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).
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 work in this area has focused on several aspects of star formation over
the years. These studies include research into the spectral signatures of accretion such as excess
continuum and emission line profiles, starspot coverage, binarity, and searches for
exoplanets around the youngest systems. I've also explored how the surrounding gas is
affected by radiation from massive young stars by imaging these regions in a variety of
molecular and atomic lines with ALMA and with large ground-based IR telescopes such
as Gemini. Another aspect to the research deals with trying to understand the causes
and implications of the time-variability of the stellar brightnesses.
A full list of refereed publications is available, as well
as a summary of research conducted in the past year.
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