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HST observations of the nebula around the central compact object in the Vela Jr. supernova remnant
Context: A handful of young (a few thousand years) supernova remnants(SNRs) host point-like X-ray sources, dubbed central compact objects(CCOs), which are thought to be radio-silent isolated neutron starsformed by the supernova explosion. So far, no CCO has been firmlydetected at other wavelengths. However, ground-based observation in theH? band detected a nebula around CXO J085201.4-461753, the CCO inthe Vela Jr. SNR. The nebula has also been detected in deep R-bandobservations performed with the Very Large Telescope (VLT).Interestingly, both its extension and its flux in the R band areconsistent with those measured in H?, suggesting that the nebulaspectrum is dominated by line emission, possibly produced by avelocity-driven bow-shock in the interstellar medium (ISM) or by itsphoto-ionisation from the neutron star. Aims: The aim of thiswork is to resolve the morphology of the H? nebula around the CCOto verify the proposed interpretations. Methods: We performedhigh-resolution imaging observations of the nebula with the Wide FieldPlanetary Camera 2 (WFPC2) of the Hubble Space Telescope (HST) throughthe 656N filter, almost exactly centred on the rest wavelength of theH? line. Results: Surprisingly enough, we did not detect thenebula in our WFPC2 image down to a 3 ? flux limit of ~3 ×10-15 erg cm-2 s-1. This limit is afactor of 10 fainter than the nebula flux measured in the discoveryground-based observations which were, however, performed with redder andbroader H? filters. Conclusions: The non-detection of thenebula in the narrower and bluer WFPC2 656N filter suggests that thepeak of the emission might actually be at longer wavelengths. Onepossibility, compatible with the bow-shock scenario only, is that theH? line is red-shifted by ~10-60 Å due to the neutron starmotion with a radial velocity 450 ? Vr ? 2700 kms-1. The other possibility is that the nebula is a knot of[NII] emission (? = 6583.6 Å) unrelated to CXOJ085201.4-461753 and possibly associated with the nearby star Wray16-30. High-resolution spectroscopy observations will help to settle thenature of the nebula.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by AURA,Inc. under contract No NAS 5-26555.
| VLT observations of the central compact object in the Vela Jr. supernova remnant
Context: X-ray observations have unveiled the existence of enigmaticpoint-like sources at the center of young (a few kyrs) supernovaremnants. These sources, known as central compact objects (CCOs), arethought to be neutron stars produced by the supernova explosion,although their X-ray phenomenology makes them markedly different fromall other young neutron stars discovered to date. Aims: The aimof this work is to search for the optical/IR counterpart of the VelaJunior CCO and to understand the nature of the associated H?nebula discovered by Pellizzoni et al. (2002). Methods: We useddeep optical (R-band) and IR (J,H,K_s-bands) observations recentlyperformed by our group with the ESO VLT to obtain the first deep, highresolution images of the field to resolve the nebula structure andpinpoint a point-like source possibly associated with the neutron star. Results: Our R-band image shows that the nebula's flux andstructure are very similar to those of, the H? one, suggestingthat the nebula spectrum is dominated by pure H? line emission.However, the nebula is not detected in our IR observations, which makesit impossible to to constrain its spectrum. A faint point-like object (J? 22.6, H ˜ 21.6 , Ks ˜ 21.4 ) compatible withthe neutron star's Chandra X-ray position is detected in our IR images(H and K_s) but not in the optical image (R ? 25.6), where it isburied by the nebula background. Conclusions: The nebula is mostlikely a bow-shock produced by the neutron star motion through the ISMor, alternatively, a photo-ionization nebula powered by UV radiationfrom a hot neutron star. A synchrotron nebula, powered by therelativistic particle wind from the neutron star, is the less likelyinterpretation because of its non-detection in the X-rays and theapparent lack of continuum emission. The candidate CCO counterpart couldbe the neutron star itself, a fallback disk left over from the supernovaexplosion, or a mid M-type star, or later, at a distance of 2 kpc.Based on observations collected at the European Southern Observatory,Paranal, Chile under programme ID 074.D-0729(A), 077.D-0764(A).
| The interior of the SNR RX J0852.0-4622 (Vela Jr) at radio wavelengths
Aims.We observed the center of the supernova remnant Vela Jr in radiocontinuum in order to search for a counterpart to the compact centralX-ray source CXOU J085201.4-461753, possibly a neutron star candidatewhich could be the remnant of the supernovaexplosion.Methods.Observations were made with the Australia TelescopeCompact Array at 13 and 20 cm. Spectral indices were obtained using fluxdensity correlations of the data which were spatially filtered to havethe same u-v coverage. A multiwavelength search for counterparts to thecompact central X-ray source was made.Results.We compiled a newcatalogue of 31 small diameter radio sources, including the previouslyknown source PMN J0853-4620, listing the integrated flux densities at 20cm and, for half of the sources, the flux densities at 13 cm with thecorresponding spectral indices. All sources are unresolved at thepresent angular resolution except for Source 18, which is clearlyelongated and lies strikingly close to CXOU J085201.4-461753. Ourobservations show no evidence for the existence of a pulsar wind drivennebula associated with the point X-ray source. Furthermore, Source 18has a thermal spectrum with index α = +0.8 ± 0.4 (S ∝να), and appears to be the counterpart of theoptical source Wray 16-30. In spite of the absence of [O iii] emissionlines as reported in the literature, we find that this object could beexplained as a low emission planetary nebula belonging to the"butterfly" morphological class.Conclusions.We conclude that if theradio source 18 is actually a planetary nebula, then CXOUJ085201.4-461753 is more likely to be related to it rather than to VelaJr.
| An H? nebula possibly associated with the central X-ray source in the G 266.2-1.2 supernova remnant
We report the discovery of a small H? nebula positionallycoincident with the candidate neutron star AX J0851.9-4617.4 located atthe center of the supernova remnant G 266.2-1.2. The nebula has aroughly circular shape with a diameter of ~ 6'' and a flux of ~10-2 photons cm-2 s-1 in the H?line. Considering the uncertainties in the distance and energy outputfrom the putative neutron star, we find that such a flux can beexplained either in a bow-shock model or assuming that the nebularemission is due to photo-ionization and heating of the ambient gas.
| The Compact Central Object in the RX J0852.0-4622 Supernova Remnant
The central region of the recently discovered supernova remnant (SNR) RXJ0852.0-4622 was observed with the Advanced CCD Imaging Spectrometerdetector aboard the Chandra X-Ray Observatory. We found only onerelatively bright source, about 4' north of the SNR center, with a fluxof ~2×10-12 ergs s-1 cm-2 in the0.5-10 keV band. The position of this pointlike source, CXOUJ085201.4-461753, rules out its association with the two bright stars inthe field, HD 76060 and Wray 16-30. Observations of the field with theCerro Tololo Inter-American Observatory 0.9 m telescope show a star(R~17, B~19) at about 2.4" from the nominal X-ray position. We considerassociation of this star with the X-ray source unlikely and estimate alimiting magnitude of the optical counterpart as B>=22.5 andR>=21.0. Based on the X-ray-to-optical flux ratio, we argue that theX-ray source is likely the compact remnant of the supernova explosionthat created the RX J0852.0-4622 SNR. The observed X-ray spectrum of thesource is softer than spectra of magnetospheric radiation ofrotation-powered pulsars, but it is harder than spectra of coolingneutron stars emitting thermal radiation from the entire surface,similar to the central compact source of the Cas A SNR. We suggest thatCXOU J085201.4-461753 belongs to the growing family of radio-quietcompact central sources, presumably neutron stars, recently discoveredin a number of SNRs.
| The X-Ray Sources at the Center of the Supernova Remnant RX J0852.0-4622
We present a BeppoSAX observation of the X-ray source AX J0851.9-4617.4.This source has been proposed as the neutron star that is associatedwith the shell-like supernova remnant RX J0852.0-4622. At E>5 keV,the image shows the presence of a new source, SAX J0852.0-4615, that wasnot resolved from AX J0851.9-4617.4 in previous observations. Theimproved positional accuracy obtained for AX J0851.9-4617.4, along withits soft thermal spectrum (kT~1.3 keV), confirms its possibleidentification with an early-type star. SAX J0852.0-4615, with itsharder X-ray spectrum and higher X-ray-to-optical flux ratio, is a morelikely candidate for the neutron star associated with the supernovaremnant.
| RX J0852.0-4622: Another Nonthermal Shell-Type Supernova Remnant (G266.2-1.2)
The newly discovered supernova remnant G266.2-1.2 (RX J0852.0-4622),along the line of sight to the Vela supernova remnant (SNR), wasobserved with ASCA for 120 ks. We find that the X-ray spectrum isfeatureless and well described by a power law, extending to three of theclass of shell-type SNRs dominated by nonthermal X-ray emission. LikeG347.3-0.5, this low-latitude remnant displays discrete regions ofenhanced emission along the rim as well as faint nonthermal emissionfrom the interior. We derive limits on the thermal content of theremnant emission, although the presence of the Vela SNR compromises ourability to seriously constrain a low-temperature component. Limitsplaced on the amount of Sc-K emission are compared with the expectedflux based on the reported 44Ti emission from G266.2-1.2. Wealso report on an unresolved X-ray source surrounded by diffuse emissionnear the center of the remnant. The properties of the source are notwell determined but appear consistent with the interpretation that thesource is a neutron star surrounded by a synchrotron nebula.Alternatively, the source may be associated with one of two starslocated within the positional error circle, but this appears somewhatunlikely.
| A HIPPARCOS Census of the Nearby OB Associations
A comprehensive census of the stellar content of the OB associationswithin 1 kpc from the Sun is presented, based on Hipparcos positions,proper motions, and parallaxes. It is a key part of a long-term projectto study the formation, structure, and evolution of nearby young stellargroups and related star-forming regions. OB associations are unbound``moving groups,'' which can be detected kinematically because of theirsmall internal velocity dispersion. The nearby associations have a largeextent on the sky, which traditionally has limited astrometricmembership determination to bright stars (V<~6 mag), with spectraltypes earlier than ~B5. The Hipparcos measurements allow a majorimprovement in this situation. Moving groups are identified in theHipparcos Catalog by combining de Bruijne's refurbished convergent pointmethod with the ``Spaghetti method'' of Hoogerwerf & Aguilar.Astrometric members are listed for 12 young stellar groups, out to adistance of ~650 pc. These are the three subgroups Upper Scorpius, UpperCentaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as VelOB2, Tr 10, Col 121, Per OB2, alpha Persei (Per OB3), Cas-Tau, Lac OB1,Cep OB2, and a new group in Cepheus, designated as Cep OB6. Theselection procedure corrects the list of previously known astrometricand photometric B- and A-type members in these groups and identifiesmany new members, including a significant number of F stars, as well asevolved stars, e.g., the Wolf-Rayet stars gamma^2 Vel (WR 11) in Vel OB2and EZ CMa (WR 6) in Col 121, and the classical Cepheid delta Cep in CepOB6. Membership probabilities are given for all selected stars. MonteCarlo simulations are used to estimate the expected number of interloperfield stars. In the nearest associations, notably in Sco OB2, thelater-type members include T Tauri objects and other stars in the finalpre-main-sequence phase. This provides a firm link between the classicalhigh-mass stellar content and ongoing low-mass star formation. Detailedstudies of these 12 groups, and their relation to the surroundinginterstellar medium, will be presented elsewhere. Astrometric evidencefor moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, CamOB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive.OB associations do exist in many of these regions, but they are eitherat distances beyond ~500 pc where the Hipparcos parallaxes are oflimited use, or they have unfavorable kinematics, so that the groupproper motion does not distinguish it from the field stars in theGalactic disk. The mean distances of the well-established groups aresystematically smaller than the pre-Hipparcos photometric estimates.While part of this may be caused by the improved membership lists, arecalibration of the upper main sequence in the Hertzsprung-Russelldiagram may be called for. The mean motions display a systematicpattern, which is discussed in relation to the Gould Belt. Six of the 12detected moving groups do not appear in the classical list of nearby OBassociations. This is sometimes caused by the absence of O stars, but inother cases a previously known open cluster turns out to be (part of) anextended OB association. The number of unbound young stellar groups inthe solar neighborhood may be significantly larger than thoughtpreviously.
| Discovery of a young nearby supernova remnant
About 200 supernova remnants have been found in the galaxy, six of whichare younger than about 1,000 years (ref. 2). Observations of these youngremnants are important for understanding of the late phases of supernovaevolution, and each new object should add substantially to our knowledgeof the processes involved. Here I report the discovery of a supernovaremnant (RXJ0852.0 - 4622), identified by its X-ray emission, at thesoutheast corner of the known Vela supernova remnant. The hightemperature (>3 × 107K) indicates an age of lessthan ~1,500yr. The observed diameter of the remnant is about 2°,which suggests a distance of less than 1kpc, based on a comparison withthe remnant of the supernova of AD1006. RXJ0852.0 - 4622 may thereforebe the nearest supernova to have occurred during recent human history.
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