Stars and star cluster heat up their environment even a million years after their birth, say scientists

BimanNath,SiddhartthaGupta-001How does the presence of a star or a gravitationally bound group of stars (star cluster), influence their environment? A previously unclear phenomenon has now been answered, thanks to a recent collaborative work from scientists at the Raman Research Institute (RRI), the Indian Institute of Science (IISc) and P.N Lebedev Physical Institute, Moscow, Russia. They have successfully developed a theoretical model to simulate the interactions between a star cluster and its surroundings, enhancing our understanding about processes that lead to the formation of stars, clusters and galaxies.

The birth of a star begins with gases accumulating under gravity until it gets hot enough to initiate nuclear fusion — a process where lighter atoms merge to form heavier atoms with an enormous outburst of energy. A strong shock wave then pushes the surrounding gas and debris into a bubble, similar to the Oort cloud surrounding our Sun. Following this, radiation from the newborn star bombards the surrounding gas to further push it away. But now, questions have been raised on the mechanism through which the stars, or clusters of stars, transfer their energy to the surrounding gas. Prevalent theories suggest that gaseous winds from the stars, and the explosions at the end of the life of stars, heat up the gas in their vicinity. This hot gas physically pushes the surrounding gas away, transferring mechanical energy in the process. But this idea did not quite explain some of the observations, thus requiring a deeper study.

The current research represents a paradigm shift. According to the new theoretical insight provided by the collaboration, pressure of the radiation dominates during the interaction between stars and the surrounding gases for the first million years of its life following which the interaction continues through the heating of the gases. Indeed, previous observations have recorded the heating of the surrounding gases. The researchers specifically predict that the heating is due to high energy photons emitted by the star cluster, which bombard the surrounding gas particles thus causing their temperature to increase.   

“What we found, something which wasn’t really anticipated, is that radiation from clusters has a two pronged effect in a way. Initially, the cluster exerts radiation pressure where it interacts by pushing against the gas. After about a million years, the radiation pressure decreases as the gas particles move far away from the cluster, but the thermal pressure starts heating the gases, causing it to expand. This thermal pressure wasn’t understood very well”, explains Biman Nath, a Professor of Astronomy and Astrophysics at RRI.

The researchers then built a computer model to simulate the effects of radiation pressure and thermal pressure from a star cluster on its surrounding environment. When they compared their results to observations from the Tarantula Nebula, also known as 30 Doradus, it was found to match closely. “Earlier observations of the nebula showed that the X-ray luminosity from the star cluster, or the brightness of X-rays, was much lower than what was theoretically predicted. Now, after incorporating our new insights, the observations match the predictions very well”, remarks Mr. Siddhartha Gupta, a postgraduate student at RRI and IISc.

The current study advances our knowledge of the formation and evolution of galaxies, but we are far from a complete understanding of the processes. “Galaxies are made of two major building blocks – gasses and stars. So it’s very important to understand the interactions between these building blocks at a micro level, to really understand how galaxies form. People have looked at the macro-scale interactions, but there are severe gaps in our understanding of these micro-level interactions. This work can be thought of as a starting point towards our understanding of the evolution of galaxies”, says Mr. Gupta about the importance of this work.

Advertisements

How does debris from supernovae make molecules? Scientists may have an answer

Milky way- Seshadri KS_1‘We are all made of stardust’ goes the common saying. The phrase is more than just rhetoric; it alludes to the formation of atoms and molecules in the universe. Most atoms and a few molecules around us were mostly formed in the bowels of exploding stars, which then went on to form planets, oceans, living organisms and everything in between. Now, a collaborative study by Raman Research Institute (RRI), Bangalore, Indian Institute of Science (IISc), Bangalore and P. N. Lebedev Physical Institute, Moscow, is studying the processes that may have led to the formation of these molecules from the debris of the exploding stars.  

Galaxies contain swirling mass of gases that eventually coalesce under gravity to form stars. “In the most common types of galaxies, like our Milky-Way, the star formation rate is between 0.5 and 1 solar masses per year, resulting in one or two supernova explosions in a century”, explains Dr. Arpita Roy, a former research student at RRI and IISc.  Occasionally however, events like close-encounters or collisions with other galaxies can shake things up within a galaxy, causing the rate of star formation to shoot up by 10 or even 100 times. Such galaxies, referred to as starburst galaxies, act as an important window into the birth and evolution of stars and galaxies. “Central regions of starburst galaxies have very high densities and are called starburst nuclei. They are the hubs for very high star formation and hence are, in general, quite violent. They are also the sources of energy, momentum, mass and heavy chemical elements”, adds Dr. Roy.

The current study focused on the processes that lead to the formation of molecules in expanding shock waves caused by supernovae, called superbubbles. “Multiple coherent supernovae in the starburst nuclei create strong shocks or superbubbles. When these strong shocks move through the interstellar medium (ISM), they sweep up ISM materials and store them in dense, thin shells behind the shocks, which further cool and form molecules. These molecular clouds could then again be sites for the formation of second generation of stars”, says Dr. Roy. “It has always been surprising to see how molecules can survive in these extreme violent environments in the central regions of the starburst galaxies. Now, there can be two situations: either these molecules are the old ones, which were originally there in the parent molecular clouds, where the massive stars were initially born, or, these are the new molecules formed in-situ in the dense superbubble shells. Our model tries to understand these issues in detail and describes that molecules in observed outflows in the central regions of the starburst galaxies can be explained by in-situ molecule formation processes” she adds.

The researchers proposed a simplified model in which superbubbles are considered to be roughly spherical in shape. Further, other factors such as the dynamics (velocity), the density and temperature of such a spherical superbubble are calculated. With these values entered in to the model, the researchers ran simulations to predict the processes, which lead to molecule formation. “We performed numerical hydrodynamic simulations with proper numerical descriptions of thermodynamics with all relevant heating (cosmic-ray heating, photo-electric heating, ionising radiation, dust emission etc.) and cooling mechanisms, which then determines the conditions for efficient molecule formation”, explains Prof. Yuri Shchekinov, a Professor at P. N. Lebedev Physical Institute. This model of molecule formation is a collective effort by Prof. Biman Nath at RRI, Prof. Prateek Sharma at IISc along with Dr. Roy and Prof. Yuri Shchekinov

Although a simple model, the simulations matched the observations of molecular outflows in superbubbles with simple spherical morphology. This further confirmed the accuracy of the proposed model and hence the processes which govern molecule formation within a starburst nuclei. The proposed model opens up the prospect of studying other aspects of galaxies and the Universe as a whole. “The detailed information of mass, energy and transport of heavy elements to the interstellar medium (ISM) help us study the overall evolution of the ISM of the host galaxies. These heavy elements may sometimes also enrich the intergalactic medium (IGM) via superbubble evolutions. Therefore, for many astrophysical purposes, such as how stars form and evolve to affect the evolution of the ISM and also the Universe in general, starburst nuclei are the most important experimental sites”, concludes Dr. Roy.

The more the merrier

Imagine two celestial objects – black holes – colliding and merging to form an even more massive black hole somewhere in deep space. Collisions and mergers such as these would be rather ubiquitous given the billions of black holes that probably dot our vast, perhaps somewhat cavernous Universe if you step just outside of Earth. But of course, with our limited technological prowess, probing the Universe’s mysteries remains out of reach – till early 2016, we had no way of directly detecting such a binary black hole merger.

Then on 11th February, 2016, the Advanced Laser Interferometer Gravitational-Wave Observatory (ALIGO) announced GW 150914, a binary black hole merger occurring on 14th September 2015 that resulted in the verification of General Relativity’s last remaining prediction – the existence of gravitational waves, powerful enough to distort our familiar four-dimensional space-time fabric. The masses involved were immense, the resultant black hole super-massive, and the energy radiated was more than the combined power of all light radiated by all stars in the thus-far observed Universe.

Physicists will tell you though that such a violent astrophysical event can hardly ever have only one tell-tale signature. 0.4 seconds after GW 150914, the Fermi Gamma-ray Burst Monitor (GBM) mounted on the Fermi Gamma-ray Space Telescope (FGST), had recorded a short gamma-ray burst (GRB).

Short GRBs are sources of extremely large amounts of energy concentrated within a very short interval of time and energetic enough to emit as much energy as our Sun does in 10 billion years. GRBs arise typically from a supernova explosion by way of which a high-mass star collapses to form a neutron star or a black hole, or perhaps from two colliding neutron stars.

Gamma-ray bursts consist of gamma-rays which are a form of electromagnetic radiation similar to X-rays, visible light or radio waves, the only difference being their underlying wavelength and the energy they bear. Along with gamma-rays though, GRBs dispense energy also in the form of highly accelerated protons. And where there are accelerated protons, can neutrinos be very far behind?

Neutrinos are uncharged, almost massless, almost non-interacting elementary particles that are everywhere around us. Forget your wonder at the dust particles you see floating in that sunbeam. A trillion neutrinos from the Sun pass through our eyes every second! Being almost non-interacting, they pass unhindered through normal matter. You need a billion kilometers of lead to detect a neutrino! But latest detectors such as the IceCube Neutrino Observatory in South Pole with its thousands of ultra-sensitive sensors buried under a cubic kilometer of dense Antarctic ice are catching up with these elusive particles.

GRBs may produce copious amounts of high-energy neutrinos (HENs). So what information do the HENs possibly associated with the GRB that was recorded 0.4s after GW150914 within the localization uncertainty limits of the GBM give us? That was precisely the question that a team of researchers from Raman Research Institute, Bengaluru, University of Johannesburg and Pennsylvania State University asked. They first theoretically modelled the GBM observation to find the amount of HENs that IceCube should detect from this short GRB and could find none. But this non-result placed an upper limit on the total energy released by this short GRB, which, incidentally turned out to be very small compared to that emitted in gravitational waves by GW150914.

No matter though, because combined HEN, GW and electromagnetic radiation studies of the Universe are here to stay – the more the merrier as the saying goes.

Dr. Nayantara Gupta, RRI says “We look forward to more instances of coinciding detections, because by combining such observations and theoretical modelling, we can learn not only from what we detect, but even what we do not.”

About the author and work:

Dr. Nayantara Gupta is an Associate Professor at the Department of Astronomy and Astrophysics at Raman Research Institute, Bengaluru. Contact: <nayan@rri.res.in>. For the publication see https://arxiv.org/pdf/1602.08436.pdf