I (Peter Plavchan) planned and proposed these observations back in 2009 with the Spitzer Space Telescope. The Spitzer Space Telescope team accepted our proposal, and made the observations in April 2010 simultaneously with a coordinated ground-based effort with four large telescopes in the Northern and Southern Hemispheres. I remember the observing runs vividly, because I was using a telescope at Kitt Peak in Arizona along with Dr. Kevin Covey, and after collecting some data we were beset by a "major" snow fall atop the mountain:
The ground-based effort was monumental - one telescope in South America was a queue based telescope, but the other two were used by a colleague in Mexico, and colleagues who had traveled to a second telescope in South America just to get this data. We were fortunate that on one night when one telescope went down we had another telescope covering the gap in data, and we all communicated online between the various telescopes to check in and see how things were going.
The project generated so much data for the Spitzer Space Telescope that it filled up the storage space onboard the spacecraft. The Spitzer Space Telescope scheduling team, for which I am forever grateful, had to space out the campaigns every-other-night instead of every night. On the days inbetween our observations, the Deep Space Network had to communicate with the Spitzer Space Telescope and download our data to make room for the next night of observations.
In 2013, Huan Meng joined me from the University of Arizona for a six month visiting graduate student fellowship at the NASA Exoplanet Science Institute where we finished analyzing the data. After a pause to work on other projects, in 2015 Huan wrote it up and submitted it for publication. While the publication of our paper this year ended one 7 year long journey in science, it opened a whole new method for studying the inner accretion environments of young stars!
Read on for Huan's story:
However, the architecture of our own Solar System may not be representative of all planetary systems. The more we learned about exoplanetary systems, the more diverse we found they are. For example, a well-known population of planets that is absent in the Solar Sytems is "hot Jupiters." The Solar System has two gas giant planets, Jupiter and Saturn, both fairly far from the Sun. Jupiter's orbit is 5.2 Astronomical Units in radius (1 AU is the average distance between the Sun and the Earth), and Saturn's is 9.5 AU. By contrast, Jupiter's hot cousins in many exoplanetary systems are merely a fraction of an AU from their central stars and are very hot in temperature. Did these "hot Jupiters" form in-situ close to the stars? Or did they form further out and migrate inward ever since? And did that happen during or after the protoplanetary disk existed? Because the structure and evolution of the protoplanetary disk sets the initial conditions for planet formation, a crucial piece of evidence in this debate is whether there is planet-building material so close to the star.
It is long known from theories and spectroscopic observations that a protoplanetary disk cannot reach the photosphere, or "surface," of its star — The disk always has a hole in the center. Two major mechanisms can make the hole. On the one hand, gas in the inner region of protoplanetary disk is ionized and interacts with the stellar magnetosphere. If getting too close, it will be diverted off the disk plane along the magnetic field lines and accreted onto the star near the stellar magnetic poles. This is also how young stars accrete mass. An important reference for this so-called "magnetospheric truncation" of a disk is the co-rotation radius, at which the orbital period of circumstellar material matches the rotation period of the star. For mass accretion onto the star to proceed, the disk inner edge has to stay interior to the co-rotation radius and the magnetospheric truncation distance also has to be smaller. On the other hand, if solid dust particles are placed inside of a distance from the star at which their temperatures would surpass the sublimation limit of the material, the solid particles will get too hot and will be vaporized. This mechanism can also truncate the disk at the "sublimation radius," which is typically outside of the co-rotation radius. These different disk truncation radii provide a diagnostic: by measuring where protoplanetary disks get truncated and comparing with the theoretical expectations, we may tell which mechanism is at work in which disk.
So, we should just measure the sizes of the inner disk holes, right? Unfortunately, this is not as simple as laying a ruler on top of a photograph. The sizes of the inner disk holes are expected to be small. For a solar-mass young star (called a "T Tauri" star) in the nearest star-forming regions, the expected disk truncation radii are hundreds of millions of times smaller than their distances to us. They are too small to be directly resolved with the current astronomical technology. Over the past decade, the only technique that can systematically explore the inner regions of protoplanetary disks is near-infrared interferometry, for which an array of designated telescopes observes the same object, combining signals to reconstruct a partial image of the object with higher spatial resolution. To obtain a measurement from the interferometric data, people have to introduce some assumptions about the disk geometry that are not necessarily justified. Such interferometric measurements have suggested that some protoplanetary disks around the most massive young stars, called "Herbig Be" stars, are truncated by magnetospheric accretion; disks around intermediate-mass "Herbig Ae" stars can be well described by a directly heated, "puffed-up" inner rim truncated at distances that corresponds to temperatures between 1500 and 2000 Kelvin (2200 to 3100 degree Fahrenheit), a typical range of silicate dust grain sublimation temperatures. However, the trend does not extrapolate down to the regime of solar-mass T Tauri stars, many of which appear to have larger-than-expected inner disk cavities. Oversized inner disk holes around T Tauri stars have raised questions about the roles of unrecognized physical processes in addition to the two major mechanisms considered above. And there are also possible problems with the model assumptions upon which the interferometric measurements are made.
Our work is a different and novel approach to this issue. Since T Tauri stars are known to be variable stars, we can simultaneously monitor the changing stellar emission at a shorter wavelength and the disk response at a longer wavelength in the infrared. Given the constant and limited speed of light, it takes time for the variable stellar emission to travel to the disk and trigger a response, just like "light echoes." Therefore, the long-wavelength disk light curve (echoes) should lag behind the short-wavelength stellar counterpart (direct light) by the amount of the additional light-travel time1. If such a time lag is detected, we can compute the corresponding light-travel distance between the central star and its inner disk rim. Compared with interferometry, such measurements are relatively independent of model assumptions and should be more robust.
The basic idea of the method, called "reverberation mapping", has been used in extragalactic astronomy for over 20 years to measure the distance between supermassive blackholes in active galactic nuclei and their surrounding moleculuar clouds ("broad line regions"). To carry out the experiment for the first time around stars, we pre-selected a field in the rho Ophiuchi cloud complex, one of the nearest star-forming regions to the Solar System, and coordinated four ground-based telescopes to observe the area simultaneously with NASA's Spitzer Space Telescope on three nights. The ground-based telescopes, in Arizona, Chile, and Mexico, were used to monitor the stars in the near-infrared H and K wavebands (1.6 and 2.2 micron), while Spitzer worked at 4.5 micron wavelength to keep an eye on the disks. To validate any time comparison, we had to first correct the light-travel time on our receiving end, especially between the Spitzer Space Telescope and the Earth. As a result, 27 young stars were observed in the common field of view. One of the T Tauri stars, called YLW 16B, was found to vary significantly in brightness rapidly and have a time lag. This is the first detection of light echoes on the stellar scale!
Detailed analysis of the data revealed that the variable signals of YLW 16B in H and K bands were synchronized all the time, consistent with both being from the accreting gas right above the stellar photosphere. The signals at 4.5 micron lagged behind both H and K by 74.5 +/- 3.2 seconds. Interestingly, YLW 16B is a known edge-on system because of its mid-infrared molecular spectrum. Taking into account the viewing geometry, our reverberation measurement of the radius of its inner disk hole was 0.084 +/- 0.004 AU. Considering the simplifications we had used to convert the time lag to a single radius, we estimated that the total error is likely larger than the nominal one by a factor of several, on the order of 0.01 AU.
We can place our measurement in the context of previous interferometric results. See our data point in red in the figure above compared to previous work. The reverberation inner radius of YLW 16B, a solar-mass T Tauri star, is consistent with a "puffed-up" inner disk rim, in the presence of backwarming, truncated at 1500 Kelvin, a typical dust sublimation temperature. This is in line with the interferometric disk sizes measured around intermediate-mass Herbig Ae stars. But unlike the old interferometric measurements of most other T Tauri stars, YLW 16B does not have an oversized inner disk hole and does not require any additional mechanism. For the planet formation question asked earlier in the article, now we have settled one more piece of the puzzle.
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Footnote 1. It may take time for the disk to respond to changing starlight. Reflection is instantaneous once allowing for light-travel time; thermal response time of isolated and exposed dust particles should be well under a second given their tiny thermal capacities, much faster than the time resolution we can achieve in real observations; radiative transfer becomes important in the bulk disk and should take days to take effect, but it has no influence on the short timescale of our interest. Hence, the disk response time can be safely neglected for our purpose.
This is such an informative post. You have a lot of really great points. I wish
ReplyDeleteI had this post as a resource when I started blogging.
Telescopes .