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Treasures of Orion.
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| Wind signatures in the X-ray emission-line profiles of the late-O supergiant ζ Orionis
X-ray line-profile analysis has proved to be the most direct diagnosticof the kinematics and spatial distribution of the very hot plasma aroundO stars. The Doppler-broadened line profiles provide information aboutthe velocity distribution of the hot plasma, while thewavelength-dependent attenuation across a line profile providesinformation about the absorption to the hot plasma, thus providing astrong constraint on its physical location. In this paper, we applyseveral analysis techniques to the emission lines in the Chandra HighEnergy Transmission Grating Spectrometer (HETGS) spectrum of the late-Osupergiant ζ Ori (O9.7 Ib), including the fitting of a simpleline-profile model. We show that there is distinct evidence forblueshifts and profile asymmetry, as well as broadening in the X-rayemission lines of ζ Ori. These are the observational hallmarks of awind-shock X-ray source, and the results for ζ Ori are very similarto those for the earlier O star, ζ Pup, which we have previouslyshown to be well fit by the same wind-shock line-profile model. The moresubtle effects on the line-profile morphologies in ζ Ori, ascompared to ζ Pup, are consistent with the somewhat lower densitywind in this later O supergiant. In both stars, the wind optical depthsrequired to explain the mildly asymmetric X-ray line profiles implyreductions in the effective opacity of nearly an order of magnitude,which may be explained by some combination of mass-loss rate reductionand large-scale clumping, with its associated porosity-based effects onradiation transfer. In the context of the recent reanalysis of thehelium-like line intensity ratios in both ζ Ori and ζ Pup, andalso in light of recent work questioning the published mass-loss ratesin OB stars, these new results indicate that the X-ray emission fromζ Ori can be understood within the framework of the standardwind-shock scenario for hot stars.
| Discovery of a strong magnetic field on the O star HD 191612: new clues to the future of θ1 Orionis C*
From observations made with the ESPaDOnS spectropolarimeter, recentlyinstalled on the 3.6-m Canada-France-Hawaii Telescope, we report thediscovery of a strong magnetic field in the Of?p spectrum variable HD191612 - only the second known magnetic O star (followingθ1 Ori C). The stability of the observed Zeemansignature over four nights of observation, together with thenon-rotational shape of line profiles, argues that the rotation periodof HD 191612 is significantly longer than the 9-d value previouslyproposed. We suggest that the recently identified 538-d spectralvariability period is the rotation period, in which case the observedline-of-sight magnetic field of -220 +/- 38 G implies a large-scalefield (assumed dipolar) with a polar strength of about -1.5 kG. Ifconfirmed, this scenario suggests that HD 191612 is, essentially, anevolved version of the near-zero-age main-sequence magnetic O starθ1 Ori C, but with an even stronger field (about 15 kGat an age similar to that of θ1 Ori C). We suggestthat the rotation rate of HD 191612, which is exceptionally slow byaccepted O-star standards, could be due to angular momentum dissipationthrough a magnetically confined wind.
| Winds from OB Stars: A Two-Component Scenario?
X-ray spectroscopy of several OB stars with massive winds has revealedthat many X-ray line profiles exhibit unexpectedly small blueshifts andare almost symmetric. Moreover, the hottest X-ray lines appear tooriginate closest to the star. These properties appear to beinconsistent with the standard model of X-rays originating in shockedmaterial in line-driven spherically symmetric winds. Here we raise thequestion, can the X-ray line data be understood in terms of atwo-component wind? We consider a scenario in which one component of thewind is a standard line-driven wind that emerges from a broad range oflatitudes centered on the equator. The second component of the windemerges from magnetically active regions in extensive polar caps. Theexistence of such polar caps is suggested by a recent model of dynamoaction in massive stars. We describe how the two-component model isconsistent with a variety of observational properties of OB star winds.
| Submillimeter imaging spectroscopy of the Horsehead nebula
We present 15 arcsecond resolution single-dish imaging of the Horseheadnebula in the Ci (1-0) and CO (4-3) lines, carried out using the CHAMParray at the Caltech Submillimeter Observatory (CSO). The data are usedtogether with supporting observations of the (2-1) transitions of the COisotopologues to determine the physical conditions in the atomic andmolecular gas via Photon Dominated Region (PDR) modeling. The CO(4-3)/(2-1) line ratio, which is an excellent tracer of the direction ofthe incoming UV photons, increases at the western and northern edges ofthe nebula, confirming that the illumination is provided mostly by thestars σ and ζ Orionis. The observed line intensities areconsistent with PDR models with an H nuclei volume density of 3{-} 7 × 104 cm-3. The models predict akinetic temperature of 12 K and a C18O fractional abundancewith respect to H atoms of 2.4 × 10-7 in the shieldedregion, which in turn imply a total molecular mass of 24 Mȯ in theC18O filament. The outer halo, devoid of C18O, buttraced by the Ci emission has a comparable density and contributesadditional 13 Mȯ of material, resulting in an upper limit of 37Mȯ for the total molecular mass of the nebula.
| 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
| Prospects for brown dwarf and extrasolar planet research with the GTC and the LMT
The topic of brown dwarf and extrasolar planet research with the GTC andthe LMT was discussed during a half day workshop at Universidad NacionalAutonoma de Mexico. This paper summarizes the contributions presented atthat Workshop plus additional contributions from members of the SpanishNetwork for extrasolar planets. Specific observational projects, whichcan be carried out with the planned instruments for the GTC, includingfirst light ones, are explained in some detail. These projects are thefollowing: our coolest neighbors, brown dwarfs in wide binaries, browndwarfs and planetary mass objects in Orion, follow-up of transitingextrasolar planets and verification of planet candidates, the search forultracool companions to nearby stars, brown dwarf binaries and disksaround brown dwarfs.
| Atlas and Catalog of Dark Clouds Based on Digitized Sky Survey I
We present a quantitative atlas and catalog of dark clouds derived byusing the optical database ``Digitized Sky Survey I''. Applying atraditional star-count technique to 1043 plates contained in thedatabase, we produced an AV map covering the entire region inthe galactic latitude range |b| ≤ 40°. The map was drawn at twodifferent angular resolutions of 6' and 18', and is shown in detail in aseries of figures in this paper. Based on the AV map, weidentified 2448 dark clouds and 2841 clumps located inside them. Somephysical parameters, such as the position, extent, and opticalextinction, were measured for each of the clouds and clumps. We alsosearched for counterparts among already known dark clouds in theliterature. The catalog of dark clouds presented in this paper lists thecloud parameters as well as the counterparts.
| The Horsehead nebula: A beautiful case
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| 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.
| X-Ray Study of Herbig Ae/Be Stars
We present ASCA results of intermediate-mass pre-main-sequence (PMS)stars, or Herbig Ae/Be (HAeBe) stars. Among the 35 ASCA pointed sources,we detect 11 plausible X-ray counterparts. X-ray luminosities of thedetected sources in the 0.5-10 keV band are in the range oflogLX~30-32 ergs s-1, which is systematicallyhigher than those of low-mass PMS stars. This fact suggests that thecontribution of a possible low-mass companion is not large. Most of thebright sources show significant time variation; in particular, two HAeBestars-MWC 297 and TY CrA-exhibit flarelike events with long decaytimescales (e-folding time ~10-60 ks). These flare shapes are similar tothose of low-mass PMS stars. The X-ray spectra are successfullyreproduced by an absorbed one- or two-temperature thin-thermal plasmamodel. The temperatures are in the range of kT~1-5 keV, significantlyhigher than those of main-sequence OB stars (kT<1 keV). These X-rayproperties are not explained by wind-driven shocks, but are more likelydue to magnetic activity. On the other hand, the plasma temperaturerises as absorption column density increases or as HAeBe stars ascend toearlier phases. The X-ray luminosity reduces after stellar age of a fewtimes 106 yr. X-ray activity may be related to stellarevolution. The age of the activity decay is apparently near thetermination of jet or outflow activity. We thus hypothesize thatmagnetic activity originates from the interaction of the large-scalemagnetic fields coupled to the circumstellar disk. We also discussdifferences in X-ray properties between HAeBe stars and main-sequence OBstars.
| The CIDA Variability Survey of Orion OB1. I. The Low-Mass Population of Ori OB1a and 1b
We present results of a large-scale, multiepoch optical survey of theOrion OB1 association, carried out with the QUEST camera at theVenezuela National Astronomical Observatory. We identify for the firsttime the widely spread low-mass, young population in the Ori OB1a andOB1b subassociations. Candidate members were picked up by theirvariability in the V band and position in color-magnitude diagrams. Weobtained spectra to confirm membership. In a region spanning ~68deg2, we found 197 new young stars; of these, 56 are locatedin the Ori OB1a subassociation and 141 in Ori OB1b. The spatialdistribution of the low-mass young stars is spatially coincident withthat of the high-mass members but suggests a much sharper edge to theassociation. Comparison with the spatial extent of molecular gas andextinction maps indicates that the subassociation Ori OB1b isconcentrated within a ringlike structure of radius ~2°(~15 pc at 440pc), centered roughly on the star ɛ Ori in the Orion belt. The ringis apparent in 13CO and corresponds to a region with anextinction AV>=1. The stars exhibiting strong Hαemission, an indicator of active accretion, are found along this ring,whereas the center is populated with weak Hα-emitting stars. Incontrast, Ori OB1a is located in a region devoid of gas and dust. Weidentify a grouping of stars within a ~3 deg2 area located inOri OB1a, roughly clustered around the B2 star 25 Ori. The Herbig Ae/Bestar V346 Ori is also associated with this grouping, which could be anolder analog of σ Ori. Using several sets of evolutionary tracks,we find an age of 7-10 Myr for Ori OB1a and of ~4-6 Myr for Ori OB1b,consistent with previous estimates from OB stars. Indicators such as theequivalent width of Hα and near-IR excesses show that the numberof accreting low-mass stars decreases sharply between Ori OB1b and OriOB1a. These results indicate that although a substantial fraction ofaccreting disks remain at ages ~5 Myr, inner disks are essentiallydissipated by 10 Myr.Based on observations obtained at the Llano del Hato NationalAstronomical Observatory of Venezuela, operated by Centro deInvestigaciones de Astronomía (CIDA) for the Ministerio deCiencia y Tecnología, and at the Fred Lawrence WhippleObservatory (FLWO) of the Smithsonian Institution.Based on observations obtained at the 3.5 m WIYN Telescope. The WIYNObservatory is a joint facility of the University of Wisconsin-Madison,Indiana University, Yale University, and the National Optical AstronomyObservatory (NOAO).
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| An XMM-Newton look at the Wolf-Rayet star WR 40. The star itself, its nebula and its neighbours
We present the results of an XMM-Newton observation of the field of theWolf-Rayet star WR 40. Despite a nominal exposure of 20 ks and the highsensitivity of the satellite, the star itself is not detected: we thusderive an upper limit on its X-ray flux and luminosity. Joining thisresult to recent reports of a non-detection of some WC stars, we suggestthat the X-ray emission from single normal Wolf-Rayet stars could oftenbe insignificant despite remarkable instabilities in the wind. On thebasis of a simple modelling of the opacity of the Wolf-Rayet wind of WR40, we show that any X-ray emission generated in the particular zonewhere the shocks are supposed to be numerous will indeed have littlechance to emerge from the dense wind of the Wolf-Rayet star. We alsoreport the non-detection of the ejecta nebula RCW 58 surrounding WR 40.Concerning the field around these objects, we detected 33 X-ray sources,most of them previously unknown: we establish a catalog of these sourcesand cross-correlate it with catalogs of optical/infrared sources.Based on observations with XMM-Newton, an ESA Science Mission withinstruments and contributions directly funded by ESA Member States andthe USA (NASA).
| Magnetic fields in massive stars: dynamics and origin
Evidence continues to accumulate in favour of the presence of magneticfields on the surfaces of massive stars. Some authors hypothesize thatsuch fields originate in a dynamo in the convective core, with buoyancybringing flux tubes to the surface. Here we show that, when realisticstellar models are used, this `core dynamo hypothesis' encounters aserious difficulty: in order for surface fields to originate in a coredynamo, the core must create magnetic fields that are much stronger thanequipartition values.As an alternative hypothesis for the origin of magnetic fields in OBstars, we suggest that a dynamo is operating in shear-unstable gas inthe radiative stellar envelope. Using a recently developed code for theevolution of rotating stars, we find that in a 10-Msolarzero-age main sequence (ZAMS) star with a typical rotation period, morethan 90 per cent of the volume of the star is subject to shearinstability. We have recently proposed that dynamo operation inshear-unstable gas helps to explain certain properties of mass loss incool giants. Here, in the context of hot stars, we show that fieldswhich originate in the shear-unstable regions of the envelope can reachthe surface without violating the constraints of equipartition. Suchfields rise to the surface on time-scales which are short compared tomain-sequence lifetimes.
| Video observation of meteors at Yunnan Observatory
In the last 20 years, with the development of the CCD and imageintensifiers, the use of small flexible video meteor observation systemshas gradually increased, with the prospect that one day videoobservation will replace the visual observation and ordinaryphotographic observations. In this paper we report on the research anddevelopment of the No.1 meteor-comet video camera system of YunnanObservatory and some preliminary observed results. The system consistsof 5 changeable modules; it has a 36° large-field camera dedicatedto the observation of meteors, with which a magnitude 6 star can berecorded on a single frame with an accuracy of about 0.2 mag. We alsopresent a comparison of the video camera system with the traditionalphotographic system, and outline the merits, possible improvements andfuture development of the video system.
| High-Resolution X-Ray Spectra of the Brightest OB Stars in the Cygnus OB2 Association
The Cyg OB2 association contains some of the most luminous OB stars inour Galaxy, the brightest of which are also among the most luminous inX-rays. We obtained a Chandra High Energy Transmission GratingSpectrometer observation centered on Cyg OB2 No. 8A, the most luminousX-ray source in the association. Although our analysis focuses on theX-ray properties of Cyg OB2 No. 8A, we also present limited analyses ofthree other OB stars (Cyg OB2 Nos. 5, 9, and 12). Applying standarddiagnostic techniques as used in previous studies of early-type stars,we find that the X-ray properties of Cyg OB2 No. 8A are very similar tothose of other OB stars that have been observed using high-resolutionX-ray spectroscopy. From analyses of the He-like ion fir emission lines(Mg XI, Si XIII, S XV, and Ar XVII), we derive radial distances of theHe-like line emission sources and find that the higher energy ions havetheir lines form closer to the stellar surface than those of lower ionstates. These fir-inferred radii are also found to be consistent withtheir corresponding X-ray continuum optical depth unity radii. Both ofthese findings are in agreement with previous O star studies and againsuggest that anomalously strong shocks or high-temperature zones may bepresent near the base of the wind. The observed X-ray emission-linewidths (HWHM~1000 km s-1) are also compatible with theobservations of other O star supergiants. Since Cyg OB2 No. 8A issimilar in spectral type to ζ Pup (the only O star that clearlyshows asymmetric X-ray emission-line profiles with large blueshifts), weexpected to see similar emission-line characteristics. Contrary to otherO star results, the emission lines of Cyg OB2 No. 8A show a large rangein line centroid shifts (~-800 to +250 km s-1). However, weargue that most of the largest shifts may be unreliable and theresulting range in shifts is much less than those observed in ζPup. Although there is one exception, the H-like Mg XII line, whichshows a blueshift of -550 km s-1, there are problemsassociated with trying to understand the nature of this isolated largeblueshifted line. To address the degree of asymmetry in these lineprofiles, we present Gaussian best-fit line profile model spectra fromζ Pup to illustrate the expected asymmetry signature in theχ2 residuals. Comparisons of the Cyg OB2 No. 8A best-fitline profile residuals with those of ζ Pup suggest that there areno indications of any statistical significant asymmetries in these lineprofiles. Both the line shift characteristics and lack of lineasymmetries are very puzzling results. Given the very high mass-lossrate of Cyg OB2 No. 8A (approximately 5 times larger than previousChandra-observed O supergiants), the emission lines from this starshould display a significant level of line asymmetry and blueshifts ascompared to other OB stars. We also discuss the implications of ourresults in light of the fact that Cyg OB2 No. 8A is a member of a rathertight stellar cluster, and shocks could arise at interfaces with thewinds of these other stars.
| Photometric Identification of the Low-Mass Population of Orion OB1b. I. The σ Orionis Cluster
We report an optical photometric survey of 0.89 deg2 of theOrion OB1b association centered on σ Ori. This region includesmost of the σ Ori cluster, the highest-density region within OrionOB1b. We have developed a statistical procedure to identify the young,low-mass, pre-main-sequence population of the association. We estimatethat the cluster has ~160 members in the mass range (0.2<=M<=1.0Msolar). The cluster has a radius of ~3-5 pc and an estimatedage of 2.5+/-0.3 Myr. We estimate that the total mass of the cluster is225+/-30 Msolar. This mass is similar to the estimated massof the ~5×105 yr old cluster NGC 2024. NGC 2024 andσ Ori appear to be a well-matched pair of clusters, except for the~2 Myr difference in their ages.
| Wind clumping and the wind-wind collision zone in the Wolf-Rayet binary γ2 Velorum observations at high and low state. XMM-Newton observations at high and low state
We present XMM-Newton observations of γ2 Velorum (WR11, WC8+O7.5III, P = 78.53 d), a nearby Wolf-Rayet binary system, at itsX-ray high and low states. At high state, emission from a hotcollisional plasma dominates from about 1 to 8 keV. At low state,photons between 1 and 4 keV are absorbed. The hot plasma is identifiedwith the shock zone between the winds of the primary Wolf-Rayet star andthe secondary O giant. The absorption at low state is interpreted asphotoelectric absorption in the Wolf-Rayet wind. This absorption allowsus to measure the absorbing column density and to derive a mass lossrate .M = 8 × 10-6 Mȯ yr-1for the WC8 star. This mass loss rate, in conjunction with a previousWolf-Rayet wind model, provides evidence for a clumped WR wind. Aclumping factor of 16 is required. The X-ray spectra below 1 keV (12Å) show no absorption and are essentially similar in both states.There is a rather clear separation in that emission from a plasma hotterthan 5 MK is heavily absorbed in low state while the cooler plasma isnot. This cool plasma must come from a much more extended region thanthe hot material. The Neon abundance in the X-ray emitting material is2.5 times the solar value. The unexpected detection of C V (25.3Å) and C VI (31.6 Å) radiative recombination continua atboth phases indicates the presence of a cool (40 000 K)recombination region located far out in the binary system.Based on observations obtained with XMM-Newton, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand the USA (NASA).
| XMM-Newton observations of the σ Ori cluster. I. The complex RGS spectrum of the hot star σ Ori AB
We present XMM-Newton observations of the young ({} 2{-}5 Myr)cluster around the hot (O9.5V) star σ Orionis AB, aimed atobtaining a high resolution RGS spectrum of the hot star as well as EPICimaging data for the whole field. We show that the RGS spectrum ofσ Ori AB may be contaminated by weaker nearby sources whichrequired the development of a suitable procedure to extract a clean RGSspectrum and to determine the thermal structure and wind properties ofthe hot star. We also report on the detection of a flare from the B2Vpstar σ Ori E and we discuss whether the flare originated from thehot star itself or rather from an unseen late-type companion. Otherresults of this observation include: the detection of 174 X-ray sourcesin the field of σ Ori of which 76 are identified as clustermembers, including very low-mass stars down to the substellar limit; thediscovery of rotational modulation in a late-type star near σ OriAB; no detectable line broadenings and shifts ( 800 kms-1) in the spectrum of σ Ori AB together with aremarkable low value of the O VII forbidden to intercombination lineratio and unusually high coronal abundances of CNO elements.
| RXTE all-sky slew survey. Catalog of X-ray sources at |b|>10o
We report results of a serendipitous hard X-ray (3-20 keV), nearlyall-sky (|b|>10o) survey based on RXTE/PCA observationsperformed during satellite reorientations in 1996-2002. The survey is80% (90%) complete to a 4σ limiting flux of ≈ 1.8 (2.5) ×10-11 erg s-1 cm-2 in the 3-20 keVband. The achieved sensitivity in the 3-8 keV and 8-20 keV subbands issimilar to and an order of magnitude higher than that of the previouslyrecord HEAO-1 A1 and HEAO-1 A4 all-sky surveys, respectively. A combined7× 103 sq deg area of the sky is sampled to flux levelsbelow 10-11 erg s-1 cm-2 (3-20 keV). Intotal 294 sources are detected and localized to better than 1 deg. 236(80%) of these can be confidently associated with a known astrophysicalobject; another 22 likely result from the superposition of 2 or 3closely located known sources. 35 detected sources remain unidentified,although for 12 of these we report a likely soft X-ray counterpart fromthe ROSAT all-sky survey bright source catalog. Of the reliablyidentified sources, 63 have local origin (Milky Way, LMC or SMC), 64 areclusters of galaxies and 100 are active galactic nuclei (AGN). The factthat the unidentified X-ray sources have hard spectra suggests that themajority of them are AGN, including highly obscured ones(NH>1023 cm-2). For the first timewe present a log N-log S diagram for extragalactic sources above4× 10-12 erg s-1 cm-2 at 8-20keV.Table 2 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/927
| High Resolution Spectra of Stellar Winds in Very Young Massive Stars
We present a series of spectra from hot stars obtained with the highenergy transmission grating spectrometer (HETGS) onboard the ChandraX-ray Observatory. Most of the selected stars are located at or near thecenters of young star-forming regions, which include IC 1396, NGC 2362,and Orion. The median ages of these clusters range from 0.3 to 12 Myr.The X-ray properties of these young, hot stars are then compared to ahighly resolved X-ray spectrum of the more evolved field star zeta Pup.Cluster stars with ages larger than 3 Myr show moderate temperatures andbroad asymmetric lines. These properties are quite similar to what hasbeen observed in zeta Pup and are consistent with X-rays produced byshock instabilities in a radiation-driven wind. Stars with agessignificantly less than 1 Myr, as found in the Orion Trapezium, showcompletely different characteristics. Here most of the X-ray flux showsvery high temperatures and the X-ray lines are narrow and unresolved.This strongly indicates that the X-ray properties are dominated bymagnetic field effects.
| High-Resolution X-Ray Spectroscopy with CHANDRA and XMM-NEWTON
The launches of the Chandra X-Ray Observatory in June 1999 and theXMM-Newton Observatory in December 1999 opened a new era in X-rayastronomy. Both of these missions incorporate novel diffraction gratingspectrometers that are providing the first high-resolution X-ray spectraof most classes of astrophysical sources. The spectra obtained to dateexhibit a wealth of discrete detail, yielding sensitive constraints onphysical conditions in the emitting plasmas. We review the essentialcharacteristics of these instruments, the basics of X-ray spectralformation in cosmic sources, and the exciting new results that haveemerged from Chandra and XMM-Newton grating observations of a widevariety of astrophysical systems.
| A Deep Chandra X-Ray Observation of the Embedded Young Cluster in NGC 2024
We present results of a sensitive 76 ks Chandra observation of the youngstellar cluster in NGC 2024, lying at a distance of ~415 pc in the OrionB giant molecular cloud. Previous infrared observations have shown thatthis remarkable cluster contains several hundred embedded young stars,most of which are still surrounded by circumstellar disks. Thus, itpresents a rare opportunity to study X-ray activity in a large sample ofoptically invisible protostars and classical T Tauri stars (CTTSs)undergoing accretion. Chandra detected 283 X-ray sources, of which 248were identified with counterparts at other wavelengths, mostly in thenear-infrared. Astrometric registration of Chandra images against theTwo Micron All Sky Survey (2MASS) resulted in positional offsets of~0.25" near field center, yielding high confidence identifications ofinfrared counterparts. The Chandra detections are characterized by hardheavily absorbed spectra and spectacular variability. Spectral analysisof more than 100 of the brightest X-ray sources yields a mean extinction~10.5 mag and typical plasma energies ~3keV. The range of variability includes rapid impulsive flares andpersistent low-level fluctuations indicative of strong magneticactivity, as well as slow rises and falls in count rate whose origin ismore obscure. Some slowly evolving outbursts reached sustainedtemperatures of kT~6-10 keV. Chandra detected all but one of a subsampleof 27 CTTSs identified from previous near- and mid-infrared photometry,and their X-ray and bolometric luminosities are correlated. We alsoreport the X-ray detection of IRS 2b, which is thought to be a massiveembedded late O or early B star that may be the ionizing source of NGC2024. Seven millimeter-bright cores (FIR 1-7) in NGC 2024 that may beprotostellar were not detected, with the possible exception of faintemission near the unusual core FIR 4.
| X-Ray Modeling of Very Young Early-Type Stars in the Orion Trapezium: Signatures of Magnetically Confined Plasmas and Evolutionary Implications
The Orion Trapezium is one of the youngest and closest star-formingregions within our Galaxy. With a dynamic age of ~3×105yr, it harbors a number of very young hot stars, which likely are on thezero-age main sequence (ZAMS). We analyzed high-resolution X-ray spectrain the wavelength range of 1.5-25 Å of three of itsX-ray-brightest members (Θ1 Ori A, C, and E) obtainedwith the High Energy Transmission Grating Spectrometer (HETGS) on boardthe Chandra X-Ray Observatory. We measured X-ray emission lines,calculated differential emission measure distributions (DEMs), andfitted broadband models to the spectra. The spectra from all three starsare very rich in emission lines, specifically from highly ionized Fe,which includes emission from Fe XVII to Fe XXV ions. A complete linelist is included. This is a mere effect of high temperatures rather thanan overabundance of Fe, which in fact turns out to be underabundant inall three Trapezium members. Similarly there is a significantunderabundance in Ne and O as well, whereas Mg, Si, S, Ar, and Ca appearclose to solar. The DEM derived from over 80 emission lines in thespectrum of Θ1 Ori C indicates three peaks located at7.9, 25, and 66 MK. The emission measure varies over the 15.4 day windperiod of the star. For the two phases observed, the low-temperatureemission remains stable, while the high-temperature emission showssignificant differences. The line widths seem to show a similarbifurcation, where we resolve some of the soft X-ray lines withvelocities up to 850 km s-1 (all widths are stated ashalf-width at half-maximum), whereas the bulk of the lines remainunresolved with a confidence limit of 110 km s-1. Thebroadband spectra of the other two stars can be fitted with severalcollisionally ionized plasma model components within a temperature rangeof 4.3-46.8 MK for Θ1 Ori E and 4.8-42.7 MK forΘ1 Ori A. The high-temperature emissivity contributesover 70% to the total X-ray flux. None of the lines are resolved forΘ1 Ori A and E with a confidence limit of 160 kms-1. The influence of the strong UV radiation field on theforbidden line in the He-like triplets allows us to set an upper limiton distance of the line-emitting region from the photosphere. The bulkof the X-ray emission cannot be produced by shock instabilities in aradiation-driven wind and are likely the result of magnetic confinementin all three stars. Although confinement models cannot explain all theresults, the resemblance of the unresolved lines and of the DEM withrecent observations of active coronae in II Peg and AR Lac during flaresis quite obvious. Thus we speculate that the X-ray production mechanismin these stars is similar, with the difference that the Orion stars maybe in a state of almost continuous flaring driven by the wind. Weclearly rule out major effects due to X-rays from a possible companion.The fact that all three stars appear to be magnetic and are near zeroage on the main sequence also raises the issue of whether the Orionstars are simply different or whether young massive stars enter the mainsequence carrying significant magnetic fields. The ratiologLX/Lbol using the ``wind'' component of thespectrum is -7 for the Trapezium stars, consistent with the expectationfrom O stars. This suggests that massive ZAMS stars generate their X-rayluminosities like normal O stars and magnetic confinement provides anadditional source of X-rays.
| High-Resolution Chandra Spectroscopy of τ Scorpii: A Narrow-Line X-Ray Spectrum from a Hot Star
Long known to be an unusual early-type star by virtue of its hard andstrong X-ray emission, τ Scorpii poses a severe challenge to thestandard picture of O-star wind-shock X-ray emission. The Chandra HETGSspectrum now provides significant direct evidence that this B0.2 stardoes not fit this standard wind-shock framework. The many emission linesdetected with the Chandra gratings are significantly narrower than whatwould be expected from a star with the known wind properties of τSco, although they are broader than the corresponding lines seen inlate-type coronal sources. While line ratios are consistent with the hotplasma on this star being within a few stellar radii of the photosphere,from at least one He-like complex there is evidence that theX-ray-emitting plasma is located more than a stellar radius above thephotosphere. The Chandra spectrum of τ Sco is harder and morevariable than those of other hot stars, with the exception of the youngmagnetized O star θ1 Ori C. We discuss these newresults in the context of wind, coronal, and hybrid wind-magnetic modelsof hot-star X-ray emission.
| Wind Accretion and State Transitions in Cygnus X-1
We present the results of a spectroscopic monitoring program (from 1998to 2002) of the Hα emission strength in HDE 226868, the opticalcounterpart of the black hole binary Cyg X-1. The feature provides animportant probe of the mass-loss rate in the base of the stellar wind ofthe supergiant star. We derive an updated ephemeris for the orbit basedon radial velocities measured from He I λ6678. We list netequivalent widths for the entire Hα emission/absorption complex,and we find that there are large variations in emission strength overboth long (years) and short (hours to days) time spans. There arecoherent orbital phase-related variations in the profiles when thespectra are grouped by Hα equivalent width. The profiles consistof (1) a P Cygni component associated with the wind of the supergiant,(2) emission components that attain high velocity at the conjunctionsand that probably form in enhanced outflows both toward and away fromthe black hole, and (3) an emission component that moves in antiphasewith the supergiant's motion. We argue that the third component forms inaccreted gas near the black hole and that the radial velocity curve ofthe emission is consistent with a mass ratio ofMX/Mopt~0.36+/-0.05. We find that there is ageneral anticorrelation between the Hα emission strength and X-rayflux (from the Rossi X-Ray Timing Explorer All Sky Monitor) in the sensethat when the Hα emission is strong (Wλ<-0.5Å) the X-ray flux is weaker and the spectrum harder. On the otherhand, there is no correlation between Hα emission strength andX-ray flux when Hα is weak. We argue that this relationship is notcaused by wind X-ray absorption nor by the reduction in Hαemissivity by X-ray heating. Instead, we suggest that the Hαvariations track changes in wind density and strength near thephotosphere. The density of the wind determines the size of X-rayionization zones surrounding the black hole, and these in turn controlthe acceleration of the wind in the direction of the black hole. Duringthe low/hard X-ray state, the strong wind is fast and the accretion rateis relatively low, while during the high/soft state, the weaker, highlyionized wind attains only a moderate velocity and the accretion rateincreases. We argue that the X-ray transitions from the normal low/hardto the rare high/soft state are triggered by episodes of decreasedmass-loss rate in the supergiant donor star.Based on data obtained at the David Dunlap Observatory, University ofToronto.
| Near-Infrared Adaptive Optics Imaging of the Embedded Cluster NGC 2024
We present the results of a high-resolution near-infrared adaptiveoptics survey of the young obscured star-forming region NGC 2024. Out ofthe total 73 stars detected in the adaptive optics survey of thecluster, we find three binaries and one triple. The resulting companionstar fraction, 7%+/-3% in the separation range of 0.35"-2.3" (145-950AU), is consistent with that expected from the multiplicity of maturesolar-type stars in the local neighborhood. Our survey was sensitive tofaint secondaries, but no companions with ΔK'>1.2mag are detected within 2" of any star. The cluster has a K' luminosityfunction that peaks at ~12, and although our completeness limit was 17.7mag at K', the faintest star we detected had a K' magnitude of 16.62.
| Observations of Star-Forming Regions with the Midcourse Space Experiment
We have imaged seven nearby star-forming regions, the Rosette Nebula,the Orion Nebula, W3, the Pleiades, G300.2-16.8, S263, and G159.6-18.5,with the Spatial Infrared Imaging Telescope on the Midcourse SpaceExperiment (MSX) satellite at 18" resolution at 8.3, 12.1, 14.7, and21.3 μm. The large angular scale of the regions imaged (~7.2-50deg2) makes these data unique in terms of the combination ofsize and resolution. In addition to the star-forming regions, twocirrus-free fields (MSXBG 160 and MSXBG 161) and a field near the southGalactic pole (MSXBG 239) were also imaged. Point sources have beenextracted from each region, resulting in the identification over 500 newsources (i.e., no identified counterparts at other wavelengths), as wellas over 1300 with prior identifications. The extended emission from thestar-forming regions is described, and prominent structures areidentified, particularly in W3 and Orion. The Rosette Nebula isdiscussed in detail. The bulk of the mid-infrared emission is consistentwith that of photon-dominated regions, including the elephant trunkcomplex. The central clump, however, and a line of site toward thenorthern edge of the cavity show significantly redder colors than therest of the Rosette complex.
| Identification of the ionizing source of NGC 2024
We propose the late-O, early-B star IRS2b as the ionizing source of theFlame Nebula (NGC 2024). It has been clear that sucha hot, massive star must be present in this heavily obscured region, andnow it has been identified. New near-infrared photometry shows thatIRS2b is the most luminous and hottest star in the young star clusterembedded in the center of NGC 2024. The near-infrared observations (5' x5') cover ~ 90% of the H II region detected in radio continuumradiation, making the probability very low that the ionizing star is notpresent in the field. A K-band spectrum of IRS2b obtained with ISAAC onthe Very Large Telescope indicates that the spectral type of IRS2b is inthe range O8 V - B2 V. Additional arguments indicate that its spectraltype is likely closer to O8 than to B2. The corresponding amount ofionizing radiation is consistent with published radio continuum andrecombination line observations.Based on observations collected at the European Southern Observatory atLa Silla and Paranal, Chile (ESO programmes 62.H-0443 and 64.H-0425).
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