PT measurements (?) of transit of HD149026 on UT 2008 Apr 15 = MJD 54571

Michael Richmond
Douglas Tucker
May 03, 2008

On the night of UT Apr 15, 2008, the SDSS Photometric Telescope ("PT" for short) took a series of exposures of HD 149026. Conditions were good, and we _may_ have detected the transit ... but at a very low confidence level.

Notes from the night

This is a chart of the field. HD149026 is the bright star indicated by the crosshairs. The labelled stars will appear in later analysis.

As a side note, the star marked by short line segments (the northwestern component of a close pair) is an eclipsing binary with a period which _might_ be about 7.7 hours.

The host star of HD149026 has a magnitude V=8.16 according to The N2K Consortium. II. A Transiting Hot Saturn Around HD 149026 With a Large Dense Core. However, there's a note in the Exoplanets Encyclopedia about this planet which states "25 Nov 06: Wright et al 2006 raise doubts about this planet."

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!

The graph below shows the amount by which instrumental magnitudes from each image needed to be shifted to match the ensemble reference (after removing the two outliers). On a clear night, this graph would show a straight horizontal line.

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

HD149026 is the fainter of the two stars near differential mag 0.0; it may show a small excess of scatter over the neighboring star of the same brightness. The "noise floor" in these measurements is about 0.006 mag -- not very good, due to the small number of bright comparison stars (and perhaps the short exposure times). The two brightest stars may be saturated slightly.

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

In this closeup, I have shifted the data for two comparison stars to move them closer in magnitude to the target.

I don't see an event.

Using Justin's page as a source for the ephemeris of this system, the ephemeris ingress of UT 2008 Apr 15 06:08:00 corresponds to JD 2,454,571.756; these is a small, brief dip near that time, but it is followed by a brief rise. Hmmm.

The ephemeris egress of UT 2008 Apr 15 09:23:00 corresponds to JD 2,454,571.891, just before the end of our observing run. It is POSSIBLE that the measurements rise at this point, but it's hard to say without a longer post-egress dataset.

You can grab the measurements for your own analysis. Below is a table with three flavors of time, plus the differential magnitude of the target 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 HD149026 made with APO PT, Apr 15, 2008 UT. 
# Each exposure 3 seconds long in SDSS i-band; 
# Tabulated times are midexposure (FITS header time - half exposure length) 
#     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
Apr15.71455      4571.71455   4571.71716   0.057  0.010 
Apr15.71565      4571.71565   4571.71826   0.055  0.007 
Apr15.71667      4571.71667   4571.71928   0.046  0.006 

Last modified 05/03/2008 by MWR.