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HD 197644


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Photometric Variability in a Sample of 187 G and K Giants
We have used three automatic photoelectric telescopes to obtainphotometric observations of 187 G, K, and (a few) M0 field giants. Wefind low-amplitude photometric variability on timescales of days toweeks on both sides of the coronal dividing line (CDL) in a total of 81or 43% of the 187 giants. About one-third of the variables haveamplitudes greater than 0.01 mag in V. In our sample the percentage ofvariable giants is a minimum for late-G spectral classes and increasesfor earlier and later classes; all K5 and M0 giants are variable. Wealso obtained high-resolution, red wavelength spectroscopic observationsof 147 of the giants, which we used to determine spectralclassifications, vsini values, and radial velocities. We acquiredadditional high-resolution, blue wavelength spectra of 48 of the giants,which we used to determine chromospheric emission fluxes. We analyzedthe photometric and spectroscopic observations to identify the cause(s)of photometric variability in our sample of giants. We show that thelight variations in the vast majority of G and K giant variables cannotbe due to rotation. For giants on the cool side of the CDL, we find thatthe variability mechanism is radial pulsation. Thus, the variabilitymechanism operating in M giants extends into the K giants up to aboutspectral class K2. On the hot side of the CDL, the variability mechanismis most likely nonradial, g-mode pulsation.

UBV Photometry of the Suspected Variable BD +18 4586
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Observation and Astrometry data

Constellation:Delphinus
Right ascension:20h44m30.16s
Declination:+19°30'51.6"
Apparent magnitude:8.056
Distance:386.1 parsecs
Proper motion RA:-6.9
Proper motion Dec:-20
B-T magnitude:9.356
V-T magnitude:8.164

Catalogs and designations:
Proper Names   (Edit)
HD 1989HD 197644
TYCHO-2 2000TYC 1642-2747-1
USNO-A2.0USNO-A2 1050-18517360
HIPHIP 102368

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