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J/ApJ/454/151 OB Associations of the Northern Milky Way (Massey+ 1995)
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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
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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:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
berk_86.dat 72 396 Catalog data for Berkeley 86/Cyg OB1 (1)
cep_ob5.dat 70 492 Catalog data for Cep OB5
ic1805.dat 74 1023 Catalog data for IC 1805/Cas OB6
ngc1893.dat 68 2986 Catalog data for NGC 1893/Aur OB2
ngc2244.dat 75 773 Catalog data for NGC 2244/Mon OB2
ngc6823.dat 75 744 Catalog data for NGC 6823/Vul OB1
ngc6871.dat 76 1960 Catalog data for NGC 6871/Cyg OB3
ngc6913.dat 75 209 Catalog data for NGC 6913/Cyg OB1 (1)
ngc7235.dat 71 563 Catalog data for NGC 7235
ngc7380.dat 70 894 Catalog data for NGC 7380/Cep OB1
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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)
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Byte-by-byte Description of file: *.dat
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Bytes Format Units Label Explanations
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1- 2 I2 h RAh Right ascension, 2000
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, 2000
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
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Note (1): These 2 columns (B-V and U-B) are inverted on the AAS CD-ROM
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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)
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(End) [CDS] 13-Nov-1996