1,580 research outputs found
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 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
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
Pre-main-sequence Lithium Depletion
In this review I briefly discuss the theory of pre-main-sequence (PMS) Li
depletion in low-mass (0.075<M<1.2 Msun) stars and highlight those uncertain
parameters which lead to substantial differences in model predictions. I then
summarise observations of PMS stars in very young open clusters, clusters that
have just reached the ZAMS and briefly highlight recent developments in the
observation of Li in very low-mass PMS stars.Comment: 8 pages, invited review at "Chemical abundances and mixing in stars
in the Milky Way and its satellites", eds. L. Pasquini, S. Randich. ESO
Astrophysics Symposium (Springer-Verlag
Discovery of a luminous white dwarf in a young star cluster in the Large Magellanic Cloud
We have identified a candidate 1-2 x 10^5 year old luminous white dwarf in
NGC 1818, a young star cluster in the Large Magellanic Cloud. This discovery
strongly constrains the boundary mass M_c at which stars stop forming neutron
stars and start forming white dwarfs, to M_c > 7.6 Msun.Comment: 4 pages, 2 figures, greyscale image available by ftp from
[email protected]. ApJLetters, accepted 17 March 199
A New Association of Post-T Tauri Stars Near The Sun
Observing ROSAT sources in 20 x 25 deg centered at the high latitude active
star ER Eri, we found evidences for a new young nearby association (~30Myr
at~60pc), the Horologium Association (HorA), formed by at least 10 probable and
6 possible members, some being Post-T Tauri stars. We examine several
requirements that characterize a young association and they, together, create a
strong evidence for the reality of the HorA. In fact, the Li line intensities
are between those of the oldest classical T Tauri stars and the ones of the
Local Association stars. The space velocities of the HorA relative to the Sun,
U= -9.5+/-1.0, V = -20.9 +/- 1.1, W = -2.1 +/- 1.9, are not far from those of
the Local Association. We suggest that some hotter and non-X-ray active stars,
with similar space velocities, could be massive members of the HorA, among
them, the nearby Be star Achernar. The maximum of the mass distribution
function of the HorA is around 0.8 solar masses. At its distance, the projected
size of the HorA, ~50 pc, would be larger than our surveyed area and many other
members could have been missed. We also observed 3 control regions, two at
northern and southern galactic latitudes and a third one in the known TW Hya
Association (TWA), and the properties and distribution of their young stars
strengthen the reality of the HorA. Contrary to the TWA, the only known
binaries in the HorA are 2 very wide systems. The HorA is much more isolated
from clouds and older than the TWA and could give some clues about the lifetime
of the disks around T Tauri stars. Actually, none of the proposed members is an
IRAS source indicating an advanced stage of the evolution of their accreting
disks. ER Eri itself was found to be a RS CVn-like system.Comment: 25 pages, 5 eps figures, to appear in Astron.
Rotation and activity in the solar-metallicity open cluster NGC2516
We report new measures of radial velocities and rotation rates (v sin i) for
51 F and early-G stars in the open cluster NGC2516, and combine these with
previously published data. From high signal-to-noise spectra of two stars, we
show that NGC2516 has a relative iron abundance with respect to the Pleiades of
delta([Fe/H])= +0.04 +/- 0.07 at the canonical reddening of E(B - V) = 0.12, in
contrast to previous photometric studies that placed the cluster 0.2 to 0.4 dex
below solar. We construct a color-magnitude diagram based on radial velocity
members, and explore the sensitivity of photometric determinations of the
metallicity and distance to assumed values of the reddening. For a metal
abundance near solar, the Hipparcos distance to NGC2516 is probably
underestimated. Finally, we show that the distribution of rotation rates and
X-ray emission does not differ greatly from that of the Pleiades, when
allowance is made for the somewhat older age of NGC2516.Comment: Accepted for publication in the Astrophysical Journal. 35 pages
including 7 figure
The Missing Link: Magnetism and Superconductivity
The effect of magnetic moments on superconductivity has long been a
controversial subject in condensed matter physics. While Matthias and
collaborators experimentally demonstrated the destruction of superconductivity
in La by the addition of magnetic moments (Gd), it has since been suggested
that magnetic fluctuations are in fact responsible for the development of
superconducting order in other systems. Currently this debate is focused on
several families of unconventional superconductors including high-Tc cuprates,
borocarbides as well as heavy fermion systems where magnetism and
superconductivity are known to coexist. Here we report a novel aspect of
competition and coexistence of these two competing orders in an interesting
class of heavy fermion compounds, namely the 1-1-5 series: CeTIn5 where T=Co,
Ir, or Rh. Our optical experiments indicate the existence of regions in
momentum space where local moments remain unscreened. The extent of these
regions in momentum space appears to control both the normal and
superconducting state properties in the 1-1-5 family of heavy fermion (HF)
superconductors.Comment: 6 pages, 2 figure
XMM-Newton survey of the low-metallicity open cluster NGC 2516
We present the first results of an XMM-Newton EPIC survey of NGC 2516, a
southern low-metallicity open cluster with an age close to the Pleiades. The
attained limiting sensitivity is of ~ 2.4 10^-15 erg sec^-1} cm^-2 in the
0.1--4.0 keV bandpass. This has been achieved by summing the data of the MOS
and PN cameras of two distinct observations for a total exposure time of ~ 33
ks and by analyzing the summed data set with the wavelet detection algorithm
developed at Osservatorio Astronomico di Palermo (OAPA) that has yielded over
200 X-ray detections. Using data just from a single exposure or from a single
camera would have reduced by a factor 2--4 our limiting sensitivity and would
have resulted in 25--40% less X-ray detections. To date, 129 detections have as
counterparts one or more of the 540 photometrically selected cluster members in
the surveyed region, for a total of 147 likely detected members, with unique
identification in 112 cases. We derive the X-ray luminosity functions (XLF) of
NGC 2516 members of different spectral types and compare them with those of the
more metal rich, approximately coeval Pleiades cluster, finding the NGC 2516
photometrically selected dG and dK stars less luminous than the Pleiades. The
XLFs of the NGC 2516 and of the Pleiades dM stars are indistinguishable. We
compare the XMM-Newton results with those recently obtained with Chandra.Comment: 8 pages, 5 figures (in 11 postscript files
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