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Discovery of an Activity Cycle in the Solar Analog HD 45184? Exploring Balmer and Metallic Lines As Activity Proxy Candidates

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Discovery of an Activity Cycle in the Solar Analog HD 45184? Exploring Balmer and Metallic Lines As Activity Proxy Candidates A&A 589, A135 (2016) Astronomy DOI: 10.1051/0004-6361/201628145 & c ESO 2016 Astrophysics Discovery of an activity cycle in the solar analog HD 45184? Exploring Balmer and metallic lines as activity proxy candidates M. Flores1; 5, J. F. González1; 3; 5, M. Jaque Arancibia1; 5, A. Buccino2; 4; 5, and C. Saffe1; 3; 5 1 Instituto de Ciencias Astronómicas, de la Tierra y del Espacio (ICATE), España Sur 1512, CC 49, 5400 San Juan, Argentina e-mail: [mflores; fgonzalez; mjaque; csaffe]@icate-conicet.gob.ar 2 Instituto de Astronomía y Física del Espacio (IAFE), Buenos Aires, Argentina e-mail: [email protected] 3 Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de San Juan, 5400 San Juan, Argentina 4 Departamento de Física, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, Argentina 5 Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina Received 16 January 2016 / Accepted 2 April 2016 ABSTRACT Context. Most stellar activity cycles similar to that found in the Sun have been detected by using the chromospheric Ca ii H&K lines as stellar activity proxies. However, it is unclear whether such activity cycles can be identified using other optical lines. Aims. We aim to detect activity cycles in solar-analog stars and determine whether they can be identified through other optical lines, such as Fe II and Balmer lines. We study the solar-analog star HD 45184 using HARPS spectra. The temporal coverage and high quality of the spectra allow us to detect both long- and short-term activity variations. Methods. We analysed the activity signatures of HD 45184 by using 291 HARPS spectra obtained between 2003 and 2014. To search for line-core flux variations, we focused on Ca ii H&K and Balmer Hα and Hβ lines, which are typically used as optical chromospheric activity indicators. We calculated the HARPS-S index from Ca ii H&K lines and converted it into the Mount Wilson scale. In addition, we also considered the equivalent widths of Balmer lines as activity indicators. Moreover, we analysed the possible variability of Fe ii and other metallic lines in the optical spectra. The spectral variations were analysed for periodicity using the Lomb-Scargle periodogram. Results. We report for the first time a long-term 5.14-yr activity cycle in the solar-analog star HD 45184 derived from Mount Wilson S index. This makes HD 45184 one of most similar stars to the Sun with a known activity cycle. The variation is also evident in the first lines of the Balmer series, which do not always show a correlation with activity in solar-type stars. Notably, unlike the solar case, we also found that the equivalent widths of the high photospheric Fe ii lines (4924 Å, 5018 Å and 5169 Å) are modulated (±2 mÅ) by the chromospheric cycle of the star. These metallic lines show variations above 4σ in the rms spectrum, while some Ba ii and Ti ii lines present variations at 3σ level, which could be considered as marginal variations. From short-term modulation of the S index we calculate a rotational period of 19.98 days, which agrees with its mean chromospheric activity level. We also clearly show that the activity cycles of HD 45184 can be detected in both Fe ii and Balmer lines. Key words. stars: activity – stars: chromospheres – stars: solar-type – stars: individual: HD 45184 1. Introduction Activity cycles have been identified in stars of spectral types F to M (even stars with exoplanets), including stars with multiple The pioneer research on stellar activity by Wilson(1978) and cycles. The Ca ii H&K lines are commonly used as optical activ- subsequent works (e.g. Vaughan et al. 1978; Duncan et al. 1991; ity proxies (e.g. Baliunas & Jastrow 1990; Baliunas et al. 1998; Gray & Baliunas 1995) have allowed to better understand the Hall et al. 2007), as are other lines, such as those of the Balmer activity phenomenon in stars other than the Sun. A main con- series (Robertson et al. 2013a; Reiners et al. 2012) or the Mg ii tribution to this subject was made by Baliunas et al.(1998), who infrared triplet (see e.g. Busà et al. 2007; Pietarila & Livingston analysed the flux variability of Ca ii H&K lines and found three 2011). However, it is unclear whether the activity cycles can be different types of behaviour. They found cycles with periods of identified using other optical lines. These activity cycles have between 2.5 and 25 yr that are often associated with stars with important implications on different fields. For instance, studying moderate activity. Very active stars display fluctuations of activ- a range of stars with physical characteristics similar to the Sun ity rather than cycles, while inactive stars seem to be in a state across a range of ages and other parameters is very useful for un- similar to the solar Maunder minimum, the time period between derstanding how typical the Sun is (e.g. Hall et al. 2007, 2009). 1645 and 1715, when the Sun deviated from its usual 11-yr ac- It also helps in particular to understand the frequency of Earth- tivity cycle. influencing behaviours (e.g. Maunder minimum). The discovery Since the pioneering surveys, several stellar cycles have been of activity cycles also helps in the important task of distinguish- reported to date (e.g. Hall et al. 2007; Metcalfe et al. 2010, 2013; ing stellar and planetary signals in radial-velocity surveys (e.g. DeWarf et al. 2010; Buccino et al. 2014; Egeland et al. 2015). Robertson et al. 2013b; Carolo et al. 2014). ? Based on data products from observations made with ESO Tele- scopes at the La Silla Paranal Observatory under programs 072.C-0488 We started a program aiming to detect activity cycles in and 183.C-0972. solar-analog stars using the extensive database of HARPS Article published by EDP Sciences A135, page 1 of6 A&A 589, A135 (2016) spectra. Our initial sample comprises close solar-analog stars taken from Nissen(2015), who carefully selected stars with 1 physical parameters very similar to the Sun (±100 K in Teff, ±0.15 dex in log g, and ±0.10 dex in [Fe/H]), and we also re- quired that the sample spectra have a very high signal-to-noise ratio S/N(≥600). The similarity between these stars and the Sun, together with the possibility to include spectra with high S/N and a homogeneous parameter determination, encouraged us to ini- tially select these objects. However, we do not discard the future possible inclusion of more stars to extend our sample. An inspection of the preliminary data showed a notable vari- ation in the chromospheric activity of G1.5V (Gray et al. 2006) star HD 45184 (=HR 2318, V = 6:39, B−V = 0:62). It is located at a distance of ∼22 pc (van Leeuwen 2007) and hosts a debris Fig. 1. Removal of telluric lines. For two observations of different runs disk of 1–2 M⊕ (Lawler et al. 2009). The reported fundamental the original spectra are shown in the top panel. Below the clean spectra parameters are Teff = 5871 ± 6 K, log g = 4:445 ± 0:012 dex, (black) are overplotted with the telluric model spectra used in each case −1 [Fe/H] = 0:047 ± 0:006 dex, and vturb = 1:06 ± 0:017 km s (light blue). (Nissen 2015). Nissen also derived a mass of 1.06 ± 0.01 M (only internal error) and age of 2.7 ± 0.5 Gyr, showing that HD 45184 is ∼1.8 Gyr younger than the Sun. Then, with the aim of detecting possible activity cycles, we collected all the Telluric features were removed by dividing each observed available HARPS spectra for this object. Multiple observations spectrum S obs by an empirical telluric spectrum T appropriately are required to properly identify both long- and short-term activ- scaled: ity variations. For HD 45184, we gathered a total of 291 spectra −α covering more than ten years of observations, which makes this S corr = S obs · T ; source a very good target in which to search for a possible ac- tivity cycle. HD 45184 has also been identified as a candidate to where α represents the relative optical thickness of the terres- trial atmosphere. This coefficient α was chosen to reproduce the host a planetary mass companion of 0:04 MJup that was detected by radial velocity (Mayor et al. 2011), and it is included in the telluric line intensities, most of which are due to water vapour Extrasolar Planets Encyclopaedia2. All this makes HD 45184 a in our spectral region. We repeated this procedure using a tem- very interesting object and encouraged us to perform the study plate of the O2 γ-band around 6300 Å. In this way, we were able presented here. to satisfactorily correct observations taken with different atmo- This work is organised as follows. In Sect. 2 we describe the spheric water vapour content. Figure1 shows an example of the observations and data reduction, while in Sect. 3 we describe our removal of telluric lines in two small spectral windows. Telluric results, and finally in Sect. 4 we present our discussion. lines in the left side of the bottom panel are due to atmospheric O2, while the right side corresponds to H2O. 2. Observations and data reduction 3. Results The spectra were obtained from the European Southern Obser- 3.1. Chromospheric Ca II H&K lines vatory (ESO) archive3 under the ESO programs 072.C-0488(E) (PI: M.
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