First Generation Of Stars Research Articles

On the origin of double main-sequence turn-offs in star clusters of the Magellanic Clouds

Recent observational studies of intermediate-age star clusters (SCs) in the Large Magellanic Cloud (LMC) have reported that a significant number of these objects show double main-sequence turn-offs (DMSTOs) in their colour-magnitude diagrams (CMDs). One plausible explanation for the origin of these DMSTOs is that the SCs are composed of two different stellar populations with age differences of ∼300 Myr. Based on analytical methods and numerical simulations, we explore a new scenario in which SCs interact and merge with star-forming giant molecular clouds (GMCs) to form new composite SCs with two distinct component populations. In this new scenario, the possible age differences between the two different stellar populations responsible for the DMSTOs are due largely to secondary star formation within GMCs interacting and merging with already-existing SCs in the LMC disc. The total gas masses being converted into new stars (i.e. the second generation of stars) during GMC-SC interaction and merging can be comparable to or larger than the masses of the original SCs (i.e. the first generation of stars) in this scenario. Our simulations show that the spatial distributions of new stars in composite SCs formed from GMC-SC merging are more compact than those of stars initially in the SCs. We discuss both advantages and disadvantages of the new scenario in explaining fundamental properties of SCs with DMSTOs in the LMC and in the Small Magellanic Cloud (SMC). We also discuss the merits of various alternative scenarios for the origin of the DMSTOs.

Influence of some reaction parameters on the synthesis of the first generation of stars with arms composed of poly(oxyethylene glycol)

The syntheses of the multiarm stars resulting from the reaction of oligoalcohols with diepoxides have been studied. MPEG 550,750, and 2000 [MPEG = monomethyl ether of poly(oxyethylene glycol)] as precursors of arms and diglycidyl ethers of ethylene (DGEG) or neopentyl glycol (DGNG) as the core forming diepoxides were used. The increase in M n , polydispersity (MWD) and the number of arms (f) in stars with the increase in the concentration of reagents and [diepoxide] o / [MPEG] o ratio, and the decrease in temperature were observed. The final content of unreacted MPEG lowers with increasing concentration of reagents, proportion of the alkoxide groups in the initiator ([RO - ]/[RO - + ROH]), length of MPEG chain, and [diepoxide] o /[MPEG] o ratio and also with decreasing reaction temperature.

Formation and dynamical evolution of multiple stellar generations in globular clusters

We study the formation and dynamical evolution of clusters with multiple stellar generations. Observational studies have found that some globular clusters host a population of second generation (SG) stars which show chemical anomalies and must have formed from gas containing matter processed in the envelopes of first generation (FG) cluster stars. We study the SG formation process by means of 1D hydrodynamical simulations, starting from a FG already in place and assuming that the SG is formed by the gas ejected by the Asymptotic Giant Branch (AGB) stars. This gas collects in a cooling flow into the cluster core, where it forms SG stars. The SG subsystem emerging from this process is initially strongly concentrated in the cluster innermost regions and its structural properties are largely independent of the FG initial properties. We also present the results of a model in which pristine gas contributes to the SG formation. In this model a very helium-rich SG population and one with a moderate helium enrichment form; the resulting SG bimodal helium distribution resembles that observed for SG stars in NGC 2808. By means of N-body simulations, we then study the two-population cluster dynamical evolution and mass loss. In our simulations, a large fraction of FG stars are lost early in the cluster evolution due to the expansion and stripping of the cluster outer layers resulting from early mass loss associated with FG SN ejecta. The SG population, initially concentrated in the innermost cluster regions, is largely unscathed by this early mass loss, and this early evolution leads to values of the number ratio of SG to FG stars consistent with observations. We also demonstrate possible evolutionary routes leading to the loss of most of the FG population, leaving an SG-dominated cluster. As the cluster evolves and the two populations mix, the local ratio of SG to FG stars, initially a decreasing function of radius, tends to a constant value in the inner parts of the cluster. Until mixing is complete, the radial profile of this number ratio is characterized by a flat inner part and a declining portion in the outer cluster regions.

The Cassiopeia–Perseus open cluster family

The observed distribution of young open clusters is far from uniform. Statistics shows that, when age, spatial distribution and kinematics are considered simultaneously, they tend to appear in clumps. These young cluster groups or families constitute unambiguously coeval, genetically related complexes associated to the underlying spiral structure. In this paper, we derive detailed physical properties for one of them: the Cassiopeia–Perseus family. With a diameter of about 600 pc, it is located 2 kpc from the Sun, embedded in the Perseus arm, and probably includes 10–20 members. It began to form 20–40 Myr ago although we find distinctive evidence for at least three generations of star formation organized in two distinct fronts, with the oldest clusters located at lower Galactic longitude than the youngest. The plane roughly defined by the structure is inclined ∼30° to the Galactic disk with most candidate members located below the disk and moving away from it. Our results for this cluster of clusters suggest that, within a coherent cloud complex, the first generation of star formation is triggered by the shock wave induced by a spiral arm. The second and subsequent generations are sustained by ionization fronts and supernova shocks created by the evolution of the first generation of massive stars. In this particular case, the front moves with average velocity of about 70 km/s in the direction of increasing Galactic longitude. The Cassiopeia–Perseus family and related objects appear to be a close relative of the cluster complexes found in the spiral galaxy M51 or perhaps a younger analog of the Gould Belt.

Protostar Formation in the Early Universe

The nature of the first generation of stars in the universe remains largely unknown. Observations imply the existence of massive primordial stars early in the history of the universe, and the standard theory for the growth of cosmic structure predicts that structures grow hierarchically through gravitational instability. We have developed an ab initio computer simulation of the formation of primordial stars that follows the relevant atomic and molecular processes in a primordial gas in an expanding universe. The results show that primeval density fluctuations left over from the Big Bang can drive the formation of a tiny protostar with a mass 1% that of the Sun. The protostar is a seed for the subsequent formation of a massive primordial star.

The search for Population III stars

AbstractPopulation III stars, the first generation of stars formed from primordial Big Bang material with a top–heavy IMF, should contribute substantially to the Universe reionization and they are crucial for understanding the early metal enrichment of galaxies. Therefore it is very important that these objects, foreseen by theories, are detected by observations. However PopIII stars, searched through the HeII 1640Å line signature, have remained elusive. We report about the search for the HeII line in a galaxy at z = 6.5, which is a very promising candidate. Unfortunately we are not yet able to show the results of this search. However we call attention to the possible detection of PopIII stars in a lensed HII dwarf galaxy at z = 3.4, which appeared in the literature some years ago, but has been overlooked.

Isolated OB Associations in Stripped HiGas Clouds

HST ACS HRC images in UV (F250W), V (F555W), and I (F814W) resolve three isolated OB associations that lie up to 30 kpc from the stellar disk of the S0 galaxy NGC 1533. Previous narrowband Hα imaging and optical spectroscopy showed these objects as unresolved intergalactic H II regions having Hα luminosities consistent with single early-type O stars. These young stars lie in stripped H I gas with column densities ranging from 1.5 to -->2.5 × 1020 cm -->−2 and velocity dispersions near 30 km s -->−1. Using the HST broadband colors and magnitudes along with previously determined Hα luminosities, we place limits on the masses and ages of each association, considering the importance of stochastic effects for faint ( -->MV > − 8) stellar populations. The upper limits to their stellar masses range from 600 to 7000 -->M☉, and ages range from 2 to 6 Myr. This analysis includes an updated calculation of the conversion factor between the ionizing luminosity and the total number of main-sequence O stars contained within an H II region. The photometric properties and sizes of the isolated associations and other objects in the HRC fields are consistent with those of Galactic stellar associations, open clusters, and/or single O and B stars. We interpret the age-size sequence of associations and clustered field objects as an indication that these isolated associations are most likely rapidly dispersing. Furthermore, we consider the possibility that these isolated associations represent the first generation of stars in the H I ring surrounding NGC 1533. This work suggests star formation in the unique environment of a galaxy's outermost gaseous regions proceeds similarly to that within the Galactic disk and that star formation in tidal debris may be responsible for building up a younger halo component.

High‐Energy Neutrino Signals from the Epoch of Reionization

We perform a new estimate of the high energy neutrinos expected from GRBs associated with the first generation of stars in light of new models and constraints on the epoch of reionization and a more detailed evaluation of the neutrino emission yields. We also compare the diffuse high energy neutrino background from Population III stars with the one from "ordinary stars" (Population II), as estimated consistently within the same cosmological and astrophysical assumptions. In disagreement with previous literature, we find that high energy neutrinos from Population III stars will not be observable with current or near future neutrino telescopes, falling below both IceCube sensitivity and atmospheric neutrino background under the most extreme assumptions for the GRB rate. This rules them out as a viable diagnostic tool for these still elusive metal-free stars.

First stars XI. Chemical composition of the extremely metal-poor dwarfs in the binary CS 22876-032

Context.Unevolved metal-poor stars constitute a fossil record of the early Galaxy, and can provide invaluable information on the properties of the first generations of stars. Binary systems also provide direct information on the stellar masses of their member stars.

NRO 45 M and ASTE 10 M observations of dense gas in giant HIIregions of M 33

Wepresent 12 CO( J = 3–2) and 12 CO( J = 1–0) observations of the supergiant Hii region NGC 604 in the nearest face-on spiral galaxy M 33 using the Atacama Submillimeter Telescope Experiment (ASTE) 10-m and the Nobeyama Radio Observatory (NRO) 45-m telescopes. We found high 12 CO( J = 3–2)/ 12 CO( J = 1–0) ratio gas with an arc-like distribution (“high-ratio gas arc”) surrounding the central star cluster of NGC 604. Our results suggest that dense gas formation and second-generation star formation occur in the surrounding gas compressed by the stellar wind and/or supernova of the first-generation stars of NGC 604, i.e. , the central star cluster of NGC 604. Thus, NGC 604 is an example of large-scale sequential star formation.

The First Stars

AbstractThe formation of the first generations of stars at redshifts z ≥ 15 − 20 signaled the transition from the simple initial state of the universe to one of ever increasing complexity. We here review recent progress in understanding the assembly process of the first galaxies, starting with cosmological initial conditions and modelling the detailed physics of star formation. In particular, we study the role of HD cooling in ionized primordial gas, the impact of UV radiation produced by the first stars, and the propagation of the supernova blast waves triggered at the end of their brief lives. We conclude by discussing how the chemical abundance patterns observed in extremely low-metallicity stars allow us to probe the properties of the first stars.

Dust distribution during reionization

Context. The dust produced by the first generation of stars will be a foreground to cosmic microwave background. Aims. In order to evaluate the effect of this early dust, we calculate the power spectrum of the dust emission anisotropies and compare it with the sensitivity limit of the Planck satellite. Methods. The spatial distribution of the dust is estimated through the distribution of dark matter. Results. At small angular scales (� 1000) the dust signal is found to be noticeable with the Planck detector for certain values of dust lifetime and production rates. The dust signal is also compared to sensitivities of other instruments. The early dust emission anisotropies are finally compared to those of local dust and they are found to be similar in magnitude at mm wavelengths.

An observational limit on the earliest gamma-ray bursts

We predict the redshift of the first observable (i.e. in our past light cone) gamma-ray burst (GRB) and calculate the GRB rate redshift distribution of the Population III stars at very early times (z = 20-60). Using the last two year of data from Swift, we place an upper limit on the efficiency (nGRB) of GRB production per solar mass from the first generation of stars. We find that the first observable GRB is most likely to have formed at redshift 60. The observed rate of extremely high redshift GRBs (XRGs) is a subset of a group of 15 long GRBs per year, with no associated redshift and no optical afterglow counterparts, detected by Swift. Taking this maximal rate, we get that nGRB < 1.1 x 10 -3 GRBs per solar mass in stars. A more realistic evaluation, for example, taking a subgroup of 5 per cent of the total sample of Swift gives an upper limit of n GRB <3.2 x 10 -4 GRBS per solar mass.

Local Radiative Feedback in the Formation of the First Protogalaxies

The formation of the first galaxies is influenced by the radiative feedback from the first generations of stars. This feedback is manisfested by the heating and ionization of the gas which lies within the H II regions surrounding the first stars, as well as by the photodissociation of hydrogen molecules within the larger Lyman-Werner (LW) bubbles that surround these sources. Using a ray-tracing method in three-dimensional cosmological simulations, we self-consistently track the formation of, and radiative feedback from, individual stars in the course of the formation of a protogalaxy. We compute in detail the H II regions of each of these sources, as well as the regions affected by their molecule-dissociating radiation. We follow the thermal, chemical, and dynamical evolution of the primordial gas, as it becomes incorporated into the protogalaxy. While the IGM is, in general, optically thick to LW photons only over physical distances of > 30 kpc at redshifts z < 20, the high molecule fraction that is built up in relic H II regions and their increasing volume-filling fraction renders even the local IGM optically thick to LW photons over physical distances of the order of a few kiloparsecs. We find that efficient accretion onto Population III relic black holes may occur after ~ 60 Myr from the time of their formation, by which time the photo-heated relic H II region gas can cool and re-collapse into the 10^6 M_solar minihalo which hosts the black hole. Also, Pop II.5 stars, postulated to have masses of the order of 10 M_solar, can likely form from this re-collapsing relic H II region gas. Overall, we find that the local radiative feedback from the first generations of stars suppresses the star formation rate by only a factor of, at most, a few.

Arclike Distribution of High CO ([FORMULA][F]J=3-2[/F][/FORMULA])/CO ([FORMULA][F]J=1-0[/F][/FORMULA]) Ratio Gas Surrounding the Central Star Cluster of the Supergiant H ii Region NGC 604

We report the discovery of a high CO(J=3-2)/CO(J=1-0) ratio gas with an arc-like distribution (``high-ratio gas arc'') surrounding the central star cluster of the supergiant HII region NGC 604 in the nearby spiral galaxy M 33, based on multi-J CO observations of a 5' $\times$ 5' region of NGC 604 conducted using the ASTE 10-m and NRO 45-m telescopes. The discovered ``high-ratio gas arc'' extends to the south-east to north-west direction with a size of $\sim$ 200 pc. The western part of the high-ratio gas arc closely coincides well with the shells of the HII regions traced by H$\alpha$ and radio continuum peaks. The CO(J=3-2)/CO(J=1-0) ratio, R_{3-2/1-0}, ranges between 0.3 and 1.2 in the observed region, and the R_{3-2/1-0} values of the high-ratio gas arc are around or higher than unity, indicating very warm (T_kin > 60 K) and dense (n(H_2) > 10^{3-4} cm^{-3}) conditions of the high-ratio gas arc. We suggest that the dense gas formation and second-generation star formation occur in the surrounding gas compressed by the stellar wind and/or supernova of the first-generation stars of NGC 604, i.e., the central star cluster of NGC 604.

Three-body recombination of hydrogen during primordial star formation

We consider the formation and destruction of H 2 and HD during the gravitational contraction of condensations of the primordial gas, which led to the formation of the first generation of stars (Population III stars). The determination of the populations of the bound rovibrational levels of molecular hydrogen is considered in detail. Initially, the rates per unit volume at which these levels are populated and depopulated are not in equilibrium. As the density increases, equilibrium between the rates of population and depopulation is established first, and then the levels gradually thermalize (i.e. their populations tend towards a Boltzmann distribution at the kinetic temperature of the gas), with the lowest energy levels thermalizing first. Ultimately, both the bound and the continuum states thermalize (i.e. attain a Saha distribution), but this process is not complete until densities n H ≈10 13 cm -3 are reached. Using the principle of microscopic reversibility, we derive an expression for the rate coefficient for three-body recombination of hydrogen which is found to differ significantly from the much used expression of Jacobs et al.

The first generation of stars in the cold dark matter cosmology

We have performed a large set of high-resolution cosmological simulations using smoothed particle hydrodynamics (SPH) to study the formation of the first luminous objects in the {Lambda}CDM cosmology. We follow the collapse of primordial gas clouds in eight early structures and document the scatter in the properties of the first star-forming clouds. Our first objects span formation redshifts from z {approx} 10 to z {approx} 50 and cover an order of magnitude in halo mass. We find that the physical properties of the central star-forming clouds are very similar in all of the simulated objects despite significant differences in formation redshift and environment. This suggests that the formation path of the first stars is largely independent of the collapse redshift; the physical properties of the clouds have little correlation with spin, mass, or assembly history of the host halo. The collapse of proto-stellar objects at higher redshifts progresses much more rapidly due to the higher densities, which accelerates the formation of molecular hydrogen, enhances initial cooling and shortens the dynamical timescales. The mass of the star-forming clouds cover a broad range, from a few hundred to a few thousand solar masses, and exhibit various morphologies: some have disk-like structures which are nearly rotational supported; others form flattened spheroids; still others form bars. All of them develop a single protostellar ''seed'' which does not fragment into multiple objects up to the moment that the central gas becomes optically thick to H{sub 2} cooling lines. At this time, the instantaneous mass accretion rate onto the centre varies significantly from object to object, with disk-like structures having the smallest mass accretion rates. The formation epoch and properties of the star-forming clouds are sensitive to the values of cosmological parameters.

A star cluster at the edge of the Galaxy

We study stars and molecular gas in the direction of IRAS06145+1455 (WB89-789) through NIR (JHK), molecular line-, and dust continuum observations. The kinematic distance of the associated molecular cloud is 11.9 kpc. With a galactocentric distance of about 20.2 kpc, this object is at the edge of the (molecular) disk of the Galaxy. The near-IR data show the presence of an (embedded) cluster of about 60 stars, with a radius ca. 1.3 pc and an average stellar surface density of ca. 12 pc^{-2}. We find at least 14 stars with NIR-excess, 3 of which are possibly Class I objects. The cluster is embedded in a 1000 Mo molecular/dust core, from which a molecular outflow originates. The temperature of most of the outflowing gas is < 40 K, and the total mass of the swept-up material is < 10 Mo. Near the center of the flow, indications of much higher temperatures are found, probably due to shocks. A spectrum of one of the probable cluster members shows a tentative likeness to that of a K3III-star (with an age of at least 20 Myr). If correct, this would confirm the kinematic distance. This cluster is the furthest one from the Galactic center yet detected. The combination of old and recent activity implies that star formation has been going on for at least 20 Myr, which is difficult to understand considering the location of this object, where external triggers are either absent or weak, compared to the inner Galaxy. This suggests that once star formation is occurring, later generations of stars may form through the effect of the first generation of stars on the (remnants of) the original molecular cloud.

From First Stars to the Spite Plateau: A Possible Reconciliation of Halo Stars Observations with Predictions from Big Bang Nucleosynthesis

Since the pioneering observations of Spite & Spite in 1982, the constant lithium abundance of metal-poor ([Fe/H]<-1.3) halo stars near the turnoff has been attributed a cosmological origin. Closer analysis revealed that the observed abundance lies at $\Delta$ Li~0.4 dex below the predictions of Big Bang Nucleosynthesis. The measurements of deuterium abundances on the lines of sight toward quasars and the recent data from the Wilkinson Microwave Anisotropy Probe independently confirmed this gap. We suggest here that part of the discrepancy is explained by the first generation of stars that efficiently depleted lithium. Assuming that the models for lithium evolution in the halo turnoff stars and $\Delta$ Li estimates are correct, we infer that between 1/3 and 1/2 of the baryonic matter of the early halo (~10^9 Mo) was processed through Population III stars. This new paradigm proposes a very economical solution to the lingering difficulty of understanding the properties of the Spite Plateau and its lack of star-to-star scatter down to [Fe/H]=-2.5. It is moreover in agreement both with the absence of lithium detection in the most metal-poor star presently known (HE 1327-2326), and also with new trends of the Plateau suggesting its low metallicity edge may be reached around [Fe/H]=-2.5. We discuss the turbulent mixing associated with enhanced supernovae explosions in the early interstellar medium in this picture. Finally we show that other chemical properties of the extremely metal-poor stars are in agreement with a significant Population III processing in the halo, provided these models include mass-loss and rotationally-induced mixing.

Formation of Primordial Stars in a ΛCDM Universe

We study the formation of the first generation of stars in the standard cold dark matter model, using a very high-resolution hydordynamic simulations. Our simulation achieves a dynamic range of 10^{10} in length scale. With accurate treatment of atomic and molecular physics, it allows us to study the chemo-thermal evolution of primordial gas clouds to densities up to n = 10^{16}/cc without assuming any a priori equation of state; a six orders of magnitudes improvement over previous three-dimensional calculations. All the relevant atomic and molecular cooling and heating processes, including cooling by collision-induced continuum emission, are implemented. For calculating optically thick H2 cooling at high densities, we use the Sobolev method. To examine possible gas fragmentation owing to thermal instability, we compute explicitly the growth rate of isobaric perturbations. We show that the cloud core does not fragment in either the low-density or high-density regimes. We also show that the core remains stable against gravitational deformation and fragmentation. We obtain an accurate gas mass accretion rate within a 10 Msun innermost region around the protostar. The protostar is accreting the surrounding hot gas at a rate of 0.001-0.01 Msun/yr. From these findings we conclude that primordial stars formed in early minihalos are massive. We carry out proto-stellar evolution calculations using the obtained accretion rate. The resulting mass of the first star is M_ZAMS = 60-100 Msun, with the exact mass dependent on the actual accretion rate.