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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.
| The atmospheric parameters, abundances and magnetic field of the AR piscium primary
The high resolution, high S/N spectra for the chromospherically activebinary AR Psc (=HD 8357) has been obtained by using the Coudéechelle spectrograph of the 2.16 m telescope of Beijing AstronomicalObservatory. The Hα lines present emission above thecontinuum and the profiles of CaII IRT2 (λ 854.2 nm) & IRT3(λ 866.2 nm) show core emission. The net chromospheric flux ofthe had been calculated by using spectral subtraction. A detailedspectroscopic analysis and model calculation have yielded theatmospheric parameters of the primary of AR Psc. The abundances of 9elements has been obtained. And, the measurement of Magnetic field havebeen made using the Stenflo Lindegren statistical analysis and theprofile-addition technique. A magnetic field strength B = 2400 ±300 G and a filling factor α = 0.45 ± 0.05 are obtained.
| 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.
| 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.
| Mass loss and orbital period decrease in detached chromospherically active binaries
The secular evolution of the orbital angular momentum (OAM), thesystemic mass (M=M1+M2) and the orbital period of114 chromospherically active binaries (CABs) were investigated afterdetermining the kinematical ages of the subsamples which were setaccording to OAM bins. OAMs, systemic masses and orbital periods wereshown to be decreasing by the kinematical ages. The first-orderdecreasing rates of OAM, systemic mass and orbital period have beendetermined as per systemic OAM, per systemic mass and per orbitalperiod, respectively, from the kinematical ages. The ratio of d logJ/dlogM= 2.68, which were derived from the kinematics of the presentsample, implies that there must be a mechanism which amplifies theangular momentum loss (AML) times in comparison to isotropic AML ofhypothetical isotropic wind from the components. It has been shown thatsimple isotropic mass loss from the surface of a component or bothcomponents would increase the orbital period.
| 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.
| Variations in D/H and D/O from New Far Ultraviolet Spectroscopic Explorer Observations
We use data obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) to determine the interstellar abundances of D I, N I, O I, Fe II,and H2 along the sight lines to WD 1034+001, BD +39 3226, andTD1 32709. Our main focus is on determining the D/H, N/H, O/H, and D/Oratios along these sight lines, with logN(H)>20.0, that probe gaswell outside of the Local Bubble. Hubble Space Telescope (HST) andInternational Ultraviolet Explorer (IUE) archival data are used todetermine the H I column densities along the WD 1034+001 and TD1 32709sight lines, respectively. For BD +39 3226, a previously published N(HI) is used. We find(D/H)×105=2.14+0.53-0.45,1.17+0.31-0.25, and1.86+0.53-0.43 and(D/O)×102=6.31+1.79-1.38,5.62+1.61-1.31, and7.59+2.17-1.76 for the WD 1034+001, BD +39 3226,and TD1 32709 sight lines, respectively (all 1 σ). The scatter inthese three D/H ratios exemplifies the scatter that has been found byother authors for sight lines with column densities in the range19.2
| A Large Millimeter Flare on the RS CVn Binary σ Geminorum
The flaring behavior of active RS CVn binaries has been studiedextensively at centimeter wavelengths, but very little has beenpublished concerning RS CVn flares in the millimeter region, where thenonthermal gyrosynchrotron emission is much more likely to be opticallythin. We present 3 mm (99 GHz) observations of a large flare on theactive binary σ Gem using the Owens Valley Millimeter Array. Two 3hr observations in 2004 February showed only 3 σ upper limits of 3mJy, while the third observation on 2004 April 22 showed a large flareoutburst that filled the 6 hr observation and reached a peak fluxdensity of 90 mJy. The strongest brightening was preceded by at least a4 hr period of significantly elevated emission at 20-40 mJy. Theseobservations demonstrate the feasibility of detailed multifrequencystudies of nonthermal electron distributions in active star coronae withthe next generation of millimeter and centimeter radio arrays.
| MERLIN Astrometry of 11 Radio Stars
We report accurate positions in the International Celestial ReferenceFrame (ICRF) for 11 radio stars. Observations were made using theMulti-Element Radio Linked Interferometer Network at a radio frequencyof 5 GHz. The positions are estimated to be accurate at the 5 mas level.Positions were obtained directly in the ICRF by phase referencing theradio stars to ICRF quasars whose positions are estimated to be accurateat the 0.25 mas level. We use our results together with results ofprevious observations to obtain proper-motion estimates for these stars.The average proper-motion uncertainties are 1.1 mas yr-1 inμαcosδ and 1.2 mas yr-1 inμδ, comparable to the Hipparcos values.
| Newly discovered active binaries in the RasTyc sample of stellar X-ray sources. I. Orbital and physical parameters of six new binaries
We present the first results from follow-up optical observations, bothphotometric and spectroscopic, of stellar X-ray sources, selected fromthe RasTyc sample, resulting from the cross-correlation of ROSAT All-SkySurvey (RASS) and TYCHO catalogues. In particular, we report on thediscovery of six late-type binaries, for which we obtained good radialvelocity curves and solved their orbits. We performed an automaticspectral classification of both single-lined and double-lined binarieswith codes developed by us and found two binaries composed of twomain-sequence stars and four binaries with an evolved (giant orsubgiant) component. Filled-in or pure emission Hα profilesindicative of a moderate or high level of chromospheric activity wereobserved. In nearly all the systems, we also detected a photometricmodulation ascribable to surface inhomogeneities that is correlated withthe orbital period, suggesting a synchronization between rotational andorbital periods. The position on the HR diagram of the components of thefive sources with a known parallax indicates three binaries containingonly main-sequence stars and two single-lined systems with a giantcomponent. The kinematical properties of two, or possibly four, of theobserved systems are consistent with a young disk population.
| Origin of the Galactic ridge X-ray emission
We analyze a map of the Galactic ridge X-ray emission (GRXE) constructedin the 3-20 keV energy band from RXTE/PCA scan and slew observations. Weshow that the GRXE intensity follows the Galactic near-infrared surfacebrightness closely and thus traces the Galactic stellar massdistribution. The GRXE consists of two spatial components that can beidentified with the bulge/bar and the disk of the Galaxy. The parametersof these components determined from X-ray data are compatible with thosederived from near-infrared data. The inferred ratio of X-ray tonear-infrared surface brightness I_3{-20 keV}(10-11 ergs-1 cm-2 deg-2)/I3.5 μm(MJy/sr) = 0.26±0.05, and the ratio of X-ray tonear-infrared luminosity L3-20 keV/L3-4 μ m=(4.1±0.3)×10-5. The corresponding ratio of the3-20 keV luminosity to the stellar mass is L_x/M= (3.5±0.5)× 1027 erg s-1Mȯ-1, which agrees within the uncertaintieswith the cumulative emissivity per unit stellar mass of point X-raysources in the Solar neighborhood, determined in an accompanying paper(Sazonov et al.). This suggests that the bulk of the GRXE is composed ofweak X-ray sources, mostly cataclysmic variables and coronally activebinaries. The fractional contributions of these classes of sources tothe total X-ray emissivity determined from the Solar neighborhood datacan also explain the GRXE energy spectrum. Based on the luminosityfunction of local X-ray sources we predict that, in order to resolve 90%of the GRXE into discrete sources, a sensitivity limit of10-16 erg s-1 cm-2 (2-10 keV) willneed to be reached in future observations.
| A search for magnetic fields in the variable HgMn star α Andromedae
Context: .The chemically peculiar HgMn stars are a class of Bp starswhich have historically been found to be both non-magnetic andnon-variable. Remarkably, it has recently been demonstrated that thebright, well-studied HgMn star α And exhibits clear Hg ii lineprofile variations indicative of a non-uniform surface distribution ofthis element. Aims: .With this work, we have conducted anextensive search for magnetic fields in the photosphere of αAnd. Methods: .We have acquired new circular polarisation spectrawith the MuSiCoS and ESPaDOnS spectropolarimeters. We have also obtainedFORS1 circular polarisation spectra from the ESO Archive, and consideredall previously published magnetic data. This extensive dataset has beenused to systematically test for the presence of magnetic fields in thephotosphere of α And. We have also examined the high-resolutionspectra for line profile variability. Results: .The polarimetricand magnetic data provide no convincing evidence for photosphericmagnetic fields. The highest-S/N phase- and velocity-resolved Stokes Vprofiles, obtained with ESPaDOnS, allow us to place a 3σ upperlimit of about 100 G on the possible presence of any undetected puredipolar, quadrupolar or octupolar surface magnetic fields (and just 50 Gfor fields with significant obliquity). We also consider and dismiss thepossible existence of more complex fossil and dynamo-generated fields,and discuss the implications of these results for explaining thenon-uniform surface distribution of Hg. The very high-quality ESPaDOnSspectra have allowed us to confidently detect variability of Hg iiλ 6149, λ 5425 and λ 5677. The profile variabilityof the Hg ii lines is strong, and similar to that of the Hg ii λ3984 line. On the other hand, variability of other lines (e.g. Mn, Fe)is much weaker, and appears to be attributable to orbital modulation,continuum normalisation differences and weak, variable fringing.
| X-ray luminosity function of faint point sources in the Milky Way
We assessed the contribution to the X-ray (above 2 keV) luminosity ofthe Milky Way by different classes of low-mass binary systems and singlestars. We began by using the RXTE Slew Survey of the sky at|b|>10° to construct an X-ray luminosity function (XLF) of nearbyX-ray sources in the range 1030 ergs-1
| 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 temperature and abundance effects during an X-ray flare on σ Geminorum
We compare quiescent and flare X-ray spectra of the RS CVn binaryσ Gem obtained with the Chandra and XMM-Newton gratingspectrometers. We find that in addition to an overall 25% flux increase,which can be ascribed to variations in the system's quiescence activityover the 15 months that passed between the observations, there is a hotplasma component of kTe 3 keV that arises with theflare. The hot component is manifested primarily by emission from highcharge states of Fe and by a vast continuum. The cooler (kTe 2 keV) plasma remains undisturbed during the flare. We find nosignificant variations in the relative abundances during the flareexcept for a slight decrease (<30%) of O and Ne.
| The `solar model problem' solved by the abundance of neon in nearby stars
The interior structure of the Sun can be studied with great accuracyusing observations of its oscillations, similar to seismology of theEarth. Precise agreement between helioseismological measurements andpredictions of theoretical solar models has been a triumph of modernastrophysics. A recent downward revision by 25-35 per cent of the solarabundances of light elements such as C, N, O and Ne (ref. 2) has,however, broken this accordance: models adopting the new abundancesincorrectly predict the depth of the convection zone, the depth profilesof sound speed and density, and the helium abundance. The discrepanciesare far beyond the uncertainties in either the data or the modelpredictions. Here we report neon-to-oxygen ratios measured in a sampleof nearby solar-like stars, using their X-ray spectra. The abundanceratios are all very similar and substantially larger than the recentlyrevised solar value. The neon abundance in the Sun is quite poorlydetermined. If the Ne/O abundance in these stars is adopted for the Sun,the models are brought back into agreement with helioseismologymeasurements.
| On the orbital period modulation of RS CVn binary systems
The Applegate hypothesis proposed to explain the orbital periodmodulation of RS Canum Venaticorum (RS CVn) close binaries (Applegate1992) is considered in the framework of a general model to treat theangular momentum exchanges within the convective envelope of amagnetically active star. This model assumes that the convection zone isstrictly adiabatic and that the Taylor-Proudman balance holds, leadingto an internal angular velocity constant over cylindrical surfacesco-axial with the rotation axis. It turns out that the angular velocityperturbations, whatever their origin, can be expressed in terms of theeigenfunctions of the equation of angular momentum conservation withstress-free boundary conditions. Moreover, a lower limit for the energydissipation rate in a turbulent convection zone can be set, thanks tothe extremal properties of the eigenfunctions. This approach allows toapply precise constraints on the amplitude and the radial profile of theangular velocity variations that are required to explain the observedorbital period changes in classical RS CVn binaries (i.e. with orbitalperiod longer than 1-2 d and a subgiant secondary component). It isfound that an angular velocity change as large as 10 per cent of theunperturbed angular velocity at the base of the stellar convection zoneis needed. Such a large change is not compatible with the observations.Moreover, it would produce an energy dissipation rate much larger thanthe typical luminosities of the active components of RS CVn systems,except in the case that fast rotation and internal magnetic fieldsreduce the turbulent viscosity by at least 2 orders of magnitude withrespect to the value given by the mixing-length theory. Therefore, themodel proposed by Applegate should be rejected, at least in the case ofclassical RS CVn close binaries. Possible alternative models are brieflydiscussed, emphasizing the effects of intense magnetic fields (~10 T) onthe internal structure of magnetically active stars and the dynamics ofclose binary systems.
| The XMM-Newton Needles in the Haystack Survey: the local X-ray luminosity function of `normal' galaxies
In this paper we estimate the local (z < 0.22) X-ray luminosityfunction of `normal' galaxies derived from the XMM-Newton Needles in theHaystack Survey. This is an on-going project that aims to identifyX-ray-selected normal galaxies (i.e. non-AGN dominated) in the localUniverse. We are using a total of 70 XMM-Newton fields covering an areaof 11 deg2 which overlap with the Sloan Digital Sky SurveyData Release 2. Normal galaxies are selected on the basis of theirresolved optical light profile, their low X-ray-to-optical flux ratio[log(fx/fo) < - 2] and soft X-ray colours. Wefind a total of 28 candidate normal galaxies to the 0.5-8keV band fluxlimit of ~2 × 10-15ergcm-2s-1.Optical spectra are available for most sources in our sample (82 percent). These provide additional evidence that our sources are bona fidenormal galaxies with X-ray emission coming from diffuse hot gas emissionand/or X-ray binaries rather than a supermassive black hole. 16 of ourgalaxies have narrow emission lines or a late-type spectral energydistribution (SED) while the remaining 12 present only absorption linesor an early-type SED. Combining our XMM-Newton sample with 18 local (z< 0.22) galaxies from the Chandra Deep Field North and South surveys,we construct the local X-ray luminosity function of normal galaxies.This can be represented with a Schechter form with a break atL*~ 3+1.4-1.0×1041ergs-1 and a slope of α~ 1.78 +/- 0.12.Using this luminosity function and assuming pure luminosity evolution ofthe form ~(1 +z)3.3 we estimate a contribution to the X-raybackground from normal galaxies of ~10-20 per cent (0.5-8keV). Finally,we derive, for the first time, the luminosity functions for early- andlate-type systems separately.
| Doppler imaging and surface differential rotation of young open cluster stars - I. HD 307938 (R58) in IC 2602
In this paper we present Doppler images of a young active G dwarf (HD307938) in the southern open cluster IC 2602. Spectroscopic data wereobtained over a four-night period in 2000 January at the 3.9-mAnglo-Australian Telescope using the University College London EchelleSpectrograph. Simultaneous photometric observations (in the V and Rbands) were obtained at the 1.0-m Australian National Universitytelescope. By applying least-squares deconvolution (LSD) to the 2500+photospheric lines in each echelle spectrum a single highsignal-to-noise ratio LSD profile was produced for each phase of thespectroscopic observations. Maximum-entropy image reconstruction,incorporating both the LSD profiles and the photometric data, was usedto produce maps of the surface features of the star, with the inclusionof the photometric data producing an increase (compared with the use ofspectroscopic data alone) in the spot occupancy in both low- andmid-latitude regions of the star. The maps show that HD 307938 possessesa large, broken polar spot extending down to ~60° latitude, as wellas lower-latitude spots similar to other rapidly rotating G dwarfs. Byincorporating a solar-like differential rotation law into the imagingprocess the surface differential rotation of HD 307938 was determined.This gave a surface shear of dΩ= 0.025 +/- 0.015 radd-1 (for an inclination angle of 60°). Thus the equatorof HD 307938 laps the poles every ~250 d and has a photospheric sheararound half that of the Sun.
| Improved astrophysical parameters for the overcontact binary FG Hydrae
Photometric data on FG Hya obtained in 2002 and 2004 are presented.Three data sets show the exchange between A-type, W-type and thevariable O'Connell effects. The photometric mass ratio (q= 0.1115 +/-0.0003) derived from B and V light curves is almost the same as thespectroscopic mass ratio (qsp= 0.112 +/- 0.004). The newphotometric solutions reveal that FG Hya is a deep overcontact binarysystem (f= 85.6 +/- 1.8 per cent) with a spotted massive component. Aperiod investigation, based on all available photoelectric or CCD timesof light minimum, shows that the O-C curve of FG Hya can be explained asa combination of a secular period decrease and a cyclic variation with aperiod of 36.4yr and an amplitude of 0.0289 d. By comparing thevariation of the depth of the primary minimum with the change of thecyclic period, it is discovered that both of them may vary with the samecycle length of 36.4yr and in the same phase. The variation of the lightcurve, the spotted primary component and the connection between thecyclic period change and the depth of the primary minimum, all maysuggest that the G0-type component displays solar-type magnetic activitywith a 36.4-yr cycle length. The long-time period decrease isinterpreted by mass transfer from the more massive component to the lessmassive one or/and angular momentum loss due to mass outflow from theouter Lagrangian point.
| Prominence Mapping of the RS CVn system HR 1099
We investigate temporal fluctuations in the H α emission profilesof the RS CVn system HR 1099 from a monitoring using the MuSiCoSspectropolarimeter (Observatoire du Pic du Midi, France) in 2001,between December 01 and December 18. Part of the observed emissionfluctuations is consistent with rotational modulation, which weinterpret as the spectral signature of a dense and complex prominencesystem trapped in the magnetosphere of HR 1099 and forced to co-rotatewith the binary system. The distribution of emitting material is mappedby means of Doppler tomography. We discuss the evolution of prominencesover the observing window.
| Stellar Activity on the Young Suns of Orion: COUP Observations of K5-7 Pre-Main-Sequence Stars
In 2003 January, the Chandra Orion Ultradeep Project (COUP) detectedabout 1400 young stars during a 13.2 day observation of the Orion NebulaCluster (ONC). This paper is a study of the X-ray properties of awell-defined sample of 28 solar-mass ONC stars based on COUP data. Ourgoals are to characterize the magnetic activity of analogs of the youngSun and thereby to improve understanding of the effects of solar X-rayson the solar nebula during the era of planet formation. Given the lengthof the COUP observation we are able to clearly distinguishcharacteristic and flare periods for all stars. We find that activeyoung suns spend 70% of their time in a characteristic state withrelatively constant flux and magnetically confined plasma withtemperatures kT2~=2.1×kT1. Duringcharacteristic periods, the 0.5-8 keV X-ray luminosity is about 0.03% ofthe bolometric luminosity. One or two powerful flares per week with peakluminosities logLX~30-32 ergs s-1 are typicallysuperposed on this characteristic emission accompanied by heating of thehot plasma component from ~=2.4 to ~=7 keV at the flare peak. The energydistribution of flares superposed on the characteristic emission levelfollows the relationship dN/dE~E-1.7. The flare rates areconsistent with the production of sufficiently energetic protons tospawn a spallogenic origin of some important short-lived radionuclidesfound in ancient meteorites. The X-rays can ionize gas in thecircumstellar disk at a rate of 6×10-9 ionizations persecond at 1 AU from the central star, orders of magnitude abovecosmic-ray ionization rates. The estimated energetic particle fluencesare sufficient to account for many isotopic anomalies observed inmeteoritic inclusions.
| Stellar Lyα Emission Lines in the Hubble Space Telescope Archive: Intrinsic Line Fluxes and Absorption from the Heliosphere and Astrospheres
We search the Hubble Space Telescope (HST) archive for previouslyunanalyzed observations of stellar H I Lyα emission lines, ourprimary purpose being to look for new detections of Lyα absorptionfrom the outer heliosphere and to also search for analogous absorptionfrom the astrospheres surrounding the observed stars. The astrosphericabsorption is of particular interest because it can be used to studysolar-like stellar winds that are otherwise undetectable. We find andanalyze 33 HST Lyα spectra in the archive. All the spectra weretaken with the E140M grating of the Space Telescope Imaging Spectrograph(STIS) instrument on board HST. The HST STIS spectra yield four newdetections of heliospheric absorption (70 Oph, ξ Boo, 61 Vir, and HD165185) and seven new detections of astrospheric absorption (EV Lac, 70Oph, ξ Boo, 61 Vir, δ Eri, HD 128987, and DK UMa), doubling theprevious number of heliospheric and astrospheric detections. Whencombined with previous results, 10 of 17 lines of sight within 10 pcyield detections of astrospheric absorption. This high detectionfraction implies that most of the ISM within 10 pc must be at leastpartially neutral, since the presence of H I within the ISM surroundingthe observed star is necessary for an astrospheric detection. Incontrast, the detection percentage is only 9.7% (3 out of 31) for starsbeyond 10 pc. Our Lyα analyses provide measurements of ISM H I andD I column densities for all 33 lines of sight, and we discuss someimplications of these results. Finally, we measure chromosphericLyα fluxes from the observed stars. We use these fluxes todetermine how Lyα flux correlates with coronal X-ray andchromospheric Mg II emission, and we also study how Lyα emissiondepends on stellar rotation.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| Stellar Coronal Abundances at Intermediate-Activity Levels: ξ UMa
We present an analysis of Chandra High Energy Transmission Grating(HETG) spectra of the corona of the critical intermediate-activity,quadruple stellar system ξ UMa. Using the maximum A/B subsystemseparation in 2001, we attempted to resolve for the first time in X-raysthe two components using the HETG Medium Energy Grating (MEG). We foundthe Aa component of the system to be at least 2 orders of magnitudefainter than the Bb component. We used the Markov Chain Monte Carlomethod to reconstruct differential emission measures (DEMs) andabundances and discuss first ionization potential (FIP) issues. Thereconstructed DEMs showed two pronounced peaks at logT~6.5 and logT~7.0K, respectively, similar to brighter solar active regions. A plot ofcoronal abundances, with respect to stellar photospheric, versus FIPreveals that the FIP behavior of ξ UMa is intermediate between lessand more active stars, commensurate with its X-ray luminosity. Theabundances of Mg, Fe, and Si are just subsolar, with Fe having thelowest abundance of these. The low-FIP elements Na and Al have similarabundances, and they are both higher than the Mg, Fe, and Si group. Theabundances show a minimum at S, rising gradually to Ne, which is higherthan photospheric, as has been seen by other authors on similar stars.Some of the FIP behavior seen here is predicted by the new ponderomotiveforce model of Laming.
| An XMM-Newton Study of the Coronae of σ2 Coronae Borealis
We present results of XMM-Newton Guaranteed Time observations of the RSCVn binary σ2 Coronae Borealis. The spectra obtainedwith the Reflection Grating Spectrometers and the European PhotonImaging Camera MOS2 were simultaneously fitted with collisionalionization equilibrium plasma models to determine coronal abundances ofvarious elements. Contrary to the solar first ionization potential (FIP)effect, in which elements with a low FIP are overabundant in the coronacompared to the solar photosphere, and contrary to the ``inverse'' FIPeffect observed in several active RS CVn binaries, coronal abundanceratios in σ2 CrB show a complex pattern, as supportedby similar findings in the Chandra HETGS analysis of σ2CrB with a different methodology by Osten and coworkers in 2003. Low-FIPelements (<10 eV) have abundance ratios relative to Fe that areconsistent with the solar photospheric ratios, whereas high-FIP elementshave abundance ratios that increase with increasing FIP. We find thatthe coronal Fe abundance is consistent with the stellar photosphericvalue, indicating that there is no metal depletion inσ2 CrB. However, we obtain a higher Fe absoluteabundance than Osten and coworkers did. Except for Ar and S, ourabsolute abundances are about 1.5 times larger than those reported byOsten and coworkers. However, a comparison of their model with ourXMM-Newton data (and vice versa) shows that both models work adequatelyin general. We find, therefore, no preference for one methodology overthe other for deriving coronal abundances. Despite the systematicdiscrepancy in absolute abundances, our abundance ratios are very closeto those obtained by Osten and coworkers. Finally, we confirm themeasurement of a low density in O VII (<4×1010cm-3) but could not confirm the higher densities measured inspectral lines formed at higher temperatures that were derived by otherstudies of σ2 CrB due to the lower spectral resolutionof the XMM-Newton grating spectrometers.
| A Far-Ultraviolet Spectroscopic Survey of Luminous Cool Stars
The Far Ultraviolet Spectroscopic Explorer (FUSE) ultraviolet spectra ofeight giant and supergiant stars reveal that high-temperature(3×105 K) atmospheres are common in luminous cool starsand extend across the color-magnitude diagram from α Car (F0 II)to the cool giant α Tau (K5 III). Emission present in thesespectra includes chromospheric H Lyβ, Fe II, C I, and transitionregion lines of C III, O VI, Si III, and Si IV. Emission lines of FeXVIII and Fe XIX signaling temperatures of ~107 K and coronalmaterial are found in the most active stars, β Cet and 31 Com. Ashort-term flux variation, perhaps a flare, was detected in β Cetduring our observation. Stellar surface fluxes of the emission of C IIIand O VI are correlated and decrease rapidly toward the cooler stars,reminiscent of the decay of magnetically heated atmospheres. Profiles ofthe C III λ977 lines suggest that mass outflow is underway atT~80,000 K and the winds are warm. Indications of outflow at highertemperatures (3×105 K) are revealed by O VI asymmetriesand the line widths themselves. High-temperature species are absent inthe M supergiant α Ori. Narrow fluorescent lines of Fe II appearin the spectra of many giants and supergiants, apparently pumped by HLyα, and formed in extended atmospheres. Instrumentalcharacteristics that affect cool star spectra are discussed.
| The Coronae of AB Doradus and V471 Tauri: Primordial Angular Momentum versus Tidal Spin-up
The zero-age main-sequence star AB Dor and the K dwarf component of theV471 Tau close binary have essentially identical rotation rates andspectral types. An analysis of their high-resolution Chandra X-rayspectra reveals remarkably similar coronal characteristics in terms ofboth temperature structure and element abundances. Both stars showdepletions of low first ionization potential (FIP) elements by factorsof ~3, with higher FIP elements showing more mild depletions. Noevidence for enhancements of very low FIP (<7 eV) elements, such asNa, Al, and Ca, as compared to other low-FIP elements, was found. Theabundance anomaly pattern for AB Dor and V471 Tau is similar to,although less extreme than, the abundance anomalies exhibited by activeRS CVn-type binaries. While we find statistically significant structurein the underlying differential emission measure distributions of thesestars over narrow temperature intervals, this structure is stronglydependent on the lines used in the analysis and is probably spurious. Onthe basis of their X-ray similarities, we conclude that the exactevolutionary state of a star has little effect on coronalcharacteristics and that the parameters that dominate coronal structureand composition are simply the rotation rate and spectral type.
| Inferring Coronal Structure from X-Ray Light Curves and Doppler Shifts: A Chandra Study of AB Doradus
The Chandra X-Ray Observatory continuously monitored the single coolstar AB Dor for a period lasting 88 ks (1.98Prot) in 2002December with the Low-Energy Transmission Grating HRC-S. The X-ray lightcurve shows rotational modulation with three peaks that repeat in twoconsecutive rotation cycles. These peaks may indicate the presence ofcompact emitting regions in the quiescent corona. Centroid shifts as afunction of phase in the strongest line profile, O VIII λ18.97,indicate Doppler rotational velocities with a semiamplitude of 30+/-10km s-1. By taking these diagnostics into account along withconstraints on the rotational broadening of line profiles (provided byarchival Chandra High-Energy Transmission Grating Fe XVII and FarUltraviolet Spectroscopic Explorer Fe XVIII profiles), we can constructa simple model of the X-ray corona that requires two components. One ofthese components is responsible for 80% of the X-ray emission and arisesfrom the pole and/or a homogeneously distributed corona. The secondcomponent consists of two or three compact active regions that causemodulation in the light curve and contribute to the O VIII centroidshifts. These compact regions account for 16% of the emission and arelocated near the stellar surface with heights of less than0.3R*. At least one of the compact active regions is locatedin the partially obscured hemisphere of the inclined star, while anotherof the active regions may be located at 40°. High-quality X-ray datasuch as these can test the models of the coronal magnetic fieldconfiguration as inferred from magnetic Zeeman Doppler imaging.
| Direct Evidence for a Polar Spot on SV Camelopardalis
We have used spectrophotometric data from the Hubble Space Telescope(HST) to eclipse-map the primary component of the RS CVn binary SV Camover nine HST orbits. We find from these observations and the Hipparcosparallax that the surface flux in the eclipsed low-latitude region ofthe primary is about 30% lower than that computed from a PHOENIX modelatmosphere at the effective temperature that best fits the spectralenergy distribution of the eclipsed flux. This can only be accounted forif about a third of the primary's surface is covered with unresolveddark starspots. Extending this to the full surface of the primary, wefind that, even taking into account this spot-filling factor, there isan additional flux deficit on the primary star. This can only beexplained if there is a large polar spot on the primary star extendingfrom the pole to latitude 48deg+/-6deg.
| Evidence for Superhumps in the Radio Light Curve of Algol and a New Model for Magnetic Activity in Algol Systems
Extensive radio data of two Algol systems and two RS CVn binaries werereanalyzed. We found evidence for a new periodicity that we interpret asa superhump in β Per (Algol), for which it may have been expectedaccording to its semidetached nature and low binary mass ratio. Theconcluded presence of an accretion disk (or an annulus) is consistentwith previous studies of optical data and numerical simulations ofβ Per. In our model, the 50 day period, previously found in theradio data of β Per, is explained as the apsidal precession of theelliptical accretion disk or annulus. If our interpretation is correct,this is the first detection of the superhump phenomenon in the radio andthe first observation of superhumps in Algol systems. According to ourresult, the accretion disk or annulus in β Per precesses in spiteof its non-Keplerian nature, and therefore, this phenomenon is notrestricted to the classical Keplerian accretion disks in compactbinaries. We propose that in Algol systems with short orbital periods,which have accretion disks or annuli, the disk is magnetically active inaddition to the cool secondary star. The magnetic field in the diskoriginates from amplification of the seed field in the magnetizedmaterial transferred from the secondary. The disk and stellar fieldsinteract with each other, with reconnection of the field lines causingflares and particle acceleration. Relativistic particles are trapped inthe field and directed toward the polar regions of the secondary starbecause of the dipole structure of its magnetic field. These systemsare, therefore, somewhat analogous to the RS CVn systems, which have twomagnetically interacting stars. Our proposed model for the magneticactivity in Algol systems provides a simple explanation for the observedproperties of β Per in the radio wavelengths, and in particular,for the presence of quiescent gyrosynchrotron emission near the polarregion of the secondary star, where electrons are difficult to confineif the field lines are open as in normal, single, magnetic stars. Itfurther explains the recent discovery that the Doppler shifts of theemission lines in the X-ray data of β Per are somewhat lower thanwhat is expected from the orbital motion of the secondary star. Wepropose that the superhump variation in the radio is generated byenhanced reconnection when the elongated side of the elliptic accretiondisk is the closest to the cool star. This leads to flares andenhancement in particle acceleration and is manifested as strongergyrosynchrotron radiation. The observed superhump period, at3.037+/-0.013 days (~6% longer than the orbital period), was used todeduce a binary mass ratio of ~0.23 in β Per, which is consistentwith previous studies. Our finding opens the possibility to extend thesuperhump phenomenon to Algol systems and to test the theories ofprecessing accretion disks in various types of interacting binaries. Itprovides a new method to estimate the mass ratios in these binaries. Italso offers new insights and improves our understanding of thecomplicated magnetic interaction and feedback between mass transferdynamics, time-dependent disk accretion, and induced magnetic activityin the Algol and related systems.
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Pozíciós és asztrometriai adatok
| Csillagkép: | Bika |
| Rektaszcenzió: | 03h36m47.30s |
| Deklináció: | +00°35'16.0" |
| Vizuális fényesség: | 5.71 |
| Távolság: | 28.969 parszek |
| RA sajátmozgás: | -21.4 |
| Dec sajátmozgás: | -162.3 |
| B-T magnitude: | 7.005 |
| V-T magnitude: | 6 |
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