Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThe Influence of Streaming Velocities on the Formation of the First Stars
109
0
0.0
(
0
)
Authors
Anna T. P. Schauer
Ask ChatGPT about the research
No Arabic abstract
How, when and where the first stars formed are fundamental questions regarding the epoch of Cosmic Dawn. A second order effect in the fluid equations was recently found to make a significant contribution: an offset velocity between gas and dark matter, the so-called streaming velocity. Previous simulations of a limited number of low-mass dark matter haloes suggest that this streaming velocity can delay the formation of the first stars and decrease halo gas fractions and the halo mass function in the low mass regime. However, a systematic exploration of its effects in a large sample of haloes has been lacking until now. In this paper, we present results from a set of cosmological simulations of regions of the Universe with different streaming velocities performed with the moving mesh code AREPO. Our simulations have very high mass resolution, enabling us to accurately resolve minihaloes as small as $10^5 : {rm M_{odot}}$. We show that in the absence of streaming, the least massive halo that contains cold gas has a mass $M_{rm halo, min} = 5 times 10^{5} : {rm M_{odot}}$, but that cooling only becomes efficient in a majority of haloes for halo masses greater than $M_{rm halo,50%} = 1.6 times 10^6 : {rm M_{odot}}$. In regions with non-zero streaming velocities, $M_{rm halo, min}$ and $M_{rm halo,50%}$ both increase significantly, by around a factor of a few for each one sigma increase in the value of the local streaming velocity. As a result, in regions with streaming velocities $v_mathrm{stream} ge 3,sigma_mathrm{rms}$, cooling of gas in minihaloes is completely suppressed, implying that the first stars in these regions form within atomic cooling haloes.
rate research
Read More
The first stars in the Universe, the so-called Population III stars, form in small dark matter minihaloes with virial temperatures $T_{rm vir} < 10^{4}$~K. Cooling in these minihaloes is dominated by molecular hydrogen (H$_{2}$), and so Population III star formation is only possible in those minihaloes that form enough H$_{2}$ to cool on a short timescale. As H$_{2}$ cooling is more effective in more massive minihaloes, there is therefore a critical halo mass scale $M_{rm min}$ above which Population III star formation first becomes possible. Two important processes can alter this minimum mass scale: streaming of baryons relative to the dark matter and the photodissociation of H$_{2}$ by a high redshift Lyman-Werner (LW) background. In this paper, we present results from a set of high resolution cosmological simulations that examine the impact of these processes on $M_{rm min}$ and on $M_{rm ave}$ (the average minihalo mass for star formation), both individually and in combination. We show that streaming has a bigger impact on $M_{rm min}$ than the LW background, but also that both effects are additive. We also provide fitting functions quantifying the dependence of $M_{rm ave}$ and $M_{rm min}$ on the streaming velocity and the strength of the LW background.
We study the influence of a high baryonic streaming velocity on the formation of direct collapse black holes (DCBHs) with the help of cosmological simulations carried out using the moving mesh code {sc arepo}. We show that a streaming velocity that is as large as three times the root-mean-squared value is effective at suppressing the formation of H$_{2}$-cooled minihaloes, while still allowing larger atomic cooling haloes (ACHs) to form. We find that enough H$_{2}$ forms in the centre of these ACHs to effectively cool the gas, demonstrating that a high streaming velocity by itself cannot produce the conditions required for DCBH formation. However, we argue that high streaming velocity regions do provide an ideal environment for the formation of DCBHs in close pairs of ACHs (the synchronised halo model). Due to the absence of star formation in minihaloes, the gas remains chemically pristine until the ACHs form. If two such haloes form with only a small separation in time and space, then the one forming stars earlier can provide enough ultraviolet radiation to suppress H$_{2}$ cooling in the other, allowing it to collapse to form a DCBH. Baryonic streaming may therefore play a crucial role in the formation of the seeds of the highest redshift quasars.
Models of the decoupling of baryons and photons during the recombination epoch predict the existence of a large-scale velocity offset between baryons and dark matter at later times, the so-called streaming velocity. In this paper, we use high resolution numerical simulations to investigate the impact of this streaming velocity on the spin and shape distributions of high-redshift minihalos, the formation sites of the earliest generation of stars. We find that the presence of a streaming velocity has a negligible effect on the spin and shape of the dark matter component of the minihalos. However, it strongly affects the behaviour of the gas component. The most probable spin parameter increases from $sim$0.03 in the absence of streaming to $sim$0.15 for a run with a streaming velocity of three times $sigma_{rm rms}$, corresponding to 1.4 km,s$^{-1}{}$ at redshift $z=15$. The gas within the minihalos becomes increasingly less spherical and more oblate as the streaming velocity increases, with dense clumps being found at larger distances from the halo centre. The impact of the streaming velocity is also mass-dependent: less massive objects are influenced more strongly, on account of their shallower potential wells. The number of halos in which gas cooling and runaway gravitational collapse occurs decreases substantially as the streaming velocity increases. However, the spin and shape distributions of gas that does manage to cool and collapse are insensitive to the value of the streaming velocity and we therefore do not expect the properties of the stars that formed from this collapsed gas to depend on the value of the streaming velocity. The spin and shape of this central gas clump are uncorrelated with the same properties measured on the scale of the halo as a whole.
The formation of globular clusters and their relation to the distribution of dark matter have long puzzled astronomers. One of the most recently-proposed globular cluster formation channels ties ancient star clusters to the large-scale streaming velocity of baryons relative to dark matter in the early Universe. These streaming velocities affect the global infall of baryons into dark matter halos, the high-redshift halo mass function, and the earliest generations of stars. In some cases, streaming velocities may result in dense regions of dark-matter-free gas that becomes Jeans unstable, potentially leading to the formation of compact star clusters. We investigate this hypothesis using cosmological hydrodynamical simulations that include a full chemical network and the formation and destruction of H$_2$, a process crucial for the formation of the first stars. We find that high-density gas in regions with significant streaming velocities -- which constitute approximately 1% of the Universe -- is indeed somewhat offset from the centers of dark matter halos, but this offset is typically significantly smaller than the virial radius. Gas outside of dark matter halos never reaches Jeans-unstable densities in our simulations. We postulate that low-level ($Z approx 10^{-3},Z_{odot}$) metal enrichment by Population III supernovae may enable cooling in the extra-virial regions, allowing gas outside of dark matter halos to cool to the CMB temperature and become Jeans-unstable. Follow-up simulations that include both streaming velocities and metal enrichment by Population III supernovae are needed to understand if streaming velocities provide one path for the formation of globular clusters in the early Universe.
The adiabatic index of H$_2,$ ($gamma_{mathrm{H_2}}$) is non-constant at temperatures between $100-10^4,mathrm{K}$ due to the large energy spacing between its rotational and vibrational modes. For the formation of the first stars at redshifts 20 and above, this variation can be significant because primordial molecular clouds are in this temperature range due to the absence of efficient cooling by dust and metals. We study the possible importance of variations in $gamma_{mathrm{H_2}}$ for the primordial initial mass function by carrying out 80 3D gravito-hydrodynamic simulations of collapsing clouds with different random turbulent velocity fields, half using fixed $gamma_{rm H_2} = 7/5$ in the limit of classical diatomic gas (used in earlier works) and half using an accurate quantum mechanical treatment of $gamma_{mathrm{H_2}}$. We use the adaptive mesh refinement code FLASH with the primordial chemistry network from KROME for this study. The simulation suite produces almost 400 stars, with masses from $0.02 - 50$ M$_odot$ (mean mass $sim 10.5,mathrm{M_{odot}}$ and mean multiplicity fraction $sim 0.4$). While the results of individual simulations do differ when we change our treatment of $gamma_{mathrm{H_2}}$, we find no statistically significant differences in the overall mass or multiplicity distributions of the stars formed in the two sets of runs. We conclude that, at least prior to the onset of radiation feedback, approximating H$_2$ as a classical diatomic gas with $gamma_{rm H_2} = 7/5$ does not induce significant errors in simulations of the fragmentation of primordial gas. Nonetheless, we recommend using the accurate formulation of the H$_2,$ adiabatic index in primordial star formation studies since it is not computationally more expensive and provides a better treatment of the thermodynamics.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
