Figure 1.5: Probability density of the rotational velocities of 496 observed OB stars (masses be-tween about 3 and 20M). Figure from Maeder & Meynet (2012), originally from Huang & Gies (2006).
(2007) have shown that the fraction of Be-type stars, whose existence is possibly linked to fast rotation, increases with decreasing metallicity.
For Pop III stars, Stacy et al. (2011) estimated their rotation speed thanks to smoothed parti-cle hydrodynamic simulations. They have recorded the angular momentum of the sink partiparti-cles falling into the growing protostar. The large amount of angular momentum suggests initial ve-locities of1000km s−1or higher for stars withMini ≥30M. A caveat in this study is that they estimated the total angular momentum accreted within a radius of 50 UA from the center of the star, meaning that they do not resolve stellar scales. This prevents to see whether some angular momentum removal processes (e.g. stellar winds, disk-locking or magnetic torques) occur in the inner region.
Recently, by studying the angular momentum transfer in primordial discs including magnetic fields, Hirano & Bromm (2018) suggested that the final rotational state of Pop III protostars should exhibit a net bimodality: either the protostar do not rotate at all, or it is a fast rotator, close to breakup speed. As mentioned in their paper, this study does not properly include MHD effects, which are likely required to self-consistently assess this possible bimodality.
Rotation and its effect in low metallicity massive stars is discussed in more details in the next chapters (especially in Sect. 2.5.3 and 3.3).
1.4 Observable signatures of the early generations of massive stars
We have many images of the Universe when it was older than a billion year. With the Cosmic Microwave Background, we also have a picture of the Universe when it was only 380000 years old.
One big challenge today is to obtain pictures in between these two periods, when the primordial Universe started to evolve into the incredibly rich zoo of objects we see today. To cite Loeb (2010):
“ the situation is similar to having a photo album of a person that begins with the first ultrasound image of him or her as an unborn baby and then skip to some additional photos of his or her years as teenager and adult ”.
CHAPTER 1. INTRODUCTION
The first generations of massive stars in the Universe have specific signatures that one can look for to reveal their nature. The present work focuses on one of these signatures (the metal-poor stars) but the list below includes also other observables, for completeness.
Integrated light of high redshift galaxies
Thousands of very distant galaxies with redshift z > 5 have been observed (e.g. Bouwens et al. 2007, 2009; Oesch et al. 2010; McLure et al. 2010). The most distant object observed today is a galaxy, GN-z11, with z = 11.1−0.12+0.08, possibly formed∼ 400Myr after the Big Bang (Oesch et al. 2016). In the future, the James Webb Space Telescope (JWST) might be able to detect up to a thousand star-bursting galaxies withz >10(Pawlik et al. 2011). Their colors and spectra could be compared with predictions of the integrated light coming from such objects in order to interpret these observations in terms of stellar populations (e.g. Schaerer 2002; Salvaterra et al. 2011).
Abundance determination of high-z objects is challenging. Past and current observations man-aged to evaluate the abundances of a few elements (generally C, N, O, Si) ofz .4objects. These objects are mostly damped Lymanα (DLA) systems with 2 < z < 3 (e.g. Lehner et al. 2016). In general, the metallicity of such objects is estimated from considerations on the nebular emission lines formed in ionized gas at the sites of star formation (e.g. Pettini & Pagel 2004; Pettini 2008).
To date, the most metal-poor DLA system is atz = 3.076and has [C/H]= −3.43±0.06, [O/H]
=−3.05±0.05, [Si/H]=−3.21±0.05and [Fe/H]≤ −2.81(Cooke et al. 2017).
One difficulty regarding high-z objects is that most of the metal lines are redshifted to the infrared range (by a factor of1 +z), a regime where the sky background is high. In space, although the current telescopes cannot reach the far infrared region (the Hubble Space Telescope, HST, goes up toλ = 2.5µm), JWST should reachλ= 28 µm, which may allow to detect the metal lines of very high-zobjects.
Reionization
As the first stars formed and radiated energy, the Universe reverted from being neutral, to being ionized once again. At the epoch of reionization and before significant expansion had oc-curred, the free electron density in the Universe was high enough for Cosmic Microwave Back-ground (CMB) photons to undergo significant Thomson scattering. This let a detectable imprint on the CMB anisotropy map. Inspection of the tiny fluctuations in the CMB polarization by the Wilkinson Microwave Anisotropy Probe (WMAP) first suggested that reionization occured be-tween 11 < z < 30 (Kogut et al. 2003). Nine years of observations by WMAP have revised this result and suggested a reionization atz ∼10.4(Hinshaw et al. 2013). More recently, Planck Col-laboration et al. (2016) found that the average redshift at which reionization occured lies between z= 7.8and8.8.
Photons from reionization also altered the excitation state of the 21-cm hyperfine line of neutral hydrogen (e.g. the reviews of Morales & Wyithe 2010; Pritchard & Loeb 2012). Models predict that the 21 cm cosmic hydrogen signal will show an absorption feature at z ' 20, coming from the Lyman-α radiation of the earliest stars. The 21-cm transition is forbidden but since hydrogen amounts to∼75% of the gas mass present in the intergalactic medium, the line intensity is enough to be detected. The line frequency is ν = ν0/(1 +z) MHz with ν0 the rest-frame frequency of 1420 MHz. For redshifts6 < z < 50the corresponding frequencies are 30 < ν < 200MHz. It corresponds to the radio frequency domain, making this line is a prime target for present and future radio interferometers like the Murchison Widefield Array, the Low Frequency Array or the Experiment to Detect the Global Epoch of Reionization Signature (EDGES). Recently, Bowman et al. (2018) reported the detection (with EDGES) of such an absorption feature peaking at 78 MHz consistent with expectations for the 21-cm signal induced by stars having existed by 180 million years (z'20) after the Big Bang. Some discrepancies between this observation and models further
1.4. Observable signatures of the early generations of massive stars
suggest that an unknown interaction between dark matter and baryons occurred at early times (Barkana 2018). It may provide new clues on the nature of dark matter.
Supernovae and Gamma-Ray Bursts in distant galaxies
Until now, the Swift Gamma-Ray Burst5mission (Gehrels et al. 2004) has detected 8 Gamma-Ray Bursts (GRB) atz > 6(reported in Table 1 of Salvaterra 2015). The [S/H] ratio of the host system of GRB 050904 (z = 6.3, Kawai et al. 2006) has been inferred to be [S/H]= −1.6±0.3 (Thöne et al. 2013), suggesting a sub-solar metallicity for this object. It remains nevertheless still far from the zero or extremely low metallicity Universe. The fact that the metallicity is likely already rather high atz= 6somewhat supports the idea of a rapid enrichment in metals. The next generation of space and ground telescopes, such as JWST, the European Extremely-Large Telescope (E-ELT) or the Giant Magellan Telescope (GMT) may not be able to directly see individual first stars (Pawlik et al. 2011). Even if such stars are gravitationally lensed, their detection will remain extremely challenging (Rydberg et al. 2013). The explosion of the first massive stars, on the other hand, should be observable with the next generation of telescopes (Tanaka et al. 2013; Whalen et al.
2013a,b,c).
Gravitational waves
The first stars were likely top-heavy and retained a significant part of their mass because of weaker stellar winds. As a consequence, they could have produced massive black holes (BH) that released gravitational waves (GW) if two of them merged. Recent results suggested that the GW background coming from Pop III binary black holes mergers might be detectable by Advanced LIGO/VIRGO detectors (Kinugawa et al. 2014; Inayoshi et al. 2016).
GW150914 was the first direct detection of gravitational waves. The signal came from the coalescence of a binary BH merger whose components had masses of36+5−4 and29+4−4 M (Abbott et al. 2016). Cosmological simulations of Hartwig et al. (2016) show that there is a probability of
&1% that GW150914 is of primordial origin. These simulations also suggest that Advanced LIGO could detect∼1primordial BH-BH merger per year. The GW background from the explosion of the first massive stars might also be detectable but with the next-generation space GW detectors, like the Decihertz Interferometer Gravitational wave Observatory (DECIGO) and the Big Bang Observer (BBO, Sandick et al. 2006; Suwa et al. 2007; Marassi et al. 2009).
Pockets of primordial material
Pockets of primordial gas might be observable in the very distant Universe, but possibly also in the relatively local Universe if such pockets escaped metal pollution. Fumagalli et al. (2011) have observed two gas clouds atz ∼3with no discernible elements heavier than hydrogen. The derived upper limit for the metallicity is Z < 10−4 Z. Simcoe et al. (2012) have reported the discovery of another similar cloud at z = 7. Such gas clouds might be used as laboratories to probe the formation of metal-free stars. However, in such clouds, the stellar formation process may be different from the formation process of true Pop III stars, formed at the beginning of the Universe: in the early Universe, it is the mass of the cloud that triggers the collapse and the star formation process. In the more mature Universe, a cloud is believed to collapse because of one of several events (e.g. nearby supernovae, collision of molecular clouds...) that compress the cloud and initiate its gravitational collapse. The way the star formation process is triggered may change the initial stellar characteristics (e.g. initial mass).
5Gamma-Ray Bursts are sudden and powerful gamma-ray flashes observed in the Universe, lasting from 0.001
sec-onds to about 15 minutes.
CHAPTER 1. INTRODUCTION
Time [Gyr]
Big bang First
stars First
supernovae 2nd generation of stars
Figure 1.6: Schematic view of the chemical evolution of the Universe. Massive first stars (Pop III) formed with the products of Big Bang nucleosynthesis and released the first metals. Then, suc-cessive stellar generations formed with the ejecta of the previous generations and released new metals. The nature of the early massive stars can be inferred by observing old low-mass stars in the nearby Universe or by observing the extremely high redshift Universe.
Nucleosynthetic imprints in low-mass metal-poor stars
As discussed in Sect. 1.1.1, Big Bang nucleosynthesis formed almost only H and He. The first metals were created and released by the first stars. As time proceeded, successive generations of stars formed, evolved and exploded as supernovae, releasing more and more metals in the Universe (see Fig. 1.6 for a schematic view). A consequence is that the global metallicity in the Universe increases with time. It also implies that the metallicity of a star can be used as a proxy for its age. From this consideration follows the assumption that the most metal-poor stars are the oldest stars and then the best candidates to probe the beginning of the stellar era6. While massive metal-poor stars have short lifetimes and are then long dead, low-mass metal-poor stars that formed very early may still be alive around us, and hence observable. These low-mass and very metal-poor stars likely formed with the material ejected by the first short-lived massive stars.
On the opposite, the Sun, which was born about 9 Gyr after the Big Bang, formed with a more metal rich material, processed by different generations of stars.
The quest for the most metal-deficient stars in our neighborhood, called stellar archaeology, has started with the HK-survey (Beers et al. 1985, 1992) and the Hambourg/ESO survey (Wisotzki et al. 1996). These surveys discovered a population of metal-poor stars in the Galactic halo. A significant fraction of these stars, especially the most iron-poor, are enriched in carbon (they are called Carbon-Enhanced Metal-Poor or CEMP stars, Beers & Christlieb 2005, see also Chapter 2).
Nowadays, several thousands of metal-poor stars are known, some of them possibly having an age close to the age of the Universe (e.g. Sneden et al. 1996, 2008). This opened a new window on the first stars: the chemical composition of the most metal-poor stars delivers valuable clues on the first nucleosynthetic events that took place in the Universe, hence on the nature of early massive stars. Chapter 2 discusses metal-poor stars in more detail.
6This assumption is however challenged by recent works suggesting that more metal-rich stars (in the Galactic bulge)
are better candidates (see the next point).