142 research outputs found
An updated catalog of OH-maser-emitting planetary nebulae
Aims. We studied the characteristics of planetary nebulae (PNe) that show
both OH maser and radio continuum emission (hereafter OHPNe). These have been
proposed to be very young PNe, and therefore, they could be key objects for
understanding the formation and evolution of PNe. Methods. We consulted the
literature searching for interferometric observations of radio continuum and OH
masers toward evolved stars, including the information from several surveys. We
also processed radio continuum and OH maser observations toward PNe in the Very
Large Array data archive. The high positional accuracy provided by
interferometric observations allow us to confirm or reject the association
between OH maser and radio continuum emission. Results. We found a total of six
PNe that present both OH maser and radio continuum emissions, as confirmed with
radio interferometric observations. These are bona fide OHPNe. The confirmed
OHPNe present a bipolar morphology in resolved images of their ionized emission
at different wavelengths, suggesting that the OH maser emission in PNe is
related to nonspherical mass-loss phenomena. The OH maser spectra in PNe
present a clear asymmetry, tending to show blueshifted emission with respect to
the systemic velocity. Their infrared colors suggest that most of these objects
are very young PNe. OHPNe do not form a homogeneous group, and seem to
represent a variety of different evolutionary stages. We suggest that OH masers
pumped in the AGB phase may disappear during the post-AGB phase, but reappear
once the source becomes a PN and its radio continuum emission is amplified by
the OH molecules. Therefore, OH maser emission could last significantly longer
than the previously assumed 1000 yr after the end of the AGB phase. This maser
lifetime may be longer in PNe with more massive central stars, which ionize a
larger amount of gas in the envelope.Comment: 16 pages, 5 figures, 4 tables. Accepted for publication by Astronomy
& Astrophysic
Position-Velocity Diagrams for the Maser Emission coming from a Keplerian Ring
We have studied the maser emission from a thin, planar, gaseous ring in
Keplerian rotation around a central mass observed edge-on. The absorption
coefficient within the ring is assumed to follow a power law dependence with
the distance from the central mass as, k=k0r^{-q}. We have calculated
position-velocity diagrams for the most intense maser features, for different
values of the exponent q. We have found that, depending on the value of q,
these diagrams can be qualitatively different. The most intense maser emission
at a given velocity can either come mainly from regions close to the inner or
outer edges of the amplifying ring or from the line perpendicular to the line
of sight and passing through the central mass (as is commonly assumed).
Particularly, when q>1 the position-velocity diagram is qualitatively similar
to the one observed for the water maser emission in the nucleus of the galaxy
NGC 4258. In the context of this simple model, we conclude that in this object
the absorption coefficient depends on the radius of the amplifying ring as a
decreasing function, in order to have significant emission coming from the
inner edge of the ring.Comment: 13 pages, 7 figures, to appear in the 2007 July 20 issue of The
Astrophysical Journa
Statistical Analysis of Water Masers in Star-Forming Regions: Cepheus A and W75 N
We have done a statistical analysis of Very Long Baseline Array (VLBA) data
of water masers in the star-forming regions (SFRs) Cepheus A and W75 N, using
correlation functions to study the spatial clustering and Doppler-velocity
distribution of these masers. Two-point spatial correlation functions show a
characteristic scale size for clusters of water maser spots < or ~1 AU, similar
to the values found in other SFRs. This suggests that the scale for water maser
excitation tends to be < or ~1 AU. Velocity correlation functions show
power-law dependences with indices that can be explained by regular velocity
fields, such as expansion and/or rotation. These velocity fields are similar to
those indicated by the water maser proper-motion measurements; therefore, the
velocity correlation functions appear to reveal the organized motion of water
maser spots on scales larger than 1 AU.Comment: 16 pages, 8 figures, and 3 tables. Accepted by The Astrophysical
Journa
Kinematics of the H2O masers at the centre of the PN K3-35
We have studied the kinematics traced by the water masers located at the
centre of the planetary nebula (PN) K3-35, using data from previous Very Large
Array (VLA) observations. An analysis of the spatial distribution and
line-of-sight velocities of the maser spots allows us to identify typical
patterns of a rotating and expanding ring in the position-velocity diagrams,
according to our kinematical model. We find that the distribution of the masers
is compatible with tracing a circular ring with a ~0.021 arcsec (~100 AU)
radius, observed with an inclination angle with respect to the line of sight of
55 degrees. We derive expansion and rotation velocities of 1.4 and 3.1 km/s,
respectively. The orientation of the ring projected on the plane of the sky, at
PA 158 degrees, is almost orthogonal to the direction of the innermost region
of the jet observed in K3-35, suggesting the presence of a disc or torus that
may be related to the collimation of the outflow.Comment: 7 pages, 4 figures, 2 tables. Accepted by MNRA
Evolution of the Outflow in the Water Fountain Source IRAS 18043-2116
We present the spectral and spatial evolution of H2O masers associated with the water fountain source IRAS 18043-2116, found in observations with the Nobeyama 45 m Telescope and the Australia Telescope Compact Array. We have found new highest-velocity components of the H2O masers (at the redshifted side V (LSR) similar or equal to 376 km s(-1) and at the blueshifted side V (LSR) similar or equal to -165 km s(-1)), and the resulting velocity spread of similar or equal to 540 km s(-1) breaks the speed record of fast jets/outflows in this type of sources. The locations of those components have offsets from the axis joining the two major maser clusters, indicating a large opening angle of the outflow (similar to 60 degrees). The evolution of the maser cluster separation of similar to 2.9 mas yr(-1) and the compact (similar to 0.\u27\u27 2) CO emission source mapped with the Atacama Large Millimeter-submillimeter Array suggest a very short (similar to 30 yr) timescale of the outflow. We also confirmed an increase in the flux density of the 22 GHz continuum source. The properties of the jet and the continuum sources and their possible evolution in the transition to the planetary nebula phase are further discussed
ALMA sub-mm maser and dust distribution of VY Canis Majoris
Cool, evolved stars have copious, enriched winds. The structure of these
winds and the way they are accelerated is not well known. We need to improve
our understanding by studying the dynamics from the pulsating stellar surface
to about 10 stellar radii, where radiation pressure on dust is fully effective.
Some red supergiants have highly asymmetric nebulae, implicating additional
forces. We retrieved ALMA Science Verification data providing images of sub-mm
line and continuum emission from VY CMa. This enables us to locate water masers
with milli-arcsec precision and resolve the dusty continuum. The 658-, 321- and
325-GHz masers lie in irregular, thick shells at increasing distances from the
centre of expansion. For the first time this is confirmed as the stellar
position, coinciding with a compact peak offset to the NW of the brightest
continuum emission. The maser shells (and dust formation zone) overlap but
avoid each other on tens-au scales. Their distribution is broadly consistent
with excitation models but the conditions and kinematics appear to be
complicated by wind collisions, clumping and asymmetries.Comment: Letter 4 pages, 5 figures plus appendix with 3 figures. Accepted by
Astronomy and Astrophysics Letter
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