Research

My current research interests range from the interactions of young stars with their circumstellar disks to exoplanet atmospheres and the interaction of a planet’s magnetic field with the wind of it’s parent star.

Publications

If you’d rather just browse papers I’ve contributed to, click this ADS link. If you don’t have access to the journals, check out pre-published (but complete) paper versions here at the arXiv.

Data

I do not currently have any of my data uploaded to the website. If you would like spectra from any of my papers, please contact me and I will send them to you.

Exoplanet atmospheres

The number of confirmed exoplanets currently stands near 3450 (NASA Exoplanet Archive; see exoplanets.org for a different count). While we have reliable radius and mass estimates for many of these objects, the atmospheres of most exoplanets are unexplored. Atmospheric content, temperature, and density are all fundamental parameters that must be know in order to fully characterize the planet.

We use high resolution optical and UV spectra to search for signatures of atomic absorption in some of the hottest (i.e., shortest orbital period) known exoplanets around nearby stars. One of the strongest signatures in a class of exoplanets called Hot Jupiters is due to atomic hydrogen, a product of hydrogen’s large abundance. Excited hydrogen has strong lines at optical wavelengths (the Balmer lines) and the shape and strength of these lines can be measured using spectrographs such as HiRes on the Keck telescopes and HRS on the Hobby Eberly Telescope at McDonald Observatory (Jensen et al. 2012, Cauley et al. 2015). Atomic sodium is also a strong absorber in Hot Jupiter atmospheres (see Redfield et al. 2008 and Jensen et al. 2011). We currently have ongoing observational programs to measure these lines in an ever increasing number of short period exoplanets.

Rotation of HD 189733 b

The rotation period of an exoplanet remains an elusive property that is not likely to be measured for many objects in the near future. This fundamental parameter of the exoplanet, however, is important in understanding the global climate and atmospheric dynamics. Thus a complete characterization of an exoplanet needs to include the rotation period, or, in other words, the length of the day of the planet, to be comprehensive. Plus, this is just a very cool thing to discover about an alien world!

Recently, a handful of papers have explored ways to measure the rotational velocity of the hot Jupiter HD 189733 b (see Louden & Wheatley 2015 and Brogi et al. 2016). The rotation of the planet broadens the observed spectral lines due to the Doppler shifts of the different pieces of the atmosphere. This is called rotational broadening and has long been used as an accurate measurement of stellar rotation rates modified by the inclination of the stellar rotation axis with respect to our line of sight. Hot Jupiter rotation periods are expected to synchronize with their orbital periods on very short timescales (~1 million years). This means that a single hemisphere of the planet faces the host star at all times, similar to the Moon’s orbit around Earth. This synchronous rotation should result in equatorial rotational velocities (the straight-line speed of the gas at the planet’s equator) of about a few kilometers per second. This is very close to what is observed for HD 189733 b by Louden & Wheatley 2015 and Brogi et al. 2016.

We are currently working on a similar analysis for the hydrogen line data that we have collected for two transits of HD 189733 b. Below is a video of the models we are using to attempt to measure the velocities in the upper atmosphere of the planet. The blue points represent matter that is moving towards us, the observers, while the red points represent stuff that’s moving away from us. The lightly colored background is the host star, HD 189733. The darkness of the colors roughly traces the magnitude of the velocity. For example, the dark blue at the beginning of the video means the planet is moving towards us at ~15 km/s. White represents material that is stationary along the line of sight. The atmosphere of the planet looks like a ring around the planet. You can watch the combination of the planet’s orbital velocity and the rotational velocity in the atmosphere produce different shades of color relative to the bulk of the planet during the transit. The velocity of the observed spectral line, a hydrogen line in this case, can be seen in the bottom panel and the line profile itself is embedded in the upper-right. The velocity of the line is suppressed when the planet first starts transiting because the rotation of the atmosphere away from us cancels some of the planet’s bulk orbital velocity towards us. The same thing happens at the end of the transit.

While the planet may not be rotating faster than the synchronous rate, large wind and jet velocities in the upper atmosphere may be probed by absorption lines in these layers. We are still exploring how useful these measurements will be so stay tuned!

Star-planet interactions (SPIs)

Planets with orbits of ~5 days or less can interact strongly with their host stars. These short period planets can raise tides on the star which can cause an increase in stellar activity. If the planet is magnetized, the planetary magnetosphere can interact with the stellar magnetosphere, injecting energy into the stellar atmosphere and potentially causing more energetic flares and CMEs (see Cuntz et al. 2000). There is some observational evidence of stellar activity that is modulated by the orbit of the star’s giant planet (e.g., see Shkolnik et al. 2008). Although we have good knowledge of the magnetic fields of many stars, we currently have very limited knowledge of exoplanetary magnetic fields.

Our recent work has focused on a promising observational technique for estimating the magnetic fields of short period planets that are moving through the wind or corona of their host star. If the planet is moving supersonically through the stellar wind or coronal plasma, a bow shock will form between the planet and the star at an angle that is determined by the relative velocity of the planet and the plasma. If the planet has a magnetosphere, the bow shock will form where the pressure between the plasma and the magnetosphere balance. For planet’s with strong magnetic fields, the bow shock can form many planetary radii ahead of the planet in it’s orbit. If the compression of the stellar wind material in the bow shock is high enough, the line-of-sight column density of material in the bow shock between us and the star can be high enough to produce a visible absorption signature in the stellar spectrum. This absorption signature occurs before the planet normally transits the star.

We are using high resolution optical spectra to look for these pre-transit signatures in Hot Jupiters. Our recent work in Cauley et al. 2015 shows that such a signature is consistent with the geometry of a bow shock ahead of the Hot Jupiter HD 189733 b. The video below shows the data from Cauley et al. 2015 (colored symbols), the transit of the bow shock material as it crosses the stellar disk, and the absorption values the model predicts (solid colored lines in the bottom panel). The upper panel shows the plane of the sky perpendicular to our line of sight. The right side panel shows a top-down view of the planet’s orbit. All of the sizes and distances are to-scale. The parameters in our model that best fit the observed absorption allow us to estimate the magnetic field of the planet. These measurements demonstrate an exciting path forward in attempting to measure exoplanet magnetic fields.

 

While the bow shock interpretation is exciting, we recently completed the analysis of a second HD 189733 b data set. While there is evidence of a pre-transit signal, the new transit does not exhibit a similar signal to that shown in the video above. This immediately tells us that the signal is variable: different things are happening during different planetary orbits! This also tells us that the strong magnetic field inferred from the bow shock model is most likely incorrect: if the field was that strong, we would expect a similar signal to appear in the pre-transit data every time that portion of the orbit is observed. So what’s causing the new pre-transit signal?

Below is a video of a different interpretation for the new data. The colors and panels are the same as the first video except this time what we’re modeling is the transit of two accretion clumps, or blobs of material that are spiraling towards the star. They produce relatively shallow transit depths and have a distinctly different transit shape compared to the bow shock geometry. We are currently investigating whether or not the pre-transit signatures can be caused purely by variations in the stellar activity level. To do this, we are attempting to take high-cadence observations of the Balmer lines outside of the pre- and in-transit phase portions of the planet’s orbit. Check back in a few months for a potential update!

In order to test whether or not the pre-transit signals are normal variations in the stellar activity level, we collected 5 nights of out-of-transit H-alpha data for HD 189733. If we see similar changes to the 2013 and 2015 pre-transit signals when the planet is not near transit then we can conclude with reasonable certainty that we are capturing fairly standard changes in the stellar activity level. On the other hand, if changes of this magnitude only occur near transit phase then we are more likely witnessing either absorption by circumplanetary material or a change in the stellar activity level caused by the planet, i.e., an SPI.

Below is the phased out-of-transit H-alpha data and a diagram of the planet’s orbit during the observations. Although there is significant scatter in the data, mainly due to the fairly low signal-to-noise of the spectra, you can see that there does not seem to be any changes similar to what is observed during the 2013 and 2015 Keck transits.

allphase_plus

Phased H-alpha data for five out-of-transit monitoring nights of HD 189733. A diagram of the planet’s orbit is shown on the right. There are no variations in the H-alpha measurement similar to what is seen for the 2013 and 2015 Keck nights.

This is confirmed if we compare the absolute deviations from the median for each night: the deviations from the out-of-transit nights are statistically different from the pre-transit signals.

 

The contrast effect for transiting exoplanets

When we observe a planet in transit, we are measuring light coming from a smaller fraction of the stellar surface than we observe when the planet is out-of-transit. Based on differences between the measured in-transit light and out-of-transit light, we can deduce things not only about the planet but also about the portion of the star that the planet is occulting. For broadband transits, astronomers have long been able to detect starspots as exoplanets pass over them during transit (see Pont et al. 2007 and Mahler-Fischer et al. 2013 for good examples): the spot is cooler than the surrounding stellar atmosphere, and so it emits less light, which results in less of the total light from the star being blocked by the planet compared to when it is not transiting a spot. This produces a bump in the transit light curve.

We can also use in-transit spectroscopic observations to learn something about the occulted portion of the star. In this case, however, we are not looking for bumps in the light curve but for changes in individual spectral lines, such as Na I or Hydrogen-alpha (Hα). Since different features (e.g., starspots, faculae and plage regions) on a star emit different types of spectra, a transiting exoplanet changes the observed spectrum, by weighting it towards one type of spectrum or another, as it occults various features on the stellar surface. When we compare individual in-transit observations with out-of-transit observations, we might see distinct changes in spectral lines due to the difference in the spectrum blocked by the planet. We call this the contrast effect and it is especially important for exoplanets transiting active stars, which have more of their surface covered by different features compared to a quiet star like our Sun.

We have been investigating the contrast effect for the active exoplanet host HD 189733 in order to determine whether or not our Hα observations can be attributed to the planet’s atmosphere or the stellar surface. Below are some transit animations showing the contrast effect for various simulations of HD 189733’s surface. In these animations, we show the planet transiting an active stellar surface and the planet has no atmosphere. In the upper-right panels, you can watch the ratio of the in-transit to out-of-transit Hα spectrum evolve as a function of transit time. In the lower-right, we show a measure of how strong the observed Hα ratio is as a function of time. Also shown in that panel are measurements from two of our papers (gray lines). The left panel shows the stellar surface and the transiting planet. We start with a less active stellar surface and move to a very active stellar surface.

While there are many details that go into the animations, you can probably see from these examples that we need a very active surface to reproduce the observed Hα profiles. This investigation is ongoing and we are currently extending the contrast work to other spectral lines, such as Ca II, so stay tuned!

 

Accretion and outflows around young stars

When our Sun was very young (a few million years old) it would have belonged to a class of objects we call T Tauri stars (TTSs) that live in star forming regions like the Orion Nebula which is featured as the background picture on this site. T Tauri stars are young, low mass stars that have not yet reached their main sequence hydrogen burning states, i.e., they are pre-main sequence stars and are still gravitationally contracting. The subclass of these young objects called classical T Tauri stars (CTTSs) are surrounded by massive disks of dust and gas which are the nurseries for planet formation. CTTSs are also strongly magnetic, hosting ordered magnetic fields that are many times stronger than our Sun.

As CTTSs contract they also accrete and eject material from their disks. The accretion can occur along magnetic field lines (i.e., magnetospheric accretion) where the material in the disk falls ballistically towards the surface of the star and rams into the atmosphere, heating up and emitting ultraviolet and x-ray photons. In addition to accretion, CTTSs show strong evidence of outflowing material. These outflows are directly tied to the accretion process, although there is ongoing research into exactly how they are launched.

My PhD thesis work focused on the difference between the accretion and outflow properties of CTTSs and another class of pre-main sequence stars called Herbig Ae/Be stars, after the late astronomer George Herbig who first identified and grouped these objects. Herbig Ae/Be stars, or HAEBES, are the intermediate mass cousins of CTTSs. Unlike CTTSs, HAEBES, in general, do not have strong magnetic fields. If accretion processes and outflows around young stars are mediated by magnetic fields, we would then expect the general accretion and outflow properties of CTTSs and HAEBES to be different. This is indeed what we find: HAEBES exhibit a lower fraction of objects with evidence of both accretion and outflows (see Cauley & Johns-Krull 2014 and Cauley & Johns-Krull 2015). More interestingly, the HAEBES in our sample which show signs of magnetospheric accretion are accreting material from deeper in their gravitational potentials than CTTSs. This results in less energy being available from the accretion flow to launch outflows from near the surface of the star. This result is in agreement with the weaker magnetic fields observed for HAEBES: weaker magnetic fields result in smaller disk truncation radii, resulting in disk material accreting from closer to the stellar surface.

One potential consequence of smaller disk truncation radii in HAEBES is the migration of planets towards the star. It was proposed by Lin et al. 1996 that inward planet migration can be halted when the planet migrates interior to the disk truncation radius. If HAEBES have smaller disk truncation radii than CTTSs, and the disk truncation radius plays a significant role in halting inward planet migration, then we would expect to see a difference in the period distribution of planets around A and B stars compared to lower mass G, K, and M stars. The number of planets known around A and B stars is currently small. It will be interesting to test this hypothesis once the statistics are more complete.

Angular momentum regulation in young stars

As described in the previous section, CTTSs have strong, ordered magnetic fields that interact with their circumstellar disks and accrete and eject material from the system. One consequence of the accretion and outflow process is that angular momentum is essentially lost from the system. The net result is to prevent the star from spinning up and rotating faster, the result that we expect if the star is left to contract on its own. Observationally, we see this effect manifested in the rotation periods distributions of CTTSs: the rotation periods are, on average, longer than they would be if the star was left to contract on its own. Thus the removal of angular momentum from the system by outflows is critical to the final spin state of the star when it emerges from the CTTS phase.

One possible result of the tug-of-war between the star and the disk is a state where the star is “locked” to the disk, i.e., disk locking. Disk-locked stars rotate at the same rate as the material in Keplerian orbit at the truncation radius. A truly disk-locked star would be in equilibrium with its disk and the net torque on the star would be zero. The more likely scenario, and one that is realized in numerical simulations of star-disk interactions, is that the star is constantly being pulled back and forth by its interaction with the disk. Thus the system is never truly in an equilibrium disk-locked state. Evidence has been found, however, that relationships between stellar and disk parameters as predicted by disk-locking theory hold in a general sense. In other words, the assumption of disk-locking more closely explains the relationships seen between the parameters than a non-disk-locked system (Johns-Krull & Gafford 2002,Cauley et al. 2012). In addition, the long-period peaks in CTTS rotation period distributions of very young clusters occur at approximately the rotation period expected for stars in a disk-locked state (~8 days; see Herbst et al. 2002Lamm et al. 2005). Thus measuring how the rotation period distribution changes with time by observing clusters of various ages can allow us to understand how young stars regulate angular momentum.

Myself, Patrick Hartigan, and Chris Johns-Krull are currently working on determining the rotation period distribution of TTSs in the very young star forming region Carina. We are using high cadence photometry taken with DECam on the CTIO 4-meter telescope to measure rotation periods for all of the X-ray objects identified in the Chandra Carina Complex Project (CCCP, for short; Townsley et al. 2011). These X-ray objects have a high probability of being low mass cluster members. By measuring the rotation periods of a large number of objects, we can compare the period distribution with models of angular momentum regulation in young stars. This unique data set is made possible by DECam’s huge field of view: we essentially observe the entire nebula every minute, resulting in simultaneous photometry for the thousands of identified X-ray sources.