The Astrophysical Journal Letters

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Measuring the Star Formation Rate with Gravitational Waves from Binary Black Holes
A measurement of the history of cosmic star formation is central to understanding the origin and evolution of galaxies. The measurement is extremely challenging using electromagnetic radiation: significant modeling is required to convert luminosity to mass, and to properly account for dust attenuation, for example. Here we show how detections of gravitational waves from inspiraling binary black holes made by proposed third-generation detectors can be used to measure the star formation rate (SFR) of massive stars with high precision up to redshifts of ∼10. Depending on the time-delay model, the predicted detection rates ranges from ∼2310 to ∼56,740 per month with the current measurement of local merger rate density. With 30,000 detections, parameters describing the volumetric SFR can be constrained at the few percent level, and the volumetric merger rate can be directly measured to 3% at z ∼ 2. Given a parameterized SFR, the characteristic delay time between binary formati...
Secondary Infall in the Seyfert’s Sextet: A Plausible Way Out of the Short Crossing Time Paradox
We used integral field spectroscopy from CALIFA DR3 and multiwavelength publicly available data to investigate the star formation histories of galaxies in the Seyfert’s Sextet (SS; HCG 79). The galaxies H79a, H79b, H79c, and H79f have low star formation rates despite showing strong signs of interaction. By exploring their individual specific star formation histories, we identified three earlier episodes of strong star formation common to these four galaxies. We use the last two episodes as markers of the epochs when the galaxies were crossing. We suggest that after the first turnaround, initially gas-rich galaxies crossed for the first time, consuming most of their gas. Hence, after the second turnaround most mergers from second crossings would be mixed or dry. The exception would be gas-rich galaxies intruding for the first time. Therefore, we suggest that SS galaxies have survived one crossing during a Hubble time. Strong Balmer absorption lines and the presence of counter-rot...
EDGE: The Origin of Scatter in Ultra-faint Dwarf Stellar Masses and Surface Brightnesses
We demonstrate how the least luminous galaxies in the universe, ultra-faint dwarf galaxies, are sensitive to their dynamical mass at the time of cosmic reionization. We select a low-mass ( {$\sim 1.5\times {10}^{9}\,{M}_{\odot }$} ) dark matter halo from a cosmological volume, and perform zoom hydrodynamical simulations with multiple alternative histories using “genetically modified” initial conditions. Earlier-forming ultra-faints have higher stellar mass today, due to a longer period of star formation before their quenching by reionization. Our histories all converge to the same final dynamical mass, demonstrating the existence of extended scatter (≥1 dex) in stellar masses at fixed halo mass due to the diversity of possible histories. One of our variants builds less than 2% of its final dynamical mass before reionization, rapidly quenching in situ star formation. The bulk of its fina...
Discovery of a Photoionized Bipolar Outflow toward the Massive Protostar G45.47+0.05
Massive protostars generate strong radiation feedback, which may help set the mass that they achieve by the end of the accretion process. Studying such feedback is therefore crucial for understanding the formation of massive stars. We report the discovery of a photoionized bipolar outflow toward the massive protostar G45.47+0.05 using high-resolution observations at 1.3 mm with the Atacama Large Millimeter/Submillimeter Array (ALMA) and at 7 mm with the Karl G. Jansky Very Large Array (VLA). By modeling the free–free continuum, the ionized outflow is found to be a photoevaporation flow with an electron temperature of 10,000 K and an electron number density of ∼1.5 × 10 7 cm −3 at the center, launched from a disk of radius of 110 au. H30 α hydrogen recombination line emission shows strong maser amplification, with G45 being one of very few sources to show such millimeter recombination line masers. The mass of the driving source is estimated to be 30–50 <...
An Ongoing Mid-infrared Outburst in the White Dwarf 0145+234: Catching in Action the Tidal Disruption of an Exoasteroid?
We report the detection of a large-amplitude MIR outburst in the white dwarf (WD) 0145+234 in the NEOWISE Survey data. The source had a stable MIR flux before 2018, and was brightened by about 1.0 magnitude in the W1 and W2 bands within half a year and has been continuously brightening since then. No significant variations are found in the optical photometry data during the same period. This suggests that this MIR outburst is caused by recent replenishing or redistribution of dust, rather than intrinsic variations of the WD. Spectral energy distribution modeling of 0145+234 suggests that there was already a dust disk around the WD in the quiescent state, and both of the temperature and surface area of the disk evolved rapidly since the outburst. The dust temperature was ≃1770 K in the initial rising phase, close to the sublimation temperature of silicate grains, and gradually cooled down to around 1150 K, while the surface area increased by a factor of about six du...
Brans–Dicke Gravity with a Cosmological Constant Smoothes Out ΛCDM Tensions
We analyze Brans–Dicke gravity with a cosmological constant, Λ, and cold dark matter (BD-ΛCDM) in the light of the latest cosmological observations on distant supernovae, Hubble rate measurements at different redshifts, baryonic acoustic oscillations, large-scale structure formation data, gravitational weak-lensing, and the cosmic microwave background under full Planck 2015 cosmic microwave background likelihood. Our analysis includes both the background and perturbations equations. We find that BD-ΛCDM is observationally favored as compared to the concordance ΛCDM model, which is traditionally defined within General Relativity (GR). In particular, some well-known persisting tensions of the ΛCDM with the data, such as the excess in the mass fluctuation amplitude {${\sigma }_{8}$} and especially the acute H 0 -tension with the local measurements, essentially disappear ...
Phase-space Correlation in Stellar Streams of the Milky Way Halo: The Clash of Kshir and GD-1
We report the discovery of a {$70^\circ $} long stellar stream in the Milky Way halo, which criss-crosses the well known “GD-1” stream. We show that this new stellar structure (“Kshir”) and GD-1 lie at similar distance, and are remarkably correlated in kinematics. We propose several explanations for the nature of this new structure and its possible association with GD-1. However, a scenario in which these two streams were accreted onto the Milky Way within the same dark matter sub-halo seems to provide a natural explanation for their phase-space entanglement, and other complexities of this coupled system.
Discovery of a New Stellar Subpopulation Residing in the (Inner) Stellar Halo of the Milky Way
We report the discovery of a unique collection of metal-poor giant stars that exhibit anomalously high levels of 28 Si, clearly above typical Galactic levels. Our sample spans a narrow range of metallicities, peaking at −1.07 ± 0.06, and exhibits abundance ratios of [Si, Al/Fe] that are as extreme as those observed in Galactic globular clusters (GCs), and Mg is slightly less overabundant. In almost all the sources we used, the elemental abundances were redetermined from high-resolution spectra, which were reanalyzed assuming LTE. Thus, we compiled the main element families, namely, the light elements (C, N), α -elements (O, Mg, Si), iron-peak element (Fe), s -process elements (Ce, Nd), and the light odd-Z element (Al). We also provide dynamical evidence that most of these stars lie on tight (inner) halo-like and retrograde orbits passing through the bulge. Such kinds of objects have been found in present-day halo GCs, providing the clearest chemical signatu...
Evidence of a Substellar Companion to AB Dor C
Studies of fundamental parameters of very low-mass objects are indispensable to provide tests of stellar evolution models that are used to derive theoretical masses of brown dwarfs and planets. However, only objects with dynamically determined masses and precise photometry can effectively evaluate the predictions of stellar models. AB Dor C (0.090 M ⊙ ) has become a prime benchmark for calibration of theoretical evolutionary models of low-mass young stars. One of the ambiguities remaining in AB Dor C is the possible binary nature of this star. We observed AB Dor C with the VLTI/AMBER instrument in low-resolution mode at the J, H , and K bands. The interferometric observables at the K band are compatible with a binary brown dwarf system with tentative components AB Dor Ca/Cb with a K -band flux ratio of 5% ± 1% and a separation of 38 ± 1 mas. This implies theoretical masses of 0.072 ± 0.013 M ⊙ and 0.013 ± 0.001 M
Rovibrational Spectral Analysis of CO 3 and C 2 O 3 : Potential Sources for O 2 Observed in Comet 67P/Churyumov–Gerasimenko
The recent ROSETTA mission to comet 67P/Churyumov–Gerasimenko detected surprisingly high levels of molecular oxygen (O 2 ; hypervolatile species) in the coma. Current models predict that considerable levels of other hypervolatiles (such as molecular nitrogen, N 2 , methane, CH 4 , and Argon) should be found at similar levels, whereas they are more depleted. One explanation explored here is that larger (less volatile) parent molecules may have been formed during radiolysis of cometary ices and, upon sublimation, are subsequently broken down within the coma into smaller, more volatile fragments. In support of this hypothesis, this work employs reliable quantum chemical techniques to provide the spectral data necessary for the detection of two candidate precursor “parent” molecules, cyclic carbon trioxide ( c -CO 3 ), and cyclic dicarbon trioxide ( c -C 2 O 3 ). Benchmark computations performed for gas-phase CO<...
The Halo Mass Function of Late-type Galaxies from H i Kinematics
We present an empirical method to measure the halo mass function (HMF) of galaxies. We determine the relation between the H i line width from single-dish observations and the dark matter halo mass ( M 200 ) inferred from rotation-curve fits in the Spitzer Photometry and Accurate Rotation Curves (SPARC) database, then we apply this relation to galaxies from the H i Parkes All Sky Survey (HIPASS) to derive the HMF. This empirical HMF is well fit by a Schechter function, and matches that expected in the Λ Cold Dark Matter (ΛCDM) model over the range 10 10.5 < M 200 < 10 12 M ⊙ . More massive halos must be poor in neutral gas to maintain consistency with the power law predicted by ΛCDM. We detect no discrepancy at low masses. The lowest halo mass probed by HIPASS, however, is just greater than the mass scale where the Local Group missing satellite problem se...
Orbital Decay in a 20 Minute Orbital Period Detached Binary with a Hydrogen-poor Low-mass White Dwarf
We report the discovery of a detached double white dwarf binary with an orbital period of ≈20.6 minutes, PTF J053332.05+020911.6. The visible object in this binary, PTF J0533+0209B, is a ≈0.17 M ⊙ mass white dwarf with a helium-dominated atmosphere containing traces of hydrogen. This object exhibits ellipsoidal variations due to tidal deformation, and is the visible component in a single-lined spectroscopic binary with a velocity semi-amplitude of K B = 618.7 ± 6.9 km s −1 . We have detected significant orbital decay due to the emission of gravitational radiation, and we expect that the Laser Interferometer Space Antenna ( LISA ) will detect this system with a signal to noise of {${8.4}_{-3.0}^{+4.2}$} after four years of operation. Because this system already has a well-determined orbital period, radial velocity semi-...
Small-scale H i Channel Map Structure Is Cold: Evidence from Na i Absorption at High Galactic Latitudes
The spatial distribution of neutral hydrogen (H i ) emission is a powerful probe of interstellar medium physics. The small-scale structure in H i channel maps is often assumed to probe the velocity field rather than real density structures. In this work we directly test this assumption, using high-resolution GALFA-H i observations and 50,985 quasar spectra from the Sloan Digital Sky Survey. We measure the equivalent widths of interstellar Na i D 1 and Na i D 2 absorption, and robustly conclude that together they depend nearly four times as strongly on the column density of small-scale structure in H i than on either the large-scale H i structure or the total H i column. This is inconsistent with the hypothesis that small-scale channel map structure is driven by ve...
Infrared Spectroscopy of the Recent Outburst in V1047 Cen (Nova Centauri 2005)
Fourteen years after its eruption as a classical nova (CN), V1047 Cen (Nova Cen 2005) began an unusual re-brightening in 2019 April. The amplitude of the brightening suggests that this is a dwarf nova (DN) eruption in a CN system. Very few CNe have had DN eruptions within decades of the main CN outburst. The 14 yr separating the CN and DN eruptions of V1047 Cen is the shortest of all instances recorded thus far. Explaining this rapid succession of CN and DN outbursts in V1047 Cen may be challenging within the framework of standard theories for DN outbursts. Following a CN eruption, the mass accretion rate is believed to remain high {$(\dot{M}\sim {10}^{-8}\,{M}_{\odot }\,{\mathrm{yr}}^{-1})$} for a few centuries, due to the irradiation of the secondary star by the still-hot surface of the white dwarf. Thus a DN eruption is not expected to occur during this high mass accretion phase a...
Propagation of Solar Energetic Particles in the Outer Heliosphere: Interplay between Scattering and Adiabatic Focusing
The turbulence and spatial nonuniformity of the guide magnetic field cause two competitive effects, namely, the scattering effect and the adiabatic focusing effect, respectively. In this work, we numerically solve the five-dimensional Fokker–Planck transport equation to investigate the radial evolutions of these important effects undergone by the solar energetic particles (SEPs) propagating through interplanetary space. We analyze the interplay process between the scattering and adiabatic focusing effects in the context of three-dimensional propagation, with special attention to the scenario of the outer heliosphere, in which some peculiar SEP phenomena are found and explained. We also discuss the radial dependence of the SEP peak intensities from the inner through the outer heliosphere, and conclude that it cannot be simply described by a single functional form such as R − α ( R is radial distance), which is often used.
Constraining Planetary Gas Accretion Rate from H α Line Width and Intensity: Case of PDS 70 b and c
Recent observations of protoplanets embedded in circumstellar disks have shed light on the planet formation process. In particular, detection of hydrogen Balmer-line (H α ) emission gives direct constraints on late-stage accretion onto gas giants. Very recently Haffert et al. measured the spectral line widths, in addition to intensities, of {${\rm{H}}\alpha $} emission from the two protoplanets orbiting PDS 70. Here, we study these protoplanets by applying radiation-hydrodynamic models of the shock-heated accretion flow onto protoplanets that Aoyama et al. have recently developed. As a result, we demonstrate that H α line widths combined with intensities lead to narrowing down the possible ranges of the protoplanetary accretion rate and/or mass significantly. While the current spectral resolution is not high enough to derive a definite conclusion regarding their accretion ...
Direct Detection of Solar Angular Momentum Loss with the Wind Spacecraft
The rate at which the solar wind extracts angular momentum (AM) from the Sun has been predicted by theoretical models for many decades, and yet we lack a conclusive measurement from in situ observations. In this Letter we present a new estimate of the time-varying AM flux in the equatorial solar wind, as observed by the Wind spacecraft from 1994 to 2019. We separate the AM flux into contributions from the protons, alpha particles, and magnetic stresses, showing that the mechanical flux in the protons is ∼3 times larger than the magnetic field stresses. We observe the tendency for the AM flux of fast wind streams to be oppositely signed to the slow wind streams, as noted by previous authors. From the average total flux, we estimate the global AM loss rate of the Sun to be 3.3 × 10 30 erg, which lies within the range of various magnetohydrodynamic wind models in the literature. This AM loss rate is a factor of ∼2 weaker than required for a Skumanich-like rotation ...
Mapping Electron Temperature Variations across a Spiral Arm in NGC 1672
We report one of the first extragalactic observations of electron temperature variations across a spiral arm. Using Multi Unit Spectroscopic Explorer mosaic observations of the nearby galaxy NGC 1672, we measure the [N ii ] λ 5755 auroral line in a sample of 80 H ii regions in the eastern spiral arm of NGC 1672. We discover systematic temperature variations as a function of distance perpendicular to the spiral arm. The electron temperature is lowest on the spiral arm itself and highest on the downstream side. Photoionization models of different metallicity, pressure, and age of the ionizing source are explored to understand what properties of the interstellar medium drive the observed temperature variations. An azimuthally varying metallicity appears to be the most likely cause of the temperature variations. The electron temperature measurements solidify recent discoveries of azimuthal variations of oxygen abundance based on ...
Molecular Gas in the Outflow of the Small Magellanic Cloud
We report the first evidence of molecular gas in two atomic hydrogen (H i ) clouds associated with gas outflowing from the Small Magellanic Cloud (SMC). We used the Atacama Pathfinder Experiment to detect and spatially resolve individual clumps of 12 CO(2 → 1) emission in both clouds. CO clumps are compact (∼10 pc) and dynamically cold (line widths {$\lesssim 1\,\mathrm{km}\,{{\rm{s}}}^{-1}$} ). Most CO emission appears to be offset from the peaks of the H i emission, some molecular gas lies in regions without a clear H i counterpart. We estimate a total molecular gas mass of {${M}_{\mathrm{mol}}\simeq {10}^{3}\mbox{--}{10}^{4}\,{M}_{\odot }$} in each cloud and molecular gas fractions up to 30% of the total...
The Shadow of a Spherically Accreting Black Hole
We explore a simple spherical model of optically thin accretion on a Schwarzschild black hole, and study the properties of the image as seen by a distant observer. We show that a dark circular region in the center—a shadow—is always present. The outer edge of the shadow is located at the photon ring radius {${b}_{\mathrm{ph}}\equiv \sqrt{27}{r}_{g}$} , where {${r}_{g}={GM}/{c}^{2}$} is the gravitational radius of the accreting mass M . The location of the shadow edge is independent of the inner radius at which the accreting gas stops radiating. The size of the observed shadow is thus a signature of the spacetime geometry and it is hardly influenced by accretion details. We briefly discuss the relevance of these results for the Event Horizon Telescope image of the supermassive black h...
Stellar Velocity Dispersion of a Massive Quenching Galaxy at z = 4.01
We present the first stellar velocity dispersion measurement of a massive quenching galaxy at z = 4. The galaxy is first identified as a massive z ≥ 4 galaxy with suppressed star formation from photometric redshifts based on deep multiband data. A follow-up spectroscopic observation with MOSFIRE on Keck revealed strong multiple absorption features, which are identified as Balmer lines, giving a secure redshift of z = 4.01. This is the most distant quiescent galaxy known to date. Thanks to the high S/N of the spectrum, we are able to estimate the stellar velocity dispersion, {$\sigma =268\pm 59\,\mathrm{km}\ {{\rm{s}}}^{-1}$} , making a significant leap from the previous highest redshift measurement at z = 2.8. Interestingly, we find that the velocity dispersion is consistent with that of massive galaxies today, implying no significant evolution in velocity ...
Dark Matter Signatures of Supermassive Black Hole Binaries
A natural consequence of the galaxy formation paradigm is the existence of supermassive black hole (SMBH) binaries. Gravitational perturbations from a far-away SMBH companion can induce high orbital eccentricities on dark matter (DM) particles orbiting the primary SMBH via the eccentric Kozai–Lidov mechanism. This process yields an influx of DM particles into the primary SMBH ergosphere, where test particles linger for long timescales. This influx results in high self-gravitating densities, forming a DM clump that is extremely close to the SMBH. In such a situation, the gravitational-wave (GW) emission between the dark matter clump and the SMBH is potentially detectable by LISA . If dark matter self-annihilates, the high densities of the clump will result in a unique codetection of GW emission and high-energy electromagnetic signatures.
The Galaxy’s Gas Content Regulated by the Dark Mark Halo Mass Results in a Superlinear M BH – M ⋆ Relation
Supermassive black holes (SMBHs) are tightly correlated with their hosts, but the origin of such connection remains elusive. To explore the cosmic buildup of this scaling relation, we present an empirically motivated model that tracks galaxy and SMBH growth down to z = 0. Starting from a random mass seed distribution at z = 10, we assume that each galaxy evolves on the star-forming “main sequence” (MS) and each BH follows the recently derived stellar mass ( M ⋆ ) dependent ratio between BH accretion rate and star formation rate, going as {$\mathrm{BHAR}/\mathrm{SFR}\propto {M}_{\star }^{0.73[+0.22,-0.29]}$} . Our simple recipe naturally describes the BH–galaxy buildup in two stages. At first, the SMBH lags behind the host that evolves along the MS. Later, as the galaxy grows in M ⋆ , our M ⋆ -dependent BHAR/SFR induces a ...
Wide Bandwidth Observations of Pulsars C, D, and J in 47 Tucanae
We report the first wide-band observations of pulsars C, D, and J in the globular cluster 47 Tucanae (NGC 104) using the Ultra-Wideband Low (UWL) receiver system recently installed on the Parkes 64 m radio telescope. The wide frequency range of the UWL receiver (704–4032 MHz), along with the well-calibrated system, allowed us to obtain flux density measurements and polarization pulse profiles. The mean pulse profiles have significant linear and circular polarization, allowing for determination of the Faraday rotation measure for each pulsar. Precise measurements of the dispersion measures show a significant deviation in the value for pulsar D compared to earlier results. Searches for new pulsars in the cluster are ongoing and we have determined optimal bands for such searches using the Parkes UWL receiver system.
Constraining X-Ray Coronal Size with Transverse Motion of AGN Ultra-fast Outflows
One of the canonical physical properties of ultra-fast outflows (UFOs) seen in a diverse population of active galactic nuclei is its seemingly very broad width (i.e., Δ v ≳ 10,000 km s −1 ), a feature often required for X-ray spectral modeling. While unclear to date, this condition is occasionally interpreted and justified as internal turbulence within the UFOs for simplicity. In this work, we exploit a transverse motion of a three-dimensional accretion disk-wind, an essential feature of nonradial outflow morphology unique to magnetohydrodynamic outflows. We argue that at least part of the observed line width of UFOs may reflect the degree of transverse velocity gradient due to Doppler broadening around a putative compact X-ray corona in the proximity of a black hole. In this scenario, line broadening is sensitive to the geometrical size of the corona, R c . We calculate the broadening factor as a function of coronal radius R ...
First Detection of Submillimeter-wave [ 13 C i] 3 P 1 – 3 P 0 Emission in a Gaseous Debris Disk of 49 Ceti with ALMA
We have detected the submillimeter-wave fine-structure transition ( 3 P 1 – 3 P 0 ) of 13 C, [ 13 C i ], in the gaseous debris disk of 49 Ceti with the Atacama Large Millimeter/submillimeter Array (ALMA). Recently, the [C i ] 3 P 1 – 3 P 0 emission has been spatially resolved in this source with ALMA. In this data set, the F = 3/2–1/2 hyperfine component of [ 13 C i ], which is blueshifted by 2.2 km s −1 from the normal species line, [C i ], has been identified in the outer part of the 49 Ceti disk, thanks to the narrow velocity widths of the gas components. The [C i ]/[ 13 C i ] line intensity ratio is found to be 12 ± 3, which is significantly lower than the 12 C/<...
Organosulfur Compounds Formed by Sulfur Ion Bombardment of Astrophysical Ice Analogs: Implications for Moons, Comets, and Kuiper Belt Objects
Carbon, hydrogen, nitrogen, oxygen, and sulfur are the main elements involved in the solid-phase chemistry of various astrophysical environments. Among these elements, sulfur chemistry is probably the least well understood. We investigated whether sulfur ion bombardment within simple astrophysical ice analogs (originating from H 2 O:CH 3 OH:NH 3 , 2:1:1) could trigger the formation of complex organosulfur molecules. Over 1100 organosulfur (CHNOS) molecular formulas (12% of all assigned signals) were detected in resulting refractory residues within a broad mass range (from 100 to 900 amu, atomic mass unit). This finding indicates a diverse, rich and active sulfur chemistry that could be relevant for Kuiper Belt objects (KBO) ices, triggered by high-energy ion implantation. The putative presence of organosulfur compounds within KBO ices or on other icy bodies might influence our view on the search of habitability and biosignatures.
The Interstellar Object ’Oumuamua as a Fractal Dust Aggregate
The first known interstellar object ’Oumuamua exhibited a nongravitational acceleration that appeared inconsistent with cometary outgassing, leaving radiation pressure as the most likely force. Barring the alien lightsail hypothesis, an ultra-low density due to a fractal structure might also explain the acceleration of ’Oumuamua by radiation pressure. In this Letter we report a decrease in ’Oumuamua’s rotation period based on ground-based observations, and show that this spin-down can be explained by the YORP effect if ’Oumuamua is indeed a fractal body with the ultra-low density of 10 −2 kg m −3 . We also investigate the mechanical consequences of ’Oumuamua as a fractal body subjected to rotational and tidal forces, and show that a fractal structure can survive these mechanical forces.
Erratum: “A New Equilibrium State for Singly Synchronous Binary Asteroids” ( 2018, ApJL, 857, L5 [] )
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A NICER Thermonuclear Burst from the Millisecond X-Ray Pulsar SAX J1808.4–3658
The Neutron Star Interior Composition Explorer ( NICER ) has extensively monitored the 2019 August outburst of the 401 Hz millisecond X-ray pulsar SAX J1808.4–3658. In this Letter, we report on the detection of a bright helium-fueled Type I X-ray burst. With a bolometric peak flux of (2.3 ± 0.1) × 10 −7 erg s −1 cm −2 , this was the brightest X-ray burst among all bursting sources observed with NICER to date. The burst shows a remarkable two-stage evolution in flux, emission lines at 1.0 and 6.7 keV, and burst oscillations at the known pulsar spin frequency, with ≈4% fractional sinusoidal amplitude. We interpret the burst flux evolution as the detection of the local Eddington limits associated with the hydrogen and helium layers of the neutron star envelope. The emission lines are likely associated with Fe, due to reprocessing of the burst emission in the accretion disk.
Inferring the Presence of Tides in Detached White Dwarf Binaries
Tidal interactions can play an important role as compact white dwarf (WD) binaries are driven together by gravitational waves (GWs). This will modify the strain evolution measured by future space-based GW detectors and impact the potential outcome of the mergers. Surveys now and in the near future will generate an unprecedented population of detached WD binaries to constrain tidal interactions. Motivated by this, I summarize the deviations between a binary evolving under the influence of only GW emission and a binary that is also experiencing some degree of tidal locking. I present analytic relations for the first and second derivative of the orbital period and braking index. Measurements of these quantities will allow the inference of tidal interactions, even when the masses of the component WDs are not well constrained. Finally, I discuss tidal heating and how it can provide complimentary information.
HSC-XD 52: An X-Ray Detected AGN in a Low-mass Galaxy at z ∼ 0.56
The properties of low-mass galaxies hosting central black holes provide clues about the formation and evolution of the progenitors of supermassive black holes. In this Letter, we present HSC-XD 52, a spectroscopically confirmed low-mass active galactic nucleus (AGN) at an intermediate redshift of z ∼ 0.56. We detect this object as a very luminous X-ray source coincident with a galaxy observed by the Hyper Suprime-Cam (HSC) as part of a broader search for low-mass AGN. We constrain its stellar mass through spectral energy distribution modeling to be LMC-like at M ⋆ ≈ 3 × 10 9 M ⊙ , placing it in the dwarf regime. We estimate a central black hole mass of M BH ∼ 10 6 M ⊙ . With an average X-ray luminosity of {${L}_{{\rm{X}}}\approx 3.5\times {10}^{43}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$} , HSC-XD ...
Varstrometry for Off-nucleus and Dual Sub-Kpc AGN (VODKA): How Well Centered Are Low- z AGN?
Off-nucleus active galactic nuclei (AGN) can be signposts of inspiraling supermassive black holes (SMBHs) on galactic scales, or accreting SMBHs recoiling after the coalescence of an SMBH binary or slingshot from three-body interactions. Because of the stochastic variability of AGN, the measured photocenter of an unresolved AGN-host system will display astrometric jitter that depends on the off-nucleus distance of the AGN, the total photometric variability of the system, and the AGN-host contrast. Here we use the precision astrometry from Gaia DR2 to constrain the off-nucleus population of a low-redshift (0.3 < z < 0.8) sample of unobscured broad-line AGN drawn from the Sloan Digital Sky Survey with significant host contribution and photometric variability. We find that Gaia DR2 already provides strong constraints on the projected off-nucleus distance in the sub-kiloparsec regime at these redshifts: 99%, 90%, and 40% of AGN must be well centered to <...
Ion Cyclotron Waves in Field-aligned Solar Wind Turbulence
The nature of the solar wind parallel fluctuations is investigated in this Letter by using magnetic helicity to characterize their polarization state at proton scales. Our aim is to assess the role of the proton cyclotron instability as a mechanism for generating ion cyclotron waves (ICWs) in solar wind turbulence. The wave polarization is found to depend strongly on the proton temperature anisotropy and on the power level of magnetic fluctuations at fluid scales. The results indicate a clear link between fluid and kinetic scales in the solar wind turbulence, allowing for a picture in which the resonant dissipation of high-frequency Alfvén waves heats protons in a direction perpendicular to the magnetic field, increasing their temperature anisotropy. The velocity distribution thus becomes unstable to the proton cyclotron instability, which then drives the local generation of ICWs in the solar wind.
The Influence of Gas-phase Chemistry on Organic Haze Formation
Although photochemically produced organic hazes are common in planetary atmospheres, there have been few experimental investigations of the gas-phase chemistry leading to organic haze formation. We report a laboratory study of the gas-phase compounds formed by far-ultraviolet irradiation of a CH 4 /N 2 mixture. Using high-resolution chemical ionization mass spectrometry (CIMS) we made in situ measurements of gas-phase products up to m/z 400. Organic nitrogen species dominate the mass spectra with smaller contributions from unsaturated hydrocarbons. Using a structural group method to estimate vapor pressures, we calculate that for compounds detected at m/z > 320, ≥50% of the total compound loading (gas + condensed phase) would be present in the condensed (aerosol) phase at 300 K. Using approximations for changes in vapor pressure with temperature, we estimate that ≥50% of the total loading for species with m/z > 110 would be in the ae...
The First Detection of a Low-frequency Turnover in Nonthermal Emission from the Jet of a Young Star
Radio emission in jets from young stellar objects (YSOs) in the form of nonthermal emission has been seen toward several YSOs. Thought to be synchrotron emission from strong shocks in the jet, it could provide valuable information about the magnetic field in the jet. Here we report on the detection of synchrotron emission in two emission knots in the jet of the low-mass YSO DG Tau A at 152 MHz using the Low-Frequency Array, the first time nonthermal emission has been observed in a YSO jet at such low frequencies. In one of the knots, a low-frequency turnover in its spectrum is clearly seen compared to higher frequencies. This is the first time that such a turnover has been seen in nonthermal emission in a YSO jet. We consider several possible mechanisms for the turnover and fit models for each of these to the spectrum. Based on the physical parameters predicted by each model, the Razin effect appears to be the most likely explanation for the turnover. From the Razin effect fit, ...
Comparing the Quenching Times of Faint M31 and Milky Way Satellite Galaxies
We present the star formation histories (SFHs) of 20 faint M31 satellites (−12 ≲ M V ≲ −6) that were measured by modeling sub-horizontal branch depth color–magnitude diagrams constructed from Hubble Space Telescope ( HST ) imaging. Reinforcing previous results, we find that virtually all galaxies quenched between 3 and 9 Gyr ago, independent of luminosity, with a notable concentration 3–6 Gyr ago. This is in contrast to the Milky Way (MW) satellites, which are generally either faint with ancient quenching times or luminous with recent (<3 Gyr) quenching times. We suggest that systematic differences in the quenching times of M31 and MW satellites may be a reflection of the varying accretion histories of M31 and the MW. This result implies that the formation histories of low-mass satellites may not be broadly representative of low-mass galaxies in general. Among the M31 satellite population we identify two distinct groups based on their SF...
Detection of CN Gas in Interstellar Object 2I/Borisov
The detection of interstellar objects passing through the solar system offers the promise of constraining the physical and chemical processes involved in planetary formation in other extrasolar systems. While the effect of outgassing by 1I/2017 U1 (’Oumuamua) was dynamically observed, no direct detection of the ejected material was made. The discovery of the active interstellar comet 2I/Borisov means spectroscopic investigations of the sublimated ices is possible for this object. We report the first detection of gas emitted by an interstellar comet via the near-UV emission of CN from 2I/Borisov at a heliocentric distance of r = 2.7 au on 2019 September 20. The production rate was found to be Q (CN) = (3.7 ± 0.4) × 10 24 s −1 , using a simple Haser model with an outflow velocity of 0.5 km s −1 . No other emission was detected, with an upper limit to the production rate of C 2 of 4 × 10 24 s −1 . The spectral re...
Detection of Pulses from the Vela Pulsar at Millimeter Wavelengths with Phased ALMA
We report on the first detection of pulsed radio emission from a radio pulsar with the Atacama Large Millimeter/submillimeter Array (ALMA) telescope. The detection was made in the Band-3 frequency range (85–101 GHz) using ALMA in the phased-array mode developed for VLBI observations. A software pipeline has been implemented to enable a regular pulsar observing mode in the future. We describe the pipeline and demonstrate the capability of ALMA to perform pulsar timing and searching. We also measure the flux density and polarization properties of the Vela pulsar (PSR J0835–4510) at millimeter wavelengths, providing the first polarimetric study of any ordinary pulsar at frequencies above 32 GHz. Finally, we discuss the lessons learned from the Vela observations for future pulsar studies with ALMA, particularly for searches near the supermassive black hole in the Galactic center, and the potential of using pulsars for polarization calibration of ALMA.
Round-trip Slipping Motion of the Circular Flare Ribbon Evidenced in a Fan-spine Jet
A solar jet that occurred on 2014 July 31, which was accompanied by a GOES C1.3 flare and a mini-filament eruption at the jet base, was studied by using observations taken by the New Vacuum Solar Telescope and the Solar Dynamic Observatory . Magnetic field extrapolation revealed that the jet was confined in a fan-spine magnetic system that hosts a null point at the height of about 9 Mm from the solar surface. An inner flare ribbon surrounded by an outer circular ribbon and a remote ribbon were observed to be associated with the eruption, in which the inner and remote ribbons, respectively located at the footprints of the inner and outer spines, while the circular one manifested the footprint of the fan structure. It is worth noting that the west part of the circular ribbon exhibited an interesting round-trip slipping motion, while the inner ribbon and the circular ribbon’s east part displayed a northward slipping motion. Our analysis results indicate that the slippi...
Four Newborn Planets Transiting the Young Solar Analog V1298 Tau
Exoplanets orbiting pre-main-sequence stars are laboratories for studying planet evolution processes, including atmospheric loss, orbital migration, and radiative cooling. V1298 Tau, a young solar analog with an age of 23 ± 4 Myr, is one such laboratory. The star is already known to host a Jupiter-sized planet on a 24 day orbit. Here, we report the discovery of three additional planets—all between the sizes of Neptune and Saturn—based on our analysis of K2 Campaign 4 photometry. Planets c and d have sizes of 5.6 and 6.4 {${R}_{\oplus }$} , respectively, and with orbital periods of 8.25 and 12.40 days reside 0.25% outside of the nominal 3:2 mean-motion resonance. Planet e is 8.7 {${R}_{\oplus }$} in size but only transited once in the K2 time series and thus has a period longe...
Intermittent Heating in the Magnetic Cloud Sheath Regions
Coherent structures such as current sheets have been usually regarded to be sites of proton heating in the solar wind. In this Letter, we statistically investigate the proton heating effects around the coherent structures within the turbulent sheath regions of magnetic clouds (MCs) based on WIND observations. It is found that the proton temperature enhancement near coherent structures in the MC sheath is not as remarkable as in the solar wind. Significant temperature increase only exists near coherent structures with great directional changes (>45°) in magnetic field or intensity changes (≥10% of the mean magnetic field magnitude), which merely account for 13% of the total of 12,426 identified intermittent events in the 71 studied MC sheaths. The temperature increment is more evident near strong current sheets with great directional changes (>45°) at smaller scales than those at larger scales. It suggests that the heating effects in the MC sheath regions are likely ...
The Evolution of Baryonic Mass Function of Galaxies to z = 3
We combine the published stellar mass function (SMF) and gas scaling relations to explore the baryonic (stellar plus cold gas) mass function (BMF) of galaxies to redshift z = 3. We find evidence that at log( M baryon / M ☉ ) > 11.3, the BMF has evolved little since z ∼ 2.2. With the evolution of BMF and SMF, we investigate the baryon net accretion rate ( {${\dot{\rho }}_{\mathrm{baryon}}$} ) and stellar mass growth rate ( {${\dot{\rho }}_{\mathrm{star}}$} ) for the galaxy population of log( M star / M ☉ ) > 10. The ratio between these two quantities, {${\dot{\rho }}_{\mathrm{baryon}}$} / ##IMG## {${\d...}
First Detection of Plasmoids from Breakout Reconnection on the Sun
Transient collimated plasma ejections (jets) occur frequently throughout the solar corona, in active regions, quiet Sun, and coronal holes. Although magnetic reconnection is generally agreed to be the mechanism of energy release in jets, the factors that dictate the location and rate of reconnection remain unclear. Our previous studies demonstrated that the magnetic breakout model explains the triggering and evolution of most jets over a wide range of scales, through detailed comparisons between our numerical simulations and high-resolution observations. An alternative explanation, the resistive-kink model, invokes breakout reconnection without forming and explosively expelling a flux rope. Here we report direct observations of breakout reconnection and plasmoid formation during two jets in the fan-spine topology of an embedded bipole. For the first time, we observed the formation and evolution of multiple small plasmoids with bidirectional flows associated with fast reconnectio...
Auroral Beads at Saturn and the Driving Mechanism: Cassini Proximal Orbits
During the Grand Finale Phase of Cassini , the Ultraviolet Imaging Spectrograph on board the spacecraft detected repeated detached small-scale auroral structures. We describe these structures as auroral beads, a term introduced in the terrestrial aurora. Those on DOY 232 2017 are observed to extend over a large range of local times, i.e., from 20 LT to 11 LT through midnight. We suggest that the auroral beads are related to plasma instabilities in the magnetosphere, which are often known to generate wavy auroral precipitations. Energetic neutral atom enhancements are observed simultaneously with auroral observations, which are indicative of a heated high pressure plasma region. During the same interval we observe conjugate periodic enhancements of energetic electrons, which are consistent with the hypothesis that a drifting interchange structure passed the spacecraft. Our study indicates that auroral bead structures are common phenomena at Earth and giant planets, which pr...
Lorentz Force Evolution Reveals the Energy Build-up Processes during Recurrent Eruptive Solar Flares
The energy release and build-up processes in the solar corona have significant implications in particular for the case of large recurrent flares, which pose challenging questions about the conditions that lead to the episodic energy release processes. It is not yet clear whether these events occur due to the continuous supply of free magnetic energy to the solar corona or because not all of the available free magnetic energy is released during a single major flaring event. In order to address this question, we report on the evolution of photospheric magnetic field and the associated net Lorentz force changes in ARs 11261 and 11283, each of which gave rise to recurrent eruptive M- and X-class flares. Our study reveals that after the abrupt downward changes during each flare, the net Lorentz force increases by (2–5) × 10 22 dyne in between the successive flares. This distinct rebuild-up of net Lorentz forces is the first observational evidence found in the evolution of a...
Collapse of the General Circulation in Shortwave-absorbing Atmospheres: An Idealized Model Study
The response of the general circulation in a dry atmosphere to various atmospheric shortwave absorptivities is investigated in a three-dimensional general circulation model with gray radiation. Shortwave absorption in the atmosphere reduces the incoming radiation reaching the surface but warms the upper atmosphere, significantly shifting the habitable zone toward the star. The strong stratification under high shortwave absorptivity suppresses the Hadley cell in a manner that matches previous Hadley cell scalings. General circulation changes may be observable through cloud coverage and superrotation. The equatorial superrotation in the upper atmosphere strengthens with the shortwave opacity, as predicted based on the gradient wind of the radiative–convective equilibrium profile. There is a sudden drop of equatorial superrotation at very low shortwave opacity. This is because the Hadley cell in those cases are strong enough to fill the entire troposphere with zero momentum air fro...
GROWTH on S190425z: Searching Thousands of Square Degrees to Identify an Optical or Infrared Counterpart to a Binary Neutron Star Merger with the Zwicky Transient Facility and Palomar Gattini-IR
The third observing run by LVC has brought the discovery of many compact binary coalescences. Following the detection of the first binary neutron star merger in this run (LIGO/Virgo S190425z), we performed a dedicated follow-up campaign with the Zwicky Transient Facility (ZTF) and Palomar Gattini-IR telescopes. The initial skymap of this single-detector gravitational wave (GW) trigger spanned most of the sky observable from Palomar Observatory. Covering 8000 deg 2 of the initial skymap over the next two nights, corresponding to 46% integrated probability, ZTF system achieved a depth of ≈21 m AB in g - and r -bands. Palomar Gattini-IR covered 2200 square degrees in J -band to a depth of 15.5 mag, including 32% integrated probability based on the initial skymap. The revised skymap issued the following day reduced these numbers to 21% for the ZTF and 19% for Palomar Gattini-IR. We narrowed 338,646 ZTF transient “alerts” over the first t...
The Nuclear Filaments inside the Circumnuclear Disk in the Central 0.5 pc of the Galactic Center
We present CS(7–6) line maps toward the central parsec of the Galactic center, conducted with the Atacama Large Millimeter/submillimeter Array. The primary goal is to find and characterize the gas structure in the inner cavity of the circumnuclear disk (CND) in high resolution (1.″3 = 0.05 pc). Our large field-of-view mosaic maps—combining interferometric and single-dish data that recover extended emission—provide a first homogeneous look to resolve and link the molecular streamers in the CND with the neutral nuclear filaments newly detected within the central cavity of the CND. We find that the nuclear filaments are rotating with Keplerian velocities in a nearly face-on orbit with an inclination angle of ∼10°–20° (radius ≤ 0.5 pc). This is in contrast to the CND which is highly inclined at ∼65°–80° (radius ∼2–5 pc). Our analysis suggests a highly warped structure from the CND to the nuclear filaments. This result may hint that the nuclear filaments and the CND were created by d...
How to Fuel an AGN: Mapping Circumnuclear Gas in NGC 6240 with ALMA
Dynamical black hole mass measurements in some gas-rich galaxy mergers indicate that they are overmassive relative to their host galaxy properties. Overmassive black holes in these systems present a conflict with the standard progression of galaxy merger–quasar evolution; an alternative explanation is that a nuclear concentration of molecular gas driven inward by the merger is affecting these dynamical black hole mass estimates. We test for the presence of such gas near the two black holes in NGC 6240 using long-baseline ALMA Band 6 observations (beam size 0.″06 × 0.″03 or 30 pc × 15 pc). We find (4.2–9.8) × 10 7 M ☉ and (1.2–7.7) × 10 8 M ☉ of molecular gas within the resolution limit of the original black hole mass measurements for the north and south black holes, respectively. In the south nucleus, this measurement implies that 6%–89% of the original black hole mass measurement actually comes from molecular gas, resolvin...
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