Brian A. Skiff

Patterns of Variation among Sun-like Stars

We examine the patterns of variation among a sample of 35 stars that includes the Sun, particularly on the timescale of the 11 yr solar activity cycle. Our investigation uses contemporaneous photometric and chromospheric HK emission time series measurements from the Lowell and Mount Wilson Observatories, and comparable solar data. We find that the photometric and HK variability of the stars in our sample can be related to their average level of chromospheric activity by power laws. The photometric variability of the Sun may be somewhat subdued for its average activity level. We find that the younger, more active stars in our sample tend to become fainter as their HK emission increases, whereas the older, less active stars tend to become brighter as their HK emission increases, as the Sun does during its activity cycle.

Flaring Behavior of the Quasar 3C 454.3 Across the Electromagnetic Spectrum

We analyze the behavior of the parsec-scale jet of the quasar 3C 454.3 during pronounced flaring in 2005-2008. Three major disturbances propagated down the jet along different trajectories with Lorentz factors Γ > 10. The disturbances show a clear connection with millimeter-wave outbursts, in 2005 May/June, 2007 July, and 2007 December. High-amplitude optical events in the R-band light curve precede peaks of the millimeter-wave outbursts by 15-50 days. Each optical outburst is accompanied by an increase in X-ray activity. We associate the optical outbursts with propagation of the superluminal knots and derive the location of sites of energy dissipation in the form of radiation. The most prominent and long lasting of these, in 2005 May, occurred closer to the black hole, while the outbursts with a shorter duration in 2005 autumn and in 2007 might be connected with the passage of a disturbance through the millimeter-wave core of the jet. The optical outbursts, which coincide with the passage of superluminal radio knots through the core, are accompanied by systematic rotation of the position angle of optical linear polarization. Such rotation appears to be a common feature during the early stages of flares in blazars. We find correlations between optical variations and those at X-ray and γ-ray energies. We conclude that the emergence of a superluminal knot from the core yields a series of optical and high-energy outbursts, and that the millimeter-wave core lies at the end of the jets acceleration and collimation zone. We infer that the X-ray emission is produced via inverse Compton scattering by relativistic electrons of photons both from within the jet (synchrotron self-Compton) and external to the jet (external Compton, or EC); which one dominates depends on the physical parameters of the jet. A broken power-law model of the γ-ray spectrum reflects a steepening of the synchrotron emission spectrum from near-IR to soft UV wavelengths. We propose that the γ-ray emission is dominated by the EC mechanism, with the sheath of the jet supplying seed photons for γ-ray events that occur near the millimeter-wave core.

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.

The Photometric Variability of Sun-like Stars: Observations and Results, 1984-1995

Using differential Stromgren b, y photometry, we monitored the brightness variations of 41 program stars and their 73 comparison stars from 1984 through 1995. The predominantly main-sequence program stars spanned ranges of temperature and mean chromospheric activity centered on solar values. About 40% of all the stars showed measurable variability, typically at levels below 0.01 mag (~1%), on both night-to-night and year-to-year timescales. The variability correlated with mean chromospheric activity and advancing spectral type. We present differential light curves and statistical descriptions of our observations.

YELLOW AND RED SUPERGIANTS IN THE LARGE MAGELLANIC CLOUD

Due to their transitionary nature, yellow supergiants (YSGs) provide a critical challenge for evolutionary modeling. Previous studies within M31 and the Small Magellanic Cloud show that the Geneva evolutionary models do a poor job at predicting the lifetimes of these short-lived stars. Here, we extend this study to the Large Magellanic Cloud (LMC) while also investigating the galaxys red supergiant (RSG) content. This task is complicated by contamination by Galactic foreground stars that color and magnitude criteria alone cannot weed out. Therefore, we use proper-motions and the LMCs large systemic radial velocity (~278?km?s?1) to separate out these foreground dwarfs. After observing nearly 2000 stars, we identified 317 probable YSGs, 6 possible YSGs, and 505 probable RSGs. Foreground contamination of our YSG sample was ~80%, while that of the RSG sample was only 3%. By placing the YSGs on the Hertzsprung-Russell diagram and comparing them against the evolutionary tracks, we find that new Geneva evolutionary models do an exemplary job at predicting both the locations and the lifetimes of these transitory objects.

A CANDIDATE YOUNG MASSIVE PLANET IN ORBIT AROUND THE CLASSICAL T TAURI STAR CI TAU

The ~2 Myr old classical T Tauri star CI Tau shows periodic variability in its radial velocity (RV) variations measured at infrared (IR) and optical wavelengths. We find that these observations are consistent with a massive planet in a ~9-day period orbit. These results are based on 71 IR RV measurements of this system obtained over 5 years, and on 26 optical RV measurements obtained over 9 years. CI Tau was also observed photometrically in the optical on 34 nights over ~one month in 2012. The optical RV data alone are inadequate to identify an orbital period, likely the result of star spot and activity induced noise for this relatively small dataset. The infrared RV measurements reveal significant periodicity at ~9 days. In addition, the full set of optical and IR RV measurements taken together phase coherently and with equal amplitudes to the ~9 day period. Periodic radial velocity signals can in principle be produced by cool spots, hot spots, and reflection of the stellar spectrum off the inner disk, in addition to resulting from a planetary companion. We have considered each of these and find the planet hypothesis most consistent with the data. The radial velocity amplitude yields an Msin(i) of ~8.1 M_Jup; in conjunction with a 1.3 mm continuum emission measurement of the circumstellar disk inclination from the literature, we find a planet mass of ~11.3 M_Jup, assuming alignment of the planetary orbit with the disk.

YELLOW SUPERGIANTS IN THE SMALL MAGELLANIC CLOUD: PUTTING CURRENT EVOLUTIONARY THEORY TO THE TEST

The yellow supergiant content of nearby galaxies provides a critical test of massive star evolutionary theory. While these stars are the brightest in a galaxy, they are difficult to identify because a large number of foreground Milky Way stars have similar colors and magnitudes. We previously conducted a census of yellow supergiants within M31 and found that the evolutionary tracks predict a yellow supergiant duration an order of magnitude longer than we observed. Here we turn our attention to the Small Magellanic Cloud (SMC), where the metallicity is 10? lower than that of M31, which is important as metallicity strongly affects massive star evolution. The SMCs large radial velocity (~160?km?s?1) allows us to separate members from foreground stars. Observations of ~500 candidates yielded 176 near-certain SMC supergiants, 16 possible SMC supergiants, along with 306 foreground stars, and provide good relative numbers of yellow supergiants down to 12 M ?. Of the 176 near-certain SMC supergiants, the kinematics predicted by the Besan?on model of the Milky Way suggest a foreground contamination of ?4%. After placing the SMC supergiants on the Hertzsprung-Russell diagram (HRD) and comparing our results to the Geneva evolutionary tracks, we find results similar to those of the M31 study: while the locations of the stars on the HRD match the locations of evolutionary tracks well, the models overpredict the yellow supergiant lifetime by a factor of 10. Uncertainties about the mass-loss rates on the main sequence thus cannot be the primary problem with the models.

HV 11423: The coolest supergiant in the SMC

We call attention to the fact that one of the brightest red supergiants in the SMC has recently changed its spectral type from K0-1 I (2004 December) to M4 I (2005 December) and back to K0-1 I (2006 September). An archival spectrum from the Very Large Telescope reveals that the star was even cooler (M4.5-M5 I) in 2001 December. By contrast, the star was observed to be an M0 I in both 1978 October and 1979 October. The M4-5 I spectral types is by far the latest type seen for an SMC supergiant, and its temperature in that state places it well beyond the Hayashi limit into a region of the H-R diagram where the star should not be in hydrostatic equilibrium. The star is variable by nearly 2 mag in V, but essentially constant in K. Our modeling of its spectral energy distribution shows that the visual extinction has varied during this time, but that the star has remained essentially constant in bolometric luminosity. We suggest that the star is currently undergoing a period of intense instability, with its effective temperature changing from 4300 to 3300 K on the timescale of months. It has one of the highest 12 ?m fluxes of any RSG in the SMC, and we suggest that the variability at V is due primarily to changes in effective temperature, and secondarily to changes in the local extinction due to creation and dissipation of circumstellar dust. We speculate that the star may be nearing the end of its life.

The activity cycle of Sigma Draconis

The temperature and granulation of several dwarf stars have been monitored since 1984. The results for σ Dra (HR 7462, HD 185144, K0 V) are discussed in this paper. Temperature is monitored using a line-depth ratio, and temperature variations: 5 K are seen. Intermediate-band photometry and Ca II H and K-line emission mimic the temperature changes: a monotonic decline from the 1984 season, a smooth minimum around 1988, followed by a rise back to the 1984 values at the current time. These types of correlated changes are similar to the solar behavior

The activity, variability, and rotation of lower main-sequence members of the Coma star cluster

Abstract : High-precision, differential, Stromgren b, y photometric observations of nine members of the Coma star cluster, spectral types F3 V to K0 V, were made. Four G-type stars in this sample were all variable on both seasonal and year-to-year time scales, and the single K-type star also showed hints of variability. In contrast, four F-type stars were not detectably variable on either time scale. The variable stars tended to become slightly bluer as they brightened. The low-level photometric variability of Coma stars appears to resemble closely that observed among similar stars in the Hyades cluster. We also measured rotation periods for the four G-type stars from modulation present in our photometric data. The rotation periods of these stars and similar stars in the Hyades are comparable. The fact that the Coma and Hyades clusters are essentially indistinguishable in terms of their activity, variability, and rotational characteristics presents difficulties for claims that the photometric Hyades anomaly is a consequence of stellar activity. Reprints.