Tonight, in addition to our own targets, we will be observing the AGN Zw 229-015 (AGN = Active Galactic Nucleus). By taking spectra using the Lick 3m telescope, the AGN team (Aaron Barth et al.) will be able to measure the Hbeta emission. This will help them to determine the mass of the black hole at the center of this AGN via a technique called reverberation mapping.
Reverberation Mapping
To understand the reverberation mapping technique, you need to start with the standard model of AGN:
The important features to note are the central accretion disk around the black hole, and the so-called "broad line region" (BLR). The BLR contains clouds which are orbiting the black hole with a velocity which corresponds directly to the mass of the black hole (M~R_BLR*V^2) where V is the velocity of the orbiting clouds, and R_BLR is the radial extent of the broad line region. The velocity of the clouds can be measured by looking at the effects of Doppler broadening on emission lines. The radial extent is what is measured using reverberation mapping. With these two numbers in hand, we can determine the mass of the central black hole which powers the AGN. Here's what the AGN looks like for real (grabbed from Aaron Barth et al. 2011):
Continuum emission from the central engine heats the broad line region clouds and causes emission lines to form. Since the accretion rate (and thus the continuum emission) from the central engine is variable, we can watch the change in the continuum level, then wait for that excess radiation to hit the broad line region some time later, which raises the emission level. By timing this lag and knowing the speed of light, we can calculate the radial extent of the BLR.
Anyway...back to our targets!
We decided yesterday that we would sacrifice the Na I D lines (which are contaminated by the street lights of San Jose) to get more of the continuum near the TiO bands (useful for determining spectral type). Ideally, we would set the central wavelength to be about 6700 angstroms (more or less centered on the Li line) so the Ha line isn't too far towards the end of the detector. Unfortunately, the maximum tilt of the grating is 25,500. This gives us a central wavelength of 6543. Good enough! (the grating tilt angle is determined from the central wavelength by an equation provided on the Lick website: 4.58*lambda - 4469 = angle).
A Note about Prioritizing...
I just wanted to mention the way that we've been prioritizing our targets. The GALEX Nearby Young Star Survey (GalNYSS) has over 2,000 low-mass stars with excess UV emission indicative of youth. While not all of those are Northern hemisphere stars, we still need a way to cut down the target list. We can make cuts based on the ra and declinations allowed by Lick at a given time of the year, but that still leaves us with hundreds of possible targets to observe. We then make a cut in proper motion - we accept only stars with total proper motion (sqrt(pmra^2 + pmdec^2)) greater than 40 mas/yr. This helps ensure that we're looking at nearby stars (which in general have higher proper motions across the sky) as opposed to distant giant stars (which would appear as luminous as nearby bright stars). This leaves us with ~300 stars at any given time of the year. We then look up each of those targets on Vizier and SIMBAD to make sure that there are no previously published measurements of lithium (usually there are not, but we sometimes find lithium measurements in Riaz et al. 2006, or Torres et al. 2006). Finally, since we are interested in finding stars close enough to Earth for the direct imaging of planets, we want to estimate their distance using their V-K color and absolute magnitude (Zuckerman & Song 2004):
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