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|Dissecting the bursts of doom.|
|Les chocs des premieres secondes.|
|Bias-free Measurement of Giant Molecular Cloud Properties|
We review methods for measuring the sizes, line widths, and luminositiesof giant molecular clouds (GMCs) in molecular-line data cubes with lowresolution and sensitivity. We find that moment methods are robust andsensitive, making full use of both position and intensity information,and we recommend a standard method to measure the position angle, majorand minor axis sizes, line width, and luminosity using moment methods.Without corrections for the effects of beam convolution and sensitivityto GMC properties, the resulting properties may be severely biased. Thisis particularly true for extragalactic observations, where resolutionand sensitivity effects often bias measured values by 40% or more. Wecorrect for finite spatial and spectral resolutions with a simpledeconvolution, and we correct for sensitivity biases by extrapolatingproperties of a GMC to those we would expect to measure with perfectsensitivity (i.e., the 0 K isosurface). The resulting method recoversthe properties of a GMC to within 10% over a large range of resolutionsand sensitivities, provided the clouds are marginally resolved with apeak signal-to-noise ratio greater than 10. We note that interferometerssystematically underestimate cloud properties, particularly the fluxfrom a cloud. The degree of bias depends on the sensitivity of theobservations and the (u,v) coverage of the observations. In an Appendixto the paper we present a conservative, new decomposition algorithm foridentifying GMCs in molecular-line observations. This algorithm treatsthe data in physical rather than observational units (i.e., parsecsrather than beams or arcseconds), does not produce spurious clouds inthe presence of noise, and is sensitive to a range of morphologies. As aresult, the output of this decomposition should be directly comparableamong disparate data sets.
|A Review of Optical Sky Brightness and Extinction at Dome C, Antarctica|
The recent discovery of exceptional seeing conditions at Dome C,Antarctica, raises the possibility of constructing an opticalobservatory there with unique capabilities. However, little is knownfrom an astronomer's perspective about the optical sky brightness andextinction at Antarctic sites. We review the contributions to skybrightness at high-latitude sites and calculate the amount of usabledark time at Dome C. We also explore the implications of the limited skycoverage of high-latitude sites and review optical extinction data fromthe South Pole. Finally, we examine the proposal of Baldry &Bland-Hawthorn to extend the amount of usable dark time through the useof polarizing filters.
|The WFPC2 Archival Pure Parallels Project|
We introduce the WFPC2 Archival Pure Parallels Project, in which themethods and procedures described here are used to obtain anear-automatic combination of WFPC2 images obtained as part of the WFPC2Archival Parallels Program. Several techniques have been developed orrefined to ensure proper alignment, registration, and combination ofoverlapping images that can be obtained at different times and withdifferent orientations. We quantify the success rate and the accuracy ofthe registration of different types of images, and we develop techniquesthat are suitable to equalize the sky background without undulyaffecting extended emission. About 600 combined images of the 1500eventually planned have already been publicly released through the STScIArchive. The images released to date are especially suited to the studyof star formation in the Magellanic Clouds, the stellar population inthe halo of nearby galaxies, and the properties of star-forming galaxiesat z~3.
|Evidence for the strong effect of gas removal on the internal dynamics of young stellar clusters|
We present detailed luminosity profiles of the young massive clustersM82-F, NGC 1569-A and NGC 1705-1 which show significant departures fromequilibrium (King and Elson, Fall & Freeman) profiles. We comparethese profiles with those from N-body simulations of clusters that haveundergone the rapid removal of a significant fraction of their mass as aresult of gas expulsion. We show that the observations and simulationsagree very well with each other, suggesting that these young clustersare undergoing violent relaxation and are also losing a significantfraction of their stellar mass.That these clusters are not in equilibrium can explain the discrepantmass-to-light ratios observed in many young clusters with respect tosimple stellar population models without resorting to non-standardinitial stellar mass functions as claimed for M82-F and NGC 1705-1. Wealso discuss the effect of rapid gas removal on the complete disruptionof a large fraction of young massive clusters (`infant mortality').Finally, we note that even bound clusters may lose >50 per cent oftheir initial stellar mass as a result of rapid gas loss (`infantweight-loss').
|Finite source effect on the polarization degree induced by a single microlens|
We investigate the effect of a single microlens on Stokes parameters.Semi-analytical formulae of the microlensed Stokes parameters arederived. The formulae not only reduce the double integrals in theestimations of those quantities but can also be approximated to a usefulform in the bypass case. By using our formulation, we show that acombination of polarimetric data with photometric data enables us toestimate not only the finite source effect but also the direction of themicrolens motion.
|Photometry of Magellanic Cloud clusters with the Advanced Camera for Surveys - II. The unique LMC cluster ESO 121-SC03|
We present the results of photometric measurements from images of theLarge Magellanic Cloud (LMC) cluster ESO 121-SC03 taken with theAdvanced Camera for Surveys (ACS) on the Hubble Space Telescope. Ourresulting colour-magnitude diagram (CMD) reaches 3 mag below themain-sequence turn-off, and represents by far the deepest observation ofthis cluster to date. We also present similar photometry from ACSimaging of the accreted Sagittarius dSph cluster Palomar 12, used inthis work as a comparison cluster. From analysis of its CMD, we obtainestimates for the metallicity and reddening of ESO 121-SC03: [Fe/H] = -0.97 +/- 0.10 and E(V - I) = 0.04 +/- 0.02, in excellent agreement withprevious studies. The observed horizontal branch (HB) level in ESO121-SC03 suggests this cluster may lie 20 per cent closer to us thandoes the centre of the LMC. ESO 121-SC03 also possesses a significantpopulation of blue stragglers, which we briefly discuss. Our newphotometry allows us to undertake a detailed study of the age of ESO121-SC03 relative to Palomar 12 and the Galactic globular cluster 47Tuc. We employ both vertical and horizontal differential indicators onthe CMD, calibrated against isochrones from the Victoria-Regina stellarmodels. These models allow us to account for the differentα-element abundances in Palomar 12 and 47 Tuc, as well as theunknown run of α-elements in ESO 121-SC03. Taking a straighterror-weighted mean of our set of age measurements yields ESO 121-SC03to be 73 +/- 4 per cent the age of 47 Tuc, and 91 +/- 5 per cent the ageof Palomar 12. Palomar 12 is 79 +/- 6 per cent as old as 47 Tuc,consistent with previous work. Our result corresponds to an absolute agefor ESO 121-SC03 in the range 8.3-9.8 Gyr, depending on the age assumedfor 47 Tuc, therefore confirming ESO 121-SC03 as the only known clusterto lie squarely within the LMC age gap. We briefly discuss a suggestionfrom earlier work that ESO 121-SC03 may have been accreted into the LMCsystem.
|The planetary nebula population of the Sagittarius dwarf spheroidal galaxy|
The identification of two new planetary nebulae (PNe) in the Sagittariusdwarf spheroidal galaxy (Sgr) is presented. This brings the total numberto four. Both new PNe were previously classified as Galactic objects.The first, StWr 2-21, belongs to the main body of Sgr, from its velocityand location. The second, the halo PN BoBn1, has a location, distanceand velocity in agreement with the leading tidal tail of Sgr. Weestimate that 10 per cent of the Galactic halo consists of Sgr debris.The specific frequency of PNe indicates a total luminosity of Sgr,including its tidal tails, of MV = -14.1. StWr 2-21 shows ahigh abundance of [O/H]= -0.23, which confirms the high-metallicitypopulation in Sgr uncovered by Bonifacio et al. The steepmetallicity-age gradient in Sgr is due to interstellar medium (ISM)removal during the Galactic plane passages, ISM reformation due tostellar mass-loss, and possibly accretion of metal-enriched gas from ourGalaxy. The ISM re-formation rate of Sgr, from stellar mass-loss, is 5× 10-4Msolaryr-1, amounting to~106Msolar per orbital period.Hubble Space Telescope images of three of the PNe reveal well-developedbipolar morphologies, and provide clear detections of the central stars.All three stars with deep spectra show WR lines, suggesting that theprogenitor mass and metallicity determines whether a PN central stardevelops a WR spectrum. One Sgr PN belongs to the class of IR-[WC]stars. Expansion velocities are determined for three nebulae. Comparisonwith hydrodynamical models indicates an initial density profile of ρ~ r-3. This is evidence for increasing mass-loss rates on theasymptotic giant branch. Peak mass-loss rates are indicated of~10-4Msolaryr-1.The IR-[WC] PN, He2-436, provides the sole direct detection of dust in adwarf spheroidal galaxy, to date.
|Carbon-rich Mira variables: kinematics and absolute magnitudes|
The kinematics of Galactic C-Miras are discussed on the basis of thebolometric magnitudes and radial velocities of Papers I and II of thisseries. Differential Galactic rotation is used to derive a zero-pointfor the bolometric period-luminosity relation which is in satisfactoryagreement with that inferred from the Large Magellanic Cloud (LMC)C-Miras. We find for the Galactic Miras, Mbol = -2.54 logP +2.06(+/-0.24), where the slope is taken from the LMC. The mean velocitydispersion, together with the data of Nordström et al. and thePadova models, leads to a mean age for our sample of C-Miras of 1.8 +/-0.4Gyr and a mean initial mass of 1.8 +/- 0.2 Msolar.Evidence for a variation of velocity dispersion with period is found,indicating a dependence of period on age and initial mass, the longerperiod stars being younger. We discuss the relation between the O- andC-Miras and also their relative numbers in different systems.
|Period-colour and amplitude-colour relations in classical Cepheid variables - IV. The multiphase relations|
The superb phase resolution and quality of the Optical GravitationalLensing Experiment (OGLE) data on the Large Magellanic Cloud (LMC) andSmall Magellanic Cloud (SMC) Cepheids, together with existing data onGalactic Cepheids, are combined to study the period-colour (PC) andamplitude-colour (AC) relations as a function of pulsation phase. Ourresults confirm earlier work that the LMC PC relation (at mean light) ismore consistent with two lines of differing slopes, separated at aperiod of 10 d. However, our multiphase PC relations reveal much newstructure which can potentially increase our understanding of Cepheidvariables. These multiphase PC relations provide insight into why theGalactic PC relation is linear but the LMC PC relation is non-linear.This is because the LMC PC relation is shallower for short (logP < 1)and steeper for long (logP > 1) period Cepheids than thecorresponding Galactic PC relation. Both of the short- and long-periodCepheids in all three galaxies exhibit the steepest and shallowestslopes at phases around 0.75-0.85, respectively. A consequence is thatthe PC relation at phase ~ 0.8 is highly non-linear. Further, theGalactic and LMC Cepheids with logP > 1 display a flat slope in thePC plane at phases close to the maximum light. When the LMCperiod-luminosity (PL) relation is studied as a function of phase, weconfirm that it changes with the PC relation. The LMC PL relation in Vand I band near the phase of 0.8 provides compelling evidence that thisrelation is also consistent with two lines of differing slopes joined ata period close to 10 d.
|Period-colour and amplitude-colour relations in classical Cepheid variables - III. The Large Magellanic Cloud Cepheid models|
Period-colour (PC) and amplitude-colour (AC) relations are studied forthe Large Magellanic Cloud (LMC) Cepheids under the theoreticalframework of the hydrogen ionization front (HIF)-photosphereinteraction. LMC models are constructed with pulsation codes thatinclude turbulent convection, and the properties of these models arestudied at maximum, mean and minimum light. As with Galactic models, atmaximum light the photosphere is located next to the HIF for the LMCmodels. However, very different behaviour is found at minimum light. Thelong-period (P > 10 d) LMC models imply that the photosphere isdisengaged from the HIF at minimum light, similar to the Galacticmodels, but there are some indications that the photosphere is locatednear the HIF for the short-period (P < 10 d) LMC models. We also usethe updated LMC data to derive empirical PC and AC relations at thesephases. Our numerical models are broadly consistent with our theory andthe observed data, though we discuss some caveats in the paper. We applythe idea of the HIF-photosphere interaction to explain recentsuggestions that the LMC period-luminosity (PL) and PC relations arenon-linear with a break at a period close to 10 d. Our empirical LMC PCand PL relations are also found to be non-linear with the F-test. Ourexplanation relies on the properties of the Saha ionization equation,the HIF-photosphere interaction and the way this interaction changeswith the phase of pulsation and metallicity to produce the observedchanges in the LMC PC and PL relations.
|Integrated-light VRI imaging photometry of globular clusters in the Magellanic Clouds|
We present accurate integrated-light photometry in Johnson/Cousins V, Rand I for a sample of 28 globular clusters in the Magellanic Clouds. Themajority of the clusters in our sample have reliable age and metallicityestimates available in the literature. The sample encompasses agesbetween 50 Myr and 7 Gyr, and metallicities ([Fe/H]) between -1.5 and0.0 dex. The sample is dominated by clusters of ages between roughly 0.5and 2 Gyr, an age range during which the bolometric luminosity of simplestellar populations is dominated by evolved red giant branch stars andthermally pulsing asymptotic giant branch (TP-AGB) stars whosetheoretical colours are rather uncertain. The VRI colours presented inthis paper have been used to calibrate stellar population synthesismodel predictions.
|Finding outlier light curves in catalogues of periodic variable stars|
We present a methodology to discover outliers in catalogues of periodiclight curves. We use a cross-correlation as the measure of `similarity'between two individual light curves, and then classify light curves withlowest average `similarity' as outliers. We performed the analysis oncatalogues of periodic variable stars of known type from the MACHO andOGLE projects. This analysis was carried out in Fourier space and weestablished that our method correctly identifies light curves that donot belong to those catalogues as outliers. We show how an approximationto this method, carried out in real space, can scale to large data setsthat will be available in the near future such as those anticipated fromthe Panoramic Survey Telescope & Rapid Response System (Pan-STARRS)and Large Synoptic Survey Telescope (LSST).
|A deep kinematic survey of planetary nebulae in the Andromeda galaxy using the Planetary Nebula Spectrograph|
We present a catalogue of positions, magnitudes and velocities for 3300emission-line objects found by the Planetary Nebula Spectrograph in asurvey of the Andromeda galaxy, M31. Of these objects, 2615 are foundlikely to be planetary nebulae (PNe) associated with M31. The surveyarea covers the whole of M31's disc out to a radius of . Beyond thisradius, observations have been made along the major and minor axes, andthe Northern Spur and Southern Stream regions. The calibrated data havebeen checked for internal consistency and compared with othercatalogues. With the exception of the very central, high surfacebrightness region of M31, this survey is complete to a magnitude limitof m5007 ~ 23.75, 3.5 mag into the PN luminosity function.We have identified emission-line objects associated with M31'ssatellites and other background galaxies. We have examined the data fromthe region tentatively identified as a new satellite galaxy, AndromedaVIII, comparing it to data in the other quadrants of the galaxy. We findthat the PNe in this region have velocities that appear to be consistentwith membership of M31 itself.The luminosity function of the surveyed PNe is well matched to the usualsmooth monotonic function. The only significant spatial variation in theluminosity function occurs in the vicinity of M31's molecular ring,where the luminosities of PNe on the near side of the galaxy aresystematically ~0.2 mag fainter than those on the far side. Thisdifference can be explained naturally by a modest amount of obscurationby the ring. The absence of any difference in luminosity functionbetween bulge and disc suggests that the sample of PNe is not stronglypopulated by objects whose progenitors are more massive stars. Thisconclusion is reinforced by the excellent agreement between the numbercounts of PNe and the R-band light.The number counts of kinematically selected PNe also allow us to probethe stellar distribution in M31 down to very faint limits. There is noindication of a cut-off in M31's disc out to beyond four scalelengths,and no signs of a spheroidal halo population in excess of the bulge outto 10 effective bulge radii.We have also carried out a preliminary analysis of the kinematics of thesurveyed PNe. The mean streaming velocity of the M31 disc PNe is foundto show a significant asymmetric drift out to large radii. Theirvelocity dispersion, although initially declining with radius, flattensout to a constant value in the outer parts of the galaxy. There are noindications that the disc velocity dispersion varies with PN luminosity,once again implying that the progenitors of PNe of all magnitudes form arelatively homogeneous old population. The dispersion profile andasymmetric drift results are shown to be mutually consistent, butrequire that the disc flares with radius if the shape of its velocityellipsoid remains invariant.
|On the correlation of short gamma-ray bursts and clusters of galaxies|
We cross-correlate gamma-ray bursts (GRBs) and X-ray selected clustersof galaxies at z<= 0.45. We find a positive 2σ signal for theangular cross-correlation function wbc(θ) on scalesθ<= 3° between short GRBs and clusters. Conversely, nocorrelation is found between clusters and the population of long GRBs.The comparison with the cluster autocorrelation function shows thatshort GRBs do not trace the cluster distribution, as not all short GRBsare found in clusters. A higher signal in wbc(θ) isfound if we only consider the cluster population up to z= 0.1. Bycomparing the short-burst autocorrelation function with modelpredictions, we then constrain short bursts to mostly originate within~270 Mpc (i.e. z<= 0.06). Our analysis also reveals that short GRBsare better correlated with `normal' galaxies. The double compact-objectmerger model for short GRBs would associate them preferentially withearly-type galaxies, but the present statistics do not allow us toexclude that at least a fraction of these events might also take placein late-type galaxies, in agreement with recent evidence.
|Near-infrared spectroscopy of a young super-star cluster in NGC 6946: chemical abundances and abundance patterns*|
Using the NIRSPEC spectrograph at Keck II, we have obtained H- andK-band echelle spectra for a young (~10-15 Myr), luminous(MV~-13.2) super-star cluster in the nearby spiral galaxy NGC6946. From spectral synthesis and equivalent width measurements weobtain for the first time accurate abundances and abundance patterns inan extragalactic super-star cluster. We find [Fe/H]=-0.45 +/- 0.08 dex,an average α-enhancement of ~+0.22 +/- 0.1 dex, and a relativelylow 12 C/13 C ~ 8 +/- 2 isotopic ratio. We alsomeasure a velocity dispersion of ~9.1 km s-1, in agreementwith previous estimates. We conclude that integrated high-dispersionspectroscopy of massive star clusters is a promising alternative toother methods for abundance analysis in extragalactic young stellarpopulations.
|GRB 050505: a high-redshift burst discovered by Swift|
We report the discovery and subsequent multiwavelength afterglowbehaviour of the high-redshift (z= 4.27) Gamma Ray Burst (GRB) 050505.This burst is the third most-distant burst, measured by spectroscopicredshift, discovered after GRB 000131 (z= 4.50) and GRB 050904 (z=6.29). GRB 050505 is a long GRB with a multipeaked γ-ray lightcurve, with a duration of T90= 63 +/- 2s and an inferredisotropic release in γ-rays of ~ 4.44 × 1053ergin the 1-104 keV rest-frame energy range. The Swift X-RayTelescope followed the afterglow for 14 d, detecting two breaks in thelight curve at 7.4 +1.5-1.5 and 58.0+9.9-15.4 ks after the burst trigger. Thepower-law decay slopes before, between and after these breaks were0.25+0.16-0.17,1.17+0.08-0.09 and1.97+0.27-0.28, respectively. The light curve canalso be fitted with a `smoothly broken' power-law model with a breakobserved at ~T+ 18.5 ks, with decay slopes of ~0.4 and ~1.8, before andafter the break, respectively. The X-ray afterglow shows no spectralvariation over the course of the Swift observations, being well fittedwith a single power law of photon index ~1.90. This behaviour isexpected for the cessation of the continued energization of theinterstellar medium shock, followed by a break caused by a jet, eitheruniform or structured. Neither break is consistent with a cooling break.The spectral energy distribution, indeed, shows the cooling frequency tobe below the X-ray but above the optical frequencies. The optical-X-rayspectrum also shows that there is significant X-ray absorption in excessof that due to our Galaxy but very little optical-ultravioletextinction, with E(B-V) ~ 0.10 for a Small Magellanic Cloud likeextinction curve.
|Planetary nebulae as tracers of galaxy stellar populations|
We address the general problem of the luminosity-specific planetarynebula (PN) number, better known as the `α' ratio, given byα=NPN/Lgal, and its relationship with theage and metallicity of the parent stellar population. Our analysisrelies on population synthesis models that account for simple stellarpopulations (SSPs), and more elaborate galaxy models covering the fullstar formation range of the different Hubble morphological types. Thistheoretical framework is compared with the updated census of the PNpopulation in Local Group (LG) galaxies and external ellipticals in theLeo group, and the Virgo and Fornax clusters.The main conclusions of our study can be summarized as follows. (i)According to the post-asymptotic giant branch (AGB) stellar core mass,PN lifetime in a SSP is constrained by three relevant regimes, driven bythe nuclear (Mcore>~ 0.57Msolar), dynamical(0.57Msolar>~Mcore>~ 0.55Msolar)and transition (0.55Msolar>~Mcore>~0.52Msolar) time-scales. The lower limit for Mcorealso sets the minimum mass for stars to reach the AGB thermal-pulsingphase and experience the PN event. (ii) Mass loss is the crucialmechanism to constrain the value of α, through the definition ofthe initial-to-final mass relation (IFMR). The Reimers mass-lossparametrization, calibrated on Pop II stars of Galactic globularclusters, poorly reproduces the observed value of α in late-typegalaxies, while a better fit is obtained using the empirical IFMRderived from white dwarf observations in the Galaxy open clusters. (iii) The inferred PN lifetime for LG spirals and irregulars exceeds10000yr, which suggests that Mcore<~ 0.65Msolarcores dominate, throughout. (iv) The relative PN deficiency inelliptical galaxies, and the observed trend of α with galaxyoptical colours, support the presence of a prevailing fraction oflow-mass cores (Mcore<~ 0.55Msolar) in the PNdistribution and a reduced visibility time-scale for the nebulae as aconsequence of the increased AGB transition time. The stellar componentwith Mcore<~ 0.52Msolar, which overrides the PNphase, could provide an enhanced contribution to hotter HB and post-HBevolution, as directly observed in M 32 and the bulge of M 31. Thisimplies that the most UV-enhanced ellipticals should also display thelowest values of α, as confirmed by the Virgo cluster early-typegalaxy population. (v) Any blue-straggler population, invoked asprogenitor of the Mcore>~ 0.7Msolar PNe inorder to preserve the constancy of the bright luminosity-functioncut-off magnitude in ellipticals, must be confined to a small fraction(a few per cent at most) of the whole galaxy PN population.
|The importance of interactions for mass loss from satellite galaxies in cold dark matter haloes|
We investigate the importance of interactions between dark mattersubstructures for the mass loss they suffer whilst orbiting within asample of high-resolution galaxy cluster mass cold dark matter (CDM)haloes formed in cosmological N-body simulations. We have defined aquantitative measure that gauges the degree to which interactions areresponsible for mass loss from substructures. This measure indicatesthat interactions are more prominent in younger systems when compared toolder more relaxed systems. We show that this is due to the increasednumber of encounters a satellite experiences and a higher mass fractionin satellites. This is in spite of the uniformity in the distributionsof relative distances and velocities of encounters between substructureswithin the different host systems in our sample.Using a simple model to relate the net force felt by a single satelliteto the mass loss it suffers, we show that interactions with othersatellites account for ~30 per cent of the total mass loss experiencedover its lifetime. The relation between the age of the host and theimportance of interactions increases the scatter about this mean valuefrom ~25 per cent for the oldest to ~45 per cent for the youngest systemwe have studied. We conclude that satellite interactions play a vitalrole in the evolution of substructure in dark matter haloes and that asignificant fraction of the tidally stripped material can be attributedto these interactions.
|Globular clusters, satellite galaxies and stellar haloes from early dark matter peaks|
The Milky Way contains several distinct old stellar components thatprovide a fossil record of its formation. We can understand theirspatial distribution and kinematics in a hierarchical formation scenarioby associating the protogalactic fragments envisaged by Searle &Zinn (1978) with the rare peaks able to cool gas in the cold dark matterdensity field collapsing at redshift z > 10. We use hierarchicalstructure formation simulations to explore the kinematics and spatialdistribution of these early star-forming structures in galaxy haloestoday. Most of the protogalaxies rapidly merge, their stellar contentsand dark matter becoming smoothly distributed and forming the innerGalactic halo. The metal-poor globular clusters and old halo starsbecome tracers of this early evolutionary phase, centrally biased andnaturally reproducing the observed steep fall off with radius. The mostoutlying peaks fall in late and survive to the present day as satellitegalaxies. The observed radial velocity dispersion profile and the localradial velocity anisotropy of Milky Way halo stars are successfullyreproduced in this model. If this epoch of structure formation coincideswith a suppression of further cooling into lower sigma peaks then we canreproduce the rarity, kinematics and spatial distribution of satellitegalaxies as suggested by Bullock, Kravtsov & Weinberg (2000).Reionization at z= 12 +/- 2 provides a natural solution to the missingsatellites problem. Measuring the distribution of globular clusters andhalo light on scales from galaxies to clusters could be used toconstrain global versus local reionization models. If reionizationoccurs contemporary, our model predicts a constant frequency of blueglobulars relative to the host halo mass, except for dwarf galaxieswhere the average relative frequencies become smaller.
|Automated analysis of eclipsing binary light curves - II. Statistical analysis of OGLE LMC eclipsing binaries|
In the first paper of this series, we presented EBAS - Eclipsing BinaryAutomated Solver, a new fully automated algorithm to analyse the lightcurves of eclipsing binaries, based on the EBOP code. Here, we apply thenew algorithm to the whole sample of 2580 binaries found in the OpticalGravitational Lensing Experiment (OGLE) Large Magellanic Cloud (LMC)photometric survey and derive the orbital elements for 1931 systems. Toobtain the statistical properties of the short-period binaries of theLMC, we construct a well-defined subsample of 938 eclipsing binarieswith main-sequence B-type primaries. Correcting for observationalselection effects, we derive the distributions of the fractional radiiof the two components and their sum, the brightness ratios and theperiods of the short-period binaries. Somewhat surprisingly, the resultsare consistent with a flat distribution in log P between 2 and 10 d. Wealso estimate the total number of binaries in the LMC with the samecharacteristics, and not only the eclipsing binaries, to be about 5000.This figure leads us to suggest that (0.7 +/- 0.4) per cent of themain-sequence B-type stars in the LMC are found in binaries with periodsshorter than 10 d. This frequency is substantially smaller than thefraction of binaries found by small Galactic radial-velocity surveys ofB stars. On the other hand, the binary frequency found by Hubble SpaceTelescope (HST) photometric searches within the late main-sequence starsof 47 Tuc is only slightly higher and still consistent with thefrequency we deduced for the B stars in the LMC.
|Automated analysis of eclipsing binary light curves - I. EBAS - a new Eclipsing Binary Automated Solver with EBOP|
We present a new algorithm, Eclipsing Binary Automated Solver (EBAS), toanalyse light curves of eclipsing binaries. The algorithm is designed toanalyse large numbers of light curves, and is therefore based on therelatively fast EBOP code. To facilitate the search for the bestsolution, EBAS uses two parameter transformations. Instead of the radiiof the two stellar components, EBAS uses the sum of radii and theirratio, while the inclination is transformed into the impact parameter.To replace human visual assessment, we introduce a new `alarm'goodness-of-fit statistic that takes into account correlation betweenneighbouring residuals. We perform extensive tests and simulations thatshow that our algorithm converges well, finds a good set of parametersand provides reasonable error estimation.
|Gas infall and stochastic star formation in galaxies in the local universe|
We study the recent star formation histories of local galaxies byanalysing the scatter in their colours and spectral properties. Wepresent evidence that the distribution of star formation historieschanges qualitatively above a characteristic stellar surface massdensity of 3 × 108Msolarkpc-2,corresponding to the transition between disc-dominated (late-type)galaxies and bulge-dominated (early-type) systems. When we average oversubpopulations of galaxies with densities below this value, we find thatsubpopulations of all masses and densities form their stars at the sameaverage rate per unit stellar mass. However, the scatter in galaxycolours, stellar absorption-line indices and emission-line strengths islarger for more compact galaxies of a given mass. This suggests thatstar formation occurs in shorter, higher amplitude events in galaxieswith smaller sizes. Above the characteristic density, galaxy growththrough star formation shuts down and the scatter in galaxy colours andspectral properties decreases. We propose that in low-density galaxies,star formation events are triggered when cold gas is accreted on to agalaxy. We have used a new high-resolution numerical simulation ofstructure formation in a `concordance' Lambda cold dark matter(ΛCDM) universe to quantify the incidence of these accretionevents, and we show that the observational data are well fitted by amodel in which the consumption time of accreted gas decreases with thesurface density of the galaxy astcons~μ-1*. The dark matter haloesthat host massive galaxies with high stellar surface mass densities arealso expected to grow through accretion, but the observations indicatethat in bulge-dominated galaxies, star formation is no longer coupled tothe hierarchical build-up of these systems.
|An ATCA radio-continuum study of the Small Magellanic Cloud - IV. A multifrequency analysis of the N66 region|
Traditional identification of supernova remnants (SNRs) include the useof radio spectral index, optical spectral studies (including strong[SII], [NII], [OI], [OII] and [OIII] lines) and X-rayco-identifications. Each of these can have significant limitationswithin the context of a particular SNR candidate and new identificationmethods are continually sought. In this paper, we explore subtractiontechniques by Ye, Turtle and Kennicutt to remove thermal emissionestimated from Hα flux from radio-continuum images. The remainingnon-thermal emission allows the identification of SNRs embedded withinthese HII regions. Subtraction images of the N66 region in the SmallMagellanic Cloud (SMC) using Hα wide-field optical CCD images fromthe Curtis Schmidt Telescope and the recent Australia Telescope CompactArray (ATCA)/Parkes radio-continuum (1420, 2370, 4800 and 8640MHz) dataare presented as an example. These show three SNRs (B0057 - 724, B0056 -724 and B0056 - 725) separated from their surrounding HII radioemission. 2.3-m dual-beam spectrograph long-slit spectra from selectedregions within N66 suggest the presence of an additional SNR with noradio or X-ray emission. Radio spectral index, [SII]/Hα ratio andarchived Chandra images of N66 combine to give a more coherent pictureof this region, confirming B0057 - 724 as an SNR. The N66 nebula complexis divided into 10 components, composed separately of these SNRs and HIIregions.
|NLTE models of line-driven stellar winds - II. O stars in the Small Magellanic Cloud|
We calculate non-local thermodynamic equilibrium (NLTE) line-driven windmodels of selected O stars in the spectral range of O4 to O9 in theSmall Magellanic Cloud (SMC). We compare predicted basic windproperties, i.e. the terminal velocity and the mass-loss rate withvalues derived from observation. We found relatively good agreementbetween theoretical and observed terminal velocities. On the other hand,predicted mass-loss rates and mass-loss rates derived from observationare in a good agreement only for higher mass-loss rates. Theoreticalmass-loss rates lower than approximately10-7Msolaryr-1 are significantly higherthan those derived from observation. These results confirm thepreviously reported problem of weak winds, since our calculatedmass-loss rates are in fair agreement with predictions of Vink et al. Westudy multicomponent models for these winds. For this purpose we developa more detailed description of wind decoupling. We show that theinstability connected with the decoupling of individual wind elementsmay occur for low-density winds. In the case of winds with very lowobserved mass-loss rates the multicomponent effects are important forthe wind structure, however this is not able to explain consistently thedifference between the predicted mass-loss rate and the mass-loss ratederived from observation for these stars. Similar to previous studies,we found the level of dependence of the wind parameters on themetallicity. We conclude that the wind mass-loss rate significantlyincreases with metallicity as , whereas the terminal velocity of wind onaverage depends on metallicity only slightly, namelyv&infy;~Z0.06 (for studied stars).
|Average extinction curves and relative abundances for quasi-stellar object absorption-line systems at 1 <=zabs < 2|
We have studied a sample of 809 MgII absorption systems with 1.0<=zabs<= 1.86 in the spectra of Sloan Digital SkySurvey quasi-stellar objects (QSOs), with the aim of understanding thenature and abundance of the dust and the chemical abundances in theintervening absorbers. Normalized, composite spectra were derived, forabundance measurements, for the full sample and several subsamples,chosen on the basis of the line strengths and other absorber and QSOproperties. Average extinction curves were obtained for the subsamplesby comparing their geometric mean spectra with those of matching samplesof QSOs without absorbers in their spectra. There is clear evidence forthe presence of dust in the intervening absorbers. The 2175-Åfeature is not present in the extinction curves, for any of thesubsamples. The extinction curves are similar to the Small MagellanicCloud (SMC) extinction curve with a rising ultraviolet (UV) extinctionbelow 2200 Å. The absorber rest-frame colour excess, E(B-V),derived from the extinction curves, depends on the absorber propertiesand ranges from <0.001 to 0.085 for various subsamples. The columndensities of MgII, AlII, SiII, CaII, TiII, CrII, MnII, FeII, CoII, NiIIand ZnII do not show such a correspondingly large variation. The overalldepletions in the high E(B-V) samples are consistent with those foundfor individual damped Lyman α systems, the depletion pattern beingsimilar to halo clouds in the Galaxy. Assuming an SMC gas-to-dust ratio,we find a trend of increasing abundance with decreasing extinction;systems with NHI~ 1020cm-2 show solarabundance of Zn. The large velocity spread of strong MgII systems seemsto be mimicked by weak lines of other elements. The ionization of theabsorbers, in general appears to be low: the ratio of the columndensities of AlIII to AlII is always less than 1/2. QSOs with absorbersare, in general, at least three times as likely to have highly reddenedspectra as compared to QSOs without any absorption systems in theirspectra.
|Selecting damped Lyman α systems through CaII absorption - I. Dust depletions and reddening at z~ 1|
We use the average E(B-V) and ZnII column densities of a sample of z~ 1CaIIλλ 3935, 3970 absorption-line systems selected fromthe fourth data release of the Sloan Digital Sky Survey (SDSS) to showthat on average, with conservative assumptions regarding metallicitiesand dust-to-gas ratios, they contain column densities of neutralhydrogen greater than the damped Lyman α (DLA) limit. We proposethat selection by CaII absorption is an effective way of identifyinghigh column densities of neutral hydrogen, and thus large samples ofDLAs at zabs<~ 1.3 from the SDSS. The number density ofstrong CaII absorbers (with rest-frame equivalent widthWλ3935>= 0.5Å), is ~20-30 per cent that ofDLAs, after correcting for the significant bias against their detectiondue to obscuration of the background quasars by dust. On average theseabsorbers have E(B-V) >~ 0.1mag the dustiest absorbers showdepletions of refractory elements at a level of the largest depletionsseen in DLAs. For the first time we can measure the dust-to-metals ratioin a sample of absorption-selected galaxies, and find values close to,or even larger than, those observed locally. All of these propertiessuggest that a substantial fraction of the CaII absorbers are morechemically evolved than typical DLAs. There is a trend of increasingdust content with Wλ3935 this trend with strong-lineequivalent width is also observed in an equivalent, but much larger,sample of MgII absorbers. Such a trend would result if the dustiersystems are hosted by more massive, or disturbed, galaxies. Follow-upimaging is required to provide conclusive evidence for or against thesescenarios. From consideration of the E(B-V) distribution in our sample,and assuming CaII absorbers represent a subset of DLAs, we calculatethat dust obscuration causes an underestimation in the number density ofDLAs by at least 8-12 per cent at these redshifts. Finally, the removalof broad absorption line (BAL) quasars from the SDSS quasar sampleincreases the sensitivity of the detection of reddening by interveningabsorbers. To this end, we describe a new, automated, principalcomponent analysis (PCA) method for identifying BAL quasars.
|The circumstellar environment of Wolf-Rayet stars and gamma-ray burst afterglows|
We study the evolution of the circumstellar medium of massive stars. Wepay particular attention to Wolf-Rayet stars that are thought to be theprogenitors of some long gamma-ray bursts (GRBs). We detail themass-loss rates we use in our stellar evolution models and how weestimate the stellar wind speeds during different phases. With thesedetails we simulate the interactions between the wind and theinterstellar medium to predict the circumstellar environment around thestars at the time of core-collapse. We then investigate how thestructure of the environment might affect the GRB afterglow. We findthat when the afterglow jet encounters the free-wind/stalled-windinterface, rebrightening occurs and a bump is seen in the afterglowlight curve. However, our predicted positions of this interface are toodistant from the site of the GRB to reach while the afterglow remainsobservable. The values of the final wind density, A*, fromour stellar models are of the same order (<~1) as some of the valuesinferred from observed afterglow light curves. We do not reproduce thelowest A* values below 0.5 inferred from afterglowobservations. For these cases, we suggest that the progenitors couldhave been a WO-type Wolf-Rayet (WR) star or a very low-metallicity star.Finally, we turn our attention to the matter of stellar wind materialproducing absorption lines in the afterglow spectra. We discuss theobservational signatures of two WR stellar types, WC and WO, in theafterglow light curve and spectra. We also indicate how it may bepossible to constrain the initial mass and metallicity of a GRBprogenitor by using the inferred wind density and wind velocity.
|High-metallicity, photoionized gas in intergalactic large-scale filaments|
We present high-resolution ultraviolet spectra of absorption-linesystems towards the low-z quasi-stellar object (QSO) HS 0624+6907(zQSO= 0.3700). Coupled with ground-based imaging andspectroscopic galaxy redshifts, we find evidence that many of theseabsorbers do not arise in galaxy haloes but rather are trulyintergalactic gas clouds distributed within large-scale structures, andmoreover, the gas is cool (T < 105 K) and has relativelyhigh metallicity (Z > 0.9Zsolar). Hubble Space Telescope(HST) Space Telescope Imaging Spectrograph (STIS) data reveal a dramaticcluster of 13 HI Lyman α (Lyα) lines within a 1000 kms-1 interval at zabs= 0.0635. We find 10 galaxiesat this redshift with impact parameters ranging from ρ=135h-170 kpc to 1.37h-170Mpc. The velocities and velocity spread of the Lyα lines in thiscomplex are unlikely to arise in the individual haloes of the nearbygalaxies; instead, we attribute the absorption to intragroup medium gas,possibly from a large-scale filament viewed along its long axis.Contrary to theoretical expectations, this gas is not the shock-heatedwarm-hot intergalactic medium (WHIM); the width of the Lyα linesall indicate a gas temperature T<< 105 K, and metallines detected in the Lyα complex also favour photoionized, coolgas. No OVI absorption lines are evident, which is consistent withphotoionization models. Remarkably, the metallicity is near-solar,[M/H]=-0.05 +/- 0.4 (2σ uncertainty), yet the nearest galaxy whichmight pollute the intergalactic medium is at least135h-170 kpc away. Tidal stripping from nearbygalaxies appears to be the most likely origin of this highly enriched,cool gas. More than six Abell galaxy clusters are found within 4° ofthe sight line suggesting that the QSO line of sight passes near a nodein the cosmic web. At z~ 0.077, we find absorption systems as well asgalaxies at the redshift of the nearby clusters Abell 564 and Abell 559.We conclude that the sight line pierces a filament of gas and galaxiesfeeding into these clusters. The absorber at zabs= 0.07573associated with Abell 564/559 also has a high metallicity with [C/H]> -0.6, but again the closest galaxy is relatively far from the sightline (ρ= 293h-170 kpc). The Doppler parametersand HI column densities of the Lyα lines observed along the entiresight line are consistent with those measured towards other low-z QSOs,including a number of broad (b > 40kms-1) Lyα lines.
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