Tartalom
Képek
Kép feltöltése
DSS Images Other Images
Kapcsolódó cikkek
Circumstellar Disk around HD 143275 and Interstellar Absorption
This paper presents a collection of spectra of the star HD 143275(δ Sco), acquired during the last 11 years. During this period,stellar absorption lines turned into emission features possiblyoriginating in the circumstellar disk. We demonstrate that thecircumstellar shell does not produce any components of interstellarabsorption features (atomic lines, molecular features, and diffuseinterstellar bands), which remain of the same equivalent widths duringthe whole period covered by our observations.
| The COMPLETE Survey of Star-Forming Regions: Phase I Data
We present an overview of data available for the Ophiuchus and Perseusmolecular clouds from Phase I of the COMPLETE Survey of Star-FormingRegions. This survey provides a range of data complementary to theSpitzer Legacy Program ``From Molecular Cores to Planet Forming Disks.''Phase I includes the following: extinction maps derived from the TwoMicron All Sky Survey (2MASS) near-infrared data using the NICERalgorithm; extinction and temperature maps derived from IRAS 60 and 100μm emission; H I maps of atomic gas; 12CO and13CO maps of molecular gas; and submillimeter continuumimages of emission from dust in dense cores. Not unexpectedly, themorphology of the regions appears quite different depending on thecolumn density tracer that is used, with IRAS tracing mainly warmer dustand CO being biased by chemical, excitation, and optical depth effects.Histograms of column density distribution are presented, showing thatextinction as derived from 2MASS NICER gives the closest match to alognormal distribution, as is predicted by numerical simulations. Allthe data presented in this paper, and links to more detailedpublications on their implications, are publicly available at theCOMPLETE Web site.
| Evolving structures of star-forming clusters
Context: .Understanding the formation and evolution of young starclusters requires quantitative statistical measures of theirstructure.Aims.We investigate the structures of observed and modelledstar-forming clusters. By considering the different evolutionary classesin the observations and the temporal evolution in models ofgravoturbulent fragmentation, we study the temporal evolution of thecluster structures.Methods.We apply different statistical methods, inparticular the normalised mean correlation length and the minimumspanning tree technique. We refine the normalisation of the clusteringparameters by defining the area using the normalised convex hull of theobjects and investigate the effect of two-dimensional projection ofthree-dimensional clusters. We introduce a new measure ξ for theelongation of a cluster. It is defined as the ratio of the clusterradius determined by an enclosing circle to the cluster radius derivedfrom the normalised convex hull. Results.The mean separation of youngstars increases with the evolutionary class, reflecting the expansion ofthe cluster. The clustering parameters of the model clusters correspondin many cases well to those from observed ones, especially when the ξvalues are similar. No correlation of the clustering parameters with theturbulent environment of the molecular cloud is found, indicating thatpossible influences of the environment on the clustering behaviour arequickly smoothed out by the stellar velocity dispersion. The temporalevolution of the clustering parameters shows that the star clusterbuilds up from several subclusters and evolves to a more centrallyconcentrated cluster, while the cluster expands slower than new starsare formed.
| The Large Magellanic Cloud: diffuse interstellar bands, atomic lines and the local environmental conditions
The Large Magellanic Cloud (LMC) offers a unique laboratory to study thediffuse interstellar bands (DIBs) under conditions that are profoundlydifferent from those in the Galaxy. DIB carrier abundances depend onseveral environmental factors, in particular the local UV radiationfield. In this paper we present measurements of twelve DIBs in fivelines of sight to early-type stars in the LMC, including the 30Doradus region. From the high resolution spectra obtained withVLT/UVES we also derive environmental parameters that characterise thelocal interstellar medium (ISM) in the probed LMC clouds. These includethe column density components (including total column density) for theatomic resonance lines of Na I, Ca II, Ti II, K I. In addition, wederive the H I column density from 21 cm line profiles, thetotal-to-selective visual extinction RV and the gas-to-dustratio N(H I)/A_V. Furthermore, from atomic line ratios we derive theionisation balance and relative UV field strength in these environments.We discuss the properties of the LMC ISM in the context of DIB carrierformation. The behaviour of DIBs in the LMC is compared to that of DIBsin different local environmental conditions in the Milky Way. A keyresult is that in most cases the diffuse band strengths are weak (up tofactor 5) with respect to Galactic lines of sight of comparablereddening, EB-V. In the line of sight towards Sk -69223 the 5780 and 5797 Å DIBs are very similar instrength and profile to those observed towards HD144217, which is typical of an environment exposed to a strongUV field. From the velocity analysis we find that DIB carriers (towardsSk -69 243) are better correlated with the ionisedspecies like Ca II than with neutrals (like Na I and CO). The mostsignificant parameter that governs the behaviour of the DIB carrier isthe strength of the UV field.
| An unbiased search for the signatures of protostars in the ρ Ophiuchi molecular cloud . II. Millimetre continuum observations
The dense cores which conceive and cradle young stars can be exploredthrough continuum emission from associated dust grains. We haveperformed a wide field survey for dust sources at 1.2 millimetres in theρ Ophiuchi molecular cloud, covering more than 1 square degree in anunbiased fashion. We detect a number of previously unknown sources,ranging from extended cores over compact, starless cores to envelopessurrounding young stellar objects of Class 0, Class I, and Class IItype. We analyse the mass distribution, spatial distribution and thepotential equilibrium of the cores. For the inner regions, the surveyresults are consistent with the findings of previous narrower surveys.The core mass function resembles the stellar initial mass function, withthe core mass function shifted by a factor of two to higher masses (forthe chosen opacity and temperature). In addition, we find no statisticalvariation in the core mass function between the crowded inner regionsand those in more isolated fields except for the absence of the mostmassive cores in the extended cloud. The inner region contains compactercores. This is interpreted as due to a medium of higher mean pressurealthough strong pressure variations are evident in each region. Thecores display a hierarchical spatial distribution with no preferredseparation scale length. However, the frequency distribution of nearestneighbours displays two peaks, one of which at 5000 AU can be the resultof core fragmentation. The orientations of the major axes of cores areconsistent with an isotropic distribution. In contrast, the relativeorientations of core pairs are preferentially in the NW-SE direction onall separation scales. These results are consistent with core productionand evolution in a turbulent environment. Finally, we report thediscovery of a new, low-mass Class 0 object candidate and its COoutflow.
| The ratio of N(C18O) and AV in Chamaeleon I and III-B. Using 2MASS and SEST
We investigate the relationship between the C18O columndensity and the visual extinction in Chamaeleon I andin a part of the Chamaeleon III molecular cloud. TheC18O column densities, N(C18O), are calculatedfrom J=1{-}0 rotational line data observed with the SEST telescope. Thevisual extinctions, A_V, are derived using {JHK} photometry from the2MASS survey and the NICER color excess technique. In contrast with theprevious results of Hayakawa et al. (2001, PASJ, 53, 1109), we find thatthe average N(C18O)/AV ratios are similar in Cha Iand Cha III, and lie close to values derived for other clouds, i.e.N(C18O) ≈ 2 × 1014 cm-2 (AV - 2 ). We find, however, clear deviations from thisaverage relationship towards individual clumps. Larger than averageN(C18O)/AV ratios can be found in clumpsassociated with the active star forming region in the northern part ofCha I. On the other hand, some regions in the relatively quiescentsouthern part of Cha I show smaller than averageN(C18O)/AV ratios and also very shallowproportionality between N(C18O) and A_V. The shallowproportionality suggests that C18O is heavily depleted inthese regions. As the degree of depletion is proportional to the gasdensity, these regions probably contain very dense, cold cores, which donot stand out in CO mappings. A comparison with the dust temperature mapderived from the ISO data shows that the most prominent of thepotentially depleted cores indeed coincides with a dust temperatureminimum. It seems therefore feasible to use N(C18O) andAV data together for identifying cold, dense cores in largescale mappings.
| XMM-Newton observations of the σ Orionis cluster. II. Spatial and spectral analysis of the full EPIC field
We present the results of an XMM-Newton observation of the young (2{-}4 Myr) cluster around the hot star σ Orionis. In a previouspaper we presented the analysis of the RGS spectrum of the central hotstar; here we discuss the results of the analysis of the full EPICfield. We have detected 175 X-ray sources, 88 of which have beenidentified with cluster members, including very low-mass stars down tothe substellar limit. We detected eleven new possible candidate membersfrom the 2MASS catalogue. We find that late-type stars have a median logL_X/L_bol -3.3, i.e. very close to the saturation limit. Wedetected significant variability in 40% of late-type members orcandidates, including 10 flaring sources; rotational modulation isdetected in one K-type star and possibly in another 3 or 4 stars.Spectral analysis of the brightest sources shows typical quiescenttemperatures in the range T_1 0.3{-}0.8 keV and T_2 1{-}3 keV,with subsolar abundances Z 0.1{-}0.3 Z_ȯ, similar to what isfound in other star-forming regions and associations. We find nosignificant difference in the spectral properties of classical andweak-lined T Tauri stars, although classical T Tauri stars tend to beless X-ray luminous than weak-lined T Tauri stars.
| Radio and X-ray variability of young stellar objects in the Coronet cluster
The Coronet cluster in the nearby R CrA dark cloud offers the rareopportunity to study at least four "class I" protostellar sources aswell as one candidate "class 0" source, a Herbig Ae star, and acandidate brown dwarf within a few square arcminutes - most of themdetected at radio- and X-ray wavelengths. These sources were observedwith the Very Large Array (VLA) at λ = 3.5 cm on nine occasionsin 1998, spread over nearly four months. The source IRS 5, shown earlierto emit circularly polarized radio emission, was observed to undergo aflux increase accompanied by changes in its polarization properties.Comparison with VLA measurements taken in January 1997 allows analysisof longer-term variability. In addition to this radio monitoring, weanalyze archival Chandra and XMM-Newton X-ray data of these sources.Three class I protostars are bright enough for X-ray spectroscopy, andwe perform a variability analysis for these sources, covering a total of154 ks spread over more than two and a half years. Also in X-rays, IRS 5shows the most pronounced variability, whilst the other two class Iprotostars IRS 1 and IRS 2 have more stable emission. X-ray data is alsoanalyzed for the recently identified candidate class 0 source IRS 7E,which shows strong variability as well as for the Herbig Ae star R CrAfor which we find extremely hot X-ray-emitting plasma. For IRS 1, 2 and5, the hydrogen column densities derived from the X-ray spectra are atabout half the values derived with near-infrared techniques, a situationsimilar to what has been observed towards some other young stellarobjects.
| Spectroscopic binary orbits from photoelectric radial velocities. Paper 180: zeta Aurigae
Not Available
| The initial conditions of isolated star formation - VI. SCUBA mappingof pre-stellar cores
Observations have been carried out with the Submillimetre Common-UserBolometer Array (SCUBA) at the James Clerk Maxwell Telescope (JCMT) ofregions of comparatively isolated star formation in molecular cloudcores. Some 52 starless cores were observed, which are molecular cloudcores that do not contain any sign of protostellar activity such asinfrared sources or bipolar outflows. These are all therefore candidatepre-stellar cores, which are believed to represent the stage of starformation that precedes the formation of a protostar. Of the 52 cores,29 were detected at 850 μm at varying levels of signal-to-noise ratiogreater than 3σ at peak, while 23 of the cores were observed butnot detected. The mean detection lower limit of the data correspondsroughly to an AV~ 15 under typical assumptions. The detectedcores were split into `bright' cores and `intermediate' cores, dependingon their peak flux density at 850 μm. Those with peak 850-μm fluxdensities greater than 170 mJy beam-1 were designated`bright' cores. Those with peak 850-μm flux densities less than thisvalue were designated `intermediate' cores. This dividing linecorresponds to a mean detection limit of 10σ at peak, and anapproximate AV~ 50 under typical assumptions. Of the 29detected cores, 13 are found to be bright and 16 are intermediate. Thedata are combined with our previously published ISO data, and thephysical parameters of the cores, such as density and temperature, arecalculated. The bright cores are detected with sufficiently highsignal-to-noise ratio to allow their structure to be mapped. Radial fluxdensity profiles of these show flattened inner regions and sharpboundaries, consistent with previous observations of pre-stellar cores.Detailed fitting of the bright core radial profiles shows that they arenot critical Bonnor-Ebert spheres, in agreement with previous findings.However, we find that intermediate cores, such as B68 (which haspreviously been claimed to be a Bonnor-Ebert sphere), may in fact beconsistent with the Bonnor-Ebert criterion, suggesting perhaps thatcores pass through such a phase during their evolution. We also findthat the masses of the bright cores have a mean value of approximatelythe same order as their virial masses. We make rough estimates of corelifetimes based on the statistics of detections and find that thelifetime of a pre-stellar core is roughly ~3 × 105 yr,while that of a bright core is ~1.5 × 105 yr.Comparisons with some models that regulate collapse using eithermagnetic fields or turbulence show that no model can match all of thedata. Models that are tuned to fit the total pre-stellar core lifetimedo not predict the relative numbers of cores seen at each stage.
| Cloud Structure and Physical Conditions in Star-forming Regions from Optical Observations. II. Analysis
To complement the optical absorption line survey of diffuse moleculargas in Paper I, we obtained and analyzed far-ultraviolet H2and CO data on lines of sight toward stars in Cep OB2 and Cep OB3.Possible correlations between column densities of different species forindividual velocity components, not total columns along a line of sightas in the past, were examined and were interpreted in terms of cloudstructure. The analysis reveals that there are two kinds of CH indiffuse molecular gas: CN-like CH and CH+-like CH. Evidenceis provided that CO is also associated with CN in diffuse molecularclouds. Different species are distributed according to gas density inthe diffuse molecular gas. Both calcium and potassium may be depletedonto grains in high-density gas, but with different dependencies onlocal gas density. Gas densities for components where CN was detectedwere inferred from a chemical model. Analysis of cloud structureindicates that our data are generally consistent with the large-scalestructure suggested by maps of CO millimeter-wave emission. On smallscales, the gas density is seen to vary by factors greater than 5.0 overscales of ~10,000 AU. The relationships between column densities of COand CH with that of H2 along a line of sight show similarslopes for the gas toward Cep OB2 and Cep OB3, but the CO/H2and CH/H2 ratios tend to differ, which we ascribe tovariation in average density along the line of sight.
| FUSE Measurements of Far-Ultraviolet Extinction. II. Magellanic Cloud Sight Lines
We present an extinction analysis of nine reddened/comparison star pairsin the Large and Small Magellanic Clouds (LMC and SMC) based onFar-Ultraviolet Spectroscopic Explorer (FUSE) FUV observations. To date,just two LMC sight lines have probed dust grain composition and sizedistributions in the Magellanic Clouds using spectral data forwavelengths as short as 950 Å. We supplement these two with datafrom four regions distinguished by their IR through UV extinction curvesand grouped as LMCAvg, LMC2, SMC bar, and SMC wing. Despite the distinctcharacters of extinction in the Clouds and Milky Way, our results aregenerally analogous to those found for Galactic curves-namely, that theFUSE portions of each extinction curve are described reasonably well byFitzpatrick & Massa curves fitted only to longer wavelength data andlack any dramatic new extinction features, and any deviations from theCardelli, Clayton, & Mathis (CCM) formalism continue into FUVwavelengths. A maximum entropy method analysis of all of these curvessuggests that LMCAvg and SMC wing sight lines, whose extinctionparameters more closely resemble those for Galactic paths, require moresilicon and/or carbon in dust than current abundance measurements wouldindicate are available. The requirements for LMC2 and SMC bar sightlines do not fully tax the available reservoirs, in part because largegrains contribute less to the extinction in these directions. Anintermediate product of this extinction analysis is the measurement ofnew H2 abundances in the Magellanic Clouds. Collectivelyconsidering Cloud sight lines that possess significant H2column densities, E(B-V)/N(H I) ratios are reduced by significantfactors relative to the Galactic mean, whereas the correspondingE(B-V)/N(H2) values more closely resemble their Galacticcounterpart. These trends reflect the fact that among these sight linesf(H2) values are lower than those common in the Milky Way forpaths with similar degrees of reddening.Based on observations with the NASA-CNES-CSA Far-UltravioletSpectroscopic Explorer, which is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS-32985.
| A Study of Far-Ultraviolet Extinction in the Upper Scorpius Cloud Using the SPINR Sounding Rocket Experiment
In this study, six new interstellar extinction curves in thefar-ultraviolet are presented using data from a sounding rocketexperiment. The sounding rocket data were combined with IUE data for sixlines of sight in the Upper Scorpius group to cover the wavelength rangeof 912-3030 Å. The extinction curves were produced using the paircomparison method with B stars of similar spectral types.Parameterizations from Fitzpatrick & Massa, Cardelli et al., andFitzpatrick were then fitted to the derived extinction curves. From thederived extinction curves, their corresponding fits, and the dust modelof Weingartner & Draine, it is concluded that the dust population inthe Upper Scorpius region exhibits a larger than average grainpopulation with a depletion of smaller grains.
| X-Ray and Infrared Observations of Embedded Young Stars in NGC 2264
Images of the NGC 2264 star-forming region, which we have acquired withthe XMM-Newton spacecraft, reveal strong X-ray emission from threedeeply embedded (AV>=10 mag) young stellar objects in thevicinity of Allen's infrared source (AFGL 989 = IRS 1) and Castelaz& Grasdalen's infrared source (RNO-EW = IRS 2). Thermal plasmamodels for the brightest source in X-rays, located 11" southwest ofAllen's star, yield a quasi-steady luminosity ofLX=1033 ergs s-1 and an extraordinarilyhigh X-ray temperature of 100 MK. The high temperature is consistentwith the presence of emission lines of Fe XXV and Fe XXVI at photonenergies of 6.7 and 6.9 keV, respectively. An even higher temperature ofnearly ~140 MK was observed during the rise phase of a powerfulimpulsive X-ray flare of another young star in the IRS 2 region.Moderate-resolution near-infrared (1-4 μm) spectra of the embeddedobjects, obtained at the NASA Infrared Telescope Facility, exhibit deepwater ice absorption bands, as well as a variety of emission andabsorption features of H I, CO, and both neutral and ionized metals.
| Optical Spectroscopy of the Surface Population of the ρ Ophiuchi Molecular Cloud: The First Wave of Star Formation
We present the results of optical spectroscopy of 139 stars obtainedwith the Hydra multiobject spectrograph. The objects extend over a 1.3deg2 area surrounding the main cloud of the ρ Ophcomplex. The objects were selected from narrowband images to haveHα in emission. Using the presence of strong Hα emission,lithium absorption, location in the Hertzsprung-Russell diagram, orpreviously reported X-ray emission, we were able to identify 88 objectsas young stars associated with the cloud. Strong Hα emission wasconfirmed in 39 objects with line widths consistent with their origin inmagnetospheric accretion columns. Two of the strongest emission-lineobjects are young, X-ray-emitting brown dwarf candidates with M8spectral types. Comparisons of the bolometric luminosities and effectivetemperatures with theoretical models suggest a median age for thispopulation of 2.1 Myr, which is significantly older than the agesderived for objects in the cloud core. It appears that these starsformed contemporaneously with low-mass stars in the Upper Scorpiussubgroup, likely triggered by massive stars in the Upper Centaurussubgroup.
| Herbig Ae/Be Stars in nearby OB Associations
We have carried out a study of the early-type stars in nearby OBassociations spanning an age range of ~3-16 Myr, with the aim ofdetermining the fraction of stars that belong to the Herbig Ae/Be class.We studied the B, A, and F stars in the nearby (<=500 pc) OBassociations Upper Scorpius, Perseus OB2, Lacerta OB1, and Orion OB1,with membership determined from Hipparcos data. We also included in ourstudy the early-type stars in the Trumpler 37 cluster, part of the CepOB2 association. We obtained spectra for 440 Hipparcos stars in theseassociations, from which we determined accurate spectral types, visualextinctions, effective temperatures, luminosities and masses, usingHipparcos photometry. Using colors corrected for reddening, we find thatthe Herbig Ae/Be stars and the classical Be (CBe) stars occupy clearlydifferent regions in the JHK diagram. Thus, we use the location on theJHK diagram, as well as the presence of emission lines and of strong 12μm flux relative to the visual, to identify the Herbig Ae/Be stars inthe associations. We find that the Herbig Ae/Be stars constitute a smallfraction of the early-type stellar population even in the youngerassociations. Comparing the data from associations with different agesand assuming that the near-infrared excess in the Herbig Ae/Be starsarises from optically thick dusty inner disks, we determined theevolution of the inner disk frequency with age. We find that the innerdisk frequency in the age range 3-10 Myr in intermediate-mass stars islower than that in the low-mass stars (<1 Msolar) inparticular, it is a factor of ~10 lower at ~3 Myr. This indicates thatthe timescales for disk evolution are much shorter in theintermediate-mass stars, which could be a consequence of more efficientmechanisms of inner disk dispersal (viscous evolution, dust growth, andsettling toward the midplane).
| Interstellar 12C/13C ratios through CH^+λλ 3957,4232 absorption in local clouds: incomplete mixing in the ISM
The 12C/13C isotope ratio is a tracer of stellaryields and the efficiency of mixing in the ISM.12CH+/13CH+ is not affectedby interstellar chemistry, and is the most secure way of measuring12C/13C in the diffuse ISM.R=12C/13C is 90 in the solar system. Previousmeasurements of 12CH+λλ3957.7,4232.3and 13CH+λλ3958.2,4232.0 absorptiontoward nearby stars indicate some variations in12C/13C, with values ranging from 40 to 90suggesting inefficient mixing. Except for the cloud toward ζOph,these R values are strongly affected by noise. With UVES on the VLT wehave improved on the previous interstellar 12C/13Cmeasurements. The weighted 12C/13C ratio in thelocal ISM is 78.27 ± 1.83, while the weighted dispersion of ourmeasurements is 12.7, giving a 6.9σ scatter. Thus we report on a6.9σ detection of 16.2% root-mean-square variations in the carbonisotopic ratio on scales of ~100 pc: R= 74.7 ± 2.3 in theζOph cloud, while R = 88.6 ± 3.0 toward HD 152235 in theLupus clouds, R = 62.2 ± 5.3 towards HD 110432 in the Coalsack,and R = 98.9 ± 10.1 toward HD 170740. The observed variations in13C/12C are the first significant detection ofchemical heterogeneity in the local ISM.
| A multiplicity survey of the ρ Ophiuchi molecular clouds
We present a volume-limited multiplicity survey with magnitude cutoff({m}K≤ 10.5 mag) of 158 young stellar objects locatedwithin or in the vicinity of the ρ Ophiuchi Dark Cloud. Withexception of eleven already well observed objects, all sources have beenobserved by us in the K-band with 3.5 m telescopes by using speckletechniques. The separation range covered by our survey is0.13''≤θ≤ 6.4'', where the lower limit is given by thediffraction limit of the telescopes and the upper limit by confusionwith background stars. The multiplicity survey is complete for fluxratios ≥ 0.1 (Δ{m}K≤ 2.5) at the diffractionlimit. After taking the background density into account the degree ofmultiplicity is 29.1%± 4.3% and thus only marginally higher thanthe value 23.5%± 4.8 % derived for the given separation range forthe main-sequence solar-like stars in the solar neighbourhood (Duquennoy& Mayor 1991). We discuss the implications of these findings.
| 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.
| Highly ionized gas in the local ISM: Some like it hot?
We present HST-STIS medium-resolution spectra (R 6.5 kms-1) of the ultraviolet interstellar absorption linesobserved towards 4 early-type stars located within the localinterstellar medium (ISM), with sight-line distances <186 pc in thegeneral direction of the Loop I superbubble (l = 330°, b =+18°). These data have been supplemented with high resolution (R 3 km s-1) visible absorption observations of the NaID-lines towards these 4 stars. Our main discovery is the detection ofhighly ionized absorption components of CIV, SiIV and NV towards the twomost distant targets, HD 127381 and HD 142256. These lines-of-sight areknown to cross both the near and far neutral interface boundaries to theLoop I cavity, in addition to intersecting the fragmented shell ofneutral and partially ionized gas that defines the boundary to the LocalBubble. However, the presently measured narrow line profile-widths andtheir measured absorption intensities are found to be incompatible withtheoretical models that predict high ion absorption due to the presenceof evaporating cloud conduction interfaces. We conclude that theformation of high ions in the local ISM is highly dependent on thelocation of the absorbing gas clouds with respect to nearby sources ofboth hot X-ray emitting gas and/or photo-ionization. Our observationshave also revealed at least 6 gas clouds with distances ranging from 5pc to 150 pc along these sight-lines. We have detected a cloud ofneutral and partially ionized gas with a velocity of -15 kms-1 and a hydrogen column density of log N(HI +HII) 19.3 cm-2 that is thought to define the boundary to the LocalBubble cavity at a distance of ~90 pc in this galactic direction. Thefar neutral boundary to the Loop I superbubble cavity is also detectedat a distance of 150-180 pc and is composed of two cold clouds moving atvelocities close to Vhelio 0 km s-1possessing a combined hydrogen column density of log N(HI + HII)≫19.5 cm-2. In contrast, we have also detected three lowdensity, warm and partially ionized diffuse clouds with averagevelocities of ~-10, -23 and -32 km s-1, that are all locatedwithin a distance of ~150 pc. The cloud component at V -23 kms-1 may be associated with the very local "G-cloud" at adistance of <5 pc, but we also provide evidence for its placement ata greater distance. The measured velocities of the majority of the gasclouds we have detected along all 4 sight-lines are consistent with aninflow of gas into the LB cavity from the direction of the Loop Isuperbubble. This gas is flowing through a region of fragmentation at adistance of ~90 pc that represents the interaction region between theLoop I and Local Bubble cavities.
| A survey for Fe 6.4 keV emission in young stellar objects in ρ Oph: The strong fluorescence from Elias 29
We report the results of a search for 6.4 keV Fe fluorescent emission inYoung Stellar Objects (YSOs) with measured accretion luminosities in theρ Oph cloud, using the existing Chandra and XMM-Newton observationsof the region. A total of nine such YSOs have X-ray data withsufficiently high S/N for the 6.4 keV line to be potentially detected ifpresent. A positive detection of the Fe 6.4 keV line is reported for oneobject, Elias 29, in both the XMM-Newton and the Chandra data. The 6.4keV line is detected in Elias 29 both during quiescent and flaringemission, unlikely all previously reported detections of 6.4 keV Fefluorescence in YSOs which were made during intense flaring. Theobserved equivalent width of the fluorescent line is large,Wα ≃ 160 eV, ruling out fluorescence from diffusecircumstellar material. It is also larger than expected for simplereflection from a solar-composition photosphere or circumstellar disk,but it is compatible with being due to fluorescence from a centrallyilluminated circumstellar disk. The X-ray spectrum of Elias 29 is alsopeculiar in terms of its high (ionized) Fe abundance, as evident fromthe very intense Fe xxv 6.7 keV line emission; we speculate on thepossible mechanism leading to the observed high abundance.
| Star-forming protoclusters associated with methanol masers
We present a multiwavelength study of five methanol maser sites whichare not directly associated with a strong (>100 mJy) radio continuumsource: G 31.28+0.06, G 59.78+0.06, G 173.49+2.42 (S231, S233IR), G188.95+0.89 (S252, AFGL5180) and G 192.60-0.05 (S255IR). Theseradio-quiet methanol maser sites are often interpreted as precursors ofultra-compact H II regions or massive protostar sites. In this work, theenvironment of methanol masers is probed from mid-IR to millimetrewavelengths at angular resolutions of 8''-34''. Spectral energydistribution (SED) diagrams for each site are presented, together withmass and luminosity estimates. Each radio-quiet maser site is alwaysassociated with a massive (>50 Mȯ), deeply embedded(Av>40 mag) and very luminous (>104Lȯ) molecular clump, with Ltotal∝Mgas0.75. These physical properties characterisemassive star-forming clumps in earlier evolutionary phases than H IIregions. In addition, colder gas clumps seen only at mm-wavelengths arealso found near the methanol maser sites. These colder clumps mayrepresent an even earlier phase of massive star formation. These resultssuggest an evolutionary sequence for massive star formation from a coldclump, seen only at mm wavelengths, evolving to a hot molecular corewith a two-component SED with peaks at far-IR and mid-IR wavelengths, toan (ultra-compact) H II region. Alternatively, the cold clumps might beclusters of low-mass YSOs, in formation near the massive star-formingclusters. Finally, the values of the dust grain emissivity index(β) range between 1.6 and 1.9.
| Anomalous dust-to-gas ratios in the Galaxy
Lines of sight with E(B-V)/N(HI) considerably smaller than the averagevalue for the solar neighbourhood have been selected from the catalogueof Diplas & Savage. In order to develop quantitative considerations,estimates of the molecular hydrogen column density were obtained usingthe relation of Savage et al. extended at E(B-V) > 0.4 with therecent data of Rachford et al. Contrary to the prevailing opinion in theliterature for sightlines with similar behaviour, we found that only 22per cent of our sample was characterized by both an average gas densitylarger than 1 cm-3 and a value of RV larger thanthat in the diffuse interstellar medium. By computing extinction models,we were able to fit the E(B-V)/N(HI) by changing the value ofRV only for some sightlines. For the remaining ones, aρd/ρH ratio different from the averageGalactic value must be invoked. The application of the Kramers-Kronigrelation to the observed extinction curves confirmed this possibility.Moreover, attempts to fit such curves with models having grain volumescorresponding to the standard ρd/ρH ratiofailed.We find a linear relation between ρd/ρHand E(B-V)/N(H) for our sightlines. The average Galactic value marks theseparation into two groups characterized by lower abundances of C and Sitrapped into the grains when E(B-V)/N(H) is smaller than the Galacticvalue, and by larger abundances when E(B-V)/N(H) is greater.
| A Uniform Database of 2.2-16.5 μm Spectra from the ISOCAM CVF Spectrometer
We present all ISOCAM circular variable filter (CVF) spectra that covermore than one-third of the 2.2-16.5 μm spectral range of theinstrument. The 364 spectra have been classified according to theclassification system of Kraemer et al., as modified by Hodge et al. toaccount for the shorter wavelength range. Prior to classification, thespectra were processed and recalibrated to create a uniform database.Aperture photometry was performed at each wavelength centered on thebrightest position in each image field and the various spectral segmentsmerged into a single spectrum. The aperture was the same for all scalesizes of the images. Since this procedure differs fundamentally fromthat used in the initial ISOCAM calibration, a recalibration of thespectral response of the instrument was required for the aperturephotometry. The recalibrated spectra and the software used to createthem are available to the community on-line via the ISO Data Archive.Several new groups were added to the KSPW system to describe spectrawith no counterparts in either the SWS or PHT-S databases: CA, E/SA,UE/SA, and SSA. The zodiacal dust cloud provides the most commonbackground continuum to the spectral features, visible in almost 40% ofthe processed sources. The most characteristic and ubiquitous spectralfeatures observed in the CVF spectral atlas are those of theunidentified infrared bands (UIR), which are typically attributed toultraviolet-excited fluorescence of large molecules containing aromatichydrocarbons. The UIR features commonly occur superimposed on thezodiacal background (18%) but can also appear in conjunction with otherspectral features, such as fine-structure emission lines or silicateabsorption. In at least 13 of the galaxies observed, the pattern of UIRemission features has been noticeably shifted to longer wavelengths.Based on observations with the Infrared Space Observatory, a EuropeanSpace Agency (ESA) project with instruments funded by ESA Member States(especially the Principal Investigator countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of theInstitute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| A Multiplicity Survey of the ρ Oph Dark Cloud
Not Available
| Stellar Coronal Astronomy
Coronal astronomy is by now a fairly mature discipline, with a quartercentury having gone by since the detection of the first stellar X-raycoronal source (Capella), and having benefitted from a series of majororbiting observing facilities. Serveral observational characteristics ofcoronal X-ray and EUV emission have been solidly established throughextensive observations, and are by now common, almost text-book,knowledge. At the same time the implications of coronal astronomy forbroader astrophysical questions (e.g.Galactic structure, stellarformation, stellar structure, etc.) have become appreciated. Theinterpretation of stellar coronal properties is however still often opento debate, and will need qualitatively new observational data to bookfurther progress. In the present review we try to recapitulate our viewon the status of the field at the beginning of a new era, in which thehigh sensitivity and the high spectral resolution provided by Chandraand SMM-Newton will address new questions which were not accessiblebefore.
| The dynamical evolution of Taurus-Auriga-type aggregates
Star formation in the Taurus-Auriga (TA) molecular clouds is producingbinary-rich aggregates containing at most a few dozen systems within aregion spanning 1 pc without massive stars. This environment is verydifferent from another well-studied star-forming event which producedthe Orion Nebula cluster (ONC). The ONC contains a few thousand systemswithin a region of 1 pc including massive stars. Differences betweenthese two environments have been found. Notably, the ONC has asignificantly smaller binary proportion but a significantly largernumber of isolated brown dwarfs (BDs) per star than TA. The aim of thepresent project is to investigate if these differences can be explainedthrough stellar-dynamical evolution alone. The stellar-dynamical issueis very relevant because dense environments destroy binaries liberatingBD companions, possibly leading to the observed difference between theTA and ONC populations. Here a series of high-precision N-body models ofTA-like embedded aggregates are presented, assuming the standardreference star formation model for the input populations according towhich stars and BDs form with the same kinematical, spatial and binaryproperties. After a discussion of the general evolution of theaggregates, it is shown that the binary population indeed remains mostlyunevolved. Therefore TA-type star formation cannot have addedsignificantly to the Galactic field population. The standard model leadsto BDs tracing the stellar distribution, apart from a high-velocity tail(v>~ 1 km s-1) which leads to a more widely distributedspatial distribution of single BDs. The slow-moving BDs, however, retaina high binary proportion, this being an important observationaldiagnostic for testing against the embryo-ejection hypothesis.Inferences about the initial mass function and the binary star orbitaldistribution functions are presented in two accompanying papers withuseful implications for star formation and the possible origin of BDs.
| Brown Dwarfs in Southern Star Forming Regions
Most of the star forming regions within 200 pc from the Sun are locatedSouth of the celestial equator. Brown dwarfs or strong candidates havebeen discovered in all of them, and have been often studied at multiplewavelengths. This paper summarizes some highlights of the research onyoung brown dwarfs in these regions.
| On the Relationship Between Stellar Rotation and Radius in Young Clusters
We have compiled data on rotational velocities for more than 1000 K andM stars in 12 young clusters ranging in age from Orion to the Hyades.These data enable a search for systematic changes in stellar rotationalvelocity vs. age. Taken together, these data show that most pre-mainsequence (PMS) stars spanning ages from about 0.1 to about 1 Myr do notappear to spin up in response to contraction down their convectivetracks, that spin up between 1 and 3 million years is modest at best,and that some stars do not appear to spin up at all.These results extend and reinforce our earlier study (Rebull et al.2002), based on observations of several hundred stars in the OrionFlanking Fields, NGC 2264, and the Orion Nebula Cluster (ONC), whichshowed that the majority of PMS stars in these three groups apparentlydo not conserve stellar angular momentum as they contract, but insteadevolve at nearly constant angular velocity. This result applies both tostars with and without near-IR I-K excesses indicative of disks.
| 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.
|
Új cikk hozzáadása
Kapcsolódó hivatkozások
- - (nincs kapcsolódó hivatkozás) -
Új link hozzáadása
Besorolás csoportokba:
|
Pozíciós és asztrometriai adatok
| Csillagkép: | Kígyótartó |
| Rektaszcenzió: | 16h25m35.20s |
| Deklináció: | -23°26'50.0" |
| Vizuális fényesség: | 5.02 |
| Távolság: | 120.919 parszek |
| RA sajátmozgás: | 35.1 |
| Dec sajátmozgás: | -31.9 |
| B-T magnitude: | 5.258 |
| V-T magnitude: | 5.067 |
Katalógusok és elnevezések:
|
INHABITED SKY
