Episodic dust formation by HD 192641 (WR 137) – II

We present new infrared photometry of the WC7-type Wolf–Rayet star HD 192641 (WR 137) from 1985 to 1999. These data track the cooling of the dust cloud formed in the 1982–84 dust-formation episode from 1985 to 1991, the increase of the infrared flux from 1994.5 to a new dust-formation maximum in 1997 and its subsequent fading. From these and earlier data we derive a period of 4765±50 d (13.05±0.15 yr) for the dust-formation episodes. Between dust-emission episodes, the infrared spectral energy distribution has the form of a power law, λF _λ ∝ λ ^−1.86. The rising branch of the infrared light curve (1994–97) differs in form from that of the episodic dust-maker WR 125. Time-dependent modelling shows that this difference can be attributed to a different time dependence of dust formation in WR 137, which occurred approximately ∝ t ² until maximum, whereas that of WR 125 could be described by a step function, akin to a threshold effect. For an adopted distance of 1.6 kpc, the rate of dust formation was found to be 5.0×10⁻⁸ M_⊙ yr⁻¹ at maximum, accounting for a fraction f _C≈1.5×10⁻³ of the carbon flowing in the stellar wind. The fading branches of the light curves show evidence for secondary 'mini-eruptions' in 1987, 1988 and 1990, behaviour very different from that of the prototypical episodic dust-maker HD 193793 (WR 140), and suggesting the presence in the WR 137 stellar wind of large-scale structures that are crossed by the wind–wind collision region.     

The Galactic hybrid Wolf–Rayet WN 7o/CE + O7V((f)) binary system WR 145

A spectroscopic study of the binary Wolf–Rayet (WR)+O system WR 145 is performed, in order to determine the radial velocity orbits of the individual stars, the angle of orbital inclination and the stellar masses. The emission and absorption components are separated from the original spectra, allowing us to confirm the spectral classification WN 7o/CE of the hybrid WR component and to derive a spectral classification O7V((f)) for the O star. A study of the wind-collision properties is performed. Fitting the radial velocity and full width at half-maximum of the excess emission with Lührs' model results in an inclination angle of   i = 63°  , leading to estimates of the stellar masses:   M _WR= 18 M_⊙  and   M _O= 31 M_⊙  . Both of these masses are compatible with those of other stars of similar types.     

Wind anisotropy revealed by dust formation around wolf-rayet stars

The presence of heated circumstellar dust around WC type Wolf-Rayet stars requires the episodic or persistent condensation of carbon grains in their stellar winds. In order to survive in the stars' strong ultraviolet radiation fields, the grains must be located at least 100AU from the stellar surfaces. The densities in isotropic winds at such large distances are too low to allow grain growth and anisotropies such as clumps, disks or wind-collision wakes in colliding-wind binary systems are required to provide grain nurseries. Observational evidence for such features in grain-forming W-R stars is examined.     

Infrared mapping of the dust disc around Vega

ISOPHOT has been used to perform high-resolution 60-μm scans of Vega, and these have been compared with those from γDra, to obtain a Gaussian width of 22±2 arcsec. The dust disc around Vega has been mapped, resolving it at 60 and 90 μm with ISOPHOT. At 90 μm a Gaussian width of 36±3 arsec has been derived. In addition, multi-filter photometry is presented, at 25, 60, 80, 100, 120, 150, 170 and 200 μm. The data are fitted by a modified blackbody with a temperature of 73 K [ Q (λ)∝1/λ^1.1]. The dust disc has a luminosity L _IR/ L _*∼3×10⁻⁵. Using a distance of 7.8 pc, 22 arcsec corresponds to a distance of 86 au, and 36 arcsec to a distance of 140 au.     

Dust Formation around Wolf-Rayet Stars

We discuss the observational evidence in favour of dust formation around massive, evolved carbon-rich Wolf-Rayet stars. Large dust shells containing (amorphous carbon) grains are observed in persistent dust makers, such as the pinwheel system WR104 (WC9+OB) and periodic/episodic dust makers, such asWR140 (WC7+O) close to periastron. Problems with the nucleation and growth of dust in such harsh environments are discussed. It is probable that mixing of carbon-rich material from the WC star plus hydrogen from an OB companion, together with efficient shielding via the shocked region of their colliding winds, provides the necessary ingredients. Many problems with the nucleation and growth remain to be addressed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Infrared spectroscopy of Nova Cassiopeiae 1993 – IV. A closer look at the dust

Nova Cassiopeiae 1993 (V705 Cas) was an archetypical dust-forming nova. It displayed a deep minimum in the visual light curve, and spectroscopic evidence for carbon, hydrocarbon and silicate dust. We report the results of fitting the infrared (IR) spectral energy distribution (SED) with the dusty code, which we use to determine the properties and geometry of the emitting dust. The emission is well described as originating in a thin shell whose dust has a carbon:silicate ratio of 2:1 by number (  ∼1.26:1  by mass) and a relatively flat size distribution. The 9.7- and 18-μm silicate features are consistent with freshly condensed dust and, while the lower limit to the grain size distribution is not well constrained, the largest grains have dimensions  ∼0.06 μm  ; unless the grains in V705 Cas were anomalously small, the sizes of grains produced in nova eruptions may previously have been overestimated in novae with optically thick dust shells. Laboratory work by Grishko & Duley may provide clues to the apparently unique nature of nova unidentified infrared (UIR) features.     

Modelling the spectra of colliding winds in the Wolf–Rayet WC7+O binaries WR 42 and WR 79

We have obtained complete phase coverage of the WC7+O binaries WR 42 = HD 97152 and WR 79 = HD 152270 with high signal-to-noise ratio (S/N), moderate-resolution spectra. Remarkable orbital phase-locked profile variations of the C  iii λ 5696 line are observed and interpreted as arising from colliding wind effects. Within this scenario, we have modelled the spectra using a purely geometrical model that assumes a cone-shaped wind–wind interaction region which partially wraps around the O star. Such modelling holds the exciting promise of revealing a number of interesting parameters for WR+O binaries, such as the orbital inclination, the streaming velocity of material in the interaction region and the ratio of wind momentum flux. Knowledge of these parameters in turn leads to the possibility of a better understanding of WR star masses, mass-loss rates and wind region characteristics.     

Scattered light models of the dust shell around HD 179821

The dust shell around the evolved star HD 179821 has been detected in scattered light in near-IR imaging polarimetry observations. Here, we subtract the contribution of the unpolarized stellar light to obtain an intrinsic linear polarization of between 30 and 40 per cent in the shell that seems to increase with radial offset from the star. The J - and K -band data are modelled using a scattering code to determine the shell parameters and dust properties. We find that the observations are well described by a spherically-symmetric distribution of dust with an r ⁻² density law, indicating that when mass-loss was occurring, the mass-loss rate was constant. The models predict that the detached nature of a spherically-symmetric, optically-thin dust shell, with a distinct inner boundary, will only be apparent in polarized flux. This is in accordance with the observations of this and other optically-thin circumstellar shells, such as IRAS 17436+5003. By fitting the shell brightness we derive an optical depth to the star that is consistent with V -band observations and that, assuming a distance of 6 kpc, gives an inner-shell radius of     , a dust number density of     at r _in and a dust mass of     . We have explored axisymmetric shell models but conclude that any deviations from spherical symmetry in the shell must be slight, with an equator-to-pole density contrast of less than 2:1. We have not been able to fit simultaneously the high linear polarizations and the small     colour excess of the shell and we attribute this to the unusual scattering properties of the dust. We suggest that the dust grains around HD 179821 either are highly elongated or consist of aggregates of smaller particles.     

Infrared spectra of evolved stars with unusual dust shells

New mid-infrared spectra are presented of a number of oxygen-rich evolved stars which have IRAS LRS (Low Resolution Spectrometer) spectra that were classified as showing SiC emission. Two of the sources, IRC−20445 and IRC−20461, show the unidentified infrared (UIR) bands superposed on silicate emission features. Both objects have been classified as M supergiants. Several other sources show three-component spectra, with peaks at 10, 11 and 13 μm. The 13-μm source FI Lyr shows a narrow emission feature at 19 μm. Emission by oxide grains may be responsible for the 11-, 13- and 19-μm features. One object, IRC−20455, shows a self-absorbed silicate feature. There is no clear evidence for SiC emission in any of the spectra: the LRS spectra were erroneously classified as showing SiC emission because of the relatively strong 11-μm emission.     

Disc wind in the HH 30 binary models

Recent interferometric observations of the young stellar object (YSO) HH 30 have revealed a low-velocity outflow in the¹²CO J =1–2 molecule line. We present here two models of the low-velocity disc winds with the aim of investigating the origin of this molecular outflow. Following Andlada et al., we treated HH 30 as a binary system. Two cases have been considered: (i) the orbital period   P = 53 yr  and (ii)   P ≤ 1 yr  . Calculations showed that in the first case the outflow cone had a spiral-like structure due to summing the velocities of the orbital motion and the disc wind. Such a structure contradicts the observations. In the second case, the outflow cone demonstrates a symmetry relatively to the system axis and agrees well with the observations.     

Optical and infrared photometry of the Type IIn SN 1998S: days 11–146

We present contemporaneous optical and infrared (IR) photometric observations of the Type IIn SN 1998S covering the period between 11 and 146 d after discovery. The IR data constitute the first ever IR light curves of a Type IIn supernova. We use blackbody and spline fits to the photometry to examine the luminosity evolution. During the first 2–3 months, the luminosity is dominated by the release of shock-deposited energy in the ejecta. After ∼100 d the luminosity is powered mostly by the deposition of radioactive decay energy from 0.15±0.05 M_⊙ of ⁵⁶Ni which was produced in the explosion. We also report the discovery of an astonishingly high IR excess, K − L '=2.5, that was present at day 130. We interpret this as being due to thermal emission from dust grains in the vicinity of the supernova. We argue that to produce such a high IR luminosity so soon after the explosion, the dust must be pre-existing and so is located in the circumstellar medium of the progenitor. The dust could be heated either by the UV/optical flash (IR echo) or by the X-rays from the interaction of the ejecta with the circumstellar material.