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Публикации по объекту
Globulettes as Seeds of Brown Dwarfs and Free-Floating Planetary-Mass Objects
Some H II regions surrounding young stellar clusters contain tiny dustyclouds, which on photos look like dark spots or teardrops against abackground of nebular emission. From our collection of Hα imagesof 10 H II regions gathered at the Nordic Optical Telescope, we found173 such clouds, which we call ``globulettes,'' since they are muchsmaller than normal globules and form a distinct class of objects. Manyglobulettes are quite isolated and located far from the molecular shellsand elephant trunks associated with the regions. Others are attached tothe trunks (or shells), suggesting that globulettes may form as aconsequence of erosion of these larger structures. None of our objectsappear to contain stellar objects. The globulettes were measured forposition, dimension, and orientation, and we find that most objects aresmaller than 10 kAU. The Rosette Nebula and IC 1805 are particularlyrich in globulettes, for which the size distributions peak at mean radiiof ~2.5 kAU, similar to what was found by Reipurth and coworkers and DeMarco and coworkers for similar objects in other regions. We estimatetotal mass and density distributions for each object from extinctionmeasures and conclude that a majority contain <13 MJ,corresponding to planetary-mass objects. We then estimate the internalthermal and potential energies and find, when also including the effectsfrom the outer pressure, that a large fraction of the globulettes couldbe unstable and would contract on short timescales, <10 6yr. In addition, the radiation pressure and ram pressure exerted on theside facing the clusters would stimulate contraction. Since theglobulettes are not screened from stellar light by dust clouds fartherin, one would expect photoevaporation to dissolve the objects. However,surprisingly few objects show bright rims or teardrop forms. Wecalculate the expected lifetimes against photoevaporation. Theselifetimes scatter around 4×106 yr, much longer thanestimated in previous studies and also much longer than the free-falltime. We conclude that a large number of our globulettes have time toform central low-mass objects long before the ionization front, drivenby the impinging Lyman photons, has penetrated far into the globulette.Hence, the globulettes may be one source in the formation of browndwarfs and free-floating planetary-mass objects in the galaxy.Based on observations made with the Nordic Optical Telescope, operatedon the island of La Palma jointly by Denmark, Finland, Iceland, Norway,and Sweden in the Spanish Observatorio del Roque de los Muchachos of theInstituto de Astrofísica de Canarias.
| 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
| Spectral observations of stars associated with nebulae. I. PP52, PP57, PP63, and PP89
Spectral observations of stars associated with nebulae, PP52, PP57,PP63, and PP89, are reported. The star PP52 is of spectral type A and isassociated with a reflecting nebula. PP57 is a nonstationary star. Aweak object (1079-0122733) immediately adjacent to PP57 is highlyvariable. It is shown that the nebulae PP63 and PP89 are cometary.
| The circumstellar environment of high mass protostellar objects. III. Evidence of infall?
The results are presented of a molecular line survey to search for thespectral signature of infall towards 77 850 μm continuum sourcesbelieved to be candidate high mass protostellar objects. Up to sixdifferent transitions, HCO+ J= 1→ 0, J= 3→ 2 and J=4→ 3, H2CO 212-111, N2H+ J=1→ 0 and H13CO+ J= 3→ 2, were observedtowards each source. Towards the peak of the 850 μm emission,N2H+ was typically strong, with a peak antenna temperature of~1.5 K, with a typical linewidth of ~2 km s-1. The goodagreement between the velocity and velocity width of the N2H+and H13CO+ emission suggests that both species aretracing similar material in the sources. With respect to the velocity ofthe N2H+, there is a statistically significant excess of blueasymmetric line profiles in both the HCO+ J= 1→ 0 andH2CO transitions. This excess reaches levels similar to that seentowards samples of low mass protostars, and suggests that the materialaround these high mass sources is infalling. We identify 22 promisingcandidate infall sources which show at least one blue asymmetric lineprofile and no red asymmetric profiles. The infall velocity is estimatedto be in the range of 0.1 km s-1 to 1 km s-1 withan implied mass accretion rate of between 2×10-4M_ȯ /yr and 10-3 M_ȯ /yr.
| Collaborative Research of Open Star Clusters
Preliminary results on observations of open clusters are pre-sented. Theproject has been initiated in the framework of the Uzbek-Taiwan andTaiwan-Baltic collaboration, mainly to upgrade and make use offacilities at Maidanak Observatory. We present detailed,multi-wavelength studies of the young cluster NGC 6823 and theassociated complex nebulosity, to diagnose the young stellar populationand star formation history in the region. In addition, 7 compact openclusters have been monitored for stellar variability. We show howobservations like these could feasibly be used to look for exoplanettransit events. We also expect to join the Whole-Earth Telescope effortin future campaigns for asteroseismology.
| The circumstellar environments of high-mass protostellar objects. II. Dust continuum models
We analyse the dust continuum emission seen towards a sample ofcandidate high-mass protostellar objects, modelling the cores werecently observed at 850 μm with a one-dimensional radiative transfercode. Fitting radial slices in a range of directions across sources, weidentify a number of objects that have non-spherical density profilesand show that for such sources fitting the azimuthal averaged emissionproduces erroneous estimates of the source properties. We find themajority of cores can be successfully modelled using envelopes ofpower-law density structure (where ρ ∝ r-α),finding a mean power-law index of overline{ α } = 1.3 ±0.4. These envelopes extend considerably further, are more dense, andhave a more shallow density profile than those bearing low-massprotostars. The majority of best-fit models have a SED resembling thecold-component dust bodies previously proposed for the sample, implyingthe short wavelength emission seen towards the HMPOs either originatesfrom a separate hot dust component(s), or involves mechanisms such asaccretion disks, stochastic heating and/or optically thin cavities notincluded in the radiative transfer model. We find evidence of smallerdust-free cavities towards some pre-UCHII sources. The modellingindicates a correlation between α and optical depth, suggestingthat the densest cores also tend to have the most strongly peakedpower-law density profiles.
| Resolution of Distance Ambiguities of Inner Galaxy Massive Star Formation Regions. I.
Fifty-four ultracompact (UC) H II regions in the GLIMPSE survey region(|b|<1deg and 30deg
| 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
| High-Mass Protostellar Candidates. II. Density Structure from Dust Continuum and CS Emission
We present a detailed 1.2 mm continuum and CS spectral line study of alarge sample of 69 massive star forming regions in very early stages ofevolution, most of them prior to building up an ultracompact H IIregion. The continuum data show a zoo of different morphologies and givedetailed information on the spatial distributions, masses, columndensities, and average densities of the whole sample. Fitting the radialintensity profiles shows that three parameters are needed to describethe spatial distribution of the sources: constant emission from thecenter out to a few arcseconds radius followed by a first power-lawintensity distribution, which steepens farther outside into a secondpower-law distribution. The inner flat region is possibly caused byfragmentation of the large-scale cores into smaller subsources, whereasthe steeper outer power-law distributions indicate finite sizes of thecores. Separating the sources into subsamples suggests that in theearliest stages prior to the onset of massive star formation, theintensity radial distributions are rather flat, resembling the structureof intensity peaks in more quiescent molecular clouds. Then in thesubsequent collapse and accretion phase the intensity distributionsbecome centrally peaked, with steep power-law indices. In thisevolutionary stage the sources show also the broadest C34Sline width. During the following phase, when ultracompact H II regionsevolve, the intensity power-law radial distributions flatten out again.This is probably caused by the ignited massive stars in the center whichdisrupt the surrounding cores. The mean inner power-law intensity indexmi (I~r-mi) is 1.2, corresponding todensity indices p (n~r-p) of 1.6. In total, the densitydistributions of our massive star formation sites seem to be not toodifferent from their low-mass counterparts, but we show that settingtight constrains on the density indices is very difficult and subject tomany possible errors. The local densities we derive from CS calculationsare higher (up to 1 order of magnitude) than the mean densities we findvia the millimeter continuum. Such inhomogeneous density distributionreflects most likely the ubiquitous phenomenon of clumping andfragmentation in molecular clouds. Line width-mass relations show adeparture from virial equilibrium in the stages of strongly collapsingcores.
| High-Mass Protostellar Candidates. I. The Sample and Initial Results
We describe a systematic program aimed at identifying and characterizingcandidate high-mass protostellar objects (HMPOs). Our candidate sampleconsists of 69 objects selected by criteria based on those establishedby Ramesh & Sridharan using far-infrared, radio continuum, andmolecular line data. IRAS and Midcourse Space Experiment data were usedto study the larger scale environments of the candidate sources anddetermine their total luminosities and dust temperatures. To derive thephysical and chemical properties of our target regions, we observedcontinuum and spectral line radiation at millimeter and radiowavelengths. We imaged the free-free and dust continuum emission atwavelengths of 3.6 cm and 1.2 mm, respectively, searched forH2O and CH3OH maser emission, and observed the COJ=2-->1 line and several NH3 lines toward all sources inour sample. Other molecular tracers were observed in a subsample. Whiledust continuum emission was detected in all sources, most of them showonly weak or no emission at 3.6 cm. Where detected, the centimeteremission is frequently found to be offset from the millimeter emission,indicating that the free-free and dust emissions arise from differentsubsources possibly belonging to the same (proto)cluster. A comparisonof the luminosities derived from the centimeter emission with bolometricluminosities calculated from the IRAS far-infrared fluxes shows that thecentimeter emission very likely traces the most massive source, whereasthe whole cluster contributes to the far-infrared luminosity. Estimatesof the accretion luminosity indicate that a significant fraction of thebolometric luminosity is still due to accretion processes. The earlieststages of HMPO evolution we seek to identify are represented by dustcores without radio emission. Line wings due to outflow activity arenearly omnipresent in the CO observations, and the molecular line dataindicate the presence of hot cores for several sources, where theabundances of various molecular species are elevated because ofevaporation of icy grain mantles. Kinetic gas temperatures of 40 sourcesare derived from NH3 (1,1) and (2, 2) data, and we comparethe results with the dust temperatures obtained from the IRAS data.Comparing the amount of dust, and hence the gas, associated with theHMPOs and with ultracompact H II (UCH II) regions, we find that the twotypes of sources are clearly separated in mass-luminosity diagrams: forthe same dust masses, the UCH II regions have higher bolometricluminosities than HMPOs. We suggest that this is an evolutionary trend,with the HMPOs being younger and reprocessing less (stellar) radiationin the IR than the more evolved UCH II regions. These results indicatethat a substantial fraction of our sample harbors HMPOs in a pre-UCH IIregion phase, the earliest known stage in the high-mass star formationprocess.
| 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
| A CCD Search for Variable Stars of Spectral TypeB in the Northern Hemisphere Open Clusters. III. NGC 6823
We present the results of variability search in the field of a veryyoung open cluster NGC 6823. Two delta Sct stars, still at the pre-mainsequence (PMS) stage of evolution, were found. These objects could beused for future testing of the evolutionary period changes in this classof variable stars. In addition, we found 13 other variables including abright cluster eclipsing binary and an SPB candidate. A few othervariables could be PMS stars of the UX Ori type. The H alpha photometry,which we made for 69 brightest stars, revealed only one object withstrong emission, discovered recently by the photographic methods. Twoother stars announced to have H alpha emission, do not show any evidencefor its presence. For a number of stars in the search field, we alsoprovide the BV(RI)_C photometry. We explain how the distribution ofabsorbing matter along the line of sight results in the unusualmorphology of the cluster color-magnitude (CM) diagram. The dereddenedCM diagram is used in the calculation of the extinction map for theobserved field, in which the E(R-I)_C color excess varies from 0.54 to0.72 mag, with the average value equal to 0.62 mag. It is also shownthat all cluster stars with spectral types later than A0 are PMSobjects. We use these stars to estimate the cluster age: 3+/-1 Myr.Using the cluster CM diagram, we compare and discuss the position of thetwo discovered delta Sct stars with respect to the theoreticalinstability strip for PMS stars of this type.
| The Medicina survey of methanol masers at 6.7 GHz
A survey of Class II methanol masers at 6.7 GHz was made in the Northernhemisphere with the 32-m Medicina radio telescope. 42 objects weredetected, 20 of them are new detections at 6.7 GHz. Our results showthat the detection rate of 6.7 GHz masers toward the inner part of theGalaxy is higher than in other directions. It is confirmed that most ofthe methanol masers are associated with faint compact HII regions. The6.7 GHz methanol masers show large velocity dispersion and largevelocity offset from the velocity of parent molecular clouds. Table 2 isonly available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| 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 search for OH maser emission from post-asymptotic giant branch stars.
We present a survey for OH maser emission at 18cm in the direction of196 IRAS point sources. The primary aim of the survey was to detect OHmaser emission from post-asymptotic giant branch (post-AGB) stars. OHemission and/or absorption was detected in the direction of 77 sourcesof which 57 OH detections were previously unknown, giving an overalldetection rate of 39 per cent. Fourteen maser sources associated withevolved stars or planetary nebulae were found, corresponding to 7 percent of the total observed. These included three planetary nebulae (twonew OH detections), nine post-AGB stars (three new OH detections) andtwo cold OH/IR stars (both previously known). Two sources were ofuncertain identification (both new OH detections). The remainder of theOH sources were identified with young stellar objects and with HIIregions. The survey included the detection of OH maser emission from HD101584, an early-type supergiant at high Galactic latitude. For severalof the post-AGB stars, changes have occurred in the OH spectral profilesover a period of several years. This is interpreted as evidence forrapid changes in the stellar winds, due either to interactions withstellar companions, novae-like events or rapid stellar evolution.
| From Ultracompact to Extended H II Regions
Abstract image available at:http://adsabs.harvard.edu/abs/1996ApJ...469..171G
| A CS(2-1) survey of IRAS point sources with color characteristics of ultra-compact HII regions.
We have made a complete survey of the CS(2-1) emission toward IRAS pointsources in the galactic plane. The sources observed were selectedaccording to their far infrared (FIR) colors, which are characteristicof UC HII regions. They have 25μm/12μm flux ratios larger than 3.7and 60μm/12μm flux ratios larger than 19.3. The survey covered aregion from b=-2deg to b=2deg from l=300deg to l=0deg and from l=0deg tol=60deg, and from b=-4deg to b=4deg elsewhere. We detected 843 sourcesout of 1427 sources observed. The distributions of detected andundetected sources in a FIR color-color diagram show some differences,suggesting that more than one type of object is present in our observedsample.
| New detections of 5_1_-6_0_A^+^-methanol masers towards IRAS sources.
We present the results of our second search for 6.7-GHz methanol maserstowards colour-selected IRAS sources. Five hundred and twenty IRASsources that meet the far-infrared colour criteria set by Wood &Churchwell for ultra-compact HII regions were searched for 6.7-GHzmethanol maser emission, to a sensitivity limit of 5Jy. Thirty one newmaser sources were detected. We also compare the FIR colours of thenewly detected maser sources with those detected by Schutte et al.(1993) and the IRAS counterparts of sources that have both methanol andhydroxyl maser emission. It was found that the average flux distributionof the newly detected sources differs significantly from that of allother known 6.7-GHz methanol maser sources. It is argued that thedifferences may be due either to intrinsic differences between the threegroups of sources or to interstellar extinction. An analysis of therelation between the 6.7-GHz maser and IRAS flux densities shows thatthe maser flux density is always less than the 100 μm flux densitywhile only three sources have a maser flux density greater than the 60μm flux density. Far-infrared pumping of the 6.7-GHz methanol masersis therefore in principle viable although it was found that the apparentmaser efficiency will exceed 10 per cent for a significant number ofsources in the case of FIR pumping by photons between 50 and 100μm.The overall detection rates on the IRAS [25-12] vs [60-12] two-colourdiagram are also presented. Possible new search strategies for masers incolour-selected IRAS sources are discussed.
| Dark Frames in Ccds
Not Available
| H2O masers from low and intermediate luminosity young stellar objects: H2O masers and YSOs
We have used the Medicina 32-m radiotelescope to search for H2O 22.2 GHzmaser emission from a sample of 68 red peculiar nebulosities associatedwith low luminosity (LIR less than 103 solarluminosity)) and intermediate luminosity (LIR approximately104 solar luminosity) Young Stellar Objects (YSOs). H2O maseremission was detected in 9 sources, with a new detection in IRAS18265+0028. Comparison with other samples indicates that YSOs have ahigher probability of hosting an H2O maser, when they are associatedwith red peculiar nebulosities. Seven of the detected sources areassociated with molecular outflows, which confirms that these twophenomena are strictly correlated. The maser sources associated with theClass I YSOs (IRAS 03225+3034, and IRAS 03245+3002, in the dark cloudsL1448 and L1455 respectively) appear overluminous with respect to theirIR luminosity. The maser emission shows a remarkable variability on timescales of months and years, which tends to be larger for lowerluminosity sources. This is indicative of unsaturated emission in lowluminosity sources.
| On the Connection Between Radial Systems of Dark Globules and Stellar Associations
We study the connection between radial systems of dark globules andstellar associations. It is shown that of the 17 systems of type 1 inTable 1 of [1] 16 radial systems are connected with known associations.A new association is found (missing from the catalogs) connected withthe remaining system (System No. 2). Four systems of the six systems oftype 2 (Table 2 of [1]) are connected with known associations. A newmethod of determining the distance to associations is proposed, usingthe mean linear thickness of dark globules of radial systems connectedwith these associations as the criterion for distance. Using this methodwe make the distance to the association Cyg OB 9 more precise and answerthe question whether several radial systems belong to the correspondingstellar associations.
| Radial Systems of Dark Globules - Part One
Not Available
| Spectral investigation of 1548 C27: object with collimated outflow.
Not Available
| Catalog of open clusters and associated interstellar matter.
Not Available
| Star-forming loops in the IRAS sky images
Loops containing diffuse and discrete emission are a feature of the IRASsky images. Some of these loops are limb-brightened shells resultingfrom supernovae or stellar winds acting on the interstellar medium.Secondary star formation appears to have occurred at the surface ofthese shells. A significant proportion of the early-type stars in thesolar neighborhood appear to have formed in stellar loops.
| The large globule LYNDS 810 as a possible member of the Vulpecula OB1 complex
A distance of 2.5 + or - 0.2 kpc is derived for the dark globule Lynds810 using photometric data for foreground stars and star counts. It isargued that this globule is likely to belong to the Vul OB1 complex. Itscometary structure, like certain other features of dust-related objectsin Vulpecula, appears to be the shock signature of a past disturbanceoriginating from a region near l = 60 deg, b = +2 deg, which lies withinthe fossil H II region Lynds 792.
| Two newly discovered S stars in a list of faint red objects
A list of 103 faint red stars in a 0.7-square-degree area located nearNGC 6820/23 on the galactic plane in the constellation Vulpecula ispresented. The stars were chosen from Near Infrared Photographic SkySurvey visual and near-IR photographic pairs and were selected for theirvisual faintness as well as their red colors. Positions, approximatemagnitudes, and finding charts for these stars are presented. It isnoted that two members of the list, 1548C858 and 1548C869, have alreadybeen determined to be faint, pure S stars.
| H II regions of the northern Milky Way: medium-large-field photographic atlas and catalogue.
Abstract image available at:http://adsabs.harvard.edu/abs/1976A&AS...25...25D
| A low-latitude galactic survey from lII=46o to 61o and 190o to 290o at 2700 MHz
Not Available
| The Effective Temperatures of the O Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1969ApJ...158..629M&db_key=AST
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