PT failure to detect transit of HD149026 on UT 2008 Apr 18 = MJD 54574

Michael Richmond
Douglas Tucker
May 04, 2008

On the night of UT Apr 18, 2008, the SDSS Photometric Telescope ("PT" for short) took a series of exposures of HD 149026. Conditions were marginal (?). I think we (again) failed to detect the transit of this system.

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 ought to show a small excess of scatter over the neighboring star of the same brightness, but doesn't. The "noise floor" in these measurements is about 0.008 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.

The lack of measurements for the target star near the start of the run is a mystery to me. The testobj-*.par files produced by mtpipe do not include any measurements for the target in these frames.

I don't see an event.

Justin's listing for this event gives the ingress time as 03:09 UT = 574.631; that is before our measurements start. His egress time is 06:24 UT = 574.767, during the gap in measurements. Given the noise in the data, I don't think we would have seen it even if there had been no gap.

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 18, 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
Apr18.72838      4574.72838   4574.73106   0.025  0.010 
Apr18.72938      4574.72938   4574.73206   0.056  0.010 
Apr18.73041      4574.73041   4574.73309   0.061  0.009 

Last modified 05/04/2008 by MWR.