Ana P. Borisova

The History of the Mysterious Eclipses of KH 15D. II. Asiago, Kiso, Kitt Peak, Mount Wilson, Palomar, Tautenburg, and Rozhen Observatories, 1954-1997

The unusual pre–main-sequence binary star KH 15D undergoes remarkably deep and long-lasting periodic eclipses. Some clues about the reason for these eclipses have come from the observed evolution of the systems light curve over the last century. Here we present UBVRI photometry of KH 15D from 1954 to 1997 based on photographic plates from various observatories. The system has been variable at the ≈1 mag level since at least 1965. There is no evidence of color variations, with a typical limit of Δ(B - V) < 0.2 mag. We confirm some previously published results that were based on a smaller sample of plates: from approximately 1965 to 1990, the total flux was modulated with the 48 day orbital period of the binary, but the maximum flux was larger, the fractional variations were smaller, and the phase of minimum flux was shifted by almost a half-cycle relative to the modern light curve. All these results are consistent with the recently proposed theory that KH 15D is being occulted by an inclined, precessing, circumbinary ring.

A long-term photometric study of V 1184 Tauri

Aims. The main purpose of our investigation is to construct the long-time light curve of the PMS star V 1184 Tau. We consider the study of the photometrical variability of PMS stars as very important for understanding stellar evolution. The unusual photometric variability of V 1184 Tau was reported in previous studies as well, but the nature of the observed deep minima is still under discussion. Methods. We present recent data from CCD photometry and from archival photographic plates. The photometric BVRI data presented in this paper were collected from November 2005 to November 2007. To construct the historical light curve of V 1184 Tau, a search for archived photographic observations in the Wide-Field Plate Database (WFPDB) was made. As a result, 412 photographic plates were found containing the field of V 1184 Tau. A part of the plates were scanned at our request and a magnitude estimation was made of V 1184 Tau. Results. Our recent photometric data suggest that the star brightness is still near the minimum. Thus the period of strong light variations that started in 2003 continues up to the present, hence more than 4 years. Our data from the archival photographic plates suggest that an unknown minimum of brightness exists in the approximate period 1980-1985. Taking all available photometric and spectroscopic data into account we must reject the hypothesis that V 1184 Tau is an FU Orionis type object. V 1184 Tau is a G type low-mass star whose spectrum is similar to WTT stars, but its photometric behavior is typical of the UX Orionis variable stars. Assuming the obscuration from orbiting dust clouds as a reason for a deep minimum, we estimate the approximate period of obscurations and the interval between two deep minima. Our calculations give a 25-28 year period between the two minima and approximately 8-10 year duration of the minima.

The different origins of magnetic fields and activity in the Hertzsprung gap stars, OU Andromedae and 31 Comae

Context: When crossing the Hertzsprung gap, intermediate-mass stars develop a convective envelope. Fast rotators on the main sequence, or Ap star descendants, are expected to become magnetic active subgiants during this evolutionary phase. Aims: We compare the surface magnetic fields and activity indicators of two active, fast rotating red giants with similar masses and spectral class but diferent rotation rates - OU And (Prot=24.2 d) and 31 Com (Prot=6.8 d) - to address the question of the origin of their magnetism and high activity. Methods: Observations were carried out with the Narval spectropolarimeter in 2008 and 2013.We used the least squares deconvolution technique to extract Stokes V and I profiles to detect Zeeman signatures of the magnetic field of the stars. We provide Zeeman-Doppler imaging, activity indicator monitoring, and a precise estimation of stellar parameters. We use stellar evolutionary models to infer the evolutionary status and the initial rotation velocity on the main sequence. Results: The detected magnetic field of OU And is a strong one. Its longitudinal component Bl reaches 40 G and presents an about sinusoidal variation with reversal of the polarity. The magnetic topology of OU And is dominated by large scale elements and is mainly poloidal with an important dipole component, and a significant toroidal component. The detected magnetic field of 31 Com is weaker, with a magnetic map showing a more complex field geometry, and poloidal and toroidal components of equal contributions. The evolutionary models show that the progenitors of OU And and 31 Com must have been rotat Conclusions: OU And appears to be the probable descendant of a magnetic Ap star, and 31 Com the descendant of a relatively fast rotator on the main sequence.

Archival light curves from the Bamberg Sky Patrol: CF Octantis, 1964-76

We use the archive of the Bamberg Sky Patrol to obtain light curves of the active K subgiant CF Octantis (HD 196818) for the interval 1964‐76. Digitized images of the field near CF Oct were obtained with a flat-bed scanner. Aperture photometry was performed of photo-positives of these images. Using a transformation to second order in plate magnitude, and first order in B − V , for nine field stars for each plate, the B magnitudes of CF Oct were obtained for just over 350 plates. The estimated precision of an individual determination of the B magnitude of CF Oct is 0.05 mag. Analysis of the resulting data reveals the known 20 d rotational variation of this star, and shows the evolution of the light curves from year to year. We obtain light curves with good phase coverage for 1964 to 1969 inclusive, partial light curves for 1970 and 1976, and a few data points from 1971. The amplitude of variation ranges from ∼0.2 to ∼0.4 mag. There is evidence that the characteristic rotation period of the star in the 1960s was slightly less than that measured from photoelectric photometry in the 1980s. Ke yw ords: techniques: photometric ‐ stars: activity ‐ stars: individual: CF Octantis (HD 196818) ‐ stars: spots ‐ stars: variables: other.

Bayesian approach to the cyclic activity of CF Oct

The Bayesian statistical methods of Gregory–Loredo and the Bretthorst generalization of the Lomb–Scargle periodogram are used to study the activity cycles of the early K-type subgiant star CF Oct. We use a ∼45-yr-long data set derived from archival photographic observations, published photoelectric photometry, the Hipparcos data series and the All Sky Automated Survey archive. We confirm the already known rotational period for the star of 20.16 d and provide evidence that it has exhibited changes from 19.90 to 20.45 d. This is an indication of stellar surface differential rotation. The Bayesian magnitude and time-residual analysis clearly reveals at least one long-term cycle. The posterior distributions of the cycle length appear to be multimodal with pronounced peaks at a period of 7.1 yr with a full width at half-maximum (FWHM) of 54 d for time-residuals and at a period of 9.8 yr with a FWHM of 184 d for magnitude data. These results are consistent with the previously postulated cycle of 9 ± 3 yr.