A&A 530, A73 (2011) Astronomy DOI: 10.1051/0004-6361/201015314 & c ESO 2011 Astrophysics Chromospheric activity of stars with planets B. L. Canto Martins, M. L. das Chagas, S. Alves, I. C. Leão, L. P. de Souza Neto, and J. R. de Medeiros Departamento de Física, Universidade Federal do Rio Grande do Norte, Natal, Brazil e-mail: [email protected] Received 30 June 2010 / Accepted 25 March 2011 ABSTRACT Context. Signatures of chromospheric activity enhancement have been found for a dozen stars, pointing to a possible star-planet interaction. Nevertheless in the coronal activity regime, there is no conclusive observational evidence of such an interaction. Does star-planet interaction manifest itself only for a few particular cases, without having a major effect on stars with planets in general? Aims. We aim to add additional observational constraints to support or reject the major effects of star-planet interactions in stellar activity, based on Ca II chromospheric emission flux. Methods. We performed a statistical analysis of Ca II emission flux of stars with planets, as well as a comparison between Ca II and X-ray emission fluxes, searching for dependencies on planetary parameters. Results. In the present sample of stars with planets, there are no significant correlations between chromospheric activity indica- tor log(RHK) and planetary parameters. Furthermore, the distribution of the chromospheric activity indicator for stars without planets is indistinguishable from the one with planets. Key words. planet-star interactions – stars: activity – stars: chromospheres – stars: coronae – stars: statistics 1. Introduction be associated to coronal manifestations of star-planet interaction. According to Poppenhaeger et al. (2010), any trends in the X-ray Since the pioneering discovery by Mayor & Queloz (1995)– luminosity versus planet orbital parameters seem to be domi- a little more than a decade ago – of a planet orbiting the star nated by selection effects. In addition, Fares et al. (2010) reports 51 Peg, more than 450 other exoplanets have been found at the no clear evidence of star-planet magnetospheric interactions in time of writing (e.g., Schneider 2010). The discovered planets HD 189733. In this context, let us recall that, more recently from have masses ranging from 4 Earth masses to 11 Jupiter masses. a comparison of the distribution of the rotation of stars with and They can be found at distances of several AU or close to the without detected planets, Alves et al. (2010) have shown that the parent star, with orbital periods ranging from a few days to v sin i distribution for these two families of stars is drawn from a few years. High eccentricity is a common parameter connect- the same population distribution function. ing these lanets (e.g., Marcy et al. 2001). Because the observational basis of stellar activity enhance- These discoveries have inspired intensive studies of the phys- ment due to star-planet interaction has clearly not yet been es- ical properties of the planets and of their parent stars, including tablished, in the present study we report a statistical analysis of a possible star-planet interaction. Indeed, in an analogy to bi- stellar CaII chromospheric emission flux for a sample of 74 stars nary stars, which show a higher activity level compared to sin- with planets, as described in the following sections. For a more gle stars, at very close distances, one might also expect plan- solid analysis of the behavior of chromospheric and coronal ac- ets to play a role in the level of activity of their host stars. tivity of stars with planets, it is essential to conduct a compar- Nevertheless, observational data, still at a very informative level, ative analysis of stars without detected planets. This is one of demonstrate that star-planet interaction appears to be more com- the major goals of the current study. In this context, we also an- plex than the widely accepted star-star interaction in close bi- alyzed the CaII chromospheric emission flux of a comparison nary systems. For instance, Shkolnik et al. (2003, 2008) report a sample of 26 stars without detected planets. The paper is orga- planet-induced chromospheric activity on two stars with planets, nized as follows. In Sect. 2 we present the characteristics of the HD 179949 and υ And, apparent from the night-to-night modu- working samples, in Sect. 3 we describe our findings with a brief lation of the CaII H and K chromospheric emission phased with discussion, and finally we outline our conclusions in Sect. 4. the hot Jupiter’s orbit. In addition, Kashyap et al. (2008) claim that stars with close-in giant planets are on average more X-ray active than those with planets that are more distant, an observa- 2. Stellar working sample and data tional result consistent with the hypothesis that giant planets in Different methods, such as radial velocity measurements (Butler close proximity to the parent stars could influence stellar mag- et al. 1996; de Medeiros et al. 2009) and photometric light curves netic activity. (Konacki et al. 2003, 2004), have been used to discover new By contrast, Poppenhaeger et al. (2010) find no significant planetary systems around other stars than the Sun. Aiming to correlations between X-ray luminosity and planetary parame- make our sample homogeneous, we only use stars that had their ters, suggesting no major average activity enhancement in the planets discovered with radial velocity method. corona of stars with planets. Indeed, these authors find no addi- The present stellar working sample consists of selected tional detectable effects in coronal X-ray luminosity that could objects from the base of extrasolar planets maintained by Article published by EDP Sciences A73, page 1 of 6 A&A 530, A73 (2011) Jean Schneider (Schneider 2010), updated on October 6, 2009, when there were 235 planets cataloged by the method in vari- ation in the radial velocity (RV). We checked which of these stars presented Ca II emission flux detected in the catalog of Wright et al. (2004). After this selection, we filtered the com- plete stellar sample to obtain a subset of main-sequence stars that are within 20 pc in the solar neighborhood. Our final work- ing sample consists of 74 stars with planets with spectral types in the F-G-K interval, as presented in Table 1. The chromospheric activity indicator, log(RHK), for all stars was computed from the Ca II H and K line-core emission in- dex listed by Wright et al. (2004), following the procedure of converting emission index S to the flux at the stellar surface pro- posed by Noyes et al. (1984). To diagnose coronal activity of the stars forming the above-mentioned stellar sample, we used the coronal activity indicator, log( LX ), that was computed us- Lbol ing the X-ray fluxes listed by Kashyap et al. (2008)and Poppenhaeger et al. (2010). Readers are referred to these works for a discussion of the observational procedures, data reduction, and error analysis. Stellar luminosities were determined as follows. First, appar- ent visual magnitudes (mv) and trigonometric parallaxes, both taken from the HIPPARCOS catalogue (ESA 1997), were com- Fig. 1. Chromospheric activity indicator log(RHK) as a function of bined to yield the absolute visual magnitude (Mv). Bolometric the planetary semi-major axis log(1/apl) for our sample of stars correction (BC), computed from Flower (1996) calibration, was with planets. applied to obtaining the bolometric magnitude, which was fi- nally converted into stellar luminosity. The effective temperature was computed using Flower (1996)(B − V)versusTeff calibra- Following the same strategy as used by Poppenhaeger et al. tion. The stellar and planetary parameters, as well as chromo- (2010) in their analysis of the coronal X-ray emission behav- spheric and coronal activity indicators for the entire sample of ior in stars with planets, we show the distribution of the chro- stars with planets, are listed in Table 1. mospheric activity indicator log(RHK) of stars with planets as a For comparative purposes, we took a sample of stars not function of planetary distance log(1/apl)(Fig.1). A close com- known to have any planetary-mass companions. This new parison of the distribution of log(RHK)versuslog(1/apl) with sample was taken from Schmitt et al. (1997), as done by the distribution of the coronal activity indicator log( LX )versus Poppenhaeger et al. (2010), and is composed of 26 F-G-K type Lbol log(1/a ), illustrated in Fig. 5 of Poppenhaeger et al. (2010), dwarf stars with all stars having Ca II emission flux measured pl shows that, at least qualitatively, the behavior of the chromo- by Wright et al. (2004) and X-ray fluxes detected by Schmitt spheric activity indicator appears to be comparable to that of the (1997). The chromospheric and coronal activity indicators were coronal activity indicator. The same aspect can be seen in Fig. 2 calculated as before for the sample of stars with planets. where we show the distribution of the chromospheric activity in- However, we should be cautious about this sample, which de- dicator log(R ) of stars with planets as a function of the product rives from a list of stars that are surveyed for planets, but for HK of the planetary mass with the reciprocal distance log(1/a ). which none have yet been found. This certainly does not mean pl Still following the strategy proposed by Poppenhaeger et al. that such stars have no planetary companions whatsoever. In fact, (2010), we also carried out a statistical analysis of both log(R ) they might host planets with very low mass and/or a long orbital HK ffi and log( LX ) activity indicators of stars with close-in planets, period that are more di cult to detect with radial velocity sur- Lbol veys.