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Search Results: 1 - 10 of 225297 matches for " Norman R. Trams "
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Obscured Asymptotic Giant Branch stars in the Magellanic Clouds III. New IRAS counterparts
Jacco Th. van Loon,Albert A. Zijlstra,Patricia A. Whitelock,L. B. F. M. Waters,Cecile Loup,Norman R. Trams
Physics , 1997,
Abstract: We have searched for near-infrared stellar counterparts of IRAS point sources in the Large Magellanic Cloud (LMC), in J and K-bands. This resulted in the detection of 21 counterparts, of which 19 are new discoveries. Using colour-magnitude and colour-colour diagrams, we identify 13 Asymptotic Giant Branch (AGB) stars with thick circumstellar dust envelopes, 7 possible early post-AGB stars or stars recovering from a thermal pulse, and 1 red supergiant or foreground star. For 10 of the IRAS targets we do not succeed in detecting and/or identifying a near-infrared counterpart. We serendipitously detect 14 other red sources, of which 2 are known Long Period Variables, and a few galaxies. The near-infrared and optical colours of the galaxies may indicate considerable interstellar extinction through the LMC, as much as A_V about 2-4 mag. The relative number of AGB carbon stars over oxygen stars is shown to decrease as the luminosity increases. Yet amongst the faintest mass-losing AGB stars oxygen-rich stars still exist, which puts constraints on current convection theories that predict the occurrence of third dredge-up and Hot Bottom Burning. We investigate the nature of some LMC stars that have infrared properties very similar to suspected Galactic post-AGB stars.
A study on the nature of the peculiar supergiant HD101584
Eric J. Bakker,Henny J. G. L. M. Lamers,L. B. F. M. Waters,Christoffel Waelkens,Norman R. Trams,Hans van Winckel
Physics , 1995,
Abstract: We present a study of low- and high-resolution ultraviolet, high-resolution optical CAT/CES spectra and ultraviolet, optical and infrared photometry of the peculiar supergiant HD101584. From the photometry we learn that the ultraviolet and optical energy distribution cannot be fitted in a consistent way and we need a model in which the UV and optical energy distribution are formed by different gas. The Geneva photometry is best fitted to a B9II Kurucz model, Teff=12000+-1000K and log g=3.0 +-1.0, with an extinction of E(B-V)=0.49+-0.05. The observed spectral features in the spectrum of HD101584 are classified in eight different categories based on the velocity, shape of profile and the identification. The high-excitation HeI(chi=20.87eV), NII(chi=18.40eV), CII (chi=14.39eV) and NI (chi=10.29eV) optical absorption lines are formed in the photosphere of a late B-star (e.g. B8-9I-II). These absorption lines show radial velocity variations which are attributed to binary motion, with the secondary being a
Detection of C2, CN, and NaID absorption in the AGB remnant of HD56126
Eric J. Bakker,L. B. F. M. Waters,Henny J. G. L. M. Lamers,Norman R. Trams,Frank L. A. Van der Wolf
Physics , 1995,
Abstract: We present the detection of molecular absorption lines in the optical spectrum of the post-AGB star HD56126. The C2 Phillips A^{1}\Pi_{u}-X^{1}\Sigma^{+}_{g} (1,0), (2,0), and (3,0); Swan d^{3}\Pi_{g}-a^{3}\Pi_{u} (0,0) and (1,0); and CN Red system A^{2}\Pi-X^{2}\Sigma^{+} (1,0), (2,0), (3,0), and (4,0) bands have been identified. From the identification of the molecular bands we find an expansion velocity of 8.5+-0.6 kms independent of excitation condition or molecular specie. On the basis of the expansion velocity, rotational temperatures, and molecular column densities we argue that the line-forming region is the AGB remnant. This is in agreement with the expansion velocity derived from the CO lines. We find column densities of log N_C2=15.3+-0.3 cm-2 and log N_CN=15.5+-0.3 cm-2, and rotational temperatures of T_rot=242+-20 K and T_rot=24+-5 K respectively for C2 and CN. By studying molecular line absorption in optical spectra of post-AGB stars we have found a new tracer of the AGB remnant. From comparison with the results of CO and IR observations it is possible to obtain information on non-spherical behavior of the AGB remnant. Using different molecules with different excitation conditions it should be possible to study the AGB remnant as a function of the distance to the star, and thus as a function of the evolutionary status of the star on the AGB.
Mass-loss rates and luminosity functions of dust-enshrouded AGB stars and red supergiants in the LMC
Jacco Th. van Loon,M. A. T. Groenewegen,A. de Koter,Norman R. Trams,L. B. F. M. Waters,Albert A. Zijlstra,Patricia A. Whitelock,Cecile Loup
Physics , 1999,
Abstract: A radiative transfer code is used to model the spectral energy distributions of 57 mass-losing Asymptotic Giant Branch (AGB) stars and red supergiants (RSGs) in the Large Magellanic Cloud (LMC) for which ISO spectroscopic and photometric data are available. As a result we derive mass-loss rates and bolometric luminosities. A gap in the luminosity distribution around M_bol = -7.5 mag separates AGB stars from RSGs. The luminosity distributions of optically bright carbon stars, dust-enshrouded carbon stars and dust-enshrouded M-type stars have only little overlap, suggesting that the dust-enshrouded AGB stars are at the very tip of the AGB and will not evolve significantly in luminosity before mass loss ends their AGB evolution. Derived mass-loss rates span a range from Mdot about 10^-7 to 10^-3 M_sun/yr. More luminous and cooler stars are found to reach higher mass-loss rates. The highest mass-loss rates exceed the classical limit set by the momentum of the stellar radiation field, L/c, by a factor of a few due to multiple scattering of photons in the circumstellar dust envelope. Mass-loss rates are lower than the mass consumption rate by nuclear burning, Mdot_nuc, for most of the RSGs. Two RSGs have Mdot >> Mdot_nuc, however, suggesting that RSGs shed most of their stellar mantles in short phases of intense mass loss. Stars on the thermal pulsing AGB may also experience episodes of intensified mass loss, but their quiescent mass-loss rates are usually already higher than Mdot_nuc.
Obscured Asymptotic Giant Branch stars in the Magellanic Clouds IV. Carbon stars and OH/IR stars
Jacco Th. van Loon,Albert A. Zijlstra,Patricia A. Whitelock,Peter te Lintel Hekkert,Jessica M. Chapman,Cecile Loup,M. A. T. Groenewegen,L. B. F. M. Waters,Norman. R. Trams
Physics , 1997,
Abstract: We present N-band photometry for a sample of 21 dust- enshrouded AGB stars in the Large Magellanic Cloud, and three additional sources in the Small Magellanic Cloud. Together with near-IR photometry, this is used to give a tentative classification into carbon and oxygen-rich atmospheres. Bolometric luminosities are also estimated for these stars. In addition, we present the results of a survey for OH masers in the LMC, which resulted in the discovery of OH maser emission from IRAS04407-7000. Spectra between 600 and 1000 nm have been obtained for two heavily obscured AGB stars in the LMC, confirming them to be highly reddened very late M-type giants. Because the dust-enshrouded stars are clearly undergoing heavy mass loss they are assumed to be very near the termination of their respective Asymptotic Giant Branch phases. The fraction of mass-losing carbon stars decreases with increasing luminosity, as expected from Hot Bottom Burning. The best candidate carbon star, with M_bol = -6.8 mag, is the most luminous mass-losing carbon star in the Magellanic Clouds, and amongst the most luminous AGB stars. At lower luminosities (M_bol = -5 mag) both oxygen and carbon stars are found. This may be explained by a range in metallicity of the individual mass-losing AGB stars.
ISO observations of obscured Asymptotic Giant Branch stars in the Large Magellanic Cloud
Norman R. Trams,Jacco Th. van Loon,L. B. F. M. Waters,Albert A. Zijlstra,Cecile Loup,Patricia A. Whitelock,M. A. T. Groenewegen,Joris A. D. L. Blommaert,Ralf Siebenmorgen,A. Heske,Michael W. Feast
Physics , 1999,
Abstract: We present ISO photometric and spectroscopic observations of a sample of 57 bright Asymptotic Giant Branch stars and red supergiants in the Large Magellanic Cloud, selected on the basis of IRAS colours indicative of high mass-loss rates. PHOT-P and PHOT-C photometry at 12, 25 and 60 $\mu$m and CAM photometry at 12 $\mu$m are used in combination with quasi-simultaneous ground-based near-IR photometry to construct colour-colour diagrams for all stars in our sample. PHOT-S and CAM-CVF spectra in the 3 to 14 $\mu$m region are presented for 23 stars. From the colour-colour diagrams and the spectra, we establish the chemical types of the dust around 49 stars in this sample. Many stars have carbon-rich dust. The most luminous carbon star in the Magellanic Clouds has also a (minor) oxygen-rich component. OH/IR stars have silicate absorption with emission wings. The unique dataset presented here allows a detailed study of a representative sample of thermal-pulsing AGB stars with well-determined luminosities.
IRAS04496-6958: A luminous carbon star with silicate dust in the Large Magellanic Cloud
Norman R. Trams,Jacco Th. van Loon,Albert A. Zijlstra,Cecile Loup,M. A. T. Groenewegen,L. B. F. M. Waters,Patricia A. Whitelock,J. A. D. L. Blommaert,Ralf Siebenmorgen,Astrid Heske
Physics , 1999,
Abstract: We describe ISO observations of the obscured Asymptotic Giant Branch (AGB) star IRAS04496-6958 in the Large Magellanic Cloud (LMC). This star has been classified as a carbon star. Our new ISOCAM CVF spectra show that it is the first carbon star with silicate dust known outside of the Milky Way. The existence of this object, and the fact that it is one of the highest luminosity AGB stars in the LMC, provide important information for theoretical models of AGB evolution and understanding the origin of silicate carbon stars.
Giving Microbes Their Due
Norman R. Pace
PLOS Biology , 2012, DOI: 10.1371/journal.pbio.1000425
Abstract:
Discrimination and reliability: Equal partners?
Geoffrey R Norman
Health and Quality of Life Outcomes , 2008, DOI: 10.1186/1477-7525-6-81
Abstract: There are several definitions of discrimination. Two, from the Webster dictionary, are: 1) the process by which two stimuli differing in some aspect are responded to differently, and 2) the quality or power of finely distinguishing. It seems to me that the manuscript by Hankins [1], in attempting to elaborate on 1), shows considerable absence of 2).To begin with, a small disclaimer: The “McMaster Framework” is hardly endorsed by all at McMaster. In fact, my co-author Dave Streiner and I, both of us originally from McMaster, in our textbook [2] specifically challenge the Kirshner and Guyatt [3] notion of different kinds of instruments for different purposes.And now to the matter at hand. Hankins [1] attempts to show that reliability is not a good measure of discrimination, and that instruments can be reliable but not discriminating, and vice versa. But, while he refers to formulas for both reliability and discrimination (more on this in a moment), he does not actually define either. This is not just pedantry; in my view, reliability is, by definition, an index of the ability of an instrument to discriminate among individuals. To quote an authority on the subject, me [1]:“..the reliability coefficients reflects the extent to which a measurement instrument can differentiate among individuals, since the magnitude of the coefficient is directly related to the variability between subjects”.An almost perfect paraphrase of the Webster definition above. Fundamentally all reliability coefficients are intraclass coefficients, and mathematically reflect the proportion of the variance in the observations that relate to real differences among subjects. The formula is:The numerator expresses variability due to different responses among individuals. The denominator expresses all variability. So reliability is a measure of the extent to which people differ, expressed as a number between 0 and 1. QED – reliability is discrimination. This is also precisely consistent with Hawkins' “te
Giving Microbes Their Due
Norman R. Pace
PLOS Biology , 2010, DOI: 10.1371/journal.pbio.1000425
Abstract:
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