IMF of First Stars: R. Schneider et al.

[wolram]

http://arxiv.org/abs/astro-ph/0510685

Authors: R. Schneider, R. Salvaterra, A. Ferrara, B. Ciardi
Comments: 11 pages, 8 figures, submitted to MNRAS

Motivated by theoretical predictions that first stars were predominantly very massive, we investigate the physics of the transition from an early epoch dominated by massive Pop III stars to a later epoch dominated by familiar low-mass Pop II/I stars by means of a numerically-generated catalogue of dark matter halos coupled with a self-consistent treatment of chemical and radiative feedback. Depending on the strength of the chemical feedback, Pop III stars can contribute a substantial fraction (several percent) of the cosmic star formation activity even at moderate redshifts, z = 5. We find that the three z = 10 sources tentatively detected in NICMOS UDFs should be powered by Pop III stars, if these are massive; however, this scenario fails to reproduce the derived WMAP electron scattering optical depth. Instead, both the UDFs and WMAP constraints can be fulfilled if stars at any time form with a more standard, slightly top-heavy, Larson IMF in the range 1 Msun < M < 100 Msun.

 

[Garth]

Thank you wolram, a paper I have been looking out for...

In the last few years, the universality of the stellar initial mass function (IMF) has been questioned by the results of theoretical studies, which consistently predict that the first stars (hereafter Pop III stars) had characteristic masses of 100-600 M_⊙

However:

Thus, our analysis suggests that Pop III stars forming with masses in the same dynamical range as Pop II/I stars, (allowing Pop III stars to form only in the conventional mass range, 1 M_⊙ < M₀ < 100 M_⊙)but possibly with higher characteristic stellar mass, appear to be favored by observations

The only problem is how to explain how such stars could form in the metal free environment, several possibilities are referred to...

However, all these processes either require very special conditions, generally related to the existence of a previous generation of very massive stars, or have failed to be realized in realistic numerical simulations. In spite of this, observations seem to indicate that either radiative feedback effects during the accretion phase on protostellar cores and/or a combination of the above mentioned processes can lead to the formation of Pop III stars with masses < 100 M_⊙. According to the results of our analysis, the latter contribution to the Pop III stellar IMF must be dominant at least at red-shifts z ≈ 10.

And if so then

If this scenario is correct, relics of the low-mass Pop III star formation mode should be still present in our Galaxy

What would such relics be? Although not as supermassive as originally thought they would still be very massive compared to the Sun and consequently would now be supernova relics - IMBHs possibly?? (Now with a mass range of ~10 - 100 M_⊙.)

Garth

 

[turbo]

And if so then
Quote:
If this scenario is correct, relics of the low-mass Pop III star formation mode should be still present in our Galaxy
What would such relics be? Although not as supermassive as originally thought they would still be very massive compared to the Sun and consequently would now be supernova relics - IMBHs possibly?? (Now with a mass range of ~10 - 100 M⊙.)

I envision the relics in our MW for such processes would appear as regions dominated by metal-poor stars - not Pop III by any stretch but poorly enriched. Is this observed?

 

[Garth]

I envision the relics in our MW for such processes would appear as regions dominated by metal-poor starts - not Pop III by any stretch but poorly enriched. Is this observed?

Although metal poor stars are observed they can be explained by the H/He rising to the photosphere and the metals lost in the stellar interiors OR they may be very early stars (Pop III) that have not synthesised much metallicity.

The question is still how would Pop III stars in this mass range form with zero metallicity in the first place? And how many - i.e. what baryon density - is needed to explain the observations of IGM metallicity and re-ionisation?

Garth

 

[SpaceTiger]

What would such relics be? Although not as supermassive as originally thought they would still be very massive compared to the Sun and consequently would now be supernova relics - IMBHs possibly?? (Now with a mass range of ~10 - 100 M_⊙.)

The "relics" he refers to are low-mass stars, not remnants of high-mass stars. He's saying that the Pop III IMF ought to extend to low masses (if their observations are to be fit) and, since low-mass stars are long-lived, there should still be some alive and kicking in our galaxy.

The observations are pretty sketchy, though, so I would be cautious in interpreting the results.

 

[Garth]

The "relics" he refers to are low-mass stars, not remnants of high-mass stars. He's saying that the Pop III IMF ought to extend to low masses (if their observations are to be fit) and, since low-mass stars are long-lived, there should still be some alive and kicking in our galaxy.
The observations are pretty sketchy, though, so I would be cautious in interpreting the results.

Thank you ST. I agree that any results from that epoch are going to be tentative, however when they said

. Instead, both the UDFs and WMAP constraints can be fulfilled if stars at any time form with a more standard, slightly top-heavy, Larson IMF in the range 1M_⊙ < M_* < 100M_⊙.

presumably the number of low mass - and still active stars - should be quite small? Would not what is left of the mass of the majority of the relics originally have been in the higher end of that scale?

Garth

 

[SpaceTiger]

however when they said presumably the number of low mass - and still active stars - should be quite small?

Yes, but the point is that they should exist. Some other theories suggest that there shouldn't be any Pop III stars still around.

Would not what is left of the mass of the majority of the relics originally have been in the higher end of that scale?

I think you're just using different (though not incorrect) terminology. From the context given in their paper, "relic" is only referring to stars still on the main sequence. If IMBHs exist, then it seems perfectly reasonable to think they might be Pop III remnants.

 

[turbo]

From the context given in their paper, "relic" is only referring to stars still on the main sequence. If IMBHs exist, then it seems perfectly reasonable to think they might be Pop III remnants.

It seems pretty speculative to propose that low-mass Pop IIIs stars might still be around in our galaxy, without first establishing the physics by which primeval stars follow their own "main sequence" and relating that to the observed main sequence of highly metallized stars. Are there any observations that support the notion that Sol-mass stars of low metallicity can live for 10-13 Gy? Most BB papers seem to ascribe the "live fast - die young" philosophy to Pop III stars, so they can supply both the observed metalicity and the reionization radiation required prior to z~6. It seems that the Pop III stars are being asked to do quite a lot in this model.

 

[Garth]

Most BB papers seem to ascribe the "live fast - die young" philosophy to Pop III stars, so they can supply both the observed metalicity and the reionization radiation required prior to z~6.

That depends on the IMF - this paper is saying that in fact observations suggest the range of Pop III masses are lower than theory predicts.

Garth

 

[Chronos]

I think [albeit it with great effort], that pop III stars were bit players in a much larger theatre in the early universe. The CMB was still fiercely hot, IMBH's were forming and merging furiously, and GRB's were popping off like fire crackers... from IMBH mergers [long bursters] and Pop III hypernovas [short bursters]. Add a dense cloud of primordial hydrogen, and no telling what might arise from that mess. There are probably some other collateral collision events that could arise from large scale effects.

 

[Garth]

and GRB's were popping off like fire crackers... from IMBH mergers [long bursters] and Pop III hypernovas [short bursters].

While I agree with the general idea - that the early environment was complex and we should keep an open mind as we have very little idea what really went on - nevertheless, should that not be the other way round? i.e. IMBH mergers [short bursters] and Pop III hypernovas [long bursters].

Garth

 

[turbo]

I think [albeit it with great effort], that pop III stars were bit players in a much larger theatre in the early universe. The CMB was still fiercely hot, IMBH's were forming and merging furiously, and GRB's were popping off like fire crackers... from IMBH mergers [long bursters] and Pop III hypernovas [short bursters]. Add a dense cloud of primordial hydrogen, and no telling what might arise from that mess. There are probably some other collateral collision events that could arise from large scale effects.

Pop III as bit player? I see a problem here that maybe you can clear up. IMBHs had to originate from something else through the processes of merger or accretion. Without massive stars (Pop III) playing that crucial role at progenitors, how do you form IMBHs?

In a reasonable heirarchical model, Pop III stars form before IMBHs, and since only a fraction of the Pop III stars will end up as IMBHs, they would have to far outnumber IMBHs.

http://arxiv.org/PS_cache/astro-ph/pdf/0410/0410526.pdf

At zero metallicity mass loss through radiatively-driven stellar winds or
nuclear-powered stellar pulsations is expected to be negligible, and Population
III stars will likely die losing only a small fraction of their mass (except for 100 < m∗ < 140 M⊙). Nonrotating very massive stars in the mass window 140∼
< m∗ ∼ < 260 M⊙ will disappear as pair-instability supernovae [4], leaving
no compact remnants and polluting the universe with the first heavy elements
[44][40]. Stars with 40 < m∗ < 140 M⊙ and m∗ > 260 M⊙ are predicted in-
stead to collapse to black holes with masses exceeding half of the initial stellar mass [20]. Barring any fine tuning of the initial mass function of Pop III stars, intermediate-mass black holes (IMBHs) – with masses above the 4–18 M⊙ range of known “stellar-mass” holes – may then be the inevitable endproduct of the first episodes of pregalactic star formation [29]

 

[Garth]

Yes, but the point is that they (Pop III) should exist.

Perhaps locally in our Milky Way's galactic ring? As here:Outer Disk Ring?

Initial observations suggest that stars in the ring are bluer than those found in the galaxy's thick disk (Yanny et al, 2003). Their initial metallicities are low (averaging around 2.5 percent of Sol's abundance of iron, and mostly ranging from 1.3 to 5.0 percent of Sol's) suggest that they are as scarce in elements heavier than hydrogen and helium as many thick disk and halo stars.

(Thank you hellfire for the link)

Garth