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1.
The theory of polar magnetic burial in accreting neutron stars predicts that a mountain of accreted material accumulates at the magnetic poles of the star, and that, as the mountain spreads equatorward, it is confined by, and compresses, the equatorial magnetic field. Here, we extend previous, axisymmetric, Grad–Shafranov calculations of the hydromagnetic structure of a magnetic mountain up to accreted masses as high as   M a= 6 × 10−4 M⊙  , by importing the output from previous calculations (which were limited by numerical problems and the formation of closed bubbles to   M a < 10−4 M⊙  ) into the time-dependent, ideal-magnetohydrodynamic code zeus-3d and loading additional mass on to the star dynamically. The rise of buoyant magnetic bubbles through the accreted layer is observed in these experiments. We also investigate the stability of the resulting hydromagnetic equilibria by perturbing them in zeus-3d . Surprisingly, it is observed that the equilibria are marginally stable for all   M a≤ 6 × 10−4 M⊙  ; the mountain oscillates persistently when perturbed, in a combination of Alfvén and acoustic modes, without appreciable damping or growth, and is therefore not disrupted (apart from a transient Parker instability initially, which expels <1 per cent of the mass and magnetic flux).  相似文献   

2.
Three-dimensional numerical magnetohydrodynamic (MHD) simulations are performed to investigate how a magnetically confined mountain on an accreting neutron star relaxes resistively. No evidence is found for non-ideal MHD instabilities on a short time-scale, such as the resistive ballooning mode or the tearing mode. Instead, the mountain relaxes gradually as matter is transported across magnetic surfaces on the diffusion time-scale, which evaluates to  τI∼ 105–108 yr  (depending on the conductivity of the neutron star crust) for an accreted mass of   M a= 1.2 × 10−4 M⊙  . The magnetic dipole moment simultaneously re-emerges as the screening currents dissipate over  τI  . For non-axisymmetric mountains, ohmic dissipation tends to restore axisymmetry by magnetic reconnection at a filamentary neutral sheet in the equatorial plane. Ideal-MHD oscillations on the Alfvén time-scale, which can be excited by external influences, such as variations in the accretion torque, compress the magnetic field and hence decrease  τI  by one order of magnitude relative to its standard value (as computed for the static configuration). The implications of long-lived mountains for gravitational wave emission from low-mass X-ray binaries are briefly explored.  相似文献   

3.
We compute the continuous part of the ideal-magnetohydrodynamic (ideal-MHD) frequency spectrum of a polar mountain produced by magnetic burial on an accreting neutron star. Applying the formalism developed by Hellsten & Spies, extended to include gravity, we solve the singular eigenvalue problem subject to line-tying boundary conditions. This spectrum divides into an Alfvén part and a cusp part. The eigenfunctions are chirped and anharmonic with an exponential envelope, and the eigenfrequencies cover the whole spectrum above a minimum ωlow. For equilibria with accreted mass  1.2 × 10−6≲ M a/M⊙≲ 1.7 × 10−4  and surface magnetic fields  1011≲ B */G ≲ 1013, ωlow  is approximately independent of   B *  , and increases with M a. The results are consistent with the Alfvén spectrum excited in numerical simulations with the zeus-mp solver. The spectrum is modified substantially by the Coriolis force in neutron stars spinning faster than ∼100 Hz. The implications for gravitational-wave searches for low-mass X-ray binaries are considered briefly.  相似文献   

4.
Modern spectropolarimeters are capable of detecting subkilogauss field strengths using the Zeeman effect in line profiles from the static photosphere, but supersonic Doppler broadening makes it more difficult to detect the Zeeman effect in the wind lines of hot stars. Nevertheless, the recent advances in observational capability motivate an assessment of the potential for detecting the magnetic fields threading such winds. We incorporate the weak-field longitudinal Zeeman effect in the Sobolev approximation to yield integral expressions for the flux of circularly polarized emission. To illustrate the results, two specific wind flows are considered: (i) spherical constant expansion with   v ( r ) = v ∞  and (ii) homologous expansion with   v ( r ) ∝ r   . Axial and split monopole magnetic fields are used to schematically illustrate the polarized profiles. For constant expansion, optically thin lines yield the well-known 'flat-topped' total intensity emission profiles and an antisymmetric circularly polarized profile. For homologous expansion, we include occultation and wind absorption to provide a more realistic observational comparison. Occultation severely reduces the circularly polarized flux in the redshifted component, and in the blueshifted component, the polarization is reduced by partially offsetting emission and absorption contributions. We find that for a surface field of approximately 100 G, the largest polarizations result for thin but strong recombination emission lines. Peak polarizations are approximately 0.05 per cent, which presents a substantial although not inconceivable sensitivity challenge for modern instrumentation.  相似文献   

5.
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6.
The binary companion to the peculiar F supergiant HD 172481 is shown to be a Mira variable with a pulsation period of 312 d. Its characteristics are within the normal range found for solitary Miras of that period, although its pulsation amplitude and mass-loss rate ̇ ∼3×10−6 M⊙ yr−1 are higher than average. Reasons are given for suspecting that the F supergiant, which has L ∼104 L⊙, is a white dwarf burning hydrogen accreted from its companion.  相似文献   

7.
A model for the angular momentum transfer within the convection zone of a rapidly rotating star is introduced and applied to the analysis of recent observations of temporal fluctuations of the differential rotation on the young late-type stars AB Doradus (AB Dor) and LQ Hydrae (LQ Hya). Under the hypothesis that the mean magnetic field produced by the stellar dynamo rules the angular momentum exchanges and that the angular velocity depends only on the distance s from the rotation axis and the time, the minimum azimuthal Maxwell stress  | BsB φ|  , averaged over the convection zone, is found to range from ∼0.04 to  ∼0.14 T2  . If the poloidal mean magnetic field   B s   is of the order of 0.01 T, as indicated by the Zeeman–Doppler imaging maps of those stars, then the azimuthal mean field   B φ  can reach an intensity of several teslas, which significantly exceeds equipartition with the turbulent kinetic energy. Such strong fields can account also for the orbital period modulation observed in cataclysmic variables and RS Canum Venaticorum systems with a main-sequence secondary component. Moreover, the model allows us to compute the kinetic energy dissipation rate during the maintenance of the differential rotation. Only in the case of the largest surface shear observed on LQ Hya may the dissipated power exceed the stellar luminosity, but the lack of a sufficient statistic on the occurrence of such episodes of large shear does not allow us to estimate their impact on the energy budget of the convection zone.  相似文献   

8.
Active galactic nuclei can produce extremely powerful jets. While tightly collimated, the scale of these jets and the stellar density at galactic centres implies that there will be many jet/star interactions, which can mass load the jet through stellar winds. Previous work employed modest wind mass outflow rates, but this does not apply when mass loading is provided by a small number of high mass-loss stars. We construct a framework for jet mass loading by stellar winds for a broader spectrum of wind mass-loss rates than has previously been considered. Given the observed stellar mass distributions in galactic centres, we find that even highly efficient (0.1 Eddington luminosity) jets from supermassive black holes of masses M BH≲ 104 M⊙ are rapidly mass loaded and quenched by stellar winds. For  104 M⊙ < M BH < 108 M⊙  , the quenching length of highly efficient jets is independent of the jet's mechanical luminosity. Stellar wind mass loading is unable to quench efficient jets from more massive engines, but can account for the observed truncation of the inefficient M87 jet, and implies a baryon-dominated composition on scales ≳2 kpc therein even if the jet is initially pair plasma dominated.  相似文献   

9.
It has been pointed out in the past that it is impossible to accelerate molecular material to velocities ≥ 25 km s−1 with gasdynamic shocks without dissociating the gas. Because of this, it has been argued that observations of molecular emission with radial velocities ∼ 20–100 km s−1 imply the presence of 'C-shocks' (which have much lower post-shock temperatures, and therefore do not dissociate the gas) and the existence of strong (∼ 10–100 μG) magnetic fields.   In this paper, we discuss an alternative mechanism for accelerating molecular material to high velocities: a high-velocity, low-density wind drives a non-dissociative shock (with shock velocity v cs ≤ 25 km s−1) into a high-density, molecular clump. Once this shock wave has gone through the clump, the molecular material is moving at a velocity ∼  v cs and has a gas pressure approximately equal to the ram pressure of the impinging wind. The compressed molecular clump can now be accelerated directly by the ram pressure of the wind (without the passage of further shocks through the molecular material), and will eventually move at the wind velocity.   This mechanism has been previously invoked to explain high-velocity molecular emission. However, numerical simulations have shown that a wind/clump interaction leads to the fragmentation of the clump before it can be accelerated to large velocities. In our numerical simulation (which includes an approximate treatment of the relevant microphysics) we find that the fragments that are produced are still largely molecular, and that they are rapidly accelerated to velocities comparable to the wind velocity. We therefore conclude that a wind/molecular clump interaction is indeed a valid mechanism for producing high-velocity molecular features.  相似文献   

10.
We argue that the first stars may have spanned the conventional mass range rather than be identified with the very massive objects  (∼100–103 M⊙)  favoured by numerical simulations. Specifically, we find that magnetic field generation processes acting in the first protostellar systems suffice to produce fields that exceed the threshold for magneto-rotational instability (MRI) to operate, and thereby allow the MRI dynamo to generate equipartition-amplitude magnetic fields on protostellar mass scales below  ∼50 M⊙  . Such fields allow primordial star formation to occur at essentially any metallicity by regulating angular momentum transfer, fragmentation, accretion and feedback in much the same way as occurs in conventional molecular clouds.  相似文献   

11.
We present high-resolution spectroscopy of a sample of 24 solar-type stars in the young (15–40 Myr), open cluster NGC 2547. We use our spectra to confirm cluster membership in 23 of these stars, to determine projected equatorial velocities and chromospheric activity, and to search for the presence of accretion discs. We find examples of both fast ( v e sin  i >50 km s−1) and slow ( v e sin  i <10 km s−1) rotators, but no evidence for active accretion in any of the sample. The distribution of projected rotation velocities is indistinguishable from the slightly older IC 2391 and IC 2602 clusters, implying similar initial angular momentum distributions and circumstellar disc lifetimes. The presence of very slow rotators indicates either that long (10–40 Myr) disc lifetimes or internal differential rotation are needed, or that NGC 2547 (and IC 2391/2602) were born with more slowly rotating stars than are presently seen in even younger clusters and associations. The solar-type stars in NGC 2547 follow a similar rotation–activity relationship to that seen in older clusters. X-ray activity increases until a saturation level is reached for v e sin  i >15–20 km s−1. We are unable to explain why this saturation level, of log( L x L bol)≃−3.3, is a factor of 2 lower than in other clusters, but rule out anomalously slow rotation rates or uncertainties in X-ray flux calculations.  相似文献   

12.
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13.
We present observations of Sakurai's Object obtained at 1–5 μm between 2003 and 2007. By fitting a radiative transfer model to an echelle spectrum of CO fundamental absorption features around  4.7 μm  , we determine the excitation conditions in the line-forming region. We find  12C/13C = 3.5+2.0−1.5  , consistent with CO originating in ejecta processed by the very late thermal pulse, rather than in the pre-existing planetary nebula. We demonstrate the existence of  2.2 × 10−6≤ M CO≤ 2.7 × 10−6 M⊙  of CO ejecta outside the dust, forming a high-velocity wind of  500 ± 80 km s−1  . We find evidence for significant weakening of the CO band and cooling of the dust around the central star between 2003 and 2005. The gas and dust temperatures are implausibly high for stellar radiation to be the sole contributor.  相似文献   

14.
Magnetic white dwarfs with fields in excess of ∼106 G (the high field magnetic white dwarfs; HFMWDs) constitute about ∼10 per cent of all white dwarfs and show a mass distribution with a mean mass of  ∼0.93 M⊙  compared to  ∼0.56 M⊙  for all white dwarfs. We investigate two possible explanations for these observations. First, that the initial–final mass relationship (IFMR) is influenced by the presence of a magnetic field and that the observed HFMWDs originate from stars on the main sequence that are recognized as magnetic (the chemically peculiar A and B stars). Secondly, that the IFMR is essentially unaffected by the presence of a magnetic field, and that the observed HFMWDs have progenitors that are not restricted to these groups of stars. Our calculations argue against the former hypothesis and support the latter. The HFMWDs have a higher than average mass because on the average they have more massive progenitors and not because the IFMR is significantly affected by the magnetic field. A requirement of our model is that ∼40 per cent of main-sequence stars more massive than  ∼4.5 M⊙  must either have magnetic fields in the range of ∼10–100 G, which is below the current level of detection, or generate fields during subsequent stellar evolution towards the white dwarf phase. In the former case, the magnetic fields of the HFMWDs could be fossil remnants from the main-sequence phase consistent with the approximate magnetic flux conservation.  相似文献   

15.
We present the results on period search and modelling of the cool DAV star KUV 02464+3239. Our observations resolved the multiperiodic pulsational behaviour of the star. In agreement with its position near the red edge of the DAV instability strip, it shows large amplitude, long-period pulsation modes, and has a strongly non-sinusoidal light curve. We determined six frequencies as normal modes and revealed remarkable short-term amplitude variations. A rigorous test was performed for the possible source of amplitude variation: beating of modes, effect of noise, unresolved frequencies or rotational triplets. Among the best-fitting models resulting from a grid search, we selected three that gave   l = 1  solutions for the largest amplitude modes. These models had masses of 0.645, 0.650 and  0.680  M ⊙  . The three 'favoured' models have   M H  between  2.5 × 10−5 and 6.3 × 10−6  M *  and give 14.2–14.8 mas seismological parallax. The  0.645  M ⊙  (11 400 K) model also matches the spectroscopic  log  g   and   T eff  within 1σ. We investigated the possibility of mode trapping and concluded that while it can explain high amplitude modes, it is not required.  相似文献   

16.
Using improved techniques, high-quality CCD uvbyVI photometry has been obtained for the eclipsing binary HV 982 in the Large Magellanic Cloud (LMC). International Ultraviolet Explorer ultraviolet spectrophotometry was also obtained. These data have been analysed using the Wilson–Devinney synthetic light-curve code and Kurucz low-metallicity model atmospheres as well as the EBOP code. The system is detached and the orbit is eccentric. Apsidal motion is detected with apsidal period 205 ± 7 yr. The effective temperatures of the components are found via flux fitting to be T eff,1 = 28 000 ± 5000 K and T eff,2 = 27 200 ± 5000 K. The large errors result from uncertainties over the appropriate interstellar extinction correction. The system plausibly comprises two ∼ 8 M stars of radius 6–7 R separated by ∼ 30 R. For pedagogical and historical interest, the near simultaneity of the eclipse minima at different wavelengths is used to constrain the constancy of the speed of light with wavelength and the mass of the photon, yielding m γ < 10−41 kg. Because of the great distance to HV 982, this limit is some 102 times smaller than previously achieved with eclipse timings, but it is nevertheless 10 orders of magnitude less stringent than that which is provided by satellite measurements of planetary magnetic fields.  相似文献   

17.
We examine a binary merger model for the formation of the mysterious triple-ring nebula surrounding Supernova 1987A, which still has not been convincingly explained in detailed hydrodynamical calculations. During the merger of 15 and  5 M⊙  binary systems, mass is ejected primarily at mid-latitudes for a sufficiently evolved primary, as demonstrated by Morris & Podsiadlowski. This material is swept up by the fast wind of the central star during its post-merger blue supergiant phase, leading to a density contrast of ∼150 in the outer rings at the time of the supernova. The equatorial ring probably formed later when the star contracted to become a blue supergiant. The asymmetry between the northern and southern outer rings can be explained by a 10 per cent asymmetry during the merger, perhaps due to a pulsational instability in the common envelope.
We present a parameter study from which we determine a mass-loss rate in the blue supergiant wind in the range  1.5–3 × 10−7 M⊙ yr−1  in agreement with previous estimates. The morphology of the best model is consistent with the well-known Hubble Space Telescope image at better than 5 per cent and is also in broad agreement with light-echo observations. The circumstellar environment on larger scales (up to 3 pc) is also investigated. We conclude with a brief discussion of the bipolar nebulae surrounding the Galactic stars, Sheridan 25, HD 168625 and η Carinae.  相似文献   

18.
High-resolution spectroscopic observations around the Hα line and BVRI photometry of the eclipsing short-period RS CVn star UV Leo are presented. The simultaneous light-curve solution and radial velocity-curve solution led to the following values of the global parameters of the binary: temperatures   T 1= 6000 ± 100 K  and   T 2= 5970 ± 20 K  ; masses   M 1= 0.976 ± 0.067 M⊙  and   M 2= 0.931 ± 0.052 M⊙  ; separation   a = 3.716 ± 0.048 R⊙  ; orbital inclination     ; radii   R 1= 1.115 ± 0.052 R⊙  and   R 2= 1.078 ± 0.051 R⊙  ; equatorial velocities   V 1= 98.8 ± 2.3 km s−1  and   V 2= 89.6 ± 2.7 km s−1  . These results lead to the conclusion that the two components of UV Leo are slightly oversized for their masses and lie within the main-sequence band on the mass–radius diagram, close to the isochrone 9 × 1010 yr.  相似文献   

19.
We study torsional Alfvén oscillations of magnetars, that is neutron stars with a strong magnetic field. We consider the poloidal and toroidal components of the magnetic field and a wide range of equilibrium stellar models. We use a new coordinate system  ( X , Y )  , where     and     and a 1 is the radial component of the magnetic field. In this coordinate system, the one+two-dimensional evolution equation describing the quasi-periodic oscillations (QPOs), see Sotani et al., is reduced to a one+one-dimensional equation where the perturbations propagate only along the y -axis. We solve the one+one-dimensional equation for different boundary conditions and the open magnetic field lines, that is magnetic field lines that reach the surface and there match up with the exterior dipole magnetic field as well as closed magnetic lines, i.e. magnetic lines that never reach the stellar surface. For the open field lines, we find two families of QPO frequencies: a family of 'lower' QPO frequencies which is located near the x -axis and a family of 'upper' frequencies located near the y -axis. According to Levin, the fundamental frequencies of these two families can be interpreted as the turning point of the continuous spectrum. We find that the upper frequencies are multiples of the lower ones by a constant equalling  2 n + 1  . For the closed lines, the corresponding factor is   n + 1  . By using these relations, we can explain both the lower and the higher observed frequencies in SGR 1806−20 and SGR 1900+14.  相似文献   

20.
We have observed the energetic binary Cygnus X-3 in both quiescent and flaring states between 4 and 16 μm using the ISO satellite. We find that the quiescent source shows the thermal free–free spectrum typical of a hot, fast stellar wind, such as from a massive helium star. The quiescent mass-loss rate arising from a spherically symmetric, non-accelerating wind is found to be in the range (0.4–2.9)×10−4 M⊙ yr−1, consistent with other infrared and radio observations, but considerably larger than the 10−5 M⊙ yr−1 deduced from both the orbital change and the X-ray column density. There is rapid, large-amplitude flaring at 4.5 and 11.5 μm at the same time as enhanced radio and X-ray activity, with the infrared spectrum apparently becoming flatter in the flaring state. We believe that non-thermal processes are operating, perhaps along with enhanced thermal emission.  相似文献   

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