Present-day Initial Mass Function Research Articles

Transition of the initial mass function in the metal-poor environments

Abstract We study star cluster formation in a low-metallicity environment using three dimensional hydrodynamic simulations. Starting from a turbulent cloud core, we follow the formation and growth of protostellar systems with different metallicities ranging from 10−6 to 0.1 Z⊙. The cooling induced by dust grains promotes fragmentation at small scales and the formation of low-mass stars with M* ∼ 0.01–0.1 M⊙ While the number of low-mass stars increases with metallicity, when Z/Z⊙ ≳ 10−5. the stellar mass distribution is still top-heavy for Z/Z⊙ ≲ 10−2 compared to the Chabrier initial mass function (IMF). In these cases, star formation begins after the turbulent motion decays and a single massive cloud core monolithically collapses to form a central massive stellar system. The circumstellar disk preferentially feeds the mass to the central massive stars, making the mass distribution top-heavy. When Z/Z⊙ = 0.1, collisions of the turbulent flows promote the onset of the star formation and a highly filamentary structure develops owing to efficient fine-structure line cooling. In this case, the mass supply to the massive stars is limited by the local gas reservoir and the mass is shared among the stars, leading to a Chabrier-like IMF. We conclude that cooling at the scales of the turbulent motion promotes the development of the filamentary structure and works as an important factor leading to the present-day IMF.

Ultra-faint dwarf galaxies: unveiling the minimum mass of the first stars

ABSTRACT The non-detection of zero-metallicity stars in ultra-faint dwarf galaxies (UFDs) can be used to constrain the initial mass function (IMF) of the first (PopIII) stars by means of a statistical comparison between available data and predictions from chemical evolution models. To this end we develop a model that follows the formation of isolated UFDs, calibrated to best reproduce the available data for the best studied system: Boötes I. Our statistical approach shows that UFDs are the best suitable systems to study the implications of the persisting non-detection of zero-metallicity stars on the PopIII IMF, i.e. its shape, the minimum mass (mmin), and the characteristic mass (mch). We show that accounting for the incomplete sampling of the IMF is essential to compute the expected number of long-lived PopIII stars in inefficiently star-forming UFDs. By simulating the colour–magnitude diagram of Boötes I, and thus take into account the mass-range of the observed stars, we can obtain even tighter constrains on mmin. By exploiting the 96 stars with measured metallicities ($\rm i \lt 19$) in the UFDs represented by our model, we demonstrate that $m_{\mathrm{ ch}} \gt 1\: \rm {M_{\odot }}$ or $m_{\mathrm{ min}} \gt 0.8 \:\rm {M_{\odot }}$ at $99\%$ confidence level. This means that a present-day IMF for PopIII stars is excluded by our model, and a top-heavy PopIII IMF is strongly favoured. We can limit $m_{\mathrm{ min}} \gt 0.8\: \rm {M_{\odot }}$ independent of the PopIII IMF shape by targeting the four UFDs Boötes I, Hercules, Leo IV, and Eridanus II with future generation instruments, such as ELT/MOSAIC ($\rm i \lt 25$), which can provide samples of >10 000 stars.

The importance of magnetic fields for the initial mass function of the first stars

ABSTRACT Magnetic fields play an important role for the formation of stars in both local and high-redshift galaxies. Recent studies of dynamo amplification in the first dark matter haloes suggest that significant magnetic fields were likely present during the formation of the first stars in the Universe at redshifts of 15 and above. In this work, we study how these magnetic fields potentially impact the initial mass function (IMF) of the first stars. We perform 200 high-resolution, three-dimensional (3D), magnetohydrodynamic (MHD) simulations of the collapse of primordial clouds with different initial turbulent magnetic field strengths as predicted from turbulent dynamo theory in the early Universe, forming more than 1100 first stars in total. We detect a strong statistical signature of suppressed fragmentation in the presence of strong magnetic fields, leading to a dramatic reduction in the number of first stars with masses low enough that they might be expected to survive to the present-day. Additionally, strong fields shift the transition point where stars go from being mostly single to mostly multiple to higher masses. However, irrespective of the field strength, individual simulations are highly chaotic, show different levels of fragmentation and clustering, and the outcome depends on the exact realization of the turbulence in the primordial clouds. While these are still idealized simulations that do not start from cosmological initial conditions, our work shows that magnetic fields play a key role for the primordial IMF, potentially even more so than for the present-day IMF.

The Low Detection Rate of Pair–instability Supernovae and the Effect of the Core Carbon Fraction

Abstract The pair-instability supernova (PISN) is a common fate of very massive stars (VMSs). Current theory predicts initial and CO core mass ranges for PISNe of ∼140–260 and ∼65–120 M ⊙, respectively, for stars that are not much affected by the wind mass loss. The corresponding relative event rate between PISNe and core-collapse supernovae is estimated to be ∼1% for the present-day initial mass function. However, no confident PISN candidate has been detected so far, despite more than 1000 supernovae being discovered every year. We investigate the evolution of VMSs with various core carbon-to-oxygen ratios for the first time by introducing a multiplication factor to the 12C(α, γ)16O reaction rate. We find that a less massive VMS with a high X(C)/X(O) develops shell convection during the core carbon-burning phase, with which the star avoids the pair-creation instability. The second result is the high explodability for a massive VMS; i.e., a star with high X(C)/X(O) explodes with a smaller explosion energy. Consequently, the initial and CO core mass ranges for PISNe are significantly increased. Finally, a PISN with high X(C)/X(O) yields a smaller amount of 56Ni. Therefore, PISNe with high X(C)/X(O) are much rarer and fainter. This result advances the first theory to decrease the PISN event rate by directly shifting the CO core mass range.

Transition of the stellar initial mass function explored using binary population synthesis

Abstract The stellar initial mass function (IMF) plays a crucial role in the determination of the number of surviving stars in galaxies, of the chemical composition of the interstellar medium and of the distribution of light in galaxies. A key unsolved question is whether the IMF is universal in time and space. Here, we use the state-of-the-art results of stellar evolution to show that the IMF of our Galaxy made a transition from an IMF dominated by massive stars to the present-day IMF at an early phase of the Galaxy formation. Updated results from stellar evolution in a wide range of metallicities have been implemented in a binary population synthesis code, and compared with the observations of carbon-enhanced metal-poor (CEMP) stars in our Galaxy. We find that the application of the present-day IMF to Galactic halo stars causes serious contradictions with four observable quantities connected with the evolution of asymptotic giant branch (AGB) stars. Furthermore, a comparison between our calculations and the observations of CEMP stars might help us to constrain the transition metallicity for the IMF, which we tentatively set at [Fe/H] ≈−2. A novelty of the current study is the inclusion of mass-loss suppression in intermediate-mass AGB stars at low metallicity. This significantly reduces the overproduction of nitrogen-enhanced stars, which was a major problem in previous studies when using the IMF dominated by high-mass stars. Our results also demonstrate that the use of the present-day IMF for all time in chemical evolution models results in the overproduction of Type I.5 supernovae. More data on stellar abundances will help us to understand how the IMF has changed, and what caused such a transition.

ON THE INITIAL MASS FUNCTION OF LOW-METALLICITY STARS: THE IMPORTANCE OF DUST COOLING

The first stars to form in the universe are believed to have distribution of masses biased toward massive stars. This contrasts with the present-day initial mass function, which has a predominance of stars with masses lower than 1 M ☉. Therefore, the mode of star formation must have changed as the universe evolved. Such a transition is attributed to a more efficient cooling provided by increasing metallicity. Especially dust cooling can overcome the compressional heating, which lowers the gas temperature thus increasing its instability to fragmentation. The purpose of this paper is to verify if dust cooling can efficiently cool the gas, and enhance the fragmentation of gas clouds at the early stages of the universe. To confirm that, we calculate a set of hydrodynamic simulations that include sink particles, which represent contracting protostars. The thermal evolution of the gas during the collapse is followed by making use of a primordial chemical network and also a recipe for dust cooling. We model four clouds with different amounts of metals (10–4, 10–5, 10–6 Z ☉, and 0), and analyze how this property affect the fragmentation of star-forming clouds. We find evidence for fragmentation in all four cases, and hence conclude that there is no critical metallicity below which fragmentation is impossible. Nevertheless, there is a clear change in the behavior of the clouds at Z <~ 10–5 Z ☉, caused by the fact that at this metallicity, fragmentation takes longer to occur than accretion, leading to a flat mass function at lower metallicities.

Early enrichment of quasars by the first stars

Studies of the broad emission-line regions (BLRs) in quasars have revealed solar or higher enrichment levels up to the highest redshifts. In combination with the presence of large amounts of dust in QSOs at z∼ 6, this implies that substantial amounts of star formation and nucleosynthesis took place at significantly earlier epochs. Here, we examine whether a top-heavy stellar initial mass function (IMF) is indicated by current data, by modelling the contributions from different regions of the IMF, including Type Ia/II and pair instability supernovae, to the metal synthesis in BLRs. We find that, in order to reproduce the observations of roughly solar values of N/C and Fe/Mg in these objects, (i) stars with a present-day IMF are sufficient, regardless of their metallicity, (ii) zero-metallicity stars with a top-heavy IMF severely underproduce N/C, and (iii) the contribution of Type Ia SNe is not strongly required by the data. Therefore, stars of mass ∼1–40 M⊙ must have existed at z∼ 10–20, possibly coeval with any hypothesized stars of masses ≳ 100 M⊙ at these epochs. This is in agreement with the nucleosynthetic abundance pattern detected in extremely metal-poor stars in the galactic halo.

The Ionizing Efficiency of the First Stars

We investigate whether a single population of first stars could have influenced both the metal enrichment and reionization of the high-redshift intergalactic medium (IGM), by calculating the generated ionizing radiation per unit metal yield as a function of the metallicity of stellar populations. We examine the relation between the ionizing radiation and carbon created by the first stars, since the evidence for the widespread enrichment of the IGM at redshifts $z$ about 3-4 comes from the detection of C IV absorption. We find that the number of ionizing photons per baryon generated in association with the detected IGM metallicity may well exceed that required for a late hydrogen reionization at $z$ of about 6, by up to a factor of 10-20 for metal-free stars in a present-day initial mass function (IMF). This would be in agreement with similar indications from recent observations of the microwave background and the high-$z$ IGM. In addition, the contribution from intermediate-mass stars to the total metal yield, neglected in past works, substantially impacts such calculations. Lastly, a top-heavy IMF is not necessarily preferred as a more efficient high-$z$ source of ionizing radiation, based on nucleosynthetic arguments in association with a given level of IGM enrichment.

A comparison of the luminosity functions in U, B, and V and their relationship to the initial mass function for the Galaxy and the Magellanic Clouds

view Abstract Citations (141) References (38) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Comparison of the Luminosity Functions in U, B, and V and Their Relationship to the Initial Mass Function for the Galaxy and the Magellanic Clouds Blaha, Cynthia ; Humphreys, Roberta M. Abstract From our compilation of luminous O and B stars and supergiants of all spectral types, we have calculated the U, B, and V luminosity functions for stars within 3 kpc in our galaxy and the Magellanic Clouds. The luminosity functions are quite similar at all wavelengths, and the slight differences observed reflect variations in the sample size and degree of completeness. In the range -9 <= M_v_ <= -7, we find that the U, B, and V luminosity functions in both our galaxy and the Clouds have a slope of roughly + 0.7. The present-day initial mass function has also been calculated for these samples of stars. Within the uncertainties, our results are consistent with ψ(M) [the number of stars per kpc per year per log (M/M_sun_)] proportional to (M)^-2^ for the 25-85 M_sun_ range. The slopes for the Magellanic Clouds may be slightly flatter than in our galaxy, but there is probably incompleteness at the lower mass intervals. The impact of a luminous young cluster on the luminosity and mass functions was studied through the addition of stars from the 30 Doradus region to our LMC sample. Although this addition nearly doubled our population of early O stars, the U, B, and V luminosity functions remained essentially unchanged. However, the slope of the initial mass function flattened with the addition of the 30 Doradus sample. This flattening may be artificially produced by the lack of faint late O and early B stars in the 30 Dor sample which have not yet been studied. Publication: The Astronomical Journal Pub Date: November 1989 DOI: 10.1086/115244 Bibcode: 1989AJ.....98.1598B Keywords: Galactic Mass; Luminosity; Magellanic Clouds; Milky Way Galaxy; Ubv Spectra; B Stars; Galactic Evolution; O Stars; Supergiant Stars; Astrophysics; STARS: LUMINOSITY FUNCTION; GALAXIES: THE GALAXY; GALAXIES: MAGELLANIC CLOUDS full text sources ADS | data products SIMBAD (4) NED (2)