**7. Future work**

There are many neglected intermediate-period interacting binary star systems that deserve studying. Two examples are Y Piscium (orbital period = 3.7 days) and RV Ophiuchus (orbital period = 3.9 days), both of which show outside-of-eclipse variations in their latest light curves, which are more than 25 years old (Walter, 1973). Can the light curves of these star systems be modelled with eccentric accretion structures and orbital inclinations that lead to apparent eclipses of the primary star by the accretion cloud? If not, what else could explain the light curve variations? If something else can explain them, can it also explain the variations seen in R Ara?

On the theoretical side of the research, it is desirable to modify the W.D. code to account for eccentric accretion structures (disks or annuli). A limitation to the "cool cloud" method used in Section 6 is that the model does not account for the emission of light by the parts of the accretion structure not in the line of sight to the primary star. This condition is quite acceptable

**0**

**2**

*Brazil*

**UV Emission and Spectral Synthesis of Accretion**

Accretion disks are present throughout the wide Universe at multiple scales. They are in the active galactic nuclei (AGN) with a disk radius (*r*disk) ∼ 0.1-1.0 parsecs (1.0 pc ∼ 206264 AU <sup>∼</sup> 3.1×1018 cm), protostars (YSOs) (*r*disk<sup>∼</sup> 1000 AU), Debris disks (*r*disk∼10-100 AU) and in close binary systems: Cataclysmic Variables (CVs), Low Mass X-Ray Binaries (LMXBs) and some symbiotic stars (*r*disk∼R�∼6.9×1010 cm). Being a very common phenomenon in the Universe, the understanding of accretion disk behavior, time scales and evolution are crucial

Among these systems, interacting binaries are the best laboratories to understand the accretion disk physics. This because of its abundance in the Galaxy, the amount of nearby systems, data availability and the existance of observational facilities to obtain them. Cataclysmic Variables present the best scenery to analyze accretion disk behavior due to the fact that its main emission region falls on ultraviolet (UV) and optical. In these wavelength bands, we have the tools with the spectral and the time resolution that allow us to see most of the disk with good detail. Also, due to the order of magnitude of CVs periods and luminosity, it is feasible to study the disk physics in a wide range of time scales, which facilities the

Cataclysmic Variables are semidetached binary systems with orbital periods between 0.28 and 18 hours (Ritter & Kolb, 2003), where a Roche-lobe filling main sequence star (secondary) transfers mass onto a white dwarf (primary). Depending on the magnitude of the primary magnetic field, this mass transfer can be performed through an accretion disk (non-magnetic CVs), a truncated disk (intermediate polars) or funnels (polars). CVs can also be classified into several types depending on their erupting behavior. System that do not show any eruptions is called "*Nova* − *Like*" (NL), systems that show one or more thermonuclear eruptions are "*Novae*" (Ne) and "*RecurrentNovae*" (RN) respectively. Also, there exists systems with a kind of "weak" periodic eruption, caused by disk instabilities. These systems are known as "*Dwar f Novae*" (DNe) (Warner, 1995). It is important to score that the types of CVs are not exclusive each other i.e. both magnetic or non-magnetic CVs can show novae eruptions. The NL systems are non-magnetic CVs with a high mass accretion rate (M˙ *<sup>a</sup>*). This fact bears a bright accretion disk, unlike DNe that show low M˙ *<sup>a</sup>* values and then weaker disks. For NL the disk emission greatly exceeds the emission of the other members of the binary system (primary plus secondary), and specially dominates the UV region of the spectrum. Also, it has

to have a clearer vision of most of the systems present in the Universe.

understanding of its evolution in a multiplicity of cases.

**1. Introduction**

**Disks in Non-Magnetic Cataclysmic Variables**

*Departamento de Astronomia. IAG. Universidade de Sao Paulo.* ˜

Raul E. Puebla and Marcos P. Diaz

for observations in UV light because, in that range of frequencies, the cool accretion structure emits very little light compared with the hot primary star. In visible, red, and infrared light, however, the accretion structure will emit more light than it does in the UV and the primary star will emit less light that is does in the UV, which makes the limitation of the "cool cloud" method more serious. Including the entire eccentric accretion structure in the light curve model will eliminate these limitations at all wavelengths are will provide a more accurate picture of the transfer of mass between these stars.

#### **8. References**

