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Results for “STELLAR ATMOSPHERES”

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 973 records · Page 54
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Outer atmospheres of cool stars. II - Mg II flux profiles and chromospheric radiative loss rates

International Ultraviolet Explorer high-resolution spectra of the Mg II lines at 2796, 2803 A in 15 stars of spectral type G2-M2 including a wide range of luminosities are presented. These spectra are calibrated in absolute flux units at earth and at the stellar surface, and the chromospheric radiative loss rates in the Mg II lines are compared with corresponding rates in the Ca II H, K, and 8542 lines. The ratio of Mg II surface flux to total surface flux is found to be independent of stellar luminosity and thus gravity; may decrease slowly with decreasing effective temperature, and increases with decreasing period among RS Canum Venaticorum binaries. The factor of 10 range in this ratio at each effective temperature may be due to differences in the fractional surface area covered by plages and may indicate that stars of all luminosity classes have chromospheric plages. In this small data sample no evidence is found that the Mg II line surface fluxes indicate whether a star possesses a transition region and hot corona.

Basri, G. S.↗
NASA NTRS1998

Global Warming on Triton

Triton, Neptune's largest moon, has been predicted to undergo significant seasonal changes that would reveal themselves as changes in its mean frost temperature. But whether this temperature should at the present time be increasing, decreasing or constant depends on a number of parameters (such as the thermal properties of the surface, and frost migration patterns) that are unknown. Here we report observations of a recent stellar occultation by Triton which, when combined with earlier results, show that Triton has undergone a period of global warming since 1989. Our most conservative estimates of the rate of temperature and surface-pressure increase during this period imply that the atmosphere is doubling in bulk every 10 years, significantly faster than predicted by any published frost model for Triton. Our result suggests that permanent polar caps on Triton play a c dominant role in regulating seasonal atmospheric changes. Similar processes should also be active on Pluto.

Elliot, J. L.↗
NASA NTRS1974

10-micron heterodyne stellar interferometer

A spatial interferometer for 10-micron wavelength which uses two independent telescopes separated by 5.5 m, heterodyne detection of the infrared radiation, and path equalization by a variable-length RF cable, has given interference fringes from radiation of the planet Mercury. Continuous fringe observations during 4000 sec indicate remarkable stability in the optical-path difference through the atmosphere and the two telescopes, fluctuations between 20-sec averages being about 1/6 of the 10-micron wavelength.

Johnson, M. A.↗
NASA NTRS1986

On the dividing line for stellar coronae

A possible explanation is proposed for the observation that late-type stars falling in a certain region of the H-R diagram exhibit no X-ray emission and hence appear not to have coronae. It is argued that, due to the low surface gravity of these stars, the hot X-ray emitting loops in their atmospheres simply become cool and emit at longer wavelengths. This argument is demonstrated quantitatively by considering the effect on a typical solar loop of scaling it up to parameters appropriate to a supergiant.

Antiochos, S. K.↗
NASA NTRS1988

The outer atmospheres and winds of T Tau and RU Lupi

Emission line fluxes were used to establish the emission measure distribution for T Tau and RU Lupi. Intersystem lines are used to constrain the electron density in the emitting regions. These are compared with the results of Alfven wave driven wind models. Observed emission line widths and absorption components formed in the stellar winds are also compared with the theoretical predictions. It is concluded that the transition region lines are not formed in the wind and that the large widths of these emission lines are caused by nonthermal turbulent motions. However, the intersystem lines formed at Te less than or = 10,000 K indicate the presence of a lower density region, identified with the wind. Alfven-wave driven winds, with T max = 10,000 K long damping lengths, and high initial wave amplitudes may be able to account for the observed characteristics.

Jordan, C.↗
NASA NTRS2019

Bright Opportunities for Atmospheric Characterization of Small Planets: Masses and Radii of K2-3 b, c, and d and GJ3470 b from Radial Velocity Measurements and Spitzer Transits

We report improved masses, radii, and densities for four planets in two bright M-dwarf systems, K2-3 and GJ3470, derived from a combination of new radial velocity and transit observations. Supplementing K2 photometry with follow-up Spitzer transit observations refined the transit ephemerides of K2-3 b, c, and d by over a factor of 10. We analyze ground-based photometry from the Evryscope and Fairborn Observatory to determine the characteristic stellar activity timescales for our Gaussian Process fit, including the stellar rotation period and activity region decay timescale. The stellar rotation signals for both stars are evident in the radial velocity data and is included in our fit using a Gaussian process trained on the photometry. We find the masses of K2-3 b, K2-3 c, and GJ3470 b to be 6.48(+0.99,-0.93), 2.14(+1.08,-1.28), and 12.58(+1.31,-1.28)Mꚛ, respectively. K2-3 d was not significantly detected and has a 3σ upper limit of 2.80 Mꚛ. These two systems are training cases for future TESS systems; due to the low planet densities (ρ < 3.7 g/cu.cm) and bright host stars (K < 9 mag), they are among the best candidates for transmission spectroscopy in order to characterize the atmospheric compositions of small planets.

Molly R. Kosiarek↗
NASA NTRS1976

OAST Space Theme Workshop. Volume 3: Working group summary. 3: Sensors (E-3). A. Statement. B. Technology needs (form 1). C. Priority assessment (form 2). D. Additional assessment

Developments required to support the space power, SETI, solar system exploration and global services programs are identified. Instrumentation and calibration sensors (rather than scientific) are needed for the space power system. Highly sophisticated receivers for narrowband detection of microwave sensors and sensors for automated stellar cataloging to provide a mapping data base for SETI are needed. Various phases of solar system exploration require large area solid state imaging arrays from UV to IR; a long focal plane telescope; high energy particle detectors; advanced spectrometers; a gravitometer; and atmospheric distanalyzer; sensors for penetrometers; in-situ sensors for surface chemical analysis, life detection, spectroscopic and microscopic analyses of surface soils, and for meteorological measurements. Active and passive multiapplication sensors, advanced multispectral scanners with improved resolution in the UV and IR ranges, and laser techniques for advanced probing and oceanographic characterization will enhance for global services.

Source record↗
NASA NTRS1981

IUE observations of pre-main-sequence stars. I - Mg II and Ca II resonance line fluxes for T Tauri stars

IUE satellite and Lick 3 m reflector image tube scanner measurements of the Mg II and Ca II resonance lines in a sample of T Tauri stars are the basis of a discussion of the Mg II h and k line emission and the Ca II H and K line emission, within the context of stellar chromospheres. Corroborative evidence is presented for the chromospheric origin of these resonance lines, and chromospheric radiative loss rates in the Mg II and Ca II resonance lines are derived. It is found that the degree of nonradiative heating present in the outer atmospheres of T Tauri stars generally exceeds that of the RS CVn systems, as well as the dMe stars and other active chromospheric dwarfs, and it is inferred that the surfaces of such pre-main sequence stars are covered by regions similar to solar plages. The mean chromospheric electron density of T Tauri stars is determined as 10 to the 11th/cu cm.

Giampapa n, M. S.↗
DOE OSTI2020

Limits on Mode Coherence in Pulsating DA White Dwarfs Due to a Nonstatic Convection Zone

The standard theory of pulsations deals with the frequencies and growth rates of infinitesimal perturbations in a stellar model. Modes which are calculated to be linearly driven should increase their amplitudes exponentially with time; the fact that nearly constant amplitudes are usually observed is evidence that nonlinear mechanisms inhibit the growth of finite amplitude pulsations. Models predict that the mass of convection zones in pulsating hydrogen-atmosphere (DAV) white dwarfs is very sensitive to temperature (i.e., M CZ α T –90 eff ), leading to the possibility that even low-amplitude pulsators may experience significant nonlinear effects. In particular, the outer turning point of finite-amplitude g-mode pulsations can vary with the local surface temperature, producing a reflected wave that is out of phase with what is required for a standing wave. This can lead to a lack of coherence of the mode and a reduction in its global amplitude. In this paper we show that: (1) whether a mode is calculated to propagate to the base of the convection zone is an accurate predictor of its width in the Fourier spectrum, (2) the phase shifts produced by reflection from the outer turning point are large enough to produce significant damping, and (3) amplitudes and periods are predicted to increase from the blue edge to the middle of the instability strip, and subsequently decrease as the red edge is approached. Furthermore, this amplitude decrease is in agreement with the observational data while the period decrease has not yet been systematically studied.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2011

Solar UV Variations During the Decline of Cycle 23

Characterization of temporal and spectral variations in solar ultraviolet irradiance over a solar cycle is essential for understanding the forcing of Earth's atmosphere and climate. Satellite measurements of solar UV variability for solar cycles 21, 22, and 23 show consistent solar cycle irradiance changes at key wavelengths (e.g. 205 nm, 250 nm) within instrumental uncertainties. All historical data sets also show the same relative spectral dependence for both short-term (rotational) and long-term (solar cycle) variations. Empirical solar irradiance models also produce long-term solar UV variations that agree well with observational data. Recent UV irradiance data from the Solar Radiation and Climate Experiment (SORCE) Spectral Irradiance Monitor (SIM) and Solar Stellar Irradiance Comparison Experiment (SOLSTICE) instruments covering the declining phase of Cycle 23 present a different picture oflong-term solar variations from previous results. Time series of SIM and SOLSTICE spectral irradiance data between 2003 and 2007 show solar variations that greatly exceed both previous measurements and predicted irradiance changes over this period, and the spectral dependence of the SIM and SOLSTICE variations during these years do not show features expected from solar physics theory. The use of SORCE irradiance variations in atmospheric models yields substantially different middle atmosphere ozone responses in both magnitude and vertical structure. However, short-term solar variability derived from SIM and SOLSTICE UV irradiance data is consistent with concurrent solar UV measurements from other instruments, as well as previous results, suggesting no change in solar physics. Our analysis of short-term solar variability is much less sensitive to residual instrument response changes than the observations of long-term variations. The SORCE long-term UV results can be explained by under-correction of instrument response changes during the first few years of measurements, rather than requiring an unexpected change in the physical behavior of the Sun.

DeLand, Matthew, T.↗
DOE OSTI2021

Gemini/GMOS Transmission Spectroscopy of the Grazing Planet Candidate WD 1856+534 b

WD 1856+534 b is a Jupiter-sized, cool giant planet candidate transiting the white dwarf WD 1856+534. Here, we report an optical transmission spectrum of WD 1856+534 b obtained from ten transits using the Gemini Multi Object Spectrograph. This system is challenging to observe due to the faintness of the host star and the short transit duration. Nevertheless, our phase-folded white light curve reached a precision of 0.12%. WD 1856+534 b provides a unique transit configuration compared to other known exoplanets: the planet is 8x larger than its star and occults over half of the stellar disk during mid-transit. Consequently, many standard modeling assumptions do not hold. We introduce the concept of a “limb darkening corrected, time-averaged transmission spectrum” and propose that this is more suitable than (R pλ /R s ) 2 for comparisons to atmospheric models for planets with grazing transits. We also present a modified radiative transfer prescription. Though the transmission spectrum shows no prominent absorption features, it is sufficiently precise to constrain the mass of WD 1856+534 b to be >0.84 M J (to 2σ confidence), assuming a clear atmosphere and a Jovian composition. High-altitude cloud decks can allow lower masses. WD 1856+534 b could have formed either as a result of common envelope evolution or migration under the Kozai–Lidov mechanism. Further studies of WD 1856+534 b, alongside new dedicated searches for substellar objects around white dwarfs, will shed further light on the mysteries of post-main-sequence planetary systems.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1979

Isotopes of titanium in cool stars

A program of stellar Ti isotopic-abundance determinations is described and related to changes that may have occurred in the Ti isotopic abundance ratios during the chemical evolution of the Galaxy, and to whether the abundance ratios are perturbed in the S and MS stars having atmospheres with enhanced abundances of s-process nuclei resulting from convective mixing after internal exposure to neutrons. High-resolution (0.07 A) Reticon spectra of portions of the TiO gamma (0,0), gamma (0,1), and delta (0,0) bands were the source material, and particular emphasis was placed on the definition of the continuum level. The isotopic abundance ratios are terrestrial in all of the sample, and errors in the (Ti-i)/(Ti-48) ratio are typically plus or minus 25% for the dwarfs and plus or minus 50% for the giants. The observations show that the magic nucleus Ti-50 is not enhanced in S and MS stars.

Clegg, R. E. S.↗
NASA NTRS1989

Helium resonance lines in the flare of 15 June 1973

Time sequences of He I and He II resonance line intensities at several sites within the flare of June 15, 1973 are derived from observations obtained with Naval Research Laboratory's Slitless Spectroheliograph on Skylab. The data are compared with predictions in six model flare atmospheres based on two values for the heating rate and three for the flux of photoionizing coronal X-rays and EUV. A peak ionizing flux more than 1000 times that in the quiet sun is indicated. Implications for the common practice of deriving stellar coronal fluxes from He II 1640 A fluxes assuming dominance of the recombination mechanism, are discussed.

Porter, Jason G.↗
NASA NTRS1990

Astrophysics from the moon; Proceedings of the Workshop, Annapolis, MD, Feb. 5-7, 1990

The present conference on astrophysics from the moon encompasses the study of the Galaxy, external planetary systems, solar physics, stars and stellar evolution, the frontiers of Galactic, extragalactic, and cosmological astronomy, an introduction to lunar-based astronomy, concepts for lunar observatories including high-energy observatories, solar observatories, and observatories for particle astrophysics and gravitational studies. Specific issues addressed include the dynamics of Jovian atmospheres, planetary magnetospheres, flare physics, exobiology and SETI from the lunar farside, and the study of interactive stars, star formation, H II regions in absorption at low frequencies, and normal galaxies. Also addressed are the potential lunar investigation of quasars, the formation epoch, and the large-scale structure of the universe, and observational issues related to X-ray large arrays, optical interferometers, VLF radio astronomy, a UV-solar reflecting coronagraph, and a heavy-nucleus detector.

Mumma, Michael J.↗
NASA NTRS1992

The origin of the extremely metal-poor post-AGB stars

Certain A-F supergiants show almost solar photospheric abundances of C, N, O, S, and Zn, while Fe, Mg, Ca, Si, Cr, and other elements are more than two orders of magnitude below solar. We suggest that the present photosphere was originally solar in composition; the missing elements were incorporated into dust grains that were separated from the atmosphere. The heavy-element-depleted gas comprises the present photosphere. There are two possible mechanisms by which this process might take place: capture by the presently visible star of the depleted gas from a binary companions, or the rapid termination of a vigorous stellar wind in a single star, so that the grains are blown outward through the expanding envelope. Besides the abundance peculiarities, the hypothesis explains why some objects have carbon-rich dust outside of an oxygen rich photosphere, why these stars have the rather narrow range of (T(eff), log g) near (7000 K, 1.0), and why some might have a great deal of circumstellar dust while others have little. Perhaps the most serious objection to the hypothesis is the possible presence of stellar winds observed in the winds of the peculiar post-AGB stars themselves.

Mathis, J. S.↗
NASA NTRS2017

Where are the r-modes? Chandra Observations of Millisecond Pulsars

We present the results of Chandra observations of two non-accreting millisecond pulsars, PSRs J1640+2224(J1640) and J1709+2313 (J1709), with low inferred magnetic fields and spin-down rates in order to constrain their surface temperatures, obtain limits on the amplitude of unstable r-modes in them, and make comparisons with similar limits obtained for a sample of accreting low-mass X-ray binary (LMXB) neutron stars. We detect both pulsars in the X-ray band for the first time. They are faint, with inferred soft X-ray fluxes(0.3-3 keV) of approx. 6 x 10(exp -15) and 3 x 10( exp -15) erg/sq cm for J1640 and J1709, respectively. Spectral analysis assuming hydrogen atmosphere emission gives global effective temperature upper limits (90% confidence) of 3.3-4.3 x 10(exp 5) K for J1640 and 3.6-4.7 x 10(exp 5) K for J1709, where the low end of the range corresponds to canonical neutron stars (M = 1.4 Stellar Mass), and the upper end corresponds to higher-mass stars (M = 2.21 Stellar Mass). Under the assumption that r-mode heating provides the thermal support, we obtain dimensionless r-mode amplitude upper limits of 3.2-4.8 x 10(exp -8) and 1.8-2.8 x 10(exp -7) for J1640 and J1709, respectively, where again the low end of the range corresponds to lower-mass, canonical neutron stars (M =1.4 Stellar Mass). These limits are about an order of magnitude lower than those we derived previously for a sample of LMXBs, except for the accreting millisecond X-ray pulsar SAX J1808.43658, which has a comparable amplitude limit to J1640 and J1709.

oscillations↗
NASA NTRS2018

The Large Ultraviolet/Optical/Infrared Surveyor

Astronomy crossed a threshold three decades ago with the discovery of planets around other stars. Compared to scientists' previous expectations set by the Solar System, exoplanets are wonderfully abundant and varied. Indirect planet discovery techniques have shown that small rocky planets residing in stellar habitable zones, where such planets may have liquid water on their surfaces, are not rare. This revelation drives us to ask more ambitious and fundamental questions, that fascinate scientists and the public alike: are there other truly Earth-like planets out there and do any of them harbour life? Today, exoplanets are largely ‘small black shadows' to us, with measurements of orbits, sizes and masses (all three in the best cases).The upcoming James Webb Space Telescope and future 30-m-class ground-based telescopes will characterize the atmospheres of habitable planet candidates orbit in glow-mass M dwarf stars. However, deeply probing atmospheres of the exoplanets most similar to the Earth, those around Sun-like stars, remains out of reach for currently planned observatories. Bringing them within our grasp is a primary motivation for the Large UV/Optical/Infrared Surveyor(LUVOIR) mission concept, currently the focus of a three-year NASA study.

exoplanets↗