Context. Pre-main sequence stars are variable sources. In diskless stars this variability is mainly due to the rotational modulation of dark photospheric spots and active regions, as in main sequence ...stars even if associated with a stronger magnetic activity. Aims. We aim at analyzing the simultaneous optical and X-ray variability in these stars to unveil how the activity in the photosphere is connected with that in the corona, to identify the dominant surface magnetic activity, and to correlate our results with stellar properties, such as rotation and mass. Methods. We analyzed the simultaneous optical and X-ray variability in stars without inner disks (e.g., class III objects and stars with transition disks) in NGC 2264 from observations obtained with Chandra/ACIS-I and CoRoT as part of the Coordinated Synoptic Investigation of NGC 2264. We searched for those stars whose optical and X-ray variability is correlated, anti-correlated, or not correlated by sampling their optical and X-ray light curves in suitable time intervals and studying the correlation between the flux observed in optical and in X-rays. We then studied how this classification is related with stellar properties. Results. Starting from a sample of 74 class III/transition disk (TD) stars observed with CoRoT and detected with Chandra with more than 60 counts, we selected 16 stars whose optical and X-ray variability is anti-correlated, 11 correlated, and 17 where there is no correlation. The remaining stars did not fall in any of these groups. We interpreted the anti-correlated optical and X-ray variability as typical of spot-dominated sources, due to the rotational modulation of photospheric spots spatially coincident to coronal active regions, and correlated variability typical of faculae-dominated sources, where the brightening due to faculae is dominant over the darkening due to spots. Conclusions. Stars with “anti-correlated” variability rotate slower and are less massive than those with “correlated” variability. Furthermore, cool stars in our sample have larger u − r variability than hot stars. This suggests that there is a connection between stellar rotation, mass, and the dominant surface magnetic activity, which may be related with the topology of the large-scale magnetic field. We thus discuss this scenario in the framework of the complex magnetic properties of weak-line T Tauri stars observed as part of recent projects.
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We measured lithium in a sample of low-mass stars (60.1-0.3 M sub((m=o.))) of the Orion Nebula cluster. We find evidence for significant Li depletion in four high-probability members, corresponding ...to nuclear ages between 615 and 30 Myr. In two cases, there is excellent agreement between the mass and age based on models of Li burning and those derived from the H-R diagram, reinforcing our early findings. For the two other stars, the nuclear age is significantly larger than the isochronal one. Several Li-depleted stars display accretion activity, veiling, and emission lines. We discuss empirical evidence in favor of the old nuclear age and the implications on the star formation history of the Orion cluster.
Aims. We present a spectroscopic study of the dynamics of the ionized and neutral gas throughout the Lagoon nebula (M 8), using VLT-FLAMES data from the Gaia-ESO Survey. The new data permit ...exploration of the physical connections between the nebular gas and the stellar population of the associated star cluster NGC 6530. Methods. We characterized through spectral fitting emission lines of Hα, N II and S II doublets, O III, and absorption lines of sodium D doublet, using data from the FLAMES-Giraffe and UVES spectrographs, on more than 1000 sightlines toward the entire face of the Lagoon nebula. Gas temperatures are derived from line-width comparisons, densities from the S II doublet ratio, and ionization parameter from Hα/N II ratio. Although doubly-peaked emission profiles are rarely found, line asymmetries often imply multiple velocity components along the same line of sight. This is especially true for the sodium absorption, and for the O III lines. Results. Spatial maps for density and ionization are derived, and compared to other known properties of the nebula and of its massive stars 9 Sgr, Herschel 36 and HD 165052 which are confirmed to provide most of the ionizing flux. The detailed velocity fields across the nebula show several expanding shells, related to the cluster NGC 6530, the O stars 9 Sgr and Herschel 36, and the massive protostar M 8East-IR. The origins of kinematical expansion and ionization of the NGC 6530 shell appear to be different. We are able to put constrains on the line-of-sight (relative or absolute) distances between some of these objects and the molecular cloud. The data show that the large obscuring band running through the middle of the nebula is being compressed by both sides, which might explain its enhanced density. We also find an unexplained large-scale velocity gradient across the entire nebula. At larger distances, the transition from ionized to neutral gas is studied using the sodium lines.
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X‐ray observations of young stellar objects (YSOs) have shown several complex phenomena at work. In recent years, a few X‐ray programs based on long, continuous, and, sporadically, simultaneous ...coordinated multiwavelength observations have paved the way to our current understanding of the physical processes at work, which very likely regulates the interaction between the star and its circumstellar disk. We will present and discuss some recent results based on a novel analysis of a few selected very large flares observed with the Chandra Orion Ultradeep Pointing, on the systematic analysis of a large collection of flares observed with the Coordinated Synoptic Investigation of NGC 2264 (CSI 2264) as well as on the Class I/II YSO Elias 29, in the rho Oph star forming region, whose data have been recently gathered as part of a joint simultaneous XMM‐Newton and NuSTAR large program.
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The X-ray properties of twenty 61 Myr old O, B, and A stars of the Orion Trapezium are examined with data from the Chandra Orion Ultradeep Project (COUP). On the basis of simple theories for X-ray ...emission, we define two classes separated at spectral type B4: hotter stars have strong winds that may give rise to X-ray emission in small-or large-scale wind shocks, and cooler stars that should be X-ray dark due to their weaker winds and absence of outer convection zones where dynamos can generate magnetic fields. Emission by late-type magnetically active companions may be present in either class. Sixteen of the 20 stars are detected with a wide range of X-ray luminosities, log L super(() sub(x) super(ergs s-1)) 6 29 -33, and X-ray efficiencies, log(L sub(x)/L sub(bol)) 6-4 to -8. Only two stars, f super(1) Ori D (B0.5) and NU Ori (Bl), show exclusively the constant soft-spectrum emission at log(L sub(x)/L sub(bol)) 6-7 expected from the standard model involving many small shocks in an unmagnetized radiatively accelerated wind. Most of the other massive O7-B3 stars exhibit some combination of soft-spectrum wind emission, hard-spectrum flaring, and/or rotational modulation indicating large-scale inhomogeneity. Magnetic confinement of winds with large-scale shocks can be invoked to explain these phenomena. This is supported in some cases by nonthermal radio emission and/or chemical peculiarities, or direct detection of the magnetic field (f Ori C). Most of the stars in the weak-wind class exhibit X-ray flares and log L sub(x) 31 ergs s super(-1), consistent with magnetic activity from known or unseen low-mass companions. In most cases, the X-ray spectra can be interpreted in terms of a two-temperature plasma model with a soft component of 3-10 MK and a hard component up to 40 MK. All nondetections belong to the weak-wind class. A group of stars exhibit hybrid properties--flarelike behavior superimposed on a constant component with log L sub(x) 6 32 ergs s super(-1)--which suggest both magnetic activity and wind emission.
We present a description of the data reduction methods and the derived catalog of more than 1600 X-ray point sources from the exceptionally deep 2003 January Chandra X-Ray Observatory (Chandra) ...observation of the Orion Nebula Cluster and embedded populations around OMC-1. The observation was obtained with Chandra's Advanced CCD Imaging Spectrometer (ACIS) and has been nicknamed the Chandra Orion Ultradeep Project (COUP). With an 838 ks exposure made over a continuous period of 13.2 days, the COUP observation provides the most uniform and comprehensive data set on the X-ray emission of normal stars ever obtained in the history of X-ray astronomy.
We report here the first results of a multiwavelength campaign focussing on magnetospheric accretion processes of the classical T Tauri star V2129 Oph. In this paper, we present spectropolarimetric ...observations collected in 2009 July with ESPaDOnS at the Canada-France-Hawaii Telescope and contemporaneous photometry secured with the SMARTS facility. Circularly polarized Zeeman signatures are clearly detected, both in photospheric absorption and accretion-powered emission lines, from time-series of which we reconstruct new maps of the magnetic field, photospheric brightness and accretion-powered emission at the surface of V2129 Oph using our newest tomographic imaging tool - to be compared with those derived from our old 2005 June data set, re-analysed in the exact same way.
We find that in 2009 July, V2129 Oph hosts octupolar and dipolar field components of about 2.1 and 0.9 kG, respectively, both tilted by about 20° with respect to the rotation axis; we conclude that the large-scale magnetic topology changed significantly since 2005 June (when the octupole and dipole components were about 1.5 and three times weaker, respectively), demonstrating that the field of V2129 Oph is generated by a non-stationary dynamo. We also show that V2129 Oph features a dark photospheric spot and a localized area of accretion-powered emission, both close to the main surface magnetic region (hosting fields of up to about 4 kG in 2009 July). We finally obtain that the surface shear of V2129 Oph is about half as strong as solar.
From the fluxes of accretion-powered emission lines, we estimate that the observed average logarithmic accretion rate (in M⊙ yr−1) at the surface of V2129 Oph is −9.2 ± 0.3 at both epochs, peaking at −9.0 at magnetic maximum. It implies in particular that the radius at which the magnetic field of V2129 Oph truncates the inner accretion disc is 0.93 and 0.50 times the corotation radius (where the Keplerian period equals the stellar rotation period) in 2009 July and 2005 June, respectively.
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Aims. We present the first extensive spectroscopic study of the global population in star clusters Trumpler 16, Trumpler 14, and Collinder 232 in the Carina nebula, using data from the Gaia-ESO ...Survey, down to solar-mass stars. Methods. In addition to the standard homogeneous survey data reduction, a special processing was applied here because of the bright nebulosity surrounding Carina stars. Results. We find about 400 good candidate members ranging from OB types down to slightly subsolar masses. About 100 heavily reddened early-type Carina members found here were previously unrecognized or poorly classified, including two candidate O stars and several candidate Herbig Ae/Be stars. Their large brightness makes them useful tracers of the obscured Carina population. The spectroscopically derived temperatures for nearly 300 low-mass members enables the inference of individual extinction values and the study of the relative placement of stars along the line of sight. Conclusions. We find a complex spatial structure with definite clustering of low-mass members around the most massive stars and spatially variable extinction. By combining the new data with existing X-ray data, we obtain a more complete picture of the three-dimensional spatial structure of the Carina clusters and of their connection to bright and dark nebulosity and UV sources. The identification of tens of background giants also enables us to determine the total optical depth of the Carina nebula along many sightlines. We are also able to put constraints on the star formation history of the region with Trumpler 14 stars found to be systematically younger than stars in other subclusters. We find a large percentage of fast-rotating stars among Carina solar-mass members, which provide new constraints on the rotational evolution of pre-main-sequence stars in this mass range.
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Aims. The main goal of this work is to characterize the mass accretion and ejection processes of the classical T Tauri star V354 Mon, a member of the young stellar cluster NGC 2264. Methods. In March ...2008, photometric and spectroscopic observations of V354 Mon were obtained simultaneously with the CoRoT satellite, the 60 cm telescope at the Observatório Pico dos Dias (LNA, Brazil) equipped with a CCD camera and Johnson/Cousins BV(RI)c filters, and the SOPHIE échelle spectrograph at the Observatoire de Haute-Provence (CNRS, France). Results. The light curve of V354 Mon shows periodical minima (P = 5.26 ± 0.50 days) that vary in depth and width at each rotational cycle. The BV(RI)c observations indicate that the system becomes slightly bluer as the flux increases. The spectra of this T Tauri star exhibit variable emission lines, with blueshifted and redshifted absorption components associated with a disk wind and with the accretion process, respectively, confirming the magnetospheric accretion scenario. From the analysis of the photometric and spectroscopic data, it is possible to identify correlations between the emission line variability and the light-curve modulation of the young system, such as the occurrence of pronounced redshifted absorption in the Hα line at the epoch of minimum flux. This is evidence that during photometric minima we see the accretion funnel projected onto the stellar photosphere in our line of sight, implying that the hot spot coincides with the light-curve minima. We applied models of cold and hot spots and a model of occultation by circumstellar material to investigate the source of the observed photometric variations. Conclusions. We conclude that nonuniformly distributed material in the inner part of the circumstellar disk is the main cause of the photometric modulation, which does not exclude the presence of hot and cold spots at the stellar surface. It is believed that the distortion in the inner part of the disk is created by the dynamical interaction between the stellar magnetosphere, inclined with respect to the rotation axis, and the circumstellar disk, as also observed in the classical T Tauri star AA Tau and predicted by magnetohydrodynamical numerical simulations.
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Context.This work is part of a systematic X-ray survey of the Taurus star-forming complex with XMM-Newton. Aims.We study the time series of all X-ray sources associated with Taurus members, to ...statistically characterize their X-ray variability, and compare the results to those for pre-main sequence stars in the Orion Nebula Cluster and to expectations arising from a model where all the X-ray emission is the result of a large number of stochastically occurring flares. Methods.The analysis of the light curves is based on a maximum likelihood algorithm that segments the time series in intervals of constant signal without the need of binning. Flares are defined with criteria that take into account the amplitude and the derivative of the segmented light curves. Variability statistics are evaluated for different classes of pre-main sequence stars (protostars, cTTS, wTTS, brown dwarfs), and for different spectral type ranges. Flare frequency and energy distribution are computed. Results.We find that roughly half of the detected X-ray sources show variability above our sensitivity limit, and in ~26% of the cases this variability is recognized as flares. Variability is more frequently detected at hard than at soft energies. The variability statistics of cTTS and wTTS are indistinguishable, suggesting a common (coronal) origin for their X-ray emission. The frequency of large flares (E $> 10^{35}$ erg) on Taurus members is 1 event per star in 800 ks. The typical duration of these flares – probably biased by the finite observing time – is about 10 ks. We have applied a rigorous maximum likelihood method in the analysis of the number distribution of flare energies on pre-main sequence stars for the first time. In its differential form this distribution follows a power law with index α = 2.4 ± 0.5, in the range typically observed for late-type stars and the Sun. Conclusions.The signature of the X-ray variability in the pre-main sequence stars in Taurus and Orion provides twofold support for coronal heating by flares: (i) the correlation between the maximum variability amplitude and the minimum emission level indicates that both flare and quiescent emissions are closely related to the coronal heating process; (ii) the power law index α derived for the flare energy distribution is large enough to explain the heating of stellar coronae by nano-flares (α > 2), albeit associated with a rather large uncertainty that leaves some doubt as to this conclusion.
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