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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
| Doppler imaging of stellar surface structure. XI. The super starspots on the K0 giant HD 12545: larger than the entire Sun
The active K0 giant HD 12545 is the star with the largest starspots everobserved. We report a record light-curve amplitude of 0\fm63 in V and0\fm17 in V-I in January 1998. At the same time, HD 12545 was at itsbrightest magnitude since the discovery of its light variability in1985. We present the first Doppler image of HD 12545 from observationsduring this state of high-amplitude light variability and find that thestar exhibits one cool high-latitude spot of gigantic dimensions (~12x20solar radii, i.e. 60 times the extension of the largest sunspot group or10,000 times larger areal coverage). Furthermore, our map shows onesmaller cool spot and one equatorial warm (bright) spot. The warm spotis situated approximately 180 degr from the large cool spot in theopposite hemisphere and is the cause of the exceptionally largelight-curve amplitude and the stellar brightening. Within our model, nolight-curve nor a reasonable line-profile solution with only cool spotswas possible. Surprisingly, the fact that HD 12545 is brighter at a timeof high spot activity is in agreement with solar analogy. Our data alsoallow to refine the orbital elements of HD 12545 and we use theHipparcos parallax to determine absolute astrophysical quantities forthis rapidly rotating K giant.
| HD 12545, A Study in Spottedness
We have solved two sets of light curves of HD 12545 at its epoch ofextreme spottedness in 1990-91, one of which coincides with anindependent set analyzed recently by Strassmeier and Olah. Even for thehuge amplitudes observed, these light-curve solutions did not givereliable determinations of several important spot properties.Specifically, we find that we could obtain acceptable solutions for awide range of inclination; that spot temperature depends on inclinationassumed, falling in the range Delta-T = T_star - T_spot = 650-1200 K forinclinations of 70-30 deg; that spot latitudes derived from the lightcurves are unreliable; and that our independent spot solutions disagreewith Strassmeier and Olah's. On a more positive note, changes in thelight curve over the past five years seem to have been caused primarilyby rearrangement of persisting spot groups, and we note that the highlevel of activity implied by its H-alpha emission, makes HD 12545 aprime candidate for a white-light flare star. (SECTION: Stars)
| On the starspot temperature of HD 12545
We have applied two different versions of the photometric spot-modelingtechnique to the spotted RS CVn-type binary HD 12545. NewUBV(RI)c photometry from early 1991 showed the star with anexceptionally large amplitude of 0.50 mag in V and 0.12 mag in V - I anda rotation period of 24.30 +/- 0.02 d which suggests a spot coverage ofapproximately 20 percent of the entire stellar surface. Such large lightand color amplitudes allowed us to separate effects ofwavelength-dependent limb darkening from spot temperature with highconfidence. This in turn makes it possible to obtain the, so far, mostprecise relative starspot temperature of 1100 +/- 35 K for a spotted,late-type star other than the sun. The U - B and B - V values and theminimum radius for HD 12545 suggest a K0III classification rather thanthe G5IV reported earlier. From a single, high-resolution spectrum of HD12545 we found a strong LiI 6707 A line with an equivalent width of 100+/- 15 mA (log n(Li) is approximately equal to 1.75), suggestive of arelatively young giant. In contrast, with our new spectral typeclassification, minimum stellar radius, and the observed mass function,we determine a space-velocity vector of 105 km/s, suggestive of an olddisk star.
| Photometry of 50 suspected variable stars
Fifty stars have been chosen as suspected variable stars and analyzedfor variability. A large portion of this sample are stars that areeither proved active chromosphere stars or are candidates for suchactivity. The photometric data base consists of differential Vmeasurements of the Vanderbilt 16 inch (41 cm) automatic photoelectrictelescope and 25 observers at 26 observatories worldwide. Publishedphotometric data have also been utilized, with proper adjustments madeto ensure that all magnitudes are differential. Searches for photometricperiod, amplitudes, and times of minimum light showed 68 percent of thesample to be photometrically variable with periods found for 34. Twostars were deemed norvariable for the period of observation. Conclusivestatements could not be made concerning the photometric variability ofthe 14 remaining stars.
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Observation and Astrometry data
| Constellation: | Ανδρομέδα |
| Right ascension: | 02h03m09.73s |
| Declination: | +36°06'38.8" |
| Apparent magnitude: | 7.808 |
| Distance: | 293.255 parsecs |
| Proper motion RA: | 29.1 |
| Proper motion Dec: | -8.1 |
| B-T magnitude: | 9.635 |
| V-T magnitude: | 7.959 |
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