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Surface Brightness Profiles for a Sample of LMC, SMC, and Fornax Galaxy Globular Clusters
We use Hubble Space Telescope archival images to measure central surfacebrightness profiles of globular clusters around satellite galaxies ofthe Milky Way. We report results for 21 clusters around the LMC, fivearound the SMC, and four around the Fornax dwarf galaxy. The profileswere obtained using a recently developed technique based on measuringintegrated light, which is tested on an extensive simulated data set.Our results show that for 70% of the sample, the central photometricpoints of our profiles are brighter than previous measurements usingstar counts with deviations as large as 2 mag arcsec-2. About40% of the objects have central profiles deviating from a flat centralcore, with central logarithmic slopes continuously distributed between-0.2 and -1.2. These results are compared with those found for a sampleof Galactic clusters using the same method. We confirm the knowncorrelation in which younger clusters tend to have smaller core radii,and we find that they also have brighter central surface brightnessvalues. This seems to indicate that globular clusters might be bornrelatively concentrated, and that a profile with an extended flat coremight not be the ideal choice for initial profiles in theoreticalmodels.
| Three clusters of the SMC from ACS/WFC HST archive data: NGC 265, K 29 and NGC 290 and their field population
Aims.We determine the age, metallicity and initial mass function ofthree clusters, namely NGC 265, K29, NGC 290, located in the main body ofthe Small Magellanic Cloud. In addition, we derive the history of starformation in the companion fields. Methods: We make use of ACS/WFC HSTarchive data. For the clusters, the age and metallicity are derivedfitting the integrated luminosity function with single synthetic stellarpopulation by means of the χ2 minimization. For thecompanion fields, the history of star formation is derived using theχ2 minimization together with the downhill-simplexmethod. Results: For the clusters we find the following ages andmetallicities: NGC 265 has log(Age)=8.5±0.3 yrand metallicity 0.004±0.003 (or [ Fe/H]=-0.62); K29 has log(Age)=8.2±0.2 yr and metallicityZ=0.003±0.002 (or [ Fe/H]=-0.75); NGC 290 haslog(Age)=7.8±0.5 yr and metallicity 0.003±0.002 (or [Fe/H]=-0.75). The superior quality of the data allows the study of theinitial mass function down to M ˜ 0.7 Mȯ. Theinitial mass function turns out to be in agreement with the standardKroupa model. The comparison of the NGC 265luminosity function with the theoretical ones from stellar models bothtaking overshoot from the convective core into account and neglectingit, seems to suggest that a certain amount of convective overshoot isrequired. However, this conclusion is not a strong one because thiscluster has a certain amount of mass segregation which makes itdifficult to choose a suitable area for this comparison. The starformation rate of the field population presents periods of enhancementsat 300-400 Myr, 3-4 Gyr and finally 6 Gyr. However it is relativelyquiescent at ages older than 6 Gyr. This result suggests that at olderages, the tidal interaction between the Magellanic Clouds and the MilkyWay was not able to trigger significant star formation events.
| Extended star formation history of the star cluster NGC 2154 in the Large Magellanic Cloud
The colour-magnitude diagram (CMD) of the intermediate-age LargeMagellanic Cloud star cluster NGC 2154 and its adjacent field has beenanalysed using Padova stellar models to determine the cluster'sfundamental parameters and its star formation history. Deep BR CCDphotometry, together with synthetic CMDs and integrated luminosityfunctions, has allowed us to infer that the cluster experienced anextended star formation period of about 1.2 Gyr, which beganapproximately 2.3 Gyr ago and ended 1.1 Gyr ago. However, the physicalreality of such a prolonged period of star formation is questionable,and could be the result of inadequacies in the stellar evolutionarytracks themselves. A substantial fraction of binaries (70 per cent)seems to exist in NGC 2154.
| Infrared photometry of Cepheids in the LMC clusters NGC 1866 and NGC 2031
Context: Near infrared (IR) studies of Cepheid variables in the LMC takeadvantage of the reduced light curve amplitude and metallicitydependence at these wavelengths. This work presents such photometry fortwo young clusters known to contain sizeable Cepheid populations:NGC 1866 and NGC 2031. Aims: Ourgoal is to determine light curves and period-luminosity (PL) relationsin the near-IR, to assess the similarity between cluster and fieldpulsators, and to examine the predictive capability of current pulsationmodels. Methods: The light curves are obtained from multiwavelengthbroadband J,H,KS photometry of Cepheids in both clusters,with periods previously established from optical photometry. Results:Mean magnitudes for the Cepheids are used to construct PL relations inthe near-IR. The properties in the PL planes are compared with thebehavior of field Cepheids in the LMC and with the predictions of recentpulsational models, both canonical and overluminous. Conclusions:.Cluster and field Cepheids are homogeneous and the inclusion of thecluster Cepheids in the field sample extends the PL relation. The slopeof the PL relation is constant over the whole period range and does notshow - at least in the adopted IR bands - the break in slope at P˜ 10~d reported by some authors. A comparison with the predictionsof pulsation models allows an estimate for the distance moduli ofNGC 1866 and NGC 2031. The twoclusters are found to lie at essentially the same distance. Fitting oftheoretical models to the data gives, for the K filter,(m-M)0 = 18.62 ± 0.10 mag if canonical models are usedand (m-M)0 = 18.42 ± 0.10 mag if overluminous modelsare used. On the basis of this result, some considerations on therelationship between the clusters and the internal structure of the LMCare presented.Based on observations collected at the Las Campanas Observatory of theCarnegie Institution of Washington, and at the European SouthernObservatory, La Silla, Chile, using SOFI at the 3.5 m NTT, withinobserving program 68.D-0287. Photometric data are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/462/599 Figures [see fulltextsee full textsee full textsee full textsee full textsee full textseefull textsee full text] and [see full textsee full textsee full textseefull textsee full textsee full textsee full textsee full text] are onlyavailable in electronic form at http://www.aanda.org
| Ages and Metallicities of Extragalactic Globular Clusters from Spectral and Photometric Fits of Stellar Population Synthesis Models
Spectra of galaxies contain an enormous amount of information about therelative mixture of ages and metallicities of constituent stars. Wepresent a comprehensive study designed to extract the maximuminformation from spectra of data quality typical in large galaxysurveys. These techniques are not intended for detailed stellarpopulation studies that use high-quality spectra. We test techniques ona sample of globular clusters, which should consist of single stellarpopulations and provide good test cases, using the Bruzual-Charlothigh-resolution stellar population synthesis models to simultaneouslyestimate the ages and metallicities of 101 globular clusters in M31 andthe Magellanic Clouds. The clusters cover a wide range of ages andmetallicities, 4 Myr
| The TP-AGB phase. Lifetimes from C and M star counts in Magellanic Cloud clusters
Using available data for C and M giants with M_bol<-3.6 in MagellanicCloud clusters, we derive limits to the lifetimes for the correspondingevolutionary phases, as a function of stellar mass. The C-star phase isfound to have a duration between 2 and 3 Myr for stars in the mass rangefrom ~1.5 to 2.8 M_ȯ. There is also an indication that the peak ofC-star lifetime shifts to lower masses (from slightly above to slightlybelow 2 Mȯ) as we move from LMC to SMC metallicities.The M-giant lifetimes also peak at ~2 Mȯ in the LMC,with a maximum value of about 4 Myr, whereas in the SMC their lifetimesappear much shorter, but, actually, they are poorly constrained by thedata. These numbers constitute useful constraints to theoretical modelsof the TP-AGB phase. We show that several models in the literatureunderestimate the duration of the C-star phase at LMC metallicities.
| Chemically peculiar stars in the Large Magellanic Cloud
Context: .The detection of magnetic chemically peculiar (CP2) stars inopen clusters of extragalactic systems can give observational answers tomany unsolved questions. For example, one can study the influence ofdifferent global as well local environments on the lack of and presenceof peculiarities. Aims: .The mean percentage of CP2 stars in theMilky Way is of the order of 5% for the spectral range from early B- toF-type, luminosity class V objects. The origin of the CP2 phenomenonseems to be closely connected to the overall metallicity and globalmagnetic field environment. The theoretical models are still only testedby observations in the Milky Way. It is therefore essential to providehigh quality observations in rather different global environments.Methods: .The young clusters NGC 2136/7 were observed in the Δ aphotometric system. This intermediate band photometric system samplesthe depth of the 520 nm flux depression by comparing the flux at thecenter with the adjacent regions with bandwidths of 11 nm to 23 nm. TheΔ a photometric system is most suitable for detecting CP2 starswith high efficiency, but is also capable of detecting a smallpercentage of non-magnetic CP objects. Furthermore, the groups of(metal-weak) λ Bootis, as well as classical Be/shell stars, canbe successfully investigated. Results: .We present high precisionphotometric Δ a observations of 417 objects in NGC 2136/7 and itssurrounding field, of which five turned out to be bona fide magnetic CPstars. In addition, we discovered two Be/Ae stars. Conclusions:.From our investigations of NGC 1711, NGC 1866, NGC 2136/7, theirsurroundings, and one independent field of the LMC population, we derivean occurrence of classical chemically peculiar stars of 2.2(6)% in theLMC, which is only half the value found in the Milky Way. The mass andage distribution of the photometrically detected CP stars is notdifferent from that of similar objects in galactic open clusters.
| Red Giant Stars in the Large Magellanic Cloud Clusters
We present deep J, H, and Ks photometry and accurate colormagnitude diagrams down to K~18.5 for a sample of 13 globular clustersin the Large Magellanic Cloud. This data set combined with the previoussample of six clusters published by our group gives the opportunity tostudy the properties of giant stars in clusters with different ages(ranging from ~80 Myr up to 3.5 Gyr). Quantitative estimates of starpopulation ratios (by number and luminosity) in the asymptotic giantbranch (AGB), the red giant branch (RGB), and the He clump have beenobtained and compared with theoretical models in the framework ofprobing the so-called phase transitions. The AGB contribution to thetotal luminosity starts to be significant at ~200 Myr and reaches itsmaximum at 500-600 Myr, when the RGB phase transition is starting. At~900 Myr the full development of an extended and well-populated RGB hasbeen completed. The occurrences of both the AGB and RGB phasetransitions are sharp events, lasting a few hundred megayears only.These empirical results agree very well with the theoretical predictionsof simple stellar population models based on canonical tracks and thefuel-consumption approach.Based on observations collected at the European Southern Observatory, LaSilla, Chile, using SOFI at the 3.5 m NTT, within the observing programs64.N-0038 and 68.D-0287.
| Mapping and Mass Measurement of the Cold Dust in NGC 205 with Spitzer
We present observations at 3.6, 4.5, 5.8, 8, 24, 70, and 160 μm ofNGC 205, the dwarf elliptical companion of M31, obtained with theSpitzer Space Telescope. The point-source-subtracted images at 8 and 24μm display a complex and fragmented infrared emission coming fromboth very small dust particles and larger grains. The extended dustemission is spatially concentrated in three main emission regions, seenat all wavelengths from 8 to 160 μm. These regions lie approximatelyalong NGC 205's semimajor axis and range from ~100 to 300 pc in size. Onthe basis of our mid-/far-infrared flux density measurements alone, wederive a total dust mass estimate on the order of3.2×104 Msolar, mainly at a temperature of~20 K. The gas mass associated with this component matches the predictedmass returned by the dying stars from the last burst of star formationin NGC 205 (~0.5 Gyr ago). Analysis of the Spitzer data combined withprevious 1.1 mm observations over a small central or ``Core'' region(18" diameter) suggests the presence of very cold (T~12 K) dust and adust mass about 16 times higher than is estimated from the Spitzermeasurements alone. Assuming a gas-to-dust mass ratio of 100, these twodata sets, i.e., with and without the millimeter observations, suggest atotal gas mass in the range from 3.2×106 to5×107 Msolar.
| A Database of 2MASS Near-Infrared Colors of Magellanic Cloud Star Clusters
The (rest-frame) near-IR domain contains important stellar populationdiagnostics and is often used to estimate masses of galaxies at low, aswell as high, redshifts. However, many stellar population models arestill relatively poorly calibrated in this part of the spectrum. Toallow an improvement of this calibration we present a new database ofintegrated near-IR JHKs magnitudes for 75 star clusters inthe Magellanic Clouds, using the Two Micron All Sky Survey (2MASS). Themajority of the clusters in our sample have robust age and metallicityestimates from color-magnitude diagrams available in the literature, andpopulate a range of ages from 10 Myr to 15 Gyr and a range in [Fe/H]from -2.17 to +0.01 dex. A comparison with matched star clusters in the2MASS Extended Source Catalog (XSC) reveals that the XSC only provides agood fit to the unresolved component of the cluster stellar population.We also compare our results with the often-cited single-channel JHKphotometry of Persson and coworkers and find significant differences,especially for their 30" diameter apertures, up to ~2.5 mag in the Kband, more than 1 mag in J-K, and up to 0.5 mag in H-K. Usingsimulations to center apertures based on maximum light throughput (asperformed by Persson et al.), we show that these differences can beattributed to near-IR-bright cluster stars (e.g., carbon stars) locatedaway from the true center of the star clusters. The wide age andmetallicity coverage of our integrated JHKs photometry sampleconstitute a fundamental data set for testing population synthesis modelpredictions and for direct comparison with near-IR observations ofdistant stellar populations.
| Astrophysics in 2005
We bring you, as usual, the Sun and Moon and stars, plus some galaxiesand a new section on astrobiology. Some highlights are short (the newlyidentified class of gamma-ray bursts, and the Deep Impact on Comet9P/Tempel 1), some long (the age of the universe, which will be found tohave the Earth at its center), and a few metonymic, for instance theterm ``down-sizing'' to describe the evolution of star formation rateswith redshift.
| Cepheid Distances to SNe Ia Host Galaxies Based on a Revised Photometric Zero Point of the HST WFPC2 and New PL Relations and Metallicity Corrections
With this paper we continue the preparation for a forthcoming summaryreport of our experiment with the HST to determine the Hubble constantusing Type Ia supernovae as standard candles. Two problems areaddressed. (1) We examine the need for, and determine the value of, thecorrections to the apparent magnitudes of our program Cepheids in the 11previous calibration papers due to sensitivity drifts and chargetransfer effects of the HST WFPC2 camera over the life time of theexperiment from 1992 to 2001. (2) The corrected apparent magnitudes areapplied to all our previous photometric data from which revised distancemoduli are calculated for the eight program galaxies that are parents tothe calibrator Ia supernovae. Two different Cepheid P-L relations areused; one for the Galaxy and one for the LMC. These differ both in slopeand zero point at a fixed period. The procedures for determining theabsorption and reddening corrections for each Cepheid are discussed.Corrections for the effects of metallicity differences between theprogram galaxies and the two adopted P-L relations are derived andapplied. The distance moduli derived here for the eight supernovaeprogram galaxies, and for 29 others, average 0.20 mag fainter (moredistant) than those derived by Gibson et al. and Freedman et al. intheir 2000 and 2001 summary papers for reasons discussed in this paper.The effect on the Hubble constant is the subject of our forthcomingsummary paper.
| On the Iron Content of NGC 1978 in the LMC: A Metal-rich, Chemically Homogeneous Cluster
We present a detailed abundance analysis of giant stars in NGC 1978, amassive, intermediate-age stellar cluster in the Large Magellanic Cloud,characterized by a high ellipticity and suspected to have a metallicityspread. We analyzed 11 giants, all cluster members, by usinghigh-resolution spectra acquired with UVES/FLAMES at the ESO Very LargeTelescope. We find an iron content of [Fe/H] = -0.38 dex with very lowσ[Fe/H]=0.07 dex dispersion, a mean heliocentric radialvelocity vr=293.1+/-0.9 km s-1, and a velocitydispersion σvr=3.1 km s-1, thusexcluding the presence of a significant metallicity, as well asvelocity, spread within the cluster.Based on observations collected at the Very Large Telescope of theEuropean Southern Observatory (ESO), Cerro Paranal, Chile, underprograms 072.D-0342 and 074.D-0369.
| 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.
| Bump Cepheids in the Magellanic Clouds: Metallicities, the Distances to the LMC and SMC, and the Pulsation-Evolution Mass Discrepancy
We use nonlinear pulsation models to reproduce the observed light andcolor curves for two samples of bump Cepheid variables, 19 from theLarge Magellanic Cloud and 9 from the Small Magellanic Cloud. Thisanalysis determines the fundamental parameters mass, luminosity,effective temperature, metallicity, distance, and reddening for thesample of stars. The use of the light-curve shape alone to determinemetallicity is a new modeling technique introduced here. Themetallicity, distance, and reddening distributions for the two samplesare in agreement with those of similar stellar populations in theliterature. The distance modulus of the Large Magellanic Cloud isdetermined to be 18.54+/-0.018, and the distance modulus of the SmallMagellanic Cloud is determined to be 18.93+/-0.024. The mean Cepheidmetallicities are Z=0.0091+/-0.0007 and 0.0050+/-0.0005 for the LMC andSMC, respectively. The masses derived from pulsation analysis aresignificantly less than those predicted by stellar evolutionary modelswith no or mild convective core overshoot. We show that this discrepancycannot be accounted for by uncertainties in our input opacities or inmass-loss physics. We interpret the observed mass discrepancy in termsof enhanced internal mixing in the vicinity of the convective coreduring the main-sequence lifetime and find that the overshoot parameterΛc rises from 0.688+/-0.009Hp at the meanLMC metallicity to 0.746+/-0.009Hp in the SMC.
| AGB stars in the Magellanic Clouds. II. The rate of star formation across the LMC
This article compares the distribution of Ks magnitudes ofLarge Magellanic Cloud (LMC) asymptotic giant branch (AGB) starsobtained from the DENIS and 2MASS data with theoretical distributions.These have been constructed using up-to-date stellar evolutioncalculations for low and intermediate-mass stars, and in particular forthermally pulsing AGB stars. A fit of the magnitude distribution of bothcarbon- and oxygen-rich AGB stars allowed us to constrain themetallicity distribution across the LMC and its star formation rate(SFR). The LMC stellar population is found to be on average 5-6 Gyr oldand is consistent with a mean metallicity corresponding to Z=0.006.These values may however be affected by systematic errors in theunderlying stellar models, and by the limited exploration of thepossible SFR histories. Instead our method should be particularly usefulfor detecting variations in the mean metallicity and SFR across the LMCdisk. There are well defined regions where both the metallicity and themean-age of the underlying stellar population span the whole range ofgrid parameters. The C/M ratio discussed in Paper I is a tracer of themetallicity distribution if the underlying stellar population is olderthan about a few Gyr. A similar study across the Small Magellanic Cloudis given in Paper III of this series.
| The Star Clusters of the Small Magellanic Cloud: Structural Parameters
We present structural parameters for 204 stellar clusters in the SmallMagellanic Cloud derived from fitting King and Elson, Fall, and Freeman(EFF) model profiles to the V-band surface brightness profiles asmeasured from the Magellanic Clouds Photometric Survey images. Both Kingand EFF profiles are satisfactory fits to the majority of the profiles,although King profiles are generally slightly superior to the softenedpower-law profiles of EFF and provide statistically acceptable fits to~90% of the sample. We find no correlation between the preferred modeland cluster age. The only systematic deviation in the surface brightnessprofiles that we identify is a lack of a central concentration in asubsample of clusters, which we designate as ``ring'' clusters. Inagreement with previous studies, we find that the clusters in the SMCare significantly more elliptical than those in the Milky Way. However,given the mean age difference and the rapid destruction of thesesystems, the comparison between the SMC and the Milky Way should notdirectly be interpreted as a difference in either the initial clusterproperties or their subsequent evolution. We find that clusterellipticity correlates with cluster mass more strongly than with clusterage. We identify several other correlations (central surface brightnessvs. local background density, core radius vs. tidal force, and size vs.distance) that can be used to constrain models of cluster evolution inthe SMC.
| Variable Stars in the Large Magellanic Cloud: Discovery of Extragalactic W UMa Binaries
We observed a field in the disk of the LMC on two consecutive nights insearch of rapid variable stars. We found two pulsating stars of typeRRab and δ Scuti and four binary stars; among the latter we foundone sdB or cataclysmic variable below the LMC blue main sequence andthree very close binary systems on the main sequence. At least one ofthe main-sequence binaries, and possibly all three, are the firstsolar-type (W UMa-type) contact binaries to be detected in anyextragalactic system and observed to obey the sameMV=MV(logP, B-V) calibration as the Galacticsystems. Given the selection effects due to small amplitudes at faintmagnitudes, the frequency of such binaries in the disk of the LMC withits large spread in population ages is not inconsistent with that in thedisk of our Galaxy and contrasts with the lack of binaries found inearlier observations of the much younger LMC cluster LW 55.Based on data obtained at Las Campanas Observatory, operated by theCarnegie Institution of Washington, during the University of Torontotime allocation.
| B, V, I photometry of the complete sample of 23 Cepheids in the field of NGC 1866 .
We present the result of BVI photometry, obtained by using FORS@VLT, ofthe Cepheids present in the field of the Large Magellanic Cloud clusterNGC 1866. We found the 22 known variables plus an additional new Cepheidlocated about 10' from the cluster center. The accuracy of thephotometry allowed us to derive B, V and I mean magnitudes with anuncertainty lower than 0.02 mag for 22 out of the 23 objects, with theexception of only one Cepheid (WS9) which presents a noisy light curvedue to the probable occurrence of image blending. As a result, weprovide accurate observational data for a substantial sample ofvariables all lying at the same distance and with a common originalcomposition. The resulting period-luminosity relations are presented andbriefly discussed.
| Near IR photometry of Cepheid variables in the LMC clusters NGC 1866 and NGC 2031.
We present JHK photometry and light curves for a sample of cepheids inthe two LMC young clusters NGC 1866 and NGC 2031. The reduced amplitudeand dependence on metallicity allow us to derive precise mean magnitudesand to draw well defined P-L relations, which we find in good agreementwith the P-L relation obtained in the IR for field cepheids.
| Distances to six Cepheids in the LMC cluster NGC1866 from the near-IR surface-brightness method .
We derive individual distances to six Cepheids in the young populousstar cluster NGC1866 in the Large Magellanic Cloud employing the near-IRsurface brightness technique. With six stars available at the exact samedistance we can directly measure the intrinsic uncertainty of themethod. We find a standard deviation of 0.11 mag, two to three timeslarger than the error estimates and more in line with the estimates fromBayesian statistical analysis by Barnes et al. \cite{Barnes05}. Usingall six distance estimates we determine an unweighted mean clusterdistance of 18.30±0.05. The observations indicate that NGC1866 isclose to be at the same distance as the main body of the LMC. If we usethe stronger dependence of the p-factor on the period as suggested byGieren et al. \cite{Gieren05} we find a distance of 18.50±0.05(internal error) and the PL relations for Galactic and MC Cepheids arein very good agreement.
| Stellar pulsation and evolution: a stepping-stone to match reality.
We discuss current status of evolutionary and pulsation predictions forintermediate-mass stars. In particular, we focus our attention on thedifferent physical mechanisms that might affect the current discrepancybetween evolutionary and pulsation estimates of Galactic and MagellanicCepheid masses. Theoretical findings and recent empirical evidenceindicate that the mass-loss may play a significan role in thislong-standing problem.
| Evidence for a universal slope of the period-luminosity relation from direct distances to Cepheids in the LMC.
We have applied the infrared surface brightness (ISB) technique toderive distances to 13 Cepheid variables in the LMC which have periodsfrom 3-42 days. The corresponding absolute magnitudes define PLrelations in VIWJK bands which agree exceedingly well with thecorresponding Milky Way relations obtained from the same technique, andare in significant disagreement with the observed LMC Cepheid PLrelations, by OGLE-II and Persson et al., in these bands. Our datauncover a systematic error in the p-factor law which transforms Cepheidradial velocities into pulsational velocities. We correct the p-factorlaw by requiring that all LMC Cepheids share the same distance.Re-calculating all Milky Way and LMC Cepheid distances with the revisedp-factor law, we find that the PL relations from the ISB technique bothin LMC and in the Milky Way agree with the OGLE-II and Persson et al.LMC PL relations, supporting the conclusion of no metallicity effect onthe slope of the Cepheid PL relation in optical/near infrared bands.
| Infrared surface brightness distances to Cepheids: a comparison of Bayesian and linear-bisector calculations.
Bayesian statistical calculations and linear-bisector calculations forobtaining Cepheid distances and radii by the infrared surface brightnessmethod have been compared for a set of 38 Cepheids. The distancesobtained by the two techniques agree to 1.5% ± 0.6% and the radiiagree to 1.1% ± 0.7%. Thus the two methods yield the samedistances and radii at the 2sigma level. This implies that the shortdistance to the LMC found in recent linear-bisector studies of Cepheidsis not a result of simplifications in the mathematical approach. Thecomputed uncertainties in distance and radius are larger in the Bayesiancalculation typically by a factor of three.
| How good are RR Lyrae and Cepheids really as distance indicators? . The observational approach
A number of recent technical developments, including the Hipparcossatellite, the Hubble Space Telescope fine guidance sensors and longbase line near-IR interferometry has made it possible to employ severallargely geometrical methods to determine direct distances to RR Lyraestars and Cepheids. The distance scale now rests on a much firmer basisand the significant differences between the distances based on RR Lyraestars (short) and Cepheids (long) to the LMC have been largelyeliminated. The effects of metallicity on the RR Lyrae period-luminosity(PL) relation in the K-band as well as on the Cepheid PL relationappears to be the main remaining issues but even here empirical resultsare beginning to show convergence. I review here some of these recentdevelopments seen from the perspective of the near-IR surface brightnessmethod.
| 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.
| New Optical and Near-Infrared Surface Brightness Fluctuation Models. II. Young and Intermediate-Age Stellar Populations
We present theoretical surface brightness fluctuation (SBF) amplitudesfor single-burst stellar populations of young and intermediate age (25Myr<=t<=5 Gyr) and metallicities Z=0.0003, 0.001, 0.004, 0.008,0.01, 0.02, and 0.04. The fluctuation magnitudes and colors as expectedin the Johnson-Cousins (UBVRIJHK) photometric system are provided. Wepay attention to the contribution of thermally pulsating asymptoticgiant branch (TP-AGB) stars. The sensitivity of the predicted SBF tochanges in the mass-loss scenario along the TP-AGB phase is examined.Below 0.6-1 Gyr both optical and near-IR SBF models exhibit a strongdependence on age and mass loss. We also evaluate SBF amplitudes usingMonte Carlo techniques to reproduce the random variation in the numberof stars experiencing bright and fast evolutionary phases (red giantbranch, AGB, TP-AGB). On these grounds we provide constraints on thefaintest integrated flux of real stellar populations required to derivereliable and meaningful SBF measurements. We analyze a technique forderiving SBF amplitudes of star clusters from the photometry ofindividual stars and estimate the uncertainty due to statisticaleffects, which may impinge on the procedure. The first optical SBFmeasurements for 11 Large Magellanic Cloud (LMC) star-rich clusters-withages ranging from a few megayears to several gigayears-are derived usingHubble Space Telescope observations. The measurements are compared toour SBF predictions, providing a good agreement with models ofmetallicity Z=0.0001-0.01. Our results suggest that, for TP-AGB stars, amass loss as a power-law function of the star luminosity is required inorder to properly reproduce the optical SBF data of the LMC clusters.Finally, near-IR models have been compared to available data, thusshowing that the general trend is well fitted. We suggest how toovercome the general problem of SBF models in reproducing the details ofthe near-IR SBF measurements of the Magellanic Cloud star clusters.
| Dust-enshrouded giants in clusters in the Magellanic Clouds
We present the results of an investigation of post-Main Sequence massloss from stars in clusters in the Magellanic Clouds, based around animaging survey in the L'-band (3.8 μm) performed with the VLT at ESO.The data are complemented with JHKs (ESO and 2MASS) andmid-IR photometry (TIMMI2 at ESO, ISOCAM on-board ISO, and data fromIRAS and MSX). The goal is to determine the influence of initialmetallicity and initial mass on the mass loss and evolution during thelatest stages of stellar evolution. Dust-enshrouded giants areidentified by their reddened near-IR colours and thermal-IR dust excessemission. Most of these objects are Asymptotic Giant Branch (AGB) carbonstars in intermediate-age clusters, with progenitor masses between 1.3and ~5 M_ȯ. Red supergiants with circumstellar dust envelopes arefound in young clusters, and have progenitor masses between 13 and 20M_ȯ. Post-AGB objects (e.g., Planetary Nebulae) and massive starswith detached envelopes and/or hot central stars are found in severalclusters. We model the spectral energy distributions of the cluster IRobjects, in order to estimate their bolometric luminosities andmass-loss rates. The IR objects are the most luminous cluster objects,and have luminosities as expected for their initial mass andmetallicity. They experience mass-loss rates in the range from a few10-6 up to 10-4 M_ȯ yr-1 (ormore), with most of the spread being due to evolutionary effects andonly a weak dependence on progenitor mass and/or initial metallicity.About half of the mass lost by 1.3-3 M_ȯ stars is shed during thesuperwind phase, which lasts of order 105 yr. Objects withdetached shells are found to have experienced the highest mass-lossrates, and are therefore interpreted as post-superwind objects. We alsopropose a simple method to measure the cluster mass from L'-band images.
| On the incidence of chemically peculiar stars in the Large Magellanic Cloud
With the aim to corroborate the result of a search for chemicallypeculiar stars in the Large Magellanic Cloud (LMC), we presentmeasurements obtained from CCD imaging of two fields, one containing ayoung open cluster (NGC 1711). While for the latter field, including itssurrounding we obtain a contribution of 3 per cent of chemicallypeculiar stars detectable by Δa photometry (i.e. the magneticobjects of this group), the second field yields about half of this valuein good accordance with Maitzen et al.'s finding for NGC 1866, thesurrounding field of which has been found to exhibit a very low value ofsuch stars (0.3 per cent). Thus, we are faced with the fact that ourincipient impression about a substantially lower appearance of magneticchemically peculiar stars in the LMC as compared to the Galaxy continuesto be valid. Most of the photometrically identified peculiar stars (fromtheir historical origin denominated Ap stars) are located in the domainof the B-type stars. However, this is a selection effect due to thelimiting magnitude of our observing conditions impeding the observationof fainter main-sequence stars. In addition to objects showing up aspositive deviators in Δa photometry, we also discuss nine starswhich appear opposite the main line of normal stars, and hence arenegative deviators. For most of them, the interpretation as emissionstars of B-type seems to be appropriate. The statistically relevantnumber of observations obtained so far in the LMC supports the view thatthe formation of magnetic peculiar stars has occurred there at asignificantly lower rate.
| The Adequacy of Stellar Evolution Models for the Interpretation of the Color-Magnitude Diagrams of Resolved Stellar Populations
Most of what we know about the stellar population of nearby, resolvedgalaxies comes from the interpretation of their color-magnitudediagrams, by comparison with stellar evolutionary models. We review howwell current stellar evolution models reproduce the properties of simplestellar populations. Emphasis is given to the regions of thecolor-magnitude diagram which are most useful for deriving age,metallicity, or distance of a population. Extensive comparison is madebetween the predictions of the most-used stellar evolution libraries, inorder to estimate how model dependent the results are. The presentreview, written from a user perspective, aims at emphasizing thestrengths and weaknesses of the models, and is intended both forobservers and theoreticians. We hope to encourage observers to providestronger observational constraints where they are needed, and tostimulate theoreticians to isolate the input physics responsible for thedifferent behavior between models and the reasons for the discrepancieswith data.
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