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Rotational Velocities for B0-B3 Stars in Seven Young Clusters: Further Study of the Relationship between Rotation Speed and Density in Star-Forming Regions
We present the results of a study aimed at assessing the differences inthe distribution of rotation speeds N(vsini) among young (1-15 Myr) Bstars spanning a range of masses 6 Msolar>1 Msolar pc-3)ensembles that will survive as rich, bound stellar clusters for ageswell in excess of 108 yr. Our results demonstrate (1) thatindependent of environment, the rotation rates for stars in this massrange do not change by more than 0.1 dex over ages t~1 to ~15 Myr; and(2) that stars formed in high-density regions lack the cohort of slowrotators that dominate the low-density regions and young field stars. Wesuggest that the differences in N(vsini) between low- and high-densityregions may reflect a combination of initial conditions andenvironmental effects: (1) the higher turbulent speeds that characterizemolecular gas in high-density, cluster-forming regions; and (2) thestronger UV radiation fields and high stellar densities thatcharacterize such regions. Higher turbulent speeds may lead to highertime-averaged accretion rates during the stellar assembly phase. In thecontext of stellar angular momentum regulation via ``disk-locking,''higher accretion rates lead to both higher initial angular momenta andevolution-driven increases in surface rotation rates as stars contractfrom the birth line to the zero-age main sequence (ZAMS). Stronger UVradiation fields and higher densities may lead to shorter disk lifetimesin cluster-forming regions. If so, B stars formed in dense clusters aremore likely to be ``released'' from their disks early during theirpre-main-sequence lifetimes and evolve into rapid rotators as theyconserve angular momentum and spin up in response to contraction. Bycontrast, the majority of their brethren in low-density,association-forming regions can retain their disks for much or all oftheir pre-main-sequence lifetimes, are ``locked'' by their disks torotate at constant angular speed, and lose angular momentum as theycontract toward the ZAMS, and thus arrive on the ZAMS as relativelyslowly rotating stars.
| NGC 7419: a young open cluster with a number of very young intermediate mass pre-MS stars
We present a photometric and spectroscopic study of the young opencluster NGC 7419, which is known to host a large number of classical Bestars for reasons not well understood. Based on CCD photometricobservations of 327 stars in UBV passbands, we estimated the clusterparameters as, reddening [E(B - V)] = 1.65 +/- 0.15 mag and distance =2900 +/- 400 pc. The turn-off age of the cluster was estimated as 25 +/-5 Myr using isochrone fits. UBV data of the stars were combined with theJHK data from Two-Micron All-Sky Survey (2MASS) and were used to createthe near-infrared (NIR) (J - H) versus (H - K) colour-colour diagram. Alarge fraction of stars (42 per cent) was found to have NIR excess andtheir location in the diagram was used to identify them as intermediatemass pre-main-sequence (MS) stars. The isochrone fits to pre-MS stars inthe optical colour-magnitude diagram showed that the turn-on age of thecluster is 0.3-3 Myr. This indicates that there has been a recentepisode of star formation in the vicinity of the cluster.Slitless spectra were used to identify 27 stars which showed Hα inemission in the field of the cluster, of which six are newidentifications. All these stars were found to show NIR excess and arelocated closer to the region populated by Herbig Ae/Be stars in the (J -H) versus (H - K) diagram. Slit spectra of 25 stars were obtained in theregion 3700-9000 Å. The spectral features were found to be verysimilar to those of Herbig Be stars. These stars were found to be morereddened than the main-sequence stars by 0.4 mag, on an average. Thus,the emission-line stars found in this cluster are more similar to theHerbig Be-type stars where the circumstellar material is the remnant ofthe accretion disc. We conclude that the second episode of starformation has led to the formation of a large number of Herbig Be starsas well as intermediate mass pre-MS stars in the field of NGC 7419, thusexplaining the presence of emission-line stars in this cluster. Thiscould be one of the young open clusters with the largest number ofHerbig Be stars.
| Caroline Herschel's catalogue of nebulae
Not Available
| Stellar Rotation: A Clue to the Origin of High-Mass Stars?
We present the results of a study aimed at assessing whether low- andhigh-mass stars form similarly. Our approach is (1) to examine theobserved projected rotational velocities among a large sample of newlyformed stars spanning a range in mass between 0.2 and 50Msolar and (2) to search for evidence of a discontinuity inrotational properties that might indicate a difference in the stellarformation process at some characteristic mass. Our database includesrecently published values of vsini for young intermediate- and low-massstars in Orion, as well as new observations of O stars located in youngclusters and OB associations. We find that the median of the quantityvobs/vc (observed rotational speed divided byequatorial breakup velocity) is typically about 0.15 and shows noevidence of a discontinuity over the full range of stellar masses, whilethe quantity Jsini/M (derived angular momentum per unit mass) exhibits aslow, monotonic rise (J/M~M0.3) with increasing mass with noevidence of a discontinuity. We suggest that these observations are mostsimply interpreted as indicative of a single stellar formation andangular momentum regulation mechanism, one that results in rotationrates well below breakup and angular momenta per unit mass that differsystematically by no more than a factor of 3-4 over a mass rangespanning a factor of 250.
| Kinematics of the Open Cluster System in the Galaxy
Absolute proper motions and radial velocities of 202 open clusters inthe solar neighborhood, which can be used as tracers of the Galacticdisk, are used to investigate the kinematics of the Galaxy in the solarvicinity, including the mean heliocentric velocity components(u1,u2,u3) of the open cluster system,the characteristic velocity dispersions(σ1,σ2,σ3), Oortconstants (A,B) and the large-scale radial motion parameters (C,D) ofthe Galaxy. The results derived from the observational data of propermotions and radial velocities of a subgroup of 117 thin disk young openclusters by means of a maximum likelihood algorithm are:(u1,u2,u3) =(-16.1+/-1.0,-7.9+/-1.4,-10.4+/-1.5) km s-1,(σ1,σ2,σ3) =(17.0+/-0.7,12.2+/-0.9,8.0+/-1.3) km s-1,(A,B) =(14.8+/-1.0,-13.0+/-2.7) km s-1 kpc-1, and (C,D) =(1.5+/-0.7,-1.2+/-1.5) km s-1 k pc-1. A discussionon the results and comparisons with what was obtained by other authorsis given.
| Caroline Herschel as observer
Not Available
| 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.
| Statistical Confirmation of a Stellar Upper Mass Limit
We derive the expectation value for the maximum stellar mass(mmax) in an ensemble of N stars, as a function of theinitial mass function (IMF) upper mass cutoff (mup) and N. Westatistically demonstrate that the upper IMF of the local massive starcensus observed thus far in the Milky Way and Magellanic Clouds clearlyexhibits a universal upper mass cutoff around 120-200 Msolarfor a Salpeter IMF, although the result is more ambiguous for a steeperIMF.
| Deepsky delights.
Not Available
| A Study of Stellar Rotation in Seven Young Clusters
The distribution of rotational velocities among young stars may provideinsight into initial conditions in star-forming regions (e.g. Wolff,Strom, & Hillenbrand 2004). In this contribution, we reportprojected rotational velocities (vsini) for a large sample of starsdrawn from 7 clusters ranging in age from 1--20 Myr (IC 1805, Cyg OB2,NGC 2244, NGC 6611, NGC 6823, NGC 7235, NGC 7380). These data providethe basis for assessing (a) the cosmic dispersion in the distribution ofvsini values, N(vsini), for late-O and early- B stars among clusters ofsimilar age; and (b) evolution-driven changes in N(vsini) via comparisonof young and ``old'' clusters in our sample.Wolff, S.C., Strom, S.E. & Hillenbrand, L.A. 2004 ApJ 601, 979
| A radio and mid-infrared survey of northern bright-rimmed clouds
We have carried out an archival radio, optical and infrared wavelengthimaging survey of 44 Bright-Rimmed Clouds (BRCs) using the NRAO/VLA SkySurvey (NVSS) archive, images from the Digitised Sky Survey (DSS) andthe Midcourse Space eXperiment (MSX). The data characterise the physicalproperties of the Ionised Boundary Layer (IBL) of the BRCs. We haveclassified the radio detections as: that associated with the ionisedcloud rims; that associated with possible embedded Young Stellar Objects(YSOs); and that unlikely to be associated with the clouds at all. Thestars responsible for ionising each cloud are identified and acomparison of the expected ionising flux to that measured at the cloudrims is presented. A total of 25 clouds display 20 cm radio continuumemission that is associated with their bright optical rims. The ionisingphoton flux illuminating these clouds, the ionised gas pressure and theelectron density of the IBL are determined. We derive internal molecularpressures for 9 clouds using molecular line data from the literature andcompare these pressures to the IBL pressures to determine the pressurebalance of the clouds. We find three clouds in which the pressureexerted by their IBLs is much greater than that measured in the internalmolecular material. A comparison of external pressures around theremaining clouds to a global mean internal pressure shows that themajority of clouds can be expected to be in pressure equilibrium withtheir IBLs and hence are likely to be currently shocked byphotoionisation shocks. We identify one source which shows 20 cmemission consistent with that of an embedded high-mass YSO and confirmits association with a known infrared stellar cluster. This embeddedcluster is shown to contain early-type B stars, implying that at leastsome BRCs are intimately involved in intermediate to high mass starformation.Figure \ref{fig:images} and Table \ref{tbl:istars1} are only availablein electronic form at http://www.edpsciences.org
| Metallicity distribution on the galactic disk
Depending mainly on UBVCCD data, the metallicities of 91 open starclusters nearby the galactic disk have been estimated using Cameron's[A&A 147 (1985b) 39] method. The metallicity radial gradient alongthe galactic plane is found to be -0.09 dex/kpc; which is in a very goodagreement with Panagia and Tosi [A&A 96 (1981) 306] and Carraro etal. [MNRAS 296 (1998) 1045]. Vertically on the galactic disk, withinabout 800 pc, the metallicity gradient is found to be so trivial. Anaverage age-metallicity relation has been examined, which confirms theprevious suggestion that the metallicity of a cluster depending mainlyon its position on the galactic disk more than its age.
| Stellar tracers of the Cygnus Arm. I. Spectroscopic study of bright photometric candidates
We present medium-resolution spectroscopy of a sample of stars in thesecond Galactic quadrant selected from the literature because theircolours suggest that they are moderately-reddened early-type stars atvery large distances. From the derived spectral types and observedcolours, we calculate distances to all these objects. For a sizablefraction of our sample, we find distances well in excess of what isexpected for Perseus Arm objects, even allowing for rather generouserrors. In the interval l=150degr -180degr , there is a large number ofobjects with distances in excess of 4 kpc, which are likely tracing theOuter or Cygnus Arm. In particular, we find that the association Cam OB3is placed on this Arm. Based on our results, the extent and definitionof the associations Cas OB4 and Aur OB2 need to be reevaluated.Based on observations made at Observatoire de Haute Provence (CNRS),France.
| 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.
| 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.
| Morphological analysis of open clusters' propertiesII. Relationships projected onto the galactic plane
A morphological analysis study of open clusters' properties has beenachieved for a sample of 160 UBVCCD open star clusters of approximately128,000 stars near the galactic plane. The data was obtained and reducedfrom using the same reduction procedures, which makes this catalogue thelargest homogeneous source of open clusters' parameters.
| Gamma-ray line emission from OB associations and young open clusters. II. The Cygnus region
Gamma-ray and microwave observations of the Cygnus region reveal anintense signal of 1.809 Me line emission, attributed to radioactivedecay of 26, that is closely correlated with 53 GHz free-freeemission, originating from the ionised interstellar medium. We modelledboth emissions using a multi-wavelength evolutionary synthesis code formassive star associations that we applied to the known massive starpopulations in Cygnus. For all OB associations and young open clustersin the field, we determined the population age, distance, and richnessas well as the uncertainties in all these quantities from publishedphotometric and spectroscopic data. We propagate the populationuncertainties in model uncertainties by means of a Bayesian method. Theyoung globular cluster Cyg OB2 turns out to be the dominant26 nucleosynthesis and ionisation source in Cygnus. Our modelreproduces the ionising luminosity of the Cygnus region very well, yetit underestimates 26 production by about a factor of 2. Weattribute this underestimation to shortcomings of currentnucleosynthesis models, and suggest the inclusion of stellar rotationas possible mechanism to enhance 26 production. We alsomodelled 60Fe nucleosynthesis in the Cygnus region, yet thesmall number of recent supernova events suggests only little60Fe production. Consequently, a detection of the 1.137 Meand 1.332 Me decay lines of 60Fe from Cygnus by the upcomingINTEGRAL observatory is not expected. Appendices A and B, and Tables 1,2, and 5 are only available in electronic form athttp://www.edpsciences.org
| Integrated photometric characteristics of galactic open star clusters
Integrated UBVRI photometric parameters of 140 galactic open clustershave been computed. Integrated I(V-R)0 and I(V-I)0colours as well as integrated parameters for 71 star clusters have beenobtained for the first time. These, in combination with published data,altogether 352 objects, are used to study the integrated photometriccharacteristics of the galactic open clusters. The I(MV)values range from -9.0 to -1.0 mag corresponding to a range in totalmass of the star clusters from ~ 25 to 4*E4 Msun.The integrated colours have a relatively narrow range, e.g., I(B-V){_0}varies from -0.4 to 1.2 mag. The scatter in integrated colours at agiven integrated magnitude can be understood in terms of differences infraction of red giants/supergiants in the clusters. The observedintegrated magnitudes and colours agree with the synthetic ones, exceptthe dependences of I(V-R)0 and I(V-I)0 colours forclusters younger than ~ 100 Myrs and also of the integrated magnitudesof oldest clusters. The large sample provides the most accurate agedependence of integrated magnitudes and colours determined so far. Theluminosity function of the I(MV) has a peak around -3.5 magand its slope indicates that only ~ 1% of the open clusters in thegalactic disc are brighter than I(MV)=-11 mag. No variationhas been found of integrated magnitude with galactocentric distance andmetallicity.
| Hα Emission Stars and Herbig-Haro Objects in the Vicinity of Bright-rimmed Clouds
Bright-rimmed clouds (BRCs) found in H II regions are probably sites oftriggered star formation due to compression by ionization/shock fronts.Many BRCs harbor IRAS point sources of low dust temperature. They alsofrequently contain a small cluster of near-IR stars that is elongatedalong the axis of the BRC. Here we present the results of our Hαgrism spectroscopy and narrowband imaging observations of BRCs in searchof candidate pre-main-sequence stars of the T Tauri, Herbig Ae/Be andrelated types, and Herbig-Haro (HH) objects. We have detected a largenumber (460) of Hα emission stars down to a limiting magnitude ofabout R=20 in and around all but two of the 28 BRCs observed. Thepresent study has, for the first time, reached down nearly to thefaintest classical T Tauri star population in OB associations. A totalof 12 new HH objects have been found. Most are of small apparent size,emphasizing the need for deep searches at high spatial resolution, butHH 588 associated with BRC 37 represents a huge HH complex composed oftwo-staged bow shocks on both sides of a tiny central knot. These starsand HH objects are concentrated near the tip of bright rims (i.e., inthe head of the BRCs and just outside the rims) and often make looseaggregates similar to those of near-IR stars, thus supporting ourhypothesis of ``small-scale sequential star formation.'' The presence ofsuch a large number of Hα emission stars in the immediatevicinities of BRCs implies that second-generation formation of low-massstars is relatively extensive and further supports the notion ofcohabitation of high- and low-mass populations in OB associations.
| Morphological analysis of open clusters' propertiesI. Properties' estimations
A sample of 160 UBVCCD observations of open star clusters near thegalactic plane has been studied, and a catalogue of their propertiesobtained. The main photometrical properties have been re-estimated selfconsistently and the results have been compared with those of Lynga[Lynga, G., 1987. Catalog of Open Cluster Data, 5th Edition, StellarData Centers, Observatoire de Strasbourg, France].
| High-mass binaries in the very young open cluster NGC 6231. Implication for cluster and star formation
New radial-velocity observations of 37 O- and B stars in the very youngopen cluster NGC 6231 confirm the high frequency of short-periodspectroscopic binaries on the upper main sequence. Among the 14 O-typestars, covering all luminosity classes from dwarfs to supergiants, 8 aredefinitively double-lined systems and all periods but one are shorterthan 7 days. Several additional binaries have been detected among theearly B-type stars. NGC 6231 is an exceptional cluster to constrain thescenarios of cluster- and binary-star formation over a large range ofstellar masses. We discuss the evidences, based on NGC 6231 and 21 otherclusters, with a total of 120 O-type stars, for a clear dichotomy in themultiplicity rate and structure of very young open clusters containingO-type stars in function of the number of massive stars. However, wecannot answer the question whether the observed characteristics resultfrom the formation processes or from the early dynamical evolution.
| Spectroscopic Binaries in Young Open Clusters
We have analysed the binarity and multiplicity characteristics of 120O-type stars in 22 very young open clusters and found marked differencesbetween the "rich" (N >= 6 O-type stars and primaries) and "poor" (N= 1) clusters. In the rich clusters, the binary frequencies vary between14% (1 SB among 7 stars) and 80% (8 SBs among 10 stars). Multiplesystems seem not to be frequent and stars are spread all over thecluster area. In poor clusters, the binary frequency of the O-typeobjects is nearly 100%, with orbital periods around 3 days. Severalbinaries are also eclipsing. Additional companions are always present.They form either hierarchical multiple stars or trapezium systems. Thesemassive multiple systems are generally found close to the clustercenter, although there are exceptions.
| 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
| On the evolutionary status of Be stars
We present a study of the incidence of Be stars in open clusters as afunction of the cluster age, using whenever possible ages determinedthrough Strömgren uvby photometry. For the first time in studies ofthis kind we have considered separately classical and Herbig Be stars.The main results can be summarized as follows: Clusters associated toemitting nebulosities and undergoing stellar formation are rich inemission line objects, which most likely are all pre main-sequencestars. No bona fide classical Be star has yet been identified amongthem. Clusters younger than 10 Myr and without associated nebulosity arealmost completely lacking Be stars, although they have a completeunevolved B main sequence. Classical Be stars appear at an age of 10Myr, and reach the maximum abundance in the age interval 13-25 Myr. Weinterpret our results in the sense that the Be phenomenon is anevolutionary effect which appears in the second half of the mainsequence lifetime of a B star. We propose that it can be related to mainstructural changes happening at this evolutionary phase, which also leadto the recently discovered non-monotonic helium abundance enhancement.The semiconvection or turbulent diffusion responsible of the surfacehelium enrichment, coupled with the high rotational velocity, cangenerate magnetic fields via the dynamo effect and thereby originate theBe phenomenon. Observational tests to this hypothesis are proposed.
| Oxygen and helium abundances in Galactic Hii regions - I. Observations
Absolute integrated line fluxes of Hii regions have been measured usinga Fabry-Perot spectrophotometer. We describe the observations andcalibration procedures. Fluxes are given for 36 Hii regions withGalactocentric distances ranging from 6.6 to 17.7kpc. Several emissionlines have been measured, mainly [Oii] λλ3726 and 3629,Hβ, [Oiii] λ5007, Hei λ5876 and Hα. The veryfaint [Oiii] λ4363 line has been measured in six regions,allowing a direct determination of the electron temperature. Newphotometric distances have been derived based on data from theliterature. A discussion of these results in terms of extinction,electron density and temperature, and oxygen and helium abundances isgiven in Paper II.
| Statistical parallaxes and kinematical parameters of classical Cepheids and young star clusters
The statistical-parallax method is applied for the first time to spacevelocities of 270 classical Cepheids with proper motions adopted fromHIPPARCOS (1997) and TRC (Hog et al. 1998) catalogs and distances basedon the period-luminosity relation by Berdnikov et al. (1996). Thedistance scale of short-period Cepheids (with periods less than 9 days)is shown to require an average correction of 15-20%, whereas statisticalparallaxes of Cepheids with periods > 9 days are found to agree wellwith photometric distances. It is shown that the luminosities ofshort-period Cepheids must have been underestimated partly due to thecontamination of this subsample by a substantial (20 to 40%) fraction offirst-overtone pulsators. The statistical-parallax technique is alsoapplied for the first time to 117 open clusters younger than 100 millionyears and with proper motions reduced to the HIPPARCOS reference system.It is concluded that a 0.12-0.15 mag increase of the distance scales ofopen clusters and Cepheids would be sufficient to reconcile thestatistical-parallax results inferred for these two types of objects.Such approach leads to an LMC distance modulus of less than 18.40 mag,which agrees, within the errors, with the short distance scale for RRLyrae variables and is at variance with the conclusions by Feast andCatchpole (1998) and Feast et al. (1998), who argue that the LMCdistance modulus should be increased to 18.70 mag. The distance scalebased on the Cepheid period-luminosity relation by Berdnikov and Efremov(1985) seems to be a good compromise. Extragalactic distances, whichrely on long-period Cepheids, seem to require no substantial correction.In addition to statistical parallaxes, kinematical parameters have beeninferred for the combined sample consisting of Cepheids andopen-clusters: solar-motion components (U0 ,V0,W0) = (9, 12, 7) km/s (+/- 1 km/s); velocity-ellipsoid axes(σU; σV; σW) = (15.0,10.3, 8.5) km/s (+/- 1 km/s); the angular velocity of rotation of thesubsystem, ω0 = 28.7 +/- 1 km/s/kpc, the Oort constantA = 17.4 +/- 1.5 km/s, and the second derivative of angular velocity,⋰ω0= 1.15 +/- 0.2 km/s/kpc3.
| BV Photometry of Northern Galactic Plane Luminous Stars. II. Faint Stars in the Second Galactic Quadrant
CCD-based BV photometry of 28 stars listed in volumes I, III, and V ofthe Case-Hamburg Luminous Stars in the Northern Milky Way catalog isreported. Twenty-four stars have B>12, including the binary-nucleusplanetary nebula M1-2.
| Pre-Main-Sequence Stars in the Young Galactic Cluster IC 4996: A CCD Photometric Study
The results of a UBV CCD photometric study of the cluster IC 4996 arepresented. We obtain new values for the cluster parameters: E(B-V) =0.71 +/- 0.08, V_0 - M_V = 11.9 +/- 0.1, and age = (7.5 +/- 3) x 10^6yr. The combined evidence from the photometric diagrams suggests thepresence of a number of pre-main-sequence (PMS) cluster members withspectral types ~A0 to F5. The interpretation of these objects as PMScandidates is further supported by independent results from CCD uvbyHβ observations. If confirmed, these stars would bridge theexisting gap in the sample of PMS stars, between the coolest Herbig AeBestars (HAeBe), and the hottest T Tauri stars. These PMS candidates arelocated some 0.5 and 1 mag above the main sequence in the V-(B-V)diagram, around the location of spectral types AF. This feature,together with their locations in the (U-B)-(B-V) diagram and theinterpretation that they are PMS members, suggest an additionalreddening law with a slope alpha = E(U-B)/E(B-V) ~= 0.55, probablycaused by circumstellar material.
| 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.
| Time-series spectropolarimetry of the short-period Wolf-Rayet+O star binary CQ Cephei
We present time-resolved, very precise linear spectropolarimetry of theshort-period WR+O star binary CQ Cephei. We find that the N IV 4058 Aline is the best spectral diagnostic of the WR orbital motion, and wederive a semiamplitude K(WR) = 290 +/- 1 km/s and a systemic velocitygamma = -72 +/- 1 km/s. We identify an antiphase spectral feature thatwe attribute to He I 4143 A absorption by the O star. A semiamplitude of360 +/- 18 km/s is found for the O star, giving a mass ratio (WR:O) of1.24 +/- 0.06. Spectropolarimetric measurements, combined with publishedphotopolarimetry, enable a determination of the orbital inclination. Theemission-line polarization vectors allow us to separate the intrinsicand interstellar polarizations, and we find that the polarizationattributable to scattering of the WR radiation in the expanding wind issmall but may be increasing towards the UV. We note that theinclinations derived from photometric analyses of eclipsing WR+Obinaries are systematically lower than those computed from polarimetricmeasurements. The light curves from Hiltner are analyzed with LIGHT2 toconfirm that the system is in a near-contact state. The resultant massesand astrophysical parameters show a normal main-sequence O star with aWR star that has parameters typical of the WN 6 type. The distancemodulus for CQ Cep derived from our results is (V0 - MV) = 12.0.
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