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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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NASA NTRS2004

Studies of Circumstellar Disk Evolution

Spitzer Space Telescope infrared data for our program on disk evolution has been taken (the main IRAC - 3-8 micron exposures; the 24 and 70 micron MIPS data are to come later). We now have deep maps in the four IRAC bands of the 3-Myr-old cluster Trumpler 37, and the 10-Myr-old cluster NGC 7160. Analysis of these data has now begun. We will be combining these data with our ground-based photometric and spectroscopic data to obtain a complete picture of disk frequency as a function of mass through this important age range, which spans the likely epoch of (giant) planet formation in most systems. Analysis of the SIRTF data, and follow-on ground-based spectroscopy on the converted MMT telescope using the wide-field, fiber-fed, multiobject spectrographs, Hectospec and Hectochelle, will be the major activity during the next year.Work was also performed on the following: protoplanetary disk mass accretion rates in very low-mass stars; the inner edge of T Tauri disks; accretion in intermediate-mass T Tauri stars (IMPS); and the near-infrared spectra of the rapidly-accreting protostellar disks FU Ori and V1057 Cyg.

Hartmann, Lee W.↗
NASA NTRS2005

Studies of Circumstellar Disk Evolution

The aim of this project is to develop a comprehensive global picture of the physical conditions in, and evolutionary timescales of, pre-main sequence accretion disks. The results of this work will help constrain the initial conditions for planet formation. To this end we are developing much larger samples of 3-10 Myr-old stars to provide better empirical constraints on protoplanetary disk evolution; measuring disk accretion rates in these systems; and constructing detailed model disk structures consistent with observations to infer physical conditions such as grain growth in protoplanetary disks.

Hartmann, Lee W.↗
NASA NTRS2007

HST/ACS Coronagraphic Observations of the HD 163296 Circumstellar Disk: Evidence of Time-Variable Self-Shadowing?

We present Hubble Space Telescope Advanced Camera for Surveys (HST/ACS) coronagraphic observations of the Herbig Ae star HD 163296. HD 163296's scattered light disk was resolved in the F606W and F814W filters in observations obtained In 2003 and in the F435W filter in observations obtained in 2004. Analysis of single-epoch data indicates that the disk (V-I) color is redder than the observed stellar (V-I) color. This spatially uniform red disk color might be indicative of either an evolution in the grain size distribution (i.e. grain growth) and/or composition. Both of these processes would be consistent with the observed flat geometry of the outer disk, as diagnosed by the observed r$(exp -3)$ power law behavior of its median azimuthally averaged disk surface brightness, which suggest that grain evolution is occurring. Comparison of ACS and STIS epoch scattered light data reveals differences in the observed disk surface brightnesses, of order 1 mag arcsec$(exp -2)$, in both V and white-light filter bandpasses. Along with the observed variability in the visibility and surface brightness of the ansa(e) in the disk, and spectropolarimetric variability of the system, these results suggest that the resolved scattered light disk is variable, a phenomenon not previously observed in any other Herbig protoplanetary system We speculate that the observed behavior might be attributable to the variable inflation of the scale height of the inner disk wall, which results in variable self-shadowing of the outer disk.

Wisniewski, J.↗
NASA NTRS2008

Observations of Circumstellar Disks with Infrared Interferometry

Star formation is arguably the area of astrophysics in which infrared interferometry has had the biggest impact. The optically thick portion of T Tauri and Herbig Ae/Be disks DO NOT extend to a few stellar radii of the stellar surface. Emission is coming from near the dust sublimation radius, but not all from a single radius. The Herbig Ae stars can be either flared or self-shadowed but very massive (early Be) stars are geometrically thin. The Herbig Ae stars can be either flared or self-shadowed but very massive (early Be) stars are geometrically thin. Observational prospects are rapidly improving: a) Higher spectral resolution will allow observations of the gas: jets, winds, accretion. b) Closure phase and imaging will help eliminate model uncertainties/dependencies.

young steller objects↗
NASA NTRS2013

Modeling Circumstellar Disks of B-Type Stars with Observations from the Palomar Testbed Interferometer

Geometrical (uniform disk) and numerical models were calculated for a set of B-emission (Be) stars observed with the Palomar Testbed Interferometer (PTI). Physical extents have been estimated for the disks of a total of15 stars via uniform disk models. Our numerical non-LTE models used parameters for the B0, B2, B5, and B8spectral classes and following the framework laid by previous studies, we have compared them to infrared K-band interferometric observations taken at PTI. This is the first time such an extensive set of Be stars observed with long-baseline interferometry has been analyzed with self-consistent non-LTE numerical disk models.

Grzenia, B. J.↗
NASA NTRS2012

The SEEDS of Planet Formation: Indirect Signatures of Giant Planets in Transitional Disks

Circumstellar disks associated with PMS stars are the site where planetesimals form and grow, and ultimately where planets are produced. A key phase in the evolution of such disks is the phase where clearing of the disk has begun, potentially enabling direct detection of giant planets, but the disk retains sufficient material that indirect signatures that these are young planetary systems are also present. After reviewing what has been learned from studies of the IR spectral energy distribution and (sub )mm-interferometry, I will discuss recent results obtained as part of the Strategic Exploration of Exoplanets and Disks with Subaru (SEEDS).

Grady, Carol↗
NASA NTRS1989

Erosion of circumstellar particle disks by interstellar dust

Circumstellar particle disks appear to be a common phenomenon; however, their properties vary greatly. Models of the evolution of such systems focus on internal mechanisms such as interparticle collisions and Poynting-Robertson drag. Herein it is shown that 'sandblasting' by interstellar dust can be an important and even dominant contributor to the evolution of circumstellar particle disks. Stars spend up to about 3 percent of their main-sequence lifetimes within atomic clouds. Among an IRAS sample of 21 nearby main-sequence A stars, beta Pictoris has the brightest disk; it also possesses the smallest random velocity and therefore the slowest predicted erosion rate.

Lissauer, Jack J.↗
NASA NTRS2011

A Collisional Algorithm for Modeling Circumstellar Debris Disks

Many planetary systems harbor circumstellar disks of dust and planetesimals thought to be debris left over from planet formation. These debris disks exhibit a range of morphological features which can arise from the gravitational perturbations of planets. Accurate models of these features, accounting for the interactions of the particles in a disk with each other and with whatever planets they contain, can act as signposts for planets in debris disks that otherwise could not be detected. Such models can also constrain the planet's mass and orbital parameters. Current models for many disks consider the gravitational and radiative effects of the star and planets on the disk, but neglect the morphological consequences of collisional interactions between the planetesimals. Many observed disk features are not satisfactorily explained by the current generation of models. I am developing a new kind of debris disk model that considers both the gravitational shaping of the disk by planets and the inelastic collisions between particles. I will use a hybrid N-body integrator to numerically solve the equations of motion for the particles and planets in the disk. To include the collisional effects, I begin with an algorithm that tests for collisions at each step of the orbit integration and readjusts the velocities of colliding particles. I am adapting this algorithm to the problem at hand by allowing each particle to represent a "swarm" of planetesimals with a range of masses. When the algorithm detects an encounter between swarms, two or three swarms are produced to approximate the range of possible trajectories of the daughter planetesimals. Here I present preliminary results from my collisional algorithm.

Nesvold, Erika↗
NASA NTRS2019

Disentangling Planets from Photoelectric Instability in Gas-rich Optically Thin Dusty Disks

Structures in circumstellar disks such as gaps and rings are often attributed to planets. This connection has been difficult to show unequivocally, as other processes may also produce these features. In particular, a photoelectric instability (PEI) has been proposed, operating in gas-rich optically thin disks, that generates structures predicted by planet–disk interactions. We examine the question of how to disentangle the planetary effects on disk structure from the effects of the PEI. We use the Pencil Code to perform 2D global hydrodynamical models of the dynamics of gas and dust in a thin disk with and without planetary perturbers. Photoelectric heating is modeled with an equation of state where pressure is proportional to dust surface density. The drag force on grains and its backreaction on the gas are included. Analyzing the situation without PEI, we find that gas–dust interactions alter the shape of the planetary gap from the dust-free case when the local dust-to-gas ratio ε approaches unity. This result also applies to primordial disks, because dust drifting inward accumulates at the edge of the planetary gap, and any initial dust-to-gas ratio eventually achieves ε = 1 if the dust reservoir is sufficient. We find a result particular to high dust-to-gas ratio disks as well: as dust drifts inward, the dust front becomes a sharp transition, and the backreaction triggers the Rossby wave instability. When PEI is included, we find that it obscures structures induced by planets unless the planet's mass is sufficiently large to carve a noticeable gap. Specifically, the instability generates arcs and rings of regular spacing: a planet is discernible when it carves a dust gap wider than the wavelength of the PEI.

Areli Castrejon↗
NASA NTRS2003

Beta Pic-like Circumstellar Gas Disk Around 2 And

This grant was awarded to support the data analysis and publication of results from our project entitled P Pic-like Circumstellar Gas Disk Around 2 And . We proposed to obtain FUSE observations of 2 And and study the characteristics and origin of its circumstellar gas. We observed 2 Andromedae with FUSE on 3-4 July 2001 in 11 exposures with a total exposure time of 21,289 seconds through the LWRS aperture. Our data were calibrated with Version 1.8.7 of the CALFUSE pipeline processing software. We corrected the wavelength scale for the heliocentric velocity error in this version of the CALFUSE software. The relative accuracy of the calibrated wavelength scale is +/- 9 km/s . We produced a co-added spectrum in the LiF 1B and LiF 2A channels (covering the 1100 to 1180 A region) by cross-correlating the 11 individual exposures and doing an exposure-time weighted average flux. The final co-added spectra have a signal-to-noise ratio in the stellar continuum near 1150 A of about 20. To obtain an absolute wavelength calibration, we cross-correlated our observed spectra with a model spectrum to obtain the best fit for the photospheric C I lines. Because the photospheric lines are very broad, this yields an absolute accuracy for the wavelength scale of approx.+/- 15 km/s. We then rebinned 5 original pixels to yield the optimal sampling of .033 A for each new pixel, because the calibrated spectra oversample the spectral resolution for FUSE+LWRS (R = 20,000 +/- 2,000).

Cheng, Patricia↗
NASA NTRS1992

Herbig Ae/Be stars - Intermediate-mass stars surrounded by massive circumstellar accretion disks

The proposition that Herbig Ae/Be stars are young intermediate mass stars surrounded by optically thick accretion disks is explored. From a study of 47 such objects, a subset of 30 stars is identified whose spectral energy distributions can be interpreted convincingly in terms of pre-main sequence stars surrounded by massive optically thick circumstellar accretion disks. Constraints on the physical properties of the disks, such as size, mass, accretion rate, lifetime, and radial structure are derived from the photometric data.

Hillenbrand, Lynne A.↗
NASA NTRS1993

Circumstellar molecular gas of the HH 34 and HH 111 exciting stars

The HH 34 and HH 111 exciting stars, two pre-main-sequence objects which are the sources of highly collimated optical jets, have been observed in the J = 1-0 transition of (C-13)O and in the 2.7 mm continuum using the Owens Valley Radio Observatory Millimeter Interferometer. The high-resolution aperture synthesis maps reveal dense molecular gas concentrations at the positions of both stars. The morphology and kinematics of the bright emission-line cores suggest that the molecular gas is distributed in circumstellar disks, each about 2000 AU in diameter, elongated perpendicular to each system's outflow axis. A fainter component of (C-13)O emission extends along each source's outflow axis. Independent mass determinations based on the dust continuum and molecular line fluxes indicate for HH 34, a disk mass of 0.2 solar mass; and for HH 111, a disk mass of 0.3 solar mass. The inferred circumstellar disks of these two jet sources are significantly more massive than those found associated with T Tauri stars and are therefore among the most massive disks known in association with low-mass pre-main-sequence stars.

Stapelfeldt, Karl R.↗
NASA NTRS2011

The SEEDS of Planet Formation: Observations of Transitional Disks

As part of its 5-year study, the Strategic Exploration of Exoplanets and Disk Systems (SEEDS) has already observed a number of YSOs with circumstellar disks, including 13 0.5- 8 Myr old A-M stars with indications that they host wide gaps or central cavities in their circumstellar disks in millimeter or far-IR observations, or from deficits in warm dust thermal emission. For 8 of the disks, the 0.15" inner working angle of HiCIAO+A0188 samples material in the millimeter or mid-IR identified cavity. In one case we reprt detection of a previously unrecognized wide gap. For the remaining 4 stars, the SEEDS data sample the outer disk: in 3 cases, we present the first NIR imagery of the disks. The data for the youngest sample members (less than 1-2 Myr) closely resemble coeval primordial disks. After approximately 3 Myr, the transitional disks show a wealth of structure including spiral features, rings, divots, and in some cases, largely cleared gaps in the disks which are not seen in coeval primordial disks. Some of these structural features are predicted consequences of lovianmass planets having formed in the disk, while others are novel features. We discuss the implications for massive planet formation timescales and mechanisms.

Grady, Carol↗