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At least 1,513 records · Page 84
All sourcesNASA NTRSDOE OSTI
NASA NTRS2016

Nanoradian Ground-Based Astrometry, Optical Navigation, and Artificial Reference Stars

Spacecraft carrying optical communication lasers can be treated as artificial stars, whose relative astrometry to Gaia reference stars provides spacecraft positions in the plane-of-sky for optical navigation. To be comparable to current Deep Space Network delta-Differential One-way Ranging measurements, thus sufficient for navigation, nanoradian optical astrometry is required. Here we describe our error budget, techniques for achieving nanoradian level ground-base astrometry, and preliminary results from a 1 m telescope. We discuss also how these spacecraft may serve as artificial reference stars for adaptive optics, high precision astrometry to detect exoplanets, and tying reference frames defined by radio and optical measurements.

optical navigation↗
NASA NTRS2020

Dim Prospects for Transmission Spectra of Ocean Earths Around M Stars

The search for water-rich Earth-sized exoplanets around low-mass stars is rapidly gaining attention because they represent the best opportunity to characterize habitable planets in the near future. Understanding the atmospheres of these planets and determining the optimal strategy for characterizing them through transmission spectroscopy with our upcoming instrumentation is essential in order to constrain their environments. For this study, we present simulated transmission spectra of tidally locked Earth-sized ocean-covered planets around late-M to mid-K stellar spectral types, utilizing GCM modeling results previously published by Kopparapu et al. (2017) as inputs for our radiative transfer calculations performed using NASA's Planetary Spectrum Generator (psg.gsfc.nasa.gov; Villanueva et al. (2018)). We identify trends in the depth of H2O spectral features as a function of planet surface temperature and rotation rate. These trends allow us to calculate the exposure times necessary to detect water vapor in the atmospheres of aquaplanets through transmission spectroscopy with the upcoming James Webb Space Telescope (JWST) as well as several future flagship space telescope concepts under consideration (LUVOIR and OST) for a target list constructed from the TESS Input Catalog (TIC). Our calculations reveal that transmission spectra for water-rich Earth-sized planets around low-mass stars will be dominated by clouds, with spectral features < 20 ppm, and only a small subset of TIC stars would allow for the characterization of an ocean planet in the Habitable Zone. We thus present a careful prioritization of targets that are most amenable to follow-up characterizations with next-generation instrumentation, in order to assist the community in efficiently utilizing precious telescope time.

exoplanet atmospheres↗
NASA NTRS1996

Cassini Star Tracking and Identification Algorithms, Scene Simulation, and Testing

The Cassini mission will use autonomous star identification for initial attitude determination and a star tracking function for maintaining attitude. Because of the complexity of the StarID software, special software simulation tools were created to simulate the Stellar Reference Unit (SRU) output as a function of commands, spacecraft attitude, and star scene, and to allow the introduction of fault conditions. This paper gives the overview of the algorithm design and SRU simulation and a description of the simulation test results and a comparison with field test results obtained using the engineering model SRU.

Cassini↗
NASA NTRS2019

Constraining the Neutron Star Mass–Radius Relation and Dense Matter Equation of State with NICER. I. The Millisecond Pulsar X-Ray Data Set

We present the set of deep Neutron Star Interior Composition Explorer (NICER) X-ray timing observations of the nearby rotation-powered millisecond pulsars PSRs J0437−4715, J0030+0451, J1231−1411, and J2124−3358, selected as targets for constraining the mass–radius relation of neutron stars and the dense matter equation of state (EoS) via modeling of their pulsed thermal X-ray emission. We describe the instrument, observations, and data processing/reduction procedures, as well as the series of investigations conducted to ensure that the properties of the data sets are suitable for parameter estimation analyses to produce reliable constraints on the neutron star mass–radius relation and the dense matter EoS. We find that the long-term timing and flux behavior and the Fourier domain properties of the event data do not exhibit any anomalies that could adversely affect the intended measurements. From phase-selected spectroscopy, we find that emission from the individual pulse peaks is well described by a single-temperature hydrogen atmosphere spectrum, with the exception of PSR J0437−4715, for which multiple temperatures are required.

Neutron stars↗
NASA NTRS2020

The Importance of Star Formation Intensity in Lyα Escape from Green Pea Galaxies and Lyman Break Galaxy Analogs

We have studied ultraviolet images of 40 Green Pea galaxies and 15 local Lyman Break Galaxy Analogs to understand the relation between Lyα photon escape and central UV photometric properties. We measured star formation intensity (SFI; star formation rate per unit area) from the central 250 pc region (S250 pc) using Cosmic Origins Spectrograph near-ultraviolet images from the Hubble Space Telescope. The measured S250 pc of our sample Green Peas ranges from 2.3–46M☉ yr−1 kpc−2, with a geometric mean of 15M☉ yr−1 kpc−2 and a standard deviation of 0.266 dex, forming a relatively narrow distribution. The Lyman Break Galaxy Analogs show a similarly narrow distribution of S250 pc (0.271 dex), though with a larger mean of 28M☉ yr−1 kpc−2. We show that while the Lyα equivalent width (EW(Lyα)) and the Lyα escape fraction ( f a esc Lyα) are not significantly correlated with the central SFI (S250 pc), both are positively correlated with the ratio of surface brightness to galaxy stellar mass (S250 pc/Mstar), with correlation coefficients (p-values) of 0.702 (1 × 10−8) and 0.529 (5 × 10−4) with EW(Lyα) and f a esc Lyα, respectively. These correlations suggest a scenario where intense central star formation can drive a galactic wind in galaxies with relatively shallow gravitational potential wells, thus clearing channels for the escape of Lyα photons.

Compact galaxies↗
NASA NTRS2022

Ultraviolet Spectropolarimetric Diagnostics of Hot Star Magnetospheres

Several space missions and instruments for UV spectropolarimetry are in preparation, such as the proposed NASA MIDEX Polstar project, the proposed ESA M mission Arago, and the Pollux instrument on the future LUVOIR-like NASA flagship mission. In the frame of Polstar, we have studied the capabilities these observatories would offer to gain information on the magnetic and plasma properties of the magnetospheres of hot stars, helping us test the fundamental hypothesis that magnetospheres should act to rapidly drain angular momentum, thereby spinning the star down, whilst simultaneously reducing the net mass-loss rate. Both effects are expected to lead to dramatic differences in the evolution of magnetic vs. non-magnetic stars.

Ultraviolet astronomy↗
NASA NTRS1996

Three Weak-Lined T Tauri Stars in Orion

We have identified three new weak lined T Tauri stars in Orion using the Double Spectrograph and 5-m telescope at Palomar Observatory. One of these is a candidate for the X-ray source RX0535-0633. H(alpha) and H(beta) line emisison are detected and equivalent widths measured in all three stars, and in V942 Ori. We also searched without success for H(alpha) line variability in V942 Ari.

stars emission line be stars flare stars V942 Ori ↗
NASA NTRS1971

Runaway stars in the Gum Nebula

It is proposed that the two pulsars PSR 0833-45 (the Vela pulsar) and MP 0835 are runaways from a common binary system originally located in the B association around gamma Velorum. Arguments are presented for a simple model of the Gum nebula in which two distinct ionized regions are present. The first consists of the Stromgren spheres of gamma Velorum and zeta Puppis, while the second is a larger, more filamentary region ionized by the supernova explosion associated with PSR 0833-45. Using this model and the available dispersion measures, the distances to the two pulsars were estimated and found to be compatible with a runaway origin. The position angle of the rotation axis of PSR 0833-45 is also compatible with this origin. The masses of the parent stars of the two pulsars can be deduced from the runaway star dynamics and an assumed age for MP 0835. It is concluded that the masses were in excess of 10 solar masses. The dynamically-determined parent star masses are in agreement with the values expected for evolved members of the B association around gamma Velorum.

Got, J. R., III↗
NASA NTRS1972

Radiation noise in a high sensitivity star sensor

An extremely accurate attitude determination was developed for space applications. This system uses a high sensitivity star sensor in which the photomultiplier tube is subject to noise generated by space radiations. The space radiation induced noise arises from trapped electrons, solar protons and other ionizing radiations, as well as from dim star background. The solar activity and hence the electron and proton environments are predicted through the end of the twentieth century. The available data for the response of the phototube to proton, electron, gamma ray, and bremsstrahlung radiations are reviewed and new experimental data is presented. A simulation was developed which represents the characteristics of the effect of radiations on the star sensor, including the non-stationarity of the backgrounds.

Parkinson, J. B.↗
NASA NTRS1972

Possibility that the far ultraviolet excess in M31 is due to main sequence stars

The far ultraviolet excess in the central region of M31, observed by OAO-2, could be due to young main sequence stars. More than enough such stars are present in the model for the M31 inner disk population derived by Tinsley and Spinrad (1971) to match line- and color-indices at longer wavelengths. If the far ultraviolet radiation of typical galaxies arises from young stars, the theoretical ultraviolet background is enhanced greatly by evolutionary effects. For evolution at the rate of Tinsley and Spinrad's model for M31, or of Arnett's (1971) linear model for our galaxy, the enhancement is a factor 2.5 to 14, depending on the Hubble constant and the spectrum at wavelengths below 1700 A.

Tinsley, B. M.↗
NASA NTRS1971

The blue stars above the turn-off in M67 - Horizontal branch or blue stragglers.

The nature of the pseudohorizontal branch in the galactic cluster M67 is investigated. A differential technique is used to determine the masses of these stars relative both to the turnoff-point stars and to the zero-age main sequence. The results obtained suggest that the pseudohorizontal-branch stars are analogs of the blue stragglers found in other galactic clusters.

Strom, S. E.↗
NASA NTRS1972

The absolute magnitudes and spectral types of the stars in the gamma Velorum system.

The ratio of the visual luminosities of the O and WC components of the binary system gamma (2) Vel has been derived by comparison of the emission-line strengths in the spectrum with those in the spectrum of a single WC8 star, HD 192103. The result is 1.4 mag, with the O star being brighter; the classification of the O star was found to be 09 I, so that the classification of gamma (2) Vel is WC8+09 I.

Conti, P. S.↗
NASA NTRS1972

Star scanner attitude determination for the OSO-7 spacecraft.

Calculation of the inertial orientation of the OSO-7 spacecraft from the times at which known stars or planets transit planes fixed in the spacecraft. Both the reference planes and the timing information are provided by the star scanner instrument aboard the spacecraft, while the star identification and the statistical estimation of a set of parameters describing the spacecraft attitude are accomplished in a ground station computer facility. A recursive least-squares determination is made of a vector of first-order differential corrections to the attitude state vector. Preliminary analysis indicates the system accuracy to be 3 arc min in each attitude Euler angle.

Mackison, D. L.↗
NASA NTRS1972

Position annihilation radiation from neutron stars

Matter accreted on the surfaces of neutron stars consists of energetic particles of a few tens to a couple hundred MeV/nucleon, depending on the neutron star mass. In addition to heat, such particles produce nuclear reactions with the surface material. It is proposed that the recently observed 473 + or - 30 keV spectral feature from the galactic center is gravitationally red-shifted positron annihilation radiation produced at the surfaces of old neutron stars. The principal observational tests of the model would be the detection of nuclear gamma ray lines from the galactic center and red-shifted positron annihilation radiation from the galactic disk.

Ramaty, R.↗
NASA NTRS1972

Pulsar observations and neutron star models

Information about the physical parameters of neutron stars is obtained from pulsar observations. The energy balance of the Crab nebula and the Vela X remnant allows derivation of limits for the masses of the Crab and Vela pulsars. Glitch observations provide further clues on the masses of these two pulsars. The degree of confidence in the derived numbers is pointed out. The possibility of observing neutron stars in binary systems as pulsating X-ray sources is discussed. The importance of observing redshifted gamma ray lines from the surface of neutron stars, and thus directly measuring either individual or statistical properties of these objects is pointed out.

Boerner, G.↗