KH 15D
My master's thesis at Wesleyan University under Prof. Bill Herbst is about the gas composition of the disk around KH 15D.
KH 15D is a 3 Myr-old K7 weak-lined T Tauri star in NGC 2264, about 760 pc away (Herbst et al. 2008). The star was first identified as unusual because of its strange lightcurve, with deep (3 mag in I), flat-bottomed eclipses and a long period (48 days), with a significant fraction (~1/3) of the lightcurve in the dimmer state (Kearns & Herbst 1998). The fraction of the lightcurve in the dimmer state has been increasing ever since then. Extensive photometric observations (Hamilton et al. 2005) combined with data from plate archives (Johnson et al. 2005) and radial velocity measurements (Johnson et al. 2004) let to the current theory: KH 15D is an eccentric binary star system gradually being occulted by an edge-on, warped, precessing circumbinary disk (Chiang & Murray-Clay 2004, Winn et al. 2004, Winn et al. 2006). KH 15D Basics
What makes this particular disk so interesting beyond the fortuitous alignment is that there is no reddening as Star A is eclipsed, as would be expected for a disk composed of small grains. (Star A is the star currently that currently appears above the disk; Star B is hidden behind the disk for the foreseeable future.) Herbst et al. 2008 calculate that the grains in the disk must be 1 mm or larger in size.
Winn et al. 2006
Winn et al. 2004
The solids in the disk have apparently condensed into chondrule-sized grains. My thesis will be to explore the density of the gas at different heights above the disk. I will use the NaI D lines from high-resolution spectra obtained at several different telescopes while Star A was at different elevations above the disk. Sodium is used as a tracer for gas in the ISM and has been used to measure gas in several debris disks: Beta Pic (Petterson & Tobin 1999), HD 32297 (Redfield 2007), Beta Car, HD 85905, and HR 10 (Redfield et al. 2007). Another reason sodium was used is because the NaI D lines are a doublet, so effectively you get two measurements for the price of one. My Master's Thesis
The phase of the spectral measurements relative to the changing lightcurve of KH 15D. Those shown with dotted lines were not used for NaI D measurements because of low S/N or bad weather.Spectra were originally taken for studying H alpha caused by gas accreting onto the young star, so many of the spectra were taken at times that were unfavorable for measuring sodium absorption. Spectra were taken using Keck, VLT, HET, and Magellan.
A spectrum of KH 15D and the comparison star spectrum after being shifted to match. The left panel shows the portion of the spectrum used to align the KH 15D and comparison star spectra, and the right panel shows the two spectra near the NaI D lines.By carefully aligning the KH 15D spectra with a comparison star spectrum using other spectral lines than sodium, and then taking the ratio between the two, we were able to measure excess NaI absorption.
Ratioed KH 15D spectrum at each of the two NaI D lines.This absorption dropped significantly at higher elevations of Star A above the disk, allowing us to measure a scale height for the gas disk.
The measured sodium column densities vs. height of Star A above the grain disk. The right panel shows the points that were used to find the scale height of the gas disk.Many questions are still left to answer. We don't know if the dust grains are supported within the warped disk by self-gravity or gas pressure. We don't know how much gas there is (a lower limit was calculated in my thesis, but this is severely limited by our lack of knowledge about the orientation of the system with respect to our line of sight). So like any good paper, this ends with a lost of questions waiting to be answered.
KH 15D has now reached the point where Star A no longer rises above the grain disk.
Here's my thesis Wordle:
