תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
A multiwavelength investigation of the temperature of the cold neutral medium
We present measurements of the HI spin temperatures (Ts) ofthe cold neutral medium (CNM) towards radio sources that are closelyaligned with stars for which published H2 ortho-paratemperatures (T01) are available from ultraviolet (UV)observations. Our sample consists of 18 radio sources close to 16 nearbystars. The transverse separation of the lines of sight of thecorresponding UV and radio observations varies from 0.1 to 12.0 pc atthe distance of the star. The UV measurements do not have velocityinformation, so we use the velocities of low ionization species (e.g.NaI/KI/CI) observed towards these same stars to make a plausibleidentification of the CNM corresponding to the H2 absorption.We then find that T01 and Ts match withinobservational uncertainties for lines of sight with H2 columndensity above 1015.8cm-2, but deviate from eachother below this threshold. This is consistent with the expectation thatin the CNM Ts tracks the kinetic temperature due tocollisions and that T01 is driven towards the kinetictemperature by proton exchange reactions.
| The Large- and Small-Scale Structures of Dust in the Star-forming Perseus Molecular Cloud
We present an analysis of ~3.5 deg2 of submillimetercontinuum and extinction data of the Perseus molecular cloud. Weidentify 58 clumps in the submillimeter map, and we identify 39structures (``cores'') and 11 associations of structures (``supercores'') in the extinction map. The cumulative mass distributions of thesubmillimeter clumps and extinction cores have steep slopes (α~2and 1.5-2, respectively), steeper than the Salpeter initial massfunction (IMF; α=1.35), while the distribution of extinction supercores has a shallow slope (α~1). Most of the submillimeter clumpsare well fit by stable Bonnor-Ebert spheres with 10 K5-7 mag), althoughBonnor-Ebert models suggest that we should have been able to detect themat lower column densities if they exist. These observations provide astronger case for an extinction threshold than that found in analysis ofless sensitive observations of the Ophiuchus molecular cloud (Johnstoneet al.). The relationship between submillimeter clumps and their parentextinction core has been analyzed. The submillimeter clumps tend to lieoffset from the larger extinction peaks, suggesting that the clumpsformed via an external triggering event, consistent with previousobservations.
| Testing Rotational Mixing Predictions with New Boron Abundances in Main-Sequence B-Type Stars
New boron abundances for seven main-sequence B-type stars are determinedfrom HST STIS spectroscopy around the B III 2066 Å line. Boronabundances provide a unique and critical test of stellar evolutionmodels that include rotational mixing, since boron is destroyed in thesurface layers of stars through shallow mixing long before otherelements are mixed from the stellar interior through deep mixing. Thestars in this study are all on or near the main sequence and are membersof young Galactic clusters. They show no evidence of mixing with gasfrom H-burning layers from their CNO abundances. Boron abundances rangefrom 12+log(B/H)<=1.0 to 2.2. The boron abundances are compared tothe published values of the stellar nitrogen abundances [all have12+log(N/H)<=7.8] and to their host cluster ages (4-16 Myr) toinvestigate the predictions from models of massive star evolution withrotational mixing effects. We find that the variations in boron andnitrogen are generally within the range of the predictions from thestellar evolution models with rotation (where predictions for modelswith rotation rates from 0 to 450 km s-1 and μ-barriersare examined), especially given their age and mass ranges. Three stars(of 34 B-type stars with detailed boron abundance determinations)deviate from the model predictions, showing either much larger borondepletions than can be explained by the rotating model predictions or aspectroscopic mass that is lower than expected, given the rotating modelpredictions for its age and abundances. The results from these threestars suggest that rotational mixing could be more efficient than thatcurrently modeled at the highest rotation rates.
| A high-resolution spectroscopy survey of β Cephei pulsations in bright stars
We present a study of absorption line-profile variations in early-B typenear-main-sequence stars without emission lines. We have surveyed atotal of 171 bright stars using the Nordic Optical Telescope (NOTSA),William Herschel Telescope (ING) and Coudé Auxiliary Telescope(ESO). Our sample contains 75% of all O9.5-B2.5 III-V non-emission-linestars brighter than 5.5 mag. We obtained high signal-to-noise,high-resolution spectra of the SiIII λ4560 triplet - for 125stars of our sample we obtained more than one spectrum - and examinedthese for pulsational-like line-profile variations and/or structure. Weconclude that about half of our sample stars show evidence forline-profile variations (LPV). We find evidence for LPV in about 65% ofour sample stars brighter than V=5.5. For stars with rotationalbroadening V sin i 100 km s-1, we find evidence for LPVin about 75% of the cases. We argue that it is likely that these LPV areof pulsational origin, and that hence more than half of thesolar-neighbourhood O9.5-B2.5 III-V stars is pulsating in modes that canbe detected with high-resolution spectroscopy. We detected LPV in 64stars previously unknown to be pulsators, and label these stars as newβ Cep candidates. We conclude that there is no obvious differencein incidence of (pulsational) LPV for early-B type near-main-sequencestars in binaries or in OB associations, with respect to single fieldstars.
| Neutral carbon and CO emission in the core and the halo of dark cloud Barnard 5
Aims.The physical conditions and chemical structure in the dark cloud ofBarnard 5 and its surrounding atomic halo is studied.The impact of the halo on the line emission emerging from the molecularcloud is investigated.Methods.We present observations of the [CI]^3P1→ ^3P0 transition of neutral carbon and the low-J transitionsof 12CO and 13CO. The CO maps extend from the core(Av 7) to the northern cloud edge and into the halo(Av 1). They are complemented by deeply integrated [CI]spectra made along a 1D cut of similar extent. Escape probability andphoton-dominated region (PDR) models are employed to interpret theobservations.Results.12CO and 13CO are detected inthe cloud and the halo, while [CI] is detected only toward the molecularcloud. This occurs even though the neutral carbon column density is5 times larger than the CO column density in the halo, but it canbe understood in terms of excitation. The [CI] excitation is governed bycollisions even at the low halo densities, while the CO excitation isdominated by the absorption of line photons emitted by the nearbymolecular cloud. The upper limit on the neutral carbon column density inthe halo is 6× 1015 cm-2. The PDR studiesshow that even small column densities of H2 and CO, such as those in theB5 halo, can significantly change the [CI] and COline emission (pre-shielding). Since this effect decreases the [CI]intensity and increases the CO intensity, the largest impact is notedfor the [CI]/CO line ratios. For the B5 cloud, a PDRmodel with a molecular hydrogen column density of ~6×1019 cm-2 in the halo matches the observed [CI]/COline ratios best. Models with no pre-shielding, in contrast, suggesthigh gas densities that are in conflict with independently deriveddensities. The PDR models with a χ<1 demonstrate that the [CI]/COratios cannot be attributed solely to a reduced FUV field.
| Multiple Outflows and Protostars in Barnard 1
Using optical (Hα and [S II]), near-IR (H2 andKs), and submillimeter (850 and 450 μm) data, we haveexamined the region surrounding the Barnard 1 (B1) core and found amultitude of new shocks from protostellar outflows. We trace severalflows, some of which are large, parsec-scale outflows with dynamic agesof order 104 yr, indicating that star formation has beentaking place in B1 for at least that long. We can confidently identifyeight protostars that are driving outflows. Of those eight protostars,one source, SMM 2 (SMM J033330+31095) is a new Class 0 source, giving B1a total of three Class 0 protostars. Based on the number of shocks andprotostars in this region, B1 appears to be a much more active region ofstar formation than previously thought. The number of shocks iscomparable to or greater than those of other active star-forming regionsin Perseus (e.g., IC 348, L1455, and L1448).
| Deep Imaging Surveys of Star-forming Clouds. III. Herbig-Haro Objects in the Perseus Molecular Cloud
We present a catalog of 72 new Herbig-Haro (HH) objects discovered inthe Perseus molecular cloud. There are 69 previously cataloged HHobjects in this region, and the new discoveries bring the total numberof known HH objects in Perseus to 141. Individual outflows often containseveral distinct HH objects. These observations demonstrate that thePerseus molecular cloud is one of the most active star-forming regionsin the solar vicinity. We explore different methods for probing themomentum injection rate of outflows and examine whether outflows candrive turbulence within the molecular cloud. On the scale of the entirePerseus cloud, the shocks produced by outflows from young stars may notinject momentum at a sufficient rate to counter the rate at whichmomentum decays. However, intense outflow activity within individualcloud cores with high star formation rates, such as NGC 1333, may besufficient to locally support or even disrupt the core.
| 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).
| Correlations between diffuse interstellar bands and atomic lines
We present and discuss correlations between strengths of the well-known,strong interstellar atomic lines of KI and CaII, and four selected,strong unidentified diffuse interstellar bands (DIBs): 5780, 5797, 5850and 6614. In order to analyse a homogeneous sample of echellehigh-resolution spectra it has been chosen to use measurements fromTerskol Observatory in Northern Caucasus plus a selected number ofhigher resolution observations performed using other instruments. Wedemonstrate that the strength of certain DIBs correlate well withneutral potassium lines and to a much lower degree with ionized calciumlines. This fact suggests that the degree of irradiation of a cloud withUV photons, capable to ionize interstellar atoms, plays a crucial rolein the formation/maintenance of certain molecular species: possiblecarriers of DIBs.
| Deep Imaging Surveys of Star-Forming Clouds II. A New Giant Herbig-Haro Flow in L1451
We report the discovery of a new giant Herbig-Haro flow near the L1451region of the Perseus molecular cloud. The east-west oriented flowcontains two known HH objects (HH 280 and HH 317), two new HH objects(HH 492 and HH 493), and is 2.1 pc long. The flow is driven by the ClassI protostar IRAS 03235+3004 embedded in a bright-rimmed, sharp-edgedcometary cloud facing northeast. The flow source is embedded in awest-facing conical reflection nebula. The cometary cloud appears tohave been shaped by O and B stars in the Per OB2 association, includingthe B0.5 star 40 Per, located along the well-defined symmetry axis ofthe cloud and with a projected separation of 26 pc. A soft-UV inducedphotoablation model is proposed to explain the properties of thecometary cloud.
| On the Hipparcos parallaxes of O stars
We compare the absolute visual magnitude of the majority of bright Ostars in the sky as predicted from their spectral type with the absolutemagnitude calculated from their apparent magnitude and the Hipparcosparallax. We find that many stars appear to be much fainter thanexpected, up to five magnitudes. We find no evidence for a correlationbetween magnitude differences and the stellar rotational velocity assuggested for OB stars by Lamers et al. (1997, A&A, 325, L25), whosesmall sample of stars is partly included in ours. Instead, by means of asimulation we show how these differences arise naturally from the largedistances at which O stars are located, and the level of precision ofthe parallax measurements achieved by Hipparcos. Straightforwardlyderiving a distance from the Hipparcos parallax yields reliable resultsfor one or two O stars only. We discuss several types of bias reportedin the literature in connection with parallax samples (Lutz-Kelker,Malmquist) and investigate how they affect the O star sample. Inaddition, we test three absolute magnitude calibrations from theliterature (Schmidt-Kaler et al. 1982, Landolt-Börnstein; Howarth& Prinja 1989, ApJS, 69, 527; Vacca et al. 1996, ApJ, 460, 914) andfind that they are consistent with the Hipparcos measurements. AlthoughO stars conform nicely to the simulation, we notice that some B stars inthe sample of \citeauthor{La97} have a magnitude difference larger thanexpected.
| On the relation between diffuse bands and column densities of H2, CH and CO molecules
Mutual relations between column densities of H2, CH and COmolecules as well as between the latter and strengths of the major 5780and 5797 diffuse bands are presented and discussed. The CH radical seemsto be a good H2 tracer, possibly better than CO. It is alsodemonstrated that the molecular fraction of the H2 moleculeis correlated with an intensity ratio of 5797 and 5780 DIBs, suggestingthe possible formation of narrow DIB carriers in denser clouds,dominated by molecular hydrogen and reasonably shielded from ionizing UVradiation by small dust grains.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/414/949
| Toward an adequate method to isolate spectroscopic families of diffuse interstellar bands
We divide some of the observed diffuse interstellar bands (DIBs) intofamilies that appear to have the spectral structure of single species.Three different methods are applied to separate such families, exploringthe best approach for future investigations of this type. Starting witha statistical treatment of the data, we found that statistical methodsby themselves give insufficient results. Two other methods of dataanalysis (`averaging equivalent widths' and `investigating the figureswith arranged spectrograms') were found to be more useful as tools forfinding the spectroscopic families of DIBs. On the basis of thesemethods, we suggest some candidates as `relatives' of 5780- and5797-Å bands.
| Observations of Rotationally Resolved C3 in Translucent Sight Lines
The rotationally resolved spectrum of theA1Πu<--X1Σ+g000-000 transition of C3, centered at 4051.6 Å, hasbeen observed along 10 translucent lines of sight. To interpret thesespectra, a new method for the determination of column densities andanalysis of excitation profiles involving the simulation and fitting ofobserved spectra has been developed. The populations of lower rotationallevels (J<=14) in C3 are best fitted by thermaldistributions that are consistent with the kinetic temperaturesdetermined from the excitation profile of C2. Just as in thecase of C2, higher rotational levels (J>14) ofC3 show increased nonthermal population distributions inclouds that have been determined to have total gas densities below ~500cm-3.
| 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.
| Observations of Star-Forming Regions with the Midcourse Space Experiment
We have imaged seven nearby star-forming regions, the Rosette Nebula,the Orion Nebula, W3, the Pleiades, G300.2-16.8, S263, and G159.6-18.5,with the Spatial Infrared Imaging Telescope on the Midcourse SpaceExperiment (MSX) satellite at 18" resolution at 8.3, 12.1, 14.7, and21.3 μm. The large angular scale of the regions imaged (~7.2-50deg2) makes these data unique in terms of the combination ofsize and resolution. In addition to the star-forming regions, twocirrus-free fields (MSXBG 160 and MSXBG 161) and a field near the southGalactic pole (MSXBG 239) were also imaged. Point sources have beenextracted from each region, resulting in the identification over 500 newsources (i.e., no identified counterparts at other wavelengths), as wellas over 1300 with prior identifications. The extended emission from thestar-forming regions is described, and prominent structures areidentified, particularly in W3 and Orion. The Rosette Nebula isdiscussed in detail. The bulk of the mid-infrared emission is consistentwith that of photon-dominated regions, including the elephant trunkcomplex. The central clump, however, and a line of site toward thenorthern edge of the cavity show significantly redder colors than therest of the Rosette complex.
| Grey extinction in the solar neighbourhood?
Some of the close O and B dwarfs appear to be fainter than indicated bytheir Hipparcos distances, intrinsic absolute magnitudes attributed totheir spectral types, and estimated selective interstellar extinction.This discrepancy is explained in the paper by the grey (neutral)interstellar extinction in the visual range of spectrum. The measure ofsuch an effect is related to discrete features of the interstellarmatter.
| Radial velocities of early-type stars in the Perseus OB2 association
We present radial velocities for 29 B- and A-type stars in the field ofthe nearby association Perseus OB2. The velocities are derived fromspectra obtained with AURELIE, via cross correlation with radialvelocity standards matched as closely as possible in spectral type. Theresulting accuracy is ~ 2-3 km s-1. We use thesemeasurements, together with published values for a few other early-typestars, to study membership of the association. The mean radial velocity(and measured velocity dispersion) of Per OB2 is 23.5 +/- 3.9 kms-1, and lies ~ 15 km s-1 away from the meanvelocity of the local disk field stars. We identify a number ofinterlopers in the list of possible late-B- and A-type members which wasbased on Hipparcos parallaxes and proper motions, and discuss thecolour-magnitude diagram of the association.Based on observations made at the Observatoire de Haute-Provence (CNRS),France.
| Interferometric Observations of the H II Region Around xi Per
Not Available
| Rotational Velocities of B Stars
We measured the projected rotational velocities of 1092 northern B starslisted in the Bright Star Catalogue (BSC) and calibrated them againstthe 1975 Slettebak et al. system. We found that the published values ofB dwarfs in the BSC average 27% higher than those standards. Only 0.3%of the stars have rotational velocities in excess of two-thirds of thebreakup velocities, and the mean velocity is only 25% of breakup,implying that impending breakup is not a significant factor in reducingrotational velocities. For the B8-B9.5 III-V stars the bimodaldistribution in V can be explained by a set of slowly rotating Ap starsand a set of rapidly rotating normal stars. For the B0-B5 III-V starsthat include very few peculiar stars, the distributions in V are notbimodal. Are the low rotational velocities of B stars due to theoccurrence of frequent low-mass companions, planets, or disks? Therotational velocities of giants originating from late B dwarfs areconsistent with their conservation of angular momentum in shells.However, we are puzzled by why the giants that originate from the earlyB dwarfs, despite having 3 times greater radii, have nearly the samerotational velocities. We find that all B-type primaries in binarieswith periods less than 2.4 days have synchronized rotational and orbitalmotions; those with periods between 2.4 and 5.0 days are rotating withina factor 2 of synchronization or are ``nearly synchronized.'' Thecorresponding period ranges for A-type stars are 4.9 and 10.5 days, ortwice as large. We found that the rotational velocities of the primariesare synchronized earlier than their orbits are circularized. The maximumorbital period for circularized B binaries is 1.5 days and for Abinaries is 2.5 days. For stars of various ages from 107.5 to1010.2 yr the maximum circularized periods are a smoothexponential function of age.
| Boron Abundances in B-Type Stars: A Test of Rotational Depletion during Main-Sequence Evolution
Boron abundances have been derived for seven main-sequence B-type starsfrom Hubble Space Telescope STIS spectra around the B III λ2066line. In two stars, boron appears to be undepleted with respect to thepresumed initial abundance. In one star, boron is detectable but isclearly depleted. In the other four stars, boron is undetectable,implying depletions of 1-2 dex. Three of these four stars are nitrogenenriched, but the fourth shows no enrichment of nitrogen. Onlyrotationally induced mixing predicts that boron depletions areunaccompanied by nitrogen enrichments. The inferred rate of borondepletion from our observations is in good agreement with thesepredictions. Other boron-depleted nitrogen-normal stars are identifiedfrom the literature. In addition, several boron-depleted nitrogen-richstars are identified, and while all fall on the boron-nitrogen trendpredicted by rotationally induced mixing, a majority have nitrogenenrichments that are not uniquely explained by rotation. The spectrahave also been used to determine iron group (Cr, Mn, Fe, and Ni)abundances. The seven B-type stars have near-solar iron groupabundances, as expected for young stars in the solar neighborhood. Wehave also analyzed the halo B-type star PG 0832+676. We find[Fe/H]=-0.88+/-0.10, and the absence of the B III line gives the upperlimit [B/H]<-2.5. These and other published abundances are used toinfer the star's evolutionary status as a post-asymptotic giant branchstar. Based on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555. These observations are associatedwith proposal GO 07400.
| Pre-main-sequence stars in the young open cluster NGC 1893. I. A spectroscopic search for candidates in the area photometrically surveyed
We present low-resolution spectroscopy of the majority of the stars inthe field of NGC 1893 which were classified asnon-members based on the photometry of Marco et al. (2001). We separatefield stars from pre-main-sequence members based on a combination ofspectral type and photometric indices which allows us to determine thereddening to the objects. For candidates to pre-main-sequence objects,we present intermediate-resolution spectra. We confirm 3 F-typeemission-line objects as massive T Tauri stars in NGC1893 and find two good candidates for being absorption-lineHerbig A stars. A B5 star is a very good candidate for anabsorption-line Herbig B star. We also investigate the nature ofemission-line B-type stars close to the cluster centre. Availableevidence suggests that both classical Be stars and Herbig Be stars arepresent in NGC 1893. Based on observations obtainedat the Asiago Observatory (Italy), the Isaac Newton Telescope (La Palma,Spain) and Observatoire de Haute Provence (CNRS, France).
| Tomographic Separation of Composite Spectra. VII. The Physical Properties of the Massive Triple System HD 135240 (δ Circini)
We present the results of a radial velocity study of the massive,double-lined, O binary HD 135240 based primarily on UV spectroscopy fromthe International Ultraviolet Explorer. Cross-correlation methodsindicate the presence of a third stationary spectral line componentwhich indicates that the system is a triple consisting of a central 3.9day close binary with a distant companion. We measured radial velocitiesfrom the cross-correlation functions after removal of the thirdcomponent, and we combined these with velocities obtained from Hαspectroscopy to reassess the orbital elements. We applied a Dopplertomography algorithm to reconstruct the individual UV spectra of allthree stars, and we determine spectral classifications of O7 III-V, O9.5V, and B0.5 V for the primary, secondary, and tertiary, respectively,using UV criteria defined by Penny, Gies, & Bagnuolo. We comparethese reconstructed spectra to standard single-star spectra to find theUV flux ratios of the components(F2/F1=0.239+/-0.022, andF3/F1=0.179+/-0.021). Hipparcos photometry revealsthat the central pair is an eclipsing binary, and we present the firstmodel fit of the light curve from which we derive an orbitalinclination, i=74deg+/-3deg. This analysisindicates that neither star is currently experiencing Roche lobeoverflow. We place the individual components in the theoretical H-Rdiagram, and we show that the masses derived from the combinedspectroscopic and photometric analysis(Mp/Msolar=21.6+/-2.0 andMs/Msolar=12.4+/-1.0) are significantly lower thanthose computed from evolutionary tracks for single stars.
| Boron Abundances in Early B Stars: Results from the B III Resonance Line in IUE Data
We have used archival International Ultraviolet Explorer high-dispersionSWP spectra to study the B III resonance line at 2065.8 Å in 44early B stars. We find a median boron abundance about half that of solarsystem meteoritic material, consistent with the values found fromprevious studies of boron in early B stars. About one-third of the starsstudied appear to have boron abundances that are a factor of 4 or morelower than this median. Many of these are stars with enhanced nitrogenabundances, confirming the belief that deep envelope mixing can occur inmain-sequence B stars. A few stars with low boron abundances have normalN/C ratios. It is unclear whether all of these can be explained as starswhere mixing depleted the boron but did not go deep enough to bringCN-processed material to the surface or if some stars were actuallyformed with an unusually low boron abundance.
| Interstellar CO towards X Persei (HD 24534) - II. Two-component model
The observations made by the Goddard High Resolution Spectrograph (GHRS)aboard the Hubble Space Telescope (HST) of molecular CO in absorbing gastowards X Persei are reported. The two-component statistical equilibriummodel incorporating radiative excitation of CO by line emission at thesame velocity that originates in nearby molecular clouds has been usedto reproduce high-resolution GHRS spectra. Earlier analysis indicatesthat the cloud has a complex structure and at least a two-componentmodel should be used to obtain accurate results. The spectra obtainedfrom the International Ultraviolet Explorer (IUE) were used tocomplement GHRS data and constrain the space of possible solutions. Thenew oscillator strengths recommended by Eidelsberg et al. for A-X bandshave been used. The results show that one of the components may beattributed to the Perseus OB2 molecular cloud, and the other componentto an extension of the Taurus dark cloud. The total CO column densityN(CO)=(1.0+/-0.2)×1016cm-2 has beendetermined. According to the results about 85per cent of the observed CObelongs to an extension of the Taurus dark cloud. The CO radiation thatoriginates in nearby molecular clouds may be the dominant excitationmechanism of the observed CO. The early results of 13CO lineanalysis indicate a 13CO/12CO ratio of about 40.
| Interstellar CO towards X Persei (HD 24534) - I. One-component model
Data on molecular CO in absorbing gas towards X Per, from theInternational Ultraviolet Explorer (IUE) and the echelle mode of theGoddard High Resolution Spectrograph (GHRS) aboard the Hubble SpaceTelescope, are reported. A statistical equilibrium model is used torecreate equivalent widths of CO bands and reproduce high-resolutionGHRS spectra. The CO column densityN(CO)=(1.0+/-0.2)x1016cm-2] is determined by theapproximation of observed GHRS high-resolution spectra. The result isrepeated using equivalent widths of CO bands (the IUE and GHRS data).The cloud pressure (Pk~2500+/-500Kcm-3) and full width athalf-maximum (FWHM) of the line profile of the cloud(ΔV=1.6+/-0.2kms-1) are obtained by approximation ofGHRS spectra. Synthetic spectra are created and compared with GHRSspectra. The data show that the cloud has more complex structure, and atleast a two-component approximation should be used to obtain moreaccurate results.
| The Perseus B5 Molecular Cloud Halo: Measurements of Pressure, Temperature, and Composition
We present GHRS observations of interstellar C I and CO toward threestars located in the direction of the Perseus region near the B5molecular cloud and discuss these together with published results from afourth sight line. The column densities of the three fine-structurelevels of C I are used together with ground-based optical data andpublished ultraviolet data to yield several properties of the gas alongthe stellar sight lines. The selected stars probe gas located in theneutral envelope surrounding the dense molecular cloud. The gas pressureis tightly constrained by our observations, yielding a value of ~2200 Kcm^-3 in the vicinity of B5, somewhat higher than in the surroundinginterstellar medium. We also present estimates of the kinetictemperature, based on observations of H_2 and C_2, yielding 20-60 K inthe vicinity of B5. Together with the pressure determination, thisimplies a local gas density of 35-100 cm^-3, very close to that whichwould be implied by the observed H_2 column density assuming a fillingfactor of unity. This result provides additional support to the notionthat the gas is part of an extensive, uniform gaseous halo surroundingthe B5 cloud. The combination of temperature and pressure suggests thatthe diffuse gas may be part of a general outflow from the densemolecular cloud.
| On the Expansion of Stellar Association Per OB2
Not Available
| A Second Catalog of Orbiting Astronomical Observatory 2 Filter Photometry: Ultraviolet Photometry of 614 Stars
Ultraviolet photometry from the Wisconsin Experiment Package on theOrbiting Astronomical Observatory 2 (OAO 2) is presented for 614 stars.Previously unpublished magnitudes from 12 filter bandpasses withwavelengths ranging from 1330 to 4250 Å have been placed on thewhite dwarf model atmosphere absolute flux scale. The fluxes wereconverted to magnitudes using V=0 for F(V)=3.46x10^-9 ergs cm^-2 s^-1Å^-1, or m_lambda=-2.5logF_lambda-21.15. This second catalogeffectively doubles the amount of OAO 2 photometry available in theliterature and includes many objects too bright to be observed withmodern space observatories.
| Classification of O Stars in the Yellow-Green: The Exciting Star VES 735
Acquiring data for spectral classification of heavily reddened starsusing traditional criteria in the blue-violet region of the spectrum canbe prohibitively time consuming using small to medium sized telescopes.One such star is the Vatican Observatory emission-line star VES 735,which we have found excites the H II region KR 140. In order to classifyVES 735, we have constructed an atlas of stellar spectra of O stars inthe yellow-green (4800-5420 Å). We calibrate spectral type versusthe line ratio He I lambda4922:He II lambda5411, showing that this ratioshould be useful for the classification of heavily reddened O starsassociated with H II regions. Application to VES 735 shows that thespectral type is O8.5. The absolute magnitude suggests luminosity classV. Comparison of the rate of emission of ionizing photons and thebolometric luminosity of VES 735, inferred from radio and infraredmeasurements of the KR 140 region, to recent stellar models givesconsistent evidence for a main-sequence star of mass 25 M_solar and ageless than a few million years with a covering factor 0.4-0.5 by thenebular material. Spectra taken in the red (6500-6700 Å) show thatthe stellar Hα emission is double-peaked about the systemicvelocity and slightly variable. Hβ is in absorption, so that theemission-line classification is ``(e)''. However, unlike the case of themore well-known O(e) star zeta Oph, the emission from VES 735 appears tobe long-lived rather than episodic.
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | פרסאוס |
| התרוממות ימנית: | 03h42m22.50s |
| סירוב: | +33°57'54.0" |
| גודל גלוי: | 4.97 |
| מרחק: | 283.286 פארסק |
| תנועה נכונה: | 3.9 |
| תנועה נכונה: | -4.9 |
| B-T magnitude: | 4.9 |
| V-T magnitude: | 4.962 |
קטלוגים וכינוים:
|
