Hartigan's Research/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
For a list of titles of papers, some available vi ftp, click
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