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Deepening the Solar/Stellar Connection for a Better Understanding of Solar and Stellar Variability

A White Paper Submitted to the Decadal Survey

Rachel A. Osten, Space Telescope Science Institute The has had enormous scientific attention paid to it due to its proximity to Earth and the important influences it has on for our increasingly technical society. Yet the Sun is only the closest of billions of in our . And while it has been the subject of daily ground- and space-based observations for decades now, the Sun provides a sample size of one for understanding stellar . The primary focus of this white paper is on solar and stellar variability. The purpose is to motivate a deepening of the solar/stellar connection, by recognizing it as a mutual relationship from which both solar physicists and stellar astronomers can benefit. With the impressively detailed nature of solar observations, one may think of the solar/stellar connection as more akin to a one-way street than a multi-lane throughway. However, by exploring the range of behaviors that stars exhibit we can access regions of parameter space which it is not possible to probe with solar observations alone. This white paper addresses four key questions which can be probed with a deeper connection between solar and stellar variability.

Question 1: How typical is the Sun as a solar-like ?

Solar flares produce emissions across the , from km- wavelength radio emission to gamma-rays. The interrelationship between various emissions can be used to infer the dynamics and energetics of the flare process, and thus gain insight into the physics involved in flares. Solar white- or visible flares are difficult to see on the Sun, due to the generally small amount of contrast between the disk-integrated light and the small physical area involved in producing white-light flare emissions. The importance of the white light flare arises from their dominance in the flare energy budget of the radiative emissions from the lower and via UV and white-light continuum emissions (Woods et al. 2006), even if the flare contrast is not as high as in other wavelength ranges (5-30% continuum enhancement in the models of Allred et al. (2005). The frequency of solar flares is distributed in energy according to a power-law in the differential flare frequency distribution dN/dE ∝E-α with α=1.7±0.1 (Kucera et al. 1997), where dN/dE is the flare frequency per unit flare energy. The largest solar flares release about 1032 erg in a matter of minutes (Woods et al. 2004) while a more typical energy release is 1029 erg. The flare frequency is such that the largest events are observed typically only a few times per of 11 years. Detailed studies of the energy budget in solar flares and coronal mass ejections (Emslie et al. 2005) led to an estimate that the total solar irradiance as diagnosed in the UV-optical bands indicated total radiated energies roughly 100 times that determined from X-ray studies alone.

Studies of stellar variability hold the key to understanding the potential extremes of solar behavior. Infrequent events can be detected much more easily on a large sample of stars than by intense scrutiny of one Sun. For example, determination of flares as large as the Carrington flare on the Sun (Cliver & Svalgaard 2004) are currently suggested to occur every 500 years, but that is an uncertain number. Schaefer et al. (2000) found evidence for superflares on solar-like stars with radiated energies up to 106 times larger than the largest solar flares, with estimated frequencies of once every several hundred years. As human society becomes more technologically advanced, we expose ourselves to increasing vulnerability to solar behavior. Having a better sense of the frequency of such events lets us know what precautions might be needed. The Kepler mission currently operating has a primary mission of finding eclipsing extrasolar , but with a monitoring campaign of >100,000 stars over 3+ years, it is collecting a vast repository of stellar data, which can be used to investigate cycles and flare variability. This is an example of where stellar input data can be used by solar physicists to gain a better perspective of the Sun as a star.

Question 2: How do flares on solar-like stars and the Sun compare to flares on extremely magnetically active stars?

While observations of stellar flares are not as numerous as those of flares on our well- studied Sun, the general consensus appears to be that stellar flares comprise the same basic physical processes occurring in solar flares. This is despite the fact that the types of stars studied so far can be significantly different from the Sun: F dwarfs (Mullan & Mathioudakis 2000), G and K giants (Ayres et al. 1999, Ayres et al. 2001, Testa et al. 2007), tidally locked RS CVn binary systems (Osten et al. 2004), dMe flare stars (Osten et al. 2005), flares on hyperactive young containing star-disk interactions (Favata et al. 2005), and very low mass stars near the substellar limit (Stelzer et al. 2006). With the advent of sensitive X-ray and radio telescopes, multi-wavelength campaigns on magnetically active stars have revealed many similarities in behavior between solar and stellar flares, which implies that common physical processes unite the phenomena. Yet, because of the observational biases in the types of stars targeted for in-depth study, our understanding of flares and associated effects may be incomplete.

Despite the generally good agreement between solar flare models and stellar flare observations, there are still areas of apparent disagreement. M dwarfs have been the subject of flare monitoring due to their propensity for frequent and extreme flaring. The white light and continuum response in a stellar flare on a dMe flare star observationally may take the form of a blackbody with temperatures near 104 K and area coverages ∼0.01% of the stellar disk area (Hawley et al. 2003), with the flare enhancement peaking to the blue end. Radiative hydrodynamic simulations of the response of a stellar flaring to the input of a beam of electrons can reproduce other features observed in stellar flares, such as line emission and observed velocity shifts and Stark broadening, but still cannot reproduce the of the observed continuum enhancements (Allred et al. 2006). This is in contrast to the same kinds of models applied to solar flares, which do show general agreement between observed white-light flares and models (Allred et al. 2005). The solar flare beam heating in an M dwarf stellar atmosphere does not produce the observed continuum increases. The electron beam cannot penetrate deeply into the photosphere to provide direct heating which would result in the observed continuum emission, and backwarming from the X-ray and EUV emission cannot provide the required energy input.

The distribution of stellar flares with energy on extremely magnetically active flaring stars appears to be similar to the distribution of large solar flare events (Osten & Brown 1999, Wolk et al. 2005), with evidence for M dwarf flare stars to have a distribution favoring smaller amplitude events (α in the flare frequency-energy distribution ≥2; Güdel et al. 2003). The typical individual large stellar flare represents energetic events at least as powerful as the largest solar flares (and often larger), by virtue of sensitivity thresholds. Power stellar flares many orders of magnitude larger than the largest solar flares have been observed: radiated energies at least 1038 erg, and possibly larger, have been observed on the most hyperactive flare stars (Favata et al. 2005, Osten et al. 2007). Flares whose at X-ray energies approach or even exceed the starʼs normal radiative output across all wavelengths (Favata et al. 2000, Osten et al. 2007, Osten et al. 2010) have been detected. These observations call out for a description of the maximum flare energy and energy storage in active regions from a theoretical perspective, which the solar community could provide.

Interpretation of stellar flares requires the spatial resolution of solar flare data to provide plausible scenarios for what might be happening on flaring stars. The interaction between stellar astronomers and solar physicists in this regard can be a fruitful one, as stellar astronomers can gain from the large solar datasets available, and solar physicists can learn how stellar flares differ from solar flares.

Question 3: What is the dependence of flare rate on stellar parameters?

Due to their dependence on the presence and dynamics of interacting magnetic fields, stellar flares are a transient diagnostic of magnetic activity, in addition to more commonly used persistent indicators such as chromospheric/coronal emissions, , and rotation. Magnetic activity is a function of depth and rotation rate, as quantified by the Rossby number (Noyes et al. 1984). This relates ultimately to the dynamo generation of magnetic fields producing the large-scale magnetic fields whose stresses and interactions cause the mechanical heating and reconnection observed as different magnetic activity phenomena.

In solar-like stars, magnetic activity is a function of age, with young stars exhibiting magnetic hyperactivity, and a consequent fall-off with increasing age accompanying spin-down. Likewise, the stellar flare rate appears to decline as a function of age. In the young suns of Orion (0.9-1.2 Msun, F7-G5 dwarfs, age ∼106 years), Wolk et al. (2005) estimated X-ray flares more energetic than 1034 erg occurred roughly once per week. This minimum flare energy is more than 100 times more energetic than the largest observed solar flares. Stelzer et al. (2000) did a systematic study of the X-ray flare rates of T Tauri stars at ages 1-10 MY, and studies of stars in the Pleiades (age ∼100 MY) and the Hyades (age ∼600 MY). They noted that the G stars in their group had the smallest flare rates by spectral type. Corrected for sensitivity biases, their flare rate for the Pleiades was 0.67±0.13%, for the Hyades 0.32±0.17% expressed as a fraction of time in the flaring state compared to total observation time.

That magnetic activity can survive to old age is evidenced by the flare observed serendipitously on the old solar neighborhood M dwarf Barnardʼs star, at an estimated age of 11-12 GY (Paulson et al. 2006). The decline of flaring with age is generally assumed to follow the trends of other activity indicators, but this is an open question. Evidence that magnetic activity may not decline monotonically at GY ages comes from a few sources: Silvestri et al. (2005) concluded that chromospheric activity in M dwarfs did not decline in the 1-10 GY range as fast as predicted based on extrapolating from objects with ages < 1 GY. Studies of chromospheric activity and its dependence on age and rotation led Pace & Pasquini (2004) to conclude that they found no evidence of decay in chromospheric activity for stars older than a few GY; instead, the major decline in activity was in objects at ages of the Hyades and earlier (0.6 GY), for clusters of 1.7 GY and older (up to 4.5 GY) the same activity level was seen. Flares as magnetic activity indicators are more sensitive to the transient build-up and release of magnetic stresses, which may happen less frequently but survive for longer than persistent levels of magnetic activity. Thus determining the existence of flares in a large sample of cool stars is a method complementary to measuring the amount of magnetic activity using other means. A large sample size can additionally constrain the dependence of flare rate on other stellar parameters, such as spectral type and rotation rate.

The primary benefit would be inputs to dynamo models, as flares are fundamentally an expression of the dynamo generation of magnetic fields.

Question 4: What are the astrobiological implications of stellar flares?

The effects of solar flares are experienced dramatically here on Earth -- disruptions to power grids and spectacular aurorae are among the many manifestations. Solar storms heat the upper atmosphere causing increased drag which accelerates the orbital decay of satellites. The Halloween solar flares of 2003 produced significant effects at Earth -- the Earthʼs Van Allen belt suffered a major distortion by the radiation and particles in the flares, being compressed by a factor of two (Baker et al. 2004). Even larger flares than observed from the Sun have potential implications for evolution and habitability. Support for the existence of large stellar flaring events at young ages comes from meteorite studies, which indicates indirectly that energetic events were common in the young Sun. The mystery of melting chondrules in meteorite studies has been suggested as due to flares in the early Sun (Feigelson & Montmerle 1999). Likewise, one hypothesis to explain isotopic abundance anomalies in meteorites uses MeV particle spallation of meteoritic material, which could arise from particle production in stellar flares. The results of Favata et al. (2005) suggest that large flares on young star can involve loops connecting the star with its disk, and flares may stimulate turbulence in the disk, which affects planetary formation. While still extremely speculative, such a scenario does cast stellar flares in a favorable role.

The renewed interest in planets around M dwarfs (Tarter et al. 2007) has enlivened the debate about the effects of stellar flares on exoplanets in such systems, due to the known high flare rate of dMe flare stars. Paradoxically, while the discovery of planets around M dwarfs (starting with Marcy et al. 1998) has led to the revitalization of the idea of the impact of flares on planets, the currently poorly constrained superflare frequency on solar-like stars has gained little attention. Smith et al. (2004) showed that a significant fraction (up to 4%) of the ionizing radiation from a transient flare event can be converted into UV emissions which reach the planetary surface, and that this is orders of magnitude higher than the fraction of the primary ionizing radiation which reaches the surface. Khodachenko et al. (2007) and Lammers et al. (2007) considered the effect of a nearly continual rate of coronal mass ejections and flaring on an exoplanet in the habitable zone around an active M dwarf. They concluded that planets with weak magnetic moments would have expanded , which coupled with compression of the magnetosphere might result in atmospheric erosion. Recently, Segura et al. (2010) calculated the effects of radiation and protons on the atmospheric chemistry of an Earth-like in the habitable zone around an M dwarf, and concluded that flares may not present a direct hazard for life.

While life did evolve around one flaring G star, the effect that flares and associated events (mass ejections, energetic particles) are predicted to have on other planets still varies widely. This is largely an result of incomplete knowledge about how to extrapolate solar flare data to stellar scenarios, as well as partial inclusion of possible effects in such calculations. With the discovery of exoplanets around a magnetically active late-type star (Lanza et al. 2010) and the coming flood of M dwarf Earth-like exoplanet discoveries, such investigations will quickly need to provide real constraints on habitability for specific systems. Solar physicists can aid in this with studies correlating key parameters which are currently unconstrainable from the stellar side.

Summary

The primary plea is for solar physicists to talk to stellar astronomers and vice versa. There is a wealth of solar data, and a relative paucity of stellar data. Despite that seeming imbalance, there are areas where interaction can be mutually beneficial. Stellar astronomers can mine solar data in ways that solar physicists might not have anticipated. This requires a funding mechanism that crosses the boundary lines between pure solar research and pure stellar research.

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