Observing Escaping Atmospheres with the Hobby-Eberly Telescope

Atmospheric escape is the dominant physical mechanism that sculpts the entire population of short-orbit planets, but it is challenging to catch planets in the act of losing their envelopes. The helium (He) triplet at 10830 Angstrom allows us to directly measure escaping helium, and is accessible from ground-based facilities including new near-infrared high-resolution instruments including the Habitable-zone Planet Finder spectrograph on the Hobby Eberly Telescope in West Texas.

Measuring this feature for any individual planet can allow us to detect an outflow from the planet, map the speed and direction of the flow, and determine the rate of mass loss. Measurements for an ensemble of planets allow us to determine the dependence of mass loss rates on system properties, like the incident high energy radiation on the planet, the surface gravity of the planet, or the age or spectral type of the host star.

Here, we present results from our pilot studies to use the HPF instrument on the HET to detect He exospheres. We confirm a previous detection of escaping helium from a hot Jupiter, demonstrating that HPF has the requisite precision to detect escaping helium. We add several additional targets to the sample.

We also present new spectra of planet-hosting stars during non-transiting times to characterize stellar variability both inside and outside the helium triplet. The He triplet varies in depth with chromospheric activity of the host star, so it is imperative that we understand how the stellar He line varies in the absence of a planet transit in order to precisely measure the effect of the planet itself and the uncertainty in the planet signal. HET, with its queue-scheduled observations, provides a unique ability to observe these bright stars at other points during the trimester, getting snapshots of the intrinsic stellar variability for a range of stellar hosts.