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We present new mid-infrared spectra for a sample of 15 targets (1 FU Orionis object, 4 Herbig Ae stars, 5 T Tauri stars and 5 Vega type stars), obtained with the TIMMI2 camera at La Silla Observatory (ESO). Three targets are members of the beta Pic moving group (HD 155555, HD 181296 and HD 319139). PAH bands are observed towards the T Tauri star HD 34700 and the Herbig Ae star PDS 144 N. For HD 34700, the band profiles indicate processed PAHs. The spectrum of the Vega-type object eta Corvi (HD 109085), for which a resolved disk at sub-mm wavelengths is known, is entirely stellar between 8--13 micron. Similarly, no indication for circumstellar matter at mid-infrared wavelengths is found towards the Vega-like stars HD 3003, HD 80951, HD 181296 and, surprisingly, the T Tauri system HD 155555. The silicate emission features of the remaining eight sources are modelled with a mixture of silicates of different grain sizes and composition. Unprocessed dust dominates FU Ori, HD 143006 and CD-43 344. Large amorphous grains are the main dust component around HD 190073, HD 319139, KK Oph and PDS 144 S. Both small grains and crystalline dust is found for the Vega-type HD 123356, with a dominance of small amorphous grains. We show that the infrared emission of the binary HD 123356 is dominated by its late-type secondary, but optical spectroscopy is still required to confirm the age of the system and the spectral class of the companion. For most targets this is their first mid-infrared spectroscopic observation. We investigate trends between stellar, disk and silicate properties and confirm correlations of previous studies. Several objects present an exciting potential for follow-up high-resolution disk studies., Comment: Accepted for publication in A&A

Context. Infrared (IR) spectroscopy and imaging provide a prime tool to study the characteristics of polycyclic aromatic hydrocarbon (PAH) molecules and the mineralogy in regions of star formation. Herbig Ae/Be stars are known to have varying amounts of natal cloud material present in their vicinity.Aims. Our aim is to study the characteristics of the mid-IR emission originating in Herbig Ae/Be stars, especially the extent of the emission and how this relates to the (proto-) stellar characteristics.Methods. Today's powerful ground-and space-based telescopes provide images and spectra at unprecedented spectral and spatial resolution. We analyse the images and spectra from four Herbig Ae/Be stars (IRAS 06084-0611, CD-42 11721, TY CrA, and HD176386), as obtained with TIMMI2 on the ESO 3.6 m telescope and VISIR on the VLT. These observations are supplemented with data from ISO-SWS and Spitzer-IRAC.Results. We find evidence for large-scale structure and extended emission in all four sources, except for HD176386, which only shows silicate emission, all sources show PAH emission in their spectra. In addition, a spatially resolved silicate and PAH spectrum could be extracted for TY CrA.Conclusions. The variety in emission scales distinguishes two classes. In the first, the morphology and spectral characteristics resemble those of reflection nebulae. In the second, the characteristics are in-line with Herbig A stars. This separation simply reflects a difference in stellar characteristics (e. g. luminosity). In Herbig B stars, dust emission from the surroundings dominates, where for Herbig A stars, the disk dominates the emission. In this scheme, IRAS 06084-0611 and CD-42 11721 resemble reflection nebulae and HD176386 a more typical Herbig Ae/Be star. TY CrA shows characteristics common to both genuine reflection nebulae and Herbig B stars. We propose a geometry for TY CrA, with most notably, a similar to 70 AU inner gap in the 340 AU circumtertiary disk cleared by a fourth stellar companion.

We present a detailed photoionization model of G29.96-0.02 (hereafter G29.96), one of the brightest Galactic Ultra Compact HII (UCHII) regions in the Galaxy. This source has been observed extensively at radio and infrared wavelengths. The most recent data include a complete ISO (SWS and LWS) spectrum, which displays a remarkable richness in atomic fine-structure lines. The number of observables is twice as much as the number available in previous studies. In addition, most atomic species are now observed in two ionization stages. The radio and infrared data on G29.96 are best reproduced using a nebular model with two density components: a diffuse (n_e~680cm-3) extended (~1 pc) component surrounding a compact (~0.1 pc) dense (n_e~57000cm-3) core. The properties of the ionizing star were derived using state-of-the-art stellar atmosphere models. CoStar models yield an effective temperature of \~30^{+2}_{-1} kK whereas more recent non-LTE line blanketed atmospheres with stellar winds indicate somewhat higher values, Teff~32--38 kK. This range in Teff is compatible with all observational constraints, including near-infrared photometry and bolometric luminosity. The range 33-36 kK is also compatible with the spectral type O5-O8 determined by Watson and Hanson (97) when recent downward revisions of the effective temperature scale of O stars are taken into account. The age of the ionizing star of G29.96 is found to be a few 10^6 yr, much older than the expected lifetime of UCHII regions. Accurate gas phase abundances are derived with the most robust results being Ne/S=7.5 and N/O=0.43 (1.3 and 3.5 times the solar values, respectively)., Accepted in Astronomy and Astrophysics

IR spectroscopy provides a valuable tool for the characterisation and identification of interstellar molecular species. Here, we present 6-9 mum spectra of a sample of reflection nebulae, HII regions, YSOs, evolved stars and galaxies that show strong unidentified infrared bands, obtained with the SWS spectrograph on board ISO. The IR emission features in this wavelength region show pronounced variations. 1) The 6.2 mum feature shifts from 6.22 to 6.3 mum and clearly shows profile variations. 2) The 7.7 mum complex is comprised of at least two subpeaks peaking at 7.6 and one longwards of 7.7 mum. In some cases the main peak can apparently shift up to 8 mum. Two sources do not exhibit a 7.7 mum complex but instead show a broad emission feature at 8.22 mum. 3) The 8.6 mum feature has a symmetric profile in all sources and some sources exhibit this band at slightly longer wavelengths. For the 6.2, 7.7 and 8.6 mum features, the sources have been classified independently based on their profile and peak position. The classes derived for these features are directly linked with each other. Sources with a 6.2 mum feature peaking at similar to6.22 mum exhibit a 7.7 mum complex dominated by the 7.6 mum component. In contrast, sources with a 6.2 mum profile peaking longwards of 6.24 mum show a 7.7 mum complex with a dominant peak longwards of 7.7 mum and a 8.6 mum feature shifted toward the red. Furthermore, the observed 6-9 mum spectrum depends on the type of object. All ISM-like sources and a few PNe and Post-AGB stars belong to the first group while isolated Herbig AeBe stars, a few Post-AGB stars and most PNe belong to the second group. We summarise existing laboratory data and theoretical quantum chemical calculations of the modes emitting in this wavelength region of PAH molecules. We discuss the variations in peak position and profile in view of the exact nature of the carrier. We attribute the observed 6.2 mum profile and peak position to the combined effect of a PAH family and anharmonicity with pure PAHs representing the 6.3 mum component and substituted/complexed PAHs representing the 6.2 mum component. The 7.6 mum component is well reproduced by both pure and substituted/complexed PAHs but the 7.8 mum component remains an enigma. In addition, the exact identification of the 8.22 mum feature remains unknown. The observed variations in the characteristics of the IR emission bands are linked to the local physical conditions. Possible formation and evolution processes that may influence the interstellar PAH class are highlighted.

Context. Spectral features between about 15-20 mu m are commonly associated with polycyclic aromatic hydrocarbons (PAHs). With the NASA Spitzer Space Telescope these features are reported routinely, and as such, warrant a deeper molecular explanation.Aims. We aim to determine the characteristics of the group of carriers of the plateau and the distinct sub-features at 15.8, 16.4, 17.4, 17.8 and 18.9 mu m and to draw astronomical implications from these spectra.Methods. We analyse and interpret the spectra of 15 different sources using the NASA Ames PAH IR spectroscopic database.Results. The bands within the 15-20 mu m region show large variations. Except for the 16.4 mu m band, there is also no connection, both in band strength and feature classification, with the mid-IR PAH bands. Of the PAH spectra considered, only those from species containing pendent rings show one "common" characteristic: a band near the astronomical 16.4 mu m position. However, coupling with the carbon skeleton's core influences its precise position in the spectrum. Compact PAHs in the size range 50-130 carbon atoms, consistently show a strong band near the astronomical 17.4 mu m band position.Conclusions. The 15-20 mu m region is the transition zone from PAH nearest neighbour modes to full-skeleton modes. We conclude that a few individual PAHs dominate the astronomical PAH family when clear features are prominent. In the few cases of a broad plateau, the PAH family would be far richer. Although PAHs containing pendent rings showed promise explaining the astronomical 16.4 mu m band, coupling with the skeleton core and the inherent strong quartet mode expected around 13.5 mu m, make it a less viable candidate. The number of large PAHs in the database becomes a limitation in studying the emission between 15-20 mu m and longward. Computation of larger PAH spectra should therefore be stimulated, especially for understanding the forthcoming far-IR data expected from Herschel, SOFIA and ALMA.