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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 685 records · Page 38
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Long-term variations in dust production in R Coronae Borealis

Several searches for periodicities in the historical visible light curves of RCB stars have found that the intervals between declines are random. This paper reexamines 70 declines of R Coronae Borealis since 1853 using one homogeneous data set, the AAVSO light curve. In this data set, pairs of consecutive declines also show a random distribution of intervals. However, if the sequence of declines is examined, there are semiperiodic variations between times of high- and low-decline activity on a time scale of a few years. Near-IR photometry of RCB stars indicates that there are large semiperiodic variations in the amount of dust being produced which have similar time scales. Possible interpretations of a semiperiodic variation in dust formation rates in RCB stars are presented. One is a magnetic activity cycle resulting in 'spots' on the star over which dust formation takes place. Such a magnetic activity cycle is similar to the solar cycle. Another is the changes in the period and amplitude of the pulsations over several years affecting the efficiency of dust production.

Clayton, Geoffrey C.↗
DOE OSTI2016

FAST RADIO BURSTS FROM THE INSPIRAL OF DOUBLE NEUTRON STARS

In this Letter, we propose that a fast radio burst (FRB) could originate from the magnetic interaction between double neutron stars (NSs) during their final inspiral within the framework of a unipolar inductor model. In this model, an electromotive force is induced on one NS to accelerate electrons to an ultra-relativistic speed instantaneously. We show that coherent curvature radiation from these electrons moving along magnetic field lines in the magnetosphere of the other NS is responsible for the observed FRB signal, that is, the characteristic emission frequency, luminosity, duration, and event rate of FRBs can be well understood. In addition, we discuss several implications of this model, including double-peaked FRBs and possible associations of FRBs with short-duration gamma-ray bursts and gravitational-wave events.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

A phenomenological wobbling model for isolated pulsars and the braking index

ABSTRACT An isolated pulsar is a rotating neutron star possessing a very high magnetic dipole moment, thus providing a powerful radiating mechanism. These stars loose rotational energy E through various processes, including a plasma wind originating from a highly magnetized magnetosphere and the emission of magnetic dipole radiation (MDR). Such phenomena produce a time decreasing angular velocity Ω(t) of the pulsar that is usually quantified in terms of its braking index. Although these mechanisms are widely acknowledged as the primary drivers of the spin evolution of isolated pulsars, it is plausible that other contributing factors influencing this effect have yet to be comprehensively investigated. Most of young isolated pulsars present a braking index different from that given by the MDR and plasma wind processes. Working in the weak field (Newtonian) limit, we take in this work a step forward in describing the evolution of such a system by allowing the star’s shape to wobble around an ellipsoidal configuration as a backreaction effect produced by its rotational deceleration. It is assumed that an internal damping of the oscillations occurs, thus introducing another form of energy loss in the system, and this phenomenon may be related to the deviation of the braking index from the models based on $\dot{E} \sim -\Omega ^4$ predictions. Numerical calculations suggest that the average braking index for typical isolated pulsars can be thus explained by a simple phenomenological model.

Araujo, E. C. A. (ORCID:0000000213520879)↗
NASA NTRS1983

The role of magnetic fields in stellar chromospheres and transition regions

Consideration is given to the different roles that magnetic fields play in the control of the structure and energy balance in the chromospheres and transition regions of late-type stars, in view of IUE and Einstein satellite observations. The rotational modulation of structures ('plages') that are bright in UV emission lines and overlie dark star spots suggests that magnetic flux tubes are the dominant structural elements in late-type stellar atmospheres, as well. It is proposed that the existence of a boundary between solar-type stars and those with little or no plasma, as well as the different types of G-K giants and supergiants, may be understood in terms of the fractional surface coverage by closed magnetic structures. Models computed for active and quiet regions of the sun are compared with those for active and quiet stars.

Linsky, J. L.↗
DOE OSTI2016

MAGNETIC CYCLES IN A DYNAMO SIMULATION OF FULLY CONVECTIVE M-STAR PROXIMA CENTAURI

The recent discovery of an Earth-like exoplanet around Proxima Centauri has shined a spot light on slowly rotating fully convective M-stars. When such stars rotate rapidly (period ≲20 days), they are known to generate very high levels of activity that is powered by a magnetic field much stronger than the solar magnetic field. Recent theoretical efforts are beginning to understand the dynamo process that generates such strong magnetic fields. However, the observational and theoretical landscape remains relatively uncharted for fully convective M-stars that rotate slowly. Here, we present an anelastic dynamo simulation designed to mimic some of the physical characteristics of Proxima Centauri, a representative case for slowly rotating fully convective M-stars. The rotating convection spontaneously generates differential rotation in the convection zone that drives coherent magnetic cycles where the axisymmetric magnetic field repeatedly changes polarity at all latitudes as time progress. The typical length of the “activity” cycle in the simulation is about nine years, in good agreement with the recently proposed activity cycle length of about seven years for Proxima Centauri. Comparing our results with earlier work, we hypothesis that the dynamo mechanism undergoes a fundamental change in nature as fully convective stars spin down with age.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1994

Radio constraints on coronal models for dMe stars

Radio data are used to test coronal models for dMe stars. Specifically, we show that photospheric magnetic field observations imply that the low corona of a dMe star should be saturated by magnetic fields with an average strength in excess of 1 kG. In such fields the hot component of the corona detected in X-ray observations (temperature of order 2 x 10(exp 7) K) would be optically thick at least up to 15 GHz due to thermal gyroresonance opacity. The resulting emission would easily be detectable by radio observations and should have a radio spectrum rising in the microwave range. We have carried out observations to test this prediction, and in the majority of cases find that the observed fluxes at 15 GHz are too low to be consistent with the assumptions. In the few cases where the stars were detected at 15 GHz, the evidence indicates that the observed emission is nonthermal. These results imply that the hot component of the X-ray-emitting plasma in the corona is not coincident with the strong magnetic fields in the lower corona. Because the hot plasma must still be confined by closed magnetic field lines, it is likely to be restricted to heights of the order of a stellar radius above the photosphere. The results seem to imply a different genesis for the two components of the X-ray-emitting corona of flare stars: the hot component may be cooling flare plasma, while the cooler component (temperature of order 3 x 10(exp 6) K) is associated with a more conventional coronal heating mechanism.

White, S. M.↗
NASA NTRS1989

A two micron polarization survey toward dark clouds

A near infrared (2.2 micron) polarization survey of about 190 sources was conducted toward nearby dark clouds. The sample includes both background field stars and embedded young stellar objects. The aim is to determine the magnetic field structure in the densest regions of the dark clouds and study the role of magnetic fields in various phases of star formation processes, and to study the grain alignment efficiency in the dark cloud cores. From the polarization of background field stars and intrinsically unpolarized embedded sources, the magnetic field structure was determined in these clouds. From the intrinsic polarization of young stellar objects, the spatial distribution was determined of circumstellar dust around young stars. Combining the perpendicularity between the disks and magnetic fields with perpendicularity between the cloud elongation and magnetic fields, it is concluded that the magnetic fields might have dominated nearly all aspects of cloud dynamics, from the initial collapse of the clouds right through to the formation of disks/tori around young stars in these low to intermediate mass star forming clouds of the Taurus, Ophiuchus, and Perseus.

Tamura, M.↗
DOE OSTI2016

THE DISCOVERY OF SOLAR-LIKE ACTIVITY CYCLES BEYOND THE END OF THE MAIN SEQUENCE?

The long-term magnetic behavior of objects near the cooler end of the stellar main sequence is poorly understood. Most theoretical work on the generation of magnetism in these ultracool dwarfs (spectral type ≥M7 stars and brown dwarfs) suggests that their magnetic fields should not change in strength and direction. Using polarized radio emission measurements of their magnetic field orientations, I demonstrate that these cool, low-mass, fully convective objects appear to undergo magnetic polarity reversals analogous to those that occur on the Sun. This powerful new technique potentially indicates that the patterns of magnetic activity displayed by the Sun continue to exist, despite the fully convective interiors of these objects, in contravention of several leading theories of the generation of magnetic fields by internal dynamos.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Radio Pulse Search and X-Ray Monitoring of SAX J1808.4−3658: What Causes Its Orbital Evolution?

The accreting millisecond X-ray pulsar SAX J1808.4−3658 shows a peculiar orbital evolution that proceeds at a very fast pace. It is important to identify the underlying mechanism responsible for this behavior because it can help to understand how this system evolves and which physical processes (such as mass loss or spin–orbit coupling) are occurring in the binary. It has also been suggested that, when in quiescence, SAX J1808.4−3658 turns on as a radio pulsar, a circumstance that might provide a link between accreting millisecond pulsars and black-widow (BW) radio pulsars. In this work, we report the results of a deep radio pulsation search at 2 GHz using the Green Bank Telescope in 2014 August and an X-ray study of the 2015 outburst with Chandra , Swift XRT, and INTEGRAL . In quiescence, we detect no radio pulsations and place the strongest limit to date on the pulsed radio flux density of any accreting millisecond pulsar. We also find that the orbit of SAX J1808.4−3658 continues evolving at a fast pace. We compare the orbital evolution of SAX J1808.4−3658 to that of several other accreting and nonaccreting binaries, including BWs, redbacks, cataclysmic variables, black holes, and neutron stars in low-mass X-ray binaries. We discuss two possible scenarios: either the neutron star has a large moment of inertia and is ablating the donor, generating mass loss with an efficiency of 40%, or the donor star has a strong magnetic field of at least 1 kG and is undergoing quasi-cyclic variations due to spin–orbit coupling.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1983

On the origin of the 6.1-ms pulsar

A quantitative model is presented to account for the observed features of the binary pulsar PSR1953+53. A lower giant branch star is suggested to have evolved on a nuclear time scale while orbiting a neutron star, with mass transfer to the neutron star commencing when the giant attained its Roche lobe. A radius-core mass relationship was used to examined the processes that occurred after the hydrogen atmosphere was drained off to the neutron star. Tidal dissipation has circularized the orbit of the companion during mass transfer, while the orbit expanded. Accretion torques caused spin-up of the neutron star, which had a magnetic moment of no more than 3 x 10 to the 27th G/cu cm. The drained star became a degenerate dwarf with very faint luminosity, which accounts for no visual companion having been observed to date.

Joss, P. C.↗
NASA NTRS2009

Polarization Properties of Rotation Powered Pulsars

Polarization measurements of rotation-powered pulsars and their nebulae have unique diagnostic potential. The polarization position angle of the pulsar wind nebula, as is know for the Crab pulsar, can tell us the orientation of the spin axis. Phase-resolved polarimetry of pulsars has had enormous diagnostic capability at radio and optical wavelengths and could also be a powerful diagnostic in the X-ray range. Measurement of the polarization properties as a function of pulse phase can therefore provide a multidimensional mapping of the pulsar emission. In the 'rotating vector' model, radiation originating near a magnetic pole is expected to show a characteristic S-shaped swing of the position angle vs. pulse phase. In this case it is possible to determine the magnetic inclination and viewing angles. Radiation originating further from the poles or further above the neutron star surface will have a more complex polarization signature, as a result of relativistic effects of aberration and time-of-flight delays and may also cause depolarization of the signal. I will discuss predicted polarization properties of pulsed emission in polar cap models, where radiation originates near the neutron star surface at the magnetic poles, and in slot gap and outer gap models, where radiation originates over a range of altitudes out to the speed-of-light cylinder.

Harding Alice K.↗
NASA NTRS1990

Magnetically controlled circumstellar matter in the helium-strong stars

This paper reports the observation and interpretation of the ultraviolet spectrum variations of nine helium-strong stars: HD 36485, 37017, 37479, 37776, 58260, 60344, 64740, 96446, and 133518. A unified model is developed to account for the observed correlation among three stellar properties: the line profile characteristics of the C IV and Si IV resonance doublets, the variations in the strength of these lines, and the inferred magnetic field geometry. It is proposed that circumstellar plasma is trapped in the stellar magnetosphere near the magnetic equator or is channeled to form jetlike outflows from the magnetic polar regions. These results, together with those of a previous study of the helium-weak sn stars, show that both helium-weak and helium-strong stars can possess magnetospherically trapped plasma, notwithstanding their different photospheric properties. New results for radii and temperatures of the helium-strong stars in Ori OB 1 and for HD 64740 from low-dispersion IUE spectra are also presented.

Shore, Steven N.↗
NASA NTRS1994

HD 129333: The Sun in its infancy

HD 129333 is a remarkable young, nearby solar-type G star which offers a unique opportunity of studying the properties of the Sun at a time very shortly after in arrived on the main sequence. Its space motion suggest that it is a member of the Pleiades moving group, with an age of approximately 70 Myr; its lithium abundance is consistent with this. HD 129333 has the highest level of Ca II emission of any G star which is not a member of a close binary. Our observations in 1983 showed it to have low-amplitude (5%) light variations implying a rotation period of about 2.7 days, or about 10 times faster than the Sun. Modeling of the photometric variations on the assumption that they are due to starspots yields a spot temperature about 500 K cooler than the photosphere, and a coverage of about 6% of the stellar surface area. ROSAT observations in 1990 revealed the star to be an X-ray source, with an X-ray luminosity in the 0.2 to 2.4 keV range about 300 times that of the Sun. We have used International Ultraviolet Explorer (IUE) observations in conjuction with ground-based photometry to examine the magnetic activity of this star. The IUE emission-line fluxes reveal a level of chromospheric activity 3 to 20 times greater than the Sun's. The transition-region activity is 20 to 100 times that of the Sun. The activity level of HD 129333 is consistent with the Skumanich law relating activity to age, and with the rotation-activity relation, although it may be near saturation level. This star can yield valuable information about the magnetic dynamo of the young Sun, as well as about stellar dynamos in general. The 1988 IUE observations covered four phases of its rotational cycle. A phase dependence of the Mg II h and k emission flux suggests a close association of chromospheric plages with starspot regions at that time. Systematic variations in the mean brightness of HD 129333 between 1983 and 1993, and in the UV emission fluxes, indicate the presence of an activity cycle of an estimated 12 years' duration. HD 129333 is the first solar-type star other than RS CVn binaries for which luminosity variations provide evidence for a spot cycle. Unlike the Sun, which is brighter at activity maximum HD 129333 appears to be fainter when more heavily spotted. Although evolutionary models for the Sun suggest that it was about 30% less luminous at age 70 Myr, they give no information about the UV flux. Accordingly, we have used the 1988 IUE observations of HD 129333 to construct a model spectrum of the infant Sun, which can be used to provide a quantitative estimate of the UV flux in the early solar system.

Dorren, J. David↗
NASA NTRS1985

The sun and nearby stars - Microwave observations at high resolution

High-resolution microwave observations are providing new insights into the nature of active regions and eruptions on the sun and nearby stars. The strength, evolution, and structure of magnetic fields in coronal loops can be determined by multiple-wavelength observations with the Very Large Array. Flare models ccan be tested with Very Large Array snapshot maps, which have angular resolutions of better than 1 second of arc in time periods as short as 10 seconds. Magnetic changes that precede solar eruptions on time scales of tens of minutes involve primarily emerging coronal loops and the interactions of two or more loops. Magnetic reconnection at the interface of two closed loops may accelerate electrons and trigger the release of microwave energy in the coronal parts of the magnetic loops. Nearby main-sequence stars of late spectral type emit slowly varying microwave radiation and stellar microwave bursts that show striking similarities to those of the sun.

Kundu, M. R.↗
NASA NTRS1985

Molecular cloud evolution and star formation

The present state of knowledge of the relationship between molecular clouds and young stars is reviewed. The determination of physical parameters from molecular line observations is summarized, and evidence for fragmentation of molecular clouds is discussed. Hierarchical fragmentation is reviewed, minimum fragment scales are derived, and the stability against fragmentation of both spherically and anisotropically collapsing clouds is discussed. Observational evidence for high-velocity flows in clouds is summarized, and the effects of winds from pre-main sequence stars on molecular gas are discussed. The triggering of cloud collapse by enhanced pressure is addressed, as is the formation of dense shells by spherical outflows and their subsequent breakup. A model for low-mass star formation is presented, and constraints on star formation from the initial mass function are examined. The properties of giant molecular clouds and massive star formation are described. The implications of magnetic fields for cloud evolution and star formation are addressed.

Silk, J.↗
NASA NTRS1989

Intermittent behavior of galactic dynamo activities

Recent observations by Beck and Golla of far-infrared and radio continuum emission from nearby spiral galaxies suggest that the galactic magnetic field strength is connected to the current star formation rate. The role of star formation on the generation of large-scale galactic magnetic field is studied in this paper. Using a simple galactic model, it is shown how the galactic dynamo depends strongly on the turbulent velocity of the interstellar medium. When the star formation efficiency is high, the ISM is churned which in turn amplifies the galactic magnetic field. Between active star formation epochs, the magnetic field is in dormant state and decays at a negligible rate. If density waves trigger star formation, then they also turn on the otherwise dormant dynamo.

Ko, C. M.↗