arXiv subjects

John N. Bahcall

Publications and source records attributed to John N. Bahcall.

At least 73 records · Page 4Linked to original sources

arXiv1997

The Be-7 Electron Capture Rate in the Sun

For solar conditions, we numerically integrate the density matrix equation for a thermal electron in the field of a Be-7 ion and other plasma ions and smeared-out electrons. Our results are in agreement with previous calculations that are based on a different physical picture, a picture which postulates the existence of distinct continuum and bound state orbits for electrons. The density matrix calculation of the electron capture rate is independent of the nature of electron states in the solar plasma. To within a 1% accuracy, the effects of screening can be described at high temperatures by a Salpeter-like factor which can be derived from the density matrix equation. The total theoretical uncertainty in the electron capture rate is about 2%.

astro-ph↗

arXiv1996

Hubble Space Telescope Images of a Sample of Twenty Nearby Luminous Quasars

Observations with the Wide-Field Camera of the Hubble Space Telescope are presented for a representative sample of 20 intrinsically luminous quasars with redshifts smaller than 0.30. These observations show that luminous quasars occur in diverse environments that include ellipticals as bright as the brightest cluster galaxies (2), apparently normal ellipticals (10), apparently normal spirals with H II regions (3), complex systems of gravitationally interacting components (3), and faint surrounding nebulosity (2). The quasar host galaxies are centered on the quasar to the accuracy of our measurements, 0.4 kpc. There are more radio quiet quasars in galaxies that appear to be ellipticals (7) than in spiral hosts (3), contrary to expectations. The average luminosity of the hosts is 2.2 magnitudes brighter than expected for a field galaxy luminosity function. We detect eight companion galaxies within projected distances of 10 kpc from quasar nuclei. Best figs. at http://www.sns.ias.edu/~jnb/Html/preprints.html

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arXiv1996

Are Standard Solar Models Reliable?

The sound speeds of solar models that include element diffusion agree with helioseismological measurements to a rms discrepancy of better than 0.2% throughout almost the entire sun. Models that do not include diffusion, or in which the interior of the sun is assumed to be significantly mixed, are effectively ruled out by helioseismology. Standard solar models predict the measured properties of the sun more accurately than is required for applications involving solar neutrinos.

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arXiv1996

Globular Cluster Photometry with the Hubble Space Telescope. VI. WF/PC-I Observations of the Stellar Populations in the Core of M13 (NGC 6205)

To probe the nature of stellar evolution in dense environments, we study the dense core of the globular cluster Messier 13 using pre-refurbishment Planetary Camera-I images obtained with the Hubble Space Telescope. We find 15 blue straggler star candidates and 10 other possible blue stragglers in this region of M13. Their specific frequency is in the range 0.04-0.07, comparable to what is observed near the centers of other dense clusters. A comparison between M13's observed V band stellar luminosity function and a theoretical model (Bergbusch & Vandenberg 1992) for the luminosity function of an old, metal-poor cluster shows that the model predicts too few of the brightest red giants (V~12.5-15) by a factor of two relative to subgiants/turnoff stars (> 6 sigma effect). The radial distributions of the red giants, blue stragglers, and subgiants are consistent with one another. Stars in the blue horizontal branch of M13, however, appear to be centrally depleted relative to other stellar types. We combine data from three dense `King model clusters', M13, M3, and 47 Tuc, and two post core collapse clusters, M30 and M15, and compare the distributions of various stellar types as a function of (r/r_half light) and (r/r_core). The horizontal branch stars in the combined sample appear to be centrally depleted relative to the giants (97% significance) - this depletion is only marginal (< about 1.5 sigma) in each of the clusters taken individually. The blue stragglers in the combined sample are centrally concentrated relative to the giants.

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arXiv1996

M Dwarfs From Hubble Space Telescope Star Counts III: The Groth Strip

We analyze the disk M dwarfs found in 31 new fields observed with the Wide Field Camera (WFC2) on the Hubble Space Telescope (HST) together with the sample previously analyzed from 22 WFC2 fields and 162 prerepair Planetary Camera (PC1) fields. The new observations, which include the 28 high-latitude fields comprising the Large Area Multi-Color Survey (``Groth Strip''), increase the total sample to 337 stars, and more than double the number of late M dwarfs (M_V>13.5) from 23 to 47. The mass function changes slope at M~0.6 Msun, from a near-Salpeter power-law index of α=-1.21 to α=0.44. In both regimes the mass function at the Galactic plane is given by ${d^3 N / d\log M d M_V d V} = 8.1\times 10^{-2}\pc^{-3} ({M / 0.59 M_\odot})^α$. The correction for secondaries in binaries changes the low-mass index from α=0.44 to α\sim 0.1. If the Salpeter slope continued to the hydrogen-burning limit, we would expect 500 stars in the last four bins (14.5<M_V<18.5), instead of the 25 actually detected. The explanation of the observed microlensing rate towards the Galactic bulge requires either a substantial population of bulge brown dwarfs or that the disk and bulge mass functions are very different for stars with M~< 0.5 Msun.

astro-ph↗

arXiv1996

Status of Solar Models

The neutrino fluxes calculated from 14 standard solar models published recently in refereed journals are inconsistent with the results of the 4 pioneering solar neutrino experiments if nothing happens to the neutrinos after they are created in the solar interior. The sound speeds calculated from standard solar models are in excellent agreement with helioseismological measurements of sound speeds. Some statements made by Dar at Neutrino 96 are answered here.

hep-ph↗

arXiv1996

Neutrino Oscillations and Moments of Electron Spectra

We show that the effects of neutrino oscillations on 8B solar neutrinos are described well by the first two moments (the average and the variance) of the energy distribution of scattered or recoil electrons. For the SuperKamiokande and the Sudbury Neutrino Observatory experiments, the differences between the moments calculated with oscillations and the standard, no-oscillation moments are greater than 3 standard deviations for a significant fraction of the neutrino mass-mixing (Delta m^2, sin^2 2 theta) parameter space.

nucl-ex↗

arXiv1996

Tests of electron flavor conservation with the Sudbury Neutrino Observatory

We analyze tests of electron flavor conservation that can be performed at the Sudbury Neutrino Observatory (SNO). These tests, which utilize $^8$B solar neutrinos interacting with deuterium, measure: 1) the shape of the recoil electron spectrum in charged-current (CC) interactions (the CC spectrum shape); and 2) the ratio of the number of charged current to neutral current (NC) events (the CC/NC ratio). We determine standard model predictions for the CC spectral shape and for the CC/NC ratio, together with realistic estimates of their errors and the correlations between errors. We consider systematic uncertainties in the standard neutrino spectrum and in the charged-current and neutral current cross-sections, the SNO energy resolution and absolute energy scale, and the SNO detection efficiencies. Assuming that either matter-enhanced or vacuum neutrino oscillations solve the solar neutrino problems, we calculate the confidence levels with which electron flavor non-conservation can be detected using either the CC spectrum shape or the CC/NC ratio, or both. If the SNO detector works as expected, the neutrino oscillation solutions that best-fit the results of the four operating solar neutrino experiments can be distinguished unambiguously from the standard predictions of electron flavor conservation.

hep-ph↗

arXiv1996

Standard Neutrino Spectrum from B-8 Decay

We present a systematic evaluation of the shape of the neutrino energy spectrum produced by beta-decay of $^8$B. We place special emphasis on determining the range of uncertainties permitted by existing laboratory data and theoretical ingredients (such as forbidden and radiative corrections). We review and compare the available experimental data on the $^8$B$(β^+){}^8$Be$(2α)$ decay chain. We analyze the theoretical and experimental uncertainties quantitatively. We give a numerical representation of the best-fit (standard-model) neutrino spectrum, as well as two extreme deviations from the standard spectrum that represent the total (experimental and theoretical) effective $\pm3σ$ deviations. Solar neutrino experiments that are currently being developed will be able to measure the shape of the $^8$B neutrino spectrum above about 5 MeV. An observed distortion of the $^8$B solar neutrino spectrum outside the range given in the present work could be considered as evidence, at an effective significance level greater than three standard deviations, for physics beyond the standard electroweak model. We use the most recent available experimental data on the Gamow--Teller strengths in the $A=37$ system to calculate the $^8$B neutrino absorption cross section on chlorine: $σ_{\rm Cl}=(1.14\pm0.11)\times10^{-42}$~cm$^2$ ($\pm3σ$ errors). The chlorine cross section is also given as a function of the neutrino energy. The $^8$B neutrino absorption cross section in gallium is $σ_{\rm Ga}=(2.46^{+2.1}_{-1.1})\times10^{-42}$ cm$^2$ ($\pm3σ$ errors).

nucl-th↗

arXiv1996

Hubble Deep Field Constraint on Baryonic Dark Matter

We use a new technique to search for faint red stars in the Hubble Deep Field (HDF) imaged by the Wide Field Camera (WFC2) on the {\it Hubble Space Telescope}. We construct a densely sampled stellar light profile from a set of undersampled images of bright stars in a low-latitude field. Comparison of this stellar light profile to densely sampled profiles of individual objects in the HDF constructed from multiple undersampled dithers allows us to distinguish unambiguously between stars and galaxies to $I=26.3$. We find no stars with $V-I>1.8$ in the outer 90\% of the volume probed. This result places strong and general constraints on the $I$ band luminosity of the constituents of the Galactic dark halo: $$M_I > 15.9 + {5\over 3}\log\biggl(f{0.5 M_\odot\over M}\biggr)\qquad (V-I>1.8),$$ where $M$ is the mass of the objects and $f$ is their density as a fraction of the local halo density, taken to be $ρ_0=9\times 10^{-3}\,M_\odot\,\pc^{-3}$. If the halo is made of white dwarfs, this limit implies that these objects have $M_V\gsim 18.4$ and $V-I\gsim 2.5$. That is, they are $\gsim 2$ magnitudes fainter than the end of the disk white dwarf sequence. Faint red dwarfs account for $<1\%$ of the Galactic dark halo for $M_I<14$, and $<6\%$ for $M_I<15$ at the 95\% confidence level. The density of inter-galactic Local Group stars is at least a factor 3000 smaller than the density of local Population II stars.

astro-ph↗

arXiv1996

Galaxy Clustering Around Nearby Luminous Quasars

We examine the clustering of galaxies around a sample of 20 luminous low redshift (z<0.30) quasars observed with the Wide Field Camera-2 on the Hubble Space Telescope. The HST resolution makes possible galaxy identification brighter than V=23.5 and as close as 2'' to the quasar. We find a significant enhancement of galaxies within a projected separation of < 100 kpc/h of the quasars. If we model the qso/galaxy correlation function as a power law with a slope given by the galaxy/galaxy correlation function, we find that the ratio of the qso/galaxy to galaxy/galaxy correlation functions is $3.8\pm 0.8$. The galaxy counts within r<15 kpc/h of the quasars are too high for the density profile to have an appreciable core radius ( > 100 kpc). Our results reinforce the idea that low redshift quasars are located preferentially in groups of 10-20 galaxies rather than in rich clusters. We see no significant difference in the clustering amplitudes derived from radio-loud and radio-quiet subsamples.

astro-ph↗

arXiv1996

The Temperature Dependence of Solar Neutrino Fluxes

By comparing neutrino fluxes and central temperatures calculated from 1000 detailed numerical solar models, we derive improved scaling laws which show how each of the neutrino fluxes depends upon the central temperature (flux $\propto T^m$); we also estimate uncertainties for the temperature exponents. With the aid of a one-zone model of the sun, we derive expressions for the temperature exponents of the neutrino fluxes. For the most important neutrino fluxes, the exponents calculated with the one-zone model agree to within 20\% or better with the exponents extracted from the detailed numerical models. The one-zone model provides a physical understanding of the temperature dependence of the neutrino fluxes. For the $pp$ neutrino flux, the one-zone model explains the (initially-surprising) dependence of the flux upon a negative power of the temperature and suggests a new functional dependence. This new function makes explicit the strong anti-correlation between the $^7$Be and $pp$ neutrino fluxes. The one-zone model also predicts successfully the average linear relations between neutrino fluxes, but cannot predict the appreciable scatter in a $Δϕ_i/ϕ_i$ versus $Δϕ_j/ϕ_j$ diagram.

astro-ph↗

arXiv1995

Observational Searches for Solar g-modes: Some Theoretical Considerations

We argue that the solar g-modes are unlikely to have caused the discrete peaks in the power spectrum of the solar wind flux observed by Thomson et al. (1995). The lower limit to the energy of individual g-modes, using the amplitudes given by Thomson et al., is estimated to be at least 10$^{36}$ erg for low order g-modes; the resulting surface velocity amplitude is at least 50 cm s$^{-1}$, larger than the observational upper limit (5 cm s$^{-1}$). We suggest that the most likely source for the excitation of solar g-modes is turbulent stresses in the convection zone. The surface velocity amplitude of low degree and low order g-modes resulting from this process is estimated to be of order 10$^{-2}$ cm s$^{-1}$. This amplitude is interestingly close to the detection threshold of the SOHO satellite. The long lifetime of g-modes ($\sim 10^6$ years for low order modes) should be helpful in detecting these small amplitude pulsations.

astro-ph↗

arXiv1995

Solar Neutrinos: Where We Are, Where We Are Going

This talk answers a series of questions. Why study solar neutrinos? What does the combined standard model (solar plus electroweak) predict for solar neutrinos? Why are the calculations of neutrino fluxes robust? What are the three solar neutrino problems? What have we learned in the first thirty years of solar neutrino research? For the next decade, what are the most important solvable problems in the physics of solar neutrinos? What are the most important problems in the astrophysics of solar neutrinos?

hep-ph↗

arXiv1995

Globular Cluster Photometry with the Hubble Space Telescope. V. WFPC2 Study of M15's Central Density Cusp

We describe images of the center of the dense globular cluster M15 (NGC 7078) obtained with the Hubble Space Telescope Wide Field and Planetary Camera 2 (WFPC2). Data taken in the F336W, F439W, and F555W filters (approximately U, B, and V) are used to study the surface density distribution of the $\sim3\times10^4$~stars detected in a 5~arcmin$^2$ region within $r<2'$ (6.7$\,$pc) of the cluster center. The surface density of stars in M15 (after correction for the effects of incompleteness and photometric bias/scatter) is well represented by a power law in radius: $N(r)\sim{r}^{-0.82\pm0.12}$, over the radial range $0.3''$ (0.017$\,$pc) to $6''$. The observed power law is remarkably similar to what is expected if the center of the cluster harbors a massive black hole. Non-parametric estimates of the density profile show a monotonic rise with decreasing radius all the way in to $r=0.3''$, the smallest radius at which the density can be reliably measured; there is no indication that the profile flattens at smaller radii. Any flat core of radius larger than $2''$ (0.11$\,$pc) in the stellar distribution is ruled out at the $>95$\% significance level. The star count profile is consistent with that expected from core-collapse models or with the predicted distribution around a massive (few times $10^3\,M_\odot$) black hole. The close triplet of bright stars, AC~214, is within $0.5''$ ($1.5σ$) of the cluster centroid position.

astro-ph↗

arXiv1995

The Apparently Normal Galaxy Hosts for Two Luminous Quasars

HST images (with WFPC2) of PHL~909\ ($z = 0.171$) and PG~0052$+$251\ ($z = 0.155$) show that these luminous radio-quiet quasars each occur in an apparently normal host galaxy. The host galaxy of PHL~909 is an elliptical galaxy ($\sim$ E4) and the host of PG~0052$+$251 is a spiral ($\sim$~Sb). Both host galaxies are several tenths of a magnitude brighter than $L^*$, the characteristic Schechter luminosity of field galaxies. The images of PHL~909 and PG~0052$+$251, when compared with HST images of other objects in our sample of 20 luminous, small-redshift ($z \leq 0.30$) quasars, show that luminous quasars occur in a variety of environments. The local environments of the luminous quasars range from luminous ellipticals, to apparently normal host galaxies, to complex systems of interacting components, to faint (and as yet undetected) hosts. The bright HII regions of the host galaxy of PG~0052$+$251 provide an opportunity to measure directly the metallicity of the host of a luminous quasar, to establish an upper limit to the mass of the nuclear AGN (i.e., the putative black hole source), and to test stringently the cosmological hypothesisthat the galaxy and the quasar are both at the distance indicated by the quasar redshift.

astro-ph↗

arXiv1995

The HST Quasar Absorption Line Key Project VII. Absorption Systems at $z_{\rm abs} \leq 1.3$

We present evidence that clumps of \lya lines are physically associated with about half of the extensive metal-line systems (absorption systems with four or more observed metal-line species) found in this paper, demonstrate that all four \lylimit systems discussed here correspond to extensive metal-line absorption systems, and present an extraordinary pair of extensive metal-line absorption systems within 2000~km/s of each other at $z~=~0.95$ that are probably an early manifestation of large scale structure. These results are obtained using ultraviolet spectra, taken with the higher-resolution gratings of the Faint Object Spectrograph of the Hubble Space Telescope, for four quasars with emission-line redshifts between 1.0 and 1.3. We also determine the evolution of \lya absorption lines at redshifts less than 1.3 by combining the results for 13 smaller redshift quasars discussed in Paper I of this series with the 4 moderate redshift quasars analyzed in the present paper. For $z_{\rm abs}~\leq~1.3$, the density of \lya lines with equivalent widths greater than 0.24~Å is adequately fit by $\left(dN/dz\right) =\left(dN/dz\right)_0 \cdot(1 + z)^γ$ with $(dN/dz)_0 = 24.3 \pm 6.6$ \lya lines per unit redshift, and $γ= 0.58 \pm 0.50$ (1-$σ$ uncertainties). This rate of evolution at low redshifts is less than the evolutionary rate inferred from several different ground-based data samples that pertain to high redshifts. The observed gaseous structures at redshifts of $0.5$ to $1.0$ with velocity dispersions of $6 \times 10^2$ \kms to $1.4 \times 10^3$ \kms (or velocity spans of $1.2 \times 10^3$ \kms to $3 \times 10^3$ \kms) constitute a constraint on cosmological models of structure formation.

astro-ph↗

arXiv1995

Solar Models with Helium and Heavy Element Diffusion

Heavy element and helium diffusion are both included for the first time in this series of papers on precise solar models, along with improvements in the input data for nuclear reaction rates, the solar luminosity, the solar age, heavy element abundances, radiative opacities, helium and metal diffusion rates, and neutrino interaction cross sections. The calculated neutrino event rates, including all of the improvements, are $9.3_{-1.4}^{+1.2}$ SNU for the \hbox{$^{37}$Cl} experimentand $137_{-7}^{+8} $ SNU for the \hbox{$^{71}$Ga} experiments. The calculated flux of \hbox{$^{7}$Be} neutrinos is $5.1(1.00_{-0.07}^{+0.06}) \times 10^{9}~{\rm cm^{-2}s^{-1}}$ and the flux of ${\rm ^8B}$ neutrinos is $6.5 (1.00_{-0.17}^{+0.14})\times 10^{6}~{\rm cm^{-2}s^{-1}}$. The present-day surface helium abundance of the model is $Y_s = 0.247$, in agreement with the helioseismological measurement of $Y_s = 0.242 \pm 0.003$ determined by Hernandez and Christensen-Dalsgaard (1994). The computed depth of the convective zone is \hbox{$R = 0.712~ R_{\odot}$} in agreement with the observed value determined from $p$-mode oscillation data of \hbox{$R = 0.713 \pm 0.003~ R_{\odot}$} found by Christensen-Dalsgaard {\it et al.}~(1991). Solar models that do not include diffusion disagree with the helioseismology measurements. Although the present results increase the predicted event rate in the four operating solar neutrino experiments by almost $1σ$ (theoretical uncertainty), they only slightly increase the difficulty of explaining the existing experiments with standard physics (i.e., by assuming that nothing happens to the neutrinos after they are created in the center of the sun).

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