Changes in the red giant and dusty environment of the recurrent nova RS Ophiuchi following the 2006 eruption

We present near-infrared spectroscopy of the recurrent nova RS Ophiuchi (RS Oph) obtained on several occasions after its latest outburst in 2006 February. The  1–5 μ  m spectra are dominated by the red giant, but the H  i , He  i and coronal lines present during the eruption are present in all our observations. From the fits of the computed infrared spectral energy distributions to the observed fluxes, we find   T _eff= 4200 ± 200   K for the red giant. The first overtone CO bands at  2.3 μ  m, formed in the atmosphere of the red giant, are variable. The spectra clearly exhibit an infrared excess due to dust emission longward of  5 μ  m; we estimate an effective temperature for the emitting dust shell of 500 K, and find that the dust emission is also variable, being beyond the limit of detection in 2007. Most likely, the secondary star in RS Oph is intrinsically variable.     

NGC 1333/IRAS 4: a multiple star formation laboratory

We present SCUBA observations of the protomultiple system NGC 1333/IRAS 4 at 450 and 850 μm. The 850-μm map shows significant extended emission which is most probably a remnant of the initial cloud core. At 450 μm, the component 4A is seen to have an elongated shape suggestive of a disc. Also we confirm that, in addition to the 4A and 4B system, there exists another component 4C, which appears to lie out of the plane of the system and of the extended emission. Deconvolution of the beam reveals a binary companion to IRAS 4B. Simple considerations of binary dynamics suggest that this triple 4A–4BI–4BII system is unstable and will probably not survive in its current form. Thus IRAS 4 provides evidence that systems can evolve from higher to lower multiplicity as they move towards the main sequence. We construct a map of spectral index from the two wavelengths, and comment on the implications of this for dust evolution and temperature differences across the map. There is evidence that in the region of component 4A the dust has evolved, probably by coagulating into larger or more complex grains. Furthermore, there is evidence from the spectral index maps that dust from this object is being entrained in its associated outflow.     

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.     

The spatial variation of the 3-μm dust features in Circinus

We report spatially resolved variations in the 3.4-μm hydrocarbon absorption feature and the 3.3-μm polycyclic aromatic hydrocarbon (PAH) emission band in the Circinus galaxy over the central few arcsec. The absorption is measured towards warm emitting dust associated with Coronal line regions to the east and west of the nucleus. There is an absorption optical depth  τ_3.4 μm∼ 0.1  in the core which decreases to the west and increases to the east. This is consistent with increased extinction out to ∼40 pc east of the core, supported by the Coronal emission line intensities which are significantly lower to the east than the west. PAH emission is measured to be symmetrically distributed out to ±4 arcsec, outside the differential extinction region. The asymmetry in the 3.4-μm absorption band reflects that seen in the 9.7-μm silicate absorption band reported by Roche et al., and the ratio of the two absorption depths remains approximately constant across the central regions, with  τ_3.4 μm/τ_9.7 μm∼ 0.06 ± 0.01  . This indicates well-mixed hydrocarbon and silicate dust populations, with no evidence for significant changes near the nucleus.     

Detection and evolution of the CO (Δv= 2) emission in Nova V2615 Ophiuchi (2007)

We present near-infrared (1–2.5 μm) spectroscopic and photometric results of Nova V2615 Ophiuchi which was discovered in outburst in 2007 March. Our observations span a period of ∼80 d starting from 2007 March 28 when the nova was at its maximum light. The evolution of the spectra is shown from the initial P Cygni phase to an emission-line phase and finally to a dust formation stage. The characteristics of the JHK spectra are very similar to those observed in a nova outburst occurring on a carbon–oxygen white dwarf. We analyse an observed line at 2.088 μm and suggest that it could be due to Fe  ii excited by Lyman α fluorescence. The highlight of the observations is the detection of the first overtone bands of carbon monoxide (CO) in the 2.29–2.40 μm region. The CO bands are modelled to estimate the temperature and mass of the emitting CO gas and also to place limits on the ¹²C/¹³C ratio. The CO bands are recorded over several epochs, thereby allowing a rare opportunity to study the evolution from a phase of constant strength through a stage when the CO is destroyed fairly rapidly. We compare the observed time-scales involved in the evolution of the CO emission and find a good agreement with model predictions that investigate the chemistry in a nova outflow during the early stages.     

Sakurai's Object: characterizing the near-infrared CO ejecta between 2003 and 2007

We present observations of Sakurai's Object obtained at 1–5 μm between 2003 and 2007. By fitting a radiative transfer model to an echelle spectrum of CO fundamental absorption features around  4.7 μm  , we determine the excitation conditions in the line-forming region. We find  ¹²C/¹³C = 3.5^+2.0_−1.5  , consistent with CO originating in ejecta processed by the very late thermal pulse, rather than in the pre-existing planetary nebula. We demonstrate the existence of  2.2 × 10⁻⁶≤ M _CO≤ 2.7 × 10⁻⁶ M_⊙  of CO ejecta outside the dust, forming a high-velocity wind of  500 ± 80 km s⁻¹  . We find evidence for significant weakening of the CO band and cooling of the dust around the central star between 2003 and 2005. The gas and dust temperatures are implausibly high for stellar radiation to be the sole contributor.     

RXTE observation of NGC 6240: a search for the obscured active nucleus

We present new data taken at 850 μm with SCUBA at the James Clerk Maxwell Telescope for a sample of 19 luminous infrared galaxies. Fourteen galaxies were detected. We have used these data, together with fluxes at 25, 60 and 100 μm from IRAS , to model the dust emission. We find that the emission from most galaxies can be described by an optically thin, single temperature dust model with an exponent of the dust extinction coefficient ( k _λ ∝ λ ^{− β} ) of β ≃1.4–2. A lower β ≃1 is required to model the dust emission from two of the galaxies, Arp 220 and NGC 4418. We discuss various possibilities for this difference and conclude that the most likely is a high dust opacity. In addition, we compare the molecular gas mass derived from the dust emission, M _850 μm, with the molecular gas mass derived from the CO emission, M _CO, and find that M _CO is on average a factor 2–3 higher than M _850 μm.     

Discovery of extended radio emission in the young cluster Wd1

We present 10-μm ISO -SWS and Australia Telescope Compact Array observations of the region in the cluster Wd1 in Ara centred on the B[e] star Ara C. An ISO -SWS spectrum reveals emission from highly ionized species in the vicinity of the star, suggesting a secondary source of excitation in the region. We find strong radio emission at both 3.5 and 6.3 cm, with a total spatial extent of over 20 arcsec. The emission is found to be concentrated in two discrete structures, separated by ∼ 14 arcsec. The westerly source is resolved, with a spectral index indicative of thermal emission. The easterly source is clearly extended and non-thermal (synchrotron) in nature. Positionally, the B[e] star is found to coincide with the more compact radio source, while the southerly lobe of the extended source is coincident with Ara A, an M2 I star. Observation of the region at 10 μm reveals strong emission with an almost identical spatial distribution to the radio emission. Ara C is found to have an extreme radio luminosity in comparison with prior radio observations of hot stars such as O and B supergiants and Wolf–Rayet stars, given the estimated distance to the cluster. An origin in a detatched shell of material around the central star is therefore suggested; however given the spatial extent of the emission, such a shell must be relatively young (τ ∼ 10³ yr). The extended non-thermal emission associated with the M star Ara A is unexpected; to the best of our knowledge this is a unique phenomenon. SAX (2–10 keV) observations show no evidence of X-ray emission, which might be expected if a compact companion were present.     

Carbon monoxide in low-mass dwarf stars

We compare high-resolution infrared observations of the CO 2–0 bands in the 2.297–2.310 μm region of M dwarfs and one L dwarf with theoretical expectations. We find a good match between the observational and synthetic spectra throughout the 2000–3500 K temperature regime investigated. None the less, for the 2500–3500 K temperature range, the temperatures that we derive from synthetic spectral fits are higher than expected from more empirical methods by several hundred kelvin. In order to reconcile our findings with the empirical temperature scale, it is necessary to invoke warming of the model atmosphere used to construct the synthetic spectra. We consider that the most likely reason for the back-warming is missing high-temperature opacity due to water vapour. We compare the water vapour opacity of the Partridge–Schwenke line list used for the model atmosphere with the output from a preliminary calculation by Barber & Tennyson. While the Partridge–Schwenke line list is a reasonable spectroscopic match for the new line list at 2000 K, by 4000 K it is missing around 25 per cent of the water vapour opacity. We thus consider that the offset between empirical and synthetic temperature scales is explained by the lack of hot water vapour used for computation of the synthetic spectra. For our coolest objects with temperatures below 2500 K, we find best fits when using synthetic spectra which include dust emission. Our spectra also allow us to constrain the rotational velocities of our sources, and these velocities are consistent with the broad trend of rotational velocities increasing from M to L.     

The light curve of the semiregular variable L2 Puppis – I. A recent dimming event from dust

The nearby Mira-like variable L₂ Pup is shown to be undergoing an unprecedented dimming episode. The stability of the period rules out intrinsic changes to the star, leaving dust formation along the line of sight as the most likely explanation. Episodic dust obscuration events are fairly common in carbon stars but have not been seen in oxygen-rich stars. We also present a 10-μm spectrum, taken with the Japanese Infrared Telescope in Space satellite, showing strong silicate emission that can be fitted with a detached, thin dust shell, containing silicates and corundum.     

Infrared polarimetry of the southern massive star-forming region G333.6−0.2

We present     spectropolarimetry, and 12- and 2-μm imaging polarimetry of the southern massive star-forming region G333.6−0.2. Spectro-polarimetry measurements show that the polarization observed towards the nebula contains a mixture of both absorptive and emissive polarizations. Model fitting to the spectra indicates that the temperature of the mid-infrared emitting dust grains is generally ∼200 K and the optical depth of the absorbing dust at 9.7 μm is ∼1.5. Fits are also made to the polarimetry spectra, which show a reasonably constant peak absorptive polarization (∼3.4 per cent at 43°) across the face of the H  ii region. This absorptive polarization position angle is consistent with that found by the 2-μm imaging polarimetry     and is most likely due to the Galactic magnetic field local to G333.6−0.2. When the absorptive polarization is subtracted from the 12-μm polarization image, the emissive polarization pattern that is intrinsic to the star-forming region is revealed. A probable magnetic field configuration implied by the intrinsic polarization suggests star formation initially influenced by the Galactic magnetic field which is eventually perturbed by the star formation process.     

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.     

Towards a spectral technique for determining material geometry around evolved stars: application to HD 179821

HD 179821 is an evolved star of unknown progenitor mass range (either post-asymptotic giant branch or post-red supergiant) exhibiting a double-peaked spectral energy distribution (SED) with a sharp rise from ∼8 to  20 μm  . Such features have been associated with ejected dust shells or inwardly truncated circumstellar discs. In order to compare SEDs from both systems, we employ a spherically symmetric radiative transfer code and compare it to a radiative, inwardly truncated disc code. As a case study, we model the broad-band SED of HD 179821 using both codes. Shortward of 40 μm, we find that both models produce equivalent fits to the data. However, longward of 40 μm, the radial density distribution and corresponding broad range of disc temperatures produce excess emission above our spherically symmetric solutions and observations. For HD 179821, our best fit consists of a   T _eff= 7000 K  central source characterized by  τ _V ∼ 1.95  and surrounded by a radiatively driven, spherically symmetric dust shell. The extinction of the central source reddens the broad-band colours so that they resemble a   T _eff= 5750 K  photosphere. We believe that HD 179821 contains a hotter central star than previously thought. Our results provide an initial step towards a technique to distinguish geometric differences from spectral modelling.     

Submillimetre continuum images of the NGC 2024 star-forming ridge

We present 450- and 800-μm images, made with the James Clerk Maxwell Telescope, of the NGC 2024 molecular ridge. The seven previously known compact cores, FIR1–7, have been detected, and FIR5 has been resolved into a compact object and an associated extended source to the east. The estimated masses of the dense cores vary between 1.6 and 5.1 M⊙ per 14-arcsec beam, assuming a dust temperature of 30 K and a dust opacity of κ_800 μm = 0.002 m² kg⁻¹. A spectral index map made from the 450- and 800-μm images shows spatial variations, with the spectral index, α ( F _ν ∝ ν^α), being systematically lower towards the dense cores. We interpret this as evidence for a lower value of the frequency dependence of the dust opacity, β, towards the denser cores relative to the surrounding molecular material. This may indicate that grain growth is occurring in the cores, prior to planetesimal formation. By comparing the high-resolution 450-μm image with interferometer maps of the integrated CS(2–1) emission, the previously reported discrepancy between dust continuum emission and molecular line emission is found to be very localized. Depletion and temperature variations are discussed as possible explanations.     

A multiwavelength infrared study of NGC 891

We present a multiwavlength infrared (IR) study of the nearby, edge-on, spiral galaxy NGC 891. We have examined 20 independent, spatially resolved IR images of this galaxy, 14 of which are newly reduced and/or previously unpublished images. These images span a wavelength regime from  λ 1.2 μ  m in which the emission is dominated by cool stars, through the mid-IR, in which emission is dominated by polycyclic aromatic hydrocarbons (PAHs), to λ 850 μm, in which emission is dominated by cold dust in thermal equilibrium with the radiation field. The changing morphology of the galaxy with wavelength illustrates the changing dominant components. We detect extraplanar dust emission in this galaxy, consistent with previously published results, but now show that PAH emission is also in the halo, to a vertical distance of   z ≥ 2.5 kpc  . We compare the vertical extents of various components and find that the PAHs (from λ 7.7 and 8 μm data) and warm dust (λ 24 μm) extend to smaller z heights than the cool dust (λ 450 μm). For six locations in the galaxy for which the signal-to-noise ratio was sufficient, we present spectral energy distributions (SEDs) of the IR emission, including two in the halo – the first time a halo SED in an external galaxy has been presented. We have modelled these SEDs and find that the PAH fraction, f _PAH, is similar to Galactic values (within a factor of 2), with the lowest value at the galaxy's centre, consistent with independent results of other galaxies. In the halo environment, the fraction of dust exposed to a colder radiation field, f _cold, is of the order of unity, consistent with an environment in which there is no star formation. The source of excitation is likely from photons escaping from the disc.     

The dual dust chemistries of planetary nebulae with [WCL] central stars

The rather rare class of central stars of planetary nebulae that show very low-excitation Wolf–Rayet spectra has been a subject of great interest, particularly in the infrared, since its discovery in the late 1960s. Further peculiarities have been found with the advent of infrared spectroscopy from ISO . Notably, these objects simultaneously betray the presence of regions of carbon-rich and oxygen-rich dust chemistry. We compare and contrast complete ISO spectra between 2 and 200 μm of a sample of six [WC8] to [WC11] central stars, finding many similarities. Among this sample, one star provides strong evidence of quasi-periodic light variations, suggestive of a dust cloud orbiting in a plane from which we view the system.     

The Infrared Evolution of Sakurai's Object

Infrared spectroscopy and photometry have revealed the remarkableevolution of Sakurai's Object from 1996 to the present. A cooling,carbon-rich photospheric spectrum was observable from 1996 to 1998.Considerable changes occured in 1998 as the continuum reddened due toabsorption and emission by newly formed dust located outside thephotosphere. In addition, a strong and broad helium 1.083 m P Cygniline developed, signifying the acceleration of an outer envelope ofmaterial to speeds as high as 1000 km s^-1. At the same time thephotosphere of the central star remained quiescent. By 1999 thephotosphere was virtually completely obscured by the dust and the heliumemission line was the only detectable spectral feature remaining in the1–5 m band. In 2000 emission by dust has become even more dominant,as the envelope continues to expand and cool and the helium line weakens.     

Discs and the 10-μm silicate spectra of young stellar objects with non-photospheric continua

Dust emission in the non-photospheric 10-μm continua of HL Tau and Taurus-Elias 7 (Haro6-10, GV Tau) is distinguished from foreground silicate absorption using a simple disc model with radial power-law temperature and mass–density distributions based on the IR–submm model of T Tauri stars by Adams, Lada & Shu with foreground extinction. The resulting 10-μm absorption profiles are remarkably similar to those of the field star Taurus-Elias 16 obtained by Bowey, Adamson & Whittet. The fitted temperature indices are 0.44 (HL Tau) and 0.33 (Elias 7) in agreement with Boss's theoretical models of the 200–300 K region, but lower than those of IR–submm discs (0.5–0.61; Mannings & Emerson); a significant fraction of the modelled 10-μm emission of HL Tau is optically thin, whilst that of Elias 7 is optically thick. We suggest that HL Tau's optically thin component arises from silicate dust within low-density layers above an optically thick disc.     

Mid-infrared imaging of WL 16: the spatial distribution of the hydrocarbon emission features

We present arcsec-resolution images at 8.2, 10.0 and 11.3 μm of the unusual young object WL 16 in Ophiuchus, which has an extended envelope of fluorescing hydrocarbon molecules. To the limit of achieved sensitivity, the faint 10.0-μm continuum has a surface-brightness distribution that is not distinguishable from those at 8.2 and 11.3 μm, where the luminosity is known to be dominated by the polycyclic aromatic hydrocarbon (PAH) emission features. We conclude that the 10-μm continuum either arises from non-equilibrium heating of small dust grains that are well mixed with the hydrocarbons or is quasi-continuous emission from the PAH particles themselves, rather than thermal equilibrium emission from macroscopic dust grains, and that there is no significant silicate absorption variation across the source. The extended hydrocarbon emission may trace a flattened, equatorial distribution of circumstellar material or arise in bipolar lobes. The former case is slightly favoured, based on currently available data, and would imply that WL 16 is a relatively evolved Herbig Ae star, the equatorial plane of which has been almost cleared of normal dust, leaving only fluorescing hydrocarbons and larger coagulated particles as a possibly transient fossil of the original circumstellar disc.     

A silhouette envelope around GGD30IR detected by Spitzer

This paper presents a study of the envelope of the young stellar object (YSO) GGD30IR. What distinguishes this from most other YSOs is the elongated absorption feature seen in silhouette against the background emission in the Spitzer Galactic Legacy Infrared Midplane Survey Extraordinaire 8 μm Infrared Array Camera image of the region. The size and the symmetrical placement of GGD30IR in the centre of this feature suggest that it is an extended envelope, perhaps the remnant of the collapse of the GGD30 core. We have used the extinction in the envelope measured from (i) the reduction in the 8 μm background intensity and (ii) field star colour excesses, to estimate the envelope mass, obtaining values of 0.6 ± 0.2 and  0.5 ± 0.3 M_⊙  , respectively. To investigate the envelope further, we have obtained Australia Telescope Compact Array 3 mm continuum and HCO+ line observations of the region. The continuum emission at 3 mm arises from both a compact (unresolved; ≤730 au) core embedded in an extended envelope ∼18 000 au × 38 000 au in extent. We estimate the core mass to be 0.11 ± 0.02  M_⊙  . The HCO+ emission is extended in a direction perpendicular to the long axis of the envelope, suggesting it comes from an outflow. The spectral energy distribution (SED) provides a 2–24 μm spectral index, α= 1.0, which places GGD30IR in the Class I YSO category. Integrating the SED provides a luminosity of   L _*≃ 25 ± 5 L_⊙  .