Оглавление
Изображения
Загрузить ваше изображение
DSS Images Other Images
Публикации по объекту
The interaction of young massive stars with their environment. A millimeter and submillimeter line study of NGC 6334 FIR II
Using the 15-m Swedish ESO Sub-millimeter Telescope (SEST), CO, HCN, andHCO+ observations of the galactic star-forming region NGC6334 FIR II are presented, complemented by [C i] ^3P1-^3P{0} and^3P2-^3P{1} data from the Atacama Pathfinder Experiment (APEX 12-mtelescope). Embedded in the extended molecular cloud and associated withthe H ii region NGC 6334-D, there is a molecular "void". [C i]correlates well with 13CO and other molecular lines and showsno rim brightening relative to molecular cloud regions farther off thevoid. While an interpretation in terms of a highly clumped cloudmorphology is possible, with photon dominated regions (PDRs) reachingdeep into the cloud, the data do not provide any direct evidence for aclose association of [C i] with PDRs. Kinetic temperatures are 40-50 Kin the molecular cloud and ⪆200 K toward the void. CO and [C i]excitation temperatures are similar. A comparison of molecular andatomic fine structure line emission with the far infrared and radiocontinuum as well as the distribution of 2.2 μm H2 emission indicatesthat the well-evolved H ii region expands into a medium that ishomogeneous on pc-scales. If the H2 emission is predominantly shockexcited, both the expanding ionization front (classified as subsonic,"D-type") and the associated shock front farther out (traced by H2) canbe identified, observationally confirming for the first time a classicalscenario that is predicted by evolutionary models of Hii regions.Integrated line intensity ratios of the observed molecules aredetermined, implying a mean C18O/C17O abundanceratio of 4.13 ± 0.13 that reflects the18O/17O isotope ratio. This ratio is consistentwith values determined in nearby clouds. Right at the edge of the void,however, the oxygen isotope ratio might be smaller.Based on observations with the Atacama Pathfinder EXperiment telescope(APEX, Chile) and the Swedish-ESO Sub-millimeter Telescope (SEST, ESO/LaSilla, Chile). Figure [see full text] is only available in electronicform at http://www.aanda.org
| Variability in red supergiant stars: pulsations, long secondary periods and convection noise
We study the brightness variations of galactic red supergiant starsusing long-term visual light curves collected by the AmericanAssociation of Variable Star Observers over the last century. The fullsample contains 48 red semiregular or irregular variable stars, with amean time-span of observations of 61 yr. We determine periods and periodvariability from analyses of power density spectra and time-frequencydistributions. We find two significant periods in 18 stars. Most ofthese periods fall into two distinct groups, ranging from a few hundredto a few thousand days. Theoretical models imply fundamental, first andpossibly second overtone mode pulsations for the shorter periods.Periods greater than 1000 d form a parallel period-luminosity relationthat is similar to the long secondary periods of the asymptotic giantbranch stars. A number of individual power spectra shows a single moderesolved into multiple peaks under a Lorentzian envelope, which weinterpret as evidence for stochastic oscillations, presumably caused bythe interplay of convection and pulsations. We find a strong 1/f noisecomponent in the power spectra that is remarkably similar in almost allstars of the sample. This behaviour fits the picture of irregularphotometric variability caused by large convection cells, analogous tothe granulation background seen in the Sun.
| Changes in the Velocity Centroid of SiO Maser Emission
The v=1, J=1-0 silicon monoxide maser emission from 85 stars wasresurveyed in 2005 and 2006. The emission from these stars hadpreviously been measured in 1979 and/or 1983. For the detected stars thevelocity centroids were determined and compared with the previousvalues. If supergiant stars, known binary stars, and S Virginis wereremoved from the data set, 76 sources remained. Seven of these sourceswere not of sufficient flux density to be claimed as detections. Themean difference in the velocity centroids of the remaining sources was0.065 km s-1, and the standard deviation was 2.00 km s-1. It is important to determine the standard deviation of SiOmaser time variations in order to identify the level of the timevariability of the sources, possible binary stars, unusual emissionpatterns, or perhaps planets. The velocity centroid of S Virginisdiffered by 13.5 km s-1 from its 1983 value. This differenceindicates that S Virginis is probably part of a multiple-star system. Anaive analysis of the present data combined with earlier publishedobservations indicates a revolution period of 14.8 yr.
| Full polarization study of SiO masers at 86 GHz
Aims.We study the polarization of the SiO maser emission in arepresentative sample of evolved stars in order to derive an estimate ofthe strength of the magnetic field, and thus determine the influence ofthis magnetic field on evolved stars. Methods: .We madesimultaneous spectroscopic measurements of the 4 Stokes parameters, fromwhich we derived the circular and linear polarization levels. Theobservations were made with the IF polarimeter installed at the IRAM 30m telescope. Results: . A discussion of the existing SiO masermodels is developed in the light of our observations. Under the Zeemansplitting hypothesis, we derive an estimate of the strength of themagnetic field. The averaged magnetic field varies between 0 and 20Gauss, with a mean value of 3.5 Gauss, and follows a 1/r law throughoutthe circumstellar envelope. As a consequence, the magnetic field mayplay the role of a shaping, or perhaps collimating, agent of thecircumstellar envelopes in evolved objects.
| Molecular line mapping of the giant molecular cloud associated with RCW 106 - I. 13CO
We present the first paper in a series detailing the results of13CO observations of a ~1 deg2 region of the giantmolecular cloud (GMC) complex associated with the HII region RCW 106.The 13CO observations are also the first stage of amultimolecular line study of the same region. These observations wereamongst the first made using the new on-the-fly mapping capability ofthe Australia Telescope National Facility Mopra Telescope. In theconfiguration used, the instrument provided a full width at half-maximum(FWHM) beam size of 33 arcsec and a velocity resolution of 0.17kms-1. The gas emission takes the form of a string of knots,oriented along an axis that extends from the north-west (NW) to thesouth-east (SE) of the field of the observations, and which issurrounded by a more extended, diffuse emission. We analyse the 2Dintegrated 13CO emission using the CLUMPFIND algorithm andidentify 61 clumps. We compare the gas data in the GMC with the dustdata provided by 21-μm Midcourse Space Experiment (MSX) and 1.2-mmSwedish European Southern Observatory Submillimetre Telescope (SEST)images that we both regridded to the cell spacing of the Mopra data andsmoothed to the same resolution. The 13CO emission is morediffuse and extended than the dust emission revealed at the latter twowavebands, which both have a much higher contrast between the peaks andthe extended emission. From comparison of their centre positions, wefind that only ~50 per cent of the 13CO clump fits to thedata are associated with any dust clumps. Using the clump fits, thetotal local thermodynamic equilibrium gas mass above the 3σ levelmeasured from the molecular data is 2.7 ×105Msolar, whereas that measured from the smoothed1.2-mm SEST dust data is 2.2 × 105Msolar.
| An empirical formula for the mass-loss rates of dust-enshrouded red supergiants and oxygen-rich Asymptotic Giant Branch stars
We present an empirical determination of the mass-loss rate as afunction of stellar luminosity and effective temperature, foroxygen-rich dust-enshrouded Asymptotic Giant Branch stars and redsupergiants. To this aim we obtained optical spectra of a sample ofdust-enshrouded red giants in the Large Magellanic Cloud, which wecomplemented with spectroscopic and infrared photometric data from theliterature. Two of these turned out to be hot emission-line stars, ofwhich one is a definite B[e] star. The mass-loss rates were measuredthrough modelling of the spectral energy distributions. We thus obtainthe mass-loss rate formula log dot{M} = -5.65 + 1.05 log ( L / 10 000{L}_ȯ ) -6.3 log ( T_eff / 3500 K ), valid for dust-enshrouded redsupergiants and oxygen-rich AGB stars. Despite the low metallicity ofthe LMC, both AGB stars and red supergiants are found at late spectraltypes. A comparison with galactic AGB stars and red supergiants showsexcellent agreement between the mass-loss rate as predicted by ourformula and that derived from the 60 μm flux density fordust-enshrouded objects, but not for optically bright objects. Wediscuss the possible implications of this for the mass-loss mechanism.
| Beam Size, Shape and Efficiencies for the ATNF Mopra Radio Telescope at 86-115 GHz
We present data characterising the performance of the Mopra RadioTelescope during the period 2000-2004, including measurements of thebeam size and shape, as well as the overall beam efficiency of thetelescope. In 2004 the full width half maximum of the beam was measuredto be 36+/-3'' at 86GHz, falling to 33+/-2'' at 115GHz. Based on ourobservations of Jupiter we measured the beam efficiency of the Gaussianmain beam to be 0.49+/-0.03 at 86GHz and 0.42+/-0.02 at 115GHz. Sourceswith angular sizes of ~80'' couple well to the main beam, while sourceswith angular sizes between ~80'' and ~160'' couple to the both the mainbeam and inner error beam. Measurements indicate that the inner errorbeam contains approximately one-third the power of the main beam. Wealso compare efficiency corrected spectra to measurements made atsimilar facilities and present standard spectra taken towards themolecular clouds Orion-KL and M17-SW.
| VLTI observations of AGB stars
We report on recent observations of AGB stars obtained with the VLTInterferometer (VLTI). We illustrate in general the potential ofinterferometric measurements to study stellar atmospheres andcircumstellar envelopes, and demonstrate in particular the advantages ofa coordinated multi-wavelength approach including near/mid-infrared aswell as radio interferometry. We report on studies of the atmosphericstructure of non-Mira and Mira variable giants. We have used VLTIobservations of the near- and mid-infrared stellar sizes and concurrentVLBA observations of the SiO maser emission. So far, this projectincludes studies of the Mira stars S Ori and RR Aql as well as of thesupergiant AH Sco. The results from our first epochs of S Orimeasurements have recently been published and the main results arereviewed here. The S Ori maser ring is found to lie at a mean distanceof approximately 2 stellar radii, a result that is virtually free of theusual uncertainty inherent in comparing observations of variable starswidely separated in time and stellar phase. We discuss the status of ourmore recent S Ori, RR Aql, and AH Sco observations, and present anoutlook on the continuation of our project.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Guilt by Association: The 13 Micron Dust Emission Feature and Its Correlation to Other Gas and Dust Features
A study of all full-scan spectra of optically thin oxygen-richcircumstellar dust shells in the database produced by the ShortWavelength Spectrometer on ISO reveals that the strength of severalinfrared spectral features correlates with the strength of the 13 μmdust feature. These correlated features include dust features at 19.8and 28.1 μm and the bands produced by warm carbon dioxide molecules(the strongest of which are at 13.9, 15.0, and 16.2 μm). The databasedoes not provide any evidence for a correlation of the 13 μm featurewith a dust feature at 32 μm, and it is more likely that a weakemission feature at 16.8 μm arises from carbon dioxide gas ratherthan dust. The correlated dust features at 13, 20, and 28 μm tend tobe stronger with respect to the total dust emission in semiregular andirregular variables associated with the asymptotic giant branch than inMira variables or supergiants. This family of dust features also tendsto be stronger in systems with lower infrared excesses and thus lowermass-loss rates. We hypothesize that the dust features arise fromcrystalline forms of alumina (13 μm) and silicates (20 and 28 μm).Based on observations with the ISO, a European Space Agency (ESA)project with instruments funded by ESA member states (especially thePrincipal Investigator countries: France, Germany, the Netherlands, andthe United Kingdom) and with the participation of the Institute of Spaceand Astronautical Science (ISAS) and the National Aeronautics and SpaceAdministration (NASA).
| Long periodic variable stars
The information on Mira-type stars and stars adjacent to them at theHertzsprung -- Russel diagram is presented. A detailed description oftheir observational characteristics is given. We give a survey ofimportant observational works concerning: multicolor photometry withspecial attention to the IR emission, maser emission, shock waves, massloss, binarity, the problem of the pulsational mode, direct measurementsof angular and linear dimensions, statistic investigations, study ofkinematic characteristics etc. The most interesting problems regardinglong periodic variable stars are specified. Some attention is given tothe classification and evolutionary stage of these objects.
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/397/997
| The neutral gas environment of the young supernova remnant SN 1006 (G327.6+14.6)
Using the Australia Telescope Compact Array, we have carried out asurvey of the H I emission in the direction of the bilateral supernovaremnant (SNR) SN 1006 (G327.6+14.6). The angular resolution of the datais 4.7' x 3.0', and the rms noise ~39 mJy/beam (~ 0.3 K). To recoverstructures at low spatial frequencies, single dish data have been addedto the interferometric images. We have also studied the 12COemission in the transitions J=1-0 and J=2-1, looking for very compactclumps of molecular gas as possible sites for the acceleration ofelectrons and nuclei to TeV energies associated with the gamma -raysource detected on the NE limb of SN 1006. These molecular gasobservations produced only marginal detections. From the presentobservations we conclude that the distribution of the surroundingneutral gas had no strong influence in shaping this SNR with a bilateralappearance. Intrinsic factors may have contributed to the presentmorphology. The remnant of SN 1006 appears to be evolving in a smoothenvironment with an atomic volume density n0 ~ 0.3cm-3. The existence of an H I concentration projected on thecenter of SN 1006 suggests an upper limit of ~-20 km s-1 onthe systemic velocity of the SNR. This limit is compatible with adistance to the SNR of about 1.7 kpc, in good agreement with previousestimates. An extended H I cloud with volume density ~0.5cm-3 is detected towards the NW border of SN 1006. Thisconcentration may be responsible for the formation of the bright Balmerfilaments observed in SN 1006. The absorbing column density towards SN1006 has been estimated to be N H ~ 6.8 × 1020cm-2, in good agreement with previous suggestions based onX-ray results.
| Numerical simulations of stellar SiO maser variability. Investigation of the effect of shocks
A stellar hydrodynamic pulsation model has been combined with a SiOmaser model in an attempt to calculate the temporal variability of SiOmaser emission in the circumstellar envelope (CE) of a model AGB star.This study investigates whether the variations in local physicalconditions brought about by shocks are the predominant contributingfactor to SiO maser variability because, in this work, the radiativepart of the pump is constant. We find that some aspects of thevariability are not consistent with a pump provided by shock-enhancedcollisions alone. In these simulations, gas parcels of relativelyenhanced SiO abundance are distributed in a model CE by a Monte Carlomethod, at a single epoch of the stellar cycle. From this epoch on,Lagrangian motions of individual parcels are calculated according to thevelocity fields encountered in the model CE during the stellar pulsationcycle. The potentially masing gas parcels therefore experience differentdensities and temperatures, and have varying line-of-sight velocitygradients throughout the stellar cycle, which may or may not be suitableto produce maser emission. At each epoch (separated by 16.6 days),emission lines from the parcels are combined to produce syntheticspectra and VLBI-type images. We report here the results for v=1, J=1-0(43-GHz) and J=2-1 (86-GHz) masers and compare synthetic lineshapes andimages with those observed. Strong SiO maser emission is calculated toform in an unfilled ring within a few stellar radii of the photosphere,indicating a tangential amplification process. The diameter of thesynthetic maser ring is dependent upon stellar phase, as clearlyobserved for TX Cam, and upon maser transition. Proper motions ofbrightly masing parcels are comparable to measurements for some masercomponents in R Aqr and TX Cam, although we are unable to reproduce allof the observed motions. Synthetic lineshapes peak at the stellarvelocity, have typical Mira linewidths and vary in intensity withstellar phase. However, the model fails quantitatively in severalrespects. We attribute these failings to (i) lack of an accurate,time-varying stellar IR field (ii) post-shock kinetic temperatures whichare too high, due to the cooling function included in our model and(iii) the lack of a detailed treatment of the chemistry of the inner CE.We expect the use of oxygen-rich hydrodynamical stellar models which arecurrently under development to alleviate these problems.
| Shocked molecular gas towards the supernova remnant G359.1-0.5 and the Snake
We have found a bar of shocked molecular hydrogen (H2)towards the OH(1720MHz) maser located at the projected intersection ofsupernova remnant (SNR) G359.1-0.5 and the non-thermal radio filamentknown as the Snake. The H2 bar is well aligned with the SNRshell and almost perpendicular to the Snake. The OH(1720MHz) maser islocated inside the sharp western edge of the H2 emission,which is consistent with the scenario in which the SNR drives a shockinto a molecular cloud at that location. The spectral line profiles of12CO, HCO+ and CS towards the maser showbroad-line absorption, which is absent in the 13CO spectraand most probably originates from the pre-shock gas. A density gradientis present across the region and is consistent with the passage of theSNR shock, while the H2 filament is located at the boundarybetween the pre-shock and post-shock regions.
| Stars with the Largest Hipparcos Photometric Amplitudes
A list of the 2027 stars that have the largest photometric amplitudes inHipparcos Photometry shows that most variable stars are all Miras. Thepercentage of variable types change as a function of amplitude. Thiscompilation should also be of value to photometrists looking forrelatively unstudied, but large amplitude stars.
| Dust features in the 10-mu m infrared spectra of oxygen-rich evolved stars
We have analyzed the 8-13.5 mu m UKIRT CGS3 spectra of 142 M-type starsincluding 80 oxygen-rich AGB stars and 62 red supergiants, with a viewto understanding the differences and similarities between the dustfeatures of these stars. We have classified the spectra into groupsaccording to the observed appearance of the infrared features. In eachcase the normalized continuum-subtracted spectrum has been compared tothose of the other stars to find similarities and form groups. The dustfeatures of the AGB stars are classified into six groups: broad AGB,where the feature extends from 8 mu m to about 12.5 mu m with littlestructure; broad+sil AGB, which consists of a broad feature with anemerging 9.7 mu m silicate bump; and four silicate AGB groups in which a``classic'' 9.7 mu m silicate feature gets progressively narrower.Likewise, the supergiant spectra have also been classified into groups,however these do not all coincide with the AGB star groups. In thesupergiant case we again have six groups: featureless, where there islittle or no emission above the continuum; broad Super, where thefeature extends from about 9 mu m to about 13 mu m; and four silicateSuper groups, which again show a progression towards the narrowest``classic'' 9.7 mu m silicate feature. We compare the mean spectrum foreach group, which yields two main results. Firstly, while the``classic'' silicate feature is essentially identical for both AGB starsand red supergiants, the broad features observed for these two stellartypes are quite different. We suggest that the dust in these twoenvironments follows different evolutionary paths, with the dust aroundMira stars, whose broad feature spectra can be fit by a combination ofalumina (Al2O3) and magnesium silicate,progressing from this composition to dust dominated by magnesiumsilicate only, while the dust around supergiants, whose broad featurecan be fit by a combination of Ca-Al-rich silicate andAl2O3, progresses from this initial composition toone eventually also dominated by magnesium silicate. The reason for thedifference in the respective broad features is not clear as yet, butcould be influenced by lower C/O ratios and chromospheric UV radiationfields in supergiant outflow environments. The second result concernsthe 12.5 - 13.0 mu m feature discovered in IRAS LRS spectra and widelyattributed to Al2O3. This feature is seenpredominantly in the spectra of semiregular variables, sometime in Mirasand only once (so far) in supergiant spectra. We argue that it isunlikely that this feature is due to Al2O3 or, ashas more recently been suggested, spinel(MgAl2O4), but could be associated with silicondioxide or highly polymerized silicates (not pyroxenes or olivines).
| Modelling of oxygen-rich envelopes using corundum and silicate grains
A set of 31 oxygen-rich stars has been modelled using corundum andsilicate grains. These stars were selected according to theirdust-envelope class, as suggested by Little-Marenin and Little in 1990.Then 16 stars classified as Sil were modelled using silicate grains; 10Broad class stars using corundum (Al2O3) grains;and 5 `Intermediate' class stars using two kinds of grainsimultaneously: corundum and silicate. The temperature of the centralstars and some characteristics of their circumstellar envelopes such astheir extinction opacities and extensions were determined by fitting theflux curves. The corundum/silicate ratios as well as the energydistributions and temperature laws have been obtained. Based on theauthors' results they suggest the existence of chemical and structuralevolution of the modelled circumstellar dust shells. The temperature ofthe central stars and the temperature of the hottest grains decreasefrom Broad to Intermediate to Sil classes, while the inner radii andoptical depths increase in this sequence.
| A search for Technetium in semiregular variables
We searched for the lines of Tc in the spectra of Semiregular variables(SRVs) in the wavelength region from 4180 to 4300 Å using highresolution spectroscopy. Tc as an s-process element is produced on thethermally pulsing AGB and is therefore a good indicator for theevolutionary status of Semiregular variables. Combining our results withprevious investigations we get a database large enough for a statisticalstudy. Tc is not found in SRVs with periods below 100 days, spectraltypes earlier than M5 and photospheric IRAS colours. These objects are`blue' SRVs in the classification system of Kerschbaum & Hron(\cite{KH94}). Among the `red' SRVs (periods longer than 100 days) thefraction of stars showing Tc in their spectra is about 15 % with aprobably lower fraction among the stars with periods above 150 days.This is significantly lower than for the typical Miras. Taking intoaccount the probable conditions for the occurence of the third dredge-upand the expected behavior of the Tc abundance along an evolutionarytrack on the AGB, our results support an evolutionary scenario from`blue' SRVs (early AGB) to `red' SRVs (early TP-AGB) and on to longperiod Miras. Only the most massive (masses above 2M_ȯ) stars showTc during the SRV stage. The luminosities of the Tc-rich SRVs and Mirasare compatible with theoretical estimates of the minimum core massrequired for the third dredge-up. Based on observations collected at theEuropean Southern Observatory, La Silla, Chile (ESO No.54.E-0350), theGerman Spanish Astronomical Centre, Calar Alto, operated by theMax-Planck-Institute for Astronomy, Heidelberg, jointly with the SpanishNational Commission for Astronomy, and Kitt Peak National Observatory,USA.
| Deuterium fractionation and the degree of ionization in the R Coronae Australis molecular cloud core
The fractionation of D and (13) C in HCO(+) was investigated in the RCoronae Australis molecular cloud core. The distributions of H(13) CO(+)and DCO(+) were found to be morphologically similar but their columndensity maxima were found to lie in different locations. The H(13) CO(+)/HC(18) O(+) abundance ratio was found to vary little from 10 within themapped region, in excellent agreement with the (13) CO/C(18) O abundanceratios derived earlier towards the cloud by Harjunpää &Mattila (1996). This corroborates the close relationship between HCO(+)and CO predicted by the chemistry models. The DCO(+) /HCO(+) abundanceratio ranges from 0.006 to 0.04, being lowest towards two locations nearthe embedded infrared source IRS 7 where the kinetic temperature, asderived from methyl acetylene (CH_3CCH) observations, is somewhatelevated. The variation of the degree of deuterium fractionation withinthe core is due to an increase in the kinetic temperature near thecluster of newly born stars. This temperature rise results in twoeffects: Firstly, the reaction H_2D(+) -> H_3(+) becomes faster; andsecondly, an intensified desorption from grain surfaces increases theabundance of neutral atoms and molecules in the gas phase leading to thedestruction of H_3(+) and H_2D(+) ions. Both processes decrease theDCO(+) /HCO(+) abundance ratio. Far from the active region the derivedabundances of neutral species indicate the presence of depletion ontograin surfaces. The observations suggest furthermore that the fractionalelectron abundance, chi (e(-)) , is lowest in the dense clump near IRS7. This region also exhibits a low degree of gas phase depletion. Infact, increased fractional abundances of neutral species such as atomicoxygen and CO lead to a decrease in the [H_3(+]/[) HCO(+]) abundanceratio which is directly proportional to chi (e(-)) .
| Dense Molecular Clouds in the Galactic Center Region. I. Observations and Data
We have observed a 160^'x40^' (lxb) region around the Galactic center inthe CS J=1-0 line with a high spatial resolution and high sensitivityusing the 45 m telescope at Nobeyama Radio Observatory. The observedarea includes the Sgr A radio arc complex, Sgr B, Sgr C, and Sgr D.About 24,000 spectra of CS J=1-0 emission, which is a good tracer ofhigh-density (n≅10^4 cm^-3) molecular clouds, were obtained. Theobserved data are presented in l-b, l-v, and b-v maps. The total CS lineflux in this area is Sdv=1.1x10^6 Jy km s^-1. Typical optical thicknessof C^32S J=1-0 emission is equal to or lower than tau=2-3. The totalmolecular mass in the Galactic center region is estimated to beM(H_2)=(3-8)x10^7 M_solar from the assumption of LTE. One-third of themolecular clouds in the Galactic center region are involved in severalcontinuous curved ridges extended along the Galactic longitude. The mostprominent ridge has a ``bow''-like structure with a length of 300 pc.
| Silicate and hydrocarbon emission from Galactic M supergiants
Following our discovery of unidentified infrared (UIR) band emission ina number of M supergiants in h and chi Per, we have obtained 10-μmspectra of a sample of 60 galactic M supergiants. Only three newsources, V1749 Cyg, UW Aql and IRC+40 427, appear to show the UIR bands;the others show the expected silicate emission or a featurelesscontinuum. The occurrence of UIR-band emission in M supergiants istherefore much higher in the h and chi Per cluster than in the Galaxy asa whole. Possible explanations for the origin and distribution of UIRbands in oxygen-rich supergiants are discussed. We use our spectra toderive mass-loss rates ranging from 10^-8 to 10^-4 M_solar yr^-1 for thenew sample, based on the power emitted in the silicate feature. Therelationship between mass-loss rate and luminosity for M supergiants isdiscussed, and correlations are explored between their mid-infraredemission properties.
| The Infrared Spectral Classification of Oxygen-rich Dust Shells
This paper presents infrared spectral classifications for a flux-limitedsample of 635 optically identified oxygen-rich variables includingsupergiants and sources on the asymptotic giant branch (AGB). Severalclasses of spectra from oxygen-rich dust exist, and these can bearranged in a smoothly varying sequence of spectral shapes known as thesilicate dust sequence. Classification based on this sequence revealsseveral dependencies of the dust emission on the properties of thecentral star. Nearly all S stars show broad emission features fromalumina dust, while most of the supergiants exhibit classic featuresfrom amorphous silicate dust. Mira variables with symmetric light curvesgenerally show broad alumina emission, while those with more asymmetriclight curves show classic silicate emission. These differences may arisefrom differences in the photospheric C/O ratio.
| Abundances of ethylene oxide and acetaldehyde in hot molecular cloud cores
We have searched for millimetre-wave line emission from ethylene oxide(c-C_2H_4O) and its structural isomer acetaldehyde (CH_3CHO) in 11molecular clouds using SEST. Ethylene oxide and acetaldehyde weredetected through multiple lines in the hot cores NGC 6334F, G327.3-0.6,G31.41+0.31, and G34.3+0.2. Acetaldehyde was also detected towardsG10.47+0.03, G322.2+0.6, and Orion 3'N, and one ethylene oxide line wastentatively detected in G10.47+0.03. Column densities and rotationalexcitation temperatures were derived using a procedure which fits theobserved line intensities by finding the minimum chi (2) -value. Theresulting rotational excitation temperatures of ethylene oxide andacetaldehyde are in the range 16 - 38 K, indicating that these speciesare excited in the outer, cooler parts of the hot cores or that theexcitation is significantly subthermal. For an assumed source size of20'', the deduced column densities are (0.6 - 1)*E(14) cm(-2) forethylene oxide and (2 - 5)*E(14) cm(-2) for acetaldehyde. The fractionalabundances with respect to H_2 are X{[c-C_2H_4O]}=(2 - 6)*E(-10) , andX{[CH_3CHO]}=(0.8 - 3)*E(-9) . The ratio X{[CH_3CHO]}/X{[c-C_2H_4O]}varies between 2.6 (NGC 6334F) and 8.5 (G327.3-0.6). We also detectedand analysed multiple transitions of CH_3OH, CH_3OCH_3, C_2H_5OH, andHCOOH. The chemical, and possibly evolutionary, states of NGC 6334F,G327.3-0.6, G31.41+0.31, and G34.3+0.2 seem to be very similar.
| Molecular gas in the Galactic center region. III. Probing shocks in molecular cores
Multiline observations of C(18) O and SiO isotopomers toward 33molecular peaks in the Galactic center region, taken at the SEST, JCMTand HHT telescopes, are presented. The C(18) O presumably traces thetotal H_2 column density, while the SiO traces gas affected by shocksand high temperature chemistry. The J =2-> 1 line of SiO is seen onlyin few regions of the Galactic disk. This line is easily detected in allGalactic center sources observed. A comparison of the strength of therare isotopomers (29) SiO and (30) SiO to the strength of the mainisotopomer (28) SiO implies that the J = 2 -> 1 transition of (28)SiO is optically thick. The (29) Si/(30) Si isotope ratio of 1.6 in theGalactic center clouds is consistent with the terrestrial value. LargeVelocity Gradient models show that the dense component (n_H_2 >=10(4) \percc) in typical molecular cores in the Galactic center is cool(\TKIN ~ 25 K), contrary to what is usually found in Giant MolecularClouds in the disk, where the densest cores are the hottest. Highkinetic temperatures, > 100 K, known to exist from NH_3 studies, areonly present at lower gas densities of a few 10(3) cm(-3) , where SiO ishighly subthermally excited. Assuming that \CEIO\ traces all of themolecular gas, it is found that in all cases but one, SiO emission iscompatible with arising in gas at higher density that is (presently)relatively cool. The relative abundance of SiO is typically 10(-9) , butdiffers significantly between individual sources. It shows a dependenceon the position of the source within the Galactic center region. Highabundances are found in those regions for which bar potential modelspredict a high likelihood for cloud-cloud collisions. These results canbe used to relate the amount of gas that has encountered shocks withinthe last ~ 10(6) years to the large scale kinematics in the inner ~ 500pc of the Galaxy. Based on observations obtained at the Swedish-ESOSubmillimeter Telescope (SEST, Project C-0518, 1996), the James ClarkMaxwell Telescope (JCMT, operated by the Joint Astronomy Centre onbehalf of the Particle Physics and Astronomy Research Council of theUnited Kingdom, the Netherlands Organisation for Scientific Research,and the National Research Council of Canada) and theHeinrich-Hertz-Telescope (HHT, operated by the the SubmillimeterTelescope Observatory).
| Infrared giants vs. supergiants. II. CO observations
We report systematic observations of millimeter CO emission from asample of 109 oxygen-rich evolved stars (AGB and supergiants),colour-selected from the IRAS Point Source Catalog (0.69 200). In most cases, the observed spread in the valuesof this ratio can be explained by a large range of luminosities. Thisleads to a new criterion to identify AGB stars: an object with {cal R}< 150 must have a low mass progenitor. Here we study the correlationsbetween {cal R} and various physical properties of the sources. Mostsources with high values of {cal R} also have low galactic latitudes,small IRAS variability indices, and early spectral types (typicallyM1-M5). Conversely, there is no dependence on the IRAS colours, nor onthe intensity of silicate 10;microns emission. However, a few AGB starsexhibit large {cal R}; other factors than luminosity are required toexplain these values. Different hypotheses, such as the possiblepresence of a chromosphere, a low (12) C abundance or a variablemass-loss rate, are examined. Considering the global high OH detectionrate ( ~ 67%), we studied the correlations with CO and OH emission. Thedetection of OH seems to be a useful discriminator of mechanisms thatenhance {cal R}.
| HIPPARCOS Astrometry of Infrared-Selected Sources and the Connection Between Optical and Infrared Reference Frames
Astrometric data from the Hipparcos satellite are reported for theoptical counterparts of 87 bright infrared sources. These sources may beuseful in defining a reference frame for infrared observations. The dataare also useful in studying the locations of circumstellar SiO masers.(SECTION: Stars)
| Classification and Identification of IRAS Sources with Low-Resolution Spectra
IRAS low-resolution spectra were extracted for 11,224 IRAS sources.These spectra were classified into astrophysical classes, based on thepresence of emission and absorption features and on the shape of thecontinuum. Counterparts of these IRAS sources in existing optical andinfrared catalogs are identified, and their optical spectral types arelisted if they are known. The correlations between thephotospheric/optical and circumstellar/infrared classification arediscussed.
| Ammonia and C^18^O in globules.
A complete and unbiased survey of 237 dust globules from the catalog ofClemens & Barvainis (1988) has been made in the (1,1) and (2,2)inversion lines of ammonia. Whereas NH_3_(1,1) emission is found in 74sources, the NH_3_(2,2) line is detected only in 17 sources. Completeammonia maps of 15 globules show angular sizes of between 45" and 85".The FWHP angular sizes of these clouds based on the C^18^O(1-0) emissionare between 2 and 9 arcminutes. Accurate determinations of the kinetictemperatures show that the temperatures of all the globules in the studyare between 8.5 and 11.5K. From NH_3_ excitation temperatures of between4.5 to 8K we determine H_2_ densities with typical values around(2-4).10^4^cm^-3^. Presumably, the more extended C^18^O emission arisesfrom lower density gas. The masses of the mapped globules range up to100 solar masses with typical values between 2 and 45 solar masses.Considerations of stability from a comparison of turbulent, thermal andgravitational energies show that all the globules are close toequilibrium.
| Carbon stars with episodic mass loss: observations and models of molecular emission from detached circumstellar shells.
We have obtained detailed CO radio line maps of the circumstellar mediumaround the bright carbon stars R Scl, U Ant, S Sct, and TT Cyg. Theyprovide direct evidence for the existence of large [radii between10-70", or (1-5)x10^17^cm], geometrically thin (we estimate that theshell widths are <~10"), over-all spherically symmetric shells of COline-emitting gas around these stars. The shells expand with velocitiesin the range 13-20km/s, i.e., their ages lie in the range(1-10)x10^3^years. Less extensive CO observations of the carbon starV644 Sco suggest that also it is surrounded by a detached shell. Theexpansion velocities of the present mass loss winds, as evidenced byweak CO emission from regions close to the stars, are considerablylower, of the order 5km/s. We conclude that the mass losscharacteristics of these, otherwise apparently normal, carbon stars havechanged significantly over the last 10^4^years. For such a shellstructure, the most reasonable cause is a short period of very intensemass loss (i.e., a mass loss eruption), although an interacting-windscenario cannot be excluded. The CO brightness distributions are verypatchy, suggesting an inhomogeneous circumstellar medium. Using a modelwhere the shell consists of a large number of small, homogeneous clumps,we estimate that the H_2_-masses of the four, spatially resolved shellsare all around 0.01Msun_ (for an adopted CO abundance withrespect to H_2_ of =~10^-3^), and that in the mass loss eruption-scenario the H_2_-mass loss rates of the stars were=~10^-5^x(10"/{DELTA}R)Msun_/yr during the formation of theshells ({DELTA}R being the unresolved shell width in arc seconds). Thepresent mass loss rates are very low, <~10^-7^Msun_/yr.These results suggest that the four stars have all gone through a typeof event that led to a dramatic change in the mass loss characteristics.The adopted model is an initial, relatively crude, attempt to provide amore realistic base for the interpretation of line emission from acircumstellar medium in which, in general, the physical conditions arevery likely quite inhomogeneous. It is the accidental overlap along theline-of-sight and in velocity space of the many small clumps that in themodel produces a clumpy appearance of the brightness distribution, atthe larger scale set by the observational resolution, that resembles theobserved ones. In the mass loss eruption -scenario the estimated lifetime of a CO line-emitting shell of the type discussed in this paper is=~10^4^years, and it is determined by the photodissociation of the COmolecules. Only shells younger than =~10^3^years are expected to beobservable in molecular radio lines other than those of CO. There is aperiod after formation when such shells should be characterized by veryanomalous line intensity ratios. For instance, in our model the lineintensity ratio between the photodissociation product CN and the parentmolecule HCN increases drastically on a time scale of hundreds of yearsas the shell recedes from the star. We suggest that the shell around RScl is in this phase, since this is the only object, among the fiveobserved, in which we have clearly detected also lines of HCN and CN,albeit with anomalous line intensity ratios.
|
Добавить новую статью
Внешние ссылки
- - Внешних ссылок не найдено -
Добавить внешнюю ссылку
Группы:
|
Наблюдательные данные и астрометрия
| Созвездие: | Скорпион |
| Прямое восхождение: | 17h11m17.02s |
| Склонение: | -32°19'30.7" |
| Видимая звёздная величина: | 7.453 |
| Расстояние: | 10000000 парсек |
| Собственное движение RA: | -4.3 |
| Собственное движение Dec: | -0.7 |
| B-T magnitude: | 10.678 |
| V-T magnitude: | 7.72 |
Каталоги и обозначения:
|
