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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 CB232 is the smallest Bok globule (~=0.6 pc) known to be associated withwater maser emission, although it would be superseded by the cases of CB65 (~=0.3 pc) and CB 199 (~=0.5 pc) if their association with YSOs isconfirmed.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.
| 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
| The Photophysics of the Carrier of Extended Red Emission
Interstellar dust contains a component that reveals its presence byemitting a broad unstructured band of light in the 540-950 nm wavelengthrange, referred to as extended red emission (ERE). The presence ofinterstellar dust and ultraviolet photons are two necessary conditionsfor ERE to occur. This is the basis for suggestions that attribute EREto an interstellar dust component capable of photoluminescence. In thisstudy, we have collected all published ERE observations withabsolute-calibrated spectra for interstellar environments, where thedensity of ultraviolet photons can be estimated reliably. In each case,we determined the band-integrated ERE intensity, the wavelength of peakemission in the ERE band, and the efficiency with which absorbedultraviolet photons are contributing to the ERE. The data show thatradiation is not only driving the ERE, as expected for aphotoluminescence process, but is modifying the ERE carrier, asmanifested by a systematic increase in the ERE band's peak wavelengthand a general decrease in the photon conversion efficiency withincreasing densities of the prevailing exciting radiation. The overallspectral characteristics of the ERE and the observed high quantumefficiency of the ERE process are currently best matched by the recentlyproposed silicon nanoparticle (SNP) model. Using the experimentallyestablished fact that ionization of semiconductor nanoparticles quenchestheir photoluminescence, we proceeded to test the SNP model bydeveloping a quantitative model for the excitation and ionizationequilibrium of SNPs under interstellar conditions for a wide range ofradiation field densities. With a single adjustable parameter, the crosssection for photoionization, the model reproduces the observations ofERE intensity and ERE efficiency remarkably well. The assumption thatabout 50% of the ERE carriers are neutral under radiation conditionsencountered in the diffuse interstellar medium leads to a prediction ofthe single-photon ionization cross section of SNPs with average diameter3.5 nm of <=3.4×10-15 cm2. The shift ofthe ERE band's peak wavelength toward larger values with increasingradiation density requires a change of the size distribution of theactively luminescing ERE carriers through a gradual removal of thesmaller particles by size-dependent photofragmentation. We propose thatheat-assisted Coulomb decay of metastable, multiply charged SNPs is sucha process, which selectively removes the smaller components of anexisting SNP size distribution.
| CS Emission from BOK Globules: Survey Results
We present the results of a survey for CS emission toward a sample of 47Bok globules selected from the catalog of Clemens & Barvainis. Theglobules were observed at the FCRAO 14 m, the SEST 15 m, and the IRAM 30m telescopes in the CS J = 2 --> 1 transition. Additionally, higherspatial resolution (11"-30") observations in the CS J = 3 --> 2 and 5--> 4 lines, as well as the C^34S J = 2 --> 1 and 3 --> 2lines, were carried out toward a subsample of 20 globules using the IRAM30 m and the CSO 10 m telescopes. Two-thirds of the globules weredetected in the CS J = 2 --> 1 line. The detection rate was higher inglobules with IRAS sources (72%) than in globules without IRAS sources(44%). The detection rate was 100% for globules with embedded Class 0and Class I infrared sources, dropping to 60% for Class II-D sources andto 40% for Class II sources. These results support the association ofdense cores with IRAS point sources in Bok globules and indicate thatthe dense gas is most apparent in the early stages. We present CS mapsof 12 globule cores and derive sizes for nine CS cores. The results ofthe CS survey are compared with the results of surveys of other linesand of dust continuum emission. The integrated intensities of the CSlines correlate with those of C^18O J = 2 --> 1, but the line widthsand FWHM sizes tend to be somewhat larger than those of C^18O. The meanFWHM size of those sources with reasonable distance estimates is 0.41+/- 0.18 pc, and the mean virial mass is 60 +/- 52 M_ȯ. The meansize places them intermediate between cores in Taurus and coresassociated with water masers and massive star formation.
| 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.
| Collapse Candidates among the BOK Globules
Abstract image available at:http://adsabs.harvard.edu/abs/1995ApJ...454..217W
| 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.
| Spectroscopy of extended red emission in reflection nebulae
This paper reports the results of a spectroscopic survey of reflectionnebulae, aimed at studying the characteristics of the broad emissionfeature responsible for the extended red emission (ERE) observed inthese objects. The ERE band was detected and its strength, centralwavelength, and width were measured in 12 nebulae, while upper limitswere recorded in 12 further objects. The maximum ERE band intensity wasfound to vary from object to object within an extremely narrow range,while the associated scattered light intensities differed by nearly twoorders of magnitude. The ERE band is interpreted as arising from thephotoluminescence of hydrogenated amorphous carbon (HAC) grains whichbecome rehydrogenated and gain luminescence efficiency in narrow H2photodissociation zones. These zones are probably thin shells around theexciting stars. Observed changes in the central wavelengths and widthsof the ERE bands detected in different parts of a given nebula, and fromnebula to nebula, support the HAC model and are explained as arisingfrom variations in the degree of hydrogenation to the solid grains.
| 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.
| Geometrical patterns of astrophysical jets
The observed morphological features of extragalactic radio jets includewiggles, pinches, and bends as well as 'C'- and 'S'-shaped symmetries.To gain understanding of these properties, several applicable models arereviewed, including helical and pinching instabilities, curvature due tovariable jet velocity, and precession. It is found that differentcombinations of these models make it possible to reproduce thesefeatures and to deduce a number of important physical parameters, suchas the Mach number, the density contrast, and the angular velocity ofthe parent galaxy.
| CCD surface photometry of bright reflection nebulae
Surface brightness measurements in the B, V, R, and I photometric bandsare presented for 14 reflection nebulae. The analysis of nebula-starcolor differences leads to the conclusion that excess emission in the Iband beyond that expected from scattering is a common phenomenon amongreflection nebulae illuminated by B stars. An ultraviolet-poweredfluorescence mechanism is suggested. Both the absolute and the relativeV surface brightnesses of the nebulae in the sample are analyzed. Thedata can be explained, if the nebulae arise in moderately denseinterstellar clouds with illuminating stars embedded at an optical depthlevel of order unity and with dust of high albedo and with a stronglyforward-directed phase function. It is concluded that bright reflectionnebulae must arise under almost optimal scattering conditions, whichapparently are found when newly formed low-mass star clusters are stillembedded in the material from which they originated.
| Reflection Nebulae: Celestial Veils
Not Available
| Untersuchungen über Reflexionsnebel am Palomar Sky Survey I. Verzeichnis von Reflexionsnebeln
Not Available
| Studies of bright diffuse galactic nebulae with special regard to their spatial distribution.
Not Available
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