The chemistry of baby stars is cloudy

When we use our eyes and common light detectors on telescopes, we see mostly what is called visible light, or rather, a very limited interval of wavelengths (in the order of hundreds of nanometers). In this regime, we can observe and study many objects in the sky, but not the ones that are shrouded by clouds of gas and dust, because visible light cannot pierce through these barriers: it is in fact absorbed or scattered. For that end, we invented infrared astronomy: by using different detectors, we can observe light in the infrared wavelengths (in the order of micrometers). And since we started using that, we’ve been able to peer through the curtain of gas and dust that obscured our vision for many regions of the sky.

Some of the most interesting objects that we can observe with infrared telescopes are protostars. These are objects that have already started collapsing and forming cores of dense material that emit radiation, and because they’re still very young, they are completely wrapped around clouds that did not collapse. The interaction between the radiation and the material around protostars produce an intriguing environment, where large and complex molecules can grow. The protostars will eventually turn into full-fledged stars, possibly with planets, comets and asteroids around them, and these objects will be populated by the complex molecules formed when the stars were only babies in their dusty cradles.

One way to understand how planets and stars are formed is through the chemistry of their beginnings, a field of study that is today known as astrochemistry. Although Hubble’s near-infrared detector and the Spitzer telescope produce some pretty cool images, in order to understand better the chemistry of the regions around protostars, we need to “see” the molecules themselves. And for that we have high-resolution spectrum detectors coupled to telescopes like the Herschel Space Observatory. By using spectra, we can observe the light emitted by molecules when they are excited by the radiation from the protostar, just like atoms do (electronic transitions). Unlike atoms, molecules emission comes from vibrations and oscillations in their structure, and they can happen in various ways, which produce various emission lines in the spectrum. Herschel is a single-dish telescope, which is basically an antenna that captures information on a single-pixel (just like the Arecibo Observatory). Herschel is amazing in observing spectra, but it has a downside: poor angular resolution. I mean, it’s observing a rather large region of the sky (with a beam size of ~20 to ~40”) in only a single-pixel!

One of the ways of doing astrochemistry is performing various experiments here on Earth, trying to simulate the conditions of space and then constructing models from these experiments and from the theory (such as radiative transfer). With these, we can try to figure out what is the chemical composition of an object in the sky just by looking at its spectra. And people have been doing that. In particular, this pre-print on arXiv caught my attention the other day (it was accepted for publication on The Astrophysical Journal). Apparently, there is a current trend of underestimation of abundances of complex organic molecules (COMs) on low-mass protostars when we try to model them through astrochemistry.

The general idea was that this could be caused by the rather large beam size of submilimeter telescopes (particularly Herschel), because the objects being observed were much smaller than the region being probed by the antenna. For instance, the hot corinos, which are the most dense regions of a low-mass protostar, have an angular size of less than 1”, generally. So the light coming from them gets averaged through beamsizes of 10 to 20”. The authors of this study decided then to observe two low-mass protostars with IRAM Plateau de Bure Interferometer (PdBi), which can achieve a spatial resolution of 1 to 2”. There’s a price to pay though: it has way less spectral resolution than Herschel’s instruments, so extreme care had to be taken when analyzing the spectra in order to study superimposing lines.

They detected a number of molecular lines from COMs (see table 2 on the pre-print), but some of them had to be discarded because of blending (caused by the low spectral resolution) and dubious identifications. They even had the first detection of glycolaldehyde in low-mass protostars other than IRAS 16293.

maps
Maps of the continuum emission of the two young stellar objects probed in the referred study (figure 1), in two different frequencies.

 

After the spectral analysis, the next step was constructing rotational diagrams. With these plots, we can derive how the molecules are being excited (through a parameter called excitation temperature) and their density on the line of sight (or column density). The first one is important to compare to other such objects and to estimate the actual [kinetic] temperature of the environment. The column densities are important to analyze the optical depth of the molecules, which in turn is used to model the emission of the object. Once we can model the emission, we can estimate abundances (a measure of how abundant is a molecule in relation to a another given molecule). In this case study, because methanol is the most abundant organic molecule in such objects, they used as the “given molecule” to compute the abundances of other COMs.

What they saw from the results was different from the previous observations done on single-dish. For one, the latter carried many uncertainties due to spectral contamination by weak lines and different calibration methods. In the end, the abundances from single-dish were generally overestimated compared to the ones from interferometry. When compared to other similar sources, the two protostars from this study showed lower abundances of methanol, but the authors argue that these discrepancies may arise from the different methods used to study such objects, instead of actually different chemical conditions between the protostars. This study also conclude that the abundance ratios of COMs stay relatively constant with the luminosity of protostars, which means that the low-mass ones may have a very similar chemistry as do their massive counterparts. This is a very important result.

But all these were results derived from observation. Actually, I cannot stress enough how absolutely amazing is that we can extract so much information from just tiny specks of light from the sky. But can we reproduce the same chemical conditions from our astrochemical models? The authors used a fairly recent model (Garrod 2013) and another not-that-recent-but-still-good (Rodgers & Charnley 2001). In the end, both of them predicted a lot less methyl formate than what was observed, and results for other molecules hints us about how some of these COMs can be formed in such environments (either in gas phase or as ices on dust grains).

Studying very young stellar objects (YSOs) is one of the most active fields in astronomy today, and it is exciting because there are so many things still unknown. And it gets even more interesting when we try to fuse other fields, such as chemistry. In the end, everything will (hopefully, if humanity doesn’t destroy itself) come together to an intricate understanding that will explain our very origins. This is the stars and their beauty in deconstruction.


Featured image: NGC 1333, the star-formation region where IRAS 2A and IRAS 4A (the objects of study in the referred paper) are located. Credit: R. A. Gutermuth (Harvard-Smithsonian CfA) et al. JPL-Caltech, NASA

 

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The chemistry of baby stars is cloudy

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