Rings are the most frequently revealed substructure in ALMA dust observations of protoplanetary disks, but their origin is still hotly debated. In this paper, we identify dust substructures in 12 disks and measure their properties to investigate how they form. This subsample of disks is selected from a high-resolution (∼ 0.12 ) ALMA 1.33 mm survey of 32 disks in the Taurus star-forming region, which was designed to cover a wide range of sub-mm brightness and to be unbiased to previously known substructures. While axisymmetric rings and gaps are common within our sample, spiral patterns and high contrast azimuthal asymmetries are not detected. Fits of disk models to the visibilities lead to estimates of the location and shape of gaps and rings, the flux in each disk component, and the size of the disk. The dust substructures occur across a wide range of stellar mass and disk brightness. Disks with multiple rings tend to be more massive and more extended. The correlation between gap locations and widths, the intensity contrast between

This paper presents Spitzer-IRS λ/∆λ ∼ 600 spectroscopy of the CO 2 15.2 µm bending mode toward a sample of 50 embedded low-mass stars in nearby star-forming clouds, taken mostly from the "Cores to Disks (c2d)" Legacy program. The average abundance of solid CO 2 relative to water in low-mass protostellar envelopes is 0.32 ± 0.02, significantly higher than that found in quiescent molecular clouds and in massive star forming regions. It is found that a decomposition of all the observed CO 2 bending mode profiles requires a minimum of five unique components. In general, roughly 2/3 of the CO 2 ice is found in a water-rich environment, while most of the remaining 1/3 is found in a CO environment with strongly varying relative concentrations of CO 2 to CO along each line of sight. Ground-based observations of solid CO toward a large subset of the c2d sample are used to further constrain the CO 2 :CO component and suggest a model in which low-density clouds form the CO 2 :H 2 O component and higher density clouds form the CO 2 :CO ice during and after the freeze-out of gas-phase CO. The abundance of the CO 2 :CO component is consistent with cosmic ray processing of the CO-rich part of the ice mantles, although a more quiescent formation mechanism is not ruled out. It is suggested that the subsequent evolution of the CO 2 and CO profiles toward low-mass protostars, in particular the appearance of the splitting of the CO 2 bending mode due to pure, crystalline CO 2 , is first caused by distillation of the CO 2 :CO component through evaporation of CO due to thermal processing to ∼ 20 − 30 K in the inner regions of infalling envelopes. The formation of pure CO 2 via segregation from the H 2 O rich mantle may contribute to the band splitting at higher levels of thermal processing ( 50 K), but is harder to reconcile with the physical structure of protostellar envelopes around low-luminosity objects.

We present an analysis of Spitzer-IRS observations of H 2 O, OH, HCN, C 2 H 2 and CO 2 emission, and Keck-NIRSPEC observations of CO emission, from a diverse sample of T Tauri and Herbig Ae/Be circumstellar disks. We find that detections and strengths of most mid-IR molecular emission features are correlated with each other, suggesting a common origin and similar excitation conditions for this mid-infrared line forest. Aside from the remarkable differences in molecular line strengths between T Tauri, Herbig Ae/Be and transitional disks discussed in Pontoppidan et al. (2010b), we note that the line detection efficiency is anti-correlated with the 13/30 µm SED spectral slope, which is a measure of the degree of grain settling in the disk atmosphere. We also note a correlation between detection efficiency and Hα equivalent width, and tentatively with accretion rate, suggesting that accretional heating contributes to line excitation. If detected, H 2 O line fluxes are correlated with the mid-IR continuum flux, and other co-varying system parameters, such as L ⋆ . However, significant sample variation, especially in molecular line ratios, remains, and its origin has yet to be explained. LTE models of the H 2 O emission show that line strength is primarily related to the best-fit emitting area, and this accounts for most source-to-source variation in H 2 O emitted flux. Best-fit temperatures and column densities cover only a small range of parameter space, near ∼ 10 18 cm −2 and 450 K for all sources, suggesting a high abundance of H 2 O in many planet-forming regions. Other molecules have a range of excitation temperatures from ∼ 500 − 1500 K, also consistent with an origin in planet-forming regions. We find molecular ratios relative to water of ∼ 10 −3 for all molecules, with the exception of CO, for which n(CO)/n(H 2 O)∼1. However, LTE fitting caveats and differences in the way thermochemical modeling results are reported make comparisons with such models difficult, and highlight the need for additional observations coupled with the use of line-generating radiative transfer codes.

From the masses of planets orbiting our Sun, and relative elemental abundances, it is estimated that at birth our Solar System required a minimum disk mass of ∼0.01 M within ∼100 AU of the star 1-4 . The main constituent, gaseous molecular hydrogen, does not emit from the disk mass reservoir 5 , so the most common measure of the disk mass is dust thermal emission and lines of gaseous carbon monoxide 6 . Carbon monoxide emission generally probes the disk surface, while the conversion from dust emission to gas mass requires knowl-1

We present a Spitzer InfraRed Spectrometer (IRS) search for 10-36 µm molecular emission from a large sample of protoplanetary disks, including lines from H 2 O, OH, C 2 H 2 , HCN and CO 2 . This paper describes the sample and data processing and derives the detection rate of mid-infrared molecular emission as a function of stellar mass. The sample covers a range of spectral type from early M to A, and is supplemented by archival spectra of disks around A and B stars. It is drawn from a variety of nearby star forming regions, including Ophiuchus, Lupus and Chamaeleon. Spectra showing strong emission lines are used to identify which lines are the best tracers of various physical and chemical conditions within the disks. In total, we identify 22 T Tauri stars with strong mid-infrared H 2 O emission. Integrated water line luminosities, where water vapor is detected, range from 5 × 10 −4 to 9 × 10 −3 L ⊙ , likely making water the dominant line coolant of inner disk surfaces in classical T Tauri stars. None of the 5 transitional disks in the sample show detectable gaseous molecular emission with Spitzer upper limits at the 1% level in terms of line-to-continuum ratios (apart from H 2 ), but the sample is too small to conclude whether this is a general property of transitional disks. We find a strong dependence on detection rate with spectral type; no disks around our sample of 25 A and B stars were found to exhibit water emission, down to 1-2% line-to-continuum ratios, in the mid-infrared, while almost 2/3 of the disks around K stars show sufficiently intense water emission to be detected by Spitzer. Some Herbig Ae/Be stars show tentative H 2 O/OH emission features beyond 20 µm at the 1-2% level, however, and one of them shows CO 2 in emission. We argue that the observed differences between T Tauri disks and Herbig Ae/Be disks is due to a difference in excitation and/or chemistry depending on spectral type and suggest that photochemistry may be playing an important role in the observable characteristics of mid-infrared molecular line emission from protoplanetary disks.