323,544 research outputs found
Chemical composition of the young open clusters IC 2602 and IC 2391
No full textContext. Galactic open clusters have been long recognized as one of the best tools to investigate the chemical content of Galactic disk and its time evolution. In the last decade, many efforts have been directed to chemically characterize the old and intermediate age population; surprisingly, the chemical content of the younger and close counterpart remains largely undetermined.
Aims. In this paper we present the abundance analysis of a sample of 15 G/K members of the young pre-main sequence clusters IC 2602 and IC 2391. Along with IC 4665, these are
the first pre-main sequence clusters for which a detailed abundance determination has been carried out so far.
Methods. We analyzed high-resolution, high spectra acquired with different instruments (UVES and CASPEC at ESO, and the echelle spectrograph at CTIO), using MOOG and equivalent width measurements. Along with metallicity ([Fe/H]), we measured Na
Chemical composition of the Taurus-Auriga association
No full textContext. The Taurus-Auriga association is perhaps the most famous prototype of a low-mass star forming region, surveyed at almost all wavelengths. Unfortunately, like several other young clusters/associations, this T association lacks an extensive abundance analysis determination.
Aims. We present a high-resolution spectroscopic study of seven low-mass members of Taurus-Auriga, including both weak-lined and classical T Tauri stars designed to help robustly determine their metallicity.
Methods. After correcting for spectral veiling, we performed equivalent width and spectral synthesis analyses using the GAIA set of model atmospheres and the 2002 version of the code MOOG.
Results. We find a solar metallicity, obtaining a mean value of [Fe/H] = −0.01 ± 0.05. The α-element Si and the Fe-peak one Ni confirm a solar composition. Our work shows that the dispersion among members is well within the observational errors at variance with previous claims. As in other star forming regions, no metal-rich members are found, reinforcing the idea that old planet-host stars form in the inner part of the Galactic disc and subsequently migrate
Accretion in the ρ-Ophiuchi pre-main sequence stars
No full textAims.The aim of this paper is to provide a measurement of the mass accretion rate in a large, complete sample of objects in the core of the star forming region ρ Oph.
Methods: .The sample includes most of the objects (104 out of 111) with evidence of a circumstellar disk from mid-infrared photometry; it covers a stellar mass range from about 0.03 to 3 M_⊙ and it is complete to a limiting mass of 0.05 M_⊙. We used J and K-band spectra to derive the mass accretion rate of each object from the intensity of the hydrogen recombination lines, Paβ or Brγ. For comparison, we also obtained similar spectra of 35 diskless objects.
Results: .The results show that emission in these lines is only seen in stars with disks, and can be used as an indicator of accretion. However, the converse does not hold, as about 50% of our disk objects do not have detectable line emission. The measured accretion rates show a strong correlation with the mass of the central object (dot M_acc ∝ M_star1.8±0.2) and a large spread, of two orders of magnitude at least, for any interval of M_star. A comparison with existing data for Taurus shows that the objects in the two regions have similar behaviour, at least for objects more massive than ∼ 0.1 M_⊙. The implications of these results are briefly discussed
The Abundance of S-Process Elements: Temporal and Spatial Trends from Open Cluster Observations
No full textSpectroscopic observations of stars belonging to open clusters, with well-determined ages and distances, are a unique tool for constraining stellar evolution, nucleosynthesis, mixing processes, and, ultimately, Galactic chemical evolution. Abundances of slow (s) process neutron capture elements in stars that retain their initial surface composition open a window into the processes that generated them. In particular, they give us information on their main site of production, i.e., the low-and intermediate-mass Asymptotic Giant Branch (AGB) stars. In the present work, we review some observational results obtained during the last decade that contributed to a better understanding of the AGB phase: the growth of s-process abundances at recent epochs, i.e., in the youngest stellar populations; the different relations between age and [s/Fe] in distinct regions of the disc; and finally the use of s-process abundances combined with those of α elements, [s/α], to estimate stellar ages. We revise some implications that these observations had both on stellar and Galactic evolution, and on our ability to infer stellar ages
The effect of heavy element opacity on pre-main sequence Li depletion
No full textContext. Recent 3-D analysis of the solar spectrum data suggests a significant change of the solar chemical composition. This may affect the temporal evolution of the surface abundance of light elements since the extension of the convective envelope is largely affected by the internal opacity value. Aims. We analyse the influence of the adopted solar mixture on the opacity in the convective envelope of pre-main sequence (PMS) stars and thus on PMS lithium depletion. The surface Li abundance depends on the relative efficiency of several processes, some of them still not known with the required precision; this paper thus analyses one of the aspects of this "puzzle". Methods. Focusing on PMS evolution, where the largest amount of Li burning occurs, we computed stellar models for three selected masses (0.8, 1.0 and 1.2 M-circle dot, with Z = 0.013, Y = 0.27, alpha = 1.9) by varying the chemical mixture, that is the internal element distribution in Z. We analysed the contribution of the single elements to the opacity at the temperatures and densities of interest for Li depletion. Several mixtures were obtained by varying the abundance of the most important elements one at a time; we then calculated the corresponding PMS Li abundance evolution. Results. We found that a mixture variation does change the Li abundance: at fixed total metallicity, the Li depletion increases when increasing the fraction of elements heavier than O
The double population of Chamaeleon I detected by Gaia DR2
No full textContext. Chamaeleon I represents an ideal laboratory to study the cluster formation in a low-mass environment. Recently, two sub-clusters spatially located in the northern and southern parts of Chamaeleon I were found with different ages and radial velocities. Aims. In this Letter we report new insights into the structural properties, age, and distance of Chamaeleon I based on the astrometric parameters from Gaia data release 2 (DR2).
Methods. We identified 140 sources with a reliable counterpart in the Gaia DR2 archive. We determined the median distance of the cluster using Gaia parallaxes and fitted the distribution of parallaxes and proper motions assuming the presence of two clusters. We derived the probability of each single source of belonging to the northern or southern sub-clusters, and compared the HR diagram of the most probable members to pre-main sequences isochrones.
Results. The median distance of Chamaeleon I is ~190 pc. This is consistent with the revised estimate using the Tycho-Gaia Astrometric Solution, but it is about 20 pc larger than the value commonly adopted in the literature. From a Kolmogorov–Smirnov test of the parallaxes and proper-motion distributions we conclude that the northern and southern clusters do not belong to the same parent population. The northern population has a distance dN = 192.7+0.4−0.4pc, while the southern one dS = 186.5+0.7−0.7pc. The two sub-clusters appear coeval, at variance with literature results, and most of the sources are younger than 3 Myr. The northern cluster is more elongated and extends towards the southern direction partially overlapping with the more compact cluster located in the south. A hint of a relative rotation between the two sub-clusters is also found
U-band study of the accretion properties in the σ Orionis star-forming region
No full textThis paper presents the results of an U band survey with FORS1/VLT of a large area in the σ Ori star-forming region. We combine the U-band photometry with literature data to compute accretion luminosity and mass accretion rates from the U-band excess emission for all objects (187) detected by Spitzer in the FORS1 field and classified by Hernandez et al. as likely members of the cluster. The sample
̇
stars range in mass from ∼0.06 to ∼1.2 M⊙; 72 of them show evidence of disks and we measure mass accretion rates Macc between
<10−11 and few 10−9 M⊙/y, using the colors of the diskless stars as photospheric templates. Our results confirm the dependence of ̇
of Macc for any value of the stellar mass is ∼2 orders of magnitude. We discuss the implications of these results in the context of ̇
Macc on the mass of the central object, which is stronger for low-mass stars and flattens out for masses larger than ∼0.3 M⊙; the spread ̇
disk evolution models. Finally, we analyze the relation between Macc and the excess emission in the Spitzer bands, and find that at ̇
Macc ∼ 10−10 M⊙/y the inner disks change from optically thin to optically thick
<i>s</i>-PROCESSING IN THE GALACTIC DISK. I. SUPER-SOLAR ABUNDANCES OF Y, Zr, La, AND Ce IN YOUNG OPEN CLUSTERS
No full textAccretion properties of T Tauri stars in σ Orionis
No full textAccretion disks around young stars evolve in time with time scales of a few million years. We present here a study of the accretion properties of a sample of 35 stars in the ∼ 3 million-year-old star-forming region σ Ori. Of these, 31 are objects with evidence of disks, based on their IR excess emission. We use near-IR hydrogen recombination lines (Paγ) to measure their mass accretion rate. We find that the accretion rates are significant lower in σ Ori than in younger regions, such as ρ Oph, consistently with viscous disk evolution. The He I 1.083 μ m line is detected (either in absorption or in emission) in 72% of the stars with disks, also providing evidence of accretion-powered activity in very low accretors, where other accretion indicators disappear
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