Large granulation cells on the surface of the giant star π1 Gruis.

Convection plays a major part in many astrophysical processes, including energy transport, pulsation, dynamos and winds on evolved stars, in dust clouds and on brown dwarfs. Most of our knowledge about stellar convection has come from studying the Sun: about two million convective cells with typical sizes of around 2,000 kilometres across are present on the surface of the Sun-a phenomenon known as granulation. But on the surfaces of giant and supergiant stars there should be only a few large (several tens of thousands of times larger than those on the Sun) convective cells, owing to low surface gravity. Deriving the characteristic properties of convection (such as granule size and contrast) for the most evolved giant and supergiant stars is challenging because their photospheres are obscured by dust, which partially masks the convective patterns. These properties can be inferred from geometric model fitting, but this indirect method does not provide information about the physical origin of the convective cells. Here we report interferometric images of the surface of the evolved giant star π1 Gruis, of spectral type S5,7. Our images show a nearly circular, dust-free atmosphere, which is very compact and only weakly affected by molecular opacity. We find that the stellar surface has a complex convective pattern with an average intensity contrast of 12 per cent, which increases towards shorter wavelengths. We derive a characteristic horizontal granule size of about 1.2 × 1011 metres, which corresponds to 27 per cent of the diameter of the star. Our measurements fall along the scaling relations between granule size, effective temperature and surface gravity that are predicted by simulations of stellar surface convection.

The temperature and chronology of heavy-element synthesis in low-mass stars.

Roughly half of the heavy elements (atomic mass greater than that of iron) are believed to be synthesized in the late evolutionary stages of stars with masses between 0.8 and 8 solar masses. Deep inside the star, nuclei (mainly iron) capture neutrons and progressively build up (through the slow-neutron-capture process, or s-process) heavier elements that are subsequently brought to the stellar surface by convection. Two neutron sources, activated at distinct temperatures, have been proposed: (13)C and (22)Ne, each releasing one neutron per α-particle ((4)He) captured. To explain the measured stellar abundances, stellar evolution models invoking the (13)C neutron source (which operates at temperatures of about one hundred million kelvin) are favoured. Isotopic ratios in primitive meteorites, however, reflecting nucleosynthesis in the previous generations of stars that contributed material to the Solar System, point to higher temperatures (more than three hundred million kelvin), requiring at least a late activation of (22)Ne (ref. 1). Here we report a determination of the s-process temperature directly in evolved low-mass giant stars, using zirconium and niobium abundances, independently of stellar evolution models. The derived temperature supports (13)C as the s-process neutron source. The radioactive pair (93)Zr-(93)Nb used to estimate the s-process temperature also provides, together with the pair (99)Tc-(99)Ru, chronometric information on the time elapsed since the start of the s-process, which we determine to be one million to three million years.

Discovery of three lead-rich stars.

About half of the stable nuclei heavier than iron are believed to be synthesized during the late stages of evolution of stars with masses in the range 0.8-8 solar masses. These elements are then expelled into the interstellar medium through stellar winds after being 'dredged up' towards the surface of the stars. These processes occur when the star is in the 'asymptotic giant branch' (AGB) phase of its life. Nuclei (mainly iron) deep inside the star slowly capture neutrons and progressively build up heavier elements (the 's-process'). For AGB stars that formed early in the history of the Galaxy, and that therefore have very low abundances of elements heavier than helium ('metals'), models predict that the s-process will accumulate synthesized material with atomic weights in the Pb-Bi region. Such stars will therefore have large overabundances of lead relative to other heavy elements. Here we report the discovery of large amounts of lead in three metal-poor stars (HD187861, HD196944 and HD224959). Our analysis shows that these stars are more enriched in lead than in any other element heavier than iron. The excellent agreement between the observed and predicted abundances reinforces our current understanding of the detailed operation of the s-process deep in the interiors of AGB stars.

The incidence of cervical intraepithelial neoplasia among women with renal transplant in relation to cyclosporine.

OBJECTIVE: To evaluate the policy of an annual smear to screen renal transplant recipients for cervical intraepithelial neoplasia and invasive carcinoma and to determine the incidence of abnormal smears and CIN before and after the introduction of cyclosporine (1983). DESIGN: A retrospective study over the period 1971 to 1992. SUBJECTS: Postmenarchial women who received renal transplants and who were on immunosuppressive treatment for at least one month. MEAN OUTCOME MEASURES: Cytology and histology results. RESULTS: A total of 144 women who received renal transplantation were eligible for our study. Observation time varied from 1 to 227 months (median 59 months) with a mean for the group transplanted before 1983 (Group A) of 103 months, and for the group transplanted after 1983 (Group B) of 46 months. Of these women, 25 had an abnormal smear. Of these, 14 were confirmed by histology and repeated smears of the other 11 patients were negative. Within the 60 women in Group A with an abnormal smear, six had CIN I or CIN II, three had CIN III and one showed adenocarcinoma of the endometrium. Among the 84 women in Group B, four had CIN I or CIN II and none had CIN III. The overall incidence of abnormal cytology was 17.3%, with no invasive cervical carcinoma in this group. CONCLUSIONS: Our policy of screening is adequate. With the introduction of cyclosporine the incidence of abnormal cytology and histology has a tendency to decrease. However, the duration of risk is not comparable yet.

[Gram staining and bacterial culture of stomach contents of the full-term neonate in prolonged status of ruptured membranes is not useful]. / Gram-preparaat en bacteriekweek van de maaginhoud van de voldragen pasgeborene bij langdurig gebroken vliezen niet zinvol.

In an effort to evaluate the management of full-term neonates born after prolonged rupture of the membranes, we studied 159 full-term newborn infants born 24 hours or more after rupture of the membranes retrospectively. Gram stain and bacterial culture of a gastric aspirate of the neonates were significantly related; however, the predictive value of the Gram stain was low, both for a positive test (46%) and for a negative test (70%). A significant relation was not found between the Gram stain (presence of bacteria or greater than or equal to 4 leucocytes per high power field) or the bacterial culture on the one hand and the occurrence of clinical symptoms of infection on the other. In all cases the decision to treat the newborn with antibiotics was made on the basis of the clinical symptoms alone. In our opinion the routine study of Gram stain and bacterial culture of a gastric aspirate of full-term neonates born after prolonged rupture of the membranes is not useful; observation alone is sufficient.