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Register a new userUltra-Short Period Double-Degenerate Binaries
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Authors
Mark Cropper
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We review the current observational status of the ROSAT sources RX J1914.4+2456 and RX J0806.3+1527, and the evidence that these are ultra-short period (<10min) binary systems. We argue that an Intermediate Polar interpretation can be ruled out, that they are indeed compact binaries with a degenerate secondary, and that the period seen in the X-ray and optical is the orbital period. A white dwarf primary is preferred, but a neutron star cannot be excluded. We examine the capability of the three current double-degenerate models (Polar, Direct Accretor and Electric Star) to account for the observational characteristics of these systems. All models have difficulties with some aspects of the observations, but none can be excluded with confidence at present. The Electric Star model provides the best description, but the lifetime of this phase requires further investigation. These ultra-short period binaries will be strong gravitational wave emitters in the LISA bandpass, and because of their known source properties will be important early targets for gravitational wave studies.
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We investigate the properties of 367 ultra-short period binary candidates selected from 31,000 sources recently identified from Catalina Surveys data. Based on light curve morphology, along with WISE, SDSS and GALEX multi-colour photometry, we identify two distinct groups of binaries with periods below the 0.22 day contact binary minimum. In contrast to most recent work, we spectroscopically confirm the existence of M-dwarf+M-dwarf contact binary systems. By measuring the radial velocity variations for five of the shortest-period systems, we find examples of rare cool-white dwarf+M-dwarf binaries. Only a few such systems are currently known. Unlike warmer white dwarf systems, their UV flux and their optical colours and spectra are dominated by the M-dwarf companion. We contrast our discoveries with previous photometrically-selected ultra-short period contact binary candidates, and highlight the ongoing need for confirmation using spectra and associated radial velocity measurements. Overall, our analysis increases the number of ultra-short period contact binary candidates by more than an order of magnitude.
We present the results of our study of the eclipsing binary systems CSS J112237.1+395219, LINEAR 1286561 and LINEAR 2602707 based on new CCD $B$, $V$, $R_c$ and $I_c$ complete light curves. The ultra-short period nature of the stars citep{Drake2014} is confirmed and the systems periods are revised. The light curves were modelled using the 2005 version of the Wilson-Devinney code. When necessary, cool spots on the surface of the primary component were introduced to account for asymmetries in the light curves. As a result, we found that CSS J112237.1+395219 is a W UMa type contact binary system belonging to W subclass with a mass ratio of $q=1.61$ and a shallow degree of contact of 14.8% where the primary component is hotter than the secondary one by $500K$. LINEAR 1286561 and LINEAR 2602707 are detached binary systems with mass ratios $q=3.467$ and $q=0.987$ respectively. These detached systems are low-mass M-type eclipsing binaries of similar temperatures. The marginal contact, the fill-out factor and the temperature difference between components of CSS J112237.1+395219 suggest that this system may be at a key evolutionary state predicted by the Thermal Relaxation Oscillation theory (TRO). From the estimated absolute parameters we conclude that our systems share common properties with others ultra-short period binaries.
We report on the discovery of four ultra-short period (P<0.18 days) eclipsing M-dwarf binaries in the WFCAM Transit Survey. Their orbital periods are significantly shorter than of any other known main-sequence binary system, and are all significantly below the sharp period cut-off at P~0.22 days as seen in binaries of earlier type stars. The shortest-period binary consists of two M4 type stars in a P=0.112 day orbit. The binaries are discovered as part of an extensive search for short-period eclipsing systems in over 260,000 stellar lightcurves, including over 10,000 M-dwarfs down to J=18 mag, yielding 25 binaries with P<0.23 days. In a popular paradigm, the evolution of short period binaries of cool main-sequence stars is driven by loss of angular momentum through magnetised winds. In this scheme, the observed P~0.22 day period cut-off is explained as being due to timescales that are too long for lower-mass binaries to decay into tighter orbits. Our discovery of low-mass binaries with significantly shorter orbits implies that either these timescales have been overestimated for M-dwarfs, e.g. due to a higher effective magnetic activity, or that the mechanism for forming these tight M-dwarf binaries is different from that of earlier type main-sequence stars.
In the last two decades, thousands of extrasolar planets were discovered based on different observational techniques, and their number must increase substantially in virtue of the ongoing and near-future approved missions and facilities. It is shown that interesting signatures of binary systems from nearby exoplanets and their parent stars can also be obtained measuring the pattern of gravitational waves that will be made available by the new generation of detectors including the space-based LISA (Laser Interferometer Space Antenna) observatory. As an example, a subset of exoplanets with extremely short periods (less than 80 min) is discussed. All of them have gravitational luminosity, $L_{GW} sim 10^{30}$ $erg/s$, strain $h sim 10^{-22}$, frequencies $f_{gw} > 10^{-4}$Hz, and, as such, are within the standard sensitivity curve of LISA. Our analysis suggests that the emitted gravitational wave pattern may also provide an efficient tool to discover ultra short period exoplanets.
We show that black-hole High-Mass X-ray Binaries (HMXBs) with O- or B-type donor stars and relatively short orbital periods, of order one week to several months may survive spiral in, to then form Wolf-Rayet (WR) X-ray binaries with orbital periods of order a day to a few days; while in systems where the compact star is a neutron star, HMXBs with these orbital periods never survive spiral-in. We therefore predict that WR X-ray binaries can only harbor black holes. The reason why black-hole HMXBs with these orbital periods may survive spiral in is: the combination of a radiative envelope of the donor star, and a high mass of the compact star. In this case, when the donor begins to overflow its Roche lobe, the systems are able to spiral in slowly with stable Roche-lobe overflow, as is shown by the system SS433. In this case the transferred mass is ejected from the vicinity of the compact star (so-called isotropic re-emission mass loss mode, or SS433-like mass loss), leading to gradual spiral-in. If the mass ratio of donor and black hole is $>3.5$, these systems will go into CE evolution and are less likely to survive. If they survive, they produce WR X-ray binaries with orbital periods of a few hours to one day. Several of the well-known WR+O binaries in our Galaxy and the Magellanic Clouds, with orbital periods in the range between a week and several months, are expected to evolve into close WR-Black-Hole binaries,which may later produce close double black holes. The galactic formation rate of double black holes resulting from such systems is still uncertain, as it depends on several poorly known factors in this evolutionary picture. It might possibly be as high as $sim 10^{-5}$ per year.
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