ATOMIUM: Probing the inner wind of evolved O-rich stars with new, highly excited H2O and OH lines
Artikel i vetenskaplig tidskrift, 2023

Context. Water (H2O) and the hydroxyl radical (OH) are major constituents of the envelope of O-rich late-type stars. Transitions involving energy levels that are rotationally or vibrationally highly excited (energies a 4000 K) have been observed in both H2O and OH. These and more recently discovered transitions can now be observed at a high sensitivity and angular resolution in the inner wind close to the stellar photosphere with the Atacama Large Millimeter/submillimeter Array (ALMA). Aims. Our goals are: (1) to identify and map the emission and absorption of H2O in several vibrational states, and of OH in I -doubling transitions with similar excitation energies; and (2) to determine the physical conditions and kinematics in gas layers close to the extended atmosphere in a sample of asymptotic giant branch stars (AGBs) and red supergiants (RSGs). Methods. Spectra and maps of H2O and OH lines observed in a 27 GHz aggregated bandwidth and with an angular resolution of ~0."02'1."0 were obtained at two epochs with the main ALMA array. Additional observations with the Atacama Compact Array (ACA) were used to check for time variability of water transitions. Radiative transfer models of H2O were revisited to characterize masing conditions. Up-to-date chemical models were used for comparison with the observed OH/H2O abundance ratio. Results. Ten rotational transitions of H2O with excitation energies ~4000'9000 K were observed in vibrational states up to (I 1,I 2,I 3) = (0,1,1). All but one are new detections in space, and from these we have derived accurate rest frequencies. Hyperfine split I -doubling transitions in I = 0, J = 27/2 and 29/2 levels of the 2Π3/2 state, as well as J = 33/2 and 35/2 of the 2Π1/2 state of OH with excitation energies of ~4780'8900 K were also observed. Four of these transitions are new detections in space. Combining our measurements with earlier observations of OH, the I = 0 and I = 1 I -doubling frequencies have been improved. Our H2O maps show compact emission toward the central star and extensions up to twelve stellar radii or more. The 268.149 GHz emission line of water in the I 2 = 2 state is time variable, tends to be masing with dominant radiative pumping, and is widely excited in AGBs and RSGs. The widespread but weaker 262.898 GHz water line in the I 2 = 1 state also shows signs of maser emission. The OH emission is weak and quasithermally excited. Emission and absorption features of H2O and OH reveal an infall of matter and complex kinematics influenced by binarity. From the OH and H2O column densities derived with nonmasing transitions in a few sources, we obtain OH/H2O abundance ratios of ~(0.7'2.8) A - 10'2.



Circumstellar matter

Instrumentation: interferometers

Stars: AGB and post-AGB

Line: identification


A. Baudry

Laboratoire d'Astrophysique de Bordeaux

K. T. Wong

Institut de Radioastronomie Millimétrique (IRAM)

Uppsala universitet

S. Etoka

University of Manchester

A.M.S. Richards

University of Manchester

Holger Muller

Universität zu Köln

F. Herpin

Laboratoire d'Astrophysique de Bordeaux

Taissa Danilovich

KU Leuven

Monash University

M. Gray

National Astronomy Research Institute of Thailand

University of Manchester

Sofia Wallström

KU Leuven

D. Gobrecht

Göteborgs universitet

KU Leuven

Theo Khouri

Chalmers, Rymd-, geo- och miljövetenskap, Astronomi och plasmafysik

L. Decin

KU Leuven

C. A. Gottlieb

Harvard-Smithsonian Center for Astrophysics

K. Menten


W. Homan

KU Leuven

T.J. Millar

Queen's University Belfast

M. Montargès

LESIA - Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique

B. Pimpanuwat

University of Manchester

J. M. C. Plane

University of Leeds

P. Kervella

LESIA - Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 674 A125


Astronomi, astrofysik och kosmologi

Atom- och molekylfysik och optik



Mer information

Senast uppdaterat