N. Ivanova

Publications (28)42.75 Total impact

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  Available from: arxiv.org

  Article: Formation of Ultracompact X-ray Binaries in Dense Star Clusters

  N. Ivanova, F. A. Rasio, Lombardi, Jr., J. C, K. L. Dooley, Z. F. Proulx
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  ABSTRACT: Bright, ultracompact X-ray binaries observed in dense star clusters, such as Galactic globular clusters, must have formed relatively recently, since their lifetimes as persistent bright sources are short (e.g., ~10^8 yr above 10^36 erg/s for a 1.4 Msun neutron star accreting from a degenerate helium companion with an initial mass of ~0.2 Msun). Therefore, we can use the present conditions in a cluster core to study possible dynamical formation processes for these sources. Here we show that direct physical collisions between neutron stars and red giants can provide a sufficient formation rate to explain the observed numbers of bright sources. These collisions produce tight, eccentric neutron star -- white dwarf binaries that decay to contact by gravitational radiation on timescales ~10^6-10^10 yr, usually shorter and often much shorter than the cluster age.
  01/2005;
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  Available from: arxiv.org

  Article: Formation and evolution of compact binaries with an accreting white dwarf in globular clusters

  N. Ivanova, F. A. Rasio
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  ABSTRACT: The population of compact binaries in dense stellar systems is affected strongly by frequent dynamical interactions between stars and their interplay with the stellar evolution. In this contribution, we consider these effects on binaries with a white dwarf accretor, in particular cataclysmic variables and AM CVns. We examine which processes can successfully lead to the creation of such X-ray binaries. Using numerical simulations, we identify predominant formation channels and predict the expected numbers of detectable systems. We discuss also why the distribution of cataclysmic variables has a weaker dependence upon the cluster density than the distribution of quiescent low-mass X-ray binaries and why dwarf nova outbursts may not occur among globular cluster cataclysmic variables.
  01/2005;
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  Available from: arxiv.org

  Article: The Evolution of Binary Fractions in Globular Clusters

  N. Ivanova, K. Belczynski, J. M. Fregeau, F. A. Rasio
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  ABSTRACT: We study the evolution of binary stars in globular clusters using a new Monte Carlo approach combining a population synthesis code (StarTrack), and a simple treatment of dynamical interactions in the dense cluster core using a new tool for computing 3-body and 4-body interactions (Fewbody). We find that the combination of stellar evolution and dynamical interactions (binary-single and binary-binary) leads to a rapid depletion of the binary population in the cluster core. The maximum binary fraction today in the core of a typical dense cluster like 47 Tuc, assuming an initial binary fraction of 100%, is only about 5-10%. We show that this is in good agreement with recent HST observations of close binaries in the core of 47 Tuc, provided that a realistic distribution of binary periods is used to interpret the results. Our findings also have important consequences for the dynamical modeling of globular clusters, suggesting that ``realistic models'' should incorporate much larger initial binary fractions than has usually been done in the past. Comment: 14 pages, MNRAS style, 9 figures, MNRAS accepted
  01/2005;
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  Available from: arxiv.org

  Article: Understanding Compact Object Formation and Natal Kicks I. Calculation Methods and the case of GRO J1655-40

  B. Willems, M. Henninger, T. Levin, N. Ivanova, V. Kalogera, K. McGhee, F. X. Timmes, C. L. Fryer
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  ABSTRACT: In recent years proper motion measurements have been added to the set of observational constraints on the current properties of Galactic X-ray binaries. We develop an analysis that allows us to consider all this available information and reconstruct the full evolutionary history of X-ray binaries back to the time of compact object formation. This analysis accounts for mass transfer through the ongoing X-ray phase, tidal circularization before the onset of Roche-lobe overflow, motion through the Galactic potential after the formation of the compact object, and binary orbital dynamics and hydrodynamic modeling of the core collapse. We apply the analysis to the soft X-ray transient GRO J1655-40 and, for the first time, use its full 3D peculiar velocity constraints right after core collapse instead of lower limits on the current space velocity given by the present-day radial velocity. We find that the system has remained within 200pc from the Galactic plane throughout its entire life time and that the mass loss and a kick possibly associated with the black hole (BH) formation imparted a kick velocity of 45-115 km/s to the binary's center of mass. Right after BH formation, the system consists of a 3.5-6.3 Msun BH and a 2.3-4 Msun main-sequence star. At the onset of the X-ray phase the donor is still on the main sequence. We find that a symmetric BH formation event cannot be formally excluded, but that the associated system parameters are only marginally consistent with the currently observed binary properties. BH formation mechanisms involving an asymmetric supernova explosion with associated BH kick velocities of a few tens of km/s, on the other hand, satisfy the constraints much more comfortably. We also derive an upper limit on the BH kick magnitude of 210 km/s. (abridged) Comment: Minor revisions. Accepted for publication in ApJ
  11/2004;
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  Available from: arxiv.org

  Article: Compact binaries in globular clusters

  N. Ivanova, F. A. Rasio
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  ABSTRACT: In dense stellar systems the frequent dynamical interactions between stars play a crucial role in the formation and evolution of compact binaries. We study these processes using a novel approach combining a state-of-the-art binary population synthesis code with a simple treatment of dynamical interactions in dense star cluster cores. Here we focus on the dynamical and evolutionary processes leading to the formation of compact binaries containing white dwarfs in dense globular clusters. We demonstrate that dynamics can increase by factors ~ 2 - 100 the production rates of interesting binaries such as cataclysmic variables, ``nonflickerers'' (He white dwarfs with a heavier dark companion), merging white dwarf binaries with total masses above the Chandrasekhar limit, and white dwarf binaries emitting gravitational waves in the LISA band.
  03/2004;
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  Available from: arxiv.org

  Article: On the Maximum Binary Fraction in Globular Cluster Cores

  N. Ivanova, K. Belczynski, J. M. Fregeau, F. A. Rasio
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  ABSTRACT: We study the evolution of binary stars in globular clusters using a novel approach combining a state-of-the-art population synthesis code with a simple treatment of dynamical interactions in the dense cluster core. We find that the combination of stellar evolution and dynamical interactions (binary-single and binary-binary) leads to a rapid depletion of the binary population in the cluster core. The maximum binary fraction today in the core of a typical dense cluster like 47 Tuc, assuming an initial binary fraction of 100%, is only about 5%. We show that this is in good agreement with recent HST observations of close binaries in the core of 47 Tuc, provided that a realistic distribution of binary periods is used to interpret the results. Our findings also have important consequences for the dynamical modeling of globular clusters, suggesting that ``realistic models'' should incorporate much larger initial binary fractions than has usually been done in the past.
  01/2004;
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  Available from: arxiv.org

  Article: An Observational Diagnostic for Ultraluminous X-Ray Sources

  V. Kalogera, M. Henninger, N. Ivanova, A. R. King
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  ABSTRACT: We consider observational tests for the nature of Ultraluminous X-ray sources (ULXs). These must distinguish between thermal-timescale mass transfer on to stellar-mass black holes leading to anisotropic X-ray emission, and accretion on to intermediate-mass black holes. We suggest that long-term transient behavior via the thermal-viscous disk instability could discriminate between these two possibilities for ULXs in regions of young stellar populations. Thermal-timescale mass transfer generally produces stable disks and persistent X-ray emission. In contrast, mass transfer from massive stars to black holes produces unstable disks and thus transient behavior, provided that the black hole mass exceeds some minimum value. This minimum mass depends primarily on the donor mass and evolutionary state. We show that it exceeds 50 solar masses for a large fraction (greater than 90%) of the mass-transfer lifetime for the most likely donors in young clusters. Thus if long-term monitoring reveals a large transient fraction among ULXs in a young stellar population, these systems would be good candidates for intermediate-mass black holes in a statistical sense; information about the donor star is needed to make this identification secure in any individual case. A transient ULX population would imply a much larger population of quiescent systems of the same type. Comment: 4 pages, 2 figure, ApJ Letters, in press (correct figure 2 included in this version)
  09/2003;
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  Available from: arxiv.org

  Article: The Role of Helium Stars in the Formation of Double Neutron Stars

  N. Ivanova, K. Belczynski, V. Kalogera, F. A. Rasio, R. E. Taam
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  ABSTRACT: We have calculated the evolution of 60 model binary systems consisting of helium stars in the mass range of M_He= 2.5-6Msun with a 1.4Msun neutron star companion to investigate the formation of double neutron star systems.Orbital periods ranging from 0.09 to 2 days are considered, corresponding to Roche lobe overflow starting from the helium main sequence to after the ignition of carbon burning in the core. We have also examined the evolution into a common envelope phase via secular instability, delayed dynamical instability, and the consequence of matter filling the neutron star's Roche lobe. The survival of some close He-star neutron-star binaries through the last mass transfer episode (either dynamically stable or unstable mass transfer phase) leads to the formation of extremely short-period double neutron star systems (with P<~0.1days). In addition, we find that systems throughout the entire calculated mass range can evolve into a common envelope phase, depending on the orbital period at the onset of mass transfer. The critical orbital period below which common envelope evolution occurs generally increases with M_He. In addition, a common envelope phase may occur during a short time for systems characterized by orbital periods of 0.1-0.5 days at low He-star masses (~ 2.6-3.3Msun). The existence of a short-period population of double neutron stars increases the predicted detection rate of inspiral events by ground-based gravitational-wave detectors and impacts their merger location in host galaxies and their possible role as gamma-ray burst progenitors. We use a set of population synthesis calculations and investigate the implications of the mass-transfer results for the orbital properties of DNS populations. Comment: 30 pages, Latex (AASTeX), 1 table, 8 figures. To appear in ApJ, v592 n1 July 20, 2003
  10/2002;