PT failure to detect transit of HD149026 on UT 2008 Feb 26 = MJD 54522

Michael Richmond
Douglas Tucker
Mar 13, 2008

On the night of UT Feb 26, 2008, the SDSS Photometric Telescope ("PT" for short) took a series of exposures of HD 149026. The results were poor, and we didn't detect any event. I suggest that we might remove this target from our list.

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 night looked decent after I removed two big outliers just before the gap in the run. 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 shows a VERY small excess of scatter over neighboring stars of the same brightness. The "noise floor" in these measurements is about 0.008 mag -- not good at all. The two brightest stars may be saturated slightly. The outlier at differential mag 3 is the eclipsing binary mentioned above.

Below are the light curves for the target (green symbols) and some comparison stars in the field. The eclipsing binary is the curve at the bottom.

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

The ingress would be at 522.84 (see ephemeris below), but I see nothing. The data is really noisy before the gap.

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
2454522.84 2008  2 26  8 13   2454522.96 2008  2 26 11  7   2454523.08 2008  2 26 14  2

The ephemeris ingress of UT 2008 Feb 26 08:13:00 corresponds to JD 2,454,522.842. The ephemeris egress of UT 2008 Feb 26 14:02:00 corresponds to JD 2,454,523.085, which falls outside our observing run.

Again, we didn't detect anything for this target on this night, and I suspect we might not be successful even under better conditions. The star may be just too bright for this telescope.

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, Feb 26, 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
Feb26.82094      4522.82094   4522.82158   0.038  0.008 
Feb26.82196      4522.82196   4522.82260   0.043  0.007 
Feb26.82300      4522.82300   4522.82364   0.027  0.008 

Last modified 03/13/2008 by MWR.