Observational Studies on the Near-Infrared Unidentified Emission Bands in Galactic H Ii Regions

Abstract

Using a large collection of near-infrared spectra (2.5-5.4 µm) of Galactic H II regions and H II region-like objects, we perform a systematic investigation of the astronomical polycyclic aromatic hydrocarbon (PAH) features. 36 objects were observed by the use of the infrared camera onboard the AKARI satellite as a part of a directer's time program. In addition to the well-known 3.3-3.6 µm features, most spectra show a relatively-weak emission feature at 5.22 µm with sufficient signal-to-noise ratios, which we identify as the PAH 5.25 µm band previously reported. By careful analysis, we find good correlations between the 5.25 µm band and both the aromatic hydrocarbon feature at 3.3 µm and the aliphatic ones at around 3.4-3.6 µm. The present results give us convincing evidence that the astronomical 5.25 µm band is associated with C-H vibrations as suggested by previous studies and show its potential to probe the PAH size distribution. The analysis also shows that the aliphatic to aromatic ratio of I 3.4−3.6 µm /I 3.3 µm decreases against the ratio of the 3.7 µm continuum intensity to the 3.3 µm band, I cont,3.7 µm /I 3.3 µm , which is an indicator of the ionization fraction of PAHs. The mid-infrared color of I 9 µm /I 18 µm also declines steeply against the ratio of the hydrogen recombination line Brα at 4.05 µm to the 3.3 µm band, I Brα /I 3.3 µm .These facts indicate possible dust processing inside or at the boundary of ionized gas. Subject headings: dust, extinction -infrared: ISM 1. Introduction H II regions are luminous gas clouds ionized by embedded massive O-type or B-type stars. Considering their extreme physical conditions (intense radiation fields and stellar winds from the central stars), H II regions are a good laboratory for investigating the evolution of materials in the I 9 µm /I 18 µm .Section 2 describes the observations and data reduction. Spectral analysis is presented in Section 3, and its results and discussion in Section 4. Summary and conclusion are given in Section 5. Observations NIR slit spectroscopy with the AKARI/IRCThe present studies are mainly based on the slit spectroscopic data obtained by the AKARI/IRC in the framework of director's time program during the AKARI post-helium mission phase (Phase 3), during which only NIR channel (2.0-5.5 µm) was in operation. All of the

Akari Near-Infrared Spectroscopy of the Extended Green Object G318.05+0.09: Detection of Co Fundamental Ro-Vibrational Emission

“…The central wavelength of the absorption peak of CO 2 ice is slightly shorter than the typical value of 4.26 μm obtained in the same study, but it is still within the uncertainty due to the large noise level and the spectral resolution of the present observation. The absorption feature at around 4.67 μm is broader than the CO ice feature (Gibb et al 2004;Mori et al 2014). We fit it with both CO gas and ice components.…”

“…We fit it with both CO gas and ice components. For CO ice, we adopt the central wavelength of 4.67 μm and the FWHM of 0.031 μm obtained from the Ns spectroscopy by Mori et al (2014) and assume the band strength reported in Gerakines et al (1995). The observed feature can be fitted by absorption of CO gas at 20 K and CO ice as shown in Figure 3(b).…”

“…8) along the slit direction for the grism and prism modes. Details of the data reduction are summarized in Mori et al (2014). Due to the relatively large number of hot pixels, a few residual hot pixels remain after the standard reduction process.…”

Ice Absorption Features in Nir Spectra of Galactic Objects

“…The data were taken from 399 lines of sight for 143 pointings, which were observed in the framework of Program DTIRC (PI: AKARI IRC team), IPYSO (PI: Takashi Shimonishi), and ISMGN (PI: Hidehiro Kaneda) during the AKARI Phase 3 period. Among them, the reduced data of http://pkas.kas.org Galactic H ii regions are released on the JAXA archive at: http://www.ir.isas.jaxa.jp/AKARI/Archive/ Catalogues/IRC_GALHII_spec/ (Mori et al, 2014). A study of part of the present dataset is also reported in Onaka et al (2016).…”

Processing of Interstellar Medium as Divulged by Akari

“…Jones et al (2013) propose an interesting scenario for the evolution of carbonaceous dust grains, where they predict that carbonaceous dust tends to be aromatized in the ISM by the interstellar radiation and aliphatic C-H bonds are converted into aromatic C-H particularly for very small grains. Mori et al (2014a) study more than 160 lines of sight towards 36 Galactic H ii regions and H ii region-like objects with IRC NIR spectroscopy and derive the intensities of the emission bands originating from aromatic bonds at 3.3 µm and aliphatic bonds for 3.4-3.5 µm. The variation of the band ratio of aliphatic to aromatic bonds I(3.4 − 3.5 µm)/I(3.3 µm) is detected and found to be correlated with the ratio of the continuum intensity at 3.7 µm to the 3.3 µm band intensity I c (3.7 µm)/I(3.3 µm) (Figure 1).…”