We present multicolor Hubble Space Telescope (HST) WFPC2 broadband observations of the Type Ic SN 1994I obtained image280 d after maximum light. We measure the brightness of the SN and, relying on ...the detailed spectroscopic database of SN 1994I, we transform the ground-based photometry obtained at early times to the HST photometric system, deriving light curves for the WFPC2 F439W, F555W, F675W, and F814W passbands that extend from 7 days before to 280 days after maximum. We use the multicolor photometry to build a quasi-bolometric light curve of SN 1994I, and compare it with similarly constructed light curves of other supernovae. In doing so, we propose and test a scaling in energy and time that allows for a more meaningful comparison of the exponential tails of different events. Through comparison with models, we find that the late-time light curve of SN 1994I is consistent with that of spherically symmetric ejecta in homologous expansion, for which the ability to trap the -rays produced by the radioactive decay of super(56)Co diminishes roughly as the inverse of time squared. We also find that by the time of the HST photometry, the light curve was significantly energized by the annihilation of positrons.
To ascertain whether repeated hypoxic stress would alter the response of the adrenal cortex to adrenocorticotropic hormone (ACTH), by premature activation of the hypothalamic-pituitary-adrenal axis, ...we studied fetal sheep subjected to daily reduction of arterial oxygen content by embolization of the fetal placental circulation with 15 microns microspheres for 8 days from about day 124 of gestation (term approximately 147 days) and sham-embolized controls. Starting before the final embolization (or sham-embolization) on day 8, and continuing for 24 h, the fetus was given an intravenous infusion of ACTH1-24 (0.5 microgram/h) or vehicle. Fetal and maternal blood samples were taken for determination of immunoreactive cortisol, and regional adrenal and fetal placental blood flows were measured by the microsphere technique at three time points: 1 h before infusion, 3 h after the start of the infusion (1 h after embolization), and after 24 h of infusion. Prior to infusion of ACTH or vehicle, fetal placental blood flow was lower in microsphere-embolized fetuses than in sham-embolized controls (199 +/ 15 vs 292 +/- 25 ml/min per 100 g tissue; mean +/- S.E.; P<0.01). However, plasma cortisol and adrenal cortical blood flow did not differ between embolized fetuses and controls. Adrenal vascular responses to the 24-h infusion of ACTH were similar in embolized and sham-embolized fetuses. Adrenal cortical blood flow increased 3-fold (P<0.05) due to decreased vascular resistance (P<0.01), with no change in adrenal medullary blood flow. Thus, while daily embolization of the fetal placental circulation caused a sustained decrease in cotyledonary blood flow, no evidence of altered responsiveness of the adrenal cortex to ACTH was found in these experiments.
Objective: To examine whether the increase in adrenal cortical blood flow induced by adrenocorticotropic hormone (ACTH) is linked to steroidogenesis, or if these effects can be separated.
Study ...design: Adrenal cortical responses to ACTH were measured in ovine fetuses at 105–112 days gestation, when the adrenal is hyporesponsive. Fetuses were given an intravenous infusion of ACTH
1–24 or of vehicle. We measured regional adrenal blood flows by the microsphere technique and plasma ACTH and cortisol levels by radioimmunoassay.
Results: After 3 and 24 h of ACTH infusion, plasma cortisol concentrations had risen from 2.8±0.8 ng/ml to 8.4±0.3 and 78.0±15.8 ng/ml (means±S.E.M.). Adrenal cortical blood flow increased from 108±25 ml/min/100 g to 319±33 and 518±51 ml/min/100 g. Plasma cortisol levels and adrenal blood flow did not change in control fetuses.
Conclusion: Since there was a rapid blood flow response to ACTH, despite the expected delay in the rise of plasma cortisol, we suggest that vascular responses to ACTH are not tightly linked to adrenal metabolic activity in the ovine fetus at 0.7 gestation.
While the broad-lined Type Ic supernovae (SN Ic-bl) associated with long-duration gamma-ray bursts (GRBs) have been studied, we do not fully understand the conditions that lead to each kind of ...explosion in a massive star. Here we show clues as to the production mechanism of GRBs by comparing the chemical abundances at the sites of 5 nearby (z < 0.25) broad-lined SN Ic that accompany nearby GRBs with those of 12 nearby (z < 0.14) broad-lined SN Ic that have no observed GRBs. We show that the oxygen abundances at the GRB sites are systematically lower than those found near ordinary broad-lined SN Ic. A unique feature of this analysis is that we present new spectra of the host galaxies and analyze the measurements of both samples in the same set of ways, using 3 independent metallicity diagnostics. We demonstrate that neither SN selection effects (SN found via targeted vs. non-targeted surveys) nor the choice of strong-line metallicity diagnostic can cause the observed trend. Though our sample size is small, the observations are consistent with the hypothesis that low metal abundance is the cause of some massive stars becoming SN-GRB. We derive a cut-off metallicity of 0.2−0.6 Z⊙, with the exact value depending on the adopted metallicity scale and solar abundance value.
PDF
