A high-sensitivity OH 5-cm line survey in late-type stars

We have undertaken a comprehensive search for 5-cm excited OH maser emission from evolved stars representative of various stages of late stellar evolution. Observed sources were selected from known 18-cm OH sources. This survey was conducted with the 100-m Effelsberg telescope to achieve high signal to noise ratio observations and a sensitivity limit of about 0.05 to 0.1 Jy. A total of 64 stellar sources were searched for both main line and satellite line emission. We confirm the previous detection of 5 cm OH in Vy 2-2, do not confirm emission from NML-Cyg and do not report any other new detection within the above sensitivity limit. Implications of these results on the pumping mechanism of the OH radical in circumstellar envelopes are briefly discussed.Comment: 8 pages, 5 figures, A&A in pres

VLBI Astrometry of the Stellar Image of U Herculis, Amplified by the 1667 OH Maser

The OH 1667 MHz maser in the circumstellar shell around the Mira variable U Herculis has been observed with the NRAO Very Long Baseline Array (VLBA) at 6 epochs, spread over 4 years. Using phase referencing techniques the position of the most blue-shifted maser spot was monitored with respect to two extra-galactic radio sources. The absolute radio positions of the maser can be compared with the stellar optical position measured by the Hipparcos satellite to 15 mas accuracy. This confirms the model in which one of the maser spots corresponds to the stellar continuum, amplified by the maser. The stellar proper motion and the annual parallax (5.3 +/- 2.1 mas) were measured.Comment: 6 pages, 4 figures; to be published in A&

Dynamics of cholesteric structures in an electric field

Motivated by Lehmann-like rotation phenomena in cholesteric drops we study the transverse drift of two types of cholesteric fingers, which form rotating spirals in thin layers of cholesteric liquid crystal in an ac or dc electric field. We show that electrohydrodynamic effects induced by Carr-Helfrich charge separation or flexoelectric charge generation can describe the drift of cholesteric fingers. We argue that the observed Lehmann-like phenomena can be understood on the same basis.Comment: 4 pages, 4 figures, submitted to PR

Ground-State SiO Maser Emission Toward Evolved Stars

We have made the first unambiguous detection of vibrational ground-state maser emission from SiO toward six evolved stars. Using the Very Large Array, we simultaneously observed the v=0, J=1-0, 43.4-GHz, ground-state and the v=1, J=1-0, 43.1-GHz, first excited-state transitions of SiO toward the oxygen-rich evolved stars IRC+10011, o Ceti, W Hya, RX Boo, NML Cyg, and R Cas and the S-type star chi Cyg. We detected at least one v=0 SiO maser feature from six of the seven stars observed, with peak maser brightness temperatures ranging from 10,000 K to 108,800 K. In fact, four of the seven v=0 spectra show multiple maser peaks, a phenomenon which has not been previously observed. Ground-state thermal emission was detected for one of the stars, RX Boo, with a peak brightness temperature of 200 K. Comparing the v=0 and the v=1 transitions, we find that the ground-state masers are much weaker with spectral characteristics different from those of the first excited-state masers. For four of the seven stars the velocity dispersion is smaller for the v=0 emission than for the v=1 emission, for one star the dispersions are roughly equivalent, and for two stars (one of which is RX Boo) the velocity spread of the v=0 emission is larger. In most cases, the peak flux density in the v=0 emission spectrum does not coincide with the v=1 maser peak. Although the angular resolution of these VLA observations were insufficient to completely resolve the spatial structure of the SiO emission, the SiO spot maps produced from the interferometric image cubes suggest that the v=0 masers are more extended than their v=1 counterparts

Full-Polarization Observations of OH Masers in Massive Star-Forming Regions: I. Data

We present full-polarization VLBA maps of the ground-state, main-line, 2 Pi 3/2, J = 3/2 OH masers in 18 Galactic massive star-forming regions. This is the first large polarization survey of interstellar hydroxyl masers at VLBI resolution. A total of 184 Zeeman pairs are identified, and the corresponding magnetic field strengths are indicated. We also present spectra of the NH3 emission or absorption in these star-forming regions. Analysis of these data will be presented in a companion paper.Comment: 111 pages, including 42 figures and 21 tables, to appear in ApJ

5cm OH masers as diagnostics of physical conditions in star-forming regions

We demonstrate that the observed characteristics of the 5 cm OH masers in star-forming regions can be explained with the same model and the same parameters as the 18 cm and the 6 cm OH masers. In our already published study of the 18 cm and the 6 cm OH masers in star-forming regions we had examined the pumping of the 5 cm masers, but did not report the results we had found because of some missing collision rate coefficients, which in principle could be important. The recently published observations on the 5 cm masers of OH encourage us to report our old calculations along with some new ones that we have performed. These calculations, in agreement with the observations, reveal the main lines at 5 cm as strong masers, the 6049 MHz satellite line as a weak maser, and the 6017 MHz satellite line as never inverted for reasonable values of the parametersComment: TeX 15 pages, 30 postscript figures, accepted by Ap

Formation of disclination lines near a free nematic interface

We have studied the nucleation and the physical properties of a -1/2 wedge disclination line near the free surface of a confined nematic liquid crystal. The position of the disclination line has been related to the material parameters (elastic constants, anchoring energy and favored anchoring angle of the molecules at the free surface). The use of a planar model for the structure of the director field (whose predictions have been contrasted to those of a fully three-dimensional model) has allowed us to relate the experimentally observed position of the disclination line to the relevant properties of the liquid crystals. In particular, we have been able to observe the collapse of the disclination line due to a temperature-induced anchoring angle transition, which has allowed us to rule out the presence of a real disclination line near the nematic/isotropic front in directional growth experiments. 61.30.Jf,61.30.G

Hot gas and dust in a protostellar cluster near W3(OH

We used the IRAM Interferometer to obtain sub-arcsecond resolution observations of the high-mass star-forming region W3(OH) and its surroundings at a frequency of 220 GHz. With the improved angular resolution, we distinguish 3 peaks in the thermal dust continuum emission originating from the hot core region about 6 arcsec (0.06 pc) east of W3(OH). The dust emission peaks are coincident with known radio continuum sources, one of which is of non-thermal nature. The latter source is also at the center of expansion of a powerful bipolar outflow observed in water maser emission. We determine the hot core mass to be 15 solar masses based on the integrated dust continuum emission. Simultaneously many molecular lines are detected allowing the analysis of the temperature structure and the distribution of complex organic molecules in the hot core. From HNCO lines, spanning a wide range of excitation, two 200 K temperature peaks are found coincident with dust continuum emission peaks suggesting embedded heating sources within them.Comment: 12 pages, 3 figure

The HI shell G132.6-0.7-25.3: A Supernova Remnant or an Old Wind-Blown Bubble?

Data from the Canadian Galactic Plane Survey reveal an abundance of HI shells and arcs in the disk of our galaxy. While their shape is suggestive of stellar winds or supernovae influence, very few of these structures have been examined in detail thus far. A fine example is an HI shell in the outer Galaxy with no continuum counterpart discovered in the survey's pilot project. Its size and kinematics suggest that it was created by the winds of a single late-type O star which has since evolved off the main sequence or by a supernova explosion. A B1 Ia star at the centre of the shell, in projection, is a possible candidate for energy source if the shell is assumed to be wind-blown. The shell's shape implies a surprisingly small scale height of less than about 30 pc for the surrounding gas if the elongation is due to evolution in a density gradient.Comment: 25 pages, 6 figures. Accepted for publication in the Astronomical Journa

Lehmann rotation of cholesteric droplets subjected to a temperature gradient: role of the concentration of chiral molecules

International audienceWe present a systematic study of the Lehmann rotation of cholesteric droplets subjected to a temperature gradient when the concentration of chiral molecules is changed. The liquid crystal chosen is an eutectic mixture of 8CB and 8OCB doped with a small amount of the chiral molecule R811. The angular velocity of the droplets strongly depend on their size and on the concentration of chiral molecules. The Lehmann coefficient is estimated by using three different methods. Our results are consistent with a Lehmann coefficient proportional to the concentration of chiral molecules. We additionally show the existence of a critical size of the droplets below which they change texture and stop rotating