Proceedings of 11th Symposium on Nuclei in the Cosmos — PoS(NIC XI)
Carbon, nitrogen, and oxygen produced in the early universe come from a variety of possible astro... more Carbon, nitrogen, and oxygen produced in the early universe come from a variety of possible astrophysical sites. Among these are early supernovae, winds of massive, rapidly-rotating, mega metal-poor stars, and intermediate mass AGB stars. Large-scale surveys such as the HK Survey of Beers and colleagues and the Hamburg/ESO Survey of Christlieb and colleagues have allowed for the identification of numerous metal-poor stars in the Galactic halo. Follow-up observations of these metal-poor stars is necessary to determine CNO abundances. Techniques have been developed such that [C/Fe], [N/Fe], and [O/Fe] can be estimated with considerable accuracy using medium-resolution observations alone. We present estimates of these species for a number of metal-poor stars based on analysis of near-UV, optical, and near-IR spectra. The data come from several different instruments on southern-hemisphere telescopes, including the Goodman HTS and OSIRIS on SOAR, GMOS-S on Gemini, and XSHOOTER on VLT. In this way, we present some of the first metal-poor stars with a full complement of CNO abundances based solely on the analysis of medium-resolution spectra.
A new moving group comprising at least four Blue Horizontal Branch (BHB) stars is identified at (... more A new moving group comprising at least four Blue Horizontal Branch (BHB) stars is identified at (l,b) = (65 deg, 48 deg). The horizontal branch at g0=18.9 magnitude implies a distance of 50 kpc from the Sun. The heliocentric radial velocity is RV = -157 +/- 4 km/s, corresponding to V(gsr) = -10 km/s; the dispersion in line-of-sight velocity is consistent with the instrumental errors for these stars. The mean metallicity of the moving group is [Fe/H] approximately -2.4, which is significantly more metal poor than the stellar spheroid. We estimate that the BHB stars in the outer halo have a mean metallicity of [Fe/H] = -2.0, with a wide scatter and a distribution that does not change much as a function of distance from the Sun. We explore the systematics of SDSS DR7 surface gravity metallicity determinations for faint BHB stars, and present a technique for estimating the significance of clumps discovered in multidimensional data. This moving group cannot be distinguished in density, and highlights the need to collect many more spectra of Galactic stars to unravel the merger history of the Galaxy.
... Spite et al. (1996) and Bonifacio & Molaro (1997) and also noted the existence of spr... more ... Spite et al. (1996) and Bonifacio & Molaro (1997) and also noted the existence of spreads of order 0.08 dex, but were content that they could be explained by uncertainties of observation and analysis. Ryan et a!. (1996), while not commenting on a general spread, noted that ...
We report the detection of the Pb I 4057.8Å line in the very metal-poor ([Fe/H]= −2.7), carbon-ri... more We report the detection of the Pb I 4057.8Å line in the very metal-poor ([Fe/H]= −2.7), carbon-rich star, LP 625-44. We determine the abundance of Pb ([Pb/Fe] = 2.65) and 15 other neutron-capture elements. The abundance pattern between Ba and Pb agrees well with a scaled solar system s-process component, while the lighter elements (Sr-Zr) are less abundant than Ba. The enhancement of s-process elements is interpreted as a result of mass transfer in a binary system from a previous AGB companion, an interpretation strongly supported by radial velocity variations of this system. The detection of Pb makes it possible, for the first time, to compare model predictions of s-process nucleosynthesis in AGB stars with observations of elements between Sr and Pb. The Pb abundance is significantly lower than the prediction of recent models (e.g., Gallino et al. 1998), which succeeded in explaining the metallicity dependence of the abundance ratios of light s-elements (Sr-Zr) to heavy ones (Ba-Dy) found in previously observed s-process-enhanced stars. This suggests that one should either (a) reconsider the underlying assumptions concerning the 13 C-rich s-processing site (13 C-pocket) in the present models, or (b) investigate alternative sites of s-process nucleosynthesis in very metal-poor AGB stars.
We present the first detailed abundance analysis of the metal-poor giant HKII 17435À00532. This s... more We present the first detailed abundance analysis of the metal-poor giant HKII 17435À00532. This star was observed as part of the University of Texas long-term project Chemical Abundances of Stars in the Halo (CASH). A spectrum was obtained with the High Resolution Spectrograph (HRS) on the Hobby-Eberly Telescope with a resolving power of R $ 15;000. Our analysis reveals that this star may be located on the red giant branch, red horizontal branch, or early asymptotic giant branch. We find that this metal-poor (½Fe/H ¼ À2:2) star has an unusually high lithium abundance [log "(Li) ¼ þ2:1], mild carbon (½C/Fe ¼ þ0:7) and sodium (½Na /Fe ¼ þ0:6) enhancement, as well as enhancement of both s-process (½Ba /Fe ¼ þ0:8) and r-process (½Eu / Fe ¼ þ0:5) material. The high Li abundance can be explained by self-enrichment through extra mixing that connects the convective envelope with the outer regions of the H-burning shell. If so, HKII 17435À00532 is the most metal-poor star in which this short-lived phase of Li enrichment has been observed. The Na and n-capture enrichment can be explained by mass transfer from a companion that passed through the thermally pulsing AGB phase of evolution with only a small initial enrichment of r-process material present in the birth cloud. Despite the current nondetection of radial velocity variations (over $180 days), it is possible that HKII 17435À00532 is in a long-period or highly inclined binary system, similar to other stars with similar n-capture enrichment patterns.
Context. Determining distances to individual stars is a necessary step towards mapping Galactic s... more Context. Determining distances to individual stars is a necessary step towards mapping Galactic structure and determining spatial variations in the chemo-dynamical properties of stellar populations in the Milky Way. Aims. We have developed a procedure that estimates distances to stars using measured spectroscopic and photometric quantities. Similar to other recent works in the literature, it employs a Bayesian approach to build the probability distribution function over stellar evolutionary models given the data, delivering estimates of expected values of model parameters (including distances) for each star individually. Our method provides several alternative distance estimates for each star in the output, along with their associated uncertainties. Methods. The code was first tested on simulations, successfully recovering input distances to mock stars with errors that scale with the uncertainties in the adopted spectro-photometric parameters, as expected. The code was then validated by comparing our distance estimates to parallax measurements from the Hipparcos mission (ESA 1997) for nearby stars (< 60 pc), to asteroseismic distances of CoRoT red giant stars, and to known distances of well-studied open and globular clusters. The photometric data of these reference samples cover both the optical and near infra-red wavelengths. The spectroscopic parameters are also based on spectra taken at various wavelengths, with varying spectral coverage and resolution: the Radial Velocity Experiment, the Sloan Digital Sky Survey programs SEGUE and APOGEE, and the ESO HARPS instrument. Results. For Hipparcos and CoRoT samples, the typical random distance scatter is 20% or less, both for the nearby and farther data. There is a trend towards underestimating the distances by < 10%. The comparison to star clusters from SEGUE and APOGEE has led to systematic differences < 5% for most cluster stars although with significant scatter. Finally, we tested our distances against those previously determined for a high quality sample of giant stars from the RAVE survey, again finding a reasonable agreement, with only a small systematic trend. Efforts are underway to provide our code to the community by running it on a public server.
Bulletin of the American Astronomical Society, May 1, 1995
ABSTRACT Theory, as well as numerical simulations, suggests that Omega_o may be observationally c... more ABSTRACT Theory, as well as numerical simulations, suggests that Omega_o may be observationally constrained by the amount of substructure observed in present-day clusters of galaxies. We have therefore begun a study of the 116 Abell clusters with richness class greater than or equal to 1 and distance class less than or equal to 4, the so-called ``volume-limited&#39;&#39; sample of Hoessel, Gunn, &amp; Thuan 1980 (ApJ 241, 486) to determine the prevalence of substructure in the clusters&#39; projected galaxy positions. We use positions of galaxies identified by the Minnesota Automated Plate Scanner to obtain contour plots of the available clusters using an adaptive kernel routine. Significance of substructure is evaluated using the 2-D Lee test as well as a likelihood-ratio test on fits made with mixtures of two-dimensional gaussians. We also present nonparametric density profile estimates obtained with the program MAPEL (Merritt and Tremblay 1994, AJ 108, 514).
We present numerical simulations to describe the evolution of pre-Galactic clouds in a model whic... more We present numerical simulations to describe the evolution of pre-Galactic clouds in a model which is motivated by cold dark matter simulations of hierarchical galaxy formation. We adopt a SN-induced star-formation mechanism within a model that follows the evolution of chemical enrichment and energy input to the clouds by Type II and Type Ia supernovae. We utilize metallicity-dependent yields for all elements at all times, and include effects of finite stellar lifetimes. We derive the metallicity distribution functions for stars in the clouds, their age-metallicity relation, and relative elemental abundances for a number of alpha-and Fe-group elements. The stability of these clouds against destruction is discussed, and results are compared for different initial mass functions. We find that the dispersion of the metallicity distribution function observed in the outer halo is naturally reproduced by contributions from many clouds with different initial conditions. The scatter in metallicity as a function of age for these stars is very large, implying that no age-metallicity relation exists in the early stages of galaxy formation. Clouds with initial masses > ∼ presently observed globular clusters are found to survive the first 0.1 Gyr from the onset of star formation, suggesting that such systems may have contributed to the formation of the first stars, and could have been selfenriched. More massive clouds are only stable when one assumes an initial mass function that is not biased towards massive stars, indicating that even if the first stars were formed according to a top-heavy mass function, subsequent star formation was likely to have proceeded with a present-day mass function, or happened in an episodic manner. The predicted relative abundances of some alpha-and Fe-group elements show reasonable agreement with the observed values down to metallicities below [Fe/H] ∼ −4 when the
Proceedings of 11th Symposium on Nuclei in the Cosmos — PoS(NIC XI)
Carbon, nitrogen, and oxygen produced in the early universe come from a variety of possible astro... more Carbon, nitrogen, and oxygen produced in the early universe come from a variety of possible astrophysical sites. Among these are early supernovae, winds of massive, rapidly-rotating, mega metal-poor stars, and intermediate mass AGB stars. Large-scale surveys such as the HK Survey of Beers and colleagues and the Hamburg/ESO Survey of Christlieb and colleagues have allowed for the identification of numerous metal-poor stars in the Galactic halo. Follow-up observations of these metal-poor stars is necessary to determine CNO abundances. Techniques have been developed such that [C/Fe], [N/Fe], and [O/Fe] can be estimated with considerable accuracy using medium-resolution observations alone. We present estimates of these species for a number of metal-poor stars based on analysis of near-UV, optical, and near-IR spectra. The data come from several different instruments on southern-hemisphere telescopes, including the Goodman HTS and OSIRIS on SOAR, GMOS-S on Gemini, and XSHOOTER on VLT. In this way, we present some of the first metal-poor stars with a full complement of CNO abundances based solely on the analysis of medium-resolution spectra.
A new moving group comprising at least four Blue Horizontal Branch (BHB) stars is identified at (... more A new moving group comprising at least four Blue Horizontal Branch (BHB) stars is identified at (l,b) = (65 deg, 48 deg). The horizontal branch at g0=18.9 magnitude implies a distance of 50 kpc from the Sun. The heliocentric radial velocity is RV = -157 +/- 4 km/s, corresponding to V(gsr) = -10 km/s; the dispersion in line-of-sight velocity is consistent with the instrumental errors for these stars. The mean metallicity of the moving group is [Fe/H] approximately -2.4, which is significantly more metal poor than the stellar spheroid. We estimate that the BHB stars in the outer halo have a mean metallicity of [Fe/H] = -2.0, with a wide scatter and a distribution that does not change much as a function of distance from the Sun. We explore the systematics of SDSS DR7 surface gravity metallicity determinations for faint BHB stars, and present a technique for estimating the significance of clumps discovered in multidimensional data. This moving group cannot be distinguished in density, and highlights the need to collect many more spectra of Galactic stars to unravel the merger history of the Galaxy.
... Spite et al. (1996) and Bonifacio &amp; Molaro (1997) and also noted the existence of spr... more ... Spite et al. (1996) and Bonifacio &amp; Molaro (1997) and also noted the existence of spreads of order 0.08 dex, but were content that they could be explained by uncertainties of observation and analysis. Ryan et a!. (1996), while not commenting on a general spread, noted that ...
We report the detection of the Pb I 4057.8Å line in the very metal-poor ([Fe/H]= −2.7), carbon-ri... more We report the detection of the Pb I 4057.8Å line in the very metal-poor ([Fe/H]= −2.7), carbon-rich star, LP 625-44. We determine the abundance of Pb ([Pb/Fe] = 2.65) and 15 other neutron-capture elements. The abundance pattern between Ba and Pb agrees well with a scaled solar system s-process component, while the lighter elements (Sr-Zr) are less abundant than Ba. The enhancement of s-process elements is interpreted as a result of mass transfer in a binary system from a previous AGB companion, an interpretation strongly supported by radial velocity variations of this system. The detection of Pb makes it possible, for the first time, to compare model predictions of s-process nucleosynthesis in AGB stars with observations of elements between Sr and Pb. The Pb abundance is significantly lower than the prediction of recent models (e.g., Gallino et al. 1998), which succeeded in explaining the metallicity dependence of the abundance ratios of light s-elements (Sr-Zr) to heavy ones (Ba-Dy) found in previously observed s-process-enhanced stars. This suggests that one should either (a) reconsider the underlying assumptions concerning the 13 C-rich s-processing site (13 C-pocket) in the present models, or (b) investigate alternative sites of s-process nucleosynthesis in very metal-poor AGB stars.
We present the first detailed abundance analysis of the metal-poor giant HKII 17435À00532. This s... more We present the first detailed abundance analysis of the metal-poor giant HKII 17435À00532. This star was observed as part of the University of Texas long-term project Chemical Abundances of Stars in the Halo (CASH). A spectrum was obtained with the High Resolution Spectrograph (HRS) on the Hobby-Eberly Telescope with a resolving power of R $ 15;000. Our analysis reveals that this star may be located on the red giant branch, red horizontal branch, or early asymptotic giant branch. We find that this metal-poor (½Fe/H ¼ À2:2) star has an unusually high lithium abundance [log "(Li) ¼ þ2:1], mild carbon (½C/Fe ¼ þ0:7) and sodium (½Na /Fe ¼ þ0:6) enhancement, as well as enhancement of both s-process (½Ba /Fe ¼ þ0:8) and r-process (½Eu / Fe ¼ þ0:5) material. The high Li abundance can be explained by self-enrichment through extra mixing that connects the convective envelope with the outer regions of the H-burning shell. If so, HKII 17435À00532 is the most metal-poor star in which this short-lived phase of Li enrichment has been observed. The Na and n-capture enrichment can be explained by mass transfer from a companion that passed through the thermally pulsing AGB phase of evolution with only a small initial enrichment of r-process material present in the birth cloud. Despite the current nondetection of radial velocity variations (over $180 days), it is possible that HKII 17435À00532 is in a long-period or highly inclined binary system, similar to other stars with similar n-capture enrichment patterns.
Context. Determining distances to individual stars is a necessary step towards mapping Galactic s... more Context. Determining distances to individual stars is a necessary step towards mapping Galactic structure and determining spatial variations in the chemo-dynamical properties of stellar populations in the Milky Way. Aims. We have developed a procedure that estimates distances to stars using measured spectroscopic and photometric quantities. Similar to other recent works in the literature, it employs a Bayesian approach to build the probability distribution function over stellar evolutionary models given the data, delivering estimates of expected values of model parameters (including distances) for each star individually. Our method provides several alternative distance estimates for each star in the output, along with their associated uncertainties. Methods. The code was first tested on simulations, successfully recovering input distances to mock stars with errors that scale with the uncertainties in the adopted spectro-photometric parameters, as expected. The code was then validated by comparing our distance estimates to parallax measurements from the Hipparcos mission (ESA 1997) for nearby stars (< 60 pc), to asteroseismic distances of CoRoT red giant stars, and to known distances of well-studied open and globular clusters. The photometric data of these reference samples cover both the optical and near infra-red wavelengths. The spectroscopic parameters are also based on spectra taken at various wavelengths, with varying spectral coverage and resolution: the Radial Velocity Experiment, the Sloan Digital Sky Survey programs SEGUE and APOGEE, and the ESO HARPS instrument. Results. For Hipparcos and CoRoT samples, the typical random distance scatter is 20% or less, both for the nearby and farther data. There is a trend towards underestimating the distances by < 10%. The comparison to star clusters from SEGUE and APOGEE has led to systematic differences < 5% for most cluster stars although with significant scatter. Finally, we tested our distances against those previously determined for a high quality sample of giant stars from the RAVE survey, again finding a reasonable agreement, with only a small systematic trend. Efforts are underway to provide our code to the community by running it on a public server.
Bulletin of the American Astronomical Society, May 1, 1995
ABSTRACT Theory, as well as numerical simulations, suggests that Omega_o may be observationally c... more ABSTRACT Theory, as well as numerical simulations, suggests that Omega_o may be observationally constrained by the amount of substructure observed in present-day clusters of galaxies. We have therefore begun a study of the 116 Abell clusters with richness class greater than or equal to 1 and distance class less than or equal to 4, the so-called ``volume-limited&#39;&#39; sample of Hoessel, Gunn, &amp; Thuan 1980 (ApJ 241, 486) to determine the prevalence of substructure in the clusters&#39; projected galaxy positions. We use positions of galaxies identified by the Minnesota Automated Plate Scanner to obtain contour plots of the available clusters using an adaptive kernel routine. Significance of substructure is evaluated using the 2-D Lee test as well as a likelihood-ratio test on fits made with mixtures of two-dimensional gaussians. We also present nonparametric density profile estimates obtained with the program MAPEL (Merritt and Tremblay 1994, AJ 108, 514).
We present numerical simulations to describe the evolution of pre-Galactic clouds in a model whic... more We present numerical simulations to describe the evolution of pre-Galactic clouds in a model which is motivated by cold dark matter simulations of hierarchical galaxy formation. We adopt a SN-induced star-formation mechanism within a model that follows the evolution of chemical enrichment and energy input to the clouds by Type II and Type Ia supernovae. We utilize metallicity-dependent yields for all elements at all times, and include effects of finite stellar lifetimes. We derive the metallicity distribution functions for stars in the clouds, their age-metallicity relation, and relative elemental abundances for a number of alpha-and Fe-group elements. The stability of these clouds against destruction is discussed, and results are compared for different initial mass functions. We find that the dispersion of the metallicity distribution function observed in the outer halo is naturally reproduced by contributions from many clouds with different initial conditions. The scatter in metallicity as a function of age for these stars is very large, implying that no age-metallicity relation exists in the early stages of galaxy formation. Clouds with initial masses > ∼ presently observed globular clusters are found to survive the first 0.1 Gyr from the onset of star formation, suggesting that such systems may have contributed to the formation of the first stars, and could have been selfenriched. More massive clouds are only stable when one assumes an initial mass function that is not biased towards massive stars, indicating that even if the first stars were formed according to a top-heavy mass function, subsequent star formation was likely to have proceeded with a present-day mass function, or happened in an episodic manner. The predicted relative abundances of some alpha-and Fe-group elements show reasonable agreement with the observed values down to metallicities below [Fe/H] ∼ −4 when the
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