By comparing three constituents of Orion A (gas, protostars, and pre-main-sequence stars), both morphologically and kinematically, we derive the following conclusions. The gas surface density near ...the integral-shaped filament (ISF) is very well represented by a power law, Σ(b) = 37 M⊙ pc-2(b/pc)−5/8, for the entire range to which we are sensitive, 0.05 pc < b < 8.5 pc, of projected separation from the filament ridge. Essentially all Class 0 and Class I protostars lie superposed on the ISF or on identifiable filament ridges farther south, while almost all pre-main-sequence (Class II) stars do not. Combined with the fact that protostars are moving ≲ 1 km s-1 relative to the filaments, while stars are moving several times faster, this implies that protostellar accretion is terminated by a slingshot-like “ejection” from the filaments. The ISF is the third in a series of identifiable star bursts that are progressively moving south, with separations of several Myr in time and 2–3 pc in space. This, combined with the observed undulations in the filament (both spatial and velocity), suggest that repeated propagation of transverse waves through the filament is progressively digesting the material that formerly connected Orion A and B into stars in discrete episodes. We construct a simple, circularly symmetric gas density profile ρ(r) = 17 M⊙ pc-3(r/pc)−13/8 consistent with the two-dimensional data. The model implies that the observed magnetic fields in this region are subcritical on spatial scales of the observed undulations, suggesting that the transverse waves propagating through the filament are magnetically induced. Because the magnetic fields are supercritical on scales of the filament as a whole (as traced by the power law), the system as a whole is relatively stable and long lived. Protostellar “ejection” (i.e., the slingshot) occurs because the gas accelerates away from the protostars, not the other way around. The model also implies that the ISF is kinematically young, which is consistent with several other lines of evidence. In contrast to the ISF, the southern filament (L1641) has a broken power law, which matches the ISF profile for 2.5 pc < b < 8.5 pc, but is shallower closer in. L1641 is kinematically older than the ISF.
I investigate the origin of arc degeneracies in satellite microlens parallax π E measurements with only late time data, e.g., t > t 0 + t E as seen from the satellite. I show that these are due to ...partial overlap of a series of osculating, exactly circular, degeneracies in the π E plane, each from a single measurement.
In events with somewhat earlier data, these long arcs break up into two arclets, or (with even earlier data) two points, because these earlier measurements give rise to intersecting rather than osculating circles. The two arclets (or points) then constitute one pair of degeneracies in the well-known four-fold degeneracy of space-based microlens parallax. Using this framework of intersecting circles, I show that next-generation microlens satellite experiments could yield good π E determinations with only about five measurements per event, i.e., about 30 observations per day to monitor 1500 events per year. This could plausibly be done with a small (hence cheap, in the spirit of Gould & Yee 2012) satellite telescope, e.g., 20 cm. KCI Citation Count: 8
I show that when the observables (πE; tE; θE; πs;μs) are well measured up to a discrete degeneracy in the microlensing parallax vector πE, the relative likelihood of the different solutions can be ...written in closed form P_i = KH_iB_i, where Hi is the number of stars (potential lenses) having the mass and kinematics of the inferred parameters of solution i and B_i is an additional factor that is formally derived from the Jacobian of the transformation from Galactic to microlensing parameters. Here tE is the Einstein timescale, θE is the angular Einstein radius, and (πs,μs) are the (parallax, proper motion) of the microlensed source. The Jacobian term B_i constitutes an explicit evaluation of the "Rich Argument", i.e., that there is an extra geometric factor disfavoring large-parallax solutions in addition to the reduced frequency of lenses given by H_i. I also discuss how this analytic expression degrades in the presence of finite errors in the measured observables. KCI Citation Count: 2
Microlensing is generally thought to probe planetary systems only out to a few Einstein radii. Microlensing events generated by bound planets beyond about 10 Einstein radii generally do not yield any ...trace of their hosts, and so would be classified as free floating planets (FFPs). I show that it is already possible, using adaptive optics (AO), to constrain the presence of potential hosts to FFP candidates at separations comparable to the Oort Cloud. With next-generation telescopes, planets at Kuiper-Belt separations can be probed. Next generation telescopes will also permit routine vetting for all FFP candidates, simply by obtaining second epochs 4--8 years after the event.At present, the search for such hosts is restricted to within the ``confusion limit'' of $\theta_\confus\sim 0.25^{\prime\prime}$, but future {\it WFIRST} (Wide Field Infrared Survey Telescope) observations will allow one to probe beyond this confusion limit as well. KCI Citation Count: 13
By extending the constant-acceleration analysis of Smith, Mao, & Paczynski to include jerk, I show that microlens parallax measurements are subject to a four-fold discrete degeneracy. The new ...degeneracy is characterized by a projected velocity image sub(j) =-(3/4) csc beta sub(ec)(image psi image beta sub(ec) +image psi ) super(3/2)v sub(o+), where beta sub(ec) is the ecliptic latitude, psi is the phase of the Earth's orbit relative to opposition at the time of the event maximum, and v sub(o+) = 30 km s super(-1) is the speed of the Earth. The degeneracy becomes important when the lens projected velocity image is of order image sub(j). For events toward the Large Magellanic Cloud, image sub(j) similar to (3/4)v sub(o+), so this degeneracy is important primarily for lenses in the Milky Way disk. In particular, it solves the puzzle of MACHO-LMC-5, whose microlens parallax measurement had yielded mass and distance determinations for the lens that were inconsistent with photometric estimates. Toward the Galactic bulge, image sub(j) ranges from approx0.2 km s super(- 1) at the summer solstice to approx200 km s super(- 1) at the equinoxes so the effect of the degeneracy depends strongly on the peak time of the event. The degeneracy applies mainly to events with Einstein timescales, t sub(E) similar to yr/2pi.
One-dimensional (1-D) microlens parallaxes can be combined with heliocentric lens-sourcerelative proper motion measurements to derive the lens mass and distance, as suggested by Ghosh et al.
(2004). ...Here I present the first mathematical anlysis of this procedure, which I show can be represented asa quadratic equation. Hence, it is formally subject to a two-fold degeneracy. I show that this degeneracycan be broken in many cases using the relatively crude 2-D parallax information that is often availablefor microlensing events. I also develop an explicit formula for the region of parameter space where it ismore difficult to break this degeneracy. Although no mass/distance measurements have yet been madeusing this technique, it is likely to become quite common over the next decade. KCI Citation Count: 20
We present an extensive and pure sample of ultrawide binary stars with separations of 0.01 s/pc 1 in the solar neighborhood. Using data from Gaia DR2, we define kinematic subpopulations via the ...systems' tangential velocities, i.e., disk-like (v ,tot ≤ 40 km s−1), intermediate (v ,tot = 40-85 km s−1), and halo-like (v ,tot ≥ 85 km s−1) binaries, presuming that these velocity cuts represent a rough ordering in the binaries' age and metallicity. Through stringent cuts on astrometric precision, we can obtain pure binary samples at such wide separations with thousands of binaries in each sample. Fitting a smoothly broken power law for the separation distribution, we find that its slope at s = 102.5-4 au is the same for all subpopulations, p(s) ∝ sγ with γ −1.54. However, the logarithmic slope of p(s) steepens at s 104 au. We find some evidences that the degree of steepening increases with the binaries' age, with a slope change of only Δγ 0.5 for disk-like stars, but Δγ 1 for halo-like stars. This trend is contrary to what might be expected if steepening at wide separations were due to gravitational perturbations by molecular clouds or stars, which would preferentially disrupt disk binaries. If we were to interpret steepening at s 104 au as a consequence of disruption by MAssive Compact Halo Objects (MACHOs), we would have to invoke a MACHO population inconsistent with other constraints. As a more plausible alternative, we propose a simple model to predict the separation distribution of wide binaries formed in dissolving star clusters. This model generically predicts γ −1.5 as observed, with steepening at larger separations due to the finite size of binaries' birth clusters.
ABSTRACT
The Gaia early Data Release 3 has delivered exquisite astrometric data for 1.47 billion sources, which is revolutionizing many fields in astronomy. For a small fraction of these sources, the ...astrometric solutions are poor, and the reported values and uncertainties may not apply. Before any analysis, it is important to recognize and excise these spurious results – this is commonly done by means of quality flags in the Gaia catalogue. Here, we devise a means of separating ‘good’ from ‘bad’ astrometric solutions that is an order of magnitude cleaner than any single flag: 99.3 per cent pure and 97.3 per cent complete, as validated on our test data. We devise an extensive sample of manifestly bad astrometric solutions, with parallax that is negative at ≥4.5σ; and a corresponding sample of presumably good solutions, including sources in healpix pixels on the sky that do not contain such negative parallaxes, and sources that fall on the main sequence in a colour–absolute magnitude diagram. We then train a neural network that uses 17 pertinent Gaia catalogue entries and information about nearby sources to discriminate between these two samples, captured in a single ‘astrometric fidelity’ parameter. A diverse set of verification tests shows that our approach works very cleanly, including for sources with positive parallaxes. The main limitations of our approach are in the very low signal-to-noise ratio and the crowded regime. Our astrometric fidelities for all of eDR3 can be queried via the Virtual Observatory, our code and data are public.
We present a study of the luminosity density distribution of the Galactic bar using number counts of red clump giants from the Optical Gravitational Lensing Experiment (OGLE) III survey. The data ...were recently published by Nataf et al.
for 9019 fields towards the bulge and have 2.94 × 106 RC stars over a viewing area of
. The data include the number counts, mean distance modulus (μ), dispersion in μ and full error matrix, from which we fit the data with several triaxial parametric models. We use the Markov Chain Monte Carlo method to explore the parameter space and find that the best-fitting model is the E
3 model, with the distance to the GC 8.13 kpc, the ratio of semimajor and semiminor bar axis scalelengths in the Galactic plane x
0, y
0 and vertical bar scalelength z
0
x
0: y
0: z
0 1.00: 0.43: 0.40 (close to being prolate). The scalelength of the stellar density profile along the bar's major axis is ∼0.67 kpc and has an angle of 29
4, slightly larger than the value obtained from a similar study based on OGLE-II data. The number of estimated RC stars within the field of view is 2.78 × 106, which is systematically lower than the observed value. We subtract the smooth parametric model from the observed counts and find that the residuals are consistent with the presence of an X-shaped structure in the Galactic Centre, the excess to the estimated mass content is ∼5.8 per cent. We estimate that the total mass of the bar is ∼1.8 × 1010 M. Our results can be used as a key ingredient to construct new density models of the Milky Way and will have implications on the predictions of the optical depth to gravitational microlensing and the patterns of hydrodynamical gas flow in the Milky Way.
We follow the microlensing approach and quantify the occurrence of Kepler exoplanets as a function of planet-to-star mass ratio, q, rather than planet radius or mass. For planets with radii ∼1-6 R⊕ ...and periods <100 days, we find that, except for a normalization factor, the occurrence rate versus q can be described by the same broken power law with a break at ∼3 × 10−5 independent of host type for hosts below 1 M . These findings indicate that the planet-to-star mass ratio is a more fundamental quantity in planet formation than planet mass. We then compare our results to those from microlensing for which the overwhelming majority satisfies the Mhost < 1 M criterion. The break in q for the microlensing planet population, which mostly probes the region outside the snowline, is ∼3-10 times higher than that inferred from Kepler. Thus, the most common planet inside the snowline is ∼3-10 times less massive than the one outside. With rocky planets interior to gaseous planets, the solar system broadly follows the combined mass-ratio function inferred from Kepler and microlensing. However, the exoplanet population has a less extreme radial distribution of planetary masses than the solar system. Establishing whether the mass-ratio function beyond the snowline is also host type independent will be crucial to build a comprehensive theory of planet formation.
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