The star formation history of Trumpler 14 and Trumpler 16

H-R diagrams are presented for the very young galactic clusters Trumpler 14 and Trumpler 16, which are the two most populous clusters in the region of vigorous star formation surrounding η Carinae. Point spread function photometry of UBV CCD images is presented down to V~19 for over 560 stars in Tr 16 and 290 stars in Tr 14. We have also obtained similar data for a local background field. After determining cluster membership through proper motions from a previous work, we find that the reddening of cluster members is significantly lower than that of the local background stars. Thus, we are able to use individual reddenings to identify likely members at far deeper levels than possible with proper motions. This work has revealed a significant population of pre-main-sequence (PMS) stars in both clusters. The location of the PMS stars in the H-R diagram indicates that the theoretical ``stellar birthline'' of Palla & Stahler follows the locus of stars far better than that of Beech & Mitalas. Comparison with both pre- and post-main-sequence isochrones also reveals that although intermediate-mass stars have been forming continuously over the last 10 Myr, the high-mass stars formed within the last 3 Myr. There is no evidence that the formation of the intermediate-mass stars was truncated by the formation of the high-mass stars

A Keck High Resolution Spectroscopic Study of the Orion Nebula Proplyds

We present the results of spectroscopy of four bright proplyds in the Orion Nebula obtained at a velocity resolution of 6 km/s. After careful isolation of the proplyd spectra from the confusing nebular radiation, the emission line profiles are compared with those predicted by realistic dynamic/photoionization models of the objects. The spectral line widths show a clear correlation with ionization potential, which is consistent with the free expansion of a transonic, ionization-stratified, photoevaporating flow. Fitting models of such a flow simultaneously to our spectra and HST emission line imaging provides direct measurements of the proplyd size, ionized density and outflow velocity. These measurements confirm that the ionization front in the proplyds is approximately D-critical and provide the most accurate and robust estimate to date of the proplyd mass loss rate. Values of 0.7E-6 to 1.5E-6 Msun/year are found for our spectroscopic sample, although extrapolating our results to a larger sample of proplyds implies that 0.4E-6 Msun/year is more typical of the proplyds as a whole. In view of the reported limits on the masses of the circumstellar disks within the proplyds, the length of time that they can have been exposed to ionizing radiation should not greatly exceed 10,000 years - a factor of 30 less than the mean age of the proplyd stars. We review the various mechanisms that have been proposed to explain this situation, and conclude that none can plausibly work unless the disk masses are revised upwards by a substantial amount.Comment: 23 pages, 8 figures, uses emulateapj.sty, accepted for publication in The Astronomical Journal (scheduled November 1999

Absolute Calibration of Image Plate for electrons at energy between 100 keV and 4 MeV

The authors measured the absolute response of image plate (Fuji BAS SR2040) for electrons at energies between 100 keV to 4 MeV using an electron spectrometer. The electron source was produced from a short pulse laser irradiated on the solid density targets. This paper presents the calibration results of image plate Photon Stimulated Luminescence PSL per electrons at this energy range. The Monte Carlo radiation transport code MCNPX results are also presented for three representative incident angles onto the image plates and corresponding electron energies depositions at these angles. These provide a complete set of tools that allows extraction of the absolute calibration to other spectrometer setting at this electron energy range

Applied Plasma Research

Contains research objectives, summary of research and reports on four research projects.National Science Foundation (Grant GK-28282X1)National Science Foundation (Grant GK-33843

Racial and Ethnic Differences Associated With Feeding- and Activity-Related Behaviors in Infants

To examine parental reports of feeding and activity behaviors in a cohort of parents of 2-month-olds and how they differ by race/ethnicity

A continental analysis of ecosystem vulnerability to atmospheric nitrogen deposition

Atmospheric nitrogen (N) deposition has been shown to decrease plant species richness along regional deposition gradients in Europe and in experimental manipulations. However, the general response of species richness to N deposition across different vegetation types, soil conditions, and climates remains largely unknown even though responses may be contingent on these environmental factors. We assessed the effect of N deposition on herbaceous richness for15,136 forest, woodland, shrubland, and grassland sites across the continental United States, to address how edaphic and climatic conditions altered vulnerability to this stressor. In our dataset, with N deposition ranging from 1 to 19 kg N·ha−1·y−1, we found a unimodal relationship; richness increased at low deposition levels and decreased above 8.7 and 13.4 kg N·ha−1·y−1 in open and closed-canopy vegetation, respectively. N deposition exceeded critical loads for loss of plant species richness in 24% of 15,136 sites examined nationwide. There were negative relationships between species richness and N deposition in 36% of 44 community gradients. Vulnerability to N deposition was consistently higher in more acidic soils whereas the moderating roles of temperature and precipitation varied across scales. We demonstrate here that negative relationships between N deposition and species richness are common, albeit not universal, and that fine-scale processes can moderate vegetation responses to N deposition. Our results highlight the importance of contingent factors when estimating ecosystem vulnerability to N deposition and suggest that N deposition is affecting species richness in forested and nonforested systems across much of the continental United States