The Activity of Possibles Maunder Minimum Candidates R

Boletim da Sociedade Astronômica Brasileira, 30, no. 1, 85-86 c SAB 2018

The activity of possibles Maunder minimum candidates R. Ferreira1, M. Castro1 & J.-D do Nascimento Jr.1,2 1 Departamento de Física Teórica e Experimental – Universidade Federal do Rio Grande do Norte CP 1641, 59072-970, Natal, Rio Grande do Norte, Brazil. e-mail: [email protected]

Abstract. Among several aspects related to the Sun activity history, there was a period of the Sun evolution with a low chromospheric activity level and a low number of observed sunspots. This period was between 1645 and 1715, (well known as Maunder Minimum period, MM), and remains as a puzzle for the stellar evolution theory. In this work, we investigate the evolutionary status of 30 sun-like type stars that are possible MM candidates due to their low chromospheric activity as compared with stars at same evolutionary status. These stars present activity index measurements along 50 yrs from the Mount Wilson program and others. They 0 have very low chromospheric activity as log RHK ≤ 5.0, < S > ' 0.15 and (B − V) between 0.61 and 0.71 (for the Sun the < S > = 0.167 and a color index (B − V) = 0.65). At this phase, we also revisited CoRoT and Kepler databases to enhance the number of Maunder minimum candidate stars presenting flat light curves and low activity profile. Resumo. Dentre os vários aspectos relacionados com o histórico de atividade do Sol, houve um período da evolução do Sol com baixa nível de atividade cromosférica e um baixo numero de manchas solares observadas. Esse período foi entre 1645 e 1715, (conhecido como mínimo de Maunder, MM), e permanece como um enigma para a teoria da evolução estelar. Neste trabalho, nós investigamos o status evolucionário de 30 estrelas do tipo solar que são consideradas possíveis candidatas MM devido aos seus baixos níveis de atividade cromosférica ao longo dos últimos 50 anos. Estas estrelas apresentam índices de atividade extremamente 0 baixos, tais como: log RHK ≤ 5.0, < S > ' 0.15 e (B − V) entre 0.61 e 0.71 (para o Sol < S > = 0.167 e o índice de cor (B − V) = 0.65). Neste estágio, nós também revisaremos as bases de dados dos satélites CoRoT e Kepler para aperfeiçoarmos o número de estrelas candidatas MM, que apresentem curvas de luz "flat" e perfil de baixa atividade.

Keywords. Stars: activity – Stars: evolution – Stars: chromospheres

1. Introduction program, it has been possible to analyze the activity evolution along the last 50 yrs. The S -index of MW is the standard index The problem of extremely low activity in main sequence stars is of activity and has been very useful to investigate the activity a puzzle for the stellar astrophysics. It is known that young stars evolution of these stars. have a strong activity. When these stars present an extremely low activity, along many years, it could be the ﬁrst evidence of

Maunder Minimum (MM hereafter), which deﬁnes a phase of Mt. Wilson stars (Duncan +1991) the solar activity during 70 yrs between XVI and XVII centuries 0.5 when almost no sunspots have been observed (see Fig.1). 0.4

0.3 MW S

0.2

All Mt. Wilson Stars 0.1 Strong MM candidates Weak MM candidates Sun 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 (B-V)

Figure 2. In this ﬁgure, we can see the MW sample stars. The inferior envelope contains the MM candidates, divided into strong MM candidates and weak MM candidates according to the literature (Ferreira et al. 2018, in prep.)

igure F 1. All measurements for sunspots along 400 yrs. The red dots This work is organized as follows: in section 2 we present show the MM period. We also have the others minimums in this ﬁgure. our sample of stars, in section 3 we show many results and discussion about some selected stars of our sample, ﬁnally the last Many surveys in Sun-like stars have been a good opportunity session is the conclusion and perspectives. to study the Sun’s activity history (Wright (2004)). They are very important to detect extremely low activity analogous to MM pe- 2. The sample riod in young stars. In fact, the MM state has been investigated since 1970’s (Eddy (1976)). Through the Mt. Wilson (MW) pro- Our sample presented in Fig. 2 is composed by Sun-like stars gram (see Fig. 2) and the California and Carnegie Planet Search (see Duncan et al. (1991); Hall et al. (2009); Isaacson & Fischer

85 R. Ferreira et al.: The activity of possibles Maunder minimum candidates

(2010); Lubin et al. (2010); Pace (2013)). We use the long- Lubin et al. (2010) analyse the MM candidates under the per- term programs for monitoring the activity along many years. The spective of lithium abundances and classiﬁed three of these stars most important was the MW project. Currently, there are some as strong MM candidates and Weak MM candidates according short-term activity programs, that observe stars for some days to the standard deviation of log n(Li). Here, we show the Lomb- only. Scargle periodogram for the "strong" MM candidates, from the activity measurements of long-term programs (see Figs. 3,4,5). The better MM candidates in the sample Star Type log n (Li) B-V < S > Age [Gyr] All S-index measurements of HD 115617 and LS periodogram All S-index measurements of HD 217014 and LS periodogram HD 10307 G2 1.92 0.618 0.152 7.00 0.30 Sun 0.20 Sun 0.28 HD 115617 HD 217014

HD 43587 G0 1.83 0.610 0.155 6.12 0.26 0.18

0.24 HD 95128 G0 1.81 0.624 0.152 7.11 0.16 0.22

HD 3651 K0 0.59 0.850 0.176 6.91 S-index 0.20 S-index 0.14 0.18 0.12 HD 115617 G5 1.20 0.709 0.163 6.53 0.16 HD 217014 G5 1.32 0.666 0.150 7.24 0.14 0.10 4000 6000 8000 10000 12000 14000 16000 0 2000 4000 6000 8000 10000 12000 14000 16000 Time [JD - 2440000] Time [JD - 2440000]

0.035 0.175

0.030 0.150

0.025 0.125

0.020 0.100

Power 0.015 Power 0.075

0.010 0.050 3. Results and discussion 0.005 0.025 0.000 0.000 0 500 1000 1500 2000 2500 3000 0 500 1000 1500 2000 2500 3000 We investigate about 30 Sun-like stars that have a flat activ- Period [Days] Period [Days] ity profile and may be considered MM candidates to according to Lubin et al. (2010) and others. From the spectroscopic S - Figure 5. All S -index spectroscopic measurements for HD 115617 index measurements variability, we calculate the Long-Scargle and HD 217014 (above) with its respectively Lomb-Scargle (LS) pe- Periodograms (Horne & Baliunas (1986)) and we found sev- riodograms (below) eral possible periods of activity. We first investigate the star HD 43587, that was a primary CoRoT target. After this, we found five more stars that can be considered MM candidates. 4. Conclusion and perspectives These stars are considered strong MM candidates by many au- All S-index measurements of HD 43587 and LS periodogram All S-index measurements of HD 95128 and LS periodogram

Sun 0.22 Sun thors and they have lithium abundance that is within the nor- 0.18 HD 43587 0.21 HD 95128 0.20 mal range for their color (Lubin et al. (2010)). The analyze of 0.17 0.19

0.18 the lithium abundances suggests that stars with normal lithium 0.16 S-index S-index 0.17

0.15 abundance and low activity proﬁle are good MM candidates. In 0.16

0.15 0.14 addition, it is not possible to determine the activity cycle period, 0.14 0 2500 5000 7500 10000 12500 15000 17500 4000 6000 8000 10000 12000 14000 16000 Time [JD - 2440000] Time [JD - 2440000] which is an indication that the star is in a long period of ﬂat ac- 0.6 0.35 tivity. We pretend to continue with the time-series spectroscopic 0.30 0.5 0.25 0.4 measurements and proposals for long-term observational cam- 0.20 0.3 Power Power 0.15 paign. 0.2 0.10

0.1 0.05

0.0 0.00 3 0 1000 2000 3000 4000 5000 6000 0 500 1000 1500 2000 2500 3000 Acknowledgements. The authors thanks GE Team (DFTE/UFRN) and Period [Days] Period [Days] CAPES/CNPq agencies for the ﬁnancial support since the beginning of this re- search. Figure 3. All S -index spectroscopic measurements for HD 43587 and HD 95128 (above) with its respectively Lomb-Scargle (LS) periodograms (below). References Wright, J. T. 2004, AJ, 128, 1273 Eddy, J. A. 1976, Science, 192, 1189 Lubin, D., Tytler, D., & Kirkman, D. 2010, ApJ, 716, 766 Duncan, D. K., Vaughan, A. H., Wilson, O. C., et al. 1991, ApJS, 76, 383 Hall, J. C., Henry, G. W., Lockwood, G. W., Skiﬀ, B. A., & Saar, S. H. 2009, AJ, All S-index measurements of HD 10307 and LS periodogram All S-index measurements of HD 3651 and LS periodogram

Sun Sun 138, 312 0.175 0.24 HD 10307 HD 3651 0.170 0.22 Isaacson, H., & Fischer, D. 2010, ApJ, 725, 875 0.165 0.20 0.160 Pace, G. 2013, A&A, 551, L8 0.18 0.155 S-index S-index 0.16 Horne, J. H., & Baliunas, S. L. 1986, ApJ, 302, 757 0.150 0.14 0.145 0.12 0.140 0.10 4000 6000 8000 10000 12000 14000 0 2000 4000 6000 8000 10000 12000 14000 16000 Time [JD - 2440000] Time [JD - 2440000]

0.30 0.8

0.25

0.6 0.20

0.4 0.15 Power Power 0.10 0.2 0.05

0.0 0.00 500 1000 1500 2000 2500 3000 0 500 1000 1500 2000 2500 3000 Period [Days] Period [Days]

Figure 4. All S -index spectroscopic measurements for HD 10307 and HD 3651 (above) with its respectively Lomb-Scargle (LS) periodograms (below)