Mixed aliphatic and aromatic composition of evaporating very small grains in NGC 7023 revealed by the 3.4/3.3 μ\mum ratio

In photon-dominated regions (PDRs), UV photons from nearby stars lead to the evaporation of very small grains (VSGs) and the production of gas-phase polycyclic aromatic hydrocarbons (PAHs). Our goal is to achieve better insight into the composition and evolution of evaporating very small grains (eVSGs) and PAHs through analyzing the infrared (IR) aliphatic and aromatic emission bands. We combined spectro-imagery in the near- and mid-IR to study the spatial evolution of the emission bands in the prototypical PDR NGC 7023. We used near-IR spectra obtained with AKARI to trace the evolution of the 3.3$\mu$m and 3.4$\mu$m bands, which are associated with aromatic and aliphatic C-H bonds on PAHs. The spectral fitting involves an additional broad feature centred at 3.45$\mu$m. Mid-IR observations obtained with Spitzer are used to discriminate the signatures of eVSGs, neutral and cationic PAHs. We correlated the spatial evolution of all these bands with the intensity of the UV field to explore the processing of their carriers. The intensity of the 3.45$\mu$m plateau shows an excellent correlation with that of the 3.3$\mu$m aromatic band (correlation coefficient R = 0.95), indicating that the plateau is dominated by the emission from aromatic bonds. The ratio of the 3.4$\mu$m and 3.3$\mu$m band intensity ($I_{3.4}/I_{3.3}$) decreases by a factor of 4 at the PDR interface from the more UV-shielded to the more exposed layers. The transition region between the aliphatic and aromatic material is found to correspond spatially with the transition zone between neutral PAHs and eVSGs. We conclude that the photo-processing of eVSGs leads to the production of PAHs with attached aliphatic sidegroups that are revealed by the 3.4$\mu$m emission band. Our analysis provides evidence for the presence of very small grains of mixed aromatic and aliphatic composition in PDRs.Comment: Accepted for publication in A&A. Abstract abridged, language editing applied in v

Effects of high-energy ionizing particles on the Si:As mid-infrared detector array on board the AKARI satellite

We evaluate the effects of high-energy ionizing particles on the Si:As impurity band conduction (IBC) mid-infrared detector on board AKARI, the Japanese infrared astronomical satellite. IBC-type detectors are known to be little influenced by ionizing radiation. However we find that the detector is significantly affected by in-orbit ionizing radiation even after spikes induced by ionizing particles are removed. The effects are described as changes mostly in the offset of detector output, but not in the gain. We conclude that the changes in the offset are caused mainly by increase in dark current. We establish a method to correct these ionizing radiation effects. The method is essential to improve the quality and to increase the sky coverage of the AKARI mid-infrared all-sky-survey map.Comment: 16 pages, 8 figures, 1 table, accepted for publication in PAS

On Ultrasmall Silicate Grains in the Diffuse Interstellar Medium

The abundance of both amorphous and crystalline silicates in very small grains is limited by the fact that the 10 micron silicate emission feature is not detected in the diffuse ISM. On the basis of the observed IR emission spectrum for the diffuse ISM, the observed ultraviolet extinction curve, and the 10 micron silicate absorption profile, we obtain upper limits on the abundances of ultrasmall (a < 15 Angstrom) amorphous and crystalline silicate grains. Contrary to previous work, as much as ~20% of interstellar Si could be in a < 15 Angstrom silicate grains without violating observational constraints. Not more than ~5% of the Si can be in crystalline silicates (of any size).Comment: Submitted to ApJ Letters, 11 pages, 4 figures, Late

Fitting of dust spectra with genetic algorithms - I. Perspectives & Limitations

Aims: We present an automatised fitting procedure for the IR range of AGB star spectra. Furthermore we explore the possibilities and boundaries of this method. Methods: We combine the radiative transfer code DUSTY with the genetic algorithm PIKAIA in order to improve an existing spectral fit significantly. Results: In order to test the routine we carried out a performance test by feeding an artificially generated input spectrum into the program. Indeed the routine performed as expected, so, as a more realistic test set-up, we tried to create model fits for ISO spectra of selected AGB stars. Here we were not only able to improve existing fits, but also to show that a slightly altered dust composition may give a better fit for some objects. Conclusion: The use of a genetic algorithm in order to automatise the process of fitting stellar spectra seems to be very promising. We were able to improve existing fits and further offer a quantitative method to compare different models with each other. Nevertheless this method still needs to be studied and tested in more detail.Comment: 9 pages, 5 figures, accepted for publication in Astronomy and Astrophysic

Electric Dipole Radiation from Spinning Dust Grains

We discuss the rotational excitation of small interstellar grains and the resulting electric dipole radiation from spinning dust. Attention is given to excitation and damping of rotation by: collisions with neutrals; collisions with ions; plasma drag; emission of infrared radiation; emission of microwave radiation; photoelectric emission; and formation of H_2 on the grain surface. We introduce dimensionless functions F and G which allow direct comparison of the contributions of different mechanisms to rotational drag and excitation. Emissivities are estimated for dust in different phases of the interstellar medium, including diffuse HI, warm HI, low-density photoionized gas, and cold molecular gas. Spinning dust grains can explain much, and perhaps all, of the 14-50 GHz background component recently observed in CBR studies. It should be possible to detect rotational emission from small grains by ground-based observations of molecular clouds.Comment: 59 pages, 19 eps figures, uses aaspp4.sty . Submitted to Ap.

Massive-Star Forming Infrared Loop around the Crab-like Supernova Remnant G54.1+0.3: Post Main-Sequence Triggered Star Formation?

We report the discovery of a star-forming loop around the young, Crab-like supernova remnant (SNR) G54.1+0.3 using the AKARI infrared satellite. The loop consists of at least eleven young stellar objects (YSOs) embedded in a ring-like diffuse emission of radius ~1'. The YSOs are bright in the mid-infrared and are also visible in the Spitzer Space Telescope Galactic plane survey images. Their Spitzer colors are similar to those of class II YSOs in [3.6]-[5.8] but significantly redder in [8]-[24], i.e., 0<[3.6]-[5.8]<1.2 and 5<[8]-[24]<9. Most of them have near-infrared counterparts in the 2MASS JHKs images, and some of them have an optical counterpart too. Their JHKs colors and magnitudes indicate that the YSOs are massive (<= 10 Msun) pre-main-sequence stars at the same distance to the SNR, i.e., 8 kpc, which supports the association of the star-forming loop with the SNR. The dereddened spectral energy distributions are similar to eraly Herbig Be stars, which are early B-type pre-main-sequence stars with inner disks that have been destroyed. The confinement to a loop structure indicates that the YSOs are young, i.e., <= 2 Myr. We propose that their formation is triggered by the progenitor star of G54.1+0.3, which has a mass of <= 15 Msun. The triggering must have occurred near the end of the progenitor's life, possibly after it had evolved off the main sequence.Comment: 6 pages, accepted for publication in the Astrophysical Journal Letters; added a reference for section

Mid-Infrared Emission Features in the ISM: Feature-to-Feature Flux Ratios

Using a limited, but representative sample of sources in the ISM of our Galaxy with published spectra from the Infrared Space Observatory, we analyze flux ratios between the major mid-IR emission features (EFs) centered around 6.2, 7.7, 8.6 and 11.3 microns, respectively. In a flux ratio-to-flux ratio plot of EF(6.2)/EF(7.7) as a function of EF(11.3)/EF(7.7), the sample sources form roughly a $\Lambda$-shaped locus which appear to trace, on an overall basis, the hardness of a local heating radiation field. But some driving parameters other than the radiation field may also be required for a full interpretation of this trend. On the other hand, the flux ratio of EF(8.6)/EF(7.7) shows little variation over the sample sources, except for two HII regions which have much higher values for this ratio due to an ``EF(8.6\um) anomaly,'' a phenomenon clearly associated with environments of an intense far-UV radiation field. If further confirmed on a larger database, these trends should provide crucial information on how the EF carriers collectively respond to a changing environment.Comment: 16 pages, 1 figure, 1 table; accepted for publication in ApJ Letter

Unidentified Infrared Emission Bands in the Diffuse Interstellar Medium

Using the Mid-Infrared Spectrometer on board the Infrared Telescope in Space and the low-resolution grating spectrometer (PHT-S) on board the Infrared Space Observatory, we obtained 820 mid-infrared (5 to 12 $\mu$m) spectra of the diffuse interstellar medium (DIM) in the Galactic center, W51, and Carina Nebula regions. These spectra indicate that the emission is dominated by the unidentified infrared (UIR) emission bands at 6.2, 7.7, 8.6, and 11.2 $\mu$m. The relative band intensities (6.2/7.7 $\mu$m, 8.6/7.7 $\mu$m, and 11.2/7.7 $\mu$m) were derived from these spectra, and no systematic variation in these ratios was found in our observed regions, in spite of the fact that the incident radiation intensity differs by a factor of 1500. Comparing our results with the polycyclic aromatic hydrocarbons (PAHs) model for the UIR band carriers, PAHs in the DIM have no systematic variation in their size distribution, their degree of dehydrogenation is independent of the strength of UV radiation field, and they are mostly ionized. The latter finding is incompatible with past theoretical studies, in which a large fraction of neutral PAHs is predicted in this kind of environment. A plausible resolution of this discrepancy is that the recombination coefficients for electron and large PAH positive ion are by at least an order of magnitude less than those adopted in past theoretical studies. Because of the very low population of neutral state molecules, photoelectric emission from interstellar PAHs is probably not the dominant source of heating of the diffuse interstellar gas. The present results imply constant physical and chemical properties of the carriers of the UIR emission bands in the DIM.Comment: 13 pages, 6 figures. Accepted for publication in Ap

Diffuse Galactic Emission from Spinning Dust Grains

Spinning interstellar dust grains produce detectable rotational emission in the 10-100 GHz frequency range. We calculate the emission spectrum, and show that this emission can account for the ``anomalous'' Galactic background component which correlates with 100um thermal emission from dust. Implications for cosmic background studies are discussed.Comment: 13 pages, 3 eps figures, uses aaspp4.sty . Accepted by Ap.J.Letters 97/12/09. Corrected typos and added 1 referenc