If the Sun has a stellar neighborhood, it can be usefully defined as a 20 parsec (65 light-years) sphere with our star at its center. Astronomers have been cataloging the stellar population in the neighborhood for decades, but it hasn't been easy because many stars are small and dim.
Despite all the challenges this endeavor presents, astronomers have made steady progress. Do we now have a complete catalog?
In a new article in the American Astronomical Society's Research Notes, two researchers from the Leibniz Institute for Astrophysics in Potsdam try to find out how complete or incomplete our catalog of the stellar environment is. The title of the article is “Do we finally know all the stellar and substellar neighbors within 10~pc of the Sun?” The authors are Ralf-Dieter Scholz and Alexey Mints.
If all stars were as bright as our sun, it would be easy to catalogue the stars in our neighborhood. But they aren't. Some are so small and dim that they are thought to be failed stars. We call them brown dwarfs or substellar objects.
When we look at the night sky with the naked eye, our view is dominated by main sequence stars and giant stars, many of which lie far outside our stellar neighborhood. Many stars are too dim to see, such as red dwarfs and brown dwarfs. In fact, Proxima Centauri, a red dwarf and our closest neighbor, was only discovered in the early 20th century.
Proxima Centauri. Image credit: ESA/Hubble and NASA
In the early days of astronomy, measurements of proper motion showed that some stars that appear to be fixed in place are closer than other stars. All stars move and have proper motion; it's just not always evident over the course of a single lifetime. Studies of stars with high proper motion led to certain stars being selected for measurements of their parallax, which helped to correctly locate more stars in space. Then, in the early 20th century, when astronomy and photography merged, photographic astrometry sparked a wave of discoveries of our solar neighbors. These efforts showed that our closest neighbors are red dwarfs (M dwarfs).
In the 1990s, infrared sky surveys began to discover more and more dim stars as technology advanced. “A second wave of discoveries began in the late 1990s as infrared sky surveys advanced,” the authors write. Missions like the Two Micron All-Sky Survey (2MASS) gave us a new, unprecedented view of the sky, discovering M dwarfs, brown dwarfs, and substellar objects such as L, T, and Y types, and even minor planets in the solar system. (The definitions of brown dwarfs and other substellar objects overlap.) In 2000, the Sloan Digital Sky Survey went online, expanding our catalog of the sky.
In 1997, Henry et al. published an important paper on the solar environment, “The Solar Environment IV: Discovery of the Twentieth Closest Star.” It showed that the discovery of LHS 1565, about 3.7 percent from Earth, spelled trouble for our inventory of the environment. “It is the twentieth closest star system and underscores the incompleteness of the sample of nearby stars, especially for objects at the end of the main sequence,” Henry et al. wrote. “Ironically, this unassuming red dwarf is a shocking reminder of how much we still have to learn about even our closest stellar neighbors.”
Since about 1997, more and more stars have been discovered in the Sun's neighborhood. The authors say that these appear to be filling in the gaps in our 10 percent neighborhood. However, some of these findings were still based on two assumptions. The first was that the survey was complete to within 5 parsecs, and the second that the density was uniform to within 10 parsecs. “The first of these assumptions is not true and the second is questionable,” the authors write.
So where are we? Up to 90 star systems could still be missing.
Artist's impression of a brown dwarf. Brown dwarfs are more massive than Jupiter, but less massive than the smallest main sequence stars. Their low brightness and mass make them difficult to see. Image: By NASA/JPL-Caltech (http://planetquest.jpl.nasa.gov/image/114) [Public domain]via Wikimedia Commons
“Using all of the neighbors, we can study the luminosity and mass functions, as well as the star-to-brown dwarf (BD) ratio,” the authors explain. Astronomers do not fully understand the relationship of brown dwarfs to other stars, but two recent papers in particular (1,2) have continued the work to better understand and catalog the dark members of our stellar neighborhood.
Earlier this year, Kirkpatrick et al. published a study claiming that a complete survey of nearby stars was possible, thanks in large part to Gaia data. They found 462 objects (including the Sun) in 339 systems within 10% of the Sun.
In previous work, the authors of this new paper added 16 more stars to the list, including late M dwarfs, some of the coolest and dimmest main sequence stars, and brown dwarfs. They also discovered a new white dwarf as a companion to an existing M dwarf.
But how complete is this latest survey?
The problem is that it is difficult to see dim stars like brown dwarfs and late M dwarfs. The further we look, the harder it is to see them. Even towards the galactic plane, they are even harder to see.
Dark objects like brown dwarfs are harder to see when looking towards the galactic plane, as this is where most of the Milky Way's mass is located. Image credit: ESA/Gaia/DPAC
The authors say that there are probably 93 star systems missing from our neighborhood star catalog, “corresponding to a deficit of ?21.5%,” they write. When it comes to individual stars, things don't look much better: “…138 missing objects, corresponding to a deficit of ?23.0%,” they write.
They broke it down even further to individual star types. We are probably missing 28.1% of AFGK stars, -31% of white dwarfs, and -27.8% of M dwarfs. There is also a higher deficit in late M dwarfs. These deficits are higher than expected. What does that mean?
“The estimated deficits of systems and individual objects within 10?pc exceed expectations, especially for the known AFGK stars,” the authors write. They conclude that the general assumption of a constant stellar density in the solar neighborhood is incorrect. They say that small-scale density fluctuations can at least partially explain the deficits.
“Our statistical estimates indicate that the probability that these discrepancies are caused by random fluctuations is about 40 percent,” the authors conclude.
There is clearly still a lot of work ahead of us.
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