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solar masses might explain their peculiar carbon stars. Indeed,the numerical models predict that stars 9 to 10 times as massiveas our Sun will become white dwarfs with oxygen-magnesiumneoncores and mostly carbon-oxygen atmospheres. Moremassive stars explode as supernovae. But scientists aren’t surewhere the dividing line is, whether stars 8, 9, 10, or 11 times asmassive as our Sun are required to create supernovae. “We don’tknow if these carbon atmosphere stars are the result of 9 or10 solar-mass star evolution, which is a key question,” Liebertsaid.The UA astronomers plan making new observations of thecarbon-atmosphere stars at the 6.5-metre MMT Observatoryon Mount Hopkins, Arizona in December to better pinpointthe stellar masses. The observations could help define the masslimit for stars dying as white dwarfs or dying as supernovae.A Short Tail for 17p/HolmesFigure 2 — A high-resolution, artistic representation of the surfaceof H1504+64.spectra exactly as observed.”The great mystery is why these carbon-atmosphere starsare found only between about 18,000 and 23,000 K. “These starsare too hot to be explained by the standard convective dredge-upscenario, so there must be another explanation,” Dufour noted.The stars might have evolved from a star like the unique, muchhotter star called H1504+65 that Pennsylvania State Universityastronomer John A. Nousek, Liebert, and others reported in 1986.If so, carbon-atmosphere stars represent a previously unknownsequence of stellar evolution. H1504+65 is a very massive starat 200,000 K. Astronomers currently suggest this star somehowviolently expelled all its hydrogen and all but a very small traceof its helium, leaving an essentially bare stellar nucleus with asurface of 50 percent carbon and 50 percent oxygen. “We thinkthat when a star like H1504+65 cools, it eventually becomeslike the pure-carbon stars,” comments Dufour. As the massivestar cools, gravity separates carbon, oxygen, and trace helium.Above 25,000 K, the trace helium rises to the top, forming a thinlayer above the much more massive carbon envelope, effectivelydisguising the star as a helium-atmosphere white dwarf. Thisbeing said, between 18,000 and 23,000 K, convection in thecarbon zone probably dilutes the thin helium layer. At thesetemperatures, oxygen, which is heavier than carbon, hasprobably sunk too deep to be dredged to the surface.Dufour and his colleagues say that models of stars 9 to 11The recent brightening of Comet Holmes has spurred a frenzyof observations both by amateur and professional astronomersalike. All these observations reveal a tailless, round, yellowishfuzzball in the constellation Perseus. Near-infrared images ofComet 17/P Holmes, obtained with the 1.6-m Ritchey-Chrétientelescope at Mont Mégantic Observatory, indicate a small taillikefeature next to the comet’s head (figures 3 and 4). Theimages were obtained by graduate student Sandie Bouchardand night assistant Bernard Malenfant on the morning of 2007October 26, using SIMON, a Near Infrared Polarimetric Imager.A preliminary analysis performed by astronomers PierreBastien and René Doyon from Université de Montréal and theCentre de recherche en astrophysique du Québec clearly showsa bright elongated feature surrounding the more luminouscomet’s coma. This elongated feature, probably a cloud of dustand gas, which resembles a small tail, is going out at a positionangle of 215 degrees (+/- 5 deg), measured from north andgoing east. This direction makes an angle of about 33 degreesrelative to the Sun-comet direction. Although the imagesdisplay tantalizing evidence of a tail, the direction of the featuredoes not point directly in the direction opposite to the Sun, asexpected.Figure 3 (left) — Comet 17P/Holmes image taken with an I filter on2007 October 26 at 04:37 edt (08:37 UTC)Figure 4 (right) — Comet 17P/Holmes image taken with a J filtertaken on 2007 October 26 at 04:10 EDT (08:10 UTC).48 Building for the International Year of Astronomy (IYA2009)JRASC April / avril 2008

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