2,497 research outputs found
Mineral chemistry of late Variscan gabbros from central Spain: constraints on crystallisation processes and nature of the parental magmas
© 2016. Universidad Complutense de Madrid. The commercial rights of the printed and online versions of Journal of Iberian Geology are property of the Universidad Complutense de Madrid (UCM) and the Consejo Superior de Investigaciones CientĂficas (CSIC), and the journal must be cited for any partial or full reproduction
WIYN/Hydra Detection of Lithium Depletion in F Stars of the Young Open Cluster M35 and Implications for the Development of the Lithium Gap
We report discovery of significant depletion of Li on the surfaces of F dwarf
stars in the 150-Myr-old open cluster M35, analagous to a feature in the
700-Myr-old Hyades cluster that has been referred to as the ``Li gap.'' We have
caught the gap in the act of forming: using high resolution, high S/N,
WIYN/Hydra observations, we detect Li in all but a few M35 F stars; the maximum
depletion lies at least 0.6-0.8 dex below minimally depleted (or undepleted)
stars. The M35 Li depletion region, a) is quite wide, with clear depletion seen
from 6000K to 6700K or hotter; b) shows a significant dispersion in Li
abundance at all T_eff, even with stars of the same T_eff; and c) contains
undepleted stars (as well as depleted ones) in the (narrow) classical Hyades
gap region, which itself shows no undepleted stars. All of these M35 Li
depletion properties support rotationally-induced slow mixing as the primary
physical mechanism that forms the gap, and argues against other proposed
mechanisms, particularly diffusion and steady main sequence mass loss. When
viewed in the context of the M35 Li depletion properties, the Hyades Li gap may
well be wider than is usually recognized.Comment: 14 Pages, 3 figures. Accepted to ApJ Letter
The NASA/MSFC global reference atmospheric model: 1990 version (GRAM-90). Part 1: Technical/users manual
A technical description of the NASA/MSFC Global Reference Atmospheric Model 1990 version (GRAM-90) is presented with emphasis on the additions and new user's manual descriptions of the program operation aspects of the revised model. Some sample results for the new middle atmosphere section and comparisons with results from a three dimensional circulation model are provided. A programmer's manual with more details for those wishing to make their own GRAM program adaptations is also presented
The Li Overabundance of J37: Diffusion or Accretion?
In September 2002 the discovery of a super Li-rich F-dwarf (J37) in NGC 6633,
an iron poor analogue of the better studied Hyades and Praecepe open clusters,
was announced. This unique star was thought to be the smoking gun for the
action of diffusion, models of which predict a narrow "Li-peak" at
approximately the correct temperature. However, with more detailed studies into
J37s abundance pattern this star provides firm evidence for the accretion of
planetesimals or other material from the circumstellar environment of new born
stars.
Thanks to the specific predictions made about the behaviour of Be abundances,
(the most striking of which being no Be in super-Li-rich dwarfs subject to
diffusion) the opposing diffusion/accretion predictions can be tested.
Initial modelling of the Be line indicates that J37 is as Be rich as it is Li
rich; log N(Be) = 2.25 +/- 0.25, and so is broadly consistent with an
accretion-fuelled enhancement. However, that both Li and Be are enhanced by
much more than the iron-peak elements (as determined in previous studies)
suggests that diffusion also plays a role in increasing the abundances of Li
and Be specifically.
Furthermore, a new data set from the UVES/UT2 combination has allowed the
elemental abundance of Iron to be measured, and the set of preliminary stellar
parameters determined; Teff ~ 7340 K, log g ~ 4.1, microturbulence ~ 4.3 km/s,
[Fe/H] ~ 0.50. This again provides distinct evidence for the effects of
accretion in J37 and requires a new synthesis of the Be doublet.Comment: 5 pages, 2 figures. Poster presented at IAU Symposium 224 "The A Star
Puzzle", 7-13 July 2004, Poprad, Slovaki
The NASA/MSFC global reference atmospheric model: 1990 version (GRAM-90). Part 2: Program/data listings
A new (1990) version of the NASA/MSFC Global Reference Atmospheric Model (GRAM-90) was completed and the program and key data base listing are presented. GRAM-90 incorporate extensive new data, mostly collected under the Middle Atmosphere Program, to produce a completely revised middle atmosphere model (20 to 120 km). At altitudes greater than 120 km, GRAM-90 uses the NASA Marshall Engineering Thermosphere model. Complete listings of all program and major data bases are presented. Also, a test case is included
Beryllium Enhancement as Evidence for Accretion in a Lithium-Rich F Dwarf
The early F dwarf star ``J37'' in the open cluster NGC6633 shows an unusual
pattern of photospheric abundances, including an order of magnitude enhancement
of lithium and iron-peak elements, but an under-abundance of carbon. As a
consequence of its thin convection zone these anomalies have been attributed to
either radiative diffusion or the accretion of hydrogen-depleted material. By
comparing high resolution VLT/UVES spectra of J37 (and other F stars in NGC
6633) with syntheses of the Be ii doublet region at 3131 Ang, we establish that
J37 also has a Be abundance (A(Be)=3.0+/-0.5) that is at least ten times the
cosmic value. This contradicts radiative diffusion models that produce a Li
over-abundance, as they also predict photospheric Be depletion. Instead, since
Be is a highly refractory element, it supports the notion that J37 is the first
clear example of a star that has accreted volatile-depleted material with a
composition similar to chondritic meteorites, although some diffusion may be
necessary to explain the low C and O abundances.Comment: Accepted for publication in MNRAS letters, 5 page
Computer solutions of the vlasov equations final report, jun. 1, 1963 - nov. 30, 1964
Computer programs for solutions of Vlasov equations for plane, cylindrical, and spherical geometr
The lithium depletion boundary and the age of NGC 2547
We present the results of a photometric and spectroscopic survey of cool M
dwarf candidates in the young open cluster NGC 2547. Using the 2dF fiber
spectrograph, we have searched for the luminosity at which lithium remains
unburned in an attempt to constrain the cluster age. The lack of a population
of individual lithium-rich objects towards the faint end of our sample places a
very strong lower limit to the cluster age of 35 Myr. However, the detection of
lithium in the averaged spectra of our faintest targets suggests that the
lithium depletion boundary lies at 9.5 < M(I) < 10.0 and that the cluster age
is < 54 Myr. The age of NGC 2547 judged from fitting isochrones to low-mass
pre-main-sequence stars in colour-magnitude diagrams is 20-35 Myr using the
same evolutionary models. The sense and size of the discrepancy in age
determined by these two techniques is similar to that found in another young
cluster, IC 2391, and in the low-mass pre main-sequence binary system, GJ
871.1AB. We suggest that the inclusion of rotation or dynamo-generated magnetic
fields in the evolutionary models could reconcile the two age determinations,
but only at the expense of increasing the cluster ages beyond that currently
indicated by the lithium depletion. Alternatively, some mechanism is required
that increases the rate of lithium depletion in young, very low-mass fully
convective stars.Comment: 14 pages, 7 figures, 3 tables, accepted by MNRA
Lithium in Blanco1: Implications for Stellar Mixing
We obtain lithium abundances for G and K stars in Blanco 1, an open cluster
with an age similar to, or slightly younger than, the Pleiades. We critically
examine previous spectroscopic abundance analyses of Blanco 1 and conclude that
while there were flaws in earlier work, it is likely that Blanco 1 is close in
overall metallicity to the older Hyades cluster and more metal-rich than the
Pleiades. However, we find Blanco 1 has Li abundances and rotation rates
similar to the Pleiades, contradicting predictions from standard stellar
evolution models, in which convective pre-main sequence (PMS) Li depletion
should increase rapidly with metallicity. If the high metallicity of Blanco 1
is subsequently confirmed, our observations imply (1) that a currently unknown
mechanism severely inhibits PMS Li depletion, (2) that additional non-standard
mixing modes, such as those driven by rotation and angular momentum loss, are
then responsible for main sequence Li depletion between the ages of Blanco 1
and the Hyades, and (3) that in clusters younger than the Hyades, metallicity
plays only a minor role in determining the amount of Li depletion among G and K
stars. These conclusions suggest that Li abundance remains a useful age
indicator among young (less than 700 Myr) stars even when metallicities are
unknown. If non-standard mixing is effective in Population I stars, the
primordial Li abundance could be significantly larger than present day
Population II Li abundances, due to prior Li depletion.Comment: 18 pages, 3 figs. To appear in ApJ Vol. 511 (Jan 20 1999
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Evaluation of peak-picking algorithms for protein mass spectrometry
Peak picking is an early key step in MS data analysis. We compare three commonly used approaches to peak picking and discuss their merits by means of statistical analysis. Methods investigated encompass signal-to-noise ratio, continuous wavelet transform, and a correlation-based approach using a Gaussian template.
Functionality of the three methods is illustrated and discussed in a practical context using a mass spectral data set created with MALDI-TOF technology. Sensitivity and specificity are investigated using a manually defined reference set of peaks. As an additional criterion, the robustness of the three methods is assessed by a perturbation analysis and illustrated using ROC curves
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