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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 91 records · Page 5
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NASA NTRS2016

Planet Formation Imager (PFI): Science Vision and Key Requirements

The Planet Formation Imager (PFI) project aims to provide a strong scientific vision for ground-based optical astronomy beyond the upcoming generation of Extremely Large Telescopes. We make the case that a breakthrough in angular resolution imaging capabilities is required in order to unravel the processes involved in planet formation. PFI will be optimised to provide a complete census of the protoplanet population at all stellocentric radii and over the age range from 0.1 to ~ 100 Myr. Within this age period, planetary systems undergo dramatic changes and the final architecture of planetary systems is determined. Our goal is to study the planetary birth on the natural spatial scale where the material is assembled, which is the “Hill Sphere” of the forming planet, and to characterise the protoplanetary cores by measuring their masses and physical properties. Our science working group has investigated the observational characteristics of these young protoplanets as well as the migration mechanisms that might alter the system architecture. We simulated the imprints that the planets leave in the disk and study how PFI could revolutionise areas ranging from exoplanet to extragalactic science. In this contribution we outline the key science drivers of PFI and discuss the requirements that will guide the technology choices, the site selection, and potential science/technology tradeoffs.

Planet formation↗
DOE OSTI2014

High-resolution submillimeter and near-infrared studies of the transition disk around Sz 91

To reveal the structures of a transition disk around a young stellar object in Lupus, Sz 91 , we have performed aperture synthesis 345 GHz continuum and CO(3-2) observations with the Submillimeter Array (∼1''-3'' resolution) and high-resolution imaging of polarized intensity at the K{sub s} -band using the HiCIAO instrument on the Subaru Telescope (0.''25 resolution). Our observations successfully resolved the inner and outer radii of the dust disk to be 65 and 170 AU, respectively, which indicates that Sz 91 is a transition disk source with one of the largest known inner holes. The model fitting analysis of the spectral energy distribution reveals an H{sub 2} mass of 2.4 × 10{sup –3} M {sub ☉} in the cold (T < 30 K) outer part at 65 AU 3 × 10{sup –9} M {sub ☉}) of hot (T ∼ 180 K) dust possibly remains inside the inner hole of the disk. The structure of the hot component could be interpreted as either an unresolved self-luminous companion body (not directly detected in our observations) or a narrow ring inside the inner hole. Significant CO(3-2) emission with a velocity gradient along the major axis of the dust disk is concentrated on the Sz 91 position, suggesting a rotating gas disk with a radius of 420 AU. The Sz 91 disk is possibly a rare disk in an evolutionary stage immediately after the formation of protoplanets because of the large inner hole and the lower disk mass than other transition disks studied thus far.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The effect of planets beyond the ice line on the accretion of volatiles by habitable-zone rocky planets

Models of planet formation have shown that giant planets have a large impact on the number, masses, and orbits of terrestrial planets that form. In addition, they play an important role in delivering volatiles from material that formed exterior to the snow line (the region in the disk beyond which water ice can condense) to the inner region of the disk where terrestrial planets can maintain liquid water on their surfaces. We present simulations of the late stages of terrestrial planet formation from a disk of protoplanets around a solar-type star and we include a massive planet (from 1 M {sub ⊕} to 1 M {sub J}) in Jupiter's orbit at ∼5.2 AU in all but one set of simulations. Two initial disk models are examined with the same mass distribution and total initial water content, but with different distributions of water content. We compare the accretion rates and final water mass fraction of the planets that form. Remarkably, all of the planets that formed in our simulations without giant planets were water-rich, showing that giant planet companions are not required to deliver volatiles to terrestrial planets in the habitable zone. In contrast, an outer planet at least several times the mass of Earth may be needed to clear distant regions of debris truncating the epoch of frequent large impacts. Observations of exoplanets from radial velocity surveys suggest that outer Jupiter-like planets may be scarce, therefore, the results presented here suggest that there may be more habitable planets residing in our galaxy than previously thought.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

N-body simulations of terrestrial planet formation under the influence of a hot Jupiter

We investigate the formation of multiple-planet systems in the presence of a hot Jupiter (HJ) using extended N-body simulations that are performed simultaneously with semianalytic calculations. Our primary aims are to describe the planet formation process starting from planetesimals using high-resolution simulations, and to examine the dependences of the architecture of planetary systems on input parameters (e.g., disk mass, disk viscosity). We observe that protoplanets that arise from oligarchic growth and undergo type I migration stop migrating when they join a chain of resonant planets outside the orbit of an HJ. The formation of a resonant chain is almost independent of our model parameters, and is thus a robust process. At the end of our simulations, several terrestrial planets remain at around 0.1 AU. The formed planets are not equal mass; the largest planet constitutes more than 50% of the total mass in the close-in region, which is also less dependent on parameters. In the previous work of this paper, we have found a new physical mechanism of induced migration of the HJ, which is called a crowding-out. If the HJ opens up a wide gap in the disk (e.g., owing to low disk viscosity), crowding-out becomes less efficient and the HJ remains. We also discuss angular momentum transfer between the planets and disk.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Searching for circumplanetary disks around LkCa 15

We present Karl G. Jansky Very Large Array (VLA) observations of the 7 mm continuum emission from the disk surrounding the young star LkCa 15. The observations achieve an angular resolution of 70 mas and spatially resolve the circumstellar emission on a spatial scale of 9 AU. The continuum emission traces a dusty annulus of 45 AU in radius that is consistent with the dust morphology observed at shorter wavelengths. The VLA observations also reveal a compact source at the center of the disk, possibly due to thermal emission from hot dust or ionized gas located within a few AU from the central star. No emission is observed between the star and the dusty ring and, in particular, at the position of the candidate protoplanet LkCa 15 b. By comparing the observations with theoretical models for circumplanetary disk emission, we find that if LkCa 15 b is a massive planet (>5 M{sub J} ) accreting at a rate greater than 10{sup –6} M{sub J} yr{sup –1}, then its circumplanetary disk is less massive than 0.1 M{sub J} , or smaller than 0.4 Hill radii. Similar constraints are derived for any possible circumplanetary disk orbiting within 45 AU from the central star. The mass estimates are uncertain by at least one order of magnitude due to the uncertainties on the mass opacity. Future ALMA observations of this system might be able to detect circumplanetary disks down to a mass of 5 × 10{sup –4} M{sub J} and as small as 0.2 AU, providing crucial constraints on the presence of giant planets in the act of forming around this young star.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Two-component secular gravitational instability in a protoplanetary disk: A possible mechanism for creating ring-like structures

The instability in protoplanetary disks due to gas-dust friction and self-gravity of gas and dust is investigated using linear analysis. In the case where the dust-to-gas ratio is enhanced and turbulence is weak, the instability grows, even in gravitationally stable disks, on a timescale of order 10{sup 4-5} yr at a radius of order 100 AU. If we ignore the dynamical feedback from dust grains in the gas equation of motion, the instability reduces to the so-called ''secular gravitational instability'', which was investigated previously to be an instability of dust in a fixed background gas flow. In this work, we solve the equations of motion for both gas and dust consistently and find that long-wavelength perturbations are stable, in contrast to the secular gravitational instability in the simplified treatment. This may indicate that we should not neglect small terms in the equation of motion if the growth rate is small. The instability is expected to form ring structures in protoplanetary disks. The width of the ring formed at a radius of 100 AU is a few tens of AU. Therefore, the instability is a candidate for the formation mechanism of observed ring-like structures in disks. Another aspect of the instability is the accumulation of dust grains, and hence the instability may play an important role in the formation of planetesimals, rocky protoplanets, and cores of gas giants located at radii ∼100 AU. If these objects survive the dispersal of the gaseous component of the disk, they may be the origin of debris disks.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Kepler-445, Kepler-446 and the occurrence of compact multiples orbiting mid-M dwarf stars

We confirm and characterize the exoplanetary systems Kepler-445 and Kepler-446: two mid-M dwarf stars, each with multiple, small, short-period transiting planets. Kepler-445 is a metal-rich ([Fe/H] = +0.25 ± 0.10) M4 dwarf with three transiting planets, and Kepler-446 is a metal-poor ([Fe/H] = –0.30 ± 0.10) M4 dwarf also with three transiting planets. Kepler-445c is similar to GJ 1214b: both in planetary radius and the properties of the host star. The Kepler-446 system is similar to the Kepler-42 system: both are metal-poor with large galactic space velocities and three short-period, likely rocky transiting planets that were initially assigned erroneously large planet-to-star radius ratios. We independently determined stellar parameters from spectroscopy and searched for and fitted the transit light curves for the planets, imposing a strict prior on stellar density in order to remove correlations between the fitted impact parameter and planet-to-star radius ratio for short-duration transits. Combining Kepler-445, Kepler-446, and Kepler-42, and isolating all mid-M dwarf stars observed by Kepler with the precision necessary to detect similar systems, we calculate that 21{sub −5}{sup +7}% of mid-M dwarf stars host compact multiples (multiple planets with periods of less than 10 days) for a wide range of metallicities. We suggest that the inferred planet masses for these systems support highly efficient accretion of protoplanetary disk metals by mid-M dwarf protoplanets.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Diversity of extrasolar planets and diversity of molecular cloud cores. II. Masses of gas giant planets

In Paper I, we suggested that the diversity of extrasolar planets might originate from the diversity of molecular cloud cores and specifically illustrated the relationship between semi-major axes of planets in a planetary system and the properties of its progenitor cloud core. In this paper, we investigate the dependence of the mass of a giant planet on the progenitor core properties. We give numerical results of planet masses as a function of the core properties by using the core accretion model of planet formation. Comparing with observations, our model could explain the range and the most frequent values of observed masses of gas giant planets. Our calculations could interpret the observed pileup of gas giants at >1 AU and the distinct deficit of gas giants from ∼0.05 to 1 AU. Our calculations indicate that it is difficult to form gas giant planets outside 20 AU and the intermediate mass planets might be found at ∼15–30 AU as the observing technique advances. We suggest a formation mechanism of super-Jupiters in the framework of the core accretion model of planet formation. A super-Jupiter can form under the condition that a disk is gravitationally unstable (high viscosity) when a protoplanet opens the deep gap. Our calculations infer that the upper limit of super-Jupiter masses, the range of semi-major axes, and the occurrence rate of super-Jupiters increase with the central star mass. We find that gas giant planets should commonly exist around various stellar masses.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

AN UNBIASED 1.3 mm EMISSION LINE SURVEY OF THE PROTOPLANETARY DISK ORBITING LkCa 15

The outer (>30 AU) regions of the dusty circumstellar disk orbiting the ∼2–5 Myr old, actively accreting solar analog LkCa 15 are known to be chemically rich, and the inner disk may host a young protoplanet within its central cavity. To obtain a complete census of the brightest molecular line emission emanating from the LkCa 15 disk over the 210–270 GHz (1.4–1.1 mm) range, we have conducted an unbiased radio spectroscopic survey with the Institute de Radioastronomie Millimétrique (IRAM) 30 m telescope. The survey demonstrates that in this spectral region, the most readily detectable lines are those of CO and its isotopologues {sup 13}CO and C{sup 18}O, as well as HCO{sup +}, HCN, CN, C{sub 2}H, CS, and H{sub 2}CO. All of these species had been previously detected in the LkCa 15 disk; however, the present survey includes the first complete coverage of the CN (2–1) and C{sub 2}H (3–2) hyperfine complexes. Modeling of these emission complexes indicates that the CN and C{sub 2}H either reside in the coldest regions of the disk or are subthermally excited, and that their abundances are enhanced relative to molecular clouds and young stellar object environments. These results highlight the value of unbiased single-dish line surveys in guiding future high-resolution interferometric imaging of disks.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

FORMATION OF SUPER-EARTH MASS PLANETS AT 125–250 AU FROM A SOLAR-TYPE STAR

We investigate pathways for the formation of icy super-Earth mass planets orbiting at 125–250 AU around a 1 M{sub ⊙} star. An extensive suite of coagulation calculations demonstrates that swarms of 1 cm–10 m planetesimals can form super-Earth mass planets on timescales of 1–3 Gyr. Collisional damping of 10{sup −2}−10{sup 2} cm particles during oligarchic growth is a highlight of these simulations. In some situations, damping initiates a second runaway growth phase where 1000–3000 km protoplanets grow to super-Earth sizes. Our results establish the initial conditions and physical processes required for in situ formation of super-Earth planets at large distances from the host star. For nearby dusty disks in HD 107146, HD 202628, and HD 207129, ongoing super-Earth formation at 80–150 AU could produce gaps and other structures in the debris. In the solar system, forming a putative planet X at a≲300 AU (a≳1000 AU) requires a modest (very massive) protosolar nebula.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Protoplanetary disks including radiative feedback from accreting planets

While recent observational progress is converging on the detection of compact regions of thermal emission due to embedded protoplanets, further theoretical predictions are needed to understand the response of a protoplanetary disk to the radiative feedback from planet formation. This is particularly important to make predictions for the observability of circumplanetary regions. In this work we use 2D hydrodynamical simulations to examine the evolution of a viscous protoplanetary disk in which a luminous Jupiter-mass planet is embedded. We use an energy equation that includes the radiative heating of the planet as an additional mechanism for planet formation feedback. Several models are computed for planet luminosities ranging from 10{sup −5} to 10{sup −3} solar luminosities. We find that the planet radiative feedback enhances the disk’s accretion rate at the planet’s orbital radius, producing a hotter and more luminous environement around the planet, independently of the prescription used to model the disk’s turbulent viscosity. We also estimate the thermal signature of the planet feedback for our range of planet luminosities, finding that the emitted spectrum of a purely active disk, without passive heating, is appreciably modified in the infrared. We simulate the protoplanetary disk around HD 100546 where a planet companion is located at about 68 AU from the star. Assuming the planet mass is five Jupiter masses and its luminosity is ∼2.5×10{sup −4} L{sub ⊙}, we find that the radiative feedback of the planet increases the luminosity of its ∼5 AU circumplanetary disk from 10{sup −5} L{sub ⊙} (without feedback) to 10{sup −3} L{sub ⊙}, corresponding to an emission of ∼1 mJy in the L{sup ′} band after radiative transfer calculations, a value that is in good agreement with HD 100546b observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Cooling requirements for the vertical shear instability in protoplanetary disks

The vertical shear instability (VSI) offers a potential hydrodynamic mechanism for angular momentum transport in protoplanetary disks (PPDs). The VSI is driven by a weak vertical gradient in the disk’s orbital motion, but must overcome vertical buoyancy, a strongly stabilizing influence in cold disks, where heating is dominated by external irradiation. Rapid radiative cooling reduces the effective buoyancy and allows the VSI to operate. We quantify the cooling timescale t{sub c} needed for efficient VSI growth, through a linear analysis of the VSI with cooling in vertically global, radially local disk models. We find the VSI is most vigorous for rapid cooling with t{sub c}<Ω{sub K}{sup −1}h|q|/(γ−1) in terms of the Keplerian orbital frequency, Ω{sub K}, the disk’s aspect-ratio, h≪1, the radial power-law temperature gradient, q, and the adiabatic index, γ. For longer t{sub c}, the VSI is much less effective because growth slows and shifts to smaller length scales, which are more prone to viscous or turbulent decay. We apply our results to PPD models where t{sub c} is determined by the opacity of dust grains. We find that the VSI is most effective at intermediate radii, from ∼5 to ∼50 AU with a characteristic growth time of ∼30 local orbital periods. Growth is suppressed by long cooling times both in the opaque inner disk and the optically thin outer disk. Reducing the dust opacity by a factor of 10 increases cooling times enough to quench the VSI at all disk radii. Thus the formation of solid protoplanets, a sink for dust grains, can impede the VSI.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Application of gas dynamical friction for planetesimals. I. Evolution of single planetesimals

The growth of small planetesimals into large planetary embryos occurs far before the dispersal of the gas from the protoplanetary disk. The planetesimal–gaseous disk interactions give rise to migration and orbital evolution of the planetesimals/planets. Small planetesimals are dominated by aerodynamic gas drag. Large protoplanets, m∼0.1M{sub ⊕}, are dominated by type I migration differential torque. There is an additional mass range m∼10{sup 21}−10{sup 25} g of intermediate-mass planetesimals (IMPs), where gravitational interactions with the disk dominate over aerodynamic gas drag, but for which such interactions were typically neglected. Here, we model these interactions using the gas dynamical friction (GDF) approach, previously used to study the disk–planet interactions at the planetary mass range. We find the critical size where GDF dominates over gas drag, and then we study the implications of GDF on single IMPs. We find that planetesimals with small inclinations rapidly become co-planar. Eccentric orbits circularize within a few Myr, provided the the planetesimal mass is large, m≳10{sup 23} g, and that the initial eccentricity is low, e≲0.1. Planetesimals of higher masses, m∼10{sup 24}−10{sup 25} g, inspiral on a timescale of a few Myr, leading to an embryonic migration to the inner disk. This can lead to an overabundance of rocky material (in the form of IMPs) in the inner protoplanetary disk (<1 AU) and induce rapid planetary growth. This can explain the origin of super-Earth planets. In addition, GDF damps the velocities of IMPs, thereby cooling the planetesimal disk and affecting its collisional evolution through quenching the effects of viscous stirring by the large bodies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

RESOLVED IMAGES OF THE PROTOPLANETARY DISK AROUND HD 100546 WITH ALMA

The disk around the Herbig Ae/Be star HD 100546 has been extensively studied and it is one of the systems for which there are observational indications of ongoing and/or recent planet formation. However, up until now, no resolved image of the millimeter dust emission or the gas has been published. We present the first resolved images of the disk around HD 100546 obtained in Band 7 with the ALMA observatory. The CO (3-2) image reveals a gas disk that extends out to 350 au radius at the 3σ level. Surprisingly, the 870 μm dust continuum emission is compact (radius <60 au) and asymmetric. The dust emission is well matched by a truncated disk with an outer radius of ≈50 au. The lack of millimeter-sized particles outside 60 au is consistent with radial drift of particles of this size. The protoplanet candidate, identified in previous high-contrast NACO/VLT L' observations, could be related to the sharp outer edge of the millimeter-sized particles. Future higher angular resolution ALMA observations are needed to determine the detailed properties of the millimeter emission and the gas kinematics in the inner region (<2''). Such observations could also reveal the presence of a planet through the detection of circumplanetary disk material.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

OPTICAL IMAGING POLARIMETRY OF THE LkCa 15 PROTOPLANETARY DISK WITH SPHERE ZIMPOL

We present the first optical (590–890 nm) imaging polarimetry observations of the pre-transitional protoplanetary disk around the young solar analog LkCa 15, addressing a number of open questions raised by previous studies. We detect the previously unseen far side of the disk gap, confirming the highly off-centered scattered-light gap shape that was postulated from near-infrared imaging, at odds with the symmetric gap inferred from millimeter interferometry. Furthermore, we resolve the inner disk for the first time and trace it out to 30 AU. This new source of scattered light may contribute to the near-infrared interferometric signal attributed to the protoplanet candidate LkCa 15 b, which lies embedded in the outer regions of the inner disk. Finally, we present a new model for the system architecture of LkCa 15 that ties these new findings together. These observations were taken during science verification of SPHERE ZIMPOL and demonstrate this facility’s performance for faint guide stars under adverse observing conditions.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Atmospheres of low-mass planets: The “boil-off”

We show that, for a low-mass planet that orbits its host star within a few tenths of an AU (like the majority of the Kepler planets), the atmosphere it was able to accumulate while embedded in the protoplanetary disk may not survive unscathed after the disk disperses. This gas envelope, if more massive than a few percent of the core (with a mass below 10M{sub ⊕}), has a cooling time that is much longer than the timescale on which the planet exits the disk. As such, it could not have contracted significantly from its original size, of the order of the Bondi radius. So a newly exposed protoplanet would be losing mass via a Parker wind that is catalyzed by the stellar continuum radiation. This represents an intermediate stage of mass-loss, occurring soon after the disk has dispersed, but before the EUV/X-ray driven photoevaporation becomes relevant. The surface mass-loss induces a mass movement within the envelope that advects internal heat outward. As a result, the planet atmosphere rapidly cools down and contracts, until it has reached a radius of the order of 0.1 Bondi radius, at which time the mass-loss effectively shuts down. Within a million years after the disk disperses, we find a planet that has only about 10% of its original envelope, and a Kelvin–Helmholtz time that is much longer than its actual age. We suggest that this “boil-off” process may be partially responsible for the lack of planets above a radius of 2.5R{sub ⊕} in the Kepler data, provided planet formation results in initial envelope masses of tens of percent.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

DETECTION OF ROTATIONAL SPECTRAL VARIATION ON THE M-TYPE ASTEROID (16) PSYCHE

The asteroid (16) Psyche is of scientific interest because it contains ∼1% of the total mass of the asteroid belt and is thought to be the remnant metallic core of a protoplanet. Radar observations have indicated the significant presence of metal on the surface with a small percentage of silicates. Prior ground-based observations showed rotational variations in the near-infrared (NIR) spectra and radar albedo of this asteroid. However, no comprehensive study that combines multi-wavelength data has been conducted so far. Here we present rotationally resolved NIR spectra (0.7–2.5 μ m) of (16) Psyche obtained with the NASA Infrared Telescope Facility. These data have been combined with shape models of the asteroid for each rotation phase. Spectral band parameters extracted from the NIR spectra show that the pyroxene band center varies from ∼0.92 to 0.94 μ m. Band center values were used to calculate the pyroxene chemistry of the asteroid, whose average value was found to be Fs{sub 30}En{sub 65}Wo{sub 5}. Variations in the band depth (BD) were also observed, with values ranging from 1.0% to 1.5%. Using a new laboratory spectral calibration method, we estimated an average orthopyroxene content of 6% ± 1%. The mass-deficit region of Psyche, which exhibits the highest radar albedo, also shows the highest value for the spectral slope and the minimum BD. The spectral characteristics of Psyche suggest that its parent body did not have the typical structure expected for a differentiated body or that the sequence of events that led to its current state was more complex than previously thought.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

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. We model the spectrally resolved HCO{sup +} (4–3), CO (3–2), and HCN (4–3) lines observed at 0.&prime;5 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{sup +} 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±0.07 M{sub ⊙}, which is inconsistent with the previously determined spectral type of K5. We also report the detection of a spatially unresolved high-velocity blueshifted excess emission feature with a measurable positional offset from the central star, consistent with a Keplerian orbit at 60 ± 20 au. Using the integrated flux of the feature in HCO{sup +} (4–3), we estimate the total H{sub 2} gas mass of this feature to be at least 1.8--8 M{sub 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.

79 ASTRONOMY AND ASTROPHYSICS↗