Of Young Stars and Ancient Planets

Since we’ve just been looking at young stars — protostars, at that — the news from Ann Arbor seems timely. Astronomers at the University of Michigan are announcing systems around UX Tau A and Lk Ca 15, young stars each, located about 450 light years away in the Taurus star formation region. What they’re actually observing at infrared wavelengths are gaps in the protoplanetary disks around these stars, the assumed result of planets sweeping the area clear of debris.

Unlike the infant star-in-the-making we looked at yesterday, UX Tau A and Lk Ca 15 are old enough — about a million years each — for planetary formation. Both are still pre-main sequence, deriving their energy from gravitational contraction instead of hydrogen-to-helium burning. To reach any conclusion about what’s happening around them, the Michigan team has to rule out photoevaporation, which is what happens when the dust and gas of a protoplanetary cloud heats up, evaporates and begins to dissipate. Catherine Espaillat rules out photoevaporation as the primary cause for what these Spitzer results have shown:

“Previously, astronomers were seeing holes at the centers of protoplanetary disks and one of the theories was that the star could be photoevaporating that material. We found that in some stars, including these two, instead of a hole, there’s a gap… It’s more like a lane has been cleared within the disk. That is not consistent with photoevaporation. The existence of planets is the most probable theory that can explain this structure.”

The paper is “On the Diversity of the Taurus Transitional Disks: UX Tau A & Lk Ca 15,” Astrophysical Journal Letters December 1, 2007 (full reference when it becomes available). An abstract is here.

Related: I sometimes ponder, as we look at systems in their infancy, whether some ancient civilization once looked on as our own Solar System formed. Some theorists, Charles Lineweaver prominent among them, have looked at how far back Earth-like planets might have formed, indicating the possibility of planets like ours fully nine billion years old, with peak planet formation about 1.8 billion years before our Sun came on the scene. So the possibility is there that we were once under infrared study from a distant world before Sol’s planets had even formed.

Or maybe not. A new paper from researchers at Southern Federal University (Russia) takes a look at star formation in terms of the appearance of metals, the latter being connected with the formation of planets. I haven’t worked through the entire paper yet (and there seem to be some problems in translation, as seen in the following excerpt), but as best I can make out, this statement from the paper’s abstract implies a different take on planetary development than Lineweaver:

Approximately 5 billion years ago average metallicity began to systematically increase, and its dispersion and the average relative magnesium abundance – to decrease. These properties may be explained by an increase in star formation rate with the simultaneous intensification of the processes of mixing the interstellar medium in the thin disk, provoke possible by interaction the Galaxy with the completely massive by satellite galaxy.

What I take from this is that the Russian team believes that stars like ours may not have been able to form in our area of the galaxy until about the time the Sun first appeared. Adam Crowl, who passed the reference along to me, points out that this would offer a take on the Fermi Paradox: They’re not here because we’re the first, or at least we’re early on the scene in a galactic neighborhood that’s relatively young in terms of living planets. It will be a while before we have a firmer grasp on just how long ago terrestrial planets might have formed in nearby space.

The paper is Marsakov, Shapovalov et al., “Star Formation History in the Galactic Thin Disk” accepted by Astrophysical Journal Letters (abstract).