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The Hot Outer Layers of a Cold Star

The Sun and all stars with outer convection zones show surprising hot, tenuous outer atmospheres. These atmospheric layers are distinguished as the chromosphere (10,000 K), transition region (100,000 K), and corona (>1,000,000 K). These regions produce emissions at radio, ultraviolet, and X-ray wavelengths much stronger than is expected from a star in radiative equilibrium. All these enhanced emissions are often referred to as "activity."

How these layers in the Sun and other stars get heated remains one of the primary outstanding questions in modern astrophysics. The strong spatial correlation between these atmospheric layers and the magnetic field on the Sun suggest that the magnetic fields play a key role, but the detailed nature of that role is uncertain. Strong explosions on the Sun, solar flares, are known to result when the magnetic field on the Sun gets tangled up, producing very high temperature gas. However, the Sun also displays persistent levels of high temperature material and it is not at clear that there are enough of these solar flares to explain all the hot material in the Sun's chromosphere, transition region, and corona. While the Sun offers many advantages in these studies due to its proximity, the Sun is actually a relatively inactive star. Many of the coolest stars in the galaxy, the M stars, display much stronger activity than the Sun, and one such star is EV Lacertae. By studying extreme cases of activity, we hope to gather insight that critically tests different ideas for how the outer atmosphere of the Sun and other stars is heated.

In this study, we use X-ray observations from the Chandra satellite, ultraviolet observations from the Hubble Space Telescope plus the FUSE and EUVE satellites, and ground based radio observations to study the detailed structure of the hot outer layers of EV Lacertae and compare this structure with model of atmospheric heating. Our study suggests that continued, low level flaring is the most likely source of heating for the chromosphere, transition region, and corona of EV Lacertae.


A solar flare observed with an H-alpha filter. This image was obtained at Big Bear Solar Observatory (http://www.bbso.njit.edu). Many small versions like this are probably responsible for heating up the chromosphere, transition region, and corona of EV Lacertae.