High-speed knots in the hourglass-shaped planetary nebula Hubble 12

We present a detailed kinematical analysis of the young compact hourglass-shaped planetary nebula Hb 12. We performed optical imaging and long-slit spectroscopy of Hb 12 using the Manchester echelle spectrometer with the 2.1-m San Pedro Mártir telescope. We reveal, for the first time, the presence of end caps (or knots) aligned with the bipolar lobes of the planetary nebula shell in a deep [N  ii ]λ6584 image of Hb 12. We measured from our spectroscopy radial velocities of  ∼120 km s⁻¹  for these knots.
We have derived the inclination angle of the hourglass-shaped nebular shell to be ∼65° to the line of sight. It has been suggested that Hb 12's central star system is an eclipsing binary which would imply a binary inclination of at least 80°. However, if the central binary has been the major shaping influence on the nebula, then both nebula and binary would be expected to share a common inclination angle.
Finally, we report the discovery of high-velocity knots with Hubble-type velocities, close to the core of Hb 12, observed in Hα and oriented in the same direction as the end caps. Very different velocities and kinematical ages were calculated for the outer and inner knots showing that they may originate from different outburst events.     

IRTF observations of white dwarfs with possible near-infrared excess

Near-infrared photometry and spectroscopy are obtained for a heterogeneous sample of nearby white dwarfs with possible excess flux as identified primarily in the Two Micron All Sky Survey. Among the sample of 43 stars are a number of white dwarfs that are either metal-rich, magnetic or binary suspects. With a few notable exceptions in four (or possibly five) distinct categories, the newly obtained JHK photometric data fail to corroborate the putative excesses, with  〈 K _IRTF− K _2MASS〉=+0.31  mag. Where available, Galaxy Evolution Explorer photometric data are used to better constrain the overall spectral energy distribution of the white dwarfs, enabling any excess near-infrared flux to stand out more readily against the expected stellar photosphere.
With superior data, a near-infrared photometric excess is confirmed at three metal-rich white dwarfs and ruled out at nine others. Several new binaries are confirmed or suggested; five white dwarf–red dwarf pairs and five double degenerates. Four apparently single magnetic white dwarfs – two DA and two DQp – display modest to strong near-infrared excess (relative to non-magnetic models), which may be better described as two effective temperatures owing to a redistribution of energy in highly magnetic or peculiar atmospheres.     

The use of genetic algorithms to model protoplanetary discs

The protoplanetary discs of T Tauri and Herbig Ae/Be stars have previously been studied using geometric disc models to fit their spectral energy distribution (SED). The simulations provide a means to reproduce the signatures of various circumstellar structures, which are related to different levels of infrared excess. With the aim of improving our previous model, which assumed a simple flat-disc configuration, we adopt here a reprocessing flared-disc model that assumes hydrostatic, radiative equilibrium. We have developed a method to optimize the parameter estimation based on genetic algorithms (GAs). This paper describes the implementation of the new code, which has been applied to Herbig stars from the Pico dos Dias Survey catalogue, in order to illustrate the quality of the fitting for a variety of SED shapes. The star AB Aur was used as a test of the GA parameter estimation, and demonstrates that the new code reproduces successfully a canonical example of the flared-disc model. The GA method gives a good quality of fit, but the range of input parameters must be chosen with caution, as unrealistic disc parameters can be derived. It is confirmed that the flared-disc model fits the flattened SEDs typical of Herbig stars; however, embedded objects (increasing SED slope) and debris discs (steeply decreasing SED slope) are not well fitted with this configuration. Even considering the limitation of the derived parameters, the automatic process of SED fitting provides an interesting tool for the statistical analysis of the circumstellar luminosity of large samples of young stars.     

Observations and modelling of spectral energy distributions of carbon stars with optically thin envelopes

We present broad-band photometry in the optical, near-infrared and submillimetre, and mid-infrared spectrophotometry of a selection of carbon stars with optically thin envelopes. Most of the observations were carried out simultaneously.   Beside the emission feature at 11.3 μ m due to silicon carbide grains in the circumstellar environment, many of our mid-infrared spectra show an emission feature at 8.6 μ m. All the observed spectral energy distributions exhibit a very large far-infrared flux excess. Both these features are indeed common to many carbon stars surrounded by optically thin envelopes.   We have modelled the observed spectral energy distributions by means of a full radiative transfer treatment, paying particular attention to the features quoted above. The peak at 8.6 μ m is usually ascribed to the presence of hydrogenated amorphous carbon grains. We find also that the feature at 8.6 μ m might be reproduced by assuming that the stars have a circumstellar environment formed of both carbon- and oxygen-rich dust grains, although this is in contrast with what one should expect in a carbon-rich environment. The far-infrared flux excess is usually explained by the presence of a cool detached dust shell. Following this hypothesis, our models suggest a time-scale for the modulation of the mass-loss rate of the order of some 10³ yr.     

A study of the 2.5–25 μm spectrum of H2O ice

We present new absorbance spectra of the 3-, 6- and 12-μm features of amorphous and crystalline H₂O ice obtained between 10 and 140 K. Three sets of measurements have been made. In series I, the ice film was initially deposited on to a CsI substrate at 10 K and successive spectra were then obtained at intermediate temperatures as the ice was warmed up to 140 K. The second set, series II, comprises spectra for ice films deposited and measured at temperatures between 10 and 140 K. In the third set of measurements, series III, spectra were obtained for an ice film deposited at 140 K and then at intermediate temperatures as the film was cooled down to 10 K. The series I and II results show that the ice undergoes an amorphous-to-crystalline phase transition in the 110–120 K range. The 3- and 12-μm bands have similar trends in full width at half-maximum (FWHM) and opposite peak wavelength shifts. The temperature behaviour of the 6-μm band is different, as no clear phase transition temperature can be discerned from its FWHM and peak wavelength position data. In the series III spectra the peak wavelength positions and FWHM of the three bands remain relatively constant, thus demonstrating the stability of the crystalline phase against thermal cycling. A comparison between the laboratory results and astronomical spectra suggests that the identification of the librational band of H₂O ice in OH 231.8 + 4.2 may be incorrect.     

S 266: a ring nebula around a Galactic B[e] supergiant?

A narrow-band Hα image and high-resolution spectroscopy have been obtained in order to investigate the nature of S 266 and its central star MWC 137. The analysis of the stellar and nebular spectra suggests that MWC 137 is a B[e] supergiant located ≥ 6 kpc away from the Sun and not a Herbig Ae/Be star, as it has been traditionally classified. Moreover, the morphology and other properties of the nebula suggest that S 266 is a ring nebula, probably produced by the interaction of stellar winds with the ambient interstellar medium or unprocessed ejected matter. This result would imply that S 266 is the first ring nebula around a B[e] supergiant known in the Galaxy.     

A new VLBI observation of an OH maser in a post-asymptotic giant branch star using the European VLBI Network

OH 17.7 − 2.0 is a post-asymptotic giant branch star that is of great interest. The 1612-MHz OH emission from OH 17.7 − 2.0 is characterized by a double-peaked spectrum. Such a line profile has been assumed to represent maser emission from an expanding circumstellar shell. A new VLBI observation of the OH maser in OH 17.7 − 2.0 has been made using the European VLBI Network, and a relative position map of the eight OH maser spots has been obtained. Using the relative position map, it is found that the eight OH maser spots are distributed on an expanding circumstellar shell. The parameters of the expanding circumstellar shell have been obtained.     

A textbook case of bow shock entrainment in a YSO outflow

Near-infrared images in H₂ line emission and submillimetre maps in CO J  = 3–2 emission illustrate the remarkable association between a molecular bow shock and the redshifted molecular outflow lobe in W75N. The flow lobe fits perfectly into the wake of the bow, as one would expect if the lobe represented swept-up gas. Indeed, these observations strongly support the 'bow shock' entrainment scenario for molecular outflows driven by young stars.   The characteristics of the bow shock and CO outflow lobe are compared with those of numerical simulations of jet-driven flows. These models successfully reproduce the bulge and limb-brightening in the CO outflow, although the model H₂ bow exhibits more structure extending back along the flow axis. We also find that the size of the flow, the high mass fraction in the flow at low outflow velocities (low γ values) and the high CO/H₂ luminosity ratio indicate that the system is evolved. We also predict a correlation, in evolved systems, between outflow age and the CO/H₂ luminosity ratio.     

Silicate and hydrocarbon emission from Galactic M supergiants

Following our discovery of unidentified infrared (UIR) band emission in a number of M supergiants in h and χ Per, we have obtained 10-μm spectra of a sample of 60 galactic M supergiants. Only three new sources, V1749 Cyg, UW Aql and IRC+40 427, appear to show the UIR bands; the others show the expected silicate emission or a featureless continuum. The occurrence of UIR-band emission in M supergiants is therefore much higher in the h and χ Per cluster than in the Galaxy as a whole. Possible explanations for the origin and distribution of UIR bands in oxygen-rich supergiants are discussed. We use our spectra to derive mass-loss rates ranging from 10⁻⁸ to 10⁻⁴ M_⊙ yr⁻¹ for the new sample, based on the power emitted in the silicate feature. The relationship between mass-loss rate and luminosity for M supergiants is discussed, and correlations are explored between their mid-infrared emission properties.