scholarly journals 3.20. OH/IR stars as signposts for ancient starburst activity in the Galactic center

1998 ◽  
Vol 184 ◽  
pp. 129-130 ◽  
Author(s):  
L.O. Sjouwerman ◽  
H.J. Habing ◽  
H.J. van Langevelde ◽  
M. Lindqvist ◽  
A. Winnberg
Keyword(s):  
Mass Loss ◽  
Galactic Center ◽  
Expansion Velocity ◽  
Late Type ◽  
Considerable Fraction ◽  
Ir Stars ◽  
Circumstellar Shells

We have surveyed the Galactic center (GC) for OH/IR stars, evolved late-type stars of different masses - and thus ages - in a short-lived stage of heavy mass-loss. By observing the 1612 MHz OH masers generated in their circumstellar shells, it is found that a considerable fraction of these OH/IR stars has the same high-valued shell expansion velocity of 19 km s−1.

scholarly journals Ice Mantle Formation in the Envelopes of OH/IR Stars

1993 ◽  
Vol 155 ◽  
pp. 329-329
Author(s):  
S.B. Charnley ◽  
R.G. Smith
Keyword(s):  
Mass Loss ◽  
Simple Theory ◽  
Late Type ◽  
Water Ice ◽  
Physical Conditions ◽  
Grain Interaction ◽  
Inner Radius ◽  
Ir Stars ◽  
Circumstellar Shells ◽  
Loss Rates

We have computed ice column densities for a sample of O-rich late-type stars.Water ice is observed in the outflows from several late-type stars (e.g. Smith et al. 1988) and it is of some observational and theoretical importance to understand how common ice mantles may be and how their formation depends upon the physical conditions in the envelope, such as the radial distributions of density and temperature. We have developed a simple theory of the gas-grain interaction in circumstellar shells (Jura & Morris 1985; Charnley et al. 1992) and used it to compute ice column densities (Nice) and mantle thicknesses (d) for a sample of well-studied OH/IR stars (Herman & Habing 1985). The models are most sensitive to the dust and gas mass loss rates, as well as to the inner radius of the envelope, Ri, which is poorly determined. Representative results are presented in the Table below where Ri was estimated according to the prescription of Herman et al. (1986).

scholarly journals Mass-Loss from Late-Type Stars: New Observational Evidence

1983 ◽  
Vol 103 ◽  
pp. 292-292
Author(s):  
P. G. Wannier ◽  
R. Sahai
Keyword(s):  
Mass Loss ◽  
Central Star ◽  
Ir Absorption ◽  
Late Type ◽  
Ir Radiation ◽  
Loss Mechanism ◽  
Continuum Source ◽  
Near Ir ◽  
Molecular Lines ◽  
Circumstellar Shells

Rapid mass-loss is observed in many late-type stars, yet the mass-loss mechanisms operating are not well understood. A survey of molecular emission from circumstellar shells has been carried out using millimeterwave molecular lines and suggests that radiation pressure alone may be inadequate to explain the observed mass-loss, especially in the case of carbon-rich objects which may display rates in excess of 10−5 M⊙/yr. Recent near-IR molecular line observations provide evidence for ejected material at several different velocities along the line-of-sight and may indicate the additional mass-loss mechanism at work. Resonantly scattered IR radiation spatially displaced from the central IR continuum source has now been observed for the first time and sheds new light on the IR absorption-line results, providing information about material within 1016 cm of the central star. These results are discussed along with recent high-resolution millimeterwave observations.

scholarly journals OH masers in oxygen-rich late-type stars

2002 ◽  
Vol 206 ◽  
pp. 319-322
Author(s):  
Sandra Etoka ◽  
A.M. Le Squeren
Keyword(s):  
Mass Loss ◽  
Radio Telescope ◽  
Loss Rate ◽  
Mass Loss Rate ◽  
Monitoring Programme ◽  
Late Type ◽  
Wide Range ◽  
Ir Stars ◽  
Circumstellar Shell

We present here some noteworthy results of two related studies on oxygen-rich late type stars. The aim of this work was to study the OH circumstellar shell properties in terms of evolution. These studies are based on an OH monitoring programme carried out with the Nançay Radio Telescope. The first study concerns seven Miras distributed along the colour-colour diagram. They were observed at two or three different epochs covering one to seven cycles over a period from 1980 to 1995 at 1612, 1667 and 1665 MHz in both circular polarizations. The second study concerns thirty objects covering a wide range of mass loss rate from Miras to OH/IR stars. They were observed in 1994 at 1665 & 1667 MHz in both circular polarizations.

scholarly journals Infrared Observations of Circumstellar Molecules

1987 ◽  
Vol 120 ◽  
pp. 327-338 ◽  
Author(s):  
Albert Betz
Keyword(s):  
Infrared Spectroscopy ◽  
Mass Loss ◽  
Abundant Species ◽  
High Resolution Spectroscopy ◽  
Late Type ◽  
Infrared Observations ◽  
Negative Results ◽  
Circumstellar Gas ◽  
Circumstellar Shells ◽  
Vantage Points

This review presents an updated listing of infrared observations of circumstellar molecules. The compendium is restricted to the more abundant species with infrared spectra accessible to groundbased observations. No analysis is offered on the chemical importance of a particular species, and no attempt is made to correlate the infrared work with the much larger body of data available from radio frequency observations. Some discussions of these topics may be found in the more comprehensive reviews listed below. The observations described here include both the successful and the unrewarded efforts. Often the negative results from attempted observations are not widely disseminated, even though such results can sometimes be of significant theoretical value. Discussions of infrared stellar and circumstellar spectroscopy from other vantage points may be found in the following reviews. in 1979 Merrill and Ridgway surveyed the contributions of infrared spectroscopy to studies of stellar photospheres and the near circumstellar environment. The following year Zuckerman (1980) summarized the current state of knowledge on circumstellar molecular clouds with attention given to both infrared and microwave observations. Ridgway and Keady (1981) subsequently noted the importance of infrared spectroscopy for studies of circumstellar gas and dust in the star IRC + 10216. in 1983 Hinkle reviewed the high-resolution spectroscopy of late-type circumstellar shells in general but emphasized chemistry and kinematics. Finally, in the proceedings of the 1984 UCLA meeting on mass-loss, Omont (1985) reviewed the physical and chemical structure of circumstellar envelopes and IRC + 10216 in particular, and Wannier (1985) discussed the significance of infrared spectroscopy for studies of mass-loss in late-type stars.

scholarly journals The Maser Strength of OH/IR Stars, the Evolution of Mass loss and the Formation of a Planetary Nebula

1983 ◽  
Vol 103 ◽  
pp. 530-530
Author(s):  
B. Baud ◽  
H. J. Habing
Keyword(s):  
Mass Loss ◽  
Planetary Nebula ◽  
Loss Rate ◽  
Large Range ◽  
Mass Loss Rate ◽  
Asymptotic Giant Branch ◽  
Expansion Velocity ◽  
Simple Modification ◽  
Ir Stars ◽  
Circumstellar Shell

From observations we find that the OH luminosity LOH of an OH/IR star increases with R2, where R is the size of the masing region. From this correlation we deduce that the mass loss rate M, the expansion velocity ve and LOH are related by LOH ~(M/ve)2. Next we consider the large range that is observed in LOH and the steep OH luminosity distribution for OH/IR stars. Both facts can be explained by the postulate that these objects undergo accelerated mass loss, and thus steadily increase their OH luminosity. We propose that OH/IR stars are at the extreme end of the Asymptotic Giant Branch and that many of them are in the process of blowing off their entire envelope in a superwind phase. Their mass loss rate during this superwind, as deduced from OH observations of the circumstellar shell, is given by a simple modification of the Reimers equation. This modification connects the superwind continuously to the Reimers wind and it provides observational evidence for the formation of a planetary nebula.

scholarly journals Cool Star Winds – Recent Observations and Theoretical Implications

1983 ◽  
Vol 6 ◽  
pp. 549-563 ◽  
Author(s):  
L. Hartmann
Keyword(s):  
Mass Loss ◽  
Absorption Lines ◽  
Late Type ◽  
Cool Star ◽  
Circumstellar Shells ◽  
Loss Rates ◽  
Loss Mechanisms

Much of our knowledge of winds from late-type stars comes from the detection of ejected material, called circumstellar shells, as observed in absorption lines of low-excitation species such as Mg II, Ca II, Na I, and K I (cf. Reimers 1977). Observations of CS shells are difficult to translate into quantitative mass loss rates, a limitation which has not helped to test various mass loss mechanisms. The data clearly demonstrate one very important fact: shell velocities are very low. In fact, they are so much lower than surface escape velocitiesthat it was not clear that material is actually being lost until Deutsch (1956) detected the existence of the CS absorption shells ejected from α Her and α Sco in the spectra of distant companion stars. Today it is possible to demonstrate the expansion of shells out to several thousand stellar radii in K I scattering (Honeycutt et al. 1980).

scholarly journals Report on a Survey of H2O Masers Towards the Galactic Center

1996 ◽  
Vol 169 ◽  
pp. 343-344
Author(s):  
Deborah A. Levine ◽  
Mark Morris
Keyword(s):  
Galactic Center ◽  
Point Sources ◽  
Late Type ◽  
Star Forming ◽  
Star Forming Regions ◽  
Circumstellar Shells

Galactic H2O masers are associated either with the circumstellar shells of late-type stars or with star-forming regions (SFRs). Previous surveys for H2O masers had revealed relatively few near the Galactic Center (GC). We report on the continuation of a survey using the VLA to make 22.2 GHz observations of IRAS point sources which lie within 2° of the GC and which have either 12 μm or 25 μm flux > 8 Jy. Taylor, Morris and Schulman (TMS, 1992, AJ, 106, 1978) previously observed 97 of 342 candidate sources. We have observed 160 more and plan to complete the survey in early 1995.

Variability of masers in circumstellar shells on timescales of decades

2007 ◽  
Vol 3 (S242) ◽  
pp. 318-319
Author(s):  
D. Engels ◽  
A. Winnberg ◽  
J. Brand ◽  
F. Jiménez-Esteban
Keyword(s):  
Spatial Distribution ◽  
Mass Loss ◽  
Spherical Symmetry ◽  
Monitoring Program ◽  
Variable Stars ◽  
Agb Stars ◽  
Spatial Asymmetry ◽  
Ir Stars ◽  
Regular Variable ◽  
Circumstellar Shells

AbstractBased on a 15–20 year long monitoring program of H2O masers and new observations of OH masers discovered 35 years ago, we studied the variability of maser emission in AGB stars over timescales of decades. The H2O maser features in the semi-regular variable stars RX Boo and SV Peg, and of the Mira stars U Her and R Cas, showed strong fluctuations superposed in case of the Mira stars on regular variations due to the pulsation of the stars. The spatial distribution of the emission regions in RX Boo and U Her showed deviations from spherical symmetry, which remained unchanged over >10 years. We conclude that the spatial asymmetry is determined by the underlying asymmetry of the mass loss process. There is no evidence that mass loss rates or the wind geometry in these stars have varied over the last 20 years. In 2005 we re-detected at 1612 MHz the 114 OH/IR stars discovered before 1978, implying an OH maser lifetime of >2800 years (1σ).

scholarly journals A study of the H2O maser emission from R Cas

2002 ◽  
Vol 206 ◽  
pp. 310-313
Author(s):  
Jan Brand ◽  
Lara Baldacci ◽  
Dieter Engels
Keyword(s):  
Mass Loss ◽  
Data Base ◽  
Monitoring Program ◽  
Optical Variability ◽  
Late Type ◽  
Small Space ◽  
Preliminary Results ◽  
The Past ◽  
Mira Variable ◽  
Ir Stars

Over the past decade we observed a sample of late-type stars (supergiants, semi-regular variables, OH/IR stars, and Mira's) 3-4 times per year in the 1.3-cm line of H2O. The observations were carried out with the Medicina 32-m and the Effelsberg 100-m telescopes. In addition, a sub-sample of these stars was observed at several epochs with the VLA. In our analysis we also use data from the literature, for instance on mass loss and optical variability. The aim is to investigate the properties of the circumstellar outflows and to elucidate the maser-pumping mechanism. In the conference poster we presented some preliminary results of a 4-star sub-set of the data base; in the small space available here, we briefly describe the monitoring program, and discuss the Mira-variable R Cas in some detail.

scholarly journals Masers in Circumstellar Shells

1987 ◽  
Vol 120 ◽  
pp. 369-377
Author(s):  
C. M. Walmsley
Keyword(s):  
Mass Loss ◽  
Mass Loss Rate ◽  
Far Infrared ◽  
Asymptotic Giant Branch ◽  
Solar Mass ◽  
Mira Variables ◽  
Ir Stars ◽  
Show Evidence ◽  
Circumstellar Shells ◽  
Infrared Wavelengths

Presumably due to a cosmic accident, circumstellar masers are almost exclusively found in oxygen rich stars with [0/C] >1. The exception here is the SiS maser found towards IRC 10216 (Henkel et al. (1985)) but I will ignore this in what follows and concentrate this review upon the circumstellar shells where OH, H2O, and SiO masers are found in profusion. Such objects are thought to be on the asymptotic giant branch (AGB) of the Herzsprung-Russell diagram and show evidence for rapid mass loss. The derived mass loss rates vary from approximately 10−7 solar masses per year for the optically well studied Mira variables in the solar neighbourhood (Distances <1 kpc) to as high as 10−4 solar masses per year for the optically invisible OH-IR stars and for some supergiants. of these objects, the supergiants appear to be more luminous than the rest (105 L rather than 104 L) and their central stars have masses of order 10 M⊙ as compared to most Mira variables whose mass is probably in the range 1–2 M⊙. Most OH-IR stars are likely to be a late stage in the evolution of solar mass type objects where the mass loss rate suddenly increases. As their name indicates, they are characterised by their strong OH-maser (1612 MHz) emission as well as by their radiation at far infrared wavelengths (20–50 μ). They may well be on the way to becoming planetary nebulae (see Pottasch, 1984 for an account of their probable evolution).

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