UNIVERSE@HOME: ULX’S, GR SOURCES, SNIA PROGENITORS
PI: Krzysztof Belczynski (Warsaw University)

This proposal aims to create the first database of the simulated stellar content of the Universe,
from the earliest stars to the most exotic black hole binaries. This comprehensive stellar library will
be a world-wide scientific resource for astronomers, and an educational asset for the public. Witha state-of-the-art suite of computational tools, we can calculate the physical properties of individual
stars, evolve them in time, and track how they interact with each other as they age. With this libraryof stellar populations that spans the known range of star formation histories and initial elemental
abundances, it is possible to tackle fundamental science questions that have been out of reach ingeneral relativity, astronomy, and cosmology. As a group we will use this database to characterize thepopulation of supernova progenitor stars that have been used to discover the mysterious dark energythat accelerates the expansion of the Universe; we can calculate how (if at all) binary evolution
facilitates the formation of ultra-luminous X-ray sources, and we can model the number and physicalproperties of neutron stars and black holes that strongly emit gravitational waves that are at theheart of research into gamma-ray bursts – the most energetic explosions known-to-date. The datathat we create can be mined not only to match current observations, but to predict how the sky
will appear to future astronomical observatories as new windows to the Universe open. Altogether,
this proposal will allow us to consolidate a strong and highly visible research group; an independent
computational center that trains a new cohort of highly educated students, and communicates withthe public via innovative science venues, and offer a fresh look at the Universe.

the remaining half is found at solar composition or
higher (Z >∼ Z⊙ ) [7]. In the bottom panel of Figure
1, we show a quantitative picture of star formation
history with its uncertainties [8].
The environment of Population I stars varries aswell. Typically, stars most commonly form as single
objects or in binaries, although higher multiplets are
known to exist. Even the dynamics of these stars aredifferent: In galaxies, most stars do not directly interact with other stars – these are the field population.
However, a small number of stars form in dense stellar
clusters, and here dynamical interactions such as collisions are frequent [9,10]. The fact that today’s stars
form and evolve in such varied circumstances meansthat they produce a wealth of information that probes
an extraordinary range of of physics. Stars emit light
across the entire electromagnetic spectrum, from radio to hard gamma rays, and even generate gravitational radiation. Various stages of stellar evolution
are connected with explosive events: X-ray flares frommagnetic low mass stars [11]; luminous blue variable
eruptions from massive single stars [12]; type Ia supernovae from binary white dwarfs [13]; X-ray bursts
from binary neutron stars [14]; gravitational radiation
bursts from mergers of neutron stars and black holes[15]; long gamma-ray bursts (GRBs) from massive,
rapidly rotating stars [16], and short GRBs from double neutron stars and black hole-neutron star mergers[17].
A number of stellar phenomena are still poorly understood and repeatedly defy our attempts to explain

1. OBJECTIVES

1.1. Context
Stars are the basic building blocks of galaxies that,in turn, make up the bulk of the baryon content of
the Universe. The first stars formed very early—
roughly 100 Myr after the Big Bang —out of the primordial hydrogen-helium mix. These stars, referred
to as Population III stars, were likely extremely massive (M ∼ 1000M⊙ ) and produced the first heavy
elements through nuclear burning and supernova explosions. These first supernovae created black holes
with masses between 10−1000M⊙ [1,2,3]. Thus, thesefirst stars provided not only the seed black holes forthe eventual formation of supermassive black holes(Mbh ∼ 106 − 109 M⊙ ), but also the metals that we
see in the second generation of stars. These stars,
known as Population II stars, began forming after the
Population III era from gas that was already polluted
with heavy elements [4]. Still, the Sun has 104 times−4
more metals than this population ( Z >∼ 10 Z⊙ ).
Population II stars most likely generated the photons
necessary to re-ionize the Universe, and set the chemical composition of the Universe we observe today.
At about 1-2 Gyr after the Big Bang, Population
I stars began forming. These stars have metallicities
comparable to that of the Sun, and presently dominate the overall star formation in the Universe [5,6].
Although most stars today are in this category, thispopulation is in no way a uniform sample. For example, the metallicity varies widely: about 50% of starformation in the past Gyr occurs at Z ∼ 0.2Z⊙ , while12
them. Our library of models will help us to solve, orat least constrain these mysteries. At the most basic
level, astronomers still lack a full description of starformation [18], convection [19], mass loss [20] and supernova explosions [21]. Since the amount of heavy elements released in stellar winds, supernovae and compact object mergers change how stars interact witheach other and their environment, these uncertainties
propagate into modeling the chemical evolution of the
Universe. We will develop physics that models these
phenomena which we will publish to facilitate comparisons with current and future observations.
As just one example of the power of this database,consider the following to put things into perspective.
Although there is a consensus that the collapse ofa massive stellar core powers a long GRB, the issueof why some stars produce GRBs and others do not
is still debated [22,23]. Currently, we do not know
which stars produce these powerful explosions [24].
The problem is even more pronounced for the case of
short GRBs where there are literally tens of alternatives to the favored merger model. Short GRBs arethought to be very distant; one is the most distant object in the Universe [25,26]. It is commonly assumed
that these most distant GRBs originate from Population III stars, and yet the debate ensued on whether
they were the result of NS-NS/BH-NS mergers or collapsars. Using our synthetic stellar database, we cangenerate stellar populations at high redshifts and incorporate the uncertainties of primordial star formation, which will provide astrophysically-based answers
for the origin and nature of these distant explosions(see Fig. 1, top panel).
1.2. Project Overview
We propose to generate synthetic data on stars andstellar remnants. The data will be the result of numerical simulations that will include detailed stellar evolutionary calculations and population synthesis predictions. The data will cover known populations (e.g., high- and low-mass X-ray binaries, double
neutron stars) that could be used for calibration andcomparisons, as well objects that are yet undetected(e.g., Population III stars and double black holes).
The project will be divided into three specific sciencetasks/categories that will be conducted in parallel,
complementing and guiding each other. Each task
will be performed with one or more young team members and external advise from a senior scientist fromabroad.
In the following section we outline three key issues that will be studied within the framework of theSynthetic Universe library during the funding period. Of course, we expect the library to extend wellbeyond the funding time. The database will be em-

ployed by small and large collaborations, that may or
may not include the local Warsaw team, and our external collaborators around the world. The data that
GRB 080913
GRB 090423

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Fig.1 – Bottom panel: Overall star formation
rate (SFR) history: all stars. We also show
the rate we obtained for PopII stars only: thepopulation is rather different for a slow (solid)
and fast (dashed line) metallicity evolution
model. The models bracket the uncertainty
on metal enrichment in the Universe. PopIII(first) stars and PopI (most recent) stars form
to the right and left of PopII, respectively. It isclearly seen that independent of the metallicity
evolution, the majority of the stars are PopIIat z <∼ 10 – this is the redshift of the farthest
spectroscopically confirmed objects in the Universe: GRB 080913 (z = 6.7) and GRB 090423(z = 8.1). Top panel: Predicted Swift detection
rates of short and long GRBs. Rates are for
GRBs originating exclusively from PopII stars.
GRB 080913 and GRB 090423 have PopII progenitors, and are both long bursts resulting
from collapsars (Belczynski et al. 2010 [8]).
we will obtain for any of the science projects will
be available in the database for any group to perform3
their own analyses and studies.
The projects within the database will not be, by
any means, limited to the list discussed below, but
represent specific interests of the team members. At
the moment, the initial results from our library are
already being used by the Einstein Telescope teamfor mock data challenges (Tania Regimbau, Universityof Nice, France), to study nucleosynthetic yields andsubsequent Galactic metal enrichment from neutronstar mergers (Stephan Rosswog, Albanova University,
Sweden; Thomas Janka, Max Planck Institute for Astrophysics, Germany) and for studies of Galactic microlensing on black hole binaries (Jeremy Schnittman,
NASA, USA).
1.3. Ultraluminous X-ray Sources
The ULX is defined as non-nuclear (not a central galactic object) X-ray point source that has an
isotropic X-ray luminosity higher than a few times
1039 erg/s. This definition has roots in early X-ray
observations (Einstein, ROSAT, ASCA) that haverevealed a population of unexpectedly bright extragalactic sources [27,28]. Since ULXs are point sourcesthat are not associated with their host galaxy centers, accreting neutron stars or black holes in close
binary systems seem to be the most natural candidates. However, the X-ray luminosity of these sources
is so high that it exceeds the critical Eddington luminosity of what was believed to be the maximum massof a stellar origin black hole. The Eddington luminosity can be written as LEdd = 1.3 × 1038 (M/M⊙ )erg/s, and it denotes the maximum bolometric luminosity attained by an accreting object of mass M. For
accreting neutron stars and black holes, most of thepower comes in the X-ray band and thus X-ray luminosity is a good approximation of bolometric luminosity. The first known stellar black holes were foundin our Galaxy and were determined to have masses
within the range 5 − 15M⊙ [29]. Therefore, the maximum luminosity that can be provided by an accreting
black hole (15M⊙ ) of the stellar origin could reach
only LEdd = 2 × 1039 erg/s. Any accreting sourcethat exceeds this limit is presently classified as an exceptional source under the ULX category.
Recent years have brought wealth of information.
Targeted programs with new generation satellites(Chandra, XMM-Newton, Suzaku, Swift) delivered
detailed ULX population X-ray spectral and timing
properties and initiated multi-band searches for theircounterparts [30]. Recent catalogs contain more than
500 point sources with X-ray luminosities as high as1039 −1042 erg/s [31,32,33]. Some ULXs exhibit strong
X-ray variations on timescales of minutes confirming
their compact nature [34,35]. The combination of
the luminosity and the compactness arguments im-

plies that accreting black holes within the mass range
10 − 10, 000M⊙ are powering ULXs. Although, somesmall contamination of the ULX catalogs by othersources cannot be excluded without extensive observational follow up. The most likely contamination is
expected from background AGNs (can be found inentire ULX luminosity range; [36]), young supernovae(Type IIn may reach 1040 erg/s; [37]) and young X-ray
pulsars (may slightly exceed 1039 erg/s; [38]). Thatsaid, most observational time and theoretical effort isdedicated to study the majority of sources that are
powered by accreting black holes. On one hand, observers provide luminosity functions [39], host galaxy
information [32], correlations between the ULX numbers and star formation/metallicity [40,41] in addition
to the multi-wavelength temporal spectral characteristic of sources [42,43,44]. On the other, theorists develop sophisticated models of accreting black holesthat include supercritical inflows, beamed emission,and outflows that can interact with the surrounding medium producing shocked nebulae [45,46,47,48].
However, despite the observational and theoretical advances the nature of ULX sources remains unknown.
The most straightforward interpretation of the ultra high X-ray luminosities comes from the Eddington
limit. If the limit is in fact not violated it means that
ULXs host black holes with masses in the range 1010,000 M⊙ . If it is further assumed that stellar black
holes can only reach mass of 10 − 20M⊙ as observedin the Galaxy, it follows that ULXs host an intermediate mass black hole population of unspecified (mostlikely dynamical formation) origin [49,50,51]. Thisidea became very popular, as it would provide themissing link between the stellar mass black holes andsupermassive black holes residing in center of various
galaxies. There are three major caveats to this interpretation. First, there are models of ”leaky” accretion disks with photon bubbles that can violate the
Eddington limit by a factor of 10 [52]. Second, theULX X-ray luminosity may be overestimated by another factor of 10 due to emission anisotropies causedby the beaming expected for high accretion rate binaries [53]. Third, the stellar mass black holes cansignificantly exceed the typical mass assumed in theabove discussion. It was recently demonstrated that if
updated wind mass loss rates are employed the stars
in low metallicity environment can form black holes
as massive as 80M⊙ [54]. This gives another factor of
about 10 in potential increase of luminosity for stellar
mass black holes. If all the above factors are combined(increase by factor of ∼ 1000), it is potentially possible, that stellar mass black holes, can explain eventhe brightest ULXs (1039 → 1042 erg/s).
Some of the above ideas were already utilized in
the ULX studies. It was argued for stellar mass black4hole origin combining the beaming and leaky disks
to explain the ULX sources with luminosities upto
1041 erg/s [55]. They could not reach the most extreme luminosities (1042 erg/s), since at the time of
their study stellar mass black holes were not known tohave masses over 10 − 20M⊙ . It is worth noting thatthe same year the so far most massive known stellar
black hole (30M⊙ ) was discovered in low metallicity
galaxy IC10 [56,57]. A few years later the formationof this most massive known black hole was explained
and used to predict the maximum mass of stellar black
holes (80M⊙ ) by our group [54]. A number of ULX
papers followed. In the first approximation, it was
postulated that if a very massive stellar origin blackhole accretes from a binary companion all (or almostall) ULX sources can be explain without invoking
intermediate mass black holes [40,58,59]. However,these results were challenged by the detailed population synthesis calculations implying that the chancefor the formation of a ULX source consisting of a very
massive stellar origin black hole and a mass transferring companion is close to nil [60,61]. This bringsback the heated conundrum: do the most luminous
ULX sources have to be powered by intermediatemass black holes or could they potentially host stellar
mass black holes?
Here, we propose the final solution to this issue.
We will test whether such evolutionary channels that
produce the brightest ULX sources powered by stellar
origin black holes exist. We will employ the only population synthesis code (StarTrack [62,63]) that at thismoment allows self-consistently for the formation of
the very massive stellar origin black holes. We will incorporate the relevant beaming and leaky disk physics
into the underlying physical model and test whether it
is possible to form a stellar black hole ULX with X-ray
luminosity reaching 1042 erg/s. The negative answer
will indicate that indeed the most luminous ULXs,
like 1042 erg/s source HLX-1 in ESO234-49 [64], must
be powered by an intermediate-mass black hole. The
positive answer will allow for the possibility that eventhe brightest ULXs may be powered by stellar origin
black holes. Obviously, our proof-of-principle study
will not be able to rule out the intermediate-mass
black hole ULX origin. However, if the answer toour hypothesis is positive, we will provide a detailed
ULX formation rate estimate. Comparison with theobserved number of ULXs in the local Universe with
our predicted number of stellar origin ULXs will indicate what fraction (potentially all?) of sources are
powered by stellar mass black holes.
The final comment is of somewhat political and of
a sensitive nature. However this point needs to bemade in order to justify the proposed line of the research. There are claims [60,61] that it is unlikely fora very massive stellar black holes to form a ULX thatare based on StarTrack population synthesis calculations. These calculations were performed on a sample
of several million massive binaries. Such a sample
corresponds to about 10% of the stellar content of alarge galaxy (e.g., Milky Way). Such a sample may
be considered relevant for most population synthesis
studies, provided that a considered population consists of tens of sources per a large galaxy. This is satisfied for most known stellar populations and such an
approach is readily justified for such cases. However,this is fundamentally different in the case of ULXs.
For one, there is no ULX in our Galaxy, and it takes
many galaxies to produce just one. In other words,
ULXs are extremely rare objects. Therefore, any population synthesis study that is focused on ULX population, needs to be performed on a very large stellar
sample. This is to ensure that, if in fact ULXs form
from binary stars, the very rare formation channels
are properly identified. One cannot simply perform
calculations on a typical population synthesis sample (although it may seem very large), find no bright
ULXs (as expected) and then extrapolate the results
to the local Universe. This is why PI of this proposal(K.Belczynski) removed his name from the author listof [60]. Here, we propose a suite of massive population synthesis calculations that will encompass a number of large galaxies, and therefore it will be ensured
that the conclusions are statistically and scientificallymeaningful.
We are also planning to undertake a study of very
massive stars within Population I/Population II stars.
Recent observations [65,66] demonstrate that stars
with significant amount of metals (30%Z⊙ ; LMC) canreach mass upto ∼ 300M⊙ . Such a possibility wasnever considered in ULX nor gravitational radiation
studies as it defies the current theoretical understanding of star formation (maximum star mass of about∼ 100M⊙ ). We will extend the Initial Mass Function(IMF) up to (or over) the currently observed limits tostudy the effects of very massive stars on ULX anddouble compact object populations. The initial estimates, performed within a large international collaboration (Duncan Brown: co-chair of LIGO coalescence group, Cole Miller: LIGO/VIRGO oversightcommittee, Chris Fryer: Los Alamos astrophysicist;
Alessandra Bunanno: the black hole relativity keyperson + Warsaw Observatory group) indicate that
despite the very low numbers (steep IMF) these very
massive stars forming intermediate-mass black holes
may totally alter the consensus on the impact of thestellar origin of black holes [67].
1.4. Gravitational Radiation Signature

5
Double compact objects, binaries consisting of black
holes, neutron stars and white dwarfs in various combinations, are of special interest in context of current
ground-based (LIGO/VIRGO) and near future (DECIGO, Einstein Telescope, eLISA) space based gravitational radiation observatories. Double compact objects are the primary sources for ground based interferometers while they will constitute the foregroundnoise for some space instruments, limiting observations of other objects. In the last decade, several
groups were working on rate predictions and characterization of double compact object populations[62,68,69]. Without exception, the gravitational wave
predictions were thus far based on observations ofstars in our Galaxy. In particular, the predictions
were based on stars with solar chemical composition –typical for Milky Way thin disk population. However,even the currently operating instruments can reach far
beyond Milky Way. In this local volume, recent deep
observations of tens of thousands of nearby galaxies
demonstrated that as much as ∼ 50% of local starformation is occurring in low metallicity environments(∼ 20%Z⊙ [7]). This fact is not yet fully recognized in
the community, but the implications of this finding are
critical. As discussed earlier, metallicity has a strong
impact on the final mass of compact objects, especially black holes (see Fig.2). If half of the star forming mass is producing heavy black holes the predicted
detection rates can increase by more than an order ofmagnitude [70] and direct detection of gravitational
waves may be much easier than previously believed.
This is supported further by empirical estimates of
BH-BH merger rates [71] and have rather deep implications the instrument (LIGO/VIRGO) development. Motivated by our estimates [70,71] Ligo Scientific Collaboration have already designed and carried out a specific signal search on the last (S6) initial
LIGO/VIRGO science run [72]. At the moment bothobservatories are being upgraded to a higher sensitivity, and our results indicate that the engineeringruns (ones that are planned before reaching targeted
sensitivity) will either result in detections or provide
robust constraints on stellar evolution. This calls fora different approach to data analysis for the first advanced LIGO/VIRGO observations.
Our theoretical predictions are supported by recent empirical estimates [71,73,74,75]. At the moment
these are the only estimates that take into accounta realistic BH/NS mass spectrum, supernovae explosions, common envelope evolution and properly consider the metallicity distribution in the local Universe[76]. Based on our calculations, we argue that thescience that can be done with gravitational wave detections shifts toward a new direction and we have already begun providing some very clear-cut examplesof what can be learned from the first gravitational
wave detections. In particular, we have demonstrated
that the detection of a single NS-NS binary within

Fig.2 – The initial-remnant mass relation fora StarTrack single star evolution for two windprescriptions: the previously used Hurley et
al. winds and newly adopted (modified) Vink
et al. winds. Top panel: Solar metallicity.
This corresponds to the stellar field populations in the Galaxy. The predicted maximum
black hole mass Mbh,max ∼ 15M⊙ for new and∼ 10M⊙ for old winds is consistent with themost massive stellar black holes observed in
our Galaxy (e.g., in GRS 1915 Mbh = 14±4M⊙ ).
Middle panel: Moderate metallicity. This corresponds to stellar populations in galaxy IC10,which hosts the most massive known stellar
black hole (Mbh = 23 − 34M⊙ ). Note that thepredicted maximum black hole mass Mbh,max ∼30M⊙ for the new winds is consistent with themeasurement in IC10, while Mbh,max ∼ 15M⊙
obtained for old winds appears to be significantly too small. Bottom panel: Very low
metallicity. This corresponds to stellar populations of Galactic globular clusters, or protogalaxies. The maximum black hole mass may
reach Mbh,max ∼ 80M⊙ for the newly adopted
wind prescriptions (Belczynski et al. 2010[54].)
the first 10 detections will put very strong constrains
on BH natal kicks [77]. Additionally, the available
electromagnetic data on known BH and NS masses is6inconclusive as to the existence of the infamous ”massgap” (the dearth of compact objects in 2 − 5M⊙ mass
range). It is not clear whether this gap is the effectof some observational biases or rather reflects on thephysics of the supernova explosion engine [78]. In a
recent study, we have identified which part of the supernova explosion physics may be responsible for themass gap and we have indicated that the final answer
will be provided by the unbiased gravitational wave
measurements of compact object masses [79].
We will provide a qualitatively new description of
double compact object populations. The synthetic
populations of double compact objects formed at various host galaxy environments (range of galaxy masses,
metallicities and star formation histories) will become
part of our library. Our model allows for evolution of
metal rich stars (Population I and II; [63]) as well as
first metal-free stars (Population III; [80,81]). Synthetic data will be provided not only as a function ofa given host galaxy type, but also as a function of redshift under an adopted cosmological model that incorporates chemical evolution [8]. The data will include
not only basic parameters for double compact objects (component masses or separations/eccentricities
at formation) but also very specific parameters (spins:
magnitude and their direction [106] or position within
the host galaxy [82]) that can allow for a number ofapplications not only within gravitational wave astronomy but also crucial for gamma-ray burst or Xray binary studies. At the moment, a number of USbased scientists are using StarTrack models to interpret Chandra observations [83,84,85,86]. At the sametime I am leading efforts together with Harvard (EdoBerger) and Boulder JILA groups (Rosalba Perna) tocharacterize populations of transients (Palomar Transient Factory) and Gamma-ray bursts (Swift) with theuse of our models.
Our results will allow for rate/parameter estimates
specific to a given gravitational wave observatory andwill not be limited to LIGO/VIRGO but will be directly applicable to any future observatory that is designated to search for double compact objects at arbitrary redshifts. This approach will facilitate the understanding of what science can be done with gravitational radiation detections with existing and planned
observatories.
1.5. Type Ia Supernovae
Supernova explosions are the fate of all massive
stars: these generate the core-collapse supernovae(SNe). SN explosions are also the final stage for a lessmassive binary system after one of the stars evolves
into a carbon-oxygen white dwarf. These generate
Type Ia SNe (SNe Ia). All SNe play a critical rolein driving the galactic gas ecosystem, in star forma-tion by heating and compressing interstellar gas, andin chemically enriching galaxies. SNe are responsible for releasing radioactive and heavy nuclei into interstellar space and spreading the building blocks ofother stars, planets, and ultimately, life. Additionally,observations of SNe Ia have led to the breakthroughdiscovery that the expansion rate of the Universe isaccelerating, due to a mysterious “Dark Energy” [87].
Despite their use as the most important “standard
candle” distance indicators, the origin of SNe Ia remains unknown. The long-standing paradigm [88] isthat their progenitors consist of a white dwarf that accretes enough matter from its binary companion suchthat its mass exceeds the Chandrasekhar mass limit(∼1.4 M⊙ ). However, the nature of the companion
star, the efficiency of matter accumulation onto thewhite dwarf, and the explosion mechanism are all thus
far unclear [89].
To date, the two most favored scenarios thought
to lead to SNe Ia are the Single Degenerate Scenario(stable mass accretion on to a white dwarf from anormal star [90,91]) and the Double Degenerate Scenario (the merger of two massive white dwarf stars[92,93]). However, other promising formation scenarios exist [94], and support for more than one formation scenario is substantiated by recent SNe Ia observations. Several different sub-classes of SNe Ia havebeen identified [95], and recent advances in theoretical SN modeling indicate that there is likely morethan one formation scenario (and explosion mechanism) contributing to the observed sample of SNe Ia[96]. This has far-reaching implications for the theory of Dark Energy: if the explosion mechanism that
leads to the formation of a SN Ia is dependent on
evolutionary scenario and/or metallicity, this could
potentially pose a challenge for cosmological studies
by introducing systematic errors [97]. Thus, determining the nature of SN Ia progenitors – both in thelocal and the most distant Universe – is one of themost important long-standing puzzles in astrophysics
that remains to be solved.
The delay time distribution (DTD) is the distribution of times in which SNe Ia explode following a(hypothetical) burst of star formation. Knowing the
DTD gives the age of the progenitor, which places
strong constraints on the different proposed formation
scenarios. If the SN Ia rate is known in addition, thenit becomes possible to rule out theoretically-predicted
evolutionary channels.
Today, we are uniquely primed to address the question of SN Ia origin. This has already been done
to some extent for Local Universe-like stellar populations [13]. Excitingly, now we are poised to determine how well SNe Ia can be considered standard candles [98], and quantify to what degree their rates and7delay times vary as a function of redshift and metallicity. In order to accomplish this, in the next step
we will combine the theoretical rates (quantity) anddelay times (feedback timescale) with nucleosynthetic
yield predictions for the leading SN Ia formation scenarios as a function of redshift (metallicity). Thisrequires generating a suite of binary evolution models
for a grid of stellar metallicities, computing explosion models with the most up-to-date nucleosynthetic
yield predictions, and calculating synthetic spectra
and light-curves. A complete pipeline is already inplace with the collaborators in our existing network:from binary star birth/evolution of SN Ia progenitors (A. Ruiter+K.Belczynski) to explosion modeling (MPA, Wurzburg, Heidelberg) to synthetic spectra calculation (MPA, Mt. Stromlo, LANL). Sucha pipeline has already been extensively tested, andshown successful in validating the importance of someof the most promising SNe Ia formation scenarios[99,100].
These efforts will enable us to assess the viability ofvarious explosion models, and gain insight into the origin of different subclasses of SNe Ia: ‘normal’ SN Ia,
sub-luminous and super-luminous SNe Ia, and otherpeculiar objects (e.g., 2002cx-likes). The Synthetic
Universe library will contain detailed information on
the formation history of the most promising SN Ia
progenitors as computed by StarTrack. Additionally, delay times and formation histories will also berecorded for core-collapse SNe; some of which are expected to be synonymous with GRBs. The synthetic
data on delay times and rates of the various SNe willserve as input for the NuGrid collaboration (see 3.2) toobtain estimates of chemical enrichment scenarios forvarious galaxy types. Such a long-term project would
involve convolving theoretical delay times for all stellar explosions with galaxy mass distribution functions
as a function of redshift, and imposing a galaxy massmetallicity dependence [5]. We will thereby provide
important input data for chemical evolution models
by supplying the relative rates (and DTDs) of various
stellar explosions on cosmological scales.
2. DISCIPLINE CONTRIBUTION

We will deliver the first library of synthetic stellar populations to the astro community. At firstwe will provide the data on the populations of GR
sources (NS-NS, BH-NS, BH-BH), ULX binary candidates (BH + stellar type companion) and potential
progenitors of Type Ia supernovae (various configurations of accreting white dwarfs in binary systems).
The database will be maintained beyond the periodof funding and new projects will be added as the needarises. Our contribution will extend beyond the scientific community. We will actively involve the public in

our research. We will provide a platform for the public to get involved in our numerical simulations andat the same time we will invest in education and our
project advertisement to a wider audience through theInternet.
3. SIGNIFICANCE

We propose to tackle three very specific stellar research projects at the frontier of stellar astrophysics.
We will provide a proof-of-principle study of the origin of ultraluminous X-ray sources and we will showwhether stellar mass black holes can or cannot explain the observed populations of these objects. We
will fully describe the populations of double compact objects (NS-NS, BH-NS, BH-BH) that are themost promising sources for gravitational-wave detectors. LIGO and VIRGO observatories will start taking data around 2015 and we will provide a scientificrationale behind the near-future detection of double
compact object mergers and help to guide the signal
search techniques. We will study various potential
progenitors of Type Ia supernovae in a quest to finallyidentify which binaries are specifically responsible forthese explosions. Our synthetic models will help toeliminate some of the proposed models, while othermodels will gain in significance, driving progenitor research. We will compare our predictions with the observed supernovae rates, delay times, light curves andspectra.
4. WORK PLAN

4.1. Database Development
The database will be placed at a local server maintained by our group, and will be visible and accessible
from the outside through the Synthetic Universe
website. The database will consist of electronic datatables accompanied by detailed descriptions of eachtable’s content. For each simulation from either theevolutionary code or population synthesis code we will
post the raw data with a full description of the simulation. A detailed code manual will be placed up
front for each code, as well as a general overview ofthe database and usage instructions. The raw data
files will serve as consistency checks and comparisonsfor groups that prefer to do the post-processing on
their own.
Post-processing involves a number of computationaltasks that are very often required to present synthetic
data in a useful way. For example, it is importantto generate a synthetic spectrum to compare to an
observed galaxy. To proceed, we will use a population synthesis code to obtain the entire stellar population in this galaxy. For each star, we will thenobtain a spectrum from one of the available stellar
spectra libraries, and co-add the spectra. These tasks

8
all require extra calculations, and external libraries.
These tasks will be performed by analysis codes that
we will create and distribute to address our specificdata needs. In this example, the final product would
be the spectrum of a synthetic galaxy that could be
downloaded from our server with the full descriptionof how it was generated.
As the database expands, we will adapt and improve our analysis codes to serve as interactive research tools. In the above example, a need may arise
to combine stellar population data to model unforeseen future observations. Since we will have a full
library of raw stellar population data within the firstyears of opening the database, we will also provide
interactive software that can allow users to combine
the data in novel ways. This can include, but is not
limited to: building galaxy models and changing cosmological parameters. Finally, we will provide visualization tools that will specifically work on information
contained within the database, requesting time snapshots, imaging, and rapid sampling of the synthetic
data. The first pilot project is already available online at: http://www.syntheticuniverse.org
4.2. Specific Schedule
1) First year. We will start preparations for all
three science projects. That will include search of
the available literature, contacting other groups thatwork on the similar subjects and we will establish
background for our calculations. We will prepare
our physical model (extra input physics, calibrations
and updates) for all three projects. In particular, we
will use the MESA code to updated criteria on common envelope development, we will implement the
primary effects of rotation into StarTrack based on
the Geneva group results, we will add various, lessthoroughly-explored though promising SN Ia models(e.g., sub-Chandrasekhar mass merger models [101],
violent white dwarf mergers [102], Symbiotic channel[103], double-detonation scenarios [104], thick windprescription [105]) and we will expand our code for relevant ULX physics (leaky disks, anisotropic emission,very massive stars). We will create and test internal (Warsaw Observatory) computing network within
the framework of the Universe@home program. We
will apply for computer time at various open Polishcomputational centers.
2) Second year. We will run numerical population
synthesis simulations relevant to all three projects.
The data will be stored at first on our local disks. At
the same time we will work on the development of our
websites (www.syntheticuniverse.org) and we will
get it ready for the arrival of the synthetic data. The
simulations will be run at US and Polish Research Institution computer clusters, and (if granted) in Polishopen computational centers. And we will launch our
Universe@home program and open it for the public.
We will begin assessing the changes imposed by effectsof rotation on relevant stellar populations.
3) Third year. The numerical simulations will continue. We expect that more and more computational resources will be acquired with time via our
Universe@home program. The first data will beplaced in our www.syntheticuniverse.org databaseand made publicly available for other research groups.
We will carry out the first analysis of our data for all
three projects and write up and submit our initialconclusions to international journals. For practicalreasons, the GR papers will be submitted to Astrophysical Journal, SN Ia papers to Monthly Notices of
the Royal Astronomical Society and ULX studies to
Astronomy & Astrophysics. We will start incorporating MESA models into the StarTrack code as an
optional tool for comparative simulations.
4) Fourth year. The numerical simulations will continue and at the end of this year the simulations will
be completed. The full data sets will be placed anddescribed in our library. We will start the final analysis and progress with the three research projects. We
will continue incorporating the Geneva models into
our population synthesis. Furthermore, we will obtainthe first population synthesis simulations that include
full effects of rotation from the MESA code.
5) Fifth year. Available computer power will be
used to obtain extra simulations that we will deem
required during our studies or that are requested by
the astro community or reviewers of our submitted
manuscripts. The entire group will focus on detailed
analysis of our results and comparisons of our simulations with (available at this stage) observations.
We will also compare our results obtained with rotating and non-rotating stellar models. We will be
writing our findings into a series of papers. Students
and graduate students will be using these results toprogress their theses. At the same time we will bealready planning further future extensions of our library. By that time we expect to have major feedback
from various groups working on the related subjects
and this will help us guide our future database development.
5. METHODOLOGY

5.1. Stellar Modeling
Although the theory of stellar evolution is quite
a mature subject, it has still to face big questions.
Among them: How do massive stars form? How does
stellar evolution influence star formation? What was
the nature and the characteristics of the first generations of stars in the Universe? What was the role ofthese first generations in the reionization of the Uni-

9verse? What physics drives the variations in the stellar populations observed in different environments?
The answers to all these questions require improving stellar models. These improvements are obtainedby including important effects, neglected until recently, such as the effects of the stellar winds, theinternal transport processes induced by axial rotation
or by other instabilities such as the thermohaline mixing, the transport of angular momentum by the gravity waves and/or by a magnetic field. All these effects
have been implemented in the Geneva and Bill Paxton
stellar models and have been tested by a whole seriesof observational features. These comparisons all support the view that extra mixing processes do indeed
occur in stars, and that models accounting for themcan much better reproduce the observations than themodels neglecting them. Including these transportmechanisms have deep impacts on the stellar populations expected at a given age and metallicity as wellas on the nucleosynthesis of the elements. Many ofthese effects can be tested using the new windows
which are now opening on stellar interiors throughastroseismology (e.g., COROT, KEPLER). Additionally,
the huge number of stars that will be observed by thesatellite GAIA will allow to probe very short evolutionary phases which so far were only in the realm of theory. Heavy mass loss of material by stellar winds, likein the Luminous Blue Variable phase, may be good
example of short phenomena that may very deeply
affect the end of the stellar life, the nucleosynthetic
outputs, the supernova type and the nature of thestellar remnant. GAIA observations will lead to strong
revisions of the stellar evolution in many parts of the
Hertzsprung-Russel diagram.
We will require two type of codes (i) population synthesis code and (ii) detailed stellar evolutionary code.
Though these codes can work independently, ideallythey should work together, delivering feedback fromone code to another. The end product of such an
approach will be a comprehensive and self-consistentpicture of stellar populations at a brand new level,
both quantitatively in terms the of a data library, andqualitatively in terms of evaluating the input physics.
5.2. Population Synthesis Models
We created the comprehensive population synthesis
tool StarTrack [62,63]. This code uniquely combines
rapid stellar evolution with the detailed physics of binary interactions and the formation and evolution ofcompact stellar remnants. The code is widely used(∼ 100 papers and 3000+ citations on StarTrack results), but only in close collaboration with the code
PI (K.Belczynski) as the code is not open source software. StarTrack is a living code, and its major
strength stems from the annual revisions and input

physics updates that allow this code to be employed
for cutting-edge and innovative research projects. To
give an example, in 2008, StarTrack was extended toinclude the evolution of black hole spins [106] (not accounted for in any other synthesis codes), in 2010 thecode was updated for the most recent wind mass loss
rates [54], in 2011 we have revised supernova explosion model [107] while in 2012 the common envelope
evolution was extended [76].
Population synthesis is claimed to lack predictive
power. The claim originates from the fact that population synthesis modeling can depend on ∼ 30 parameters. These parameters bracket our lack of knowledgeof the physics of star formation, evolution, and death.
This may be perceived as a major weakness, especially
if the method is used “blindly” and there are many
examples of population synthesis papers of this sort.
This is why StarTrack is not an open source code.
However, from another perspective, one can argue
that this is the unchallenged strength of population
synthesis. By definition, population synthesis allows
us to tackle a number of problems that are not approachable by other means—simply because we lackthe full physical picture of many phenomena. The
predictions can then be contrasted with observations
to either lend support for or rule out various models that attempt to describe the unknowns. As a result, population synthesis can guide the developmentof first-principle-studies, delivering invaluable insights
on problems that are at the edge of our understanding.
It is an art to provide predictions that are realistic anddo not depend (in any significant way) on the method
uncertainties – but it is not an impossible art. For
example, the revision of wind mass loss rates has led
to a very significant increase in the black hole masses
that can be formed by stars in the local Universe.
Although, the revised winds had been acknowledged
for several years, this striking implication was somehow missed. In Figure 2 we show how stellar black
holes can reach ∼ 80M⊙ , up to 8 times larger thancommonly believed. This finding does not depend on
any population synthesis parameters, although it was
obtained with the StarTrack code.
Population synthesis codes most commonly consistof two major parts; one deals with single stellar evolution and one with physical interactions of two stars
in a binary. The StarTrack binary physics part is, asmentioned earlier, constantly updated to reflect newadvances. As for the stellar evolutionary part, thecode employs rapid analytic formulae based on the
updated Eggleton stellar evolutionary code [108]. The
major update planned for this proposal is to extend
StarTrack to include rotation in stellar models. Currently, most stellar models exclude the effects of rotation on a star’s evolution. Both, the Geneva group10
and Bill Paxton (MESA) have begun producing the firstmodels with rotation, however the models are still too
limited in parameter space to employ them in population synthesis. To probe mass, metallicity, and rotation adequately, we would need approximately 3000
stellar models. At first stage, we plan to use the published Geneva models and use MESA code to generaterotating stellar models to recalibrate our evolutionary
formulae within the StarTrack code. Such approach
will allow us, for example, to test how the rotation affects the compact object mass. On one hand rotation
will increase the star’s core mass and on the other thewind mass loss rates are higher for rotating models
and it is not clear how these two opposing effects influence the final compact object mass. At the second
stage, we will accumulate all available rotating models and we will construct the grid (mass-metallicityrotation) to be employed as a tool for population synthesis of binary stars with rotating components.
This is high computational cost task. However, sucha task is feasible and realistic, and the science profits
will by far repay the effort. This is the final step before adopting the full and detailed evolutionary binarymodels in population synthesis calculations. This effort is cutting-edge: such a symbiosis of population
synthesis with rotating stellar models has never been
attempted.
5.3. Detailed Stellar Evolutionary Models
For a detailed stellar evolutionary code we will
adopt and modify an open source MESA code [109] forthe database needs. The code will be used to provide single stellar models when an in depth characterization of a single star is required, such as its internal structure, mass profile, or instantaneous composition. The details of stellar evolution are needed toboth guide the population synthesis calculations andto characterize populations of single stars or the components of binaries. This is useful to estimate cluster
ages, or to predict the existence and range of habitable zones for extrasolar planets, for example.
Similar to Geneva group models [110], MESA allows
to include rotation. Rotation is the last major missing parameter in massive stellar evolution modeling.
The Geneva group was the first to introduce the effects of rotation on stellar evolution, noting significant
changes in mixing and in mass loss rates compared
to non-rotating models. Since many stars form either in close binaries where tidal spinup occurs, orwith high initial spin, introducing stellar rotation in
massive population synthesis calculations is very important and is likely to produce interesting and unexpected results.
To enhance the chemical evolution modeling, we
will partner with the NuGrid collaboration(http://forum.astro.keele.ac.uk:8080/nugrid)and help to provide nucleosynthesis yields. As a firstscience goal, the NuGrid collaboration will generate
yields from single stellar models. However, to deliveruseful input for chemical evolution of stellar groups
and cosmological volumes, more realistic stellar populations will need to be considered. Chris Fryer (Los
Alamos), who is participating in both Synthetic
Universe and NuGrid projects, will act as the emissary between these groups. On our side we will provide realistic properties of interacting binary stars
and double compact objects that feed elements into
the interstellar medium quite differently than single
stars. We will also provide calibration factors, such as
the estimated star formation, supernova and merger
rates, that will serve for post-processing of NuGrid
calculations.
5.4. Computational Resources
The proposed project is overall computationally intensive. We will need thousands of CPU hours foreach full simulation. However, each simulation, or
say given realization of stellar populations in Universe, will be used to publish at least one paper by
our group and will be used by other researchers to
conduct their own studies.
Ideally, we would need to have our own computer cluster (500-1000 CPUs), dedicated for this very
project. However, such cluster is rather expensive(500,000 PLN or more). Also, it was indicated that
our computations could be performed in various Polish open access computer centers. There is a potential issue with applying for CPU time for generally
accessible large machines. As it happens architecture of most of such machines is dedicated to performlarge parallel simulations (focus is laid on speed ofCPUs communication and on short tasks with the useof multi-hundred processors). Our evolutionary andpopulation synthesis simulations do not require parallel computing, but simple sheer CPU force (or large
number of single CPUs to run for prolonged time).
Therefore, it may be perceived that our application
for time on these open-access machines is not justified
or is not an optimal use of the available resources.
We will proceed along three different routes toensure that we have enough CPU time to perform
our simulations and we are only asking for minimal(in comparison with the cost of the dedicated computer cluster) funds to purchase 2 laptops for the
youngest team members, and the dedicated serverwith large hard-drive memory and UPS system tohost our database (at the moment our initial site is
run from a desktop computer).
1) We will apply for time to open-access machineswith the science-rationale provided in this proposal.

11
We have identified the following computational resources to be used: ICM, CYFRONET, AstroGridPL).
Predicted success rate: ∼ 50%.
2) We will use PI established connections to perform simulations on various (non-open access) computer clusters. We have been given access and permission to perform calculations on 3 machines: FUTURO
at University of Texas at Brownsville, USA (contact:
Matt Benacquista, benacquista@phys.utb.edu), PSK
at Copernicus Center, PAN, Warsaw, Poland (contact: Zbyszek Loska, zbylo@camk.edu.pl), SUGAR at
Syracuse University, USA (contact: Duncan Brown,
dabrown@physics.syr.edu).
3) We will employ the Berkeley Open Infrastructure
for Network Computing (BOINC) software to run calculations on personal computers. We will seek volunteers within general public to join our program:
Universe@home – and allow their home computers to
run our software (StarTrack set for a small part ofa given simulation) at times when a their computers CPUs are idle. This is similar to projects like
Einstein@home or Pulsars@home that are successfully operated to perform data analysis of VIRGO
data and search for pulsars in Arecibo data, respectively. At the moment, there are about 40 such programs maintained around the planet. Two of them
are based in Poland, but these are run for personal
use and mostly dedicated to breaking codes. Our
initiative will be the first Polish application of thatsort for scientific use. The initial test program will
be launched within Warsaw University Observatory.
We will use our Synthetic Universe server to run
the BOINC software with the integrated StarTrackcode. We will request students and graduate students within the Observatory (about 20 desktops,
many with dual or quad core CPUs) to join the initial Universe@home network and participate in one
simulation. At parallel, we will run a control simulation at one of our available clusters (see above). Once
we are sure that our initial configuration runs without any problems and delivers correct results, we will
launch the main program. To advertise the programand attract the public we will use: (i) our websites(both personal and professional), (ii) social networks(facebook, twitter) and (iii) science venues (conference presentations, departmental talks, research visits). As proven by other programs of that sort, there
is abundant public interest to participate in scientificresearch. However, it is obvious that advertisement isa critical factor to make such a program successful. As
indicated above we will follow several routes to advertise. Additionally, we have initiated talks with Copernicus Science Center in Warsaw (contact: Weronika
Sliwa,weronika.sliwa@kopernik.org.pl) to advertise our program to a broader audience that the Cen-

ter attracts. This avenue to CPU resources is relatively inexpensive (advertisement costs, website maintenance and broad-band fast network connection) but
can offer a great deal of computational power. As afront side seen by public we will run images of Xray (Chandra), gamma-ray (Swift) and gravitationalradiation (predicted only at the moment) sky backedup by popular stories that connect our science goals
with the particular calculations performed by the program participants. We hope not only to attract people to participate in our simulations but to make themunderstand, appreciate and entertain the ideas behind
our science project.
6. APPOINTING A NEW SCIENTIFIC TEAM

1) Ashley Ruiter, Ph.D. (Max Planck Institute for
Astrophysics, Germany: postdoctoral fellow): supernova type Ia expert, stellar evolution of low and intermediate mass stars, white dwarf physics. Tasks:
search for supernova Ia progenitors, expanding the
population synthesis with new explosion models. External expertise/advise: Wolfgang Hillebrandt (MPA,
Germany).
2) Michal Dominik, M.Sc. (Warsaw Observatory:
graduate 3rd year student): expected to be Ph.D. by
the time of funding. Gravitational radiation source
expert. Tasks: studies of BH-BH, BH-NS, NS-NS
binaries and adding rotation to population synthesis
model. Science and technical Support of Synthetic
Universe website. External expertise/advise: Ilya
Mandel (University of Birmingham, England)3) Grzegorz Wiktorowicz, Ms.C. (Warsaw Observatory: graduate 3rd year student): expected to be
Ph.D. During first year of funding. X-ray modelingand studies of the formation of High- and Low-mass
X-ray binaries. Tasks: Search for ULX stellar sources.
Implementation of new ULX physics into the population synthesis code. Science and technical supportof Universe@home. External expertise/advise: Chris
Fryer (Los Alamos National Laboratory, USA).
4) Paulina Karczmarek, Ms.C. (Warsaw Observatory: graduate 1st year student): expected to obtain
Ph.D. at the end of the funding period. Currently
working with the MESA code. Tasks: generating stellar models with rotation and StarTrack calibration
for low- and intermediate-mass stars. P.Karczmarek
will work closely with A.Ruiter and M.Dominik. External expertise/advise: Grzegorz Pietrzynski (Warsaw University, Poland)
5) Marek Walczak (Warsaw Observatory: last year
undergraduate student): expected to be Ph.D. student during entire period of funding. Currently
working on intermediate-mass black holes. Tasks:
study of intermediate-mass BHs as potential sources
for LIGO/VIRGO and as alternative explanation to12
the origin of ULXs. M.Walczak will work closelywith M.Dominik and G.Wiktorowicz. External expertise/advise: Cole Miller (University of Maryland,USA)6) Krzysztof Belczynski, Ph.D. (Warsaw Observatory): primary investigator for this proposal. Expertise in population synthesis, double compact ob-

jects, X-ray binaries, gamma-ray burst and supernova progenitors. Tasks: lead of the above groupand coordination of the entire project. Also population synthesis code (input physics) development.
Lead and development of Synthetic Universe website and Universe@home program.

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