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XMM-Newton and the Pleiades - I. Bright coronal sources and the X-ray emission from intermediate-type stars
We present results of X-ray spectral and timing analyses of solar-like(spectral types F5-K8) and intermediate-type (B4-F4) Pleiads observed ina 40-ks XMM-Newton EPIC exposure, probing X-ray luminosities(LX) up to a factor 10 fainter than previous studies usingthe ROSAT PSPC. All eight solar-like members have`quasi-steady'LX>~ 1029erg s-1,consistent with the known rotation-activity relation and four exhibitflares. Using a hydrodynamic modelling technique, we derive loophalf-lengths for the two strongest flares, on H II 1032 and H II 1100.Near the beginning of its flare, the light curve of H II 1100 shows afeature with a profile suggestive of a total occultation of the flaringloop. Eclipse by a substellar companion in a close orbit is possible butwould seem an extraordinarily fortuitous event; absorption by afast-moving cloud of cool material requires NH at least twoorders of magnitude greater than any solar or stellar prominence. Anoccultation may have been mimicked by the coincidence of two flares,though the first, with its decay time being shorter than its rise timeand suggestive of , would be unusual.Spectral modelling of the quasi-steady emission shows a rising trend incoronal temperature from F and slowly rotating G stars to K stars tofast-rotating G stars, and a preference for low coronal metallicity.These features are consistent with those of nearby solar-like stars,although none of the three stars showing `saturated' emission bears thesignificant component at 2 keV seen in the saturated coronae of AB Dorand 47 Cas. Of five intermediate-type stars, two are undetected(LX < 4 × 1027erg s-1) andthree show X-ray emission with a spectrum and LX consistentwith origin from an active solar-like companion.
| Search for X-ray flares in the Pleiades using SoHO LASCO C3 images.
Not Available
| The Lithium-Rotation Correlation in the Pleiades Revisited
The dispersion in lithium abundance at fixed effective temperature inyoung cool stars like the Pleiades has proved a difficult challenge forstellar evolution theory. We propose that Li abundances relative to amean temperature trend, rather than the absolute abundances, should beused to analyze the spread in abundance. We present evidence that thedispersion in Li equivalent widths at fixed color in cool singlePleiades stars is at least partially caused by stellar atmosphereeffects (most likely departures from ionization predictions of modelphotospheres) rather than being completely explained by genuineabundance differences. We find that effective temperature estimates fromdifferent colors yield systematically different values for active stars.There is also a strong correlation between stellar activity and Liexcess, but not a one-to-one mapping between unprojected stellarrotation (from photometric periods) and Li excess. Thus, it is unlikelythat rotation is the main cause for the dispersion in the Li abundances.Finally, there is a significant correlation between detrended Li excessand potassium excess but not calcium-perhaps supporting incompleteradiative transfer calculations (and overionization effects inparticular) as an important source of the Li scatter. Other mechanisms,such as very small metallicity variations and magnetic fields, whichinfluence pre-main-sequence Li burning may also play a role. Finally, wefind no statistical evidence for a decrease in dispersion in the coolestPleiades stars, contrary to some previous work.
| Statistics of binaries in the Pleiades cluster
We present a statistical analysis of binaries in galactic clusters,based on photometric properties. Synthetic clusters are used to solvethe deconvolution problem. If the colour-magnitude relation for singlestars is given and if triple stars are treated as binaries, thedistribution of systems in a colour-magnitude diagram (CMD) can be usedto determine cluster properties. They include the maximum number ofsingle stars N1max which is compatible with the distributionof systems, a standard deviation sigma (describing the scatter of starson the main sequence), a critical mass ratio q_c, and the mass ratiodistribution in the case of N_1=N1max. In the general case(N_1<= N1max) the mass ratio distribution can bedetermined for q>q_c, and in favourable cases (if N_1 is sufficientlylarge) for all mass ratios. A first application concerns thePleiades cluster in the colour range 0.2<= B-V<0.98. The concentration of systems near the main sequence in the CMDis used to derive an approximation for the colour-magnitude relation. Asmall positive number c is involved as a parameter. The mass ratiodistribution depends sensitively on c and increases towards small massratios, at least up to q =~ 0.5 and probably up to q =~ 0.3. Thedistribution is bimodal, with a peak at q=1. Photometric arguments showthat c<~ 0.02. A binary frequency of 60-70% as expected from clustersimulations (Kroupa \cite{krou}) requires c<~ 0.03. An adjustment oftwo parameters (c=0.02 and a 70% binary frequency) is sufficient toreproduce the mass ratio distribution for binaries in the galacticfield. This suggests that the mass ratio distribution in the Pleiades issimilar to the distribution in the field, in accordance with aconjecture of Bouvier et al. (\cite{brn}, BRN).
| The Pleiades and alpha Persei Clusters
The upper-main-sequence members of the Pleiades and alpha Perseiclusters, considered as members of the Local Association, yield meanparallaxes that are only 4% larger than the mean values from Hipparcosobservations. The (log T_eff, M_V) diagram reveals that in thetemperature range from 6000 to 8000 K, the Hyades and alpha Perseimain-sequence members are nearly identical and several tenths of amagnitude brighter than similar stars on the Pleiades main sequence. Thedeparture of the Pleiades main sequence cannot be traced to either ageor heavy-element abundance differences in the range thought to apply tothese clusters. A 50% increase in the helium abundance of Pleiades overHyades stars could account for the luminosity difference. Alternativeexplanations are that the Pleiades cluster is rejected from superclustermembership and/or that the Hipparcos parallax results for the Pleiadesare in error by some 10%.
| Initial Results of a Comprehensive Ultrasoft Survey of the Einstein IPC Database: Source List and Confirmation of the Selection Procedure
We have carried out a survey of the entire Einstein IPC database (atotal of 5934 sources) to select objects with strong components ofultrasoft X-ray emission (i.e., emission below 0.5 keV). The selectioncriteria are based on ratios of counts within three broad energy bands(i.e., X-ray ``colors''). A total of 516 objects have been selected,with 202 (39%) having been identified through correlations with otherdatabases. Of the currently identified objects, 45% are active galacticnuclei, 28% are stellar objects, 6% are ``normal'' galaxies and galaxyclusters, and the remaining 21% are ``nonstellar'' Galactic objects suchas cataclysmic variables, pulsars, and white dwarfs. We present adiscussion of the survey selection procedure, the list of selectedsources, and discussions of interesting objects from the aforementionedgroups. We discuss the remarkable success of our rather ``lowresolution'' method of estimating spectral shape and present plans forfollow-up studies of the as yet unidentified sources.
| ROSAT Observations of the Pleiades. I. X-Ray Characteristics of a Coeval Stellar Population
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJS..102...75M&db_key=AST
| High-precision positions and proper motions of 441 stars in the Pleiades astrometric standard region.
Not Available
| Spectral and Temporal Characteristics of X-Ray--bright Stars in the Pleiades
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...450..217G&db_key=AST
| Photometric Survey Near the Main Galactic Meridian - Part One - Photoelectric Stellar Magnitudes and Colours in the UBVR System
Not Available
| Membership probabilities in the Pleiades field.
A catalogue of proper motions and photographic B, V magnitudes for starsup to B=19mag within a region centered near Alcyone is presented. Thecatalogue is based on MAMA measurements of 8 plates taken with theTautenburg Schmidt telescope. The survey includes ca. 14500 stars andcovers a total field of about 9 square degrees. For stars brighter thanB=18mag an internal accuracy of 0.05mag and 2.5mas/year has beenestimated for photometric data and proper motions, respectively.Membership probabilities, proper motions and B, V magnitudes are listedfor 442 stars up to B=19mag in the Pleiades field.
| A deep imaging survey of the Pleiades with ROSAT
We have obtained deep ROSAT images of three regions within the Pleiadesopen cluster. We have detected 317 X-ray sources in these ROSAT PositionSensitive Proportional Counter (PSPC) images, 171 of which we associatewith certain or probable members of the Pleiades cluster. We detectnearly all Pleiades members with spectral types later than G0 and within25 arcminutes of our three field centers where our sensitivity ishighest. This has allowed us to derive for the first time the luminosityfunction for the G, K, amd M dwarfs of an open cluster without the needto use statistical techniques to account for the presence of upperlimits in the data sample. Because of our high X-ray detection frequencydown to the faint limit of the optical catalog, we suspect that some ofour unidentified X-ray sources are previously unknown, very low-massmembers of Pleiades. A large fraction of the Pleiades members detectedwith ROSAT have published rotational velocities. Plots ofLX/LBol versus spectroscopic rotational velocityshow tightly correlated `saturation' type relations for stars with ((B -V)0) greater than or equal to 0.60. For each of several colorranges, X-ray luminosities rise rapidly with increasing rotation rateuntil c sin i approximately equal to 15 km/sec, and then remainsessentially flat for rotation rates up to at least v sin i approximatelyequal to 100 km/sec. The dispersion in rotation among low-mass stars inthe Pleiades is by far the dominant contributor to the dispersion inLX at a given mass. Only about 35% of the B, A, and early Fstars in the Pleiades are detected as X-ray sources in our survey. Thereis no correlation between X-ray flux and rotation for these stars. TheX-ray luminosity function for the early-type Pleiades stars appears tobe bimodal -- with only a few exceptions, we either detect these starsat fluxes in the range found for low-mass stars or we derive X-raylimits below the level found for most Pleiades dwarfs. The X-ray spectrafor the early-type Pleiades stars detected by ROSAT areindistinguishable from the spectra of the low-mass Pleiades members. Webelieve that the simplest explanation for this behavior is that theearly-type Pleiades stars are not themselves intrinsic X-ray sources andthat the X-ray emission actually arises from low-mass companions tothese stars.
| The structure of the galactic halo outside the solar circle as traced by the blue horizontal branch stars
A complete sample of blue horizontal branch (BHB) stars in the magnituderange 13.0 less than or equal to V less than or equal to 16.5 isisolated in two Galactic fields that have previously been searched forRR Lyrae variables: SA 57 in the Northern Polar Cap and the LickAstrograph field RR 7 in the Anticenter (l = 183 deg, b = +37 deg).These BHB stars are a subset of the AF stars found in the CaseLow-Dispersion Northern Survey; lists of these AF stars were madeavailable by the late Nick Sanduleak. In the color range 0.00 less thanor equal to (B - V)0 less than or equal to +0.20, we candistinguish the BHB stars among these AF stars by comparing them bothwith well known local field horizontal branch (FHB) stars and also theBHB members of the halo globular clusters M3 and M92. The criteria forthis comparison include (1) a (u - B)K color index (derivedfrom photoelectric observations using the Stroemgren u filter and theJohnson B and V filters) that measures the size of the Balmer jump, (2)a spectrophotometric index lambda that measures the steepness of theBalmer jump, and (3) a parameter D0.2 that is the mean widthof the H-delta and H-gamma Balmer lines measured at 20 percent of thecontinuum level. These criteria give consistent results in separatingBHB stars from higher gravity main sequence AF stars in the color range0.00 less than or equal to (B - V)0 less than or equal to+0.20. All three photometric and spectrophotometric criteria weremeasured for 35 stars in the SA 57 field and 37 stars in the RR 7 fieldthat are in the color range (B - V)0 less than or equal to+0.23 and in the magnitude range 13.0 less than or equal to V less thanor equal to 16.5. For a small number of additional stars only (u -B)K was obtained. Among the AF stars that are fainter than B= 13 and bluer than (B - V)0 = +0.23, about half of those inthe SA 57 field and about one third of those in the lower latitude RR 7field are BHB stars. Isoabundance contours were located empirically inplots of the pseudoequivalent width versus (B - V)0 for thelines of Mg II lambda-4481 A, Ca II lambda-3933 A, and Fe I lambda-4272A. Solar abundances were defined by the data from main sequence stars inthe Pleiades and Coma open clusters. Data from the BHB stars in M3 andM92 defined the (Fe/H) = -1.5 and -2.2 isoabundance contours,respectively. Metallicities of all stars were estimated by interpolatingthe measured pseudoequivalent widths in these diagrams at the observed(B - V)0. The distribution of (Fe/H) found for the BHB starsin this way is very similar to that which we found for the RR Lyraestars in the same fields using the Preston Delta-S method. The spacedensities of these BHB stars were analyzed both separately and togetherwith earlier observations of field BHB stars. This analysis supports atwo-component model for the halo of our Galaxy that is similar in manyrespects to that proposed by Hartwick although our discussion refersonly to the region outside the solar circle.
| Rotation and chromospheric emission among F, G, and K dwarfs of the Pleiades
High-resolution echelle spectra of more than 100 F, G, and K dwarfs inthe Pleiades are reported. Chromospheric activity in these stars ismeasured via comparisons of the profiles of H-alpha and the Ca II IRtriplet to chromospherically inactive field stars. Consistent dereddenedcolors are determined from the available photometry and temperatures arederived. Most G and K dwarfs in the Pleiades rotate slowly, but about 20percent of the stars are ultrafast rotators (UFRs). That fraction ofUFRs is independent of color, and the highest rotation rates are foundamong the K dwarfs. The Pleiades exhibit a broad range in the strengthof chromospheric emission at any one color. Most G and K dwarfs in thePleiades show H-alpha and the IR triple in absorption, with filling inof the line cores.
| The evolution of the lithium abundances of solar-type stars. III - The Pleiades
New measurements of lithium in more than 100 Pleiades F, G, and K dwarfsare reported. Abundances are determined from spectrum synthesis fits tothe data as well as from use of new covers of growth from the Li 6708-Afeature. It is argued that most Late-F and early-G dwarfs in thePleiades are consistent with the tight N(Li) vs mass relation seen inthe Hyades in the same mass range. Most Li-rich stars have abundances ator near the primordial level for Population I, and none exceed thatlevel by a significant amount. At any given color the stars that rotatefast have the most Li and have the strongest chromospheric activity.Ways in which an apparent spread in N(Li) could arise from anintrinsically tight n(Li)-mass relation are considered, and it isconcluded that the spread is probably real and is not an artifact ofline formation conditions or inhomogeneous atmospheres on the stars.
| CA II H and K measurements made at Mount Wilson Observatory, 1966-1983
Summaries are presented of the photoelectric measurements of stellar CaII H and K line intensity made at Mount Wilson Observatory during theyears 1966-1983. These results are derived from 65,263 individualobservations of 1296 stars. For each star, for each observing season,the maximum, minimum, mean, and variation of the instrumental H and Kindex 'S' are given, as well as a measurement of the accuracy ofobservation. A total of 3110 seasonal summaries are reported. Factorswhich affect the ability to detect stellar activity variations andaccurately measure their amplitudes, such as the accuracy of the H and Kmeasurements and scattered light contamination, are discussed. Relationsare given which facilitate intercomparison of 'S' values with residualintensities derived from ordinary spectrophotometry, and for convertingmeasurements to absolute fluxes.
| Radial velocity measurements in the Pleiades
High-resolution radial velocities for 71 stars of both early and latespectral types in the Pleiades cluster have been obtained in order toinvestigate the possibility of establishing an early-type star velocitysystem by bootstrapping late-type velocity standards to early-type onesin young clusters. It is shown that in the absence of early-typevelocity standards the velocity scale of the early-type spectra in thecluster can be tied into the IAU velocity system by examining thevelocity distributions of both the early-type and the late-type stars,since the internal velocity dispersion in an open cluster is less thanabout 1 km/s. It is found that the cluster velocity of the Pleiades is6.0 km/s, with an observed velocity dispersion of 1 km/s. Based on thesevelocity results, a list of potentially constant velocity B and A starsis proposed for further studies to confirm the constancy of theirvelocities.
| X-ray studies of coeval star samples. II - The Pleiades cluster as observed with the Einstein Observatory
Coronal X-ray emission of the Pleiades stars is investigated, andmaximum likelihood, integral X-ray luminosity functions are computed forPleiades members in selected color-index ranges. A detailed search isconducted for long-term variability in the X-ray emission of those starsobserved more than once. An overall comparison of the survey resultswith those of previous surveys confirms the ubiquity of X-ray emissionin the Pleiades cluster stars and its higher rate of emission withrespect to older stars. It is found that the X-ray emission from dA andearly dF stars cannot be proven to be dissimilar to that of Hyades andfield stars of the same spectral type. The Pleiades cluster members showa real rise of the X-ray luminosity from dA stars to early dF stars.X-ray emission for the young, solarlike Pleiades stars is about twoorders of magnitude more intense than for the nearby solarlike stars.
| Determination of the Proper Motions for Stars in the Region of the Pleiades - Part One
Not Available
| Lithium in old open clusters - NGC 188
Echelle spectra which include the Li I line at 6707 A are reported forseven main-sequence stars and one subgiant in NGC 188. The Li I line isdetected in five of the six dwarfs which are highly probable clustermembers. The derived atmospheric Li/H ratios exceed the solar value byfactors ranging approximately from 10 to 40, although these apparentlyclosely solarlike stars are about twice as old as the sun. The variationof the lithium abundance with stellar mass along the main sequences ofthe Pleiades, the Hyades, NGC 752, and NGC 188 are compared. Theresulting evolutionary pattern indicates that the lithium fraction inthe Galactic gas has shown no appreciable change from Li/H of roughly 10to the -9th since the birth of NGC 188 about 10 Gyr ago, except that theabundance could have been higher by an uncertain but possiblyappreciable factor at the beginning of that epoch.
| The abundance of lithium in pleiades F stars
The abundance of lithium has been determined for 18 stars in thePleiades cluster with spectral types from A7V to G0V. The pronounced dipin the lithium abundance among the mid-F stars which has been reportedfor other, older star clusters is not present in the Pleiades. Theremoval of lithium from the surfaces of middle-F dwarfs therefore occursprincipally after about 100 Myr on the main sequence.
| Interstellar extinction in the direction of the Merope dark cloud in Pleiades
The dependence of interstellar extinction on distance in the directionof the dark cloud south of Merope is determined using photoelectricphotometry of 93 stars in the Vilnius photometric system. The cloudfront edge is detected at 120-130 pc from the sun, and the distance ofthe Pleiades cluster is found to be 127 pc. Mean extinction, A(V), inthe Merope cloud is of the order of 1.0 mag. There is no evidence ofextinction at distances exceeding the Merope cloud distance. Thevariable extinction method yields R = A(V)/E(B-V) = 3.6, while themaximum polarization wavelength method gives the value 3.4. SomePleiades stars are suspected to be unresolved binaries.
| The distance of the Pleiades cluster and the calibration of photometric luminosities for early-type stars
An examination of the lower main-sequence (mode-A) stars in the Pleiadescluster suggests an Fe/H abundance ratio between 0.0 and 0.1 dex with aresulting modulus of 5.65 + or - 0.1 mag, and fundamental defects in thecalculation of Balona and Shobbrook (1984), with an adopted modulus of5.50 mag, are discussed. It is suggested that the ZAMS of Balona andShobbrook, and of Mermilliod (1981), are too bright due to theirassumption that the color-luminosity arrays of such clusters as thePleiades represent isochrones, leading to uncertainties in the ZAMS,particularly with respect to slope. Several recently publishedphotometric luminosity calibrations for early-type stars may beincorrect due to their failing to recognize the probable presence of atleast two evolutionary modes and the apparent absence of ZAMS stars nearthe sun.
| The Pleiades cluster. III - Polarization, reddening, and the unusual distribution of interstellar matter
A polarization survey of Pleiades cluster members and nonmembers hasbeen carried out together with a determination of interstellar reddeningfor cluster members. The ISM in front of and inside the cluster leads tovariable polarization and reddening across the cluster. The optical datacorrelate well with available radio and millimeter features. Twodistinct ISM concentrations are isolated: (1) a uniform foreground sheetin front of the whole cluster which is seen as constant reddening andpolarization in the eastern part of the cluster; and (2) additionalintracluster material, located mainly in the western part of thecluster.
| A VBLUW photometric survey of the Pleiades cluster
Photometric data are presented for 390 known or suspected members of thePleiades cluster. The data were obtained in 1979 at ESO using theWalraven VBLUW photometer and the Dutch 91-cm telescope. A comparisonwas made with subsets obtained with the same telescope andinstrumentation at the former Leiden Southern Station at the SAAO annexduring 1976 and 1977, and with data obtained at ESO during 1980 and1981. The much improved performance of the telescope and the photometerat their new site is obvious from these comparisons. The stars measuredcomprise the selection of possible members by Hertzsprung (1947)brighter than m(pg) = 14.5 and a selection of possible members in theouter region of the cluster by Pels et al. (1975). Of the starsselected, 66 were found not to be members. A few of these are possiblyescaping members. The present data set provides a well determined mainsequence over the range K2V to B9V, as well as data on some of theprobably premain-sequence K2-to-K5 and postmain-sequence B8-to-B6 stars.Finding charts for the stars selected by Pels are presented in anappendix.
| Rotational studies of lower main sequence stars in nearby open galactic clusters. I - Velocity distributions and age dependence
More than 300 new v sin i values are determined by means of thespectrometer CORAVEL in the following four nearby open galacticclusters: Pleiades, Hyades, Praesepe, and Coma Ber. After excluding allvariable stars and known binaries the sample still contains about 200stars. A determination of the velocity distribution in each cluster ispresented which shows that for the Hyades the velocity distribution isconsistent with a Dirac function. By comparing the mean rotationalvelocity of the different clusters it is shown that there is equal timedependence of rotation for stars of spectral type F8 V to K3 V and thatthe t to the -1/2 law for the rotational decay does not fit well thedata. The v sin i determined for some photometric variable K dwarfs ofthe Pleiades seem to be in disagreement with a rotational modulation ofluminosity.
| Refined Data for Parallax Stars
Not Available
| The Pleiades and the zero-age main sequence.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1976PASP...88..144M&db_key=AST
| Four-color and H-beta photometry for open clusters. XI - The Pleiades
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1976AJ.....81..419C&db_key=AST
| A spectroscopic investigation of the Pleiades.
Not Available
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