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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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS2010

The Origin of Dust in the Early Universe: Probing the Star Formation History of Galaxies by Their Dust Content

Two distinct scenarios for the origin of the approximately 4 x 10(exp 8) Solar Mass of dust observed in the high-redshift (z = 6.4) quasar J1148+5251 have been proposed. The first assumes that this galaxy is much younger than the age of the universe at that epoch so that only supernovae, could have produced this dust. The second scenario assumes a significantly older galactic age, so that the dust could have formed in lower-mass AGB stars. Presenting new integral solutions for the chemical evolution of metals and dust in galaxies, we offer a critical evaluation of these two scenarios. ^N;"(,, show that the AGB scenario is sensitive to the details of the galaxy's star formation history (SFH), which must consist of an early intense starburst followed by a period of low stellar activity. The presence or absence of massive amounts of dust in high-redshift galaxies can therefore be used to infer their SFH. However, a problem with the AGB scenario is that it produces a stellar mass that is significantly larger than the inferred dynamical mass of J1148+5251, an yet unresolved discrepancy. If this problem persists, then additional sites for the growth or formation of dust, such as molecular clouds or dense clouds around active galactic nuclei, must be considered.

Dwek, Eli↗
NASA NTRS2004

Removing Activity-Related Radial Velocity Noise to Improve Extrasolar Planet Searches

We have made significant progress towards the proposal goals of understanding the causes and effects of magnetic activity-induced radial velocity (v_r) jitter and developing methods for correcting it. In the process, we have also made some significant discoveries in the fields of planet-induced stellar activity, planet detection methods, M dwarf convection, starspot properties, and magnetic dynamo cycles. We have obtained super high resolution (R approximately 200,000), high S / N (greater than 300) echelle study of joint line bisector and radial velocity variations using the McDonald 2-D coude. A long observing run in October 2002 in particular was quite successful (8 clear nights). We now have close to three years of data, which begins to sample a good fraction of the magnetic cycle timescales for some of our targets (e.g., kappa Ceti; P_cyc = 5.6 yrs). This will be very helpful in unraveling the complex relationships between plage and radial velocity (v-r) changes which we have uncovered. Preliminary analysis (Saar et al. 2003) of the data in hand, reveals correlations between median line bisector displacement and v_r. The correlation appears to be specific the the particular star being considered, probably since it is a function of both spectral type and rotation rate. Further analysis and interpretation will be in the context of evolving plage models and is in progress.

Saar, Steven↗
NASA NTRS1984

The sun as a star - 1982 June 14-August 13

During a 60-day period in the summer of 1982, a joint observing project between the High Altitude Observatory and the University of Hawaii was undertaken to collect a set of solar photospheric, chromospheric, and coronal data. From these data, the relationships between quantities recognized to be related to solar activity are specified. The amplitude of rotational modulation of the diagnostic parameters derived for the full disk was approximately equal to the modulation observed over an entire solar cycle. Those integrated solar data have been used as diagnostics of generalized stellar activity. A relationship between the Ca II line K flux and total inferred coronal mass of the sun existed during this observation period, and this fact leads to the hypothesis that ca II flux modulation in other stars may be used to infer stellar coronal conditions.

Fisher, R.↗
NASA NTRS2022

Firefly: The Case for a Holistic Understanding of the Global Structure and Dynamics of the Sun and the Heliosphere

This white paper presents the current knowledge gaps in understanding how magnetic fields control solar (and by extension, stellar) activity in timescales from minutes to years and discusses a transformative observational approach to fill those gaps. The solar activity drives space weather as the result of dynamic magnetic fields forming in the solar interior and evolving continuously until reaching levels of complexity in the atmosphere that trigger eruptions. However, we do not fully understand how solar and, more generally, stellar magnetic fields are generated and how they evolve through the eruptive states. The major obstacle is our reliance on observations from a single viewpoint, particularly in the ecliptic plane. This vantage point can only provide limited information for (i) understanding the generation of solar magnetic fields deep in the convection zone; (ii) determining the origin of the solar cycle and predicting its timing and strength; (iii) explaining the causes of solar activity and their triggers; (iv) reliably predicting when and how CMEs will impact Earth and other planets; (v) fathoming the structure and dynamics of the corona as it creates the heliosphere; (vi) understanding the energization and transport of energetic particles; etc.

Nour E. Raouafi↗
NASA NTRS2021

A Sub-Neptune and a Non-Transiting Neptune-Mass Companion Unveiled by ESPRESSO Around the Bright Late-F Dwarf HD 5278 (TOI-130)

Context. Transiting sub-Neptune-type planets, with radii approximately between 2 and 4R⊕, are of particular interest as their study allows us to gain insight into the formation and evolution of a class of planets that are not found in our Solar System. Aims. We exploit the extreme radial velocity (RV) precision of the ultra-stable echelle spectrograph ESPRESSO on the VLT to unveil the physical properties of the transiting sub-Neptune TOI-130 b, uncovered by the TESS mission orbiting the nearby, bright, late F-typestar HD 5278 (TOI-130) with a period of Pb=14.3 days. Methods. We used 43 ESPRESSO high-resolution spectra and broad-band photometry information to derive accurate stellar atmospheric and physical parameters of HD 5278. We exploited the TESS light curve and spectroscopic diagnostics to gauge the impact of stellar activity on the ESPRESSO RVs. We performed separate as well as joint analyses of the TESS photometry and the ESPRESSORVs using fully Bayesian frameworks to determine the system parameters. Results. Based on the ESPRESSO spectra, the updated stellar parameters of HD 5278 are Teff=6203±64K, logg=4.50±0.11dex, [Fe/H] =−0.12±0.04dex,M?=1.126+0.036−0.035M, and R?=1.194+0.017−0.016R. We determine HD 5278 b’s mass and radius to be Mb=7.8+1.5−1.4M⊕ and Rb=2.45±0.05R⊕. The derived mean density, %b=2.9+0.6−0.5g cm−3, is consistent with the bulk composition of a sub-Neptune with a substantial (∼30%) water mass fraction and with a gas envelope comprising ∼17% of the measured radius. Given the host brightness and irradiation levels, HD 5278 b is one of the best targets orbiting G-F primaries for follow-up atmospheric characterization measurements with HST and JWST. We discover a second, non-transiting companion in the system, with a period of Pc=40.87+0.18−0.17days and a minimum mass of Mcsinic=18.4+1.8−1.9M⊕. We study emerging trends in parameters space (e.g., mass, radius, stellar insolation, and mean density) of the growing population of transiting sub-Neptunes, and provide statistical evidence for a low occurrence of close-in,10−15M⊕companions around G-F primaries withTeff&5500K.

planetary systems↗
NASA NTRS2023

Heliocentric Distance and Solar Activity Dependence of Sustained Quasi-radial Interplanetary Magnetic Field Occurrence

Planets close to their stars experience an interplanetary magnetic field (IMF) that is dominantly quasi-radial. Our solar system serves as a laboratory to study how the occurrence of quasi-radial IMF varies away from the star and under different stellar activities. Furthermore, on time and spatial scales relevant to magnetospheric physics, solar wind variability prevails in the form of structures generated both at the Sun and locally in the interplanetary space. The stationary Parker spiral model only approximates the large-scale structure of the IMF. Deviations from the Parker spiral often result in strongly radial magnetic fields that give rise to kinetic foreshock turbulence, which in turn can impact planetary magnetospheres. The relative significance of this type of interaction can be estimated statistically based on the occurrence rate of cases where the IMF is directed along the radial direction, leading to the entire day-side magnetosphere being downstream of the ion foreshock. We use observations covering radial distances from 0.1 to 10 au and more than 2 solar cycles to quantify the prevalence of radial IMFs throughout the heliosphere. Near Earth's orbit, it is found that the occurrence rates of quasi-radial and southward IMF orientations are similar, and that the Pearson correlation coefficient is ${{ \mathcal R }}_{{xy}}\sim -0.7$ calculated between quasi-radial IMF occurrence rate and solar activity. A negative correlation is demonstrated for radial distances extending to at least Mars but not to Saturn.

Brandon L Burkholder↗
NASA NTRS1987

The onset of chromospheric activity among the A- and F- type stars

IUE observations of C II lambda1335 and C IV lambda1549 and ground-based observations of He I lambda5876 have previously discovered intense levels of chromospheric activity among early F type stars. Virtually all F dwarfs show stronger chromospheric and transition region emission than do the cooler and more deeply convective dwarf stars like the Sun. The IUE spectra and those of He lambda5876 place the onset of stellar activity along the main sequence near a color B - V = 0.28, which corresponds approximately to spectral type FO and an effective temperature of 7300 K. However, existing X-ray observations of A and F stars suggest that coronal activity may reach a peak blueward of this high temperature boundary at B - V = 0.28 before vanishing among the early and mid A-type stars. Discussed are preliminary results of a new effort to refine the location of the high temperature boundary to chromospheric activity among A- and F- type stars, making use of low dispersion short-wavelength spectra from the IUE archives from which the strengths of C IV, C II, and Lyman alpha emission have been measured.

Simon, Theodore↗
NASA NTRS1998

Photospheric Fluorescence and Resonance Scattering: Non Classical Diagnostics and the Future of X-ray Stellar Spectroscopy

High resolution AXAF and XMM observations of stellar coronae will yield a wealth of X-ray plasma line diagnostics that will provide a giant leap forward in our understanding of coronal densities, abundance anomalies and emission measure distributions. Unfortunately, there is one very basic unanswered question in the physics of active stellar coronae that the usual plasma diagnostics cannot address directly: What are the spatial characteristics of stellar coronae-the scale height and filling factor? What do other stellar coronae actually look like? I will discuss two novel diagnostics of coronal geometry and their application to future X-ray spectra: photospheric fluorescence and resonance line optical depths.

Drake, Jeremy↗
NASA NTRS1993

Magnetic activity in low-mass stars

The manifestations of magnetic activity in low-mass stars, particularly M dwarfs, can be used as a tool to study their evolution, the operation of the interior dynamo with changing interior conditions, and the structure of their outer atmospheres. Extensive background material on the current understanding of low-mass stellar activity is presented. Two new surveys are described which will greatly increase the number of active low-mass stars known in the field and in nearby open clusters. These surveys will define the characteristics of the activity in low-mass stars, and how the activity changes with a number of parameters of interest, including mass, effective temperature, and age. The data will also allow a rigorous determination of a possible age-activity relation among the low-mass M dwarfs. Theoretical models of M dwarf atmospheres, and their connection to the understanding of the observations, are also discussed.

Hawley, Suzanne L.↗
NASA NTRS1989

Multifrequency survey of the intergalactic cloud in the M96 group

The intergalactic cloud of neutral hydrogen in the M96 group are examined for signs of emission over a wide range of frequencies, from radio waves to X rays. Past or present stellar activity in the gas might have been expected to produce detectable visual infrared, CO, OH, or radio recombination-line emission. None was detected. The limits are used to study physical conditions in the intergalactic gas. In particular, B and V band limits on starlight and IRAS limits on the presence of dust strongly constrain the presence of stars or stellar by-products. However, given the uncertainties about physical conditions in the intergalactic environment, it is difficult to rule out entirely the presence of stellar-processed materials. Results of neutral hydrogen mapping from a large-scale survey of the intergalactic cloud and surrounding region are also presented. These observations confirm that the gas is confined to a large ringlike structure. The simplest interpretation remains that the intergalactic gas in Leo is primordial.

Schneider, Stephen E.↗
NASA NTRS2016

Rapidly Rotating, X-Ray Bright Stars in the Kepler Field

We present Kepler light curves and optical spectroscopy of twenty X-ray bright stars located in the Kepler field of view. The stars, spectral type F-K, show evidence for rapid rotation including chromospheric activity 100 times or more above the Sun at maximum and flaring behavior in their light curves. Eighteen of our objects appear to be (sub)giants and may belong to the class of FK Com variables, which are evolved rapidly spinning single stars with no excretion disk and high levels of chromospheric activity. Such stars are rare and are likely the result of W UMa binary mergers, a process believed to produce the FK Com class of variable and their descendants. The FK Com stage, including the presence of an excretion disk, is short lived but leads to longer-lived stages consisting of single, rapidly rotating evolved (sub)giants with high levels of stellar activity.

stars: chromospheres – stars: evolution – star↗
NASA NTRS1984

Rotational modulation of the chromospheric activity in Chi(1) Orionis (G0 V)

The age and rotation period of the solar type star Chi(1) Ori recommend it for a rotational modulation study of its strong UV emission lines. IUE observations have accordingly been made at four different phases during each of three separate rotation cycles, over a time span of 325 days. C IV, He II, and Mg II observations are consistent with a uniform spatial distribution and brightness for the Mg II emission network, and a more complex distribution of the regions producing the UV transmission region emission. Alternatively, if the area coverage of the chromospheric and transition region emission is nearly identical, the surface brightness contrast between stellar active regions and normal ones must increase by almost an order of magnitude in the higher temperature lines.

Simon, T.↗
NASA NTRS2016

The Habitability of Proxima Centauri b: I. Irradiation, Rotation and Volatile Inventory From Formation to the Present

Proxima b is a planet with a minimum mass of 1.3 M(sub ⨁) orbiting within the habitable zone (HZ) of Proxima Centauri, a very low-mass, active star and the Sun’s closest neighbor. Here we investigate a number of factors related to the potential habitability of Proxima b and its ability to maintain liquid water on its surface. We set the stage by estimating the current high-energy irradiance of the planet and show that the planet currently receives 30 times more extreme-UV radiation than Earth and 250 times more X-rays. We compute the time evolution of the star’s spectrum, which is essential for modeling the flux received over Proxima b’s lifetime. We also show that Proxima b’s obliquity is likely null, and its spin is either synchronous or in a 3:2 spin-orbit resonance, depending on the planet’s eccentricity and level of triaxiality. Next, we consider the evolution of Proxima b’s water inventory. We use our spectral energy distribution to compute the hydrogen loss from the planet with an improved energy-limited escape formalism. Despite the high level of stellar activity, we find that Proxima b is likely to have lost less than an Earth ocean’s worth of hydrogen (EO(sub H)) before it reached the HZ 100–200 Myr after its formation. The largest uncertainty in our work is the initial water budget, which is not constrained by planet formation models. We conclude that Proxima b is a viable candidate habitable planet.

Proxima Cen↗
NASA NTRS1980

Ultraviolet spectroscopy of the outer layers of stars

Ultraviolet observations of the extended atmospheres and circumstellar envelopes of early-type and cool stars are reviewed. UV spectra of OB stars have shown that mass loss occurs in virtually all these stars and in many Be stars, and the Copernicus satellite has also provided information on the physical conditions and variability of the winds. The winds have been interpreted in terms of hot coronal wind models, imperfect flow models and radiation pressure models, and estimates of mass loss ranging from 10 to the -10th to 9 x 10 to the -6th solar masses/year have been obtained. Further UV data of faint stars, additional wavelengths and time variability are required. Recent UV and X-ray experiments have detected stellar chromospheres, transition regions coronas and winds in cool stars. Semiempirical line-profile and line-flux models and purely theoretical atmospheric models have been constructed to explain chromosphere, corona and wind data, and future observations of the physical properties of stellar chromospheres, coronas and winds, terms in the energy balance equation, stellar activity and its long-term variability and atmospheric modifications due to the presence of companion stars are proposed for such missions as Spacelab.

Snow, T. P., Jr.↗
NASA NTRS1982

Einstein observations of BY Draconis variables and RS CVn binaries

Observations of five BY Draconis variables and four RS Canum Venaticorum systems with the Einstein Observatory are presented. All nine stars were detected. Although the BY Draconis stars have an average X-ray luminosity one to two orders of magnitude lower than the RS CVn binaries, at least three of these systems fit the X-ray/bolometric luminosity ratio-vs-period relation first described by Walter and Bowyer (1981) for RS CVn's. Since one of the BY Draconis stars is almost certainly single, this result lends strong support to the suggestion that rapid rotation, and not duplicity, is the determining factor in defining the level of stellar activity in such stars. In addition, the X-ray data are consistent with the suggestion, made by Vogt and Fekel (1979), that the BY Draconis stars are in the final stages of pre-main-sequence evolution.

Caillault, J.-P.↗
NASA NTRS1982

Einstein observations of cool stars

Observations of cool stars with the Einstein Observatory (HEAO-2) have brought about a fundamental change in our knowledge and understanding of stellar coronae. The existence of X-ray emission from stars throughout the H-R diagram, the wide range of X-ray luminosity within a given spectral and luminosity class, and the strong correlation of X-ray luminosity with stellar age and rotation are among the more significant Einstein results. These results are strong evidence for the influence of stellar dynamo action on the formation and heating of stellar coronae. A discussion of relevant consortium and guest observations is given. The Hyades cluster, in particular, serves as an example to demonstrate the usefulness of X-ray observations in the study of stellar activity and coronal evolution.

Stern, R. A.↗
NASA NTRS1985

The future of solar physics

Outstanding problems for the future of solar physics and stellar physics are examined. The physics of stellar interiors has been called into serious question by the very low measured neutrino flux from the sun. The Ga-71 neutrino detection experiment is the next step in unravelling this mystery. The new methods of helioseismology, for probing the interior of the sun, have already found the primordial rapid rotation of the central core. The forthcoming worldwide helioseismology observing network will permit fuller exploitation of the method, promising to provide the first direct sounding of the interior of a star, hitherto known to us only through theoretical inference and the discrepant neutrino emission. An essential step in developing the physics of stellar activity will be the Solar Optical Telescope (presently planned by NASA to be launched early in the next decade) to permit a 'microscopic' examination of the surface of the sun to study the source of the action. The activity and X-ray emission of other stars depend on much the same effects, so that the study of the sun is essential to determining the significance of the X-ray emission from other stars.

Parker, E. N.↗
NASA NTRS1986

Narrow-band radio flares from red dwarf stars

VLA observations of narrow-band behavior in 20 cm flares from two red dwarf stars, L726 - 8A and AD Leo, are reported. The flare on L726 - 8A was observed at 1415 and 1515 MHz; the flux and the evolution differed significantly at the two frequencies. The flare on AD Leo lasted for 2 hr at 1415 MHz but did not appear at 1515 MHz. The AD Leo flare appears to rule out a source drifting through the stellar corona and is unlikely to be due to plasma emission. In the cyclotron maser model the narrow-band behavior reflects the range of magnetic fields present within the source. The apparent constancy of this field for 2 hr is difficult to understand if magnetic reconnection is the source of energy for the flare. The consistent polarization exhibited by red dwarf flares at 20 cm may be related to stellar activity cycles, and changes in this polarization will permit measuring the length of these cycles.

White, Stephen M.↗