PT measurement (?) of transit of HD149026 on UT 2008 Apr 12 = MJD 54568

Michael Richmond
Douglas Tucker
May 03, 2008

On the night of UT Apr 12, 2008, the SDSS Photometric Telescope ("PT" for short) took a series of exposures of HD 149026. Conditions were good, and I think we may have detected the ingress of this transit.

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 shows a small excess of scatter over neighboring stars of the same brightness. The "noise floor" in these measurements is about 0.005 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, but if I had to guess at an ingress, it would be around 568.89.

Since the ephemeris from seems to be very different from the one Justin is using , I'll use Justin's times here.

The ephemeris ingress of UT 2008 Apr 12 09:07:00 corresponds to JD 2,454,568.880, which is close to my guess at a dip in the data. Could we have detected the ingress?

The ephemeris egress of UT 2008 Apr 12 12:22:00 corresponds to JD 2,454,569.015, which falls far outside our observing run.

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 12, 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
Apr12.83524      4568.83524   4568.83777   0.048  0.007 
Apr12.83627      4568.83627   4568.83880   0.049  0.006 
Apr12.83727      4568.83727   4568.83980   0.061  0.006 

Last modified 05/03/2008 by MWR.