Clues to NaCN formation Quintana-Lacaci, G.; Cernicharo, J.; Velilla Prieto, L. ...
Astronomy & astrophysics,
11/2017, Letnik:
607
Journal Article
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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 CH3CN, current photochemical models fail to reproduce this CN reservoir. We also found that the abundance peak of NaCN appears at a radius of 3 × 1015 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.
We report on the detection of hot CO
in the O-rich AGB star R Leo based on high spectral resolution observations in the range 12.8 - 14.3
m carried out with the Echelon-cross-Echelle Spectrograph ...(EXES) mounted on the Stratospheric Observatory for Infrared Astronomy (SOFIA). We have found ≃ 240 CO
emission lines in several vibrational bands. These detections were possible thanks to a favorable Doppler shift that allowed us to avoid contamination by telluric CO
features. The highest excitation lines involve levels at an energy of ≃ 7000 K. The detected lines are narrow (average deconvolved width ≃ 2.5 km s
) and weak (usually ≲ 10% the continuum). A ro-vibrational diagram shows that there are three different populations, warm, hot, and very hot, with rotational temperatures of ≃ 550, 1150, and 1600 K, respectively. From this diagram, we derive a lower limit for the column density of ≃ 2.2 × 10
cm
. Further calculations based on a model of the R Leo envelope suggest that the total column density can be as large as 7 × 10
cm
and the abundance with respect to H
- 2.5 × 10
. The detected lines are probably formed due to de-excitation of CO
molecules from high energy vibrational states, which are essentially populated by the strong R Leo continuum at 2.7 and 4.2
m.
We present the detection of C4H2 for first time in the envelope of the C-rich AGB star IRC+10216, based on high spectral resolution mid-infrared observations carried out with the Texas ...Echelon-cross-Echelle Spectrograph mounted on the Infrared Telescope Facility. The obtained spectrum contains 24 narrow absorption features above the detection limit, identified as lines of the ro-vibrational C4H2 band . The analysis of these lines through a ro-vibrational diagram indicates that the column density of C4H2 is (2.4 1.5) × 1016 cm−2. Diacetylene is distributed in two excitation populations accounting for 20% and 80% of the total column density and with rotational temperatures of 47 7 and 420 120 K, respectively. This two-folded rotational temperature suggests that the absorbing gas is located beyond 0 4 20 R from the star, with a noticeable cold contribution outwards from 10″ 500 R . This outer shell matches up with the place where cyanoacetylenes and carbon chains are known to form due to the action of the Galactic dissociating radiation field on the neutral gas coming from the inner layers of the envelope.
High spectral resolution mid-IR observations of ethylene ( ) toward the AGB star IRC+10216 were obtained using the Texas Echelon Cross Echelle Spectrograph (TEXES) at the NASA Infrared Telescope ...Facility (IRTF). 80 ro-vibrational lines from the 10.5 m vibrational mode with J 30 were detected in absorption. The observed lines are divided into two groups with rotational temperatures of 105 and 400 K (warm and hot lines). The warm lines peak at −14 km s−1 with respect to the systemic velocity, suggesting that they are mostly formed outwards from . The hot lines are centered at −10 km s−1 indicating that they come from a shell between 10 and . 35% of the observed lines are unblended and can be fitted with a code developed to model the emission of a spherically symmetric circumstellar envelope. The analysis of several scenarios reveals that the abundance relative to H2 in the range 5−20R is on average and it could be as high as 1.1 × 10−7. Beyond , it is 8.2 × 10−8. The total column density is (6.5 3.0) × 1015 cm−2. is found to be rotationally under local thermodynamical equilibrium (LTE) and vibrationally out of LTE. One of the scenarios that best reproduce the observations suggests that up to 25% of the molecules at could condense onto dust grains. This possible depletion would not significantly influence the gas acceleration although it could play a role in the surface chemistry on the dust grains.
This work presents a detailed analysis of the laboratory and astrophysical spectral data available for 28SiC2, 29SiC2, 30SiC2, Si13CC, and Si2C. New data on the rotational lines of these species ...between 70 and 350 GHz have been obtained with high spectral resolution (195 kHz) with the IRAM 30 m telescope in the direction of the circumstellar envelope IRC +10216. Frequency measurements can reach an accuracy of 50 kHz for features observed with a good signal to noise ratio. From the observed astrophysical lines and the available laboratory data new rotational and centrifugal distortion constants have been derived for all the isotopologues of SiC2, allowing us to predict their spectrum with an estimated accuracy better than 50 kHz below 500 GHz and around 50–100 kHz for the strong lines above 500 GHz. Improved rotational and centrifugal distortion constants have also been obtained for disilicon carbide, Si2C. This work shows that observations of IRC +10216 taken with the IRAM 30 m telescope, with a spectral resolution of 195 kHz, can be used for any molecular species detected in this source to derive, or improve, its rotational constants. Hence, IRC +10216 in addition to be one the richest sources in molecular species in the sky, can also be used as a spectroscopy laboratory in the millimetre and submillimetre domains.
We present high spectral resolution mid-IR observations of SiS towards the C-rich AGB star IRC+10216 carried out with the Texas Echelon-cross-Echelle Spectrograph mounted on the NASA Infrared ...Telescope Facility. We have identified 204 ro-vibrational lines of 28Si32S, 26 of 29Si32S, 20 of 28Si34S, and 15 of 30Si32S in the frequency range 720–790 cm−1. These lines belong to bands v = 1–0, 2–1, 3–2, 4–3, and 5–4, and involve rotational levels with J
low ≲ 90. About 30 per cent of these lines are unblended or weakly blended and can be partially or entirely fitted with a code developed to model the mid-IR emission of a spherically symmetric circumstellar envelope composed of expanding gas and dust. The observed lines trace the envelope at distances to the star ≲35R
⋆ (≃0.7 arcsec). The fits are compatible with an expansion velocity of 1 + 2.5(r/R
⋆ − 1) km s−1 between 1 and 5R
⋆, 11 km s−1 between 5 and 20R
⋆, and 14.5 km s−1 outwards. The derived abundance profile of 28Si32S with respect to H2 is 4.9 × 10−6 between the stellar photosphere and 5R
⋆, decreasing linearly down to 1.6 × 10−6 at 20R
⋆ and to 1.3 × 10−6 at 50R
⋆. 28Si32S seems to be rotationally under local thermodynamic equilibrium (LTE) in the region of the envelope probed with our observations and vibrationally out of LTE in most of it. There is a red-shifted emission excess in the 28Si32S lines of band v = 1−0 that cannot be found in the lines of bands v = 2−1, 3–2, 4–3, and 5–4. This excess could be explained by an enhancement of the vibrational temperature around 20R
⋆ behind the star. The derived isotopic ratios 28Si/29Si, and 32S/34S are 17 and 14, compatible with previous estimates.
The nucleosynthesis production of fluorine (F) is still a matter of debate. Asymptotic giant branch (AGB) stars are one of the main candidates for F production. However, their contribution to the ...total F budget is not fully known due to the lack of observations. In this paper, we report the detection of aluminium monofluoride (AlF) line emission, one of the two main carriers of F in the gas-phase in the outflow of evolved stars, towards five nearby oxygen-rich (M-type) AGB stars. We studied the Atacama large millimetre/sub-millimetre array (ALMA) observations of AlF (
v
= 0,
J
= 4—3, 9–8, 10–9, and 15–14) and (
v
= 1,
J
= 7–6) line emission towards o Ceti, and (
v
= 0,
J
= 7–6 and 15–14) lines towards R Leo. We also report a tentative detection of AlF (
v
= 0,
J
= 7–6) line in IK Tau, (
v
= 0,
J
= 15–14) line towards R Dor, and (
v
= 0,
J
= 7–6 and
J
= 15–14) lines in W Hya. From spatially resolved observations, we estimated the AlF emitting region with a radius ~11
R
⋆
for
o
Ceti and ~9
R
⋆
for R Leo. From population diagram analysis, we report the AlF column densities of ~5.8 × 10
15
cm
−2
and ~3 × 10
15
cm
−2
for o Ceti and R Leo, respectively, within these regions. For
o
Ceti, we used the C
18
O (
v
= 0,
J
= 3–2) observations to estimate the H
2
column density of the emitting region. We found a fractional abundance of
f
AIF/H2
~ (2.5 ± 1.7) × 10
−8
. This gives a lower limit on the F budget in
o
Ceti and is compatible with the solar F budget
f
F/H2
= (5 ± 2) × 10
−8
. For R Leo, a fractional abundance
f
AIF/H2
= (1.2 ± 0.5) × 10
−8
is estimated. For other sources, we cannot precisely determine the emitting region based on the available data. Assuming an emitting region with a radius of ~11
R
⋆
and the rotational temperatures derived for
o
Ceti and R Leo, we crudely approximated the AlF column density to be ~(1.2−1.5) × 10
15
cm
−2
in W Hya, ~(2.5−3.0) × 10
14
cm
−2
in R Dor, and ~(0.6−1.0) × 10
16
cm
−2
in IK Tau. These result in fractional abundances within a range of
f
AIF/H2
~ (0.1 − 4) × 10
−8
in W Hya, R Dor, and IK Tau.
Low-mass evolved stars are major contributors to interstellar medium enrichment as a consequence of the intense mass-loss process these stars experience at the end of their lives. The study of the ...gas in the envelopes surrounding asymptotic giant branch (AGB) stars through observations in the millimetre wavelength range provides information about the history and nature of these molecular factories. Here we present ALMA observations at subarsecond resolution, complemented with IRAM-30 m data, of several lines of SiO, SiS, and CS towards the best-studied AGB circumstellar envelope, IRC + 10°216. We aim to characterise their spatial distribution and determine their fractional abundances mainly through radiative transfer and chemical modelling. The three species display extended emission with several enhanced emission shells. CS displays the most extended distribution reaching distances up to approximately 20′′. SiS and SiO emission have similar sizes of approximately 11′′, but SiS emission is slightly more compact. We have estimated fractional abundances relative to H2, which on average are equal to f(SiO) ~10−7, f(SiS) ~10−6, and f(CS) ~10−6 up to the photo-dissociation region. The observations and analysis presented here show evidence that the circumstellar material displays clear deviations from an homogeneous spherical wind, with clumps and low density shells that may allow UV photons from the interstellar medium (ISM) to penetrate deep into the envelope, shifting the photo-dissociation radius inwards. Our chemical model predicts photo-dissociation radii compatible with those derived from the observations, although it is unable to predict abundance variations from the starting radius of the calculations (~10 R*), which may reflect the simplicity of the model. We conclude that the spatial distribution of the gas proves the episodic and variable nature of the mass loss mechanism of IRC + 10°216, on timescales of hundreds of years.
Context. The variability in IRC+10216, the envelope of the asymptotic giant branch (AGB) star CW Leo, has attracted increasing attention in recent years. Studying the details of this variability in ...the molecular emission required a systematic observation program. Aims. We aim to reveal and characterize the periodical variability of the rotational lines from several molecules and radicals in IRC+10216, and to compare it with previously reported IR variability. Methods. We carried out systematic monitoring within the ~80–116 GHz frequency range with the IRAM 30 m telescope. Results. We report on the periodical variability in IRC+10216 of several rotational lines from the following molecules and radicals: HC3N, HC5N, CCH, C4H, C5H, and CN. The analysis of the variable molecular lines provides periods that are consistent with previously reported IR variability, and interesting phase lags are revealed that point toward radiative transfer and pumping, rather than chemical effects. Conclusions. This study indicates that observations of several lines of a given molecule have to be performed simultaneously or at least at the same phase in order to avoid erroneous interpretation of the data. In particular, merging ALMA data from different epochs may prove to be difficult, as shown by the example of the variability we studied here. Moreover, radiative transfer codes have to incorporate the effect of population variability in the rotational levels in CW Leo.
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