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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 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).
| CO observations of Southern molecular clouds. Outflows from young stellar objects GRV 8 and GRV 16
12CO (1-0) observations of two Southern dark clouds (globules)associated with cometary nebulae GRV 8 (a biconical nebula) and GRV 16(a conelike nebula) are presented. GRV 8 shows an outflow from thecentral part of the nebula (where in 2MASS images a star is located,which is perhaps responsible for this outflow); however, both lobes ofthe outflow are redshifted with a velocity of +1.95 km/s with respect tothe molecular cloud. The two opposite redshifted lobes are a rather rarephenomenon that could be explained by the presence of a double starinstead of a single one as the engine responsible for the outflow. Thetwo lobes are almost parallel to the axis of symmetry of the biconicalnebula. In the case of the conelike nebula GRV 16 we observe a bipolaroutflow, where the eastern blueshifted lobe has a velocity of -4 km/swith respect to the molecular cloud, and the western redshifted one hasa velocity +2.5 km/s. The outflow has a direction almost coinciding withthe axis of symmetry of the conelike nebula. The star associated withthe conelike nebula is responsible for this outflow.
| A Study of the Star-Forming Group RNO 129
We have studied the compact star-forming group RNO 129 in the cloudL1228. New images and both slit and integrated spectroscopy for severalnebulas and HH objects are presented. We have detected several new HHobjects, including two blobs embedded in a bright reflection nebula. Thecentral star of this nebula is binary and is ejecting at least onecollimated jet. Some peculiarities in the spectrum of the nebulaindicate its similarity with NGC 2261.
| A study of high velocity molecular outflows with an up-to-date sample
A statistical study of the properties of molecular outflows is performedbased on an up-to-date sample. 391 outflows were identified in publishedarticles or preprints before February 28, 2003. The parameters ofposition, morphology, mass, energy, outflow dynamics and central sourceluminosity are presented for each outflow source. Outflow lobe polarityis known for all the sources, and 84% are found to be bipolar. Thesources are divided into low mass and high mass groups according toeither the available bolometric luminosity of the central source or theoutflow mass. The pace of discovery of outflows over the past sevenyears has increased much more rapidly than in previous periods. Surveysfor outflows are still continuing. The number of high-mass outflowsdetected (139) has considerably increased, showing that they arecommonly associated with massive as well as low mass stars. Energeticmass ejection may be a common aspect of the formation of high mass aswell as low mass stars. Outflow masses are correlated strongly withbolometric luminosity of the center sources, which was obtained for thefirst time. There are also correlations between the central sourceluminosity and the parameters of mechanical luminosity and the thrust orforce necessary to drive the outflow. The results show that flow mass,momentum and energy depend on the nature of the central source. Despitetheir similarity, there are differences between the high mass and lowmass outflows. Low mass outflows are more collimated than high massoutflows. On average, the mass of high mass sources can be more than twoorders of magnitude larger than those of low mass outflows. The relationbetween flow mass and dynamical time appears to differ for the two typesof outflows. Low mass sources make up 90% of outflows associated with HHobjects while high mass outflows make up 61% of the sources associatedwith H_2O masers. Sources with characteristics of collapse or infallcomprise 12% of the entire outflow sample. The spatial distribution ofthe outflow sources in the Galaxy is presented and the local occurrencerate is compared with the stellar birth rate.Tables 1a and 1b are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/426/503
| 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.
| Various manifestations of the outflow connected to the cometary nebula GM 3-12 (RNO 124)
We present the results of imaging and scanning Fabry-Pérotinterferometry of the cometary nebula GM 3-12 (RNO 124) and Herbig-Haroobject HH 376A. The nebula is cone-shaped with the star IRAS 20359+6745at the apex, straight walls and two probable helical arms. HH 376A isthe bow shock structure in the collimated high velocity flow, directedby the axis of the cone nebula, with shock velocity about 70-80 kms-1. The spatial and kinematical separation between bow shockand Mach disk in HH 376A was not detected, which makes this objectsimilar to HH 111. The Hα profiles in the vicinity of the sourceare double peaked. The high velocity component can be attributed to thecollimated flow.Based on the observations collected with the 2.6 m telescope of theByurakan Astrophysical Observatory (BAO) of the National Academy ofSciences of Armenia.
| Diffraction-limited Bispectrum Speckle Interferometry
Not Available
| 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
| Diffraction-limited bispectrum speckle interferometry of the Herbig Be star R Mon
We explore the structures immediately surrounding the intermediate-massyoung stellar object R Mon with bispectrum speckle interferometry,conventional near-infrared imaging and by analyzing optical HST archivedata. Our near-infrared speckle images with unprecedenteddiffraction-limited resolution of 55 mas ( ~ 44 AU; H-band) and 76 mas (~ 61 AU; K-band) represent the highest resolution R Mon images obtainedso far and exhibit previously unseen complex structures. While thebinary companion R Mon B appears as an unresolved point source in ourspeckle images, the image of the primary R Mon A is marginally extendedin the K-band and significantly extended in the H-band. The mostprominent new feature is a bright arc-shaped structure, pointing awayfrom R Mon in north-western direction. We interpret this feature as thesurface of a dense structure near the thick circumstellar disk or torusaround R Mon. Our images also reveal several twisted filaments ofhelical shape which are similar to the twisted filaments in the outerparts of the nebula. We identify structures which probably areresponsible for casting pronounced shadows in the outer regions of theNGC 2261 reflection nebula. Finally, we discuss the relation of theobserved features, in particular the arc-shaped speckle feature, to thewind and outflow activity (Herbig-Haro objects and jets) of R Mon. Partof the results presented in this paper are based on observationsobtained at the German-Spanish Astronomical Centre, Calar Alto, operatedby the Max-Planck-Institute for Astronomy, Heidelberg, jointly with theSpanish National Commission for Astronomy. The speckle data werecollected at the Special Astrophysical Observatory with the 6 mtelescope.
| A twisted jet from R Mon
We present the results of long-slit and integral-field spectroscopy ofthe R Mon Herbig-Haro jet. The data were obtained with the 6 mtelescope. A slightly curved jet of 16\arcsec in length is evident in [SIi] lines. Maps of radial velocities and electron densities are shown.The difference between radial velocities on the sides of the jet as wellas the density distribution can be considered as an indication ofpresence of the helical shock structure in the jet. This structure,having a DNA-like appearance, abruptly changes direction near the edges,which could account for the observed velocity variations. Radialvelocities were determined also for the several knots in the HH 39group. The kinematics of the system as a whole also suggests theprecession of the outflow. Based on observations collected with the 6 mtelescope of the Special Astrophysical Observatory (SAO) of the RussianAcademy of Sciences (RAS) which is operated under the financial supportof Science Department of Russia (registration number 01-43).
| A global jet/circulation model for young stars
Powerful, highly collimated jets, surrounded by bipolar molecularoutflows, are commonly observed near Young Stellar Objects (YSOs). Inthe usual theoretical picture of star formation, a jet is ejected from amagnetized accretion disk, with a molecular outflow being driven eitherby the jet or by a wider wind coming from the disk. Here, we propose analternative global model for the flows surrounding YSOs. In addition toa central accretion-ejection engine driving the jet, the molecularoutflow is powered by the infalling matter and follows a circulationpattern around the central object without necessarily being entrained bya jet. It is shown that the model produces a heated pressure-drivenoutflow with magneto-centrifugal acceleration and collimation. We reportsolutions for the three different parts of this self-similar model, i.e.the jet, the infalling envelope and the circulating matter thateventually forms the molecular outflow. This new picture of theaccretion/outflow phase provides a possible explanation for severalobserved properties of YSO outflows. The most relevant ones are thepresence of high mass molecular outflows around massive protostars, anda realistic fraction (typically 0.1) of the accretion flow that goesinto the jet.
| Multiple scattering of polarized radiation by non-spherical grains: First results
We present the first numerical radiative transfer simulation of multiplelight scattering in dust configurations containing aligned non-spherical(spheroidal) dust grains. Such models are especially important if onewants to explain the circular polarization of light, observed in avariety of astronomical objects. The optical properties of thespheroidal grains are calculated using the method of separation ofvariables developed by Voshchinnikov & Farafonov (\cite{vf93}). Theradiative transfer problem is solved on the basis of the Monte Carlomethod. Test simulations, confirming the correct numericalimplementation of the scattering mechanism, are presented. As a firstapplication, we investigate the linear and circular polarization oflight coming from a spherical circumstellar shell. This shell containsperfectly aligned prolate or oblate spheroidal grains. We investigatethe dependence of the results on the grain parameters (equivolumeradius, aspect ratio) and the shell parameters (inner/outer radius,optical thickness). The most remarkable features of the simulated linearpolarization maps are so-called polarization null points where thereversal of polarization occurs. They appear in the case when the grainalignment axis is perpendicular to the line of sight. The position ofthese points may be used for the estimation of grain shape andgeometrical structure of the shell. The origin of null points lies inthe physics of light scattering by non-spherical particles and is notrelated to the cancellation of polarization as was discussed in previousmodels. The maps of circular polarization have a sector-like structurewith maxima at the ends of lines inclined to the grain alignment axis by+/- 45degr .
| Diffraction-limited bispectrum speckle interferometry of the Herbig Be star R Mon.
Not Available
| Spectra of PV Cep and GM1-29 (RNO 125) in 1976-1997
The main spectral characteristics of PV Cep, a T Tau star, and of thevariable nebula GM1-29 associated with it are considered. Theirvariations during 1976-1997, based on observations on the 6-m telescopeand literature data, are described. In the period of maximum brightness(1977-1979), the star had a well-developed emission spectrum, typical ofclassical T Tau stars, with clearly defined P Cyg components in the Hand H lines, strong Na D absorption features shifted in the bluedirection, and well-defined fluorescence in Fe I emission features.After the end of the latter eruption, the star's spectrum has remainedmore or less in a stable state; emission lines are considerably weaker.The ejected matter continues to be observed in the form of shiftedcomponents in emission lines. There are also indications of the possibleexistence of a collimated jet 1.5-2 long in the star's immediatevicinity. Also clearly seen is HH215 P1 — the first HH knot in thegiant jet emerging from PV Cep. At the epoch of the object's maximumbrightness, there were clear signs of spectral asymmetry of the stellarenvelope in the nebula's spectrum, due to the directional outflow,manifested in variation of the profiles and intensities of Balmer lines.PV Cep and GM1-29 thereby prove to be another object in which thephenomenon first noted in R Mon and NGC 2261 is observed.
| Polarization models of young stellar objects - II. Linear and circular polarimetry of R Coronae Australis
Near-infrared linear imaging polarimetry of the young stellar objects RCrA and T CrA in the J, H and Kn bands, and circular imaging polarimetryin the H band, is presented. The data are modelled with the Clark andMcCall scattering model. The R CrA and T CrA system is shown to be aparticularly complex scattering environment. In the case of R CrA thereis evidence that the wavelength dependence of polarization changesacross the nebula. MRN dust grain models do not explain this behaviour.Depolarization by line emission is considered as an alternativeexplanation. The dust grain properties could also be changing across thenebula. Although surrounded by reflection nebulosity, there is a regionof particularly low polarization surrounding R CrA that is best modelledby the canonical bipolar outflow being truncated by an evacuatedspherical cavity surrounding the star. The symmetry axis of the nebulaappears inclined by 50° to the plane of the sky. The H-band circularpolarimetry of R CrA clearly shows a quadrupolar structure of positiveand negative degrees of circular polarization that reach peak magnitudesof ~5per cent within our limited map. It is shown that spherical MRNgrains are incapable of producing this circular polarization given theobserved linear polarization of the R CrA system. Instead, scatteringfrom aligned non-spherical grains is proposed as the operatingmechanism. T CrA is a more archetypical bipolar reflection nebula, andthis object is modelled as a canonical parabolic reflection nebula thatlies in the plane of the sky. The wavelength independence of linearpolarization in the T CrA reflection nebula suggests that the scatteringparticles are Rayleigh sized. This is modelled with the MRN interstellargrain size distribution.
| Disintegrating Multiple Systems in Early Stellar Evolution
An analysis of the multiplicity of 14 sources driving giant Herbig-Haroflows has revealed an observed binary frequency between 79% and 86%, ofwhich half are higher order multiples. These sources represent thehitherto youngest sample of stars examined for binarity. I postulatethat the dynamical decay of triple or multiple systems leads to strongoutflow activity. It is well known that a large fraction ofnonhierarchical triple systems rapidly break up and eject the lightestmember. At the same time a closer binary in a highly eccentric orbit isformed. Massive disk truncation results, accompanied by large-scaleaccretion, with a consequent burst of outflow activity, which producesthe observed giant HH bow shocks. Some of the material culled from theindividual circumstellar disks may settle into a circumbinary diskaround the newly bound stellar pair. The small remaining and truncatedcircumstellar disks are fed from the circumbinary disk through gasstreams, and this as well as other dynamical effects cause the binaryorbit to shrink. Gas streams together with disk interactions atperiastron drive cyclic accretion modulated on an orbital timescale. Asthe stellar components gradually spiral toward each other, theincreasingly frequent mass-loss events form chains of HH objects untileventually the binary has a semimajor axis of only 9-12 AU, at whichpoint the closely spaced shocked ejecta appear as a finely collimatedjet. Thus, such HH flows can be read as a fossil record of the evolutionof orbital motions of a binary, newly formed in a triple disintegrationevent, as it shrinks from a typical separation of 100 AU or more to 10AU or less. When the triple system disintegrates and a single star isejected, the newly formed binary recoils, and as a result bothcomponents (star and close binary) leave their nascent envelope. Whileone component becomes visible as a T Tauri star, the other will beobscured for a while by the envelope and will appear as a brightnear-infrared object. For typical parameters, this geometry persists foronly 5000 yr or so. If the ejected star does not escape, cyclic motionof a hierarchical triple begins. This explains the so-called IRCbinaries that are infrequently found in star-forming regions. Thestandard model of early stellar evolution states that young starsgradually and smoothly make the transitions from Class 0 through Class Iand II objects to eventually become Class III objects. In contrast,stars born in multiple systems can abruptly transit from a Class 0 or Iobject to a visible T Tauri star. The main accretion phase may beterminated by the stochastic process of triple decay. Depending on themoment of triple disintegration, the ejected objects can range fromstellar embryos, which will emerge as very low mass stars or even browndwarfs, to essentially fully built-up stars. In this picture, theinitial mass function toward its low-mass end has an importantstochastic component that can only be described by the half-life of thedecay processes. Because the ejected stars can take only limitedcircumstellar material with them, they will soon lose their classical TTauri characteristics and join the halo of weak-line T Tauri stars thatsurround star-forming clouds. Differences in ejection may explain whytwo apparently similar T Tauri stars of about the same age can havemajor differences in the size of their circumstellar disks.
| The Director's Choice: Mellish, Hubble and the discovery of the variable nebula
In the summer of 1915, amateur astronomer John Edward Mellish joined thestaff of Yerkes Observatory as an unpaid observer. Soon after arriving,Mellish discovered what he thought was a comet in the dawn twilight.Yerkes director Edwin Brant Frost promptly notified Harvard Observatoryof the discovery only to learn later in the day, too late to preventdistribution of an international telegram, that the object Mellishobserved was actually the diffuse nebula NGC 2261. Edwin Powell Hubble,a graduate student in his first year at Yerkes, was assigned the task ofdetermining whether, as Mellish insisted, the nebula had changed. Thisled to Hubble's first professional papers and his initial fame as thediscoverer of `Hubble's Variable Nebula.' Frost's choice, assigning theinvestigation to Hubble rather than Mellish, reflected his irritationwith Mellish over matters that went well beyond the mistaken cometdiscovery. When Mellish discovered another comet a few weeks later,Frost delayed his notification to Harvard for several days to allowphotographic confirmation of the discovery by George Van Biesbroeck,another newcomer at Yerkes. These events highlight staffing problems atYerkes in 1915, problems that were common to other Americanobservatories. Mellish and Van Biesbroeck were likely the last twoamateur astronomers to have an opportunity to `try out' as professionalsat Yerkes. By 1915 a stronger requirement for educational credentialswas emerging in the astronomical community. On the other hand, likeother observatory directors, Frost was experiencing considerabledifficulty employing graduate astronomers. With S. W. Burnham alreadyretired, Frost adopted stopgap measures for staffing as E. E. Barnardand others from an earlier generation prepared for retirement. Theassignment of the nebula investigation to Hubble indicates that Frosthad likely already concluded that Mellish would not be an acceptablesubstitute for a degreed professional.
| A High-Resolution Polarimetry Map of the Circumbinary Disk around UY Aurigae
We have obtained J-band (1.2 μm) polarimetry observations of thecircumbinary disk around UY Aurigae. These observations were madepossible by the use of the University of Hawaii 36 element adaptiveoptics instrument, Hokupa'a, at the 3.35 m CFHT. The deep (120 minute),high-resolution (0.15") polarization images reveal a centrosymmetricpolarization signature from the light scattered off the circumbinarydust disk which is ~106 times fainter than the stars in thebinary system. A comparison with a Mie scattering model of thecircumbinary disk in UY Aurigae suggests that the polarization signatureis dominated by the smallest grains in the disk (~0.03 μm) andfurther supports the hypothesis that the resolved light seen in theoptical and infrared originates from a large flattened disk of dustsurrounding both stars.
| Dust emission from star-forming regions. VI. The submillimetre YSO cluster in NGC 2264
We present new submillimetre and millimetre continuum observations ofNGC 2264, in the region of Allen's infra-red source IRS1, at 350, 450,800 and 1300 μm taken with the 15-m JCMT and the IRAM 30-m MRT. Weobserve a ridge of bright submillimetre and millimetre emission which wedesignate NGC 2264 MM. Within this ridge we have discovered a cluster offive submillimetre sources which we label NGC 2264 MMS1-5. The masses ofthe sources lie in the range ~ 10-50M_sun and each source has a FWHM of<=0.1pc. The total mass of the ridge is ~ 730M_sun. We identify MMS3as harbouring the driving source of a previously discovered bipolaroutflow. We note that MMS5 is positionally coincident with a previouslydiscovered HST near-ir source, but we conclude that this is a chancealignment, since typical extinction values within the ridge lie in therange A_V ~ 200-1600. The cluster which we have discovered is of highermass and extends over a larger area than the low-mass sub-clusterdiscovered by the HST surrounding IRS1. Our continuum maps show agreater dynamic range than molecular spectral line maps, due to gasfreezing out onto dust grains at the densities of these sources. Wesuggest that each of the sources MMS1-5 is forming one or moreintermediate or high mass stars.
| Density and Excitation Mapping of M2-9
We have acquired imaging of the bipolar outflow M2-9 in the transitions[O III] λ5007, H I λ6563, [N II] λ6584, and [S II]λλ6717, 6731. As a result, we are able to map bothexcitation and density over the face of the nebula. We find thatunderlying densities in the lobes increase tone~1.6x103 cm-3 at distancesrp~12"-15" from the central star, after which they againdecrease to ne<200 cm-3 forrp>27". Superposed upon this are various density knots,associated with the visual condensations, in which peak values ofne are of order 6.3x103 cm-3. Theseappear to be responsible for low-excitation shadowing of the outerenvelope. All of these features (the density peaks, and broad underlyingdensity variation) are shown to be consistent with a particular model ofshock excitation, in which an interior wind is shock-interacting with anexternal neutral envelope. Such a model appears also to be consistentwith trends in [O III]/Hα and [S II]/Hα, which (for thenorthern lobe at least) appear most consistent with shock excitation.
| Acquisition and analysis of adaptive optics imaging polarimetry data
The process of data taking, reduction and calibration of near-infraredimaging polarimetry data taken with the ESO Adaptive Optics SystemADONIS is described. The ADONIS polarimetric facility is provided by arotating wire grid polarizer. Images were taken at increments of22.5(deg) of polarizer rotation from 0 to 180(deg) , over-sampling thepolarization curve but allowing the effects of photometric variations tobe assessed. Several strategies to remove the detector signature aredescribed. The instrumental polarization was determined, by observationsof stars of negligible polarization, to be 1.7% at J, H and K bands. Thelack of availability of unpolarized standard stars in the IR, inparticular which are not too bright as to saturate current IR detectors,is highlighted. The process of making polarization maps is described.Experiments at restoring polarimetry data, in order to reach diffractionlimited polarization, are outlined, with particular reference to data onthe Homunculus reflection nebula around eta Carinae.
| Polarimetry and magnetic field structure of the Herbig-Haro objects HH1 and HH2 and their environment
We present continuum and Hα imaging polarimetry of the Herbig-Haroobjects HH1 and HH2, the faint nebulosity which surrounds them, andnearby stars. We also discuss IRAS observations of the area. We findevidence for a strongly dichroic interstellar medium throughout thisregion, which polarizes local Hα emission and severely distortsthe underlying polarization pattern arising in light scattered from thecentral object VLA1. It also polarizes nearby stars. This dichroism canbe explained by alignment of dust in a magnetic field which, on largescales, is aligned at PA = 126 deg but, on smaller scales, is distortedwithin the HH1-HH2 system. Its geometry appears consistent with modelsof star formation in which a large-scale rotating disc drives an outflowvia twisted axial field lines. Both HH1 and HH2 are also associated withsmall-scale polarization patterns. These can best be understood asresulting from internal dichroic extinction arising in a concentrationof well-aligned dust grains lying within the shocked Hα-emittingregions of these objects. The polarization of HH1 exhibits spiralstructure, which may offer support to magnetized jet models, but thepolarization of HH2 is more disordered. Our results suggest that furtheremission-line polarimetry may offer new insights into the structure ofHH objects.
| A Variable Asymmetry in the Circumstellar Disk of HH 30
We report Hubble Space Telescope observations of variability within thereflection nebulosity of HH 30, a compact bipolar nebula that is anearly edge-on accretion disk system. A dramatic lateral asymmetryappeared in the upper reflection nebula in the spring of 1998, but waslargely absent in 1994 and 1995 images. The variability timescale ismuch shorter than disk dynamical timescales at the projected radius ofthe asymmetry, which indicates that its origin is a variableillumination pattern projected onto the outer disk by changes in theinner disk or the central star. Orbital motion of coherent clumps orvoids in the inner disk at radii of a few AU might produce such aneffect. Another possibility recently proposed is accretion hot spotsnear the star's magnetic poles which produce broad beams of lightsweeping across the disk as the star rotates. Simulated images of a diskilluminated by such a central ``lighthouse'' are a reasonable match tothe bright lateral asymmetry in the upper nebula of HH 30. However, amodel with identical opposed hot spots is excluded by the absence of aprominent asymmetry in the lower reflection nebula. Further temporalmonitoring of the system is needed to better characterize the nebularvariability and establish its physical origin.
| Simultaneous Polarimetry and Photometry of the Young Stellar Object R Monocerotis
Linear polarization and flux of the young stellar object R Mon wereobserved simultaneously at seven bands in the optical wavelength from0.36 to 0.76 μm in 1991-1997. During the observational period, the Vmagnitude of R Mon changed by about 0.7 mag in a diaphragm of 18". Wehave found a strong positive correlation between the degree of linearpolarization p and the V magnitude and have found that the coefficientof correlation is 0.92 for the data in 1993-1997. The values of p and Vin 1991-1992 did not follow this correlation. The colors B-V, V-R_C,V-R_J, and R_C-I_C are also found to increase with the V magnitudeduring the whole observational period. However, in 1991 when the objectwas in the faintest phase, only the color U-B decreased. Thesecorrelations are similar to those observed in other Herbig Ae/Be stars.The position angle theta of the linear polarization also showssignificant variation with time, though the correlation between thetaand other quantities is weak. The observed correlations betweenmagnitude, colors, and polarization degree can be explained by thecombination of extinction by the clouds orbiting around R Mon andscattering by diffusely surrounding medium, as originally proposed byGrinin for Herbig Ae/Be stars. Since the timescale of variation is short(~10 days), the orbiting clouds should be in the vicinity, that is,<~10 AU, of the star. The extinction by those clouds less depends onwavelength than that observed in the diffuse interstellar medium. Thesize of grains in the clouds may be larger than those in the diffuseinterstellar clouds. Our Mie calculations show that the radius of grainsmay be in the range from 0.08 to 0.50 mum. Alternatively, if the cloudsare dense and opaque, ``gray'' extinction may occur and explain theobservation.
| L43: the late stages of a molecular outflow
Our new 21-arcsec resolution CO J=2-->1 map of the L43 dark cloudshows a poorly collimated molecular outflow, with little evidence forwings at velocities10 km s^-1. The outflow appears not to be currentlydriven by a jet: its structure can instead be modelled as a slowlyexpanding shell. The shell may be compressed either by a wide-angledwind catching up with an existing shell (as in the case of planetarynebulae), or by the thermal pressure of a hot low-emissivity mediuminterior to the shell. The outflow is most probably in a late stage ofevolution, and appears to be in the process of blowing away itsmolecular cloud. We also present a 45-arcsec resolution CO J=1-->0map of the whole molecular cloud, showing that the outflow structure isclearly visible even in the integrated intensity of this low excitationline, and suggesting that rapid mapping may prove useful as a way offinding regions of outflow activity. We also examine the immediatesurroundings of the driving source with 450 μm imaging: this confirmsthat the outflow has already evacuated a bay in the vicinity of theyoung stellar object.
| Adaptive Optics Infrared Imaging Polarimetry and Optical HST Imaging of Hubble's Variable Nebula (R Monocerotis/NGC 2261): A Close Look at a Very Young Active Herbig Ae/Be Star
We present high-resolution (FWHM = 0."2) near-IR (J, H, and K') adaptiveoptics images of the Herbig Ae/Be star R Monocerotis. Optical HubbleSpace Telescope (HST) WFPC2 PC camera archival images are alsopresented. For the first time, adaptive optics was utilized to makehigh-resolution (FWHM = 0."2) IR-imaging polarimetry maps of R Mon. Inaddition, the first mid-IR array images (at 11.7 and 20.8 mu m) of R Monhave been obtained. We also present new 3.16, 3.93, and 4.67 mu mimages. We have found that R Mon is a 0."69 binary star with a companionthat dereddens onto the classical T Tauri locus. Based on thenear-infrared photometry of this companion we believe it is a 1.5Mȯ, very young (<3 x 105 yr) classical T Tauri star. The closepresence of a young companion suggests that R Mon itself is a rareexample of a very young isolated massive star. At the highestresolutions, R Mon is revealed to be extended by ~0."1 east-west, and~0."05 north-south in the visible. The young R Mon star is not directlyvisible in the optical but appears as a resolved conical reflectionnebula in scattered light. At infrared wavelengths, the densecircumstellar dust is penetrated and R Mon appears to be an unresolvedpoint source located at 0."06 +/- 0."02 south of the peak optical flux.The large-scale optical-IR morphology of R Mon and its large reflectionnebula (NGC 2261) suggests a thin bipolar parabolic shell of dust. Theappearance of the parabolic shell is consistent with an inclination of20 deg +/- 10 deg from the plane of the sky. This inclination impliesthat R Mon is located 760^{+800}_{-280} pc distant based on previousproper-motion and radial velocity measurements of R Mon's jet. Ourhigh-resolution (FWHM ~ 0."2) adaptive optics infrared polarimetry mapsagree with the current interpretation that NGC 2261 is a reflectionnebula illuminated by R Mon. Interior to the parabolic shell there is acomplex of twisted filaments along the eastern edge. These filamentsresemble a double-helical structure which is well described by a powerlaw from ~103 to 105 AU from R Mon. This double helix may trace atwisted magnetic field above R Mon. Based on H I emission-line ratios,we find the direct extinction toward R Mon to be AV = 13.1 mag in theinfrared ( lambda > 1.28 mu m), falling to a lower value of AV = 3.6mag in the optical ( lambda < 1.28 mu m), where scattered lightincreasingly lowers the effective extinction in the line ratios. Thelarge AV = 13.1 extinction is likely due to the dusty atmosphere of aninclined R ~ 100 AU optically thick accretion disk surrounding R Mon. Asimple model of such an accretion disk + star system (with Macc ~ 8 x10-5 Mȯ yr-1, M* = 10.4 Mȯ, R* = 2 Rȯ, and T* ~ 3.5 x 104K) reproduces the observed dereddened R Mon spectral energy distribution(SED) from the optical (0.4 mu m) to the millimeter region.Consideration of the lower extinction (AV = 3.6) on the path followed bythe scattered visible light eliminated any need for an inner "gap" inthe accretion disk model to reproduce the SED. In general, young stellarobjects (YSOs) that are obscured in the optical but directly visible inthe infrared will have different effective optical and infraredextinctions. Infrared extinctions derived from optical observationsdominated by scattered light will be underestimates of the true IRextinction along the direct path. The use of an independent estimator ofboth the optical and infrared extinctions such as common upper-level H Irecombination lines is highly desirable. The utilization of the correctoptical and infrared extinctions may relieve the need for optically thininner-disk gaps to explain YSO near-IR SEDs.
| Photodissociated H I in NGC 2023
We report the detection of photodissociated H I toward the reflectionnebula NGC 2023. The Very Large Array observations reveal compact (~ 0.3pc) emission associated with the source. The observed spectrum has adouble-peaked profile that we believe is caused by absorbing foregroundneutral gas. We corrected the observed spectrum for the absorption, anddetermined with this absorption-corrected profile an H I column densityand mass of 1.7 ×1021 cm-2 and 1.2Modot, respectively. This column density agrees well withtheoretical model predictions. The H I 21 cm map shows a morphology verysimilar to that present in the images of [C II] at 158 μm, HIRES 60μm IRAS, and POSS optical.
| High resolution imaging and spectroscopy of the Serpens reflection nebula (SRN). Evidence of a latitude-dependent wind.
An optical study (high resolution images and long-slit spectra in theHα range) of the Serpens reflection nebula (SRN) is presented. TheSRN is a bipolar nebula illuminated by the low mass pre-main-sequence(PMS) star Serpens/SVS 2. The Hα profile of Serpens/SVS 2 is shownto be very broad (the width at a 10% of the peak intensity is13.1Å (~600km/s)). The profile has three emission peaks centeredat -137, 5 and 100km/s. The relative strength of the peaks varies withthe slit orientation. The blue and redshifted components have similarintensities at low inclinations (5 and 0) while at high inclinations theblueshifted component is weaker than the redshifted suggesting asignificant contribution of absorption by low latitude outflowing gas.These profiles could be produced in a rotating, latitude dependent windwith the outflow axis parallel to the disk axis. The nebular Hαprofile is double peaked; it has a blue and a redshifted component atthe same velocities as the star. The profile shows no significantvariations along a given PA; the emission is best explained by singlescattering of the stellar radiation. The absence of the 0-velocityemission component is suggestive of the presence of warm absorbing gaswithin a few stellar radii. There are several knots of gas and dustembedded within the north-western (NW) nebular lobe. These knots areconnected by a faint emission, defining a helical path around the majoraxis of the nebula. This area is also characterized by an unusuallylarge polarization that reaches a 30-40% at 0.95μm that is consistentwith a concentration of large and reflecting dust grains that arepresumably ice coated carbon grains. This region coincides with a ridgeof hot (T=~35K) dust detected by IRAS along the major axis of the SRN.
| Variability and a vanishing YSO in the Serpens cloud core.
This letter compares data from a recent near-IR study of the Serpenscloud core with data from previously published studies. It is found thatone object, irs 81 has completely vanished, fading by more than 3.8magnitudes at K, while a total of 10 of the observed objects have variedby over 0.5 magnitudes in H or K, with little change in colour. We havealso discovered 13 new objects in the cloud core which were below themagnitude limit of previous studies.
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