[Triumf-seminars] TRIUMF Seminar today at 14:00

Mon, 2 Oct 2006 05:00:03 -0700

---------------------- multipart/mixed attachment
Date/Time: Mon 2006-10-02 at 14:00

Location:  Auditorium          

Speaker:   Randall Cooper (Harvard University)

Title:     15O(alpha,gamma)19Ne and the Onset of Type I X-ray Bursts on Accreting Neutron Stars

Abstract: Type I X-ray bursts are thermonuclear explosions that occur on accreting neutron stars.  They are triggered by thermally unstable He burning near the stellar surface.  Nearly all theoretical models of type I X-ray bursts predict that bursts should occur for all accretion rates up to approximately the Eddington limit.  However, observations indicate that bursts do not occur for accretion rates above roughly 30% of the Eddington limit.  In this talk, I will discuss a proposed solution to this longstanding problem.  I show that H burning via the temperature-independent hot CNO cycle, augmented by the extra CNO nuclei produced from stable He burning, significantly lowers the temperature sensitivity of the total nuclear energy generation rate to such an extent as to suppress a thermonuclear runaway and hence a type I X-ray burst for accretion rates above roughly 30% of the Eddington limit, in accord with observations.  For relatively large 15O(a,g)19Ne trial reaction rates in theoretical burst models, alpha capture onto 15O competes favorably with beta decay.  These alpha captures quell hot CNO cycle H burning and thereby increase the temperature sensitivity of the total nuclear energy generation rate. Consequently, bursts occur for all accretion rates up to approximately the Eddington limit. Since the latter scenario disagrees with observations, I suggest that the true 15O(a,g)19Ne reaction rate is lower than the fiducial rate of Caughlan & Fowler (1988).

Stimulants available 15 minutes before the talk. Please note the unusual day of the week.

______________________________

Subscription information available at http://admin.triumf.ca/netdata/seminars/list

---------------------- multipart/mixed attachment--