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Near-infrared observations of the Fornax dwarf galaxy. I. The red giant branch
Aims.We present a study of the evolved stellar populations in the dwarfspheroidal galaxy Fornax based on wide-area near-infrared observations,aimed at obtaining new independent estimates of its distance andmetallicity distribution. Assessing the reliability of near-infraredmethods is most important in view of future space- and ground-based deepnear-infrared imaging of resolved stellar systems. Methods: We haveobtained JHK imaging photometry of the stellar populations in Fornax.The observations cover an 18.5×18.5 arcmin2 centralarea with a mosaic of SOFI images at the ESO NTT. Our data sample allthe red giant branch (RGB) for the whole area. Deeeper observationsreaching the red clump of helium-burning stars have also been obtainedfor a 4.5×4.5 arcmin2 region. Results: Near-infraredphotometry led to measurements of the distance to Fornax based on theK-band location of the RGB tip and the red clump. Once corrected for themean age of the stellar populations in the galaxy, the derived distancemodulus is (m-M)0 = 20.74±0.11, corresponding to adistance of 141 Kpc, in good agreement with estimates from optical data.We have obtained a photometric estimate of the mean metallicity of redgiant stars in Fornax from their (J-K) and (V-K) colors, using severalmethods. The effect of the age-metallicity degeneracy on the combinedoptical-infrared colors is shown to be less important than for opticalor infrared colors alone. By taking age effects into account, we havederived a distribution function of global metallicity [M/H] fromoptical-infrared colors of individual stars. Our photometric MetallicityDistribution Function covers the range -2.0 < [M/H] < -0.6, with amain peak at [M/H]≃ -0.9 and a long tail of metal-poor stars, andless metal-rich stars than derived by recent spectroscopy. Ifmetallicities from Ca II triplet lines are correct, this result confirmsa scenario of enhanced metal enrichment in the last 1-4 Gyr.Based on data collected at the European Southern Observatory, La Silla,Chile, Proposals No. 65.N-0167, 66.B-0247. Full Tables 2 and 3 are onlyavailable in electronic format at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/467/1025
| Metallicity and age gradients in round elliptical galaxies
Aims.We probe the stellar population age and metallicity distributionsin nearby elliptical galaxies over the largest extension to date. Methods: Long-slit spectroscopy is made by using the spectrographSCORPIO of the 6-m telescope of the Special Astrophysical Observatory ofthe Russian Academy of Sciencies. The Lick indices Hβ, Mg b, Fe5270, and Fe 5335 are calculated along the slit up to radii of 1.3 to3re in 4 galaxies and up to 0.5re in the fifthone. The comparison with evolutionary synthesis models of simple stellarpopulations allows us to disentangle age and metallicity and to measureboth. Results: We have found that the mean stellar age is constantalong the radius only in one galaxy out of 5. The other 4 galaxiesdemonstrate quite different behaviour of the mean stellar age: the outerparts are older than the centres in 3 cases and younger - in one case.The metallicity gradients cannot be approximated by a single power lawover the full radial extension in 4 galaxies of 5. The inner metallicitygradients within 0.5re are all rather steep, steeper than-0.4 metallicity dex per radius dex, and are inconsistent with theorigin of the elliptical galaxies by a major merger.Based on observations collected with the 6 m telescope of the SpecialAstrophysical Observatory (SAO) of the Russian Academy of Sciences (RAS)which is operated under the financial support of Science Department ofRussia (registration number 01-43).
| Where the Blue Stragglers Roam: Searching for a Link between Formation and Environment
The formation of blue stragglers is still not completely understood,particularly the relationship between formation environment andmechanism. We use a large, homogeneous sample of blue stragglers in thecores of 57 globular clusters to investigate the relationships betweenblue straggler populations and their environments. We use a consistentdefinition of ``blue straggler'' based on position in thecolor-magnitude diagram and normalize the population relative to thenumber of red giant branch stars in the core. We find that thepreviously determined anticorrelation between blue straggler frequencyand total cluster mass is present in the purely core population. We findsome weak anticorrelations with central velocity dispersion and withhalf-mass relaxation time. The blue straggler frequency does not showany trend with any other cluster parameter. Even though collisions maybe expected to be a dominant blue straggler formation process inglobular cluster cores, we find no correlation between the frequency ofblue stragglers and the collision rate in the core. We also investigatedthe blue straggler luminosity function shape and found no relationshipbetween any cluster parameter and the distribution of blue stragglers inthe color-magnitude diagram. Our results are inconsistent with somerecent models of blue straggler formation that include collisionalformation mechanisms and may suggest that almost all observed bluestragglers are formed in binary systems.
| The INTEGRAL Galactic bulge monitoring program: the first 1.5 years
Aims.The Galactic bulge region is a rich host of variable high-energypoint sources. Since 2005, February 17 we are monitoring the sourceactivity in the Galactic bulge region regularly and frequently, i.e.,about every three days, with the instruments onboard INTEGRAL. Thanks tothe large field of view, the imaging capabilities and the sensitivity athard X-rays, we are able to present for the first time a detailedhomogeneous (hard) X-ray view of a sample of 76 sources in the Galacticbulge region. Methods: We describe the successful monitoring programand show the first results from the start of the monitoring up to 2006,April 21, i.e., for a period of about one and a half year, during threevisibility seasons. We focus on the short (hour), medium (month) andlong-term (year) variability in the hard X-ray bands, i.e., 20-60 keVand 60-150 keV. When available, we discuss the simultaneous observationsin the soft X-ray, 3-10 keV and 10-25 keV, bands. Results: Almost allthe sources in the Galactic bulge region we detect in the 20-60 keV and60-150 keV bands are variable. During the last two and a half weeks ofthe third visibility season most of the known persistent (hard) X-raysources in the Galactic Center region were not detected. Of our sampleof sources, per visibility season we detect 32/33 sources in the 20-60keV band and 8/9 sources in the 60-150 keV band above a signal to noiseof 7. On average, we find per visibility season one active bright(⪆100 mCrab, 20-60 keV) black-hole candidate X-ray transient andthree active weaker (⪉25 mCrab, 20-60 keV) neutron star X-raytransients. Most of the time a clear anti-correlation can be seenbetween the soft and hard X-ray emission in some of the X-ray bursters.Hard X-ray flares or outbursts in X-ray bursters, which have a durationof the order of weeks are accompanied by soft X-ray drops. On the otherhand, hard X-ray drops can be accompanied by soft X-rayflares/outbursts. During the course of our program we found a number ofnew sources, IGR J17354-3255, IGR 17453-2853, IGR J17454-2703, IGRJ17456-2901b, IGR J17536-2339, and IGR J17541-2252. We report here onsome of the high-energy properties of these sources. Conclusions: Thehigh-energy light curves of all the sources in the field of view, andthe high-energy images of the region, are made available through theWWW, as soon as possible after the observations have been performed, athttp://isdc.unige.ch/Science/BULGE/.Appendices are only available in electronic form at http://www.aanda.org
| Near-Infrared Properties of 24 Globular Clusters in the Galactic Bulge
We present near-IR color-magnitude diagrams and physical parameters fora sample of 24 Galactic globular clusters toward the bulge. In thispaper we discuss the properties of 12 new clusters (out of the 24) inaddition to those previously studied and published by our group. Thecompilation includes measurements of the cluster reddening, distance,photometric metallicity, horizontal branch red clump, and red giantbranch morphological (e.g., mean ridgelines) and evolutionary (e.g.,bump and tip) features. The compilation is available in electronic formon the World Wide Web, and it will be updated regularly.Based on data taken at the ESO New Technology Telescope, within theobserving programs 73.D-0313, 75.D-0372, and 77.D-0757.
| Six new candidate ultracompact X-ray binaries
Ultracompact X-ray binaries (UCXBs) appear able to sustain accretiononto the compact accretor at rates lower than in wider X-ray binaries.This may be understood by the smaller accretion disks in UCXBs: a lowerX-ray luminosity suffices to keep a disk completely ionized throughirradiation and, thus, keep the viscosity at a sufficiently high levelto allow effective transport of matter to the compact object. We employthis distinguishing factor on data from RXTE and BeppoSAX to identifysix new candidate UCXBs, thus increasing the population by one quarter.The candidates are drawn from the population of persistently accretingand type-I X-ray bursting low-mass X-ray binaries. The X-ray burstsestablish the low-mass X-ray binary nature and provide a handle on theaccretion rate. We find that the low accretion rates are supported bythe long burst recurrence times and the hard X-ray spectra of thepersistent emission as derived from the 2nd INTEGRAL catalog of softγ-ray sources. We discuss the peculiar light curves of some newUCXB candidates.
| INTEGRAL Spectral Variability Study of the Atoll 4U 1820-30: First Detection of Hard X-Ray Emission
We study the 4-200 keV spectral and temporal behavior of the low-massX-ray binary 4U 1820-30 with INTEGRAL during 2003-2005. This source asbeen observed in both the soft (banana) and hard (island) spectralstates. A high-energy tail, above 50 keV, in the hard state has beenobserved for the first time. This places the source in the category ofX-ray bursters showing high-energy emission. The tail can be modeled asa soft power-law component, with the photon index of ~=2.4, on top ofthermal Comptonization emission from a plasma with electron temperaturekTe~=6 keV and optical depth τ~=4. Alternatively, but ata poorer goodness of fit, the hard-state broadband spectrum can beaccounted for by emission from a hybrid, thermal-nonthermal, plasma.During this monitoring the source spent most of the time in the softstate, usual for this source, and the >~4 keV spectra are describedby thermal Comptonization with kTe~=3 keV and τ~=6-7.INTEGRAL is an ESA project with instruments and Science Data Centrefunded by ESA member states (especially the PI countries: Denmark,France, Germany, Italy, Switzerland, and Spain), Czech Republic, andPoland, and with the participation of Russia and the USA.
| Integrated-Light Two Micron All Sky Survey Infrared Photometry of Galactic Globular Clusters
We have mosaicked Two Micron All Sky Survey (2MASS) images to derivesurface brightness profiles in J, H, and Ks for 104 Galacticglobular clusters. We fit these with King profiles and show that thecore radii are identical to within the errors for each of these IRcolors and are identical to the core radii at V in essentially allcases. We derive integrated-light colors V-J, V-H, V-Ks, J-H,and J-Ks for these globular clusters. Each color shows areasonably tight relation between the dereddened colors and metallicity.Fits to these are given for each color. The IR - IR colors have verysmall errors, due largely to the all-sky photometric calibration of the2MASS survey, while the V-IR colors have substantially largeruncertainties. We find fairly good agreement with measurements ofintegrated-light colors for a smaller sample of Galactic globularclusters by M. Aaronson, M. Malkan, and D. Kleinmann from 1977. Ourresults provide a calibration for the integrated light of distantsingle-burst old stellar populations from very low to solarmetallicities. A comparison of our dereddened measured colors withpredictions from several models of the integrated light of single-burstold populations shows good agreement in the low-metallicity domain forV-Ks colors but also shows an offset at a fixed [Fe/H] of~0.1 mag in J-Ks, which we ascribe to photometric systemtransformation issues. Some of the models fail to reproduce the behaviorof the integrated-light colors of the Galactic globular clusters nearsolar metallicity.
| Chandra Detection of Fe XVII in Absorption: Iron Abundance in the Hot Gaseous Interstellar Medium
The iron depletion level and the gas-phase iron abundance in the hot(~106 K) interstellar medium (ISM) are critical to ourunderstanding of its energy balance as well as the thermal sputtering,cooling, and heating processes of dust grains. Here, we report on thefirst detection of the Fe XVII absorption line at 15.02 Å from thehot ISM in the spectrum of the low-mass X-ray binary 4U 1820-303observed with the Chandra X-Ray Observatory. By jointly analyzing thisabsorption line with those from O VII, O VIII, and Ne IX ions in thesame spectrum, we obtain an abundance ratio as Fe/Ne=0.8(0.4,2.1) inunits of the Anders & Grevesse solar value (90% confidenceintervals). We find that the result is robust with respect to differentassumed gas temperature distributions. The obtained Fe/Ne abundanceratio, albeit with large uncertainties, is consistent with the solarvalue, indicating that there is very little or no iron depleted intodust grains, i.e., most of or all of the dust grains have been destroyedin the hot ISM.
| A Comparison of Ultraluminous X-Ray Sources in NGC 1399 and the Antennae Galaxies (NGC 4038/4039)
The temporal and spectral properties of ultraluminous X-ray sources(ULXs, LX>2×1039 ergs s-1) andbright X-ray sources (LX>3×1038 ergss-1) are examined and compared in two extremely differenthost environments: the old elliptical galaxy NGC 1399 and the young,star-forming Antennae galaxies (NGC 4038/4039). ULXs in NGC 1399 showlittle variability on either long or short timescales. Only 1 of 8 ULXsand 10 of 63 bright sources in NGC 1399 are variable at a confidencelevel of 90%. On long timescales, the NGC 1399 sources are steadier thanmost Galactic black hole X-ray binaries, but are similar to GRS1915+105. The outburst duration of the NGC 1399 sources is about 20 yr,again, similar to that of GRS 1915+105. The bright X-ray sources in NGC1399 may be black hole X-ray binaries with giant-star companions similarto GRS 1915+105. The brightest ULX (PSX-1) in NGC 1399 is coincidentwith a globular cluster, shows a hard spectrum with a photon indexaround 1.5, and has a nearly constant luminosity around5×1039 ergs s-1. It may be anintermediate-mass black hole (IMBH) in a hard spectral state. Incontrast to NGC 1399, the ULXs in the Antennae are all variable, and alarge fraction of the bright sources (9 of 15) are also variable. Thevariability and luminosity of ULXs in the Antennae suggest that they areblack hole high-mass X-ray binaries accreting via Roche lobe overflow. Aflare with a duration of about 5 ks is found from Antennae X-42. Themost luminous ULX, X-16, with a very hard spectrum (Γ=1.0-1.3) anda luminosity that varies by a factor of 10, could be an IMBH candidate.
| On the maximum amplitude and coherence of the kilohertz quasi-periodic oscillations in low-mass X-ray binaries
I study the behaviour of the maximum rms fractional amplitude,rmax, and the maximum coherence, Qmax, of thekilohertz quasi-periodic oscillations (kHz QPOs) in a dozen low-massX-ray binaries. I find that (i) the maximum rms amplitudes of the lower-and upper-kHz QPOs, rlmax andrumax, respectively, decrease more or lessexponentially with increasing luminosity of the source; (ii) the maximumcoherence of the lower-kHz QPO, Qlmax, firstincreases and then decreases exponentially with luminosity, at a fasterrate than both rlmax andrumax; (iii) the maximum coherence of theupper-kHz QPO, Qumax, is more or less independentof luminosity; and (iv) rmax and Qmax show theopposite behaviour with hardness of the source, consistent with the factthat there is a general anticorrelation between luminosity and spectralhardness in these sources. Both rmax and Qmax inthe sample of sources, and the rms amplitude and coherence of the kHzQPOs in individual sources show a similar behaviour with hardness. Thissimilarity argues against the interpretation that the drop of coherenceand rms amplitude of the lower-kHz QPO at high QPO frequencies inindividual sources is a signature of the innermost stable circular orbitaround a neutron star. I discuss possible interpretations of theseresults in terms of the modulation mechanisms that may be responsiblefor the observed variability.
| The remarkable stability of probable black hole low-mass X-ray binaries in nearby galaxies
The most luminous X-ray sources in nearby elliptical galaxies are likelyblack hole low-mass X-ray binaries (BHLMXBs). In the Milky Way, suchsystems are always transient, and with the exception of GRS1915+105 haveburst durations on the order of weeks or months. However, the low dutycycle of short-duration outburst BHLMXBs makes it improbable that anyone source would be caught in an outburst during a single snapshotobservation. Long-duration outburst BHLMXBs, although much rarer, wouldbe detectable in a series of snapshot observations separated by severalyears. Our analysis of multi-epoch Chandra observations of the giantelliptical galaxies NGC 1399 and M87 separated by 3.3 and 5.3 yr,respectively, finds that all 37 luminous (>8 × 1038ergs-1) X-ray sources that were present in the first epochobservations were still in outburst in all of the followingobservations. Many of these probable long-duration outburst BHLMXBsreside within globular clusters of the galaxies. Conversely, nodefinitive short-duration outburst BHLMXBs were detected in any of theobservations. This places an upper limit on the ratio ofshort-to-long-duration outbursters that is slightly lower, butconsistent with what is seen in the Milky Way. The fact that none of theluminous sources turned off between the first and last epochs places a95 per cent lower limit of 50 yr on the mean burst duration of thelong-duration outburst sources. The most likely scenario for the originof these sources is that they are long-period (>30 d) black holebinaries with a red giant donor, much like GRS1915+105. However, unlikeGRS1915+105, most of the sources show only modest variability from epochto epoch.
| Global fitting of globular cluster age indicators
Context: .Stellar models and the methods for the age determinations ofglobular clusters are still in need of improvement. Aims: .Weattempt to obtain a more objective method of age determination based oncluster diagrams, avoiding the introduction of biases due to thepreference of one single age indicator. Methods: .We compute newstellar evolutionary tracks and derive the dependence of age indicatingpoints along the tracks and isochrone - such as the turn-off or bumplocation - as a function of age and metallicity. The same criticalpoints are identified in the colour-magnitude diagrams of globularclusters from a homogeneous database. Several age indicators are thenfitted simultaneously, and the overall best-fitting isochrone isselected to determine the cluster age. We also determine thegoodness-of-fit for different sets of indicators to estimate theconfidence level of our results. Results: .We find that ourisochrones provide no acceptable fit for all age indicators. Inparticular, the location of the bump and the brightness of the tip ofthe red giant branch are problematic. On the other hand, the turn-offregion is very well reproduced, and restricting the method to indicatorsdepending on it results in trustworthy ages. Using an alternative set ofisochrones improves the situation, but neither leads to an acceptableglobal fit. Conclusions: .We conclude that evolutionary tracks oflow-mass metal-poor stars are far from reproducing all aspects ofglobular cluster colour-magnitude diagrams and that the determination ofcluster ages still depends on the favourite method or indicator chosen.
| Surface Brightness Profiles of Galactic Globular Clusters from Hubble Space Telescope Images
The Hubble Space Telescope (HST) allows us to study the central surfacebrightness profiles of globular clusters at unprecedented detail. Wehave mined the HST archives to obtain 38 WFPC2 images of Galacticglobular clusters with adequate exposure times and filters, which we useto measure their central structure. We outline a reliable method toobtain surface brightness profiles from integrated light that we test onan extensive set of simulated images. Most clusters have central surfacebrightness about 0.5 mag brighter than previous measurements made fromground-based data, with the largest differences around 2 mag. Includingthe uncertainties in the slope estimates, the surface brightness slopedistribution is consistent with half of the sample having flat cores andthe remaining half showing a gradual decline from 0 to -0.8[dlogΣ/dlogr)]. We deproject the surface brightness profiles in anonparametric way to obtain luminosity density profiles. Thedistribution of luminosity density logarithmic slopes shows similarfeatures, with half of the sample between -0.4 and -1.8. These resultsare in contrast to our theoretical bias that the central regions ofglobular clusters are either isothermal (i.e., flat central profiles) orvery steep (i.e., luminosity density slope approximately -1.6) forcore-collapse clusters. With only 50% of our sample having centralprofiles consistent with isothermal cores, King models appear torepresent most globular clusters in their cores poorly.
| Instantaneous radio spectra of giant pulses from the crab pulsar from decimeter to decameter wavelengths
The results of simultaneous multifrequency observations of giant radiopulses from the Crab pulsar, PSR B0531+21, at 23, 111, and 600 MHz arepresented and analyzed. Giant pulses were detected at a frequency as lowas 23 MHz for the first time. Of the 45 giant pulses detected at 23 MHz,12 were identified with counterparts observed simultaneously at 600 MHz.Of the 128 giant pulses detected at 111 MHz, 21 were identified withcounterparts observed simultaneously at 600 MHz. The spectral indicesfor the power-law frequency dependence of the giant-pulse energies arefrom -3.1 to -1.6. The mean spectral index is -2.7 ± 0.1and is the same for both frequency combinations (600 111 MHz and 600 23MHz). The large scatter in the spectral indices of the individual pulsesand the large number of unidentified giant pulses suggest that thespectra of the individual giant pulses do not actually follow a simplepower law. The observed shapes of the giant pulses at all threefrequencies are determined by scattering on interstellar plasmainhomogeneities. The scatter-broadening of the pulses and its frequencydependence were determined as τ sc =20(ν/100)-3.5±0.1 ms, where frequency ν is in MHz.
| Multivariate analysis of globular cluster horizontal branch morphology: searching for the second parameter
Aims.The interpretation of globular cluster horizontal branch (HB)morphology is a classical problem that can significantly blur ourunderstanding of stellar populations. Methods: .We present a newmultivariate analysis connecting the effective temperature extent of theHB with other cluster parameters. The work is based on Hubble SpaceTelescope photometry of 54 Galactic globular clusters. Results: .The present study reveals the important role of the total mass of theglobular cluster on its HB morphology. More massive clusters tend tohave HBs more extended to higher temperatures. For a set of three inputvariables including the temperature extension of the HB, [Fe/H] and M_V,the first two eigenvectors account for 90% of the total samplevariance. Conclusions: . Possible effects of clusterself-pollution on HB morphology, stronger in more massive clusters,could explain the results derived here.
| An Empirical Calibration of the Mixing-Length Parameter α
We present an empirical calibration of the mixing-length free parameterα based on a homogeneous infrared database of 28 Galactic globularclusters spanning a wide metallicity range (-2.15<[Fe/H]<-0.2).Empirical estimates of the red giant effective temperatures have beenobtained from infrared colors. Suitable relations linking thesetemperatures to the cluster metallicity have been obtained and comparedto theoretical predictions. An appropriate set of models for the Sun andPopulation II giants has been computed by using both the standard solarmetallicity (Z/X)solar=0.0275 and the most recently proposedvalue (Z/X)solar=0.0177. We find that when the standard solarmetallicity is adopted, a unique value of α=2.17 can be used toreproduce both the solar radius and the Population II red gianttemperature. Conversely, when the new solar metallicity is adopted, twodifferent values of α are required: α=1.86 to fit the solarradius and α~2.0 to fit the red giant temperatures. However, itmust be noted that regardless the adopted solar reference, theα-parameter does not show any significant dependence onmetallicity.Based on observations collected at the European Southern Observatory(ESO), La Silla, Chile. Also based on observations made with the ItalianTelescopio Nazionale Galileo (TNG) operated on the island of La Palma bythe Fundacion Galileo Galilei of the INAF (Istituto Nazionale diAstrofisica) at the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofisica de Canarias.
| Surface-brightness fluctuations in stellar populations. IAC-star models for the optical and near-IR wavelengths
Aims.A new theoretical calibration of surface-brightness fluctuations(SBF) for single age, single metallicity stellar populations ispresented for the optical and near-IR broad-band filters, as well as forthe HST WFPC2 and ACS filters. Methods: .The IAC-star code isused. Two Padua and the Teramo stellar evolution libraries have beenconsidered. A set of single-burst stellar populations (SSP) with a widerange of ages (3 Gy-15 Gy) and metallicities (Z = 0.0001-0.03) have beencomputed using each one of the three considered stellar evolutionlibraries. For each SSP, color indexes and SBF magnitudes are given forthe filters U, B, V, R, I, J, H, K, {F218W}, {F336W}, {F439W}, {F450W},{F555W} and {F814W}, and for the first time, an uncertainty has beenestimated for the SBF theoretical calibration. Results: .Althoughsome differences might be addressed, the Padua and Teramo stellarevolution libraries provide comparable SBF results. A detailedcomparison of the present SBF calibrations with both previouscalibrations and observational data is also presented. Comparing thedifferent models with observational data, Padua based models reproducefairly well the optical data for globular clusters, while Teramo basedmodels fits both optical galaxies and globular clusters data, as well.In the near-IR wavelengths, the Teramo based models provide the only SBFtheoretical calibration to date able to properly reproduce theobservational data for superclusters, with intermediate-to-lowmetallicity. As a conclusion, Teramo based models work better than anyother calibration reproducing observational data for the near-IRwavelengths. Furthermore, the age-metallicity degeneracy is broken forlow metallicity (Z≤0.0037) stellar populations. Finally, a clearrelation between the B SBF absolute magnitude of a stellar populationand its metallicity is found for intermediate to old populations, so theB-band fluctuation magnitude is proposed as a metallicity tracer. Thepresent theoretical calibration shows that the analysis of SBF providesa very powerful tool in the study and characterization of unresolvedstellar populations.
| X-Ray Absorption Spectroscopy of the Multiphase Interstellar Medium: Oxygen and Neon Abundances
X-ray absorption spectroscopy provides a powerful tool in determiningthe metal abundances in various phases of the interstellar medium (ISM).We present a case study of the sight line toward 4U 1820-303, based onChandra grating observations. The detection of O I, O II, O III, O VII,O VIII, and Ne IX Kα absorption lines allows us to measure theatomic column densities of the neutral, warm ionized, and hot phases ofthe ISM through much of the Galactic disk. By comparing thesemeasurements with the 21 cm hydrogen emission and with the pulsardispersion measure, we estimate the mean oxygen abundances in theneutral and total ionized phases as 0.3(0.2, 0.6) and 2.2(1.1, 3.5) inunits of Anders & Greversse's solar value (90% confidenceintervals). This significant oxygen abundance difference is apparently aresult of molecule/dust grain destruction and recent metal enrichment inthe warm ionized and hot phases. We also measure the column density ofneon from its absorption edge and obtain a solar value of the Ne/O ratioaccounting for the expected oxygen contained in molecules and dustgrains. From a joint analysis of the O VII, O VIII, and Ne IX lines, weobtain the Ne/O abundance ratio of the hot phase as 1.4(0.9, 2.1) solar,which is not sensitive to the exact hot gas temperature distributionassumed. These comparable ISM Ne/O ratios for the different phases arethus considerably less than the value recently inferred from coronaemission of solar-like stars.
| Green Bank Telescope Studies of Giant Pulses from Millisecond Pulsars
We have conducted a search for giant pulses from four millisecondpulsars using the 100 m Green Bank Telescope. Coherently dedispersedtime series from PSR J0218+4232 were found to contain giant pulses ofvery short intrinsic duration whose energies follow power-lawstatistics. The giant pulses are in phase with the two minima of theradio integrated pulse profile but are phase-aligned with the peaks ofthe X-ray profile. Historically, individual pulses more than 10-20 timesthe mean pulse energy have been deemed to be ``giant pulses.'' As only 4of the 155 pulses had energies greater than 10 times the mean pulseenergy, we argue the emission mechanism responsible for giant pulsesshould instead be defined through: (1) intrinsic timescales ofmicrosecond or nanosecond duration; (2) power-law energy statistics; and(3) emission occurring in narrow phase windows coincident with the phasewindows of nonthermal X-ray emission. Four short-duration pulses withgiant-pulse characteristics were also observed from PSR B1957+20. As theinferred magnetic fields at the light cylinders of the millisecondpulsars that emit giant pulses are all very high, this parameter haspreviously been considered to be an indicator of giant-pulse emissivity.However, the frequency of giant-pulse emission from PSR B1957+20 issignificantly lower than for other millisecond pulsars that have similarmagnetic fields at their light cylinders. This suggests that theinferred magnetic field at the light cylinder is a poor indicator of therate of emission of giant pulses.
| Globular cluster system and Milky Way properties revisited
Aims.Updated data of the 153 Galactic globular clusters are used toreaddress fundamental parameters of the Milky Way, such as the distanceof the Sun to the Galactic centre, the bulge and halo structuralparameters, and cluster destruction rates. Methods: .We build areduced sample that has been decontaminated of all the clusters youngerthan 10 Gyr and of those with retrograde orbits and/or evidence ofrelation to dwarf galaxies. The reduced sample contains 116 globularclusters that are tested for whether they were formed in the primordialcollapse. Results: .The 33 metal-rich globular clusters([Fe/H]≥-0.75) of the reduced sample basically extend to the Solarcircle and are distributed over a region with the projected axial-ratiostypical of an oblate spheroidal, Δ x:Δ y:Δz≈1.0:0.9:0.4. Those outside this region appear to be related toaccretion. The 81 metal-poor globular clusters span a nearly sphericalregion of axial-ratios ≈1.0:1.0:0.8 extending from the central partsto the outer halo, although several clusters in the external regionstill require detailed studies to unravel their origin as accretion orcollapse. A new estimate of the Sun's distance to the Galactic centre,based on the symmetries of the spatial distribution of 116 globularclusters, is provided with a considerably smaller uncertainty than inprevious determinations using globular clusters, R_O=7.2±0.3 kpc.The metal-rich and metal-poor radial-density distributions flatten forR_GC≤2 kpc and are represented well over the full Galactocentricdistance range both by a power-law with a core-like term andSérsic's law; at large distances they fall off as R-3.9. Conclusions: .Both metallicity components appearto have a common origin that is different from that of the dark matterhalo. Structural similarities between the metal-rich and metal-poorradial distributions and the stellar halo are consistent with a scenariowhere part of the reduced sample was formed in the primordial collapseand part was accreted in an early period of merging. This applies to thebulge as well, suggesting an early merger affecting the central parts ofthe Galaxy. The present decontamination procedure is not sensitive toall accretions (especially prograde) during the first Gyr, since theobserved radial density profiles still preserve traces of the earliestmerger(s). We estimate that the present globular cluster populationcorresponds to ≤23±6% of the original one. The fact that thevolume-density radial distributions of the metal-rich and metal-poorglobular clusters of the reduced sample follow both a core-likepower-law, and Sérsic's law indicates that we are dealing withspheroidal subsystems at all scales.
| Galactic Globular Clusters with Luminous X-Ray Binaries
Luminous X-ray binaries (LMXBs; >1034 ergs s-1)have a neutron star or black hole primary, and in globular clusters,most of these close binaries are expected to have evolved from widerbinaries through dynamical interactions with other stars. We attempt tofind a predictor of this formation rate that is representative of theinitial properties of globular clusters rather than of the highlyevolved core quantities. Models indicate the half-light quantities bestreflect the initial conditions, so we examine whether the associateddynamical interaction rate, proportional toL1.5/r2.5h, is useful for understandingthe presence of luminous LMXBs in the Galactic globular cluster system.We find that while LMXB clusters with large values ofL1.5/r2.5h preferentially host LMXBs,the systems must also have half-mass relaxation times belowth,relax~109 yr. This relaxation time effectprobably occurs because several relaxation times are required to modifybinary separations, a timescale that must be shorter than cluster ages.The likelihood of finding an LMXB cluster is enhanced if the cluster ismetal-rich and if it is close to the bulge region. The dependence onmetallicity is most likely either due to differing initial massfunctions at the high-mass end or because bulge systems evolve morerapidly from tidal interactions with the bulge. This approach can beused to investigate globular cluster systems in external galaxies, wherecore properties are unresolved.
| UBVI CCD Photometry of the Old Open Cluster Berkeley 17
Photometric UBVI CCD photometry is presented for NGC 188 and Berkeley17. Color-magnitude diagrams (CMDs) are constructed and reach well pastthe main-sequence turnoff for both clusters. Cluster ages are determinedby means of isochrone fitting to the cluster CMDs. These fits areconstrained to agree with spectroscopic metallicity and reddeningestimates. Cluster ages are determined to be 7.0+/-0.5 Gyr for NGC 188and 10.0+/-1.0 Gyr for Berkeley 17, where the errors refer touncertainties in the relative age determinations. These ages arecompared to the ages of relatively metal-rich inner halo/thick-diskglobular clusters and other old open clusters. Berkeley 17 and NGC 6791are the oldest open clusters, with ages of 10 Gyr. They are 2 Gyryounger than the thick-disk globular clusters. These results confirm thestatus of Berkeley 17 as one of the oldest known open clusters in theMilky Way, and its age provides a lower limit to the age of the Galacticdisk.
| Nearby Spiral Globular Cluster Systems. I. Luminosity Functions
We compare the near-infrared (JHK) globular cluster luminosity functions(GCLFs) of the Milky Way, M31, and the Sculptor Group spiral galaxies.We obtained near-infrared photometry with the Persson's AuxiliaryNasmyth Infrared Camera on the Baade Telescope for 38 objects (mostlyglobular cluster candidates) in the Sculptor Group. We also havenear-infrared photometry from the Two Micron All Sky Survey (2MASS)-6Xdatabase for 360 M31 globular cluster candidates and aperture photometryfor 96 Milky Way globular cluster candidates from the 2MASS All-Sky andSecond Incremental Release databases. The M31 6X GCLFs peak at absolutereddening-corrected magnitudes of MJ0=-9.18,MH0=-9.73, and MK0=-9.98.The mean brightness of the Milky Way objects is consistent with that ofM31 after accounting for incompleteness. The average Sculptor absolutemagnitudes (correcting for relative distance from the literature andforeground reddening) are MJ0=-9.18,MH0=-9.70, and MK0=-9.80.NGC 300 alone has absolute foreground-dereddened magnitudesMJ0=-8.87, MH0=-9.39, andMK0=-9.46 using the newest Gieren et al. distance.This implies either that the NGC 300 GCLF may be intrinsically fainterthan that of the larger galaxy M31 or that NGC 300 may be slightlyfarther away than previously thought. Straightforward application of ourM31 GCLF results as a calibrator gives NGC 300 distance moduli of26.68+/-0.14 using J, 26.71+/-0.14 using H, and 26.89+/-0.14 using K.Data for this project were obtained at the Baade 6.5 m telescope, LasCampanas Observatory, Chile.
| The bimodal metallicity distribution function of the globular clusters in the Galaxy: halo disc complementarity
Aims.Our aim in this paper is to present an explanatory scenario for theformation of the observed relatively metal rich globular clustersassociated with the thick disc of the Galaxy, distinct from the mode offormation of the lower metallicity halo clusters.Methods.Theobservations to be accounted for here are the two peaks in themetallicity distribution of the thick disc globular clusters, at [ Fe/H] -0.7 and at [ Fe/H] -0.4. The first step is to verify thestatistical significance of these peaks, and the insignificance of amuch smaller peak at [ Fe/H] -0.2. The basic model assumption isthat these globular clusters were formed as the most massive long termsurvivors of a much larger set of open clusters whose epochs offormation coincided with the main epochs of star formation in the thindisc. These latter are identified using established data sets giving thelocal stellar frequency distribution in time based on stellar activityindices.Results.Our simple stellar accretion model accounts reasonablyfor the presence of the observed peaks in the cluster metallicitydistribution, and the long time constant for the accretion as a massivecluster moves through the stellar environment explains qualitatively whythe most recent peak in the local star formation rate has not yet givenrise to a corresponding peak in the globular cluster distribution. Italso explains in broad terms how a uniform process of cluster formationoriginating both open clusters and disc globular clusters can yield theobserved high numbers of open clusters and the few surviving globulars.
| Explaining the Color Distributions of Globular Cluster Systems in Elliptical Galaxies
The colors of globular clusters in most large elliptical galaxies arebimodal. This is generally taken as evidence for the presence of twocluster subpopulations that have different geneses. However, here wefind that, because of the nonlinear nature of the metallicity-to-colortransformation, a coeval group of old clusters with a unimodalmetallicity spread can exhibit color bimodality. The models of clustercolors indicate that horizontal-branch stars are the main drivers behindthe empirical nonlinearity. We show that the scenario gives simple andcohesive explanations for all the key observations and could simplifytheories of elliptical galaxy formation.
| RR Lyrae-based calibration of the Globular Cluster Luminosity Function
We test whether the peak absolute magnitude MV(TO) of theGlobular Cluster Luminosity Function (GCLF) can be used for reliableextragalactic distance determination. Starting with the luminosityfunction of the Galactic Globular Clusters listed in Harris catalogue,we determine MV(TO) either using current calibrations of theabsolute magnitude MV(RR) of RR Lyrae stars as a function ofthe cluster metal content [Fe/H] and adopting selected cluster samples.We show that the peak magnitude is slightly affected by the adoptedMV(RR)-[Fe/H] relation, with the exception of that based onthe revised Baade-Wesselink method, while it depends on the criteria toselect the cluster sample. Moreover, grouping the Galactic GlobularClusters by metallicity, we find that the metal-poor (MP) ([Fe/H]<-1.0, <[Fe/H]>~-1.6) sample shows peak magnitudes systematicallybrighter by about 0.36mag than those of the metal-rich (MR) ([Fe/H]>-1.0, (<[Fe/H]>~-0.6) one, in substantial agreement with thetheoretical metallicity effect suggested by synthetic Globular Clusterpopulations with constant age and mass function. Moving outside theMilky Way, we show that the peak magnitude of the MP clusters in M31appears to be consistent with that of Galactic clusters with similarmetallicity, once the same MV(RR)-[Fe/H] relation is used fordistance determination. As for the GCLFs in other external galaxies,using Surface Brightness Fluctuations (SBF) measurements we giveevidence that the luminosity functions of the blue (MP) GlobularClusters peak at the same luminosity within ~0.2mag, whereas for the red(MR) samples the agreement is within ~0.5mag even accounting for thetheoretical metallicity correction expected for clusters with similarages and mass distributions. Then, using the SBF absolute magnitudesprovided by a Cepheid distance scale calibrated on a fiducial distanceto Large Magellanic Cloud (LMC), we show that the MV(TO)value of the MP clusters in external galaxies is in excellent agreementwith the value of both Galactic and M31 ones, as inferred by an RR Lyraedistance scale referenced to the same LMC fiducial distance. Eventually,adopting μ0(LMC) = 18.50mag, we derive that the luminosityfunction of MP clusters in the Milky Way, M31, and external galaxiespeak at MV(TO) =-7.66 +/- 0.11, - 7.65 +/- 0.19 and -7.67 +/-0.23mag, respectively. This would suggest a value of -7.66 +/- 0.09mag(weighted mean), with any modification of the LMC distance modulusproducing a similar variation of the GCLF peak luminosity.
| Hot Populations in M87 Globular Clusters
To explore the production of UV-bright stars in old, metal-richpopulations like those in elliptical galaxies, we have obtained HubbleSpace Telescope (HST) Space Telescope Imaging Spectrograph far- andnear-UV photometry of globular clusters (GCs) in four fields in thegiant elliptical (gE) galaxy M87. To a limit of mFUV~25 wedetect a total of 66 GCs in common with the deep HST optical-band studyof Kundu et al. Despite strong overlap in V- and I-band properties, theM87 GCs have UV-optical properties that are distinct from clusters inthe Milky Way and in M31. M87 clusters, especially metal-poor ones,produce larger hot horizontal-branch populations than do Milky Wayanalogs. In color plots including the near-UV band, the M87 clustersappear to represent an extension of the Milky Way sequence. Cluster massis probably not a factor in these distinctions. The most metal-rich M87GCs in our sample are near solar metallicity and overlap the local Egalaxy sample in estimated Mg2 line indices. Nonetheless, theclusters produce much more UV light at a given Mg2, being upto 1 mag bluer than any gE galaxy in (FUV-V) color. The M87 GCs do notappear to represent a transition between Milky Way-type clusters and Egalaxies. The differences are in the correct sense if the clusters aresignificantly older than the E galaxies.Comparisons with Galactic open clusters indicate that the hot stars lieon the extreme horizontal branch, rather than being blue stragglers, andthat the extreme horizontal branch becomes well populated for ages>~5 Gyr. Existing model grids for clusters do not match theobservations well, due to poorly understood giant branch mass loss orperhaps high helium abundances. We find that 41 of our UV detectionshave no optical-band counterparts. Most appear to be UV-brightbackground galaxies seen through M87. Eleven near-UV variable sourcesdetected at only one epoch in the central field are probably classicalnovae. Two recurrent variable sources have no obvious explanation butcould be related to activity in the relativistic jet.
| Resolved Massive Star Clusters in the Milky Way and Its Satellites: Brightness Profiles and a Catalog of Fundamental Parameters
We present a database of structural and dynamical properties for 153spatially resolved star clusters in the Milky Way, the Large and SmallMagellanic Clouds, and the Fornax dwarf spheroidal. This databasecomplements and extends others in the literature, such as those ofHarris and Mackey & Gilmore. Our cluster sample comprises 50 ``youngmassive clusters'' in the LMC and SMC, and 103 old globular clustersbetween the four galaxies. The parameters we list include central andhalf-light-averaged surface brightnesses and mass densities; core andeffective radii; central potentials, concentration parameters, and tidalradii; predicted central velocity dispersions and escape velocities;total luminosities, masses, and binding energies; central phase-spacedensities; half-mass relaxation times; and ``κ-space'' parameters.We use publicly available population-synthesis models to computestellar-population properties (intrinsic B-V colors, reddenings, andV-band mass-to-light ratios) for the same 153 clusters plus another 63globulars in the Milky Way. We also take velocity-dispersionmeasurements from the literature for a subset of 57 (mostly old)clusters to derive dynamical mass-to-light ratios for them, showing thatthese compare very well to the population-synthesis predictions. Thecombined data set is intended to serve as the basis for futureinvestigations of structural correlations and the fundamental plane ofmassive star clusters, including especially comparisons between thesystemic properties of young and old clusters.The structural and dynamical parameters are derived from fitting threedifferent models-the modified isothermal sphere of King; an alternatemodified isothermal sphere based on the ad hoc stellar distributionfunction of Wilson; and asymptotic power-law models withconstant-density cores-to the surface-brightness profile of eachcluster. Surface-brightness data for the LMC, SMC, and Fornax clustersare based in large part on the work of Mackey & Gilmore, but includesignificant supplementary data culled from the literature and importantcorrections to Mackey & Gilmore's V-band magnitude scale. Theprofiles of Galactic globular clusters are taken from Trager et al. Weaddress the question of which model fits each cluster best, finding inthe majority of cases that the Wilson models-which are spatially moreextended than King models but still include a finite, ``tidal'' cutoffin density-fit clusters of any age, in any galaxy, as well as or betterthan King models. Untruncated, asymptotic power laws often fit about aswell as Wilson models but can be significantly worse. We argue that theextended halos known to characterize many Magellanic Cloud clusters maybe examples of the generic envelope structure of self-gravitating starclusters, not just transient features associated strictly with youngage.
| An Ultracompact X-Ray Binary in the Globular Cluster M15 (NGC 7078)
We have used the Advanced Camera for Surveys on board the Hubble SpaceTelescope to image the core of the globular cluster M15 in thefar-ultraviolet (FUV) wave band. Based on these observations, weidentify the FUV counterpart of the recently discovered low-mass X-raybinary M15 X-2. Our time-resolved FUV photometry shows a modulation with0.062+/-0.004 mag semiamplitude, and we clearly detect a period of22.5806+/-0.0002 minutes. We have carried out extensive Monte Carlosimulations, which show that the signal is consistent with beingcoherent over the entire observational time range of more than 3000cycles. This strongly suggests that it represents the orbital period ofthe binary system. M15 X-2 is FUV-bright (FUV~=17 mag) and ischaracterized by an extremely blue spectral energy distribution(Fλ~λ-2.0). We also find evidencefor an excess of flux between 1500 and 1600 Å and probably between1600 and 2000 Å that might be due to C IV 1550 Å and He II1640 Å emission lines. We also show that M15 X-2's X-rayluminosity can be powered by accretion at the rate expected for masstransfer driven by gravitational waves at this binary period. Theobserved FUV emission appears to be dominated by an irradiated accretiondisk around the neutron star primary, and the variability can beexplained by irradiation of the low-mass white dwarf donor if theinclination of the system is ~34°. We conclude that allobservational characteristics of M15 X-2 are consistent with its beingan ultracompact X-ray binary, only the third confirmed such object in aglobular cluster.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS 5-26555.
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