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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 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
NASA NTRS2013

Suzaku View of the Neutron Star in the Dipping Source 4U 1822-37

The dipping X-ray source 4U 1822−37 was observed by Suzaku on 2006 Octrober 20 for a net exposure of 37 ks. The source was detected with the XIS at a 1-10 keV flux of 5.5 ×10(exp −10) erg per square centimeter per second, and with the HXD (HXD-PIN) at a 10-50 keV flux of 8.9 ×10(exp −10) erg per square centimeter per second. With HXD-PIN, the pulsation was detected at a barycentric period of 0.592437 seconds, and its change rate was reconfirmed as −2.43 × 10(exp −12) seconds per second. The 1-50 keV spectra of 4U 1822-37 were found to be very similar to those of Her X- 1 in the slopes, cutoff and iron lines. Three iron lines (Fe Kalpha, Fe XXV, and Fe XXVI) were detected, on top of a 1-50 keV continuum that is described by an NPEX model plus a soft blackbody. In addition, a cyclotron resonance scattering feature was detected significantly ( greater than 99% confidence), at an energy of 33+/-2 keV with a depth of 0.4(sup +0.6)/(sub −0.3). Therefore, the neutron star in this source is concluded to have a strong magnetic field of 2.8 × 10(exp 12) G. Further assuming that the source has a relatively high intrinsic luminosity of several times 10(exp 37) erg per second, its spectral and timing properties are consistently explained.

Stars: magnetic field↗
DOE OSTI2017

Distortion of Magnetic Fields in a Starless Core II: 3D Magnetic Field Structure of FeSt 1-457

Three-dimensional (3D) magnetic field information on molecular clouds and cores is important for revealing their kinematical stability (magnetic support) against gravity, which is fundamental for studying the initial conditions of star formation. In the present study, the 3D magnetic field structure of the dense starless core FeSt 1-457 is determined based on the near-infrared polarimetric observations of the dichroic polarization of background stars and simple 3D modeling. With an obtained angle of line-of-sight magnetic inclination axis θ{sub inc} of 45{sup ∘}±10{sup ∘} and previously determined plane-of-sky magnetic field strength B{sub pol} of 23.8 ± 12.1 μG, the total magnetic field strength for FeSt 1-457 is derived to be 33.7 ± 18.0 μG. The critical mass of FeSt 1-457, evaluated using both magnetic and thermal/turbulent support is M{sub cr}=3.70±0.92 M{sub ⊙}, which is identical to the observed core mass, M{sub core}=3.55±0.75 M{sub ⊙}. We thus conclude that the stability of FeSt 1-457 is in a condition close to the critical state. Without infalling gas motion and no associated young stars, the core is regarded to be in the earliest stage of star formation, i.e., the stage just before the onset of dynamical collapse following the attainment of a supercritical condition. These properties could make FeSt 1-457 one of the best starless cores for future studies of the initial conditions of star formation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

The X-ray luminosity function of M37 and the evolution of coronal activity in low-mass stars

We use a 440.5 ks Chandra observation of the ≈500 Myr old open cluster M37 to derive the X-ray luminosity functions of its ≤1.2 M{sub ⊙} stars. Combining detections of 162 M37 members with upper limits for 160 non-detections, we find that its G, K, and M stars have a similar median (0.5–7 keV) X-ray luminosity L{sub X}=10{sup 29.0} erg s{sup −1}, whereas the L{sub X}-to-bolometric-luminosity ratio (L{sub X}/L{sub bol}) indicates that M stars are more active than G and K stars by ≈1 order of magnitude at 500 Myr. To characterize the evolution of magnetic activity in low-mass stars over their first ≈600 Myr, we consolidate X-ray and optical data from the literature for stars in six other open clusters: from youngest to oldest they are, the Orion Nebula Cluster (ONC), NGC 2547, NGC 2516, the Pleiades, NGC 6475, and the Hyades. For these, we homogenize the conversion of instrumental count rates to L{sub X} by applying the same one-temperature emission model as for M37, and obtain masses using the same empirical mass-absolute magnitude relation (except for the ONC). We find that for G and K stars X-ray activity decreases ≈2 orders of magnitude over their first 600 Myr, and for M stars, ≈1.5. The decay rate of the median L{sub X} follows the relation L{sub X}∝ t{sup b}, where b=−0.61±0.12 for G stars, −0.82 ± 0.16 for K stars, and −0.40 ± 0.17 for M stars. In L{sub X}/L{sub bol} space, the slopes are −0.68 ± 0.12, −0.81 ± 0.19, and −0.61 ± 0.12, respectively. These results suggest that for low-mass stars the age-activity relation steepens after ≈625 Myr, consistent with the faster decay in activity observed in solar analogs at t>1 Gyr.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1971

Magnetism in white dwarfs

White dwarf stars broadband circular polarization and normal/quadratic Zeeman effect

Angel, J. R. P.↗
NASA NTRS2006

Hubble Space Telescope NICMOS Polarization Measurements of OMC-1

We present 2 micrometer polarization measurements of positions in the BN region of the Orion Molecular Cloud (OMC-1) made with NICMOS Camera 2 (0.2" resolution) on Hubble Space Telescope. Our goals are to seek the sources of heating for IRc2, 3, 4, and 7, identify possible young stellar objects (YSOs), and characterize the grain alignment in the dust clouds along the lines-of-sight to the stars. Our results are as follows: BN is approximately 29% polarized by dichroic absorption and appears to be the illuminating source for most of the nebulosity to its north and up to approximately 5" to its south. Although the stars are probably all polarized by dichroic absorption, there are a number of compact, but non-point-source, objects that could be polarized by a combination of both dichroic absorption and local scattering of star light. We identify several candidate YSOs, including an approximately edge-on bipolar YSO 8.7" east of BN, and a deeply-embedded IRc7, all of which are obviously self-luminous at mid-infrared wavelengths and may be YSOs. None of these is a reflection nebula illuminated by a star located near radio source I, as was previously suggested. Other IRc sources are clearly reflection nebulae: IRc3 appears to be illuminated by IRc2-B or a combination of the IRc2 sources, and IRc4 and IRc5 appear to be illuminated by an unseen star in the vicinity of radio source I, or by Star n or IRc2-A. Trends in the magnetic field direction are inferred from the polarization of the 26 stars that are bright enough to be seen as NICMOS point sources. Their polarization ranges from N less than or equal to 1% (all stars with this low polarization are optically visible) to greater than 40%. The most polarized star has a polarization position angle different from its neighbors by approximately 40 degrees, but in agreement with the grain alignment inferred from millimeter polarization measurements of the cold dust cloud in the southern part of OMC-1. The polarization position angle of another highly-polarized, probable star also requires a grain alignment and magnetic field orientation substantially different from the general magnetic field orientation of OMC-1.

Simpson, Janet P.↗
DOE OSTI2021

Rotational Velocities of Am and Non-chemical-peculiar Stars Based on Kepler and LAMOST DR5

Rotation is a critical physical process operating in the formation of Am stars. There is a strong correlation between low-velocity rotation and chemical peculiarity. However, the existence of many non-chemical-peculiar slow rotators challenges the understanding of Am stars. The purpose of our work is to search for low-velocity rotating non-chemical-peculiar A-type stars and Am stars and to make a comparative analysis. In this paper, we pick out a sample from the LAMOST–Kepler project, including 21 Am stars, 125 non-chemical-peculiar slow rotators, and 53 non-chemical-peculiar fast rotators. We calculate the rotational frequencies through a periodic change of light curves caused by inhomogeneous stellar surfaces and then obtain the rotational velocities. For slow rotators, the age of Am stars is statistically younger than that of non-chemical-peculiar stars in the same temperature bin. In the comparison of the period, the average amplitude, and stellar mass of Am and non-chemical-peculiar stars, we discover that there is no difference in the photometric variability between Am and non-chemical-peculiar stars, which implies similar inhomogeneities on the surfaces. The average amplitude of non-chemical-peculiar stars has a downward trend with the increase of effective temperature and stellar mass, which is consistent with the theoretical prediction caused by weak dynamo-generated magnetic fields in A-type stars. In addition, we confirm four non-chemical-peculiar stars that have flares by checking field-of-view images, pixel images, and pixel-level light curves.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

MODELING THE EARLY AFTERGLOW IN THE SHORT AND HARD GRB 090510

The bright, short, and hard GRB 090510 was detected by all instruments aboard the Fermi and Swift satellites. The multiwavelength observations of this burst presented similar features to the Fermi -LAT-detected gamma-ray bursts. In the framework of the external shock model of early afterglow, a leptonic scenario that evolves in a homogeneous medium is proposed to revisit GRB 090510 and explain the multiwavelength light curve observations presented in this burst. These observations are consistent with the evolution of a jet before and after the jet break. The long-lasting LAT, X-ray, and optical fluxes are explained in the synchrotron emission from the adiabatic forward shock. Synchrotron self-Compton emission from the reverse shock is consistent with the bright LAT peak provided that the progenitor environment is entrained with strong magnetic fields. It could provide compelling evidence of magnetic field amplification in the neutron star merger.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2008

Magnetar-Like Emission from the Young Pulsar in Kes 75

We report the detection of magnetar-like x-ray bursts from the young pulsar PSR J1846-0258, at the center of the supernova remnant Kes 75. This pulsar, long thought to be exclusively rotation-powered, has an inferred surface dipolar magnetic field of 4.9deg 10(exp 13) gauss, which is higher than those of the vast majority of rotation-powered pulsars, but lower than those of the approximately 12 previously identified magnetars. The bursts were accompanied by a sudden flux increase and an unprecedented change in timing behavior. These phenomena lower the magnetic and rotational thresholds associated with magnetar-like behavior and suggest that in neutron stars there exists a continuum of magnetic activity that increases with inferred magnetic field strength.

Gavriil, F. P.↗
NASA NTRS1988

Theory of magnetic cataclysmic binary X-ray sources

The theory of magnetic cataclysmic binary X-ray sources is reviewed. The physics of the accretion torque for disk and for stream accretion is described, and the magnetic field strengths of DQ Her stars inferred from their spin behavior and of AM Her stars from direct measurement are discussed. The implications of disk and stream accretion for the geometry of the emission region and for the X-ray pulse profiles are considered. The physicl properties of the X-ray emission region and the expected infrared, optical, soft X-ray, and hard X-ray spectra are described. The orientations of the magnetic moment in AM Her stars inferred from the circular and linear polarization of the optical light and the optical light curve are commented on.

Lamb, Don Q.↗
NASA NTRS2002

Chandra Observations of Magnetic White Dwarfs and Their Theoretical Implications

Observations of cool DA and DB white dwarfs have not yet been successful in detecting coronal X-ray emission but observations of late-type dwarfs and giants show that coronae are common for these stars. To produce coronal X-rays, a star must have dynamo-generated surface magnetic fields and a well-developed convection zone. There is strong observational evidence that the DA star LHS 1038 and the DB star GD 358 have weak and variable surface magnetic fields. Since these fields are likely to be generated by dynamo action and since both stars have well-developed convection zones, theory predicts detectable levels of coronal X-rays from these white dwarfs. However, we present analysis of Chandra observations of both stars showing no detectable X-ray emission. The derived upper limits for the X-ray fluxes provide strong constraints on theories of formation of coronae around magnetic white dwarfs.

Musielak, Z. E.↗
NASA NTRS1990

The wavelength dependence of broadband linear polarization in cool stars

A model is developed to investigate the effect of wavelength dependence of magnetically generated broadband linear polarization (BLP) in late-type stars as a function of line blanketing, observation filters, and other variables. Linear polarization is assumed to be a single 'average' line and the line opacity, magnetic area filling factor, and continuum intensity are incorporated to establish an equation for disk-generated BLP. The method is investigated by computing special models for the flare star BY Dra and the sun which are compared to previous calculations. The model is shown to agree with the calculations of BLP, and the magnetic BLP dependent on wavelengths varies significantly from the simple power law dependences on wavelength. The model sets lower limits to the area occupied by magnetic regions, and an upper limit to the expected BLP also results from the formulation.

Saar, Steven H.↗
NASA NTRS2003

Chandra Observations of Magnetic White Dwarfs and their Theoretical Implications

Observations of cool DA and DB white dwarfs have not yet been successful in detecting coronal X-ray emission, but observations of late-type dwarfs and giants show that coronae are common for these stars. To produce coronal X-rays, a star must have dynamo-generated surface magnetic fields and a well-developed convection zone. There is some observational evidence that the DA star LHS 1038 and the DB star GD 358 have weak and variable surface magnetic fields. It has been suggested that such fields can be generated by dynamo action, and since both stars have well-developed convection zones, theory predicts detectable levels of coronal X-rays from these white dwarfs. However, we present analysis of Chandra observations of both stars showing no detectable X-ray emission. The derived upper limits for the X-ray fluxes provide strong constraints on theories of formation of coronae around magnetic white dwarfs. Another important implication of our negative Chandra observations is the possibility that the magnetic fields of LHS 1038 and GD 358 are fossil fields.

Musielak, Z. E.↗
NASA NTRS1995

Circinus X-1 revisited: Fast-timing properties in relation to spectral state

We have studied the X-ray spectral and fast-timing variations of Cir X-1 by performing a homogenous analysis of all EXOSAT ME data on this source using X-ray hardness-intensity diagrams (HIDs), color-color diagrams (CDs), and power spectra. Cir X-1 exhibits a wide range of power spectral shapes and a large variety in X-ray spectral shapes. At different epochs the power spectra variously resemble those of an atoll source, a Z source, a black-hole candidate, or are unlike any of these. At some epochs one-dimensional connected-branch patterns are seen in HID and CD, and at other times more complex structures are found. We interpret the complex behavior of Cir X-1 in terms of a model where accretion rate, orbital phase and epoch are the main determinants of the source behavior, and where the unique properties of the source are due to two special circumstances: (1) the source is the only known atoll source (accreting neutron star with a very low magnetic field) that can reach the Eddington critical accretion rate, and (2) it has a unique, highly eccentric and probably precessing orbit. Property (1) makes Cir X-1 a very important source for our understanding of the similarities in the observable properties of neutron stars and black holes as it allows to separate out black hole signatures from properties that are merely due to the presence of accretion compact with a low magnetic field.

Oosterbroek, T.↗
NASA NTRS1985

Activity-driven structure variations as a cause of period changes in RS Canis Venaticorum-like systems

Magnetic activity cycles of a convective star in a close binary system may lead to orbit period changes through tidal spin-orbit coupling. An increase in the mean magnetic field throughout the convection zone provides an additional pressure support and increases the star's moment of inertia. In order for the system to conserve angular momentum instantaneously, the convective star must reduce its rotation rate. When this happens, the system is no longer synchronous; in particular, the equilibrium tide now suffers a phase lag. A tidal torque must act, transferring angular momentum from the star's rotation to the orbit, lengthening the period. When the field decays, these events reverse and the period shortens. Observed period changes in RS CVn-like systems are of the order delta p/p = 10 to the -6th, implying variations in the mean field strength at the base of the convection zone of approximately 1000 G over time scales of 10-30 yr. It is predicted that the strength of magnetic activity indicators will track orbital period changes in a given system: the period will be largest during the active portion of the magnetic cycle and smallest during the quiescent part.

Van Buren, D.↗
DOE OSTI2015

The r-process nucleosynthesis in the various jet-like explosions of magnetorotational core-collapse supernovae

The r-process nucleosynthesis in core-collapse supernovae (CC-SNe) is studied, with a focus on the explosion scenario induced by rotation and strong magnetic fields. Nucleosynthesis calculations are conducted based on magneto-hydrodynamical explosion models with a wide range of parameters for initial rotation and magnetic fields. The explosion models are classified in two different types: prompt-magnetic-jet and delayed-magnetic-jet, for which the magnetic fields of proto-neutron stars (PNSs) during collapse and the core-bounce are strong and comparatively moderate, respectively. Following the hydrodynamical trajectories of each explosion model, we confirmed that r-processes successfully occur in the prompt-magnetic-jets, which produce heavy nuclei including actinides. On the other hand, the r-process in the delayed-magnetic-jet is suppressed, which synthesizes only nuclei up to the second peak (A∼130). Thus, the r-process in the delayed-magnetic-jets could explain only “weak r-process” patterns observed in metal-poor stars rather than the “main r-process,” represented by the solar abundances. Our results imply that CC-SNe are possible astronomical sources of heavy r-process elements if their magnetic fields are strong enough, while weaker magnetic explosions may produce “weak r-process” patterns (A≲130). We show the potential importance and necessity of magneto-rotational SNe for explaining the galactic chemical evolution, as well as abundances of r-process enhanced metal-poor stars. We also examine the effects of the remaining uncertainties in the nature of PNSs due to weak interactions that determine the final neutron-richness of ejecta. Additionally, we briefly discuss radioactive isotope yields in primary jets (e.g., {sup 56}Ni), with relation to several optical observation of SNe and relevant high-energy astronomical phenomena.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Magnetar-powered Supernovae in Two Dimensions. II. Broad-line Supernovae Ic

Nascent neutron stars (NSs) with millisecond periods and magnetic fields in excess of 10{sup 16} Gauss can drive highly energetic and asymmetric explosions known as magnetar-powered supernovae. These exotic explosions are one theoretical interpretation for supernovae Ic-BL, which are sometimes associated with long gamma-ray bursts. Twisted magnetic field lines extract the rotational energy of the NS and release it as a disk wind or a jet with energies greater than 10{sup 52} erg over ∼20 s. What fraction of the energy of the central engine go into the wind and the jet remain unclear. We have performed two-dimensional hydrodynamical simulations of magnetar-powered supernovae (SNe) driven by disk winds and jets with the CASTRO code to investigate the effect of the central engine on nucleosynthetic yields, mixing, and light curves. We find that these explosions synthesize less than 0.05 M{sub ⊙} of {sup 56}Ni and that this mass is not very sensitive to central engine type. The morphology of the explosion can provide a powerful diagnostic of the properties of the central engine. In the absence of a circumstellar medium, these events are not very luminous, with peak bolometric magnitudes of M{sub b}∼−16.5 due to low {sup 56}Ni production.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1996

Test of an Acoustic Mechanism for Atmospheric Heating in Dynamo-Deficient F Stars

In a qualitative sense, the heating of chromospheres and coronae has long been ascribed to either acoustic or magnetic heating. However, quantitative discussions of the energy balance with detailed comparison to the fluxes of chromospheric emission lines have begun to appear only recently. The aim of this work is to observe F stars where magnetic effects might be expected to be rather small, thereby allowing us hopefully to access acoustically heated atmospheres.

Mullan, D. J.↗
DOE OSTI2017

Searching for Magnetar-powered Merger-novae from Short GRBS

The merger of a double neutron star (NS–NS) binary may result in a rapidly rotating massive NS with an extremely strong magnetic field (i.e., a millisecond magnetar). In this case, the magnetic spin-down of the NS remnant provides an additional source of sustained energy injection, which would continuously power the merger ejecta. The thermal emission from the merger ejecta would give rise to a bright optical “magnetar-powered merger-nova.” In this work, we carry out a complete search for magnetar-powered merger-nova from a Swift short gamma-ray burst sample. We focus on short GRBs with extended emission or internal plateau, which may signify the presence of magnetars as the central engine. We eventually find three candidates of magnetar-powered merger-nova from the late observations of GRB 050724, GRB 070714B, and GRB 061006. With standard parameter values, the magnetar remnant scenario could well interpret the multi-band data of all three bursts, including the extended emission and their late chromatic features in the optical and X-ray data. The peak luminosities of these merger-novae reach several times 10{sup 42} erg s{sup −1}, more than one order of magnitude brighter than the traditional “kilo-novae” with peak luminosity of ∼10{sup 41} erg s{sup −1}. Intense, multi-color, late-time observations of short GRBs are encouraged to identify more merger-novae in the future.

79 ASTRONOMY AND ASTROPHYSICS↗