Transit Photometry of the Core# Dominated Planet HD 149026B
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Transit Photometry of the Core# dominated Planet HD 149026b The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters Citation Charbonneau, David, Joshua N. Winn, David W. Latham, Gaspar Bakos, Emilio E. Falco, Matthew J. Holman, Robert W. Noyes, et al. 2006. “Transit Photometry of the Core#dominated Planet HD 149026b.” The Astrophysical Journal 636 (1): 445–52. https:// doi.org/10.1086/497959. Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:41397427 Terms of Use This article was downloaded from Harvard University’s DASH repository, and is made available under the terms and conditions applicable to Other Posted Material, as set forth at http:// nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of- use#LAA The Astrophysical Journal, 636:445–452, 2006 January 1 A # 2006. The American Astronomical Society. All rights reserved. Printed in U.S.A. TRANSIT PHOTOMETRY OF THE CORE-DOMINATED PLANET HD 149026b David Charbonneau, Joshua N. Winn,1 David W. Latham, Ga´spa´r Bakos,1 Emilio E. Falco, Matthew J. Holman, Robert W. Noyes, and Bala´zs Csa´k2 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138; [email protected] Gilbert A. Esquerdo and Mark E. Everett Planetary Science Institute, 1700 East Fort Lowell, Tucson, AZ 85719 and Francis T. O’Donovan California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125 Received 2005 August 1; accepted 2005 September 8 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 þ0:00085 þ0:00012 of the transit ephemeris to Tc ¼ (2;453;527:87455À0:00091) þ (2:87598À0:00017)N (HJD). Assuming that the star has a radius of 1:45 Æ 0:10 R and a mass of 1:30 Æ 0:10 M , we estimate the planet radius to be (0:726 Æ 0:064)RJup, þ0:42 À3 which implies a mean density of 1:07À0:30 gcm . 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 con- firm 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. Subject headings:g planetary systems — stars: individual (HD 149026) — techniques: photometric Online material: machine-readable table 1. INTRODUCTION (Bundy & Marcy 2000; Moutou et al. 2001, 2003; Vidal-Madjar Sato et al. (2005) recently presented the discovery of a plan- et al. 2003, 2004; Winn et al. 2004; Narita et al. 2005), (5) the measurement of the angle between the sky-projected orbit nor- etary companion to the bright G0 IV star HD 149026. The star mal and stellar rotation axis (Queloz et al. 2000; Winn et al. exhibits a time-variable Doppler shift that is consistent with a 2005), (6) the search for spectroscopic features near the times of sinusoid of amplitude K ¼ 43 m sÀ1 and period P ¼ 2:9 days, secondary eclipse (Richardson et al. 2003a, 2003b), and (7) the which would be produced by the gravitational force from an or- direct detection of thermal emission from the planet (Charbonneau biting planet with M sin i ¼ 0:36M . Furthermore, at the pre- P Jup et al. 2005; Deming et al. 2005b). Charbonneau (2004) reviewed dicted time of planet-star conjunction, the star’s flux declines by these techniques and related investigations. 0.3% in the manner expected of an eclipse by a planet of radius Second, the planet is the smallest and least massive of the eight 0.72R (given an estimate of the stellar radius, 1.45 R , which Jup known transiting extrasolar planets.3 This makes HD 149026b is based on the stellar parallax and effective temperature). Sato an important test case for theories of planetary structure. Sato et al. (2005) observed three such eclipses. This discovery is ex- et al. (2005) argued that, once the effects of stellar insolation are traordinary for at least two reasons. included, the small planetary radius implies that the planet has First, the occurrence of eclipses admits this system into the elite club of bright stars with detectable planetary transits. Of a large and dense core. In particular, assuming a core density 5:5gcmÀ3, their model demands a prodigious core mass all the previously known transiting systems, only HD 209458 c ¼ of 78 Earth masses, or 74% of the total mass of the planet. This, (Charbonneau et al. 2000; Henry et al. 2000) and TrES-1 (Alonso in turn, would seemingly prove that the planet formed through et al. 2004; Sozzetti et al. 2004) have parent stars brighter than core accretion, as opposed to direct collapse through a gravita- V ¼ 12, and therefore only they are amenable to a number of tional instability. fascinating measurements requiring a very high signal-to-noise A system of such importance should be independently con- ratio. Among these studies are (1) the search for satellites and firmed, and the determination of its basic parameters should be rings (Brown et al. 2001), (2) the search for period variations due refined through multiple observations. With this as motivation, to additional companions (Wittenmyer et al. 2005), (3) the de- we performed photometry of HD 149026 on two different nights tection of (or upper limits on) atmospheric absorption features in transmission (Charbonneau et al. 2002; Brown et al. 2002; when transits were predicted by Sato et al. (2005). These obser- vations and the data reduction procedures are described in 2. Deming et al. 2005a), (4) the characterization of the exosphere x 3 In addition to HD 209458b and TrES-1, the OGLE photometric survey 1 Hubble Fellow. (Udalski et al. 2002, 2004) and spectroscopic follow-up efforts have located five 2 SAO Predoctoral Fellow. Home institution: Department of Experimental such objects. Recent estimates of the planetary radii have been given by Bouchy Physics and Astronomical Observatory, University of Szeged, Do´mte´r 9, 6720 et al. (2004), Holman et al. (2006), Konacki et al. (2003, 2005), Moutou et al. Szeged, Hungary. (2004), and Pont et al. (2004) and references therein. 445 446 CHARBONNEAU ET AL. The model that we used to determine the system parameters is sky-subtracted apertures. Normalization and residual extinction described in x 3, and the results are discussed in x 4. Our data are corrections are described in x 3. available in digital form in the electronic version of this article and from the authors upon request. 2.2. TopHAT V Photometry at FLWO TopHAT is an automated telescope located at Mt. Hopkins, 2. THE OBSERVATIONS AND DATA REDUCTION Arizona, which was designed to perform multicolor photometric 2.1. FLWO 1.2 m g and r Photometry follow-up of transiting extrasolar planet candidates identified by the HAT network (Bakos et al. 2004). Since TopHAT has not We observed HD 149026 (V ¼ 8:16, B À V ¼ 0:56) on UT previously been described in the literature, we digress briefly to 2005 June 6 and July 2 with the 48 inch (1.2 m) telescope of the outline the principal goals and features of the instrument. F. L. Whipple Observatory (FLWO) located at Mount Hopkins, Wide-field transit surveys must contend with a large rate of Arizona. We used Minicam, an optical CCD imager with two astrophysical false positives, which result from stellar systems 2048 ; 4608 chips. In order to increase the duty cycle of the that contain an eclipsing binary and precisely mimic the single- observations, we employed 2 ; 2 binning, which reduced the color photometric light curve of a Jupiter-sized planet transiting readout and overhead time to 20 s. Each binned pixel subtends a Sun-like star (Brown 2003; Charbonneau et al. 2004; Mandushev approximately 0B6 on the sky, giving an effective field of view of et al. 2005; Torres et al. 2004). Although multiepoch radial ve- about 100 ; 230 for each CCD. Fortunately, there exists a nearby locity follow-up is an effective tool for identifying these false object of similar brightness and color (HD 149083; V ¼ 8:05, positives (e.g., Latham 2003), instruments such as TopHATand B À V ¼ 0:40, Á ¼ 5A1, Á ¼170), which we employed Sherlock (Kotredes et al. 2004) can be fully automated and thus as an extinction calibrator. We selected the telescope pointing offer a very efficient means of culling the bulk of such false so that both stars were imaged simultaneously. We defocused positives. TopHAT is a 0.26 m diameter f/5 commercially avail- the telescope so that the FWHM of a stellar image was typically able Baker Ritchey-Chre´tien telescope on an equatorial fork mount 15 binned pixels (900), and we used automatic guiding to en- developed by Fornax Inc. A 1N25 square field of view is imaged sure that the centroid of the stellar images drifted no more than onto a 2k ; 2k Peltier-cooled, thinned CCD detector, yielding a 3 binned pixels over the course of the night. In addition to en- pixel scale of 2B2. The time for image readout and associated abling longer integration times, this served to mitigate the effects overheads is 25 s. Well-focused images have a typical FWHM of pixel-to-pixel sensitivity variations that were not perfectly of 2 pixels.