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| INTRODUCTION.
In cataclismic
variables (CVs) a late spectral type star (quasi-main sequence) filling
its Roche lobe, transfers mass to a more massive white dwarf (WD) (main
component). The way how the gas travels from the donor star (secondary
component) depends on the magnetic field intensity of the WD. In the case
of weakly magnetic WDs, the material forms a disk around the white dwarf
into which the plasma falls spiraling towards the surface of the star.
On the contrary, when the field is very strong, the material is forced
to follow the magnetic field lines, rendering imposible the formation of
an accretion disk. Finally, there's an intermediate case, in which the
gas is transported through a disc truncated by the magnetic field in its
innermost regions (i.e. nearest to the WD).
Analyzing the distribution of the number of CVs as a function of its orbital period, a big clump is observed in the interval of periods between 3 and 4 hours. One third part of CVs in this interval show peculiar spectroscopic characteristic compared to the rest and are denominated SW Sextantis systems (see e.g. Rodríguez-Gil & Martínez-Pais 2002 for further details). In these cataclismics, the mass transfer rate from the donor star is unusually big, and the temperature of the WDs is atypicaly high, both facts with no explanation in the frame of the standard theory of the evolution of CVs. Contrary to what was thought in the late 1990s, there's as well observational evidence supporting the presence of magnetic WDs in the SW Sextantis systems (Rodríguez-Gil et al. 2001; Hameury & Lasota 2002). All these characteristics have made these systems, considered before as an exception, to become the roule in the interval of orbital periods from 3 to 4 hours. Unfortunately, the high mass transfer rate renders the accretion structures very luminous dazzling any other structure of the binary system (WD, donor star, etc.). From the evolutionary point of view, the dynamic characterization of SW Sextantis system is fundamental to check whether the masses of the stellar components involved are compatible or not with the rest of CVs within the same range of orbital periods. To do that, it is neccesary to detect the spectrum of the donor star, which is impossible because of the brightness of the accretion structures. Fortunately, the SW Sextantis systems undergo phases of low rate gas transfer (as well with no explanation as yet), in this phases the brightness of the accretion disk disminishes to the point where the faint donor star becomes visible. In this moments it is possible to carry out spectroscopic and photometric observations dedicated to the study of the secondary star and the WD separately. As shown if Fig. 1, the drop of brightness may reach up to 3-4 magnitudes, forcing the use of big aperture telescopes.
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| Figure 1. Ligth curves in V filter of the systems V533 Herculis (left) and DW Ursae Majoris (right), where it is patent the falls in brightness occurring in a quasi-periodic manner in both SW Sextantis systems. In the case of DW UMa, two falls can be seen separated by ~6-7 years. Figure adapted from Honeycutt & Kafka (2004). |
| THE “M1 GROUP” LABOUR.
Though these states of low brightness do not happen often (they use to happen once every 5-10 years in a particular system), the growing number of new SW Sextantis systems being discovered makes it possible to detect 1 or 2 events per year. On the other hand, the photometric monitoring of a great number of stars is an impossible task for professional observatories. For this reason, in april 2004 a collaboration of the members of the M1 group was started with the main goal of performing routine photometric monitoring of a big number of SW Sextantis systems to try and detect states of low brightness. Currently, about 30 systems are being photometrically monitored, since April 2004 four systems have been detected in low states (HS 0220+0603 in October 2004; V442 Oph, March 2005; KUV 03580+0614, September 2005; and HS 0455+8315, January 2006) with magnitudes between 2 and 4 fainter than usual (see Fig. 2), demonstrating the high efficiency of the monitoring program.
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| Figure 2. Light curves of the SW Sextantis systems detected in a state of low rate of mass transfer by the observers of the M1 Group. From top to bottom : HS 0220+0603, V442 Oph, KUV 03580+0614 y HS 0455+8315. The date is represented in the horizontal axis and the magnitude in V band is shown in the vertical. The normal magnitude of V442 Oph is V ~ 13.5, indicating that the observations were done in the state of low brigthness. |
| The first observations by the VLT telescope one month after the drop show the spectrum of both, the donor star and the white dwarf, this is the first observation of the spectrum in visible light of both stellar componets of an SW Sextantis system ever. The analysis of these spectra (see fig. 3) indicate the presence of a secondary star of a spectral type M3-4 V and a white dwarf unusually hot, with Teff > 25000K, the distance to the binary is estimated in d ~ 700 – 1000 pc. |
| Figure 3. Spectrum of HS 0220+0603 in low brightness state taken with the VLT/FORS2 on 2004 November the 21th (gray). The spectrum of the donor star is clearly seen in red and a raising continuum in blue originated by the photosphere of the white dwarf. The decomposition of the spectrum shows the presence of a donor star of spectral type M3-4 V (red), a white dwarf with Teff > 25000 (blue) and a distance of d ~ 700-1000 pc. The addition of both components is shown in black. |
| The time
resolved spectroscopic data obtained with the WHT has returned the amplitude
of the curve of radial speed of the companion star, i.e. the orbital velocity
around the mass center as seen from the Earth. With the same telescope,
very high resolution time resolved photometry was carried out on 2005 August
with the camera ULTRACAM (see Fig. 4), they show for the first time ever
the fast ocultation of the white dwarf by the donor star. This data, together
with the spectroscopy and the already existing data of the system in its
normal state, will enable the precise calculation of the masses of the
system components, other orbital parameters, and the temperature of the
white dwarf. We are working currently to that end.
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| Figure 4. Light curves of HS 0220+0603 obtained with WHT/ULTRACAM on 2005 August the 9th. From top to bottom the filters i, g and u are represented. The steep drop in brightness corresponds to the occultation of the white dwarf by the donor star, lasting about 15min. |
| A LONG
TERM COLLABORATION.
The only manner of measuring the orbital parameters of the SW Sextantis system directly, and thus, of obtaining as time permits a significant statistical sample of them, is to perform observations, as those described here, during low brigthness states. Hence, the long term monitoring by the M1 Group is essential for the succes of the project. The labour of the M1 Group is also very important when applying for observation time in big aperture telescopes, it would not be possible to obtain it without a reliable monitoring program. In this sense, we have been granted with observation time in target of opportunity regime in the VLT, WHT and NOT telescopes, the observations will be activated just after the M1 alert reception and subsequent confirmation by the IAC80 telescope of the Observatorio del Teide in Tenerife. The success of this collaboration has moved us to extend it to the southern hemisphere, there're already some telescopes there searching this low brightness states, in SW Sextantis systems, only visible from these latitudes. REFERENCES
- Honeycutt R. K., Kafka S., 2004, The Astronomical Journal, 128, 1279.
- Rodríguez-Gil P., Casares J., Martínez-Pais I. G., Hakala P., Steeghs D., 2001, The Astrophysical Journal, 548, L49.
- Rodríguez-Gil P., Martínez-Pais I. G., 2002, Monthly Notices of the Royal Astronomical Society, 337, 209.
- Rodríguez-Gil P., 2005, en The Astrophysics of Cataclysmic Variables and Related Objects, ASP Conference Vol. 330, eds. J.-M. Hameury y J.-P. Lasota. San Francisco: Astronomical Society of the Pacific, p. 335 ACKNOWLEDGMENTS.
From these pages I'd like to express my gratitude and congratulate all the astronomers collaborating or that have collaborated actively in this joint project, specially to Diego Rodríguez. I want to reflect here my gratitude for his work and effort together with that of Julio Castellano, Ramón Naves, Esteban Reina, Sensi y José Antonio de Observamurcia, Tomás Luis Gómez, Teófilo Arranz, José Ripero, Alfredo González Herrera, Miguel Rodríguez Marco, Juan Lacruz, Joaquín Sánchez, and José Castillo. I'd also like to thank Álex Oscoz (Telescope operations chief) and the groups of telescope operators and support astronomers at the Observatorio del Teide for their quick response confirming alerts raised by the M1 Group. Pablo Rodríguez
Gil is Phd in astrophysics by the Universidad de La Laguna (Tenerife).
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