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Observations of the 6 cm Lines of OH in Evolved (OH/IR) Stars
Recent observational and theoretical advances have called into questiontraditional OH maser pumping models in evolved (OH/IR) stars. Thedetection of excited-state OH lines would provide additional constraintsto discriminate among these theoretical models. In this Letter, wereport on VLA observations of the 4750 and 4765 MHz lines of OH toward45 sources, mostly evolved stars. We detect 4765 MHz emission in thestar-forming regions Mon R2 and LDN 1084, but we do not detectexcited-state emission in any evolved stars. The flux density andvelocity of the 4765 MHz detection in Mon R2 suggest that a new flaringevent has begun.
| Long-term evolution of FU Orionis objects at infrared wavelengths
We investigate the brightness evolution of seven FU Orionis systems inthe 1-100 μm wavelength range using data from the Infrared SpaceObservatory (ISO). The ISO measurements were supplemented by 2MASS andMSX observations performed in the same years as the ISO mission(1995-98). The spectral energy distributions (SEDs) based on these datapoints were compared with earlier ones derived from the IRAS photometryas well as from ground-based observations carried out around the epoch1983.In three cases (Z CMa, Parsamian 21, V1331 Cyg) no difference betweenthe two epochs was seen within the measurement uncertainties. V1057 Cyg,V1515 Cyg and V1735 Cyg have become fainter at near-infrared wavelengthswhile V346 Nor has become slightly brighter. V1057 Cyg exhibits asimilar flux change in the mid-infrared. At λ≥ 60 μm mostof the sources remained constant; only V346 Nor seems to fade. Our dataon the long-term evolution of V1057 Cyg agree with the model predictionsof Kenyon & Hartmann (\cite{Kenyonh91}) and Turner et al.(\cite{Turner97}) at near- and mid-infrared wavelengths, but disagree atλ>25 μm. We discuss if this observational result atfar-infrared wavelengths could be understood in the framework of theexisting models.Based on observations with ISO, an ESA project with instruments fundedby ESA member states (especially the PI countries France, Germany, theNetherlands and the United Kingdom) with participation of ISAS and NASA.
| IRAS 05436-0007 and the Emergence of MCNeil's Nebula
We present a study of McNeil's Nebula, a newly appeared reflectionnebula in the L1630 cloud, together with photometry and spectroscopy ofits source. New IR photometry compared with earlier Two Micron All SkySurvey data shows that the star has brightened by about 3 mag in thenear-infrared, changing its location in a J-H/H-K' diagramprecisely along a reddening vector. A Gemini Near InfraRed Imager K-bandspectrum shows strong CO band head emission, and Brγ is inemission, indicative of strong accretion. A Gemini Multi-ObjectSpectrograph optical spectrum shows only a red, heavily veiledcontinuum, with Hα strongly in emission and displaying apronounced P Cygni profile, with an absorption trough reachingvelocities up to 600 km s-1. This implies significant massloss in a powerful wind. However, no evidence is found for any shocks,as commonly seen in collimated outflows from young stars. Apparently theeruption has dispersed a layer of extinction, and this, together withthe intrinsic brightening of the IRAS source, has allowed an earlieroutflow cavity to be flooded with light, thus creating McNeil's Nebula.
| Observations of Star-Forming Regions with the Midcourse Space Experiment
We have imaged seven nearby star-forming regions, the Rosette Nebula,the Orion Nebula, W3, the Pleiades, G300.2-16.8, S263, and G159.6-18.5,with the Spatial Infrared Imaging Telescope on the Midcourse SpaceExperiment (MSX) satellite at 18" resolution at 8.3, 12.1, 14.7, and21.3 μm. The large angular scale of the regions imaged (~7.2-50deg2) makes these data unique in terms of the combination ofsize and resolution. In addition to the star-forming regions, twocirrus-free fields (MSXBG 160 and MSXBG 161) and a field near the southGalactic pole (MSXBG 239) were also imaged. Point sources have beenextracted from each region, resulting in the identification over 500 newsources (i.e., no identified counterparts at other wavelengths), as wellas over 1300 with prior identifications. The extended emission from thestar-forming regions is described, and prominent structures areidentified, particularly in W3 and Orion. The Rosette Nebula isdiscussed in detail. The bulk of the mid-infrared emission is consistentwith that of photon-dominated regions, including the elephant trunkcomplex. The central clump, however, and a line of site toward thenorthern edge of the cavity show significantly redder colors than therest of the Rosette complex.
| 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
| Multifrequency behaviour of fuor systems.
Not Available
| A photometric catalogue of southern emission-line stars
We present a catalogue of previously unpublished optical and infraredphotometry for a sample of 162 emission-line objects and shell starsvisible from the southern hemisphere. The data were obtained between1978 and 1997 in the Walraven (WULBV), Johnson/Cousins(UBV(RI)c) and ESO and SAAO near-infrared (JHKLM) photometricsystems. Most of the observed objects are Herbig Ae/Be (HAeBe) stars orHAeBe candidates appearing in the list of HAeBe candidates of Théet al. (1994), although several B[e] stars, LBVs and T Tauri stars arealso included in our sample. For many of the stars the data presentedhere are the first photo-electric measurements in the literature. Theresulting catalogue consists of 1809 photometric measurements. Opticalvariability was detected in 66 out of the 116 sources that were observedmore than once. 15 out of the 50 stars observed multiple times in theinfrared showed variability at 2.2 mu m (K band). Based on observationscollected at the European Southern Observatory, La Silla, Chile and onobservations collected at the South African Astronomical Observatory.Tables 2-4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/380/609
| On the Transience of High-Latitude OH/IR Stars. II. Thermal Pulse Link
High-latitude, |b|>=10deg, asymptotic (AGB) stars in theirsuperwind phase have distinctive IR colors, which make them easy toidentify and count. But the expansion velocity (Ve) versuscolor plots of these stars are a puzzle, with features implying (1) astep increase in dM/dt and (2) short-term changes in the circumstellarshell that are enhanced by increased metallicity. The net duration oftheir superwind phase is ~3700 yr when estimated from the relativefrequency of associated proto-planetary nebulae and the ~103 yrexpansion age of one, IRAS 18095+2704. Since these stars lose on average~0.04 Msolar during the superwind phase and have progenitormasses circa 1 Msolar, they must lose more than 0.3Msolar prior to the AGB. A single 3700 yr superwind episode,however, cannot explain the 2-18 km s-1 range of expansionvelocities exhibited by these stars. This range is best understood asresulting from a cyclical superwind, wherein stars lose most of theirenvelope mass in three to four discrete episodes of ever largerVe, with each episode being triggered by a sudden onset inthe coupling of photon momentum by dust to gas near a thermal pulse. Theresulting newly accelerated shell thereafter quickly expands beyond theprotective dust shroud of the prior shell, which allows interstellar UVto degrade its molecules. This turns the shell into an OH/IR star colormimic (a shell with the dM/dt of an OH/IR star without 1612 MHz masers),until enough dust is again in place beyond the shell to allow moleculesthe longevity needed to support masers. Two by-products of this modelare the natural explanation it offers for why Ve(CO)>Ve(OH), as well as for the occurrence and integration ofmimics into the normal development of a shell. The circumstellar shellsof high-latitude OH/IR stars are thus cyclically transient phenomena.
| On the Similarity of FU Orionis Stars to Class I Protostars: Evidence from the Submillimeter
On the basis of analysis of new submillimeter maps of a sample of mostknown FU Orionis stars, we confirm that these stars are extremely young,more similar to Class I protostars than to Class II T Tauri stars. Thesubmillimeter maps reveal that FU Orionis stars have accretion disksthat are larger and contain an order of magnitude more mass than thoseof T Tauri stars but are comparable in mass to those seen around Class Isources. The disks around the FU Orionis stars are sufficiently massiveto act as reservoirs of material for replenishing the mass of the disksafter eruptions, in which large amounts of material are believed to fallonto the underlying stars. We have identified a number of FU Orionisstars that are surrounded by extended cloud emission, and in severalcases we find nearby submillimeter sources that have no optical ornear-infrared counterparts. These sources most likely are youngprotostellar Class I or Class 0 sources. In at least in one case, V1735Cyg, the dominant submillimeter source is the protostar, not theoptically visible FU Orionis star.
| 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
| The submillimetre colour of young stellar objects
A survey is presented of the submillimetre continuum spectra from dustaround 73 young stellar objects (YSOs) associated with Herbig-Haroobjects and molecular outflows. Spectra at millimetre to submillimetrewavelengths taken with similar beamsizes have been obtained for 42 ofthe objects, and the remainder either have been detected or have upperlimits at 800mum. The data are combined with far-infrared IRAS fluxes,and then compared with published submillimetre data from other types ofYSO; this results in a data set of ~150 submillimetre spectra. We havethen employed colour-colour diagrams to investigate the differences insubmillimetre continuum spectra between classes of YSOs. The majority oftargets are optically obscured YSOs (generally Lada Class I); all arefound to have a submillimetre spectral index (alpha_mm) of ~3.5. None ofthese objects has alpha_mm<=3.0. This is significantly different frompublished optically visible T Tauri stars (Class II-III), many of whichhave alpha_mm<=2.5. The few main-sequence stars so far observed inthe submillimetre generally also have a relatively low value ofalpha_mm. We find that objects which might be classified as T Tauristars, but which are not directly visible, have higher values ofalpha_mm. Assuming isothermal optically thin dust emission, this impliesa dust emissivity index beta of ~1.5 for embedded objects, and ~0.5 forvisible objects. The reduced alpha_mm in low optical extinction sourcescan be attributed to either grain growth or the formation of a compactoptically thick circumstellar disc, and we discuss these two options inthe light of the new data. If, as the results suggest, large fractalgrains are present in the more evolved systems, then these will havehigher millimetre mass opacity, implying that the determination of thecircumstellar mass evolution from submillimetre continuum fluxes musttake into account the evolution of the dust itself. High-mass embeddedYSOs also have spectra very similar to their low-mass counterparts, butwith higher dust temperature (again, under the isothermal optically thinassumption). Also, most Class 0 YSOs have high values of alpha_mm,although three have flatter spectra more typical of fractal dust, whichis difficult to explain under simple grain coagulation models. Thesubmillimetre continuum flux is also compared with the intensity of twomolecular lines: C^18O J=2-1 and H_2CO 3_03-2_02. The integratedemission from the higher density tracer, H_2CO, is well correlated withcontinuum flux. The C^18O line shows a less significant correlation,particularly at low masses, suggesting that this may not be a goodmolecule to search for low-mass YSOs. By comparing T Tauri stars andyounger YSOs with the same continuum flux, we find evidence that T Tauristars are unusually weak in C^18O emission, and we give some possibleexplanations.
| Infrared Spectroscopy of Herbig-Haro Energy Sources
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....114.2700R
| 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.
| 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.
| 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.
| Stellar density enhancements associated with IRAS sources in L1641
We obtained H and K' images of 59 Infrared Astronomical Satellite (IRAS)sources associated with dense molecular gas in L1641. Some of thesources were also imaged in narrow-band L and M. Using these near-IRimages and photometry, we are able to identify the near-IR counterpartsfor most of the IRAS sources. The spectral energy distributions of thesources suggest that all of them are young stellar objects (class I andII sources). Most importantly, we find in this study that 14 IRASsources are associated with small (but statistically significant)groupings of bright near-IR sources defined as stellar densityenhancements (SDEs). The spatial distribution of young stars in theOrion A molecular cloud can be characterized by a range of stellardensities, from the Trapezium Cluster, to the SDEs, to individual stars.We conjecture that the SDEs are regions of continuous star formationwithin or around dense molecular cores and that they may represent animportant mode of star formation in L1641. If true, adjustments to thestandard star formation model may be required.
| An IRAS Sample from Co-added Images of L1641
Abstract image available at:http://adsabs.harvard.edu/abs/1993ApJ...416..235C
| The Exciting Sources of Herbig-Haro Objects - Part One - a Catalogue of 1-MICRON to 20-MICRON Observations
Abstract image available at:http://adsabs.harvard.edu/abs/1993A&AS..101...59M
| An OH Satellite Line Maser Survey of Cool IRAS Sources and Circumstellar Envelope Evolution
Abstract image available at:http://adsabs.harvard.edu/abs/1993A&A...277..453D
| The discovery of two FU Orionis objects in L1641
We have obtained spectra of the reflection nebulosity illuminated by twoheavily embedded IRAS sources in the L1641 molecular cloud, Re 50 and IC430 (V883 Ori). Examination of these spectra in combination with theirother properties indicates that these objects are FU Orionis objects.Our spectrum of L1551 IRS 5 confirms the FU Ori classification made forthis star by Mundt et al. (1985).
| Infrared luminosity functions for the young stellar population associated with the L1641 molecular cloud
Results are reported of a deep near-IR imaging survey which samples 0.77sq deg of the L1641 star-forming complex and reaches 5 sigma limits at J(1.25 micron,), H (1.65 micron), and K (2.2 microns) of 16.8, 15.8, and14.7 mag, respectively. A population of about 1500 stars spreadthroughout the cloud, seven small aggregates comprised of 10-50 starswhose typical projected surface densities exceed that of the distributedpopulation by factors of 4-10, and a heretofore unknown, partiallyembedded dense cluster comprised of about 150 stars are identified.Analysis of the distribution of (J-H), (H-K), and (R-I) colors for thesegroups suggest that, in all cases, the stellar populations are dominatedby solar-type PMS stars which appear to contain a mix of objectsanalogous to weak-line T Tauri stars. The present observed luminosityfunctions are based on reddening-corrected J-band magnitude.
| The association of (C-13)O molecular gas with IRAS sources in L1641
(C-13)O observations with 2.7 arcsec angular resolution were made ofIRAS sources in L1641 to test the association of (C-13)O dense coreswith young stellar objects. The sample includes 40 IRAS sources, eightof which are associated with CO molecular outflows, and the other 32 arerandomly selected from an IRAS catalog by Strom et al. (1989). We findthat six of eight outflow sources are associated with molecular densecores, compared to only five of 32 nonoutflow sources. We also find thatthe cold IRAS sources, including seven of the eight outflow sources,tend to be more luminous and associated with higher (C-13)O columndensity than the rest. Our results suggest that the dense cores aredissipated by the high-velocity molecular outflow in the early stage ofstar formation. This is consistent with the star formation scenarioproposed by Shu et al. (1987).
| An OH mainline maser survey of IRAS circumstellar envelope sources
We present the results of a survey in the OH mainline at 1667 MHz.Because this line is generally weak in the OH/IR stars, systematicsurveys of the mainline maser have not been previously performed. Atotal of 131 sources were detected out of which 108 were previouslyunknown. The 1667 MHz profile in OH/IR stars is similar in form to thatof the 1612 MHz emission, and the relevant maser properties have beenmeasured under the assumption of the habitual radially expandingenvelope model for circumstellar envelopes. We show that the mainlinemaser is still present in circumstellar envelopes with cool (60/25)colors; its detection is, in fact, higher for such colors. Summarydetection rates are given.
| Infrared spectra and circumstellar emission of IRAS sources with ten-micron silicate absorption
Results are presented of near-infrared photometry carried out in the J,H, K, and L bands for 33 IRAS sources with a 10-micron silicateabsorption features. The observed sources were found to be mostlycomprised of two groups of stars which showed very different spectralcharacteristics from each other between 1 and 100 microns. These starswere either evolved OH/IR stars, which generally showed IR photometricspectra similar to black-body spectra; or they were young stellarobjects showing much broader spectral energy distributions thanblack-body spectra.
| A search for methanol masers at 44 GHz
Results of an extensive survey of young stellar objects in the methanolline 7(0) - 6(1)A(+) (44 GHz) are presented. Three new masers weredetected towards cold IRAS sources in the dark clouds L 291 (GGD 27), L379, and IC 1396 N. The new masers were also observed in 4(-1) - 3(0) E(36 GHz) and 1(0) - 0(0)A(+) (48 GHz) methanol transitions. Arelationship between methanol masers and high-velocity flows with densedisks around central sources is proposed, and a possible correlationbetween maser emission and their intensity in the FIR is suggested.
| (C-13)O and HCO(+) observations of IRAS sources in L1641
Twenty-eight IRAS sources selected from the IRAS co-added images inL1641 were observed in the (C-13)O (J = 1-0) and HCO(+) (J = 1-0) usingthe 45 m telescope at the Nobeyama Radio Observatory. The (C-13)O andHCO(+) column densities toward the IRAS sources correlated with eachother, and both column densities also correlated with the IRAS colordefined by the 12-25-micron flux ratio. Three outflow sources mapped inthis study showed well-defined HCO(+) cores with a size of 0.1 pc and amass of 3-30 solar masses. The most probable explanation of theseresults is that the molecular dense cores are dissipated by bothaccretion and outflow during an early stage of stellar evolution.
| Molecular outflows associated with young stellar objects in the L1641 region of Orion
The results of a follow-up survey of a flux-limited sample of IRASsources located in the L1641 region south of the Orion Nebula arepresented. The molecular outflow sources identified include allpreviously known molecular outflows and, reported for the first time,three additional molecular outflows. None of the observed molecularoutflows appear to be oriented along a preferred direction, and they arealso distributed nonuniformly throughout the L1641 region. Maps of themolecular outflows suggest that the direction of the molecular outflowmay be shaped by the ambient molecular cloud. The estimated momentum ofthe observed molecular outflows is found to be sufficient to support themolecular cloud over the dissipation time scale against gravitationalcollapse.
| Molecular outflows in the L1641 region of Orion
Searches for high-velocity C-12O emission from possiblemolecular-outflow candidates, and CS emission from dense cores in thevicinity of each IRAS source, were conducted during a mm-wave survey ofa flux-limited sample of IRAS sources within a 4 sq deg region of theL1641 portion of the Orion A molecular cloud. The C-12O line emissionwidth is noted to be correlated with the strength of the integrated CSemission. Attention was given to the IR properties of the embeddedsources; the molecular outflow candidate sources have generally redder12-25 micron colors and bluer 60-100 micron colors than the nonmolecularoutflow candidates.
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