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CO in OH/IR stars close to the Galactic centre  [PDF]
A. Winnberg,S. Deguchi,M. J. Reid,J. Nakashima,H. Olofsson,H. J. Habing
Physics , 2009, DOI: 10.1051/0004-6361/200911655
Abstract: Aims: A pilot project has been carried out to measure circumstellar CO emission from three OH/IR stars close to the Galactic centre. The intention was to find out whether it would be possible to conduct a large-scale survey for mass-loss rates using, for example, the Atacama Large Millimeter Array (ALMA). Such a survey would increase our understanding of the evolution of the Galactic bulge. Methods: Two millimetre-wave instruments were used: the Nobeyama Millimeter Array at 115 GHz and the Submillimeter Array at 230 GHz. An interferometer is necessary as a `spatial filter' in this region of space because of the confusion with interstellar CO emission. Results: Towards two of the stars, CO emission was detected with positions and radial velocities coinciding within the statistical errors with the corresponding data of the associated OH sources. However, for one of the stars the line profile is not what one expects for an unresolved expanding circumstellar envelope. We believe that this CO envelope is partially resolved and that this star therefore is a foreground star not belonging to the bulge. Conclusions: The results of the observations have shown that it is possible to detect line profiles of circumstellar CO from late-type stars both within and in the direction of the Galactic bulge. ALMA will be able to detect CO emission in short integrations with sensitivity sufficient to estimate mass-loss rates from a large number of such stars.
Discrete Source Survey of 6 GHz OH emission from PNe & pPNe and first 6 GHz images of K 3-35  [PDF]
J. -F. Desmurs,A. Baudry,P. Sivagnanam,C. Henkel,A. M. S. Richards,I. Bains
Physics , 2010, DOI: 10.1051/0004-6361/200913387
Abstract: The aim of this study is to investigate the physical properties of molecular envelopes of planetary nebulae in their earliest stages of evolution. Using the 100m telescope at Effelsberg, we have undertaken a high sensitivity discrete source survey for the first excited state of OH maser emission (J=5/2, 2PI3/2 at 6GHz) in the direction of planetary and proto-planetary nebulae exhibiting 18cm OH emission (main and/or satellite lines), and we further validate our detections using the Nan\c{c}ay radio telescope at 1.6-1.7GHz and MERLIN interferometer at 1.6-1.7 and 6GHz. Two sources have been detected at 6035MHz (5cm), both of them are young (or very young) planetary nebulae. The first one is a confirmation of the detection of a weak 6035MHz line in Vy 2-2. The second one is a new detection, in K 3-35, which was already known to be an exceptional late type star because it exhibits 1720MHz OH emission. The detection of 6035MHz OH maser emission is confirmed by subsequent observations made with the MERLIN interferometer. These lines are very rarely found in evolved stars. The 1612MHz masers surround but are offset from the 1720 and 6035MHz masers which in turn lie close to a compact 22GHz continuum source embedded in the optical nebula.
86 GHz SiO maser survey of late-type stars in the Inner Galaxy. I. Observational data  [PDF]
M. Messineo,H. J. Habing,L. O. Sjouwerman,A. Omont,K. M. Menten
Physics , 2002, DOI: 10.1051/0004-6361:20021017
Abstract: We present 86 GHz (v = 1, J = 2 -1) SiO maser line observations with the IRAM 30-m telescope of a sample of 441 late-type stars in the Inner Galaxy (-4 degr < l < +30 degr). These stars were selected on basis of their infrared magnitudes and colours from the ISOGAL and MSX catalogues. SiO maser emission was detected in 271 sources, and their line-of-sight velocities indicate that the stars are located in the Inner Galaxy. These new detections double the number of line-of-sight velocities available from previous SiO and OH maser observations in the area covered by our survey and are, together with other samples of e.g. OH/IR stars, useful for kinematic studies of the central parts of the Galaxy.
Obscured Asymptotic Giant Branch stars in the Magellanic Clouds IV. Carbon stars and OH/IR stars  [PDF]
Jacco Th. van Loon,Albert A. Zijlstra,Patricia A. Whitelock,Peter te Lintel Hekkert,Jessica M. Chapman,Cecile Loup,M. A. T. Groenewegen,L. B. F. M. Waters,Norman. R. Trams
Physics , 1997,
Abstract: We present N-band photometry for a sample of 21 dust- enshrouded AGB stars in the Large Magellanic Cloud, and three additional sources in the Small Magellanic Cloud. Together with near-IR photometry, this is used to give a tentative classification into carbon and oxygen-rich atmospheres. Bolometric luminosities are also estimated for these stars. In addition, we present the results of a survey for OH masers in the LMC, which resulted in the discovery of OH maser emission from IRAS04407-7000. Spectra between 600 and 1000 nm have been obtained for two heavily obscured AGB stars in the LMC, confirming them to be highly reddened very late M-type giants. Because the dust-enshrouded stars are clearly undergoing heavy mass loss they are assumed to be very near the termination of their respective Asymptotic Giant Branch phases. The fraction of mass-losing carbon stars decreases with increasing luminosity, as expected from Hot Bottom Burning. The best candidate carbon star, with M_bol = -6.8 mag, is the most luminous mass-losing carbon star in the Magellanic Clouds, and amongst the most luminous AGB stars. At lower luminosities (M_bol = -5 mag) both oxygen and carbon stars are found. This may be explained by a range in metallicity of the individual mass-losing AGB stars.
Maser and Infrared Studies of Oxygen-Rich Late/Post-AGB Stars and Water Fountains: Development of a New Identification Method  [PDF]
Bosco Yung,Jun-ichi Nakashima,Christian Henkel
Physics , 2014, DOI: 10.1088/0004-637X/794/1/81
Abstract: We explored an efficient method to identify evolved stars with oxygen-rich envelopes in the late AGB or post-AGB phase of stellar evolution, which include a rare class of objects - the "water fountains". Our method considers the OH and H2O maser spectra, the near infrared Q-parameters (these are colour indices accounting for the effect of extinction), and far-infrared AKARI colours. Here we first present the results of a new survey on OH and H2O masers. There were 108 colour-selected objects: 53 of them were observed in the three OH maser lines (1612, 1665, and 1667 MHz), with 24 detections (16 new for 1612 MHz); and 106 of them were observed in the H2O maser line (22 GHz) with 24 detections (12 new). We identify a new potential water fountain source, IRAS19356+0754, with large velocity coverages of both OH and H2O maser emission. In addition, several objects with high velocity OH maser emission are reported for the first time. The Q-parameters as well as the infrared [09]-[18] and [18]-[65] AKARI colours of the surveyed objects are then calculated. We suggest that these infrared properties are effective in isolating aspherical from spherical objects, but the morphology may not necessarily be related to the evolutionary status. Nonetheless, by considering altogether the maser and infrared properties, the efficiency of identifying oxygen-rich late/post-AGB stars could be improved.
Bipolar outflows in OH/IR stars  [PDF]
Albert A. Zijlstra,J. M. Chapman,P. te Lintel Hekkert,L. Likkel,F. Comeron,R. P. Norris,F. J. Molster,R. J. Cohen
Physics , 2000, DOI: 10.1046/j.1365-8711.2001.04113.x
Abstract: We investigate the development of bipolar outflows during the early post-AGB evolution. A sample of ten OH/IR stars is observed at high angular resolution, including bipolar nebulae (OH231.8+4.2), bright post-AGB stars (HD 101584) and reflection nebulae (e.g. Roberts 22). The IRAS colour--colour diagram separates the sample into different types of objects. One group may contain the progenitors to the (few) extreme bipolar planetary nebulae. Two objects show colours and chemistry very similar to the planetary nebulae with late IR-[WC] stars. One object is a confirmed close binary. A model is presented consisting of an outer AGB wind which is swept up by a faster post-AGB wind, with either wind being non-spherically symetric. The interface of the two winds is shown to exhibit a linear relation between velocity and distance from the star. The OH data confirms the predicted linear velocity gradients, and reveals torus-like, uniformly expanding components. All sources are discussed in detail using optical/HST images where available. ISO data for Roberts 22 reveal a chemical dichotomy, with both crystalline silicates and PAHs features being present. IRAS 16342-3814 shows a dense torus; HST data shows four point-like sources located symmetrically around the nebula, near the outer edge of the dense torus. Lifetimes for the bipolar OH/IR stars are shown to be in excess of 10^4 yr, longer than normal post-AGB timescales. This suggests that the disks are near-stationary. We suggest that accretion from such a disk slows down the post-AGB evolution. Such a process could explain the link between the long-lived bipolar nebular geometry and the retarded star.
The Onset of Cluster Formation around Intermediate Mass Stars  [PDF]
Leonardo Testi,Francesco Palla,Antonella Natta
Physics , 2000,
Abstract: We present the results of a recent near-infrared survey of the fields surrounding a large sample of intermediate-mass pre-main-sequence (Herbig Ae/Be) stars. While late-type Be and Ae stars are never associated with conspicuous groups of young lower mass stars, early-type Be stars are usually found within rich clusters. This finding has been tested against possible biases due to different mass sensitivity of the observations or dynamical dissipation of the clusters around older stars. Our results suggest that massive stars are preferentially produced in dense stellar clusters, possibly by dynamical interaction rather than by standard gas accretion as in the case of lower mass stars. The possibility that the observed correlation between maximum stellar mass and cluster richness could be the results of random sampling the cluster size spectrum and the stellar IMF is also discussed. Future observational tests capable of discriminating between these two competitive models are outlined.
The linear polarisation of southern bright stars measured at the parts-per-million level  [PDF]
Daniel V. Cotton,Jeremy Bailey,Lucyna Kedziora-Chudczer,Kimberly Bott,P. W. Lucas,J. H. Hough,Jonathan P. Marshall
Physics , 2015, DOI: 10.1093/mnras/stv2185
Abstract: We report observations of the linear polarisation of a sample of 50 nearby southern bright stars measured to a median sensitivity of $\sim$4.4 $\times 10^{-6}$. We find larger polarisations and more highly polarised stars than in the previous PlanetPol survey of northern bright stars. This is attributed to a dustier interstellar medium in the mid-plane of the Galaxy, together with a population containing more B-type stars leading to more intrinsically polarised stars, as well as using a wavelength more sensitive to intrinsic polarisation in late-type giants. Significant polarisation had been identified for only six stars in the survey group previously, whereas we are now able to deduce intrinsic polarigenic mechanisms for more than twenty. The four most highly polarised stars in the sample are the four classical Be stars ($\alpha$ Eri, $\alpha$ Col, $\eta$ Cen and $\alpha$ Ara). For the three of these objects resolved by interferometry, the position angles are consistent with the orientation of the circumstellar disc determined. We find significant intrinsic polarisation in most B stars in the sample; amongst these are a number of close binaries and an unusual binary debris disk system. However these circumstances do not account for the high polarisations of all the B stars in the sample and other polarigenic mechanisms are explored. Intrinsic polarisation is also apparent in several late type giants which can be attributed to either close, hot circumstellar dust or bright spots in the photosphere of these stars. Aside from a handful of notable debris disk systems, the majority of A to K type stars show polarisation levels consistent with interstellar polarisation.
Polarization properties of OH masers in AGB and post-AGB stars  [PDF]
Pawe? Wolak,Marian Szymczak,Eric Gérard
Physics , 2011, DOI: 10.1051/0004-6361/201117263
Abstract: Context: Ground-state OH maser emission from late-type stars is usually polarized and remains a powerful probe of the magnetic field structure in the outer regions of circumstellar envelopes if observed with high angular and spectral resolutions. Observations in all four Stokes parameters are quite sparse and this is the most thorough, systematic study published to date. Aims: We aim to determine polarization properties of OH masers in an extensive sample of stars that show copious mass loss and search for candidate objects that are well-suited for high angular resolution studies. Methods: Full-polarization observations of the OH 1612 and 1667 MHz maser transitions were carried out for a sample of 117 AGB and post-AGB stars. Several targets were also observed in the 1665 MHz line. Results: Polarized features occur in more than 75% of the sources in the complete sample and there is no intrinsic difference in the occurrence of polarized emission between the three classes of objects of different infrared characteristics. The highest fractional polarization occurs for the post-AGB+PN and the Mira+SR classes at 1612 and 1667 MHz, respectively. Differences in the fractional polarization between the sources at different evolutionary stages appear to be related to depolarization caused by blending. The alignment of the polarization angles at the extreme sides of the shell implies a regular structure of the magnetic field of a strength of 0.3-2.3 mG. Conclusions: Polarized OH maser features are widespread in AGB and post-AGB stars. The relationship between the circular and linear fractional polarizations for a representative sample are consistent with the standard models of polarization for the Zeeman splitting higher than the Doppler line width, whereas the polarized features are the sigma components.
The Pattern Speed of the OH/IR Stars in the Milky Way  [PDF]
Victor P. Debattista,Ortwin Gerhard,Maartje N. Sevenster
Physics , 2002, DOI: 10.1046/j.1365-8711.2002.05500.x
Abstract: We show how the continuity equation can be used to determine pattern speeds in the Milky Way Galaxy (MWG). This method, first discussed by Tremaine & Weinberg in the context of external galaxies, requires projected positions, $(l,b)$, and line-of-sight velocities for a spatially complete sample of relaxed tracers. If the local standard of rest (LSR) has a zero velocity in the radial direction ($u_{\rm LSR}$), then the quantity that is measured is $\Delta V \equiv \Omega_p R_0 - V_{\rm LSR}$, where $\Omega_p$ is the pattern speed of the non-axisymmetric feature, $R_0$ is the distance of the Sun from the Galactic centre and $V_{\rm LSR}$ is the tangential motion of the LSR, including the circular velocity. We use simple models to assess the reliability of the method for measuring a single, constant pattern speed of either a bar or spiral in the inner MWG. We then apply the method to the OH/IR stars in the ATCA/VLA OH 1612 MHz survey of Sevenster et al, finding $\Delta V = 252 \pm 41$ km/s, if $u_{\rm LSR} = 0$. Assuming further that $R_0 = 8$ kpc and $V_{\rm LSR} = 220$ \kms, this gives $\Omega_p = 59\pm 5$ km/s/kpc with a possible systematic error of perhaps 10 km/s/kpc. The non-axisymmetric feature for which we measure this pattern speed must be in the disc of the MWG.
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