YOUNG STELLAR OBJECTS IN THE IRAS POINT-SOURCE CATALOG

The IRAS Point Source Catalog (IPSC), providing an infrared survey of 96% of the sky, is searched for low mass young stellar objects (YSOs). Three different statistical approaches are pursued to classify an initial set of 69436 sources out of the total of 245889 IPSC objects. The performance of the classifiers is assessed by means of false alarm and hit rates. The similarity, in infrared properties, to a combined sample of YSOs taken from Beichman et al. (1986) and Myers et al. (1987) is exploited by two simple classifiers to produce rank-ordered lists of YSO candidates. A third, two-stage classifier utilizes the infrared properties of galactic and extragalactic objects in conjunction with the clustering properties of YSOs on the sky. This combination considerably improves the discrimination between YSOs and other galactic sources and selects a sample of 5 825 YSO candidates lying outside of confused regions (viz. the galactic plane and the Magellanic clouds). According to our performance analysis 87% or 5068 of these YSO candidates are indeed YSOs (implying a contamination of 13% or 757 misclassified sources), while the selection reaches a completeness of 85%. From these figures-of-merit we can estimate that the initial sample contains in total 5962 YSOs outside the confused areas. The quality of this classification is empirically verified by examining IPSC sources from the initial sample in the well-studied Chamaeleon I star forming region

EXTENDED SOFT-X-RAY EMISSION IN THE SEYFERT-2 GALAXY NGC-4388 DISCOVERED WITH THE ROSAT HRI

Extended (R similar to 4.5 kpc) X-ray emission in the Seyfert 2 galaxy NGC 4388 has been discovered with the ROSAT HRI. No evidence for a nuclear, unresolved component has been found; the upper limit for such a component is about 20% of the total flux. The interpretation of the extended emission in terms of nuclear radiation scattered by a plasma ionized by the nuclear photons can be excluded on the basis of a simple photoionization calculation. The two most likely alternatives, i.e. thermal emission from a collisionally heated plasma and the sum from many unresolved discrete sources, are briefly discussed

HDL cholesterol levels are an important factor for determining the lifespan of erythrocytes

Objective. Scavenger receptor class B, type I (SR-BI) is a multifunctional receptor that promotes the selective uptake of cholesteryl esters from high-density lipoprotein (HDL). Disruption of SR-BI in mice results in a dramatic increase in HDL cholesterol. Interestingly, mice lacking SR-BI also develop anemia, as evidenced by accumulation of reticulocytes in the circulation. The objective of the current study was to delineate the mechanism underlying development of anemia in the absence of SR-BI. Methods. Expression of important mediators of erythropoiesis, as well as key enzymes in the degradation of erythrocytes, were analyzed using real-time polymerase chain reaction in SR-BI wild-type and SR-BI knockout mice. In addition, in vivo studies were performed using bio-tinylated erythrocytes to determine erythrocyte survival. Results. mRNA expression of TAL-1, GATA-1, FOG-1, erythropoietin receptor, and ferrochelatase, important mediators of erythropoiesis, was increased in spleens of SR-BI-deficient mice. In addition, the relative amount of early Ter119(high)CD71(high) -expressing erythroblasts was increased in SR-BI-deficient spleens. Interestingly, also expression of hemeoxygenase 1 and biliverdin reductase, enzymes involved in the degradation of erythrocytes, was increased. Furthermore, an elevated amount of conjugated bilirubin, the breakdown product of hemoglobin, was found in bile. Using biotinylated erythrocytes, we show that survival of erythrocytes was decreased in SR-BI-deficient mice. Thus, the observed increased erythropoiesis in the SR-BI-deficient mice is most likely a direct response to the reduced erythrocyte lifespan. Finally, we show that increased HDL cholesterol levels due to SR-BI deficiency induce erythrocyte cholesterol: phospholipid ratios, resulting in decreased deformability and increased osmotic fragility, thereby providing an explanation for the observed reduced lifespan. Conclusions. SR-BI is not only essential for HDL cholesterol homeostasis and atherosclerosis susceptibility, but also for maintaining normal erythrocyte lifespan. (c) 2005 Internatioinal Society for Experimental Hematology. Published by Elsevier Inc

The Evolution of Neutron Star Magnetic Fields

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