Study of stellar streams in the galaxy

Abstract

Our Galaxy, also called the Milky Way, is a spiral galaxy consisting of three major distinct components: bulge, disc and halo. They differ from each other in their structural as well as kinematical and chemical properties. Bulge contains both the young and old stars, and large amounts of gas and dust. The region is known for high star formation rate. On the other hand halo is very sparse containing mostly old stars (>10 Gyr) and stars from accreted satellite galaxies. Disc of the Galaxy contains stars of very young age (few Myrs) and stars as old as 10 Gyr. Spiral structure of the Galaxy forms the disc. Apart from the large scale structures, the Milky Way has small scale structures which include open clusters, OB associations, globular clusters, and star forming regions. Some of these structures are gravitationally tightly bound in a smaller region in the space (globular clusters, Open Clusters) and some are loosely bound in relatively larger regions (OB associations etc). As they rotate about the center of the Galaxy, the spatially coherent groups disintegrate over time due to interactions with large structures such as molecular clouds. Probably, most of the stars in the field were once part of the spatially coherent groups. Also, there may be stars in the field resulting from the disintegrated merged satellite galaxies. The field stars are spatially unbound. The subject of our study is not of the spatially bound or unbound stars but of stars of kinematically coherent groups. There are stars in the field which are spatially incoherent but coherent in their kinematics. In general, field stars within the disc or in the halo are expected to exhibit random motion such as three dimensional Schwarzschild velocity distribution. Contrary to this general expectation, it has been known in the literature for more than a century, there exist stars that form clumps in the velocity space sharing common motion. The kinematically coherent groups are known as moving groups or stellar streams. Stellar streams are defined as the tight clumps or over-densities of stars present in the velocity space over and above the large scale structure that Schwarzschild distribution can reproduce in the solar neighborhood. The pioneering work of identification of such moving groups in the Galaxy was done by O. J. Eggen (Eggen 1996 and references therein) who thought the moving groups are the resultant of cluster disintegration or cluster dispersion. Thus, many of the moving groups are named after the clusters or bright stars in the sky which share kinematics with the moving group stars. For example, the Sirius moving group is named after a bright star Sirius and the Hyades moving group is named after a young open cluster Hyades. Though, the concept of stellar streams has been around in the literature for long, their origin and evolution within the Galaxy remained very poorly understood. There are three principal hypothesis for the existence of the moving groups: a) cluster disruption, b) satellite accretion, c) dynamical perturbations. According to cluster disruption hypothesis, open clusters while orbiting around the Galactic center get disrupted due to encounters with the giant molecular clouds. The dispersed cluster members retain parental kinematics, thus forming moving groups in the Galaxy. As per the satellite accretion scenario, the Milky Way accretes its dwarf satellite galaxies and the debris of such accreted satellites remain in the Galaxy as stellar streams or moving groups. Finally, dynamical perturbation scenario says that the dynamical perturbations introduced by non-axisymmetric components of the Galaxy such as the bar and/or spiral arms or the merging satellite galaxies produce moving groups. In this thesis we studied a few identified moving groups based on chemical tagging of their member stars. The chemical tagging of stars (Freeman & Bland-Hawthorn 2002) is an innovative method to understand stars’ past history as their chemistry remains unaltered, to a large extent, from the chemical composition of the natal clouds from which the stars formed. We performed abundance analysis of member stars of the moving groups in search of clues to their origin. Understanding of the moving groups has far reaching significance in decoding the chronological evolution of the Galaxy. We chose five well defined moving groups for this study. Of which two moving groups (Arcturus and AF06) belong to the thick disc and three (Hercules, Hyades and Sirius moving groups) belong to the thin disc component of the Galaxy. Main sequence member stars of the moving groups of the thick disc are chosen from the study of Arifyanto & Fuchs (2006). The Red giant member stars of the moving groups of the thin disc are chosen from the study of Famaey et al. (2005). All together, a total of 169 stars (44 main sequence dwarfs and 125 red giants) are subjected to the abundance study. Chemical tagging of stars requires accurate photospheric elemental abundance determination which in turn requires high resolution spectra of individual stars. The high resolution (R 60,000) and high quality (S/N 100) spectra of member stars of the selected moving groups are obtained using two telescopes - 2.7 m Harlan J. Smith Telescope, located at the McDonald Observatory, Texas, USA and the 2.3 m Vainu Bappu Telescope located at the Vainu Bappu Observatory, Kavalur, India. The raw spectral data are reduced and calibrated using various spectral image processing tools. Determination of stellar abundances requires determination of photospheric parameters : the effective surface temperature (Te f f ), the surface gravity (log g), microturbulent velocity ( t), and the metallicity ([Fe/H]). The parameters are determined using both photometry and spectroscopy. The photometric determinations involve optical and in frared colours and empirical colour-temperature relations (Alonso et al. 1996, Alonso et al. 1999). Spectroscopic determination of stellar atmospheric parameters require the equivalent widths measured from high resolution spectra, atomic data, stellar theoretical model atmosphers (Kurucz 1998) and spectral synthesis and analysis package. Parameters determined from photometry and spectroscopy agree very well within uncertainties. For the abundance study we adopted parameters determined from spectroscopy. For each member star, abundances of 16 elements of different nucleosynthesis groups- Fe-peak, - process, and s-process are obtained. Ages are estimated using colour-luminosity diagram and the computed stellar evolutionary tracks (Demarque et al. 2004). To extract clues of the origin of moving groups, we combined the abundance information with the kinematic information. The kinematic motion (U; V;W) for each of the member star is derived using radial velocity derived from the observed spectrum and the astrometry (parallax and proper motion) taken from HIPPARCOS catalogue (van Leeuwen 2007). Here, the velocity U is the radial component towards or away from the Galactic center, V is the tangential component, positive in the direction of Galactic rotation, and W is the vertical component, positive towards the North Galactic Pole. Distribution of member stars in the velocity spaces (U 􀀀 V, W 􀀀 V etc.) studied in comparision with the member stars of the background thick and thin disc. Other parameters such as angular momentum components per unit mass - Jx, Jy and Jz are derived to segregate groups more easily. The orbital parameters - The apogalactic distance (Rmax), perigalactic distance (Rmin), the maximum and minimum distances away from the Galactic plane (Zmax and Zmin), eccentricity of the orbit (e) are calculated by integrating U; V;W components of the stars over the Galactic potential. The probabilities with which stars belong to any one of three major components of the Galaxy (thin disc, thick disc, halo) are calculated using derived kinematic motion and the recipe given in Reddy et al. (2006) (see references there in). Arcturus stream and AF06 stream are high velocity streams. Kinematic results show that member stars of both the moving groups belong to the thick disc. Abundance results show that member stars of Arcturus and AF06 are metal poor and very old (10-14 Gyr). Other abundance trends ([Fe/H] versus [X/Fe], where X is any element other than Fe) are very similar to the background thick disc stars. Wide range in metallicity and ages rule out the possibility of the stars originated from the dispersed open clusters. Kinematics and abundance results suggest that the two streams might have formed as a result of resonant interactions of spirals/bar with stellar orbits. Moving groups being originated from accreted satellite dwarf galaxies is also unlikely as none of the nearby dwarf galaxies abundances are similar to abundances of the moving groups. Hercules, Hyades-Pleiades and Sirius are the low velocity streams. Kinematic results show that the groups revolve around the Galactic center in near circular orbits and belong to the thin disc component of the Galaxy. Abundance results suggest relatively a wide range of metallicity and ages compared to what one would expect for member stars of spatially coherent groups such as open clusters and associations. However, we find a fraction of stars exhibiting very similar ages and abundances which are characteristics of open clusters. For example, many of the Sirius moving group stars seems to be originated from the Sirius super cluster which is in a particular stage of disintegration. Contrary to the recent results, results in this study suggest that the Hercules moving group consists of only thin disc stars and not a mixture of thin and thick disc stars. Results also suggest that it is highly unlikely that any of the streams originated outside of the MilkyWay. It is very likely that dynamical perturbations caused either by external forces and/or by interactions of bar and spirals with stellar orbits within the Galaxy are playing the role in creating moving clusters. In that sense, the moving groups may be a transient phenomenon.The major findings of the thesis are : 1. None of the five streams studied here show chemical homogeneity and coevality, the defining properties of spatially coherent groups. 2. None of the streams studied here show any sort of similarity with the dwarf satellite galaxies of the Milky Way galaxy. 3. Abundance patterns are indistinguishable from that of the background Galactic disc (thin or thick disc) field stars. 4. A fraction of member stars of the moving groups (in particular Sirius, Hyades) seems to be originated from the cluster dispersion.