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

Optical, infrared and millimetre-wave properties of Vega-like systems - III. Models with thermally spiking grains

Vega-like stars are main-sequence stars that exhibit excess IR emission due to circumstellar dust grains which are probably distributed in discs. We have recently published an obser- vational data base for a large sample of candidate Vega-like systems, comprising optical, near- IR and mm/submm-wave photometry, and mid-IR spectra. In a previous paper we presented radiative transfer models of eight sources from our sample that had low fractional excess luminosities. Here we present models of a further eight sources, all with large fractional excess luminosities dominated by excess emission at near-IR wavelengths. It was found that no single distribution of dust grains at thermal equilibrium in a disc could simultaneously match the excess emission at near-IR and longer wavelengths. We attempted to model the near-IR emission as due to thermally spiking small grains, which can temporarily attain the high temperatures required to produce excess near-IR emission. A near-IR spectrum of SAO 186777 shows the 3.3-μm UIR emission band, confirming our earlier detection of UIR emission at longer wavelengths, and suggesting that small carbonaceous particles are responsible for some of the near-IR emission. The thermally spiking models were only partially successful and many of the sources required the presence of grains emitting in thermal equilibrium at ∼ 1000- 1500 K. These grains must either be located very close to the stars (<1 au), or else be powered by accretion luminosity. Calculations of the optical depths of the model discs suggest the discs are optically thick at visual wavelengths; optically thick modelling of these sources is desirable. The discs are optically thin at mm wavelengths, allowing us to confirm the presence of large grains in the discs. The stars presented in this paper may well be younger than the prototype Vega-like stars

Silicate dust in a Vega-excess system

The 10-µm spectrum of the K5V star SAO 179815 ( = HD 98800) is presented, and conclusively demonstrates the presence of small silicate dust grains (probably in a disc) around this star. The 9.7-µm silicate dust feature is unusually broad and shallow in this system. This, together with the slow fall-off of flux at longer wavelengths, constrains the size and density distributions of dust grains in models of the disc. We find that there must be a significant population of small grains (radii at least as small as 0.01 µm), as well as a population of large grains (radii at least as large as 100 µm) in order to explain all the observed properties of the disc

The nature of the silicon carbide in carbon star outflows

We present 7.5-13.5 μm UKIRT CGS3 spectra of 32 definite or candidate carbon stars. In addition to the extreme carbon star AFGL 3068, the only carbon star previously known to show the 11-μm silicon carbide (SiC) feature in absorption, we have discovered three further examples of sources that show SiC in net absorption, namely IRAS 02408 + 5458, AFGL 2477 and AFGL 5625. We investigate the mineralogy of carbon star SiC and its relationship to meteoritic dust by using a χ2-minimization routine to fit the observed SiC features, and laboratory optical constants that have been published for a variety of SiC samples. With the exception of R For, all of the observed SiC features are best fitted by α-SiC grains. Excluding V414 Per, all of the sources with 8–13 μm colour temperatures > 1200 K (corresponding to mass-loss rates at the bottom end of the range) are best fitted by μ-SiC in pure emission, whereas all but one of the sources with 8–13 μm colour temperatures < 1200 K (corresponding to higher mass-loss rates) are best fitted using self-absorbed a-SiC emission. The four sources whose SiC features are in net absorption (and which have the lowest 8–13 μm colour temperatures and therefore presumably the highest mass-loss rates) are also well fitted by self-absorbed α-SiC emission, but with higher optical depths. Given that β-SiC is the form most commonly found in meteorites, we have searched for evidence of β-SiC in the circumstellar shells of all these carbon stars. However, our observations provide no unambiguous evidence for the presence of β-SiC around these stars, with all of the observed SiC features being best explained in terms of α-SiC grains. The self- absorption that we find in the observed SiC emission features has not previously been taken into account in radiative transfer modelling, and so the amount of SiC present in the outflows has probably been underestimated in the past

UIR-band emission from M supergiants

We have obtained 10−μm spectra of 16 M supergiants, 15 of them in the h and χ Per association. All of the stars exhibit silicate emission features, but in addition seven of the stars show narrow UIR (unidentified infrared) band emission features, at 11.3 μm⁠, 8.65 μm and other wavelengths, which are normally associated with carbon-rich media. Not only are these the coolest objects to have been found to exhibit UIR-band emission, but the outflows from these classical oxygen-rich stars should form only O-rich particles according to equilibrium condensation theory. We interpret our results in terms of the non-equilibrium chemistry model by Beck et al., whereby chromospheric UV radiation can liberate some atomic carbon via the photodissociation of CO molecules, enabling the formation of carbon-rich species as well as silicates. Such a chromospheric UV radiation field could also provide the photons needed to excite the observed UIR-band emission

Methanol Ice in the Protostar GL 2136

We present ground-based spectra in the 10 and 20 micron atmospheric windows of the deeply embedded protostar GL 2136. These reveal narrow absorption features at 9.7 and 8.9 microns, which we ascribe to the CO-stretch and CH3 rock (respectively) of solid methanol in grain mantles. The peak position of the 9.7 micron band implies that methanol is an important ice mantle component. However, the CH3OH/H2O abundance ratio derived from the observed column densities is only 0.1. This discrepancy suggests that the solid methanol and water ice are located in independent grain components. These independent components may reflect chemical differentiation during grain mantle formation and/or partial outgassing close to the protostar

The nature of dust around the post-asymptotic giant branch objects HD 161796 and HD 179821

Ground-based 7.4-24-micron spectra of two post-AGB objects, HD 161796 and HD 179821, are reported, and they are compared to those of other preplanetary nebulae. HD 161796 and HD 17982 show emission features at 10-12 microns and at 10 microns, and they exhibit a very rapid increase in flux between 13 and 15.5 microns. In view of the O-rich photosphere of HD 161796 and the presence of OH maser emission around all three objects, these features are ascribed to various oxides. The observed spectral features are quite different from the canonical silicate features observed in most O-rich giants. It is argued that HD 161796 and the bipolar nebulae Roberts 22 and NGC 6302 have all undergone the third dredge-up, with most of the dredged-up carbon having been converted to nitrogen by envelope-burning. It is concluded that carbon-rich grain material, produced during the interval between the end of the third dredge-up and the moment when envelope burning finally reduced the C/O ratio below unity again, could be responsible for the UIR bands now being excited in Roberts 22 and NGC 6302

A compact, variable radio nebula around P Cygni

We present high spatial resolution images, at a wavelength of 6 cm, of the luminous blue variable star P Cygni. The images fully resolve the core of the stellar wind, and show that it is very clumpy. Two images were taken, separated in time by approximately a month, during which the structure in the wind has changed radically. The total flux observed has also changed significantly. We show that the clump sizes and electron densities are consistent with the radio variability being due to recombination of the ionized gas within the clumps, after key cooling lines become optically thin, causing the free-free emission to disappear. This is the first time that resolved radio images of a hot star wind have been obtained. It also represents important confirmation of a previous observation that thermal free-free wind emission can vary rapidly in a hot stellar wind

A large radio nebula around P Cygni

We present a large set of radio observations of the luminous blue variable P Cygni. These include two 6-cm images obtained with MERLIN which spatially resolve the 6-cm photosphere, monitoring observations obtained at Jodrell Bank every few days over a period of two months, and VLA observations obtained every month for seven years. This combination of data shows that the circumstellar environment of P Cyg is highly inhomogeneous, that there is a radio nebula extending to almost an arcminute from the star at 2 and 6 cm, and that the radio emission is variable on a time-scale no longer than one month, and probably as short as a few days. This short-time-scale variability is difficult to explain. We present a model for the radio emission with which we demonstrate that the star has probably been losing mass at a significant rate for at least a few thousand years, and that it has undergone at least two major outbursts of increased mass loss during the past two millenia

CO emission from shock and PDR in C-rich PN and post-AGB objects

The LWS full grating scans of the PN, NGC 7027, and post-AGB objects, GL618 and GL2688 reveal a forest of lines which are identified as CO rotational lines. These lines are used as diagnostics for warm gas around these objects. For NGC 7027 and GL 618, the hot central star is the source of the ionizing photons, creating a PDR. GL2688 is a cooler post-AGB star with evidence of a fast wind which results in shock heated gas. From the CO observations, we can estimate the density of the molecular layer. In agreement with earlier work, we found that the molecular layer is warm (T~ 350-600 K) and dense (n~ 107 cm-3). This may have implications on mass loss during the last stage of the evolution before stars evolve off the AGB