiling') to measure mass accretion rates. By measuring the strength of the forbidden emission lines of oxygen, we can estimate the mass outflow rate as well. These studies also provide insight into how material accretes onto the star, and to what extent accretion affets the angular momentum of the star. I have also been involved with several surveys of dark clouds to better define the initial mass function of T Tauri stars and to quantify what fraction of young stars that possess circumstellar disks.

For a list of titles of papers, some available vi ftp, click here.

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Patrick Hartigan