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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.
| Hubble's top 10.
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| Dissecting the bursts of doom.
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| A Redetermination of the LMC Proper Motion using Four Quasar Fields
The proper motion (pm) of the Large Magellanic Cloud (LMC) relative tofour background quasi-stellar objects is determined using CCD dataobtained at the Cassegrain focus of the CTIO 1.5-m telescope, between1989 and 2001. The background quasar projected at the center of eachfield is used as a fiducial point of reference to obtain the pm of thecenter of the LMC.
| Can Sterile Neutrinos Be Ruled Out as Warm Dark Matter Candidates?
We present constraints on the mass of warm dark matter (WDM) particlesfrom a combined analysis of the matter power spectrum inferred from theSloan Digital Sky Survey Lyman-α flux power spectrum at2.2
| 40Ca(α,γ)44Ti Reaction in the Energy Regime of Supernova Nucleosynthesis
The 44Ti(t1/2=59 yr) nuclide, an importantsignature of supernova nucleosynthesis, has recently been observed aslive radioactivity by γ-ray astronomy from the Cas A remnant. Weinvestigate in the laboratory the major 44Ti productionreaction 40Ca(α,γ)44Ti(Ec.m.0.6 1.2 MeV/u) by direct off-line counting of44Ti nuclei. The yield, significantly higher than inferredfrom previous experiments, is analyzed in terms of a statistical modelusing microscopic nuclear inputs. The associated stellar rate hasimportant astrophysical consequences, increasing the calculatedsupernova 44Ti yield by a factor 2 over previousestimates and bringing it closer to Cas A observations.
| Star Formation in the Era of the Three Great Observatories
This paper summarizes contributions and suggestions as presented at theChandra Workshop Star Formation in the Era of Three Great Observatoriesconducted in July 2005. One of the declared goals of the workshop was toraise recognition within the star formation research community about thesensible future utilization of the space observatories Spitzer, Hubble,and Chandra in their remaining years of operation to tackle imminentquestions of our understanding of stellar formation and the earlyevolution of stars. A white paper was generated to support thecontinuous and simultaneous usage of observatory time for star formationresearch. The contents of this paper have been presented and discussedat several other meetings during the course of 2005 and January 2006.
| 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.
| A Double-Mode RR Lyrae Star with a Strong Fundamental-Mode Component
NSVS 5222076, a 13th magnitude star in the Northern Sky VariabilitySurvey, was identified by L. Oaster as a possible new double-mode RRLyrae star. We confirm the double-mode nature of NSVS 5222076,supplementing the survey data with new V-band photometry. NSVS 5222076has a fundamental-mode period (P0) of 0.4940 days and afirst-overtone period (P1) of about 0.3668 days, giving aperiod ratio of P1/P0=0.743. In most double-modeRR Lyrae stars, the amplitude of the first-overtone mode's pulsation isgreater than that of the fundamental-mode pulsation. That is not truefor this star. Its fundamental-mode light curve has an amplitude twiceas large as that of the first-overtone mode, a ratio very rarely seeneven among the double-mode RR Lyrae stars that have relatively strongfundamental-mode pulsation. Data from the literature are used to discussthe location in the Petersen diagram of double-mode RR Lyrae starshaving strong fundamental-mode pulsation. Such stars tend to occurtoward the short-period end of the Petersen diagram, and NSVS 5222076 isno exception to this rule.
| The Light Echo around Supernova 2003gd in Messier 74
We confirm the discovery of a light echo around the Type II-plateausupernova 2003gd in Messier 74 (NGC 628), seen in images obtained withthe High Resolution Channel of the Advanced Camera for Surveys on boardthe Hubble Space Telescope (HST) as part of a larger Snapshot program onthe late-time emission from supernovae. The analysis of the echo wepresent suggests that it is due to the SN light pulse scattered by asheet of dust grains located ~113 pc in front of the SN, and that thesegrains are not unlike those assumed to be in the diffuse Galacticinterstellar medium, both in composition and in size distribution. Theecho is less consistent with scattering off carbon-rich grains, and ifanything, the grains may be somewhat more silicate rich than theGalactic dust composition. The echo also appears to be more consistentwith a SN distance closer to 7 Mpc than to 9 Mpc. This further supportsthe conclusion we reached elsewhere that the initial mass for the SNprogenitor was relatively low (~8-9 Msolar). The HST shouldbe used to continue to monitor the echo in several bands, particularlyin the blue, to better constrain its origin.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute (STScI),which is operated by the Association of Universities for Research inAstronomy (AURA), Inc., under NASA contract NAS 5-26555.
| A Multitransition CO Study in the 30 Doradus Complex in the Large Magellanic Cloud
We have made a multitransition CO study of the largest H II complex, the30 Doradus nebula, in the Large Magellanic Cloud (LMC). This is the mostluminous example of a starburst region in the Local Group. We havesearched for 12CO J=7-->6 emission toward the 30 Doraduscomplex with the Antarctic Submillimeter Telescope and RemoteObservatory (AST/RO), located at 2847 m altitude at the Amundsen-ScottSouth Pole Station. As a result, we have detected a 12COJ=7-->6 emitting cloud near the 30 Doradus complex. The12CO J=7-->6/12CO J=4-->3 line temperatureratio in this region is approximately a factor of 2 higher than thatobserved near the Sgr B2 complex. A radiative transfer calculation usingthe line ratios shows that the core of massive star formation in the LMCis much warmer and denser than that of the Milky Way.
| OSIRIS: AO-assisted integral-field spectroscopy at the Keck Observatory
OSIRIS (OH-Suppressing Infra-Red Integral-field Spectrograph) is a newfacility instrument for the Keck Observatory. After seeing first lightin February 2005, OSIRIS is currently undergoing commissioning. OSIRISprovides the capability of performing three-dimensional spectroscopy inthe near-infrared z, J, H, and K bands at the resolution limit of theKeck II telescope, which is equipped with adaptive optics and a laserguide star. The science case for OSIRIS is summarized, and theinstrument and associated data reduction software are described.
| Outflows from evolved stars with OASIS, NAOMI and GLAS
I discuss the potential of integral-field spectroscopy (IFS) withadaptive optics in the study of the outflows from evolved stars ofdifferent masses. With IFS, detailed 3-D spatio-kinematical models ofthe outflows can be built, providing excellent observational datasets tobe confronted with the existing dynamical theories. In addition, ifmulti-epoch observations are able to resolve the apparent expansion ofthe nebulae in the plane of the sky, then their dynamics can be furtherconstrained, and other basic quantities like the distance via theexpansion parallax, can be determined. The kind of results that can beobtained are illustrated by recent HST and VLT observations of the ringnebula around the symbiotic nova He 2-147. Given the presentcapabilities of the OASIS integral-field spectrograph of the IsaacNewton Group of Telescopes (ING), classical novae ejecta are the mostappealing targets for such kind of studies, provided that its AO systemNAOMI is complemented with the forthcoming laser guide star system GLAS.IFS+AO is also a powerful technique to detect faint ionized nebulaearound bright stars, like for instance the outflows from luminous bluevariables.
| Near-infrared integral-field spectroscopy of violent starburst environments
Near-infrared (NIR) integral-field spectroscopy (IFS) of violentstarburst environments at high spatial (and spectral) resolution has thepotential to revolutionise our ideas regarding the local interactionsbetween the newly formed massive stars and the interstellar medium (ISM)of their host galaxies. To illustrate this point, I present NIR IFSanalysis of the central starburst region of NGC 1140, obtained withCIRPASS on Gemini-South. While strong [FeII] emission is foundthroughout the galaxy, higher-order Brackett emission is predominantlyassociated with the northern starburst region. Based on the spatialdistributions of the [FeII] versus Brackett line emission, I concludethat a galaxy-wide starburst was induced several ×107yr ago, with more recent starburst activity concentrated around thenorthern starburst region. I look forward and discuss the excitingprospects that IFS at higher spatial (and spectral) resolution willallow us trace (i) the massive outflows (“superwinds”)expected to originate in the dense, young massive star clusters commonlyfound in intense starburst environments, and (ii) their impact on thegalaxy’s ISM.
| Long γ-ray bursts and core-collapse supernovae have different environments
When massive stars exhaust their fuel, they collapse and often producethe extraordinarily bright explosions known as core-collapse supernovae.On occasion, this stellar collapse also powers an even more brilliantrelativistic explosion known as a long-duration γ-ray burst. Onewould then expect that these long γ-ray bursts and core-collapsesupernovae should be found in similar galactic environments. Here weshow that this expectation is wrong. We find that the γ-ray burstsare far more concentrated in the very brightest regions of their hostgalaxies than are the core-collapse supernovae. Furthermore, the hostgalaxies of the long γ-ray bursts are significantly fainter andmore irregular than the hosts of the core-collapse supernovae. Togetherthese results suggest that long-duration γ-ray bursts areassociated with the most extremely massive stars and may be restrictedto galaxies of limited chemical evolution. Our results directly implythat long γ-ray bursts are relatively rare in galaxies such as ourown Milky Way.
| Albert Jones, discoverer of SN 1987A.
Not Available
| Spitzer observations of acetylene bands in carbon-rich asymptotic giant branch stars in the Large Magellanic Cloud
We investigate the molecular bands in carbon-rich asymptotic giantbranch (AGB) stars in the Large Magellanic Cloud (LMC), using theInfrared Spectrograph (IRS) onboard the Spitzer Space Telescope (SST)over the 5-38 μm range. All 26 low-resolution spectra show acetylene(C2H2) bands at 7 and 14 μm. The hydrogencyanide (HCN) bands at these wavelengths are very weak or absent. Thisis consistent with low nitrogen abundances in the LMC. The observed 14μm C2H2 band is reasonably reproduced by anexcitation temperature of 500 K. There is no clear dilution of the 14μm C2H2 band by circumstellar dust emission.This 14-μm band originates from molecular gas in the circumstellarenvelope in these high mass-loss rate stars, in agreement with previousfindings for Galactic stars. The C2H2columndensity, derived from the 13.7 μm band, shows a gas mass-loss rate inthe range 3 × 10-6 to 5 ×10-5Msolaryr-1. This is comparable withthe total mass-loss rate of these stars estimated from the spectralenergy distribution. Additionally, we compare the line strengths of the13.7 μm C2H2 band of our LMC sample with thoseof a Galactic sample. Despite the low metallicity of the LMC, there isno clear difference in the C2H2 abundance amongLMC and Galactic stars. This reflects the effect of the third dredge-upbringing self-produced carbon to the surface, leading to highcarbon-to-oxygen ratio at low metallicity.
| A multiresolution analysis of the radio-FIR correlation in the Large Magellanic Cloud
We investigate the local correlation between the 1.4-GHz radio continuumand 60-μm far-infrared (FIR) emission within the Large MagellanicCloud (LMC) on spatial scales between 0.05 and 1.5 kpc. On scales below~1 kpc, the radio-FIR correlation is clearly better than the correlationof the cold gas tracers (CO and HI) with either the radio or the FIRemission. For the LMC as a whole, there is a tight correlation betweenthe radio and FIR emission on spatial scales above ~50 pc. Bydecomposing the radio emission into thermal and non-thermal components,however, we show that the scale on which the radio-FIR correlationbreaks down depends on the thermal fraction of the radio emission;regions that show a strong correlation to very small scales are the sameregions where the thermal fraction of the radio emission is high.Contrary to previous studies of the local radio-FIR correlation in theLMC, we show that the slope of the relation between the radio and FIRemission is non-linear. In bright star-forming regions, the radioemission increases faster than linearly with respect to the FIR emission(power-law slope of ~1.2), whereas a flatter slope of ~0.6-0.9 appliesmore generally across the LMC. Our results are consistent with ascenario in which the ultraviolet photons and cosmic rays in the LMChave a common origin in massive star formation, but the cosmic rays areable to diffuse away from their production sites. Our results do notprovide direct evidence for coupling between the magnetic field and thelocal gas density, but we note that synchrotron emission may not be agood tracer of the magnetic field if cosmic rays can readily escape theLMC.
| Nuclear embedded star clusters in NGC 7582*
We report on the discovery of several compact regions of mid-infraredemission in the star-forming circumnuclear disc of the starburst/Seyfert2 galaxy NGC 7582. The compact sources do not have counterparts in theoptical and near-infrared, suggesting that they are deeply embedded indust. We use the [NeII] 12.8-μm line emission to estimate theemission measure of the ionized gas, which in turn is used to assess thenumber of ionizing photons. Two of the brighter sources are found tohave ionizing fluxes of ~2.5 × 1052, whereas thefainter ones have ~1 × 1052 photon s-1.Comparing with a 1-Myr-old starburst, we derive stellar masses in therange 3-5 × 105Msolar and find that thenumber of O stars in each compact source is typically 0.6-1.6 ×103. We conclude that the compact mid-infrared sources arelikely to be young, embedded star clusters, of which only a few areknown so far. Our observation highlights the need for high-resolutionmid-infrared imaging to discover and study embedded star clusters in theproximity of active galactic nuclei.
| 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').
| Strong magnetic field in W75N OH maser flare
A flare of OH maser emission was discovered in W75N in 2000. Itslocation was determined with the Very Long Baseline Array (VLBA) to bewithin 110 au from one of the ultracompact HII regions, Very Large Array2 (VLA2). The flare consisted of several maser spots. Four of the spotswere found to form Zeeman pairs, all of them with a magnetic fieldstrength of about 40mG. This is the highest ever magnetic field strengthfound in OH masers, an order of magnitude higher than in typical OHmasers. Three possible sources for the enhanced magnetic field arediscussed: (i) the magnetic field of the exciting star dragged out bythe stellar wind; (ii) the general interstellar field in the gascompressed by the magnetohydrodynamic shock; and (iii) the magneticfield of planets which orbit the exciting star and produce maseremission in gaseous envelopes.
| Methods for analysing structure in molecular clouds
We have previously reported a dimensionless measure, , which can bothquantify, and distinguish between, the extent to which a star cluster iscentrally concentrated, and the extent to which it contains small-scalesubclusters. is the ratio of the normalized correlation length, , (i.e.the mean projected separation between stars, divided by the overallradius of the cluster), to the mean length, , of the segments of aminimal spanning tree (MST) joining all star positions: .In this paper, we attempt to adapt the correlation-length method to thecharacterization of gas clouds, with a view to comparing directly thestructures of gas clouds and star clusters. We also compare the resultsof the correlation-length method with fractal dimensions estimated usingthe more familiar perimeter-area method whereby the lengths of closedcontours are plotted against the areas they enclose, on a log-log plot.We find that the normalized correlation length, when modified to dealwith pixellated grey-scale data, is a robust indicator of either centralconcentration or fractal subclustering of gas clouds, but cannotdistinguish between the two types of structure. It is, however,extremely reliable, easy to implement and works accurately at all scalesand over all dynamic ranges, even with poorly sampled data. Itimplicitly incorporates edge effects, so all the data in the completecloud are used, and it therefore provides a useful method for comparingthe structures of molecular clouds and star clusters.The normalized correlation length produces comparable results to theperimeter-area method when used on molecular cloud data. However, theperimeter-area method is unable to distinguish the degree of clusteringin three-dimensional objects with fractal dimensions greater than 2.0.It also suffers from measurement noise and lack of objectivity,particularly if only a few contours are selected for analysis. It cannotbe used to compare clouds with star clusters.It is not found possible to construct an MST algorithm which worksreliably for grey-scale data and is immune to scaling problems. Thepreviously reported parameter is therefore not useful when consideringgas clouds.
| On the mass of dense star clusters in starburst galaxies from spectrophotometry
The mass of unresolved young star clusters derived fromspectrophotometric data may well be off by a factor of 2 or more oncethe migration of massive stars driven by mass segregation is accountedfor. We quantify this effect for a large set of cluster parameters,including variations in the stellar initial mass function (IMF), theintrinsic cluster mass, and mean mass density. Gas-dynamical modelscoupled with the Cambridge stellar evolution tracks allow us to derive ascheme to recover the real cluster mass given measured half-lightradius, one-dimensional velocity dispersion and age. We monitor theevolution with time of the ratio of real to apparent mass through theparameter η. When we compute η for rich star clusters, we findnon-monotonic evolution in time when the IMF stretches beyond a criticalcut-off mass of 25.5Msolar. We also monitor the rise ofcolour gradients between the inner and outer volume of clusters: we findtrends in time of the stellar IMF power indices overlapping well withthose derived for the Large Magellanic Cloud cluster NGC 1818 at an ageof 30Myr. We argue that the core region of massive Antennae clustersshould have suffered from much segregation despite their low ages. Weapply these results to a cluster mass function, and find that the peakof the mass distribution would appear to observers shifted to lowermasses by as much as 0.2dex. The star formation rate derived for thecluster population is then underestimated by from 20 to 50 per cent.
| SN 2004A: another Type II-P supernova with a red supergiant progenitor
We present a monitoring study of SN 2004A and probable discovery of aprogenitor star in pre-explosion Hubble Space Telescope (HST) images.The photometric and spectroscopic monitoring of SN 2004A show that itwas a normal Type II-P which was discovered in NGC 6207 about two weeksafter explosion. We compare SN 2004A to the similar Type II-P SN 1999emand estimate an explosion epoch of 2004 January 6. We also calculatethree new distances to NGC 6207 of 21.0 +/- 4.3,21.4 +/- 3.5 and 25.1+/- 1.7Mpc. The former was calculated using the Standard Candle Method(SCM) for SNeII-P, and the latter two from the brightest supergiantsmethod (BSM). We combine these three distances with existing kinematicdistances, to derive a mean value of 20.3 +/- 3.4Mpc. Using thisdistance, we estimate that the ejected nickel mass in the explosion is0.046+0.031-0.017Msolar. The progenitorof SN 2004A is identified in pre-explosion WFPC2 F814W images with amagnitude of mF814W = 24.3 +/- 0.3, but is below thedetection limit of the F606W images. We show that this was likely a redsupergiant (RSG) with a mass of9+3-2Msolar. The object is detected at4.7σ above the background noise. Even if this detection isspurious, the 5σ upper limit would give a robust upper mass limitof 12Msolar for a RSG progenitor. These initial masses arevery similar to those of two previously identified RSG progenitors ofthe Type II-P SNe 2004gd (8+4-2Msolar)and 2005cs (9+3-2Msolar).
| 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.
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