A Near-Infrared Study of the Highly-Obscured Active Star-Forming Region W51B

We present wide-field JHKs-band photometric observations of the three compact HII regions G48.9-0.3, G49.0-0.3, and G49.2-0.3 in the active star-forming region W51B. The star clusters inside the three compact HII regions show the excess number of stars in the J-Ks histograms compared with reference fields. While the mean color excess ratio E(J-H)/E(H-Ks) of the three compact HII regions are similar to ~ 2.07, the visual extinctions toward them are somewhat different: ~ 17 mag for G48.9-0.3 and G49.0-0.3; ~ 23 mag for G49.2-0.3. Based on their sizes and brightnesses, we suggest that the age of each compact HII region is =< 2 Myr. The inferred total stellar mass, ~ 1.4 x 10^4 M_sun, of W51B makes it one of the most active star forming regions in the Galaxy with the star formation efficiency of ~ 10 %.Comment: 12 pages, 10 eps figures, uses jkas.st

He abundance of Dense Circumstellar Clumps in the Cassiopeia A Supernova Remnant

We report on the result of He abundance analysis of dense circumstellar clumps in the young supernova remnant Cassiopeia A. These clumps, which are called quasi-stationary flocculi (QSFs), are known from previous optical studies to be enriched in He along with N, but the degree of He overabundance relative to H has remained uncertain. For several QSFs with near-infrared spectroscopic data, we have analyzed their He I 1.083 $\mu$m/Pa$\gamma$ ratios together with the ratios of [Fe II] lines by using the Raymond shock code. According to our analysis, He is overabundant relative to H by a factor of $\lesssim 3$ in most of these QSFs. This He abundance of QSFs is consistent with the previous conclusion from the N overabundance that QSFs were ejected when a substantial amount of the H envelope of the progenitor star had been stripped off. We discuss the mass-loss history of the progenitor star and the origin of QSFs.Comment: 20 pages, 8 figure

Phosphorus in the Young Supernova Remnant Cassiopeia A

Phosphorus (^(31)P), which is essential for life, is thought to be synthesized in massive stars and dispersed into interstellar space when these stars explode as supernovae (SNe). Here, we report on near-infrared spectroscopic observations of the young SN remnant Cassiopeia A, which show that the abundance ratio of phosphorus to the major nucleosynthetic product iron (^(56)Fe) in SN material is up to 100 times the average ratio of the Milky Way, confirming that phosphorus is produced in SNe. The observed range is compatible with predictions from SN nucleosynthetic models but not with the scenario in which the chemical elements in the inner SN layers are completely mixed by hydrodynamic instabilities during the explosion

Optical Spectroscopy of Supernova Remnants in M81 and M82

We present spectroscopy of 28 SNR candidates as well as one H II region in M81, and two SNR candidates in M82. Twenty six out of the M81 candidates turn out to be genuine SNRs, and two in M82 may be shocked condensations in the galactic outflow or SNRs. The distribution of [N II]/H{\alpha} ratios of M81 SNRs is bimodal. M81 SNRs are divided into two groups in the spectral line ratio diagrams: an [O III]-strong group and an [O III]-weak group. The latter have larger sizes, and may have faster shock velocity. [N II]/H{\alpha} ratios of the SNRs show a strong correlation with [S II]/H{\alpha} ratios. They show a clear radial gradient in [N II]/H{\alpha} and [S II]/H{\alpha} ratios: dLog ([N II]/H{\alpha})/dLog R = -0.018 {\pm} 0.008 dex/kpc and dLog ([S II]/H{\alpha})/dLog R = -0.016 {\pm} 0.008 dex/kpc where R is a deprojected galactocentric distance. We estimate the nitrogen and oxygen abundance of the SNRs from the comparison with shock-ionization models. We obtain a value for the nitrogen radial gradient, dLog(N/H)/dLogR = -0.023 {\pm} 0.009 dex/kpc, and little evidence for the gradient in oxygen. This nitrogen abundance shows a few times flatter gradient than those of the planetary nebulae and H II regions. We find that five SNRs are matched with X-ray sources. Their X-ray hardness colors are consistent with thermal SNRs.Comment: 19 pages, 24 figures, 5 tables, ApJ accepte