A&A 379, 905-916 (2001)
DOI: 10.1051/0004-6361:20011406
P. Mathias1 -
C. Aerts2 - M. Briquet3 - P. De Cat2 - J. Cuypers4 - H. Van
Winckel2, - J. M. Le Contel1
1 - Observatoire de la Côte d'Azur,
Departement Fresnel, UMR 6528, BP 4229, 06304 Nice Cedex 4, France
2 -
Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200
B, 3001 Leuven, Belgium
3 - Institut d'Astrophysique et de Géophysique,
Avenue de Cointe 5, 4000 Liège, Belgium
4 - Koninklijke Sterrenwacht van
België, Ringlaan 3, 1180 Brussel, Belgium
Received 6 July 2001 / Accepted 27 September 2001
Abstract
A one-year follow-up campaign of
high-resolution, high-signal-to-noise spectroscopy for 10 candidate slowly
pulsating B stars, which were discovered from the HIPPARCOS astrometric mission,
shows that all stars exhibit line-profile variability. From our data, and from
the HIPPARCOS photometry, we conclude that all but one of the targets provide
evidence of multiperiodicity, with periods of the order of days, confirming
their pulsational nature. Thus they are confirmed slowly
pulsating B stars. We summarize the pulsation periods and Q-values
and select the most interesting targets for very-long-term
follow-up observations with the goal of performing
asteroseismology.
Key words: stars: variables: slowly pulsating B stars - stars: oscillations - line: profiles - stars: binaries: spectroscopic
In the coming decade, a large step forward is expected in the detailed knowledge of the internal structure of stars through the technique of asteroseismology. The goal of this, relatively new, research domain is to derive the internal processes of stars with unprecedented precision through a detailed study of their non-radial oscillations. The predictive power of helioseismology performed on SoHO data has given rise to the development of several future space missions devoted to seismological studies of stars across the whole HR diagram. These missions, of which the launches are foreseen in the time frame 2002-2004, are currently in full preparation.
This paper deals with a class of non-radial pulsators along the main
sequence, namely the slowly pulsating B stars (hereafter termed SPBs - see
Waelkens 1991 for the definition of the class of variables). These stars
are multiperiodic high-order low-degree gravity-mode oscillators with masses in
the range
.
The HIPPARCOS mission led to the discovery of many new SPB candidates (Waelkens et al. 1998, PaperI). In view of their potential for asteroseismology, Aerts et al. (1999, PaperII) selected twelve bright stars of this sample, together with five previously known SPBs, for spectroscopic and photometric monitoring in the Southern Hemisphere. Of these seventeen stars, with spectral types ranging from B2 up to B9, two were misclassified since their line-profile variability indicates chemical inhomogeneities at the rotating stellar surface rather than pulsation. Additionally, one star is an ellipsoidal variable with an orbital period of 1.7 days. These stars are not SPBs. About half of the remaining stars in the sample presented in PaperII turns out to be close spectroscopic binaries. Their orbital parameters were derived by De Cat et al. (2000, PaperIII). De Cat (2001) performed a detailed frequency analysis and a first attempt of mode identification for all targets. All true SPBs selected in PaperII exhibit clear line-profile variability. Thus a study of the latter in principle allows us to derive the characteristics of the g-mode pulsations in full detail, provided that the overall beat-period is well covered. This is a large observational challenge, but is feasible if one has permanent access to a moderate-size telescope equipped with an Echelle spectrograph.
The main goal of this paper is to derive the basic pulsational characteristics of a sample of bright northern SPBs to increase the sample selected in PaperII. To confirm the pulsational nature of the targets, we have started a long-term spectroscopic campaign with the 1.52 m telescope, equipped with the spectrograph AURELIE, at the Observatoire de Haute-Provence over 1.5 years. In this paper, we report on the first results from this campaign for the 10 target stars, as well as on a search for multiperiodicity in the HIPPARCOS photometry. A second paper will be devoted to the detailed study of the line-profile variations of one target, with the goal of identifying the pulsation modes.
Period | Nights | Observer(s) |
February 98 | 7 | Mathias |
June 98 | 5 | Mathias |
July 98 | 8 | De Cat |
August 98 | 6 | Van Winckel |
October 98 | 6 | Mathias |
December 98 | 8 | Mathias |
April 99 | 7 | Aerts/Mathias |
May 99 | 6 | Briquet/Le Contel |
HD | HR (name) | Spectra | Range | Exposure | S/N |
1976 | 91 | 26 | 197 | 58 | 140 |
21071 | 1029 | 35 | 436 | 64 | 80 |
25558 | 1253 (40Tau) | 25 | 313 | 59 | 140 |
28114 | 1397 | 17 | 305 | 64 | 90 |
138764 | 5780 | 16 | 185 | 58 | 130 |
140873 | 5863 | 14 | 186 | 60 | 110 |
147394 | 6092 (![]() |
280 | 460 | 15 | 190 |
182255 | 7358 (3Vul) | 53 | 348 | 48 | 150 |
206540 | 8292 | 25 | 159 | 77 | 100 |
208057 | 8356 (16Peg) | 36 | 201 | 52 | 140 |
HD | ST | V |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
1976 | B5IV | 5.6 | 4.20 | 2.92 | 4.07 | 5.0 | 3.41 | <140 |
21071 | B7V | 6.1 | 4.15 | 2.53 | 4.36 | 4.1 | 2.21 | <62 |
25558 | B3V | 5.3 | 4.23 | 2.81 | 4.21 | 5.1 | 2.93 | <22 |
28114 | B6IV | 6.1 | 4.16 | 3.02 | 4.03 | 5.0 | 3.57 | <11 |
138764 | B6IV | 5.2 | 4.15 | 2.69 | 4.24 | 4.3 | 2.60 | <13 |
140873 | B8III | 5.4 | 4.15 | 2.44 | 4.37 | 3.9 | 2.13 | - |
147394 | B5IV | 3.9 | 4.17 | 3.33 | 4.08 | 5.6 | 3.57 | <36 |
182255 | B6III | 5.2 | 4.15 | 2.60 | 4.28 | 4.2 | 2.46 | <14 |
206540 | B5IV | 6.1 | 4.14 | 2.81 | 4.14 | 4.5 | 2.99 | <10 |
208057 | B3Ve | 5.1 | 4.23 | 2.87 | 4.12 | 5.2 | 3.28 | <110 |
Once the different radial velocity variations are obtained, a frequency analysis was performed on them, as well as on the HIPPARCOS photometry. The latter data were specifically used to search for multiperiodicity. Four different period-search methods, adapted to unequally spaced data, were used to perform the frequency analyses: Fourier analysis, CLEAN (Roberts et al. 1987), Vanicek (1971), and PDM (Stellingwerf 1978). Each periodogram was computed in the [0; 3]cd-1 interval. For the CLEAN method, 100 iterations were performed, with a 0.5 gain. For the PDM method, the bin structure was (5, 2). We emphasize that the CLEAN and Vanicek algorithms are not independent. Moreover, they are both based on Fourier analysis. Both methods try to deconvolve alias frequencies from the periodogram. Since there is always the risk of removing a real frequency instead of its alias, we preferred to use both methods. The PDM methods is based on a different principle and offers an independent check of the results. We accepted a frequency when it was found by each of the four methods.
We also point out that the variable exposure times do not introduce smoothing effects on the frequency spectra, since the temporal resolution, i.e. the ratio of the exposure time to the pulsation period, is less than 5.2% for each measurement.
The HIPPARCOS data do not lead to severe alias problems. Moreover they have a
time base of some 3.3 years, which is much longer than for the spectra. The
space photometry is therefore most useful for the frequency analysis. As shown
by De Cat (2001) through his detailed frequency analysis of data of bright
southern SPBs, the HIPPARCOS data allow the detection of multiperiodicity for
bright stars. In the following we provide the significance of the frequencies
based on the HIPPARCOS data by means of the formula provided by Kovacs
(1981):
In the following parts of this section, we describe the results for each target star. All stars exhibit line-profile variations. We chose to show line profiles only for those stars whose variations are clear from visual inspection.
![]() |
Figure 1:
Phase diagrams of the ![]() |
Open with DEXTER |
None of the HIPPARCOS frequencies are dominant in the different power spectra of the frequency analyses of the radial velocity curves. No well-defined peaks are present at all. There is some indication for the second frequency f2 to be present in the line-profile variations. A formal periodic fit with the frequency f2 found in the HIPPARCOS data leads to an amplitude of 2.7 kms-1 and hardly reduces the standard deviation.
Hence, the radial-velocity variations of this spectroscopic binary seem to be dominated by a different frequency than the HIPPARCOS photometry. Our spectra are not sufficiently numerous, and have a too low resolution, to identify the pulsation modes.
![]() |
Figure 2:
Phase diagrams of the ![]() |
Open with DEXTER |
![]() |
Figure 3:
Line-profile variations of the Si II doublet
![]() |
Open with DEXTER |
The frequency analysis applied to the radial-velocity data
leads to a common peak around 1.14 cd-1, close to the f2 value
derived from the photometry.
Since our spectroscopic data set is not
very large, we imposed the two frequencies f1 and f2 for a harmonic fit.
This leads to amplitudes of 3.3 and 0.9 kms-1 for, respectively, f1 and
f2.
The phase diagram of the velocity variations for f1 is
displayed in Fig. 4.
![]() |
Figure 4: Phase diagram of the mean heliocentric velocities for the star HD21071 for f1. The main uncertainty in the radial velocity data is 2.0 kms-1. |
Open with DEXTER |
Thus this SPB is clearly multiperiodic. More and higher quality spectroscopic data are needed to derive the spherical wavenumbers of the pulsation.
![]() |
Figure 5:
Phase diagram of the ![]() |
Open with DEXTER |
![]() |
Figure 6: Same as Fig.3, but for the star HD25558. |
Open with DEXTER |
![]() |
Figure 7: Same as Fig.4, but for HD25558 and for f1. The uncertainty is 1.9 kms-1. Note that the two points that have a high value near phase 1 correspond to asymmetric and broad profiles. |
Open with DEXTER |
![]() |
Figure 8:
Phase diagram of the ![]() |
Open with DEXTER |
![]() |
Figure 9: Same as Fig.3, but for the star HD28114. |
Open with DEXTER |
Wolff (1978) claims, from 9 low-resolution spectrograms (13.6Åmm-1), that the star is a spectroscopic binary, with a 0.2-0.4 cd-1 orbital frequency and a peak-to-peak variation of 13 kms-1. It is very likely that she misinterpreted the radial-velocity variations originating from the line-profile variability as being caused by a companion. Our 17 spectra are by far too few to disentangle the pulsational behaviour, but the clear profile variability allows us to conclude that the star pulsates with a velocity amplitude compatible with the one reported by Wolff (1978).
![]() |
Figure 10: Same as Fig.3 but concerning the star HD138764. |
Open with DEXTER |
![]() |
Figure 11: Same as Fig.4, but for HD138764 and for f1. Uncertainty on velocity data is 1.4 kms-1. |
Open with DEXTER |
![]() |
Figure 12: Same as Fig. 3, but for the star HD140873. Lines of the secondary are sometimes visible. |
Open with DEXTER |
![]() |
Figure 13: Same as Fig.4, but for HD140873 and for f1. The velocity data have an uncertainty of 2.1 kms-1. |
Open with DEXTER |
![]() |
Figure 14:
Phase diagram of the ![]() |
Open with DEXTER |
Our much more extensive dataset of 280 spectra
provides a better view of
the line-profile variations (see Fig.15).
![]() |
Figure 15: Same as Fig.3, but for the star HD147394. For a better viszualisation, only a small fraction of the spectra are represented. |
Open with DEXTER |
![]() |
Figure 16: Same as Fig.4, but for HD147394 and for f1. The uncertainty on velocity data is 1.1 kms-1. |
Open with DEXTER |
Briquet et al. (in preparation) present a much more detailed analysis and mode identification for this star.
![]() |
Figure 17:
Phase diagrams of the ![]() |
Open with DEXTER |
Line-profile variations of this star were first reported by
Hube & Aikman (1991), who observed "traveling bumps'' in the
Si II doublet profiles. This discovery led them to classify the star as a
member of the 53Per class introduced by Smith (1977).
Some of the 53Per stars of Smith's original sample (see e.g. Chapellier et al.
1998) are the line-profile-variable analogues of the original sample of
photometrically discovered SPBs by Waelkens (1991). All SPBs turn
out to be line-profile variables as well (see PaperII). Therefore, HD182255
must be regarded as an already known, confirmed SPB before the present study.
Its clear line-profile variations (Fig.18) are
easily seen
thanks to the low projected rotation velocity. These variations are
superposed on line shifts due to orbital motion.
![]() |
Figure 18: Same as Fig.3, but for the star HD182255. |
Open with DEXTER |
The frequency analysis after removal of the orbital motion leads to the two
frequencies f1 and f2.
The first frequency found in the radial velocity
data is f1 and after
prewhitening we find f2 in the residuals.
The amplitude of f1 amounts to 3.5 kms-1 and the one of
f2 is around 2.1 kms-1.
The other frequencies are not significantly present in the
spectroscopic data.
The resulting phase diagram for f1, free of the
binary motion, is represented in Fig. 19.
![]() |
Figure 19: Same as Fig.4, but for HD182255 and for f1. Note that the orbital motion has been removed first. Velocity uncertainty is 1.2 kms-1. |
Open with DEXTER |
![]() |
Figure 20:
Phase diagrams of the ![]() |
Open with DEXTER |
![]() |
Figure 21: Same as Fig.3, but for the star HD206540. |
Open with DEXTER |
![]() |
Figure 22: Same as Fig.4, but for HD206540 and for f1'. The uncertainty on velocities is 1.5 kms-1. |
Open with DEXTER |
![]() |
Figure 23:
Phase diagrams of the ![]() |
Open with DEXTER |
HD | T | N | ![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
1976 | 1173 | 188 | 0.0106 | 0.0055 | 0.0062 | 0.93914 | 0.39934 | 0.38 |
0.39934 | 0.89 | |||||||
21071 | 905 | 85 | 0.0175 | 0.0060 | 0.0059 | 1.18843 | 1.18843 | 0.52 |
1.14942 | 0.54 | |||||||
25558 | 766 | 90 | 0.0142 | 0.0045 | 0.0057 | 0.65284 | 0.65284 | 0.69 |
0.7318? | 0.61 | |||||||
1.9298? | 0.23 | |||||||
28114 | 919 | 56 | 0.0132 | 0.0065 | 0.0076 | 0.79104 | 0.79104 | 0.42 |
147394 | 1192 | 116 | 0.0095 | 0.0036 | 0.0059 | 0.80027 | 0.80027 | 0.44 |
0.7813 | ||||||||
182255 | 1040 | 204 | 0.0192 | 0.0054 | 0.0086 | 0.79220 | 0.79220 | 0.67 |
0.97191 | 0.97191 | 0.55 | ||||||
0.47233 | 1.13 | |||||||
1.14708? | 0.46 | |||||||
0.65933? | 0.81 | |||||||
206540 | 1084 | 98 | 0.0132 | 0.0063 | 0.0076 | 0.65359 | 0.76237 | 0.63 |
0.76237 | 0.54 | |||||||
208057 | 1113 | 88 | 0.0114 | 0.0047 | 0.0058 | 0.80172 | 0.89045 | 0.48 |
0.89045 | 0.43 |
HD28114 is the only star found to be monoperiodic in the space photometry. However, while the photometric frequency reduces the standard deviation to the level of the noise in the Hipparcos data, this is not the case for spectroscopic variations. It would be safer to conclude that, if other frequencies exist, photometry of higher precision is needed to detect them. De Cat (2001) showed that some monoperiodic SPB candidates had to be reclassified as chemically peculiar stars, the variation being attributed to rotation. On the other hand, Adelman & Philip (1996) found the abundances of HD28114 to be within the range of values seen for superficially normal, main-sequence stars of similar temperature. Therefore, HD28114 should be still considered as an SPB candidate only. We also remark that all of our other targets have never been mentioned as CP stars.
Table 4 summarizes the pulsation frequencies of the stars.
The pulsation constant Q has been computed using the
and
values derived from the HIPPARCOS
photometry provided in Table 3.
Except for HD1976 and HD208057,
the main photometric frequency is also
the main spectroscopic frequency. This result shows that each star
has a dominant non-radial low-degree g-mode.
Both exceptions are rapid rotators,
leading to a very bad determination of
the velocity curve, and are a member or a suspected member of a
binary system.
The frequency found to be "dominant'' in the spectra
also accounts in
both cases for less than 20% of the variation.
Before concluding that for these
two stars a low-degree non-radial g-mode is mainly responsible for the
photometric variability while the spectral variations are dominated by a
higher-degree mode, additional spectroscopic observations are necessary.
Compatibility between the photometric and spectroscopic variations was in
general also found by De Cat (2001) in his sample of southern SPBs.
For the pulsation constants ,
we find a range of
0.23-1.13d with an average of 0.58d. Again this is compatible with the
results of De Cat (2001), who has treated a larger sample of SPBs
with more extensive and higher quality data sets.
Our study clearly shows that very long-term spectroscopic monitoring is necessary to derive the frequencies and the character of the excited pulsation modes. Although our new spectroscopic data set has a time base of 1.5 years, we have sufficient spectra for only one star, HD147394, to find multiperiodic signals in the line profiles and use these to undertake a mode identification (Briquet et al., in preparation).
In general, the time basis and sampling is a crucial point to perform
asteroseismology, and the only way to obtain the observational requirements
is either to join efforts and perform international longitude campaigns such as
those developed for the Scuti stars (DELPHI) or the white dwarfs and
sdB pulsators (WET) or to use dedicated telescopes during long observation runs.
Acknowledgements
The authors want to thank our referee, Dr. S. J. Adelman, for his constructive remarks.