1 research outputs found
ALMA and VLA reveal the lukewarm chromospheres of the nearby red supergiants Antares and Betelgeuse
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 (i.e., 0.35 above the
photosphere) to a peak value of 3800 K at 2.5 , after which it
then more gradually decreases to 1650 K at 11.6 . The rise in gas
temperature between 1.35 and 2.5 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 between 0.7 mm
and 1.4 cm, which we associate with chromosphere-dominated emission, while the
flux density between 4.3 cm and 10 cm, which we
associate with wind-dominated emission. 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., K)
chromospheric photoionization component with a much smaller filling factor must
also exist throughout the chromospheres of these stars.Comment: 12 pages, 6 figures, accepted for publication in A&