arXiv subjects

Frank Verbunt

Publications and source records attributed to Frank Verbunt.

48 records · Page 3Linked to original sources

arXiv2000

Population synthesis for double white dwarfs I.Close detached systems

We model the population of double white dwarfs in the Galaxy and find a better agreement with observations compared to earlier studies, due to two modifications. The first is the treatment of the first phase of unstable mass transfer and the second the modelling of the cooling of the white dwarfs. A satisfactory agreement with observations of the local sample of white dwarfs is achieved if we assume that the initial binary fraction is ~ 50% and that the lowest mass white dwarfs (M < 0.3 Msun) cool faster than the most recently published cooling models predict. With this model we find a Galactic birth rate of close double white dwarfs of 0.05 yr^{-1}, a birth rate of AM CVn systems of 0.005 yr^{-1}, a merger rate of pairs with a combined mass exceeding the Chandrasekhar limit (which may be progenitors of SNe Ia) of 0.003 yr^{-1} and a formation rate of planetary nebulae of 1 yr^{-1}. We estimate the total number of double white dwarfs in the Galaxy as 2.5 10^8. In an observable sample with a limiting magnitude V_lim = 15 we predict the presence of ~855 white dwarfs of which ~220 are close pairs. Of these 10 are double CO white dwarfs of which one has a combined mass exceeding the Chandrasekhar limit and will merge within a Hubble time.

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arXiv2000

Reconstructing the evolution of double helium white dwarfs: envelope loss without spiral-in

The unique core-mass - radius relation for giants with degenerate helium cores enables us to reconstruct the evolution of three observed double helium white dwarfs with known masses of both components. The last mass transfer phase in their evolution must have been a spiral-in. In the formalism proposed by Webbink (1984), we can constrain the efficiency of the deposition of orbital energy into the envelope to be $1 \la α\la 6$, for an envelope structure parameter $λ=0.5$. We find that the two standard mass transfer types (stable mass transfer and spiral-in) are both unable to explain the first phase of mass transfer for these three binaries. We use a parametric approach to describe mass transfer in low-mass binaries, where both stars are of comparable mass and find that the orbital characteristics of the observed double helium white dwarfs can be well reproduced if the envelope of the primary is lost with ~1.5 times the specific angular momentum of the initial binary. In this case no substantial spiral-in occurs.

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arXiv2000

Multiple and variable X-ray sources in the globular clusters omega Cen, NGC6397, NGC6752, and Liller 1

We detect stars from the Hipparcos and Tycho Catalogues in the field of view of observations with the ROSAT HRI of three globular clusters. We use the positions of these stars to reduce the systematic error in the positions of X-ray sources in the clusters to ~2'' for omega Cen and NGC6752, and 1'' for NGC6397. We detect three X-ray sources in the core of omega Cen, and four in the core of NGC6752; the data for the center of NGC6397 may be fitted with five or six sources. Outside the cores, but within the half-mass radius of the clusters, we detect two sources in omega Cen, one in NGC6397 and two in NGC6752; these may or may not be cluster members. A ROSAT HRI observation of Liller 1 does not detect a low-luminosity source, at a limit below a detection with ASCA. We discuss the nature of the low-luminosity X-ray sources in globular clusters in the light of these new results.

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arXiv1999

X-rays from old star clusters

A brief overview is given of X-ray observations of old clusters. Most X-ray sources in old open clusters are interacting binaries, formed via evolution of a primordial binary, and emitting X-rays because of magnetic activity; however, a sizable fraction of the cluster sources is not well understood, including some of the most luminous ones. Globular clusters appear to contain fewer magnetically active X-ray sources than expected if one scales from old open clusters by mass.

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arXiv1999

Optical spectroscopy of X-ray sources in the old open cluster M67

We have obtained optical spectra of seven stars in the old galactic cluster M67 that are unusual sources of X-rays, and investigate whether the X-ray emission is due to magnetic activity or to mass transfer. The two binaries below the giant branch S1063 and S1113, the giant with the white dwarf companion S1040 and the eccentric binary on the subgiant branch S1242 show magnetic activity in the form of Ca II H&K emission and H-alpha emission, suggesting that their X-rays are coronal. The reason for the enhanced activity level in S1040 is not clear. The two wide, eccentric binaries S1072 and S1237 and the blue straggler S1082 do not show evidence for Ca II H&K emission. A second spectral component is found in the spectrum of S1082, most clearly in the variable H-alpha absorption profile. We interpret this as a signature of the proposed hot subluminous companion.

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arXiv1999

X-ray observations through the outburst cycle of the dwarf nova YZ Cnc

We have observed YZ Cnc at two day intervals from 6 to 24 April 1998, covering two full outburst cycles. The 0.1-2.4 keV flux is lower during optical outburst than in quiescence, and lowest at the end of the outburst. The decline of the X-ray flux in the quiescent interval appears to be in contrast to prediction of simple models for accretion-disk instabilities. Variability on hour time scales is present, but appears not related to the orbital phase. YZ Cnc was less luminous in X-rays during our 1998 observations than in earlier ROSAT observations.

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arXiv1999

X-ray sources in old stellar clusters: the contribution of ROSAT

Two new bright X-ray sources in globular clusters, and many less luminous ones in globular clusters and in old open clusters, have been discovered with ROSAT. Accurate positions obtained with ROSAT help identification with optical objects, which however is still very incomplete in globular clusters. One dim globular cluster source has been identified with a recycled radio pulsar; several others may be cataclysmic variables. The four brightest X-ray sources identified in the old open cluster M67 are puzzling, as we do not understand why they emit X-rays. In comparison with the two old open clusters studied so far, globular clusters are remarkably underluminous in X-rays.

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arXiv1998

Observations of accretion disks in X-ray binaries

A brief overview is given of the observations of accretion disks in X-ray binaries, discussing their geometry, their effect on the rotation period of the accreting object, their optical and X-ray spectra, time variability, and radio emission and jets. Some outstanding questions for disk theory are listed.

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arXiv1997

Formation of Low-mass Black Hole X-ray Transients

We study the formation of low-mass X-ray binaries with a black hole as accreting object. The performed semi-analytic analysis reveals that the formation rate of black holes in low-mass X-ray binaries is about two orders of magnitude smaller than that of systems with a neutron star as accretor. This is contradicted by the six observed systems, which are all transients, which suggest that the majority of these systems has not been seen jet. The birthrate for both type of objects are expected to be similar (for reviews see Cowley 1992, Tanaka & Lewin 1995). Solution of this conundrum requires either that stars with masses less than 20~\msun can evolve into black holes, or that stellar wind from a member of a binary is accompanied by a much larger loss of angular momentum than hitherto assumed.

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arXiv1997

Star Cluster Ecology II: Binary evolution with single-star encounters

Three-body effects greatly complicate stellar evolution. We model the effects of encounters of binaries with single stars, based on parameters chosen from conditions prevalent in the cores of globular clusters. For our three-body encounters, we start with a population of primordial binaries, and choose incoming stars from the evolving single star populations of Portgies Zwart, Hut and Verbunt (1997). In addition, we study the formation of new binaries through tidal capture among the single stars. In subsequent papers in this series, we will combine stellar evolution with the dynamics of a full N-body system.

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arXiv1997

Star Cluster Ecology I: A Cluster Core with Encounters between Single Stars

Star clusters with a high central density contain an ecological network of evolving binaries, affected by interactions with passing stars, while in turn affecting the energy budget of the cluster as a whole by giving off binding energy. This is the first paper in a series aimed at providing the tools for increasingly realistic simulations of these ecological networks.

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arXiv1997

The formation of black-holes in low-mass X-ray binaries

We calculate the formation rates of low-mass X-ray binaries with a black hole. Both a semi-analytic and a more detailed model predict formation rates two orders of magnitude lower than derived from the observations. Solution of this conundrum requires either that stars with masses less than 20M$_\odot$ can evolve into a black hole, or that stellar wind from a member of a binary is accompanied by a much larger loss of angular momentum than hitherto assumed.

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