Blog Post 19, Free Form Post, A Cepheid Anomaly

For this blog post, I had to read an article from either Astrobites or astro-ph that I found interesting, and summarize and discuss the result. I chose this article, titled A Cepheid Anomaly, that summarizes the paper The Strange Evolution of Large Magellanic Cloud Cepheid OGLE-LMC-CEP1812 by Hilding R. Neilson, Robert G. Izzard, Norbert Langer, and Richard Ignace.

On Worksheet 5.2, we looked at Cepheid Variable stars, a class of stars that pulsate radially in a predictable way. As there is a relationship between the Cepheid's luminosity and period of pulsation, Cepheid variable stars can be used as standard candles to determine the distances to objects. This article discusses an apparent discrepancy between predicted masses of Cepheids: the mass predicted by stellar pulsation models of a Cepheid with a set luminosity and temperature varies from that predicted by stellar evolution models. This discrepancy has been difficult to resolve, as it is not so easy to measure the mass of a star. However, one rather interesting method of measuring star masses has come in handy, known as an eclipsing binary.

An eclipsing binary is a system of two stars in which one star crosses in front of our line of sight to the other. As it turns out, if the orbits of the two stars are very close to edge-on to our point of view, we can use the variations in the light observed during this eclipse to determine the period, speed of stars in the orbit, and the size of the orbit, which can be used to find the mass using Kepler's laws! (Read more about it here) Unfortunately, eclipsing binary systems in the right orientation are very rare, and the chances of finding one with a Cepheid are even lower - there have only been four found in the LMC.

This paper focuses on a specific Cepheid, CEP1812, in a system with a red giant. While both stars are expected to be the same age, the Cepheid instead appears to be approximately 100 million years younger than the red giant. This paper suggests that this age discrepancy could be because of the Cepheid's history - they suggest that the Cepheid formed from a merging event of two smaller, main sequence stars. This would make the Cepheid appear younger, as the star formed as a result of the merger would evolve like a star that was born with the combined mass of the two smaller stars, but look younger.

The authors believe that if this is indeed the case - the Cepheid formed from a merger of two stars - then it should have a relatively short period with a mass of approximately 1 \(M_\odot\). However, using the above method of determining the Cepheid's mass, it has been found that CEP1812 has a mass of around 3.8 \(M_\odot\)! This discovery will force astronomers to reevaluate their Cepheid classifications, and proves that while we may know a lot about Cepheid variable stars and their characteristics, there is still much to learn.

See the article here: http://astrobites.org/2015/08/28/a-cepheid-anomaly/