By Rachel Osten, AURA Astronomer at STScI
A stellar flare is the release of energy that occurs during a magnetic reconnection event in the upper atmosphere of the star. Flares involve the release of substantial amounts of ionizing radiation, and are the most dramatic forms of energy release that cool stars will undergo during their time on the main sequence. Stellar flares appear to be an extension of the same phenomena observed in great detail on our own star, the Sun, despite the large difference in the energies involved between the two: the largest solar flares have energies of about 1032 erg, while stellar flares can be a thousand to a million times more energetic. From the solar perspective, flares are part of a triad: the flare, a coronal mass ejection, and highly energetic particles. Even though they all contribute to space weather, the latter two are the most important for determining how damaging a solar eruptive event might be. These are also the two for which we have the least some constraints on occurrence in a stellar context.
Figure: Artist’s conception of a large flare observed on a nearby M dwarf flare star. The flare involves all layers of the star’s atmosphere, from the photosphere to the chromosphere to the tenuous, hot corona. These large stellar flares may be accompanied by coronal mass ejections, which would affect the stellar environment around active stars.
While stellar flares have been studied for decades — non-solar stellar flares were first described by Ejnar Hertzsprung in a 1924 article entitled “Note on a peculiar variable star or nova of short duration” — it has been the discovery of planets around other stars which has provided much recent astrophysical motivation for studying stellar flares. Several recent papers (Segura et al. 2010, Khodachenko et al. 2007) have speculated about the possible impacts on close-in exoplanets of flares and associated events like accelerated particles and coronal mass ejections. The results suggested that the flares themselves were not the main worrisome aspect of magnetic activity that could affect habitability; rather, it was the other “messengers”, namely the coronal mass ejection and the energetic particles, which had drastic implications for habitability. In the Segura paper, the energetic particles accompanying the flare were responsible for widespread destruction of the ozone layer in the atmosphere of a terrestrial exoplanet in the habitable zone, which took several years to recover to the pre-flare levels. In the Khodachenko paper, the frequent coronal mass ejections acted like an enhanced stellar wind, compressing the planetary magnetosphere and exposing the exoplanet’s atmosphere to enhanced levels of ionizing radiation, which can cause atmospheric loss.
Stellar flares are a multi-wavelength phenomenon. They involve all layers of the star’s atmosphere, from the photosphere to the magnetically heated chromosphere and corona, and involve a variety of physical processes, including plasma heating and particle acceleration. Stellar flares are typically observed in a piecemeal fashion, usually covering only one wavelength region, and for short periods of time. Imagine how little we would know about the Sun’s flares if we were limited to observing it for only a few days every year or so. And that’s if your proposal got accepted! Even missions like Kepler, which stared at one patch of sky for multiple years and observed many thousands of stellar flares, observed in only one wavelength region; having a way to relate the energy released in one bandpass to the energy released by a flare across all wavelength regions, the so-called bolometric flare energy, is important for an intercomparison of heterogeneously observed stellar flares.
Recent work on solar flares has taken a global view of the energetics of large solar eruptive events (Kretzschmar 2011, Emslie et al. 2012). These papers established how radiated energy is partitioned in the dominant mechanisms, namely chromospheric and transition region line emission, continuum emission from a hot blackbody presumed to form in the photosphere, and coronal emission from plasma heated to temperatures in excess of a million degrees. This solar work has also demonstrated a near equipartition between the total amount of radiated energy (the bolometric flare energy), and the kinetic energy in the associated coronal mass ejection (Emslie et al. 2012, Drake et al. 2013).
My recent paper (Osten & Wolk 2015) unites these two concepts with application to stellar flares. When considering the potential impact of stellar eruptive events to exoplanets, one needs to “follow the photons”; current astronomical limitations means that we have few options for direct detection of coronal mass ejections, and none for the existence of these very energetic particles expected to be produced in stellar flares as they are in solar flares. So if we want to gain an initial grasp of the potential influence of stellar eruptive events, the easiest way to do so is to look at the flares from a holistic standpoint, and relate them to coronal mass ejections using a physically motivated way of connecting them.
In order to do the latter — that is, connect the total radiated flare energy with the coronal mass ejection’s kinetic energy — we need a way to “correct” for the flare energy in a given bandpass to the bolometric amount. Using previously published papers of a few well-observed stellar flares, I established that these flares appear to have similar fractions of energy as aggregates of solar flares (see the Table). So far, so good. This is another confirmation of the basic approach, in treating solar and stellar flares as originating from the same basic physical process, despite the orders of magnitude difference in energy release. Being able to relate flares observed in one wavelength range to the total amount of radiated energy released enables a more global intercomparison of those flares.
Table: Fraction of total radiated energy in Solar and stellar flares released in particular bandpasses. The numbers for solar flares and those for active stars are remarkably similar, and confirm the expectation that the same physical process is occurring. This energy partition allows for a better intercomparison of flares observed in different wavelength regions.
We observe that stellar flares of differing energies have an occurrence that is a power-law in frequency as a function of size. Relating the kinetic energy of coronal mass ejections to the bolometric energy of an accompanying flare (assuming equipartition, based on solar flare studies) means that we have a way to estimate the cumulative impact of the transient mass loss that is occurring in these flare-related coronal mass ejections. To make a long story short (if you want the long story, read the paper!), we find that this cumulative effect can be large. We examined published flare frequency distributions from a variety of different types of flaring stars: young solar-mass stars; a 70 MY young solar analog; recently observed superflaring Sun-like stars; young low mass stars; nearby hyperactive M dwarfs with flare frequency distributions measured at both optical and X-ray wavelengths; and inactive early- and mid-M dwarfs. At the high end, the flare-related transient mass loss may be as much as three orders of magnitude higher than the present-day total solar mass loss rate of 2×10-14 Msun/yr.
So what are the implications of these findings? The main impact is on the stellar environment, not the star itself. The cumulative mass loss rate, though potentially enhanced compared to the Sun’s current total rate of mass loss, is not enough to change the course of the star’s evolution, as the total mass lost is not an appreciable fraction of the total stellar mass. As mentioned before, a high rate of coronal mass ejections means that any close-in exoplanet would be subjected to enhanced compression of its magnetosphere and resulting exposure of its atmosphere to ionizing flare radiation. The interaction between an enhanced stellar wind and an exoplanet’s magnetic field can generate a magnetospheric field that would have a negative feedback on the planet’s internal dynamo, according to one calculation. Frequent strong CMEs could quench dynamo growth in the planet, leading to weak planetary magnetic fields and a reduced ability to protect the atmosphere from exposure to ionizing radiation. Other impacts on the stellar environment include seeding the planet-forming disk with processed stellar material, or removing material from the debris disk at a later stage in the stellar system’s evolution. Results from young solar analogs suggest that the Sun’s youth was likely marked by such frequent strong CMEs. While these results are suggestive, direct constraints on the existence and occurrence of stellar coronal mass ejections are a vital next step to a fuller understanding of how stars affect their surroundings.
Drake, J. et al. 2013 ApJ 764, 170
Emslie, A.G. et al. 2012 ApJ 759, 71
Hertzsprung, E. 1924 BAN, 2, 87
Khodachenko, M. et al. 2007 Astrobioloy 7, 167
Kretzschmar, M. 2011 A&A 530, A84
Osten, R. A. & Wolk, S. J. 2015
Segura, A. et al. 2010 Astrobiology 10, 751