2,636 research outputs found
A lithium depletion boundary age of 21 Myr for the Beta Pictoris moving group
Optical spectroscopy is used to confirm membership for 8 low-mass candidates
in the young Beta Pic moving group (BPMG) via their radial velocities,
chromospheric activity and kinematic parallaxes. We searched for the presence
of the Li I 6708A resonance feature and combined the results with literature
measurements of other BPMG members to find the age-dependent lithium depletion
boundary (LDB) -- the luminosity at which Li remains unburned in a coeval
group. The LDB age of the BPMG is 21 +/- 4 Myr and insensitive to the choice of
low-mass evolutionary models. This age is more precise, likely to be more
accurate, and much older than that commonly assumed for the BPMG. As a result,
substellar and planetary companions of BPMG members will be more massive than
previously thought.Comment: Accepted for MNRAS Letter
An XMM-Newton observation of the young open cluster NGC 2547: coronal activity at 30 Myr
We report XMM-Newton observations of the young open cluster NGC 2547 which
allow us to characterise coronal activity in solar-type stars at an age of 30
Myr. X-ray emission peaks among G-stars at luminosities (0.3-3keV) of
Lx~10^{30.5} erg/s and declines to Lx<=10^{29.0} erg/s among M-stars. Coronal
spectra show evidence for multi-temperature differential emission measures and
low coronal metal abundances (Z~0.3). The G- and K-type stars follow the same
relationship between X-ray activity and Rossby number established in older
clusters and field stars, although most solar-type stars in NGC 2547 exhibit
saturated/super-saturated X-ray activity levels. Median levels of Lx and
Lx/Lbol in the solar-type stars of NGC 2547 are similar to T-Tauri stars of the
Orion Nebula cluster (ONC), but an order of magnitude higher than in the older
Pleiades. The spread in X-ray activity levels among solar-type stars in NGC
2547 is much smaller than in older or younger clusters. Coronal temperatures
increase with Lx, Lx/Lbol and surface X-ray flux. Active solar-type stars in
NGC 2547 have coronal temperatures between those in the ONC and the most active
older ZAMS stars. A flaring rate (for total flare energies [0.3-3keV] >10^{34}
erg) of 1 every 350^{+350}_{-120} ks was found for solar-type stars, similar to
rates found in the ONC and Pleiades. Comparison with ROSAT HRI data taken 7
years previously reveals that only 10-15 percent of solar-type stars or stars
with Lx>3x10^{29} erg/s exhibit X-ray variability by more than a factor of two.
The similar levels of X-ray activity and rate of occurrence for large flares in
NGC 2547 and the ONC demonstrate that the X-ray radiation environment around
young solar-type stars remains relatively constant over their first 30 Myr
(abridged).Comment: Accepted for publication in MNRAS. Electronic tables available from
the autho
Transition probabilities in OH A 2 sigma + - X 2 pi i: Bands with v prime = 0 and 1, v double prime = 0 to 4
Experimental results for relative vibrational band transition probabilities for v prime = 0 and 1, and v double prime = 0 to 4 in the A-X electronic system of OH are presented. The measurements, part of a larger set involving v prime = 0 to 4 and v double prime = 0 to 6, were made using spectrally dispersed laser-induced fluorescence (LIF) in the burnt gases of a flame. These Einstein coefficients will be useful in dynamics experiments for quantitative LIF determinations of OH radical concentrations in high v double prime
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A manual for assessing post-harvest fisheries losses
This manual describes three loss assessment methods developed in the artisanal postharvest fishery sector for use by researchers working for the development of artisanal fisheries in countries worldwide. The methods are the Informal Fish Loss Assessment Method (IFLAM), Load Tracking (LT) and the Questionnaire Loss Assessment Method (QLAM). They are designed to give the user the quantitative and qualitative understanding of post-harvest fish losses required for planning loss reduction measures. The improved understanding generated by application of the methods will also inform the fisheries planner and policy-maker with regard to decisions concerning the development of the artisanal post-harvest sector and the improvement of the livelihoods of those working in the sector
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
Lithium abundances from the 6104A line in cool Pleiades stars
Lithium abundances determined by spectral synthesis from both the 6708A
resonance line and the 6104 subordinate line are reported for 11 late-type
Pleiades stars, including spectra previously analysed by Russell (1996). We
report a 0.7 dex scatter in the abundances from 6708A, and a scatter at least
as large from the 6104A line. We find a reasonable correllation between the
6104A and 6708A Li abundances, although four stars have 6104A-determined
abundances which are significantly larger than the 6708-determined values, by
up to 0.5 dex, suggesting problems with the homogeneous, one-dimensional
atmospheres being used. We show that these discrepancies can be explained,
although probably not uniquely, by the presence of star spots with plausible
coverage fractions. The addition of spots does not significantly reduce the
apparent scatter in Li abundances, leaving open the possibility that at least
some of the spread is caused by real star-to-star differences in pre-main-
sequence Li depletion.Comment: 13 pages, 7 figures; Accepted by A&A 17/05/0
Ages of young stars
Determining the sequence of events in the formation of stars and planetary
systems and their time-scales is essential for understanding those processes,
yet establishing ages is fundamentally difficult because we lack direct
indicators. In this review we discuss the age challenge for young stars,
specifically those less than ~100 Myr old. Most age determination methods that
we discuss are primarily applicable to groups of stars but can be used to
estimate the age of individual objects. A reliable age scale is established
above 20 Myr from measurement of the Lithium Depletion Boundary (LDB) in young
clusters, and consistency is shown between these ages and those from the upper
main sequence and the main sequence turn-off -- if modest core convection and
rotation is included in the models of higher-mass stars. Other available
methods for age estimation include the kinematics of young groups, placing
stars in Hertzsprung-Russell diagrams, pulsations and seismology, surface
gravity measurement, rotation and activity, and lithium abundance. We review
each of these methods and present known strengths and weaknesses. Below ~20
Myr, both model-dependent and observational uncertainties grow, the situation
is confused by the possibility of age spreads, and no reliable absolute ages
yet exist. The lack of absolute age calibration below 20 Myr should be borne in
mind when considering the lifetimes of protostellar phases and circumstellar
material.Comment: Accepted for publication as a chapter in Protostars and Planets VI,
University of Arizona Press (2014), eds. H. Beuther, R. Klessen, C.
Dullemond, Th. Hennin
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
Low-mass members of the young cluster IC 4665 and pre-main-sequence lithium depletion
We have used fibre spectroscopy to establish cluster membership and examine
pre-main-sequence (PMS) lithium depletion for low-mass stars (spectral types F
to M) in the sparse young (~30 Myr) cluster IC 4665. We present a filtered
candidate list of 40 stars that should contain 75 per cent of single cluster
members with V of 11.5 to 18 in the central square degree of the cluster.
Whilst F- and G-type stars in IC 4665 have depleted little or no lithium, the
K- and early M-type stars have depleted more Li than expected when compared
with similar stars in other clusters of known age. An empirical age estimate
based on Li-depletion among the late-type stars of IC 4665 would suggest it is
older than 100 Myr. This disagrees entirely with ages determined either from
the nuclear turn-off, from isochronal matches to low-mass stars or from the
re-appearance of lithium previously found in much lower mass stars (the
``lithium depletion boundary''). We suggest that other parameters besides age,
perhaps composition or rotation, are very influential in determining the degree
of PMS Li-depletion in stars with M greater than 0.5 Msun. Further work is
required to identify and assess the effects of these additional parameters,
particularly to probe conditions at the interface between the sub-photospheric
convection zone and developing radiative core. Until then, PMS Li depletion in
F- to early M-type stars cannot be confidently used as a precise age indicator
in young clusters, kinematic groups or individual field stars.Comment: Accepted for publication in MNRA
Elemental abundances in the Blanco 1 open cluster
High resolution spectroscopy is used to determine the detailed chemical
abundances of a group of eight F- and G-type stars in the young open cluster
Blanco 1. An average [Fe/H] of +0.04 +/- 0.02 (internal error) +/- 0.04
(external error) is found, considerably lower than a previous spectroscopic
estimate for this cluster. The difference is due mainly to our adoption of
significantly cooler temperatures which are consistent with both photometric
and spectroscopic constraints. Blanco~1 exhibits sub-solar [Ni/Fe] (-0.18 +/-
0.01 +/- 0.01), [Si/Fe] (-0.09$ +/- 0.02 +/- 0.03), [Mg/Fe] (-0.14 +/- 0.02 +/-
0.03) and [Ca/Fe] (-0.09 +/- 0.03 +/- 0.03); ratios which are not observed
among nearby field stars. The material from which Blanco 1 formed may not have
been well mixed with interstellar matter in the galactic disc, which tallies
with its current location about 240pc below the galactic plane. A simultaneous
deficit of Ni and alpha elements with respect to Fe is hard to reconcile with
most published models of yields from supernovae of types Ia and II. The revised
abundances for Blanco 1 indicate that overall radiative opacities in its stars,
and hence convective zone properties at a given mass, are similar to those in
the Pleiades at approximately the same age. This can explain a previous
observation that the Li depletion patterns of G- and K-type stars in the two
clusters are indistinguishable. The lower overall metallicity of Blanco 1 now
make it less attractive as a target for discovering transiting, short period
exoplanets.Comment: Accepted for MNRAS, 12 page
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