b'Carnegie Science|Summer 2019 17Can Rare SupernovaCarnegie team members on the supernovae research are (left to right) Juna Kollmeier, Nidia Morrell, Tony Piro, Mark Phillips, and Josh Simon. Resolve Origin Debate? Images courtesy Matthew Scott, Wiki Creative Commons CC0, Sandy Huffaker Jr. Photography, Mark Phillips, Josh SimonDetection of a supernova with an unusual chemical signature mayAstronomers eagerly study the chemical signatures of the hold the key to solving the longstanding mystery of the source ofmaterial ejected during these explosions to understand the these violent explosions. This supernova, ASASSN-18tb, was foundmechanism and players involved in creating Type la supernovae. by a team of astronomers led by Carnegies Juna Kollmeier, includingIn recent years, astronomers have discovered a small number Carnegies Nidia Morrell, Anthony Piro, Mark Phillips, and Joshof rare Type la supernovae that are cloaked in a large amount of Simon. Observations with the Magellan telescopes at Carnegies Lashydrogen. But in several respects, ASASSN-18tb is different.Campanas Observatory in Chile were crucial to detecting theIts possible that the hydrogen we see when studying hydrogen emission that makes supernova ASASSN-18tb soASASSN-18tb is like these previous supernovae, but there are some distinctive. Monthly Notices of the Royal Astronomical Society striking differences that arent so easy to explain, said Kollmeier. published the work. First, in all previous cases these hydrogen-cloaked Type la The brilliance of Type la supernovae explosions allows them tosupernovae were found in young, star-forming galaxies where be seen across great distances and to be used as cosmic mileplenty of hydrogen-rich gas may be present. But ASASSN-18tb markers. Additionally, the explosions synthesize elements, whichoccurred in a galaxy with old stars. Second, the amount of populate the galaxy to generate future stars and stellar systems. hydrogen ejected by ASASSN-18tb is significantly less than that Although hydrogen is the most abundant element, it is almostseen surrounding those other Type la supernovae, which is about never seen in Type la supernova. In fact, the lack of hydrogen is oneone-hundredth the mass of our Sun. of the defining features of this supernovae category and is thoughtOne exciting possibility is that we are seeing material being to be key to understanding what came before their explosions. So thestripped from the exploding white dwarfs companion star as the hydrogen coming from ASASSN-18tb was particularly surprising.supernova collides with it, said Anthony Piro. If this is the case, it Type la supernovae originate from the thermonuclear explosionwould be the first-ever observation of such an occurrence. of a white dwarf that is part of a binary system. But what triggers theI have been looking for this signature for a decade! said explosion of the white dwarf, the dead core after a Sun-like starcoauthor Josh Simon. We finally found it, but its so rare, which is exhausts its nuclear fuel, is a great puzzle. A prevailing idea is thatan important piece of the puzzle for solving the mystery of how the white dwarf gains matter from its companion star, a process thatType la supernovae originate.may eventually trigger the explosion, but this has been hotly debatedNidia Morrell was observing that night, and she immediately for decades. reduced the data coming off the telescope and circulated them to This puzzle led the research team to begin a major survey ofthe team, including Ph.D. student Ping Chen, who works on 100lAS Type la supernovae. The survey, called 100lAS, was launched whenfor his thesis, and Jos Luis Prieto of Universidad Diego Portales, a Kollmeier was discussing these supernovae origins with studyveteran supernova observer. Chen was the first to notice that this coauthors Subo Dong of Peking University and Doron Kushnir of thewas not a typical spectrum. All were completely surprised by what Weizmann Institute of Science. Kushnir, along with Weizmannthey saw in ASASSN-18tbs spectrum. colleague Boaz Katz, put forward a new theory for Type la explosionsI was shocked, and I thought to myself could this really be that involves the violent collision of two white dwarfs. hydrogen? recalled Morrell.To discuss the observation, Morrell met with team member This cartoon illustrates three possibilities for the origin of the mysterious hydrogen emissions fromMark Phillips, a pioneer in establishing the relationship informally the Type Ia supernova called ASASSN-18tb thatnamed after him that allows Type la supernovae to be used as were observed by Carnegie astronomers. Starting from the top and going clockwise: The collision ofstandard rulers. Phillips was convinced: It is hydrogen youve the explosion with a hydrogen-rich companionfound; no other possible explanation.star, the explosion triggered by two colliding This is an unconventional supernova program, but I am an white dwarf stars subsequently collidingwith a third hydrogen-rich star, or the unconventional observera theorist, in fact, said Kollmeier. Its an explosion interacting with circumstellar extremely painful project for our team to carry out. Observing these hydrogen material.Image courtesy Anthony Piro things is like catching a knife, because by definition they get fainter and fainter with time! Its only possible at a place like Carnegie SUPPORT: where access to the Magellan telescopes allows us to do time-The National Natural Science Foundationintensive and sometimes arduous, but extremely important, cosmic of China supported this research in part.experiments. No pain, no gain. '