A&A 456, 261-268 (2006)
DOI: 10.1051/0004-6361:20065211
E. Rodríguez1 - V. Costa1 - A.-Y. Zhou2 -
A. Grigahcène3 - M. A. Dupret4 -
J. C. Suárez1,4, -
A. Moya1,4 - M. J. López-González1 - J.-Y. Wei2
- Y. Fan2
1 - Instituto de Astrofísica de Andalucía, CSIC,
PO Box 3004, 18080 Granada, Spain
2 -
National Astronomical Observatories, Chinese Academy of Sciences,
Beijing 100012, PR China
3 -
CRAAG, Algiers Observatory, BP 63 Bouzareah 16340, Algiers, Algeria
4 -
LESIA, Observatoire de Paris-Meudon, UMR 8109, 92190 Meudon, France
Received 15 March 2006 / Accepted 10 May 2006
Abstract
The variability of HD 239276
was suspected photometrically nearly twenty years ago, but was confirmed with
new observations obtained in 2001 during a two-site photometric campaign
carried out from Spain, in uvby
Strömgren-Crawford photometry,
and China, using the Johnson V filter. Two low-dispersion spectra were also
collected. The results establish this star as a new multiperiodic
Dor-type pulsator with deficiency in metallicity.
Its possible
Boo nature is discussed.
The frequency analysis shows three
pulsational frequencies as significant, but some more are probably present
among the residuals.
The method based on phase shifts and amplitude ratios in multicolour
photometry is used to identify the excited modes with non-adiabatic
time-dependent convection models. A very
good agreement between the theoretical and observed amplitude ratios is
obtained and the two main modes are identified as l=1 modes.
Nevertheless, our results do not allow us to discriminate
between a solar abundance and a metal deficient nature for this star.
The frequency ratio method is further used for the identification of the
modes. The results suggest low metallicity for this star, but a
Boo nature may be not ruled out.
Key words: stars: variables: general - stars: individual: HD 239276 - stars: oscillations - techniques: photometric
The Dor-type variables constitute a recently recognized group of
main-sequence long-period pulsating stars. They were firstly proposed as a
group by Krisciunas (1993) and officially named as
Dor variables
during the meeting on Stellar Pulsation held in Cape Town, in 1995.
These
stars reside in the zone close to the cool border of the classical instability
strip, partially overlapping with the
Sct-type pulsators.
Their relatively long periods, between about 0.3 and 3 days, and
small amplitudes,
between a few millimagnitudes and some hundredths, are produced by
excited nonradial gravity (g) modes of high radial (n) and
low angular (l) orders (Handler 1999; Handler & Shobbrook 2002;
Henry et al. 2005).
The star HD 239276 (SAO 32177, V=9.1, A3, Simbad 2006) was
one of the check stars used for photometric uvby
observations on the high amplitude SX Phe star XX Cyg carried out,
at the Sierra Nevada Observatory (SNO) (Spain) during July, 1986 and
the period June-July, 1987. During these
observations a smooth variability was detected in HD 239276, with an
amplitude of a few hundredths of magnitude (from peak to peak),
a probable main period longer than 10 h (Fig. 1) and multiperiodic
behaviour.
Although the calibration of its derived Strömgren-Crawford colour
indices (Table 1) places this star much later than A3, and close to the
cool border of the
Sct pulsational region, it is still
inside the instability strip (Fig. 2).
Binarity as a cause of
the variations was discarded because the colour indices were found to vary
accordingly with the light curves. These variations probably indicate
pulsations. However, the pulsations were in disagreement with the
shorter-period
Sct-type pulsations expected for this star.
Interestingly, at the same time (summer of 1987), similar variations were
photometrically detected in the star HR 8799 (=HD 218396) which was
proposed as a new member of the Dor group by Rodríguez &
Zerbi (1995). A more detailed study
on HR 8799 was published in Zerbi et al. (1999).
Intriguingly, the two stars present very similar photometric
characteristics (Table 1 and Fig. 2): their uvby
colour indices
are very similar as are their locations in the H-R
diagram, the two stars are also deficient in metal abundances
(the corresponding [Me/H] values are about -0.4). Furthermore, the frequency
contents are similar: the main periods are very close to 0
5
(0
51 in HR 8799, Zerbi et al. 1999; and 0
47 in HD 239276, see below) with both variables showing a multiperiodic behaviour.
Consequently, the 1986-87 observations suggested HD 239276 as a new
Dor-type
variable. In order to confirm this, we undertook a detailed study on its
nature and performed a two-site photometric
campaign during September-October of 2001 from China and Spain.
The results of
this investigation are presented here, including an asteroseismologic study of
this new variable.
![]() |
Figure 1: Observed light variations of HD 239276 in the Strömgren v band during the years 1986 and 1987. |
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Figure 2:
Location of HD 239276 (star) in the H-R diagram together with
HR 8799 (solid circle) and the sample of ![]() ![]() |
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Table 1:
uvby
indices obtained for HD 239276 during the
1986-87 run and comparison with those available in the bibliography for
HR 8799. Sources: 1) present work; 2) Shuster & Nissen (1986); 3) Zerbi et al. (1999).
The observations obtained in China consisted of 3 nights of Johnson V photometry collected with the three-channel high-speed photoelectric photometer P45-A mounted on the 85-cm Cassegrain telescope at Xinglong Station of the Beijing Astronomical Observatory (BAO). This photometer is commonly used for the Whole Earth Telescope multisite campaigns (WET, Nather et al. 1990) and is equipped with standard Johnson UBV filters and an ST-6 CCD camera for guiding. The exposures were continuous with short integration times of only 5 s for simultaneously measuring sky, variable and one comparison star in the field close to the variable. The data were reduced as magnitude differences relative to the comparison star and averaged every 10 measurements to diminish the noise level.
The observations in Spain consisted of 18 nights of simultaneous observations
in the four uvby filters of the Strömgren system. The measurements
were carried out with the six-channel uvby
spectrophotometer
attached to the 90-cm telescope at SNO (Rodríguez et al.
1997). A few H
measurements were also collected for calibration
purposes. In these observations, the comparison stars used were C1 = HD 189296
(V = 6
16, A4V, Simbad 2006),
C2 = HD 191096 (
,
F4V) and C3 = HD 186760
(V = 6
30, G0V) with an observational sequence of C1, C2, C3, Var with sky
measurements every 2 or 3 cycles. The final data were calculated
as magnitude differences of each object relative to C1 and no sign of
periodicity was found here for any of the comparison stars.
Besides the photometric observations, two low-dispersion spectra (200 Å/mm)
were obtained for the variable using the Cassegrain spectrometer attached to
the 2.16 m telescope at BAO (Fig. 3). According to the intensities of
H,
H
and other characteristic lines and their ratio
values, the spectrum of HD 239276 is similar to that of an around A9V star.
This is later than the A3 listed in the Simbad database (Simbad 2006), but
agrees well with F0V, typical of a
Dor-type variable, as
suggested by its colour indices (Tables 1 and 5)
and location in the H-R diagram using the calibrations of Gray & Garrison
(1989).
![]() |
Figure 3: Spectrum of HD 239276 = SAO 32177. |
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The data, as magnitude differences of variable minus main comparison star
versus Heliocentric Julian Date are presented in Tables 2 (uvby
observations collected in 2001), 3 (Johnson V photometry) and 4 (uvby
data collected during the 1986-87 run). In the later case, SAO 32139 was the
comparison star used. These three tables are available in electronic
form at the CDS and can also be requested from the authors. To transform our
instrumental data into the standard uvby
system,
we followed the method described in Rodríguez et al. (1997).
In the case of the 1986-87 run, a more detailed description is given in
Rodríguez et al. (1993).
Table 5:
uvby
indices of HD 239276 and
comparison stars obtained during the 2001 run.
The pairs below the star names mean the number of points
collected for each object in uvby and
,
respectively.
The numbers below the magnitudes and colour indices mean the
corresponding error bars.
Table 6: Reddening and derived physical parameters for HD 239276.
As seen from Tables 1 and 5, the two sets of derived colour indices for the
variable are in very good agreement each with each other. Table 5 also lists
the indices derived for the comparison stars assuming the
values listed for C1 in the homogeneous
catalogue of Olsen
(1996) as the zero point. The error bars listed in this table mean standard
deviations of magnitude differences relative to C1. The results obtained for
C2 and C3 are in good agreement with
those shown in the same catalogue or in that of Hauck & Mermilliod (1998),
although there is a discrepancy with respect to the V and c1 values of C3
listed in these catalogues (
and
).
In order to estimate the physical parameters of HD 239276 we followed
the method
described in Rodríguez et al. (2001) using suitable calibrations
available in the literature for
photometry. The results
are summarized in Table 6 which place the star well in the
Dor
region as shown in Fig. 2 and slightly deficient in metal content
([Me/H] = -0.40) as mentioned in Sect. 1.
The error bars listed in this table are typical taking into account our
observational uncertainties in deriving the colour indices and the adopted
relations to determine the different parameters.
The evolutionary tracks of Claret & Giménez (1995),
with Z=0.01, were used to estimate the mass and age of this
star. As expected for
Dor-type variables, HD 239276 is in the
main-sequence with a mass of 1.42(
0.1)
and age of
1.7(
0.1) Gyr as shown in Fig. 4.
However, as discussed in Sect. 1, the photometric characteristics
of HD 239276 and HR 8799 are very similar. It is possible that the metal
deficiency in HD 239276 is a sign of a Boo nature as was
already found for HR 8799 by Gray & Kaye (1999). This possibility
was also pointed out for the multiperiodic
Dor star
HD 218427 (Rodríguez et al. 2006). Indeed, the three stars are located
inside the
Boo region of both (m1, b-y) and ([m1],
)
diagrams (Gray 1988; Gray & Corbally 1993). If so, these stars should
constitute a small subgroup of metal-poor
Dor objects similar
to the
Boo stars that are pulsating as
Sct-type variables.
If we accept a
Boo nature for HD 239276, we can assume a mass of
and an age of 1.0 Gyr for this star
by using the evolutionary tracks of
Claret (1995) with Z=0.02. Nevertheless, high-resolution spectra are neccesary
in order to clarify this point.
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Figure 4: Position of HD 239276 in the HR diagram with associated error bars and evolution tracks for Z=0.01 (solid lines) (Claret & Giménez 1995) and Z=0.02 (dotted lines) (Claret 1995). |
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The analysis of the pulsational content was carried out using the method described in Rodríguez et al. (1998) where Fourier and least-squares algorithms are combined in the same computation program package. When a new peak is found as significant in the periodograms, this together with all the previously determined frequencies are simultaneously optimized and extracted from the data together with the corresponding amplitudes and phases that minimize the residuals. Then, the method does not depend on successive prewhitenings of the data. The results thus obtained were checked using the new version of the computation package PERIOD (PERIOD04) (Lenz & Breger 2005). The two methods yield identical results.
This analysis stops when the new peaks suggested in the periodograms are not formally significant. Following Breger et al. (1993, 1999), a peak is considered as significant when the amplitude signal/noise (S/N) ratio is larger than 4.0 for independent frequencies or larger than 3.5 for harmonics or frequency combinations. The noise level was calculated by averaging the amplitudes of the residuals over 5 cd-1 boxes around the frequency under consideration.
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Figure 5: Amplitude spectra of C2 = HD 191096 and C3 = HD 186760 in the b band. |
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Before analysing the periodograms of the variable, those corresponding to
the comparison stars were checked in order to avoid misinterpretations
in our results. Although C2 and C3 are placed beyond the cool border of the
Dor-region, periodicities produced by binarity effects or spots
might take place in them. On the other hand, the time distribution of the
observations was very similar for the variable and check stars. Thus, the
investigation of the C2-C1 and C2-C3 Fourier spectra is of special importance
in the low frequency domain where the instrumental and/or atmospheric problems
generally manifest themselves, producing spurious peaks. In this sense,
simultaneous multicolour photometry plays an important role: the pulsational
amplitudes and phases in different filters must follow defined rules which
make it possible to distinguish intrinsic from spurious peaks. Finally, taking
into account that the main comparison star C1 lies within the
Sct
limits, it is also very important to verify the constancy of the C2-C1 and
C3-C1 periodograms in the region of the high frequencies where
Sct-type variability could occur.
In the present work, none such effects seem to be present. Only a very small increase in power is shown in the periodograms of the comparison stars, as seen in Fig. 5 for the b band, probably produced by nightly zero-point shifts. No periodicities are photometrically detected in the low region larger than about 1.4 mmag (or 0.6 mmag in the region beyond 10 cd-1. These limits are lower than the internal error per single measurement collected for the variable (about 2.0 mmag in v and b). Hence, the limit in our investigation for the variable is defined by the significance level in the corresponding amplitude spectra. A peak can be considered as intrinsic to this star when it is found significant in its periodograms.
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Figure 6: Power spectra of HD 239276 in the combined vby band corresponding to the original data and residuals after removing different sets of simultaneously optimized peaks. |
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Although the main periodicities expected from the observed light
curves of HD 239276 are typical of Dor-type variables, the
possibility of short-period pulsations, typical of
Sct-type stars,
cannot be ruled out taking into account the location of this star in the
H-R diagram. The discovery of new examples with both types of pulsations in
the same star is, presently, a very exciting task.
Thus, in order to check this possibility, the
amplitude spectra were investigated in the full range of possible frequencies,
but no significant peaks were found in the high frequency domain. In the
region higher than 10 cd-1, the periodograms always appear flat,
resembling white noise similar to that shown in the right parts
of Fig. 6.
For consistency in our results, the vby data were combined
following the method described in Rodríguez et al. (2001). This way,
the by data were transformed to simulate the v amplitudes and
the measurements collected at each instant in filters v, b and y were averaged with weights according to their internal precision.
Thus, a combined vby "filter'' was built. In addition, the Johnson V data were also aligned. The results of this analysis are presented in
Fig. 6 and Table 7 with three close frequencies found as significant around
2 cd-1. Besides them, a fourth peak is detected at 1.56 cd-1, but
with
,
which is lower than the formal limit
of 4.0. In fact, when the 4-frequency solution is applied to the four
uvby filters for the data collected at SNO,
the S/N values are in all cases smaller than 3.0. Then, although there are
some indications that favour the reliability of this peak, we
will only consider the 3-frequency solution as definitive for our data set.
Table 7: Frequencies, amplitudes and amplitude signal/noise ratios obtained for the combined vby filter.
Nevertheless, the residuals of the 3-frequency solution (
mmag)
are too large (much higher than expected). This is also shown in the bottom
panel of Fig. 6. In particular, the power in the low frequency region is much
higher than that at higher frequencies. This suggests that more periodicities,
with smaller amplitudes, remain among the residuals, but our data set
is not long enough to resolve them. Figure 7 shows the data phased at each
of the three frequencies after prewhitening for the other two.
Table 8 shows the results when applying our 3-frequency solution to
each of the
four uvby data. As seen, the residuals are much larger than expected.
Moreover, the residuals decrease from filters v to b and y,
that is, following the same sense of decreasing pulsational amplitudes.
This confirms
the fact that some frequencies are still present in the residual light
curves.
Table 9 lists the corresponding phase shifts and amplitude ratios, relative
to the y filter, for the three main frequencies. The results are
consistent, in all cases, with Dor-type pulsation
(Rodríguez 2005).
Table 8: Results from the Fourier analysis applied to the four uvby filters.
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Figure 7: Phase diagrams for f1=2.1066 cd-1 ( top), f2=1.8694 cd-1 ( middle) and f3=2.2124 cd-1 ( bottom). In each panel, the data have been prewhitened to remove the other two frequencies. |
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The theoretical monochromatic magnitude variation of a
non-radial mode is given by:
Two important coefficients of Eq. (1) are the
normalized amplitude ()
and phase (
)
of local effective temperature variation. These two quantities
can only be theoretically determined by non-adiabatic models.
Dupret et al. (2005) showed that time-dependent convection
models (Grigahcene et al. 2005)
must be used to reach a good agreement with observations.
As a first step, we consider
in this section
and
as free parameters and
determine which values are required to get the best agreement between
theory and observations. This will give us a model independent mode
identification of the degree l of the modes. In the next section,
we will determine
which theoretical models are able to reproduce these values.
In Fig. 8, we show the amplitude ratios obtained for
different values of .
For the modes f1 and f2, we see that
good agreement with
observations can only be obtained with l=1 modes. The best values for
are
for f1 and
for
f2.
is required to get a good agreement with the observed
phase differences. For the mode f3, the best agreement is found for an
l=2 mode with
but because of the large observational
error bars, l=1 cannot be completely discarded.
As shown in Aerts et al. (2004) and Dupret et al. (2005),
we see that very small values of
are required for
Dor stars.
Table 9: Observed phase shifts and amplitude ratios.
In this section, we use stellar models computed with the Liege stellar
evolution code CLES (Code Liégeois d'Évolution Stellaire) that uses:
standard MLT for convection calculations;
the OPAL opacities (Iglesias & Rogers 1996) completed at low temperatures
with the opacities of Alexander & Ferguson (1994); the CEFF equation of state
(Christensen-Dalsgaard & Däppen 1992); and the atmosphere models of
Kurucz (1998) as boundary conditions.
Non-adiabatic computations using TDC models (Grigahcène et al. 2005) enable
us to determine the stability of the modes and the theoretical values of
and
for different models. These values can be used to
determine the photometric amplitude ratios and compare them with observations.
We begin with the results of our stability analysis.
The frequencies of HD 239276 are relatively high compared to those
of typical
Dor stars. For all the theoretical TDC models considered
in this study, l=1 modes are near the limit between stability and
instability for
c/d. For
,
the modes are always
predicted to be unstable in the
observed range.
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Figure 8:
Strömgren photometric amplitude ratios obtained for different values
of ![]() ![]() ![]() ![]() |
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Figure 9:
Position of HD 239276 in the HR diagram with associated error
bars and evolution tracks
for Z=0.01 (solid lines) and Z=0.02 (dotted lines) using the CLES code.
Some of our best models for Z=0.01 (![]() |
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We consider now the values found for
and the corresponding
amplitude ratios.
In Fig. 9, we give the location of HD 239276 in the HR diagram,
as deduced from Table 6, that is,
K and log
.
As expected, the new models using the CLES code provide slightly
different masses to those derived in Sect. 3 as consequence of the existing
differences in some ingredients used.
In particular, in the case of HD 239276, the new masses are
about 4% and 7% smaller than those derived from Claret's models for Z=0.020
and Z=0.010, respectively.
We begin by considering the results obtained
for a model in the centre of this photometric error box with Z=0.02(
,
).
Figure 10 shows the comparison between the theoretical and
observed amplitude ratios for this model, for l=1 and l=2 modes.
The l=3 modes are not given as the associated amplitude ratios
completely
disagree with observations. We see that the agreement is very good for f1
and f2, identified as l=1 modes.
It is more difficult to obtain agreement for f3 (with larger error bars).
![]() |
Figure 10:
Strömgren photometric amplitude ratios obtained with our TDC treatment,
for a model with
![]() ![]() |
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As a second step, we consider models with different masses, metallicities and
ages and determine those giving the required values for .
All these models are with mixing-length parameter
.
In Table 10, the values of
obtained for
these models are listed.
The values found for
are around [
,
].
More precisely, we give the values of
obtained for the l=1 g-mode with a frequency closest to that of f1,
for the l=1 g-mode with frequency closest to that of f2
and for the l=2 mode with frequency closest to that of f3.
We get the required values
for f1 and
for f2 for a
,
Z=0.02 model with
the photometrically determined effective temperature and luminosity
(Fig. 10).
We consider this model (boldfaced in Table 10)
as our best one.
It is also possible to obtain the required values of
with other
models.
These models are represented by + and
in Fig. 9
and we see that they are out of the photometric error box.
As an example, the deduced amplitude ratios obtained
for one of our best models with Z=0.01 are plotted in Fig. 11.
However, none of these best models gives the required
high value of
for the mode f3 (l=2); higher mass
would be required as shown by the
,
result of
Table 10.
For these best models,
the l=1 theoretical predictions agree with observations better than the
l=2 ones for f3, but discrepancy is still present and we do not
obtain conclusive results for this mode.
As conclusion, in our photometric mode identification using TDC models, f1
and f2 are identified as l=1 modes. No conclusive mode
identification is found for f3.
The differences found between our Z=0.01 and
Z=0.02 theoretical results are small and do not allow us to discriminate
between a Bootis and a submetallic nature for this star.
![]() |
Figure 11:
Strömgren photometric amplitude ratios obtained with our TDC
treatment, model with
![]() ![]() ![]() |
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Table 10:
Theoretical values of .
Following Moya et al. (2005) the frequency ratio method (FRM) was applied
to HD 239276 in order to obtain information on: 1) possible identification
of the radial order n and degree l of the modes corresponding to
the observed frequencies and 2) an estimate of the integral of the buoyancy
frequency (Brunt-Väisälä) weighted over the stellar radius
along the radiative zone (). We follow the same scheme adopted in
Rodríguez et al. (2006) for the
Dor star HD 218427, and assume
that all the excited modes correspond to the same azimuthal order m=0.
From the multicolour photometry results given in the previous section, a
possible mode degree l identification is provided for each observed
mode. This allows us to apply the FRM without assuming the hypothesis of equal
l, that is, using the generalized form given by Eq. (6) of Moya et al.
(2005). In particular, multicolour photometry analysis predicts f1 and
f2 identified as
modes and f3 as
modes. We can thus
apply the FRM searching for natural number sets fullfilling
In Table 11, the FRM solutions are listed. For each
possible solution, an estimate of the observed Brunt-Väisälä
integral
is also provided. This new observable allows us to
obtain a model constraint by placing it in a
diagram
as described in Moya et al. (2005), where
represents the theoretical
Brunt-Väisälä integral calculated from equilibrium models.
In Fig. 12, such a diagram is depicted for the solutions
given in Table 11 with the corresponding error boxes.
Notice that the only solution for which the observed
integrals lies close to the solar metallity tracks region is the one
assuming all the modes having
,
which contradicts
the multicolour analysis results. When considering f3 as
a
mode (f1 and f2 as
modes), the solutions are
compatible with the observed metallicity ([Me/H] = -0.4). For f3 as
mode (f1 and f2 as
modes), no solutions
are found. For the
case of the three modes are identified as
modes, the
solutions are compatible with models around [Me/H] = -0.6 dex.
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Figure 12:
Theoretical Brunt-Väisälä frequency integral as
a function of
the effective temperature for representative models with masses
in the range of
M=1.2-1.6 ![]() ![]() ![]() |
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Table 11:
List of possible (n, l,
)
identifications
provided by the FRM applied to the observed frequencies of HD 239276. The
first three columns represent the resulting (n, l) identifications.
The last column corresponds to the observed Brunt-Väisälä integral.
Therefore, for three of four possible
combinations studied,
the FRM provides solutions compatible with the observed metallicity
dex. However, there is no evidence that ensures
the discrimination between these three solutions.
Nevertheless, the case of the three observed frequencies beeing identified as
modes is in contradiction with the results from our multicolour
photometry analysis given in the previous section. Then, the results obtained
by both mode discrimination methods agree for only the two solutions with the
lowest metallicities. This suggests a metal-deficient abundance in HD 239276.
However, a more detailed study of this star is needed in order to
confirm our results. As discussed in previous sections HD 239276 is a
suspected Boo-type star and therefore its metallic composition could
be underestimated. The metal particularities of
Boo-type stars may
induce changes in the stellar structure, for instance by
means of some microscopic diffusion, which may change the results in
the application of our identification modes. It is
interesting to note that similar discordant results are also found
when applying the method to the confirmed
Boo star HR 8799
(Suárez et al. 2006).
In this work, we present the results of two-site photometric observations
carried out during the autumn of 2001 from China and Spain in order to
confirm and study the variability of the star HD 239276. The observations
collected in Spain consisted of simultaneous uvby measurements together
with data obtained in the H-Crawford system, while those
obtained in China consisted of Johnson V photometry.
The analysis of its colour indices places this star in the Dor
instability region. This is confirmed by the spectral type A9V derived from
two low-dispersion spectra obtained in this work. This is much later than that
of A3 listed in the
Simbad database (Simbad 2006), but agrees with the long-period
variability (about 0.5 days) observed in this star.
The physical parameters of HD 239276 were determined using the
uvby
indices derived in this work together with the most
suitable Strömgren photometric calibrations available in the literature,
including atmospheres and stellar evolution models. The results establish
this star as a
Dor-type pulsator with deficiency in metallicity,
similar to other already known
Dor stars, such as HR 8799 (Gray &
Kaye 1999) and HD 218427 (Rodríguez et al. 2006). The possibility of a
Boo nature is discussed. In such a
case, the metallicity content of HD 239276 could be underestimated.
Nevertheless, high-resolution spectra are neccesary to clarify this point.
The frequency analysis of our data shows the existence of at least three
pulsation frequencies, and some more periodicities probably remain
among the residuals. New observations from a
further coordinated multisite photometric campaign will be of crucial
importance to study the full pulsational behaviour of this variable.
Nevertheless, the already derived phase shifts and amplitude ratios between
different filters for the three main periodicities are in all cases consistent
with Dor-type pulsation.
In order to perfom an identification of the spherical degree l of the
excited modes, two recently developed methods suitable for Dor-type
pulsators have been used: 1) that based on the observed phase shifts and
amplitude ratios among different filtersin multicolour photometry using a
non-adiabatic time-dependent convection (TDC) treatment (Dupret et al. 2005;
Grigahcène et al. 2005) and 2) the frequency-ratio method (FRM) (Moya et
al. 2005) based on
the observed frequency ratios which is particularly useful when at least three
oscillation frequencies are present in the light curves. When using the former
method, a very good agreement is obtained between the theoretical and
observed amplitude ratios and the two main modes are identified as l=1
modes. This is compatible with the results derived using the FRM. Moreover,
the joint results suggest low metallicity for HD 239276, but a
Boo
nature is not ruled out.
Acknowledgements
This research was partially supported by the Junta de Andalucía and the Dirección General de Investigación (DGI) under projects AYA2003-4651 and ESP2004-03855-C03-01. J.C.S. acknowledges the financial support of the European Marie Curie action MERG-CT-2004-513610 and the Spanish Consejería de Innovación, Ciencia y Empresa, from the Junta de Andalucía local government. M.A.D. acknowledges financial support from the CNRS. This research has made use of the Simbad database, operated at CDS, Strasbourg, France.