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|VLT K-band spectroscopy of massive young stellar objects in (ultra-)compact HII regions|
High-quality K-band spectra of strongly reddened point sources, deeplyembedded in (ultra-)compact H II, have revealed a population of 20 youngmassive stars showing no photospheric absorption lines, but sometimesstrong Brγ emission. The Brγ equivalent widths occupy a widerange (from about 1 to over 100 Å); the line widths of 100-200 kms-1 indicate a circumstellar rather than a nebular origin.The K-band spectra exhibit one or more features commonly associated withmassive young stellar objects (YSOs) surrounded by circumstellarmaterial: a very red colour (J-K) 2, CO bandhead emission,hydrogen emission lines (sometimes doubly peaked), and Fe II and/or MgII emission lines. The large number of objects in our sample allows amore detailed definition and thorough investigation of the properties ofthe massive YSOs. In the (K, J-K) colour-magnitude diagram (CMD) themassive YSO candidates are located in a region delimited by the OBzero-age main sequence, Be stars, Herbig Ae and Be stars, and B[e]supergiants. The massive YSO distribution in the CMD suggests that themajority of the objects are of similar spectral type as the Herbig Bestars, but some of them are young O stars. The spectral properties ofthe observed objects do not correlate with the location in the CMD. TheCO emission must come from a relatively dense ( 1010cm-3) and hot (T 2000-5000 K) region, sufficientlyshielded from the intense UV radiation field of the young massive star.The hydrogen emission is produced in an ionised medium exposed to UVradiation. The best geometrical solution is a dense and neutralcircumstellar disk causing the CO bandhead emission, and an ionisedupper layer where the hydrogen lines are produced. We present argumentsthat the circumstellar disk is more likely a remnant of the accretionprocess than the result of rapid rotation and mass loss such as inBe/B[e] stars.
|VLT K-band spectroscopy of massive stars deeply embedded in IRAS sources with UCHII colours|
We have obtained high resolution (R = 10 000) K-band spectra ofcandidate young massive stars deeply embedded in (ultra-) compact H IIregions (UCHIIs). These objects were selected from a near-infraredsurvey of 44 fields centered on IRAS sources with UCHII colours. Often,the near-infrared counterpart of the IRAS source is a young embeddedcluster hosting massive stars. In these clusters, three types of objectsare identified. The first type (38 objects) consists of "naked" OB starswhose K-band spectra are dominated by photospheric emission. We classifythe K-band spectra of the OB-type cluster members using near-infraredclassification criteria. A few of them have a very early (O3-O4 V)spectral type, consistent with a young age of the embedded clusters. Thespectral classification provides an important constraint on the distanceto the embedded cluster. The ionising power of the population thusderived is compared to the information obtained from the infrared andradio flux of these sources. In most cases these two differentdeterminations of the ionising flux are consistent, from which weconclude that we have identified the ionising star(s) in about 50% ofthe embedded clusters. The second type (7 objects) are point sourcesassociated with UCHII radio emission, that exhibit nebular emissionlines in the near-infrared. Six of the objects in this group produce HeI emission indicative of an embedded O-type star. These objects are moreembedded than the OB stars and probably do not dominate the infraredflux as measured by IRAS. They may emit the bulk of their reprocessed UVradiation at mm wavelengths. The third type (20 objects) ischaracterised by broad (100-200 km s-1) Brγ emissionand no photospheric absorption profiles. Bik et al. (2005, A&A,submitted) show that these objects are massive YSO candidates surroundedby dense circumstellar disks.
|Near infrared observations of the star formation region GGD12-15|
Near infrared imaging observations in JHK broad bands and HZ v=10 S(1)narrow band of the star formation region GGD12-15 are presented in thisarticle. The images reveal young compact infrared clusters and infrarednebulae in association with infrared sources, as well as outflowactivities characterized by H2 emission knots. Most of theinfrared point sources are invisible in the optical wave band. TheJHK′ photometric results of 76 infrared point sources contain 32with infrared excess, and 5 with the characteristics of protostars,indicating very active star formation activities in the molecularclouds. Massive stars (defined as those with spectral types earlier thanB8) account for ˜ 10 26 per cent of all stars of the clusters.The K′ magnitude distribution of GGD12-15 peaks at 15.0 mag, andspreads broadly from 13.0 mag to 16.0 mag. The peak of the [H-K′lcolor distribution occurs at ˜0.7 mag, above which the reddercomponents of the cluster make up 70 per cent of the total. Thenewly-discovered H2 emission knots are concentrated in the central areaof the clusters, and the spatial distribution has obvious relation toviolent star formation activities. Five of the emission knots arelocated in the central part of the molecular outflow, indicating thatthey may have been excited by the same source as the molecular outflow.
|Mid-infrared images of the star forming region GGD 14 (IRAS 06084-0611)|
Mid-infrared images at 8.7, 9.7 and 12.5 mu m are presented of the starforming region GGD 14 associated with the source IRAS 06084-0611. In anarea of ~ 30arcsec around the IRAS position, two mid-infrared sourceswere found. One was identified with the cometary compact HII region VLA1 and the second with the faint and unresolved radio continuum sourceVLA 4. The mid-infrared morphology of VLA 1 is very similar to that ofthe radio continuum suggesting a direct interaction between the ionizedgas and the dust. The infrared source associated with VLA 4 shows alarge infrared excess and its derived infrared luminosity is L1-20mu m = 350 Lsun, indicating the presence of a youngstar later than B3. In addition, its spectrum between 2.47 and 11.62 mum observed by ISOPHOT shows the presence of infrared emission bands at3.3, 6.2, 7.7, 8.6 and 11.2 mu m with relative intensity ratios typicalof compact HII regions. Finally, the radio continuum source VLA 2,undetected on our mid-infrared images, shows an infrared luminosity of ~ 8.5 Lsun confirming its nature of an embedded T-Tauri star.Based on observations collected at the Observatorio AstronómicoNacional, San Pedro Mártir, B.C, Mexico, and ISOPHOT data fromthe ISO Data Archive (TDT No.70901306).
|Merged catalogue of reflection nebulae|
Several catalogues of reflection nebulae are merged to create a uniformcatalogue of 913 objects. It contains revised coordinates,cross-identifications of nebulae and stars, as well as identificationswith IRAS point sources.The catalogue is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (22.214.171.124) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/141
|Mid-infrared images of compact and ultracompact HII regions: W51 and W75N.|
A mid-infrared imaging survey at 8.7, 9.7 and 12.5 mu m of a selectednumber of compact and ultracompact (UC HII) regions has been undertakenwith the 2.1m telescope of the OAN at San Pedro Martir, Mexico. Wepresent a detailed plane of the observations, and we discuss thepreliminary results of two massive star-forming regions; W51 and W75N.
|The Nature of the Cluster of Radio Sources in GGD 14|
We present sensitive radio continuum observations at 3.6 and 6 cm madequasi-simultaneously toward the star-forming region GGD 14. Theobservations reveal the presence of nine extremely compact (<=0.3")and faint radio sources in a region of ~30" around the bright cometary HII region, VLA 1. Most of these sources show variability at radiowavelengths, and their spectral indices are characteristically negative.Four of the faint sources have an infrared counterpart. We propose thatgyrosynchrotron radiation from an active magnetosphere explains theemission from almost all the faint compact sources, suggesting that theGGD 14 region harbors a cluster of low-mass, pre-main-sequence stars.Two sources that do not show time variability are VLA 1, the cometary HII region, and VLA 7. The derived spectral index for VLA 7 (+0.6+/-0.3)fits the stellar wind model better than other possibilities, supportingthe idea that VLA 7 is the powering source of the molecular outflowobserved toward the GGD 14 region.
|The Excitation and Metallicity of Galactic H II Regions from Infrared Space Observatory SWS Observations of Mid-Infrared Fine-Structure Lines|
We present mid-infrared Infrared Space Observatory Short-WavelengthSpectrometer (ISO-SWS) observations of the fine-structure emissionslines [Ne II] 12.8 μm, [Ne III] 15.6 μm, [Ne III] 36.0 μm, [ArII] 6.99 μm, [Ar III] 8.99 μm, [S III] 18.7 μm, [S III] 33.5μm, and [S IV] 10.5 μm and the recombination lines Brα andBrβ in a sample of 112 Galactic H II regions and 37 nearbyextra-Galactic H II regions in the LMC, SMC, and M33. We selected oursources from archival ISO-SWS data as those showing prominent [Ne II]12.8 μm or [Ne III] 15.6 μm emissions. The Galactic sources have awide range in galactocentric distance (0kpc<~Rgal<~18kpc), which enables us to study excitation and metallicity variationsover large Galactic scales. We detect a steep rise in the [Ne III] 15.6μm/[Ne II] 12.8 μm, [Ar III] 8.99 μm/[Ar II] 6.99 μm, and [SIV] 10.5 μm/[S III] 33.5 μm excitation ratios from the innerGalaxy outward, and a moderate decrease in metallicity, from ~2Zsolar in the inner Galaxy to ~1 Zsolar in theouter disk. The extra-Galactic sources in our sample show low gasdensity, low metallicity, and high excitation. We find a goodcorrelation between [Ne III] 15.6 μm/[Ne II] 12.8 μm and [Ar III]8.99 μm/[Ar II] 6.99 μm excitation ratios in our sample. Theobserved correlation is well reproduced by theoretical nebular modelsthat incorporate new-generation wind-driven non-LTE model stellaratmospheres for the photoionizing stars. In particular, the non-LTEatmospheres can account for the production of [Ne III] emission in the HII regions. We have computed self-consistent nebular and stellaratmosphere models for a range of metallicities (0.5-2Zsolar). We conclude that the increase in nebular excitationwith galactocentric radius is due to an increase in stellar effectivetemperature (as opposed to a hardening of the stellar spectral energydistributions due to the metallicity gradient). We estimate anintegrated [Ne III] 15.6 μm/[Ne II] 12.8 μm ratio for the Galaxyof 0.8, which puts it well inside the range of values for starburstgalaxies. The good fit between observations and our models support theconclusion of Thornley and coworkers that the low [Ne III] 15.6μm/[Ne II] 12.8 μm ratios observed in extra-Galactic sources aredue to global aging effects. Based on observations with ISO, an ESAproject with instruments funded by ESA member states (especially the PIcountries: France, Germany, the Netherlands, and the United Kingdom)with the participation of ISAS and NASA.
|A survey of the 6.7 GHz methanol maser emission from IRAS sources. I. Data|
We report the first results of a search for 6.7 GHz methanol masers inthe direction of 1399 IRAS objects north of declination-20deg with the flux densities greater than 100 Jy at 60 mu mand the flux density ratio F60/F25>1.Observations were made with the sensitivity of 1.7 Jy and the velocityresolution of 0.04 km s-1 using the 32-m Toruń radiotelescope. Maser emission was found in 182 sources, including 70 newdetections. 32 new sources were identified with objects of radioemission associated with star-forming regions. Comparison of the presentdata set with other observations suggests that about 65% of methanolmasers exhibit moderate or strong variations on time-scales of about 4and 8 years. Table 2 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (126.96.36.199) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
|Observations of OH 4765-MHz maser emission from star-forming regions|
We report observations of the 4765-MHz maser transition of OH(2Π1/2, J=1/2, F=1->0) towards 57star-forming regions, taken with the 32-m Toruń telescope. Ninemaser sources were detected, of which two had not been reportedpreviously. The newly discovered sources in W75N and Cep A and fourpreviously known sources were monitored over periods ranging from a fewweeks to six months. Significant variations of the maser intensityoccurred on time-scales of one week to two months. The relationshipsbetween the flux density and the linewidth for the new sources,established during the rise and fall phases of bursts that lasted 6-8weeks, are consistent with a model of saturated masers.
|Survey of bipolar outflows and methanol masers in the C(32) S (2-1) and C(34) S (2-1) lines in the Northern sky|
A survey of 158 sources (bipolar outflows and methanol masers) wascarried out in the CS(2-1) and C(34) S(2-1) lines to measure and comparedensities in a large number of bipolar outflows and in Class I and ClassII methanol masers, both associated with and unrelated to bipolaroutflows. The statistical characteristics of the regions, forming ClassI methanol masers differ from those both in the centres of bipolaroutflows and in the regions forming Class II methanol masers. It ispossible that physical conditions in bipolar outflows are closer tophysical conditions of Class II methanol masers. Figures i and ii areonly available in electronic form at http://www.edpsciences.com
|High density molecular clumps around protostellar candidates.|
There are several indications that strong H_2O maser emission arises atthe very beginning of the evolution of a massive star and disappearswhen an Hii region becomes detectable in the radio continuum. If this isthe case, one expects to find dense hot molecular gas surroundingembedded far IR sources coincident with H_2O masers. In order to testthis hypothesis, we have used the Pico Veleta 30-m radiotelescope tosearch for molecular line emission towards a sample of 12 H_2O maserslocated in regions of massive star formation, but not directlyassociated with bright compact radio Hii regions, with the intention toidentify the sites of newly born massive (proto)stars in their earlieststages and to study their properties. Our main goals were: a) to confirmthe hypothesis that the H_2O masers not associated with compact radiocontinuum emission are indeed located at the centre of high densityclumps within a molecular cloud; b) to use several molecular transitions(namely: (12) COI(2-1), CS(3-2), C(34) S(2-1), C(34) S(3-2), C(34)S(5-4), HCN(1-0), CH_3CN(8-7), CH_3CN(12-11), HCO(+) (1-0), CH_3OH(3-2),CH_3OH(5-4)) in order to derive information on the size, kinematics,temperature, density, and ionisation degree of the molecular gas in theplaces where star formation has just begun, as well as to search for thepresence of outflows on scale sizes of 10arcsec -30arcsec . In thispaper we present the large amount of data obtained at Pico Veleta in acompressed way, but still sufficiently ample to give usable informationsfor further studies. General results from a first analysis of the dataare also presented. Our first goal is amply verified since in all casesand in molecules tracing high density gas we find a barely resolved peakat the position of the maser, confirming the validity of our selectioncriteria. Our sample thus provides a valid reference list of regions ofmassive star formation in their earliest phases. As far as the secondgoal is concerned, the large variety of intensity ratios of differentmolecules, as well as of other derived parameters, point out that themolecular clumps where star formation is taking place are far fromidentical and that chemical evolution and influence of the newborn starmay amply affect the line intensity ratios. In some cases small scale(seconds of arc) outflows were detected, not necessarily related to theminute of arc scale outflows present in the same regions. More detailedstudies of each region are presented in separate papers.
|Studies of ultracompact HII regions - II. High-resolution radio continuum and methanol maser survey|
High spatial resolution radio continuum and 6.67-GHz methanol spectralline data are presented for methanol masers previously detected by Walshet al. (1997). Methanol maser and/or radio continuum emission is foundin 364 cases towards IRAS-selected regions. For those sources withmethanol maser emission, relative positions have been obtained to anaccuracy of typically 0.05 arcsec, with absolute positions accurate toaround 1 arcsec. Maps of selected sources are provided. The intensity ofthe maser emission does not seem to depend on the presence of acontinuum source. The coincidence of water and methanol maser positionsin some regions suggests there is overlap in the requirements formethanol and water maser emission to be observable. However, there is astriking difference between the general proximity of methanol and watermasers to both cometary and irregularly shaped ultracompact (UC) Hiiregions, indicating that, in other cases, there must be differingenvironments conducive to stimulating their emission. We show that themethanol maser is most likely present before an observable UC Hii regionis formed around a massive star and is quickly destroyed as the UC Hiiregion evolves. There are 36 out of 97 maser sites that are linearlyextended. The hypothesis that the maser emission is found in acircumstellar disc is not inconsistent with these 36 maser sites, but isunlikely. It cannot, however, account for all other maser sites. Analternative model which uses shocks to create the masing spots can morereadily reproduce the maser spot distributions.
|Studies of ultracompact HII regions - I. Methanol maser survey of IRAS-selected sources|
A survey of ultracompact (UC) Hii regions has been carried out bysearching for 6.669-GHz methanol maser emission from a sample of 535IRAS-selected candidates. A total of 201 candidates exhibit methanolemission. These sources have been used, in conjunction with previouslyidentified UC Hii regions, to provide a base for further studies of suchregions. Estimates of distances have indicated that the identified UCHii regions tend to have some Galactic structure but it is not clearwhether they lie in or between the spiral arms of the Galaxy. Theregions are tightly constrained to the plane of the Galaxy. Comparisonof identified regions and IRAS sources selected by Wood & Churchwellindicates that there there is some degree of contamination, which couldbe due to an older phase in the life on an UC Hii region where methanolmaser emission is not apparent. Luminosities and spectral types havebeen derived for many of the regions. The maximum number of maser spotsobserved seems to increase with increasing peak maser luminosity, whichindicates that the maser emission is more dependent on the abundance ofmethanol than the availability of far-infrared radiation.
|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.
|A survey of star-forming regions in the 5 CM lines of OH.|
We have undertaken a comprehensive search for 5 cm OH masers in regionsvisible from Effelsberg with the 100-m telescope (i.e., those withdeclinations above -29^o^). Our sample is based on bright sources takenfrom the IRAS Point Source Catalog and/or from previous 18 cm OH and H_2_O surveys. Among the 165 sources observed, 16 are new (15 exhibitemission and one source shows both absorption and emission). Two newabsorption line sources have been tentatively detected. The generalproperties of the 5 cm OH sources are presented and discussed. The6035MHz line is more often detected than the 6031MHz line. Nevertheless,the latter line is frequently present, a fact that is not explained bypresent OH excitation models. Simple calculations tend to show thatthere are enough FIR photons to pump the 6035 and 6031MHz masers, and wefound that the statistics of the ratio S_ radio_/S_ IR_ at 18, 5 and 6cm suggest that the maser pumping efficiency decreases with increasingOH excitation. Variability on short (months) or long (years) time-scalesis a common feature in many 5 cm OH sources. We also presentobservations of the 6 GHz satellite lines and report, besides W3(OH), ontwo certain and perhaps two newly detected weak sources. Someimplications on the excitation of OH are briefly discussed. Clearly,there is no efficient maser mechanism for the satellite lines. We derivethe percentage of circular polarization in detected 6035 and 6031 MHzemission sources and suggest the identifications of possible Zeemanpatterns. We have found several features having > 90 % polarization.Combining our magnetic field measurements with other published resultswe find that the dominant field alignment is consistent with thedirection of the Galactic rotation. However, there is no convincingcorrelation of the field direction with the Galactic spiral pattern, andit is possible that the field direction is not preserved in the starformation process.
|Infrared Images and Millimeter Data from Cold Southern IRAS Sources|
We present near-infrared (H, K'), CO (2--1), CS (2--1), and 1.3 mmcontinuum data for 31 southern objects [ delta (1950) <= 10 deg]known to have extremely red IRAS colors [F nu (100 mu m) > F nu (60mu m) > F nu (25 mu m) > 20 x F nu (12 mu m)]. The data are meantto help reveal new, very young stellar objects. K'-band near-infraredcounterparts to the IRAS point sources are detected in 22 of 25 good K'images. Most K' counterparts are multiples. Eighteen of 21 objects weredetected in CS, implying the presence of dense gas. Completing the setof CS (2--1) spectra by including the data of Bronfman, Nyman, &Ray, we still find only three nondetections among all 31 objects; thesethree were also not detected in K'. Wings indicative of outflows arefound in a large fraction (20/30) of CO spectra. Twenty-six of 31observations in the millimeter continuum were detections and point tothe presence of large amounts of circumstellar matter. Most of theobjects have 103--105 times solar luminosity; we speculate that mostcontain at least one massive star capable of producing acompact/ultracompact H II region.
|Detection of an absorption feature at the position of the 4.27-μm band of solid CO_2_.|
We report observations of 13 young infrared sources associated withmolecular clouds with ISO's spectrophotometer ISOPHOT-S in thewavelength range from 2.5 to 11.6μm. In addition to a number ofwell-known interstellar bands due to various molecular ices, a new bandat 4.27μm was detected, which is very likely attributed to C=Ostretching vibrations of solid CO_2_. This new band is seen in all oursources. Thus, CO_2_ seems to be a common constituent of icy grainmantles in molecular clouds. The strength of the band increases with thestrengths of the CO and H_2_O bands. We estimate that CO_2_ and CO maybe present in comparable amounts in the mantles. The implications forthe models of interstellar chemistry are briefly discussed.
|High-Velocity Molecular Gas from High-Mass Star Formation Regions|
Abstract image available at:http://adsabs.harvard.edu/abs/1996ApJ...457..267S
|Dusty Plasmas in Interstellar Clouds and Star Forming Regions|
|A study of the relation between 22 GHz H2O masers and CO outflows in intense far-infrared sources on the stages of star formation.|
|Studies of Embedded Far Infrared Sources in the Vicinity of H2O Masers - Part One - Observations|
Abstract image available at:http://adsabs.harvard.edu/abs/1995MNRAS.276.1024J
|Exploring the engines of molecular outflows. Radio continuum and H_2_O maser observations.|
We present A-configuration VLA observations of the 22GHz H_2_O maserline and 8.4GHz continuum emission of 22 selected CO bipolar outflowsassociated with water masers. These observations allow us to study theregion within 10^4^AU of the engine powering the outflow. The positionsof the maser spots are compared with those of ultra-compact (UC)continuum sources found in our observations, with IRAS data and withdata from the literature on the molecular outflows. Weak unresolvedcontinuum sources are found in several cases associated with the maser.Most probably they represent the ionized envelope surrounding the youngstellar object (YSO) which powers the maser and the outflow. These weakradio continuum sources are not necessarily associated with the IRASsources, which are more representative of the global emission from thestar forming region. A comparison of the velocity pattern of the COoutflow with those of the maser spots detected with the VLA is alsomade. Asymmetries in the H_2_O velocities are found on opposite sides ofthe YSO, suggesting that the outflow acceleration begins from the YSOitself. In a few cases we find evidence for two outflows in differentevolutionary stages. The H_2_O masers in these sources are always foundat the centre of the younger outflow. The degree of variability of eachmaser is derived from single dish observations obtained with theMedicina radiotelescope before and after the VLA observations. Velocitydrifts of some features are interpreted as acceleration of the maser.
|Excited OH 4.7-GHz masers associated with IRAS far-infrared sources - II|
A sample of 85 far-infrared sources selected from the IRAS Point SourceCatalog has been searched for maser emission from excited OH in the^2Pi_1/2 J=1/2 state, in the three transitions at 4765, 4750 and 4660MHz. Four new detections were made at 4765 MHz, including one sourcewhere ground-state OH maser emission has not been reported. One newdetection was made at 4660 MHz. This source OH 351.78-0.54 is remarkablein that it was detected only at 4660 MHz, and not at 4765 or 4750 MHz.Positions were measured for the new detections and for six previouslyknown 4.7-GHz maser sources. In some cases the excited OH 4.7-GHzpositions are significantly different from the positions of previouslyknown ground-state OH 18-cm masers. The 4.7-GHz sources are among thebrighter far-infrared sources in the IRAS sample. In most cases they arealso OH 18-cm masers. There is a strong association between OH 4.7-GHzemission and OH 18-cm maser emission in the 1720-MHz transition. Morethan half of the 4.7-GHz masers that were examined for variabilityshowed strong variations on a time-scale of a year.
|Ultracompact H II regions. 2: New high-resolution radio images|
Radio continuum observations were made of 59 Infrared AstronomySatellite (IRAS) sources that have 100 micrometers flux densitiesgreater than or equal to 1000 Jy and far-infrared colors identified withultracompact (UC) H II regions. Eighty percent were found to haveassociated compact radio sources. Seventy-five sources were detected atless than or approximately equal to 1 sec resolution at 3.6 and 2 cmwavelengths, for which we provide contour plots and flux densitydistributions ranging from the radio to the near-infrared. Over half areunresolved and their morphologies undetermined. The remaining sourcescan be described by only five morphological classes, whose frequency ofoccurrence is consistent with that of the Wood and Churchwell survey. Wecalculate physical properties of the nebulae and show that they areconsistent with UC photoionized regions. Alternative explanations areexplored and found to be unlikely. The correlation of UC H II regionpositions with proposed spiral arms is examined and found to be wellcorrelated only for the local spiral arm or `spur.' No obviousenhancement of UC H II regions is apparent along the proposedSagittarius and Scutum arms, probably because of inaccuracies in thekinematic distances. We find the latitude distribution of UC H IIregions to lie in the range of absolute value of bFWHMgreater than or equal to 0.5 deg and less than or equal to 0.8 deg. Nocorrelation between size and density of cometary and core-halo UC H IIregions is found, consistent with the bow shock interpretation of thesemorphologies. Spherical and unresolved UC H II regions, however, appearto show a trend toward lower densities with increasing size, as expectedfor expanding H II regions. The observed ratios of far-infrared to radioflux densities of UC H II regions lie in the range 103 togreater than or approximately equal to 105. By applying theresults of model atmospheres, it is shown that this ratio depends onspectral type, ranging from approximately equal to 103 for anO4 star to greater than or approximately equal to 105 for aB3 star. We find that many of the UC H II regions in our sample must beexcited by a cluster of stars, and most probably contain significantamounts of dust.
|Classification and Statistical Properties of Galactic H2O Masers|
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&AS..101..153P&db_key=AST
|Infrared spectroscopy of solid CO - The Rho Ophiuchi molecular cloud|
Infrared spectra centered on the solid CO feature at 4.76 microns arepresented for seven embedded objects in the Rho Oph molecular cloud.Synthetic spectra based on laboratory data for various ice mixturescontaining CO are compared with the observations, in an attempt toconstrain the abundances of both CO and other grain mantle constituents.The profiles indicate that the CO is embedded in only two types ofmatrix: pure (or nearly pure) CO, and H2O. A large abundance of CO2mixed with CO in the mantles is ruled out. There are at least twocomponents to the absorption in most cases, attributed to two or moredifferent grain environments. Column densities of solid CO in theseregions are presented, along with upper limits on the depth of the COfeature for a further eight lines of sight in the cloud. The featuresreflect varying physical and chemical conditions within the Rho Ophcloud. The composite features are fundamentally different within currentobservational limits from those seen in the Taurus dark cloud.
|Infrared spectra and circumstellar emission of IRAS sources with ten-micron silicate absorption|
Results are presented of near-infrared photometry carried out in the J,H, K, and L bands for 33 IRAS sources with a 10-micron silicateabsorption features. The observed sources were found to be mostlycomprised of two groups of stars which showed very different spectralcharacteristics from each other between 1 and 100 microns. These starswere either evolved OH/IR stars, which generally showed IR photometricspectra similar to black-body spectra; or they were young stellarobjects showing much broader spectral energy distributions thanblack-body spectra.
|Maser search towards young stellar objects|
In order to characterize the maser properties of massive young stellarobjects (YSOs), a systematic 1.3-cm wavelength H2O maser search in aflux-limited sample of bright IRAS sources has been made with the 100-mEffelsberg telescope. The sample also included other YSOs thought to bevery young massive stars. Thirty four sources (10 new detections) weredetected in a sample of 88 objects. In the case of the flux-limitedsample, nearly complete information about the molecular outflowproperties is available. It was found that 80 percent of the CO outflowsources examined are associated with H2O masers. It is found that thedetection probability for H2O masers depends on the IRAS color indexR(12/25). This seems to be not a pure color effect, but to be probablyrelated to the luminosity of the sources. Relations between the maserparameters and the outflow and IRAS source properties are analyzed.
|A survey of circumstellar CO emission from a sample of IRAS point sources|
The first results from a survey of circumstellar CO(1-0) emission arepresented. The sources were selected from the IRAS point source catalogaccording to the IRAS color criteria described in van der Veen andHabing (1988). The sources have good quality fluxes at 12, 25, and 60microns, flux densities larger than 20 Jy at 25 microns, and aresituated more than 5 deg away from the Galactic plane. The survey isundertaken to study the relationship between mass loss rates, dustproperties, and the evolution along the AGB. The sample consists of 787sources and contains both oxygen and carbon-rich stars, including Miravariables, OH/IR objects, protoplanetary nebulae, planetary nebulae, and60-micron excess sources. So far, 519 objects, situated on both thenorthern and the southern sky, have been observed; 163 sources werefound to have circumstellar CO emission, and in 58 of these CO emissionhas not previously been detected.
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