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Huge Flares from a Tiny Star

Just a few days ago we looked at evidence that Kepler-438b, thought in some circles to be a possibly habitable world, is likely kept out of that category by flare activity and coronal mass ejections from the parent star. These may well have stripped the planet’s atmosphere entirely (see A Kepler-438b Caveat – and a Digression). Now we have another important study, this one out of the Harvard-Smithsonian Center for Astrophysics, taking a deep look at the red dwarf TVLM 513–46546 and finding flare activity far stronger than anything our Sun produces.

Led by the CfA’s Peter Williams, the team behind this work used data from the Atacama Large Millimeter/submillimeter Array (ALMA), examining the star at a frequency of 95 GHz. Flares have never before been detected from a red dwarf at frequencies as high as this. Moreover, although TVLM 513 is just one-tenth as massive as Sol, the detected emissions are fully 10,000 times brighter than what our star produces. The four-hour observation window was short, which may be an indication that we’re looking at a star that is frequently active.

Now considered an M9 dwarf, TVLM 513 is about 35 light years away in the constellation Boötes. It is believed to be on the borderline between red and brown dwarfs, with a radius 0.11 that of the Sun, a temperature of 2500 K, and a rotation rate of a scant two hours (the Sun takes almost a month for a complete rotation). For a habitable planet to exist here — one with temperatures allowing liquid water on the surface — it would need to orbit at about 0.02 AU. That’s obviously a problem, as Williams explains in this CfA news release:

“It’s like living in Tornado Alley in the U.S. Your location puts you at greater risk of severe storms. A planet in the habitable zone of a star like this would be buffeted by storms much stronger than those generated by the Sun.”

2015-26

Image: Artist’s impression of red dwarf star TVLM 513-46546. ALMA observations suggest that it has an amazingly powerful magnetic field (shown by the blue lines), potentially associated with a flurry of solar-flare-like eruptions. Credit: NRAO/AUI/NSF; Dana Berry / SkyWorks.

Another unusual aspect of TVLM 513 is its magnetic field. Data from the Very Large Array in New Mexico had previously shown a magnetic field several hundred times stronger than the Sun’s. The paper argues that the emissions observed in the ALMA data are the result of synchrotron emission — radiation generated by the acceleration of high-velocity charged particles through magnetic fields — associated with the small star’s magnetic activity.

We have a lot to learn about small stars, their magnetic fields and their flare processes, and even in this study, the paper offers a caveat:

… confident inferences based on the broadband radio spectrum of TVLM 513 are precluded because the ALMA observations were not obtained contemporaneously with observations at longer wavelengths, and TVLM 513’s radio luminosity, and possibly its radio spectral shape, are variable. Additional support from the Joint ALMA Observatory to allow simultaneous observations with other observatories would be highly valuable.

The authors add that while it has long been known that both stars and gas giant planets have magnetic fields, the mechanisms at work are different and it is unclear what kind of magnetic activity we should expect from objects of intermediate size. Learning more about magnetic processes in small stars should help us understand more about exoplanets and their magnetic activity. This first result at millimeter wavelengths thus points to the work ahead:

Modern radio telescopes are capable of achieving ∼µJy sensitivities at high frequencies (≿20 GHz), raising the possibility of probing the means by which particles are accelerated to MeV energies by objects with effective temperatures of ≾2500 K.

So we’re going to learn a lot more about small red dwarfs as we study whether or not such stars can host habitable planets. The argument against red dwarfs and astrobiology used to focus on tidal lock and the problems of atmospheric circulation, but we’re now wondering whether, particularly in young red dwarfs, flare activity may not be the key factor. If TVLM 513 is representative of a category of flare-spitting stars, the smallest red dwarfs may be hostile to life.

The paper is Williams et al., “The First Millimeter Detection of a Non-Accreting Ultracool Dwarf,” in press at The Astrophysical Journal (preprint).

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Comments on this entry are closed.

  • Michael November 24, 2015, 14:45

    These stars may be dangerous to the emergence of life in their systems but they may be great for a technological species. I mean that magnetic field sweeps around it at the surface at around 60 km/s, go further out and it is whipping around a lot faster, getting large fractions of the speed of light and would harbour an enormous amount of energy to fling objects to other Stars! The reason that these little bundles of tantrums may have such powerful magnetic fields is probably due to an efficient convection system.

    Just off topic I have just noticed something about the rotation of the magnetic field of our Sun and the Star velocities of our galaxy, they look a lot a like.

    http://3.bp.blogspot.com/_okIcsBieX4U/STNFGLHxq0I/AAAAAAAAACc/zFTYwQOsBtw/s1600-h/velocities.png

    http://en.citizendium.org/images/3/31/GalaxyRotationCurve.png

    Is dark matter nothing more than a galactic magnetic field imposing its effect on star forming materials that eventually form the Stars that we see and the strange effect of the velocity conundrum mentioned by Fritz Zwicky?

    I think I need more coffee or sleep!

  • Andrew Palfreyman November 24, 2015, 16:05

    @Michael: That won’t work because the velocity profiles for a Lorentz-type force start off wrong in any case. One would have to postulate a wholly unnatural magnetic field shape in order to effect the kind of affect you describe.

  • andy November 24, 2015, 16:16

    M-dwarfs span a (fractional) mass range approximately 3 times greater than the mass range of FGK dwarfs combined. Late-type M-dwarfs have significant differences compared to early-type M-dwarfs: they have convective cores and there is some evidence that this may be linked to the transition between active and inactive states occurring at longer rotation periods for late-type M-dwarfs (e.g. this paper). Lower mass stars also reach faster rotation periods and have slower spindown timescales (see section 8 of this paper, and also the point in the conclusion that there is a large jump in active lifetime between M3V and M5V).

    While I remain cautiously optimistic about the prospects for habitable planets around early-type M-dwarfs, I wouldn’t be surprised if the situation around the late-type M-dwarfs was too hostile. Of course, there’s no substitution for doing the actual observations to find out what the actual situation is for insolation-habitable-zone planets around such stars.

  • ljk November 24, 2015, 17:08

    We always look upon radiation as a bad thing for living creatures, at least most organisms on Earth – with the exception of this literal little bugger:

    https://en.wikipedia.org/wiki/Deinococcus_radiodurans

    Could there be higher, much more intelligent organic beings who not only can resist high levels of radiation but actually THRIVE in such an environment? Where cosmic radiation is actually an energy supply for them. It would certainly be an evolutionary advantage to a being that lives in space as they would have ample food sources all the time.

    Scientists always like to say things like “Life AS WE KNOW IT could not survive on a world as harsh as Venus.” Well, considering how tough life can be in the harsher environments of Earth – way deep underground, around hydrothermal vents, swimming in boiling hot geysers – all part of the whole survival of the fittest thing, why not beings who have adapted to the biggest environment of all – outer space – and thrived because what is deadly to us is dinner to them.

    Just food for thought.

  • Ashley Baldwin November 24, 2015, 17:14

    There is no doubt that on scales that rotation plays a big role in either planetary or stellar magnetic field production. Mass does too, and ironically for stars the metallicity too. Smaller M class stars like TVLM 513 are fully convective so rather than the classic dipole field of larger stars ( bigger than M3.5) they have a “distributed” magnetic field with flux lines going off in all directions , getting entwined and in doing so trapping stellar wind and any other emissions . Tightening the tourniquet until it finally snaps and releases a huge coronal mass ejection constituting a flare . This is how all stellar magnetic fields create such phenomena , but the complex magnetic fields of late M dwarfs do so to an even greater extent and most potently when the star is relatively young with rotation as quick as two days . ( ironically the interaction of this field with the stellar wind ultimately acts as a brake slowing the star significantly in a few billion years and consequently reducing its activity though there is big aviation from one star to another with some M dwarfs even less active than the Sun which is somewhat atypically inactive ) . In later life it’s larger “early” M dwarf stars that tend to be more active due to mass and “metallicity” . All of this is explored in excellent articles on arxiv by Robertson et al in Nov 2012 and Gomes da Silva in Feb of the same year. This work also shows the disruptive effect of close in companions be they stellar or planetary and a 10 mass Jupiter exoplanet in a 2 day orbit would help stir things up further as seen so clearly with TVLM 513 . With a two day orbit, the planet and it’s field, will sit directly within the stellar flux lines . Being tidally locked the planet will rotate also quickly thus creating its ownpotent field and flux lines to the complicated mishmash of the star’s. A veritable Gordian knot that as with its mythological twin , can only be relieved by being severed . Here the simalirity ends and instead of opening up for Alexander the Great a monster stellar flare bursts forth.
    In terms of planets the effect of rapid rotation on the electrically conducting outer liquid core of a rocky exoplanet ( or liquid hydrogen of a Gas Giant ) is dramatic with the same relationship evident with increasing rotation leading to increased field size . Zuluaga in Nov 2011 showed that rotation rates of less than 1.5 days produced the largest and longest lived fields . It not difficult to imagine this happening with both star and planet with the two interacting for further complexity. For planets there is a precipitous fall in field strength and longevity after 2.5 days.

  • Ashley Baldwin November 24, 2015, 18:37

    Good points all round. Next year will be a big year for M dwarfs and especially Proxima Centauri. No planets discovered yet though bigger planets further out largely ruled out. Difficult to do RV spectroscopy for discovery because of the activity so well illustrated in this star . Proxima is an M5 star so not much larger and despite its age , still capable of regular flares despite a sedate 84 day rotation. So even though a large and very sensitive spectrometer like upcoming ESPRESSO on the VLT might be able to spot the tiny spectroscopic changes created by Earth sized rocky planets in close in orbits ( “HBZ “for Proxima is about 11-18 days) , the star’s noise is likely to make it difficult to verify as a planet.

    However , next year is the year that the first astrometry results of the Gaia survey come in . They will obviously be most precise for nearer stars . Combined with the already numerous and high quality astrometry studies done on Proxima they could pick out smaller planets with close orbits , especially if there is a pointer from a positive RV study.

    One UK based RV group have indeed found such a signal , with a provisional period of 10-20 days ( but strongest around 12 days) and, msini allowing, a minimum mass of 1.3 times that of Earth. It is currently being confirmed by extended consecutive viewing with the HARPS high resolution planetary spectroscope and two other separate telescope networks . It possible further groups have seen this too and are sitting on their data till it can be verified beyond challenge by astrometry based around the first Gaia data set. This will also help determine its exact mass too. Difficult, but I’m told by reliable astronomer sources it is certainly possible.

    So 2016 could be year of Proxima and the year that astrometry finally came of age. China are currently developing the 1.2m STEP high precision astrometry telescope ( a three mirror anastigmat for those interested and very similar to the THEIA telescope concept ) that will search nearby stars for down to Earth mass planets. They are pushing for a 2020 launch , so expertise allowing ( something that has traditionally dogged many of their missions ) we could have dedicated planetary observatory operational within just 5 years.

  • Alex Tolley November 24, 2015, 21:53

    The high radiation levels only apply to the surface. Organisms may well survive in ocean depths or the lithosphere. Just don’t expect terrestrial life abundant on the planet’s surface.

    Since some micro organisms can feed directly on electric currents, it may be possible that such an environment could sustain such life, where generated electric currents substitute for sunlight for primary producers.

  • P November 24, 2015, 22:08

    What Andy said. The difference in internal structure between M0-M4 stars with a radiative zone and fully convective stars below that is so great I wonder if they shouldn’t have be given a different spectral class. I also remain hopeful about habitable planets in the former (and I believe these are the most numerous as a tail0ff in numbers begins around M5-6?).

    All this doesn’t mean we shouldn’t be looking for planets around the smallest stars (and brown dwarfs!) – or that we shouldn’t expect to be surprised by what we find. Sometimes here over-emphasis is placed on the conventional habitable zone. Maybe fully convective M dwarfs have a lot of icy Europa-like moons?
    I’ll be cheering when and if they find a planet around Prox cen, even if it isnt ‘habitable’.

    Phil

  • Eniac November 24, 2015, 23:27

    “It’s like living in Tornado Alley in the U.S. Your location puts you at greater risk of severe storms. A planet in the habitable zone of a star like this would be buffeted by storms much stronger than those generated by the Sun.”

    I find this a little overdramatic. Solar storms hardly “buffet” us, as far as surface life is concerned, and if they were 10,000 times stronger, we would most likely still not notice them, except perhaps for spectacular northern lights at night, way up north.

    Secondly, they measured a 10,000 fold increased radio emissions, but as far as I can tell there is no evidence that this is accompanied by any other equally elevated emissions. To the contrary, it seems this star has an unusually strong magnetic field, which may well fully account for the observed stronger radio emissions.

    Lastly, liquid surface water requires an atmosphere, and even a moderately thick atmosphere (on the scale between Mars and Venus) would shield the surface from any amount of radiation. Thus rendering radiation a non-issue for habitability. Now, it could still be true that high stellar activity would erode the atmosphere of an Earth-sized planet, in which case lack of liquid water would be more of a problem for life than radiation. However, then all we need is a larger planet, of which there are plenty.

    What we need is measurements of atmospheres, not bold predictions about the circumstances under which they would not exist. I predict that findings in the coming decades will surprise us, especially those of us who have strong, unsubstantiated notions about what is and is not possible around Red Dwarfs.

  • Eniac November 24, 2015, 23:57

    Michael makes an interesting point about potential technologies made possible by strong stellar fields. I like to muse about a world in which strong, varying magnetic fields make the discovery of electricity much easier, and turn a simple loop of metallic wire into a inexhaustible source of energy. It may be a world where animal life derives its energy from environmental electromagnetic fields instead of the chemical oxidation of organic matter. A technological civilization could evolve on such a world through the use of induction currents in metallic loops instead of fire.

    This would make a great old-fashioned hard science fiction story that has not been done yet. Or has it?

  • Eniac November 25, 2015, 0:05

    LJK:

    Could there be higher, much more intelligent organic beings who not only can resist high levels of radiation but actually THRIVE in such an environment? Where cosmic radiation is actually an energy supply for them. It would certainly be an evolutionary advantage to a being that lives in space as they would have ample food sources all the time.

    It is very difficult to harvest much of the energy of ionizing radiation, technologically and even more so biologically. Even if you could harvest it, the energy density of even the most damaging radiation is minimal. Not “ample”, by any means. Much less than sunlight, for sure.

  • Mark Zambelli November 25, 2015, 10:17

    Given the hard radiation environments around these M-dwarfs seem to spell disaster for atmosphere retention and the possibility of finding an ‘hospitable’ surface, what does this mean for sub-terraneanregions? How far below ground would the radiation fall off to just background levels… a few cm or maybe a few tens of metres?

    For Earth, 90% of the total biomass is found within the top 2 to 3 km of the crust, with the other 10% summing the total of all lifeforms that take advantage of the surface/oceans. If organic life arises deep down and makes a break to colonize planetary surfaces when it can, then maybe simple life could thrive around M-dwarfs. So no intelligent surface dwelling civs (that evolved locally anyway) probably but does that mean they’ll be sterile? If not then one very far-fetched musing could be that spacefarers seek out M-dwarf Earth-analogues to not only harness the magnetic field strength but also for zoology… maybe these worlds are as attractive to them for their ‘bugs’ as the mold-ridden loam soils of our rainforests are to pharmaceutical companies.

  • Michael November 25, 2015, 10:26

    @Ashley Baldwin

    ‘No planets discovered yet though bigger planets further out largely ruled out. Difficult to do RV spectroscopy for discovery because of the activity so well illustrated in this star . Proxima is an M5 star so not much larger and despite its age , still capable of regular flares despite a sedate 84 day rotation…’

    It is interesting that the rotation is so long, it could be an indicator of planets as they like to rob Stars of angular momentum.

  • Michael November 25, 2015, 13:10

    @Andrew Palfreyman November 24, 2015 at 16:05

    ‘That won’t work because the velocity profiles for a Lorentz-type force start off wrong in any case. One would have to postulate a wholly unnatural magnetic field shape in order to effect the kind of affect you describe.’

    The galactic magnetic field can’t ignored and does have a bearing on the star velocity profile but depending on whose model is used it can account for most if not all of the profile if errors are taken into consideration or not enough by far, still early days yet.

    https://ned.ipac.caltech.edu/level5/March01/Battaner/node31.html

  • Ashley Baldwin November 25, 2015, 15:01

    Michael. You’re absolutely right. I’ve read up on this area for a while and even though “Hot Jupiters” were amongst the first planets discovered in ridiculously small orbits , it’s only been recently that there could be interaction both ways , especially magnetic. Not just the star on planet. As it appears that the typical planetary system has lots of planets in sub Mercury distance orbits there is likely to be a lot of these two way magnetic interactions possibly inducing flare activity in already active stars like late M dwarfs . Eighty four days is a slow rotation period yet this star is still very active so there are clearly other factors in play including a close in large planet . Unlikely to be a gas giant as their incidence in M dwarf systems appears to be very low.

    I’ve spoken to stellar experts who struggle to describe the mechanism why a small fully convective star like a late M dwarf can produce a potent magnetic field . As I understand it theory says that a stellar magnetic field requires a base (normally the convective /radiation zone boundary ,the” tachicline” ) on which to fix itself. Later than M3 stars are all fully convective though so no such base exists , suggesting an alternative mechanism. A so called “distributed ” magnetic field has been posited rather than a conventional di pole field for a star like the Sun. Flare activity or coronal mass ejections occur when stellar matter gets trapped within the magnetic field lines as they tangle up due to rotation , which ultimately snap thus releasing their content.

    I guess that if the field is of an irregular distributed variety , strengthened by interaction with a close in binary or planet it can form independently of a conventional rotation induced dynamo style thus allowing much large volumes of stellar material to build up before the slow rotation finally twists the flux lines enough to break them and release their contents as a flare.

  • Mervl November 25, 2015, 18:13

    @ Eniac November 24, 2015 at 23:57
    “A technological civilization could evolve on such a world through the use of induction currents in metallic loops instead of fire.
    This would make a great old-fashioned hard science fiction story that has not been done yet. Or has it?”

    It has indeed, in a short story I read many years ago. A planet with a continuous ring of copper all round it, providing the inhabitants with all the power they needed. Would-be colonists from Earth arrived and, surprise, surprise, wanted to destroy this ring to impose their will on the said inhabitants. Unfortunately, I have no idea who wrote it or where and have been looking for it online with no success. Does anybody else know this story? I’d love to read it again.

  • andy November 27, 2015, 20:58

    Proxima is an M5 star so not much larger and despite its age , still capable of regular flares despite a sedate 84 day rotation.

    Which fits nicely with the abstract of the first paper I linked in my previous post – relevant quote: “we find that all M dwarfs with rotation periods shorter than 26 days (early-type; M1-M4) and 86 days (late-type; M5-M8) are magnetically active”