CO in OH/IR stars close to the Galactic centre

Winnberg, A.; Deguchi, Shuji; Reid, M. J.; Nakashima, Jun-ichi; Olofsson, H.; Habing, H. J.

doi:10.1051/0004-6361/200911655

Cited by 4 publications

(25 citation statements)

References 16 publications

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“…Five inner-Bulge OH/IR stars, with a metallicity most likely somewhat higher than the solar value, have been detected in CO line emission by Winnberg et al (2009) and Sargent et al (2013). The estimated mass-loss rates lie on the high side, and the gas-to-dust-mass ratios center around 200.…”

Section: The Galactic Bulge and Halomentioning

confidence: 95%

Mass loss of stars on the asymptotic giant branch

Höfner

Olofsson

2018

Astron Astrophys Rev

471

376

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As low-and intermediate-mass stars reach the asymptotic giant branch (AGB), they have developed into intriguing and complex objects that are major players in the cosmic gas/dust cycle. At this stage, their appearance and evolution are strongly affected by a range of dynamical processes. Large-scale convective flows bring newlyformed chemical elements to the stellar surface and, together with pulsations, they trigger shock waves in the extended stellar atmosphere. There, massive outflows of gas and dust have their origin, which enrich the interstellar medium and, eventually, lead to a transformation of the cool luminous giants into white dwarfs. Dust grains forming in the upper atmospheric layers play a critical role in the wind acceleration process, by scattering and absorbing stellar photons and transferring their outward-directed momentum to the surrounding gas through collisions. Recent progress in high-angularresolution instrumentation, from the visual to the radio regime, is leading to valuable new insights into the complex dynamical atmospheres of AGB stars and their windforming regions. Observations are revealing asymmetries and inhomogeneities in the photospheric and dust-forming layers which vary on time-scales of months, as well as more long-lived large-scale structures in the circumstellar envelopes. High-angularresolution observations indicate at what distances from the stars dust condensation occurs, and they give information on the chemical composition and sizes of dust grains in the close vicinity of cool giants. These are essential constraints for building B Susanne Höfner

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Section: The Galactic Bulge and Halomentioning

confidence: 95%

Mass loss of stars on the asymptotic giant branch

Höfner

Olofsson

2018

Astron Astrophys Rev

471

376

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“…CO is very useful in determining gas mass-loss rates from evolved stars (see Ramstedt et al -3 -2008;De Beck et al 2010). Winnberg et al (2009) observed a small sample of OH/IR stars close to the Galactic Center in the CO J=1-0 and J=2-1 lines. Using Equation A.1 from Ramstedt et al (2008), Winnberg et al (2009) calculated the gas mass-loss rates for a couple of the stars in their sample.…”

Section: Introductionmentioning

confidence: 99%

“…Winnberg et al (2009) observed a small sample of OH/IR stars close to the Galactic Center in the CO J=1-0 and J=2-1 lines. Using Equation A.1 from Ramstedt et al (2008), Winnberg et al (2009) calculated the gas mass-loss rates for a couple of the stars in their sample. CO J=2-1 and higher level transitions of CO, such as J=3-2, are useful in constraining the mass-loss rate history of AGB and OH/IR stars (e.g., Decin et al 2006).…”

Section: Introductionmentioning

confidence: 99%

“…We follow Winnberg et al (2009) and observe a sample of OH/IR and AGB stars in the inner Galactic Bulge (also known as the Nuclear Bulge; see Mezger et al 1996;Schultheis et al 2003) in the CO J=2-1 line to investigate their gas mass-loss rates using the Submillimeter Array (SMA; Ho et al 2004). The objects in the inner Galactic Bulge are in an extreme environment for star formation and evolution (e.g., see Mezger et al 1996).…”

Section: Introductionmentioning

confidence: 99%

“…There is the additional issue that, though our sample may be located near the Galactic Bulge on the sky, individual stars in the sample may not all be located 8.0 ± 0.3 kpc away. This type of issue is specifically discussed by Winnberg et al (2009) for both OH 359.762 +0.120 (also known as [SLO2003] E51) and OH 359.971 -0.119 and by Blommaert et al (1998) for OH 359.762 +0.120.…”

Section: Introductionmentioning

confidence: 99%

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COJ= 2-1 EMISSION FROM EVOLVED STARS IN THE GALACTIC BULGE

Sargent¹,

Patel²,

Meixner³

et al. 2013

ApJ

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We observe a sample of 8 evolved stars in the Galactic Bulge in the CO J=2-1 line using the Submillimeter Array (SMA) with angular resolution of 1-4 arcseconds. These stars have been detected previously at infrared wavelengths, and several of them have OH maser emission. We detect CO J=2-1 emission from three of the sources in the sample: OH 359.943 +0.260, [SLO2003] A12, and [SLO2003] A51. We do not detect the remaining 5 stars in the sample because of heavy contamination from the galactic foreground CO emission. Combining CO data with observations at infrared wavelengths constraining dust mass loss from these stars, we determine the gas-to-dust ratios of the Galactic Bulge stars for which CO emission is detected. For OH 359.943 +0.260, we determine a gas mass-loss rate of 7.9 (± 2.2) ×10 −5 M ⊙ yr −1 and a gas-to-dust ratio of 310 (± 89). For [SLO2003] A12, we find a gas mass-loss rate of 5.4 (± 2.8) ×10 −5 M ⊙ yr −1 and a gas-to-dust ratio of 220 (± 110). For [SLO2003] A51, we find a gas massloss rate of 3.4 (± 3.0) ×10 −5 M ⊙ yr −1 and a gas-to-dust ratio of 160 (± 140), reflecting the low quality of our tentative detection of the CO J=2-1 emission from A51. We find the CO J=2-1 detections of OH/IR stars in the Galactic Bulge require lower average CO J=2-1 backgrounds.-2 -Subject headings: circumstellar matter -infrared: stars IntroductionAt the end of a star's life, it returns material back to the interstellar medium from which it emerged. This mass loss begins in the first red giant phase of a star's life (e.g., Groenewegen 2012). Observations show gas mass loss from red giant stars (Dupree et al. 2009), but very little in the way of dust mass loss from red giants (McDonald et al. 2011). During the red giant phase, the mass loss is relatively weak, but it increases dramatically in the Asymptotic Giant Branch (AGB) phase. The dust grains that form around AGB stars experience radiation pressure from the star, pushing them outwards, and they, in turn, push on the circumstellar gas, driving the gas outwards along with the dust (e.g., Forrest 1974). Detailed axisymmetric models by Woitke (2006) confirm this picture for Carbon-rich (C-rich) AGB stars. Bladh & Höfner (2012) confirm this idea also works for Oxygen-rich (O-rich) AGB stars, which produce silicate dust, but require the dust grains to be Mg-rich, being highly transparent in the infrared, and to grow to 0.1-1 µm sizes in order to have the large scattering cross-sections necessary to experience radiation pressure sufficient to push the grains outward to drive the mass outflow. The mass-loss rate is an important parameter in characterizing the life of a star and also in constructing galaxy evolution models. For AGB stars, we have been estimating dust mass-loss rates through fitting optical through mid-infrared spectral energy distributions (SEDs; plots of flux versus wavelength) for stars in the Large Magellanic Cloud (Riebel et al. 2012) using radiative transfer models of emission from dust shells around stars (e.g., see the "GRAMS" models constructed by...

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CO line observations of OH/IR stars in the inner Galactic Bulge: Characteristics of stars at the tip of the AGB

Olofsson

Khouri

Sargent

et al. 2022

A&A

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Context. OH/IR stars are examples of late stellar evolution on the asymptotic giant branch (AGB), and they are, as such, important objects to study. They are also excellent probes of stellar populations, in particular in regions of high interstellar extinction such as the central regions of our Galaxy. Aims. Our goal is to characterise the stellar and circumstellar properties of high-mass-loss-rate OH/IR stars in the inner Galactic Bulge using the Atacama Large Millimeter/submillimeter Interferometer (ALMA). Methods. Rotational lines of 12 CO and 13 CO, as well as a millimetre-wave continuum, have been observed for a sample of 22 OH/IR stars in directions within 2 • of the Galactic Centre. Photometry data (≈ 1−30 µm) have been gathered from the literature to construct spectral energy distributions (SEDs) and to determine pulsational variability. Radiative transfer models have been used to interpret the line and photometry data. Results. All stars in the sample were detected in at least one CO line, and eight objects were detected in 324 GHz continuum. Based on luminosity criteria, the sample is divided into 17 objects that most likely lie within the inner Galactic Bulge, and five objects that are most likely foreground objects. The median luminosity of the inner-Galactic-Bulge sub-sample, 5600 L , corresponds to an initial mass in the range 1.2 − 1.6 M , indicating that these inner-Galactic-Bulge OH/IR stars descend from solar-type stars. The objects in this sub-sample are further divided into two classes based on their SED characteristics: Eleven objects have SEDs that are well matched by models invoking dust envelopes extending from a few stellar radii and outwards, while six objects are better modelled as having detached dust envelopes with inner radii in the range 200 − 600 au and warmer central stars. The former objects have periodic variability, while the latter objects are predominantly non-periodic. The median gas-mass-loss rate, gas terminal expansion velocity, gas-to-dust mass ratio, and circumstellar 12 CO/ 13 CO abundance ratio have been estimated to be 2×10 −5 M yr −1 , 18 km s −1 , 200 (excluding the sources with detached dust envelopes, which show markedly lower gas-to-dust ratios), and 5, respectively, for the inner-Galactic-Bulge sub-sample. All line brightness distributions are resolved at an angular scale of ≈ 0 . 15, but only two objects show a structure in their circumstellar envelopes at our resolution and sensitivity. In both cases, this structure takes the form of a cavity and a bipolar morphology. Conclusions. The inner-Galactic-Bulge sub-sample consists of high mass-loss-rate stars that descend from solar-type progenitors and that lie near the tip of the AGB. Some of the sample stars may have recently ceased mass loss and, hence, have begun to evolve beyond the AGB, as evidenced by a change in circumstellar characteristics and indications of warmer central stars. The inferred very low stellar 12 C/ 13 C isotope ratios are indicative of CNO-cycle nuclear processing, and they are most like...

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CO in OH/IR stars close to the Galactic centre

Cited by 4 publications

References 16 publications

Mass loss of stars on the asymptotic giant branch

Mass loss of stars on the asymptotic giant branch

COJ= 2-1 EMISSION FROM EVOLVED STARS IN THE GALACTIC BULGE

CO line observations of OH/IR stars in the inner Galactic Bulge: Characteristics of stars at the tip of the AGB

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