Background

Spy in the Sky: The Herschel Space Telescope with which the measurements of TW Hydrae were accomplished.

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.

HOR / MP

Go to Editor View