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Centro de Astrofísica da Universidade do Porto

CAUP Researchers: Pedro C. V. R. M. Almeida, Nuno C. Santos, Jorge Filipe Gameiro
Team at CAUP: Origin and Evolution of Stars and Planets
Other Researchers: Claudio H. F. Melo (ESO), Matthias Ammler-von Eiff (PT), Carlos A. O. Torres (BR), Germano R. Quast (BR), Michael F. Sterzik (ESO)

Search for associations containing young stars (SACY)
II. Chemical abundances of stars in 11 young Associations in the Solar neighborhood
Astronomy and Astrophysics, Volume 501, pp. 965 (2009)

The recently discovered coeval, moving groups of young stellar objects in the solar neighborhood represent invaluable laboratories for studying recent star formation and searching for high metallicity stars that can be included in future exo-planet surveys. In this study, we derived through an uniform and homogeneous method stellar atmospheric parameters and abundances for iron, silicium, and nickel in 63 post-T Tauri stars from 11 nearby young associations. We further compare the results with two different pre-main sequence (PMS) and main sequence (MS) star populations. The stellar atmospheric parameters and the abundances presented here were derived using the equivalent width of individual lines in the stellar spectra by assuming the excitation/ionization equilibrium of iron. Moreover, we compared the observed Balmer lines with synthetic profiles calculated for model atmospheres with a different line-formation code. We found that the synthetic profiles agree reasonably well with the observed profiles, although the Balmer lines of many stars are substantially filled-in, probably by chromospheric emission. Solar metallicity is found to be a common trend in all the nearby young associations studied. The low abundance dispersion within each association strengthens the idea that the origin of these nearby young associations is related to the nearby star-forming regions (SFR). Abundances of elements other than iron are consistent with previous results for Main Sequence stars in the solar neighborhood. The chemical characterization of the members of the newly found nearby young associations, performed in this study and intended to proceed in subsequent works, is essential to understanding and testing the context of local star formation and the evolutionary history of the galaxy.

Figure 1 | Metallicity-weighted distribution of the 6 SFR studied by SA08 (light salmon-colored area), of our 11 associations (yellow-colored area) and of the 450 MS stars from SOU08 (blue-colored area) in the solar neighborhood plotted as functions of [Fe/H].

Since 1992, several loose associations of young stars have been found in the solar neighborhood, providing us unique opportunities to study recent and local star formation. However, in order to better understand the origin of these associations, we must compare them with typical stellar populations (both main sequence (MS) and pre-main sequence (PMS) stars), which requires the determination of their chemical abundances. Furthermore, there seems to be a connection between metallicity and the presence of planets around these stars, stressing the need to determine chemical abundances of these post-T Tauri stars (PTTS). In this study, we present stellar parameters and chemical abundances for 63 PTTS in 11 young nearby associations and compare them with samples of PMS and MS populations in the solar neighborhood taken from the literature.

The 63 stars chosen for this study were selected based on the following criteria: (i) they have been classified as a highly probable or probable member of a given association; (ii) they have vsini lower than 15 km/s to avoid strong line blending; (iii) to the best of our knowledge, they are all single stars; and (iv) they are young (younger or of Pleiades age) based on the equivalent width of their Li I 6708 line. High-resolution (R = λ/Δλ = 50 000 in both arms) spectroscopic observations were performed using the DIC#1 390+580 mode with the UVES spectrograph at the VLT/UT2 8.2-m Kueyen Telescope. The final spectra present high SNR (~150-200) and cover a wavelength interval from 4800 to 7000 Å with a gap between ~5755 and 5833 Å. The comparison data for MS dwarfs used in this paper were taken from Sousa et al. (2008, hereafter SOU08) and Gilli et al. (2006, hereafter GI06), whereas the weak-lined T Tauri stars comparison sample comes from Santos et al. (2008, hereafter SA08).

The stellar parameters and iron abundances were determined using the same methodology described in Santos el al. (2004). The four fundamental parameters, effective temperature (Teff), surface gravity, microturbulence and the iron abundance ([Fe/H]) were derived by imposing the Fe excitation/ionization equilibrium. We also derived errors for these parameters, which were computed with the method used by Gonzalez (1998).

We found that the derived microturbulence values were higher than the ones typically exhibited by MS stars, which has also been reported by other studies of PMS stars; however, the reason for this is still unknown. We also noted that the iron abundances tend to increase with increasing Teff in the range 4500 - 5500 K for any given association; again, this effect was already reported by other studies. However, it is reasonable to assume that stars which originated within the same cloud would show a similar abundance pattern and, therefore, we believe that this trend arises from systematic errors.

It has been shown that H-line profiles are very sensitive to temperature variations, but quite insensitive to gravity and metallicity variations, which allows them to be used for checking the consistency of the stellar parameters estimation. By performing this check for 10 stars, we found no clear and strong systematic difference between temperatures derived from H-lines and from the iron excitation/ionization equilibrium. A small systematic temperature difference may be present in the data, although not higher than ~100 K from peek to peek. However, this error could account, at most, for ~0.06 dex in the derived iron abundances and, thus, can only explain around 50% of the [Fe/H] trend.

The results obtained for the iron abundances, summarized in Fig. 1, present low standard deviations and are within the range of the [Fe/H] uncertainties. From the comparison of the obtained results with the SA08 and SOU08 samples, also performed in Fig.1, one realizes that nearby young associations and the PMS sample present very similar distributions, while MS stars present a broader range of metallicities. Thus, the nearby young associations and the SFRs share the same metallicity. Within the errors, both show solar metallicities with very little (below 0.1 dex) dispersion around the mean [Fe/H] value, which brings further evidence supporting the claim that the young stellar associations are related to the nearby SFRs. Therefore, these results also support the idea suggested by SA08 that super metal-rich stars (with metallicity clearly above solar) found in the solar vicinity may have been formed in inner Galactic disk regions.

It has been shown that the structural components of the Milky Way present characteristic abundance patterns for α-elements and iron-peak elements with respect to iron and oxygen. Thus, we derived nickel (iron-peak element) and silicium (α-element) abundances for the stars studied in this paper and concluded that they are compatible with the abundances measured in field stars with similar metallicities from GI06. This confirms a similar conclusion, in SA08, regarding the nearby SFRs.

Thus, the determination of the iron, silicium and nickel abundances of members of 11 young nearby associations showed that no metal overabundance exists in our sample, as well as in the sample from SA08. As, in both samples, no significant spatial displacement has yet occurred, this implies that the high-metallicity stars harboring planets were not formed in the solar neighborhood but were instead carried to their present position by Galactic dynamical forces.

This highlighted paper was written as part of the Ph.D. project of Pedro Viana Almeida who, thus, provided the major contribution to this work.

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