Dr. Catrina Hamilton-Drager
Mt. Holyoke

University of Florida Astronomy Colloquium - Apr 19th, 2006

Resolving an Accretion Flow: The H-alpha Emission Line Profiles of KH 15D

KH 15D, located in the young (2-4 Myr) open cluster NGC 2264 (d ? 760 pc), is an extraordinary pre-main-sequence eclipsing binary system with a strongly eccentric orbit (e ~ 0.7). The observable star in this system is a K6/K7, which undergoes deep eclipses (~ 4 mag in I) every 48.37 days. The eclipse depth and duration have been steadily increasing over the last decade. The very slow advance of an occulting edge explains the evolution of the light curve of the system from the 1950~Rs until the present. A physical model of the system has been proposed in which the advance of this edge is the result of nodal precession of a circumbinary disk or ring. The currently visible star is spending an increasingly large fraction of its orbital cycle (currently > 50%) obscured by the same material that obscures its companion 100% of the time. The system has all of the indications of a typical T Tauri star jet/outflow source, including forbidden optical emission lines, very broad H-alpha profiles, inverse P-cygni profiles and shocked molecular hydrogen emission. The K6/K7 star is an X-ray source but a very weak one for its mass and age. The edge of the occulting matter is remarkably sharp, and as it cuts across the visible star and its inner disk, it acts like a "natural coronagraph", progressively covering or revealing structure within a few stellar radii of the photosphere. During each ingress and egress we get convolved information on its density structure as well as the distribution of luminous material in the vicinity of the star. In the context of the model of a circumbinary disk, the sharpness of the occulting edge implies that dust has settled into a vertically thin and dense layer. The properties of this dusty layer resemble those imagined for the formation of planetesimals. Furthermore, it is now quite clear that there is an extended (blue) nebula around the visible star and probably around the invisible star. This could be an inner disk or ring, a magnetically channeled accretion zone, an inner part of the jet or outflow, or a corona.

In my talk, I will present high-resolution echelle spectra (Keck/HIRES, VLT/ UVES) of KH 15D in each of the four previous observing seasons. Our observations focus heavily on the phases of ingress and egress in an effort to exploit the natural coronagraphic behavior of this system. The near IR colors of KH 15D show no signs of an inner dust disk; however, our line profile observations clearly indicate ongoing accretion and outflow, and possibly provide direct detection of gas in the circumbinary occulting disk. An absorption feature in Na I D appears at or close to the systemic velocity and is, therefore, presumably arising in gas associated with the circumbinary material.