Imaging the water snowline in protostellar envelopes

Imaging the water snowline in protostellar envelopes

Author:

van 't Hoff

Abstract:

Determining the locations of the major snowlines in protostellar environments is crucial to fully understand the planet formation process and its outcome. Despite being located far enough from the central star to be spatially resolved with ALMA, the CO snowline remains difficult to detect directly in protoplanetary disks. Instead, its location can be derived from N2H+ emission, when chemical effects like photodissociation of CO and N2 are taken into account. The water snowline is even harder to observe than that for CO, because in disks it is located only a few AU from the protostar, and from the ground only the less abundant isotopologue H182O can be observed. Therefore, using an indirect chemical tracer, as done for CO, may be the best way to locate the water snowline. A good candidate tracer is HCO+, which is expected to be particularly abundant when its main destructor, H2O, is frozen out. Comparison of H182O and H13CO+ emission toward the envelope of the Class 0 protostar IRAS2A shows that the emission from both molecules is spatially anticorrelated, providing a proof of concept that H13CO+ can indeed be used to trace the water snowline in systems where it cannot be imaged directly.

Candidate Snowlines Found Around T Tauri Stars

CANDIDATE WATER VAPOR LINES TO LOCATE THE H2O SNOWLINE THROUGH HIGH-DISPERSION SPECTROSCOPIC OBSERVATIONS. I. THE CASE OF A T TAURI STAR

Authors:

Notsu et al

Abstract:

Inside the H2O snowline of protoplanetary disks, water evaporates from the dust-grain surface into the gas phase, whereas it is frozen out onto the dust in the cold region beyond the snowline. H2O ice enhances the solid material in the cold outer part of a disk, which promotes the formation of gas-giant planet cores. We can regard the H2O snowline as the surface that divides the regions between rocky and gaseous giant planet formation. Thus observationally measuring the location of the H2O snowline is crucial for understanding the planetesimal and planet formation processes, and the origin of water on Earth. In this paper, we find candidate water lines to locate the H2O snowline through future high-dispersion spectroscopic observations. First, we calculate the chemical composition of the disk and investigate the abundance distributions of H2O gas and ice, and the position of the H2O snowline. We confirm that the abundance of H2O gas is high not only in the hot midplane region inside the H2O snowline but also in the hot surface layer of the outer disk. Second, we calculate the H2O line profiles and identify those H2O lines that are promising for locating the H2O snowline: the identified lines are those that have small Einstein A coefficients and high upper state energies. The wavelengths of the candidate H2O lines range from mid-infrared to sub-millimeter, and they overlap with the regions accessible to the Atacama Large Millimeter/sub-millimeter Array and future mid-infrared high-dispersion spectrographs (e.g., TMT/MICHI, SPICA).

The Snowline is NOT Static Over the Life of a Star

Imaging the water-snow line during a protostellar outburst

Authors:

Cieza et al

Abstract:

A snow-line is the region of a protoplanetary disk at which a major volatile, such as water or carbon monoxide, reaches its condensation temperature. Snow-lines play a crucial role in disk evolution by promoting the rapid growth of ice-covered grains. Signatures of the carbon monoxide snow-line (at temperatures of around 20 kelvin) have recently been imaged in the disks surrounding the pre-main-sequence stars TW Hydra and HD163296, at distances of about 30 astronomical units (au) from the star. But the water snow-line of a protoplanetary disk (at temperatures of more than 100 kelvin) has not hitherto been seen, as it generally lies very close to the star (less than 5 au away for solar-type stars). Water-ice is important because it regulates the efficiency of dust and planetesimal coagulation, and the formation of comets, ice giants and the cores of gas giants. Here we report ALMA images at 0.03-arcsec resolution (12 au) of the protoplanetary disk around V883 Ori, a protostar of 1.3 solar masses that is undergoing an outburst in luminosity arising from a temporary increase in the accretion rate. We find an intensity break corresponding to an abrupt change in the optical depth at about 42 au, where the elevated disk temperature approaches the condensation point of water, from which we conclude that the outburst has moved the water snow-line. The spectral behaviour across the snow-line confirms recent model predictions: dust fragmentation and the inhibition of grain growth at higher temperatures results in soaring grain number densities and optical depths. As most planetary systems are expected to experience outbursts caused by accretion during their formation our results imply that highly dynamical water snow-lines must be considered when developing models of disk evolution and planet formation.

Kepler-421b may be Alone in its System

No Timing Variations Observed in Third Transit of Snow-Line Exoplanet Kepler-421b

Authors:

Dalba et al

Abstract:

We observed Kepler-421 during the anticipated third transit of the snow-line exoplanet Kepler-421b in order to constrain the existence and extent of transit timing variations (TTVs). Previously, the Kepler Spacecraft only observed two transits of Kepler-421b leaving the planet's transit ephemeris unconstrained. Our visible light, time-series observations from the 4.3-meter Discovery Channel Telescope were designed to capture pre-transit baseline and the partial transit of Kepler-421b barring significant TTVs. We use the light curves to assess the probabilities of various transit models using both the posterior odds ratio and the Bayesian Information Criterion (BIC) and find that a transit model with no TTVs is favored to 3.6-sigma confidence. These observations suggest that Kepler-421b is either alone in its system or is only experiencing minor dynamic interactions with an unseen companion. With the Kepler-421b ephemeris constrained, we calculate future transit times and discuss the opportunity to characterize the atmosphere of this cold, long-period exoplanet via transmission spectroscopy. Our investigation emphasizes the difficulties associated with observing long-period exoplanet transits and the consequences that arise from failing to refine transit ephemerides.

The Carbon monoxide Snowline is at 17-23 AU for TW Hydrae's Protoplanetary Disk

The Radial Distribution of H2 and CO in TW Hya as Revealed by Resolved ALMA Observations of CO Isotopologues

Authors:

Schwartz et al

Abstract:

CO is widely used as a tracer of molecular gas. However, there is now mounting evidence that gas phase carbon is depleted in the disk around TW Hya. Previous efforts to quantify this depletion have been hampered by uncertainties regarding the radial thermal structure in the disk. Here we present resolved ALMA observations of 13CO 3-2, C18O 3-2, 13CO 6-5, and C18O 6-5 emission in TW Hya, which allow us to derive radial gas temperature and gas surface density profiles, as well as map the CO abundance as a function of radius. These observations provide a measurement of the surface CO snowline at ~30 AU and show evidence for an outer ring of CO emission centered at 53 AU, a feature previously seen only in less abundant species. Further, the derived CO gas temperature profile constrains the freeze-out temperature of CO in the warm molecular layer to < 21 K. Combined with the previous detection of HD 1-0, these data constrain the surface density of the warm H2 gas in the inner ~30 AU. We find that CO is depleted by two orders of magnitude from R=10-60 AU, with the small amount of CO returning to the gas phase inside the surface CO snowline insufficient to explain the overall depletion. Finally, this new data is used in conjunction with previous modeling of the TW Hya disk to constrain the midplane CO snowline to 17-23 AU.

Looking for Exoplanets Past the Snowline

Transiting Planet Candidates Beyond the Snow Line Detected by Visual Inspection of 7557 Kepler Objects of Interest

Authors:

Uehara et al

Abstract:

We visually inspected the light curves of 7557 Kepler Objects of Interest (KOIs) to search for single transit events (STEs) possibly due to long-period giant planets. We identified 28 STEs in 24 KOIs, among which 14 events are newly reported in this paper. We estimate the radius and orbital period of the objects causing STEs by fitting the STE light curves simultaneously with the transits of the other planets in the system or with the prior information on the host star density. As a result, we found that STEs in seven of those systems are consistent with Neptune- to Jupiter-sized objects of orbital periods ranging from a few to ∼ 20yr. We also estimate that ≳20% of the compact multi-transiting systems host cool giant planets with periods ≳3yr on the basis of their occurrence in the KOIs with multiple candidates, assuming the small mutual inclination between inner and outer planetary orbits.

Dust Properties Across the CO Snowline in HD 163296's Protoplanetary Disk

Dust properties across the CO snowline in the HD 163296 disk from ALMA and VLA observations

Authors:

Guidi et al

Abstract:

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 (≥1 cm) in the inner regions of the disk (inside ∼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 ∼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.

Collisions of CO2 Ice Grains in Planet Formation

Collisions of CO2 Ice Grains in Planet Formation

Authors:

Musiolik et al

Abstract:

In protoplanetary disks, CO2 is solid ice beyond its snow line at ∼10AU. Due to its high abundance, it contributes heavily to the collisional evolution in this region of the disk. For the first time, we carried out laboratory collision experiments with CO2 ice particles and a CO2-covered wall at a temperature of 80 K. Collision velocities varied between 0 - 2.5 m/s. Particle sizes were on the order of ∼ 100 μm. We find a threshold velocity between the sticking and the bouncing regime at 0.04 m/s. Particles with greater velocities but below 1 m/s bounce off the wall. For yet greater velocities, fragmentation occurs. We give analytical models for the coefficients of restitution and fragmentation strength consistent with the experimental data. Set in context, our data show that CO2 ice and silicate dust resemble each other in the collisional behavior. Compared to water ice the sticking velocity is an order of magnitude smaller. One immediate consequence as example is that water ice particles mantled by CO2 ice lose any "sticking advantage." In this case, preferential planetesimal growth attributed to the sticking properties of water ice will be limited to the region between the H2O ice line and the CO2 ice line.

There may be Multiple Carbon monoxide Snow-lines in Protoplanetary Disks

Multiple Carbon monoxide Snow-lines in Disks Sculpted by Radial Drift

Author:

Ilsedore Cleeves

Abstract:

Observations of protoplanetary disks suggest that the gas and dust follow significantly different radial distributions. This finding can be theoretically explained by a combination of radial drift and gas drag of intermediate-sized dust grains. Using a simple parametric model to approximate the different distributions of the gas and dust components, we calculate and examine the impact of radial drift on the global dust temperature structure. We find that the removal of large grains beyond the "truncation radius" allows this region to become significantly warmer from reprocessed stellar radiation shining down from the disk upper layers, increasing the outer disk temperature by ∼10−30%. This change is sufficient to raise the local temperature to a value exceeding the CO desorption temperature. These findings imply that the disk density structures induced by radial drift are able to create multiple CO snow-lines. The inner disk CO is in the gas phase, freezing out near the classical snow-line at R∼20−40 AU. Moving outward, the CO sublimates once again beyond the truncation radius (80 AU in our models) and subsequently re-freezes out at sufficiently large stellar distances, beyond R≳130−200 AU. We find that thermal desorption of CO in the outer disk becomes competitive with external UV photodesorption and that this additional transition from solid state CO to the gas-phase has significant implications for the C/O ratio in the outer disk.

Measuring the Water Snowline in Protoplanetary Disks

Measurements of water surface snow lines in classical protoplanetary disks

Authors:

Blevins et al

Abstract:

We present deep Herschel-PACS spectroscopy of far-infrared water lines from a sample of four protoplanetary disks around solar-mass stars, selected to have strong water emission at mid-infrared wavelengths. By combining the new Herschel spectra with archival Spitzer-IRS spectroscopy, we retrieve a parameterized radial surface water vapor distribution from 0.1-100 AU using two-dimensional dust and line radiative transfer modeling. The surface water distribution is modeled with a step model comprising of a constant inner and outer relative water abundance and a critical radius at which the surface water abundance is allowed to change. We find that the four disks have critical radii of ∼3−11 AU, at which the surface water abundance decreases by at least 5 orders of magnitude. The measured values for the critical radius are consistently smaller than the location of the surface snow line, as predicted by the observed spectral energy distribution. This suggests that the sharp drop-off of the surface water abundance is not solely due to the local gas-solid balance, but may also be driven by the de-activation of gas-phase chemical pathways to water below 300 K. Assuming a canonical gas-to-dust ratio of 100, as well as coupled gas and dust temperatures Tgas=Tdust, the best-fit inner water abundances become implausibly high (0.01-1.0 H2−1). Conversely, a model in which the gas and dust temperatures are decoupled leads to canonical inner disk water abundances of ∼10−4H−12, while retaining gas-to-dust ratios of 100. That is, the evidence for gas-dust decoupling in disk surfaces is stronger than for enhanced gas-to-dust ratios.

Terrestrial Worlds Could Acquire Water Through Pebble Accretion Past the Snowline and MIgrate

On the water delivery to terrestrial embryos by ice pebble accretion

Authors:

Sato et al

Abstract:

Standard accretion disk models suggest that the snow line in the solar nebula migrated interior to the Earth's orbit in a late stage of nebula evolution. In this late stage, a significant amount of ice could have been delivered to 1 AU from outer regions in the form of mm to dm-sized "pebbles." This raises the question why the present Earth is so depleted of water (with the ocean mass being as small as 0.023% of the Earth mass). Here we quantify the amount of icy pebbles accreted by terrestrial embryos after the migration of the snow line assuming that no mechanism halts the pebble flow in outer disk regions. We use a simplified version of the coagulation equation to calculate the formation and radial inward drift of icy pebbles in a protoplanetary disk. The pebble accretion cross section of an embryo is calculated using analytic expressions presented by recent studies. We find that the final mass and water content of terrestrial embryos strongly depends on the radial extent of the gas disk, the strength of disk turbulence, and the time at which the snow lines arrives at 1 AU. The disk's radial extent sets the lifetime of the pebble flow, while turbulence determines the density of pebbles at the midplane where the embryos reside. We find that the final water content of the embryos falls below 0.023 wt% only if the disk is compact (less than 100 AU), turbulence is strong at 1 AU, and the snow line arrives at 1 AU later than 2-4 Myr after disk formation. If the solar nebula extended to 300 AU, initially rocky embryos would have evolved into icy planets of 1-10 Earth masses unless the snow-line migration was slow. If the proto-Earth contained water of ~ 1 wt% as might be suggested by the density deficit of the Earth's outer core, the formation of the proto-Earth was possible with weaker turbulence and with earlier (greater than 0.5-2 Myr) snow-line migration.

From Microlensing, how Common are Snowline Region Exoplanets?

The frequency of snowline-region planets from four-years of OGLE-MOA-Wise second-generation microlensing

Authors:

Shvartzvald et al

Abstract:

We present a statistical analysis of the first four seasons from a "second-generation" microlensing survey for extrasolar planets, consisting of near-continuous time coverage of 8 deg2 of the Galactic bulge by the OGLE, MOA, and Wise microlensing surveys. During this period, 224 microlensing events were observed by all three groups. Over 12% of the events showed a deviation from single-lens microlensing, and for ∼1/3 of those the anomaly is likely caused by a planetary companion. For each of the 224 events we have performed numerical ray-tracing simulations to calculate the detection efficiency of possible companions as a function of companion-to-host mass ratio and separation. Accounting for the detection efficiency, we find that 55+34−22% of microlensed stars host a snowline planet. Moreover, we find that Neptunes-mass planets are ∼10 times more common than Jupiter-mass planets. The companion-to-host mass ratio distribution shows a deficit at q∼10−2, separating the distribution into two companion populations, analogous to the stellar-companion and planet populations, seen in radial-velocity surveys around solar-like stars. Our survey, however, which probes mainly lower-mass stars, suggests a minimum in the distribution in the super-Jupiter mass range, and a relatively high occurrence of brown-dwarf companions.

Direct Imaging of the Water Snow Line int HL Tauri's Protoplanetary Disk by ALMA

Direct imaging of the water snow line at the time of planet formation using two ALMA continuum bands

Authors:

Banzatti et al

Abstract:

Molecular snow lines in protoplanetary disks have been studied theoretically for decades because of their importance in shaping planetary architectures and compositions. The water snow line lies in the planet formation region at less than 10 AU, and so far its location has been estimated only indirectly from spatially-unresolved spectroscopy. This work presents a proof-of-concept method to directly image the water snow line in protoplanetary disks through its physical and chemical imprint in the local dust properties. We adopt a physical disk model that includes dust coagulation, fragmentation, drift, and a change in fragmentation velocities of a factor 10 between dry silicates and icy grains as found by laboratory work. We find that the presence of a water snow line leads to a sharp discontinuity in the radial profile of the dust emission spectral index {\alpha}_mm, due to replenishment of small grains through fragmentation. We use the ALMA simulator to demonstrate that this effect can be observed in protoplanetary disks using spatially-resolved ALMA images in two continuum bands. We explore the model dependence on the disk viscosity and find that the spectral index reveals the water snow line for a wide range of conditions, with opposite trends when the emission is optically thin rather than thick. If the disk viscosity is low ({\alpha}_visc less than 10^-3) the snow line produces a ring-like structure with a minimum at {\alpha}_mm ~ 2 in the optically thick regime, possibly similar to what has been measured with ALMA in the innermost region of the HL Tau disk.

Exoplanetary Systems Without Gas Giants Will be More Water Rich

Fossilized condensation lines in the Solar System protoplanetary disk

Authors:

Morbidelli et al

Abstract:

The terrestrial planets and the asteroids dominant in the inner asteroid belt are water poor. However, in the protoplanetary disk the temperature should have decreased below water condensation level well before the disk was photoevaporated. Thus, the global water depletion of the inner Solar System is puzling. We show that, even if the inner disk becomes cold, there cannot be direct condensation of water. This is because the snowline moves towards the Sun more slowly than the gas itself. The appearance of ice in a range of heliocentric distances swept by the snowline can only be due to the radial drift of icy particles from the outer disk. However, if a sufficiently massive planet is present, the radial drift of particles is interrupted, because the disk acquires a superKeplerian rotation just outside of the planetary orbit. From this result, we propose that the precursor of Jupiter achieved about 20 Earth masses when the snowline was still around 3 AU. This effectively fossilized the snowline at that location. Although cooling, the disk inside of the Jovian orbit remained ice-depleted because the flow of icy particles from the outer system was intercepted by the planet. This scenario predicts that planetary systems without giant planets should be much more rich in water in their inner regions than our system. We also show that the inner edge of the planetesimal disk at 0.7AU, required in terrestrial planet formation models to explain the small mass of Mercury and the absence of planets inside of its orbit, could be due to the silicate condensation line, fossilized at the end of the phase of streaming instability that generated the planetesimal seeds. Thus, when the disk cooled, silicate particles started to drift inwards of 0.7AU without being sublimated, but they could not be accreted by any pre-existing planetesimals.

The Differences Between the Water, Carbon monoxide and Carbon dioxide Snowlines in Protoplanetary Disks

C/O and Snowline Locations in Protoplanetary Disks: The Effect of Radial Drift and Viscous Gas Accretion

Authors:

Piso et al

Abstract:

The C/O ratio is a defining feature of both gas giant atmospheric and protoplanetary disk chemistry. In disks, the C/O ratio is regulated by the presence of snowlines of major volatiles at different distances from the central star. We explore the effect of radial drift of solids and viscous gas accretion onto the central star on the snowline locations of the main C and O carriers in a protoplanetary disk, H2O, CO2 and CO, and their consequences for the C/O ratio in gas and dust throughout the disk. We determine the snowline locations for a range of fixed initial particle sizes and disk types. For our fiducial disk model, we find that grains with sizes ~0.5 cm less than s less than 7 m for an irradiated disk, and ~0.001 cm less than s less than 7 m for an evolving and viscous disk, desorb at a size-dependent location in the disk, which is independent of the particle's initial position. The snowline radius decreases for larger particles, up to sizes of ~7 m. Compared to a static disk, we find that radial drift and gas accretion in a viscous disk move the H2O snowline inwards by up to 40%, the CO2 snowline by up to 60%, and the CO snowline by up to 50%. We thus determine an inner limit on the snowline locations when radial drift and gas accretion are accounted for.

Imaging the Carbon monoxide Snowline in HD 163296's Protoplanetary Disk

Chemical Imaging of the CO Snow Line in the HD 163296 Disk

Authors:

Qi et al

Abstract:

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.

The Problematic Volatile Snowlines In Embedded Disks

Volatile snowlines in embedded disks around low-mass protostars

Authors:

Harsono et al

Abstract:

Models of the young solar nebula assume a hot initial disk with most volatiles are in the gas phase. The question remains whether an actively accreting disk is warm enough to have gas-phase water up to 50 AU radius. No detailed studies have yet been performed on the extent of snowlines in an embedded accreting disk (Stage 0). Quantify the location of gas-phase volatiles in embedded actively accreting disk system. Two-dimensional physical and radiative transfer models have been used to calculate the temperature structure of embedded protostellar systems. Gas and ice abundances of H2O, CO2, and CO are calculated using the density-dependent thermal desorption formulation. The midplane water snowline increases from 3 to 55 AU for accretion rates through the disk onto the star between 10−9-10−4 M⊙ yr−1. CO2 can remain in the solid phase within the disk for M˙≤10−5 M⊙ yr−1 down to ∼20 AU. Most of the CO is in the gas phase within an actively accreting disk independent of disk properties and accretion rate. The predicted optically thin water isotopolog emission is consistent with the detected H182O emission toward the Stage 0 embedded young stellar objects, originating from both the disk and the warm inner envelope (hot core). An accreting embedded disk can only account for water emission arising from R<50 0="" 30="" 50="" a="" accretion="" alma="" and="" au="" be="" blockquote="" can="" chemical="" compared="" content="" decreases="" deeply="" disks="" during="" early="" embedded="" emission="" envelope="" extent="" for="" from="" have="" high="" hot="" however="" in="" inherited="" limit.="" low="" measured="" nebula="" not="" observations="" occurred="" of="" only="" our="" out="" periods="" phase="" radial="" rapidly="" rates.="" reset="" solar="" stage="" sublimate="" system.="" t-tauri="" the="" this="" thus="" to="" volatiles="" with="" young="">

KMT-2015-1b: a 2 Jupiter Mass Exoplanet Orbiting a M Dwarf Beyond the Snowline

KMT-2015-1b: a Giant Planet Orbiting a Low-mass Dwarf Host Star Discovered by a New High-cadence Microlensing Survey with a Global Telescope Network

Authors:

Hwang et al

Abstract:

We report the discovery of an extrasolar planet, KMT-2015-1b, that was detected using the microlensing technique. The planetary lensing event was observed by KMTNet survey that has commenced in 2015. With dense coverage by using network of globally distributed telescopes equipped with very wide-field cameras, the short planetary signal is clearly detected and precisely characterized. We find that KMT-2015-1b is a giant planet orbiting a low-mass M-dwarf host star. The planet has a mass about twice that of Jupiter and it is located beyond the snow line of the host star. With the improvement of existing surveys and the advent of new surveys, future microlensing planet samples will include planets not only in greatly increased number but also in a wide spectrum of hosts and planets, helping us to have a better and comprehensive understanding about the formation and evolution of planets.

Detection of N2H+ Ring Around TW Hyadrae

Analytical Formulas of Molecular Ion Abundances and N2H+ Ring in Protoplanetary Disks

Authors:

Aikawa et al

Abstract:

We investigate the chemistry of ion molecules in protoplanetary disks, motivated by the detection of N2H+ ring around TW Hya. While the ring inner radius coincides with the CO snow line, it is not apparent why N2H+ is abundant outside the CO snow line in spite of the similar sublimation temperatures of CO and N2. Using the full gas-grain network model, we reproduced the N2H+ ring in a disk model with millimeter grains. The chemical conversion of CO and N2 to less volatile species (sink effect hereinafter) is found to affect the N2H+ distribution. Since the efficiency of the sink depends on various parameters such as activation barriers of grain surface reactions, which are not well constrained, we also constructed the no-sink model; the total (gas and ice) CO and N2 abundances are set constant, and their gaseous abundances are given by the balance between adsorption and desorption. Abundances of molecular ions in the no-sink model are calculated by analytical formulas, which are derived by analyzing the full-network model. The N2H+ ring is reproduced by the no-sink model, as well. The 2D (R-Z) distribution of N2H+, however, is different among the full-network model and no-sink model. The column density of N2H+ in the no-sink model depends sensitively on the desorption rate of CO and N2, and the flux of cosmic ray. We also found that N2H+ abundance can peak at the temperature slightly below the CO sublimation, even if the desorption energies of CO and N2 are the same.

Can the Solar System's Architecture be Explained by the Snowline?

The great dichotomy of the Solar System: small terrestrial embryos and massive giant planet cores

Authors:

Morbidelli et al

Abstract:

The basic structure of the solar system is set by the presence of low-mass terrestrial planets in its inner part and giant planets in its outer part. This is the result of the formation of a system of multiple embryos with approximately the mass of Mars in the inner disk and of a few multi-Earth-mass cores in the outer disk, within the lifetime of the gaseous component of the protoplanetary disk. What was the origin of this dichotomy in the mass distribution of embryos/cores? We show in this paper that the classic processes of runaway and oligarchic growth from a disk of planetesimals cannot explain this dichotomy, even if the original surface density of solids increased at the snowline. Instead, the accretion of drifting pebbles by embryos and cores can explain the dichotomy, provided that some assumptions hold true. We propose that the mass-flow of pebbles is two-times lower and the characteristic size of the pebbles is approximately ten times smaller within the snowline than beyond the snowline (respectively at heliocentric distance r less than rice and r greater than rice, where rice is the snowline heliocentric distance), due to ice sublimation and the splitting of icy pebbles into a collection of chondrule-size silicate grains. In this case, objects of original sub-lunar mass would grow at drastically different rates in the two regions of the disk. Within the snowline these bodies would reach approximately the mass of Mars while beyond the snowline they would grow to ∼20 Earth masses. The results may change quantitatively with changes to the assumed parameters, but the establishment of a clear dichotomy in the mass distribution of protoplanets appears robust, provided that there is enough turbulence in the disk to prevent the sedimentation of the silicate grains into a very thin layer.