Context. Young open clusters (ages of less than 200 Myr) have been observed to exhibit several peculiarities in their chemical compositions. These anomalies include a slightly sub-solar iron content, ...super-solar abundances of some atomic species (e.g. ionised chromium), and atypical enhancements of Ba/Fe, with values up to ~0.7 dex. Regarding the behaviour of the other s-process elements like yttrium, zirconium, lanthanum, and cerium, there is general disagreement in the literature: some authors claim that they follow the same trend as barium, while others find solar abundances at all ages. Aims. In this work we expand upon our previous analysis of a sample of five young open clusters (IC 2391, IC 2602, IC 4665, NGC 2516, and NGC 2547) and one star-forming region (NGC 2264), with the aim of determining abundances of different neutron-capture elements, mainly Cu I, Sr I, Sr II, Y II, Zr II, Ba II, La II, and Ce II. For NGC 2264 and NGC 2547 we present the measurements of these elements for the first time. Methods. We analysed high-resolution, high signal-to-noise spectra of 23 solar-type stars observed within the Gaia-ESO survey. After a careful selection, we derived abundances of isolated and clean lines via spectral synthesis computations and in a strictly differential way with respect to the Sun. Results. We find that our clusters have solar Cu/Fe within the uncertainties, while we confirm that Ba/Fe is super-solar, with values ranging from +0.22 to +0.64 dex. Our analysis also points to a mild enhancement of Y, with Y/Fe ratios covering values between 0 and +0.3 dex. For the other s-process elements we find that X/Fe ratios are solar at all ages. Conclusions. It is not possible to reconcile the anomalous behaviour of Ba and Y at young ages with standard stellar yields and Galactic chemical evolution model predictions. We explore different possible scenarios related to the behaviour of spectral lines, from the dependence on the different ionisation stages and the sensitivity to the presence of magnetic fields (through the Landé factor) to the first ionisation potential effect. We also investigate the possibility that they may arise from alterations of the structure of the stellar photosphere due to the increased levels of stellar activity that affect the spectral line formation, and consequently the derived abundances. These effects seem to be stronger in stars at ages of less than ~ 100 Myr. However, we are still unable to explain these enhancements, and the Ba puzzle remains unsolved. With the present study we suggest that other elements, for example Sr, Zr, La, and Ce, might be more reliable tracer of the s-process at young ages, and we strongly encourage further critical observations.
We present results of a long-baseline interferometry campaign using the PAVO beam combiner at the CHARA Array to measure the angular sizes of five main-sequence stars, one subgiant and four red giant ...stars for which solar-like oscillations have been detected by either Kepler or CoRoT. By combining interferomettic angular diameters, Hipparcos parallaxes, asteroseismic densities, bolometric fluxes, and high-resolution spectroscopy, we derive a full set of near-model-independent fundamental properties for the sample. We first use these properties to test asteroseismic scaling relations for the frequency of maximum power (v sub(max)) and the large frequency separation ( Delta v). We find excellent agreement within the observational uncertainties, and empirically show that simple estimates of asteroseismic radii for main-sequence stars are accurate to lap4%. We furthermore find good agreement of our measured effective temperatures with spectroscopic and photometric estimates with mean deviations for stars between T sub(eff) = 4600-6200 K of -22 + or - 32 K (with a scatter of 97 K) and -58 + or - 31 K (with a scatter of 93 K), respectively. Finally, we present a first comparison with evolutionary models, and find differences between observed and theoretical properties for the metal-rich main-sequence star HD 173701. We conclude that the constraints presented in this study will have strong potential for testing stellar model physics, in particular when combined with detailed modeling of individual oscillation frequencies.
Spectroscopic study of the open cluster NGC 6811 Molenda-Żakowicz, J; Brogaard, K; Niemczura, E ...
Monthly notices of the Royal Astronomical Society,
12/2014, Letnik:
445, Številka:
3
Journal Article
Recenzirano
Odprti dostop
The NASA space telescope Kepler has provided unprecedented time series observations which have revolutionized the field of asteroseismology, i.e. the use of stellar oscillations to probe the interior ...of stars. The Kepler-data include observations of stars in open clusters, which are particularly interesting for asteroseismology. One of the clusters observed with Kepler is NGC 6811, which is the target of this paper. However, apart from high-precision time series observations, sounding the interiors of stars in open clusters by means of asteroseismology also requires accurate and precise atmospheric parameters as well as cluster membership indicators for the individual stars. We use medium-resolution (R ∼ 25 000) spectroscopic observations, and three independent analysis methods, to derive effective temperatures, surface gravities, metallicities, projected rotational velocities and radial velocities, for 15 stars in the field of the open cluster NGC 6811. We discover two double-lined and three single-lined spectroscopic binaries. Eight stars are classified as either certain or very probable cluster members, and three stars are classified as non-members. For four stars, cluster membership could not been assessed. Five of the observed stars are G-type giants which are located in the colour–magnitude diagram in the region of the red clump of the cluster. Two of these stars are surely identified as red clump stars for the first time. For those five stars, we provide chemical abundances of 31 elements. The mean radial velocity of NGC 6811 is found to be +6.68 ± 0.08 km s−1 and the mean metallicity and overall abundance pattern are shown to be very close to solar with an exception of Ba which we find to be overabundant.
Lithium abundance in most of the warm metal-poor main sequence stars shows a constarnt plateau (A(Li) ~ 2.2 dex) and then the upper envelope of the lithium vs. metallicity distribution increases as ...we approach solar metallicity. Meteorites, which carry information about the chemical composition of the interstellar medium (ISM) at the solar system formation time, show a lithium abundance A(Li) ~ 3.26 dex. This pattern reflects the Li enrichment history of the ISM during the Galaxy lifetime. After the initial Li production in big bang nucleosynthesis, the sources of the enrichment include asymptotic giant branch (AGB) stars, low-mass red giants, novae, type II supernovae, and Galactic cosmic rays. The total amount of enriched Li is sensitive to the relative contribution of these sources. Thus different Li enrichment histories are expected in the Galactic thick and thin disc. We investigate the main sequence stars observed with UVES in Gaia-ESO Survey iDR4 catalogue and find a Li- α/Fe anticorrelation independent of Fe/H, Teff, and log (g). Since in stellar evolution different α enhancements at the same metallicity do not lead to a measurable Li abundance change, the anticorrelation indicates that more Li is produced during the Galactic thin disc phase than during the Galactic thick disc phase. We also find a correlation between the abundance of Li and s-process elements Ba and Y, and they both decrease above the solar metallicity, which can be explained in the framework of the adopted Galactic chemical evolution models.
Recent evidence indicates that glutamate homeostasis and neurotransmission are altered in major depressive disorder, but the nature of the disruption and the mechanisms by which it contributes to the ...syndrome are unclear. Glutamate can act via AMPA, NMDA, or metabotropic receptors. Using targeted mutagenesis, we demonstrate here that mice with deletion of the main AMPA receptor subunit GluR-A represent a depression model with good face and construct validity, showing behavioral and neurochemical features of depression also postulated for human patients. GluR-A⁻/⁻ mice display increased learned helplessness, decreased serotonin and norepinephrine levels, and disturbed glutamate homeostasis with increased glutamate levels and increased NMDA receptor expression. These results correspond well with current concepts regarding the role of AMPA and NMDA receptors in depression, postulating that compounds that augment AMPA receptor signaling or decrease NMDA receptor functions have antidepressant effects. GluR-A⁻/⁻ mice represent a model to investigate the pathophysiology underlying the depressive phenotype and to identify changes in neural plasticity and resilience evoked by the genetic alterations in glutamatergic function. Furthermore, GluR-A⁻/⁻ mice may be a valuable tool to study biological mechanisms of AMPA receptor modulators and the efficacy of NMDA antagonists in reducing behavioral or biochemical changes that correlate with increased helplessness.--Chourbaji, S., Vogt, M. A., Fumagalli, F., Sohr, R., Frasca, A., Brandwein, C, Hörtnagl, H., Riva, M. A., Sprengel, R., Gass, P. AMPA receptor subunit 1 (GluR-A) knockout mice model the glutamate hypothesis of depression.
Substellar companions at wide separation around stars hosting planets or brown dwarfs (BDs) yet close enough for their formation in the circumstellar disc are of special interest. In this Letter we ...report the discovery of a wide (projected separation ∼16″.0, or 2400 AU, and position angle 114.61°) companion of the GQ Lup A-B system, most likely gravitationally bound to it. A VLT/X-shooter spectrum shows that this star, 2MASS J15491331-3539118, is a bonafide low-mass (∼0.15
M
⊙
) young stellar object (YSO) with stellar and accretion/ejection properties typical of Lupus YSOs of similar mass, and with kinematics consistent with that of the GQ Lup A-B system. A possible scenario for the formation of the triple system is that GQ Lup A and 2MASS J15491331-3539118 formed by fragmentation of a turbulent core in the Lup I filament, while GQ Lup B, the BD companion of GQ Lup A at 0″.7, formed in situ by the fragmentation of the circumprimary disc. The recent discoveries that stars form along cloud filaments would favour the scenario of turbulent fragmentation for the formation of GQ Lup A and 2MASS J15491331-3539118.
Aims.
Very recently, a second companion on a wider orbit has been discovered around GQ Lup. This is a low-mass accreting star that is partially obscured by a disk seen at high inclination. If ...detected, this disk may be compared to the known disk around the primary.
Methods.
We detected this disk on archive HST and WISE data.
Results.
The extended spectral energy distribution provided by these data confirms the presence of accretion from H
α
emission and UV excess, and shows an IR excess attributable to a warm disk. In addition, we resolved the disk on the HST images. It is found to be roughly aligned with the disk of the primary. Both of them are roughly aligned with the Lupus I dust filament containing GQ Lup.
Context. Young stellar systems actively accrete from their circumstellar disk and simultaneously launch outflows. The physical link between accretion and ejection processes remains to be fully ...understood. Aims. We investigate the structure and dynamics of magnetospheric accretion and associated outflows on a scale smaller than 0.1 au around the young transitional disk system GM Aur. Methods. We devised a coordinated observing campaign to monitor the variability of the system on timescales ranging from days to months, including partly simultaneous high-resolution optical and near-infrared spectroscopy, multiwavelength photometry, and low-resolution near-infrared spectroscopy, over a total duration of six months, covering 30 rotational cycles. We analyzed the photometric and line profile variability to characterize the accretion and ejection processes. Results. The optical and near-infrared light curves indicate that the luminosity of the system is modulated by surface spots at the stellar rotation period of 6.04 ± 0.15 days. Part of the Balmer, Paschen, and Brackett hydrogen line profiles as well as the HeI 5876 Å and HeI 10830 Å line profiles are modulated on the same period. The Paβ line flux correlates with the photometric excess in the u′ band, which suggests that most of the line emission originates from the accretion process. High-velocity redshifted absorptions reaching below the continuum periodically appear in the near-infrared line profiles at the rotational phase in which the veiling and line fluxes are the largest. These are signatures of a stable accretion funnel flow and associated accretion shock at the stellar surface. This large-scale magnetospheric accretion structure appears fairly stable over at least 15 and possibly up to 30 rotational periods. In contrast, outflow signatures randomly appear as blueshifted absorption components in the Balmer and HeI 10830 Å line profiles. They are not rotationally modulated and disappear on a timescale of a few days. The coexistence of a stable, large-scale accretion pattern and episodic outflows supports magnetospheric ejections as the main process occurring at the star-disk interface. Conclusions. Long-term monitoring of the variability of the GM Aur transitional disk system provides clues to the accretion and ejection structure and dynamics close to the star. Stable magnetospheric accretion and episodic outflows appear to be physically linked on a scale of a few stellar radii in this system.
Context. The origin and dynamical evolution of star clusters is an important topic in stellar astrophysics. Several models have been proposed in order to understand the formation of bound and unbound ...clusters and their evolution, and they can be tested by examining the kinematical and dynamical properties of clusters over a wide range of ages and masses. Aims. We use the Gaia-ESO Survey products to study four open clusters (IC 2602, IC 2391, IC 4665, and NGC 2547) that lie in the age range between 20 and 50 Myr. Methods. We employ the gravity index γ and the equivalent width of the lithium line at 6708 Å together with effective temperature Teff and the metallicity of the stars in order to discard observed contaminant stars. Then we derive the cluster radial velocity dispersions σc, the total cluster mass Mtot, and the half mass radius rhm. Using the Gaia-DR1 TGAS catalogue, we independently derive the intrinsic velocity dispersion of the clusters from the astrometric parameters of cluster members. Results. The intrinsic radial velocity dispersions derived by the spectroscopic data are higher than those derived from the TGAS data, possibly due to the different masses of the considered stars. Using Mtot and rhm we derive the virial velocity dispersion σvir and we find that three out of four clusters are supervirial. This result is in agreement with the hypothesis that these clusters are dispersing, as predicted by the “residual gas expulsion” scenario. However, recent simulations show that the virial ratio of young star clustersmay be overestimated if it is determined using the global velocity dispersion, since the clusters are not fully relaxed.
We report the discovery of Kepler-77b (alias KOI-127.01), a Saturn-mass transiting planet in a 3.6-day orbit around a metal-rich solar-like star. We combined the publicly available Kepler photometry ...(quarters 1−13) with high-resolution spectroscopy from the Sandiford at McDonald and FIES at NOT spectrographs. We derived the system parameters via a simultaneous joint fit to the photometric and radial velocity measurements. Our analysis is based on the Bayesian approach and is carried out by sampling the parameter posterior distributions using a Markov chain Monte Carlo simulation. Kepler-77b is a moderately inflated planet with a mass of Mp = 0.430 ± 0.032 MJup, a radius of Rp = 0.960 ± 0.016 RJup, and a bulk density of ρp = 0.603 ± 0.055 g cm-3. It orbits a slowly rotating (Prot = 36 ± 6 days) G5 V star with M⋆ = 0.95 ± 0.04 M⊙, R⋆ = 0.99 ± 0.02 R⊙, Teff = 5520 ± 60 K, M/H = 0.20 ± 0.05 dex, that has an age of 7.5 ± 2.0 Gyr. The lack of detectable planetary occultation with a depth higher than ~10 ppm implies a planet geometric and Bond albedo of Ag ≤ 0.087 ± 0.008 and AB ≤ 0.058 ± 0.006, respectively, placing Kepler-77b among the gas-giant planets with the lowest albedo known so far. We found neither additional planetary transit signals nor transit-timing variations at a level of ~0.5 min, in accordance with the trend that close-in gas giant planets seem to belong to single-planet systems. The 106 transitsobserved in short-cadence mode by Kepler for nearly 1.2 years show no detectable signatures of the planet’s passage in front of starspots. We explored the implications of the absence of detectable spot-crossing events for the inclination of the stellar spin-axis, the sky-projected spin-orbit obliquity, and the latitude of magnetically active regions.
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