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Large Aperture O I 6300 Å Observations of Comet Hyakutake: Implications for the Photochemistry of OH and O I Production in Comet Hale-Bopp
In previous work (Morgenthaler et al. and Glinski et al.), we proposed arevision to the standard OH photochemistry of van Dishoeck &Dalgarno in order to explain the anomalously bright [O I] 6300 Åemission observed by several instruments in comet C/1995 O1 (Hale-Bopp).In order to test the validity of revisions to the OH photochemistry, wepresent wide-field Fabry-Pérot [O I] 6300 Å observations ofcomet C/1996 B2 (Hyakutake) and review similar observations recorded for1P/Halley and 1989 c1 (Austin). Conventional long-slit spectroscopicobservations of several comets in [O I] are also reviewed. The bulk ofthe evidence suggests that the OH photochemical rates of van Dishoeck& Dalgarno are, on the whole, correct and that the bright [O I]emission observed in Hale-Bopp, particularly at cometocentric distancesbeyond ~3×104 km, was unique to that comet, or possiblyany comet with such a large production rate.
| On the Distance and Structure of W80
The continuum radiation of the Galactic region located atl=85deg, b=-0.5deg is well defined by its 11 cmemission. The region, extended 3° in diameter, is cataloged as W80,and its optical images show the North America and Pelican Nebulaecomplexes. In this paper we present new distance measurements andphysical parameters obtained from radio recombination line observationsat a frequency near 1.4 GHz. Four structures are identified in theregion under study: a near structure is located at a distance of about0.7 kpc, another structure lies on the east side of W80 at a distance of1.7 kpc, there is a concentration of ionized gas on the west side at adistance of 2.7 kpc, and a fourth structure is found further away at 3.3kpc, which does not seem to belong to the W80 complex. The results arepresented using the (X, Y, Z)-coordinates of the Galactic plane. Themodel used assumes a constant electron temperature Te=6800 Kand also assumes homogeneity. Our results seem to indicate that theionized hydrogen of W80 spreads along the line of sight instead ofclumping at one distance. We obtain values of the electron density andthe ionization parameter between ne=5 and 16 cm-3and U=51 and 110 cm-2 pc, respectively.
| 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.
| A new view of the Cygnus X region. KOSMA 13CO 2 to 1, 3 to 2, and 12CO 3 to 2 imaging
Context: .The Cygnus X region is one of the richest star formation sitesin the Galaxy. There is a long-standing discussion about whether theregion is a chance superposition of several complexes along the line ofsight or a single coherent complex at a distance of 1.5 to 2 kpc. Aims: . Combining a 13CO 2 to 1 survey taken with the KOSMA 3m telescope with mid-IR images from MSX provides a way to improve ourunderstanding of the spatial structure of the complex. The physicalproperties of the molecular gas can be derived in more detail as it wasdone in former studies. Methods: . Cygnus X has been mapped in13CO J =2to1 (10.8 deg^2) at an angular resolution of 130'',as well as for smaller areas in 12CO and 13CO J =3 to 2 (90''), using the KOSMA 3 m submm-telescope. Results: .Weidentified 91 clumps in 13CO 2 to 1 that have a typicalexcitation temperature of 10-30 K, an average density of1.3×103 cm-3, radii of 1-8 pc, and masses ofa few hundred to several ten thousand M_ȯ. The main cloudcomplexes, the northern part (M≃2.8×10^5 M_ȯ) includingDR21 and W75N and the southern region (M≃4.5×10^5 M_ȯ)with IC 1318 b/c and AFGL2591, show differences in their physicalproperties. The 13CO emission is closely associated withmid-IR emission seen with MSX. We find evidence that Cygnus OB2 andCygnus OB9 are affecting the molecular material in Cygnus X. Conclusions: . Since essentially all molecular cloud complexes in CygnusX form groups that are connected by molecular emission (visible inchannel and position-velocity maps) and partly show evidence ofinteraction with UV radiation, we conclude that most of the objects seenin this region are located at the same distance, i.e., that of the OB2cluster at 1.7 kpc, which is also consistent with the distances ofother OB associations (OB9, OB1) in Cygnus X.
| 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.
| CCD Photometry and Classification of Stars in the North America and Pelican Nebulae Region. III. The Dark Cloud L935
Magnitudes and color indices of 430 stars down to V ˜ 17.5 mag inthe eight-color Vilnius + I photometric system were obtained in fourareas of diameter 20' within the dark cloud L935 separating the NorthAmerica and Pelican nebulae. Spectral types, interstellar colorexcesses, extinctions and distances of stars were determined from thephotometric data. The plot of extinction vs. distance shows that thedark cloud begins at a distance of 520±50 pc. About 40 stars inthe cloud, mostly K and M dwarfs, are suspected to have Hαemission; these stars also exhibit infrared excesses. Four of them areknown pre-main-sequence stars. Our star set contains J205551.3+435225 (V= 13.24) which, according to Camerón and Pasquali (2005), is theO5 V type star ionizing the North America and Pelican nebulae. If thisspectral type is confirmed, the star would have an extinctionAV between 9 and 10 magnitudes (depending on the acceptedextinction law) and a distance which is not very different from the dustcloud distance.
| An Investigation of Diffuse Interstellar Gas toward a Large, Low-Extinction Window into the Inner Galaxy
Hα and Hβ spectroscopy with the Wisconsin Hα Mapper(WHAM) reveals a strong concentration of high-velocity emission in a~5deg×5deg area centered near (l,b)=(27deg, -3deg), known as the Scutum cloud. Thehigh velocities imply that we are detecting optical emission from nearthe plane of the Galaxy out to the tangent point at heliocentricdistances of Dsolar>~6 kpc, assuming that the gasparticipates in circular Galactic rotation. The ratio of the Hα toHβ emission as a function of velocity suggests that dust alongthese lines of sight produces a total visual extinction ofAV~3 at Dsolar~6 kpc. This makes it possible touse optical emission lines to explore the physical conditions of ionizedgas in the inner Galaxy. At a Galactocentric distance RG~4kpc, for example, we find that the H+ has an rms midplanedensity of ~1 cm-3 with a vertical scale height of ~300 pc.We also find evidence for an increase in the flux of Lyman continuumphotons and an increase in the ratio of ionized to neutral hydrogentoward the inner Galaxy. We have extended the measurements of E(B-V) inthis direction to distances far beyond what has been accessible throughstellar photometry and find E(B-V)/NH to be near the localmean of 1.7×10-22 cm2 mag, with evidence foran increase in this ratio at RG~4 kpc. Finally, ourobservations of [N II] λ6583, [S II] λ6716, and [O III]λ5007 toward the window reveal that in the inner Galaxy thetemperature of the gas and the ionization state of oxygen increase withincreasing height from the midplane.
| Is G84.0+0.8 a high mass star formation site near the edge of the Pelican nebula?
We present visible and near-infrared observations of the G84.0+0.8 HIIregion, a bright compact knot projected within the boundaries of the W80complex dominated by the North America and Pelican nebulae. The spectrumof the nebula indicates a temperature of the ionizing stellar spectrumT* ≃ 40 000{-}45 000 K (corresponding to a O7-O5 star)and a density of the HII region n ≃ 460 cm-3, with aforeground extinction of AV ≃ 5.9 mag. A comparison ofnarrow-band near-infrared images through the Brγ and the H2 S(1)v=1 → 0 filters shows that G84.0+0.8 consists of a fan-shapedcavity in a molecular cloud at least partly bounded by aphotodissociation region, filled with Brγ-emitting ionized gas,and with a compact cluster at the tip of the fan. The brightest star atthe position of the cluster is found to be a late G-type interloper.While membership of G84.0+0.8 in the local arm is well established fromexisting radial velocity measurements of the ionized gas, we find thatthe ionizing flux estimated from the size and density of the nebula onthe one hand, and the radio continuum properties of the nebula on theother hand, are well below the expected ionizing flux of a mid, or evenlate, O-type star. We consider the possibility that G84.0+0.8 might beexternally ionized by a nearby mid-O star. Currently availableobservations do not definitely confirm or reject the membership ofG84.0+0.8 in the W80 complex, although a larger distance seems favoredby the available data. Nevertheless, we can firmly rule out thepossibility that it represents a massive star forming site in thatcomplex, as its appearance as a compact HII region containing anembedded cluster may lead one to think.
| The T Tauri Star Population of the Young Cluster NGC 2264
An Hα emission survey of the young cluster NGC 2264 in the Mon OB1association resulted in the detection of 490 Hα emission stars ina 25'×40' field approximately centeredbetween the O7 V multiple star S Mon and the Cone Nebula. The survey wascarried out with the wide-field grism spectrograph (WFGS) on theUniversity of Hawaii 2.2 m telescope on Mauna Kea. X-ray observationsmade with the European Photon Imaging Camera on board the European SpaceAgency's XMM-Newton satellite observatory will be discussed in asubsequent paper. Optical (BVRCIC) photometry wasobtained for selected fields to supplement similar data from theliterature. Spectra covering the 6000-8000 Å region at aresolution of R~3000 (adequate for the determination of Li Iλ6708 line strengths) were obtained for 150 Hα and X-rayemission sources with the Gemini Multi-Object Spectrograph.Near-infrared spectra (1-2.5 μm) of a number of T Tauri stars (TTSs),X-ray sources, and LHα25 (W90) were also obtained using SpeX onthe Infrared Telescope Facility. Ages and masses for the Hαemitters were inferred from the isochrones and evolutionary tracks ofD'Antona & Mazzitelli. The median age for the TTS population isabout 1.1 Myr, but a considerable dispersion, from 0.1 to 5 Myr, existsfor individual objects. Several fields in the cluster were observed withthe WFGS on more than one occasion, permitting an examination ofHα variability over long baselines in time. About 90% of theclassical T Tauri stars (CTTSs) showed changes in W(Hα) of atleast 10%, while 57% varied at levels of 50% or more. No evidence wasfound for a significant pool of dormant Hα emitters. Summing themasses of the TTSs and the OB stellar population of NGC 2264, a lowerlimit for the total stellar mass content of the cluster is about 430Msolar. This is less than 1% of the total mass of the atomicand molecular gas believed to be associated with NGC 2264. Evidence forhierarchical structure within the cluster is suggested by the spatialdistribution of TTSs. Four concentrations of Hα emitters areevident: two near S Mon and two near the Cone Nebula. The median age ofthe TTSs in the immediate vicinity of S Mon was found to be greater thanthat of the TTSs near Allen's infrared source (IRS-1), but a significantdispersion is present. From the rotational data of Lamm et al. andMakidon et al., 241 of the TTSs are periodic variables, 150 weak-line TTauri stars (WTTSs) and 91 CTTSs, while 123 stars are irregularvariables (30 WTTSs and 93 CTTSs). A weak-to-moderate positivecorrelation is found between H-K color and Prot for theCTTSs, in the sense that stars having longer periods tend to have largerH-K colors. A similar positive correlation is found betweenLHα and Prot among the CTTSs. Nostatistically significant correlation is found between Protand theoretical age or between Prot and LX. Othertopics discussed include the fraction of Hα emitters that areWTTSs, f(WTTS)=N(WTTS)/N(TTS), for clusters of different ages; therelative detectability of Hα emission using WFGS and narrowbandfilter imaging techniques; and the correlation of W(Li I) withTe, age, H-K color, and W(Hα).
| The ionizing star of the North America and Pelican nebulae
We present the results of a search for the ionizing star of the NorthAmerica (NGC 7000) and the Pelican (IC 5070) nebulae complex. Theapplication of adequate selection criteria to the 2MASS JH KSbroad-band photometry allows us to narrow the search down to 19preliminary candidates in a circle of 0o 5 radius containingmost of the L935 dark cloud that separates both nebulae. Follow-upnear-infrared spectroscopy shows that most of these objects are carbonstars and mid-to-late-type giants, including some AGB stars. Two of thethree remaining objects turn out to be later than spectral type B andthus cannot account for the ionization of the nebula, but a thirdobject, 2MASS J205551.25+435224.6, has infrared properties consistentwith it being a mid O-type star at the distance of the nebulae complexand reddened by AV ≃ 9.6. We confirm its O5V spectraltype by means of visible spectroscopy in the blue. This star has thespectral type required by the ionization conditions of the nebulae andphotometric properties consistent with the most recent estimates oftheir distance. Moreover, it lies close to the geometric center of thecomplex that other studies have proposed as the most likely location forthe ionizing star, and is also very close to the position inferred fromthe morphology of cloud rims detected in radio continuum. Given thefulfillment of all the conditions and the existence of only one star inthe whole search area that satisfies them, we thus propose 2MASSJ205551.25+435224.6 as the ionizing star of the North America/Pelicancomplex.Based on observations collected at the Centro AstronómicoHispano-Alemán (CAHA) at Calar Alto, operated jointly by theMax-Planck Institut für Astronomie and the Instituto deAstrofísica de Andalucía (CSIC).Figure 1 is only available in electronic form athttp://www.edpsciences.org
| A radio and mid-infrared survey of northern bright-rimmed clouds
We have carried out an archival radio, optical and infrared wavelengthimaging survey of 44 Bright-Rimmed Clouds (BRCs) using the NRAO/VLA SkySurvey (NVSS) archive, images from the Digitised Sky Survey (DSS) andthe Midcourse Space eXperiment (MSX). The data characterise the physicalproperties of the Ionised Boundary Layer (IBL) of the BRCs. We haveclassified the radio detections as: that associated with the ionisedcloud rims; that associated with possible embedded Young Stellar Objects(YSOs); and that unlikely to be associated with the clouds at all. Thestars responsible for ionising each cloud are identified and acomparison of the expected ionising flux to that measured at the cloudrims is presented. A total of 25 clouds display 20 cm radio continuumemission that is associated with their bright optical rims. The ionisingphoton flux illuminating these clouds, the ionised gas pressure and theelectron density of the IBL are determined. We derive internal molecularpressures for 9 clouds using molecular line data from the literature andcompare these pressures to the IBL pressures to determine the pressurebalance of the clouds. We find three clouds in which the pressureexerted by their IBLs is much greater than that measured in the internalmolecular material. A comparison of external pressures around theremaining clouds to a global mean internal pressure shows that themajority of clouds can be expected to be in pressure equilibrium withtheir IBLs and hence are likely to be currently shocked byphotoionisation shocks. We identify one source which shows 20 cmemission consistent with that of an embedded high-mass YSO and confirmits association with a known infrared stellar cluster. This embeddedcluster is shown to contain early-type B stars, implying that at leastsome BRCs are intimately involved in intermediate to high mass starformation.Figure \ref{fig:images} and Table \ref{tbl:istars1} are only availablein electronic form at http://www.edpsciences.org
| Optical photometry and spectral classification in the field of the open cluster NGC 6996 in the North America Nebula
We present and discuss broad band CCD UBV(I)_C photometry and lowresolution spectroscopy for stars in the region of the open cluster NGC6996, located in the North America Nebula. The new data allow us totightly constrain the basic properties of this object. We revise thecluster size, which in the past has been significantly underestimated.The width of the Main Sequence is mainly interpreted in terms ofdifferential reddening, and indeed the stars' color excessEB-V ranges from 0.43 to 0.65, implying the presence of asignificant and evenly distributed dust component. We cross-correlateour optical photometry with near infrared photometry from 2MASS, and bymeans of spectral classification we are able to build extinction curvesfor an handful of bright members. We find that the reddening slope andthe total to selective absorption ratio R_V toward NGC 6996 areanomalous. Moreover the reddening-corrected colors and magnitudes allowus to derive estimates for the cluster distance and age, which turn outto be 760 ± 70 pc (V0-MV = 9.4 ±0.2) and 350 Myr, respectively. Based on our results, we suggestthat NGC 6996 is located in front of the North America Nebula, and doesnot seem to have any apparent relationship with it.Based on observations carried out at Asiago and Teramo Observatories,Italy.Photometry 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/419/149
| The Wisconsin Hα Mapper Northern Sky Survey
The Wisconsin Hα Mapper (WHAM) has surveyed the distribution andkinematics of ionized gas in the Galaxy above declination -30°. TheWHAM Northern Sky Survey (WHAM-NSS) has an angular resolution of 1°and provides the first absolutely calibrated, kinematically resolved mapof the Hα emission from the warm ionized medium (WIM) within~+/-100 km s-1 of the local standard of rest. LeveragingWHAM's 12 km s-1 spectral resolution, we have modeled andremoved atmospheric emission and zodiacal absorption features from eachof the 37,565 spectra. The resulting Hα profiles reveal ionizedgas detected in nearly every direction on the sky with a sensitivity of0.15 R (3 σ). Complex distributions of ionized gas are revealed inthe nearby spiral arms up to 1-2 kpc away from the Galactic plane.Toward the inner Galaxy, the WHAM-NSS provides information about the WIMout to the tangent point down to a few degrees from the plane. Ionizedgas is also detected toward many intermediate velocity clouds at highlatitudes. Several new H II regions are revealed around early B starsand evolved stellar cores (sdB/O). This work presents the details of theinstrument, the survey, and the data reduction techniques. The WHAM-NSSis also presented and analyzed for its gross properties. Finally, somegeneral conclusions are presented about the nature of the WIM asrevealed by the WHAM-NSS.
| T Tauri Stars
The observable characteristics of young solar-mass stars (T Tauri stars)are reviewed. Modern theoretical concepts and models of these stars arediscussed briefly.
| Irradiated Jets and Outflows in the Pelican Nebula
We report the discovery of new Herbig-Haro objects in the Pelican Nebula(IC 4050). HH 555 is a bipolar jet emerging from the tip of a majorelephant trunk protruding into the Pelican Nebula from the adjacentmolecular cloud. Both beams of HH 555 bend toward the west, indicatingdeflection by a side wind. A chain of three nearly equally spaced bowshocks, HH 563, HH 564, and HH 565, trace a bent flow bursting out ofthe southern rim of the Pelican molecular cloud, possibly driven by themoderate-luminosity Class I protostar IRAS 20489+4406. Object HH 570 isa highly collimated jet emerging from a compact cloud located about 15'southeast of the Pelican molecular cloud. A parallel outflow, possiblydriven by IRAS 20496+4354, powers the bright bow shock HH 569. Theseobservations demonstrate that vigorous star formation is still occurringwithin the clouds that surround the evolved North America/Pelican Nebulacomplex.
| Near-Infrared Interferometric Measurements of Herbig Ae/Be Stars
We have observed the Herbig Ae/Be sources AB Aur, VV Ser, V1685 Cyg (BD+40°4124), AS 442, and MWC 1080 with the Palomar TestbedInterferometer, obtaining the longest baseline near-IR interferometricobservations of this class of objects. All of the sources are resolvedat 2.2 μm with angular size scales generally <~5 mas, consistentwith the only previous near-IR interferometric measurements of HerbigAe/Be stars, by Millan-Gabet and collaborators. We determine the angularsize scales and orientations predicted by uniform-disk, Gaussian, ring,and accretion disk models. Although it is difficult to distinguishdifferent radial distributions, we are able to place firm constraints onthe inclinations of these models, and our measurements are the firstthat show evidence for significantly inclined morphologies. In addition,the derived angular sizes for the early-type Herbig Be stars in oursample, V1685 Cyg and MWC 1080, agree reasonably well with thosepredicted by the face-on accretion disk models used by Hillenbrand andcollaborators to explain observed spectral energy distributions. Incontrast, our data for the later-type sources AB Aur, VV Ser, and AS 442are somewhat inconsistent with these models and may be explained betterthrough the puffed-up inner disk models of Dullemond and collaborators.
| A Search for Ionized Gas in the Draco and Ursa Minor Dwarf Spheroidal Galaxies
The Wisconsin Hα Mapper has been used to set the first deep upperlimits on the intensity of diffuse Hα emission from warm ionizedgas in the Draco and Ursa Minor dwarf spheroidal galaxies (dSphs).Assuming a velocity dispersion of 15 km s-1 for the ionizedgas, we set limits of IHα<=0.024 R andIHα<=0.021 R for the Draco and Ursa Minor dSphs,respectively, averaged over our 1° circular beam. Adopting a simplemodel for the ionized interstellar medium, these limits translate toupper bounds on the mass of ionized gas of <~10% of the stellar mass,or ~10 times the upper limits for the mass of neutral hydrogen. Notethat the Draco and Ursa Minor dSphs could contain substantial amounts ofinterstellar gas, equivalent to all of the gas injected by dying starssince the end of their main star-forming episodes >~8 Gyr in thepast, without violating these limits on the mass of ionized gas.
| Galactic environment and the 10-μm silicate feature of young stellar objects
Disc and sphere dust models are used to fit 8-13 μm flux spectra of19 low-mass young stellar objects (YSOs) and five Herbig AeBe stars. The13 non-photospheric low-mass YSOs in quiescent environments and the fiveHerbig AeBe stars have mean disc temperature indices of 0.4, indicatingthat the emission arises from optically thin layers above a flaredoptically thick disc; 10 out of 14 of the low-mass YSO and four out offive of the Herbig AeBe features contain an optically thin silicateemission component. The radius of the peak 10-μm emission for nineout of the 13 low-mass YSOs is 10-130 au, and three out of the fiveHerbig AeBe stars are 10-30 au in size. In contrast, the five YSOs fromdisrupted molecular clouds that have been shaped by expanding supernovaremnants have temperature indices of between 0.3 and 0.8; four out ofthe five are optically thick and three out of the five have radii <~2au. The photosphere-like continuum of Taurus-Elias 18 could be fittedonly with truncated optically thick models, implying the presence of avoid between the >500 K and cold (<~100 K) foreground dust.Silicates surrounding low-mass YSOs in quiescent molecular clouds aresimilar to those in the Trapezium region of the Orion Nebula except whenAV<~ 2 mag. In the low-AV case and in low-massYSOs in disrupted molecular clouds the silicates are similar tocircumstellar dust around the evolved star μ Cephei.
| 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.
| Digitization and Scientific Exploitation of the Italian and Vatican Astronomical Plate Archives
There is a widespread interest to digitize the precious informationcontained in the astronomical plate archives, both for the preservationof their content and for its fast distribution to all interestedresearchers in order to achieve their better scientific exploitation.This paper presents the first results of our large-scale project todigitize the archive of plates of the Italian Astronomical Observatoriesand of the Specola Vaticana. Similar systems, composed by commercialflat-bed retro-illuminated scanners plus dedicated personal computersand acquisition and analysis software, have been installed in allparticipating Institutes. Ad-hoc codes have been developed to acquirethe data, to test the suitability of the machines to our scientificneeds, and to reduce the digital data in order to extract theastrometric, photometric and spectroscopic content. Two more elementscomplete the overall project: the provision of high quality BVRI CCDsequences in selected fields with the Campo Imperatore telescopes, andthe distribution of the digitized information to all interestedresearchers via the Web. The methods we have derived in the course ofthis project have been already applied successfully to plates taken byother Observatories, for instance at Byurakan and at Hamburg.
| Measurements of [O I] λ6300/Hα Line Intensity Ratios for Four O Star H II Regions
We have used the Wisconsin Hα Mapper facility to measure the [O I]λ6300/Hα line intensity ratios for four O star H IIregions: S27 (observation coordinates l=6.3d,b=+23.6d), S252(l=190.1d,b=+0.6d), S261 (l=194.1d,b=-1.9d), and S264(l=195.1d,b=-12.0d). We find that the ratios range from 0.0015 to0.0053. These results are roughly a factor of 10 lower than measured [OI]/Hα ratios in directions that sample the warm ionized componentof the interstellar medium. This difference implies a significantlylower hydrogen ionization ratio n(H+)/n(H0) orhigher electron temperature in the diffuse ionized gas compared withthat in the bright discrete O star H II regions.
| Structure Function Scaling of a 2MASS Extinction Map of Taurus
We compute the structure function scaling of a 2MASS extinction map ofthe Taurus molecular cloud complex. The scaling exponents of thestructure functions of the extinction map follow Boldyrev's velocitystructure function scaling of supersonic turbulence. This confirms ourprevious result based on a spectral map of 13CO J=1-0covering the same region and suggests that supersonic turbulence isimportant in the fragmentation of this star-forming cloud.
| A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in Translucent Clouds
We report the first ensemble results from the Far UltravioletSpectroscopic Explorer survey of molecular hydrogen in lines of sightwith AV>~1 mag. We have developed techniques for fittingcomputed profiles to the low-J lines of H2, and thusdetermining column densities for J=0 and J=1, which contain >~99% ofthe total H2. From these column densities and ancillary datawe have derived the total H2 column densities, hydrogenmolecular fractions, and kinetic temperatures for 23 lines of sight.This is the first significant sample of molecular hydrogen columndensities of ~1021 cm-2, measured through UVabsorption bands. We have also compiled a set of extinction data forthese lines of sight, which sample a wide range of environments. We havesearched for correlations of our H2-related quantities withpreviously published column densities of other molecules and extinctionparameters. We find strong correlations between H2 andmolecules such as CH, CN, and CO, in general agreement with predictionsof chemical models. We also find the expected correlations betweenhydrogen molecular fraction and various density indicators such askinetic temperature, CN abundance, the steepness of the far-UVextinction rise, and the width of the 2175 Å bump. Despite therelatively large molecular fractions, we do not see the values greaterthan 0.8 expected in translucent clouds. With the exception of a fewlines of sight, we see little evidence for the presence of individualtranslucent clouds in our sample. We conclude that most of the lines ofsight are actually composed of two or more diffuse clouds similar tothose found toward targets like ζ Oph. We suggest a modification interminology to distinguish between a ``translucent line of sight'' and a``translucent cloud.''
| A Highly Ordered Faraday Rotation Structure in the Interstellar Medium
We describe a Faraday rotation structure in the interstellar mediumdetected through polarimetric imaging at 1420 MHz from the CanadianGalactic Plane Survey (CGPS). The structure, at l=91.8d,b=-2.5d, has anextent of ~2°, within which polarization angle varies smoothly overa range of ~100°. Polarized intensity also varies smoothly, showinga central peak within an outer shell. This region is in sharp contrastto its surroundings, where low-level chaotic polarization structureoccurs on arcminute scales. The Faraday rotation structure has nocounterpart in radio total intensity and is unrelated to known objectsalong the line of sight, which include a Lynds Bright Nebula, LBN 416,and the star cluster M39 (NGC 7092). It is interpreted as a smoothenhancement of electron density. The absence of a counterpart, in eitheroptical emission or total intensity, establishes a lower limit to itsdistance. An upper limit is determined by the strong beam depolarizationin this direction. At a probable distance of 350+/-50 pc, the size ofthe object is 10 pc, the enhancement of electron density is 1.7cm-3, and the mass of ionized gas is 23 Msolar. Ithas a very smooth internal magnetic field of strength 3 μG, slightlyenhanced above the ambient field. G91.8-2.5 is the second such object tobe discovered in the CGPS, and it seems likely that such structures arecommon in the magneto-ionic medium.
| Hα Emission Stars and Herbig-Haro Objects in the Vicinity of Bright-rimmed Clouds
Bright-rimmed clouds (BRCs) found in H II regions are probably sites oftriggered star formation due to compression by ionization/shock fronts.Many BRCs harbor IRAS point sources of low dust temperature. They alsofrequently contain a small cluster of near-IR stars that is elongatedalong the axis of the BRC. Here we present the results of our Hαgrism spectroscopy and narrowband imaging observations of BRCs in searchof candidate pre-main-sequence stars of the T Tauri, Herbig Ae/Be andrelated types, and Herbig-Haro (HH) objects. We have detected a largenumber (460) of Hα emission stars down to a limiting magnitude ofabout R=20 in and around all but two of the 28 BRCs observed. Thepresent study has, for the first time, reached down nearly to thefaintest classical T Tauri star population in OB associations. A totalof 12 new HH objects have been found. Most are of small apparent size,emphasizing the need for deep searches at high spatial resolution, butHH 588 associated with BRC 37 represents a huge HH complex composed oftwo-staged bow shocks on both sides of a tiny central knot. These starsand HH objects are concentrated near the tip of bright rims (i.e., inthe head of the BRCs and just outside the rims) and often make looseaggregates similar to those of near-IR stars, thus supporting ourhypothesis of ``small-scale sequential star formation.'' The presence ofsuch a large number of Hα emission stars in the immediatevicinities of BRCs implies that second-generation formation of low-massstars is relatively extensive and further supports the notion ofcohabitation of high- and low-mass populations in OB associations.
| Extinction with 2MASS: Star Counts and Reddening toward the North America and Pelican Nebulae
We propose a general method for mapping the extinction in densemolecular clouds using Two Micron All Sky Survey near-infrared data. Thetechnique is based on the simultaneous utilization of star counts andcolors. These two techniques provide independent estimations of theextinction, and each method reacts differently to foreground starcontamination and to star clustering. We take advantage of both methodsto build a large-scale extinction map (2.5d×2.5d) of the NorthAmerica and Pelican Nebulae complex. With Ks star counts andH-Ks color analysis the visual extinction is mapped up to 35mag. Regions with visual extinction greater than 20 mag account for lessthan 3% of the total mass of the cloud. Color is generally a betterestimator for the extinction than star counts. Nine star clusters areidentified in the area, seven of which were previously unknown.
| 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.
| CCD Photometry and Classification of Stars in the North America and Pelican Nebulae Region. I. Molėtai Photometry
Magnitudes and color indices in the Vilnius seven-color system aremeasured for 690 stars down to ~ 13.2 mag in the area of the NorthAmerica and Pelican nebulae. Spectral types, absolute magnitudes, colorexcesses, interstellar extinctions and distances of the stars aredetermined. The plots of interstellar extinction A_V versus distance forthe North America Nebula and for the dark cloud L935 show that bothareas are covered by the same absorbing cloud, situated at a distance of600 pc. The maximal extinction in the area of the nebula is ~ 3 mag,while in the dark cloud L935 it is much greater.
| Large-Aperture [O I] 6300 Å Photometry of Comet Hale-Bopp: Implications for the Photochemistry of OH
Large-aperture photometric observations of comet Hale-Bopp (C/1995 O1)in the forbidden red line of neutral oxygen ([O I] 6300 Å) withthe 150 mm dual-etalon Fabry-Pérot spectrometer that comprisesthe Wisconsin Hα Mapper and a 50 mm dual-etalon Fabry-Pérotspectrometer at the McMath-Pierce main telescope from 1997 late Februaryto mid April yield a total metastable O(1D) production rateof (2.3-5.9)×1030 s-1. Applying the standardH2O and OH photodissociation branching ratios found inHuebner, Keady, & Lyon and van Dishoeck & Dalgarno, we derive awater production rate, Q(H2O), of(2.6-6.1)×1031 s-1, which disagrees withQ(H2O)~1×1031 s-1 determined byindependent H2O, OH, and H measurements. Furthermore, our own[O I] 6300 Å observations of the inner coma (<30,000 km) usingthe 3.5 m Wisconsin-Indiana-Yale-NOAO telescope Hydra and Densepakmultiobject spectrographs yieldQ(H2O)~1×1031 s-1. Using our [OI] 6300 Å data, which cover spatial scales ranging from 2,000 to1×106 km, and a complementary set of wide-fieldground-based OH images, we can constrain the sources of the apparentexcess O(1D) emission to the outer coma, wherephotodissociation of OH is assumed to be the dominant O(1D)production mechanism. From production rates of other oxygen-bearingvolatiles (e.g., CO and CO2), we can account for at most 30%of the observed excess O(1D) emission. Since even lessO(1D) should be coming from other sources (e.g., electronexcitation of neutral O and distributed nonnuclear sources ofH2O), we hypothesize that the bulk of the excessO(1D) is likely coming from photodissociating OH. Using theexperimental OH photodissociation cross section of Nee & Lee atLyα as a guide in modifying the theoretical OH cross sections ofvan Dishoeck & Dalgarno, we can account for ~60% of the observedO(1D) excess without requiring major modifications to theother OH branching ratios or the total OH photodissociation lifetime.
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