Mothra is a system of two supergiant stars in one of the extremely magnified galaxies behind the lensing galaxy cluster MACS J0416.1-2403.
Mothra is best described as a binary system with two supergiant stars, a hot one with temperature T ≈ 14,000 K, and a cooler one with T ≈ 5,000 K. Image credit: NASA / ESA / CSA / STScI / Jose M. Diego, IFCA / Jordan C.J. D’Silva, UWA / Anton M. Koekemoer, STScI / Jake Summers, ASU / Rogier Windhorst, ASU / Haojing Yan, University of Missouri.
“The discovery of ‘MACS J1149 Lensed Star 1,’ informally known as Icarus, represented the birth of a new branch of astronomy dedicated to the study of luminous stars at cosmological distances,” said Instituto de Física de Cantabria astronomer Jose Diego and his colleagues.
“This feat is only possible thanks to the boost provided by extreme magnification factors, with values over 1,000 at least for short periods of time.”
“In the case of Icarus, the object caught the attention of astronomers because it brightened by more than a magnitude in observations two years apart.”
“The search for lensed stars currently covers a wide range in redshift and includes objects that do not show variability,” they added.
“One example is Godzilla, which was identified thanks to its anomalous magnification.”
“Godzilla was identified as a monster star because of its extraordinary brightness. At z = 2.37 (10.9 billion light-years from Earth), it has apparent magnitude AB ≈ 22 in the visible bands and is within reach of modest ground-based telescopes.”
“Another example is Earendel at an estimated z = 6.2 (12.9 billion light-years from Earth) and currently holding the record for the most distant star ever observed.”
The newly-detected star is located in a galaxy with spectroscopic redshift z = 2.091 (10.5 billion light-years from Earth).
“We nicknamed the star system Mothra in a nod to its monster nature, being both extremely bright and extremely magnified,” the astronomers said.
“The more official name is EMO J041608.838-240358.60, where the acronym EMO refers to extremely magnified object.”
Mothra is magnified by a factor of at least 4,000 by both the foreground galaxy cluster MACS J0416.1-2403 and an additional object that likely weighs about as much as a globular cluster.
“Interestingly, Mothra is also visible in the Hubble observations that were taken nine years previously,” the researchers said.
“This is unusual, because a very specific alignment between the foreground galaxy cluster and the background star is needed to magnify a star so greatly.”
“The mutual motions of the star and the cluster should have eventually eliminated that alignment.”
“The most likely explanation is that there is an additional object within the foreground cluster that is adding more magnification.”
The authors were able to constrain its mass to be between 10,000 and 1 million times the mass of our Sun.
“The exact nature of this so-called milli-lens remains unknown,” they said.
“The most likely explanation is a globular star cluster that’s too faint for Webb to see directly.”
“But we don’t know the true nature of this additional lens yet.”
The discovery is reported in a paper in the journal Astronomy & Astrophysics.
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Jose M. Diego et al. 2023. JWST’s PEARLS: Mothra, a new kaiju star at z = 2.091 extremely magnified by MACS0416, and implications for dark matter models. A&A 679, A31; doi: 10.1051/0004-6361/202347556
