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A WIYN Lithium Survey for Young Stars in the λ Orionis Star-Forming Region
We have found 72 pre-main-sequence (PMS) stars near the center of theλ Orionis star-forming region by spectroscopically testing amagnitude-limited sample for the presence of lithium λ6708absorption, a diagnostic of youth. All of these stars show large lithiumequivalent widths and radial velocities consistent with Orionmembership, but only two were discovered previously via Hα orX-ray surveys. Comparison with PMS evolutionary tracks show that thelow-mass star formation did not begin prior to the initiation ofhigh-mass star formation 5-7 Myr ago. However, the subsequent detailedstar formation history is model dependent. Baraffe et al. isochronesimply that high- and low-mass stars began to form together 5-7 Myr ago,with the low-mass stellar formation ceasing abruptly 1 Myr ago. On theother hand, D'Antona & Mazzitelli isochrones indicate a narrowspread of PMS ages, which suggests a burst of low-mass star formation1-2 Myr ago. Furthermore, kinematic arguments require that the parentmolecular cloud gravitationally bound the stars together until recently,but at present the requisite gas mass is not visible. This leads us toconjecture that both the high- and low-mass stars were in a tightlybound cluster until a supernova blast about 1 Myr ago disrupted theparent cloud. This supernova also impacted on the PMS formation processby either (1) ceasing formation through removal of the gas supply or (2)triggering star births via cloud compression, depending on choice ofstellar evolution models. Finally, we find that despite their youth,only four of the 72 PMS stars have T Tauri-like Hα emission,suggesting the absence of accretion disks. We conjecture that this maybe the result of photoevaporation of the disks while the low-mass starswere in much closer proximity to the OB stars prior to becominggravitationally unbound.
| An Einstein Observatory SAO-based catalog of B-type stars
About 4000 X-ray images obtained with the Einstein Observatory are usedto measure the 0.16-4.0 keV emission from 1545 B-type SAO stars fallingin the about 10 percent of the sky surveyed with the IPC. Seventy-fourdetected X-ray sources with B-type stars are identified, and it isestimated that no more than 15 can be misidentified. Upper limits to theX-ray emission of the remaining stars are presented. In addition tosummarizing the X-ray measurements and giving other relevant opticaldata, the present extensive catalog discusses the reduction process andanalyzes selection effects associated with both SAO catalog completenessand IPC target selection procedures. It is concluded that X-rayemission, at the level of Lx not less than 10 exp 30 ergs/s, is quitecommon in B stars of early spectral types (B0-B3), regardless ofluminosity class, but that emission, at the same level, becomes lesscommon, or nonexistent, in later B-type stars.
| Dust ring around Lambda Orionis
Sky maps of a 12 deg x 12 deg region obtained by IRAS reveal a dust ringwhich has a diameter of about 9 deg and is centered on the H II region S264 and its exciting star, Lambda Orionis. The 60/100 micron colortemperature in the ring with an assumed dust emissivity law nu exp 1.5is on average 25 K. The luminosity in the IRAS wavelength range and thetotal far-infrared luminosity for the ring are 38,000 and 63,000 solarluminosities, respectively. The radiation field responsible for theheating of dust in the ring contains an enhanced UV component with anenergy density 2 to 3 times as large as that of the general interstellarradiation field. This enhancement of the UV component is ascribed to theO star Lambda Ori and several other early B stars. A careful analysis ofthe energetics of the ring shows that its total infrared luminosity canbe explained by the energy supply from the Lambda Ori OB association, HD34989 and the interstellar radiation field.
| Ultraviolet photometry from the Orbiting Astronomical Observatory. XX - The ultraviolet extinction bump
Ultraviolet extinction bumps are investigated in the interstellarextinction curves between 1800 and 3600 A for 36 stars which have (B-V)excesses ranging from 0.03 to 0.55 and are mostly confined to thebrighter OB associations distributed along the galactic plane. Eachextinction curve is found to have a broad bump which peaks near 2175 Aand whose position and profile appear to be constant among all thestars. It is shown that the bump is probably interstellar in origin andthat the constancy of its position and shape places such severerestrictions on grain geometrical parameters that classical scatteringtheory cannot be used to explain the feature unless the dust grains inwidely separated regions of space and with very different physicalconditions are assumed to have nearly identical size and shapedistributions. Three extinction curves which extend to 1100 A areexamined and found to have the same general characteristics as theothers. Several extinction curves are analyzed for fine structure, butno convincing evidence is found in the present interval. Some processesare discussed which may be responsible for the bumps.
| Photoelectric distances of 461 Northern OB-stars and galactic structure from Hγ- luminosities Author: Beer, A.
Abstract image available at:http://adsabs.harvard.edu/abs/1964MNRAS.128..261B
| A Catalogue of H II Regions.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1959ApJS....4..257S&db_key=AST
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| Сазвежђа: | Орион |
| Ректацензија: | 05h35m58.25s |
| Deклинација: | +09°31'54.2" |
| Apparent магнитуда: | 8.627 |
| Даљина: | 403.226 parsecs |
| Proper motion RA: | -0.6 |
| Proper motion Dec: | -1.6 |
| B-T magnitude: | 8.595 |
| V-T magnitude: | 8.625 |
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