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Unraveling the Origins of Nearby Young Stars
A systematic search for close conjunctions and clusterings in the pastof nearby stars younger than the Pleiades is undertaken, which mayreveal the time, location, and mechanism of formation of these oftenisolated, disconnected from clusters and star-forming regions, objects.The sample under investigation includes 101 T Tauri, post-TT, andmain-sequence stars and stellar systems with signs of youth, culled fromthe literature. Their Galactic orbits are traced back in time and nearapproaches are evaluated in time, distance, and relative velocity.Numerous clustering events are detected, providing clues to the originof very young, isolated stars. Each star's orbit is also matched withthose of nearby young open clusters, OB and TT associations andstar-forming molecular clouds, including the Ophiuchus, Lupus, CoronaAustralis, and Chamaeleon regions. Ejection of young stars from openclusters is ruled out for nearly all investigated objects, but thenearest OB associations in Scorpius-Centaurus, and especially, the denseclouds in Ophiuchus and Corona Australis have likely played a major rolein the generation of the local streams (TWA, Beta Pic, andTucana-Horologium) that happen to be close to the Sun today. The core ofthe Tucana-Horologium association probably originated from the vicinityof the Upper Scorpius association 28 Myr ago. A few proposed members ofthe AB Dor moving group were in conjunction with the coeval Cepheus OB6association 38 Myr ago.
| Magnetic flux transport on active cool stars and starspot lifetimes
Context: .Many rapidly rotating cool stars show signatures of largemagnetic regions at all latitudes. Mid-latitude starspots and magneticregions have characteristic lifetimes of 1 month or less, as indicatedby observations using (Zeeman-) Doppler imaging techniques. Aims:.We aim to estimate the lifetimes of bipolar magnetic regions andstarspots on the surfaces of cool stars. We consider different possibleconfigurations for starspots and compare their flux variations andlifetimes based on a magnetic flux transport model. Methods: .Wecarry out numerical simulations of the surface evolution of bipolarmagnetic regions (BMRs) and magnetic spots on stars, which have radiiand surface rotational shears of AB Doradus, the Sun,and the HR 1099 primary. The surface flux transportmodel is based on the magnetic induction equation for radial fieldsunder the effects of surface differential rotation, meridional flow, andturbulent diffusion due to convective flow patterns. We calculate theflux evolution and the lifetimes of BMRs and unipolar starspots, varyingthe emergence latitude, surface shear rate, and tilt angle.Results: .For BMRs comparable to the largest observed on the Sun, wefind that varying the surface flows and the tilt angle modifies thelifetimes over a range of one month. For very large BMRs (area ~10% ofthe stellar surface) the assumption of a tilt angle increasing withlatitude leads to a significant increase of lifetime, as compared to thecase without tilt. Such regions can evolve to polar spots that live morethan a year. Adopting the observed weak latitudinal shear and the radiusof the active subgiant component of HR 1099, we find longer BMRlifetimes as compared to the more strongly sheared ABDor case. Random emergence of six additional tilted bipoles inan activity belt at 60° latitude enhanced the lifetimes of polarcaps up to 7 years. We have also compared the evolution and lifetime ofmonolithic starspots with those of conglomerates of smaller spots ofsimilar total area. We find similar decay patterns and lifetimes forboth configurations.
| Improved age constraints for the AB Doradus quadruple system. The binary nature of AB Doradus B
We present resolved NACO photometry of the close binary AB Dor B in H-and Ks-band. AB Dor B is itself known to be a wide binary companion toAB Dor A, which in turn has a very low-mass close companion named AB DorC. These four known components make up the young and dynamicallyinteresting system AB Dor, which will likely become a benchmark systemfor calibrating theoretical pre-main sequence evolutionary mass tracksfor low-mass stars. However, for this purpose the actual age has to beknown, and this subject has been a matter of discussion in the recentscientific literature. We compare our resolved photometry of AB Dor Baand Bb with theoretical and empirical isochrones in order to constrainthe age of the system. This leads to an age estimate of about 50 to 100Myr. We discuss the implications of such an age range for the case of ABDor C, and compare with other results in the literature.Based on observations collected at the European Southern Observatory,Chile (NACO SDI commissioning run, February 2004).
| Evidence of Stellar-driven Outflows from the Classical T Tauri Star RY Tau
RY Tau is a rapidly rotating classical T Tauri star observed close toedge-on. The combination of new HST/STIS observations obtained in 2001with HST/GHRS archive data from 1993 has allowed us to get, for thefirst time, information on the thermal structure and velocity of thewind. The repeated observations of the Si III] and C III] lines show alack of changes with time on the blue side of the profile (dominated bythe wind contribution). Very high temperature plasma(logTe=4.8) is detected at densities of9.5<=logne(cm-3)<=10.2 associated with thewind. The emitting volumes are ~(0.35 Rsolar)3,suggesting a stellar origin. The wind kinematics derived from theprofiles (Si III], C III], and [O II]) does not satisfy the theoreticalpredictions of MHD centrifugally driven disk winds. The profiles'asymmetry, large velocity dispersions, and small variability as well asthe small emitting volumes are best explained if the wind is produced bythe contributions of several outflows from atmospheric open-fieldstructures like those observed in the Sun.
| Magnetic activity on AB Doradus: temporal evolution of star-spots and differential rotation from 1988 to 1994.
Surface brightness maps for the young K0 dwarf AB Doradus arereconstructed from archival data sets for epochs spanning 1988 to 1994.By using the signal-to-noise enhancement technique of Least-SquaresDeconvolution, our results show a greatly increased resolution of spotfeatures than obtained in previously published surface brightnessreconstructions. These images show that for the exception of epoch1988.96, the starspot distributions are dominated by a long-lived polarcap, and short-lived low to high latitude features. The fragmented polarcap at epoch 1988.96 could indicate a change in the nature of the dynamoin the star. For the first time we measure differential rotation forepochs with sufficient phase coverage (1992.05, 1993.89, 1994.87). Thesemeasurements show variations on a timescale of at least one year, withthe strongest surface differential rotation ever measured for AB Doroccurring in 1994.86. In conjunction with previous investigations, ourresults represent the first long-term analysis of the temporal evolutionof differential rotation on active stars.
| The coronal structure of Speedy Mic - II. Prominence masses and off-disc emission
Observations of stellar prominences on young rapidly rotating starsprovide unique probes of their magnetic fields out to many stellarradii. We compare two independently obtained data sets of the K3 dwarfSpeedy Mic (BO Mic, HD 197890) using the Anglo-Australian Telescope(AAT) and the European Southern Observatory (ESO) Very Large Telescope(VLT). Taken more than a fortnight apart, they provide the first insightinto the evolution of the prominence system on such a young rapidlyrotating star. The largest prominences observed transiting the stellardisc are found at very similar rotational phases between the epochs.This suggests that the magnetic structures supporting the prominencesretain their identity on a two to three week time-scale. By takingadvantage of the high signal-to-noise ratio and large wavelength rangeof the VLT observations, we identify prominences as transient absorptionfeatures in all lines of the hydrogen Balmer series down toH10. We use the ratios of the prominence equivalent widths(EWs) in these lines to determine their column densities in the firstexcited state of hydrogen. We determine the optical depths, findingprominences to be rather optically thick (τ ~ 20) in the Hαline. The total hydrogen column density and thus the prominence massesare determined via observations of the CaII H&K lines. We findtypical masses for four of the largest prominences to be in the range0.5-2.3 × 1014 kg, slightly larger than giant solarprominence masses. Rotationally modulated emission is seen outside ofthe Hα line. These loops of emission are shown to be caused byprominences seen off the stellar disc. We find that all of the largeemission loops can be associated with prominences we see transiting thestellar disc. This, combined with the fact that many prominences appearto eclipse the off-disc emission of others, strongly suggests that theprominence system is highly flattened and likely confined to low stellarlatitudes.Based on observations made with the ESO telescopes obtained from the ESOScience Archive Facility.E-mail: njd2@st-andrews.ac.uk
| Rotationally modulated X-ray emission from T Tauri stars
We have modelled the rotational modulation of X-ray emission from TTauri stars assuming that they have isothermal, magnetically confinedcoronae. By extrapolating surface magnetograms, we find that T Tauricoronae are compact and clumpy, such that rotational modulation arisesfrom X-ray emitting regions being eclipsed as the star rotates. Emittingregions are close to the stellar surface and inhomogeneously distributedabout the star. However, some regions of the stellar surface, whichcontain wind bearing open field lines, are dark in X-rays. Fromsimulated X-ray light curves, obtained using stellar parameters from theChandra Orion Ultradeep Project, we calculate X-ray periods and makecomparisons with optically determined rotation periods. We find thatX-ray periods are typically equal to, or are half of, the opticalperiods. Further, we find that X-ray periods are dependent upon thestellar inclination, but that the ratio of X-ray to optical period isindependent of stellar mass and radius.
| On the time dependence of differential rotation in young late-type stars
A model for the angular momentum transfer within the convection zone ofa rapidly rotating star is introduced and applied to the analysis ofrecent observations of temporal fluctuations of the differentialrotation on the young late-type stars AB Doradus (AB Dor) and LQ Hydrae(LQ Hya). Under the hypothesis that the mean magnetic field produced bythe stellar dynamo rules the angular momentum exchanges and that theangular velocity depends only on the distance s from the rotation axisand the time, the minimum azimuthal Maxwell stress|BsBϕ|, averaged over the convection zone, isfound to range from ~0.04 to ~0.14T2. If the poloidal meanmagnetic field Bs is of the order of 0.01T, as indicated bythe Zeeman-Doppler imaging maps of those stars, then the azimuthal meanfield Bϕ can reach an intensity of several teslas, whichsignificantly exceeds equipartition with the turbulent kinetic energy.Such strong fields can account also for the orbital period modulationobserved in cataclysmic variables and RS Canum Venaticorum systems witha main-sequence secondary component. Moreover, the model allows us tocompute the kinetic energy dissipation rate during the maintenance ofthe differential rotation. Only in the case of the largest surface shearobserved on LQ Hya may the dissipated power exceed the stellarluminosity, but the lack of a sufficient statistic on the occurrence ofsuch episodes of large shear does not allow us to estimate their impacton the energy budget of the convection zone.
| Mass accretion on to T Tauri stars
It is now accepted that accretion on to classical T Tauri stars iscontrolled by the stellar magnetosphere, yet to date most accretionmodels have assumed that their magnetic fields are dipolar. Byconsidering a simple steady state accretion model with both dipolar andcomplex magnetic fields, we find a correlation between mass accretionrate and stellar mass of the form , with our results consistent withinobserved scatter. For any particular stellar mass there can be severalorders of magnitude difference in the mass accretion rate, withaccretion filling factors of a few per cent. We demonstrate that thefield geometry has a significant effect in controlling the location anddistribution of hotspots, formed on the stellar surface from the highvelocity impact of accreting material. We find that hotspots are oftenat mid to low latitudes, in contrast to what is expected for accretionto dipolar fields, and that particularly for higher mass stars, theaccretion flow is predominantly carried by open field lines.
| A radio-emitting outflow in the quiescent state of A0620-00: implications for modelling low-luminosity black hole binaries
Deep observations with the Very Large Array of A0620-00, performed in2005 August, resulted in the first detection of radio emission from ablack hole binary at X-ray luminosities as low as 10-8.5times the Eddington limit. The measured radio flux density, of 51 +/- 7μJy at 8.5 GHz, is the lowest reported for an X-ray binary system sofar, and is interpreted in terms of partially self-absorbed synchrotronemission from outflowing plasma. Making use of the estimated outeraccretion rate of A0620-00 in quiescence, we demonstrate that theoutflow kinetic power must be energetically comparable to the totalaccretion power associated with such rate, if it was to reach the blackhole with the standard radiative efficiency of 10 per cent. This favoursa model for quiescence in which a radiatively inefficient outflowaccounts for a sizable fraction of the missing energy, and, in turn,substantially affects the overall dynamics of the accretion flow.Simultaneous observations in the X-ray band, with Chandra, confirm thevalidity of a non-linear radio/X-ray correlation for hard state blackhole binaries down to low quiescent luminosities, thereby contradictingsome theoretical expectations. Taking the mass term into account, theA0620-00 data lie on the extrapolation of the so-called FundamentalPlane of black hole activity, which has thus been extended by more thantwo orders of magnitude in radio and X-ray luminosity. With the additionof the A0620-00 point, the plane relation provides an empirical prooffor the scale invariance of the jet-accretion coupling in accretingblack holes over the entire parameter space observable with currentinstrumentation.
| 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.
| Surface differential rotation and photospheric magnetic field of the young solar-type star HD 171488 (V889 Her)
We present spectropolarimetric observations of the young, single earlyG-dwarf HD 171488. These observations were obtained over a five-nightperiod in 2004 September at the 3.9-m Anglo-Australian Telescope usingthe SEMPOL spectropolarimeter visitor instrument. Using the technique ofleast-squares deconvolution to increase the signal-to-noise ratio of thedata, we have applied Zeeman Doppler imaging to reconstruct brightnessand magnetic surface topologies of the star. The brightness image showsa large polar spot with weaker low- to mid-latitude features, confirmingan earlier Doppler imaging observation. The reconstruction of thesurface magnetic field shows regions of radial field at all latitudes(except near the pole) and regions of azimuthal field predominantly athigh latitudes (60°-70°), with the azimuthal field almostforming a ring around the polar regions.We have incorporated a solar-like differential rotation law into theimaging process to determine the surface differential rotation of coolspots on HD 171488. This gives an equatorial rotation rate of 1.313 +/-0.004 d and a surface shear of dΩ = 0.402 +/- 0.044radd-1. This means that the equator of HD 171488 laps thepoles every ~16 +/- 2 d and that HD 171488 has a photospheric shearapproximately seven times the solar value. This is the largestmeasurement of surface differential rotation yet obtained using theDoppler imaging method and is over twice the value of previouslyobserved early G-dwarfs.
| X-ray and optical bursts and flares in YSOs: results from a 5-day XMM-Newton monitoring campaign of L1551
We present the results of a five-day monitoring campaign with XMM-Newtonof six X-ray bright young stellar objects (YSOs) in the star-formingcomplex L1551 in Taurus. All stars present significant variability onthe five-day time scale. Modulation of the light curve on time scalescomparable with the star's rotational period appeared to be present inthe case of one weak-lined T Tauri star. Significant spectral variationsbetween the 2000 and the 2004 observations were detected in the(unresolved) classical T Tauri binary system XZ Tau: a hot plasmacomponent which was present in the X-ray spectrum in 2000 hadsignificantly weakened in 2004. As XZ Tau N was undergoing a strongoptical outburst in 2000, which had terminated since then, we speculateon the possible relationship between episodic, burst accretion, andX-ray heating. The transition object HL Tau underwent a strong flarewith a complex temperature evolution, which is indicative of an eventconfined within a very large magnetic structure (few stellar radii),similar to the ones found in YSOs in the Orion Nebula Cluster.
| Star Formation in the Era of the Three Great Observatories
This paper summarizes contributions and suggestions as presented at theChandra Workshop Star Formation in the Era of Three Great Observatoriesconducted in July 2005. One of the declared goals of the workshop was toraise recognition within the star formation research community about thesensible future utilization of the space observatories Spitzer, Hubble,and Chandra in their remaining years of operation to tackle imminentquestions of our understanding of stellar formation and the earlyevolution of stars. A white paper was generated to support thecontinuous and simultaneous usage of observatory time for star formationresearch. The contents of this paper have been presented and discussedat several other meetings during the course of 2005 and January 2006.
| Optical spectroscopy of the dwarf nova U Geminorum
The dwarf nova U Gem is unique in having a direct measurement of the Kvelocity of its white dwarf from Hubble Space Telescope (HST) spectra(K1 = 107 +/- 2kms-1 from Long et al.). We presenthigh-resolution optical spectra of U Gem in quiescence taken to test theaccuracy to which the HST value can be recovered from optical data. Wefind that, even with data of very high signal-to-noise ratio on thisarchetypal system, we cannot recover Long et al.'s value to better thanabout 20 per cent by any method. Contamination by neighbouring emissionlines seems a likely culprit. Our data reveal a number of new features:Doppler tomograms show emission at low velocity, close to the centre ofmass, and a transient, narrow absorption feature is seen in the Balmerlines near the line centres at the time of eclipse. We suggest thatstellar prominences, as previously invoked for the dwarf novae IP Pegand SS Cyg in outburst, may explain both of these features. The HeII4686.75 Å line emission is dominated by the gas stream/disc impactregion. Two distinct spots are seen in Doppler maps, the first beingvery narrow and showing a velocity close to that of the accretion discin the impact region, and the second much broader and located betweenthe velocities of the (ballistic) stream and the (Keplerian) disc. Wepresent tentative evidence of weak spiral structure, which may supportexplanations for `spiral shocks' based upon three-body effects. We findno evidence of stream-disc overflow in the system. Our data suggest aninclination angle >70°, favouring a mass <1.1Msolarfor the white dwarf, in disagreement with measurements of itsgravitational redshift. The mass donor is clearly seen in the Dopplermaps, with emission concentrated towards its poles, and mainly on theside facing the white dwarf. This suggests irradiation with shielding bythe disc from which we estimate a height-to-radius ratio (H/R) between0.15 and 0.25.
| The Nearest Young Moving Groups
The latest results in the research of forming planetary systems have ledseveral authors to compile a sample of candidates for searching forplanets in the vicinity of the Sun. Young stellar associations areindeed excellent laboratories for this study, but some of them are notclose enough to allow the detection of planets through adaptive opticstechniques. However, the existence of very close young moving groups cansolve this problem. Here we have compiled the members of the nearestyoung moving groups, as well as a list of new candidates from ourcatalog of late-type stars that are possible members of young stellarkinematic groups, studying their membership through spectroscopic andphotometric criteria.
| The Unusual X-Ray Spectrum of FU Orionis
FU Orionis objects (FUors) have undergone strong optical outbursts andare thought to be young low-mass stars accreting at high rates of up toM˙acc~10-4 Msolar yr-1.FUors have been extensively studied at optical and infrared wavelengths,but little is known about their X-ray properties. We have thus initiateda program aimed at searching for and characterizing their X-rayemission. First results are presented here for the prototype star FUOrionis based on observations obtained with XMM-Newton. Its CCD X-rayspectrum is unusual compared to those of accreting classical T Tauristars (CTTSs). The cool and hot plasma components typically detected inCTTSs are present but are seen through different absorption columndensities. The absorption of the cool component is consistent withAV~2.4 mag anticipated from optical studies, but theabsorption of the hot component is at least 10 times larger. The originof the excess absorption is uncertain, but cold accreting gas or astrong near-neutral wind are likely candidates. The hot plasma componentaccounts for most of the observed X-ray flux, and thermal models givevery high temperatures kT>=5 keV. The most prominent feature in theX-ray spectrum is an exceptionally strong Fe K emission line at 6.67keV, and weak emission from fluorescent Fe I at ~6.4 keV may also bepresent. The high plasma temperature clearly demonstrates that theemission is dominated by magnetic processes. We discuss possible originsof the unusual X-ray spectrum in the context of a complex physicalenvironment that likely includes disk accretion, a strong wind, magneticactivity, and close binarity.
| Roche tomography of cataclysmic variables - III. Star-spots on AE Aqr
We present a Roche tomography reconstruction of the secondary star inthe cataclysmic variable AE Aqr. The tomogram reveals several surfaceinhomogeneities that are due to the presence of large, cool star-spots.In addition to a number of lower latitude spots, the maps also show thepresence of a large, high-latitude spot similar to that seen in Dopplerimages of rapidly rotating isolated stars, and a relative paucity ofspots at a latitude of 40°. In total, we estimate that some 18 percent of the Northern hemisphere of AE Aqr is spotted.We have also applied the entropy landscape technique to determineaccurate parameters for the binary system. We obtain optimal massesM1 = 0.74Msolar, M2 =0.50Msolar, a systemic velocity γ = -63kms-1and an orbital inclination i = 66°.Given that this is the first study to successfully image star-spots onthe secondary star in a cataclysmic variable, we discuss the role thatfurther studies of this kind may play in our understanding of thesebinaries.
| A Comparative Study of Flaring Loops in Active Stars
Dynamo activity in stars of different types is expected to generatemagnetic fields with different characteristics. As a result, adifferential study of the characteristics of magnetic loops in a broadsample of stars may yield information about dynamo systematics. In theabsence of direct imaging, certain physical parameters of a stellarmagnetic loop can be extracted if a flare occurs in that loop. In thispaper we employ a simple nonhydrodynamic approach introduced by Haisch,to analyze a homogeneous sample of all of the flares we could identifyin the EUVE DS database: a total of 134 flares that occurred on 44 starsranging in spectral type from F to M and in luminosity class from V toIII. All of the flare light curves that have been used in the presentstudy were obtained by a single instrument (EUVE DS). For each flare, wehave applied Haisch's simplified approach (HSA) in order to determineloop length, temperature, electron density, and magnetic field. For eachof our target stars, a literature survey has been performed to determinequantitatively the extent to which our results are consistent withindependent studies. The results obtained by HSA are found to be wellsupported by results obtained by other methods. Our survey suggeststhat, on the main sequence, short loops (with lengths<=0.5R*) may be found in stars of all classes, while thelargest loops (with lengths up to 2R*) appear to be confinedto M dwarfs. Based on EUVE data, the transition from small to largeloops on the main sequence appears to occur between spectral types K2and M0. We discuss the implications of this result for dynamo theories.
| On the Temperature-Emission Measure Distribution in Stellar Coronae
Strong peaks in the emission measure-temperature (EM-T ) distributionsin the coronae of some binary stars are associated with the presence ofhot (107 K), dense (up to 1013 cm -3)plasma. These peaks are very reminiscent of those predicted to arise inan impulsively heated solar corona. A coronal model comprised of manyimpulsively heated strands is adapted to stellar parameters. It is shownthat the properties of the EM-T distribution can be accounted for ingeneral terms provided the emission comes from many very small loops(length under 103 km) with intense magnetic fields (1 kG)distributed across part of the surface of the star. The heating requiresevents that generally dissipate between 1026 and 1028 ergs, which is in the range of solar microflares. This impliesthat such stars must be capable of generating regions of localizedintense magnetic fields.
| Photometric study of the active binary star V1430 Aquilae
New BVR light curves and a photometric analysis of the eclipsing binarystar V1430 Aql are presented. The light curves were obtained at theÇanakkale Onsekiz Mart University Observatory in 2004. The lightcurves are generally those of detached eclipsing binaries, but there arelarge asymmetries between maxima. New BVR light curves were analysedwith an ILOT procedure. Light curve asymmetries of the system wereexplained in terms of large dark starspots on the primary component. Theprimary star shows a long-lived and quasi-poloidal spot distributionwith active longitudes in opposite hemispheres. Absolute parameters ofthe system were derived. We also discuss the evolution of the system:the components are likely to be pre-main sequence stars, but a post-mainsequence stage cannot be ruled out. More observations are needed todecide this point.
| Spot patterns and differential rotation in the eclipsing pre-cataclysmic variable binary, V471 Tau
We present surface spot maps of the K2V primary star in thepre-cataclysmic variable binary system, V471 Tau. The spot maps show thepresence of large high-latitude spots located at the sub-white dwarflongitude region. By tracking the relative movement of spot groups overthe course of four nights (eight rotation cycles), we measure thesurface differential rotation rate of the system. Our results revealthat the star is rotating rigidly with a surface shear rate, dΩ=1.6 +/- 6mradd-1. The single active star AB Dor has a similarspectral type, rotation period and activity level as the K star in V471Tau, but displays much stronger surface shear (46 < dΩ <58mradd-1). Our results suggest that tidal locking mayinhibit differential rotation; this reduced shear, however, does notaffect the overall magnetic activity levels in active K dwarfs.
| X-ray emission from T Tauri stars
We have modelled the X-ray emission of T Tauri stars assuming that theyhave isothermal, magnetically confined coronae. These coronae extendoutwards until either the pressure of the hot coronal gas overcomes themagnetic field, or, if the corona interacts with a disc before thishappens, by the action of the disc itself. This work is motivated by theresults of the Chandra Orion Ultradeep Project that show an increase inthe X-ray emission measure (EM) with increasing stellar mass. We findthat this variation (and its large scatter) results naturally from thevariation in the sizes of the stellar coronae. The reduction in themagnitude of the X-ray emission due to the presence of a disc strippingthe outer parts of the stellar corona is most pronounced for the lowermass stars. The higher mass stars with their greater surface gravitieshave coronae that typically do not extend out as far as the inner edgeof the disc and so are less affected by it. For these stars, accretiontakes place along open field lines that connect to the disc. Byextrapolating surface magnetograms of young main-sequence stars, we haveexamined the effect on the X-ray emission of a realistic degree of fieldcomplexity. We find that the complex fields (which are more compact)give densities of some (2.5-0.6) × 1010cm-3.This is consistent with density estimates of (1-8) ×1010cm-3 from modelling of individual flares. Asimple dipole field in contrast gives densities typically an order ofmagnitude less. For the complex fields, we also find surface hotspots ata range of latitudes and longitudes with surface-filling factors of onlya few per cent. We find that the dipolar fields give a relationshipbetween X-ray EM and stellar mass that is somewhat steeper thanobserved, while the complex fields give a relation that is shallowerthan observed. This may suggest that T Tauri stars have coronal fieldsthat are slightly more extended than their main-sequence counterparts,but not as extended as a purely dipolar field.
| Simulated X-ray cycles in rapidly rotating solar-like stars
It is generally accepted that the presence of a hot magnetic coronaprovides the source of X-ray emission in cool stars. With thisconnection one could expect to see the variation of magnetic flux in theactivity cycle of a star mirrored by a similar variation in the starsX-ray emission. Using magnetic maps produced from flux emergence andtransport simulations and assuming a potential field for the corona, wecan extrapolate the coronal magnetic field and hence calculate thevariation of the X-ray emission. We consider three types of activitycycle that successfully reproduce the pattern of intermingled magneticflux at high latitudes, a feature observed with Zeeman-Doppler imaging.The three different cycles take the form of (1) an enhanced butterflypattern where flux emergence is extended to a latitude of 70°, (2)an extended emergence profile as before but with an overlap of 4 yr inthe butterfly diagram and (3) where no butterfly diagram is used. Thecyclic variation in the X-ray emission is around two orders of magnitudefor cases (1) and (3), but less than one order of magnitude for case(2). For all three cases, the rotational modulation of the X-rayemission is greatest at cycle minimum, but the emission measure weighteddensity varies little over the cycle. For cases (1) and (2) the fractionof the total flux that is open (along which a wind can escape) varieslittle over the cycle, but for case (3) this is three times larger atcycle minimum than at maximum. Our results clearly show that althoughmagnetic cycles may exist for stars they are not necessarily observablein the X-ray emission.
| Starspot activity in late stars: Methods and results
Three types of methods for studying the surface inhomogeneities of coolstars and the results of their use on type BY Dra, RS CVn, FK Com, and TTau variables are discussed. The current relevance of traditionalphotometric methods and the advantages of the zonal spottedness modelare pointed out. Dependences of the maximum total areas, averagelatitudes, and temperatures of spots on the global parameters of thestars are given. Analogs of the solar cycle in the variations of theareas and latitudes of starspots are examined, as well as the effects ofdifferential rotation and active longitudes.
| The Remarkable Be Star HD 110432 (BZ Crucis)
HD 110432 (B1e) has gained considerable recent attention because it is ahard, variable X-ray source with local absorption and also because itsoptical spectrum is affected by an extensive Be disk. From time-serialechelle data obtained over 2 weeks during 2005 January and February, wehave discovered several remarkable characteristics in the star's opticalspectrum. The line profiles show rapid variations on some nights, whichcan most likely be attributed to irregularly occurring and short-livedmigrating subfeatures. Such features have been found in spectra ofγ Cas and AB Dor, two stars for which it is believed magneticfields force circumstellar clouds to corotate over the star's surface.The star's optical spectrum also exhibits a number of mainly Fe II andHe I emission features with double-lobed profiles typical of anoptically thin circumstellar disk viewed nearly edge-on. Using spectralsynthesis techniques for the January data, we find that its temperatureand column density are close to 9800 K and roughly3×1022 cm-2, respectively. Its projecteddisk size covers remarkably large 100 stellar areas, and the emittingvolume resides at a surprisingly large distance of 1 AU from the star.Surprisingly, we also find that the absorption wings of the strongestoptical and UV lines in the spectrum extend to at least +/-1000 kms-1, even though the rotational velocity is 300-400 kms-1. We are unable to find a satisfactory explanation forthese extreme line broadenings. Otherwise, HD 110432 and γ Casshare similarly peculiar X-ray and optical characteristics. Theseinclude a high X-ray temperature, erratic X-ray variability ontimescales of a few hours, optical metallic emission lines, andsubmigrating features in optical line profiles. Because of thesesimilarities, we suggest that HD 110432 is a member of a select newclass of ``γ Cas analogs.''
| Testing Theoretical Evolutionary Models with AB Doradus C and the Initial Mass Function
We assess the constraints on the evolutionary models of young low-massobjects that are provided by the measurements of the companion AB Dor Cby Close and coworkers and by a new comparison of model-derived IMFs ofstar-forming regions to the well-calibrated IMF of the solarneighborhood. After performing an independent analysis of all of theimaging and spectroscopic data for AB Dor C that were obtained by Close,we find that AB Dor C (which has no methane) is not detected at asignificant level (S/N~1.2) in the SDI data when one narrowband image issubtracted from another but that it does appear in the individual SDIframes, as well as the images at J, H, and Ks. Although ourbroadband photometry for AB Dor C is consistent with that of Close, theuncertainties that we measure are larger. Using the age of τ=75-150Myr recently estimated for AB Dor by Luhman and coworkers, theluminosity predicted by the models of Chabrier and Baraffe is consistentwith the value that we estimate from the photometry for AB Dor C. Wemeasure a spectral type of M6+/-1 from the K-band spectrum of AB Dor C,which is earlier than the value of M8+/-1 reported by Close and isconsistent with the model predictions when a dwarf temperature scale isadopted. In a test of these evolutionary models at much younger ages, weshow that the low-mass IMFs that they produce for star-forming regionsare similar to the IMF of the solar neighborhood. If the masses of thelow-mass stars and brown dwarfs in these IMFs of star-forming regionswere underestimated by a factor of 2 as suggested by Close, then the IMFcharacterizing the current generation of Galactic star formation wouldhave to be radically different from the IMF of the solar neighborhood.Based on observations made with ESO telescopes at the ParanalObservatories under program ID 60.A-9026. This publication makes use ofdata products from the Two Micron All Sky Survey, which is a jointproject of the University of Massachusetts and the Infrared Processingand Analysis Center/California Institute of Technology, funded by theNational Aeronautics and Space Administration and the National ScienceFoundation.
| Photometric evidence for two-temperature photospheric inhomogeneities on magnetically active K dwarf stars
We derive the properties of spotted regions in late-type active starsfrom V- and B-band photometric data. Specifically, we compare theamplitudes of spot-induced light and color variations with modelamplitudes, which depend on the area, temperature and distribution ofspotted regions over the stellar surface. Our analysis is applied tothree well-known chromospherically active K dwarf stars: DXLeonis, AB Doradus and LQHydrae, and indicates that all three stars show significantvariations of the average spot temperature from epoch to epoch. Theobserved temporal variations of the starspot temperatures are bestexplained as arising from the contemporary presence on the stellarphotosphere of two-component (two-temperature) active regions. Theseconsist of either dark spots and hot faculae and/or dark umbrae andpenumbrae and the variations over time arise from change of their arealratio.
| On the dynamics of the AB Doradus system
We present an astrometric analysis of the binary systems AB Dor A /ABDor C and AB Dor Ba /AB Dor Bb. These two systems of well-knownlate-type stars are gravitationally associated and they constitute thequadruple AB Doradus system. From the astrometric data available atdifferent wavelengths, we report: (i) a determination of the orbit of ABDor C, the very low mass companion to AB Dor A, which confirms the massestimate of 0.090 M_ȯ reported in previous works; (ii) ameasurement of the parallax of AB Dor Ba, which unambiguously confirmsthe long-suspected physical association between this star and AB Dor A;and (iii) evidence of orbital motion of AB Dor Ba around AB Dor A, whichplaces an upper bound of 0.4 M_ȯ on the mass of the pair AB Dor Ba/AB Dor Bb (50% probability). Further astrometric monitoring of thesystem at all possible wavelengths would determine with extraordinaryprecision the dynamical mass of its four components.
| The coronal structure of Speedy Mic - I. A densely packed prominence system beyond corotation
We present new observations of the prominence system on the K3 dwarfSpeedy Mic (BO Mic, HD 197890). Using an improved technique to track theabsorption features in Hα we find a very active prominence systemwith approximately 10 prominences on the observable hemisphere perrotation. From a total of 25 prominences, we find an average axialdistance of (2.85 +/- 0.54) R* which is twice the corotationradius above the stellar surface. We discuss the consequences of theseobservations on the nature of the supporting magnetic structures. Twoconsecutive nights, with complete phase coverage, combined with afurther night after a three-night gap allow us to study the evolution ofthe prominence system on two different time-scales. Several of theprominences have counterparts at similar phases on consecutive nights.During this interval, many prominences show evidence for evolution intheir heights and phases of observation. Five nights (13 rotationcycles) later, we recover many prominences at approximately the samephases. Whilst individual prominences change axial distances orappear/reappear from night-to-night, the underlying prominencesupporting structures appear to be stable over as many as 13 stellarrotations.
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