Seeking the real Cyg OB2 using IPHASselected A stars

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Seeking the real Cyg OB2 using IPHAS-selected A
stars
Janet Drew, Imperial College London
-- test of a general technique in a tough
location --
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(from Knoedlseder 2000)
The area around Cyg OB2 Contours CO
intensity Ellipse Reddish, Lawrence Pratt
(1966) location of Cyg OB2 l 80o.1, b 0.o9
gt300 MS OB stars, typical reddening 4-6 mags,
distance 2 kpc, total mass 2-6 x 104 M8 young
globular cluster
Gal. Equator
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Since 1966, up to Hanson (2003) favoured DM
11.2 (1.8 kpc) based on OB star spec.parallax
Knoedlseder (2000) proposed doubling of extent
and mass- 2600 /- 400 OB members, 8600 /-
earlier than F3V
2MASS K
DSS (optical)
(figure from Knoedlseder 2000)
New TeV source
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BUT Hanson (2003) revisited Cyg OB2 DM using
new OB calibrations-
and challenged membership of early-type stars
in expanded cluster
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What IPHAS can enable .selection
and appraisal of A0-5 dwarfs in the region
Important data- Ha absorption EW maximal A0 A5
V (checked against Indo-US
library) Median representative type A2V
(from Miller-Scalo mass
function) Expected absolute magnitude MV
1.6, Dispersion at given type s(MV) 0.39, or
FWHM 0.9 (HIPPARCOS,
Houk et al 1997)
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21 IPHAS nnnn/nnnno overlaps examined
Nos. in each box top no. of paired sources,
bottom fraction selected as A dwarf candidates
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5985/5985o (NW of centre)
6010/6010o (SE of centre)
6001/6001o (S of centre by 0.7o)
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Selection from the IPHAS (r-Ha,r-i) plane
everything below consistently-placed early-A
reddening line
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The cautious approach Require 2MASS J,K
measurements plus good optical/IR reddening
agreement, and sp-type appropriate colours Trial
for 6010/6010o, requiring AV estimates to match
within 0.5 mags-
Sp. Type- B8 B9 A0
A1 A2 A3 A5 A7 A9
Mean (Observed Expected)- (r-J)o
-0.09 -0.04 0.02 0 0
0.01 0.01 0.19 0.25 (i-J)o
-0.09 -0.04 0.02 0
0 0.01 0 0.19
0.25 matched stars 43 43 49
49 50 49 43
56 56
good agreement in expected sp type range BUT
only uses under half A-dwarf candidates available
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5985/5985o ..the other Cyg OB2 central field
(most densely populated)
6001/6001o ..to the south, the field containing
MWC 349A more reddened, and less well-populated
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Result of selection and dereddening of IPHAS data
to A2V norm (with minimal 2MASS constraints
applied)-
1504 A-dwarf candidates shown
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Results on individual field pairs (a)
6010/6010o, SE of Cyg OB2 centre 113 stars
Main concentration (DM 10.7)
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(b) 6001/6001o, south of centre 58
stars (redder, more sparse, consistent DM wrt
6010/6010o)
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(c) 5985/5985o, NW of Cyg OB2 centre 278
stars (seeing much further through the reddening
hole)
Slight overdensity at DM 11.1, AV of 4 5
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Two clusters? not one
(Malmquist effect)
Cyg OB2 A B separated by 400
pc
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The whole region
DM ranges 10.3 10.8 10.8 11.3 11.3
11.8 gt11.8
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Conclusion- there is structure single
cluster unlikely Astar data hint at A- at
DM10.6 (1.3 kpc), 1o across, to south of
CygOB2 8 trapezium B- at DM 11.1 or more (gt1.7
kpc), Reddishs cluster reinstated, offset to
NW Knoedlseders (2000) A0?F3 V population
estimate for Cyg OB2 too high .not a young
GC, but sequential star formation seen down the
barrel .difficult region for verifying
O-star calibrations
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