Research Report for 2016

Professor Patrick Hartigan's research involved the formation of stars, stellar jets, laboratory astrophysics, emission nebulae, and the physics of shock waves in 2016. Press releases of selected research papers and a full list of publications are available on the Web.

Extrasolar Planets and Star Spots: Led by Rice faculty member Chris Johns-Krull, Hartigan and a team of scientists and students from Rice and Lowell Observatory continued a search for radial velocity perturbations that might be caused by planets around young stars. Sensitive searches for older planets have turned up many such objects, but finding them around young stars is difficult because starspots, which are common when stars are young, often mimic the radial velocity signatures of planets.

Using telescopes at Kitt Peak National Observatory, Mauna Kea, and McDonald Observatory, our team discovered that small periodic variations of radial velocities exist in the infrared spectra of the young star CI Tau. These variations have no optical counterpart as they would if spots were responsible, implying that the most likely explanation is the existence of a massive planet close to this star. A paper published in the Astrophysical Journal in 2016 summarizes the observations in support of this result.

Laboratory Experiments: Hartigan continued to lead a collaboration with scientists at the University of Michigan, University of Rochester, MIT, and Imperial College, to study shock waves in the laboratory. Funded by the DOE, one project uses the Omega laser to launch jets and observe what happens when strong shock waves intersect. A separate project explores what happens to shock waves in a plasma where magnetic field strengths are large. We completed the intersecting shock work at Omega in 2015, and published a paper summarizing several years of work in the Astrophysical Journal in 2016. Graduate student Andy Liao continued his Ph.D. thesis work using numerical models to help design laboratory experiments of magnetized flows. Of particular interest are diagnostics of magnetic fields made possible with proton radiography, and we have obtained some very interesting radiographs in the past year that use magnetic fields as a tracer to image areas of shock fronts with much better spatial resolution than is possible using simple optical imaging. Our team is currently analyzing additional data that use spatially-resolved Thomson scattering to follow the dynamics of the gas as it flows through a strong shock. Hartigan was part of a complementary effort led by Chikang Li at MIT that resulted in a paper published in Nature Communications in 2016.

Another major ongoing experimental effort to study shock waves uses the pulsed-power machine at Imperial College. Much of this effort is devoted to understanding the physics of thermal instabilities and turbulence in shock waves. Both of these processes generate dense clumps in the hot postshock gas, something we observe quite frequently in high-resolution images of astrophysical jets. Together with the team led by Suzuki-Vidal, Hartigan published a paper in the Astrophysical Journal reporting the best example to-date of a thermal instability in a laboratory shock wave. By studying the behavior of such systems in the lab we can better understand and identify this process when we observe it within the strong shock waves that are commonplace throughout the interstellar medium, in star-forming regions, and within the remnants of supernovae explosions.

Massive Star Forming Regions: As part of a large work related to molecular hydrogen emission in the Carina star-formation region published in the Astronomical Journal in 2015, a group led by Hartigan noticed that there was a consistent offset between emission from molecular hydrogen and that from atomic hydrogen. Additional data on the CygOB2 and IC 1396 region confirm this result as a general characteristic of photodissociation regions. Rice undergraduate Scott Carlsten wrote his senior thesis on this topic (winning the Departmental Heaps Prize for it), and we are working on a paper that describes these results.

ALMA Studies of Photodissociation Regions: With Rice assistant professor Andrea Isella, Hartigan obtained deep maps of a bright region of photodissociation in the Carina Nebula at millimeter and sub-mm wavelengths with the Atacama Large Millimeter Array in Chile. The array has recently been configured to operate at high enough frequencies to observe emission from the 600-micron line of C I. Together with existing H2 observations and new ALMA CO and CS maps of the region, we now have the first high-resolution maps that trace gas from where it is molecular deep within the dark cloud, to its atomic stage, to where it becomes ionized at the dissociation front, a process which occurs in all regions of massive star formation. The data have just been acquired, and we are looking to analyze it fully in 2017. Additional high-resolution images should become available this spring, as a proposal led by Hartigan for time on the 8-meter Gemini adaptive optics imager was just accepted. The adaptive optics imager should provide the sharpest image ever of a photodissociation region, and will rival that achievable with the James Webb Space Telescope when it comes on-line.

Numerical Simulations of Jets and Shock Fronts: Hartigan has been collaborating with professor A. Frank and University of Rochester graduate student Eddie Hansen to develop models of magnetohydrodynamic flows of pulsed supersonic jets, and to investigate the intersection surfaces of overlapping bow shocks. Two refereed papers resulted from this work appeared in 2015, and another was just accepted for publication in the Astrophysical Journal and should appear in 2017. This new paper compares the simulation results directly with observations from the Hubble Space Telescope. We are also working on another work to compare non-LTE emission line cooling routines in 3-D codes with those predicted from steady-state 1-D codes.

Large Synoptic Survey Telescope and a future UV Space Telescope: Hartigan is part of the Transient and Variable Stars group of the Large Synoptic Survey Telescope, a facility under construction in Chile that will survey the entire sky visible from that location every three days. These data will usher in a new era of `time-domain astronomy', requiring astronomers to sort through vast amounts of data in a short time. The LSST subgroups are tasked with optimizing the time cadences for their objects, and Hartigan led a white paper on the subject related to young stars.

A separate effort, led by Paul Scowen at ASU, proposes to replace the Hubble Space Telescope with another orbiting spacecraft optimized for optical and ultraviolet observations. Hartigan is a member of that group, and has helped to define what the proposed telescope would contribute to knowledge in the field of nebular astrophysics.

Campus Observatory Activities: It was a another busy year at the campus observatory, with a great deal of public interest focused on the `supermoon' of November 2016. The event was interesting, but not nearly so remarkable as the press made it out to be. In an effort to clarify the situation, Hartigan wrote up a summary article on the supermoon that was picked up by some newspapers. We also ran several open houses in 2016 and will continue to do so in 2017. We are attempting to host a public talk before the open houses as a means for the Houston public to interact directly with Rice astronomers.

The big news for the public in 2017 will be the lovely total solar eclipse that will cut across the central part of the US on August 21. Hartigan pland to give a talk regarding this event sometime in the spring of 2017. A summary of all the lunar eclipses visible from the Houston area out to the year 2060 is also available.

The Rice University Campus Observatory (RUCO), located on the roof of the Brockman Physics building continues to be the workhorse for our undergraduate major and nonmajor courses. We now have a new CCD imager we will be testing this year that complements our existing spectrograph. These are used students as part of their undergraduate training. After many decades of having equipment scattered in multiple locations, we are excited about consolidating our equipment in a single storage location, a process that is nearing completion. Details about current observatory schedules can be found on the observatory website.

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Patrick Hartigan