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Hubble Space Telescope and United Kingdom Infrared Telescope Observations of the Center of the Trifid Nebula: Evidence for the Photoevaporation of a Proplyd and a Protostellar Condensation
The Trifid Nebula (M20) is a well-known prominent optical H II regiontrisected by bands of obscuring dust lanes and excited by an O7.5 star,HD 164492A. Previous near-IR, mid-IR, and radio continuum observationsof the cluster of stars at the center of the Trifid Nebula indicated thepresence of circumstellar disks associated with hot stars with envelopesthat are photoionized externally by the UV radiation from the hotcentral star, HD 164492A. Using the WFPC2 on the Hubble Space Telescope,we present evidence of a resolved proplyd in Hα and [S II] lineemission from a stellar source emitting cool dust emission. Using theUnited Kingdom Infrared Telescope, an infrared observation of thestellar source with a proplyd indicates a late F to mid-G spectral type.We also note a remarkable complex of filamentary and sheetlikestructures that appear to arise from the edge of a protostellarcondensation. These observations are consistent with a picture in whichthe bright massive star HD 164492A is responsible for thephotoevaporation of protoplanetary disks of other less massive membersof the cluster, as well as the closest protostellar condensation facingthe central cluster. Using the evidence for a proplyd, we argue that themassive and intermediate-mass members of the cluster, HD 164492C (B6star) and HD 164492 (Herbig Be star), have disks associated with them.
| Proper Motion of the Irradiated Jet HH 399 in the Trifid Nebula
HH 399 is one of the first Herbig-Haro flows recognized to be irradiatedby the UV radiation of the massive O7.5 star in the Trifid Nebula. Wepresent the proper motion of the first irradiated jet, based on twoepochs of Hubble Space Telescope (HST) observations of HH 399 separatedby nearly 5 yr, using Hα and [S II] line filters. High propermotion with continuous velocities between 200+/-55 and 528+/-24 kms-1 is detected in both lines along the 18" extent of the jetaxis. The irradiated fully ionized jet consists of numerous knots alongthe jet but also shows evidence for a number of isolated blob likestructures running immediately outside the jet with lower transversevelocities. The transverse velocities combined with radial velocitymeasurements indicate that the jet axis lies away from the plane of thesky by only a few degrees. We argue that the jet is fully ionized, basedon a [S II]/Hα line ratio, as well as radio continuum emissiondetected from the full extent of the jet at a 3.6 cm wavelength. Thestellar mass-loss rate producing HH 399 is estimated to be~2×10-6 Msolar yr-1.
| Chandra Observation of the Trifid Nebula: X-Ray Emission from the O Star Complex and Actively Forming Pre-Main-Sequence Stars
The Trifid Nebula, a young star-forming H II region, was observed for 16hr by the ACIS-I detector on board the Chandra X-Ray Observatory. Wedetected 304 X-ray sources, 30% of which are hard sources and 70% ofwhich have near-infrared counterparts. Chandra resolved the HD 164492multiple system into a number of discrete X-ray sources. X-ray emissionis detected from components HD 164492A (an O7.5 III star that ionizesthe nebula), B and C (a B6 V star), and possibly D (a Be star).Component C is blended with an unidentified source to the northwest. HD164492A has a soft spectrum (kT~0.5 keV), while the component C blendshows much harder emission (kT~6 keV). This blend and other hard sourcesare responsible for the hard emission and Fe K line seen by ASCA, whichwas previously attributed entirely to HD 164492A. The soft spectrum ofthe O star is similar to emission seen from other single O stars and isprobably produced by shocks within its massive stellar wind. Lack ofhard emission suggests that neither a magnetically confined wind shocknor colliding wind emission is important in HD 164492A. A dozen starsare found to have flares in the field, and most of them arepre-main-sequence stars (PMS). Six sources with flares have both opticaland Two Micron All Sky Survey counterparts. These counterparts are notembedded, and thus it is likely that these sources are in a later stageof PMS evolution, possibly Class II or III. Two flare sources did nothave any near-IR, optical, or radio counterparts. We suggest that theseX-ray flare stars are in an early PMS stage (Class I or earlier). Wealso detected X-ray sources apparently associated with two massivestar-forming cores, TC 1 and TC 4. The spectra of these sources showhigh extinction and X-ray luminosities of (2-5)×1031ergs s-1. If these sources are Class 0 objects, it is unclearwhether their X-ray emission is due to solar-type magnetic activities,as in Class I objects, or to some other mechanism.
| Disks around hot stars in the Trifid nebula
We report on mid-IR observations of the central region in the Trifidnebula, carried out with ISOCAM in several broad-band infrared filtersand in the low resolution spectroscopic mode provided by the circularvariable filter. Analysis of the emission indicates the presence of ahot dust component (500 to 1000 K) and a warm dust component at lowertemperatures (~ 150-200 K) around several members of the clusterexciting the H II region, and other stars undetected at opticalwavelengths. Complementary VLA observations suggest that the mid-IRemission could arise from a dust cocoon or a circumstellar disk,evaporated under the ionization of the central source and the excitingstar of the nebula. In several sources the 9.7 kern 0.20em mu m silicateband is seen in emission. One young stellar source shows indications ofcrystalline silicates in the circumstellar dust.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Radio Continuum Emission from the Central Stars of M20, and the Detection of a New Supernova Remnant near M20
The Trifid Nebula (M20) is a well-known prominent optical H II regiontrisected by obscuring dust lanes. Radio continuum VLA observations ofthis nebula show free-free emission at λ=3.6 and 6 cm from threestellar sources lying close to the O7 V star at the center of thenebula. We argue that neutral material associated with these stars isphotoionized externally by the UV radiation from the hot central star.We also report the discovery of a barrel-shaped supernova remnant, SNRG7.06-0.12, at the northwest rim of the nebula, and two shell-likefeatures, G6.67-0.42 and G6.83-0.21, adjacent to W28 and M20. We discussthe nature of these features and their possible relationship to thepulsar PSR 1801-2306 and W28 OH (1720 MHz) masers.
| Photometry and spectroscopy of the central star of the Trifid nebula
UBV photometry of the central star of the Trifid nebula - HD 164492,which has so far been suspected of photometric as well as of radialvelocity variability - is presented. The results of our photometry donot confirm any variability. Moreover, based on new high resolution CCDspectra any radial velocity variability can be discarded. Photometry ofseveral other members of the multiple stellar system is included;according to this photometry, the system is a physical one. Equivalentwidths for the main component of HD 164492 are given. Positions measuredon CCD exposures are presented too. Based on observations collected atthe European Southern Observatory, La Silla, Chile.
| 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.
| A Radial Velocity Database for Stephenson-Sanduleak Southern Luminous Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....113..823R&db_key=AST
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| A spectroscopic database for Stephenson-Sanduleak Southern Luminous Stars
A database of published spectral classifications for objects in theStepenson-Sanduleak Luminous Stars in the Southern Milky Way catalog hasbeen compiled from the literature. A total of 6182 classifications for2562 stars from 139 sources are incorporated.
| A study of visual double stars with early type primaries. IV Astrophysical data
Astrophysical parameters (MK class, color excess, absolute magnitude,distance, effective temperature, mass, and age) are derived fromcalibrations of the uvby-beta indices for the members of 253 doublestars with O or B type primaries and faint secondaries. The photometricspectral classification is compared to the MK classes, and the agreementis very good. The derived data together with spectroscopic and JHKL dataare used for deciding which pairs are likely to be physical and whichare optical, and it is shown that 98 (34 percent) of the secondaries arelikely to be members of physical systems. For 90 percent of the physicalpairs the projected separation between the components is less than25,000 AU. A majority of the physical secondaries are late-type stars,and 50 percent of them are contracting towards the zero-agemain-sequence. Also presented are new uvby-beta data for 43 secondariesand a computer program for determining astrophysical parameters fromuvby-beta data.
| UBV photometry of the stars in the fields of emission nebulae. I. M 20.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1975PASJ...27..119O&db_key=AST
| Near infrared magnitudes of 248 early-type emission-line stars and related objects.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1973MNRAS.161..145A&db_key=AST
| Hα-EMISSION Stars and Planetary Nebulae in the Vicinity of m8 and M20 and in VELA from L = 230° to 241° Along the Galactic Equator.
Abstract image available at:http://adsabs.harvard.edu/abs/1957ApJ...126..302V
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Observation and Astrometry data
| Constellation: | Sagittaire |
| Right ascension: | 18h02m23.13s |
| Declination: | -23°02'00.2" |
| Apparent magnitude: | 8.655 |
| Distance: | 10000000 parsecs |
| Proper motion RA: | 0.3 |
| Proper motion Dec: | -11.5 |
| B-T magnitude: | 8.657 |
| V-T magnitude: | 8.656 |
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INHABITED SKY
