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NASA NTRS1994

The helium-to-hydrogen ratio of the UV-bright star Barnard 29 of Messier 13

Barnard 29 is the most luminous UV-bright star of the globular cluster Messier 13. Comparison of its reddening corrected optical region spectrophotometry and IUE low dispersion fluxes and its H-gamma profile with the predictions of metal-poor model atmospheres having the metallicity of M 13 indicated T(sub eff) = 20250 K, log g = 3.15. Observations of He I lambda(4026) taken with echelle spectrograph of the Multi-Mirror telescope using a Reticon detector show that He/H = 0.055 +/- 0.020. Some astrophysical implications of this result are discussed.

Adelman, S. J.↗

NASA NTRS1994

Comparative pulsation calculations with OP and OPAL opacities

Comparative linear nonadiabatic pulsation calculations are presented using the OPAL and Opacity Project opacities. The two sets of opacities include effects due to intermediate coupling and fine structure as well as new abundances. We used two mass luminosity (M-L) relations, one standard (BIT), and one employing substantial convective core overshoot (COV). The two sets of opacities cannot be differentiated on the basis of the stellar pulsation calculations presented here. The BIT relation can model the beat and bump Cepheids with masses between 4 and 7 solar mass, while if the overshoot relation is used, masses between 2 and 6 solar mass are required. In the RR Lyrae regime, we find the inferred masses of globular cluster RRd stars to be little influenced by the choice of OPAL or OP. Finally, the limited modeling we have done is not able to constrain the Cepheid M-L relation based upon period ratios observed in the beat and bump stars.

Kanbur, Shashi M.↗

NASA NTRS1994

A possible optical counterpart for the triple pulsar system PSR B1620-26

We have searched deep optical images of the globular cluster M4 for a possible optical counterpart for the third body in the PSR B1620-26 system. We identify as a possible optical counterpart a star located within 0.3 sec of the nominal pulsar position with V approximately equal to 20. This magnitude is consistent with a main-sequence cluster member with M approximately equal to 0.45 solar mass. However, this star may be blended with fainter stars, and the probability of a chance superposition is nonnegligible.

Bailyn, Charles D.↗

NASA NTRS1995

Very low mass stars and white dwarfs in NGC 6397

Deep Wide Field/Planetary Camera 2 (WFPC2) images in wide bands centered at 606 and 802 nm were taken with the Hubble Space Telescope (HST) 4.6 min from the center of the galactic globular cluster NGC 6397. The images were used to accurately position approximately 2120 stars detected in the field on a color magnitude diagram down to a limiting magnitude m(sub 814) approximately = m(sub I) approximately = 26 determined reliably and solely by counting statistics. A white dwarf sequence and a rich, narrow cluster main sequence are detected for the first time, the latter stretching from m(sub 814) = 18.5 to m(sub 814) = 24.0 where it becomes indistinguishable from the field population. Two changes of slope of the main sequence at m(sub 814) approximately = 20 and m(sub 814) approximately = 22.5 are evident. The corresponding luminosity function increases slowly from M(sub 814) approximately = 6.5 to 8.5 are expected from ground-based observations but then drops sharply from there dwon to the measurement limit. The corresponding mass function obtained bu using the only presently available mass-luminosity function for the cluster's metallicity rises to a plateau between approximately 0.25 and approximately 0.15 solar mass, but drops toward the expected mass limit of the normal hydrogen burning main sequence at approximately 0.1 solar mass. This result is in clear contrast to that obtained from the ground and implies either a substantial modification of the cluster's initial mass function due to dynamical evolution in its lifetime, or that very low mass stars are not produced in any dynamically significant amount by clusters of this type. The white dwarf sequence is in reasonable agreement with a cooling sequence of models of mass 0.5 solar mass at the canonical distance of NGC 6397 with a scatter that is most likely due to photometric errors, but may also reflect real differences in mass or chemical composition. Contamination from unresolved galaxies, which cannot be reliably identified with our filters, makes it difficult to meaningfully compare the observed white dwarf luminosity function with its theoretical counterpart.

Paresce, Francesco↗

NASA NTRS1993

DOPHOT, a CCD photometry program: Description and tests

The design considerations and operational features of DOPHOT, a point-spread function (PSF) fitting photometry program, are described. Some relevant details of the PSF fitting are discussed. The quality of the photometry returned by DOPHOT is assessed via reductions of an 'artificial' globular cluster generated from a list of stars with known magnitudes and colors. Results from comparative tests between DOPHOT and DAOPHOT using this synthetic cluster and real data are also described.

Schechter, Paul L.↗

NASA NTRS1993

The stellar population and luminosity function in M31 bulge and Inner Disk Fields

We report infrared photometry and stellar identifications for stars in five fields in the M31 bulge located from 2 to 11 arcmin from the nucleus. These fields have been chosen such that the bulge/disk star ratio predicted from Kent's (1989) small bulge model varies from 7:1 to 1:5, allowing a study of near pure disk and near pure bulge stellar populations. We reject the hypothesis of Davies et al. (1991) that luminous stars found within 500 pc of the nucleus are due to a contaminating disk population. We find that the bulge contains stars in excess of M(sub bol) = -5 mag and that the bulge luminosity function has a distinct shape different from the disk fields. We find many stars redder than (J-K) = 2 mag, and suggest that these stars may be the counterparts of the IRAS-selected Galactic bulge Miras studied by Whitelock et at. (1991). The number of bright stars (M(sub bol) is less than -5 mag) falls off more rapidly than the r band surface brightness. By building model fields out of a bulge luminosity function and artificial stars, we are able to show that the change in the luminosity function toward the center cannot be explained simply by the mismeasurement of overcrowded star images. However, these tests also raise the possibility that the asymptotic giant branch (AGB) tip may be approximately equal to 1 mag fainter than actually measured in our most crowded field, reaching only M(sub bol) = -5. We compare observed counts of AGB stars with those predicted from theoretical lifetimes using a technique of general interest for this problem, the Fuel Consumption Theorem of Renzini & Buzzoni (1986) Spectral Evolution of Galaxies (Reidel, Dordrecht). Our methodology is generally applicable to the study of other resolved extragalactic stellar populations. The number of observed stars per magnitude up to a luminosity of M(bol) = -5.5 mag is consistent with AGB evolution of the whole population of the innermost bulge field with the standard lifetime on the AGB of 1.3 Myr/mag. We advance the possibility that the bulge of M31 may be younger than the oldest Galactic globular clusters. We note that M33 has recently been found to have an r(exp 1/4)-law spheroid consisting of intermediate-age stars; bulges can form later than the old halo population.

Rich, R. Michael↗

NASA NTRS1994

Detecting bimodality in astronomical datasets

We discuss statistical techniques for detecting and quantifying bimodality in astronomical datasets. We concentrate on the KMM algorithm, which estimates the statistical significance of bimodality in such datasets and objectively partitions data into subpopulations. By simulating bimodal distributions with a range of properties we investigate the sensitivity of KMM to datasets with varying characteristics. Our results facilitate the planning of optimal observing strategies for systems where bimodality is suspected. Mixture-modeling algorithms similar to the KMM algorithm have been used in previous studies to partition the stellar population of the Milky Way into subsystems. We illustrate the broad applicability of KMM by analyzing published data on globular cluster metallicity distributions, velocity distributions of galaxies in clusters, and burst durations of gamma-ray sources. FORTRAN code for the KMM algorithm and directions for its use are available from the authors upon request.

Ashman, Keith A.↗

NASA NTRS1995

Sigma observations of the low mass X-ray binaries of the galactic bulge

The soft gamma-ray telescope (35-1300 keV) SIGMA aboard the high energy GRANAT space observatory has been monitoring the Galactic Bulge region for more than 2000 h of effective time since March 1990. In the resulting average 35-75 keV image we detected ten sources at a level of greater than 5 standard deviations, 6 of which can be identified with low mass X-ray binaries (LMXB). Among them, one is the 1993 X-ray nova in Ophiuchus (GRS 1726-249), one is an X-ray pulsar (GX 1+4), two are associated with X-ray bursters (GX 354-0 and A 1742-294) and two with bursting X-ray binaries in the globular clusters Terzan 2 and Terzan 1. Their spectral and long term variability behavior as measured by SIGMMA are presented and discussed.

Goldwurm, A.↗

NASA NTRS1993

A high-frequency radio observation of NGC 6624

We report a high-frequency Very Large Array (VLA) observation of the globular cluster NGC 6624. No source is detected at 8 GHz, which implies that the radio source seen at 1.4 GHz (Johnston & Kulkarni 1992) is in fact one of the pulsars, and not the X-ray binary.

Johnston, Helen M.↗

NASA NTRS1995

White dwarfs and mass segregation in the core of 47 Tucanae

The core of the galactic globular cluster (GC) 47 Tucanae was observed by the Faint Object Camera (FOC) on the refurbished Hubble Space Telescope (HST) for the first time through medium-band F253M and F346M filters centered at 255 and 348 nm. A total of 442 stars down to the detection limit at m(sub 346) approximately equal to 24 in the 7 sec x 7 sec field were accurately placed on a m(sub 346) versus m(sub 253) - M(sub 346) magnitude diagram to characterize the stellar populations in the core. Approximately 100 objects above and to the right of the main-sequence (MS) turnoff (TO) are the same as those classified earlier with the aberrated HST, but the rest below it are new objects seen now for the first time thanks to the substantial improvement in sensitivity. The new objects form a well-defined main sequence whose luminosity function (LF) turns over dramatically at m(sub 346) approximately equal to 19 well before the completeness limit and well before the end of the slowly increasing luminosity function for the outer fields measured from the ground by Hesser et al. We interpret this premature drop in the number of stars below approximately 0.8 solar masses in the core as the effect of mass segregation due to two-body relaxation. The position of nine objects in the range 20 less than m(sub 346) less than 24 and -1.5 less than m(sub 253) - m(sub 346) less than 0 is consistent with the expected location of a approx. a half solar mass white dwarf cooling sequence and luminosity function.

Paresce, Francesco↗

NASA NTRS1994

A CCD survey for faint high-latitude carbon stars

We describe a wide-area CCD survey to search for faint high-latitude carbon (FHLC) stars. Carbon giants provide excellent probes of the structure and kinematics of the outer Galactic halo. We use two-color photometric selection with large-format CCDs to cover 52 sq deg of sky to a depth of about V = 18. Of 94 faint C star candidates from our own CCD survey, one highly ranked V = 17 candidate was found to have a strong carbon and CN bands. We estimate that, to a depth of V = 18, the surface density of FHLC stars is 0.02 deg(exp -2). An updated FHLC sample is used to constrain halo kinematic and structural parameters. Although larger samples are needed, the effective radius of FHLC giants, assuming a de Vancouleurs law distribution, is larger than that for Galactic globular clusters.

Green, Paul J.↗

NASA NTRS1995

On blue straggler information by direct collisions of main sequence stars

We report the results of new smoothed particle hydrodynamics calculations of parabolic collisions between main-sequence (MS) stars. The stars are assumed to be close the MS turnoff point in a globular cluster and are therefore modeled as n = 3, Gamma = 5/3 polytropes. We find that the high degree of central mass concentration in these stars has a profound effect on the hydrodynamics. In particular, very little hydrodynamic mixing occurs between the dense, helium-rich inner cores and the outer envelopes. As a result, and in contrast to what has been assumed in previous studies, blue stragglers formed by direct stellar collisions are not necessarily expected to have anomalously high helium abundances in their envelopes or to have their cores replenished with fresh hydrogen fuel.

Lombardi, James, C. jr.↗

NASA NTRS1995

A determination of the thick disk chemical abundance distribution: Implications for galaxy evolution

We present a determination of the thick disk iron abundance distribution obtained from an in situ sample of F/G stars. These stars are faint, 15 less than or approximately = V less than or approximately = 18, selected on the basis of color, being a subset of the larger survey of Gilmore and Wyse designed to determine the properties of the stellar populations several kiloparsecs from the Sun. The fields studied in the present paper probe the iron abundance distribution of the stellar populations of the galaxy at 500-3000 pc above the plane, at the solar Galactocentric distance. The derived chemical abundance distributions are consistent with no metallicity gradients in the thick disk over this range of vertical distance, and with an iron abundance distribution for the thick disk that has a peak at -0.7 dex. The lack of a vertical gradient argues against slow, dissipational settling as a mechanism for the formation of the thick disk. The photometric and metallicity data support a turn-off of the thick disk that is comparable in age to the metal-rich globular clusters, or greater than or approximately = 12 Gyr, and are consistent with a spread to older ages.

Gilmore, Gerard↗

NASA NTRS1995

GAIA and beyond: The perspective from the milliarcseconds of Hubble and Hipparcos

Astrometry with the Hubble space telescope and Hipparcos satellite advanced the optical astrometric accuracies to the milliarcsec level. The global astrometric interferometer for astrophysics (GAIA) satellite and other proposed optical interferometry space missions would advance the optical astrometric accuracy to better than 10 microarcsec. A catalog figure of merit is defined which allows a quantitative comparison to be carried out for astrometric catalogs. Using two specific astrophysical problems, the level of the expected scientific contribution from a GAIA-type mission is assessed. The two problems are: the age of the globular clusters in relation to the age of the Universe, and the initial mass function compared with fluctuations in the star formation rate with time.

Hemenway, Paul D.↗

NASA NTRS1997

Hubble Space Telescope Astrometry Science Team

Topics considered include: Calibration and Astrometry Pipeline; Astrometry Anomaly Analysis; The STAT pipeline and software analysis tools; GTO Program; Parallaxes of Astrophysically Interesting Objects; HST- Hipparcos Extragalactic Link; Ground-Based Observations of Radial Velocities; GTO Globular Cluster Internal Motions; and GTO Hyades Cluster Member FGS Parallaxes.

Benedict, George F.↗

NASA NTRS1998

Period Derivative of the M15 X-Ray Binary AC211/X2127+119

We have combined Rossi X-ray Timing Explorer observations of X2127+119, the low-mass X-ray binary in the globular cluster M15, with archival X-ray lightcurves to study the stability of the 17.1 hr orbital period. We find that the data cannot be fit by the Ilovaisky ephemeris, and requires either a 7.sigma change to the period or a period derivative P prime/P approximately 9 x 10(exp -7)/yr. Given its remarkably low L(sub X)/L(sub opt) such a P prime lends support to models that require super-Eddington mass transfer in a q approximately 1 binary.

Homer, L.↗

NASA NTRS1999

Proceedings of the Workshop on the Spectrophotometric Dating of Stars and Galaxies

In the past decade, we have seen an avalanche of new observational results from space observatories and ground-based observatories. These observations have revealed young globular clusters in the cores of merger galaxies, elliptical galaxies at redshifts up to z=1.5, and starburst galaxies at high redshift. Analyses of the detailed spectra or color- magnitude diagrams of these systems promise to give a new understanding of evolutionary processes and to provide a check on cosmological ages. At the same time, these new spectro-photometric data present new challenges to current methods of spectral analysis and modeling.At the Workshop, we will discuss these new opportunities and challenges on spectro-photometric dating of stars and galaxies.

Hubeny, Ivan↗

NASA NTRS1999

Physical Parameters of Hot Horizontal-Branch Stars in NGC 6752: Deep Mixing and Radiative Levitation

Atmospheric parameters (T(sub eff), log g and log n(sub He)/n(sub H-dot)) are derived for 42 hot horizontal branch (HB) stars in the globular cluster NGC 6752. For 19 stars Mg II and Fe II lines are detected indicating an iron enrichment by a factor 50 on average with respect to the cluster abundance whereas the magnesium abundances are consistent with the cluster metallicity. This finding adds to the growing evidence that radiative levitation plays a significant role in determining the physical parameters of blue HB stars. Indeed, we find that iron enrichment can explain part, but not all, of the problem of anomalously low gravities along the blue HB. Thus the physical parameters of horizontal branch stars hotter than about 11,500 K in NGC 6752, as derived in this paper, are best explained by a combination of helium mixing and radiative levitation effects.

Moehler, S.↗