A&A 597, A22 (2017)
DOI: 10.1051/0004-6361/201628541
c
ESO 2016
Astronomy
&
Astrophysics
The IACOB project
III. New observational clues to understand macroturbulent broadening
in massive O- and B-type stars
⋆
S. Simón-Díaz
1, 2
, M. Godart
1, 2, 3
, N. Castro
4, 5
, A. Herrero
1, 2
, C. Aerts
6, 7
, J. Puls
8
, J. Telting
9
, and L. Grassitelli
4
1
Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
e-mail: ssimon@iac.es
2
Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
3
Institut d’Astrophysique et de Géophysique, Université de Liège, 17 allée du 6 Août, 4000 Liège, Belgium
4
Argelander Institut für Astronomie, Auf den Hägel 71, 53121 Bonn, Germany
5
Department of Astronomy, University of Michigan, 1085 S. University Avenue, Ann Arbor, MI 48109-1107, USA
6
Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
7
Department of Astrophysics/IMAPP, Radboud University Nijmegen, 6500 GL Nijmegen, The Netherlands
8
LMU Munich, Universitäts-Sternwarte, Scheinerstr. 1, 81679 München, Germany
9
Nordic Optical Telescope, Rambla José Ana Fernández Pérez 7, 38711 Breña Baja, Spain
Received 17 March 2016 / Accepted 16 August 2016
ABSTRACT
Context. The term macroturbulent broadening is commonly used to refer to a certain type of non-rotational broadening affecting the
spectral line profiles of O- and B-type stars. It has been proposed to be a spectroscopic signature of the presence of stellar oscillations;
however, we still lack a definitive confirmation of this hypothesis.
Aims. We aim to provide new empirical clues about macroturbulent spectral line broadening in O- and B-type stars to evaluate its
physical origin.
Methods. We used high-resolution spectra of 430 stars with spectral types in the range O4 – B9 (all luminosity classes) compiled in the
framework of the IACOB project. We characterized the line broadening of adequate diagnostic metal lines using a combined Fourier
transform and goodness-of-fit technique. We performed a quantitative spectroscopic analysis of the whole sample using automatic
tools coupled with a huge grid of fastwind models to determine their effective temperatures and gravities. We also incorporated
quantitative information about line asymmetries into our observational description of the characteristics of the line profiles, and
performed a comparison of the shape and type of line-profile variability found in a small sample of O stars and B supergiants with
still undefined pulsational properties and B main-sequence stars with variable line profiles owing to a well-identified type of stellar
oscillations or to the presence of spots in the stellar surface.
Results. We present a homogeneous and statistically significant overview of the (single snapshot) line-broadening properties of stars
in the whole O and B star domain. We find empirical evidence of the existence of various types of non-rotational broadening agents
acting in the realm of massive stars. Even though all these additional sources of line-broadening could be quoted and quantified as
a macroturbulent broadening from a practical point of view, their physical origin can be different. Contrarily to the early- to late-B
dwarfs and giants, which present a mixture of cases in terms of line-profile shape and variability, the whole O-type and B supergiant
domain (or, roughly speaking, stars with M
ZAMS
& 15 M
⊙
) is fully dominated by stars with a remarkable non-rotational broadening
component and very similar profiles (including type of variability). We provide some examples illustrating how this observational
dataset can be used to evaluate scenarios aimed at explaining the existence of sources of non-rotational broadening in massive stars.
Key words. stars: early-type – stars: fundamental parameters – stars: massive – stars: rotation – stars: oscillations –
techniques: spectroscopic
1. Introduction
Line profiles in optical spectra of OB stars
1
are not only
broadened by rotation. This statement has been known since
⋆
Full Table 1 is only available at the CDS via anonymous ftp to
cdsarc.u-strasbg.fr (130.79.128.5) or via
http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/597/A22
1
Throughout this paper, and following
Reed (2003), we use the term
OB stars to refer to O- and early-B type stars on the main sequence
and their evolved descendants, the B supergiants. The remaining B-type
stars (dwarfs and giants) are considered as a separate group.
the first large spectroscopic surveys of massive stars in
the late 1950s (
Slettebak 1956; Conti & Ebbets 1977). First
indirect suspicions, supported by the absence of narrow-line
stars of this type, were soon confirmed with the advent
of high-resolution spectrographs: the V shape of some of
the profiles did not correspond to an exclusively rotationally
broadened line. More recently, the use of Fourier transform
techniques (cf.
Gray 1976), in combination with profile
fitting techniques, has allowed us to have access to actual
projected rotational velocities (3 sin i) in OB stars and quantify
the relative contribution of the non-rotational and rotational
Article published by EDP Sciences A22, page 1 of 17
A&A 597, A22 (2017)
broadenings (e.g.,
Simón-Díaz & Herrero 2007, 2014, and
references therein).
The extra source of line broadening found in OB stars
has been commonly quoted as macroturbulent broadening (and
quantified as a macroturbulent velocity, 3
mac
). However, its
connection with large-scale turbulent motions of material in
the line-formation region
2
is highly improbable (see, e.g.,
Simón-Díaz et al. 2010). An alternative scenario proposed for
the O and B star domain relates the above mentioned extra
broadening to the effect of stellar oscillations on the line profiles
(e.g., Lucy 1976; Howarth 2004; Aerts et al. 2009). In this
context, not only the most commonly considered heat-driven
non-radial modes, but also other spectroscopic variability
phenomena identified and/or predicted in massive stars may play
a role (e.g., rotational modulation, strange modes, stochastically
excited non-radial modes and/or convectively driven internal
gravity waves).
The presence of a variable pulsational broadening
component is well known in observed line profiles of B
dwarfs and giants located in the β-Cep and SPB (slowly
pulsating B-type star) instability domains (e.g.,
Aerts et al.
2014, and references therein). Indirect arguments presented
in recent years (Aerts et al. 2009; Simón-Díaz et al. 2010)
indicates that this may also be the case for B supergiants
(Sgs). However, the macroturbulent-pulsational broadening
connection in the whole OB star domain, which is a region of
the Hertzsprung-Russel diagram that to-date is (by far) less
explored and understood from an asteroseismic point of view,
still requires direct (observational) confirmation. Indeed, even if
we assume this likely connection, the exact driving mechanism
(or mechanisms) of the type of oscillations that might result in
the observed (macroturbulent) profiles remains undefined (but
see
Aerts et al. 2009; Cantiello et al. 2009; Samadi et al. 2010;
Shiode et al. 2013; Sundqvist et al. 2013a; Simón-Díaz 2015;
Aerts & Rogers 2015; Grassitelli et al. 2015a).
With the aim of providing a set of empirical constraints that
could be used to assess the pulsational (or any other) hypothesis
to explain the physical origin of macroturbulent broadening in
OB stars, in 2008 we started the compilation of a high-quality
spectroscopic database including multi-epoch observations of
a large sample of bright (V < 9) northern O- and B-type
stars. This observational material, which has now become part
of the IACOB spectroscopic database (
Simón-Díaz et al. 2011a,
2015), presently comprises (a) high-resolution spectra of ≈620
Galactic stars covering spectral types between O4 and B9 and
all luminosity classes, and (b) time-series spectra with a time
span of several years and various types of time coverage for a
selected sample of targets.
In
Simón-Díaz et al. (2010) we used part of these
observations to present first empirical evidence for the existence
of a correlation between the macroturbulent broadening and
photospheric line-profile variations in a sample of 13 OB
Sgs. In
Simón-Díaz & Herrero (2014) we concentrated on
the line-broadening analysis of ≈200 O and early-B stars
to investigate the impact of other sources of non-rotational
broadening on the determination of projected rotational
velocities in OB stars. In this paper we benefit from a much
larger and extended (in terms of spectral type coverage)
2
This concept of macroturbulent broadening was initially introduced
and studied in the context of cool stars (see, e.g., the review by
Gray
1978
, and other historical references therein). Even though the use of
this term has been extended to other star domains, it does not necessarily
refer to the same type of broadening mechanism or the same physical
origin.
spectroscopic dataset to provide new observational clues to step
forward in our understanding of macroturbulent broadening in
massive stars. In particular, with this work we increase and
improve the available information about the single snapshot
properties of this enigmatic line broadening in the whole O and B
star domain, which is presently fragmentary and not necessarily
homogeneous (e.g.,
Ryans et al. 2002; Dufton et al. 2006;
Lefever et al. 2007, 2010; Fraser et al. 2010; Bouret et al. 2012;
Simón-Díaz & Herrero 2014; Markova et al. 2014; Martins et al.
2015; Mahy et al. 2015).
In this paper, we focus on the global aspects of the
line-broadening properties of the large sample we composed.
Subsequent work will be tuned toward time-series analysis
for a subsample of ≈70−100 targets. Sections
2 and 3
describe the observational dataset and spectroscopic analysis
tools we used to extract the level of line-broadening,
line-asymmetry, and spectroscopic parameters. The variety of
profiles found in the sample is illustrated and discussed in
Sect.
4.1, while the distribution of stars in the 3 sin i – 3
mac
and
spectroscopic Hertzsprung-Russell (sHR; Langer & Kudritzki
2014; Castro et al. 2014) diagrams is discussed in Sect. 4.2.
An extensive discussion on how these observations, once
combined with further information about line-profile skewness
and variability, can help us to definitely identify the origin of the
various sources of non-rotational broadening found in massive
stars, is presented in Sect. 5. Last, a summary of our work,
including the main conclusions and some future prospects, are
presented in Sect.
6.
2. Observations
The main observational sample discussed in this paper comprises
high-resolution, single snapshot spectra of 430 Galactic stars.
Basically, from the 618 O- and B-type stars in the IACOB
spectroscopic database as for November 2015, we discarded:
– 80 stars showing clear signatures of being a double line
spectroscopic binary, a multiple system or, more generally,
a composite spectrum in at least one of the IACOB spectra;
– 25 stars in which all the main diagnostic lines considered to
obtain information about 3 sin i and 3
mac
(see Sect.
3.1) are
weak, absent or present strong spectroscopic peculiarities,
among this subsample one can find stars with a spectral type
earlier than O4, very fast rotators and some Oe and Be stars;
– 83 stars having a projected rotational velocity larger than
200 km s
−1
(see explanation in Sect. 4.2.1).
The IACOB database includes spectra from two different
instruments: the FIES (Telting et al. 2014) and HERMES
(
Raskin et al. 2011) spectrographs attached to the 2.56 m
Nordic Optical Telescope and the 1.2 m Mercator telescope,
respectively. Both instruments provide a complete wavelength
coverage between 3800 and 7000 Å (9000 Å for the case of
HERMES spectra), and the associated resolving power (R) of
the spectra is 25 000, 46 000 (FIES) and 85 000 (HERMES).
By default, all the spectra in the IACOB database are reduced
using the corresponding available pipelines (FIEStool
3
and
HermesDRS
4
, respectively) and they are normalized by means
of our own procedures implemented in IDL.
3
http://www.not.iac.es/instruments/fies/fiestool/
FIEStool.html
4
http://www.mercator.iac.es/instruments/hermes/
hermesdrs.php
A22, page 2 of 17
S. Simón-Díaz et al.: New observational clues to understand macroturbulent broadening in massive O- and B-type stars
Table 1. Stars considered for this paper, including information about line broadening, stellar parameters, and the quantity RS k (relative skewness,
see Eq. (
1)).
Target SpC Line S/N
c
EW 3 sin i 3
mac
RS k σ
RSk
log T
eff
log L /L
⊙
FT GOF GOF
. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . .
HD 16582 B2 IV Si iii 183 138 9 9 19 0.02 0.05 4.34 2.94
HD 17081 B7 V Mg ii 195 294 18 19 24 −0.03 0.02 4.12 2.40
HD 17603 O7.5 Ib(f) O iii 239 317 109 99 115 −0.01 0.02 4.53 4.21
HD 17743 B8 III Mg ii 157 214 48 47 22 0.00 0.06 4.13 2.21
HD 18409 O9.7 Ib O iii 256 179 131 128 <88 0.08 0.16 4.51 3.99
HD 18604 B6 III Mg ii 184 305 131 132 <50 0.07 0.08 4.11 2.44
HD 19820 O8.5 III(n)((f)) O iii 316 251 144 147 <54 −0.22 0.11 4.51 3.91
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Notes. The line used to determine the line-broadening parameters is also indicated, along with its equivalent width and the signal-to-noise ratio of
the adjacent continuum. Spectral classifications indicated in Col. 2 must be handled with caution since they come from various sources, not all of
which are equally reliable. EW in mÅ, 3 sin i and 3
mac
in km s
−1
, T
eff
in K. The full table is available at the CDS.
The best signal-to-noise ratio (S/N) spectrum per considered
star was selected for the purposes of this study. The typical S/N
is in the range 150−300. In many cases, especially for targets
brighter than V = 6, the same star was observed with both
instruments. For these stars, we preferred the HERMES over the
FIES spectrum whenever the associated S/N is similar because
of the larger resolving power.
This main (single snapshot) spectroscopic dataset is
complemented with high-resolution spectroscopic time series of
a sample of eight selected targets (Sect.
5.2.3). Half of these stars
are well-known pulsators and stars with spots located in the B
main-sequence star domain (selected from the sample described
and analyzed in
Aerts et al. 2014, and references therein), the
other four stars correspond to bright O stars and B Sgs surveyed
by the IACOB project (as an extension of the observations
presented in Simón-Díaz et al. 2010). In all cases we rely on
more than 90 spectra gathered during more than four years.
These observations are used to provide a first comparison of
the type of line-profile variability present in OB stars with an
important contribution of the macroturbulent broadening and B
main-sequence stars with well-identified causes of spectroscopic
variability.
3. Tools and methods
In this section we describe the strategy we followed to extract
from the spectra information about (a) 3 sin i and 3
mac
; (b) the
amount of asymmetry of the diagnostic lines considered to
derive the line-broadening parameters; and (c) the stellar
parameters that allow us to locate the stars in the spectroscopic
HR diagram. The results from the analysis, which is used for the
discussion in Sects.
4 and 5, are summarized in Table 1, where
the meaning of each column is explained below.
3.1. Line-broadening parameters
We applied the iacob-broad tool (Simón-Díaz & Herrero
2014, in the following SDH14) to the O iii λ5591, Si iii λ4552,
Mg ii λ4481, or C ii λ4267 line, depending on the spectral
type of the star, to derive 3 sin i and 3
mac
in the whole
sample. We followed the strategy described in SDH14; namely,
the line-broadening analysis is based on a combined Fourier
transform (FT) plus goodness-of-fit (GOF) methodology where
we consider a radial-tangential definition for the macroturbulent
profile, with equal radial and tangential components, and the
starting intrinsic profile for the GOF computation is simplified
by a δ function.
In most cases, the outcome of the global rectification applied
to all IACOB spectra was fairly good when zooming into
the smaller spectral windows associated with the lines to be
analyzed. However, whenever necessary, we activated the option
available in iacob-broad to locally improve the normalization
before performing the line-broadening analysis.
Results from the line-broadening analysis of the 430 stars are
indicated in Cols. 6–8 of Table
1 along with some information
about the equivalent width (EW, Col. 5) of the considered
diagnostic line (Col. 3) and the S/N of the adjacent continuum
(Col. 4). We provide both FT and GOF solutions for 3 sin i, but
only the latter, together with the associated 3
mac
are considered
in this paper. The main reason for this is that we found a general
good agreement between the 3 sin i derived by means of both
methods (better than ±10 km s
−1
in most cases; see Fig.
1).
But we found that, first, the GOF approach is less affected by
the subjectivity in the selection of the first zero of the Fourier
transform in some complicated cases and, second, the GOF
solution allows us to detect cases in which we can only provide
upper limits to 3
mac
. The latter refers to situations in which the
3
mac
value associated with the best-fitting solution (minimum χ
2
)
is higher than zero, but 3
mac
= 0 is still an acceptable solution
(below the 1-σ confidence level). As we show and discuss in
Sect.
4, these cases normally correspond to profiles in which
the rotational broadening component dominates. Those cases
in which 3
mac
could not be properly determined are quoted in
Table 1 by providing the value corresponding to the best-fitting
solution as an upper limit.
3.2. Line-profile asymmetry
Most of the previous work on macroturbulent broadening in
OB stars concentrates on the characterization of the line profiles
via 3 sin i and 3
mac
. However, as is well known in the context
of stellar oscillations (cf. Chapts. 5 and 6 of Aerts et al. 2010b),
such a simplistic two-parameter description lacks an important
piece of information about the properties of the profiles: the
amount of line asymmetry. In this respect,
Schrijvers et al.
(1997) is a particularly illustrative study in which the predicted
variability of line profiles from adiabatic non-radial pulsations
in rotating stars is shown for a large variety of oscillation
A22, page 3 of 17
A&A 597, A22 (2017)
Fig. 1. Comparison of the 3 sin i values determined by means of the FT
and GOF methodologies using the iacob-broad tool. The agreement
is better than ±10 km s
−1
for 95% of the sample, and better than
±20 km s
−1
for the remaining stars.
and rotation parameters. This reference work contains various
diagnostics used to describe the snapshot characteristics of the
line profiles, a combination of which we adopt here as the
relative skewness (RS k). We define this quantity as
RS k ≡ h3
3
i/h3
2
i
3/2
, (1)
where
h3
n
i =
R
∞
−∞
(3 − h3i)
n
(1 − F(3))d3
R
∞
−∞
(1 − F(3))d3
for n = 2, 3 (2)
are the second and third normalized central moments of a
spectral line denoted as (3, F(3)), adopting the definition used
in
Schrijvers et al. (1997), and
h3i =
R
∞
−∞
3 (1 − F(3))d3
R
∞
−∞
(1 − F(3))d3
· (3)
The dimensionless quantity defined by Eq. (
1) – which, in
statistical terms, is quoted as the third standardized moment
or (Pearson’s) moment coefficient of skewness – represents the
amount of skewness of the line profile relative to its total width.
This allows us to compare the amount of asymmetry for lines
with very different widths in a meaningful way.
We used the same line profiles considered to derive the
line-broadening parameters (Sect.
3.1) for the computation of
the first three moments and, ultimately, the quantity RS k. To
compute the latter, we considered various integration limits and
found the best overall choice to be an integration centered on
h3i and extending over ±2.5
p
h3
2
i. These integration limits were
selected as the best compromise between including as much
information as possible from the extended wings of the V-shaped
line profiles, while avoiding spurious subtleties associated with
the noise of the adjacent continuum.
The computed values of RS k for the whole sample of stars,
along with their corresponding uncertainties, are presented in
Cols. 9 and 10 in Table
1. These uncertainties result from
the formal propagation of errors for Eqs. (1)–(3), assuming
that the only source of uncertainty is the noise associated
with the normalized flux of the line profile, which is inversely
proportional to the S/N of the adjacent continuum (S/N
c
). Given
the stability of the spectrographs and the level of precision of the
wavelength calibration, this approximation is fully justified.
3.3. Stellar parameters
The spectroscopic stellar parameters of the sample were
obtained by means of iacob-gbat (Simón-Díaz et al. 2011b,
O stars) or an updated version of the tool described in
Castro et al. (2012, B stars). These tools, aimed at performing
quantitative spectroscopic analyses of O- and B-type stars in
a fast, automatic way, are based on a huge, pre-computed
grid of fastwind (Santolaya-Rey et al. 1997; Puls et al.
2005; Rivero González et al. 2012) synthetic spectra and a
χ
2
minimization line-profile fitting technique. Some notes on
the followed strategy can be found in Lefever et al. (2007),
Castro et al. (2012), and Sabín-Sanjulián et al. (2014). In this
case, we fixed 3 sin i and 3
mac
to the values resulting from
the line-broadening analysis described above. From the whole
set of output parameters provided by the automatic tools, we
concentrate on the derived T
eff
and log g because they allow us
to locate the studied stars in the sHR diagram.
We provide log T
eff
and log L /L
⊙
(where L := T
eff
4
/g,
Langer & Kudritzki 2014) in the two last columns of Table 1
to facilitate the identification of stars in the corresponding
diagrams. In addition, we highlight below some important points
regarding the compiled set of stellar parameters:
– Given the quality of the spectroscopic observations and the
strategy we have followed for the spectroscopic analysis, we
can assume ≈5% and ≈0.15 dex as rough estimations for the
uncertainties in the derived T
eff
and log g, respectively.
– There are ≈50 stars for which we do not provide stellar
parameters in Table
1. Most of these stars are late-B stars
whose parameters lay outside our grid of fastwind models.
The lower T
eff
boundary of the grid is 11 000 K and
this implies that our analysis tools do not provide reliable
parameters for stars with T
eff
≤ 12 000 K.
– We have not checked one-by-one all the analyzed spectra in
detail. Therefore, the stellar parameters quoted in Table 1
must be considered with caution for purposes other than
those discussed in this paper, especially concerning the
investigation of individual stars.
4. Results
4.1. Line profiles in O and B stars: a qualitative overview
Figures 2 and 3 show some representative examples of the
various types of line profiles found in stars in the IACOB sample.
In all cases the observed profiles, colored and labeled following
the guidelines described in Sect. 4.2.1, are complemented
with information from the outcome of the line-broadening and
line-asymmetry analyses (see Sect.
3). We also overplot, for
guidance purposes, synthetic profiles with the same equivalent
width as the observed profiles, convolved with the indicated
3 sin i (dotted lines) and 3 sin i + 3
mac
values (dashed lines). The
comparison between the dotted lines and the observed profiles
allows us to visualize the effect of the non-rotational broadening
on the shape of the lines for different 3 sin i−3
mac
combinations.
The dashed lines can be used to assess the quality of the final fit
and visually identify line profiles that are clearly asymmetric or
have a bumpy shape (see, e.g., Fig.
3 and description below).
A22, page 4 of 17
S. Simón-Díaz et al.: New observational clues to understand macroturbulent broadening in massive O- and B-type stars
Fig. 2. Illustrative examples of the various types of line profiles found
in the IACOB sample of O- and B-type stars. The values of 3 sin i and
3
mac
resulting from the line-broadening and line-asymmetry analyses
(see Sect.
3) are quoted in the lower right corners. Each profile was
selected to be a representative of the six regions indicated in Fig.
4
(the same color and A)−B) label codes are used here). The profiles
are organized from bottom to top and from left to right, following an
increasing sequence of 3
mac
and 3 sin i, respectively.
As illustrated by Figs.
2 and 3, the width of the profiles
ranges from narrow to broad, and its shape varies from clearly
roundish, as expected in a rotationally dominated case, to
triangular, where the so-called macroturbulent broadening is
dominating. Many different combinations of width and shape
occur. Most of the profiles are smooth (i.e., do not have any
detectable substructure) and symmetric from visual inspection,
having similar characteristics as those shown in Fig. 2;
however, there is also a non-negligible number of stars showing
asymmetric profiles and/or spectroscopic signatures that could
be associated with the effect of certain type of pulsations on the
line profiles, spots, a magnetosphere, and/or undetected binarity
(see some examples in Fig.
3). In particular, regarding pulsating
stars, the most clearly detectable cases from single snapshot
spectra are those associated with coherent pressure modes as
in β Cep stars with moderate projected rotational velocities
(see, e.g., Panel B.3 in Fig. 3 and Telting et al. 2006, for more
illustrative examples) or coherent gravity modes that occur in
so-called SPBs (
De Cat & Aerts 2002).
4.2. Line broadening in O and B stars: A quantitative
overview
Outside the asteroseismology community, it is common to
summarize the information about the width and shape of
spectral line profiles by means of only two time-independent,
line-broadening parameters: 3 sin i and 3
mac
. While the first
parameter has a well-defined physical meaning, 3
mac
is only
a tuning parameter used to quantify the effect of any other
broadening mechanism that is not rotation. For example, as
indicated by
Aerts et al. (2014), periodic line-profile variability
Fig. 3. Same as Fig. 2 but for line profiles that are more asymmetric
(A.1, A.3, B.2) or have a more complex structure (A.2, B.3). A case for
which only an upper limit of 3
mac
can be obtained is also illustrated in
B.1. In the latter, iacob-broad gives (3 sin i, 3
mac
) = (134, 47; dashed
line) as the best fitting solution, but (134, 0; dotted line) is an equally
acceptable solution.
caused by surface inhomogeneities or by oscillations in
main-sequence B stars can be mimicked by a combination
of time-dependent rotational and macroturbulent broadening.
Moreover, the value derived for this parameter depends on
specific assumptions on the definition of the macroturbulent
profile (e.g., radial-tangential versus isotropic Gaussian and the
percentage of radial versus tangential components). In addition,
as highlighted in SDH14 (see also
Sundqvist et al. 2013b;
Markova et al. 2014), there is empirical evidence indicating that
the methodology described in Sect. 3.1 for disentangling rotation
from other sources of line broadening may be failing in some
specific cases. This mainly refers to potential limitations, of
still unclear origin, in having access to the actual value of the
projected rotational velocity in stars with line profiles dominated
by the so-called macroturbulent broadening. Therefore, we must
handle any quantitative interpretation of the macroturbulent
broadening in O and B stars in terms of 3
mac
with care, especially
when combining measurements from different sources that
might not be using the same techniques or assumptions.
A detailed investigation of the shortcomings of the
application of the FT and GOF methodologies in the whole
O and B star domain, following some guidelines presented in
SDH14, is planed for a subsequent paper of this series (see
also some notes in Sect.
5.2.3). In the meantime, we rely on
the measurements resulting from the application of the methods
described in Sect. 3.1 to provide a complete homogeneous
overview of the single snapshot line-broadening characteristics
of stars in the whole O and B star domain. Even though some
of the 3 sin i and 3
mac
values quoted in Table 1 may change in
the future, the main conclusions presented in these sections will
remain since the followed approach provides a representation of
the global shape of the line profiles that are valid enough for the
purposes of this work.
A22, page 5 of
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