Astronomy & Astrophysics
Research >> Astronomy & Astrophysics
The research and development activities of ARIES in the field of Astronomy and Astrophysics can be classified into the following areas:
- Binary stars
- Stellar Activity
- Galactic and extra-galactic astronomy
- Instrumentation for astronomy
- Polarization, ISM, Dust
- Solar system
- Star-formation and stellar evolution in Galactic star forming regions
- Transient objects - Gamma-ray burst, Supernovae
- Variable stars
Electromagnetic spectrum of static stars is emitted from the photosphere (visible surface), and does not directly provide information on the internal structure. On the other hand, oscillating stars provide such opportunity, just as terrestrial seismology determine the internal structure of the earth. In India we lead in the field of Asteroseismology and one major ongoing research projects is described below in brief.
Nainital -Cape Survey
In 1978, Kurtz, D. W. (Kurtz, D. W., 1978, Ap.J., 221, 867) found that some of the cool stars belongs to A-peculiar (Ap) group show periodic photometric variations in the period range of 4 - 16 minutes. He called this group of stars as rapidly oscillating Ap (roAp) stars. As most of the research work was done for the stars that lie in the Southern hemisphere. Therefore, to search and study these peculiar objects in the Northern hemisphere, a collaborative project was started among four Institutes namely ARIES, Nainital, India; ISRO Satellite Application Center (ISAC), Bangalore, India; SAAO, South Africa and University of Central Lancashire, U. K.
The main aim of this project is to search for new roAp stars in the Northern hemisphere from a list of over 200 probable candidates which are selected on the basis of their Stromgren photometric colours. For this survey a three channel photometer has developed at ARIES along with ISAC. Among our target stars some of them are showing variations, but they have to be confirmed with more observations in future. Six Delta- Scuti stars and one roAp star have been discovered so far.
The properties of a star are characterised by a set of some fundamental parameters such as mass, radious, surface temperature and chemical compositions. Photometric observation of eclipsing binary stars together with their spectroscopic parameters are the main source of our current knowledge of mass, radius and evolutionary status of stars single as well as binary.
Studies in the field of eclipsing binary stars began in the institute as early as 1956 with the spectrographic study of the Wolf-Rayet eclipsing binary HD 214419. However, the photoelectric photometry of eclipsing binaries commenced in the year 1957-58 on 25-cm Cooke refractor using the B, V and interference filters. Some of the stars selected for observations were eclipsing binaries HD 193576, Beta Lyrae, Delta Orionis and AR Cassiopiaea.
The year 1964 marked a systematic study of eclipsing binaries with observations of complete light curves in the U, B and V filters of Johnson and Morgan in order to obtain the absolute elements i.e. masses and radii of the system. ZZ Cephei is the first eclipsing system for which geometrical elements and absolute dimensions were determined using V light of the system. There are 45 eclipsing systems that have been studied at the institute and they include 20 CVn, 32c Cyg, AE Cas, AO Mon, AR Lac, AS Cam, AW Uma, AV Hya, AY Cam, AZ Eri, BZ Eri, CC Cas, CD Eri, CD Tau, CU Eri, DI Oeg, DX Aqr, Delta Cep, ER Vul, EE Aqu, FW Mon, GG Cas, GH Peg, IM Mon, IZ Per, MW Cas, RI Cep, RW Cet, RS Cvn, RZ Eri, RT Per, ST Per, TW Cas, TX Leo, UW Boo, V364 Cas, VZ Hya, WW Cep, WX Eri and ZZ Cep. Some of the observed peculiarities among these systems are the following :
- Flares detected in AR Lac during primary eclipse.
- Intrinsic light variability of the hotter component in MM Cas and
- Primary component of DI Peg is surrounded by a disc of circum-stellar material
Twenty-six binaries have been analyzed for geometrical elements by using either the Russel & Merrill method or the Kopal's frequency domain method. Besides photometric observations, spectrophotometric studies of binary stars have also been carried using the 104-cm telescope. The binary star thus observed include AR Lac, Beta Lac, Delta Cap, GG Cas, KX and RZ Cas, UX Ari, and V711 Tau.
Masses and radii have been calculated for 19 binaries, nine of which are detached and the others are semi detached. Using the derived absolute dimensions, the evolutionary status of these binaries have been studied. The main conclusions are (i) the cooler components in semi-detached binaries are over luminous and (ii) the cooler component of GG Cas is significantly under-luminous.
Period studies have been made for AL Cam, AR Lac, AW Uma, AX Dra, BZ Eri, CQ Cep, Delta Cap, DI Hya, DI Peg, DX Aqr, EE Aqr, EI Cap GG Cas, GH Peg, IT Per, IZ Per, RW Com, ST Per, SW Ceg, SZ Ari, TV Cas, TX Cet, V364 Cas, V444 Cyg, V450 Her, VZ Hya, XX Cas, XX Leo and XY Cet. Using the observed times of minimum light derived from the photometric observations, the mass loss and the mass transfer rates have been computed for seven binary systems, i.e. AR Lac, CQ Cep, DI Peg, SW Cyg, TV Cas and V444Cyg. On the basis of the computed mass transfer rates in TV Cas and SW Cyg, it has been concluded that
- the Roche-Lobe of the secondary components in Algols are shrinking as mass transfer proceeds
- the average mass transfer in Algols corresponds to the thermal time scale of the mass losing star and
- the orbital angular momentum is not conserved during the mass transfer.
Period-mass ratio relations for eclipsing binaries with periods not exceeding five days have been obtained and an empirical relation between systematic mass and angular momentum have been determined. It has been found that for the same mass, a semi-detached binary system has greater orbital angular momentum than a contact binary and less than a detached system. For this study, 91 binaries have been considered. Gravitational radiation and spiraling time relations for close binary systems have also been obtained.
On the basis of non-conservation of orbital angular momentum, the evolutionary process in semi-detached binary stars has been studied. It is found that all the semi-detached binary stars originated through case B mass transfer. Further, 333 double lined spectroscopic binaries were examined in the bi-logarithmic plots of M versus A with respect to the critical separation AAB and ABA and it was noted that binary stars having separations corresponding to case A evolution cannot form.
Galactic and extra-galactic astronomy
Wolf Rayet Galaxy:
Wolf Rayet (WR) galaxies are extragalactic objects whose integrated spectra shows most commonly He II (4686 Å) emission line originating in the stellar winds of Wolf Rayet stars. They are star burst galaxies showing strong nebular recombination lines, indicating the presence of a large number of O type stars. They are found among a large variety of morphological types, from low mass blue compact dwarfs (BCD's), irregular galaxies, to massive spirals & luminous merging IRAS galaxies and Seyfert galaxies. WR galaxies are an important class of objects, both in terms of understanding massive star evolution and star burst phenomenon. These galaxies are in the stage of very recent star formation that produce massive stars evolving to W-R phase. Conti (1991) was the first person who catalogues the WR galaxies which contains only 37 galaxies. However, a recent catalogue by Schaerer et al., (1999) and the references therein, shows a total of 139 WR galaxies. Using ARIES 104 cm Sampurnanand Telescope few of them have been observed to study the star burst properties like age, duration of burst and star formation rate.
An extragalactic radio source identified with an optical galaxy, whose radio power output lies in the range 1035 to 10 38 watt. These sources often show double structure and may be distinguished from normal galaxies, such as M31, whose radio emissions roughly follow the optical contours of the galaxy and radio powers are much lower, in the range 1030 to 10 32 watt. Powerful radio sources are always located in elliptical galaxies, many of its are the central dominant member of a star cluster of galaxies. Radio galaxies with diameter greater than 1 MPc are now known. Radio galaxies are a form of active galaxy. High luminosity radio galaxies sometimes share some characteristics of quasars, like broad optical emission lines and a bright optical nucleus in the galaxy. Such galaxies are used to be commonly termed as broad line radio galaxies (BLRG). A multi wavelength study of a large number of giant radio galaxy has been initiated at ARIES to study different physical parameters.
Dark matter in the Galaxies:
One of the most important problem in cosmology is to identify dark matter in galactic halos as overwhelming evidences suggest that there is more gravitating mass in the universe than can be counted in the form of luminous stellar and interstellar matter. The nature of this dark matter remains a mystery.
In recent years, attention has also been centered on the possibility that the dark matter consists of some sort of astrophysical objects, generically termed as MACHOs and gravitational lensing technique is used to search them. To search for micro lensing events towards M 31, a lensing technique called the Pixel Method has been developed under a collaborative program AGAPE (Andromeda Gravitational Amplification Pixel Experiment). As the observations are required for a short duration each night but for a long period, since 1998 we started observations using CCD mounted at the ARIES 104 cm Sampurnanand Telescope. Observations for four years have been collected and a few possible micro-lensing events have been detected.
Polarization, ISM, Dust
Polarimetry of star light plays a vital role in understanding the structure of ISM, properties of interstellar dust as well as the contribution of magnetic fields in the dynamics of ISM. In cloud, extinction of star light coming from background star is due to the dust particle, which produces linear polarization if the dust grains are non-spherical and aligned. Polarization of star light due to interstellar dust was first observed by Hall et al. (1949). In the year 1951 Davis and Greenstein (Davis et al. 1951) proposed a physical mechanism for grain alignment in cloud. According to that the non-spherical dust grains of paramagnetic material are aligned due to magnetic torques induced by ambient galactic magnetic field.
From June 2006, we have been carrying out a broad band optical polarimetric survey program for a number of galactic open cluster located in Perseus spiral arm by using ARIES Imaging Polarimeter (AIMPOL) as a backend instrument of 104 cm ARIES Sampurnanand Telescope. The aim of this program is to find out the magnetic map of Perseus spiral arm and some clues about the characteristic of dust grains responsible for the polarization and extinction of star light (Medhi et al. 2007).
Star formation and stellar evolution in Galactic star forming regions
Star formation is a key issue in many branches of Astrophysics. It is known that the stars are form in dense molecular clouds where clouds collapse under gravity, increases density and temperature until nuclear reactions begin. In recent years, it is found that the bipolar flows and circumstellar disks are frequent features of the star formation process. However, the detailed physical processes from the collapse of the molecular cloud to the end product of a main sequence star are not well understood. Another very important and most fundamental quantity in the study of star formation is Initial Mass Function (IMF), i.e., distribution of stellar mass at a star formation event. Theoretically, the shape of the IMF would depend on the chemical composition (metallicity) of the star forming cloud, it's physical state and environmental conditions. Due to dense molecular clouds, the detailed processes are hidden from optical observations. Infra-red, millimeter and radio observations are needed to penetrate the clouds. However, optical observations of young star clusters in combinations with near-infrared provides valuable information about IR-excess which is the indication of the presence of circumstellar disk.
One of the major impact of near-infrared observations will be to probe the giant molecular clouds and large dark clouds in search of embedded young stellar objects, in order to derive the IMF and the Star Forming Efficiency (SFE). The near IR range is very sensitive to young stellar objects because of their high luminosity and low extinction of the surrounding interstellar dust. The recent use of the new near-IR detectors and survey like 2MASS, IRAS have allowed statistical analysis of some star forming regions including the Orion Nebula, Rho Oh and Taurus-Auriga complexes. The observations cover only a small fraction (~20%) of the total star forming regions.
A group of gravitationally bound stars formed nearly at the same time from the same molecular cloud is known as star cluster. They are therefore located at the same distance and also considered to have same primordial chemical composition. Consequently, HR-diagram of a star cluster reflects the evolutionary dispersion of approximately equally old stars of different masses. Open clusters and the globular clusters therefore provide a wealth of information regarding star formation and evolutionary processes and act as a test-bed for the theory of stellar evolution. Comparison between the theoretical and observed HR-diagrams leads to the determination of the age and chemical composition of stars in the cluster. Observed colour-colour and colour-magnitude diagrams are also used to determine interstellar reddening, age and distance to the clusters.
The globular clusters in our Galaxy are amongst the oldest star known, reflecting it's early evolution while open clusters are relatively young objects. To understand the evolution of these systems, it is important to obtain photometric and spectroscopic study of the faintest stars. The young star clusters are also excellent tools for tracing the evolution of the Galaxy and it's present dynamical state. ARIES 104 cm Sampurnanand Telescope has been optimally used for obtaining important observations of large number of open clusters and a few Galactic globular clusters to fulfill the above goal. The observations have led to the studies of open clusters pertaining to their star formation efficiency, age distribution, mass function and luminosity function. Spatial structure of clusters and interstellar extinction in young open star clusters have also been studied. The slope of the mass function above 1 solar mass for young (age < 100 Myr) star clusters is found to be universal with a slope of Saltpeter value.
Solar system studies
Study of Comets:
The first comet observed from the institute was comet Arend Roland in 1957. Polarization measurements were carried out. The systematic spectrophotometric studies of comets, however began in 1970 with the observation of Comet Bennett. The other 17 comets studied at the institute are comet Austin, Comet Bradfield, Comet Crommelin, Comet C/2002VI(NEAT), Comet C/2000WMI(LINEAR), Comet Encke, Comet Hale-Bopp, Comet Halley, Comet Hartley-Good, Comet Hayakutake, Comet Ikeya-Zhang, Comet Kohoutek, Comet Levy, Comet Okazaki-Levy-Rodenko, Comet Swift-Tutle, Comet West and Comet Wilson.
Occultation of stars by planets and minor planets:
The occultation of the star SAO 158687 by Uranus in March 1977 was observed photometrically and four dips in the pre-occultation phase were recorded. These dips confirmed the discovery of rings around the Uranus. From the observations of the Uranus occulting the star BD-190 4222 on 26 April, 1981 and that of emission of the star Hyd-200 51695 on 01 May 1982, the temperature profiles of the Uranian upper atmosphere have been obtained. A comparison with the previous temperature estimates indicates an increase during 1977-1980 and a decrease during 1980-1982. The occultation of the star MKE 31 by Neptune on 12 September 1983 showed the extension of the latter's possible ring system from 6400km to 64190km in its equatorial plane.
The occultation of the star SAO 158913 by saturn observed simultaneously from Nainital and Kavalur on 24 and 25 March 1984, showed symmetric dips in the light curves indicative of the presence of occulting material in orbit around Saturn at a planetocentric distance of ~12.5 saturn radii. Using phtoelectric observations of the star SAO 158763 on 12 May 1984 a ring system of Saturn at 19 saturn radii was dected. The ring structure is approximately 56,000km wide and extends from 19.41 to 20.36 Saturn radii. The orbit of Titan is located within the width of the ring structure. Light variations show that the ring can be divided between two broader zones, having width of about 29,000 km and 26,400 km.
Transient objects - Gamma-ray burst, Supernovae
Gamma Ray Bursts (GRBs) are short and intense flashes of cosmic high energy (~100 KeV - 1MeV) photons. They release ~1051 - 1053 ergs or more in a few seconds and thus becomes the most (electromagnetically) luminous objects in the universe. The origin of GRBs is still a mystery. Considering astrophysical importance of optical observations of the GRB afterglows, astronomers from ARIES obtained broadband photometric CCD observations of a number of afterglows. In fact, the first Indian optical observation of an afterglow of a GRB was made at ARIES on 23 January 1999. Since then observations of around 30 more afterglows have been successfully carried out. Out of these afterglows detected, earliest optical observations of GRB 000301C (Sagar et al. 2000), GRB 030329/SN 2003dh (Resmi et al. 2005) and GRB 060124 (Misra et al. 2007) around the globe have been carried out using the 104-cm Sampurnanand Telescope at ARIES. The afterglow radiation is a broad-band non-thermal synchrotron emission. The observations support the Fireball model and the beaming effects in a GRB reduce the budget of the emitted isotropic gamma ray energy ~ 1051 ergs, a value compatible with the current popular stellar death models for the origin of the GRBs.
In this project we have both national and international scientific collaborations for the multi-wavelength study of GRB afterglows, our work is recognised internationally as they are cited in the leading journals of Astronomy and Astrophysics.
Interacting binary systems:Cataclysmic variables (CVs) are semi-detached binaries with one companion, usually a dwarf star, filling its Roche-lobe and second star, a degenerate white dwarf. Matter is transferred from the lobe-filling star in to the white dwarf. Due to conservation of angular momentum this mass does not fall directly onto the degenerate component but forms an accretion disk around it. At the collision point between disk and mass stream an energetic shock front is created. The strength and the structure of the magnetic field of the white dwarf also strongly influence the nature of the accretion. Such systems exhibit an enormously wide range of physical phenomena. The interacting binary systems play a crucial role in the study of the structure, evolution and instabilities of accretion discs because they (1) are nearby, (2) enable much more detailed study by virtue of being in a binary and (3) in a few cases are eclipsing binaries which provide additional constraints on the geometry of the system and the structure of the accretion disc. The interaction between different sources-mass-lobe filling star, accretion disk and shock front-results in a rather complicated observational picture: Quasi-periodic light fluctuations of several tenths of a magnitude on time scales of several minutes are superimposed on the orbital variation of the two components which ranges from about 1.5 to 10 hours. Optical and near-IR photometric and spectroscopic data allow us to derive essential physical parameters for these systems, such as orbital periods, masses and sizes of the components, mass transfer rates and temperature structure of the accretion disk and shock-front. This information can be put into a larger frame to examine the evolution of these systems, and to bring them into perspective with other semi-detached binaries and close binaries in general. We therefore plan to study the CVs using simultaneous optical and near-IR observations.
Variability of blue stragglers in star clusters:
In some clusters a few stars are found on an extension of the MS above the turn-off point, and are referred to as blue stragglers. Their origin is still uncertain, though there is growing evidence that they are either binary stars or the result of a merger of two stars in the late stage of the development of close binary. Since the binary or merged binary should be more massive than single cluster stars, it should preferentially be found in the central regions of relaxed cluster. Evidence that a cluster star is a binary or is a merged binary would be strengthened by the discovery of variability. A number of variable blue stragglers have been discovered so far but little is known of the characteristics of their variability. A close study of these stars could be expected to classify some aspects of the late stages of stellar evolution which are only imperfectly understood at present. A few galactic open and globular clusters have been studied by us keeping the above points in mind.
It is proposed to monitor selected galactic globular and open clusters for variability on time scales of a few tens of minutes to a few days. Spectroscopic studies of the confirmed and interesting variables can also be undertaken.
Variable stars towards the bulge of M31:
As a backup result of the AGAPE experiment, we have discovered more than dozen Cepheids, a few Nova-like objects and several long-period variables. The Cepheid variables in M31 are valuable as they are used a standard candle for determining the extragalactic distance scales. The variables discovered under this project will be studied in detail during the next few years.
Studies of EUV-bright and soft X-ray sources:
Many new soft X-ray sources have recently been discovered in surveys with the Einstein and ROSAT. The optical, radio and X-ray observations indicate that these systems may be RS CVn like objects. However, evidence for binarity and also the optical photometric properties of these objects are not known. A list of about twenty five such northern hemisphere objects has recently been compiled, and they are being monitored with ARIES telescopes using CCD Camera. Optical variability has been detected in 5 of the ten objects observed so far. This work is in collaboration with scientists from RRI, Bangalore, TIFR, Mumbai.
We would propose to spectroscopically identify and study the optical counterparts of several bright EUV sources, and some unusual soft X-ray sources, discovered in the ROSAT all-sky surveys. A large fraction of these objects are probable new accreting binaries, either magnetic CVs or Low Mass X-ray Binaries. Our aim will be to identify the optical counter part and then to carry out detailed optical/near-IR photometric and spectroscopic studies.
Experience gained in this area before 2008, the launch of ASTROSAT, will be valuable since we shall be ready to participate in the optical follow-up observations desired for the ASTROSAT sources.