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Spatially Resolved Spectroscopy Across Stellar Surfaces. III

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Spatially Resolved Spectroscopy Across Stellar Surfaces. III Astronomy & Astrophysics manuscript no. Dravins_etal_Spatially_resolved_III c ESO 2018 June 4, 2018 Spatially resolved spectroscopy across stellar surfaces. III. Photospheric Fe I lines across HD 189733A (K1 V) Dainis Dravins1, Martin Gustavsson1, and Hans-Günter Ludwig2 1 Lund Observatory, Box 43, SE-22100 Lund, Sweden e-mail: [email protected] 2 Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl, DE–69117 Heidelberg, Germany e-mail: [email protected] Received March 13, 2018; accepted May 15, 2018 ABSTRACT Context. Spectroscopy across spatially resolved stellar surfaces reveals spectral line profiles free from rotational broadening, whose gradual changes from disk center toward the stellar limb reflect an atmospheric fine structure that is possible to model by 3-D hydrodynamics. Aims. Previous studies of photospheric spectral lines across stellar disks exist for the Sun and HD 209458 (G0 V) and are now extended to the planet-hosting HD 189733A to sample a cooler K-type star and explore the future potential of the method. Methods. During exoplanet transit, stellar surface portions successively become hidden and differential spectroscopy between various transit phases uncovers spectra of small surface segments temporarily hidden behind the planet. The method was elaborated in Paper I, in which observable signatures were predicted quantitatively from hydrodynamic simulations. Results. From observations of HD 189733A with the ESO HARPS spectrometer at λ/∆λ ∼115,000, profiles for stronger and weaker Fe i lines are retrieved at several center-to-limb positions, reaching adequate S/N after averaging over numerous similar lines. Conclusions. Retrieved line profile widths and depths are compared to synthetic ones from models with parameters bracketing those of the target star and are found to be consistent with 3-D simulations. Center-to-limb changes strongly depend on the surface granulation structure and much greater line-width variation is predicted in hotter F-type stars with vigorous granulation than in cooler K-types. Such parameters, obtained from fits to full line profiles, are realistic to retrieve for brighter planet-hosting stars, while their hydrodynamic modeling offers previously unexplored diagnostics for stellar atmospheric fine structure and 3-D line formation. Precise modeling may be required in searches for Earth-analog exoplanets around K-type stars, whose more tranquil surface granulation and lower ensuing microvariability may enable such detections. Key words. stars: atmospheres – stars: solar-type – techniques: spectroscopic – stars: line profiles – exoplanets: transits 1. Introduction rated in Paper I, in which observable signatures were predicted quantitatively. In Paper II (Dravins et al. 2017b), a first observa- Three-dimensional and time-dependent hydrodynamic simula- tional demonstration of this method was presented for the G0 V tions provide realistic descriptions for the photospheres of the star HD 209458. various classes of stars. The combination of stellar spectra com- This Paper III extends such studies to another bright planet- puted from such models, with corresponding models of planetary hosting star but of lower temperature, the mV = 7.7, K1 V atmospheres, constrain the properties of both the star and its ex- star HD 189733A, circled by its planet HD 189733Ab. To avoid oplanets. To test and evolve such models, more detailed observa- constantly repeating their (easily intermixed) numbers from the tions than the ordinary spectrum of integrated starlight are desir- Henry Draper catalog, for the purposes of this paper we refer to able. Spectral line syntheses from 3-D atmospheres display nu- HD 189733A as ‘Alopex’1, and to HD 189733Ab as simply ‘the merous features in the gradual changes of photospheric line pro- planet’. file strengths, shapes, asymmetries, and wavelength shifts from Besides probing a cooler stellar atmosphere, there is a signif- the stellar disk center toward the stellar limb. Direct compar- arXiv:1806.00012v1 [astro-ph.SR] 31 May 2018 icance for exoplanet searches since the surfaces of K-type stars isons between theory and spectral-line observations were in the are predicted to be quieter, with lesser amplitudes in velocity and past carried out only for the spatially resolved Sun (e.g., Lind et temperature than in hotter and more solar-like stars. al. 2017). This project extends such studies to other stars as well. Searches for exoplanets that mimic the Earth in terms of In Paper I (Dravins et al. 2017a), theoretically predicted spatially mass and orbit are limited by intrinsic stellar microvariability resolved signatures were examined for a group of main-sequence in either apparent radial velocity or photometric brightness (Fis- stellar models with temperatures between 3960 and 6730 K. Cor- cher et al. 2016), and therefore may appear less challenging responding observations are feasible during exoplanet transits when small stellar surface portions successively become hidden, 1 This name – Greek Aλωπηξ´ – is taken from the original taxonomic and differential spectroscopy between different transit phases name for the arctic fox (Alopex Lagopus) given by Linnaeus in 1758, provide spectra of small surface segments temporarily hidden here relating to the namesake of its constellation Vulpecula (Latin for behind the planet. The observational requirements were elabo- ‘little fox’). It is also the Greek name for that constellation. Article number, page 1 of 14 A&A proofs: manuscript no. Dravins_etal_Spatially_resolved_III around K-type stars. This requires adequate modeling of their granulation images for stars at different temperature, see Magic surface convection so that, for example, correlations between et al. (2013), Nordlund & Dravins (1990), and Tremblay et al. stellar fluctuations in brightness, color, and radial velocity are (2013). adequately understood and may be compensated for. However, given the often greater magnetic activity and increased spottiness on such stars, separate treatments of magnetically sensitive fea- 2.2. K-dwarf surface spectroscopy tures over the timescales of their variability may also be needed As stressed in Paper I, the simulation of 3-D atmospheric hy- to disentangle exoplanet signatures. drodynamics is only a first part of the problem since most astro- Further, analyses of exoplanet atmospheres by transmitted physical deductions about stellar or planetary properties require starlight require the background stellar spectrum to be precisely synthetic spectra for comparison; see, for example, Asplund et known. That is not the averaged spectrum of the entire stellar al. (2000), Asplund (2005) and Chiavassa et al. (2018). Synthetic disk but that from only the small area temporarily behind the spectral lines from such K-type stellar models have been com- planet, such that the varying stellar spectrum along the chord of puted by Dravins & Nordlund (1990b) and Ramírez et al. (2009). transit has to be known for any more precise studies (Rackham The lower amplitudes of convective motion and lesser tempera- et al. 2018). ture contrast result in smaller convective wavelength blueshifts Besides applying this method to a new target star, the aim and weaker curvatures of stellar bisectors, as compared to, for in- is to explore its general practicality. Currently, large transiting stance, the solar case. Some observations have been made to seek planets with bright host stars are actively being sought by several evidence of the surface structure on Alopex and other K-type photometric surveys. Recent and forthcoming high-resolution dwarfs. Thus, Ramírez et al. (2008) obtained high-resolution spectrometers at major telescopes are likely to observe such sys- spectra of several K-type dwarfs to identify asymmetries in dif- tems for exoplanet research; using this method, the same data ferently strong Fe i line profiles that have C-shaped bisectors as can be used to probe also the host star. signatures of granulation. Specifically for Alopex, observational confrontations with 3-D models have been carried out in analyzing photometric light 2. Hydrodynamic atmospheres and their spectra curves during transit, searching for effects to segregate between 2.1. Three-dimensional structures on different stars limb-darkening functions predicted from 1-D and 3-D model atmospheres. The differences are small, however, and largely Three-dimensional and time-variable model photospheres are masked by the great amount of absorption lines, whose differ- now established as realistic descriptions of stellar outer lay- ent and complex center-to-limb behavior largely masks the un- ers. Originating from initial modeling of the solar photosphere derlying temperature structure (Hayek et al. 2012). If extremely (e.g., Nordlund et al. 2009), simulations have now been ex- good photometric precision could be achieved, polarimetric ob- tended to stars with widely different parameters over much of the servations during transit could offer further constraints on atmo- Hertzsprung-Russel diagram (Magic et al. 2013; Tremblay et al. spheric structure, as simulated by Kostogryz et al. (2017). 2013). Although the basic physical processes are similar in gi- An ambitious study to deduce the center-to-limb variation of ants, dwarfs, and even white dwarfs, synthetic surface images re- the strong absorption lines of Ca ii H & K and Na i D1 &D2 veal the greatly changing granulation sizes on stars with different lines was carried out by Czesla et al. (2015), recording transit surface gravity,
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