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Stellar Activity Mimics a Habitable-Zone Planet Around

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Stellar Activity Mimics a Habitable-Zone Planet Around To appear in The Astrophysical Journal Letters (ApJL) Preprint typeset using LATEX style emulateapj v. 5/2/11 STELLAR ACTIVITY MIMICS A HABITABLE-ZONE PLANET AROUND KAPTEYN'S STAR Paul Robertson1;2, Arpita Roy1;2;3, and Suvrath Mahadevan1;2;3 To appear in The Astrophysical Journal Letters (ApJL) ABSTRACT Kapteyn's star is an old M subdwarf believed to be a member of the Galactic halo population of stars. A recent study has claimed the existence of two super-Earth planets around the star based on radial velocity (RV) observations. The innermost of these candidate planets{Kapteyn b (P = 48 days){resides within the circumstellar habitable zone. Given recent progress in understanding the impact of stellar activity in detecting planetary signals, we have analyzed the observed HARPS data for signatures of stellar activity. We find that while Kapteyn's star is photometrically very stable, a suite of spectral activity indices reveals a large-amplitude rotation signal, and we determine the stellar rotation period to be 143 days. The spectral activity tracers are strongly correlated with the purported RV signal of \planet b," and the 48-day period is an integer fraction (1=3) of the stellar rotation period. We conclude that Kapteyn b is not a planet in the Habitable Zone, but an artifact of stellar activity. 1. INTRODUCTION pear to be ubiquitous at the RV ≤ 1 m/s level. M dwarfs are very attractive targets in the search for Kapteyn's star (= GJ 191) is a halo star with high potentially habitable terrestrial planets. Their compara- proper motion, which may originate from outside the tively small masses and radii make the detection of low- Galaxy (see review by Kotoneva et al. 2005). At 3.9 mass planets in the habitable zone (HZ; Kopparapu et pc from the Sun, it is the nearest halo star. The star al. 2013) via radial velocity (RV) and transit photome- is a target of the HARPS exoplanet search program, try feasible even now. The observational advantages of and Bonfils et al. (2013) reported preliminary solutions M dwarf planets also makes them amenable to atmo- of possible planet detections. More recently, Anglada- spheric characterization (e.g. GJ 1214b; Kreidberg et al. Escud´eet al. (2014, hereafter A14) reported the detec- 2014), and there is a possibility that JWST could place tion of two super-Earth exoplanets at P = 48:6 (planet constraints on the atmospheric composition{including b) and P = 121:5 (planet c) days, based partially on biosignatures{of a terrestrial planet in the HZ of a nearby those same HARPS observations. The proposed orbit for M dwarf (Seager et al. 2013; Cowan et al. 2015). planet b places it in the HZ. Assuming a stellar age typ- An insidious problem for RV detection of low mass ical of halo stars, the existence of an HZ planet around planets around M dwarfs, however, is the presence of RV Kapteyn's star allows for the fascinating possibility of signals created by stellar activity. Starspots, plage, fil- life nearly as old as the Universe itself. Considering the aments, convective suppression, and activity cycles can ramifications of such a discovery, it is doubly important all create apparent RV shifts by altering the balance be- to ensure Kapteyn b truly exists. tween red- and blueshifted portions of the stellar sur- A14 considered the possibility that activity creates sig- face (e.g. Dumusque et al. 2014, and references therein). nals in the RVs of Kapteyn's star. However, two of Activity-induced RV signals appear preferentially at the the activity tracers they examined{the Ca II H&K lines stellar rotation period and its integer fractions (Boisse (SHK ) and V -band photometry{are known to be subop- et al. 2011), so for old M stars with typical rotation pe- timal for M dwarfs (e.g. K¨urster et al. 2003; D´ıazet al. riods around 100 days (Engle & Guinan 2011), periods 2007; Gomes da Silva et al. 2011). Here, we have reexam- associated with the HZ will be plagued by activity sig- ined activity on Kapteyn's star using the Hα and Na I D nals. We have recently shown examples (Robertson et al. lines. We find that these lines offer a more significant de- 2014; Robertson & Mahadevan 2014) in which RV peri- tection of the P ∼ 140-day periodicity observed by A14 arXiv:1505.02778v2 [astro-ph.EP] 1 Jun 2015 odicities believed to be low-mass planets in or near the in SHK and FWHM. Based on the improved significance HZ were actually caused by activity. Furthermore, for of the detection, and the mass and likely age for the star, Barnard's star (K¨ursteret al. 2003) and GJ 581 (Robert- we conclude the 143-day periodicity must be the stellar son et al. 2014), the activity signals which drove RV shifts rotation period. This signal is strongly correlated with did not create a detectable photometric periodicity, sug- the proposed RV signal at P = 48 days. We conclude gesting that certain magnetic phenomena{probably mag- that the stellar rotation induces an RV signature which, netic suppression of convection{may only be apparent in when observed at the sparse cadence of the present RV spectral activity tracers. Magnetic activity signals ap- set, creates the signal attributed to planet b. 1 Department of Astronomy and Astrophysics, The Pennsyl- 2. DATA vania State University 2 Center for Exoplanets & Habitable Worlds, The Pennsylva- A14 claimed the detection of planets Kapteyn b and nia State University c based on RVs derived from 135 spectra obtained from 3 The Penn State Astrobiology Research Center, The Penn- the HARPS, HIRES, and PFS spectrographs. Our anal- sylvania State University ysis focused on the 95 HARPS spectra, which are all 2 Robertson et al. 50 40 FAP = 1% 30 Power α 20 H 10 40 30 20 10 Na D Power 40 30 20 10 FWHM Power 20 Power 10 HK S 15 10 5 BIS Power 0 1 10 100 1000 10000 Period (days) Fig. 1.| Periodograms of the HARPS spectral activity tracers. The vertical scales are not identical. The peak near P = 140 days (highlighted) is present in every tracer except BIS. The dashed lines indicate the power required for a FAP of 1%. publicly available4. We also considered the 20 (of 32) cance when using our reduction, their qualitative behav- HIRES spectra where extracted spectra containing Hα iors are identical for the original HARPS values. were available in the Keck Observatory Archive. PFS In addition to quantities derived from the HARPS spectra are not publicly available, and only account for spectra, we examined photometry of Kapteyn's star from 8 of the available RVs. We adopted RVs from A14, the Hipparcos (ESA 1997) and ASAS (Pojmanski 1997) which are derived using the HARPS-TERRA algorithm archives. Photometry of Kapteyn's star from KELT- (Anglada-Escud´e& Butler 2012) and a HIRES iodine-RV South (Pepper et al. 2012) also exists, but the star's algorithm using these same spectra. proper motion is so high that it moves between pix- We considered an array of stellar activity indicators els throughout the observations, causing large offsets on for Kapteyn's star. For the HARPS spectra, we mea- the same timescale as the stellar rotation period (J. Ro- sured the activity indices for the Na I D resonance lines driguez, private comm.). We therefore do not incorpo- (ID; Gomes da Silva et al. 2011) and the Hα line (IHα; rate the KELT-S data into this analysis. Robertson et al. 2013), which have been shown to be par- We find that SHK and ID are clearly correlated with ticularly sensitive to activity in M dwarf stars. We also each other, and anticorrelated with Hα. There are three 5 consider the SHK values provided in A14. For HIRES spectra for which the spectral indices do not follow these spectra, only IHα is available. correlations. We excluded these spectra from our analy- While the HARPS reduced data archive provides val- sis, as they likely correspond either to flare events, other ues for the line-shape characteristic quantities Bisec- abnormal activity, or errors in the measurement of one tor Inverse Slope (BIS) and full-width at half-maximum or more lines. (FWHM), we have computed our own values for these ac- 3. ANALYSIS tivity tracers using the HARPS CCF (cross-correlation function) files, which are a product of the cross- 3.1. Determining the stellar rotation period correlation between stellar spectra and a tailored numer- We began our analysis by examining the time series of ical mask. As in Robertson et al. (2014), we summed the activity indices for any periodic signals which might the individual CCFs of orders in the wavelength range leave an imprint on the RVs. In particular, for the peri- between ∼5400 { 6200 A˚ to obtain an \average" CCF, ods of the proposed planets, stellar rotation is the most then calculated BIS and FWHM according to Wright et likely source of astrophysical noise to create impostor al. (2013). Although the periodicites and correlations signals. We computed the generalized Lomb-Scargle pe- observed herein for BIS and FWHM have higher signifi- riodogram (Zechmeister & K¨urster2009) to search for periodicities in the activity tracers. 4 Data obtained from the ESO Science Archive Facility under request numbers 115242 and 149297 5 BJD = 2454879.556, 2456424.446, 2456428.462 Stellar activity mimics a habitable-zone planet around Kapteyn's star 3 Barycentric Julian Date 2453000 2454000 2455000 2456000 0.078 0.076 α H I 0.074 0.072 0.078 0.078 0.076 0.076 α α H H I I 0.074 0.074 0.072 0.072 -0.4 -0.2 0 0.2 0.4 6300 6400 6500 6600 6700 Phase BJD - 2450000 Fig.
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