My research history can be summarized in a number of steps, which have led to the design and then further development of the population synthesis code StarTrack. The code at the present time is a state-of-the-art tool targeted for studies of binaries containing compact objects: black holes, neutron stars and white dwarfs.
At first I focused on studies of binaries with white dwarf accretors, in particular cataclysmic variables and symbiotic stars. In the next step I investigated the black holes and heavy neutron stars in binary configurations that may possibly be connected to short Gamma-ray burst progenitors.
This naturally led to binary compact objects, and calculation of their physical properties and merger rates. Since the binary compact objects are the most promising candidates for ground-based gravitational-wave detectors I was invited to join the Laser Interferometer Gravitational-wave Observatory (LIGO) scientific collaboration. Eventually, my interests shifted to X-ray binaries. I have customized the most recent theoretical developments connected to formation and evolution of X-ray binaries for the population synthesis method.
At present, StarTrack is applied to the analysis of Chandra observations of Galactic and extragalactic point X-ray sources. Finally, I am also leading the studies of: Type Ia SN progenitors; gravitational radiation sources for the Laser Interferometer Space Antenna (LISA) observatory; and research on the formation and evolution of black holes in cluster environments.

GENERAL OVERVIEW

• Achievements
  

  The StarTrack population synthesis code was already widely recognized in the community (∼ 100 citations on the first code description paper [28]) and was successfully used in a number of studies (≥ 30 refereed publications). Additionally, over the last couple of years, the code has undergone major revisions and updates [1], and now StarTrack is one of the best population synthesis codes on the market. The results based on the StarTrack calculations have attracted attention at many international meetings. Besides a number of contributed talks, I was invited to review extragalactic X-ray binary modeling at an AAS meeting, and twice (LIGO and LISA targeted workshops) to review double compact object modeling at the Aspen Center for Physics. Also, several colaborations have been initiated following various presentations of StarTrack results. These include: studies of nearby starburst galaxies with A.Zezas (Harvard-Smithsonian CfA); research on early Universe metal-free stars and their potential role in context of Gamma-ray bursts (GRBs) and gravitational radiation (GR) sources with C.Fryer and A.Heger (Los Alamos National Lab.) or LISA-related projects with M.Benacquista (Univ.of Montana) and S.Larson (Penn State). Although the code was primarily developed to study compact objects: white dwarfs (WDs), neutron stars (NSs) and black holes (BHs), its design and flexibility allow for research in very different areas, which are in a broad sense connected to stellar evolution.

• Education/Teaching
  

  I began working with students 3 years ago. During my stay at Northwestern I helped with the undergraduate projects supervised by the Theory group faculty [5,6,10,19]. Currently, I am independently advising a student who is finishing his Master degree thesis at Warsaw Univ. (A.Sadowski; [4,22]) and a student who is working toward her Ph.D. at New Mexico State Univ. (A.Ruiter; [2,3,8]). Also, I have just started advising two undergrads on smaller research projects.
  StarTrack is an ideal research tool for students (both on under- and graduate levels) to conduct their own research. Once acquainted with the code, I usually start students on advanced science projects. Along the way they learn about stellar evolution, physics of compact objects and additionally they acquire experience in massive computations, modeling and data processing.
  Recently, we have recognized the need to update graduate students in our department on programming/computational skills. In the spring semester 2006 I will be teaching a course on computational astrophysics (ASTR698; for syllabus see my Teaching page). I am also interested in teaching undergraduate level courses (like Introduction to Astronomy) to attract the best students from other related departments (like physics, engineering, math) for short research projects in astrophysics. Such an early selection offers a good chance of getting the best (and tested) prospective students for graduate programs.

• Future Perspectives
  

  I am planning to form an independent group working in the general area of theoretical astrophysics of compact objects, high-energies and stellar evolution. Of course, such a group will strongly interact with other faculty members and their teams. I am very interested to work with observers who have access to or collect data on binaries with white dwarfs, neutron stars and black holes. Additionally I also seek a direct research contact/interaction with theorists working in GRB and GR fields.
  I have already made the first steps toward creating an initial group. As indicated above there are several students working on various projects with me. I was also successful in securing extra funds in the form of research grants (see my CV) to support both students and the research we carry out. In the near future (next NSF cycle) I plan to apply for larger funds that would allow me to expand the group with new members (e.g., a post-doc). Also, since population synthesis modeling requires substantial computer power, I plan to secure funds for development and maintenance of a small computer cluster (32 or 64 processors).

In the following I describe my past and current research in a greater detail. Some of the near future plans are also outlined. However, I usually catch opportunities of research as they appear in the field, so it is indeed very hard to predict precisely what I will be working on in the years to come. Please, note the varied range of topics covered in my publications, along with a general drift of expanding interests with passing years.

CATACLYSMIC/SYMBIOTIC STARS

I began work on symbiotic stars with my Master Degree thesis: ``Light curve analysis of symbiotic stars''. The work included collection of archival data, period estimation, and light curve synthesis for several symbiotics. Careful study of T CrB light curves revealed that the hot component of this system is a massive WD and not a main sequence star, and solved the long standing argument about its nature [39]. Later, I prepared A Catalog of Symbiotic Stars [34]. This new catalog had been long-awaited as many new symbiotic stars were discovered and a wealth of new data had been collected since 1986, when the preceding catalog was published. As a byproduct of this work we have found new orbital periods and estimated binary parameters for two symbiotic stars: FN Sgr [12] and AE Ara.

I have also undertaken a study of a supersoft X-ray prototype and cataclysmic variable V Sge, a quizzical system, for which many different models were proposed throughout the years. We have reanalyzed the old and new light curves together with spectroscopic data showing inconsistencies of some other proposed models and attempted to build a self-consistent picture for this binary and explain its behavior [32].

GAMMA-RAY BURSTS

In 1998, I started a project on binary population synthesis in context of GRBs. First, I have constructed a simple code to evolve massive stars and generate NS-NS/BH-NS binaries. These systems were suggested as progenitors of short-duration GRBs. The studies were set to check if the distribution of merger sites of these systems may be reconciled with observed GRB sites in respect to their host galaxies. In this first approach we have also studied the influence of different evolutionary parameters on the population of potential GRB progenitors [38,36].

Later on, I created the first version of StarTrack and results of population synthesis were combined with star formation history and a cosmological model to calculate the change of the GRB rates with redshift. Comparison of my predictions has allowed me to put an upper limit of ∼ 4 degrees on GRB collimation. Comparison of merger sites (with respect to their host galaxies) of various proposed binary models has allowed me to exclude them as possible long-duration GRB progenitors, with the exception of helium mergers [30] which was later confirmed by the connection of long GRBs with SNe of massive stars.

However, for short GRBs the mergers of compact objects are still the most preferable model. Contrary to previous predictions, I found that the majority of NS-NS binaries are expected to merge within their host galaxies. Previous studies have neglected final evolutionary stages of progenitor systems, and in particular the last mass transfer episode driven by the He-rich donor (additional orbital shrinkage). Inclusion of this effect led to the formation of a very short-lived NS-NS, which do not live long enough to escape their host galaxies, despite their large center-of-mass velocities [33,27,24].
The corresponding observational afterglow characteristics of the systems merging within the host galaxies were then computed for the StarTrack models [29]. It is interesting to note that now the SWIFT team (e.g., Fox et al. 2005, Nature 437, 845) was able to locate a short GRB (which was connected to NS-NS merger) within its host Galaxy. I have obtained exactly the same result on theoretical grounds with the StarTrack calculation, which at that time was rather controversial and in clear contradiction with other findings.

GRAVITATIONAL WAVE SOURCES

Mergers of NS-NS, BH-NS and BH-BH are expected to be the most promising candidates for ground-based gravitational wave observatories such as LIGO or VIRGO. I have investigated binary evolution leading to the formation of double compact objects. This led me to recognize new channels for NS-NS formation [33,27]. Distinctive physical properties for the new group of NS-NS were predicted (short lifetimes, high contribution of non-recycled pulsars), and the increase correction factors were suggested for the rate estimates based on the observed sample of NS-NS.

I have preformed a comprehensive study of compact object binaries with the use of StarTrack, in which special care was given to population method uncertainties (over 30 models investigated). A conclusion was reached that a successful detection is not expected during the initial LIGO stage (current status), but it is very likely during the advanced stage (∼ 2009). I also showed that the results of StarTrack are in agreement with the most recent observationally based NS-NS merger rate estimates. The results are of importance for groups performing gravitational-wave signal and hydro-dynamical merger calculations, e.g., they were used in the LIGO project [40,41,42,43,44,45,46,47,48].

I have aimed a number of studies to assess the observational characteristics of sources for a given detector. Predictions of chirp masses of merging binaries have been made. My calculations have shown that despite the overall dominance of NS-NS binaries, BH-BH systems will be much easier to detect due to their higher chirp masses. Realistic NS/BH mass distributions and the change of the merger rate with redshift and other cosmological parameters were taken into account.
I also made first attempts to predict what could be learned from the initial GR observations. Moreover, it was shown that some parameters describing the uncertain evolutionary processes may be constrained with only several tens of detections [25,21,18] and also a non-uniform distribution of galaxies within a sampled volume was taken into account [19].

X-RAY BINARIES

My recent research involves studies of the X-ray binaries (XRBs), and it reflects the wealth of public Chandra and XMM Newton X-ray observations available for theoretical analysis. Several projects are already finished, several others are being conducted.

Recent RXTE and Chandra discoveries of low-mass X-ray binaries with ultra-short orbital periods have initiated theoretical work on the origins of these peculiar systems. Using the StarTrack code I analyzed the formation and evolution of X-ray ultracompact binaries (UCBs) in the Galactic field. The relative number of UCBs with a NS or a BH accretor populating our Galaxy was predicted. I demonstrated that standard evolutionary scenarios involving primordial binaries can be sufficient to produce the UCBs in the Galactic field without requiring additional dynamical processes associated with the dense cluster environments. In contrast to previous studies it was found that the majority of the UCB progenitors are formed through the accretion induced collapse of the heavy ONeMg WD to a NS. The most resent results on the accretion physics and mass accumulation were used to obtain this result [16].

I constructed the first synthetic XRB populations for direct comparison with the X-ray luminosity functions (XLFs). I also carried out the first, and successful, comparison with Chandra observations of a nearby starburst galaxy NGC~1569. A self-consistent model of XRB which incorporated the most recent advances in the field was created. Then the main goal was to examine whether it is possible to reproduce the XLF shape with the StarTrack models (and test their validity), given the current knowledge for the star-formation history of this galaxy.
It was found that, for typical binary evolution parameters, it is indeed possible to closely match the observed XLF shape. The robust match is achieved for a hybrid synthetic population consisting of the young and old stars formed in two starbursts, in agreement with HST observations of NGC~1569. In view of this encouraging first step, I have discussed the implications of XRB models and their potential as tools to study binary populations in other galaxies [15].

Recent deep Chandra surveys of the Galactic center region have revealed the existence of a low luminosity hard X-ray point source population. It has been proposed that a majority of these sources can be explained by a population of a neutron star fed by a stellar wind of a main sequence companion. In my investigation, a population synthesis study of the Galactic center region has been carried out, and it was found that transient systems in quiescence, composed of low mass WDs transferring matter via Roche lobe overflow to their NS/BH companions dominate over wind-fed XRBs [14].
However, I also found that the populations of wind-fed XRBs and quiescent transients are not large enough to explain the bulk of observed faint X-ray sources in the Galactic center (∼ 2000; Muno et al. 2003, ApJ 509, 225). The StarTrack X-ray model was expanded to include cataclysmic variables (CVs), and the population synthesis of the Galactic center was repeated. The first pilot study [2] shows that indeed a subpopulation of the brightest CVs (intermediate polars) may explain both the number and luminosities of the faint Galactic center sources.

I have undertaken a comprehensive, long-term theoretical study of XRB populations, targeting point sources in different stellar environments (starburst, spiral and elliptical galaxies). The existing and new Chandra observations are being used for comparison to my theoretical models with the aim of deriving constraints on the formation and evolution of various types of X-ray binary systems in order to reveal and understand the underlying X-ray binary populations formed in different stellar environments. Among others the high-mass X-ray binaries, low-mass X-ray binaries, ultra-luminous X-ray sources, CVs and pulsars are included in the studies. A population synthesis method, based on statistical analysis concepts of large stellar ensembles, is used and is perfectly suited for the task.
There are two major steps in the project. The initial step is to use the available X-ray data for the model calibration, for the first time, of the various subclasses of X-ray binary systems. This calibration will impose new constraints on the binary evolution leading to the X-ray phases and will provide insights on the main physical processes important for the X-ray binary systems. The second step is to use the X-ray binary calibrated code to constrain the stellar environments in different galaxies with observed point source populations. This will yield information on the SF histories, initial mass functions, content of current X-ray populations and compositions of stars in other galaxies.

OTHER ONGOING PROJECTS

I recently started studies of Type Ia SNe and their unknown origins. Type Ia SNe are the prime distance indicators used in a modern cosmology, and it is crucial to understand their origin. Connecting Type Ia SNe to one or several different progenitors may resolve the Type Ia SN standard candle issue. Using the most recent observations of delay times of Type Ia SNe up to high redshifts, we were able to demonstrate that most likely Type Ia SNe originate from a single progenitor: merger of two carbon oxygen white dwarfs, confirming the validity of Type Ia SN standard candle assumption. First results were already communicated [8].

Close WD-WD binaries are predicted to be the source of confusion noise for the LISA, a low frequency gravitational radiation detector. It is very important to understand the characteristic level of this noise (which will hinder a detection of other important sources, like extreme mass ratio inspirals) especially during the design phase. Also a number of nearby WD-WD systems will be resolved with LISA, presenting a great potential for testing our models of common envelope evolution or a number of issues (e.g., tidal heating) involved in WD-WD mass transfer calculations. I initiated comprehensive theoretical studies of the white dwarf populations, and the first results have been already obtained [3]. The next set of results is coming in a near future (Ruiter et al., in preparation).

Investigations of black hole populations in young cluster environments are also underway. So far we only presented the results which can serve as the initial input for detailed dynamical studies of dense cluster environments [4,22]. However, we will also present follow up studies with full dynamical interactions (Sadowski et al., in preparation). These studies are targeted to address a number of interesting issues connected with the combined effects of stellar evolution and dynamics in clusters (e.g., enhanced cluster formation of XRBs, or intermediate-mass BH formation).

The first population metal-free stars (Pop III) have attracted a great deal of interest since they are believed to be responsible for re-ionization and metal enrichment in the early Universe. Additionally, they can also form (due to their initial high mass and lack of mass loss) massive BH remnants (≥ 100 M _sun). These intermediate mass BHs may have played a crucial role in forming todays supermassive BHs. I plan on performing detailed modeling of the evolution of the first stars with a special emphasis on massive BH formation. Evolutionary consequences include GR signals [17] and high-redshift GRBs.

For references see my List of Publications.