Living Rev. Solar Phys., 4, (2007), 3 http://www.livingreviews.org/lrsp-2007-3 The Sun in Time: Activity and Environment Manuel Gu¨del Paul Scherrer Institute, Wu¨renlingen and Villigen, CH-5232 Villigen PSI, Switzerland and Max Planck Institute for Astronomy, Ko¨nigstuhl 17, D-69117 Heidelberg, Germany email: Imprint / Terms of Use Living Reviews in Solar Physics are published by the Max Planck Institute for Solar System Research, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany. ISSN 1614-4961 This review is licensed under a Creative Commons Attribution-Non-Commercial-NoDerivs 2.0 Germany License: http://creativecommons.org/licenses/by-nc-nd/2.0/de/ Because a Living Reviews article can evolve over time, we recommend to cite the article as follows: Manuel Gu¨del, “The Sun in Time: Activity and Environment”, Living Rev. Solar Phys., 4, (2007), 3. 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Contents 1 Introduction 7 2 What is a Solar-Like Star? 10 3 The Sun in Time 12 3.1 Goals of the “Sun in Time” project . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.2 Overview of stellar sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 4 The Solar Magnetic Field in Time 16 4.1 The young solar photosphere: Large, polar spots . . . . . . . . . . . . . . . . . . . 16 4.1.1 Doppler imaging of young solar analogs . . . . . . . . . . . . . . . . . . . . 16 4.1.2 Polar spots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 4.2 Coronal structure of the young Sun . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 4.2.1 Magnetic loop models and active regions . . . . . . . . . . . . . . . . . . . . 20 4.2.2 Inferences from coronal density measurements . . . . . . . . . . . . . . . . . 21 4.2.3 Inferences from rotational modulation . . . . . . . . . . . . . . . . . . . . . 21 4.2.4 Inferences from eclipses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4.2.5 Photospheric-field extrapolation to the corona . . . . . . . . . . . . . . . . . 23 4.2.6 Summary on coronal structure . . . . . . . . . . . . . . . . . . . . . . . . . 23 4.3 Activity cycles in the young Sun . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 4.3.1 Starspot cycles of solar analogs . . . . . . . . . . . . . . . . . . . . . . . . . 25 4.3.2 X-ray cycles of solar analogs . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 5 Solar Radiation and Wind in Time 29 5.1 The solar wind in time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 5.2 The solar spin in time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 5.3 The ultraviolet Sun in time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 5.4 The far-ultraviolet Sun in time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 5.5 The extreme-ultraviolet and X-ray Sun in time . . . . . . . . . . . . . . . . . . . . 35 5.5.1 The solar X-ray corona in time . . . . . . . . . . . . . . . . . . . . . . . . . 35 5.5.2 The coronal temperature in time . . . . . . . . . . . . . . . . . . . . . . . . 37 5.6 Putting it all together: The XUV Sun in time . . . . . . . . . . . . . . . . . . . . . 42 5.7 The radio Sun in time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 5.7.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 5.7.2 Observational results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 5.8 Coronal flares in time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 5.8.1 Flare energy distributions and coronal heating . . . . . . . . . . . . . . . . 52 5.8.2 Phenomenological evidence . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 5.8.3 Stellar flare energy distributions . . . . . . . . . . . . . . . . . . . . . . . . 54 5.8.4 Stochastic flares and coronal observations . . . . . . . . . . . . . . . . . . . 55 5.8.5 Summary: The importance of stochastic flares . . . . . . . . . . . . . . . . 56 5.9 The solar coronal composition in time . . . . . . . . . . . . . . . . . . . . . . . . . 56 5.9.1 Abundances in stellar coronae . . . . . . . . . . . . . . . . . . . . . . . . . . 57 5.9.2 The composition of the young solar corona . . . . . . . . . . . . . . . . . . 58 5.9.3 The Ne/O abundance ratio: Subject to evolution? . . . . . . . . . . . . . . 58 5.10 Summary: The young, active Sun . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 6 Further Back in Time: The Pre-Main Sequence Sun 61 6.1 Where was the cradle of the Sun? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 6.2 New features in the pre-main sequence Sun . . . . . . . . . . . . . . . . . . . . . . 61 6.2.1 Evolutionary stages: Overview . . . . . . . . . . . . . . . . . . . . . . . . . 61 6.2.2 New features: Accretion, disks, and jets . . . . . . . . . . . . . . . . . . . . 62 6.2.3 New emission properties: Solar-like or not? . . . . . . . . . . . . . . . . . . 62 6.3 The T Tauri Sun . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 6.3.1 The magnetic field of the T Tauri Sun . . . . . . . . . . . . . . . . . . . . . 63 6.3.2 The ultraviolet T Tauri Sun . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 6.3.3 The X-ray T Tauri Sun in time . . . . . . . . . . . . . . . . . . . . . . . . . 66 6.3.4 Coronal excesses and deficits induced by activity? . . . . . . . . . . . . . . 68 6.3.5 X-ray flaring of the T Tauri Sun . . . . . . . . . . . . . . . . . . . . . . . . 70 6.3.6 The radio T Tauri Sun in time . . . . . . . . . . . . . . . . . . . . . . . . . 70 6.3.7 The composition of the T Tauri Sun’s corona . . . . . . . . . . . . . . . . . 71 6.4 The protostellar Sun . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 6.4.1 Magnetic activity in the protostellar Sun . . . . . . . . . . . . . . . . . . . 72 6.4.2 Magnetic flaring of the protostellar Sun . . . . . . . . . . . . . . . . . . . . 73 6.4.3 Radio emission from the protostellar Sun . . . . . . . . . . . . . . . . . . . 73 6.5 The pre-main sequence Sun’s environment in time . . . . . . . . . . . . . . . . . . 73 6.5.1 Circumstellar disk ionization . . . . . . . . . . . . . . . . . . . . . . . . . . 73 6.5.2 Circumstellar disk heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 6.5.3 Observational evidence of disk irradiation . . . . . . . . . . . . . . . . . . . 76 6.6 The T Tauri Sun’s activity and meteoritics . . . . . . . . . . . . . . . . . . . . . . 77 6.7 Summary: The violent pre-main sequence Sun . . . . . . . . . . . . . . . . . . . . . 79 7 The Solar System in Time: Solar Activity and Planetary Atmospheres 80 7.1 The Faint Young Sun Paradox: Greenhouse or deep freeze? . . . . . . . . . . . . . 80 7.1.1 The relevance of greenhouse gases . . . . . . . . . . . . . . . . . . . . . . . 81 7.1.2 A bright young Sun? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 7.1.3 Cosmic rays and a stronger solar wind . . . . . . . . . . . . . . . . . . . . . 84 7.2 The Sun’s activity in the young solar system . . . . . . . . . . . . . . . . . . . . . 84 7.2.1 Planetary atmospheric chemistry induced by high-energy radiation . . . . . 84 7.2.2 High-energy radiation and the planetary biospheres in habitable zones . . . 86 7.2.3 Planetary atmospheric loss and high-energy radiation and particles . . . . . 87 7.2.4 Mercury: Erosion of atmosphere and mantle? . . . . . . . . . . . . . . . . . 89 7.2.5 Venus: Where has the water gone? . . . . . . . . . . . . . . . . . . . . . . . 89 7.2.6 Earth: A CO2 atmosphere and magnetic protection . . . . . . . . . . . . . 90 7.2.7 Mars: Once warmer and wetter? . . . . . . . . . . . . . . . . . . . . . . . . 91 7.2.8 Venus, Earth, Mars: Similar start, different results? . . . . . . . . . . . . . 92 7.2.9 Titan: Early atmospheric blow-off? . . . . . . . . . . . . . . . . . . . . . . . 93 7.2.10 Hot Jupiters: Mass evolution by evaporation? . . . . . . . . . . . . . . . . . 94 7.3 Summary: The high-energy young solar system . . . . . . . . . . . . . . . . . . . . 95 8 Summary and Conclusions 96 9 Acknowledgements 99 References 100 List of Tables 1 Symbols and units used throughout the text. . . . . . . . . . . . . . . . . . . . . . . 11 2 The “Sun in Time” project: Relevant observations. . . . . . . . . . . . . . . . . . . 13 a 3 The “Sun in Time” sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 −2 −1 4 Integrated fluxes (in units of erg cm s ) of strong emission features normalized to a distance of 1 AU and the radius of a one solar mass star. UV and EUV fluxes 1 1 of π UMa and χ Ori have been averaged. Data for solar analogs are from Telleschi et al. (2005) and Ribas et al. (2005), and for TW Hya from Herczeg et al. (2002), Herczeg et al. (2004), Kastner et al. (2002), and Stelzer and Schmitt (2004); the radius of TW Hya is 1R⊙ and its distance is 56 pc, see Herczeg et al. (2004). Fluxes of TW Hya have not been corrected for (small) photoabsorption and extinction. . . 44 5 Parameters of the power-law fits to the measured integrated fluxes and individual a line fluxes from MS solar analogs (data from Telleschi et al. 2005 and Ribas et al. 2005). The parameters α and β are defined in Equation 17. . . . . . . . . . . . . . 47 a 6 Enhancement factors of X-ray/EUV/XUV/FUV fluxes in solar history . . . . . . . 49 The Sun in Time: Activity and Environment 7 1 Introduction The study of the past of our Sun and its solar system has become an interdisciplinary effort be- tween stellar astronomy, astrophysics of star and planet formation, astrochemistry, solar physics, geophysics, planetology, meteoritical science and several further disciplines. The interest in un- derstanding the past evolution of our star is obvious; the Sun’s radiative energy, the solar wind, and various forms of transient phenomena (e.g., shock waves, high-energy particle streams during flares) are key factors in the formation and evolution of the planets and eventually the biosphere on Earth. The Sun is, like almost all cool stars, a “magnetic star” that produces magnetic fields through dynamo operation in the interior. These fields reach the surface where their presence is noticed in the form of sunspots. However, magnetic activity has much more far-reaching consequences: Solar magnetic fields control essentially the entire outer solar atmosphere, they heat coronal gas to millions of degrees, they produce flares whose by-products such as shock waves and high-energy particles travel through interplanetary space to eventually interact with planetary atmospheres; the solar wind is guided by open magnetic fields; this magnetized-wind complex forms a large bubble, an “astrosphere” in interstellar space containing the entire solar system and protecting it from a high dose of cosmic rays. Was the Sun’s magnetic activity different in its infancy when planets and their atmospheres formed, or when it was still surrounded by an accretion disk? Accumulated direct and indirect evidence indeed points to a much higher level of magnetic activity in the young Sun, in particular in its pre-main sequence (PMS) phase and the subsequent epoch of its settling on the main sequence (MS). Direct evidence includes meteoritic traces and isotopic anomalies that require much higher proton fluxes at early epochs at least partly from within the solar system (Section 6.5 below); indirect evidence comes from systematic comparisons of the contemporary Sun with solar analogs of younger age that unequivocally show a strong trend toward elevated activity at younger ages (Section 5 below). Interestingly, planetary atmospheres offer further clues to strongly elevated activity levels: Evidence for a warmer early climate on Mars or the extremely arid atmosphere of Venus – a sister planet of the water-rich Earth – call for explanations, and such explanations may be found in the elevated activity of the young Sun (see Section 7.2 below). The study of the early solar activity is the theme of the present review article. The main goal of this article is therefore to demonstrate evidence for a much more active young Sun, and to study the consequences this might have had for the development of the solar environment, including the formation and evolution of planets. Our discussion will therefore take us through the following three major issues: • The young Sun’s more rapid rotation induced an internal magnetic dynamo that was much more efficient than the present-day Sun’s. Consequently, stronger surface magnetic fields and/or higher surface magnetic filling factors should have induced enhanced “activity” in all its variations, from larger surface spots to a stronger, extended solar wind. If we can observationally probe the outer magnetic activity of the Sun, we obtain invaluable diagnostics for a deeper theoretical understanding of the internal dynamo. • The solar output largely controls planetary atmospheres and their climates. While this is obviously true for the dominant optical and infrared emissions of a star like the Sun, the irradiation of planetary atmospheres by higher-energy ultraviolet and X-ray photons as well as interactions with high-energy particles and the solar wind leads to atmospheric alterations that have been recognized and numerically simulated only recently. The much higher mag- netic activity of the young Sun and the resulting higher levels of ultraviolet, X-ray, and particle irradiation were therefore of prime importance for the early evolution of the planets. Living Reviews in Solar Physics http://www.livingreviews.org/lrsp-2007-3 8 Manuel Gu¨del The discovery of extrasolar planets in particular around Sun-like stars has also spurred inter- est in star-planet interactions, e.g., erosion of atmospheres or photochemical reactions, that profit from detailed studies in the solar system. • Similarly, at still younger stages of a star’s evolution, its environment is rich in molecular gas and dust, both in the form of a large envelope (in the youngest, protostellar phases of evolution) and a circumstellar gas and dust disk (including the T Tauri phase when planets were forming). Star-disk interactions are manifold, and their role is fundamental in various respects. The optical and ultraviolet radiation heats the disk and therefore primarily determines disk structure and the formation and evolution of planetary systems. High-energy emission, in particular X-ray radiation, further heats and ionizes parts of the circumstellar disk. Even moderate disk ionization will lead to accretion instabilities if weak magnetic fields are present. Disk heating by X-rays may also produce extreme temperature gradients across the disk that drive complicated chemical networks relevant for the later processing of the disk material into forming planets and planetary atmospheres. The focus of this review is therefore, on the one hand, on signatures of magnetic activity across the electromagnetic spectrum, representing physical processes in the photosphere, the chromo- sphere, and the thermal and non-thermal corona of a solar-like star. I will mostly use young solar-like stars to infer conditions that – by analogy – might have prevailed on the young Sun. On the other hand, I will also discuss traces that the elevated activity of the young Sun might have left behind in meteorites and in planetary atmospheres, thus collecting “in-situ” information about the distant past of our own solar system. While this article focuses on the conditions on the young Sun and in the early solar system, it has proven convenient to study the solar evolution in time systematically from young to old, because a number of trends become evident that can be calibrated with the contemporaneous Sun. We thus not only learn about the young Sun, but we uncover the systematics that made it different from what it is today. This is the approach I adopt in the present work. This article will not address issues on the formation and evolution of the Sun that are related to its internal constitution, with the exception of cursory reference to the magnetic dynamo that is, of course, at the origin of all solar magnetic activity. I will treat the PMS Sun in separate chapters for three related reasons: First, fundamental properties of the PMS Sun were largely different from those of the contemporaneous Sun (for example, its spectral type, or its photospheric effective temperature). Second, new features not present in the modern Sun become dominant key players related to activity and environment, among them accretion disks, accretion streams, star-disk magnetospheres, outflows, and jets. And third, the PMS behavior of the Sun cannot be assessed in detail judged from the present-day solar parameters; we can only discuss the range of potential evolutionary scenarios now observed in a wide sample of PMS stars (e.g., with respect to mass accretion rate, disk mass, disk dispersal time, rotation period, etc.). Numerous review articles are available on subjects related to the present one. Without intention to be complete, I refer here in particular to the collection of papers edited by Sonett et al. (1991) and Dupree and Benz (2004), the Cool Stars Workshop series (the latest volume edited by van Belle 2007), and the Protostars and Planets series (in particular the latest volumes by Mannings et al. 2000 and Reipurth et al. 2007). An early overview of solar variability (including that of its activity) can be found in Newkirk Jr (1980). Walter and Barry (1991) specifically reviewed knowledge of the long-term evolution of solar activity as known in the early nineties, in a similar spirit as the present review; numerous older references can be found in that work. For summaries of stellar X-ray and radio emission, I refer to Gu¨del (2004) and Gu¨del (2002), respectively. Feigelson and Montmerle (1999) and Feigelson et al. (2007) have summarized PMS aspects of magnetic activity. Glassgold et al. (2005) have reviewed the influence of the magnetic activity of the PMS Sun on its environment, in particular on its circumstellar disk where our planets were forming. Wood (2004) Living Reviews in Solar Physics http://www.livingreviews.org/lrsp-2007-3 The Sun in Time: Activity and Environment 9 has discussed evidence for winds emanating from solar-like stars, and Goswami and Vanhala (2000) have summarized findings related to radionuclides in meteorites and inferences for the young solar system; the most recent developments in this field have been reviewed by Wadhwa et al. (2007). Kulikov et al. (2007) and Lundin et al. (2007) have provided summaries on interactions between solar high-energy radiation and particles with planetary atmospheres, in particular those of Venus, Earth, and Mars. Living Reviews in Solar Physics http://www.livingreviews.org/lrsp-2007-3 10 Manuel Gu¨del 2 What is a Solar-Like Star? The present Sun is a G2 V star with a surface effective temperature of approximately 5780 K. Stellar evolution theory indicates, however, that the Sun has shifted in spectral type by several subclasses, becoming hotter by a few hundred degrees and becoming more luminous (the bolometric luminosity of the Sun in its zero-age main-sequence [ZAMS] phase amounted to only about 70% of the present-day output; Siess et al. 2000). In understanding the solar past, we must therefore also consider stars of mid-to-late spectral class G. On the other hand, alternative evolutionary scenarios have suggested continuous mass loss from the young Sun at a high rate that would require a somewhat earlier spectral classification of the young Sun (Sackmann and Boothroyd, 2003). In any case, magnetic activity in the outer stellar atmospheres is predominantly controlled by the depth of the stellar convection zone and stellar rotation, both of which also evolve during stellar evolution. For our understanding of magnetic activity, the precise spectral subclass is rather likely to play a minor role. When discussing “solar analogs”, I will therefore concentrate on stars mostly of early-to-mid-G spectral types but will occasionally also consider general information from outer atmospheres of somewhat lower-mass stars if available. The situation is more complex for stars in their PMS stage. The Sun spent much of its PMS life as a mid-K (K5 IV) star when it moved down the Hayashi track. But again, the precise spectral subtype matters even less for magnetic activity in this stage, the more important key parameters being the age of the star (controlling its total luminosity, its radius, and the development and therefore the depth of the convection zone), the presence and dispersal of a circumstellar disk (controlling mass accretion and, via magnetic fields, the spin of the star), and the presence and strength of outflows (controlling, together with accretion, the final evolution of the stellar mass). A somewhat more generous definition of “pre-main sequence solar analogs” is clearly in order, given that the Sun’s history of rotation, accretion, the circumsolar disk, and the solar mass loss cannot be precisely assessed. Quite generally, I will take solar-like stars in the PMS phase to be, from the perspective of “magnetic activity”, stars with masses of roughly 0.5 – 1.5 solar masses, covering spectral classes from early G to late K/early M. The expression “solar twin” (Cayrel de Strobel and Bentolila, 1989) is occasionally used. This term should be used solely in the context of a solar analog with an age close to the Sun’s, i.e., of order 4 – 6 Gyr, an age range in which the internal structure and the rotation period of a 1M⊙ (and therefore, its activity level) evolve only insignificantly. Efforts toward identifying real solar twins have been important in the context of putting our Sun into a wider stellar context; nearby solar analogs that are essentially indistinguishable from the Sun with regard to spectral type, effective temperature, gravity, luminosity, age, rotation, and magnetic activity (Porto de Mello and da Silva, 1997) prove that the Sun can be robustly used as an anchor to calibrate evolutionary trends – the Sun is not an exception but is representative of its age and mass, a conclusion also reached by 1 Gustafsson (1998) from a rather general comparison of the Sun with sun-like stars. Table 1 gives a list of terms, symbols, and acronyms used throughout the text. 1The star claimed to be “the closest ever solar twin” (Porto de Mello and da Silva, 1997), HR 6060, shows parameters nearly indistinguishable from solar values indeed: L/L⊙ = 1.05 ± 0.02; spectral type G2 Va (Sun: G2 V); B − V = 0.65 (Sun: 0.648); U − B = 0.17 (Sun: 0.178); Teff = 5789 K (Sun: 5777 K; ∆Teff = 12 ± 30 K); log g = 4.49 (Sun: 4.44; ∆ log g = 0.05 ± 0.12), microturbulence velocity ξ = 1.54 km s−1 (Sun: 1.52 km s−1; ∆ξ = [0.02 ± 0.04] km s−1); element abundances solar within 1σ, in particular [Fe/H] = 0.05 ± 0.06; Mount Wilson activity index < S >= 0.174 (Sun: 0.177 in 1980); rotational velocity v sin i < 3.0 km s−1 (Sun: v = 2 km s−1). Living Reviews in Solar Physics http://www.livingreviews.org/lrsp-2007-3