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A PHOTOMETRIC STUDY of the ORION -OB1 ASSOCIATION Ill

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A PHOTOMETRIC STUDY of the ORION -OB1 ASSOCIATION Ill X-680-77-251 A PHOTOMETRIC STUDY OF THE ORION -OB 1 ASSOCIATION Ill. SUBGROUP ANALYSES (NASA-TM-X-71407) A PHOTOMETRIC STUDY OF N177-34062 THE ORION OB 1 ASSOCIATION. 3: SUBGROUP ANALYSES (NASA) 231 p HC All/MF A01 CSCL 03A Unclas G3/89 50336 WAYNE H. WARREN, JR. JAMES E.>, HESSER OCTOBER-1977 GODDARD SPACE FLIGHT GREENBELT, MARYLAND Forinf6ratiorconerning-availabi ty, of'this-documentcontat Tednkcal Information-& Administrative SupportDivision, Code 250 Go~ddard-SpaceFlightCenter- Gr enbelt; Marylafid 20771 (Telephone 301-982-4-88) "This-paper'presents the viws of theauthor s , and,'does iot-necessarily, reflectthe iews of the-Goddard Space Flight-Center, or NASA." A PHOTOMETRIC STUDY OF THE ORION'OB 1 ASSOCIATION. III. SUBGROUP-ANALYSES WAYNE H. WARREN JR.* Laboratory for Astronomy and Solar Physics NASA-Goddard Space Flight Center and JAMES E. HESSER Cerro Tololo Inter-American Observatory * Visiting Astronomer, Kitt Peak National Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation. NAS-NRC Resident Research Associate. The Cerro Tololo Inter-American Observatory is supported by the National Science Foundation under contract NSF-C866. Accepted by Astophy4siaa Jounat Supptement Sitse4 -2- ABSTRACT The four principal subgroups of the association are exam­ ined in detail using individual distances and reddening values determined for their B-type members from uvby data presented in the previous papers of this series (Warren and Hesser (1977ab). Subgroup la appears not to show a spread in age as suggested by a previous study; nor does it show a syste­ matic distance increase with right ascension when fainter members are considered, although such a trend in declination indicates a possible connection of this subgroup with the X Orionis clustering to the north. An eastwardly increase in distance is found for subgroup lb, in agreement with previous studies, but the reddening law for the east Belt appears normal. The small subclusterings in the vicinity of the Orion Nebula appear not to differ in evolutionary state, but their ages are considerably greater than those of stars in the nebula and its associated cluster. Both uniform differences and large individual discrepan­ cies in the mo index, similar to those found in a study of the Upper Scorpius region of the association Sco OB 2, have been found for the young stars surrounding the Orion Nebula. They appear to be a result of several effects connected with line and/or continuum anomalies and mechanisms associated with local absorption. Several pre-main-sequence candidates are found among the late B- and A-type stars in the nebula region, and the-lower main-sequence turn-up occurs at approximately spectral type AO, beyond which the A- and F-type stars define a uniform sequence running parallel to the main sequence, but above it by about 1.3 mag. A clear gap in the F-star sequence is found in the range 0.21 < (b-y)o 0.26, indicating possible implications for the theory of convection in stellar envelopes of pre-main-sequence stars. The distribution of reddened B stars in the northern and southern sections of the association is indicative of absorp­ tion by circumstellar remnant dust for stars near the pre­ main-sequence turn-up. New age estimates for the association subgroups, made using intrinsic colors of their bluest main­ sequence stars, are in substantial agreement with earlier determinations, but'the accuracy can be improved by correcting for rotational-velocity effects if and when rotating models for higher mass stars become available. Subject headings: clusters: associations--nebulae: Orion Nebula - photometry - stars: early-type ­ stars: evolution - stars: pre-main-sequence - 4- I. INTRODUCTION New photoelectric data on the uvbyf and UBV systems have been presented for 526 stars in the vicinity of the young stellar association Orion OB 1 in Paper I of this series (Warren and Hesser 1977a), where we also established cri­ teria for membershio in the association based upon proper­ motion, radial-velocity, and photometric data. The procedures adopted for analysis of the photometric data in terms of color excesses, absolute visual magnitudes, rota­ tional-velocity effects and spectral types were discussed in Paper II (Warren and Hesser 1977b), where we also examined the photometry with regard to polarization and infrared excesses. We now turn our attention to the detailed analyses of the individual subgroups of the association, with respect to both their individual properties and the relationships among them. Analyses of subgroups la (Northwest region), lb (Belt region), lc (outer Sword) and ld (Orion-Nebula cluster) are presented in 9 IT; these subassociations closely resemble those defined by Blaauw (1964) except for addi­ tional subdivisions into which we have divided the lb, lc and ld subgroups (see Fig. 6 of Paper I). As in Paper II, our discussion of each subgroup is separated into sections for the early group (0- and B-type stars), the -5­ intermediate group (% AO-A3) and the late group (A-F stars), a division which follows naturally from the procedures used to analyze the uvbyo photometry (Strtmgren 1966). An im­ portant feature of these discussions is the presentation of extensive summary tables for this investigation, containing our mean values of the original photometric data, plus the relevant intrinsic parameters derived from the reductions, such as intrinsic colors, color excesses, absolute visual magnitudes and related spectroscopic data. In § III we intercompare the distances and properties of the individual subgroups, while their ages are briefly discussed in § IV. Finally, in § V3 we summarize the major conclusions of this study and discuss possibilities for future work on the association which might solve some of the problems encountered in the present work. -6- II. PHOTOMETRIC PROPERTIES OF INDIVIDUAL SUBGROUPS The principal results of this investigation are contained in Tables 5-11 and their associated notes: for convenience of future reference we have grouped these tables in the Appendix and urge the reader to briefly examine them before proceeding with the detailed discussion following. a) Subgroup la (Northwest region) i) Previous Investigations The la group is located to the west and north of 6 Orionis, being the most dispersed and presumably the oldest in the Orion OB 1 association. It contains the bright stars n, V, and w Orionis and extends westward to approximately 5 hours right ascension and northward to about +6 degrees declination in the present program. Although this subgroup was not distinguished from the lb group (Belt region) by Sharpless (1962)3 Blaauw (1964) found that its color-magnitude diagram shows a higher degree of evolution away from the top of the zero-age main sequence than that of the Belt. The internal motions and MK spectral types of the bright­ est members were studied by Lesh (1968a), who found a significantly smaller mean distance modulus for la than for the Belt and Sword (1c) subgroups. The spectral types of the brightest B stars indicate a more advanced evolutionary -7­ state and the internal motions show a rapid rate of expansion. The kinematic results of Lesh have been questioned by Steffey (1973) on the basis that an important condition for the use of Blaauw's method of finding expansion (which Lesh used) is not met by her results; namely, that the expansion rates must be nearly equal in two coordinates. [Lesh found expan­ sion rates of p = 0U00074 deg - I year - 1 ± 000027 (p.e.), P8 = 0'00029 deg - I year - I ± 0'J00049 (p.e.)]. Another prob­ lem with the derived expansion is that a systematic distance increase across a group can mimic expansion and Steffey finds just such an increase from west to east for the bright B stars used by Lesh. He also notes that the MV, (U-B)o diagram indicates a large range of ages for the bright stars studied by Lesh, and even suggests that the la subgroup may consist of mostly peripheral members of the nearby lb and X Orionis groups. Considering the possible effects caused by rotation on the MV, (U-B)o diagram, the latter argument may not be valid; nevertheless, Steffey's analysis suggests that we examine our data for any implications regarding the existence of the la subgroup. ii) Distance Modulus Tables 5a, b, c of the Appendix contain the intrinsic parameters for our program stars in the Northwest region. Figure 1 shows a plot of the m0 index against for all stars listed in these tables. The diagram demonstrates the well-known effect that the group classification (B, I, AF) depends primarily on the mi index, and shows that the B-type stars define a fairly tight sequence. This correlation is expected for early-type stars due to the inclusion of the HS line in the v filter, but reference to the , W(Hy) diagram (Fig. 4, Paper II) shows that the scatter is greater than that expected if the m, index is dependent upon H6 strength alone. 1 This dispersion suggests that the mi index is 1 The very high m0 index for M78B (WH503) may be a result of its high reddening and/or contamination from a nearby F-type optical companion which was included in the measurement. sensitive to composition differences in early-type stars a through its detection of metal lines in the 14100 A region. This is supported by the abnormally high indices usually found for Ap stars (Cameron 1967, Warren 1973), whose spectra contain many weak lines in this region. Evidence for abundance sensitivity of the m, index has also been found for the B stars of the Sco-Cen association by Glaspey (1971b). Figure 2 shows the corrected apparent magnitude V0 plot­ ted against R for the B- and I-type stars in subgroup la.
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