J/A+A/623/A83 Blue stragglers Wd1-27 and Wd1-30a spectra (Clark+, 2019)
A VLT/FLAMES survey for massive binaries in Westerlund 1.
V. The X-ray selected blue stragglers Wd1-27 and -30a.
Clark J.S., Najarro F., Negueruela I., Ritchie B.W., Gonzalez-Fernandez C.,
Lohr M.E.
<Astron. Astrophys. 623, A83 (2019)>
=2019A&A...623A..84C 2019A&A...623A..84C (SIMBAD/NED BibCode)
ADC_Keywords: Stars, double and multiple ; Spectroscopy
Keywords: blue stragglers - binaries: general - stars: early-type -
stars: evolution - stars: individual: Wd1-27 -
stars: individual: Wd1-30a
Abstract:
Recent observational studies indicate that a large number of OB stars
are found within binary systems which may be expected to interact
during their lifetimes. Significant mass transfer or indeed merger of
both components is expected to modify evolutionary pathways,
facilitating the production of exceptionally massive stars which will
present as blue stragglers. Identification and characterisation of
such objects is crucial if the efficiency of mass transfer is to be
established; a critical parameter in determining the outcomes of
binary evolutionary channels.
The young and coeval massive cluster Westerlund 1 hosts a rich
population of X-ray bright OB and Wolf-Rayet stars where the emission
is attributed to shocks in the wind collision zones of massive
binaries. Motivated by this, we instigated a study of the extremely
X-ray luminous O supergiants Wd1-27 and -30a.
We subjected a multi-wavelength and -epoch photometric and
spectroscopic dataset to quantitative non-LTE model atmosphere and
time-series analysis in order to determine fundamental stellar
parameters and search for evidence of binarity. A detailed examination
of the second Gaia data release was undertaken to establish cluster
membership.
Both stars were found to be early/mid-O hypergiants with luminosities,
temperatures and masses significantly in excess of other early stars
within Wd1, hence qualifying as massive blue stragglers. The binary
nature of Wd1-27 remains uncertain but the detection of radial
velocity changes and the X-ray properties of Wd1-30a suggest that it
is a binary with an orbital period ≤10 days. Analysis of Gaia proper
motion and parallactic data indicates that both stars are cluster
members; we also provide a membership list for Wd1 based on this
analysis.
The presence of hypergiants of spectral types O to M withinWd1 cannot
be understood solely via single-star evolution. We suppose that the
early-B and mid-O hypergiants formed via binary-induced mass-stripping
of the primary and mass-transfer to the secondary, respectively. This
implies that for a subset of objects massive star-formation may be
regarded as a two-stage process, with binary-driven mass-transfer or
merger yielding stars with masses significantly in excess of their
initial 'birth' mass.
Description:
A single spectrum of Wd1-27 was obtained on 2006 February 17 with the
NTT/EMMI with grism #6 covering the range 5800-8650Å at a
resolution R∼1500.
Subsequently one and three epochs of I-band observations were made
during 2011 and 2013 April-September, respectively, with VLT/FLAMES.
We utilised the GIRAFFE spectrograph in MEDUSA mode with setup HR21 to
cover the 8484-9001Å range with resolution R∼16,200
A further three spectra were obtained between 2016 May-June with
VLT/UVES.
Turning to Wd1-30a and eleven epochs of I-band spectroscopy, also
utilising the GIRAFFE+HR21 configuration of VLT/FLAMES, were made
between 2008 June - 2009 August, with a further five epochs between
2013 April - September. These were supplemented by a single
observation with VLT/FLAMES with the low-resolution grating LR6,
yielding an R-band spectrum with a wavelength range of 6438-7184Å
at a resolution of R∼8500.
Finally, K-band spectra of Wd1-27, 30a and other evolved stars
withinWd1 were obtained with VLT/ISAAC between 2004 August 1-3 under
programme 073.D-0837 and were extracted from the ESO archive.
Objects:
-----------------------------------------------------------
RA (2000) DE Designation(s)
-----------------------------------------------------------
16 47 05.1 -45 50 41 Wd1-27 = Cl* Westerlund 1 W 27
16 47 04.1 -45 50 39 Wd1-30a = Cl* Westerlund 1 W 30a
-----------------------------------------------------------
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
w27spec.dat 28 12115 Wd1-27 spectrum
w30aspec.dat 26 14681 Wd1-30a spectrum
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Byte-by-byte Description of file: w27spec.dat w30aspec.dat
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Bytes Format Units Label Explanations
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6- 15 F10.4 0.1nm lambda Wavelength
19- 28 F10.8 --- NFlux ?=- Normalized flux (relative to the continuum)
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Acknowledgements:
Somon Clark, simon.clark(at)open.ac.uk
References:
Clark et al., Paper I 2009A&A...507.1585R 2009A&A...507.1585R
Ritchie et al., Paper II 2010A&A...520A..48R 2010A&A...520A..48R
Clark et al., Paper III 2011A&A...531A..28C 2011A&A...531A..28C
Clark et al., Paper IV 2014A&A...565A..90C 2014A&A...565A..90C
(End) Patricia Vannier [CDS] 06-Mar-2019