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Onsala, Sweden

Chong S.-N.,Kagoshima University | Kwok S.,University of Hong Kong | Imai H.,Kagoshima University | Tafoya D.,Onsala Space Observatory | Chibueze J.,Kagoshima University
Astrophysical Journal

Planetary nebulae (PNe) have diverse morphological shapes, including point-symmetric and multipolar structures. Many PNe also have complicated internal structures such as tori, lobes, knots, and ansae. A complete accounting of all the morphological structures through physical models is difficult. A first step toward such an understanding is to derive the true three-dimensional structure of the nebulae. In this paper, we show that a multipolar nebula with three pairs of lobes can explain many such features, if orientation and sensitivity effects are taken into account. Using only six parameters - the inclination and position angles of each pair - we are able to simulate the observed images of 20 PNe with complex structures. We suggest that multipolar structure is an intrinsic structure of PNe and the statistics of multipolar PNe have been severely underestimated in the past. © 2012. The American Astronomical Society. All rights reserved.. Source

Maercker M.,Albanova University Center | Olofsson H.,Albanova University Center | Eriksson K.,Uppsala University | Gustafsson B.,Uppsala University | Schoier F.L.,Onsala Space Observatory
Astronomy and Astrophysics

Context: Geometrically thin, detached shells of gas have been found around a handful of carbon stars. The current knowledge on these shells is mostly based on CO radio line data. However, imaging in scattered stellar light adds important new information as well as allows studies of the dust shells. Aims: Previous observations of scattered stellar light in the circumstellar medium around the carbon star U Ant were taken through filters centred on the resonance lines of K and Na. These observations could not separate the scattering by dust and atoms. The aim of this paper is to remedy this situation. Methods: We have obtained polarization data on stellar light scattered in the circumstellar medium around U Ant through filters which contain no strong lines, making it possible to differentiate between the two scattering agents. Kinematic, as well as spatial, information on the gas shells were obtained through high-resolution echelle spectrograph observations of the KI and NaD lines. Results: We confirm the existence of two detached shells around U Ant. The inner shell (at a radius of ≈43″ and a width of ≈2″) consists mainly of gas, while the outer shell (at a radius of ≈50″ and a width of ≈7″) appears to consist exclusively of dust. Both shells appear to have an over-all spherical geometry. The gas shell mass is estimated to be 2 × 10-3 M⊙, while the mass of the dust shell is estimated to be 5 × 10-5 M⊙. The derived expansion velocity, from the KI and NaD lines, of the gas shell, 19.5 kms-1, agrees with that obtained from CO radio line data. The inferred shell age is 2700 years. There is structure, e.g. in the form of arcs, inside the gas shell, but it is not clear whether these are due to additional shells. Conclusions: Our results support the hypothesis that the observed geometrically thin, detached shells around carbon stars are the results of brief periods of intense mass loss, probably associated with thermal pulses, and subsequent wind-wind interactions. The separation into a gas and a dust shell, with different widths, is most likely the effect of different dynamical evolutions of the two media after their ejection. © 2010 ESO. Source

Kalenskii S.V.,RAS Lebedev Physical Institute | Johansson L.E.B.,Onsala Space Observatory
Astronomy Reports

A spectral survey of the W51 e1/e2 star-forming region at 84-115 GHz has yielded detections of 105 molecules and their isotopic species, from simple diatomic or triatomic molecules, such as CO, CS, HCN, up to complex organic compounds, such as CH3OCH3, CH3COCH3, and C2H5OOCH. Ninety-three lines that are absent from the Lovas list of molecular lines observed from space were detected, and approximately half of these were identified. A significant number of the detectedmolecules are typical for hot cores. These include the neutral molecules CH3OCHO, C2H5OH, CH3COCH3 etc., which are currently believed to exist in the gas phase only in hot cores and shock-heated gas. In addition, vibrationally excited SiO, C4H, HCN, l-C3H, HCCCN, CH3CN, CH3OH, H2O, and SO2 lines with upper-level temperatures of several hundred Kelvin were found. Such lines can arise only in hot gas with temperatures of the order of 100 K or higher. Apart from neutral molecules, various molecular ions were also detected. Some of these (N2H+, HCO+, HCS+) usually exist in molecular clouds with high visual extinctions AV. At the same time, the CF+ ion should be observed in photon-dominated regions with AV values of about unity or lower. An interesting result is the tentative detection of two molecules that have thus far been observed only in the atmospheres of late-type giant stars-MgCN and NaCN. This suggests that the conditions in the hottest W51 regions (probably, in the vicinities of protostars) are close to those in the atmospheres of giant stars. It would be desirable to search for other lines of these molecules to verify these tentative detections. Analysis of the radial velocities of the detected molecules suggests that the contribution from the e2 core dominates the emission of some O-bearing molecules (CH3OCHO, CH3CH2OH), while the contribution of the e1 core dominates the emission of some N-bearing molecules (e. g., CH3CH2CN). Thus, the molecular composition of the e2 core may be closer to the composition of the "Compact Ridge" in OMC-1, while the composition of the e1 core is closer to that for the "Hot Core" in the same cloud. © 2010 Pleiades Publishing, Ltd. Source

Rushton A.,Onsala Space Observatory | Rushton A.,European Southern Observatory | Spencer R.,University of Manchester | Fender R.,University of Southampton | And 2 more authors.
Astronomy and Astrophysics

Recent studies of different X-ray binaries (XRBs) have shown a clear correlation between the radio and X-ray emission. We present evidence of a close relationship found between the radio and X-ray emission at different epochs for GRS 1915+105, using observations from the Ryle Telescope and Rossi X-ray Timing Explorer satellite. The strongest correlation was found during the hard state (also known as the "plateau" state), where a steady AU-scale jet is known to exist. Both the radio and X-ray emission were found to decay from the start of most plateau states, with the radio emission decaying faster. An empirical relationship of was then fitted to data taken only during the plateau state, resulting in a power-law index of ξ ~ 1.7 ± 0.3, which is significantly higher than in other black hole XRBs in a similar state. An advection-flow model was then fitted to this relationship and compared to the universal XRB relationship as described by Gallo et al. (2003, MNRAS, 344, 60). We conclude that either (I) the accretion disk in this source is radiatively efficient, even during the continuous outflow of a compact jet, which could also suggest a universal turn-over from radiatively inefficient to efficient for all stellar-mass black holes at a critical mass accretion rate (M c≈1018.5 g/s); or (II) the X-rays in the plateau state are dominated by emission from the base of the jet and not the accretion disk (e.g. via inverse Compton scattering from the outflow). © 2010 ESO. Source

Sakamoto K.,Academia Sinica, Taiwan | Aalto S.,Onsala Space Observatory | Evans A.S.,University of Virginia | Evans A.S.,U.S. National Radio Astronomy Observatory | And 2 more authors.
Astrophysical Journal Letters

Infrared pumping and its effect on the excitation of HCN molecules can be important when using rotational lines of HCN to probe dense molecular gas in galaxy nuclei. We report the first extragalactic d tection of (sub)millimeter otational lines of vibrationally excited HCN, in the dust-enshrouded nucleus of the luminous infrared galaxy GC 4418. We estimate the excitation temperature of Tvib ≈ 230 K between the vibrational ground and excited (v 2 = 1) states. This excitation is most likely due to infrared radiation. At this high vibrational temperature the path through the v 2 = 1 state must have a strong impact on the rotational excitation in the vibrational ground level, although it may not be dominant for all rotational levels. Our observations also revealed nearly confusion-limited lines of CO, HCN, HCO+, H13CN, HC15N, CS, N 2H+, and HC3N at λ ∼ 1 mm. Their relative intensities may also be affected by the infrared pumping. 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A. Source

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