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At least 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
NASA NTRS1973

The effect of Fe I absorption on the Mg II emission at 2795 A in alpha Orionis

The Fe I absorption line at 2795.01 A is derived from the observations of Mg II resonance emission in alpha Orionis by Kondo et al (1972). A spectrum is synthesized, including Mg II emission and self-absorption and also Fe I absorption, which accounts for the observed asymmetry in the Mg II 2795.5 line and for the discrepancy between the theoretical and observed intensity ratios of the two Mg II lines. The astrophysical significance of this strong Fe I absorption is discussed.

Modisette, J. L.↗
NASA NTRS1975

Ultraviolet observations of cool stars. IV - Intensities of Lyman-alpha and Mg II in epsilon Pegasi and epsilon Eridani, and line width-luminosity correlations

A spectrometer on the Copernicus satellite has been used to confirm the existence of a line width-luminosity relation for the Ly-alpha and Mg II 2800-A chromospheric emission lines in K-type stars by observation of a K2 dwarf (epsilon Eri) and a K2 supergiant (epsilon Peg). Combined with previously reported observations of lines in three K giants (alpha Boo, alpha Tau, and beta Gem), the data are consistent with an identical dependence of line width on absolute visual magnitude for the Ca II K, Ly-alpha, and Mg II 2795-A lines. Surface fluxes of Ly-alpha, Mg II 2800-A, and O V 1218-A (upper limit) for epsilon Eri, and of Mg II 2800-A for epsilon Peg are also compared with values reported previously for the three giant stars.

Mcclintock, W.↗
NASA NTRS1977

The Mg II features near 2800A and spectral classification

Relatively high-resolution (about 0.4 A) observations of the Mg II resonance lines at 2795.523 and 2802.698 A as well as their respective subordinate lines at 2797.989 and 2790.768 A are discussed for main-sequence stars, giants, and supergiants. The general behavior of the Mg II lines as a function of spectral type is described. An expression is obtained for the relationship between absolute magnitude and Mg II emission widths. The variation of the absorption widths of the Mg II features with spectral type and luminosity class is briefly summarized.

Kondo, Y.↗
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 NTRS1984

Physical conditions in the cool parts of prominences. II - The Mg triplet lines

The statistical equilibrium of Mg under prominence conditions is considered. It is shown that the principal lines of the triplet system can serve as an equivalent to the Na D lines for spectral diagnostic purposes. Like the Na D lines, these Mg lines are relatively easy to measure in prominences and exhibit small optical depths. It is demonstrated that potential complications due to possible radiative transfer effects in the Mg resonance transition (3 1S0 - 3 1P1) have little consequence for the triplet populations. The results are used to repeat the spectral analysis of Landman (1983) using the Mg multiplets in place of the Na ones. The results of the two analyses are in general agreement. The finding of a low degree of H ionization in the cool prominence regions and a correspondingly high gas pressure is confirmed.

Landman, D. A.↗
NASA NTRS1986

Short-term spectral variability in AB Aurigae - Clues for activity in Herbig Ae stars. I - The ultraviolet lines of Mg II and Fe II

IUE satellite monitoring of the Herbig Ae star AB Aur in 1982 and 1984, with attention to the variability of both the Mg II 2795 A wind profile and a number of Fe II UV lines, has indicated temporal variations in the Mg II blue wing velocity by 30-50 percent at both epochs. By contrast, the Fe II spectrum was constant in 1982, but variable in 1984. There is no evidence of photometric variability in AB Aur during either period. An analysis of the Mg II blue wing velocity vs time shows that the variability can be fitted with a sine curve of period 45 + or - 6 yr, which is interpreted to be the stellar rotation period. The observed rotational modulation of the blue wing velocity of Mg II 2795 A in AB Aur's wind is the first direct evidence of a nonaxisymmetric wind in an A-type premain sequence object.

Praderie, F.↗
NASA NTRS1991

A statistical study of high coronal densities from X-ray line-ratios of Mg XI

An X-ray line-ratio density diagnostic was applied to 50 Mg XI spectra of flaring active regions on the sun recorded by the Flat Crystal Spectrometer on the SMM. The plasma density is derived from R, the flux ratio of the forbidden to intercombination lines of the He-like ion, Mg XI. The R ratio for Mg XI is only density sensitive when the electron density exceeds a critical value (about 10 to the 12th/cu cm), the low-density limit (LDL). This theoretical value of the low-density limit is uncertain as it depends on complex atomic theory. Reported coronal densities above 10 to the 12th/cu cm are uncommon. In this study, the distribution of R ratio values about the LDL is estimated and the empirical values are derived for the 1st and 2nd moments of this distribution from 50 Mg XI spectra. From these derived parameters, the percentage of observations is derived which indicated densities above this limit.

Linford, G. A.↗
NASA NTRS1991

Effects of Mg II and Ca II ionization on ab-initio solar chromosphere models

Acoustically heated solar chromosphere models are computed considering radiation damping by (non-LTE) emission from H(-) and by Mg II and Ca II emission lines. The radiative transfer equations for the Mg II k and Ca II K emission lines are solved using the core-saturation method with complete redistribution. The Mg II k and Ca II K cooling rates are compared with the VAL model C. Several substantial improvements over the work of Ulmschneider et al. (1987) are included. It is found that the rapid temperature rises caused by the ionization of Mg II are not formed in the middle chromosphere, but occur at larger atmospheric heights. These models represent the temperature structure of the 'real' solar chromosphere much better. This result is a major precondition for the study of ab-initio models for solar flux tubes based on MHD wave propagation and also for ab-initio models for the solar transition layer.

Rammacher, W.↗
NASA NTRS1993

A sample of lobe-dominated quasars with Mg II absorption lines

We present a sample of quasars which are lobe-dominated in their radio emission and contain Mg II absorption line systems. This sample will be useful in a search for redshifted H I 21 cm absorbers which are along lines of sight different from the optical. These systems will allow the measurement of the linear size and related physical parameters of the absorption line clouds. We have defined a set of criteria for selecting these objects and obtained the necessary data to assemble a sample based on these criteria. Out of 4234 QSOs we find 31 lobe-dominated quasars which are appropriate for optical and radio absorption line study in the Northern Hemisphere. We have obtained or located optical spectra for 29 of these 31 quasars. In the optical spectra we find 17 Mg II absorption systems with redshifts between 0.3 and 1.4 and Mg II 2796 A rest equivalent widths between 0.3 and 2.0 A. Based on statistics of 21 cm absorption in Mg II selected QSOs we expect this sample of 17 systems to yield one or two new H I absorbers. In this paper we give an analysis of our criteria, discuss the sample, and present new VLA observations of 16 quasars.

Aldcroft, Thomas L.↗
NASA NTRS1993

Excitation cross sections for the ns 2S yields np 2P resonance transitions in Mg(+) (n = 3) and Zn(+) (n = 4) using electron-energy-loss and merged-beams methods

Electron-excitation cross sections are reported for the 3s 2S yields 3p 2P(h, k) resonance transition in Mg(+) at energies from threshold (4.43 eV) to approximately 9 times threshold (40.0 eV). The electron-energy-loss merged-beams technique used in these measurements is described in detail. In addition, the method of separating contributions of the elastically scattered (Coulomb) and the inelastically scattered electrons in the present Mg(+) case and previously reported Zn(+) results is described. Comparisons in the experimental energy range are made for Mg(+) with the two five-state close-coupling theoretical calculations carried out herein, and with other published close-coupling, distorted-wave, and semiempirical calculations. The present Mg(+) cross sections and Zn(+) cross sections from earlier measurements are tabulated.

Smith, Steven J.↗
NASA NTRS1994

The Mg 2 lines as diagnostic of PMS nature in Herbig Ae/Be stars

The main objective of this program was to observe some newly discovered and previously unobserved Herbig Ae/Be (HAeBe) stars with IUE. A previous archival study suggested that there was a full range of profiles in the Mg II lines for these objects; ranging from the double peak emission to the narrow absorption lines. It has been found that by studying the IUE data of main sequence B and classical Be stars that the P Cygni profiles in the Mg II lines in HAeBe stars are truly unique and are the signature of accelerating winds in their extended circumstellar material. The arguement, for the purposes of this proposal, that the structure of the Mg II lines was a superior diagnostic of the dynamical activity and ultimately the PMS nature of these objects. This program was a follow up study of several interesting intermediate-mass stars with the goal of classifying these objects based on the shape of their MG II lines. A list of publications resulting from this research is included.

Source record↗
NASA NTRS1994

Extreme Mg-26 and O-17 enrichments in an Orgueil corundum: Identification of a presolar oxide grain

A corundum (Al2O3) grain from the Orgueil meteorite is greatly enriched in O-17 and (Mg-26)(sup *). The measured O-16/O-17 is 1028 +/- 11 compared to solar(O-16/O-17) = 2610. This is the largest O-17 excess so far observed in any meteoritic material. The Mg-26 excess ((Mg-26)(sup *)) is most plausibly due to in situ decay of Al-26. The inferred (Al-26/Al-27)(sub 0) ratio of 8.9 x 10(exp -4) is approximately 18 times larger than the 5 x 10(exp -5) value commonly observed in refractory inclusions formed in the solar system. The large O-17 excess and high (Mg-26)(sup *) Al-27 ratio unambiguously identify this corundum as a presolar oxide grain. Enrichments in O-17 and Al-26 are characteristic of H-burning and point to red giant or asymptotic giant branch (AGB) stars as likely sources.

Hutcheon, I. D.↗
NASA NTRS2007

Investigation of Synthetic Mg(1.3)V(1.7)O4 Spinel with MgO Inclusions: Case Study of a Spinel with an Apparently occupied Interstitial Site

A magnesium vanadate spinel crystal, ideally MgV2O4, synthesized at 1 bar, 1200 C and equilibrated under FMQ + 1.3 log f(sub o2) condition, was investigated using single-crystal X-ray diffraction, electron microprobe, and electron backscatter (EBSD). The initial X-ray structure refinements gave tetrahedral and octahedral site occupancies, along with the presence of 0.053 apfu Mg at an interstitial octahedral site . Back-scattered electron (BSE) images and electron microprobe analyses revealed the existence of an Mg-rich phase in the spinel matrix, which was too small (less than or equal to 3microns) for an accurate chemical determination. The EBSD analysis combined with X-ray energy dispersive spectroscop[y (XEDS) suggested that the Mg-rich inclusions are periclase oriented coherently with the spinel matrix. The final structure refinements were optimized by subtracting the X-ray intensity contributions (approx. 9%) of periclase reflections, which eliminated the interstitial Mg. This study provides insight into possible origins of refined interstitial cations reported in the the literature for spinel, and points to the difficulty of using only X-ray diffraction data to distinguish a spinel with interstitial cations from one with coherently oriented MgO inclusions.

Uchida, Hinako↗
DOE OSTI2022

Atomic Layer Deposition for Materials-Based H2 Storage: Mg(BH4)2 as a Case Study

To meet the requirements for vehicular solid-state hydrogen (H2) storage, novel materials such as metal borohydrides have increasingly been investigated, in particular, magnesium borohydride (Mg(BH4)2). While these materials have a high H2 capacity (> 14 wt%), poor hydrogenation-dehydrogenation cyclability and material degradation, (e.g., loss of boron), need to be overcome. Prior research has indicated that nano-encapsulation and chemical additives can address these challenges. Therefore, we pursued these two strategies simultaneously with atomic layer deposition (ALD) on Mg(BH4)2. We investigated the use of metal-oxides (e.g., Al2O3, TiO2, CeO2), Pt- group metals (e.g., Pd, Ru) as well as pulsing only one precursor molecule (e.g., Al(CH3)3, BBr3, TiCl4), and assessed these modified Mg(BH4)2 in terms of their H2 storage properties. This presentation will also present the benefits and limitations of using vapor-phase techniques to modify the properties of Mg(BH4)2.

atomic layer deposition↗
DOE OSTI2020

Influences of Mg Ions and Al Ions on Deposition Behavior of Supersaturated Silicic Acid - 20261

Silicic acid can change into various species such as deposit, colloidal, and soluble form under certain conditions: for example, gradual pH changes around geological disposal repositories. In particular, silicic acid deposits narrow groundwater flow paths, thereby restricting the flow of groundwater. Such effects may contribute to a delay in radionuclide migration. Previous studies have reported that silicic acid supersaturated in sodium and calcium ions can narrow the flow paths. This study examined the effect of magnesium (Mg) and aluminum (Al) ions on the deposition of supersaturated silicic acid on the solid phase. The deposition behavior of supersaturated silicic acid on the solid phase was investigated in a batch system while changing the initial supersaturation concentration, the coexisting ion concentration, and the amount of added solid phase. In the presence of Mg ions, silicic acid precipitation increased with increasing Mg ion concentration due to the influence of the electrolyte, and crosslinking reactions. The coexistence of Al ions significantly suppressed silicic acid deposition due to the formation of a covering on the solid surface during Al ion hydrolysis under conditions of high Al concentration (1.4 mM). On the basis of these experimental results, the apparent deposition rate constant of silicic acid was estimated to be in the range from 10{sup -11} to 10{sup -10} m/s. The Damkoehler number (the dimensionless number expressed by the ratio of the characteristic time of mass transfer and the chemical reaction constant) was sufficient to cause the clogging of the host rock by deposition. This suggests that supersaturated silicic acid deposition, regardless of the presence of Mg ions (and Al ions), contributes to improving barrier performance against radionuclide migration in the host rock. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2020

Materials Data on Mg(PO3)2 by Materials Project

Mg(PO3)2 crystallizes in the monoclinic C2/c space group. The structure is three-dimensional. there are two inequivalent Mg2+ sites. In the first Mg2+ site, Mg2+ is bonded to six O2- atoms to form MgO6 octahedra that share corners with six PO4 tetrahedra and edges with two equivalent MgO6 octahedra. There are a spread of Mg–O bond distances ranging from 2.01–2.19 Å. In the second Mg2+ site, Mg2+ is bonded to six O2- atoms to form MgO6 octahedra that share corners with six PO4 tetrahedra and edges with two equivalent MgO6 octahedra. There are two shorter (2.05 Å) and four longer (2.12 Å) Mg–O bond lengths. There are two inequivalent P5+ sites. In the first P5+ site, P5+ is bonded to four O2- atoms to form PO4 tetrahedra that share corners with three MgO6 octahedra and corners with two equivalent PO4 tetrahedra. The corner-sharing octahedra tilt angles range from 19–58°. There are a spread of P–O bond distances ranging from 1.48–1.62 Å. In the second P5+ site, P5+ is bonded to four O2- atoms to form PO4 tetrahedra that share corners with three MgO6 octahedra and corners with two equivalent PO4 tetrahedra. The corner-sharing octahedra tilt angles range from 21–58°. There are a spread of P–O bond distances ranging from 1.48–1.61 Å. There are six inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted bent 120 degrees geometry to two P5+ atoms. In the second O2- site, O2- is bonded in a bent 150 degrees geometry to one Mg2+ and one P5+ atom. In the third O2- site, O2- is bonded in a distorted trigonal planar geometry to two Mg2+ and one P5+ atom. In the fourth O2- site, O2- is bonded in a bent 150 degrees geometry to two P5+ atoms. In the fifth O2- site, O2- is bonded in a distorted bent 150 degrees geometry to one Mg2+ and one P5+ atom. In the sixth O2- site, O2- is bonded in a distorted trigonal planar geometry to two Mg2+ and one P5+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mg(AlH4)2 by Materials Project

Mg(AlH4)2 crystallizes in the trigonal P-3m1 space group. The structure is two-dimensional and consists of one Mg(AlH4)2 sheet oriented in the (0, 0, 1) direction. Mg2+ is bonded to six equivalent H1- atoms to form MgH6 octahedra that share corners with six equivalent AlH4 tetrahedra. All Mg–H bond lengths are 1.90 Å. Al3+ is bonded to four H1- atoms to form AlH4 tetrahedra that share corners with three equivalent MgH6 octahedra. The corner-sharing octahedral tilt angles are 14°. There is one shorter (1.60 Å) and three longer (1.63 Å) Al–H bond length. There are two inequivalent H1- sites. In the first H1- site, H1- is bonded in a linear geometry to one Mg2+ and one Al3+ atom. In the second H1- site, H1- is bonded in a single-bond geometry to one Al3+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mg(H2N)2 by Materials Project

Mg(NH2)2 crystallizes in the tetragonal I4_1/acd space group. The structure is three-dimensional and consists of two Mg(NH2)2 frameworks. Mg2+ is bonded to four N3- atoms to form corner-sharing MgN4 tetrahedra. There are a spread of Mg–N bond distances ranging from 2.09–2.12 Å. There are three inequivalent N3- sites. In the first N3- site, N3- is bonded to two equivalent Mg2+ and two H1+ atoms to form distorted corner-sharing NMg2H2 tetrahedra. Both N–H bond lengths are 1.03 Å. In the second N3- site, N3- is bonded to two equivalent Mg2+ and two equivalent H1+ atoms to form distorted corner-sharing NMg2H2 tetrahedra. Both N–H bond lengths are 1.03 Å. In the third N3- site, N3- is bonded to two equivalent Mg2+ and two equivalent H1+ atoms to form distorted corner-sharing NMg2H2 tetrahedra. Both N–H bond lengths are 1.03 Å. There are four inequivalent H1+ sites. In the first H1+ site, H1+ is bonded in a single-bond geometry to one N3- atom. In the second H1+ site, H1+ is bonded in a single-bond geometry to one N3- atom. In the third H1+ site, H1+ is bonded in a single-bond geometry to one N3- atom. In the fourth H1+ site, H1+ is bonded in a single-bond geometry to one N3- atom.

36 MATERIALS SCIENCE↗