233 research outputs found
The Shapes of AGB Envelopes as Probes of Binary Companions
We describe how the large scale geometry of the circumstellar envelopes of
asymptotic giant branch stars can be used to probe the presence of unseen
stellar companions. A nearby companion modifies the mass loss by
gravitationally focusing the wind towards the orbital plane, and thereby
determines the shape of the envelope at large distances from the star. Using
available simulations, we develop a prescription for the observed shapes of
envelopes in terms of the binary parameters, envelope orientation, and type of
observation. The prescription provides a tool for the analysis of envelope
images at optical, infrared, and millimetre wavelengths, which can be used to
constrain the presence of companions in well observed cases. We illustrate this
approach by examining the possible role of binary companions in triggering the
onset of axi-symmetry in planetary nebula formation. If interaction with the
primary leads to axi-symmetry, the spherical halos widely seen around newly
formed nebulae set limits on the companion mass. Only low mass objects may
orbit close to the primary without observable shaping effects: they remain
invisible until the interaction causes a sudden change in the mass loss
geometry.Comment: 11 pages, 7 figures, to appear in MNRA
Solar-Like Cycle in Asymptotic Giant Branch Stars
I propose that the mechanism behind the formation of concentric semi-periodic
shells found in several planetary nebulae (PNs) and proto-PNs, and around one
asymptotic giant branch (AGB) star, is a solar-like magnetic activity cycle in
the progenitor AGB stars. The time intervals between consecutive ejection
events is about 200-1,000 years, which is assumed to be the cycle period (the
full magnetic cycle can be twice as long, as is the 22-year period in the sun).
The magnetic field has no dynamical effects; it regulates the mass loss rate by
the formation of magnetic cool spots. The enhanced magnetic activity at the
cycle maximum results in more magnetic cool spots, which facilitate the
formation of dust, hence increasing the mass loss rate. The strong magnetic
activity implies that the AGB star is spun up by a companion, via a tidal or
common envelope interaction. The strong interaction with a stellar companion
explains the observations that the concentric semi-periodic shells are found
mainly in bipolar PNs.Comment: 10 pages, submitted to Ap
The low wind expansion velocity of metal-poor carbon stars in the Halo and the Sagittarius stream
We report the detection, from observations using the James Clerk Maxwell
Telescope, of CO J 3 2 transition lines in six carbon stars, selected
as members of the Galactic Halo and having similar infrared colors. Just one
Halo star had been detected in CO before this work. Infrared observations show
that these stars are red (J-K 3), due to the presence of large dusty
circumstellar envelopes. Radiative transfer models indicates that these stars
are losing mass with rather large dust mass-loss rates in the range 1--3.3
Myr, similar to what can be observed in the
Galactic disc. We show that two of these stars are effectively in the Halo, one
is likely linked to the stream of the Sagittarius Dwarf Spheroidal galaxy (Sgr
dSph), and the other three stars certainly belong to the thick disc. The wind
expansion velocities of the observed stars are low compared to carbon stars in
the thin disc and are lower for the stars in the Halo and the Sgr dSph stream
than in the thick disc. We discuss the possibility that the low expansion
velocities result from the low metallicity of the Halo carbon stars. This
implies that metal-poor carbon stars lose mass at a rate similar to metal-rich
carbon stars, but with lower expansion velocities, as predicted by recent
theoretical models. This result implies that the current estimates of mass-loss
rates from carbon stars in Local Group galaxies will have to be reconsidered.Comment: 10 pages, 7 figures, accepted for publication in MNRA
A Detailed Analysis of the Dust Formation Zone of IRC+10216 Derived from Mid-IR Bands of C2H2 and HCN
A spectral survey of IRC+10216 has been carried out in the range 11 to 14 um
with a spectral resolution of about 4 km s^-1. We have identified a forest of
lines in six bands of C2H2 involving the vibrational states from the ground to
3nu5 and in two bands of HCN, involving the vibrational states from the ground
up to 2nu2. Some of these transitions are observed also in H13CCH and H13CN. We
have estimated the kinetic, vibrational, and rotational temperatures, and the
abundances and column densities of C2H2 and HCN between 1 and 300 R* (1.5E16
cm) by fitting about 300 of these ro-vibrational lines. The envelope can be
divided into three regions with approximate boundaries at 0.019 arcsec (the
stellar photosphere), 0.1 arcsec (the inner dust formation zone), and 0.4
arcsec (outer dust formation zone). Most of the lines might require a large
microturbulence broadening. The derived abundances of C2H2 and HCN increase by
factors of 10 and 4, respectively, from the innermost envelope outwards. The
derived column densities for both C2H2 and HCN are 1.6E19 cm^-2. Vibrational
states up to 3000 K above ground are populated, suggesting pumping by
near-infrared radiation from the star and innermost envelope. Low rotational
levels can be considered under LTE while those with J>20-30 are not
thermalized. A few lines require special analysis to deal with effects like
overlap with lines of other molecules.Comment: 8 pages, 16 figures, 2 machine-readable tables, accepted in the
Astrophysical Journa
Water vapor emission from IRC+10216 and other carbon-rich stars: model predictions and prospects for multitransition observations
We have modeled the emission of H2O rotational lines from the extreme C-rich
star IRC+10216. Our treatment of the excitation of H2O emissions takes into
account the excitation of H2O both through collisions, and through the pumping
of the nu2 and nu3 vibrational states by dust emission and subsequent decay to
the ground state. Regardless of the spatial distribution of the water
molecules, the H2O 1_{10}-1_{01} line at 557 GHz observed by the Submillimeter
Wave Astronomy Satellite (SWAS) is found to be pumped primarily through the
absorption of dust-emitted photons at 6 m in the nu2 band. As noted by
previous authors, the inclusion of radiative pumping lowers the ortho-H2O
abundance required to account for the 557 GHz emission, which is found to be
(0.5-1)x10^{-7} if the presence of H2O is a consequence of vaporization of
orbiting comets or Fischer-Tropsch catalysis. Predictions for other
submillimeter H2O lines that can be observed by the Herschel Space Observatory
(HSO) are reported. Multitransition HSO observations promise to reveal the
spatial distribution of the circumstellar water vapor, discriminating among the
several hypotheses that have been proposed for the origin of the H2O vapor in
the envelope of IRC+10216. We also show that, for observations with HSO, the
H2O 1_{10}-1_{01} 557 GHz line affords the greatest sensitivity in searching
for H2O in other C-rich AGB stars.Comment: 35 pages, 12 figures, to be published in The Astrophysical Journa
Imaging of detached shells around the carbon stars R Scl and U Ant through scattered stellar light
We present the first optical images of scattered light from large, detached
gas/dust shells around two carbon stars, R Scl and U Ant, obtained in narrow
band filters centred on the resonance lines of neutral K and Na, and in a Str.
b filter (only U Ant). They confirm results obtained in CO radio line
observations, but also reveal new and interesting structures. Towards R Scl the
scattering appears optically thick in both the K and Na filters, and both
images outline almost perfectly circular disks with essentially uniform
intensity out to a sharp outer radius of 21". These disks are larger -- by
about a factor of two -- than the radius of the detached shell which has been
marginally resolved in CO radio line data. In U Ant the scattering in the K
filter appears to be, at least partially, optically thin, and the image is
consistent with scattering in a geometrically thin (3") shell (radius 43") with
an overall spherical symmetry. The size of this shell agrees very well with
that of the detached shell seen in CO radio line emission. The scattering in
the Na filter appears more optically thick, and the image suggests the presence
of at least one, possibly two, shells inside the 43" shell. There is no
evidence for such a multiple-shell structure in the CO data, but this can be
due to considerably lower masses for these inner shells. Weak scattering
appears also in a shell which is located outside the 43" shell. The present
data do not allow us to conclusively identify the scattering agent, but we
argue that most of the emission in the K and Na filter images is to due to
resonance line scattering, and that there is also a weaker contribution from
dust scattering in the U Ant data. Awaiting new observational data, our
interpretation must be regarded as tentative.Comment: 14 pages, 11 figures, to be published in A&
Dust temperature and density profiles of AGB and post-AGB stars from mid-infrared observations
First mid-infrared images of a sample of AGB and post-AGB carbon stars (V
Hya, IRC +10216, CIT 6 and Roberts 22) obtained at La Silla Observatory (ESO,
Chile) are reported. CIT 6 presents a cometary-like feature clearly seen in the
9.7m image, Roberts 22 shows an envelope slightly elongated in the
north-east direction while images of V Hya and IRC+10216 are roughly
spherically symmetric. Using inversion technique, the dust emissivity was
derived from the observed intensity profiles, allowing a determination of the
grain temperature and density distributions inside the envelope for these
stars. Dust masses and mass-loss rates were estimated for V Hya and IRC +10216.
Our results are comparable to those obtained in previous studies if dust grains
have dimensions in the range 0.01 - 0.2 m. Color maps suggest the
presence of temperature inhomogeneities in the central regions of the dust
envelopes. In the case of V Hya, an eccentric hot point, which direction
coincides with the jet previously seen in [SII] emission, suggest that we are
observing a material ejected in a previous mass-loss event. Bipolar lobes are
clearly seen in the color maps of Roberts 22 and IRC +10216.Comment: accepted for publication in Astronomy and Astrophysic
The mass-loss rates of red supergiants and the de Jager prescription
Mass loss of red supergiants (RSG) is important for the evolution of massive
stars, but is not fully explained. Several empirical prescriptions have been
proposed, trying to express the mass-loss rate (Mdot) as a function of
fundamental stellar parameters (mass, luminosity, effective temperature). Our
goal is to test whether the de Jager et al. (1988) prescription, used in some
stellar evolution models, is still valid in view of more recent mass-loss
determinations. By considering 40 Galactic RSGs presenting an infrared excess
and an IRAS 60-mu flux larger than 2 Jy, and assuming a gas-to-dust mass ratio
of 200, it is found that the de Jager rate agrees within a factor 4 with most
Mdot estimates based on the 60-mu signal. It is also in agreement with 6 of the
only 8 Galactic RSGs for which Mdot can be measured more directly through
observations of the circumstellar gas. The two objects that do not follow the
de Jager prescription (by an order of magnitude) are mu Cep and NML Cyg. We
have also considered the RSGs of the Magellanic Clouds. Thanks to the works of
Groenewegen et al. (2009) and Bonanos et al. (2010), we find that the RSGs of
the SMC have Mdots consistent with the de Jager rate scaled by
(Z/Zsun)**(alpha), where Z is the metallicity and alpha is 0.7. The situation
is less clear for the LMC RSGs. In particular, for luminosties larger than
1.6E+05 Lsun, one finds numerous RSGs (except WOH-G64) having Mdot
significantly smaller than the de Jager rate, and indicating that Mdot would no
longer increase with L. Before this odd situation is confirmed through further
analysis of LMC RSGs, we suggest to keep the de Jager prescription unchanged at
solar metallicity in the stellar evolutionary models and to apply a
(Z/Zsun)**0.7 dependence.Comment: 13 pages, 9 figures. Accepted by Astronomy and Astrophysic
Crossing the `Yellow Void' -- Spatially Resolved Spectroscopy of the Post- Red Supergiant IRC+10420 and Its Circumstellar Ejecta
IRC +10420 is one of the extreme hypergiant stars that define the empirical
upper luminosity boundary in the HR diagram. During their post--RSG evolution,
these massive stars enter a temperature range (6000-9000 K) of increased
dynamical instability, high mass loss, and increasing opacity, a
semi--forbidden region, that de Jager and his collaborators have called the
`yellow void'. We report HST/STIS spatially resolved spectroscopy of IRC +10420
and its reflection nebula with some surprising results. Long slit spectroscopy
of the reflected spectrum allows us to effectively view the star from different
directions. Measurements of the double--peaked Halpha emission profile show a
uniform outflow of gas in a nearly spherical distribution, contrary to previous
models with an equatorial disk or bipolar outflow. Based on the temperature and
mass loss rate estimates that are usually quoted for this object, the wind is
optically thick to the continuum at some and possibly all wavelengths.
Consequently the observed variations in apparent spectral type and inferred
temperature are changes in the wind and do not necessarily mean that the
underlying stellar radius and interior structure are evolving on such a short
timescale. To explain the evidence for simultaneous outflow and infall of
material near the star, we propose a `rain' model in which blobs of gas
condense in regions of lowered opacity outside the dense wind. With the
apparent warming of its wind, the recent appearance of strong emission, and a
decline in the mass loss rate, IRC +10420 may be about to shed its opaque wind,
cross the `yellow void', and emerge as a hotter star.Comment: To appear in the Astronomical Journal, August 200
HI and CO in the circumstellar environment of the oxygen-rich AGB star RX Lep
Circumstellar shells around AGB stars are built over long periods of time
that may reach several million years. They may therefore be extended over large
sizes (~1 pc, possibly more), and different complementary tracers are needed to
describe their global properties. In the present work, we combined 21-cm HI and
CO rotational line data obtained on an oxygen-rich semi-regular variable, RX
Lep, to describe the global properties of its circumstellar environment. With
the SEST, we detected the CO(2-1) rotational line from RX Lep. The line profile
is parabolic and implies an expansion velocity of ~4.2 km/s and a mass-loss
rate ~1.7 10^-7 Msun/yr (d = 137 pc). The HI line at 21 cm was detected with
the Nancay Radiotelescope on the star position and at several offset positions.
The linear shell size is relatively small, ~0.1 pc, but we detect a trail
extending southward to ~0.5 pc. The line profiles are approximately Gaussian
with an FWHM ~3.8 km/s and interpreted with a model developed for the detached
shell around the carbon-rich AGB star Y CVn. Our HI spectra are well-reproduced
by assuming a constant outflow (Mloss = 1.65 10^-7 Msun/yr) of ~4 10^4 years
duration, which has been slowed down by the external medium. The spatial offset
of the HI source is consistent with the northward direction of the proper
motion, lending support to the presence of a trail resulting from the motion of
the source through the ISM, as already suggested for Mira, RS Cnc, and other
sources detected in HI. The source was also observed in SiO (3 mm) and OH (18
cm), but not detected. The properties of the external parts of circumstellar
shells around AGB stars should be dominated by the interaction between stellar
outflows and external matter for oxygen-rich, as well as for carbon-rich,
sources, and the 21-cm HI line provides a very useful tracer of these regions.Comment: 15 pages, 9 figures, accepted for publication in A&
- …