Ruprecht-Karls-Universität Heidelberg
Research

Research

Scientific background of the STARLIGHT project

The environment of high-redshift star formation

The appearance of the first stars ended the so-called cosmic dark ages and played a key role in the metal enrichment and reionization of the Universe, thereby shaping the galaxies we see today (Bromm & Larson 2004, Glover 2005, Ciardi & Ferrara 2005, Bromm et al. 2009). Studying stellar birth in the early Universe is a relatively young area of astrophysical sciences. Only with the advent of new algorithms and powerful supercomputers did the numerical modeling of early star formation become feasible. As a consequence, there is still little consensus on the physical processes that govern the birth of stars at high redshifts. There is a debate over how and when the star formation process turns over from the intermediate to high-mass mode that is thought to dominate in the truly primordial Universe (Bromm et al. 2009) to the low-mass mode we see today (Kroupa 2002, Chabrier 2003). The field of “stellar archeology” (Beers & Christlieb 2005) attempts to address this issue by compiling observational databases of stars with extremely low metallicities in the hope to learn more about the origin of the very first and second generations of stars.
Within the STARLIGHT collaboration we will address the question of stellar birth in the early Universe with multi-scale and multi-physics theoretical models and state-of-the-art numerical calculations and compare our findings with the available observational data. It is our goal to formulate a quantitative model of the initial mass function (IMF) and other key characteristics of the first and second generations of stars in our Universe and to derive precise and reliable predictions of their observable properties and their detectability with current and future instruments.

The first generation of stars

The first generation of stars, the so-called population III (or Pop III) build up from truly metal-free primordial gas. They have long been thought to live short, solitary lives, with only one extremely massive star with about 100 solar masses or more forming in each minihalo (Abel et al. 2002, Bromm et al. 2002, Yoshida et al. 2006, O’Shea & Norman 2007). This picture needs to be revised! The introduction of numerical and theoretical concepts from present-day star formation (such as sink particles) allows us to follow the build-up and long-term evolution of accretion disks around the first stars and to model their fragmentation into dense stellar clusters (see e.g. Clark et al. 2008, 2011a, Greif et al. 2011). We have begun to realize that the gas flow in high-redshift halos is highly turbulent (e.g. Wise & Abel 2007, Greif et al. 2011, Turk et al. 2012) with important consequences for star formation (for an overview for present-day star formation, see the reviews by Mac Low & Klessen 2004, and McKee & Ostriker 2007). Recent calculations in our group (Clark et al. 2011b) strongly suggest that Pop III stars formed as members of multiple stellar systems with separations as small as the distance between the Earth and the Sun. With sufficient numerical resolution one finds that the accretion disks around the first stars are violently unstable to fragmentation, as illustrated in Figure 1 beside (see also Turk et al. 2009, and Stacy et al. 2010).

The second generation of stars

Second generation stars, sometimes termed Pop II.5 stars, have formed from material that has been enriched from the debris of the first stars. Unlike the very first stars, for which we have no direct detections yet, members of the second generation have already been found in surveys looking for extremely metal-poor stars in our Milky Way and neighboring satellite galaxies (Beers & Christlieb 2005, Frebel et al. 2007). The recent discovery of the truly primitive star SDSSJ1029151+172927 in the constellation of Leo (Caffau et al. 2011) has provided strong constraints on the physical processes that governed its formation. We know now that dust-induced fragmentation determines the mass spectrum of the second generation of stars (Omukai et al. 2005, Schneider et al. 2002, 2012, Klessen et al. 2012). Detailed simulations by Clark et al. (2008) and Dopcke et al. (2011) indeed demonstrated that dust cooling leads to a stellar mass spectrum comparable to the present-day IMF down to metallicities 10-5 -10-6 of the solar value (see also Tsuribe & Omukai 2008).

Observational constraints and expected numbers of metal-free and metal-poor stars

The direct detection of individual stars in the early Universe is next to impossible, even with the next generation of spaceborne or earthbound telescopes. High-redshift observations can provide only indirect contraints on the physical properties (mass, luminosity, frequency, etc.) of the first and second generations of stars, e.g. by looking at their influence on reionization or cosmic metal enrichment (Ciardi & Ferrara 2005). Alternative pathways are provided by high-redshift gamma-ray bursts (GRBs), which could originate from first star binaries (Bromm & Loeb 2006). The proposed high binary fraction of Pop III stars (Clark et al. 2011b) supports to this argument. Even more intriguing, if metal-free Pop III stars with masses of around half a solar mass or below exist, they must have survived until the present day. This opens up the exciting prospect of probing the earliest stages of star and galaxy formation directly in our cosmic backyard.

Until recently, the deepest survey searching for metal-poor or even metal-free stars was the Hamburg-ESO survey (Christlieb et al. 2008). With low-resolution SDSS spectra (http://www.sdss.org/) we get a better estimate of the metallicity and can select a more constraint sample of extremely metal-poor star candidates. This lead to the recent discovery of a truly pristine star with ~10-5 Zsun in the constellation of Leo (Caffau et al. 2011, 2012). The fact that it was found out of only six candidates suggests that these stars may not be as rare as previously thought. Indeed, the inferred metallicity distribution in the SDSS catalogue shows a low metallicity tail that seems to extend down to zero metallicities, hinting at the existence of a population of truly metal-free low-mass stars in the Milky Way. Clearly, this needs to be confirmed by high-resolution follow-up spectroscopy. To do so, members of the STARLIGHT consortium are part of the ESO large program ToPoS to find and characterize ~100 of the most metal-poor stars in our vicinity (PI is E. Caffau in Heidelberg). This unique database will provide valuable information about the oldest and most primitive stellar population in the Galaxy and help us to guide and evaluate our theoretical and numerical models.

Primary research objectives

The main objective of the STARLIGHT project is to understand the origin of stars in the early Universe and to determine their physical characteristics. We build our research consortium around four key questions:

  1. What are the physical processes that governed the birth and evolution of the first and second generations of stars in the Universe?
  2. What are their statistical characteristics and how do these depend on environmental conditions?
  3. What are the observational signatures of the first and second generations of stars and how can we study their properties with current earthbound and spaceborne instruments
  4. How did these stars affect their birth habitat and influence subsequent cosmic evolution?

In an international collaboration with key researchers in early star formation studies we plan to set up a cluster of expertise at Heidelberg University in this cutting-edge field of modern astrophysics and address the above questions with the help of numerical simulations complemented by high-precision observations. Our goal is to perform the first ever simulations of Pop III and Pop II.5 star formation to self-consistently model not only the formation of the initial protostar, but also the fragmentation of the halo gas and subse- quent accretion flow onto all protostars in the nascent cluster, as well as treating the effects of all relevant forms of feedback from these protostars that determine the characteristics of subsequent stellar generations.

We will set up a comprehensive theoretical and computational framework and use it to model high-redshift star formation with unprecedented detail and high predictive power. We will do so for a wide range of envi- ronmental conditions. We will make clear predictions of the physical characteristics of the first and second generations of stars and compare our results with observations of the high-redshift Universe as well as with detailed analyses of extremely metal-poor stars in the Milky Way. Within the ESO large program mentioned above we will build up an observational database of the oldest and most metal-poor stellar population which will provide guidance for our theoretical and numerical activities in STARLIGHT. All our results and tools will become openly available as part of a VO-compliant web-based data system.

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Figure 1: View down onto the accretion disk that surrounds the newly born central star at the time when a second fragment forms. Clearly visible are spiral arms of enhanced gas density which fragment to build up secondary stars (from Clark et al. 2011).