are there He-core WDs ??

[Widdekind]

According to reference,

 

When the H fuel is exhausted, star puffs up and becomes red giant. Therefore, on H-R Diagram, star leaves main sequence and moves up and to the right (or, if mass is very low, star turns directly into white dwarf, left in H-R Diagram, bypassing red giant phase).

Does that mean, that some WDs, are composed of He (as the spent cores of low mass stars), not CNO ??

[csmyth3025]

According to reference,

 

 

Does that mean, that some WDs, are composed of He (as the spent cores of low mass stars), not CNO ??

According to the reference you cited:

 

4. The Smallest Stars: Brown and Red Dwarfs

Some protostars don't even quite make it to star-hood: if its mass is less than about 0.08 M_Sun, a ball of H and He gas won't have enough gravity to produce the temperature and pressure necessary for nuclear fusion. These luke-warm failed stars are called brown dwarfs. If the star is just massive enough to ignite nuclear fusion in its core, but not much more, what happens when it uses up its hydrogen? It will contract and heat up -- but not enough to fuse helium atoms together -- winding up as a small, hot, glowing helium ember, emitting black-body radiation and cooling over billions of years: a white dwarf.

 

Note that very low-mass stars like red dwarfs are fully convective: they mix up their insides constantly like boiling water. Therefore H gets used up throughout the star, not just in the core.

(ref. http://www.astro.uma...life_death.html )

 

Your same reference states in the previous section, however, that:

1. If the mass is much lower than 1 M_Sun (red dwarfs), the star could live on the main sequence for hundreds of billions of years, much longer than the current age of the Universe! Eventually, it will either blow away its mass (mostly in the form of helium) or contract and turn into a small, hot, ember: a white dwarf.

 
and in the subsequent section:
 

If a star has a mass between about 0.4 M_Sun and 7 M_Sun, something different happens when the H runs out in a star's core
 
-and-
 
When core T finally reaches 100,000,000 K, helium can now fuse to form carbon -- there's a new burst of energy, the helium flash. Star enters new phase of relative stability (though much shorter than main sequence) converting helium to carbon and oxygen in core.

 
Also, by definition, red dwarf stars have masses no greater than 0.4 solar masses. (ref. http://en.wikipedia....characteristics )
 
So, as you can see, although red dwarf stars can end their lives as (mostly) helium white dwarfs, there hasn't been enough time since the beginning of the universe for any red dwarf to have reached this stage of its evolution.
 
Chris

[Widdekind]

So, seemingly, small WDs are "spent He-cores", perhaps up to [math]\approx 0.5 M_{\odot}[/math].

[csmyth3025]

So, seemingly, small WDs are "spent He-cores", perhaps up to [math]\approx 0.5 M_{\odot}[/math].

As I noted in my earlier post, the link you supplied says (in part):

If the mass is much lower than 1 M_Sun (red dwarfs), the star could live on the main sequence for hundreds of billions of years, much longer than the current age of the Universe!

 

Your description of small white dwarfs as "spent HE-cores" is probably appropriate. Although there has not been enough time since the big bang for a <0.5 solar mass red dwarf star to naturally evolve to a HE white dwarf, the HE core of a larger star might remain if a nearby companion "stole" hydrogen from the outer layers of the progenitor star to the extent that it could no longer sustain nuclear fusion:

 

If the mass of a main-sequence star is lower than approximately half a solar mass, it will never become hot enough to fuse helium at its core. It is thought that, over a lifespan exceeding the age (~13.7 billion years) of the Universe, such a star will eventually burn all its hydrogen and end its evolution as a helium white dwarf composed chiefly of helium-4 nuclei. Owing to the time this process takes, it is not thought to be the origin of observed helium white dwarfs. Rather, they are thought to be the product of mass loss in binary systems or mass loss due to a large planetary companion.

(ref. http://en.wikipedia....h_very_low_mass )

 

Chris

[Widdekind]

So, seemingly, small WDs will be "spent He-cores", billions of years far into our future, once those slow-burning, M-class 'red dwarves' ([math]M_i < 0.4 M_{\odot}[/math]) finally fuse their last H atom. (That is an important caveat)

 

 

 

EDIT: this link looks relevant

Edited August 18, 2011 by Widdekind

[Widdekind]

Low-mass stars are 'slow all the way around' -- they not only burn thru their initial H fuel more slowly, but also form more slowly, from their proto-stellar cloud precursors:

 

Because the rate of the cloud's contraction depends upon how fast it radiates heat into space, the higher luminosity of more massive clouds causes them to contract much faster than less massive clouds (in the example given, one hundred times faster, because heat is generated a hundred times faster, but radiated away ten thousand times faster than in the less massive cloud); and as a result, massive clouds can become stars in only tens or hundreds of thousands of years, while less massive clouds become stars in millions or tens of millions of years (seligman).

QUESTION -- If cosmic 'Reionization' occurred, comparatively quickly, billions of years ago, would all that ionizing radiation, from high-mass 'Pop. III' stars, have quenched the birth of low-mass-and-slow-assembling stars ? For example, 2 billion years later, low-mass-and-slow-assembling galaxies were similarly quenched by Quasars, during secondary (He) reionization.

Edited August 19, 2011 by Widdekind