Last

rasc.ca

insidethisissue - The Royal Astronomical Society of Canada

of the identified science builds on the large-scale surveyscurrently being done with the Arecibo L-band Feed Array(ALFA). These include (1) a pulsar survey that is sensitive tocompact relativistic binary pulsars and millisecond pulsars; (2)surveys for extragalactic hydrogen that are determining the HImass function in the local Universe among many other results;(3) galactic surveys of hydrogen to probe turbulence in theinterstellar medium; (4) Faraday tomography of galactic andextragalactic magnetic fields; and (5) SETI.” Taylor, at the U ofC, is involved with a survey of galactic magnetic fields dubbedthe GALFA Continuum Transit Survey (GALFACTS). This is afive-year project, which “promises a transformational advancein our understanding of the magnetic field of the Milky Wayand to serve as a ‘pathfinder’ to the Square Kilometre Array(SKA) in the area of cosmic magnetism.”NAIC still has lots of hurdles to overcome, but progressis being made. The outcome of the US National ScienceFoundation’s ten-year review process in 2010 will be central tothe future of Arecibo, but for now it looks like the professionalsdo not need the help of the RASC, so we can focus our efforts onsaving the DDO.References1. www.naic.edu/alfa/2. www.naic.edu/~ehowell/newslet/dec2007/Newsletter42-shortened.pdf3. www.ucalgary.ca/ras/GALFACTS/Figure 1 — The newly painted Arecibo telescope receiver platformhousing ALFA. Image from NAIC Newsletter # 42.Stars with Pure Carbon AtmospheresAstronomers have discovered white dwarf stars with purecarbon atmospheres. The stars are possibly part of a previouslyunknown sequence of stellar evolution. They may have evolvedfrom stars that are not quite massive enough to explode assupernovae but are just on the borderline. All but the mostmassive two or three percent of stars eventually die as whitedwarfs rather than explode as supernovae.When a star burns helium, it leaves “ashes” of carbon andoxygen. When its nuclear fuel is exhausted, the star then diesto become a white dwarf. Astronomers believe that most whitedwarf stars have a core made of carbon and oxygen, which ishidden from view by a surrounding atmosphere of hydrogen orhelium. They didn’t expect, therefore, to find stars with carbonatmospheres. “We’ve found stars with no detectable traces ofhelium and hydrogen in their atmospheres,” said Universityof Arizona Steward Observatory astronomer Patrick Dufour.“We might actually be observing directly a bare stellar core. Wepossibly have a window on what used to be the star’s nuclearfurnace and are seeing the ashes of the nuclear reaction thatonce took place.”Dufour, along with James Liebert (Université de Montréal)and colleagues at the Paris Observatory, published the resultsin the November 22 issue of Nature. The stars were discoveredamong 10,000 new white-dwarf stars found in the Sloan DigitalSky Survey. The survey, known as the SDSS, found about fourtimes as many white-dwarf stars as previously known. Liebertidentified a few dozen of the newfound stars as DQ white dwarfsin 2003. When observed in optical light, DQ stars appear to bemostly helium and carbon. Astronomers believe that convectionin the helium zone dredges up carbon from the star’s carbonoxygencore.Dufour developed a model to analyze the atmospheresof DQ stars as part of his doctoral research at the Universitéde Montréal. His model simulated cool DQ stars — stars attemperatures between 5000 and 12,000 K. For reference, ourSun’s surface temperature is around 5780 K. When Dufourjoined Steward Observatory in January, he updated his codeto analyze hotter stars, stars as hot as 24,000 K. “When I firststarted modeling the atmospheres of these hotter DQ stars,my first thought was that these are helium-rich stars withtraces of carbon, just like the cooler ones,” Dufour said. “Butas I started analyzing the stars with the higher temperaturemodel, I realized that even if I increased the carbon abundance,the model still didn’t agree with the SDSS data.” In May 2007,“out of pure desperation, I decided to try modeling a purecarbonatmosphere. It worked,” Dufour added. “I found that if Icalculated a pure carbon atmosphere model, it reproduces theThe Royal Astronomical Society of Canada is dedicated to the advancement of astronomy and its related sciences; the Journal espouses thescientific method and supports dissemination of information, discoveries, and theories based on that well-tested method.April / Avril 2008JRASCBuilding for the International Year of Astronomy (IYA2009)47

More magazines by this user
Similar magazines