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A survey of OH masers towards high mass protostellar objects
Context: Masers are important tracers of the early evolution of younghigh mass stars, but the relationship between different types of maserand the evolutionary state of the exciting source remains unclear. Aims: To determine whether OH masers are common towards candidate highmass protostellar objects. Methods: We present a survey of OH maseremission towards a sample of high mass protostellar objects made usingthe Nançay and GBT telescopes. Results: OH maser emission wasdetected towards 63 objects with 36 new detections. There are 56star-forming regions and 7 OH/IR candidates. Nearly half of the detectedsources have OH flux densities ⪉1 Jy. There is no evidence thatsources with OH masers have a different range of luminosities from thenon-maser sources. The results of this survey are compared with previousH2O and class II CH3OH maser observations of the same objects. Some ofthe detected sources are only associated with OH masers and some sourcesare only associated with the 1720 MHz OH maser line. The velocity rangeof the maser emission suggests that the water maser sources may bedivided into two groups. The detection rates and velocity range of theOH and Class II CH3OH masers support the idea that there is a spatialassociation of the OH and Class II CH3OH masers. The sources span a widerange in R, the ratio of the methanol maser peak flux to OH 1665 MHzmaser peak flux, however there are only a few sources with intermediatevalues of R, 8
| 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).
| Spectral and Photoelectric Studies of the Herbig Ae/Be Star HD 259431
Spectral and photoelectric (ubvy, H, H) observations of the Herbig Ae/Bestar HD 259431 are reported. It is found that as its brightness fades,this star becomes bluer in the Paschen continuum and the intensity andequivalent width of the hydrogen emission lines increase. The spectralobservations reveal significant variations in the intensity of the Mg II4481 Å photospheric absorption line. A rise and fall in theluminosity by 0m.04 within a period of 5-7 minutes was recorded. Radicalvariations in the H lineshape ("double" "P Cyg") and flare activity arenot only observed in this star, but also in a number of HAEBE stars. Itis suggested that flare activity may initiate a change in the velocitygradient at the base of the wind and, thereby, induce "double P Cyg" or"P Cyg single" transitions. The nonradial pulsations of this star arealso discussed.
| 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
| Testing Models of Low-Excitation Photodissociation Regions with Far-Infrared Observations of Reflection Nebulae
This paper presents Kuiper Airborne Observatory observations of thephotodissociation regions (PDRs) in nine reflection nebulae. Theseobservations include the far-infrared atomic fine-structure lines of [OI] 63 and 145 μm, [C II] 158 μm, and [Si II] 35 μm and theadjacent far-infrared continuum to these lines. Our analysis of thesefar-infrared observations provides estimates of the physical conditionsin each reflection nebula. In our sample of reflection nebulae, thestellar effective temperatures are 10,000-30,000 K, the gas densitiesare 4×102-2×104 cm-3, thegas temperatures are 200-690 K, and the incident far-ultravioletintensities are 300-8100 times the ambient interstellar radiation fieldstrength (1.2×10-4 ergs cm-2 s-1sr-1). Our observations are compared with current theory forlow-excitation PDRs. The [C II] 158 μm to [O I] 63 μm line ratiodecreases with increasing incident far-ultraviolet intensity. This trendis due in part to a positive correlation of gas density with incidentfar-ultraviolet intensity. We show that this correlation arises from abalance of pressure between the H II region and the surrounding PDR. The[O I] 145 to 63 μm line ratio is higher (greater than 0.1) thanpredicted and is insensitive to variations in incident far-ultravioletintensity and gas density. The stellar temperature has little effect onthe heating efficiency that primarily had the value3×10-3, within a factor of 2. This result agrees with amodel that modifies the photoelectric heating theory to account forcolor temperature effects and predicts that the heating efficiencieswould vary by less than a factor of 3 with the color temperature of theilluminating field. In addition to the single-pointing observations, an[O I] 63 μm scan was done across the molecular ridge of one of oursample reflection nebulae, NGC 1977. The result appears to supportprevious suggestions that the ionization front of this well-studied PDRis not purely edge-on.
| The Dusty Circumstellar Environments of Ae/Be Protoplanetary Disk Candidates
We have obtained high spatial resolution mid-IR observations of fourintermediate-mass pre-main-sequence stars: HD 259431, AS 310, LkHα234, and MWC 1080. Our observations reveal the morphology of thecircumstellar (500-10,000 AU) environments of these young stars. Weresolve companion stars and complex diffuse emission from dust that canaccount for on average 50% of the IR emission detected in previouslow-resolution observations. We calculate new spectral indices for theseobjects and show that they differ from those expected from simplegeometrically flat disk models. We also show mid-IR spectral energydistributions that reveal the presence of silicate emission andabsorption features for several objects. Finally, temperature mapsconstructed from our observations suggest the presence of transientlyheated grains, and optical depth maps point to the locations of the mostembedded objects.
| 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.
| Membership in the Region of the Open Cluster NGC 2244
A double elliptic bivariate model has been developed to solve themembership problem in the region of the open cluster NGC 2244. Once thephysical cluster members were found a fit to the ZAMS was made in orderto find the NGC 2244 distance modulus.
| Crystalline silicon nanoparticles as carriers for the Extended Red Emission
In an attempt to determine the carrier of the Extended Red Emission(ERE), we have investigated a series of amorphous and crystallinematerials: natural coal, amorphous hydrogenated carbon, amorphoushydrogenated silicon carbide, porous silicon, and crystalline siliconnanoparticles. The photoluminescence (PL) behavior of various samples ofthese materials upon excitation with UV light was studied at roomtemperature focusing on both the wavelength dependence of thephotoluminescence and the PL yield. For some samples the yield is by fartoo low, other samples do not comply with the characteristic wavelengthrange of ERE. Only the samples of nanocrystalline silicon (poroussilicon and silicon nanoparticles) reveal PL properties that arecompatible with the astronomical observations. Besides this experimentalevidence, we will supply additional arguments leading to the conclusionthat silicon nanoparticles should be seriously considered as anattractive carrier for the Extended Red Emission.
| A survey of the 6.7 GHz methanol maser emission from IRAS sources. I. Data
We report the first results of a search for 6.7 GHz methanol masers inthe direction of 1399 IRAS objects north of declination-20deg with the flux densities greater than 100 Jy at 60 mu mand the flux density ratio F60/F25>1.Observations were made with the sensitivity of 1.7 Jy and the velocityresolution of 0.04 km s-1 using the 32-m Toruń radiotelescope. Maser emission was found in 182 sources, including 70 newdetections. 32 new sources were identified with objects of radioemission associated with star-forming regions. Comparison of the presentdata set with other observations suggests that about 65% of methanolmasers exhibit moderate or strong variations on time-scales of about 4and 8 years. Table 2 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Herbig Ae/Be stars with the IRAS low-resolution spectra.
Not Available
| Revisiting Hipparcos data for pre-main sequence stars
We cross-correlate the Herbig & Bell and Hipparcos Catalogues inorder to extract the results for young stellar objects (YSOs). Wecompare the distances of individual young stars and the distance oftheir presumably associated molecular clouds, taking into accountpost-Hipparcos distances to the relevant associations and usingHipparcos intermediate astrometric data to derive new parallaxes of thepre-main sequence stars based on their grouping. We confirm that YSOsare located in their associated clouds, as anticipated by a large bodyof work, and discuss reasons which make the individual parallaxes ofsome YSOs doubtful. We find in particular that the distance of TaurusYSOs as a group is entirely consistent with the molecular clouddistance, although Hipparcos distances of some faint Taurus-Auriga starsmust be viewed with caution. We then improve some of the solutions forthe binary and multiple pre-main sequence stars. In particular, weconfirm three new astrometric young binaries discovered by Hipparcos:RY Tau, UX Ori, and IXOph. Based on observations made with the ESA Hipparcosastrometry satellite
| Transitional YSOs: candidates from flat-spectrum IRAS sources
We are searching for Young Stellar Objects (YSOs) near the boundarybetween protostars and pre-main-sequence objects, what we termTransitional YSOs. We have identified a sample of 125 objects ascandidate transitional YSOs on the basis of IRAS colors and the opticalappearance on POSS plates. We have obtained optical and near-IR imagingof 82 objects accessible from the Northern Hemisphere and optical imagesof 62 sources accessible from the South. We also created deconvolved 60mu m IRAS images of all sources. We have classified the objects on thebasis of their morphology in the optical and near-IR images. We findthat the majority of our objects are associated with star-formingregions, confirming our expectation that the bulk of these objects areYSOs. Of the 125 objects, 28 have a variety of characteristics verysimilar to other transitional YSOs, while another 22 show some of thesecharacteristics. Furthermore we have found seven objects to be goodcandidates for members of the Herbig Ae/Be stellar group, of which threeare newly identified as such. We have placed a set of images for each ofthe objects in the archives of the Centre de Données astronomiquede Strasbourg (CDS). Based on observations collected at the EuropeanSouthern Observatory, La Silla, Chile.
| HIPPARCOS photometry of Herbig Ae/Be stars
The photometric behaviour of a sample of 44 Herbig Ae/Be (HAeBe)candidate stars was studied using a uniform set of optical photometryobtained by the Hipparcos mission. Astrophysical parameters (distance,temperature, luminosity, mass, age) of this sample of stars were derivedas well by combining the astrometric data provided by Hipparcos withdata from literature. Our main conclusions can be summarized as follows:(1) More than 65% of all HAeBe stars show photometric variations with anamplitude larger than 0\fm05; (2) HAeBes with a spectral type earlierthan A0 only show moderate (amplitude < 0\fm5) variations, whereasthose of later spectral type can (but not necessarily have to) showvariations of more than 2\fm5. We explain this behaviour as being due tothe fact that stars with lower masses become optically visible, andhence recognizable as Herbig Ae stars, while still contracting towardsthe zero-age main sequence (ZAMS), whereas their more massivecounterparts only become optically visible after having reached theZAMS; (3) The Herbig stars with the smallest infrared excesses do notshow large photometric variations. This can be understood by identifyingthe stars with lower infrared excesses with the more evolved objects inour sample; (4) No correlation between the level of photometricvariability and the stellar v sin i could be found. If the largephotometric variations are due to variable amounts of extinction by dustclouds in the equatorial plane of the system, the evolutionary effectsprobably disturb the expected correlation between the two. Based on datafrom the Hipparcos astrometry satellite.
| Classification and Identification of IRAS Sources with Low-Resolution Spectra
IRAS low-resolution spectra were extracted for 11,224 IRAS sources.These spectra were classified into astrophysical classes, based on thepresence of emission and absorption features and on the shape of thecontinuum. Counterparts of these IRAS sources in existing optical andinfrared catalogs are identified, and their optical spectral types arelisted if they are known. The correlations between thephotospheric/optical and circumstellar/infrared classification arediscussed.
| A new CO survey of the Monoceros OB1 region.
A new, fully sampled, sensitive CO survey of 52.5 square degrees towardsthe Monoceros OB1 (Mon OB1) region has been completed using the CfA 1.2m millimeter-wave radio telescope. This survey extends from b=-1.5°to b=+3.5° and from l=196.0° to l=206.5° on a uniformlyspaced grid of 3.75' in l and b, and has substantially bettersensitivity than previous surveys of the region. CO is detected in 80%of the area, 60% of which is weak with integrated intensities less than5K.km/s. The large-scale kinematics are dominated by the Perseus andLocal spiral arms. Within the Local arm the molecular gas appears tohave been strongly influenced by supernova remnants and expanding HIIregions with the strongest emission at velocities ranging from -17 to+10km/s, and a total mass of 1.3x10^5^Msun_. Weak CO emissionfrom Perseus arm clouds shows that they have a mean kinematic distanceof 3.5kpc and are comparable in size and mass to local molecular clouds.The coincident positions of colour selected IRAS point sources indicatesthat these distant clouds are undergoing massive star formation.
| A search for precursors of ultracompact HII regions in a sample of luminous IRAS sources. I. Association with ammonia cores.
We present observations of NH_3_ (1,1) and (2,2) lines in twoflux-limited samples of IRAS sources selected according to colourcriteria which should result in a high fraction of Young StellarObjects. The first sample contains sources (named 'Low') whoseevolutionary status is essentially unknown, while the second samplecontains sources (named 'High') possibly associated with ultracompactHII regions, the distinction being based on the IRAS [25-12] colour.Indications from a previous study of H_2_O maser emission suggest thatthe sources in the first group may be in an evolutionary phase prior tothe appearance of an HII region, thus being among the youngest knownhigh-mass forming objects. Low sources were detected in ammonia with alower rate than High sources (45% and 80% respectively); the onlydifference between the two groups is in the linewidths: the (1,1) linesare generally narrower than (2,2) lines in High sources (FWHMs medianvalues are 1.81km/s and 2.00km/s respectively), while the opposite istrue in Low sources (FWHMs median values are 1.72km/s and 1.33km/s forthe (1,1) and (2,2) lines respectively). We propose that the Low groupconsists of two distinct populations of evolutionary different objects,based on the (non-) association with ammonia emission. The Low sourcesshowing ammonia emission are characterized by more quiescent envelopesthan those surrounding High sources, and the relationships betweenrelevant physical quantities derived from our observations and the IRAScolours suggest that High sources, contrary to Low, may harbour objectswhich dominate the physical and dynamical properties of the clump, thuspossibly implying the relative youth of this subgroup of the Lowsources.
| Infrared observation and study of young stellar objects
This paper presents a near-infrared photometry of 36 young stellarobjects of different types. Combining the JHK densities with otheroptical, near-infrared, IRAS, submillimetre and millimetre emission fluxdata, we have calculated their infrared colour indices, spectral indicesand luminosities. Colour-colour diagrams of the various sources in boththe near and far infrared are constructed. For the low-masspre-main-sequence stars we improved the radiative models and calculatedand analyzed their circumstellar structure.
| Discovery of Three Radio Pulsars from an X-Ray--selected Sample
Abstract image available at:http://adsabs.harvard.edu/abs/1996ApJ...456..305Z
| MWC147: A Suspect of a Pre-Main Sequence Binary Star
Not Available
| A new catalogue of members and candidate members of the Herbig Ae/Be (HAEBE) stellar group
A new up-to-date catalogue of Herbig Ae/Be (HAEBE) stars and relatedobjects is certainly needed, for both well-seasoned researchers and, inparticular, for new investigators starting to study the many interestingastrophysical properties of these very young objects. We present a briefdiscussion of the current observational characteristics that distinguishthis class from their main sequence counterparts. The HAEBE and relatedstars are listed in five tables, containing 287 objects. Table 1contains all Ae and Be stars which historically are recognized as trueHAEBE stars or potential candidate members. Table 2 gives the stars ofspectral type Fe, and emission line stars with very uncertain or unknownspectral type. In Table 3 are given all known Extreme Emission LineObjects (EELOs), of which most have not been identified to belong to anyspecific group. Table 4a and b list other Bep or B[e] stars with strongIR-excess and unknown spectral type. Table 5 contains the non-emissionline possible young objects. Furthermore, Table 6 contains 35 starsrejected from former published lists of HAEBE stars. In these tables weare including coordinates, spectral types, visual magnitudes, ranges inphotometric variability and references of several key publicationsrelated to each object. Relevant remarks, such as the presence of anebula in the vicinity of an object, are also given.
| Not Available
Not Available
| Early-type emission-line stars with large infrared excesses
A catalog is presented of early-type emission-line stars obtained bycross-identification between Wackerling's catalog and the IRAS catalogof point sources. A study of the distribution in space of the starsshows that these stars belong to the extreme Population I; thus thepresent compilation provides a rather complete sample for further studyof the evolution of pre- and post-main sequence stars of medium and highmasses.
| Water masers associated with dense molecular clouds and ultracompact H II regions
Results are reported from a study surveying the occurrence of the H2O22GHz maser emission from bright IR sources in star-forming regions. Asample of 260 sources was selected from the IRAS Point Source Catalog,following the color selection criteria suggested by Richards et al.(1987), to identify dense molecular clouds where star formation might beat the earliest stages. Water vapor emission toward 32 sources wasdetected. Twelve new maser sources were found, all characterized byrather low integrated fluxes and narrow components. On the basis of FIRcolor indices and the maser detection rates, and from a comparison withother classes of objects, it is suggested that the sources included inthe sample not associated with ultracompact H II regions contain a highfraction of candidate molecular clouds with embedded high massprotostars still in the accretion phase.
| The young open clusters NGC 2244 and NGC 2264
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1991RMxAA..22...99P&db_key=AST
| Emission from dust in visual reflection nebulae at infrared and submillimeter wavelengths
Far-IR and submillimeter images of five bright visual reflectionnebulae, IC 446, NGC2247, NGC 2245, NGC 7023, and CED 201 are presentedand used with composite IR spectra to derive parameters such as thefraction of nebulae emission attributed to molecule-sized grains, therange of nebulae grain albedos, gas densities, and gas cloud geometries.The results show that 30-45 percent of the nebulae emission lies atwavelengths of less than 30 microns. The variation in IR luminosity maybe related to variations in nebulae gas density and less than optimalgas cloud geometries rather than to anomalous grain albedos. Relativeextinction efficincies of roughly 1000-5000 are inferred for stellarphotospheres with effective wavelengths of roughly 0.25-0.5 micron. Theresultant mass-extinction coefficient is roughly 10-50 sq cm/g.
| A catalog of pre-main-sequence emission-line stars with IRAS source associations
To aid in finding premain-sequence (PMS) emission-line stars that mighthave dusty circumstellar environments, 361 PMS stars that are associatedwith 304 separate IRAS sources were identified. These stars include 200classical T Tauri stars, 25 weak-lined (naked) T Tauri stars, 56 HerbigAe/Be stars, six FU Orionis stars, and two SU Aurigae stars. All six ofthe FU Orionis stars surveyed by IRAS were detected. Of the PMS-IRASPoint Source Catalog (PSC) associations, 90 are new and are not noted inthe PSC. The other 271 entries include 104 that are correctly identifiedin the PSC but have not yet appeared in the literature, 56 more that canbe found in both the PSC and in the published and unpublished iterature,and 111 that are in the literature but not in the PSC. Spectral slopediagrams constructed from the 12-, 25-, and 60-micron flux densitiesreveal unique distributions for the different PMS subclasses; thesediagrams may help identify the best candidate PMS stars for observationsof circumstellar dust.
| Emission From Dust in Visual Reflection Nebulae at Infrared and Submilimeter Wavelengths
Not Available
| 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 survey of infrared features in H II regions, planetary nebulae and proto-planetary nebulae from the IRAS-LRS data base
The paper reports systematic investigation of IR emission and absorptionfeatures in H II regions, planetary nebulae, and protoplanetary nebulaefrom the IRAS-Low Resolution Spectra data base. The resulting selectionconstitutes a new data base (containing 284 sources) which iscomplemented by all published lines in the range 2-20 microns.Information on atomic, molecular, and solid state material is provided.A preliminary analysis of the data set is presented, emphasizing the PAHemission features.
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