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Do we have any information on when the white dwarf member of the 40 Eridani system would have gone thorugh its red giant stage? Perhaps what type of star it originally was?

I've managed to find a couple of references stating that the system is 4.0 billion years old, though nothing official. But assuming this, could there possibly be a habitable world in the A component system? Would the white dwarf have formed early enough to predate multicellular life?

...John...

You mean this one? http://www.solstation.com/stars/40erida3.htm

That's a somewhat weird system. A KV star orbited by a far binary WD/M V pair? I found this page suggesting it's 4 billion years old, though to be honest they may be referring to just the K V star in the system.

If the system (and therefore the WD) is 4 Ga old, then assuming solar metallicity we're probably looking at a progenitor star that was between 1.3 and 1.5 solar masses. It would have started off as an F1-F5 V, spending 2.5-3 Ga as a main sequence star. Then it would have been a subgiant for a few hundred million years, go through a rapid RGB stage (5 - 10 Ma) where it would have ended up as an M1 II red giant about 0.5 AU in radius and with a luminosity of about 2000 Sol. Then it'd settle down for about 0.1 Ga on its horizontal branch as a K3 III giant before swelling up again to as an AGB giant to end its life as a Mira-type M1 II again (but this time slightly more luminous and larger than the RGB stage). So that's about 3-4 Ga of total lifespan.

If the WD progenitor was more massive then it would have got through that faster, but I think given that the companion is an M V star it's more probable that it was originally an F V - IIRC you don't usually have THAT big a mass difference between companion stars.

So I'd guess based on this scenario that probably there's been about 500Ma - 1 Ga between the white dwarf forming and today?

Excellent, thanks for that information!

So, let's assume that any habitable world orbiting the (now) primary K V star would have followed a similar time line to Earth's development of life. During the first 3 billion years or so, life would have been confined to the oceans as simple single-cell forms. It would not be until after the white dwarf had formed that life would develop into multicellular forms.

Now, assuming that the orbital parameters between the double pair and the K V primary have not changed (yes, a big assumption, but as far as I know there is no way to determine if there HAS been an orbital change or what it might have been), would the progenitor F-star, its whole red giant phase, or even the planetary nebula stage have had any kind of major impact on a habvitable world orbiting the K V star? In essence, would it have made life impossible on such a world?

For my part, I have no idea. It *seems* the distance would be enough to shield the planet from any effects, but then I simply don't have the knowledge base to make much of an educated guess.

...John...

Dollan wrote:Excellent, thanks for that information!

That was all gleaned from the Geneva Stellar Evolution Grids in case you're curious. So it may not be 100% realistic but it's on the right track (AFAIK they're still valid anyway).

Now, assuming that the orbital parameters between the double pair and the K V primary have not changed (yes, a big assumption, but as far as I know there is no way to determine if there HAS been an orbital change or what it might have been), would the progenitor F-star, its whole red giant phase, or even the planetary nebula stage have had any kind of major impact on a habvitable world orbiting the K V star? In essence, would it have made life impossible on such a world?

I think there'd be some change in orbital parameters. The WD would have had to shed about 0.8-1.0 solar masses to get down to 0.5 solar masses today. I think that means that the M V companion would have been closer to it in the past than it is today. From the POV of the K V primary (which wouldn't actually have been the primary back then I guess, since it's less massive than the F V progenitor), the binary pair went from having a total mass of about 1.5 solar masses to having a total mass of about 0.7 solar masses, so again it probably moved further from the pair after the WD formed.

But being 8000 AU away, I don't think the RGB or AGB phases would have affected the planet's temperature - it's too far away, at most I think it adds less than 0.01K to the average temperature. And I'd imagine that the K V's magnetic field would have shielded the planet from the material shed by the AGB when it turned into a WD.

Wonderful information. I'll be printing this out for when I *do* get around to designing this system. Thanks much!

...John...

It's an 8000 year period, not an 8000 AU separation... the separation between A and BC is about 440 AU... I don't think the eccentricity of the orbit has been determined though.

I still don't think that a red giant at 440 AU would have much effect on the temperature of an Earth-type planet orbiting the A component, but if it swings in closer, it might have more of an effect.

I've been more concerned over the effects of the planetary nebula on a potentially habitable world. I wonder how far, if at all, it would compress the heliosphere, and what the expelled gas itself might do to a planet.

For that matter -- and this just occured to me -- would the planetary nebula stage erode the mass at all of any outer gas giants?

...John...

Oops, you're right about the distance.

Even at 440 AU, a 2000L red giant only increases the temperature of the planet by about 1 K. So it makes a little difference to the weather patterns maybe, but not much in the long run.

I'd presume that the outer gas giants were within the primary's magnetosphere. Or that their own magnetic fields would protect them.

Don't planetary nebulae result in the expulsion of a lot of gas and dust, including carbon and or silicon? It might be possible to detect the dust given off by the star at various stages in the rocks of a terrestrial planet orbiting Keid A. But I doubt it would cause anything spectacular like an extinction event.

Here is the OA version of this system.
http://www.orionsarm.com/worlds/Twilight.html

I suspect life would not have a serious problem with a red giant as described. If the life is single-celled life living in the oceans during that time, it would evolve fast enough to cope with the secular changes in the environment caused by the stellar evolution of the red giant.

The given mass of the white dwarf is suspect. In many online sources for white dwarfs that I have consulted, a mass of 0.5 solar masses is used for all white dwarfs. If the white dwarf is from a 1.5 solar mass star, it would have a mass significantly greater than 0.5 solar masses, given the Sun is theoretically predicted to produce a white dwarf with a mass of 0.6 solar masses. The mass estimate is unlikely to come from the orbital motion with the small companion star, because the orbital period of the BC-pair is a couple of hundred years.

As to the evolution of the orbit over time, there will be some changes due to the mass loss from the red giant evolving to a white dwarf. This mass loss would make the orbits bigger, so when the star was a red giant the orbits would have been smaller. Other than these changes, there's no good reason to suppose that other orbital changes must have occurred in the 4-billion-year history of the system. There may have been other companion stars that were ejected from the system, but these would likely have taken place early in the lifetime of the system and such ejected companion stars would most likely have been distant low-mass stars of low luminosity. It's therefore reasonable to assume there haven't been other orbital changes for most of the history of the system that would adversely affect the development of life on a hypothetical planet orbiting the A-component.

bdm wrote:I suspect life would not have a serious problem with a red giant as described. If the life is single-celled life living in the oceans during that time, it would evolve fast enough to cope with the secular changes in the environment caused by the stellar evolution of the red giant.

Keep in mind that the changes happen over tens or hundreds of millions of years, and most of the time the planet doesn't even feel them because the massive progenitor star is too far away. It's only when it's in its shortlived RGB and AGB phases that the temperature of the planet is affected at all, and even then it's a very slight change. Even if the red giant goes a bit variable near the end it's still really not going to affect anything on a world in the habitable zone of the K V star.

One thing I'm not entirely sure of now I come to think of it - I may be mis-remembering this, but I thought that a freshly exposed white dwarf might be putting out an awful lot of UV radiation. That might do nasty things to an ozone layer around a nearby planet, if it's powerful enough?

If a white dwarf DOES put out that much UV, and life on the planet is still relegated to oceanic environments anyway, then I don't think that it would matter. I forget what depth, but water should provide an adequate shield if there's enough of it.

I think....

...John...

That is true now I come to think of it; a white dwarf starts off incredibly hot, and cools down over a period of hundreds of thousands of years to merely very hot...
The blackbody radiation from a newly created white dwarf will be full of very energetic UV rays, the sort of stuff that is hardly present in Sunlight at all.
From the Stellar Database website it lookslike the white dwarf currently shines less brightly than a crescent Moon as seen from Keid A; but when newly formed this tiny point of light would be pumping out very dangerous far UV.
Almost certainly it would cause visual damage if looked at directly.

eburacum45 wrote:That is true now I come to think of it; a white dwarf starts off incredibly hot, and cools down over a period of hundreds of thousands of years to merely very hot...
The blackbody radiation from a newly created white dwarf will be full of very energetic UV rays, the sort of stuff that is hardly present in Sunlight at all.
From the Stellar Database website it lookslike the white dwarf currently shines less brightly than a crescent Moon as seen from Keid A; but when newly formed this tiny point of light would be pumping out very dangerous far UV.
Almost certainly it would cause visual damage if looked at directly.

So that's probably going to do nasty things to ozone layers etc? Do new WDs emit any other kind of radiation (eg gamma rays?).

Young ones emit soft X-rays according to Wikipedia;
I have also heard that young WDs cool by neutrino emissions (an interesting factoid that lead to the imaginary OA neutrino cooling technology for use in stellar atmospheres).

eburacum45 wrote:Young ones emit soft X-rays according to Wikipedia;
I have also heard that young WDs cool by neutrino emissions (an interesting factoid that lead to the imaginary OA neutrino cooling technology for use in stellar atmospheres).

Well going by this month's Astronomy magazine, intense gamma rays can break down N2 and O2 in habitable atmospheres to make NO2 smogs that can cause climatic disasters (the article was about GRBs and how they'd affect Earth). So that might be another thing to worry about.