Fundamental parameters of Cepheids. V. Additional photometry and radial velocity for southern Cepheids

I present photometric and radial velocity data for Galactic Cepheids, most of them being in the southern hemisphere. There are 1250 Geneva 7-color photometric measurements for 62 Cepheids, the average uncertainty per measurement is better than 0.01 mag. A total of 832 velocity measurements have been obtained with the CORAVEL radial velocity spectrograph for 46 Cepheids. The average accuracy of the radial velocity data is 0.38 km/s. There are 33 stars with both photometry and radial velocity data. I discuss the possible binarity or period change that these new data reveal. I also present reddenings for all Cepheids with photometry. The data are available electronically.Comment: To appear in ApJS. Data available electronically at ftp://cfa-ftp.harvard.edu/pub/dbersier

The Rigidly Rotating Magnetosphere of Sigma Ori E

We attempt to characterize the observed variability of the magnetic helium-strong star sigma Ori E in terms of a recently developed rigidly rotating magnetosphere model. This model predicts the accumulation of circumstellar plasma in two co-rotating clouds, situated in magnetohydrostatic equilibrium at the intersection between magnetic and rotational equators. We find that the model can reproduce well the periodic modulations observed in the star's light curve, H alpha emission-line profile, and longitudinal field strength, confirming that it furnishes an essentially correct, quantitative description of the star's magnetically controlled circumstellar environment.Comment: 4 pages, 3 figures, accepted by Ap

The Shape and Scale of Galactic Rotation from Cepheid Kinematics

A catalog of Cepheid variables is used to probe the kinematics of the Galactic disk. Radial velocities are measured for eight distant Cepheids toward l = 300; these new Cepheids provide a particularly good constraint on the distance to the Galactic center, R_0. We model the disk with both an axisymmetric rotation curve and one with a weak elliptical component, and find evidence for an ellipticity of 0.043 +/- 0.016 near the Sun. Using these models, we derive R_0 = 7.66 +/- 0.32 kpc and v_circ = 237 +/- 12 km/s. The distance to the Galactic center agrees well with recent determinations from the distribution of RR Lyrae variables, and disfavors most models with large ellipticities at the solar orbit.Comment: 36 pages, LaTeX, 10 figure

Detection of Magnetic Massive Stars in the Open Cluster NGC 3766

A growing number of observations indicate that magnetic fields are present among a small fraction of massive O- and B-type stars, yet the origin of these fields remains unclear. Here we present the results of a VLT/FORS1 spectropolarimetric survey of 15 B-type members of the open cluster NGC 3766. We have detected two magnetic B stars in the cluster, including one with a large field of nearly 2 kG, and we find marginal detections of two additional stars. There is no correlation between the observed longitudinal field strengths and the projected rotational velocity, suggesting that a dynamo origin for the fields is unlikely. We also use the Oblique Dipole Rotator model to simulate populations of magnetic stars with uniform or slightly varying magnetic flux on the ZAMS. None of the models successfully reproduces our observed range in B_l and the expected number of field detections, and we rule out a purely fossil origin for the observed fields.Comment: 6 pages ApJ emulate style; accepted to Ap

The pecular magnetic field morphology of the white dwarf WD 1953-011: evidence for a large-scale magnetic flux tube?

We present and interpret new spectropolarimetric observations of the magnetic white dwarf WD 1953-011. Circular polarization and intensity spectra of the H$\alpha$ spectral line demonstrate the presence of two-component magnetic field in the photosphere of this star. The geometry consists of a weak, large scale component, and a strong, localized component. Analyzing the rotationally modulated low-field component, we establish a rotation period $P_{rot} = 1.4480 \pm 0.0001$ days. Modeling the measured magnetic observables, we find that the low-field component can be described by the superposition of a dipole and quadrupole. According to the best-fit model, the inclination of the stellar rotation axis with respect to the line of sight is $i \approx 20^\circ$, and the angle between the rotation axis and the dipolar axis is $\beta \approx 10^\circ$. The dipole strength at the pole is about 180 kG, and the quadrupolar strength is about 230 kG. These data suggest a fossil origin of the low-field component. In contrast, the strong-field component exhibits a peculiar, localized structure (``magnetic spot'') that confirms the conclusions of Maxted and co-workers. The mean field modulus of the spot ($|B_{spot}| = 520 \pm 7$ kG) together with its variable longitudinal magnetic field having a maximum of about +400 kG make it difficult to describe it naturally as a high-order component of the star's global poloidal field. Instead, we suggest that the observed strong-field region has a geometry similar to a magnetic flux tube.Comment: 11 figure

First HARPSpol discoveries of magnetic fields in massive stars

In the framework of the Magnetism in Massive Stars (MiMeS) project, a HARPSpol Large Program at the 3.6m-ESO telescope has recently started to collect high-resolution spectropolarimetric data of a large number of Southern massive OB stars in the field of the Galaxy and in many young clusters and associations. In this Letter, we report on the first discoveries of magnetic fields in two massive stars with HARPSpol - HD 130807 and HD 122451, and confirm the presence of a magnetic field at the surface of HD 105382 that was previously observed with a low spectral resolution device. The longitudinal magnetic field measurements are strongly varying for HD 130807 from $\sim$-100 G to $\sim$700 G. Those of HD 122451 and HD 105382 are less variable with values ranging from $\sim$-40 to -80 G, and from $\sim$-300 to -600 G, respectively. The discovery and confirmation of three new magnetic massive stars, including at least two He-weak stars, is an important contribution to one of the MiMeS objectives: the understanding of origin of magnetic fields in massive stars and their impacts on stellar structure and evolution.Comment: 4 pages, 2 figures, accepted for publication in A&A Lette

The Luminosities and Distance Scales of Type II Cepheid and RR Lyrae variables

Infrared and optical absolute magnitudes are derived for the type II Cepheids kappa Pav and VY Pyx from revised Hipparcos parallaxes and for kappa Pav, V553 Cen and SW Tau from pulsation parallaxes. Phase-corrected JHK mags are given for 142 RR Lyrae variables based on 2MASS data. RR Lyrae itself is overluminous compared with LMC RR Lyraes at the classical Cepheid modulus (18.39) consistent with a prediction of Catalan and Cortes. V553 Cen and SW Tau deviate by only 0.02 mag in the mean from the Matsunaga PL(K) relation for globular cluster type II Cepheids with a zero-point based on the same LMC modulus. Comparing directly these two stars with type II Cepheids in the LMC and in the Galactic Bulge leads to an LMC modulus of 18.37\pm0.09 and a distance to the Galactic Centre of 7.64\pm 0.21kpc. Kappa Pav may be a binary. V553 Cen and SW Tau show that at optical wavelengths PL relations are wider for field stars than for those in globular clusters (abridged).Comment: 29 pages, 13 figures, accepted for MNRA