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What Are Flare Stars?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

M-Dwarfs

The M-dwarf (dM) variant of red dwarfs, also known as UV Ceti variables, are one of the smallest classes of stars out there, existing on the lower right side of the HR Diagram. They are typically 1/3 the mass of our Sun (with the usual range of 10 to 60% that of our Sun), range in temperature between 2500 and 4000 Kelvin, and have 1% the luminosity of the Sun, yet are likely the most common type of star in the Universe. In fact, they are “probably more numerous than those of all other stellar types put together" (Nicastro 67, Templeton, Joy).

But they are extremely faint and so getting proper estimates is challenging. But something they do that is not hard to spot are their impressive flares, packed with an energy output per minute that rivals what they put out over their entire surface over 1 hour. But what is going on with these flares, and what do they tell us about conditions across the Universe (Ibid)?

Well the flares themselves are brighter than their host stars by 1-6 magnitudes of order and usually hit their peak brightness a few seconds after the flare starts, then slowly dwindle over a 5-10 minute period. It has the same light curve as a nova or supernova but a more gradual shape as well as “no prominent initial spike in luminosity.” Flares seem to be random, with no pattern to them unlike most regular variable stars (Nicastro 69, Templeton, Joy).

But like most energetic events in the Universe, UV rays offer the best way to examine flares, especially because their host object isn’t energetic. With an average surface temperature of 3000 K, M-dwarfs put out mainly IR rays, so it a flare with a ton of energy occurs then the UV portion would be the least utilized, giving a great comparison in hunting for the flares (Ibid)?

What Else Could It Be?

But is it possible that the flare actually isn’t happening but is instead some other change to a star? Well, based on the energy released, it would require the entire surface of the star to increase in temperature by 40 to 50 K, and wholescale changes are more unlikely than a localized event like a flare, which can go past 10, 000 K. It would require such dynamic changes in such a short span of time as to render any such model ridiculous (Nicastro 69, 71; Templeton, Joy).

Besides, we have a well-known flare model right here in our solar system: The Sun. Though its flare mechanism are still mysterious, the general picture seems to be as follows Sunspots are regions where the surface of the Sun is 100 to 200 K cooler and also happen to be where magnetic field loops emerge. The fields themselves prevent convention from occurring and thus reduce the temperature at these spots (Ibid).

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This cooling effect allows low energy photons through and blocks high energy ones. When magnetic field lines get twisted, eventually they break as they reduce to lower strength fields and the material travelling along them is released to space (which interacts with material around, reclaiming free elections and giving off UV rays) along with the previously trapped high energy photons (even more UV rays). So with this and our observational data of the Sun, we have a method with direct comparisons to our flare stars. Therefore, scientists are confident in their flare model (Ibid).

Emission M-Dwarfs

Now, only some dM stars do flare, but the emission M-dwarf (dMe) are the easiest to spot because of their emission spectrums and are by far the main type of flare star we know of. They also process magnetic fields up to 10, 000 Gauss in strength while dM stars hardly have any at all. This again ties into our comparison of solar flares to red dwarfs. As far as how many of each type of M-dwarf there are, about ½ of all emission lines seen from M-dwarfs are split between the dM2.5-dM5 group and dMe (Nicastro 69-71, Joy).

Using this along with current estimates of the M-dwarf population give a rough value of 5*1011 dMe stars out there. Each dMe star gives off about 2*10-13 solar masses in one year. Combine that with the solar wind these stars release and now we are up to 1.2*10-12 solar masses lost in a year. Now, taking that and the total number into account gives a yearly loss of 0.6 solar masses amongst all the dMe stars. That makes them the number one exporter of materials to the Universe. In fact, it outpaces the next three sources (M II giants, OB stars, and supernova) combined (Ibid).

This makes dMe stars great energy sources for the interstellar medium, up to 5*1041 Joules of energy per year. In terms of mass loss of the host galaxy, their dMe stars don’t impact them greatly. That is, as long as the host galaxy is a spiral or an irregular, for they have large haloes that can collect much of that lost material and are massive enough to effectively counter the escape velocity of the ejected material. But elliptical galaxies are a different story. They not only lack this halo of material but they are generally less massive than other galaxy types. The ejected material would act like a galactic wind, clearing the area of spare gas, something which does match with observational data (Nicastro 71).

Works Cited

Joy, Alfred H. “Stellar Flares.” Astronomical Society of the Pacific Leaflets, Vol. 10, No. 456, No. 456, p. 41-48 (1967).

Nicastro, Anthony. “Flare Stars.” Astronomy. June. 1981. Print. 67, 69-71.

Templeton, Matthew. “UV Ceti and the flare stars.”aavso.org. AAVSO. Web. 01 Mar. 2021.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2022 Leonard Kelley

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