Temporal evolution of the size and temperature of Betelgeuse’s extended atmosphere

Abstract

Spatially resolved multi-wavelength centimeter continuum observations of cool evolved stars can not only constrain the morphology of the radio emitting regions, but can also directly probe the mean gas temperature at various depths of the star's extended atmosphere. Here, we use the Very Large Array (VLA) in the A configuration with the Pie Town (PT) Very Long Baseline Array (VLBA) antenna to spatially resolve the extended atmosphere of Betelgeuse over multiple epochs at 0.7, 1.3, 2.0, 3.5, and 6.1 cm. The extended atmosphere deviates from circular symmetry at all wavelengths while at some epochs we find possible evidence for small pockets of gas significantly cooler than the mean global temperature. We find no evidence for the recently reported e-MERLIN radio hotspots in any of our multi-epoch VLA/PT data, despite having sufficient spatial resolution and sensitivity at short wavelengths, and conclude that these radio hotspots are most likely interferometric artefacts. The mean gas temperature of the extended atmosphere has a typical value of 3000 K at 2 R and decreases to 1800 K at 6 R , in broad agreement with the findings of the single epoch study from Lim et al. (1998, Nature, 392, 575). The overall temperature profile of the extended atmosphere between 2 R r 6 R can be described by a power law of the form T gas (r) ∝ r −0.6 , with temporal variability of a few 100 K evident at some epochs. Finally, we present over 12 yr of V band photometry, part of which overlaps our multi-epoch radio data. We find a correlation between the fractional flux density variability at V band with most radio wavelengths. This correlation is likely due to shock waves induced by stellar pulsations, which heat the inner atmosphere and ionize the more extended atmosphere through radiative means. Stellar pulsations may play an important role in exciting Betelgeuse's extended atmosphere.

The inhomogeneous submillimeter atmosphere of Betelgeuse

“…We find the global spectral index to be α = 1.67 ± 0.09 by separately imaging the lower (at 331.86 GHz) and upper (at 344.09 GHz) portions of the bandpass and calculating the integrated flux density in each image. This is slightly larger than the α = 1.57 ± 0.05 that can be derived from previous unresolved millimeter observations (Altenhoff et al 1994;Harper et al 2009;O'Gorman et al 2012) and is much larger than the α = 1.33 ± 0.01 that can be derived from multi-epoch centimeter studies (O'Gorman et al 2015a). We note that the NE-peak has a spectral index of 2 in the spectral index image (from CASA's clean parameter nterms = 2) but the error values per pixel may be underestimated due to the small fractional bandwidth coverage.…”

“…2 along with our ALMA measurement of 2760 ± 140 K at ∼1.3 R . This value is below the photospheric effective temperature of 3690 ± 54 K (Ohnaka et al 2011) and most of the temperature The red filled circles and blue filled diamonds represent the gas temperature derived from multi-epoch spatially resolved radio observations (Lim et al 1998;O'Gorman et al 2015a). The large black dashed rectangle is the approximate location and temperature range of the MOLsphere.…”

“…At these frequencies the free-free opacity varies as roughly ν 2 , so multi-frequency observations allow the temperature profile to be constructed. This method has been used by Lim et al (1998) and O'Gorman et al (2015a) to show that the mean gas temperature of Betelgeuse's extended atmosphere declines from approximately 3600 K at 2 R to 1400 K at 6 R , and is not as hot (∼8000 K) as previously thought (e.g., Hartmann & Avrett 1984). These values are plotted in Fig.…”

The close circumstellar environment of Betelgeuse

“…The near-infrared MOLsphere thus closely corresponds to the ALMA continuum radius R star , while the ALMA line emission reaches approximately twice the near-infrared continuum radius of the star. O'Gorman et al (2015O'Gorman et al ( , 2017 established that the apparent continuum radius of Betelgeuse decreases as a power law with wavelength between 6.1 and 0.7 cm, and breaks down at λ = 0.9 mm, the wavelength of the present ALMA observations.…”

ALMA sub-mm maser and dust distribution of VY Canis Majoris

“…Using a matching beam size 160 × 64 mas 2 , C and VY had peak flux densities of 133.9 and 71.7 (σ rms 0.9) mJy beam −1 at 321 GHz and of 474 and 296 (σ rms 4) mJy beam −1 at 658 GHz. The continuum emission is analysed further by O'Gorman et al (2014).…”