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AAVSO Alert Notice 370

GJ 436 Extrasolar Planet Transit Campaign
February 12, 2008


Observations of the extrasolar planetary system around GJ 436 
are requested to help explain anomalous changes in the 
transiting properties of a "hot Neptune" type planet (GJ 
436b). These perturbations manifest themselves in changes in 
the larger planet's transit properties. Specifically, the 
transit time, duration and amplitude may be affected. So far, 
only transit timing variations (changes in the transit 
midpoint) may have been detected.

A recent paper published to ArXiv by Ribas, Font-Ribera and 
Beaulieu suggests that a previously undetected 5-Earth mass 
planet may exist elsewhere in the GJ 436 system and that its 
gravity may be the cause of the anomalies. If the smaller planet 
does exist, it would be the smallest planet yet detected in a solar 
system around a main sequence star.

However, that is still a big *if* as there are possibly other 
astrophysical explanations as well. The theory is provocative 
enough that it should be checked. The key will be to get 
large amounts of quality data to confirm or negate this dual 
planet hypothesis.

This system is not as bright (V=~10.7) and the expected 
transit length (~1h) is not as long as other extrasolar 
transit campaigns we have conducted. So this may be a good 
entry point for those who have not participated in an 
extrasolar planet campaign before. However, the transit depth 
is still very low (<.01 mag) so it will be a challenging 
system. Please follow our photometric guidelines (below) 
carefully and take your time when reducing the photometry.

Dr. Greg Laughlin (transitsearch.org and University of 
California, Santa Cruz) has analyzed recent data and 
measurements from both observers and the literature to come 
up with the following transit period and ephemeris:

 P = 2.643901 days
 T_mid = 2454222.6157 HJD (02:46:36.48 UT May 2, 2007)

Dr. Laughlin has agreed to list anyone who contributes 
quality data of a transit as a co-author of any paper that 
may result from this campaign.


Photometric observations of future transits of the system are 

 The next transit is scheduled for February 14, 2008 from 
06:49-07:47 UT. The actual transit may occur 10 to 15 minutes 
later. Transits occur about every 2.64385d. A list of 
subsequent transit dates can be found here:

GJ 436 is located at R.A. = 11:42:11.1, Dec. = +26:42:24 
(2000). Finder charts can be made using the Variable Star 
Plotter (http://www.aavso.org/observing/charts/vsp/) and 
using "GJ 436" as the star's "name". Please submit your data 
to the AAVSO using the same name.

The GJ 436 M3V parent star is very red. As a result, it will 
be very important to submit your airmass values. They can be 
computed with our Airmass calculator at 
http://www.aavso.org/observing/programs/ccd/airmass.shtml .

A spectra of the object from Maness, H. L., Marcy, G. W., 
Ford, E. B. et al. (2007PASP..119...90M) has been posted at 
this URL: http://www.aavso.org/news/gj436_spectra.jpg

GJ436 itself has these magnitude and colors:
        V     B-V    V-Rc   Rc-Ic   V-Ic
     10.702  1.489  1.073   1.349   2.462

Note that it is intrinsicly very red, so the spectrum is full 
of molecular bands, and the star is *much* brighter redwards 
of V (Ic=8.240).  The comparison stars in the field are 
typically much bluer than the target star (the reddest has 
V-Ic = 1.08).  This is good, in that the comparison stars are 
unlikely to be variable; it is bad, in that everyone's 
photometry will be on a different "system" unless 
transformed, and second-order extinction will be important.

The bottom line is that you are probably better off not 
transforming your data, but be sure that you submit long time 
series so that we can detrend the photometry for airmass 
effects, and offset your results so that your data will be on 
the same scale as other observers. Luckily, the transits are 
short so you are likely to cover an entire transit and data 
from two or more observers will not be necessary to get 
ingress and egress timings.  Don't just observe the hour-long 
transit; be sure you have a long lead-in and exit segment for 
our detrending needs.

Your best bet is to observe filtered, and with B as your 
first preference and V as your second preference.  B is less 
affected by molecular absorption, and the star will be 
fainter at B.  There are two advantages to this. First, being 
fainter means you can take longer exposures, which will 
reduce the effect of scintillation.  Second, and perhaps more 
important, there will be a smaller magnitude difference 
between GJ436 and its nearby comparison stars.  This means 
that an ensemble of these comparison stars will yield better 
results.  V-band is a good second choice, but do not go any 
redder than this.

With a 0.006 magnitude transit depth, every small error gets 
magnified. Even at 8 micron (SST) depth is 7.3 +/- 0.3 mmag, 
with no dependence upon filter (so far). When possible, 
observe when the transit is expected to occur near meridinal 
crossing so that airmass changes are minimized.  Use large 
apertures so that centroiding of stars does not cause a 
measurement error.  Get lots of signal/noise in your flats (1 
million electrons per pixel is the minimum you should have, 
usually from a large stack of flats).  Try to keep the field 
in the same spot on your chip (autoguiding really helps for 
this project).  Use ensemble techniques for the photometry if 
at all possible.  Keep accurate time, and make sure your 
submitted photometry uses the exposure midpoint for its time. 
I would suggest binning your data in chunks no larger than 30 
seconds elapsed time to get good time resolution on your 
light curve.


Info on the system and on a recent, previous campaign we have 
run on GJ 436 is here: 

Bruce Gary has created a web page with more detailed 
information on this system and some recent light curves: 

The Ribas, Font-Ribera and Beaulieu ArXiv paper is available at:

Please consult the AAVSO-Photometry Discussion Group for 
questions, advice and updates regarding photometry of this 
system. The URL for archives and subscription is: 

This Alert Notice was written by: A. Price & A. Henden


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