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A search for previously unrecognized metal-poor subdwarfs in the Hipparcos astrometric catalogue
We have identified 317 stars included in the Hipparcos astrometriccatalogue that have parallaxes measured to a precision of better than 15per cent, and the location of which in the(MV,(B-V)T) diagram implies a metallicitycomparable to or less than that of the intermediate-abundance globularcluster M5. We have undertaken an extensive literature search to locateStrömgren, Johnson/Cousins and Walraven photometry for over 120stars. In addition, we present new UBV(RI)C photometry of 201of these candidate halo stars, together with similar data for a further14 known metal-poor subdwarfs. These observations provide the firstextensive data set of RCIC photometry ofmetal-poor, main-sequence stars with well-determined trigonometricparallaxes. Finally, we have obtained intermediate-resolution opticalspectroscopy of 175 stars. 47 stars still lack sufficient supplementaryobservations for population classification; however, we are able toestimate abundances for 270 stars, or over 80 per cent of the sample.The overwhelming majority have near-solar abundance, with theirinclusion in the present sample stemming from errors in the colourslisted in the Hipparcos catalogue. Only 44 stars show consistentevidence of abundances below [Fe/H]=-1.0. Nine are additions to thesmall sample of metal-poor subdwarfs with accurate photometry. Weconsider briefly the implication of these results for clustermain-sequence fitting.
| Time-series photometric spot modeling. IV. The multi-periodic K5Ve binary V833 Tauri
We analysed all the existing photometric observations of V833 Tau andfound long-term variability on time scales of ~70 years, 6.5 years and2.4 years. Using V and IC-band data from 1990-2000, weobtained starspot surface distributions from 20 suitable light curves.We found that the spot-area changes generally follow the long-term lightvariations. Spot temperatures are determined for each of the datasetsand an average value of 1230 K+/-270 K relative to the photosphere isfound. This value agrees with previous results from TiO modeling (1330K+/-110 K). Small scale spot temperature and area changes are recoveredduring 1997-1999, that we explain with a variable spot/faculae ratio. Apowerful flare in UBV(RI)C was observed in 1993 November. A10 000 K fit yielded a total emitted flare energy of more than1034 ergs from an area of about 1.3% of the stellar surface,60% of it in the V(RI)C bands. The flare characteristics arecompared to another, 100-times smaller flare observed earlier in 1983.
| Starspot photometry with robotic telescopes. UBV(RI)_C and by light curves of 47 active stars in 1996/97
We present continuous multicolor photometry for 47 stars from October1996 through June 1997. Altogether, 7073 V(RI)_c, UBV, and by datapoints, each the average of three individual readings, were acquiredwith three automatic photoelectric telescopes (APTs) at FairbornObservatory in southern Arizona. Most of our targets arechromospherically active single and binary stars of spectral type G to Kbut there are also four pre-main-sequence objects and three pulsatingstars in our sample. The light variability is generally due torotational modulation of an asymmetrically spotted stellar surface andtherefore precise rotational periods and their seasonal variations aredetermined from Fourier analysis. We also report on photometricvariations of gamma CrB (A0V) with a period of 0.44534 days. All dataare available in numerical form. All data are available from CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The spottedness of V775 Her, V833 Tau, and YZ CMi from long-term photometric observations
Photographic measurements and UBVRI photometry are used to calculate thephotospheric spottedness of the three active red dwarf stars V775 Her,V833 Tau, and YZ CMi in the framework of a zonal spot model. The spottedregions can cover large fractions of the stellar surfaces: up to 42% ofV775 Her, 68% of V833 Tau, and 21% of YZ CMi. The differences in thetemperatures of the quiescent photosphere and spots are 900, 500, and420 K, respectively.
| The Henry Draper Extension Charts: A catalogue of accurate positions, proper motions, magnitudes and spectral types of 86933 stars
The Henry Draper Extension Charts (HDEC), published in the form offinding charts, provide spectral classification for some 87000 starsmostly between 10th and 11th magnitude. This data, being highlyvaluable, as yet was practically unusable for modern computer-basedastronomy. An earlier pilot project (Roeser et al. 1991) demonstrated apossibility to convert this into a star catalogue, using measurements ofcartesian coordinates of stars on the charts and positions of theAstrographic Catalogue (AC) for subsequent identification. We presenthere a final HDEC catalogue comprising accurate positions, propermotions, magnitudes and spectral classes for 86933 stars of the HenryDraper Extension Charts.
| A new optical extinction law and distance estimate for the Taurus-Auriga molecular cloud
This paper presents optical spectrophotometry of field stars projectedon the Taurus-Auriga molecular cloud. We derive extinction laws forheavily reddened stars in this sample and show that the reddening lawthrough the dark cloud is nearly identical to the standard law forlambda-lambda(3600-6100) and AV approximately less than 3mag. Our spectroscopic parallaxes suggest a distance of 140 +/- 10 pcfor the northern portion of the cloud and show no compelling evidencefor a substantial variation in distance across the leading edge of thecloud.
| Long-term behaviour of starspots. I - Polar starspots, rotational modulation and flare activity of V 833 Tauri
V 833 Tau is a late-type (dK5e) single lined spectroscopic binary wherethe primary is much brighter than the secondary. This paper presentsmulticolor photometry from 1983 and 1987-1990 to study the lightvariability of the star. In addition, the discovery of flare activity ofV 833 Tau is announced, and it is determined that 0.1 percent of theU-filter flux is due to flaring. Long-term changes on a time scale ofmonths with amplitudes of 0.1-0.2 mag in different filters were found.The system has a low inclination (about 20 deg), so this variability maybe interpreted as due to a changing polar spot area. Small amplitude(0.04 mag) periodic variations were discovered in 1983, presumablycaused by lower latitude spots rotating in and out of view. Superimposedon the longterm variability, such rotational modulation was alsodetected in 1989 and 1990, when the polar spot had a reduced size (i.e.during bright star intervals). The observed magnitude ranges fit thelong-term trend found by Hartmann et al. (1981). The rotational periodof the star is about 1.79 days.
| Magnetic field structure in the Taurus dark cloud
Optical and infrared polarimetry of sources in the direction of theTaurus cloud are obtained in order to study the magnetic field in thiscloud and its possible role in the cloud's evolution. Most of the starsare background giant stars whose light shines through the cloud and ispolarized by the cloud material. The transverse component of themagnetic field, as delineated by the polarization vectors, is generallyperpendicular to the galactic plane, and the stratified structure of thecloud could be due to the effect of the magnetic field during the earlystages of collapse. Three of the 13 embedded stars are stronglypolarized with position angles nearly perpendicular to those of nearbyfield stars. The polarization of these stars is most likely intrinsic,and the direction of polarization indicates that the materialsurrounding these stars may be magnetic i.e., that the magnetic field isfrozen in this material.
| Polarimetric investigation of background stars in the region of T and RY Tau
Electropolarimetric observations of 97 background stars in the TTauri/RY Tauri region, obtained with the 40-cm Cassegrain telescope ofthe Biurakan Astrophysical Observatory in December 1982, are reportedand interpreted in terms of local-magnetic-field effects oncometary-nebula bending. The data are presented in a table, and thedependence of polarization position angle on declination is establishedin a graph and attributed to a continuous variation in the direction ofthe magnetic field. From the magnitude of the effect, however, it isconcluded that the observed bending of the cometary nebulae in theregion results from the combined influence of the local magnetic momentand the magnetic moment of the star itself (as proposed by Vardanian,1983) rather than from the local moment alone.
| U, B, V, R, I stellar photometry in the field of the Taurus dark clouds
A program of photoelectric stellar photometry has been carried out inthe field of the Taurus dark clouds with a view to establishing thedistance of the clouds and the absorption law in that region of the sky.It is found that the distance of the dark clouds accompanying the Tau T1and Tau T3 associations is 132 plus or minus 10 pc. Within theabsorption region 0-4 m, the Taurus dark clouds are characterized by anearly normal absorption law (to within 5-10%). In the field of the darkclouds a group of F0-G0 stars close to the main sequence is found whosespace density is three to eight times that in the solar neighborhood.
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