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  1. Abstract

    Upcoming new coronographs with deeper contrast limits, together with planned and current high-contrast imaging campaigns, will push the detectability limit of protoplanets. These planet-hunting campaigns present a new opportunity to characterize protoplanets and their surrounding environments. However, there are clear uncertainties as to what the extinction levels are at different regions of protoplanetary disks, which will impede our ability to characterize young planets. A correct understanding of the expected extinction, together with multiple photometric observations, will lead to constraints on the extinction levels, dust growth, disk evolution, and protoplanetary accretion rates. In this work, we used hydrodynamic simulations and protoplanetary disk observational constraints obtained from both dust and gas emission to explore the expected extinction maps for continuum filters associated with strong hydrogen lines as tracers of accretion and key broadband photometric filters. We provide a scaling relationship for the extinction as a function of planetary separation and disk mass for three different gas giant masses. We also report values for a subset of disks of interest targeted by multiple imaging campaigns. The described values will be useful for the optimal design of future planet-hunting surveys and for giving context to nondetections in protoplanetary disks and the observed fluxes of point sources along with the birth conditions of protoplanets.

     
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  2. Abstract

    We present the first detection of13CCH in a protoplanetary disk (TW Hya). Using observations of C2H, we measure CCH/13CCH = 65 ± 20 gas with a CO isotopic ratio of12CO/13CO = 21 ± 5. The TW Hya disk exhibits a gas phase C/O that exceeds unity, and C2H is the tracer of this excess carbon. We confirm that the TW Hya gaseous disk exhibits two separate carbon isotopic reservoirs, as noted previously. We explore two theoretical solutions for the development of this dichotomy. One model represents TW Hya today with a protoplanetary disk exposed to a cosmic-ray ionization rate that is below interstellar as consistent with current estimates. We find that this model does not have sufficient ionization in cold (T< 40 K) layers to activate carbon isotopic fractionation. The second model investigates a younger TW Hya protostellar disk exposed to an interstellar cosmic-ray ionization rate. We find that the younger model has sources of ionization deeper in a colder disk that generates two independent isotopic reservoirs. One reservoir is12C-enriched carried by methane/hydrocarbon ices, and the other is13C-enriched carried by gaseous CO. The former potentially provides a source of methane/hydrocarbon ices to power the chemistry that generates the anomalously strong C2H emission in this (and other) disk systems in later stages. The latter provides a source of gaseous13C-rich material to generate isotopic enrichments in forming giant planets, as recently detected in the super-Jupiter TYC 8998-760-1 b by Zhang et al.

     
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  3. Abstract

    Molecular emission is used to investigate both the physical and chemical properties of protoplanetary disks. Therefore, to derive disk properties accurately, we need a thorough understanding of the behavior of the molecular probes upon which we rely. Here we investigate how the molecular line emission of N2H+, HCO+, HCN, and C18O compare to other measured quantities in a set of 20 protoplanetary disks. Overall, we find positive correlations between multiple line fluxes and the disk dust mass and radius. We also generally find strong positive correlations between the line fluxes of different molecular species. However, some disks do show noticeable differences in the relative fluxes of N2H+, HCO+, HCN, and C18O. These differences occur even within a single star-forming region. This results in a potentially large range of different disk masses and chemical compositions for systems of similar age and birth environment. While we make preliminary comparisons of molecular fluxes across different star-forming regions, more complete and uniform samples are needed in the future to search for trends with birth environment or age.

     
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  4. Abstract

    Observations of substructure in protoplanetary disks have largely been limited to the brightest and largest disks, excluding the abundant population of compact disks, which are likely sites of planet formation. Here, we reanalyze ∼0.″1, 1.33 mm Atacama Large Millimeter/submillimeter Array (ALMA) continuum observations of 12 compact protoplanetary disks in the Taurus star-forming region. By fitting visibilities directly, we identify substructures in six of the 12 compact disks. We then compare the substructures identified in the full Taurus sample of 24 disks in single-star systems and the ALMA DSHARP survey, differentiating between compact (Reff,90%< 50 au) and extended (Reff,90%≥50 au) disk sources. We find that substructures are detected at nearly all radii in both small and large disks. Tentatively, we find fewer wide gaps in intermediate-sized disks withReff,90%between 30 and 90 au. We perform a series of planet–disk interaction simulations to constrain the sensitivity of our visibility-fitting approach. Under the assumption of planet–disk interaction, we use the gap widths and common disk parameters to calculate potential planet masses within the Taurus sample. We find that the young planet occurrence rate peaks near Neptune masses, similar to the DSHARP sample. For 0.01MJ/MMp/M*≲0.1MJ/M, the rate is 17.4% ± 8.3%; for 0.1MJ/MMp/M*≲1MJ/M, it is 27.8% ± 8.3%. Both of them are consistent with microlensing surveys. For gas giants more massive than 5MJ, the occurrence rate is 4.2% ± 4.2%, consistent with direct imaging surveys.

     
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  5. Abstract We explore terrestrial planet formation with a focus on the supply of solid-state organics as the main source of volatile carbon. For the water-poor Earth, the water ice line, or ice sublimation front, within the planet-forming disk has long been a key focal point. We posit that the soot line, the location where solid-state organics are irreversibly destroyed, is also a key location within the disk. The soot line is closer to the host star than the water snow line and overlaps with the location of the majority of detected exoplanets. In this work, we explore the ultimate atmospheric composition of a body that receives a major portion of its materials from the zone between the soot line and water ice line. We model a silicate-rich world with 0.1% and 1% carbon by mass with variable water content. We show that as a result of geochemical equilibrium, the mantle of these planets would be rich in reduced carbon but have relatively low water (hydrogen) content. Outgassing would naturally yield the ingredients for haze production when exposed to stellar UV photons in the upper atmosphere. Obscuring atmospheric hazes appear common in the exoplanetary inventory based on the presence of often featureless transmission spectra. Such hazes may be powered by the high volatile content of the underlying silicate-dominated mantle. Although this type of planet has no solar system counterpart, it should be common in the galaxy with potential impact on habitability. 
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  6. Abstract

    The radial transport, or drift, of dust has taken a critical role in giant planet formation theory. However, it has been challenging to identify dust drift pileups in the hard-to-observe inner disk. We find that the IM Lup disk shows evidence that it has been shaped by an episode of dust drift. Using radiative transfer and dust dynamical modeling we study the radial and vertical dust distribution. We find that high dust drift rates exceeding 110MMyr−1are necessary to explain both the dust and CO observations. Furthermore, the bulk of the large dust present in the inner 20 au needs to be vertically extended, implying high turbulence (αz≳ 10−3) and small grains (0.2–1 mm). We suggest that this increased level of particle stirring is consistent with the inner dust-rich disk undergoing turbulence triggered by the vertical shear instability. The conditions in the IM Lup disk imply that giant planet formation through pebble accretion is only effective outside of 20 au. If such an early, high-turbulence inner region is a natural consequence of high dust drift rates, then this has major implications for understanding the formation regions of giant planets including Jupiter and Saturn.

     
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  7. Abstract

    We present MIRI Medium-resolution Spectrograph observations of the large, multi-gapped protoplanetary disk around the T Tauri star AS 209. The observations reveal hundreds of water vapor lines from 4.9–25.5μm toward the inner ∼1 au in the disk, including the first detection of rovibrational water emission in this disk. The spectrum is dominated by hot (∼800 K) water vapor and OH gas, with only marginal detections of CO2, HCN, and a possible colder water vapor component. Using slab models with a detailed treatment of opacities and line overlap, we retrieve the column density, emitting area, and excitation temperature of water vapor and OH, and provide upper limits for the observable mass of other molecules. Compared to MIRI spectra of other T Tauri disks, the inner disk of AS 209 does not appear to be atypically depleted in CO2nor HCN. Based on Spitzer Infrared Spectrograph observations, we further find evidence for molecular emission variability over a 10 yr baseline. Water, OH, and CO2line luminosities have decreased by factors of 2–4 in the new MIRI epoch, yet there are minimal continuum emission variations. The origin of this variability is yet to be understood.

     
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  8. Abstract An understanding of abundance and distribution of water vapor in the innermost region of protoplanetary disks is key to understanding the origin of habitable worlds and planetary systems. Past observations have shown H 2 O to be abundant and a major carrier of elemental oxygen in disk surface layers that lie within the inner few astronomical units of the disk. The combination of high abundance and strong radiative transitions leads to emission lines that are optically thick across the infrared spectral range. Its rarer isotopologue H 2 18 O traces deeper into this layer and will trace the full content of the planet-forming zone. In this work, we explore the relative distribution of H 2 16 O and H 2 18 O within a model that includes water self-shielding from the destructive effects of ultraviolet radiation. In this Letter we show that there is an enhancement in the relative H 2 18 O abundance high up in the warm molecular layer within 0.1–10 au due to self-shielding of CO, C 18 O, and H 2 O. Most transitions of H 2 18 O that can be observed with JWST will partially emit from this layer, making it essential to take into account how H 2 O self-shielding may effect the H 2 O to H 2 18 O ratio. Additionally, this reservoir of H 2 18 O -enriched gas in combination with the vertical “cold finger” effect might provide a natural mechanism to account for oxygen isotopic anomalies found in meteoritic material in the solar system. 
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  9. Abstract

    The chemical composition of the inner region of protoplanetary disks can trace the composition of planetary-building material. The exact elemental composition of the inner disk has not yet been measured and tensions between models and observations still exist. Recent advancements have shown UV shielding to be able to increase the emission of organics. Here, we expand on these models and investigate how UV shielding may impact chemical composition in the inner 5 au. In this work, we use the model from Bosman et al. and expand it with a larger chemical network. We focus on the chemical abundances in the upper disk atmosphere where the effects of water UV shielding are most prominent and molecular lines originate. We find rich carbon and nitrogen chemistry with enhanced abundances of C2H2, CH4, HCN, CH3CN, and NH3by >3 orders of magnitude. This is caused by the self-shielding of H2O, which locks oxygen in water. This subsequently results in a suppression of oxygen-containing species like CO and CO2. The increase in C2H2seen in the model with the inclusion of water UV shielding allows us to explain the observed C2H2abundance without resorting to elevated C/O ratios as water UV shielding induced an effectively oxygen-poor environment in oxygen-rich gas. Thus, water UV shielding is important for reproducing the observed abundances of hydrocarbons and nitriles. From our model result, species like CH4, NH3, and NO are expected to be observable with the James Webb Space Telescope (JWST).

     
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  10. Abstract

    Carbon dioxide is an important tracer of the chemistry and physics in the terrestrial planet-forming zone. Using a thermochemical model that has been tested against the mid-infrared water emission, we reinterpret the CO2emission as observed with Spitzer. We find that both water UV-shielding and extra chemical heating significantly reduce the total CO2column in the emitting layer. Water UV-shielding is the more efficient effect, reducing the CO2column by ∼2 orders of magnitude. These lower CO2abundances lead to CO2-to-H2O flux ratios that are closer to the observed values, but CO2emission is still too bright, especially in relative terms. Invoking the depletion of elemental oxygen outside of the water midplane ice line more strongly impacts the CO2emission than it does the H2O emission, bringing the CO2-to-H2O emission in line with the observed values. We conclude that the CO2emission observed with Spitzer-IRS is coming from a thin layer in the photosphere of the disk, similar to the strong water lines. Below this layer, we expect CO2not to be present except when replenished by a physical process. This would be visible in the13CO2spectrum as well as certain12CO2features that can be observed by JWST-MIRI.

     
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