Dust cloud determines distance
Normally, what you see in the sky are bright objects, and these are also easy to spot with telescopes. However, sometimes astronomers take a different view and look at objects that are detectable through their shadow. An international team of researchers from Australia, Germany, Italy, Japan, and the USA have now focused on dust in the remains of an exploded star (also called a supernova remnant, SNR in short). This object, with the name RX J1713.7-3946, is one of the brightest emitters of very high-energy radiation and the origin of this has been debated since its discovery.
This schematic shows how the scientists measured the distance to the supernova remnant. The light of different background stars gets attenuated through the dust cloud, which helps to determine the cloud’s position. In this figure, one can infer that the dust cloud is between d2 and d4.
Rather than investigating the high-energy radiation directly, the scientists investigated the shadows cast by intergalactic dust clouds on stars in the background. They reconstructed the dust extinction in the region of the SNR using a Gaussian process and more than 260 000 stars. While the resulting map indicates the presence of several clouds along the line of sight, only one of the clouds appears to be in the vicinity of the exploded star.
By using distance estimates to the individual stars in the direction of RX J1713.7-3946 provided by the European Space Agency Gaia space mission, the researchers were able to constrain the distance to the dust cloud, and therefore to the supernova remnant, to a 1% accuracy, about 20 times more precisely than previous estimates.
The dust clouds found in this study are depicted here as a virtual observer would see them from above the galaxy (top panel) and east of the remnant (bottom panel). The white lines indicate where the supernova remnant is known to be located. The only dust cloud that overlaps with its position is found to be at 1120pc (or 3750 light-years away).
This is an important milestone, as it helps to understand the energetic processes taking place at and around the supernova remnant. Precise distances are among the most challenging measurements in Astronomy. Without their knowledge, it is impossible to be sure whether an object is intrinsically brighter and further away, or intrinsically fainter but closer.
A precise distance determination is crucial for this supernova remnant, as it is at the origin of some of the highest energy photons that we see on Earth. Yet, the mechanism by which these photons (or gamma-rays) are generated is still not fully understood. One possible explanation for the high-energy radiation are proton-proton collisions, but previous estimates of the gas content fell short. With an estimated mass of about 7000 times the mass of the Sun, the dust cloud could be big enough to contain the missing protons, but further studies are needed to confirm this.
