THE IMPACT OF STELLAR ROTATION ON THE DETECTABILITY OF HABITABLE PLANETS AROUND M DWARFS The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Newton, Elisabeth R. et al. “THE IMPACT OF STELLAR ROTATION ON THE DETECTABILITY OF HABITABLE PLANETS AROUND M DWARFS.” The Astrophysical Journal 821.1 (2016): L19. © 2016 The American Astronomical Society As Published http://dx.doi.org/10.3847/2041-8205/821/1/l19 Publisher IOP Publishing Version Final published version Citable link http://hdl.handle.net/1721.1/108367 Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. The Astrophysical Journal Letters, 821:L19 (6pp), 2016 April 10 doi:10.3847/2041-8205/821/1/L19 © 2016. The American Astronomical Society. All rights reserved. THE IMPACT OF STELLAR ROTATION ON THE DETECTABILITY OF HABITABLE PLANETS AROUND M DWARFS Elisabeth R. Newton1, Jonathan Irwin1, David Charbonneau1, Zachory K. Berta-Thompson2, and Jason A. Dittmann1 1 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA 2 Kavli Institute for Astrophysics and Space Research and Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA Received 2016 February 8; accepted 2016 March 9; published 2016 April 11 ABSTRACT Stellar activity and rotation frustrate the detection of exoplanets through the radial velocity technique. This effect is particularly of concern for M dwarfs, which can remain magnetically active for billions of years. We compile rotation periods for late-type stars and for the M dwarf planet-host sample in order to investigate the rotation periods of older field stars across the main sequence. We show that for stars with masses between 0.25 and 0.5 M (M4V–M1V), the stellar rotation period typical of field stars coincides with the orbital periods of planets in the habitable zone. This will pose a fundamental challenge to the discovery and characterization of potentially habitable planets around early M dwarfs. Due to the longer rotation periods reached by midM dwarfs and the shorter orbital period at which the planetary habitable zone is found, stars with masses between 0.1 and 0.25 M (M6V–M4V) offer better opportunities for the detection of habitable planets via radial velocities. Key words: stars: low-mass – stars: rotation 1. INTRODUCTION Bonfils et al. (2013) identified a super-Earth around Gl 667C, with a second found by Anglada-Escudé et al. (2012b) Radial velocities are a powerful tool for the discovery and and Delfosse et al. (2013). Anglada-Escudé et al. (2013b) characterization of exoplanetary systems. However, stellar identified a total of six Keplerian signals at a range of periods. magnetic activity and rotation affect the measured radial Though long-period signals were seen by Delfosse et al. (2013) velocities, as surface inhomegeneities cross the stellar surface and by Makarov & Berghea (2014), they were postulated to be and change with time. Starspots diminish light received from ∼ the limbs of a rotating star, causing changes in thespectral line related to the 105 day stellar rotation period. Independent ( ) analyses by Feroz & Hobson (2013) and Robertson & centroid e.g., Saar & Donahue 1997 . Plages have a similar ( ) fi influence, but with the opposite sign (e.g., Meunier et al. 2010). Mahadevan 2014 did not nd evidence for planets exterior fi to Gl 667C b and c. Strong localized magnetic elds, typically associated with ( ) spots, modify convective flows and cause a net blueshift (e.g., Anglada-Escude et al. 2014 reported two planets with ) periods of 48 and 120 days around Gl 191. Robertson et al. Gray 2009; Meunier et al. 2010 . Disentangling these stellar ( ) activity signals from the planetary reflex motion is challenging, 2015b showed that the 48 day signal is correlated with stellar and typically requires modeling activity-induced changes to activity, and suggested a non-planetary origin. They also spectral line profiles (e.g., Queloz et al. 2009; Boisse express concern that the period of the outer planet is close to et al. 2011; Dumusque et al. 2011, 2012; Rajpaul the 143 day stellar rotation period. et al. 2015). This is particularly pertinent for M dwarf stars, These examples illustrate that stellar activity signals, which which retain high levels of magnetic activity for longer than appear at the stellar rotation period and its harmonics, can mimic the effect of a planet at or near these orbital periods. The Sun-like stars. Studies of exoplanets around the nearby M fi dwarfs Gl 581, Gl 667C, and Gl 191 (Kapteynʼs star) have typical rotation periods of eld-age dwarf stars varies by almost an order of magnitude across the lower main sequence, recently highlighted this concern. ( ) Gl 581 is an M3V (Henry et al. 1997)3 star. Bonfils et al. increasing from about 25 days for early G stars Barnes 2007 , ( (2005) first discovered the hot Neptune Gl 581 b orbiting this to greater than 100 days for mid-to-late M dwarfs Newton ) star, while Udry et al. (2007) identified a second short-period et al. 2016 . Over the same stellar mass range, the orbital period planet, Gl 581 c. The number of additional planets that orbit corresponding to a habitable planet decreases from approxi- this star has been a matter of recent debate. Udry et al. (2007) mately 365 days to 10 days. There thus exists a range of stellar further identified planet d, and Mayor et al. (2009) planet e. masses where the stellar rotation and planetary habitable-zone ʼ Vogt et al. (2010) found evidence for a long-period planet f, periods overlap. This came to our team s attention in pursuit of and a potentially habitable planet g. Analyses by other groups follow-up observations of K2-3, an early M dwarf with three fi ( fi ) quickly cast doubt on planets f (Forveille et al. 2011; Tuomi con rmed transiting super-Earths Cross eld et al. 2015 :we ∼ 2011) and g (Gregory 2011; Baluev 2012; Hatzes 2013). noted that the stellar rotation period of 40 days is very close Baluev (2012) also raised concerns about planet d. Robertson to the 44 day orbital period of K2-3 d, which is near the inner et al. (2014) looked at the Hα line, a magnetic activity edge of the planetary habitable zone. indicator. They found support for the existence of the three There are several current and near-future radial velocity fi innermost planets (b, c, and e), but indications that the outer surveys committed to nding planets around M dwarf stars, ( ) planets (d, g, and f) are the result of stellar activity and rotation. including CARMENES Quirrenbach et al. 2014 , HPF (Mahadevan et al. 2012), and SPIRou (Artigau et al. 2014). 3 Included in the RECONS compilation: http://www.recons.org/TOP100. It is therefore critical that we understand how stellar rotation posted.htm. may impact the discovery of the potentially habitable planets 1 The Astrophysical Journal Letters, 821:L19 (6pp), 2016 April 10 Newton et al. around these stars. In this Letter, we leverage new rotation Figure 1 includes stars of a range of ages. As single stars age, periods of mid-to-late M dwarfs in the solar neighborhood from they lose angular momentum and their rotation periods our recent work in Newton et al. (2016) to investigate this increase. Angular momentum loss is massdependent, with problem for stars across the M spectral class. lower-mass stars taking longer to spin down, but eventually reaching longer rotation periods. The long-period envelope of the rotation period distribution is thought to represent an old 2. STELLAR ROTATION PERIODS AND MASSES field population,and the fast rotators at a given mass will ACROSS THE MAIN SEQUENCE eventually evolve toward these long periods.4 The envelope To assess stellar rotation across the main sequence, we comprises stars of similar age to the Sun and older, which was compile literature measurements for the rotation periods of field established by Skumanich (1972) and Barnes (2003) for Sun- stars, using data from Baliunas et al. (1996), Kiraga & Stepien like stars. We quantified this for M dwarfs by using galactic (2007), Hartman et al. (2011), Suárez Mascareño et al. (2015), kinematics to estimate that mid M dwarfs with rotation periods and Newton et al. (2016). The latter supplies most of the around 100 days are on average 5 Gyr old (Newton et al. 2016). rotation period measurements for midand lateM dwarfs, and results from our analysis of photometry from the MEarth-North transit survey (Berta et al. 2012; Irwin et al. 2015). The rotation 3. STELLAR ROTATION AND THE HABITABLE ZONE periods from Hartman et al. (2011) and Kiraga & Stepien ( ) When radial velocity variations induced by stellar rotation 2007 also come from broadband optical or red-optical and activity coincide with the planetary orbital period, it can be photometry. On the other hand, Baliunas et al. (1996) and fi ( ) dif cult to identify the planetary signal. This stellar signal can Suárez Mascareño et al. 2015 measure periodicities in appear not just at the rotation period itself, but at higher-order chromospheric activity features. ( ) ( ) harmonics as well. Boisse et al. 2011 tested cases where the We use stellar mass M* as the independent parameter for photosphere is dominated by one or two spotsand found that this study. For G and early K dwarfs, we use B−V colors to ( ) power appears primarily at one-half and one-third the stellar estimate effective temperature Teff , and then use isochrones to ( ) B−V rotation period see also Aigrain et al.