Maria Mitchell Association

Animals with polyploid, hybrid nuclei offer a challenge for models of gene expression and regulation during embryogenesis. To understand how such organisms proceed through development, we examined the timing and prevalence of mortality among embryos of unisexual salamanders in the genus Ambystoma.

Our regional field surveys suggested that heightened rates of embryo mortality among unisexual salamanders begin in the earliest stages of embryogenesis. Although we expected elevated mortality after zygotic genome activation in the blastula stage, this is not what we found among embryos which we reared in the laboratory. Once embryos entered the first cleavage stage, we found no difference in mortality rates between unisexual salamanders and their bisexual hosts. Our results are consistent with previous studies showing high rates of unisexual mortality, but counter to reports that heightened embryo mortality continues throughout embryo development.

Possible causes of embryonic mortality in early embryogenesis suggested by our results include abnormal maternal loading of RNA during meiosis and barriers to insemination. The surprising survival rates of embryos post-cleavage invites further study of how genes are regulated during development in such polyploid hybrid organisms.

The Fermi Bubbles are two giant gamma-ray emitting lobes extending 55° above and below the Galactic center. While the Northern Bubble has been extensively studied in ultraviolet (UV) absorption, little is known about the gas kinematics of the southern Bubble. We use UV absorption-line spectra from the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope to probe the southern Fermi Bubble using a sample of 17 background AGNs projected behind or near the Bubble. We measure the incidence of high-velocity clouds (HVC), finding that 4 out of 6 sightlines passing through the Bubble show HVC absorption, versus 6 out of 11 passing outside. We find strong evidence that the maximum absolute LSR velocity of the HVC components decreases as a function of galactic latitude within the Bubble, for both blueshifted and redshifted components, as expected for a decelerating outflow. We explore whether the column density ratios Si iv/Si iii, Si iv/Si ii, and Si iii/Si ii correlate with the absolute galactic latitude within the Bubble. These results demonstrate the use of UV absorption-line spectroscopy to characterize the kinematics and ionization conditions of embedded clouds in the Galactic center outflow.

New optical spectra have been obtained with VLT/FORS2 of the final helium shell flash (FF) star, V605 Aql, which peaked in brightness in 1919.New models suggest that this star is experiencing a very late thermal pulse.The evolution to a cool luminous giant and then back to a compact hot star takes place in only a few years.V605 Aql, the central star of the planetary nebula (PN) A58, has evolved from T_eff ∼ 5000 K in 1921 to ∼95,000 K today.There are indications that the new FF star, Sakurai's object (V4334 Sgr), which appeared in 1996, is evolving along a similar path.The abundances of Sakurai's object today and V605 Aql 80 years ago mimic the hydrogen-deficient R Coronae Borealis (RCB) stars, with 98% He and 1% C.The new spectra show that V605 Aql has stellar abundances similar to those seen in Wolf-Rayet [WC] central stars of PNe, with ∼55% He, and ∼40% C.The stellar spectrum of V605 Aql can be seen even though the star is not directly detected.Therefore, we may be seeing the spectrum in light scattered around the edge of a thick torus of dust seen edge-on.In the present state of evolution of V605 Aql, we may be seeing the not too distant future of Sakurai's object.