We present the Ðrst results of a large, systematic adiabatic survey of the pulsation properties o... more We present the Ðrst results of a large, systematic adiabatic survey of the pulsation properties of models of subdwarf B (sdB) stars. This survey is aimed at providing the most basic theoretical data with which to analyze the asteroseismological properties of the recently discovered class of pulsating sdB stars (the EC 14026 stars). Such a theoretical framework has been lacking up to now. In this paper, the Ðrst of a series of three, an adiabatic pulsation code is used to compute, in the 80È1500 s period window, the radial (l \ 0) and nonradial (from l \ 1 up to l \ 3) oscillation modes for a representative evolutionary model of subdwarf B stars. Quantities such as the periods, kinetic energies, Ðrst-order rotational splitting coefficients, eigenfunctions, and weight functions are given by the code, providing a complete set of very useful diagnostic tools with which to study the mode properties. The main goal is to determine how these quantities relate to the internal structure of B subdwarfs, a crucial and necessary step if one wants to eventually apply the tools of asteroseismology to EC 14026 stars. All modes (p, f, and g) were considered in order to build the most complete picture we can have on pulsations in these stars. In that context, we show that g-modes are essentially deep interior modes oscillating mainly in the radiative helium-rich core (but not in the convective nucleus), while p-modes are shallower envelope modes. We demonstrate that g-modes respond to a trapping/conÐnement phenomenon induced mainly by the He/H chemical transition between the H-rich envelope and the He-rich core of subdwarf B stars. This phenomenon is very similar in nature to the g-mode trapping and conÐnement mechanisms observed in pulsating white dwarf models. We emphasize that p-modes may also experience distortions of their period distribution due to this He/H transition, although these are not as pronounced as in the g-mode case. These phenomena are of great interest as they can potentially provide powerful tools for probing the internal structure of these objects, in particular, with respect to constraining the mass of their H-rich envelope. The results given in this Ðrst paper form the minimal background on pulsation mode characteristics in sdB stars. Upcoming discussions on additional mode properties in subdwarf B star models (Paper II and Paper III of this series) will strongly rely on these basic results since they provide essential guidance in understanding mode period behaviors as functions of B subdwarf stellar parameters and/or evolution.
We are embarking on a three-year Hubble Treasury program, GO-9455, whose goal is to place the det... more We are embarking on a three-year Hubble Treasury program, GO-9455, whose goal is to place the determination of ages and metallicities old stellar systems on a quantitative basis. Our program will provide the tools to analyze their composite integrated-light spectra in the near ultraviolet (UV). We will calculate an extensive grid of stellar spectra from 2280A to 3120A from first
We present an analysis of the far-ultraviolet upturn phenomenon (UVX) observed in elliptical gala... more 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. Such objects have Zero Age Horizontal Branch envelope masses M 0 env < 0:05M. Local examples of EHB stars exist in some globular clusters and in the Galactic disk field and serve both as a guide and constraint. 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. There are several important distinctions between clusters and galaxies. They do not occupy a single Mg 2-color sequence. 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. It also implies that the UVX in galaxies is not produced by metal poor subpopulations similar to the clusters. We develop a simple spectral synthesis formulation for all phases of single star evolution from the 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 HB morphologies in the age range 2 < t < 20 Gyr and for 6 metallicities in the range 2:26 < [Fe=H] < 0:58; we also consider the effect of enhanced Y in metal rich models. UV properties of the models are predicted using the Kurucz (1991) atmospheres. The maximum lifetime UV output is produced by EHB stars with M 0 env 0:02M , 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 0 env , P(M 0 env), on the ZAHB. The UV output is not very sensitive to [Fe=H] or to Y , but it can change very rapidly with M 0 env. Thus, it is extremely sensitive to the precise nature of giant branch mass loss. Because this process is not well understood physically, we choose to leave mass loss as an implicit free parameter. Our models use simple descriptions of P(M 0 env) 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. Clusters with "blue HB" morphologies do not require the hotter EHB stars to explain their UV colors, although a small EHB population is consistent with our models. The largest known population of these stars in a cluster, as a fraction of the total HB, is 20% in ! Cen. For [Fe=H] > 0:5, however, blue HB stars will be rare. As a consequence, we find that models with [Fe=H] > 0 which do not contain EHB stars cannot reproduce the colors of most of the galaxies. However, only small EHB fractions are required: < 5% for the bulk of the E galaxies and 20% for those with the strongest UVX. These results are independent of the assumed [Fe/H]. The EHB fraction required for most galaxies is comparable to the fraction of hot subdwarfs in the Galactic disk. Most of these are EHB stars, and their existence considerably strengthens the case for EHB populations as the source of elliptical galaxy UV light. The models also predict that the fraction of the far-UV light from P-AGB stars, which are spatially resolvable in nearby galaxies, is 70% and 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 strong limits of h[Fe=H]i > 0:5 and < 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. However, the 25 V colors of the galaxies other than the strong UVX systems are too blue to be consistent with [Fe=H] > 0:2 for any age. This may be additional evidence that [Mg/Fe] > 0 in elliptical galaxies. UV colors for M32 are consistent with the solar abundance, intermediate age (4-6 Gyr) population inferred from optical/IR observations.
The goal of our Hubble Treasury Program is to provide observational and theoretical spectral temp... more The goal of our Hubble Treasury Program is to provide observational and theoretical spectral templates for interpreting the age metallicity and population of old stellar systems. To this end we are calculating stellar mid-UV and optical spectra from first principles for comparison with observed spectra. Here we present the Peterson et al. (2003) comparison of coadditions of such spectral calculations to archival mid-UV and optical spectra of the globular cluster G1 in Andromeda (M31). Our composite spectra were generated by coadding the fluxes calculated for four to six individual stellar models weighted by the model radius squared times the relative number of stars represented by the model. We attain an excellent match to the Ponder et al. (1998) FOS observations but only when the calculations include stars on the cool blue horizontal branch (BHB) and hot extreme horizontal branch (EHB). Our metallicity [Fe/H] = -0.8 and BHB fraction both agree well with the results of Rich et al. (1996) from the G1 color-magnitude diagram. Our G1 turnoff temperature is 6000K - 6100K that of the Galactic halo. We conclude that G1 is old and mildly metal-poor but nonetheless harbors both BHB and EHB stars. Only the UV can discern them.
The globular cluster M79 was observed with the Hopkins Ultraviolet Telescope (HUT) during the Ast... more The globular cluster M79 was observed with the Hopkins Ultraviolet Telescope (HUT) during the Astro-1 space shuttle mission in 1990 December. The cluster's far-UV integrated spectrum shows strong absorption in the Lyman lines of atomic hydrogen. We seek to use this spectrum, together with optical photometry, to constrain the stellar mass distribution along its zero-age horizontal branch (ZAHB). We find that a Gaussian distribution of ZAHB masses, with a mean of 0.59M ⊙ and standard deviation 0.05M ⊙ , is able to reproduce the cluster's (B, V) color-magnitude diagram when subsequent stellar evolution is taken into account, but cannot reproduce the cluster's far-UV spectrum. Model stellar spectra fit directly to the HUT data indicate a surprising distribution of atmospheric parameters, with surface gravities (and thus implied masses) significantly lower than are predicted by canonical HB evolutionary models. This result is consistent with the findings of Moehler et al. [A&A, 294, 65 (1995)] for individual HB stars in M15. Further progress in understanding the mass distribution of the HB must await resolution of the inconsistencies between the derived stellar atmospheric parameters and the predictions of HB evolutionary models. Improved stellar spectral models, with higher spectral resolution and non-solar abundance ratios, may prove useful in this endeavor.
Until now there have been few spectroscopic studies of cepheids in globular clusters. In this pre... more Until now there have been few spectroscopic studies of cepheids in globular clusters. In this preliminary report we present sample spectra of the stars Vl and V29 in w Cen. Eventually, we hope to use the abundance patterns, masses, and period changes of cepheids to better understand post-horizontal branch evolution in globular clusters.
The present semianalytical treatment for aspects of horizontal branch (HB) star core evolution co... more The present semianalytical treatment for aspects of horizontal branch (HB) star core evolution considers a criterion for the existence of a partial mixing zone in a general stellar model. This criterion implies that there must be a zone with a composition gradient. Attention is given to the consequences of the assumption for the behavior of the core close to the central He exhaustion; with the degree of mixing implied by the constraint on the temperature gradient, core opacity tends to a limit late in evolution, and then declines. The central opacity and flux decreases give the convective region a strong tendency to shrink.
The globular cluster ω Centauri contains the largest known population of very hot horizontal bran... more The globular cluster ω Centauri contains the largest known population of very hot horizontal branch (HB) stars. We have used the Hubble Space Telescope to obtain a far-UV/optical color-magnitude diagram of three fields in ω Cen. We find that over 30% of the HB objects are "extreme" HB or hot post-HB stars. The hot HB stars are not concentrated toward the cluster center, which argues 1 Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract No. NAS5-26555.
We have used WFPC2 to construct B, V color-magnitude diagrams of four metal-rich globular cluster... more We have used WFPC2 to construct B, V color-magnitude diagrams of four metal-rich globular clusters, NGC 104 (47 Tuc), NGC 5927, NGC 6388, and NGC 6441. All four clusters have well populated red horizontal branches (RHB), as expected for their metallicity. However, NGC 6388 and 6441 also exhibit a prominent blue horizontal-branch (BHB) extension, including stars reaching as faint in V as the turnoff luminosity. This discovery demonstrates directly for the first time that a major population of hot horizontal-branch (HB) stars can exist in old, metal-rich systems. This may have important implications for the interpretation of the integrated spectra of elliptical galaxies. The cause of the phenomenon remains uncertain. We examine the possibility that NGC 6388 and 6441 are older than the other clusters, but a simple difference in age may not be sufficient to produce the observed distributions along the HB. The high central densities in NGC 6388 and 6441 suggest that the existence of the BHB tails might be caused by stellar interactions in the dense cores of these clusters, which we calculate to have two of the highest collision rates among globular clusters in the Galaxy. Tidal collisions might act in various ways to enhance loss of envelope mass and therefore populate the blue side of the HB. However, the relative frequency of tidal collisions does not seem large enough (compared to that of the clusters with pure RHBs) to account for such a drastic difference in HB morphology. While a combination of an age difference and dynamical interactions may help, prima facie the lack of a radial gradient in the BHB͞RHB star ratio seems to argue against dynamical effects playing a role. Subject headings: Galaxy: kinematics and dynamics-globular clusters: individual (NGC 104, 47 Tucanae, NGC 5927, NGC 6388, NGC 6441)-stars: horizontal-branch 1 Based on observations with the NASA͞ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.
Atmospheric parameters (T eff , log g), and radial velocities are derived for twelve candidate bl... more Atmospheric parameters (T eff , log g), and radial velocities are derived for twelve candidate blue horizontal branch (HB) stars in the globular clusters 47 Tuc and NGC 362, which so far have been known to contain primarily red HB stars. The spectroscopic targets were selected from the catalog of hot stars detected in these clusters at 1600Å using the Ultraviolet Imaging Telescope (UIT). Spectroscopic analyses of these stars reveal, however, that one of the four HB candidates targets in 47 Tuc, and five out of the eight targets in NGC 362 are probably background stars belonging to the Small Magellanic Cloud. With the exception of the photometric binary MJ38529 in 47 Tuc, the parameters of those stars that are probable members of 47 Tuc and NGC 362 agree well with canonical HB evolution. The three hot stars in 47 Tuc all have 10,000 K < T eff < 15,000 K and include one photometric binary, which suggests that they might have a different physical origin than the dominant red HB population. The somewhat cooler blue HB stars in NGC 362 show more continuity with the dominant red HB population and might simply arise from red giants with unusually high mass loss.
ABSTRACT This is a progress report for our Hubble Treasury program GO-10094. Our goal is to provi... more ABSTRACT This is a progress report for our Hubble Treasury program GO-10094. Our goal is to provide both theoretical and observational spectral templates for the mid-ultraviolet, from 2200Å to 3150Å and beyond, for a quantitative interpretation of the spectrum of a stellar system such as a globular cluster or elliptical galaxy. This is aimed at determining the age and metallicity of systems as young as 1 Gyr. We are now calculating grids of theoretical spectral templates for stars all across the color-magnitude diagram, from red giants to turn-off stars to blue stragglers to extreme blue horizontal branch stars. The grids span a wide range of metallicities, and incorporate both the solar mix of elemental abundances and mixes with enhanced abundances of light elements such as magnesium. At the same time we are calculating stellar tracks and isochrones for these metallicities and mixtures. We will then generate theoretical spectral templates for single-age, single-metallicity systems by coadding the stellar spectra with weights derived from the isochrones. We will make public the stellar and composite grids, as well as broad-band indices and colors derived from them. We present examples of the excellent matches between observed and theoretical spectra achieved to date, in both the ultraviolet and the optical spectral regions. We also illustrate the application of these theoretical spectra to several problems, from the determination of very heavy elements produced by the s- and r-process in individual stars, to the age and metallicity of extragalactic globular clusters and remote galaxies.
We present the Ðrst results of a large, systematic adiabatic survey of the pulsation properties o... more We present the Ðrst results of a large, systematic adiabatic survey of the pulsation properties of models of subdwarf B (sdB) stars. This survey is aimed at providing the most basic theoretical data with which to analyze the asteroseismological properties of the recently discovered class of pulsating sdB stars (the EC 14026 stars). Such a theoretical framework has been lacking up to now. In this paper, the Ðrst of a series of three, an adiabatic pulsation code is used to compute, in the 80È1500 s period window, the radial (l \ 0) and nonradial (from l \ 1 up to l \ 3) oscillation modes for a representative evolutionary model of subdwarf B stars. Quantities such as the periods, kinetic energies, Ðrst-order rotational splitting coefficients, eigenfunctions, and weight functions are given by the code, providing a complete set of very useful diagnostic tools with which to study the mode properties. The main goal is to determine how these quantities relate to the internal structure of B subdwarfs, a crucial and necessary step if one wants to eventually apply the tools of asteroseismology to EC 14026 stars. All modes (p, f, and g) were considered in order to build the most complete picture we can have on pulsations in these stars. In that context, we show that g-modes are essentially deep interior modes oscillating mainly in the radiative helium-rich core (but not in the convective nucleus), while p-modes are shallower envelope modes. We demonstrate that g-modes respond to a trapping/conÐnement phenomenon induced mainly by the He/H chemical transition between the H-rich envelope and the He-rich core of subdwarf B stars. This phenomenon is very similar in nature to the g-mode trapping and conÐnement mechanisms observed in pulsating white dwarf models. We emphasize that p-modes may also experience distortions of their period distribution due to this He/H transition, although these are not as pronounced as in the g-mode case. These phenomena are of great interest as they can potentially provide powerful tools for probing the internal structure of these objects, in particular, with respect to constraining the mass of their H-rich envelope. The results given in this Ðrst paper form the minimal background on pulsation mode characteristics in sdB stars. Upcoming discussions on additional mode properties in subdwarf B star models (Paper II and Paper III of this series) will strongly rely on these basic results since they provide essential guidance in understanding mode period behaviors as functions of B subdwarf stellar parameters and/or evolution.
We are embarking on a three-year Hubble Treasury program, GO-9455, whose goal is to place the det... more We are embarking on a three-year Hubble Treasury program, GO-9455, whose goal is to place the determination of ages and metallicities old stellar systems on a quantitative basis. Our program will provide the tools to analyze their composite integrated-light spectra in the near ultraviolet (UV). We will calculate an extensive grid of stellar spectra from 2280A to 3120A from first
We present an analysis of the far-ultraviolet upturn phenomenon (UVX) observed in elliptical gala... more 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. Such objects have Zero Age Horizontal Branch envelope masses M 0 env < 0:05M. Local examples of EHB stars exist in some globular clusters and in the Galactic disk field and serve both as a guide and constraint. 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. There are several important distinctions between clusters and galaxies. They do not occupy a single Mg 2-color sequence. 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. It also implies that the UVX in galaxies is not produced by metal poor subpopulations similar to the clusters. We develop a simple spectral synthesis formulation for all phases of single star evolution from the 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 HB morphologies in the age range 2 < t < 20 Gyr and for 6 metallicities in the range 2:26 < [Fe=H] < 0:58; we also consider the effect of enhanced Y in metal rich models. UV properties of the models are predicted using the Kurucz (1991) atmospheres. The maximum lifetime UV output is produced by EHB stars with M 0 env 0:02M , 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 0 env , P(M 0 env), on the ZAHB. The UV output is not very sensitive to [Fe=H] or to Y , but it can change very rapidly with M 0 env. Thus, it is extremely sensitive to the precise nature of giant branch mass loss. Because this process is not well understood physically, we choose to leave mass loss as an implicit free parameter. Our models use simple descriptions of P(M 0 env) 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. Clusters with "blue HB" morphologies do not require the hotter EHB stars to explain their UV colors, although a small EHB population is consistent with our models. The largest known population of these stars in a cluster, as a fraction of the total HB, is 20% in ! Cen. For [Fe=H] > 0:5, however, blue HB stars will be rare. As a consequence, we find that models with [Fe=H] > 0 which do not contain EHB stars cannot reproduce the colors of most of the galaxies. However, only small EHB fractions are required: < 5% for the bulk of the E galaxies and 20% for those with the strongest UVX. These results are independent of the assumed [Fe/H]. The EHB fraction required for most galaxies is comparable to the fraction of hot subdwarfs in the Galactic disk. Most of these are EHB stars, and their existence considerably strengthens the case for EHB populations as the source of elliptical galaxy UV light. The models also predict that the fraction of the far-UV light from P-AGB stars, which are spatially resolvable in nearby galaxies, is 70% and 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 strong limits of h[Fe=H]i > 0:5 and < 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. However, the 25 V colors of the galaxies other than the strong UVX systems are too blue to be consistent with [Fe=H] > 0:2 for any age. This may be additional evidence that [Mg/Fe] > 0 in elliptical galaxies. UV colors for M32 are consistent with the solar abundance, intermediate age (4-6 Gyr) population inferred from optical/IR observations.
The goal of our Hubble Treasury Program is to provide observational and theoretical spectral temp... more The goal of our Hubble Treasury Program is to provide observational and theoretical spectral templates for interpreting the age metallicity and population of old stellar systems. To this end we are calculating stellar mid-UV and optical spectra from first principles for comparison with observed spectra. Here we present the Peterson et al. (2003) comparison of coadditions of such spectral calculations to archival mid-UV and optical spectra of the globular cluster G1 in Andromeda (M31). Our composite spectra were generated by coadding the fluxes calculated for four to six individual stellar models weighted by the model radius squared times the relative number of stars represented by the model. We attain an excellent match to the Ponder et al. (1998) FOS observations but only when the calculations include stars on the cool blue horizontal branch (BHB) and hot extreme horizontal branch (EHB). Our metallicity [Fe/H] = -0.8 and BHB fraction both agree well with the results of Rich et al. (1996) from the G1 color-magnitude diagram. Our G1 turnoff temperature is 6000K - 6100K that of the Galactic halo. We conclude that G1 is old and mildly metal-poor but nonetheless harbors both BHB and EHB stars. Only the UV can discern them.
The globular cluster M79 was observed with the Hopkins Ultraviolet Telescope (HUT) during the Ast... more The globular cluster M79 was observed with the Hopkins Ultraviolet Telescope (HUT) during the Astro-1 space shuttle mission in 1990 December. The cluster's far-UV integrated spectrum shows strong absorption in the Lyman lines of atomic hydrogen. We seek to use this spectrum, together with optical photometry, to constrain the stellar mass distribution along its zero-age horizontal branch (ZAHB). We find that a Gaussian distribution of ZAHB masses, with a mean of 0.59M ⊙ and standard deviation 0.05M ⊙ , is able to reproduce the cluster's (B, V) color-magnitude diagram when subsequent stellar evolution is taken into account, but cannot reproduce the cluster's far-UV spectrum. Model stellar spectra fit directly to the HUT data indicate a surprising distribution of atmospheric parameters, with surface gravities (and thus implied masses) significantly lower than are predicted by canonical HB evolutionary models. This result is consistent with the findings of Moehler et al. [A&A, 294, 65 (1995)] for individual HB stars in M15. Further progress in understanding the mass distribution of the HB must await resolution of the inconsistencies between the derived stellar atmospheric parameters and the predictions of HB evolutionary models. Improved stellar spectral models, with higher spectral resolution and non-solar abundance ratios, may prove useful in this endeavor.
Until now there have been few spectroscopic studies of cepheids in globular clusters. In this pre... more Until now there have been few spectroscopic studies of cepheids in globular clusters. In this preliminary report we present sample spectra of the stars Vl and V29 in w Cen. Eventually, we hope to use the abundance patterns, masses, and period changes of cepheids to better understand post-horizontal branch evolution in globular clusters.
The present semianalytical treatment for aspects of horizontal branch (HB) star core evolution co... more The present semianalytical treatment for aspects of horizontal branch (HB) star core evolution considers a criterion for the existence of a partial mixing zone in a general stellar model. This criterion implies that there must be a zone with a composition gradient. Attention is given to the consequences of the assumption for the behavior of the core close to the central He exhaustion; with the degree of mixing implied by the constraint on the temperature gradient, core opacity tends to a limit late in evolution, and then declines. The central opacity and flux decreases give the convective region a strong tendency to shrink.
The globular cluster ω Centauri contains the largest known population of very hot horizontal bran... more The globular cluster ω Centauri contains the largest known population of very hot horizontal branch (HB) stars. We have used the Hubble Space Telescope to obtain a far-UV/optical color-magnitude diagram of three fields in ω Cen. We find that over 30% of the HB objects are "extreme" HB or hot post-HB stars. The hot HB stars are not concentrated toward the cluster center, which argues 1 Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract No. NAS5-26555.
We have used WFPC2 to construct B, V color-magnitude diagrams of four metal-rich globular cluster... more We have used WFPC2 to construct B, V color-magnitude diagrams of four metal-rich globular clusters, NGC 104 (47 Tuc), NGC 5927, NGC 6388, and NGC 6441. All four clusters have well populated red horizontal branches (RHB), as expected for their metallicity. However, NGC 6388 and 6441 also exhibit a prominent blue horizontal-branch (BHB) extension, including stars reaching as faint in V as the turnoff luminosity. This discovery demonstrates directly for the first time that a major population of hot horizontal-branch (HB) stars can exist in old, metal-rich systems. This may have important implications for the interpretation of the integrated spectra of elliptical galaxies. The cause of the phenomenon remains uncertain. We examine the possibility that NGC 6388 and 6441 are older than the other clusters, but a simple difference in age may not be sufficient to produce the observed distributions along the HB. The high central densities in NGC 6388 and 6441 suggest that the existence of the BHB tails might be caused by stellar interactions in the dense cores of these clusters, which we calculate to have two of the highest collision rates among globular clusters in the Galaxy. Tidal collisions might act in various ways to enhance loss of envelope mass and therefore populate the blue side of the HB. However, the relative frequency of tidal collisions does not seem large enough (compared to that of the clusters with pure RHBs) to account for such a drastic difference in HB morphology. While a combination of an age difference and dynamical interactions may help, prima facie the lack of a radial gradient in the BHB͞RHB star ratio seems to argue against dynamical effects playing a role. Subject headings: Galaxy: kinematics and dynamics-globular clusters: individual (NGC 104, 47 Tucanae, NGC 5927, NGC 6388, NGC 6441)-stars: horizontal-branch 1 Based on observations with the NASA͞ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.
Atmospheric parameters (T eff , log g), and radial velocities are derived for twelve candidate bl... more Atmospheric parameters (T eff , log g), and radial velocities are derived for twelve candidate blue horizontal branch (HB) stars in the globular clusters 47 Tuc and NGC 362, which so far have been known to contain primarily red HB stars. The spectroscopic targets were selected from the catalog of hot stars detected in these clusters at 1600Å using the Ultraviolet Imaging Telescope (UIT). Spectroscopic analyses of these stars reveal, however, that one of the four HB candidates targets in 47 Tuc, and five out of the eight targets in NGC 362 are probably background stars belonging to the Small Magellanic Cloud. With the exception of the photometric binary MJ38529 in 47 Tuc, the parameters of those stars that are probable members of 47 Tuc and NGC 362 agree well with canonical HB evolution. The three hot stars in 47 Tuc all have 10,000 K < T eff < 15,000 K and include one photometric binary, which suggests that they might have a different physical origin than the dominant red HB population. The somewhat cooler blue HB stars in NGC 362 show more continuity with the dominant red HB population and might simply arise from red giants with unusually high mass loss.
ABSTRACT This is a progress report for our Hubble Treasury program GO-10094. Our goal is to provi... more ABSTRACT This is a progress report for our Hubble Treasury program GO-10094. Our goal is to provide both theoretical and observational spectral templates for the mid-ultraviolet, from 2200Å to 3150Å and beyond, for a quantitative interpretation of the spectrum of a stellar system such as a globular cluster or elliptical galaxy. This is aimed at determining the age and metallicity of systems as young as 1 Gyr. We are now calculating grids of theoretical spectral templates for stars all across the color-magnitude diagram, from red giants to turn-off stars to blue stragglers to extreme blue horizontal branch stars. The grids span a wide range of metallicities, and incorporate both the solar mix of elemental abundances and mixes with enhanced abundances of light elements such as magnesium. At the same time we are calculating stellar tracks and isochrones for these metallicities and mixtures. We will then generate theoretical spectral templates for single-age, single-metallicity systems by coadding the stellar spectra with weights derived from the isochrones. We will make public the stellar and composite grids, as well as broad-band indices and colors derived from them. We present examples of the excellent matches between observed and theoretical spectra achieved to date, in both the ultraviolet and the optical spectral regions. We also illustrate the application of these theoretical spectra to several problems, from the determination of very heavy elements produced by the s- and r-process in individual stars, to the age and metallicity of extragalactic globular clusters and remote galaxies.
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Papers by Ben Dorman