A&A 628, L1 (2019) Astronomy https://doi.org/10.1051/0004-6361/201936030 & c ESO 2019 Astrophysics LETTER TO THE EDITOR Ross 128 – GL 447 A possible activity cycle for a slow-rotating fully convective star R. V. Ibañez Bustos1,2?, A. P. Buccino1,3, M. Flores4,5, and P. J. D. Mauas1,3 1 Instituto de Astronomía y Física del Espacio (CONICET-UBA), C.C. 67 Sucursal 28, C1428EHA Buenos Aires, Argentina e-mail: [email protected] 2 Departamento de Física. FI, Universidad de Buenos Aires, Buenos Aires, Argentina 3 Departamento de Física. FCEyN, Universidad de Buenos Aires, Buenos Aires, Argentina 4 Instituto de Ciencias Astronómicas, de la Tierra y del Espacio (ICATE-CONICET), San Juan, Argentina 5 Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de San Juan, San Juan, Argentina Received 5 June 2019 / Accepted 6 July 2019 ABSTRACT Context. Long-term chromospheric activity in slow-rotating fully convective stars has scarcely been explored. Ross 128 (Gl 447) is a slow-rotator and inactive dM4 star that has been extensively observed. It hosts the fourth closest extrasolar planet. Aims. Ross 128 is an ideal target to test dynamo theories in slow-rotating low-mass stars. Methods. To characterize the magnetic activity of Ross 128, we studied the S K -indexes derived from CASLEO, HARPS, FEROS, UVES, and X-shooter spectra. Using the generalized Lomb-Scargle and CLEAN periodograms, we analyzed the whole S K time-series obtained between 2004 and 2018. We performed a similar analysis for the Na I-index, and we analyzed its relation with the S K -index. Results. From both indexes, we obtain a possible activity cycle with a period of about five years, which is one of a small handful of activity cycles that have been reported for a slow-rotating fully convective star. Key words. stars: activity – stars: late-type – techniques: spectroscopic 0 1. Introduction observed in log(RHK)(Astudillo-Defru et al. 2017). Moreover, multiwavelength observations of slow-rotator stars confirmed Cool main-sequence stars later than M3.5-4V (with masses that they have strong magnetic fields (Hawley & Pettersen 1991; ∼ lower than 0.35 M ) are thought to be fully convective Reiners et al. 2009). Three-dimensional dynamo simulations (Chabrier & Baraffe 1997). For decades, many authors therefore without a tachocline (Browning 2008; Yadav et al. 2015, 2016) assumed that a standard solar-αΩ dynamo could not operate in also show that it is possible to produce large-scale magnetic these stars due to the absence of a tachocline (Chabrier & Küker fields in stellar convection layers. All these studies suggests that 2006; Browning 2008; Reiners & Basri 2010). In this context, an αΩ dynamo can operate for slow- and fast-rotator fully con- Pizzolato et al.(2003) and Wright et al.(2011) studied the X-ray vective stars even without a tachocline. The detection of activ- rotation-activity relationship in late-type stars through LX=Lbol ity cycles in fully convective stars might therefore help to better versus Prot and LX=Lbol versus Ro diagrams. In both diagrams understand the different dynamos that are at work. they found two regimes: a saturated and a non-saturated regime. On the other hand, in the last decade, M stars have become Wright et al.(2011) suggested that this bimodality might be favorable targets to search for Earth-like planets in the solar caused by two different dynamos: a convective dynamo parame- neighborhood because of their low mass and the appreciable terized by the bolometric luminosity for fast-rotator stars, and star-planet contrast ratio. Around 200 exoplanets have been an interface-type dynamo parameterized by the Rossby num- detected orbiting M-dwarf stars through both transit and radial ber for slow rotators (Prot > 10 days). However, it was still velocity (RV) methods (Bonfils et al. 2005, 2018; Udry et al. unclear why two different dynamo configurations would exist in 2007; Mayor et al. 2009; Anglada-Escudé et al. 2013, 2016; response to two regimes whose transition seemed to be rather Crossfield et al. 2015; Gillon et al. 2017; Dittmann et al. 2017; smooth. Recently, Wright et al.(2018) extended this analysis Díaz et al. 2019). However, it is well known that different mag- by including slow-rotating fully convective stars in the LX=Lbol netic phenomena such as starspots, plages, and activity cycles diagram. The authors concluded that the rotation-activity rela- can induce RV shifts that disturb the detectability of planetary tionship for fully convective stars (both slow and fast-rotators) signals (Dumusque et al. 2014). The detection of activity cycles present a similar behavior to partly convective stars, as also in exostars allows us to distinguish between stellar and planetary ? Based on data obtained at Complejo Astronómico El Leoncito, signals (Robertson et al. 2013; Carolo et al. 2014; Flores et al. operated under agreement between the Consejo Nacional de Inves- 2018). Several theoretical studies have also addressed the effect tigaciones Científicas y Técnicas de la República Argentina and the of stellar activity on planetary atmospheres and habitability National Universities of La Plata, Córdoba and San Juan. (Cuntz et al. 2000; Ip et al. 2004; Buccino et al. 2006, 2007; Article published by EDP Sciences L1, page 1 of6 A&A 628, L1 (2019) Cohen et al. 2010; Dumusque et al. 2017). M-dwarfs stars, with Table 1. CASLEO spectra for Ross 128 from the HKα project. particularly strong flares, are ideal to study these effects. In the last three years, Earth-like planets orbiting late- Label xJD t (s) type M dwarfs have been reported: Proxima Centauri (M5.5Ve; Anglada-Escudé et al. 2016), TRAPPIST-1 (M8; Gillon et al. 0307 4165 7200 2017), LHS 1140 (M4.5V; Dittmann et al. 2017), and Ross 128 0308 4550 7200 (M4V; Bonfils et al. 2018). In particular, the last three stars 0309 4904 7200 are inactive (i.e., they do not emit in Hα). In addition, Prox- 0609 4990 5400 ima Centauri, LHS 1140, and Ross 128 are slow rotators. Of 0612 6089 5400 these, activity cycles have only been detected in Proxima Cen- 0313 6353 7200 tauri (Cincunegui et al. 2007a; Suárez Mascareño et al. 2016; 0513 6435 7260 Wargelin et al. 2017). 0614 6834 5400 To determine the long-term chromospheric activity cycles 0315 7109 5400 of late stars, we have developed the HKα Project in 1999. It 0315 7110 6000 is dedicated to periodically obtain mid-resolution echelle spec- 0316 7468 5400 tra of fully and partly convective stars. Throughout these 20 0417 7858 6000 years, we have found evidence of cyclic activity in M stars 0318 8203 6000 in different regimes: the fully convective star Proxima Centauri (Cincunegui et al. 2007a), partly convective stars (GJ 229 A and Notes. Column 1: Date of each observing run (MMYY). Column 2: Julian date xJD = JD–2450000. Column 3: Exposure time in seconds. GJ 752 A, Buccino et al. 2011; AU Mic, Ibañez Bustos et al. 2019) and in the convective threshold (the binary system GJ 375, Table 2. ID programs of the Ross 128 observations. Díaz et al. 2007a; AD Leo, Buccino et al. 2014). We also found activity cycles in RS CVn (Buccino & Mauas 2009) and in the K stars Eri (Metcalfe et al. 2013) and ι Hor (Flores et al. 2017). HARPS FEROS UVES X-shooter In this paper we present a long-term activity study of Ross 072.C-0488(E) 191.C-0873(D) 074.D-0016(A) 091.D-0296(A) 084.D-0795(A) 128, which has been observed for more than a decade by 183.C-0437(A) 191.C-0873(E) 090.A-9003(A) 098.D-0222(A) the HKα Project. This star is a fully convective (M4V) flare 191.C-0873(A) 191.C-0873(F) star (Lee & Hoxie 1972) with a mass ∼0.168 M and a radius ∼0.1967 R . It is a slow rotator with a period of about 121 days. Bonfils et al.(2018) reported a planet orbiting Ross 128 with an We complement our data with public observations obtained orbital period of 9.9 days, which, due to its proximity to the Sun with several ESO spectrographs, by the programs listed in (3.4 parsec), is the fourth closest extrasolar planet. This star is an Table2. Forty-one spectra were observed by HARPS, mounted ideal target to study both the magnetic dynamo in slow-rotating at the 3.6 m telescope (R ∼ 115 000), during two time inter- low-mass stars and the impact of stellar activity on planetary vals, 2004−2006 and 2012−2016; 3 spectra were obtained in atmospheres. We organized this work as follows: in Sect.2 we 2005 and 2013 by FEROS, placed on the 2.2 m telescope (R ∼ present our spectroscopic observations obtained at the Complejo 48 000); 3 spectra were taken in 2013 with UVES, attached Astronómicómico El Leoncito (CASLEO), which are comple- to the Unit Telescope 2 (UT2) of the Very Large Telescope mented by public spectroscopic data obtained from the European (VLT) (R ∼ 80 000); and 11 mid-resolution spectra (R ∼ 8900) Southern Observatory (ESO). In Sect.3 we analyze the decadal were obtained in 2014 and 2017 with X-shooter, mounted at time-series in different activity indicators, and finally, we outline the UT2 Cassegrain focus, in the UVB wavelength range (300– our conclusions in Sect.4. 559.5 nm). HARPS and FEROS spectra have been automatically processed by the respective pipelines3;4. UVES and X-shooter data were reduced with the corresponding procedure5;6. 2. Observations In 1999 we started the HKα Project to study long-term chro- 3.