15 research outputs found
Study of CS, SiO, and SiS abundances in carbon star envelopes: Assessing their role as gas-phase precursors of dust
Aim: We aim to determine the abundances of CS, SiO, and SiS in a large sample
of carbon star envelopes covering a wide range of mass loss rates to
investigate the potential role that these molecules could play in the formation
of dust in the surroundings of the central AGB star. Methods: We surveyed a
sample of 25 carbon-rich AGB stars in the 2 mm band, using the IRAM
30 m telescope. We performed excitation and radiative transfer calculations
based on the LVG method to model the observed lines of the molecules and to
derive their fractional abundances in the observed CSEs. Results: We detected
CS in all 25 CSEs, SiO in 24 of them, and SiS in 17 sources. We found that CS
and SiS have similar abundances in carbon star envelopes, while SiO is present
with a lower abundance. We also found a strong correlation in which the denser
the envelope, the less abundant are CS and SiO. The trend is however only
tentatively seen for SiS in the range of high mass loss rates. Furthermore, we
found a relation in which the integrated flux of the MgS dust feature at 30 um
increases as the fractional abundance of CS decreases. Conclusions: The decline
in the fractional abundance of CS with increasing density could be due to
gas-phase chemistry in the inner envelope or to adsorption onto dust grains.
The latter possibility is favored by a correlation between the CS fractional
abundance and the 30 um feature, which suggests that CS is efficiently
incorporated onto MgS dust around C-rich AGB stars. In the case of SiO, the
observed abundance depletion with increasing density is most likely caused by
an efficient incorporation onto dust grains. We conclude that CS, SiO (very
likely), and SiS (tentatively) are good candidates to act as gas-phase
precursors of dust in C-rich AGB envelopes.Comment: Accepted for publication in A&A, 19 pages, 11 figures, adbridged
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The abundance of S- and Si-bearing molecules in O-rich circumstellar envelopes of AGB stars
Aims: We aim to determine the abundances of SiO, CS, SiS, SO, and SO in a
large sample of oxygen-rich AGB envelopes covering a wide range of mass loss
rates to investigate the potential role that these molecules could play in the
formation of dust in these environments.
Methods:We surveyed a sample of 30 oxygen-rich AGB stars in the 2
mm band using the IRAM 30m telescope. We performed excitation and radiative
transfer calculations based on the LVG method to model the observed lines of
the molecules and to derive their fractional abundances in the observed
envelopes.
Results:We detected SiO in all 30 targeted envelopes, as well as CS, SiS, SO,
and SO in 18, 13, 26, and 19 sources, respectively. Remarkably, SiS is not
detected in any envelope with a mass loss rate below M
yr, whereas it is detected in all envelopes with mass loss rates above
that threshold. From a comparison with a previous, similar study on C-rich
sources, it becomes evident that the fractional abundances of CS and SiS show a
marked differentiation between C-rich and O-rich sources, being two orders of
magnitude and one order of magnitude more abundant in C-rich sources,
respectively, while the fractional abundance of SiO turns out to be insensitive
to the C/O ratio. The abundance of SiO in O-rich envelopes behaves similarly to
C-rich sources, that is, the denser the envelope the lower its abundance. A
similar trend, albeit less clear than for SiO, is observed for SO in O-rich
sources.
Conclusions: The marked dependence of CS and SiS abundances on the C/O ratio
indicates that these two molecules form more efficiently in C- than O-rich
envelopes. The decline in the abundance of SiO with increasing envelope density
and the tentative one for SO indicate that SiO and possibly SO act as gas-phase
precursors of dust in circumstellar envelopes around O-rich AGB stars.Comment: 22 pages, 3 figures, Accepted for publications in Astronomy &
Astrophysic
The Abundance of SiC2 in Carbon Star Envelopes: Evidence that SiC2 is a gas-phase precursor of SiC dust
Silicon carbide dust is ubiquitous in circumstellar envelopes around C-rich
AGB stars. However, the main gas-phase precursors leading to the formation of
SiC dust have not yet been identified. The most obvious candidates among the
molecules containing an Si--C bond detected in C-rich AGB stars are SiC2, SiC,
and Si2C. We aim to study how widespread and abundant SiC2, SiC, and Si2C are
in envelopes around C-rich AGB stars and whether or not these species play an
active role as gas-phase precursors of silicon carbide dust in the ejecta of
carbon stars. We carried out sensitive observations with the IRAM 30m telescope
of a sample of 25 C-rich AGB stars to search for emission lines of SiC2, SiC,
and Si2C in the 2 mm band. We performed non-LTE excitation and radiative
transfer calculations based on the LVG method to model the observed lines of
SiC2 and to derive SiC2 fractional abundances in the observed envelopes. We
detect SiC2 in most of the sources, SiC in about half of them, and do not
detect Si2C in any source, at the exception of IRC +10216. Most of these
detections are reported for the first time in this work. We find a positive
correlation between the SiC and SiC2 line emission, which suggests that both
species are chemically linked, the SiC radical probably being the
photodissociation product of SiC2 in the external layer of the envelope. We
find a clear trend in which the denser the envelope, the less abundant SiC2 is.
The observed trend is interpreted as an evidence of efficient incorporation of
SiC2 onto dust grains, a process which is favored at high densities owing to
the higher rate at which collisions between particles take place. The observed
behavior of a decline in the SiC2 abundance with increasing density strongly
suggests that SiC2 is an important gas-phase precursor of SiC dust in envelopes
around carbon stars.Comment: Published in A&A. 16 pages and 10 figure
Clues to NaCN formation
ALMA is providing us essential information on where certain molecules form.
Observing where these molecules emission arises from, the physical conditions
of the gas, and how this relates with the presence of other species allows us
to understand the formation of many species, and to significantly improve our
knowledge of the chemistry that occurs in the space. We studied the molecular
distribution of NaCN around IRC +10216, a molecule detected previously, but
whose origin is not clear. High angular resolution maps allow us to model the
abundance distribution of this molecule and check suggested formation paths. We
modeled the emission of NaCN assuming local thermal equilibrium (LTE)
conditions. These profiles were fitted to azimuthal averaged intensity profiles
to obtain an abundance distribution of NaCN. We found that the presence of NaCN
seems compatible with the presence of CN, probably as a result of the
photodissociation of HCN, in the inner layers of the ejecta of IRC +10216.
However, similar as for CH 3 CN, current photochemical models fail to reproduce
this CN reservoir. We also found that the abundance peak of NaCN appears at a
radius of 3 x 10 15 cm, approximately where the abundance of NaCl, suggested to
be the parent species, starts to decay. However, the abundance ratio shows that
the NaCl abundance is lower than that obtained for NaCN. We expect that the LTE
assumption might result in NaCN abundances higher than the real ones. Updated
photochemical models, collisional rates, and reaction rates are essential to
determine the possible paths of the NaCN formation.Comment: 7 pages, 10 figures. Accepted for publication in A&A letter
The first detection of SiC in the interstellar medium
We report the first detection of SiC in the interstellar medium. The
molecule was identified through six rotational transitions toward
G\,+0.6930.027, a molecular cloud located in the Galactic center. The
detection is based on a line survey carried out with the GBT, the Yebes 40m,
and the IRAM 30m telescopes covering a range of frequencies from 12 to 276 GHz.
We fit the observed spectra assuming local thermodynamic equilibrium and derive
a column density of ( cm, which gives a
fractional abundance of with respect to H, and an
excitation temperature of K. We conclude that SiC can be formed
in the shocked gas by a reaction between the sputtered atomic silicon and
CH, or it can be released directly from the dust grains due to
disruption. We also search for other Si-bearing molecules and detect eight
rotational transitions of SiS and four transitions of SiO. The derived
fractional abundances are and ,
respectively. All Si-bearing species toward G\,+0.6930.027 show fractional
abundances well below what is typically found in late-type evolved stars.Comment: 8 pages, 5 figure
Multi-frequency high spectral resolution observations of HCN toward the circumstellar envelope of Y CVn
High spectral resolution observations toward the low mass-loss rate C-rich,
J-type AGB star Y CVn have been carried out at 7.5, 13.1 and 14.0 um with
SOFIA/EXES and IRTF/TEXES. Around 130 HCN and H13CN lines of bands v2, 2v2,
2v2-v2, 3v2-2v2, 3v2-v2, and 4v2-2v2 have been identified involving lower
levels with energies up to ~3900 K. These lines have been complemented with the
pure rotational lines J=1-0 and 3-2 of the vibrational states up to 2v2
acquired with the IRAM 30 m telescope, and with the continuum taken with ISO.
We have analyzed the data with a ro-vibrational diagram and a code which models
the absorption and emission of the circumstellar envelope of an AGB star. The
continuum is produced by the star with a small contribution from dust grains
comprising warm to hot SiC and cold amorphous carbon. The HCN abundance
distribution seems to be anisotropic. The ejected gas is accelerated up to the
terminal velocity (~8 km/s) from the photosphere to ~3R* but there is evidence
of higher velocities (>9-10 km/s) beyond this region. In the vicinity of Y CVn,
the line widths are as high as ~10 km/s, which implies a maximum turbulent
velocity of 6 km/s or the existence of other physical mechanisms probably
related to matter ejection that involve higher gas expansion velocities than
expected. HCN is rotationally and vibrationally out of LTE throughout the whole
envelope. A difference of about 1500 K in the rotational temperature at the
photosphere is needed to explain the observations at 7.5 and 13-14 um. Our
analysis finds a total HCN column density that ranges from ~2.1E+18 to 3.5E+18
cm^{-2}, an abundance with respect to H2 of 3.5E-05 to 1.3E-04, and a 12C/13C
isotopic ratio of ~2.5 throughout the whole envelope.Comment: 24 pages, 11 figures, 3 tables, accepted for publication in A&
Ionize Hard: Interstellar PO+ Detection
We report the first detection of the phosphorus monoxide ion (PO+) in the interstellar medium. Our unbiased and very sensitive spectral survey toward the G+0.693–0.027 molecular cloud covers four different rotational transitions of this molecule, two of which (J = 1–0 and J = 2–1) appear free of contamination from other species. The fit performed, assuming local thermodynamic equilibrium conditions, yields a column density of N=(6.0 \ub1 0.7)
7 1011\ua0cm−2. The resulting molecular abundance with respect to molecular hydrogen is 4.5
7 10–12. The column density of PO+ normalized by the cosmic abundance of P is larger than those of NO+ and SO+, normalized by N and S, by factors of 3.6 and 2.3, respectively. The N(PO+)/N(PO) ratio is 0.12 \ub1 0.03, more than one order of magnitude higher than that of N(SO+)/N(SO) and N(NO+)/N(NO). These results indicate that P is more efficiently ionized than N and S in the ISM. We have performed new chemical models that confirm that the PO+ abundance is strongly enhanced in shocked regions with high values of cosmic-ray ionization rates (10–15 − 10–14 s−1), as occurring in the G+0.693–0.027 molecular cloud. The shocks sputter the interstellar icy grain mantles, releasing into the gas phase most of their P content, mainly in the form of PH3, which is converted into atomic P, and then ionized efficiently by cosmic rays, forming P+. Further reactions with O2 and OH produces PO+. The cosmic-ray ionization of PO might also contribute significantly, which would explain the high N(PO+)/N(PO) ratio observed. The relatively high gas-phase abundance of PO+ with respect to other P-bearing species stresses the relevance of this species in the interstellar chemistry of P
Abundance Estimates in Carbon Star Envelopes
2 pags., 1 fig. -- Why Galaxies Care About AGB Stars: A Continuing Challenge through Cosmic Time Proceedings IAU Symposium No. 343, 2019The synthesis of dust grains mostly takes place in the circumstellar envelopes (CSEs) of asymptotic giant branch (AGB) stars. What are the precursor seeds of condensation nuclei and how do these particles evolve toward the micrometer sized grains that populate the interstellar medium? These are key questions of the NANOCOSMOS project. In this study, we carried out an observational study to constrain what the main gas-phase precursors of dust in C-rich AGB stars are
The abundance of SiC2 in carbon star envelopes
Poster presented at the Symposia Astrochemistry VII - Through the Cosmos from Galaxies to Planets, held in Puerto Varas (Chile) on march 20-24th, 2017
Clues to NaCN formation
[Context] ALMA is providing us essential information on where certain molecules form. Observing where these molecules emission arises from, the physical conditions of the gas, and how this relates with the presence of other species allows us to understand the formation of many species, and to significantly improve our knowledge of the chemistry that occurs in the space.[Aims] We studied the molecular distribution of NaCN around IRC +10216, a molecule detected previously, but whose origin is not clear. High angular resolution maps allow us to model the abundance distribution of this molecule and check suggested formation paths.[Methods] We modeled the emission of NaCN assuming local thermal equilibrium (LTE) conditions. These profiles were fitted to azimuthal averaged intensity profiles to obtain an abundance distribution of NaCN.[Results] We found that the presence of NaCN seems compatible with the presence of CN, probably as a result of the photodissociation of HCN, in the inner layers of the ejecta of IRC +10216. However, similar as for CHCN, current photochemical models fail to reproduce this CN reservoir. We also found that the abundance peak of NaCN appears at a radius of 3 × 10 cm, approximately where the abundance of NaCl, suggested to be the parent species, starts to decay. However, the abundance ratio shows that the NaCl abundance is lower than that obtained for NaCN. We expect that the LTE assumption might result in NaCN abundances higher than the real ones. Updated photochemical models, collisional rates, and reaction rates are essential to determine the possible paths of the NaCN formation.The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007–2013)/ERC Grant Agreement No. 610256 (NANOCOSMOS). We would also like to thank the Spanish MINECO for funding support from grants CSD2009-00038, AYA2012-32032 & AYA2016-75066-C2-1-P. M.A. also ackowledges funding support from the Ramón y Cajal programme of Spanish MINECO (RyC-2014-16277)