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A Sensitive Survey for Water Maser Emission Toward Bok Globules Using the Robledo 70 m Antenna
We report on the most sensitive water maser survey toward Bok globulesto date, performed using NASA's 70 m antenna at Robledo de Chavela(Spain). We observed 207 positions within the Clemens and Barvainiscatalog with a higher probability of harboring a young star, using asselection criteria the presence of radio continuum emission (fromsubmillimeter to centimeter wavelengths), geometric centers of molecularoutflows, peaks in maps of high-density gas tracers (NH3 orCS), and IRAS point sources. We have obtained seven maser detections,six of which (in CB 34, CB 54, CB 65, CB 101, CB 199, and CB 232) arereported for the first time here. Most of the water masers we detectedare likely to be associated with young stellar objects (YSOs), exceptfor CB 101 (probably an evolved object) and CB 65 (uncertain nature).The water maser in CB 199 shows a relatively high shift (~=30 kms-1) of its velocity centroid with respect to the cloudvelocity, which is unusual for low-mass YSOs. We speculate thathigh-velocity masers in this kind of object could be related to episodesof energetic mass loss in close binaries. Alternatively, the maser in CB199 could be pumped by a protoplanetary or a young planetary nebula. CB232 is the smallest Bok globule (~=0.6 pc) known to be associated withwater maser emission, although it would be superseded by the cases of CB65 (~=0.3 pc) and CB 199 (~=0.5 pc) if their association with YSOs isconfirmed. All our selection criteria have statistically compatibledetection rates, except for IRAS sources, which tend to be somewhatworse predictors for the presence of maser emission.
| Optical spectroscopic and 2MASS measurements of Stephenson Halpha stars
We present the results of spectroscopic observations for 52 objects fromthe list of Halpha emission stars of Stephenson(\cite{Ste86}). Out of six known T Tauri stars observed, five showedHalpha in emission and in one (StHa 40), Halpha changed from being in absorption to emission over a period of two years,accompanied by photometric and spectral type variability. We confirm theT Tauri nature of one Stephenson object (StHa 48) on the basis of thepresence of Halpha and Hbeta in emission, Li Ilambda6708 in absorption, infrared excess and X-ray emission. Among the52 objects observed, there were other emission line objects: 1 Ke star,1 BQ[ ] star, 2 galaxies and 2 Be stars. We present a higher-resolutionspectrum of StHa 62 showing permitted and forbidden lines in emissiontypical of BQ[ ] stars. Twenty five out of 30 newly observed objectsfailed to show Halpha in emission. We also present 2MASSobservations for 112 StHa objects. We suggest three Stephenson objects(StHa 52, 125 and 129) to be YSOs on the basis of 2MASS, IRAS and ROSATobservations. These and all other known YSOs amongst StHa stars arefound in regions of star-forming clouds in Taurus, Orion and Ophiuchus.YSOs at high galactic latitudes in other parts of the sky are thereforerare.Table 2 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/402/963
| 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
| Remnant molecular clouds in the ORI OB 1 association
We suggest and give some evidence that as an HII region expands and theO star(s) evolve(s) into B giant(s) the remains of the molecular cloudsfirst appear as bright-rimmed clouds, then as cometary globules andfinally as small clouds which are visible by the reflected light fromthe B giants. We propose to call the last one `reflection clouds' andall three categories collectively `remnant clouds'. A list is presentedof about 80 objects of these remnant clouds in the Ori OB 1 association.In the Belt region there is a beautiful spatial sequence frombright-rimmed clouds through cometary globules to reflection clouds. Wesuspect that retarded star formation in remnant clouds can explain thepresence of so-called dispersed T Tau stars in the peripheries of OBassociations.
| HCN in BOK Globules: A Good Tracer of Collapsing Cores
We have conducted an HCN (J = 1-0) survey of dense cores in a sample of24 star-forming and quiescent Bok globules. HCN emission was detectedtoward 11 of 13 globules containing embedded point sources, with no HCNdetection among 11 starless globules. As in other dark clouds, the J =1-0 hyperfine line intensity ratios vary from globule to globule andalso with position toward the same globule, suggesting the presence ofeither a scattering envelope surrounding the core or a complex densitystructure. We find that the J = 1-0 transition of the HCN molecule canbe used effectively to search for early stages of star formation insmall molecular clouds: in the sample of star-forming cores, for three(of five) sources mapped, there is good spatial coincidence (better than6") between the position of the peak integrated HCN emission and thelocation of the associated embedded source. Furthermore, this transitionis well correlated with the young stellar object (YSO) class, detectingpreferentially class I YSOs. Our results indicate that detecting strong(>1 K km s^-1) HCN emission from a molecular cloud core seems toimply the presence of an embedded protostar and thus, indirectly, thatof a collapsing core.
| Two new T Tauri stars and a candidate FU Orionis star associated with BOK globules.
We present photometric and spectroscopic evidence of two new T Tauristars formed in the conditions of isolated small Bok globules. Thespectral energy distributions of these objects display excess infraredemission, they are associated with optical reflection nebulae, and theiroptical spectra reveal Balmer emission lines and the Li Iλ6707Å absorption line. Additionally, we report thediscovery of what may be a new FU Orionis star seen towards Bok globuleCB 34. The star is about 4 magnitudes brighter than it appears on thePalomar plates and is associated with the aggregate of young stellarobjects forming in Bok globule CB 34.
| A Search for Radio Continuum Emission From Young Stellar Objects in BOK Globules
Abstract image available at:http://adsabs.harvard.edu/abs/1996AJ....111..841Y
| Near-infrared imaging survey of young stellar objects in BOK globules
We performed a near-infrared imaging survey toward 34 Bok globulescontaining Infrared Astronomy Satellite (IRAS) point sources, which wereyoung stellar objects (YSOs) candidates. We used state-of-the-art NICMOS3 and SQIID cameras for this survey. Visual examination of the imagesrevealed that 20 globules showed evidence of nebular emission or veryred stellar objects located at the position of the YSO candidates. TheIRAS 12/25 micrometers spectral indices of these 20 objects aredistinctively different from those of the 14 globules which showed nonebulosity, in the sense that more than 50 percent of the 20 nebular orvery red objects have negative IRAS 12/25 micrometers spectral indices,while only 20 percent of the 14 non-nebular objects show such red IRAScolors. Analysis of the near-infrared nebulosities present in the imagesrevealed the following: (1) these nebulosities generally contain astellarlike source surrounded by an extended component; (2) severalpossible binaries with separations of about 10" were found to reside incommon infrared nebulosity; (3) infrared reflection nebulae, seen at 2.2micrometers, are good tracers of CO mass outflow morphology. The groupof objects displaying nebulosities, when ordered by their increasing12/25 micrometers indices, seems to form an evolutionary sequence. Largenegative 12/25 micrometers indices seem to indicate objects deeplyembedded in their clouds (by showing nebulosities mostly in the K band,having associated molecular outflows, and no optical counterparts). Asthese objects reach later stages of their pre-main-sequence evolution(by showing nebulosity in the J band only, no detected molecularoutflow, and having optical counterparts or optical nebulae), their12/25 micrometers increase and become positive.
| A new water maser source in LBN594
The detection of a new water maser source on the 6(16)-5(23) transitionin the Bok globule LBN594 is presented. Besides the main feature, anumber of velocity components were detected. Spectra taken within a fewdays interval suggest possible maser variability. Maser emission wasalso confirmed for two Ae/Be Herbig stars. Upper limits (3sigma) aregiven for the 79 observed globules and 12 Herbig stars.
| BOK globules and small molecular clouds - Deep IRAS photometry and (C-12)O spectroscopy
The entire sample of small molecular clouds cataloged by Clemens andBarvainis (1988) has been probed using deep, coadded IRAS survey dataanalysis and (C-12)O spectroscopy. The far-IR colors are similar tothose found for other amorphous Galactic constituents. The mean 12.25micron color was 0.01, the mean 25/60 micron color was 0.68, and themean 60/100 micron color was 0.70. The bulk of the dust mass was colderthan 30 K. A very strong anticorrelation was found between dust opticaldepth and 60/100 micron color temperature. The mean far-IR luminosity ofthe sample is about 6.4 solar luminosities, the mean cloud mass is about11 solar, the radius is around 0.35 pc, and the density is around 1000H2/cu cm. Upper and lower limits for the number of these clouds in theGalaxy are 160,000 and 650,000. A total Galactic mass in Bok globules ofabout 3.5 million solar and a total luminosity of about two millionsolar is found.
| A catalog of small, optically selected molecular clouds - Optical, infrared, and millimeter properties
A catalog of small, optically selected molecular clouds is presented.The relevant selection criteria were small angular size, large centralopacity, and freedom from connecting opaque material. The meanellipticity of the clouds in the catalog is 2.0, with a maximumellipticity of 7. The position angle of the clouds is uncorrelated withthe direction of the Galactic plane. About 60 percent of the catalogedclouds have associated IRAS Point Source Catalog (PSC) sources in theircores or envelopes. The likelihood of a detection of a PSC source issome 6.1 times higher for the core regions of the clouds than for theenvelopes, indicating a strong association of FIR sources and Bokglobules. Of a sample of 97 of the clouds observed in the J = 2-1 lineof (C-12)O, 70 percent have cold gas kinetic temperatures andapproximately sonic line widths. The remaining 30 percent are aboutequally divided between those clouds likely to be radiatively heated bynearby stars and those undergoing active protostellar collapse oroutflows with no bulk gas heating.
| The large system of molecular clouds in Orion and Monoceros
Emission is noted over about one-eighth of an 850-sq deg region centeredon Orion and Monoceros that has been surveyed in the J = 1 to 0 line ofCO; most of the emission arises from giant molecular clouds associatedwith Orion A and B, and Mon R2. A much smaller area was surveyed forC-13O emission. A comparison of cloud masses obtained by threeindependent methods indicates that CO luminosity is as accurate ameasure of cloud mass as other indicators. The possible relationshipsamong clouds in the survey are discussed, including the conjecture thatthe overall Orion complex of clouds is a much larger system thanpreviously considered, incorporating most of the clouds in the presentsurvey.
| Radio Emission from Small Galactic Nebulae at 606 MHz
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1972AJ.....77..115T&db_key=AST
| Untersuchungen über Reflexionsnebel am Palomar Sky Survey I. Verzeichnis von Reflexionsnebeln
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| Studies of bright diffuse galactic nebulae with special regard to their spatial distribution.
Not Available
| A general study of diffuse galactic nebulae.
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