arXiv subjects

U. Kolb

Publications and source records attributed to U. Kolb.

62 records · Page 4Linked to original sources

arXiv2000

On the late spectral types of cataclysmic variable secondaries

We investigate why the spectral type of most cataclysmic variable (CV) secondaries is significantly later than that of a ZAMS star with the same mean density. Using improved stellar input physics, tested against observations of low-mass stars at the bottom of the main sequence, we calculate the secular evolution of CVs with low-mass donors. We consider sequences with different mass transfer rates and with a different degree of nuclear evolution of the donor prior to mass transfer. Systems near the upper edge of the gap ($P \sim 3 - 6$ h) can be reproduced by models with a wide range of mass transfer rates from $1.5 \times 10^{-9} \msolyr$ to $10^{-8} \msolyr$. Evolutionary sequences with a small transfer rate and donors that are substantially evolved off the ZAMS (central hydrogen content $0.05-0.5$) reproduce CVs with late spectral types above $P \simgr$ 6 h. Systems with the most discrepant (late) spectral type should have the smallest donor mass at any given $P$. Consistency with the period gap suggests that the mass transfer rate increases with decreasing donor mass for evolved sequences above the period gap. In this case, a single-parameter family of sequences with varying $\xc$ and increasing mass transfer rate reproduces the full range of observed spectral types. This would imply that CVs with such evolved secondaries dominate the CV population.

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arXiv1999

A mystery solved: the mass ratio of the dwarf nova EM Cygni

We have discovered that the spectrum of the well-known dwarf nova EM Cyg is contaminated by light from a K2-5V star (in addition to the K-type mass donor star). The K2-5V star contributes approximately 16 per cent of the light from the system and if not taken into account has a considerable effect upon radial velocity measurements of the mass donor star. We obtain a new radial velocity amplitude for the mass donor star of K2 = 202 +/- 3 km/s, which compares with the value of K2 = 135 +/- 3 km/s obtained in Stover, Robinson & Nather's classic 1981 study of EM Cyg. The revised value of the amplitude combined with a measurement of rotational broadening of the mass donor vsini = 140 +/- 6 km/s, leads to a new mass ratio of q = M2/M1 = 0.88 +/- 0.05. This solves a long standing problem with EM Cyg because Stover et al.'s measurements indicated a mass ratio q > 1, a value which should have led to dynamically unstable mass transfer for the secondary mass deduced by Stover et al. The revised value of the mass ratio combined with the orbital inclination i = 67 +/- 2 degrees leads to masses of 0.99 +/- 0.12 Msun and 1.12 +/- 0.08 Msun for the mass donor and white dwarf respectively. The mass donor is evolved, since it has a later spectral type (K3) than its mass would imply. We discuss whether the K star could be physically associated with EM Cyg or not, and present the results of the spectroscopic study.

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arXiv1999

The evolution of black-hole mass and angular momentum

We show that neither accretion nor angular momentum extraction are likely to lead to significant changes in the mass M_1 or angular momentum parameter a_* of a black hole in a binary system with realistic parameters. Current values of M_1 and a_* therefore probably reflect those at formation. We show further that sufficiently energetic jet ejection powered by the black hole's rotational energy can stabilize mass transfer in systems with large adverse mass ratios, and even reduce the mass transfer rate to the point where the binary becomes transient.

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arXiv1998

Are the red dwarfs in cataclysmic variables main-sequence stars?

We show that the secondaries in short-period cataclysmic variables with orbital periods $P < 3$ hr are close to the solar-abundance main sequence defined by single field stars. In cataclysmic variables with $P > 3$ hr, the earliest spectral types at a given period correspond to main sequence stars, while the majority of secondaries have later spectral types. Possible causes are nuclear evolution prior to mass transfer and lack of thermal equilibrium due to mass transfer. A comparison with evolutionary sequences obtained with up--to--date stellar models implies unusually high transfer rates and a large fraction of systems with evolved donors. There is no evidence for a secondary of low metallicity in any of the well-studied cataclysmic variables.

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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

The Formation of Low-Mass Transient X-Ray Binaries

We consider constraints on the formation of low-mass X-ray binaries containing neutron stars (NLMXBs) arising from the presence of soft X-ray transients among these systems. We show that in short-period systems driven by angular momentum loss these constraints require the secondary at the beginning of mass transfer to have a mass > 1.2 M_sun, and to be significantly nuclear-evolved. As a consequence a comparatively large fraction of such systems appear as soft X-ray transients even at short periods, as observed. Moreover the large initial secondary masses account for the rarity of NLMXBs at periods less than 3 hr. In contrast, NLMXB populations forming with large kick velocities would not have these properties, suggesting that the kick velocity is generally small compared to the pre-SN orbital velocity in a large fraction of systems. We derive constraints on progenitor system parameters and on the strength of magnetic braking.

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