(Please feel free to discuss the possibility of collaborating!)

Space weather for every star in the night sky

PhD Advisor: Nicholas Law


Evryscope observes stars that flare!

Like the Sun, stars flare and emit coronal mass ejections (CME) that impact planet atmospheres. Active stars can even totally evaporate an Earth-like planet’s atmosphere! With NASA missions like TESS looking for Earth-like planets orbiting nearby stars, the space weather environment of each discovered planet is an essential factor in whether it could support life. Furthermore, stars very similar to our Sun sometimes emit very energetic superflares that could threaten life on our planet. By studying the population of nearby Sun-like stars, we can better understand why these events occur.

To measure the stellar activity for each TESS star, I apply statistics and machine learning techniques to find stellar flares in the Evryscope dataset of millions of high cadence, multi-year lightcurves. By measuring how often flares of various energies occur on each star, I can predict how often that star emits a life-threatening superflare.

Flare candidates seen by the Evryscope on TESS target stars. The number on each flare is the number of g-band magnitudes the star brightened. The color of each flare corresponds to the spectral type that best fits* the star (i.e. A=blue, F=green, G=yellow, K=purple, M=red). * fit assumes main sequence & does not account for blending or reddening

For example, Proxima Cen, the nearest star to our solar system, hosts a likely-rocky planet (Proxima b) in its habitable zone (where surface water can be liquid). We observed a superflare on March 18 2016, during which Proxima increased in brightness by a factor of ~100X and became just visible to the naked-eye under ideal conditions.

The Proxima superflare, observed by Evryscope in March 2016, during which the star increased in brightness by ~100X and irradiated the potentially-habitable planet Proxima b with lethal doses of UV radiation

We also observe 23 other large flares on Proxima and use these to demonstrate Proxima b experiences 2-5 superflares in a year, in agreement with a previous estimate by Dr. James Davenport using the MOST satellite. We plot the cumulative number of flares at a given energy (or higher) that occur in a day, versus the energy of the  flares; we plot both large Evryscope flares and lower-energy flares seen by the MOST satellite to measure how often we can expect a superflare.

 

Making Robo-AO go faint: laser-only adaptive optics

Ground-based telescopes have to look through a lot of atmosphere when observing the night sky. To get high resolution images, complex adaptive optics systems can correct for atmospheric blurring by comparing the wavefront of incoming starlight from a nearby star (called a guide star) to the expected wavefront and adjusting. These fortuitous guide stars must be fairly bright, and are not always found near the desired target star. By reflecting a laser signal at very high altitudes, a laser guide star AO (LGS AO) system can do away with the need for a natural guide star (NGS) for all but one aspect of the atmospheric correction.

This remaining aspect is called tip-tilt error, and it still requires a NGS, albeit with less stringent requirements on its brightness, allowing a higher fraction of the sky to be covered by the AO system. Tip-tilt is the random apparent motion of the star on the sky due to atmospheric effects, and removing it without a sufficiently bright NGS severely limits the sky coverage of LGS AO systems.

Typical performance of laser-only AO (no tip-tilt correction!)

Robo-AO, the first autonomous LGS AO system, observes hundreds of targets a night due to its low observation overheads, a magnitude increase in observing efficiency over classic LGS AO systems. I reduced 42,000 Robo-AO observations with no tip-tilt correction at all to test the typical improvement in effective seeing possible for laser-only AO. Turns out it’s a 39+/-19% improvement in FWHM!  Furthermore, 50% encircled-energy performance without tip-tilt correction remains comparable to diffraction-limited, standard Robo-AO performance. Faint-target science programs primarily limited by 50% encircled-energy (e.g. those employing integral field spectrographs placed behind the AO system) may see significant benefits to sky coverage from employing laser-only AO.

Check out this recent paper detailing this work: 1711.04375

Cold Atomic and Molecular Hydrogen Cloud Structure in the Galaxy:

Research Advisor: Stephen Gibson

Visual summary of our results. Image credit: Stephen Gibson

Visual summary of our results. Image credit: Stephen Gibson

It is thought that cold, dense molecular hydrogen clouds in the Milky Way condense from cold atomic clouds (HISA), but this relationship is not well understood. Molecular clouds in the interstellar medium (ISM) do not really emit light, so we “trace” them with CO emission to see where the molecular clouds lie. I compared the relative distributions of  cold neutral atomic hydrogen cloud self-absorption (HISA) in the 21.1cm-line using International Galactic Plane Survey (IGPS) data, and molecular clouds traced by CO emitting at the 2.6mm-line with other radio survey data. Significantly, while we’d expect CO tracing molecular clouds to only show up where there’s atomic gas, instead we found that the correspondence between them may be an observational effect and not real at all. Clearly, more work to understand the ISM is necessary! 🙂

IAU poster here: poster_2015_iau_gibson_full

SESAPS Talk here: