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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 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

PHOTCAL: The IRAF photometric calibration package

The IRAF photmetric calibration package PHOTCAL is discussed. PHOTCAL is a set of tasks designed to derive the transformation from the instrumental photometric system to the standard photometric system, and apply the transformation to the observations. The PHOTCAL package contains tasks for: (1) creating and/or editing standard star catalogs and observations catalogs, (2) creating, checking and editing the configuration file which specifies the format of the standard star and observations catalogs and the form of the transformation equations, (3) solving the transformation equations interactively or non-iteractively using non-linear least squares fitting routines, and (4) applying the transformation to the observations. PHOTCAL standard star and observations catalogs are simple text files, whose columns are delimited by whitespace, and whose first column contains the star names. This format makes it relatively easy to interface the output of non-IRAF photometry programs as well as the output of the IRAF APPHOT and DAOPHOT photometry packages to PHOTCAL. PHOTCAL maintains a standard star catalog directory for the convenience of the user, but users can easily create their own standard star catalogs and/or define their own standard star catalog directory. Separate observations files produced by APPHOT, DAOPHOT or a user program containing data for stellar fields taken through different filters, can be combined into observations catalogs using one of the PHOTCAL preprocessor tasks. The input configuration file required by PHOTCAL is a text file, consisting of a series of instructions written by the user in a mini-language understood by the PHOTCAL parser. These instructions: (1) assign names to the input data columns in the standard star and observations catalogs, (2) assign names and initial values to the parameters to be fit, (3) define and describe how to solve the transformation equations. The mini-language approach permits great flexibility in the format of the input catalogs and the form of the transformation equations.

Davis, L. E.↗
NASA NTRS1994

Nonthermal accretion disk models around neutron stars

We consider the structure and emission spectra of nonthermal accretion disks around both strongly and weakly magnetized neutron stars. Such disks may be dissipating their gravitational binding energy and transferring their angular momentum via semicontinuous magnetic reconnections. We consider specifically the structure of the disk-stellar magnetospheric boundary where magnetic pressure balances the disk pressure. We consider energy dissipation via reconnection of the stellar field and small-scale disk turbulent fields of opposite polarity. Constraints on the disk emission spectrum are discussed.

Tavani, M.↗
NASA NTRS2001

Kuiper Belt Mapping Radar

Since their initial discovery in 1992, to date only a relatively small number of Kuiper Belt Objects (KBO's) have been discovered. Current detection techniques rely on frame-to-frame comparisons of images collected by optical telescopes such as Hubble, to detect KBO's as they move against the background stellar field. Another technique involving studies of KBO's through occultation of known stars has been proposed. Such techniques are serendipitous, not systematic, and may lead to an inadequate understanding of the size, range, and distribution of KBO's. In this paper, a future Kuiper Belt Mapping Radar is proposed as a solution to the problem of mapping the size distribution, extent, and range of KBO's. This approach can also be used to recover radar albedo and object rotation rates. Additional information is contained in the original extended abstract.

Freeman, A.↗
NASA NTRS2017

Spitzer Photometry of Approximately 1 Million Stars in M31 and 15 Other Galaxies

We present Spitzer IRAC 3.6-8 micrometer and Multiband Imaging Photometer 24 micrometer point-source catalogs for M31 and 15 other mostly large, star-forming galaxies at distances approximately 3.5-14 Mpc, including M51, M83, M101, and NGC 6946. These catalogs contain approximately 1 million sources including approximately 859,000 in M31 and approximately 116,000 in the other galaxies. They were created following the procedures described in Khan et al. through a combination of pointspread function (PSF) fitting and aperture photometry. These data products constitute a resource to improve our understanding of the IR-bright (3.6-24 micrometer) point-source populations in crowded extragalactic stellar fields and to plan observations with the James Webb Space Telescope.

Photometry↗
NASA NTRS1994

Initial mass functions from ultraviolet stellar photometry: A comparison of Lucke and Hodge OB associations near 30 Doradus with the nearby field

UV stellar photometry is presented for 1563 stars within a 40 minutes circular field in the Large Magellanic Cloud (LMC), excluding the 10 min x 10 min field centered on R136 investigated earlier by Hill et al. (1993). Magnitudes are computed from images obtained by the Ultraviolet Imaging Telescope (UIT) in bands centered at 1615 A and 2558 A. Stellar masses and extinctions are estimated for the stars in associations using the evolutionary models of Schaerer et al. (1993), assuming the age is 4 Myr and that the local LMC extinction follows the Fitzpatrick (1985) 30 Dor extinction curve. The estimated slope of the initial mass function (IMF) for massive stars (greater than 15 solar mass) within the Lucke and Hodge (LH) associations is Gamma = -1.08 +/- 0.2. Initial masses and extinctions for stars not within LH associations are estimated assuming that the stellar age is either 4 Myr or half the stellar lifetime, whichever is larger. The estimated slope of the IMF for massive stars not within LH associations is Gamma = -1.74 +/- 0.3 (assuming continuous star formation), compared with Gamma = -1.35, and Gamma = -1.7 +/- 0.5, obtained for the Galaxy by Salpeter (1955) and Scalo (1986), respectively, and Gamma = -1.6 obtained for massive stars in the Galaxy by Garmany, Conti, & Chiosi (1982). The shallower slope of the association IMF suggests that not only is the star formation rate higher in associations, but that the local conditions favor the formation of higher mass stars there. We make no corrections for binaries or incompleteness.

Hill, Jesse K.↗
NASA NTRS1984

Comparisons of a standard galaxy model with stellar observations in five fields

Modern data on the distribution of stellar colors and on the number of stars as a function of apparent magnitude in five directions in the Galaxy are analyzed. It is found that the standard model is consistent with all the available data. Detailed comparisons with the data for five separate fields are presented. The bright end of the spheroid luminosity function and the blue tip of the spheroid horizontal branch are analyzed. The allowed range of the disk scale heights and of fluctuations in the volume density is determined, and a lower limit is set on the disk scale length. Calculations based on the thick disk model of Gilmore and Reid (1983) are presented.

Bahcall, J. N.↗
DOE OSTI2025

Error field measurements and correction on MUSE permanent magnet stellarator

The magnetic field topology and error fields are measured in the MUSE permanent magnet stellarator with an electron beam and fluorescent rod. The effective magnetization of the magnets is determined by measuring rational flux surface locations. Electron magnetic drift effects must be considered self-consistently due to the small magnetic shear on MUSE. Error fields are characterized by flux surface shape deviation and magnetic island chain width and phase. Error fields are then corrected by adjusting the permanent magnet (PM) holders as rigid bodies. Resonant error fields are on the order of or smaller than 3 x 10 -6 of the toroidal field after correction and surface shape matches well with the design. Resonant components of the quasi-symmetry deviation are estimated, using a novel magnetic drift measurement, to be in the 10 −4 range. The PM approach is validated as a low-cost and precise way to build optimized stellarators.

electron beam mapping↗
NASA NTRS1978

Disk accretion by magnetic neutron stars

A model for disk accretion by a rotating magnetic neutron star is proposed which includes a detailed description of matter flow in the transition region between the disk and the magnetosphere. It is shown that the disk plasma cannot be completely screened from the stellar magnetic field and that the resulting magnetic coupling between the star and the disk exerts a significant torque on the star. On the assumption that the distortion of the residual stellar field lines threading the disk is limited by reconnection, the total accretion torque on the star is calculated. The calculated torque gives period changes in agreement with those observed in the pulsating X-ray sources and provides a natural explanation of why a fast rotator like Her X-1 has a spin-up rate much below the conventional estimate for slow rotators. It is shown that for such fast rotators, fluctuations in the mass-accretion rate can produce fluctuations in the accretion torque about 100 times larger. For sufficiently fast rotators or, equivalently, for sufficiently low accretion rates, the star experiences a braking torque even while accretion continues and without any mass ejection from its vicinity.

Ghosh, P.↗
NASA NTRS1993

Resistive dissipation and magnetic field topology in the stellar corona

Tangential discontinuities, or current sheets, in a magnetic field embedded in a fluid with vanishing resistivity are created by discontinuous fluid motion. Tangential discontinuities are also created when a magnetic field is allowed to relax to magnetostatic equilibrium after mixing by fluid motions (either continuous or discontinuous) into any but the simplest topologies. This paper shows by formal examples that the current sheets arising solely from discontinuous fluid motions do not contribute significantly to the dissipation of magnetic free energy when a small resistivity is introduced. Dissipation that is significant under coronal conditions occurs only by rapid reconnection, which arises when, and only when, the current sheets are required by the field topology. Hence it is topological dissipation that is primarily responsible for heating tenuous coronal gases in astronomical settings, whether the fluid displacements of the field are continuous or discontinuous.

Parker, E. N.↗
DOE OSTI2024

Debiasing with Diffusion: Probabilistic Reconstruction of Dark Matter Fields from Galaxies with CAMELS

Abstract Galaxies are biased tracers of the underlying cosmic web, which is dominated by dark matter (DM) components that cannot be directly observed. Galaxy formation simulations can be used to study the relationship between DM density fields and galaxy distributions. However, this relationship can be sensitive to assumptions in cosmology and astrophysical processes embedded in galaxy formation models, which remain uncertain in many aspects. In this work, we develop a diffusion generative model to reconstruct DM fields from galaxies. The diffusion model is trained on the CAMELS simulation suite that contains thousands of state-of-the-art galaxy formation simulations with varying cosmological parameters and subgrid astrophysics. We demonstrate that the diffusion model can predict the unbiased posterior distribution of the underlying DM fields from the given stellar density fields while being able to marginalize over uncertainties in cosmological and astrophysical models. Interestingly, the model generalizes to simulation volumes ≈500 times larger than those it was trained on and across different galaxy formation models. The code for reproducing these results can be found athttps://github.com/victoriaono/variational-diffusion-cdm✎.

Astronomy & Astrophysics↗
NASA NTRS1988

Semi-empirical models of the wind in cool supergiant stars

A self-consistent semi-empirical model for the wind of the supergiant in zeta Aurigae type systems is proposed. The damping of the Alfven waves which are assumed to drive the wind is derived from the observed velocity profile. Solution of the ionization balance and energy equation gives the temperature structure for given stellar magnetic field and wave flux. Physically acceptable solutions of the temperature structure place limits on the stellar magnetic field. A crude formula for a critical mass loss rate is derived. For a mass loss rate below the critical value the wind cannot be cool. Comparison between the observed and the critical mass loss rate suggests that the proposed theory may provide an explanation for the coronal dividing line in the Hertzsprung-Russell diagram. The physical explanation may be that the atmosphere has a cool wind, unless it is physically impossible to have one. Stars which cannot have a cool wind release their nonthermal energy in an outer atmosphere at coronal temperatures. It is possible that in the absence of a substantial stellar wind the magnetic field has less incentive to extend radially outward, and coronal loop structures may become more dominant.

Kuin, N. P. M.↗
NASA NTRS1983

Magnetic fields and activity of the sun and stars - An overview

Recent work on the observation and theory of solar and stellar magnetic field activity and its relation to stellar activity is reviewed, emphasizing those aspects relevant to the problem of activity of red dwarf stars. New observational facts relevant to understanding the root cause of stellar surface activity are summarized and theoretical questions concerning the underlying physical basis for the observed correlations between stellar activity, rotation, and magnetic fields are addressed. These include dyanamo theory and the rotation-activity connection as well as flux tube dynamics and plasma heating.

Rosner, R.↗
NASA NTRS1979

Accretion by rotating magnetic neutron stars. II - Radial and vertical structure of the transition zone in disk accretion

The radial and vertical structure of the transition zone at the magnetospheric boundary of an aligned rotating neutron star accreting matter from a Keplerian disk are calculated. The results obtained indicate that: (1) the inner edge of the disk is located where the integrated magnetic stress acting on the disk plasma becomes comparable to the integrated material stress associated with its inward radial drift and orbital motion; (2) the stellar magnetic field threads the disk near its inner edge via the Kelvin-Helmholtz instability, turbulent diffusion, and reconnection, producing a broad transition zone between the unperturbed disk flow and corotating magnetosphere; (3) the transition zone consists of two qualitatively different regions, viz., a broad outer transition zone where the motion is Keplerian and a narrow inner zone, or boundary layer, where the departure from Keplerian motion is substantial; (4) the stellar magnetic field is largely but not entirely screened by currents flowing in the boundary layer; and (5) there are no steady-flow solutions for sufficiently fast stellar rotation.

Ghosh, P.↗
NASA NTRS1985

Radiative Amplification of Acoustic Waves in Hot Stars

The discovery of broad P Cygni profiles in early type stars and the detection of X-rays emitted from the envelopes of these stars made it clear, that a considerable amount of mechanical energy has to be present in massive stars. An attack on the problem, which has proven successful when applied to late type stars is proposed. It is possible that acoustic waves form out of random fluctuations, amplify by absorbing momentum from stellar radiation field, steepen into shock waves and dissipate. A stellar atmosphere was constructed, and sinusoidal small amplitude perturbations of specified Mach number and period at the inner boundary was introduced. The partial differential equations of hydrodynamics and the equations of radiation transfer for grey matter were solved numerically. The equation of motion was augmented by a term which describes the absorption of momentum from the radiation field in the continuum and in lines, including the Doppler effect and allows for the treatment of a large number of lines in the radiative acceleration term.

Wolf, B. E.↗
NASA NTRS1987

Stellar winds and magnetic fields in the helium peculiar stars

This note discusses the consequences of the interaction of radiatively driven mass loss with magnetic fields in upper-main-sequence helium-peculiar stars. The field constrains mass loss to occur in narrow cones above the magnetic poles. These jets can explain the observed behavior of UV resonance lines in the helium-peculiar stars, especially the sn and helium-rich stars. Suppression of mass outflow at the magnetic equator produces a region in which radiatively driven diffusive mass transport can levitate helium in the hottest stars, producing helium enrichment of the atmosphere. In the cooler stars, the surface becomes progressively more helium poor.

Shore, Steven N.↗