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Публикации по объекту
Spectral properties of X-ray bright variable sources in the Taurus molecular cloud
Aims.We analyze 19 bright variable X-ray sources detected in theXMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST), tocharacterize the variations with time of their coronal properties and toderive information on the X-ray emitting structures. Methods: Weperformed time-resolved spectroscopy of the EPIC PN and MOS spectra ofthe XEST sources, using a model with one or two thermal components, andwe used the time evolution of the temperatures and emission measuresduring the decay phase of flares to derive the size of the flaringloops. Results: The light curves of the selected sources showdifferent types of variability: flares, long-lasting decay or risethrough the whole observation, and slow modulation or complex flare-likevariability. Spectral analysis shows typical quiescent plasmatemperatures of ~5-10 MK and ~15-35 MK; the cool component generallyremains constant, while the observed flux changes are due to variationsof the hot component. During flares the plasma reaches temperatures upto 100 MK and luminosities up to ~1031 erg s-1.Loop sizes inferred from flare analysis are generally smaller than orcomparable to the stellar radius.Table [see full textsee full text] is only available in electronic format http://www.aanda.org
| High-resolution X-ray spectroscopy of T Tauri stars in the Taurus-Auriga complex
Context: Differences have been reported between the X-ray emission ofaccreting and non-accreting stars. Some observations have suggested thataccretion shocks could be responsible for part of the X-ray emission inclassical T Tauri stars (CTTS). Aims: We present high-resolutionX-ray spectroscopy for nine pre-main sequence stars in order to test theproposed spectroscopic differences between accreting and non-accretingpre-main sequence stars. Methods: We used X-ray spectroscopy fromthe XMM-Newton Reflection Grating Spectrometers and the EPICinstruments. We interpret the spectra using optically thin thermalmodels with variable abundances, together with an absorption columndensity. For BP Tau and AB Aur we derive electron densities from the Ovii triplets. Results: Using the O vii/O viii count ratios as adiagnostic for cool plasma, we find that CTTS display a soft excess(with equivalent electron temperatures of ≈2.5-3 MK) when comparedwith WTTS or zero-age main-sequence stars. Although the O vii triplet inBP Tau is consistent with a high electron density (3.4 ×1011 cm-3), we find low density for the accretingHerbig star AB Aur (ne < 1010 cm-3).The element abundances of accreting and non-accreting stars are similar.The Ne abundance is found to be high (4-6 times the Fe abundance) in allK and M-type stars. In contrast, for the three G-type stars (SU Aur, HD283572, and HP Tau/G2), we find an enhanced Fe abundance (0.4-0.8 timessolar photospheric values) compared to later-type stars. Conclusions: Adding the results from our sample to formerhigh-resolution studies of T Tauri stars, we find a soft excess in allaccreting stars, but in none of the non-accretors. On the other hand,high electron density and high Ne/Fe abundance ratios do not seem to bepresent in all accreting pre-main sequence stars.
| The X-ray activity-rotation relation of T Tauri stars in Taurus-Auriga
Context: The Taurus-Auriga star-forming complex hosts the onlypopulation of T Tauri stars in which an anticorrelation of X-rayactivity and rotation period has been observed. Aims: We aim toexplain the origin of the X-ray activity-rotation relation inTaurus-Auriga. We also aim to put the X-ray activity of these stars intothe context of the activity of late-type main-sequence stars and T Tauristars in the Orion Nebula Cluster. Methods: We have usedXMM-Newton's European Photon Imaging Cameras to perform the mostsensitive survey to date of X-ray emission (0.3-10 keV) from young starsin Taurus-Auriga. We investigated the dependences of X-ray activitymeasures - X-ray luminosity, L_X, its ratio with the stellar luminosity,L_X/Lstar, and the surface-averaged X-ray flux, F_XS - onrotation period and compared them with predictions based solely on theobserved dependence of LX on a star's Lstar andwhether it is accreting or not. We tested for differences in thedistributions of L_X/Lstar of fast and slow rotators,accretors and non-accretors, and compared the dependence ofL_X/Lstar on the ratio of the rotation period and theconvective turnover timescale, the Rossby number, with that of late-typemain-sequence stars. Results: We found significantanticorrelations of LX and F_XS with rotation period, butthese could be explained by the typically higher stellar luminosity andeffective temperature of fast-rotators in Taurus-Auriga and anear-linear dependence of LX on Lstar. We found noevidence for a dependence of L_X/Lstar on rotation period,but for accretors to have lower L_X/Lstar than non-accretorsat all rotation periods. The Rossby numbers of accretors andnon-accretors were found to be the same as those of late-typemain-sequence stars showing saturated X-ray emission. Conclusions:Non-accreting T Tauri stars show X-ray activity entirely consistent withthe saturated activity of fast-rotating late-type main-sequence stars.Accreting T Tauri stars show lower X-ray activity, but this cannot beattributed to their slower rotation.
| Very Large Array Plus Pie Town Astrometry of 46 Radio Stars
We have used the Very Large Array, linked with the Pie Town Very LongBaseline Array antenna, to determine astrometric positions of 46 radiostars in the International Celestial Reference Frame (ICRF). Positionswere obtained in the ICRF directly through phase referencing of thestars to nearby ICRF quasars whose positions are accurate at the 0.25mas level. Radio star positions are estimated to be accurate at the 10mas level, with position errors approaching a few milliarcseconds forsome of the stars observed. Our measured positions were combined withprevious measurements taken from as early as 1978 to obtainproper-motion estimates for all 46 stars with average uncertainties of~1.7 mas yr-1. We compared our radio star positions andproper motions with the Hipparcos Catalogue data and found consistencyin the reference frames produced by each data set on the 1 σlevel, with errors of ~2.7 mas per axis for the reference frameorientation angles at our mean epoch of 2003.78. No significant spin isfound between our radio data frame and the Hipparcos Celestial ReferenceFrame, with the largest rotation rates of +0.55 and -0.41 masyr-1 around the x- and z-axes, respectively, with 1 σerrors of 0.36 mas yr-1. Thus, our results are consistentwith a nonrotating Hipparcos frame with respect to the ICRF.
| Mid-Infrared Observations of T Tauri Stars: Probing the Star-Disk Connection in Rotational Evolution
We present mid-IR N-band (λeff=10.2 μm) photometryof a carefully selected sample of T Tauri stars thought to be singlefrom the Taurus-Auriga molecular cloud. Infrared excesses in these starsare generally attributed to circumstellar dust disks. Combiningobservations at 2.16 (Ks band) and 10.2 μm (N band) weprobe a region in the circumstellar dust disk from a few stellar radiithrough the terrestrial planet zone (0.02-1.0 AU). By analyzing thedistribution of the (Ks-N) color index with respect topreviously measured photometric rotation periods we investigate whatrole circumstellar disks play in the rotational evolution of the centralstar. The resulting positive correlation between these two variables isconsistent with the notion that a star-disk interaction facilitates theregulation of angular momentum during the T Tauri stage. We alsodemonstrate how including nonsingle stars in such an analysis weakensany correlation in the relation between (Ks-N) color andperiod. To further understand disk properties we also present spectralenergy distributions for a few objects with new ground-based M-band(λeff=4.8 μm) and Q-band (λeff=20μm) data and compare them to a geometrically thin, optically thickdisk model.
| A kinematic study of the Taurus-Auriga T association
Aims.This is the first paper in a series dedicated to investigating thekinematic properties of nearby associations of young stellar objects.Here we study the Taurus-Auriga association, with the primary objectiveof deriving kinematic parallaxes for individual members of this low-massstar-forming region. Methods: .We took advantage of a recentlypublished catalog of proper motions for pre-main sequence stars, whichwe supplemented with radial velocities from various sources found in theCDS databases. We searched for stars of the Taurus-Auriga region thatshare the same space velocity, using a modified convergent point methodthat we tested with extensive Monte Carlo simulations. Results:.Among the sample of 217 Taurus-Auriga stars with known proper motions,we identify 94 pre-main sequence stars that are probable members of thesame moving group and several additional candidates whose pre-mainsequence evolutionary status needs to be confirmed. We derive individualparallaxes for the 67 moving group members with known radial velocitiesand give tentative parallaxes for other members based on the averagespatial velocity of the group. The Hertzsprung-Russell diagram for themoving group members and a discussion of their masses and ages arepresented in a companion paper.
| Modeling of PMS Ae/Fe stars using UV spectra
Context: .Spectral classification of AeFe stars, based on visualobservations, may lead to ambiguous conclusions. Aims: . We aimto reduce these ambiguities by using UV spectra for the classificationof these stars, because the rise of the continuum in the UV is highlysensitive to the stellar spectral type of A/F-type stars. Methods: . We analyse the low-resolution UV spectra in terms of a3-component model, that consists of spectra of a central star, of anoptically-thick accretion disc, and of a boundary-layer between the discand star. The disc-component was calculated as a juxtaposition of Planckspectra, while the 2 other components were simulated by thelow-resolution UV spectra of well-classified standard stars (taken fromthe IUE spectral atlases). The hot boundary-layer shows strongsimilarities to the spectra of late-B type supergiants (see Appendix A). Results: . We modeled the low-resolution UV spectra of 37 AeFestars. Each spectral match provides 8 model parameters: spectral typeand luminosity-class of photosphere and boundary-layer, temperature andwidth of the boundary-layer, disc-inclination and circumstellarextinction. From the results of these analyses, combined with availabletheoretical PMS evolutionary tracks, we could estimate their masses andages and derive their mass-accretion rates. For a number of analysed PMSstars we calculated the corresponding SEDs and compared these with theobserved SEDs. Conclusions: . All stars (except βPic) showindications of accretion, that affect the resulting spectral type of thestellar photosphere. Formerly this led to ambiguities in classificatonof PMS stars as the boundary-layer was not taken into consideration. Wegive evidence for an increase of the mass-accretion rate with stellarmass and for a decreases of this rate with stellar age.
| A Survey and Analysis of Spitzer Infrared Spectrograph Spectra of T Tauri Stars in Taurus
We present mid-infrared spectra of T Tauri stars in the Taurusstar-forming region obtained with the Spitzer Infrared Spectrograph(IRS). For the first time, the 5-36 μm spectra of a large sample of TTauri stars belonging to the same star-forming region is studied,revealing details of the mid-infrared excess due to dust incircumstellar disks. We analyze common features and differences in themid-IR spectra based on disk structure, dust grain properties, and thepresence of companions. Our analysis encompasses spectral energydistributions from the optical to the far-infrared, a morphologicalsequence based on the IRS spectra, and spectral indices in IRS wavebands representative of continuum emission. By comparing the observedspectra to a grid of accretion disk models, we infer some basic diskproperties for our sample of T Tauri stars and find additional evidencefor dust settling.
| Astrophysics in 2005
We bring you, as usual, the Sun and Moon and stars, plus some galaxiesand a new section on astrobiology. Some highlights are short (the newlyidentified class of gamma-ray bursts, and the Deep Impact on Comet9P/Tempel 1), some long (the age of the universe, which will be found tohave the Earth at its center), and a few metonymic, for instance theterm ``down-sizing'' to describe the evolution of star formation rateswith redshift.
| Rotational periods of T Tauri stars in Taurus-Auriga, south of Taurus-Auriga, and in MBM12
Context: .The ROSAT All-Sky Survey detected many young objects outsideany known star forming region. Their formation is yet unclear.Aims: .In order to improve the knowledge about these X-ray brightobjects we aimed at measuring their rotational properties, which arefundamental stellar parameters, and at comparing them to young objectsinside molecular clouds. Methods: .We monitored photometricvariations of 5 T Tauri stars in MBM12 and of 26 young objects in theTaurus-Auriga molecular cloud and south of it. Among the 26 youngobjects there are 17 weak-line T Tauri stars, 7 zero age main-sequencestars and 2 of unknown type. In addition, 2 main-sequence K-type starswere observed, and one comparison star turned out to be an eclipsingbinary. Results: .We found periodic variations for most of thetargets. The measured periods of the T Tauri stars range from 0.57 to7.4 days. The photometric variation can be ascribed to rotationalmodulation caused by spots. For a few of the periodic variables, changesof the light curve profile within several weeks are reported. For onestar such changes have been observed in data taken two years apart. Theexceptions are two eclipsing systems. One so far unknown system - GSC2.2N3022313162 - shows a light curve with full phase coverage having bothprimary and secondary minima well resolved. It has an orbital period of0.59075 days. From our spectroscopic observations we conclude that it isa main sequence star of spectral type F2 ± 4. We further comparedthe off-cloud weak-line T Tauri stars to the weak-line T Tauri starsinside the molecular cloud in terms of rotational period distribution.Statistical analysis of the two samples shows that both groups arelikely to have the same period distribution.
| Evidence for J- and H-Band Excess in Classical T Tauri Stars and the Implications for Disk Structure and Estimated Ages
We argue that classical T Tauri stars (CTTSs) possess significantnonphotospheric excess in the J and H bands (1.25 and 1.66 μm,respectively). We first show that normalizing the spectral energydistributions (SEDs) of CTTSs to the J band leads to a poor fit of theoptical fluxes (which are systematically overestimated), whilenormalizing the SEDs to the IC band (0.8 μm) produces abetter fit to the optical bands and in many cases reveals the presenceof a considerable excess at J and H. Near-infrared spectroscopic veilingmeasurements from the literature support this result. We find that J-and H-band excesses correlate well with the K-band (2.2 μm) excessand that the J-K and H-K colors of the excess emission are consistentwith that of a blackbody at the dust sublimation temperature (~1500-2000K). We propose that this near-IR excess originates at a hot inner rim,analogous to those suggested to explain the ``near-IR bump'' in the SEDsof Herbig Ae/Be stars. To test our hypothesis, we use the modelpresented by Dullemond and coworkers to fit the photometry data between0.5 and 24 μm of 10 CTTSs associated with the Chamaeleon II molecularcloud. We find that simple models that include luminosities calculatedfrom IC-band magnitudes and an inner rim may account for thereported J- and H-band excesses. The models that best fit the data arethose in which the inner radius of the disk is larger than expected fora rim in thermal equilibrium with the photospheric radiation fieldalone. In particular, we find that large inner rims are necessary toaccount for the mid-infrared fluxes (3.6-8.0 μm) obtained by theSpitzer Space Telescope (Spitzer). The large radius could be explainedif, as proposed by D'Alessio and colleagues, the UV radiation from theaccretion shock significantly affects the sizes of the inner holes indisks around CTTSs. Finally, we argue that deriving the stellarluminosities of CTTSs by making bolometric corrections to the J-bandfluxes, which is the ``standard'' procedure for obtaining CTTSluminosities, systematically overestimates these luminosities. Theoverestimated luminosities translate into underestimated ages when thestars are placed in the H-R diagram. Thus, the results presented hereinhave important implications for the dissipation timescale of inneraccretion disks.
| Bright X-Ray Flares in Orion Young Stars from COUP: Evidence for Star-Disk Magnetic Fields?
We have analyzed a number of intense X-ray flares observed in theChandra Orion Ultradeep Project (COUP), a 13 day observation of theOrion Nebula Cluster (ONC), concentrating on the events with the higheststatistics (in terms of photon flux and event duration). Analysis of theflare decay allows to determine the physical parameters of the flaringstructure, particularly its size and (using the peak temperature andemission measure of the event) the peak density, pressure, and minimumconfining magnetic field. A total of 32 events, representing the mostpowerful ~=1% of COUP flares, have sufficient statistics and aresufficiently well resolved to grant a detailed analysis. A broad rangeof decay times are present in the sample of flares, withτlc (the 1/e decay time) ranging from 10 to 400 ks. Peakflare temperatures are often very high, with half of the flares in thesample showing temperatures in excess of 100 MK. Significant sustainedheating is present in the majority of the flares. The magneticstructures that are found, from the analysis of the flare's decay, toconfine the plasma are in a number of cases very long, with semilengthsup to ~=1012 cm, implying the presence of magnetic fields ofhundreds of G (necessary to confine the hot flaring plasma) extending tocomparable distance from the stellar photosphere. These very large sizesfor the flaring structures (length L>>R*) are not foundin more evolved stars, where, almost invariably, the same type ofanalysis results in structures with L<=R*. As the majorityof young stars in the ONC are surrounded by disks, we speculate that thelarge magnetic structures that confine the flaring plasma are actuallythe same type of structures that channel the plasma in themagnetospheric accretion paradigm, connecting the star's photospherewith the accretion disk.
| Circumstellar Dust Disks in Taurus-Auriga: The Submillimeter Perspective
We present a sensitive, multiwavelength submillimeter continuum surveyof 153 young stellar objects in the Taurus-Auriga star formation region.The submillimeter detection rate is 61% to a completeness limit of ~10mJy (3 σ) at 850 μm. The inferred circumstellar disk masses arelognormally distributed with a mean mass of ~5×10-3Msolar and a large dispersion (0.5 dex). Roughly one-third ofthe submillimeter sources have disk masses larger than the minimalnebula from which the solar system formed. The median disk-to-star massratio is 0.5%. The empirical behavior of the submillimeter continuum isbest described as Fν~ν2.0+/-0.5 between 350μm and 1.3 mm, which we argue is due to the combined effects of thefraction of optically thick emission and a flatter frequency behavior ofthe opacity compared to the interstellar medium. The latter effect couldbe due to a substantial population of large dust grains, whichpresumably would have grown through collisional agglomeration. In thissample, the only stellar property that is correlated with the outer diskis the presence of a companion. We find evidence for significantdecreases in submillimeter flux densities, disk masses, andsubmillimeter continuum slopes along the canonical infrared spectralenergy distribution evolution sequence for young stellar objects. Thefraction of objects detected in the submillimeter is essentiallyidentical to the fraction with excess near-infrared emission, suggestingthat dust in the inner and outer disks is removed nearly simultaneously.
| XMM-Newton spectroscopy of the metal depleted T Tauri star TWA 5
We present results of X-ray spectroscopy for TWA 5, amember of the young TW Hydrae association, observed with XMM-Newton.TWA 5 is a multiple system which shows Hαemission, a signature typical of classical T Tauri stars, but noinfrared excess. From this analysis of the RGS and EPIC spectra, we havederived the emission measure distribution vs. temperature of the X-rayemitting plasma, its abundances, and the electron density. Thecharacteristic temperature and density of the plasma suggest a coronasimilar to that of weak-line T Tauri stars and active late-type mainsequence stars. TWA 5 also shows low iron abundance(~0.1 times the solar photospheric one) and a pattern of increasingabundances for elements with increasing first ionization potentialreminiscent of the inverse FIP effect observed in highly active stars.The especially high ratio Ne/Fe10 is similar to that of theclassical T Tauri star TW Hya, where the accretingmaterial has been held responsible for the X-ray emission. We discussthe possible role of an accretion process in this scenario. Since all TTauri stars in the TW Hydrae association studied so far have very highNe/Fe ratios, we also propose that environmental conditions may causethis effect.
| Pre-main sequence star Proper Motion Catalogue
We measured the proper motions of 1250 pre-main sequence (PMS) stars andof 104 PMS candidates spread over all-sky major star-forming regions.This work is the continuation of a previous effort where we obtainedproper motions for 213 PMS stars located in the major southernstar-forming regions. These stars are now included in this present workwith refined astrometry. The major upgrade presented here is theextension of proper motion measurements to other northern and southernstar-forming regions including the well-studied Orion and Taurus-Aurigaregions for objects as faint as V≤16.5. We improve the precision ofthe proper motions which benefited from the inclusion of newobservational material. In the PMS proper motion catalogue presentedhere, we provide for each star the mean position and proper motion aswell as important photometric information when available. We providealso the most common identifier. The rms of proper motions vary from 2to 5 mas/yr depending on the available sources of ancient positions anddepending also on the embedding and binarity of the source. With thiswork, we present the first all-sky catalogue of proper motions of PMSstars.
| Coronal properties of G-type stars in different evolutionary phases
We report on the analysis of XMM-Newton observations of three G-typestars in very different evolutionary phases: the weak-lined T Tauri starHD 283572, the Zero Age Main Sequence starEK Dra and the Hertzsprung-gap giant star 31Com. They all have high X-ray luminosity (~1031 ergs-1 for HD 283572 and 31 Com and ~1030 ergs-1 for EK Dra). We compare the Emission MeasureDistributions (EMDs) of these active coronal sources, derived fromhigh-resolution XMM-Newton grating spectra, as well as the pattern ofelemental abundances vs. First Ionzation Potential (FIP). We alsoperform time-resolved spectroscopy of a flare detected by XMM from EKDra. We interpret the observed EMDs as the result of the emission ofensembles of magnetically confined loop-like structures with differentapex temperatures. Our analysis indicates that the coronae of HD 283572and 31 Com are very similar in terms of dominant coronal magneticstructures, in spite of differences in the evolutionary phase, surfacegravity and metallicity. In the case of EK Dra the distribution appearsto be slightly flatter than in the previous two cases, although the peaktemperature is similar.
| X-ray astronomy of stellar coronae
X-ray emission from stars in the cool half of the Hertzsprung-Russelldiagram is generally attributed to the presence of a magnetic coronathat contains plasma at temperatures exceeding 1 million K. Coronae areubiquitous among these stars, yet many fundamental mechanisms operatingin their magnetic fields still elude an interpretation through adetailed physical description. Stellar X-ray astronomy is thereforecontributing toward a deeper understanding of the generation of magneticfields in magnetohydrodynamic dynamos, the release of energy in tenuousastrophysical plasmas through various plasma-physical processes, and theinteractions of high-energy radiation with the stellar environment.Stellar X-ray emission also provides important diagnostics to study thestructure and evolution of stellar magnetic fields from the first daysof a protostellar life to the latest stages of stellar evolution amonggiants and supergiants. The discipline of stellar coronal X-rayastronomy has now reached a level of sophistication that makes tests ofadvanced theories in stellar physics possible. This development is basedon the rapidly advancing instrumental possibilities that today allow usto obtain images with sub-arcsecond resolution and spectra withresolving powers exceeding 1000. High-resolution X-ray spectroscopy has,in fact, opened new windows into astrophysical sources, and has played afundamental role in coronal research.
| The Status of ROSAT X-ray Active Young Stars toward Taurus-Auriga
We present an astrometric study of the candidates of T Tauri stars (TTS)and non-TTS X-ray sources around Tau-Aur, based on the HipparcosCatalogue and the ACT Reference Catalogue. The ROSAT selected X-raysources are found to be a mixed population. A few of them areassociated with the Tau-Aur or Orion Star Forming Regions (SFR). Some,with distances similar to that of Tau-Aur but with discrepant propermotions, are probable or sure Pleiades super-cluster members or otherlate type young active stars with unresolved nature, more likely tooriginate in rapidly moving cloudlets, or else having originated fromdifferent sites other than Tau-Aur and moved to the present locations. Agood many of the non-TTS X-ray sources are considered as Hyades clustermembers. Some TTS candidates could be foreground pre-main sequencestars or actually young dwarfs not yet depleted of their Lithium. Underthe hypothesis that the sources we studied are representative of theROSAT selected TTS candidates discovered in the outskirts of the Tau-Aurregion, we conclude that only up to one third of the weak-line TTScandidates could be expected to be physically associated with theTau-Aur association. Along with the parallax and proper motion analysisof the non-TTS X-ray sources around the Tau-Aur SFR, our result suggeststhat the vast majority of the young active X-ray sources within anangular diameter of about 30 ° of the Tau-Aur SFR, belong to fourmain subgroups that are spatially separate.
| The Impact of Space Experiments on our Knowledge of the Physics of the Universe
With the advent of space experiments it was demonstrated that cosmicsources emit energy practically across all the electromagnetic spectrumvia different physical processes. Several physical quantities givewitness to these processes which usually are not stationary; thosephysical observable quantities are then generally variable. Thereforesimultaneous multifrequency observations are strictly necessary in orderto understand the actual behaviour of cosmic sources. Space experimentshave opened practically all the electromagnetic windows on the Universe.A discussion of the most important results coming from multifrequencyphotonic astrophysics experiments will provide new inputs for theadvance of the knowledge of the physics, very often in its more extremeconditions. A multitude of high quality data across practically thewhole electromagnetic spectrum came at the scientific community'sdisposal a few years after the beginning of the Space Era. With thesedata we are attempting to explain the physics governing the Universeand, moreover, its origin, which has been and still is a matter of thegreatest curiosity for humanity. In this paper we will try to describethe last steps of the investigation born with the advent of spaceexperiments, to note upon the most important results and open problemsstill existing, and to comment upon the perspectives we can reasonablyexpect. Once the idea of this paper was well accepted by ourselves, wehad the problem of how to plan the exposition. Indeed, the exposition ofthe results can be made in different ways, following several points ofview, according to: - a division in diffuse and discrete sources; -different classes of cosmic sources; - different spectral ranges, whichimplies in turn a sub-classification in accordance with differenttechniques of observations; - different physical emission mechanisms ofelectromagnetic radiation; - different vehicles used for launching theexperiments (aircraft, balloons, rockets, satellites, observatories). Inorder to exhaustively present The Impact of Space Experiments on ourKnowledge of the Physics of the Universe it would then have beennecessary to write a kind of Encyclopaedia of the Astronomical SpaceResearch, which is not our desire. On the contrary, since our goal is toprovide an useful tool for the reader who has not specialized in spaceastrophysics and for the students, we decided to write this paper in theform of a review, the length of which can be still consideredreasonable, taking into account the complexity of the argumentsdiscussed. Because of the impossibility of realizing a complete pictureof the physics governing the Universe, we were obliged to select how toproceed, the subjects to be discussed the more or the less, or those tobe rejected. Because this work was born in the Ph.D. thesis of one of us(LSG) (Sabau-Graziati, 1990) we decided to follow the `astronomicaltradition' used there, namely: the spectral energy ranges. Although suchenergy ranges do not determine physical objects (even if in many casessuch ranges are used to define the sources as: radio, infrared, optical,ultraviolet, X-ray, γ-ray emitters), they do determine themethods of study, and from the technical point of view they define thetechnology employed in the relative experiments. However, since then wehave decided to avoid a deep description of the experiments, satellites,and observatories, simply to grant a preference to the physical results,rather than to technologies, however fundamental for obtaining thoseresults. The exposition, after an introduction (Section 1) and somecrucial results from space astronomy (Section 2), has been focussed intothree parts: the physics of the diffuse cosmic sources deduced fromspace experiments (Section 3), the physics of cosmic rays from ground-and space-based experiments (Section 4), and the physics of discretecosmic sources deduced from space experiments (Section 5). In this firstpart of the paper we have used the logic of describing the main resultsobtained in different energy ranges, which in turn characterize theexperiments on board space vehicles. Within each energy range we havediscussed the contributions to the knowledge of various kind of cosmicsources coming from different experiments. And this part is mainlyderived by the bulk of the introductory part of LSG's Ph.D. thesis. Inthe second part of the paper, starting from Section 6, we have preferredto discuss several classes of cosmic sources independently of the energyranges, mainly focussing the results from a multifrequency point ofview, making a preference for the knowledge of the physics governing thewhole class. This was decided also because of the multitude of new spaceexperiments launched in the last fifteen years, which would haverendered almost impossible a discussion of the results divided intoenergy ranges without weakening the construction of the entire puzzle.We do not pretend to cover every aspect of every subject consideredunder the heading of the physics of the universe. Instead a crosssection of essays on historical, modern, and philosophical topics areoffered and combined with personal views into tricks of the spaceastrophysics trade. The reader is, then, invited to accept this papereven though it obviously lacks completeness and the arguments discussedare certainly biased by a selection effect owed essentially to ourknowledge, and to it being of a reasonable length. Some parts of itcould seem, in certain sense, to belong to an older paper, in which the`news' is not reported. But this is owed to our own choice, just in fullaccord with the goals of the text: we want to present those resultswhich have, in our opinion, been really important, in the development ofthe science. These impacting results do not necessarily constitute thelast news. This text was formally closed just on the day of the launchof the INTEGRAL satellite: October 17, 2002. After that date onlyfinishing touches have been added.
| Multisite observations of SU Aurigae
We present results from the 1996 Multi-Site Continuous Spectroscopy(MUSICOS) campaign on the T Tauri star SU Aurigae. We find a 2.7-dperiodicity in the HeI (587.6 nm) line, and somewhat longer, lesswell-pronounced periodicities in the Balmer lines and in Na D. Ourobservations support the suggestion that the wind and infall signaturesare out of phase on SU Aur. We present Doppler images of SU Aur thathave been obtained from least-squares deconvolved profiles. Images takenabout one rotation apart show only limited overlap, in particular at lowlatitudes. This is due in part to limitations in signal-to-noise ratio,and in part to line-profile deformations that arise from short-livedand/or non-surface features. The agreement at high latitudes is betterand suggests that at least some longer-lived features are present. Theanalysis of Stokes V profiles yields a marginal magnetic field detectionduring one of the phases.
| XMM-Newton EPIC observations of stellar clusters and star forming regions
We report on observations of open clusters (OCs) and star formingregions (SFRs) obtained with the EPIC camera as part of the MissionScientist Guaranteed Time on XMM-Newton. These observations provide apowerful tool to investigate the evolution of coronal activity inlate-type convective stars and its dependence on magnetic fieldgeneration by dynamo processes. We discuss the motivations for thisprogram and present some results for the SFRs sigma Orionis (2-5Myr) and Taurus-Auriga (1-10 Myr) as well as for the OCs IC 2602(30 Myr), alpha Persei (50 Myr), Praesepe (600 Myr) andthe Hyades (600 Myr). We discuss imaging and spectral data providedby the EPIC MOS and PN detectors focussing on the determination of thecluster X-ray luminosity function and of the temperature structure,chemical abundances and time variability of cluster stars.Based on observations collected with the ESA mission XMM-Newton as partof the Mission Scientist (R. Pallavicini) Guaranteed Time
| Stellar Coronal Astronomy
Coronal astronomy is by now a fairly mature discipline, with a quartercentury having gone by since the detection of the first stellar X-raycoronal source (Capella), and having benefitted from a series of majororbiting observing facilities. Serveral observational characteristics ofcoronal X-ray and EUV emission have been solidly established throughextensive observations, and are by now common, almost text-book,knowledge. At the same time the implications of coronal astronomy forbroader astrophysical questions (e.g.Galactic structure, stellarformation, stellar structure, etc.) have become appreciated. Theinterpretation of stellar coronal properties is however still often opento debate, and will need qualitatively new observational data to bookfurther progress. In the present review we try to recapitulate our viewon the status of the field at the beginning of a new era, in which thehigh sensitivity and the high spectral resolution provided by Chandraand SMM-Newton will address new questions which were not accessiblebefore.
| VLA Radio Positions of Stars: 1978-1995
VLA astrometric positions of the radio emission from 52 stars arereported, from observations obtained between 1978 and 1995. Thepositions of these stars have been obtained and reduced in a uniformmanner. Based on our measurements, the offset of the optical (Hipparcos)frame from the radio reference frame is in agreement with the Hipparcosextragalactic link results, within their mean errors. Comparison of theVLA measurements with the Hipparcos optical positions confirms earlierestimates of the accuracy of these positions as 30 mas. Long-termmeasurements of UX Ari have improved its proper motion.
| Detections of Rovibrational H2 Emission from the Disks of T Tauri Stars
We report the detection of quiescent H2 emission in thev=1-->0 S(1) line at 2.12183 μm in the circumstellar environmentof two classical T Tauri stars, GG Tau A and LkCa 15, in high-resolution(R~=60,000) spectra, bringing to four, including TW Hya and theweak-lined T Tauri star DoAr 21, the number of T Tauri stars showingsuch emission. The equivalent widths of the H2 emission linelie in the range 0.02-0.10 Å, and in each case the centralvelocity of the emission line is centered at the star's systemicvelocity. The line widths range from 9 to 14 km s-1, inagreement with those expected from gas in Keplerian orbits incircumstellar disks surrounding K-type stars at distances >=10 AUfrom the sources. UV fluorescence and X-ray heating are likely candidatemechanisms responsible for producing the observed emission. We presentmass estimates from the measured line fluxes and show that the estimatedmasses are consistent with those expected from the possible mechanismsresponsible for stimulating the observed emission. The high temperaturesand low densities required for significant emission in the v=1-->0S(1) line suggests that we have detected reservoirs of hot H2gas located in the low-density upper atmospheres of circumstellar disksof these stars.
| Star Formation in Space and Time: Taurus-Auriga
To understand the formation of stellar groups, one must first documentcarefully the birth pattern within real clusters and associations. Inthis study of Taurus-Auriga, we combine pre-main-sequence ages from ourown evolutionary tracks with stellar positions from observationalsurveys. Aided by the extensive millimeter data on the molecular clouds,we develop a picture of the region's history. Star formation began, at arelatively low level and in a spatially diffuse manner, at least107 yr in the past. Within the last few million years, newstars have been produced at an accelerating rate, almost exclusivelywithin a confined group of striated cloud filaments. The gas both insideand around the filaments appears to be in force balance. Thus, theappearance of the filaments is due to global, quasi-static contractionof the parent cloud material. Gravity drives this contraction and shockdissipation mediates it, but the internal motion of the gas does notappear to be turbulent. The accelerating nature of recent star formationmeans that the condensation of cloud cores is a threshold phenomenon,requiring a minimum background density. Other, nearby cloud regions,including Lupus and Chamaeleon, contain some locales that have attainedthis density, and others that have not. In the latter, we find extensiveand sometimes massive molecular gas that is still devoid of young stars.
| Flows, Fragmentation, and Star Formation. I. Low-Mass Stars in Taurus
The remarkably filamentary spatial distribution of young stars in theTaurus molecular cloud has significant implications for understandinglow-mass star formation in relatively quiescent conditions. The largescale and regular spacing of the filaments suggests that small-scaleturbulence is of limited importance, which could be consistent with thedriving on large scales by flows that produced the cloud. The smallspatial dispersion of stars from gaseous filaments indicates that thelow-mass stars are generally born with small velocity dispersionsrelative to their natal gas, of the order of the sound speed or less.The spatial distribution of the stars exhibits a mean separation ofabout 0.25 pc, comparable to the estimated Jeans length in the densestgaseous filaments, and is consistent with roughly uniform density alongthe filaments. The efficiency of star formation in filaments is muchhigher than elsewhere, with an associated higher frequency of protostarsand accreting T Tauri stars. The protostellar cores are generallyaligned with the filaments, suggesting that they are produced bygravitational fragmentation, resulting in initially quasi-prolate cores.Given the absence of massive stars that could strongly dominate clouddynamics, Taurus provides important tests of theories of dispersedlow-mass star formation and numerical simulations of molecular cloudstructure and evolution.
| Doppler images of starspots
I present a literature survey of the currently available Doppler imagesof cool stars. The 65 individual stars with Doppler images consist of 29single stars and 36 components in close binaries. Out of the total, 31were observed only once but 12 stars are (or were) being monitored foryears. Each image for each star is identified with the time when it wasobserved, whether photometry was used in the imaging, the inclination ofthe stellar rotation axis, the vsin i, the stellar rotation period, andwhether a polar spot and/or a high-latitude or low-latitude spot wasseen. The type of variable star and its M-K spectral classification isalso listed to identify the evolutionary status. The sample consists of3 classical T Tauri stars, 8 weak-lined T Tauri's, 27 main-sequencestars, 9 subgiants, and 18 giants. The total number of Doppler images is245 as of June 2002.
| Measuring starspots on magnetically active stars with the VLTI
We present feasibility studies to directly image stellar surfacefeatures, which are caused by magnetic activity, with the Very LargeTelescope Interferometer (VLTI). We concentrate on late typemagnetically active stars, for which the distribution of starspots onthe surface has been inferred from photometric and spectroscopic imaginganalysis. The study of the surface spot evolution during consecutiverotation cycles will allow first direct measurements (apart from theSun) of differential rotation which is the central ingredient ofmagnetic dynamo processes. The VLTI will provide baselines of up to 200m, and two scientific instruments for interferometric studies at near-and mid-infrared wavelengths. Imaging capabilities will be made possibleby closure-phase techniques. We conclude that a realistically modeledcool surface spot can be detected on stars with angular diametersexceeding ~ 2 mas using the VLTI with the first generation instrumentAMBER. The spot parameters can then be derived with reasonable accuracy.We discuss that the lack of knowledge of magnetically active stars ofthe required angular size, especially in the southern hemisphere, is acurrent limitation for VLTI observations of these surface features.
| Variability of the Photospheric Radiation of Active K-M Dwarfs and Their X-ray Luminosities
Variability of the photospheric radiation of 40 (dKe-dMe) dwarfs in thesolar neighborhood due to variations in the spottedness of theirsurfaces is analyzed based on the behavior of their mean annualbrightnesses over long time intervals. The amplitudes and characteristictime scales of the variations of the mean annual brightness are taken tobe indicators of photospheric activity and were used to infer the levelsof photospheric activity in the stars studied. The influence of axialrotation on the development of cyclic activity in young red dwarfs andF-M main-sequence stars is analyzed. The durations and amplitudes of thephotospheric variability of rapidly rotating (dK0e-dK5e) stars testifiesto a higher level of photospheric activity among red dwarfs andsolar-type stars. The X-ray luminosities of these stars grow with theamplitude of the variations of the mean annual brightness. However, thisis not typical of rapidly rotating M dwarfs, for which the X-rayemission varies by more than two orders of magnitude, although theirdegrees of spottedness are all virtually the same. A linear relationshipbetween the X-ray and bolometric luminosities is observed for young(dKe-dMe) stars, with their ratios log(L x/L bol) being about -3. Theseproperties can be used to determine whether a red dwarf is a young staror is already on the main sequence.
| Coronal structure geometries on pre-main sequence stars
Using a hydrodynamic model we have re-analyzed large flaring events onthree different categories of pre-main sequence (PMS) stars: the youngstellar object (YSO) YLW 15, the classical T Tauri star (CTTS)LkHα 92, the weak-line T Tauri star (WTTS) V773 Tau, and the WTTSHD 283572 (the first three objects were observed by ASCA, the last byROSAT; all observations have been previously reported in theliterature). The first three flares were previously analyzed on thebasis of a quasi-static model mostly used up to now, consistentlyyielding large loops (L >~ R*) and no evidence ofsustained heating. Our hydrodynamic modeling approach, however, showsthat the size of the flaring regions must be much smaller (L <~R*) and moreover this method shows in all cases evidence ofvigorous sustained heating during the flare decay, so that the decay ofthe observed light curve actually reflects the temporal profile of theheating rather than that of the free decay of the heated loop(s). Theevents on the protostar YLW 15 have durations comparable to the stellarrotation period, so that their limited size and their lack ofself-eclipses give evidence of a polar location on the star. This is incontrast with the recently advanced hypothesis that these flares are dueto long loops spanning the region between the star and the accretiondisk. In general, the present analysis shows that flaring coronae on PMSstars have a structure similar to the coronae on older active stars.
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Наблюдательные данные и астрометрия
| Созвездие: | Телец |
| Прямое восхождение: | 04h21m58.85s |
| Склонение: | +28°18'06.5" |
| Видимая звёздная величина: | 9.035 |
| Собственное движение RA: | 7.7 |
| Собственное движение Dec: | -28.3 |
| B-T magnitude: | 9.988 |
| V-T magnitude: | 9.114 |
Каталоги и обозначения:
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