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Three Very Young HGMN Stars in the Orion OB1 Association
We report the detection of three mercury-manganese stars in the OrionOB1 association. HD 37886 and BD -0 deg984 are in the approximately 1.7million year old Orion OB1b. HD 37492 is in the approximately 4.6million year old Orion OB1c. Orion OB1b is now the youngest cluster withknown HgMn star members. This places an observational upper limit on thetimescale needed to produce the chemical peculiarities seen inmercury-manganese stars, which should help in the search for the causeor causes of the peculiar abundances in HgMn and other chemicallypeculiar upper main-sequence stars.
| Far-ultraviolet stellar photometry: A field in Orion
Far-ultraviolet photometry for 625 objects in Orion is presented. Thesedata were extracted from electrographic camera images obtained duringsounding rocket flights in 1975 and 1982. The 1975 images were centeredclose to the belt of Orion while the 1982 images were centeredapproximately 9 deg further north. One hundred and fifty stars fell inthe overlapping region and were observed with both cameras. Sixty-eightpercent of the objects were tentatively identified with known starsusing the SIMBAD database while another 24% are blends of objects tooclose together to separate with our resolution. As in previous studies,the majority of the identified ultraviolet sources are early-type stars.However, there are a significant number for which no such identificationwas possible, and we suggest that these are interesting objects whichshould be further investigated. Seven stars were found which were brightin the ultraviolet but faint in the visible. We suggest that some ofthese are nearby white dwarfs.
| Studies of the local interstellar medium. VIII - Morphology and kinematics of the diffuse interstellar clouds toward Orion
Interstellar clouds in the direction of the Orion association show onlypositive velocities for target stars within 190 pc of the sun, and bothpositive and negative velocities for more distant target stars,confirming an earlier prediction by Cowie, Songaila, and York (1979).The nearby positive velocity cloud, designated here as Orion-Lepus 70(OL 70), is a standard diffuse interstellar cloud: it is subject to theambient galactic radiation field, with properties consistent with Tabout equal to 100 K and n about equal to 3/cu cm. Combined with acolumn density log N(H) = 19.8-20.0/sq cm, these values imply a cloudthickness of about 7 pc. The kinematics of OL 70 are consistent witheither an origin as part of the expanding Loop I superbubble shell, oras part of Lindbald's expanding ring, or a synthesis of the two models.The negative velocity interstellar components seen in stars at d notless than 200 pc are caused by interstellar matter accelerated by theexpanding Ori-Eri superbubble. Relatively dense interstellar gas atpositive LSR velocities is also found within the Orion association, sothat it is difficult to pick out OL 70 components in the spectra of thedistant stars.
| Luminosities, masses, and ages of B-type stars
Measurements of H-gamma lines and photometry of the Balmer discontinuityare combined with models of stellar atmospheres and interiors to derivevalues of T(eff) log g, mass, and M(bol) for main-sequence B-type starsin the Alpha Per and the Pleiades clusters and in the Orion, Sco-Gen,and I Lac associations. The masses as a function of T(eff) agree withthe analyses of eclipsing binary orbits. The luminosities yielddistances for the observed clusters and associations that are in goodagreement with independent determinations. The observed main sequence isbroader than predicted by standard models and suggests that mixing mayplay a role in extending the main-sequence lifetime. Stars within eachcluster and association show a range of ages, a result that suggeststhat star formation occurred over a period of at least 1-3 x 10 to the7th yr.
| Walraven photometry of nearby southern OB associations
Homogeneous Walraven (VBLUW) photometry is presented for 5260 stars inthe regions of five nearby southern OB associations: Scorpio Centaurus(Sco OB2), Orion OB1, Canis Major OB1, Monoceros OB1, and Scutum OB2.Derived V and (B - V) in the Johnson system are included.
| A photometric study of the Orion OB 1 association. III - Subgroup analyses
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1978ApJS...36..497W&db_key=AST
| Spectral types for early-type stars observed by SKYLAB
MK spectral types are presented for 246 early-type stars observed withthe S-019 ultraviolet stellar astronomy experiment on Skylab. K-linetypes are also given where applicable, and various peculiar stars areidentified. The peculiar stars include five silicon stars, a shell star,a helium-rich star, a silicon-strontium star, a chromium-europium star,and two marginal metallic-line stars.
| A photometric study of the Orion OB 1 association. I - Observational data.
A catalog of observational data is presented for stars in the region ofthe young stellar association Orion OB 1. Photoelectric observationsobtained in the uvby-beta and UBV systems are compiled along withprevious photoelectric and spectroscopic data for all these stars aswell as for several bright members of the association with availablephotometric indices. Mean weighted values are computed for thephotometric data and summarized in tables expected to be reasonablycomplete for association members earlier than spectral type A0.Membership criteria are derived, and qualitative membershipprobabilities summarized, for the 526 stars in the final program. Theanalytical procedures are discussed for association stars of B,intermediate, and AF types. Effects of the nebular environment andvarious calibrations of Balmer-line and four-color indices areconsidered for the determination of absolute magnitudes for the B-typestars.
| Interaction between the Barnard Loop Nebula and the interstellar magnetic field
New observations of the linear interstellar polarization of starlightare used to study the geometric structure of the interstellar magneticfield in the vicinity of the Barnard Loop Nebula in Orion. The observedcomplex field structure cannot be explained by a simple radial expansionof a conducting gaseous shell into an initially parallel field. Asatisfactory fit of the observations can be obtained by assuming thatthe interstellar gas in Orion is suspended in an interstellar 'magneticpocket'. It is suggested that the formation of the observed denseinterstellar clouds and very young stars in the Orion aggregate is aresult of the local structure of the interstellar magnetic field.
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