arXiv subjects

I. Thompson

Publications and source records attributed to I. Thompson.

At least 37 records · Page 2Linked to original sources

arXiv2012

Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. V. Evidence for a wide age distribution and a complex MDF

Based on high-resolution spectra obtained during gravitational microlensing events we present a detailed elemental abundance analysis of 32 dwarf and subgiant stars in the Galactic bulge. [ABRIDGED], we now have 58 microlensed bulge dwarfs and subgiants that have been homogeneously analysed. The main characteristics of the sample and the findings that can be drawn are: (i) The metallicity distribution (MDF) is wide and spans all metallicities between [Fe/H]=-1.9 to +0.6; (ii) The dip in the MDF around solar metallicity that was apparent in our previous analysis of a smaller sample (26 microlensed stars) is no longer evident; instead it has a complex structure and indications of multiple components are starting to emerge. [ABRIDGED]; (iii) The stars with [Fe/H]<-0.1 are old with ages between 10 and 12 Gyr; (iv) The metal-rich stars with [Fe/H]>-0.1 show a wide variety of ages, ranging from 2 to 12 Gyr with a distribution that has a dominant peak around 4-5 Gyr and a tail towards higher ages; (v) There are indications in the [alpha/Fe] - [Fe/H] that the "knee" occurs around [Fe/H] = -0.3 to -0.2, which is a slightly higher metallicity as compared to the "knee" for the local thick disk. This suggests that the chemical enrichment of the metal-poor bulge has been somewhat faster than what is observed for the local thick disk. The results from the microlensed bulge dwarf stars in combination with other findings in the literature, in particular the evidence that the bulge has cylindrical rotation, indicate that the Milky Way could be an almost pure disk galaxy. The bulge would then just be a conglomerate of the other Galactic stellar populations (thin disk, thick disk, halo, and ...?), residing together in the central parts of the Galaxy, influenced by the Galactic bar.

astro-ph.GA↗

arXiv2012

Pulsation models for the 0.26M_sun star mimicking RR Lyrae pulsator. Model survey for the new class of variable stars

We present non-linear hydrodynamic pulsation models for OGLE-BLG-RRLYR-02792 - a 0.26M_sun pulsator, component of the eclipsing binary system, analysed recently by Pietrzynski et al. The star's light and radial velocity curves mimic that of classical RR Lyrae stars, except for the bump in the middle of the ascending branch of the radial velocity curve. We show that the bump is caused by the 2:1 resonance between the fundamental mode and the second overtone - the same mechanism that causes the Hertzsprung bump progression in classical Cepheids. The models allow to constrain the parameters of the star, in particular to estimate its absolute luminosity (approx 33L_sun) and effective temperature (approx 6970K, close to the blue edge of the instability strip). We conduct a model survey for the new class of low mass pulsators similar to OGLE-BLG-RRLYR-02792 - products of evolution in the binary systems. We compute a grid of models with masses corresponding to half (and less) of the typical mass of RR Lyrae variable, 0.20M_sun<=M<=0.30M_sun, and discuss the properties of the resulting light and radial velocity curves. Resonant bump progression is clear and may be used to distinguish such stars from classical RR Lyrae stars. We present the Fourier decomposition parameters for the modelled light and radial velocity curves. The expected values of the phi_31 Fourier phase for the light curves differ significantly from that observed in RR Lyrae stars, which is another discriminant of the new class.

astro-ph.SR↗

arXiv2012

UNEDF: Advanced Scientific Computing Collaboration Transforms the Low-Energy Nuclear Many-Body Problem

The demands of cutting-edge science are driving the need for larger and faster computing resources. With the rapidly growing scale of computing systems and the prospect of technologically disruptive architectures to meet these needs, scientists face the challenge of effectively using complex computational resources to advance scientific discovery. Multidisciplinary collaborating networks of researchers with diverse scientific backgrounds are needed to address these complex challenges. The UNEDF SciDAC collaboration of nuclear theorists, applied mathematicians, and computer scientists is developing a comprehensive description of nuclei and their reactions that delivers maximum predictive power with quantified uncertainties. This paper describes UNEDF and identifies attributes that classify it as a successful computational collaboration. We illustrate significant milestones accomplished by UNEDF through integrative solutions using the most reliable theoretical approaches, most advanced algorithms, and leadership-class computational resources.

nucl-th↗

arXiv2012

Signatures of an intermediate-age metal-rich bulge population

We have determined detailed elemental abundances and stellar ages for a sample of now 38 microlensed dwarf and subgiant stars in the Galactic bulge. Stars with sub-solar metallicities are all old and have enhanced alpha-element abundances -- very similar to what is seen for local thick disk stars. The metal-rich stars on the other hand show a wide variety of stellar ages, ranging from 3-4 Gyr to 12 Gyr, and an average around 7-8 Gyr. The existence of young and metal-rich stars are in conflict with recent photometric studies of the bulge which claim that the bulge only contains old stars.

astro-ph.GA↗

arXiv2011

He II 4686 in eta Carinae: collapse of the wind-wind collision region during periastron passage

The periodic spectroscopic events in eta Carinae are now well established and occur near the periastron passage of two massive stars in a very eccentric orbit. Several mechanisms have been proposed to explain the variations of different spectral features, such as an eclipse by the wind-wind collision boundary, a shell ejection from the primary star or accretion of its wind onto the secondary. All of them have problems explaining all the observed phenomena. To better understand the nature of the cyclic events, we performed a dense monitoring of eta Carinae with 5 Southern telescopes during the 2009 low excitation event, resulting in a set of data of unprecedented quality and sampling. The intrinsic luminosity of the He II 4686 emission line (L~310 Lsun) just before periastron reveals the presence of a very luminous transient source of extreme UV radiation emitted in the wind-wind collision (WWC) region. Clumps in the primary's wind probably explain the flare-like behavior of both the X-ray and He II 4686 light-curves. After a short-lived minimum, He II 4686 emission rises again to a ne

astro-ph.SR↗

arXiv2011

The Araucaria Project. Accurate determination of the dynamical mass of the classical Cepheid in the eclipsing system OGLE-LMC-CEP-1812

We have analyzed the double-lined eclipsing binary system OGLE-LMC-CEP-1812 in the LMC and demonstrate that it contains a classical fundamental mode Cepheid pulsating with a period of 1.31 days. The secondary star is a stable giant. We derive the dynamical masses for both stars with an accuracy of 1.5%, making the Cepheid in this system the second classical Cepheid with a very accurate dynamical mass determination, following the OGLE-LMC-CEP-0227 system studied by Pietrzynski et al. (2010). The measured dynamical mass agrees very well with that predicted by pulsation models. We also derive the radii of both components and accurate orbital parameters for the binary system. This new, very accurate dynamical mass for a classical Cepheid will greatly contribute to the solution of the Cepheid mass discrepancy problem, and to our understanding of the structure and evolution of classical Cepheids.

astro-ph.SR↗

arXiv2011

Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. IV. Two bulge populations

[ABRIDGED] Based on high-resolution (R~42000 to 48000) and high signal-to-noise (S/N~50 to 150) spectra obtained with UVES/VLT, we present detailed elemental abundances (O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Zn, Y, and Ba) and stellar ages for 26 microlensed dwarf and subgiant stars in the Galactic bulge. The analysis is based on equivalent width measurements and standard 1-D LTE MARCS model stellar atmospheres. We also present NLTE Li abundances based on line synthesis of the 7Li line at 670.8 nm. We show that the bulge metallicity distribution (MDF) is double-peaked; one peak at [Fe/H]= -0.6 and one at [Fe/H]=+0.3, and with a dearth of stars around solar metallicity. This is in contrast to the MDF derived from red giants in Baade's window, which peaks at this exact value. A simple significance test shows that it is extremely unlikely to have such a gap in the microlensed dwarf star MDF if the dwarf stars are drawn from the giant star MDF. To resolve this issue we discuss several possibilities, but we can not settle on a conclusive solution for the observed differences. We further find that the metal-poor bulge dwarf stars are predominantly old with ages greater than 10\,Gyr, while the metal-rich bulge dwarf stars show a wide range of ages. The metal-poor bulge sample is very similar to the Galactic thick disk in terms of average metallicity, elemental abundance trends, and stellar ages. Speculatively, the metal-rich bulge population might be the manifestation of the inner thin disk. If so, the two bulge populations could support the recent findings, based on kinematics, that there are no signatures of a classical bulge and that the Milky Way is a pure-disk galaxy. Also, recent claims of a flat IMF in the bulge based on the MDF of giant stars may have to be revised based on the MDF and abundance trends probed by our microlensed dwarf stars.

astro-ph.GA↗

arXiv2011

UNEDF: Advanced Scientific Computing Transforms the Low-Energy Nuclear Many-Body Problem

The UNEDF SciDAC collaboration of nuclear theorists, applied mathematicians, and computer scientists is developing a comprehensive description of nuclei and their reactions that delivers maximum predictive power with quantified uncertainties. This paper illustrates significant milestones accomplished by UNEDF through integration of the theoretical approaches, advanced numerical algorithms, and leadership class computational resources.

physics.comp-ph↗

arXiv2009

The stellar content of the Hamburg/ESO survey. V. The metallicity distribution function of the Galactic halo

We determine the metallicity distribution function (MDF) of the Galactic halo by means of a sample of 1638 metal-poor stars selected from the Hamburg/ESO objective-prism survey (HES). The sample was corrected for minor biases introduced by the strategy for spectroscopic follow-up observations of the metal-poor candidates, namely "best and brightest stars first". [...] We determined the selection function of the HES, which must be taken into account for a proper comparison between the HES MDF with MDFs of other stellar populations or those predicted by models of Galactic chemical evolution. The latter show a reasonable agreement with the overall shape of the HES MDF for [Fe/H] > -3.6, but only a model of Salvadori et al. (2007) with a critical metallicity for low-mass star formation of Z_cr = 10^{-3.4} * Z_Sun reproduces the sharp drop at [Fe/H] ~-3.6 present in the HES MDF. [...] A comparison of the MDF of Galactic globular clusters and of dSph satellites to the Galaxy shows qualitative agreement with the halo MDF, derived from the HES, once the selection function of the latter is included. However, statistical tests show that the differences between these are still highly significant. [ABSTRACT ABRIDGED]

astro-ph↗

arXiv2009

Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. Detailed abundance analysis of OGLE-2008-BLG-209S

AIMS. Our aims are twofold. First we aim to evaluate the robustness and accuracy of stellar parameters and detailed elemental abundances that can be derived from high-resolution spectroscopic observations of microlensed dwarf and subgiant stars. We then aim to use microlensed dwarf and subgiant stars to investigate the abundance structure and chemical evolution of the Milky Way Bulge. [ABRIDGED] METHODS. We present a detailed elemental abundance analysis of OGLE-2008-BLG-209S, the source star of a new microlensing event towards the Bulge, for which we obtained a high-resolution spectrum with the MIKE spectrograph on the Magellan Clay telescope. We have performed four different analyses of OGLE-2008-BLG-209S. [ABRIDGED] We have also re-analysed three previous microlensed dwarf stars OGLE-2006-BLG-265S, MOA-2006-BLG-099S, and OGLE-2007-BLG-349S with the same method. This homogeneous data set, although small, enables a direct comparison between the different stellar populations. RESULTS. We find that OGLE-2008-BLG-209S is a subgiant star that has a metallicity of [Fe/H] ~-0.33. It possesses [alpha/Fe] enhancements similar to what is found for Bulge giant stars at the same metallicity, and what also is found for nearby thick disc stars at the same metallicity. In contrast, the previous three microlensing dwarf stars have very high metallicities, [Fe/H]>+0.4, and more solar-like abundance ratios, i.e. [alpha/Fe]~0. The decrease in the [alpha/Fe] ratio with [Fe/H] is the typical signature of enrichment from low and intermediate mass stars. We furthermore find that the results for the four Bulge stars, in combination with results from studies of giant stars in the Bulge, seem to favour a secular formation scenario for the Bulge.

astro-ph.GA↗

arXiv2009

The Araucaria Project. Determination of the LMC Distance from Late-Type Eclipsing Binary Systems: I. OGLE-051019.64-685812.3

We have analyzed the double-lined eclipsing binary system OGLE-051019.64-685812.3 in the LMC which consists of two G4 giant components with very similar effective temperatures. A detailed analysis of the OGLE I-band light curve of the system, radial velocity curves for both components derived from high-resolution spectra, and near-infrared magnitudes of the binary system measured outside the eclipses has allowed us to obtain an accurate orbit solution for this eclipsing binary, and its fundamental physical parameters. Using a surface brightness-(V-K) color relation for giant stars we have calculated the distance to the system and obtain a true distance modulus of 18.50 mag, with an estimated total uncertainty of ~ 3 %. More similar eclipsing binary systems in the LMC which we have discovered and for which we are currently obtaining the relevant data will allow us to better check on the systematics of the method and eventually provide a distance determination to the LMC accurate to 1 percent, so much needed for the calibration of the distance scale.

astro-ph.CO↗

arXiv2007

An Update on the 0Z Project

We give an update on our 0Z Survey to find more extremely metal poor (EMP) stars with [Fe/H] < -3 dex through mining the database of the Hamburg/ESO Survey. We present the most extreme such stars we have found from ~1550 moderate resolution follow up spectra. One of these, HE1424-0241, has highly anomalous abundance ratios not seen in any previously known halo giant, with very deficient Si, moderately deficient Ca and Ti, highly enhanced Mn and Co, and low C, all with respect to Fe. We suggest a SNII where the nucleosynthetic yield for explosive alpha-burning nuclei was very low compared to that for the hydrostatic alpha-burning element Mg, which is normal in this star relative to Fe. A second, less extreme, outlier star with high [Sc/Fe] has also been found. We examine the extremely metal-poor tail of the HES metallicity distribution function (MDF). We suggest on the basis of comparison of our high resolution detailed abundance analyses with [Fe/H](HES) for stars in our sample that the MDF inferred from follow up spectra of the HES sample of candidate EMP stars is heavily contaminated for [Fe/H](HES) < -3 dex; many of the supposed EMP stars below that metallicity are of substantially higher Fe-metallicity, including most of the very C-rich stars, or are spurious objects.

astro-ph↗

arXiv2007

Calibration of the Relationship between Precipitable Water Vapor and 225 GHz Atmospheric Opacity via Optical Echelle Spectroscopy at Las Campanas Observatory

We report precipitable water vapor (PWV) measurements made at Las Campanas Observatory using optical spectra of H2O lines obtained with the Magellan echelle spectrograph, and calculated using a robust technique that is accurate to 5-10%. Calibration of the relationship between our PWV measurements and opacity values at 225 GHz was made possible by simultaneous tipping radiometer observations. Based on this calibration, we present Las Campanas Observatory winter-time precipitable water vapor statistics, measured using the tipping radiometer, during a two month campaign. The median value of 2.8 +- 0.3 mm is consistent with that measured at the nearby La Silla Observatory during the VLT site survey. We conclude that, in the Southern hemisphere winter months, we can expect good conditions for infrared observing (<~1.5 mm) approximately 10% of the time at Las Campanas Observatory.

astro-ph↗

arXiv2007

The Curious Case of NGC6908

The object NGC6908 was once thought to be simply a surface-brightness enhancement in the eastern spiral arm of the nearby spiral galaxy NGC6907. Based on an examination of near-infrared imaging, the object is shown in fact to be a lenticular S0(6/7) galaxy hidden in the optical glare of the disk and spiral structure of the larger galaxy. New radial velocities of NGC6908 (3,060+/-16 (emission); 3,113+/-73 km/s (absorption)) have been obtained at the Baade 6.5m and the duPont 2.5m telescopes at Las Campanas, Chile placing NGC6908 at the same expansion-velocity distance as NGC6907 (3,190+/-5 km/s), eliminating the possibility of a purely chance line-of-sight coincidence. The once-enigmatic asymmetries in the disk and outer spiral structure of NGC6907 are now explained as being due to an advanced merger event. Newly discovered tails and debris in the outer reaches of this galaxy further support the merger scenario for this system. This pair of galaxies is a rather striking example of two objects discovered over 100 years ago, whose true nature was lost until modern detectors operating at infrared wavelengths gave us a new (high-contrast) look. Other examples of embedded merger remnants may also reveal themselves in the growing samples of near-infrared imaging of nearby galaxies; and a pilot study does reveal several other promising candidates for follow-up observations.

astro-ph↗

arXiv2002

Clusters AgeS Experiment. Hot subdwarfs and luminous white dwarf candidates in the field of the globular cluster M4

We present UBV color magnitude diagrams (CMDs) for the globular cluster M4. The CMDs show a sequence of four luminous blue stars (V<20, U-V<-0.6) which appear to be cluster hot subdwarfs. We present spectra for the three brightest ones. We also note the presence of a population of faint blue objects, likely to be hot, young white dwarfs (WDs) belonging to the cluster. We have selected five objects above V=22 mag, bright enough for follow-up ground-based spectroscopy and present their coordinates and finding charts. We show a spectrum for variable V46 (Kaluzny et al. 1997) which suggests that it is a hot subdwarf, along with a new light curve obtained with the ISIS image subtraction package (Alard 2000). The light curve is unstable, but only one period of variability is apparent. Two new variables have been discovered, both located on the cluster red giant branch (RGB). We also present a differential E(B-V) reddening map and a fiducial sequence for the main sequence, subgiant branch and red giant branch on the V/B-V CMD for a selected region with uniform reddening. Based on a comparison with the M5 fiducial sequence we obtain a reddening estimate of E(B-V)=0.41 mag towards M4, consistent with previous determinations.

astro-ph↗

arXiv2001

Photometry and Spectroscopy of Eclipsing Binaries in omega Centauri

Detached eclipsing double line spectroscopic binaries are accurate distance indicators. They can be used for the direct determination of distances to globular clusters in which such systems can be identified. The results of major photometric surveys aimed at identification of variable stars in omega Cen are summarized. Over 43 eclipsing binaries have been identified so far in the cluster field of which only 8 objects are true detached systems. Distance and age determinations are presented for the system OGLEGC-17. The apparent distance modulus is (m-M)_{V}=14.09\pm 0.04 and the age of this binary is tau=11.8\pm 0.6 Gyr.

astro-ph↗

arXiv2000

UBVI Color-Magnitude Diagrams in Baade's Window: Metallicity Range, Implications for the Red Clump Method, Color ``Anomaly'' and the Distances to the Galactic Center and the Large Magellanic Cloud

(Abridged) We analyze the UBVI color-magnitude diagrams towards the Galactic bulge in a relatively low-reddening region of Baade's Window. The dereddened red giant branch is very wide [~1.0 mag in (U-B)_0 and ~0.4 mag in (B-V)_0 and (V-I)_0], indicating a significant dispersion of stellar metallicities, which by comparison with the theoretical isochrones and data for Galactic clusters we estimate to lie between approximately -0.7< [Fe/H] < +0.3. We also discuss the metallicity dependence of the red clump I-band brightness and we show that it is between 0.1-0.2 mag dex^{-1}. This agrees well with the previous empirical determinations and the models of stellar evolution. The de-reddened (V-I)_0 color of the red clump in the observed bulge field is <(V-I)_0> = 1.066, sigma_{(V-I)_0} = 0.14, i.e. 0.056 mag redder than the local stars with good parallaxes measured by Hipparcos. When we use our data to re-derive the red clump distance to the Galactic center, we obtain the Galactocentric distance modulus μ_{0,GC}=14.69 +- 0.1 mag (R_0=8.67 +- 0.4 kpc), with error dominated by the systematics of photometric calibration. We then discuss the systematics of the red clump method and how they affect the red clump distance to the Large Magellanic Cloud. We argue that the value of distance modulus μ_{0,LMC}=18.24 +- 0.08 (44.5 +- 1.7 kpc), recently refined by Udalski, is currently the most secure and robust of all LMC distance estimates. This has the effect of increasing any LMC-tied Hubble constant by about 12%, including the recent determinations by the HST Key Project and Sandage et al. The UBVI photometry is available through the anonymous ftp service.

astro-ph↗

arXiv2000

Variable stars in the field of the globular cluster E3

We present the results of a search for variable stars in the faint sparse globular cluster E3. We have found two variable stars: an SX Phe variable (V1) and a W UMa eclipsing binary (V2). We have applied period-luminosity and period-color-luminosity relations to the variables to obtain their distance moduli. V1 seems to be a blue straggler belonging to E3, based on its distance modulus and location on the CMD. V2 is probably located behind the cluster, in the Milky Way halo. We also present V/B-V and V/V-I color magnitude diagrams of E3.

astro-ph↗