arXiv subjects

Edwin A. Bergin

Publications and source records attributed to Edwin A. Bergin.

At least 127 records · Page 7Linked to original sources

arXiv2019

The Disk Gas Mass and the Far-IR Revolution

The gaseous mass of protoplanetary disks is a fundamental quantity in planet formation. The presence of gas is necessary to assemble planetesimals, it determines timescales of giant planet birth, and it is an unknown factor for a wide range of properties of planet formation, from chemical abundances (X/H) to the mass efficiency of planet formation. The gas mass obtained from traditional tracers, such as dust thermal continuum and CO isotopologues, are now known to have significant (1 - 2 orders of magnitude) discrepancies. Emission from the isotopologue of H2, hydrogen deuteride (HD), offers an alternative measurement of the disk gas mass. Of all of the regions of the spectrum, the far-infrared stands out in that orders of magnitude gains in sensitivity can be gleaned by cooling a large aperture telescope to 8 K. Such a facility can open up a vast new area of the spectrum to exploration. One of the primary benefits of this far-infrared revolution would be the ability to survey hundreds of planet-forming disks in HD emission to derive their gaseous masses. For the first time, we will have statistics on the gas mass as a function of evolution, tracing birth to dispersal as a function of stellar spectral type. These measurements have broad implications for our understanding of the time scale during which gas is available to form giant planets, the dynamical evolution of the seeds of terrestrial worlds, and the resulting chemical composition of pre-planetary embryos carrying the elements needed for life. Measurements of the ground-state line of HD requires a space-based observatory operating in the far-infrared at 112 microns.

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arXiv2018

Science with an ngVLA: Tracing the Water Snowline in Protoplanetary disks with the ngVLA

The water snowline in protoplanetary disks is one of the most pivotal locations for planet formation: it sets a critical boundary of chemical composition in the disk and likely serves as a favorable site of planetesimal growth and planet formation. However, the water snowline at the mid-plane cannot be directly traced by water line observations as the dust emission around the water snowline is highly optically thick at these line frequencies. Alternatively, the water snowline maybe traced through the sharp changes in physical properties (surface density and dust size distribution) at this transition or through the snowlines of other molecules that sublimate co-spatially with water. In this chapter, we discuss the ngVLA's capability of tracing the mid-plane water snowline through mapping sharp transitions in the continuum spectral index caused by changes of dust size distributions and by using the NH3 snowline as a proxy. We found that with the current design of the ngVLA ability, a sharp transition in spectral index across the water snowline can be readily traced in relatively bright disks. A detection of the NH3 snowline is challenging under the current design, but may still be possible if the NH3-to-water abundance in the disk is close to that of the ISM ices. In summary, the proposed ngVLA is the only facility can provide sufficient sensitivity and spatial resolution to trace the evolution of water snowline at the disk mid-plane and to reveal its role in accelerating the planetesimal formation and in building the architecture of planetary systems.

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arXiv2018

Exploring the Origins of Earth's Nitrogen: Astronomical Observations of Nitrogen-bearing Organics in Protostellar Environments

It is not known whether the original carriers of Earth's nitrogen were molecular ices or refractory dust. To investigate this question, we have used data and results of Herschel observations towards two protostellar sources: the high-mass hot core of Orion KL, and the low-mass protostar IRAS 16293-2422. Towards Orion KL, our analysis of the molecular inventory of Crockett et al. (2014) indicates that HCN is the organic molecule that contains by far the most nitrogen, carrying $74_{-9}^{+5}\%$ of nitrogen-in-organics. Following this evidence, we explore HCN towards IRAS 16293-2422, which we consider a solar analog. Towards IRAS 16293-2422, we have reduced and analyzed Herschel spectra of HCN, and fit these observations against "jump" abundance models of IRAS 16293-2422's protostellar envelope. We find an inner-envelope HCN abundance $X_{\textrm{in}} = 5.9\pm0.7 \times 10^{-8}$ and an outer-envelope HCN abundance $X_{\textrm{out}} = 1.3 \pm 0.1 \times 10^{-9}$. We also find the sublimation temperature of HCN to be $T_{\textrm{jump}} = 71 \pm 3$~K; this measured $T_{\textrm{jump}}$ enables us to predict an HCN binding energy $E_{\textrm{B}}/k = 3840 \pm 140$~K. Based on a comparison of the HCN/H2O ratio in these protostars to N/H2O ratios in comets, we find that HCN (and, by extension, other organics) in these protostars is incapable of providing the total bulk N/H2O in comets. We suggest that refractory dust, not molecular ices, was the bulk provider of nitrogen to comets. However, interstellar dust is not known to have 15N enrichment, while high 15N enrichment is seen in both nitrogen-bearing ices and in cometary nitrogen. This may indicate that these 15N-enriched ices were an important contributor to the nitrogen in planetesimals and likely to the Earth.

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arXiv2018

Protoplanetary Disk Sizes and Angular Momentum Transport

In young circumstellar disks, accretion--the inspiral of disk material onto the central star--is important for both the buildup of stellar masses and the outcome of planet formation. Although the existence of accretion is well documented, understanding the angular momentum transport mechanism that enables disk accretion has proven to be an enduring challenge. The leading theory to date, the magnetorotational instability, which redistributes angular momentum within the disk, is increasingly questioned, and magnetothermal disk winds, which remove angular momentum from the disk, have emerged as an alternative theoretical solution. Here we investigate whether measurements of disk radii can provide useful insights into which, if either, of these mechanisms drive disk accretion, by searching for evidence of viscous spreading in gaseous disks, a potential signature of "in disk" angular momentum transport. We find that the large sizes of most Class II (T Tauri) gas disks compared to those of their earlier evolutionary counterparts, Class I gas disks, are consistent with expectations for viscous spreading in the Class II phase. There is, however, a large spread in the sizes of Class II gas disks at any age, including a population of very small Class II gas disks. Their small sizes may result from processes such as photoevaporation, disk winds, or truncation by orbiting low mass companions.

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arXiv2018

Transport of CO in Protoplanetary Disks: Consequences of Pebble Formation, Settling, and Radial Drift

Current models of (exo)planet formation often rely on a large influx of so-called `pebbles' from the outer disk into the planet formation region. In this paper, we investigate how the formation of pebbles in the cold outer regions of protoplanetary disks and their subsequent migration to the inner disk can alter the gas-phase CO distribution both interior and exterior to the midplane CO snowline. By simulating the resulting CO abundances in the midplane as well as the warm surface layer, we identify observable signatures of large-scale pebble formation and migration that can be used as `smoking guns' for these important processes. Specifically, we find that after $1\mathrm{~Myr}$, the formation and settling of icy pebbles results in the removal of up to $80\%$ of the CO vapor in the warm ($T>22\mathrm{~K}$) disk layers outside the CO snowline, while the radial migration of pebbles results in the generation of a plume of CO vapor interior the snowline, increasing the CO abundance by a factor ${\sim}2{-}6$ depending on the strength of the turbulence and the sizes of the individual pebbles. The absence of this plume of CO vapor in young nearby disks could indicate efficient conversion of CO into a more refractory species, or a reduction in the radial mass flux of pebbles by, for example, disk inhomogeneities or early planetesimal formation.

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arXiv2018

Chemistry During the Gas-rich Stage of Planet Formation

In this chapter we outline some of the basic understanding of the chemistry that accompanies planet formation. We discuss the basic physical environment which dictates the dominant chemical kinetic pathways for molecule formation. We focus on three zones from both observational and theoretical perspectives: (1) the planet forming midplane and ice/vapor transition zones (snow-lines), (2) the warm disk surface that is shielded from radiation, which can be readily accessed by todays observational facilities, and (3) the surface photodissociation layers where stellar radiation dominates. We end with a discussion of how chemistry influences planet formation along with how to probe the link between formation and ultimate atmospheric composition for gas giants and terrestrial worlds.

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arXiv2018

The determination of protoplanetary disk masses

In this article we review the methods used to determine the gas and dust masses of protoplanetary disks, with an emphasis on the lesser characterized total gas mass. Our review encompasses all the indirect tracers and the methodology that is be used to isolate the hidden H2 via dust, CO, and HD emission. We discuss the overall calibration of gaseous tracers which is based on decades of study of the dense phases of the interstellar medium. At present, disk gas masses determined via CO and HD are (in a few instances) different by orders of magnitude, hinting at either significant evolution in total disk mass or in the CO abundance . Either of these would represent a fundamental physical or chemical process that appears to dominate the system on ~million year timescales. Efforts to reconcile these differences using existing and future facilities are discussed.

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arXiv2018

First Detection of the Simplest Organic Acid in a Protoplanetary Disk

The formation of asteroids, comets and planets occurs in the interior of protoplanetary disks during the early phase of star formation. Consequently, the chemical composition of the disk might shape the properties of the emerging planetary system. In this context, it is crucial to understand whether and what organic molecules are synthesized in the disk. In this Letter, we report the first detection of formic acid (HCOOH) towards the TW Hydrae protoplanetary disk. The observations of the trans-HCOOH 6$_{(1,6)-5(1,5)}$ transition were carried out at 129~GHz with ALMA. We measured a disk-averaged gas-phase t-HCOOH column density of $\sim$ (2-4)$\times$10$^{12}$~cm$^{-2}$, namely as large as that of methanol. HCOOH is the first organic molecules containing two oxygen atoms detected in a protoplanetary disk, a proof that organic chemistry is very active even though difficult to observe in these objects. Specifically, this simplest acid stands as the basis for synthesis of more complex carboxylic acids used by life on Earth.

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arXiv2018

CO in Protostars (COPS): $Herschel$-SPIRE Spectroscopy of Embedded Protostars

We present full spectral scans from 200-670$μ$m of 26 Class 0+I protostellar sources, obtained with $Herschel$-SPIRE, as part of the "COPS-SPIRE" Open Time program, complementary to the DIGIT and WISH Key programs. Based on our nearly continuous, line-free spectra from 200-670 $μ$m, the calculated bolometric luminosities ($L_{\rm bol}$) increase by 50% on average, and the bolometric temperatures ($T_{\rm bol}$) decrease by 10% on average, in comparison with the measurements without Herschel. Fifteen protostars have the same Class using $T_{\rm bol}$ and $L_{\rm bol}$/$L_{\rm submm}$. We identify rotational transitions of CO lines from J=4-3 to J=13-12, along with emission lines of $^{13}$CO, HCO$^+$, H$_{2}$O, and [CI]. The ratios of $^{12}$CO to $^{13}$CO indicate that $^{12}$CO emission remains optically thick for $J_{\rm up}$ < 13. We fit up to four components of temperature from the rotational diagram with flexible break points to separate the components. The distribution of rotational temperatures shows a primary population around 100 K with a secondary population at $\sim$350 K. We quantify the correlations of each line pair found in our dataset, and find the strength of correlation of CO lines decreases as the difference between $J$-level between two CO lines increases. The multiple origins of CO emission previously revealed by velocity-resolved profiles are consistent with this smooth distribution if each physical component contributes to a wide range of CO lines with significant overlap in the CO ladder. We investigate the spatial extent of CO emission and find that the morphology is more centrally peaked and less bipolar at high-$J$ lines. We find the CO emission observed with SPIRE related to outflows, which consists two components, the entrained gas and shocked gas, as revealed by our rotational diagram analysis as well as the studies with velocity-resolved CO emission.

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arXiv2018

The need for a far-infrared cold space telescope to understand the chemistry of planet formation

At a time when ALMA produces spectacular high resolution images of gas and dust in circumstellar disks, the next observational frontier in our understanding of planet formation and the chemistry of planet-forming material may be found in the mid- to far-infrared wavelength range. A large, actively cooled far-infrared telescope in space will offer enormous spectroscopic sensitivity improvements of 3-4 orders of magnitude, making it possible to uniquely survey certain fundamental properties of planet formation. Specifically, the Origins Space Telescope (OST), a NASA flagship concept to be submitted to the 2020 decadal survey, will provide a platform that allows complete surveys of warm and cold water around young stars of all masses and across all evolutionary stages, and to measure their total planet-forming gas mass using the ground-state line of HD. While this white paper is formulated in the context of the NASA Origins Space Telescope concept, it can be applied in general to inform any future space-based, cold far-infrared observatory.

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arXiv2018

Unlocking CO Depletion in Protoplanetary Disks I. The Warm Molecular Layer

CO is commonly used as a tracer of the total gas mass in both the interstellar medium and in protoplanetary disks. Recently there has been much debate about the utility of CO as a mass tracer in disks. Observations of CO in protoplanetary disks reveal a range of CO abundances, with measurements of low CO to dust mass ratios in numerous systems. One possibility is that carbon is removed from CO via chemistry. However, the full range of physical conditions conducive to this chemical reprocessing is not well understood. We perform a systematic survey of the time dependent chemistry in protoplanetary disks for 198 models with a range of physical conditions. We varying dust grain size distribution, temperature, comic ray and X-ray ionization rate, disk mass, and initial water abundance, detailing what physical conditions are necessary to activate the various CO depletion mechanisms in the warm molecular layer. We focus our analysis on the warm molecular layer in two regions: the outer disk (100 au) well outside the CO snowline and the inner disk (19 au) just inside the midplane CO snow line. After 1 Myr, we find that the majority of models have a CO abundance relative to H$_2$ less than $10^{-4}$ in the outer disk, while an abundance less than $10^{-5}$ requires the presence of cosmic rays. Inside the CO snow line, significant depletion of CO only occurs in models with a high cosmic ray rate. If cosmic rays are not present in young disks it is difficult to chemically remove carbon from CO. Additionally, removing water prior to CO depletion impedes the chemical processing of CO. Chemical processing alone cannot explain current observations of low CO abundances. Other mechanisms must also be involved.

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arXiv2018

Imaging the water snowline in a protostellar envelope with H$^{13}$CO$^+$

Snowlines are key ingredients for planet formation. Providing observational constraints on the locations of the major snowlines is therefore crucial for fully connecting planet compositions to their formation mechanism. Unfortunately, the most important snowline, that of water, is very difficult to observe directly in protoplanetary disks due to its close proximity to the central star. Based on chemical considerations, HCO$^+$ is predicted to be a good chemical tracer of the water snowline, because it is particularly abundant in dense clouds when water is frozen out. This work maps the optically thin isotopologue H$^{13}$CO$^+$ ($J=3-2$) toward the envelope of the low-mass protostar NGC1333-IRAS2A (observed with NOEMA at ~0.9" resolution), where the snowline is at larger distance from the star than in disks. The H$^{13}$CO$^+$ emission peaks ~2" northeast of the continuum peak, whereas the previously observed H$_2^{18}$O shows compact emission on source. Quantitative modeling shows that a decrease in H$^{13}$CO$^+$ abundance by at least a factor of six is needed in the inner ~360 AU to reproduce the observed emission profile. Chemical modeling predicts indeed a steep increase in HCO$^+$ just outside the water snowline; the 50% decrease in gaseous H$_2$O at the snowline is not enough to allow HCO$^+$ to be abundant. This places the water snowline at 225 AU, further away from the star than expected based on the 1D envelope temperature structure for NGC1333-IRAS2A. In contrast, DCO$^+$ observations show that the CO snowline is at the expected location, making an outburst scenario unlikely. The spatial anticorrelation of the H$^{13}$CO$^+$ and H$_2^{18}$O emission provide a proof of concept that H$^{13}$CO$^+$ can be used as a tracer of the water snowline.

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arXiv2017

Destruction of Refractory Carbon in Protoplanetary Disks

The Earth and other rocky bodies in the inner solar system contain significantly less carbon than the primordial materials that seeded their formation. These carbon-poor objects include the parent bodies of primitive meteorites, suggesting that at least one process responsible for solid-phase carbon depletion was active prior to the early stages of planet formation. Potential mechanisms include the erosion of carbonaceous materials by photons or atomic oxygen in the surface layers of the protoplanetary disk. Under photochemically generated favorable conditions, these reactions can deplete the near-surface abundance of carbon grains and polycyclic aromatic hydrocarbons by several orders of magnitude on short timescales relative to the lifetime of the disk out to radii of ~20-100+ au from the central star depending on the form of refractory carbon present. Due to the reliance of destruction mechanisms on a high influx of photons, the extent of refractory carbon depletion is quite sensitive to the disk's internal radiation field. Dust transport within the disk is required to affect the composition of the midplane. In our current model of a passive, constant-alpha disk, where alpha = 0.01, carbon grains can be turbulently lofted into the destructive surface layers and depleted out to radii of ~3-10 au for 0.1-1 um grains. Smaller grains can be cleared out of the planet-forming region completely. Destruction may be more effective in an actively accreting disk or when considering individual grain trajectories in non-idealized disks.

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arXiv2017

Formation of Wide Binaries by Turbulent Fragmentation

Understanding the formation of wide binary systems of very low mass stars (M $\le$ 0.1 Msun) is challenging. The most obvious route is via widely separated low-mass collapsing fragments produced through turbulent fragmentation of a molecular core. However, close binaries/multiples from disk fragmentation can also evolve to wide binaries over a few initial crossing times of the stellar cluster through tidal evolution. Finding an isolated low mass wide binary system in the earliest stage of formation, before tidal evolution could occur, would prove that turbulent fragmentation is a viable mechanism for (very) low mass wide binaries. Here we report high resolution ALMA observations of a known wide-separation protostellar binary, showing that each component has a circumstellar disk. The system is too young to have evolved from a close binary and the disk axes are misaligned, providing strong support for the turbulent fragmentation model. Masses of both stars are derived from the Keplerian rotation of the disks; both are very low mass stars.

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arXiv2017

Variable H$^{13}$CO$^+$ Emission in the IM Lup Disk: X-ray Driven Time-Dependent Chemistry?

We report the first detection of a substantial brightening event in an isotopologue of a key molecular ion, HCO$^+$, within a protoplanetary disk of a T Tauri star. The H$^{13}$CO$^+$ $J=3-2$ rotational transition was observed three times toward IM Lup between July 2014 and May 2015 with the Atacama Large Millimeter Array. The first two observations show similar spectrally integrated line and continuum fluxes, while the third observation shows a doubling in the disk integrated $J=3-2$ line flux compared to the continuum, which does not change between the three epochs. We explore models of an X-ray active star irradiating the disk via stellar flares, and find that the optically thin H$^{13}$CO$^+$ emission variation can potentially be explained via X-ray driven chemistry temporarily enhancing the HCO$^+$ abundance in the upper layers of the disk atmosphere during large or prolonged flaring events. If the HCO$^+$ enhancement is indeed caused by a X-ray flare, future observations should be able to spatially resolve these events and potentially enable us to watch the chemical aftermath of the high-energy stellar radiation propagating across the face of protoplanetary disks, providing a new pathway to explore ionization physics and chemistry, including electron density, in disks.

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arXiv2017

Mass inventory of the giant-planet formation zone in a solar nebula analog

The initial mass distribution in the solar nebula is a critical input to planet formation models that seek to reproduce today's Solar System. Traditionally, constraints on the gas mass distribution are derived from observations of the dust emission from disks, but this approach suffers from large uncertainties in grain growth and gas-to-dust ratio. On the other hand, previous observations of gas tracers only probe surface layers above the bulk mass reservoir. Here we present the first partially spatially resolved observations of the $^{13}$C$^{18}$O J=3-2 line emission in the closest protoplanetary disk, TW Hya, a gas tracer that probes the bulk mass distribution. Combining it with the C$^{18}$O J=3-2 emission and the previously detected HD J=1-0 flux, we directly constrain the mid-plane temperature and optical depths of gas and dust emission. We report a gas mass distribution of 13$^{+8}_{-5}\times$(R/20.5AU)$^{-0.9^{+0.4}_{-0.3}}$ g cm$^{-2}$ in the expected formation zone of gas and ice giants (5-21AU). We find the total gas/millimeter-sized dust mass ratio is 140 in this region, suggesting that at least 2.4M_earth of dust aggregates have grown to >centimeter sizes (and perhaps much larger). The radial distribution of gas mass is consistent with a self-similar viscous disk profile but much flatter than the posterior extrapolation of mass distribution in our own and extrasolar planetary systems.

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arXiv2017

Composition of Early Planetary Atmospheres II: Coupled Dust and Chemical Evolution in Protoplanetary Disks

We present the next step in a series of papers devoted to connecting the composition of the atmospheres of forming planets with the chemistry of their natal evolving protoplanetary disks. The model presented here computes the coupled chemical and dust evolution of the disk and the formation of three planets per disk model. Our three canonical planet traps produce a Jupiter near 1 AU, a Hot Jupiter and a Super-Earth. We study the dependency of the final orbital radius, mass, and atmospheric chemistry of planets forming in disk models with initial disk masses that vary by 0.02 $M_\odot$ above and below our fiducial model ($M_{disk,0} = 0.1 ~M_\odot$). We compute C/O and C/N for the atmospheres formed in our 3 models and find that C/O$_{\rm planet}\sim$ C/O$_{\rm disk}$, which does not vary strongly between different planets formed in our model. The nitrogen content of atmospheres can vary in planets that grow in different disk models. These differences are related to the formation history of the planet, the time and location that the planet accretes its atmosphere, and are encoded in the bulk abundance of NH$_3$. These results suggest that future observations of atmospheric NH$_3$ and an estimation of the planetary C/O and C/N can inform the formation history of particular planetary systems.

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arXiv2017

Large-Scale Spectroscopic Mapping of the $ρ$ Ophiuchi Molecular Cloud Complex I. The C$_{2}$H to N$_2$H$^+$ Ratio as a Signpost of Cloud Characteristics

We present 2.5-square-degree C$_{2}$H N=1-0 and N$_2$H$^+$ J=1-0 maps of the $ρ$ Ophiuchi molecular cloud complex. These are the first large-scale maps of the $ρ$ Ophiuchi molecular cloud complex with these two tracers. The C$_{2}$H emission is spatially more extended than the N$_2$H$^+$ emission. One faint N$_2$H$^+$ clump Oph-M and one C$_{2}$H ring Oph-RingSW are identified for the first time. The observed C$_{2}$H to N$_{2}$H$^{+}$ abundance ratio ([C$_{2}$H]/[N$_{2}$H$^{+}$]) varies between 5 and 110. We modeled the C$_{2}$H and N$_2$H$^+$ abundances with 1-D chemical models which show a clear decline of [C$_2$H]/[N$_2$H$^+$] with chemical age. Such an evolutionary trend is little affected by temperatures when they are below 40 K. At high density (n$_H$ $>$ 10$^5$ cm$^{-3}$), however, the time it takes for the abundance ratio to drop at least one order of magnitude becomes less than the dynamical time (e.g., turbulence crossing time $\rm \sim$10$^5$ years). The observed [C$_2$H]/[N$_2$H$^+$] difference between L1688 and L1689 can be explained by L1688 having chemically younger gas in relatively less dense regions. The observed [C$_{2}$H]/[N$_{2}$H$^{+}$] values are the results of time evolution, accelerated at higher densities. For the relative low density regions in L1688 where only C$_2$H emission was detected, the gas should be chemically younger.

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