Engineering Papers⌕ Search

SEARCH · Engineering Papers

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 847 records · Page 47
All sourcesNASA NTRSDOE OSTI
NASA NTRS2020

The Featureless Transmission Spectra of Two Super-puff Planets

The Kepler mission revealed a class of planets known as "super-puffs," with masses only a few times larger than Earth's but radii larger than Neptune, giving them very low mean densities. All three of the known planets orbiting the young solar-type star Kepler 51 are super-puffs. The Kepler 51 system thereby provides an opportunity for a comparative study of the structures and atmospheres of this mysterious class of planets, which may provide clues about their formation and evolution. We observed two transits each of Kepler 51b and 51d with the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope. Combining new WFC3 transit times with reanalyzed Kepler data and updated stellar parameters, we confirmed that all three planets have densities lower than 0.1 g/cu.cm. We measured the WFC3 transmission spectra to be featureless between 1.15 and 1.63 μm, ruling out any variations greater than 0.6 scale heights (assuming a H/He-dominated atmosphere), thus showing no significant water absorption features. We interpreted the flat spectra as the result of a high-altitude aerosol layer (pressure <3 mbar) on each planet. Adding this new result to the collection of flat spectra that have been observed for other sub-Neptune planets, we find support for one of the two hypotheses introduced by Crossfield & Kreidberg, that planets with cooler equilibrium temperatures have more high-altitude aerosols. We strongly disfavor their other hypothesis that the H/He mass fraction drives the appearance of large-amplitude transmission features.

Jessica E. Libby-Roberts↗
NASA NTRS1987

Research relative to atmosphere physics and spacecraft applications studies

Progress is reported in several areas of research. Brief descriptions are given in each of the following areas: Spacelab data analysis; San Marco activity; Molecular physics; Stellar energy analysis; Troposphere data analysis; Voyager encounter analysis; Laser activity; Gravity wave study; Venus studies; and Shuttle environmental studies.

Greenwood, Stuart W.↗
NASA NTRS1995

The threshold for stellar winds in hot main-sequence stars

The profiles of ultraviolet resonance lines of C IV were surveyed in a sample of 29 cluster and association members in the spectral type range O9-B2 III-V, together with a few field stars of interest. The temperatures and gravities of the stars were taken from the model atmosphere analysis by Grigsby, Morrison, & Anderson (1992), and the luminosities were estimated on the basis of cluster and association distances from the recent literature. A parameter P(sub w) was defined in order to describe the degree and assymetry of the C IV profile. This parameter, together with total C IV equivalent width, was found to be well correlated with stellar luminosity and temperature. A few anomalous stars were noted: tau Sco, HD 66665, HD 13621, and the ON stars HD12323 and HD 201345. The results suggest a sudden onset of observable mass loss at T(effective) = 27,500 +/- 500 K, log (L/solar luminosity) = 4.4 +/- 0.12, in agreement with the previous study by Prinja (1989). At T(effective) = 28,000 K and log g = 4, our non-LTE model atmospheres show an enhancement in the ground-state population of C(+3) in their topmost layer, which could be responsible for initiation of the winds via radiation pressure on the C(+3) ions, or for the onset of visibility of C(+3) ions in the wind because of an increase in the optical depth in the C IV lines in the outermost layers.

Grigsby, James A.↗
NASA NTRS1978

A white-light amplitude interferometer with 180-degree rotational shear

The paper reports on the fabrication, alignment, and test techniques of an equal-path, wave-front, folding amplitude interferometer with 180-degree rotational shear. The device may be used for measuring atmospheric phase-perturbing properties and designing optical filters used in processing spatial information. The instrument has been used to photograph the Michelson Stellar interferometer fringes.

Breckinridge, J. B.↗
NASA NTRS1986

The energy distributions of B supergiants in the Large Magellanic Cloud

It is shown that line-blanketed, LTE, plane-parallel model atmosphere calculations provide excellent fits to the ultraviolet-through-visual energy distributions of B supergiants in the Large Magellanic Cloud. The models were computed using Kurucz's (1979) ATLAS atmosphere program, but with lower gravities than were contained in Kurucz's published model grid. The ultraviolet continua of low gravity stars are found to be sensitive to changes in temperature and gravity. Measurements of Teff and log g for ten LMC B supergiants from model atmosphere fits to the energy distributions yield estimates of their radii, luminosities, and masses. Model atmosphere fits suggest that the late B supergiants have significantly lower masses than the earlier B types of the same luminosity, contrary to stellar evolution theory which predicts that B supergiants are in a post-core hydrogen burning phase and should evolve very quickly and at essentially constant mass.

Fitzpatrick, Edward L.↗
NASA NTRS1993

Solar maximum mission/ultraviolet spectrometer and polarimeter studies

This final report for NASA Contract No. NAS8-35921 describes various studies performed for the Ultraviolet Spectrometer and Polarimeter (UVSP) experiment, one of several instruments on the Solar Maximum Mission (SMM) satellite which was launched on 14 February 1980. The UVSP consisted primarily of a Gregorian telescope and an Ebert-Fastie spectrometer with a polarimeter that could be inserted into the light path. The spacecraft and most of the instruments, including the UVSP, operated successfully until 23 November 1980, when part of the SMM attitude control system (fine pointing control) failed. The UVSP was then unable to observe the Sun until 18 April 1984, when the SMM was visited by the space shuttle and the attitude control module was replaced by astronauts. The SMM mission ended when the spacecraft reentered the atmosphere of the Earth and was thereby destroyed on 2 December 1989. The topics covered in this report include the following: (1) ultraviolet stellar polarimetry (probably the first such attempted measurement); no polarization as detected and the upper limits, based on the sensitivity as determined by the observed count rate, are rather high; (2) an investigation into the possible position of the UVSP wavelength drive after it became stuck on 26 April 1985; (3) fast timing tests for sit-and-state observations involving one or two detectors; (4) development of computer subroutines to allow the calculation of the component of the SMM spacecraft orbital velocity along the line of sight to the Sun at any desired time during the 1984/1985 period when the UVSP wavelength drive was operating properly; (5) listing of published research papers; (6) description of the UVSP catalog of observations; and (7) description of UVSP calibration report and data users guide.

Henze, William, Jr.↗
NASA NTRS1994

Research in astrophysical processes

Work completed under this grant is summarized in the following areas:(1) radio pulsar turn on and evaporation of companions in very low mass x-ray binaries and in binary radio pulsar systems; (2) effects of magnetospheric pair production on the radiation from gamma-ray pulsars; (3) radiation transfer in the atmosphere of an illuminated companion star; (4) evaporation of millisecond pulsar companions;(5) formation of planets around pulsars; (6) gamma-ray bursts; (7) quasi-periodic oscillations in low mass x-ray binaries; (8) origin of high mass x-ray binaries, runaway OB stars, and the lower mass cutoff for core collapse supernovae; (9) dynamics of planetary atmospheres; (10) two point closure modeling of stationary, forced turbulence; (11) models for the general circulation of Saturn; and (12) compressible convection in stellar interiors.

Ruderman, Malvin A.↗
NASA NTRS1994

The metallicity of M4: Accurate spectroscopic fundamental parameters for four giants

High-quality spectra, covering the wavelength range 5480 to 7080 A, have been obtained for four giant stars in the intermediate-metallicity CN-bimodal globular cluster M4 (NGC 6121). We have employed a model atmosphere analysis that is entirely independent from cluster parameters, such as distance, age, and reddening, in order to derive accurate values for the stellar parameters effective temperature, surface gravity, and microturbulence, and for the abundance of iron relative to the Sun, (Fe/H), and of calcium, Ca/H, for each of the four stars. Detailed radiative transfer and statistical equilibrium calculations carried out for iron and calcium suggest that departures from local thermodynamic equilibrium are not significant for the purposes of our analysis. The spectroscopically derived effective temperatures for our program stars are hotter by about 200 K than existing photometric calibrations suggest. We conclude that this is due partly to the uncertain reddening of M4 and to the existing photometric temperature calibration for red giants being too cool by about 100 K. Comparison of our spectroscopic and existing photometric temperatures supports the prognosis of a significant east-west gradient in the reddening across M4. Our derived iron abundances are slightly higher than previous high-resolution studies suggested; the differences are most probably due to the different temperature scale and choice of microturbulent velocities adopted by earlier workers. The resulting value for the metallicity of M4 is (Fe/H )(sub M4) = -1.05 + or - 0.15. Based on this result, we suggest that metallicities derived in previous high-dispersion globular cluster abundance analyses could be too low by 0.2 to 0.3 dex. Our calcium abundances suggest an enhancement of calcium, an alpha element, over iron, relative to the Sun, in M4 of (Ca/H) = 0.23.

Drake, J. J.↗
NASA NTRS2013

Dust Production and Mass Loss in Cool Evolved Stars

Following the red giant branch phase and the subsequent core He-burning phase, the low- to intermediate-mass stars (0.8<M/M solar mass <8) begin to ascend the asymptotic giant branch (AGB). Pulsations levitate material from the stellar surface and provide density enhancements and shocks, which can encourage dust formation and re-processing. The dust composition depends on the atmospheric chemistry (abundance of carbon relative to oxygen), which is altered by dredging up newly formed carbon to the surface of the star. I will briefly review the current status of models that include AGB mass loss and relate them to recent observations of AGB stars from the Surveying the Agents of Galaxy Evolution (SAGE) Spitzer surveys of the Small and Large Magellanic Clouds, including measures of the total dust input to the interstellar medium from AGB stars.

Boyer, M. L.↗
NASA NTRS2024

Stellar Occultation Observations to Constrain the Stratosphere of Uranus for Aerocapture

Background on Uranus: Voyager 2 (V2) UV stellar and solar occultations at Uranus detected a warm stratosphere and extremely hot thermosphere [1, 2], far in excess of solar irradiance [3, 4] and internal heating [5, 6]. New theories to explain similar heating at Jupiter [7] and Saturn [8] cannot be tested at Uranus due to a dearth of reliable measurements. In [9, 10], we reprocessed 26 archival Earth-based stellar occultations by Uranus (1977-1996), finding stratospheric temperatures (~200 K) warmer than the original (~100 K), but in stark tension with V2 (~300-500 K). In [10], we built a physics-based, 1-D atmospheric model (see Fig 1) that finds a nearly isothermal stratosphere and a dynamic heat sink in the lower thermosphere. Aerocapture: Aerocapture is spacecraft maneuver that uses a single deep dip to enter orbit. It could decrease cruise time and launch mass for a Uranus mission, but the greatest impediment is uncertain stratospheric densities of Uranus [11]. Aims: 1) Observe and process many high S/N Uranus stellar occultations in the next decade; 2) constrain stratospheric densities for aerocapture; 3) better understand the energy balance; 4) determine stratospheric changes since 1996. Upcoming Occultations: We will present our observing plan for the 2025 April Uranus occultation (K mag 8) and discuss the best-in-a-century 2031 event (K mag 4). We will discuss other events in the early 2030s [12] [13], predicted constraints on density, and simulations of aerocapture for UOP. We will discuss the Shadow Chaser, a small satellite concept for observing occultations from Earth orbit [12]. Conclusions: New stellar occultations can vastly improve profiles of the stratosphere of Uranus; this is critical for understanding energy circulation and constraining densities for using aerocapture on UOP. References: [1] Herbert, F. et al. (1987). JGR. [2] Stevens, M. et al. (1993). Icarus. [3] Marley, M. & McKay, C. (1999). Icarus, [4] Li, C. et al. (2018). JQRST. [5] Pearl, J. et al. (1990). Icarus. [6] Melin, H (2020). Nat Astron. [7] O’Donoghue, J. et al. (2021) Nature. [8] Mueller-Wodarg, I. et al. (2019) GRL. [9] Saunders, W. et al. (2023). PSJ. [10] Saunders, W. et al. (2024). PSJ. [11] Report of the Aerocapture Demonstration Relevance Assessment Team (2023). [12] Saunders, W. et al. (2022). P&SS. [13] French, R. & Souami, D. (2023) PSJ.

William Saunders↗
NASA NTRS1980

The Barnes-Evans color-surface brightness relation: A preliminary theoretical interpretation

Model atmosphere calculations are used to assess whether an empirically derived relation between V-R and surface brightness is independent of a variety of stellar paramters, including surface gravity. This relationship is used in a variety of applications, including the determination of the distances of Cepheid variables using a method based on the Beade-Wesselink method. It is concluded that the use of a main sequence relation between V-R color and surface brightness in determining radii of giant stars is subject to systematic errors that are smaller than 10% in the determination of a radius or distance for temperature cooler than 12,000 K. The error in white dwarf radii determined from a main sequence color surface brightness relation is roughly 10%.

Shipman, H. L.↗
NASA NTRS1983

The C2H, C2, and CN electronic absorption bands in the carbon star HD 19557

Infrared spectrophotometry of the R-type carbon star HD 19557 is presented. Two unusual spectroscopic features are seen: a 3.1 micron band is lacking and a 2.8 micron band is present. Identifications are proposed for three previously unreported stellar absorption bands with electronic sequences of C2, CN, and C2H. The latter is proposed to be responsible for the 2.8 micron feature. The atmospheric structure of the star is studied with synthetic spectra, and an effective temperature between 2600 K and 3000 K is suggested. No SiC emission is seen at 11.3 microns, indicating that grain formation is not a viable process around the star. The lack of dust in R stars may suggest a salient difference between R and N types.

Goebel, J. H.↗
NASA NTRS1987

Magnetic flux tubes as sources of wave generation

The structure of solar, and very likely stellar, surface magnetic fields is highly inhomogeneous: at the photospheric level, the fields are locally strong, and show concentration into a flux tube structure. In this case, the wave energy generated in stellar convection zones may be largely carried away by flux tube waves, which can then become important sources for the heating of the outer atmospheric layers. Such flux tube wave generation may help to explain the UV and X-ray fluxes observed by the IUE and Einstein observatories. The generation of longitudinal tube waves in magnetic flux tubes embedded in an otherwise magnetic field-free, turbulent, and stratified medium was considered. It is shown that compressible tube waves are generated by dipole emission and that the generation efficiency is a strong function of the magnetic field strength. Energy flux calculations are presented for different magnetic flux tubes, and show how the results depend on the magnetic field strength and the characteristics of the convective turbulence.

Musielak, Z. E.↗
NASA NTRS1990

The Fe XI-excited fluorescent cascade in Ne IV

A theoretical spectroscopic investigation of the fluorescent cascade to be expected in nitrogen-like Ne IV, when it is resonantly photoexcited by Fe XI in the solar atmosphere, symbiotic stars, or novae, is described. Primary and secondary cascade intensities are obtained as functions of photoexcitation rate, and expected absolute intensities in the solar atmosphere are derived on the basis of observed Fe XI and Ne EUV emission. Comparisons between the spatially resolved solar situation and spatially unresolved stellar cases of cataclysmic variables, in which these ions have been found to coexist, are made. An interesting possibility of periodic time-dependent fluorescence exists for the binary cataclysmics.

Bhatia, A. K.↗
NASA NTRS1991

Non-LTE analysis of the Ofpe/WN9 star HDE 269227 (R84)

The paper presents the results of a spectral analysis of the Ofpe/WN9 star HD 269227 (R84), which assumes a spherically expanding atmosphere to find solutions for equations of radiative transfer. The spectra of hydrogen and helium were predicted with a non-LTE model. Six stellar parameters were determined for R84. The shape of the velocity law is empirically found, since it can be probed from the terminal velocity of the wind. The six stellar parameters are further employed in a hydrodynamic model where stellar wind is assumed to be directed by radiation pressure, duplicating the mass-loss rate and the terminal wind velocity. The velocity laws found by computation and analysis are found to agree, supporting the theory of radiation-driven stellar wind. R84 is surmised to be a post-red supergiant which lost half of its initial mass, possibly during the red-supergiant phase. This mass loss is also suggested by its spectroscopic similarity to S Doradus.

Schmutz, Werner↗
NASA NTRS1992

Iron abundance in the hot DA white dwarfs Feige 24 and G191 B2B

Attention is given to model calculations of the far- and extreme-UV line spectra of highly ionized Fe species (Fe IV, Fe V, and Fe VI) for hot high-gravity H-rich stars. A spectral analysis of 31 hr of exposure of the DA white dwarf Feige 24 with IUE in the echelle mode reveals the presence of Fe with an abundance relative to H by number of (5-10) x 10 exp -6 with an uncertainty dominated by the determination of stellar parameters. An analysis of IUE data from the white dwarf G191 B2B results in a similar Fe abundance if this star shares similar atmospheric parameters (Teff, g) with Feige 24. Fe is thus the second most abundant photospheric element in hot DA white dwarfs.

Vennes, Stephane↗
NASA NTRS1998

Determination of an Infrared Reference Frame

The mission of the European Space Agency's Hipparcos satellite was to provide high precision astrometric and photometric data for over 100,000 stars and somewhat lower precision data on over a million additional stars. These observations were made in the optical band (340 - 850 nm) and resulted in median precisions of better than 1 milli-arcsec (mas) in position and parallax and I mas/yr in proper motion. The Hipparcos frame is inertial within an estimated uncertainty of +/- 0.25 mas/yr. The other important wavelength band for astrometry has been the radio band. A reference frame has been defined based on absolute Very Long Baseline Interferometry (VLBI) positions of several hundreds of radio sources. There have been a number of projects to refine the link between the optical and radio reference frames. In recent years there has been increasing interest in the astrometry of infrared sources. For example, stars with circumstellar shells can be observed in both the optical and radio (e.g. SiO maser emission). The infrared emission from such sources is dominated by thermal dust emission. The spatial distribution of radiation in the three bands is expected to be quite different. In 1982 we proposed a number of infrared-selected sources (S(sub 10 micron) greater than or approx. equal to 100 J(sub y)) for inclusion in the Hipparcos Input Catalog. Most of those sources were observed, and in Sutton (1997) we report the optical astrometric results (position, proper motion, and parallax) for 87 such sources. These sources were selected to have reasonable agreement between infrared and optical positions, and so may be used as primary standards for future infrared astrometry. They are well distributed across the sky, but exhibit some bias towards the galactic plane and the northern hemisphere. We have also obtained a set of 1480 secondary standards (unpublished) A comparison between optical positions and 86 GHz SiO maser positions for 10 sources indicates coincidence at the 0.15 arcsec level, consistent with current uncertainties in the SiO positions. Improved measurements of the SiO maser positions are needed. VLBI data show that SiO maser spots can be distributed in broken rings of radii 10 - 30 mas, presumably centered on the stellar position. However the SiO flux may be distributed asymmetrically, in which case the flux-weighted center of the SiO emission can be different from the stellar position. We have also published a related paper on phase fluctuations in millimeter interferometry (Sutton and Hueckstaedt 1996). Radiometric monitoring of emission from atmospheric water vapor may be a method for correcting for such fluctuations. This is relevant to positional determinations at centimeter and millimeter wavelengths. The integration of visible, infrared, and radio astrometry is one of our principal goals.

Sutton, Edmund C.↗
NASA NTRS2013

Interannual Variations of MLS Carbon Monoxide Induced by Solar Cycle

More than eight years (2004-2012) of carbon monoxide (CO) measurements from the Aura Microwave Limb Sounder (MLS) are analyzed. The mesospheric CO, largely produced by the carbon dioxide (CO2) photolysis in the lower thermosphere, is sensitive to the solar irradiance variability. The long-term variation of observed mesospheric MLS CO concentrations at high latitudes is likely driven by the solar-cycle modulated UV forcing. Despite of different CO abundances in the southern and northern hemispheric winter, the solar-cycle dependence appears to be similar. This solar signal is further carried down to the lower altitudes by the dynamical descent in the winter polar vortex. Aura MLS CO is compared with the Solar Radiation and Climate Experiment (SORCE) total solar irradiance (TSI) and also with the spectral irradiance in the far ultraviolet (FUV) region from the SORCE Solar-Stellar Irradiance Comparison Experiment (SOLSTICE). Significant positive correlation (up to 0.6) is found between CO and FUVTSI in a large part of the upper atmosphere. The distribution of this positive correlation in the mesosphere is consistent with the expectation of CO changes induced by the solar irradiance variations.

carbon monoxide↗