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| Wonderful Cool Miras Anchor an Independent Rate of Cosmic Expansion | ||
In the seventeenth century, Mira aka Omicron Ceti was the first variable star known to exhibit periodic brightness variations. Mira (meaning the wonderful in Latin) is now the archetype of the class of stars that expand and contract regularly with pulsation periods in the range between 100 and 1000 days. These cool (surface temperature of ~3000K), giant stars are at the late stage of their evolution, and exhibit a strong correlation between their luminosity and period of pulsation. This relation makes Miras excellent stellar standard candles to measure extragalactic distances.
The extragalactic distance ladder utilizes different standard candles to reach farther and farther distances, eventually reaching the Hubble flow where the expansion rate of the Universe is determined. The present expansion rate of the Universe, the Hubble constant, is a crucial parameter in cosmology to map the age and size of the Universe. There is an ongoing “Hubble Tension”, a large discrepancy in the Hubble constant values determined using Cepheid and Type Ia Supernovae as standard candles versus its model-dependent inference based on observations of the early universe. The Hubble tension suggests that the cosmic expansion rate in the local universe is significantly faster than predicted by the standard cosmological model.
IUCAA faculty Prof. Anupam Bhardwaj recently led an international study of 40 Oxygen-rich Mira variables in 18 stellar clusters of our Galaxy. Through long-term monitoring of these Miras, their mean observed luminosities and periods were established. The European Space Agency's Gaia mission provided accurate geometric distances to these star clusters between 13000 and 55000 light years from us, enabling an absolute calibration of their stellar luminosities. The absolute period-luminosity relations for cluster Miras provided a Cepheid-independent calibration of the supernovae in the distance ladder. This led to a 3.7% precise determination of the Hubble constant.
“We used Miras in our Galaxy as anchors for the first time to determine the most precise cosmic expansion rate based on these cool stars. Similar to Cepheids, these cluster Miras in our Galaxy allowed a three-anchor baseline calibration of the extragalactic distance ladder with Miras in two external galaxies. The three-anchor solution showed that the effect of metal-abundance on Mira luminosity is three times smaller than Cepheids, thus making them a promising alternative for the Hubble constant determination.” says Prof. Bhardwaj, lead author in the study published in the Astrophysical Journal.
“The consistency between Cepheid and Mira anchored Hubble constant values further suggests the Hubble tension is unlikely due to the measurement errors, and points to a more fundamental cause including the possibility of new physics.” says Nobel Laureate Adam Riess of the Space Telescope Science Institute and the Johns Hopkins University in Baltimore, USA, a co-author in the study.
“This study combines the fields of stellar astrophysics and cosmology. I would expect it to have a long-term impact as it ensures our understanding of the potential of Mira variable stars as a new well calibrated anchor for the Hubble constant determination.” says Dr. Marina Rejkuba, staff astronomer at the European Southern Observatory, Garching, Germany.
The absolute calibration of Miras at the first step of the distance ladder has reached a precision comparable to Cepheids. But the total error on the Mira based Hubble constant is currently dominated by the availability of only two supernovae host galaxies with known Miras. A large number of Miras are expected to be discovered in supernovae host galaxies with Rubin observatory, opening up a new way to precisely map the age and the size of the Universe.
The extragalactic distance ladder utilizes different standard candles to reach farther and farther distances, eventually reaching the Hubble flow where the expansion rate of the Universe is determined. The present expansion rate of the Universe, the Hubble constant, is a crucial parameter in cosmology to map the age and size of the Universe. There is an ongoing “Hubble Tension”, a large discrepancy in the Hubble constant values determined using Cepheid and Type Ia Supernovae as standard candles versus its model-dependent inference based on observations of the early universe. The Hubble tension suggests that the cosmic expansion rate in the local universe is significantly faster than predicted by the standard cosmological model.
IUCAA faculty Prof. Anupam Bhardwaj recently led an international study of 40 Oxygen-rich Mira variables in 18 stellar clusters of our Galaxy. Through long-term monitoring of these Miras, their mean observed luminosities and periods were established. The European Space Agency's Gaia mission provided accurate geometric distances to these star clusters between 13000 and 55000 light years from us, enabling an absolute calibration of their stellar luminosities. The absolute period-luminosity relations for cluster Miras provided a Cepheid-independent calibration of the supernovae in the distance ladder. This led to a 3.7% precise determination of the Hubble constant.
“We used Miras in our Galaxy as anchors for the first time to determine the most precise cosmic expansion rate based on these cool stars. Similar to Cepheids, these cluster Miras in our Galaxy allowed a three-anchor baseline calibration of the extragalactic distance ladder with Miras in two external galaxies. The three-anchor solution showed that the effect of metal-abundance on Mira luminosity is three times smaller than Cepheids, thus making them a promising alternative for the Hubble constant determination.” says Prof. Bhardwaj, lead author in the study published in the Astrophysical Journal.
“The consistency between Cepheid and Mira anchored Hubble constant values further suggests the Hubble tension is unlikely due to the measurement errors, and points to a more fundamental cause including the possibility of new physics.” says Nobel Laureate Adam Riess of the Space Telescope Science Institute and the Johns Hopkins University in Baltimore, USA, a co-author in the study.
“This study combines the fields of stellar astrophysics and cosmology. I would expect it to have a long-term impact as it ensures our understanding of the potential of Mira variable stars as a new well calibrated anchor for the Hubble constant determination.” says Dr. Marina Rejkuba, staff astronomer at the European Southern Observatory, Garching, Germany.
The absolute calibration of Miras at the first step of the distance ladder has reached a precision comparable to Cepheids. But the total error on the Mira based Hubble constant is currently dominated by the availability of only two supernovae host galaxies with known Miras. A large number of Miras are expected to be discovered in supernovae host galaxies with Rubin observatory, opening up a new way to precisely map the age and the size of the Universe.
Figure: The left panels show light curves of two cluster Miras with 542.6 and 261.3 days periodic brightness variations. The luminosity (in magnitudes) in near-infrared (JHKs) photometric systems is offset for clarity. The right panels display the absolute calibration of the Period-Luminosity relation for cluster Mira variables in the Hubble Space Telescope photometric system. The residuals are shown as a function of the metallicity for cluster Miras
Reference:
Absolute Calibration of Cluster Mira Variables to Provide a New Anchor for the Hubble Constant DeterminationAnupam Bhardwaj, Noriyuki Matsunaga, Caroline D. Huang, Adam G. Riess, Marina Rejkuba
The Astrophysical Journal - [ https://iopscience.iop.org/article/10.3847/1538-4357/adf20b ]
Research contacts:
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Prof. Anupam Bhardwaj IUCAA, Pune E-mail: anupam.bhardwaj_at_iucaa.in |
