Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThe not so simple stellar system Omega Cen. II. Evidence in support of a merging scenario
382
0
0.0
(
0
)
Added by
Annalisa Calamida Dr.
Publication date
2020
fields
Physics
and research's language is
English
Ask ChatGPT about the research
No Arabic abstract
We present multi-band photometry covering $sim$ 5deg $times$ 5deg across $omega$ Cen collected with the Dark Energy Camera, combined to Hubble Space Telescope and Wide Field Imager data for the central regions. The unprecedented photometric accuracy and field coverage allowed us to confirm the different spatial distribution of blue and red main-sequence stars, and of red-giant branch (RGB) stars with different metallicities. The ratio of the number of blue to red main-sequence stars shows that the blue main-sequence sub-population has a more extended spatial distribution compared to the red main-sequence one, and the frequency of blue main-sequence stars increases at a distance of $sim$ 20 arcmin from $omega$ Cen center. Similarly, the more metal-rich RGB stars show a more extended spatial distribution compared to the more metal-poor ones in the outskirts of the cluster. Moreover, the centers of the distributions of metal-rich and metal-poor RGB stars are shifted in different directions with respect to the geometrical center of $omega$ Cen. We constructed stellar density profiles for the blue and red main-sequence stars; they confirm that the blue main-sequence sub-population has a more extended spatial distribution compared to the red main-sequence one in the outskirts of $omega$ Cen, as found based on the star number ratio. We also computed the ellipticity profile of $omega$ Cen, which has a maximum value of 0.16 at a distance of $sim$ 8 arcmin from the center, and a minimum of 0.05 at $sim$ 30 arcmin; the average ellipticity is $sim0.10$. The circumstantial evidence presented in this work suggests a merging scenario for the formation of the peculiar stellar system $omega$ Cen.
rate research
Read More
[Abbreviated] We have investigated the color-magnitude diagram of Omega Centauri and find that the blue main sequence (bMS) can be reproduced only by models that have a of helium abundance in the range Y=0.35-$0.40. To explain the faint subgiant branch of the reddest stars (MS-a/RG-a sequence), isochrones for the observed metallicity ([Fe/H]approx0.7) appear to require both a high age (~13Gyr) and enhanced CNO abundances ([CNO/Fe]approx0.9$). Y~0.35 must also be assumed in order to counteract the effects of high CNO on turnoff colors, and thereby to obtain a good fit to the relatively blue turnoff of this stellar population. This suggest a short chemical evolution period of time (<1Gyr) for Omega Cen. Our intermediate-mass (super-)AGB models are able to reproduce the high helium abundances, along with [N/Fe]~2 and substantial O depletions if uncertainties in the treatment of convection are fully taken into account. These abundance features distinguish the bMS stars from the dominant [Fe/H] $approx1.7$ population. The most massive super-AGB stellar models (M_zams>=6.8M_sun, M_He,core>=1.245M_sun) predict too large N-enhancements, which limits their role in contributing to the extreme populations. We show quantitatively that highly He- and N-enriched AGB ejecta have particularly efficient cooling properties. Based on these results and on the reconstruction of the orbit of Omega Cen with respect to the Milky Way we propose the galactic plane passage gas purging scenario for the chemical evolution of this cluster. Our model addresses the formation and properties of the bMS population (including their central location in the cluster). We follow our model descriptively through four passage events, which could explain not only some key properties of the bMS, but also of the MS-a/RGB-a and the s-enriched stars.
We examine the effects of gas expulsion on initially sub-structured and out-of-equilibrium star clusters. We perform N-body simulations of the evolution of star clusters in a static background potential before adjusting that potential to model gas expulsion. We investigate the impact of varying the rate at which the gas is removed, and the instant at which gas removal begins. Reducing the rate at which the gas is expelled results in an increase in cluster survival. Quantitatively, this dependency is approximately in agreement with previous studies, despite their use of smooth, and virialised initial stellar distributions. However, the instant at which gas expulsion occurs is found to have a strong effect on cluster response to gas removal. We find if gas expulsion occurs prior to one crossing time, cluster response is poorly described by any global parameters. Furthermore in real clusters the instant of gas expulsion is poorly constrained. Therefore our results emphasis the highly stochastic and variable response of star clusters to gas expulsion.
The RAdial Velocity Experiment (RAVE) is a spectroscopic survey of the Milky Way. We use the subsample of spectra with spectroscopically determined values of stellar parameters to determine the distances to these stars. The list currently contains 235,064 high quality spectra which show no peculiarities and belong to 210,872 different stars. The numbers will grow as the RAVE survey progresses. The public version of the catalog will be made available through the CDS services along with the ongoing RAVE public data releases. The distances are determined with a method based on the work by Breddels et al.~(2010). Here we assume that the star undergoes a standard stellar evolution and that its spectrum shows no peculiarities. The refinements include: the use of either of the three isochrone sets, a better account of the stellar ages and masses, use of more realistic errors of stellar parameter values, and application to a larger dataset. The derived distances of both dwarfs and giants match within ~21% to the astrometric distances of Hipparcos stars and to the distances of observed members of open and globular clusters. Multiple observations of a fraction of RAVE stars show that repeatability of the derived distances is even better, with half of the objects showing a distance scatter of simlt 11%. RAVE dwarfs are ~300 pc from the Sun, and giants are at distances of 1 to 2 kpc, and up to 10 kpc. This places the RAVE dataset between the more local Geneva-Copenhagen survey and the more distant and fainter SDSS sample. As such it is ideal to address some of the fundamental questions of Galactic structure and evolution in the pre-Gaia era. Individual applications are left to separate papers, here we show that the full 6-dimensional information on position and velocity is accurate enough to discuss the vertical structure and kinematic properties of the thin and thick disks.
The origin of multiple stellar populations in Globular Clusters (GCs) is one of the greatest mysteries of modern stellar astrophysics. N-body simulations suggest that the present-day dynamics of GC stars can constrain the events that occurred at high redshift and led to the formation of multiple populations. Here, we combine multi-band photometry from the Hubble Space Telescope (HST) and ground-based facilities with HST and Gaia Data Release 2 proper motions to investigate the spatial distributions and the motions in the plane of the sky of multiple populations in the type II GCs NGC 5139 ($omega,$Centauri) and NGC 6656 (M 22). We first analyzed stellar populations with different metallicities. Fe-poor and Fe-rich stars in M 22 share similar spatial distributions and rotation patterns and exhibit similar isotropic motions. Similarly, the two main populations with different iron abundance in $omega,$Centauri share similar ellipticities and rotation patterns. When analyzing different radial regions, we find that the rotation amplitude decreases from the center towards the external regions. Fe-poor and Fe-rich stars of $omega,$Centauri are radially anisotropic in the central region and show similar degrees of anisotropy. We also investigate the stellar populations with different light-element abundances and find that their N-rich stars exhibit higher ellipticity than N-poor stars. In $omega,$Centauri Centauri both stellar groups are radially anisotropic. Interestingly, N-rich, Fe-rich stars exhibit different rotation patterns than N-poor stars with similar metallicities. The stellar populations with different nitrogen of M 22 exhibit similar rotation patterns and isotropic motions. We discuss these findings in the context of the formation of multiple populations.
Using Subaru/Suprime-Cam wide-field imaging and both Keck/ESI and LBT/MODS spectroscopy, we identify and characterize a compact star cluster, which we term NGC 3628-UCD1, embedded in a stellar stream around the spiral galaxy NGC 3628. The size and luminosity of UCD1 are similar to $omega$ Cen, the most luminous Milky Way globular cluster, which has long been suspected to be the stripped remnant of an accreted dwarf galaxy. The object has a magnitude of $i=19.3$ mag (${rm L}_{rm i}=1.4times10^{6}~{rm L}_{odot}$). UCD1 is marginally resolved in our ground-based imaging, with a half-light radius of $sim10$ pc. We measure an integrated brightness for the stellar stream of $i=13.1$ mag, with $(g-i)=1.0$. This would correspond to an accreted dwarf galaxy with an approximate luminosity of ${rm L}_isim4.1times10^{8}~{rm L}_{odot}$. Spectral analysis reveals that UCD1 has an age of $6.6$ Gyr , $[rm{Z}/rm{H}]=-0.75$, an $[{alpha}/rm{Fe}]=-0.10$. We propose that UCD1 is an example of an $omega$ Cen-like star cluster possibly forming from the nucleus of an infalling dwarf galaxy, demonstrating that at least some of the massive star cluster population may be created through tidal stripping.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
