Aims. We present a spectroscopic and photometric analysis of the rapid rotator KIC 8429280, discovered by ourselves as a very young star and observed by the NASA Kepler mission, designed to determine ...its activity level, spot distribution, and differential rotation. Methods. We use ground-based data, such as high-resolution spectroscopy and multicolor broad-band photometry, to derive stellar parameters (vsini, spectral type, Teff, log g, and Fe/H), and we adopt a spectral subtraction technique to highlight the strong chromospheric emission in the cores of hydrogen Hα and Ca ii H&K and infrared triplet (IRT) lines. We then fit a robust spot model to the high-precision Kepler photometry spanning 138 days. Model selection and parameter estimation is performed in a Bayesian manner using a Markov chain Monte Carlo method. Results. We find that KIC 8429280 is a cool (K2 V) star with an age of about 50 Myr, based on its lithium content, that has passed its T Tau phase and is spinning up approaching the ZAMS on its radiative track. Its high level of chromospheric activity is clearly indicated by the strong radiative losses in Ca ii H&K and IRT, Hα, and Hβ lines. Furthermore, its Balmer decrement and the flux ratio of Ca ii IRT lines imply that these lines are mainly formed in optically-thick regions similar to solar plages. The analysis of the Kepler data uncovers evidence of at least seven enduring spots. Since the star’s inclination is rather high – nearly 70° – the assignment of the spots to either the northern or southern hemisphere is not unambiguous. We find at least three solutions with nearly the same level of residuals. Even in the case of seven spots, the fit is far from being perfect. Owing to the exceptional precision of the Kepler photometry, it is not possible to reach the noise floor without strongly enhancing the degrees of freedom and, consequently, the non-uniqueness of the solution. The distribution of the active regions is such that the spots are located around three latitude belts, i.e. around the star’s equator and around ± (50°–60°), with the high-latitude spots rotating slower than the low-latitude ones. The equator-to-pole differential rotation dΩ ≃ 0.27 rad d-1 is at variance with some recent mean-field models of differential rotation in rapidly rotating main-sequence stars, which predict a much smaller latitudinal shear. Our results are consistent with the scenario of a higher differential rotation, which changes along the magnetic cycle, as proposed by other models.
The Gaia ESO Public Spectroscopic Survey (GES) is providing the astronomical community with high-precision measurements of many stellar parameters including radial velocities (RVs) of stars belonging ...to several young clusters and star-forming regions. One of the main goals of the young cluster observations is to study their dynamical evolution and provide insight into their future, revealing whether they will eventually disperse to populate the field rather than evolve into bound open clusters. In this paper we report the analysis of the dynamical state of L1688 in the ρ Ophiuchi molecular cloud using the dataset provided by the GES consortium. We performed the membership selection of the more than 300 objects observed. Using the presence of the lithium absorption and the location in the Hertzspung-Russell diagram, we identify 45 already known members and two new association members. We provide accurate RVs for all 47 confirmed members. A dynamical analysis, after accounting for unresolved binaries and errors, shows that the stellar surface population of L1688 has a velocity dispersion σ ~ 1.14 ± 0.35 km s-1 that is consistent with being in virial equilibrium and is bound with a ~80% probability. We also find a velocity gradient in the stellar surface population of ~1.0 km s-1 pc-1 in the northwest-southeast direction, which is consistent with that found for the pre-stellar dense cores, and we discuss the possibility of sequential and triggered star formation in L1688.
Context. The Li abundance observed in pre-main sequence and main sequence late-type stars is strongly age-dependent, but also shows a complex pattern depending on several parameters, such as ...rotation, chromospheric activity, and metallicity. The best way to calibrate these effects, and with the aim of studying Li as an age indicator for FGK stars, is to calibrate coeval groups of stars, such as open clusters (OCs) and associations. Aims. We present a considerable target sample of 42 OCs and associations – with an age range from 1 Myr to 5 Gyr – observed within the Gaia-ESO survey (GES), and using the latest data provided by GES iDR6 and the most recent release of Gaia that was then available, EDR3. As part of this study, we update and improve the membership analysis for all 20 OCs presented in our previous article. Methods. We perform detailed membership analyses for all target clusters to identify likely candidates, using all available parameters provided by GES, complemented with detailed bibliographical searches, and based on numerous criteria: from radial velocity distributions, to the astrometry (proper motions and parallaxes) and photometry provided by Gaia, to gravity indicators (log g and the γ index), Fe/H metallicity, and Li content in diagrams of (Li equivalent widths) EW(Li) versus Teff. Results. We obtain updated lists of cluster members for the whole target sample, as well as a selection of Li-rich giant contaminants obtained as an additional result of the membership process. Each selection of cluster candidates was thoroughly contrasted with numerous existing membership studies using data from Gaia to ensure the most robust results. Conclusions. These final cluster selections will be used in the third and last paper of this series, which reports the results of a comparative study characterising the observable Li dispersion in each cluster and analysing its dependence on several parameters, allowing us to calibrate a Li–age relation and obtain a series of empirical Li envelopes for key ages in our sample.
Context. Knowledge of radial velocities, elemental abundances, and accretion properties of members of star-forming regions is important for our understanding of stellar and planetary formation. While ...infrared observations reveal the evolutionary status of the disk, optical spectroscopy is fundamental to acquire information on the properties of the central star and on the accretion characteristics. Aims. Existing 2MASS archive data and the Spitzer c2d survey of the Chamaeleon II dark cloud have provided disk properties of a large number of young stars. We complement these data with optical spectroscopy with the aim of providing physical stellar parameters and accretion properties. Methods. We use FLAMES/UVES and FLAMES/GIRAFFE spectroscopic observations of 40 members of the Chamaeleon II star-forming region to measure radial velocities through cross-correlation technique, lithium abundances by means of curves of growth, and for a suitable star elemental abundances of Fe, Al, Si, Ca, Ti, and Ni using the code MOOG. From the equivalent widths of the Hα, Hβ, and the He i λ5876, λ6678, λ7065 Å emission lines, we estimate the mass accretion rates, Ṁacc, for all the objects. Results. We derive a radial velocity distribution for the Chamaeleon II stars, which is peaked at ⟨Vrad⟩ = 11.4 ± 2.0 km s-1. We find dependencies of Ṁacc ∝ M⋆1.3 and of Ṁacc ∝ Age-0.82 in the ~0.1−1.0 M⊙ mass regime, as well as a mean mass accretion rate for Chamaeleon II of Ṁacc ~ 7-5+26 × 10-10 M⊙ yr-1. We also establish a relationship between the He i λ7065 Å line emission and the accretion luminosity. Conclusions. The radial velocity distributions of stars and gas in Chamaeleon II are consistent. The spread in Ṁacc at a given stellar mass is about one order of magnitude and can not be ascribed entirely to short timescale variability. Analyzing the relation between Ṁacc and the colors in Spitzer c2d and 2MASS bands, we find indications that the inner disk changes from optically thick to optically thin at Ṁacc ~ 10-10 M⊙ yr-1. Finally, the disk fraction is consistent with the age of Chamaeleon II.
Discovering wide companions of stellar systems allows us to constrain the dynamical environment and age of the latter. We studied four probable wide companions of four different stellar systems. The ...candidates were selected mainly based on their similar kinematic properties to the central star using
Gaia
DR2. The central stars are V4046 Sgr, HIP 74865, HIP 65426, and HIP 73145, and their probable wide companions are 2MASS J18152222-3249329, 2MASS J15174874-3028484, 2MASS J13242119-5129503, and 2MASS J14571503-3543505 respectively. V4046 Sgr is a member of
β
-Pictoris Moving Group while the rest of the stellar systems are acknowledged as members of the Scorpius-Centaurus association. The selected stellar systems are particularly interesting because all of them are already known to possess a low-mass companion and/or a spatially resolved disk. Identifying wider companions of these stars can improve their eligibility as benchmarks for understanding the formation channels of various triple systems, and can help us to determine the orbits of their possibly undiscovered inner, wider companions in case of higher multiplicity. By analyzing the X-shooter spectra of the wide companion candidates of these stars, we obtained their stellar parameters and determined their ages. We find that 2MASS J15174874-3028484 (0.11
M
⊙
, 7.4 ± 0.5 Myr), an already recognized pre-main sequence (PMS) member of Scorpius-Centaurus association, is a highly probable wide companion of HIP 74865. 2MASS J13242119-5129503 (0.04
M
⊙
, 16 ± 2.2 Myr) is ruled out as a plausible wide companion of HIP 65426, but confirmed to be a new sub-stellar member of the Scorpius-Centaurus association. 2MASS J14571503-3543505 (0.02
M
⊙
, 17.75 ± 4.15 Myr) is a probable sub-stellar member of the same association, but we cannot confirm whether or not it is gravitationally bound to HIP 73145. 2MASS J18152222-3249329 (0.3
M
⊙
, older than 150 Myr) is determined to be a mildly active main sequence star, much older than members of
β
-Pictoris Moving Group, and unbound to V4046 Sgr despite their similar kinematic features. PMS wide companions such as 2MASS J15174874-3028484 might have formed through cascade fragmentation of their natal molecular core, hinting at high multiplicity in shorter separations which can be confirmed with future observations.
Context.
NGC 2264 is a young cluster whose accretion properties can be investigated in detail by taking advantage of the FLAMES data in the context of the
Gaia
-ESO Survey. In fact, the analysis of ...the H
α
emission line profile can provide us with information about the accretion and ejection activity of young stars. However, a strong nebular emission that contributes to the H
α
emission can alter the profiles, with consequences for their physical interpretation.
Aims.
Our study is aimed at investigating the accretion and ejection properties of NGC 2264 by applying a proper treatment of the sky contribution to the H
α
and forbidden emission lines (FELs; SII and NII doublets).
Methods.
We developed a tool, the OH
α
NA-method, to handle the strong nebular contribution and spectra with spurious profiles of the H
α
and FELs, namely altered H
α
profiles or absorption features artificially created where emission lines (FELs) are expected. We derived the quantitative measurements of relevant parameters to describe the accretion and ejection processes in young members of NGC 2264, focusing on reliable quantities derived from the width of the lines, which is relatively unaffected by the nebular emission, unlike the intensity peak, which can be altered significantly.
Results.
We derive the quantitative measurements related to the H
α
emission line and discuss the comparison between the original and sky-subtracted spectra. We thus reveal possible profile alterations with consequences for their physical interpretation. Furthermore, we show the analysis of the variability for multi-epoch observations, also deriving the velocity of the infalling and outflowing plasma from the wings of the broad H
α
emission line (in accreting stars). We also explore the mass accretion rate versus full width at zero intensity of the H
α
line, namely
Ṁ
versus FWZI(H
α
), a correlation based on the width of the emission line, which is expected to be more robust with respect to any measurement derived from the peak (e.g., H
α
10%
) and possibly altered by the nebular contribution.
Conclusions.
We are able to ascertain that more than 20% of the confirmed accretors, which have already been identified in NGC 2264, are affected by the alteration of their line profiles due to the contribution of the nebular emission. Therefore, this is an important issue to consider when investigating accretion and ejection processes in young stellar clusters. While a small fraction of spectra can be unequivocally classified as either unaffected by nebular emission or dominated by nebular emission, the majority (> 90%) represent intermediate cases whose spectral features have to be investigated in detail to derive reliable measurements of the relevant parameters and their physical implications.
Context.
Multiple stellar systems play a fundamental role in the formation and evolution of stellar populations in galaxies. Recent and ongoing large ground-based multi-object spectroscopic surveys ...significantly increase the sample of spectroscopic binaries (SBs) allowing analyses of their statistical properties.
Aims.
We investigate the repeated spectral observations of the
Gaia
-ESO Survey internal data release 5 (GES iDR5) to identify and characterise SBs with one visible component (SB1s) in fields covering mainly the discs, the bulge, the CoRot fields, and some stellar clusters and associations.
Methods.
A statistical
χ
2
-test is performed on spectra of the iDR5 subsample of approximately 43 500 stars characterised by at least two observations and a signal-to-noise ratio larger than three. In the GES iDR5, most stars have four observations generally split into two epochs. A careful estimation of the radial velocity (RV) uncertainties is performed. Our sample of RV variables is cleaned from contamination by pulsation- and/or convection-induced variables using
Gaia
DR2 parallaxes and photometry. Monte-Carlo simulations using the SB9 catalogue of spectroscopic orbits allow to estimate our detection efficiency and to correct the SB1 rate to evaluate the GES SB1 binary fraction and its relation to effective temperature and metallicity.
Results.
We find 641 (resp., 803) FGK SB1 candidates at the 5
σ
(resp., 3
σ
) level. The maximum RV differences range from 2.2 km s
−1
at the 5
σ
confidence level (1.6 km s
−1
at 3
σ
) to 133 km s
−1
(in both cases). Among them a quarter of the primaries are giant stars and can be located as far as 10 kpc. The orbital-period distribution is estimated from the RV standard-deviation distribution and reveals that the detected SB1s probe binaries with log
P
d ⪅ 4. We show that SB1s with dwarf primaries tend to have shorter orbital periods than SB1s with giant primaries. This is consistent with binary interactions removing shorter period systems as the primary ascends the red giant branch. For two systems, tentative orbital solutions with periods of 4 and 6 d are provided. After correcting for detection efficiency, selection biases, and the present-day mass function, we estimate the global GES SB1 fraction to be in the range 7–14% with a typical uncertainty of 4%. A small increase of the SB1 frequency is observed from K- towards F-type stars, in agreement with previous studies. The GES SB1 frequency decreases with metallicity at a rate of (−9 ± 3)% dex
−1
in the metallicity range −2.7 ≤ Fe/H ≤ +0.6. This anticorrelation is obtained with a confidence level higher than 93% on a homogeneous sample covering spectral types FGK and a large range of metallicities. When the present-day mass function is accounted for, this rate turns to (−4 ± 2)% dex
−1
with a confidence level higher than 88%. In addition we provide the variation of the SB1 fraction with metallicity separately for F, G, and K spectral types, as well as for dwarf and giant primaries.
We report the discovery of periodic dips in the multiband light curve of ISO-ChaI 52, a young stellar object in the Chamaeleon I dark cloud. This is one of the peculiar objects that display very low ...or negligible accretion in their UV continuum and spectral lines, although they present a remarkable infrared excess emission characteristic of optically thick circumstellar disks. We have analyzed a spectrum obtained at the Very Large Telescope with the X-shooter spectrograph with the tool ROTFIT to determine the stellar parameters. The latter, along with photometry from our campaign with the Rapid Eye Mount telescope and from the literature, have allowed us to model the spectral energy distribution and to estimate the size and temperature of the inner and outer disk. Based on the rotational period of the star-disk system of 3.45 days, we estimate a disk inclination of 36°. The depth of the dips in different bands has been used to gain information about the occulting material. A single extinction law is not able to fit the observed behavior, while a two-component model of a disk warp composed of a dense region with a gray extinction and an upper layer with an extinction as in the interstellar medium provides a better fit to the data.
CAOS spectroscopy of Am stars Kepler targets Catanzaro, G; Ripepi, V; Biazzo, K ...
Monthly notices of the Royal Astronomical Society,
07/2015, Letnik:
451, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The Kepler space mission and its K2 extension provide photometric time series data with unprecedented accuracy. These data challenge our current understanding of the metallic-lined A stars (Am stars) ...for what concerns the onset of pulsations in their atmospheres. It turns out that the predictions of current diffusion models do not agree with observations. To understand this discrepancy, it is of crucial importance to obtain ground-based spectroscopic observations of Am stars in the Kepler and K2 fields in order to determine the best estimates of the stellar parameters. In this paper, we present a detailed analysis of high-resolution spectroscopic data for seven stars previously classified as Am stars. We determine the effective temperatures, surface gravities, projected rotational velocities, microturbulent velocities and chemical abundances of these stars using spectral synthesis. These spectra were obtained with CAOS, a new instrument recently installed at the observing station of the Catania Astrophysical Observatory on Mt Etna. Three stars have already been observed during quarters Q0–Q17, namely: HD 180347, HD 181206 and HD 185658, while HD 43509 was already observed during K2 C0 campaign. We confirm that HD 43509 and HD 180347 are Am stars, while HD 52403, HD 50766, HD 58246, HD 181206 and HD 185658 are marginal Am stars. By means of non-LTE (local thermodynamic equilibrium) analysis, we derived oxygen abundances from O i λ7771–5 Å triplet and we also discussed the results obtained with both non-LTE and LTE approaches.
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