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Register a new userThe Universality of the Companion Mass Ratio Distribution
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We present new results regarding the companion mass-ratio distribution (CMRD) of stars, as a follow-up of our previous work. We used a maximum-likelihood-estimation method to re-derive the field CMRD power law avoiding dependence on the arbitrary binning. We also considered two new surveys of multiples in the field for solar-type stars and M dwarfs to test the universality of the CMRD. We found no significant differences in the CMRD for M dwarfs and solar-type stars compared with previous results over the common mass ratio and separation range. The new best-fit power law of the CMRD in the field, combining two previous sets of data, is $dN/dq propto q^{beta}$, with $beta=0.25pm0.29$.
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We present a statistical comparison of the mass ratio distribution of companions, as observed in different multiplicity surveys, to the most recent estimate of the single object mass function (Bochanski et al. 2010). The main goal of our analysis is to test whether or not the observed companion mass ratio distribution (CMRD) as a function of primary star mass and star formation environment is consistent with having been drawn from the field star IMF. We consider samples of companions for M dwarfs, solar type and intermediate mass stars, both in the field as well as clusters or associations, and compare them with populations of binaries generated by random pairing from the assumed IMF for a fixed primary mass. With regard to the field we can reject the hypothesis that the CMRD was drawn from the IMF for different primary mass ranges: the observed CMRDs show a larger number of equal-mass systems than predicted by the IMF. This is in agreement with fragmentation theories of binary formation. For the open clusters {alpha} Persei and the Pleiades we also reject the IMF random- pairing hypothesis. Concerning young star-forming regions, currently we can rule out a connection between the CMRD and the field IMF in Taurus but not in Chamaeleon I. Larger and different samples are needed to better constrain the result as a function of the environment. We also consider other companion mass functions (CMF) and we compare them with observations. Moreover the CMRD both in the field and clusters or associations appears to be independent of separation in the range covered by the observations. Combining therefore the CMRDs of M and G primaries in the field and intermediate mass primary binaries in Sco OB2 for mass ratios, q = M2/M1, from 0.2 to 1, we find that the best chi-square fit follows a power law dN/dq propto q^{beta}, with {beta} = -0.50 pm 0.29, consistent with previous results.
There is a population of stars with velocities in excess of 500 km s$^{-1}$ relative to the Galactic center. Many, perhaps most, of these hyper-velocity stars (HVSs) are B stars, similar to the disk and S stars in a nuclear cluster around a super-massive black hole (SMBH) near $rm Sgr~A^{star}$. In the paper I of this series, we showed that the eccentricity of the stars emerged from a hypothetical disk around the SMBH can be rapidly excited by the secular perturbation of its intermediate-mass companion (IMC), and we suggested IRS 13E as a potential candidate for the IMC. Here, we show that this process leads to an influx of stars on parabolic orbits to the proximity of $rm Sgr~A^{star}$ on a secular timescale of a few Myr. This timescale is much shorter than the diffusion timescale into the lost cone through either the classical or the resonant relaxation. Precession of the highly-eccentric stars longitude of periastron, relative to that of the IMC, brings them to its proximity within a few Myr. The IMCs gravitational perturbation scatters a fraction of the stars from nearly parabolic to hyperbolic orbits, with respect to the SMBH. Their follow-up close encounters with the SMBH induce them to escape with hyper-velocity. This scenario is a variant of the hypothesis proposed by Hills based on the anticipated breakup of some progenitor binary stars in the proximity of the SMBH, and its main objective is to account for the limited lifespan of the known HVSs. We generalize our previous numerical simulations of this process with a much wider range of orbital configurations. We demonstrate the robustness and evaluate the efficiency of this channel of HVS formation. From these numerical simulations, we infer observable kinematic properties for the HVSs.
We present optical spectroscopy for 18 halo white dwarfs identified using photometry from the Canada-France Imaging Survey and Pan-STARRS1 DR1 3$pi$ survey combined with astrometry from Gaia DR2. The sample contains 13 DA, 1 DZ, 2 DC, and two potentially exotic types of white dwarf. We fit both the spectrum and the spectral energy distribution in order to obtain the temperature and surface gravity, which we then convert into a mass, and then an age, using stellar isochrones and the initial-to-final mass relation. We find a large spread in ages that is not consistent with expected formation scenarios for the Galactic halo. We find a mean age of 9.03$^{+2.13}_{-2.03}$ Gyr and a dispersion of 4.21$^{+2.33}_{-1.58}$ Gyr for the inner halo using a maximum likelihood method. This result suggests an extended star formation history within the local halo population.
The circumstellar disk of the Herbig Fe star HD 142527 is host to several remarkable features including a warped inner disk, a 120 au-wide annular gap, a prominent dust trap and several spiral arms. A low-mass companion, HD 142527 B, was also found orbiting the primary star at $sim$14 au. This study aims to better characterize this companion, which could help explain its impact on the peculiar geometry of the disk. We observed the source with VLT/SINFONI in $H$+$K$ band in pupil-tracking mode. Data were post-processed with several algorithms based on angular differential imaging (ADI). HD 142527 B is conspicuously re-detected in most spectral channels, which enables us to extract the first medium-resolution spectrum of a low-mass companion within 0.1 from its central star. Fitting our spectrum with both template and synthetic spectra suggests that the companion is a young M2.5$pm$1.0 star with an effective temperature of $3500pm100$ K, possibly surrounded with a hot (1700 K) circum-secondary environment. Pre-main sequence evolutionary tracks provide a mass estimate of $0.34pm0.06 M_{odot}$, independent of the presence of a hot environment. However, the estimated stellar radius and age do depend on that assumption; we find a radius of $1.37 pm 0.05 R_{odot}$ (resp. $1.96 pm 0.10 R_{odot}$) and an age of $1.8^{+1.2}_{-0.5}$ Myr (resp. $0.75 pm 0.25$ Myr) in the case of the presence (resp. absence) of a hot environment contributing in $H$+$K$. Our new values for the mass and radius yield a mass accretion rate of $sim$5 $times 10^{-9} M_{odot}$ yr$^{-1}$ (2-3% that of the primary). Our results illustrate thus the potential for SINFONI+ADI to characterize faint close-in companions. The new spectral type makes HD 142527 B a twin of the well known TW Hya T-Tauri star, and the revision of its mass to higher values further supports its role in shaping the disk.
We report the detection of a radial velocity companion to the extremely low mass white dwarf LP400-22. The radial velocity of the white dwarf shows variations with a semi-amplitude of 119 km/s and a 0.98776 day period, which implies a companion mass of M > 0.37 Msun. The optical photometry rules out a main sequence companion. Thus the invisible companion is another white dwarf or a neutron star. Using proper motion measurements and the radial velocity of the binary system, we find that it has an unusual Galactic orbit. LP400-22 is moving away from the Galactic center with a velocity of 396 km/s, which is very difficult to explain by supernova runaway ejection mechanisms. Dynamical interactions with a massive black hole like that in the Galactic center can in principle explain its peculiar velocity, if the progenitor was a triple star system comprised of a close binary and a distant tertiary companion. Until better proper motions become available, we consider LP400-22 to be most likely a halo star with a very unusual orbit.
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