433 research outputs found
Dense molecular clumps in the envelope of the yellow hypergiant IRC+10420
The circumstellar envelope of the hypergiant star IRC+10420 has been traced
as far out in SiO J=2-1 as in CO J = 1-0 and CO J = 2-1, in dramatic contrast
with the centrally condensed (thermal) SiO- but extended CO-emitting envelopes
of giant and supergiant stars. Here, we present an observation of the
circumstellar envelope in SiO J=1-0 that, when combined with the previous
observation in {\sioii}, provide more stringent constraints on the density of
the SiO-emitting gas than hitherto possible. The emission in SiO peaks at a
radius of 2\arcsec\ whereas that in SiO J=2-1 emission peaks at a smaller
radius of 1\arcsec, giving rise to their ring-like appearances. The ratio
in brightness temperature between SiO J=1-0 and SiO J=2-1 decreases from a
value well above unity at the innermost measurable radius to about unity at
radius of 2\arcsec, beyond which this ratio remains approximately
constant. Dividing the envelope into three zones as in models for the CO J =
1-0 and CO J = 2-1 emission, we show that the density of the SiO-emitting gas
is comparable with that of the CO-emitting gas in the inner zone, but at least
an order of magnitude higher by comparison in both the middle and outer zones.
The SiO-emitting gas therefore originates from dense clumps, likely associated
with the dust clumps seen in scattered optical light, surrounded by more
diffuse CO-emitting interclump gas. We suggest that SiO molecules are released
from dust grains due to shock interactions between the dense SiO-emitting
clumps and the diffuse CO-emitting interclump gas.Comment: Accepted for publication in Ap
Molecular shells in IRC+10216: Evidence for non-isotropic and episodic mass loss enhancement
We report high angular-resolution VLA observations of cyanopolyyne molecules
HCN and HCN from the carbon rich circumstellar envelope of IRC+10216.
The observed low-lying rotational transitions trace a much more extended
emitting region than seen in previous observations at higher frequency
transitions. We resolve the hollow quasi-spherical distribution of the
molecular emissions into a number of clumpy shells. These molecular shells
coincide spatially with dust arcs seen in deep optical images of the IRC+10216
envelope, allowing us to study for the first time the kinematics of these
features. We find that the molecular and dust shells represent the same density
enhancements in the envelope separated in time by 120 to 360 yrs.
From the angular size and velocity spread of the shells, we estimate that each
shell typically covers about 10% of the stellar surface at the time of
ejection. The distribution of the shells seems to be random in space. The good
spatial correspondance between HCN and HCN emissions is in qualitative
agreement with a recent chemical model that takes into account the presence of
density-enhanced shells. The broad spatial distribution of the cyanopolyyne
molecules, however, would necessitate further study on their formation.Comment: 16 pages, 5 figures, accepted for publication in Ap
The peculiar molecular envelope around the post-AGB star IRAS 08544--4431
Circumbinary disks have been hypothesized to exist around a number of binary
post-AGB stars. Although most of the circumbinary disks have been inferred
through the near IR excess, a few of them are strong emitters of molecular
emission. Here we present high angular resolution observations of the emission
of CO and its isotopomer CO J=2--1 line from the circumstellar
envelope around the binary post-AGB star IRAS 085444431, which is one of the
most prominent members of this class of objects. We find that the envelope is
resolved in our observations and two separate components can be identified: (a)
a central extended and strong component with very narrow linewidth between 2 -
6 \kms; (b) a weak bipolar outflow with expansion velocity up to 8 \kms. The
central compact component possesses low and variable CO/CO J=2--1
line ratio, indicating optically thick emission of the main isotope. We
estimate a molecular gas mass of 0.0047 M for this component based on
the optically thinner CO J=2--1 line. We discuss the relation of the
molecular envelope and the circumbinary disk inferred from near IR excess and
compare with other known cases where the distribution of molecular gas has been
imaged at high angular resolution.Comment: 14 pages, 4 figures. Accepted for publication in Astrophysical
Journa
The shaping effect of collimated fast outflows in the Egg nebula
We present high angular resolution observations of the HCN J=5--4 line
from the Egg nebula, which is the archetype of protoplanetary nebulae. We find
that the HCN emission in the approaching and receding portion of the
envelope traces a clumpy hollow shell, similar to that seen in normal carbon
rich envelopes. Near the systemic velocity, the hollow shell is fragmented into
several large blobs or arcs with missing portions correspond spatially to
locations of previously reported high--velocity outlows in the Egg nebula. This
provides direct evidence for the disruption of the slowly--expanding envelope
ejected during the AGB phase by the collimated fast outflows initiated during
the transition to the protoplanetary nebula phase. We also find that the
intersection of fast molecular outflows previously suggested as the location of
the central post-AGB star is significantly offset from the center of the hollow
shell. From modelling the HCN distribution we could reproduce qualitatively
the spatial kinematics of the HCN J=5--4 emission using a HCN shell
with two pairs of cavities cleared by the collimated high velocity outflows
along the polar direction and in the equatorial plane. We infer a relatively
high abundance of HCN/H 3x10 for an estimated mass--loss
rate of 3x10 M yr in the HCN shell. The high
abundance of HCN and the presence of some weaker J=5--4 emission in the
vicinity of the central post-AGB star suggest an unusually efficient formation
of this molecule in the Egg nebula.Comment: 22 pages, 6 figures, submitted to the Astrophysical Journa
On the theory of astronomical maser. II. Polarization of maser radiation
In this paper we investigate the polarization property of the radiation
amplified by astronomical masers in the presence of a strong magnetic field.
Our model explicitly takes into account the broadband nature of the radiation
field and the interaction of the radiation with the maser transition J=1--0.
The amplification of different realisations of the background continuum
radition by the maser is directly simulated and the Stokes parameters of the
radiation field are then obtained by averaging over the ensemble of emerging
maser radiation. For isotropic pumping and partially saturated masers we find
that the maser radiation is linearly polarized in two representative cases
where the magnetic field {\bf B} makes an angle =30 and
=90 to the maser axis. The linear polarization for maser radiation
obtained in our simulations for both cases are in agreement with the results of
the standard model. Furthermore, no instability during amplification is seen in
our simulations. Therefore, we conclude that there is no problem with the
previous numerical investigations of maser polarization in the unsaturated and
partially saturated regime.Comment: 17 pages, 7 figures, to appear on MNRA
On the theory of astronomical maser. I. Statistics of maser radiation
In this paper we re-analyse the amplification process of broadband continuum
radiation by astronomical masers in one-dimensional case. The basic equations
appropriate for the scalar maser and the random nature of the maser radiation
field are derived from basic physical principles. Comparision with the standard
radiation transfer equation allows us to examine the underlying assumptions
involved in the current theory of astronomical masers. Simulations are carried
out to follow the amplification of different realisations of the broadband
background radiation by the maser. The observable quantities such as intensity,
spectral line profile are obtained by averaging over an ensemble of the
emerging radiation corresponding to the amplified background radiation field.
Our simulations show that the fluctuations of the radiation field inside the
astronomical maser deviates significantly from Gaussian statistics even when
the maser is only partially saturated. Coupling between different frequency
modes and the population pulsing are shown to have increasing importance in the
transport of maser radiation as the maser approaches saturation. Our results
suggest that the standard formulation of radiation transfer provides a
satisfactory description of the intensity and the line narrowing effect in the
unsaturated and partially saturated masers within the framework of
one-dimensional model. Howerver, the application of the same formulation to the
strong saturation regime should be considered with caution.Comment: 16 pages, 4 figures, to appear on MNRA
Herschel/HIFI deepens the circumstellar NH3 enigma
Circumstellar envelopes (CSEs) of a variety of evolved stars have been found
to contain ammonia (NH3) in amounts that exceed predictions from conventional
chemical models by many orders of magnitude. The observations reported here
were performed in order to better constrain the NH3 abundance in the CSEs of
four, quite diverse, oxygen-rich stars using the NH3 ortho J_K = 1_0 - 0_0
ground-state line. We used the Heterodyne Instrument for the Far Infrared
aboard Herschel to observe the NH3 J_K = 1_0 - 0_0 transition near 572.5 GHz,
simultaneously with the ortho-H2O J_Ka,Kc = 1_1,0 -1_0,1 transition, toward VY
CMa, OH 26.5+0.6, IRC+10420, and IK Tau. We conducted non-LTE radiative
transfer modeling with the goal to derive the NH3 abundance in these objects'
CSEs. For the latter two stars, Very Large Array imaging of NH3
radio-wavelength inversion lines were used to provide further constraints,
particularly on the spatial extent of the NH3-emitting regions. Results. We
find remarkably strong NH3 emission in all of our objects with the NH3 line
intensities rivaling those obtained for the ground state H2O line. The NH3
abundances relative to H2 are very high and range from 2 x 10-7 to 3 x 10-6 for
the objects we have studied. Our observations confirm and even deepen the
circumstellar NH3 enigma. While our radiative transfer modeling does not yield
satisfactory fits to the observed line profiles, it leads to abundance
estimates that confirm the very high values found in earlier studies. New ways
to tackle this mystery will include further Herschel observations of more NH3
lines and imaging with the Expanded Very Large Array.Comment: 4+2 page
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