Our galaxy, the Milky Way came into being about fourteen giga years ago and since then it is continuously forming stars. Our Sun is five giga years old and it is an ordinary star amongst a total of about hundred billion stars in the whole Milky Way. Existence and preservance of life on Earth depends on the star formation history of solar neighborhood in the the Milky way. Investigating the content of our own Milky Way galaxy forms one of the thrust research areas of the astronomy and astrophysics division at ARIES.
The astrophysical problems under this category include : How does a star form ? Do they form in groups or in isolation ? Do the low and high mass stars form differently ? How does it evolve and interact with its surroundings ? What is the mass distribution of a newly formed stellar systems ? Do the Sun-like stars have more tendency to be formed and in what conditions ? What is the internal structure of stars, and high energy astrophysical processes occurring during accretion of matter from one star onto the other ?
To address these questions, astronomers at ARIES are perusing observations of a variety of astronomical objects in the Milky Way, for example, star clusters, commentary globules, hydrogen clouds, X-ray binaries, black holes, chromospherically active stars and many more. ARIES scientists are using optical telescopes of the institute equipped with modern instruments capable of detecting milli-magnitude variations in the starlight and making deep detections of sources. Additionally observations from various space and ground based facilities encompassing the full electromagnetic spectrum are also carried out.
A variable star is the celestial object that changes its brightness from milli-magnitude to order of magnitude over a period of time varying from few minutes to years. The observation of these stars can be used to infer valuable stellar parameters such as their masses, radii and luminosities. These parameters are essential to investigate the relationships between rotation, stellar activity, age and masses and also impose constraints on stellar pulsation models. The research in stellar variability was continued since the establishment of the observatory when spectrographic observations of an eclipsing binary star HD 214419 was successfully obtained. Since last two decades, the observing facilities at ARIES are being extensively used to study the stellar variability and investigate internal structure and evolution of stars.
Following types of variable stars are being studied in detail:
Pulsational Variables
The pulsation in a star arises due to periodic expansion and contraction. The pulsation or oscillations phenomena of a star allow us to investigate the interior and evolution of stars, the technique known as the asteroseismology. The Nainital-Cape survey is one of the dedicated survey programme initiated by ARIES in 1999 for the asteroseismology of chemically peculiar stars to probe their atmosphere in the presence of various physical process such as rotation, magnetic field.
Rotational Variables
Like our Sun, distant stars are rotating spheres of hot gas. Stars, however, do not rotate like solid spheres: regions at different latitudes rotate at different rates. For example in case of nearest star the Sun, a full rotation near the equator takes approximately 25 days while near the pole it takes approximately 31 days. Using the highly precise data one can determine their rotational profiles at different latitudes and underline physical phenomena.
Binary Stars
A binary system is simply a set of two stars which orbit around a common center of mass and both are gravitationally bound to each other. Such systems are of importance for astronomers as they allow to determine masses of stars accurately. Study of binary stars such eclipsing, cataclysmic are the main research interest.
Variability in Open and Globular Star Clusters
Star clusters are widely investigated to explore stellar variability as they are birth place of different types of variable stars depending upon their evolutionary stages. While young clusters allow the study of pre-main sequence objects, the intermediate age clusters are important stellar populations to probe the short-period variables such as Delta-Scuti and Gamma-Dor types of variables. At ARIES, Nainital, a long-term observational programme to search and characterize the variable stars in galactic open and globular clusters was started in 2009 using various 1 to 2 m class optical telescopes located in different part of India. The main objective of this programme is to study the physical properties of the clusters under study and investigate the stellar and dynamical evolution.
The formation of stars and their evolution with time is always a central point to the curiosity of human minds. For such studies, the open and globular star clusters are used as test-beds. These research topics are being pursued at the institute from the observatory eras and the institute has made significant contributions and is known internationally. Some key aspects are highlighted below.
Star formation
(Top Left) GMRT 1280 radio contours + YSOs overlaied on the Herchel column density maps. (Top right) GMRT 680 radio contours + YSOs overlaied on the Herchel column density maps. The contour levels are at 1.0, 2.5, 5, 12, 25, 40, 60 and 90 % of the peak flux density. The peak fluxes at 610 is 0.162 Jy/beam. (Bottom Left) Spectral index map. (Bottom Right): Corresponding error map retrived from the AIPS task COMB representing pixel uncertainity.
Recent advances in longer wavelength observations (such as near-infrared, mid-infrared, far-infrared, sub-millimeter, millimeter, and centimeter) with very good spatial resolution, help us to study star formation in great details. The star clusters and H II regions in our Galaxy, constitute the nearest laboratories for the direct astronomical investigation of the physical processes of star formation. Our group has been using Indian (ARIES 104-cm ST / 1.3-m DFOT / 3.6-m DOT, 2-m HCT and GMRT), International (4-m Blanco, 3.6-m TNG, 2.2-m ESO, 1-m KISO) observing facilities along with the archival data from Chandra, Panstar, Gaia, 2MASS, UKIDSS, Spitzer, WISE, Herchel, NVSS, and CGPS to study these regions. The group is specially interested to study the mode of star formation in different regions of our Galaxy, the effect of massive star(s) on star formation, initial mass function and K-band luminosity function of the star clusters etc. The highlights of the research work of group members are : 1) Star formation in star clusters is non-coeval and may continue for more than five million years. It does not cease after the formation of massive star in the cluster. 2). Radiation feedback from massive star may further triggers the next generation of star formation in their vicinity. 3) IMF of young clusters in the mass range about ~30 - 0.3 solar mass can be represented by power law with a break around 1-2 solar mass. Mass segregation is also observed in these young clusters that may be due to star formation process itself.
Stellar Evolution
The fragmentation of giant molecular clouds often culminates in the formation of star clusters containing hundreds to thousands stars in a bound system. It is known that most of the stars in the Galaxy are in fact formed within the star clusters. To understand the detailed structure of the Galaxy, we study different components of the Galaxy such as individual stars, star clusters, interstellar gas and dust, etc. Star clusters are located in disc and halo of the Galaxy hence can be considered as the best tool to investigate the structure and evolution of the Galaxy. Star clusters are of two types, one which are loosely bound and relatively young are called open star clusters while those which are tightly bound and quite old are called globular clusters. While study of the open clusters is vital for understanding the history of star formation and nature of the parent star clusters, an in-depth knowledge of globular cluster is important to probe the kinematics of the Galaxy. The systematic studies of star clusters thus offer unique opportunities to understand large-scale star formation processes which is essential to probe the entire Galaxy including Galactic buldge, Galactic disk and Galactic halo. The kinematical study of star clusters has a very important role in many areas of astrophysics. Among them are the stellar evolution, stellar dynamics, and Galactic evolution. The proper motion in star clusters can be useful in many ways. One of the basic goals of the proper motion is to identify the cluster members and produce a color-magnitude diagram free from field stars. Since most of the open clusters are embedded in the Galactic disk and are likely to be affected by field star contamination, it is essential to discriminate between members and non-members of the clusters. The general aim of the internal kinematical study of the star cluster is to test the stellar dynamical model of the cluster. Internal velocity dispersion can be useful to estimate the cluster mass.
Group Members
Faculties : J. C. Pandey, Neelam Panwar,R. K. S. Yadav, Santosh Joshi, Saurabh, Sneh Lata, Y. C. Joshi
