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Register a new userALMA imaging of the CO snowline of the HD 163296 disk with DCO+
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The high spatial and line sensitivity of ALMA opens the possibility of resolving emission from molecules in circumstellar disks. With an understanding of physical conditions under which molecules have high abundance, they can be used as direct tracers of distinct physical regions. In particular, DCO+ is expected to have an enhanced abundance within a few Kelvin of the CO freezeout temperature of 19 K, making it a useful probe of the cold disk midplane. We compare ALMA line observations of HD 163296 to a grid of models. We vary the upper- and lower-limit temperatures of the region in which DCO+ is present as well as the abundance of DCO+ in order to fit channel maps of the DCO+ J=5-4 line. To determine the abundance enhancement compared to the general interstellar medium, we carry out similar fitting to HCO+ J=4-3 and H13CO+ J=4-3 observations. ALMA images show centrally peaked extended emission from HCO+ and H13CO+. DCO+ emission lies in a resolved ring from ~110 to 160 AU. The outer radius approximately corresponds to the size of the CO snowline as measured by previous lower resolution observations of CO lines in this disk. The ALMA DCO+ data now resolve and image the CO snowline directly. In the best fitting models, HCO+ exists in a region extending from the 19 K isotherm to the photodissociation layer with an abundance of 3x10^-10 relative to H2. DCO+ exists within the 19-21 K region of the disk with an abundance ratio [DCO+] / [HCO+] = 0.3. This represents a factor of 10^4 enhancement of the DCO+ abundance within this narrow region of the HD 163296 disk. Such a high enhancement has only previously been seen in prestellar cores. The inferred abundances provide a lower limit to the ionization fraction in the midplane of the cold outer disk (approximately greater than 4x10^-10), and suggest the utility of DCO+ as a tracer of its parent molecule H2D+. Abridged
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To characterize the mechanisms of planet formation it is crucial to investigate the properties and evolution of protoplanetary disks around young stars, where the initial conditions for the growth of planets are set. Our goal is to study grain growth in the disk of the young, intermediate mass star HD163296 where dust processing has already been observed, and to look for evidence of growth by ice condensation across the CO snowline, already identified in this disk with ALMA. Under the hypothesis of optically thin emission we compare images at different wavelengths from ALMA and VLA to measure the opacity spectral index across the disk and thus the maximum grain size. We also use a Bayesian tool based on a two-layer disk model to fit the observations and constrain the dust surface density. The measurements of the opacity spectral index indicate the presence of large grains and pebbles ($geq$1 cm) in the inner regions of the disk (inside $sim$50 AU) and smaller grains, consistent with ISM sizes, in the outer disk (beyond 150 AU). Re-analysing ALMA Band 7 Science Verification data we find (radially) unresolved excess continuum emission centered near the location of the CO snowline at $sim$90 AU. Our analysis suggests a grain size distribution consistent with an enhanced production of large grains at the CO snowline and consequent transport to the inner regions. Our results combined with the excess in infrared scattered light found by Garufi et al. (2014) suggests the presence of a structure at 90~AU involving the whole vertical extent of the disk. This could be evidence for small scale processing of dust at the CO snowline.
The condensation fronts (snow lines) of H2O, CO and other abundant volatiles in the midplane of a protoplanetary disk affect several aspects of planet formation. Locating the CO snow line, where the CO gas column density is expected to drop substantially, based solely on CO emission profiles is challenging. This has prompted an exploration of chemical signatures of CO freeze-out. We present ALMA Cycle 1 observations of the N2H+ J=3-2 and DCO+ J=4-3 emission lines toward the disk around the Herbig Ae star HD~163296 at ~0.5 (60 AU) resolution, and evaluate their utility as tracers of the CO snow line location. The N2H+ emission is distributed in a ring with an inner radius at 90 AU, corresponding to a midplane temperature of 25 K. This result is consistent with a new analysis of optically thin C18O data, which implies a sharp drop in CO abundance at 90 AU. Thus N2H+ appears to be a robust tracer of the midplane CO snow line. The DCO+ emission also has a ring morphology, but neither the inner nor the outer radius coincides with the CO snow line location of 90 AU, indicative of a complex relationship between DCO+ emission and CO freeze-out in the disk midplane. Compared to TW Hya, CO freezes out at a higher temperature in the disk around HD 163296 (25 vs. 17 K in the TW Hya disk), perhaps due to different ice compositions. This highlights the importance of actually measuring the CO snow line location, rather than assuming a constant CO freeze-out temperature for all disks.
We report Submillimeter Array (SMA) observations of CO (J=2--1, 3--2 and 6--5) and its isotopologues (13CO J=2--1, C18O J=2--1 and C17O J=3--2) in the disk around the Herbig Ae star HD 163296 at ~2 (250 AU) resolution, and interpret these data in the framework of a model that constrains the radial and vertical location of the line emission regions. First, we develop a physically self-consistent accretion disk model with an exponentially tapered edge that matches the spectral energy distribution and spatially resolved millimeter dust continuum emission. Then, we refine the vertical structure of the model using wide range of excitation conditions sampled by the CO lines, in particular the rarely observed J=6--5 transition. By fitting 13CO data in this structure, we further constrain the vertical distribution of CO to lie between a lower boundary below which CO freezes out onto dust grains (T ~ 19 K) and an upper boundary above which CO can be photodissociated (the hydrogen column density from the disk surface is ~ 10^{21} cm-2). The freeze-out at 19 K leads to a significant drop in the gas-phase CO column density beyond a radius of ~155 AU, a CO snow line that we directly resolve. By fitting the abundances of all CO isotopologues, we derive isotopic ratios of 12C/13C, 16O/18O and 18O/17O that are consistent with quiescent interstellar gas-phase values. This detailed model of the HD 163296 disk demonstrates the potential of a staged, parametric technique for constructing unified gas and dust structure models and constraining the distribution of molecular abundances using resolved multi-transition, multi-isotope observations.
We report Very Large Array observations at 7 mm, 9 mm, and 3 cm toward the pre-transitional disk of the Herbig Ae star HD 169142. These observations have allowed us to study the mm emission of this disk with the highest angular resolution so far ($0rlap.12times0rlap.09$, or 14 au$times$11 au, at 7 mm). Our 7 and 9 mm images show a narrow ring of emission at a radius of $sim25$ au tracing the outer edge of the inner gap. This ring presents an asymmetric morphology that could be produced by dynamical interactions between the disk and forming planets. Additionally, the azimuthally averaged radial intensity profiles of the 7 and 9 mm images confirm the presence of the previously reported gap at $sim45$ au, and reveal a new gap at $sim85$ au. We analyzed archival DCO$^+$(3-2) and C$^{18}$O(2-1) ALMA observations, showing that the CO snowline is located very close to this third outer gap. This suggests that growth and accumulation of large dust grains close to the CO snowline could be the mechanism responsible for this proposed outer gap. Finally, a compact source of emission is detected at 7 mm, 9 mm, and 3 cm toward the center of the disk. Its flux density and spectral index indicate that it is dominated by free-free emission from ionized gas, which could be associated with either the photoionization of the inner disk, an independent object, or an ionized jet.
This work aims to understand which midplane conditions are probed by the DCO$^+$ emission in the disk around the Herbig Ae star HD 169142. We explore the sensitivity of the DCO$^+$ formation pathways to the gas temperature and the CO abundance. The DCO$^+$ $J$=3-2 transition was observed with ALMA at a spatial resolution of 0.3. The HD 169142 DCO$^+$ radial intensity profile reveals a warm, inner component at radii <30 AU and a broad, ring-like structure from ~50-230 AU with a peak at 100 AU just beyond the millimeter grain edge. We modeled DCO$^+$ emission in HD 169142 with a physical disk structure adapted from the literature, and employed a simple deuterium chemical network to investigate the formation of DCO$^+$ through the cold deuterium fractionation pathway via H$_2$D$^+$. Contributions from the warm deuterium fractionation pathway via CH$_2$D$^+$ are approximated using a constant abundance in the intermediate disk layers. Parameterized models show that alterations to the midplane gas temperature and CO abundance of the literature model are both needed to recover the observed DCO$^+$ radial intensity profile. The best-fit model contains a shadowed, cold midplane in the region z/r < 0.1 with an 8 K decrease in gas temperature and a factor of five CO depletion just beyond the millimeter grain edge, and a 2 K decrease in gas temperature for r > 120 AU. The warm deuterium fractionation pathway is implemented as a constant DCO$^+$ abundance of 2.0$times$10$^{-12}$ between 30-70 K. The DCO$^+$ emission probes a reservoir of cold material in the HD 169142 outer disk that is not revealed by the millimeter continuum, the SED, nor the emission from the 12CO, 13CO, or C18O $J$=2-1 lines.
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