3,999 research outputs found
Cool stars in NGC 2547 and pre main sequence lithium depletion
We present the results of a spectroscopic survey of X-ray selected, low-mass
candidate members of the young open cluster NGC 2547. Using a combination of
photometry, spectroscopic indices and radial velocities we refine our candidate
list and then use our spectroscopy to study the progression of lithium
depletion in low-mass pre main sequence stars. We derive lithium abundances or
upper limits for all our candidate members, which have effective temperatures
in the range 5000>Teff>3200K, and compare these with predictions for lithium
burning and depletion provided by a number of models and also with the lithium
depletion seen in younger and older stars. We find that some models can
reproduce the lithium abundance pattern of NGC 2547 if the cluster has an age
of ~20-35Myr, which is also indicated by fits to low-mass isochrones in the
Hertzsprung-Russell diagram. But the lack of significant further lithium
depletion between NGC 2547 and older clusters argues for an age of at least
50Myr, more in keeping with the lack of lithium observed in even fainter NGC
2547 candidates. We show that reconciliation of these age estimates may require
additions to the physics incorporated in current generations of pre main
sequence models.Comment: Accepted for publication in MNRAS (better version of Fig.1 available
at http://www.astro.keele.ac.uk/~rdj/
Pacific bonito management information document
Management of Pacific bonito in California is examined in this Management Information Document by a State-Federal team of scientists.
Abundance of Pacific bonito in southern California has fallen dramatically between the 1963-1969 period and the 1974-1977 period. Since 1976 the commercia1 fleet has found few large fish in southern California, and has caught fish in the size range of 15 to 57 cm (1.2 to 4.7 pounds). This fact, coupled with the low abundance indices, point out the need for a more active management regime.
To develop management measures for the California bonito
fishery both a surplus yield analysis and a yield-per-recruit analysis were performed. A maximum sustained yield of 10,000 short tons was estimated for the fishery in southern California, while the whole fishery, including Baja California, has an estimated MSY of 13,000 tons. In order to achieve this level of catch, however, the stock abundance must be increased by a factor of five.
Yield-per-recruit considerations suggest that a minimum
size limit in the commercial fishery has two important effects. A three-pound size limit could result in a slight increase in yield-per-recruit. If the size limit is increased to 5 or 7.5 lbs, the yield-per-recruit would fall significantly. Offsetting the effect on yield-per-recruit, however, would be a substantial increase in average amount of spawning per recruit which should result in a proportional increase in recruitment. With the current depressed stock abundance both a reduced annual take and
a minimum size limit on commercial catch would confer
substantial benefits in the form of an increase in the future stock size.
After considering seven different types of management
measures, the team finds that three types -- an annual commercial catch quota, a commercial size limit, and a recreational bag limit -- appear desirable.
Re-establishment of the stock in southern California was
the major consideration in this evaluation because the stock is currently depressed. All segments of the fishery will benefit from a more abundant resource. The difficult issues for policy, however, concern the rate of rebuilding, the degree of risk that is acceptable, and the distribution of benefits among user groups. By judicious choice among the options discussed here, a variety of positions can be established with respect to these issues. The greater the
size limit, for instance, the more benefit is provided the
recreational sector while difficulties are imposed upon commercial fishermen. The higher the quotas adopted, the
slower the stock rebuilding and the greater the risk of continued stock depletion. A final reconciliation of the management options involves social, political and legal considerations which must be thoroughly incorporated by decision-makers before adoption of a management plan. (93pp.
Ages, Distances, and the Initial Mass Functions of Stellar Clusters
We provide a review of the current status of several topics on the ages,
distances, and mass functions of open clusters, with a particular emphasis on
illuminating the areas of uncertainty. Hipparcos has obtained parallaxes for
nearby open clusters that have expected accuracies much better than has been
previously achievable. By using the lithium depletion boundary method and
isochrone fitting based on much improved new theoretical evolutionary models
for low mass stars, it is arguable that we will soon have have much better age
scales for clusters and star-forming regions. With improved optical and near-IR
cameras, we are just now beginning to extend the mass function of open clusters
like the Pleiades into the regime below the hydrogen burning mass limit.
Meanwhile, observations in star-forming regions are in principle capable of
identifying objects down to of order 10 Jupiter masses.Comment: 13 pages, including 3 embedded figures (4 EPS files). To appear in
"11th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun," ed. R.
J. Garcia Lopez, R. Rebolo, and M. R. Zapatero Osori
The lower mass function of the young open cluster Blanco 1: from 30 M_(Jup) to 3 M_â
Aims. We performed a deep wide field optical survey of the young (~100â150 Myr) open cluster Blanco 1 to study its low mass population well down into the brown dwarf regime and estimate its mass function over the whole cluster mass range.
Methods. The survey covers 2.3 square degrees in the I and z-bands down to I â z â 24 with the CFH12K camera. Considering two different cluster ages (100 and 150 Myr), we selected cluster member candidates on the basis of their location in the (I, I â z) CMD
relative to the isochrones, and estimated the contamination by foreground late-type field dwarfs using statistical arguments, infrared photometry and low-resolution optical spectroscopy.
Results. We find that our survey should contain about 57% of the cluster members in the 0.03â0.6 M_â mass range, including 30â40 brown dwarfs. The candidateâs radial distribution presents evidence that mass segregation has already occured in the cluster. We took it into account to estimate the cluster mass function across the stellar/substellar boundary. We find that, between 0.03 M_â
and 0.6 M_â, the cluster mass distribution does not depend much on its exact age, and is well represented by a single power-law, with an index α = 0.69 ± 0.15. Over the whole mass domain, from 0.03 M_â to 3 M_â, the mass function is better fitted by a log-normal function with m_0 = 0.36 ± 0.07 M_â and Ï = 0.58 ± 0.06.
Conclusions. Comparison between the Blanco 1 mass function, other young open clustersâ MF, and the galactic disc MF suggests that
the IMF, from the substellar domain to the higher mass part, does not depend much on initial conditions. We discuss the implications
of this result on theories developed to date to explain the origin of the mass distribution
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