Astronomy & Astrophysics manuscript no. main © ESO 2020
December 14, 2020
A dearth of young and bright massive stars in the Small Magellanic
Cloud
A. Schootemeijer
1
, N. Langer
1,2
, D. Lennon
3,4
, C. J. Evans
5
, P. A. Crowther
6
, S. Geen
7
, I. Howarth
8
, A. de Koter
7
, K.
M. Menten
2
, and J. S. Vink
9
1
Argelander-Institut f
¨
ur Astronomie, Universit
¨
at Bonn, Auf dem H
¨
ugel 71, 53121 Bonn, Germany
e-mail: aschoot@astro.uni-bonn.de
2
Max-Planck-Institut f
¨
ur Radioastronomie, Auf dem H
¨
ugel 69, 53121 Bonn, Germany
3
Instituto de Astrof
´
ısica de Canarias, E-38200 La Laguna, Tenerife, Spain
4
Departamento de Astrof
´
ısica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain
5
UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
6
Department of Physics and Astronomy, University of Sheffield, Sheffield, S3 7RH, UK
7
Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
8
University College London, Gower Street, London WC1E 6BT, UK
9
Armagh Observatory, College Hill, Armagh BT61 9DG, UK
Received September – ; accepted –
ABSTRACT
Context. Massive star evolution at low metallicity is closely connected to many fields in high-redshift astrophysics, but is poorly
understood so far. Because of its metallicity of ∼0.2 Z
, its proximity, and because it is currently forming stars, the Small Magellanic
Cloud (SMC) is a unique laboratory in which to study metal-poor massive stars.
Aims. We seek to improve the understanding of this topic using available SMC data and a comparison to stellar evolution predictions.
Methods. We used a recent catalog of spectral types in combination with GAIA magnitudes to calculate temperatures and luminosities
of bright SMC stars. By comparing these with literature studies, we tested the validity of our method, and using GAIA data, we
estimated the completeness of stars in the catalog as a function of luminosity. This allowed us to obtain a nearly complete view of
the most luminous stars in the SMC. We also calculated the extinction distribution, the ionizing photon production rate, and the star
formation rate.
Results. Our results imply that the SMS hosts only ∼30 very luminous main-sequence stars (M ≥ 40 M
; L & 3 · 10
5
L
), which
are far fewer than expected from the number of stars in the luminosity range 3 · 10
4
< L/L
< 3 · 10
5
and from the typically
quoted star formation rate in the SMC. Even more striking, we find that for masses above M & 20 M
, stars in the first half of their
hydrogen-burning phase are almost absent. This mirrors a qualitatively similar peculiarity that is known for the Milky Way and Large
Magellanic Cloud. This amounts to a lack of hydrogen-burning counterparts of helium-burning stars, which is more pronounced for
higher luminosities. We derived the H I ionizing photon production rate of the current massive star population. It agrees with the H α
luminosity of the SMC.
Conclusions. We argue that a declining star formation rate or a steep initial mass function are unlikely to be the sole explanations for
the dearth of young bright stars. Instead, many of these stars might be embedded in their birth clouds, although observational evidence
for this is weak. We discuss implications for the role that massive stars played in cosmic reionization, and for the top end of the initial
mass function.
Key words. Stars: massive – Stars: early-type – Stars: evolution – Galaxies: star formation – Galaxies: stellar content
1. Introduction
Massive stars in low-metallicity environments are linked to
spectacular astrophysical phenomena, such as mergers of two
black holes (Abbott et al. 2016), long-duration gamma-ray
bursts (Graham & Fruchter 2017), and superluminous super-
novae (Chen et al. 2017). Moreover, massive stars are thought
to have played a crucial role in providing ionizing radiation and
mechanical feedback in galaxies in the early universe (Hopkins
et al. 2014).
Massive star evolution at low metallicity therefore is of
great importance, but it is also poorly understood. For exam-
ple, some theoretical predictions suggest that very massive stars
are more more likely to form in low-metallicity environments
(e.g., Larson & Starrfield 1971; Abel et al. 2002), although the
arguments for this are debated (Keto & Wood 2006; Krumholz
et al. 2009; Bate 2009; Kuiper & Hosokawa 2018). This in turn
would have important consequences for the extent to which these
astrophysical phenomena take place. While studies of individ-
ual stars in the early universe are currently not feasible, a re-
cent analysis of the starburst region 30 Doradus in the nearby
Large Magellanic Cloud (half the solar metallicity) did indeed
find an overabundance of very massive stars (Schneider et al.
2018b). This overabundance would be in line with the predic-
tions mentioned above, but it is currently unknown if metallicity
effects and/or environmental effects cause this excess of massive
stars. Neither do we know whether this trend continues toward
lower metallicity. Because it has only about one-fifth of the solar
metallicity (Hill et al. 1995; Korn et al. 2000; Davies et al. 2015),
a better understanding of the Small Magellanic Cloud (SMC) is
a crucial stepping stone. Earlier studies have been unable to find
indications of an overabundance of massive stars in the SMC,
1
arXiv:2012.05913v1 [astro-ph.GA] 10 Dec 2020
A. Schootemeijer et al.: A dearth of young and bright massive stars in the Small Magellanic Cloud
however (Blaha & Humphreys 1989; Massey et al. 1995). For
field stars in the SMC, Lamb et al. (2013) derived an exponent
of Γ = −2.3 for the initial mass function (IMF), which is much
steeper than the canonical Salpeter exponent of Γ = −1.35 (with
dN ∝ M
Γ
d log M, where N is the number of stars that are born,
and M is the stellar mass).
In addition to the IMF, there is much to gain from a more
complete picture of the massive star content of the SMC. A
prevalence of blue supergiants can constrain internal mixing
(Schootemeijer et al. 2019; Higgins & Vink 2020) and bi-
nary interaction (Justham et al. 2014). Moreover, models of
gravitational wave progenitors (e.g., de Mink & Mandel 2016;
Marchant et al. 2016; Belczynski et al. 2016; Kruckow et al.
2018), of which the physical assumptions are extrapolated to-
ward low metallicity, can be put to the test. Furthermore, we can
investigate the amount of ionizing radiation that is emitted by
massive stars at low metallicity.
A main tool that is used to test stellar evolution predictions is
the Hertzsprung-Russell diagram (HRD). This has been applied
for the SMC massive star population in dedicated studies pub-
lished about two to three decades ago. They used two different
approaches: photometry, and spectroscopy. Massey (2002) and
Zaritsky et al. (2002) took the first approach and mapped U BV
magnitudes of SMC sources, which are indications for the lo-
cation of these sources in the HRD. While these studies can be
expected to be highly complete, they lack accuracy in predict-
ing the effective temperatures and luminosities of hot stars (e.g.,
Massey 2003). Effective temperatures can be predicted more ac-
curately from spectral types, which then also reduces the uncer-
tainty in luminosity. Blaha & Humphreys (1989) and Massey
et al. (1995) have used this method to compile HRDs of bright
SMC stars. Inevitably, the completeness of their input catalogs is
lower than for the photometry catalogs. The main focus of these
studies was the IMF, which they found to be consistent with the
IMF in the Milky Way (see the discussion above).
Since then, various developments in the field of observa-
tional astronomy have taken place that allow a major leap for-
ward. First, spectral types of many more SMC stars have be-
come available, which have been compiled in the catalog of
Bonanos et al. (2010), hereafter referred to as B10. Second, the
second data release (DR2) of the all-sky GAIA survey (Gaia
Collaboration et al. 2018) provides a spatially complete catalog
of SMC sources with information on their magnitudes and mo-
tions, thereby yielding information about which sources are in
the foreground. Third, detailed atmosphere analyses on subsets
of massive SMC stars (Trundle et al. 2004; Trundle & Lennon
2005; Mokiem et al. 2006; Hunter et al. 2008b; Bouret et al.
2013; Dufton et al. 2019; Ramachandran et al. 2019) can im-
prove our understanding of how effective temperature correlates
with spectral type. They also enable a systematic verification of
the reliability of the HRD positions based on spectral types. In
summary, new observations allow a large improvement in terms
of sample size, accuracy, completeness assessment, and identifi-
cation of foreground sources.
Our goal is to fully exploit these new observations. We use
them to provide an extinction distribution, ionizing photon emis-
sion rates, and the relations of spectral type and temperature for
massive SMC stars, and most importantly, also the improved
HRD. This paper is organized in the following way. In Sect. 2 we
describe our methods and the catalogs we used. Then, in Sect. 3,
we present the general properties of the stars in our sample. In
Sect. 4 we perform an in-depth analysis of features of the mas-
sive star population in the SMC. We find that it contains only
a few bright stars and young stars. This is the main result of
this paper. We further discuss our main result in Sect. 5, where
we compare numbers. In Sect. 6 we consider possible explana-
tions for our main result: a steeper IMF, model uncertainties, star
formation history, observational biases, unresolved binaries, and
embedding in birth clouds. Finally, we present our conclusions
in Sect. 7.
2. Methods
To achieve our goal of providing a more complete picture of lu-
minous SMC stars, we employ three data sets in this study. We
describe them in detail in Appendix A. The first and most essen-
tial data set is retrieved from the B10 spectral type catalog (their
table 1). We then cross-correlate it with the GAIA DR2 catalog.
Out of the 5324 B10 sources, we find a match in 5304 cases. All
of these have listed G magnitudes in GAIA DR2. We note that
only ∼3000 sources have a V magnitude listed in the B10 cata-
log. As a result, the use of G magnitudes improves the number of
sources for which we can calculate a luminosity. Out of the 5304
matched sources, 5269 pass our foreground test (Appendix A).
We discuss potential biases in the B10 catalog in Sect. 6.1.4.
The second data set is retrieved from GAIA DR2 alone
(again, see Appendix A for details). The completeness of GAIA
DR2 (see Gaia Collaboration et al. 2018; Arenou et al. 2018) is
essentially 100% in the magnitude range of our sources of inter-
est, which extends to G ≈ 16. Therefore we can use this data set
to estimate the completeness of the B10 catalog.
The third data set is a compilation of literature data in which
atmosphere analyses have been performed on a sample of bright
SMC stars. This data set is referred to as the various spectro-
scopic studies (VSS) sample, and is extracted from the studies of
Trundle et al. (2004), Trundle & Lennon (2005), Mokiem et al.
(2006), Hunter et al. (2008b), Bouret et al. (2013), Dufton et al.
(2019), and Ramachandran et al. (2019). The VSS data set con-
tains temperatures and luminosities, while the other two do not.
It consists of 545 sources. Of these, 160 fall in the luminosity
range of our main HRD (log(L/L
) > 4.5; Fig. 5).
Our aim is to also calculate temperatures and luminosities for
stars in the B10 data set, which contains many more stars than
the VSS sample. We use relations of spectral type and temper-
ature to calculate effective temperatures of the stars in the B10
data set. For OB-type stars, we derive our own relations based on
data from the VSS sample. For A-types and later, we use exist-
ing relations. Then, we use their GAIA G magnitudes to obtain
a luminosity and place the B10 sources in an HRD. We describe
these procedures in more detail in Sect. 2.1, and we explain our
estimation of the B10 completeness with the GAIA data set in
Sect. 2.2.
2.1. Deriving effective temperature and luminosity
We used the existing studies from the VSS sample of SMC stars
(last paragraph of Appendix A), which provide spectral types as
well as temperatures, to derive empirical relations of spectral
types and effective temperatures (T
eff
). For each spectral type,
we took the average derived temperature. We did this separately
for stars with luminosity class (LC) V+IV, LC III+II, and LC
I. When only one star of a certain spectral type was available,
the temperature that we adopted was the average of its derived
temperature and the temperatures we found for the two neigh-
boring types. As an example: for spectral type B0, a neighbor-
ing type is O9.7, and a neighbor of a neighbor would be O9.5.
Next, for all spectral types that have neighbors and neighbors-of-
neighbors at either side, we smoothed the relations of spectral
2
A. Schootemeijer et al.: A dearth of young and bright massive stars in the Small Magellanic Cloud
O2 O4 O7 B0 B5 B9
Spectral type
10
15
20
25
30
35
40
45
50
T
eff
[kK]
P13
LC V&IV
LC III&II
LC I
Fig. 1. Derived relations of spectral types and effective tempera-
tures (crosses) for different luminosity classes (LCs). The solid
gray line shows the relation for Galactic dwarf stars (P13; Pecaut
& Mamajek 2013).
type and temperature. Illustrated for type B0, the applied for-
mula is as follows: T
eff,B0,smooth
= (0.25T
eff,O9.5
+ 0.5T
eff,O9.7
+
T
eff,B0
+ 0.5T
eff,B0.2
+ 0.25T
eff,B0.5
)/2.5.
The resulting spectral type - T
eff
relations are shown in Fig. 1
and Table A.1. For stars of A-type and later, we used the SMC
spectral type - temperature relations of Evans & Howarth (2003)
and Tabernero et al. (2018). We compared our results with rela-
tions for Galactic dwarf (LC V) stars (from Pecaut & Mamajek
2013, their extended table 5)
1
. The evolved LC I stars are cooler
than dwarf stars that have the same spectral type. Moreover,
early-type dwarfs at low metallicity are hotter than their Galactic
counterparts with the same spectral type. These two trends are in
line with the trends shown in Trundle et al. (2007). The trend for
types earlier than O7 is not in line with these trends; the line for
LC III and II stars lies above the values for LC V and IV stars.
However, the LC III and II line there has only one data point as
a consequence of how rare such stars are, so that some scatter
can be expected. The typical differences between the Pecaut &
Mamajek (2013) spectral type - T
eff
relations and ours do not
exceed 1 − 2 kK.
We were unable to convert the spectral type of 114 sources
in the B10 data set into a temperature (e.g., ‘Be? + XRB’). We
applied the spectral type - T
eff
relations to infer effective tem-
peratures for the remaining 5155 sources. When the temperature
was known, the bolometric correction (BC) in the G band was
taken from the MIST (Dotter 2016; Choi et al. 2016) website
2
.
Because the surface gravity of most of the sources in the B10
data set is unknown, we used the BCs for log g = 3. At higher
and lower log g, these BCs match temperature values that are
typically well within 1 kK. In the B10 data set, 116 sources have
the label ‘binary’. We used the spectral type that is mentioned
first for them and further treated them as single stars. We high-
light them in Fig. B.10.
For stars in the VSS sample, we compared the GAIA col-
ors that are predicted for their effective temperatures to their
observed GAIA colors. The predicted colors are calculated
as BC(G
BP
) − BC(G
RP
) as given by MIST. We find that the
predicted and observed colors match best for a reddening of
1
http://www.pas.rochester.edu/
˜
emamajek/EEM_dwarf_
UBVIJHK_colors_Teff.txt
2
http://.waps.cfa.harvard.edu/MIST/model_grids.html,
where we take those with the label ‘DR2Rev’
4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0 6.2
log(
L
B10
/
L
)
0.4
0.2
0.0
0.2
0.4
0.6
0.8
1.0
log(
L
VSS
/
L
) log(
L
B10
/
L
)
Fig. 2. Difference between the logarithm of the luminosities re-
ported in the various spectroscopic studies (VSS) sample and
those derived by our method using spectral types of B10 as
a function of log(L
B10
/L
). Each dot represents an individual
source. The dotted line shows where L
B10
= L
VSS
; the solid line
is a linear fit to the scatter points.
E(G
BP
−G
RP
) = 0.14. This value corresponds to an extinction of
A
G
= 0.28 in the G band and A
V
= 0.35 in the V band (table 3
from Wang & Chen 2019, see also Gordon et al. 2003). We use
these extinction values throughout the paper.
Furthermore, we adopted a distance modulus (DM) of the
SMC of 18.91 (Hilditch et al. 2005). Then we calculated the
absolute bolometric magnitude of a source using
M
bol
= m
G
+ BC − DM − A
G
, (1)
which is translated into luminosity using log(L/L
) =
−0.4(M
bol
− M
bol,
). Here, we adopt a solar value of M
bol,
=
4.74.
To test this method, we show the luminosities of the sources
brighter than log(L/L
) = 4.5 in Fig. 2 for the sources that are
included in the VSS sample and in the B10 data set. On the y-
axis we show the difference between the luminosity reported in
the VSS sample and the luminosity we obtained with our method
based on the B10 data set. The black line, showing a linear fit,
indicates that there is no systematic offset between the luminosi-
ties derived by our B10 method and the VSS literature values.
The values of log(L
VSS
/L
) − log(L
B10
/L
) have a standard de-
viation of σ = 0.13 dex.
2.2. Investigating the completeness with GAIA photometry
Although the B10 catalog contains more stars than the VSS
sample, it still does not contain all of the brightest stars in the
SMC. We can expect the GAIA data to be much more complete.
This is supported by the fact that we found a GAIA counter-
part for 99.6% of the B10 sources; see also a discussion of this
in Appendix B.1. Unfortunately, however, GAIA colors are less
accurate in determining effective temperatures (and therefore lu-
minosities) than spectral types, and the results strongly depend
on extinction. This is especially true for the hotter stars.
Using GAIA colors and magnitudes, we therefore cannot in-
dividually derive reliable luminosities for our sources. However,
the colors and magnitudes can be used to estimate the complete-
ness of an ensemble of stars, in this case, the B10 catalog. With
‘completeness’ we mean the completeness fraction of hot, bright
stars that can be identified as such by the relevant observational
approaches, that is, spectroscopy, and/or photometry in the opti-
cal.
3
A. Schootemeijer et al.: A dearth of young and bright massive stars in the Small Magellanic Cloud
3.63.84.04.24.44.64.85.0
log(
T
eff, phot
/
K
)
4.6
4.8
5.0
5.2
5.4
5.6
5.8
6.0
6.2
log(
L
phot
/
L
)
15 19
27 35
88 131
130 231
236 563
284 732
N
B10
N
GAIA
A
V
= 0.35
GAIA
B10
Fig. 3. Hertzsprung-Russell diagram of bright SMC sources con-
structed with GAIA photometry. Black markers indicate sources
from the GAIA sample. The smaller red markers are sources in
the B10 sample. Therefore, red points with black edges (all but
one of the red points, see Appendix A) are sources that are listed
in both samples. We count the number of sources that are hotter
than 10
3.8
K for both samples in different luminosity intervals.
The numbers are displayed on the left side of the plot. The lu-
minosity intervals are indicated by dashed blue lines. For com-
pleteness, we also show the cool sources with crosses.
Table 1. Number of stars with T
eff
& 10
3.8
K counted in different
luminosity intervals.
log(L
B10
/L
) N
B10
N
GAIA, phot
N
B10
/N
GAIA, phot
5.75+ 15 19 0.79
5.50-5.75 27 35 0.77
5.25-5.50 88 131 0.67
5.00-5.25 130 231 0.56
4.75-5.00 236 563 0.42
4.50-4.75 284 732 0.39
Total 780 1640 0.47
We used GAIA photometry, and again MIST BCs, to calcu-
late the photometric temperature and luminosity of the B10 and
GAIA sources
3
: T
eff, phot
and L
phot
. We then plotted them in an
HRD that we refer to as the ‘photometric HRD’ (Fig. 3). Next,
we used this HRD to estimate the completeness of the B10 cat-
alog as a function of luminosity. For this estimate we also used
an HRD for which we used B10 spectral types instead of GAIA
colors (Fig. 5), as we explain below. We only considered temper-
atures above T
eff
= 10
3.8
K because for cooler temperatures we
rely on the results of Davies et al. (2018) on cool, bright SMC
stars. The way we operate is described below.
1. We counted the number of sources in Fig. 5 in different lu-
minosity intervals. The numbers are listed under N
B10
in
Table 1. For example, in Fig. 5 we count 15 stars above
log(L
B10
/L
) = 5.75.
3
The observed GAIA color is used to infer a temperature. Apart from
this, the procedure is the same as described in Sect. 2.1.
4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0 6.2
log(
L
phot
/
L
)
4.50
4.75
5.00
5.25
5.50
5.75
6.00
6.25
log(
L
/
L
)
VSS
B10
Fig. 4. Correlation between the luminosity of sources as calcu-
lated using their GAIA color and magnitude (L
phot
) and the lu-
minosity of the same source derived in various spectroscopic
studies (VSS) or its luminosity based on the temperature derived
from the spectral type listed in B10.
2. Then we examined the photometric HRD (Fig. 3), from high
to low luminosity. In this example we considered the 15
brightest stars in B10, which represent the log(L
B10
/L
) >
5.75 bin.
3. We counted the number of GAIA sources until we found 15
sources that were also in the B10 catalog. We took this ap-
proach to have the same number of B10 sources in each lumi-
nosity bin. This counting in Fig. 3 took place in the intervals
that are separated by dashed blue lines. They do not by def-
inition coincide exactly with the luminosity intervals listed
in the left column of Table 1. In this example at the bright
end, we count N
GAIA, phot
= 19. For the log(L
B10
/L
) > 5.75
bin, we thus estimate that it has a completeness fraction of
15/19 = 0.79.
4. We repeated this for each luminosity interval (i.e., also for
5.50 < log(L
B10
/L
) < 5.75 with the 16th to 42nd bright-
est star from the B10 catalog method, etc.) listed in Table 1
to calculate the completeness fraction of the B10 catalog:
N
B10
/N
GAIA, phot
.
Fig. 3 and Table 1 show that the completeness level of the
B10 catalog is 70% to 80% for the brightest sources. The com-
pleteness drops below 40% around log(L/L
) = 4.5.
For this estimate to be reliable, the values of L
B10
and L
phot
need to be similar for most of the stars. We note that if L
phot
had no predictive power, we would expect to measure the same
completeness in all luminosity bins. We investigate this further
in Fig. 4, where we compare L
phot
of the sources to their VSS lu-
minosity and the luminosity calculated using their B10 spectral
type. This figure shows that while the scatter is large, the lumi-
nosities obtained with GAIA photometry on average give a good
indication in which luminosity segment most sources belong.
In Appendix B.1 we provide another (simpler) test in which
we count blue sources in the B10 and GAIA DR2 catalogs. The
trends in this second test are very similar to those described in
this section.
Both of our completeness tests imply a higher completeness
than the completeness quoted for O stars in B10 itself (∼4%).
However, their number is based on an estimate of 2800 O stars of
M > 20 M
from the conference proceedings of Massey (2010).
This number is in turn based on U BV photometry from Massey
(2002). In their table 8b, all 70 blue stars that are known to have
O types have a photometry-derived temperature in the O-star
regime. Because of the uncertainties in determining the tem-
4
A. Schootemeijer et al.: A dearth of young and bright massive stars in the Small Magellanic Cloud
3.63.84.04.24.44.64.8
log(
T
eff
/
K
)
4.50
4.75
5.00
5.25
5.50
5.75
6.00
6.25
log(
L
/
L
)
0.5
t
MS
W1
W2
W3
W4
W5
W6(O)
W7(O)
W8(O)
W9
W10
W11
W12
A
V
= 0.35
sc
= 10
ov
= 0.33
16M
20M
25M
32M
40M
50M
80M
Fig. 5. Hertzsprung-Russell diagram of luminous stars in the SMC. Red dots represent sources from the B10 data set. Open dark
red squares are red supergiants from Davies et al. (2018), where the T
eff
is obtained using relations from Tabernero et al. (2018).
We show the Wolf-Rayet stars (Hainich et al. 2015; Shenar et al. 2016, 2017) labeled with a ‘W’ and their identifying number. Whe
they have an O-star companion that is brighter in the V-band, we show this instead (indicated by ‘(O)’). We also show evolutionary
tracks of Schootemeijer et al. (2019) with a semiconvection parameter of α
sc
= 10, and an overshooting parameter of α
ov
= 0.33.
Solid lines indicate hydrogen- and helium-core burning phases; dashed lines indicate the in between phase. The dash-dotted gray
line shows the location of these models halfway through their main-sequence (MS) lifetime, t
MS
.
peratures of hot stars with photometry (e.g., Massey 2003), this
means that flagging many B-type stars as O-type stars seems un-
avoidable with this method. This has also been argued by Smith
(2019), and we refer to Appendix B.1 for a quantitative discus-
sion that supports this view. It therefore seems likely that the
O-star completeness fraction quoted in B10 is severely underes-
timated.
In the Simbad catalog
4
, we inspected the four most luminous
sources in Fig. 3 that are not in the B10 catalog. From high to low
luminosity in Fig. 3, these are i) Sk 177 (Sanduleak 1969) with
the unusable spectral type information of ‘OB’; ii) the O5.5 V
star of log(L/L
) ≈ 5.3 in Lamb et al. (2013); iii) Sk 183, which
4
http://simbad.u-strasbg.fr
is an O3 V star with log(L/L
) = 5.66 (Evans et al. 2012); and
iv) a source without information on spectral type.
We return to the luminosity-dependent completeness fraction
presented in Table 1 in Sect. 4.4. There we discuss the luminos-
ity distribution of stars in the B10 sample and compare it with
theoretical predictions.
3. General population properties
Fig. 5 shows the distribution of the B10 sources in the HRD. It
contains 780 stars with T
eff
> 10
3.8
K that are more luminous
than 10
4.5
L
. Below T
eff
= 10
3.8
K, we rely on the red super-
giant (RSG) sample of Davies et al. (2018), which we overplot
on the HRD. To avoid duplicates, the B10 sources in this low-
5
