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Soneira, R. M.

Publications and source records attributed to Soneira, R. M..

16 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Detection of binaries with projected separations as large as 0.1 parsec

Repeated, accurate radial velocity measurements have been made of the 19 candidate wide binary stellar pairs of Bahcall and Soneira. There are 16 unambiguous cases of which six pairs are physical binaries. Projected separations of the binaries are estimated using the observed angular separations and well-determined spectroscopic parallaxes. The projected separations vary from about 0.002 pc to 0.08 pc. This validation of the statistical techniques used in identifying the candidate wide binaries opens up the possibility of large-scale systematic studies of the characteristics of wide binaries.

Latham, D. W.↗
NASA NTRS1984

Comparisons of a standard galaxy model with stellar observations in five fields

Modern data on the distribution of stellar colors and on the number of stars as a function of apparent magnitude in five directions in the Galaxy are analyzed. It is found that the standard model is consistent with all the available data. Detailed comparisons with the data for five separate fields are presented. The bright end of the spheroid luminosity function and the blue tip of the spheroid horizontal branch are analyzed. The allowed range of the disk scale heights and of fluctuations in the volume density is determined, and a lower limit is set on the disk scale length. Calculations based on the thick disk model of Gilmore and Reid (1983) are presented.

Bahcall, J. N.↗
NASA NTRS1984

A supercluster catalog

The largest available complete redshift sample of rich clusters of galaxies is used to construct a three-dimensional catalog of superclusters complete to redhsift of about 0.08 or less. The criterion of membership in the catalog is the density enhancement of clusters over the mean density; the catalog is presented for various values of this parameter. The dependence of the identified sample of superclusters on the overdensity is studied, and the sensitivity of the identified superclusters to various quantities and selection procedures is investigated. The validity of the identification procedure is established by comparisons with random catalogs.

Bahcall, N. A.↗
NASA NTRS1983

Some constraints on the color-magnitude diagram of giants in the galactic spheroid

The color-magnitude diagram of giants in the Galactic spheroid is shown to be important in determining the number-color histogram of stars brighter than B = 19 mag. This result is demonstrated by comparing a standard Galaxy model with observations of 391 stars in a field in the direction of Aquarius (l = 36.5 deg, b = -51.1 deg). More than 80 percent of the spheroid stars and 40 percent of all stars in this magnitude range and direction are predicted to be giants. At most, a few percent of the spheroid stars in the current sample can lie on the main sequence bluer than the turn-off onto the giant branch near B - V approximately 0.4. The available observations suggest that the blue tip of the horizontal branch of the spheroid must be sparsely populated about a factor of 10 less than would be expected if the color-magnitude diagram of the spheroid were the same as diagrams for any of the globular clusters M3, M13, or M92. The total dispersion in colors (measurement errors and intrinsic dispersion) has a standard deviation in B - V color that is less than 0.2 mag.

Bahcall, J. N.↗
NASA NTRS1983

The spatial correlation function of rich clusters of galaxies

A series of objective statistical estimators is applied to directly study the three-dimensional distribution of rich clusters, using a recently completed redshift sample of 104 Abell clusters out to distance class four and galactic latitude of 30 deg or more. The sample and the relevant positional and redshift selection effects are discussed, and the two-point angular correlation function and nearest-neighbor distribution of clusters in the sample are determined, as well as in the larger D = 5+6 sample and in several subsamples of different richness classes and latitude zones. The redshift correlation function of clusters for various angular separations on the sky is examined as a test for the reality of the observed angular correlations. The two-point spatial correlation function of clusters in the sample is determined, and the consistency among the three observed correlation functions is tested. A model that describes all the observed correlations is presented.

Bahcall, N. A.↗
NASA NTRS1983

The galactic spheroid

Analytic approximations and numerical simulations are used to derive the characteristic behavior of star counts in a galactic spheroidal population, whose visible stars' stellar luminosity function is obtainable for galactocentric distances between about 4 and 12 kpc from star count observations above 30 deg galactic latitude. The total densities of stars and of mass in the spheroid at the solar position are evaluated using different assumed luminosity functions, in order to extrapolate the measured values to a wider range of absolute magnitudes. The upper limits to the frequency of intermediate population stars are derived for the absolute visual magnitude range of 5-8. If such stars occur in either a flat disk with a scale height of 3 kpc or a spheroid with an ellipticity of 0.5, their local surface density is less than 1.8 times that of the spheroid.

Bahcall, J. N.↗
NASA NTRS1983

The distribution of stars in the Galaxy

A discussion is presented of the basic ideas underlying the use of star counts to determine galactic parameters. Previous work using star counts is reviewed, and possible future observational programs that will be important in increasing the knowledge of the stellar content of the galaxy are indicated. The techniques used in star counts are illustrated by applications to the differential number-magnitude counts in SA 57, the frequency-color diagrams in SA 57 and SA 68, the paucity of intermediate population stars in the range M(V) = 5-8, and the M/L ratio for the unseen matter in the halo.

Bahcall, J. N.↗
NASA NTRS1982

An approximately 300 Mpc void of rich clusters of galaxies

Kirshner et al. (1981) have reported an apparent absence of relatively bright galaxies over an enormous volume in the direction of Bootes. This result, if statistically significant, has important implications for theories of galaxy clustering and formation. In connection with the study of the correlation function of rich clusters of galaxies, Bahcall and Soneira (1982) have noticed a large area in the sky, extending approximately 100 deg across, which is completely void of nearby, but not distant, rich Abell clusters. This apparent absence of rich clusters may represent a physical void. If this interpretation is correct, the void extends approximately 150 Mpc/h along the line of sight. The sample studied is Abell's (1958) statistical sample of clusters of galaxies. The distribution of rich clusters is discussed, taking into account observed versus expected characteristics. Attention is also given to aspects of absorption and galaxy counts in the considered region.

Bahcall, N. A.↗
NASA NTRS1982

Large-scale superclusters surrounding the giant galaxy void in Bootes

The overdensity of galaxies observed by Kirshner et al. (1981) on both redshift sides of the galaxy void in Bootes is found to coincide in redshift space with similar overdensities of clusters and superclusters of galaxies in the northern hemisphere. The main contributors to these overdensities are the rich Hercules and Corona Borealis superclusters. It is suggested that large-scale density enhancements of galaxies associated with the Hercules and Corona Borealis superclusters may extend to more than about 100-150/h Mpc in projection and surround the giant volume of galaxy depletion in Bootes. Previous observational evidence, together with the results presented here, suggests that galaxy voids may generally be associated with surrounding galaxy excesses.

Bahcall, N. A.↗
NASA NTRS1982

Stellar luminosity functions in the R, I, J, and K bands obtained by transformation from the visual band

The stellar luminosity function that has been measured in the visual band into the R, I, J, and K bands, where it has not yet been obtained directly, is transformed. The transformation is effected by subdividing the total visual function, which includes all stars, into subluminosity functions for each luminosity class (supergiants through white dwarfs), applying the known (V - D) color, D = R, I, J, K for each spectral type, and then summing the resultant transformed subluminosity functions into a total luminosity function for the band D. Simple analytic forms that accurately fit the transformed luminosity functions are given. Consideration is also given to the possibility of a systematic error resulting from the existence of a very red stellar population not accounted for in the visual band luminosity function

Mamon, G. A.↗
NASA NTRS1981

Predicted star counts in selected fields and photometric bands Applications to galactic structure, the disk luminosity function, and the detection of a massive halo

Tables are presented of predicted star counts in 17 selected directions on the sky as a function of apparent B, V, R, and I magnitudes and absolute visual magnitudes calculated from the Bahcall-Soneira Galaxy model. It is noted that eight of the fields were selected ab initio for their usefulness in determining from star counts either parameters of galactic structure, the faint end of the disk luminosity function, or the detection of a massive stellar halo. Star counts in specified fields to m sub I = 19 mag are able to reveal the faint end of the disk luminosity function down to the end of the hydrogen-burning main sequence. It is thought that star counts at high galactic latitudes to m sub I = 22 mag should reveal the stellar constituents of a massive halo if they are massive enough to be on the hydrogen-burning main sequence.

Bahcall, J. N.↗
NASA NTRS1981

Guide star probabilities

Probabilities are calculated for acquiring suitable guide stars (GS) with the fine guidance system (FGS) of the space telescope. A number of the considerations and techniques described are also relevant for other space astronomy missions. The constraints of the FGS are reviewed. The available data on bright star densities are summarized and a previous error in the literature is corrected. Separate analytic and Monte Carlo calculations of the probabilities are described. A simulation of space telescope pointing is carried out using the Weistrop north galactic pole catalog of bright stars. Sufficient information is presented so that the probabilities of acquisition can be estimated as a function of position in the sky. The probability of acquiring suitable guide stars is greatly increased if the FGS can allow an appreciable difference between the (bright) primary GS limiting magnitude and the (fainter) secondary GS limiting magnitude.

Soneira, R. M.↗
NASA NTRS1980

Star counts as an indicator of galactic structure and quasar evolution

A detailed model of the stellar content of the Galaxy is described briefly. Illustrative applications of the model are made, using existing data, to indicate how star counts can be used to determine some parameters of galactic structure, to detect a massive (stellar) halo, and to constrain models of quasar evolution.

Bahcall, J. N.↗
NASA NTRS1979

A cosmological redshift-distance square law

This paper directly examines the claims of Segal (1976) that the (m,z) Hubble diagram is fitted best by a square law z = Kr-squared rather than by the traditional Hubble law z = Hr in the low-redshift range, z no more than about 0.01, corresponding to galaxies brighter than 14th mag. Segal attempts to fit a distance relation to the (m,z) scatter diagram in which each individual galaxy is plotted. The exact relation between the mean redshift for all galaxies in a small magnitude interval and the apparent magnitude is calculated. This relation is independent of luminosity function and peculiar velocity distribution about the general expansion, and is not affected by sample incompleteness as a function of apparent magnitude or the clustering of galaxies in the sample. Segal's method is affected by all of these and requires a highly sophisticated statistical analysis to deal with the non-Gaussian pointwise scatter. The present analysis favors the Hubble law and conclusively rules out the square law for the small redshift region.

Soneira, R. M.↗