The Evolving Radio Photospheres of Long-period Variable Stars

Matthews, L. D.; Reid, M. J.; Menten, K. M.; Akiyama, Kazunori

doi:10.3847/1538-3881/aac491

Cited by 25 publications

(36 citation statements)

References 68 publications

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“…Assuming that the star has a brightness of 4800 L and a temperature of 3236 K (Winters et al 2007), Homan et al (2018b) estimated the continuum stellar flux contribution (by assuming black-body emission in a bandwidth 1.6 GHz around the rest frequency of 230.5 GHz) to be 22.7 mJy. It is known that the observable extent of an AGB star increases with wavelength (Reid & Menten 1997;Matthews et al 2018). So, for EP Aqr, we can assume photospheric radius R of 166 R , or about 6 mas at 135 pc, as estimated by Dumm & Schild (1998).…”

Section: Alma Continuum Emissionmentioning

confidence: 99%

A detailed view on the circumstellar environment of the M-type AGB star EP Aquarii

Homan

Cannon

Montargès

et al. 2020

A&A

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Cool evolved stars are known to be significant contributors to the enrichment of the interstellar medium through their dense and dusty stellar winds. High resolution observations of these outflows have shown them to possess high degrees of morphological complexity. We observed the asymptotic giant branch (AGB) star EP Aquarii with ALMA in band 6 and VLT/SPHERE/ZIMPOL in four filters the visible. Both instruments had an angular resolution of 0.025″. These are follow-up observations to the lower-resolution 2016 ALMA analysis of EP Aquarii, which revealed that its wind possesses a nearly face-on, spiral-harbouring equatorial density enhancement, with a nearly pole-on bi-conical outflow. At the base of the spiral, the SiO emission revealed a distinct emission void approximately 0.4″ to the west of the continuum brightness peak, which was proposed to be linked to the presence of a companion. The new ALMA data better resolve the inner wind and reveal that its morphology as observed in CO is consistent with hydrodynamical companion-induced perturbations. Assuming that photodissociation by the UV-field of the companion is responsible for the emission void in SiO, we deduced the spectral properties of the tentative companion from the size of the hole. We conclude that the most probable companion candidate is a white dwarf with a mass between 0.65 and 0.8 M⊙, though a solar-like companion could not be definitively excluded. The radial SiO emission shows periodic, low-amplitude perturbations. We tentatively propose that they could be the consequence of the interaction of the AGB wind with another much closer low-mass companion. The polarised SPHERE/ZIMPOL data show a circular signal surrounding the AGB star with a radius of ∼0.1″. Decreased signal along a PA of 138° suggests that the dust is confined to an inclined ring-like structure, consistent with the previously determined wind morphology.

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Section: Alma Continuum Emissionmentioning

confidence: 99%

A detailed view on the circumstellar environment of the M-type AGB star EP Aquarii

Homan

Cannon

Montargès

et al. 2020

A&A

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“…They coined the term radio photosphere to describe the region of the atmosphere at ∼2 R from which the radio emission emanates, with the opacity being generated from electrons from metals with low-ionization potentials. These properties for the radio photospheres of AGB stars have now been confirmed by multiple subsequent studies (Reid & Menten 2007;Menten et al 2012;Matthews et al 2015Matthews et al , 2018Planesas et al 2016;Vlemmings et al 2019).…”

Section: Rsgs Versus Agb Stars At Radio Wavelengthsmentioning

confidence: 54%

“…Size and shape of the Antares atmosphere A number of relatively simple models were fit to the ALMA and VLA visibilities of Antares. The models ranged from uniform intensity circular and elliptical disks to slightly more complex circular and elliptical disks with one and two superimposed point sources, Gaussians and rings; all of which have been used to model radio emission from other evolved stars (e.g., Lim et al 1998;O'Gorman et al 2017;Matthews et al 2018). Uniformintensity circular disks provided the best fits to all of the VLA data, although this could be due to the fact that the star is only marginally resolved at most VLA wavelengths, but it is better resolved with ALMA, which provides better distinction between simple and more complex models.…”

Section: Resultsmentioning

confidence: 99%

ALMA and VLA reveal the lukewarm chromospheres of the nearby red supergiants Antares and Betelgeuse

O’Gorman

Harper

Ohnaka

et al. 2020

A&A

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We first present spatially resolved ALMA and VLA continuum observations of the early-M red supergiant Antares to search for the presence of a chromosphere at radio wavelengths. We resolve the free-free emission of the Antares atmosphere at 11 unique wavelengths between 0.7 mm (ALMA band 8) and 10 cm (VLA S band). The projected angular diameter is found to continually increase with increasing wavelength, from a low of 50.7 mas at 0.7 mm up to a diameter of 431 mas at 10 cm, which corresponds to 1.35 and 11.6 times the photospheric angular diameter, respectively. All four ALMA measurements show that the shape of the atmosphere is elongated, with a flattening of 15% at a similar position angle. The disk-averaged gas temperature of the atmosphere initially rises from a value of 2700 K at 1.35 R⋆ (i.e., 0.35 R⋆ above the photosphere) to a peak value of 3800 K at ∼2.5 R⋆, after which it then more gradually decreases to 1650 K at 11.6 R⋆. The rise in gas temperature between 1.35 R⋆ and ∼2.5 R⋆ is evidence for a chromospheric temperature rise above the photosphere of a red supergiant. We detect a clear change in the spectral index across the sampled wavelength range, with the flux density Sν ∝ ν1.42 between 0.7 mm and 1.4 cm, which we associate with chromosphere-dominated emission, while the flux density Sν ∝ ν0.8 between 4.3 cm and 10 cm, which we associate with wind-dominated emission. We show that the Antares MOLsphere is transparent at our observed wavelengths, and the lukewarm chromosphere that we detect is therefore real and not just an average of the cool MOLsphere and hot ultraviolet emitting gas. We then perform nonlocal thermal equilibrium modeling of the far-ultraviolet radiation field of another early-M red supergiant, Betelgeuse, and find that an additional hot (i.e., > 7000 K) chromospheric photoionization component with a much smaller filling factor must also exist throughout the chromospheres of these stars.

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“…The continuum emission can be described by two Gaussian-like components: one compact and one extended (Table 1). We can estimate the size of the continuum source at 7 mm at the same pulsation phase as during our observations (≃ 0.50) from the results by Matthews et al (2018) assuming first an uniform disk fit, using linear interpolations, and finally scaling the result to get a Gaussian size. The result is ≃(0·″035±0·″006)×(0·″025±0·″003), smaller than the size of our compact source (0·″045×0·″030). This only can occur if the star is surrounded by a continuum source that emits more at 1 mm than at 7 mm like a dusty shell (Norris et al 2012).…”

Section: Resultsmentioning

confidence: 93%

“…The ellipticity of this component is e = 1 − θ min / θ maj ≃ 0.83, where θ maj and θ min are the deconvolved major and minor axes (Table 1). Since no extended emission has been discovered at longer wavelengths (Reid & Menten 2007; Matthews et al 2018) the extended component would be produced by dust.…”

Section: Resultsmentioning

confidence: 99%

Gas infall and possible circumstellar rotation in R Leonis

Fonfría

Santander-García

Cernicharo

et al. 2019

A&A

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We present new interferometer molecular observations of R Leo taken at 1.2 mm with the Atacama Large Millimeter Array with an angular resolution up to ≃0·″026. These observations permit us to resolve the innermost envelope of this star revealing the existence of a complex structure that involves extended continuum emission and molecular emission showing a non-radial gas velocity distribution. This molecular emission displays prominent red-shifted absorptions located right in front to the star typical of material infall and lateral gas motions compatible with the presence of a torus-like structure.

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The Evolving Radio Photospheres of Long-period Variable Stars

Cited by 25 publications

References 68 publications

A detailed view on the circumstellar environment of the M-type AGB star EP Aquarii

A detailed view on the circumstellar environment of the M-type AGB star EP Aquarii

ALMA and VLA reveal the lukewarm chromospheres of the nearby red supergiants Antares and Betelgeuse

Gas infall and possible circumstellar rotation in R Leonis

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