From the standpoint of planetary detections, the small red stars called M dwarfs are all but ideal. Their size is an advantage because radial velocity and transit methods should find it easier to pull the signature of smaller planets out of the statistical noise. Not so long ago, that wouldn’t have seemed important because the search for terrestrial worlds seemed confined to G- and K-class stars not too different from our Sun. But more and more theory is piling up as to why a terrestrial-sized planet in the habitable zone of an M dwarf could harbor life.
So these are important stars, especially when you add in the fact that they account for 75 percent or so of all the stars in the Milky Way (that statistic is admittedly subject to change as we learn more about other stars, especially brown dwarfs). And that makes the recent flare on EV Lacertae quite interesting. Some sixteen light years from Earth, the star is young (300 million years), dim (shining with one percent of Sol’s light) and small (its mass and diameter being a third that of the Sun). And although far too dim to pick out with the naked eye under normal circumstances, the recent monster flare it emited would have made it easy to see.
Once again we can thank the Swift satellite for the detection. Although intended to hunt gamma ray bursts (GRBs), Swift often does double duty, as in the recent case of a supernova caught just as it exploded. When the satellite detected the EV Lacertae flare, detailed measurements followed. The flare was thousands of times stronger than solar flares in our own system, of a magnitude that Rachel Olsten (NASA GSFC) calls “unprecedented.” Osten adds: “This star has a record of producing flares, but this one takes the cake.”
Image : An artist’s depiction of the incredibly powerful flare that erupted from the red dwarf star EV Lacertae. Credit: Casey Reed/NASA.
So now we are developing the ability to watch flares on other stars as they develop, with the help of Swift and other space-based resources like Chandra and XMM-Newton. That’s useful data as we ponder life’s chances around M dwarfs, where intense magnetic activity can generate flares like this one, capable of damaging a planetary atmosphere. This seems to be the thorniest issue of all, for although we can develop plausible scenarios for habitable climates on such worlds, their sheer proximity to their parent star could make frequent flares an evolutionary wildcard, if not prohibiting the development of life altogether. The range of flare activity possible on M dwarfs — some are far more benign than others — should be a factor as we fine-tune our target lists for future space-based observatories.
What with the flares problem, and the issue of the high velocities of in-situ accretion, it may well be that the typical habitable planet around an M-dwarf is going to be a massive ocean planet that migrated in from the outer system, rather than a more traditional terrestrial planet, which may well get baked dry.
That scenario certainly makes sense to me, particularly for the more active M-dwarfs.
It would be interesting to know which proportion of M stars are flare stars, since these stars are the most common in the universe by far, and becoming even more dominant as the universe ages. These flare stars are probably not very conducive to life, because of the enormous amounts of X-ray/gamma ray emitted during flares.
Other question: are flare stars generally metal-poor or metal-rich? Since the metallicity of red dwarfs increases with increasing age of the universe.
Hi Folks;
Speaking of stellar flares and the energy available in the corona-sphere, atmosphere, etc., of stars, my brother John once brought to my attention a novel propulsion concept wherein a space craft would orbit a star not far from its corona-sphere wherein the space craft would extract light energy and/or other electromagnetic energy from the star in conjunction with mechanisms for reacting against the stellar plasma in proximity to the star in an electro-dynamic-plasma-hydrodynamic drive system.
Accordingly, the craft would continue to accelerate around the star while vectoring its thrust in such a way that the craft would be pushed in circular orbit around the star. Eventually, when the velocity of the craft was significantly relativistic, the craft would disengage the thrust vectoring system and fly away from the star at speeds approaching C.
Obviously, some means would be required to cancel out the centripetal acceleration induced forces; perhaps electro-dynamic means of magnetizing the contents of the craft and/or electrically charging the contents of the craft and then using reacting fields to cancel out the effective G forces induced by the intense angular acceleration of the craft.
Using large red super giant stars (because of their diameter as great as several multiples of 1 AU) would reduced the required angular acceleration. Blue super giant stars would allow reduced angular acceleration along with much higher light flux densities which might be collected thru some sort of exotic photovoltaic materials as the craft went highly relativistic.
I remember hearing of this concept before my brother John mentioned it, but his brilliant insight into explaining this potentially wonderful concept to achieve high gamma factors intrigued me and I have fallen in love with this concept ever since he first mentioned it to me.
I will have more to say on this concept in the coming days. There must be some good use for the electro-dynamic-plasma energy around stars.
Thanks;
Jim
Flaring red dwarfs are an astounding discovery and would give planetary searchers pause for concern.
Andy makes a good point about possible massive oceanic worlds orbiting M-type stars, but how about Earth-like moons orbiting gas giants in such a solar system?
I’m not a planetologist, so what are the chances of M stars having gas giants? Is it feasible?
dad2059, we do have some evidence of gas giants around M-dwarfs. GJ 849 b, for example, is a gas giant in an orbit about 2.35 AU out from its star; it’s about eighty percent as massive as Jupiter. Planets like this are interesting because they help us tune up our understanding of the planetary formation process. A less massive protoplanetary disk may make the formation of planets much larger than Neptune-class unlikely, but so far our data isn’t terribly good for planets outside the 2 AU range in M-dwarfs. Lots of work ahead on that question.
dad2059: both of the gas giants in the Gliese 876 system are located in the habitable zone, at least according to this paper. A moon around one of these planets would have the advantage of a day-night cycle.
Andy: According to the table at the end of the paper, the author doesn’t hold out much hope for a habitable moon around the Gliese 876 gas giants, even in the hypothetical HZ.
Most of the table is full of stars with HZs though and their computer models work on most of them.
Hope springs eternal.
Hi Folks;
Regarding the concept of the electro-dynamic-plasma-hydrodynamic drive space-craft I mentioned previously above, one could conceivably imagine such a space craft having all or part of its forward facing and sunward facing surface area covered with negative index of refraction materials wherein the space craft would use the pull of the incident starlight to help shape its continued circular motion around the star as well as its continued acceleration in terms of its angular velocity around the star being utilized.
A combination of PV and negative index of refraction materials could be used to power the space craft’s angular velocity increase. As the space craft accelerated, the frequency of the incident light would become Doppler blue shifted with respect to the space craft which would result in greater incident optical power for both the PV systems and the negative index of refraction systems to operate on perhaps permitting greatly increased kinetic energy gain for the space craft and resulting greater gamma factors.
Perhaps much more massive craft could utilize the plasma in accretion disks around super-massive black holes for their electro-dynamic-plasma-hydrodynamic drive propulsion systems. There must be tremendous energy locked up within the accretion disk of the massive black hole located within the center of the Milky Way Galaxy. The multibillion solar mass black holes that appear to power QUAZARS should have even more stored energy within their accretion disks. Imagine in theory how large of a space craft that these huge accretion disks might be able to boost to very high gamma factors.
Thanks;
Jim
dad2059: as far as I can tell, that paper does not consider moons of the planets, it just tests whether an Earthlike planet in the habitable zone would be stable… since the Gliese 876 habitable zone already contains two gas giants, it isn’t a surprise they rule it out.
M dwarfs: effective temperatures, radii and metallicities
Authors: L. Casagrande, C. Flynn, M. Bessell
(Submitted on 15 Jun 2008)
Abstract: We empirically determine effective temperatures and bolometric luminosities for a large sample of nearby M dwarfs, for which high accuracy optical and infrared photometry is available.
We introduce a new technique which exploits the flux ratio in different bands as a proxy of both effective temperature and metallicity. Our temperature scale for late type dwarfs extends well below 3000 K (almost to the brown dwarf limit) and is supported by interferometric angular diameter measurements above 3000 K.
Our metallicities are in excellent agreement (usually within 0.2 dex) with recent determinations via independent techniques. A subsample of cool M dwarfs with metallicity estimates based on hotter Hipparcos common proper-motion companions indicates our metallicities are also reliable below 3000 K, a temperature range unexplored until now.
The high quality of our data allow us to identify a striking feature in the bolometric luminosity versus temperature plane, around the transition from K to M dwarfs. We have compared our sample of stars with theoretical models and conclude that this transition is due to an increase in the radii of the M dwarfs, a feature which is not reproduced by theoretical models.
Comments: 26 pages, 14 figures. Accepted by MNRAS. Landscape table available online at this http URL
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0806.2471v1 [astro-ph]
Submission history
From: Luca Casagrande [view email]
[v1] Sun, 15 Jun 2008 21:30:30 GMT (390kb)
http://arxiv.org/abs/0806.2471
Hi Folks;
Regarding instabilities of stars and their potential to end civilizations, it occurred to me to ponder the hypothetical formation of two stars located at a good distance from each other and not too close to any perturbative source of gravitation, cold dark matter, interstellar baryoinic gas, electric or magnetic fields, or electromagnetic fields and the like.
If such stars were of the G type or larger, even to the extent of having a mass that would lead to the stars going supernova, I began to wonder yesterday as to whether or not the stars would have the same or essentially the same lifetimes, especially for potential supernova, the same lifetime down to the seconds, minutes, or hours before exploding. I am curious as to what extent quantum scale random perturbations and background noise would be washed out or to what extent such small scale effects would grow into unique acoustic waves, density fluctuations, magnetic field flux patterns, coronal mass ejections and solar flares, and the like which could conceivably cause a significant but short difference within the lifetimes of such stars.
We have all heard of the classical chaotic information concept of the proverbial butterfly flapping its wings in South America leading to the formation of a hurricane in the Atlantic Ocean, but I wonder to what extent these classical scale fluctuations, perhaps brought on the quantum scale fluctuations, would amplify to produce global stellar acoustic patterns and other global scale or large sub-scale thermodynamic and mass-energy-transport states which might cause the life of the stars to differ by several years if not hundreds of years.
Perhaps these classical scale fluctuations largely wash out by random statistics so that the two stars would explode within hours or less from each other. Either way, modeling such effects with such precision would be an awesome computer undertaking perhaps by an Earth massed computronium computer. The knowledge of such fine temporal scale details by future human and ETI civilizations or any current ETI planetary civilizations might be very important in order to plan an orderly evacuation of their solar systems for cases where their parent star(s) or nearby stars become unstable.
Thanks;
Jim
Hi Folks;
Note that in the above comments, I meant to state that the two hypothetical stars would have the same mass, same initial temperature distribution, and same elemental/isotopic matter distribution. I assume however, that most if not all of the readership would infer this initial identicality of stars. I apologize for any lack of clarity on my part.
Thanks;
Jim
Solar Flare Surprise?
NASA Science News for December 15, 2008
Solar flares are supposed to obliterate everything in their vicinity, yet one of the most powerful flares of the past 30 years has done just the opposite, emitting a beam of pure and unbroken hydrogen atoms. Researchers think this strange event could yield vital clues to the inner workings of solar flares.
FULL STORY at
http://science.nasa.gov/headlines/y2008/15dec_solarflaresurprise.htm?list1094208
Check out our RSS feed at http://science.nasa.gov/rss.xml!