The Be/X-ray transient V0332+53: evidence for a tilt between the orbit and the equatorial plane?

We present optical and infrared observations of BQ Cam, the optical counterpart to the Be/X-ray transient system V0332+53. BQ Cam is shown to be an O8–9Ve star, which places V0332+53 at a distance of ∼7 kpc. H α spectroscopy and infrared photometry are used to discuss the evolution of the circumstellar envelope. Owing to the low inclination of the system, parameters are strongly constrained. We find strong evidence for a tilt of the orbital plane with respect to the circumstellar disc (presumably on the equatorial plane). Even though the periastron distance is only ≈10 R _*, during the present quiescent state the circumstellar disc does not extend to the distance of periastron passage. Under these conditions, X-ray emission is effectively prevented by centrifugal inhibition of accretion. The circumstellar disc is shown to be optically thick at optical and infrared wavelengths, which, together with its small size, is taken as an indication of tidal truncation.     

One hundred years of observations of the Be star HDE 245770 (the X-ray binary A0535+26/V725 Tau): The end of an active phase?

UBV observations of the X-ray binary system A0535+26/V725 Tau at the Crimean Station of the Sternberg Astronomical Institute in 1980–1998 are presented. Based on our and published data, we analyze the photometric history of the star from 1898. Over a period of 100 years, the star apparently showed all three activity phases (B, Be, Be-shell) of Be stars. We conclude that the X-ray activity of the object is attributable to the 1970–1997 outburst of the Be star due to envelope ejection. The star's colors during the minimum light of 1998 and its 1953–1956 colors (before the outburst) correspond to the spectral type B0–B1 at the color excesses E _B-V = 0.74 and E _U-B = 0.48, in agreement with the current spectral type O9.7. The minimum light of 1998 and the color excesses are used to determine the colors of the additional radiation, analyze their evolution during the 1973–1997 outburst, and refine the distance to the object (3 kpc). The colors of the additional radiation at maximum light of the star (1973–1980) match the colors of a hydrogen plasma with T _e = 1.5 × 10⁴ K which is optically thick in the Balmer continuum. The brightness decline corresponds to a decrease in the optical depth of the plasma; at $V \simeq 9_.^m 1$ , it becomes optically thin in the Balmer continuum with T _e = 10⁴ K and N _e = 10¹⁰ ? 10¹² cm^?3. This conclusion is consistent with the model of a circumstellar envelope but is inconsistent with the existence of an accretion disk around the neutron star. All the additional radiation responsible for the optical variability is produced by a single source. The intensity of the Hα emission line at maximum light (1975–1980) is triple its intensity in 1987–1997, when quasi-periodic light fluctuations with P ≈ 1400^d were observed [1]. At this time, the line intensity correlated with brightness. The Hα line was in absorption at the minimum of 1998, and, at present, the star's active phase appears to have ended.     

Long-term variability of the Be/X-ray binary EXO 2030+375

We present a multi-wavelength study of the Be/X-ray binary system EXO 2030+375. We report that the Be companion is currently in a low-activity phase as indicated by the notable decrease of the infrared and optical emission. If this trend continues the source will lose its circumstellar envelope. Infrared spectroscopy in the IJHK bands is presented for the first time, along with optical and X-ray observations. These infrared spectra agree with the optical companion being an early-type (B0) main-sequence star. When active EXO 2030+375 shows an X-ray outburst at each periastron passage of the neutron star. In addition to the maximum X-ray luminosity displayed at orbital phase ∼0.0, we find a smaller maximum in the light curve at phase ∼0.5. This second intensity peak may be explained if the velocity of the wind is lower than or comparable to the orbital velocity of the neutron star at apastron. We also comment on the relation between the optical/infrared behaviour and the X-ray emission and argue that the X-ray inactive period observed between 1993 August and 1996 April is a result of centrifugal inhibition of accretion of matter rather than a low-activity circumstellar disc.     

An optical and X-ray study of the counterpart to the Small Magellanic Cloud X-ray binary pulsar system SXP327

Optical and X-ray observations are presented here of a newly reported X-ray transient system in the Small Magellanic Cloud. The data reveal many previously unknown X-ray detections of this system and clear evidence for a 45.99 d binary period. In addition, the optical photometry shows recurring outburst features at the binary period which may be well indicative of the neutron star interacting with a circumstellar disc around a Be star.     

The continuum re-processing efficiency of Be‐star circumstellar discs

We have calculated the total flux emitted in H α , P α and Br α by the circumstellar envelope of both an early Be star, γ Cas, and a late Be star, 1 Delphini, assuming the central star is the only source of energy input into the circumstellar envelope. These estimates are based on the Be-star models of Millar & Marlborough which have self-consistent temperature distributions determined by equating the local rates of energy gain and energy loss in the envelopes. We find that an additional source of ionizing photons, as argued by Apparao, is not necessary to account for the observed emission.     

The symbiotic star CH Cygni – I. Non-thermal bipolar jets

Very Large Array surface brightness and spectral index maps of the evolving extended emission of the triple symbiotic star CH Cygni are presented. These are derived from observations at 4.8, 8.4 and 14 GHz between 1985 and 1999. The maps are dominated by thermal emission around the central bright peak of the nebula, but we also find unambiguous non-thermal emission associated with the extended regions. Our observations confirm that this is a jet. The central region has been associated with the stellar components through Hubble Space Telescope imaging. If the jets are the result of ejection events at outburst, expansion velocities are consistent with those from other measurement methods. We propose that the non-thermal emission is caused by material ejected in the bipolar jets interacting with the circumstellar wind envelope. The resulting shocks lead to local enhancements in the magnetic field from the compact component of the order of 3 mG.     

Evolution of red Nova V4332 Sagittarii remnant

We present the multicolor BV RI photometry and BTA/SCORPIO spectroscopy for the red nova V4332 Sgr, performed in 2005–2012. We have analyzed the behavior of the nova remnant, considering our observations along with all the published observations and archival photographic photometry. The atomic and molecular emission spectra show an exponential weakening. Since 2003, the line fluxes have on the average decreased 30-fold. The continuum of the M-type giant in the spectrum has weakened abruptly between 2006 and 2008, twice in the red and by 4 times in the blue range. The variations in the spectral energy distribution of the stellar continuum after the outburst correspond to a decrease in the surface temperature of the M-type giant by 1000 K. The presence of a faint star of about 20^m is possible. The star can be either a member of the system or a field star. It is found that the phenomenon of the red nova in this system is not related with the M-type star. The progenitor of the explosion could be a blue straggler, which has disappeared from the spectral energy distribution after the explosion of 1994. It is most likely that this object was a contact binary system. To explain the “cold explosion” of V4332 Sgr, we have accepted the hypothesis of merging components of a contact binary star in a hierarchical triple or multiple system. There are evidences of dynamical destruction of the outburst remnant and accretion of its matter to the M-type giant. The cause of the red nova phenomenon is thought to be a sudden explosive energy release in the center of a star having a massive envelope, and a subsequent expansion of the envelope in the conditions close to adiabatic. As a result of the explosion, thermal energy reaches the surface of the envelope a year or a few years after the outburst, whereas the envelope already has a large surface area. The cause of the explosion in the center of the star can be both a merger event of the nuclei of two stars in the contact system with a formed common massive envelope and instability in the core of a single massive star. Thus, the red novae can be heterogeneous objects at different evolutionary stages.     

Near infrared observations and analyses of molecular outflows

We present results of near infrared observation of 21 molecular outflow sources and two non-outflow sources with compact cores. Combined with IRAS and other surface station observations we analyse their spectra and find that the outflow sources have, on average, steeper spectral gradients than the non-outflow sources in the range 2.28–25 μ. Most of the bipolar outflow sources have gradients greater than 2.0. Using a revised blackbody photosphere model we calculate the contributions to the JHK fluxes by the central young star and the circumstellar envelope. For the sources with known bolometric luminosity we derive the photospheric temperature of the central star and the circumstellar extinction. Results show that most of the young stars associated with molecular outflows are probably T Tauri stars (5000–7000 K) or emission line stars (9000–26000 K). The circumstellar extinction in JHK is around 10 to 20 magnitudes. These facts show that molecular outflow sources are young objects still embedded deep inside or around the interior of compact cores. Fitting the 3.5– 25 μ and 60–100 μ spectra with a λ^-1 dust emission model to five source gives a negative power law for the temperature profile of the circumstellar shell with exponents between 0.39 and 0.48, close to the theoretical results for molecular clouds associated with HI I regions.     

Formation rate of triple stars relative to double stars

A series of our and McAlisteret al.'s speckle observations of spectroscopic binaries show that certain fractions of them are triple systems. A typical configuration of those systems is that of two stars separated by about 0.1 AU with a third component in an orbit at a distance about 10 AU from the central stars. These results suggest that the third star was formed from the outer circumstellar envelope after forming the central double.     

Hα spectropolarimetry of the Herbig Ae star AB Aurigae

We present spectropolarimetric observations, obtained at H α , of the Herbig Ae star AB Aurigae. Changes in linear polarization across the H α line probe structure in the immediate circumstellar environment of the central star, down to scales of the order of one to a few stellar radii. In the case of AB Aurigae the observed polarimetric signature is complex. After applying a correction for foreground continuum polarization, we find that there is a linear-polarized H α emission component intrinsic to the source. Rotation of the angle of polarization through the emission-line profile suggests scattering in a rotating circumstellar disc. The magnetic accretor model commonly applied to T Tauri stars shows promise of explaining these data.     

Full polarization structure of the OH main-line maser envelopes of W Hydrae

Simultaneous MERLIN observations of the OH 1665- and 1667-MHz maser lines in the circumstellar envelope of the semiregular star W Hya have been taken in all Stokes parameters. The 1665-MHz emission comes from two elongated clusters located 80 au from the star. The 1667-MHz emission arises in an incomplete shell of radius 130 au, with the blueshifted features located in the northern part of the envelope and the redshifted components clustered south of the centre. The circularly polarized maser components exhibit spatial separation along the north–south direction. The linearly polarized components were found from the near side of the envelope. Their polarization position angles indicate that the projected axis of the magnetic field at PA ≃ −20° is consistent with spatial segregation of circular polarization. The intensity of the magnetic field, estimated from a tentative measurement of Zeeman splitting, is about 0.6 mG at the location of the 1667-MHz emission, with the field pointing away from the observer. A small change of position angles of linear polarization observed in both maser lines is interpreted as a weak Faraday effect in the maser regions with an electron density of about 2 cm⁻³. The overall polarization structure of the envelope suggests an ellipsoidal or weak bipolar geometry. In such a configuration, the circumstellar magnetic field may exert a non-negligible influence on mass loss. The velocity field in the circumstellar envelope recovered from observations of SiO, H₂O, OH and CO lines at five radial distances reveals a logarithmic velocity gradient of 0.25 and 0.21 in the 1665- and 1667-MHz maser regions respectively. The acceleration within tens of stellar radii cannot be explained by the classical model of radiation pressure on dust.     

Is LkHα 264 like a young, extremely active Sun?

We combine calibrated International Ultraviolet Explorer ( IUE ) archive data and new low-resolution optical data for the T Tauri star LkH α 264 covering the region from 1200 to 7000 Å. The UV continuum is well fitted by the combination of a blackbody at 4300 K plus hydrogenic free–free and free–bound emission from a dense plasma at 3.5×10⁴ K plus the emission by a second blackbody. This last component is at T ≈8700 K and covers about 4 per cent of the stellar surface. We interpret this last component to be the result of emission from one or various hotspots. The interesting result is that this combined emission also fits the observed optical continuum well. We conclude that this star is an analogue of the Sun, however displaying a much higher level of activity.     

ISO LWS Observations of H2O from R Cas: A Consistent Model For Its Circumstellar Envelope

We present an ISO LWS 43-197 μm grating spectrum of the oxygen-rich AGB star R Cas. The spectrum is rich of isolated and blended H2O lines. For their identification and in order to determine the physical parameters of the circumstellar envelope, we have constructed a model which treats radiative transfer, chemical exchange and photodissociation reactions, and various heating and cooling processes in a consistent manner. By fitting the observed line fluxes and using stellar parameters based upon the Hipparcos distance, we derived a mass-loss rate of M = 1 10-6 M⊙ yr-1 which is close to the value 6 10-7 M⊙ yr-1 previously derived for W Hya, another oxygen-rich AGB star. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ultraviolet observations of the hydrogen-deficient variable star MV sagittarii

IUE observations of the hydrogen-deficient irregular variable star MV Sgr obtained in 1980 June-October and also in 1979 November are discussed. These observations show a prominent λ 2200 absorption feature. A value ofE(B — V) = 0.55 is deduced from the strength of λ 2200 band assuming that this absorption is caused by interstellar medium. The dereddened continuum obtained at different times can be fitted to a theoretical energy distribution of a helium star model with T_eff = 18000 K and log g = 2.5, similar to that of BD + 10^‡ 2179. This theoretical energy distribution, after applying interstellar extinction, givesV = 12.7 mag, agreeing with the observed visual magnitude of ≃ 13 in 1979 November and 1980 June-October. Even though there was no change in the continuum flux, the ultraviolet line-spectrum shows variations. The IUE spectra of 1980 October show enhanced (circumstellar) absorption lines of Fe II, Si II, O I, C I and others along with the absorption lines of a B star. In view of the similarity of the spectroscopic phenomena of MV Sgr with that of α Sco system, a model is proposed in which a cool companion star, surrounded by dust, occasionally blows gas towards the hotter hydrogen-poor B star. This model explains the irregular light variations and the spectroscopic phenomena. Based on observations obtained with IUE satellite at the Villafranca Satellite Tracking Station of the European Space Agency.     

Large molecules in the envelope surrounding IRC+10216

A new chemical model of the circumstellar envelope surrounding the carbon-rich star IRC+10216 is developed that includes carbon-containing molecules with up to 23 carbon atoms. The model consists of 3851 reactions involving 407 gas-phase species. Sizeable abundances of a variety of large molecules including carbon clusters, unsaturated hydrocarbons and cyanopolyynes have been calculated. Negative molecular ions of chemical formulae and C _n H (7 n 23) exist in considerable abundance, with peak concentrations at distances from the central star somewhat greater than their neutral counterparts. The negative ions might be detected in radio emission, or even in the optical absorption of background field stars. The calculated radial distributions of the carbon-chain C _n H radicals are looked at carefully and compared with interferometric observations.     

Continuum Energy Distribution of Bright Be Stars

This paper presents the spectrophotometric observations of five Be stars belonging to spectral type from B2.5 to B5 and luminosity class from III to V. The continuum energy distribution data of these stars are extracted from the spectrophotometric measurements over the wavelength range of λλ 3200–8000 Å. The observed data of continuum energy distribution are compared with the theoretical energy distribution curves to derive the value of effective temperatures of these stars. We have also examined the effect of circumstellar envelopes of Be stars on their continuum energy distribution. It has been found that the extended circumstellar envelope of Be stars affect the underlying continua in the near-ultraviolet and near-infrared regions. Three Be stars are found to exhibit near-ultraviolet and near-infrared flux deficiency in their continua. For one of the Be, namely star HR 1289, we report the first continuum energy distribution measurements. Another normal B star HR 1363, which was observed as comparison star, was found to exhibit near-ultraviolet and excess emission, which are signatures of a Be star, thus putting this star in the category of Be stars.     

LO Pegasi: an investigation of multiband optical polarization

We present BVR polarimetric study of the cool active star LO Pegasi (LO Peg) for the first time. LO Peg was found to be highly polarized among the cool active stars. Our observations yield average values of polarization in LO Peg:   P_B = 0.387 ± 0.004 per cent, θ_B= 88°± 1°; P_V = 0.351 ± 0.004 per cent, θ_V= 91°± 1°  and   P_R = 0.335 ± 0.003 per cent, θ_R= 91°± 1°  . Both the degree of polarization and the position angle are found to be variable. The semi-amplitude of the polarization variability in B, V and R bands is found to be  0.18 ± 0.02, 0.13 ± 0.01  and  0.10 ± 0.02  per cent, respectively. We suggest that the levels of polarization observed in LO Peg could be the result of scattering of an anisotropic stellar radiation field by an optically thin circumstellar envelope or scattering of the stellar radiation by prominence-like structures.     

Axisymmetry in protoplanetary nebulae: using imaging polarimetry to investigate envelope structure

We use ground-based imaging polarimetry to detect and image the dusty circumstellar envelopes of a sample of protoplanetary nebulae (PPNe) at near-infrared wavelengths. This technique allows the scattered light from the faint envelope to be separated from the glare of the bright central star, and is particularly well suited to this class of object. We detect extended (up to 9-arcsec diameter) circumstellar envelopes around 15 out of 16 sources with a range of morphologies including bipolars and shells. The distribution of scattered light in combination with its polarization (up to 40 per cent) provides unambiguous evidence for axisymmetry in 14 objects, showing this to be a common trait of PPNe. We suggest that the range of observed envelope morphologies results from the development of an axisymmetric dust distribution during the superwind phase at the end of the AGB. We identify shells seen in polarized light with scattering from these superwind dust distributions, which allows us to provide constraints on the duration of the superwind phase. In one object (IRAS 19475+3119) the circumstellar envelope has a two-armed spiral structure, which we suggest results from the interaction of the mass-losing star with a binary companion.     

On the initial phase of interaction between expanding stellar envelopes and surrounding medium

The initial stages of deceleration in the circumstellar medium of a stellar envelope, thrown off by a shock wave, are investigated. The equations of spherical-symmetric adiabatic hydrodynamics are shown to have a similarity solution in the case of the density of the expanding envelope being approximated by a reasonable power law. The overall flow pattern has such a form that the stellar material is decelerated in the internal shock wave while another shock propagates through the circumstellar matter. Between the shocks there is a contact discontinuity separating the circumstellar and stellar matter. The characteristics of the similarity solution are calculated for various exponents in the density laws of an expanding envelope and circumstellar matter and for two values of the adiabatic index (=5/3, 4/3). Some parts of the flow exhibit Rayleigh-Taylor instability.Special attention is paid to the validity of the hydrodynamics. In full agreement with D'yachenkoet al. (1969), we conclude that the kinetic and collisionless processes are of great importance if the initial stages of stellar envelope deceleration are to be properly monitored.The results obtained can also be employed to describe the interaction between the exploding core of a red giant star and its rarefied envelope. This is of interest for explosive nucleosynthesis.The similarity solution is applied to the envelopes expelled both by type-II supernovae and by rapid novae. In particular, the thermalization time-scale of circumstellar plasma is estimated. For SNii this time-scale proves to be of the order of 60 yr. This confirms with the observational data on the moment of the maximum radio-emission of young SNRs. In the case of rapid novae, this time is less by a factor of 10. Therefore, the peak radio and X-ray (2 keV) lumnosity may occur several years after the rapid nova outburst. The explosion of a degenerate carbon core is found to result in the heating of the hydrogen-helium envelope of a red giant star up to 3×10⁶ K.     

A model for the dust envelope of V1853 Cygni

We computed a model for the dust envelope of the protoplanetary nebula V1853 Cyg by using data on its fluxes from the ultraviolet to the far infrared. The spherically symmetric envelope was assumed to be composed of silicate grains with the standard MRN size distribution; their number per unit volume is inversely proportional to the distance squared. The optical depth of the envelope, whose inner boundary lies at a distance of 7.6×10¹⁶ cm from the central star, is 0.18 at a wavelength of 0.55 μm. The grain temperature at the inner boundary of the envelope is 110 K. The distance to V1853 Cyg is estimated to be 4.1 kpc. The current mass-loss rate of the object was found by solving a self-consistent problem of radiative transfer and dust motion in the envelope to be 2.2 × 10^?5M_⊙ yr^?1.