Садржај
Слике
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
Mid- and Far-Infrared Photometry of Galactic Planetary Nebulae with the AKARI All-Sky Survey
We provide mid- and far-infrared photometry of 857 Galactic planetarynebulae (PNe) using data derived from the AKARI All-Sky Survey. Theseinclude fluxes at 9 and 18 ?m obtained with the Infrared Camera(IRC), and at 65, 90, 140 and 160 ?m using the far-Infrared Surveyor(FIS). It is noted that the IR luminosities of the youngest PNe arecomparable to the total luminosities of the central stars, andsubsequently decline to 5×102 L&sun; whereD > 0.08 pc. This is consistent with an evolution of PNe dustopacities, and appreciable absorption in young and proto-PNe. We alsonote that there is little evidence for the evolution in IR/radio fluxratios suggested by previous authors. The fall-off of dust temperatureswith increasing nebular diameter is similar to that determined inprevious studies, whilst levels of Ly? heating are <0.5 of thetotal energy budget of the grains. There appears to be an evolution inthe infrared excess (IRE) as nebulae expand, with the largest valuesoccurring in the most compact PNe.
| From pre- to young planetary nebulae: radio continuum variability
Searching for variability, we have observed a sample of hotpost-asymptotic giant branch stars and young planetary nebula candidateswith the Very Large Array at 4.8, 8.4 and 22.4 GHz. The sources had beenpreviously detected in the radio continuum, which is a proof that thecentral stars have started ionizing their circumstellar envelopes, andan increase in radio flux with time can be expected as a result of theprogression of the ionization front. Such a behaviour has been found inIRAS 18062+2410, whose radio modelling has allowed us to determine thatits ionized mass has increased from 10-4 to 3.3 ×10-4 M&sun; in 8 yr and its envelope has becomeoptically thin at lower frequencies.Different temporal behaviours have been found for three other sources.IRAS 17423-1755 has shown a possibly periodic pattern and an inversionof its radio spectral index, as expected from a varying stellar wind. Weestimate that the radio flux arises from a very compact region aroundthe central star (˜1015 cm) with an electron density of2 × 106 cm-3. IRAS 22568+6141 and IRAS17516-2525 have decreased their radio flux densities by about 10 percent per year over 4 yr.While a linear increase in the flux density with time points to theprogression of the ionization front in the envelope, decreases as wellas quasi-periodic patterns may indicate the presence of unstable stellarwinds/jets or thick dusty envelopes absorbing ionizing photons.
| Central stars of planetary nebulae: New spectral classifications and catalogue
Context. There are more than 3000 confirmed and probable known Galacticplanetary nebulae (PNe), but central star spectroscopic information isavailable for only 13% of them. Aims: We undertook aspectroscopic survey of central stars of PNe at low resolution andcompiled a large list of central stars for which information wasdispersed in the literature. Methods: We observed 45 PNs usingthe 2.15 m telescope at Casleo, Argentina. Results: We present acatalogue of 492 confirmed and probable CSPN and provide a preliminaryspectral classification for 45 central star of PNe. This revisesprevious values of the proportion of CSPN with atmospheres poor inhydrogen in at least 30% of cases and provide statistical informationthat allows us to infer the origin of H-poor stars.Based on data collected at the Complejo Astronómico El Leoncito(CASLEO), which is operated under agreement between the Consejo Nacionalde Investigaciones Científicas y Técnicas de laRepública Argentina y Universidades Nacionales de La Plata,Córdoba y San Juan, Argentina.
| Spitzer Detection of Polycyclic Aromatic Hydrocarbons and Silicate Features in Post-AGB Stars and Young Planetary Nebulae
We have observed a small sample of hot post-asymptotic giant branch(AGB) stars with the Infrared Array Camera (IRAC) and the InfraredSpectrograph (IRS) on board the Spitzer Space Telescope. The stars wereselected from the literature on the basis of their far-infrared (IR)excess (i.e., post-AGB candidates) and B spectral type (i.e., close tothe ionization of the envelope). The combination of our IRACobservations with Two Micron All Sky Survey and IRAS catalog data, alongwith previous radio observations in the cm range (where available)allowed us to model the spectral energy distributions of our targets andfind that in almost all of them at least two shells of dust at differenttemperatures must be present, the hot dust component ranging up to103 K. In several targets, grains larger than 1 μm areneeded to match the far-IR data points. In particular, in IRAS17423-1755 grains up to 100 μm must be introduced to match theemission in the millimeter range. We obtained IRS spectra to identifythe chemistry of the envelopes and found that more than one-third of thesources in our sample have mixed chemistry, showing both mid-IR bandsattributed to polycyclic aromatic hydrocarbons (PAHs) and silicatefeatures. The analysis of the PAH features indicates that thesemolecules are located in the outflows, far away from the central stars.We consider the larger than expected percentage of mixed-chemistrytargets as a selection bias toward stars with a disk or torus aroundthem. Our results strengthen the current picture of mixed chemistrybeing due to the spatial segregation of different dust populations inthe envelopes.
| The Origin and Shaping of Planetary Nebulae: Putting the Binary Hypothesis to the Test
Planetary nebulae (PNs) are circumstellar gas ejected during an intensemass-losing phase in the lives of asymptotic giant branch stars. PNshave a stunning variety of shapes, most of which are not sphericallysymmetric. The debate over what makes and shapes the circumstellar gasof these evolved, intermediate mass stars has raged for two decades.Today the community is reaching a consensus that single stars cannottrivially manufacture PNs and impart to them nonspherical shapes andthat a binary companion, possibly even a substellar one, might be neededin a majority of cases. This theoretical conjecture has, however, notbeen tested observationally. In this review we discuss the problem fromboth the theoretical and observational standpoints, explaining theobstacles that stand in the way of a clean observational test and waysto ameliorate the situation. We also discuss indirect tests of thishypothesis and its implications for stellar and galactic astrophysics.
| The Magellanic Cloud Calibration of the Galactic Planetary Nebula Distance Scale
Galactic planetary nebula (PN) distances are derived, except in a smallnumber of cases, through the calibration of statistical properties ofPNs. Such calibrations are limited by the accuracy of individual PNdistances, which are obtained with several nonhomogeneous methods, eachcarrying its own set of liabilities. In this paper we use the physicalproperties of the PNs in the Magellanic Clouds and their accuratelyknown distances to recalibrate the Shklovsky/Daub distance technique.Our new calibration is very similar (within 1%) to the commonly useddistance scale by Cahn et al., although there are important differences.We find that neither distance scale works well for PNs with classic(``butterfly'') bipolar morphology, and while the radiation-bounded PNsequences in both the Galactic and the Magellanic Cloud calibration havesimilar slopes, the transition from optically thick to optically thinappears to occur at higher surface brightness and smaller size than inthat adopted by Cahn et al. The dispersion in the determination of thescale factor suggests that PN distances derived by this method areuncertain by at least 30%, and that this dispersion cannot be reducedsignificantly by using better calibrators. We present a catalog ofGalactic PN distances using our recalibration, which can be used forfuture applications, and compare the best individual Galactic PNdistances to our new and several other distance scales, both in theliterature and newly recalibrated by us, finding that our scale is themost reliable to date.
| Variability and rapid evolution of the protoplanetary object IRAS 18062+2410=V886 her
We present our long-term photometric and spectroscopic observations of ahigh-latitude B supergiant with an infrared excess—theprotoplanetary nebula IRAS 18062+2410. Our U BV observations in 20002006 have confirmed the rapid irregular photometric variability of thestar with a maximum amplitude as high as 0{./ m }4in V that we found previously. The B— V and U— B colorindices vary with amplitudes as high as 0{./ m }10and 0{./ m }25, respectively, and show no clearcorrelation with the brightness. Our V-band CCD observations on 11nights in 2006 have revealed brightness trends during the night. Thevariability of IRAS18062+2410 is similar in pattern to the lightvariations in other hot post-AGB objects and some of the nuclei of youngplanetary nebulae. We assume that pulsations and a variable stellar windcan be responsible for the variability of these stars. In addition tothe rapid variability, our 12-year-long observations have revealed asystematic decline in the mean brightness of IRAS 18062+2410. This maybe related to a rise in the temperature of the star at constantluminosity as a result of its evolution. Low-resolution spectroscopicobservations have shown a systematic increase in the equivalent widthsof the H α, H β, [NII]λ6584 Å, OI λ8446Å, and [OII] λ7320 7330 Å emission lines. The changesin the star’s emission line spectrum are probably caused by anincrease in the degree of ionization of the gas shell due to a rise inthe temperature of the ionizing star. Our photometric and spectroscopicobservations of IRAS 18062+2410 confirm the previously made assumptionsthat the star evolves very rapidly to the region of planetary nebulae.
| New Estimates of the Solar-Neighborhood Massive Star Birthrate and the Galactic Supernova Rate
The birthrate of stars of masses >=10 Msolar is estimatedfrom a sample of just over 400 O3-B2 dwarfs within 1.5 kpc of the Sunand the result extrapolated to estimate the Galactic supernova ratecontributed by such stars. The solar-neighborhood Galactic-plane massivestar birthrate is estimated at ~176 stars kpc-3Myr-1. On the basis of a model in which the Galactic stellardensity distribution comprises a ``disk+central hole'' like that of thedust infrared emission (as proposed by Drimmel and Spergel), theGalactic supernova rate is estimated at probably not less than ~1 normore than ~2 per century and the number of O3-B2 dwarfs within the solarcircle at ~200,000.
| Planetary nebula distances re-examined: an improved statistical scale
The distances of planetary nebulae (PNe) are still quite uncertain.Although observational estimates are available for a small proportion ofPNe, based on statistical parallax and the like, such distances are verypoorly determined for the majority of galactic PNe. In particular,estimates of so-called `statistical' distance appear to differ byfactors of ~2.7.We point out that there is a well-defined correlation between the 5-GHzluminosity of the sources, L5, and their brightnesstemperatures, TB. This represents a different trend to thoseinvestigated in previous statistical analyses, and permits us todetermine independent distances to a further 449 outflows. Thesedistances are shown to be closely comparable to those determined using aTB-R correlation, providing that the latter trend is taken tobe non-linear.This non-linearity in the TB-R plane has not been noted inprevious analyses, and is likely responsible for the broad (andconflicting) ranges of distance that have previously been published.Finally, we point out that there is a close accord between observedtrends within the L5-TB and TB-Rplanes, and the variation predicted through nebular evolutionarymodelling. This is used to suggest that observational biases areprobably modest, and that our revised distance scale is reasonablytrustworthy.
| Circumstellar dust shells of hot post-AGB stars
Parameters of the circumstellar dust shells of 15 hot post-AGB starshave been derived using a radiative transfer code (DUSTY). Combining theoptical, near and far-infrared (ISO, IRAS) data of the stars, we havereconstructed their spectral energy distributions (SEDs) and estimatedthe dust temperatures, mass loss rates, angular radii of the innerboundary of the dust envelopes and the distances to these stars. Themass loss rates (10-6-10-5 Mȯyr-1) are intermediate between stars at the tip of the AGBand the PN phase. We have also studied the ISO spectra of 7 of thesestars. Amorphous and crystalline silicate features were observed in IRAS14331-6435 (Hen3-1013), IRAS 18062+2410 (SAO 85766) and IRAS 22023+5249(LSIII +5224) indicating oxygen-rich circumstellar dust shells. Thepresence of unidentified infrared (UIR) band at 7.7 μ, SiC emissionat 11.5 μ and the ``26 μ" and ``main 30 μ" features in the ISOspectrum of IRAS 17311-4924 (Hen3-1428) suggest that the central starmay be carbon-rich. The ISO spectrum of IRAS 17423-1755 (Hen3-1475)shows a broad absorption feature at 3.1 μ due toC2H2 and/or HCN which is usually detected in thecircumstellar shells of carbon-rich stars.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.
| Indications of a Large Fraction of Spectroscopic Binaries among Nuclei of Planetary Nebulae
Previous work indicates that about 10% of planetary-nebula nuclei (PNNi)are photometrically variable short-period binaries with periods of hoursto a few days. These systems have most likely descended fromcommon-envelope (CE) interactions in initially much wider binaries.Population-synthesis studies suggest that these very close pairs couldbe the short-period tail of a much larger post-CE binary population withperiods of up to a few months. We have initiated a radial-velocity (RV)survey of PNNi with the WIYN 3.5 m telescope and Hydra spectrograph,which is aimed at discovering these intermediate-period binaries. Wepresent initial results showing that 10 out of 11 well-observed PNNihave variable RVs, suggesting that a significant binary population maybe present. However, further observations are required because we haveas yet been unable to fit our sparse measurements with definite orbitalperiods and because some of the RV variability might be due tovariations in the stellar winds of some of our PNNi.
| The relation between Zanstra temperature and morphology in planetary nebulae
We have created a master list of Zanstra temperatures for 373 galacticplanetary nebulae based upon a compilation of 1575 values taken from thepublished literature. These are used to evaluate mean trends intemperature for differing nebular morphologies. Among the most prominentresults of this analysis is the tendency forη=TZ(HeII)/TZ(HeI) to increase with nebularradius, a trend which is taken to arise from the evolution of shelloptical depths. We find that as many as 87 per cent of nebulae may beoptically thin to H ionizing radiation where radii exceed ~0.16 pc. Wealso note that the distributions of values η and TZ(HeII)are quite different for circular, elliptical and bipolar nebulae. Acomparison of observed temperatures with theoretical H-burning trackssuggests that elliptical and circular sources arise from progenitorswith mean mass ≅ 1 Msolar(although the elliptical progenitors are probably more massive).Higher-temperature elliptical sources are likely to derive fromprogenitors with mass ≅2 Msolar, however, implying thatthese nebulae (at least) are associated with a broad swathe ofprogenitor masses. Such a conclusion is also supported by trends in meangalactic latitude. It is found that higher-temperature ellipticalsources have much lower mean latitudes than those with smallerTZ(HeII), a trend which is explicable where there is anincrease in with increasing TZ(HeII).This latitude-temperature variation also applies for most other sources.Bipolar nebulae appear to have mean progenitor masses ≅2.5Msolar, whilst jets, Brets and other highly collimatedoutflows are associated with progenitors at the other end of the massrange (~ 1 Msolar). Indeed it ispossible, given their large mean latitudes and low peak temperatures,that the latter nebulae are associated with the lowest-mass progenitorsof all.The present results appear fully consistent with earlier analyses basedupon nebular scale heights, shell abundances and the relativeproportions of differing morphologies, and offer further evidence for alink between progenitor mass and morphology.
| Galactic Planetary Nebulae and their central stars. I. An accurate and homogeneous set of coordinates
We have used the 2nd generation of the Guide Star Catalogue (GSC-II) asa reference astrometric catalogue to compile the positions of 1086Galactic Planetary Nebulae (PNe) listed in the Strasbourg ESO Catalogue(SEC), its supplement and the version 2000 of the Catalogue of PlanetaryNebulae. This constitutes about 75% of all known PNe. For these PNe, theones with a known central star (CS) or with a small diameter, we havederived coordinates with an absolute accuracy of ~0\farcs35 in eachcoordinate, which is the intrinsic astrometric precision of the GSC-II.For another 226, mostly extended, objects without a GSC-II counterpartwe give coordinates based on the second epoch Digital Sky Survey(DSS-II). While these coordinates may have systematic offsets relativeto the GSC-II of up to 5 arcsecs, our new coordinates usually representa significant improvement over the previous catalogue values for theselarge objects. This is the first truly homogeneous compilation of PNepositions over the whole sky and the most accurate one available so far.The complete Table \ref{tab2} is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/408/1029}
| UV(IUE) spectra of hot post-AGB candidates
Analysis of the low resolution UV(IUE) spectra (1150 to 3200 Å) of15 hot post-AGB candidates is presented. The UV(IUE) spectra of 10 starssuggest partial obscuration of the hot stars due to circumstellar dust.The reddened continua of these 10 stars were used to model and estimatethe circumstellar extinction. The circumstellar extinction law was foundto be linear in lambda -1 in the case of IRAS 13266-5551(CPD-55 5588), IRAS 14331-6435 (Hen3-1013), IRAS 16206-5956 (SAO243756), IRAS 17074-1845 (Hen3-1347), IRAS 17311-4924 (Hen3-1428), IRAS18023-3409 (LSS 4634), IRAS 18062+2410 (SAO 85766), IRAS 18371-3159 (LSE63), IRAS 22023+5249 (LSIII +5224) and IRAS 22495+5134 (LSIII +5142).There seems to be no significant circumstellar extinction in the case ofIRAS 17203-1534, IRAS 17460-3114 (SAO 209306) and IRAS 18379-1707 (LSS5112). The UV(IUE) spectrum of IRAS 12584-4837 (Hen3-847) shows severalemission lines including that of HeII. It may be a massive youngOB-supergiant or a low mass star in the post-AGB phase of evolution.IRAS 16206-5956 (SAO 243756) and IRAS 18062+2410 (SAO 85766) showvariability in the UV which in addition to stellar pulsations may beattributed to a dusty torus in motion around the hot central stars. TheUV spectrum of the bipolar PPN, IRAS 17423-1755 (Hen3-1475) indicatesthat the central B-type star is obscured by a dusty disk. The stars wereplaced on the log g-log Teff diagram showing the post-AGBevolutionary tracks of Schönberner. Terminal wind velocities of thestars were estimated from the CIV and NV stellar wind features. Thepresence of stellar wind in some of these stars indicates ongoingpost-AGB massloss.Based on observations obtained with the International UltravioletExplorer (IUE), retrieved from the Multimission Archive at STScI.Complete Figs. \ref{fig3} and \ref{fig4} and Appendix A are onlyavailable in electronic form at http://www.edpsciences.org
| The 77th Name-List of Variable Stars
We present the next regular Name-List of variable stars containinginformation on 1152 variable stars recently designated in the system ofthe General Catalogue ofVariable Stars. New designations are alsointroduced for two ``old'' variables whose corrected coordinates bringthem to other constellations.
| Catalog of Galactic OB Stars
An all-sky catalog of Galactic OB stars has been created by extendingthe Case-Hamburg Galactic plane luminous-stars surveys to include 5500additional objects drawn from the literature. This work brings the totalnumber of known or reasonably suspected OB stars to over 16,000.Companion databases of UBVβ photometry and MK classifications forthese objects include nearly 30,000 and 20,000 entries, respectively.
| Central Stars of Young Planetary Nebulae - A New Class of Variables
Not Available
| Version 2000 of the Catalogue of Galactic Planetary Nebulae
The ``Catalogue of Galactic Planetary Nebulae (Version 2000)'' appearsin Abhandlungen aus der Hamburger Sternwarte, Band XII in the year 2001.It is a continuation of CGPN(1967) and contains 1510 objects classifiedas galactic PNe up to the end of 1999. The lists of possible pre-PNe andpossible post-PNe are also given. The catalogue is restricted only tothe data belonging to the location and identification of the objects. Itgives identification charts of PNe discovered since 1965 (published inthe supplements to CGPN) and those charts of objects discovered earlier,which have wrong or uncertain identification. The question ``what is aplanetary nebula'' is discussed and the typical values of PNe and oftheir central stars are summarized. Short statistics about thediscoveries of PNe are given. The catalogue is also available in theCentre de Données, Strasbourg and at Hamburg Observatory viainternet. The Catalogue is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/378/843
| The Galactic disc distribution of planetary nebulae with warm dust emission features - I
We investigate the Galactic disc distribution of a sample of planetarynebulae characterized in terms of their mid-infrared spectral features.The total number of Galactic disc PNe with 8-13μm spectra is broughtup to 74 with the inclusion of 24 new objects, the spectra of which wepresent for the first time. 54 PNe have clearly identified warm dustemission features, and form a sample that we use to construct thedistribution of the C/O chemical balance in Galactic disc PNe. The dustemission features complement the information on the progenitor massesbrought by the gas-phase N/O ratios: PNe with unidentified infraredemission bands have the highest N/O ratios, while PNe with the silicatesignature have either very high N enrichment or close to none. We find atrend for a decreasing proportion of O-rich PNe towards the third andfourth Galactic quadrants. Two independent distance scales confirm thatthe proportion of O-rich PNe decreases from 30\pm 9 per cent inside the solar circle to 14\pm 7 per cent outside. PNe with warm dustare also the youngest. PNe with no warm dust are uniformly distributedin C/O and N/O ratios, and do not appear to be confined to C/O\sim 1. They also have higher 6-cmfluxes, as expected from more evolved PNe. We show that the IRAS fluxesare a good representation of the bolometric flux for compact andIR-bright PNe, which are probably optically thick. Selection of objectswith \fontshape{it}{F}(12\hphantom{0}\mu m)>0.5\hphantom{0} Jyshould probe a good portion of the Galactic disc for these young, denseand compact nebulae, and the dominant selection effects are rooted inthe PN catalogues.
| Stars with the Largest Hipparcos Photometric Amplitudes
A list of the 2027 stars that have the largest photometric amplitudes inHipparcos Photometry shows that most variable stars are all Miras. Thepercentage of variable types change as a function of amplitude. Thiscompilation should also be of value to photometrists looking forrelatively unstudied, but large amplitude stars.
| The dust content of planetary nebulae: a reappraisal
We have performed a statistical analysis using broad band IRAS data onabout 500 planetary nebulae with the aim of characterizing their dustcontent. Our approach is different from previous studies in that it usesan extensive grid of photoionization models to test the methods forderiving the dust temperature, the dust-to-gas mass ratio and theaverage grain size. In addition, we use only distance independentdiagrams. With our models, we show the effect of contamination by atomiclines in the broad band IRAS fluxes during planetary nebula evolution.We find that planetary nebulae with very different dust-to-gas massratios exist, so that the dust content is a primordial parameter for theinterpretation of far infrared data of planetary nebulae. In contrastwith previous studies, we find no evidence for a decrease in thedust-to-gas mass ratio as the planetary nebulae evolve. We also showthat the decrease in grain size advocated by Natta & Panagia(\cite{NattaPanagia}) and Lenzuni et al. (\cite{Lenzuni}) is an artefactof their method of analysis. Our results suggest that the timescale fordestruction of dust grains in planetary nebulae is larger than theirlifetime. Table~1 is only accessible 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
| Infrared Planetary Nebulae in the NRAO VLA Sky Survey
In order to construct a sample of planetary nebulae (PNe) unbiased bydust extinction, we first selected the 1358 sources in the IRAS PointSource Catalog north of J2000 declination delta=-40^deg having measuredS(25 μm)>=1 Jy and colors characteristic of PNe: detections orupper limits consistent with both S(12 μm)<=0.35S(25 μm) andS(25 μm)>=0.35S(60 μm). The majority are radio-quietcontaminating sources such as asymptotic giant branch stars. Free-freeemission from genuine PNe should make them radio sources. The 1.4 GHzNRAO VLA Sky Survey (NVSS) images and source catalog were used to rejectradio-quiet mid-infrared sources. We identified 454 IRAS sources withradio sources brighter than S~2.5 mJy beam^-1 (equivalent to T~0.8 K inthe 45" FHWM NVSS beam) by positional coincidence. They comprise 332known PNe in the Strasbourg-ESO Catalogue of Galactic Planetary Nebulaeand 122 candidate PNe, most of which lie at very low Galactic latitudes.Exploratory optical spectroscopic observations suggest that most ofthese candidates are indeed PNe optically dimmed by dust extinction,although some contamination remains from H II regions, Seyfert galaxies,etc. Furthermore, the NVSS failed to detect only 4% of the known PNe inour infrared sample. Thus it appears that radio selection can greatlyimprove the reliability of PN candidate samples withoutsacrificingcompleteness.
| Variable central stars of young Planetary Nebulae. A photometric study of the central star of M 2-54
We acquired 63.8 hours of time-series photometry of the variable centralstar of the young Planetary Nebula M 2-54. This object exhibits lightvariations with a peak-to-peak amplitude of up to 0.3 mag in Johnson V.Two different time scales (several days and several hours) are present.While the long-term variations appear to be nonperiodic, the short-termmodulations are (quasi)periodic with a time scale of either 8.9 or 14.3hours. An analysis of the HIPPARCOS photometry of this object did notallow us to infer which of these two time scales is the correct one. Thepossible causes for the observed variability are examined. The slowvariations can be explained by either a spot hypothesis or variations inthe stellar mass loss, while the short-term modulations are mostconsistent with stellar pulsation. All this behaviour is strikinglysimilar to that of best studied representative of this class of variablestar, the central star of IC 418, strongly suggesting that the physicalcause of the variability of these two objects is the same. While itappears quite attractive to suspect that we are in the presence of a newclass of pulsating variables, further work is needed to confirm orreject this. Consequently, some suggestions in this direction are given.Table 2 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Catalogue of H-alpha emission stars in the Northern Milky Way
The ``Catalogue of Stars in the Northern Milky Way Having H-alpha inEmission" appears in Abhandlungen aus der Hamburger Sternwarte, Band XIin the year 1997. It contains 4174 stars, range {32degr <= l() II< 214degr , -10degr < b() II < +10degr } having the Hαline in emission. HBH stars and stars of further 99 lists taken from theliterature till the end of 1994 were included in the catalogue. We givethe cross-identification of stars from all lists used. The catalogue isalso available in the Centre de Données, Strasbourg ftp130.79.128.5 or http://cdsweb.u-strasbg.fr and at the HamburgObservatory via internet.
| A sample of planetary nebulae observed by HIPPARCOS
By using HIPPARCOS data (magnitudes, parallaxes, proper motions), (1) wedetermine new values of the apparent luminosity of the central stars of19 planetary nebulae, (2) we discuss their distance and position on theT/L diagram. By comparison with the distances determined by individualor statistical methods, we see that most of these ``ground" distanceslook overestimated when we trust the HIPPARCOS trigonometric parallaxes.It seems that for compact nebulae, the nebula itself could influence theparallax measurement. In particular, the very small HIPPARCOS distancesto SwSt 1 and Hu 2-1 are unexpected and are not trustworthy. Peculiarmotions are analyzed, in terms of astrophysical parameters of the binarysystem A 35, and in relation with asymmetric morphology.
| Planetary Nebulae in the NRAO VLA Sky Survey
The 1.4 GHz NRAO VLA Sky Survey (NVSS) images and source catalog wereused to detect radio emission from the 885 planetary nebulae north ofJ2000 declination delta = -40 deg in the Strasbourg-ESO Catalogue ofGalactic Planetary Nebulae. We identified 680 radio sources brighterthan about S = 2.5 mJy beam-1 (equivalent to T ~ 0.8 K in the 45" FWHMNVSS beam) with planetary nebulae by coincidence with accurate opticalpositions measured from Digitized Sky Survey (DSS) images. Totalextinction coefficients c at lambda = 4861 Angstroms were calculated forthe 429 planetary nebulae with available H beta fluxes and low free-freeoptical depths at 1.4 GHz. The variation of c with Galactic latitude andlongitude is consistent with the extinction being primarily interstellarand not intrinsic.
| A New Distance Indicator to Galactic Planetary Nebulae Based upon IRAS Fluxes
It is well known that the IRAS colors of planetary nebulae (PNs) arevery similar to blackbody colors. Taking account of this characteristic,we define a new method to obtain some information about the distance toPNs by blackbody fitting of IRAS four-band fluxes, assuming these fluxesare due to thermal emission from the nebular dust envelope. The fitshould have two free parameters-the dust temperature T_D and thedistance-dependent ``scaling factor'' A-under the assumption of uniformdust mass. We find that the A-values have a good correlation withpublished distances. The scaling factor A could be a more effectivedistance scale than others, because many PNs have been detected as IRASsources and the extinction of IRAS fluxes is not so severe, and isknown. We also find that T_D is concentrated between 100 and 200 K, witha typical value of about 150 K.
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
| On the mass distribution of planetary nebulae central stars.
We apply a method, described in Gorny et al. (1997A&A...318..256G),to derive the masses of 125 central stars of planetary nebulae (PN).This method is self-consistent and distance-independent. It requires theknowledge of the nebular Hβ fluxes, angular radii and expansionvelocities, as well as the stellar visual magnitudes. This method isbased on a simple model for the evolution of planetary nebulae, in whichthe central stars evolve according to the theoretical models of Bloecker(1995A&A...299..755B) and Schoenberner (1983ApJ...272..708S). Theresults are dependent on the assumed total nebular mass. Nevertheless,for any reasonable total nebular mass distribution, we find that therange in planetary nebulae central star masses is very restricted: morethan 80% of the objects have a central star mass between 0.55 and0.65Msun_. We show how to convert, in this mass range, theobserved PN central star mass distribution into a zero-age post-AGB starmass distribution.
| 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.
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
| Сазвежђа: | Гуштер |
| Ректацензија: | 22h51m38.92s |
| Deклинација: | +51°50'42.5" |
| Apparent магнитуда: | 11.784 |
| Proper motion RA: | -2.2 |
| Proper motion Dec: | -2 |
| B-T magnitude: | 12.065 |
| V-T magnitude: | 11.808 |
Каталог и designations:
|
