Flare is a phenomenon of a sudden brightening on the surface of the Sun. Akin to solar flares, the phenomenon was also discovered on the stars other than the Sun. However, only very few stellar flare cases had been identified, and even fewer for Sun-like stars.
In 2009, the Kepler space telescope was launched. This mission has observed the light curves of a large volume of stars. These light-curve data provide a massive number of stellar flares.
Based on the light-curve data of Kepler, a research team led by Prof. HE Han from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) has revealed the characteristic time of stellar flares on Sun-like stars.
A flare has two distinct phases: the rise phase and the decay phase, which can be seen from the light curves of solar flares. “The rise phase generally represents a rapid release of magnetic field energy through a magnetic reconnection process, while the decay phase generally demonstrates a prolonged term comprising the whole cooling process,” said Prof. HE, a corresponding author of the study. The timescales of the flare rise phase and decay phase are important in flare study.
The research selected star samples that have the stellar parameters approximate to the Sun and identified 184 stellar flares from the short-cadence (SC) light curves of the Sun-like stars. The duration times of the flare rise phase and decay phase were determined for each flare sample based on the flare light-curve profile, and then a statistical analysis was performed on the obtained rise times and decay times of the flare samples.
“For the stellar flares on Sun-like stars, the median values of the flare rise time and decay times are 5.9 min and 22.6 min, respectively. These time values for stellar flares are similar to the timescale of solar flares, which supports the idea that stellar flares and solar flares have the same physical mechanism,” said Dr. YAN Yan from NAOC, the other corresponding author of the study.
The researchers also found that both the rise time and the decay time of the stellar flares follow a lognormal distribution, showing a peak-shaped head and a long tail.
The statistical results obtained for Sun-like stars can be a benchmark of flare characteristic times when compared with other types of stars.
In addition, stellar flare radiation is a key factor in the habitability of exoplanets within a stellar system. The result obtained in this work can act as an important input element for analyzing the impact of stellar flares to the atmosphere, space environment, and habitability of exoplanets.
Reference: Y Yan, H He, C Li, A Esamdin, B L Tan, L Y Zhang, H Wang, Characteristic time of stellar flares on Sun-like stars, Monthly Notices of the Royal Astronomical Society: Letters, Volume 505, Issue 1, July 2021, Pages L79–L83, https://doi.org/10.1093/mnrasl/slab055
Regular flares are first of their kind witnessed by humans.
You’ve heard of Old Faithful, the Yellowstone National Park geyser that erupts every hour or two, a geological phenomenon on a nearly predictable schedule.
Now, an international group of scientists who study space have discovered an astronomical “Old Faithful” – an eruption of light flashing about once every 114 days on a nearly predictable schedule. The researchers believe it is a tidal disruption event, a phenomenon that happens when a star gets so close to a black hole that the black hole “rips” away pieces of the star, causing the flare.
Video: Old Faithful In Space Simulation by NASA; video editing by Aaron Nestor
The team made the discovery using data from NASA and from a network of telescopes operated by The Ohio State University.
“It’s really exciting, because we’ve seen black holes do a lot of things, but we’ve never seen them do something like this – cause this regular eruption of light – before,” said Patrick Vallely, a co-author of the study and National Science Foundation Graduate Research Fellow at Ohio State. “It’s like an extra-galactic Old Faithful.”
But in this case, scientists think the star is circling around a supermassive black hole, getting closer, then zooming away again. As the star approaches the black hole each time, the black hole pulls a little bit of the star away, accreting that part into the black hole. That accretion sends out a flare of light each time. That flare – and the fact that it happened regularly – gave scientists their first clue that this was no ordinary space phenomenon.
Scientists have named the regular outbursts of light ASASSN-14ko, after the All-Sky Automated Survey for Supernovae (commonly called ASAS-SN), a network of 20 robotic telescopes headquartered at Ohio State. Data from ASAS-SN allowed Anna Payne, lead author of the paper and a NASA Fellow at the University of Hawai’i at Mānoa, to identify that something strange was happening inside that galaxy.
The ASAS-SN network first detected the flare on Nov. 14, 2014. Astronomers initially suggested the outburst was a supernova, but a supernova is a one-time event. In early 2020, Payne examined all of ASAS-SN’s data on the galaxy and noticed a series of 17 flares spaced 114 days apart. Flares with any kind of regularity had never been seen before.
Payne and her colleagues predicted that the galaxy would flare again on May 17, 2020, so they coordinated joint observations with ground- and space-based facilities, including multiwavelength measurements with Swift. ASASSN-14ko happened right on schedule. The team predicted and observed flares on Sept. 7 and Dec. 26, 2020.
The team also used TESS data for a detailed look at a past flare. TESS observes swaths of the sky, called sectors, for about a month at a time. During the mission’s first two years, the cameras collected a full sector image every 30 minutes. These snapshots created a precise timeline of a single flare that began on Nov. 8, 2018, from dormancy to rise, peak, and decline.
“TESS provided a very thorough picture of that particular flare, but because of the way the mission images the sky, it can’t observe all of them,” Vallely said. “ASAS-SN collects less detail on individual outbursts, but provides a longer baseline, which was crucial in this case. The two surveys complement one another.”
Scientists say the black hole that is causing the flares is very large – about 20 times the size of the black hole in the center of our Milky Way.
Chris Kochanek, Ohio Eminent Scholar, astronomy professor at Ohio State and co-lead of the ASAS-SN project, said there is evidence that a second supermassive black hole exists in that galaxy.
“The galaxy that hosts this object is something of a ‘trainwreck’ consisting of two galaxies in the process of merging into one,” he said.
And while astronomers observed the eruptions recently, they actually happened about 600 million years ago. Because the galaxy is so far away, the light took that long to reach us.
“There was life on Earth, but it was all very primitive,” said Kris Stanek, a co-author on the paper and university distinguished professor of astronomy at Ohio State.
Astronomers classify galaxies with unusually bright and variable centers as active galaxies. These objects produce much more energy than the combined contribution of all their stars, which can include excesses at visible, ultraviolet and X-ray wavelengths. Astrophysicists think the extra emission comes from near the galaxy’s central supermassive black hole, where a swirling disk of gas and dust accumulates and heats up because of gravitational and frictional forces. The black hole slowly consumes the material, which creates low-level, random changes in the disk’s emitted light.
Astronomers have been searching for periodic emissions from active galaxies, which might signal theoretically suggested but observationally elusive cosmic phenomena. The 2020 Nobel Prize in Physics was awarded in part to astronomers studying the supermassive black hole in the Milky Way.
“In general, we really want to understand the properties of these black holes and how they grow,” Stanek said. Because the eruptions from this black hole happen regularly and predictably, Stanek said, “it gives us a truly unique opportunity to better understand the phenomenon of episodic mass accretion on supermassive black holes. The ability to exactly predict the timing of the next episode allows us to take data that we could not otherwise take, and we are taking such data already.”
ASAS-SN is supported by Las Cumbres Observatory and funded in part by the Gordon and Betty Moore Foundation, the National Science Foundation, the Mt. Cuba Astronomical Foundation, the Center for Cosmology and AstroParticle Physics at Ohio State, the Chinese Academy of Sciences South American Center for Astronomy and the Villum Fonden in Denmark.
Reference: Anna V. Payne, Benjamin J. Shappee, Jason T. Hinkle, Patrick J. Vallely, Christopher S. Kochanek, Thomas W.-S. Holoien, Katie Auchettl, K. Z. Stanek, Todd A. Thompson, Jack M. M. Neustadt, Michael A. Tucker, James D. Armstrong, Joseph Brimacombe, Paulo Cacella, Robert Cornect, Larry Denneau, Michael M. Fausnaugh, Heather Flewelling, Dirk Grupe, A.N. Heinze, Laura A. Lopez, Berto Monard, Jose L. Prieto, Adam C. Schneider, Scott S. Sheppard, John L. Tonry, Henry Weiland, “ASASSN-14ko is a Periodic Nuclear Transient in ESO 253-G003”, ArXiv, pp. 1-26, 2021. https://arxiv.org/abs/2009.03321
Robust stellar flares might not prevent life on exoplanets, could facilitate its detection.
Although violent and unpredictable, stellar flares emitted by a planet’s host star do not necessarily prevent life from forming, according to a new Northwestern University study.
Emitted by stars, stellar flares are sudden flashes of magnetic imagery. On Earth, the sun’s flares sometimes damage satellites and disrupt radio communications. Elsewhere in the universe, robust stellar flares also have the ability to deplete and destroy atmospheric gases, such as ozone. Without the ozone, harmful levels of ultraviolet (UV) radiation can penetrate a planet’s atmosphere, thereby diminishing its chances of harboring surface life.
By combining 3D atmospheric chemistry and climate modeling with observed flare data from distant stars, a Northwestern-led team discovered that stellar flares could play an important role in the long-term evolution of a planet’s atmosphere and habitability.
“We compared the atmospheric chemistry of planets experiencing frequent flares with planets experiencing no flares. The long-term atmospheric chemistry is very different,” said Northwestern’s Howard Chen, the study’s first author. “Continuous flares actually drive a planet’s atmospheric composition into a new chemical equilibrium.”
“We’ve found that stellar flares might not preclude the existence of life,” added Daniel Horton, the study’s senior author. “In some cases, flaring doesn’t erode all of the atmospheric ozone. Surface life might still have a fighting chance.”
The study will be published on Dec. 21 in the journal Nature Astronomy. It is a joint effort among researchers at Northwestern, University of Colorado at Boulder, University of Chicago, Massachusetts Institute of Technology and NASA Nexus for Exoplanet System Science (NExSS).
Horton is an assistant professor of Earth and planetary sciences in Northwestern’s Weinberg College of Arts and Sciences. Chen is a Ph.D. candidate in Horton’s Climate Change Research Group and a NASA future investigator.
Importance of flares
All stars — including our very own sun — flare, or randomly release stored energy. Fortunately for Earthlings, the sun’s flares typically have a minimal impact on the planet.
“Our sun is more of a gentle giant,” said Allison Youngblood, an astronomer at the University of Colorado and co-author of the study. “It’s older and not as active as younger and smaller stars. Earth also has a strong magnetic field, which deflects the sun’s damaging winds.”
Unfortunately, most potentially habitable exoplanets aren’t as lucky. For planets to potentially harbor life, they must be close enough to a star that their water won’t freeze — but not so close that water vaporizes.
“We studied planets orbiting within the habitable zones of M and K dwarf stars — the most common stars in the universe,” Horton said. “Habitable zones around these stars are narrower because the stars are smaller and less powerful than stars like our sun. On the flip side, M and K dwarf stars are thought to have more frequent flaring activity than our sun, and their tidally locked planets are unlikely to have magnetic fields helping deflect their stellar winds.”
Chen and Horton previously conducted a study of M dwarf stellar systems’ long term climate averages. Flares, however, occur on an hours- or days-long timescales. Although these brief timescales can be difficult to simulate, incorporating the effects of flares is important to forming a more complete picture of exoplanet atmospheres. The researchers accomplished this by incorporating flare data from NASA’s Transiting Exoplanet Satellite Survey, launched in 2018, into their model simulations.
Using flares to detect life
If there is life on these M and K dwarf exoplanets, previous work hypothesizes that stellar flares might make it easier to detect. For example, stellar flares can increase the abundance of life-indicating gasses (such as nitrogen dioxide, nitrous oxide and nitric acid) from imperceptible to detectable levels.
“Space weather events are typically viewed as a detriment to habitability,” Chen said. “But our study quantitatively shows that some space weather can actually help us detect signatures of important gases that might signify biological processes.”
This study involved researchers from a wide range of backgrounds and expertise, including climate scientists, exoplanet scientists, astronomers, theorists and observers.
“This project was a result of fantastic collective team effort,” said Eric T. Wolf, a planetary scientist at CU Boulder and a co-author of the study. “Our work highlights the benefits of interdisciplinary efforts when investigating conditions on extrasolar planets.”
* Planetary habitability, defined by a planet’s ability to sustain liquid water on its surface, is one of the most important concepts in exoplanet science. * Exoplanets (planets that orbit stars outside of our solar system) are subject to space weather in the form of stellar flares, emissions of radiation from stars. * These emissions consist of extreme ultraviolet (XUV) photons and charged particles and can alter the upper atmosphere of the exoplanet. Current methods to determine a planet’s ability to support life do not take stellar activity into consideration.
Abu Dhabi, UAE, November 9, 2020: In a new study researchers, led by Research Scientist Dimitra Atri of the Center for Space Science at NYU Abu Dhabi (NYUAD), identified which stars were most likely to host habitable exoplanets based on the calculated erosion rates of the planetary atmospheres.
In the paper titled Stellar flares versus luminosity: XUV-induced atmospheric escape and planetary habitability, published in the journal Monthly Notices of Royal Astronomical Society: Letters, Atri and graduate student Shane Carberry Mogan present the process of analyzing flare emission data from NASA’s TESS (Transiting Exoplanet Survey Satellite) observatory.
It was found that more frequent, lower energy flares had a greater impact on an exoplanet’s atmosphere than less frequent, higher energy flares. The researchers also determined how different types of stars extreme ultraviolet radiation (XUV) through stellar flares, and how nearby planets are affected.
The ability to sustain an atmosphere is one of the most important requirements for a habitable planet. This research provides new insights into the habitability of exoplanets, as the effects of stellar activity were not well understood. This study also highlights the need for better numerical modeling of atmospheric escape – how planets release atmospheric gases into space – as it can lead to the erosion of atmosphere and the diminishment of the planet’s habitability.
“Given the close proximity of exoplanets to host stars, it is vital to understand how space weather events tied to those stars can affect the habitability of the exoplanet,” said Atri. “The next research step would be to expand our data set to analyze stellar flares from a larger variety of stars to see the long-term effects of stellar activity, and to identify more potentially habitable exoplanets.”
New research shows that sunspots and other active regions can change the overall solar emissions. The sunspots cause some emissions to dim and others to brighten; the timing of the changes also varies between different types of emissions. This knowledge will help astronomers characterize the conditions of stars, which has important implications for finding exoplanets around those stars.
An international research team led by Shin Toriumi at the Japan Aerospace Exploration Agency added up the different types of emissions observed by a fleet of satellites including “Hinode” and the “Solar Dynamics Observatory” to see what the Sun would look like if observed from far away as a single dot of light like other stars.
The team investigated how features like sunspots change the overall picture. They found that when a sunspot is near the middle of the side of the Sun facing us, it causes the total amount of visible light to dim. In contrast, when the sunspots are near the edge of the Sun the total visible light brightens because at that viewing angle bright structures known as faculae surrounding the sunspots are more visible than the dark centers.
In addition, X-rays which are produced in the corona above the solar surface grow brighter when a sunspot is visible. The coronal loops extending above the sunspots are magnetically heated, so this brightening appears before the sunspot itself rotates into view and persists even after the sunspot has rotated out of view.
Because the changes in the overall solar emissions and their timings carry information about the location and structure of features on the surface of the Sun, astronomers hope to be able to deduce the surface features of other stars such as starspots and magnetic fields. This will help astronomers to better recognize dimming caused by the shadow of an exoplanet. With better knowledge about the effects of starspots, we can estimate the parameters, such as the radii and orbits, of exoplanets more accurately.
As in-depth investigations into the Sun proceed, a better understanding of the detailed mechanisms of atmospheric heating and flare eruptions will be gained. Toriumi comments, “To this end, the next-generation solar-observing satellite Solar-C(EUVST), being developed by Japan in close collaboration with US and European partners, aims to observe the Sun in emissions that probe the chromosphere, transition region, and corona as a single system.”
References: Shin Toriumi, Vladimir S. Airapetian, Hugh S. Hudson, Carolus J. Schrijver, Mark C. M. Cheung, and Marc L. DeRosa, “Sun-as-a-star Spectral Irradiance Observations of Transiting Active Regions”, The Astrophysical Journal, Volume 902, Number 1, 2020. https://iopscience.iop.org/article/10.3847/1538-4357/abadf9