arXiv subjects

John N. Bahcall

Publications and source records attributed to John N. Bahcall.

At least 91 records · Page 5Linked to original sources

arXiv1995

Disk M Dwarf Luminosity Function From HST Star Counts

We study a sample of 257 Galactic disk M dwarfs (8 12 (M<0.25 \ms), decreasing by a factor \gsim 3 by M_V~14 (M~0.14\ms). This decrease in the LF is in good agreement with the ground-based photometric study of nearby stars by Stobie et al. (1989), and in mild conflict with the most recent LF measurements based on local parallax stars by Reid et al. (1995). The local LF of the faint Galactic disk stars can be transformed into a local mass function using an empirical mass-M_V relation. The mass function can be represented analytically over the mass range 0.1\ms<M<1.6\ms by \log(ϕ)=-1.35-1.34\log(M/\ms)-1.85 [\log(M/\ms)]^2 where ϕis the number density per logarithmic unit of mass. The total column density of M stars is only Σ_M=11.8\pm 1.8\ms\pc^{-2}, implying a total `observed' disk column density of Σ_\obs~=39\ms\pc^{-2}, lower than previously believed, and also lower than all estimates with which we are familiar of the dynamically inferred mass of the disk. The measured scale length for the M-star disk is 3.0\pm 0.4 kpc. The optical depth to microlensing toward the LMC by the observed stars in the Milky Way disk is τ~1x10^{-8}, compared to the observed optical depth found in ongoing experiments τ_\obs~ 10^{-7}. The M-stars show evidence for a population with characteristics intermediate between thin disk and spheroid populations. Approximating what may be a continuum of populations by two separate component, we find characteristic exponential scale heights of ~210 pc and ~740 pc.

astro-ph↗

arXiv1995

Progress and Prospects in Neutrino Astrophysics

The neutrino deficits observed in four solar neutrino experiments, relative to the theoretical predictions, have led to fresh insights into neutrino and solar physics. Neutrino emission from distant, energetic astronomical systems may form the basis for a new field of astronomy. Additional experiments are needed to test explanations of the solar neutrino deficits and to allow the detection of more distant sources.

astro-ph↗

arXiv1995

Solar Neutrinos: Radiative Corrections in Neutrino-Electron Scattering Experiments

Radiative corrections to the electron recoil-energy spectra and to total cross sections are computed for neutrino-electron scattering by solar neutrinos. Radiative corrections change monotonically the electron recoil spectrum for incident \b8 neutrinos, with the relative probability of observing recoil electrons being reduced by about 4 \% at the highest electron energies. For $p-p$ and $\be7$ neutrinos, the recoil spectra are not affected significantly. Total cross sections for solar neutrino-electron scattering are reduced by about 2 \% compared to previously computed values. We also calculate the recoil spectra from $^{13}$N and $^{15}$O neutrinos including radiative corrections.

astro-ph↗

arXiv1995

HST Images of Nearby Luminous Quasars II: Results for Eight Quasars and Tests of the Detection Sensitivity

HST observations of eight intrinsically luminous quasars with redshifts between $0.16$ and~$0.29$ are presented. Seven companion galaxies brighter than $M_V = -16.5$ lie within a projected distance of 25 kpc of the quasars; three of the companions are located closer than $3''$ (6 kpc projected distance) from the quasars, well within the volume that would be enclosed by a typical $L^*$ host galaxy. The observed association of quasars and companion galaxies is statistically significant and may be an important element in the luminous-quasar phenomenon. Evidence for candidate host galaxies is presented for the three most promising cases: PG 1116+215, 3C 273, and PG 1444+407, but additional observations are required before the characteristics of the candidate hosts can be regarded as established. Upper limits are placed on the visual-band brightnesses of representative galactic hosts for all of the quasars. These limits are established by placing galaxy images obtained with HST underneath the quasars and measuring at what faintness level the known galaxies are detected. On average, the HST spirals would have been detected if they were one magnitude fainter than $L^*$ or brighter and the early-type galaxies could have been detected down to a brightness level of about $L^*$, where $L^*$ is the Schechter characteristic luminosity of field galaxies.

astro-ph↗

arXiv1994

HST Images of Nearby Luminous Quasars

Strong upper limits are placed on the visual-band brightnesses of galactic hosts for four luminous, radio-quiet quasars with redshifts between $0.16$ and~$0.24$ that were studied with the HST's Wide Field/Planetary Camera-2. Typical upper limits on the luminosities of galactic hosts are about $1.4$ mag fainter than $L^*$ for spirals and about $0.5$ mag fainter than $L^*$ for ellipticals. The galactic hosts of the quasars are more than a magnitude and a half fainter than the median integrated absolute magnitude of Seyfert galaxies. If the detection limits are determined using featureless simulated galaxies instead of observed galaxy images, then the detection limits for spirals are 0.5--1.0 mag less stringent. These results are consistent with the hypothesis that the quasar phenomenon corresponds to the early stages of galaxy formation, before extensive star formation occurs.

astro-ph↗

arXiv1994

Detection of Compact Ultraviolet Nuclear Emission in LINER Galaxies

Low-ionization nuclear emission-line regions (LINERs), which exist in a large fraction of galaxies, may be the least luminous manifestation of quasar activity. The nature of LINERs has, however, remained controversial because an AGN-like nonstellar continuum source has not been directly observed in them. We report the detection of bright, unresolved (FWHM $\ltorder 0.1''$) point sources of UV ($\sim 2300$ Å) emission in the nuclei of nine nearby galaxies from a complete sample of 110 nearby galaxies imaged with {\it HST}. Ground-based optical spectroscopy reveals that five of the nuclei are LINERs, three are starburst nuclei, and one is a Seyfert nucleus. The observed UV flux in each of the five LINERs implies an ionizing flux that is sufficient to account for the observed emission lines through photoionization. The detection of a strong UV continuum in the LINERs argues against shock excitation as the source of the observed emission lines, and supports the idea that photoionization excites the lines in at least some objects of this class. Among the Northern-hemisphere galaxies in the sample, 26 are LINERs, of which only the above five LINERs have a detected nuclear UV source. There are no obvious differences in the optical line intensity ratios between the UV-bright and UV-dark LINERs. If all LINERs are photoionized, then the continuum source is unobscured along our line of sight in $5/26\approx 20\%$ of LINERs. Alternatively, spectrally-similar LINERs may be produced by various excitation mechanisms, with photoionization responsible in only about 20\% of the cases. The high angular resolution allows us to set upper limits, typically several parsecs, on the physical size of the compact star-cluster or AGN-type continuum source that is emitting the UV light in these objects.

astro-ph↗

arXiv1994

How Many Solar Neutrino Experiments Are Wrong?

Ten recently-published solar models give $\7be$ neutrino fluxes that lie within a range of $\pm 10$\% of the average value, a convergence that is independent of uncertainties in the measured laboratory rate of the $\7be(p,γ)\8b$ reaction. If nothing happens to solar neutrinos after they are created ({\it a la} standard electroweak theory) and the operating solar neutrino experiments are correct, then the $\7be$ solar neutrino flux must be less than 50\% of the solar model value. At least three of the four existing solar neutrino experiments must be wrong {\it if}: (1) standard electroweak theory is correct, and (2) the true $\7be$ neutrino flux lies within the range predicted by standard solar models.

astro-ph↗

arXiv1994

Solar Neutrinos: Sensitivity to Pre-Main Sequence Evolution and to the Depth of the Convective Zone

Predicted rates for solar neutrino experiments that are obtained with a modified stellar evolution code originally developed to study the advanced stages of stellar evolution are shown to be in agreement with other recently-calculated precise solar models to about 2% (0.2 SNU for the chlorine experiment). Different scenarios for pre-main sequence evolution are explored and are found to change the predicted rates for solar neutrino experiments by less than or of order 1%. The influence of the depth of the solar convective zone on the predicted solar neutrino fluxes is established by direct calculation. It is shown that a change in the calculated depth of the convective zone that is five times larger than the quoted helioseismological measurement uncertainty determined by Christensen-Dalsgaard, Gough, & Thompson causes a change in the predicted $^8$B neutrino flux of less than 7% and a change in the $^7$Be neutrino flux of less than 4%. Additionally, it is shown that the radiative opacities near the depth of the convective zone cannot differ from the standard OPAL values by more than about 12% without causing the calculated and measured depths of the convective zone to differ by more than five times the helioseismological measurement uncertainty.

astro-ph↗

arXiv1994

M Dwarfs, Microlensing, and the Mass Budget of the Galaxy

We show that faint red stars do not contribute significantly to the mass budget of the Galaxy or to microlensing statistics. Our results are obtained by analyzing two long exposures of a high-latitude field taken with the Wide Field Camera on the newly repaired {\it Hubble Space Telescope}. Stars are easily distinguished from galaxies essentially to the limiting magnitudes of the images. We find five stars with $2.0 3.0$. Therefore, main-sequence stars with $M_I>10$ that are above the hydrogen-burning limit in the dark halo or the spheroid contribute $<6%$ of the unseen matter. Faint red disk stars, M-dwarfs, contribute at most $ 15%$ to the mass of the disk. We parameterize the faint end of the cumulative distribution of stars, $Φ$, as a function of luminosity $L_V$, $dΦ/d \ln L_V \propto L_V^{-γ}$. For spheroid stars, $γ<0.32$ over the range $6<M_V<17$, with 98% confidence. The disk luminosity function falls, $γ<0$, for $15\lsim M_V\lsim 19$. Faint red stars in the disk or thick disk, and stars with $M_V<16$ in the spheroid contribute $τ< 10^{-8}$ to the optical depth to microlensing toward the Large Magellanic Cloud.

astro-ph↗

arXiv1994

Solar Neutrinos: Where We Are, Where We Are Going

There are four important facts about solar neutrinos. They are listed in order of importance in this abstract and discussed more in the text of the talk. First, solar neutrinos have been detected in four experiments with approximately the energies and fluxes predicted by the standard solar model, confirming the hypothesis that the energy source for the solar luminosity is the fusion of light elements. Second, the measured event rates are significantl less than the event rates predicted by the combined standard solar and electroweak models in all four experiments. Third, a comparison of the event rates measured in the chlorine experiment (threshold 0.8 Mev) and the neutrino-electron scattering experiment (Kamiokande II, threshold 7.5 MeV) indicates that the deficiency of electron-type neutrinos at the earth is energy dependent, if the rates and the uncertainties in both experiments have been correctly understood. The inference that the deficiency is energy-dependent conflicts with the simplest version of standard electroweak theory. Fourth, experiments are being constructed that have the capabilities to determine conclusively if new neutrino physics is required.

astro-ph↗

arXiv1994

Has A Standard Model Solution to the Solar Neutrino Problem Been Found?

The claim by Dar and Shaviv that they have found a standard model solution to the solar neutrino problem is based upon an incorrect assumption made in extrapolating nuclear cross sections and the selective use of a small fraction of the nuclear physics and of the neutrino data. In addition, five different solar model codes show that the rate obtained for the chlorine experiment using the Dar-Shaviv stated parameters differs by at least $14 σ$ from the observed rate.

astro-ph↗

arXiv1994

The $^7$Be Solar Neutrino Line: A Reflection of the Central Temperature Distribution of the Sun

A precise test of the theory of stellar evolution can be performed by measuring the average difference in energy between the neutrino line produced by ${\rm ^7Be}$ electron capture in the solar interior and the corresponding neutrino line produced in a terrestrial laboratory. This energy shift is calculated to be 1.29~keV (to an accuracy of a few percent) for the dominant ground-state to ground-state transition. The energy shift is approximately equal to the average temperature of the solar core, computed by integrating the temperature over the solar interior with a weighting factor equal to the locally-produced $^7$Be neutrino emission. The characteristic modulation of the ${\rm ^7Be}$ line shape that would be caused by either vacuum neutrino oscillations or by matter-enhanced (MSW) neutrino oscillations is shown to be small. Other frequently-discussed weak interaction solutions to the solar neutrino problem are also not expected to change significantly the line profile. Therefore, a measurement of the energy shift is a measurement of the central temperature distribution of the sun.

astro-ph↗

arXiv1993

Solar Neutrinos: What We Have Learned

The four operating solar neutrino experiments confirm the hypothesis that the energy source for solar luminosity is hydrogen fusion. However, the measured rate for each of the four solar neutrino experiments differs significantly (by factors of 2.0 to 3.5) from the corresponding theoretical prediction that is based upon the standard solar model and the simplest version of the standard electroweak theory. If standard electroweak theory is correct, the energy spectrum for \b8 neutrinos created in the solar interior must be the same (to one part in $10^5$) as the known laboratory \b8 neutrino energy spectrum. Direct comparison of the chlorine and the Kamiokande experiments, both sensitive to \b8 neutrinos, suggests that the discrepancy between theory and observations depends upon neutrino energy, in conflict with standard expectations. Monte Carlo studies with 1000 implementations of the standard solar model confirm that the chlorine and the Kamiokande experiments cannot be reconciled unless new weak interaction physics changes the shape of the \b8 neutrino energy spectrum. The results of the two gallium solar neutrino experiments strengthen the conclusion that new physics is required and help determine a relatively small allowed region for the MSW neutrino parameters.

astro-ph↗

arXiv1993

Vacuum-Polarization Corrections to Solar-Fusion Rates

The vacuum-polarization corrections to rates for nuclear-fusion reactions in the $pp$ chain and in the CNO cycle are calculated. For the reactions of particular importance to the solar-neutrino problem, the $\Hethree$, $\Hefour$, $\Be$, and $\Ni$ reactions, we find the magnitude of the effect to be less than 2\%. The effect of VP on all the other reaction rates is expected to be of a similar order of magnitude. We discuss how these results affect the predicted fluxes of solar neutrinos.

astro-ph↗

arXiv1993

The Central Temperature of the Sun can be Measured via the $^7$Be Solar Neutrino Line

A precise test of the theory of stellar evolution can be performed by measuring the difference in average energy between the neutrino line produced by ${\rm ^7Be}$ electron capture in the solar interior and the corresponding neutrino line produced in a terrestrial laboratory. The high temperatures in the center of the sun broaden the line asymmetrically, FWHM = 1.6~keV, and cause an average energy shift of 1.3~keV. The width of the $^7$Be neutrino line should be taken into account in calculations of vacuum neutrino oscillations.

hep-ph↗

arXiv1993

Solar Neutrino Experiments: New Physics?

Physics beyond the simplest version of the standard electroweak model is required to reconcile the results of the chlorine and the Kamiokande solar neutrino experiments. None of the 1000 solar models in a full Monte Carlo simulation is consistent with the results of the chlorine or the Kamiokande experiments. Even if the solar models are forced articficially to have a ${}^8 B$ neutrino flux in agreement with the Kamiokande experiment, none of the fudged models agrees with the chlorine observations. This comparison shows that consistency of the chlorine and Kamiokande experiments requires some physical process that changes the shape of the ${}^8 B$ neutrino energy spectrum. The GALLEX and SAGE experiments, which currently have large statistical uncertainties, differ from the predictions of the standard solar model by $2 σ$ and $3 σ$, respectively. The possibility that the neutrino experiments are incorrect is briefly discussed.

astro-ph↗

arXiv1993

The Rate of the Proton-Proton Reaction

We re-evaluate the matrix element for the proton-proton reaction which is important for stellar-evolution calculations and for the solar-neutrino problem. We self-consistently determine the effect of vacuum polarization on the matrix element by first correcting the low-energy scattering data to account for vacuum polarization. We then calculate the proton-proton wave function by integrating the Schrodinger equation with vacuum polarization included. We use improved data for proton-proton scattering and for the deuteron wave function. We evaluate the uncertainties that are due to experimental error and estimate those that are due to theoretical inadequacies. We estimate the theoretical uncertainty by using six different deuteron potentials and five different proton-proton potentials. Vacuum polarization decreases the calculated value by $0.6_{-0.4}^{+0.1}$\%. The complete result is $Λ^2=6.92\times(1\pm0.002^{+0.014}_{-0.009})$ where the first uncertainty is due to experimental errors and the second uncertainty is due to theoretical uncertainties. Our value of $Λ^2$ is 2\% smaller than the value obtained in 1969 by Bahcall and May. The improved calculations of the rate of the $pp$ reaction described here increase slightly the predicted event rates for the chlorine and the Kamiokande solar-neutrino experiments.

astro-ph↗

arXiv1993

Is Large Lepton Mixing Excluded?

The original \bnum -(or $\barν_τ$-) energy spectrum from the gravitational collapse of a star has a larger average energy than the spectrum for \bnue since the opacity of \bnue exeeds that of \bnum (or $ν_τ$). Flavor neutrino conversion, \bnue $\leftrightarrow$ \bnum, induced by lepton mixing results in partial permutation of the original \bnue and \bnum spectra. An upper bound on the permutation factor, $p \leq 0.35$ (99$\%$ CL) is derived using the data from SN1987A and the different models of the neutrino burst. The relation between the permutation factor and the vacuum mixing angle is established, which leads to the upper bound on this angle. The excluded region, $\sin^2 2θ> 0.7 - 0.9$, covers the regions of large mixing angle solutions of the solar neutrino problem: ``just-so" and, partly, MSW, as well as part of region of $ν_{e} - ν_μ$ oscillation space which could be responsible for the atmospheric muon neutrino deficit. These limits are sensitive to the predicted neutrino spectrum and can be strengthened as supernova models improve.

hep-ph↗