HD 131835 is a relatively young star, with an age of 15 million years [1, 2], located at a distance of 423 light years (129.7 pc) [3, 4] in the large and rather complicated constellation of Centaurus. The star is part of the Scorpius-Centaurus OB Association [1, 2], which sounds like it could be the name of an obstetrics clinic but which is actually a group of very young stars that include very bright, very blue stars with the classifications O and B. However, this OB association contains fainter white, yellow, and red stars as well. These are all stars that formed out of the same interstellar cloud of gas very recently, but the group is not gravitationally bound like an open cluster. One of the special things about the Scorpius-Centaurus OB Association is that it is the closest OB association to Earth [1, 2]. However, the stars are spread across several large constellations, so it's not an interesting object to look at for amateur astronomy.
Anyway, maybe the random number generator will select the Scorpius-Centaurus OB Association for a future episode. Let's go back to HD 131835. It's classified as an A2 type star [5], which means that it's a whitish star about a couple times the mass of the Sun that, like the Sun, is powered by the fusion of hydrogen into helium in its core. What makes this star stand out is that it is surrounded by a circumstellar disk of gas and dust [6, 7, 8, 9, 10] which I am initially going to very carefully refer to as a circumstellar disk because the terminology can become a little complicated, and if I use the wrong terminology, someone who knows more about the topic than I do will spontaneously appear in my office and mildly criticize me for it. So, let's go on a magical journey of astrophysical semantics as we explore the wonders of the HD 131835 circumstellar disk.
This star system was first identified as potentially containing a circumstellar disk in a study published in 2006 where the researchers had looked through archival data from the Infrared Astronomical Satellite and Infrared Space Observatory to identify stars with unusually strong infrared emission [11]. Astronomers typically refer to infrared emission as either near-infrared, mid-infrared, or far-infrared based on how close the wavelengths are to what we can see with our own eyes. Near-infrared light, which has wavelengths just slightly longer than red light, is primarily produced by stars, while mid-infrared light, which have wavelengths several times larger than visible light, and far-infrared light, which has wavelengths about 100 times larger than visible light or longer, is primarily produced by very small dust grains in space. Since the astronomers working with the archival infrared data found an excessive amount of mid-infrared and far-infrared light coming from the HD 131835 star system, they thought it was likely that the star was surrounded by a dust disk [11].
In 2008 and 2009, astronomers used the Gemini South Telescope to perform follow-up high-resolution infrared observations of the system, and they found that the mid-infrared emission was indeed extended [6]. The emission even looked somewhat disk-shaped. This was the first definitive proof that HD 131835 had a circumstellar dust disk. However, that was just the beginning, as the Gemini South images were really blurry, and astronomers wanted to make much sharper images of the disk.
Circumstellar dust not only emits infrared radiation but also reflects starlight as well, although astronomers prefer to use the term scattering for that phenomenon. A couple of different groups of people used special near-infrared cameras to image the starlight scattered by HD 131835's dust disk [8, 9]. These instruments used little black disks called coronagraphs to block out the star at the center of the HD 131835 system so that astronomers could see the other things next to the star. The best near-infrared images, which came from the unimaginatively-named Very Large Telescope, revealed that the HD 131835 dust disk contains a set of concentric rings [9]. The outermost ring is located at a distance of about 100 Astronomical Units (AU) from the star, while the innermost ring is located at a distance of about 40 AU from the star [9]. For reference, the distance from the Sun to the Earth is defined as 1 AU, and the distance from the Earth to Neptune is 30 AU, so these rings would look really large if they were in our Solar System.
This is now the part of the podcast where the magic of semantics becomes really important. Many of the reference papers that I read about HD 131835 referred to its disk as a debris disk, but those may have been, strictly speaking, the wrong words to use. A debris disk is a specific type of disk structure caused when things like asteroids or comets or even small planet-like objects called planetesimals collide with each other and produce lots of small dust particles [12]. If HD 131835 had just one ring, and if that ring contained primarily dust, this would indicate that the disk was produced through exactly this process of collisions [9]. However, the HD 131835 circumstellar disk contains several rings. Moreover, observations at wavelengths slightly shorter than 1 millimeter have revealed the presence of a substantial amount of gas in the circumstellar disk [9, 10], and it seems a little unlikely that all of this gas was created by smashing small planetary objects together.
This has lead at least some astronomers to the conclusion that HD 131835 is not surrounded by what could strictly be called a debris disk. Now, you may recall that I said earlier that HD 131835 looked like it has an age of about 15 million years, which is relatively young in astronomy terms. It might just be the case that at least part of HD 131835's disk is a leftover protostellar or protoplanetary disk [9, 10]. (You can legally use the terms "protostellar disk" and "protoplanetary disk" interchangeably as far as I know, but I think that "protoplanetary" would imply that planets were forming in the disk. Also, do not confuse this term with "protoplanetary nebula", which is a term for a completely different thing that does not involve planets.) Anyway, both "protostellar disk" and "protoplanetary disk" are terms used to describe a disk of gas and dust that surrounds a star that is in the process of actually forming. A protostellar or protoplanetary disk is the type of circumstellar disk that planetary objects would form out of, which makes it distinctly different from a debris disk, which forms when planetary objects get destroyed.
So, the most popular current hypothesis about HD 131835 is that its circumstellar disk is some type of weird debris disk / protostellar disk hybrid [9, 10]. If that is the case, then it would say something quite interesting about how stars and their planetary systems form and evolve. At the very least, it says that a star can be a fully functional hydrogen-fusing star for about 15 million years and still retain part of the gas and dust disk that it formed out of, but you can also expect a lot of collisions between planetesimals at this stage. All of this has implications for how planets form.
A few other systems with debris disk / protostellar disk hybrid have also been identified [10], but its still not entirely clear as to what's happening in these star systems. This is still a topic of ongoing research, and HD 131835 is going to be a central part of it.