The Secrets of Hot Subdwarf Stars Revealed

Hot subdwarf stars are like the cool kids of the stellar community. These stars, which are quite different from your average star, are typically in the later stages of their lives. They are mainly Helium-core or helium-shell burning stars with very thin hydrogen layers. Why do they exist in such a unique state? Well, most of them have to go through some serious binary interactions to get there!

In the vast universe, different types of hot subdwarfs have different backstories. The goal of studying these stars is to uncover the reasons behind their varying behaviors, particularly when it comes to Radial Velocity (RV) variability. RV variability can help us learn more about how these stars are formed and how they interact with their surroundings.

#What Are Hot Subdwarf Stars?

Let's start from the beginning. Hot subdwarf stars are special types of stars with a unique combination of properties. They are often found at the extreme blue end of the Hertzsprung-Russell diagram, which is a fancy way of saying we can see them shining brightly in one specific area of the night sky.

Most of these stars have masses around 0.5 times that of our sun and their hydrogen envelopes are extremely thin. Their effective temperatures range from about 20,000 to 80,000 K. This range means they are hotter than most of the stars that we can easily see and study.

Hot subdwarfs are important for several reasons. Firstly, they contribute to the ultraviolet light emitted by elliptical galaxies, which is a bit like the frosting on the cosmic cake. Secondly, they are considered potential progenitors for type Ia supernovae, which are powerful explosions caused when certain stars run out of fuel. Thirdly, they can also be valuable sources for gravitational wave studies, which sounds quite fancy but basically helps us learn more about the fabric of space-time!

#The Variety of Hot Subdwarfs

Now, let's take a closer look at the different types of hot subdwarfs. They can be classified mainly into two categories: single-lined and composite stars.

Single-lined hot subdwarfs show the spectroscopic features of hot subdwarfs without any visible companions, while composite stars have companions that can be detected, usually by looking for infrared signals in their light. These companions can range from main-sequence stars to white dwarfs or even brown dwarfs.

Interestingly, a large portion of hot subdwarfs is found in short-period binary systems, where two stars are very close together and orbit each other. In fact, about one-third of all hot subdwarfs are in these types of systems, and they typically showcase various light curves due to the gravitational interactions between the stars.

#How Do We Measure Radial Velocity Variability?

To study the RV variability of hot subdwarfs, scientists utilize various measurements. One common method involves using spectral data from telescopes. By examining the light emitted by these stars, astronomers can analyze shifts in spectral lines caused by the Doppler effect. This effect essentially tells us how fast an object is moving toward or away from us.

The cross-correlation function method is a popular technique in RV measurement. This involves comparing observed spectra with template spectra of known stars to identify how their velocities change over time. By studying the changes in spectral lines, researchers can track RV variations in hundreds of hot subdwarfs.

#The Findings: RV Variability Fractions

In a recent investigation involving 434 hot subdwarfs, researchers found some intriguing results. Of the single-lined He-rich hot subdwarfs, only about 6% showed significant RV variability, which was considerably lower than the 31% found in single-lined He-poor sdB stars. It seemed that being a He-rich star might mean less motion at the level of RV variability.

For single-lined sdB stars with effective temperatures between 25,000 - 33,000 K, the RV-variability fraction was around 34%. However, cooler single-lined sdB stars (below 25,000 K) showed a lower fraction of 11%. This suggests that temperature has a significant role in determining how variable these stars can be.

Interestingly, stars located just above the extreme horizontal branch (EHB) and with effective temperatures of 35,000 – 45,000 K exhibited a similar RV-variability fraction to the 25,000 – 33,000 K sdB stars. However, single-lined stars with temperatures above 45,000 K showed a much lower RV-variability fraction of only 10%.

Moreover, single-lined hot subdwarfs positioned below the canonical EHB displayed the highest RV-variability fraction at a remarkable 51%. It appears that these stars are more active or dynamic compared to their peers.

Composite hot subdwarfs, on the other hand, presented an even lower RV-variability fraction of just 9%. Since many of these composite systems are long-period binaries, this finding is expected because they typically exhibit lower RV amplitudes.

#The Evolutionary Connection

The RV findings provide clues about how different types of hot subdwarfs evolve. For instance, most single-lined He-rich hot subdwarfs may form through merger channels, while composite stars likely originate from stable Roche-lobe overflow in binary systems.

Stars located above the EHB and those cooler than 25,000 K may have evolutionary links. For example, the cooler sdB stars could evolve from iHe-rich hot subdwarfs through processes such as helium diffusion over millions of years.

In contrast, the differences in RV variability fractions for different subclasses of hot subdwarfs imply that their formation channels might differ significantly. Understanding these channels helps astronomers piece together the puzzle of how stars evolve and interact with one another in binary systems.

#The Importance of Observations

The study of RV variability in hot subdwarfs not only reveals their dynamic nature but also helps astronomers refine their knowledge about stellar evolution. Continuous monitoring of these stars through high-quality observations, such as those from space-based missions, is crucial in enhancing our understanding of their behavior.

Additionally, light curves obtained from missions like TESS and K2 offer deeper insights into the properties of hot subdwarfs. By combining light curves with spectroscopic data, researchers can further probe the relationships and dynamics between these fascinating stellar objects.

#Conclusion

In summation, hot subdwarf stars are intriguing celestial bodies that challenge our understanding of stellar evolution. Through studying their RV variability, we gain valuable insights into their formation channels and the unique conditions that shape their lifetimes.

From the difference in RV variability fractions to their respective classifications, each discovery strengthens our knowledge of the universe. As technology advances and we gather more observations, it will be exciting to see how our comprehension of hot subdwarfs continues to unfold.

So, next time you look up at the night sky, remember that among those twinkling stars, some have intriguing stories to tell. They are more than just shiny dots; they are telling us about the grand cosmic dance that is happening far beyond our reach.