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A SPALLATION MODEL FOR CASSIOPEIA AAMI R OUYED, RACHID OUYED, AN D DENIS LEAHY

DEPARTMENT OF PHYSICS AND ASTRONOMY, UNIVERSITY OF CALGARY }

NTRODUCTION

Cassiopeia A is a unusual supernova remnant. Nuclear decay lines of44Ca and4Sc detected by COMPTEL and BEPPO SAX indicate that the SN detonation synthe-ized a very large mass for 44Ti of(0.8 2.5) 104 M [1]. A large amount of44Timplies a very bright explosion, but the 1680 description by Flamsteed indicates a 6th

magnitude. Furthermore, Cassiopeia A has been the only SN where 44Ti decay lineshave been detected [2].

While other explanations, like dust extinction [2], are adequate to elucidate abouthe low luminosity, they are inadequate when explaining the large 44Ti mass. In this

poster we propose an alternative explanation.We assume the Ti mass was not created by the SN detonation, rather, it was created

hrough the spallation of neutrons against the 56Ni layer of the SN envelope. Theneutrons are produced by the explosion of a collapsing neutron star (Quark Nova)4, 5, 6]. The destruction of56Ni would explain the low luminosity, while the creationf44Ti would explain the Titanium nuclear decay lines [3].

RESULTSA0 56,tdelay 4 days

TiHe

H

H

LIBeB

Combined

0 20 40

Atomic Mass A

0.0

0.2

0.4

0.6

PrA

A0 56,tdelay 5 days

TiC

He

Combined

0 20 40

Atomic Mass A

0.0

0.1

0.2

0.3

PrA

A0 56,tdelay 6 days

TiO

Combined

0 20 40

Atomic Mass A

0.0

0.2

0.4

PrA

FIG. 1: Spallation products in successive layers (back to front) from 56Ni for tdelay =4, 5, 6 days. The overall distribution is shown in the front layer labelled combined[3].

2 4 6 8 10

0

50

100

150

2 4 6 8 10

1

FIG. 2: Mass Yields (upper panel) andtotal spallation nucleons (bottom panel)versus time delay for 56Ni target. In

the bottom panel, spallation layers arenumbered 0-9, with spallation effectivelyceasing after 10 layers for tdelay = 2 days[3].

DISCUSSIONNi poor, Ti rich: In FIG 1, the case for tdelay = 5 days is particularly interesting

for CasA, because we can clearly see a rich Ti abundance and a poor Ni abundance,whichis consistent with CasAs Tiexcess andlow luminosity. Theuniqueness of CasAcould be due to the constraints set up by tdelay = 5 days, and the conditions necessaryfor a QN. We observe in FIG 2, that for 3 days < tdelay < 7 days, the result is Ni-

poor, Ti-rich, C-rich debris. This C abundance of our model could explain the C richatmosphere of CasAs compact object [8]. Delayed Hydrogen signatures: We observe in FIG 2, that although some hy-

drogen is formed through spallation, it contributes for less than 1 percent of total massof the target layer. Other hydrogen will be formed by E 73 MeV spallation protonsturning into H through recombination, and neutrons decaying into protons (whichwill also recombine into H). However, the hydrogen signature will be delayed be-cause the trapped recombination and thermal continuum radiation will ionize H. Thehydrogen sign will only appear after the SN becomes transparent [3]. Light curve: The late time, radioactive tail of the SN should carry a QN sig-

nature, because spallation destroys 56Ni, but also creates new radioactive isotopes.The buildup of isotopes like 44Ti and 22Na from the destruction of 56Ni could shifthigher the values for the bolometric intensity of a SN after 800 days of its detona-tion, providing an alternative, simpler explanation, to the so called "freeze-out" phasein SN1987a [9]. Possible QN signature: Plausible signatures of the QN in Cas A have re-

cently been suggested by observations [10]. Other distinct spallation signatures in

dual-shock QNe have been discussed in [11].

SPALLATION MODELFollowing a lab analogy, we describe our model as composed of a beam and a

arget [3]. The beam, a pulse of relativistic, E 10 GeV neutrons, is produced by a

neutron star explosion (Quark Nova, or QN) [4, 5, 6]. The target, is the expanding,nnermost 56 Ni layer of a core collapse SN. The targets distance from the neutron stars Rin = vtdelay, where v is the velocity of the Ni layer (v c), and tdelay is a parameterhat acts as time delay between the QN and the SN. We use MA as the mass of the Niayer, as the mean free path of the beams neutrons, and a semi-empirical, spallationross section sp = 45A0.7 mb [7] to derive an expression for the number of collision

Ncoll made by an incoming neutron on the Ni layer [3],

Ncoll =R

2.75

MA,0.1MA0.356

(v5000km/stdelay,5 days)2, (1)

where the lower index indicates the units, for example MA,0.1M is in units of0.1M.For the multiplicity of nucleons generated by a collision, we use a semi-empirical

ormula [7]. If we treat protons and neutrons identically, and take into account thatpallation ceases at E 73 MeV we find an expression for the average nucleon multi-

plicity of,

(E, A) 4.67A56(1 + 0.38lnE)Ynp, (2)

where 1.25 < Ynp < 1.67 [7].To simulate the destruction of heavy nuclei and the construction of lighter

lements, we divide the Ni width into Ncoll imaginary layers, where k = 0 is thennermost layer. A given Ni nucleus will be hit multiple times (Nhits) by neutrons,esulting in the product nucleus [3],

A1 = A0

N0hits1

j=0

0(E0, Aj). (3)

We draw N0hits and 0 from Poisson distributions peaking at N

0

hits and 0

, where

N0

hits = spN0(1 e1)/(4R2in) [3].

MODEL ASSUMPTIONSOurmodelhas some finetuning that is unavoidable dueto uncertainties regarding

the nature of the hadron-quark phase transition. Reference [8] gives mass-radius con-straints for thecompactobject in CasA, which effectivelyrule outlow-massQSs basedon noninteracting quark equations of state. However, large and heavy QSs may exist,so long as the quark super- conducting gap and strong coupling corrections are takeninto account (e.g., [12, 13, 14, 15]). The issue of the mass-radius relation for quark starsis still a matter of debate. Spectral fitting of Cas A agrees very well with theoreticalcooling models for NSs, when superfluidity and pair-breaking effects are taken intoaccount [15]. It is unlikely that a QS would exhibit exactly the same cooling behavioras a NS, which is a problem for our model, but there are no comprehensive coolingsimulation studies of QS and it might be purely coincidental that at this particularlyyoung age, a NS and a QS have the same surface temperature. Studies of cooling ofQSs which include similar attention to physics details (e.g., color superconductivity)are needed to determine whether they could be at all consistent with Cas A.

URL:

http://quarknova.ucalgary.ca

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