16 research outputs found
Mapping the Linearly Polarized Spectral Line Emission around the Evolved Star IRC+10216
We present spectro-polarimetric observations of several molecular lines
obtained with the Submillimeter Array (SMA) toward the carbon rich AGB star
IRC+10216. We have detected and mapped the linear polarization of the CO 3-2,
SiS 19-18 and CS 7-6 lines. The polarization arises at a distance of ~450 AU
from the star and is blueshifted with respect the Stokes I. The SiS 19-18
polarization pattern appears to be consistent with a locally radial magnetic
field configuration. However, the CO 3-2 and CS 7-6 line polarization suggests
an overall complex magnetic field morphology within the envelope. This work
demonstrates the feasibility of using spectro-polarimetric observations to
carry out tomographic imaging of the magnetic field in circumstellar envelopes.Comment: Accepted for publication in The Astrophysical Journal Letters. 10
pages, 3 figure
High-J v=0 SiS Maser Emission in IRC+10216: A New Case of Infrared Overlaps
We report on the first detection of maser emission in the J=11-10, J=14-13
and J=15-14 transitions of the v=0 vibrational state of SiS toward the C-rich
star IRC+10216. These masers seem to be produced in the very inhomogeneous
region between the star and the inner dust formation zone, placed at 5-7 R*,
with expansion velocities below 10 km/s. We interpret the pumping mechanism as
due to overlaps between v=1-0 ro-vibrational lines of SiS and mid-IR lines of
C2H2, HCN and their 13C isotopologues. The large number of overlaps found
suggests the existence of strong masers for high-J v=0 and v=1 SiS transitions,
located in the submillimeter range. In addition, it could be possible to find
several rotational lines of the SiS isotopologues displaying maser emission.Comment: 4 pages, 1 figure, published in the ApJ Letter
Understanding the chemical complexity in Circumstellar Envelopes of C-Rich AGB stars: The case of IRC +10216
Abstract The circumstellar envelopes of carbon-rich AGB stars show a chemical complexity that is exemplified by the prototypical object IRC +10216, in which about 60 different molecules have been detected to date. Most of these species are carbon chains of the type C n H, C n H 2 , C n N, HC n N. We present the detection of new species (CH 2 CHCN, CH 2 CN, H 2 CS, CH 3 CCH and C 3 O) achieved thanks to the systematic observation of the full 3 mm window with the IRAM 30m telescope plus some ARO 12m observations. All these species, known to exist in the interstellar medium, are detected for the first time in a circumstellar envelope around an AGB star. These five molecules are most likely formed in the outer expanding envelope rather than in the stellar photosphere. A pure gas phase chemical model of the circumstellar envelope is reasonably successful in explaining the derived abundances, and additionally allows to elucidate the chemical formation routes and to predict the spatial distribution of the detected species
The abundance of SiS in circumstellar envelopes around AGB stars
New SiS multi-transition (sub-)millimetre line observations of a sample of
AGB stars with varying photospheric C/O-ratios and mass-loss rates are
presented. A combination of low- and high-energy lines are important in
constraining the circumstellar distribution of SiS molecules. A detailed
radiative transfer modelling of the observed SiS line emission is performed,
including the effect of thermal dust grains in the excitation analysis. We find
that the circumstellar fractional abundance of SiS in these environments has a
strong dependence on the photospheric C/O-ratio as expected from chemical
models. The carbon stars (C/O>1) have a mean fractional abundance of 3.1E-6,
about an order of magnitude higher than found for the M-type AGB stars (C/O<1)
where the mean value is 2.7E-7. These numbers are in reasonable agreement with
photospheric LTE chemical models. SiS appears to behave similar to SiO in terms
of photodissociation in the outer part of the circumstellar envelope. In
contrast to previous results for the related molecule SiO, there is no strong
correlation of the fractional abundance with density in the CSE, as would be
the case if freeze-out onto dust grains were important. However, possible
time-variability of the line emission in the lower J transitions and the
sensitivity of the line emission to abundance gradients in the inner part of
the CSE may mask a correlation with the density of the wind. There are
indications that the SiS fractional abundance could be significantly higher
closer to the star which, at least in the case of M-type AGB stars, would
require non-equilibrium chemical processes.Comment: Accepted for publication in A&A (14 pages, 7 figures
Submillimeter narrow emission lines from the inner envelope of IRC+10216
A spectral-line survey of IRC+10216 in the 345 GHz band has been undertaken
with the Submillimeter Array. Although not yet completed, it has already
yielded a fairly large sample of narrow molecular emission lines with
line-widths indicating expansion velocities of ~4 km/s, less than 3 times the
well-known value of the terminal expansion velocity (14.5 km/s) of the outer
envelope. Five of these narrow lines have now been identified as rotational
transitions in vibrationally excited states of previously detected molecules:
the v=1, J=17--16 and J=19--18 lines of Si34S and 29SiS and the v=2, J=7--6
line of CS. Maps of these lines show that the emission is confined to a region
within ~60 AU of the star, indicating that the narrow-line emission is probing
the region of dust-formation where the stellar wind is still being accelerated.Comment: 5 pages, 5 figures, Accepted for publication in Ap
Understanding the Chemical Complexity in Circumstellar Envelopes of C-rich AGB Stars: the Case of IRC +10216
The circumstellar envelopes of carbon-rich AGB stars show a chemical
complexity that is exemplified by the prototypical object IRC +10216, in which
about 60 different molecules have been detected to date. Most of these species
are carbon chains of the type CnH, CnH2, CnN, HCnN. We present the detection of
new species (CH2CHCN, CH2CN, H2CS, CH3CCH and C3O) achieved thanks to the
systematic observation of the full 3 mm window with the IRAM 30m telescope plus
some ARO 12m observations. All these species, known to exist in the
interstellar medium, are detected for the first time in a circumstellar
envelope around an AGB star. These five molecules are most likely formed in the
outer expanding envelope rather than in the stellar photosphere. A pure gas
phase chemical model of the circumstellar envelope is reasonably successful in
explaining the derived abundances, and additionally allows to elucidate the
chemical formation routes and to predict the spatial distribution of the
detected species.Comment: 4 pages, 4 figures; to appear in Astrophysics and Space Science,
special issue of "Science with ALMA: a new era for Astrophysics" conference,
November, 13-17 2006, ed. R. Bachille
AstrofĂsica molecular: caracterizaciĂłn de envolturas circunestelares y procesos colisionales en el laboratorio
Esta investigaciĂłn estĂĄ basada en la utilizaciĂłn de la ecuaciĂłn maestra para el estudio de un gas de N2 natural en expansiĂłn con el fin de imponer restricciones experimentales
a un conjunto de tasas de transferencia calculadas numĂ©ricamente. La informaciĂłn sobre el gas obtenida gracias a la tĂ©cnica no intrusiva de la espectroscopĂa Raman lineal ha permitido determinar las condiciones fĂsicas (densidad absoluta y temperatura rotacional) del N2 en una serie de puntos del eje de 4 expansiones supersĂłnicas generadas a presiones distintas. Utilizando estas dos magnitudes se han calculado la temperatura traslacional del gas, su velocidad de expansiĂłn y la derivada temporal de las poblaciones del N2 a lo largo del eje de la expansiĂłn con precisiones lo suficientemente buenas como para imponer las
restricciones a las tasas de transferencia