At Crust09: Bob's Take Away

Something for the observers: still wondering what exactly Bob's famous quest is?

Since June 2007 Bob's quest is available for download. Try it out, and prepare your answer, before Bob asks!

Many thanks to Erik Kuulkers for the mp3.

Richard Cyburt: X-ray burst reaction flows and ash composition

Richard compared results for XRB lightcurves coming from two different methods: Multizone and single zone calculations. Overall the two methods agree quite well, both for composition and light curves. Both compositions peak in the interval A =60,80.
Richard shared us a very useful piece of information about the JINA REACLIB project. REACLIB is a huge data base with nuclear reaction rates. Users can create their own libraries there too.

Jordi Jose: Hydrodynamic simulations of type I X-ray bursts: metallicity effects

Jordi described his groups simulations of type I X-ray bursts using the 1D SHIBA hydro code coupled to a large network. In models with solar abundance of metals he finds very little 12C in the ashes (~1% by mass), peak nuclei with A~60, and, in general, results that approximately match those found by Heger et al. Sub-solar models (Z=0.001) had less energetic but longer bursts, lower alpha (alpha~30), synthesized heavier species (A~100), and smaller 12C mass fractions (~0.1%). More runs are in progress. Among the take home points (better constraints on Mdot, L, reaction rates), Jordi suggested we take home some local jewelry and handmade artwork :).

Jeff Blackmon: Nuclear Physics on Accreting Neutron Stars

Jeff Blackmon stressed the importance of experimental measurements of (p,g) and (a,p) reactions to accreting neutron stars. Current experiments are using indirect techniques to locate important resonances for these reactions. Specifically mentioned two direct measurements, the first was performed at Oak Ridge studying f17(p,g). Important resonance was measured, but direct capture rate still needs to be constrained. Secondly, he mentioned o14(a,p) measured at CRIB (RIKEN) which is 50-80% different from the recommended REACLIB rate.

To measure many of these reactions more low energy rare isotope beams are needed. Combined with new high resolution detectors such as ANASEN these important X-ray burst ignition rates can be better constrained.

Andrew Melatos: Crustal magnetic field in an accreting neutron star

Andrew gave us a lucid discussion of magnetic field evolution and its consequences in accreting neutron stars.  

Magnetic field burial and flux freezing together distort the initial dipole and decrease the magnetic moment; however, the local field strength may increase!  ZEUS says that the magnetic structures are Parker unstable but the instability is suppressed: In the end, the substrate exhibits global MHD oscillations.  The interesting claim is that the end state could be a 10^15 G (!!!) magnetic field "wall" that could affect type I X-ray bursts, for example.  The idea is exciting because there are severe discrepancies between observations and X-ray burst models that are difficult to explain through nuclear physics uncertainties alone, but the presence of such a structure, particularly at the ignition pressures of X-ray bursts (which are << B^2/8pi for B = 10^15 G), are difficult to believe.

A more plausible, yet no less exciting, idea is that asymmetric field burial could produce a quadrupole; the resultant gravitational radiation would limit the possible neutron star spin frequency.  This could explain why all neutron stars have spin frequencies <~ 700 Hz, well below the break-up spin.

Alexander Heger: An Overview of X-Ray Burst Simulations

Alex began with an overview of type I X-ray bursts and their various accretion rate regimes. He then described his numerical simulations of bursts using the 1D Kepler hydro code which he coupled to a large reaction network. Runs with H/He accreting at high Mdot with low Z show long (~200s) tails due to rp-process burning and little 12C in the ashes of burning (X12~0.001). In runs with low Mdot and high Z, the ignition takes place in a pure He layer, leading to a very quick burst rise (~1 ms) and near Eddington luminosity at peak. Runs with very low Mdot undergo weak hydrogen flashes.

Some of the results may be sensitive to uncertainties in the nuclear physics and reaction rates and are a subject of future study.

At Crust09: Alex Heger Opens the Tuesday Session

In Santa Fe...

Bob Rutledge performs a delicate scattering experiment.

Coming Talks: Tuesday May 19

Alexander Heger	An Overview of X-Ray Burst Simulations	8:45
Andrew Melatos	Crustal magnetic field in an accreting neutron star	9:30
Jeff Blackmon	Nuclear Physics on Accreting Neutron Stars	9:55
Jordi Jose	Hydrodynamic simulations of type I X-ray bursts: metallicity effects	11:00
Richard Cyburt	X-ray burst reaction flows and ash composition	11:25
Livius Trache	Nuclear reaction rates for explosive H-burning from experiments with rare nuclear beams. Indirect methods	11:50
K. Ernst Rehm	Recent Results for (p,g) and (a,p) Reactions on the rp-Procerss Path	12:15
Uwe Greife	Proton Capture Measurements with Radioactive Ion Beams	2:00
Anuj Parikh	Nuclear Physics Uncertainties in Models of Type I X-Ray Bursts	2:25
Giuseppe Lorusso	beta deay study in the Sn100 region	2:50
Wanpeng Tan	measurement of alpha-induced nuclear reactions for X-ray Bursts	3:15
Jerome Chenevez	The INTEGRAL view of intermediate long X-ray bursts	4:10
Laurens Keek	X-ray bursts with too short recurrence times	4:35
Craig Heinke	Progress & Problems from Quiescent LMXBs	5:00
Schatz, Brown, Haensel, Horowitz	Discussion, Accreting Neutron Stars	5:30

In Santa Fe: The Shed

Ed Brown discusses burning models with Nevin Weinberg and Andrew
Cumming. The Shed is a "new Mexican" restaurant, one block from La
Fonda. Map.

Milan Matos: Mass Measurements of Exotic Nuclei

Milan gave a nice summary of mass measurement techniques. Milan mentioned the importance of masses to neutron star modeling. He then summarized current techniques including direct measurements such as Penning traps, Schottky method, time-of-flight, and indirect measurements done with transfer reactions. Milan also mentioned capabilities of different facilities producing isotopes in different regions of the chart of nuclides.

Dmitry Yakovlev: Pyconuclear reactions in the crust

Dmitry Yakovlev gives an interesting and entertaining overview of pyconuclear reactions. These density sensitive nuclear fusion processes are manifestations of the QED vacuum, which involves Kindergarten physics and the problem is therefore explained by a young Russian girl. The reaction rate of pyconuclear reactions is independent of temperature, but increases exponentially with increasing density. These processes are thought to allow for the formation of large nuclei in the neutron star crust.

(The image shows George Gamow at young age).

Dany Page: Crust cooling of strange stars vs neutron stars

Dany described how as cooling depends on the thickness of the crust it should be possible to distinguish between neutron stars and strange stars - a strange star has a much thinner crust (a few hundred metres as opposed to km).  So, relaxation of the crust should be a good test.  However, he hasn't yet had the chance to do the calculations.......answers may be coming in the near future.

Sanjib Gupta: Nuclear reactions in the crust and implications for superburst ignition and cooling

Some talked about nuclear reactions near the stellar surface; others talked about those in the core.   Sanjib talked about reactions in between.  Such reactions in the crust are a figurative bridge between surface heating and core neutrino cooling.

These nuclear reactions, electron captures, neutron emissions, and pycnonuclear reactions, are of utmost importance because they set both the heating and compositional profile.

In the outer crust, electrons can capture into excited states of nuclei; the subsequent radiative de-excitation can release ~4 times more energy than previous models predicted, which assumed electron capture into the ground state.  Interestingly, Daligault & Gupta (2009) find that the outer crust is amorphous, i.e. it does not form a lattice!  This result is exciting, but conflicts with previous calculations (like Chuck Horowitz's simulations).  Does it conflict with observations?  Unclear right now, but it would be useful to find out (hint, hint...).

A multi-component inner crust may form a lattice, as some leaked, free neutrons redistribute themselves among the other nuclei and thereby "homogenize" the plasma.  This neutron rearrangement can affect pycnonuclear heating as well.