Stellar Variability of the Exoplanet Hosting Star HD 63454
Stephen R. Kane, Diana Dragomir, David R. Ciardi, Jae-Woo Lee, Gaspare Lo Curto, Christophe Lovis, Dominique Naef, Suvrath Mahadevan, Genady Pilyavsky, Stephane Udry, Xuesong Wang, Jason Wright
aa r X i v : . [ a s t r o - ph . E P ] M a y Submitted for publication in the Astrophysical Journal
Preprint typeset using L A TEX style emulateapj v. 03/07/07
STELLAR VARIABILITY OF THE EXOPLANET HOSTING STAR HD 63454
Stephen R. Kane , Diana Dragomir , David R. Ciardi , Jae-Woo Lee , Gaspare Lo Curto , Christophe Lovis ,Dominique Naef , Suvrath Mahadevan , Genady Pilyavsky , Stephane Udry , Xuesong Wang , Jason Wright Submitted for publication in the Astrophysical Journal
ABSTRACTOf the hundreds of exoplanets discovered using the radial velocity technique, many are orbitingclose to their host stars with periods less than 10 days. One of these, HD 63454, is a young active Kdwarf which hosts a Jovian planet in a 2.82 day period orbit. The planet has a 14% transit probabilityand a predicted transit depth of 1.2%. Here we provide a re-analysis of the radial velocity data toproduce an accurate transit ephemeris. We further analyse 8 nights of time series data to search forstellar activity both intrinsic to the star and induced by possible interactions of the exoplanet withthe stellar magnetospheres. We establish the photometric stability of the star at the 3 millimag leveldespite strong Ca II emission in the spectrum. Finally, we rule out photometric signatures of bothstar–planet magnetosphere interactions and planetary transit signatures. From this we are able toplace constraints on both the orbital and physical properties of the planet.
Subject headings: planetary systems – techniques: photometric – techniques: radial velocities – stars:individual (HD 63454) INTRODUCTION
The number of known exoplanets has now well ex-ceeded 500, revealing a large diversity in both plane-tary properties and orbital characteristics. In the earlydays of exoplanet discoveries, one of the first surpriseswas that of very short-period planets, the so-called hotJupiters. Studies have been undertaken which attempt tofind star–planet interactions between these hot Jupitersand their host stars, and correlations of stellar activ-ity with planetary emission spectra (Knutson et al. 2010)and surface gravities (Hartman 2010) have been detected.Evidence has also been found for a general increasein chromospheric activity of stars which harbor short-period planets (Canto Martins et al. 2011) and surveyshave been undertaken which evaluate such activity in po-tential planet search targets (Arriagada 2011). Most ofthese effects are caused by interactions between coronalmagnetic fields and the magnetospheres of the close-inplanets (Cohen et al. 2009; Lanza 2009). Searches forobservable signatures of such interactions have been un-dertaken for HD 189733 (Fares et al. 2010) and CoRoT-6(Lanza et al. 2011) but the evidence has been inconclu-sive. Shkolnik et al. (2008) observed synchronicity of theCa II H and K emission for both HD 179949 and υ Andwith the rotation of their respective short-period planets,likely due to interactions with the stellar magnetic fields.
Electronic address: [email protected] NASA Exoplanet Science Institute, Caltech, MS 100-22, 770South Wilson Avenue, Pasadena, CA 91125 Department of Physics & Astronomy, University of BritishColumbia, Vancouver, BC V6T1Z1, Canada Department of Astronomy & Space Science, Sejong University,143-747 Seoul, Korea ESO, Karl-Schwarzschild Strasse, 2, Garching bei M¨unchen,Germany Observatoire de Gen`eve, Universit´e de Gen`eve, 51 ch.des Mail-lettes, 1290 Sauverny, Switzerland Department of Astronomy and Astrophysics, PennsylvaniaState University, 525 Davey Laboratory, University Park, PA 16802 Center for Exoplanets & Habitable Worlds, Pennsylvania StateUniversity, 525 Davey Laboratory, University Park, PA 16802
The 2.82 day period planet orbiting HD 63454(HIP 37284, TYC 9385-1045-1) was first detected byMoutou et al. (2005) using radial velocity data obtainedwith the High-Accuracy Radial-velocity Planet Searcher(HARPS) mounted on the ESO 3.6m telescope. The hoststar is a relatively young ( ∼ ∼
20 days. This star is extremely southern indeclination ( − ◦ ) and so follow-up observations of thesystem have been minimal since the planet’s discovery.Here we present the results of photometrically moni-toring HD 63454 as part of the Transit Ephemeris Re-finement and Monitoring Survey (TERMS; Kane et al.(2009)). The star was observed over a two week periodin order to extract variability properties of the star. Inparticular, we are interested in variability that may cor-respond with the radial velocity jitter and/or the influ-ence of the planet on the star. We find no evidence ofsuch correlations which places limits on the causation ofstellar activity due to the interactions of the planet. Wepresent additional HARPS data which refines the periodand redetermines the phase of the planet. We subse-quently monitored transit windows which confirms thatthis planet does not transit the host star. With such asmall orbital period, we use this result to place a lowerlimit on the mass of the planet and an upper limit on theradius of the planet. KEPLERIAN ORBIT AND TRANSIT EPHEMERIS
The original discovery of HD 63454b presented byMoutou et al. (2005) included 26 radial velocity mea-surements acquired using the HARPS instrument andwere subject to an additional analysis by Babu et al.(2010). Here we present 8 additional measurements fromHARPS acquired since then which have been used to re-fine the orbital parameters and redetermine the phase of Stephen R. Kane et al.
TABLE 1HARPS Radial Velocities
Date Radial Velocity Uncertainty(JD – 2440000) (km s − ) (km s − )13047.625113 33.78298 0.0032113060.629178 33.90876 0.0043513061.601527 33.79109 0.0030813063.592466 33.89097 0.0029013064.637653 33.78317 0.0036613066.590014 33.87237 0.0027113145.500240 33.85952 0.0026813146.479839 33.78204 0.0024013147.463245 33.89531 0.0055113151.455026 33.83624 0.0032313152.452481 33.82294 0.0034513153.473472 33.91101 0.0019913156.443421 33.90046 0.0031613158.466001 33.85960 0.0048513295.880778 33.78365 0.0017713314.841071 33.83579 0.0016113340.808816 33.79013 0.0017313342.779095 33.85122 0.0023213344.787166 33.90508 0.0017513346.788259 33.79824 0.0014013369.736742 33.86615 0.0014213371.762067 33.78346 0.0018613372.728689 33.88211 0.0017813375.769216 33.89765 0.0019413377.779025 33.80591 0.0032113400.742286 33.85003 0.0019313402.640683 33.77902 0.0030913403.736539 33.88467 0.0015713405.746675 33.77627 0.0018113406.654971 33.89017 0.0016413408.668892 33.80302 0.0017313468.516837 33.87624 0.0023215260.618227 33.83241 0.0009515262.516387 33.76660 0.00077 this planet. All 34 measurements are shown in Table 1.The stellar mass according to Moutou et al. (2005) is M ⋆ = 0 . M ⊙ and the surface gravity is log g = 4 . T eff = 4840 ±
66 K, log g = 4 . ± .
16 cm s − , and[Fe / H] = 0 . ± .
03 respectively. Using the polynomialrelations of Torres et al. (2010), we derive revised stellarparameters of M ⋆ = 0 . M ⊙ and R ⋆ = 1 . R ⊙ forHD 63454.We fit a single-planet Keplerian solution to the RV datausing the techniques described in Howard et al. (2010)and the partially linearized, least-squares fitting proce-dure described in Wright & Howard (2009). The inclu-sion of a linear trend to the solution reduced the χ from 30.68 to 10.14 and the RMS of the residuals from10.87 to 6.84 m s − . While an offset between the bulk ofthe data and the final two measurements of ∼
20 m s − would produce comparable reduction in the χ , HARPSis known to be extremely stable and such an offset is notconsidered plausible. This leads us to favour the solu-tion which includes a trend. Further radial velocity datais required to ascertain the precise source of the trend,whether it be due to the magnetic cycle of the star or thepresence of an additional companion within the system.The adopted solution with the trend is shown in Table 2.The parameter uncertainties were determined from thesampling distribution of each parameter through a non- TABLE 2Keplerian Fit Parameters
Parameter Value P (days) 2 . ± . T c a (JD – 2440000) 15583 . ± . T p b (JD – 2440000) 13342 . ± . e . ± . K (m s − ) 64 . ± . ω (deg) 87 . ± . dv/dt (m s − yr − ) − . ± . χ − ) 6.84 a Time of transit. b Time of periastron passage.
Fig. 1.—
Radial velocity measurements of HD 63454 along withthe best-fit orbital solution (solid line). The shaded region showsthe extent of the 1 σ transit window. parametric bootstrap analysis (Freedman 1981). Thefolded data and adopted model with the trend removedare shown in Figure 1.Using the aforementioned stellar mass, we derive aplanetary mass of M p sin i = 0 . M J and a semi-majoraxis of a = 0 . R p = 1 . R J . This results in a predicted transit proba-bility of 14.3%, a depth of 1.2%, and a transit duration of0.13 days. A preliminary search by Moutou et al. (2005)found no evidence for transits of this planet. In Figure 1we include a shaded region which indicates the calculatedsize of the transit window (Kane et al. 2009) at the timeof acquiring our photometry. This is described further inSection 5 where we present re-phased photometry whichplaces limits on transits and the inclination or size of theplanet. PHOTOMETRY
Photometry from Hipparcos
We investigated the low-frequency photometric stabil-ity of HD 63454 using observations from the
Hipparcos satellite.
Hipparcos observed the star during its three-year mission and acquired a photometric data set con-sisting of 124 measurements spanning a period of 1180days (Perryman et al. 1997), shown in Figure 2. The 1 σ RMS scatter of the 124 HD 63454 measurements is 0.031mag, while the mean of the measurement uncertaintiesis 0.019. The scatter is roughly 50% higher than the ex-tellar Variability of HD 63454 3
Fig. 2.—
Photometry of HD 63454 from the
Hipparcos mission. pected uncertainty of a single observation, but the range,0.345 mag, is significantly more than that expected froma constant star. Consequently, the
Hipparcos
Cataloguedescribed by Perryman et al. (1997) lists the variabilitytype for HD 63454 as a blank, indicating that the star“could not be classified as variable or constant.” We per-formed a Fourier analysis of the
Hipparcos data and donot detect any significant periodic variability. However,this only rules out activity above the 3% level. Addition-ally, the Nyquist frequency of the data is 0.0525 days − which is slightly above the predicted frequency of thestellar rotation, thus resulting in substantial aliasing atsmaller periods. The strongest peaks in the periodogramoccur at 0.25 and 0.27 days but the power of these peaksare very low. Photometry from CTIO
Observations of HD 63454 were carried out at theCerro Tololo Inter-American Observatory (CTIO) 1.0mtelescope using the Y4KCam Detector , which is a 4k × V band filter for 8 nightsduring the period 22–30 January 2011. An additionalnight of data was acquired using this telescope on thenight of 5 April 2011 in order to complete phase cover-age of the transit window (see Section 5). The brightnessof the target ( V = 9 .
37) led to exposure times of 8–12seconds, high enough to eliminate the effects of shuttererrors. The principal target and comparison stars werecarefully placed on cosmetically clean regions of the CCDand kept in exactly the same place during the monitoringsequences to avoid inter-pixel sensitivities.The target star HD 63454 is known in the 2MASScatalog as 2MASS J07392187-7816442 (Skrutskie et al.2006). There is a nearby fainter star 6.14 arcsecs away(2MASS J07391989-7816428) for which the
JHK mag-nitudes are 10.636, 12.523, and 12.350 respectively. Ac-cording to the photometric quality flags of the 2MASScatalog, the J value represents an upper limit on themagnitude (i.e., represents a minimum brightness for thestar). The H − K value of 0.173 means the star is anearly M star if on the main sequence.Aperture photometry was performed on each star byextracting small regions from the image, ±
100 pixelsfrom the estimated center of the stellar point-spreadfunction (PSF). The size of the photometric aperture waslimited to restrict light contamination from the nearbystar which was particularly important during nights ofbad seeing which may cause the PSF to spread furtherinto the aperture. To achieve sufficient precision to de- tect low-amplitude variability including transit signals,we performed relative photometry using the methods de-scribed in Everett & Howell (2001). The resulting pho-tometry were binned into equal time intervals of 5 min-utes each and are shown in the top panel of Figure 3. Formost nights the 1 σ RMS was less than 3 millimags, butthe combined dataset has a 1 σ RMS of 3.4 millimags. PHOTOMETRIC FOURIER ANALYSIS
Here we describe an analysis of the photometry for thepurposes of studying the stability of the star. To investi-gate the high-frequency variability of HD 63454, we useda weighted Lomb-Scargle (L-S) fourier analysis, similarto that described by Kane et al. (2007). In particular,we are interested in activity related to the magnetic cycleand interactions of the magnetic field and chromospherewith the planet on the short timescales of its orbital pe-riod. Investigation of the line bisector inverse slope byMoutou et al. (2005) found no correlation with the or-bital period. In the bottom-left panel of Figure 3 weshow the complete CTIO dataset folded on the orbitalperiod from Table 2. Phase zero in this figure is thelocation of the predicted transit time of the planet.As described by Dawson & Fabrycky (2010), aliases inperiodograms result from discrete sampling times whichoccur to a lesser degree with unevenly sampled data. Theperiodogram of the January 2011 photometry is shown inthe bottom-right panel of Figure 3. There are significantaliases at periods less than 1 day which are harmonics ofthe observing schedule, such as 0.20, 0.26, 0.60, and 1.50days. Of note is the strongest peak located at 0.26 dayssince this lies between the two strongest peaks locatedin the Hipparcos data (see Section 3.1). This is assumedto be the result of the cadence and resulting Nyquist fre-quency in each dataset, but we note it here as a possibleindicator of low-amplitude high-frequency activity. Ob-servations each night lasted ∼ . R ⋆ which, for the relatively young K dwarf( ∼ PLANETARY TRANSIT EXCLUSION
Moutou et al. (2005) state that their photometry Stephen R. Kane et al.
Fig. 3.—
Top panel: Photometry of HD 63454 from the January observing run at CTIO, where the data has been binned into 5 minuteintervals. Bottom-left panel: All CTIO photometry folded on the best fit period from Table 2 with the predicted transit time at phasezero. Bottom-right: Weighted L-S periodogram of the Janurary CTIO photometry where dotted lines indicate thresholds of false-alarmprobabilities. “showed no planetary transit” although they do notpresent the photometry or the precision of the measure-ments. Here we present our photometry acquired dur-ing transit windows based upon the revised orbital pa-rameters and ephemeris and discuss limits on the im-plied properties of the planet and orbital inclination. Todemonstrate the importance of refining orbital parame-ters, the observations during the January CTIO run weredesigned to cover two transit windows based upon the or-bital parameters of Moutou et al. (2005). However, theorbital fit to the updated HARPS data shifted the pre-dicted windows into the observing gaps in orbital phase,thus necessitating the additional night of data in April.Figure 4 shows a zoomed-in version of the lower-leftpanel of Figure 3, where the data once again has beenphased on a zero-point which is the location of the pre-dicted transit mid-point. The vertical dashed lines in-dicate the 1 σ extent of the transit window which is thepredicted duration plus twice the transit mid-point un-certainty (see Section 2). In this case, the transit win-dow size is evenly split between the duration and mid-point uncertainty yielding a total transit window size of0.26 days = 0.094 orbital phase. We calculated the pre-dicted transit signature based upon the analytic modelsof Mandel & Agol (2002), overplotted as a solid line inthe figure.The conditions on the observing night in April 2011were exceptional which produced the photometry thatdominates the right-hand side of Figure 4. The scat- Fig. 4.—
Zoom-in of the CTIO photometry phased on the orbitalperiod with the time of mid-transit at phase zero. The solid line isthe predicted transit signature for the predicted stellar/planetaryradii and the dashed lines indicate the 1 σ extent of the transitwindow. ter in these data is larger than what is expected fromphoton counting statistics. This excess may be due tothe nearby faint star but is more likely due to stellarphotometric variations. The photometry for that nighthas a 1 σ RMS scatter of 2.3 millimags. The predictedtransit depth (1.2%) is therefore ruled out at the 5.4 σ level. This means that, for a non-transiting planet, theorbital inclination of the planet is restricted to i < . ◦ which results in a lower limit of the planetary mass of M p > . M J . On the other hand, if the planet doestellar Variability of HD 63454 5transit then the photometric precision rules out plane-tary radii of R p > . R J . A radius just below thisthreshold would yield a density of 1.16 g cm − , resultingin the planet having remarkably similar properties to thelarge-cored planet HD 149026b (Sato et al. 2005), bothin terms of orbital parameters and planetary character-istics. CONCLUSIONS
Understanding the star–planet interaction for systemswith hot Jupiters presents an opportunity to furthercharacterize these planets, particularly for transiting ex-oplanets where both the mass and radius of the planet areknown. Detection of magnetic field interactions wouldyield insight into the internal structure and rotation rateof these planets. The study of HD 63454 presented herewas conducted as part of the Transit Ephemeris Refine-ment and Monitoring Survey (TERMS) in order to detector rule out both stellar variability and transit signaturesdue to the presence of the planet which has 14% transitprobability and a predicted transit depth of 1.2%.This study includes new HARPS radial velocity datain order to redetermine the phase of the planet dur-ing times of photometric monitoring. The requirementfor additional photometry in order to rule out a tran-sit based upon the revised orbital parameters demon-strates the need for careful examination of the planetaryphase when monitoring predicted transit windows. The
Hipparcos photometry reveals no long-term variability of the star, although the sampling frequency and photomet-ric precision are inadequate to detect periodicity relatedto the rotational timescale. The lack of high-frequencyvariability at the 3 millimag level may indicate that theplanet is (a) outside the magnetosphere of the star, or(b) the planetary magnetosphere is very small, or (c) theinteraction of the planetary magnetosphere bow shockwith the stellar magnetic field is best detected at eitherhigher precision or longer wavelengths (radio). The lackof a transit signature detection indicates that either themass of the planet is larger than 0.402 M J or that theradius is less than 0.77 R J . In the case of the latter, thiswould imply that the planet has very similar propertiesto HD 149026b. ACKNOWLEDGEMENTS