arXiv subjects

H. Ritter

Publications and source records attributed to H. Ritter.

27 records · Page 2Linked to original sources

arXiv2002

A source of high-velocity white dwarfs

We investigate whether the recently-observed population of high-velocity white dwarfs can be derived from a population of binaries residing initially within the thin disk of the Galaxy. In particular we consider binaries where the primary is sufficiently massive to explode as a type II supernova. A large fraction of such binaries are broken up when the primary then explodes as a supernova owing to the combined effects of the mass loss from the primary and the kick received by the neutron star on its formation. For binaries where the primary evolves to fill its Roche lobe, mass transfer from the primary leads to the onset of a common envelope phase during which the secondary and the core of the primary spiral together as the envelope is ejected. Such binaries are the progenitors of X-ray binaries if they are not broken up when the primary explodes. For those systems which are broken up, a large number of the secondaries receive kick velocities ~100 - 200 km/s and subsequently evolve into white dwarfs. We compute trajectories within the Galactic potential for this population of stars and relate the birthrate of these stars over the entire Galaxy to those seen locally with high velocities relative to the LSR. We show that for a reasonable set of assumptions concerning the Galactic supernova rate and the binary population, our model produces a local number density of high-velocity white dwarfs compatible with that inferred from observations. We therefore propose that a population of white dwarfs originating in the thin disk may make a significant contribution to the observed population of high-velocity white dwarfs.

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arXiv2000

On the spin-up of neutron stars to millisecond pulsars in long-period binaries

We study the accretion efficiency of neutron stars in long-period binaries, i.e. with periods longer than about 200d, which accrete from a giant companion. Using alpha-disc models and taking into account the effect of irradiation of the accretion disc by the central accretion light source we derive explicit expressions for the duty cycle and the accretion efficiency in terms of the parameters of the binary system and the disc instability limit cycle. We show that the absence of millisecond pulsars in wide binaries with circular orbits and periods longer than about 200d can be understood as a consequence of the disc instability if the duration of the quiescent phase between two subsequent outbursts is at least a few decades.

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arXiv2000

Irradiation and mass transfer in low-mass compact binaries

We study the reaction of low-mass stars to anisotropic irradiation and its importance for the long-term evolution of compact binaries binaries. We show by means of a simple homology model that if the energy outflow through the surface layers of a low-mass main sequence star is blocked over a fraction s_eff < 1 of its surface it will inflate only modestly and that the maximum contribution to mass transfer is s_eff times what one obtains in the isotropic case. The duration of this mass transfer phase is roughly -0.1 ln(1-s_eff) times the thermal time scale of the convective envelope. Numerical computations confirm these results. Second, we present a simple analytic one-zone model for computing the blocking effect by irradiation which gives results in acceptable quantitative agreement with detailed numerical computations. Third, we show that unless mass transfer is strongly enhanced by consequential angular momentum losses, cataclysmic variables are stable against mass transfer if the mass of the main sequence donor is less than about 0.7 solar masses. Otherwise systems may be unstable, subject to the efficiency of irradiation. We argue that low-mass X-ray binaries are even less susceptible to this instability. In case of instability, mass transfer must evolve through a limit cycle in which phases of high mass transfer alternate with phases of small (or no) mass transfer.

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arXiv1999

Analytical solution for the evolution of a binary with stable mass transfer from a giant

We derive a simple analytical solution for the evolution of a close binary with nuclear time-scale driven mass transfer from a giant. This solution is based on the well-known fact that the luminosity and the radius of a giant scale to a good approximation as simple power laws of the mass M_c of the degenerate helium core. Comparison with results of numerical calculations by Webbink, Rappaport & Savonije (1983) show the analytical solution and the power law approximation to be quite accurate. The analytical solution presented does also allow (in parametrized form) for non-conservative mass transfer. Furthermore it is shown that the near constancy of the mass transfer rate over most of the mass transfer phase seen in the results by Webbink, Rappaport & Savonije is not a generic feature of this type of evolution but rather a consequence of a particular choice of parameters. The analytical solution also demonstrates that the level of mass transfer is largely set by the core mass of the giant at the onset of mass transfer. Finally we show that the model is selfconsistent and discuss its applicability to low-mass X-ray binaries.

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arXiv1999

Cygnus X-2, super-Eddington mass transfer, and pulsar binaries

We consider the unusual evolutionary state of the secondary star in Cygnus X-2. Spectroscopic data give a low mass ($M_2 \simeq 0.5 - 0.7\msun$) and yet a large radius ($R_2 \simeq 7\rsun$) and high luminosity ($L_2 \simeq 150\lsun$). We show that this star closely resembles a remnant of early massive Case B evolution, during which the neutron star ejected most of the $\sim 3\msun$ transferred from the donor (initial mass $M_{\rm 2i}\sim 3.6\msun$) on its thermal time-scale $\sim 10^6$ yr. As the system is far too wide to result from common-envelope evolution, this strongly supports the idea that a neutron star efficiently ejects the excess inflow during super--Eddington mass transfer. Cygnus X-2 is unusual in having had an initial mass ratio $q_{\rm i} = M_{\rm 2i}/M_1$ in a narrow critical range near $q_{\rm i}\simeq 2.6$. Smaller $q_{\rm i}$ lead to long-period systems with the former donor near the Hayashi line, and larger $q_{\rm i}$ to pulsar binaries with shorter periods and relatively massive white dwarf companions. The latter naturally explain the surprisingly large companion masses in several millisecond pulsar binaries. Systems like Cygnus X-2 may thus be an important channel for forming pulsar binaries.

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arXiv1998

The CV period gap: still there

We consider a recently-proposed alternative explanation of the CV period gap in terms of a revised mass-radius relation for the lower main sequence. We show that no such thermal-equilibrium relation is likely to produce a true gap. Using population synthesis techniques we calculate a model population that obeys the claimed equilibrium mass-radius relation. A theoretical period histogram obtained from this population shows two prominent period spikes rather than a gap. We consider also recent arguments suggesting that the period gap itself may not be real. We argue that, far from demonstrating a weakness of the interrupted-braking picture, the fact that most CV subtypes prefer one side of the gap or the other is actually an expected consequence of it.

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arXiv1997

Transients Among Binaries with Evolved Low-Mass Companions

We show that stable disk accretion should be very rare among low-mass X-ray binaries and cataclysmic variables whose evolution is driven by the nuclear expansion of the secondary star on the first giant branch. Stable accretion is confined to neutron-star systems where the secondary is still relatively massive, and some supersoft white dwarf accretors. All other systems, including all black-hole systems, appear as soft X-ray transients or dwarf novae. All long-period neutron-star systems become transient well before most of the envelope mass is transferred, and remain transient until envelope exhaustion. This complicates attempts to compare the numbers of millisecond pulsars in the Galactic disk with their LMXB progenitors, and also means that the pulsar spin rates are fixed in systems which are transient rather than steady, contrary to common assumption. The long-period persistent sources Sco X-2, LMC X-2, Cyg X-2 and V395 Car must have minimum companion masses > 0.75 Msun if they contain neutron stars, and still larger masses if they contain black holes. The companion in the neutron-star transient GRO J1744-2844 must have a mass <0.87 Msun. The existence of any steady sources at all at long periods supports the ideas that (a) the accretion disks in many, if not all, LMXBs are strongly irradiated by the central source, and (b) mass transfer is thermally unstable in long-period supersoft X-ray sources.

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arXiv1997

Mass Transfer Cycles in Close Binaries with Evolved Companions

We give a global analysis of mass transfer variations in low-mass X-ray binaries and cataclysmic variables whose evolution is driven by the nuclear expansion of the secondary star. We show that limit cycles caused by irradiation of the secondary by the accreting primary are possible in a large class of these binaries. In the high state the companion transfers a large fraction of its envelope mass on a thermal timescale. In most cases this implies super-Eddington transfer rates, and would thus probably lead to common-envelope evolution and the formation of an ultrashort-period binary. Observed systems with (sub)giant secondaries stabilize themselves against this possibility either by being transient, or by shielding the secondary from irradiation in some way.

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arXiv1996

Illumination in binaries

We give a simple, but accurate method that can be used to account for illumination in compact binary systems which have a low-mass companion, even if spherically symmetric illumination of the secondary star (not necessarily on the main sequence) is not assumed. This is done by introducing a multiplicative factor Phi in the Stefan-Boltzmann surface boundary condition, which accounts for the blocking of the intrinsic secondary flux by X-ray heating of the photospheric layers. Numerical fits and tables for Phi are given for unperturbed effective temperatures in the range 2500 - 5600 K and log g in the range 1.0 - 5.0

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