Imaging the Radio Photospheres of Long-Period Variable Stars

A series of radio observations will be carried out to study the atmospheres of old stars with cool, dusty winds. The process that drives the winds is unknown, but understanding it is important, and the new observations obtained in this program will help. The amount of mass lost in dusty winds can determine the star's fate, and the material shed from the star eventually spreads into the interstellar medium. It is now possible to measure the size, shape, and gas motions in the atmospheres of these stars using the Jansky Very Large Array (JVLA) radio telescope in New Mexico. The PI and her collaborators will make such measurements and use them to understand how the cool, dusty winds are launched. An educational podcast centered on the theme "Radio Stars" (the fourth in an ongoing series) will share results from this program with the general public. The PI will mentor an undergraduate student from Haystack Observatory's Research Experience for Undergraduate program.

Using recent observations obtained with the JVLA, this program will produce the most sensitive (by an order of magnitude) images to date of the radio photospheres of a sample of five of the nearest, best-studied AGB stars. A special calibration technique will be employed to "freeze" atmospheric phase variations, providing near perfect, diffraction-limited radio seeing, and yielding images with an angular resolution of ~40 mas (~4 AU at a distance of 100 pc). The extraordinary sensitivity will allow accurate measurements of fundamental stellar parameters, including the radius and brightness temperature, which will be compared with model predictions and other multi-wavelength observations. Four of the sample stars have been imaged previously at centimeter wavelengths, and two show clear evidence for deviation from spherical symmetry. The new observations (taken at different phases of the stellar pulsation cycle) will reveal whether the photospheric shape is time-variable and will be used to distinguish between possible causes of non-spherical shapes. For the stars exhibiting SiO maser emission, simultaneous observations will provide complementary information on the atmospheric dynamics. These will provide insight into the maser pumping and excitation processes and radial variations in density and temperature.