Hydrogenated polycyclic aromatic hydrocarbons and the 2940 and 2850 wavenumber (3.40 and 3.51 micron) infrared emission features

The 3150-2700 cm-1 (3.17-3.70 microns) range of the spectra of a number of Ar-matrix-isolated PAHs containing excess H atoms (Hn-PAHs) are presented. This region covers features produced by aromatic and aliphatic C-H stretching vibrations as well as overtone and combination bands involving lower lying fundamentals. The aliphatic C-H stretches in molecules of this type having low to modest excess H coverage provide excellent fits to a number of the weak emission features superposed on the plateau between 3080 and 2700 cm-1 (3.25 and 3.7 microns) in the spectra of many planetary nebulae, reflection nebulae, and H II regions. Higher H coverage is implied for a few objects. We compare these results in context with the other suggested identifications of the emission features in the 2950-2700 cm-1 (3.39-3.70 microns) region and briefly discuss their astrophysical implications.     

The discovery of a new infrared emission feature at 1905 wavenumbers (5.25 microns) in the spectrum of BD +30 degrees 3639 and its relation to the polycyclic aromatic hydrocarbon model

We have discovered a new IR emission feature at 1905 cm-1 (5.25 microns) in the spectrum of BD +30 degrees 3639. This feature joins the family of well-known IR emission features at 3040, 2940, 1750, 1610, "1310," 1160, and 890 cm-1 (3.3, 3.4, 5.7, 6.2, "7.7," 8.6, and 11.2 microns). The origin of this new feature is discussed and it is assigned to an overtone or combination band involving C-H bending modes of polycyclic aromatic hydrocarbons (PAHs). Laboratory work suggests that spectral studies of the 2000-1650 cm-1 (5.0-6.1 microns) region may be very useful in elucidating the molecular structure of interstellar PAHs. The new feature, in conjunction with other recently discovered spectral structure, suggests that the narrow IR emission features originate in PAH molecules rather than large carbon grains. Larger species are likely to be the source of the broad underlying "plateaus" seen in many of the spectra.     

Thermal emission spectra of silicates from planetary nebulae

Calculations of the grain equilibrium temperature and of the expected infrared spectra of IC 418, BD+30° 3639, NGC 6572 and NGC 7027 have been performed using dielectric constants of lunar silicates. The results have been compared with previous work on pure graphite and ice-mantle grains. Lα heating of dust followed by thermal re-emission is consistent with the large infrared excesses detected in planetary nebulae. An extra source of heating is, nevertheless, necessary to fit correctly the experimental results. It appears from the calculations that, for each object, it is possible to define theoretically the most probable nature of the emitting dust.     

Infrared photometry of eight planetary nebulae

We discuss the infrared (IR) (1.25–5 µm) photometry of eight planetary nebulae performed in 1999–2006. For all of the nebulae under study, we have firmly established IR brightness and color variations on time scales shorter than one year and up to 6–8 years. The greatest IR brightness variations were observed in IC 2149, IC 4997, and NGC 7662. Their J magnitudes varied within 0 _. ^m 2–0 _. ^m 25. In the remaining objects, the J magnitude variations did not exceed 0 _. ^m 15. All of the planetary nebulae under study exhibited IR color variations. Based on the IR photometry, we have classified the central regions of the planetary nebula NGC 1514 and of the northern part of NGC 7635 seen through a 12″ aperture as a B(3–7) main-sequence star (NGC 1514) and a ～O9.5 upper-main-sequence star (NGC 7635). The nebulae IC 4997 and NGC 7027 exhibited an excess emission (with respect to the emission from a hot source) at λ > 2.5 µm.     

Magnetic fields in planetary nebulae and post-AGB nebulae

Magnetic fields are an important but largely unknown ingredient of planetary nebulae. They have been detected in oxygen-rich asymptotic giant branch (AGB) and post-AGB stars, and may play a role in the shaping of their nebulae. Here we present SCUBA submillimetre polarimetric observations of four bipolar planetary nebulae and post-AGB stars, including two oxygen-rich and two carbon-rich nebulae, to determine the geometry of the magnetic field by dust alignment. Three of the four sources (NGC 7027, 6537 and 6302) present a well-defined toroidal magnetic field oriented along their equatorial torus or disc. NGC 6302 may also show field lines along the bipolar outflow. CRL 2688 shows a complex field structure, where part of the field aligns with the torus, whilst an other part approximately aligns with the polar outflow. It also presents marked asymmetries in its magnetic structure. NGC 7027 shows evidence for a disorganized field in the south-west corner, where the SCUBA shows an indication for an outflow. The findings show a clear correlation between field orientation and nebular structure.     

Planetary Nebulae

This review summarizes spectroscopic and imaging results obtained on board the Infrared Space Observatory (ISO) on well-known planetary nebulae: the young nebula NGC 7027, and two fully evolved nearby nebulae, the Dumbbell and the Helix. This revised version was published online in August 2006 with corrections to the Cover Date.     

NGC 7027: analysis of 7.8–20 micron continuum,silicate and PAH grain distributions

Images of mid-infrared (5 - 20µm) circumstellar dust sources have been obtained with a new 58 × 62 pixel infrared array camera system. A seven-color imaging study of the bright planetary nebula NGC 7027 challenges the assertion that polycyclic aromatic hydrocarbons (PAH) may extend further from the center of the nebula than the continuum emission from silicate dust grains. It appears that the overall distributions are nearly identical, ruling out differences in intensity between the PAH emission and the general “silicate” dust material. A rigorous comparison between the infrared image data and new visible CCD images of NGC 7027 is made.     

X-ray emission from O-type stars: DH Cep and HD 97434

We present X-ray emission characteristics of the massive O-type stars DH Cep and HD 97434 using archival XMM-Newton observations. There is no convincing evidence for short-term variability in the X-ray intensity during the observations. However, the analysis of their spectra reveals X-ray structure being consistent with two-temperature plasma model. The hydrogen column densities derived from X-ray spectra of DH Cep and HD 97434 are in agreement with the reddening measurements for their corresponding host clusters NGC 7380 and Trumpler 18, indicating that the absorption by stellar wind is negligible. The X-ray emission from these hot stars is interpreted in terms of the standard instability-driven wind-shock model.     

Circumstellar PAHs and Carbon Stardust

The SWS on ISO has revealed the incredible richness of IR emission features in the spectra of post-AGB objects and planetary nebulae. Besides the well known features at 3.3, 6.2, 7.7, 8.6, and 11.3 μm, a wealth of weaker features is present as well. These studies also reveal detailed variations from source to source, which may be of key importance in the molecular identification process. The emission from Polycyclic Aromatic Hydrocarbon molecules and carbon dust are contrasted with an emphasis on the expected temperature and the feature-to-continuum ratio. Graphite stardust, isolated from carbonaceous meteorites, consist of a core of microcrystalline aromatic moieties mantled by crystalline graphite. This structure suggests the importance of two distinct growth stages or processes. The cores resemble terrestrial soot and indicate formation at relatively low temperature where kinetic factors played an important role, presumably involving PAH formation through reactions of C2H2 and its radical progeny, followed by coagulation of these PAHs as well as further chemical growth. The mantle seems to reflect slow chemical growth at higher temperatures of relatively few PAHs accreted onto these cores. This revised version was published online in September 2006 with corrections to the Cover Date.     

Insoluble organic material of the Orgueil carbonaceous chondrite and the unidentified infrared bands

Deuterium abundance measurements and the 13C NMR spectrum of the HF/HCl insoluble residue of the Orgueil carbonaceous chondrite indicate the presence of an extensive component of polycyclic aromatic hydrocarbons (PAH) that is of possible interstellar origin or is similar to PAH hypothesized to exist in interstellar space. Infrared spectra have been obtained using an FTIR spectrometer of the acid insoluble residue, the residue after heating in vacuum, and condensate. Bulk acid insoluble residue was pressed into KBr pellets and also heated under high vacuum to sublimate a volatile component onto KBr disks. The remaining non-volatile organic component of the Orgueil meteorite from such evaporations pressed into KBr pellets exhibits a spectral signature similar to that observed in emission from the Orion Nebula and found in Raman spectra of interplanetary dust particles (IDPs). In addition it has an 11.3 microns (885 cm-1) band indicating PAH having single hydrogens per peripheral aromatic ring. We conclude the nonvolatile material is similar to interstellar PAH because the observed 11.3 microns (885 cm-1) unidentified infrared (UIR) emission suggests interstellar PAH have single hydrogens per peripheral aromatic ring. The volatile fraction of the Orgueil shows an aliphatic CH stretch feature and its spectrum in the 2-25 microns range is similar to that of the bulk residue.     

Low frequency observations of emission nebulae and their electron temperatures

Radio observations of emission nebulae at 74 MHz are reported. Flux densities for NGC 1499, IC 405, IC 410, NGC 2024, NGC 2175, NGC 2237-46, and NGC 6820 are given. Most of the observed nebulae are found to be optically thick at 74 MHz. Combining the low frequency results with previously measured flux densities at high frequencies, it is shown that it is possible to derive the mean electron temperatures of the observed nebulae. The results show that the range in temperature is from 3200 to 6800 K.     

Internal radial velocities of some selected small diameterHii regions

A photographic Fabry-Pérot interferometer was built for the University of Maryland's 20-in. telescope for the study of the radial velocities of diffuse emission nebulae. Observations of S101, S112, IC5146, NGC7380, NGC281, and IC1795 were made and radial velocities at many points within each nebula were determined with an estimated error of ±3 km/sec. Average heliocentric radial velocities were obtained for each nebula and they agree well with those found by other observers.For S112 and IC5146, a systematic difference of 6–8 km/sec in the radial velocities between the east and west sides of the nebulae was observed. This phenomenon is attributed to the braking effect of cold gas which appears to be associated with the western side of both nebulae.Ring half-widths were obtained for S112, IC5146 and NGC281, and found to be consistent with expansion velocities of 5 km/sec or less. For IC5146 the average Doppler temperature is only 5500±1000 K, which indicates an even lower electron temperature.     

Pseudo-resonance absorption lines in planetary nebulae: Discovery?

The possibility of the formation of pseudo-resonance absorption lines in planetary nebulae is predicted only theoretically. However, this has not been confirmed by direct observations. In the present article an attempt has been made to show, as a result of careful analysis of IUE spectral recordings for a group of planetary nebulae, that the existence of one pseudo-resonance line 1300 SiIII is without doubt at least in spectra of three planetary nebulae: NGC 2610, NGC 3587, and NGC 6891. The presence of this line in the spectra of the other three planetary nebulae, NGC 6210, IC 3568, and IC 4776, seems to be probable. The role of the interstellar selective absorption, the blending effect by the resonance lines of SiII,Oi, andSi in the formation of the pseudo-resonance line 1300 SiIII as well as the possibility of formation of this line in the photosphere of central star of nebula were analysed.     

Shock modelling of Planetary Nebulae

The kinematics of Planetary Nebulae are analyzed in terms of the solutions to the equations of hydrodynamic equilibrium developed by J. Cantó. We apply our analysis to the Planetary Nebulae NGC 6905 and NGC 6537. A detailed spectroscopic study of these objects reveals the existence of high nuclear velocities, together with complex kinematic structures and unusual emission line intensities. Shock ionization clearly plays a key role in these nebulae. Remarkably good agreement is obtained when comparing the synthetic maps and spectra resulting from the shock solutions with the observational data.     

Hα line measurements from ten diffuse galactic sources using the DEFPOS facility

The hydrogen Balmer‐α emission line spectrum of ten diffuse ionization sources in the Milk Way – NGC 40 (WC8), NGC 2022, NGC 6210, NGC 6618 (M17, Sh2‐45), NGC 6720 (M57), NGC 6781, NGC 6888 (Sh2‐105), NGC 6992 (Sh2‐103), NGC 7635 (Sh2‐162,) and IC 1848 (Sh2‐199) – has been investigated using a dual etalon Fabry‐Pérot optical spectrometer (DEFPOS) aatached to the 150 cm RTT150 telescope at TUBITAK National Observatory (TUG, Antalya, Turkey: 36° 51′ N; 30° 20′ E; elevation: 2547 m). All of our galactic Hα observations discussed in this paper were carried out during the nights of 2013 June 21–24 with exposure time of 3600 s. As main results the intensity, the full width at half maximum, and the radial velocity with respect to the LSR have been determined for each data set. The intensities, the radial velocities, and the line widths of the Hα emission line vary from 59.15 to 8923.44 R, –46.72 to +54.07 km s^–1, and 31.4 to 48.01 km s^–1, respectively. The radial velocities and the half‐widths of the Hii regions and planetary nebulae determined from our measurements are found tobe consistent with values given in literature, especially with those in Schneider et al. (1983) and Fich et al. (1990). (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Science verification results from PMAS

PMAS, the Potsdam Multi‐Aperture Spectrophotometer, is a new integral field instrument which was commissioned at the Calar Alto 3.5 m Telescope in May 2001. We report on results obtained from a science verification run in October 2001. We present observations of the low‐metallicity blue compact dwarf galaxy SBS0335‐052, the ultra‐luminous X‐ray Source X‐1 in the Holmberg II galaxy, the quadruple gravitational lens system Q2237+0305 (the “Einstein Cross”), the Galactic planetary nebula NGC7027, and extragalactic planetary nebulae in M31. PMAS is now available as a common user instrument at Calar Alto Observatory. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The infrared emission bands. II. A spatial and spectral study of the Orion Bar

We have studied the spectral and spatial distribution across the Orion Bar of the 3-14 micrometers emission, including hydrogen Brackett alpha and 12.8 micrometers [Ne II] emission lines and several "dust" emission features. The data indicate that the "dust" consists of three components; (1) "classical" dust with a temperature of approximately 60 K accounting for emission longward of 20 micrometers, (2) amorphous carbon particles or polycyclic aromatic hydrocarbon (PAH) clusters (approximately 400 C atoms) which produce broad emission features in the 6-9 and 11-13 micrometers bands, and (3) free PAHs which emit in sharper bands (most strongly at 3.3, 6.2, 7.7, 8.6, and 11.3 micrometers). The 3.3 and 11.3 micrometers features, which are due to C-H modes, are well correlated spatially, while the 7.7 micrometers band, due to C=C modes, has a different distribution than the 3.3 and 11.3 micrometers bands. We conclude that the sharp emission bands arise in the photodissociation transition region between the H II region and the molecular cloud and are not present in the H II region. The broad continuum feature extending from 11-13 micrometers is strong in both regions. Previous broad-band observations of the 10 and 20 micrometers flux distributions, which show that the 10 micrometers radiation extends farther into the neutral gas to the south than the 20 micrometers radiation, suggest that some of the 10 micrometers flux is supplied via a nonthermal mechanism, such as fluorescence.     

UIR Bands in Carbon Star Spectra

Unidentified infrared emission bands (UIR bands) have been attributed to polycyclic aromatic hydrocarbons (PAHs), which are believed to require ultraviolet radiation in order for the UIR bands to be excited. If, in addition to amorphous carbon and hydrogenated amorphous carbon (HAC) particles, PAHs are able to form in the outflows of cool carbon-rich stars (Cherchneff et al. 1991), then the weak UV radiation field from such stars would be unlikely to be able to excite the UIR bands and so the PAH species could remain undetected in the spectra of C-stars. However, cool carbon stars with hot companions might be exposed to strong enough UV radiation fields for UIR-band emission to be excited from PAHs. Buss et al. (1991) reported the detection of the 8 μm UIR-band (C-C stretch) in the IRAS LRS spectrum of HD 38218 (TU Tau), a carbon star with a hotter A2III companion. To investigate the phenomenon further, we have therefore obtained UKIRT CGS3 10 μm spectra of three carbon stars with hot companions, TU Tau, UV Aur and CS776. It was found that TU Tau showed the 11.25 μm and 8.6 μm UIR-bands (both attributed to C-H bend modes) at good contrast, while UV Aur clearly exhibited the 11.25 μm UIR band. No narrow UIR-band emission was detected in the spectrum of CS776. We have fitted these 10 μm region spectra using a χ²-minimization program equipped to fit stellar and dust emission continua together with the broad SiC feature and the narrow UIR-bands. The features seen in the spectra of TU Tau and UV Aur can be well fitted by a narrow 11.25 μm UIR-band sitting on top of a broad, self-absorbed 11.3 μm silicon carbide feature. Our results therefore provide strong support for the supposition that PAHs can form in carbon star outflows. This revised version was published online in September 2006 with corrections to the Cover Date.     

The near- and far-infrared colours of MASH planetary nebulae

We have analysed the near-infrared (NIR) and far-infrared (FIR) colours of MASH I and MASH II (the Macquarie/AAO/Strasbourg surveys) planetary nebulae (PNe), using data deriving from the Two-Micron All-Sky Survey and Infrared Astronomical Satellite . We were able to identify ∼5 per cent of the sources in the NIR, and a slightly larger fraction (∼12 per cent) in the FIR. It is concluded that whilst the NIR colours of these nebulae are consistent with those of less evolved (and higher surface brightness) PNe, their FIR colours are markedly different. This disparity is likely to arise as a result of an evolution in dust temperatures, in their line emission characteristics, and in the relative contributions of the 8.6 and 11.3 μm polycyclic aromatic hydrocarbon emission features. A rump of ∼9 per cent of the detected sources have values  log[ F (25 μm)/ F (60 μm)]  which are lower than can be explained in terms of normal nebular evolution, however. If these are comparable in nature to the undetected PNe, then this would argue that ∼1 in 10 of MASH I and II nebulae may represent galactic H  ii regions, Stromgren spheres, symbiotic nebulae or other unrelated categories of source.