Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Tracing mixing in stars: new beryllium observations of the open clusters NGC 2516, Hyades, and M 67
Context: Determinations of beryllium abundance in stars, together withlithium, provide a key tool to investigate the so far poorly understoodextra-mixing processes at work in stellar interiors. Aims: We measuredBe in three open clusters, complementing existing Be surveys, and aimingat gathering a more complete empirical scenario of the evolution of Beas a function of stellar age and temperature. Methods: We analyzedVLT/UVES spectra of members of NGC 2516, theHyades, and M 67 to determinetheir Be and Li abundances. In the first two clusters we focused onstars cooler than 5400 K, while the M 67 sample includes stars warmerthan 6150 K, as well as two subgiants and two blue stragglers. We alsocomputed the evolution of Be for a 0.9 Mȯ star based onstandard evolutionary models. Results: We find different behavioursfor stars in different temperature bins and ages. Stars warmer than 6150K show Be depletion and follow a Be vs. Li correlation, while Be isundepleted in stars in the 6150{-}5600 K range. NGC 2516 members coolerthan 5400 K have not depleted any Be, while older Hyades of similartemperature show some depletion. Be is severely depleted in thesubgiants and blue stragglers. Conclusions: The results for warm starsare in agreement with those of previous studies, supporting thehypothesis that mixing in this temperature regime is driven by rotation.The same holds for the two subgiants that have evolved from the“Li gap”. This mechanism is instead not the dominant one forsolar-type stars. We show that Be depletion of cool Hyades cannot simplybe explained by the effect of increasing depth of the convective zone.Finally, the different Be content of the two blue stragglers suggeststhat they have formed by two different processes (i.e., collisions vs.binary merging).Based on observations collected at ESO-VLT, Paranal Observatory, Chile,Programme numbers 65.L-0427, 68.D-0491, 69.D-0454.
| The Monitor project: rotation of low-mass stars in the open cluster NGC2516
We report on the results of an i-band time-series photometric survey ofNGC2516 using the Cerro Tololo Inter-American Observatory (CTIO) 4-mBlanco telescope and 8k Mosaic-II detector, achieving better than 1 percent photometric precision per data point over 15 <~ i <~ 19.Candidate cluster members were selected from a V versus V - Icolour-magnitude diagram over 16 < V < 26 (covering masses from0.7Msolar down to below the brown dwarf limit), finding 1685candidates, of which we expect ~1000 to be real cluster members, takinginto account contamination from the field (which is most severe at theextremes of our mass range). Searching for periodic variations in thesegave 362 detections over the mass range 0.15 <~ M/Msolar<~ 0.7. The rotation period distributions were found to show aremarkable morphology as a function of mass, with the fastest rotatorsbounded by P > 0.25d, and the slowest rotators for M <~0.5Msolar bounded by a line of P ~ M3, with thosefor M >~ 0.5Msolar following a flatter relation closer toP ~ constant. Models of the rotational evolution were investigated,finding that the evolution of the fastest rotators was well reproducedby a conventional solid body model with a mass-dependent saturationvelocity, whereas core-envelope decoupling was needed to reproduce theevolution of the slowest rotators. None of our models were able tosimultaneously reproduce the behaviour of both populations.
| On the common origin of the AB Doradus moving group and the Pleiades cluster
AB Doradus (AB Dor) is the nearest identified moving group. As withother such groups, the age is important for understanding of several keyquestions. It is important, for example, in establishing the origin ofthe group and also in comparative studies of the properties of planetarysystems, eventually surrounding some of the AB Dor group members, withthose existing in other groups. For AB Dor two rather differentestimates for its age have been proposed: the first one, of the order of50 Myr, by Zuckerman and coworkers from a comparison with theTucana/Horologium moving group and a second one of about 100-125 Myr byLuhman and coworkers from colour-magnitude diagrams. Using this lastvalue and the closeness in velocity space of AB Dor and the Pleiadesgalactic cluster, Luhman and coworkers suggested coevality for thesesystems. Because strictly speaking such a closeness does not stillguarantee coevality, here we address this problem by computing andcomparing the full 3D orbits of AB Dor, Pleiades, α Persei and IC2602. The latter two open clusters have estimated ages of about 85-90and 50 Myr. The resulting age 119 +/- 20 Myr is consistent with AB Dorand Pleiades being coeval. Our solution and the scenario of open clusterformation proposed by Kroupa and collaborators suggest that the AB Dormoving group may be identified with the expanding subpopulation (GroupI) present in this scenario. We also discuss other related aspects asiron and lithium abundances, eventual stellar mass segregation duringthe formation of the systems and possible fraction of debris discs inthe AB Dor group.
| Unraveling the Origins of Nearby Young Stars
A systematic search for close conjunctions and clusterings in the pastof nearby stars younger than the Pleiades is undertaken, which mayreveal the time, location, and mechanism of formation of these oftenisolated, disconnected from clusters and star-forming regions, objects.The sample under investigation includes 101 T Tauri, post-TT, andmain-sequence stars and stellar systems with signs of youth, culled fromthe literature. Their Galactic orbits are traced back in time and nearapproaches are evaluated in time, distance, and relative velocity.Numerous clustering events are detected, providing clues to the originof very young, isolated stars. Each star's orbit is also matched withthose of nearby young open clusters, OB and TT associations andstar-forming molecular clouds, including the Ophiuchus, Lupus, CoronaAustralis, and Chamaeleon regions. Ejection of young stars from openclusters is ruled out for nearly all investigated objects, but thenearest OB associations in Scorpius-Centaurus, and especially, the denseclouds in Ophiuchus and Corona Australis have likely played a major rolein the generation of the local streams (TWA, Beta Pic, andTucana-Horologium) that happen to be close to the Sun today. The core ofthe Tucana-Horologium association probably originated from the vicinityof the Upper Scorpius association 28 Myr ago. A few proposed members ofthe AB Dor moving group were in conjunction with the coeval Cepheus OB6association 38 Myr ago.
| The Monitor project: searching for occultations in young open clusters
The Monitor project is a photometric monitoring survey of nine young(1-200Myr) clusters in the solar neighbourhood to search for eclipses byvery low mass stars and brown dwarfs and for planetary transits in thelight curves of cluster members. It began in the autumn of 2004 and usesseveral 2- to 4-m telescopes worldwide. We aim to calibrate the relationbetween age, mass, radius and where possible luminosity, from the Kdwarf to the planet regime, in an age range where constraints onevolutionary models are currently very scarce. Any detection of anexoplanet in one of our youngest targets (<~10Myr) would also provideimportant constraints on planet formation and migration time-scales andtheir relation to protoplanetary disc lifetimes. Finally, we will usethe light curves of cluster members to study rotation and flaring inlow-mass pre-main-sequence stars.The present paper details the motivation, science goals and observingstrategy of the survey. We present a method to estimate the sensitivityand number of detections expected in each cluster, using a simplesemi-analytic approach which takes into account the characteristics ofthe cluster and photometric observations, using (tunable) best-guessassumptions for the incidence and parameter distribution of putativecompanions, and we incorporate the limits imposed by radial velocityfollow-up from medium and large telescopes. We use these calculations toshow that the survey as a whole can be expected to detect over 100 younglow and very low mass eclipsing binaries, and ~3 transiting planets withradial velocity signatures detectable with currently availablefacilities.
| New catalogue of blue stragglers in open clusters
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/789
| The Distances to Open Clusters from Main-Sequence Fitting. III. Improved Accuracy with Empirically Calibrated Isochrones
We continue our series of papers on open cluster distances with acritical assessment of the accuracy of main-sequence fitting usingisochrones that employ empirical corrections to the color-temperaturerelations. We use four nearby open clusters with multicolor photometryand accurate metallicities and present a new metallicity for Praesepe([Fe/H]=+0.11+/-0.03) from high-resolution spectra. The internalprecision of distance estimates is about a factor of 5 better than thecase without the color calibrations. After taking into account all majorsystematic errors, we obtain distances accurate to about 2%-3% whenthere exists a good metallicity estimate. Metallicities accurate tobetter than 0.1 dex may be obtained from BVICKsphotometry alone. We also derive a helium abundance for the Pleiades ofY=0.279+/-0.015, which is equal within the errors to the Sun's initialhelium abundance and that of the Hyades. Our best estimates of distancesare (m-M)0=6.33+/-0.04, 8.03+/-0.04, and 9.61+/-0.03 toPraesepe, NGC 2516, and M67, respectively. Our Pleiades distance at thespectroscopic metallicity,(m-M)0=5.66+/-0.01(internal)+/-0.05(systematic), is inexcellent agreement with several geometric distance measurements. Wehave made calibrated isochrones for -0.3<=[Fe/H]<=+0.2 availableonline.
| Spitzer 24 μm Observations of Open Cluster IC 2391 and Debris Disk Evolution of FGK Stars
We present 24 μm Spitzer MIPS photometric observations of the ~50 Myropen cluster IC 2391. Thirty-four cluster members ranging in spectraltype from B3 to M5 were observed in the central square degree of thecluster. Excesses indicative of debris disks were discovered around oneA star, six FGK stars, and possibly one M dwarf. For the cluster membersobserved to their photospheric limit, we find a debris disk frequency of10+17-3% for B-A stars and31+13-9% for FGK stars using a 15% relative excessthreshold. Relative to a model of decaying excess frequency, thefrequency of debris disks around A-type stars appears marginally low forthe cluster's age while that of FGK stars appears consistent. Scenariosthat may qualitatively explain this result are examined. We concludethat planetesimal activity in the terrestrial region of FGK stars iscommon in the first ~50 Myr and decays on timescales of ~100 Myr.Despite luminosity differences, debris disk evolution does not appear todepend strongly on stellar mass.
| A Photometric Search for Planets in the Open Cluster NGC 7086
In an attempt to discover short-period, Jupiter-mass planets orbitingsolar-type stars in open clusters, we searched for planetary transits inthe populous and relatively unstudied open cluster NGC 7086. Acolor-magnitude diagram constructed from new B and V photometry ispresented, along with revised estimates of the cluster's color excess,distance modulus, and age. Several turnoff stars were observedspectroscopically in order to determine a color excess ofE(B-V)=0.83+/-0.02. Empirically fitting the main sequences of two youngopen clusters and the semiempirical zero-age main sequence of Vandenbergand Poll yielded a distance modulus of (V-MV)=13.4+/-0.3 mag.This corresponds to a true distance modulus of (m-M)0=10.8mag or a distance of 1.5 kpc to NGC 7086. These values were used withisochrones from the Padova group to obtain a cluster age of 100 Myr.Eleven nights of R-band photometry were used to search for planetarytransits. Differential magnitudes were constructed for each star in thecluster. Light curves for each star were produced on a night-to-nightbasis and inspected for variability. No planetary transits wereapparent; however, some interesting variable stars were discovered: apulsating variable that appears to be a member of the γ Dor classand four possible eclipsing binary stars, one of which actually may be amultiple system.
| New constrains on Gliese 86 B. VLT near infrared coronographic imaging survey of planetary hosts
Aims.We present the results of multi epoch imaging observations of thecompanion to the planetary host Gliese 86. Associated with radialvelocity measurements, this study aimed at dynamically characterizingwith the orbital properties and the mass of this companion (hereafterGliese 86 B), but also at investigating the possible history of thisparticular system. Methods: .We used the adaptive opticsinstrument NACO at the ESO Very Large Telescope to obtain deepcoronographic imaging to obtain new photometric and astrometricmeasurements of Gliese 86 B. Results: .Part of the orbit isresolved. The photometry of Gl 86 B indicates colorscompatible with a ~70 Jupiter mass brown dwarf or a white dwarf. Bothtypes of objects fit the available, still limited astrometric data. Ifwe attribute the long term radial velocity residual drift observed forGl 86 A to B, then the mass of the latter object is≃0.5 M_ȯ. We analyse both astrometric and radial velocity datato propose first orbital parameters for Gl 86 B.Assuming Gl 86 B is a ≃0.5 M_ȯ white dwarf,we explore the constraints induced by this hypothesis and refine theparameters of the system.
| On the origin of the ultramassive white dwarf GD50
We argue on the basis of astrometric and spectroscopic data that theultramassive white dwarf GD50 is associated with the star formationevent that created the Pleiades and is possibly a former member of thiscluster. Its cooling age (~60 Myr) is consistent with it having evolvedessentially as a single star from a progenitor with a mass M >6Msolar, so we find no need to invoke a white dwarf-whitedwarf binary merger scenario to account for its existence. This resultmay represent the first direct observational evidence that single-starevolution can produce white dwarfs with M > 1.1Msolar, aspredicted by some stellar evolutionary theories. On the basis of itstangential velocity, we also provisionally identify the ultramassive (M~ 1.2Msolar) white dwarf PG0136 + 251 as being related to thePleiades. These findings may help to alleviate the difficulties inreconciling the observed number of hot nearby ultramassive white dwarfswith the smaller number predicted by binary evolution models under theassumption that they are the products of white dwarf mergers.Based on observations collected at the European Southern Observatory,Chile. ESO No. 072.D-0362.E-mail: pdd@star.le.ac.uk
| Spitzer Observations of the Orion OB1 Association: Second-Generation Dust Disks at 5-10 Myr
We report new Spitzer observations of intermediate-mass stars in tworegions of the Orion OB1 association located in the subassociations OB1a(~10 Myr) and OB1b (~5 Myr). In a representative sample of stars earlierthan F5 of both stellar groups, we find a population of stars surroundedof debris disks, without excess in the IRAC bands and without emissionlines in their optical spectra, but with a varying degree of 24 μmexcess. Comparing our samples with 24 μm observations ofintermediate-mass stars in other stellar groups, spanning a range ofages from 2.5 to 150 Myr, we find that debris disks are more frequentand have larger 24 μm excess at 10 Myr (OB1a). This trend agrees withpredictions of models of evolution of solids in the outer regions ofdisks (>30 AU), where large icy objects (~1000 km) begin to form at~10 Myr; the presence of these objects in the disk initiates acollisional cascade, producing enough dust particles to explain therelatively large 24 μm excess observed in OB1a. The dust luminosityobserved in the stellar groups older than 10 Myr declines roughly aspredicted by collisional cascade models. Combining Spitzer observations,optical spectra, and 2MASS data, we found a new Herbig Ae/Be star (HD290543) and a star (HD 36444) with a large 24 μm excess, both inOB1b. This last object could be explained as a intermediate stagebetween HAeBe and true debris systems or as a massive debris diskproduced by a collision between two large objects (>1000 km).
| On the current status of open-cluster parameters
We aim to characterize the current status of knowledge on the accuracyof open-cluster parameters such as the age, reddening and distance.These astrophysical quantities are often used to study the globalcharacteristics of the Milky Way down to the very local stellarphenomena. In general, the errors of these quantities are neglected orset to some kind of heuristic standard value. We attempt to give somerealistic estimates for the accuracy of available cluster parameters byusing the independently derived values published in the literature. Intotal, 6437 individual estimates for 395 open clusters were used in ourstatistical analysis. We discuss the error sources depending ontheoretical as well as observational methods and compare our resultswith those parameters listed in the widely used catalogue by Dias et al.In addition, we establish a list of 72 open clusters with the mostaccurate known parameters which should serve as a standard table in thefuture for testing isochrones and stellar models.
| X-Ray and Infrared Point Source Identification and Characteristics in the Embedded, Massive Star-Forming Region RCW 38
We report on results of a 96.7 ks Chandra observation of one of theyoungest, most embedded, and most massive young stellar clusters studiedin X-rays: RCW 38. We detect 460 sources in the field, of which 360 areconfirmed to be associated with the RCW 38 cluster. The cluster membersrange in luminosity from 1030 to 1033.5 ergss-1. Over 10% of the cluster members with over 100 countsexhibit flares, while about 15% of the cluster members with over 30counts are variable. Of the sources identified as cluster members, 160have near-infrared (NIR) counterparts either in the Two Micron All SkySurvey database or detected via Very Large Telescope observations. Ofthese, about 20% appear to have optically thick disks. An additional 353members are identified through NIR observations, of which at least 50%possess optically thick disks. We fit over 100 X-ray sources as absorbedRaymond-Smith-type plasmas and find that the column to the clustermembers varies from 1021.5 to 1023cm-2. We compare the gas to dust absorption signatures inthese stars and findNH=AV(2×1021) cm-2. Wefind that the cluster contains 31 candidate OB stars and is centeredabout 10" (0.1 pc) west of the primary source of the ionization, the O5star IRS 2. The cluster has a peak central density of about 400 X-raysources pc-2. We estimate that the total cluster membershipexceeds 2000 stars.
| X-ray variability of NGC 2516 stars in the XMM-Newton observations
We present the characteristics of the X-ray variability of stars in thecluster NGC 2516 as derived from XMM-Newton/EPIC/pn data. The X-rayvariations on short (hours), medium (months), and long (years) timescales have been explored. We detected 303 distinct X-ray sources byanalysing six EPIC/pn observations; 194 of them are members of thecluster. Stars of all spectral types, from the early-types to the late-Mdwarfs, were detected. The Kolmogorov-Smirnov test applied to the X-rayphoton time series shows that, on short time scales, only a relativelysmall fraction (ranging from 6% to 31% for dG and dF, respectively) ofthe members of NGC 2516 are variable with a confidence level ≥99%;however, it is possible that the fraction is small only because of thepoor statistics. The time X-ray amplitude distribution functions (XAD)of a set of dF7-dK2 stars, derived on short (hours) and medium (months)time scales, seem to suggest that medium-term variations, if present,have a much smaller amplitude than those on short time scales; a similarresult is also obtained for dK3-dM stars. The amplitude variations oflate-type stars in NGC 2516 are consistent with those of the coevalPleiades stars. Comparing these data with those of ROSAT/PSPC, collected7-8 years earlier, and of ROSAT/HRI, just 4-5 years earlier, we find noevidence of significant variability on the related time scales,suggesting that long-term variations due to activity cycles similar tothe solar cycle are not common among young stars. Indications ofspectral variability was found in one star whose spectra at three epochswere available.
| The Monitor project: rotation of low-mass stars in the open cluster M34
We report on the results of a V- and i-band time-series photometricsurvey of M34 (NGC 1039) using the Wide Field Camera (WFC) on the IsaacNewton Telescope (INT), achieving better than 1 per cent precision perdata point for 13 <~ i <~ 17. Candidate cluster members wereselected from a V versus V - I colour-magnitude diagram over 14 < V< 24 (0.12 <~ M/Msolar <~ 1.0), finding 714candidates, of which we expect ~400 to be real cluster members (takinginto account contamination from the field). The mass function wascomputed, and found to be consistent with a lognormal distribution indN/d logM. Searching for periodic variable objects in the candidatemembers gave 105 detections over the mass range 0.25 ~ 5d), consistent with the work ofother authors at very low masses. Our results are interpreted in thecontext of previous work, finding that we reproduce the same generalfeatures in the rotational period distributions. A number of rapidrotators were found with velocities ~ a factor of 2 lower than in thePleiades, consistent with models of angular momentum evolution assumingsolid body rotation without needing to invoke core-envelope decoupling.
| HD 65949: the highest known mercury excess of any CP star?
ESO spectra of HD 65949 show it to be unlike any of the well-known typeswithin its temperature range ≈13 600 K. It is neither a silicon, nora mercury-manganese star, though it has a huge Hg II line atλ3984. We estimate log (Hg/H) + 12.0 ≈ 7.4. This is higherthan any published stellar mercury abundance. HD 65949 is a member of anearby open cluster, NGC 2516, which is only slightly older than thePleiades, and has been of recent interest because of its numerous X-rayemission stars, including HD 65949 itself, or a close companion. Alongitudinal magnetic field of the order of -290 Gauss at the 4.7σlevel was very recently diagnosed from accurate circularspectropolarimetric observations with FORS 1 at the VLT. The spectrallines are sharp, allowing a thorough identification study. Secondspectra of Ti, Cr, and Fe are rich. Mn II is well identified but notunusually strong. Numerous lines of S II and P II are found, but not GaII. The resonance lines of Sr II are strong. While many Y II lines areidentified, and Nb II is very likely present, no Zr II lines were found.Xe II is well identified. Strong absorptions from the third spectra ofthe lanthanides Pr, Nd, and Ho are present, but lines from the secondspectra of lanthanides are extremely weak or absent. Among lines fromthe heavier elements, those of Pt II are clearly present, and theheaviest isotope, 198Pt, is indicated. The uncommon spectrumof Re II is certain, while Os II and Te II are highly probable. Severalof the noted anomalies are unusual for a star as hot as HD 65949.
| On the difference between nuclear and contraction ages
Context: .Ages derived from low mass stars still contracting onto themain sequence often differ from ages derived from the high mass onesthat have already evolved away from it. Aims: .We investigate thegeneral claim of disagreement between these two independent agedeterminations by presenting UBVRI photometry for the young galacticopen clusters NGC 2232, NGC 2516, NGC 2547 and NGC 4755, spanning theage range ~10-150 Myr Methods: .We derived reddenings, distances,and nuclear ages by fitting ZAMS and isochrones to color-magnitudes andcolor-color diagrams. To derive contraction ages, we used four differentpre-main sequence models, with an empirically calibratedcolor-temperature relation to match the Pleiades cluster sequence.Results: .When exclusively using the V vs. V-I color-magnitude diagramand empirically calibrated isochrones, there is consistency betweennuclear and contraction ages for the studied clusters. Although thecontraction ages seem systematically underestimated, in none of thecases do they deviate by more than one standard deviation from thenuclear ages.
| New Praesepe white dwarfs and the initial mass-final mass relation
We report the spectroscopic confirmation of four further white dwarfmembers of Praesepe. This brings the total number of confirmed whitedwarf members to 11, making this the second largest collection of theseobjects in an open cluster identified to date. This number is consistentwith the high-mass end of the initial mass function of Praesepe beingSalpeter in form. Furthermore, it suggests that the bulk of Praesepewhite dwarfs did not gain a substantial recoil kick velocity frompossible asymmetries in their loss of mass during the asymptotic giantbranch phase of evolution. By comparing our estimates of the effectivetemperatures and the surface gravities of WD0833+194, WD0840+190,WD0840+205 and WD0843+184 to modern theoretical evolutionary tracks, wehave derived their masses to be in the range 0.72-0.76 Msolarand their cooling ages ~300 Myr. For an assumed cluster age of 625 +/-50 Myr, the inferred progenitor masses are between 3.3 and 3.5Msolar. Examining these new data in the context of theinitial mass-final mass relation, we find that it can be adequatelyrepresented by a linear function (a0 = 0.289 +/-0.051,a1 = 0.133 +/- 0.015) over the initial mass range 2.7-6Msolar. Assuming an extrapolation of this relation to largerinitial masses is valid and adopting a maximum white dwarf mass of 1.3Msolar, our results support a minimum mass for core-collapsesupernovae progenitors in the range ~6.8-8.6 Msolar.
| Kinematics of the Open Cluster System in the Galaxy
Absolute proper motions and radial velocities of 202 open clusters inthe solar neighborhood, which can be used as tracers of the Galacticdisk, are used to investigate the kinematics of the Galaxy in the solarvicinity, including the mean heliocentric velocity components(u1,u2,u3) of the open cluster system,the characteristic velocity dispersions(σ1,σ2,σ3), Oortconstants (A,B) and the large-scale radial motion parameters (C,D) ofthe Galaxy. The results derived from the observational data of propermotions and radial velocities of a subgroup of 117 thin disk young openclusters by means of a maximum likelihood algorithm are:(u1,u2,u3) =(-16.1+/-1.0,-7.9+/-1.4,-10.4+/-1.5) km s-1,(σ1,σ2,σ3) =(17.0+/-0.7,12.2+/-0.9,8.0+/-1.3) km s-1,(A,B) =(14.8+/-1.0,-13.0+/-2.7) km s-1 kpc-1, and (C,D) =(1.5+/-0.7,-1.2+/-1.5) km s-1 k pc-1. A discussionon the results and comparisons with what was obtained by other authorsis given.
| Evolution of magnetic fields in stars across the upper main sequence: I. Catalogue of magnetic field measurements with FORS 1 at the VLT
To properly understand the physics of Ap and Bp stars it is particularlyimportant to identify the origin of their magnetic fields. For that, anaccurate knowledge of the evolutionary state of stars that have ameasured magnetic field is an important diagnostic. Previous resultsbased on a small and possibly biased sample suggest that thedistribution of magnetic stars with mass below 3 M_ȯ in the H-Rdiagram differs from that of normal stars in the same mass range (Hubriget al. 2000). In contrast, higher mass magnetic Bp stars may well occupythe whole main-sequence width (Hubrig, Schöller & North 2005b).In order to rediscuss the evolutionary state of upper main sequencemagnetic stars, we define a larger and bias-free sample of Ap and Bpstars with accurate Hipparcos parallaxes and reliably determinedlongitudinal magnetic fields. We used FORS 1 at the VLT in itsspectropolarimetric mode to measure the magnetic field in chemicallypeculiar stars where it was unknown or poorly known as yet. In thisfirst paper we present our results of the mean longitudinal magneticfield measurements in 136 stars. Our sample consists of 105 Ap and Bpstars, two PGa stars, 17 HgMn stars, three normal stars, and nine SPBstars. A magnetic field was for the first time detected in 57 Ap and Bpstars, in four HgMn stars, one PGa star, one normal B-type star and fourSPB stars.
| Deep X-ray survey of the young open cluster NGC 2516 with XMM-Newton
Aims.We report a deep X-ray survey of the young (~140 Myr), rich opencluster NGC 2516 obtained with the EPIC camera on board the XMM-Newtonsatellite. Methods: .By combining data from six observations, ahigh sensitivity, greater than a factor of 5 with respect to recentChandra observations, has been achieved. Kaplan-Meier estimators of thecumulative X-ray luminosity distribution are built, statisticallycorrected for non members contaminants and compared to those of thenearly coeval Pleiades. The EPIC spectra of the X-ray brightest starsare fitted using optically thin model plasma with one or two thermalcomponents. Results: .We detected 431 X-ray sources and 234 ofthem have as optical counterparts cluster stars spanning the entire NGC2516 Main Sequence. On the basis of X-ray emission and opticalphotometry, we indicate 20 new candidate members of the cluster; at thesame time we find 49 X-ray sources without known optical or infraredcounterpart. The X-ray luminosities of cluster stars span the range logLX (erg s-1) = 28.4-30.8. The representativetemperatures span the 0.3-0.6 keV (3.5-8 MK) range for the coolcomponent and 1.0-2.0 keV (12-23 MK) for the hot one; similar values arefound in other young open clusters like the Pleiades, IC 2391, andBlanco 1. While no significant differences are found in X-ray spectra,NGC 2516 solar type stars are definitely less luminous in X-rays thanthe nearly coeval Pleiades. The comparison with a previous ROSAT surveyreveals the lack of variability amplitudes larger than a factor of 2 insolar type stars in a 11 yr time scale of the cluster and thusactivity cycles like in the Sun are probably absent or have a differentperiod and amplitude in young stars.
| Searching for links between magnetic fields and stellar evolution. I. A survey of magnetic fields in open cluster A- and B-type stars with FORS1
Context: .About 5% of upper main sequence stars are permeated by astrong magnetic field, the origin of which is still matter of debate. Aims: . With this work we provide observational material to studyhow magnetic fields change with the evolution of stars on the mainsequence, and to constrain theory explaining the presence of magneticfields in A and B-type stars. Methods: . Using FORS1 inspectropolarimetric mode at the ESO VLT, we have carried out a survey ofmagnetic fields in early-type stars belonging to open clusters andassociations of various ages. Results: . We have measured themagnetic field of 235 early-type stars with a typical uncertainty of 100 G. In our sample, 97 stars are Ap or Bp stars. For thesetargets, the median error bar of our field measurements was 80 G.A field has been detected in about 41 of these stars, 37 of which werenot previously known as magnetic stars. For the 138 normal A and B-typestars, the median error bar was 136 G, and no field was detected in anyof them.
| An XMM-Newton observation of the young open cluster NGC 2547: coronal activity at 30 Myr
We report on XMM-Newton observations of the young open cluster NGC 2547which allow us to characterize coronal activity in solar-type stars, andstars of lower mass, at an age of 30Myr. X-ray emission is seen fromstars at all spectral types, peaking among G stars at luminosities(0.3-3keV) of Lx~= 1030.5ergs-1 anddeclining to Lx<= 1029.0ergs-1 amongM stars with masses >=0.2Msolar. Coronal spectra showevidence for multi-temperature differential emission measures and lowcoronal metal abundances of Z~= 0.3. The G- and K-type stars of NGC 2547follow the same relationship between X-ray activity and Rossby numberestablished in older clusters and field stars, although most of thesolar-type stars in NGC 2547 exhibit saturated or even supersaturatedX-ray activity levels. The median levels of Lx andLx/Lbol in the solar-type stars of NGC 2547 arevery similar to those in T-Tauri stars of the Orion Nebula cluster(ONC), but an order of magnitude higher than in the older Pleiades. Thespread in X-ray activity levels among solar-type stars in NGC 2547 ismuch smaller than in older or younger clusters.Coronal temperatures increase with Lx,Lx/Lbol and surface X-ray flux. The most activesolar-type stars in NGC 2547 have coronal temperatures intermediatebetween those in the ONC and the most active older zero-agemain-sequence (ZAMS) stars. We show that simple scaling argumentspredict higher coronal temperature in coronally saturated stars withlower gravities. A number of candidate flares were identified among thelow-mass members and a flaring rate [for total flare energies (0.3-3keV)> 1034erg] of one every350+350-120ks was found for solar-type stars,which is similar to rates found in the ONC and Pleiades. Comparison withROSAT High Resolution Imager (HRI) data taken 7yr earlier reveals thatonly 10-15 per cent of solar-type stars or stars with Lx >3 × 1029ergs-1 exhibit X-ray variability bymore than a factor of 2. This is comparable with clusters of similar agebut less than in both older and younger clusters. The similar medianlevels of X-ray activity and rate of occurrence for large flares in NGC2547 and the ONC demonstrate that the X-ray radiation environment aroundyoung solar-type stars remains relatively constant over their first30Myr.
| Exploring the lower mass function in the young open cluster IC 4665
We present a study of the young (30-100 Myr) open cluster IC 4665 withthe aim to determine the shape of the mass function well into the browndwarf regime. We photometrically select 691 low-mass stellar and 94brown dwarf candidate members over an area of 3.82 square degreescentred on the cluster. K-band follow-up photometry and Two-MicronAll-Sky Survey data allow a first filtering of contaminant objects fromour catalogues. A second filtering is performed for the brightest starsusing proper motion data provided by the Tycho-2 and UCAC2 publiccatalogues. Contamination by the field population for the lowest massobjects is estimated using same latitude control fields. We fit observedsurface densities of various cluster populations with King profiles andfind a consistent tidal radius of 1.0°. The presence of possiblemass segregation is discussed. In most respects investigated, IC 4665 issimilar to other young open clusters at this age: (1) a power law fit tothe mass function between 1 and 0.04 Mȯ results in bestfit for a slope of -0.6; (2) a cusp in the mass function is noticed atabout the substellar boundary with respect to the power law description,the interpretation of which is discussed; (3) a fraction between 10-19%for BDs with M 0.03 Mȯ to total members; (4) abest-fit lognormal function to the full mass distribution shows anaverage member mass of 0.32 Mȯ, if IC 4665 has an age of50 Myr.
| Proper motion determination of open clusters based on the UCAC2 catalogue
We present the kinematics of hundreds of open clusters, based on theUCAC2 Catalogue positions and proper motions. Membership probabilitieswere obtained for the stars in the cluster fields by applying astatistical method uses stellar proper motions. All open clusters withknown distance were investigated, and for 75 clusters this is the firstdetermination of the mean proper motion. The results, including the DSSimages of the cluster's fields with the kinematic members marked, areincorporated in the Open Clusters Catalogue supported on line by ourgroup.
| Spectral synthesis analysis and radial velocity study of the northern F-, G- and K-type flare stars
In this paper, we present a study of the general physical and chemicalproperties and radial velocity monitoring of young active stars. Wederive temperatures, logg, [Fe/H], v sini and Rspec valuesfor eight stars. The detailed analysis reveals that the stars are nothomogeneous in their principal physical parameters or in the agedistribution. In 4/5, we found a periodic radial velocity signal whichoriginates in surface features; the fifth is surprisingly inactive andshows little variation.
| Monitor: transiting planets and brown dwarfs in star forming regions and young open clusters
The Monitor project is a large scale photometric monitoring survey often star forming regions and open clusters aged between 1 and 200 Myrusing wide-field optical cameras on 2-4 m telescopes worldwide. Theprimary goal of the project is to search for close-in planets and browndwarfs at young ages through the detection of transit events. Suchdetections would provide unprecedented constraints on planet formationand migration time-scales, as well as on evolutionary models of planetsand brown dwarfs in an age range where such constraints are very scarce.Additional science goals include rotation period measurements and theanalysis of flares and accretion-related variability.Workshop ``Ultralow-mass star formation and evolution'', see AN 326, No. 10www.ast.cam.ac.uk/suz/monitor/monitor.php
| Kinematics of the Gould belt based on open clusters.
Not Available
| The lower mass function of young open clusters: clues to (sub)stellar formation
We report the present day mass functions (PDMFs) of 4 young openclusters over a mass range from 30 Jupiter masses to3Mȯ. Three of these clusters have been chosen to have asimilar age of ˜100 Myr. Their PDMFs are remarkably similar andare comparable to the field mass function. This suggests little impactof initial conditions (stellar density, metallicity) on the massdistribution and raises some issues concerning the currently debatedstar and brown dwarf formation theories. The fourth cluster is older(600 Myr) which allow us to investigate the effect of the clusterdynamical evolution on the shape of the mass function.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
Cataloghi e designazioni:
|
