Omicron-2 (40) Eridani System Question...

[Dollan]

Or rather, I'm looking for opinions, perhaps even links to papers and such...

As most of you likely know, Omicron-2 Eridani is made up of a K-type sun, quite within the solar norms, and a distant companion set of a red and white dwarf pair. It is estimated that when it was on the main sequence, the white dwarf was a star just a bit more massive than the Sun (making it likely that this is an old star system).

So, if we take it that the system is about 8 billion years old, it is possible (if one is almost unrealistically optomistic!) that both the now gone G-star and the current K-star could have had Earth-like worlds. Obvisously, such a world about the G-star would have been destroyed when that sun expanded into a red giant.

My question is: what do you think would have happened to the Earth-like world about the still stable K-star? Would the red giant have had any appreciable impact on the planet's biosphere? Would the collapse into a white dwarf have done any damage?

I'm trying to come up with a realistic system about the K-star, and I was hoping to have a habitable planet there. But....

Any thoughts?

...John...

[granthutchison]

I guess it depends on how closely the red-giant/red-dwarf pair approached the K star - we don't have an orbit for them (because the period must be thousands of years long).
The original (pre-red giant) star must've been in the same ball-park as the Sun: I see SolStation estimates the mass of Omicron2 Eri B at 0.43 to just over half a solar mass, and the predicted mass for the Sun's eventual white dwarf is 0.51-0.58 (see Our Sun. III. Present and Future).
If we assume the current distance of 418AU, and a peak luminosity of 5200 times the Sun (the predicted max for our own Sun's asymptotic red giant phase), then that's going to drop about 3% of a solar constant into the habitable zone of the K star - enough the shove the temperature of a planet in that region up by a couple of degrees Celsius. But it's possible the red giant could have been much closer - mass loss during the red giant phase is going to have made the mutual orbit of the K star and the red-giant/red-dwarf pair increase in semi-major axis, and we've no idea how elliptical the current orbit is, or how close to pericentre the stars are at present.

I can't think of any major effects from the red giant mass loss that would impinge directly on a planet in the habitable zone of the K star. The mass-loss is steady, rather than catastrophic, and it's happening beyond the heliopause of the K star. (But I wonder if it might shove the heliopause all the way into the inner solar system?)

Grant

[Evil Dr Ganymede]

That's something I've always wondered, actually - if the red giant shell could actually get all the way through to a planet with an atmosphere, what would it do to it? Would it be like a whole load of dust landing on the planet?

I know there are a few stars that are "Barium Dwarfs" near Earth: HD147513 and Chi1 Orionis, apparently they were part of the same system when another star became a white dwarf - the shell enriched the other two stars with unusual elements. So sometimes, it does get through to the inner system and the stars themselves (the WD star is currently over 4000 AU from HD 147513A too - I dunno if it always was that far away, but if it was then it seems this can happen from quite a distance)

[Dollan]

Well, this is the current, tentative description of the primary planet in the system; visuals will follow as the project evolves:

"RETUNDA: A world that is a shadow of its former self. A once thriving world, much of its atmosphere was lost in the face of the distant B component's evolution into a red giant and the subsequent collapse into a white dwarf. Geology has since re-thickened the atmosphere, but it remains perhaps a third thinner than before this ancient series of events. Likewise, the oceans have decreased in volume, and as a consequence the continents are edged by great cliffs -- the edges of the continental shelves. Life has recovered with time, but it is marked by far less diversity, and forms which have adapted to less resources. Surface conditions tend to be extreme."

This will probably suffice, at least until new data becomes available (which might well be some time!).

...john...

[Evil Dr Ganymede]

You have seen the Solstation page for this system, right?
http://www.solstation.com/stars/40erida3.htm


And this too:
http://www.projectrho.com/vulsun.htm

(seems this is the system that Vulcan is supposed to be located in, from Star Trek!).

Apparently the system is only about 4 billion years old, which would make it more likely for Retunda to still be geologically active after it lost some of it atmosphere.

[Dollan]

Oh yes. This is one of the three primary sites I consult....

You have seen the Solstation page for this system, right?
http://www.solstation.com/stars/40erida3.htm

[granthutchison]

Apparently the system is only about 4 billion years old, which would make it more likely for Retunda to still be geologically active after it lost some of it atmosphere.

Odd. The K star is said to be 4 billion years old in your Star Trek reference, but that's not enough time for the white dwarf to have formed, unless the original star was considerably more massive than the Sun.

Grant

[granthutchison]

Likewise, the oceans have decreased in volume, and as a consequence the continents are edged by great cliffs

Continental shelf margins aren't necessarily cliff-like - although here and there on Earth you can find steep cliffs (like along the Pacific coast of South America), in many places the slope is quite gentle; the average gradient is 1:15 (= 4 degrees, which wouldn't even slow down your granny very much), and in places it's as low as 1:40. It's another one of those spurious images generated by the fact that textbooks sometimes miss out the essential phrase "vertical scale exaggerated" ...

Grant

[Dollan]

Finding any reliable reference on a star's age is so difficult; the Trek reference, too, is rather dated. If the B component was estimated to be just over a solar mass, then the system is probably at least 7 to 8 billion years old. However, all the references that I can find range from 4 to 4.5 billion years. If THAT is true, then maybe the initial mass was somewhere around 1.4 of Sol?

...John...

Odd. The K star is said to be 4 billion years old in your Star Trek reference, but that's not enough time for the white dwarf to have formed, unless the original star was considerably more massive than the Sun.

Grant

[Dollan]

Yeah, a little research brought that up. Still, some interesting geological morphology could be present. Large rivers might provide some deep canyons. I wonder if, given enough time, natural erosional processes might "even out" the continental margins, leaving an edge no different from its earlier incarnation?

Of course, going from my strict requirements for habitable stars (for my own setting), Omicron-2 Eri A falls below the required luminosity level of 0.4xSol. I may have to make this a terraformed world anyway....

...John...

Continental shelf margins aren't necessarily cliff-like - although here and there on Earth you can find steep cliffs (like along the Pacific coast of South America), in many places the slope is quite gentle; the average gradient is 1:15 (= 4 degrees, which wouldn't even slow down your granny very much), and in places it's as low as 1:40. It's another one of those spurious images generated by the fact that textbooks sometimes miss out the essential phrase "vertical scale exaggerated" ...

Grant

[granthutchison]

However, all the references that I can find range from 4 to 4.5 billion years. If THAT is true, then maybe the initial mass was somewhere around 1.4 of Sol?

Or we're seeing a system in which the K evolved separately and was then captured. Maybe it's the captured component of a double system that had a close encounter with the white-dwarf/red-dwarf pair. That pair have quite an eccentric orbit, which would fit with some sort of gravitational interaction in the past ... my recollection is that slow mass-loss from one star tends to circularize orbits (in contrast to sudden mass-loss, which leaves an eccentric system behind). If that's correct and I'm not misremembering, then we perhaps need to account for the current eccentricity of the white-dwarf/red-dwarf pair.

Grant

[Dollan]

Now THAT'S an interesting thought. I'll do some research on age indicators for the BC pair. I know that red dwarf flares are often thought of as an indication of youth, but I'm not certain if that is a rule of thumb anymore. Besides, I suppose that the white dwarf could be affecting the red dwarf in this manner... not to mention perhaps messing with rotational rates (another age indicator... some times).

This is turning out to be a much more complex system than I had initially thought!

...John...

Or we're seeing a system in which the K evolved separately and was then captured. ...

Grant

[Evil Dr Ganymede]

Finding any reliable reference on a star's age is so difficult; the Trek reference, too, is rather dated. If the B component was estimated to be just over a solar mass, then the system is probably at least 7 to 8 billion years old. However, all the references that I can find range from 4 to 4.5 billion years. If THAT is true, then maybe the initial mass was somewhere around 1.4 of Sol?

Well, the Geneva stellar evolution grids say that a star with 1.25 times the mass of the sun and similar metallicity will start off as an F7 V and finish its entire lifecycle and turn into a white dwarf in only 4.98 billion years. A 1.5 solar mass star will become a white dwarf 2.83 billion years after it forms. So if that's right, it sounds like the progenitor star could have been around 1.3 solar masses (starting as an F5 V?)?

Plus the red dwarf in the system is a Flare star, which (I think?) implies that the system isn't too old...

[eburacum45]

I captured this image using the data in the Nearby Stars add-on