Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
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.
| 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.
| New Relativistic Atomic Data for Fe IX
A large-scale configuration interaction (CI) calculation using ProgramCIV3 of Hibbert is performed for the lowest 87 fine-structure levels ofargon-like iron ions. We have calculated energy levels, oscillatorstrengths, and transition probabilities for all electric-dipole-allowedand intercombination transitions between the levels of3s23p6, 3s23p53d,3s3p63d, 3s23p54s,3s23p54p, 3s23p54d,3s3p64s, 3s23p54f, 3s3p64p,3s23p55s, 3s23p55p, and3s23p55d states of Fe IX in the LSJ couplingscheme. Lifetimes of the 3d levels are also determined. The calculationsinclude the major correlation effects. We attempt to correct theinaccuracies in the CI coefficients in the wave functions, which wouldlead to inaccuracy in transition probabilities by applying a``fine-tuning'' technique. The relativistic effects are incorporated byadding mass correction, Darwin term, and spin-orbit interaction terms tothe nonrelativistic Hamiltonian in the Breit-Pauli approximations. Thepresent results are in good agreement with other available calculationsand experiments. We predict new data for several levels where no othertheoretical or experimental results are available. We hope our extensivecalculations will be useful to experimentalists in identifying thefine-structure levels in their future work.
| The Far Ultraviolet Spectroscopic Explorer Survey of O VI Emission in the Milky Way
We present a survey of O VI λ1032 emission in the Milky Way usingdata from the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite.The observations span the period from launch in 1999 to 2003 July. Oursurvey contains 112 sight lines, 23 of which show measurable O VIλ1032 emission. The O VI λ1032 emission feature wasdetected at all latitudes and exhibits intensities of 1900-8600 photonss-1 cm-2 sr-1. Combined with valuesfrom the literature, these emission measurements are consistent with thepicture derived from recent O VI absorption surveys: high-latitude sightlines probe O VI-emitting gas in a clumpy, thick disk or halo, whilelow-latitude sight lines sample mixing layers and interfaces in the thindisk of the Galaxy.
| Soft X-Ray Emission Lines of Fe XV in Solar Flare Observations and the Chandra Spectrum of Capella
Recent calculations of atomic data for Fe XV have been used to generatetheoretical line ratios involving n=3-4 transitions in the soft X-rayspectral region (~52-83 Å), for a wide range of electrontemperatures and densities applicable to solar and stellar coronalplasmas. A comparison of these with solar flare observations from arocket-borne spectrograph (X-Ray Spectrometer/Spectrograph Telescope[XSST]) reveals generally good agreement between theory and experiment.In particular, the 82.76 Å emission line in the XSST spectrum isidentified, for the first time to our knowledge in an astrophysicalsource, as the3s3d3D3-3s4p3P2 transitionof Fe XV. Most of the Fe XV transitions that are blended have had thespecies responsible clearly identified, although there remain a fewinstances in which this has not been possible. The line ratiocalculations are also compared with a co-added spectrum of Capellaobtained with the Chandra satellite, which is probably the highestsignal-to-noise ratio observation achieved for a stellar source in the~25-175 Å soft X-ray region. Good agreement is found betweentheory and experiment, indicating that the Fe XV lines are reliablydetected in Chandra spectra and hence may be employed as diagnostics todetermine the temperature and/or density of the emitting plasma.However, the line blending in the Chandra data is such that individualemission lines are difficult to measure accurately, and fluxes may onlybe reliably determined via detailed profile fitting of the observations.The co-added Capella spectrum is made available to hopefully encouragefurther exploration of the soft X-ray region in astronomical sources.
| Chandra Detection of Local Warm-Hot Gas toward Markarian 279
We report the Chandra detection of O VII Kα absorption at z=0 inthe direction of the z=0.03 Seyfert 1 galaxy Mrk 279. The high-velocitycloud Complex C lies along this line of sight, with H I 21 cm emissionand O VI λ1032 absorption both observed at velocities ofroughly-150 km s-1 relative to the local standard of rest. Wepresent an improved method for placing limits on the Doppler parameterand column density of a medium when only one unresolved line can bemeasured; this method is applied to the O VII absorption seen here,indicating that the O VII Doppler parameter is inconsistent with that ofany low-velocity (Galactic thick disk) or high-velocity O VI [(OVI)HV] component. Direct association of the O VII with the (OVI)HV is further ruled out by the high temperatures requiredto produce the observed (O VI)HV/O VII ratio and thesignificant velocity difference between the O VII and (OVI)HV lines. If the O VII absorption is associated with avery broad, undetected O VI component, then the absorption must bebroadened by primarily nonthermal processes. The large velocitydispersion and possible slight redshift of the O VII absorption (as wellas limits on the absorber's temperature and density) may be indicativeof a local intergalactic medium origin, although absorption from a hot,low-density Galactic corona cannot be ruled out.
| Chandra/HETGS Observations of the Capella System: The Primary as a Dominating X-Ray Source
Using the Chandra/High Energy Transmission Grating Spectrometer (HETGS),we have detected the Doppler motion of Capella's X-ray emission lines inthe 6-25 Å wave band. The observed motion follows the expectedorbital motion of Capella's primary. This finding implies that theprimary G8 III star, not the secondary G1 III star in the Hertzsprunggap, has been the dominant source of hot 106.8-107K plasma at least in the last 6 years. In addition, the resultsdemonstrate the long-term stability of the HETGS, and they demonstratesmall uncertainties of 25 and 33 km s-1 in the velocitydetermination with the High and Medium Energy Gratings (HEG and MEG),respectively.
| 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.
| 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
| Coronal Density Diagnostics with Si X: Chandra LETGS Observations of Procyon, α Centauri A and B, Capella, and ɛ Eridani
Electron density diagnostics based on the line intensity ratio of Si Xare applied to the X-ray spectra of Procyon, α Cen A and B,Capella, and ɛ Eri measured with the Low Energy TransmissionGrating Spectrometer combined with the High Resolution Camera on boardthe Chandra X-Ray Observatory. The ratio R1 of theintensities of the Si X lines at 50.524 and 50.691 Å is adopted. Acertain temperature effect in R1 appears near the low-densitylimit region, which is due to the contamination of the Si X line at50.703 Å. Using the emission measure distribution model derived byAudard and coworkers for Capella and emissivities calculated with theAstrophysical Plasma Emission Code model by Smith and coworkers, wesuccessfully estimate the contributions of the Fe XVI lines at 50.367and 50.576 Å (73% and 62%, respectively). A comparison between theobserved ratios and theoretical predictions constrains the (logarithmic)electron densities for Procyon to be8.61+0.24-0.20 cm-3, while for αCen A and B, Capella, and ɛ Eri they are8.81+0.27-0.23,8.60+0.39-0.32, 9.30-0.48, and9.11+1.40-0.38 cm-3, respectively. Thecomparison of our results with those constrained by the triplet ofHe-like carbon shows good agreement. For normal stars, our resultsdisplay a narrow uncertainty, while for active stars, a relativelylarger uncertainty due to contamination from Fe XVI lines is found.Another possible reason may be the uncertainty of the continuum level,since the emission lines of Si X become weak for active stars. Forɛ Eri, an electron density in the C V-forming region was firstestimated through Si X emission.
| A Mid-Infrared Study of the Class 0 Cluster in LDN 1448
We present ground-based mid-infrared observations of Class 0 protostarsin LDN 1448. Of the five known protostars in this cloud, we detectedtwo, L1448N:A and L1448C, at 12.5, 17.9, 20.8, and 24.5 μm, and athird, L1448 IRS 2, at 24.5 μm. We present high-resolution images ofthe detected sources and photometry or upper limits for all five Class 0sources in this cloud. With these data we are able to augment existingspectral energy distributions for all five objects and place them on anevolutionary status diagram.
| A compact flare eclipsed in the corona of SV Camelopardalis
The eclipsing active binary SV Cam (G0V/K6V, Porb=0.593071 d)was observed with XMM-Newton during two campaigns in 2001 and 2003. Noeclipses in the quiescent emission are clearly identified, but a flarewas eclipsed during the 2001 campaign, allowing us to stronglyconstrain, from purely geometrical considerations, the position and sizeof the event: the flare is compact and it is formed at a latitude below65°. The size, temperature and Emission Measure of the flare implyan electron density of log ne (cm-3)10.6{-}13.3 and a magnetic field of 65-1400 G in order to confine theplasma, consistent with the measurements that are obtained fromdensity-sensitive line ratios in other similar active stars. Averageemission seems to come from either extended or polar regions because oflack of eclipses. The Emission Measure Distribution, coronal abundancesand characteristics of variability are very similar to other activestars such as AB Dor (K1V).
| 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.
| An X-ray emission-line spectrum of Nova V382Velorum 1999
We report on the analysis of an X-ray grating spectrum of the ClassicalNova V382Vel (1999), obtained with the Low Energy Transmission Grating(LETG)+HRC-S instrument onboard Chandra, which shows emission linesdominating over any continuum. Lines of Si, Mg, Ne, O, N and C areidentified, but no Fe lines are detected. The total luminosity in thelines is ~4 × 1027ergs-1 (corrected forNH= 1.2 × 1021cm-2). The lineshave broad profiles with full width at half-maximum corresponding to avelocity ~2900 +/-200kms-1. Some structure is identified inthe profiles, but for different elements we find different profilestructures. While lines of O show a broadened Gaussian profile, those ofNe are double-peaked, suggesting a fragmented emitting plasma. Using theemission measure distribution, we derive relative element abundances andfind abundances of Ne and N that are significantly enhanced relative tothat of O, while Fe is not overabundant. The lack of any source emissionlongwards of 50Åand the OVIIILyα/Lyβ line ratio supports previousvalues of the hydrogen column density. We find weak continuum emissionfrom the white dwarf, consistent with a blackbody spectrum with an upperlimit to the temperature of T= 3 × 105K, assuming asource radius of 6000km. The upper limit for the integrated blackbodyluminosity is 2 × 1036ergs-1. The BeppoSAXand Chandra ACIS observations of V382Vel show that the nova was brightand in the Super-Soft phase as late as 1999 December 30. Our LETGobservation obtained 6 weeks later, as well as all subsequent X-rayobservations, showed a remarkable fading to a nearly pure emission linephase which suggests that nuclear burning on the white dwarf had turnedoff by February. In the absence of a photoionizing source, the emissionlines were formed in a collisionally ionized and excited expandingshell.
| Visual Star Colours from Instrumental Photometry
In order to display graphically the visual colours of stars and otherastronomical objects, photometric broadband R, V, B colours are used toproxy for the r, g, b colours of the three visual sensors of the eye.From photometric Johnson B-V and V-R colour indices, R, V, and Bmagnitudes (V = 0) are calculated, and from these the respectivebrightnesses (r, v = 1 = g, and b) are calculated. After suitablenormalization these are then placed in a ternary diagram having r, g,and b as the vertices. All B-V and V-R are adjusted so that the Sunfalls in the same place as a blackbody at 5800 K. The resulting ternaryplot shows all of its objects (stars, planets) in their visual coloursat their relative positions in the ternary diagram. The star coloursdisplayed on a computer monitor screen or as a print with a colourprinter are more vivid than the usual visual impressions of isolatedstars, undoubtedly because of properties of the dark-adapted eye, butdouble-star pairs with contrasting colours correspond nicely totelescopic visual impressions.
| Wer beobachtet mit: BK Aurigae.
Not Available
| 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.
| Accurate Wavelength Measurements and Modeling of Fe XV to Fe XIX Spectra Recorded in High-Density Plasmas between 13.5 and 17 Å
Iron spectra have been recorded from plasmas created at three differentlaser plasma facilities: the Tor Vergata University laser in Rome(Italy), the Hercules laser at ENEA in Frascati (Italy), and the CompactMultipulse Terawatt (COMET) laser at LLNL in California (USA). Themeasurements provide a means of identifying dielectronic satellite linesfrom Fe XVI and Fe XV in the vicinity of the strong 2p-->3dtransitions of Fe XVII. About 80 Δn>=1 lines of Fe XV (Mg-like)to Fe XIX (O-like) were recorded between 13.8 and 17.1 Å with ahigh spectral resolution (λ/Δλ~4000) about 30 ofthese lines are from Fe XVI and Fe XV. The laser-produced plasmas hadelectron temperatures between 100 and 500 eV and electron densitiesbetween 1020 and 1022 cm-3. The HebrewUniversity Lawrence Livermore Atomic Code (HULLAC) was used to calculatethe atomic structure and atomic rates for Fe XV-XIX. HULLAC was used tocalculate synthetic line intensities at Te=200 eV andne=1021 cm-3 for three differentconditions to illustrate the role of opacity: optically thin plasmaswith no excitation-autoionization/dielectronic recombination (EA/DR)contributions to the line intensities, optically thin plasmas thatincluded EA/DR contributions to the line intensities, and opticallythick plasmas (optical depth ~200 μm) that included EA/DRcontributions to the line intensities. The optically thick simulationbest reproduced the recorded spectrum from the Hercules laser. However,some discrepancies between the modeling and the recorded spectra remain.
| Measurement of Absolute Cross Sections for Excitation of the 2s2 1S --> 2s2p 1Po Transition in O+4
Experimental cross sections are reported for the1s22s21S-->1s22s2p1Potransition in O+4 located at 19.689 eV. Use is made of theelectron energy-loss method, using a merged electron-ion beam geometry.The center-of-mass interaction energies for the measurements in the1S-->1Po transition are in the range18 eV (below the threshold) to 30 eV. Data are compared with otherprevious electron energy-loss measurements and with results of a 26 termR-matrix calculation that includes fine structure explicitly via theBreit-Pauli Hamiltonian. Clear resonance enhancement is observed in allexperimental and theoretical results near the threshold for this1S-->1Po transition.
| Bulk Velocities, Chemical Composition, and Ionization Structure of the X-Ray Shocks in WR 140 near Periastron as Revealed by the Chandra Gratings
The Wolf-Rayet WC7+O4-5 binary WR 140 went through the periastronpassage of its 8 yr eccentric binary orbit in early 2001 as the twostars made their closest approach. Both stars have powerful supersonicstellar winds that crash into each other between the stars to produceX-rays. Chandra grating observations were made when the X-rays were attheir peak, making WR 140 the brightest hot-star X-ray source in the skyand giving the opportunity to study the velocity profiles of lines, allof which were resolved and blueshifted before periastron. In the generalcontext of shock physics, the measurements constrain the flow of hot gasand where different ions were made. The brightness of lines relative tothe strong continuum in conjunction with plasma models gives interimabundance estimates for eight different elements in WC-type materialincluding an Ne/S ratio in good agreement with earlier long-wavelengthmeasurements. The lower velocity widths of cool ions imply a plasma thatwas not in equilibrium, probably due to the collisionless nature of theshock transitions and the slow character of both the postshock energyexchange between ions and electrons and subsequent ionization. Electronheat conduction into fast-moving preshock gas was absent, probablysuppressed by the magnetic field involved in WR 140's synchrotronemission. After periastron, the spectrum was weaker due mainly toabsorption by cool Wolf-Rayet star material.
| Can Life Develop in the Expanded Habitable Zones around Red Giant Stars?
We present some new ideas about the possibility of life developingaround subgiant and red giant stars. Our study concerns the temporalevolution of the habitable zone. The distance between the star and thehabitable zone, as well as its width, increases with time as aconsequence of stellar evolution. The habitable zone moves outward afterthe star leaves the main sequence, sweeping a wider range of distancesfrom the star until the star reaches the tip of the asymptotic giantbranch. Currently there is no clear evidence as to when life actuallyformed on the Earth, but recent isotopic data suggest life existed atleast as early as 7×108 yr after the Earth was formed.Thus, if life could form and evolve over time intervals from5×108 to 109 yr, then there could behabitable planets with life around red giant stars. For a 1Msolar star at the first stages of its post-main-sequenceevolution, the temporal transit of the habitable zone is estimated to beseveral times 109 yr at 2 AU and around 108 yr at9 AU. Under these circumstances life could develop at distances in therange 2-9 AU in the environment of subgiant or giant stars, and in thefar distant future in the environment of our own solar system. After astar completes its first ascent along the red giant branch and the Heflash takes place, there is an additional stable period of quiescent Hecore burning during which there is another opportunity for life todevelop. For a 1 Msolar star there is an additional109 yr with a stable habitable zone in the region from 7 to22 AU. Space astronomy missions, such as proposed for the TerrestrialPlanet Finder (TPF) and Darwin, that focus on searches for signatures oflife on extrasolar planets, should also consider the environments ofsubgiants and red giant stars as potentially interesting sites forunderstanding the development of life. We performed a preliminaryevaluation of the difficulty of interferometric observations of planetsaround red giant stars compared to a main-sequence star environment. Weshow that pathfinder missions for TPF and Darwin, such as Eclipse andFKSI, have sufficient angular resolution and sensitivity to search forhabitable planets around some of the closest evolved stars of thesubgiant and red giant class.
| Radio Photosphere and Mass-Loss Envelope of VY Canis Majoris
We have used the VLA to detect emission from the supergiant VY CMa atradio wavelengths and have constructed 3000-4500 K isothermal outeratmospheres constrained by the data. These models produce a radiophotosphere at 1.5-2 R*. An extrapolation of the model canaccount for the observed total mass-loss rate of the star. We alsopresent mid-infrared imaging of the supergiant which suggests that warmdust is extended in the same direction as the near-infrared reflectionnebula around VY CMa. The origin of the asymmetries in the outflowremains an unsolved problem.
| An Assessment of the Fe XVIII and Fe XIX Line Ratios from the Chandra Grating Observations of Capella
Observations of Fe XVIII and Fe XIX X-ray, extreme-UV, and far-UV lineemission, formed at the peak of Capella's (α Aurigae's) emissionmeasure distribution and ubiquitous in spectra of many cool stars andgalaxies, provide a unique opportunity to test the robustness of FeXVIII and Fe XIX spectral models. The Astrophysical Plasma Emission Code(APEC) is used to identify over 35 lines from these two ions alone, andto compare model predictions with spectra obtained with the Chandra LowEnergy Transmission Grating and High Energy Transmission GratingSpectrometers, the Far Ultraviolet Spectroscopic Explorer (FUSE), andthe Extreme Ultraviolet Explorer. Some flux discrepancies larger than afactor of 2 are found between observations of Fe XVIII and Fe XIX linesand predictions by APEC and other models in common use. In particular,the X-ray resonance lines for both ions are stronger than predicted byall models relative to the EUV resonance lines. The multiwavelengthobservations demonstrate the importance of including dielectronicrecombination and proton-impact excitation, and of using accuratewavelengths in spectral codes. These ions provide important diagnostictools for 107 K plasmas currently observed with Chandra,XMM-Newton, and FUSE.
| O/H in the Local Bubble
We present new measurements of the oxygen gas-phase abundance along foursight lines probing gas inside the Local Bubble, using data obtainedwith the Far Ultraviolet Spectroscopic Explorer (FUSE) and the HubbleSpace Telescope (HST). Combining these with seven previously publishedN(O I) and N(H I) measurements, we determine a mean O/H ratio for theLocal Bubble, (O/H)LB=(3.45+/-0.19)×10-4 (1σ in the mean). Our result is virtually identical toO/H=(3.43+/-0.15)×10-4 derived from data for sightlines probing gas well outside the Local Bubble. In contrast to the D/Oand D/H ratios, which seem to have different values beyond the LocalBubble, our results show that the O/H ratio is constant and singlevalued both inside and outside the Local Bubble for low-density sightlines with column densities up to log N(H)~=21. In addition, thesimilarity of the two values above suggests that the net effect of theuncertain O I f-values in the FUSE bandpass is not significant. Takinginto account the latest determinations of the solar abundance of oxygen,our result implies that ~25% of the oxygen in the Local Bubble isdepleted onto dust grains. The similarity of the value for O/H along lowaverage density sight lines in the Local Bubble with that of densersight lines beyond may permit a limit on the diluting effects ofinfalling low-metallicity gas.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by The Johns HopkinsUniversity under NASA contract NAS5-32985. Based also on observationsmade with the NASA/ESA Hubble Space Telescope and obtained from the DataArchive at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555.
| Emission Measure Distribution in Loops Impulsively Heated at the Footpoints
This work is prompted by evidence of sharply peaked emission measuredistributions in active stars and by the claims of isothermal loops insolar coronal observations, at variance with the predictions ofhydrostatic loop models with constant cross section and uniform heating.We address the problem with loops heated at the footpoints. Since steadyheating does not allow static loop model solutions, we explore whetherpulse-heated loops can exist and appear as steady loops on a timeaverage. We simulate pulse-heated loops, using the Palermo-Harvard 1-Dhydrodynamic code, for different initial conditions corresponding totypical coronal temperatures of stars ranging from intermediate toactive [T~(3-10)×106 K]. We find long-livedquasi-steady solutions even for heating concentrated at the footpointsover a spatial region of the order of ~1/5 of the loop half-length andbroader. These solutions yield an emission measure distribution with apeak at high temperature, and the cool side of the peak is as steep as~T5, in contrast to the usual ~T3/2 of hydrostaticmodels with constant cross section and uniform heating. Such peaks aresimilar to those found in the emission measure distribution of activestars around 107 K.
| 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.
| Flare X-Ray Observations of AB Doradus: Evidence of Stellar Coronal Opacity
X-ray spectra of the late-type star AB Dor obtained with the XMM-Newtonsatellite are analyzed. AB Dor was particularly active during theobservations. An emission measure reconstruction technique is employedto analyze flare and quiescent spectra, with emphasis on the Fe XVII15-17 Å wavelength region. The Fe XVII 16.78 Å/15.01 Åline ratio increases significantly in the hotter flare plasma. Thischange in the ratio is opposite to the theoretical predictions and isattributed to the scattering of 15.01 Å line photons from the lineof sight. The escape probability technique indicates an optical depth of~0.4 for the 15.01 Å line. During the flare, the electron densityis 4.4+2.7-1.6×1010cm-3, and the fractional Fe abundance is 0.5+/-0.1 of thesolar photospheric value. Using these parameters, a path length of ~8000km is derived. There is no evidence of opacity in the quiescent X-rayspectrum of the star.
| 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.
| Infrared Imaging of Capella with the IOTA Closure Phase Interferometer
We present infrared aperture synthesis maps produced with the upgradedInfrared Optical Telescope Array interferometer. Michelsoninterferograms on the close binary system Capella (α Aur) wereobtained in the H band between 2002 November 12 and 16 using the IONIC3beam combiner. With baselines of 15m<=B<=38 m, we were able todetermine the relative position of the binary components withmilliarcsecond precision and to track their movement along the ~14°arc covered by our observation run. We briefly describe the algorithmsused for visibility and closure phase estimation. Three different hybridmapping and bispectrum fitting techniques were implemented within onesoftware framework and used to reconstruct the source brightnessdistribution. By dividing our data into subsets, the system could bemapped at three epochs, revealing the motion of the stars. The preciseposition of the binary components was also determined with model fits,which in addition revealed IAa/IAb=1.49+/-0.10 andapparent stellar uniform-disk diameters ofΘAa=8.9+/-0.6 mas and ΘAb=5.8+/-0.8mas. To improve the (u,v)-plane coverage, we compensated this orbitalmotion by applying a rotation-compensating coordinate transformation.The resulting model-independent map with a beam size of 5.4mas×2.6mas allows the resolution of the stellar surfaces of the Capella giantsthemselves.
| Deuterium Depletion and Magnesium Enhancement in the Local Disk
The local disk deuterium is known to be depleted in comparison to thelocal bubble. We used Hubble Space Telescope (HST) spectra to obtaincolumn densities of Si, Mg and Fe. We compared normalized columndensities of these elements in the directions with high and lowdeuterium abundances.We show, that the lines of sight that are depleted in deuterium, areenhanced in magnesium. This observation implicates that astration isresponsible for both deuterium depletion and magnesium enhancement.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
Cataloghi e designazioni:
|
