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the extremely precise 33 hour phase function measurement of hd  189733b ( * ? ? ? * knutson et al .  ( 2007 ) ) observed with the 8 micron irac camera on the spitzer space telescope yielded the most precise times of transit and secondary eclipse for any extrasolar planet , 6 and 24 seconds , respectively .
this led us to propose to observe an additional six transits and eclipses of this system over time with the goal of measuring :    * precise transit - timing ( ( * ? ? ? * agol et al .
2005 ) , ( * ? ? ?
* holman & murray 2005 ) ) to search for the presence of resonant ( or near - resonant ) terrestrial - mass planets captured by migration ( ( * ? ? ?
* e.g.  mandell , raymond & sigurdsson 2007 ) ) or on longer timescales precession of an eccentric orbit ( ( * ? ? ?
* miralda - escud 2002 ) , ( * ? ? ?
* heyl & gladman 2007 ) ; also fabrycky and wolfe , these proceedings ) ; * variations in the depth of the secondary eclipses with time which might be caused by large - scale variable atmospheric features ( ( * ? ? ?
* rauscher et al . 2007 ) ; also showmand and dobbs - dixon , these proceedings ) ; * precise transit depth for comparison with atmospheric - absorption models to contrain the molecular composition ( ( * ? ? ?
* e.g. tinetti et al .
2007 ) ; also tinetti et al .
, fortney et al . , and hubeny et al . , these proceedings
) ; * improved system parameters for better characterization of the planet , host - star , and orbit properties ( ( * ? ? ?
* winn et al .  ( 2007 ) ) ; also winn , these proceedings )
.    for librating planets in a low - order mean motion resonance , the times of transit vary with an amplitude : @xmath0 and libration period of @xmath1 where @xmath2 are masses of the earth , jupiter , and host star , @xmath3 is the period of the transiting planet , and the numbers have been estimated for the libration amplitude for planets starting on circular orbits with exact commensurability ( * ? ? ?
* ( agol et al .  2005 ) ) ; the actual value depends on the libration amplitude .
this timescale requires observations separated by months with @xmath4 seconds precision in timing , and in principle could be sensitive to sensitive to mars mass planets .
so far spitzer has observed four transits and four secondary eclipses of hd  189733 for this program with 44,000 exposures of 0.4 second each over 5 hours each .
additionally we re - analyzed the data from ( * ? ? ?
* knutson et al .  ( 2007 ) ) .
we utilized the irac 8 microns as it has been demonstrated to be the most stable irac band . due to the brightness of the host star we made the observations in sub - array mode .
we carried out aperture photometry with a 3.5 pixel radius . due to the small limb - darkening , stable instrument ( thanks to the earth - trailing orbit of spitzer which leads to stable thermal properties and no occultation of targets by the earth , as occurs with hst ) we obtained 0.5% precision per 0.4 second exposure . in the shot - noise limit
we expect a 3-second precision for transit times .
figure [ fig01 ] shows a gallery of the transits and secondary eclipses obtained . the strong  ramp " which causes the flux to change by about 1% is a well known feature of the irac camera when observing bright sources .
we fit the ramp with the function @xmath5 , where @xmath6 is the start of the observation and @xmath7 are constants .
this appears to remove any trace of the ramp in the corrected data .
we assign each data point an error bar which equals the scatter in residuals of the data outside of transit / eclipse .
in two initial fits we include one ephemeris for the transits , one ephemeris for the secondary eclipses , and required all other parameters describing the transits and eclipses to be the same for all 10 transits / eclipses to determine their mean values .
we performed one of these fits with stellar limb - darkening set to zero , while for a second fit we allowed the linear coefficient of the stellar limb - darkening to freely vary ( in all cases we assume the planet to be uniform in surface brightness ) .
we allowed the ramp parameters to vary independently for each transit / eclipse .
a sum of the five transits corrected for the ramp is shown in figure [ fig02a](a ) without limb darkening , and figure [ fig02b](b ) with limb darkening .
the five secondary eclipses are shown in figure [ fig02c ] .    without limb darkening we find a best - fit @xmath8 for 231508 degrees of freedom ( 231516 data points with 8 model parameters ) . with linear limb darkening the @xmath9
improves by @xmath10 for 231507 degrees of freedom , with a best - fit limb darkening parameter of @xmath11 ; thus limb - darkening is detected at 12@xmath12 - this can be seen by eye by comparing figures [ fig02a](a ) and [ fig02b](b ) .
this best - fit value for limb - darkening gives a limb - darkening profile which is very close to that predicted by a kurucz model with parameters appropriate for hd  189733a ( effective temperature @xmath13 k and surface gravity @xmath14 = 4.5 $ ] ) .
we find a best - fit planet - star radius ratio of @xmath15 , which translates into a best - fit area ratio of @xmath16% , while the best - fit eclipse depth is @xmath17% - note that this is a 72@xmath12 detection of a secondary eclipse ! the errors on these parameters ( and throughout the rest of the paper ) are computed by generating one hundred synthetic data sets from the best fit light curve ( for each model ) added to the residuals shifted by a random number of data points , re - fitting the model to each synthetic data set , and then taking the standard deviation of the 100 synthetic parameter sets to determine each parameter s error .
we have measured the best - fit ephemeris to our data , separately for the transits and eclipses .
we find a transit ephemeris of @xmath18 hjd and @xmath19 days , while for the eclipses we find @xmath20 hjd and @xmath21 days , assuming that they occur exactly one half period after primary transit .
the periods are consistent within @xmath22 , while the central eclipse times differ by @xmath23 seconds , indicating that the secondary eclipses occur later .
about 31 seconds of this difference can be accounted for by the light travel time across the system , while the other 26 seconds is likely due to a slight orbital eccentricity .
we have performed a second set of fits allowing the central times of transit / secondary eclipse to vary .
the transit timing variations are plotted in figure [ fig03 ] for the transits and secondary eclipses , compared to the best - fit transit ephemeris ( and assuming the secondary eclipse is offset by 1/2 an orbit ) .
we find no significant deviations from a uniform period by more than 5 seconds for either the transits or secondary eclipses ; once the final four data sets are obtained we will carry out a more detailed analysis .
poster 79 by miller - ricci and poster 87 by neale gibson at this iau symposium also present new transit - timing data for this system .
in a third set of fits we held the ephemerides fixed , but allowed the depths of secondary eclipse to vary .
figure [ fig04 ] shows the variations in the five observed eclipse depths in units of the flux of the star .
we find that the fractional variations in the eclipse depth are smaller than about 10% ; this is about the level of variation predicted by ( * ? ? ?
* rauscher et al .  ( 2007 ) ) .
a fit to the eclipse depths assuming a constant depth gives a @xmath24 for 4 degrees of freedom ( 5 eclipses minus one free - parameter , the mean eclipse depth ) .
once the final two secondary eclipse observations are obtained ( in june and july 2008 ) , a more comprehensive analysis will be carried out of the limits on secondary eclipse depth variation .
the depth of primary transit we have obtained contains five times as much data as that in ( * ? ? ?
* knutson et al .  ( 2007 ) ) , providing more precise constraints on the spectral energy distribution in the mid - infrared .
we have combined this data point with transit depths measured by ( * ? ? ?
* pont et al .  ( 2008 ) ) in the optical with hst , ( * ? ? ?
* swain et al .  ( 2008 ) ) in the near - infrared with hst , ( * ? ? ?
* beaulieu et al .  ( 2008 ) ) at 3.6 and 5.8 microns with irac , and ( * ? ? ?
* knutson et al .  ( 2008 ) ) at 24 microns with mips ( also knutson et al . , these proceedings ) .
the data are plotted in figure [ fig05 ] along with the best - fitting model of ( * ? ? ?
* tinetti et al .  ( 2007 ) ) which does not have enough methane to fit the near - infrared data .
we find that our 8 micron data point lies above the tinetti model ; as the 8 micron band covers a region in which methane absorption is quite strong ( @xmath25 @xmath26 per molecule ) , the model may be brought back into agreement with the data with a higher abundance of methane , as is already required by the near - infrared data ( ( * ? ? ?
* ; * ? ? ?
* swain et al .  2008 ) ) .
it also clear that the model is a poor fit to the optical data ; the fit may be improved by including rayleigh scattering ( lecavelier des etanges et al .
2008 , and these proceedings ) .
error bars.,width=491 ]
the five transits we have observed are consistent with no transit timing variations greater than 5 seconds .
the five secondary eclipses are consistent with no eclipse depth variations at greater than the 10% level . from just an analysis of the lightcurves
we can place constraints on the eccentricity and longitude of periastron : @xmath27 and @xmath28 ; similar to the results of ( * ? ? ?
* winn et al .  ( 2007 ) ) .
we are collecting two more transits and two more secondary eclipses in summer 2008 ; thus the results presented here are preliminary and require further more careful analysis .    2005 , _ mnras _ 359 , 567 , 2008 , _ apj _ , 677 , 1343 2007 , _ mnras _ , 377 , 1511 2005 , _ science
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we present limits on transit timing variations and secondary eclipse depth variations at 8 microns with the spitzer space telescope irac camera . due to the weak limb darkening in the infrared and uninterrupted observing , 
 spitzer provides the highest accuracy transit times for this bright system , in principle providing sensitivity to secondary planets of mars mass in resonant orbits . 
 finally , the transit data provides tighter constraints on the wavelength - dependent atmospheric absorption by the planet .