Astrophysics studies physics phenomena that occur in space and govern the evolution of the Universe. It started to blossom in the middle of the past century and developed very rapidly since then, to become today the most dynamic branch of contemporary physics.
Many fields of modern physics converge to astrophysics: nuclear physics governs the evolution of stars; particle physics governs the evolution of the Universe at its very beginning, just after the Big Bang; atomic and molecular physics are at the heart of spectroscopy – the main tool of observation of the Universe; ionised gas volumes are ubiquitous, requiring plasma physics for their study; condensed matter physics governs the formation of dust and later of planets, etc.
However, in many cases, extreme conditions of temperature and pressure, far away from conditions that can be realized on Earth, are achieved in the Universe, making it a unique laboratory for the study of the laws of nature. Neutron stars are made of the same nuclear matter that makes nuclei, but at a scale of some thousand cubic kilometres; White dwarfs, with sizes at the Earth’s scale, are in a quantum state where the electrons and nuclei of atoms have split in two very distinct families; Black holes offer extreme conditions of pressure resulting in densities exceeding the limits of current knowledge; velocities of celestial objects are often a significant fraction of the light velocity, requiring special relativity to describe the kinematics and gravitation dominates the landscape, requiring general relativity to describe the dynamics; nuclear fusion occurs in stars in a smooth and slow fashion, heat being confined to the interior of their huge volumes, in conditions that are impossible to realize in the much smaller terrestrial reactors; the very large distances and very long time scales that dominate the Universe allow for the observations of rare phenomena, such as the 21 cm hyperfine transition of neutral hydrogen, which could not be observed on Earth, etc.
A consequence is that the most puzzling unanswered questions that challenge modern physics are at the heart of astrophysics: quantum physics and gravitation are known to be incompatible under the conditions of pressure and temperature that dominate just after the Big bang, in the so called inflation period, requiring a complete revision of our understanding of physics at what is called the Planck scale; an unknown form of matter, called dark matter, having only gravitational interactions, dominates the dynamics of galaxies and accounts for a quarter of the energy content of the Universe; most of the remaining three quarters are also not understood, we call them dark energy: they dominate the scene at very large distances, causing in particular an acceleration of the rate of expansion of the Universe, but have negligible effects at galactic distances. Indeed, what we know and understand, namely stars and gas, amounts for only a few percent of the energy content of the Universe, the rest being still a complete mystery.
An important factor of the rapid development of astrophysics has been the ability to observe the Universe from space. Absorption in the Earth atmosphere causes ground observatories to be blind to electromagnetic radiations other than visible and radio, up to millimetre wavelengths. Observation from space has opened new windows on the Universe, at microwave, infrared, ultraviolet, X and gamma wavelengths. As a result, many laboratories have developed, hosting together space technology and astrophysics, NASA being the most emblematic example.
What about Vietnam?
The recent history of our country has prevented the development of fundamental research for most of the past century. Even today, the priorities implied by the socio-economic development of the nation seriously limit the amount of resources that can be devoted to fundamental research. Yet, several Viet Kieu astrophysicists doing active research in observatories abroad, have encouraged the promotion of astrophysics in the country. Professor Nguyen Quang Rieu, a radio astronomer at the Paris-Meudon Observatory, has been the most active in this respect and can be considered as the founding father of what exists today, with essentially three nuclei of active research: one in TPHCM, with Professor Phan Bao Ngoc at the International University, and two in Ha Noi, with Professor Dinh Van Trung at the Institute of Physics and a team of half a dozen PhDs at the DAP of VNSC, both under the Vietnam Academy of Sciences and technology (VAST). In addition, astronomy lectures are organized by the departments of physics of the universities of education in Ha Noi and TPHCM, giving the lecturers a chance to do some research in collaboration with foreign astrophysicists.
The DAP was originally hosted at the Institute for nuclear science and technology (INST), under VINATOM, where it focused on cosmic ray research during the first decade of the century, using data from the Pierre Auger observatory in Argentina, the most advanced facility in the world for the study of the highest energy cosmic rays. At the instigation of professor Nguyen Quang Rieu, and recognizing the very long time scales that will be required to progress beyond what had been achieved at the Pierre Auger observatory, its members gradually switched to radio astronomy, much better suited to the progress of a young research team in a developing country.
In April 2014, the INST team organized a colloquium that brought together the Vietnamese astrophysics research community together with prestigious astrophysicists from France, Japan, Korea and China. It was on this occasion that Dr Pham Anh Tuan, who presented to the participants the activities of VNSC and of the future Vietnam Space Centre, invited the INST team to join. As VNSC was offering it a much better environment to progress, in contact with engineers and scientists sharing similar motivations and interests, the decision was promptly taken and the transfer took place on January 1st, 2015.
The 2.6 m radio telescope: an excellent training tool
Research in modern astrophysics implies the use of very large facilities, operated jointly by several countries. A typical example is the ALMA radio interferometer, which recently started operation in Chile, the most sensitive tool today for the exploration of the Universe at the frontier of millimetre/submillimeter wavelengths. It is jointly operated by American, European and Asian member states. Having access to such facilities, meaning the ability to propose and exploit observations, implies, for Vietnamese scientists, collaborating with foreign teams. In parallel with such research, it is important to have at home instruments, of a much more modest scale, offering an opportunity to become familiar with the methods and techniques in use on the larger international facilities.
Velocity Doppler spectra have been collected along the disk of the Milky Way over three quarters of the galactic longitude. They have been reduced into peaks associated with different clouds of atomic hydrogen and show evidence for differential rotation. A map of atomic hydrogen in the Milky Way disk has been constructed and compared with its known arm and bar structure (published in Comm. Phys. Vietnam).
The Sun, which recently went over a maximum of its 11 year cycle of activity, has been tracked for several months. A joint analysis of the collected data with similar data taken by the Learmonth Solar Observatory in Australia has shown the presence of flares and has given evidence for correlated mHz oscillations at the percent level (published in Solar Physics). These oscillations are now understood as having an instrumental origin. They are caused by interferences between the direct signal detected in the main lobe and its specular reflection on ground detected in a side lobe. We have shown that this causes the observed correlations between the Ha Noi and Learmonth oscillations. Essentially, as the period of multipathing oscillations depends on the velocity of the source, and as the speed of Earth rotation is the same in Ha Noi and at Learmonth, the observed periods measured simultaneously by both telescopes have to be correlated (published in PASA). The observation of solar flares and the comparison with the Learmonth observations, have revealed the occurrence of very large polarizations (~70%), that could be detected because the Ha Noi and Learmonth telescopes use different feeds, helical in Ha Noi and dipole in Learmonth, implying the detection of different polarization components of the incident wave (published in Comm. Phys. Vietnam).
Right: Oscillation signals detected in Hanoi (upper panel) and Learmonth (lower panel).
We have performed detailed studies of the performance of the instrument (published in Comm. Phys. Vietnam). In particular, we have measured the pointing accuracy as 0.11o×0.22o, the beam width as σ=2.3o, the frequency resolution as 7.8 kHz, the gain drop vs amplitude as ~5 ppm/K of antenna temperature, the gain drop vs frequency as ~70 ppm/kHz at 1420.4 MHz, increasing to ~130 ppm/kHz at 1415 MHz, the antenna efficiency factor as ~65%, the conversion factor as ~1.25±0.09 K/kJy (7%) and the sensitivity limit at ~300 Jyresulting from gain instabilities and human interferences rather than noise. The latter was the object of a long campaign of observation of the Moon, at the limit of sensitivity of the instrument, that gave a measurement of its black body temperature, 207±40 K (published in Comm. Phys. Vietnam).
The telescope has now been transferred to the roof of the University of Science and Technology of Hanoi (USTH) building, close to VNSC, where it will be used to train students and young researchers, from both VNSC and USTH. We are currently looking into a possible successor, having more ambitious aims, and preparing a report to the VNSC directorate that must be handed to them by end of June, 2015. The acquisition of efficient training tools is an essential task for any research team: it must be done with great care in order to optimize the ratio between the quality of the training that it provides and the resources, material and human, that are necessary for its acquisition, maintenance, operation and exploitation.
The study of evolved stars
Stars die after having converted most of their hydrogen into helium, which, for stars such as our Sun, takes several billions of years. Their central core condenses in a very dense object – a White dwarf – while their envelope grows to dimensions at the scale of the solar system and finally gets diluted into space, recycling matter that will be used again later on for the formation of new stars. The time spent in such agony is at the scale of millions of years and implies a succession of different periods during which nuclear reactions produce various light and intermediate nuclei, in particular oxygen and carbon. Such evolved stars, after having become what is called Red giants, spend time on the so-called Asymptotic Giant Branch (AGB) before fading away in the form of Planetary Nebulae surrounding the White dwarf.
In collaboration with the Observatoire de Paris, we have studied the CO and hydrogen (HI) emissions of such AGB stars, using observations of the IRAM radio interferometer at Plateau de Bure, the IRAM 30 m telescope at Pico Veleta and the Very Large Array in New Mexico.
We developed new analysis tools with the study of RS Cnc, an Asymptotic Giant Branch star in its thermal pulsing phase. The star features a bipolar molecular outflow having a velocity of ~8 km/s, superimposed on a slower wind (~2 km/s). Detailed studies of the velocity spectra allow for constructing a model of the star atmosphere accounting for most of the observed features (published in AA). A simultaneous analysis of CO(1-0) and CO(2-1) data has revealed a lower temperature than previously assumed and asymmetries between the two jets (published in RAA). Better agreement between observations and model has been obtained, providing useful information on the transition between the Red giant and Planetary nebula phases.
The same model was applied to the study of another AGB star, at an earlier stage of evolution, EP Aqr, revealing a similar bipolar outflow oriented along the line of sight within a few degrees. Such special geometry makes it easier to study the morphology and kinematics of the gas volume around the star and revealed a strong dependence of the temperature over the star latitude (publication in preparation).
We took advantage of the open access policy of ALMA, that gives access to its observations one year after completion, to study the CO emission of a particularly spectacular post-AGB star, called the Red Rectangle. We were able to evaluate the density, temperature and velocity of the gas, giving evidence for a bipolar outflow separated from a rotating equatorial region, with a strong temperature enhancement at the interface between the two gas volumes. As the proponents of these observations have not yet completed their own analysis of the data, it was for us an opportunity to publish first many new results that provide important information on the physics of such post-AGB stars(submitted for publication in RAA).
The HI emission of several low mass loss rate AGB stars, measured at the Nançay observatory and using the VLA, has been studied, allowing observations at much larger distances from the star than possible for CO molecules that are soon dissociated from the interstellar UV radiation (publication in preparation).
Having now acquired some expertise in the study of evolved stars, we shall pursue their observation using the most performing facilities available in the world, either alone or in collaboration with foreign teams. In particular we intend to take advantage of a cooperation agreement existing between VAST and the French CNRS to strengthen our collaboration with Observatoire de Paris on such studies. We are also planning to propose observations in HI using the giant FAST antenna in current construction in nearby China.
High redshift galaxies
In collaboration with University Paul Sabatier in Toulouse, we studied the CO(7-6) emission of RX J0911, host galaxy of a gravitationally lensed high red shift quasar (z~2.8, look back of 11.3 Gyr). The line measures its gas content and the continuum underneath its dust content, providing useful information on the star formation rate in such early galaxies. The lensing mechanism has been studied in Hanoi (published in RAA). Our results show that the source has a radius of 850±120 pc on the line (~7 s.d.) and 260±130 pc (only 2 s.d.) in the continuum. They also provide evidence for ellipticity (3.3 standard deviations away from circular) and for a significant velocity gradient (molecular outflow and/or rotation, 4.5 standard deviations) (published in Astronomy and Astrophysics Letters). The very narrow CO(7-6) line implies a low dynamical mass and both gas and dust mass evaluations fall on the low side of the normal high-zquasar host population. The large star formation efficiency is on the high side of both low-z and high-z galaxies: much of the gas has been exhausted after an intense star formation period, leaving the galaxy at the border between high-zand low-z quasar hosts.
We shall actively pursue this line of research, in particular using the outstanding potential offered by ALMA for the study of galaxies of the early Universe. To this aim, we are currently exploring possibilities of collaboration with foreign teams, particularly Japanese teams, which should be eased by the existing Japan-Vietnam cooperation agreement within which framework VNSC is operating.
Teaching, training, outreach
From the beginning we have given high priority to the promotion of astrophysics in Vietnam, from where it is essentially absent. Members of the team give lectures at the Hanoi university of sciences, at the Hanoi university of education and at USTH. They also contribute lab work and tutoring to the USTH master course on Space & Applications.
We do our utmost to establish close contacts with astronomers and astrophysicists in the region and abroad. In this context, with the help of some International Astronomer Union (IAU) support, we organised a colloquium in April 2014, which brought together the whole Vietnamese community and several astrophysicists from abroad. We take an active part in conferences and schools organised at regional level, either south-east Asia, Asia-Pacific or others, and maintain close contacts with the astrophysics communities of such countries, particularly with radio astronomers and young astronomers.
We are acting for the reinsertion of Vietnam as full member of IAU, with VAST as representative organization, and are keeping close and friendly contact with many of their members at the highest levels.
We are sensitive to the importance of outreach to make school students and the general public aware of the progress of modern astrophysics and of space science and of the fascination that it exerts on us. We also encourage activities of astronomy clubs by giving them active support. We have played an essential role in convincing the Ministry of Sciences and Technology to establish a yearly Science day during which Vietnamese laboratories open their doors to the public.
Perspectives
The presence of the team within the main Vietnamese space organization provides optimal conditions for its growth and progress. Cross-fertilization between astrophysics and space technology is known to be very fruitful. Indeed, radio astronomy owes many of its successes to radio engineers who have been showing curiosity and interest in the physics of the Universe, starting with Grote Reber and Karl Jansky at the very beginning, then with radar engineers in the wake of World War II, culminating with the discovery in 1964 of the Cosmic microwave background by Arno Penzias and Robert Wilson.
The VNSC and VAST environments make it easier for the team to take part in international collaborations, in particular within the framework of existing cooperation agreements, and increase its visibility in the science community, both international and Vietnamese.
The main lines of research that the team is contemplating cover the two topics that have been discussed above, the study of evolved stars and of distant galaxies. While we are busy consolidating the former, we need to seek new collaborators for optimizing the latter, which we are currently actively doing. At the same time, we spend time on the study of possible implications in other programmes on a longer range. In this context, we are actively considering participation in the FAST science programme, whether on HI emission of evolved stars or on millisecond pulsars. Moreover, as mentioned earlier, we presently review possible radio astronomy facilities that could be optimally used as training facilities, with the writing of a report to be handed over to the VNSC directorate by end of June 2015.
Finally, we shall keep devoting significant time and effort to teaching, training and outreach activities, with the aim to promote the interest for space science and technology within the Vietnamese population.
