Quantum mechanics

The disc instability model of dwarf novae and low-mass X-ray binary transients

The VIIth catalogue of galactic Wolf–Rayet stars

Subdwarf B (sdB) stars (and related sdO/sdOB stars) are believed to be helium-core-burning objects with very thin hydrogen-rich envelopes. In recent years it has become increasingly clear from observational surveys that a large fraction of these objects are members of binary systems. To understand their formation better, we present the results of a detailed investigation of the three main binary evolution channels that can lead to the formation of sdB stars: the common-envelope (CE) ejection channel, the stable Roche lobe overflow (RLOF) channel, and the double helium white dwarfs (WDs) merger channel. The CE ejection channel leads to the formation of sdB stars in short-period binaries with typical orbital periods between 0.1 and 10 d, very thin hydrogen-rich envelopes and a mass distribution sharply peaked around similar to0.46 M-.. On the other hand, under the assumption that all mass transferred is soon lost, the stable RLOF channel produces sdB stars with similar masses but long orbital periods (400-1500 d) and with rather thick hydrogen-rich envelopes. The merger channel gives rise to single sdB stars whose hydrogen-rich envelopes are extremely thin but which have a fairly wide distribution of masses (0.4-0.65 M-.). We obtained the conditions for the formation of sdB stars from each of these channels using detailed stellar and binary evolution calculations where we modelled the detailed evolution of sdB stars and carried out simplified binary population synthesis simulations. The observed period distribution of sdB stars in compact binaries strongly constrains the CE ejection parameters. The best fits to the observations are obtained for very efficient CE ejection where the envelope ionization energy is included, consistent with previous results. We also present the distribution of sdB stars in the T (eff) -log g diagram, the Hertzsprung-Russell diagram and the distribution of mass functions.

The origin of subdwarf B stars – I. The formation channels

We have carried out a detailed binary population synthesis (BPS) study of the formation of subdwarf B (sdB) stars and related objects (sdO, sdOB stars) using the latest version of the BPS code developed by Han and co-workers. We systematically investigate the importance of the five main evolutionary channels in which the sdB stars form after one or two common-envelope (CE) phases, one or two phases of stable Roche lobe overflow (RLOF) or as the result of the merger of two helium white dwarfs (WDs). Our best BPS model can satisfactorily explain the main observational characteristics of sdB stars, in particular their distributions in the orbital period-minimum companion mass (log P-M-comp) diagram and in the effective temperature-surface gravity (T-eff- log g) diagram, their distributions of orbital period, log (gtheta(4))(theta = 5040 K / T-eff) and mass function, their binary fraction and the fraction of sdB binaries with WD companions, their birth rates and their space density. We obtain a Galactic formation rate for sdB stars of 0.014- 0.063 yr(-1) with a best estimate of similar to0.05 yr(-1) and a total number in the Galaxy of 2.4-9.5 x 10(6) with a best estimate of similar to6 x 10(6); half of these may be missing in observational surveys owing to selection effects. The intrinsic binary fraction is 76-89 per cent, although the observed frequency may be substantially lower owing to the selection effects. The first CE ejection channel, the first stable RLOF channel and the merger channel are intrinsically the most important channels, although observational selection effects tend to increase the relative importance of the second CE ejection and merger channels. We also predict a distribution of masses for sdB stars that is wider than is commonly assumed and that some sdB stars have companions of spectral type as early as B. The percentage of A-type stars with sdB companions can in principle be used to constrain some of the important parameters in the binary evolution model. We conclude that (i) the first RLOF phase needs to be more stable than is commonly assumed, either because the critical mass ratio q(crit) for dynamical mass transfer is higher or because of tidally enhanced stellar wind mass loss; (ii) mass transfer in the first stable RLOF phase is non-conservative, and the mass lost from the system takes away a specific angular momentum similar to that of the system; and (iii) common-envelope ejection is very efficient.

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The origin of subdwarf B stars – II

We present spectra of the black hole binary A0620-00 taken at Hα and Hβ to study the distribution of emission-line flux from its accretion disc. Using the spectra of Hα, we have measured the K-type companion star's radial velocity semi-amplitude and rotational broadening to be K 2 =433±3 km s −1 and v sin i=83±5 km s −1 . Accounting for the Roche-lobe geometry of the companion, the rotational broadening gives the mass ratio q=M 2 /M 1 =0.06 7±0.01. The disc contributes 6±3 per cent of the light at Hα and 17±3 per cent at Hβ, with the K star providing the rest. The masses of the compact object and K star derived from our values of K 2 and q are M 1 =(3.09±0.09)sin −3 i M ○. and M 2 =(0.21±0.04)sin −3 i M ○.

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Spectroscopy of $A0620-00$: the mass of the black hole and an image of its accretion disc

/pdf/cataclysmic-variables-and-related-objects-1xmpsx5kq1.pdf

Cataclysmic variables and related objects

We present the results of high time resolution UV spectroscopy and simultaneous high-speed UBVR photometry of AE Aqr. The UV spectra were obtained with the Faint Object Spectrograph aboard the Hubble Space Telescope (HST), and the photometry was carried out with the 82 sec telescope at McDonald Observatory. Our study focuses on the coherent 33 sceond oscillations, whose amplitude is found to be very large in the UV (40% of the mean quiescent level). The mean pulse profile has two broad unequal peaks spaced by half an oscillation cycle. The pulse profiles in the UV and optical bands appear quite similar in shape, with no discernible shifts. The orbital delay curve of the UV pulses establishes the white dwarf as their origin. The (UV+optical) spectrum of the pulsations is well described by a white dwarf atmosphere model with a temperature of about 26,000 K. We find no oscillations in the UV emission-line fluxes, nor in their velocities, down to a limit of 800 km/s. Based on the properties of the UV and optical pulsations we suggest that they originate in the X-ray heated magnetic polar caps of the white dwarf. Under this assumption we produce maximum entropy maps of the brightness distribution of the white dwarf surface. Using this model we are able to reproduce the observed mean pulse profile and interpret fluctuations in the oscillation amplitude as small fluctuations in the accretion rate. We find that the amplitudes and profiles of the pulses are not strongly affectd by the large aperiodic flares exhibited by the system. This suggests that the large flares are not related to the process of depositing material onto the white dwarf and argues against models that place their origin at the white dwarf magnetosphere.

The ultraviolet pulsations of the cataclysmic variable ae aquarii as observed with the hubble space telescope

We report on a 1 day ASCA observation and ROSAT HRI observations of the eclipsing dwarf nova HT Cas. The presence of the X-ray eclipse, originally detected in a ROSAT PSPC observation, is confirmed at a much higher significance level. The quality of the ASCA light curve is high enough to allow detailed investigation of the eclipse depth, width, and shape. The eclipse is found to be deep, compatible with being total. The eclipse width is comparable to that of the white dwarf, as derived from optical light curves, and in fact may be narrower. The eclipse transition is also found to be short, which puts a limit of 1.15 times the white dwarf radius as the total size of the X-ray emission region. The out-of-eclipse spectrum of HT Cas is found to be consistent with a single temperature (kT ~ 10 keV), absorbed (NH ~ 3.3 × 1021 cm-2), thermal model.

The X-ray eclipse of the dwarf nova HT cassiopeiae: Results from ASCA and ROSAT HRI observations

We present optical spectroscopy of the post-common envelope binary HW Vir covering λ =3704-8667 A We use the low-resolution blue spectra to determine the atmospheric parameters of the sdB primary star and the medium resolution Hα observations to measure its radial velocity We find K1=823 ± 40 km s-1 and γ = 29 ± 31 km s-1 The effective temperature, gravity and helium abundance of the sdB star are Te= 28488 ± 208 K, log g=563 ± 003 and N(He)/N(H)=00066 ± 00004 These characteristics are typical of a classical sdB star, as is our derived mass of M1=048 ± 009 M⊙ We find the distance to HW Vir is 171 ± 19 pc The predicted value of the projected orbital velocity of the secondary star is 284 ± 21 km s-1



Using previously published photometric light curves of HW Vir we corrected our spectra to absolute fluxes and extracted the spectrum of the secondary star This spectrum shows Hα absorption lines at phases close to the maximum of the reflection effect, with a measured radial velocity semi-amplitude of 275±15 km s−1 We attribute these lines to irradiation of the face of the secondary star closest to the sdB star

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Janet H. Wood

Papers

Spectroscopy of $A0620-00$: the mass of the black hole and an image of its accretion disc

Cataclysmic variables and related objects

The ultraviolet pulsations of the cataclysmic variable ae aquarii as observed with the hubble space telescope

The X-ray eclipse of the dwarf nova HT cassiopeiae: Results from ASCA and ROSAT HRI observations

Spectroscopy of the post-common envelope binary HW Virginis