Robert T. Rood

Ultraviolet radiation from evolved stellar populations. I: Models

This series of papers comprises a systematic exploration of the hypothesis that the far-ultraviolet radiation from star clusters and elliptical galaxies originates from extremely hot horizontal-branch (HB) stars and their post-HB progeny. This first paper presents an extensive grid of calculations of stellar models from the zero-age horizontal branch (ZAHB) through to a point late in post-HB evolution or a point on the white dwarf cooling track. The grid will be used to produce synthesized UV fluxes for the interpretation of existing and future short-wavelength (900-3000 A) observations

Ultraviolet radiation from evolved stellar populations. 2: The ultraviolet upturn phenomenon in elliptical galaxies

We discuss the far-ultraviolet upturn phenomenon (UVX) observed in elliptical galaxies and spiral galaxy bulges. Our premise is the UV radiation from these systems emanates primarily from extreme horizontal branch (EHB) stars and their progeny. We derive the broad-band UV colors 1500-V and 2500-V for globular clusters and elliptical galaxies from the available satellite data and investigate color-color and color-line strength correlation. Clusters can be bluer than any galaxy in 15-V and 25-V, implying larger hot star populations, but galaxies are significantly bluer than clusters in 15-25 at a given 15-V. We attribute this primarily to the effect of metal abundance on the mid-UV (2500 A) light. These redder colors of the galaxies also imply that the UVX in galaxies is not produced by metal-poor subpopulations similar to the clusters. We devlop a simple spectral synthesis formulation for all phases of single star evolution from the zero-age main sequence (ZAMS) to the white dwarf cooling track that requires only one or two parameters for each choice of age and abundance. We provide the ingredients necessary for constructing models with arbitrary horizontal branch (HB) morphologies in the age range 2 less than t less than 20 Gyr and for six metallicities in the range -2.26 less than (Fe/H) less than 0.58; we also consider the efect of enhanced Y in metal-rich models. The maximum lifetime UV output is produced by EHB stars with (M(sub env))(sup 0) approximately 0.02 solar mass and can be up to 30 times higher than for post-asymptotic giant branch (P-AGB) stars. The ultraviolet output of old populations is governed primarily by the distribution of (M(sub env))(sup 0)P(M(sub env))(sup 0), on the ZAHB. The UV output is not very sensitive to (Fe/H) or to Y, but it can change very rapidly with (M(sub env))(sup 0). Thus it is extremely sensitive to the precise nature of giant-branch mass loss. Our models use simple descriptions of P(M(sub env))(sup 0) to bracket the colors produced from any real distribution of stars. Our models accurately predict the range of UV colors observed for the globular clusters, given known constraints on their age, abundances, and HB morphologies. We find that models with (Fe/H) greater than or = 0 that do not contain EHB stars cannot reproduce the colors of most of the galaxies. The models also predict that the fraction of the far-UV light from P-AGB stars, which are spatially resolvable in nearby galaxies, is approximately 70% and approximately 20% for moderate UVX and strong UVX systems, respectively. We find that 25-V, but not 15-V, is sensitive to the age and abundance, though these cannot always be cleanly distinguished. The galaxy colors place limits of (Fe/H) greater than -0.5 and less than 15% on the contribution of globular cluster-type populations to the V light. Galaxy colors are consistent with solar-abundance models with ages in the range 6-14 Gyr. We discuss several implications of the observations and the models, including the question of light metal versus iron peak enhancements in galaxies, whether the UV color-Mg(sub 2) correlation is continuous or discrete, effects of helium abundnace on the UVX, and the key question of whether red giant branch mass loss can be large enough to produce the necessary EHB population in the strong UVX galaxies.

Constraints on Cosmology and Neutrino Physics from Big Bang Nucleosynthesis

New calculations of primordial nucleosynthesis are presented and compared with the abundances of D, /sup 4/He, and /sup 7/Li to derive constraints to the number of two-component neutrino types (N/sub L/), the time-variation of the gravitational constant (Gapprox.t/sup -x/), and the present nucleon density (rho/sub N/=2 x 10/sup -29/..cap omega../sub N/h/sub 0//sup 2/). The observational and theoretical considerations related to the helium abundance are reviewed, and a primordial mass fraction Y< or approx. =0.25 is proposed. It follows from this limit to the /sup 4/He mass fraction that N/sub L/< or approx. =3,x<0.005,..cap omega../sub N/h/sub 0//sup 2/< or approx. =0.1. Independent considerations from the abundances of D, /sup 4/He, and /sup 7/Li all lead to the conclusion that the universe cannot be closed by nucleons.

THE GREEN BANK TELESCOPE H II REGION DISCOVERY SURVEY. II. THE SOURCE CATALOG

The Green Bank Telescope (GBT) H II Region Discovery Survey has doubled the number of known H II regions in the Galactic zone 343 degrees \textless= l \textless= 67 degrees with vertical bar b vertical bar \textless= 1 degrees. We detected 603 discrete hydrogen radio recombination line (RRL) components at 9 GHz (3 cm) from 448 targets. Our targets were selected based on spatially coincident mid-infrared and 20 cm radio continuum emission. Such sources are almost invariably H II regions; we detected hydrogen RRL emission from 95% of our target sample. The sensitivity of the GBT and the power of its spectrometer together made this survey possible. Here, we provide a catalog of the measured properties of the RRL and continuum emission from the survey nebulae. The derived survey completeness limit, 180 mJy at 9 GHz, is sufficient to detect all H II regions ionized by single O-stars to a distance of 12 kpc. These recently discovered nebulae share the same distribution on the sky as does the previously known census of Galactic HIT regions. On average, however, the new nebulae have fainter continuum fluxes, smaller continuum angular sizes, fainter RRL intensities, and smaller RRL line widths. Though small in angular size, many of our new nebulae show little spatial correlation with tracers associated with extremely young H II regions, implying that our sample spans a range of evolutionary states. We discovered 34 first quadrant negative-velocity H II regions, which lie at extreme distances from the Sun and appear to be part of the Outer Arm. We found RRL emission from 208 Spitzer GLIMPSE 8.0 mu m “bubble” sources, 65 of which have been cataloged previously. It thus appears that nearly all GLIMPSE bubbles are H II regions and that similar to 50% of all Galactic H II regions have a bubble morphology at 8.0 mu m.

Blue Straggler Stars: The Spectacular Population in M80*

Using Hubble Space Telescope WFPC2 observations in two ultraviolet (UV) filters (F225W and F336W) of the central region of the high-density Galactic globular cluster (GGC) M80, we have identified 305 blue straggler stars (BSS), which represents the largest and most concentrated population of BSS ever observed in a GGC. We also identify the largest clean sample of evolved BSS yet found. The high stellar density alone cannot explain the BSS, and we suggest that in M80 we are witnessing a transient dynamical state, during which stellar interactions are delaying the core-collapse process leading to an exceptionally large population of collisional BSS.

Multimodal Distributions along the Horizontal Branch

We report on the Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) U, V, and far-ultraviolet observations of three galactic globular clusters (GGCs), NGC 5272 = M3, NGC 6205 = M13, and NGC 6093 = M80. Two of these clusters (namely, M13 and M80) have horizontal-branch (HB) tails that extend to the helium-burning main sequence, with the hottest stars reaching theoretical effective temperatures above 35,000 K. In both clusters, groups of stars are found to be separated by narrow gaps along the blue HB sequence. These gaps appear at similar locations in the color-magnitude diagrams of the two clusters. While stochastic effects may give rise to variations in the color distribution along the HB, the coincidence of gaps in different clusters effectively rules this out as the primary cause. The comparison among the clusters strongly suggests that there are separate physical processes operating during the earlier red giant phase of evolution to produce mass loss.

The Electron Temperature Gradient in the Galactic Disk

We derive the electron temperature gradient in the Galactic disk, using a sample of H II regions that spans Galactocentric distances of 0-17 kpc. The electron temperature was calculated using high-precision radio recombination line and continuum observations for more than 100 H II regions. Nebular Galactocentric distances were calculated in a consistent manner, using the radial velocities measured by our radio recombination line survey. The large number of nebulae widely distributed over the Galactic disk, together with the uniformity of our data, provide a secure estimate of the present electron temperature gradient in the Milky Way. Because metals are the main coolants in the photoionized gas, the electron temperature along the Galactic disk should be directly related to the distribution of heavy elements in the Milky Way. Our best estimate of the electron temperature gradient is derived from a sample of 76 sources for which we have the highest quality data. The present gradient in electron temperature has a minimum at the Galactic center and rises at a rate of 287 ± 46 K kpc-1. There are no significant variations in the value of the gradient as a function of Galactocentric radius or azimuth. The scatter we find in the H II region electron temperatures at a given Galactocentric radius is not due to observational error, but rather to intrinsic fluctuations in these temperatures, which are almost certainly due to fluctuations in the nebular heavy-element abundances. Comparing the H II region gradient with the much steeper gradient found for planetary nebulae suggests that the electron temperature gradient evolves with time, becoming flatter as a consequence of the chemical evolution of the Milky Ways disk.

Discovery of Another Peculiar Radial Distribution of Blue Stragglers in Globular Clusters: The Case of 47 Tucanae*

We have used the high-resolution Wide Field Planetary Camera (WFPC2) on the Hubble Space Telescope (HST) and wide-field ground-based observations to construct a catalog of blue straggler stars (BSSs) in the globular cluster 47 Tuc spanning the entire radial extent of the cluster. The BSS distribution is highly peaked in the cluster center, rapidly decreases at intermediate radii, and finally rises again at larger radii. The observed distribution closely resembles that discovered in M3 by Ferraro and coworkers. To date, complete BSS surveys covering the full radial extent (from HST for the center and wide-field CCD, ground-based observations for the exterior) have only been performed for these two clusters. Both show a bimodal radial distribution despite their different dynamical properties. BSS surveys covering the full spatial extent of more globular clusters are clearly required to determine how common bimodality is and what its consequences are for theories of BSS formation and cluster dynamics.

Blue Straggler Stars: A Direct Comparison of Star Counts and Population Ratios in Six Galactic Globular Clusters

The central regions of six Galactic globular clusters (GGCs) (M3, M80, M10, M13, M92, and NGC 288) have been imaged using HST-WFPC2 and the ultraviolet filters (F255W and F336W). The selected sample covers a large range in both central density (log ρ0) and metallicity ([Fe/H]). In this paper, we present a direct cluster-to-cluster comparison of the blue straggler star (BSS) population as selected from (m255, m255-m336) color magnitude diagrams. We have found (1) that BSSs in three of the clusters (M3, M80, M92) are much more concentrated toward the center of the cluster than the red giants; because of the smaller BSS samples for the other clusters, we can only note that the BSS radial distributions are consistent with central concentration; and (2) that the specific frequency of BSSs varies greatly from cluster to cluster. The most interesting result is that the two clusters with largest BSS specific frequency are at the central density extremes of our sample: NGC 288 (lowest central density) and M80 (highest). This evidence, together with the comparison with theoretical collisional models, suggests that both stellar interactions in high-density cluster cores and at least one other alternate channel operating in low-density GGCs play an important role in the production of BSSs. We also note a possible connection between horizontal-branch morphology and blue straggler luminosity functions in these six clusters.

The Contribution of Primordial Binaries to the Blue Straggler Population in 47 Tucanae

The recent observation (Ferraro et al. 2003b) of the blue str aggler population in 47 Tucanae gives the first detailed characterization of their spatial distribution i n the cluster over its entire volume. Relative to the light distribution, blue stragglers appear to be overabundant in the core and at large radii. The observed surface density profile shows a central peak, a zone of avoidance and a rise bey ond twenty core radii. In light of these findings we explored the evolution of blue stragglers mimicking their dynamics in a multi-mass King model for 47 Tucanae. We find that the observed spatial distribution can not be expl ained within a purely collisional scenario in which blue stragglers are generated exclusively in the core throu gh direct mergers. An excellent fit is obtained if we require that a sizable fraction of blue stragglers is generate d in the peripheral regions of the cluster inside primordial binaries that evolve in isolation experiencing mass-trans fer. Subject headings: stars: blue stragglers - binaries: general - globular clust ers: individual (47 Tuc)