Low-mass Binary Systems Research Articles

White dwarfs with unresolved substellar companions and debris disks in the UKIDSS Survey

We present a near-infrared (NIR) photometric search for substellar companions and debris disks around white dwarfs in the UKIRT Infrared Deep Sky Survey (UKIDSS). We cross-correlate the SDSS DR4 and McCook & Sion catalogues of white dwarfs with the UKIDSS DR3 database producing 408 and 133 matches respectively. Models are then fitted to Sloan photometry to identify those with NIR photometric excesses consistent with an unresolved sub-stellar companion or a debris disk. We present follow up photometry from targets previously identified in UKIDSS DR2 and the first results from DR3. In total we identify 8 potential white dwarf + very low mass binary systems, 2 potential white dwarf + disk systems, and 2 systems which require further investigation to clarify the source of the excess.

UV observations of Cataclysmic Variables

Cataclysmic Variables are the most abundant population of galactic compact interacting binaries. They represent test objects to improve our knowledge of accretion theory as well as evolution of low-mass close binary systems. The UV domain represents a key range, linking the high energy emission to that of the optical domain, to understand the complex interplay of accretion and outflow processes, as well as to infer the physical status of the accreting primaries. Some of the major results and future needs in this range are discussed.

Observing the positions of spinning binary systems using LISA

We present an analysis of the expected angular and distance uncertainties of spinning binary systems as measured by the space-based gravitational-wave observatory LISA. We focus on two distinct cases: supermassive coalescing black holes at high redshift and nearby binary star systems with roughly constant orbital frequency. We calculate the uncertainties using least-squares parameter estimation of simulated waveforms that are accurate to 4/2 post-Newtonian order. Our results show that the presence of spin can decrease massive black hole distance and positional uncertainties by as much as factors of 10 and 100, respectively. In optimal circumstances, we find that LISA's massive black hole merger resolution can be as low as 10{sup -6} steradians. However, in low-mass binary systems the presence of spin tends to degrade LISA's resolution by anywhere from a factor of 3 to a factor of 100.

SMA CO (2-1) Observations of CG 30: A Protostellar Binary System with a High-Velocity Quadrupolar Molecular Outflow

We present interferometric observations in the12CO (2-1) line and at 1.3 mm dust continuum of the low-mass protostellar binary system in the cometary globule CG 30, using the Submillimeter Array. The dust continuum images resolve two compact sources (CG 30N and CG 30S), with a linear separation of ~8700 AU and total gas masses of ~1.4 and ~0.6 M☉, respectively. With the CO images, we discover two high-velocity bipolar molecular outflows, driven by the two sources. The two outflows are nearly perpendicular to each other, showing a quadrupolar morphology. The northern bipolar outflow extends along the southeast (redshifted, with a velocity up to ~23 km s−1) and northwest (blueshifted, velocity up to ~30 km s−1) directions, while the southern pair has an orientation from southwest (blueshifted, velocity up to ~13 km s−1) to northeast (redshifted, velocity up to ~41 km s−1). The outflow mass of the northern pair, driven by the higher mass source CG 30N, is ~9 times larger than that of the southern pair. The discovery of the quadrupolar molecular outflow in the CG 30 protobinary system, as well as the presence of other quadrupolar outflows associated with binary systems, demonstrate that the disks in (wide) binary systems are not necessarily co-aligned after fragmentation.

1E 161348−5055 in the Supernova Remnant RCW 103: A Magnetar in a Young Low‐Mass Binary System?

We suggest that the unique X-ray source 1E 161348?5055 at the center of the supernova remnant RCW 103 consists of a neutron star in close orbit with a low-mass main-sequence star. The time signature of 6.67 hr is interpreted as the neutron star's spin period. This requires the neutron star to be endowed with a high surface magnetic field of ~1015 G. Magnetic or/and material (propeller) torques are able to rapidly spin the young neutron star down to an asymptotic, equilibrium spin period in close synchronism with the orbital period, similar to what happens in the Polar cataclysmic variables. 1E 161348?5055 could be the first case of a magnetar born in a young low-mass binary system.

Tests of Bayesian model selection techniques for gravitational wave astronomy

The analysis of gravitational wave data involves many model selection problems. The most important example is the detection problem of selecting between the data being consistent with instrument noise alone, or instrument noise and a gravitational wave signal. The analysis of data from ground based gravitational wave detectors is mostly conducted using classical statistics, and methods such as the Neyman-Peterson criteria are used for model selection. Future space based detectors, such as the Laser Interferometer Space Antenna (LISA), are expected to produce rich data streams containing the signals from many millions of sources. Determining the number of sources that are resolvable, and the most appropriate description of each source poses a challenging model selection problem that may best be addressed in a Bayesian framework. An important class of LISA sources are the millions of low-mass binary systems within our own galaxy, tens of thousands of which will be detectable. Not only are the number of sources unknown, but so are the number of parameters required to model the waveforms. For example, a significant subset of the resolvable galactic binaries will exhibit orbital frequency evolution, while a smaller number will have measurable eccentricity. In the Bayesian approach to model selection one needs to compute the Bayes factor between competing models. Here we explore various methods for computing Bayes factors in the context of determining which galactic binaries have measurable frequency evolution. The methods explored include a reverse jump Markov chain Monte Carlo algorithm, Savage-Dickie density ratios, the Schwarz-Bayes information criterion, and the Laplace approximation to the model evidence. We find good agreement between all of the approaches.

16-20 M Jup Radial Velocity Companion Orbiting the Brown Dwarf Candidate Cha Hα 8

We report the discovery of a 16-20 Jupiter mass radial velocity companion around the very young (~3 Myr) brown dwarf candidate ChaHa8 (M5.75-M6.5). Based on high-resolution echelle spectra of ChaHa8 taken between 2000 and 2007 with UVES at the VLT, a companion was detected through RV variability with a semi-amplitude of 1.6 km/s. A Kepler fit to the data yields an orbital period of the companion of 1590 days and an eccentricity of e=0.49. A companion minimum mass M2sini between 16 and 20 Jupiter masses is derived when using model-dependent mass estimates for the primary. The mass ratio q= M2/M1 might be as small as 0.2 and, with a probability of 87%, it is less than 0.4. ChaHa8 harbors most certainly the lowest mass companion detected so far in a close (~ 1 AU) orbit around a brown dwarf or very low-mass star. From the uncertainty in the orbit solution, it cannot completely be ruled out that the companion has a mass in the planetary regime. Its discovery is in any case an important step towards RV planet detections around BDs. Further, ChaHa8 is the fourth known spectroscopic brown dwarf or very low-mass binary system with an RV orbit solution and the second known very young one.

The LuckyCam survey for very low mass binaries – II. 13 new M4.5–M6.0 binaries

We present results from a high angular resolution survey of 77 very low mass (VLM) binary systems with 6.0 ≤V−K colour ≤ 7.5 and proper motion ≥0.15 arcsec yr−1. 21 VLM binaries were detected, 13 of them new discoveries. The new binary systems range in separation between 0.18 and 1.3 arcsec. The distance-corrected binary fraction is 13.6+6.5−4 per cent, in agreement with previous results. Nine of the new binary systems have projected separations >10 au, including a new wide VLM binary with 27 au projected orbital separation. One of the new systems forms two components of a 2300 au separation triple system. We find that the projected separation distribution of the binaries with V−K < 6.5 in this survey appears to be different from that of redder (lower mass) objects, suggesting a possible rapid change in the orbital radius distribution at around the M5 spectral type. The target sample was also selected to investigate X-ray activity among VLM binaries. There is no detectable correlation between excess X-ray emission and the frequency and binary properties of the VLM systems.

Discovery of the Widest Very Low Mass Binary

We report the discovery of a very low mass binary system (primary mass <0.1 Msol) with a projected separation of ~5100 AU, more than twice that of the widest previously known system. A spectrum covering the 1-2.5 microns wavelength interval at R ~1700 is presented for each component. Analysis of the spectra indicates spectral types of M6.5V and M8V, and the photometric distance of the system is ~62 pc. Given that previous studies have established that no more than 1% of very low mass binary systems have orbits larger than 20 AU, the existence of such a wide system has a bearing on very low mass star formation and evolution models.

Comparing η Carinae with the Red Rectangle

I compare the structures of the bipolar nebulae around the massive binary system Eta Carinae and around the low mass binary system HD 44179. While Eta Carinae is on its way to become a supernova, the Red Rectangle is on its way to form a planetary nebula. Despite the two orders of magnitude difference in mass, these two systems show several similarities, both in the properties of the stellar binary systems and the nebulae. From this comparison and further analysis of the accretion process during the 20 years Great Eruption of Eta Carinae, I strengthen the binary model for the formation of its bipolar nebula--the Homunculus. In the binary model a large fraction of the mass lost by the primary star during the Great Eruption was transferred to the secondary star (the companion); An accretion disk was formed around the companion, and the companion launched two opposite jets. I show that the gravitational energy of the mass accreted onto the secondary star during the Great Eruption can account for the extra energy of the Great Eruption, both the radiated energy and the kinetic energy in the Homunculus. I also conclude that neither the proximity of the primary star in Eta Car to the Eddington luminosity, nor the rotation of the primary star are related directly to the shaping of the Homunculus. I speculate that the Great Eruption of Eta Carinae was triggered by disturbance in the outer boundary of the convective region, most likely by magnetic activity, that expelled the outer radiative zone.

Resolved Hubble space spectroscopy of ultracool binary systems

Using the low-resolution mode of the Space Telescope Imaging Spectrograph (STIS) aboard the \emph{Hubble Space Telescope} (HST), we have obtained spatially resolved spectra of 20 ultracool dwarfs. 18 of them belong to 9 known very low-mass binary systems with angular separations in the range 0.37-0.098 arcseconds. We have derived spectral types in the range dM7.5 to dL6 from the PC3 index, and by comparing our STIS spectra with ground-based spectra of similar spectral resolution from Mart{\'\i}n et al. (1999). We have searched for H$_\alpha$ emission in each object but it was clearly detected in only 2 of them. We find that the distribution of H$_\alpha$ emission in our sample is statistically different from that of single field dwarfs, suggesting an intriguing anticorrelation between chromospheric activity and binarity for M7--M9.5 dwarfs. We provide measuments of the strength of the main photospheric features and the PC3 index, and we derive calibrations of spectral subclasses versus F814W and K-band absolute magnitudes for a subset of 10 dwarfs in 5 binaries that have known trigonometric parallaxes.

Collapse of Neutron Stars to Black Holes in Binary Systems: A Model for Short Gamma-Ray Bursts

The accretion of ≈0.1-1 M☉ of material by a neutron star through Roche lobe overflow of its companion or through white dwarf-neutron star coalescence in a low-mass binary system could be enough to exceed the critical mass of a neutron star and trigger its collapse to a black hole, leading to the production of a short gamma-ray burst (SGRB). In this model, SGRBs would often be found in early-type galaxies or in globular cluster environments, though they could also be formed through stellar wind accretion in high-mass binary systems of star-forming galaxies. Although the collapse event is likely to be of short (1 s) duration, afterglow complexity could be produced by neutron decoupling in the fireball and subsequent accretion of the companion remnant, for example, the disrupted white dwarf core.

X-ray luminosity function of faint point sources in the Milky Way

We assess the contribution to the X-ray (above 2 keV) luminosity of the Milky Way from different classes of low-mass binary systems and single stars. We begin by using the RXTE Slew Survey of the sky at |b|>10 to construct an X-ray luminosity function (XLF) of nearby X-ray sources in the range 10^30<Lx(erg/s)<10^34 (where Lx is the luminosity over 2-10 keV), occupied by coronally active binaries (ABs) and cataclysmic variables (CVs). We then extend this XLF down to Lx~10^27.5 erg/s using the Rosat All-Sky Survey in soft X-rays and available information on the 0.1-10 keV spectra of typical sources. We find that the local cumulative X-ray (2-10 keV) emissivities (per unit stellar mass) of ABs and CVs are (2.0+/-0.8)x10^27 and (1.1+/-0.3)x10^27 erg/s/Msol, respectively. In addition to ABs and CVs, representing old stellar populations, young stars emit locally (1.5+/-0.4)x10^27 erg/s/Msol. We finally attach to the XLF of ABs and CVs a high luminosity branch (up to ~10^39 erg/s) composed of neutron-star and black-hole low-mass X-ray binaries (LMXBs), derived in previous work. The combined XLF covers ~12 orders of magnitude in luminosity. The estimated combined contribution of ABs and CVs to the 2-10 keV luminosity of the Milky Way is ~2x10^38 erg/s, or ~3% of the integral luminosity of LMXBs (averaged over nearby galaxies). The XLF obtained in this work is used elsewhere (Revnivtsev et al.) to assess the contribution of point sources to the Galactic ridge X-ray emission.

Production and evolution of millisecond X-ray and radio pulsars

Observations using the Rossi X-ray Timing Explorer ( RXTE) have discovered dozens of accreting neutron stars with millisecond spin periods in low-mass binary star systems. Eighteen are millisecond X-ray pulsars powered by accretion or nuclear burning or both. These stars have magnetic fields strong enough for them to become millisecond rotation-powered (radio) pulsars when accretion ceases. Few, if any, accretion- or rotation-powered pulsars have spin rates higher than 750 Hz. There is strong evidence that the spin-up of some accreting neutron stars is limited by magnetic spin-equilibrium whereas the spin-up of others is halted when accretion ends. Further study will show whether the spin rates of some accretion- or rotation-powered pulsars are or were limited by emission of gravitational radiation.

Multiplicity at the Stellar/Substellar Boundary in Upper Scorpius

We present the results of a high-resolution imaging survey of 12 brown dwarfs and very low mass stars in the closest (~145 pc) young (~5 Myr) OB association, Upper Scorpius. We obtained images with the Advanced Camera for Surveys High Resolution Channel on HST through the F555W (V), F775W (i'), and F850LP (z') filters. This survey discovered three new binary systems, including one marginally resolved pair with a projected separation of only 4.9 AU, resulting in an observed binary fraction of 25% ± 14% at separations ≳4 AU. After correcting for detection biases assuming a uniform distribution of mass ratios for m_s/m_p > 0.6, the estimated binary fraction is 33% ± 17%. The binary fraction is consistent with that inferred for higher mass stars in Upper Sco, but the separation and mass ratio distributions appear to be different. All three low-mass binary systems in Upper Sco are tight ( 10M_J at projected separations >20 AU or masses >5M_J at projected separations >40 AU orbiting any of the low-mass (0.04-0.10 M☉) objects in our sample.

Discovery of a Wide, Low-Mass Binary System in Upper Scorpius

Using the near-infrared spectrometer SpeX and its slit-viewing camera at the IRTF, I have resolved a low-mass member of the Upper Scorpius OB association into a double star. From K-band images of the pair, DENIS-P J161833.2-251750.4 A and B, I measure a separation of 0.96 and a magnitude difference of dK=0.42 mag. I present resolved 0.8-2.5 micron spectroscopy of the two objects, both of which exhibit signatures of youth in the shape of their H- and K-band continua, demonstrating that both are members of Upper Scorpius rather than field stars. In addition, through a comparison to optically-classified pre-main-sequence objects, I derive a spectral type near M5 for each component, corresponding to a mass of ~0.15 Msun with the evolutionary models of Chabrier and Baraffe. The probability that this pair is composed of unrelated M-type members of Upper Scorpius is ~10^-5. When added to the recent discoveries of other wide, easily disrupted low-mass binaries, this new system further establishes that the formation of low-mass stars and brown dwarfs does not require ejection from multiple systems. These observations also indicate that wide low-mass binaries can form in OB associations as well as in smaller clusters where the previously known wide pairs have been found. Thus, the available data show no perceptible effect of star-forming environment on the prevalence of loosely bound low-mass systems.

Steps towards a solution of the FS Aurigae puzzle - I. Multicolour high-speed photometry with ULTRACAM

We present the analysis of high-speed photometric observations of FS Aur taken with the 4.2-m William Herschel Telescope and the high-speed camera ULTRACAM in late 2003. These observations were intended to determine whether there was any evidence of photometric variability with a period in the range 50-100 seconds. The discovery of such variations would help to explain existence, in FS Aur, of the very coherent photometric period of 205.5 min that exceeds the spectroscopic period by 2.4 times. Such a discrepancy in the photometric and spectroscopic periods is an unusual for a low mass binary system that is unambiguously identified as a Cataclysmic Variable. Using various methods, including wavelet-analysis, we found that with exception of the 205.5-minute periodicity, the main characteristic of variability of FS Aur is usual flickering and Quasi-Periodic Oscillations. However, we detected variability with a period of ~101 and/or ~105 sec, seen for a short time every half of the orbital period. These oscillations may be associated with the spin period of the white dwarf, not ruling out the possibility that we are observing a precessing rapidly rotating white dwarf in FS Aur.

Discovery of a widely separated binary system of very low mass stars

We report our discovery of a nearby wide binary system, LP 714-37 AB. Phan-Bao et al. identified LP 714-37 as a mid-M dwarf during a cross-identification of the NLTT and DENIS catalogues. Our CCD images resolve the system into a binary with a projected angular separation of 1.8´´, or 33 AU, and low-resolution optical spectra give spectral types of M5.5 (LP 714-37A) and M7.5 (LP 714-37B). This makes LP 714-37 AB one of very few widely separated (separation >30 AU) very low-mass binary systems known in the field, and we discuss it in the context of the multiplicity properties of very low-mass stars and brown dwarfs.

A Strong X‐Ray Burst from the Low‐Mass X‐Ray Binary EXO 0748−676

We have observed an unusually strong X-ray burst as a part of our regular eclipse timing observations of the low-mass binary system EXO 0748-676. The burst peak flux was 5.2 × 10-8 ergs cm-2s-1, approximately 5 times the normal peak X-ray burst flux observed from this source by RXTE. Spectral fits to the data strongly suggest that photospheric radius expansion occurred during the burst. In this paper we examine the properties of this X-ray burst, which is the first example of a radius expansion burst from EXO 0748-676 observed by RXTE. We find no evidence for coherent burst oscillations. Assuming that the peak burst luminosity is the Eddington luminosity for a 1.4 M☉ neutron star, we derive a distance to EXO 0748-676 of 7.7 kpc for a helium-dominated burst photosphere and 5.9 kpc for a hydrogen-dominated burst photosphere.

Evolutionary Processes in Low-Mass Binary Systems

Evolutionary Processes in Low-Mass Binary Systems