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The Orbit and Occultations of KH 15D
The unusual flux variations of the pre-main-sequence binary star KH 15Dhave been attributed to occultations by a circumbinary disk. We testwhether or not this theory is compatible with newly available data,including recent radial velocity measurements, CCD photometry over thepast decade, and photographic photometry over the past 50 years. We findthe model to be successful, after two refinements: a more realisticmotion of the occulting feature and a halo around each star thatprobably represents scattering by the disk. The occulting feature isexceptionally sharp edged, raising the possibility that the dust in thedisk has settled into a thin layer and providing a tool for fine-scalemapping of the immediate environment of a T Tauri star. However, thewindow of opportunity is closing, as the currently visible star may behidden at all orbital phases by as early as 2008.
| X-Ray and Infrared Observations of Embedded Young Stars in NGC 2264
Images of the NGC 2264 star-forming region, which we have acquired withthe XMM-Newton spacecraft, reveal strong X-ray emission from threedeeply embedded (AV>=10 mag) young stellar objects in thevicinity of Allen's infrared source (AFGL 989 = IRS 1) and Castelaz& Grasdalen's infrared source (RNO-EW = IRS 2). Thermal plasmamodels for the brightest source in X-rays, located 11" southwest ofAllen's star, yield a quasi-steady luminosity ofLX=1033 ergs s-1 and an extraordinarilyhigh X-ray temperature of 100 MK. The high temperature is consistentwith the presence of emission lines of Fe XXV and Fe XXVI at photonenergies of 6.7 and 6.9 keV, respectively. An even higher temperature ofnearly ~140 MK was observed during the rise phase of a powerfulimpulsive X-ray flare of another young star in the IRS 2 region.Moderate-resolution near-infrared (1-4 μm) spectra of the embeddedobjects, obtained at the NASA Infrared Telescope Facility, exhibit deepwater ice absorption bands, as well as a variety of emission andabsorption features of H I, CO, and both neutral and ionized metals.
| The Disappearing Act of KH 15D: Photometric Results from 1995 to 2004
We present results from the most recent (2002-2004) observing campaignsof the eclipsing system KH 15D, in addition to rereduced data obtainedat Van Vleck Observatory (VVO) between 1995 and 2000. Phasing 9 yr ofphotometric data shows substantial evolution in the width and depth ofthe eclipses. The most recent data indicate that the eclipses are nowapproximately 24 days in length, or half the orbital period. Theseresults are interpreted and discussed in the context of the recentmodels for this system put forward by Winn and coworkers and Chiang& Murray-Clay. A periodogram of the entire data set yields a highlysignificant peak at 48.37+/-0.01 days, which is in accord with thespectroscopic period of 48.38+/-0.01 days determined by Johnson andcoworkers. Another significant peak, at 9.6 days, was found in theperiodogram of the out-of-eclipse data at two different epochs. Weinterpret this as the rotation period of the visible star and argue thatit may be tidally locked in pseudosynchronism with its orbital motion.If so, application of Hut's theory implies that the eccentricity of theorbit is e=0.65+/-0.01. Analysis of the Ultraviolet and Visual EchelleSpectrograph VLT spectra obtained by Hamilton and coworkers shows thatthe vsini of the visible star in this system is 6.9+/-0.3 kms-1. Using this value of vsini and the measured rotationperiod of the star, we calculate the lower limit on the radius to beR=(1.3+/-0.1) Rsolar, which concurs with the value obtainedby Hamilton and coworkers from its luminosity and effective temperature.Here we assume that i=90deg, since it is likely that the spinand orbital angular momenta vectors are nearly aligned. One unusuallybright data point obtained in the 1995/1996 observing season at VVO isinterpreted as the point in time when the currently hidden star B madeits last appearance. Based on this datum, we show that star B is0.46+/-0.03 mag brighter than the currently visible star A, which isentirely consistent with the historical light curve. Finally,well-sampled VJ and IJ data obtained at the CerroTololo Inter-American Observatory Yale 1 m telescope during 2001/2002show an entirely new feature: the system becomes bluer by a small butsignificant amount in very steady fashion as it enters eclipse and showsan analogous reddening as it emerges from eclipse. This suggests anextended zone of hot gas located close to but above the photosphere ofthe currently visible star. The persistence of the bluing of the lightcurve shows that its length scale is comparable to a stellar radius.
| The T Tauri Star Population of the Young Cluster NGC 2264
An Hα emission survey of the young cluster NGC 2264 in the Mon OB1association resulted in the detection of 490 Hα emission stars ina 25'×40' field approximately centeredbetween the O7 V multiple star S Mon and the Cone Nebula. The survey wascarried out with the wide-field grism spectrograph (WFGS) on theUniversity of Hawaii 2.2 m telescope on Mauna Kea. X-ray observationsmade with the European Photon Imaging Camera on board the European SpaceAgency's XMM-Newton satellite observatory will be discussed in asubsequent paper. Optical (BVRCIC) photometry wasobtained for selected fields to supplement similar data from theliterature. Spectra covering the 6000-8000 Å region at aresolution of R~3000 (adequate for the determination of Li Iλ6708 line strengths) were obtained for 150 Hα and X-rayemission sources with the Gemini Multi-Object Spectrograph.Near-infrared spectra (1-2.5 μm) of a number of T Tauri stars (TTSs),X-ray sources, and LHα25 (W90) were also obtained using SpeX onthe Infrared Telescope Facility. Ages and masses for the Hαemitters were inferred from the isochrones and evolutionary tracks ofD'Antona & Mazzitelli. The median age for the TTS population isabout 1.1 Myr, but a considerable dispersion, from 0.1 to 5 Myr, existsfor individual objects. Several fields in the cluster were observed withthe WFGS on more than one occasion, permitting an examination ofHα variability over long baselines in time. About 90% of theclassical T Tauri stars (CTTSs) showed changes in W(Hα) of atleast 10%, while 57% varied at levels of 50% or more. No evidence wasfound for a significant pool of dormant Hα emitters. Summing themasses of the TTSs and the OB stellar population of NGC 2264, a lowerlimit for the total stellar mass content of the cluster is about 430Msolar. This is less than 1% of the total mass of the atomicand molecular gas believed to be associated with NGC 2264. Evidence forhierarchical structure within the cluster is suggested by the spatialdistribution of TTSs. Four concentrations of Hα emitters areevident: two near S Mon and two near the Cone Nebula. The median age ofthe TTSs in the immediate vicinity of S Mon was found to be greater thanthat of the TTSs near Allen's infrared source (IRS-1), but a significantdispersion is present. From the rotational data of Lamm et al. andMakidon et al., 241 of the TTSs are periodic variables, 150 weak-line TTauri stars (WTTSs) and 91 CTTSs, while 123 stars are irregularvariables (30 WTTSs and 93 CTTSs). A weak-to-moderate positivecorrelation is found between H-K color and Prot for theCTTSs, in the sense that stars having longer periods tend to have largerH-K colors. A similar positive correlation is found betweenLHα and Prot among the CTTSs. Nostatistically significant correlation is found between Protand theoretical age or between Prot and LX. Othertopics discussed include the fraction of Hα emitters that areWTTSs, f(WTTS)=N(WTTS)/N(TTS), for clusters of different ages; therelative detectability of Hα emission using WFGS and narrowbandfilter imaging techniques; and the correlation of W(Li I) withTe, age, H-K color, and W(Hα).
| A Very Large Array Search for Water Masers in Six H II Regions: Tracers of Triggered Low-Mass Star Formation
We present a search for water maser emission at 22 GHz associated withyoung low-mass protostars in six H II regions-M16, M20, NGC 2264, NGC6357, S125, and S140. The survey was conducted with the NRAO Very LargeArray from 2000 to 2002. For several of these H II regions, ours are thefirst high-resolution observations of water masers. We detected 16 watermasers: eight in M16, four in M20, three in S140, and one in NGC 2264.All but one of these were previously undetected. No maser emission wasdetected from NGC 6357 or S125. There are two principle results to ourstudy. (1) The distribution of water masers in M16 and M20 does notappear to be random but instead is concentrated in a layer of compressedgas within a few tenths of a parsec of the ionization front. (2)Significantly fewer masers are seen in the observed fields than expectedbased on other indications of ongoing star formation, indicating thatthe maser-exciting lifetime of protostars is much shorter in H IIregions than in regions of isolated star formation. Both of theseresults confirm predictions of a scenario in which star formation isfirst triggered by shocks driven in advance of ionization fronts andthen truncated ~105 yr later when the region is overrun bythe ionization front.
| H2 Emission Nebulosity Associated with KH 15D
An H2 emission filament is found in close proximity to theunique object KH 15D using the adaptive optics system of the SubaruTelescope. The morphology of the filament, the presence of spectroscopicoutflow signatures observed by Hamilton et al., and the detection ofextended H2 emission from KH 15D by Deming, Charbonneau,& Harrington suggest that this filament arises from shockedH2 in an outflow. The filament extends about 15" to the northof KH 15D.Based on data collected at Subaru Telescope, which is operated by theNational AstronomiObservatory of Japan.
| Spectropolarimetry and Modeling of the Eclipsing T Tauri Star KH 15D
KH 15D is a strongly variable T Tauri star in the young star cluster NGC2264 that shows a decrease in flux of 3.5 mag lasting for 18 days andrepeating every 48 days. The eclipsing material is likely due toorbiting dust or rocky bodies in a partial ring or warped disk thatperiodically occults the star. We measured the polarized spectrum in andout of eclipse at the Keck and Palomar observatories. Outside of theeclipse, the star exhibited low polarization consistent with zero.During eclipse, the polarization increased dramatically to ~2% acrossthe optical spectrum, while the spectrum had the same continuum shape asoutside of eclipse and exhibited emission lines of much largerequivalent width, as previously seen. From the data, we conclude that(1) the scattering region is uneclipsed, (2) the scattering is nearlyachromatic, and (3) the star is likely completely eclipsed so that theflux during eclipse is entirely due to scattered light, a conclusionalso argued for by the shape of the ingress and egress. We argue thatthe scattering is not due to electrons but may be due to large dustgrains of size ~10 μm, similar to the interplanetary grains thatscatter the zodiacal light. We construct a warped-disk model with anextended dusty atmosphere that reproduces the main features of the lightcurve, namely, (1) a gradual decrease before ingress due to extinctionin the atmosphere (similar for egress), (2) a sharper decrease withiningress due to the optically thick base of the atmosphere, and (3) apolarized flux during eclipse that is 0.1% of the total flux outside ofeclipse, which requires no fine-tuning of the model. The inclination ofthe warp is set by the duration of ingress and egress assuming that thewarp is located at the Keplerian radius, and the inclination of theobserver is then determined by the duration of the eclipse.
| The History of the Mysterious Eclipses of KH 15D: Asiago Observatory, 1967-1982
We are gathering archival observations to determine the photometrichistory of the unique and unexplained eclipses of the pre-main-sequencestar KH 15D. Here we present a light curve from 1967 to 1982, based onphotographic plates from Asiago Observatory. During this time, thesystem alternated periodically between bright and faint states, asobserved today. However, the bright state was 0.9 mag brighter than themodern value, and the fractional variation between bright and faintstates (ΔI=0.7 mag) was smaller than observed today (3.5 mag). Apossible explanation for these findings is that the system contains asecond star that was previously blended with the eclipsing star but isnow completely obscured.Based on data from the digitized Italian photographic archives, producedunder contract MIUR/Cofin 2002 to C. Barbieri, Dipartimento diAstronomia, Università di Padova.
| Limits on Eclipses of the Pre-Main-Sequence Star KH 15D in the First Half of the 20th Century
Over the last decade, the pre-main-sequence star KH 15D has exhibitedperiodic eclipses that are surprisingly deep (~3 mag) and long-lasting(~40% of the 48.4 day period). The cause of the eclipses is unknown, butit could be a feature in a nearly edge-on protoplanetary disk. Here wereport on an analysis of archival photographs of KH 15D from the HarvardCollege Observatory plate collection, most of which were taken duringthe years 1913-1951. During this time range, the data are consistentwith no eclipses; the duty cycle of 1 mag eclipses was less than 20%.The decadal timescale of this change in eclipse behavior is compatiblewith the expected timescale of protoplanet/disk interactions. Archivalimages from more recent epochs should reveal the onset of the eclipses.
| Circumstellar Disk Candidates Identified in NGC 2264
We present an optical and near-infrared study of a 45'×45' fieldin NGC 2264, which includes both S Mon and the Cone Nebula. We reportphotometry at optical (UBVRCIC) and near-infrared(JHK) wavelengths for ~5600 stars and spectroscopic classifications for~400 of these stars. We identify circumstellar disk candidates usingthree techniques: excess ultraviolet (U-V) emission, excess near-IR (I-Kand H-K) emission, and Hα emission-line equivalent widths forthose stars with spectra. We find generally good correlation betweendisk indicators thought to originate from different physical processes.We find little if any evolution of disk fraction with stellar age ormass. However, when we derive mass accretion rates (M) from the excessemission at U, we find that M decreases with age over the age rangespanned by our data, ~0.1-5 Myr, and increases with mass over the range~0.25-1 Msolar. These findings are comparable to resultsfound previously by us in the Orion Nebula cluster flanking fields.
| ASCA Observations of the NGC 2264 Molecular Cloud
We have made ASCA observations of the molecular cloud associated withNGC 2264, and detected nine and five sources in the soft and hard X-rayband images, respectively. Most of them are identified with knownpre-main-sequence stars or protostellar objects. Two Class I sourceswith intermediate luminosities and one Class II source associated withmolecular outflow are promising candidates for X-ray emitters. Anoptically thin hot-plasma model with two components can reproduce theGIS X-ray spectra, including both of the Class I sources. The hardX-rays probably arise from these Class I sources, and are intrinsicallyvery luminous, >1032 erg s-1 in the 0.7-10.0keV energy band, and their luminosities lie between low-mass Class Isources and that of the W 3 massive core region. Since near-infraredsurvey data reveal that Class I sources are surrounded by embedded youngclusters, the soft X-ray components could be explained by aggregates oflow-mass pre-main-sequence stars.
| Accelerating Star Formation in Clusters and Associations
We use our own, recently developed pre-main-sequence evolutionary tracksto investigate the star formation histories of relatively nearbyassociations and clusters. We first employ published luminosities andeffective temperatures to place the known members of each region in theH-R diagram. We then construct age histograms detailing that region'shistory. The groups studied include Taurus-Auriga, Lupus, Chamaeleon,ρ Ophiuchi, Upper Scorpius, IC 348, and NGC 2264. This study is thefirst to analyze a large number of star-forming regions with the sameset of theoretical tracks. Our investigation corroborates and extendsour previous results on the Orion Nebula Cluster. In all cases, we findthat star formation began at a relatively low level some 107yr in the past and has more recently undergone a steep acceleration.This acceleration, which lasts several million years, is usuallycontinuing through the present epoch. The one clear exception is the OBassociation Upper Scorpius, where the formation rate climbed upward,peaked, and has now died off. Significantly, this is also the onlyregion of our list that has been largely stripped of molecular gas. Theacceleration represents a true physical phenomenon that cannot beexplained away by incompleteness of the samples; nor is the pattern ofstellar births significantly affected by observational errors or thepresence of unresolved binaries. We speculate that increasing starformation activity arises from contraction of the parent cloud. Despitethe short timescale for acceleration, the cloud is likely to evolvequasi-statically. Star formation itself appears to be a criticalphenomenon, occurring only in locations exceeding some thresholddensity. The cloud's contraction must reverse itself, and the remnantgas dissipate, in less than 107 yr, even for aggregatescontaining no massive stars. In this case, molecular outflows from thestars themselves presumably accomplish the task, but the actualdispersal mechanism is still unclear.
| The Pre-Main-Sequence Stars and Initial Mass Function of NGC 2264
UBVRI and Hα CCD photometry has been performed in the southernregion of NGC 2264 around the Cone Nebula. A nearly complete list ofpre-main-sequence (PMS) members in the cluster has been made from thecompilation of Hα emission stars identified in this study plusthose selected in previous investigations, together with stars withX-ray emission. Using the H-R diagram, we tested a set of four PMSevolution models based on the PMS age and age spread estimate for thecluster and the mass-age relationship among individual stars. Theresultant initial mass function (IMF) is in good agreement with thefield star IMF of the solar neighborhood, with an IMF slope Γ=-1.7in the mass range 0.3<=logm<=0.8.
| Five-colour photometry of OB-stars in the Southern Hemisphere
Observations of OB-stars, made in 1959 and 1960 at the Leiden SouthernStation near Hartebeespoortdam, South Africa, with the VBLUW photometerattached to the 90 cm light-collector, are given in this paper. They arecompared with photometry obtained by \cite[Graham (1968),]{gra68}\cite[Walraven & Walraven (1977),]{wal77} \cite[Lub & Pel(1977)]{lub77} and \cite[Van Genderen et al. (1984).]{gen84} Formulaefor the transformation of the present observations to those of\cite[Walraven & Walraven (1977)]{wal77} and \cite[Lub & Pel(1977)]{lub77} are given. Table 4 is only 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
| BVRI photometry of the star-forming region NGC 2264: the initial mass function and star-forming rate
The pre-main sequence (PMS) population in the mass range between ~ 0.2and ~ 3 M_sun in southern part of the star-forming region NGC 2264 hasbeen studied, determining both the Initial Mass Function in the regionas well as the star-formation rates for different mass ranges. Thesample is a composite one, derived through the union of samples obtainedthrough different techniques and each suffering from different biases:previously known PMS stars in the region from the literature,photometrically-selected T Tauri candidates (from our own photometricdata, discussed in detail in the present paper) and X-ray selected PMScandidates (discussed in detail in a companion paper) have been joinedto form a sample which we show to be statistically complete (i.e. freefrom the biases which affect each of the parent samples) down to =~ 0.6M_sun (while being incomplete at lower masses). Individual masses andages have been derived by placing the individual stars on evolutionarytracks, allowing us to derive both the IMF and the star formation rate.The Initial Mass Function thus derived for NGC 2264 shows evidence for abimodal distribution of masses, with a break in the IMF at around 1M_sun. Based on observations with the VATT: the Alice P. LennonTelescope and the Thomas J. Bannan Astrophysics Facility. Table~3 isavailable in electronic form at CDS via ftp 130.79.128.5
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
| New Perspectives on AX Monocerotis
AX Moncerotis is a 232d, noneclipsing, interacting binary star thatconsists of a K giant, a Be-like giant, and large amounts ofcircumstellar material. The K star is almost certainly a synchronousrotator and is probably in contact with its critical lobe. The Be starwas believed to be a rapid rotator based on extremely wide absorptionlines, but new spectra show that these lines arise from thecircumstellar environment. Hydrogen emission, also circumstellar, ismany times stronger than the continuum. Near-ultraviolet light curvesexhibit a 0.5 mag dip near phase 0.75, but there is no such variabilityat longer wavelengths. Gas flow trajectories from the cusp of the K startoward the Be star provide a simple explanation for the photometric andspectroscopic behavior. We may have found a decreasing orbital period,but more data are necessary to confirm this result. We present severalmodels for AX Mon based on (1) new and archival visible photometry, (2)archival ultraviolet spectroscopy, (3) new and archival visiblespectroscopy, (4) new visible polarimetry, and (5) new radio photometry.Future observations, including optical interferometry, are proposed.
| 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.
| 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.
| 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.
| Uvby-Beta and JHKLM Photometry of Peculiar Stars in the Galactic Cluster NGC2264
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&AS..102..201N&db_key=AST
| Optical studies of interstellar material in low density regions of the Galaxy. I - A survey of interstellar NA I and CA II absorption toward 57 distant stars
We present high-resolution spectra of the Na I D and Ca II K linestoward 57 late-O and early-B stars along extended (d greater than 1 kpc)low-density paths through the Milky Way disk and halo. The sight linespreferentially sample diffuse gas in the interstellar medium (ISM) alonginterarm, Galactic center, and high latitude directions. We measureequivalent widths, apparent column densities, and absorption componentstructure. The Ca II to Na I ratios presented as a function of velocityfor each sight line exhibit variations due to elemental depletion,ionization, and density enhancements. Absorption along high latitudesight lines is kinematically simpler than it is along interarm andGalactic center sight lines. Galactic rotation noticeably broadens theabsorption profiles of distant stars located in these latter directions.Along several sight lines, we see Ca II absorption at velocitiescorresponding to large distances (/z/ about 1 kpc) from the Galacticplane. The effects of differences in the Ca II and Na I scale heightsand nonzero velocity dispersions are readily apparent in the data. Briefnotes are given for several sight lines with interesting absorptionproperties.
| 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.
| More radial-velocity measurements in young open clusters
Further high resolution radial-velocity measurements are reported in 23young open clusters using the Kitt Peak CCD coude spectrograph on the0.9-m feed telescope. The radial velocities for the cluster stars arederived with the technique of cross correlation. The internal precisionof the velocity measurements is typically 2 km/s for early type stars.From these new data and previously published velocities, the observedstars in two clusters, NGC 663 and NGC 2287, were found to show arelatively small dispersion in the measured mean velocities. Furtherobservations of stars in young clusters will be useful in helping toestablish an early-type-star-velocity standard system.
| VBLUW photometry of the very young open cluster NGC 2264
This study presents and discusses VBLUW photometry of 112 stars with Vless than 13 in the area of the very young cluster NGC 2264.Temperatures, gravities, and reddening for stars hotter than 8400 K aredetermined. The reddening and the distance are found to be in goodagreement with the results of other references; to the existing list ofpresumable nonmembers, six stars are tentatively added as possiblenonmembers.
| Absolute magnitudes of B emission line stars - Correlation between the luminosity excess and the effective temperature
A new determination of the visual absolute magnitude of Be stars iscarried out. For this, a new calibration of visual absolute magnitudesof B stars of luminosity classes, V, IV, and III is first obtained froma sample of 215 stars. The absolute luminosity excess in the visual isdetermined for a sample of 49 Be stars. It is found that this excess iscorrelated with the effective temperature of the underlying stars. Awell defined correlation between this excess and the emission in thefirst two Balmer lines is established. From these results, using asimple model of circumstellar envelope, it is inferred that the zones ofthe circumstellar envelope contributing to the emission in the continuumand in the lines have to be rather small. It is also deduced that theemission measure of the envelope is correlated with the temperature ofthe central star and that the irregular photometric variations of Bestars are an envelope-opacity phenomenon.
| Remarks to Lapicz positions of stars in NGC 2264.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1990RMxAA..20..113L&db_key=AST
| 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.
| UVBY beta photometry of stars in the field of NGC 2244 and NGC 2264
Results are presented from uvby beta photometric observations of 34stars in the fields of NGC 2244 and NGC 2264. Photometric andastrometric indicators suggest that about a third of the sample arenonmembers of the clusters. Cluster color excesses are determined forthe two clusters. For the stars examined, log g and effectivetemperature are estimated. Also, stellar radii, masses, and positions inthe H-R diagram are calculated. The cluster ages and the main-sequencelifetime for massive stars show that star formation in the region hasnot been restricted to a single epoch.
| Equatorial coordinates of double and multiple star components measured on GPO astrographic plates
Equatorial coordinates are given for some of the multiple starcomponents having incomplete or inconsistent information in the IndexCatalog of Visual Double Stars. Two photographic plates were taken foreach system with incomplete or inconsistent information using the GPOastrograph. The plate measurement and reduction techniques used aredescribed.
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Observation and Astrometry data
| Constellation: | Μονόκερως |
| Right ascension: | 06h41m09.60s |
| Declination: | +09°27'57.5" |
| Apparent magnitude: | 7.172 |
| Proper motion RA: | 1.5 |
| Proper motion Dec: | -1.9 |
| B-T magnitude: | 6.921 |
| V-T magnitude: | 7.152 |
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