We analysed high-resolution UVES spectra of six stars belonging to the subgiant branch of omega Centauri, and derived abundance ratios of 19 chemical elements (namely Al, Ba, C, Ca, Co, Cr, Cu, Fe, La, Mg, Mn, N, Na, Ni, Sc, Si, Sr, Ti, and Y). A comparison with previous abundance determinations for red giants provided remarkable agreement and allowed us to identify the sub-populations to which our targets belong. We found that three targets belong to a low-metallicity population at [Fe/H]~-2.0 dex, [alpha/Fe]~+0.4 dex and [s/Fe]~0 dex. Stars with similar characteristics were found in small amounts by past surveys of red giants. We discuss the possibility that they belong to a separate sub-population that we name VMP (very metal-poor, at most 5% of the total cluster population), which - in the self-enrichment hypothesis - is the best-candidate first stellar generation in omega Cen. Two of the remaining targets belong to the dominant metal-poor population (MP) at [Fe/H]~-1.7 dex, and the last one to the metal-intermediate (MInt) one at [Fe/H]~-1.2 dex. The existence of the newly defined VMP population could help to understand some puzzling results based on low-resolution spectroscopy (Sollima et al., Villanova et al.) in their age differences determinations, because the metallicity resolution of these studies was probably not enough to detect the VMP population. The VMP could also correspond to some of the additional substructures of the subgiant-branch region found in the latest HST photometry (Bellini et al.). After trying to correlate chemical abundances with substructures in the subgiant branch of omega Cen, we found that the age difference between the VMP and MP populations should be small (0+/-2 Gyr), while the difference between the MP and MInt populations could be slightly larger (2+/-2~Gyr).
Forward asteroseismic modelling plays an important role towards a complete understanding of the physics taking place in deep stellar interiors. With a dynamical mass in the range over which models develop convective cores while in the main sequence, the solar-like oscillator $α$ Centauri A presents itself as an interesting case study. We address the impact of varying the metallicity mixture on the determination of the energy transport process at work in the core of $α$ Centauri A. We find that $\gtrsim$ 70$\%$ of models reproducing the revised dynamical mass of $α$ Centauri A have convective c…
Cosmic rays are an important factor of space weather determining radiation conditions near the Earth and it seems to be essential to clarify radiation conditions near extrasolar planets too. Last year a terrestrial planet candidate was discovered in an orbit around Proxima Centauri. Here we present our estimates on parameters of stellar wind of the Parker model, possible fluxes and fluencies of galactic and stellar cosmic rays based on the available data of the Proxima Centauri activity and its magnetic field. We found that galactic cosmic rays will be practically absent near Proxima~b up to e…
High spectral resolution and high signal-to-noise ratio optical spectra of red giants in the globular cluster Omega Centauri are analysed for stellar parameters and chemical abundances of 15 elements including helium by either line equivalent widths or synthetic spectrum analyses. The simultaneous abundance analysis of MgH and Mg lines adopting theoretical photospheres and a combination of He/H$-$ratios proved to be the only powerful probe to evaluate helium abundances of red giants cooler than 4400 K, wherein otherwise helium line transitions (He {\scs I} 10830 and 5876 Å) present for a direc…
Proxima and Alpha Centauri AB have almost identical distances and proper motions with respect to the Sun. Although the probability of such similar parameters is in principle very low, the question as to whether they actually form a single gravitationally bound triple system has been open since the discovery of Proxima one century ago. Owing to HARPS high precision absolute radial velocity measurements and the recent revision of the parameters of the Alpha Cen pair, we show that Proxima and Alpha Cen are gravitationally bound with a high degree of confidence. The orbital period of Proxima is ap…
By using high-resolution spectra acquired with FLAMES-GIRAFFE at the ESO/VLT, we measured radial and rotational velocities for 110 Blue Straggler stars (BSSs) in Omega Centauri, the globular cluster-like stellar system harboring the largest known BSS population. According to their radial velocities, 109 BSSs are members of the system. The rotational velocity distribution is very broad, with the bulk of BSSs spinning at less than ~40 km/s (in agreement with the majority of such stars observed in other globular clusters) and a long tail reaching ~200 km/s. About 40% of the sample has vsini >40 k…
The bright, nearby binary $α$ Centauri provides an excellent laboratory for testing stellar evolution models, as it is one of the few stellar systems for which we have high-precision classical (mass, radius, luminosity) and asteroseismic ($p$-mode) observations. Stellar models are created and fit to the classical and seismic observations of both stars by allowing for the free variation of convective mixing length parameter $α_{\text{MLT}}$. This system is modeled using five different sets of assumptions about the physics governing the stellar models. There are 31 pairs of tracks (out of ${\sim…
Asteroseismic measurements enable inferences of the underlying stellar structure, such as the density and the speed of sound at various points within the interior of the star. This provides an opportunity to test stellar evolution theory by assessing whether the predicted structure of a star agrees with the measured structure. Thus far, this kind of inverse analysis has only been applied to the Sun and three solar-like main-sequence stars. Here we extend the technique to stars on the subgiant branch, and apply it to one of the best-characterized subgiants of the Kepler mission, HR 7322. The ob…
Understanding the physical process responsible for the transport of energy in the core of $α$ Centauri A is of the utmost importance if this star is to be used in the calibration of stellar model physics. Adoption of different parallax measurements available in the literature results in differences in the interferometric radius constraints used in stellar modelling. Further, this is at the origin of the different dynamical mass measurements reported for this star. With the goal of reproducing the revised dynamical mass derived by Pourbaix & Boffin, we modelled the star using two stellar grids …
We present a study of white light flares from the active M5.5 dwarf Proxima Centauri using the Canadian microsatellite MOST. Using 37.6 days of monitoring data from 2014 and 2015, we have detected 66 individual flare events, the largest number of white light flares observed to date on Proxima Cen. Flare energies in our sample range from $10^{29}$-$10^{31.5}$ erg. The flare rate is lower than that of other classic flare stars of similar spectral type, such as UV Ceti, which may indicate Proxima Cen had a higher flare rate in its youth. Proxima Cen does have an unusually high flare rate given it…
We report the discovery of two Li-rich giant stars (fainter than the red giant branch bump) in the stellar system Omega Centauri using GIRAFFE-FLAMES spectra. These two stars have A(Li)=1.65 and 2.40 dex and they belong to the main population of the system ([Fe/H]=--1.70 and --1.82, respectively). The most Li-rich of them (#25664) has [Na/Fe]=+0.87 dex that is ~0.5 dex higher than those measured in the most Na-rich stars of Omega Centauri of similar metallicity. The chemical abundances of Li and Na in #25664 can be qualitatively explained by deep extra mixing efficient within the star during i…
The nearest stellar system consists of the stars Proxima, alpha Centauri A and B and at least one planet Proxima b. The habitability of Proxima b and any other planets are likely to be significantly influenced by the orbital evolution of the system. To study the dynamical evolution of the system, we simulate the motions of Proxima and alpha Centauri A and B due to the perturbations from the Galactic tide and stellar encounters in a Monte Carlo fashion. From 100 clones, we find that 74% orbits of Proxima Centauri are bound to alpha Centauri A and B while 17% and 9% orbits become unbound in the …
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 ($ω\…
$α$ Centauri A is the closest solar-type star to the Sun and offers the best opportunity to find and ultimately to characterize an Earth-sized planet located in its Habitable Zone (HZ). Here we describe initial results from an ALMA program to search for planets in the $α$ Cen AB system using differential astrometry at millimeter wavelengths. Our initial results include new absolute astrometric measurements of the proper motion, orbital motion, and parallax of the $α$ Cen system. These lead to an improved knowledge of the physical properties of both $α$ Cen A and B. Our estimates of ALMA's rela…
Spatial Property of Multiple Metallic Populations in the Tidal Stream of ω Centauri
Sun Yat-sen University（中山大学）；CSST Science Center for the Guangdong-Hong Kong-Macau Greater Bay Area（CSST粤港澳大湾区科学中心）；Instituto de Astronomía, Universidad Católica del Norte（北方天主教大学天文学研究所）
Near UV Stellar Activity and Brightness Fluctuations of the Alpha Centauri AB Star System from Weeks to Decades -- Inputs for Reflected Light Spectroscopy with HWO
Laboratory for Atmospheric and Space Physics, University of Colorado Boulder（科罗拉多大学博尔德分校大气与空间物理实验室）；Center for Astrophysics and Space Astronomy, University of Colorado Boulder（科罗拉多大学博尔德分校天体物理学与空间天文中心）；Department of Physics, Ashoka University（阿肖卡大学物理系）；Applied Physics Laboratory, Johns Hopkins University（约翰斯·霍普金斯大学应用物理实验室）；Space Research Institute, Austrian Academy of Sciences（奥地利科学院空间研究所）
We present a chemical abundance analysis of a metal-poor star, ROA 276, in the stellar system omega Centauri. We confirm that this star has an unusually high [Sr/Ba] abundance ratio. Additionally, ROA 276 exhibits remarkably high abundance ratios, [X/Fe], for all elements from Cu to Mo along with normal abundance ratios for the elements from Ba to Pb. The chemical abundance pattern of ROA 276, relative to a primordial omega Cen star ROA 46, is best fit by a fast-rotating low-metallicity massive stellar model of 20 Msun, [Fe/H] = -1.8, and an initial rotation 0.4 times the critical value; no ot…
From the CMD of Omega Centauri and (super-)AGB stellar models to a Galactic plane passage gas purging chemical evolution scenario
我们研究了半人马座欧米茄的色星等图，发现蓝色主序（bMS）只能由氦丰度在Y=0.35- $0.40. To explain the faint subgiant branch of the reddest stars ("MS-a/RG-a" sequence), isochrones for the observed metallicity ([Fe/H]\approx0.7) appear to require both a high age (~13Gyr) and enhanced CNO abundances ([CNO/Fe]\approx0.9$范围内的模型再现。还必须假设Y~0.35，以抵消高CNO对关闭颜色的影响，从而获得与该恒星星族的相对蓝色关闭的良好拟合。这表明Omega Cen的化学演化时间较短（<1 Gyr）。我们的中等质量（超）AGB模型能够再现高氦丰度，以及[N/Fe]~2和大量的O贫化，如果充分考虑对流处理的不确定性。这些丰度特征将bMS星与占主导地位的[Fe/H] $\approx1.7$星族区分开来。最大质量的超AGB恒星模型（M_zams>=6.8M_sun，M_He，核心>=1.245M_sun）预测的N增强太大，这限制了它们对极端人口的贡献。我们定量地表明，高度He和N-富集AGB喷出物具有特别有效的冷却性能。基于这些结果和重建的轨道欧米茄…
Brown dwarfs can serve as both clocks and chemical tracers of the evolutionary history of the Milky Way due to their continuous cooling and high sensitivity of spectra to composition. We focus on brown dwarfs in globular clusters that host some of the oldest coeval populations in the galaxy. Currently, no brown dwarfs in globular clusters have been confirmed, but they are expected to be uncovered with advanced observational facilities such as JWST. In this paper we present a new set of stellar models specifically designed to investigate low-mass stars and brown dwarfs in $ω$ Centauri -- the la…
We have performed multivariate statistical analyses of photometric and chemical abundance parameters of three large samples of stars in the globular cluster $ω$ Centauri. The statistical analysis of a sample of 735 stars based on seven chemical abundances with the method of Maximum Parsimony (cladistics) yields the most promising results: seven groups are found, distributed along three branches with distinct chemical, spatial and kinematical properties. A progressive chemical evolution can be traced from one group to the next, but also within groups, suggestive of an inhomogeneous chemical enr…
The galactic globular cluster Omega Centauri is the most massive of its kind, with a complex mix of multiple stellar populations and several kinematic and dynamical peculiarities. Different mean proper motions have been detected among the three main sub-populations, implying that the most metal-rich one is of accreted origin. This particular piece of evidence has been a matter of debate because the available data have either not been sufficiently precise or limited to a small region of the cluster to ultimately confirm or refute the result. Using astrometry from the second Gaia data release an…
We present [K/Fe] abundance ratios for a sample of 450 stars in Omega Centauri, using high resolution spectra acquired with the multi-object spectrograph FLAMES@VLT. Abundances for Fe, Na and Mg were also derived. We detected intrinsic K variations in the analysed stars. Moreover, [K/Fe] shows a significant correlation with [Na/Fe] and anti-correlation with [Mg/Fe]. The presence of a clear-cut Mg-K anti-correlation makes Omega Centauri the third stellar system, after NGC 2419 and NGC 2808, hosting a sub-population of stars with [Mg/Fe]<0.0 dex, K-enriched in the case of Omega Centauri by ~0.3 …
The globular cluster Omega Centauri is the most massive and luminous cluster in the Galaxy. The $γ$-ray source FL8Y J1326.7$-$4729 is coincident with the core of the cluster, leading to speculation that hitherto unknown radio pulsars or annihilating dark matter may be present in the cluster core. Here we report on the discovery of five millisecond pulsars in Omega Centauri following observations with the Parkes radio telescope. Four of these pulsars are isolated with spin periods of 4.1, 4.2, 4.6 and 6.8 ms. The fifth has a spin period of 4.8 ms and is in an eclipsing binary system with an orb…
In the fourth paper of this series, we present -- and publicly release -- the state-of-the-art catalogue and atlases for the two remaining parallel fields observed with the Hubble Space Telescope for the large programme on omega Centauri. These two fields are located at ~12' from the centre of the globular cluster (in the West and South-West directions) and were imaged in filters from the ultraviolet to the infrared. Both fields were observed at two epochs separated by about 2 years that were used to derive proper motions and to compute membership probabilities.
The M dwarf Proxima~Centauri is known to be magnetically active and it hosts a likely Earth-like planet in its habitable zone. Understanding the characteristics of stellar radiation by understanding the properties of the emitting plasma is of paramount importance for a proper assessment of the conditions on Proxima~Centauri~b and exoplanets around M dwarfs in general. We determine the temperature structure of the coronal and transition region plasma of Proxima Centauri from simultaneous X-ray and far-ultraviolet (FUV) observations. The differential emission measure distribution (DEM) was const…
