Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
An X-Ray Search for Compact Central Sources in Supernova Remnants. II. Six Large-Diameter SNRs
We present the second in a series of studies in which we have searchedfor undiscovered neutron stars in supernova remnants (SNRs). This paperdeals with the six largest SNRs in our sample, too large for Chandra orXMM-Newton to cover in a single pointing. These SNRs are nearby, withtypical distances of <1 kpc. We therefore used the ROSAT BrightSource Catalog and past observations in the literature to identify X-raypoint sources in and near the SNRs. Out of 54 sources, we wereimmediately able to identify optical/IR counterparts to 41 from existingdata. We obtained Chandra snapshot images of the remaining 13 sources.Of these, 10 were point sources with readily identified counterparts,two were extended, and one was not detected in the Chandra observationbut is likely a flare star. One of the extended sources may be a pulsarwind nebula, but if so it is probably not associated with the nearbySNR. We are then left with no identified neutron stars in these six SNRsdown to luminosity limits of ~1032 ergs s-1. Theselimits are generally less than the luminosities of typical neutron starsof the same ages, but are compatible with some lower luminosity sourcessuch as the neutron stars in the SNRs CTA 1 and IC 443.
| Effective temperature scale and bolometric corrections from 2MASS photometry
We present a method to determine effective temperatures, angularsemi-diameters and bolometric corrections for population I and II FGKtype stars based on V and 2MASS IR photometry. Accurate calibration isaccomplished by using a sample of solar analogues, whose averagetemperature is assumed to be equal to the solar effective temperature of5777 K. By taking into account all possible sources of error we estimateassociated uncertainties to better than 1% in effective temperature andin the range 1.0-2.5% in angular semi-diameter for unreddened stars.Comparison of our new temperatures with other determinations extractedfrom the literature indicates, in general, remarkably good agreement.These results suggest that the effective temperaure scale of FGK starsis currently established with an accuracy better than 0.5%-1%. Theapplication of the method to a sample of 10 999 dwarfs in the Hipparcoscatalogue allows us to define temperature and bolometric correction (Kband) calibrations as a function of (V-K), [m/H] and log g. Bolometriccorrections in the V and K bands as a function of T_eff, [m/H] and log gare also given. We provide effective temperatures, angularsemi-diameters, radii and bolometric corrections in the V and K bandsfor the 10 999 FGK stars in our sample with the correspondinguncertainties.
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are 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/418/989
| 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.
| Searching for old neutron stars with ROSAT. II. Soft X-ray sources in galactic dark clouds
This is the second in a series of three papers constraining the numberof detectable old neutron stars in the Galaxy. Here, I present thestatistical analysis of a sample of X-ray sources coincident with areasof dark clouds in the Galactic plane. I compare this sample with allsources in the ROSAT All-Sky Survey bright source catalog within 20(deg)of the Galactic plane. I present the results of an identificationprogram of a subset of sources that are compatible with a soft, thermalX-ray spectrum and an effective source temperature of less than 70eV.The three brightest sources in this sample form an intriguing subgroup.One of them is a previously identified candidate for an accretingneutron star. I identify the other two sources with hot white dwarfstars. I find no new accreting neutron star candidate in this sample.Based on this result, I derive an upper limit to the space density ofaccreting neutron stars in fields of Galactic dark clouds of ~ 2sr(-1)at a count rate >0.05s(-1) .
| Far-ultraviolet stellar photometry - A field in Monoceros
FUV photometry of stars in a field in Monoceros in the wavelength rangefrom 1230 to 1600 A has been carried out using data from anelectrographic Schmidt camera carried on a sounding rocket. Ultravioletmagnitudes were extracted for 602 objects in the field. Fifty-eightpercent were tentatively identified with visible stars using the SIMBADdata base while another 25 percent are blends of objects too closetogether to separate with our resolution. Eleven of the UV objectscoincide with parts of the star clusters NGC 2169, NGC 2244, and NGC2264 in which individual stars cannot be resolved. As in previousstudies, the majority of the identified ultraviolet sources areidentified with early-stars. However, there are a significant number forwhich no such identification was possible, and we suggest that many ofthese are nearby white dwarfs.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Unicorno |
| Ascensione retta: | 06h29m36.88s |
| Declinazione: | +08°29'32.8" |
| Magnitudine apparente: | 7.624 |
| Distanza: | 50.15 parsec |
| Moto proprio RA: | -16.6 |
| Moto proprio Dec: | -13.6 |
| B-T magnitude: | 8.271 |
| V-T magnitude: | 7.678 |
Cataloghi e designazioni:
|
