Gamma Pavonis/Dingir Help

[rfmcdpei]

Hi!

As an exercise, I'm trying to generate statistics for a planetary system at Gamma Pavonis
<http://www.solstation.com/stars/gpavonis.htm>, home to Traveller's world of Dingir, using Tyge Sjöstrands's world generation system.

I'm having problems with some seemingly contradictory data about the star, though. Sol Station gives it a mass of eight-tenths Sol, a
spectral classificaion of F6-8, a luminosity of ~1.5 Sol, and an age possibly in excess of 9.1 billion years. The Stellar Database gives
similar results <http://www.stellar-database.com/Scripts/search_star.exe?Name=gamma+pavonis>. Both sources identify it as a main-sequence star.

From what I know about stellar evolution, it's impossible for a main-sequence star with a mass eight-tenths that of Sol to have a spectral classification and total luminosity greatly in excess of Sol. The age, too, is another problem--late F-class main sequence stars are supposed to have a life span barely more than half of 9.1 billion years. Gamma Pavonis should look more like Tau Ceti than a stabler metal-poor Procyon.

The only explanation I can think of is that Gamma Pavonis is beginning to move off the main sequence and heating up; but then, surely it should be getter redder, not bluer.

Is there something I'm missing?

 

[Malenfant]

Yeah, you're missing the metallicity. It's an old disk star, so it has less metals than Sol does. It's not quite low enough to be a subdwarf though. What this means is that the star is bluer than a star with the same mass that has a Sol-like metallicity.

I suspect it's reaching the end of its main sequence life though. It probably started off as an early G V star with low metallicity and has been getting bluer (well, yellower/whiter in this case) as it's aged. The low mass (0.8 Sols) means it can have a relatively long lifespan, but the low metallicity shortens that, so it doesn't last as long as a solar-metallicty star with 0.8 masses would.

 

[rfmcdpei]

Yeah, you're missing the metallicity. It's an old disk star, so it has less metals than Sol does. It's not quite low enough to be a subdwarf though. What this means is that the star is bluer than a star with the same mass that has a Sol-like metallicity.

Ah, so it's burning hotter than a star of its mass with normal metallicity? Out of curiosity, why's that?

I suspect it's reaching the end of its main sequence life though. It probably started off as an early G V star with low metallicity and has been getting bluer (well, yellower/whiter in this case) as it's aged. The low mass (0.8 Sols) means it can have a relatively long lifespan, but the low metallicity shortens that, so it doesn't last as long as a solar-metallicty star with 0.8 masses would.]

So Gamma Pavonis would be on the verge of becoming a subgiant, then? This works.

 

[Malenfant]

IIRC, lower metallicity stars are less opaque to radiation. I think this means energy can get out of them easier, which somehow means that their lifespans are shorter and they're bluer than main sequence stars.

And yes, I think Gamma Pav is probably about to become a Subgiant ("about" being "within the next few million years, probably").

 

[Straybow]

Eh? 0.8M¤ should be almost to K. It would have to be very, very late in life to brighten to F8 (normally over 2L¤), much less F6 (typically 3L¤ or so). Might go (sub-) giant within foreseeable human future (but well past OTU timeline).

 

[Malenfant]

0.8 M would be K if the star had solar metallicity. Since it's a low metallicity star, it would actually start off as a G V star (probably mid G, actually, not early). And then it brightens and gets bluer with old age.

 

[Straybow]

Yikes, I didn't think metallicity would make that much difference. From what I've seen, stars double in brightness over main sequence lifespan. More or less. This one has gone from something less than 1L¤ to 3L¤, which is more like a factor of 4.

 

[Malenfant]

Yeah... in theory, a 0.8 solar mass and metallicity star would go from 0.27 to 0.77 Sols luminosity from star to end of its main sequence.

A bona fide subdwarf (with 5% the metallicity of Sol) with the same mass goes from 0.5 to 3.5 Sols luminosity!

So I'd imagine a star with intermediate metals should increase in luminosity by a factor of abot 4 or 5.