H. G. Shan

Photometric study of galaxies with OH megamasers in the infrared

OH megamasers are the most luminous masers in the universe. The total of 109 OH megamaser (OHM) galaxies known so far have been collected, and cross-identifications of those OHM galaxies with Two Micron All Sky Survey (2MASS) sources are made in this paper. Using 2MASS and IRAS data, the infrared properties of OHM-detected and nondetected sources are compared. It is shown that the main difference between those two kinds of sources is the infrared color from 25 to 60 mu m. Statistically, the OHM-detected sources have redder colors and steeper spectral indices in the 25-60 mu m region than the nondetected sources. In addition, considering the spectral types, the properties of the OHM galaxies in the near-infrared and far-infrared are discussed. It is found that Seyfert 1 and Seyfert 2 sources with hidden broad-line regions (BLRs) have redder colors and steeper spectral indices than other sources, including LINER, composite spectrum nucleus, and H II sources in the near-infrared region, whereas the latter have redder colors and steeper spectral indices than the Seyfert 1 and Seyfert 2 sources with hidden BLRs in the far-infrared. It is also found that almost all sources either with or without an identified spectral type show a starburst-dominated feature in the far-infrared, while a considerable number of sources, in particular Seyfert 1 and Seyfert 2 sources with hidden BLRs, show an active galactic nucleus-dominated feature in the near-infrared. Furthermore, as far-infrared radiation at 35 and 53 mu m is likely responsible for the OH maser pumping in OHM galaxies, the pumping efficiencies in the infrared are also analyzed in this paper. It is found that, statistically, the pumping efficiencies increase with far-infrared color. It is also found that, statistically, the OH peak fluxes are not proportional to the related infrared fluxes at either 35 or 53 mu m. This result implies that the pump lines are optically thin and the pump sources are unsaturated.

Infrared spectral evolution of carbon stars

We collected almost all Highly Processed Data Products (HPDP) of ISO SWS01 spectra for the Galactic visual carbon stars, infrared carbon stars, extreme carbon stars and carbon-rich proto-planetary nebulae (PPNs). Those infrared spectra are primarily analyzed and discussed. It is shown that either spectral shapes/peaks, or main molecular/dust features are evidenced to change in the sequence of visual carbon stars, infrared carbon stars, extreme carbon stars and carbon-rich PPNs. Statistically, in this sequence, continua are gradually changed from blue to red and locations of spectral peaks of continua are also gradually changed from short wavelengths to long wavelengths. In addition, in this sequence, intensities of main molecular/dust features are also gradually changed from prominent in the short wavelengths to prominent in the long wavelengths. Furthermore, from 2MASS and IRAS photometric data, the sequence is also proved. Results in this paper strongly support the previous suggestion for the evolution sequence of carbon-rich objects in our Galaxy, that is the sequence of visual carbon stars → infrared carbon stars → extreme carbon stars → carbon-rich PPNs.

Discussion on candidates for the M stars with 11.2 μm SiC feature

The oxygen-rich stars with SiC emission features are unexpected according to the evolutionary theory in the late stage of stellar evolution. After the careful re-examination of such samples reported previously and searching for new samples, seven stars are found to be the candidates of the M stars with 11.2 mum SiC emission feature. However, with the IRAS low-resolution spectrum, the IRAS two colour diagram and other information, it is seen that no strong candidates for oxygen-rich AGB stars with carbon-rich circumstellar dust could be found so far