J/AJ/110/2715 UBV photometry in NGC 6822, M31 and M33 (Massey+ 1995)
Hot, luminous stars in selected regions of NGC 6822, M31 and M33
Massey P., Armandroff T.E., Pyke R., Patel K., Wilson C.D.
<Astron. J. 110, 2715 (1995)>
=1995AJ....110.2715M 1995AJ....110.2715M
ADC_Keywords: Stars, luminous ; Photometry, UBV
Abstract:
We investigate the massive star content of the three Local Group
galaxies NGC 6822, M31, and M33 using crowded-field CCD UBV photometry
in selected regions to identify the most luminous and massive stars.
Optical spectroscopy is presented for many of these stars, allowing
construction of accurate H-R diagrams and the first meaningful
characterization of the massive star populations in these galaxies.
The spectral types also allow investigation of the internal reddenings
within these systems and provide fine candidates for stellar-wind
studies in the UV. The early-type stars identified include O-type in
all three systems, and we call attention to a new Luminous Blue
Variable candidate in M33. Our spectroscopy of extreme B supergiants
(M_V=-7.5) shows the expected changes with metallicity in comparison
to similar objects in the Milky Way, LMC, and SMC, although the metal
lines in the NGC 6822 stars are considerably weaker than that expected
for a metallicity intermediate between that of the LMC and SMC,
suggesting that this galaxy is more metal poor than usually supposed.
There is considerable internal reddening within all three galaxies,
including even the dwarf irregular NGC 6822, where the color excesses
show a systematic spatial trend from E(B-V)=0.26 near the edges to
0.45 in the middle. The slope of the reddening curve is normal in NGC
6822 and M33, with E(U-B)/E(B-V)∼0.72, but in M31 we find that this
ratio is 0.4-0.5 in all three of our fields. We spectroscopically
confirm that stars of high mass (>80M☉) and luminosity
(Mbol~-11) are found in M31 and M33. We have not found stars of
similar high mass or luminosity in NGC 6822, where the most luminous
star present has Mbol=-10 and an inferred mass of 60M☉.
Similarly, none of the OB associations in NGC 6822 are as impressive
in terms of the number of massive stars as the rich associations of
the Milky Way and Magellanic Clouds. However, OB78 (=NGC 206) and OB48
in M31 both contain 9-15 stars of mass >40M☉, making them
comparable to impressive sites of star formation in the Milky Way and
Magellanic Clouds. M33 contains regions that are even more extreme,
with M33-OB127 and M33-OB21 containing 20-30 such stars. The low
number of very massive stars in NGC 6822 is consistent with the
overall star-formation rate inferred by other means, but the M33
results suggest that the formation of high-mass stars may be favored.
The massive star content of individual associations would not have
been inferred on the basis of Halpha flux, meaning that the nebulae
associated with a number of these OB associations are density rather
than radiation bounded, probably due to holes blown in the H I. We
have also found that the ratio of the number of very massive
(>40M☉) stars to the number of WR stars is constant within all
the Local Group galaxies we have studied, suggesting that (a) the
effect of metallicity on the evolution of massive stars is subtle if
present, and (b) that WR stars make excellent tracers of the massive
star populations. The fact that this ratio is roughly 3, rather than
the 10 expected given the relative H- and He-burning lifetimes, argues
that either our samples are (uniformly?) incomplete or that some
fraction of WR stars are H-burning objects.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
ngc6822.dat 80 167 Photometry of NGC 6822 stars
m33.dat 80 490 Photometry of M33 stars
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See also:
J/AJ/121/861 : BVI photometry of central part of M33 (Macri+, 2001)
J/AJ/121/870 : Variables in the central part of M33 BVI phot. (Macri+, 2001)
J/A+A/366/498 : HST VI photometry of M33 star clusters (Chandar+, 2001)
J/AJ/110/1649 : HST VI photometry near the M33 nucleus (Mighell+ 1995)
Byte-by-byte Description of file: ngc6822.dat m33.dat
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Bytes Format Units Label Explanations
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1- 11 A11 --- Star Star identification
13- 14 I2 h RAh Right Ascension (J2000)
16- 17 I2 min RAm Right Ascension (J2000)
19- 23 F5.2 s RAs Right Ascension (J2000)
25 A1 --- DE- Declination sign (J2000)
26- 27 I2 deg DEd Declination (J2000)
29- 30 I2 arcmin DEm Declination (J2000)
32- 35 F4.1 arcsec DEs Declination (J2000)
37- 41 F5.2 mag Vmag V magnitude
43- 47 F5.2 mag U-B U-B color
49- 53 F5.2 mag B-V B-V color
55- 59 F5.2 mag Q Q = U-B - 0.72x(B-V)
61- 80 A20 --- Comment *Spectral type, cross id's and/or comments
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Note on Comment:
References for cross-id's:
NGC6822 CW: Wilson (1992AJ....104.1374W 1992AJ....104.1374W)
WR=Wolf-Rayet from Armandroff & Massey (1985ApJ...291..685A 1985ApJ...291..685A)
M33 B or HS are id's from Humphreys & Sandage (1980ApJS...44..319H 1980ApJS...44..319H)
RW=Regan & Wilson (1993AJ....105..499R 1993AJ....105..499R)
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Origin: AAS CD-ROM series, Volume 6, 1996 Lee Brotzman [ADS] 19-Feb-1996
(End) [CDS] 05-Sep-1996