Variability of the H<sub>2</sub>O maser associated with U Orionis

Rudnitskij, G. M.; Lekht, E. E.; Mendoza-Torres, J. E.; Pashchenko, M. I.; Berulis, I. I.

doi:10.1051/aas:2000278

Cited by 13 publications

(14 citation statements)

References 26 publications

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“…Lekht et al (2005) detected slightly less flux than we did for S Per in 1994, and more in 1999. Rudnitskij et al (2000) show that we detected a similar amount of maser emission from U Ori as they did in 1994 and 1999. The single-dish flux of RT Vir within a few months of our observations (Mendoza-Torres et al 1997;Lekht et al 1999) was lower than the closest interferometry measurements, but both sets of observations show that it is highly variable, changing flux density by a factor of 2 in a few months.…”

Section: Observationssupporting

confidence: 84%

“…Stronger shocks are likely to propagate out from the stellar surfaces of U Ori and U Her, which could flatten maser clouds. Rudnitskij et al (2000) describes signs of the impact of stellar pulsations apparent from long-term spectral monitoring of H 2 O masers around U Ori. Moreover, both U Ori and U Her have undergone OH maser flares likely to have originated in the inner CSE (Pataki & Kolena 1974;Chapman & Cohen 1985;Etoka & Le Squeren 1997).…”

Section: Matter-bounded Masersmentioning

confidence: 99%

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Observational evidence for the shrinking of bright maser spots

Richards

Elitzur

Yates

2010

A&A

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Context. The nature of maser emission means that the apparent angular size of an individual maser spot is determined by the local amplification process as well as by the instrinsic size of the emitting cloud. Highly sensitive radio interferometry images made using MERLIN spatially and spectrally resolve water maser clouds around evolved stars. Aims. We used measurements of the cloud properties, around the red supergiant S Per and the AGB stars IK Tau, RT Vir, U Her and U Ori, to test maser beaming theory. In particular, spherical clouds are expected to produce an inverse relationship between maser intensity and apparent size, which would not be seen from very elongated (cylindrical or slab-like) regions. Methods. We analysed the measured properties of the maser emission in order to estimate the saturation state. We analysed the variation of observed maser spot size with intensity and across the spectral line profiles. Results. Circumstellar masers emanate from discrete clouds from about one to 20 AU in diameter depending on the star. Most of the maser features have negative excitation temperatures close to zero and modest optical depths, showing that they are mainly unsaturated. Around S Per and (at most epochs) RT Vir and IK Tau, the maser component size shrinks with increasing intensity, although in some cases the slope is shallower than predicted, probably due to shape irregularities and the presence of velocity gradients within clouds. In contrast, the masers around U Ori and U Her tend to increase in size, with a larger scatter. Conclusions. The water masers from S Per, RT Vir and IK Tau are mainly beamed into spots with an observed angular size much smaller than the emitting clouds. The brighter spots at the line peaks are smaller than those in the wings. This suggests that the masers are amplification-bounded, emanating from clouds which can be approximated as spheres. Many of the masers around U Her and U Ori have apparent sizes which are more similar to the emitting clouds and have less or no dependence on intensity, which suggests that these masers are matter-bounded. This is consistent with an origin in flattened clouds and these two stars have shown other behaviour indicating the presence of shocks which could produce this effect.

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Section: Observationssupporting

confidence: 84%

Section: Matter-bounded Masersmentioning

confidence: 99%

Observational evidence for the shrinking of bright maser spots

Richards

Elitzur

Yates

2010

A&A

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“…Four epochs of MERLIN observations from 1994-2001 show that the water masers always fall within a hollow shell bounded by radii of ∼ 8 and ∼ 35 AU, wherein the brightest emission is always at a similar distance from the star, although at different position angles. The weighted mean velocity of the brightest masers has drifted by less than 1 km s −1 in 20 yr [113]. Unusually, the apparent size of individual bright maser components is not inversely proportional to the brightness temperature as expected for beaming from approximately spherical clouds, but increases, as shown in Fig.…”

Section: Pos(8thevn)042mentioning

confidence: 63%

Beaming in on some stimulating issues in stellar evolution

Richards

2007

Proceedings of 8th European VLBI Network Symposium — PoS(8thEVN)

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This review discusses some recent maser observations and their interpretation, with the emphasis on physical processes in star formation and mass loss from evolved stars. I draw attention to issues where high-resolution observations have led to new, surprising or controversial developments. I also mention some results and prospects for maser studies arising from new instruments and surveys. Highlights include measurements of Zeeman splitting in non-paramagnetic species such as water and methanol. The water maser results often indicate a much stronger magnetic field than is seen by nearby OH masers, suggesting that the former arise from overdense clumps. Maser scattering or Faraday rotation implicates a significant ionised fraction. The increasing number of axisymmetric outflows from evolved stars, often with no signs of rotation, are likely to be shaped by magnetic fields, but there is no agreement on their origin. Extraordinary thousand-AU filaments have been seen in e.g. W3(OH) in OH and methanol, and in S128 in water. At the other extreme, compact 6-GHz OH and methanol are associated on sub-arcsec, but not milli-arcsec scales; does this indicate chemical chronology or competitive maser amplification? Another surprise is the increasing number of weak methanol and water masers associated with low-mass young stellar objects. Masers are still an ideal tool to probe the chemistry, kinematics, density, magnetic field and ionised fraction, and hence the rate and clumping scales of star formationbut the process is intrinsically a lot more complex than it once seemed.

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“…According to our data of March and June 1980, this star was 'silent' in H 2 O with the peak flux density below 10-12 Jy. However, between June and October 1980 the U Ori maser brightened to more than 1000 Jy (Berulis et al 1983;Rudnitskij et al 2000). Similar events took place in the H 2 O maser associated with the Mira-like semiregular variable W Hya in 1981 (Berulis et al 1983) and in 2001.…”

Section: Observationsmentioning

confidence: 82%

“…This may be due to a periastron shock wave episode of a planet in a highly eccentric orbit with a period P ∼ 15 years. Some other stars (e.g., U Ori, Rudnitskij et al 2000) have already shown some hints of H 2 O maser 'superperiodicity'of 12-15 years, which could be associated with planetary revolution periods.…”

Section: Effects Of a Relic Planetary System Around A Red Giantmentioning

confidence: 99%

Molecular Masers in Variable Stars

Rudnitskij¹

2002

Publ. – Astron. Soc. Aust

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When a star with a mass of one to a few solar masses enters the red giant stage of its evolution, the radius of its atmosphere reaches several astronomical units. Pulsational instability is typical for this stage. Most stars become Mira-type or semiregular variables with light cycles of a few hundred days. Red giants lose mass at a rate M = 10−7−10−5M⊙ yr−1. Extensive gas–dust circumstellar envelopes form. These envelopes contain various molecular species. Some of these molecules (OH, H2O, SiO, HCN) manifest themselves in maser radio emission. Data on the H2O maser variability and its connection with the stellar brightness variations are discussed. In the H2O line circumstellar masers can be divided into ‘stable’ (showing persistent emission — R Aql, U Her, S CrB, X Hya) and ‘transient’ (appearing in the H2O line once per 10–15 stellar light cycles — R Leo, R Cas, U Aur). Physical mechanisms of the maser variability are discussed. The most probable process explaining the observed visual–H2O correlation is the influence of shock waves on the masing region. Usually it is assumed that shocks in Mira atmospheres are driven by stellar pulsations. Here an alternative explanation is proposed. If a star during its main sequence life possessed a planetary system, similar to the solar system, the planets will be embedded in a rather dense and hot medium. Effects of a planet revolving around a red giant at a short distance (inside its circumstellar envelope) are discussed. A shock produced by the supersonic motion of a planet can account for the correlated variability of the Hα line emission and H2O maser. If the planetary orbit is highly eccentric, then the connected Hα–H2O flare episodes may be explained by the periastron passage of the planet. New tasks for the upgraded ATCA are discussed.

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Variability of the H₂O maser associated with U Orionis

Cited by 13 publications

References 26 publications

Observational evidence for the shrinking of bright maser spots

Observational evidence for the shrinking of bright maser spots

Beaming in on some stimulating issues in stellar evolution

Molecular Masers in Variable Stars

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Variability of the H2O maser associated with U Orionis

Cited by 13 publications

References 26 publications

Observational evidence for the shrinking of bright maser spots

Observational evidence for the shrinking of bright maser spots

Beaming in on some stimulating issues in stellar evolution

Molecular Masers in Variable Stars

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Product

Resources

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Variability of the H₂O maser associated with U Orionis