Stephen M. Henry

Patterns of Photometric and Chromospheric Variation among Sun-like Stars: A 20 Year Perspective

We examine patterns of variation of 32 primarily main-sequence Sun-like stars [selected at project onset as stars on or near the main sequence and color index 0.42 ≤ (B - V) ≤ 1.4], extending our previous 7-12 yr time series to 13-20 yr by combining Stromgren b, y photometry from Lowell Observatory with similar data from Fairborn Observatory. Parallel chromospheric Ca II H and K emission data from the Mount Wilson Observatory span the entire interval. The extended data strengthen the relationship between chromospheric and brightness variability at visible wavelengths derived previously. We show that the full range of photometric variation has probably now been observed for a majority of the program stars. Twenty-seven stars are deemed variable according to an objective statistical criterion. On a year-to-year timescale, young active stars become fainter when their Ca II emission increases, while older less active stars such as the Sun become brighter when their Ca II emission increases. The Suns total irradiance variation, scaled to the b and y stellar filter photometry, still appears to be somewhat smaller than stars in our limited sample with similar mean chromospheric activity, but we now regard this discrepancy as probably due mainly to our limited stellar sample.

Spectroscopy and Photometry of Nearby Young Solar Analogs

We present new photometry and spectroscopy of 34 stars from a catalog of 38 nearby (d < 25 pc) G and K dwarfs selected as analogs to the early Sun. We report that the least active star in our sample is also slowly rotating and probably of solar age. Two other stars appear to be evolved objects that have recently acquired angular momentum. A fourth star may be a spectroscopic binary. Many of the other stars belong to previously identified common proper-motion groups. Space motions, lithium abundances, and Ca II emission of these stars suggest ages between 70 and 800 Myr.

PHOTOMETRIC VARIABILITY IN A SAMPLE OF 187 G AND K GIANTS

We have used three automatic photoelectric telescopes to obtain photometric observations of 187 G, K, and (a few) M0 —eld giants. We —nd low-amplitude photometric variability on timescales of days to weeks on both sides of the coronal dividing line (CDL) in a total of 81 or 43% of the 187 giants. About one-third of the variables have amplitudes greater than 0.01 mag in V. In our sample the percentage of variable giants is a minimum for late-G spectral classes and increases for earlier and later classes; all K5 and M0 giants are variable. We also obtained high-resolution, red wavelength spectroscopic observations of 147 of the giants, which we used to determine spectral classi—cations, v sin i values, and radial veloci- ties. We acquired additional high-resolution, blue wavelength spectra of 48 of the giants, which we used to determine chromospheric emission —uxes. We analyzed the photometric and spectroscopic obser- vations to identify the cause(s) of photometric variability in our sample of giants. We show that the light variations in the vast majority of G and K giant variables cannot be due to rotation. For giants on the cool side of the CDL, we —nd that the variability mechanism is radial pulsation. Thus, the variability mechanism operating in M giants extends into the K giants up to about spectral class K2. On the hot side of the CDL, the variability mechanism is most likely nonradial, g-mode pulsation. Subject headings: stars: fundamental parametersstars: late-typestars: oscillations ¨ stars: rotationstars: spotsstars: variables: other

Eleven New γ Doradus Stars

We present new high-dispersion spectroscopic and precise photometric observations to identify 11 new γ Doradus variables. Seven of these new γ Doradus stars appear to be single, three are primaries of single-lined binaries, and one has two distant visual companions; none are double-lined or close visual binaries. Several of the stars show spectroscopic line-profile and low-amplitude radial velocity variability indicative of pulsation. All 11 stars are photometrically variable with amplitudes between 8 and 93 mmag in Johnson B and periods between 0.398 and 2.454 days. One star is monoperiodic; the rest have between two and five independent periods. The variability at all periods approximates a sinusoid, although three of the stars exhibit cycle-to-cycle variation in the level of maximum brightness, similar to the Blazhko effect observed in some RR Lyrae stars. We provide a new tabulation of all 54 γ Doradus stars confirmed to date and list some of their properties. All are dwarfs or subgiants and lie within a well-defined region of the H-R diagram that overlaps the cool edge of the δ Scuti instability strip. Four of the new γ Doradus variables from this paper also lie within the δ Scuti instability strip but do not exhibit the additional higher frequency variability typical of δ Scuti stars. The variability type of several of these stars given in the General Catalog of Variable Stars and in SIMBAD should now be revised.

HD 207651: A TRIPLE SYSTEM WITH SCUTI AND ELLIPSOIDAL VARIATIONS BUT NO DORADUS PULSATIONS

We examine HD 207651 as a possible example of a star exhibiting both γ Doradus and δ Scuti type pulsations. We find photometric periods of 0.06479 and 0.06337 days with peak-to-peak amplitudes in Johnson B of 21 and 13 mmag, respectively, clearly indicating δ Scuti pulsations. Additional light variation with a period of 0.73540 days and an even larger amplitude of 31 mmag is within the range of γ Doradus pulsation periods but results instead from the ellipticity effect. HD 207651 has a composite spectrum with a weak, narrow absorption line superposed near the center of each broad metal line. The broad-lined component is the primary of a short-period, single-lined binary, which has a period of 1.4708 days, twice the period of the ellipsoidal variations seen in the photometry. We determine the primary to be an A8 giant and estimate the unseen secondary of the short-period binary to be a mid-M dwarf. The narrow-lined star, an F7: dwarf, shows velocity variability with a period of months or perhaps years. It is thus a more distant companion to the binary, making HD 207651 a triple system. All light variations come from the A8 giant primary star. Since the 0.73540 day variation results from the ellipticity effect, HD 207651 is not an example of a star that exhibits both δ Scuti and γ Doradus pulsations. The growing number of confirmed γ Doradus stars that also occur within the δ Scuti instability strip but fail to show additional δ Scuti variability makes it increasingly unlikely that the two types of pulsation can coexist in the same star.

Chromospherically Active Stars. XX. The Giant Single-Lined Binary HD 161570

Spectroscopy of HD 161570 shows it to be a single-lined spectroscopic binary with a period of 45.623 days and a nearly circular orbit. The primary star has spectral type G7 III, and the secondary is most likely a G or K dwarf. A high-resolution spectrum of the 3950 A region confirms that the primary has Ca II H and K emission lines. The logarithm of the giants lithium abundance is less than 0.9, indicating that it is not lithium-rich. From photometric observations covering six seasons, we detected periodic short-term light variability with an amplitude ranging from 0.02 to 0.04 mag. We interpret these short-term variations as due to stellar rotation and find that the rotation period varied somewhat from season to season, with a representative value of 64.1 days. Thus, the very different orbital and rotation periods make HD 161570 an asynchronous rotator. Low-amplitude, long-term photometric variability was also seen. The giants weak Ca II H and K emission, low-amplitude photometric variations, and relatively low v sin i of 7.0 km s-1 indicate that this star is only modestly chromospherically active. This result, plus the spectral type and asynchronous rotation of the primary, suggest that it is just beginning to ascend the red giant branch.