Above: Participants at the 6th Meeting on Hot Subdwarf Stars (sdOB6) in Tucson, AZ (May 2013)         Credit: S. Jeffery

The Enigmatic Hot Subdwarf Stars

What are hot subdwarf stars?

When a star like the Sun runs out of available hydrogen fuel for fusion into helium, its core will collapse while its outer layers expand. Consequently, from the outside, the star appears to expand in size and cool off, becoming a “red giant” star. If you could peer inside such a star, you would see a dense core made of helium surrounding by a “fluffy” outer envelope made of hydrogen. Now imagine, for whatever reason, that you suddenly removed all of the outer hydrogen. The remaining object would have a core of helium surrounded by a thin layer of hydrogen; it has approximately 50% the mass of the Sun and is 20% its size. Such a star will fuse helium to carbon in its core for approximately 100 million years, after which it will directly evolve into a white dwarf. The object just described is what a “hot subdwarf” star looks like to astronomers. The question is: how in the world do you remove all of that hydrogen mass?

How are they formed?

Interestingly, almost every hot subdwarf we observe appears to be a member of a binary star system (in orbit with another star). Therein lies the secret to their formation: perhaps a neighboring star can strip off the outer layers of a red giant, leaving behind a “hot subdwarf” star???

Theoretical models show that, indeed, binary interactions might be responsible for the formation of hot subdwarf stars. In essence, as the red giant star expands, it ends up filling what’s called its “Roche Lobe,” and material transfers from the red giant to the companion star. In some cases, they end up sharing an atmosphere for a period time, while in other cases, the red giant just simply transfers its mass over to the other star. Figure 2 illustrates the former process. Once this interesting evolutionary phase is over, a hot subdwarf will form from what started as a red giant branch star.

Why study them?

There are many reasons to study hot subdwarf stars. They dominate surveys of faint blue objects in the Milky Way, outnumbering even the white dwarfs at magnitudes brighter than 18, and have been found in the Galactic bulge, halo, and disk. Outside of our own Galaxy, they have been identified in giant elliptical galaxies, globular clusters, and other evolved stellar populations. Many giant elliptical galaxies, which are supposed to be “red and dead” emit vast quantities of UV light; the hot subdwarfs may explain some fraction of this so called “UV Upturn.” Similarly, these stars may help explain the “second parameter” problem of globular cluster morphology. Lastly, recent studies have shown that determining the periods of hot subdwarf binaries is an excellent way to constrain the parameterizations in Binary Population Synthesis (BPS) codes, which are used throughout astrophysics to model exotic phenomena, including low-mass X-ray binaries, Type 1A supernovae, black hole binaries, and cataclysmic variables.

Quick Facts

Properties of Hot Subdwarf B Stars:
  • post-red giant branch star that was stripped of most of its outer hydrogen layer
  • essentially a helium core with a very thin hydrogen venier
  • ~half the mass of the sun
  • approximately one-fifth the size of the sun
  • high surface temperatures (20000-40000 K)
  • fuse helium in their cores
  • populate the "extended horizontal branch" on Hertzsprung-Russell (H-R) diagram
  • live approximately 100 million years
  • will evolve directly into white dwarfs

FIGURE 1: H-R diagram (luminosity versus temperature) showing the position of hot subdwarf stars (sdB's and sdO's) relative to the main sequence, white dwarfs, etc.)         Credit: Heber (2009, ARAA, 47, 211)

FIGURE 2: Example formation channel for hot subdwarfs. In this scenario, a red giant branch star is in a binary with a main sequence star. As the red giant expands, it fills its Roche lobe and begins transfering mass to the main sequence star. Eventually, a common envelope is formed and ejected, leaving behind a hot subdwarf with main sequence companion.         Credit: Heber (2009, ARAA, 47, 211)