Browsing by Subject ": formation"
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Item The c2d Spitzer Spectroscopic Survey Of Ices Around Low-Mass Young Stellar Objects. III. CH4(2008-05) Oberg, Karin I.; Boogert, Adwin C. A.; Pontoppidan, Klaus M.; Blake, Geoffrey A.; Evans, Neal J.; Lahuis, Fred; van Dishoeck, Ewine F.; Evans, Neal J.CH4 is proposed to be the starting point of a rich organic chemistry. Solid CH4 abundances have previously been determined mostly toward high-mass star-forming regions. Spitzer IRS now provides a unique opportunity to probe solid CH4 toward low-mass star-forming regions as well. Infrared spectra from the Spitzer Space Telescope are presented to determine the solid CH4 abundance toward a large sample of low-mass young stellar objects. A total of 25 out of 52 ice sources in the "Cores to Disks'' (c2d) Legacy program have an absorption feature at 7.7 mu m, attributed to the bending mode of solid CH4. The solid CH4/H2O abundances are 2%-8%, except for three sources with abundances as high as 11%-13%. The latter sources have relatively large uncertainties due to small total ice column densities. Toward sources with H2O column densities above 2 x 10(18) cm(-2), the CH4 abundances (20 out of 25) are nearly constant at 4.7% +/- 1.6%. Correlation plots with solid H2O, CH3OH, CO2, and CO column densities and abundances relative to H2O reveal a closer relationship of solid CH4 with CO2 and H2O than with solid CO and CH3OH. The inferred solid CH4 abundances are consistent with models where CH4 is formed through sequential hydrogenation of C on grain surfaces. Finally, the equal or higher abundances toward low-mass young stellar objects compared with high-mass objects and the correlation studies support this formation pathway as well, but not the two competing theories: formation from CH3OH and formation in gas phase with subsequent freezeout.Item Massive Z Similar To 1.3 Evolved Galaxies Revealed(2003-01) Saracco, P.; Longhetti, M.; Severgnini, P.; Della Ceca, R.; Mannucci, F.; Bender, R.; Drory, N.; Feulner, G.; Ghinassi, F.; Hopp, U.; Maraston, C.; Drory, N.We present the results of TNG near-IR low resolution spectroscopy of two (S7175_254 and S7F5_45) sources belonging to a complete sample of 15 EROs with K' < 18 and R - K' > 5 selected from the MUNICS Survey. Both the spectra show a sharp drop in the continuum which can be ascribed only to the Balmer break. This places them at 1.2 < z < 1.5. Their rest-frame z = 1.2 K-band absolute magnitude is M-K similar or equal to -26.6 (L - 7L*). The comparison of the spectra and the photometric data with a grid of synthetic template spectra provides a redshift z similar or equal to 1.22(+0.02)(-0.07) for S7175_254 and z similar or equal to 1.46 +/- 0.02 for S7175_45. The resulting lower limits to their stellar mass are M-stars(min) = 6 x 10(11) M. and M-stars(min) = 4 x 10(11) M.. The minimum age of the last burst of star formation in S7F5_254 is 3.5 Gyr while it is 0.5 Gyr in S7F5_45 implying a minimum formation redshift z(f) greater than or similar to 3.5 and z(f) greater than or similar to 2 for the two EROs respectively.