Photometric survey searching for pulsations in sdO stars
Cristina Rodríguez López1, Ana Ulla1, Rafael Garrido2, Minia Manteiga3
1. Departamento de Física Aplicada, Universidade de Vigo, E-36200 Vigo (Spain). email: [email protected], [email protected]
2. Instituto de Astrofísica de Andalucía-CSIC. E-18008 Granada (Spain). email: [email protected]
3. Departamento de Ciencias de la Navegación y de la Tierra, Universidade de A Coruña, E-15011 A Coruña. (Spain). email: [email protected]
INTRODUCTION
The O-type hot subdwarfs, sdOs, are faint blue objects in an advanced state of evolution. With temperatures in the range 40 000-100 000K, and log g between 4.0 and 6.5, they have not yet reached the WDs cooling
sequence. They are found in the Hertzprung-Russell diagram in both the post-AGB and the post-EHB locations, and are thought to be predecessors of low mass WDs (Heber, 1986).
Among the multiple unanswered questions concerning sdOs are their spectral classification, binary origin and evolutionary state. We present here the preliminary results of an ongoing study designed to search for
possible oscillations in these objects.
The study of oscillations in sdOs was first proposed by Greenstein (1957). Since then, only the studies of Bartolini et al. (1982, 2001) reported to have found qualitative microvariability in 4 sdOs, not confirmed to date as
far as we know.
The simultaneous experimental discovery (Kilkenny et al., 1997) and theoretical prediction (Charpinet et al., 1996) of oscillations in B-type hot subdwarfs (sdBs), and the absence of similar studies for sdOs motivate this
search for possible pulsations in them. This study was started with a sample of 11 sdOs and 1 sdOB (Rodríguez López, 2003), in which one candidate for pulsations, PG1036+433 with a marginal frequency at 1.04 mHz,
having a ratio of amplitude signal to noise of 3σ, was noted.
The discovery of pulsations in sdOs would allow us to use asteroseismological techniques to scrutinize their interiors, with the final aim of elucidating their evolutionary state.
OBSERVATIONS
Twenty-five more sdOs (see Table 1), mostly from the Palomar Green Catalogue have been observed. They were chosen with a variety of physical parameters and B magnitude ranging from 10.7 to 15.8.
Twenty objects were observed in December 2003-January 2004 with the 1.5 m telescope at the Sierra Nevada Observatory (OSN). Five more objects were observed in May 2004 at the 2.56 m Nordic Optical Telescope
(NOT) at Roque de los Muchachos Observatory (ORM). High-speed CCD photometry with a B Johnson filter was performed for the OSN objects, while the ALFOSC camera in fast photometry mode with a wide filter (an hybrid
of the V and B Jonhson filters) was used for the objects observed at NOT.
The data were reduced with standard procedures. Bias and flat-field corrections where applied to all the images. Differential photometry was done provided that the comparison stars in the field were not too faint making
the Fourier spectra noisier.
For OSN objects aperture photometry was performed with the IRAF package (http://iraf.noao.edu). The pulsation frequency analysis was done with Fourier techniques with a programme implemented by Garrido (IAA)
based on the Deeming (1975) method.
In the case of ORM objects, the programs 'rtp' for bias and flat-field correction, 'rtcorr' for extinction correction and 'rtft' for doing the Fourier transform were used (http://www.ing.iac.es/~roy/software/rtp-guide.html).
Figure 1 shows the light curves of all the observed objects. Figure 2 shows their amplitude spectra. The dotted horizontal line is 4 times the average amplitude in the whole frequency range. We would consider a
frequency statstically significant should this level be reached (Breger, 1993).
Objets
Bmag
HZ3
12.8
Observatory
LSV+2238
11.5
OSN
PG0226+151
15.31
OSN
PG1134+463
15.33
OSN
PG0208+016
12.34
OSN
HZ1
12.60
OSN
PG0217+155
14.56
OSN
PG0310+149
15.8
OSN
PG0920+297
14.58
OSN
PG1239+178
10.95
OSN
PG0836+619
14.45
OSN
PG0921+311
14.36
OSN
PG0909+275
10.74
OSN
HS0231+0505
15.8
OSN
PG0039+135
12.05
OSN
PG1038+510
14.82
OSN
PG1102+499
13.86
OSN
PG2215+151
13.88
OSN
PG0844+232
14.6
OSN
FBS0759+413
15.38
OSN
PG1301+270
15.23
ORM
PG1401+289
14.87
ORM
PG1427+196
13.85
ORM
PG1528+029
15.16
ORM
PG1624+382
14.96
ORM
OSN
Figure 1. Light curves
Table1. Sample objects
Figure 2. Amplitude spectra. The dotted horizontal line is 4 times
the average amplitude in the whole frequency range
PRELIMINARY CONCLUSIONS
An evidence of the presence of pulsations in a sdO has not been obtained so far. Marginal peaks on the 3σ level have been noted, especially in the low frequency range . Nevertheless, they can not be
confirmed as real pulsation frequencies unless with longer runs to improve the signal to noise ratio. A precision as low as 0.3 mmag was achieved for the objects with lower noise.
We note the objects PG 1427+196 with a frequency at 1.97 mHz in the 4σ level, and PG 1301+270 and PG 1624+382 with low frequencies marginal detections. These objects will be reobserved at
OSN in longer runs, along with PG 1036+433 named above, in order to asses or discard their possible frequencies of oscillation. Should the eventual sdOs pulsational frequencies mimic that of the sdBs
discovered to date, in the range 1.8-12 mHz, we would have one of our candidates lying in this range. However, no predictions of a theoretical instability strip for sdOs, in case of existence, have been done
so far.
FUTURE WORK
We will extend our search to a number of northern and southern hemisphere sdOs more. For choosing them we will bear in mind the pulsating spectral class known as PG 1159 (or GW Vir) stars. Their
physical parameters (Teff and log g) overlap in some range with those of sdOs. About half of the PG 1159 are pulsating stars with the driving of the modes due to the κ-mechanism associated with the opacity
bump due to the partial ionization zones of the K-shell electrons of C and O (Starrfield et al. 1983, 1984). Hence, the C and O rich objects are more likely to present pulsations. Therefore, might an
evolutionary connection exist between sdOs and PG 1159 stars, we should expect the most likely pulsating sdO candidates to be C-rich. A selection of targets has already been made from the literature and
private communications, and observing campaigns for them are in progress.
REFERENCES
Bartolini, C., Bonifazi, A., D'Antona, F. et al., 1982, Ap&SS, 83, 287
Bartolini, C., Bernabei, S., Bruni, I. et al., 2001, Mem.S.A.It., vol. 72, 4
Breger, M., Stich, J., Garrido, R. et al., 2003, A&A, 271, 482
Charpinet, S., Fontaine, G., Brassard, P. & Dorman, B., 1996, ApJ, 471, 103
Deeming, J. T., 1975, ApSS, 36, 137
Greenstein, J. L., 1957, IAUS, 3, 41
Heber, U., 1986, A&A, 155, 33
Kilkenny, D., Koen, C., O'Donoghue, D. & Stobie, R. S., 1997, MNRAS, 285, 640
Rodríguez López, C., 2003, Ms. Thesis, Univ. de Vigo
Starrfield, S., Cox, A. N., Hodson, S. W. & Pesnell, W. D., 1983, ApJ, 268, L27
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ACKNOWLEDGEMENTS
We thank A. Sota and V. Casanova for doing the observations at OSN (operated by IAA), and R. Oreiro and R. ∅stensen for observing at NOT (operated by IAC). We also
thank the Centro de Supercomputación de Galicia (CESGA) for the use made of their computational facilities. This work was supported by the Spanish Ministerio de
Ciencia y Tecnología under projects AYA 2000-1691 and AYA 2003-09499.