To study the physical and chemical evolution of ices in solar-mass systems, a spectral survey is conducted of a sample of 41 low-luminosity YSOs (L $ 0:1Y10 L ) using 3Y38 m Spitzer and ground-based spectra. The sample is complemented with previously published Spitzer spectra of background stars and with ISO spectra of well-studied massive YSOs (L $ 10 5 L ). The long-known 6.0 and 6.85 m bands are detected toward all sources, with the Class 0Y type YSOs showing the deepest bands ever observed. The 6.0 m band is often deeper than expected from the bending mode of pure solid H 2 O. The additional 5Y7 m absorption consists of five independent components, which, by comparison to laboratory studies, must be from at least eight different carriers. Much of this absorption is due to simple species likely formed by grain surface chemistry, at abundances of 1%Y30% for CH 3 OH, 3%Y8% for NH 3 , 1%Y5% for HCOOH, $6% for H 2 CO, and $0.3% for HCOO À relative to solid H 2 O. The 6.85 m band has one or two carriers, of which one may be less volatile than H 2 O. Its carrier(s) formed early in the molecular cloud evolution and do not survive in the diffuse ISM. If an NH þ 4 -containing salt is the carrier, its abundance relative to solid H 2 O is $7%, demonstrating the efficiency of low-temperature acid-base chemistry or cosmic-rayYinduced reactions. Possible origins are discussed for enigmatic, very broad absorption between 5 and 8 m. Finally, the same ices are observed toward massive and low-mass YSOs, indicating that processing by internal UV radiation fields is a minor factor in their early chemical evolution.

We have mapped over 50 massive, dense clumps with four dense gas tracers: HCN J = 1 − 0 and 3 − 2; and CS J = 2 − 1 and 7 − 6 transitions. Spectral lines of optically thin H 13 CN 3-2 and C 34 S 5-4 were also obtained towards the map centers. These maps usually demonstrate single well-peaked distributions at our resolution, even with higher J transitions. The size, virial mass, surface density, and mean volume density within a well-defined angular size (FWHM) were calculated from the contour maps for each transition. We found that transitions with higher effective density usually trace the more compact, inner part of the clumps but have larger linewidths, leading to an inverse linewidth-size relation using different tracers. The mean surface densities are 0.29, 0.33, 0.78, 1.09 g cm −2 within FWHM contours of CS 2-1, HCN 1-0, HCN 3-2 and CS 7-6, respectively. We find no correlation of L IR with surface density and a possible inverse correlation with mean volume density, contrary to some theoretical expectations. Molecular line luminosities L ′ mol were derived for each transition. We see no evidence in the data for the relation between L ′ mol and mean density posited by modelers. The correlation between L ′ mol and the virial mass is roughly linear for each dense gas tracer. No obvious correlation was found between the line luminosity ratio and infrared luminosity, bolometric temperature, or the L IR /M V ir ratio. A nearly -2linear correlation was found between the infrared luminosity and the line luminosity of all dense gas tracers for these massive, dense clumps, with a lower cutoff in luminosity at L IR = 10 4.5 L ⊙ . The L IR -L ′ HCN 1−0 correlation agrees well with the one found in galaxies. These correlations indicate a constant star formation rate per unit mass from the scale of dense clumps to that of distant galaxies when the mass is measured for dense gas. These results support the suggestion that starburst galaxies may be understood as having a large fraction of gas in dense clumps.

Infrared $5-35 m spectra for 40 solar mass T Tauri stars and 7 intermediate-mass Herbig Ae stars with circumstellar disks were obtained using the Spitzer Space Telescope as part of the c2d IRS survey. This work complements prior spectroscopic studies of silicate infrared emission from disks, which were focused on intermediate-mass stars, with observations of solar mass stars limited primarily to the 10 m region. The observed 10 and 20 m silicate feature strengths/shapes are consistent with source-to-source variations in grain size. A large fraction of the features are weak and flat, consistent with micron-sized grains indicating fast grain growth (from 0.1 to 1.0 m in radius). In addition, approximately half of the T Tauri star spectra show crystalline silicate features near 28 and 33 m, indicating significant processing when compared to interstellar grains. A few sources show large 10-to-20 m ratios and require even larger grains emitting at 20 m than at 10 m. This size difference may arise from the difference in the depth into the disk probed by the two silicate emission bands in disks where dust settling has occurred. The 10 m feature strength versus shape trend is not correlated with age or H equivalent width, suggesting that some amount of turbulent mixing and regeneration of small grains is occurring. The strength versus shape trend is related to spectral type, however, with M stars showing significantly flatter 10 m features ( larger grain sizes) than A / B stars. The connection between spectral type and grain size is interpreted in terms of the variation in the silicate emission radius as a function of stellar luminosity, but could also be indicative of other spectral-type-dependent factors (e.g., X-rays, UV radiation, and stellar/disk winds).

We have mapped 63 regions forming high-mass stars in CS J ¼ 5 ! 4 using the CSO. The CS peak position was observed in C 34 S J ¼ 5 ! 4 toward 57 cores and in 13 CS J ¼ 5 ! 4 toward the nine brightest cores. The sample is a subset of a sample originally selected toward water masers; the selection on maser sources should favor sources in an early stage of evolution. The cores are located in the first and second Galactic quadrants with an average distance of 5:3 AE 3:7 kpc and were well detected with a median peak signalto-noise ratio in the integrated intensity of 40. The integrated intensity of CS J ¼ 5 ! 4 correlates very well with the dust continuum emission at 350 lm. For 57 sufficiently isolated cores, a well-defined angular size (FWHM) was determined. The core radius (R CS ), aspect ratio [ða=bÞ obs ], virial mass (M vir ), surface density (AE), and the luminosity in the CS J ¼ 5 ! 4 line (LðCS54Þ) are calculated. The distributions of size, virial mass, surface density, and luminosity are all peaked with a few cores skewed toward much larger values than the mean. The median values, l 1/2 , are as follows: l 1/2 ðR CS Þ ¼ 0:32 pc, l 1/2 ðða=bÞ obs Þ ¼ 1:20, l 1/2 ðM vir Þ ¼ 920 M , l 1/2 ðAEÞ ¼ 0:60 g cm À2 , l 1/2 ðLðCS54ÞÞ ¼ 1:9 Â 10 À2 L , and l 1/2 ðL bol =M vir Þ ¼ 165 ðL=MÞ . We find a weak correlation between C 34 S line width and size, consistent with Dv $ R 0:3 . The line widths are much higher than would be predicted by the usual relations between line width and size determined from regions of lower mass. These regions are very turbulent. The derived virial mass agrees within a factor of 2-3 with mass estimates from dust emission at 350 lm after corrections for the density structure are accounted for. The resulting cumulative mass spectrum of cores above 1000 M can be approximated by a power law with a slope of about À0.9, steeper than that of clouds measured with tracers of lower density gas and close to that for the total masses of stars in OB associations. The median turbulent pressures are comparable to those in UCH ii regions, and the pressures at small radii are similar to those in hypercompact H ii regions (P=k $ 10 10 K cm À3 ). The filling factors for dense gas are substantial, and the median abundance of CS is about 10 À9 . The ratio of bolometric luminosity to virial mass is much higher than the value found for molecular clouds as a whole, and the correlation of luminosity with mass is tighter.

The relation between ices in the envelopes and disks surrounding YSOs and those in the quiescent interstellar medium is investigated. For a sample of 31 stars behind isolated dense cores, ground-based and Spitzer spectra and photometry in the 1-25 µm wavelength range are combined. The baseline for the broad and overlapping ice features is modeled, using calculated spectra of giants, H 2 O ice and silicates. The adopted extinction curve is derived empirically. Its high resolution allows for the separation of continuum and feature extinction. The extinction between 13-25 µm is ∼50% relative to that at 2.2 µm. The strengths of the 6.0 and 6.85 µm absorption bands are in line with those of YSOs. Thus, their carriers, which, besides H 2 O and CH 3 OH, may include NH + 4 , HCOOH, H 2 CO and NH 3 , are readily formed in the dense core phase, before stars form. The 3.53 µm C-H stretching mode of solid CH 3 OH was discovered. The CH 3 OH/H 2 O abundance ratios of 5-12% are larger than upper limits in the Taurus molecular cloud. The initial ice composition, before star formation occurs, therefore depends on the environment. Signs of thermal and energetic processing that were found toward some YSOs are absent in the ices toward background stars. Finally, the peak optical depth of the 9.7 µm band of silicates relative to the continuum extinction at 2.2 µm is significantly shallower than in the diffuse interstellar medium. This extends the results of Chiar et al. (2007) to a larger sample and higher extinctions.