|Photo by Robert Gendler|
INTRODUCTION AND BASIC DATA
KH 15D is a variable star near the Cone Nebula in the extremely young cluster NGC 2264. It was first recognized as interesting and, indeed, unique by Kearns & Herbst (1998). They discovered its 48 day periodicity and extremely deep (~3.5 mag) minima. They also discovered its peculiar return to a brighter level near the middle of the eclipse.
The light curves over the first few seasons, based on data obtained at Wesleyan are shown below.
The fundamental nature of the star was determined by Hamilton et al. (2001) based on their spectral analysis. It is a K7 pre-main sequence member of NGC 2264, with a mass of 0.5-1 solar mass and an age of 2-4 My. Basic information from Table 1 of their paper, modified with updated results, is reproduced here.
A BRIEF HISTORY
An international campaign to monitor the star was organized for the 2001/2002 observing season by Wesleyan physics Ph.D. candidate Catrina Hamilton. Participants were:William Herbst, Wesleyan University
Catrina Hamilton, Wesleyan University/Connecticut College
Frederick Vrba, USNO, Flagstaff Station
Mansur Ibraghimov, Tashkent, Maidanak Observatory
Coryn Bailer-Jones, MPIA-Heidelberg
Reinhard Mundt, MPIA-Heidelberg
Markus Lamm, MPIA-Heidelberg
Eric Williams, Wesleyan University
Andrew Rhodes, Wesleyan University
Tsevi Mazeh, Tel Aviv University, Wise Observatory
Zoe Webster, UC Berkeley
Tom Balonek, Colgate University
Richard Crowe, U Hawaii-Hilo
Karl Haisch, NASA Ames
Alex Scholz, Tautenberg
Arno Riffeser, Munich
In addition, spectra were obtained in and out of eclipse with the UVES high resolution spectrograph attached to the Ver Large Telescope. The light curve during 2001/2002 is shown below. The spectra revealed a "natural coronagraphic effect". During eclipse, it was clear that the star was completely obscured by the cloud and we viewed it only by reflected light. The surrounding nebulosity became much brighter by contrast, such that KH 15D transitions from a weak-line T Tauri star during maximum light to a classical T Tauri star during minimum light. It is clearly still an accreting system which is driving an outflow. (Herbst et al. 2002; Hamilton et al. 2003).
More Recent Light Curves
The light curves have continued to evolve in the sense that the minima are now deeper and longer. They now extend for ~half of the 48.35 day period. Data from Tenagra Observatory over the period 2001-2004 is shown below.
Historical Light Curves and a ModelInvestigations of the historical light curve of KH 15D based on archival photographs from the last century through 1982 showed that the deep eclipses began only recently, probably in the 1990's. Prior to then the star was brighter and the eclipses were either non-existent or were shallower and occurring nearly 180 degrees out of phase with the current ephemeris (Winn et al. 2003; Johnson & Winn 2004). These discoveries led to a binary model for KH 15D in which the currently visible star is in an eccentric orbit which allows it to peak above a circumstellar disk. The disk may be precessing, causing the gradual evolution of the light curve. (Winn et al. 2004; Chiang & Murray-Clay 2004). The binary model has been confirmed in a radial velocity study by Johnson et al. (2004). The most detailed recent study of the binary model is by Winn et al. (2006). Herbst et al. (2008) present a different interpretation for the reflected light component and argue that significant grain growth has occurred within the disk (see also Agol et al. 2004).
Other Recent Results and SummaryKH 15D is unusual in other respects. It is apparently the source of a filamentary nebula visible in the light of shocked molecular hydrogen (Tokunaga et al. 2004, Deming et al. 2004). This suggests that it is, indeed, still active in producing a wind or jet outflow from either a disk or from the star. Furthermore, the star is a remarkably weak X-ray emitter for a T Tauri star of its effective temperature (Herbst & Moran, 2005). The star deserves continued and intensive observation because it has unique potential to allow us to probe the inner disk environment of a solar-like star of planet-forming age. The results may well have implications for the origin of the Earth and other planets of our own Solar System.
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