Stars

Pin It
Whatsapp

23-Aug-2022: Study of dust from cosmic dance of a white dwarf and companion star could unravel mysteries behind start of life

In the winter months of 2007, astrophysicists from all over the world had made a beeline to observatories on mountaintops to observe a bright explosion born out of the cosmic dance of a white dwarf and its companion star resulting in thick dust around an imploding novae.

Dr R K Das, scientist from SN Bose Centre for Basic Science (SNBCBS) who had stationed himself at Mount Abu Observatory, and his team, observed the imploding novae called Nova V1280 Scorpii and found that a thick dust formed around it after a month and lasted for about 250 days.

The team from SNBCBS, an autonomous institute of Department of Science and Technology (DST) used the observed data on infrared spectra of the imploding novae and constructed simple models which helped them estimate its parameters like hydrogen density, temperature, luminosity and elemental abundances during pre- and post-dust phase. They have found high abundance of certain elements like carbon, nitrogen and oxygen in the ejecta along with a mixture of small amorphous carbon dust grains and large astrophysical silicate dust grains.

The dust formation was observed in parallel by international collaborators of the team from the Very Large Telescope Interferometer in Chile. This helped them take precision measurements of the rate of expansion of the dust shell around a nova for the first time.

The stellar event which was golden opportunity for scientists to study the exploding stellar matter was an example of space-dust collisions which could propel organisms over enormous distances between planets to start life on a planet. Their study of novae dust could help in understanding the nature and characteristics of the dust and associated processes.

Cosmic dust or extra-terrestrial dust formation in the hostile environment of novae ejection has been an open question for many years. Hundreds of kilograms of such dust fall on the Earth every day. However, formation, nature and composition of the dust are not properly understood yet. Dr. Das explained that dust formation in novae ejecta is not a common phenomenon. It has been observed only in a few novae within 30 to 100 days after an outburst, as compared to interstellar dust, which typically takes a few thousand years to form and hence provided opportunity to study the dust formation process in novae.

The team varied the parameters over a wide range and constructed more than fifty thousand models generating spectrum for each model. Finally, they fit the observed spectrum with the model generated ones. From the best fit, they estimated the parameters during pre- and post-dust phase. Such a detailed modelling of a dust forming Nova had never been done before. The entire process took a couple of years.

Besides high abundance of isotopes certain elements like carbon, nitrogen and oxygen in predust phase of the imploding novae as compared to solar values, the scientists found a mixture of small amorphous carbon dust grains and large astrophysical silicate dust grains present in the ejecta in the postdust phase. Some complex organic compounds like amorphous organic solids with a mixed aromatic–aliphatic structure were found which play an important role in formation of molecular cloud in stars and planets. The study was published in the journal Astrophysical Journal recently.

The team has suggested that as the expanding dust shell of V1280 Scorpii Nova continued to expand these dust grains will eventually mix with interstellar matter. But that will take thousands of years - a small time in the cosmic time scale.

17-Jan-2022: Motion of some stars holds clue of dark matter shape in barred galaxies

Dark matter forms the skeleton on which galaxies form, evolve, and merge. Scientists investigating how the shape of dark matter halo affects the motion of stars in stellar bars (found at the centre of some galaxies) have found that out-of-plane bending events of the bar explain the shape of dark matter halos in barred galaxies. Out of plane bending of the bar in barred galaxies (central bar-shaped structure composed of stars) is a rare violent bar thickening mechanism known as buckling.

The trillions of galaxies in our Universe have different shapes and sizes, which are determined by the motion of their stars. Our own galaxy, the Milky Way, is a disk galaxy made up of stars moving in circular orbits around the center in a flattened disk, with a dense collection of the stars at the center called the bulge. These bulges can have shapes ranging from nearly spherical to as flat as the galaxy disk.  Milky Way has a flat boxy or peanut-shaped bulge in its center. Such bulges are formed due to thickening of the stellar bars in galaxies. One of the interesting and violent thickening mechanism is buckling, where bar bends out of the plane of the galaxy disk.  Many recent numerical and observational studies suggest that dark matter halos are spherical, prolate (a sphere squashed from the sides), or oblate (a sphere squashed from the top and bottom)   in shape. However, its effect on stellar kinematics in the bulges and bars of galaxies is not well understood.

In the current work led by Ankit Kumar, a Ph.D. student at the Indian Institute of Astrophysics, an autonomous institute of the Department of Science & Technology, Govt. of India, and co-authored by Prof. Mousumi Das of IIA and Dr. Sandeep Kumar Kataria of Shanghai Jiao Tong University, the team investigated the dynamical evolution of the galaxies using state-of-art numerical simulations at IIA. Their simulations demonstrate that bars in prolate dark matter halos undergo three prominent bar buckling (out of plane bending) events in 8 billion years which make them detectable for longer time. It is the first time that three-bar buckling events have been reported in any study. The boxy/peanut shape bulges, which formed as a result of bar buckling, are stronger in prolate dark matter halo, and the signatures of bar buckling are the most durable in them.  This work has been published in the peer-reviewed journal “Monthly Notices of the Royal Astronomical Society”.

They concluded that the rarity of observed buckling events along with multiple buckling event in our prolate halo (a sphere squashed from the sides) simulation indicate that the shapes of dark matter halos in most of the barred galaxies maybe oblate (a sphere squashed from the top and bottom) or spherical.

“We have studied the effect of non-spherical dark matter halos on the shape of the disk galaxies by generating realistic mock galaxies and evolving them in time using the supercomputing facility available at IIA, Bengaluru,” said Ankit Kumar, the lead author of the paper.

“In our Universe, detection of the ongoing buckling events is very rare. To our knowledge, there are only 8 galaxies in the observations which are currently going through buckling. Our study suggests that most barred galaxies may have more oblate or spherical halos rather than prolate halos,” the authors added.

They explained that each event of the buckling thickens the bar further. During the first buckling, the innermost region of the bar gets thicker, while in the subsequent buckling events outer region of the bar gets thicker. Since the bar in prolate halo shows three distinct buckling events, the bar becomes the thickest in prolate halo. As a result, most strong boxy/peanut bulge formed in prolate halo.” said Prof. Mousumi Das of IIA. Dr. Sandeep Kataria pointed out that shape of dark matter halos is important for understanding halo spin, which is an area of research being studied by many galaxy simulation groups over the world.

7-Jan-2022: A star with a heartbeat & without a magnetic field discovered

A group of Indian and international scientists have spotted a peculiar binary star that shows heartbeat but no pulsations contrary to the norm of binary stars of sporting both heartbeats as well as pulsations. This star is called HD73619 in Praesepe (M44), located in the Cancer constellation, one of the closest open star clusters to the Earth.

A total of about 180 heartbeat stars are known to date. The name 'Heartbeat' stems from the resemblance of the path of the star to an electrocardiogram of the human heart. These are the binary star systems where each star travels in a highly elliptical orbit around the common centre of mass, and the distance between the two stars varies drastically as they orbit each other. When the stars are at closest passage of binary systems, a sudden increase in integrated brightness with amplitude of the order of several parts-per-thousand (ppt) is observed. As the components move apart, the light variation falls and finally becomes flat, indicating that combined flux is reduced, resulting in alternating peaks and troughs in their light curves. The pulsation activity of such stars is due to the oscillations in the component stars when they are at their closest approach.

A team of 33 scientists, led by Dr. Santosh Joshi from Aryabhatta Research Institute of Observational Sciences (ARIES), an autonomous institution under the Department of Science & Technology (DST), Govt of India, carried out analysis of photometric and high-resolution spectroscopic observations of HD73619 obtained using 8 ground-based telescopes located in different parts of the globe. They have found that HD73619 is the first member of heartbeat systems in binary chemically peculiar stars that does not show any pulsation/vibrational activity at their closest approach. Chemically peculiar stars are those stars which have an unusual abundance of elements heavier than hydrogen and helium on the surface. Their data also revealed that the newly discovered heartbeat star exhibits either very weak or no magnetic field. Absence of weak magnetic field means that any dark spots on the HD73619 may have different and hitherto unknown origin as compared to sunspots which are created by strong magnetic field. The findings have been accepted for publication by Monthly Notices of Royal Astronomical Society, a scientific journal of Oxford University Press.

The discovery is of vital importance for the study of inhomogeneities due to spots in non-magnetic stars and to investigate the origin of the pulsation variability. The research has been the result of the Nainital-Cape survey, one of the longest ground-based surveys to search and study the pulsation variability in a sample of CP stars which was initiated about two decades ago by astronomers of ARIES, Nainital, and South African Astronomical Observatory SAAO, Cape Town. As part of this survey, the group had monitored a few members of Praesepe in the past. The other members of this wide collaboration are from Uganda, Thailand, United States of America, Russia, Belgium, United Kingdom, France, Spain, South Africa, Poland, and Turkey. This joint work is supported by the Department of Science & Technology (DST), Govt. of India, and the Belgian Federal Science Policy Office (BELSPO), Govt. of Belgium under the Belgo-Indian Network for Astronomy and Astrophysics (BINA), project.

Pin It
Whatsapp

23-Aug-2022: Study of dust from cosmic dance of a white dwarf and companion star could unravel mysteries behind start of life

In the winter months of 2007, astrophysicists from all over the world had made a beeline to observatories on mountaintops to observe a bright explosion born out of the cosmic dance of a white dwarf and its companion star resulting in thick dust around an imploding novae.

Dr R K Das, scientist from SN Bose Centre for Basic Science (SNBCBS) who had stationed himself at Mount Abu Observatory, and his team, observed the imploding novae called Nova V1280 Scorpii and found that a thick dust formed around it after a month and lasted for about 250 days.

The team from SNBCBS, an autonomous institute of Department of Science and Technology (DST) used the observed data on infrared spectra of the imploding novae and constructed simple models which helped them estimate its parameters like hydrogen density, temperature, luminosity and elemental abundances during pre- and post-dust phase. They have found high abundance of certain elements like carbon, nitrogen and oxygen in the ejecta along with a mixture of small amorphous carbon dust grains and large astrophysical silicate dust grains.

The dust formation was observed in parallel by international collaborators of the team from the Very Large Telescope Interferometer in Chile. This helped them take precision measurements of the rate of expansion of the dust shell around a nova for the first time.

The stellar event which was golden opportunity for scientists to study the exploding stellar matter was an example of space-dust collisions which could propel organisms over enormous distances between planets to start life on a planet. Their study of novae dust could help in understanding the nature and characteristics of the dust and associated processes.

Cosmic dust or extra-terrestrial dust formation in the hostile environment of novae ejection has been an open question for many years. Hundreds of kilograms of such dust fall on the Earth every day. However, formation, nature and composition of the dust are not properly understood yet. Dr. Das explained that dust formation in novae ejecta is not a common phenomenon. It has been observed only in a few novae within 30 to 100 days after an outburst, as compared to interstellar dust, which typically takes a few thousand years to form and hence provided opportunity to study the dust formation process in novae.

The team varied the parameters over a wide range and constructed more than fifty thousand models generating spectrum for each model. Finally, they fit the observed spectrum with the model generated ones. From the best fit, they estimated the parameters during pre- and post-dust phase. Such a detailed modelling of a dust forming Nova had never been done before. The entire process took a couple of years.

Besides high abundance of isotopes certain elements like carbon, nitrogen and oxygen in predust phase of the imploding novae as compared to solar values, the scientists found a mixture of small amorphous carbon dust grains and large astrophysical silicate dust grains present in the ejecta in the postdust phase. Some complex organic compounds like amorphous organic solids with a mixed aromatic–aliphatic structure were found which play an important role in formation of molecular cloud in stars and planets. The study was published in the journal Astrophysical Journal recently.

The team has suggested that as the expanding dust shell of V1280 Scorpii Nova continued to expand these dust grains will eventually mix with interstellar matter. But that will take thousands of years - a small time in the cosmic time scale.

17-Jan-2022: Motion of some stars holds clue of dark matter shape in barred galaxies

Dark matter forms the skeleton on which galaxies form, evolve, and merge. Scientists investigating how the shape of dark matter halo affects the motion of stars in stellar bars (found at the centre of some galaxies) have found that out-of-plane bending events of the bar explain the shape of dark matter halos in barred galaxies. Out of plane bending of the bar in barred galaxies (central bar-shaped structure composed of stars) is a rare violent bar thickening mechanism known as buckling.

The trillions of galaxies in our Universe have different shapes and sizes, which are determined by the motion of their stars. Our own galaxy, the Milky Way, is a disk galaxy made up of stars moving in circular orbits around the center in a flattened disk, with a dense collection of the stars at the center called the bulge. These bulges can have shapes ranging from nearly spherical to as flat as the galaxy disk.  Milky Way has a flat boxy or peanut-shaped bulge in its center. Such bulges are formed due to thickening of the stellar bars in galaxies. One of the interesting and violent thickening mechanism is buckling, where bar bends out of the plane of the galaxy disk.  Many recent numerical and observational studies suggest that dark matter halos are spherical, prolate (a sphere squashed from the sides), or oblate (a sphere squashed from the top and bottom)   in shape. However, its effect on stellar kinematics in the bulges and bars of galaxies is not well understood.

In the current work led by Ankit Kumar, a Ph.D. student at the Indian Institute of Astrophysics, an autonomous institute of the Department of Science & Technology, Govt. of India, and co-authored by Prof. Mousumi Das of IIA and Dr. Sandeep Kumar Kataria of Shanghai Jiao Tong University, the team investigated the dynamical evolution of the galaxies using state-of-art numerical simulations at IIA. Their simulations demonstrate that bars in prolate dark matter halos undergo three prominent bar buckling (out of plane bending) events in 8 billion years which make them detectable for longer time. It is the first time that three-bar buckling events have been reported in any study. The boxy/peanut shape bulges, which formed as a result of bar buckling, are stronger in prolate dark matter halo, and the signatures of bar buckling are the most durable in them.  This work has been published in the peer-reviewed journal “Monthly Notices of the Royal Astronomical Society”.

They concluded that the rarity of observed buckling events along with multiple buckling event in our prolate halo (a sphere squashed from the sides) simulation indicate that the shapes of dark matter halos in most of the barred galaxies maybe oblate (a sphere squashed from the top and bottom) or spherical.

“We have studied the effect of non-spherical dark matter halos on the shape of the disk galaxies by generating realistic mock galaxies and evolving them in time using the supercomputing facility available at IIA, Bengaluru,” said Ankit Kumar, the lead author of the paper.

“In our Universe, detection of the ongoing buckling events is very rare. To our knowledge, there are only 8 galaxies in the observations which are currently going through buckling. Our study suggests that most barred galaxies may have more oblate or spherical halos rather than prolate halos,” the authors added.

They explained that each event of the buckling thickens the bar further. During the first buckling, the innermost region of the bar gets thicker, while in the subsequent buckling events outer region of the bar gets thicker. Since the bar in prolate halo shows three distinct buckling events, the bar becomes the thickest in prolate halo. As a result, most strong boxy/peanut bulge formed in prolate halo.” said Prof. Mousumi Das of IIA. Dr. Sandeep Kataria pointed out that shape of dark matter halos is important for understanding halo spin, which is an area of research being studied by many galaxy simulation groups over the world.

7-Jan-2022: A star with a heartbeat & without a magnetic field discovered

A group of Indian and international scientists have spotted a peculiar binary star that shows heartbeat but no pulsations contrary to the norm of binary stars of sporting both heartbeats as well as pulsations. This star is called HD73619 in Praesepe (M44), located in the Cancer constellation, one of the closest open star clusters to the Earth.

A total of about 180 heartbeat stars are known to date. The name 'Heartbeat' stems from the resemblance of the path of the star to an electrocardiogram of the human heart. These are the binary star systems where each star travels in a highly elliptical orbit around the common centre of mass, and the distance between the two stars varies drastically as they orbit each other. When the stars are at closest passage of binary systems, a sudden increase in integrated brightness with amplitude of the order of several parts-per-thousand (ppt) is observed. As the components move apart, the light variation falls and finally becomes flat, indicating that combined flux is reduced, resulting in alternating peaks and troughs in their light curves. The pulsation activity of such stars is due to the oscillations in the component stars when they are at their closest approach.

A team of 33 scientists, led by Dr. Santosh Joshi from Aryabhatta Research Institute of Observational Sciences (ARIES), an autonomous institution under the Department of Science & Technology (DST), Govt of India, carried out analysis of photometric and high-resolution spectroscopic observations of HD73619 obtained using 8 ground-based telescopes located in different parts of the globe. They have found that HD73619 is the first member of heartbeat systems in binary chemically peculiar stars that does not show any pulsation/vibrational activity at their closest approach. Chemically peculiar stars are those stars which have an unusual abundance of elements heavier than hydrogen and helium on the surface. Their data also revealed that the newly discovered heartbeat star exhibits either very weak or no magnetic field. Absence of weak magnetic field means that any dark spots on the HD73619 may have different and hitherto unknown origin as compared to sunspots which are created by strong magnetic field. The findings have been accepted for publication by Monthly Notices of Royal Astronomical Society, a scientific journal of Oxford University Press.

The discovery is of vital importance for the study of inhomogeneities due to spots in non-magnetic stars and to investigate the origin of the pulsation variability. The research has been the result of the Nainital-Cape survey, one of the longest ground-based surveys to search and study the pulsation variability in a sample of CP stars which was initiated about two decades ago by astronomers of ARIES, Nainital, and South African Astronomical Observatory SAAO, Cape Town. As part of this survey, the group had monitored a few members of Praesepe in the past. The other members of this wide collaboration are from Uganda, Thailand, United States of America, Russia, Belgium, United Kingdom, France, Spain, South Africa, Poland, and Turkey. This joint work is supported by the Department of Science & Technology (DST), Govt. of India, and the Belgian Federal Science Policy Office (BELSPO), Govt. of Belgium under the Belgo-Indian Network for Astronomy and Astrophysics (BINA), project.

2021

31-Dec-2021: Carbon-rich stars steal heavy elements from their low mass companions

Scientists have long been intrigued by the presence of much higher fraction of elements heavier than iron than is expected in carbon-rich stars.  A new research by Indian astronomers has now traced its origin to the low mass companions of these stars, from which the materials have been stolen.

Though there has been significant progress in understanding the formation and evolution of several chemical elements and their isotopes in the Universe,  the origin and evolution of heavy elements, that is, elements heavier than iron, in the  Universe is far from being clearly understood.

A group of astronomers from the Indian Institute of Astrophysics, Bengaluru, an autonomous institute of the Department of Science & Technology, Govt. of India led by Prof Aruna Goswami and her doctoral students Meenakshi P. and Shejeelammal J. at IIA have analyzed the surface chemical composition of several Carbon Enhanced Metal-Poor (CEMP) stars and have achieved a significant advancement in unraveling this puzzle. This work has been published in ‘The Astrophysical Journal’ recently.

CEMP stars are characterized by diverse heavy elements abundance patterns and are primarily classified into four groups, based on which groups of heavy elements are more abundant. These are mostly dwarf stars, sub-giant stars, or giant stars, and stars that belong to these evolutionary stages cannot produce elements heavier than iron.

“At the evolutionary stages in which the stars exist, they are not expected to produce heavy elements. However, the surface chemical composition of these stars exhibit abundances of heavy elements those are about 100 to 1000 times higher than that of the Sun. So, we looked for the origin of the observed enhanced abundances of the heavy elements, deriving possible clues from the stellar surface chemical abundance signatures,” Prof. Goswami explained.

The team analyzed high quality, high-resolution spectra of the stars acquired using 2-m Himalayan Chandra Telescope (HCT) at the Indian Astronomical Observatory, Hanle, 1.52-m Telescope at the European Southern Observatory at La Silla, Chile, and the  8.2-m SUBARU Telescope at the summit of Mauna kea, Hawaii, operated by the National Astronomical  Observatory of Japan, to resolve this puzzle.

They used elemental abundance ratios of certain key elements such as carbon, magnesium, strontium, barium, europium, lanthanum, etc., as diagnostics that gave important clues to the origin of the enhanced abundances.

In two recent papers, both published in the Astrophysical Journal, they have shown that the enhanced heavy elements observed in the CEMP stars are actually produced by their low-mass stellar companions in a phase of evolution called Asymptotic Giant Branch (AGB) and transferred the products to the CEMP stars through various mass transfer mechanisms. The low-mass companions have further evolved to white dwarfs that are no longer detectable. The scientists employed a set of classification schemes and used spectroscopic techniques to examine if the stars show variability in radial velocity and found that most of the stars are indeed binaries.

“Analysis have provided further support and confirmed that the companions are of low-mass and also of low-metallicity.” Shejeelammal and Meenakshi pointed out.