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Search Results: 1 - 10 of 143614 matches for " Robert T. Rood "
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Ultraviolet Radiation from Evolved Stellar Populations: II. The Ultraviolet Upturn Phenomenon in Elliptical Galaxies
Ben Dorman,Robert W. O'Connell,Robert T. Rood
Physics , 1994, DOI: 10.1086/175428
Abstract: We present an analysis of the far-ultraviolet upturn phenomenon (UVX) observed in elliptical galaxies and spiral galaxy bulges. Our premise is that the UV radiation from these systems emanates primarily from extreme horizontal branch (EHB) stars and their progeny. We re-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 correlations. We also provide the ingredients necessary for constructing models with arbitrary HB morphologies.
Age and Abundance Discrimination in Old Stellar Populations Using Mid-Ultraviolet Colors
Ben Dorman,Robert W. O'Connell,Robert T. Rood
Physics , 2003, DOI: 10.1086/375413
Abstract: The restframe mid-ultraviolet spectral region (2000-3200 A) is important in analyzing the stellar populations of the "red envelope" systems observed at high redshifts. Here, we explore the usefulness of the mid-UV for determining ages and abundances of old populations. A mid-UV to optical/IR wavelength baseline provides good separation of population components because the main sequence turnoff dominates the integrated light between 2500 and 4000 A. We find a six magnitude difference in the mid-UV continuum level over the metallicity range -1.5 < log Z/Z_o < +0.5 and a comparable difference (per unit log t) for ages in the range 4-16 Gyr. Logarithmic derivatives of mid-UV colors with respect to age or metal abundance are 3-10 times larger than for the UBV region. Most of the spectral information on old populations therefore resides below 4000 A. We investigate the capability of UBV and mid-UV broad-band colors to separately determine age and abundance, taking into account precision in the color measurements. We find that the mid-UV improves resolution in (log t,log Z) space by about a factor of 3 for a given observational precision. Contamination by hot horizontal branch phases can seriously affect mid-UV spectra, reaching over 80% in some cases. However, this is straightforward to remove as long as far-UV measurements are available. Finally, we show that a 4 Gyr, solar abundance model based on empirical spectra provides an excellent fit to the mid-UV spectrum of the E galaxy M32. This indicates that the poorer results obtained from theoretical spectra arise from limitations of the synthesis models for individual stars. [Condensed]
Blue Straggler Stars: The Spectacular Population in M80
Francesco R. Ferraro,Barbara Paltrinieri,Robert T. Rood,Ben Dorman
Physics , 1999, DOI: 10.1086/307700
Abstract: Using HST-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.
Modeling Mid-Ultraviolet Spectra. I. Temperatures of Metal-Poor Stars
Ruth C. Peterson,Ben Dorman,Robert T. Rood
Physics , 2000, DOI: 10.1086/322294
Abstract: Determining the properties of old stellar systems using evolutionary population synthesis requires a library of reliable model stellar fluxes. Empirical libraries are limited to spectra of stars in the solar neighborhood, with nearly solar abundances and abundance ratios. We report here a first step towards providing a flux library that includes nonsolar abundances, based on calculations from first principles that are calibrated empirically. We have started with main-sequence stars, whose light dominates the mid-ultraviolet spectrum of an old stellar system. We have calculated mid-ultraviolet spectra for the Sun and nine nearby, near-main-sequence stars spanning metallicities from less than 1/100 solar to greater than solar, encompassing a range of light-element abundance enhancements. We first determined temperatures of eight of the stars by analyzing optical echelle spectra together with the mid-ultraviolet. Both could be matched at the same time only when models with no convective overshoot were adopted, and only when an approximate chromosphere was incorporated near the surface of relatively metal-rich models. Extensive modifications to mid-UV line parameters were also required, notably the manual assignment of approximate identifications for mid-UV lines missing from laboratory linelists. Without recourse to additional missing opacity, these measures suffice to reproduce in detail almost the entire mid-UV spectrum of solar-temperature stars up to one-tenth solar metallicity, and the region from 2900A to 3100A throughout the entire metallicity range. Ramifications for abundance determinations in individual metal-poor stars and for age-metallicity determinations of old stellar systems are briefly discussed, with emphasis on the predictive power of the calculations.
A Superwind from Early Post-Red Giant Stars?
Noam Soker,Marcio Catelan,Robert T. Rood,Amos Harpaz
Physics , 2001, DOI: 10.1086/338443
Abstract: We suggest that the gap observed at 20,000 K in the horizontal branches of several Galactic globular clusters is caused by a small amount of extra mass loss which occurs when stars start to "peel off" the red giant branch (RGB), i.e., when their effective temperature starts to increase, even though they may still be on the RGB. We show that the envelope structure of RGB stars which start to peel off is similar to that of late asymptotic giant branch stars known to have a super-wind phase. An analogous super-wind in the RGB peel-off stars could easily lead to the observed gap in the distribution of the hottest HB stars.
The Origin of Extreme Horizontal Branch Stars
Noella Lambert d'Cruz,Ben Dorman,Robert T. Rood,Robert W. O'Connell
Physics , 1995,
Abstract: Strong mass loss on the red giant branch (RGB) can result in the formation of extreme horizontal branch (EHB) stars. The EHB stars spend most of their He core and shell burning phase at high temperatures and produce copious ultraviolet flux. They have very small hydrogen envelopes and occupy a small range in mass. We have computed evolutionary RGB models with mass loss for stars with a range of metallicities at initial masses < 1.1 Msun corresponding to populations ages between 12.5 and 14.5 Gyr. We used the Reimers formula to characterize mass loss, but investigated a larger range of the mass loss efficiency parameter, eta, than is common. To understand how the number of EHB stars varies with metallicity in a stellar population we considered how the zero-age horizontal branch (ZAHB) is populated. The range in eta producing EHB stars is comparable to that producing `mid-HB' stars. Somewhat surprisingly, neither the range nor magnitude of eta producing EHB stars varies much metallicity. In contrast, the range of eta producing mid-HB stars decreases with increasing metallicity. Hence the HB of populations with solar metallicity and higher, such as expected in elliptical galaxies and spiral bulges, will be bimodal if the distribution covers a sufficiently large range in eta.
HII Region Metallicity Distribution in the Milky Way Disk
Dana S. Balser,Robert T. Rood,T. M. Bania,L. D. Anderson
Physics , 2011, DOI: 10.1088/0004-637X/738/1/27
Abstract: The distribution of metals in the Galaxy provides important information about galaxy formation and evolution. HII regions are the most luminous objects in the Milky Way at mid-infrared to radio wavelengths and can be seen across the entire Galactic disk. We used the NRAO Green Bank Telescope (GBT) to measure radio recombination line and continuum emission in 81 Galactic HII regions. We calculated LTE electron temperatures using these data. In thermal equilibrium metal abundances are expected to set the nebular electron temperature with high abundances producing low temperatures. Our HII region distribution covers a large range of Galactocentric radius (5 to 22 kpc) and samples the Galactic azimuth range 330 degree to 60 degree. Using our highest quality data (72 objects) we derived an O/H Galactocentric radial gradient of -0.0383 +/- 0.0074 dex/kpc. Combining these data with a similar survey made with the NRAO 140 Foot telescope we get a radial gradient of -0.0446 +/- 0.0049 dex/kpc for this larger sample of 133 nebulae. The data are well fit by a linear model and no discontinuities are detected. Dividing our sample into three Galactic azimuth regions produced significantly different radial gradients that range from -0.03 to -0.07 dex/kpc. These inhomogeneities suggest that metals are not well mixed at a given radius. We stress the importance of homogeneous samples to reduce the confusion of comparing data sets with different systematics. Galactic chemical evolution models typically derive chemical evolution along only the radial dimension with time. Future models should consider azimuthal evolution as well.
The Green Bank Telescope HII Region Discovery Survey: III. Kinematic Distances
L. D. Anderson,T. M. Bania,Dana S. Balser,Robert T. Rood
Physics , 2012, DOI: 10.1088/0004-637X/754/1/62
Abstract: Using the HI Emission/Absorption method, we resolve the kinematic distance ambiguity and derive distances for 149 of 182 (82%) HII regions discovered by the Green Bank Telescope HII Region Discovery Survey (GBT HRDS). The HRDS is an X-band (9GHz, 3cm) GBT survey of 448 previously unknown HII regions in radio recombination line and radio continuum emission. Here we focus on HRDS sources from 67deg. > l > 18deg., where kinematic distances are more reliable. The 25 HRDS sources in this zone that have negative recombination line velocities are unambiguously beyond the orbit of the Sun, up to 20kpc distant. They are the most distant HII regions yet discovered. We find that 61% of HRDS sources are located at the far distance, 31% at the tangent point distance, and only 7% at the near distance. "Bubble" HII regions are not preferentially at the near distance (as was assumed previously) but average 10kpc from the Sun. The HRDS nebulae, when combined with a large sample of HII regions with previously known distances, show evidence of spiral structure in two circular arc segments of mean Galactocentric radii of 4.25 and 6.0kpc. We perform a thorough uncertainty analysis to analyze the effect of using different rotation curves, streaming motions, and a change to the Solar circular rotation speed. The median distance uncertainty for our sample of HII regions is only 0.5kpc, or 5%. This is significantly less than the median difference between the near and far kinematic distances, 6kpc. The basic Galactic structure results are unchanged after considering these sources of uncertainty.
The Green Bank Telescope HII Region Discovery Survey II. The Source Catalog
L. D. Anderson,T. M. Bania,Dana S. Balser,Robert T. Rood
Physics , 2011, DOI: 10.1088/0067-0049/194/2/32
Abstract: The Green Bank Telescope HII Region Discovery Survey has doubled the number of known HII regions in the Galactic zone 343deg.\leql\leq67deg. with |b|\leq1deg. We detected 603 discrete hydrogen radio recombination line (RRL) components at 9GHz (3cm) from 448 targets. Our targets were selected based on spatially coincident mid-infrared and 20cm radio continuum emission. Such sources are almost invariably HII regions; we detected hydrogen RRL emission from 95% of our target sample. The sensitivity of the Green Bank Telescope 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, 180mJy at 9GHz, is sufficient to detect all HII regions ionized by single O-stars to a distance of 12kpc. These recently discovered nebulae share the same distribution on the sky as does the previously known census of Galactic HII 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 HII regions, implying that our sample spans a range of evolutionary states. We discovered 34 first quadrant negative-velocity HII regions, which lie at extreme distances from the Sun and appear to be part of the Outer Arm. We found RRL emission from 207 Spitzer GLIMPSE 8.0{\mu}m "bubble" sources, 65 of which have been cataloged previously. It thus appears that nearly all GLIMPSE bubbles are HII regions and that \sim50% of all Galactic HII regions have a bubble morphology at 8.0{\mu}m.
What's The Problem With ^3He?
Keith A. Olive,Robert T. Rood,David N. Schramm,James Truran,Elisabeth Vangioni-Flam
Physics , 1994, DOI: 10.1086/175640
Abstract: We consider the galactic evolutionary history of \he3 in models which deplete deuterium by as much as a factor of 2 to $\sim$ 15 from its primordial value to its present day observed value in the ISM. We show that when \he3 production in low mass stars (1 -- 3 $M_\odot$) is included over the history of the galaxy, \he3 is greatly over-produced and exceeds the inferred solar values and the abundances determined in galactic \hii regions. Furthermore, the ISM abundances show a disturbing dispersion which is difficult to understand from the point of view of standard chemical evolution models. In principle, resolution of the problem may lie in either 1) the calculated \he3 production in low mass stars; 2) the observations of the \he3 abundance; or 3) an observational bias towards regions of depleted \he3. Since \he3 observations in planetary nebula support the calculated \he3 production in low mass stars, option (1) is unlikely. We will argue for option (3) since the \he3 interstellar observations are indeed made in regions dominated by massive stars in which \he3 is destroyed. In conclusion, we note that the problem with \he3 seems to be galactic and not cosmological.
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