TW Hydrae: There's more to astronomers' favorite planetary nursery than previously thought
Using ESA's Herschel Space Telescope, astronomers including Thomas Henning from the Max Planck Institute for Astronomy in Heidelberg have used a new method to determine the mass of the planetary nursery around the star TW Hydrae. At a distance of merely 176 light-years from Earth, this is the closest star that is currently forming new planets – hence one of the most important objects for astronomers studying planet formation. The precise new measurement shows a much larger mass for TW Hydrae's disk than in previous studies, indicating that the system could be forming planets similar to those of our own Solar System. The study is published in the January 31 issue of the journal Nature.
Figure 1: Artist's impression of the gas and dust disk around the young star
TW Hydrae. New measurements using the Herschel space telescope have shown
that the mass of the disk is greater than previously thought.
Where Egyptologists have their Rosetta Stone and geneticists their Drosophila fruit flies, astronomers studying planet formation have TW Hydrae: A readily accessible sample object with the potential to provide foundations for an entire area of study. TW Hydrae is a young star with about the same mass as the Sun. It is surrounded by a protoplanetary disk: a disk of dense gas and dust in which small grains of ice and dust clump to form larger objects and, eventually, into planets. This is how our Solar System came into being more than 4 billion years ago.
What is special about the TW Hydrae disk is its proximity to Earth: at a distance of 176 light-years from Earth, this disk is two-and-a-half times closer to us than the next nearest specimens, giving astronomers an unparalleled view of this highly interesting specimen – if only figuratively, because the disk is too small to show up on an image; its presence and properties can only be deduced by comparing light received from the system at different wavelengths (that is, the object's spectrum) with the prediction of models.
In consequence, TW Hydrae has one of the most frequently observed protoplanetary disks of all, and its observations are a key to testing current models of planet formation. That's why it was especially vexing that one of the fundamental parameters of the disk remained fairly uncertain: The total mass of the molecular hydrogen gas contained within the disk. This mass value is crucial in determining how many and what kinds of planets can be expected to form.
Previous mass determinations were heavily dependent on model assumptions; the results had significant error bars, spanning a mass range between 0.5 and 63 Jupiter masses. The new measurements exploit the fact that not all hydrogen molecules are created equal: Some very few of them contain a deuterium atom – where the atomic nucleus of hydrogen consists of a single proton, deuterium has an additional neutron. This slight change means that these "hydrogen deuteride" molecules consisting of one deuterium and one ordinary hydrogen atom emit significant infrared radiation related to the molecule's rotation.
The Herschel Space Telescope provides the unique combination of sensitivity at the required wavelengths and spectrum-taking ability ("spectral resolution") required for detecting the unusual molecules. The observation sets a lower limit for the disk mass at 52 Jupiter masses, with an uncertainty ten times smaller than previous result. While TW Hydrae is estimated to be relatively old for a stellar system with disk (between 3 and 10 million years), this shows that there is still ample of matter in the disk to form a planetary system larger than our own (which arose from a much lighter disk).
On this basis, additional observations, notably with the millimeter/submillimeter array ALMA in Chile, promise much more detailed future disk models for TW Hydrae – and, consequently, much more rigorous tests of theories of planet formation.
The observations also throw an interesting light on how science is done – and how it shouldn't be done. Thomas Henning explains: "This project started in casual conversation between Ted Bergin, Ewine van Dishoeck and me. We realized that Herschel was our only chance to observe hydrogen deuteride in this disk – way too good an opportunity to pass up. But we also realized we would be taking a risk. At least one model predicted that we shouldn't have seen anything! Instead, the results were much better than we had dared to hope."
TW Hydrae holds a clear lesson for the committees that allocate funding for scientific projects or, in the case of astronomy, observing time on major telescopes – and which sometimes take a rather conservative stance, practically requiring the applicant to guarantee their project will work. In Henning's words: "If there's no chance your project can fail, you're probably not doing very interesting science. TW Hydrae is a good example of how a calculated scientific gamble can pay off."
The results have been published as E. A. Bergin et al., "An Old Disk That Can Still Form a Planetary System" in the January 31 edition of Nature. Original article
The co-authors are Edwin A. Bergin, L. Ilsedore Cleeves (both University of Michigan), Uma Gorty (SETI Institute and NASA Ames Research Center), Ke Zhang, Geoffrey A. Blake (both Caltech), Joel D. Green (University of Texas, Austin), Sean M. Andrews (Harvard-Smithsonian Center for Astrophysics [CfA]), Neal J. Evans II (University of Texas, Austin), Thomas Henning (Max Planck Institute for Astronomy), Karin Öberg (CfA), Klaus Pontoppidan (Space Telescope Science Institute), Chunhua Qi (CfA), Colette Salyk (NOAO), and Ewine F. van Dishoeck (Max Planck Institute for Extraterrestrial Physics and Leiden Observatory).
Herschel Observations were performed as part of the Herschel Open Time Programme "A New Method to Determine the Gas Mass in Protoplanetary Disks" led by Edwin Bergin.
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. PACS has been developed by a consortium of institutes led by MPE (Germany) and including: UVIE (Austria); KU Leuven, CSL, IMEC (Belgium); CEA, LAM (France); MPIA (Germany); INAF-IFSI/OAA/OAP/OAT, LENS, SISSA (Italy); IAC (Spain). This development has been supported by the funding agencies BMVIT (Austria), ESA-PRODEX (Belgium), CEA/CNES (France), DLR (Germany), ASI/INAF (Italy), and CICYT/MCYT (Spain).
What was the problem with previous mass measurements?
Whenever astronomers want to estimate the abundance of some compound, they search for characteristic light announcing the compound's presence. But this doesn't work for molecular hydrogen, as hydrogen molecules do not emit detectable radiation. Previous methods relied on indirect tracers to deduce the amount of hydrogen present – measuring either the abundance of carbon monoxide or of dust, and using additional measurements and models to infer the abundance of molecular hydrogen.
Mass estimates based on the thermal emission from dust grains in the disk require assumptions about the opacity of the dust; this value, however, can change dramatically as dust clumps into larger and larger grains, leading to large uncertainty. Adding to the uncertainty are assumptions of the gas-to-dust ratio, a correction factor derived from measurements of the interstellar medium.
Mass estimates based on the presence of CO are troubled by the fact that the disk is opaque to this type of radiation. Observations can only show the surface of the disk; their relation to the bulk of the disk then must be inferred using a suitable model. Depending on the model chosen, the widely varying mass values cited in the main text are obtained (between 0.5 and 63 Jupiter masses).
How was the new mass measurement made?
The new measurements exploit the fact that, while ordinary hydrogen molecules do not emit measurable radiation, hydrogen deuteride – hydrogen molecules in which one of the atoms is deuterium – emit radiation associated with rotational degrees of freedom which is a million times stronger than for ordinary molecular hydrogen. Its intensity depends on the temperature of the gas; this temperature was measured via ALMA observations of carbon monoxide (CO J = 3 → 2).
The ratio of deuterium to hydrogen appears to be constant in our cosmic neighbourhood, as a survey of objects with distances of less than about 300 light-years from the Sun shows (Linsky 1998). Detect the hydrogen deuteride and multiply by this ratio, and you will get a good estimate for the total amount of molecular hydrogen present. Should some of the deuterium atoms be hidden in more complex molecules (notably polycyclic aromatic hydrocarbon) or in molecular ice, or should parts of the disk be opaque for this kind of radiation, the estimate will be too low; that is one reason why the current result is presented as a lower limit.
The temperature estimate is derived from CO lines and thus is likely to be somewhat too low – it probes material near the surface of the disk, which, if anything, should have a higher temperature than the deeper regions from which the hydrogen deuteride lines originate. In this way, all the corrective factors serve to push the mass above the given conservative limits; this is why, as a lower limit, the current mass estimate is very reliable.
Why was Herschel important for this kind of measurement?
The fundamental line of hydrogen deuteride (J = 1 → 0) has a wavelength of 112 µm, placing it firmly in the far-infrared region of the spectrum. This kind of radiation is absorbed by water vapour in the atmosphere, and can only be observed from space or from the stratosphere, leaving the Herschel Space Telescope and the flying observatory SOFIA.
With SOFIA, observations of this particular line could be possible under optimal conditions and scheduling ample of observing time (which would have been unlikely to get approval, given that some models predicted such observations would see nothing). With Herschel, the combination of 36 observations with a total exposure time of nearly 7 hours on November 20, 2011, detected the J = 1 → 0 line unambiguously (at the 9σ level).
The observations used Herschel's instrument PACS ("Photodetector Array Camera & Spectrometer"), a combination of camera and spectrograph for wavelengths between 57 and 210 µm. The instrument was developed and constructed by a consortium led by the Max Planck Institute for Extraterrestrial Physics in Garching, with key contributions by the Max Planck Institute for Astronomy in Heidelberg.
Is this kind of measurement likely to establish itself as a standard method?
Lines of this kind are difficult to detect. This is only the second time hydrogen deuteride has been detected outside our Solar System, the first being an observation with the ISO satellite within the Orion nebula (Wright et al. 1999). Thus, the present is result to remain a special case – albeit with far-reaching consequences for our understanding of planet formation.