Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
A Comparison between Anomalous 6 cm H2CO Absorption and CO(1-0) Emission in the L1204/S140 Region
We report observations of the dust cloud L1204 with the Onsala 25 mtelescope in the 6 cm (111-110) transition ofH2CO. The observed region includes the S140 Hα arc.This spectral line is seen here in absorption against the cosmicmicrowave background, indicating the presence of widespread warmmolecular gas at intermediate densities. Overall, the distributions ofH2CO and CO (taken from the literature) are fairly similar,although significant differences exist at small scales. Most notably,while the CO peak is nearly coincident with the S140 Hα arc, themaximum H2CO absorption is clearly separated from it by afull 10' beam (~3 pc). We argue that these differences result fromdiffering abundances and excitation requirements. The CO(1-0) line ismore optically thick and more biased toward warm gas than the H 2CO 6 cm line. On the other hand, formaldehyde is more easilyphotodissociated and is, therefore, a poorer tracer of the molecular gaslocated immediately behind photon-dominated regions.
| Line profiles of water for the photon dominated region and embedded sources in the S140 region
H_2O is a key ingredient in many interstellar environments, like photondominated regions and star forming clouds. It plays an important role inthe oxygen chemistry and can act as a coolant in dense interstellarclouds and shocks. Observations and modelling of water lines thusprovide powerful diagnostics of the physical conditions in interstellaremission zones. A radiative transfer method for the treatment ofmolecular lines is presented. We apply this method to previous SWAS andISO observations of water vapor in the source S140 in order to makemodels to plan for, and to interpret, HIFI data. Level populations arecalculated with the use of a three-dimensional (multi-zone) escapeprobability method and with a long characteristic code that usesMonte-Carlo techniques with fixed directions. Homogeneous andinhomogeneous models are used to compute the differences between H_2Oline profiles across the S140 region. We find that when an outflow orinfall velocity field with a gradient of a few km s-1 isadopted, line profiles with a FWHM of 6 km s-1 are found, inagreement with observations. Inhomogeneous models are favoured toproduce a single-peaked line profile. When zooming in on smaller regionswithin the PDR, the shapes of the line profiles start to differ due tothe different temperature and density distributions there. The embeddedsources are traced by high excitation lines, e.g.,321{-}221, 303{-}212,212{-}101 and 220{-}111. Thecomputed intensities are roughly consistent with existing ISOobservations. Water emission in a PDR source like S140 requires acombination of a pure PDR and an embedded source in order to match theobservations. Because of its good angular resolution, HIFI will be ableto distinguish between a dense star forming region or a more diffuse gascomponent. It is therefore important for future observing programs toconsider both in their predictions of the emission characteristics ofwater in these environments.
| Modeling the Neutral Hydrogen Interstellar Medium: A Better Kinematic Distance Tool
An advanced approach to the kinematic distance method is developed. Themethod is applicable to second- and third-quadrant Galactic objects withknown velocities. It is based on fitting a model of the density andvelocity features in an isothermal H I disk to observed H I data. Thevelocity field of the gas is modeled with a power law for basic circularrotation, underlying noncircular motions from a two-armed density wavepattern. With a reasonable number of adjustable parameters andconstraints the model reproduces observations toward many Galacticobjects, and accurate distances are found from the modeled velocityfield. High-resolution H I spectral line data from the Canadian GalacticPlane Survey (CGPS; Taylor et al.) are used to discriminate clouds fromthe intercloud medium (the ``stratum'') for which the model is intended.The ability of the model to reproduce these data is demonstrated in one-[Tb(v)] and two- [Tb(l, v)] dimensional fits.Distances to 22 H II regions and SNRs calculated by the fitted velocityfield compare extremely well with other kinematically independentdistances.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| The excitation of water in the S140 photon dominated region
We consider the excitation of water in the Photon Dominated Region (PDR)S140. With the use of a three-dimensional escapeprobability method we compute the level populations of ortho- andpara-H2O up to ~350 K (i.e., 8 levels), as well as line intensities forvarious transitions. Homogeneous and inhomogeneous models are presentedwith densities of 104-105 cm-3 and thedifferences between the resulting intensities are displayed. Density,temperature, and abundance distributions inside the cloud are computedwith the use of a self-consistent physi-chemical (in)homogeneous modelin order to reproduce the line intensities observed with SWAS, and tomake predictions for various lines that HIFI will probe in the future.Line intensities vary from ~10-13 erg cm-2s-1 sr-1 to a few times 10-6 ergcm-2 s-1 sr-1. We can reproduce theintensity for the 110 → 101 line observed bythe SWAS satellite. It is found that the 212 →101 line is the strongest, whereas the 312 →221 line is the weakest, in all the models. It is found thatthe 110 → 101 line probes the total column,while higher excitation lines probe the higher density gas (e.g.,clumps).
| Ion Chemistry in Photon-dominated Regions: Examining the [HCO+]/[HOC+]/[CO+] Chemical Network
HOC+ and CO+ have been detected toward twowell-known photon-dominated regions (PDRs), S140 and NGC 2023, using theArizona Radio Observatory 12 m telescope. The J=1-->0 transition ofHOC+ at 89 GHz and the two spin components of the N=2-->1line of CO+ near 236 GHz were observed, as well as theJ=1-->0 transitions of H13CO+ andHC18O+. The J=3-->2 line of HOC+ at268 GHz was also mapped across the Orion Bar. The[HCO+]/[HOC+] ratios determined in S140 and NGC2023 were ~12,408 and 1913, respectively, values indicative of quiescentmolecular gas rather than PDR sources, where the abundance ofHOC+ is thought to be enhanced. However, the beam in boththese measurements may contain material from the adjoining molecularcloud, favoring HCO+. Alternatively, the[HCO+]/[HOC+] ratio may vary with Av inPDR regions. The [CO+]/[HOC+] ratio in S140 andNGC 2023, at several positions in the Orion Bar, and in other PDRs, onthe other hand, falls uniformly in the range ~1-10. In addition, theline profiles of CO+ and HOC+ in the Orion Barlook remarkably similar. The syntheses of HOC+ andCO+ appear to be correlated in PDRs, most likely through thecommon precursor, C+. The reaction of C+ andH2O is thought to preferentially create HOC+, asopposed to HCO+, and C++OH or O2 leadsfavorably to CO+.
| A Very Large Array Search for Water Masers in Six H II Regions: Tracers of Triggered Low-Mass Star Formation
We present a search for water maser emission at 22 GHz associated withyoung low-mass protostars in six H II regions-M16, M20, NGC 2264, NGC6357, S125, and S140. The survey was conducted with the NRAO Very LargeArray from 2000 to 2002. For several of these H II regions, ours are thefirst high-resolution observations of water masers. We detected 16 watermasers: eight in M16, four in M20, three in S140, and one in NGC 2264.All but one of these were previously undetected. No maser emission wasdetected from NGC 6357 or S125. There are two principle results to ourstudy. (1) The distribution of water masers in M16 and M20 does notappear to be random but instead is concentrated in a layer of compressedgas within a few tenths of a parsec of the ionization front. (2)Significantly fewer masers are seen in the observed fields than expectedbased on other indications of ongoing star formation, indicating thatthe maser-exciting lifetime of protostars is much shorter in H IIregions than in regions of isolated star formation. Both of theseresults confirm predictions of a scenario in which star formation isfirst triggered by shocks driven in advance of ionization fronts andthen truncated ~105 yr later when the region is overrun bythe ionization front.
| A 13CO and C18O Survey of the Molecular Gas Around Young Stellar Clusters within 1 Kiloparsec of the Sun
As the first step of a multiwavelength investigation into therelationship between young stellar clusters and their environment, wepresent fully sampled maps in the J=1-0 lines of 13CO andC18O and the J=2-1 line of C18O for a selectedgroup of 30 young stellar groups and clusters within 1 kpc of the Sun.This is the first systematic survey of these regions to date. Theclusters range in size from several stars to a few hundred stars. Thirtyfields ranging in size from 8'×8' to 30'×60' were mappedwith 47" resolution simultaneously in the two J=1-0 lines at the FiveCollege Radio Astronomy Observatory. Seventeen sources were mapped overfields ranging in size from 3'×3' to 13'×13' in the J=2-1line with 35" resolution at the Submillimeter Telescope Observatory. Wecompare the cloud properties derived from each of the three tracers inorder to better understand systematic uncertainties in determiningmasses and line widths. Cloud masses are determined independently usingthe 13CO and C18O transitions; these masses rangefrom 30 to 4000 Msolar. Finally, we present a simplemorphological classification scheme, which may serve as a roughindicator of cloud evolution.
| The association of IRAS sources and 12CO emission in the outer Galaxy
We have revisited the question of the association of CO emission withIRAS sources in the outer Galaxy using data from the FCRAO Outer GalaxySurvey (OGS). The availability of a large-scale high-resolution COsurvey allows us to approach the question of IRAS-CO associations from anew direction - namely we examined all of the IRAS sources within theOGS region for associated molecular material. By investigating theassociation of molecular material with random lines of sight in the OGSregion we were able to construct a quantitative means to judge thelikelihood that any given IRAS-CO association is valid and todisentangle multiple emission components along the line of sight. Thepaper presents a list of all of the IRAS-CO associations in the OGSregion. We show that, within the OGS region, there is a significantincrease ( ~ 22%) in the number of probable star forming regions overprevious targeted CO surveys towards IRAS sources. As a demonstration ofthe utility of the IRAS-CO association table we present the results ofthree brief studies on candidate zone-of-avoidance galaxies with IRAScounterparts, far outer Galaxy CO clouds, and very bright CO clouds withno associated IRAS sources. We find that ~ 25% of such candidate ZOAGsare Galactic objects. We have discovered two new far outer Galaxystar-forming regions, and have discovered six bright molecular cloudsthat we believe are ideal targets for the investigation of the earlieststages of sequential star formation around HII regions. Finally, thispaper provides readers with the necessary data to compare othercatalogued data sets with the OGS data.Tables 1, 2 and A1 are only available in electronic form at the CDS viaanonymous ftp to\ cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/1083
| Photon-dominated Regions in Low-Ultraviolet Fields: A Study of the Peripheral Region of L1204/S140
We have carried out an in-depth study of the peripheral region of themolecular cloud L1204/S140, where the far-ultraviolet radiation and thedensity are relatively low. Our observations test theories ofphoton-dominated regions (PDRs) in a regime that has been littleexplored. Knowledge of such regions will also help to test theories ofphotoionization-regulated star formation. [C II] 158 μm and [O I] 63μm lines are detected by the Infrared Space Observatory at all 16positions along a one-dimensional cut in right ascension. Emission fromH2 rotational transitions J=2-->0 andJ=3-->1 at 28 and 17 μm, respectively, was also detected atseveral positions. The [C II], [O I], and H2intensities along the cut show much less spatial variation than do therotational lines of 12CO and other CO isotopes. The average[C II] and [O I] intensities and their ratio are consistent with modelsof PDRs with low values of far-ultraviolet radiation (G0) anddensity. The best-fitting model has G0~15 and densityn~103 cm-3. Standard PDR models underpredict theintensity in the H2 rotational lines by up to anorder of magnitude. This problem has also been seen in bright PDRs andattributed to factors, such as geometry and gas-grain drift, that shouldbe much less important in the regime studied here. The fact that we seethe same problem in our data suggests that more fundamental solutions,such as higher H2 formation rates, are needed.Also, in this regime of low density and small line width, the [O I] lineis sensitive to the radiative transfer and geometry. Using theionization structure of the models, a quantitative analysis oftimescales for ambipolar diffusion in the peripheral regions of the S140cloud is consistent with a theory of photoionization-regulated starformation. Observations of [C II] in other galaxies differ from boththose of high-G0 PDRs in our galaxy and the low-G0regions we have studied. The extragalactic results are not easilyreproduced with mixtures of high- and low-G0 regions.
| The outflow activity of the protostars in S140 IRS
The S140/L1204 cloud contains a deeply embedded region of star formationand a powerful molecular outflow. In this paper, we present images ofthe S140 region obtained in the light of the 2.12 mu m molecularhydrogen emission line and adjacent continuum. Our images reveal severalknots of H2 line emission originating from shocked materialclose to IRS 1 as well as further out. Strong H2 shockemission is found north-east of IRS1 (at position angle of ~20°-30°), as well as to the south-west of IRS1 (at positionangles around ~ 190°-220°), clearly demonstrating the presenceof outflow activity in the north-east/south-west direction. We also findpatches of H2 emission several arcminutes away from IRS1 at aposition angles of ~ 150° and 340degr , i.e. in directionsconsistent with the previously known north-west/south-east molecularoutflow. Our results therefore provide evidence for the existence of twodistinct bipolar outflow systems originating simultaneously from IRS 1.We also discuss general aspects of the star formation process in theS140 region. An inferred high ratio of stellar to gas mass suggests thatthe outflows have dispersed most of the cloud mass. Based onobservations obtained at the German-Spanish Astronomical Centre, CalarAlto, operated by the Max-Planck-Instiute for Astronomy, Heidelberg,jointly with the Spanish National Commission for Astronomy.
| Long-term study of water maser emission associated with young stellar objects. I. The database
We present the results of more than 10 years of monitoring of the watervapor maser emission in 14 star forming regions obtained with theMedicina 32-m radiotelescope. The sample of objects covers a large rangeof luminosities of the associated FIR sources. In order to present in acompact and representative way the large amount of data available, wegive for each source: a time-velocity-flux density plot, the timedependent integrated flux, the lower and upper envelopes of the maseremission, the frequency of the maser occurrence as a function ofvelocity, and the mean velocity averaged over the observing period. Abrief morphological description of the environment of the maser sourceis also given. The present data form the basis for a discussion of themain properties of the water vapor maser emission to be presented in aforthcoming paper.
| Dust Characteristics of Massive Star-forming Sites in the Mid-Infrared
Four massive star-forming regions were imaged in the mid-infrared withthe MIRAC3 instrument: W51 IRS 2, Mon R2, DR 21, and S140. We obtainedhigh spatial resolution (~1") images at several wavelengths from 7.8 to13.2 μm with the circular variable filter, as well as narrow-bandcontinuum images at 12.5 and 20.6 μm toward each region. In eachmassive star-forming region, one or more sources show deep silicateabsorption. For at least two of the massive star-forming regions, W51IRS 2 and Mon R2, the absorbing material is highly localized and may becircumstellar material in disks or shells. The silicate absorptionoccurs at least as often around massive young stars as around youngstars of lower mass (which are more often observed). The estimatedoptical depths of the silicate features are consistent with thosepredicted by radiative transfer models toward ultracompact H II regions,but substantially higher than observed toward T Tauri stars and otherlow-mass young stellar objects. There is no consistent correspondencebetween silicate absorption and either the dust color temperature or the12.5 μm opacity. In W51 IRS 2, the two previously known mid-infraredsources have been resolved into at least six subsources. Infraredcounterparts are newly reported for two radio-continuum sources in S140.Also, new mid-infrared sources have been detected in both W51 IRS 2 andS140. We suggest that the infrared source in the southwest of DR 21 maynot be self-luminous, but may instead be heated by the three nearbyradio continuum sources. The gas density in the ring at Mon R2 supportsthe blister scenario for the IRS 1 H II region.
| The Abundance and Emission of H2O and O2 in Clumpy Molecular Clouds
Recent observations with the Submillimeter Wave Astronomy Satellite(SWAS) indicate abundances of gaseous H2O and O2in dense molecular clouds that are significantly lower than those foundin standard homogeneous chemistry models. We present here results forthe thermal and chemical balance of inhomogeneous molecular cloudsexposed to ultraviolet radiation in which the abundances ofH2O and O2 are computed for various densitydistributions, radiation field strengths, and geometries. It is foundthat an inhomogeneous density distribution lowers the column densitiesof H2O and O2 compared with the homogeneous caseby more than an order of magnitude at the same AV.O2 is particularly sensitive to the penetrating ultravioletradiation, more so than H2O. The S140 and ρ Ophiuchiclouds are studied as relevant test cases of star-forming and quiescentregions. The SWAS results of S140 can be accommodated naturally in aclumpy model with a mean density of 2×103cm-3 and an enhancement of IUV=140 compared withthe average interstellar radiation field, in agreement with observationsof [C I] and 13CO of this cloud. Additional radiativetransfer computations suggest that this diffuse H2O componentis warm, ~60-90 K, and can account for the bulk of the110-101 line emission observed by SWAS. The ρOph model yields consistent O2 abundances but too muchH2O, even for [C]/[O]=0.94, if IUV<10(respectively <40) for a mean density of 103 (respectively104 cm-3). It is concluded that enhancedphotodissociation in clumpy regions can explain the low H2Oand O2 abundances and emissivities found in the large SWASbeam for extended molecular clouds but that additional freezeout ofoxygen onto grains is needed in dense cold cores.
| Classification and properties of UV extinction curves
The catalog of Savage et al. (\cite{ref27}) reporting colour excesses of1415 stars from ANS photometry offers the opportunity to deeplyinvestigate the characteristics of UV extinction curves which differfrom the standard extinction of the diffuse interstellar medium. To thisaim we have selected a sample of 252 curves, which have been comparedwith the relations derived by Cardelli et al. (\cite{ref4}; CCM in thefollowing) for a variety of R_V values in the range 2.4-5 and have beenclassified as normal if they fit at least one of the CCM curves oranomalous otherwise. We find that normal curves with small R_V are justas numerous as those with large R_V. The anomalous objects are arrangedinto two groups according to the strength of the bump at 0.217 mu . Fora given value of c_2 this increases along the sequence: type Aanomalous, normals and type B anomalous, suggesting that this sequenceshould correspond to an increase of the amount of small grains along thesightline. Considerations concerning the environmental characteristicsindicate that the anomalous behaviour is not necessarily tied to theexistence of dense gas clouds along the line of sight.
| A HIPPARCOS Census of the Nearby OB Associations
A comprehensive census of the stellar content of the OB associationswithin 1 kpc from the Sun is presented, based on Hipparcos positions,proper motions, and parallaxes. It is a key part of a long-term projectto study the formation, structure, and evolution of nearby young stellargroups and related star-forming regions. OB associations are unbound``moving groups,'' which can be detected kinematically because of theirsmall internal velocity dispersion. The nearby associations have a largeextent on the sky, which traditionally has limited astrometricmembership determination to bright stars (V<~6 mag), with spectraltypes earlier than ~B5. The Hipparcos measurements allow a majorimprovement in this situation. Moving groups are identified in theHipparcos Catalog by combining de Bruijne's refurbished convergent pointmethod with the ``Spaghetti method'' of Hoogerwerf & Aguilar.Astrometric members are listed for 12 young stellar groups, out to adistance of ~650 pc. These are the three subgroups Upper Scorpius, UpperCentaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as VelOB2, Tr 10, Col 121, Per OB2, alpha Persei (Per OB3), Cas-Tau, Lac OB1,Cep OB2, and a new group in Cepheus, designated as Cep OB6. Theselection procedure corrects the list of previously known astrometricand photometric B- and A-type members in these groups and identifiesmany new members, including a significant number of F stars, as well asevolved stars, e.g., the Wolf-Rayet stars gamma^2 Vel (WR 11) in Vel OB2and EZ CMa (WR 6) in Col 121, and the classical Cepheid delta Cep in CepOB6. Membership probabilities are given for all selected stars. MonteCarlo simulations are used to estimate the expected number of interloperfield stars. In the nearest associations, notably in Sco OB2, thelater-type members include T Tauri objects and other stars in the finalpre-main-sequence phase. This provides a firm link between the classicalhigh-mass stellar content and ongoing low-mass star formation. Detailedstudies of these 12 groups, and their relation to the surroundinginterstellar medium, will be presented elsewhere. Astrometric evidencefor moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, CamOB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive.OB associations do exist in many of these regions, but they are eitherat distances beyond ~500 pc where the Hipparcos parallaxes are oflimited use, or they have unfavorable kinematics, so that the groupproper motion does not distinguish it from the field stars in theGalactic disk. The mean distances of the well-established groups aresystematically smaller than the pre-Hipparcos photometric estimates.While part of this may be caused by the improved membership lists, arecalibration of the upper main sequence in the Hertzsprung-Russelldiagram may be called for. The mean motions display a systematicpattern, which is discussed in relation to the Gould Belt. Six of the 12detected moving groups do not appear in the classical list of nearby OBassociations. This is sometimes caused by the absence of O stars, but inother cases a previously known open cluster turns out to be (part of) anextended OB association. The number of unbound young stellar groups inthe solar neighborhood may be significantly larger than thoughtpreviously.
| Origin and Evolution of the Cepheus Bubble
We have imaged a 10 deg x 10 deg region of the Cepheus bubble in the J =1-0 line of CO and the 21 cm line of atomic hydrogen. The CO emissiondefines a giant expanding shell 120 pc in diameter, which is similar tothat seen in the IRAS sky maps. We estimate the total gas mass in theregion to be ~4 x 10^5 M_ȯ. The total kinetic energy from theobserved spread of velocities of the molecular clouds is ~10^51 ergs. Wesuggest that the members of earlier generations of massive stars in NGC7160 are responsible for the origin of the Cepheus bubble. These starscreated an expanding compressed shell of gas that became gravitationallyunstable at an age of ~7 Myr. The members of the Cepheus OB2 associationcomprise the second, intermediate generation of stars in this regionthat formed as a consequence of this instability. The numerous colorselected IRAS point sources represent the third and youngest generationof stars in this region. Our observations suggest the great importanceof sequentially triggered star formation in the region of the Cepheusbubble.
| Surface brightness measurements of extended galactic nebulae
We discuss the importance of digitized sky-limited Schmidt B- andR-band, and H-alpha images of extended translucent (A_B approx. 1 - 5mag) and dark (A_B greater than 5 mag) clouds. In translucent clouds(e.g. high-latitude cirrus) photons can penetrate through the wholecloud and control the physical and chemical processes. Comparison of theB images with far-infrared/submillimetre CO, CI and CII emissionprovides important information on the non-homogeneous cloud structure,carbon chemistry, and energy balance. In dark clouds the interior iscompletely shielded from the interstellar radiation field. L1204 is adark cloud with at its edge the well-known HII rim S140. The R-bandimage shows S140 as well as the diffuse extended HII region around theexciting B-type star. However, the sensitivity for H-alpha in the R-bandis rather low and there is contamination with diffusely scattered redlight from dust. Deep H-alpha and R-band imaging of an approx. 20' x 20'field has recently been done with the 1 m Hoher List telescope of BonnUniversity. A wealth of small-scale structures can been seen oncontinuum subtracted images which provide new insight on the geometry ofthis cloud.
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
| The photon dominated regions associated with NGC 2023 and S 140.
We have used the Effelsberg 100-m telescope to observe the C91αradio recombination line towards the photon-dominated regions associatedwith NGC 2023 and S 140. This has been supplemented with observations ofthe C^18^O (2-1) and (3-2) lines using the 3-m KOSMA telescope. We haveanalyzed the C91α observations using 1-dimensional homogeneousmodels in order to derive estimates of the density and ultravioletradiation field. Towards both sources, our radio line data suggestdensities of 10^5^cm^-3^ although we have evidence for a drop-off to avalue of around 10^4^cm^-3^ at offsets of 0.4 pc from the exciting starin NGC 2023. The molecular line data towards NGC 2023 are alsoconsistent with high densities (certainly above 10^4^ and probably oforder 10^5^cm^-3^). We discuss the influence of clumping on these modelsand conclude that densities as high as 10^6^cm^-3^ are possible in boththe molecular and carbon line emitting gas.
| Extended fine structure and continuum emission from S 140/L 1204.
Grating spectra, covering the wavelength range 45 to 187μm have beentaken with the ISO Long Wavelength Spectrometer (LWS) at a series ofpointing positions over the S 140 region, centred on the cluster ofembedded young stellar objects at the south-west corner of the L1204molecular cloud. Extended emission from [CII]158μm and [OI]63μm isseen, peaking near the position of the embedded stars. The measurementsof the fine structure lines are interpreted in terms of PDR models forthe emission, as well as the underlying thermal continuum for the heatedgas and dust.
| H_2_ infrared line emission from S 140: a warm PDR.
ISO SWS spectra of H_2_ pure rotational and ro-vibrational transitions,as well as [FeII], and [SiII] fine structure lines were obtained for oneposition on the photodissociation region (PDR) adjacent to the brightrim of the S 140 HII region. The gas density is =~10^4^cm^-3^ and theincident UV flux χ=~400 at the location of the ionization front. OurPDR model analysis shows that the relative H_2_ line intensities arecharacteristic of low-density fluorescent emission and that the gastemperature is T>500K in the partially dissociated gas. A comparisonof predicted and observed fine structure line intensities permits anestimate of the abundances of Fe and Si.
| Derivation of the Galactic rotation curve using space velocities
We present rotation curves of the Galaxy based on the space-velocitiesof 197 OB stars and 144 classical cepheids, respectively, which rangeover a galactocentric distance interval of about 6 to 12kpc. Nosignificant differences between these rotation curves and rotationcurves based solely on radial velocities assuming circular rotation arefound. We derive an angular velocity of the LSR of{OMEGA}_0_=5.5+/-0.4mas/a (OB stars) and {OMEGA}_0_=5.4+/-0.5mas/a(cepheids), which is in agreement with the IAU 1985 value of{OMEGA}_0_=5.5mas/a. If we correct for probable rotations of the FK5system, the corresponding angular velocities are {OMEGA}_0_=6.0mas/a (OBstars) and {OMEGA}_0_=6.2mas/a (cepheids). These values agree betterwith the value of {OMEGA}_0_=6.4mas/a derived from the VLA measurementof the proper motion of SgrA^*^.
| An infrared study of the Sharpless H II regions S140, S141 and S142.
The infrared emission behavior of the S140, S141 and S142 H IIregion-molecular cloud complexes is examined through the IRAS Sky SurveyAtlases. The IRAF and Skyview software are used, and the contours of theinfrared intensity, color temperature and optical depths of each sourceare presented. The total infrared luminosity and the distribution ofdust in the complexes are also shown. The dense areas within themolecular clouds, where star formation is probably going on, areconcentrated. The exciting star of the S141 region is also analyzed.
| Giant Molecular Cloud Complexes with Optical H II Regions: 12CO and 13CO Observations and Global Cloud Properties
Abstract image available at:http://adsabs.harvard.edu/abs/1996ApJ...463..630H
| ^13^CO and C^18^O observations of S140: delineation. of the outflow structure, a study of fractionation effects and comparison with CI observations
The outflow and photon-dominated region (PDR) associated with the S140complex have been observed at high resolution (~14 arcsec) in the ^13^COand C^18^O J=3->2 lines. The C^18^O map confirms earlier C^17^OJ=3->2 line observations (Minchin et al. 1994) that show an `arc' ofemission observed to the south of the peak, and also reveals a similar(and more prominent) arc feature to the east, a region not covered bythe C^17^O map. This is a particularly fine example of the classic`tuning fork' morphology, where emission at the ambient cloud velocityis tracing the outflow cavity wall of the blueshifted lobe. TheN(^13^CO)/N(C^18^O) ratio has been plotted against extinction and fitsthe power law relation N(^13^CO)/N(C^18^O)=21A_v_^-0.35^. The highestvalues, as expected, occur for observed positions towards the PDR, withN(^13^CO)/N(C^18^O) exceeding the terrestrial value (5.5) forA_v_=<40 magnitudes. In the outermost parts of the cloud (A_v_=<10magnitudes) the N(^13^CO)/N(C^18^O) ratio is largest, up to 20. Theincreased fractionation may be due to higher photoionization of theoptically thinner isotope, C^18^O. There is a close correlation betweenN(CI)/N(CO) and visual extinction over a wide extinction range(A_v_=3-100 mags.). The best fit power law is N(CI)/N(CO)=4.2A_v_^-0.9^.For positions toward the outflow (A_v_~50-100) N(CI)/N(CO)~0.1(0.07-0.12). N(CI)/N(CO) increases with decreasing extinction to ~1for A_v_=<5 mags., corresponding to positions near the edge of thecloud. A detailed comparison of antenna temperatures and linewidths forthe ^13^CO, C^18^O and CI lines is presented. The ^13^CO and C^18^Oantenna temperatures and linewidths are closely correlated, and implythe emission, for both isotopes, emanates from gas that is in LTE and iswell mixed. The CI emission from the PDR implies that here the atomiccarbon is in LTE, but occupies a different volume of gas than theisotopic CO. Towards the outflow the CI linewidths are systematicallybroadened relative to those for the isotopic CO lines. This isinterpreted as evidence that atomic carbon is produced by the effect ofshocks on the chemical and physical processes at the interface between astellar wind and the outflow cavity wall.
| Far-Ultraviolet (912--1900 Angstrom ) Energy Distribution in Early-Type Main-Sequence Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...449..280C&db_key=AST
| The S 140/L 1204 complex: radio recombination lines of hydrogen, carbon and sulphur.
We have used the Effelsberg 100-m telescope to observe the hydrogen andcarbon 165α and 166α radio recombination lines towards the S140/L 1204 complex. In addition to the transitions known from earlierobservations, we detect a feature due to ionized sulphur. On the basisof the observed H line parameters, we conclude that the emission nebulaS 140 is only a part of a larger partially obscured H II region whichhas an electron temperature of 6400K and an electron density of order10cm^-3^. The obscured fraction of the H II region appears to be eitherbehind or partially immersed in the molecular and C II regions. Wederive parameters for the C II region by combining our results withmeasurements of the C II 158μm fine structure line by Boreiko et al.(1990) and conclude that the temperature is in the range 75-200K whereasthe electron density is in the range 0.5-9cm^-3^. If there is pressureequilibrium between H II and C II gas, then the lower values of thetemperature and density are preferable.
| C I emission from the outflow and PDR in S 140
We present high resolution maps (10-14 arcsec full width at half maximum(FWHM)) of the atomic carbon (CI) (3)P1 (492.1607 GHz) fine structureline and the C(17)O J = 3 goes to 2 (337.0611 GHz) rotational line fromthe outflow and photon-dominated region (PDR) in S 140. Theseobservations reveal remarkable, previously unresolved structure. Thereis a marked variation in CI line profiles across the mapped region.Towards the outflow the lines are broad (FWHM approximately 5-6 km/s)with peak values of Tmb approximately 10-12K, yet towards thePDR the lines are distinctly narrower (approximately 3-4 km/s) withhigher values of peak Tmb up to 18K. Both the CI and C(17)Ointergrated intensity maps show a similar morphology. The emision peaklies towards the molecular outflow source, and there is an arc ofemission extending from the peak towards the South. The CI and C(17)Oarcs are offset, with the CI arc offset to the northeast. This CI arcfeature is observed at blue and redshifted velocities, while the C(17)arc feature is only observed in blueshifted emission. This implies theCI emission lies on the inner edge of the blueshifted molecular outflowwall (traced by the C(17)O emission). Towards the molecular outflowsource the abundance ratio N(CI)/N(CO) = 0.14, with a similar ratiofound for the blue wing and core velocity intervals (0.12). The red winghas a particularly high abundance, N(CI)/N(CO) = 0.67. The mostplausible mechanism for producing the CI emission is the effect ofshocks on the chemical and physical processes at the interface betweenthe stellar wind and the blueshifted outflow cavity wall. The PDR is aclumpy, narrow (approximately 0.1-0.15pc) elongated ridge-like feature,adjacent to the south-western edge of the molecular cloud. Emission fromthe PDR is observed across a relatively narrow velocity range, -10 to -6km/s. There is a localized intergrated emission peak (at offset position-28, -42 arcsec), which we have designated PDRc1 (PDR clump 1). Theabundance of CI is particularly high, N(CI)/N(CO) = 0.29 at the positionof PDRc1. The observed column density (9.7 +/- 0.8 x 1017/sq.cm) and emergent intensity (8.7 +/- .07 x 10-6 erg/sq.cm/s/sr) towards PDRc1 are in close agreement with recent modelling oflow-density PDRs.
| Observations of large-scale (C I) emission from S140
We have mapped the large-scale distribution of the 492 GHz (C I) (3P1 to3P0) and 220 GHz (13)CO J = 2 to 1 lines across the S140 molecularcloud. The observations employed the University of Texas Gaussian FocalReducer on the Caltech Submillimeter Observatory telescope to produce ahigh surface accuracy off-axis telescope with a 3 min beam. The (C I)emission covers a greater than or equal to 8 by approximately 10 pcregion which closely matches the extent of the S140 molecular cloud asseen in (13)CO. The line shapes of (C I) and (13)CO are also quitesimilar. Typical column densities of C0 are approximately 15times those of (13)CO. In this source, the neutral carbon emission mostlikely arises in the photon-dominated surface layers of molecularclumps. The layer in which C0 is the dominant form of gaseouscarbon contains greater than or equal to 20% of all the neutral materialin the extended cloud. The ratio of the (C I) (3P1 to 3P0) to CO J = 2to 1 cooling integrated over the cloud is approximately 1.3, similar tothe global value of this ratio in our Galaxy and to the value in theinner 100 pc of IC 342 and M82.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
| Takýmyýldýz: | Kral-Sefe |
| Sað Açýklýk: | 22h18m27.80s |
| Yükselim: | +63°13'22.4" |
| Görünürdeki Parlaklýk: | 7.773 |
| Uzaklýk: | 355.872 parsek |
| özdevim Sað Açýklýk: | -1.4 |
| özdevim Yükselim: | -4.7 |
| B-T magnitude: | 8.069 |
| V-T magnitude: | 7.798 |
Kataloglar ve belirtme:
|
INHABITED SKY
