Issue |
A&A
Volume 496, Number 2, March III 2009
|
|
---|---|---|
Page(s) | 469 - 473 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361/20079257 | |
Published online | 30 January 2009 |
The planet-hosting subdwarf B star V 391 Pegasi is a hybrid pulsator![[*]](/icons/foot_motif.gif)
R. Lutz1,4 - S. Schuh1 - R. Silvotti2 - S. Bernabei3 - S. Dreizler1 - T. Stahn4 - S. D. Hügelmeyer1
1 - Institut für Astrophysik, Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
2 - INAF - Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
3 - INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
4 - Max-Planck-Institut für Sonnensystemforschung,
Max-Planck-Straße 2, 37191 Katlenburg-Lindau, Germany
Received 17 December 2007 / Accepted 19 December 2008
Abstract
Context. A noticeable fraction of subdwarf B stars shows either short-period (p-mode) or long-period (g-mode) luminosity variations, with two objects so far known to exhibit hybrid behaviour, i.e. showing both types of modes at the same time. The pulsating subdwarf B star V 391 Pegasi (or HS 2201+2610), which is close to the two known hybrid pulsators in the -
plane, has recently been discovered to host a planetary companion.
Aims. In order to learn more about the planetary companion and its possible influence on the evolution of its host star (subdwarf B star formation is still not well understood), an accurate characterisation of the host star is required. As part of an ongoing effort to significantly improve the asteroseismic characterisation of the host star, we investigate the low-frequency behaviour of HS 2201+2610.
Methods. We obtained rapid high signal-to-noise photometric CCD (B-filter) and PMT (clear-filter) data at 2 m-class telescopes and carried out a careful frequency analysis of the light curves.
Results. In addition to the previously known short-period luminosity variations in the range 342 s-367 s, we find a long-period variation with a period of 54 mn and an amplitude of 0.15 per cent. This can most plausibly be identified with a g-mode pulsation, so that HS 2201+2610 is a new addition to the short list of hybrid sdB pulsators.
Conclusions. Along with the previously known pulsating subdwarf B stars HS 0702+6043 and Balloon 090100001 showing hybrid behaviour, the new hybrid HS 2201+2610 is the third member of this class. This important property of HS 2201+2610 can lead to a better characterisation of this planet-hosting star, helping the characterisation of its planetary companion as well. Current pulsation models cannot yet reproduce hybrid sdBV stars particularly well and improved pulsation models for this object have to include the hybrid behaviour.
Key words: subdwarfs - stars: horizontal-branch - stars: oscillations - stars: individual: HS 2201+2610
1 Introduction
Subdwarf B stars (sdBs) are evolved objects with masses
around half a solar mass and are thought to be core helium burning. Having
lost most of their original hydrogen envelope in earlier stages of
their evolution, the remaining H-rich envelope is too thin to sustain
hydrogen shell burning. The sdBs populate the extreme horizontal
branch (EHB) at effective temperatures of 20 000 to 40 000 K
and surface gravities
between 5.0 and
6.2. Instead of evolving towards the asymptotic giant branch, the
sdBs are predicted to follow tracks leading directly towards the white
dwarf region after leaving the extreme horizontal branch.
A fraction of the sdB stars shows pulsations: the first
pulsating sdB star was discovered in 1997. Variable subdwarf B stars
(sdBV stars) can be divided into the classes of rapid p-mode pulsators
(sdBV
)
and slow g-mode pulsators (sdBV
),
with two objects known so far to belong to both classes simultaneously
(hybrid pulsators, sdBV
).
The p-mode pulsators show low amplitudes (few ten mmag) and short periods
(few minutes) at higher temperatures. The acoustic pressure waves are of low
degree and low radial order. In contrast, the g-mode pulsators have even
lower amplitudes (few mmag) and longer periods (30 to 90 mn) at lower
temperatures. The class prototypes are
EC 14026-2647 (Kilkenny et al. 1997) and PG 1716+426
(Green et al. 2003), respectively. Both mode types are thought to be
driven by the same -mechanism due to an opacity bump of iron
and nickel (Fontaine et al. 2003; Charpinet et al. 1997; Jeffery & Saio 2006). Hybrid sdB pulsators show p- and
g-modes simultaneously and are of particular interest since the two mode
types probe different regions within the star. In the
-
diagram, the hybrids are located at the interface
of the p-mode and g-mode instability regions. The known hybrids are
HS 0702+6043 (Lutz et al. 2008; Schuh et al. 2006,2005) and Balloon 090100001
(Baran et al. 2005; Oreiro et al. 2005).
This paper presents the third member of this group: HS 2201+2610 is a 14.3 B-magnitude star having an effective temperature of
K and a surface gravity
of
in cgs units (Østensen et al. 2001). Based on
these spectroscopic parameters, which are very similar to those of HS 0702+6043 and Balloon 090100001, HS 2201+2610 was a natural further target to search for simultaneous g-modes in a known p-mode
![]() |
Figure 1: Light curves from the September/November 2006 runs at Calar Alto. The relative flux is set to 1.0 for the 27. Sep. 2006 light curve and previous (following) nights are shifted upwards (downwards) by 0.1 units. |
Open with DEXTER |
In Sect. 2 we present the photometric data obtained specifically to investigate the gravity mode frequency range, as well as suitable confirmation data obtained as part of our long-term monitoring. The data reduction and frequency analysis are described in Sect. 3, and as a result of our investigation we will present HS 2201+2610 as a new member of the group of hybrid pulsating sdB stars. We discuss the implications of this discovery for the ongoing asteroseismic characterisation of HS 2201+2610, in the context of our efforts to constrain the mass of its planetary companion, in Sect. 4. Finally, we present a concluding outlook in Sect. 5.
2 Observations
The light curves (see Fig. 1) were obtained at Calar Alto
Observatory in September and November 2006. The instrumental setup was chosen
to consist of the Calar Alto Faint Object Spectrograph (CAFOS) focal reducer
attached to the 2.2 m telescope. A 2k 2k CCD chip recorded the
science frames through a Johnson B filter with a 2
2 binning and an
exposure time of ten seconds with an average cycle time of 25.5 s. In
order to sample the short-period p-modes as well as to detect very
low-amplitude g-modes, one has to find a good compromise with the exposure
time. Ten seconds turned out to be sufficient to resolve the p-mode cycles as
well as provide a S/N good enough to resolve low-amplitude signals. We
have a total of about 26 h of photometric data.
The data reduction was carried out using the time resolved imaging photometry package (TRIPP, see Schuh et al. 2003), which is designed to handle the aperture photometry of large CCD sets. This includes bias-, flatfield- and background corrections and finally yields extracted light curves in the form of relative flux (in comparison to four stable reference stars) as a function of geocentric Julian date. Additionally, a barycentric time correction was applied. Differential extinction effects on a night-by-night basis were corrected using second-order-polynomials.
In addition to our Calar Alto data from 2006, one of us (R. S.) provided older data from August/September 2003, which were part of the whole earth telescope extended coverage 23 (WET Xcov23). The data taken at Loiano Observatory (LOI, 1.5 m telescope and 3 channel photometer) and Observatoire de Haute-Provence (OHP, 1.9 m telescope and 3 channel photometer) were provided as extracted light curves (total length of 28 h) and we used them to cross-check our Calar Alto results.
Table 1: Results of our frequency analysis of HS 2201+2610.
3 Results
We carried out a frequency analysis on both data sets with a
program called sinfit, which is part of TRIPP. This
includes least-squares non-linear sinusoidal fits to the light curves and calculation of the
corresponding periods, amplitudes and phases. The amplitudes are given in
milli modulation intensity units (mmi). In this context 10 mmi means that
the luminosity has changed by one per cent with respect to the mean brightness level. We used the Scargle algorithm (Scargle 1982) to calculate lomb-scargle-periodograms
(LSPs) in the form of power spectral density (psd) as a function of frequency,
together with confidence levels, which are based on false alarm probabilities
(faps). See Sect. 4 in Dreizler et al. (2002) for a more detailed
description. In our case, we derived the faps from 158 000 white noise
simulations. This yields a confidence level of 4,
judging a signal above this level to be real with a probability of 99.99366%.
Table 1 summarizes the results of our frequency
analyses of both data sets. We refer to the pulsation frequency identifications
obtained by Silvotti et al. (2002) and also list the mode degree (l)
identification suggested there in Fig. 9. For the Calar Alto data we estimate the errors in the frequency to be 0.2 Hz (FWHM of the central
peak in the window function) and having a detection limit of 1.1 mmi, which was calculated as four times
the mean noise level in the residual DFT.
We used (f4) and (f5) in the prewhitening, because low-amplitude
signals at these frequency ranges were
also found in other runs (Silvotti et al. 2002).
Regarding the OHP/LOI data, we calculate a
frequency resolution of 0.54
Hz (FWHM of the central peak in the window
function) and a detection limit of 0.97 mmi (four times the mean noise level
in the residual DFT). From the WET Xcov23 runs, the OHP data from
August 22/23 and the LOI data from August 29/31 and September 3/4 have been
included. The frequencies of the ``mean'' solution
refer to BJD 2452788, which corresponds to the middle of the 7-year
long data set in Silvotti et al. (2007). Estimating the formal frequency
resolution of the 7-year ``mean'' solution yields 0.0045
Hz.
We can clearly recover the three strongest modes found in an extensive run published by Silvotti et al. (2002), here labeled with f1 to f3. These frequencies are assigned to pressure driven modes. The original and residual LSPs of our Calar Alto data in the p-mode regime are displayed in Fig. 2. In addition, we find a long-period luminosity variation (f7, shown in Fig. 3) of about 54 mn.
![]() |
Figure 2:
Lomb Scargle Periodograms (LSPs) of the p-mode regime. The top
panel shows the complete Calar Alto data. The bottom panel displays the
LSP after simultaneous subtraction of five frequencies. Horizontal dotted
lines indicate 4 |
Open with DEXTER |
As mentioned before, we looked at suitable WET Xcov23 data with the focus on the g-mode domain to confirm our detection of a long-period luminosity variation found in the Calar Alto data (left plots in Fig. 3). The periodogram of these additional data is shown in the right plots of Fig. 3. The OHP/LOI results (also in Table 1) clearly confirm the detection of a low frequency oscillation in HS 2201+2610.
As can be seen in the middle right plot in
Fig. 3, there is still further residual power above 4
in the OHP/LOI data LSP after subtraction of the long-period variation f7. However, this is not seen
in the Calar Alto data (which were taken three years later) so that further
high-quality data are required to test the persistence of these residual peaks.
4 Discussion
We identify the long-period low-amplitude luminosity variation in
HS 2201+2610
with a gravity driven mode. The presence of both types
of modes within HS 2201+2610 would therefore define this object to
be a member of the hybrid sdB pulsator group, beside the previously known
hybrids HS 0702+6043 and Balloon 090100001.
Other interpretations for the long-period variation can be excluded with a high
probability with similar arguments as for HS 0702+6043
(Schuh et al. 2006).
The same long-period variation, in
frequency as well as amplitude, is present in data sets three years
apart. This rules out sky or transparency variations, as well
as rotating star spots. The fast rotation rate that would be required for the latter
interpretation is further unlikely since rotation periods of single
sdB stars are typically much longer.
Rotational splitting (for l>0 modes) has not yet been detected in
HS 2201+2610, but the split components often have lower
amplitudes and might be below our detection limit.
With the light-travel time effect used in the detection of the planet
candidate around HS 2201+2610 in its wide orbit, we would
not be sensitive to a further hypothetical companion in a very close
orbit with a 54 mn period. A binary origin of the long-period
variation is unlikely for the following reasons.
A 54 mn orbital period would be at the extreme low limit of the observed and
theoretical orbital period distributions (see e.g. Fig. 2 in
Morales-Rueda et al. 2006). Additionally, a low-mass main-sequence
(M-dwarf) companion as the source of the 54 mn signal can be excluded by
geometrical considerations (Roche-Limits vs. separation in Keplerian
orbits). The same is true for a sdB+WD configuration in a 54 mn orbit down
to very low WD-masses (below 0.2
). In the case of deformation of the sdB
by a hypothetical WD companion, causing ellipsoidal variation, the underlying
orbital period would be 108 mn, but we would then expect a signal at least ten
times stronger down to low inclinations, dropping to the observed very
low amplitude at only about 10
.
A WD companion as the source of the
signal is therefore also very unlikely.
For less massive and hence harder to detect companions it would be
increasingly hard to prevent mass transfer in the system.
Binary orbital motion as the cause of the long-period variation will
immediately and conclusively be ruled out if its multi-periodicity
is confirmed.
![]() |
Figure 3:
Results of our frequency analysis in the g-mode
regime. Left: September/November 2006 Calar Alto data. The
top panel is a Lomb Scargle Periodogram of the three best nights
of the full light curve where five frequencies have already been subtracted (f1 to f5, see
Table 1). Fitting a long-periodic luminosity
variation (f7) to this subset of the data results in the residual LSP displayed in the
middle. The window function is shown in the bottom panel. Right: August/September 2003 OHP/LOI data. The top panel displays the Lomb Scargle Periodogram of the light curve after pre-whitening of three frequencies (f1 to f3, see Table 1). The residual LSP after subtraction of the
long-period variation f7 is shown in the middle and the window
function is illustrated in the bottom panel. Horizontal dashed lines
indicate 4 |
Open with DEXTER |
The period and amplitude fall within the typical range for
g-mode sdB pulsations, strongly suggesting
that the variation is in fact due to a g-mode.
Finally, in comparison to the location of the other two known hybrid
sdB pulsators in the -
diagram of sdBV stars
(see e.g. Fig. 1 in Lutz et al. 2008), the very
similar parameters of HS 2201+2610 again imply that hybrid
behaviour may plausibly be expected. All three objects are located at
the interface between the empirical p-mode and g-mode
instability regions. Hybrid pulsators have also been shown to exist in
the partially overlapping regions of the
Cep / [SPB] stars, which resemble
the p/g-mode sdB pulsators in many ways
(e.g. Pigulski & Pojmanski 2008b; Handler et al. 2004,2006; Jerzykiewicz et al. 2005; Pigulski & Pojmanski 2008a). Hybrid behaviour is also observed in the less closely related
Sct /
Dor stars (e.g. Henry & Fekel 2005).
From a theoretical point of view, modeling p-mode sdB pulsators has been
very successful. Fontaine et al. (2006) report on the determination of
the basic structural parameters of twelve p-mode sdB pulsators.
Regarding the g-mode sdB pulsators there are still issues.
The ``second-generation models'' presented by
Charpinet et al. (1997) and Fontaine et al. (2003) diverge by
some 4000 K from the observations, i.e. they fall short of the
empirical blue edge of the g-mode pulsators. A solution to this
problem has been suggested by
Jeffery & Saio (2007,2006): In their models,
using opacity project (OP) instead of OPAL opacity tables and including
Ni in addition to Fe to the opacity
profile shifts the theoretical blue edge close to the empirical
one. Both aspects of this opacity problem are, in close analogy, also
relevant for the Cep and [SPB] pulsators (Miglio et al. 2007). In fact,
the hybrid objects (both in the main-sequence as well as in the
extreme horizontal branch pulsators groups) in particular
represent excellent test cases for the adequateness of the
opacities used, and should provide strong constraints.
From previous experience, it appears to be clear that radiative
levitation and self-consistent diffusion of the
Fe/ Ni group elements need to be included
in the EHB models. It will be interesting to see if ``third-generation
models'' incorporating both radiative levitation/diffusion and improved
opacities
will finally deliver an adequate description of the hybrid sdB
pulsators while still reproducing the behaviour of the p-mode
pulsators as consistently as before, and at the same time correctly
describing the extent of the empirical g-mode instability region.
5 Concluding outlook
As recently published by Silvotti et al. (2007), HS 2201+2610
has a planetary companion which was detected using asteroseismological
techniques. Long-term monitoring of the strongest p-mode
pulsations made it possible to derive secular variations of the two main pulsation
frequencies, leading to an evolutionary timescale of around 5-
years, which matches theoretical evolutionary models of extreme horizontal
branch stars. Periodic residuals in the O-C diagram (observed -calculated) analysis, attributable to light-travel time effects, revealed the
presence of an
companion around
HS 2201+2610 in a 3.2 yr orbit.
We are in the process of continuing our regular long term photometric
monitoring of HS 2201+2610 beyond the current coverage of
two orbit cycles. In order to further characterise the companion, especially to
determine its mass, it is necessary to constrain the orbital inclination
of the system and to derive a more accurate asteroseimic
mass of the host star.
The mass of the host star is in principle accessible through asteroseismology, and Silvotti et al. (2002) already presented one possible solution. The determination of the fundamental parameters of the star has however only weakly been constrained in that work due to the small number of modes detected. Now that we have added HS 2201+2610 to the list of hybrid sdB pulsators, this finding might provide the additional constraints necessary for a reliable and accurate asteroseismic characterisation. At this point, we emphasize again the current need to improve on the model calculations first. For the individual object HS 2201+2610, it will be a special challenge to additionally drive g-modes in an improved version of one of the possible models found by Silvotti et al. (2002).
Acknowledgements
The Loiano and OHP data were taken during the Whole Earth Telescope extended coverage run 23; we thank the WET Xcov23 team: R. Janulis, J.-E. Solheim, R. Østensen, T. D. Oswalt, I. Bruni, R. Gualandi, A. Bonanno, G. Vauclair, M. Reed, C.-W. Chen, E. Leibowitz, M. Paparo, A. Baran, S. Charpinet, N. Dolez, S. Kawaler, D. Kurtz, P. Moskalik, R. Riddle, S. Zola. Part of the observations at Calar Alto were carried out in service mode by U. Thiele and A. Guijarro. The observations at Calar Alto were supported through Deutsche Forschungsgemeinschaft (DFG) travel grant SCHU 2249/3-1.
References
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Footnotes
- ... pulsator
- Based on observations collected at the Centro Astronómico Hispano Alemán (CAHA) at Calar Alto, operated jointly by the Max-Planck Institut für Astronomie and the Instituto de Astrofísica de Andalucía (CSIC).
All Tables
Table 1: Results of our frequency analysis of HS 2201+2610.
All Figures
![]() |
Figure 1: Light curves from the September/November 2006 runs at Calar Alto. The relative flux is set to 1.0 for the 27. Sep. 2006 light curve and previous (following) nights are shifted upwards (downwards) by 0.1 units. |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Lomb Scargle Periodograms (LSPs) of the p-mode regime. The top
panel shows the complete Calar Alto data. The bottom panel displays the
LSP after simultaneous subtraction of five frequencies. Horizontal dotted
lines indicate 4 |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Results of our frequency analysis in the g-mode
regime. Left: September/November 2006 Calar Alto data. The
top panel is a Lomb Scargle Periodogram of the three best nights
of the full light curve where five frequencies have already been subtracted (f1 to f5, see
Table 1). Fitting a long-periodic luminosity
variation (f7) to this subset of the data results in the residual LSP displayed in the
middle. The window function is shown in the bottom panel. Right: August/September 2003 OHP/LOI data. The top panel displays the Lomb Scargle Periodogram of the light curve after pre-whitening of three frequencies (f1 to f3, see Table 1). The residual LSP after subtraction of the
long-period variation f7 is shown in the middle and the window
function is illustrated in the bottom panel. Horizontal dashed lines
indicate 4 |
Open with DEXTER | |
In the text |
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