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A Search for Formaldehyde 6 cm Emission toward Young Stellar Objects. II. H2CO and H110α Observations
We report the results of our second survey for Galactic H2COmaser emission toward young stellar objects. Using the GBT and the VLAin the A configuration we observed 58 star-forming regions anddiscovered the fifth H2CO 6 cm maser region in the Galaxy(G23.71-0.20). We have discussed the detection toward G23.71-0.20 in aprevious paper. Here we present all the other results from our survey,including detection of H2CO absorption features toward 48sources, detection of the H110α recombination line toward 29sources, detection (including tentative detections) of the carbonrecombination line C110α toward 14 sources, subarcsecond angularresolution images of 6 cm continuum emission toward five sources, andobservations of the H2CO masers in IRAS 18566+0408 and NGC7538. In the case of NGC 7538, we detected the two main H2COmaser components, and our observations confirm variability of theblueshifted component recently reported by Hoffman et al. Thevariability of both maser components in NGC 7538 could be caused by ashock wave that reached the redshifted component approximately 14 yrbefore reaching the blueshifted component. If that were the case, wewould expect to detect an increase in the flux density rate of change ofthe blueshifted component sometime after the year 2009. Our data alsosupport the use of H2CO/H110α observations as a tool toresolve the kinematic distance ambiguity of massive star-forming regionsin the Galaxy.
| A Multiwavelength Optical Emission Line Survey of Warm Ionized Gas in the Galaxy
We report on observations of several optical emission lines toward avariety of newly revealed faint, large-scale Hα-emitting regionsin the Galaxy. The lines include [N II] λ6583, [N II]λ5755, [S II] λ6716, [O III] λ5007, and He Iλ5876 obtained with the Wisconsin Hα Mapper (WHAM) towardsight lines that probe superbubbles, high-latitude filamentary features,and the more diffuse warm ionized medium (WIM). Our observations includemaps covering thousands of square degrees toward the well-knownOrion-Eridanus bubble, a recently discovered60deg×20deg bipolar superbubble centered inPerseus, plus several classical H II regions surrounding OB stars andhot evolved stellar cores. We use the emission-line data to explore thetemperature and ionization conditions within the emitting gas and theirvariations between the different emission regions. We find that in thediffuse WIM and in the faint high-latitude filamentary structures theline ratios of [N II]/Hα and [S II]/Hα are generally high,while [O III]/Hα and He I/Hα are generally low compared tothe bright classical H II regions. This suggests that the gas producingthis faint widespread emission is warmer, in a lower ionization state,and ionized by a softer spectrum than gas in classical H II regionssurrounding O stars, the presumed ionization source for the WIM. Inaddition, we find differences in physical conditions between the largebubble structures and the more diffuse WIM, suggesting that theionization of superbubble walls by radiation from interior Oassociations does not account entirely for the range of conditions foundwithin the WIM, particularly the highest values of [N II]/Hα and[S II]/Hα.
| Star formation associated with H II regions
Star formation associated with H II regions is briefly reviewed. Specialemphasis is laid on our series of observational studies on bright-rimmedclouds (BRCs), in which we found a phenomenon called "small-scalesequential star formation." In addition a new hypothesis is advocated onthe two modes of star formation associated with H II regions, i.e., thecluster and dispersed modes. The former gives birth to a rich clusterand in the associated H II region BRCs are formed only at a later stageof its evolution in the peripheries. In the latter mode no clusters oronly loose ones are formed, but BRCs can appear at earlier stages ininner part of the H II region. Presumably these modes depend on theinitial density distribution of the natal molecular cloud.
| Abundance Gradients in the Galaxy
Six H II regions at galactocentric distances of R=10-15 kpc have beenobserved in the far-IR emission lines of [O III] (52 μm, 88 μm),[N III] (57 μm), and [S III] (19 μm) using the Kuiper AirborneObservatory. These observations have been combined with Very Large Arrayradio continuum observations of these sources to determine theabundances of O++, N++, and S++relative to hydrogen. In addition, eight of the most recent sets ofmeasurements of ionic line strengths in H II regions have beenreanalyzed in order to attempt to reconcile differences in opticalversus far-IR abundance determinations. We have in total 168 sets ofobservations of 117 H II regions in our analysis. The new analysisincluded updating the atomic constants (transition probabilities andcollision cross sections), recalculation of some of the physicalconditions in the H II regions (ne and Te), andthe use of new photoionization models to determine stellar effectivetemperatures of the exciting stars. We also use the most recent dataavailable for the distances for these objects, although for most westill rely on kinematic distance determinations. Our analysis findslittle indication of differences between optical and infraredobservations of the nitrogen abundances, but some differences are seenin the oxygen and sulfur abundances. A very significant offset continuesto be seen between optical and infrared measurements of the N/Oabundance ratio.
| Astrophysical parameters of Galactic open clusters
We present a catalogue of astrophysical data for 520 Galactic openclusters. These are the clusters for which at least three most probablemembers (18 on average) could be identified in the ASCC-2.5, a catalogueof stars based on the Tycho-2 observations from the Hipparcos mission.We applied homogeneous methods and algorithms to determine angular sizesof cluster cores and coronae, heliocentric distances, mean propermotions, mean radial velocities, and ages. For the first time we derivedistances for 200 clusters, radial velocities for 94 clusters, and agesof 196 clusters. This homogeneous new parameter set is compared withearlier determinations, where we find, in particular, that the angularsizes were systematically underestimated in the literature.
| A general catalogue of 6.7-GHz methanol masers. I. Data.
Methanol masers are often detected in regions of intense star formation.Several studies in the last decade indicate that they may even be theearliest signpost of a high-mass star-forming region. Their powerfulemission make them very good candidates for observations using bothsingle-dish telescopes and interferometers, the latter allows detailedstructural and dynamical studies of these objects. We have prepared acatalogue of all known 6.7-GHz methanol masers, discovered both bysurveys that targeted possible associated objects and unbiased surveyscovering a large fraction of galactic longitudes across the Galacticplane (-0.5 ° ≤ b ≤ 0.5 ° for most of the regions). Thecatalogue contains 519 sources which are listed with their kinematic(galactocentric and heliocentric) distances as well as possiblyassociated IR objects. We find that 6.7-GHz methanol masers clearlytrace the molecular ring of our Galaxy, where most of the OBassociations are located. The present list of masers also reportsdetections of other masing transitions of methanol as furtherinformation for the study of the maser phenomenon. In a furtherpublication we will address some statistical considerations.The complete catalogue (Table [see full text]) is available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/432/737 Figures 1 and 2are only available in electronic form at http://www.edpsciences.org
| Long-term monitoring of 6.7-GHz methanol masers
A sample of 54 6.7-GHz methanol masers was monitored using theHartebeesthoek 26-m telescope during the period 1999 January - 2003March. The observations were taken at 1-2 week intervals, with dailyobservations when possible if a source was seen to be varying rapidly.It was found that the majority of the sources display a significantlevel of variability. The time-range of variations range from a few daysup to several years. The types of behaviour observed includednon-varying, monotonic increases or decreases, as well as aperiodic,quasi-periodic and periodic variations. Seven sources show clearevidence of periodicity, with periods ranging from 132 d up to 520 d.
| 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
| Far-infrared loops in the 2nd Galactic Quadrant
We present the results of an investigation of the large-scale structureof the diffuse interstellar medium in the 2nd Galactic Quadrant(90°≤l≤180°). 145 loops were identified on IRAS-basedfar-infrared maps. Our catalogue lists their basic physical properties.The distribution clearly suggests that there is an efficient processthat can generate loop-like features at high Galactic latitudes.Distances are provided for 30 loops. We also give an observationalestimate of the volume filling factor of the hot gas in the Local Arm,4.6%≤f2nd<6.4%.Appendices A-C are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/131
| Physical Conditions of High-Luminosity Star-Forming Regions Observed by ISO-LWS
We present here the results obtained with the Infrared SpaceObservatory's Long Wavelength Spectrometer on a sample ofhigh-luminosity star forming regions, characterized by the presence ofstrong far ultraviolet stellar fields causing atomic ionization. We showhow photoionization models can explain the observed fine structure lineluminosities and where other components, such as photodissociationemission and shock excitation by outflows, have to be included toaccount for the observed spectra.Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially, the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.
| Distances to Cepheid open clusters via optical and K-band imaging
We investigate the reddening and main-sequence-fitted distances to 11young, Galactic open clusters that contain Cepheids. Each clustercontains or is associated with at least one Cepheid variable star.Reddening to the clusters is estimated using the U-B:B-V colours of theOB stars and the distance modulus to the cluster is estimated via B-V:Vand V-K:V colour-magnitude diagrams. Our main-sequence fitting assumesthat the solar-metallicity zero-age main sequence of Allen appliesuniversally to all the open clusters, although this point iscontroversial at present. In this way we proceed to calibrate theCepheid period-luminosity (PL) relation and find MV=-2.87× logP- 1.243 +/- 0.09, MK=-3.44 × logP- 2.21 +/-0.10 and absolute distance moduli to the Large Magellanic Cloud (LMC) of18.54 +/- 0.10 from the V-band and 18.48 +/- 0.10 from the K-band givingan overall distance modulus to the LMC of μ0= 18.51 +/-0.10. This is in good agreement with the previous Cepheid PL-K result ofLaney & Stobie at μ0= 18.51 +/- 0.09 and with theHipparcos parallax-calibrated Cepheid PL-K estimate of Feast &Catchpole at μ0= 18.66 +/- 0.10 when no account is takenof the LMC metallicity.We also find that the two-colour U-B:B-V diagrams of two importantclusters are not well fitted by the standard main-sequence line. In onecase, NGC 7790, we find that the F stars show a UV excess and in thesecond case, NGC 6664, they are too red in U-B. Previous spectroscopicestimates of the metallicity of the Cepheids in these clusters appear tosuggest that the effects are not due to metallicity variations. Otherpossible explanations for these anomalies are positional variations inthe dust reddening law and contamination by foreground or backgroundstars.
| Long-term study of water masers associated with Young Stellar Objects. II. Analysis
We present the analysis of the properties of water maser emission in 14star forming regions (SFRs), which have been monitored for up to 13years with a sampling rate of about once every 2-3 months. The 14regions were chosen to span a range in luminosity Lfir of theassociated Young Stellar Object (YSO) between 20 Lsun and 1.8x 106 Lsun. The general scope of the analysis isto investigate the dependence of the overall spectral morphology of themaser emission and its variability on the luminosity of the YSO. We findthat higher-luminosity sources tend to be associated with stronger andmore stable masers. Higher-luminosity YSOs can excite more emissioncomponents over a larger range in velocity, yet the emission thatdominates the spectra is at a velocity very near that of the molecularcloud in which the objects are embedded. For Lfir >~ 3 x104 Lsun the maser emission becomes increasinglystructured and more extended in velocity with increasingLfir. Below this limit the maser emission shows the samevariety of morphologies, but without a clear dependence onLfir and with a smaller velocity extent. Also, for sourceswith Lfir above this limit, the water maser is always presentabove the 5sigma -level; below it, the typical 5sigma detection rate is75-80%. Although the present sample contains few objects with low YSOluminosity, we can conclude that there must be a lower limit toLfir (<~ 430 Lsun), below which the associatedmaser is below the detection level most of the time. These results canbe understood in terms of scaled versions of similar SFRs with differentYSO luminosities, each with many potential sites of maser amplification,which can be excited provided there is sufficient energy to pump them,i.e. the basic pumping process is identical regardless of the YSOluminosity. In SFRs with lower input energies, the conditions of maseramplification are much closer to the threshold conditions, andconsequently more unstable.We find indications that the properties of the maser emission may bedetermined also by the geometry of the SFR, specifically by the beamingand collimation properties of the outflow driven by the YSO.For individual emission components the presence of velocity gradientsseems to be quite common; we find both acceleration and deceleration,with values between 0.02 and 1.8 km s-1yr.From the 14 ``bursts'' that we looked at in some detail we derivedurations of between 60 and 900 days and flux density increases ofbetween 40% and >~ 1840% (with an absolute maximum of ~ 820 Jy over63 days). The ranges found in burst- intensity and -duration are biasedby our minimum sampling interval, while the lifetime of the burst isfurthermore affected by the fact that bursts of very long duration maynot be recognized as such.In addition to the flux density variations in individual emissioncomponents, the H2O maser output as a whole is found toexhibit a periodic long-term variation in several sources. This may be aconsequence of periodic variations in the wind/jets from the excitingYSO.
| N2H+(1-0) survey of massive molecular cloud cores
We present the results of N2H+(1-0) observationsof 35 dense molecular cloud cores from the northern and southernhemispheres where massive stars and star clusters are formed. Lineemission has been detected in 33 sources, for 28 sources detailed mapshave been obtained. Peak N2H+ column densities liein the range: 3.6x 1012-1.5x 1014 cm-2.Intensity ratios of (01-12) to (23-12) hyperfine components are slightlyhigher than the LTE value. The optical depth of (23-12) component towardpeak intensity positions of 10 sources is ~ 0.2-1. In many cases thecores have elongated or more complex structures with several emissionpeaks. In total, 47 clumps have been revealed in 26 sources. Their sizeslie in the range 0.3-2.1 pc, the range of virial masses is ~ 30-3000Msun. Mean N2H+ abundance for 36 clumpsis 5x 10-10. Integrated intensity maps with axial ratios<2 have been fitted with a power-law radial distribution ~r-p convolved with the telescope beam. Mean power-law indexfor 25 clumps is close to 1.3. For reduced maps where positions of lowintensity are rejected mean power-law index is close to unitycorresponding to the ~ r-2 density profile providedN2H+ excitation conditions do not vary insidethese regions. In those cases where we have relatively extensive andhigh quality maps, line widths of the cores either decrease or stayconstant with distance from the center, implying an enhanced dynamicalactivity in the center. There is a correlation between total velocitygradient direction and elongation angle of the cores. However, the ratioof rotational to gravitational energy is too low (4x10-4-7.1x 10-2) for rotation to play a significantrole in the dynamics of the cores. A correlation between mean linewidths and sizes of clumps has been found. A comparison with physicalparameters of low-mass cores is given.
| Optical Line Profile Variability of the B1+Neutron Star Binary System LS I +65 010 (2S 0114+650)
We present high-resolution spectroscopic observations of the massiveX-ray binary system LS I +65 010 (2S 0114+650) in the optical wavelengthregion. A correlation between equivalent width and radial velocity ofphotospheric absorption lines is found. The systemic velocity, inferredfrom the weaker lines, is V[helio] -31+/-5kms-1, which, if attributedsolely to the Galactic rotation curve, implies that LS I +65 010 lieswithin 3 kpc from the Sun. The ISM Na I D lines display 2 resolved highvelocity components at V[helio] -70, -48kms-1, possibly associated withgas surrounding the binary system, in addition to the -24 and -8kms-1ISM features due to the Orion and Perseus arm regions, respectively.Strong photospheric line profile variability is present on anight-to-night timescale, with He I 5875 Å displaying anadditional blue-shifted absorption in some of the spectra, similar towhat is observed in the optical counterpart of Vela X-1. A connectionbetween the extended blue wing and X-ray maximum is suggested. Shorttimescale variations in line profiles are detected on only two nights,but the evidence that these variations occur on the 2.78 hour X-rayflaring period is marginal.
| On the oxygen abundance in our Galaxy
The compilation of published spectra of Galactic H II regions withavailable diagnostic [OIII]lambda 4363 line information has been carriedout. Our list contains 71 individual measurements of 13 H II regions inthe range of galactocentric distances from 6.6 to 14.8 kpc. The oxygenabundances in all the H II regions were recomputed in the same way,using the classic T_e-method. The oxygen abundance at the solargalactocentric distance traced by those H II regions is in agreementwith the oxygen abundance in the interstellar medium in the solarvicinity derived with high precision from the interstellar absorptionlines towards stars. The derived radial oxygen abundance distributionwas compared with that for H II regions from the Shaver et al.(\cite{Shaver83}) sample which is the basis of many models for thechemical evolution of our Galaxy. It was found that the original Shaveret al.'s oxygen abundances are overestimated by 0.2-0.3 dex. Oxygenabundances in H II regions from the Shaver et al. sample have beenredetermined with the recently suggested P-method. The radialdistribution of oxygen abundances from the Shaver et al. sampleredetermined with the P-method is in agreement with our radialdistribution of (O/H)T_e abundances.
| A molecular cloud complex above the galactic plane.. I. Extended CO observations of the NGC 281 region
We have mapped the entire extent of the molecular cloud complexassociated with NGC281, both in 12CO (J=1-0) and 13CO (J=1-0),using the Taeduk Radio Astronomy Observatory (TRAO) 14 m telescope. Wealso observed a few selected fields with bright 13COemission, in CS (J=2-1) and HCO+ (J=1-0). This region isdistinguished by its vertical height of ~320 pc above the midplane ofthe Perseus spiral arm, at a distance of 2.9 kpc. The molecular gas inthe mapped region is found to extend far beyond the Hii region NGC 281 (Sh184). The southern boundary of the CO emission closely follows thesouthern boundary of the 100 μm dust emission. The molecular cloudcomplex consists of two main clouds bisected by the Hii region, as wellas several other clouds distributed over a range of 90 pc along thegalactic longitude direction. Except for one cloud atVLSR=-43.9 km s-1, all these clouds occupy asmall, contiguous velocity range centered at VLSR=-30 kms-1. Although these clouds are weakly connected, they clearlyshow distinct, knotted features along the galactic longitude direction.We assigned names to each cloud (A-H), derive their various physicalparameters, and discuss their physical properties. These clouds do notshow any specific, systematic behavior in their velocity field, exceptfor a small gradient along galactic longitude (for A-E). The brightest12CO and 13CO emissions are detected toward cloudA, which is located farthest from the two main clouds. We found starformation to be ongoing in clouds A and B, though these are not asactive as D and E. The associated newborn stars have FIR propertiessimilar to those of Herbig Ae/Be stars, and several outflows are alsoseen. We estimated the cloud masses in three different ways, and discussthe discrepancies between the resulting values. We estimate the totalmass of the mapped region to be ~3.7×104Msolar, using a conversion factor from CO luminosity to gasmass. About half of the clouds seem to be gravitationally bound.
| Observational Evidence for Triggered Star Formation on Scales of 1 to 300 pc
We present evidence for triggered star formation on two differentspatial scales in the NGC 281 molecular cloud complex. This remarkablecomplex is situated 300 pc above the Galactic plane, and appears to bepart of a 270 pc diameter ring of atomic and molecular clouds expandingat 22 km /s. We suggest that the ring has formed in a superbubbleblowout driven by OB stars in the plane of the Galaxy. Within the cloudcomplex, combined optical, NIR, mm and cm data detailing the interactionof a young O star with neighboring molecular cores, provide evidence oftriggered star formation inside the cloud complex on a few parsec scale.These data suggest that two modes of triggered star formation areoperating in the NGC 281 complex--the initial supernovae triggeredformation of the entire complex and, after the first generation of Ostars formed, the subsequent triggering of star formation byphotoevaporation-driven molecular core compression.
| Water Maser Survey toward Low-Mass Young Stellar Objects in the Northern Sky with the Nobeyama 45 Meter Telescope and the Very Large Array
We give a detailed presentation of all the data from a multiepoch 22 GHzH2O maser survey mainly toward low-mass young stellar objects(YSOs) using the Nobeyama 45 m telescope and the Very Large Array (VLA).Our major results are already published (our Paper I). The Nobeyamasurvey is the first complete H2O maser survey toward knownClass 0 sources in the northern sky (δ>-35deg) andis one of the most sensitive surveys ever performed. The survey wasconducted from 1996 May through 1999 March over 32 periods. A total of606 observations were carried out toward 173 YSOs, including 36 unknownluminosity sources, and toward 31 preprotostellar cores (PPSCs) in theOphiuchus star-forming region. We detected 149 spectra toward 39 YSOsand zero spectra toward the 31 PPSCs. Subsequent to the Nobeyama survey,we performed a follow-up interferometric survey with the VLA in order toassociate 19 maser sources detected by the 45 m telescope withindividual YSOs. In this paper we compile properties of 142 YSOstogether with their H2O maser activity. On the basis of thisdata set, we use properties of water maser emission as a probe of jetphenomena in low-mass stars.
| 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.
| ISO -LWS two-colour diagram of young stellar objects
We present a [60-100] versus [100-170]μm two-colour diagram for asample of 61 young stellar objects (YSOs) observed with the LongWavelength Spectrometer (LWS) on-board the Infrared Space Observatory(ISO ). The sample consists of 17 Class 0 sources, 15 Class I, nineBright Class I (L bol >104 Lsolar )and 20 Class II (14 Herbig Ae/Be stars and six T Tauri stars). We findthat each class occupies a well-defined region in our diagram withcolour temperatures increasing from Class 0 to Class II. Therefore the[60-100] versus [100-170] two-colour diagram is a powerful and simpletool to derive from future (e.g. with the Herschel Space Observatory )photometric surveys the evolutionary status of YSOs. The advantage overother tools already developed is that photometry at other wavelengths isnot required: three flux measurements are enough to derive theevolutionary status of a source. As an example we use the colours of theYSO IRAS 18148-0440 to classify it as Class I. The main limitation ofthis work is the low spatial resolution of the LWS which, for someobjects, causes a high uncertainty in the measured fluxes due tobackground emission or to source confusion inside the LWS beam.
| On the oxygen abundances in H II regions of the Galaxy
Oxygen abundances in H II regions of the Galaxy were derived with theclassical Te-method within the framework of the two-zone H IIregion model using published spectro-photometric data (69 spectra of 11H II regions in the range of galactocentric distances from 6.6 to 14.8kpc). The derived radial distribution of the oxygen abundance wascompared with that from Shaver et al. (1983), which is widely used inconstructing the model of the chemical evolution of the Galaxy. It wasfound that the oxygen abundances given by Shaver et al. areoverestimated by 0.2-0.3 dex.
| 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.
| Mapping of Bipolar Outflows and Methanol Masers in the CS(2-1) Line
Eighteen regions (bipolar outflows and methanol masers) are mapped inthe CS(2-1) line using the 20-m Onsala radio telescope. The coordinatesof the CS emission peaks are refined. The sizes and masses of denseregions are estimated for 13 maps. Measurement of the angular sizes ofregions of emission indicates that all the sources were resolved by theOnsala radio telescope. The lower limit for the linear dimensions of theCS condensations studied is 0.2 2.1 pc. The hydrogen densities andmasses of the CS condensations are estimated to be n(H2)=(0.313.1)×104 cm-3 and (M ≈ 7 2800M ȯ). Methanol masers areassociated with denser and more massive regions, whether or not themaser condensation is connected with a bipolar outflow.
| On the oxygen abundance determination in HII regions. High-metallicity regions
This is our second paper devoted to the problem of line intensity -oxygen abundance calibration starting from the idea of McGaugh(\cite{mcg91}) that the strong oxygen lines ([OII] lambda lambda 3727,3729 and [OIII] lambda lambda 4959, 5007) contain the necessaryinformation to determine accurate abundances in HII regions. In theprevious study (Pilyugin 2000) the corresponding relations were obtainedfor the low-metallicity HII regions (12+log O/H <= 7.95, the lowerbranch of the O/H - R23 diagram). The high-metallicity HIIregions (12+log O/H >= 8.2, the upper branch of the O/H -R23 diagram) are considered in the present study. A relationof the type O/H=f(P, R23) between oxygen abundance and thevalue of abundance index R23, introduced by Pagel et al.(\cite{pag79}), and the excitation parameter P (which is defined here asthe contribution of the radiation in [OIII] lambda lambda 4959, 5007lines to the ``total" oxygen radiation) has been derived empiricallyusing the available oxygen abundances determined via measurement of atemperature-sensitive line ratio [OIII]4959,5007/[OIII]4363(Te-method). By comparing oxygen abundances inhigh-metallicity HII regions derived with the Te-method andthose derived with the suggested relations (P-method), it was found thatthe precision of oxygen abundance determination with the P-method isaround 0.1 dex (the mean difference for the 38 HII regions considered is~ 0.08 dex) and is comparable to that of the Te-method. Arelation of the type Te=f(P, R23) between electrontemperature and the values of abundance index R23 and theexcitation parameter P was derived empirically using the availableelectron temperatures determined via measurement oftemperature-sensitive line ratios. The maximum value of differencesbetween electron temperatures determined via measurement oftemperature-sensitive line ratios and those derived with the suggestedrelation is around 1000 K for HII regions considered here, the meanvalue of differences for 38 HII regions is ~ 500 K, which is the sameorder of magnitude as the uncertainties of electron temperaturedeterminations in high-metallicity HII regions via measuredtemperature-sensitive line ratios.
| Detection of [N II] λ5755 Emission from Low-Density Ionized Interstellar Gas
The extremely faint, temperature-sensitive ``auroral'' emission line [NII] λ5755 has been detected from the low-density ionized gasalong the sight line toward l=130.0d, b=-7.5d using the WisconsinHα Mapper. The intensity of this emission line, relative to thered nebular line [N II] λ6584, is found to be twice that observedin bright, classical H II regions surrounding O stars. This implies thatthe electron temperature of the ionized gas along this sight line isabout 2000 K higher than the H II regions and that the enhanced [N II]λ6584/Hα and [S II] λ6716/Hα intensity ratiosin this low-density gas are due at least in part to an elevatedtemperature.
| Far infrared observations of the southern Galactic star forming complex around IRAS 09002-4732
The Galactic star forming region in the southern sky, associated withIRAS 09002-4732 has been mapped simultaneously in two far infrared bands(lambda eff = 148 & 209 mu m), with ~ 1 arcmin angularresolution. Fifteen sources, including IRAS 08583-4719, 08589-4714,09002-4732 and 09014-4736 have been detected, some of which are wellresolved. Taking advantage of similar beams in the two bands, a reliabledust temperature [T(148/209)] map has been obtained, which detectscolder dust (< 30 K) in this region. The HIRES processed IRAS maps at12, 25, 60 and 100 mu m, have also been used for comparison. The opticaldepth maps, tau 200 and tau 100, generated fromthese far-IR data quantify the spatial distribution of the dust. Thediffuse emission from this entire region has been found to be 35% of thetotal FIR luminosity. The slope of the IMF in the mass range 4-16Msun has been estimated to be-1.25+0.75-0.65 for this star forming complex.Radiative transfer models in spherical geometry have been explored tofit available observations of the 4 IRAS sources and extract variousphysical parameters for corresponding dust-gas clouds. Whereas aconstant (r0) radial density distribution is favoured in IRAS08583-4719, 08589-4714 and 09002-4732, the r-1 law isinferred for IRAS 09014-4736. The dust composition is found to besimilar (Silicate dominated) in all four IRAS sources modelled. Theluminosity per unit mass is found to be in the narrow range of 44 - 81Lsun/Msun for these star forming regions.
| HNCO in massive galactic dense cores
We surveyed 81 dense molecular cores associated with regions of massivestar formation and Sgr A in the JK-1K-1 =505-404 and 10010-909 linesof HNCO. Line emission was detected towards 57 objects. Selectedsubsamples were also observed in the 101-000,404-303, 707-606,15015-14014, 16016-15015 and21021-20020 lines, covering a frequency range from22 to 461 GHz. HNCO lines from the K-1 = 2,3 ladders weredetected in several sources. Towards Orion-KL, K-1 = 5transitions with upper state energies Eu/k ~ 1100 and 1300 Kcould be observed. Five HNCO cores were mapped. The sources remainspatially unresolved at 220 and 461 GHz (10010-909and 21010-20020 transitions) with beam sizes of24'' and 18\arcsec, respectively. The detection of hyperfine structurein the 101-000 transition is consistent withoptically thin emission under conditions of Local ThermodynamicEquilibrium (LTE). This is corroborated by a rotational diagram analysisof Orion-KL that indicates optically thin line emission also fortransitions between higher excited states. At the same time a tentativedetection of interstellar HN13CO (the100,10-90,9 line at 220 GHz toward G 310.12-0.20)suggests optically thick emission from some rotational transitions.Typical HNCO abundances relative to H2 as derived from apopulation diagram analysis are ~ 10-9. The rotationaltemperatures reach ~ 500 K. The gas densities in regions of HNCOK-1=0 emission should be n>~ 106cm-3 and in regions of K-1>0 emission about anorder of magnitude higher even for radiative excitation. HNCO abundancesare found to be enhanced in high-velocity gas. HNCO integrated lineintensities correlate well with those of thermal SiO emission. Thisindicates a spatial coexistence of the two species and may hint at acommon production mechanism, presumably based on shock chemistry. Basedon the observations collected at the European Southern Observatory, LaSilla, Chile and on observations with the Heinrich-Hertz-Telescope(HHT). The HHT is operated by the Submillimeter Telescope Observatory onbehalf of Steward Observatory and the MPI für Radioastronomie.Tables 1, 2, 5, 6 are also available in electronic form and Tables 7-14are only 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
| Far-Infrared Study of IRAS 00494+5617 and IRAS 05327-0457
High angular resolution far-infrared observations at 143 and 185 μm,using the Tata Institute of Fundamental Research 1 m balloon-bornetelescope, are presented for two Galactic star-forming complexesassociated with IRAS 00494+5617 and IRAS 05327-0457. The latter map alsoreveals the cold dust in OMC-3. Both these regions are illuminated atthe edges by high-mass stars with substantial UV flux. The HIRES(High-Resolution processing using Maximum Correlation Method) processedIRAS maps at 12, 25, 60, and 100 μm have also been presented forcomparison. The present study is aimed at quantifying the role of thenearby stars vis-à-vis embedded young stellar objects in theoverall heating of these sources. Based on the FIR observations at 143and 185 μm carried out simultaneously with almost identical angularresolution, reliable dust temperature and optical depth maps have beengenerated for the brighter regions of these sources. Radiative transfermodeling in spherical geometry has been carried out to extract physicalparameters of these sources by considering the observationalconstraints, such as spectral energy distribution, angular size atdifferent wavelengths, dust temperature distribution, etc. It isconcluded that for both IRAS 00494+5617 and IRAS 05327-0457, theembedded energy sources play the major role in heating them with finitecontribution from the nearby stars. The best-fit model for IRAS00494+5617 is consistent with a simple two-phase clump-interclumppicture with ~5% volume filling factor (of clumps) and a densitycontrast of ~80.
| Mid-and far-infrared studies of galactic compact H II regions
Several Galactic H II region complexes have been observed in the farinfrared (FIR) wavebands using the TIFR 1-m balloon borne telescope, andin the mid infrared (MIR) using the Infrared Space Observatory (ISO).Many of these FIR maps show interesting structural details. The MIRobservations have clearly detected PAH features in two compact H IIregions and one molecular clump. A self consistent radiative transferscheme (1-D) has been developed which has been successful in extractingimportant details like geometric sizes, radial density distribution,dust composition, etc, from these and other similar measurements. A newscheme (2-D) has been developed in cylindrical geometry to model IRAS19181 + 1349, which has been resolved into two sources. The modelparameters in this scheme have been constrained by the observed spectralenergy distribution (SED) and radial profiles at MIR & FIRwavebands. Non-equilibrium processes have also been incorporated inspherical geometry, to explain the PAH emission features. This schemehas successfully explained high resolution spectra (ISO-SWS) of severalGalactic compact H II regions.
| Oxygen and Helium Abundances in Galactic H II Regions: Abundance Gradients
Absolute integrated fluxes have been obtained for 34 H II regions, withGalactocentric distances RG in the 6.6-17.7 kpc range, in theemission lines O II λ3726 and λ3729, O III λ4363and λ5007, HeI\ λ5876, Hα and Hβ. These fluxesare analyzed to derive the physical parameters, the ionic abundancesO+/H+, O++/H+ andHe+/H+ and the O/H abundances. Accurate electrontemperatures have been derived in six H II regions with RGbetween 6.6 kpc and 14.8 kpc, widening the RG range for whichreliable Te measurements exist. Our O/H relationship for 5kpc=180) measured in the Galaxy.
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