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Search for X-ray flares in the Pleiades using SoHO LASCO C3 images.
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| The Tokyo PMC catalog 90-93: Catalog of positions of 6649 stars observed in 1990 through 1993 with Tokyo photoelectric meridian circle
The sixth annual catalog of the Tokyo Photoelectric Meridian Circle(PMC) is presented for 6649 stars which were observed at least two timesin January 1990 through March 1993. The mean positions of the starsobserved are given in the catalog at the corresponding mean epochs ofobservations of individual stars. The coordinates of the catalog arebased on the FK5 system, and referred to the equinox and equator ofJ2000.0. The mean local deviations of the observed positions from theFK5 catalog positions are constructed for the basic FK5 stars to comparewith those of the Tokyo PMC Catalog 89 and preliminary Hipparcos resultsof H30.
| High-precision positions and proper motions of 441 stars in the Pleiades astrometric standard region.
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| COYOTES II: SPOT properties and the origin of photometric period variations in T Tauri stars.
We present the results of a new multi-site campaign (COYOTES II) tomonitor the light variations of T Tauri stars (TTS) of the Taurus-Aurigadark cloud. The UBVRI light curves of 19 TTS were obtained over a twomonths period to search for rotational modulation by spots. We reportnew period detections for IQ Tau (6.25d), LkCa-3 (7.2d), and LkCa-14(3.35d) and confirm previously detected periods for DF Tau (9.8d), DRTau (9.0d), GM Aur (11.9d), and TAP 26 (2.58d). We also report tentativeperiods for CW Tau (8.2d), CY Tau (7.9d), HP Tau (5.9d), and XZ Tau(2.6d). No periods were found in the present data set for CI Tau, DGTau, DQ Tau, GH Tau, RY Tau, Hubble 4, TAP 45, and TAP 57NW. Altogether,the results of this new campaign confirm the main conclusion of COYOTESI that classical T Tauri stars (CTTS) on average have longer rotationalperiods than weak-line TTS (WTTS). The present study also confirms thatrotational modulation in WTTS is due to spots cooler than thephotosphere and we show that the amplitude of the modulation primarilyreflects the amount of areal coverage by spots. The amplitude of thelight variations, and hence the spot size, is found to increase withboth rotation rate and advancing spectral type, as expected if WTTS coolspots correspond to photospheric regions of strong dynamo-generatedmagnetic fields. Finally, combined with previous studies, these newresults provide further evidence for temporal variations of thephotometric periods of CTTS. Such variations seem to occurpreferentially in CTTS whose rotational modulation is dominated by hotspots and we therefore argue that the observed period changes are linkedto the magnetospheric accretion process rather than to surfacedifferential rotation.
| 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.
| COYOTES - Part One - Multisite UBVRI Photometry of 24 Pre Main Sequence Stars of the Taurus-Auriga Cloud
Abstract image available at:http://adsabs.harvard.edu/abs/1993A&AS..101..485B
| Analysis of Neckel and Chini standard stars in the UBVRI photometric system
This paper studies the list of 60 faint standard stars of Neckel andChini (1980) in the UBVRI Johnson photometric system. We have re-reducedthe standard stars using our own method of reduction from a large numberof selected observations made at the Observatorio Astronomico Nacionaland at the Centro Astronomico Hispano-Aleman, both at Calar Alto, Spain,and at the Observatorio del Roque de los Muchachos, La Palma Island,Spain. The reduction method used is explained and the results are givenfor the four colors used: (B-V), (U-B), (V-R), and (V-I), together withthe V magnitude, that is, we have corrected many standard stars for eachcolor and magnitude. These results are supported with graphs whereresidual color is plotted against corresponding color (or magnitude).Finally, our (V-R) and (V-I) results are compared to those of Taylor etal. (1989), with a good correspondence.
| Simultaneous high-speed UBVRI-photometry of RW Tri in 1985-1990.
Not Available
| The properties of the Neckel-Chini VRI system
Cousins (1980) data for 54 of the standard stars of Neckel and Chini(1980) and published measurements are used to investigate the propertiesof the Neckel-Chini VRI system. For red stars, this system diverges fromthe Johnson (1962) system, despite frequent claims of identity betweenthe two. The Neckel-Chini and Cousins systems, however, are closelycomparable. Both of these conclusions were previously reached in a paperby Bessell (1983); fair to good quantitative agreement with his resultsare obtained. Reddening ratios, the scatter in the Neckel-Chinistandard-star data, and the effect of this scatter on publishedmeasurements for program stars, are discussed. Transformations from theNeckel-Chini system to the Cousins system are given.
| Determination of the Proper Motions for Stars in the Region of the Pleiades - Part One
Not Available
| Sixty faint UBVRI standards
A list of 60 faint UBVRI standards is presented. Their UBV magnitudesare in the system introduced by Johnson (1955) and their V-R and V-Icolors are in the system of the Arizona Tonantzintla catalogue byIriarte et al. (1965). These stars are spread over an equatorial belt atan angle less than 30 deg, so that they can be observed as well fromnorthern as from southern sites.
| A spectroscopic investigation of the Pleiades.
Not Available
| Catalog of Indidual Radial Velocities, 0h-12h, Measured by Astronomers of the Mount Wilson Observatory
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1970ApJS...19..387A&db_key=AST
| Accurate positions of 502 stars in the region of the Pleiades
Not Available
| Star in the group of the Pleiades but not belonging to the physical system.
Not Available
| Photovisual magnitudes of the stars in the Pleiades.
Not Available
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