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A tale of two cores: triggered massive star formation in the bright-rimmed cloud SFO 75
Context: Bright-rimmed clouds (BRCs) are isolated molecular cloudslocated on the edges of evolved HII regions. Star formation within theBRCs may have been triggered through the propagation ofphotoionisation-induced shocks driven by the expansion of the HIIregion. Aims: The main focus of this paper is to investigate thecurrent level of star formation within one of these clouds and evaluateto what extent, if any, star formation may have been triggered. Methods: We present a detailed multi-wavelength study of the BRC SFO 75,including 1.3 cm and 1.2 mm continuum, and 13CO and ammoniaspectral line observations. To build up a comprehensive picture of thelocal environment we complement our observations with archival data fromthe 2MASS, GLIMPSE and IRAS surveys. Results: The 13CO and1.2 mm emission reveals the presence of a dense core located behind thebright rim of the cloud which is approximately coincident with that ofthe IRAS point source. From an analysis of the IRAS and 1.2 mm fluxes wederive a dust temperature of 30 K, a luminosity of L{bol} =1.6×104 L_ȯ and estimate the core mass to be 570M_ȯ. The higher resolution ammonia observations resolve the 1.2 mmcore into two distinct cores, one directly behind the cloud's rim (CoreA) and the second located slightly farther back (Core B). These havemasses of 8-15 M_ȯ and 3.5-7 M_ȯ for Core A and Core Brespectively, which are significantly larger than their virial masses.Comparing the morphology of Core A with that of the photon-dominatedregion and ionised boundary layer leaves little doubt that it is beingstrongly affected by the ionisation front. 2MASS and GLIMPSE archivedata which reveal a small cluster of three deeply embedded(A{v}˜ 20 mag) high- and intermediate-mass youngstellar objects towards Core A leads us to conclude that the starformation found towards this core has been triggered. In stark contrast,Core B appears to have a much simpler, almost spherical, morphology. Nostars are found towards Core B. We find evidence supporting the presenceof shocked gas within the surface layers of the cloud which appears toextend to midway between the two ammonia cores. Conclusions: Thescenario that emerges from our analysis is one where the two ammoniacores pre-date the arrival of the ionisation front. Since its arrivalthe over-pressure of the ionised gas at the surface of the cloud hasdriven shocks into the surface layers of the cloud. The propagation ofthese shocks through Core A have triggered the formation of a smallcluster of massive stars, however, the shock front has not yetpropagated deeply enough into the cloud to have affected the evolutionof Core B.
| Radio continuum and molecular line observations of four bright-rimmed clouds
Aims.To search for evidence of triggered star formation within fourbright-rimmed clouds, SFO 58, SFO 68, SFO 75 and SFO 76. Methods:.We present the results of radio continuum and molecular lineobservations conducted using the Mopra millimetre-wave telescope andAustralia Telescope Compact Array. We use the J=1{-}0 transitions of12CO, 13CO and C18O to trace thedistribution of molecular material and to study its kinematics. Theradio continuum data is used to trace the distribution of the ionisedgas and to derive its parameters. Combining these observations witharchival data allows us to build up a comprehensive picture of thecurrent state of star formation within these clouds. Results:.These observations reveal the presence of a dense core(n{H_2}>104 cm-3) embedded withineach cloud, and the presence of a layer of hot ionised gas coincidedwith their bright-rims. The ionised gas has electron densitiessignificantly higher than the critical density (>25 cm-3)above which an ionised boundary layer can form and be maintained,strongly supporting the hypothesis that these clouds are beingphotoionised by the nearby OB star(s). Using a simple pressure-basedargument, photoionisation is shown to have a profound effect on thestability of these cores, leaving SFO 58 and SFO 68 on the edge ofgravitational stability, and is also likely to have rendered SFO 75 andSFO 76 unstable to gravitational collapse. From an evaluation of thepressure balance between the ionised and molecular gas, SFO 58 and SFO68 are identified as being in a post-pressure balance state, while SFO75 and SFO 76 are more likely to be in a pre-pressure balance state. Wefind secondary evidence for the presence of ongoing star formationwithin SFO 58 and SFO 68, such as molecular outflows, OH, H2O andmethanol masers, and identify a potential embedded UC HII region, butfind no evidence for any ongoing star formation within SFO 75 and SFO76. Conclusions: .Our results are consistent with the starformation within SFO 58 and SFO 68 having been triggered by theradiatively driven implosion of these clouds.
| A Galactic O Star Catalog
We have produced a catalog of 378 Galactic O stars with accuratespectral classifications that is complete for V<8 but includes manyfainter stars. The catalog provides cross-identifications with othersources; coordinates (obtained in most cases from Tycho-2 data);astrometric distances for 24 of the nearest stars; optical (Tycho-2,Johnson, and Strömgren) and NIR photometry; group membership,runaway character, and multiplicity information; and a Web-based versionwith links to on-line services.
| A Compact Array imaging survey of southern bright-rimmed clouds
We have carried out a radio-wavelength imaging survey of 45bright-rimmed clouds (BRCs), using the Australia Telescope Compact Arrayto characterise the physical properties in their ionised boundarylayers. We detected radio emission from a total of 25 clouds and using acombination of Digitised Sky Survey and mid-infrared MSX 8 \mum imagesclassified the emission into that associated with the ionised cloudrims, that associated with embedded possible massive YSOs and thatunlikely to be associated with the clouds at all. A total of 18 cloudsdisplay radio emission clearly associated with the cloud rim and wedetermine the ionising photon flux illuminating these clouds and theelectron density and pressure of their ionised boundary layers. Using aglobal estimate for the interior molecular pressure of these clouds weshow that the majority are likely to be in pressure equilibrium andhence are currently being shocked by photoionisation-induced shocks. Weidentify those clouds where the predicted ionising photon flux isinconsistent with that derived from the observations and show thateither the spectral types of the stars illuminating the BRCs are earlierthan previously thought or that there must be additional ionisingsources within the HII regions. Finally, we identify the radio sourcesembedded within the clouds with infrared stellar clusters and show thatthey contain late O and early B-type stars, demonstrating that a numberof BRCs are intimately involved with high to intermediate-mass starformation.Full Figs. \ref{fig:images} and \ref{fig:sfo86dss} are only available inelectronic form at http://www.edpsciences.org
| Observations of OB-stars at the former Leiden Southern Station
About 700 stars, mostly OB-stars, were observed by the author at theformer Leiden Southern Station at Hartebeespoortdam, South Africa, inthe observing seasons 1965, 1968, 1969, 1974, 1977, 1978. Observationswere made in the five channels of the Walraven photometric system. Dueto weathering of the telescope mirror the W channel gave no reliableresults for the faintest stars (m = 11 mag); in these cases the U-Wcolour index is not given. The change in sensitivity in the V channel,supposedly having occurred in 1968, was not recognised. Table~5 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/369/527
| Two-colour photometry for 9473 components of close Hipparcos double and multiple stars
Using observations obtained with the Tycho instrument of the ESAHipparcos satellite, a two-colour photometry is produced for componentsof more than 7 000 Hipparcos double and multiple stars with angularseparations 0.1 to 2.5 arcsec. We publish 9473 components of 5173systems with separations above 0.3 arcsec. The majority of them did nothave Tycho photometry in the Hipparcos catalogue. The magnitudes arederived in the Tycho B_T and V_T passbands, similar to the Johnsonpassbands. Photometrically resolved components of the binaries withstatistically significant trigonometric parallaxes can be put on an HRdiagram, the majority of them for the first time. Based on observationsmade with the ESA Hipparcos satellite.
| Molecular Clouds and Star Formation in the Southern H II Regions
We have carried out extensive 13CO(J = 1-0) observationstoward 23 southern H II regions associated with bright-rimmed clouds. Intotal, 95 molecular clouds have been identified to be associated withthe H II regions. Among the 95, 57 clouds \ are found to be associatedwith 204 IRAS point sources which are candidates for young stellarobjects. There is a significant increase of star-formation efficiency onthe side facing to the H II regions; the luminosity-to-mass ratio,defined as the ratio of the stellar luminosity to the molecular cloudmass, is higher by an order of magnitude on the near side of the H II \regions than that on the far side. This indicates that molecular gasfacing to the H II regions is more actively forming massive s\ tarswhose luminosity is >~ 103 LO . In addition, the numberdensity of the IRAS point sources increases by a factor of 2 on the nearside of the H II regions compared with on the far side. These resultsstrongly suggest that the active formation of massive stars on the nearside of the H II regions is due to the effects of the H II regions, suchas the compression of molecular material by the ionization/shock fronts.For the whole Galaxy, we estimate that the present star-formation rateunder such effects is at least 0.2-0.4 MO yr-1, correspondingto a few 10% by mass.
| 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.
| Wolf-Rayet stars and O-star runaways with HIPPARCOS. I. Kinematics
Reliable systemic radial velocities are almost impossible to secure forWolf-Rayet stars, difficult for O stars. Therefore, to study the motions- both systematic in the Galaxy and peculiar - of these two relatedtypes of hot, luminous star, we have examined the Hipparcos propermotions of some 70 stars of each type. We find that (a) both groupsfollow Galactic rotation in the same way, (b) both have a similarfraction of ``runaways'', (c) mean kinetic ages based on displacementand motion away from the Galactic plane tend to slightly favour thecluster ejection over the the binary supernova hypothesis for theirformation, and (d) those with significant peculiar supersonic motionrelative to the ambient ISM, tend to form bow shocks in the direction ofthe motion. Based on data from the ESA Hipparcos astrometry satellite.Table~1 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
| 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
| 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.
| Deep Hα survey of the Milky Way. II. The l=328deg area.
The detailed velocity field of the ionized hydrogen over an area ofabout 4x3deg around galactic longitude 328deg has been obtained in theframe of an Hα Survey of the galactic plane carried out at ESO LaSilla. The detailed analysis of the Hα profiles shows that thereare several layers of ionized hydrogen with different velocities.Besides individual HII regions there are two diffuse nebular emissionsdetected all over the studied area with velocities -20km/s and -40km/s.The HII regions exhibit complex structures; according to theirvelocities they can be divided into two groups around -40km/s and-47km/s. We have studied in detail the structure and velocities of thebright HII regions. The well known narrow lines of RCW94 are confirmed.The distances of the O-B3 stars in this area have been calculated fromthe literature data, and combined with radio data and our Hα dataallow us to determine the distances of the nebular components. We findthat the faint ionized gas layer at -20km/s, never detected before,neither at optical nor at radio wavelengths, is at about 1kpc anddefines the Sagittarius Carina arm at longitude 328deg. This impliesthat the main HII regions in this area and the diffuse ionized hydrogenin which they are immersed belong to a second arm (probably the ScutumCrux arm). This arm appears split at this longitude with two groups at2.3 and 3kpc. The radiosource 327.759-0.351 at H109αV_LSR_=-72km/s (d_kin_=4.5kpc) has been detected at Hα wavelengthbehind the other Hα emissions.
| Exciting stars and the distances of the diffuse nebulae
Not Available
| The spiral structure of our Galaxy determined from H II regions
An attempt is made to establish a coherent scheme of the spiralstructure of the Galaxy on the basis of a study of H II regions andtheir exciting stars. The data used for this include the distances ofthe exciting stars, H-alpha radial velocities, and H109-alpha radialvelocities. A detailed model of galactic rotation is developed whichexhibits deviations from circular motion in certain sectors, thusallowing the stellar and kinematic distances to be matched correctly.The resulting spiral pattern is found to have two symmetrical pairs ofarms (four altogether) with a pitch angle of 12 deg. These areidentified as the Sagittarius-Carina Arm (the major arm or arm No. 1),the Scutum-Crux Arm (the intermediate arm or arm No. 2), the Norma Arm(the internal arm or arm No. 1'), and the Perseus Arm (the external armor arm No. 2'). It is concluded that the morphological type of theGalaxy is much closer to Sc than to Sb.
| A catalogue of galactic O stars. The ionization of the low density interstellar medium by runaway stars.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1974RMxAA...1..211C&db_key=AST
| Spectroscopic observations of stars in HII regions.
Not Available
| Faint O - B3 stars in a region in Norma.
Abstract image available at:http://adsabs.harvard.edu/abs/1973AJ.....78..924M
| Radial velocities of southern B stars determined at the Radcliffe observatory 6.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1972MNRAS.158...85C&db_key=AST
| Observations of stars in HII regions : spectral classification and UBVphotometry.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1971AJ.....76..260C&db_key=AST
| Régions H II de l'hémisphère austral
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1970A&AS....3....1G&db_key=AST
| Studies of the Milky Way from Centaurus to Norma. III. OB stars.
Not Available
| A survey of southern HII regions.
Not Available
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Observation and Astrometry data
| Constellation: | Règle |
| Right ascension: | 15h55m39.58s |
| Declination: | -54°38'36.6" |
| Apparent magnitude: | 10.958 |
| Proper motion RA: | -3 |
| Proper motion Dec: | -3.7 |
| B-T magnitude: | 11.509 |
| V-T magnitude: | 11.004 |
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INHABITED SKY
