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New Observations and a Possible Detection of Parameter Variations in the Transits of Gliese 436b
We present ground-based observations of the transiting Neptune-massplanet Gl 436b obtained with the 3.5 m telescope at Apache PointObservatory and other supporting telescopes. Included in this is anobserved transit in early 2005, over 2 years before the earliestreported transit detection. We have compiled all available transit datato date and perform a uniform modeling using the JKTEBOP code. We do notdetect any transit timing variations of amplitude greater than ~1 minuteover the ~3.3 year baseline. We do however find possible evidence for aself-consistent trend of increasing orbital inclination, transit width,and transit depth, which supports the supposition that Gl 436b is beingperturbed by another planet of <~12 M? in anonresonant orbit.
| A Hubble Space Telescope transit light curve for GJ 436b
We present time series photometry for six partial transits of GJ 436bobtained with the Fine Guidance Sensor instrument on the Hubble SpaceTelescope (HST). Our analysis of these data yields independent estimatesof the host star's radius Rstar =0.505+0.029-0.020 R&sun;, and theplanet's orbital period P = 2.643882^+0.000060-0.000058 d,orbital inclination i = 85.80°+0.21°-0.25°, mean central transit timeTc = 2 454 455.279241+0.00026-0.00025HJD, and radius Rp = 4.90+0.45-0.33R?. The radius we determine for the planet is largerthan the previous findings from analyses of an infrared light curveobtained with the Spitzer Space Telescope. Although this discrepancy hasa 92% formal significance (1.7?), it might be indicative ofsystematic errors that still influence the analyses of even thehighest-precision transit light curves. Comparisons of all the measuredradii to theoretical models suggest that GJ 436b has a H/He envelope of10% by mass. We point out the similarities in structure between thisplanet and Uranus and Neptune and discuss possible parallels betweenthese planets' formation environments and dynamical evolution. We alsofind that the transit times for GJ 436b are constant to within 10 s overthe 11 planetary orbits that the HST data span. However, the ensemble ofpublished values exhibits a long-term drift and our mean transit time is128 s later than that expected from the Spitzer ephemeris. Thesparseness of the currently available data hinders distinguishingbetween an error in the orbital period or perturbations arising from anadditional object in the system as the cause of the apparent trend.Assuming the drift is due to an error in the orbital period we obtain animproved estimate for it of P = 2.643904 ± 0.000005 d. This valueand our measured transit times will serve as important benchmarks infuture studies of the GJ 436 system.Table 2 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/486/1039
| A finding-list of stars of spectral type F2 and earlier in a north galactic pole region.
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Observation and Astrometry data
| Constellation: | Löwe |
| Right ascension: | 11h42m20.29s |
| Declination: | +26°45'37.0" |
| Apparent magnitude: | 10.638 |
| Proper motion RA: | 9.5 |
| Proper motion Dec: | 7.2 |
| B-T magnitude: | 11.131 |
| V-T magnitude: | 10.679 |
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