arXiv subjects

D. N. Schramm

Publications and source records attributed to D. N. Schramm.

11 recordsLinked to original sources

arXiv1996

On the significance of population II Li6 abundances

We explore possible depletion factors for Li6 in the three stars where it has been detected. To this end, we assume that 6Li/H scales as 16O/H throughout the galactic evolution, an assumption motivated by the observations of a similar scaling for 9Be. We examine the uncertainties attached to this modeling of 6Li galactic evolution; we also incorporate a recent evaluation of the primordial production of 6Li. The depletion factor D6, for 6Li, in HD84937, is constrained to be less than 4 at 95% c.l.. This depletion factor is an extreme upper limit on the depletion factor for 7Li. We conclude that the primordial abundance of 7Li cannot be higher that twice the observed value, since no current stellar model is able to reproduce the observed trends of 7Li with temperature and metallicity, and yet deplete 7Li by more than a factor 2, while satisfying D6<4. All three 6Li observations are in excellent agreement with all standard expectations: Big-Bang nucleosynthesis with 2<$η_{10}$<6.5, and standard isochrones of 6Li survival. We discuss possible deviations from our assumption of 6Li/H scaling with 16O/H, due to alpha-alpha creation of 6Li in various sites. We notably reject stellar flares as a significant source of 6Li in HD84937.

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arXiv1996

Low Mass Stars and the He3 Problem

The prediction of standard chemical evolution models of higher abundances of He3 at the solar and present-day epochs than are observed indicates a possible problem with the yield of He3 for stars in the range of 1-3 solar masses. Because He3 is one of the nuclei produced in Big Bang Nucleosynthesis (BBN), it is noted that galactic and stellar evolution uncertainties necessarily relax constraints based on He3. We incorporate into chemical evolution models which include outflow, the new yields for He3 of Boothroyd & Malaney (1995) which predict that low mass stars are net destroyers of He3. Since these yields do not account for the high \he3/H ratio observed in some planetary nebulae, we also consider the possibility that some fraction of stars in the 1 - 3 solar mass range do not destroy their He3 in theirpost main-sequence phase. We also consider the possibility that the gas expelled by stars in these mass ranges does not mix with the ISM instantaneously thus delaying the He3 produced in these stars, according to standard yields, from reaching the ISM. In general, we find that the Galactic D and He3 abundances can be fit regardless of whether the primordial D/H value is high (2 x 10^{-4}) or low (2.5 x 10^{-5}).

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arXiv1995

The Local Abundance of $^3$He: A Confrontation Between Theory and Observation

Determinations of the \he3 concentrations in Galactic matter serve to impose interesting and important constraints both on cosmological models and on models of Galactic chemical evolution. At present, observations of \he3 in the solar system and in the interstellar medium today suggest that the \he3 abundance has not increased significantly over the history of the Galaxy, while theoretical models of Galactic chemical evolution (utilizing current nucleosynthesis yields from stellar evolution and supernova models) predict a rather substantial increase in \he3. We consider the possibility that the solar \he3 abundance may have been affected by stellar processing in the solar neighborhood prior to the formation of the solar system. Such a discrepancy between solar abundances and average galactic abundances by as much as a factor of two, may be evidenced by several isotopic anomalies. Local destruction of \he3 by a similar amount could serve to help to reconcile the expected increase in the \he3 abundance predicted by models of galactic chemical evolution. We find however, that the production of heavier elements, such as oxygen, places a strong constraint on the degree of \he3 destruction. We also explore the implications of both alternative models of Galactic chemical evolution and the stellar yields for \he3 in low mass stars, which can explain the history of the \he3 concentration in the Galaxy.

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arXiv1995

A Break in the Highest Energy Cosmic Ray Spectrum: A Signature of New Physics?

Recent experimental data from the Fly's Eye and the Akeno array seem to indicate significant structure in the ultrahigh energy cosmic ray spectrum above $10^{18}\eV$. A statistically significant dip has been established at about $5\times10^{18}\eV$. In addition, each experiment observed a different superhigh energy event above $10^{20}\eV$ separated from the rest of the data by about half a decade in energy. In this article we discuss what this implies for the existence or non-existence of the ``Greisen-Zatsepin-Kuz'min cutoff'', a long lasting and still open question in cosmic ray physics. This cutoff, caused by energy losses in the cosmic microwave background, is predicted to occur at a few times $10^{19}\eV$ if cosmic rays are produced by shock acceleration of lower energy particles at extragalactic distances. We show that from the spectral point of view, sources nearer than a few $\Mpc$ are still consistent with the data at the $1σ$ level, provided these sources accelerate particles beyond $3\times10^{20}\eV$. However, persistence of the apparent gap in the existing data at the level of a 4 times higher total exposure would rule out a wide range of acceleration models at $98\%$ C.L., whether they rely on nearby or extragalactic sources. This might hint to the existence of a ``top down'' mechanism which produces an additional hard component of ultrahigh energy particles directly, say, by decay from some higher energy scale in contrast to bottom up acceleration of charged particles. In this scenario a cutoff followed by a pronounced spectral flattening and possibly even a gap could naturally be formed.

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arXiv1995

HELIUM PHOTODISINTEGRATION AND NUCLEOSYNTHESIS: IMPLICATIONS FOR TOPOLOGICAL DEFECTS, HIGH ENERGY COSMIC RAYS, AND MASSIVE BLACK HOLES

We consider the production of $^3$He and $^2$H by $^4$He photodisintegration initiated by non-thermal energy releases during early cosmic epochs. We find that this process cannot be the predominant source of primordial $^2$H since it would result in anomalously high $^3$He/D ratios in conflict with standard chemical evolution assumptions. We apply this fact to constrain topological defect models of highest energy cosmic ray (HECR) production. Such models have been proposed as possible sources of ultrahigh energy particles and gamma-rays with energies above $10^{20}$eV. The constraints on these models derived from $^4$He-photodisintegration are compared to corresponding limits from spectral distortions of the cosmic microwave background radiation (CMBR) and from the observed diffuse gamma-ray background. It is shown that for reasonable primary particle injection spectra superconducting cosmic strings, unlike ordinary strings or annihilating monopoles, cannot produce the HECR flux at the present epoch without violating at least the $^4$He-photodisintegration bound. The constraint from the diffuse gamma-ray background rules out the dominant production of HECR by the decay of Grand Unification particles in models with cosmological evolution assuming standard fragmentation functions. Constraints on massive black hole induced photodisintegration are also discussed.

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arXiv1994

On Solar Model Solutions to the Solar Neutrino Problem

Without assuming any solar models and neutrino flavor conversions, $^8$B neutrinos seen by the Kamiokande experiment should contribute 2.6$\pm 0.45$ SNU to the chlorine experiment. When this rate is compared with the total event rate of 2.3$\pm 0.2$ SNU observed by the Homestake experiment which should inlude a 0.2 SNU contribution from uncertainty-free pep neutrinos, there may still be a possible evidence that $^7$Be neutrinos are more severely suppressed than the $^8$B neutrinos with respect to the predictions of standard solar models, which cannot be explained by any known astrophysics solution. Given a Kamiokande event rate of larger than 36$\%$ (2$σ$) of the prediciton of Bahcall and Pinsonneault's standard solar model, variations of standard solar models yield minimal rates of 3.6 SNU for the Homestake experiment and 114 SNU for GALLEX and SAGE. Therefore, variations of standard solar models as solutions to the solar neutrino problem are so far inconsistent with the Homestake experiment and only marginally allowed by the gallium experiments. If the gallium experiments confirm a flux significantly below 114 SNU, it would imply unconventional physics.

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arXiv1994

Possible Systematic Decreases in the Age of Globular Clusters

The ages of globular clusters inferred from observations depends sensitively on assumptions like the initial helium abundance and the mass loss rate. A high helium abundance (e.g., $Y\approx$0.28) or a mass loss rate of $\sim$10$^{-11}M_\odot$ yr$^{-1}$ near the main sequence turn-off region lowers the current age estimate from 14 Gyr to about 10--12 Gyr, significantly relaxing the constraints on the Hubble constant, allowing values as high as 60km/sec/Mpc for a universe with the critical density and 90km/sec/Mpc for a baryon-only universe. Possible mechanisms for the helium enhancement in globular clusters are discussed, as are arguments for an instability strip induced mass loss near the turn-off. Ages lower than 10 Gyr are not possible even with the operation of both of these mechanisms unless the initial helium abundance in globular clusters is $>0.30$, which would conflict with indirect measurements of helium abundances in globular clusters.

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arXiv1994

On the Origin of Highest Energy Cosmic Rays

In this paper we show that the conventional diffusive shock acceleration mechanism for cosmic rays associated with relativistic astrophysical shocks in active galactic nuclei (AGNs) has severe difficulties to explain the highest energy cosmic ray events. We show that protons above around $2\times10^{20}\eV$ could have marginally been produced by this mechanism in an AGN or a rich galaxy cluster not further away than around $100\Mpc$. However, for the highest energy Fly's Eye and Yakutsk events this is inconsistent with the observed arrival directions. Galactic and intergalactic magnetic fields appear unable to alter the direction of such energetic particles by more than a few degrees. We also discuss some other options for these events associated with relativistic particles including pulsar acceleration of high $Z$ nuclei. At the present stage of knowledge the concept of topological defects left over from the early universe as the source for such events appears to be a promising option. Such sources are discussed and possible tests of this hypothesis are proposed.

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arXiv1993

Constraints on Neutrino Oscillations from Big Bang Nucleosynethesis

We discuss in detail the effect of neutrino oscillations in Big Bang nucleosynthesis, between active and sterile neutrinos, as well as between active and active neutrinos. We calculate the constraints on mixings between active and sterile neutrinos from the present observation of the primordial helium abundance and discuss the potential implications on various astrophysical and cosmological problems of such oscillations. In particular, we show that large angle sterile neutrino mixing seems to be excluded as a MSW solution to the solar neutrino situation or a solution to the atmospheric neutrino mixing hinted at in some underground experiments. We show how with this constraint, the next generation of solar neutrino experiments should be able to determine the resolution of the solar neutrino problem. It is also shown how sterile neutrinos remain a viable dark matter candidate.

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arXiv1993

Testing for Gaussianity Through the Three Point Temperature Correlation Function

One of the crucial aspects of density perturbations that are produced by the standard inflation scenario is that they are Gaussian where seeds produced by topological defects tend to be non-Gaussian. The three point correlation function of the temperature anisotropy of the cosmic microwave background radiation (CBR) provides a sensitive test of this aspect of the primordial density field. In this paper, this function is calculated in the general context of various allowed non-Gaussian models. It is shown that by COBE and the forthcoming South Pole and Balloon CBR anisotropy data may be able to test provide a crucial test of Gaussianity.

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arXiv1993

Constraints on Dirac Neutrinos from SN 1987A

The Livermore Supernova Explosion Code was used to calculate the effect of a massive Dirac neutrino on neutrino emission from SN 1987A in a fully self-consistent manner. Spin-flip interactions lead to the copious emission of sterile, right-handed neutrinos and cool the core faster than the observed neutrino emission time for Dirac masses exceeding about 3$\,$keV. This limit is relaxed to $7\,$keV if pion emission processes in the core are neglected. These limits are compared with the previous less stringent limits of Burrows et al.

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