I am thinking about using Groombridge 1618 as a setting for my world. It is a main sequence star of spectral type K7.5 Ve, 67% of the Sun's mass. The habitable zone is at a radius of 0.26–0.56 AU. There is a possibility that it has a gas giant around it. It also might have a cold debris disk which I can use for raw materials.
It looks like a great location for my story and fits my plot nicely. However, Groombridge 1618 is a flare star. I am not sure how it would affect a human colony on a moon of a gas giant and terraforming attempts.
What should I be concerned about?
Is there something that could possibly make long-term human survival (thousands of years) in this system impossible?
There are a great many ways to deal with radiation and flares, so starting in order of difficulty:
Distance. Any spacefaring civilization can settle any body in free space or in orbit, so settling in cometary bodies in the star's Oort cloud puts you at sufficient distance from the star that flares become a minor annoyance. The ices of comets should contain sufficient Deuterium to run fusion reactors for thousands of years, and gigantic mirrors to concentrate solar energy can also be erected nearby, although the focal point should never be pointed at the colony.
To be totally realistic, the star would be so far away it would simply be the brightest star in the sky and not show a visible disc
Digging in. If the colonists are closer to the star, they can simply burrow into the body in question until there is sufficient mass between them and the star to provide protection. On an asteroid, they could take the additional step of aligning the long axis to point at the star and burrow into the "dark" pole to put the maximum amount of mass between them and the star. Solar mirrors and energy collection devices can be placed on the "hot" pole facing the star. This strategy works for moons and planets orbiting close to the star as well.
An artificial cavern would look the same almost anywhere
Artificial magnetic fields. With sufficient energy, artificial magnetospheres can be erected on or near planets, moons, asteroids or even artificial bodies in free space. While the L1 point may not be convenient with the star so close to the planet (typical planets orbiting Red Dwarf stars are found inside where Mercury orbits the Sun, for example), the magnetic device could be maintained in position using solar sails or other technology.
Keeping the magnetic field generator in place will be an issue
Modifying the star itself. Star Lifting technology can be used to remove material from the star, but since it operates by modifying and applying magnetic fields around the star, it could potentially be used to control the magnetic fields around the star itself, or direct flares away from the planet to regions beneficial to the star lifting civilization.
Star lifting used to pump plasmas out of the stellar pole regions. This could also be used to redirect flares
Okay so to protect from the radiation a flare puts out you need a strong magnetic field. It just so happens that the habitable moon of a large gas giant probably has not just one but two strong magnetic fields between it and the primary; if the gas giant is similar to Jupiter then the moon is probably within it's planetary magnetic field. Such large magnetic fields trap and ionise particles so a habitable moon will need it's own magnetic field, at reverse polarity to the local planetary field, to keep such particles largely out of it's atmosphere the same way Earth's magnetic field keeps normal solar winds from stripping our atmosphere. The net effect is that such a habitable moon will be very well insulated from stellar activity. They won't be completely covered mind, the colonists may still lose the odd crop to excessive surface radiation and would be well advised to keep Geiger Counters pointed at the sky and radiation bunkers in every town and isolated farmhouse, much like storm shelters in tornado alley. So constant monitoring, good food reserves and well shielded shelters for food, people, and livestock will be essential to maintaining a colony on such a moon but the problems will be minimal.