Søren S. Larsen

Preoperative Staging of Lung Cancer with Combined PET–CT

BACKGROUND Fast and accurate staging is essential for choosing treatment for non-small-cell lung cancer (NSCLC). The purpose of this randomized study was to evaluate the clinical effect of combined positron-emission tomography and computed tomography (PET-CT) on preoperative staging of NSCLC. METHODS We randomly assigned patients who were referred for preoperative staging of NSCLC to either conventional staging plus PET-CT or conventional staging alone. Patients were followed until death or for at least 12 months. The primary end point was the number of futile thoracotomies, defined as any one of the following: a thoracotomy with the finding of pathologically confirmed mediastinal lymph-node involvement (stage IIIA [N2]), stage IIIB or stage IV disease, or a benign lung lesion; an exploratory thoracotomy; or a thoracotomy in a patient who had recurrent disease or death from any cause within 1 year after randomization. RESULTS From January 2002 through February 2007, we randomly assigned 98 patients to the PET-CT group and 91 to the conventional-staging group. Mediastinoscopy was performed in 94% of the patients. After PET-CT, 38 patients were classified as having inoperable NSCLC, and after conventional staging, 18 patients were classified thus. Sixty patients in the PET-CT group and 73 in the conventional-staging group underwent thoracotomy (P=0.004). Among these thoracotomies, 21 in the PET-CT group and 38 in the conventional-staging group were futile (P=0.05). The number of justified thoracotomies and survival were similar in the two groups. CONCLUSIONS The use of PET-CT for preoperative staging of NSCLC reduced both the total number of thoracotomies and the number of futile thoracotomies but did not affect overall mortality. (ClinicalTrials.gov number, NCT00867412.)

Hubble Space Telescope Observations of Star Clusters in NGC 1023: Evidence for Three Cluster Populations?*

Using Hubble Space Telescope images we have carried out a study of cluster populations in the nearby S0 galaxy NGC 1023. In two WFPC2 pointings we have identified 221 cluster candidates. The small distance (~9 Mpc) combined with deep F555W and F814W images allows us to reach about 2 mag below the expected turnover of the globular cluster luminosity function. NGC 1023 appears to contain at least three identifiable cluster populations: the brighter clusters show a clearly bimodal color distribution with peaks at (V-I)0 = 0.92 and at (V-I)0 = 1.15, and in addition there are a number of fainter, more extended objects with predominantly red colors. Among the brighter clusters, we find that the blue clusters have somewhat larger sizes than the red ones with mean effective radii of Re ~ 2 and Re ~ 1.7 pc, respectively. These clusters have luminosity functions (LFs) and sizes consistent with what is observed for globular clusters in other galaxies. Fitting Gaussians to the LFs of the blue and red compact clusters, we find turnover magnitudes of MTO(blue) = -7.58 and MTO(red) = -7.37 in V and dispersions of σV(blue) = 1.12 and σV(red) = 0.97. The fainter, more extended clusters have effective radii up to Re ~ 10–15 pc, and their LF appears to rise at least down to MV ~ -6, few of them being brighter than MV = -7. We suggest that these fainter objects may have a formation history distinct from that of the brighter GCs.

Structure and Mass of a Young Globular Cluster in NGC 6946

Using the Wide Field Planetary Camera 2 on board the Hubble Space Telescope, we have imaged a luminous young star cluster in the nearby spiral galaxy NGC?6946. Within a radius of 65 pc, the cluster has an absolute visual magnitude, MV = -13.2, comparable to the most luminous young super star clusters in the Antennae merger galaxy. UBV colors indicate an age of about 15 Myr. The cluster has a compact core (radius ~1.3 pc) surrounded by an extended envelope with a power-law luminosity profile. The outer parts of the cluster profile gradually merge with the general field, making it difficult to measure a precise half-light radius Re, but we estimate Re ~ 13 pc. Combined with population synthesis models, the luminosity and age of the cluster imply a mass of 8.2 ? 105 M? for a Salpeter initial mass function (IMF) extending down to 0.1 M?. If the IMF is lognormal below 0.4 M?, then the mass decreases to 5.5 ? 105 M?. Depending on model assumptions, the central density of the cluster is between 5.3 ? 103 and 1.7 ? 104 M? pc-3, comparable to other high-density star-forming regions. We also estimate a dynamical mass for the cluster using high-dispersion spectra from the HIRES spectrograph on the Keck I telescope. The HIRES data indicate a velocity dispersion of 10.0 ? 2.7 km s-1 and imply a total cluster mass within 65 pc of ? 106 M?. Comparing the dynamical mass with the mass estimates based on the photometry and population synthesis models, we find that the mass-to-light ratio is at least as high as for a Salpeter IMF extending down to 0.1 M?, although a turnover in the IMF at 0.4 M? is still possible within the ~1 ? errors. The cluster will presumably remain bound, evolving into a globular cluster-like object.

Chemical composition and constraints on mass loss for globular clusters in dwarf galaxies: WLM and IKN

Aims. We determine the metallicities of globular clusters (GCs) in the WLM and IKN dwarf galaxies, using VLT/UVES and Keck/ESI spectroscopy. These measurements are combined with literature data for field stars to constrain GC formation scenarios. For the WLM GC, we also measure detailed abundance ratios for a number of light, , Fe-peak, and n-capture elements, which are compared with literature data for the Fornax dSph and the Milky Way. Methods. The abundances are derived by computing synthetic integrated-light model spectra and adjusting the input composition until the best fits to the observed spectra are obtained. Results. We find low metallicities of [Fe=H] = 2:0 and 2:1 for the WLM GC and the GC IKN-5, respectively. We estimate that 17%‐31% of the stars with [Fe=H] 2 in WLM belong to the GC, and IKN-5 may even contain a similar number of metal-poor stars as the whole of the IKN dwarf itself. While these fractions are much higher than in the Milky Way halo, we have previously found a similarly high ratio of metal-poor GCs to field stars in the Fornax dSph. The overall abundance patterns in the WLM GC are similar to those observed for GCs in the Fornax dSph: the [Ca/Fe] and [Ti/Fe] ratios are super-Solar at about +0:3 dex, while [Mg/Fe] is less elevated than [Ca/Fe] and [Ti/Fe]. The [Na/Fe] ratio is similar to the averaged [Na/Fe] ratios in Milky Way GCs, but higher (by 2 ) than those of Milky Way halo stars. Iron-peak (Mn, Sc, Cr) and heavy elements (Ba, Y, La) generally follow the trends seen in the Milky Way halo. Conclusions. The GCs in the WLM and IKN dwarf galaxies resemble those in the Fornax dSph by being significantly more metalpoor than a typical halo GC in the Milky Way and other large galaxies. They are also substantially more metal-poor than the bulk of the field stars in their parent galaxies. It appears that only a small fraction of the Milky Way GC system could have been accreted from galaxies similar to these dwarfs. The relatively high Na abundance in the WLM GC suggests that the [Na/O] anti-correlation is present in this cluster, while the high ratios of metal-poor GCs to field stars in the dwarfs are in tension with GC formation scenarios that require GCs to have lost a very large fraction of their initial mass.

Evidence for an Intermediate-Age, Metal-rich Population of Globular Clusters in NGC 4365*

We present spectroscopy for globular clusters (GCs) in the elliptical galaxy NGC 4365, obtained with the Low-Resolution Imaging Spectrograph on the Keck I telescope. Previous studies have shown that the optical color distribution of GCs in NGC 4365 lacks the bimodal structure that is common in globular cluster systems, showing only a single broad peak. Measurements of Balmer line indices (Hβ, Hγ, and Hδ) on the GC spectra support recent suggestions by Puzia et al. on the basis of optical and near-infrared photometry that some of the clusters in NGC 4365 are intermediate-age (2-5 Gyr) and metal-rich (-0.4 [Z/H] 0) rather than old (~10-15 Gyr) and metal-poor. We also find some genuinely metal-poor, old clusters, suggesting that the ages and metallicities of the two populations conspire to produce the single broad distribution observed in optical colors.

Resolved photometry of extragalactic young massive star clusters

Aims. We present colour–magnitude diagrams (CMDs) of young massive star clusters in several galaxies located well beyond the Local Group. The richness of these clusters allows us to obtain large samples of post-main sequence stars and test how well the observed CMDs are reproduced by canonical stellar isochrones. Methods. We use imaging of seven clusters in the galaxies NGC 1313, NGC 1569, NGC 1705, NGC 5236 and NGC 7793 obtained with the Advanced Camera for Surveys on board the Hubble Space Telescope and carry out PSF-fitting photometry of individual stars in the clusters. The clusters have ages in the range ∼(5−50) × 10 6 years and masses of ∼10 5 M� –10 6 M� . Although crowding prevents us from obtaining photometry in the inner regions of the clusters, we are still able to measure up to 30–100 supergiant stars in each of the richest clusters. The resulting CMDs and luminosity functions are compared with photometry of artificially generated clusters, designed to reproduce the photometric errors and completeness as realistically as possible. Results. In agreement with previous studies, our CMDs show no clear gap between the H-burning main sequence and the Heburning supergiant stars, contrary to predictions by common stellar isochrones. In general, the isochrones also fail to match the observed number ratios of red-to-blue supergiant stars, although the difficulty of separating blue supergiants from the main sequence complicates this comparison. In several cases we observe a large spread (1–2 mag) in the luminosities of the supergiant stars that cannot be accounted for by observational errors. We find that this spread can be reproduced by including an age spread of ∼(10−30) × 10 6 years in the models. However, age spreads cannot fully account for the observed morphology of the CMDs and other processes, such as the evolution of interacting binary stars, may also play a role. Conclusions. Colour–magnitude diagrams can be successfully obtained for massive star clusters out to distances of at least 4–5 Mpc. Comparing such CMDs with models based on canonical isochrones we find several areas of disagreement. One interesting possibility is that an age spread of up to ∼30 Myr may be present in some clusters. The data presented here may provide useful constraints on models for single and/or binary stellar evolution.

Hubble Space Telescope Imaging of a Peculiar Stellar Complex in NGC 6946

The stellar populations in a stellar complex in NGC 6946 are analyzed on images taken with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. The complex is peculiar because of its very high density of stars and clusters and semicircular shape. Its physical dimensions are about the same as for the local Gould Belt, but the stellar density is 1-2 orders of magnitude higher. In addition to an extremely luminous, ~15 Myr old cluster discussed in an earlier paper, accounting for about 17% of the integrated V-band light, we identify 18 stellar clusters within the complex with luminosities similar to the brightest open clusters in the Milky Way. The color-magnitude diagram of individual stars in the complex shows a paucity of red supergiants compared to model predictions in the 10-20 Myr age range for a uniform star formation rate. We thus find tentative evidence for a gap in the dispersed star formation history, with a concentration of star formation into a young globular cluster during this gap. Confirmation of this result must, however, await a better understanding of the late evolution of stars in the corresponding mass range (12 M☉). A reddening map based on individual reddenings for 373 early-type stars is presented, showing significant variations in the absorption across the complex. These may be responsible for some of the arclike structures previously identified on ground-based images. We finally discuss various formation scenarios for the complex and the star clusters within it.

The spatially resolved stellar populations of isolated early-type galaxies

ABSTRACT We present radial stellar population parameters for a subsample of 12 galaxies from the36 isolated early-type galaxies of Reda et al. Using new long-slit spectra, central valuesand radial gradients for the stellar age, metallicity [Z/H] and α-element abundance[E/Fe] are measured. Similarly, the central stellar population parameters are derivedfor a further 5 isolated early-type galaxies using their Lick indices from the literature.On average, the seventeen isolated galaxies have mean central [Z/H] o and [E/Fe] o of0.29 ±0.03 and 0.17 ±0.03 respectively and span a wide range of ages from 1.7 to15 Gyrs. We find that isolated galaxies follow similar scaling relations between centralstellar population parameters and galaxy velocity dispersion to their counterparts inhigh density environments. However, we note a tendency for isolated galaxies to haveslightly younger ages, higher metallicities and lower abundance ratios. Such propertiesare qualitatively consistent with the expectation of an extended star formation historyfor galaxies in lower density environments. Generally we measure constant age and[E/Fe] radial gradients. However, three galaxies show remarkable positive age gradientsand two galaxies have negative age gradients. We find that the age gradients anti-correlatewith the centralgalaxyage.Thus asa youngstarburstevolves,the age gradientflattens from positive to almost zero. Metallicity gradients range from near zero tostrongly negative. For our high mass galaxies (σ > 160 km/s) metallicity gradientsare shallower with increasing mass. Such behaviour is not predicted in dissipationalcollapse models but might be expected in multiple mergers. The metallicity gradientsare also found to be correlated with the central age and metallicity, as well as to the agegradients. In conclusion, our stellar population data for a sample of isolated early-typegalaxies are more compatible with an extended merger/accretion history than earlydissipative collapse.Key words: Galaxies: elliptical and lenticular, cD - galaxies: abundances - galaxies:formation - galaxies: evolution - galaxies: kinematic and dynamics

Structural Parameters and Dynamical Masses for Globular Clusters in M33

Using high-dispersion spectra from the HIRES echelle spectrograph on the Keck I telescope, we measure velocity dispersions for four globular clusters (GCs) in M33. Combining the velocity dispersions with integrated photometry and structural parameters derived from King-Michie model fits to Wide Field Planetary Camera 2 images, we obtain mass-to-light ratios for the clusters. The mean value is M/LV = 1.53 ± 0.18, very similar to the M/LV of Milky Way and M31 GCs. The M33 clusters also fit very well into the fundamental plane and binding energy–luminosity relations derived for Milky Way GCs. Dynamically and structurally, the four M33 clusters studied here appear virtually identical to Milky Way and M31 GCs.

A Young Globular Cluster in the Galaxy NGC 6946

A globular cluster D15 Myr old that contains D 5 ) 105 of stars inside an D11 pc radius has M _ been found in the nearby spiral galaxy NGC 6946, surrounded by clouds of dust and smaller young clusters, inside a giant circular bubble 300 pc in radius. At the edge of the bubble is an arc of regularly spaced clusters that could have been triggered during the bubbles formation. The region is at the end of a spiral arm, suggesting an origin in the asymmetric collapse of spiral-arm gas. This globular cluster is one of the nearest examples of a cluster that is similar to the massive old globular clusters in the Milky Way. We consider the energetics of the bubble and possible formation mechanisms for the globular cluster, including the coalescence of smaller clusters. Subject headings: stars: formationISM: bubblesglobular clusters: general During a recent search for massive young clusters in the nearby spiral galaxy NGC 6946, Larsen & Richtler (1999) found a circular bubble containing numerous small clusters and a bright compact cluster that resembled a young globu- lar cluster. Some of the small clusters in the bubble are organized into arclike shapes. This feature was found and sketched —rst by Hodge (1967) as the only positive detection in his search for multiple-arc structures similar to that in the Constellation III region of the LMC. The diUuse object inside the bubble was noted later by Efremov (1999), who suggested that it was a cluster causally related to the arcs in the same way that the LMC massive cluster NGC 1978