15-Dec-2021: Smaller solar storms in the last decade baffles scientists
The Sun, an explosive celestial object, has been much quieter between 2008 and 2019 than it was between 1996 and 2007, and scientists have quantified that radial size of its Coronal Mass Ejections (CMEs) are two-thirds the radial size of CMEs in the last decade. There has been a significant decrease in the mass, size as well as internal pressure of explosive phenomena. Surprisingly, this was also accompanied with decrease in the average radial size of Coronal Mass Ejections (CMEs) – contrary to the expectation that decrease of pressure in the interplanetary medium will be accompanied with increase in radial size of CMEs.
The Sun is known to be very active with sunspots, solar flares, and CMEs-- episodic expulsion of huge magnetized plasma from the Sun out into space. Understanding such activity of the Sun, particularly the propagation of CMEs, is important since they cause major perturbations in the Earth’s magnetosphere. They effect the near-Earth space environment disturbing the orbit of satellites in low-earth orbits, Global Positioning Signals (GPS), long-distance radio communications, and power grids. Intensity of such solar activity is known to vary in 11-year long periodic cycles. It had earlier been traced that Cycle 24 (2008-2019) was weaker than Cycle 23 (1996-2007), and the Sun was weakest in 2019 during the last 100 years.
Since CMEs and other events propagate in this interplanetary space, astronomers expected that weakening of the solar cycle 24 will be reflected in the properties of these CMEs as well. They looked at the radial extent of the CMEs when they reach the Earth, over both the cycles 23 and 24, to investigate their differences.
Scientists led by Dr. Wageesh Mishra of the Indian Institute of Astrophysics (IIA), Bengaluru, an autonomous institute of the Department of Science & Technology, Government of India, have shown that the average radial size of CMEs in the last decades (during solar cycle 24 from 2008-2019) is only two-thirds of its value in the previous cycle. This was baffling to the scientists as, for them, reduced ambient pressure implied that CMEs were expanding into an interplanetary space to a significantly larger size, expectedly giving rise to large radial size. However, they found the opposite happening.
Explaining this unexpected finding, Dr. Wageesh Mishra suggested, “The reduced pressure in the interplanetary space in cycle 24 is compensated by a reduced magnetic content inside CMEs, which did not allow the CMEs to expand enough in the later phase of their propagation”. The scientist’s explanation is further strengthened by the fact that the lack of stronger and bigger CMEs arriving at the Earth during solar cycle 24 had caused reduced geomagnetic perturbations compared to cycle 23.
The team also established that the gas pressure in the interplanetary space in cycle 24 was only 40% of the pressure in cycle 23. Besides, the rate at which the Sun was losing its mass through these episodic ejections was 15% less in cycle 24 than cycle 23. Additionally, the rate of loss of quasi-steady matter by the Sun was also 10% lower in cycle 24.
This work has been published in the Frontiers in Astronomy and Space Sciences journal and is co-authored by Prof. Nandita Srivastava from Physical Research Laboratory, Udaipur, and Urmi Doshi from the M. S. University of Baroda, Vadodara, India. In this research, the team studied the Earth-directed CMEs and interplanetary counterparts of CMEs (ICMEs) using publicly available observations from Solar and Heliospheric Observatory (SOHO) and Advanced Composition Explorer (ACE), both of them missions launched by NASA in 1995 and 1997, respectively.
The expansion history of CMEs which is governed primarily by the difference in the total pressure between the CMEs and the ambient space around them, are difficult to understand. The study used expansion speeds of CMEs close to the Sun and at Earth.
The scientists said that CMEs need to be observed at different distances from the Sun to better understand the evolution of their radial sizes and expansion behaviour. Such a study would be possible using observations from several space missions such as Aditya-L1 to be launched in the coming year by ISRO, India, as well as the Parker Solar Probe launched by NASA, Solar Orbiter launched by ESA.
30-Nov-2021: The mystery behind the high abundance of Lithium in some evolved stars traced
Scientists have found a clue to the mystery behind the high abundance of Lithium— a trace element on Earth, and a key component of rechargeable batteries, in some evolved stars.
For more than four decades, Astronomers have known that a class of stars have an anomalous amount of Lithium on their surface. The reason and processes behind the high abundance of Lithium in about one percent red giants has remained a puzzle since the models of how stars evolve predict the Lithium must have been destroyed in the hot plasma of the star.
Mr. Deepak from the Indian Institute of Astrophysics (IIA) Bangalore, an autonomous institute of the Department of Science & Technology (DST), Government of India and Professor Emeritus David L. Lambert from the University of Texas at Austin and an Honorary Fellow of IIA Bangalore have for the first time confirmed that all the lithium-rich stars are burning helium in their core. They speculated in their paper published in the journal MNRAS that lithium production is linked to the violent helium-core flash.
“About four decades ago, a red giant with extraordinarily high lithium abundance at its surface was discovered. In all other respects, this red giant was of normal composition. Early follow-up investigation of lithium among red giants showed that just about one percent of sun-like red giants had a lithium-enriched surface. The questions on processes that led to a 100-fold or so increase in the lithium abundance in this exceptional red giant and reason behind this selective enrichment of lithium in the one percent of red giants intrigued us,” Deepak explains.
The authors drew on a large survey of the compositions of red giants undertaken in Australia at the Australian National University with observations gathered by on the 3.9 m Anglo-Australian Telescope at the Australian Astrophysical Observatory. The survey GALAH - named after a common Australian bird -- provided a collection of about 500,000 stars with well-determined physical and chemical properties, including lithium abundances.
To find if the enrichment of lithium in red giants favours any particular mass and metallicity, they separated GALAH's stars into different mass and metallicity ranges and then searched for lithium-rich giants among these groups. This exercise, done for the first time on such a large scale and across a wide range of mass and metallicity, reveals the rare presence of lithium-rich giants in all the Sun-like low-mass stars.
They created virtual stars of various masses and metallicity and compared the properties of these virtual stars with that of real stars from the GALAH survey. These comparisons confirmed that all the lithium-rich stars are burning helium in their core.
In a separate study, the researchers combined information about oscillations in stars' interiors with their lithium abundances to find the origin of lithium-rich giant stars. For this study, they collected astero-seismic data (i.e., information about oscillations in stars' interiors) for giant stars with measured lithium abundances. They found that all the lithium-rich giant stars are burning helium in their core.
Years ago, nuclear astrophysicists proposed a simple and short sequence of nuclear reactions involving a collision between the two stable helium isotopes which led to a stable lithium isotope. This reaction is supposed to occur during the initial violent ignition of helium-burning in the hydrogen-depleted core just prior to the onset of stable helium burning in the red giant. The authors adopt this idea and call for theoretical astrophysicists interested in the interior structure of red giants to provide quantitative estimates of lithium production during the ignition of helium burning in red giants.
12-Nov-2021: Indian astronomers develop methodology to understand the Exoplanets accurately
Indian astronomers have developed an algorithm that can increase the accuracy of data from exoplanets by reducing the contamination by the Earth’s atmosphere and the disturbances due to instrumental effects and other factors. This algorithm, called the critical noise treatment algorithm, can help to study the environment of exoplanets with better precision.
The understanding of physical properties of exoplanets with extreme accuracy can help to explore the ones that could be similar to planet Earth and hence might be habitable. With this purpose, a group of astronomers at Indian Institute of Astrophysics, Bangalore has been using the ground-based optical telescopes available in India and the data obtained by the space telescope “Transiting Exoplanet Survey Satellite” or TESS.
Prof. Sujan Sengupta of Indian Institute of Astrophysics and his Ph.D. students Aritra Chakrabarty and Suman Saha have been using the Himalayan Chandra Telescope at Indian Astronomical Observatory, Hanle and the Jagadish Chandra Bhattacharyya Telescope at Vainu Bappu Observatory, Kavalur in order to obtain signals of Exoplanets. Following the photometric transit method, they have acquired photometric data from several planet hosting stars.
However, the transit signals are heavily affected by the noise due to various sources that pose a challenge to estimate the physical parameters of the planets accurately. The team lead by Prof. Sengupta have developed a critical noise treatment algorithm that can treat the transit signals detected by both ground- and space-based telescopes with much better precision than ever before.
Recently, Saha and Sengupta have demonstrated the effectiveness of this algorithm by critically analysing the data of TESS (Transiting Exoplanet Survey Satellite) space telescope, reduced the instrumental noise and the disturbances arising from variability and pulsation of the host stars and estimated the physical parameters of a few Exoplanets accurately. The work has been published in The Astronomical Journal, a peer-reviewed scientific journal by the American Astronomical Society (AAS).