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A survey of OH masers towards high mass protostellar objects
Context: Masers are important tracers of the early evolution of younghigh mass stars, but the relationship between different types of maserand the evolutionary state of the exciting source remains unclear. Aims: To determine whether OH masers are common towards candidate highmass protostellar objects. Methods: We present a survey of OH maseremission towards a sample of high mass protostellar objects made usingthe Nançay and GBT telescopes. Results: OH maser emission wasdetected towards 63 objects with 36 new detections. There are 56star-forming regions and 7 OH/IR candidates. Nearly half of the detectedsources have OH flux densities ⪉1 Jy. There is no evidence thatsources with OH masers have a different range of luminosities from thenon-maser sources. The results of this survey are compared with previousH2O and class II CH3OH maser observations of the same objects. Some ofthe detected sources are only associated with OH masers and some sourcesare only associated with the 1720 MHz OH maser line. The velocity rangeof the maser emission suggests that the water maser sources may bedivided into two groups. The detection rates and velocity range of theOH and Class II CH3OH masers support the idea that there is a spatialassociation of the OH and Class II CH3OH masers. The sources span a widerange in R, the ratio of the methanol maser peak flux to OH 1665 MHzmaser peak flux, however there are only a few sources with intermediatevalues of R, 8
| Searching for massive pre-stellar cores through observations of N2H+ and N2D+
Aims.We have measured the deuterium fractionation and the CO depletionfactor (ratio between expected and observed CO abundance) in a sample ofhigh-mass protostellar candidates, in order to understand whether theearliest evolutionary stages of high-mass stars have chemicalcharacteristics similar to those of low-mass ones. It has been foundthat low-mass starless cores on the verge of star formation have largevalues both of the column density ratio N(N_2D^+)/N(N_2H^+) and of theCO depletion factor. Methods: .With the IRAM-30 m telescope andthe JCMT we have observed two rotational lines of N{2}H+ andN{2}D+, the (2-1) line of C17O andDCO+, and the sub-millimeter continuum towards a sample of 10high-mass protostellar candidates. Results: .We have detectedN{2}D+ emission in 7 of the 10 sources of our sample, andfound an average value N(N_2D^+)/N(N_2H^+)˜ 0.015. This value is 3 orders of magnitude larger than the interstellar D/H ratio, indicatingthe presence of cold and dense gas, in which the physical-chemicalconditions are similar to those observed in low-mass pre-stellar cores.The integrated CO depletion factors show that in the majority of thesources the expected CO abundances are larger than the observed values,with a median ratio of 3.2. Conclusions: .In principle, the coldgas that generates the N{2}D+ emission can be the remnant ofthe massive molecular core in which the high-mass (proto-)star was born,not yet heated up by the central object. If so, our results indicatethat the chemical properties of the clouds in which high-mass stars areborn are similar to their low-mass counterparts. Alternatively, thiscold gas could be located in one (or more) starless core (cores) nearthe protostellar object. Due to the poor angular resolution of our data,we cannot distinguish between the two scenarios.
| CCS and NH3 Emission Associated with Low-Mass Young Stellar Objects
In this work we present a sensitive and systematic single-dish survey ofCCS emission (complemented with ammonia observations) at 1 cm, toward asample of low- and intermediate-mass young star-forming regions known toharbor water maser emission, made with NASA's 70 m antenna at Robledo deChavela, Spain. Out of the 40 star-forming regions surveyed in the CCS(21-10) line, only six low-mass sources show CCSemission: one transitional object between the prestellar andprotostellar Class 0 phase (GF9-2), three Class 0 protostars(L1448-IRS3, L1448C, and B1-IRS), a Class I source (L1251A), and a youngT Tauri star (NGC 2071 North). Since CCS is considered an ``early-time''(<~105 yr) molecule, we explain these results by eitherproposing a revision of the classification of the age of NGC 2071 Northand L1251A, or suggesting the possibility that the particular physicalconditions and processes of each source affect thedestruction/production of the CCS. No statistically significantrelationship was found between the presence of CCS and parameters of themolecular outflows and their driving sources. Nevertheless, we found asignificant relationship between the detectability of CCS and theammonia peak intensity (higher in regions with CCS), but not with itsintegrated intensity. This tendency may suggest that the narrowerammonia line widths in the less turbulent medium associated with youngercores may compensate for the differences in ammonia peak intensity,rendering differences in integrated intensity negligible. From the CCSdetection rate we derive a lifetime of this molecule of~=(0.7-3)×104 yr in low-mass star-forming regions.
| The youngest stellar clusters. Clusters associated with massive protostellar candidates
We report on the identification of 54 embedded clusters around 217massive protostellar candidates of which 34 clusters are new detections.The embedded clusters are identified as stellar surface densityenhancements in the 2 μm All Sky Survey (2MASS) data. Because theclusters are all associated with massive stars in their earliestevolutionary stage, the clusters should also be in an early stage ofevolution. Thus the properties of these clusters should reflectproperties associated with their formation rather than their evolution.For each cluster, we estimate the mass, the morphological type, thephotometry and extinction. The clusters in our study, by theirassociation with massive protostars and massive outflows, reinstate thenotion that massive stars begin to form after the first generation oflow mass stars have completed their accretion phase. Further, theobserved high gas densities and accretion rates at the centers of theseclusters is consistent with the hypothesis that high mass stars form bycontinuing accretion onto low mass stars.
| A Millimeter Continuum Survey for Massive Protoclusters in the Outer Galaxy
Our search for the earliest stages of massive star formation turned up12 massive pre-protocluster candidates plus a few protoclusters. Forthis search, we selected 47 FIR-bright IRAS sources in the outer Galaxy.We mapped regions of several square arcminutes around the IRAS source inthe millimeter continuum in order to find massive cold cloud corespossibly being in a very early stage of massive star formation. Massesand densities are derived for the 128 molecular cloud cores found in theobtained maps. We present these maps together with near-infrared,mid-infrared, and radio data collected from the 2MASS, MSX, and NVSScatalogs. Further data from the literature on detections of high-densitytracers, outflows, and masers are added. The multiwavelength data setsare used to characterize each observed region. The massive cloud cores(M>100 Msolar) are placed in a tentative evolutionarysequence depending on their emission at the investigated wavelengths.Candidates for the youngest stages of massive star formation areidentified by the lack of detections in the above-mentionednear-infrared, mid-infrared, and radio surveys. Twelve massive coresprominent in the millimeter continuum fulfill this requirement. Sinceneither FIR nor radio emission have been detected from these cloudcores, massive protostars must be very deeply embedded in these cores.Some of these objects may actually be pre-protocluster cores: an up tonow rare object class, where the initial conditions of massive starformation can be studied.
| Near-Infrared Imaging of the Star Formation Region AFGL 5142
Near-infrared JHK' and H2 v=1-0 S(1) imaging observations ofthe star-forming region AFGL 5142 are presented. A cluster of youngstars is confirmed to be embedded in the dense molecular cloud core.Many point sources are newly detected. The cluster's K'-magnitudedistribution and [H-K'] color peak have brighter and reddermagnitudes than those outside the cluster. Many of the cluster sourcesexhibit infrared excesses typical of T Tauri stars, Herbig Ae/Be stars,and protostars. The observations also reveal new H2 lineemission around the cluster, exhibiting several jets, filaments, and afaint elliptical structure. The jets are associated with the CO outflowsin the region. The powering sources of the jets, the CO outflows, andthe small faint elliptical structure are identified and analyzedindividually. The strong H2 jets and faint diffuse emissionfurther confirm that the cluster is in an early stage of evolution.
| Search for CO Outflows toward a Sample of 69 High-Mass Protostellar Candidates. II. Outflow Properties
We present a study of molecular outflows toward a sample of 69 luminousIRAS point sources. The sample is associated with dense molecular gasand has far-infrared luminosities ranging from 102 to105 Lsolar, indicating these objects as regionslikely forming high-mass stars. Mapping in the CO J=2-1 line shows thatmolecular outflows are ubiquitous in these regions. Most of the outflowshave masses of tens of Msolar. The typical dynamicaltimescale of the flow, without correcting for inclination of the flowaxis, is a few times 104 yr. The typical energy in theoutflows is 1046 ergs, comparable to the turbulent energy inthe core. Nearly half of the outflows show spatially resolved bipolarlobes. This indicates that low-mass young stars that coexist in theregion are not responsible for the bipolar outflows observed. It is themore massive stars that drive the outflow. The large detection rate ofoutflows in the region favors an accretion process in the formation ofmassive stars. The maximum mass-loss rate in the wind is about10-4 Msolar yr-1. If these outflows aredriven via accretion, the accretion rate should be as high as a fewtimes 10-4 Msolar yr-1. We compare COoutflows with images at near-infrared wavelengths from the Two MicronAll Sky Survey (2MASS) archive and find that some outflows areassociated with extended emission in the K band, which may be partly dueto vibrationally excited H2 emission at 2.12 μm.
| A study of high velocity molecular outflows with an up-to-date sample
A statistical study of the properties of molecular outflows is performedbased on an up-to-date sample. 391 outflows were identified in publishedarticles or preprints before February 28, 2003. The parameters ofposition, morphology, mass, energy, outflow dynamics and central sourceluminosity are presented for each outflow source. Outflow lobe polarityis known for all the sources, and 84% are found to be bipolar. Thesources are divided into low mass and high mass groups according toeither the available bolometric luminosity of the central source or theoutflow mass. The pace of discovery of outflows over the past sevenyears has increased much more rapidly than in previous periods. Surveysfor outflows are still continuing. The number of high-mass outflowsdetected (139) has considerably increased, showing that they arecommonly associated with massive as well as low mass stars. Energeticmass ejection may be a common aspect of the formation of high mass aswell as low mass stars. Outflow masses are correlated strongly withbolometric luminosity of the center sources, which was obtained for thefirst time. There are also correlations between the central sourceluminosity and the parameters of mechanical luminosity and the thrust orforce necessary to drive the outflow. The results show that flow mass,momentum and energy depend on the nature of the central source. Despitetheir similarity, there are differences between the high mass and lowmass outflows. Low mass outflows are more collimated than high massoutflows. On average, the mass of high mass sources can be more than twoorders of magnitude larger than those of low mass outflows. The relationbetween flow mass and dynamical time appears to differ for the two typesof outflows. Low mass sources make up 90% of outflows associated with HHobjects while high mass outflows make up 61% of the sources associatedwith H_2O masers. Sources with characteristics of collapse or infallcomprise 12% of the entire outflow sample. The spatial distribution ofthe outflow sources in the Galaxy is presented and the local occurrencerate is compared with the stellar birth rate.Tables 1a and 1b are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/426/503
| Upgrade Procedure for the Delingha 13.7-m Telescope
The 13.7-m millimeter-wave radio telescope of Purple MountainObservatory operates at 3200-m above the sea level near Delingha,Qinghai Province, China. Equipped with a superconducting SIS receiver,the telescope is used in the millimeter-wave band ranging from 85 to 115GHz. An upgrade procedure is reported here which includes asuperconducting SIS receiver, a new phase-locked local oscillator, adedicated multi-line backend system, and a new control system based onindustrial computer with PCI bus. With the dedicated multi-line backendsystem, the CO and isotopic lines around 110 GHz are obtainedsimultaneously. In recent years, scientific activities with thistelescope have been focused on studies of Galactic molecular clouds andstar formation regions, including surveys of molecular lines from IRASsources and large-scale map of molecular clouds. Other programs includestudies of the circumstellar envelope of late-type stars and interactionof Galactic supernova remnants with dense molecular gas.
| 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.
| Automated Classification of 2000 Bright IRAS Sources
An artificial neural network (ANN) scheme has been employed that uses asupervised back-propagation algorithm to classify 2000 bright sourcesfrom the Calgary database of Infrared Astronomical Satellite (IRAS)spectra in the region 8-23 μm. The database has been classified into17 predefined classes based on the spectral morphology. We have beenable to classify over 80% of the sources correctly in the firstinstance. The speed and robustness of the scheme will allow us toclassify the whole of the Low Resolution Spectrometer database,containing more than 50,000 sources, in the near future.
| Dense Cloud Cores in Massive Star-forming Regions
Not Available
| New detection of near-infrared H2 line emission in AFGL 5157
Narrow-band H2 v=1-0 S (1) and broad-band Karcmin imagingobservations towards AFGL 5157 revealed a number of new H2knots associated with the NH3 core in the region, implyingthat the multiple H2 outflows are driven by a cluster ofembedded point sources in the core. The shell around the infraredcluster is more clearly seen than before, showing a few of newlydetected ridge-like features. A string of H2 knots newlyrevealed along one lobe of the known CO outflow suggests a parsec-scaledjet emanating from a young source in the core. These H2outflows and jet, together with the known compact H II region, radiocontinuum source, and H2O masers further imply an extremelyembedded cluster containing massive stars within it. The evidence showsthat the massive young stars within the cluster are in a strong outflowstage of evolution, and together with the nearby infrared cluster in NGC1985, further confirms that star forming process propagates from thewest to the east in AFGL 5157. The two types of H2 lineemission, knots and shell, may be indicators of two different stages ofstar formation in clusters: the shocked H2 knots are drivenby the young stellar sources embraced in the younger cluster, while thediffuse shell-like structure comes from UV photons of relatively evolvedcluster. The near-infrared nebula in AFGL 5157 is found to be onlyassociated with the evolved cluster but not observed in the embeddedcluster.
| 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 (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/141
| Embedded Clusters in Molecular Clouds
Stellar clusters are born embedded within giant molecular clouds (GMCs)and during their formation and early evolution are often only visible atinfrared wavelengths, being heavily obscured by dust. Over the past 15years advances in infrared detection capabilities have enabled the firstsystematic studies of embedded clusters in galactic molecular clouds. Inthis article we review the current state of empirical knowledgeconcerning these extremely young protocluster systems. From a survey ofthe literature we compile the first extensive catalog of galacticembedded clusters. We use the catalog to construct the mass function andestimate the birthrate for embedded clusters within 2 kpc of the sun. Wefind that the embedded cluster birthrate exceeds that of visible openclusters by an order of magnitude or more indicating a high infantmortality rate for protocluster systems. Less than 4-7% of embeddedclusters survive emergence from molecular clouds to become boundclusters of Pleiades age. The vast majority (90%) of stars that form inembedded clusters form in rich clusters of 100 or more members withmasses in excess of 50 Mȯ. Moreover, observations ofnearby cloud complexes indicate that embedded clusters account for asignificant (70-90%) fraction of all stars formed in GMCs. We review therole of embedded clusters in investigating the nature of the initialmass function (IMF) that, in one nearby example, has been measured overthe entire range of stellar and substellar mass, from OB stars tosubstellar objects near the deuterium burning limit. We also review therole embedded clusters play in the investigation of circumstellar diskevolution and the important constraints they provide for understandingthe origin of planetary systems. Finally, we discuss current ideasconcerning the origin and dynamical evolution of embedded clusters andthe implications for the formation of bound open clusters.
| The Formation of Massive Stars. I. High-Resolution Millimeter and Radio Studies of High-Mass Protostellar Candidates
We used the Owens Valley Millimeter Array and the Very Large Array toobtain interferometric maps at millimeter and centimeter wavelengths inboth the continuum and various lines [HCO+ (1-0),H13CO+ (1-0), SiO (v=0, J=2-1) andH13CN (1-0)] toward a sample of 11 high-mass protostellarcandidates. These sources are known from a previous study to beassociated with dense gas and dust and to not be associated with H IIregions. All 11 sources were detected in HCO+ (1-0), nine inthe millimeter continuum and five (of eight observed) in the centimetercontinuum. The derived physical parameters confirm the high-mass natureof these molecular clumps and suggest they are gravitationally bound.Molecular outflows were detected toward six sources, with flow massesand momenta much higher than in low-mass young stellar objects. In manyof the sources the molecular emission is organized in substructures,resolved both spatially and in velocity. We find that the sources may becharacterized by their degree of fragmentation, turbulence, and outflowactivity, with the sample dividing into two groups: group 1 cores havemultiple peaks but with a clearly dominant component and larger linewidths and are systematically associated with outflows, while group 2cores have several comparable subentities, smaller line widths, and noassociation with outflows. We speculate that more massive cores may formfrom smaller cores via coalescence or competitive accretion. Evenconservative estimates of outflow mass-loss rates, however, indicatethat accretion is the dominant process in the later formation of massiveprotostars from such cores. We find a flattening of the outflow massspectra with increasing flow velocities, at variance with previousstudies that suggest a steepening with increasing flow velocities. Inthe light of this result we suggest a reevaluation of the wide-anglewind momentum-driven flow models to describe the acceleration ofoutflows in the earliest stages of massive star formation.
| New C18O (1-0) emission of massive young stellar objects
Observations of C18O (1-0) line emission toward 74 massive young stellarobjects (YSOs) or its candidates were carried out by using the 13.7mmillimeter wave telescope of Purple Mountain Observatory at the QinghaiStation. 63 observed objects show the C18O (1-0) emission. 57 of themwere first detected in C18O (1-0) lines. According to the line radiationtemperature (TR*) and the line half width (? V), the C18O(1-0) opticaldepths and C18O column densities were derived for all with LTE method.Besides, we discussed the line intensity ratio and integral intensityratio for 13CO(1-0) and C18O (1-0).
| A single distance sample of molecular outflows from high-mass young stellar objects
We have made 12CO 2-1 and 1-0 maps of eleven molecularoutflows associated with intermediate to high-mass young stellar objects(YSOs) in order to establish whether the correlations between outflowparameters and source bolometric luminosity hold in the high-massregime. It is important to consider the effects of Malmquist-type biaseswhen looking at high-mass YSOs, as they are generally much more distantthan their low mass counterparts. We therefore chose only objectslocated at ~2 kpc. We find that the relations show much more scatterthan is seen in similar studies of low-mass YSOs. We also find that themass-spectrum is significantly steeper in high-mass outflows, indicatinga larger mass-fraction at lower velocities, a low collimation factor(~1-2) and no Hubble-like relationship.
| 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 (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Dense Cores Mapped in Ammonia: A Database
We present a database of 264 cores mapped in the (J,K)=(1,1) and (2,2)lines of NH3. We list the core gas properties-peak positions,total ammonia column densities, intrinsic line widths, kinetictemperatures, volume densities, core sizes, aspect ratios, and velocitygradients, as well as the properties of associated young stellar objects(YSOs)-associated IRAS sources along with their luminosities andcore-YSO distances, outflow velocities, and SIMBAD and clusterassociations. We also present the results of our statistical analysisand enumerate important pairwise correlations among the various gas andYSO properties. The results indicate that the association of stellarclusters with star-forming cores has a greater impact on theirproperties than does the presence of associated YSOs within these cores,although the latter influence is also statistically significant. Inother words, the difference in core properties (nonthermal line widths,kinetic temperatures, and core sizes) between cores with and withoutassociated YSOs is less significant when compared with the difference inthese properties between cores with and without cluster associations.Furthermore, core gas and YSO properties show a significant dependenceon the star-forming region in which the core is located. For instance,cores in Orion have larger line widths, higher kinetic temperatures, andlarger sizes compared with cores in Taurus. Similarly, YSOs in Orion aremore luminous than those in Taurus. These cluster and regionaldependencies seem important enough that they ought to be accounted forin any self-consistent theory of star formation. Finally, the ratio ofstarless to stellar cores is too small (8:12 in Taurus, 2:41 in Orion A)to be consistent with ambipolar diffusion timescales that predict ratiosas high as 3-30. This result is true even for regions that are known tobe well surveyed and not to suffer from significant sample biases.
| 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
| J=1-0 HCN toward bright far-infrared sources in the outer Galaxy
Results of the J=1-0 HCN observations toward 34 bright far-infraredsources selected from the IRAS Point Source Catalog are reported.Together with 17 sources observed in this line earlier (Pirogov et al.,1996) they form a complete sample of the sources with flux densitiesS(100 mu m)>500 Jy and delta > 0degr in the outer Galaxy. The HCNdata are compared with the HCO(+) , NH_3, CS and CO data taken fromliterature. Prominent correlations with nearly similar slopes of ~ 1 arerevealed between line integrated intensities of the molecules known tobe high density tracers (HCN, HCO(+) , NH_3 and CS). The correlationsbecome higher after adding the data for dark clouds, small globules andcirrus cores implying similar excitation and formation mechanisms of theconsidered molecules. Collisional excitation in regions with differentdensities as well as different molecular abundances and velocitydispersions in different types of cores seem to be important inproducing these correlations. The following relations hold on theaverage over ~ 3 orders of magnitude of integrated intensities:I(HCN)>~ I(HCO(+) ~ ) I(CS) > I(NH_3) where ammonia integratedintensities are several times lower than HCN ones. Correlations are alsofound between HCN and CO integrated intensities for the sample sourcesas well as between HCN line widths and those of other species. The HCNlines have the same widths as the HCO(+) ones and are larger than CS andespecially NH_3 line widths. Weak correlations are found between HCNline widths and luminosities of IRAS sources as well as between HCNintegrated intensities, IRAS flux densities at 100mu m and luminositiesof IRAS sources divided by distance squared. The sources with mostintense HCN lines have associated water masers and molecular outflowswhile the lack of associated maser and outflow implies weak or no HCNemission. In order to reproduce the anomalies of the J=1-0 HCN hyperfinestructure (R12 < 0.6) detected in several sources togetherwith suprathermal line widths the model calculations are performed. Twomodels have been considered: a microturbulent one with a smooth densitydistribution and a clumpy model. It is found that in the parameter spaceexplored a clumpy model is preferable in comparison with amicroturbulent one due to the absence of self-reversals on calculatedprofiles and the possibility of reproducing HCN profile anomalies andsuprathermal line widths. Detailed clumpy model simulations have beenperformed to fit the observed HCN and H(13) CN spectra toward S140 IRS1.The best fit model parameters are calculated, including radialdependencies of clump density and volume filling factor.
| Near-Infrared Imaging of the Star-forming Region AFGL 5157
We present near-infrared images of the star-forming region AFGL 5157 inthe JHK' broadband filters and H_2 v=1-0 S(1) narrowband filter. Theimages reveal a dense cluster of stars and infrared nebulositiesassociated with previously known infrared sources. Of 54 near-infraredsources detected in the nebula, NGC 1985, 12 exhibit infrared excessestypical of T Tauri stars, Herbig Ae/Be stars, and protostars. Themagnitude and color distribution of the cluster of stars in the nebulaare found to be different from those outside the nebular region. TheK'-magnitude distribution of the cluster is quite flat, while thenoncluster is peaked toward the low magnitude. The [H-K'] color of thecluster also displays 0.3 mag redder than that of the noncluster. Theinfrared nebula displays a bright nucleus with two spirals extended tothe north and south. In light of the color properties of the nebula, wepropose a shell model for the nebular structure that could be formed bystar-forming activity of the central cluster. Many sources with infraredexcesses are found to be embedded in the shell structure. Twelve shockedknots in H_2 emission are observed in the region. The nonaxisymmetricdistribution of the knots indicates the presence of multiple outflows inthe region. Although we failed to identify the powering sourcesresponsible for some of the HH-like objects, the relationship of the H_2emission with infrared sources shows that there must be severalspatially separated sources exciting the shocked H_2 emission as well asthe previously observed H_2O masers and molecular outflow. Diffuse H_2emission is also detected on the shell structure, which supports theshell model of the nebula. This diffuse emission could result fromfluorescence by relatively evolved stars in the cluster.
| The Medicina survey of methanol masers at 6.7 GHz
A survey of Class II methanol masers at 6.7 GHz was made in the Northernhemisphere with the 32-m Medicina radio telescope. 42 objects weredetected, 20 of them are new detections at 6.7 GHz. Our results showthat the detection rate of 6.7 GHz masers toward the inner part of theGalaxy is higher than in other directions. It is confirmed that most ofthe methanol masers are associated with faint compact HII regions. The6.7 GHz methanol masers show large velocity dispersion and largevelocity offset from the velocity of parent molecular clouds. Table 2 isonly available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| A survey of SiO emission towards interstellar masers. I. SiO line characteristics
We have conducted a survey of SiO emission towards galactic H_2O and OHmasers and ultracompact HII regions using the 15-m SEST and the 20-mOnsala telescope. With the SEST the transitions (v=0,J=2-1) and(v=0,J=3-2) of SiO at 3 and 2 mm were measured simultaneously. WithOnsala only the (v=0,J=2-1) line was accessible. Altogether 369 objectswere observed and SiO was detected towards 137 of them. The detectionrate is highest towards the most intense H_2O masers, which probablyrequire powerful shocks to be excited. The SiO detection rate correlatesalso with the integrated far-infrared flux density and the FIRluminosity of the associated IRAS point source, indicating that theoccurrence of shocks is related to the amount of radiation from thecentral stellar source(s). For flux and luminosity limited samples theSiO detection rate is higher in the inner 7 kpc from the galactic centrethan elsewhere. This suggests that dense cores belonging to the socalled ``molecular ring" provide particularly favourable conditions forthe production of gaseous SiO. The full widths above 2sigma of theSiO(J=2-1) lines, which are likely to be related to the associated shockvelocities, range from 2 to 60 km s(-1) except for the line in Ori KLwhich has a full width of about 100 km s(-1) . The median of our sampleis 19 km s(-1) . The SiO lines are single-peaked and the peak velocitiesare always close to the ambient cloud velocity as determined frompublished CS observations. These line characteristics are compared withthe predictions of kinematical bow-shock models. The SiO line shapescorrespond with the model of \cite[Raga & Cabrit (1993)]{R93} wherethe emission arises from turbulent wakes behind bow-shocks. However, thenumber of symmetric, relatively narrow profiles indicates that at leastin some of the observed sources SiO emission arises also from thequiescent gas component. We suggest that this is due to evaporation ofsilicon compounds from grain mantles and their reprocessing to SiO indense quiescent gas according to the model of \cite[McKay (1995,1996).]{M95} These reactions may be initiated and sustained by ionizingradiation from shocks, in the same way as the enhancement of HCO(+) nearHerbig-Haro objects has been explained in the model of \cite[Wolfire& Koenigl (1993).]{W93} The excitation temperatures of SiO(J=2-1)and (J=3-2) transitions were determined towards three strong sourcesusing measurements in isotopically substituted SiO. In all three sourcesthe transitions are clearly subthermally excited, implying moderatedensities (< 3 10(6) cm(-3) ) in the SiO emission regions. The entireTables~B.1 and B.2 and the spectra of all the detected SiO sources areonly available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| A search for precursors of ultracompact H II regions in a sample of luminous IRAS sources. II. VLA observations
We have used the Very Large Array (VLA) to search for radio continuumemission towards a sample of 67 IRAS sources selected from a previousstudy. All observed sources are associated with high density moleculargas, exhibit an infrared spectral energy distribution characteristic ofvery cold young stellar objects and many of them are associated withH_2O masers. The observed sample is divided into two groups of sources:High, with IRAS spectral energy distributions resembling those ofultracompact Hii regions, and Low, for which previously collectedevidence suggests that they may contain a higher fraction ofprotostellar objects than the High group; such objects might not havestarted hydrogen burning yet. Radio continuum emission was detectedtowards 37 sources (55%), although only in 22 cases an association withthe IRAS source is established. Of the latter, 9 (24%) objects belong tothe Low type and 13 (43%) to the High type. Thus, we find that 76% ofLow and 57% of High sources are not associated with a radio counterpart.Because the majority of the sources have luminosities above ~ 10(4)L_sun, corresponding to central stars of spectral type between B2 andO7, the lack of radio emission is interpreted as being due to the actionof accreting matter that chokes off the expansion of the ionised gas. Weshow that this requires only moderate mass accretion rates, below ~10(-4) M_sun yr(-1) . Alternatively, dust absorption can alsoeffectively absorb UV photons and the gas column density implied by ourobservations indicates values in excess of 10(22) cm(-2) . The physicalproperties of IRAS sources with associated radio counterpart derivedfrom the present observations do not distinguish between High and Lowsources. These sources are likely to be ZAMS stars with variable amountsof dust within the ionised region which acts as UV field absorber. Thelarge majority of detected sources (75%) have spherical or unresolvedmorphology, while 15% are irregular or multiply peaked and only 10% havea core-halo structure. These results agree with the known properties ofultracompact Hii regions, even though the average luminosity of thepresent sample is an order of magnitude lower than that in previousstudies. The tables, and the complete set of radio maps in the Appendixis available in electronic form at CDS via ftp 130.79.128.5
| A 13CO Survey of Molecular Clouds in Gemini and Auriga
A large-scale survey for molecular clouds in 13CO(J = 1-0) was performedtoward the Gemini and Auriga regions (170 deg < l <= 196 deg and-10 deg <= b < 10 deg) with velocity coverages of -30 < VLSR< +30 km s-1 and -20 < VLSR < +40 km s-1 for 170 deg < l<= 188 deg and 188 deg < l <= 196 deg, respectively, by usingthe two 4 m millimeter-wave telescopes at Nagoya University. An area of520 deg2 was covered at an 8' grid spacing with a 2.'7 beam, and 29,250positions were observed. Significant 13CO emission (>=1.2 K km s-1 =3 sigma ) is detected at 1032 positions, and 139 distinct 13CO cloudsare identified. Physical properties such as molecular column density,size, and mass are derived for each cloud. Comparison with known H IIregions and other associated visible objects indicates that 98 of the139 clouds are located at ~2 kpc, while the rest lie at <~ 1 kpc. Thecandidates for protostars are selected from IRAS point sources and 141sources are identified as protostellar candidates associated with the13CO clouds. A statistical study of the 13CO clouds and of thecandidates for protostars shows the following results: 1. The massspectra of the 13CO clouds are well represented by a power law,Ncloud(>=Mcloud) = 1.4 x 104(Mcloud/Mȯ)-0.83 - 1.4 for theclouds at ~2 kpc and Ncloud(>=Mcloud) = 3.5 x102(Mcloud/Mȯ)-0.64 - 2.4 for the rest. 2. The line width, DeltaVcomp, and the size, R, of the 13CO clouds show a power-law relationwith an index 0.24 +/- 0.06, where the dispersion in the fitting isfairly large. 3. A virial analysis made for the 13CO clouds indicatesthat the relation between the virial mass, Mvir, and the mass measuredin 13CO, Mcloud, is aproximated well by (Mvir/Mȯ) = 2.0 x101(Mcloud/Mȯ)0.72, which suggests that smaller clouds tend to bemore weakly bound gravitationally than larger clouds or are dispersingif the external pressure is negligible. This is probably the cuase of asmall index value of the line width-size relation. 4. The luminosityfunction of the IRAS point sources associated with 13CO clouds are wellrepresented by a power law withN_⋆(>=L_⋆)=4.0x102L^{-0.29}_⋆-1.9x10 forthose at ~2 kpc and N_⋆(>=L_⋆)=2.3x10L^{-0.27}_⋆-3.2for the rest. 5. The luminosity of the most luminous IRAS point sourcein a given molecular cloud increases systematically with the mass of theassociated cloud. 6. The 13CO clouds associated with IRAS point sources,which are regarded as ongoing star-forming clouds, tend to be moremassive and larger in size and to have higher column densities thanthose without any sign of star formation. These relations are found tobe consistent with those derived in Cygnus by Dobashi, Bernard, &Fukui and in Cepheus-Cassiopeia by Yonekura et al. In order to study thestar formation activities, we studied the ratio of virial mass and themass measured in 13CO, Mvir/Mcloud, and its relation with starformation. It is clearly seen that the star-forming 13CO clouds have lowMvir/Mcloud, and all the clouds with high Mvir/Mcloud exhibit no sign ofstar formation. This suggests that star formation rarely occurs inclouds with overall kinetic energy higher than the gravitational energy.
| CS observations of star-forming regions
We have mapped the CS (J = 1-0) emission in five star-forming regions,HH 43, AFGL 6366S, L673, and L1251 using the 14-m Yebes telescope, andW75S using the 37-m Haystack telescope. Additionally, we have observedC-34S (J = 1-0) emission at selected positions in these regions. Thesesources have all been previously mapped with similar angular resolutionin the (J,K) = ( 1,1) inversion transition of NH3. The comparison of theemissions shows a separation between the CS and NH3 peaks of about 0.2pc. The regions traced by the CS molecule are, in general, larger thanthose traced by the NH3. However, our data suggest that the size ratiodepends on the size of the source. The line width of the CS lines is, onaverage, about 0.5 km/s larger than that of the NH3 lines. Thisdifference in line width can be attributed to the different sizes of theregions traced by each molecule. The difference in the spatialdistribution of the CS and NH3 emission is interpreted in terms of thedifferent evolutionary stages of the emitting clumps.
| Infrared emission and H-R diagram of bright far-infrared sources during star formation
Young stellar objects (YSOs) were studied using a set of bright FIRsources selected from IRAS-PSC and associated with high-mass YSOs. Amodified blackbody photospheric model, double-shell dust emission model,and a detailed radiation transfer model were used to fit the observedinfrared fluxes, and some physical parameters, such as the photospherictemperature of the central star, the circumstellar extinction, andthermal structure of dust shell were derived. Using the derivedphotospheric temperatures and corresponding stellar luminosities, wehave determined the positions on an extended H-R diagram and theevolution properties of the investigated bright FIR sources. We havefound that there is a trend of continuous increase of the infraredspectral slope S (12-60) from 12 to 60 microns along the direction tomain sequence stars at the same mass line of H-R diagram. For the YSOs,which are evolving at the same time, there is also a trend of increaseof S (12-60) with mass. We analyzed the nature of the bright FIRsources. It seems that the extended H-R diagram presented here is a goodmeans to study this kind of objects, which are deeply embedded inmolecular clouds and dust envelopes.
| 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.
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