Detection of highly excited OH towards AGB stars

Aims. We characterise the gas in the extended atmospheres of the oxygen-rich asymptotic giant branch (AGB) stars W Hya and R Dor using high angular resolution ALMA observations. Methods. We report the detection and investigate the properties of high-excitation Λ-doubling line emission of hydroxyl (OH). Results. The OH lines are produced very close to the central stars and seem optically thin and with no maser effect. We analyse the molecular excitation using a population diagram and find rotational temperatures of ∼2500 K and column densities of ∼1019 cm−2 for both sources. For W Hya, we observe emission from vibrationally excited H2O arising from the same region as the OH emission. Moreover, CO v = 1, J = 3 − 2 emission also shows a brightness peak in the same region. Considering optically thin emission and the rotational temperature derived for OH, we find a CO column density ∼15 times higher than that of OH, within an area of (92 × 84) mas2 centred on the OH emission peak. These results should be considered tentative because of the simple methods employed. The observed OH line frequencies differ significantly from the predicted transition frequencies in the literature, and provide the possibility of using OH lines observed in AGB stars to improve the accuracy of the Hamiltonian used for the OH molecule. We predict stronger OH Λ-doubling lines at millimetre wavelengths than those we detected. These lines will be a good probe of shocked gas in the extended atmosphere and are possibly even suitable as probes of the magnetic field in the atmospheres of close-by AGB stars through the Zeeman effect.

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Resolving the extended stellar atmospheres of asymptotic giant branch stars at (sub)millimetre wavelengths

Astronomy and Astrophysics ◽

10.1051/0004-6361/201935329 ◽

2019 ◽

Vol 626 ◽

pp. A81

Author(s):

W. H. T. Vlemmings ◽

T. Khouri ◽

H. Olofsson

Keyword(s):

Initial Conditions ◽

Stellar Atmospheres ◽

Asymptotic Giant Branch ◽

High Angular Resolution ◽

Inhomogeneous Layer ◽

Temperature Structure ◽

Asymptotic Giant Branch Stars ◽

Agb Stars ◽

Brightness Temperatures ◽

Stellar Photosphere

Context. The initial conditions for mass loss during the asymptotic giant branch (AGB) phase are set in their extended atmospheres, where, among others, convection and pulsation driven shocks determine the physical conditions. Aims. High resolution observations of AGB stars at (sub)millimetre wavelengths can now directly determine the morphology, activity, density, and temperature close to the stellar photosphere. Methods. We used Atacama Large Millimeter/submillimeter Array (ALMA) high angular resolution observations to resolve the extended atmospheres of four of the nearest AGB stars: W Hya, Mira A, R Dor, and R Leo. We interpreted the observations using a parameterised atmosphere model. Results. We resolve all four AGB stars and determine the brightness temperature structure between 1 and 2 stellar radii. For W Hya and R Dor we confirm the existence of hotspots with brightness temperatures > 3000 to 10 000 K. All four stars show deviations from spherical symmetry. We find variations on a timescale of days to weeks, and for R Leo we directly measure an outward motion of the millimetre wavelength surface with a velocity of at least 10.6 ± 1.4 km s−1. For all objects but W Hya we find that the temperature-radius and size-frequency relations require the existence of a (likely inhomogeneous) layer of enhanced opacity. Conclusions. The ALMA observations provide a unique probe of the structure of the extended AGB atmosphere. We find highly variable structures of hotspots and likely convective cells. In the future, these observations can be directly compared to multi-dimensional chromosphere and atmosphere models that determine the temperature, density, velocity, and ionisation structure between the stellar photosphere and the dust formation region. However, our results show that for the best interpretation, both very accurate flux calibration and near-simultaneous observations are essential.

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SMA Spectral Line Survey of the Proto-Planetary Nebula CRL 618

Proceedings of the International Astronomical Union ◽

10.1017/s1743921318008104 ◽

2018 ◽

Vol 14 (S343) ◽

pp. 483-484

Author(s):

Nimesh A. Patel ◽

Carl Gottlieb ◽

Ken Young ◽

Tomasz Kaminski ◽

Michael McCarthy ◽

...

Keyword(s):

Spectral Line ◽

Planetary Nebula ◽

Line Emission ◽

Asymptotic Giant Branch ◽

60 Ghz ◽

Agb Stars ◽

Complex Molecules ◽

Compact Region ◽

Line Survey ◽

Pn Stage

AbstractCarbon-rich Asymptotic Giant Branch (AGB) stars are major sources of gas and dust in the interstellar medium. During the brief (∼1000 yr) period in the evolution from AGB to the Planetary Nebula (PN) stage, the molecular composition evolves from mainly diatomic and small polyatomic species to more complex molecules. Using the Submillimeter Array (SMA), we have carried out a spectral line survey of CRL 618, covering a frequency range of 281.9 to 359.4 GHz. More than 1000 lines were detected in the ∼60 GHz range, most of them assigned to HC3N and c-C3H2, and their isotopologues. About 200 lines are unassigned. Lines of CO, HCO+, and CS show the fast outflow wings, while the majority of line emission arises from a compact region of ∼1” diameter. We have analyzed the lines of HC3N, c-C3H2, CH3CN, and their isotopologues with rotation temperature diagrams.

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Detection of CI line emission towards the oxygen-rich AGB star omi Ceti

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833080 ◽

2018 ◽

Vol 612 ◽

pp. L11 ◽

Cited By ~ 4

Author(s):

M. Saberi ◽

W. H. T. Vlemmings ◽

E. De Beck ◽

R. Montez ◽

S. Ramstedt

Keyword(s):

Uv Radiation ◽

Line Emission ◽

Asymptotic Giant Branch ◽

High Angular Resolution ◽

Circumstellar Envelope ◽

Velocity Shift ◽

Compact Region ◽

Fractional Abundance ◽

Stellar Velocity ◽

First Time

We present the detection of neutral atomic carbon CI(3P1–3P0) line emission towards omi Cet. This is the first time that CI is detected in the envelope around an oxygen-rich M-type asymptotic giant branch (AGB) star. We also confirm the previously tentative CI detection around V Hya, a carbon-rich AGB star. As one of the main photodissociation products of parent species in the circumstellar envelope (CSE) around evolved stars, CI can be used to trace sources of ultraviolet (UV) radiation in CSEs. The observed flux density towards omi Cet can be reproduced by a shell with a peak atomic fractional abundance of 2.4 × 10−5 predicted based on a simple chemical model where CO is dissociated by the interstellar radiation field. However, the CI emission is shifted by ~4 km s−1 from the stellar velocity. Based on this velocity shift, we suggest that the detected CI emission towards omi Cet potentially arises from a compact region near its hot binary companion. The velocity shift could, therefore, be the result of the orbital velocity of the binary companion around omi Cet. In this case, the CI column density is estimated to be 1.1 × 1019 cm−2. This would imply that strong UV radiation from the companion and/or accretion of matter between two stars is most likely the origin of the CI enhancement. However, this hypothesis can be confirmed by high-angular resolution observations.

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Abundance of SiC2 in carbon star envelopes

Astronomy and Astrophysics ◽

10.1051/0004-6361/201732038 ◽

2018 ◽

Vol 611 ◽

pp. A29 ◽

Cited By ~ 11

Author(s):

S. Massalkhi ◽

M. Agúndez ◽

J. Cernicharo ◽

L. Velilla Prieto ◽

J. R. Goicoechea ◽

...

Keyword(s):

Silicon Carbide ◽

Gas Phase ◽

Line Emission ◽

Carbon Star ◽

Active Role ◽

Asymptotic Giant Branch ◽

Agb Stars ◽

Carbon Stars ◽

Clear Trend ◽

Phase Precursor

Context. Silicon carbide dust is ubiquitous in circumstellar envelopes around C-rich asymptotic giant branch (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. To date, the ring molecule SiC2 has been observed in a handful of evolved stars, while SiC and Si2C have only been detected in the C-star envelope IRC +10216.Aim. 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.Methods. We carried out sensitive observations with the IRAM 30 m telescope of a sample of 25 C-rich AGB stars to search for emission lines of SiC2, SiC, and Si2C in the λ 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.Results. We detect SiC2 in most of the sources, SiC in about half of them, and do not detect Si2C in any source except 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 is probably the photodissociation product of SiC2 in the external layer of the envelope. We find a clear trend where the denser the envelope, the less abundant SiC2 is. The observed trend is interpreted as evidence of efficient incorporation of SiC2 onto dust grains, a process that is favored at high densities owing to the higher rate at which collisions between particles take place.Conclusions. 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.

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Search for surface magnetic fields in Mira stars: first results on χ Cyg

Proceedings of the International Astronomical Union ◽

10.1017/s1743921314002579 ◽

2013 ◽

Vol 9 (S302) ◽

pp. 385-388 ◽

Cited By ~ 1

Author(s):

Agnès Lèbre ◽

Michel Aurière ◽

Nicolas Fabas ◽

Denis Gillet ◽

Fabrice Herpin ◽

...

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Zeeman Effect ◽

Stellar Atmosphere ◽

Asymptotic Giant Branch ◽

Photospheric Level ◽

First Results ◽

Weak Surface ◽

Surface Fields ◽

The Magnetic Field

AbstractSo far, surface magnetic fields have never been reported on Mira stars, while observational facilities allowing detection and measurement of weak surface fields through the Zeeman effect have become available. Then, in order to complete the knowledge of the magnetic field and of its influence during the transition from Asymptotic Giant Branch (AGB) to Planetary Nebulae (PN) stages, we have undertaken a search for magnetic fields at the surface of Miras. We present the first spectropolarimetric observations (performed with the Narval instrument at Télescope Bernard Lyot-TBL, Pic du Midi, France) of the S-type Mira star χ Cyg. We have detected a polarimetric signal in the Stokes V spectra and we have established its Zeeman origin. We claim that it is likely to be related to a weak magnetic field present at the photospheric level and in the lower part of the stellar atmosphere. The origin of this magnetic field is discussed in the framework of shock waves periodically propagating throughout the atmosphere of a Mira.

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Maser emission from the CO envelope of the asymptotic giant branch star W Hydrae

Astronomy and Astrophysics ◽

10.1051/0004-6361/202141656 ◽

2021 ◽

Vol 654 ◽

pp. A18

Author(s):

W. H. T. Vlemmings ◽

T. Khouri ◽

D. Tafoya

Keyword(s):

Mass Loss ◽

Radiation Field ◽

Asymptotic Giant Branch ◽

Small Scale ◽

High Angular Resolution ◽

Circumstellar Envelope ◽

Agb Stars ◽

Maser Emission ◽

Co Emission ◽

Loss Rates

Context. Observation of CO emission around asymptotic giant branch (AGB) stars is the primary method to determine gas mass-loss rates. While radiative transfer models have shown that molecular levels of CO can become mildly inverted, causing maser emission, CO maser emission has yet to be confirmed observationally. Aims. High-resolution observations of the CO emission around AGB stars now have the brightness temperature sensitivity to detect possible weak CO maser emission. Methods. We used high angular resolution observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the small-scale structure of CO J = 3−2 emission around the oxygen-rich AGB star W Hya. Results. We find CO maser emission amplifying the stellar continuum with an optical depth τ ≈−0.55. The maser predominantly amplifies the limb of the star because CO J = 3−2 absorption from the extended stellar atmosphere is strongest towards the centre of the star. Conclusions. The CO maser velocity corresponds to a previously observed variable component of high-frequency H2O masers and with the OH maser that was identified as the amplified stellar image. This implies that the maser originates beyond the acceleration region and constrains the velocity profile since we find the population inversion primarily in the inner circumstellar envelope. We find that inversion can be explained by the radiation field at 4.6 μm and that the existence of CO maser emission is consistent with the estimated mass-loss rates for W Hya. However, the pumping mechanism requires a complex interplay between absorption and emission lines in the extended atmosphere. Excess from dust in the circumstellar envelope of W Hya is not sufficient to contribute significantly to the required radiation field at 4.6 μm. The interplay between molecular lines that cause the pumping can be constrained by future multi-level CO observations.

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Vibrationally excited water emission at 658 GHz from evolved stars

Astronomy and Astrophysics ◽

10.1051/0004-6361/201731694 ◽

2017 ◽

Vol 609 ◽

pp. A25 ◽

Cited By ~ 2

Author(s):

A. Baudry ◽

E. M. L. Humphreys ◽

F. Herpin ◽

K. Torstensson ◽

W. H. T. Vlemmings ◽

...

Keyword(s):

Vibrational State ◽

Line Emission ◽

Central Star ◽

Rotational Transition ◽

Asymptotic Giant Branch ◽

Time Variability ◽

Vibrationally Excited ◽

Physical Conditions ◽

Evolved Stars ◽

Water Layers

Context. Several rotational transitions of ortho- and para-water have been identified toward evolved stars in the ground vibrational state as well as in the first excited state of the bending mode (v2 = 1 in (0, 1, 0) state). In the latter vibrational state of water, the 658 GHz J = 11,0−10,1 rotational transition is often strong and seems to be widespread in late-type stars. Aims. Our main goals are to better characterize the nature of the 658 GHz emission, compare the velocity extent of the 658 GHz emission with SiO maser emission to help locate the water layers and, more generally, investigate the physical conditions prevailing in the excited water layers of evolved stars. Another goal is to identify new 658 GHz emission sources and contribute in showing that this emission is widespread in evolved stars. Methods. We have used the J = 11,0−10,1 rotational transition of water in the (0, 1, 0) vibrational state nearly 2400 K above the ground-state to trace some of the physical conditions of evolved stars. Eleven evolved stars were extracted from our mini-catalog of existing and potential 658 GHz sources for observations with the Atacama Pathfinder EXperiment (APEX) telescope equipped with the SEPIA Band 9 receiver. The 13CO J = 6−5 line at 661 GHz was placed in the same receiver sideband for simultaneous observation with the 658 GHz line of water. We have compared the ratio of these two lines to the same ratio derived from HIFI earlier observations to check for potential time variability in the 658 GHz line. We have compared the 658 GHz line properties with our H2O radiative transfer models in stars and we have compared the velocity ranges of the 658 GHz and SiO J = 2−1, v = 1 maser lines. Results. Eleven stars have been extracted from our catalog of known or potential 658 GHz evolved stars. All of them show 658 GHz emission with a peak flux density in the range ≈50–70 Jy (RU Hya and RT Eri) to ≈2000–3000 Jy (VY CMa and W Hya). Five Asymptotic Giant Branch (AGB) stars and one supergiant (AH Sco) are new detections. Three AGBs and one supergiant (VY CMa) exhibit relatively weak 13CO J = 6−5 line emission while o Ceti shows stronger 13CO emission. We have shown that the 658 GHz line is masing and we found that the 658 GHz velocity extent tends to be correlated with that of the SiO maser suggesting that both emission lines are excited in circumstellar layers close to the central star. Broad and stable line profiles are observed at 658 GHz. This could indicate maser saturation although we have tentatively provided first information on time variability at 658 GHz.

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Diffuse LINER-type emission from extended disc regions of barred galaxies

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa1369 ◽

2020 ◽

Vol 496 (1) ◽

pp. 36-48 ◽

Cited By ~ 1

Author(s):

S M Percival ◽

P A James

Keyword(s):

Line Emission ◽

Elliptical Galaxies ◽

Asymptotic Giant Branch ◽

Agb Stars ◽

Population Synthesis ◽

Full Sample ◽

Barred Galaxies ◽

Horizontal Branch Stars ◽

Ambient Gas ◽

Barred Spiral Galaxies

ABSTRACT We present a spectroscopic analysis of the central disc regions of barred spiral galaxies, concentrating on the region that is swept by the bar but not including the bar itself (the ‘star formation desert’ or SFD region). New spectroscopy is presented for 34 galaxies, and the full sample analysed comprises 48 SBa–SBcd galaxies. These data confirm the full suppression of SF within the SFD regions of all but the latest type (SBcd) galaxies. However, diffuse [N ii] and H α line emission is detected in all galaxies. The ubiquity and homogeneous properties of this emission from SBa to SBc galaxies favour post-asymptotic giant branch (p-AGB) stars as the source of this line excitation, rather than extreme blue horizontal branch stars. The emission-line ratios strongly exclude any contribution from recent SF, but are fully consistent with recent population synthesis modelling of p-AGB emission by other authors, and favour excitation dominated by ambient gas of approximately solar abundance, rather than ejecta from the AGB stars themselves. The line equivalent widths are also larger than those observed in many fully passive (e.g. elliptical) galaxies, which may also be a consequence of a greater ambient gas density in the SFD regions.

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Magnetic fields in AGB stars and (proto-) Planetary Nebulae

Proceedings of the International Astronomical Union ◽

10.1017/s1743921309030051 ◽

2008 ◽

Vol 4 (S259) ◽

pp. 47-52 ◽

Cited By ~ 5

Author(s):

Fabrice Herpin ◽

A. Baudy ◽

E. Josselin ◽

C. Thum ◽

H. Wiesemeyer

Keyword(s):

Magnetic Field ◽

Mass Loss ◽

Zeeman Effect ◽

Asymptotic Giant Branch ◽

High Mass ◽

Asymptotic Giant Branch Star ◽

High Mass Loss ◽

Pn Stage ◽

Draw Conclusion ◽

The Magnetic Field

AbstractDuring its quick transition to the Planetary Nebula stage, the Asymptotic Giant Branch star will completely change its geometry. This AGB stellar evolution stage is characterized by a high mass loss driven by the radiation pressure. Strong magnetic field may rule the mass loss geometry and the global shaping of these objects. Following our previous work on the polarization of the SiO maser emission in a representative sample of O-rich evolved stars, we present here a study towards C-rich objects and PPN/PN objects to obtain unbiased conclusions. Using Xpol at the IRAM-30 m telescope, we have conducted CN N=1-0 observations to investigate the Zeeman effect in this molecule and draw conclusion on the evolution of the magnetic field and its influence during the transition of an AGB star to the PN stage. Following the analysis described by Crutcher et al. (1996) we derive an estimate of the magnetic field.

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ALMA’s Acute View of pPNe: Through the Magnifying Glass... and What We Found There

Galaxies ◽

10.3390/galaxies6030094 ◽

2018 ◽

Vol 6 (3) ◽

pp. 94 ◽

Cited By ~ 2

Author(s):

Carmen Contreras ◽

Javier Alcolea ◽

Valentín Bujarrabal ◽

Arancha Castro-Carrizo

Keyword(s):

Angular Resolution ◽

Asymptotic Giant Branch ◽

High Angular Resolution ◽

Agb Stars ◽

Rotating Disks ◽

Circumstellar Envelopes ◽

Intermediate Mass ◽

Intermediate Mass Stars ◽

Shaping Process ◽

Late Stages

We present recent Atacama Large Millimeter/submillimeter Array (ALMA)-based studies of circumstellar envelopes (CSEs) around Asymptotic Giant Branch (AGB) stars and pre-Planetary Nebulae (pPNe). In only a few years of operation, ALMA is revolutionising the field of AGB-to-PN research by providing unprecedentedly detailed information on the complex nebular architecture (at large but also on small scales down to a few ∼10 AU from the centre), dynamics and chemistry of the outflows/envelopes of low-to-intermediate mass stars in their late stages of the evolution. Here, we focus on continuum and molecular line mapping studies with high angular resolution and sensitivity of some objects that are key to understanding the complex PN-shaping process. In particular, we offer (i) a brief summary of ALMA observations of rotating disks in post-AGB objects and (ii) report on ALMA observations of OH 231.8+4.2 providing the most detailed and accurate description of the global nebular structure and kinematics of this iconic object to date.

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