Recently, Tiwari, Chaubey, & Pandey (2007) detected the bright component of the visual binary HD151878 to exhibit rapid photometric oscillations through a Johnson B filter with a period of 6 min (2.78 mHz) and a high, modulated amplitude up to 22 mmag peak-to-peak, making this star by far the highest amplitude roAp star known. As a new roAp star, HD151878 is of additional particular interest as a scarce example of the class in the northern sky, and only the second known case of an evolved roAp star - the other being HD 116114. We used the FIES spectrograph at the Nordic Optical Telescope to obtain high time resolution spectra at high dispersion to attempt to verify the rapid oscillations. We show here that the star at this epoch is spectroscopically stable to rapid oscillations of no more than a few tens of m/s. The high-resolution spectra furthermore show the star to be of type Am rather than Ap and we show the star lacks most of the known characteristics for rapidly oscillating Ap stars. We conclude that this is an Am star that does not pulsate with a 6-min period. The original discovery of pulsation is likely to be an instrumental artefact.
We present preliminary calculations of electron scattering polarizations from models of structured cool star envelopes. We note that net polarizations from unresolved sources can result from non-spherical scattering envelopes and/or anisotropic illumination from a photosphere that has brightness variations. The resultant polarizations are quite small (hundreths of a percent); however, Rayleigh scattering from molecular opacity and/or dust scattering from the more extended envelope under similar considerations may produce higher polarizations.
Most of the X-ray emitting gas in early-type galaxies probably originates from red giant mass loss and here we model the interaction between this stellar mass loss and the hot ambient medium. Using two-dimensional hydrodynamic simulations, we adopt a temperature for the ambient medium of 3E6 K along with a range of ambient densities and stellar velocities. When the stellar velocity is supersonic relative to the ambient medium, a bow shock occurs, along with a shock driven into the stellar ejecta, which heats only a fraction of the gas. Behind the bow shock, a cool wake develops but the fast fl…
We test our capability of deriving stellar physical parameters of giant stars by analysing a sample of field stars and the well studied open cluster IC 4651 with different spectroscopic methods. The use of a technique based on line-depth ratios (LDRs) allows us to determine with high precision the effective temperature of the stars and to compare the results with those obtained with a classical LTE abundance analysis. (i) For the field stars we find that the temperatures derived by means of the LDR method are in excellent agreement with those found by the spectral synthesis. This result is ext…
A High Resolution Study of the Hydra A Cluster with Chandra: Comparison of the Core Mass Distribution with Theoretical Predictions and Evidence for Feedback in the Cooling Flow
We present a spectroscopic analysis of over 38,000 low-mass stars from the Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5). Analysis of this unprecedentedly large sample confirms the previously detected decrease in the fraction of magnetically active stars (as traced by H-alpha emission) as a function of vertical distance from the Galactic Plane. The magnitude and slope of this effect varies as a function of spectral type. Using simple 1-D dynamical models, we demonstrate that the drop in activity fraction can be explained by thin disk dynamical heating and a rapid decrease in magnetic ac…
We present a detailed analysis of a large spectroscopic and photometric sample of DZ white dwarfs based on our latest model atmosphere calculations. We revise the atmospheric parameters of the trigonometric parallax sample of Bergeron, Leggett, & Ruiz (12 stars) and analyze 147 new DZ white dwarfs discovered in the Sloan Digital Sky Survey. The inclusion of metals and hydrogen in our model atmosphere calculations leads to different atmospheric parameters than those derived from pure helium models. Calcium abundances are found in the range from log (Ca/He) = -12 to -8. We also find that fits of…
We present the discovery of pulsational variations in the cool magnetic Ap star HD 115226 -- the first high-amplitude rapidly oscillating Ap star discovered with time-series spectroscopy. Using high-resolution spectra obtained with the HARPS instrument at the ESO 3.6-m telescope, we detect radial velocity variations with a period of 10.86 min in Pr III, Nd III, Dy III lines and in the narrow cores of hydrogen lines. Pulsational amplitudes exceed 1 km/s in individual lines of Nd III. The presence of running waves in the stellar atmosphere is inferred from a phase shift between the radial veloci…
We present a high-resolution spectroscopic analysis of two samples of evolved stars selected in the field and in the intermediate-age open cluster IC 4651, for which detailed measurements of chemical composition were made in the last few years. Applying the Gray's method based on ratios of line depths, we determine the effective temperature and compare our results with previous ones obtained by means of the curves of growth of iron lines. The knowledge of the temperature enables us to estimate other fundamental stellar parameters, such as color excess, age, and mass.
We report the detection and monitoring of transient substructures in the radiation-driven winds of five massive, hot stars in different evolutionary stages. Clumping in the winds of these stars shows up as variable, narrow subpeaks superposed on their wide, wind-broadened (optical) emission lines. Similar patterns of emission-line profile variations are detected in the Of stars zeta Puppis and HD93129A, in the more evolved hydrogen-rich, luminous, Of-like WN stars HD93131 and HD93162, and in the more mass-depleted WC star in gamma2 Velorum. These observations strongly suggest that stochastic w…
A search of the Sloan Digital Sky Survey spectroscopic database has turned up 23 new ultra-cool subdwarfs, low-mass metal-poor stars of spectral subtype M 7.0 or later. Spectra from these red objects all show very strong molecular bands of CaH but relatively weak bands of TiO, indicative of a cool, metal-poor atmosphere. Five of the stars are formally classified as M subdwarfs (sdM7.0-sdM8.5), 13 as more metal-poor extreme subdwarfs (esdM7.0-esdM8.0), and five as extremely metal-poor ultra subdwarfs (usdM7.0-usdM7.5). In the [H_r,r-z] reduced proper motion diagram, these subdwarfs clearly popu…
We present an analysis of optical and ultraviolet Hubble Space Telescope photometry for evolved stars in the core of the distant massive globular cluster NGC 2419. We characterize the horizontal branch (HB) population in detail including corrections for incompleteness on the long blue tail. We present a method for removing (to first order) lifetime effects from the distribution of HB stars to facilitate more accurate measurements of helium abundance for clusters with blue HBs and to clarify the distribution of stars reaching the zero-age HB. The population ratio R = N_HB / N_RGB implies there …
