arXiv subjects

Jonathan P. Williams

Publications and source records attributed to Jonathan P. Williams.

At least 109 records · Page 6Linked 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

The Ophiuchus DIsc Survey Employing ALMA (ODISEA) - I : project description and continuum images at 28 au resolution

We introduce the Ophiuchus DIsc Survey Employing ALMA (ODISEA), a project aiming to study the entire population of Spitzer-selected protoplanetary discs in the Ophiuchus Molecular Cloud (~300 objects) from both millimeter continuum and CO isotopologues data. Here we present 1.3 mm/230 GHz continuum images of 147 targets at 0.2" (28 au) resolution and a typical rms of 0.15 mJy. We detect a total of 133 discs, including the individual components of 11 binary systems and 1 triple system. Fifty-three of these discs are spatially resolved. We find clear substructures (inner cavities, rings, gaps, and/or spiral arms) in 8 of the sources and hints of such structures in another 4 discs. We construct the disc luminosity function for our targets and perform comparisons to other regions. A simple conversion between flux and dust mass (adopting standard assumptions) indicates that all discs detected at 1.3 mm are massive enough to form one or more rocky planets. In contrast, only ~50 discs (~1/3 of the sample) have enough mass in the form of dust to form the canonical 10 M_Earth core needed to trigger runaway gas accretion and the formation of gas giant planets, although the total mass of solids already incorporated into bodies larger than cm scales is mostly unconstrained. The distribution in continuum disc sizes in our sample is heavily weighted towards compact discs: most detected discs have radii < 15 au, while only 23 discs (~15% of the targets) have radii > 30 au.

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arXiv2018

Scaling Relations Associated with Millimeter Continuum Sizes in Protoplanetary Disks

We present a combined, homogenized analysis of archival Submillimeter Array (SMA) and Atacama Large Millimeter/submillimeter Array (ALMA) observations of the spatially resolved 340 GHz (870 $μ$m) continuum emission from 105 nearby protoplanetary disks. Building on the previous SMA survey, we infer surface brightness profiles using a simple model of the observed visibilities to derive the luminosities ($L_{\rm mm}$) and effective sizes ($R_{\rm eff}$) of the continuum emission. With this sample, we confirm the shapes, normalizations, and dispersions for the strong correlations between $L_{\rm mm}$, $M_\ast$ (or $L_\ast$), and $\dot{M}_\ast$ found in previous studies. We also verify the continuum size--luminosity relation determined from the SMA survey alone (extending to an order of magnitude lower $L_{\rm mm}$), demonstrating that the amount of emission scales linearly with the emitting surface area. Moreover, we identify new, although weaker, relationships between $R_{\rm eff}$ and the host and accretion properties, such that disks are larger around more massive hosts with higher accretion rates. We explore these inter-related demographic properties with some highly simplified approximations. These multi-dimensional relationships can be explained if the emission is optically thick with a filling factor of $\sim$0.3, or if the emission is optically thin and disks have roughly the same optical depth profile shapes and normalizations independent of host properties. In both scenarios, we require the dust disk sizes to have a slightly sub-linear relationship with the host mass and a non-negligible dispersion ($\sim$0.2 dex at a given $M_\ast$).

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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

The Distinct Evolutionary Nature of two Class 0 Protostars in Serpens Main SMM4

We have observed the submillimeter continuum condensation SMM4 in Serpens Main using the Atacama Large Millimeter/submillimeter Array (ALMA) during its Cycle 3 in 1.3 mm continuum, 12CO J=2-1, SO J_N=6_5-5_4, and C18O J=2-1 lines at angular resolutions of ~0.55" (240 au). The 1.3 mm continuum emission shows that SMM4 is spatially resolved into two protostars embedded in the same core: SMM4A showing a high brightness temperature, 18 K, with little extended structure and SMM4B showing a low brightness temperature, 2 K, with compact and extended structures. Their separation is ~2100 au. Analysis of the continuum visibilities reveals a disk-like structure with a sharp edge at r~240 au in SMM4A, and a compact component with a radius of 56 au in SMM4B. The 12CO emission traces fan-shaped and collimated outflows associated with SMM4A and SMM4B, respectively. The blue and red lobes of the SMM4B outflow have different position angles by ~30 deg. Their inclination and bending angles in the 3D space are estimated at i_b~ 36 deg, i_r~70 deg, and alpha~40 deg, respectively. The SO emission traces shocked regions, such as cavity walls of outflows and the vicinity of SMM4B. The C18O emission mainly traces an infalling and rotating envelope around SMM4B. The C18O fractional abundance in SMM4B is ~50 times smaller than that of the interstellar medium. These results suggest that SMM4A is more evolved than SMM4B. Our studies in Serpens Main demonstrate that continuum and line observations at millimeter wavelengths allow us to differentiate evolutionary phases of protostars within the Class 0 phase.

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arXiv2018

New insights into the nature of transition disks from a complete disk survey of the Lupus star forming region

Transition disks with large dust cavities around young stars are promising targets for studying planet formation. Previous studies have revealed the presence of gas cavities inside the dust cavities hinting at recently formed, giant planets. However, many of these studies are biased towards the brightest disks in the nearby star forming regions, and it is not possible to derive reliable statistics that can be compared with exoplanet populations. We present the analysis of 11 transition disks with large cavities (>20 AU radius) from a complete disk survey of the Lupus star forming region, using ALMA Band 7 observations at 0.3" (22-30 AU radius) resolution of the 345 GHz continuum, 13CO and C18O 3-2 observations and the Spectral Energy Distribution of each source. Gas and dust surface density profiles are derived using the physical-chemical modeling code DALI. This is the first study of transition disks of large cavities within a complete disk survey within a star forming region. The dust cavity sizes range from 20-90 AU radius and in three cases, a gas cavity is resolved as well. The deep drops in gas density and large dust cavity sizes are consistent with clearing by giant planets. The fraction of transition disks with large cavities in Lupus is ~11%, which is inconsistent with exoplanet population studies of giant planets at wide orbits. Furthermore, we present a hypothesis of an evolutionary path for large massive disks evolving into transition disks with large cavities.

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arXiv2017

The ALMA early science view of FUor/EXor objects - V: continuum disc masses and sizes

Low-mass stars build a significant fraction of their total mass during short outbursts of enhanced accretion known as FUor and EXor outbursts. FUor objects are characterized by a sudden brightening of ~5 magnitudes at visible wavelengths within one year and remain bright for decades. EXor objects have lower amplitude outbursts on shorter timescales. Here we discuss a 1.3 mm ALMA mini-survey of eight outbursting sources (three FUor, four EXor, and the borderline object V1647 Ori) in the Orion Molecular Cloud. While previous papers in this series discuss the remarkable molecular outflows observed in the three FUor objects and V1647 Ori, here we focus on the continuum data and the differences and similarities between the FUor and EXor populations. We find that FUor discs are significantly more massive (~80-600 M_JUP) than the EXor objects (~0.5-40 M_JUP ). We also report that the EXor sources lack the prominent outflows seen in the FUor population. Even though our sample is small, the large differences in disc masses and outflow activity suggest that the two types of objects represent different evolutionary stages. The FUor sources seem to be rather compact (Rc < 20-40 au) and to have a smaller characteristic radius for a given disc mass when compared to T Tauri stars. V1118 Ori, the only known close binary systemin our sample, is shown to host a disc around each one of the stellar components. The disc around HBC 494 is asymmetric, hinting at a structure in the outer disc or the presence of a second disc.

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arXiv2017

ALMA observations of Elias 2-24: a protoplanetary disk with multiple gaps in the Ophiuchus Molecular Cloud

We present ALMA 1.3 mm continuum observations at 0.2" (25 au) resolution of Elias 2-24, one of the largest and brightest protoplanetary disks in the Ophiuchus Molecular Cloud, and report the presence of three partially resolved concentric gaps located at ~20, 52, and 87 au from the star. We perform radiative transfer modeling of the disk to constrain its surface density and temperature radial profile and place the disk structure in the context of mechanisms capable of forming narrow gaps such as condensation fronts and dynamical clearing by actively forming planets. In particular, we estimate the disk temperature at the locations of the gaps to be 23, 15, and 12 K (at 20, 52, and 87 au respectively), very close to the expected snow-lines of CO (23-28 K) and N2 (12-15 K). Similarly, by assuming that the widths of the gaps correspond to 4-8 x the Hill radii of forming planets (as suggested by numerical simulations), we estimate planet masses in the range of 0.2-1.5 M_Jup, 1.0-8.0 M_Jup, and 0.02-0.15 M_Jup for the inner, middle, and outer gap, respectively. Given the surface density profile of the disk, the amount of "missing mass" at the location of each one of these gaps (between 4 and 20 M_Jup) is more than sufficient to account for the formation of such planets.

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arXiv2017

Origin of Interstellar Object A/2017 U1 in a Nearby Young Stellar Association?

We suggest that the kinematics and properties of interstellar object A/2017 U1 point towards its formation in a protoplanetary disk in the ~45 Myr-old Carina or Columba young stellar associations, and subsequent ejection with a low peculiar velocity (1-2 km/sec) during or soon after planet formation inside the ice line. This scenario predicts that the Solar System will encounter more such objects with radiants similar to that of A/2017 U1.

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arXiv2017

ALMA Observations of SMM11 Reveal an Extremely Young Protostar in Serpens Main Cluster

We report the discovery of an extremely young protostar, SMM11, located in the associated submillimeter condensation SMM11 in the Serpens Main cluster using the Atacama Large Millimeter/submillimeter Array (ALMA) during its Cycle 3 at 1.3 mm and an angular resolution of ~0.5"~210 AU. SMM11 is a Class 0 protostar without any counterpart at 70 um or shorter wavelengths. The ALMA observations show 1.3 mm continuum emission associated with a collimated 12CO bipolar outflow. Spitzer and Herschel data show that SMM11 is extremely cold (T_bol=26 K) and faint (L_bol<~0.9 Lsun). We estimate the inclination angle of the outflow to be ~80 deg, almost parallel to the plane of the sky, from simple fitting using wind-driven-shell model. The continuum visibilities consist of Gaussian and power-law components, suggesting a spherical envelope with a radius of ~600 AU around the protostar. The estimated low C18O abundance, X(C18O)=1.5-3x10^-10, is also consistent with its youth. The high outflow velocity, a few 10 kms^-1 at a few 1000 AU, is much higher than theoretical simulations of first hydrostatic cores and we suggest that SMM11 is a transitional object right after the second collapse of the first core.

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arXiv2017

How do stars gain their mass? A JCMT/SCUBA-2 Transient Survey of Protostars in Nearby Star Forming Regions

Most protostars have luminosities that are fainter than expected from steady accretion over the protostellar lifetime. The solution to this problem may lie in episodic mass accretion -- prolonged periods of very low accretion punctuated by short bursts of rapid accretion. However, the timescale and amplitude for variability at the protostellar phase is almost entirely unconstrained. In "A JCMT/SCUBA-2 Transient Survey of Protostars in Nearby Star Forming Regions", we are monitoring monthly with SCUBA-2 the sub-mm emission in eight fields within nearby (<500 pc) star forming regions to measure the accretion variability of protostars. The total survey area of ~1.6 sq.deg. includes ~105 peaks with peaks brighter than 0.5 Jy/beam (43 associated with embedded protostars or disks) and 237 peaks of 0.125-0.5 Jy/beam (50 with embedded protostars or disks). Each field has enough bright peaks for flux calibration relative to other peaks in the same field, which improves upon the nominal flux calibration uncertainties of sub-mm observations to reach a precision of ~2-3% rms, and also provides quantified confidence in any measured variability. The timescales and amplitudes of any sub-mm variation will then be converted into variations in accretion rate and subsequently used to infer the physical causes of the variability. This survey is the first dedicated survey for sub-mm variability and complements other transient surveys at optical and near-IR wavelengths, which are not sensitive to accretion variability of deeply embedded protostars.

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arXiv2017

ALMA Survey of Lupus Protoplanetary Disks I: Dust and Gas Masses

We present the first high-resolution sub-mm survey of both dust and gas for a large population of protoplanetary disks. Characterizing fundamental properties of protoplanetary disks on a statistical level is critical to understanding how disks evolve into the diverse exoplanet population. We use ALMA to survey 89 protoplanetary disks around stars with $M_{\ast}>0.1~M_{\odot}$ in the young (1--3~Myr), nearby (150--200~pc) Lupus complex. Our observations cover the 890~$μ$m continuum and the $^{13}$CO and C$^{18}$O 3--2 lines. We use the sub-mm continuum to constrain $M_{\rm dust}$ to a few Martian masses (0.2--0.4~$M_{\oplus}$) and the CO isotopologue lines to constrain $M_{\rm gas}$ to roughly a Jupiter mass (assuming ISM-like $\rm {[CO]/[H_2]}$ abundance). Of 89 sources, we detect 62 in continuum, 36 in $^{13}$CO, and 11 in C$^{18}$O at $>3σ$ significance. Stacking individually undetected sources limits their average dust mass to $\lesssim6$ Lunar masses (0.03~$M_{\oplus}$), indicating rapid evolution once disk clearing begins. We find a positive correlation between $M_{\rm dust}$ and $M_{\ast}$, and present the first evidence for a positive correlation between $M_{\rm gas}$ and $M_{\ast}$, which may explain the dependence of giant planet frequency on host star mass. The mean dust mass in Lupus is 3$\times$ higher than in Upper Sco, while the dust mass distributions in Lupus and Taurus are statistically indistinguishable. Most detected disks have $M_{\rm gas}\lesssim1~M_{\rm Jup}$ and gas-to-dust ratios $<100$, assuming ISM-like $\rm {[CO]/[H_2]}$ abundance; unless CO is very depleted, the inferred gas depletion indicates that planet formation is well underway by a few Myr and may explain the unexpected prevalence of super-Earths in the exoplanet population.

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arXiv2017

An ALMA Survey of Protoplanetary Disks in the $σ$ Orionis Cluster

The $σ$ Orionis cluster is important for studying protoplanetary disk evolution, as its intermediate age ($\sim$3-5 Myr) is comparable to the median disk lifetime. We use ALMA to conduct a high-sensitivity survey of dust and gas in 92 protoplanetary disks around $σ$ Orionis members with $M_{\ast}\gtrsim0.1 M_{\odot}$. Our observations cover the 1.33 mm continuum and several CO $J=2-1$ lines: out of 92 sources, we detect 37 in the mm continuum and six in $^{12}$CO, three in $^{13}$CO, and none in C$^{18}$O. Using the continuum emission to estimate dust mass, we find only 11 disks with $M_{\rm dust}\gtrsim10 M_{\oplus}$, indicating that after only a few Myr of evolution most disks lack sufficient dust to form giant planet cores. Stacking the individually undetected continuum sources limits their average dust mass to 5$\times$ lower than that of the faintest detected disk, supporting theoretical models that indicate rapid dissipation once disk clearing begins. Comparing the protoplanetary disk population in $σ$ Orionis to those of other star-forming regions supports the steady decline in average dust mass and the steepening of the $M_{\rm dust}$-$M_{\ast}$ relation with age; studying these evolutionary trends can inform the relative importance of different disk processes during key eras of planet formation. External photoevaporation from the central O9 star is influencing disk evolution throughout the region: dust masses clearly decline with decreasing separation from the photoionizing source, and the handful of CO detections exist at projected separations $>1.5$ pc. Collectively, our findings indicate that giant planet formation is inherently rare and/or well underway by a few Myr of age.

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arXiv2017

ALMA Observations of Asymmetric Molecular Gas Emission from a Protoplanetary Disk in the Orion Nebula

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of molecular line emission from d216-0939, one of the largest and most massive protoplanetary disks in the Orion Nebula Cluster (ONC). We model the spectrally resolved HCO$^+$ (4--3), CO (3--2), and HCN (4--3) lines observed at 0\farcs5 resolution to fit the temperature and density structure of the disk. We also weakly detect and spectrally resolve the CS (7--6) line but do not model it. The abundances we derive for CO and HCO$^+$ are generally consistent with expected values from chemical modeling of protoplanetary disks, while the HCN abundance is higher than expected. We dynamically measure the mass of the central star to be $2.17\pm0.07\,M_\odot$ which is inconsistent with the previously determined spectral type of K5. We also report the detection of a spatially unresolved high-velocity blue-shifted excess emission feature with a measurable positional offset from the central star, consistent with a Keplerian orbit at $60\pm20\,\mathrm{au}$. Using the integrated flux of the feature in HCO$^+$ (4--3), we estimate the total H$_2$ gas mass of this feature to be at least $1.8-8\,M_\mathrm{Jupiter}$, depending on the assumed temperature. The feature is due to a local temperature and/or density enhancement consistent with either a hydrodynamic vortex or the expected signature of the envelope of a forming protoplanet within the disk.

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arXiv2016

Lupus disks with faint CO isotopologues: low gas/dust or large carbon depletion?

An era has started in which gas and dust can be observed independently in protoplanetary disks, thanks to the recent surveys with ALMA. The first near-complete high-resolution disk survey in both dust and gas in a single star-forming region has been carried out in Lupus, finding surprisingly low gas/dust ratios. The goal of this work is to fully exploit CO isotopologues observations in Lupus, comparing them with physical-chemical model results, in order to obtain gas masses for a large number of disks. We have employed physical-chemical models to analyze continuum and CO isotopologues observations of Lupus disks, including isotope-selective processes and freeze-out. Employing also the ALMA 13CO-only detections, disk gas masses have been calculated for a total of 34 sources, expanding the sample of 10 disks studied by Ansdell et al. (2016), where also C18O was detected. We confirm that overall gas-masses are very low, often smaller than 1 $M_{\rm J}$, if volatile carbon is not depleted. Accordingly, global gas/dust ratios predominantly between 1 and 10. Low CO-based gas masses and gas/dust ratios may indicate rapid loss of gas, or alternatively chemical evolution, e.g. via sequestering of carbon from CO to more complex molecules, or carbon locked up in larger bodies. Current ALMA observations cannot distinguish between these two hypotheses. We have simulated both scenarios, but chemical model results do not allow us to rule out one of the two. Assuming that all Lupus disks have evolved mainly due to viscous processes over the past few Myr, the observed correlation between the current mass accretion rate and dust mass found by Manara et al. (2016) implies a constant gas-to-dust ratio, which is close to 100 based on the observed $M_{\rm disk}/\dot{M}_{\rm acc}$ ratio. This in turn points to a scenario in which carbon depletion is responsible for the low CO isotopologue line luminosities.

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arXiv2016

A VLA Survey For Faint Compact Radio Sources in the Orion Nebula Cluster

We present Karl G. Janksy Very Large Array (VLA) 1.3 cm, 3.6 cm, and 6 cm continuum maps of compact radio sources in the Orion Nebular Cluster. We mosaicked 34 square arcminutes at 1.3 cm, 70 square arcminutes at 3.6 cm and 109 square arcminutes at 6 cm, containing 778 near-infrared detected YSOs and 190 HST-identified proplyds (with significant overlap between those characterizations). We detected radio emission from 175 compact radio sources in the ONC, including 26 sources that were detected for the first time at these wavelengths. For each detected source we fit a simple free-free and dust emission model to characterize the radio emission. We extrapolate the free-free emission spectrum model for each source to ALMA bands to illustrate how these measurements could be used to correctly measure protoplanetary disk dust masses from sub-millimeter flux measurements. Finally, we compare the fluxes measured in this survey with previously measured fluxes for our targets, as well as four separate epochs of 1.3 cm data, to search for and quantify variability of our sources.

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arXiv2016

Imaging the water-snow line during a protostellar outburst

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.

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