An XMM-Newton observation of the Lagoon Nebula and the very young open cluster NGC 6530
We report the results of an XMM-Newton observation of the Lagoon Nebula(M 8). Our EPIC images of this region reveal a cluster of point sources,most of which have optical counterparts inside the very young opencluster NGC 6530. The bulk of these X-ray sources are probablyassociated with low and intermediate mass pre-main sequence stars. Oneof the sources experienced a flare-like increase of its X-ray fluxmaking it the second brightest source in M 8 after the O4 star 9 Sgr.The X-ray spectra of most of the brightest sources can be fitted withthermal plasma models with temperatures of kT ~ a few keV. Only a few ofthe X-ray selected PMS candidates are known to display Hα emissionand were previously classified as classical T Tauri stars. This suggeststhat most of the X-ray emitting PMS stars in NGC 6530 are weak-line TTauri stars. In addition to 9 Sgr, our EPIC field of view contains alsoa few early-type stars. The X-ray emission from HD 164816 is found to betypical for an O9.5 III-IV star. At least one of the known Herbig Bestars in NGC 6530 (LkHα 115) exhibits a relatively strong X-rayemission, while most of the main sequence stars of spectral type B1 andlater are not detected. We also detect (probably) diffuse X-ray emissionfrom the Hourglass Region that might reveal a hot bubble blown by thestellar wind of Herschel 36, the ionizing star of the Hourglass Region.Based on observations with XMM-Newton, an ESA Science Mission withinstruments and contributions directly funded by ESA Member states andthe USA (NASA).
|
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
|
UBVRI and Hα Photometry of the Young Open Cluster NGC 6530
New UBVRI and Hα photometry has been obtained for the young opencluster NGC 6530 located 10' east of the Lagoon Nebula (M8). We found 37pre-main-sequence (PMS) stars with Hα emission and nine PMScandidates using the R-Hα color, a measure of Hα emission.From this new photometry we derived the cluster parameters(V0-MV=11.25+/-0.1 mag and =0.35mag), typical age (1.5 Myr), and an age spread of about 5 Myr. Weconfirmed the presence of a small amount of differential reddeningacross the cluster. We also verified the abnormal reddening law for thecluster, especially for a heavily embedded O7 V star, Herschel 36.Comparisons were made for several PMS evolution models with regard tothe resulting age distribution and the initial mass function.
|
A multiwavelength study of star formation in the very young open cluster NGC 6530
The distance and formation history of the very young open cluster NGC6530 were investigated by studying a total of 132 probable member starsof this open cluster. It was concluded that the distance to NGC 6530 is1.8 +/- 0.2 kpc, and its interstellar reddening E(B-V) is 0\fm30.Furthermore, we conclude that the extinction law of the intraclustermaterial is normal (i.e. R_V = 3.1), but anomalous extinction laws werefound for several more embedded stars in the cluster. Of the 132 starsincluded in this study, 11 are suspected to be variable, five showHα in emission and nine show an infrared excess. Among the memberstars of NGC 6530, three were found to be part of the Herbig Ae/Bestellar class, whereas two others are possible members of this stellargroup as well. Also, one cluster member is probably a new massivepost-AGB star, whereas the same could possibly apply to another memberstar. Finally, from the distribution of post- and pre-main sequencestars in the cluster's HR-diagram, it was concluded that the process ofstar formation in NGC 6530 must have started a few times 10^7 years agoand, for the less massive stars, is probably still going on today. Basedon observations collected at the European Southern Observatory, LaSilla, Chile, on observations collected at Cerro Tololo Inter-AmericanObservatory, and on spectral data obtained with the InternationalUltraviolet Explorer. Tables 1, 2 and 5 are available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/Abstract.html
|
The Tokyo PMC catalog 90-93: Catalog of positions of 6649 stars observed in 1990 through 1993 with Tokyo photoelectric meridian circle
The sixth annual catalog of the Tokyo Photoelectric Meridian Circle(PMC) is presented for 6649 stars which were observed at least two timesin January 1990 through March 1993. The mean positions of the starsobserved are given in the catalog at the corresponding mean epochs ofobservations of individual stars. The coordinates of the catalog arebased on the FK5 system, and referred to the equinox and equator ofJ2000.0. The mean local deviations of the observed positions from theFK5 catalog positions are constructed for the basic FK5 stars to comparewith those of the Tokyo PMC Catalog 89 and preliminary Hipparcos resultsof H30.
|
Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
|
The Henry Draper Extension Charts: A catalogue of accurate positions, proper motions, magnitudes and spectral types of 86933 stars
The Henry Draper Extension Charts (HDEC), published in the form offinding charts, provide spectral classification for some 87000 starsmostly between 10th and 11th magnitude. This data, being highlyvaluable, as yet was practically unusable for modern computer-basedastronomy. An earlier pilot project (Roeser et al. 1991) demonstrated apossibility to convert this into a star catalogue, using measurements ofcartesian coordinates of stars on the charts and positions of theAstrographic Catalogue (AC) for subsequent identification. We presenthere a final HDEC catalogue comprising accurate positions, propermotions, magnitudes and spectral classes for 86933 stars of the HenryDraper Extension Charts.
|
An IUE survey of interstellar H I LY alpha absorption. 1: Column densities
We measure Galactic interstellar neutral hydrogen column densities byanalyzing archival interstellar Ly alpha absorption line data toward 554B2 and hotter stars observed at high resolution with the IUE satellite.This study more than doubles the number of lines of sight with measuresof N(H I) based on Ly alpha. We have included the scattered lightbackground correction algorithm of Bianchi and Bohlin in our datareduction. We use the correlation between the Balmer discontinuity(c1) index and the stellar Ly alpha absorption in order toassess the effects of stellar Ly alpha contamination. Approximately 40%of the B stars with measured (c1) index, exhibit seriousstellar Ly alpha contamination. One table contains the derived values ofthe interstellar N(H I) for 393 stars with at most small amounts ofstellar contamination. Another lists the observed values of total N(H I)for 161 stars with suspected stellar Ly alpha contamination and/oruncertain stellar parameters.
|
Ultraviolet gas absorption and dust extinction toward M8
Interstellar absorption lines are analyzed using high-resolution IUEspectra of 11 stars in the young cluster NGC 6530 located in the M8region. High-velocity clouds at -35 km/s and -60 km/s are seen towardall cluster stars. The components arise in gases that are part of largeinterstellar bubbles centered on the cluster and driven by stellar windsof the most luminous members. Absorption lines of species of differentionization states are separated in velocity. The velocity stratificationis best explained as a 'champagne' flow of ionized gas away from thecluster. The C IV/Si IV ratios toward the hotter cluster members areconsistent with simple photoionization models if the gas-phase C/Siratio is increased by preferential accretion onto dust grains. High ioncolumn densities in the central cluster decline with distance from W93,suggesting that radiation from a hot source near W93 has photoionizedgas in the central cluster.
|
Getting to the bottom of the lagoon - Dust, magnetism, and star formation
A V-band polarimetric survey of stars associated with the Lagoon Nebula(M8; NGC 6523) and the embedded cluster NGC 6530 has been conducted. Thedata have been combined with existing photometric and spectroscopicobservations in order to study the dust in the immediate vicinity and toinvestigate the alignment of magnetic field lines with identifiablesymmetry axes. Using the cluster method, it has been possible toseparate multiple stars from single stars and thereby derive acccuratevalues for the ratio of total to selective extinction and the distancemodulus. The value of R for the associated dust is 4.6 + or - 0.3.Anomalous reddening is a characteristic of the region, not just Herschel36. Electric vectors show a high degree of alignment perpendicular tothe direction of the extension of the large-scale distributions ofionized gas and massive stars. The overall collapse of molecular gasappears to have been asymmetrical, proceeding most rapidly along adirection parallel to magnetic field lines. A lot of the dust mixed withthe ionized gas has been destroyed, so the large-scale uniformity ofreddening laws and polarization position angles indicates that much ofthe dust responsible for extinction and polarization is associated withneutral gas.
|
Chemically peculiar stars in open clusters. I - The catalog
The largest existing compilation is presented of Ap and Am open clusterstars. The catalog contains information on 381 chemically peculiar (CP)stars of the upper main sequence in 79 open clusters. The catalog iscomposed of the following tables: (1) the main body, which lists CP (orsuspected CP) stars which are kinematical (or suspected kinematical)members of open clusters; (2) the list of CP (or suspected CP) starssometimes numbered among cluster members but which are actuallykinematical nonmembers; (3) the list of stars sometimes designated as'peculiar' but, in fact, probably not CP; (4) references for numberingsystems of cluster stars; (5) references for membership; and (6)references for spectral and/or peculiarity types.
|
Ultraviolet interstellar extinction toward NGC 6530 and the intrinsic energy distribution of 9 Sagittarii and HD 165052
The ultraviolet interstellar extinction curve is determined toward 12early B main-sequence stars and one O8 V star in NGC 6530. Theextinction is found to be uniform across the cluster. At shortwavelengths, the extinction curve of NGC 6530 falls below the averageGalactic curve by a full magnitude, indicating that the population ofsmall grains relative to larger grains is lower than normal. Longward of2830 A the curve is almost coincident with the mean Galactic curve. The2175 A feature is narrower than the average. The derived extinctioncurve is used to deredden the observed fluxes of two early O-type starsin NGC 6530:9 Sgr and HD 165052. The intrinsic continua are compared tomodel atmospheres in the region 1200-8200 A. The LTE, line-blanketedmodels of Kurucz (1979) fit the observations better than the non-LTE,unblanketed models of Mihalas (1972) suggesting that line-blanketingeffects are more important than the non-LTE effects in the atmospheresof these stars. The temperatures of the models that best fit theobservations are lower than the Zanstra temperatures and thetemperatures estimated from the strengths of the He I and He II lines,by 10,000 to 20,000 K.
|
UBV and H-beta observations of stars towards M8
New and more accurate UBV magnitudes have been obtained for 106 starswith known proper motions in the field of the emission nebular M8. Thecolor-magnitude diagram of the embedded cluster NGC 6530 consists of amain sequence from B1 to about B9 with a distance modulus of 11.4 mag.Spectral types earlier than B1 and later than B9 fall above the mainsequence. Evidence is given that the so-called pre-main sequence of NGC6530 is not related to this cluster but consists of field stars. H-betaobservations strongly support previous results that many cluster membersare intrinsic Be stars.
|
Study of the extremely young cluster NGC 6530.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1978MNRAS.184..467S&db_key=AST
|
Broad-band photometry of NGC 6530.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1977MNRAS.178..423K&db_key=AST
|
Membership in the extremely young open cluster NGC 6530(M8).
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1972A&A....20..425V&db_key=AST
|
Studies in Spectral Classification. II. The HR Diagram of NGC 6530.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1965ApJ...141..183H&db_key=AST
|
A Catalogue of H II Regions.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1959ApJS....4..257S&db_key=AST
|
Studies of Extremely Young Clusters. II. NGC 6530.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1957ApJ...125..636W&db_key=AST
|
A Catalogue of Emission Nebulae Near the Galactic Plane.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1953ApJ...118..362S&db_key=AST
|
Bright nebulae and star clusters in Sagittarius and Scutum photographed with the 60-inch reflector.
Not Available
|
