R. de Grijs

VISTA Variables in the Via Lactea (VVV): The public ESO near-IR variability survey of the Milky Way

Original article can be found at: http://www.sciencedirect.com/science/journal/13841076 Copyright Elsevier B.V.

The Star Cluster Population of M51: II. Age distribution and relations among the derived parameters

We use archival Hubble Space Telescope observations of broad-band images from the ultraviolet (F255W- filter) through the near infrared (NICMOS F160W-filter) to study the star cluster population of the interacting spiral galaxy M51. We obtain age, mass, extinction, and effective radius estimates for 1152 star clusters in a region of � 7.3 × 8.1 kpc centered on the nucleus and extending into the outer spiral arms. In this paper we present the data set and exploit it to determine the age distribution and relationships among the fundamental parameters (i.e. age, mass, effective radius). We show the critical dependence of the age distribution on the sample selection, and confirm that using a constant mass cut-off, above which the sample is complete for the entire age range of interest, is essential. In particular, in this sample we are complete only for masses above 5×10 4 M⊙ for the last 1 Gyr. Using this dataset we find: i) that the cluster formation rate seems to have had a large increase � 50-70 Myr ago, which is coincident with the suggested second passage of its companion, NGC 5195, ii) a large number of extremely young (< 10 Myr) star clusters, which we interpret as a population of unbound clusters of which a large majority will disrupt within the next �10 Myr, and iii) that the distribution of cluster sizes can be well approximated by a power-law with exponent, � = 2.2 ± 0.2, which is very similar to that of Galactic globular clusters, indicating that cluster disruption is largely independent of cluster radius. In addition, we have used this dataset to search for correlations among the derived parameters. In particular, we do not find any strong trends between the age and mass, mass and effective radius, nor between the galactocentric distance and effective radius. There is, however, a strong correlation between the age of a cluster and its extinction, with younger clusters being more heavily reddened than older clusters.We present the age and mass distribution of star clusters in M51. The structural parameters are found by fitting cluster evolution models to the spectral energy distribution consisting of 8 HST-WFPC2 pass bands. There is evidence for a burst of cluster formation at the moment of the second encounter with the companion NGC5195 (50-100 Myr ago) and a hint for an earlier burst (400-500 Myr ago). The cluster IMF has a power law slope of -2.1. The disruption time of clusters is extremely short (< 100 Myr for a 10^4 Msun cluster).

Star cluster formation and evolution in nearby starburst galaxies — II. Initial conditions

We use the ages, masses and metallicities of the rich young star cluster systems in the nearby starburst galaxies NGC 3310 and NGC 6745 to derive their cluster formation histories and subsequent evolution. We further expand our analysis of the systematic uncertainties involved in the use of broad-band observations to derive these parameters (Paper I) by examining the effects of a priori assumptions on the individual cluster metallicities. The age (and metallicity) distributions of both the clusters in the circumnuclear ring in NGC 3310 and of those outside the ring are statistically indistinguishable, but there is a clear and significant excess of higher-mass clusters in the ring compared to the non-ring cluster sample; it is likely that the physical conditions in the starburst ring may be conducive for the formation of higher-mass star clusters, on average, than in the relatively more quiescent environment of the main galactic disc. For the NGC 6745 cluster system we derive a median age of � 10 Myr. NGC 6745 contains a significant population of high-mass “super star clusters”, with masses in the range 6.5 . log(Mcl/M⊙) . 8.0. This detection supports the scenario that such objects form preferentially in the extreme environments of interacting galaxies. The age of the cluster populations in both NGC 3310 and NGC 6745 is significantly lower than their respective characteristic cluster disruption time-scales, respectively log(t dis /yr) = 8.05 and 7.75, for 10 4 M⊙ clusters. This allows us to obtain an independent estimate of the initial cluster mass function slope, � = 2.04(±0.23) +0.13 −0.43 for NGC 3310, and 1.96(±0.15) ± 0.19 for NGC 6745, respectively, for masses Mcl & 10 5 M⊙ and Mcl & 4 × 10 5 M⊙. These mass function slopes are consistent with those of other

Analysing observed star cluster SEDs with evolutionary synthesis models: systematic uncertainties

We discuss the systematic uncertainties inherent to analyses of observed (broad-band) Spectral Energy Distributions (SEDs) of star clusters with evolutionary synthesis models. We investigate the effects caused by restricting oneself to a limited number of available passbands, choices of various passband combinations, finite observational errors, non-continuous model input parameter values, and restrictions in parameter space allowed during analysis. Starting from a complete set of UBVRIJH passbands (respectively their Hubble Space Telescope/WFPC2 equivalents) we investigate to which extent clusters with different combinations of age, metallicity, internal extinction and mass can or cannot be disentangled in the various evolutionary stages throughout their lifetimes and what are the most useful passbands required to resolve the ambiguities. We find the U and B bands to be of the highest significance, while the V band and near-infrared data provide additional constraints. A code is presented that makes use of luminosities of a star cluster system in all of the possibly available passbands, and tries to find ranges of allowed age-metallicity-extinction-mass combinations for individual members of star cluster systems. Numerous tests and examples are presented. We show the importance of good photometric accuracies and of determining the cluster parameters independently without any prior assumptions.

New Galactic star clusters discovered in the VVV survey

Context. VISTA Variables in the V´oa Lactea (VVV) is one of the six ESO Public Surveys operating on the new 4-meter Visible and Infrared Survey Telescope for Astronomy (VISTA). VVV is scanning the Milky Way bulge and an adjacent section of the disk, where star formation activity is high. One of the principal goals of the VVV Survey is to find new star clusters of different ages. Aims. In order to trace the early epochs of star cluster formation we concentrated our search in the directions to those of known star formation regions, masers, radio, and infrared sources. Methods. The disk area covered by VVV was visually inspected using the pipeline processed and calibrated KS-band tile images for stellar overdensities. Subsequently, we examined the composite JHKS and ZJKS color images of each candidate. PSF photometry of 15 × 15 arcmin fields centered on the candidates was then performed on the Cambridge Astronomy Survey Unit reduced images. After statistical field-star decontamination, color-magnitude and color-color diagrams were constructed and analyzed. Results. We report the discovery of 96 new infrared open clusters and stellar groups. Most of the new cluster candidates are faint and compact (with small angular sizes), highly reddened, and younger than 5Myr. For relatively well populated cluster candidates we derived their fundamental parameters such as reddening, distance, and age by fitting the solar- metallicity Padova isochrones to the color-magnitude diagrams.

The VMC survey - IV. The LMC star formation history and disk geometry from four VMC tiles

We derive the star formation history (SFH) for several regions of the Large Magellanic Cloud (LMC), using deep near-infrared data from the VISTA near-infrared Y JKs survey of the Magellanic system (VMC). The regions include three almost-complete 1.4 deg 2 tiles located ∼ 3.5 ◦ away from the LMC centre in distinct directions. They are split into 21.0 ′ × 21.5 ′ (0.12 deg 2 ) subregions, and each of these is analysed independently. To this dataset, we add two 11.3 ′ × 11.3 ′ (0.036 deg 2 ) subregions selected based on their small and uniform extinction inside the 30 Doradus tile. The SFH is derived from the simultaneous reconstruction of two different colour‐magnitude diagrams (CMDs), using the minimization code StarFISH together with a database of “partial models” representing the CMDs of LMC populations of various ages and metallicities, plus a partial model for the CMD of the Milky Way foreground. The distance modulus (m− M)0 and extinction AV is varied within intervals∼ 0.2 and∼ 0.5 mag wide, respectively, within which we identify the best-fitting star formation rate SFR( t) as a function of lookback time t, age‐metallicity relation (AMR), (m− M)0 and AV. Our results demonstrate that VMC data, due to the combination of depth and little sensitivity to differential reddening, allow the derivation of the space-reso lved SFH of the LMC with unprecedented quality compared to previous wide-area surveys. In particular, the data clea rly reveal the presence of peaks in the SFR(t) at ages log(t/yr)≃ 9.3 and 9.7, which appear in most of the subregions. The most recent SFR(t) is found to vary greatly from subregion to subregion, with the general trend of being more intense in the innermost LMC, except for the tile next to the N11 complex. In the bar region, the SFR(t) seems remarkably constant over the time interval from log(t/yr)≃ 8.4 to 9.7. The AMRs, instead, turn out to be remarkably similar across the LMC. Thanks to the accuracy in determining the distance modulus for every subregion ‐ with typical errors of just∼ 0.03 mag ‐ we make a first attempt to derive a spatial model of the LMC disk. The fields studied so far are fit extremel y well by a single disk of inclination i = 26.2± 2.0 ◦ , position angle of the line of nodesθ0 = 129.1± 13.0 ◦ , and distance modulus of (m− M)0 = 18.470± 0.006 mag (random errors only) up to the LMC centre. We show that once the (m− M)0 values or each subregion are assumed to be identical to those derived from this best-fitting plane, systematic errors in t he SFR(t) and AMR are reduced by a factor of about two.

Star cluster formation and evolution in nearby starburst galaxies – I. Systematic uncertainties

The large majority of extragalactic star cluster studies done to date have essentially used two or three-passband aperture photometry, combined with theoretical stellar population synthesis models, to obtain age, mass and extinction estimates, and sometimes also metallicities. The accuracy to which this can be done depends on the choice of (broad-band) passband combination and, crucially, also on the actual wavelengths and the wavelength range covered by the observations. Understanding the inherent systematic uncertainties (the main aim of this paper) is of the utmost importance for a well-balanced interpretation of the properties of extragalactic star cluster systems. We simultaneously obtain ages, metallicities and extinction values for ∼ 300 clusters in the nearby starburst galaxy NGC 3310, based on archival Hubble Space Telescope observations from the ultraviolet (UV) to the near-infrared (NIR). We show that, for ages 6 . log(age/yr) . 9, and if one can only obtain partial coverage of the spectral energy distribution (SED), an optical passband combination of at least four filters including both blue and red passbands results in the most representative age distribution, as compared to the better constrained ages obtained from the full UV–NIR SED coverage. We find that while blue-selected passband combinations lead to age distributions that are slightly biased towards younger ages due to the well-known age–metallicity degeneracy, red-dominated passband combinations should be avoided. NGC 3310 underwent a (possibly extended) global burst of cluster formation ∼ 3×10 7 yr ago. This coincides closely with the last tidal interaction or merger with a low-metallicity galaxy that likely induced the formation of the large fraction of clusters with (significantly) subsolar metallicities. The logarithmic slope of the Vband cluster luminosity function, for clusters in the range 17.7 . F606W . 20.2

Mass segregation in young compact star clusters in the Large Magellanic Cloud — I. Data and luminosity functions

We have undertaken a detailed analysis of HST/WFPC2 and STIS imaging observations, and of supplementary wide-field ground-based observations obtained with the ESO New Technology Telescope (NTT) of two young (10-25 Myr) compact star clusters in the LMC, NGC 1805 and 1818. The ultimate goal of our work is to improve our understanding of the degree of primordial mass segregation in star clusters. This is crucial for the interpretation of observational luminosity functions (LFs) in terrns of the initial mass function (IMF), and for constraining the universality of the IMF. We present evidence for strong luminosity segregation in both clusters. The LF slopes steepen with cluster radius; in both NGC 1805 and 1818 the LF slopes reach a stable level well beyond the core of the clusters or half-light radii. In addition, the brightest cluster stars are strongly concentrated within the inner 4R h l . The global cluster LF, although strongly non-linear, is fairly well approximated by the core or half-light LF; the (annular) LFs at these radii are dominated by the segregated high-luminosity stars, however. We present tentative evidence for the presence of an excess number of bright stars surrounding NGC 1818, for which we argue that they are most probably massive stars that have been collisionally ejected from the cluster core. We therefore suggest that the cores of massive young stars clusters undergo significant dynamical evolution, even on time-scales as short as 25 Myr.

Unveiling the X-ray point source population of the Young Massive Cluster Westerlund 1 ,

Aims. We investigate the nature of the X-ray point source population within the Young Massive Cluster Westerlund 1. Methods. Chandra observations of 18 ks and 42 ks were used to determine the X-ray properties of emitters within Wd 1, while a comprehensive multiwavelength dataset was employed to constrain their nature. Results. We find X-ray emission from a multitude of different stellar sources within Wd 1, including both evolved high mass and low mass pre-MS stars. We attribute the X-ray emission from the high mass component to both single stars and colliding wind binaries on the basis of their observed flux and spectral properties, with binaries being systematically harder and more luminous than single stars. We are able to infer a high binary fraction for both WN (10/16) and WC stars (7/8), resulting in a combined Wolf Rayet binary fraction of >70%. These represent the most stringent limits currently placed on the binary fraction of very massive (>45 M� ) stars. We place the first observational constraints on X-ray emission from stars transitioning between the Main Sequence and Wolf Rayet phases, finding that both hot (B hypergiants) and cool (yellow hypergiants and red supergiants) spectral types appear to be intrinsically X-ray faint. The B[e] star W9 is found to be X-ray bright and shows similarities to both the X-ray binary SS433 and the Luminous Blue Variable η Carinae. Globally, we find the point source population to be systematically fainter than those found in younger massive star forming regions such as NGC 3603 and R136/30 Doradus, consistent with a loss of the most massive stars to SNe and a reduction in emissivity from the low mass pre-Main Sequence stars. No unambiguous evidence for X-ray emission due to accretion onto relativistic objects of any mass is found, although the current data do not exclude the presence of either a High Mass X-ray Binary or an Intermediate Mass Black Hole accreting at a low rate. Finally, we suggest the progenitor mass for the magnetar CXOU J164710.2-455216 is comparable to that of SGR 1806-20 (∼55 M� ), while that for SGR 1900+14 appears significantly lower (∼15 M� ), implying that magnetars may form from stars with a wide range of initial masses.

Exciting the magnetosphere of the magnetar CXOU J164710.2-455216 in Westerlund 1

We describe XMM-Newton observations taken 4.3 days prior to and 1.5 days subsequent to two remarkable events that were detected with Swift on 2006 September 21 from the candidate magnetar CXOU J164710.2-455216: (1) a 20 ms burst with an energy of 1e37 erg (15-150 keV), and (2) a rapid spin-down (glitch) with a fractionap period change of 1e-4. We find that the luminosity of the pulsar increased by a factor of 100 in the interval between observations, from 1e33 to 1e35 erg/s (0.5-8.0 keV), and that its spectrum hardened. The pulsed count rate increased by a factor of 10 (0.5-8.0 keV), but the fractional rms amplitude of the pulses decreased from 65 to 11 per cent, and their profile changed from being single-peaked to exhibiting three peaks. Similar changes have been observed from other magnetars in response to outbursts, such as that of 1E 2259+586 in 2002 June. We suggest that a plastic deformation of the neutron stars crust induced a very slight twist in the external magnetic field, which in turn generated currents in the magnetosphere that were the direct cause of the X-ray outburst.