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1.
We critically re-examine the available data on the spectral types, masses and radii of the secondary stars in cataclysmic variables (CVs) and low-mass X-ray binaries (LMXBs), using the new catalogue of Ritter &38; Kolb as a starting point. We find there are 55 reliable spectral type determinations and only 14 reliable mass determinations of CV secondary stars (10 and 5, respectively, in the case of LMXBs). We derive new spectral type–period, mass–radius, mass–period and radius–period relations, and compare them with theoretical predictions. We find that CV secondary stars with orbital periods shorter than 7–8 h are, as a group, indistinguishable from main-sequence stars in detached binaries. We find that it is not valid, however, to estimate the mass from the spectral type of the secondary star in CVs or LMXBs. We find that LMXB secondary stars show some evidence for evolution, with secondary stars which are slightly too large for their mass. We show how the masses and radii of the secondary stars in CVs can be used to test the validity of the disrupted magnetic braking model of CV evolution, but we find that the currently available data are not sufficiently accurate or numerous to allow such an analysis. As well as considering secondary star masses, we also discuss the masses of the white dwarfs in CVs, and find mean values of − M  = 0.69 ± 0.13 M⊙ below the period gap, and − M  = 0.80 ± 0.22 M⊙ above the period gap.  相似文献   

2.
An analysis of X-ray and optical light curves of the magnetic cataclysmic variable (MCV) BY Cam is presented. This system is one of three MCVs in which the spin period of the white dwarf and the binary orbital period differ by ∼1 per cent. As such these 'BY Cam' stars are important objects with which to probe the field structure of the magnetic white dwarf and ultimately the nature of synchronization of AM Her binaries. We confirm asynchronous rotation of the magnetic white dwarf with respect to the binary. We find evidence that the accretion stream accretes directly on to the white dwarf as in AM Her systems, but further, the stream impacts on to different magnetic poles over the course of the beat period. We present evidence that the optical and hard X-ray light curves modulate in phase, but together they are out of phase with the soft X-ray light curve. We confirm the spin down of the white dwarf which is expected to lead to the synchronization of the spin and orbital periods of BY Cam.  相似文献   

3.
We report on the identification of cyclical changes in the orbital period of the eclipsing dwarf novae V2051 Ophiuchi and V4140 Sagittarii. We used sets of white dwarf mid-eclipse timings to construct observed-minus-calculated diagrams covering, respectively, 25 and 16 yr of observations. The V2051 Oph data present cyclical variations that can be fitted by a linear plus sinusoidal function with period of  22 ± 2 yr  and amplitude of  17 ± 3 s  . The statistical significance of this period by an F-test is larger than 99.9 per cent. The V4140 Sgr data present cyclical variations of similar amplitude and period of  6.9 ± 0.3 yr  which are statistically significant at the 99.7 per cent level. We derive upper limits for secular period changes of     and     for V2051 Oph and V4140 Sgr, respectively.
We have combined our results with those in the literature to construct a diagram of the amplitude versus period of the modulation for a sample of 11 eclipsing cataclysmic variables (CVs). If the cyclical period changes are the consequence of a solar-type magnetic activity cycle in the secondary star, then magnetic activity is a widespread phenomenon in CVs, being equally common among long- and short-period systems. This gives independent evidence that the magnetic field (and activity) of the secondary stars of CVs do not disappear when they become fully convective. We also find that the fractional cycle period changes of the short-period CVs are systematically smaller than those of the long-period CVs.  相似文献   

4.
We present medium-resolution VLT/FORS2 spectroscopy of six cataclysmic variables (CVs) discovered by the Sloan Digital Sky Survey (SDSS). We determine orbital periods for  SDSS J023322.61+005059.5 (96.08 ± 0.09 min), SDSS J091127.36+084140.7 (295.74 ± 0.22 min), SDSS J103533.02+055158.3 (82.10 ± 0.09 min)  and SDSS J121607.03+052013.9 (most likely 98.82 ± 0.16 min, but the one-day aliases at 92 and 107 min are also possible) using radial velocities measured from their Hα and Hβ emission lines. Three of the four orbital periods measured here are close to the observed 75–80 min minimum period for CVs, indicating that the properties of the population of these objects discovered by the SDSS are substantially different to those of the CVs found by other means. Additional photometry of SDSS J023322.61+005059.5 reveals a periodicity of approximately 60 min which we interpret as the spin period of the white dwarf, suggesting that this system is an intermediate polar with a low accretion rate. SDSS J103533.02+055158.3 has a period right at the observed minimum value, a spectrum dominated by the cool white dwarf primary star and exhibits deep eclipses, so is an excellent candidate for an accurate determination of the parameters of the system. The spectroscopic orbit of SDSS J121607.03+052013.9 has a velocity amplitude of only  13.8 ± 1.6 km s−1  , implying that this system has an extreme mass ratio. From several physical constraints we find that this object must contain either a high-mass white dwarf or a brown-dwarf-mass secondary component or both.  相似文献   

5.
Intermediate polars (IPs) are cataclysmic variables which contain magnetic white dwarfs with a rotational period shorter than the binary orbital period. Evolutionary theory predicts that IPs with long orbital periods evolve through the 2–3 h period gap, but it is very uncertain what the properties of the resulting objects are. Whilst a relatively large number of long-period IPs are known, very few of these have short orbital periods. We present phase-resolved spectroscopy and photometry of SDSS J233325.92+152222.1 (SDSS J2333) and classify it as the IP with the shortest-known orbital period (83.12 ± 0.09 min), which contains a white dwarf with a relatively long spin period (41.66 ± 0.13 min). We estimate the white dwarf's magnetic moment to be μWD≈ 2 × 1033 G cm3, which is not only similar to three of the other four confirmed short-period IPs but also to those of many of the long-period IPs. We suggest that long-period IPs conserve their magnetic moment as they evolve towards shorter orbital periods. Therefore, the dominant population of long-period IPs, which have white dwarf spin periods roughly 10 times shorter than their orbital periods, will likely end up as short-period IPs like SDSS J2333, with spin periods a large fraction of their orbital periods.  相似文献   

6.
We present low–medium resolution optical spectroscopy of the eclipsing AM Her system MN Hya (RX J0929–24). We determine the magnetic field strength at the primary accretion region of the white dwarf to be 42 MG from the spacing of cyclotron features visible during π ∼ 0.4–0.7. From spectra taken during the eclipse we find that the secondary has an M3–4 spectral type. Combined with the eclipse photometry of Sekiguchi, Nakada &38; Bassett and an estimate of the interstellar extinction we find a distance of ∼300–700 pc. We find unusual line variations at π ∼ 0.9: Hα is seen in absorption and emission. This is at the same point in the orbital phase at which a prominent absorption dip is seen in soft X-rays.  相似文献   

7.
We discuss the properties of 137 cataclysmic variables (CVs) which are included in the Sloan Digital Sky Survey (SDSS) spectroscopic data base, and for which accurate orbital periods have been measured. 92 of these systems are new discoveries from SDSS and were followed-up in more detail over the past few years. 45 systems were previously identified as CVs because of the detection of optical outbursts and/or X-ray emission, and subsequently re-identified from the SDSS spectroscopy. The period distribution of the SDSS CVs differs dramatically from that of all the previously known CVs, in particular it contains a significant accumulation of systems in the orbital period range 80–86 min. We identify this feature as the elusive 'period minimum spike' predicted by CV population models, which resolves a long-standing discrepancy between compact binary evolution theory and observations. We show that this spike is almost entirely due to the large number of CVs with very low accretion activity identified by SDSS. The optical spectra of these systems are dominated by emission from the white dwarf photosphere, and display little or no spectroscopic signature from the donor stars, suggesting very low mass companion stars. We determine the average absolute magnitude of these low-luminosity CVs at the period minimum to be  〈 Mg 〉= 11.6 ± 0.7  . Comparison of the SDSS CV sample to the CVs found in the Hamburg Quasar Survey and the Palomar Green Survey suggests that the depth of SDSS is the key ingredient resulting in the discovery of a large number of intrinsically faint short-period systems.  相似文献   

8.
Archival data on permanent superhump systems are compiled to test the thermal stability of their accretion discs. We find that their discs are almost certainly thermally stable as expected. This result confirms Osaki's suggestion that permanent superhump systems form a new subclass of cataclysmic variables (CVs), with relatively short orbital periods and high mass-transfer rates. We note that if the high accretion rates estimated in permanent superhump systems represent their mean secular values, then their mass-transfer rates cannot be explained by gravitational radiation, therefore, either magnetic braking should be extrapolated to systems below the period gap or they must have mass-transfer cycles. Alternatively, a new mechanism that removes angular momentum from CVs below the gap should be invoked.
We suggest applying the nova cycle scenarios offered for systems above the period gap to the short orbital period CVs. Permanent superhumps have been observed in the two non-magnetic ex-novae with binary periods below the gap. Their post-nova magnitudes are brighter than their pre-outburst values. In one case (V1974 Cyg) it has been demonstrated that the pre-nova should have been a regular SU UMa system. Thus, it is the first nova whose accretion disc was observed to change its thermal stability. If the superhumps in this system indicate persistent high mass-transfer rates rather than a temporary change induced by irradiation from the hot post-nova white dwarf, it is the first direct evidence for mass-transfer cycles in CVs. The proposed cycles are driven by the nova eruption.  相似文献   

9.
One method of obtaining the mass of the white dwarf in magnetic cataclysmic variables (mCVs) is through their hard X-ray spectra. However, previous mass estimates using this method give lower limits because the temperature of the plasma in the post-shock region (where the hard X-rays are emitted) is lower than the temperature of the shock itself. In AM Her systems, the additional cooling of the post-shock plasma by cyclotron emission will further lower the derived mass. Here we present estimates of the masses of the white dwarf in 13 mCVs derived using Ginga data and a model in which X-rays are emitted from a multi-temperature emission region with the appropriate temperature and density profile. We include in the model reflection from the surface of the white dwarf and a partially ionized absorber. We are able to achieve good fits to the data. We compare the derived masses with previous estimates and the masses for larger samples of isolated white dwarfs and those in CVs.  相似文献   

10.
We argue that the period distribution of AM Herculis binaries in the enlarged sample incorporating results from the recent ROSAT X-ray survey differs significantly from that of other cataclysmic variables. In particular, there is no evidence for a pronounced period gap at 2—3 hr and the significance of the period spike at about 114 min is brought into question. We present an alternative evolutionary scenario for the AM Herculis binaries based on the hypothesis that magnetic braking by the stellar wind of the M star secondary either ceases or is severely curtailed when the rotation of the magnetic white dwarf becomes synchronised with the orbital motion, The orbital evolution of the AM Herculis binaries is thereafter driven mainly by angular momentum loss due to gravitational radiation. This scenario not only explains the higher proportion of AM Herculis binaries in the period gap when compared with other cataclysmic variables but also provides a natural explanation for the low mass transfer rates in these binaries and for the existence of an apparent upper limit for the surface magnetic fields of their white dwarfs.  相似文献   

11.
Intermediate polars (IPs) are a group of cataclysmic variables (CVs) which are thought to contain white dwarfs which have a magnetic field strength in the range ∼0.1–10 MG. A significant fraction of the X-ray sources detected in recent deep surveys has been postulated to consist of IPs. Until now two of the defining characteristics of IPs have been the presence of high (and complex) absorption in their X-ray spectra and the presence of a stable modulation in the X-ray light curve which is a signature of the spin period, or the beat period, of the accreting white dwarf. Three CVs, V426 Oph, EI UMa and LS Peg, have characteristics which are similar to IPs. However, there has been only tentative evidence for a coherent period in their X-ray light curve. We present the results of a search for coherent periods in XMM–Newton data of these sources using an autoregressive analysis which models the effects of red noise. We confirm the detection of a ∼760 s period in the soft X-ray light curve of EI UMa reported by Reimer et al. and agree that this represents the spin period. We also find evidence for peaks in the power spectrum of each source in the range 100–200 s which are just above the 3σ confidence level. We do not believe that they represent genuine coherent modulations. However, their X-ray spectra are very similar to those of known IPs. We believe that all three CVs are bona fide IPs. We speculate that V426 Oph and LS Peg do not show evidence for a spin period since they have closely aligned magnetic and spin axes. We discuss the implications that this has for the defining characteristics of IPs.  相似文献   

12.
We present observations and an analysis of the X-ray source 1RXS J0832.6–2525 which shows it to be a low field magnetic white dwarf with an unusual high mass. This is the second magnetic white dwarf for which a determination of a spectroscopic mass has been possible, and both stars belong to the growing class of ultramassive white dwarfs ( M  ≥ 1.1 M⊙).  相似文献   

13.
I present pointed ROSAT PSPC observations of the pre-cataclysmic binary V471 Tauri. The hard X-ray emission (>0.4 keV) is not eclipsed by the K star, demonstrating conclusively that this component cannot be emitted by the white dwarf. Instead I show that its spectrum and luminosity are consistent with coronal emission from the tidally spun-up K star. The star is more active than other K stars in the Hyades, but equally active as K stars in the Pleiades with the same rotation periods, demonstrating that rotation — and not age — is the key parameter in determining the level of stellar activity.   The soft X-ray emission (<0.4 keV) is emitted predominately by the white dwarf and is modulated on its spin period. I find that the pulse profile is stable on time-scales of hours and years, supporting the idea that it is caused by the opacity of accreted material. The profile itself shows that the magnetic field configuration of the white dwarf is dipolar and that the magnetic axis passes through the centre of the star.   There is an absorption feature in the light curve of the white dwarf, which occurs at a time when our line of sight passes within a stellar radius of the K star. The column density and duration of this feature imply a volume and mass for the absorber that are similar to those of coronal mass ejections of the Sun.   Finally I suggest that the spin–orbit beat period detected in the optical by Clemens et al. may be the result of the interaction of the K-star wind with the magnetic field of the white dwarf.  相似文献   

14.
We show that the recently discovered short period supersoft source in M31 is probably a progenitor of a magnetic cataclysmic variable (CV). The white dwarf spins asynchronously because of the current high accretion rate. However its fieldstrength is typical of an AM Herculis system, which is what it will ultimately become. We discuss the relevance of this system to CV evolution, and its relation to some particular CVs with special characteristics.  相似文献   

15.
Near-infrared photometry and spectroscopy are obtained for a heterogeneous sample of nearby white dwarfs with possible excess flux as identified primarily in the Two Micron All Sky Survey. Among the sample of 43 stars are a number of white dwarfs that are either metal-rich, magnetic or binary suspects. With a few notable exceptions in four (or possibly five) distinct categories, the newly obtained JHK photometric data fail to corroborate the putative excesses, with  〈 K IRTF− K 2MASS〉=+0.31  mag. Where available, Galaxy Evolution Explorer photometric data are used to better constrain the overall spectral energy distribution of the white dwarfs, enabling any excess near-infrared flux to stand out more readily against the expected stellar photosphere.
With superior data, a near-infrared photometric excess is confirmed at three metal-rich white dwarfs and ruled out at nine others. Several new binaries are confirmed or suggested; five white dwarf–red dwarf pairs and five double degenerates. Four apparently single magnetic white dwarfs – two DA and two DQp – display modest to strong near-infrared excess (relative to non-magnetic models), which may be better described as two effective temperatures owing to a redistribution of energy in highly magnetic or peculiar atmospheres.  相似文献   

16.
Using improved, up-to-date stellar input physics tested against observations of low-mass stars and brown dwarfs, we calculate the secular evolution of low-mass donor cataclysmic variables (CVs), including those that form with a brown-dwarf donor. Our models confirm the mismatch between the calculated minimum period ( P min70 min) and the observed short-period cut-off (80 min) in the CV period histogram. We find that tidal and rotational corrections applied to the one-dimensional stellar structure equations have no significant effect on the period minimum. Theoretical period distributions synthesized from our model sequences always show an accumulation of systems at the minimum period, a feature absent from the observed distribution. We suggest that non-magnetic CVs become unobservable as they are effectively trapped in permanent quiescence before they reach P min, and that small-number statistics may hide the period spike for magnetic CVs.  相似文献   

17.
The Sloan Digital Sky Survey has been instrumental in obtaining a homogeneous sample of the rare AM CVn stars: mass-transferring binary white dwarfs. As part of a campaign of spectroscopic follow-up on candidate AM CVn stars from the Sloan Digital Sky Survey, we have obtained time-resolved spectra of the   g = 20.2  candidate SDSS J155252.48+320150.9 on the Very Large Telescope of the European Southern Observatory. We report an orbital period   P orb= 3376.3 ± 0.3 s  , or 56.272 ± 0.005 min, based on an observed 'S-wave' in the helium emission lines of the spectra. This confirms the ultracompact nature of the binary. Despite its relative closeness to the orbital period minimum for hydrogen-rich donors, there is no evidence for hydrogen in the spectra. We thus classify SDSS J1552 as a new bona fide AM CVn star, with the second-longest orbital period after V396 Hya  ( P = 65.5 min)  . The continuum of SDSS J1552 is compatible with either a blackbody or helium atmosphere of   T eff= 12 000–15 000 K  . If this represents the photosphere of the accreting white dwarf, as is expected, it puts the accretor at the upper end of the temperature range predicted by thermal evolution models. This suggests that SDSS J1552 consists of (or formerly consisted of) relatively high-mass components.  相似文献   

18.
We present the discovery of the widest known ultracool dwarf–white dwarf binary. This binary is the first spectroscopically confirmed widely separated system from our target sample. We have used the Two-Micron All-Sky Survey (2MASS) and SuperCOSMOS archives in the southern hemisphere, searching for very widely separated ultracool dwarf–white dwarf binaries, and find one common proper motion system, with a separation of 3650–5250 au at an estimated distance of 41–59 pc, making it the widest known system of this type. Spectroscopy reveals 2MASS J0030−3740 is a DA white dwarf with   T eff= 7600 ± 100 K, log( g ) = 7.79–8.09  and   M WD= 0.48–0.65 M⊙  . We spectroscopically type the ultracool dwarf companion (2MASS J0030−3739) as M9 ± 1 and estimate a mass of  0.07–0.08 M⊙,  T eff= 2000–2400 K  and  log( g ) = 5.30–5.35  , placing it near the mass limit for brown dwarfs. We estimate the age of the system to be >1.94 Gyr (from the white dwarf cooling age and the likely length of the main-sequence lifetime of the progenitor) and suggest that this system and other such wide binaries can be used as benchmark ultracool dwarfs.  相似文献   

19.
We report spectroscopic orbital periods of 0.147 d (=3.53 h) for V533 Her, 0.207 d (=4.97 h) for V446 Her and 1.478 d for X Ser. V533 Her (Nova Herculis 1963) shows absorption features in its He  i and Balmer lines which appear only in a limited range of orbital phase, suggesting that it is a low-inclination SW Sextantis star. V446 Her is unusual in that it has started normal dwarf nova eruptions after a nova outburst, but we find nothing else unusual about it – in particular, a distance estimate based on its dwarf nova outbursts agrees nicely with another based on the rate of decline of its nova eruption, both giving d ∼1 kpc. In X Ser, unlike in other old novae with long periods, no spectral features of the secondary star are visible. This and its outburst magnitude both suggest that it is quite distant and luminous, and at least 1 kpc from the Galactic plane.  相似文献   

20.
We present time-resolved spectroscopy and circular spectropolarimetry of the SW Sex star RX J1643.7+3402. We find significant polarization levels exhibiting a variability at a period of  19.38 ± 0.39  min. In addition, emission-line flaring is found predominantly at twice the polarimetric period. These two findings are strong evidences in favour of the presence of a magnetic white dwarf in the system. We interpret the measured periodicities in the context of our magnetic accretion model for SW Sex stars. In contrast with LS Pegasi – the first SW Sex star discovered to have modulated circular polarization – the polarization in RX J1643.7+3402 is suggested to vary at  2(ω−Ω)  , while the emission lines flare at  (ω−Ω)  . However, a  2ω/ω  interpretation cannot be ruled out. Together with LS Peg and V795 Her, RX J1643.7+3402 is the third SW Sex star known to exhibit modulated circular polarization.  相似文献   

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