Title: Magnetic Activity of radio stars based on TESS and LAMOST surveys
Authors: Yinpeng Wang, Liyun Zhang, Tianhao Su, Xianming L. Han, and Prabhakar Misra
First Author’s Institution: College of Physics, Guizhou University, 550025 Guiyang, PR China
Status: Published in Astronomy & Astrophysics [open access]
There are several ways to describe how magnetically active a star is. Some describe stellar magnetic activity by how frequently a star impulsively releases magnetic energy as electromagnetic radiation through a flare. Sometimes activity is described by the distribution of the star’s flares as a function of their energy. Sometimes activity isn’t defined by flares directly, but by properties of the star that indicate stellar flares are prevalent, like the star’s X-ray luminosity or through emission lines like H-alpha and Calcium, all of which can tell you how much heating is happening in the star’s atmosphere.
Understanding magnetic activity and its effects on the surrounding environment is crucial for understanding the habitability of worlds around these stars, called exoplanets. If a star is constantly releasing flares tens, hundreds, or thousands times stronger than the flares we see from the Sun (and releasing similarly energetic particles in the form of coronal mass ejections) how habitable do we expect the exoplanet to actually be?
Before we can start talking about habitability, we first need to understand stellar magnetic activity. To investigate, today’s authors examine the radio emission from stars. Radio emission can arise from a variety of different mechanisms and not all of them are related to flares. So, to determine the possible relation between the radio emission and magnetic activity, the authors also look at stellar surveys containing information on other activity indicators like flares and H-alpha emission.
The Radio Star Sample
The authors have two samples of radio-emitting stars that they use to inform their search for other activity indicators. The first, Sample 1, consists of 3,699 stars that have been observed in the radio up until 1994 and are associated with 1,537 stars from the Transiting Exoplanet Survey Satellite (TESS). The much smaller Sample 2 consists of 146 stars that have been observed across three different radio surveys: the Low Frequency Array Two-metre Sky Survey (LoTSS), the Very Large Array Sky Survey (VLASS), and surveys from the Australian Square-Kilometre Array Pathfinder Telescope (ASKAP) and are associated with 98 stars in TESS. These stars cover a variety of stellar temperatures and stages in stellar evolution, as shown in figure 1.
It is important to acknowledge two key details about these two samples. The first is that Sample 1 includes targeted observations (observations that focus on a single star rather than collecting data over the entire sky). These primarily focused on stars that were already known to be active; this introduces a bias to the dataset that will impact the statistical analysis between the two samples. The other key detail is that Sample 2 consists of stars from very different surveys—these surveys look at stars at different declinations, different frequency ranges, different durations, and with different sensitivities. This all means that the stars in Sample 2 do not represent a uniform sample (meaning a star emitting in one sample does not have the same probability of being observed in another sample). This does not mean that this analysis is not meaningful, but rather highlights a gap that has existed in stellar radio astronomy that has only recently begun to be addressed with these expansive surveys.
The Other Data
After identifying their stellar radio samples, the authors investigated other activity indicators– primarily looking for flares in TESS data. Although this telescope was designed to look for exoplanets around cold stars, it has been key in studying stellar activity because of its continuous monitoring of a colossal quantity of stars. This allowed the authors to identify more than 16,000 flares across their collection of about 1,600 stars.
The authors also looked for spectra from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) that were associated with the radio stars, allowing them to study the H-alpha line. This gives them a sense of the stellar activity even without observing flares.
The Results
Although Sample 1 and 2 somewhat differ statistically, they broadly represented the same trends. Namely, how frequently flares occurred on stars decreased as stellar rotation period increased, aligning with the idea that as stars age, their rotation slows down and they become less active. They also find that stars in binaries tend to flare more than stars they identified as isolated stars. This is consistent with the idea that stars in binaries can spin each other up to become more active and interact through their magnetic fields to drive stronger flares.
Surprisingly, more than 30% of the flaring stars that the authors present here also present highly energetic flares, more than 1000 times stronger than the strongest solar flare. However, the vast majority of flares were found to occur at lower energies and, in fact, the occurrence rates of various flare energies were found to be consistent with other flare rate studies, including those for the Sun.
Finally the H-alpha lines from LAMOST, as well as other spectral activity indicators, show that stars that were found to flare in TESS showed higher levels of spectral activity than those that didn’t have flares. Despite these correlations that seem to support conclusions of previous studies, none of the data from TESS or LAMOST appear to have any correlation with quantities coming from the radio data.
The Conclusion
This final result may be related to the non-uniformity of the radio data we mentioned earlier. It may also be related to the non-uniformity of the stellar sample– this work showed activity across a menagerie of stellar types, and it may be bold to assume they all emit the same type of radio emission, even if the majority of them exhibit flares.
In the future, it will be exciting to see how the three radio surveys used here can synergize to give a broader picture of the activity these stars experience. In the meantime, we are left with the question: what is the nature of the relationship between stars that are both active in the radio and at other wavelengths, and what data are we missing to understand the radio star?
Astrobite edited by Diana Solano-Oropeza
Featured image credit: adapted from NASA/SDO
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