Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
The Evolution of Outflow-Envelope Interactions in Low-Mass Protostars
We present multiline and continuum observations of the circumstellarenvironment within 104 AU of a sample of protostars toinvestigate how the effects of outflows on their immediate environmentchange over time. 12CO (1-0) emission probes thehigh-velocity molecular outflows near the protostars and demonstratesthat the outflow opening angle widens as the nascent star evolves. Mapsof the 13CO (1-0) and HCO+ (1-0) outflow emissionshow that protostellar winds erode the circumstellar envelope throughthe entrainment of the outer envelope gas. The spatial and velocitydistribution of the dense circumstellar envelope, as well as its mass,is traced by the C18O (1-0) emission and also displaysevolutionary changes. We show that outflows are largely responsible forthese changes and propose an empirical model for the evolution ofoutflow-envelope interactions. In addition, some of the outflows in oursample appear to affect the chemical composition of the surroundingenvironment, enhancing the HCO+ abundance. Overall, ourresults confirm that outflows play a major role in the star formationprocess through their strong physical and chemical impacts on theenvironments of the young protostars.
| Molecular Hydrogen Fluorescence in the Eridanus Superbubble
The first far-ultraviolet (1350-1750 Å) spectral imagingobservations of the Eridanus superbubble, obtained with the SPEAR/FIMSmission, have revealed distinct fluorescent emission from molecularhydrogen. Here we compare the observed emission features with those froma photodissociation region model with assumed illuminating stellarfields. The result shows rather high line ratiosI1580/I1610, which may imply the existence ofhigh-temperature molecular clouds in the region. The H2fluorescence intensity shows a proportional correlation with Hαemission, indicating that the fluorescence and the recombinationemission have similar physical origins.
| The Homogeneity of Interstellar Elemental Abundances in the Galactic Disk
We present interstellar elemental abundance measurements derived fromSpace Telescope Imaging Spectrograph echelle observations of 47 sightlines extending up to 6.5 kpc through the Galactic disk. These pathsprobe a variety of interstellar environments, covering ranges of nearly4 orders of magnitude in molecular hydrogen fraction f(H2)and more than 2 in mean hydrogen sight-line density. Coupling the current data with Goddard HighResolution Spectrograph data from 17 additional sight lines and thecorresponding Far Ultraviolet Spectroscopic Explorer and Copernicusobservations of H2 absorption features, we explore magnesium,phosphorus, manganese, nickel, copper, and germanium gas-phase abundancevariations as a function of : density-dependentdepletion is noted for each element, consistent with a smooth transitionbetween two abundance plateaus identified with warm and cold neutralinterstellar medium depletion levels. The observed scatter with respectto an analytic description of these transitions implies that totalelemental abundances are homogeneous on length scales of hundreds ofparsecs, to the limits of abundance measurement uncertainty. Theprobable upper limit we determine for intrinsic variability at any is 0.04 dex, aside from an apparent 0.10 dexdeficit in copper (and oxygen) abundances within 800 pc of the Sun.Magnesium dust abundances are shown to scale with the amount of siliconin dust, and in combination with a similar relationship between iron andsilicon, these data appear to favor the young F and G star values ofSofia & Meyer as an elemental abundance standard for the Galaxy.Based on observations with the NASA/ESA.
| First Optical Images of Circumstellar Dust Surrounding the Debris Disk Candidate HD 32297
Near-infrared imaging with the Hubble Space Telescope recently revealeda circumstellar dust disk around the A star HD 32297. Dust-scatteredlight is detected as far as 400 AU radius, and the linear morphology isconsistent with a disk ~10° away from an edge-on orientation. Herewe present the first optical images that show the dust-scattered lightmorphology from 560 to 1680 AU radius. The position angle of theputative disk midplane diverges by ~31°, and the color of dustscattering is most likely blue. We associate HD 32297 with a wall ofinterstellar gas and the enigmatic region south of the Taurus molecularcloud. We propose that the extreme asymmetries and blue disk colororiginate from a collision with a clump of interstellar material as HD32297 moves southward, and discuss evidence consistent with an age of 30Myr or younger.
| Triggered Star Formation in the Orion Bright-rimmed Clouds
We have developed an empirical and effective set of criteria, based onthe Two Micron All Sky Survey (2MASS) colors, to select candidateclassical T Tauri stars (CTTSs). This provides a useful tool to studythe young stellar population in star-forming regions. Here we presentour analysis of the bright-rimmed clouds (BRCs) B35, B30, IC 2118, LDN1616, LDN 1634, and Ori East to show how massive stars interact withmolecular clouds to trigger star formation. Our results support theradiation-driven implosion model, in which the ionization fronts from OBstars compress a nearby cloud until the local density exceeds thecritical value, thereby inducing the cloud to collapse to form stars. Wefind that only BRCs associated with strong IRAS 100 μm emission (atracer of high density) and Hα emission (a tracer of ionizationfronts) show signs of ongoing star formation. Relevant timescales,including the ages of O stars, expanding H II regions, and the ages ofCTTSs, are consistent with sequential star formation. We also find thatCTTSs are only seen between the OB stars and the BRCs, with those closerto the BRCs being progressively younger. There are no CTTSs leading theionization fronts, i.e., within the molecular clouds. All of thesefindings provide strong evidence of triggered star formation and showthe major roles massive stars play in sustaining the star-formingactivities in the region.
| Pushing the Envelope: The Impact of an Outflow at the Earliest Stages of Star Formation
We present new multiline and 3 mm continuum interferometer observationsof the circumstellar environment within 2×104 AU of theclass 0 protostar IRAS 05295+1247, the source of the RNO 43 outflow. Thehigh-resolution molecular line and continuum images enable a thoroughstudy of the circumstellar envelope, the molecular outflow, and itsimpact on the surroundings. We detect a highly collimated bipolarmolecular outflow seen in the 12CO (1-0) map, which weinterpret to be a new outflow episode, undetected in previousobservations of the RNO 43 outflow. The medium-density gas of the outercircumstellar envelope, which is traced by the 13CO (1-0)emission, is accelerated over an extended volume and not confined to thenarrow 12CO outflow axis. Our data also suggest that theoutflow affects the chemical composition of the surroundings, enhancingthe HCO+ (1-0) abundance along its edges. Most importantly,the kinetic energy injected by the outflow is sufficient to produce avelocity gradient in the dense inner circumstellar envelope traced bythe C18O (1-0) and H13CO+ (1-0)emission. Such a strong perturbation to the envelope will clearly havean important effect on the mass-assembling process in IRAS 05295+1247.
| 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.
| A near-infrared survey for new low-mass members in α Per
We present a near-infrared (K'-band) survey of 0.7 square degree area inthe α Persei open cluster (age = 90 Myr, distance = 182 pc)carried out with the Omega-Prime camera on the Calar Alto 3.5-mtelescope. Combining optical data (Rc and I_c) obtained withthe KPNO/MOSA detector and presented in Stauffer et al. (1999) with theK' observations, a sample of new candidate members has been extractedfrom the optical-infrared colour-magnitude diagram. The location ofthese candidates in the colour-colour diagram suggests that two-thirdsof them are actually reddened background giants. About 20 new candidatemembers with masses between 0.3 and 0.04 Mȯ are added tothe ~400 known α Per cluster members. If they are indeed αPer members, four of the new candidates would be brown dwarfs. Wediscuss the advantages and drawbacks of the near-infrared survey ascompared to the optical selection method. We also describe the outcomeof optical spectroscopy obtained with the Twin spectrograph on the CalarAlto 3.5-m telescope for about 30 candidates, including selected membersfrom the optical sample presented in Barrado y Navascués et al.(2002) and from our joint optical/infrared catalogue. These resultsargue in favour of the optical selection method for this particularcluster.
| On the local birth place of Geminga
Using estimates of the distance and proper motion ofGeminga and the constraints on its radial velocityposed by the shape of its bow shock, we investigate its birth place bytracing its space motion backwards in time. Our results exclude theλ Ori association as the origin site becauseof the large distance between both objects at any time. Our simulationsplace the birth region at approximately 90-240 pc from the Sun, between197° and 199° in Galactic longitude and -16° and -8° inlatitude, most probably inside the Cas-Tau OBassociation or the Ori OB1a association. We discardthe possibility of the progenitor being a massive field star. Theassociation of Geminga with either stellarassociation implies an upper limit of M ≈ 15 Mȯ forthe mass of its progenitor. We also propose new members for theCas-Tau and Ori OB1 associations.
| The Homogeneity of Interstellar Oxygen in the Galactic Disk
We present an analysis of high-resolution Hubble Space Telescope (HST)Space Telescope Imaging Spectrograph (STIS) observations of O Iλ1356 and H I Lyα absorption in 36 sight lines that probe avariety of Galactic disk environments and include paths that range overnearly 4 orders of magnitude in f(H2), over 2 orders ofmagnitude in , and that extend up to 6.5 kpc inlength. Since the majority of these sight lines have also been observedby the Far Ultraviolet Spectroscopic Explorer (FUSE), we have undertakenthe study of gas-phase O/H abundance ratio homogeneity using the currentsample and previously published Goddard High Resolution Spectrograph(GHRS) results. Two distinct trends are identified in the 56 sight linesample: an apparent decrease in gas-phase oxygen abundance withincreasing mean sight-line density () and a gapbetween the mean O/H ratio for sight lines shorter and longer than about800 pc. The first effect is a smooth transition between two depletionlevels associated with large mean density intervals; it is centered near=1.5cm-3 and is similar to trendsevident in gas-phase abundances of other elements. Paths less dense thanthe central value exhibit a mean O/H ratio of log10(O/H)=-3.41+/-0.01 (or 390+/-10ppm), which is consistent with averages determined for several longlow-density paths observed by STIS (André et al. 2003) and shortlow-density paths observed by FUSE (Moos et al. 2002). Sight lines ofhigher mean density exhibit an average O/H value of log10(O/H)=-3.55+/-0.02 (284+/-12ppm). The data points for low- paths are scatteredmore widely than those for denser sight lines, because O/H ratios forsuch paths shorter than 800 pc are generally about 0.10 dex lower thanthe values for longer ones. Scenarios that would be consistent withthese results include a recent infall of metal-poor gas onto the localGalactic disk and an interstellar environment toward Orion that isconducive to reducing the apparent gas-phase oxygen abundance.Based on observations with the NASA/ESA Hubble Space Telescope (HST) andthe NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer (FUSE). HSTspectra were obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS5-26555 FUSE is operated for NASA by theJohns Hopkins University under NASA contract NAS5-32985.
| The Substellar Population of the Young Cluster λ Orionis
By collecting optical and infrared photometry and low-resolutionspectroscopy, we have identified a large number of low-mass stars andbrown dwarf candidates belonging to the young cluster (~5 Myr)associated with the binary star λ Orionis. The lowest mass objectfound is an M8.5 with an estimated mass of 0.02 Msolar (~0.01Msolar for objects without spectroscopic confirmation). Forthose objects with spectroscopy, the measured strength of the Hαemission line follows a distribution similar to other clusters with thesame age range, with larger equivalent widths for cooler spectral types.Three of the brown dwarfs have Hα emission equivalent widths oforder 100 Å, suggesting that they may have accretion disks andthus are the substellar equivalent of classical T Tauri stars. We havederived the initial mass function for the cluster. For the substellarregime, the index of the mass spectrum is α=0.60+/-0.06, verysimilar to other young associations.
| Reanalysis of Copernicus Measurements of Interstellar Carbon Monoxide
We used archival data acquired with Copernicus to reexamine CO columndensities, as self-consistent oscillator strengths are now available.Our focus is on lines of sight containing modest amounts of molecularspecies. Our resulting column densities are small enough thatself-shielding from photodissociation does not occur in the cloudsprobed by the observations. While our sample shows that the columndensities of CO and H2 are related, no correspondence withthe CH column density is evident. The case for the CH+ columndensity is less clear. Recent chemical models for these sight linessuggest that CH is mainly a by-product of CH+ synthesis inlow-density gas. The models are most successful in reproducing theamounts of CO in the densest sight lines. Thus, much of the COabsorption must arise from denser clumps along the line of sight toaccount for the trend with H2.
| Interstellar Carbon in Translucent Sight Lines
We report interstellar C II column densities or upper limits determinedfrom weak absorption of the 2325.4029 Å intersystem transitionobserved in six translucent sight lines (AV>~1) with theSpace Telescope Imaging Spectrograph (STIS). The sight lines sample awide range of interstellar characteristics, including total to selectiveextinction, RV=2.6-5.1 average hydrogen density along thesight line, =3-14 cm-3 and fraction of H inmolecular form, 0-~40%. Four of the sight lines, those toward HD 37021,HD 37061, HD 147888, and HD 207198, have interstellar gas-phaseabundances that are consistent with the diffuse sight line ratio of161+/-17 carbon atoms in the gas per million hydrogen nuclei. We notethat while it has a gas-phase carbon abundance that is consistent withthe other sight lines, a large fraction of the C II toward HD 37061 isin an excited state. The sight line toward HD 152590 has a measuredinterstellar gas-phase carbon abundance that is well above the diffusesight line average; the column density of C in this sight line may beoverestimated because of noise structure in the data. Toward HD 27778 wefind a 3 σ abundance upper limit of <=108 C atoms in the gasper million H, a substantially enhanced depletion of C as compared tothe diffuse sight line value. The interstellar characteristics toward HD27778 are otherwise not extreme among the sample, except for anunusually large abundance of CO molecules in the gas.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 NAS 5-26555.
| Searches for HH-objects and Emission Stars in Star Formation Regions. II. The Region of GM1-61 and V453 Ori
Observations of objects lying in the dark cloud L1582A, which containsthe cometary nebulae GM1-61 and V453 Ori, are reported. Five previouslyunknown HH-objects are discovered in this region; they appear to formseveral flows. The morphology of the nebulae is examined and possiblesources of the HH-flows are discussed. Another reflecting nebula isfound that is visible mainly in the infrared.
| A radio and mid-infrared survey of northern bright-rimmed clouds
We have carried out an archival radio, optical and infrared wavelengthimaging survey of 44 Bright-Rimmed Clouds (BRCs) using the NRAO/VLA SkySurvey (NVSS) archive, images from the Digitised Sky Survey (DSS) andthe Midcourse Space eXperiment (MSX). The data characterise the physicalproperties of the Ionised Boundary Layer (IBL) of the BRCs. We haveclassified the radio detections as: that associated with the ionisedcloud rims; that associated with possible embedded Young Stellar Objects(YSOs); and that unlikely to be associated with the clouds at all. Thestars responsible for ionising each cloud are identified and acomparison of the expected ionising flux to that measured at the cloudrims is presented. A total of 25 clouds display 20 cm radio continuumemission that is associated with their bright optical rims. The ionisingphoton flux illuminating these clouds, the ionised gas pressure and theelectron density of the IBL are determined. We derive internal molecularpressures for 9 clouds using molecular line data from the literature andcompare these pressures to the IBL pressures to determine the pressurebalance of the clouds. We find three clouds in which the pressureexerted by their IBLs is much greater than that measured in the internalmolecular material. A comparison of external pressures around theremaining clouds to a global mean internal pressure shows that themajority of clouds can be expected to be in pressure equilibrium withtheir IBLs and hence are likely to be currently shocked byphotoionisation shocks. We identify one source which shows 20 cmemission consistent with that of an embedded high-mass YSO and confirmits association with a known infrared stellar cluster. This embeddedcluster is shown to contain early-type B stars, implying that at leastsome BRCs are intimately involved in intermediate to high mass starformation.Figure \ref{fig:images} and Table \ref{tbl:istars1} are only availablein electronic form at http://www.edpsciences.org
| Multi-wavelength observations of the star forming region in L1616
We present the results of a multi-wavelength study of the star formingregion in L1616. Our observations include ROSAT All-Sky Survey (RASS)and High Resolution Imager (HRI) X-ray observations, optical wide-fieldimaging and near-IR imaging data and optical long-slit and multi-objectspectroscopic follow-up. 22 new low-mass pre-main sequence (PMS) starsare found to be distributed mainly to the East of the L1616 cometarycloud, in about a one-square-degree field. We find that the class-IIIinfrared sources outnumber the class-II infrared sources by a factor ofabout three. The X-ray properties of the PMS stars in L1616 are quitesimilar to those of PMS stars detected in the Orion Nebula Cluster. Thecomparison of the position of the L1616 PMS stars in the HR diagram withtheoretical PMS evolutionary tracks yields an average age of 1-2 Myr,with a very small age spread of about 1 Myr. Unlike the fossil starforming regions in Orion, L1616 appears to be a region of on-going starformation relatively far from the Orion A and B clouds. Given the smallage spread, the spatial distribution of the PMS stars relative to thehead of the cloud, as well as its cometary shape and high star formationefficiency, we conclude that the star formation in L1616 was most likelyinduced by a single event, the impact of the winds of the massive starsof the Orion OB association or a supernova explosion being the possibletriggers. The Initial Mass Function (IMF) in L1616 is roughly consistentwith that of the field in the mass range 0.3< M/Mȯ< 2.5. Several faint objects, detected in our optical images, aregood candidates for young Brown Dwarfs (BDs). We might expect the numberof BDs in L1616 to be intermediate between Taurus and the Trapezium.Based on observations carried out at the European Southern Observatory,La Silla, Chile under proposals numbers 56.E-0566 and 64.I-0355, and atthe Calar Alto observatory.
| The Homogeneity of Interstellar Krypton in the Galactic Disk
We present an analysis of high-resolution Hubble Space Telescope SpaceTelescope Imaging Spectrograph (STIS) observations of Kr I λ1236absorption in seven sight lines that probe a variety of interstellarenvironments. In combination with krypton and hydrogen column densitiesderived from current and archival STIS and Far-Ultraviolet SpectroscopicExplorer data, the number of sight lines with reliable Kr/H ISMabundance ratios has been increased by 50% to 26-including paths thatsample a range of nearly 5 orders of magnitude in f(H2) andover 2 orders of magnitude in , and extend up to4.8 kpc in length. For sight lines contained entirely within the localspiral arm (the Orion spur), the spread of Kr/H ratios about the mean oflog10[N(Kr)/N(H)]ISM=-9.02+/-0.02is remarkably tight (0.06 dex), less than the typical data-pointuncertainty. Intriguingly, the only two sight lines that extend throughneighboring structures, in particular gas associated with theCarina/Sagittarius arm, exhibit relatively large, near-solar kryptonabundances (log10[N(Kr)/N(H)]combined=-8.75+0.09-0.11).Although these deviations are only measured at the 2 σ level, theysuggest the possibility that krypton abundances beyond the Orion spurmay differ from the local value.Based on observations with the NASA/ESA Hubble Space Telescope (HST) andthe NASA-CNES-CSA Far-Ultraviolet Spectroscopic Explorer (FUSE). HSTspectra were obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc. under NASA contract NAS5-26555 FUSE is operated for NASA by theJohns Hopkins University under NASA contract NAS-32985.
| Potential Variations in the Interstellar N I Abundance
We present Far Ultraviolet Spectroscopic Explorer (FUSE) and SpaceTelescope Imaging Spectrograph observations of the weak interstellar N Iλ1160 doublet toward 17 high-density sight lines[N(Htot)>=1021 cm-2]. When combinedwith published data, our results reveal variations in the fractional N Iabundance showing a systematic deficiency at large N(Htot).At the FUSE resolution (~20 km s-1), the effects ofunresolved saturation cannot be conclusively ruled out, although O Iλ1356 shows little evidence of saturation. We investigated thepossibility that the N I variability is due to the formation ofN2 in our mostly dense regions. The 0-0 band of thec'41Σ+u-X1Σ+gtransition of N2 at 958 Å should be easily detected inour FUSE data; for 10 of the denser sight lines, N2 is notobserved at a sensitivity level of a few times 1014cm-2. The observed N I variations are suggestive of anincomplete understanding of nitrogen chemistry.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer, which is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS 5-32985, and the NASA/ESA HubbleSpace Telescope, obtained from the Multimission Archive at the SpaceTelescope Science Institute, which is operated by the Association ofUniversities for Research in Astronomy, Inc., under the NASA contractNAS 5-26555.
| High-Resolution Spectroscopy of FU Orionis Stars
High-resolution spectroscopy was obtained of the FU orionis stars FU Oriand V1057 Cyg between 1995 and 2002 with the SOFIN spectrograph at theNordic Optical Telescope and with HIRES at Keck I. During these years FUOri remained about 1 mag (in B) below its 1938-39 maximum brightness,but V1057 Cyg (B~10.5 at peak in 1970-1971) faded from about 13.5 to14.9 and then recovered slightly. Their photospheric spectra resemblethat of a rotationally broadened, slightly veiled supergiant of abouttype G0 Ib, with veqsini=70 km s-1 for FU Ori, and55 km s-1 for V1057 Cyg. As V1057 Cyg faded, P Cyg structurein Hα and the IR Ca II lines strengthened and a complexshortward-displaced shell spectrum of low-excitation lines of theneutral metals (including Li I and Rb I) increased in strength,disappeared in 1999, and reappeared in 2001. Several SOFIN runs extendedover a number of successive nights so that a search for rapid and cyclicchanges in the spectra was possible. These spectra show rapidnight-to-night changes in the wind structure of FU Ori at Hα,including clear evidence of sporadic infall. The equivalent width of theP Cyg absorption varied cyclically with a period of 14.8 days, withphase stability maintained over three seasons. This is believed to bethe rotation period of FU Ori. The internal structure of itsphotospheric lines also varies cyclically, but with a period of 3.54days. A similar variation may be present in V1057 Cyg, but the data aremuch noisier and that result uncertain. As V1057 Cyg has faded and thecontinuum level fallen, the emission lines of a preexistinglow-excitation chromosphere have emerged. Therefore we believe that the``line doubling'' in V1057 Cyg is produced by these central emissioncores in the absorption lines, not by orbital motion in an inclinedKeplerian disk. No convincing dependence of veqsini onwavelength or excitation potential was detected in either FU Ori orV1057 Cyg, again contrary to expectation for a self-luminous accretiondisk. It was found also that certain critical lines in the near infraredare not accounted for by synthetic disk spectra. It is concluded that arapidly rotating star near the edge of stability, as proposed by Larson,can better account for these observations. The possibility is alsoconsidered that FUor eruptions are not a property of ordinary T Tauristars but may be confined to a special subspecies of rapidly rotatingpre-main-sequence stars having powerful quasi-permanent winds.
| 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.
| An Ultra-high-Resolution Survey of the Interstellar 7Li/6Li Isotope Ratio in the Solar Neighborhood
In an effort to probe the extent of variations in the interstellar7Li/6Li ratio seen previously,ultra-high-resolution (R~360,000), high signal-to-noise spectra of starsin the Perseus OB2 and Scorpius OB2 associations were obtained. Thesemeasurements confirm our earlier findings of an interstellar7Li/6Li ratio of about 2 toward ο Per, thevalue predicted from models of Galactic cosmic-ray spallation reactions.Observations of other nearby stars yield limits consistent with theisotopic ratio of ~12 seen in carbonaceous chondrite meteorites. If thisratio originally represented the gas toward ο Per, then todecrease the original isotope ratio to its current value an order ofmagnitude increase in the Li abundance is expected, but it is not seen.The elemental K/Li ratio is not unusual, although Li and K are formedvia different nucleosynthetic pathways. Several proposals to account forthe low 7Li/6Li ratio were considered, but noneseems satisfactory. Analysis of the Li and K abundances from our surveyhighlighted two sight lines where depletion effects are prevalent. Thereis evidence for enhanced depletion toward X Per, since both abundancesare lower by a factor of 4 when compared to other sight lines. Moreover,a smaller Li/H abundance is observed toward 20 Aql, but the K/Habundance is normal, suggesting enhanced Li depletion (relative to K) inthis direction. Our results suggest that the7Li/6Li ratio has not changed significantly duringthe last 4.5 billion years and that a ratio of ~12 represents most gasin the solar neighborhood. In addition, there appears to be a constantstellar contribution of 7Li, indicating that one or twoprocesses dominate its production in the Galaxy.
| XMM-Newton high-resolution X-ray spectroscopy of the Wolf-Rayet object WR 25 in the Carina OB1 association
We report the analysis of the first high-resolution X-ray spectra of theWolf-Rayet (WR) object WR 25 (HD 93162, WN6ha+O4f) obtained with theReflection Grating Spectrometers (RGS) and the European Photon ImagingCameras (EPIC-MOS and PN) CCD spectrometers on board the XMM-Newtonsatellite. The spectrum exhibits bright emission lines of the H- andHe-like ions of Ne, Mg, Si and S, as well as Fe XVII i to Fe XX and FeXXV lines. Line fluxes have been measured. The RGS and EPIC spectrahave been simultaneously fitted to obtain self-consistent temperatures,emission measures, and elemental abundances. Strong absorption by thedense WR stellar wind and the interstellar medium (ISM) is observedequivalent to NH = 7 x 1021 cm-2.Multi-temperature (DEM) fitting yields two dominant components aroundtemperatures of 7.0 and 32 MK, respectively. The XMM intrinsic (i.e.unabsorbed, corrected for the stellar wind absorption and the absorptionof ISM) X-ray luminosity of WR 25 is Lx(0.5-10 keV) = 1.3 x1034 erg s-1, and Lx(0.5-10 keV) = 0.85x 1034 erg s-1, (when correcting for the ISM only)assuming d=3.24 kpc. The obtained chemical abundances are subsolar,except for S. This may be real, but could equally well be due to a weakcoupling to the continuum, which is strongly influenced by theabsorption column density and the subtracted background. The expectedhigh N-abundance, as observed in the optical wavelength region, couldnot be confirmed due to the strong wind absorption, blocking out itsspectral signature. The presence of the Fe XXV emission-line complex at~ 6.7 keV is argued as being indicative for colliding winds inside aWR+O 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).
| Star Formation in Space and Time: Taurus-Auriga
To understand the formation of stellar groups, one must first documentcarefully the birth pattern within real clusters and associations. Inthis study of Taurus-Auriga, we combine pre-main-sequence ages from ourown evolutionary tracks with stellar positions from observationalsurveys. Aided by the extensive millimeter data on the molecular clouds,we develop a picture of the region's history. Star formation began, at arelatively low level and in a spatially diffuse manner, at least107 yr in the past. Within the last few million years, newstars have been produced at an accelerating rate, almost exclusivelywithin a confined group of striated cloud filaments. The gas both insideand around the filaments appears to be in force balance. Thus, theappearance of the filaments is due to global, quasi-static contractionof the parent cloud material. Gravity drives this contraction and shockdissipation mediates it, but the internal motion of the gas does notappear to be turbulent. The accelerating nature of recent star formationmeans that the condensation of cloud cores is a threshold phenomenon,requiring a minimum background density. Other, nearby cloud regions,including Lupus and Chamaeleon, contain some locales that have attainedthis density, and others that have not. In the latter, we find extensiveand sometimes massive molecular gas that is still devoid of young stars.
| A Photometric Study of the Young Stellar Population throughout the λ Orionis Star-Forming Region
We present VRI photometry of 320,917 stars with 11<~R<~18throughout the λ Ori star-forming region. Using the morespatially limited spectroscopic surveys of Dolan & Mathieu to definethe color-magnitude domain of young low-mass members of the associationand removing statistically the field stars in this domain, we use ourphotometry to identify a representative pre-main-sequence (PMS)population throughout the interior of the molecular ring. The spatialdistribution of this population shows a concentration of PMS starsaround λ Ori and in front of the B35 dark cloud. However, few PMSstars are found outside these pockets of high stellar density,suggesting that star formation was concentrated in an elongated cloudextending from B35 through λ Ori to the B30 cloud. We find alower limit for the global stellar mass of about 500 Msolar.We find that the global ratio of low- to high-mass stars is similar tothat predicted by the field initial mass function, but this ratio variesstrongly as a function of position in the star-forming region. Locally,the star formation process does not produce a universal initial massfunction. Using our derived stellar ages across the region, we constructa history of the star-forming complex. This history incorporates arecent supernova to explain the distribution of stars and gas today. Weinfer that most of the present molecular ring was formed by ejecta fromthe center driven by the supernova blast about 1 Myr ago. However, wesuggest that the B30 and B35 clouds were primordial, and massive enoughto be mostly little disturbed by the shock. The stars that we see todaytrace the former extent of the cloud complex. Given the kinematics ofthe stellar population, we predict that the association will disperseinto the field within a few tens of megayears.
| Distances of absorbing features in the LDN 1622 direction. An application of Tycho-2 photometry and Michigan Classification
With Tycho-2 BT and VT photometry of Hipparcosstars with π > 8.0 mas, σπ/π < 15% andwith spectral type and luminosity classification from the MichiganCatalogues we have a presumably unreddened sample for establishing a(BT-VT_0- MV_T main sequence relationin the range from A0 to G5. We discuss the location of the median mainsequence relation relative to published ZAMS relations for the A0-G5range in some detail since the applications of this relation will haveto assume that the evolutionary status of the calibration sample isrepresentative of the local disk. If the ZAMS is defined as the lowerenvelope of the class V stars the published versions we discuss seem tobe too bright by about half a magnitude in the F0-G5 range. Theresulting intrinsic color and absolute magnitude estimates have errorsof the mean of 0.03m and 0.3m-0.4m respectively. From this relation anygiven star with Tycho-2 photometry, spectral classification andluminosity class V can have its color excess E(BT -VT) and distance modulus estimated. This means that the localinterstellar extinction may be estimated for the ~50% of the sky whereMichigan classification presently is available. The individual colorexcesses and distance moduli may not be particularly accurate but wepropose that they may be used to indicate extinction discontinuitiescaused by interstellar material at well defined distances. We haveapplied this new technique on stars from the Michigan Catalogue in thedirection of LDN 1622 (l, b) = (204.7dg, -11.8dg) supposed to beassociated to the Orion B region 400-500 pc away but color excessesE(BT - VT) exceeding 0.15m start appearing alreadyin the distance slot 160-200 pc. Presently we cannot decide whether thisnearby dust is related to LDN 1622 or whether this cloud is associatedto Orion B. In a final comparison we study the color excess -- distancevariation in a 4 deg region centered on LDN 1622 from Hipparcos --Tycho-1 -- Michigan data and the dust at ~160 pc is confirmed.
| The evolved early-type binary HDE 228766 revisited
We use an extensive set of spectroscopic observations to reinvestigatethe properties of the massive binary HDE 228766. Conventionalclassification criteria suggest that HDE 228766 consists of an O7primary and an Of+ secondary. However, several spectralfeatures of the secondary, such as the simultaneous presence of N III, NIV and N V emissions, make it a rather unusual object. We find that theorbital motion of the secondary is probably best described by the radialvelocities of the narrow N III emission lines. Our orbital solutionyields m1 sin 3i = 31.7 and m2 sin3i = 25.5 Msun for the primary and secondaryrespectively. The He II lambda 4542 absorption in the secondary'sspectrum appears considerably blue-shifted with respect to the narrowemission lines, indicating that the absorption is probably formed in theaccelerating part of the secondary's wind. We use a tomographictechnique to investigate the profile variability of the broader emissionlines. In addition to a strong emission from the secondary, the Hαline displays a weak emission feature that is probably associated with awind interaction region located near the surface of the primary star.Finally, our analysis of the spectrum with a non-LTE code indicates thatthe secondary is an evolved object that exhibits some CNO processedmaterial in its atmosphere and has a large mass loss rate. Assuming adistance of 3.5 kpc (which follows from adopting MV, s = -6for the secondary) we infer dot {M} =~ 10-5 Msunyr-1 for the secondary. Our results suggest that HDE 228766could be in a post-Roche lobe overflow stage. The secondary may beclassified as WN8ha and is currently in a transition stage between a``normal'' Of star and a WNL-type Wolf-Rayet star. Based on observationscollected at the Observatoire de Haute Provence (France) and theObservatorio Astronómico Nacional of San Pedro Mártir(Mexico).
| Binary Star Observations in Selected Instants of Good Seeing
Video recordings of images of binary stars at the focus of a 0.36mtelescope have been used to select images recorded in instants of goodseeing. The selected images have been analysed to give separations andposition angles for the binary systems which are in good agreement withvalues predicted from previous observations. In these exploratoryobservations it has been shown that separations of 0.9 arcseconds can bemeasured with an accuracy of ~2% and position angles to ~1-2 degreeswhen the average seeing was ~1.3 arcseconds. These observationsdemonstrated that the diffraction limit of the telescope could bereached when the seeing was a factor of 2-3 greater than it. A binarywith three magnitudes difference in the brightness of its components hasbeen measured with comparable accuracy although difficulties areanticipated for binaries with components closer than ~2 arcseconds withthis magnitude difference. The limiting magnitude is determined by theneed to limit exposure times of individual frames to be comparable withor less than the atmospheric coherence time.
| Space Telescope Imaging Spectrograph Observations of Interstellar Oxygen and Krypton in Translucent Clouds
We have obtained high-resolution Hubble Space Telescope (HST) SpaceTelescope Imaging Spectrograph (STIS) observations of O I λ1356and Kr I λ1236 absorption in 11 sight lines characterized by highextinction, large H I column densities, and/or long path lengths.Previous Goddard High Resolution Spectrograph (GHRS) measurements ofthese weak features in seven relatively nearby diffuse clouds have shownno evidence for density-dependent depletion of either oxygen or kryptonand have yielded a weighted mean gas-phase abundance ratio oflog[N(O)/N(Kr)]GHRS=5.56+/-0.04. Our STIS measurements yielda lower weighted mean of log[N(O)/N(Kr)]STIS=5.48 thedifference is due primarily to several translucent sight lines in theSTIS data set that diverge from the GHRS value. These translucent cloudsight lines pass near dense, star-forming regions, notably the ρOph, Orion, and Taurus molecular clouds. Since Kr, as a noble gas,should not be depleted much into grains, these cases suggest a trendtoward the enhanced oxygen depletion predicted for denser ISM clouds.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 NAS 5-26555.
| Submillimeter Mapping and Analysis of Cold Dust Condensations in the Orion M42 Star-forming Complex
We present here the continuum submillimeter maps of the molecular cloudaround the M42 Nebula in the Orion region. These have been obtained infour wavelength bands (200, 260, 360, and 580 μm) with the ProgrammeNational d'Observations Submillimétriques 2 m balloon-bornetelescope. The area covered is 7 pc wide (50' at a distance of 470 pc)with a spatial resolution of about 0.4 pc. Thanks to the highsensitivity to faint surface brightness gradients, we have found severalcold condensations, with temperatures ranging from 12 to 17 K, within 3pc of the dense ridge. The statistical analysis of the temperature andspectral index spatial distribution shows an evidence of an inversecorrelation between these two parameters. Being invisible in the IRAS100 μm survey, some cold clouds are likely to be the seeds for futurestar formation activity going on in the complex. We estimate theirmasses, and we show that two of them have masses higher than their Jeansmasses and may be gravitationally unstable.
| The Milky Way in Molecular Clouds: A New Complete CO Survey
New large-scale CO surveys of the first and second Galactic quadrantsand the nearby molecular cloud complexes in Orion and Taurus, obtainedwith the CfA 1.2 m telescope, have been combined with 31 other surveysobtained over the past two decades with that instrument and a similartelescope on Cerro Tololo in Chile, to produce a new composite CO surveyof the entire Milky Way. The survey consists of 488,000 spectra thatNyquist or beamwidth (1/8deg) sample the entire Galactic plane over astrip 4deg-10deg wide in latitude, and beamwidthor 1/4deg sample nearly all large local clouds at higher latitudes.Compared with the previous composite CO survey of Dame et al. (1987),the new survey has 16 times more spectra, up to 3.4 times higher angularresolution, and up to 10 times higher sensitivity per unit solid angle.Each of the component surveys was integrated individually using clippingor moment masking to produce composite spatial and longitude-velocitymaps of the Galaxy that display nearly all of the statisticallysignificant emission in each survey but little noise. The composite mapsprovide detailed information on individual molecular clouds, suggestrelationships between clouds and regions widely separated on the sky,and clearly display the main structural features of the molecularGalaxy. In addition, since the gas, dust, and Population I objectsassociated with molecular clouds contribute to the Galactic emission inevery major wavelength band, the precise kinematic information providedby the present survey will form the foundation for many large-scaleGalactic studies. A map of molecular column density predicted fromcomplete and unbiased far-infrared and 21 cm surveys of the Galaxy wasused both to determine the completeness of the present survey and toextrapolate it to the entire sky at |b|<32deg. The closeagreement of the observed and predicted maps implies that only ~2% ofthe total CO emission at |b|<32deg lies outside ourcurrent sampling, mainly in the regions of Chamaeleon and the GumNebula. Taking into account this small amount of unobserved emission,the mean molecular column density decreases from~3×1020 cm-2 at |b|=5deg to~0.1×1020 cm-2 at |b|=30deg thisdrop is ~6 times steeper than would be expected from a plane-parallellayer, but is consistent with recent measurements of the mean molecularcolumn density at higher latitudes. The ratio of the predicted molecularcolumn density map to the observed CO intensity map provides acalibration of the CO-to-H2 mass conversion factorX≡NH2/WCO. Out of the Galacticplane (|b|>5deg), X shows little systematic variation withlatitude from a mean value of (1.8+/-0.3)×1020cm-2 K-1 km-1 s. Given the large skyarea and large quantity of CO data analyzed, we conclude that this isthe most reliable measurement to date of the mean X value in the solarneighborhood.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
Catàlegs i designacions:
|
