J/AJ/145/125 Spectroscopy and HST photometry in Westerlund 2 (Vargas+, 2013)
The distance to the massive Galactic cluster Westerlund 2 from a spectroscopic
and HST photometric study.
Vargas Alvarez C.A., Kobulnicky H.A., Bradley D.R., Kannappan S.J.,
Norris M.A., Cool R.J., Miller B.P.
<Astron. J., 145, 125 (2013)>
=2013AJ....145..125V 2013AJ....145..125V
ADC_Keywords: Clusters, open ; Stars, early-type ; Photometry, HST ;
Cross identifications ; Extinction ; Spectral types
Keywords: open clusters and associations: individual: Westerlund 2 -
stars: early-type - stars: fundamental parameters
Abstract:
We present a spectroscopic and photometric determination of the
distance to the young Galactic open cluster Westerlund 2 using WFPC2
imaging from the Hubble Space Telescope (HST) and ground-based optical
spectroscopy. HST imaging in the F336W, F439W, F555W, and F814W
filters resolved many sources previously undetected in ground-based
observations and yielded photometry for 1136 stars. We identified 15
new O-type stars, along with two probable binary systems, including
MSP188 (O3+O5.5). We fit reddened spectral energy distributions based
on the Padova isochrones to the photometric data to determine
individual reddening parameters RV and AVfor O-type stars in Wd2.
We find average values <RV≥3.77±0.09 and <AV≥6.51±0.38mag,
which result in a smaller distance than most other spectroscopic and
photometric studies. After a statistical distance correction
accounting for close unresolved binaries (factor of 1.08), our
spectroscopic and photometric data on 29 O-type stars yield that
Westerlund 2 has a distance
=4.16±0.07(random)+0.26(systematic)kpc. The cluster's age remains
poorly constrained, with an upper limit of 3Myr. Finally, we report
evidence of a faint mid-IR polycyclic aromatic hydrocarbon ring
surrounding the well-known binary candidate MSP 18, which appears to
lie at the center of a secondary stellar grouping within Westerlund 2.
Description:
The cluster Westerlund 2 was observed on 2005 May 19 and on 2005 May
20 using the Wide-Field Planetary Camera 2 (WFPC2) on board HST (with
the filters F336W, F439W, F555W, and F814W; Table2). The observations
were performed during Cycle 13, proposal ID 10276, PI H. Kobulnicky,
and HST program 8zt. In Table2, UBVI magnitudes are transformed from
the HST to Johnson system.
Longslit spectra were obtained for 10 sources from Moffat et al.
(1991AJ....102..642M 1991AJ....102..642M) with the 6.5m Magellan I Walter Baade Telescope
(located at Las Campanas Observatory, Chile) and the 4.1m SOAR
(Southern Astrophysical Research) telescope, using the
Inamori-Magellan Areal Camera and Spectrograph (IMACS) and the Goodman
High Throughput Spectrograph (GHTS), respectively. Spectra for six
stars (528, 640, 714, 738, 857, and 906 in our numeration system) were
obtained on 2011 April 6/7 with the Magellan Telescope. The sources
137, 549, 1004, and 1031 were observed on 2011 June 9 and 10 with
SOAR. We obtained spectra of stars 714 and 738 with the Magellan
Inamori Kyocera Echelle (MIKE) spectrograph on the Magellan II Clay
telescope on 2011 November 14.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table2.dat 117 1136 HST photometry
table3.dat 79 27 New spectral classification
table4.dat 113 30 Reddening summary
table5.dat 177 30 Reddening table for the HST filters
table6.dat 112 32 Distance calculation
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See also:
III/195 : Atlas of Optical Spectral Classification OB Stars (Walborn+ 1990)
J/A+A/555/A50 : UBVIc photometry of stars in Westerlund 2 (Carraro+, 2013)
J/A+A/466/137 : Westerlund 2 JHKs photometry (Ascenso+, 2007)
J/A+A/463/981 : BV CCD photometry in Westerlund 2 (Rauw+, 2007)
J/ApJ/663/320 : IR-through-UV extinction curve (Fitzpatrick+, 2007)
J/A+AS/127/423 : VI photometry of Westerlund 1 and 2 (Piatti+ 1998)
Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 4 I4 --- Star [1/1136] Running star number
(<[VKB2013] NNNN> in Simbad)
6- 8 I3 --- M+91 ? Moffat et al. (1991AJ....102..642M 1991AJ....102..642M) identifier
9 A1 --- m_M+91 Multiplicity flag on M+91 (a, b, c, d) (G1)
11- 19 F9.5 deg RAdeg Right Ascension in decimal degrees (J2000)
21- 29 F9.5 deg DEdeg Declination in decimal degrees (J2000)
31- 36 F6.3 mag F336W ?=99.999 HST/WFPC2 F336W band magnitude
38- 42 F5.3 mag e_F336W ? The 1σ uncertainty in F336W
44- 49 F6.3 mag F439W ?=99.999 HST/WFPC2 F439W band magnitude
51- 55 F5.3 mag e_F439W ? The 1σ uncertainty in F439W
57- 62 F6.3 mag F555W ?=99.999 HST/WFPC2 F555W band magnitude
64- 68 F5.3 mag e_F555W ? The 1σ uncertainty in F555W
70- 75 F6.3 mag F814W ?=99.999 HST/WFPC2 F814W band magnitude
77- 81 F5.3 mag e_F814W ? The 1σ uncertainty in F814W
83- 88 F6.3 mag Umag ?=99.999 Standard U band magnitude
90- 95 F6.3 mag Bmag ?=99.999 Standard B band magnitude
97-102 F6.3 mag Vmag ?=99.999 Standard V band magnitude
104-109 F6.3 mag Imag ?=99.999 Standard I band magnitude
111-115 F5.1 --- S/N Signal-to-Noise ratio
117 A1 --- D [sl] Detection of the star (s=on short exposure
image, l=on long exposure image) (1)
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Note (1): A two-orbit HST imaging program was performed. On the first orbit
the cluster was centered on the PC1 chip, and a series of three exposures,
dubbed the "long exposures," were obtained in each of the filters F336W
(160s), F439W (40s), F555W (8s), and F814W (20s). On the second orbit the
cluster was centered on the WF3 chip, and a series of three exposures,
dubbed the "short exposures," were obtained in the same four filters with
exposure times of 50s, 12s, 2s, and 2s, respectively.
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Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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1- 2 A2 --- Gr Group of stars (A, OB, F, U) (1)
4- 7 I4 --- Star [137/1031] Running star number
8- 10 A3 --- f_Star Spectroscopic flag on Star (a, b, or/and c) (2)
12- 14 I3 --- M+91 ? Moffat et al. (1991AJ....102..642M 1991AJ....102..642M) identifier
15 A1 --- m_M+91 Multiplicity flag on M+91 (a, b, c) (G1)
17- 22 F6.3 mag Vmag [13.464/18.982] The V band magnitude
24- 28 F5.3 mag B-V [0.373/1.66] The B-V color index
30 A1 --- l_EWII [<] Upper limit flag on EWII
31- 35 F5.2 0.1nm EWII ? Measured equivalent width for HeII lines
(λ5411); in Å (3)
37- 40 F4.2 0.1nm e_EWII ? Uncertainty in EWII
42- 46 F5.2 0.1nm EWI ? Measured equivalent width for HeI lines
(λ5876); in Å (3)
48- 51 F4.2 0.1nm e_EWI ? Uncertainty in EWI
53- 56 F4.2 --- REW ? EWII/EWI Ratio (4)
58- 61 F4.2 --- e_REW ? Uncertainty in REW
63- 67 I5 K Teff ? Stellar effective temperature (5)
69- 72 I4 K e_Teff ? Uncertainty in Teff
74- 77 A4 --- SpT MK spectral type (6)
79 A1 --- Tel Telescope (7)
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Note (1): Group of stars as follows:
A = Stars with only Absorption lines;
OB = Stars believed to be late O/early B;
F = Field star;
U = Stars Unable to be identified.
Note (2): Flag as follows:
a = This spectrum also exhibits Hα emission;
b = This spectrum is a blend of two or more stars. The labeled one is the
dominant source on the slit;
c = This source exhibits a MgII absorption line.
Note (3): Having detections above 2σ. Negative values correspond to
emission lines.
Note (4): From Equation(1):
EWII/EWI=(1.16208*10-12T3)-(1.19205*10-7T2)+(4.22137*10-3T)
-50.5093.
Note (5): Corresponding to the ratio as determined by the relation in
Equation(1).
Note (6): Determined from the temperature approximation to one subtype
(Martins et al., 2005A&A...436.1049M 2005A&A...436.1049M).
Note (7): Source of the spectroscopic data as follows:
M = 6.5m Magellan Telescope (at Las Campanas Observatory, Chile);
S = SOAR (Southern Astrophysical Research) Telescope.
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Byte-by-byte Description of file: table4.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 4 I4 --- Star [137/1039]? Running star number, as in table2
(blank for average)
5 A1 --- f_Star [ab] Photometric flag on Star (a or b) (G2)
7- 11 A5 --- SpT Adopted MK spectral type
13 A1 --- A [A] A=the following columns give the Average RV
and AV values for the cluster
15- 18 F4.2 mag Rv1 Best-fitting ratio of selective to total
extinction using the method of Cardelli et al.
(1989ApJ...345..245C 1989ApJ...345..245C) (RV=AV/E(B-V))
20- 23 F4.2 mag e_Rv1 Uncertainty in Rv1
25- 28 F4.2 mag Rv2 Best-fitting ratio of selective to total
extinction using the method of Fitzpatrick
(2004ASPC..309...33F 2004ASPC..309...33F) (RV=AV/E(B-V))
30- 33 F4.2 mag e_Rv2 Uncertainty in Rv2
35- 38 F4.2 mag Rv3 Best-fitting ratio of selective to total
extinction using the method of Fitzpatrick &
Massa (2007, J/ApJ/663/320) (RV=AV/E(B-V))
40- 43 F4.2 mag e_Rv3 Uncertainty in Rv3
45- 48 F4.2 mag Rv4 Best-fitting ratio of selective to total
extinction using the method of Fitzpatrick &
Massa (2009ApJ...699.1209F 2009ApJ...699.1209F) (RV=AV/E(B-V))
50- 53 F4.2 mag e_Rv4 Uncertainty in Rv4
55- 58 F4.2 mag <Rv> The average ratio of selective to total extinction
60- 63 F4.2 mag e_<Rv> Uncertainty in <Rv>
65- 68 F4.2 mag Av1 Best-fitting extinction using the method of
Cardelli et al. (1989ApJ...345..245C 1989ApJ...345..245C)
70- 73 F4.2 mag e_Av1 Uncertainty in Av1
75- 78 F4.2 mag Av2 Best-fitting extinction using the method of
Fitzpatrick (2004ASPC..309...33F 2004ASPC..309...33F)
80- 83 F4.2 mag e_Av2 Uncertainty in Av2
85- 88 F4.2 mag Av3 Best-fitting extinction using the method of
Fitzpatrick & Massa (2007, cat. J/ApJ/663/320)
90- 93 F4.2 mag e_Av3 Uncertainty in Av3
95- 98 F4.2 mag Av4 Best-fitting extinction using the method of
Fitzpatrick & Massa (2009ApJ...699.1209F 2009ApJ...699.1209F)
100-103 F4.2 mag e_Av4 Uncertainty in Av4
105-108 F4.2 mag <Av> [5.75/7.49] The average extinction
110-113 F4.2 mag e_<Av> Uncertainty in <Av>
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Byte-by-byte Description of file: table5.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 4 I4 --- Star [137/1039]? Running star number, as in table2
(blank for average)
5 A1 --- f_Star [ab] Photometric flag on Star (a or b) (G2)
7- 11 A5 --- SpT MK spectral type
13 A1 --- A [A] A=the following columns give the Average
extinction values for the cluster
15- 19 F5.2 mag A1 The F336W filter extinction for Cardelli et al.
(1989ApJ...345..245C 1989ApJ...345..245C)
21- 24 F4.2 mag e_A1 Uncertainty in A1
26- 30 F5.2 mag A2 The F336W filter extinction for Fitzpatrick
(2004ASPC..309...33F 2004ASPC..309...33F)
32- 35 F4.2 mag e_A2 Uncertainty in A2
37- 41 F5.2 mag A3 The F336W filter extinction for Fitzpatrick &
Massa (2007, cat. J/ApJ/663/320)
43- 46 F4.2 mag e_A3 Uncertainty in A3
48- 52 F5.2 mag [8.7/11.35] The F336W filter average extinction
54- 57 F4.2 mag e_ Uncertainty in
59- 62 F4.2 mag A4 The F439W filter extinction for Cardelli et al.
(1989ApJ...345..245C 1989ApJ...345..245C)
64- 67 F4.2 mag e_A4 Uncertainty in A4
69- 72 F4.2 mag A5 The F439W filter extinction for Fitzpatrick
(2004ASPC..309...33F 2004ASPC..309...33F)
74- 77 F4.2 mag e_A5 Uncertainty in A5
79- 82 F4.2 mag A6 The F439W filter extinction for Fitzpatrick &
Massa (2007, cat. J/ApJ/663/320)
84- 87 F4.2 mag e_A6 Uncertainty in A6
89- 92 F4.2 mag [7.46/9.79] The F439W filter average extinction
94- 97 F4.2 mag e_ Uncertainty in
99-102 F4.2 mag A7 The F555W filter extinction for Cardelli et al.
(1989ApJ...345..245C 1989ApJ...345..245C)
104-107 F4.2 mag e_A7 Uncertainty in A7
109-112 F4.2 mag A8 The F555W filter extinction for Fitzpatrick
(2004ASPC..309...33F 2004ASPC..309...33F)
114-117 F4.2 mag e_A8 Uncertainty in A8
119-122 F4.2 mag A9 The F555W filter extinction for Fitzpatrick &
Massa (2007, cat. J/ApJ/663/320)
124-127 F4.2 mag e_A9 Uncertainty in A9
129-132 F4.2 mag [5.85/7.68] The F555W filter average extinction
134-137 F4.2 mag e_ Uncertainty in
139-142 F4.2 mag A10 The F814W filter extinction for Cardelli et al.
(1989ApJ...345..245C 1989ApJ...345..245C)
144-147 F4.2 mag e_A10 Uncertainty in A10
149-152 F4.2 mag A11 The F814W filter extinction for Fitzpatrick
(2004ASPC..309...33F 2004ASPC..309...33F)
154-157 F4.2 mag e_A11 Uncertainty in A11
159-162 F4.2 mag A12 The F814W filter extinction for Fitzpatrick &
Massa (2007, cat. J/ApJ/663/320)
164-167 F4.2 mag e_A12 Uncertainty in A12
169-172 F4.2 mag [3.65/6.62] The F814W filter average extinction
174-177 F4.2 mag e_ Uncertainty in
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Byte-by-byte Description of file: table6.dat
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Bytes Format Units Label Explanations
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1- 4 I4 --- Star [137/1039] Running star number
6- 8 I3 --- M+91 ? Moffat et al. (1991AJ....102..642M 1991AJ....102..642M) number
9 A1 --- m_M+91 Multiplicity flag on M+91 (a, b, c, d) (G1)
11- 23 A13 --- SpT MK spectral type
24 A1 --- f_SpT [*] Determined by Rauw et al. (2007, cat.
J/A+A/463/981; 2011A&A...535A..40R 2011A&A...535A..40R)
26- 31 F6.3 mag F555 Observed apparent HST/WFPC2 F555W magnitude
33- 37 F5.3 mag e_F555 Uncertainty in F555W
39- 43 F5.3 mag (B-V) The observed (F439W-F555W) color index
45- 49 F5.3 mag e_(B-V) Uncertainty in 439-555
51- 56 F6.3 mag M555 absolute HST/WFPC2 F555W magnitude
58- 62 F5.3 mag e_M555 Uncertainty in 555Mag
64- 69 F6.3 mag (B-V)0 The intrinsic (F439W-F555W) color index
71- 75 F5.3 mag e_(B-V)0 Uncertainty in 439-555i
77- 81 F5.3 mag A555 The F555W filter extinction (AF555W)
83- 87 F5.3 mag e_A555 Uncertainty in A555
89- 94 F6.3 mag DM [11.177/14.526] Distance modulus
96-100 F5.3 mag e_DM Uncertainty in DM
102-106 F5.3 kpc d [1.72/8.04] Distance
108-112 F5.3 kpc e_d Uncertainty in d
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Global notes:
Note (G1): In many instances the angular resolution of HST reveals several
sources within a ∼2'' radius of the nominal star from Moffat et al.. These
are labeled with an alphanumeric flag "a" for the brightest component, "b"
for the next brightest component, as measured in the F555W band, etc.
Note (G2): Source as follows:
a = The IR photometry of these stars comes from the 2MASS PSC instead of
Ascenso et al. (2007, cat. J/A+A/466/137);
b = This star lacks J photometry;
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History:
From electronic version of the journal
(End) Greg Schwarz [AAS], Sylvain Guehenneux [CDS] 19-May-2014