effects of Metallicity on stars ?

[Widdekind]

Metals can cool the clouds, in the ISM, from which stars form. And, the properties of those clouds could be characteristic, of the outer envelopes, of stars:

 

The cooling and heating processes that occur in the ISM are highly dependent on metallicity (the abundance of heavy elements). As one example, gas can cool by radiation from collisional excitation of heavy elements. Electrons in atoms are excited to higher energy levels by thermal collisions with other atoms. When those electrons fall back down, they emit photons that effectively carry some thermal energy out of the nebula. H and He have energy level spacings (excitation potentials) that are typically greater than the characteristic thermal energy kT, so they are rarely excited by collisions. It’s the heavier elements, the nucleosynthetic products of past generations of stars, that are then most responsible for this form of cooling (astrobites).

Indeed, for a given 'core operating power', and hence, ultimately, luminosity, more metals makes stars colder (redder), but bigger -- shifting the MS to the left, on the HRD:

 

(source:

mnsu

)

Indeed again, more metals in the stellar mixture modifies post-MS evolution, shifting He-core-burning stars, from the 'Horizontal Branch' (low Z), leftwards, to the 'Red Clump' (high Z) (access-science).

 

The bigger, bloated, higher metallicity stars also (therefore?) rotate less rapidly; and, have higher mass-loss rates (~Z^1/2) (supersci). In turn, these properties of stars, during their lives on the MS, affect their post-stellar evolution off of the MS -- in particular, the SNe / HNe / GRBs of high-mass stars:

 

Less mass loss and a more compact progenitor favors larger angular momentum at death...

 

• Stars will be more massive at death, and possibly more difficult to explode.
  
• Fall back may be more important, and black hole formation, common.
  
• Rotation rates in the inner core may be higher.
  

...Low metallicity stars will die with higher masses – potentially greater nucleo-synthesis in more massive stars

 

• But – the heavier members will be more difficult to explode and will experience greater amounts of fall back
  
• Rotationally enhanced mixing may be increased and the effects of angular momentum more pronounced during the late stages
  
• More black holes will be made
  

That is, low-metallicity stars are smaller, more compact, have hotter envelopes, and rotate more rapidly. They also have higher core-masses (He thru Fe), which cores re-attract more 'fall-back' material, during SNe. And, indeed, faster-rotating & more-massive remnants, re-accreting more material, could account for the 'collapsar' model of (long-type) GRB generation, during the initial phases, of the HNe, of hyper-massive stars, in the early universe.

Edited August 24, 2011 by Widdekind

[Widdekind]

According to the Virial Theorem, for an idealized, isothermal, spherical inter-stellar cloud, 2 K_T + U_G = 0, s.t.:

 

[math]\frac{3 k_B T}{\mu m_H} = \frac{3}{5} \frac{G M}{R}[/math]

where the mean molecular weight is [math]\mu^{-1} = 13/16[/math] (primordial atomic gas) & [math]\mu^{-1} = 27/16[/math] (primordial ionized gas). Thus,

 

[math]T_{vir} \approx 5 \times 10^6 K \times \left(\frac{M/M_{\odot}}{R/R_{\odot}} \right)[/math]

for fully atomic gas (e.g. envelopes of low-mass stars). Thus, stars on the MS appear to be highly sub-virial, by roughly three orders-of-magnitude. But, such explains the "high-mass-loss-and-PNe-pocket", in the upper right-hand corner, of the HRD, (1) wherefrom stars descend, on their Hyashi tracks, towards the MS; (2) whereto stars ascend, off MS, on their various GBs. There, stars like Antares & Betelgeuse reside, having radii [math]\approx 10^3 R_{\odot}[/math], so that they have bloated up to become quasi-virial, with K_T ~ U_G. Thus, since those stellar envelopes have roughly as much thermal energy, as GBE, it is easy for them to be blown off, into circum-stellar mass-loss nebulae, or PNe.

 

Thus, the pattern of tracking, across the HRD, by stars ascending on their GBs, when their cores are gravitationally collapsing, and their 'decoupled' envelopes expanding, represents a 'return' towards that initial state of quasi-virialization. Indeed, if-and-when core does not re-ignite (e.g., up AGB), such that the core keeps contracting, and dumping its GBE into its over-lying envelope, then even as the core "keeps on contracting", the envelope "keeps on expanding", eventually lifting up-and-off the star core completely, creating PNe.

[Widdekind]

According to James Kaler's Astronomy!, pg.429, as middling-mass stars rise up the AGB, over the course of ~1 Gyr, their contracting He cores, non-fusing & degenerate, but surrounded by a H-burning shell, slowly heat up, from MS core temperatures of 15-20M K, to 100M K. At that point, the He-core re-ignites, in the 'Helium flash', eventually halting core collapse, and causing a 'bounce', wherein the core expands, and escapes degeneracy. Similarly, after core-He-burning stops, the now-CNO core, surrounded by a He-burning shell, re-contracts, evidently heating up from 100-140M K. Those temperatures can be inferred, from the ~T⁴ and ~T¹⁷ temperature dependences, of H-burning & He-burning, respectively. Evidently, the contracting cores "carry down in" their circum-core still-fusing shells, heating them up, and causing the observed ~1000x increases in Luminosity. At PNe phase, the He-shell-burning, CNO-core apparently 'blows its lid', expelling outlying envelope layers, and perhaps losing 'Grav. pressure confinement', quenching fusion? In any event, on the AGB, and GB -> WD tracks, the stellar-core is degenerate; whereas, from Helium-flash to HB/Red-Clump, the core is expanding & non-degenerate. The expansion, or 're-virialization', of the envelope, and its eventual expulsion, may arise from the increasing luminosity, and resulting radiation pressure, of the increasingly efficient core.

Edited August 29, 2011 by Widdekind

[Enthalpy]

And why should gas cold down, hence contract, only by emitting light? Couldn't it expel its hottest atoms and molecules instead, and let the coldest sink? As long as the mean free path is big enough, this must necessarily happen.

 

Also, the even spread of thermal energy means that heavier nuclides have less speed hence must fall down (or diffuse down if density is big enough). As gravitation potential is many times stronger than heat there, random movements can delay this but not prevent it. Turbulence could hinder that, or too slow diffusion - this must depend on density.