In Search of Dark Stars | Hacker News Reader

In Search of Dark Stars

simonsfoundation.org

47 pointsdigital5510 comments

So back when I was running numerical simulations of pop III stellar evolution (which is rather a long time ago now), the interesting thing is that, in comparison to pop II and I, the extreme metal paucity means that you can have (a) stable and extremely long-lived very dim, very small stars, verging on being warmed up gas giants, and (b) very large and extremely short-lived stars, which will supernova or otherwise give way to instability on the order of a few million years after core ignition.

You can then work back to speculative mass functions (distribution of mass in the population) by asking (a) how many large, short-lived stars you need to generate the observed metallicity of pop II, and (b) how many extremely small dim objects you'd need to fill out the missing mass without having so many that someone would have seen them by now. The latter is an interesting line of thinking; what is the upper density bound on (for example) 0.1M objects to make both the solar system's continued existence and an inability to see these things plausible.

You can also look at the paucity of candidates for ~1M extremely low metallicity stars (there are just a couple out there known that could be pop III candidates) as another constraint on the pop III mass function. Any pop III ~1M stars would have a lifetime longer than that of the universe to date, or at least such was the case in my models.

Research on missing mass and pop III has moved on considerably from those days of course.

> Her move to Stockholm, she said, is motivated in part by the cultural acceptance of women in science there. “On the entire planet, as far as I can tell, the best place for a woman to do science is Scandinavia,” she said.

Am I wrong to think that Sweden is as wonderfully progressive as it seems?

>> Am I wrong to think that Sweden is as wonderfully progressive as it seems?

There are advantages/disadvantages with everything.

A consensus culture where the boss goes around and talks to people before setting goals is very different for Americans. Like all other cultures it works better for some areas, worse in others.

Think politically correct, group oriented. (Have you seen that anywhere else?)

Edit: Let me paraphrase it like this: "The worst of worlds, the best of worlds". Let me give an example. It isn't good for someone to have the "wrong" opinions on some sensitive point in a consensus culture; a low roof. [The Swedish term is "the opinion corridor", i.e. that only a small bandwidth of opinions are tolerated.]

Edit 2: I think that was the first time in my life I've seen the Stockholm people described as "friendly". :-)

I was surprised as well, regarding edit 2. Then again, my heart belongs to Gothenburg. :-)

> We pointed out that because the Earth moves around the sun, any dark matter signal you see should go up and down with the time of year, peaking in June, with a minimum in December.

Why June and December? (Perihelion is around January 3, if that's relevant.)

if i remember it is explained by the move against and along the dark matter "wind" in our galaxy. Personally i think there are a lot of "wind" to be accounted for - out galaxy moves, our Sun moves inside galaxy and the Earth moves around Sun. So Earth definitely hits some "wind". Is it dark matter "wind"?

Ah, that makes sense. So the "wind" would peak when the Earth is moving against the galactic background.

Because of the motion of the Earth relative to the galaxy. Our Solar System is in orbit around the galaxy, and thus traveling at considerable speed (200 km/s or so) around the galactic center. The dark matter halo of the Milky Way, however, does not travel in the same way, so there is a persistent relative velocity between the Earth and dark matter (the WIMP cloud), this is called the "WIMP wind". However, the Earth itself orbits our Sun in a circular motion, which causes it's velocity relative to the Sun and the overall Solar System to change from about 30 km/s in one direction to 30 km/s in the opposite direction, a difference of about 60 km/s, which is significant compared to the 200 km/s galactic motion. Due to the orientation of the Solar System the relative speed of the "WIMP wind" is highest in June and lowest in December. This has nothing to due with Earth's orbital nodes or with the seasons, it merely has to do with the coincidental orientation of Earth's orbital plane relative to the galaxy. Because Earth's orbit is not aligned with the Sun's orbit through the galaxy (it's actually tilted at about 60 deg.) the difference is not the full 60 km/s, it's closer to just 30 km/s (cos(60 deg.) = 1/2), but that's still significant.

There are more details than you'd ever care to learn about here: http://arxiv.org/abs/1209.3339

This is the same Freese who has proposed detecting dark matter using gold and DNA. I have no idea if she is right or wrong, but reading about this kind of researching is exciting to me.

I know this is off-topic, but anyone else had "Benson, Arizona" ringing through their head when they read the title?

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