Context. The line profiles of the stars with v(e) sin i below a few km s(-1) can reveal direct signatures of local velocity fields such as convection in stellar atmospheres. This effect is well established in cool main sequence stars, and has been detected and studied in three A stars.Aims. This paper reports observations of main sequence B, A and F stars (1) to identify additional stars with sufficiently low values of v(e) sin i to search for spectral line profile signatures of local velocity fields and (2) to explore how the signatures of the local velocity fields in the atmosphere depend on stellar parameters such as effective temperature and peculiarity type.Methods. We have carried out a spectroscopic survey of B and A stars of low v(e) sin i at high resolution. Comparison of model spectra with those observed allows us to detect signatures of the local velocity fields such as asymmetric excess line wing absorption, best-fit v(e) sin i parameter values that are found to be larger for strong lines than for weak lines, and discrepancies between observed and modelled line profile shapes.Results. Symptoms of local atmospheric velocity fields are always detected through a non-zero microturbulence parameter for main sequence stars having T-e below about 10 000 K, but not for hotter stars. Direct line profile tracers of the atmospheric velocity field are found in six very sharp-lined stars in addition to the three reported earlier. Direct signatures of velocity fields are found to occur in A stars with and without the Am chemical peculiarities, although the amplitude of the effects seems larger in Am stars. Velocity fields are also directly detected in spectral line profiles of A and early F supergiants, but without significant line asymmetries.Conclusions. We confirm that several atmospheric velocity field signatures, particularly excess line wing absorption which is stronger in the blue line wing than in the red, are detectable in the spectral lines of main sequence A stars of sufficiently low v(e) sin i. We triple the sample of A stars known to show these effects, which are found both in Am and normal A stars. We argue that the observed line distortions are probably due to convective motions reaching the atmosphere. These data still have not been satisfactorily explained by models of atmospheric convection, including numerical simulations.

Three recent surveys of 21-cm line emission in the Galactic plane, combining single dish and interferometer observations to achieve resolution of 1 arcmin to 2 arcmin, 1 km/s, and good brightness sensitivity, have provided some 650 absorption spectra with corresponding emission spectra for study of the distribution of warm and cool phase H I in the interstellar medium. These emission-absorption spectrum pairs are used to study the temperature of the interstellar neutral hydrogen in the outer disk of the Milky Way, outside the solar circle, to a radius of 25 kpc. The cool neutral medium is distributed in radius and height above the plane with very similar parameters to the warm neutral medium. In particular, the ratio of the emission to the absorption, which gives the mean spin temperature of the gas, stays nearly constant with radius to 25 kpc radius. This suggests that the mixture of cool and warm phases is a robust quantity, and that the changes in the interstellar environment do not force the H I into a regime where there is only one temperature allowed. The mixture of atomic gas phases in the outer disk is roughly 15% to 20% cool (40 K to 60 K), the rest warm, corresponding to mean spin temperature 250 to 400 K. The Galactic warp appears clearly in the absorption data, and other features on the familiar longitude-velocity diagram have analogs in absorption with even higher contrast than for 21-cm emission. In the third and fourth Galactic quadrants the plane is quite flat, in absorption as in emission, in contrast to the strong warp in the first and second quadrants. The scale height of the cool gas is similar to that of the warm gas, and both increase with Galactic radius in the outer disk.

Aims. We investigated a sample of 28 well-known spectroscopically-identified magnetic Ap/Bp stars, with weak, poorly-determined or previously undetected magnetic fields. The aim of this study is to explore the weak part of the magnetic field distribution of Ap/Bp stars. Methods. Using the MuSiCoS and NARVAL spectropolarimeters at Telescope Bernard Lyot (Observatoire du Pic du Midi, France) and the cross-correlation technique Least Squares Deconvolution (LSD), we obtained 282 LSD Stokes V signatures of our 28 sample stars, in order to detect the magnetic field and to infer its longitudinal component with high precision (median σ = 40 G). Results. For the 28 studied stars, we obtained 27 detections of StokesV Zeeman signatures from the MuSiCoS observations. Detection of the Stokes V signature of the 28th star (HD 32650) was obtained during science demonstration time of the new NARVAL spectropolarimeter at Pic du Midi. This result clearly shows that when observed with sufficient precision, all firmly classified Ap/Bp stars show detectable surface magnetic fields. Furthermore, all detected magnetic fields correspond to longitudinal fields which are significantly greater than some tens of G. To better characterise the surface magnetic field intensities and geometries of the sample, we phased the longitudinal field measurements of each star using new and previously-published rotational periods, and modeled them to infer the dipolar field intensity (Bd, measured at the magnetic pole) and the magnetic obliquity (β). The distribution of derived dipole strengths for these stars exhibits a plateau at about 1 kG, falling off to larger and smaller field strengths. Remarkably, in this sample of stars selected for their presumably weak magnetic fields, we find only 2 stars for which the derived dipole strength is weaker than 300 G. We interpret this “magnetic threshold” as a critical value necessary for the stability of large-scale magnetic fields, and develop a simple quantitative model that is able to approximately reproduce the observed threshold characteristics. This scenario leads to a natural explanation of the small fraction of intermediate-mass magnetic stars. It may also explain the near-absence of magnetic fields in more massive B and O-type stars.

Aims. In this paper we confirm the presence of a globally-ordered, kG-strength magnetic field in the photosphere of the young O star theta(1) Orionis C, and examine the properties of its optical line profile variations.Methods. A new series of high-resolution MuSiCoS Stokes V and I spectra has been acquired which samples approximately uniformly the rotational cycle of theta(1) Orionis C. Using the Least-Squares Deconvolution ( LSD) multiline technique, we have succeeded in detecting variable Stokes V Zeeman signatures associated with the LSD mean line profile. These signatures have been modeled to determine the magnetic field geometry. We have furthermore examined the profile variations of lines formed in both the wind and photosphere using dynamic spectra.Results. Based on spectrum synthesis fitting of the LSD profiles, we determine that the polar strength of the magnetic dipole component is 1150 less than or similar to B-d less than or similar to 1800 G and that the magnetic obliquity is 27 degrees less than or similar to beta less than or similar to 68 degrees, assuming i = 45 +/- 20 degrees. The best-fit values for i = 45 degrees are B-d = 1300 +/- 150 G (1 sigma) and beta = 50 degrees +/- 6 degrees (1 sigma). Our data confirm the previous detection of a magnetic field in this star, and furthermore demonstrate the sinusoidal variability of the longitudinal field and accurately determine the phases and intensities of the magnetic extrema. The analysis of "photospheric" and "wind" line profile variations supports previous reports of the optical spectroscopic characteristics, and provides evidence for infall of material within the magnetic equatorial plane.

Aims.This paper reports the photospheric, magnetic and circumstellar gas characteristics of the magnetic B8p star 36 Lyncis (HD 79158). Methods: .Using archival data and new polarised and unpolarised high-resolution spectra, we redetermine the basic physical properties, the rotational period and the geometry of the magnetic field, and the photospheric abundances of various elements. Results: .Based on magnetic and spectroscopic measurements, we infer an improved rotational period of 3.83475± 0.00002 d. We determine a current epoch of the longitudinal magnetic field positive extremum (HJD 2 452 246.033), and provide constraints on the geometry of the dipole magnetic field (i? 56°, 3210~G? B_d? 3930 G, ? unconstrained). We redetermine the effective temperature and surface gravity using the optical and UV energy distributions, optical photometry and Balmer line profiles (T_eff=13 300± 300 K, log g=3.7-4.2), and based on the Hipparcos parallax we redetermine the luminosity, mass, radius and true rotational speed (L=2.54± 0.16~L_?, M=4.0± 0.2~M_?, R=3.4± 0.7~R_?, v_eq=45-61.5 km s-1). We measure photospheric abundances for 21 elements using optical and UV spectra, and constrain the presence of vertical stratification of these elements. We perform preliminary Doppler Imaging of the surface distribution of Fe, finding that Fe is distributed in a patchy belt near the rotational equator. Most remarkably, we confirm strong variations of the H? line core which we interpret as due to occultations of the star by magnetically-confined circumstellar gas.

We describe the HI component of the Southern Galactic Plane Survey (SGPS). The SGPS is a large-scale project to image at arcminute resolution the HI spectral line and 21 cm continuum emission in parts of the plane of the Milky Way. The survey covers Galactic longitudes 253 deg < l < 358 deg and latitudes |b| <1.5 deg (SGPS I), plus a first quadrant extension covering 5 deg < l < 20 deg and |b| <1.5 deg (SGPS II). The survey combines data from the Australia Telescope Compact Array and the Parkes Radio Telescope for sensitivity to angular scales ranging from 2 arcminutes to several degrees. The combined data cover 325 sq-deg and have an rms sensitivity of 1.6 K. Here we describe the HI observations and data reduction in detail, and present examples from the final data products. The complete dataset is publicly available through the Australia Telescope National Facility's HI Surveys archive. This dataset provides an unprecedented view of the neutral component of interstellar hydrogen in the inner Milky Way.

Forty-five new measurements of the mean longitudinal magnetic fields and mean equivalent widths of 4 roAp stars have obtained using the MuSiCoS spectropolarimeter at Pic du Midi observatory. These new high-precision data have been combined with archival measurements in order to constrain the rotational periods of HD 12098, HD 24712 = HR 1217, HD 122970 and HD 176232 = 10 Aql. We report a revised rotational period for HD 24712 (P-rot = 12.45877 +/- 0.00016 d, crucial for interpretation of upcoming MOST observations of this star), new rotational periods for HD 12098 and HD 122970 (P-rot = 5.460 +/- 0.001 d and P-rot = 3.877 +/- 0.001 d, respectively) and evidence for an extremely long period for HD 176232.

We present spectropolarimetric observations of the FK Com star HD 199178 obtained between 1998 December and 2003 August at the Telescope Bernard Lyot (Observatoire du Pic du Midi, France). We report the detection of a photospheric magnetic field and reconstruct its distribution by means of Zeeman-Doppler imaging. We observe large regions where the magnetic field is mainly azimuthal, suggesting that the dynamo processes generating the magnetic activity of HD 199178 may be active very close to the stellar surface. We investigate the rapid evolution of surface brightness and magnetic structures from a continuous monitoring of the star over several weeks in 2002 and 2003. We report that significant changes occur in the distribution of cool-spots and magnetic regions on typical time-scales of the order of two weeks. Our spectropolarimetric observations also suggest that the surface of HD 199178 is sheared by differential rotation, with a difference in rotation rate between equatorial and polar regions of the order of 1.5 times that of the Sun.

We present a 21 cm emission-absorption study toward extragalactic sources in the Canadian Galactic Plane Survey (CGPS). We have analyzed H I spectra toward 437 sources with Sν ≥ 150 mJy, giving us a source density of 0.6 sources per square degree at arcminute resolution. We present the results of a first analysis of the H I temperatures, densities, and feature statistics. Particular emphasis is placed on five features with observed spin temperatures below 40 K. We find most spin temperatures in the range 40-300 K. A simple H I two-component model constrains the bulk of the cold component to temperatures (Tc) between 40 and 100 K. We find that Tc peaks in the Perseus arm region, and clearly drops off with Galactocentric radius R beyond that. The H I density follows this trend, ranging from a local value of 0.4 cm-3 to less than 0.1 cm-3 at R = 20 kpc. We find that H I emission alone on average traces about 75% of the total H I column density, as compared to the total inferred by the emission and absorption. Comparing the neutral hydrogen absorption to CO emission, no correlation is found in general, but all strong CO emission is accompanied by a visible H I spectral feature. Finally, the number of spectral H I absorption features per kiloparsec drops off exponentially with increasing R.

We are conducting a magnetic survey of a sample of about 12 spectroscopicallyidentified A Ρ stars (selected from the HD catalogue), but with faint or previously undetected magnetic fields. We use the MuSiCoS spectropolarimeter at Telescope Bernard Lyot (Pic du Midi Observatory, France) and the cross-correlation technique Least Squares Deconvolution (LSD; Donati et al, 1997). For 10 studied stars, we have obtained the detections of Stokes V Zeeman signatures (data quality and phase coverage may explain our lack of detection of any field in some objects). Our results suggest that all Ap stars are magnetic and, furthermore that there may exist a minimum field strength for which Ap-type characteristics are produced.

We present here spectropolarimetric observations of the RS CVn system HR 1099 (V711 Tau) secured from 1998 February to 2002 January with the spectropolarimeter MuSiCoS at the T'elescope Bernard Lyot (Observatoire du Pic du Midi, France). We apply Zeeman- Doppler imaging and reconstruct surface brightness and magnetic topologies of the K1 primary subgiant of the system, at five different epochs. We confirm the presence of large, axisymmetric regions where the magnetic field is mainly azimuthal, providing further support to the hypothesis that dynamo processes may be distributed throughout the whole convective zone in this star.We study the short- term evolution of surface structures from a comparison of our images with observations secured at close- by epochs by Donati et al. at the Anglo- Australian Telescope. We conclude that the small- scale brightness and magnetic patterns undergo major changes within a time- scale of 4- 6 weeks, while the largest structures remain stable over several years.We report the detection of a weak surface differential rotation (both from brightness and magnetic tracers) indicating that the equator rotates faster than the pole with a difference in rotation rate between the pole and the equator about four times smaller than that of the Sun. This result suggests that tidal forces also affect the global dynamic equilibrium of convective zones in cool active stars.

We are conducting a magnetic survey of a sample of about 30 spectroscopically identified Ap stars, with weak or previously undetected magnetic fields. For 28 studied stars, we have obtained 25 detections of Stokes V Zeeman signatures. Our results suggest that all Ap stars are magnetic. Further there may exist a minimum field strength for which Ap-type characteristics are produced.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present here spectropolarimetric observations of the RS CVn system HR 1099 (V711 Tau) secured from 1998 February to 2002 January with the spectropolarimeter MuSiCoS at the Télescope Bernard Lyot (Observatoire du Pic du Midi, France). We apply Zeeman–Doppler imaging and reconstruct surface brightness and magnetic topologies of the K1 primary subgiant of the system, at five different epochs. We confirm the presence of large, axisymmetric regions where the magnetic field is mainly azimuthal, providing further support to the hypothesis that dynamo processes may be distributed throughout the whole convective zone in this star. We study the short-term evolution of surface structures from a comparison of our images with observations secured at close-by epochs by Donati et al. at the Anglo-Australian Telescope. We conclude that the small-scale brightness and magnetic patterns undergo major changes within a time-scale of 4–6 weeks, while the largest structures remain stable over several years. We report the detection of a weak surface differential rotation (both from brightness and magnetic tracers) indicating that the equator rotates faster than the pole with a difference in rotation rate between the pole and the equator about four times smaller than that of the Sun. This result suggests that tidal forces also affect the global dynamic equilibrium of convective zones in cool active stars.