Research Report for 2018

Professor Patrick Hartigan's research involved the formation of stars, stellar jets, laboratory astrophysics, emission nebulae, the physics of shock waves, and time-domain astronomy in 2018. A full list of publications is available on the Web, as are overview pages that describe research areas of young stars and stellar jets.

Laboratory Experiments: Hartigan concluded 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, the project explored how shock waves shape the geometry of magnetic fields. Rice graduate student Andy Liao finished his Ph.D. thesis work using numerical models to help design laboratory experiments of magnetized flows. Of particular interest were diagnostics of magnetic fields made possible with proton radiography. The experimental radiographs show how the flow sweeps up magnetic fields into concentrations that mark the locations of shocks in the flows. These fields serve as tracers to image areas of shock fronts with much better spatial resolution than is possible using simple optical cameras. The results were presented in a special issue of High Power Laser Science and Engineering.

Proton images of magnetized shock waves (see Liao at al 2018)

Massive Star Forming Regions/JWST: 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 published the results of this work in the Astrophysical Journal in 2018. Hartigan is a collaborator on one of the fifteen large proposals accepted in the first round for the James Webb Space Telescope (JWST; O. Berne PI). The proposal will study the Orion Bar region as a means to test a variety of instruments on the telescope once it launches.

Comparison of spatial profiles of H2 and H line emission along an irradiated globule in the Cygnus OB2 star-forming region (Carlsten and Hartigan 2018).

Hubble Observations of Stellar Jets: Hartigan is currently actively involved with two projects that use the Hubble Space Telescope. The first of these imaged the stellar jet HH 7-11 to determine electron densities and excitations everywhere along the flow with the spatial resolution of HST. This science can only be done by employing a rarely-used set of filters on the WFC3 camera that isolate individual emission lines. The images reveal a remarkable cavity evacuated by the flow, and show that the jet punches through sheets of material as it propagates and then deflects to the side in response to these encounters. The end of the flow is fascinating example of a combination of a molecular and atomic shock front, where the cooling zones are well-resolved. By comparing with previous images we were able to watch how the shock waves moved in real time, a great aid for interpreting the dynamics. A paper on these results is nearing completion, and should be submitted in the spring of 2019.

The second proposal, led by B. Nisini in Rome, will use HST to image the regions very close to protostars in an attempt to learn how jets are launched. The images were just acquired as of early 2019, and appear to be of high quality. These images can be combined with future studies with JWST that will probe even further through the dense, dusty disks that surround these systems.

Physical conditions within the HH 7 shock complex as observed by HST (from Hartigan et al 2019).

ALMA and Gemini 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. Graduate student Maxwell Hummel is studying the cores in the Carina region with these data.

In a parallel project, we have just acquired additional high-resolution infrared images with the 8-meter Gemini adaptive optics imager. These images are the sharpest ever of a photodissociation region, and rival that achievable with the James Webb Space Telescope when it comes on-line. Part of the science here is to understand the interface shapes, and Hartigan has been working to devise a new analysis technique along those lines.

Images of the Western Wall in Carina: left - optical/IR composite ; right - ALMA continuum and CO map of a dense core

Numerical Simulations of Jets and Shock Fronts: Hartigan collaborated with professor A. Frank and University of Rochester graduate student Eddie Hansen to develop models of supersonic magnetohydrodynamic jets, and to investigate the intersection surfaces of overlapping bow shocks. A refereed paper from this work appeared in the Astrophysical Journal in 2017. A second paper, with collaborator John Raymond at the Center for Astrophysics, tested a new cooling routine devised by Hartigan for 3D MHD codes against a well-tested 1D code that included all the atomic physics. The results were excellent, and the new cooling routine matches the 1D results while being fast enough to run in full 3D MHD mode. A paper on this work was published in 2018 in the Monthly Notices of the Royal Astronomical Society. Hartigan is working to produce a large grid of radiative shock models that can be used by researchers to predict emission line ratios. These are particularly helpful for interpreting forbidden lines, as will be needed by many studies once the James Webb Space Telescope is operational.

Comparison of 1D and 3D radiative shock models (from Hansen et al. 2018)

Forbidden Lines in Young Stars: Emission-line profiles of [O I] and [S II] forbidden lines are particularly useful diagnostics of outflow conditions close to young stars. Hartigan was a Co-I on a 2018 Astrophysical Journal paper by Fang et al. that improved upon a previous study by Hartigan et al. (1995) by acquiring and analyzing more sensitive and higher resolution spectra. The results show both the broad, and narrow-line components to the profiles as had been observed previously, but the new observations reveal the narrow-component itself is comprised of two differing components. The paper interprets these results in terms of modern disk-wind models for stellar jets.

A sample set of emission line profiles for a T Tauri star (from Fang et al. 2018)

Time Domain Studies: For the observing season 2019A, Hartigan led a team that successfully acquired a full month of time on the Blanco NOAO 4-m telescope in Chile to study variability in the Carina star formation region with the Dark Energy Camera (DECam). This imager has a full field of view of over 2 degrees, and is ideal for monitoring light variations of the thousands of young stars present in this region. Variability holds clues to many aspects of the star formation process, including rotational properties, obscuration by dusty envelopes, starspot coverage, accretion and flares. Results from the project will presage what the Large Synoptic Survey telescope may accomplish in this arena.

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. Hartigan and Co-PI Rosario led a white paper on the subject related to young stars. The entire document is now available on-line.

Star Spots: With Rice professor C. Johns-Krull, Hartigan initiated a study of the light curves of young stars with the Kepler satellite. Although Kepler has lost two of its reaction wheels, it can still be used to obtain high precision, uninterrupted light curves over the course of a couple of months. This ability is a vast improvement from analogous ground-based studies, which suffer both from much higher noise and from gaps in the temporal coverage caused by daylight and weather. The preliminary data look quite interesting, with multiple flares as well as sinusoidal fluctuations. There should be enough flares detected to provide a good comparison with solar phenomenon. Young stars have particularly strong flaring activity, and this activity will affect how the atmospheres of newly-formed planets evolve.

Campus Observatory Activities: 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 continue to provide regular public viewing opportunities througout the semester, and give general astronomy talks before each observing session. All the lectures and observing opportunities are free to the public. There was an interesting total lunar eclipse on Jan 31, 2018, but the weather here was poor. A summary of all the lunar eclipses visible from the Houston area out to the year 2060 is available. Hartigan hosted an open house on May 19, 2018 to coincide with the transits and eclipses of Jupiter's moons, and we were able to clearly see the shadow of Europa across the Jovian disk. Details about current observatory schedules can be found on the observatory website.

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Patrick Hartigan