* Astronomy

Blobrana · L

SW Sex Stars

Blobrana · L

RE: SW Sextantis Stars

Blobrana · L

Title: The SW Sex phenomenon as an evolutionary stage of Cataclysmic Variables
Authors: Linda Schmidtobreick

From recent large observing campaigns, one finds that nearly all non- or weakly magnetic cataclysmic variables in the orbital period range between 2.8 and 4 hours are of SW Sex type and as such experience very high mass transfer rates. The evolution of cataclysmic variables as for any interacting binary is driven by angular momentum loss which results in a decrease of the orbital period on evolutionary time scales. In particular, all long-period systems need to cross the SW Sex regime of the orbital period distribution before entering the period gap. This makes the SW Sex phenomenon an evolutionary stage in the life of a cataclysmic variable. Here, I present a short overview of the current state of research on these systems.

Read more (80kb, PDF)

Blobrana · L

Title: The SW Sex enigma
Authors: V. S. Dhillon, D. A. Smith, T. R. Marsh

The SW Sex stars are a class of cataclysmic variables, originally identified because they shared a number of enigmatic properties - most notably, single-peaked emission lines instead of the double-peaked lines one would expect from their high-inclination accretion discs. We present high time-resolution spectrophotometry of the eclipsing nova-like variables SW Sex and DW UMa, two of the founding members of the SW Sex class. Both systems show single-peaked Balmer and HeII 4686A emission lines that appear to originate from a region in the disc that lies close to, but downstream of, the bright spot. The emission-line light curves are consistent with the finding from X-ray and ultraviolet observations that we predominantly see the flared disc rim and the unobscured back portion of the disc in these systems. In DW UMa, the HeII 4686A emission line originates from close to the white dwarf and exhibits flaring. Such flares have been used to argue for magnetically-channelled accretion, as in the intermediate polars, but the lack of a clear periodicity in the flares argues for a simpler model in which we are viewing the central regions of the disc through the non-uniform upper edge of a flared disc rim. We also observe narrow, blue-shifted, transient absorption features in the Balmer lines of DW UMa, which we attribute to blobs of material ejected from the system, possibly by a magnetic propeller, that happen to be passing between us and the binary. Our results suggest that the solution to the SW Sex enigma is a combination of dominant bright-spot emission and a self-occulting disc. We also propose a simplified classification scheme for nova-like variables.

Read more (1586kb, PDF)

Blobrana · L

Title: A Study of the Orbital Periods of Deeply Eclipsing SW Sextantis Stars
Authors: David Boyd

Results are presented of a five-year project to study the orbital periods of eighteen deeply eclipsing novalike cataclysmic variables, collectively known as SW Sextantis stars, by combining new measurements of eclipse times with published measurements stretching back in some cases over fifty years. While the behaviour of many of these binary systems is consistent with a constant orbital period, it is evident that in several cases this is not true. Although the time span of these observations is relatively short, evidence is emerging that the orbital periods of some of these stars show cyclical variation with periods in the range 10-40 years. The two stars with the longest orbital periods, V363 Aur and BT Mon, also show secular period reduction with rates of -6.6 x 10^{-8} days/year and -3.3 x 10^{-8} days/year. New ephemerides are provided for all eighteen stars to facilitate observation of future eclipses.

Read more (498kb, PDF)

Blobrana · L

Title: The Search for SW Sex Type Stars
Authors: Linda Schmidtobreick, Pablo Rodriguez-Gil, Boris T. Gaensicke

All eclipsing nova-likes in the 2.8-4h orbital period range belong to the group of SW Sex stars, and as such experience very high mass transfer rates. Since the physical properties of a star should be independent of the inclination it is observed at, this suggests that all or at least a large fraction of the non- or weakly-magnetic cataclysmic variables in this period range are physically SW Sex stars. We here present preliminary results of a large campaign to search for SW Sex characteristic features in the spectra of such stars. We find that 14 out of the 18 observed non-eclipsing cataclysmic variables belong to the group of SW Sex stars the classification of the other four is uncertain from our data. This confirms the domination of SW Sex stars in the period range of 2.8-4 h just above the period gap. Since all long-period systems need to cross this range before entering the gap, the SW Sex phenomenon is likely to be an evolutionary stage in the life of a cataclysmic variable.

Read more (971kb, PDF)