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1.
WD 1704+481 is a visual binary in which both components are white dwarfs. We present spectra of the H α line of both stars which show that one component (WD 1704+481.2=Sanduleak B=GR 577) is a close binary with two white dwarf components. Thus, WD 1704+481 is the first known triple degenerate star. From radial velocity measurements of the close binary we find an orbital period of 0.1448 d, a mass ratio, q M bright M faint, of 0.70±0.03 and a difference in the gravitational redshifts of 11.5±2.3 km s−1. The masses of the close pair of white dwarfs predicted by the mass ratio and gravitational redshift difference combined with theoretical cooling curves are 0.39±0.05 and 0.56±0.07 M⊙. WD 1704+481 is therefore also likely to be the first example of a double degenerate in which the less massive white dwarf is composed of helium and the other white dwarf is composed of carbon and oxygen.  相似文献   

2.
We present light curves of four binary subdwarf B stars (sdB), Ton 245, Feige 11, PG 1432+159 and PG 1017−086. We also present new spectroscopic data for PG 1017−086 from which we derive its orbital period,   P =0.073 d  , and the mass function,   f m=0.0010±0.0002 M⊙.  This is the shortest period for an sdB binary measured to date. The values of P and f m for the other sdB binaries have been published elsewhere. We are able to exclude the possibility that the unseen companion stars to Ton 245, Feige 11 and PG 1432+159 are main-sequence stars or subgiant stars from the absence of a sinusoidal signal, which would be caused by the irradiation of such a companion star, i.e. they show no reflection effect. The unseen companion stars in these binaries are likely to be white dwarf stars. In contrast, the reflection effect in PG 1017−086 is clearly seen. The lack of eclipses in this binary combined with other data suggests that the companion is a low-mass M-dwarf or, perhaps, a brown dwarf.  相似文献   

3.
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   

4.
We report a study of the photospheric composition of the hot DA white dwarf WD 2218+706, which is also the central star of the old planetary nebula DeHt5. Helium is detected in the far-UV spectrum. In addition, the star clearly contains significant quantities of elements heavier than He at abundances generally a factor of 2 to 10 higher than those found in the archetypal heavy element-rich DA G191−B2B. This is the first detection of trace He using the He  ii λ 1640 line in an isolated DA white dwarf, but the low surface gravity is more indicative of a binary evolution route from the red giant branch rather than a path along the asymptotic giant branch (AGB) as a single star. However, the absence of any evidence for a companion star and the uncertainty in the measured mass for WD 2218+706 still allow the possibility of an origin along an AGB evolutionary track.
We reanalyse the existing optical spectra of WD 2218+706 using our latest pure H and heavy element-rich model atmospheres, obtaining a good match between the observed and synthetic spectra with either set of models. We find little evidence of any inconsistency in the temperature required to fit individual Balmer lines, as reported elsewhere for this star. Any discrepancies we see are confined to the H α line and the core of H β but they do not compromise our analysis.  相似文献   

5.
We report the discovery, in an Extreme Ultraviolet Explorer ( EUVE ) short-wavelength spectrum, of an unresolved hot white dwarf companion to the 5th magnitude B5Vp star HR 2875. This is the first time that a non-interacting white dwarf+B star binary has been discovered: previously, the earliest type of star known with a white dwarf companion was Sirius (A1V). As the white dwarf must have evolved from a main-sequence progenitor with a mass greater than that of a B5V star (≯6.0 M⊙), this places a lower limit on the maximum mass for white dwarf progenitors, with important implications for our knowledge of the initial–final mass relation. Assuming a pure-hydrogen atmospheric composition, we constrain the temperature of the white dwarf to be between 39 000 and 49 000 K. We also argue that this degenerate star is likely to have a mass significantly greater than the mean mass for white dwarf stars (≈0.55 M⊙). Finally, we suggest that other bright B stars (e.g. θ Hya) detected in the extreme ultraviolet surveys of the ROSAT Wide Field Camera and EUVE may also be hiding hot white dwarf companions.  相似文献   

6.
We present a parallax measurement for the very cool degenerate WD 0346+246, the serendipitous discovery of which was reported by Hambly et al. We find an absolute parallax of 36±5 mas, yielding a distance estimate of 28±4 pc. The resulting absolute visual magnitude of the object is M V =16.8±0.3, making it the second-lowest luminosity white dwarf currently known. We use the distance estimate and measured proper motion to show that the object has kinematics consistent with membership of the Galactic halo. WD 0346+246 is therefore by far the coolest and least luminous of only a handful of plausible halo white dwarf candidates. As such, the object has relevance to the ongoing debate concerning the results of microlensing experiments and the nature of any baryonic dark matter component to the Galactic halo residing in stellar remnants.  相似文献   

7.
We have examined the evolution of merged low-mass double white dwarfs which become low-luminosity (or high-gravity) extreme helium stars. We have approximated the merging process by the rapid accretion of matter, consisting mostly of helium, on to a helium white dwarf. After a certain mass is accumulated, a helium shell flash occurs, the radius and luminosity increase and the star becomes a yellow giant. Mass accretion is stopped artificially when the total mass reaches a pre-determined value. As the helium-burning shell moves inwards with repeating shell flashes, the effective temperature gradually increases as the star evolves towards the helium main sequence. When the mass interior to the helium‐burning shell is approximately 0.25 M⊙, the star enters a regime where it is pulsationally unstable. We have obtained radial pulsation periods for these models.
These models have properties very similar to those of the pulsating helium star V652 Her. We have compared the rate of period change of the theoretical models with that observed in V652 Her, as well as with its position on the Hertzsprung–Russell diagram. We conclude that the merger between two helium white dwarfs can produce a star with properties remarkably similar to those observed in at least one extreme helium star, and is a viable model for their evolutionary origin. Such helium stars will evolve to become hot subdwarfs close to the helium main sequence. We also discuss the number of low-luminosity helium stars in the Galaxy expected for our evolution scenario.  相似文献   

8.
A spectroscopic analysis of Sloan Digital Sky Survey (SDSS) J160043.6+074802.9, a binary system containing a pulsating subdwarf-O (sdO) star with a late-type companion, yields   T eff= 70 000 ± 5000 K  and  log  g = 5.25 ± 0.30  , together with a most likely type of K3 V for the secondary star. We compare our results with atmospheric parameters derived by Fontaine et al. and in the context of existing evolution models for sdO stars. New and more extensive photometry is also presented which recovers most, but not all, frequencies found in an earlier paper. Therefore, it seems probable that some pulsation modes have variable amplitudes. A non-adiabatic pulsation analysis of uniform metallicity sdO models show those having  log  g > 5.3  to be more likely to be unstable and capable of driving pulsation in the observed frequency range.  相似文献   

9.
We present FUSE H Lyman series spectroscopy of the hot white dwarf companion to the 4th magnitude A1 III star β  Crt, which shows that it has an unusually low mass,     , and has almost certainly evolved through binary interaction. This system could be a long-sought remnant of Algol-type evolution, although radial velocity measurements appear to show that the pair are not close. Instead, micro-variations in the proper motion of β  Crt as measured by Hipparcos suggest that the period could be as high as ∼10 yr. However, a low-mass white dwarf in a system with a period ≳3 yr is difficult to explain by conventional models for binary evolution. We speculate on alternative models for the evolution of this system which involve an eccentric binary or multiple components.  相似文献   

10.
We present spectroscopy and photometry of GD 448, a detached white dwarf – M dwarf binary with a period of 2.47 h. We find that the Na  I  8200-Å feature is composed of narrow emission lines, owing to irradiation of the M dwarf by the white dwarf, within broad absorption lines that are essentially unaffected by heating. Combined with an improved spectroscopic orbit and gravitational redshift measurement from spectra of the Hα line, we are able to derive masses for the white dwarf and M dwarf directly (0.41 ± 0.01 and 0.096 ± 0.004 M⊙, respectively). We use a simple model of the Ca II emission lines to establish the radius of the M dwarf assuming the emission from its surface to be proportional to the incident flux per unit area from the white dwarf. The radius derived is 0.125 ± 0.020 R⊙. The M dwarf appears to be a normal main-sequence star in terms of its mass and radius, and is less than half the size of its Roche lobe. The thermal time-scale of the M dwarf is much longer than the cooling age of the white dwarf, so we conclude that the M dwarf was unaffected by the common-envelope phase. The anomalous width of the Hα emission from the M dwarf remains to be explained, but the strength of the line may be due to X-ray heating of the M dwarf owing to accretion on to the white dwarf from the M dwarf wind.  相似文献   

11.
We report additional photometric CCD observations of KPD 0422+5421, a binary with an orbital period of 2.16 h which contains a subdwarf B star (sdB) and a white dwarf. There are two main results of this work. First, the light curve of KPD 0422+5421 contains two distinct periodic signals, the 2.16-h ellipsoidal modulation discovered by Koen, Orosz & Wade and an additional modulation at 7.8 h. This 7.8-h modulation is clearly not sinusoidal: the rise time is about 0.25 in phase, whereas the decay time is 0.75 in phase. Its amplitude is roughly half of the amplitude of the ellipsoidal modulation. Secondly, after the 7.8-h modulation is removed, the light curve folded on the orbital period clearly shows the signature of the transit of the white dwarf across the face of the sdB star and the signature of the occultation of the white dwarf by the sdB star. We have used the Wilson–Devinney code to model the light curve to obtain the inclination, the mass ratio and the Ω potentials, and a Monte Carlo code to compute confidence limits on interesting system parameters. We find component masses of     and     ( M total     , 68 per cent confidence limits). If we impose an additional constraint and require the computed mass and radius of the white dwarf to be consistent with a theoretical mass–radius relation, we find     and     (68 per cent confidence limits). In this case the total mass of the system is less than 1.4 M⊙ at the 99.99 per cent confidence level. We briefly discuss possible interpretations of the 7.8-h modulation and the importance of KPD 0422+5421 as a member of a rare class of evolved binaries.  相似文献   

12.
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   

13.
Intermediate-resolution (0.5–1 Å) optical spectroscopy of the cataclysmic variable (CV) SY Cnc reveals the spectrum of the donor star. Our data enable us to resolve the orbital motion of the donor and provide a new orbital solution, binary mass ratio and spectral classification. We find that the donor star has spectral-type G8 ± 2 V and orbits the white dwarf with   P = 0.382 3753 ± 0.000 0003  d,   K 2= 88.0 ± 2.9  km s−1 and   V sin  i = 75.5 ± 6.5  km s−1. Our values are significantly different from previous works and lead to   q = M 2/ M 1= 1.18 ± 0.14  . This is one of the highest mass ratios known in a CV and is very robust, because it is based on resolving the rotational broadening over a large number of metallic absorption lines. The donor could be a slightly evolved main sequence or descendant from a massive star which underwent an episode of thermal time-scale mass transfer.  相似文献   

14.
We have measured the radial velocity variation of the white dwarf secondary in the binary system containing the millisecond pulsar PSR J 1012 + 5307. Combined with the orbital parameters of the radio pulsar, we infer a mass ratio q (≡ M 1/ M 2) = 10.5 ± 0.5. Our optical spectroscopy has also allowed us to determine the mass of the white dwarf companion by fitting the spectrum to a grid of DA model atmospheres: we estimate M 2 = 0.16 ± 0.02 M⊙, and hence the mass of the neutron star is 1.64 ± 0.22 M⊙, where the error is dominated by that of M 2. The orbital inclination is 52 ± 4°. For an initial neutron star mass of ∼ 1.4 M⊙, only a few tenths of a solar mass at most has been successfully accreted over the lifetime of the progenitor low-mass X-ray binary. If the initial mass of the secondary was ∼ 1 M⊙, our result suggests that the mass transfer may have been non-conservative.  相似文献   

15.
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   

16.
We report on the results from a five-night campaign of high-speed spectroscopy of the 17-min binary AM Canum Venaticorum (AM CVn), obtained with the 4.2-m William Herschel Telescope on La Palma.
We detect a kinematic feature that appears to be entirely analogous to the 'central spike' known from the long-period, emission-line AM CVn stars GP Com, V396 Hya and SDSS J124058.03−015919.2, which has been attributed to the accreting white dwarf. Assuming that the feature indeed represents the projected velocity amplitude and phase of the accreting white dwarf, we derive a mass ratio   q = 0.18 ± 0.01  for AM CVn. This is significantly higher than the value found in previous, less direct measurements. We discuss the implications for AM CVn's evolutionary history and show that a helium star progenitor scenario is strongly favoured. We further discuss the implications for the interpretation of AM CVn's superhump behaviour, and for the detectability of its gravitational-wave signal with the Laser Interferometer Space Antenna ( LISA ).
In addition, we demonstrate a method for measuring the circularity or eccentricity of AM CVn's accretion disc, using stroboscopic Doppler tomography. We test the predictions of an eccentric, precessing disc that are based on AM CVn's observed superhump behaviour. We limit the effective eccentricity in the outermost part of the disc, where the resonances that drive the eccentricity are thought to occur, to   e = 0.04 ± 0.01  , which is smaller than previous models indicated.  相似文献   

17.
We have examined the evolution of merged low-mass double white dwarfs that become luminous helium stars. We have approximated the merging process by the rapid accretion of matter, consisting mostly of helium, on to a carbon–oxygen (CO) white dwarf. After a certain mass is accumulated, a helium shell flash occurs, the radius and luminosity increase and the star becomes a yellow giant. Mass accretion is stopped artificially when the total mass reaches a pre-determined value. When the mass above the helium-burning shell becomes small enough, the star evolves blueward almost horizontally in the Hertzsprung–Russell diagram. The theoretical models for the merger of a 0.6-M⊙ CO white dwarf with a 0.3-M⊙ He white dwarf agree very well with the observed locations of extreme helium stars in the  log  T eff–log  g   diagram, with their observed rates of blueward evolution, and with luminosities and masses obtained from their pulsations. Together with predicted merger rates for  CO+He  white dwarf pairs, the evolutionary time-scales are roughly consistent with the observed numbers of extreme helium stars. Predicted surface carbon and oxygen abundances can be consistent with the observed values if carbon and oxygen produced in the helium shell during a previous asymptotic giant branch phase are assumed to exist in the helium zone of the initial CO white dwarfs. These results establish the  CO+He  white dwarf merger as the best, if not only, viable model for the creation of extreme helium stars and, by association, the majority of R Coronae Borealis stars.  相似文献   

18.
Simultaneous spectroscopic and photometric observations of the Z Cam type dwarf nova SY Cancri were used to obtain absolute flux calibrations. A comparison of the photometric calibration with a wide-slit spectrophotometric calibration showed that either method is equally satisfactory. A radial velocity study of the secondary star, made using the far-red Na  i doublet, yielded a semi-amplitude of   K 2= 127 ± 23 km s−1  . Taking the published value of  86 ± 9 km s−1  for K 1 gives a mass ratio of   q = M 2/ M 1= 0.68 ± 0.14  ; this is very different from the value of  1.13 ± 0.35  quoted in the literature. Using the new lower mass ratio, and constraining the mass of the white dwarf to be within reasonable limits, then leads to a mass for the secondary star that is substantially less than would be expected for its orbital period if it satisfied a main-sequence mass–radius relationship. We find a spectral type of M0 that is consistent with that expected for a main-sequence star of the low mass we have found. However, in order to fill its Roche lobe, the secondary must be significantly larger than a main-sequence star of that mass and spectral type. The secondary is definitely not a normal main-sequence star.  相似文献   

19.
We present a detailed calculation of the evolution of low-mass (<0.25 M⊙) helium white dwarfs. These white dwarfs (the optical companions to binary millisecond pulsars) are formed via long-term, low-mass binary evolution. After detachment from the Roche lobe, the hot helium cores have a rather thick hydrogen layer with mass between 0.01 and 0.06 M⊙. As a result of mixing between the core and outer envelope, the surface hydrogen content ( X surf) is 0.5–0.35 , depending on the initial value of the heavy element Z and the initial secondary mass. We found that the majority of our computed models experience one or two hydrogen shell flashes. We found that the mass of the helium dwarf in which the hydrogen shell flash occurs depends on the chemical composition. The minimum helium white dwarf mass in which a hydrogen flash takes place is 0.213 M⊙ ( Z =0.003), 0.198 M⊙ ( Z =0.01), 0.192 M⊙ ( Z =0.02) or 0.183 M⊙ ( Z =0.03). The duration of the flashes (independent of chemical composition) is between a few ×106 and a few ×107 yr. In several flashes the white dwarf radius will increase so much that it forces the model to fill its Roche lobe again. Our calculations show that the cooling history of the helium white dwarf depends dramatically on the thickness of the hydrogen layer. We show that the transition from a cooling white dwarf with a temporarily stable hydrogen-burning shell to a cooling white dwarf in which almost all residual hydrogen is lost in a few thermal flashes (via Roche lobe overflow) occurs between 0.183 and 0.213 M⊙ (depending on the heavy element value).  相似文献   

20.
We present time-resolved spectroscopy and photometry of the double-lined eclipsing cataclysmic variable V347 Pup (=LB 1800). There is evidence of irradiation on the inner hemisphere of the secondary star, which we correct for using a model to give a secondary-star radial velocity of   K R= 198 ± 5 km s−1  . The rotational velocity of the secondary star in V347 Pup is found to be   v sin  i = 131 ± 5 km s−1  and the system inclination is   i = 840 ± 23  . From these parameters we obtain masses of   M 1= 0.63 ± 0.04 M⊙  for the white dwarf primary and   M 2= 0.52 ± 0.06 M⊙  for the M0.5V secondary star, giving a mass ratio of   q = 0.83 ± 0.05  . On the basis of the component masses, and the spectral type and radius of the secondary star in V347 Pup, we find tentative evidence for an evolved companion. V347 Pup shows many of the characteristics of the SW Sex stars, exhibiting single-peaked emission lines, high-velocity S-wave components and phase-offsets in the radial velocity curve. We find spiral arms in the accretion disc of V347 Pup and measure the disc radius to be close to the maximum allowed in a pressureless disc.  相似文献   

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