We consider the long-term collisional and dynamical evolution of solid material orbiting in a narrow annulus near the Roche limit of a white dwarf. With orbital velocities of 300 , systems of solids ...with initial eccentricity generate a collisional cascade where objects with radii are ground to dust. This process converts 1-100 km asteroids into 1 m particles in 102−106 yr. Throughout this evolution, the swarm maintains an initially large vertical scale height H. Adding solids at a rate enables the system to find an equilibrium where the mass in solids is roughly constant. This equilibrium depends on and , the radius of the largest solid added to the swarm. When 10 km, this equilibrium is stable. For larger , the mass oscillates between high and low states; the fraction of time spent in high states ranges from 100% for large to much less than 1% for small . During high states, the stellar luminosity reprocessed by the solids is comparable to the excess infrared emission observed in many metallic line white dwarfs.
We consider the long-term evolution of gaseous disks fed by the vaporization of small particles produced in a collisional cascade inside the Roche limit of a 0.6 white dwarf. Adding solids with ...radius at a constant rate into a narrow annulus leads to two distinct types of evolution. When , the cascade generates a fairly steady accretion disk where the mass transfer rate of gas onto the white dwarf is roughly and the mass in gas is g, where T0 is the temperature of the gas near the Roche limit and is the dimensionless viscosity parameter. If , the system alternates between high states with large mass transfer rates and low states with negligible accretion. Although either mode of evolution adds significant amounts of metals to the white dwarf photosphere, none of our calculations yield a vertically thin ensemble of solids inside the Roche limit. X-ray observations can place limits on the mass transfer rate and test this model for metallic line white dwarfs.
We present the final sample of 98 detached double white dwarf (WD) binaries found in the Extremely Low Mass (ELM) Survey, a spectroscopic survey targeting <0.3 M He-core WDs completed in the Sloan ...Digital Sky Survey footprint. Over the course of the survey we observed ancillary low-mass WD candidates like GD 278, which we show is a P = 0.19 day double WD binary, as well as candidates that turn out to be field blue straggler/subdwarf A-type stars with luminosities too high to be WDs given their Gaia parallaxes. Here, we define a clean sample of ELM WDs that is complete within our target selection and magnitude range 15 < g0 < 20 mag. The measurements are consistent with 100% of ELM WDs being 0.0089 < P < 1.5 day double WD binaries, 35% of which belong to the Galactic halo. We infer that these are mostly He+CO WD binaries given the measurement constraints. The merger rate of the observed He+CO WD binaries exceeds the formation rate of stable mass-transfer AM CVn binaries by a factor of 25, and so the majority of He+CO WD binaries must experience unstable mass transfer and merge. The systems with the shortest periods, such as J0651+2844, are signature LISA verification binaries that can be studied with gravitational waves and light.
We use a semianalytic circumstellar disk model that considers movement of the snow line through evolution of accretion and the central star to investigate how gas giant frequency changes with stellar ...mass. The snow line distance changes weakly with stellar mass; thus, giant planets form over a wide range of spectral types. The probability that a given star has at least one gas giant increases linearly with stellar mass from 0.4 to 3 M unk. Stars more massive than 3 M unk evolve quickly to the main sequence, which pushes the snow line to 10-15 AU before protoplanets form and limits the range of disk masses that form giant planet cores. If the frequency of gas giants around solar mass stars is 6%, we predict occurrence rates of 1% for 0.4 M unk stars and 10% for 1.5 M unk stars. This result is largely insensitive to our assumed model parameters. Finally, the movement of the snow line as stars unk 2.5 M unk move to the main sequence may allow the ocean planets suggested by Leger et al. to form without migration.
We use new Gaia measurements to explore the origin of the highest velocity stars in the hypervelocity star (HVS) survey. The measurements reveal a clear pattern in B-type stars. Halo stars dominate ...the sample at speeds of 100 km s−1 below Galactic escape velocity. Disk runaway stars have speeds up to 100 km s−1 above Galactic escape velocity, but most disk runaways are bound. Stars with speeds 100 km s−1 above Galactic escape velocity originate from the Galactic center. Two bound stars may also originate from the Galactic center. Future Gaia measurements will enable a large, clean sample of Galactic center ejections for measuring the massive black hole ejection rate of HVSs, and for constraining the mass distribution of the Milky Way dark matter halo.
ABSTRACT We estimate the merger rate of double degenerate binaries containing extremely low mass (ELM; M ) white dwarfs (WDs) in the Galaxy. Such WDs are detectable for timescales of 0.1-1 Gyr in the ...ELM Survey; the binaries they reside in have gravitational wave merger times of 0.001-100 Gyr. To explain the observed distribution requires that most ELM WD binary progenitors detach from the common envelope phase with <1 hr orbital periods. We calculate the local space density of ELM WD binaries and estimate a merger rate of 3 × 10−3 yr−1 over the entire disk of the Milky Way; the merger rate in the halo is 10 times smaller. The ELM WD binary merger rate exceeds by a factor of 40 the formation rate of stable mass transfer AM CVn binaries, marginally exceeds the rate of underluminous supernovae, and is identical to the formation rate of R CrB stars. On this basis, we conclude that ELM WD binaries can be the progenitors of all observed AM CVn and possibly underluminous supernovae; however, the majority of He+CO WD binaries go through unstable mass transfer and merge, e.g., into single massive ∼1 M WDs.
We present the discovery of 15 extremely low-mass (5 < log 7 >) white dwarf (WD) candidates, 9 of which are in ultra-compact double-degenerate binaries. Our targeted extremely low-mass Survey sample ...now includes 76 binaries. The sample has a lognormal distribution of orbital periods with a median period of 5.4 hr. The velocity amplitudes imply that the binary companions have a normal distribution of mass with 0.76 M sub(middot in circle) mean and 0.25 M sub(middot in circle) dispersion. Thus extremely low-mass WDs are found in binaries with a typical mass ratio of 1:4. Statistically speaking, 95% of the WD binaries have a total mass below the Chandrasekhar mass, and thus are not type Ia supernova progenitors. Yet half of the observed binaries will merge in less than 6 Gyr due to gravitational wave radiation; probable outcomes include single massive WDs and stable mass transfer AM CVn binaries.
The TNG300-1 run of the IllustrisTNG simulations includes 1697 clusters of galaxies with
M
200c
> 10
14
M
⊙
covering the redshift range 0.01 − 1.04. We built mock spectroscopic redshift catalogs ...of simulated galaxies within these clusters and applied the caustic technique to estimate the cumulative cluster mass profiles. We computed the total true cumulative mass profile from the 3D simulation data, calculated the ratio of caustic mass to total 3D mass as a function of cluster-centric distance, and identified the radial range where this mass ratio is roughly constant. The ratio of 3D to caustic mass on this plateau defines ℱ
β
. The filling factor, ℱ
β
= 0.41 ± 0.08, is constant on a plateau that covers a wide cluster-centric distance range, (0.6 − 4.2)
R
200c
. This calibration is insensitive to redshift. The calibrated caustic mass profiles are unbiased, with an average uncertainty of 23%. At
R
200c
, the average
M
C
/
M
3D
= 1.03 ± 0.22; at 2
R
200c
, the average
M
C
/
M
3D
= 1.02 ± 0.23. Simulated galaxies are unbiased tracers of the mass distribution. IllustrisTNG is a broad statistical platform for application of the caustic technique to large samples of clusters with spectroscopic redshifts for ≳200 members in each system. These observations will allow extensive comparisons with weak-lensing masses and will complement other techniques for measuring the growth rate of structure in the Universe.
Abstract
The formation of planets like Earth is expected to conclude with a series of late-stage giant impacts that generate warm dusty debris, the most anticipated visible signpost of terrestrial ...planet formation in progress. While there is now evidence that Earth-sized terrestrial planets orbit a significant fraction of solar-type stars, the anticipated dusty debris signature of their formation is rarely detected. Here we discuss several ways in which our current ideas about terrestrial planet formation imply transport mechanisms capable of erasing the anticipated debris signature. A tenuous gas disk may be regenerated via
takeout
(i.e., the liberation of planetary atmospheres in giant impacts) or
delivery
(i.e., by asteroids and comets flung into the terrestrial planet region) at a level sufficient to remove the warm debris. The powerful stellar wind from a young star can also act, its delivered wind momentum producing a drag that removes warm debris. If such processes are efficient, terrestrial planets may assemble inconspicuously, with little publicity and hoopla accompanying their birth. Alternatively, the rarity of warm excesses may imply that terrestrial planets typically form very early, emerging fully formed from the nebular phase without undergoing late-stage giant impacts. In either case, the observable signposts of terrestrial planet formation appear more challenging to detect than previously assumed. We discuss observational tests of these ideas.
We analyze radial velocity observations of the 12 extremely low-mass (ELM), with <=0.25 M , white dwarfs (WDs) in the MMT Hypervelocity Star Survey. Eleven of the twelve WDs are binaries with orbital ...periods shorter than 14 hr; the one non-variable WD is possibly a pole-on system among our non-kinematically selected targets. Our sample is unique: it is complete in a well-defined range of apparent magnitude and color. The orbital mass functions imply that the unseen companions are most likely other WDs, although neutron star companions cannot be excluded. Six of the eleven systems with orbital solutions will merge within a Hubble time due to the loss of angular momentum through gravitational wave radiation. The quickest merger is J0923+3028, a g = 15.7 ELM WD binary with a 1.08 hr orbital period and a <=130 Myr merger time. The chance of a supernova Ia event among our ELM WDs is only 1%-7%, however. Three binary systems (J0755+4906, J1233+1602, and J2119-0018) have extreme mass ratios and will most likely form stable mass-transfer AM CVn systems. Two of these objects, SDSS J1233+1602 and J2119-0018, are the lowest surface gravity WDs ever found; both show Ca II absorption likely from accretion of circumbinary material. We predict that at least one of our WDs is an eclipsing detached double WD system, important for constraining helium core WD models.
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