J/ApJ/469/629 UV-brightest stars of M33 and its nucleus (Massey+ 1996)
The UV-brightest stars of M33 and its nucleus: discovery, photometry,
and optical spectroscopy
Massey P., Bianchi L., Hutchings J.B., Stecher T.P.
<Astrophys. J. 469, 629 (1996)>
=1996ApJ...469..629M 1996ApJ...469..629M
ADC_Keywords: Photometry, UBV ; Photometry, ultraviolet
Mission_Name: UIT
Keywords: galaxies: individual (M33) - galaxies: stellar content -
stars: early-type - stars: Wolf-Rayet - ultraviolet: stars
Abstract:
We investigate the UV-brightest sources in the nearby galaxy M33. Our
catalog of 356 sources is constructed from far-ultraviolet (FUV;
1500A) and near-ultraviolet (NUV; 2400A) images obtained with the
Ultraviolet Imaging Telescope (UIT) matched with ground-based UBV
data. We find that our survey is limited by the FUV flux and is
complete to F_1500=2.5x10-15ergs/cm2^/s/A, other than in the most
crowded regions; this corresponds roughly to Mbol=-9.2 to -10.0 (or
masses of 40-60M☉), for Teff=50,000° to 10,000°. We use
Hubble Space Telescope (HST) WFPC2 images of several M33 fields to
conclude that at least one-half of our sample is uncontaminated by
unresolved neighbors, at least at the 0.1" (0.4pc) level, a resolution
similar to that achieved in the LMC from the ground. Spectral types
have been obtained for 131 of our objects. We discuss the spatial
distribution of the UIT sources, finding that they provide an
excellent tracer of the spiral arm pattern and confirm that star
formation continues in the nuclear region to the present day. Our
survey has found a large number of O and early B-type supergiants,
including stars as early as O6, but the optical spectroscopic sample
is dominated by later type B supergiants, as these are the visually
brighter. Among the brightest stars (both at 1500A and at V) are the
"superluminous" Wolf-Rayet stars first discovered by Conti & Massey in
the largest H II regions of M33; these objects are now known to be
small groups of stars in modest analog to R136 in 30 Dor. In general,
our survey has failed to detect the known W-R stars, as they are too
faint, but we did find several new late-type WN stars and composite
systems, which are brighter. Two stars of high absolute visual
magnitude (M_v~-9.0) are found to be B I + WN binaries, similar to HDE
269546 in the LMC; one of these is multiple at HST resolution. Most
interesting, perhaps, is our finding six Ofpe/WN9 "slash" stars, five
of them newly discovered. These stars show properties intermediate
between those of Of and WN stars and are believed to be a quiescent
form of luminous blue variables (LBVs). Our spectroscopy found five
additional stars that are spectroscopically similar to the known LBVs
of M33. One of these stars has recently been shown to be
spectroscopically variable, and we suggest that all of these stars
deserve continued scrutiny. The nucleus of M33 is the visually
brightest object in our survey, and its UV colors are indicative of a
hotter component than its optical photometry or spectral type would
suggest. We discuss the possibility that the pointlike nucleus may
contain a few interesting hot stars that dominate the light in the UV,
and we make the comparison to the cluster of He I emission-line stars
found near the center of the Milky Way. We comment on which
color-magnitude and color-color plots make the best diagnostic tools
for studying the hot, massive star population of a galaxy like M33.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 120 356 UIT sources in M33
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Byte-per-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 6 A6 --- Star Source name
8- 9 I2 h RAh Right ascension, J2000
11- 12 I2 min RAm Right ascension, J2000
14- 18 F5.2 s RAs Right ascension, J2000
19 A1 --- DE- Declination sign, always blank
20- 21 I2 deg DEd Declination, J2000
23- 24 I2 arcmin DEm Declination, J2000
26- 29 F4.1 arcsec DEs Declination, J2000
31- 32 I2 --- N3 *Number of stars within 3" of V counterpart
34- 35 I2 --- N5 *Number of stars within 5" of V counterpart
37- 41 F5.2 mag FUV FUV magnitude
43- 47 F5.2 mag FUV-NUV FUV-NUV color
49- 53 F5.2 mag Vmag V magnitude
55- 59 F5.2 mag U-B U-B color index
61- 65 F5.2 mag B-V B-V color index
67- 71 F5.2 mag FUV-V FUV-V color index
73- 87 A15 --- Sp Spectral type
89-120 A32 --- Comment *Comments, cross IDs, indication of crowding
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Note on N3:
Total number of stars found on V image within 3" of the UIT V counterpart
position.
Note on N5:
Total number of stars found on V image within 5" of the UIT V counterpart
position.
Note on Comment:
Cross-identifications given in the Comments are:
Halpha designation is from the unpublished Halpha survey described by
Neese et al. (1991, IAU Symp. 143), the full text of which is in preparation
B: stars from the list of blue stars in M33 by
Humphreys & Sandage, 1980ApJS...44..319H 1980ApJS...44..319H.
HSanon: unlisted blue stars on their finding charts, which also
form the basis of describing a star as being inside a given OB association.
obnn-xxx: stars from Massey et al., 1995AJ....110.2715M 1995AJ....110.2715M (Cat. J/AJ/110/2715)
MC: refers to Massey & Conti, 1983ApJ...273..576M 1983ApJ...273..576M, W-R stars
CM: refers to Conti & Massey, 1981ApJ...249..471C 1981ApJ...249..471C, stars in H II regions
W91: refers to stars listed by Wilson, 1991AJ....101.1663W 1991AJ....101.1663W
AM: stars are W-R stars found by Armandroff & Massey, 1985ApJ...291..685A 1985ApJ...291..685A
MJ: refers to a recent deep survey for W-R stars by
Massey & Johnson, 1996, in preparation
N: refers to H II regions with NGC designations.
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Origin: AAS CD-ROM series, Volume 7, 1996 Lee Brotzman [ADS] 22-Oct-96
(End) [CDS] 03-Feb-1997