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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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NASA NTRS2017

Sonora: A New Generation Model Atmosphere Grid for Brown Dwarfs and Young Extrasolar Giant Planets

Brown dwarf and giant planet atmospheric structure and composition has been studied both by forward models and, increasingly so, by retrieval methods. While indisputably informative, retrieval methods are of greatest value when judged in the context of grid model predictions. Meanwhile retrieval models can test the assumptions inherent in the forward modeling procedure. In order to provide a new, systematic survey of brown dwarf atmospheric structure, emergent spectra, and evolution, we have constructed a new grid of brown dwarf model atmospheres. We ultimately aim for our grid to span substantial ranges of atmospheric metallilcity, C/O ratios, cloud properties, atmospheric mixing, and other parameters. Spectra predicted by our modeling grid can be compared to both observations and retrieval results to aid in the interpretation and planning of future telescopic observations. We thus present Sonora, a new generation of substellar atmosphere models, appropriate for application to studies of L, T, and Y-type brown dwarfs and young extrasolar giant planets. The models describe the expected temperature-pressure profile and emergent spectra of an atmosphere in radiative-convective equilibrium for ranges of effective temperatures and gravities encompassing 200 less than or equal to T(sub eff) less than or equal to 2400 K and 2.5 less than or equal to log g less than or equal to 5.5. In our poster we briefly describe our modeling methodology, enumerate various updates since our group's previous models, and present our initial tranche of models for cloudless, solar metallicity, and solar carbon-to-oxygen ratio, chemical equilibrium atmospheres. These models will be available online and will be updated as opacities and cloud modeling methods continue to improve.

emergent spectra↗
NASA NTRS2022

Moist Convection in the Giant Planet Atmospheres

The outer planets of our Solar System display a myriad of interesting cloud features, of different colors and sizes. The differences between the types of observed clouds suggest a complex interplay between the dynamics and chemistry at play on these atmospheres. Particularly, the stark difference between the banded structures of Jupiter and Saturn, vs the sporadic clouds on the ice giants highlights the varieties in dynamic, chemical and thermal processes that shape these atmospheres. Since the early explorations of these planets by spacecrafts, such as Voyager and Voyager 2, there are many outstanding questions about the long term stability of the observed features. One hypothesis is that the internal heat generated during the formation of these planets is transported to the upper atmosphere through latent heat release from convecting clouds (i.e., moist convection). In this review, we present evidences of moist convective activity on the gas giant atmospheres of our Solar System from remote sensing data, both from ground- and space-based observations. We detail the processes that drive moist convective activity, both in terms of the dynamics as well as the microphysical processes that shape the resulting clouds. Finally, we also discuss the effects of moist convection on shaping the large scale dynamics (such as jet structures on these planets).

Planetary Science↗
NASA NTRS1972

New theory for giant loops.

Four very large, shell-shaped features are known from the surveys of radio emission from the Galaxy. It is suggested that the giant loops represent a class of nebulae, induced by supernova radiation as opposed to supernova ejecta. The model considered provides a qualitative explanation for the spectral and polarization properties of the radio emission from giant loops. The radio emitting electrons are part of the general cosmic ray gas and need not arise specifically in the supernova that produced the fossil Stroemgren sphere.

Brandt, J. C.↗
NASA NTRS1972

Abundances in late-type giants.

The narrow-band photometry of Spinrad and Taylor (1969) is analyzed by the synthetic spectra technique. Surface gravities, Doppler broadening velocities, effective temperatures, and metal abundances are determined for 131 K giants. Evolutionary tracks and gravities determined from the photometry are used to give space positions, space velocities, and absolute magnitudes of the 131 stars. We conclude that a class of stars exists with metals and/or nitrogen significantly enhanced with respect to the sun; further that these stars have average space motions similar to those of moderately old Population I stars. The Doppler broadening velocity is found to be a strong function of temperature for K giants, and in some cases, possibly a function of surface gravity.

Gottlieb, D. M.↗
NASA NTRS1974

Lunar mascons as consequences of giant impacts

An investigation is conducted regarding the effect of a giant impact on the thermal evolution of the moon. A model is proposed for the formation of the lunar mascons. The model takes into account the evidence of late volcanism during a period from about 3.7 to about 3.2 b.y. ago. It is found that a giant impact produces extensive fracturing in the region surrounding the basin and reduces the average thermal conductivity of the upper 20 km of this region by a factor of about 2. The less conductive ejecta blanket and the fractured zone behave as a thermal insulator. The lithosphere beneath the basin thickens monotonically and it becomes strong enough to support the mascon.

Arkani-Hamed, J.↗
NASA NTRS1974

A search for soft X-ray emission from red-giant coronae

Hills has pointed out that if red-giant coronae are weak sources of soft X-rays, then the problems of the identification of the local component of the soft X-ray background and the observed lack of gas in globular clusters may be simultaneously resolved. Using instrumentation aboard OAO Copernicus, we have searched unsuccessfully for emission in the 10-100 A band from four nearby red giants. In all cases, our upper limits are of the order of the minimum theoretically predicted fluxes.

Margon, B.↗
NASA NTRS1978

Physics of the primitive solar nebula and of giant gaseous protoplanets

It has been proposed that the supernova responsible for injecting Al-26 into the early solar system was in fact responsible for triggering the collapse of an interstellar cloud in order to produce a system of stars, one of which would be the solar system. Details concerning the mechanism involved in such a process are discussed. Attention is given to the evolution of the primitive solar nebula, the instabilities in the primitive solar nebula, and the giant gaseous protoplanets. The principal conclusion to be drawn from the material presented is that the primitive solar nebula was a rather chaotic place, highly turbulent, with the multiple formation of giant gaseous protoplanets.

Cameron, A. G. W.↗
NASA NTRS1979

Meridional circulation and CNO anomalies in red giant stars

The possibility is investigated that meridional circulation driven by internal rotation might lead to the mixing of CNO-processed material from the vicinity of the hydrogen shell into the envelope of a red giant star. This theory of meridional mixing is found to be generally consistent with available data and to be capable of explaining a number of observational results without invoking a radical departure from the standard physics of stellar interiors. It is suggested that meridional circulation must be a normal characteristic of a rotating star and that meridional mixing provides a reasonable framework for understanding many of the CNO anomalies exhibited by weak-G-band and CN-strong stars as well as the low C-12/C-13 ratios measured among field red giants.

Sweigart, A. V.↗
NASA NTRS1980

Mass loss from warm giants: Magnetic effects

Among warm giant stars, rapid mass loss sets in along a well defined velocity dividing line (VDL). Hot corona also disappear close to the VDL and thermal pressure cannot drive the observed rapid mass loss in these stars. The VDL may be associated with magnetic fields changing from closed to open. Such a change is consistent with the lack of X-rays from late-type giants. A magnetic transition locus based on Pneuman's work on helmet streamer stability agrees well with the empirical VDL. The change from closed to open fields not only makes rapid mass loss possible, but also contributes to energizing the mass loss in the form of discrete bubbles.

Mullan, D. J.↗
NASA NTRS1979

Origin of the magnetic fields in the giant planets

The paper discusses origin of the magnetic fields in the giant planets. Recent data on the generation of these fields provide conclusions on giant planets; the Jovian magnetic field can be of primordial origin or generated by a thermally driven dynamo, and the expected Saturnian field can be accounted for thermally or by a precessionally driven dynamo. The presence of a 0.1 gauss field on Uranus presents a problem because the so far unobserved thermal flux and convection may be too low, and if such a dynamo were to operate then the field should show seasonal variations. A conductive shell on Neptune similar to that on Uranus appears to be much thinner, but it is likely that Neptune has a magnetic field which is too weak to lead to observable electromagnetic variations.

Smoluchowski, R.↗
NASA NTRS1981

Observations of circumstellar carbon monoxide and evidence for multiple ejections in red giants

Observations of the fundamental 4.6 micron band of CO in nine red giants are presented. A common feature is multiple absorption lines which are identified as products of separate components or shells. Column densities are derived; the relative values should be free of the uncertainties inherent in determining the absolute scale. These column densities are well fitted by single excitation temperatures for each absorption component; these excitation temperatures are identified with the local kinetic temperatures. There is no correlation of CO column density with either gas or dust column density nor of the expansion velocity of the component with its distance from the star. The evidence is reviewed, and it is concluded that mass loss from red giants is most likely episodic in nature.

Bernat, A. P.↗
NASA NTRS1982

Density sensitive C 2 lines in cool giant stars

The density sensitivity of the emission lines within the ultraviolet 0.01 multiplet of C II near 2325 A was examined in additional late type giants and supergiants with deep LWR high dispersion exposures. The new data support the original contention based on these lines that noncoronal red giants posses geometrically extended chromospheres.

Stencel, R. E.↗
NASA NTRS1981

Far-ultraviolet fluorescence of carbon monoxide in the red giant Arcturus

Evidence is presented that many of the weak features observed with International Ultraviolet Explorer (IUE) in the far-ultraviolet (1150-2000 A) spectrum of the archetype red giant Arcturus (K2 III) are A-X fourth positive bands of carbon monoxide excited by chromospheric emissions of O I, C I, and H I. The appearance of fluorescent CO bands near the wavelength of commonly used indicators of high-temperature (T greater than 20,000 K) plasma, such as C II at wavelength 1335 and C IV at wavelength 1548, introduces a serious ambiguity in diagnosing the presence of hot material in the outer atmospheres of the cool giants by means of low-dispersion IUE spectra.

Ayres, T. R.↗
NASA NTRS1983

A giant X-ray flare in the Hyades

The Einstein Observatory has observed a giant stellar flare in the Hyades binary HD 27130 whose peak X-ray luminosity is greater than 10 to the 31st ergs/sec. This is several thousand times brighter than the most intense solar flares. HD 27130, first detected as an X-ray source in the central Hyades survey of Stern et al (1981), has recently been determined to be a double-lined eclipsing binary with a 5.6-d period. The primary and secondary are G and K dwarfs, respectively. The estimated 40 million K temperature of the flare decay suggests that it is solar-like, but its deduced size scales are much larger than those observed for typical solar flares. It is suggested that giant flares may be typical of young or rapidly rotating systems.

Stern, R. A.↗
NASA NTRS1984

Consequences of a chromospheric temperature gradient on the width of H Alpha in late-type giants

An analytic expression for the integrated H alpha optical depth profile is derived for a one dimensional slab geometry model chromosphere, with electron temperature increasing as a power law with height. The formula predicts H alpha opacity and profile width to be sensitive functions of the thermal gradient. Application of the model to observation reveals that broad H alpha absorption widths in G and K giant stars are consistent with a mean H alpha chromospheric optical depth of 50, while narrower widths in M stars indicate slightly lower opacities. It is proposed that differences in H alpha width between late-type giants of similar spectral type may be due, in part, to differences in their chromospheric thermal gradient, and associated H alpha opacity.

Zarro, D. M.↗
NASA NTRS1983

First detection of winds in red giants by microwave continuum techniques

Eight red giants and supergiants have been observed at 4885 MHz (6 cm) with the Very Large Array in an attempt to detect continuum emission. The bright giant Alpha-1 Her (M5 II) was detected at an average flux density of 0.9 + or - 0.13 mJy. Since the likely source of this emission is an ionized, optically thick component of a stellar wind, this detection implies a mass loss rate of 2 x 10 to the -9th solar masses per yr for the ionized gas. The fraction of the outflow in Alpha-1 Her that is ionized (0.002-0.02) seems to be similar to that previously found for Alpha Ori and Alpha Sco A. Alpha Boo (K2 IIIp) and Beta Gem (K0 III) are probable and definite detections, respectively. The derived ionized mass loss rates for these two stars are about 1 x 10 to the -10th solar masses per yr, implying in the case of Alpha Boo that the wind is largely ionized.

Drake, S. A.↗
NASA NTRS1984

Chromospheres and mass loss in metal-deficient giant stars

Semiempirical atmospheric models indicate that the characteristic emission in the wings of the H-alpha line observed in Population II giant stars can arise naturally within static chromospheres. Radial expansion gives an asymmetric, blueshifted H-alpha core accompanied by greater emission in the red line wing than in the blue wing. Wind models with extended atmospheres suggest mass loss rates much smaller than 2 x 10 to the -9th solar mass per yr. Thus H-alpha provides no evidence that steady mass loss can significantly affect the evolution of stars on the red giant branch of globular clusters.

Dupree, A. K.↗