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1.
We present new high- and low-resolution spectroscopic and photometric data of nine members of the young association CMa R1. All the stars have circumstellar dust at some distance, as could be expected from their association with reflection nebulosity. Four stars (HD 52721, HD 53367, LkH α  220 and LkH α  218) show H α emission and we argue that they are Herbig Be stars with discs. Our photometric and spectroscopic observations of these stars reveal new characteristics of their variability. We present first interpretations of the variability of HD 52721, HD 53367 and the two LkH α stars in terms of a partially eclipsing binary, a magnetic activity cycle and circumstellar dust variations, respectively. The remaining five stars show no clear indications of H α emission in their spectra, although their spectral types and ages are comparable with those of HD 52721 and HD 53367. This indicates that the presence of a disc around a star in CMa R1 may depend on the environment of the star. In particular we find that all H α emission stars are located at or outside the arc-shaped border of the H  ii region, which suggests that the stars inside the arc have lost their discs through evaporation by UV photons from nearby O stars, or from the nearby (<25 pc) supernova, about 1 Myr ago.  相似文献   

2.
We present the first measurements of surface differential rotation on a pre-main-sequence binary system. Using intensity (Stokes I) and circularly polarized (Stokes V) time-series spectra, taken over 11 nights at the Anglo-Australian Telescope (AAT), we incorporate a solar-like differential rotation law into the surface imaging process. We find that both components of the young, 18 Myr, HD 155555 (V824 Ara, G5IV + K0IV) binary system show significant differential rotation. The equator–pole lap times as determined from the intensity spectra are 80 d for the primary star and 163 d for the secondary. Similarly, for the magnetic spectra we obtain equator–pole lap times of 44 and 71 d, respectively, showing that the shearing time-scale of magnetic regions is approximately half of that found for stellar spots. Both components are therefore found to have rates of differential rotation similar to those of the same spectral-type main-sequence single stars. The results for HD 155555 are therefore in contrast to those found in other, more evolved, binary systems where negligible or weak differential rotation has been discovered. We discuss two possible explanations for this: first that at the age of HD 155555 binary tidal forces have not yet had time to suppress differential rotation and secondly that the weak differential rotation previously observed on evolved binaries is a consequence of their large convection zone depths. We suggest that the latter is the more likely solution and show that both temperature and convection zone depth (from evolutionary models) are good predictors of differential rotation strength. Finally, we also examine the possible consequences of the measured differential rotation on the interaction of binary star coronae.  相似文献   

3.
Spruit has shown that an astrophysical dynamo can operate in the non-convective material of a differentially rotating star as a result of a particular instability in the magnetic field (the Tayler instability). By assuming that the dynamo operates in a state of marginal instability, Spruit has obtained formulae which predict the equilibrium strengths of azimuthal and radial field components in terms of local physical quantities. Here, we apply Spruit's formulae to our previously published models of rotating massive stars in order to estimate Tayler dynamo field strengths. There are no free parameters in Spruit's formulae. In our models of 10- and  50-M⊙  stars on the zero-age main sequence, we find internal azimuthal fields of up to 1 MG, and internal radial components of a few kG. Evolved models contain weaker fields. In order to obtain estimates of the field strength at the stellar surface, we examine the conditions under which the Tayler dynamo fields are subject to magnetic buoyancy. We find that conditions for Tayler instability overlap with those for buoyancy at intermediate to high magnetic latitudes. This suggests that fields emerge at the surface of a massive star between magnetic latitudes of about 45° and the poles. We attempt to estimate the strength of the field which emerges at the surface of a massive star. Although these estimates are very rough, we find that the surface field strengths overlap with values which have been reported recently for line-of-sight fields in several O and B stars.  相似文献   

4.
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   

5.
We have produced brightness and magnetic field maps of the surfaces of CV Cha and CR Cha: two actively accreting G- and K-type T Tauri stars in the Chamaeleon I star-forming cloud with ages of 3–5 Myr. Our magnetic field maps show evidence for strong, complex multipolar fields similar to those obtained for young rapidly rotating main-sequence stars. Brightness maps indicate the presence of dark polar caps and low-latitude spots – these brightness maps are very similar to those obtained for other pre-main-sequence and rapidly rotating main-sequence stars.
Only two other classical T Tauri stars have been studied using similar techniques so far: V2129 Oph and BP Tau. CV Cha and CR Cha show magnetic field patterns that are significantly more complex than those recovered for BP Tau, a fully convective T Tauri star.
We discuss possible reasons for this difference and suggest that the complexity of the stellar magnetic field is related to the convection zone; with more complex fields being found in T Tauri stars with radiative cores (V2129 Oph, CV Cha and CR Cha). However, it is clearly necessary to conduct magnetic field studies of T Tauri star systems, exploring a wide range of stellar parameters in order to establish how they affect magnetic field generation, and thus how these magnetic fields are likely to affect the evolution of T Tauri star systems as they approach the main sequence.  相似文献   

6.
From Zeeman–Doppler images of the young, rapidly-rotating K0 dwarf AB Doradûs, we have created a potential approximation to the observed radial magnetic field and have evolved it over 30 d subject to the observed surface differential rotation , meridional flow and various diffusion rates. Assuming that the dark polar cap seen in Doppler images of this star is caused by the presence of a unipolar field, we have shown that the observed differential rotation will shear this field to produce the observed high-latitude band of unidirectional azimuthal field. By cross-correlating the evolved fields with the initial field each day we have followed the decay with time of the cross-correlation function. Over 30 d it decays by only 10 per cent. This contrasts with the results of Barnes et al. , who show that on this time-scale the spot distribution of He699 is uncorrelated. We propose that this is due to the effects of flux emergence changing the spot distributions.  相似文献   

7.
The visual triple system HD 9770 (BB Scl) has been the subject of a four-year programme of UVB ( RI )C photometry and H α échelle spectroscopy. Analysis of the data obtained over that period shows that star B, and probably also star A, of HD 9770 is a binary. The A system comprises a K1V star, which may be in a binary system with another K dwarf. The B system is an eclipsing binary of the BY Dra type in which both stars are chromospherically active. An orbital period of 0.476 525±0.000 013 d has been derived from the light curve in V . Physical parameters derived from analysis of the light curves in UBV ( RI )C are presented.  相似文献   

8.
Decline onset times were measured in long-term visual light curves for five R Coronae Borealis (RCB) variable stars. These included RY Sgr and V854 Cen, the two RCB stars previously reported to have a relationship between dust formation events and pulsational variations. Analysis of the decline epochs showed that all decline onsets for a given star obey a linear ephemeris tied to the object's dominant radial pulsation period. Thus, in addition to confirming the pulsation–decline correlation for RY Sgr and V854 Cen, this same behaviour was demonstrated in UW Cen, R CrB and S Aps for the first time. This general result firmly establishes the connection between radial pulsation and mass loss in the RCB stars. The dominant pulsation period of ≈40 d for all of these objects therefore represents the characteristic time-scale on which these stars produce dust.  相似文献   

9.
We present a Roche tomography reconstruction of the secondary star in the cataclysmic variable AE Aqr. The tomogram reveals several surface inhomogeneities that are due to the presence of large, cool star-spots. In addition to a number of lower latitude spots, the maps also show the presence of a large, high-latitude spot similar to that seen in Doppler images of rapidly rotating isolated stars, and a relative paucity of spots at a latitude of 40°. In total, we estimate that some 18 per cent of the Northern hemisphere of AE Aqr is spotted.
We have also applied the entropy landscape technique to determine accurate parameters for the binary system. We obtain optimal masses   M 1= 0.74 M⊙, M 2= 0.50 M⊙  , a systemic velocity  γ=−63 km s−1  and an orbital inclination   i = 66°  .
Given that this is the first study to successfully image star-spots on the secondary star in a cataclysmic variable, we discuss the role that further studies of this kind may play in our understanding of these binaries.  相似文献   

10.
In this paper, we present new spectropolarimetric observations of the planet-hosting star τ Bootis, using ESPaDOnS and Narval spectropolarimeters at Canada–France–Hawaii Telescope and Telescope Bernard Lyot, respectively.
We detected the magnetic field of the star at three epochs in 2008. It has a weak magnetic field of only a few gauss, oscillating between a predominant toroidal component in January and a dominant poloidal component in June and July. A magnetic polarity reversal was observed relative to the magnetic topology in 2007 June. This is the second such reversal observed in 2 years on this star, suggesting that τ Boo has a magnetic cycle of about 2 years. This is the first detection of a magnetic cycle for a star other than the Sun. The role of the close-in massive planet in the short activity cycle of the star is questioned.
τ Boo has a strong differential rotation, a common trend for stars with shallow convective envelope. At latitude 40°, the surface layer of the star rotates in 3.31 d, equal to the orbital period. Synchronization suggests that the tidal effects induced by the planet may be strong enough to force at least the thin convective envelope into corotation.
τ Boo shows variability in the Ca  ii H & K and Hα throughout the night and on a night-to-night time-scale. We do not detect enhancement in the activity of the star that may be related to the conjunction of the planet. Further data are needed to conclude about the activity enhancement due to the planet.  相似文献   

11.
Sequences of Doppler images of the young, rapidly rotating late-type stars AB Dor and LQ Hya show that their equatorial angular velocity and the amplitude of their surface differential rotation vary versus time. Such variations can be modelled to obtain information on the intensity of the azimuthal magnetic stresses within stellar convection zones. We introduce a simple model in the framework of the mean-field theory and discuss briefly the results of its application to those solar-like stars. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

12.
Very-high-resolution ( R ∼160 000) spectroscopic observations are presented for the early B-type star, HD 83206. Because it has very sharp metal lines, this star affords an opportunity to test theories of model atmospheres and line formation. Non-LTE model atmosphere calculations have been used to estimate the atmospheric parameters and absolute metal abundances (C, N, O, Mg and Si); an LTE analysis was also undertaken to investigate the validity of this simpler approach and to estimate an iron abundance. For the non-LTE calculations, there is excellent agreement with observations of the Balmer lines H α and H δ and the lines of Si  ii and Si  iii for atmospheric parameters of T eff≃21 700±600 K and log  g ≃4.00±0.15 dex. The agreement is less convincing for the LTE calculations, and a higher gravity is deduced. Careful comparison of the metal line profiles with non-LTE calculations implies that the projected rotational and microturbulent velocities have maximum values of ≃5 and ≃2 km s−1, respectively. The latter value is smaller than has often been adopted in LTE model atmosphere analyses of main-sequence stars. Non-LTE absolute metal abundances are estimated, and a comparison with those for normal B-type stars (deduced using similar non-LTE techniques) shows no significant differences. A comparison of the abundances deduced using non-LTE and LTE calculations implies systematic differences of 0.1–0.2 dex, showing the importance of using a non-LTE approach when accurate absolute abundances are required. Its location in the Hertzsprung–Russell diagram and normal metal abundance lead us to conclude that HD 83206 is probably a main-sequence B-type star. As such, it is among the sharpest-lined young B-type star discovered to date.  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 discuss the orbital elements of the multiple system Tr 16-104 which is usually believed to be a member of the open cluster Trumpler 16 in the Carina complex. We show that Tr 16-104 could be a hierarchical triple system consisting of a short-period (2.15 d) eclipsing O7 V+O9.5 V binary bound to a B0.2 IV star. Our preliminary orbital solution of the third body indicates that the B star most probably describes an eccentric orbit with a period of ∼285 or ∼1341 d around the close binary. Folding photometric data from the literature with our new ephemerides, we find that the light curve of the close binary exhibits rather narrow eclipses indicating that the two O stars must be well inside their Roche lobes. Our analysis of the photometric data yields a lower limit on the inclination of the orbit of the close binary of i ≥77° . The stellar radii and luminosities of the O7 V and O9.5 V stars are significantly smaller than expected for stars of this spectral type. Our results suggest that Tr 16-104 lies at a distance of the order of 2.5 kpc and support a fainter absolute magnitude for zero-age main-sequence O stars than usually adopted. We find that the dynamical configuration of Tr 16-104 corresponds to a hierarchical system that should remain stable provided that it suffers no strong perturbation. Finally, we also report long-term temporal variations of high-velocity interstellar Ca  ii absorptions in the line of sight towards Tr 16-104.  相似文献   

15.
We report the results of our search for magnetic fields in a sample of 16 field Be stars, the binary emission‐line B‐type star υ Sgr, and in a sample of fourteen members of the open young cluster NGC3766 in the Carina spiral arm. The sample of cluster members includes Be stars, normal B‐type stars and He‐strong/He‐weak stars. Nine Be stars have been studied with magnetic field time series obtained over ∼1 hour to get an insight into the temporal behaviour and the correlation of magnetic field properties with dynamical phenomena taking place in Be star atmospheres. The spectropolarimetric data were obtained at the European Southern Observatory with the multi‐mode instrument FORS1 installed at the 8m Kueyen telescope. We detect weak photospheric magnetic fields in four field Be stars, HD 62367, μ Cen, o Aqr, and ε Tuc. The strongest longitudinal magnetic field, 〈Bz〉 = 117 ± 38 G, was detected in the Be star HD 62367. Among the Be stars studied with time series, one Be star, λ Eri, displays cyclic variability of the magnetic field with a period of 21.12 min. The binary star υ Sgr, in the initial rapid phase of mass exchange between the two components with strong emission lines in the visible spectrum, is a magnetic variable star, probably on a timescale of a few months. The maximum longitudinal magnetic field 〈Bz〉 = –102 ± 10 G at MJD 54333.018 was measured using hydrogen lines. The cluster NGC3766 seems to be extremely interesting, where we find evidence for the presence of a magnetic field in seven early B‐type stars out of the observed fourteen cluster members (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

16.
We present surface spot maps of the K2V primary star in the pre-cataclysmic variable binary system, V471 Tau. The spot maps show the presence of large high-latitude spots located at the sub-white dwarf longitude region. By tracking the relative movement of spot groups over the course of four nights (eight rotation cycles), we measure the surface differential rotation rate of the system. Our results reveal that the star is rotating rigidly with a surface shear rate,  dΩ= 1.6 ± 6 mrad d−1  . The single active star AB Dor has a similar spectral type, rotation period and activity level as the K star in V471 Tau, but displays much stronger surface shear  (46 < dΩ < 58 mrad d−1)  . Our results suggest that tidal locking may inhibit differential rotation; this reduced shear, however, does not affect the overall magnetic activity levels in active K dwarfs.  相似文献   

17.
We present new multicolour photometry and simultaneous high-dispersion spectroscopy for the δ Scuti star 1 Mon. The two main periodicities in the star are still present, but the third known period is not directly detected in the new radial velocities or light variations. However, it is detected in the periodogram of the second moment of the line profile variations. We use the cross-correlation function as an approximation for the line profile variations. By computing theoretical profiles for a given mode and comparing them with phased cross-correlation profiles, we are able to determine a goodness-of-fit criterion and estimate the most probable spherical harmonic degree,     the azimuthal order, m , of the pulsation and also the angle of inclination. We then compare the relative amplitudes and phases of the photometric variations in five wavebands and obtain the best estimates of     for the two visible periodicities. We confirm the earlier determinations that the main periodicity is a radial mode and that the other periodicity is probably         We show that the line profile variations and light variations give consistent results. We point out the importance of a long wavelength range when using the photometric mode identification technique. Finally, we attempt to match the two periods with unstable modes from linear, non-adiabatic calculations. We are able to show that the principal period is well matched by either the fundamental or first overtone radial mode, but could not find a satisfactory fit to the     mode. We discuss implications for mode identification of δ Scuti stars based on what we have learned from this star.  相似文献   

18.
We study the formation of the absorption features, called the cyclotron–annihilation lines, in the γ-spectra of the neutron stars (pulsars), owing to the fundamental quantum-electrodynamic effect of the one–photon pair creation in magnetized vacuum. As a result, we substantiate a new method for the determination of the neutron star magnetic fields B based on measuring the interval between the main annihilation and the first cyclotron–annihilation absorption lines. It is found that these lines may be easily resolved, and, consequently, the method is surely applicable if the following conditions are satisfied. (i) A γ-source has to be compact enough and located near a star, but not close to its magnetic poles. For instance, it may be a disc in the plane of a star magnetic equator with latitudinal angular width less than     and radial extent up to 25 per cent of the star radius. (ii) The source is to produce detectable γ-radiation at large angles ≳60° to the local magnetic field. Being situated in a closed field line region and having a broad radiation pattern, such a source is not what is usually considered in the context of the polar cap and outer gap models of the pulsar γ-emission dealing with open field lines only. (iii) Magnetic field strength must lie in a certain narrow interval with the centre at  ∼(3–4) × 1012  G. Its width depends on the star orientation and disc radial extend and in the most favourable case is about 20–30 per cent of its lower boundary. Finally, the influence of the star rotation on this method employment is considered and new possibilities arising from forthcoming polarization observations are briefly discussed.  相似文献   

19.
We analyse a light curve (LC) of the symbiotic star BF Cyg, covering 114 yr of its photometric history. The star had a major outburst around the year 1894. Since then the mean optical brightness of the system is in steady decline, reaching only in the last few years its pre-outburst value. Superposed on this general decline are some six less intense outbursts of 1–2 mag and duration of 2000–5000 d. We find a cycle of 6376 d, or possibly twice this period, in the occurrence of these outbursts. We suggest that the origin of the system outbursts is in some magnetic cycle in the outer layers of the giant star of the system, akin to the less intense 8000-d magnetic cycle of our Sun. We further find, that in addition to its well-known binary period of 757.3 d, BF Cyg possesses also another photometric period of 798.8 d. This could be the rotation period of the giant star of the system. If it is, the beat period of these two periodicities, 14 580 d, is the rotation period of a tidal wave on the surface of the giant. A fourth period of 4436 d, the beat period of the 14 580-d and the 6376-d cycles is possibly also present in the LC. We predict that BF Cyg will be at the peak of its next outburst around the month of May in the year 2007. The newly discovered 798.8-d period explains the disappearance of the orbital modulation at some epochs in the LC. The 757.3-d oscillations will be damped again around the year 2013.  相似文献   

20.
Be stars are rapidly spinning B stars surrounded by an outflowing disc of gas in Keplerian rotation. Be star/X-ray binary systems contain a Be star and a neutron star. They are found to have non-zero eccentricities and there is evidence that some systems have a misalignment between the spin axis of the star and the spin axis of the binary orbit. The eccentricities in these systems are caused by a kick to the neutron star during the supernova that formed it. Such kicks would also give rise to misalignments. In this paper, we investigate the extent to which the same kick distribution can give rise to both the observed eccentricity distribution and the observed misalignments. We find that a Maxwellian distribution of velocity kicks with a low velocity dispersion,  σk≈ 15 km s−1  , is consistent with the observed eccentricity distribution but is hard to reconcile with the observed misalignments, typically   i ≥ 25°  . Alternatively, a higher velocity kick distribution,  σk= 265 km s−1  , is consistent with the observed misalignments but not with the observed eccentricities, unless post-supernova circularization of the binary orbits has taken place. We discuss briefly how this might be achieved.  相似文献   

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