Herbig Ae/Be Stars in nearby OB Associations
We have carried out a study of the early-type stars in nearby OBassociations spanning an age range of ~3-16 Myr, with the aim ofdetermining the fraction of stars that belong to the Herbig Ae/Be class.We studied the B, A, and F stars in the nearby (<=500 pc) OBassociations Upper Scorpius, Perseus OB2, Lacerta OB1, and Orion OB1,with membership determined from Hipparcos data. We also included in ourstudy the early-type stars in the Trumpler 37 cluster, part of the CepOB2 association. We obtained spectra for 440 Hipparcos stars in theseassociations, from which we determined accurate spectral types, visualextinctions, effective temperatures, luminosities and masses, usingHipparcos photometry. Using colors corrected for reddening, we find thatthe Herbig Ae/Be stars and the classical Be (CBe) stars occupy clearlydifferent regions in the JHK diagram. Thus, we use the location on theJHK diagram, as well as the presence of emission lines and of strong 12μm flux relative to the visual, to identify the Herbig Ae/Be stars inthe associations. We find that the Herbig Ae/Be stars constitute a smallfraction of the early-type stellar population even in the youngerassociations. Comparing the data from associations with different agesand assuming that the near-infrared excess in the Herbig Ae/Be starsarises from optically thick dusty inner disks, we determined theevolution of the inner disk frequency with age. We find that the innerdisk frequency in the age range 3-10 Myr in intermediate-mass stars islower than that in the low-mass stars (<1 Msolar) inparticular, it is a factor of ~10 lower at ~3 Myr. This indicates thatthe timescales for disk evolution are much shorter in theintermediate-mass stars, which could be a consequence of more efficientmechanisms of inner disk dispersal (viscous evolution, dust growth, andsettling toward the midplane).
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The Angular Momentum Evolution of 0.1-10 Msolar Stars from the Birth Line to the Main Sequence
Projected rotational velocities (vsini) have been measured for a sampleof 145 stars with masses between 0.4 and greater than 10Msolar (median mass 2.1 Msolar) located in theOrion star-forming complex. These measurements have been supplementedwith data from the literature for Orion stars with masses as low as 0.1Msolar. The primary finding from analysis of these data isthat the upper envelope of the observed values of angular momentum perunit mass (J/M) varies as M0.25 for stars on convectivetracks having masses in the range ~0.1 to ~3 Msolar. Thispower law extends smoothly into the domain of more massive stars (3-10Msolar), which in Orion are already on the zero-age mainsequence. This result stands in sharp contrast to the properties ofmain-sequence stars, which show a break in the power law and a sharpdecline in J/M with decreasing mass for stars with M<2Msolar. A second result of our study is that this break isseen already among the pre-main-sequence stars in our Orion sample thatare on radiative tracks, even though these stars are only a few millionyears old. A comparison of rotation rates seen for stars on either sideof the convective-radiative boundary shows that stars do not rotate assolid bodies during the transition from convective to radiative tracks.As a preliminary demonstration of how observations can be used toconstrain the processes that control early stellar angular momentum, weshow that the broad trends in the data can be accounted for by simplemodels that posit that stars (1) lose angular momentum before they aredeposited on the birth line, plausibly through star-disk interactions;(2) undergo additional braking as they evolve down their convectivetracks; and (3) are subject to core-envelope decoupling during theconvective-radiative transition.
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ROSAT PSPC observations of the Orion Trapezium area II. Source variability and revised source list
A deep ROSAT PSPC image centred on the Orion Trapezium has been reduceda second time using an improved version of the PSF to fit the data. Theouter rim of the field of view was also included. The new cataloguecontains 316 X-ray sources which are easily identified with pre-mainsequence stars of the Ori OB1 Ic and Id association. All 316 sourceswere tested for variability. No variations were found inside the singleexposures of about 45 minutes length each. Between the 4 exposuresspaced over 5 days about 1/3 of the sources show signs of activities ofvarious forms. As above 25% of these have somewhat regular lights curves(monotonically rising or falling or hill-shaped) we infer that at leastsome outbursts with time scales longer than a day are present and thatpast searches for X-ray flares of pre-main sequence stars were biasedtowards shorter time scales.
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A calibration of Geneva photometry for B to G stars in terms of Teff, log G and [M/H]
We have used recent Kurucz models and numerous standard stars to improvethe calibration of the Geneva photometric system proposed a few yearsago. A new photometric diagram for the classification of intermediatestars (8500 <= Teff <= 11000 K) is proposed and fills agap that the previous calibration had left open. Evidence is given for aclear inadequacy of the new Kurucz models in the region of the parameterspace where convection begins to take over radiation in the star'satmosphere. This problem makes the determination of the surface gravitydifficult, but leaves that of the other parameters apparentlyunaffected. The determination of metallicity is considerably improved,thanks to the homogeneous spectroscopic data published recently by\cite[Edvardsson et al. (1993)]{ref23}. Instead of showing thetraditional diagrams, we chose to publish the diagrams of the physicalparameters with the inverted grids inside, i.e. the lines of constantphotometric parameters.
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Proper motions of stars in the region of the Orion Nebula cluster (C 0532-054).
Relative proper motions and membership probabilities for 333 starswithin an area of 1.6deg by 1.8deg centred on the Orion Nebula M 42 aredetermined using plates taken over a period of 83 years with the doubleastrograph of Shanghai Observatory (scale of 30"/mm). The plates weremeasured with the ASTROSCAN automatic plate-measuring machine of LeidenObservatory. The average proper motion accuracy obtained for stars inthe photographic magnitude range 7 to 14 is 0.3mas/yr. Errors aresomewhat larger towards fainter and brighter magnitude, but the majoritylie well below 1mas/yr. 64% of the stars have been measured successfullyon at least 13 out of 18 plates. The number of stars with membershipprobabilities higher than 0.7 is 184. It is shown by a detaileddiscussion that the proper motions and membership probabilities of thestars determined in this paper are in good agreement with the resultspresented recently by other authors. Although there is a clearconcentration in the proper motion diagram, both the remainingdispersion of the internal motions and the distribution of "members" asprojected on the sky indicate that the stars in this region are notbound as one system, but do have a common origin. A similar conclusioncan be drawn from a comparison with spectroscopic and radial velocitydata.
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The complete Einstein Observatory X-ray survey of the Orion Nebula region.
We have analyzed archival Einstein Observatory images of a roughly 4.5square degree region centered on the Orion Nebula. In all, 245 distinctX-ray sources have been detected in six High Resolution Imager (HRI) and17 Imaging Proportional Counter (IPC) observations. An optical databaseof over 2700 stars has been assembled to search for candidatecounterparts to the X-ray sources. Roughly half the X-ray sources areidentified with a single Orion Nebula cluster member. The 10main-sequence O6-B5 cluster stars detected in Orion have X-ray activitylevels comparable to field O and B stars. X-ray emission has also beendetected in the direction of four main-sequence late-B and early-A typestars. Since the mechanisms producing X-rays in late-type coronae andearly-type winds cannot operate in the late-B and early-A typeatmospheres, we argue that the observed X-rays, with LXapproximately = 3 x 1030 ergs/s, are probably produced in thecoronae of unseen late-type binary companions. Over 100 X-ray sourceshave been associated with late-type pre-main sequence stars. The upperenvelope of X-ray activity rises sharply from mid-F to late-G, withLx/Lbol in the range 10-4 to 2 x10-3 for stars later than approximately G7. We have lookedfor variability of the late-type cluster members on timescales of a dayto a year and find that 1/4 of the stars show significantly variableX-ray emission. A handful of the late-type stars have publishedrotational periods and spectroscopic rotational velocities; however, wesee no correlation between X-ray activity and rotation. Thus, for thissample of pre-main-sequence stars, the large dispersion in X-rayactivity does not appear to be caused by the dispersion in rotation, incontrast with results obtained for low-mass main-sequence stars in thePleiades and pre-main-sequence stars in Taurus-Auriga.
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Far-ultraviolet stellar photometry: A field in Orion
Far-ultraviolet photometry for 625 objects in Orion is presented. Thesedata were extracted from electrographic camera images obtained duringsounding rocket flights in 1975 and 1982. The 1975 images were centeredclose to the belt of Orion while the 1982 images were centeredapproximately 9 deg further north. One hundred and fifty stars fell inthe overlapping region and were observed with both cameras. Sixty-eightpercent of the objects were tentatively identified with known starsusing the SIMBAD database while another 24% are blends of objects tooclose together to separate with our resolution. As in previous studies,the majority of the identified ultraviolet sources are early-type stars.However, there are a significant number for which no such identificationwas possible, and we suggest that these are interesting objects whichshould be further investigated. Seven stars were found which were brightin the ultraviolet but faint in the visible. We suggest that some ofthese are nearby white dwarfs.
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An Einstein Observatory SAO-based catalog of B-type stars
About 4000 X-ray images obtained with the Einstein Observatory are usedto measure the 0.16-4.0 keV emission from 1545 B-type SAO stars fallingin the about 10 percent of the sky surveyed with the IPC. Seventy-fourdetected X-ray sources with B-type stars are identified, and it isestimated that no more than 15 can be misidentified. Upper limits to theX-ray emission of the remaining stars are presented. In addition tosummarizing the X-ray measurements and giving other relevant opticaldata, the present extensive catalog discusses the reduction process andanalyzes selection effects associated with both SAO catalog completenessand IPC target selection procedures. It is concluded that X-rayemission, at the level of Lx not less than 10 exp 30 ergs/s, is quitecommon in B stars of early spectral types (B0-B3), regardless ofluminosity class, but that emission, at the same level, becomes lesscommon, or nonexistent, in later B-type stars.
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Machine-readable version of the Parenago catalogue of stars in the area of the Orion nebula
Not Available
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Early type high-velocity stars in the solar neighborhood. IV - Four-color and H-beta photometry
Results are presented from photometric obaservations in the Stromgrenuvby four-color and H-beta systems of early-type high-velocity stars inthe solar neighborhood. Several types of photometrically peculiar starsare selected on the basis of their Stromgren indices and areprovisionally identified as peculiar A stars, field horizontal-branchstars, metal-poor stars near the Population II and old-disk turnoffs,metal-poor blue stragglers, or metallic-line A stars. Numerousphotometrically normal stars were also found.
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Walraven photometry of nearby southern OB associations
Homogeneous Walraven (VBLUW) photometry is presented for 5260 stars inthe regions of five nearby southern OB associations: Scorpio Centaurus(Sco OB2), Orion OB1, Canis Major OB1, Monoceros OB1, and Scutum OB2.Derived V and (B - V) in the Johnson system are included.
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A new calibration of Geneva photometry in terms of Te, log G and mass for B stars
A new, semiempirical calibration of the two reddening-free parameters Xand Y of Geneva photometry in terms of effective temperature and gravityis proposed. The calibration is based on Kurucz's grid of atmospheremodels and on a set of fundamental stars; it is valid for main sequencestars with Te equal to or greater than 10,000 K. Theoreticalevolutionary tracks were then used to calibrate the Y vs X diagram interms of stellar mass, for M equal to or greater than 2.5 - 3.0 solarmasses.
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A complex investigation of the star formation region in the Orion constellation
The proper motions of 54 young stars in the Ori OB1 cluster are refined.Cluster contraction with a characteristic time of 3 x 10 to the 6th yris found as well as cluster rotation with a period of 6 x 10 to the 6thyr. A cluster velocity of -10 km/s is determined from the spatialvelocities of 26 stars, indicating that about 1.4 x 10 to the 7th yrago, the cluster was situated in the Galactic symmetry plane; thiscoincides with the age of the oldest subcluster of the association.
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Interstellar grain size. II - Infrared photometry and polarization in Orion
Infrared colors, polarization wavelength dependence and criteria forcluster membership are used to examine the interstellar extinction lawin the Orion region, in light of 0.36-0.82 micron linear polarimetry andinfrared photometry observations of stars within a degree of the OrionNebula. The stars inside the dust clouds are examined on a star-by-starbasis, and the different intracluster grain size indicators are found tobe in excellent agreement. Normal radius and maximum wavelength valuesof about 3.0 and 0.55 microns are observed in stars lying outside thetwo main regions of nebulosity, indicating normal grain sizes. For acarefully selected group of stars inside the nebulosity that ischaracterized by extinction by large interstellar grains, thecircumstellar emission does not significantly affect the values of Rderived from the V-K and V-L colors.
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A photometric study of the Orion OB 1 association. III - Subgroup analyses
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1978ApJS...36..497W&db_key=AST
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Two sparse open clusters in the region of Collinder 132.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1977PASP...89..803C&db_key=AST
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Spectral types in the ORI OB1 association.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1977PASP...89..797A&db_key=AST
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A photometric study of the Orion OB 1 association. I - Observational data.
A catalog of observational data is presented for stars in the region ofthe young stellar association Orion OB 1. Photoelectric observationsobtained in the uvby-beta and UBV systems are compiled along withprevious photoelectric and spectroscopic data for all these stars aswell as for several bright members of the association with availablephotometric indices. Mean weighted values are computed for thephotometric data and summarized in tables expected to be reasonablycomplete for association members earlier than spectral type A0.Membership criteria are derived, and qualitative membershipprobabilities summarized, for the 526 stars in the final program. Theanalytical procedures are discussed for association stars of B,intermediate, and AF types. Effects of the nebular environment andvarious calibrations of Balmer-line and four-color indices areconsidered for the determination of absolute magnitudes for the B-typestars.
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A polarization survey of stars near the Orion Nebula
A polarization survey of over 200 young stars near the Orion Nebulaindicates that 25 percent of the sample shows linear polarizationsignificantly above the average interstellar value for this cluster.Several regions of high polarization and reddening exist in the Orioncluster, although not all the stars in these regions are highlypolarized. Position angles of polarization are not randomly distributed,which suggests that an external polarization mechanism operates for mostof the polarized stars. Polarization is always accompanied by thepresence of infrared excesses. Finally, the amounts and origins ofpolarization are considered as a function of stellar spectral type andcolor.
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Studies of extremely young clusters. V. Stars in the vicinity of the Orion nebula
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1969ApJ...155..447W&db_key=AST
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Interstellar extinction in the Orion association
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1968ApJ...152..913L&db_key=AST
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Classificazione spettrale con prisma obiettivo di stelle del complesso Orione
Not Available
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KURZE MITTEILUNG. Zur Expansion der Assoziation Cep III
Not Available
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Axial Rotation of Orion Stars of Spectral Type B5-B9.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1963ApJ...137..316M&db_key=AST
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Two-Dimensional Spectral Classification by Narrow-Band Photometry for B Stats in Clusters and Associations.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1958ApJ...128..185C&db_key=AST
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Catalogue of stars in the area of the Orion Nebula.
Not Available
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A Study of the Orion Aggregate of Early-Type Stars.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1952ApJ...116..251S&db_key=AST
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