PT photometry of the field around TrES-3

Michael Richmond
Sep 27, 2007

On the night of UT Sep 16, 2007, the SDSS Photometric Telescope ("PT" for short) took a series of exposures of the field around TrES-3 . The series included only the start of the transit, ending before egress.

Notes from the night

This is a chart of the field. TrES-3 is the bright star indicated by the crosshairs. I have marked one bright star nearby which will appear (together with other stars not on this chart) later in graphs.

The star marked "C" has Tycho-2 magnitudes of Bt=12.95, Vt=11.83. The host star of TrES-2 itself has a magnitude V=12.40, according to TrES-3: A Nearby, Massive, Transiting Hot Jupiter in a 31-Hour Orbit.

Following the procedures outlined by Kent Honeycutt's article on inhomogeneous ensemble photometry, I used all stars available in each image to define a reference frame, and measured each star against this frame. You can find the software package used to do the ensemble photometry online; it's free!

Below is a graph of the scatter in differential magnitude versus magnitude in the ensemble solution.

TrES-3 is the star at differential mag 2.1; note its slightly elevated scatter. The "noise floor" in these measurements is about 0.003 mag -- very nice!

Below are the light curves for the target (green symbols) and four comparison stars in the field.

You can see that TrES-3 becomes dimmer in the latter stages of this run. The transit is obvious when we zoom in: in this closeup, I have shifted the data for star C to move them closer in magnitude to the target.

An ephemeris grabbed from predicts for this night

 Begin Transit Window           PREDICTED CENTRAL TRANSIT     End Transit Window
                                     All Times UT

                               HJD        Year M  D  H  M
 2454359.60 2007  9 16  2 17   2454359.63 2007  9 16  3 12   2454359.67 2007  9 16  4  7

The ingress start of UT 2007 Sep 16 02:17:00 corresponds to JD 2,454,359.5951. That seems a bit earlier than the light curve above would suggest -- the light curve indicates ingress at about 359.605, some 14 minutes later than the ephemeris. However, since the PT images don't start much earlier than this time, there isn't a really strong baseline for the pre-transit brightness of the target star.

You can grab the measurements for your own analysis. Below is a table with three flavors of time, plus the differential magnitude of TrES-3 and an estimate of the uncertainty in each measurement. I show the first few lines of the file to give you an idea of its format.

# Measurements of TrES-3 made with APO PT, Sep 16, 2007 UT. 
# Each exposure 30 seconds long in SDSS i-band; 
# Tabulated times are midexposure and accurate only to +/- 1 second (??). 
# 'mag' is a differential magnitude based on ensemble photometry 
#    using a circular aperture of radius 5.25 arcseconds. 
# UT day      JD-2,450,000  HJD-2,450,000   mag    uncert
Sep16.59593      4359.59593   4359.59613   2.196  0.004 
Sep16.59723      4359.59723   4359.59743   2.198  0.004 
Sep16.59856      4359.59856   4359.59876   2.199  0.004 

Warning: the times in this datafile have a small systematic error. I assumed the FITS header time was the shutter open time, so added half the exposure time to it. The FITS header time is instead roughly the shutter CLOSING time. My calculated times are therefore later than correct by about the duration of one exposure.

Last modified 9/17/2007 by MWR.