127 research outputs found
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
Beryllium abundances in stars hosting giant planets
We have derived beryllium abundances in a wide sample of stars hosting
planets, with spectral types in the range F7V-K0V, aimed at studying in detail
the effects of the presence of planets on the structure and evolution of the
associated stars. Predictions from current models are compared with the derived
abundances and suggestions are provided to explain the observed
inconsistencies. We show that while still not clear, the results suggest that
theoretical models may have to be revised for stars with Teff<5500K. On the
other hand, a comparison between planet host and non-planet host stars shows no
clear difference between both populations. Although preliminary, this result
favors a ``primordial'' origin for the metallicity ``excess'' observed for the
planetary host stars. Under this assumption, i.e. that there would be no
differences between stars with and without giant planets, the light element
depletion pattern of our sample of stars may also be used to further
investigate and constraint Li and Be depletion mechanisms.Comment: A&A in press -- accepted on the 22/02/2002 (11 pages, 6 figures
included
A low upper-limit on the lithium isotope ratio in HD140283
We have obtained a high-S/N (900-1100), high-resolving-power (R=95000)
spectrum of the metal-poor subgiant HD 140283 in an effort to measure its
6Li/7Li isotope ratio. From a 1-D atmospheric analysis, we find a value
consistent with zero, 6Li/7Li = 0.001, with an upper limit of 6Li/7Li < 0.026.
This measurement supersedes an earlier detection (0.040 +/- 0.015(1sigma)) by
one of the authors. HD 140283 provides no support for the suggestion that
Population II stars may preserve their 6Li on the portion of the subgiant
branch where 7Li is preserved. However, this star does not defeat the
suggestion either; being at the cool end of subgiant branch of the Spite
plateau, it may be sufficiently cool that 6Li depletion has already set in, or
the star may be sufficiently metal poor that little Galactic production of 6Li
had occurred. Continued investigation of other subgiants is necessary to test
the idea. We also consider the implications of the HD 140283 upper limit in
conjunction with other measurements for models of 6Li production by cosmic rays
from supernovae and structure formation shocks.Comment: 8 pages, 4 figures; accepted for publication in Astronomy and
Astrophysic
Correlated Depletion and Dilution of Lithium and Beryllium Revealed by Subgiants in M 67
The surface content of lithium (Li) and beryllium (Be) provide insights into
the mixing and circulation mechanisms in stellar interiors. The old open
cluster, M 67, has been well-studied for Li abundances in both main-sequence
and evolved stars. The Be abundances give us a probe to a deeper level in
stars. We have taken high-resolution spectra with Keck I with HIRES to
determine Be abundances along the subgiant branch of M 67, where there are
dramatic depletions of Li. These subgiants range in mass from 1.26 to 1.32
M and have evolved from main-sequence stars that would have occupied
the region of the Li-Be dip found in younger clusters. Lithium abundances have
been adjusted to the same scale for 103 stars in M 67 by Pace et al. The more
massive stars now the coolest and furthest-evolved from the main-sequence
show a drop in Li by a factor of 400 across the subgiant branch. Our new Be
abundances also show a decline, but by a factor of 50. The two elements
decline together with Li showing a steeper decline in these subgiants than it
does in the Li-Be dip stars. The relative decline in Be abundance compared to
Li is remarkably well fit by the models of Sills & Deliyannis, made
specifically for the subgiants in M 67. Those models include the effects of
mixing induced by stellar rotation. These M 67 subgiants show the effects of
both main-sequence depletion and post-main-sequence dilution of both Li and Be.Comment: 31 pages with 3 tables, 13 figures Accepted by Ap. J. 10/7/201
Combining Interaction Design and Gaming Technologies for the Development of Interactive Archaeological Content Presentation Systems
Our main objective is to produce state-of-the-art edutainment and serious game end-systems, which satisfy the requirements of all three parties involved in the development process: content experts, end-users and application developers. Their requirements are often cross disciplinary, as each party involved in the process requires solutions to a number of problems which need to be answered in a systematic and complete manner. The ultimate goal of this process is to introduce an efficient, extendable and aesthetically pleasing end-system. In order to achieve these goals, we address and attempt to resolve the most common presentation design issues that arise during the process of interaction design. Completion of this process enables the actual system development to commence with a precise and complete specification of content features and system characteristics
Lithium and Beryllium in One Solar Mass Stars
The surface content of lithium (Li) and beryllium (Be) in stars can reveal
important information about the temperature structure and physical processes in
their interior regions. This study focuses on solar-type stars with a sample
that is more precisely defined than done previously. Our selection of stars
studied for Be is constrained by five parameters: mass, temperature, surface
gravity, metallicity, and age to be similar to the Sun and is focussed on stars
within +-0.02 of 1 M_sun. We have used the Keck I telescope with HIRES to
obtain spectra of the Be II spectral region of 52 such stars at high spectral
resolution (45,000) and high signal-to-noise ratios. While the spread in
Li in these stars is greater than a factor of 400, the spread in Be is only 2.7
times. Two stars were without any Be, perhaps due to a merger or a mass
transfer with a companion. We find a steep trend of Li with temperature but
little for Be. While there is a downward trend in Li with [Fe/H] from -0.4 to
+0.4 due to stellar depletion, there is a small increase in Be with Fe from
Galactic Be enrichment. While there is a broad decline in Li with age, there
may be a small increase in Be with age, though age is less well-determined. In
the subset of stars closest to the Sun in temperature and other parameters we
find that the ratio of the abundances of Be to Li is much lower than predicted
by models; there may be other mixing mechanisms causing additional Li
depletion.Comment: total 29 pages including 12 figures, 5 tables Accepted for
Astrophysical Journa
Lithium in a Short-Period Tidally Locked Binary of M67: Implications for Stellar Evolution, Galactic Lithium Evolution, and Cosmology
In open clusters, late-F stars exhibit a Li maximum (the Li \u27peak\u27 region) at lower abundance with age, which could be due either to stellar depletion or Galactic Li enrichment (or some other cause). We have observed a short-period tidally locked binary (SPTLB) on the Li peak region in the old cluster M67 to distinguish between alternatives. SPTLBs which synchronized in the early pre-main sequence would avoid the rotational mixing which, according to Yale models, may be responsible for depleting Li with age in open cluster dwarfs. We find that both components of the M67 SPTLB have a Li abundance lying about a factor of 2 or more above any other M67 single star and about a factor of 3 or more above the mean Li peak region abundance in M67. Our results suggest that the initial Li abundance in M67 is at least as high as approximately 3.0 = 12 + log (NLi/NH). Our high M67 SPTLB Li abundance and those in other clusters support the combination of Zahn\u27s tidal circularization and the Yale rotational mixing theories and may indicate that the halo Li plateau (analogous to the cluster Li peak region) abundance has been depleted from a higher primordial value. Implications are discussed
Superflares on Ordinary Solar-Type Stars
Short duration flares are well known to occur on cool main-sequence stars as
well as on many types of `exotic' stars. Ordinary main-sequence stars are
usually pictured as being static on time scales of millions or billions of
years. Our sun has occasional flares involving up to ergs which
produce optical brightenings too small in amplitude to be detected in
disk-integrated brightness. However, we identify nine cases of superflares
involving to ergs on normal solar-type stars. That is,
these stars are on or near the main-sequence, are of spectral class from F8 to
G8, are single (or in very wide binaries), are not rapid rotators, and are not
exceedingly young in age. This class of stars includes many those recently
discovered to have planets as well as our own Sun, and the consequences for any
life on surrounding planets could be profound. For the case of the Sun,
historical records suggest that no superflares have occurred in the last two
millennia.Comment: 16 pages, accepted for publication in Ap
Beryllium anomalies in solar-type field stars
We present a study of beryllium (Be) abundances in a large sample of field
solar-type dwarfs and sub-giants spanning a large range of effective
temperatures. The analysis shows that Be is severely depleted for F stars, as
expected by the light-element depletion models. However, we also show that
Beryllium abundances decrease with decreasing temperature for stars cooler than
6000 K, a result that cannot be explained by current theoretical models
including rotational mixing, but that is, at least in part, expected from the
models that take into account internal wave physics. In particular, the light
element abundances of the coolest and youngest stars in our sample suggest that
Be, as well as lithium (Li), has already been burned early during their
evolution. Furthermore, we find strong evidence for the existence of a Be-gap
for solar-temperature stars. The analysis of Li and Be abundances in the
sub-giants of our sample also shows the presence of one case that has still
detectable amounts of Li, while Be is severely depleted. Finally, we compare
the derived Be abundances with Li abundances derived using the same set of
stellar parameters. This gives us the possibility to explore the temperatures
for which the onset of Li and Be depletion occurs.Comment: 16 pages, 13 figures, accepted for publication in Astronomy &
Astrophysic
- …