Transit Photometry of the Core‐dominated Planet HD 149026b

Abstract

We report g, V, and r photometric time series of HD 149026 spanning predicted times of transit of the Saturn-mass planetary companion, which was recently discovered by Sato and collaborators. We present a joint analysis of our observations and the previously reported photometry and radial velocities of the central star. We refine the estimate of the transit ephemeris to T c ¼ (2;453;527:87455 þ0:00085 À0:00091 ) þ (2:87598 þ0:00012 À0:00017 )N ( HJD). Assuming that the star has a radius of 1:45 AE 0:10 R and a mass of 1:30 AE 0:10 M , we estimate the planet radius to be (0:726 AE 0:064)R Jup , which implies a mean density of 1:07 þ0:42 À0:30 g cm À3 . This density is significantly greater than predicted for models that include the effects of stellar insolation and in which the planet has only a small core of solid material. Thus, we confirm that this planet likely contains a large core and that the ratio of core mass to total planet mass is more akin to that of Uranus and Neptune than to either Jupiter or Saturn.

THE 8 μm PHASE VARIATION OF THE HOT SATURN HD 149026b

“…It has a mass comparable to that of Saturn (Sato et al 2005;Winn et al 2008;Carter et al 2009) but its small radius and correspondingly high average density (Sato et al 2005;Charbonneau et al 2006;Winn et al 2008;Nutzman et al 2009;Carter et al 2009) suggest that, unlike Saturn, an incredible 50%-90% of this planet's mass must exist in the form of a solid icy or rocky core (Sato et al 2005;Fortney et al 2006;Ikoma et al 2006;Broeg & Wuchterl 2007;Burrows et al 2007a). Together with the Neptune-mass planets GJ 436b (Butler et al 2004;Gillon et al 2007) and HAT-P-11b (Bakos et al 2009), this places HD 149026b in a class that is distinct from that of the more massive "hot Jupiter" transiting planets with their primarily hydrogen-helium compositions.…”

The Search for an Atmospheric Signature of the Transiting Exoplanet HD 149026b1

“…As a first step, we identified the in-transit and out-of-transit spectra by phase-folding the radial velocity (RV) data with a period of P = 2.875965 + 0.000085 − 0.000135 days. This value was calculated by taking the mean of the values provided by Charbonneau et al (2005), and Wolf et al (2006) which agree with each other within the stated uncertainties. The time at the center of transit, T c (HJD) = 2453527.87455 + 0.00085 − 0.00091, was adopted from Charbonneau et al (2005).…”

“…This value was calculated by taking the mean of the values provided by Charbonneau et al (2005), and Wolf et al (2006) which agree with each other within the stated uncertainties. The time at the center of transit, T c (HJD) = 2453527.87455 + 0.00085 − 0.00091, was adopted from Charbonneau et al (2005). Figure 1 represents an update to the phased radial velocity data for HD 149026 presented in Sato et al (2005).…”

A Precise Estimate of the Radius of HD 149026b

“…Figure 2 also indicates that, at 15 minute resolution (bins of roughly 2,000 images), the photometric precision dips below 2×10 −4 . In order to achieve the strongest possible constraints on the system parameters, we combined our data with previously published ground-based photometry (Sato et al 2005, Charbonneau et al 2006.…”