Circumstellar matter in Algols and Serpentids

A systematic search is reported for phenomena associated with circumstellar matter in interacting binary systems of the Algol and W Serpentis type. Ultraviolet emission lines have been detected by the author in 10 Algols and 6 Serpentids. No conspicuous difference exists between systems with disk accretion and those with direct stream impact. Evidence is indicated that the lines are formed predominantly by scattering in an induced stellar wind. Relative intensity of Fe II and Fe III emissions appears to be correlated with the spectral type of the gainer. It is suggested that in the Serpentids we see a thick disk nearly edge-on, but that the emission lines indicate the nature of the gainer inside.     

Theoretical modelling of Algol disks

A brief review of various theoretical approaches to model accretion disks is presented. Emphasis is given to models that determine self-consistently the structure of a disk together with the radiation field. It is argued that a proper treatment of the vertical structure is essential for calculating theoretical spectra to be compared with observations. In particular, it is shown that hot layers above an accretion disk (sometimes called disk “chromospheres” or “coronae”), whose presence is indicated by recent UV observations of strong emission lines of highly ionized species, may be explained using simple energy balance arguments.     

X-ray spectrum of the Tycho SNR observed with EXOSAT

The Gas Scintillation and Medium Energy instruments onboard EXOSAT measured the continuum and line emission spectrum in the Tycho supernova remnant. For the first time the iron line at 6.53 keV is clearly resolved from the continuum, in addition to the lines from S, Ar and Ca. The comparison with a non-equilibrium ionisation model (Hamilton et al 1983) confirms the over-abundance of S and Ca, whereas the Fe abundance is found to be solar, which provides additional support that the predicted 0.1–1 M₀ Fe in type I supernovae is presently not shocked to X-ray emitting temperatures.     

Nonthermal radio emission from the Galaxy

Synchrotron radio emission from interstellar space has long been recognized as a useful tool to probe into the galactic distribution of high energy electrons and magnetic fields. We first review the results obtained from the local (<2kpc distant) region of the Galaxy and conclude that the observed local synchrotron emissivity is consistently explained by the measured cosmic ray electron spectrum and the interstellar magnetic field if some reasonable assumptions are allowed. The large scale distribution of radio emissivity shows evidence for spiral structure and is likely to originate in two distinct disk systems: a thin disk (thickness 250 pc in the inner Galaxy) formed by population I objects which emits about 10% of the galactic radio luminosity and a thick disk (2.5 kpc thick in the inner Galaxy) which constitutes the truly diffuse emission and produces 90% of the total luminosity.     

Time-resolved spectroscopy of accretion disks in Algols

Time-resolved spectroscopy during the eclipse of short-period Algol systems, has shown their accretion disks to be small, turbulent structures with non-Keplerian velocity fields and asymmetries between the leading and trailing sides of the disk. These transient disks are produced by the impact of the gas stream on the mass-gaining star, and occur in systems where the star is just large enough to ensure the stream collision is complete. These emission line disks and the excess continuum emission do not always occur together. The permanent accretion disks in at least a few of the long-period Algol systems have features in common with the transient disks including non-Keplerian velocity fields.     

Time-resolved spectroscopy of accretion disks in Algols

Time-resolved spectroscopy during the eclipse of short-period Algol systems, has shown their accretion disks to be small, turbulent structures with non-Keplerian velocity fields and asymmetries between the leading and trailing sides of the disk. These transient disks are produced by the impact of the gas stream on the mass-gaining star, and occur in systems where the star is just large enough to ensure the stream collision is complete. These emission line disks and the excess continuum emission do not always occur together. The permanent accretion disks in at least a few of the long-period Algol systems have features in common with the transient disks including non-Keplerian velocity fields.     

Stellar chromospheres

Observations indicating the presence of stellar chromospheres, that is hot envelopes around stars are summarized. Undisputed indicators (called type I) for hot stellar envelopes are spectral lines of highly ionized atoms, Fe ii emission lines and flares in late type stars and the presence of the He i10830 Å line. Whether indicators (called type II) like emission cores in the Ca ii H and K and Mg ii h and k lines or mass loss signify the presence of stellar chromospheres is still somewhat debated, although the discussion points in favour of the usefulness of these indicators. The combined evidence to date shows that all non degenerate type stars have chromospheres except possibly the A stars. There are however theoretical reasons for expecting chromospheres in A stars. Empirical chromosphere models for a rapidly growing sample of stars have recently been constructed on the basis of Ca ii and Mg ii line observations. A discussion of possible heating mechanisms is given and the relative importance of these mechanisms is evaluated. For the low and middle chromosphere the short period acoustic heating mechanism seems to be the dominant process although there are still uncertainties. Both steady state and time dependent theoretical models of stellar chromospheres, based on the short period acoustic heating theory, are discussed, and predictions of these models are compared with results from empirical models. This relatively favourable comparison shows that the explanation of the Wilson-Bappu effect might be at hand.     

The structure of low-mass X-ray binaries

The present knowledge of the structure of low-mass X-ray binary systems is reviewed. We examine the orbital period distribution of these sources and discuss how the orbital periods are measured. There is substantial observational evidence that the accretion disks in low-mass X-ray binaries are thick and structured. In a number of highly inclined systems, the compact X-ray emitting star is hidden from direct view by the disk and X-radiation is observed from these only because photons are scattered into the line of sight by material above and below the disk plane. In such systems the X-ray emission can appear extended with respect to the companion star, which can lead to partial X-ray eclipses. There are substantial variations in the thickness of the disk rim with azimuth. These give rise to the phenomenon of irregular dips in the X-ray flux which recur with the orbital period, or to an overall binary modulation of the X-ray flux if the source is extended. The X-ray spectra of low-mass X-ray binaries can be used to probe the innermost emission regions surrounding the compact star. The spectra of the bright Sco X-1 variables can be fitted with two components which are provisionally identified as originating in the inner disk and the boundary layer between the disk and the neutron star respectively. The characteristic energy dependent flaring of the Sco X-1 sub-class may be a geometric effect triggered by an increase in the thickness of the inner disk or boundary layer. The X-ray spectra of the lower luminosity systems, including the bursters, are less complex, and in many cases can be represented by a single power law with, in some sources, a high energy cut-off. Iron line emission is a characteristic of most low-mass X-ray binaries, irrespective of luminosity.     

Future instrumental capabilities in the energy range of nuclear transitions

Gamma-ray lines are the fingerprints of nuclear transitions, carrying the memory of high energy processes in the universe. Setting out from what is presently known about line emission in gamma-ray astronomy, requirements for future telescopes are outlined. The inventory of observed line features shows that sources with a wide range of angular and spectral extent have to be handled: the scientific objectives for gamma-ray spectroscopy are spanning from compact objects as broad class annihilators, over longlived galactic radioisotopes with hotspots in the degree-range to the extremely extended galactic disk and bulge emission of the narrow e^-e⁺ line.The instrumental categories which can be identified in the energy range of nuclear astrophysics have their origins in the different concepts of light itself: geometrical optics is the base of coded aperture systems — these methods will continue to yield adequate performances in the near future. Beyond this, focusing telescopes and Compton telescopes, based on wave- and quantum- optics respectively, may be capable to further push the limits of resolution and sensitivity.     

The Hα emitting regions of the accretion disks in Algols

The circumstellar plasma that produces Hα emission in Algol binaries has been investigated using phase-resolved, high dispersion data acquired from CCD and image tube detectors. Results are summarized in this paper, including discussions of the disk geometry and size, asymmetry in the distribution of material, long-term or non-phase dependent variability, mass outflow, the mean electron density, and how the latter properties vary with the system's period or location in the r-q diagram. Five systems which display permanent emission with periods ranging from 4.5 to 261 days (SW Cyg, UX Mon, TT Hya, AD Her, and RZ Oph) are intercompared. If P < 4.5 days, no permanent disks are observed, while if P > 6 days, stable disks with only slight long-term variations in their Hα brightness are seen. The most variable systems appear to be those in the 5 – 6 day range, but the star's position in the r-q diagram has the largest influence on its behavior. The trailing side of the accretion disk, where the gas stream impacts the inner disk, is usually brighter, and the leading side is often times more extended. The disk extends out to at least 95% of the Roche surface of the primary and is highly flattened (≤R_P). Mass outflow near phase 0.5 is commonplace.

     

Who is who in Algol-land ? - Part II -

In part I (De Greve and Packet) we have investigated the occurrence of reversed phases of mass-transfer during Case A evolution in close binaries. If the initial period of a system is shorter than 1–2 days (Early Case A) the reversed phase starts before core hydrogen exhaustion of the gainer (part I). This type of evolution is characterized by at least two phases of slow mass-transfer.We have computed the evolution of four Early Case A systems with initial masses of the loser equal to 3 Mo and 5 Mo. These four systems start mass-exchange when Xc of the primary has decreased to 0.525 (75% of its initial value). They all experience two phases of slow mass-transfer.We find that both phases have about the same duration for all systems. The mass ratios are clearly distinct, being closer to unity during the first phase. In the Hertzsprung-Russell, mass-radius and mass-luminosity diagrams both components remain close to the main-sequence band during slow mass-transfer. Evolution as an Algol is ended when both components overflow their outer critical surface after a second reversal of the mass-transfer.Observed Algol systems evolving in Early Case A are scarce. A search thruogh the catalogue by Giuricin et al. gives us the following candidates: X Tri, SX Aur and V Pup. Based on their mass ratios, SX Aur can tentatively be assigned to the first phase of slow mass transfer and X Tri to the second phase. For V Pup (which is more massive) this choice can not be made with certainty.     

Observed redshifts of transition region/corona lines

Solar UV observations reveal a redshifted emission at transition region temperatures, commonly interpreted as a net downflow of plasma. In earlier investigations the magnitude of the redshift has been found to increase with temperature, reaching a maximum at T=10⁵ K, and then to decrease towards higher temperatures. These observations, mostly from Skylab, suggested no significant shift of the O V line at 1218 Å formed at 2.4×10⁵ K. The variation of the downflow velocity with temperature is, however, uncertain since there are few reliable observations of lines formed at higher temperatures.Using spectrograms from the High Resolution Telescope and Spectrograph — HRTS we find an average net redshift of the O V lines at 1218 Å and 1371 Å at all locations extending from disk center to solar limb. A discrepancy between the observed flow velocity in the two lines is probably caused by uncertainty in the available laboratory wavelength of the intercombination line at 1218 Å (2s^{2 1}S₀-2s2p³P₁).The observed shift in O V is compared with corresponding measurements of lines formed at other temperatures (Si IV, C IV, N IV, O IV, and Fe XII). Large variations in the shift are found along the instrument slit. Thus, blueshifts are also observed with the sites of the largest upflow located in the sunspot umbrae and in a quiet region close to an active region.     

Who is who in Algol-land ? - Part II -

In part I (De Greve and Packet) we have investigated the occurrence of reversed phases of mass-transfer during Case A evolution in close binaries. If the initial period of a system is shorter than 1–2 days (Early Case A) the reversed phase starts before core hydrogen exhaustion of the gainer (part I). This type of evolution is characterized by at least two phases of slow mass-transfer. We have computed the evolution of four Early Case A systems with initial masses of the loser equal to 3 Mo and 5 Mo. These four systems start mass-exchange when Xc of the primary has decreased to 0.525 (75% of its initial value). They all experience two phases of slow mass-transfer. We find that both phases have about the same duration for all systems. The mass ratios are clearly distinct, being closer to unity during the first phase. In the Hertzsprung-Russell, mass-radius and mass-luminosity diagrams both components remain close to the main-sequence band during slow mass-transfer. Evolution as an Algol is ended when both components overflow their outer critical surface after a second reversal of the mass-transfer. Observed Algol systems evolving in Early Case A are scarce. A search thruogh the catalogue by Giuricin et al. gives us the following candidates: X Tri, SX Aur and V Pup. Based on their mass ratios, SX Aur can tentatively be assigned to the first phase of slow mass transfer and X Tri to the second phase. For V Pup (which is more massive) this choice can not be made with certainty.     

Coronal Hole Properties Observed with Sumer

We analyze SUMER spectra of 14 lines belonging to 12 ions, obtained on both sides of the boundary of polar coronal holes as well as at other locations along the limb. We compare line intensities, shifts and widths in coronal holes with values obtained in the quiet Sun. We find that with increasing formation temperature, spectral lines show an increasingly stronger blueshift in coronal holes relative to the quiet Sun at an equal heliospheric angle. The width of the lines is generally larger (by a few km/s) inside the coronal hole. Intensity measurements show the presence of the coronal hole in Ne VIII lines as well as in Fe XII, with evidence for a slightly enhanced emission in polar coronal holes for lines formed below 10⁵ K. This revised version was published online in June 2006 with corrections to the Cover Date.     

螺旋桨滑流对带后缘襟翼机翼气动特性影响的数值分析

采用动量理论结合有限体积法求解N-S方程的方法,进行了螺旋桨滑流对带后缘襟翼机翼气动特性干扰的数值模拟分析.螺旋桨模型简化为一个激励盘,即一个无厚度可穿透的圆盘,以给定的圆盘拉力载荷,即压强差来模拟螺旋桨滑流的效果.采用嵌套网格技术处理桨盘与机翼间复杂的几何关系.算例计算表明,该计算方法能够较好的模拟滑流对带后缘襟翼机翼气动特性的影响.     

CCD spectroscopy of the W Serpentis binaries KX And and RX Cas

Initial results are presented from a study of H _γ profiles in the two interacting binaries KX And and RX Cas of W Serpentis type. The used CCD spectra with a resolution of 0.13Å/px were obtained with the 2.2m telescope and the Coudé spectrograph at the German-Spanish Astronomical Center at Calar Alto/Spain. KX And. This star is probably a non-eclipsing member of the W Serpentis type interactive binaries and has a period of P = 38.908 days. Our seven spectra of KX And were obtained at phase 0.54 – 0.75. The P Cyg profiles of the H _γ line during our observations indicate an expanding shell. The asymetry becomes blue-sided at phase 0.67 and increases thereafter. This points toward a strong outflow of matter in the vicinity of the L3 point. RX Cas. According to the model of Andersen et al. (1988) the primary is a mid-B type star with M = 5.8M _⊙ and R = 2.5R _⊙. The star is completely obscured by a geometrically and optically thick disk, which is supplied by mass transfer from the other component. The secondary is a K1 giant with M = 1.8M _⊙ and R = 23.5R _⊙ and fills out his critical Roche lobe. Radiative and geometrical properties of the disk are variable and its structure is probably not homogenous. Five spectra of RX Cas were obtained during the primary eclipse (phase 0.95 – 0.19). The observed double-peak emission is seen only after the eclipse with a separation of ≈ 250 km/s peak-to-peak, while during the eclipse an asymetric line profile can be observed with a red-shifted emission always presented. Also, a central emission at φ = 0.94 should be noticed, probably originating in the vicinity of L1. The observations of both systems indicate that we are dealing with strongly interacting binaries. Further observations are planned for better covering of phase.     

Magnetic energy release near accreting black holes

The observed non-thermal emission from accreting compact objects is often understood in terms of the expected magnetic activity of accretion disks. This review discusses the constraints on this view point that can be obtained from, principally, the X-ray spectra and the X-ray variability of black hole candidates.Furthermore, the traditional view of an accretion disk corona, put forward as the source of the non-thermal emission, analogous to the solar corona is shown to be wrong on a few important points. Firstly, the density in the equilibrium accretion disk corona is extremely low. A reasonable plasma density is retained by pair production processes similar to those existing in the pulsar magnetosphere. Secondly, the dominant resistivity in the accretion the disk on the current carrying electrons.     

Tenma observations of bright binary X-ray sources

Spectroscopic study of bright binary X-ray sources, performed with the gas scintillation proportional counters on board Tenma, is reviewed. Properties of an iron emission line from two classes of bright binary X-ray sources: X-ray pulsars and low-mass binary sources, are first presented. It is shown that a most likely candidate for the line emitting region is an Alfven shell in case of X-ray pulsars, whereas that of low mass binary sources is an outer accretion disk. Next, nature of the continuum emission from low-mass binary sources is consistently interpreted by a picture that an optically thick accretion disk extends down to very near the surface of a weakly magnetized neutron star. Origin of ultrasoft spectra of black hole candidate sources is also discussed.