The venerable Hubble Space Telescope has given us a lot during its service history (32 years, 7 months, 6 days and counting!) Even after all these years, the versatile and sophisticated observatory is still pulling its weight together with a more recent addition. , such as the James Webb Space Telescope (JWST) and other members of NASA’s Large Observatories family. In addition to how it is still conducting observing campaigns, astronomers and astrophysicists are sifting through the volumes of data Hubble has accumulated over the years to find even more hidden gems.
A Caltech-led team recently made some very interesting findings in the Hubble archives, where they looked at the sites of six supernovae to learn more about their parent stars. Their observations were part of the Hubble Space Telescope’s Snapshot program, where astronomers use HST images to trace the life cycle and evolution of stars, galaxies and other celestial objects. From this, they were able to place constraints on the size, mass and other key characteristics of the progenitor stars and what they experienced before they experienced core collapse.
The team was led by Dr. Schuyler D. Van Dyk, a senior research scientist at Caltech’s Infrared Processing and Analysis Center (IPAC). His teammates included researchers from the University of California, Berkeley, the Space Telescope Science Institute, the University of Arizona’s Steward Observatory, the University of Hawaii Institute for Astronomy ‘i and the School of Physics and Astronomy of the University of Minnesota. Their findings were published in a paper entitled “The disappearance of six supernovae progenitors” to appear in the Monthly Notices of the Royal Astronomical Society.
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Hubble’s Ultra Deep Field seen in ultraviolet, visible and infrared light. Image credit: NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI)
As they indicate in their paper, the targets of their study were all nearby core-collapse supernovae (SNe) that Hubble imaged at high spatial resolutions. The images were part of the Hubble Snapshot program, created by the Space Telescope Science Institute (STScI) to provide a large sample of images for various targets. Each target is observed in a single Hubble orbit around Earth between other observing programs, allowing a degree of flexibility not possible with other observatories.
For their study, Van Dyk and his colleagues examined images of six extragalactic supernovae before and after the explosion, designated SN 2012A, SN 2013ej, SN 2016gkg, SN 2017eaw, SN 2018zd and SN 2018aoq. With extragalactic targets, astronomers have a hard time knowing whether the stars they identified were progenitors of the supernova, given the distance involved. As Van Dyk told Universe Today via email, the only way to be sure is to wait for the supernova to darken and then confirm that the parent star is gone:
“Because the supernova explosion is so luminous, we have to wait a few years until it has faded enough to be less luminous than the progenitor. In some of the cases we show in our paper, there is no doubt that the star that was there before the explosion is gone. In the other cases, we’re reasonably sure, but the supernova is still detectable and faint enough to infer that the progenitor is gone.”
In a previous study, Van Dyk and several colleagues who were co-authors of this study investigated another supernova (iPTF13bvn) whose progenitor star disappeared. In this case, the research team relied on data obtained by Hubble from the SN site, as part of the Ultraviolet Ultra Deep Field (UVUDF) campaign, approximately 740 days after the star’s explosion. In 2013, Van Dyk led a study that used images from an earlier Snapshot program to confirm that the progenitor of SN 2011dh in the Whirlpool Galaxy (Messier 51) was gone.
The Whirlpool Galaxy (Spiral Galaxy M51, NGC 5194), a classical spiral galaxy located in the constellation Canes Venatici, and its companion NGC 5195. Credit: NASA/ESA
These and other papers over the years have shown that progenitor candidates can be identified directly from pre-explosion images. In this most recent study, Van Dyk and his colleagues observed supernovae in the final stages of their evolution to learn what mechanisms fuel them. In many cases, the mechanism is the disintegration of radioactive nuclei (in particular, radioactive nickel, cobalt and iron) that were synthesized by the enormous energy of the explosion. But, he explained, they suspected other mechanisms might be involved:
“However, we have indications that some supernovae inevitably have additional energy sources; one possibility is that the light from the supernova was scattered by interstellar dust immediately after the explosion, in the form of a ‘light echo’ “; another more likely possibility is that the shock wave associated with the explosion is interacting with gas that was deposited around the progenitor star by the star itself during the course of the star’s life, in the form of a wind or burst, i.e. circumstellar matter. The ejecta from the explosion moving and interacting with this circumstellar matter can give rise to luminous energy that can persist for years, even for decades”.
In short, the team was trying to estimate how many of the supernovae they observed evolved through radioactive decay versus more exotic power mechanisms. Their results showed that SN 2012A, SN 2018zd, and SN 2018aoq had faded to the point where they were no longer detectable in the Hubble Snapshot images, while SN 2013ej, SN 2016gkg, and SN 2017eaw had faded enough. Therefore, they were able to infer in all six cases that the parents had disappeared. However, not all were the result of the core collapse of a single massive star.
In the case of SN 2016gkg, the images acquired by Hubble’s Wide Field Camera 3 (WFC3) were of much higher resolution and spatial sensitivity than images of the host galaxy previously taken by the now retired WFC2. This allowed them to theorize that SN 2016gkg was not the result of a single core-collapse supernova, but of a progenitor star interacting with a neighboring star. Van Dyk said:
“So in the old image, the progenitor looked like a ‘star’, whereas in the new images, we could see that the progenitor must have been spatially different from the neighboring star. So we were able to get a better estimate of the luminosity and color of the progenitor, now uncontaminated by the neighbor, and from this we have been able to make some new inferences about the global properties of the progenitor, or in this case the progenitor system, since we characterized the new results using existing models of binary star systems.”
Artist’s impression of a supernova remnant. Credit: ESA/Hubble
Specifically, they determined that the progenitor belonged to the class of “stripped envelope” (SESNe) supernovae, in which the H-rich outer envelope of the parent star has been significantly or completely removed. They further estimated that the progenitor was the main one and that its companion was probably a main-sequence star. They even put constraints on their respective pre-explosion masses (4.6 and 17-20.5 solar masses, respectively).
After checking images taken at the same time by another Snapshot program, they also noticed something interesting about SN 2017eaw. These images indicated that this supernova was particularly bright in the UV band (an “ultraviolet excess”). By combining these images with their data, Va Dyk and his team speculated that SN 2017eaw had an excess of light in the UV at the time it was observed, which was likely caused by the interaction between the shock of the supernova and the circumstellar medium around this progenitor.
The team also noted that dust created by a supernova explosion is a complicating factor because of how it cools as it expands outward. That dust, Van Dyk said, can obscure light from distant sources and lead to complications with observations:
“The caveat here, then, is that the star we saw before the explosion might not be the progenitor at all, for example, and again, because of the distances to the host galaxies, that star is at fractions of a pixel from the actual progenitor (physically, in the immediate neighborhood of the progenitor), so if the supernova has dusted, that dust effectively covers both the supernova and the neighboring star. This is possible, but not excessively likely. And it becomes a harder argument to make in those few cases where nothing is seen at the position of the supernova years later, as we point out in the paper, that would require enormous amounts of dust, which is probably physically not possible”.
Tracing the origins of supernovae is one of the many ways astronomers can learn more about the life cycle of stars. With improved instruments, data collection and flexibility, they are able to reveal more about how our Universe evolved and will continue to change over time.
Further reading: arXiv
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