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|A new method for ISOCAM data reduction - I. Application to the European Large Area ISO Survey Southern Field: method and results|
We develop a new data reduction technique for ISOCAM LW data and applyit to the European Large Area ISO Survey (ELAIS) LW3 (15-μm)observations in the southern hemisphere (S1). This method, known as LARItechnique and based on the assumption of the existence of two differenttime-scales in ISOCAM transients (accounting for either fast or slowdetector response), is particularly designed for the detection of faintsources. In the ELAIS S1 field we obtain a catalogue of 462 15-μmsources with signal-to-noise ratios >=5 and flux densities in therange 0.45-150mJy (filling the whole flux range between the Deep ISOCAMSurveys and the IRAS Faint Source Survey). The completeness at differentflux levels and the photometric accuracy of this catalogue are testedwith simulations. Here we present a detailed description of the methodand discuss the results obtained by its application to the S1 LW3 data.
|Rotation and activity in the solar-type stars of NGC 2547|
We present high-resolution spectroscopy of a sample of 24 solar-typestars in the young (15-40Myr), open cluster NGC 2547. We use our spectrato confirm cluster membership in 23 of these stars, to determineprojected equatorial velocities and chromospheric activity, and tosearch for the presence of accretion discs. We find examples of bothfast (vesini>50kms-1) and slow(vesini<10kms-1) rotators, but no evidence foractive accretion in any of the sample. The distribution of projectedrotation velocities is indistinguishable from the slightly older IC 2391and IC 2602 clusters, implying similar initial angular momentumdistributions and circumstellar disc lifetimes. The presence of veryslow rotators indicates either that long (10-40Myr) disc lifetimes orinternal differential rotation are needed, or that NGC 2547 (and IC2391/2602) were born with more slowly rotating stars than are presentlyseen in even younger clusters and associations. The solar-type stars inNGC 2547 follow a similar rotation-activity relationship to that seen inolder clusters. X-ray activity increases until a saturation level isreached for vesini>15-20kms-1. We are unable toexplain why this saturation level, oflog(LxLbol)~=-3.3, is a factor of 2 lower than inother clusters, but rule out anomalously slow rotation rates oruncertainties in X-ray flux calculations.
|A Revised and Extended Catalog of Magellanic System Clusters, Associations, and Emission Nebulae. II. The Large Magellanic Cloud|
A survey of extended objects in the Large Magellanic Cloud was carriedout on the ESO/SERC R and J Sky Survey Atlases, checking entries inprevious catalogs and searching for new objects. The census provided6659 objects including star clusters, emission-free associations, andobjects related to emission nebulae. Each of these classes containsthree subclasses with intermediate properties, which are used to infertotal populations. The survey includes cross identifications amongcatalogs, and we present 3246 new objects. We provide accuratepositions, classification, and homogeneous measurements of sizes andposition angles, as well as information on cluster pairs andhierarchical relation for superimposed objects. This unification andenlargement of catalogs is important for future searches of fainter andsmaller new objects. We discuss the angular and size distributions ofthe objects of the different classes. The angular distributions show twooff-centered systems with different inclinations, suggesting that theLMC disk is warped. The present catalog together with its previouscounterpart for the SMC and the inter-Cloud region provide a totalpopulation of 7847 extended objects in the Magellanic System. Theangular distribution of the ensemble reveals important clues on theinteraction between the LMC and SMC.
|X-ray emission and low-mass stars in the young open cluster NGC 2547|
We present the results of deep ROSAT High Resolution Imager X-rayobservations and broad-band BVI_C CCD photometry of the young opencluster NGC 2547. We have been able to find cluster counterparts for themajority of the 102 significant X-ray sources detected, confirming thepower of high spatial resolution X-ray observations to find low-mass,magnetically active members of distant open clusters. The age of thecluster has been determined by fitting the main-sequence turn-off andthe positions of low-mass stars on their pre-main-sequence tracks. Thetwo methods yield 55+/-25 and 14+/-4Myr respectively, but we considerthe lower age to be more reliable because of the comparatively largenumber of low-mass stars that constrain the fit. We deduce that any agespread among the low-mass stars is <10Myr and any small spread thatexists could be attributable to binarity and starspots. The distributionof X-ray activity levels in the solar-type stars of NGC 2547 liesconsiderably above our threshold of sensitivity, and it is probable thatour X-ray-selected sample is complete at these masses. For lower massstars our sample is likely to be incomplete. Comparison with initialmass functions indicates there are still of order 100, low-activity Mdwarfs yet to be found. When X-ray activity is gauged in terms of X-rayto bolometric flux ratio, L_x/L_bol, we find that there are G stars inthe older (age 52Myr) alpha Per cluster that are less active than theirleast active counterparts in NGC 2547. This is consistent with thecurrent rotation-activity paradigm if there is modest angular momentumloss between 14 and 52Myr. There are no G or early K stars in NGC 2547that reach the saturated level L_x/L_bol=10^-3, seen in older clusters,whereas saturated late K and M stars are observed. From this, we deducethat there are no fast-rotating G and early K stars in NGC 2547 withequatorial velocities >20 kms^-1. If this is confirmed then, evenwith no angular momentum loss, the fastest rotating NGC 2547 stars couldnot evolve into the fastest rotating stars in the alpha Per cluster.These results cast doubt on the assumption that rotation rates andmagnetic activity seen in one cluster are representative of similarstars at the same age. We hypothesize that the solar-type stars of NGC2547 either still possess, or have recently lost, circumstellaraccretion discs which regulate their angular momentum. This wouldrequire longer disc lifetimes than commonly measured or assumed forother young stars, and we speculate on possible reasons for this.
|Integrated UBV Photometry of 624 Star Clusters and Associations in the Large Magellanic Cloud|
We present a catalog of integrated UBV photometry of 504 star clustersand 120 stellar associations in the LMC, part of them still embedded inemitting gas. We study age groups in terms of equivalent SWB typesderived from the (U-B) X (B-V) diagram. The size of the spatialdistributions increases steadily with age (SWB types), whereas adifference of axial ratio exists between the groups younger than 30 Myrand those older, which implies a nearly face-on orientation for theformer and a tilt of ~45^deg^ for the latter groups. Asymmetries arepresent in the spatial distributions, which, together with thenoncoincidence of the centroids for different age groups, suggest thatthe LMC disk was severely perturbed in the past.
|The cluster system of the Large Magellanic Cloud|
A new catalog of clusters in the Large Magellanic Cloud has beenconstructed from searches of the IIIa-J component of the ESO/SERCSouthern Sky Atlas. The catalog contains coordinate and diametermeasurements of 1762 clusters in a 25 deg x 25 deg area of sky centeredon the LMC, but excluding the very crowded 3.5 sq deg region around theBar. The distribution of these clusters appears as two superimposedelliptical systems. The higher density inner system extends over about 8deg; the lower density outer system can be represented by a 13 deg x 10deg disk inclined at 42 deg to the line of sight. There are suggestionsof two weak 'arms' in the latter.
|A Catalogue of Clusters in The LMC|
|A catalogue of clusters in the outer parts of the Large Magellanic Cloud|
Abstract image available at:http://adsabs.harvard.edu/abs/1963MNRAS.127...31L
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