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Distances of absorbing features in the LDN 1622 direction. An application of Tycho-2 photometry and Michigan Classification
With Tycho-2 BT and VT photometry of Hipparcosstars with π > 8.0 mas, σπ/π < 15% andwith spectral type and luminosity classification from the MichiganCatalogues we have a presumably unreddened sample for establishing a(BT-VT_0- MV_T main sequence relationin the range from A0 to G5. We discuss the location of the median mainsequence relation relative to published ZAMS relations for the A0-G5range in some detail since the applications of this relation will haveto assume that the evolutionary status of the calibration sample isrepresentative of the local disk. If the ZAMS is defined as the lowerenvelope of the class V stars the published versions we discuss seem tobe too bright by about half a magnitude in the F0-G5 range. Theresulting intrinsic color and absolute magnitude estimates have errorsof the mean of 0.03m and 0.3m-0.4m respectively. From this relation anygiven star with Tycho-2 photometry, spectral classification andluminosity class V can have its color excess E(BT -VT) and distance modulus estimated. This means that the localinterstellar extinction may be estimated for the ~50% of the sky whereMichigan classification presently is available. The individual colorexcesses and distance moduli may not be particularly accurate but wepropose that they may be used to indicate extinction discontinuitiescaused by interstellar material at well defined distances. We haveapplied this new technique on stars from the Michigan Catalogue in thedirection of LDN 1622 (l, b) = (204.7dg, -11.8dg) supposed to beassociated to the Orion B region 400-500 pc away but color excessesE(BT - VT) exceeding 0.15m start appearing alreadyin the distance slot 160-200 pc. Presently we cannot decide whether thisnearby dust is related to LDN 1622 or whether this cloud is associatedto Orion B. In a final comparison we study the color excess -- distancevariation in a 4 deg region centered on LDN 1622 from Hipparcos --Tycho-1 -- Michigan data and the dust at ~160 pc is confirmed.
| 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
| 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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