J/ApJ/952/113 MIKE sp. of 6 yellow supergiants of Westerlund 1 (Beasor+, 2023)
Don't believe the hype(r): the yellow supergiants of Westerlund 1.
Beasor E.R., Smith N., Andrews J.E.
<Astrophys. J., 952, 113 (2023)>
=2023ApJ...952..113B 2023ApJ...952..113B
ADC_Keywords: Stars, giant; Spectra, optical; Equivalent widths
Keywords: Massive stars ; Stellar evolution ; Supergiant stars
Abstract:
Yellow hypergiants (YHGs) are often presumed to represent a
transitional post-red supergiant (RSG) phase for stars ∼30-40M☉.
Here we present visual-wavelength echelle spectra of six YHG
candidates in the Galactic cluster Westerlund 1, and we compare them
to known YHGs, IRC +10420 and Hen 3-1979. We find that the six YHG
candidates do not exhibit any metal emission lines, nor do they show
strong Hα emission, and as such do not meet the criteria
necessary to be classified as YHGs. In conjunction with their moderate
luminosities of log(L/L☉)=4.7-5.4 estimated from
optical/infrared photometry, we suggest instead that they are normal
yellow supergiants (YSGs) with more modest initial masses around
15-20M☉. This adds additional support to the hypothesis that Wd1
is a multiage cluster with an older age than previously assumed, and
is not a ∼5Myr old cluster caught at a very specific transitional
point when single-star evolution might yield Wolf-Rayet stars,
luminous blue variables, RSGs, and YHGs in the same cluster.
Nevertheless, the population of YSGs in Wd1 is very unusual, with YSGs
outnumbering RSGs, but with both spanning a large luminosity range.
Here, we discuss evolutionary scenarios that might have led to the
high fraction of YSGs. The number of YSGs and their significant
luminosity spread cannot be explained by simple population synthesis
models with single or binary stars. Even with multiple ages or a large
age spread, the high YSG/RSG ratio remains problematic. We suggest
instead that the objects may experience a prolonged YSG phase due to
evolution in triple systems.
Description:
We obtained optical echelle spectra of the six Yellow hypergiant (YHG)
candidates in the Galactic cluster Westerlund 1 (Wd1) on 2021 July 31,
using the MIKE spectrograph, which is a double echelle spectrograph
designed for use at the Magellan Telescopes at Las Campanas
Observatory in Chile. The observations consisted of 3x300s exposures
(except for W16a, for which the exposures were 3x600s) with the 1"x5"
slit.
Objects:
----------------------------------------------------------
RA (ICRS) DE Designation(s)
----------------------------------------------------------
16 47 02.40 -45 51 07.2 Wd1 = Cl Westerlund 1
----------------------------------------------------------
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 31 6 EWs for OI7774Å FeI7748Å in each of the
yellow supergiants (YSGs)
sp/* . 6 Individual spectra in FITS format
(Data behind Figure 2)
--------------------------------------------------------------------------------
See also:
J/A+A/537/A146 : Stellar models with rot. 0.8<M<120, Z=0.014 (Ekstrom+, 2012)
J/other/Sci/337.444 : RV curves of Galactic massive O stars (Sana+, 2012)
J/MNRAS/478/3138 : Luminous supergiants in Magellanic Clouds (Davies+, 2018)
J/A+A/635/A187 : Massive binaries in Westerlund 1. VII. (Clark+, 2020)
J/MNRAS/516/1289 : Gaia-EDR3 distance to the Wd1 Complex (Navarete+, 2022)
J/A+A/664/A146 : Westerlund 1 under light of Gaia EDR3 (Negueruela+, 2022)
Byte-by-byte Description of file: table1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 4 A4 --- Name Name of the star
6- 9 F4.2 --- EWOI [2.1/2.94] OI 7774Å equivalent width
11- 14 F4.2 --- e_EWOI [0.04/0.06] EWOI uncertainty
16- 20 F5.3 --- EWFeI [0.015/0.3]? FeI 7748Å equivalent width
22- 26 F5.3 --- e_EWFeI [0.01/0.05]? EWFeI uncertainty
28- 31 F4.2 --- EWOI-C05 [2.1/3] OI 7774Å equivalent width from
Clark+ 2005A&A...434..949C 2005A&A...434..949C
--------------------------------------------------------------------------------
History:
From electronic version of the journal
(End) Emmanuelle Perret [CDS] 10-Sep-2025