A&A 489, 1255-1262 (2008)
DOI: 10.1051/0004-6361:200809587
N. Nardetto1 - A. Stoekl2 - D. Bersier3 - T. G. Barnes4,5
1 - Max-Planck-Institut für Radioastronomie, Auf dem Hügel
69, 53121 Bonn, Germany
2 - CRAL, Université de Lyon, CNRS
(UMR5574), École Normale Supérieure de Lyon, 69007 Lyon,
France
3 - Astrophysics Research Institute, Liverpool John Moores
University, Twelve Quays House, Egerton Wharf, Birkenhead, CH41 1LD,
UK
4 - University of Texas at Austin, McDonald Observatory, 1
University Station, C1402, Austin, TX 78712-0259, USA
5 - currently
on assignment to the National Science Foundation, 4201 Wilson
Boulevard, Arlington, VA 22230, USA
Received 15 February 2008 / Accepted 1 April 2008
Abstract
Context. Galactic Cepheids in the vicinity of the Sun have a residual line-of-sight velocity, or -velocity, which shows a systematic blueshift of about 2 km s-1 compared to an axisymmetric rotation model of the Milky Way. This term is either related to the space motion of the star and, consequently, to the kinematic structure of our Galaxy, or it is the result of the dynamical structure of the Cepheids' atmosphere.
Aims. We aim to show that these residual -velocities are an intrinsic property of Cepheids.
Methods. We observed eight galactic Cepheids with the HARPS spectroscope, focusing specifically on 17 spectral lines. For each spectral line of each star, we computed the
-velocity (resp.
-asymmetry) as an average value of the interpolated radial velocity (resp. line asymmetry) curve.
Results. For each Cepheid in our sample, a linear relation is found between the -velocities of the various spectral lines and their corresponding
-asymmetries, showing that residual
-velocities stem from the intrinsic properties of Cepheids. We also provide a physical reference to the stellar
-velocity: it should be zero when the
-asymmetry is zero. Following this definition, we provide very precise and physically calibrated estimates of the
-velocities for all stars of our sample [in km s-1]:
[R TrA],
[S Cru],
[Y Sgr],
[
Dor],
[
Gem],
[RZ Vel],
[
Car],
[RS Pup]. Finally, we investigated several physical explanations for these
-asymmetries like velocity gradients or the relative motion of the line-forming region compared to the corresponding mass elements. However, none of these hypotheses seems to be entirely satisfactory to explain the observations.
Conclusions. To understand this very subtle -asymmetry effect, further numerical studies are needed. Cepheids' atmosphere are strongly affected by pulsational dynamics, convective flows, nonlinear physics, and complex radiative transport. Hence, all of these effects have to be incorporated simultaneously and consistently into the numerical models to reproduce the observed line profiles in detail.
Key words: techniques: spectroscopic - stars: atmospheres - stars: oscillations - stars: variables: Cepheids - stars: distances
Cepheids are very important astrophysical objects due to their well-known period-luminosity (PL) relation. Based on this relation, a multi-decade work allowed us to determine the kinematic structure of the Milky Way (in particular its rotation) and to reach cosmologically significant extragalactic distances (see Hubble Space Telescope Key Project, Freedman et al. 2001). In the second paper of this series, Nardetto et al. (2007) (hereafter Paper II) established a clear link between the distance scale and the dynamical structure of Cepheids' atmosphere through a period-projection factor (Pp) relation. Similarly, studies concerning the kinematics of the Milky Way might be closely related to the dynamical structure of Cepheids' atmosphere.
Concerning the distance scale, near-infrared interferometry
currently provides a new, quasi-geometrical way to determine the
distance of galactic Cepheids up to 1 kpc (see e.g. Sasselov &
Karovska 1994; and Kervella et al. 2004).
The basic principle of the Interferometric Baade-Wesselink method
(IBW) is to compare the linear and angular size variation of a
pulsating star in order to derive its distance through a simple
division. The key point is that interferometric measurements in
the continuum lead to angular diameters corresponding to the photospheric layer, while the linear stellar radius variation is
deduced by spectroscopy, i.e., based on line-forming regions which
form higher in the atmosphere. Thus, radial velocities
,
which are derived from line profiles, include
the integration in two directions: over the stellar surface through
limb-darkening and over the atmospheric layers through velocity
gradients. All these phenomena are currently merged in one specific
quantity, generally considered constant with time: the projection
factor p, defined as
,
where
is defined as the photospheric pulsation
velocity (Nardetto et al. 2004).
is then
integrated with time to derive the photospheric radius variation.
The precision in the distance currently obtained with the IBW method
is a few percent. However, it remains strongly dependent on the
projection factor. If a constant projection factor is used
(generally p=1.36 for all stars) to derive the PL relation,
errors of 0.10 and 0.03 on the slope and zero-point of the
PL relation can be introduced. This means that distances can
be overestimated by 10% for long-period Cepheids (Paper II).
In Paper II, we divide the projection factor into three sub-topics:
(1) a geometrical effect related to the limb-darkening; (2) the
velocity gradient within the atmosphere; and (3) the differential
motion of the ``optical'' pulsating photosphere compared to the
corresponding mass elements, called
.
Even if the
Pp relation was recently confirmed by HST observations
(Fouqué et al. 2007), the
is relatively
uncertain and currently entirely based on hydrodynamical
calculations. Nevertheless, a key point is that this quantity should
be, in principle, related to the so-called
-velocity term.
As Sabbey et al. (1995) noted, the changing depth of the spectral
line-forming region over a Cepheid pulsation cycle leads to unequal
extrema in the line asymmetry curve. Such phenomenon was also
observed in the first paper of this series (Nardetto et al. 2006,
hereafter Paper I). Spectral lines are thus not associated with the
same gas particles during the pulsation cycle. Formally, they do not
comply with path conservation, stated as
The state of the art of the BW methods, concerning the
-velocity is the following: the
-velocity is
generally removed, i.e. the average value of the radial velocity
curve is forced to zero before integrating. However, the
differential motion between the line-forming region and the gas also
modifies the velocity amplitude. The
was
introduced, based on hydrodynamical models in Paper II, in order to
correct this velocity amplitude effect. The next step is to find a
correlation between
and the
-velocity. In
this paper, we will provide some indications, but the problem
remains unsolved.
Table 1:
Linear relations between the -velocities and the
-asymmetries
for all stars. The last column corresponds to the
final physically calibrated
-velocities for each star (see
Sect. 3.1 for details). The subscripts give the
uncertainty.
The -velocities are also of great importance for the
determination of the kinematic structure of the Milky Way. Galactic
Cepheids in the solar vicinity show a residual line-of-sight
velocity (a ``K-term'') in their radial velocities which is
systematically blueshifted of about 2 km s-1 compared to an
axisymmetric rotation model of the Milky Way (Camm 1938, 1944;
Parenago 1945; Stibbs 1956; Wielen 1974;
Caldwell & Coulson 1987;
Moffett & Barnes 1987; Wilson et al. 1991;
Pont et al. 1994). Wielen (1974) found no correlation of the K-term with any
obvious parameter such as period, amplitude or distance, and
concluded that the K-term is an intrinsic property of Cepheid
atmospheres. On the other hand, Pont et al. (1994) tried to revive
Camm (1944) and Parenago's (1945) suggestion that the K-term is due
to a real effect in the dynamics of the Galaxy. In addition, Butler
et al. (1996) found a
-velocity reduced by 2 km s-1 by
introducing velocity gradients in hydrostatic stellar atmospheres
models. The K-term problem is still a matter of debate today.
Based on very high quality HARPS observations and careful
methodology (Sect. 2), we will show that
-velocities are due to intrinsic properties of Cepheids
(Sect. 3). Finally, we discuss our results in
Sect. 4.
Ten stars have been observed with the HARPS spectrometer
(R=120 000): R Tra, S Cru, Y Sgr,
Dor,
Gem, Y Oph, RZ Vel,
Car, RS Pup and X Sgr. X Sgr is an atypical
Cepheid presenting several components in the spectral line profiles.
It was studied separately by Mathias et al. (2006). Y Oph is not
studied here in detail due to its insufficient phase coverage (see
Paper II, Fig. 3). We thus consider 8 Cepheids in this paper.
![]() |
Figure 1:
Fe I 4896.4 (
![]() ![]() ![]() ![]() |
Open with DEXTER |
Using Kurucz models 1992 we have identified about 150 unblended spectral lines. In Paper II, we carefully selected 17 spectral lines following two criteria: (1) the continuum must to be perfectly defined for all pulsation phases of all stars, in order to avoid bias in the determination of the line depth; (2) the selected sample of lines has to cover a large range of depth. The spectral lines selected are presented in Table 1 of Paper II.
As in Paper I, we use bi-Gaussian fits to derive line asymmetries.
We repeat here the main equations in order to show that there is no
a priori link between the line asymmetry and our
centroid method of the radial velocity determination.
The centroid radial velocity (
), or the first moment
of the spectral line profile, has been estimated as
The analytic line profile is defined by
We insist on the
definition of the radial velocity
since it is absolutely required to allow direct comparisons between
-velocities of different spectral lines from different
Cepheids. Indeed, it is the only method which provides a
radial velocity independent of the rotation (projected on the line
of sight) and the natural width of the spectral line (Burki et al. 1982, and Paper I).
The
and A quantities for 17 selected spectral
lines for each of the 8 stars have been interpolated over the
pulsation phase using a periodic cubic spline function. The
interpolation is performed either directly on the observational
points (e.g.
Dor) or using arbitrary pivot points (e.g. RZ Vel). In the latter case, a classical minimization process
between the observations and the interpolated curve is used to
optimize the position of the pivot points. All interpolated curves
presented in this study were derived using one of these two methods.
Finally,
and
are calculated by
averaging the
and
interpolated curves, where
is the pulsation phase. The
corresponding uncertainties are defined as the average values of
individual uncertainties on the observational points.
The aim of this section is to study the -velocities and to
show that they consist of two components: one related to the space
motion of the star itself, and one (the K-term) related to the
dynamical structure of Cepheids' atmosphere.
To introduce our methodology we discuss the exemplary case of
Dor in detail.
First, interpolated radial velocity curves derived from all spectral
lines were corrected by the -velocity found in the Galactic
Cepheid Database of the David Dunlap
Observatory
(Fernie et al. 1995, hereafter
,
see
Table 1).
Figure 1 shows the spectral line profile of two metallic
lines Fe I 4896.4 (
% of the continuum) and
Fe I 6024.1 (
%) as a function of the pulsation
phase. We translated wavelengths into velocities for comparison. We
used spectral lines with different line depths for clarity, but the
results are actually independent of the line depth. Two
qualitative key-points concerning Fig. 1 should be
mentioned: (1) the Fe I 4896.4 spectral line (smaller line
depth) seems to be systematically redshifted compared to the
Fe I 6024.1 spectral line (look, for instance, at the pixel
minimum of each profile) and (2) its asymmetry is also
systematically larger (in absolute value) from phase
to
,
while it is systematically lower (in absolute value)
from phase
to
.
This leads us to the idea of
a correlation between the
-velocity and the
-asymmetry. In Figs. 2a,b we present the
corresponding interpolated radial velocity and asymmetry curves for
these two spectral lines. Another line of intermediate depth is also
presented (Fe I 5373, solid line). From the interpolated
curves, we now calculate the
-velocities and -asymmetries
corresponding to each spectral line (horizontal lines). An
anti-correlation is clearly seen.
In Fig. 2c,
plots and the
corresponding (
,
)
average values (crosses)
for the three different lines are presented. Although the
plots have different shapes (and this is
confirmed for all spectral lines), which is the result of the
dynamical structure of the Cepheid atmosphere, the average values
(big crosses) are aligned, confirming the correlation found in
Figs. 2a,b.
Figure 2d is a generalization of diagram (c) for all
spectral lines. The
curves are not included for
clarity. Upper values correspond to residual
-velocities
of the 17 selected spectral lines i after the GCD
-velocity correction. A linear fit is performed, and we find
the relation
In principle, the line asymmetry and the -asymmetry are
supposed to be the result of the dynamical structure of the Cepheid
atmosphere only. The
(hereafter
)
correlation found is then a strong
indication that residual
-velocities (after correction by
the GCD
-velocities) are related to intrinsic physical
properties of Cepheids' atmosphere and not to a real effect in
dynamics of the Galaxy (see discussion).
and
were derived for all spectral lines
and for all stars of our sample using the same method as presented
in the case of
Dor.
Table 1 gives
,
the slope
of the interpolated
curves,
the correction
applied, and our final
-velocities
for each star. The reduced
,
defined as
with N being the number
of spectral lines and
the number of degrees of freedom, is
also indicated. The
plots are shown in
Fig. 3. Linear correlation curves between
and
are found for all stars of our sample, and
our
corrections range from 0.2 to 3.6 km s-1. The average
value is
km s-1, which is consistent with the 2 km s-1 ``K-term'' found in the literature. We discuss these important
observational results in the next section.
The linear relation between
and
can be
easily understood. The basic principle is demonstrated in
Fig. 1 for the pulsation phase
.
For
clarity, we only present the argument for one pulsation phase.
The solid line is the bi-Gaussian fit of the Fe I 6024.1
spectral line. Artificially decreasing the bi-Gaussian asymmetry of
this spectral line by the average asymmetry of the
Fe I 6024.1 line,
,
as shown in
Fig. 1 with a dashed-line, we find that the
centroid-velocity increases (i.e. is redshifted) by an amount of
+1.45 km s-1. To illustrate this, we included in Fig. 1
the computed line positions
by vertical lines
before (solid line) and after (dot-dashed line) modification of the
bi-Gaussian asymmetry. Generally, we find that forcing the average
line asymmetry
to zero by uniformly changing the line
asymmetry by the same amount at all phases alters the derived
centroid velocities
in such a way that the
resulting
-velocity becomes zero. For instance, the point
(
%,
km s-1) of the
linear curve of
Dor (Fig. 3) translates into (
%,
km s-1).
This result is obtained universally, regardless which spectral line
or star is considered. Basically, we can conclude that the
-velocities are a side-effect of the line asymmetries and
related to the problem of determining line positions of asymmetric
lines. This view is also supported by the high conformity of the
slopes
of the
relation for
all sample stars (see Table 1) which (almost) agree
within
uncertainties.
Although this method of arbitrarily modifying the line asymmetry is
a rather ad hoc method, it still allowed us to gain some
insight into the origin of the -velocities. In particular,
it shows (1) that the residual
-velocities are related to
the shapes of the spectral lines and, consequently, to an intrinsic
property of Cepheids; and (2) that the main physical question is not
to understand the
-velocities, but to understand the
-asymmetries.
Another important point is that this interpretation provides a
physical meaning to the -velocities and, hence, a physical
reference. The relation between
-asymmetry and
-velocity allows us to compute the contribution of the
dynamical Cepheid's atmosphere to the
-velocity. We can thus
really provide a
-velocity corresponding to the space motion
of the star itself (
), independently of the
dynamical structure of its atmosphere. From Table 1,
we find an average systematic red-shift correction of
km s-1 (averaged over 8 stars) between
our physically calibrated
-velocities and the ones found in
the GCD, commonly used by the community. Consequently, the K-term
(blueshifted) found in the literature is not due to the kinematic
structure of the Galaxy, but to a bias in the previous methods of
deriving the
-velocities (cross-correlation, Gaussian fit of
the spectral line), most likely due to these
linear relations. By using only one metallic line to derive the
-velocity, one can make, for instance, an error (or find
inconsistencies) ranging from -2 to +1 km s-1. Finally, our results
are thus consistent with an axisymmetric rotation model of the Milky
Way.
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Figure 2:
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
Open with DEXTER |
![]() |
Figure 3:
Same as Fig. 2d for all Cepheids of our
sample. The origin of the
![]() ![]() ![]() ![]() ![]() |
Open with DEXTER |
Different aspects of the -asymmetries must be pointed out.
![]() |
Figure 4:
Radius (in solar radii) as a function of the pulsation
phase for spectral lines of our sample. The radial velocity curves
have been corrected from their ![]() |
Open with DEXTER |
![]() |
Figure 5:
![]() |
Open with DEXTER |
We found -asymmetries varying from one spectral line to
another, as well as a global dependency on the period of the star
or, correspondingly, on the spatial extension of the Cepheids'
atmosphere. Right now, we have no clear physical explanations for
this effect. Most likely, it results from a combination of several
effects in the dynamical structure of the Cepheids' atmosphere, such
as phase- and wavelength-dependence of the intensity distribution
within the different spectral lines, velocity gradients, non-linear
pulsational effects, shock fronts, and relative motions between
line-forming regions (specific to each spectral line considered) and
the material.
In order to further investigate the line asymmetries, improved
numerical models are required. Since Cepheids' atmospheres are not
in a hydrostatic state but characterized by pulsational dynamics,
one has to perform non-linear, time-dependent simulations of the
underlying pulsation. Snapshots from this temporal evolution
(including the velocity field) can then be used to compute a
detailed frequency-dependent radiative transport. In order to
resolve narrow features such as shock fronts or sharp ionization
regions, high spatial resolution, especially in the line-forming
regions, is needed. Although convective transport plays only a minor
role in the stellar structure of Cepheids in that temperature range
(
,
see Table 2 in Paper II), there can still be
considerable convective velocities (some km s-1). As a consequence,
the consistent inclusion of the convective velocity field
- and of the interaction of convection with pulsation - in the numerical models
might be crucial for the computation of line asymmetries. Dynamics
in the circumstellar envelope (Kervella et al. 2006; Mérand
et al. 2006, 2007; Nardetto et al. 2008) is also important. Confronting
such models with observations (spectral line profiles, spatial- and
spectral- visibility curves from interferometry) may finally lead
the way to a complete picture of the relevant effects in Cepheids'
atmospheres.
However, even now, we already have a clear evidence from our
observed linear
relation that the residual
-velocities (or K-term) seen in Cepheids are the result of
the dynamical structure of their atmosphere. This provides a
physical meaning to the
-velocities and a physical
reference: the
-velocity should be zero when the
-asymmetry is zero. This definition of the
-velocities could be used for kinematical studies of the
Galaxy, even though this method requires high signal-to-noise and
high-resolution spectroscopic observations. Using only one metallic
line to derive the
-velocity can inflict errors (or
inconsistencies) ranging from -2 to 1 km s-1.
Acknowledgements
Based on observations collected at La Silla observatory, Chile, in the framework of European Southern Observatory's programs 072.D-0419 and 073.D-0136. We thanks Kervella for having provided the HARPS data and Fekety for her careful English correction of the paper. This research has made use of the SIMBAD and VIZIER databases at CDS, Strasbourg (France). This material is based in part upon work by TGB while serving at the National Science Foundation. Any opinions, findings, and conclusionsor recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. N.N. acknowledges the Max Planck Institut for Radioastronomy for financial support.
Table 2:
and
for all
spectral lines of all stars.