Engineering topics

Lambert, D. L.

Publications and source records attributed to Lambert, D. L..

At least 19 records

NASA NTRS1994

Chemical transitions for interstellar C2 and CN in cloud envelopes

Observations were made of absorption from CH, C2, and CN toward moderately reddened stars in Sco, OB2, Ceo OB3, and Taurus/Auriga. For these directions, most of the reddening is associated with a single cloud complex, for example, the rho Ophiuchus molecular cloud, and as a result, the observations probe moderately dense material. When combined with avaliable data for nearby directions, the survey provides the basis for a comprehensive analysis of the chemistry for these species. The chemical transitions affecting C2 and CN in cloud envelopes were analyzed. The depth into a cloud at which a transition takes place was characterized by tau(sub uv), the grain optical depth at 1000 A. One transition at tau(sub uv) approx. = 2, which arises from, the conversion of C(+) into CO, affects the chemistries for both molecules because of the key role this ion plays. A second one involving production terms in the CN chemistry occurs at tau(sub uv) of approx. = 3; neutral reactions which C2 and CH is more important at larger values for tau(sub uv). The transition from photodissociation to chemical destruction takes place at tau(sub uv) approx. = 4.5 for C2 and CN. The observational data for stars in Sco OB2, Cep OB3, and Taurus/Auriga were studied with chemical rate equations containing the most important production and destruction mechanisms. Because the sample of stars in Sco OB2 includes sight lines with A(sub v) ranging from 1-4 mag, sight lines dominated by photochemistry could be analyzed separately from those controlled by gas-phase destruction. The analysis yielded values for two poorly known rate constants for reactions involved in the production of CN; the reactions are C2 + N yields CN + C and C(+) + NH yields all products. The other directions were analyzed with the inferred values. The predicted column densities for C2 and CN agree with the observed values to better than 50%, and in most instances 20%. When combining the estimates for density and temperature derived from chemical modeling and molecular excitation for a specific cloud, such as the rho Ophiuchus molecular cloud, the portion of the cloud envelope probed by C2 and CN absorption was found to be in pressure equilibrium.

Federman, S. R.↗

NASA NTRS1993

Detection of boron, cobalt, and other weak interstellar lines toward Zeta Ophiuchi

Numerous weak lines from interstellar atomic species toward Zeta Ophiuchi were observed with the Goddard High-Resolution Spectrograph. Of particular note are the first interstellar detection of cobalt and the detection of boron in this sight line. These measurements provide estimates for the amount of depletion for the two elements. Boron, a volatile, and cobalt, a refractory element, display the depletion pattern found by Savage et al. (1992). The abundance of phosphorus in the H II region associated with the star was obtained from a detection of P III. Additional weak lines from S I, C I, Ni II, and Cu II were detected for the first time; these lines provide the basis for refinements in oscillator strength and column density. Analysis of the neutral sulfur data indicates that the atomic gas is more widely distributed than the molecular material in the main component.

Federman, S. R.↗

NASA NTRS1992

High-velocity interstellar gas toward HD 169454

The interstellar spectrum of HD 169454 reveals Na D and Ca K absorption at about + 100 km/s. Measurements made at high resolution resolve 4 narrow (about 2-3 km/s) features whose N(Ca II)/N(Na I) ratios are small. This high-velocity gas is, therefore, unique in that it resembles local quiescent gas, not shocked gas. Redshifted gas is also seen in moderate-resolution spectra of faint stars lying in the same portion of the sky as HD 169454. The resulting picture is one in which a supernova explosion or stellar winds within the Sct OB3 association at a distance of about 1.5 kpc is pushing quiescent gas away from the sun. The gas is seen only in stars estimated to be at most 3 kpc, but greater than 2 kpc, away. If HD 169454 is at this greater distance, its absolute magnitude would be about -9, making it the second most luminous B star in the Galaxy.

Federman, S. R.↗

NASA NTRS1991

Vibration-rotation bands of CH in the solar infrared spectrum and the solar carbon abundance

High resolution solar spectra obtained from the ATMOS Fourier Transform Spectrometer (Spacelab 3 flight on April 29-May 6, 1985) have made it possible to identify and measure a large number of lines of the vibration-rotation fundamental bands of the X2 Pi state of CH. From about 100 lines of the 1-0, 2-1, and 3-2 bands and adopting theoretical transition probabilities, a solar carbon abundance of 8.60 + or - 0.05 is derived. This value is compared with new results inferred from other carbon abundance indicators. The final recommended solar abundance of carbon is 8.60 + or - 0.05.

Grevesse, N.↗

NASA NTRS1990

Identification of solar vibration-rotation lines of NH and the solar nitrogen abundance

High resolution solar spectra obtained from the ATMOS Fourier Transform Spectrometer (Spacelab 3 flight on April 29 - May 6, 1985) made it possible to detect for the first time vibration-rotation lines of NH from the X3 Sigma(-) state near 3 microns. Using recent theoretical results for the transition probabilities of 23 selected measured lines of the 1-0 and 2-1 bands, a solar abundance of nitrogen of 8.06 + or - 0.07 is derived. This value is compared with the result derived from N I lines and agrees with the abundance obtained from the 3s-3p, 3p-3d and 3s-prime - 3p-prime transitions, A(N) = 8.00 + or - 0.09. The final recommended solar abundance of nitrogen is A(N) = 8.00 + or - 0.05.

Grevesse, N.↗

NASA NTRS1988

Possible identifications of the 3.4 micrometer feature

A feature at 3.4 micrometer was first detected in Comet Halley by the IKS spectrometer on board the Vega 1 probe; and subsequently from the ground. The feature has since been reported in Comet Wilson. The presence of the feature is of considerable interest for a number of reasons. First, it may represent the detection of a new parent molecule, and when combined with data from Giotto and Vega yield new information on cometary chemistry and the early solar system composition. Secondly, it may represent a link to the interstellar medium, the feature corresponds in wavelength and shape with an interstellar feature seen in absorption in a luminous star, towards the Galactic center known as GC-IRS7. The feature in turn is thought to be related with a growing family of unidentified infrared emission features seen in stellar objects, planetary nebulae, reflection nebulae, HII regions and extra galactic sources. These features occur at wavelengths 3.3, 3.4, 3.5, 6.2, 7.7, 8.6, and 11.25 micrometers. Further identification theory is given.

Danks, Anthony C.↗

NASA NTRS1988

The carbon chemistry in interstellar clouds toward moderately reddened stars

New data for C2 toward X Per, HD 206267, HD 207198, and Gamma Cep, for CH and CN toward X Per, and for CO toward HD 207198 have been obtained. The column densities of CH, C2, CN, and CO toward the stars in the Cepheus OB2 association are similar to reddened directions in Perseus and in Ophiuchus, indicating a similarity in physical conditions for the foreground clouds. The available data for other directions have been analyzed and the resulting data applied to study the transition from a photochemical regime to a chemical regime. The data for N(CN), N(C2), and N(CO) have been plotted against N(CH) to elucidate the chemistry of carbon-bearing molecules more clearly. The observed trends for CN and C2 suggest a change in slope at N(CH) of roughly 5 x 10 to the 13th/sq cm. Below this value, photodestruction is predicted to dominate and the slope is determined by the photochemistry. For directions with more N(CH), a linear correlation consistent with destruction by chemical reactions is expected.

Federman, S. R.↗

NASA NTRS1986

Epsilon Aurigae in eclipse. II - Optical absorption lines from the secondary

Optical high-resolution 2.7-m McDonald Observatory observations of Epsilon Aur obtained during the 1982-1985 eclipse are discussed, with focus on the K I resonance lines, some Fe II and Ti II lines in the blue, and the near-infrared N I excitation lines. The N I lines are immune to contamination by the secondary, and their radial velocity decline over the three-year interval fits the predicted velocity curve. The 7664 A and 7699 A K I lines strengthen greatly during the eclipse due to gas in orbit around the secondary passing in front of the primary, and the velocity pattern analysis indicates a mass of the secondary to solar mass ratio of between about 3 and 6 and a primary mass of 3 solar masses. The data are consistent with the picture of a low-mass post-AGB primary having lost mass through a wind and by Roche-lobe transfer to the secondary, and which will evolve to a white dwarf in less than 10,000 years.

Lambert, D. L.↗

NASA NTRS1986

A spectroscopic view of Epsilon Aurigae

Spectroscopic observations from the ultraviolet, visible, and infrared obtained at the recent eclipse are discussed. The rotation curve for the disk around the secondary suggests that secondary is a low mass star (or binary). This result with the known mass function suggests that the primary may be a low mass star leaving the AGB and evolving to the white dwarf region.

Lambert, D. L.↗

NASA NTRS1984

The CN radical in diffuse interstellar clouds

A survey of 15 lines of sight for the CN B2Sigma(+) X2Sigma(+) interstellar absorption lines shows that the CN column density in diffuse interstellar clouds follows the relation log N(CN) is proportional to m log N(H2), where m is approximately equal to 3. This result is reproduced by a reaction network in which CN is produced primarily from C2 by the neutral-neutral reaction C2 + N yields CN + C, and photodissociation is the main destruction pathway for the neutral molecules CH, C2, and CN. The CN radical is the first molecular species observed in diffuse clouds that requires a neutral-neutral reaction for its formation in the gas phase. The network also reproduces the observed ratio N(CN)/N(H2).

Federman, S. R.↗

NASA NTRS1984

IUE observations of DQ Herculis and its nebula, and the nature of the cold nova shells

The nebula ejected in the 1934 outburst of the classical nova DQ Her is remarkable for its unprecedentedly low temperature of Te 500 K as measured by Williams et al. (1978). In this paper, IUE observations are combined with Steward optical spectra. It is confirmed that the gas is quite cold. It is further shown that the gas is ionized by the radiation field of the central object. X-ray, ultraviolet, optical and infrared observations of the underlying binary are combined with the extreme-ultraviolet continuum deduced from the level of ionization of the nebula to obtain a composite energy distribution for the central object. This energy distribution bears no resemblance to that predicted by theoretical models of accretion disks. Photoionization models of the nebula using the deduced continuum, as well as theoretical accretion disk continua, are presented to show that the low electron temperature is the result of the very high metal abundances which characterize nova shells. Infrared fine-structure lines are efficient coolants, and low temperatures are achieved for a wide variety of radiation fields. The implications of these results for nebulae surrounding other old novae are discussed.

Ferland, G. J.↗

NASA NTRS1983

Epsilon Aurigae in eclipse. I - Ultraviolet spectroscopy during ingress and totality

Epsilon Aurigae is a long-period eclipsing system containing a F0 Ia supergiant and an unseen slightly less-massive secondary. A primary eclipse began in mid-1982 with the unseen companion passing in front of the supergiant. Low-resolution ultraviolet (IUE) spectra of Epsilon Aur in 1982 and early 1983 provide eclipse light curves extending into the total phase of the current eclipse. The depth of eclipse from 3000 A to 1700 A is slightly deeper than at visual wavelengths (0.8 m). The depth declines for wavelengths less than 1700 A and is just 0.2 m at wavelengths less than 1300 A. The disappearance of the eclipse at wavelengths less than 1300 A may be attributed to a hot star or spot within the disk-shaped secondary. A main-sequence star of spectral type B0 accounts for the observations. However, an alternative site of the ultraviolet excess may be the primary's upper photosphere or chromosphere.

Parthasarathy, M.↗

NASA NTRS1983

Do all barium stars have a white dwarf companion?

International Ultraviolet Explorer short-wavelength, low-dispersion spectra were analyzed for four barium, two mild barium, and one R-type carbon star in order to test the hypothesis that the barium and related giants are produced by mass transfer from a companion now present as a white dwarf. An earlier tentative identification of a white dwarf companion to the mild barium star Zeta Cyg is confirmed. For the other stars, no ultraviolet excess attributable to a white dwarf is seen. Limits are set on the bolometric magnitude and age of a possible white dwarf companion. Since the barium stars do not have obvious progenitors among main-sequence and subgiant stars, mass transfer must be presumed to occur when the mass-gaining star is already on the giant branch. This restriction, and the white dwarf's minimum age, which is greater than 8 x 10 to the 8th yr, determined for several stars, effectively eliminates the hypothesis that mass transfer from an asymptotic giant branch star creates a barium star. Speculations are presented on alternative methods of producing a barium star in a binary system.

Dominy, J. F.↗

NASA NTRS1982

Physical conditions in the accretion disk of V603 Aquilae

Ultraviolet and optical spectra of the old nova V603 Aql are discussed. The UV-optical continuum is dominated by emission from the accretion disk. Emission lines from ions of H, He, C, N, and O are identified. These lines are probably formed in a circumstellar shell with radius comparable to the binary separation, density ten billion per cu cm, and a roughly solar chemical composition. This corona is probably heated by radiation emitted by the underlying accretion disk. Photoionization calculations of the structure and emission-line spectrum of the corona are presented, and the effects of this gas on the X-ray continuum are discussed.

Ferland, G. J.↗

NASA NTRS1982

Binary stars: Mass transfer and chemical composition

It is noted that mass exchange (and mass loss) within a binary system should produce observable changes in the surface chemical composition of both the mass losing and mass gaining stars as a stellar interior exposed to nucleosyntheses is uncovered. Three topics relating mass exchange and/or mass loss to nucleosynthesis are sketched: the chemical composition of Algol systems; the accretion disk of a cataclysmic variable fed by mass from a dwarf secondary star; and the hypothesis that classical Ba II giants result from mass transfer from a more evolved companion now present as a white dwarf.

Lambert, D. L.↗

NASA NTRS1982

Ultraviolet properties of the symbiotic stars

A general discussion of the UV spectra of symbiotic stars, including both the emission lines and the continua, is presented, with AG Pegasi considered as an illustrative example. It is noted that the IUE observations of the symbiotics have revealed UV properties which rival the diversity of the optical features. Nevertheless, the UV data have for the first time permitted the hot component to be studied relatively uncontaminated by the giant companion, which dominates the optical regime. The UV observations provide convincing evidence that many of the symbiotics have hot stellar companions embedded in the enshrouding nebula or accretion shell formed from the wind from one or possibly both of the components.

Slovak, M. H.↗

NASA NTRS1981

A high resolution IUE spectrum of the G0-G5Ia supergiant HR8752

A high-resolution long-wavelength IUE spectrum of the G0-G5Ia supergiant HR 8752 and its B main sequence companion is discussed. The supergiant, which dominates for wavelengths greater than approximately 2700 A, appears to have a normal spectrum. The spectrum of the B star contains several deep asymmetric absorption lines. It is argued that these lines are interstellar and not circumstellar in origin.

Stickland, D. J.↗

NASA NTRS1981

The ultraviolet spectrum of the twin-degenerate interacting binary G61-29

A low-resolution IUE spectrum (1200 A-1900 A) of the twin-degenerate white-dwarf binary star G61-29 is presented. A comparison is made to similar IUE spectra of the old novae DQ Her and V603 Aql. G61-29 shows N V 1240 A and He II 1640 A in emission. Unlike the old novae, the C IV 1550 A doublet is not present. It is proposed that the mass-losing He white dwarf (with a mass of 0.02 solar mass) is the H-exhausted, He-rich core of an old star in which the original CNO nuclei have been converted to N-14 by the CNO-cycles.

Lambert, D. L.↗