Optical, infrared and millimetre-wave properties of Vega-like systems – IV. Observations of a new sample of candidate Vega-like sources

Photometric observations at optical and near-infrared wavelengths are presented for members of a new sample of candidate Vega-like systems, or main sequence stars with excess infrared emission due to circumstellar dust. The observations are combined with IRAS fluxes to define the spectral energy distributions of the sources. Most of the sources show only photospheric emission at near-IR wavelengths, indicating a lack of hot (∼1000 K) dust. Mid-infrared spectra are presented for four sources from the sample. One of them, HD 150193, shows strong silicate emission, while another, HD 176363, was not detected. The spectra of two stars from our previous sample of Vega-like sources both show UIR-band emission, attributed to hydrocarbon materials. Detailed comparisons of the optical and IRAS positions suggest that in some cases the IRAS source is not physically associated with the visible star. Alternative associations are suggested for several of these sources. Fractional excess luminosities are derived from the observed spectral energy distributions. The values found are comparable to those measured previously for other Vega-like sources.     

On the nature of the proposed blob in the inner preplanetary disk of the Herbig Ae/Be star HD141569: evidence for a giant vortex?

Recently, Brittain and Rettig, using the cryogenic echelle spectrograph at the Infrared Telescope Facility to study the infrared emission from the inner preplanetary disk of the Herbig Ae/Be star HD141569, detected CO and H₃⁺ ion emission. This emission has been tentatively interpreted as due to the existence of a forming gas giant planet. The suggested protoplanetary blob appears to be orbiting its host star at about 7 AU being perhaps 2 AU across and roughly five times the mass of Jupiter. Based on numerical modeling of the evolution of the dust disk we show that their observational results are compatible with the presence of an evolved giant vortex in the disk. Our calculations suggest that vortices formed in disks similar to the one found around HD141569 are more effective at capturing solid material than equivalent structures around solar-like stars. On the other hand, we investigate the possibility to find evidence for large-scale vortices in preplanetary disks by submillimeter interferometry. Disks around Herbig Ae/Be stars may be primary targets for giant vortex detection using this technique.     

A survey of 10-μm silicate emission from dust around young sun-like stars

We obtained low resolution (R=100) mid-infrared (8–13 μm wavelengths) spectra of 8 nearby young main sequence stars with the Keck 1 telescope and Long-Wavelength Spectrometer (LWS) to search for 10 μm silicate (Si–O stretch) emission from circumstellar dust. No stars exhibited readily apparent emission: Spectra were then analyzed by least-squares fitting of a template based on a spectrum of Comet Hale-Bopp. Using this technique, we were able to constrain the level of silicate emission to a threshold 10 times below what was previously possible from space. We found one star, HD 17925, with a spectrum statistically different from its calibrator and consistent with a silicate emission peak of 7% of the photosphere at a wavelength of 10 μm. Excess emission at 60 μm from this star has already been reported.     

Non-emission-line young stars of intermediate mass

We present optical spectra of four intermediate-mass candidate young stellar objects that have often been classified as Herbig Ae/Be stars. Typical Herbig Ae/Be emission features are not present in the spectra of these stars. Three of them, HD 36917, HD 36982 and HD 37062, are members of the young Orion nebula cluster (ONC). This association constrains their ages to be ≲1 Myr. The lack of appreciable near-infrared excess in them suggests the absence of hot dust close to the central star. However, they do possess significant amounts of cold and extended dust as revealed by the large excess emission observed at far-infrared wavelengths. The fractional infrared luminosities  ( L _ir/ L _★)  and the dust masses computed from IRAS fluxes are systematically lower than those found for Herbig Ae/Be stars but higher than those for Vega-like stars. These stars may thus represent the youngest examples of the Vega phenomenon known so far. In contrast, the other star in our sample, HD 58647, is more likely to be a classical Be star, as is evident from the low   L _ir/ L _★  , the scarcity of circumstellar dust, the low polarization, the presence of H α emission and near-infrared excess, and the far-infrared spectral energy distribution consistent with free–free emission similar to other well-known classical Be stars.     

Mid‐infrared spectroscopy of refractory inclusions (CAIs) in CV and CO chondrites

Abstract— We present laboratory mid‐infrared absorption spectra (2.5 urn to 16.0 μm) of powdered calcium‐aluminum‐rich inclusions (CAIs) and matrix separated from the carbonaceous chondrites Allende (CV3.2), Vigarano (CV3.3), and Ornans (C03.3). Two groups of spectra with different features were found for the CAI: in the first group spectra are dominated by spinel, pyroxene, and sodalite ± nepheline, where main features occur at 9.3 μm, 10.3 μm, and 11.3 μm. In the second group, characteristic minerals are spinel and melilite with typical band maxima at 11.0 μm and 12.3 μm, and a broad feature between 14.0 μn and 15.0 μn. The position of the broad spinel feature probably depends on its iron content. Comparison of band positions in spectra from the CAI components to observed circumstellar emission spectra indicates the potential occurrence of CAI‐like material. Pyroxene‐ and spinel‐rich features could occur in spectra of dust around the Herbig Ae star HD104237, the T Tauri star Hen3‐600 and the post‐AGB star R Sge. Melilite‐ and spinel‐rich components possibly appear in the spectrum of HD 104237, Hen3‐600, 04187_1927, R Sge, and the planetary nebula Hb 12. There is also indication for a spinel component in dust from the Herbig Ae/Be star HD 179218. The spectra of the AGB stars R Cas and θ Aps show no features of CAl‐type spinel.     

The Evolution of the Angular Momentum Distribution during Star Formation

We have estimated the ages of eight late-type Vega-like stars by using standard age-dating methods for single late-type stars, e.g., location on the color-magnitude diagram, Li lambda6708 absorption, Ca ii H and K emission, X-ray luminosity, and stellar kinematic population. With the exception of the very unusual pre-main-sequence star system HD 98800, all the late-type Vega-like stars are the same age as the Hyades cluster (600-800 Myr) or older.     

SCUBA photometry of candidate Vega-like sources

New SCUBA measurements at millimetre wavelengths are presented for a sample of Vega-like stars. Six stars were detected, while sensitive upper limits were obtained for a further 11 sources. Most of the sample selected from a recent catalogue of Vega-like stars have infrared excesses similar to those of the prototype Vega-like stars α Lyr and α PsA. Their IR–submm spectral indices are steep, indicating that the submm emission from the discs is dominated by grains which are smaller than the wavelength of observation and that only small grains exist in those dusty discs. HD 98800 has an IR–submillimetre spectral index of less than two, which suggests that grains have grown to more than 0.3 mm in size. Hipparcos parallax data for HD 42137 and HD 123160 suggest that these two stars are giants rather than dwarfs, similar to the situation previously found for HD 233517. Dust masses, or upper limits, were derived for the sample; these indicate that most of the sources do not have as much dust as Herbig Ae/Be or T Tauri stars, but are likely to have dust masses comparable to those of the prototype Vega-like stars.     

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 Gas-to-Dust Relation in the Dark Cloud L1523 - Observational Evidence for CO Gas Depletion

Correlation between gas and dust column density has been studied for the dark globule L1523. The 13CO(J= 1→0) emission is used for tracing the gas, and the IR emissions, for tracing the dust constituent. In order to match the beam resolution between the images, a beam de-convolution algorithm based on the Maximum Correlation Method (MCM) was applied on the Infrared Astronomical Satellite (IRAS) data. The morphology of 13CO column density map shows a close correlation to that of 100μm dust optical depth. The distribution of the optical depth at 100 μm follows that of gas column density more closely than does the flux map at either 60 or 100μm. The ratio of the 13CO column density to the 100μm optical depth shows a decreasing trend with increasing dust optical depth in the central part, indicating possible molecular gas condensation onto dust particles. The excessive decrease in the CO column density in the envelope may most probably be due to the photo-dissociation of CO molecules.     

Palomar and Table Mountain observations of 9P/Tempel 1 during the Deep Impact encounter: First results

We present the first results of the Palomar Adaptive Optics observations taken during the Deep Impact encounter with 9P/Tempel 1 in July 2005. We have combined the Palomar near-IR imaging data with our visual wavelength images obtained simultaneously at JPL's Table Mountain Observatory to cover the total wavelength range from 0.4 to 2.3 μm in the B, V, R, I, J, H, and K filter bands, spanning the dates from 2005 July 03-07. We also include in our overall analysis images taken on the pre-encounter dates of June 1 and June 15, 2005. The broad wavelength range of our observations, along with high temporal resolution, near-IR sensitivity, and spatial resolution of our imaging, have enabled us to place constraints on the temperature of the impact flash and incandescent plume of >700 K, and to provide mean dust velocities of order approximately 1.25 h after impact derived from our 1.64 μm observations. Our ejected dust mass estimates, as derived from our near-IR observations, are an order of magnitude less than those previously reported for visual wavelength observations.     

The ney-allen nebula: an unambiguous example of circumstellar dust emission

Diffraction-limited array images of the Trapezium/Ney Allen infrared nebula have been obtained at six wavelengths between 7.8 and 12.4 microns, including the 9.7 micron silicate feature. Extended emission from warm dust shows significant differences in structure around each of the four Trapezium stars. The most dramatic infrared source is associated with ¹ Orionis D, where the bright mid-infrared emission is found to be a distinct crescent-shaped ridge or shell, concentric with the O star. This unambiguous relationship between a known type stellar luminosity source and a distinct circumstellar dust cloud of known distance and dimensions provides a unique opportunity to test the predictions of dust grain emission models for circumstellar infrared sources.     

Identification of SH Deltav=1 Ro-vibrational Lines in R Andromedae

We report the detection of a broad 22 μm emission feature in the Carina Nebula H ii region by the Infrared Space Observatory (ISO) short-wavelength spectrometer. The feature shape is similar to that of the 22 μm emission feature of newly synthesized dust observed in the Cassiopeia A supernova remnant. This finding suggests that both of the features are arising from the same carrier and that supernovae are probably the dominant production sources of this new interstellar grain. A similar broad emission dust feature is also found in the spectra of two starburst galaxies from the ISO archival data. This new dust grain could be an abundant component of interstellar grains and can be used to trace the supernova rate or star formation rate in external galaxies. The existence of the broad 22 μm emission feature complicates the dust model for starburst galaxies and must be taken into account correctly in the derivation of dust color temperature. Mg protosilicate has been suggested as the carrier of the 22 μm emission dust feature observed in Cassiopeia A. The present results provide useful information in studies on the chemical composition and emission mechanism of the carrier.     

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.     

Photodissociation regions and star formation in the Carina nebula

We have obtained wide-field thermal infrared (IR) images of the Carina nebula, using the SPIREX/Abu telescope at the South Pole. Emission from polycyclic aromatic hydrocarbons (PAHs) at 3.29 μm, a tracer of photodissociation regions (PDRs), reveals many interesting well-defined clumps and diffuse regions throughout the complex. Near-IR images  (1–2 μm)  , along with images from the Midcourse Space Experiment ( MSX ) satellite  (8–21 μm)  have been incorporated to study the interactions between the young stars and the surrounding molecular cloud in more detail. Two new PAH emission clumps have been identified in the Keyhole nebula, and have been mapped in  ¹²CO(2–1)  and  (1–0)  using the Swedish–ESO Submillimetre Telescope (SEST). Analysis of their physical properties reveals that they are dense molecular clumps, externally heated with PDRs on their surfaces and supported by external pressure in a similar manner to the other clumps in the region. A previously identified externally heated globule containing IRAS 10430−5931 in the southern molecular cloud shows strong 3.29-, 8- and 21-μm emission, the spectral energy distribution (SED) revealing the location of an ultracompact (UC) H  ii region. The northern part of the nebula is complicated, with PAH emission intermixed with mid-IR dust continuum emission. Several point sources are located here, and through a two-component blackbody fit to their SEDs we have identified three possible UC H  ii regions as well as a young star surrounded by a circumstellar disc. This implies that star formation in this region is ongoing and not halted by the intense radiation from the surrounding young massive stars.     

Star formation in starburst galaxies from near-IR spectroscopy and evolutionary population synthesis

We show that the combined study of the near-IR Stellar Energy Distribution (SED) and emission lines in IR luminous starburst galaxies, using evolutionary population synthesis, gives constraints both on the star formation (SF) parameters and on the relative spatial distribution of the components (burst stars, evolved stars, H II regions, dust). We also note that the He I λ 2.06/Br _γ ratio, used to constrain the stellar upper mass limit, is sensitive to a variety of other factors that should be accounted for.     

A Submillimeter HCN Laser in IRC +10216

We present subarcsecond thermal infrared imaging of HD 98800, a young quadruple system composed of a pair of low-mass spectroscopic binaries separated by 0&farcs;8 (38 AU), each with a K-dwarf primary. Images at wavelengths ranging from 5 to 24.5 μm show unequivocally that the optically fainter binary, HD 98800B, is the sole source of a comparatively large infrared excess on which a silicate emission feature is superposed. The excess is detected only at wavelengths of 7.9 μm and longer, peaks at 25 μm, and has a best-fit blackbody temperature of 150 K, indicating that most of the dust lies at distances greater than the orbital separation of the spectroscopic binary. We estimate the radial extent of the dust with a disk model that approximates radiation from the spectroscopic binary as a single source of equivalent luminosity. Given the data, the most likely values of disk properties in the ranges considered are Rin=5.0+/-2.5 AU, DeltaR=13+/-8 AU, lambda0=2+4-1.5 μm, gamma=0+/-2.5, and sigmatotal=16+/-3 AU2, where Rin is the inner radius, DeltaR is the radial extent of the disk, lambda0 is the effective grain size, gamma is the radial power-law exponent of the optical depth tau, and sigmatotal is the total cross section of the grains. The range of implied disk masses is 0.001-0.1 times that of the Moon. These results show that, for a wide range of possible disk properties, a circumbinary disk is far more likely than a narrow ring.     

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.     

The circumstellar disc around the Herbig AeBe star HD169142

We present 7 mm and 3.5 cm wavelength continuum observations towards the Herbig AeBe star HD169142 performed with the Very Large Array (VLA) with an angular resolution of ≃1 arcsec. We find that this object exhibits strong (≃4.4 mJy), unresolved (≲1 arcsec) 7 mm continuum emission, being one of the brightest isolated Herbig AeBe stars ever detected with the VLA at this wavelength. No emission is detected at 3.5 cm continuum, with a 3σ upper limit of ≃0.08 mJy. From these values, we obtain a spectral index α≳ 2.5 in the 3.5 cm to 7 mm wavelength range, indicating that the observed flux density at 7 mm is most likely dominated by thermal dust emission coming from a circumstellar disc. We use available photometric data from the literature to model the spectral energy distribution (SED) of this object from radio to near-ultraviolet frequencies. The observed SED can be understood in terms of an irradiated accretion disc with low mass accretion rate,     , surrounding a star with an age of ≃10 Myr. We infer that the mass of the disc is ≃0.04 M_⊙, and is populated by dust grains that have grown to a maximum size of 1 mm everywhere, consistent with the lack of silicate 10 μm emission. These features, as well as indications of settling in the wall at the dust destruction radius, led us to speculate that the disc of HD169142 is in an advanced stage of dust evolution, particularly in its inner regions.     

Searching for hidden Wolf–Rayet stars in the Galactic plane – 15 new Wolf–Rayet stars

We report the discovery of 15 previously unknown Wolf–Rayet (WR) stars found as part of an infrared (IR) broad-band study of candidate WR stars in the Galaxy. We have derived an empirically based selection algorithm which has selected ∼5000 WR candidate stars located within the Galactic plane drawn from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (mid-IR) and Two-Micron All-Sky Survey (near-IR) catalogues. Spectroscopic follow-up of 184 of these reveals 11 nitrogen-rich (WN) and four carbon-rich (WC) WR stars. Early WC subtypes are absent from our sample and none shows evidence for circumstellar dust emission. Of the candidates which are not WR stars, ∼120 displayed hydrogen emission-line features in their spectra. Spectral features suggest that the majority of these are in fact B supergiants/hypergiants, ∼40 of these are identified Be/B[e] candidates.
Here, we present the optical spectra for six of the newly detected WR stars, and the near-IR spectra for the remaining nine of our sample. With a WR yield rate of ∼7 per cent and a massive star detection rate of ∼65 per cent, initial results suggest that this method is one of the most successful means for locating evolved, massive stars in the Galaxy.