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J/ApJ/454/151        OB Associations of the Northern Milky Way (Massey+ 1995)

The Initial Mass Function and Massive Star Evolution in the OB Associations of the Northern Milky Way Massey P., Johnson K.E., De Gioia-Eastwood K. <Astrophys. J. 454, 151 (1995)> =1995ApJ...454..151M
ADC_Keywords: Associations, stellar ; Stars, early-type ; Photometry, UBV Keywords: Galaxy: open clusters and associations: general - stars: early-type - stars: evolution - stars: luminosity function, mass function - stars: Wolf-Rayet Abstract: We investigate the massive star content of Milky Way clusters and OB associations in order to answer three questions: (1) How coeval is star formation? (2) How constant is the initial mass function (IMF)? (3) What is the progenitor mass of Wolf-Rayet stars? Our sample includes NGC 6823/Vul OB1, NGC 6871/Cyg OB3, Berkeley 86/Cyg OB1, NGC 6913/Cyg OB1, NGC 7235, NGC 7380/Cep OB1, Cep OB5, IC 1805/Cas OB6, NGC 1893/Aur OB2, and NGC 2244/Mon OB2. Large-field CCD imaging and multiobject, fiber spectroscopy has resulted in UBV photometry for >10,000 stars and new spectral types for ∼200 stars. These data are used to redetermine distances and reddenings for these regions and to help exclude probable nonmembers in constructing the H-R diagrams. We reanalyze comparable data previously published on Cyg OB2, Tr 14/16, and NGC 6611 and use all of these to paint a picture of star formation and to measure the IMFs. We find the following: (1) Most of the massive stars are born during a period Delta(Tau) < 3 Myr in each association. Some star formation has clearly preceded this event, as evidenced by the occasional presence of evolved (Tau ∼ 10 Myr) 15 Msun stars despite a typical age Tau ∼ 2 Myr for the more massive population. However, all these regions also show evidence of 5-10 Msun pre-main-sequence stars (Tau < 1 Myr), demonstrating that some star formation at lower masses does continue for at least 1 Myr after the formation of high-mass stars. (2) There is no statistically significant difference in IMF slopes among these clusters, and the average value is found to be Gamma = -1.1 ± 0.1 for stars with masses > 7 Msun. A comparison with similarly studied OB associations in the Magellanic Clouds reveals no difference in IMF slope, and hence we conclude that star formation of massive stars in clusters proceeds independently of metallicity, at least between z = 0.02 and z = 0.002. The masses of the highest mass stars are approximately equal in the Milky Way, LMC, and SMC associations, contrary to the expectation that this value should vary by a factor of 3 over this metallicity range. We conclude that radiation pressure on grains must not limit the mass of the highest mass star that can form, in accord with the suggestion of Wolfire & Cassinelli that the mere existence of massive stars suggests that shocks or other mechanisms have disrupted grains in star-forming events. (3) The four Wolf-Rayet stars in our sample have come from stars more massive than 40 Msun; one WC star and one late-type WN star each appear to have come from very massive (∼100 Msun) progenitors. File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file berk_86.dat 76 396 Catalog data for Berkeley 86/Cyg OB1 (1) cep_ob5.dat 76 492 Catalog data for Cep OB5 ic1805.dat 76 1023 Catalog data for IC 1805/Cas OB6 ngc1893.dat 76 2986 Catalog data for NGC 1893/Aur OB2 ngc2244.dat 76 773 Catalog data for NGC 2244/Mon OB2 ngc6823.dat 76 744 Catalog data for NGC 6823/Vul OB1 ngc6871.dat 76 1960 Catalog data for NGC 6871/Cyg OB3 ngc6913.dat 76 209 Catalog data for NGC 6913/Cyg OB1 (1) ngc7235.dat 76 563 Catalog data for NGC 7235 ngc7380.dat 76 894 Catalog data for NGC 7380/Cep OB1
Note (1): the file names of these two files (berk_86.dat and ngc6913.dat) are inverted on the AAS CD-ROM In ngc6913.dat, two misprints corrected on 13-Nov-1996: Star at 20 23 59.47 +38 31 47.6 is BD+38_4067 (not BD+38_406) Star at 20 23 45.88 +38 30 2.6 is H HDE229221 (not HDE227621)
Byte-by-byte Description of file: *.dat
Bytes Format Units Label Explanations
1- 2 I2 h RAh Right ascension, J2000 3- 5 I3 min RAm R.A. 6- 11 F6.2 s RAs R.A. 13 A1 --- DE- Declination sign 14- 15 I2 deg DEd Declination, J2000 16- 18 I3 arcmin DEm Dec. 19- 23 F5.1 arcsec DEs Dec. 24- 29 F6.2 mag Vmag V magnitude 30 A1 --- u_Vmag V uncertainty flag 31- 36 F6.2 mag B-V B-V color (1) 37 A1 --- u_B-V B-V uncertainty flag 38- 43 F6.2 mag U-B U-B color (1) 44 A1 --- u_U-B U-B uncertainty flag 45- 54 A10 --- SpNew Newly determined spectral type, if any 56- 65 A10 --- ID Other identification, if any 67- 76 A10 --- SpLit Spectral type from the literature, if any
Note (1): These 2 columns (B-V and U-B) are inverted on the AAS CD-ROM
Origin: AAS CD-ROM series, Volume 5, 1995 Lee Brotzman [ADS] 05-Nov-95 History: * 08-Nov-1996: the file names corresponding to Berkeley 86 and NGC 6913 have been corrected (see Note(1) in File Summary above) * 18-Nov-1996: column names B-V/U-B inverted in the Byte-by-byte description (see Note(1) in Byte-by-byte description)
(End) [CDS] 13-Nov-1996
The document above follows the rules of the Standard Description for Astronomical Catalogues.From this documentation it is possible to generate f77 program to load files into arrays or line by line

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