364 research outputs found
Rotational modulation of the chromospheric activity in the young solar-type star, X-1 Orionis
The IUE satellite was used to observe one of the youngest G stars (GO V) for which Duncan (1981) derives an age of 6 x 10 to the 8th power years from the Li abundance. Rotational modulation was looked for in the emission flux in the chromospheric and transition region lines of this star. Variations in the Ca 11 K-lines profile were studied with the CHF telescope at Mauna Kea. Results show that the same modulation of the emission flux of Ca 11 due to stellar rotation is present in the transition region feature of C IV and probably of He II. For other UV lines the modulation is not apparent, due to a more complex surface distribution of the active areas or supergranulation network, or a shorter lifetime of the conditions which give rise to these features, or to the uncertainities in the measured line strengths. The Mg II emission flux is constant to within + or - 3.4% implying a rather uniform distribution of Mg II emission areas. The Ca II emission not only shows a measurable variation in intensity but also variations in detailed line profile shape when observed at high resolution
IUE observations of the chromospheric activity-age relation in young solar-type stars
Ultraviolet data obtained with the IUE spacecraft are presented for a dozen solar-type stars in the field. The stars are of spectral type F6 V - G1 V; on the basis of their high Li content, they range in age from 0.1 to 2.8 Gyr. The evolution of transition regions and chromospheric emission with stellar age is studied along with the surface distribution of magnetically active regions as revealed by rotational modulation of UV emission line fluxes
Beryllium and Alpha-Element Abundances in a Large Sample of Metal-Poor Stars
The light elements, Li, Be, and B, provide tracers for many aspects of
astronomy including stellar structure, Galactic evolution, and cosmology. We
have taken spectra of Be in 117 metal-poor stars ranging in metallicity from
[Fe/H] = -0.5 to -3.5 with Keck I + HIRES at a resolution of 42,000 and
signal-to-noise ratios of near 100. We have determined the stellar parameters
spectroscopically from lines of Fe I, Fe II, Ti I and Ti II. The abundances of
Be and O were derived by spectrum synthesis techniques, while abundances of Fe,
Ti, and Mg were found from many spectral line measurements. There is a linear
relationship between [Fe/H] and A(Be) with a slope of +0.88 +-0.03 over three
orders of magnitude in [Fe/H]. We fit the relationship between A(Be) and [O/H]
with both a single slope and with two slopes. The relationship between [Fe/H]
and [O/H] seems robustly linear and we conclude that the slope change in Be vs.
O is due to the Be abundance. Although Be is a by-product of CNO, we have used
Ti and Mg abundances as alpha-element surrogates for O in part because O
abundances are rather sensitive to both stellar temperature and surface
gravity. We find that A(Be) tracks [Ti/H] very well with a slope of 1.00
+-0.04. It also tracks [Mg/H] very well with a slope of 0.88 +-0.03. We find
that there are distinct differences in the relationships of A(Be) and [Fe/H]
and of A(Be) and [O/H] for our dissipative stars and our accretive stars. We
suggest that the Be in the dissipative stars was primarily formed by GCR
spallation and Be in the accretive stars was formed in the vicinity of SN II.Comment: Accepted for Ap.J. Nov. 10, 2011, v. 741 70 pages, 27 figures, 5
table
Lithium in the Intermediate-Age Open Cluster, NGC 3680
High-dispersion spectra centered on the Li 6708 A line have been obtained for
70 potential members of the open cluster NGC 3680, with an emphasis on stars in
the turnoff region. A measurable Li abundance has been derived for 53 stars, 39
of which have radial velocities and proper motions consistent with cluster
membership. After being transferred to common temperature and abundance scales,
previous Li estimates have been combined to generate a sample of 49 members, 40
of which bracket the cluster Li-dip. Spectroscopic elemental analysis of 8
giants and 5 turnoff stars produces [Fe/H] = -0.17 +/- 0.07 (sd) and -0.07 +/-
0.02 (sd), respectively. We also report measurements of Ca, Si and Ni which are
consistent with scaled-solar ratios within the errors. Adopting [Fe/H] = -0.08
(Sect. 3.6), Y^2 isochrone comparisons lead to an age of 1.75 +/- 0.10 Gyr and
an apparent modulus of (m-M) = 10.30 +/- 0.15 for the cluster, placing the
center of the Li-dip at 1.35 +/- 0.03 solar masses. Among the giants, 5 of 9
cluster members are now known to have measurable Li with A(Li) near 1.0. A
combined sample of dwarfs in the Hyades and Praesepe is used to delineate the
Li-dip profile at 0.7 Gyr and [Fe/H] = +0.15, establishing its center at 1.42
+/- 0.02 solar masses and noting the possible existence of secondary dip on its
red boundary. When evolved to the typical age of the clusters NGC 752, IC 4651
and NGC 3680, the Hyades/Praesepe Li-dip profile reproduces the observed
morphology of the combined Li-dip within the CMD's of the intermediate-age
clusters while implying a metallicity dependence for the central mass of the
Li-dip given by Mass = (1.38 +/-0.04) + (0.4 +/- 0.2)[Fe/H]. The implications
of the similarity of the Li-dichotomy among giants in NGC 752 and IC 4651 and
the disagreement with the pattern among NGC 3680 giants are discussed.Comment: Latex ms. is 56 pages, including 10 figures and 4 tables. Accepted
for the Astronomical Journa
Identifying Young Planet Host Stars from Keck-HIRES Spectra of Lithium
The lithium doublet at 6708 \AA\ provides an age diagnostic for main sequence
FGK dwarfs. We measured the abundance of lithium in 1305 stars with detected
transiting planets from the Kepler Mission using high-resolution spectroscopy.
Our catalog of lithium measurements from this sample have a range of abundance
from A(Li) = 3.11 0.07 to an upper limit of 0.84 dex. For a
magnitude-limited sample that comprises 960 of the 1305 stars, our Keck-HIRES
spectra have a median S/N = 45 per pixel at 6700 \AA\ with spectral
resolution = = 55,000. We identify 80
young stars that have A(Li) values greater than the Hyades at their respective
effective temperatures; these stars are younger than 650 Myr old, the
approximate age of the Hyades. We then compare the distribution of A(Li) with
planet size, multiplicity, orbital period, and insolation flux. We find larger
planets preferentially in younger systems, with an A-D two-sided test p-value =
0.002, a confidence that the older and younger planet samples do not
come from the same parent distribution. This is consistent with planet
inflation/photoevaporation at early ages. The other planet parameters (
planet multiplicity, orbital period, and insolation flux) are uncorrelated with
age.Comment: 22 pages, 15 figures, 3 tables. Accepted for publication in ApJ. For
a brief video explaining this paper, see https://youtu.be/TkO-ef28Va
Fiducial Stellar Population Sequences for the VJKs Photometric System
We have obtained broad-band near-infrared photometry for seven Galactic star
clusters (M92, M15, M13, M5, NGC1851, M71 and NGC6791) using the WIRCam
wide-field imager on the Canada-France-Hawaii Telescope, supplemented by images
of NGC1851 taken with HAWK-I on the VLT. In addition, 2MASS observations of the
[Fe/H] ~ 0.0 open cluster M67 were added to the cluster database. From the
resultant (V-J)-V and (V-Ks)-V colour-magnitude diagrams (CMDs), fiducial
sequences spanning the range in metallicity, -2.4 < [Fe/H] < +0.3, have been
defined which extend (for most clusters) from the tip of the red-giant branch
(RGB) to ~ 2.5 magnitudes below the main-sequence turnoff. These fiducials
provide a valuable set of empirical isochrones for the interpretation of
stellar population data in the 2MASS system. We also compare our newly derived
CMDs to Victoria isochrones that have been transformed to the observed plane
using recent empirical and theoretical colour-Teff relations. The models are
able to reproduce the entire CMDs of clusters more metal rich than [Fe/H] ~
-1.4 quite well, on the assumption of the same reddenings and distance moduli
that yield good fits of the same isochrones to Johnson-Cousins BV(RI)C
photometry. However, the predicted giant branches become systematically redder
than the observed RGBs as the cluster metallicity decreases. Possible
explanations for these discrepancies are discussed.Comment: 18 pages, 20 figures, Accepted for publication in A
Standard Cosmic Ray Energetics and Light Element Production
The recent observations of Be and B in metal poor stars has led to a
reassessment of the origin of the light elements in the early Galaxy. At low it
is metallicity ([O/H] < -1.75), it is necessary to introduce a production
mechanism which is independent of the interstellar metallicity (primary). At
higher metallicities, existing data might indicate that secondary production is
dominant. In this paper, we focus on the secondary process, related to the
standard Galactic cosmic rays, and we examine the cosmic ray energy
requirements for both present and past epochs. We find the power input to
maintain the present-day Galactic cosmic ray flux is about 1.5e41 erg/s = 5e50
erg/century. This implies that, if supernovae are the sites of cosmic ray
acceleration, the fraction of explosion energy going to accelerated particles
is about 30%, a value which we obtain consistently both from considering the
present cosmic ray flux and confinement and from the present 9Be and 6Li
abundances. Using the abundances of 9Be (and 6Li) in metal-poor halo stars, we
extend the analysis to show the effect of the interstellar gas mass on the
standard galactic cosmic ray energetic constraints on models of Li, Be, and B
evolution. The efficiency of the beryllium production per erg may be enhanced
in the past by a factor of about 10; thus the energetic requirement by itself
cannot be used to rule out a secondary origin of light elements. Only a clear
and undisputable observational determination of the O-Fe relation in the halo
will discriminate between the two processes. (abridged)Comment: 24 pages, LaTeX, uses aastex macro
Lithium and Lithium Depletion in Halo Stars on Extreme Orbits
We have determined Li abundances in 55 metal-poor (3.6 < [Fe/H] < -0.7) stars
with extreme orbital kinematics. We find the Li abundance in the Li-plateau
stars and examine its decrease in low-temperature, low-mass stars. The Li
observations are primarily from the Keck I telescope with HIRES (spectral
resolution of ~48,000 and median signal-to-noise per pixel of 140). Abundances
or upper limits were determined for Li for all the stars with typical errors of
0.06 dex. Our 14 stars on the Li plateau give A(Li) = log N(Li)/N(H) + 12.00 of
2.215 +-0.110, consistent with earlier results. We find a dependence of the Li
abundance on metallicity as measured by [Fe/H] and the Fe-peak elements [Cr/H]
and [Ni/H], with a slope of ~0.18. We also find dependences of A(Li) with the
alpha elements, Mg, Ca, and Ti. For the n-capture element, Ba, the relation
between A(Li) and [Ba/H] has a shallower slope of 0.13; over a range of 2.6 dex
in [Ba/H], the Li abundance spans only a factor of two. We examined the
possible trends of A(Li) with the characteristics of the orbits of our halo
stars, but find no relationship with kinematic or dynamic properties. The stars
cooler than the Li plateau are separated into three metallicity subsets. The
decrease in A(Li) sets in at hotter temperatures at high metallicities than at
low metallicities; this is in the opposite sense of the predictions for Li
depletion from standard and non-standard models.Comment: 29 pages including 3 tables and 12 figures Accepted by The
Astrophysical Journal, for the 1 November 2005 issue, v. 63
Early Universe Constraints on Time Variation of Fundamental Constants
We study the time variation of fundamental constants in the early Universe.
Using data from primordial light nuclei abundances, CMB and the 2dFGRS power
spectrum, we put constraints on the time variation of the fine structure
constant , and the Higgs vacuum expectation value leads to a variation
in the electron mass, among other effects. Along the same line, we study the
variation of and the electron mass . In a purely phenomenological
fashion, we derive a relationship between both variations.Comment: 18 pages, 12 figures, accepted for publication in Physical Review
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