Helium absorption and emission towards Θ1 Ori C

The He I absorption and emission systems towards Θ1 Ori C, the exciting star of the Orion Nebula, are investigated. To this end, high-resolution near-infrared long-slit spectra centred on the He I 1.083-μm and Brγ lines and an ultra-high-resolution (R ~ 106) spectrum of the optical He I 3889-Å line have been obtained. These data are supplemented by blue high spectral resolution echelle observations of Θ1 Ori C and the other members of the Trapezium. Even at R ~ 106, the He I absorption profile, associated with foreground gas at a heliocentric velocity of +3 km s−1, is very smooth, suggesting a simple broadening mechanism and homogeneity. The combination of the He I 23 S column density, deduced from the 3889-Å line, with the non-detection of Brγ emission at the same velocities sets an upper limit on the election density in this medium of 1010 m−3. The He I 1.083- μm long-slit spectrum shows the familiar background nebular emission, while a second blueshifted component is visible off-star at the same velocities as the absorption on-star. Several mechanisms to explain this emission are explored. We conclude that it is most probably emission formed behind, and leaking through, the absorbing gas layer. A clue to the origin of this emission is found in its spatial distribution: unlike the bright background nebular emission, the blueshifted component peaks symmetrically around the position of Θ1 Ori C, suggesting a physical association with the star. A possible model for the blueshifted emission is that it arises in a large-scale, dense shock front, provoked in some way by the wind of Θ1 Ori C

The coordinated radio and infrared survey for high-mass star formation – IV. A new radio-selected sample of compact galactic planetary nebulae

We present a new radio-selected sample of PNe from the CORNISH survey. This is a radio continuum survey of the inner Galactic plane covering Galactic longitude, 10 1000 K) and located closer than 7 kpc. Within this sample is a water-maser PN with a spectral index of - 0.55 ± 0.08, which indicates non-thermal radio emission. Such a radio-selected sample, unaffected by extinction, will be particularly useful to compare with population synthesis models and should contribute to the understanding of the formation and evolution of PNe

Spatially resolving polycyclic aromatic hydrocarbons in Herbig Ae disks with VISIR-NEAR at the VLT

Context. The emission from polycyclic aromatic hydrocarbons (PAHs) arises from the uppermost layers of protoplanetary disks, higher than the optical/near-infrared scattered light and similar to the emission from the highly thick 12CO millimeter lines. The PAH intensity profiles trace the gas distribution and can constrain the penetration depth of UV radiation. Aims. We aim to constrain the spatial intensity profiles of the four strongest PAH emission features in the telluric N-band spectral region. Thereby, we seek to constrain the dependence of PAH properties on the (radial) location in the disk, such as charge state, the interrelation with the presence and dynamics of small silicate grains, and the correlation of PAH emission with gas or dust. Methods. We used the long-slit spectroscopy mode of the VISIR-NEAR experiment to perform diffraction-limited observations of eight nearby Herbig Ae protoplanetary disks. We extracted spectra for various locations along the slit with a spectral resolution of R ≈ 300 and performed a compositional fit at each spatial location using spectral templates of silicates and the four PAH bands. This yields the intensity versus location profiles of each species. Results. We obtained spatially resolved intensity profiles of the PAH emission features in the N band for five objects (AB Aurigae, HD 97048, HD 100546, HD 163296, and HD 169142). We observe two kinds of PAH emission geometry in our sample: centrally peaked (HD 97048) and ring-like (AB Aurigae, HD 100546, HD 163296, and potentially HD 169142). Comparing the spatial PAH emission profiles with near-infrared scattered light images, we find a strong correlation in the disk substructure but a difference in radial intensity decay rate. The PAH emission shows a less steep decline with distance from the star. Finally, we find a correlation between the presence of (sub)micron-sized silicate grains and the depletion of PAH emission within the inner regions of the disks. Conclusions. In this work we find the following: (1) PAH emission traces the extent of Herbig Ae disks to a considerable radial distance. (2) The correlation between the presence of silicate emission within the inner regions of disks and the depletion of PAH emission can result from dust-mixing and PAH coagulation mechanisms and competition over UV photons. (3) For all objects in our sample, PAHs undergo stochastic heating across the entire spatial extent of the disk and are not saturated. (4) The difference in radial intensity decay rates between the PAHs and scattered-light profiles may be attributed to shadowing and dust-settling effects, which impact the scattering grains more so than the PAHs

Time-resolved spectroscopy of the peculiar H alpha variable Be star HD 76534

We present time-resolved spectroscopy of the Be star HD 76534, which was observed to have an Ha outburst in 1995, when the line went from photospheric absorption to emission at a level of more than two times the continuum within 2.5 hours. To investigate the short-term behaviour of the spectrum of HD 76534 we have obtained 30 spectra within two hours real-time and searched for variations in the spectrum.Within the levels of statistical significance, no variabilitywas found. Rather than periodic on short time scales, the H behaviour seems to be commonly episodic on longer (> 1 year) time scales, as an assessment of the existing data on theH line and the Hipparcos photometry suggests. HD 76534 underwent only 1 photometric outburst in the 3 year span that the star was monitored by the Hipparcos satellite.Peer reviewe

Diffuse interstellar bands in the spectra of massive young stellar objects

Copyright Blackwell PublishingWe have compared the B−V colour excess, E(B–V), obtained for a sample of five optically visible massive YSOs both from diffuse interstellar bands (DIBs) in their spectra and from their optical continuum slopes. Our targets are HD 200775, BD+40o4124, MWC 1080, MWC 297 and MWC 349A. First, E(B–V) towards each of the targets is derived by dereddening the observed continua to match those of B-type standard stars. A survey of DIBs in the spectra of the massive YSOs, and a control field star, then reveals that the DIBs are significantly weaker in the former than would be expected based on the total E(B–V) values. This result is strengthened by the finding that the DIBs in the control field star, HD 154445, have on average the strength expected from its continuum E(B–V). A rough estimate of the foreground reddening of intervening diffuse interstellar medium shows it to be smaller than the DIB- E(B–V), implying that at least part of the DIB carriers are formed within the parental molecular clouds in which the YSOs are embedded. The formation efficiency of the DIBs varies strongly however from cloud to cloud. The DIB- E(B–V) compares favourably with the total E(B–V) towards BD+40o4124, but is almost negligible in the line of sight towards MWC 297. Despite this general, but not unexpected, deficit we provide evidence that the DIB at 5849°A is a good tracer of total extinction in these lines of sight.Peer reviewe

Spectropolarimetry of Young and Evolved Stars

Circumstellar disks play an important role in many stages of the evolution of stars. However, it is only possible to directly image circumstellar disks for a few of the nearest stars. For massive stars, the situation is even more di±cult, as they are on average further away than the more numerous low mass stars. In order to shed light on such disks we are pursuing a programme of spectropolarimetry. This technique can reveal the presence of disks on a spatial scale of order stellar radii, while it has the potential for further modeling. Here we present our results on two groups of stars, on young, intermediate mass, premain sequence Herbig Ae/Be stars and on a sample of massive evolved objects, the Luminous Blue Variables. We also present initial results obtained at nearinfrared wavelengths

Ha spectropolarimetry of B[e] and Herbig Be stars

The definitive version is available at www.blackwell-synergy.com. Copyright Blackwell Publishing DOI : 10.1046/j.1365-8711.1999.02383.xWe present the results of medium resolution ( v 60 km s−1) spectropolarimetric observations across H of a sample of B[e] and Herbig Be objects. A change in linear polarization across H is detected in a large fraction of the objects, with characteristics ranging from simple depolarization in a couple of Herbig Be stars, to more complex behaviour in the probable post main sequence B[e] stars. H in the spectra of HD 37806 and HD 50138 each consist of a double-peaked polarized line and a superposed unpolarized single emission peak, suggesting two distinct line-forming regions.Multiple observations of HD 45677 allow for the separation of electron and dust scattering effects for the first time: the difference between derived intrinsic polarization angles of the two components indicate that the dust-scattering region is clumpy. Two unexpected results are the non-detections of H polarization changes in ! Ori, where depolarization has previously been detected, and in MWC 297, which exhibits source elongation at radio wavelengths. In ! Ori time variability is probably responsible such that this star’s electron-scattering disk was much weakened at the time of observation. Two hypotheses are advanced that might explain the MWC 297 result.Peer reviewe