Stars, Planets, and Metal

One of the characteristics of most stars with planets is that they tend to be towards the higher end of metallicity (they have more heavy elements). This should make sense since planets are thought to have rocky cores and you can't have a rocky core without heavy elements.

So what happens when a study of low mass stars in the nearby galaxy shows that all the M class stars are metal poor?

Obviously something is seriously wrong. There's two ways this could be taken: Either the study is wrong, or our theory of planetary formation has a major flaw in it.

Much to the disappointment of young Earthers, it's much more likely that the study. The reason is that for M class stars, finding accurate metallicity is not an easy task. To understand why, let's take a look at the ways we measure metal content of stars.

The most accurate method is via spectroscopy. This method is accurate because you can get the relative strengths of all the elements present by looking at the depth of their absorption lines. That's very nifty since the way we calculate metallicity relies on those ratios.

But the difficulty with all spectroscopy is that it must be performed on each star individually, unlike photometry which allows for quicker data, but with larger errors without careful calibration.

Fortunately, there exist photometric methods by which we can determine metal content. In the photometry I talked about in the post I just linked to, the filters used are carefully selected to avoid as many of the absorption lines as possible to get accurate measurements of the blackbody spectrum. However, if we instead choose filters to hit the pits of these absorption lines for the elements we're interested in, we can use those to determine our metallicity.

The trouble is that for small, cool stars, like the M class stars in question, there's lots of absorption lines. In fact, there's so many, there's almost no distinguishable continuum. Lines can overlap which makes spectroscopy, let alone photometry, exceptionally difficult.

To help try to improve things, the authors of the study used the fact that there's a relation between how much metal a star has and how much it's moved off the main sequence (metals tend to make a star slightly redder and scoot it right on the HR diagram). Piecing this together with photometric data, they argued that they had reliable metallicity estimates.

However, a recent paper is suggesting that the estimates may not be so reliable after all. It tested the older study to see if it could accurately estimate the metallicity of several stars for which they had higher quality measurements. The new study found the old significantly underestimated the metallicity!

Whew! Planetary formation theory is safe!

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Bonfils, X., Delfosse, X., Udry, S., Santos, N., Forveille, T., & Ségransan, D. (2005). Metallicity of M dwarfs Astronomy and Astrophysics, 442 (2), 635-642 DOI: 10.1051/0004-6361:20053046

Johnson, J., & Apps, K. (2009). ON THE METAL RICHNESS OF M DWARFS WITH PLANETS The Astrophysical Journal, 699 (2), 933-937 DOI: 10.1088/0004-637X/699/2/933