Search

Planet formation is strongly influenced by the composition and distribution of volatiles within protoplanetary disks. With JWST, it is now possible to obtain direct observational constraints on disk ices, as recently demonstrated by the detection of ice absorption features towards the edge-on HH 48 NE disk as part of the Ice Age Early Release Science program. Here, we introduce a new radiative transfer modeling framework designed to retrieve the composition and mixing status of disk ices using their band profiles, and apply it to interpret the H2O, CO2, and CO ice bands observed towards the HH 48 NE disk. We show that the ices are largely present as mixtures, with strong evidence for CO trapping in both H2O and CO2 ice. The HH 48 NE disk ice composition (pure vs. polar vs. apolar fractions) is markedly different from earlier protostellar stages, implying thermal and/or chemical reprocessing during the formation or evolution of the disk. We infer low ice-phase C/O ratios around 0.1 throughout the disk, and also demonstrate that the mixing and entrapment of disk ices can dramatically affect the radial dependence of the C/O ratio. It is therefore imperative that realistic disk ice compositions are considered when comparing planetary compositions with potential formation scenarios, which will fortunately be possible for an increasing number of disks with JWST., Comment: Accepted to ApJ. 24 pages, 15 figures

Abridged: Very Low Luminosity Objects (VeLLOs) are deeply embedded, and extremely faint objects and are thought to be in the quiescent phase of the episodic accretion process. They fill an important gap in our understanding of star formation. The VeLLO in the isolated DC3272+18 cloud has undergone an outburst, and is thus an ideal target for investigating the chemical inventory in the gas phase of an object of its type. Observations with the Atacama Pathfinder EXperiment (APEX) in four spectral windows in the frequency range of 213.6--272.4~GHz have been carried out to identify molecules that can be directly linked to the past outburst, utilize the line fluxes, column densities, and the abundance ratios of the detected species to characterize the different physical components of the VeLLO, and probe for the presence of complex organic molecules. Nitric oxide (NO) is detected for the first time in a source of this type, and its formation could be induced by the sublimation of grain-surface species during the outburst. A pathway to form NO directly in the gas phase is from the photodissociation products created after the sublimation of H$_2$O and NH$_3$ from the ices. While the present time water snowline has likely retreated to pre-outburst small radius, the volatile NO species is still extensively present in the gas phase, as evident by its high column density relative to methanol in the observations. This suggests that NO could be potentially used to trace the water snowline in outbursting sources. In order to rule out non-thermal desorption processes that could also have led to the formation of NO, this proposition has to be verified with future observations at higher spatial resolution, and by searching for NO in additional targets., Comment: 19 pages, 5 figures, 5 tables. Accepted for publication in A&A

Here we report the detection of dust depletion in a misaligned inner disk around UX Tau A using JWST MIRI spectra. Mid-infrared (MIR) continuum "seesaw" variability was detected in this disk by Spitzer and attributed to variable shadows cast on the outer disk by the inner disk. The JWST MIRI spectrum of UX Tau A also shows seesaw variability, but with a significant decrease of emission shortwards of 10 micron to nearly photospheric levels. We argue that UX Tau A's MIR continuum variability is due to depletion of dust in a misaligned inner disk. We speculate that this dust depletion occurs because the inner disk is misaligned from the outer disk, which can disrupt the replenishment of the inner disk from the outer disk. Using contemporaneous measurements of the mass accretion rate of UX Tau A and estimating the amount of dust necessary to produce the MIR excess in the Spitzer observations, we estimate a minimum dust depletion timescale of ~0.1 yr. These observations show that we can indirectly detect the signatures of misaligned inner disks through MIR continuum variability and that in some cases the inner disk may be significantly depleted of dust and become optically thin., Comment: accepted to ApJL

Ice-coated dust grains provide the main reservoir of volatiles that play an important role in planet formation processes and may become incorporated into planetary atmospheres. However, due to observational challenges, the ice abundance distribution in protoplanetary disks is not well constrained. We present JWST/MIRI observations of the edge-on disk HH 48 NE carried out as part of the IRS program Ice Age. We detect CO$_2$, NH$_3$, H$_2$O and tentatively CH$_4$ and NH$_4^+$. Radiative transfer models suggest that ice absorption features are produced predominantly in the 50-100 au region of the disk. The CO$_2$ feature at 15 micron probes a region closer to the midplane (z/r = 0.1-0.15) than the corresponding feature at 4.3 micron (z/r = 0.2-0.6), but all observations trace regions significantly above the midplane reservoirs where we expect the bulk of the ice mass to be located. Ices must reach a high scale height (z/r ~ 0.6; corresponding to modeled dust extinction Av ~ 0.1), in order to be consistent with the observed vertical distribution of the peak ice optical depths. The weakness of the CO$_2$ feature at 15 micron relative to the 4.3 micron feature and the red emission wing of the 4.3 micron CO$_2$ feature are both consistent with ices being located at high elevation in the disk. The retrieved NH$_3$ abundance and the upper limit on the CH$_3$OH abundance relative to H$_2$O are significantly lower than those in the interstellar medium (ISM), but consistent with cometary observations. Full wavelength coverage is required to properly study the abundance distribution of ices in disks. To explain the presence of ices at high disk altitudes, we propose two possible scenarios: a disk wind that entrains sufficient amounts of dust, thus blocking part of the stellar UV radiation, or vertical mixing that cycles enough ices into the upper disk layers to balance ice photodesorption., Comment: 16 pages, 11 figures, accepted in A&A

We present JWST MIRI MRS observations of the edge-on protoplanetary disk around the young sub-solar mass star Tau 042021, acquired as part of the Cycle 1 GO program "Mapping Inclined Disk Astrochemical Signatures (MIDAS)." These data resolve the mid-IR spatial distributions of H$_2$, revealing X-shaped emission extending to ~200 au above the disk midplane with a semi-opening angle of $35 \pm 5$ degrees. We do not velocity-resolve the gas in the spectral images, but the measured semi-opening angle of the H$_2$ is consistent with an MHD wind origin. A collimated, bipolar jet is seen in forbidden emission lines from [Ne II], [Ne III], [Ni II], [Fe II], [Ar II], and [S III]. Extended H$_2$O and CO emission lines are also detected, reaching diameters between ~90 and 190 au, respectively. Hot molecular emission is not expected at such radii, and we interpret its extended spatial distribution as scattering of inner disk molecular emission by dust grains in the outer disk surface. H I recombination lines, characteristic of inner disk accretion shocks, are similarly extended, and are likely also scattered light from the innermost star-disk interface. Finally, we detect extended PAH emission at 11.3 microns co-spatial with the scattered light continuum, making this the first low-mass T Tauri star around which extended PAHs have been confirmed, to our knowledge. MIRI MRS line images of edge-on disks provide an unprecedented window into the outflow, accretion, and scattering processes within protoplanetary disks, allowing us to constrain the disk lifetimes and accretion and mass loss mechanisms., Comment: Accepted for publication in ApJL on March 13th, 2024

We report the first detection of variability in the mid-infrared neon line emission of a protoplanetary disk by comparing a JWST MIRI MRS spectrum of SZ Cha taken in 2023 with a Spitzer IRS SH spectrum of this object from 2008. We measure the [Ne III]-to-[Ne II] line flux ratio, which is a diagnostic of the high-energy radiation field, to distinguish between the dominance of EUV- or X-ray-driven disk photoevaporation. We find that the [Ne III]-to-[Ne II] line flux ratio changes significantly from $\sim1.4$ in 2008 to $\sim0.2$ in 2023. This points to a switch from EUV-dominated to X-ray-dominated photoevaporation of the disk. We present contemporaneous ground-based optical spectra of the Halpha emission line that show the presence of a strong wind in 2023. We propose that this strong wind prevents EUV radiation from reaching the disk surface while the X-rays permeate the wind and irradiate the disk. We speculate that at the time of the Spitzer observations, the wind was suppressed and EUV radiation reached the disk. These observations confirm that the MIR neon emission lines are sensitive to changes in high-energy radiation reaching the disk surface. This highlights the [Ne III]-to-[Ne II] line flux ratio as a tool to gauge the efficiency of disk photoevaporation in order to provide constraints on the planet-formation timescale. However, multiwavelength observations are crucial to interpret the observations and properly consider the star-disk connection., Comment: Accepted to ApJL

Ices are the main carriers of volatiles in protoplanetary disks and are crucial to our understanding of the chemistry that ultimately sets the organic composition of planets. The ERS program Ice Age on the JWST follows the ice evolution through all stages of star and planet formation. JWST/NIRSpec observations of the edge-on Class II protoplanetary disk HH~48~NE reveal spatially resolved absorption features of the major ice components H$_2$O, CO$_2$, CO, and multiple weaker signatures from less abundant ices NH$_3$, OCN$^-$, and OCS. Isotopologue $^{13}$CO$_2$ ice has been detected for the first time in a protoplanetary disk. Since multiple complex light paths contribute to the observed flux, the ice absorption features are filled in by ice-free scattered light. The $^{12}$CO$_2$/$^{13}$CO$_2$ ratio of 14 implies that the $^{12}$CO$_2$ feature is saturated, without the flux approaching 0, indicative of a very high CO$_2$ column density on the line of sight, and a corresponding abundance with respect to hydrogen that is higher than ISM values by a factor of at least a few. Observations of rare isotopologues are crucial, as we show that the $^{13}$CO$_2$ observation allows us to determine the column density of CO$_2$ to be at an order of magnitude higher than the lower limit directly inferred from the observed optical depth. Radial variations in ice abundance, e.g., snowlines, are significantly modified since all observed photons have passed through the full radial extent of the disk. CO ice is observed at perplexing heights in the disk, extending to the top of the CO-emitting gas layer. We argue that the most likely interpretation is that we observe some CO ice at high temperatures, trapped in less volatile ices like H$_2$O and CO$_2$. Future radiative transfer models will be required to constrain the implications on our current understanding of disk physics and chemistry., Comment: 16 pages, 8 figures, accepted for publication in A&A

Interstellar ices are largely composed of frozen water. It is important to derive fundamental parameters for H$_2$O ice such as absorption and scattering opacities for which accurate complex refractive indexes are needed. The primary goal of this work is to derive ice-grain opacities based on accurate H$_2$O ice complex refractive indexes and to assess their impact on the derivation of ice column densities and porosity in space. We use the \texttt{optool} code to derive ice-grain opacities values based on new mid-IR complex refractive index measurements of H$_2$O ice. Next, we use those opacities in the \texttt{RADMC-3D} code to run a radiative transfer simulation of a protostellar envelope containing H$_2$O ice. This is used to calculate water ice column densities. We find that the real refractive index in the mid-IR of H$_2$O ice at 30~K is $\sim$14\% lower than previously reported in the literature. This has a direct impact on the ice column densities derived from the simulations of embedded protostars. We find that ice porosity plays a significant role in the opacity of icy grains and that the H$_2$O libration mode can be used as a diagnostic tool to constrain the porosity level. Finally, the refractive indexes presented here allow us to estimate a grain size detection limit of 18~$\mu$m based on the 3~$\mu$m band whereas the 6~$\mu$m band allows tracing grain sizes larger than 20~$\mu$m. Based on radiative transfer simulations using new mid-IR refractive indexes, we conclude that H$_2$O ice leads to more absorption of infrared light than previously estimated. This implies that the 3 and 6~$\mu$m bands remain detectable in icy grains with sizes larger than 10~$\mu$m. Finally, we propose that also the H$_2$O ice libration band can be a diagnostic tool to constrain the porosity level of the interstellar ice, in addition to the OH dangling bond, which is routinely used for this purpose., Comment: Accepted for publication in A&A, 12 pages, 15 figures

The abundance and distribution of ice in protoplanetary disks (PPD) is critical to understand the linkage between the composition of circumstellar matter and the composition of exoplanets. Edge-on PPDs are a useful tool to constrain such ice composition and its location in the disk, as ice spectral signatures can be observed in absorption against the continuum emission arising from the warmer central disk regions. The aim of this work is to model ice absorption features in PPDs and determine how well the abundance of the main ice species across the disk can be determined within the uncertainty of the physical parameter space. The edge-on PPD around HH 48 NE, a target of the JWST ERS program IceAge, is used as a reference system. We use RADMC-3D to raytrace the mid-infrared continuum. Using a constant parameterized ice abundance, ice opacities are added to the dust opacity in regions wherever the disk is cold enough for the main carbon, oxygen and nitrogen carriers to freeze out. The global abundance of the main ice carriers in HH 48 NE can be determined within a factor of 3, when taking the uncertainty of the physical parameters into account. Ice features in PPDs can be saturated at an optical depth <1, due to local saturation. Spatially observed ice optical depths cannot be directly related to column densities due to radiative transfer effects. Vertical snowlines will not be a clear transition due to the radially increasing height of the snowsurface, but their location may be constrained from observations using radiative transfer modeling. Radial snowlines are not really accesible. Not only the ice abundance, but also inclination, settling, grain size distribution and disk mass have strong impact on the observed ice absorption features in disks. Relative changes in ice abundance can be inferred from observations only if the source structure is well constrained, Comment: 18 pages, 12 figures, accepted for publication in Astronomy & Astrophysics

Context. Observations of edge-on disks are an important tool for constraining general protoplanetary disk properties that cannot be determined in any other way. However, most radiative transfer models cannot simultaneously reproduce the spectral energy distributions (SEDs) and resolved scattered light and submillimeter observations of these systems, due to the differences in geometry and dust properties at different wavelengths. Aims. We simultaneously constrain the geometry of the edge-on protoplanetary disk HH 48 NE and the characteristics of the host star. HH 48 NE is part of the JWST early release science program Ice Age. This work serves as a stepping stone towards a better understanding of the disk physical structure and icy chemistry in this particular source. This kind of modeling lays the groundwork for studying other edge-on sources to be observed with the JWST. Methods. We fit a parameterized dust model to HH 48 NE by coupling the radiative transfer code RADMC-3D and an MCMC framework. The dust structure was fitted independently to a compiled SED, a scattered light image at 0.8 ${\mu}$m and an ALMA dust continuum observation at 890 ${\mu}$m. Results. We find that 90% of the dust mass in HH 48 NE is settled to the disk midplane, less than in average disks, and that the atmospheric layers of the disk contain exclusively large grains (0.3-10 ${\mu}$m). The exclusion of small grains in the upper atmosphere likely has important consequences for the chemistry due to the deep penetration of high-energy photons. The addition of a relatively large cavity (ca. 50 au in radius) is necessary to explain the strong mid-infrared emission, and to fit the scattered light and continuum observations simultaneously., Comment: 16 pages, 8 figures, accepted for publication in Astronomy & Astrophysics

Icy grain mantles are the main reservoir of the volatile elements that link chemical processes in dark, interstellar clouds with the formation of planets and composition of their atmospheres. The initial ice composition is set in the cold, dense parts of molecular clouds, prior to the onset of star formation. With the exquisite sensitivity of JWST, this critical stage of ice evolution is now accessible for detailed study. Here we show the first results of the Early Release Science program "Ice Age" that reveal the rich composition of these dense cloud ices. Weak ices, including, $^{13}$CO$_2$, OCN$^-$, $^{13}$CO, OCS, and COMs functional groups are now detected along two pre-stellar lines of sight. The $^{12}$CO$_2$ ice profile indicates modest growth of the icy grains. Column densities of the major and minor ice species indicate that ices contribute between 2 and 19% of the bulk budgets of the key C, O, N, and S elements. Our results suggest that the formation of simple and complex molecules could begin early in a water-ice rich environment., Comment: To appear in Nature Astronomy on January 23rd, 2023. 33 pages, 16 figures, 3 tables; includes extended and supplemental data sections. Part of the JWST Ice Age Early Release Science program's science enabling products. Enhanced spectra downloadable on Zenodo at the following DOI: 10.5281/zenodo.7501239

The total disk gas mass and elemental C, N, O composition of protoplanetary disks are crucial ingredients for our understanding of planet formation. Measuring the gas mass is complicated, since H$_2$ cannot be detected in the cold bulk of the disk and the elemental abundances with respect to hydrogen are degenerate with gas mass in all disk models. We present new NOEMA observations of CO, $^{13}$CO, C$^{18}$O and optically thin C$^{17}$O $J$=2-1 lines, and use additional high angular resolution Atacama Large Millimeter Array millimeter continuum and CO data to construct a representative model of LkCa 15. The transitions that constrain the gas mass and carbon abundance most are C$^{17}$O 2-1, N${_2}$H$^+$ 3-2 and HD 1-0. Using these three molecules we find that the gas mass in the LkCa 15 disk is $M_\mathrm{g}=0.01 ^{+0.01}_{-0.004} M_{\odot}$, a factor of six lower than estimated before. The carbon abundance is C/H = ($3 \pm 1.5) \times10^{-5}$, implying a moderate depletion of elemental carbon by a factor of 3-9. All other analyzed transitions also agree with these numbers, within a modeling uncertainty of a factor of two. Using the resolved \ce{C2H} image we find a C/O ratio of $\sim$1, which is consistent with literature values of H$_2$O depletion in this disk. The lack of severe carbon depletion in the LkCa 15 disk is consistent with the young age of the disk, but contrasts with the higher depletions seen in older cold transition disks. Combining optically thin CO isotopologue lines with N$_2$H$^+$ is promising to break the degeneracy between gas mass and CO abundance. The moderate level of depletion for this source with a cold, but young disk, suggests that long carbon transformation timescales contribute to the evolutionary trend seen in the level of carbon depletion among disk populations, rather than evolving temperature effects and presence of dust traps alone., Comment: 16 pages, 8 figures. Accepted for publication in A&A

High quality vibrational spectra of solid-phase molecules in ice mixtures and for temperatures of astrophysical relevance are needed to interpret infrared observations toward protostars and background stars. Over the last 25 years, the Laboratory for Astrophysics at Leiden Observatory has provided more than 1100 spectra of diverse ice samples. Timely with the recent launch of the James Webb Space Telescope, we have fully upgraded the Leiden Ice Database for Astrochemistry (LIDA) adding recently measured spectra. The goal of this manuscript is to describe what options exist to get access to and work with a large collection of IR spectra, and the UV/vis to mid-infrared refractive index of H2O ice and astronomy-oriented online tools to support the interpretation of IR ice observations. LIDA uses Flask and Bokeh for generating the web pages and graph visualization, respectively, SQL for searching ice analogues within the database and Jmol for 3D molecule visualization. The infrared data in the database are recorded via transmission spectroscopy of ice films condensed on cryogenic substrates. The real UV/vis refractive indices of H2O ice are derived from interference fringes created from the simultaneous use of a monochromatic HeNe laser beam and a broadband Xe-arc lamp, whereas the real and imaginary mid-IR values are theoretically calculated. LIDA also offers online tools. The first tool, SPECFY, used to create a synthetic spectrum of ices towards protostars. The second tool aims at the calculation of mid-infrared refractive index values. LIDA allows to search, download and visualize experimental data of astrophysically relevant molecules in the solid phase, as well as to provide the means to support astronomical observations. As an example, we analyse the spectrum of the protostar AFGL 989 using the resources available in LIDA and derive the column densities of H2O, CO and CO2 ices., Comment: Accepted for publication in A&A. 29 pages, 9 figures. Online database at https://icedb.strw.leidenuniv.nl/

The composition of planets may be largely determined by the chemical processing and accretion of icy pebbles in protoplanetary disks. Recent observations of protoplanetary disks hint at wide-spread depletion of gaseous carbon. The missing volatile carbon is likely frozen in CO and/or CO$_2$ ice on grains and locked into the disk through pebble trapping in pressure bumps or planetesimals. We present the results of the first successful ACA (Atacama Compact Array) [C I] $J$ = 1-0 mini-survey of seven protoplanetary disks. Using tailored azimuthally symmetric DALI (Dust And LInes) thermo-chemical disk models, supported by the [C I] $J$ = 1-0 and resolved CO isotopologue data, we determine the system-averaged elemental volatile carbon abundance in the outer disk of three sources. Six out of seven sources are detected in [C I] $J$ = 1-0 with ACA, four of which show a distinct disk component. Based on the modeling we find severe cold gaseous carbon depletion in the outer disk of DL Tau and moderate depletion in the outer disks of DR Tau and DO Tau. Combining the outer and inner disk carbon abundances, we demonstrate definitive evidence for radial drift in the disk of DL Tau, where the existence of multiple dust rings points to either short lived or leaky dust traps. We find dust locking in the compact and smooth disks of DO Tau and DR Tau, hinting at unresolved dust substructure. Comparing our results with stars of different ages and luminosities, we identify an observational evolutionary trend in gaseous carbon depletion that is consistent with dynamical models of CO depletion processes. Transport efficiency of solids in protoplanetary disks can significantly differ from what we expect based on the current resolved substructure in the continuum observations. This has important implications for our understanding of the impact of radial drift and pebble accretion on planetary compositions., Comment: 23 pages, 19 figures, accepted to A&A

(Abridged) Recent surveys of young star formation regions have shown that the average Class II object does not have enough dust mass to make the cores of giant planets. Younger Class 0/I objects have enough dust in their embedded disk, which begs the questions: can the first steps of planet formation occur in these younger systems? The first step is building the first planetesimals, generally believed to be the product of the streaming instability. Hence the question can be restated: are the physical conditions of embedded disks conducive to the growth of the streaming instability? Here we model the collapse of a `dusty' proto-stellar cloud to show that if there is sufficient drift between the falling gas and dust, regions of the embedded disk can become sufficiently enhanced in dust to drive the streaming instability. We include four models, three with different dust grain sizes and one with a different initial cloud angular momentum to test a variety of collapse trajectories. We find a `sweet spot' for planetesimal formation for grain sizes of a few 10s of micron since they fall sufficiently fast relative to the gas to build a high dust-to-gas ratio along the disk midplane, but have slow enough radial drift speeds in the embedded disk to maintain the high dust-to-gas ratio. Unlike the gas, which is held in hydrostatic equilibrium for a time due to gas pressure, the dust can begin collapsing from all radii at a much earlier time. The streaming instability can produce at least between 7-35 M$_\oplus$ of planetesimals in the Class 0/I phase of our smooth embedded disks, depending on the size of the falling dust grains. This first generation of planetesimals could represent the first step in planet formation, and occurs earlier in the lifetime of the young star than is traditionally thought., Comment: Accepted for publication in A&A, 22 pages, 19 figures

In this era of spatially resolved observations of planet forming disks with ALMA and large ground-based telescopes such as the VLT, Keck and Subaru, we still lack statistically relevant information on the quantity and composition of the material that is building the planets, such as the total disk gas mass, the ice content of dust, and the state of water in planetesimals. SPICA is an infrared space mission concept developed jointly by JAXA and ESA to address these questions. The key unique capabilities of SPICA that enable this research are (1) the wide spectral coverage 10-220 micron, (2) the high line detection sensitivity of (1-2) 10-19 W m-2 with R~2000-5000 in the far-IR (SAFARI) and 10-20 W m-2 with R~29000 in the mid-IR (SMI, spectrally resolving line profiles), (3) the high far-IR continuum sensitivity of 0.45 mJy (SAFARI), and (4) the observing efficiency for point source surveys. This paper details how mid- to far-IR infrared spectra will be unique in measuring the gas masses and water/ice content of disks and how these quantities evolve during the planet forming period. These observations will clarify the crucial transition when disks exhaust their primordial gas and further planet formation requires secondary gas produced from planetesimals. The high spectral resolution mid-IR is also unique for determining the location of the snowline dividing the rocky and icy mass reservoirs within the disk and how the divide evolves during the build-up of planetary systems. Infrared spectroscopy (mid- to far-IR) of key solid state bands is crucial for assessing whether extensive radial mixing, which is part of our Solar System history, is a general process occurring in most planetary systems and whether extrasolar planetesimals are similar to our Solar System comets/asteroids. ... (abbreviated), Comment: accepted for publication in PASA

Gas-rich circumstellar disks are the cradles of planet formation. As such, their evolution will strongly influence the resulting planet population. In the ESO DESTINYS large program, we study these disks within the first 10 Myr of their development with near-infrared scattered light imaging. Here we present VLT/SPHERE polarimetric observations of the nearby class II system SU Aur in which we resolve the disk down to scales of ~7 au. In addition to the new SPHERE observations, we utilize VLT/NACO, HST/STIS and ALMA archival data. The new SPHERE data show the disk around SU Aur and extended dust structures in unprecedented detail. We resolve several dust tails connected to the Keplerian disk. By comparison with ALMA data, we show that these dust tails represent material falling onto the disk. The disk itself shows an intricate spiral structure and a shadow lane, cast by an inner, misaligned disk component. Our observations suggest that SU Aur is undergoing late infall of material, which can explain the observed disk structures. SU Aur is the clearest observational example of this mechanism at work and demonstrates that late accretion events can still occur in the class II phase, thereby significantly affecting the evolution of circumstellar disks. Constraining the frequency of such events with additional observations will help determine whether this process is responsible for the spin-orbit misalignment in evolved exoplanet systems., Comment: 18 pages, 12 figures, published in ApJL on 18-02-2021

Volatile molecules are critical to terrestrial planetary habitability, yet difficult to observe directly where planets form at the midplanes of protoplanetary disks. It is unclear whether the inner 1 AU of disks are volatile-poor or if this region is resupplied with ice-rich dust from colder disk regions. Dust traps at radial pressure maxima bounding disk gaps can cut off the inner disk from such volatile reservoirs. However, the trap retention efficiency and atomic composition of trapped dust have not been measured. We present a new technique to measure the absolute atomic abundances in gas accreting onto T Tauri stars and infer the bulk atomic composition and distribution of midplane solids retained in the disk around the young star TW Hya. We identify line emission from gas-phase material inside the dust sublimation rim of TW Hya. Gaussian decomposition of the strongest H Paschen lines isolates the inner disk hydrogen emission. We measure several key elemental abundances, relative to hydrogen, using a chemical photoionization model and infer dust retention in the disk. With a 1D transport model, we determine approximate radial locations and retention efficiencies of dust traps for different elements. Volatile and refractory elements are depleted from TW Hya's hot gas by factors of ~10^2 and up to 10^5, respectively. Dust traps beyond the CO and N2 snowline cumulatively sequester 96% of the total dust, while the trap at 2 AU retains 3% of the initial dust mass. The high depletions of Si, Mg, and Ca are explained by a third trap at 0.3 AU. TW Hya has a significant volatile reservoir rich in C- and N-ices in its outer ring structure. However, the lack of C resupply may leave the terrestrial planet-forming region dry and carbon-poor. Any planets that form within the silicate dust trap at 0.3 AU may resemble Earth in terms of the degree of their volatile depletion., Comment: Accepted for publication in A&A Letters. 6 pages, 3 figures, plus appendices

The carbon content of protoplanetary disks is an important parameter to characterize planets formed at different disk radii. There is some evidence from far-infrared and submillimeter observations that gas in the outer disk is depleted in carbon, with a corresponding enhancement of carbon-rich ices at the disk midplane. Observations of the carbon content inside of the inner sublimation rim could confirm how much carbon remains locked in kilometer size bodies in the disk. Using near-infrared spectra, I self-consistently determine the stellar, extinction, veiling, and accretion properties of the 26 stars in my sample. From these values, I extract the inner disk excess of the target stars and identify a series of C^0 recombination lines in 18 of these disks and use the CHIANTI atomic line database with an optically thin slab model to constrain the average n_e, T_e, and n_C for these lines in the five disks with a complete set of lines. By comparing these values with other slab models of the inner disk using the Cloudy photoionization code, I also constrain n_H and the carbon abundance, X_C, and hence the amount of carbon `missing' from the slab. The inner disks are extremely dense (n_H~10^{16} cm^{-3}), warm (T_e~4500 K), and moderately ionized (logX_e~3.3). Three of the five modeled disks show robust carbon depletion <=42 relative to the solar value. I discuss multiple ways in which the `missing' carbon could be locked out of the accreting gas. Given the high-density inner disk gas, evidence for radial drift, and lack of obvious gaps in these three systems, their carbon depletion is most consistent with the `missing' carbon being sequestered in kilometer size bodies. For DR Tau, nitrogen and silicon are also depleted by factors of 45 and 4, respectively, suggesting that the kilometer size bodies into which the grains are locked were formed beyond the N_2 snowline., Comment: 14 pages, 7 figures, appendices, accepted to A&A July 16th, 2019

The point at which a protoplanetary disk becomes a debris disk is difficult to identify. To better understand this, here we study the $\sim$40~AU separation binary T~54 in the Chamaeleon I cloud. We derive a K5 spectral type for T~54~A (which dominates the emission of the system) and an age of $\sim$2~Myr. However, the dust disk properties of T~54 are consistent with those of debris disks seen around older and earlier-type stars. At the same time, T~54 has evidence of gas remaining in the disk as indicated by [Ne II], [Ne III], and [O I] line detections. We model the spectral energy distribution of T~54 and estimate that $\sim$3$\times$10$^{-3}$ Earth-masses of small dust grains ($<$0.25~$\mu$m) are present in an optically thin circumbinary disk along with at least $\sim$3$\times$10$^{-7}$ Earth-masses of larger ($>$10~$\mu$m) grains within a circumprimary disk. Assuming a solar-like mixture, we use Ne line luminosities to place a minimum limit on the gas mass of the disk ($\sim$3$\times$10$^{-4}$ Earth-masses) and derive a gas-to-dust mass ratio of $\sim$0.1. We do not detect substantial accretion, but we do see H$\alpha$ in emission in one epoch, suggestive that there may be intermittent dumping of small amounts of matter onto the star. Considering the low dust mass, the presence of gas, and young age of T~54, we conclude that this system is on the bridge between the protoplanetary and debris disk stages., Comment: 15 pages, 9 figures, accepted to ApJ

In order to look for signs of on-going planet formation in young disks, we carried out the first J-band polarized emission imaging of the Herbig Ae/Be stars HD 150193, HD 163296, and HD 169142 using the Gemini Planet Imager (GPI), along with new H band observations of HD 144432. We confirm the complex "double ring" structure for the nearly face-on system HD 169142 first seen in H-band, finding the outer ring to be substantially redder than the inner one in polarized intensity. Using radiative transfer modeling, we developed a physical model that explains the full spectral energy distribution (SED) and J- and H-band surface brightness profiles, suggesting that the differential color of the two rings could come from reddened starlight traversing the inner wall and may not require differences in grain properties. In addition, we clearly detect an elongated, off-center ring in HD 163296 (MWC 275), locating the scattering surface to be 18 AU above the midplane at a radial distance of 77 AU, co-spatial with a ring seen at 1.3mm by ALMA linked to the CO snow line. Lastly, we report a weak tentative detection of scattered light for HD 150193 (MWC 863) and a non-detection for HD 144432; the stellar companion known for each of these targets has likely disrupted the material in the outer disk of the primary star. For HD 163296 and HD 169142, the prominent outer rings we detect could be evidence for giant planet formation in the outer disk or a manifestation of large-scale dust growth processes possibly related to snow-line chemistry., Comment: Accepted Astrophysical Journal

We present our investigation of 319 Class II objects in Orion A observed by $Spitzer$/IRS. We also present the follow-up observation of 120 of these Class II objects in Orion A from IRTF/SpeX. We measure continuum spectral indices, equivalent widths, and integrated fluxes that pertain to disk structure and dust composition from IRS spectra of Class II objects in Orion A. We estimate mass accretion rates using hydrogen recombination lines in the SpeX spectra of our targets. Utilizing these properties, we compare the distributions of the disk and dust properties of Orion A disks to those of Taurus disks with respect to position within Orion A (ONC and L1641) and to the sub-groups by the inferred radial structures, such as transitional disks vs. radially continuous full disks. Our main findings are as follows. (1) Inner disks evolve faster than the outer disks. (2) Mass accretion rate of transitional disks and that of radially continuous full disks are statistically significantly displaced from each other. The median mass accretion rate of radially continuous disks in ONC and L1641 is not very different from that in Taurus. (3) Less grain processing has occurred in the disks in ONC compared to those in Taurus, based on analysis of the shape index of the 10 $\mu$m silicate feature ($F_{11.3}/F_{9.8}$). (4) The 20-31 $\mu$m continuum spectral index tracks the projected distance from the most luminous Trapezium star, $\theta^{1}$ Ori C. A possible explanation is the effect of UV ablation of the outer part of the disks., Comment: 152 pages, 28 figures, 11 tables

We present Herschel Space Observatory PACS spectra of T Tauri stars, in which we detect amorphous and crystalline water ice features. Using irradiated accretion disk models, we determine the disk structure and ice abundance in each of the systems. Combining a model-independent comparison of the ice feature strength and disk size with a detailed analysis of the model ice location, we estimate that the ice emitting region is at disk radii >30AU, consistent with a proto-Kuiper belt. Vertically, the ice emits most below the photodesorption zone, consistent with Herschel observations of cold water vapor. The presence of crystallized water ice at a disk location a) colder than its crystallization temperature and b) where it should have been re-amorphized in ~1 Myr suggests that localized generation is occurring; the most likely cause appears to be micrometeorite impact or planetesimal collisions. Based on simple tests with UV models and different ice distributions, we suggest that the SED shape from 20 to 50 micron may probe the location of the water ice snow line in the disk upper layers. This project represents one of the first extra-solar probes of the spatial structure of the cometary ice reservoir thought to deliver water to terrestrial planets., Comment: Accepted to ApJ on Nov. 20th 2014; 16 pages, including 11 figures and 4 tables

We present sensitive, sub-arcsecond resolution Submillimeter Array observations of the protoplanetary disk around the nearby, pre-main sequence spectroscopic binary V4046 Sgr. We report for the first time a large inner hole (r=29 AU) spatially resolved in the 1.3 mm continuum emission and study the structure of this disk using radiative transfer calculations to model the spectral energy distribution (SED), continuum visibilities, and spectral line emission of CO and its main isotopologues. Our modeling scheme demonstrates that the majority of the dust mass is distributed in a narrow ring (centered at 37 AU with a FWHM of 16 AU) that is ~5 times more compact than the gas disk. This structure implies that the dust-to-gas mass ratio has a strong spatial variation, ranging from a value much larger than typical of the interstellar medium (ISM) at the ring to much smaller than that of the ISM at larger disk radii. We suggest that these basic structural features are potentially observational signatures of the accumulation of solids at a local gas pressure maximum. These models also require a substantial population of ~micron-sized grains inside the central disk cavity. We suggest that this structure is likely the result of dynamical interactions with a low-mass companion, although photoevaporation may also play a secondary role., Comment: 31 pages, 8 figures, accepted for publication in ApJ

The dust sublimation walls of disks around T Tauri stars represent a directly observable cross-section through the disk atmosphere and midplane. Their emission properties can probe the grain size distribution and composition of the innermost regions of the disk, where terrestrial planets form. Here we calculate the inner dust sublimation wall properties for four classical T Tauri stars with a narrow range of spectral types and inclination angles and a wide range of mass accretion rates to determine the extent to which the walls are radially curved. Best-fits to the near- and mid-IR excesses are found for curved, 2-layer walls in which the lower layer contains larger, hotter, amorphous pyroxene grains with Mg/(Mg+Fe)=0.6 and the upper layer contains submicron, cooler, mixed amorphous olivine and forsterite grains. As the mass accretion rates decrease from 10^(-8) to 10^(-10) Msol/yr, the maximum grain size in the lower layer decreases from 3 to 0.5 microns. We attribute this to a decrease in fragmentation and turbulent support for micron-sized grains with decreasing viscous heating. The atmosphere of these disks is depleted of dust with dust-gas mass ratios 1x10^(-4) of the ISM value, while the midplane is enhanced to 8 times the ISM value. For all accretion rates, the wall contributes at least half of the flux in the optically thin 10 micron silicate feature. Finally, we find evidence for an iron gradient in the disk, suggestive of that found in our solar system., Comment: 18 pages, 12 figures, 1 table, accepted to ApJ

Using NASA IRTF SpeX data from 0.8 to 4.5 $\mu$m, we determine self-consistently the stellar properties and excess emission above the photosphere for a sample of classical T Tauri stars (CTTS) in the Taurus molecular cloud with varying degrees of accretion. This process uses a combination of techniques from the recent literature as well as observations of weak-line T Tauri stars (WTTS) to account for the differences in surface gravity and chromospheric activity between the TTS and dwarfs, which are typically used as photospheric templates for CTTS. Our improved veiling and extinction estimates for our targets allow us to extract flux-calibrated spectra of the excess in the near-infrared. We find that we are able to produce an acceptable parametric fit to the near-infrared excesses using a combination of up to three blackbodies. In half of our sample, two blackbodies at temperatures of 8000 K and 1600 K suffice. These temperatures and the corresponding solid angles are consistent with emission from the accretion shock on the stellar surface and the inner dust sublimation rim of the disk, respectively. In contrast, the other half requires three blackbodies at 8000, 1800, and 800 K, to describe the excess. We interpret the combined two cooler blackbodies as the dust sublimation wall with either a contribution from the disk surface beyond the wall or curvature of the wall itself, neither of which should have single-temperature blackbody emission. In these fits, we find no evidence of a contribution from optically thick gas inside the inner dust rim., Comment: Accepted to ApJ, 22 pages, 2 appendices, 12 figures, 7 tables

High-energy radiation from T Tauri stars (TTS) influences the amount and longevity of gas in disks, thereby playing a crucial role in the creation of gas giant planets. Here we probe the high-energy ionizing radiation from TTS using high-resolution mid-infrared (MIR) Spitzer IRS Neon forbidden line detections in a sample of disks from IC 348, NGC 2068, and Chamaeleon. We report three new detections of [Ne III] from CS Cha, SZ Cha, and T 54, doubling the known number of [Ne III] detections from TTS. Using [Ne III]-to-[Ne II] ratios in conjunction with X-ray emission measurements, we probe high-energy radiation from TTS. The majority of previously inferred [Ne III]/[Ne II] ratios based on [Ne III] line upper limits are significantly less than 1, pointing to the dominance of either X-ray radiation or soft Extreme-Ultraviolet (EUV) radiation in producing these lines. Here we report the first observational evidence for hard EUV dominated Ne forbidden line production in a T Tauri disk: [Ne III]/[Ne II]~1 in SZ Cha. Our results provide a unique insight into the EUV emission from TTS, by suggesting that EUV radiation may dominate the creation of Ne forbidden lines, albeit in a minority of cases., Comment: 16 pages, accepted ApJ

We present Herschel Space Observatory PACS spectra of GQ Lup, a protoplanetary disk in the Lupus star-forming region. Through SED fitting from 0.3{\mu}m to 1.3mm, we construct a self-consistent model of this system's temperature and density structures, finding that although it is 3 Myr old, its dust has not settled to the midplane substantially. The disk has a radial gradient in both the silicate dust composition and grain size, with large amorphous grains in the upper layers of the inner disk and an enhancement of submicron, crystalline grains in the outer disk. We detect an excess of emission in the Herschel PACS B2A band near 63{\mu}m and model it with a combination of {\sim}15 to 70{\mu}m crystalline water ice grains with a size distribution consistent with ice recondensation-enhanced grain growth and a mass fraction half of that of our solar system. The combination of crystalline water ice and silicates in the outer disk is suggestive of disk-wide heating events or planetesimal collisions. If confirmed, this would be the first detection of water ice by Herschel., Comment: 8 pages, 3 figures, 1 table, accepted to ApJ Letters

We present high resolution H-band polarized intensity (PI; FWHM = 0."1: 14 AU) and L'-band imaging data (FWHM = 0."11: 15 AU) of the circumstellar disk around the weak-lined T Tauri star PDS 70 in Centaurus at a radial distance of 28 AU (0."2) up to 210 AU (1."5). In both images, a giant inner gap is clearly resolved for the first time, and the radius of the gap is ~70 AU. Our data show that the geometric center of the disk shifts by ~6 AU toward the minor axis. We confirm that the brown dwarf companion candidate to the north of PDS 70 is a background star based on its proper motion. As a result of SED fitting by Monte Carlo radiative transfer modeling, we infer the existence of an optically thick inner disk at a few AU. Combining our observations and modeling, we classify the disk of PDS 70 as a pre-transitional disk. Furthermore, based on the analysis of L'-band imaging data, we put an upper limit mass of companions at ~30 to ~50MJ within the gap. Taking account of the presence of the large and sharp gap, we suggest that the gap could be formed by dynamical interactions of sub-stellar companions or multiple unseen giant planets in the gap., Comment: accepted by APJL

We report on the {\lambda} = 5-36{\mu}m Spitzer Infrared Spectrograph spectra of 79 young stellar objects in the very young nearby cluster NGC 1333. NGC 1333's youth enables the study of early protoplanetary disk properties, such as the degree of settling as well as the formation of gaps and clearings. We construct spectral energy distributions (SEDs) using our IRS data as well as published photometry and classify our sample into SED classes. Using "extinction-free" spectral indices, we determine whether the disk, envelope, or photosphere dominates the spectrum. We analyze the dereddened spectra of objects which show disk dominated emission using spectral indices and properties of silicate features in order to study the vertical and radial structure of protoplanetary disks in NGC 1333. At least nine objects in our sample of NGC 1333 show signs of large (several AU) radial gaps or clearings in their inner disk. Disks with radial gaps in NGC 1333 show more-nearly pristine silicate dust than their radially continuous counterparts. We compare properties of disks in NGC 1333 to those in three other well studied regions, Taurus-Auriga, Ophiuchus and Chamaeleon I, and find no difference in their degree of sedimentation and dust processing., Comment: 67 pages, 20 figures, accepted to The Astrophysical Journal Supplement Series

Circumstellar disks are thought to experience a rapid "transition" phase in their evolution that can have a considerable impact on the formation and early development of planetary systems. We present new and archival high angular resolution (0.3" = 40-75 AU) Submillimeter Array (SMA) observations of the 880 micron dust continuum emission from 12 such transition disks in nearby star-forming regions. In each case, we directly resolve a dust-depleted disk cavity around the central star. Using radiative transfer calculations, we interpret these dust disk structures in a homogeneous, parametric model framework by reproducing their SMA visibilities and SEDs. The cavities in these disks are large (R_cav = 15-73 AU) and substantially depleted of small (~um-sized) dust grains, although their mass contents are still uncertain. The structures of the remnant material at larger radii are comparable to normal disks. We demonstrate that these large cavities are common among the millimeter-bright disk population, comprising at least 20% of the disks in the bright half of the millimeter luminosity (disk mass) distribution. Utilizing these results, we assess some of the physical mechanisms proposed to account for transition disk structures. As has been shown before, photoevaporation models do not produce the large cavity sizes, accretion rates, and disk masses representative of this sample. It would be difficult to achieve a sufficient decrease of the dust optical depths in these cavities by particle growth alone: substantial growth (to meter sizes or beyond) must occur in large (tens of AU) regions of low turbulence without also producing an abundance of small particles. Given those challenges, we suggest instead that the observations are most commensurate with dynamical clearing due to tidal interactions with low-mass companions --young brown dwarfs or giant planets on long-period orbits., Comment: ApJ, in press

We present 5 to 36 micron mid-infrared spectra of 82 young stars in the ~2 Myr old Chamaeleon I star-forming region, obtained with the Spitzer Infrared Spectrograph (IRS). We have classified these objects into various evolutionary classes based on their spectral energy distributions and the spectral features seen in the IRS spectra. We have analyzed the mid-IR spectra of Class II objects in Chamaeleon I in detail, in order to study the vertical and radial structure of the protoplanetary disks surrounding these stars. We find evidence for substantial dust settling in most protoplanetary disks in Chamaeleon I. We have identified several disks with altered radial structures in Chamaeleon I, among them transitional disk candidates which have holes or gaps in their disks. Analysis of the silicate emission features in the IRS spectra of Class II objects in Chamaeleon I shows that the dust grains in these disks have undergone significant processing (grain growth and crystallization). However, disks with radial holes/gaps appear to have relatively unprocessed grains. We further find the crystalline dust content in the inner (< 1-2 AU) and the intermediate (< 10 AU) regions of the protoplanetary disks to be tightly correlated. We also investigate the effects of accretion and stellar multiplicity on the disk structure and dust properties. Finally, we compare the observed properties of protoplanetary disks in Cha I with those in slightly younger Taurus and Ophiuchus regions and discuss the effects of disk evolution in the first 1-2 Myr., Comment: 80 pages, 32 Figures, Accepted for publication in the Astrophysical Journal Supplement Series

Using the Spitzer Infrared Spectrograph, we have performed mid-infrared spectroscopy on the young binary brown dwarf 2MASS J04414489+2301513 (15 AU) in the Taurus star-forming region. The spectrum exhibits excess continuum emission that likely arises from a circumstellar disk around the primary. Silicate emission is not detected in these data, indicating the presence of significant grain growth. This is one of the few brown dwarf disks at such a young age (~1 Myr) that has been found to lack silicate emission. To quantitatively constrain the properties of the disk, we have compared the spectral energy distribution of 2MASS J04414489+2301513 to the predictions of our vertical structure codes for irradiated accretion disks. Our models suggest that the remaining atmospheric grains of moderately depleted layers may have grown to a size of $\gtrsim5$ micron. In addition, our model fits indicate an outer radius of 0.2-0.3 AU for the disk. The small size of this circumprimary disk could be due to truncation by the secondary. The absence of an outer disk containing a reservoir of small, primordial grains, combined with a weak turbulent mechanism, may be responsible for the advanced grain growth in this disk., Comment: Accepted for publication in the ApJL, 14 pages, 3 figures

In the past few years, several disks with inner holes that are empty of small dust grains have been detected and are known as transitional disks. Recently, Spitzer has identified a new class of "pre-transitional disks" with gaps; these objects have an optically thick inner disk separated from an optically thick outer disk by an optically thin disk gap. A near-infrared spectrum provided the first confirmation of a gap in the pre-transitional disk of LkCa 15 by verifying that the near-infrared excess emission in this object was due to an optically thick inner disk. Here we investigate the difference between the nature of the inner regions of transitional and pre-transitional disks using the same veiling-based technique to extract the near-infrared excess emission above the stellar photosphere. We show that the near-infrared excess emission of the previously identified pre-transitional disks of LkCa 15 and UX Tau A in Taurus as well as the newly identified pre-transitional disk of ROX 44 in Ophiuchus can be fit with an inner disk wall located at the dust destruction radius. We also model the broad-band SEDs of these objects, taking into account the effect of shadowing by the inner disk on the outer disk, considering the finite size of the star. The near-infrared excess continua of these three pre-transitional disks, which can be explained by optically thick inner disks, are significantly different from that of the transitional disks of GM Aur, whose near-infrared excess continuum can be reproduced by emission from sub-micron-sized optically thin dust, and DM Tau, whose near-infrared spectrum is consistent with a disk hole that is relatively free of small dust. The structure of pre-transitional disks may be a sign of young planets forming in these disks and future studies of pre-transitional disks will provide constraints to aid in theoretical modeling of planet formation., Comment: Accepted for publication in ApJ on May 10, 2010; 29 pages

We analyze samples of Spitzer Infrared Spectrograph (IRS) spectra of T Tauri stars in the Ophiuchus, Taurus, and Chamaeleon I star-forming regions, whose median ages lie in the <1 to 2 Myr range. The median mid-infrared spectra of objects in these three regions are similar in shape, suggesting, on average, similar disk structures. When normalized to the same stellar luminosity, the medians follow each other closely, implying comparable mid-infrared excess emission from the circumstellar disks. We use the spectral index between 13 and 31 micron and the equivalent width of the 10 micron silicate emission feature to identify objects whose disk configuration departs from that of a continuous, optically thick accretion disk. Transitional disks, whose steep 13-31 micron spectral slope and near-IR flux deficit reveal inner disk clearing, occur with about the same frequency of a few percent in all three regions. Objects with unusually large 10 micron equivalent widths are more common (20-30%); they could reveal the presence of disk gaps filled with optically thin dust. Based on their medians and fraction of evolved disks, T Tauri stars in Taurus and Chamaeleon I are very alike. Disk evolution sets in early, since already the youngest region, the Ophiuchus core (L1688), has more settled disks with larger grains. Our results indicate that protoplanetary disks show clear signs of dust evolution at an age of a few Myr, even as early as ~1 Myr, but age is not the only factor determining the degree of evolution during the first few million years of a disk's lifetime., Comment: 23 pages, 20 figures; accepted for publication in ApJ

Mid-infrared spectra of 65 T Tauri stars (TTS) taken with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope are modeled using dust at two temperatures to probe the radial variation in dust composition in the uppermost layers of protoplanetary disks. Most spectra indicating crystalline silicates require Mg-rich minerals and silica, but a few suggest otherwise. Spectra indicating abundant enstatite at higher temperatures also require crystalline silicates at temperatures lower than those required for spectra showing high abundance of other crystalline silicates. A few spectra show 10 micron complexes of very small equivalent width. They are fit well using abundant crystalline silicates but very few large grains, inconsistent with the expectation that low peak-to-continuum ratio of the 10 micron complex always indicates grain growth. Most spectra in our sample are fit well without using the opacities of large crystalline silicate grains. If large grains grow by agglomeration of submicron grains of all dust types, the amorphous silicate components of these aggregates must typically be more abundant than the crystalline silicate components. Crystalline silicate abundances correlate positively with other such abundances, suggesting that crystalline silicates are processed directly from amorphous silicates and that neither forsterite, enstatite, nor silica are intermediate steps when producing either of the other two. Disks with more dust settling typically have greater crystalline abundances. Large-grain abundance is somewhat correlated with greater settling of disks. The lack of strong correlation is interpreted to mean that settling of large grains is sensitive to individual disk properties. Lower-mass stars have higher abundances of large grains in their inner regions., Comment: 84 pages, 27 figures, submitted to the Astrophysical Journal on 7 November, 2008