Astronomers Discover Stars with Carbon Atmospheres

Astronomers at the University of Arizona have discovered a new class of white dwarfs, ones with carbon atmospheres. Most stars end their lives as white dwarfs (with only a few of them ending up exploding as supernovae). It is believed that most white dwarfs have cores consisting of oxygen and carbon which are obscured by the hydrogen and helium atmosphere surrounding them.

Recently however, astronomers have found stars with "no detectable traces of helium and hydrogen in their atmospheres" (Patrick Dufour). Dufour along with other astronomers at the University of Arizona believe that they may be seeing the bare cores of these white dwarfs. Found using the Sloan Digital Sky Survey (SDSS), white dwarfs such as these (which displayed carbon features) are dubbed DQ's (carbon white dwarfs). When observed via optical light, DQ white dwarf stars appear to be mostly carbon and helium. It is hypothesized that convection within the star causes carbon from the core to be dredged-up to the surface.

So far, eight carbon-dominated atmosphere DQ white dwarfs have been found among the 200 that Dufour and his team have searched. However, these stars lie at temperatures between 18,000 and 23,000 Kelvin, much hotter than what can be explained using the standard (convection/dredged-up) model for DQ white dwarfs. It is thought that these stars could be the next evolutionary step for stars such as H1504+65 (a hot pre-white dwarf star which is thought to have a surface of 50% carbon and 50% oxygen), which was discovered by John Nousek (of Pennsylvania State University), Leibert (of UA), and others. Currently, H1504+65 has a temperature of about 200,000 K; however, it is thought that when this star cools, it could become a pure-carbon star. The picture to the left is an artists conception of what the surface H1504+65 looks like.

For stars above 25,000 K, Dufour and his colleagues say that traces of helium rise up to the top of the star, which forms a thin layer and thus, disguises the star as a helium white dwarf. However, between the temperatures of 18,000 and 23,000 K, it is thought that convection dilutes the helium, the heavier oxygen sinks to the core, and the carbon is seen. It is also thought that a mass of nine to eleven solar masses could explain the carbon atmosphere white dwarfs found.

I think that this find is interesting because it could represent a new branch in stellar evolution. This could change the way we look at the formation and death of stars in the universe. The more unusual stars that are found, the more we can learn about stellar dynamics which are taking place now (and possibly about ones that occurred in the past).


http://uanews.org/node/17027

Scientists Discover Record Fifth Planet Orbiting Nearby Star

Astronomers have recently discovered a fifth planet around the star 55 Cancri. This star lies 41 light-years away (or 12.6 parsecs) towards the constellation of Cancer, the Crab. 55 Cancri is a binary star system with one star being a G-type star (the same as our sun) and the other being a red dwarf. The image to the right is an artists conception of what a planet around 55 Cancri might look like.

There are multiple methods for discovering extrasolar planets. The main methods for detection are astrometry, radial velocity (or doppler spectroscopy), pulsar timing, transits, gravitational microlensing, circumstellar disks, and direct imaging. The fifth star around 55 Cancri was discovered using the radial velocity method. In this method, one looks for doppler shifts in the spectrum of the star to determine whether or not there is an orbiting planet(s).

The planet, which was discovered by astronomers at San Francisco State University, University of California (Berkeley), and a team of other collaborators, is about 45 times the mass of the Earth and could be similar to Saturn (in terms of composition and appearance). It is the fourth planet from the star and has an orbital period of 260 days (or about .71 Earth years). Located in the habitable zone, a region of space where conditions are favorable for life, the planet lies 116.7 million kilometers (or .78 AU [where Earth is 1 AU away from the Sun]). The image to the left shows the 55 Cancri system on top and our own solar system (for comparison) on the bottom. The green areas in the pictures depict the location of the habitable zones in each system.

Thus far, all the planets discovered around 55 Cancri are gas giant-like planets ranging in size from Neptune to larger than Jupiter. These planets have nearly circular orbits with a Jupiter like planet orbiting at nearly the same distance as our Jupiter. These findings, along with others like them, leave hope for finding Earth-like planets around other stars and life elsewhere in the galaxy.

I think that finding planets around other stars is always a good thing. Finding more planets around stars allows us to get a better picture of how our own solar system formed and the evolution of solar systems in general.

NASA article:
http://www.jpl.nasa.gov/news/news.cfm?release=2007-128
Space.com article:
http://www.space.com/scienceastronomy/071106-five-planets.html