AM CVn Stars:Status and Challenge

If you're interested on Solheim paper and don't want to read it, here is the 1 page review of the review!! LOL

J.-E SOLHEIM paper review:
AM CVn stars are short-period binary stars in which a white dwarf (WD) accretes material from a low-mass donor star. Initially they're detached, but they evolved into semidetached contact under certain conditions. These are among the first possible detectable sources of low frequency gravitational wave (GW) radiation, since their evolution toward shorter periods is driven by angular momentum lost due GW radiation. They also represents the terminal phase of a binary star evolution, as been compact and hydrogen low systems by studying these we can study physics under extreme conditions.

There are 3 possible channels for donor evolution: The WD Channel is when Roche lobe overflow happens between 2 WDs, where the donor star is the smallest and helium rich. For this channel there is a Direct-Impact phase. With small mass ratios, the distance between the WDs become shorter because of GW radiation and the accretion stream hit directly on the accretor's surface. Observation of accretor is possible during this phase.
If The Helium-Star Channel the donor star is the nondegenerate core of a 2.5-5 solar masses star that may be at different stages of He exhaustion in the core at the time of contact. Finally, The Hydrogen Star Channel is the less probable. It happen when in a regular Cataclysmic Variable star, the donor lost most of its hydrogen so it's the remnant of a low-mass main-sequence star.

A possible outcome of AM CVn stars is explosive events, such as SN Ia type or SN .Ia supernova, being SN 2005E, a helium accreting binary WD supernova, a good candidate for the first observed SN .Ia with AM CVn progenitor.

The Am CVn objects can be divided in 4 groups according to disk properties: 1.-Ultrashort periods an no disk (P<12 m), 2.-Large stable disks in superoutburst state (12
<20 m), 3.- Variable-size disk, with outburst and occasional superoutburst (20
<40 m) and 4.- Small stable disk (P>40 m). For the last one, the spectra changes from a high state with absorption lines to a low state with emission lines.

In last 10 years AM CVn stars number had increased because of the large observation campaigns, rising them up to 25 systems. Thanks to this, density and population estimations have improved. There are 2 evolution scenarios: the optimistic case and the pessimistic case. Roelofs et al. predicted sky surface density of Am CVn stars based on T_eff, M_g, Galaxy population models and WD and helium progenitors assumed evolution. The predicted density of AM CVn stars resulted to be highest for low Galactic latitudes, with an error lower than a factor 2. But the predicted density for optimistic case and the pessimistic case are far from reality. This is telling us that something may be overlooked and there still work to be done.

"The strange star SS433 may be a black hole whose accretion disk is highly inclined with respect to its orbit about a companion star. Digital, cover, February 2005 "Astronomy" magazine."

Image used with the permission of Don Dixon, FIAAA
Scientific Illustration

http://cosmographica.com/gallery/portfolio2007/content/index_28.html



Very soon in Spanish as well (hopfully!) XD enjoy