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AAVSO Alert Notice 350: Exoplanet Transit Search For GJ 436 AAVSO HOME > publications > alert notice

AAVSO Alert Notice 350

Exoplanet Transit Search For GJ 436
June 6, 2007

Object: 1137+27 GJ 436
    RA (2000) DEC           V     B-V    V-Rc   Rc-Ic  V-Ic
 11:42:11.58 +26:42:17.7  10.702  1.489  1.073  1.349  2.462

INTRODUCTION

Dr. Greg Laughlin (UC Santa Cruz/Lick Observatory & Transitsearch.org) has
requested help in observing GJ 436 to look for transits of previously
detected and undetected extrasolar planets.

A Neptune mass planet was recently discovered transiting this M2V class 
star with a photometric depth of 0.6% and a period of 2.64 days. This is
 the planet which has been mentioned in the popular press recently as a
"hot water ice" planet because it likely contains water solidified by
atmospheric pressure.

According to Dr. Laughlin: "The radial velocity data set for the star
indicates that the transiting planet has a significant eccentricity. Given
the fact that it has a Neptune-like composition, the tidal circularization
timescale is quite short, and the presence of an eccentric orbit thus
strongly indicates the presence of additional planets in the system. These
can potentially be detected through transit, or alternately, and more
provocatively through precise timing of the transits of the known transiting planet.

"This is probably the most important opportunity that has come around for
small-telescope observers in a long time."

PHOTOMETRIC GUIDELINES & ADVICE

Since this is a red star and observations will have to both precise and 
accurate, careful photometry will be critical. Precision of less than 0.006 
will not be easy. So please take your time and be as careful as possible. 
We recommend this project for experienced observers only. Consider this a
challenge to push your system's capabilities.

Arne has proposed the following photometric procedures and guidelines for
this project:

"Since we are looking for extremely small transit depths, I'd suggest 
using ensemble techniques if your software permits. Use the set of 8 
comparison stars for the field as your ensemble.

If you use the standard comparison and check scheme, the best stars look 
like two stars in the grouping just NNW of the target:

                                  V      B-V   V-Rc   Rc-Ic  V-Ic
comp   11:42:28.04 +26:49:42.6  10.684  0.986  0.582  0.506  1.082
check  11:42:12.08 +26:46:07.4  11.370  1.071  0.562  0.503  1.060
          
These are about as close in color as possible to the target, but there will
still be systematic differences between observers if data is not 
transformed. Beware that the 11.370 star has a fainter companion about
15arcsec to the NW - try to use an aperture small enough to exclude this 
companion.

GJ436 is near 11th magnitude, so exposures will be relatively long in  
comparison to some of the other transiting targets. Scintillation will not 
be as important. If you can do two filters, it is strongly advised to do so 
in order that your data can be transformed. Choosing B and V would be your 
best choice, as B is less influenced by molecular lines and, since GJ436 is 
fainter in the blue, will enable longer exposures so that scintillation is 
even less important. Remember that exoplanet transits are 'grey', meaning 
there is little advantage for any specific bandpass in maximizing transit 
depth. Instead, use filters to: standardize your observations; cut down the 
amount of light so that you can use longer exposures; and to remove the 
influence of spectral features like molecular absorption lines that will 
plague unfiltered measurements.

We will provide instructions as to how to determine your transformation 
coefficients in the near future."

A comparison star sequence w/BVRI photometry has been added to the Variable 
Star Plotter. Visit the URL below and use "GJ 436" as the star name. Note
that GJ 436 is a high proper motion system, so the "dot" to the immediate 
northwest of the crosshairs is the position of GJ 436 when the NOMAD 
database was created. Thus the dot is not real and GJ 436 is not a double 
star.

            http://www.aavso.org/observing/charts/vsp/
            
An ephemeris for transits is located at the URL below. However, we need
observations at all times, not just during transits. So please don't 
restrict your observing to transit windows alone.

             http://www.ucolick.org/%7elaugh/GJ436____b.transits.txt
             
It would be most useful if we had many observers working on the same 
evening. If you can, post your observing session dates and times to the
AAVSO-Photometry Discussion Group and try to schedule together. We will 
keep an eye on the incoming data and post periodic updates and light curves 
to the same discussion group.

DATA ANALYSIS HELP

There is an opportunity for non-observers to also help out with this 
campaign. Dr. Laughlin and his team have developed a tool that does 
characterization and dynamical analysis of published datasets for known 
extrasolar planets. Volunteers are needed to work with the data of this and 
other extrasolar planetary systems. The software, called Systemic Console, 
runs on Windows, OS X and Linux with a java-based GUI. Info is here:

                    http://oklo.org/?page_id=86
                    
This campaign will be coordinated by: A. Price.




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