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Register a new userMetallicity of low-mass stars in Orion
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V. DOrazi
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Determining the metal content of low-mass members of young associations provides a tool that addresses different issues, such as triggered star formation or the link between the metal-rich nature of planet-host stars and the early phases of planet formation. The Orion complex is a well known example of possible triggered star formation and is known to host a rich variety of proto-planetary disks around its low-mass stars. Available metallicity measurements yield discrepant results. We analyzed FLAMES/UVES and Giraffe spectra of low-mass members of three groups/clusters belonging to the Orion association. Our goal is the homogeneous determination of the metallicity of the sample stars, which allows us to look for [Fe/H] differences between the three regions and for the possible presence of metal-rich stars. Nine members of the ONC and one star each in the $lambda$ Ori cluster and OB1b subgroup were analyzed. After the veiling determination, we retrieved the metallicity by means of equivalent widths and/or spectral synthesis using MOOG. We obtain an average metallicity for the ONC [Fe/H]=-0.01pm 0.04. No metal-rich stars were detected and the dispersion within our sample is consistent with measurement uncertainties. The metallicity of the $lambda$ Ori member is also solar, while the OB1b star has an [Fe/H] significantly below the ONC average. If confirmed by additional [Fe/H] determinations in the OB1b subgroup, this result would support the triggered star formation and the self-enrichment scenario for the Orion complex.
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Among Carbon-Enhanced Metal-Poor (CEMP) stars, some are found to be enriched in s-process elements (CEMP-s), in r-process elements (CEMP-r) or in both s- and r-process elements (CEMP-rs). The origin of the abundance differences between CEMP-s and CEMP-rs stars is presently unknown. It has been claimed that the i-process, whose site still remains to be identified, could better reproduce CEMP-rs abundances than the s-process. We analyze high-resolution spectra of 25 metal-poor stars, observed with the high-resolution HERMES spectrograph mounted on the Mercator telescope, La Palma, or with the UVES/VLT and HIRES/KECK spectrographs. We propose a new, robust classification method for CEMP-s and CEMP-rs stars using eight heavy element abundances. The abundance profiles of CEMP-s and CEMP-rs stars are derived and there appears to be an abundance continuum between the two stellar classes. CEMP-rs stars present most of the characteristics of extrinsic stars such as CEMP-s, CH, Barium and extrinsic S stars, with an even larger binarity rate among CEMP-rs stars than among CEMP-s stars. Stellar evolutionary tracks of an enhanced carbon composition (consistent with our abundance determinations) are necessary to explain the position of CEMP-s and CEMP-rs stars in the HR diagram using Gaia DR2 parallaxes; they are found to lie mostly on the RGB. CEMP-rs stars can be explained as being polluted by a low-mass, low-metallicity TP-AGB companion experiencing i-process nucleosynthesis after proton ingestion during its first convective thermal pulses. The global fitting of our i-process models to CEMP-rs stars is as good as the one of our s-process models to CEMP-s stars. As such, CEMP-rs stars could be renamed as CEMP-sr stars, since they represent a particular manifestation of the s-process at low-metallicities. For these objects a call for an exotic i-process site may not necessarily be required anymore.
The Chandra High Energy Transmission Gratings (HETG) Orion Legacy Project (HOLP) is the first comprehensive set of observations of a very young massive stellar cluster which provides high resolution X-ray spectra of very young stars over a wide mass range (0.7 - 2.3 Msun). In this paper, we focus on the six brightest X-ray sources with T Tauri stellar counterparts which are well-characterized at optical and infra-red wavelengths. All stars show column densities which are substantially smaller than expected from optical extinction indicating that the sources are located on the near side of the cluster with respect to the observer as well as that these stars are embedded in more dusty environments. Stellar X-ray luminosities are well above $10^{31}$ erg/s, in some cases exceeding $10^{32}$ erg/s for a substantial amount of time. The stars during these observations show no flares but are persistently bright. The spectra can be well fit with two temperature plasma components of 10 MK and 40 MK, of which the latter dominates the flux by a ratio 6:1 on average. The total EMs range between 3 - 8$times10^{54}$ cm$^{-3}$ and are comparable to active coronal sources. Limits on the forbidden to inter-combination line ratios in the He-Like K-shell lines show that we observe a predominantely optically thin plasma with electron densities below $10^{12}$ cm$^{-3}$. Observed abundances compare well with active coronal sources underlying the coronal nature of these sources. The surface flux in this sample of 0.6 to 2.3 Msun classical T Tauri stars shows that coronal activity and possibly coronal loop size increase significantly between ages 0.1 to 10 Myrs.
Context. X-ray flares are common phenomena in pre-main sequence stars. Their analysis gives insights into the physics at work in young stellar coronae. The Orion Nebula Cluster offers a unique opportunity to study large samples of young low mass stars. This work is part of the Chandra Orion Ultradeep project (COUP), an ~10 day long X-ray observation of the Orion Nebula Cluster (ONC). Aims. Our main goal is to statistically characterize the flare-like variability of 165 low mass (0.1-0.3 M_sun) ONC members in order to test and constrain the physical scenario in which flares explain all the observed emission. Methods. We adopt a maximum likelihood piece-wise representation of the observed X-ray light curves and detect flares by taking into account both the amplitude and time derivative of the count-rate. We then derive the frequency and energy distribution of the flares. Results. The high energy tail of the energy distribution of flares is well described by a power-law with index 2.2. We test the hypothesis that light curves are built entirely by overlapping flares with a single power law energy distribution. We constrain the parameters of this simple model for every single light curve. The analysis of synthetic light curves obtained from the model indicates a good agreement with the observed data. Comparing low mass stars with stars in the mass interval (0.9-1.2M_sun), we establish that, at ~1 Myr, low mass and solar mass stars of similar X-ray luminosity have very similar flare frequencies. Conclusions. Our observational results are consistent with the following model/scenario: the light curves are entirely built by over- lapping flares with a power-law intensity distribution; the intense flares are individually detected, while the weak ones merge and form a pseudo-quiescent level, which we indicate as the characteristic level.
Using asteroseismic data from the Kepler satellite, we explore the systematic uncertainties arising from changes in the input physics used when constructing evolution models of solar-type stars. We assess the impact of including atomic diffusion and of varying the metallicity mixture on the determination of global stellar parameters (i.e., radius, mass, and age). We find significant systematic uncertainties on global stellar parameters when diffusion is included in stellar grids. Furthermore, we find the systematic uncertainties on the global stellar parameters to be comparable to the statistical uncertainties when a different metallicity mixture is employed in stellar grids.
We investigate metal pollution onto the surface of low-mass population III stars (Pop. III survivors) via interstellar objects floating in the Galactic interstellar medium. Only recently, Tanikawa et al. analytically estimated how much metal should collide to an orbiting Pop. III survivor encouraged by the recent discovery of Oumuamua and suggested that ISOs are the most dominant contributor of metal enrichment of Pop. III survivors. When we consider a distribution of interstellar objects in the Galactic disc, Pop. III survivors orbits are significant properties to estimate the accretion rate of them though Tanikawa et al. assumed one modelled orbit. To take more realistic orbits into calculating the accretion rate, we use a high-resolution cosmological $N$-body simulation that resolves dark matter minihaloes. Pop. III survivors located at solar neighbourhood have a number of chances of ISO($> 100$ m) collisions, typically $5times10^6$ times in the last $5$ Gyr, which is one order of magnitude greater than estimated in the previous study. When we assume a power-law parameter $alpha$ of the ISO cumulative number density with size greater than $D$ as $n propto D^{-alpha}$, $0.80 , M_{odot}$ stars should be typically polluted [Fe/H]$sim -2$ for the case of $alpha=2.0$. Even in the cases of $0.70$ and $0.75 , M_{odot}$ stars, the typical surface metallicity are around [Fe/H]$=-6 sim -5$. From the presence of stars with their [Fe/H], we can constrain on the lower limit of the power $alpha$, as $alpha gtrsim 2.0$, which is consistent with $alpha$ of km-size asteroids and comets in the solar system. Furthermore, we provide six candidates as the ISO-polluted Pop. III stars in the case of $alpha sim 2.5$. Metal-poor stars so far discovered are possible to be metal-free Pop. III stars on birth.
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