Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Caroline Herschel's catalogue of nebulae
Not Available
| The Compact Circumstellar Material around OH 231.8+4.2
We have observed the bipolar post-asymptotic giant branch candidate OH231.8+4.2, using the mid-infrared interferometer MIDI and the infraredcamera with the adaptive optics system NACO on the Very Large Telescope.An unresolved core (<200 mas in FWHM) is found at the center of OH231.8+4.2 in the 3.8 μm image. This compact source is resolved withthe interferometer. We used two 8 m telescopes with four differentbaselines, which cover projected baseline lengths from 62 to 47 m, andprojected position angles from 112° to 131° that are almostperpendicular to the bipolar outflow. Fringes from 8 to 9 μm and from12 to 13.5 μm were clearly detected, while strong silicateself-absorption allows only marginal detection of visibilities between 9and 12 μm. The fringes from the four baselines consistently show thepresence of a compact circumstellar object with an inner radius of 30-40mas, which is equivalent to 40-50 AU at 1.3 kpc. This clearly shows thatthe mid-infrared compact source is not the central star (3 AU) butcircumstellar material. The measured size of the circumstellar materialis consistent with the size of such disks calculated by hydrodynamicmodels, implying that the circumstellar material may be in a diskconfiguration.Based on observations made with the VLT and the VLTI (programs065.L-0395, 072.D-0766, and 074.D-0405).
| Searching for Planetary Transits in Galactic Open Clusters: EXPLORE/OC
Open clusters potentially provide an ideal environment for the searchfor transiting extrasolar planets, since they feature a relatively largenumber of stars of the same known age and metallicity at the samedistance. With this motivation, over a dozen open clusters are now beingmonitored by four different groups. We review the motivations andchallenges for open cluster transit surveys for short-period giantplanets. Our photometric monitoring survey of Galactic southern openclusters, the Extrasolar Planet Occultation Research/Open Clusters(EXPLORE/OC) project, was designed with the goals of maximizing thechance of finding and characterizing planets and of providing astatistically valuable astrophysical result in the case of nodetections. We use the EXPLORE/OC data from two open clusters, NGC 2660and NGC 6208, to illustrate some of the largely unrecognized issuesfacing open cluster surveys, including severe contamination by Galacticfield stars (>80%) and the relatively low number of cluster membersfor which high-precision photometry can be obtained. We discuss how acareful selection of open cluster targets under a wide range of criteriasuch as cluster richness, observability, distance, and age can meet thechallenges, maximizing chances to detect planet transits. In addition,we present the EXPLORE/OC observing strategy to optimize planetdetection, which includes high-cadence observing and continuouslyobserving individual clusters rather than alternating between targets.
| Caroline Herschel as observer
Not Available
| Magnetically Driven Winds from Post-Asymptotic Giant Branch Stars: Solutions for High-Speed Winds and Extreme Collimation
This paper explores the effects of post-asymptotic giant branch (AGB)winds driven solely by magnetic pressure from the stellar surface. It isfound that winds can reach high speeds under this assumption and lead tothe formation of highly collimated proto-planetary nebulae. Bipolarknotty jets with periodic features and constant velocity are wellreproduced by the models. Several wind models with terminal velocitiesfrom a few tens of km s-1 up to 103 kms-1 are calculated, yielding outflows with linear momenta inthe range 1036-1040 g cm s-1, andkinetic energies in the range 1042-1047 ergs.These results are in accord with recent observations of proto-planetarynebulae that have pointed out serious energy and momentum deficits ifradiation pressure is considered as the only driver for these outflows.Our models strengthen the notion that the large mass loss rates ofpost-AGB stars, together with the short transition times from the lateAGB to the planetary nebula stage, could be directly linked with thegeneration of strong magnetic fields during this transition stage.
| Astrophysical parameters of Galactic open clusters
We present a catalogue of astrophysical data for 520 Galactic openclusters. These are the clusters for which at least three most probablemembers (18 on average) could be identified in the ASCC-2.5, a catalogueof stars based on the Tycho-2 observations from the Hipparcos mission.We applied homogeneous methods and algorithms to determine angular sizesof cluster cores and coronae, heliocentric distances, mean propermotions, mean radial velocities, and ages. For the first time we derivedistances for 200 clusters, radial velocities for 94 clusters, and agesof 196 clusters. This homogeneous new parameter set is compared withearlier determinations, where we find, in particular, that the angularsizes were systematically underestimated in the literature.
| Astrophysical supplements to the ASCC-2.5. II. Membership probabilities in 520 Galactic open cluster sky areas
We present a catalogue (CSOCA ) of stars residing in 520 Galactic opencluster sky areas which is the result of the kinematic (proper motion)and photometric member selection of stars listed in the homogeneousAll-sky Compiled Catalogue of 2.5 Million Stars (ASCC-2.5). We describethe structure and contents of the catalogue, the selection procedureapplied, and the proper motion and photometric membership constraintsadopted. In every cluster area the CSOCA contains the complete list ofthe ASCC-2.5 stars regardless of their membership probability. Forevery star the CSOCA includes accurate J2000 equatorial coordinates,proper motions in the Hipparcos system, BV photometric data in theJohnson system, proper motion and photometric membership probabilities,as well as angular distances from the cluster centers for about 166 000ASCC-2.5 stars. If available, trigonometric parallaxes, spectral types,multiplicity and variability flags from the ASCC-2.5, and radialvelocities with their errors from the Catalogue of Radial Velocities ofGalactic Stars with high precision Astrometric Data (CRVAD) are alsogiven.
| MHD Solutions for Proto-Planetary Nebulae
This paper provides solutions for the origin of post-AGB winds, theiracceleration up to high speed, and the subsequent formation of extremelycollimated proto-planetary nebulae. Several wind models with terminalvelocities from a few tens of voidb @xkms-1 up to 10^3 voidb @xkms-1are calculated, which produce proto-planetary nebulae with linearmomentum in the range 10^36 - 10^40 voidb @xgcms-1 and with kineticenergies in the range 10^42 - 10^47 erg. These results match availableobservations of proto-planetary nebulae. In the present simplisticscheme, the driver of the wind is just the magnetic pressure at thestellar surface. Other forces are not taken into account in this study,except gravity. We conclude that mass-loss rates of post-AGB stars andtransition times from late AGB up to planetary nebula central starscould be directly linked with the production of magnetic field at thestellar core. As an example, mass-loss rates as large as 8×10^-5M[ scriptstyle sun ]yr-1 and transition times as short as 5,000 yearsare predicted.
| Winds, Bubbles, ...but Magnetized: Solutions for High Speed Post-AGB Winds and Their Extreme Collimation
This paper provides solutions for the origin of post-AGB winds, theiracceleration up to high speed, and the subsequent formation of extremelycollimated proto-planetary nebulae. Several wind models with terminalvelocities from a few tens of km/s up to 10^3km/s are calculated, whichproduce proto-planetary nebulae with linear momenta in the range 10^36to 10^40gcm/s and with kinetic energies in the range 10^42 to 10^47 erg.These results match available observations of proto-planetary nebulae.In the present simplistic scheme, the driver of the wind is just themagnetic pressure at the stellar surface. Other forces are not takeninto account in this study, except gravity. We conclude that mass-lossrates of post-AGB stars and transition times from late AGB up toplanetary nebula central stars could be directly linked with theproduction of magnetic field at the stellar core. As an example,mass-loss rates as large as 8×10^-5 M[ sun ]/yr and transitiontimes as short as 5000 years are predicted.
| Proper Motions of Open Star Clusters and the Rotation Rate of the Galaxy
The mean proper motions of 167 Galactic open clusters withradial-velocity measurements are computed from the data of the Tycho-2catalog using kinematic and photometric cluster membership criteria. Theresulting catalog is compared to the results of other studies. The newproper motions are used to infer the Galactic rotation rate at the solarcircle, which is found to be ω0=+24.6±0.8 km s-1 kpc-1.Analysis of the dependence of the dispersion of ω0 estimates onheliocentric velocity showed that even the proper motions of clusterswith distances r>3 kpc contain enough useful information to be usedin kinematic studies demonstrating that the determination of propermotions is quite justified even for very distant clusters.
| On the Galactic Disk Metallicity Distribution from Open Clusters. I. New Catalogs and Abundance Gradient
We have compiled two new open cluster catalogs. In the first one, thereare 119 objects with ages, distances, and metallicities available, whilein the second one, 144 objects have both absolute proper motion andradial velocity data, of which 45 clusters also have metallicity dataavailable. Taking advantage of the large number of objects included inour sample, we present an iron radial gradient of about -0.063+/-0.008dex kpc-1 from the first sample, which is quite consistentwith the most recent determination of the oxygen gradient from nebulaeand young stars, about -0.07 dex kpc-1. By dividing clustersinto age groups, we show that the iron gradient was steeper in the past,which is consistent with the recent result from Galactic planetarynebulae data, and also consistent with inside-out galactic diskformation scenarios. Based on the cluster sample, we also discuss themetallicity distribution, cluster kinematics, and space distribution. Adisk age-metallicity relation could be implied by those properties,although we cannot give conclusive result from the age- metallicitydiagram based on the current sample. More observations are needed formetal-poor clusters. From the second catalog, we have calculated thevelocity components in cylindrical coordinates with respect to theGalactic standard of rest for 144 open clusters. The velocitydispersions of the older clusters are larger than those of youngclusters, but they are all much smaller than that of the Galactic thickdisk stars.
| Urban Astronomy: Observing the Messier Objects from the City
Not Available
| Radial velocities, binarity, and kinematic membership in open clusters with blue straggler candidates
Not Available
| Abundance Gradient from Open Clusters and Implications for the Galactic Disk Evolution
We compile a new sample of 89 open clusters with ages, distances andmetallicities available. We derive a radial iron gradient of about-0.099±0.008 dexkpc (unweighted) for the whole sample, which issomewhat greater than the most recent determination of oxygen gradientfrom nebulae and young stars. By dividing the clusters into age groups,we show that the iron gradient was steeper in the past and has evolvedslowly in time. Current data show a substantial scatter of the clustermetallicities indicating that the Galactic disk has undergone a veryrapid, inhomogeneous enrichment.Also, based on a simple, but quitesuccessful model of chemical evolution of the Milky Way disk, we make adetailed calculation of the iron abundance gradient and its timeevolution. The predicted current iron gradient is about -0.072 dexkpc.The model also predicts a steady flattening of the iron gradient withtime, which agrees with the result from our open cluster sample.
| The Massive Disk around OH 231.8+4.2
We have obtained 11.7 and 17.9 μm images at the Keck I telescope ofthe circumstellar dust emission from OH 231.8+4.2, an evolvedmass-losing red giant with a well-studied bipolar outflow. We detectboth a central unresolved point source with a diameter of less than 0.5"producing Fν(17.9μm)=60 Jy and emission extended morethan 1" away from the star, which is aligned with the bipolar outflowseen on larger scales. We find that the unresolved central source can beexplained by an opaque, flared disk with an outer radius of~5×1015 cm and an outer temperature of ~130 K. Onepossible model to explain this flaring is that the material in the diskis orbiting the central star and not simply undergoing a radialexpansion.
| HST observations of the protoplanetary nebula OH 231.8+4.2: The structure of the jets and shocks
We present high-resolution images obtained with the WFPC2, on board theHST, of the protoplanetary nebula (PPN) OH 231.8+4.2. Hα and NIIline emission and scattered light in the continuum at 6750 and 7910Å were observed. We also discuss NIR NICMOS images from the HSTarchive. The images show with high accuracy the shape and excitationstate of the shocks developed in the nebula. Our high-resolution images(and data from other works) allow a very detailed and quantitativedescription of the different nebular components and of the physicalconditions in them. We interpret specific structures identified in ourimages using existing models of shock interaction. In the center of thenebula, there is a dense torus- or disk-like condensation continued byan hourglass-like structure, with relatively high densities ( ~105-106 cm-3) and temperatures ( ~ 30K). Inside this torus we have identified the location of the centralstar, from SiO maser observations. Two shock regions are detected fromthe optical line emission images, respectively in the north and southlobes. In both regions, a forward and a backward shock are identified.The densities of this hot gas vary between 40 and 250 cm-3,with the densest clumps being placed in the reverse shocks. The totalmass of the shocked hot gas is ~ 2x10-3 Msun, bothlobes showing similar masses in spite of their different extents. Therelatively collimated jet that impinges on an originally slow shell, soproducing the shocks, is identified from the scattered light images andin CO maps. This flow is significantly denser and cooler than theshocked Hα regions. Its density decreases with the distance to thestar, with typical values ~ 105-104cm-3, and its temperature ranges between about 25 and 8 K. Weexplain the high Hα emission of the backward shock assuming thatit propagates in a diffuse gas component, entrained by the observedcollimated flow and sharing its axial movement. The existence of shocksalso in the collimated densest flow is suggested by the high abundanceof some molecules like HCO+ and its structure and kinematicsin certain regions, but they are not seen in Hα emission, probablybecause of the absence of (well developed) hot components in this denseflow. We think that the exceptionally detailed and quantitative imagederived for the wind interaction regions in OH 231.8+4.2 is a challengeto check and improve hydrodynamical models of wind interaction in PPNe.
| Proper motions of open clusters based on the TYCHO2 Catalogue. II. Clusters farther than 1 kpc
We determined the mean absolute proper motion of 94 open clusterssituated farther than 1 kpc from the Sun. The results are derived fromthe stellar proper motion data given in the Tycho2 Catalogue. The meanproper motion of the clusters and membership probability of individualstars were obtained from the proper motion data by applying thestatistical method proposed by Sanders (\cite{Sanders1971}). Themeasurements made use of a large number of stars, usually several tens,for each cluster. The total number of stars investigated in the fieldsof the clusters is 4864 of which 2021 were considered members. For 55clusters, this is the first determination of the proper motion. Based onobservations of the ESA Hipparcos satellite. Tables 1 to 95 are 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/388/168
| A study of spatial structure of galactic open star clusters
In order to study the relation between the core and corona in galacticstar clusters, the spatial structure of 38 rich open star clusters hasbeen studied using radial density profiles derived from the photometricdata of the Digital Sky Survey. The shape of the radial density profileindicates that the corona, most probably, is the outer region around thecluster. It can exist from the very beginning of the cluster formationand dynamical evolution is not the reason for its occurrence. The studydoes not find any relation between cluster size and age but indicatesthat the clusters with galacto-centric distances >9.5 kpc have largersizes. Further, we find that the average value of the core radius is1.3+/- 0.7 pc and that of annular width of the corona is 5.6+/- 1.9 pc,while average values of densities of cluster members in the core andcorona are 15.4+/- 9.9 star/pc2 and 1.6+/- 0.99star/pc2 respectively. Average field star contaminations inthe core and corona are ~ 35% and 80% respectively. In spite of smallerdensities in the coronal region, it contains ~ 75% of the clustermembers due to its larger area in comparison to the core region. Thisclearly demonstrates the importance of the coronal region in studiesdealing with the entire stellar contents of open star clusters as wellas their dynamical evolution. In contrast to the cluster cores, thestructure of coronal regions differs significantly from one cluster toother.
| The highly collimated bipolar outflow of OH 231.8+4.2
We present high spatial resolution observations of the CO molecularemission (J=1-0 and J=2-1 lines) in the post-AGB bipolar nebula OH231.8ler. High-quality NIR images (J, H, K' bands) of light scattered bygrains were also obtained. Our observations probe the bulk of thenebular material, providing maps with a resolution ~ 1 arcsec of themass distribution, both CO and NIR images being very closely coincident.The combination of the two 12CO lines has been used tomeasure the distribution of the kinetic temperature in the nebula, whichis found to be very low, ranging between 8 K, in the outer southernclumps, and 35 K, in the central region. A relative temperature increaseis found in the northernmost condensation, probably associated to astrong bow-like shock. Since velocities are also measured in CO, thedynamic parameters (kinetic momentum and energy) are also measured withhigh resolution. Most of the nebular mass ( ~ 0.64 Msun) islocated in the central condensation and flows at expansion velocities<=40 km s-1. The rest of the gas, ~ 0.3 Msunalmost equally distributed in the two lobes, flows along the nebularaxis at high velocities, that increase proportionally to the distance tothe central star reaching values as large as 430 km s-1, as aresult of a sudden acceleration happened about 770 yr ago. The generalmass distribution in OH 231.8ler\ is found to be clumpy and veryelongated, with a length/width ratio reaching a factor 20 in thesouthern tail. In the center, however, we find a double hollow-lobestructure, similar to those found in other well studied protoplanetarynebulae. We stress the enormous kinetic linear momentum carried by themolecular nebula, about 27 Msun,km s-1 (5.5 x1039 g cm s-1). The kinetic energy is also veryhigh, ~ 1700 Msun,km2 s-4 ~ 3.4 x1046 erg. Given the short time during which the accelerationof the molecular outflow took place, we conclude that the linearmomentum carried by the stellar photons is about a factor 100 smallerthan that carried by the outflow, even if the effects of multiplescattering are taken into account. We independently argue that radiationpressure directly acting onto grains (the mechanism thought to beresponsible for the mass ejection in AGB envelopes) cannot explain theobserved bipolar flow, since this would produce a significant shiftbetween the dust and gas features that is not observed. Finally, wereview the uncertain nature and evolutionary status of this uniqueobject. Based on observations carried out with the IRAM Plateau de BureInterferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany)and IGN (Spain).
| Homogeneous Photometry for Star Clusters and Resolved Galaxies. II. Photometric Standard Stars
Stars appearing in CCD images obtained over 224 nights during the courseof 69 observing runs have been calibrated to the Johnson/Kron-CousinsBVRI photometric system defined by the equatorial standards of Landolt(1992, AJ, 104, 340). More than 15,000 stars suitable for use asphotometric standards have been identified, where ``suitable'' meansthat the star has been observed five or more times during photometricconditions and has a standard error of the mean magnitude less than 0.02mag in at least two of the four bandpasses, and shows no significantevidence of intrinsic variability. Many of these stars are in the samefields as Landolt's equatorial standards or Graham's (1982, PASP, 94,244) southern E-region standards but are considerably fainter. Thisenhances the value of those fields for the calibration of photometryobtained with large telescopes. Other standards have been defined infields containing popular objects of astrophysical interest, such asstar clusters and famous galaxies, extending Landolt-system calibratorsto declinations far from the equator and to stars of subsolar chemicalabundances. I intend to continue to improve and enlarge this set ofphotometric standard stars as more observing runs are reduced. The fullcurrent database of photometric indices is being made freely availablevia a site on the World Wide Web or via direct request to the author.Although the contents of the database will evolve in detail, at anygiven time it should represent the largest sample of precise BVRIbroadband photometric standards available anywhere.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Astrophysics in 1997
Martian marvels, a gamma-ray burster with a redshift, Galileo converseswith Ganymede, a record galactic redshift of 4.92, and much else. Fiscal1997 was definitely an exciting year for astronomers. We have tried hardto hit all the obvious highlights, but also to report more gradualprogress on traditional problems of understanding planets, stars,galaxies, and the universe. Though the year was saddened by the loss ofmany valued colleagues, we nevertheless indulge in occasionalsoupçons of frivolity.
| Search for star clustering: methodology and application to the Two Micron Galactic Survey
A new approach to the study of the large-scale stellar clusterdistribution in the Galaxy based on two-point correlation techniques ispresented. The basic formalism for this method is outlined, and itsapplications are then investigated by the use of a simple model ofcluster distribution in the Galaxy. This provides an estimate of thepotentials of the two-point correlation function for indicatingclustering in the measured star positions, which can be related to thepresence of star clusters in the observed field. This technique is thenapplied to several areas of the Two Micron Galactic Survey catalogue,from which information is obtained on the distribution of clustersaccording to position in the Galaxy, as well as about age, density ofstars, etc.
| Parameters of open star clusters from uvby-beta photometry.
Not Available
| Some Revised Observational Constraints on the Formation and Evolution of the Galactic Disk
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114.2556T&db_key=AST
| A Search for Variables at Goethe Link Observatory
The 25-cm Cooke astrograph and blink comparator at Indiana University'sGoethe Link Observatory are being used to conduct a successfulphotographic search for new variable stars. Some difficulties inrecognizing stellar variability from photographic images are discussed.
| Study of the planetary nebula NGC 2438. I. Spectroscopy of the nebula and of some cluster stars
We found 36 emission lines in the spectrum of the planetary nebula NGC2438, and we determined its exc. class to be about 6-7. The nebularRV_hel = 60.3+/- 3.6 km/s (5 spectra) is in agreement with RV_hel = 60.8+/- 4.0 km/s of 4 cluster stars (10 spectra). We conclude that contraryto earlier statements the nebula is probably associated with thecluster.
| From Guerrilla Astronomer to Department Chair
Not Available
| Chemical Evolution of the Galactic Disk: Evidence for a Gradient Perpendicular to the Galactic Plane
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....110.2813P&db_key=AST
| Catalogue of blue stragglers in open clusters.
An extensive survey of blue straggler candidates in galactic openclusters of both hemispheres is presented. The blue stragglers wereselected considering their positions in the cluster colour-magnitudediagrams.They were categorized according to the accuracy of thephotometric measurements and membership probabilities. An amount of 959blue straggler candidates in 390 open clusters of all ages wereidentified and classified. A set of basic data is given for everycluster and blue straggler. The information is arranged in the form of acatalogue. Blue stragglers are found in clusters of all ages. Thepercentage of clusters with blue stragglers generally grows with age andrichness of the clusters. The mean ratio of the number of bluestragglers to the number of cluster main sequence stars is approximatelyconstant up to a cluster age of about 10^8.6^ yr and rises for olderclusters. In general, the blue stragglers show a remarkable degree ofcentral concentration.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
Cataloghi e designazioni:
|
