Stars spend the majority of their lives converting hydrogen into helium. We refer to this as the main sequence phase of a star. Once a star depletes its core hydrogen, it has to start fusing heavier elements to continue functioning as a star. When this occurs, physical features of the star can change dramatically, signaling the start of the post-main sequence. High mass stars (with masses ~ten to hundreds of times bigger than the Sun), undergo extreme phases during the post-main sequence where the star can lose an appreciable fraction of its mass before finally ending its life as a supernova. Some of these post-main sequence phases are poorly understood because they are very rare. In particular, the Luminous Blue Variable (LBV) phase is an interesting phase which is very short-lived and violent. LBVs are able to eject multiple solar masses of material from their surface which results in a large shell of gas and dust surrounding the star (see the Pistol star below).

After going through the LBV phase, some stars evolve to become Wolf-Rayet (WR ) stars. WR stars are very bright (~a few hundred thousand times the luminosity of the Sun) and drive powerful winds (~1000-3000 km/s). WR stars often show strong emission lines from helium, carbon, nitrogen, and sometimes oxygen, which makes them somewhat easy to identify spectroscopically. However, a small subset of these objects are known to be very efficient dust producers. The exact process in which the dust is formed in these systems is poorly understood, but binarity appears to be a key ingredient. It is thought that the wind-collision zone between a WR star and O or B main sequence star in a binary system could provide a suitable environment for dust formation, though many specific details in this process are unclear. High-resolution imaging of some of these systems have revealed interesting dust spirals which trace out dust formed in the binary. An excellent overview on these types of objects can be found on Prof. Peter Tuthill's research page.

The center of our galaxy is an excellent place to study massive evolved stars because a large fraction of the massive stars in our galaxy reside there. My 2016 paper was focused on studying infrared emission from dusty WR stars in the Quintuplet cluster. From the FORCAST maps of the region, we inferred that Q9 is much dustier than the other quintuplet stars. Q9's designation as a dusty WR star is somewhat uncertain since it has not been spectroscopically confirmed. Interestingly, there appears to be a bow-shock like structure associated with this object, which is visible in high resolution mid-infrared images. Q9 also has a inferred mass-loss rate that is a factor of ~10 time larger than other dusty WR stars. Because of these unusual features, we have followed up our earlier observations of Q9 with additional observations to better characterize this source. These results will be featured in an upcoming paper.

In addition to studying objects in the Galactic center, I am also actively involved in efforts to study other dusty WR stars with SOFIA and other mid-infrared facilities like the Very Large Telescope (VLT). I have created a simple dust emission model to reproduce the spiral pattern produced by these types of objects. My models have been used in observation planning for JWST and long baseline interferometry with the VLT-Interferometer. Both of these observatories use cutting edge technologies that allow us to study these types of objects in unprecedented detail. Our goal is to better understand how the dust is produced in these systems and determine how much of the dust survives as it propagates outward from the central binary system and into the surrounding interstellar medium.

Studying WR stars and other types of massive evolved stars have a broad range of application to different fields in astronomy. For one, massive evolved binary systems are the progenitors of binary black hole systems, which can produce gravitational waves. Observatories like LIGO are heavily invested in detecting gravitational signals from these types of systems, and studying massive post-main sequence binaries can better help us understand the population of objects that produce these types of events.

Additionally, studying the dust produced by WR stars and other post-main sequence phases helps us better understand the origin dust in galaxies, which is not well understood at present. This issue, sometimes referred to as the 'dust budget', examines the total amount of dust in a galaxy and then looks at the sources of dust production from various types of stars and supernovae. Recent studies have found that the expected sources of dust do not produce enough dust to explain the observed amount of dust in nearby galaxies (e.g. Boyer et al. 2012). Dust from massive evolved stars could be an important source of dust in galaxies, though it is frequently not considered because these types of objects are rare and short-lived.

Dust production from high-mass stars could be very important in galaxies in the very early universe. Low-mass stars, which are a major source of dust in the local universe, would not have had enough time to evolve off the main sequence. Therefore, low-mass stars would not be a source of dust in these early galaxies. Instead, massive binary stars could produce systems similar to dusty WR stars in the local universe, even in very low metallicity environments. Binaries in particular orbital configurations can produce WR stars though envelope stripping and mass transfer between the stars in the binary system. Many of these binary systems could also produce conditions suitable for dust formation via the colliding wind mechanism that works in binaries in the local universe. Because of these features, some of the earliest dust in the universe may have come from massive binaries. There are many details that still need to be worked out in this scenario, and I am pursuing these topics.